MSc Chemistry

Track Molecular Sciences

Literature Thesis

Recent advances in chemical recycling techniques of waste

polyester/cotton blends

by

Floris Blom

11296062

9 July 2021

Research Institute

Van ‘t Hoff Institute for Molecular Sciences

Research Group

Industrial Sustainable Chemistry

Supervisor

prof. Gert-Jan Gruter

Second examiner

prof. Jan van Maarseveen

Daily Supervisor

dr. Gerard van Klink

2

Abstract

Over the past decades the world population has grown steadily and the textile industry has grown

accordingly with it. The large growth of textile use and the linearity of the system pose an environmental

threat by both its production and its growing generation of waste. Therefore reuse and recycling of textile

waste and moving towards a circular economy is necessary. However, mechanical recycling is difficult

due to diverse amounts of fiber blends on the market. In this literature research the recently emerging

chemical recycling methods of mixtures made from polyester and cotton have been reviewed. Four

different methods were identified: depolymerization of polyester, depolymerization of cellulose,

dissolution of polyester and dissolution of cellulose. In all of these methods the other material remained

mostly intact. Though in some cases breakdown to (reusable) smaller constituents was observed.

Adaptation of existing processes of wood pulp cellulose fiber generation has also been investigated,

especially for the viscose and lyocell processes. Besides these processes, hydrolysis was predominantly

researched in the depolymerization methods including acid, alkaline or enzymatic processes.

Furthermore, commercialized methods have also been discussed as well as life cycle assessments on the

environmental impact of some of the researched methods. Direct dissolution by ionic liquids, a class of

solvents based on molten salts, was shown to be the most promising technique, specifically guanidine-

based ionic liquids. Although, there are still some limitations before this technique will be adapted on a

large scale. Future developments may include more research on the life cycle and economic benefits of

the guanidine-based ionic liquids. Moreover, a combination of reuse, open loop and closed loop

recycling should be implemented in textile waste recycling to utilize textile waste as efficiently as

possible, in which almost all described methods could play an important role.

3

Table of Contents

1. Introduction _____________________________________________________________ 4

1.1 Textile waste ________________________________________________________________________ 4 1.2 Reuse and recycling __________________________________________________________________ 5 1.3 Scope and Outline ____________________________________________________________________ 8

2 Chemical recycling methods _________________________________________________ 9

2.1 Cellulose Recycling ___________________________________________________________ 9 2.1.1 Cellulose structure __________________________________________________________________ 9 2.1.2 Fiber Spinning processes ____________________________________________________________ 11 2.1.3 Cotton Depolymerization and Regeneration ____________________________________________ 12 2.1.4 Enzymatic hydrolysis _______________________________________________________________ 14 2.1.5 Alkaline pretreatment for enzymatic hydrolysis _________________________________________ 15 2.1.6 Acid pretreatment for enzymatic hydrolysis ____________________________________________ 17 2.1.7 Alkaline hydrolysis _________________________________________________________________ 18 2.1.8 Acid Hydrolysis ____________________________________________________________________ 18

2.2 Recycling with Conservation of Polyester and Cellulose. ____________________________ 22 2.2.1 Adaptation of the Lyocell Method for Recycling of Polycotton ______________________________ 22 2.2.2 Dissolution of cellulose in ionic liquids _________________________________________________ 23 2.2.3 Methods to increase cellulose dissolution in ionic liquids __________________________________ 26 2.2.4 Cellulose regeneration as fibers from IL processes _______________________________________ 27

2.3 Polyester Recycling __________________________________________________________ 30 2.3.1 PET structure and virgin PET Synthesis _________________________________________________ 30 2.3.2 PET fiber spinning _________________________________________________________________ 30 2.3.3 PET Depolymerization with Conservation of Cellulose ____________________________________ 31

2.4 Commercialized recycling processes ____________________________________________ 34 2.4.1 Cellulose recycling _________________________________________________________________ 34 2.4.2 Polyester recycling _________________________________________________________________ 38

3 Life Cycle Assessments _____________________________________________________ 41

3.1 Viscose and Lyocell LCA ______________________________________________________________ 42 3.2 Pretreatment of enzymatic hydrolysis LCA _______________________________________________ 43 3.3 Enzymatic hydrolysis LCA _____________________________________________________________ 43 3.4 NMMO & IL LCA ____________________________________________________________________ 45 3.5 LCA of commercialized processes ______________________________________________________ 45

4 Discussion _______________________________________________________________ 49

4.1 Polycotton waste separation __________________________________________________________ 50 4.2 Cellulose fiber regeneration ___________________________________________________________ 52 4.3 Limitations ________________________________________________________________________ 54

5 Concluding remarks and future outlook _______________________________________ 56

6. References ______________________________________________________________ 57

4

1. Introduction The world population has grown steadily over the past decades and therefore the textile industry as well.

In the coming 10 years the use of textile fibers is expected to increase with 3% annually.1 This increase

in demand has already led to a market supply of over 120 million tons of virgin textile fibers in 2020.1,2

Worldwide per capita use of textile has increased from 5.9 kg in 1975 to 16 kg in 2019. Synthetic (mostly

petroleum based) fibers account for approximately 60 wt% of the market share of textile production,

with cotton coming in second at around 24 wt%.3 The largest growth in textile use comes from

households with increased income, particularly from developing countries.4 Another, more recent,

pattern of consumption that heavily influences the purchase of textiles is the concept of ‘fast fashion’.3

This phenomenon drives the clothing and apparel industry to design and deliver less durable clothing,

which in turn increases production of textiles drastically.5

Furthermore, the growth of the textile industry has led to outsourcing of textile production by

western companies towards developing countries. In these countries, (underaged) workers work under

extremely harsh conditions with long hours and low pay.6,7 Despite many efforts to improve these

working conditions it is still a relevant topic in current times, mostly due to the emergence of fast

fashion.

The growth in textile production is also closely correlated to environmental challenges, such as

solid waste generation, high water usage, high energy demand, and pesticide and fertilizer use.3,5,8

Moreover, the industry uses mostly non-renewable resources, pollutes the ocean with microfibers and

discharges water streams containing hazardous compounds.9 The growing demand also leads to

production issues of renewable natural fibers. For the production of cotton, for instance, it is not

expected that the demands can be met, due to restrictions on irrigation and farmland use.10

1.1 Textile waste The generated solid textile waste can be divided into two main groups: post-consumer textile

waste and pre-consumer textile waste.3 The former is the waste created during usage and disposal by

consumers, which creates the largest part of the textile waste. This waste consists of different blends of

fibrous materials. The latter, pre-consumer textile waste, refers to all of the waste created during the

industrial textile production process. It is estimated that around 10 to 15 percent of solid textile waste is

generated during cutting in pre-consumer processes (Figure 1).9,11 Pre-consumer textile waste is related

closely to the raw materials used in the processes. These raw materials are divided in three main groups:

cellulose fiber, protein fiber and synthetic fiber.12 Cellulose fibers consist of plant materials such as

cotton, hemp and ramie. Protein fibers are produced from animals and include wool, cashmere and silk.

Synthetic fibers are produced from petroleum-based compounds and include polyesters (e.g.

polyethylene terephthalate (PET)), nylon and polypropene). Blends of these raw materials are common

in the textile industry, due to their differences in characteristics.13 For instance, the mechanical properties

of cotton include softness and high water absorbency, while the properties of PET contribute to

durability and strength. These contrasting and consequently complementary properties make blends of

PET and cotton the most commonly produced textile materials. Which exist in varying compositions,

ranging from 99:1 and 1:99 cotton to polyester ratios, with 65/35 and 50/50 cotton to polyester ratio

being the most common to balance durability and price.14 These materials are generally referred to as

polycotton.9,13,15 All of the mentioned raw materials require specific chemicals and materials during their

production processes and, thereby create different by-products and streams of textile waste.12

One of the largest problems with textile waste is that the textile system is very linear (e.g.

clothing life cycles). In 2015 the assumption was made that less than 1% of materials used to produce

clothing was recycled into new clothing.9 Figure 1 shows an overview of the general life cycle of textile.

Roughly 60% of textile fibers are used in the apparel industry, the rest is used in other applications such

as household textiles (e.g. carpets, drapes, and towels), or workplace textiles (e.g. backpacks, tents, and

cleaning rags). Almost all materials come from virgin feedstocks and, as mentioned before, roughly 12%

of fibers and/or (finished) products are lost during the manufacturing process. After the use phase a

small portion is used for recycling, which can be divided into two groups: recycling of clothes for new

clothes and recycling of clothes for other applications such as insulation material or mattress stuffing.

5

However most of the collected clothing ends up in landfills (e.g. in the United States) or is incinerated

to produce energy (e.g. in Western European Countries).15 These percentages should change drastically

in order to meet the rising demand of textiles and diminish the environmental impact of the production

processes.

Figure 1: Global material flows in garment industry in 2015.9 60% of annual fiber production is used

in clothing (53 million tons), the rest is used in other applications such as household textiles (e.g.

carpets, drapes, and towels), or workplace textiles (e.g. backpacks, tents, and cleaning rags).

1.2 Reuse and recycling One possible way to accomplish this is through the concept of the circular economy. This concept has

gained more attention over the past few years and has become an important tool to move towards a more

sustainable future.16 This concept is based on the Lansink Ladder, proposed by Dutch politician Ad

Lansink. The Lansink Ladder shows a hierarchical system for waste management.17,18 On top it starts

with waste prevention, by design of new materials and process innovation. Followed up by preparation

for reuse. The next step would be recycling, if reuse is not possible. Finally, when recycling is not

possible either, incineration could be used as a form of energy recovery. Landfill should be avoided,

however, it is necessary when none of the aforementioned methods have an effect on the waste. Waste

prevention and reuse should have priority within circular textile cycles. Some companies have put

forward some initiatives to achieve this (e.g. H&M19, Patagonia20, PUMA21).

Figure 2: The Lansink Ladder (shaped as staircase): A hierarchy for waste management proposed by

the Dutch politician Ad Lansink in 1979, with the highest priority options on the top.18 Adapted from

van Meerbeek17

6

Reuse refers to an existing product being used again within that particular product chain. When taking

a closer look at recycling we can see that the term recycling, on the other hand, refers to the breakdown

of products into their raw materials in order for that raw material to be reclaimed and used in new

products.8 Textile recycling may involve the aforementioned pre-consumer and post-consumer waste.

Four different recycling approaches can be identified: primary, secondary, tertiary, and quaternary

recycling.22 Primary recycling includes pre-consumer waste recycling, mostly from industrial cutoffs,

which are reintroduced into the manufacturing process. Secondary recycling involves collection and

recycling of post-consumer textile wastes such as household textiles and clothing. After collection of

these textile wastes they are separated and sorted. High quality items are considered for resale, while

the rest is used in mechanical recycling processes. Tertiary recycling involves chemical breakdown of

the textiles into monomers or fuels, this requires sorted, clean synthetic materials such as PET or nylon,

since these are suitable for repolymerization without loss of characteristics. Quaternary recycling refers

to energy recovery of the textiles by incineration.

After recycling, the raw materials can enter a new product stream. Which new stream the

products enter can be further classified as closed-loop recycling or open-loop recycling.8 Open-loop

recycling comprises a system in which the raw material is broken down at the end of its life cycle and

then used as starting material for a second, often unrelated product system. In general this second product

is not recycled but disposed in either landfill or incineration (Figure 3).23 It is the most common

technique used in textile recycling, contributing to recycling of nearly 12% of the created textile waste

(Figure 1).9 This process is commonly known as downcycling, since the recycled products are usually

of lower value than the original fibers or clothing.19 Examples of downcycling include recycling of PET

bottles or food packaging to clothing,25 as well as downcycling used denim for insulation purposes26

Figure 3: Open-loop recycling process. A recycling process in which the material at the end of the life

cycle of product A is used as the raw input material of product B (the ‘cradle’. At the end of the life

cycle of product B the material is disposed in either landfill or incineration (the ‘grave’). 8

In contrast, closed-loop recycling refers to the collection of textiles and reprocessing them into new

fibers that can be re-used in new garments, thereby re-entering the same production cycle from which

the textile came.27 There are three different ways of closed-loop recycling in the apparel industry: pre-

consumer waste, post-consumer waste and cradle to cradle (C2C) recycling. Reuse also falls under

closed-loop recycling. In the C2C approach waste is collected, separated and then used again in products

of the same or higher value (Figure 4).28 In this approach waste is processed by either biological or

technical methods. Biological waste can be composted, while technical waste can be reused by

mechanical or chemical recycling. Technical closed-loop recycling uses mostly synthetic products that

are not biodegradable. Polymer based fibers such as polyester, acrylic and nylon are some examples of

this in textile recycling.8 However, cellulose fibers such as cotton can also fall under technical recycling,

when they are not able to biodegrade safely due to use of toxins or dyes in their production processes.

7

Figure 4: Closed-loop recycling, the cradle to cradle approach. In this process the waste of a product

at the end of the life cycle is collected, separated and then used as input material in products of the

same or higher value.8,28

Through the open-loop and closed-loop recycling the value of textile products can be captured at

different levels (Figure 5).9,25 This means that, depending on context, a given product can contain a

recycling route that may be referred to as either closed- or open-loop recycling. For example, in a

business to business context, a fiber or a fabric is considered a final product as it is traded in the same

form. However, in the eyes of the consumer or a retailer, that same fiber or fabric can be defined as a

starting material, since garments are key textile products in that setting.25 This latter view of the product

implies that the closed-loop recycling relies for instance on a pair of jeans being remade into another

pair of jeans. In contrast, when a more lax definition is handled for closed-loop recycling it could imply

that a material category (i.e. packaging), is recycled into a the same material category instead of another

material category (i.e. textiles, as seen in the aforementioned PET bottle to fiber recycling).

Sandin and Peters25 arrange recycling routes based on the disassembly of the recovered material.

When the fabric of a product is reused in new products this is referred to as fabric recycling. In the case

that the fabric is disassembled but the original fibers are preserved and reused this is called fiber

recycling. This is mostly performed by sorting garments and shredding them in a mechanical process,

of which a large part is used in downcycling processes. Polymer recycling is the process where the fibers

are taken back to the polymer level, which destroys the fibers but leaves the chemical structure of the

material intact.9 There are two distinguishable variants in terms of output quality and methods.25 The

first is mechanical polymer recycling, which is carried out by melting and extrusion of textiles made

from synthetic, mono-plastic based fibers. This process is unable to filter out dyes and contaminants.

The second method is chemical polymer recycling, in which textiles are dissolved with chemicals after

the garments have been de-zipped and shredded. The final process is also referred to as monomer

recycling if the textiles are completely broken down to their constituent monomers, by a combination of

mechanical and/or chemical methods.

These monomers can be applied for the assembly of new, virgin quality fibers, as well as being

applied to other fields such as biofuel synthesis29, concrete fabrication30, and green packaging.31 An

overview of these different methods is shown in Figure 5.

8

Figure 5: Different reuse and recycling methods applicable in the textile industry. Recycling can be

divided into different categories based on the level of decomposition (i.e. fabric, fibre, polymer, or

monomer recycling).25

The aforementioned processes of monomer, and polymer recycling are also referred to as chemical

recycling. In chemical recycling the three different fiber types (i.e. cellulose, protein and synthetic) are

recycled via separate routes. The cellulosic and protein fibers are dissolved, while the synthetic fibers

are depolymerized. Dissolving cellulosic fibers happens through alkaline/urea,32 or ionic liquid (IL)

solvent systems.33 In this way the degree of polymerization (DP) of the cellulose is decreased.34 For

synthetic fiber depolymerization the most common techniques are acid,35 alkaline,15,36 or enzymatic

hydrolysis.37 These techniques are especially widely studied and reviewed for PET recycling, due to the

large volumes of PET bottles and their subsequent waste problem.29,38,39 Other techniques such as

alcoholysis40, glycolysis41 and the use of quaternary ammonium salts42 have also been of academic

interest.

1.3 Scope and Outline There is an important aspect of textiles to take into account when discussing recycling: more than a third

of post-consumer textile waste consists of blended materials.43 There are several types of blending, such

as intimate blending, where a specific weight ratio of fibers is used in the manufacturing of spun yarns.15

Other blending methods are fabric mixtures where different yarns are used for the manufacturing

processes of warp and weft and mixture yarns, which are made by plying yarns together. Consequently

these blended textiles are difficult to be reused or recycled. As mentioned before, blends of polyester

and cotton (polycotton) are the most common.13 Therefore this review will focus on the recent chemical

recycling methods for polycotton blends with varying compositions. Chemical recycling of polycotton

is possible, however, to achieve this cotton and PET must be separated, which requires relatively pure

waste streams for commercial recycling processes.44 There are four main separation strategies employed

in research. First is dissolution of PET by using a suitable solvent.45 Second and third are

depolymerization of one of the two polycotton components using acidic, alkaline or enzymatic catalysts,

in this way the other component is left unaffected by the catalyst.13 The fourth strategy is dissolution of

9

the cellulose in a suitable solvent, followed up by PET separation and regeneration of the dissolved

cellulose using a suitable anti-solvent.34

Looking deeper in the recycling methods there is an increased cost and environmental impact.

For instance mechanical shredding to recycle fibers requires a lot less energy and water than chemical

recycling to create monomers or polymers.46 On the other hand these monomer and polymer recycling

methods can create higher quality fibers that will last longer during their second use phase.9 Therefore

methods to assess the environmental benefits and impacts of various textile reuse and recycling

processes have been developed. One of those methods is life cycle assessment (LCA). LCAs are widely

used in different contexts and have a range of parameters that can be assessed.25 These parameters

include, but are not limited to, climate change, energy use, water use/depletion and human toxicity. In

this way the environmental impact of the different processes and product life cycles can be compared to

each other. Furthermore it can give both quantitative and qualitative information on the assessed

approaches.8 Most LCAs on textiles have focused on the mechanical recycling and reuse of textiles.

However in recent years more LCAs were conducted that have focused on chemical recycling, both

closed- and open-looped.44

In line with this recent focus on chemical recycling this literature research aims to give an

overview of the emerging chemical techniques developed for polycotton recycling from textiles and

compare their environmental impact. This will be done by giving a detailed explanation of processes

developed in the past decade by both companies and academic research institutes. This will be followed

by a comparison of available LCAs for the described methods. Finally the techniques are critically

discussed, by showing the up- and downsides of each and finding out if there are knowledge gaps and

limitations. This is done to provide a summary of the current knowledge and point out areas for further

research.

2 Chemical recycling methods As mentioned before there are multiple separation strategies for chemical polycotton recycling. The

three main separation strategies are dissolution of PET, depolymerization of one of the two polycotton

components, and dissolution of cellulose.34 The most researched method is cellulose depolymerization,

although dissolution of cellulose has been adapted in some industrial processes already, albeit in the

form of biomass processing (e.g. wood pulp). An emerging method is the use of ionic liquids for

cellulose dissolution.47 All of these methods will be described more thoroughly throughout the next

section. First recent advances in cellulose depolymerization will be discussed, followed up by those in

ILs, and finally PET depolymerization techniques will be elaborated upon.

2.1 Cellulose Recycling

2.1.1 Cellulose structure

Figure 6: Microstructure of cotton fiber at various scales. Underneath the cuticle are primary and

secondary walls (segregated by a winding layer), which protect the lumen. The secondary walls

contain microfibrils, made up of glycoproteins and glucose polymers in the form of α-cellulose.48,49

10

Cotton, as mentioned before, is the most widely used natural fiber in textile products.3 The virgin fiber

structure is shown in Figure 6. A cotton fiber contains a multi-layered cell wall structure, which is further

defined in a microstructure. This microstructure and most important physiochemical properties have

been explained in great detail.48,49 A small summary, however, could be useful to better understand the

recycling methods described in the following sections. As shown in Figure 6 the lumen, the inner part

of the fiber originally called the ‘living’ part of the cell, is filled with liquid and protected by the cuticle

(or epidermis), primary wall, a winding layer, and secondary walls.3 In every layer of the cell wall, the

microfibrils, fiber-like strands made up of glycoproteins and glucose, form into spiraling bundles which

align at different directions of the cotton cell.48 The lumen in matured cotton cells (i.e. dried cells)

becomes a hollow space inside the broken down cell wall.

Figure 7: Chemical structure of (a) Cellulose, with n repeating cellobiose units between brackets, and

(b) Hemicellulose. Adapted from Tezara et al.50

The major component of the cotton fiber is α-cellulose, ranging between 88.0 and 96.5% of the total

fiber composition. Most of it is found in the secondary walls, with a high DP of 14000 and contributing

to almost 100% of the material in these walls. Conversely, the primary wall of the cotton fiber is made

up of 30% α-cellulose, with a DP between 2000 and 6000. The rest of the primary wall is made up of

non-cellulosic polymers, sugars and various proteins.49

Both α-cellulose and hemicellulose are biopolymers containing polysaccharide chains of

glucose. However, xylose, mannose, galactose, rhamnose, and arabinose are also incorporated in the

polymer chain of hemicellulose, while this is not the case for α-cellulose.50 Hemicellulose is more

branched than α-cellulose, as shown above by their chemical structures in Figure 7.

Figure 8: Configuration of order (crystalline) and disordered (amorphous) regions in cellulose

microfibrils. Adapted from Tayeb et al.51

The long cellulose chains are constructed in different orders of microfibrils, shown in Figure 8. In the

crystalline regions, the cellulose chains are highly organized and linked together in a specific order by

hydrogen bonds and van der Waals forces.52 In contrast, the amorphous region consists of microfibrils

that are either twisted or more disordered. Amorphous cellulose is therefore more easily hydrolyzed by

enzymes, such as cellulase, and these regions have often been considered to be the ‘weak points’ of the

11

cellulose part of fiber in higher plants.53 The chain of α-cellulose consists of anhydroglucose units

(AGUs) which are bonded by intermolecular hydrogen bonds, forming repeating cellobiose, a dimer of

glucose molecules, as seen in between the brackets of Figure 6 and Figure 7a.

Cellulose can be divided into two types based on the orientation of inter- and intramolecular

hydrogen bonds: cellulose I and cellulose II, as shown in Figure 9.54 These inter- and intramolecular

hydrogen bonds make the polymer chains group together in a highly ordered structure. Cellulose I and

II differ by these hydrogen bonds, resulting in different packings: parallel and antiparallel, respectively.

The main intramolecular O3H–O5’ hydrogen bond is shared by both polymorphs. The intramolecular

O2H–O6’ hydrogen bond only occurs in cellulose I. Cellulose I possesses O6H–O3” intermolecular

hydrogen bonds whereas cellulose II contains O6H–O2” intermolecular hydrogen bonds.55,56 The crystal

(or lattice) system of Cellulose I contains a triclinic unit cell including one polymer chain.54,57 In contrast,

cellulose II contains a monoclinic unit cell including two antiparallel polymer chains. Cellulose II is

considered to be more stable and can be irreversibly formed from cellulose I by mercerization, a process

which improves dye uptake and tear strength and also reduces fabric shrinkage by treatment of the

cellulose with alkali reagents.

Figure 9: Supramolecular distinction between cellulose I and cellulose II lies in inter- and

intramolecular hydrogen bonds.54 Cellulose I and II differ by these hydrogen bonds, resulting in

different packings: parallel and antiparallel, respectively.

2.1.2 Fiber Spinning processes In order to recycle cotton into fibers they have to be re-spun, after the dissolution or depolymerization

of cellulose. This spinning to new fibers can be performed in two different ways: wet spinning and dry-

jet wet spinning.58 In the former, cellulose fibers are formed from precipitation of dissolved polymers

through immersion in a coagulation liquid, an anti-solvent for the cellulose, where phase inversion

occurs. The liquid is extruded from a spinneret at elevated temperatures and immediately submerged in

the coagulation bath and is converted into two phases. After coming out of the bath the fibers precipitate

and solidify. Subsequently the fibers are stretched on a rotating drum. The dry-jet wet spinning on the

other hand is a type of wet spinning process where the filaments pass through a short airgap before

entering the coagulation bath from which the fibers are extracted. Both methods are shown in Figure 10.

Many parameters influence the mechanical properties of the fibers, such as the DP of cellulose,

the concentration of the spinning dope, rheology of the cellulose solution, and the drawing ratio in the

air gap.59 Besides that, the spinnability of the solution is influenced by the distribution of cellulose chains

and the dynamic modulus of the spinning dope (the ratio of stress to strain under shear, compression, or

elongation conditions, also known as E modulus).60 Furthermore, the molar mass distribution, the dope

concentration, and the stretching rate also affect the fiber properties.61,62 Moreover, fiber tenacity, a

physical property of fibers describing their sturdiness (expressed in cN tex-1,with tex as a unit for fiber

weight over length), could be affected by temperature.63 Increasing the temperature of the coagulation

bath, for instance, leads to a decrease in fiber tenacity.

12

Figure 10: Common systems of a) wet

spinning, and b) dry-jet wet spinning. In

dry-jet wet spinning, the polymer solution

is extruded through an air gap before the

coagulation bath, resulting in higher

molecular alignment compared to

conventional wet spinning.64

2.1.3 Cotton Depolymerization and Regeneration In search of an expansion of fiber production in the textile industry, with cotton showing limited growth

potential, man-made cellulosic fibers were researched. Early research on cotton depolymerization

focused on two different processes: derivatization and direct dissolution (non-derivatizing).65 Examples

of the former are viscose and cellulose nitrite (Figure 11). An example of the latter is the lyocell process.

In the derivatizing processes the chemical structure of the starting cellulose is modified by

addition of a reagent that forms an intermediate such as cellulose acetate or xanthate (i.e. viscose).66

Subsequently these intermediate compounds are processed and dissolved followed by regeneration of

the original cellulose fibers. Regenerated fibers in these processes are derived from cellulose from cotton

linter, wood, and bamboo.34 However, the viscose and cellulose acetate process are the only two

derivatizing processes that are commercially used. Only the viscose process will be discussed here in

more detail, because in 2014 almost 80% of the cellulose acetate was used in cigarette filters and not in

textiles.65

Figure 11: Classification of cellulose regeneration processes. In derivatizing processes, the chemical

structure is modified by addition of reagents to form soluble intermediates. In direct dissolution

processes the solvents are able to dissolve the cellulose directly without need for this derivation.65

13

Viscose fibers are among the most versatile of all man-made textile fibers, because they can be

engineered chemically and structurally in many ways. The process consists of various steps (Figure 12).

In the first step swelling and dissolution of pulp takes place, sodium hydroxide (NaOH) forms hydrates

with water that are able to break the inter- and intra-molecular hydrogen bonds, which causes swelling

of the cellulose fibers.67 First the amorphous region of the cellulose is swelled, after which alkali ions

can diffuse into the crystalline regions of the cellulose, resulting in the formation of an intermediate

known as alkali cellulose (or akcell). This intermediate shows irreversible swelling and is a phenomenon

also known as mercerization.68 In the mercerization process, native cellulose changes its crystalline

structure from cellulose I to cellulose II.69 This step is followed by the xanthogenation process, in which

the mercerized akcell reacts with CS2 vapor to form sodium cellulose xanthate. This derivative is soluble

in dilute caustic soda and forms a homogeneous viscose dope that can be used in a wet spinning

process.65 However, the generation of viscose fibers is an environmentally harmful process, which uses

carbon disulfide and dihydrogen sulfide for cellulose derivatization into xanthate.66

Figure 12: Schematic overview of the viscose process. Xanthogenation is one of the key steps in this

process. Here CS2 is added to the mercerized cellulose to form xanthate, which is soluble in caustic

soda to form the viscose dope that can be used in the eventual fiber spinning.65

On the contrary, in the non-derivatizing processes cellulose is directly dissolved in (in)organic solvent

without modification that forms an intermediate before regenerating fibers. Many direct dissolution

methods have been investigated in the past, such as LiCl/N,N-dimethylacetamide (DMAc),70

dimethylsulfoxide/tetrabutylammonium fluoride (DMSO/TBAF),71 cuprammonium,72 ethylene

diamine/potassium thiocyanate (EDA/KSCN),73 and inorganic molten salts (e.g. LiSCN·2H2O).74

However, these methods have shown to be either using too expensive starting materials or toxic solvents

and have not been commercialized.65

Therefore the search for alternative processes to generate cellulosic fibers has continued over

time. The most promising of these alternative approaches with direct dissolution was the amine oxide

procedure which uses the mono-hydrate of N-methylmorpholine-N-oxide (NMMO), or more generally:

a mixture of NMMO and water, to directly dissolve wood pulp without need for initial derivatization of

the cellulose.75 This approach is referred to as the lyocell process. Due to their inherent properties,

lyocell fibers have been applied in many different products, including textiles and various consumer and

industrial products, as shown in Figure 13.76

14

Figure 13: Applications of Lyocell fibers in many industries including the apparel and home textile

industries.76

In the Lyocell process cellulose fibers are directly obtained from the cellulose solution in NMMO,

therefore no derivatization, such as alkalization and xanthation is required.62 The process starts with

thorough mixing of a disintegrated bleached chemical pulp (usually paper or dissolving pulp depending

on which fiber is used) which contains a lowered DP and is dissolved in an aqueous NMMO solution.77

After water is evaporated the cellulose is completely dissolved in a monohydrate NMMO. The resulting

doped solution is degassed, filtered and extruded through a climatized airgap, here the fiber filaments

are drawn. These liquid filaments then enter a coagulation bath which contains a highly diluted aqueous

solution of NMMO. Staple fibers are produced from this and after washing, cutting, finishing, and drying

steps the final product is obtained. Very few process chemicals are applied and ideally NMMO and

water are completely recycled, which is also an important economic factor.75 In the Lyocell process

NMMO can already dissolve up to 30% of the cellulose and its recovery reached over 99% yield in

commercial large-scale systems.75,77 In comparison to virgin cotton and viscose, the Lyocell process has

a significantly lower specific environmental challenge, although it has a higher process cost (which may

be compensated by higher product durability).78

2.1.4 Enzymatic hydrolysis Despite the ability of the viscose and lyocell methods to extract and regenerate cellulose from wood

pulp, they have not been used for the direct chemical recycling of cotton waste. Chemical recycling can

be achieved with a few different methods, with early research focusing on hydrolysis targeting the inter

and intra-molecular hydrogen bonds of the cellulose.79,80 This hydrolysis can be performed by three

different methods: acid, alkaline, and enzymatic. Enzymes, due to their high specificity, would allow

step-wise recovery of the components of blended materials under environmentally friendly conditions.

An example is cellulase, an enzyme complex that is composed of several mono-enzymes that

are highly specific for enzymatic hydrolysis of cellulose, i.e. β-1,4-endoglucanases (EGs), exoglucanase

or β-1,4-cellobiohydrolase (CBH), and β-glucosidase (BGs).81 Some of the key cellulase mono-enzymes

and the areas they affect at the cellulose fiber are shown in Figure 14. The cellulase enzymes work

synergistically: EGs cleave the β-1,4-glucosidic bonds when binding randomly to the amorphous

cellulose region. This reaction creates new chain ends in the microfibrils, which can be targeted by CBH

at both the reducing and non-reducing ends of the cellulose chain to further cut the bonds along the chain

15

and gradually release cellobiose in the hydrolysate. Finally, this cellobiose is reduced to glucose by

BGs.81,82

Early advances in enzymatic hydrolysis techniques for cellulose in cotton-synthetic blends have

been made around a decade ago.83,84 Vasconcelos and Cavaco-Paulo84 removed cotton from blended

polycotton fabric with a yield of 80% of insoluble microfibrillar material by a combination of

mechanical beating and cellulase hydrolysis. They used commercial crude cellulase, derived from the

fungus Trichoderma reesei or Cellusoft L, as the working enzyme in the process. Despite their high

yield, the cotton could not be reduced to monomeric glucose and the process required high cellulase

concentrations of 200 mg/g at elevated temperatures of 50 °C with reaction times of 9 h. Therefore,

using a proper pretreatment prior to hydrolysis is essential for efficient hydrolysis of cellulosic part of

waste textile.85

Figure 14: Schematic diagram of enzymatic hydrolysis processes with a) native cellulose I before

pretreatment, b) cellulose pretreated by acid, and c) regenerated cellulose II after alkaline

pretreatment.3

2.1.5 Alkaline pretreatment for enzymatic hydrolysis Cotton has the advantage over other lignocelluloses that there is no lignin or hemicellulose covering the

glucan polymers.86 However, enzymatic hydrolysis is hindered by the high crystallinity of cotton.

Pretreatment is a primary step to increase the accessibility of the cellulose fibers to cellulase.87 Alkali

pretreatment processes were developed to lower the crystallinity and create more surface area for the

hydrolysis.86,88 The most important effect of the alkaline pretreatment is the crystallographic

modification of the fibers from cellulose I to cellulose II, which has a major impact on the digestibility

of cotton.89

Alkaline pretreatment is considered as one of the low cost and environmentally friendly

pretreatments which is also among the most efficient processes of disintegration of the cellulose

structure.90 This pretreatment shows similarities to mercerization.57 The key property of the alkaline

reagent is the swelling effect of cellulose, which breaks down inter- and intramolecular bonds of both

the amorphous and crystalline parts of the cellulose, resulting in more amorphous regions available for

enzymatic hydrolysis.91,92 Commonly used chemicals for the alkaline pretreatment include sodium

hydroxide (NaOH)86,93–95, potassium hydroxide (KOH)96, and calcium hydroxide (Ca(OH)2).97

Öztürk et al.96 showed that KOH pretreatments of lyocell fibers could be used to lower

fibrillation in lyocell, while only slightly lowering elongation at break (the ratio between changed length

and initial length after breakage of the fiber) without a loss in tensile strength. Nevertheless, this

16

technique was found only to be suitable for bleaching or vat dyeing processes. It could therefore be used

as a pretreatment step to improve cellulose purity before enzymatic hydrolysis takes place.

On the other hand, research by Jeihanipour and Taherzadeh86 has shown that cotton based waste

textiles could be enzymatically hydrolyzed with NaOH pretreatment. They converted cotton linter and

blue jeans, with respective cotton contents of 98% and 93%, into glucose by cellulase and β-glucosidase

in 24 h with subsequent simultaneous saccharification and fermentation (SSF) for 4 days to synthesize

ethanol. Without the alkali pretreatment hydrolysis of the textiles resulted in only 24% digestion.

However, when the materials were treated with NaOH (0-20 wt%) for 3 h, the digestion rate went up

massively. The most successful pretreatment occurred at 0 °C and with 12 wt% NaOH, with 99.1%

glucose yield after subsequent enzymatic hydrolysis at 45 °C.

Gholamzad et al.95 took it a step further and investigated enzymatic hydrolysis of 40/60

polyester/cotton blend and the pretreatment effects of NaOH/(thio)urea compositions consisting of

different alkali solutions of NaOH (12 wt%), NaOH/urea (7/12 wt%), NaOH/thiourea (9.5/4.5 wt%) and

NaOH/urea/thiourea (8/8/6.4 wt%). All of these pretreatments were performed at –20, 0, 23, and 100 °C

for 1 h and resulted in improvement of enzymatic hydrolysis yield to 91.0%, while it was 46.3% for the

untreated textile based on the mass of cellulose. The alkali solution was then removed by filtration, the

solid product washed with water, air dried, and finally treated with an acidic buffered solution of

cellulase (pH = 4.8) at different temperatures for 72 h. The highest glucose yield (91.0%) was reached

through saccharification after enzymatic hydrolysis of the NaOH/urea pretreated textile at –20 °C for 72

h. After the enzymatic hydrolysis the polyester was recovered by sieving with a yield of 98.0% while

this was 51.5% for the untreated textile. It was also reported that the recovery of polyester was not

significantly influenced by changes in the NaOH/(thio)urea composition. In order to demonstrate the

advantage of the pretreatment process, subsequent SSF to glucose and ethanol was conducted on

polycotton fibers with and without pretreatment. Glucose was fermented to ethanol as an open-loop

recycling method with a yield of 70%, while it yielded only 38% without the pretreatment. Despite the

yield improvement by making use of the pretreatment, more investigations in the subsequent

fermentation of ethanol are necessary. An overview of the total process is shown in Figure 15.

Figure 15: Enzymatic hydrolysis of polycotton to glucose and PET fibers with alkali pretreatment and

subsequent separation and fermentation to ethanol.95

In more recent research Li et al.98 investigated the influence of the key factors related to hydrolysis of

40/60 polyester/cotton textile waste, such as substrate loading, temperature and pH. Regarding enzyme

inputs, the dosages of cellulase and β-glucosidase were also investigated. Two types of pretreatment

methods were investigated in this work, which included chemical pretreatment by NaOH/urea and

mechanical pretreatment by milling. For the freezing NaOH/urea method, textile waste was soaked in

an alkaline mixture of 7 weight/volume% NaOH and 12 w/v% urea at − 20 °C for 6 h. The highest

glucose yield for this pretreatment method was 98.3%, obtained with 20 FPU/g of cellulase dosage and

10 U/g of β-glucosidase dosage at 3 w/v% substrate loading, at 50 °C and pH 5. FPU/g and U/g are

common units of measurement for enzyme loading, abbreviations for filter-paper units per gram (FPU/g)

and enzyme units per gram (U/g) respectively, related to their activity.

17

For comparison, in the work of Gholamzad et al.95, in which a maximum yield of 91.0% was

reached, pretreatment with NaOH/urea lasted only 1 h at –20 °C and hydrolysis was performed with 30

FPU cellulase and 60 U β-glucosidase per gram of cellulose for 72 h (vide supra).

Furthermore, Li et al.98 found that the efficiency of the enzymatic hydrolysis was highly

dependent on multiple factors, such as temperature, pH, substrate loading, enzyme dosages and

structural features of the substrate. Therefore lower enzyme loading was used, which was compensated

by longer reaction times, thereby reaching a higher glucose yield.

2.1.6 Acid pretreatment for enzymatic hydrolysis Other pretreatment methods to gain more accessibility of the cellulose fibers for the cellulase enzymes

have also been investigated. In their research on enzymatic hydrolysis Vasconcelos and Cavaco-Paulo84

also compared the enzymatic hydrolysis to sulfuric acid treatment. Figure 16 shows the treatment of the

polycotton blends with either cellulase enzyme (Figure 16b) or 75% sulfuric acid (Figure 16c). The

enzymatic treatment showed less degradation than the concentrated sulfuric acid treatment, due to the

ability of the acid to dissolve the cotton only the polyester ‘skeleton’ remained. The enzyme-treated

cellulose shows some remaining cotton fibers, as is visible in the smaller inter-yarn space in Figure 16b.

This could be attributed to the presence of polyester in the fabric blend, which will considerably reduce

the surface area of the cellulose exposed to enzyme attack. It was concluded that the enzymes adsorbed

competitively on polyester fibers in the presence of cotton, approximately 30% more enzyme was

adsorbed on the polyester fibers than on the cotton fibers. However, the obtained result was very similar

to the sample treated with the acid.

Figure 16: SEM pictures of polycotton samples a) without treatment, b) treated with Cellusoft L and

c) treated with sulfuric acid.84

Therefore, the more recent research efforts in enzymatic hydrolysis have been focused on the use of an

acid pretreatment step in combination with enzymatic hydrolysis or saccharification.98–100 The key

mechanism of acid pretreatment is the decomposition of the cellulose fiber microstructure, which is

illustrated in Figure 14. During this process the amorphous region of the cellulose can be hydrolyzed to

short saccharide chains, cellobioses and some glucose, while the crystalline regions can be exposed to

the enzymes, subsequently facilitating the enzymatic degradation process.

There are two preferable concentration ranges for sulfuric acid pretreatment, based on the

applications and feedstock that has to be treated.101 The concentrated pretreatment usually occurs with

acid concentrations between 60 and 90%, while dilute acid concentrations typically range between only

0.5 and 15% acid in the pretreatment liquid.

Following the research of Vasconcelos and Cavaco-Paulo a two-step process was developed by

Schimper et al.83 using the cellulase enzyme. Here a pretreatment step, pad-batch pre-hydrolysis, was

introduced to loosen up the fabric and increase the rate of hydrolysis. In this pretreatment step the fabric

samples were padded with enzyme-liquor and rested for 12 h at either room temperature (21 °C) or 55

°C. After this pretreatment the regular batch hydrolysis step was performed, resulting in a weight loss

of 25-28% for the two-step process, based on the total weight of the sample. At room temperature this

pretreatment step resulted in higher overall hydrolysis rate while it slightly lowered activity at 55 °C.

The process efficiency suffers from the total reaction time due to the long pretreatment step.

Shen et al.99 developed a process for both sugar and polyester recovery from polycotton textile

waste by using phosphoric acid (H3PO4). Phosphoric acid can be used as a solvent for crystalline

cellulose dissolution and regeneration, which has been investigated for nearly 90 years.92 Phosphoric

acid can be characterized as relatively non-corrosive and non-toxic when compared to other acids

frequently used in pretreatment processes.102 Furthermore it has the ability to dissolve crystalline

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cellulose at atmospheric pressure with moderate temperatures, indicating that the polyester can be

separated from the cotton-based part. By diluting the phosphoric acid the cellulose can be recovered

with a decrease in the degree of polymerization and crystallinity.99 Hereafter separation of the polyester

and cotton occurs, in turn followed by the enzymatic hydrolysis, which takes place in 96 h. The best

result recovered polyester with 100% yield and sugar with a yield of 79.2% after 85% phosphoric acid

pretreatment for 7 h at 50 °C. The 100% polyester recovery was reached by increasing the temperature,

time, phosphoric acid concentration or decreasing the ratio of textiles to acid, however this reduced the

cellulose recovery significantly, from 91% to 79.2% respectively.

Jeihanipour and Taherzadeh86 also used concentrated phosphoric acid as a pretreatment for

enzymatic hydrolysis of cotton linter and cotton jeans. Despite reaching lower conversion than their

alkaline pretreatment, the acid pretreatment still resulted in a glucose yield of 63.4 and 60.7%

respectively.

Sasaki et al.103 recently investigated cotton hydrolysis with sulfuric acid pretreatment assisted

by microwave treatment. The hydrolysis of the cotton, when the cotton was impregnated with

concentrated (although not the typically used concentration of 60-90 wt%) 51 wt% sulfuric acid for 30

min, by microwave at 200 °C for 2 min resulted in 30.9% glucose yield. When the enzymatic step was

tested the glucose yield reached 74.2% by microwave treatment at 180 °C for 3 min with subsequent

enzymatic hydrolysis.

2.1.7 Alkaline hydrolysis Conversely, there have been investigations in hydrolysis methods for which enzymes are unnecessary.

Therefore research on the direct dissolution of cellulose in aqueous NaOH has been conducted, although

it was shown that NaOH/water is not a thermodynamically favorable solvent for cellulose.69 It also has

the disadvantage of inducing gelation of the solution when temperatures or reaction times are increased. 104 Accordingly, cellulose can only be dissolved in narrow ranges of NaOH concentrations (7-10%) and

freezing temperatures (–4 to –8 °C).105 Due to these disadvantages the addition of urea to aqueous NaOH

in order to better dissolve cellulose was investigated. Zhang and coworkers106,107 developed a variety of

alkali/urea solvents to dissolve cellulose within 2 minutes at low temperatures. Cellulose pulps could be

dissolved in 4.6-7 wt% NaOH/12-15 wt% urea aqueous solutions, when precooled to –12 °C. In the

dissolution mechanism of this process, NaOH ‘hydrates’ could form new hydrogen bond networks with

cellulose chains that are relatively stable at lower temperatures. At the same time the urea hydrates form

a shell surrounding the NaOH-cellulose hydrogen bonds to form an inclusion complex leading to the

dissolution of cellulose.67 Even though the NaOH/urea mixture was able to dissolve cellulose quickly,

these solutions are only stable for one week at 0-5 °C and heating to temperatures higher than 20 °C

lead to irreversible gelation of the cellulosic material.108 It has also been reported that the dissolved

cellulose could be spun back into fibers with tenacities of 15-20 cN tex -1, similar to that of the viscose

process.109

2.1.8 Acid Hydrolysis Besides alkaline hydrolysis, concentrated acid hydrolysis is another widely researched method that

could be used as a direct hydrolyzing agent instead of a pretreatment for enzymatic hydrolysis only. 29,35,110–113 The first examples of acid-catalyzed depolymerization of the cellulosic compound of

polycotton date back to the 1970’s and early 1980’s.114 Here, the polycotton was treated with 5-10 wt%

of aqueous sulfuric acid at 80-100 °C for 30 min after which a mixture of PET and cellulosic powder

with a DP of approximately 100 remained. The cellulose powder was separated from the PET fibers by

filtration, which could be recovered through sedimentation. The PET could then be (re-)used in yarn

manufacturing, or fabrication of nonwoven materials.

Instead of sulfuric acid, gaseous hydrochloric acid (HCl) was also used for 30 min at 49-71

°C.115 After removal of the HCl gas by compressed air and washing with water, cellulose could be

separated as a fine powder while PET remained in the vessel.

Boerstoel et al.116 investigated phosphoric acid for the direct dissolution of cellulose to form

liquid crystalline solutions. By the use of a mixture of two or more components from orthophosphoric

acid (H3PO4), pyrophosphoric acid (H4P2O7), polyphosphoric acid (H6P4O13), and phosphorus pentoxide

(P2O5) with a concentration of 74 wt% in water (also referred to as superphosphoric acid) anhydrous

19

conditions could be obtained. The use of superphosphoric acid resulted in fast (i.e. minutes) cellulose

dissolution and direct formation of liquid crystalline cellulose solutions at 42 °C, which could be

performed with cellulose concentrations between 7.5 and 38 wt%. After the dissolution solutions

containing 19 wt% of dry polymer were filtered, heated, and dry-jet wet-spun, with subsequent

neutralization by sodium carbonate. The fibers produced with this method showed a tenacity of 70 cN

tex-1.117

Meanwhile, Sun et al.118 used formic acid in combination with HCl for the hydrolysis of

cellulose. Formic acid was found to be useful in breaking down the intermolecular hydrogen bonds,

resulting in swelling of the fibers and therefore the rigid framework of the crystalline lattice could be

crushed to form microfibrils. After this step the HCl was added to hydrolyze the cellulose further to

glucose at 65 °C for 5 h, which resulted in a glucose yield of 22.5%. Both acids could be effectively

recovered and reused.

In more recent research the focus laid on complete degradation of the cellulosic components

into monomeric sugars or sugar derivatives, in order to contribute to open-looped recycling of cotton,

mostly for energy applications such as biofuel generation.119,120 There was also more focus on reducing

the environmental impact of the processes. For instance Chu et al.29 showed that a concentration of 55%

sulfuric acid was able to hydrolyze virgin cotton into sugars at a temperature of 40 °C with a yield of

73.9%. When the sulfuric acid concentrations were lower than 45% the yield declined to 8.8%.

On the other hand, Ouchi et al.111 developed a two-step process to obtain cellulose powder from

various cellulosic fabrics (e.g. from mercerized cotton, lyocell, rayon, or polycotton). The first step is a

10 min acid treatment with either 10 M aqueous sulfuric acid or 10 M hydrochloric acid, both at a

temperature of 95°C. Followed by a mechanical beating process, ranging from 15 min to 4 h, in water

at room temperature. Afterwards the cellulose powder is separated from the PET by decantation of the

suspension of generated cotton powder and filtration of the remaining cloths, as shown in Figure 17. In

order to reduce the amount of acid required for this process, the fabric was padded in the sulfuric acid

and the hydrolysis was performed in toluene. The results showed faster hydrolysis in toluene than in the

aqueous acid, with the DP of the cellulose decreasing from ~2200 to 160-170 for the powder. Hydrolysis

with HCl gave somewhat similar results regarding the DP, despite lower cellulose powder recovery.

They did report a complete separation of both components of the polycotton fabric.

Figure 17: Two-step procedure for the separation of polycotton fabrics to PET fibers and low DP

cellulose powder. After acid treatment at 95°C a mechanical beating process at room temperature is

performed. Subsequent decantation and filtration results in the separated compounds.111

Ling et al.112 investigated a slightly different approach to create a more environmentally friendly process,

using phosphotungstic acid (H3PW12O40, HPW). HPW is a solid strong acid with a melting point of 89

°C, that hydrolyzes cotton to microcrystalline cellulose (MCC). After the process the acid catalyst can

be recycled by diethyl ether extraction.113 In their process the waste blended polycotton fabric (WBF,

65% polyester and 35% cotton) was treated with an aqueous solution of HPW in an autoclave at 120-

170 °C for a period of 3 to 8 h. By filtration the solid residues of the WBF components, PET and MCC,

were recovered, while the filtrate was captured to recycle the HPW. Subsequently the PET was sieved

to remove the MCC powder by using a 2 mm sieve and rinsing with deionized water. The final step in

this process was hydrolysis of PET with water at 250 °C for purification of the terephthalic acid.112 An

overview of this process is shown in Figure 18.

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Figure 18: Procedure of WBF separation by phosphotungstic acid to produce MCC and TPA. The

polyester is hydrolyzed to pure TPA, after the filtration step to separate cellulose (in the form of MCC)

and polyester.112

Sanchis-Sebastiá et al.110 very recently developed a two-step process for the acid hydrolysis of cotton

waste textiles. The acid hydrolysis proved successful in converting cotton waste into glucose with a

yield between 80 and 90%, which could be further open-loop recycled into valuable chemicals or fuels.

First investigations looked into the one-step acid hydrolysis with sulfuric acid at different acid

concentrations (5, 32.5, or 60 wt%), temperatures (30, 80 or 130 °C), and residence times (1 or 6 h).

When compared to the results of Chu (73.9% yield with acid concentration of 55%)29 the results of the

waste cotton for this one-step hydrolysis were significantly lower with a maximum glucose yield less

than 20%.110 Despite concentrated sulfuric acid being able to dissolve 80% of the waste textile, the

glucose yield was as low as the dilute sulfuric acid treatments. It was implied that the glucose produced

in this process was rapidly degraded into other by-products, such as hydroxymethylfurfural (HMF) and

levulinic acid. Due to the high concentration of protons the glucose would react further to the

aforementioned by-products. As seen before in the process of Shen et al, the concentrated acid could be

used to separate the cellulosic fibers from PET in polycotton99, however, it could not be used to recycle

the cotton via glucose production.110 Therefore, a two-step process was developed, combining both

dilute and concentrated acid with the advantages of each concentration, i.e. good dissolution of cellulose

at high acid concentrations and low degradation at low acid concentrations, respectively. When 70-80

wt% sulfuric acid solutions were used the highest degree of dissolution and degree of hydrolysis were

reached, as shown in Figure 19a. With higher loading of cellulose the glucose yield decreased, especially

when higher loading than 0.74 g waste textiles per g of sulfuric acid solution was used, as seen in Figure

19b. The maximum glucose concentration that was reached was 40g/L in this two-step process.

a) b)

Figure 19: Graphs showing a) influence of concentration H2SO4 versus degree of hydrolysis and solid

recovery and b) solid cellulose loading versus glucose yield and concentration.110

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Hou et al.121 tried to find an environmentally friendly recycling method to efficiently separate the

polycotton fabrics. They investigated a method using hydrothermal degradation, followed up by mild

acid hydrolysis in order to attenuate the corrosion of equipment. In the hydrothermal process, water was

used as the solvent for the hydrolysis reaction under high temperature and pressure. It was found that

under the acid catalysis conditions the cotton fibers hydrolyzed at 120 °C while the polyester started to

hydrolyze at 180 °C. Therefore this difference in hydrolysis temperature could be used to separate the

polycotton waste textile. The WBFs that were used contained blue 65/35 cotton/polyester fabric, which

was crushed into small pieces (5x2 cm2) mechanically and washed and dried before the recycling process

started. After the small polycotton pieces were treated hydrothermally with dilute HCl (0.5-2.5 wt%), at

150 °C for 3 h, PET was recovered with 96.2% yield. The recovered PET only showed a slight decrease

in crystallinity and viscosity. MCC and glucose were recovered with 49.3% and 15.6% yield

respectively. Furaldehyde also formed as the major side product, resulting in a total cellulose conversion

of 84.5%. When the temperature was increased to 170 °C the yield of cellulose powder decreased

significantly. This method showed some promising results, however not all of the cellulose could be

hydrolyzed to monomeric glucose. An overview of the process is shown in Figure 20.

Figure 20: Flowchart of a hydrothermal process to recycle 65/35 cotton/polyester WBF into polyester

and MCC. The WBFs were cut into small pieces before being treated in the high-pressure batch

stainless steel reactor with dilute HCl at 150 °C to recover PET, MCC and glucose.121

Similar to the hydrothermal treatment of Hou et al. a two-step procedure was developed by Sankauskaite

et al.122, with a pretreatment step between 20 and 130 °C for 30 min, followed up by heat treatment at

150-180 °C. Polycotton blends (50:50) were pretreated with neutral (aluminum sulfate, Al2(SO4)3) or

weakly acidic (citric acid) aqueous solutions with or without addition of magnesium chloride (MgCl2).

Polycotton pretreated at 20 °C with a MgCl2/Al2(SO4)3 mixture (20 g/l MgCl2 and 4 g/l Al2(SO4)3),

followed by heating to 180 °C, resulted in a cotton depolymerization yield of 95.5%. Pretreatment with

MgCl2/citric acid (200 g/l MgCl2 and 2 g/l citric acid) at 130 °C followed by heating to 180 °C resulted

in a slightly lower depolymerization yield of 91.4%. This indicates that higher concentrations of MgCl2

and higher temperatures were required when Al2(SO4)3 was switched out for citric acid.

22

In summary, the research in cotton depolymerization has shown that the major part of cotton consists of

α-cellulose type I, which consists of crystalline and amorphous regions. In order to re-form fibers from

cotton the cellulose has to be regenerated. Commonly used regeneration processes are derivatizing

processes (e.g. viscose) and direct dissolution processes (e.g. lyocell). Besides these processes, chemical

recycling methods have been developed based on hydrolysis. During hydrolysis cellulose can be broken

down into monomeric glucose, ethanol, or MCC, depending on the methods used. These methods

include acid, alkaline, and enzymatic hydrolysis, of which enzymatic hydrolysis is usually performed

with either an acid or alkaline pretreatment to improve the results. Some of the hydrolysis methods could

only handle pure cotton waste (i.e. enzymatic without pretreatment, NaOH, sulfuric acid), while others

were able to hydrolyze cotton from WBFs with conservation of PET during the process (i.e. enzymatic

with NaOH/urea or phosphoric acid pretreatment, HPW, hydrochloric acid, two-step sulfuric acid).

Furthermore, it was found that the efficiency of enzymatic hydrolysis was highly dependent on multiple

factors such as pretreatment temperature, pH, substrate loading, enzyme dosages and structural features

of the substrate. Finally, hydrothermal treatments were combined with mild acid hydrolysis to reduce

the use of corrosive chemicals. This method could be enhanced with the use of a pretreatment with a

mixture of MgCl2/Al2(SO4)3

2.2 Recycling with Conservation of Polyester and Cellulose.

2.2.1 Adaptation of the Lyocell Method for Recycling of Polycotton As mentioned before, NMMO employed in the lyocell method could be used industrially as a more

environmentally friendly preparation of virgin fibers from wood pulp.75 However, it has not been

industrially used for the recycling of textile waste. It has been demonstrated academically that

pretreatment of highly crystalline cotton-based waste textiles in NMMO can be very effective to improve

enzymatic saccharification rates and yield of cellulose for ethanol production.123–126 As the result of

transformation of crystalline cellulose to amorphous cellulose has shown, the regenerated cellulose from

NMMO/water solutions was three times more reactive in the hydrolysis reaction than untreated

cellulosic material.126

Jeihanipour et al.127 were one of the first to investigate biogas production from cellulose in

WBFs with the use of NMMO pretreatment before enzymatic hydrolysis. In their research 50/50

polycotton waste textiles were used. Pretreatment was performed with concentrated NMMO/water

solutions (85 wt%) at 120°C for 2 h. The undissolved polymers were then separated with a 1 mm sieve,

and washed with another batch of NMMO solution to remove the remaining cellulosic part. Afterwards

boiling water was added to regenerate the cellulose, with subsequent enzymatic hydrolysis at 4 °C after

separation of the cellulose by vacuum filtration. This resulted in cellulose recovery up to 90% yield and

glucose yields up to 95.8% from this cellulose recovery resulting in a total glucose yield of 86% based

on the initial cellulose. Furthermore, the effect of using recycled solvent instead of fresh solvent was

investigated, as well as the addition of an antioxidant (n-propyl gallate) to the NMMO solution. The use

of one time recycled NMMO did not result in significant differences for either the dissolution of the

cotton or the glucose yield. Even though the rate of enzymatic hydrolysis and the ethanol production

during antioxidant addition showed no significant difference, it did, on the other hand, result in only

40% recovery of cellulose and was therefore appointed as a cause for lower solubility of the cotton.

These results indicate that dissolving cellulose in NMMO and then precipitating is enough to change its

structure and make it easier for biodegradation by enzymes.88 Fibers spun from NMMO treated cellulose

typically possess tenacities in the range of 40-42 cN tex-1.

However, despite the high yields reached by this process, NMMO suffers from side reactions

and dangerous runaway reactions, which affect the final properties of the fibers and requires a stabilizer

to reduce these effects.75,77,128 Other downsides are related to the fiber spinning process, such as high

degrees of thermal instability during spinning and energy consumption. In addition, the created lyocell

fibers fibrillate under wet abrasion, (i.e. fibers split at surfaces and produce a hairy appearance)32

Furthermore, the high viscosity of the cellulose solution in the NMMO monohydrate, at moderate

polymer concentrations of 13 wt%, sets limits in both the process economy and strength properties of

the fibers.62

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2.2.2 Dissolution of cellulose in ionic liquids From a safety, environmental and economic point of view, it is very attractive to use solvents that can

dissolve cellulose in a non-derivatizing process with higher thermal and chemical stability comprising

lower solution viscosity than NMMO to use with the Lyocell spinning technology.62 One of the solvent

classes that attracted research interest in the past two decades are ionic liquids (ILs).129–132 ILs describe

a class of salts in the liquid state, typically with melting points below 100°C.133 Due to their properties,

such as high chemical stability, low vapor pressure, and tunable solvent properties, they have attracted

interest in various research fields (i.e. extraction processes, chemical reactions, electrochemical

processes).49,130,134,135 Besides synthetic applications, some of the ILs have been identified as direct

solvents for (ligno)cellulosic materials, as early as 1934 by Graenacher136, despite not gaining a lot of

attention. He applied N-alkylpyridinium salts (Figure 21) for the dissolution of cellulose and as medium

for homogeneous chemical reactions.

Figure 21: Two examples of N-alkylpyridinium salts: benzylpyridinium chloride (a) and

allylpyridinium chloride (b) used for the dissolution of cellulose by Graenacher136

Unfortunately, direct solvation of cellulose by ILs was not widely researched until 2002, when Swatloski

et al.129 initiated research into specific ILs to substitute NMMO as a direct solvent for cellulose. They

researched the direct dissolution of cotton in 1-butyl-3-methylimidazolium cations [BMIM]+ with a

range of anions, from small, hydrogen-bond acceptors (i.e. Cl–) to large, noncoordinating anions (i.e.

[PF6]–). At room temperature the cellulose could not be dissolved, however at elevated temperatures of

100-110 °C cellulose did dissolve slowly to yield increasingly viscous solutions. Solutions containing

up to 25 wt% cellulose could be formed, although lower concentrations between 5 and 10 wt% were

more readily prepared. The solutions with 25 wt% were obtained as high viscous pastes. Cellulose could

be precipitated in various forms (i.e. monoliths, fibers, films, powdery floc, and fibers) from the IL

solution by addition of water, or other polar anti-solvents. This research has shown that ILs can be used

as a non-derivatizing solvents for cellulose and that the formation of liquid crystalline solutions of

cellulose may have useful applications for the generation of new, advanced materials.

Consequently, more research groups started working on the dissolution of cellulose in ionic

liquids.137–139 Imidazolium based ILs are the most frequently employed, however pyridinium-based ILs,

low-melting quaternary phosphonium and ammonium compounds have also been described in the

literature 140,141. Most cellulose dissolving ILs contain 1-alkyl-3-methylimidazolium cations [RMIM]+

with different n-alkyl chain.142,143 The most frequently used alkyl chains are allyl, ethyl-, and butyl-side

chains. In addition, imidazole-based ILs with benzyl- and methylnapthyl substituents are able to dissolve

cellulose.144 The most commonly employed anions of imidazolium ILs as solvents for cellulose

processing are acetate and chloride.145 Other anions employed include, hexafluorophosphate [PF6]-,

tetrafluoroborate [BF4]-, nitrate [NO3]-, mesylate [CH3SO3]-, diethyl ether phosphonate [(EtO)2PO2]

(DEP), triflate [CF3SO3]-, and bis-(trifluoromethanesulfonyl)amide [Tf2N]-.134 The ability to dissolve

cellulose is not an inherent property of this broad class of compounds. Forsyth et al.146 estimated that

24

the number of potential ion combinations could potentially result in 1012 ILs. ILs are formed from bulky,

asymmetrical ions with a delocalized charge in order to achieve a low melting temperature.134 However

only a minor part, among the vast number of ILs reported so far, are able to dissolve cellulose. Especially

the anions are restricted to a small number of suitable ones.142 An overview of the most common cations

and anions used for ionic liquids and their abbreviations is shown in Figure 22.

Figure 22 : Molecular structures and abbreviations of typical anions and cations used in ionic liquids

and low-melting organic salts, reported for dissolution of cellulose.142

Subsequently, the mechanism of cellulose dissolution in ionic liquids has been studied extensively in

the following years.134,141,145,147,148 This mechanism will be discussed only briefly here.

A schematic representation of the hydrogen bonding was shown before in Figure 9. As can be

seen there, the AGUs are bonded by their intermolecular hydrogen bonds, in particular between the

hydroxy group of C6 and the oxygen atom of C3 of an adjacent cellulose chain of cellulose I.54

Therefore, the cellulose molecules are linked in a layer and these layers are held together by hydrophobic

25

interactions and weak hydrogen bonds between hydroxyl hydrogen atoms and oxygen atoms as shown

in Figure 23.149

Figure 23: Side and top view of hydrophilic and hydrophobic sites of cellulose.149

It became apparent that disruption of the inter- and intramolecular H-bonds between the hydroxyl groups

of the AGUs and the van der Waals interactions was required for physical dissolution of cellulose.150

The ability of ILs to dissolve cellulose is closely related to their unique molecular structures. However,

due to the broad structural diversity of this class, it is an exhausting task to propose a general dissolution

mechanism.146 It has been generally accepted that ILs are non-derivatizing cellulose solvents in which

the anion should be a strong hydrogen bond acceptor, namely with high Lewis basicity, in order to

facilitate dissolution of the polysaccharide chain.151–153 The major contribution that leads to dissolution

is the interaction of the anions with hydroxyl groups of the polysaccharide backbone, which causes

breaking of the strong intramolecular hydrogen bond network. Due to electrostatic repulsion, the

negatively charged cellulose-anion complexes start to separate.

Ohno and Fukaya154 have shown the importance of basicity and hardness of the anion. When

the same cation is used (e.g. 1,3-disubstituted imidazolium), carboxylate anions (such as acetate or

propionate) and fluorides dissolved cellulose better than the chlorides. Moreover, most bromides do not

dissolve cellulose. An example of the importance of anion volume is that [BMIM]pivalate dissolves less

cellulose than the corresponding acetate [BMIM]AcO.

The investigation of cellulose in chloride-based IL using NMR spectroscopy revealed that the

solvation of cellulose by [BMIM]Cl involves a stoichiometric amount of hydrogen bonding between the

hydroxyl protons of the cellulose and the chloride ions of the IL.155 Furthermore, the solvation of

cellobiose, in 1-ethyl-3-methylimidazoliumacetate ([EMIM]AcO) under a range of cellobiose

concentrations and temperatures suggest that hydrogen bond formation occurred between the hydroxyls

of cellobiose and both the anions and the cations of [EMIM]AcO.156

Nonetheless, interaction of the cations with cellulose, anions and cations themselves has to be

considered as well.145 The separation of the cellulose-anion complex is enhanced by “condensation” of

the cations in order to maintain electric charge neutrality. Which in turn leads to increased steric

repulsion between the chains of the cellulose-IL complex, with accompanying disruption of the van der

Waals interactions between the AGUs.34,141

Lu et al.157 investigated the effect of cations on the dissolution of cellulose. They prepared 13

ILs with an identical anion, but a different cation. It was concluded that the cations significantly affected

cellulose dissolution, either by increasing the cellulose solubility through hydrogen bond formation with

the oxygen atom of the hydroxyl moiety and the oxygen atom of the ether moiety of cellulose, or by

decreasing the solubility through anion competition or steric hindrance.

It has also been generally accepted that cellulose dissolution is favored by decreasing cation

size, due to their easier intercalation between the cellulose chains.144 In summary, the charge density,

the volume, rigidity, Lewis acidity, and hydrophobic character of the cation determine the level of

cellulose dissolution.145 Therefore, it is generally agreed upon that anions and cations are both

synergistically involved in the dissolution of cellulose through various inter-molecular interactions. A

simplified scheme of this process with the interactions of cellulose with the components of the IL is

shown in Figure 24. Wang, Gurau and Rogers158 created an extensive overview of the solubility of

26

cellulose from varying sources in a wide range of ILs. To view these tables their work should be

consulted.

Figure 24: Schematic representation of the proposed dissolution mechanisms for cellulose in

imidazolium-based ionic liquids: (A) interactions between the basic anion and the cellulose hydroxyl

groups break up the hydrogen bond network between the cellulose chains. (B) Hydrophobic

interactions take place between the hydrophobic face of cellulose and the imidazolium ring. (C) The

acidic proton at the C2 position of the imidazolium ring interacts with the cellulose hydroxyl

groups.159

2.2.3 Methods to increase cellulose dissolution in ionic liquids Despite the many advantages of ILs as cellulose solvators, these advantages are combined with some

limitations, as depicted in Figure 25a and b: high viscosity of the ILs and the corresponding cellulose

solutions and limited miscibility of the ILs with hydrophobic reagents and cellulose derivatives.160

Especially at room temperature, cellulose-IL solutions are so viscous that they do not flow, which makes

sufficient mixing with derivatizing agents very difficult and results in sluggish and irreproducible

reactions.

Hence, mixtures of ILs and dipolar aprotic solvents are used to enhance miscibility and

simultaneously lower the solution viscosity (Figure 25c and d). Addition of polar, aprotic co-solvents

(e.g. DMAc, DMSO, DMF, NMP, etc.) to ILs would enhance their solvating power.141,161,162 These co-

solvents do not significantly influence the specific interactions between the cations and anions of the IL

or between the ILs and cellulose, but do accelerate mass transport and decrease solvent viscosity.163–166

According to Gericke et al.160 suitable co-solvents should be polar in order to obtain high miscibility and

have only weak hydrogen bond donation capabilities since cellulose would otherwise precipitate, due to

competitive hydrogen bonding interactions between the hydrogen bond accepting IL and the hydrogen

bond donating co-solvent that disrupt the IL/cellulose interactions. Furthermore, high basicity values

are favorable, since increasing basicity also increases the amount of co-solvent tolerated in cellulose/IL

solutions.

27

Figure 25: Visualization of the beneficial effects of co-solvents during chemical modification of

cellulose dissolved in ILs. With a+b) high viscous and hydrophilic cellulose/ionic liquid solutions that

are not suited for derivatization and c+d) less viscous and less hydrophilic cellulose/ionic liquid

solutions are formed by the addition of a co-solvent.160

Zhu et al.167 recently described a dry-jet wet spinning process where low molecular weight

microcrystalline cellulose (MCC) was dissolved in [EMIM]DEP combined with DMSO as co-solvent,

which made it possible to dissolve cellulose up to 23.6 wt% and obtain highly aligned fibers.

Phadagi et al.168 performed both experimental and theoretical research on the dissolution of

cellulose by ILs with DMF as co-solvent. This study revealed that among the ionic liquids considered,

[BMIM] and 1-butyl-3-methylpyridinium ([BMPy]) cations would be better candidates for the

dissolution process of cellulose than the [AMIM] cation. It was found that ILs with DMF as co-solvent

are more effective than pure ILs in the dissolution of cellulose.

Besides co-solvents, other methods were found to significantly increase cellulose dissolution,

e.g. microwave-assistance,129,169 sonication-assistance,169,170 as well as addition of lithium salts.171–173 In

addition, the source of cellulose, the water content of ILs and cellulose, and the temperature greatly

affect the cellulose dissolution.59

2.2.4 Cellulose regeneration as fibers from IL processes As discussed in the previous sections, the choice of cation and anion affects cellulose dissolution and

impacts the environmental aspects and sustainability of this process.158 A lot of research has been put in

regeneration of the dissolved cellulose in different physical forms, such as micro- or nanoparticles, films

and membranes, nonwoven materials, and fibers. The latter will be discussed in more detail. Reviews

on the other products can be found elsewhere.34,59,145,174

The two techniques of fiber spinning mentioned earlier in this research are also applicable with

ILs as solvents. Kosan, Michels and Meister130 investigated some of the most common imidazolium-

based ILs (i.e. imidazolium chlorides and acetates: [BMIM]Cl, [BMIM][OAc], [EMIM][Cl] and

[EMIM][OAc]), which were compared with NMMO as spin dopes for fiber generation from cellulose.

The cellulose was dissolved in concentrations of 13-16 wt%, depending on the molar mass of the solvent.

Subsequently the fibers were formed by a dry-jet wet spinning process. The spinning conditions were

adjusted for each IL solution and were changed accordingly for the NMMO spinning process,

28

specifically the temperature and conditions at the air gap were varied. Solutions of cellulose in ILs

possessed higher viscosities and were shaped into fibers with higher tenacities than those spun from

NMMO. The fiber DP was comparable between all five solvents, ranging between 479 and 520.

Instead of the dry-jet process the wet spinning process could also be used to fabricate cellulose

fibers. This process results in fiber tenacities between 20 and 30 cN tex -1, since the viscosity of the

solution had to be reduced.175,176 These tenacities are lower than the dry-jet wet spinning fibers and

similar to those spun by the viscose process. The viscosity is decreased by using different methods, such

as decreasing the dope concentration,177,178 increasing the spinning temperature,179 and, as mentioned

before, using co-solvents.176,180 Additionally, the fibrillation of fibers that occurs during the dry-jet wet

spinning process could be avoided.177

Continued research effort has been put into dissolution of cellulose in imidazolium-based ILs

and fiber regeneration from these solvents, due to their molecular and structural flexibility and their

efiiciency.59,181 Jiang et al.182 obtained cellulose fibers from a [BMIM]Cl solution with a tensile strength

of 42.1 cN tex-1. Despite similar properties to the NMMO solvent, BMIM halides suffered from some

drawbacks, that is, their corrosive nature, severe cellulose decomposition, and relatively high melting

points (above 70°C).181 The ILs with chloride or acetate anions showed significant differences in

cellulose concentrations. Therefore dopes at higher cellulose concentrations were available for acetate

containing ILs, resulting in a more efficient solvent for cellulose dissolution and shaping processes.130

Olsson et al.180 showed that in [EMIM]OAc, the highest tensile strength of the regenerated fibers

was 35 cN tex-1, slightly lower than those regenerated from [BMIM]Cl. Such tensile strengths exceed

the standard of viscose fibers (~18.5 cN tex-1) and are comparable to that of lyocell (~40 cN tex-1).59

De Silva et al.183 investigated the separation of polycotton blends by 1-allyl-3-

methylimidazolium chloride ([AMIM]Cl). In this research yarn samples were oven dried at 105°C for

24 h prior to dissolution in the IL, before being added with 2-10 wt% to the [AMIM]Cl at temperatures

between 80 and 120 °C for 6 h. Water was used as the coagulated solvent for the regeneration process.

It was found that the cotton was almost completely dissolved from the polycotton mixtures and could

be used for yarn spinning. 13C NMR and FTIR did indicate that some small amount (less than 2%) of

cotton remained in the recovered polyester. The scheme below (Figure 26) shows the process for

simultaneous recovery of cellulose and PET from polycotton using ([AMIM]Cl)

Figure 26: Separation of polycotton through dissolution of cotton using AMIMCl with

subsequent filtration, by using an anti-solvent the cotton could be regenerated from solution.183

Since 2006 another class of ILs was discovered and thoroughly researched: distillable acid-base

conjugated ILs, which were made by combining organic acid with a range of superbases (i.e. 1,1,3,3-

tetra-methylguanidine (TMG), 1,5-diazabicyclo-[4.3.0]non-5-ene (DBN), 1,8-

diazabicyclo[5.4.0]undec-7-ene (DBU), Figure 22 and 27).127,184–189 This class of ILs did not only

dissolve cellulose rapidly in high concentrations, but could also be distilled with recovery and purity

over 99%.59,190

29

Figure 27: Structures of the superbase based ionic liquids.60

During the last decade Sixta and coworkers produced a string of research papers on the use of the IL

[DBNH]OAc for the production of regenerated cellulose fibers from blended cotton waste via the

Ioncell-F process.188,191–193 The synthesis of [DBNH]OAc is a neutralization reaction of a stochiometric

amount of DBN and glacial acetic acid. During the addition of acetic acid, permanent cooling is

necessary due to the exothermic nature of the reaction. [DBNH]OAc does not require the addition of

any stabilizers and thus allows for adjustment of the process conditions.194 Therefore solutions

containing between 13–15 wt% cellulose could be obtained. After hydraulic pressure filtration (70 °C,

1–8 MPa, with a 5–6 mm metal filter mesh), the resulting cellulose solution could be used directly as a

spinning dope for dry-jet wet spinning to obtain Lyocell-type fibers with tenacities ranging from 27–48

cN tex-1. 62,128,195 Spinning conditions depended on the composition of the solution, however spinning

temperatures were generally in the range of 65–80 °C.127,196 Direct dissolution of the cellulose and

coagulation in water makes this process interesting for up-scaling and commercialization, however there

are still challenges concerning the efficiency of IL recovery.34

Haslinger et al.194 studied the degradation of the recovered PET from polycotton waste in the

Ioncell process. As mentioned before, in this process the cellulose solutions could be directly subjected

to the dry-jet wet spinning process directly after they were solidified. In this case the cellulose solutions

were solidified by storage at 8 °C for a couple of days. On the other hand, recovered polyester fibers

were still contaminated with residual cellulose and required further purification steps. Although the PET

filaments suffered no substantial weight loss in longer IL treatment periods, their tenacity was cut into

less than half over a treatment time of 4 h. This decrease from 38 to 18 cN tex-1 exhibits the largest drop

already after 30 min, while the degrading effect of the IL flattens out with an increased treatment time.

Therefore the polyester residue was extracted with a fresh batch of IL for 1 h at 80 °C. After which the

solvent was removed by filtration and subsequent washing with deionized water.

To summarize the findings on the recycling of polycotton with conservation of both components: the

research on adaption of the lyocell process for polycotton recycling has shown that NMMO has been

employed as a pretreatment for enzymatic hydrolysis of WBFs, resulting in high cellulose recovery and

subsequent glucose production (>90%), as well as recovery of PET. Despite high yields, the NMMO

process suffered from side reactions and dangerous runaway reactions. Therefore a new class of solvent

compounds have been investigated: ionic liquids. It was found that imidazolium based ILs (i.e.

[BMIM]AcO, [BMIM]Cl, and [AMIM]Cl) could be adapted for use in the lyocell process. Despite their

advantages, some limitations such as high viscosity and limited miscibility with hydrophobic reagents

occurred, especially at room temperature. This problem was overcome in two different ways: addition

of polar and aprotic co-solvents (e.g. DMAc, DMSO, DMF, NMP, etc.) or using distillable acid-base

ILs (i.e. [DBNH]OAc, [DBUH]OAc). In this way the limitations were resolved and less viscous

solutions could be produced for spinning of regenerated cellulose fibers. All of the regenerated fibers

from the ILs possessed tensile strengths comparable to that of the NMMO process and higher than that

of the viscose process. A drawback might be the diminished tensile strength of the separated PET and

recovery of ILs after the process.

30

2.3 Polyester Recycling

2.3.1 PET structure and virgin PET Synthesis A totally different approach to polycotton recycling has been of interest to academic and industrial

research as well: polyester depolymerization. Polyesters can be defined as manufactured fibers in which

the fiber forming substance is any long chain synthetic polymer, composed of at least 85 wt% of esters

derived from terephthalic acid and a dihydric alcohol (such as glycol).197 The most widely used polyester

is PET, a product of the (poly)condensation reaction between ethylene glycol (EG) and terephthalic acid

(TPA), which is commonly simply referred to as polyester.198 Two different grades of PET (fiber-grade

and bottle-grade) are dominant in the global market.199 These two PET grades possess differences in

molecular weight and intrinsic viscosity (IV), optical properties and production recipes.200

PET can be produced via two different pathways. The first is the esterification reaction in which

TPA reacts with EG to form bis(hydroxethyl)terephthalate (BHET) as a pre-polymer. The second

pathway also forms BHET, but from a transesterification reaction of dimethyl terephthalate (DMT) with

the EG. After formation of BHET the polycondensation reactions can take place to form PET.199 The

two different pathways of esterification (a and b) and the final condensation reaction (c) are shown in

Figure 28.

The TPA process has been the most widely applied in PET production and requires temperatures

between 240 and 260 °C. Due to high versatility and chemical properties, such as high tensile strength,

high E-modulus, chemical resistance, clarity, and relatively high thermal stability, PET has been

employed in a wide range of applications.197,199 Although, more than 60% is applied in the textile

industry.

Figure 28: PET synthesis reactions a) esterification reaction of TPA and EG to BHET, b)

transesterification of DMT with EG to form BHET c) Final condensation reaction of BHET to form

PET.199

2.3.2 PET fiber spinning When used in the textile industry polyester can be spun into filaments, staple fibers or textured yarns by

a process called melt spinning.201 In this process, a viscous melt of polymer is extruded through a

spinneret into a quenching chamber, where a blast of cold air or gas is directed onto the surface of the

fibers for solidification.202 After cooling and application of a spin finish the filaments or staple fibers are

drawn on a winder. An overview of this method is shown in Figure 29.

31

Figure 29: Typical melt spinning process. Polyester pellets are fed into a vessel. By simple heating the

pellets are melted to a viscous fluid. Through a gear pump the polymer melt is continuously put out to

a spinneret. During air cooling the fibers are solidified and spun onto a winder.203

2.3.3 PET Depolymerization with Conservation of Cellulose As PET shows chemical and thermal stability a problem arises: PET is so stable that it is essentially

non-bio-degradable when discarded as waste.204 Therefore recycling methods have been employed in

the past decades to reduce the amount of plastic waste including both mechanical and chemical

recycling. Mechanical recycling of PET can be performed by sorting, separation and size reduction of

the materials, before being able to be re-melted and spun back into fibers.205 However, these processes

usually result in downcycled products that are used in insulation for external double walls or buildings

or as reinforcing materials in polymer concrete, due to the lowered IV and molecular weight of the

PET.206 In order to increase closed-loop recycling of PET, research into chemical recycling has emerged

over the past two decades. In chemical recycling the backbone of the polymer chain is either degraded

into monomeric units by depolymerization or ruptured into larger fragments by random chain scission.38

These processes include acid, alkaline or enzymatic hydrolysis,15,35–37,207,208 alcoholysis40,

methanolysis,209 glycolysis,41,210 aminolysis,211 and the use of quaternary ammonium salts.42 Overviews

of these recycling methods can be found in the papers of different authors.38,211,212 Out of these different

depolymerization methods only the hydrolysis and alcoholysis methods degrade PET to monomeric EG

and TPA.13 Therefore these are the most interesting techniques for blended textile recycling.

Despite the large amount of research on the chemical recycling of PET, not much research has

focused on the recycling of polyester from WBFs.15 The earliest approaches employed alcoholysis and

hydrolysis.213,214 Negulescu et al.214 created a recycling process for 50/50 blends of polycotton by

alkaline hydrolysis of PET by 3M NaOH at 95-108 °C. Afterwards TPA could be filtrated subsequent

to acidification of the aqueous phase, while the filtrate was concentrated by evaporation to yield an

aqueous solution of EG. Cellulose was also separated from this mixture and re-spun into fibers after

dissolution in NMMO.

Palme215 investigated polycotton recycling by alkaline hydrolysis. This process used 5-15 wt%

aqueous NaOH and was tested both with and without the addition of a phase transfer catalyst (PTC),

namely benzyltributylammonium chloride (BTBAC). The hydrolysis took place at 70-95 °C for 1-3 h,

which resulted in the formation of disodium terephthalate salt and EG. Both products are soluble in the

32

aqueous phase. During this process cellulose was not dissolved and therefore filtrated from the mixture.

After the reaction has finished the aqueous phase is acidified to a pH of 2.5-3, which causes both

formation and precipitation of TPA. This process is shown in Figures 30 and 31.

Without the addition of the PTC, the hydrolysis is relatively slow since the hydrophilic

hydroxide ions are dissolved in the aqueous phase and PET is hydrophobic solid material. The PTC

transfers the hydroxide ions to the surface of the PET to facilitate the reaction.216 Most commonly used

PTCs are quaternary ammonium or phosphonium salts with lipophilic side chains. The use of BTBAC

as the PTC in the process of Palme resulted in complete hydrolysis of PET in 40 min in 10% NaOH at

90 °C and was recovered as pure TPA.215 It was also concluded that the isolation of cotton and TPA as

pure streams implied that PET fibers could be regenerated with either virgin or recycled EG, despite

separation and purification of EG was not considered in her research. The retrieved cotton fibers could

also be used in the production of regenerated cellulosic fibers.

Figure 30: (a) Hydrolysis of PET with NaOH into disodium terephthalate salt and EG. and (b)

Formation of TPA from the disodium terephthalate salt through acidification with sulfuric acid.13

Figure 31: Overview of the separation process of polycotton textiles by polyester hydrolysis with the

use of a phase transfer catalyst. During the alkaline hydrolysis of polycotton PET was depolymerized.

The cotton compound was separated through filtration, after which the PET filtrate was acidified to

precipitate TPA.13

Besides alkaline hydrolysis, some research in neutral and enzymatic PET hydrolysis methods was

conducted.217,218 Neutral hydrolysis is carried out by using water (or steam) at a pressure between 1 and

4 MPa and temperatures between 200 and 300 °C.217 Cutinases, a class of enzymes belonging to the α/β-

hydrolase family, showed the ability to hydrolyze the ester bonds in PET, albeit in low percentages.219,220

It was reported that only PET with low crystallinity could be completely hydrolyzed by cutinases.

Complete hydrolysis of high crystalline PET, on the other hand, was either very difficult or

impossible.221–223

Therefore Quartinello et al.218 developed a process in which the neutral hydrolysis treatment

was combined with enzymatic hydrolysis. In this process pure PET was first heated with water to 250

°C at a pressure of 39 bar to reduce it to a whitish powder. This whitish powder consisted of 85% TPA

according to HPLC and 1H-NMR analysis. Since it was not possible to hydrolyze the PET any further

33

than 85%, despite further increases in temperature and pressure, the powder was incubated with different

concentrations of Humicola insolens cutinase (HiC) at 50 °C for 24 h to hydrolyze the remaining

oligomers. This incubation resulted in an increase of TPA yield to 97%. It was implied that future

research should focus on the use of the recovered TPA to regenerate PET fibers.

Very recently Yousef et al.224 took a different approach to chemical recycling of polyester from

waste jeans, with the use of a Switchable Hydrophilicity Solvent (SHS). In earlier research SHS was

used to separate all layers of multilayer food packaging, including polyester.225 The polyester and

organic components were extracted by dissolving the layers in the SHS and then using addition of CO2

to alter the hydrophilicity of the solution. This approach was then used in the research of Yousef et al.224

to create a process for recovery of both cotton and polyester fibers from textile waste. The first step

involved leaching of textile dyes from the 80/20 cotton/polyester waste jeans by nitric acid (HNO3),

followed by dissolution of the polyester in the SHS, N,N-Dimethylcyclohexylamine (DMCA). HNO3

was regenerated by removing the dyes with activated carbon. After the dissolution of polyester, the

cotton fibers were liberated from the SHS in the second step. Subsequently, to recover the polyester

from the SHS solution, distilled water was added at double the volume of the solution followed by

cooling in an ice bath for 1 h, after which CO2 was added. In the final step filtration was performed to

extract the polyester, while the remaining solution was heated overnight at 45 °C to remove CO2 and

revert the SHS back to its original hydrophobic state. This approach resulted in a polyester and cotton

recycling rate of 96.3% and an acid and SHS regeneration of 99.5%, although it was not tested for

solvent reuse in the process. An overview of this process is shown in Figure 32. This process leads to a

decrease in power consumption and polymer degradation, as well as leading to lower gas emissions

normally caused by heating for regeneration in case of other, more traditional solvents. Additionally,

the potential of this process for industrial application was defined by investigation of the economic

benefits of this new technology. It was found that economic returns of waste were up to 1629 $/ton and

the carbon footprint of the waste was reduced by 1440 kg of CO2-eq/ton.

Figure 32: Separation flowchart of waste jeans (80/20 cotton/polyester). After a leaching treatment to

remove textile dyes the polyester was dissolved in DMCA and separated from the cotton fibers

through filtration. By use of CO2 the hydrophilicity of the solvent was changed and the polyester was

regenerated.224

34

In summary, research in PET depolymerization from WBFs has shown that PET could be depolymerized

to its monomers EG and TPA. Methods to depolymerize PET from textile waste included alkaline

hydrolysis (i.e. NaOH), enzymatic hydrolysis (i.e. cutinase), and the use of switchable hydrophilicity

solvents (i.e. DMCA). In the case of alkaline hydrolysis by NaOH a PTC was used (i.e. BTBAC) to

improve the hydrolysis rate by transferring hydroxide ions to the surface of PET to facilitate the reaction.

Pure TPA was recovered and after polymerization with EG could subsequently be used for melt spinning

to regenerate PET fibers. In the case of the switchable hydrophilicity solvent, distilled water and CO2

were added to precipitate the PET after separation from the cellulose material. Moreover, it was found

that economic and environmental benefits could be retrieved from this process.

2.4 Commercialized recycling processes Industrial application of recycling processes is a big part of reaching economic viability, which in turn

is needed for the general population to accept the processes of the circular economy.46 Therefore this

section will give an overview of consortia and (start-up) companies working on polycotton recycling

processes that have either patented these processes or made efforts to commercialize them.

2.4.1 Cellulose recycling

Re:newcell

Re:newcell, a Swedish company working on textile recycling, has invented a process of cotton recycling

which they call circulose.226 In the circulose process cotton is shredded down mechanically, after which

the cellulose is dissolved in 4-18% NaOH. After this the solution is filtered and the cellulose is thereafter

regenerated following the viscose process.227 An overview of the process is shown in Figure 33. This

process may result in lower fiber properties than that of the virgin viscose process, therefore blends of

virgin and regenerated materials could be produced to increase properties.226 Re:newcell has partnered

with H&M to produce 50/50 circulose/viscose yarns, and a pilot plant has been built in 2018 in

Sweden.228

Figure 33: Re:newcell process of recycling cotton fibers. Cotton waste is shredded mechanically after

which the cellulose is dissolved in an alkaline solutions. The steps of the aforementioned viscose

process are followed to regenerate the cellulose from this solution.226

35

Infinited Fiber Company

Infinited Fiber Company is a Finnish company that developed a technology for recycling of cotton

textile waste by cellulose carbamate (CCA) technology. Their process consists of three major steps.229

First, shredding of the waste with subsequent removal of silicates with NaOH, bleaching with ozone,

and removal of metal by acidic treatment before drying.230 The chemical pretreatment was also used

with high temperatures and pressures to remove polyester residues from the waste and finalize fiber

separation.229 This fiber separation takes place by alkali and urea treatment at elevated temperatures and

CCA is formed in the process. To improve dissolution of cellulose into aqueous NaOH, carbamate

groups are added into the cellulose backbone in the carbamation process (Figure 34). In the final step,

the CCA is dissolved in a spinning dope consisting of an aqueous solution of NaOH containing zinc

oxide at 10-25 °C, after which the solution was filtered through a sieve to remove residual polyester.

Polyester residues are difficult to remove from the spinning dope by filtration, therefore multiple

filtration steps with high efficiency filters were required before filaments can be spun. A pilot plant has

been built in 2019 capable of processing 50 kt per annum and they have created garments together with

H&M and Adidas.231

Figure 34: Cellulose Carbamate formation reaction. Urea reacts with cellulose in the presence of

NaOH to form cellulose carbamate, which is more soluble than cellulose in NaOH.232

Blocktexx

Australia based Blocktexx is another interesting company that has developed one of the recycling

methods mentioned before, acid hydrolysis.233 Their S.O.F.T. (Separation of fiber technology). process

involves mechanical shredding of polycotton blends, followed up with mixing of the waste with a

solution of 2.5% sulfuric acid in water. In the next step, the aqueous solution with the textile waste is

heated to 120-150 °C at a pressure of 5 bar to catalyze the acid hydrolysis. The mixture is then dewatered,

with subsequent washing to retrieve a mixture containing polyester fiber and MCC particles. This

mixture is then filtered by a decantation centrifuge and the liquid, still partially containing MCC, is

reused as a supplement to mix with another batch of polycotton waste. This process is repeated two

more times and in the final stage the recovered cellulose particles and polyester fibers can be used to

generate new fibers. This process is shown in Figure 35. A pilot plant has been built in Queensland,

processing textile wastes up to 10 kt per annum with a recovery rate of 95%.234,235

36

Figure 35: S.O.F.T. process flowchart for separating polycotton textiles. In this process dilute sulfuric

acid is used at high pressure and temperature to depolymerize cotton from the mechanically shredded

polycotton. After separation polyester fibers and MCC are retrieved.233

Circ

In a recent patent, the startup company Circ (formerly Tyton Biosciences LLC) has developed a process

of polycotton recycling by subcritical water in the hydrothermal treatment.236 Subcritical water refers to

liquid water at temperatures between the atmospheric boiling point and the critical temperature (374 °C)

that possesses unique properties (e.g. density, dielectric constant, ion concentration and solubility).

Furthermore, subcritical water is a non-toxic, environmentally benign, inexpensive, and therefore green

solvent that can be used alternatively to chemicals traditionally used in textile recycling. TPA and EG

are produced by treatment at 105°C to 190°C with a pressure of 40-300 psi at a pH range of 10-14. In

this stage a cellulose with a degree of polymerization of about 150-2500 is also formed. With further

hydrothermal treatment by subcritical water, cellulose could be recovered with a purity of 94-98%,

confirmed by HPLC analysis. However, no cellulose yields or possible side products were reported.

Polymerization of TPA with EG could be performed in an industry standard batch autoclave at 235-

290°C with appropriate catalyst, such as antimony or a titanium catalyst.

This process has been used in apparel of Patagonia237 and Circ is part of the Fashion For Good

concern, consisting of multiple corporate partners (i.e. Adidas, C&A, Chanel, Zalando, etc.) and

affiliated partners producing the materials (i.e. Mistra Future Fashion, Worn Again, Amsterdam Fashion

Institute etc.).238

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SaXcell

As mentioned before, viscose and NMMO have been two of the commercialized strategies for textile

recycling, both being used for cellulose regeneration. However, the viscose process has seen a decline

in use due to its environmentally harmful process as well as the derivatization into xanthate.66 Therefore

the NMMO process, without need for initial derivatization of the cellulose, has seen more use.75 This

approach is referred to as the lyocell process and commercialized under the name Tencel®.239,240 The

lyocell process has been adapted for the wet spinning of waste cotton from NMMO monohydrate.

SaXcell BV, a Dutch spin-off company from Saxion University of Applied Sciences, has

patented this process.241 In their process polycotton textile waste is treated with a mixture of NMMO

and water, which does dissolve the cellulose but does not degrade it. The solution is then heated to 85-

100 °C. The composition is preferably made up of 10-17 wt% cellulose, 5-15 wt% water and the rest

NMMO. NMMO in this process can be recycled up to 98% and polyester is not degraded in the process.

The DP of the cellulose in solution ranges between a DP of 400 and 800. After a development stage of

5 years SaXcell worked together with a consortium of local textile companies to open a pilot plant in

November 2020, capable of processing 25 tonnes textile waste per annum. 241,242

Ioncell-F

The lyocell process has also been adapted in different research efforts mentioned before, such as ionic

liquids. For instance the Ioncell-F process of Sixta and coworkers has started their commercialization

phase in 2018 by patenting the Ioncell technology.243 Furthermore, a roadmap (Figure 36) shows that

plans were made to start the build of a pilot plant in 2020, which will have to obtain proof of concept

until 2023, after which the start of commercialization will happen in 2025.244,245

Figure 36: Roadmap of commercialization of Ioncell-F project. 244

Elsayed et al.246 investigated the recycling of ILs in the Ioncell-F process, in order to reach the pilot

scale. By using pre-hydrolysis cellulose kraft pulp as cellulose input the recyclability of the ILs was

tested. It was found that with an increasing number of cycles in a closed-loop operation, there is a higher

risk that alterations in the solvent composition negatively affect the quality and rheology of the cellulose

solution and therefore also affect the stability of the fiber spinning process. Besides the previously

mentioned [DBNH][OAC] the Ioncell process was also performed with the guanidine-based 7-methyl-

1,5,7- triazabicyclo[4.4.0]dec-5-enium acetate, [mTBDH]OAc. After a five-cycle trial the deviations of

the recovered [mTBDH]OAc showed no impairment in its ability to dissolve up to 13 wt% of cellulose.

On the other hand the [DBNH]OAc already showed significant loss of dissolution capability after the

first cycle. Therefore it was concluded that [mTBDH]OAc should be used for the commercialization of

the Ioncell-F process. An overview of the ioncell process can be found in Figure 37.

38

Figure 37: General overview of the Ioncell-F process showing cellulose dissolution by [DBNH]OAc,

and fiber regeneration by the use of an anti-solvent. Recycling of the diluted IL occurs through

coagulation bath filtration and centrifuge evaporation. In two subsequent evaporation stages (TFE-1

and TFE-2) the IL is further concentrated and the distillate is fed back to the coagulation bath. The

recovered IL is fed back to the kneader without addition of fresh IL.246

Evrnu

Seattle based company Evrnu invented NuCyclTM, recycled polyester and cotton fibers, and partnered

with Levi’s and Adidas to create limited edition garments.247 In their process a wide variety of methods

are described, ranging from enzymatic hydrolysis or acidic hydrolysis, to ionic liquids or alkaline

hydrolysis. In all cases the cellulose is first dissolved and separated from the polyester, after which new

fibers can be spun from the regenerated cellulose. The polyester component remains and can be

hydrolyzed by NaOH or other highly alkaline reagents. A phase transfer catalyst such as BTBAC was

also mentioned to improve the reaction rate.

2.4.2 Polyester recycling

Blend Re:wind

The aforementioned polyester depolymerization process with NaOH and BTBAC as phase transfer

catalyst has been developed by a partnership between the Swedish Textile Research Institute (RISE),

Mistra, and different companies to create a consortium based program called Mistra Future Fashion.9,215

The project ran from 2011 until 2019 and has been named the Blend Re:wind process.15 Despite not

being commercialized, the research has continued after the project ended.248 Researchers from RISE

have investigated other methods of polyester recycling from textile waste through glycolysis.249 Guo et

al.250 showed that BHET could be formed by glycolysis catalyzed by Mg-Al double oxides with a yield

of 80%. After three cycles the catalyst had to be re-activated by heat-treatment. The PET fibers

regenerated from this process showed similar spinnability and mechanical properties when compared to

virgin PET.

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Worn Again Technologies LTD

UK based Worn Again Technologies LTD has developed a process for polycotton recycling that can

use two different waste sources.251 The first is PET from bottles and packaging. The second is polycotton

textile waste consisting of 40/60 cotton/polyester. In their process polycotton waste is first

decontaminated and shredded to small pieces. In a second step the PET is dissolved from the blend in

either 1,3-dimethylimidazolidinone (DMI) or butyl benzoate at 70-110 °C for 1 to 2 h (Figure 38).

Subsequently the cotton can be separated by filtration and the solution cooled down to precipitate the

PET. The PET could be recovered with yields of 39-81%.

Figure 38: The most common solvents used in the Worn Again process251

Worn Again has been part of the Fashion for Good program and in 2018 the company has been awarded

a grant as the first recycling technology to receive a Cradle to Cradle certification.252 In 2020 a pilot

plant was opened at CPI, a technology and innovation center in Redcar, England, capable of processing

5 kt of polycotton waste per annum.2 Their process is shown in Figure 39.

Figure 39: Waste processing flowchart of the Worn Again process. During pre-processing the waste

polycotton waste is cleaned and shredded, after which PET is dissolved from the blend by 1,3-

dimethylimidazolidinone (DMI) or butyl benzoate at elevated temperatures. By filtration the cellulose

can be separated from the solution containing PET. After a cooling period the PET precipitates and

both compounds can be used to spin new fibers.253

CuRe, Unific Inc., Carbios & Jeplan

Also in the field of polyester recycling, CuRe technology has been developed as a joint effort by

Cumapol, DSM-Niaga, and Morssinkhof.254 CuRe is able to hydrolyze PET from different sources such

as bottles, textiles and non-wovens. A patent has been applied for the recycling of carpets by glycolysis

or methanolysis, however no firm description of the process was given.255 Moreover, a pilot plant has

been built in Emmen, the Netherlands to demonstrate the process.254

American based Unifi Inc. has shown to be a leading company in processing PET bottles in an

open-looped recycling process to form polyester fibers under the name REPREVE®, which is used by

a number of clothing companies (e.g. Patagonia, Quicksilver, and O’neill).256

40

Carbios, a French company, has developed a technology for recycling of different waste sources

with a PET fiber content of 65-95%, the rest being cotton, viscose or nylon.257 Their technology is based

on enzymatic hydrolysis of PET. Cutinases are used to depolymerize the polyester into EG and TPA or

BHET. A plastic binding protein is also used to facilitate the depolymerase binding to polyester.

Afterwards the generated monomers can be reused to create new PET fibers or non-wovens.258 A

demonstration plant has been built in Lyon, France in 2020 with an estimated capacity of 50-100 kt per

annum.

Japanese based company Jeplan created a process called BRING TechnologyTM for production

of BHET from a solution of PET with EG as solvent at 220 °C.259 Sodium methylate is added to act as

an ester exchange catalyst. After repolymerization PET bottles and PET fibers could be reformed from

BHET and EG. A pilot plant has been built in Japan in 2017, however no numbers on capacity of textile

waste recycling were published.260

Other companies

Other companies that have commercialized recycling of polyester, cotton or polycotton include Convert,

I:CO, Wolkat, Pure Waste, Rífo, Refibra, and Södra.231 Refibra and Södra have been using the viscose

process to recycle pre-consumer cotton waste combined with wood pulp to create new fibers. The other

companies mentioned here have used mechanical recycling as their recycling processes. An overview

of almost all European based companies that are involved in polycotton recycling can be seen in Figure

40.

Figure 40: Overview of European (poly)cotton recycling companies. 231

As shown in Figure 40 there have been a lot of European companies working on recycling of textile

wastes. It can be concluded that those that work on chemical recycling could be divided in three different

categories: pure cotton recycling (i.e. Re:newcell, Infinited fiber), pure PET recycling (i.e. CuRe,

Carbios, and Jeplan), and polycotton blend recycling. Polycotton blend recycling can happen by cotton

depolymerization through acid hydrolysis (i.e. Blocktexx) and hydrothermal treatment (i.e. Circ),

adaption of the lyocell process with NMMO (i.e. SaXcell) or ILs (i.e. Ioncell-F), or by PET

depolymerization (i.e. Blend Re:wind and Worn again).

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3 Life Cycle Assessments With the different methods of chemical recycling that have been or will be developed, it is important to

find out what the effect is of implementing these recycling methods on the life cycle of polycotton. As

mentioned in the introduction, life cycle assessment (LCA) is a method of comparing the environmental

impact of the different production/recycling methods with each other. In the next section an overview

of LCAs is given after which the aforementioned recycling processes, that have been evaluated by LCA

studies, will be discussed.

According to the International Organization for Standardization (ISO) guidelines, the LCA

methodology comprises of four steps: goal and scope definition, inventory analysis, impact assessment,

and interpretation (Figure 41).261 The first is the step in which the study objectives, product function,

functional unit (FU), and the system boundaries are defined. The FU is described as a quantification of

the performance of a product system for use as a reference unit (e.g. a kg or tonne of starting material

or final product). The system boundary is a set of criteria to define which processes are included in the

assessment of the product system. The second step, life cycle inventory analysis (LCI), is the step in

which an inventory of material and energy flows is constructed, subsequently these flows are quantified.

The third step, life cycle impact assessment (LCIA), is a phase in which additional information is

gathered and aggregation of the environmental impacts are quantified into distinguishable factors known

as impact categories. An example of this is the impact category ‘global warming potential’ (GWP) which

uses kg CO2 equivalents as its corresponding indicator. This indicator can be used to show the impact

of a process step on greenhouse gas emissions. In this way impact categories provide quantifiable

information on the processes and makes comparisons between different processes possible. In the final

step, interpretation, the results of either an LCI or LCIA (or both) are summarized and discussed. This

can be used to form a basis for conclusions, recommendations and decision-making. However, this is

not a linear process, interim interpretation can be made during the other three steps.

Figure 41: The four steps of life cycle assessment. During the first step, Goal and Scope Definition, the system

boundaries and FU and study objectives are set. The second step, LCI, is the step in which an inventory of

material and energy flows is constructed, subsequently these flows are quantified. The third step, LCIA, is a

phase in which additional information is gathered and aggregation of the environmental impacts are quantified

into distinguishable factors known as impact categories. In the fourth step, these impact categories are

interpreted. 261

Different variants of an LCA can be conducted, depending on the goal, scope, and system boundary

definition step. The standard LCA addresses the environmental aspects throughout a product’s total life

cycle from raw material obtainment through production, use, end-of-life treatment, recycling, and

disposal.261 This is called a cradle-to-grave LCA. However, in some cases a different approach for the

LCA is necessary. For instance when no information is available on one or more of the stages in a

product’s life cycle, or an assessment focusses on the production or recycling process only. The latter is

42

usually the case for LCAs on textile recycling processes, since the recycling process itself usually does

not impact the use phase of a textile product (except when certain properties of a final product have to

be incorporated in the recycling process, such as composite materials or blends) and the use phase

contains many uncertainties.23 This mostly results in a cradle-to-gate approach, where the use and

disposal phases are not considered for the assessment. 44,262–264 Other, more unconventional approaches

include gate-to-gate analysis and gate-to-cradle analysis.265,266 The gate-to-gate approach includes the

assessment of only one or two steps of the process (e.g. pretreatment of textile waste), while a gate-to-

cradle approach accounts for the environmental impacts from the point of textile waste generation to

conversion into a new fiber product.

3.1 Viscose and Lyocell LCA Reference LCAs on the known viscose, lyocell, and PET production processes have to be discussed in

order to compare the LCAs of the textile recycling methods. Shen and Patel267 have performed a cradle-

to-gate LCA on man-made cellulose fibers from Lenzing AG (i.e. viscose and lyocell) and compared

that to cotton and PET fibers. The processes of these man-made fibers are shown in Figure 42. The FU

that was used in this study was one metric tonne of staple fibers. The impact categories used in this

assessment were identified as: GWP, abiotic depletion, ozone layer depletion, human toxicity, fresh

water aquatic ecotoxicity, terrestrial ecotoxicity, photochemical oxidation, acidification, and

eutrophication. The results showed that the man-made cellulose fibers required less energy use than

PET, but higher than that of cotton. Since cotton fiber generation is not an energy intensive process, its

high environmental impact is based on land and water use as well as the ecotoxicity caused by pesticides

used in cotton cultivation. Additionally, the fertilizer use causes a large eutrophication potential. In all

of these impact categories the man-made cellulose fibers score better than cotton and were better than

or comparable to the impact of PET. The most harmful impacts from the viscose process were attributed

to CS2 use, SO2 and NOx emissions, and the sourcing of the chemicals used in the process. Compared to

the other processes, the lyocell process benefited from low energy consumption, low chemical use (i.e.

recycling of NMMO), low CO2 and SO2 emissions, and low water consumption. It was therefore

concluded that all man-made cellulose fibers had better environmental profiles than PET and cotton,

with the lyocell method as the least environmentally harmful. Some of the LCAs conducted on the

recycling of cotton, polyester or polycotton used the data of Shen et al. as their reference.226,262,266

Figure 42: The viscose and lyocell processes. During the viscose process mercerized cellulose pulp is

derivatized by xanthation after which a viscose dope can be spun into fibers. In the Lyocell process the cellulose

pulp is dissolved in NMMO and directly coagulated in a spin bath, after which the Lyocell fibers are spun and

finished. In both processes chemicals are (partly) recovered.267

43

3.2 Pretreatment of enzymatic hydrolysis LCA The first LCA of the recycling processes that will be discussed is that of Rosson and Byrne265, who

produced a comparative gate-to-gate LCA on the alkali and acid pretreatment step in the recycling of

cotton. It was acknowledged that their method had limitations before and after the pretreatment, by not

accounting for environmental impacts caused during the initial production of virgin cotton and the use

phase of the waste cotton prior to the pretreatment, nor accounting for the impacts associated with the

use of pretreated materials compared to virgin materials (Figure 43). However, it was deemed

unnecessary to account for these impacts to draw a quantitative comparison between the acidic and

alkaline processes. In the alkaline pretreatment an aqueous solution of 10 wt% NaOH was heated to 90

°C for 15 h. The treated cotton was then filtered, washed, and dried. Citric acid powder was added to

neutralize the pH in these steps. The acidic pretreatment, on the other hand, required a significantly

lower concentration with an aqueous solution of 0.5 wt% sulfuric acid heated to 75 °C for 1 h. This

cotton was also washed filtered and dried in similar fashion as the NaOH pretreatment. Although, here

addition of sodium carbonate powder to the filtrate was used to neutralize the pH instead of citric acid.

Eleven different impact categories were selected in the assessment, of which six were dominated by the

electricity required in the heating of the solutions. It was concluded that the impacts of the alkali method

were between a factor 13.6 and 33.1 higher than that of the acid method (a GWP of 6.70 kg CO2-eq. for

the alkali method vs 0.49 kg CO2-eq. for the acid method and ozone layer depletion of 4.67×10−8 kg

CFC-11-eq. vs 1.41×10−9 kg CFC-11-eq. respectively), mostly due to the higher temperature and longer

pretreatment time during the alkali method. Furthermore, in the alkali method more energy for

transportation and manufacturing of raw materials was needed per functional unit. Therefore the acid

pretreatment was deemed to be more sustainable than the alkali pretreatment process.

Figure 43: System boundary of a pretreatment step in the chemical recycling of cotton. Waste cotton

comes into the system which is then pretreated with either an alkali or acid solution. The dissolution

and fiber spinning step are not accounted for in the system boundaries, since these steps are the same

for both processes.265

3.3 Enzymatic hydrolysis LCA Subramanian et al.266 conducted a gate-to-cradle LCA study on the environmental impact of the work

from their research group on enzymatic hydrolysis of cotton to glucose with PET fiber recovery,

conducted by Li et al.98, Hu et al.100 and processed by Kwan et al. 268 in a techno-economic analysis. The

inputs of the LCA were based on the primary input data of the aforementioned studies.

For this approach the FU was determined as 1 kg of recovered PET fibers, while the input of

textile waste consisted of a 50/50 polyester/cotton mixture. 266 Furthermore, the processes included in

44

the study were crushing, pretreatment, enzymatic hydrolysis, hydrolysate purification, and melt-

spinning. The hydrolysate conversion into a high viscous glucose-rich syrup for further usage in

different bio-based products (such as bio-plastics, bio-chemicals in industries, and bio-surfactants) is

also included. An overview of the system boundaries is given in Figure 44. The preliminary results of

the inventory step were divided over 18 midpoint impact categories which impact the environment

negatively (e.g. climate change, ozone depletion, terrestrial acidification etc.). These midpoint impact

categories were then grouped together in three different categories indicating the overall damage (i.e.

ecosystem, human health, and resource depletion). The impacts of these steps are expressed in

percentages of the total impact of the entire process, mentioned in brackets behind the respective impact

category. It was found that the alkali pretreatment and melt-spinning were environmentally the most

impactful in the bio-recycling process. The pretreatment step generates higher impact than the other

steps due to energy intensive sub-processes such as cooling, heating, neutralization as well as production

of urea, which is employed during the pretreatment step. Therefore pretreatment contributed the most

to ecosystem quality (59%) and human health impacts (61%). Hard coal mine operation and natural gas

production during the production of urea contributed the most to resource depletion. Conversely, melt-

spinning mainly affects the resource depletion category (34%) and is less impactful on ecosystems

(14%) and human health (15%). In addition, the re-spinning process of PET fibers required an input of

PET bottle flakes of 80 wt%, which are added to obtain industrial-grade PET fibers suitable for textile

application. Making this an open loop recycling method. Xylene used in the flake production is

responsible for the fossil resource scarcity of this step. Enzymatic hydrolysis was found to have a

relatively low impact on human health (12%) and resource depletion (8%). However, it mostly impacts

the ecosystem quality (14%), which is caused by the high impacts of enzymatic hydrolysis on marine

eutrophication (64%) and land use (49%), due to the use of citric acid and cellulase enzyme.

Furthermore, pre-treatment (207 MJ) is also the most energy-intensive step per FU, followed by melt-

spinning (98.5 MJ) and enzymatic hydrolysis (44.8 MJ).

Figure 44: Illustration of the LCA system boundary. Cotton-polyester textile waste blends at its end of

life are biologically processed (pre-treatment, hydrolysis, purification, and melt-spinning) and PET

fiber is recovered as output. The recovered PET fiber becomes the feedstock for a new garment

production cycle represented by blue arrows.266

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3.4 NMMO & IL LCA Opposed to the previous LCA, Zamani et al.269 conducted a gate-to-grave study on the recycling of

textile waste with NMMO, starting with the moment the collected textile waste enters the recycling

facility as the ‘gate’ and the residues generated in the recycling system as the ‘grave’. The FU of this

study was 1 tonne of household textile waste, consisting of a 50/50 polyester/cotton mixture. The

processes included in this assessment were the separation of polyester from cotton and the recycling of

polyester. These processes were then compared to remanufacturing for reuse and incineration for energy

recovery. The impact categories that were chosen for this research were GWP and energy use. The

analysis showed that per tonne household textile waste GWP savings could reach up to 8 tons of CO2

eq. and primary energy use savings up to 164 GJ, which resulted mainly from avoided textile production

from virgin sources.

A more complete assessment of the NMMO process was conducted and compared to the ionic

liquid dissolution process by Righi et al.264 They performed a comparative cradle-to-gate LCA of cotton

dissolution by the ionic liquid [BMIM]Cl and NMMO/H2O in the lyocell process. Both dissolution

processes and all the upstream processes were analyzed. Since the main purpose of the system in this

LCA was to dissolve cellulose pulp to obtain textile fibers the FU was 1 kg of dissolved cellulose. The

boundaries for this system included all upstream processes such as cellulose pulp production,

transportations necessary for the process, and energy and chemical production. No downstream

processes were included in the system since it was expected that these would contribute the same for

both processes. In the study ten impact categories were defined of which GWP, freshwater aquatic

ecotoxicity, and human toxicity potentials were deemed to possess the most significant environmental

impacts. It was concluded that dissolution in IL appeared more promising from an environmental point

of view. Although the most significant increase in environmental impacts resulted from photochemical

ozone creation potential, volatile organic compound emissions and freshwater ecotoxicity. On the other

hand, a small reduction in toxic compound emission was observed. In both the IL dissolution and the

NMMO/H2O dissolution the major environmental impacts originate from precursor synthesis. These

syntheses are characterized by long supply chains from natural resources to the end product, requiring

large quantities of energy, solvents and organic compounds. Furthermore these processes involve

harmful emissions to air and water. For the IL route specifically, the synthesis and precursor use led to

most eutrophication potential of the process (80%) and freshwater aquatic ecotoxicity (99%) due to

discharge of formaldehyde into wastewater as well as nitrogen oxide, ammonia, and ion emissions

containing nitrogen into the air and wastewater. It was also found that solvent recovery is a key

parameter in the environmental assessment. In both cases the cellulose dissolution step contributions

were less than 10%.

3.5 LCA of commercialized processes

Infinited Fiber LCA

Paunonen et al.262 conducted a cradle-to-gate LCA on the previously mentioned process of cotton

recycling through the CCA process from the Infinited Fiber Company. In the CCA process shredded

polycotton waste is treated with NaOH and urea at elevated temperatures. To improve the solubility of

the cellulose into aqueous NaOH, carbamate groups are added to the polymer backbone. It should be

noted that during the conduction of this LCA the method was still in the development phase and not yet

commercialized. Two different scenarios for the LCA were conducted, one where a separate factory for

the recycling process was built and another where the CCA process was integrated in an existing pulping

mill. Cotton and viscose fibers were chosen as reference fibers. The FU of this process was 1000 kg of

finished fibers, the input of textile waste to create these 1000 kg were estimated at 1267 kg, of which

254 kg was lost during the pretreatment, filtration, and spinning due to impurities the other 13 kg was

lost due to buttons, zippers etc., that were removed during the cutting process. The system boundaries

(Figure 45) included the sorting, transportation, pretreatment, CCA process, and fiber spinning. In the

case of the integrated CCA scenario, the chemicals and water used for the processes were partially

derived from the pulp mill in which the process was integrated. The impact categories of this assessment

46

were limited to GWP and water scarcity and use. It was shown that the total GWP of the integrated

process was one third of the standalone process. Using the circulation and recycling of the chemicals in

the integrated process could potentially save up to 4000 kg CO2 eq. per FU. Furthermore, the GWP of

the integrated process was comparable to that of the viscose process and half of that of virgin cotton. In

the case of water usage and scarcity both scenarios showed very low impact when compared to the

viscose and cotton processes. It was therefore concluded that, despite still being in the development

phase and requiring pretreatment, the environmental impact of the CCA process was comparable to that

of viscose fibers from virgin wood pulp, without the harmful side-effects of using CS2. Integration of

the CCA process in a wood pulping mill showed the greatest advantages, especially when NaOH could

be recycled.

Figure 45: System boundaries for CCA fiber production in a stand-alone mill and in an integrated

pulping mill. In both scenarios sorting and transportation of discarded cotton textiles are included. In

the integrated process heat and cooling water come from outside the system boundary, while this is

included in the stand-alone factory.262

SaXcell LCA

Oelerich et al.263 conducted a cradle-to-gate LCA on the SaXcell process of cotton regeneration. The FU

of this process was determined at 100 kg of SaXcell pulp. The LCA included two scenarios for the pulp

generation: 100% cotton waste and a mixture of 90% cotton/10% PET waste. The assessment included

the first five steps of the SaXcell process (Figure 46) and the results of five impact categories were

reported, even though eighteen categories were calculated. These impact categories included GWP,

human toxicity, agricultural land use, urban land occupation, and water depletion. Furthermore, the

SaXcell process with 100% cotton waste was compared with that of a commercial sulphate pulping

method. The results showed that dissolving pulp from 100% cotton textile had a low impact on GWP

and water use. However when PET was included in the textile waste, the use of necessary process

chemicals increased, due to the need for separation of the two components, therefore the GWP doubled.

Compared to the sulphate method the 100% cotton waste pulping method showed comparable results in

all categories except agricultural land use, since the land use of SaXcell is 0.01 m2a, compared to 4.82

m2a for the Sulphate pulping process. In LCA, land use occupation is measured as area time (m2a).270 It

was concluded that the use of cotton waste as pulp feedstock for regenerated cellulose fibers has clear

advantages compared to other pulping processes.263

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Figure 46: The SaXcellTM recycling process and subsequent fabric production. NMMO is used as the

preferred cellulose solvent in this process to adjust the DP. Mixed textile waste is the input for this

system, while finished products are the output.263

Blend Re:wind LCA

Peters, Sandin and Spak44 conducted a cradle-to-gate LCA on the aforementioned Blend Re:wind

process of polycotton recycling and compared that to the production of virgin cotton and the viscose

process. The FU of this LCA was the recycling of 850 tonnes of mixed textile waste. Two different

scenarios were created with and without EG recycling, because this purification process appeared to be

an environmental hotspot in initial calculations. The boundaries were set to the collection of waste until

the production of cellulose and polyester before yarn spanning (Figure 47). It was assumed that the

recycled fibers were of sufficient quality to replace virgin fibers in a 1:1 ratio. In the LCA eight different

impact categories were considered including acidification potential, eutrophication, freshwater

ecotoxity, primary energy use, and water scarcity. The results showed that with the 850 tons of textile

waste, 280 tons of cellulosic fiber, and 350 tons of polyester could be produced, leading to a loss of 26%

of the original mass. The majority of this loss (19%) was associated with the mechanical shredding

processes where non-recyclable components (e.g. zippers and buttons) were separated from the main

waste stream, this may include small parts of fibers attached to these non-recyclable components. In

terms of contributions to climate change and primary energy consumption the Blend Re:wind process

was harmed by the EG separation process and NaOH production. Conversely, for the virgin products

the CO2 emissions during polyester production were the largest contributors to climate change.

Additionally, in terms of water scarcity the recycling processes were comparable to viscose and 4 times

less impactful than virgin cotton production. It was concluded that the alkaline hydrolysis process was

competitive with the alternative methods (virgin cotton and viscose production). Although almost half

of the indicators favor the single use scenarios, however the calculated differences were less than the

uncertainty associated with them in most cases. Furthermore, the scenario without EG purification

turned out to have relatively little benefit over the scenario with EG separation and recycling. The

emissions contributing to the GWP during the separation of EG were comparable to the emissions from

virgin glycol production. However, the recycling scenario including EG separation does not require EG

from virgin sources, which would implicate that it does have some environmental benefit in terms of

resource scarcity. Overall, the recycling methods appeared to be competitive with the single use

scenarios, despite the latter having the advantage of being completely optimized for large scale

production.

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Figure 47: System boundaries for the Blend Re:wind process. Mixed polycotton waste is collected

and shredded, after which the PET hydrolysis can be performed. Output of cellulosic and polyester

fibers are the end of the system boundary. In this scenario the EG is separated and purified for reuse to

create new polyester fibers. Another scenario without separation and purification of EG was also

used.44

Re:newcell LCA

Youhanan226 took another approach to quantify the implications of the Re:newcell process, since not

enough data was available due to confidentiality issues. Instead of conducting an LCA on the Re:newcell

process, an environmental assessment was conducted by using data from comparable processes, when

applicable (e.g. viscose process, PET recycling processes), and combining these to form a conclusion

on the environmental impacts of the Re:newcell process. The FU was determined at 1 tonne of mixed

household textile waste (including the complete range of polycotton waste). Environmental impacts

included in the study were recovery rate, energy use, water use, chemicals, emissions and waste

generation. This was compared to other recycling processes, incineration, and biogas production. The

energy required for the Re:newcell process was comparable to that of the polyester recycling process.

This was also shown for chemical use. Conversely, in terms of water use the polyester recycling and

biogas production were more sustainable. It was concluded that the Re:newcell process reduces the

impact of textile production on the environment more than other recovery methods. However, since it

replaces viscose instead of virgin cotton the quality of the fibers is lowered.

In summary, the research has shown that different variants of an LCA can be conducted to assess the

environmental impacts of (part of) a textile manufacturing process. Cradle-to-gate LCA is the most used

approach in textile recycling, since it was assumed that use phase and disposal phase for all methods are

more or less equal. Therefore, cradle-to-gate LCAs have been conducted on the enzymatic hydrolysis

with and without pretreatments, and on NMMO and ILs in the lyocell process. Additionally, cradle-to-

gate LCAs were conducted on some of the commercialized processes (i.e. Infinited fiber, SaXcell, Blend

Re:wind, and Re:newcell). In particular the LCA on the Re:newcell process was approached differently

49

to assess the environmental impacts, namely through use of data from comparable processes instead of

calculated data from the actual process. In the next section the LCAs will be compared and the processes

on which no LCAs are available will also be discussed.

4 Discussion As has been shown in the previous sections, various methods of chemical recycling are being researched,

both academically and industrially (i.e. cellulose hydrolysis, PET hydrolysis, and dissolution of

cellulose in NMMO or ILs). An overview of these methods can be seen in Table 1, while Table 2 gives

an overview of the commercialized methods researched by companies. Furthermore, Table 3 shows the

mechanical properties of the regenerated fibers of some of the discussed methods. In this section the

different methods will be compared to each other and the advantages and limitations of these methods

will be discussed.

Table 1: Published methods of (poly)cotton waste separation and recovery discussed in this review.

Treatment Method Polycotton compositiona Results (yield %)

Enzymatic Hydrolysis

Cellusoft L84 34/66 80% insoluble microfibrillar material

NaOH pretreatment86 93-98% cotton 99.1% glucose

NaOH pretreatment95 40/60 91% glucose and 89% polyester

NaOH/urea pretreatment98 40/60 98% glucose and 99% polyester

Sulfuric Acid pretreatment83 100% cottonb 25-28% loss of cellulose weight

Sulfuric Acid

(microwave pretreatment)103 87.8% cotton 74.2% glucose

Phosphoric Acid pretreatment99 85/15 79.2% glucose and 100% polyester

Phosphoric Acid pretreatment86 100% cottonb 63.4% glucose

NMMO pretreatment124 50/50 95% glucose

Cutinase218 100% polyester 97% TPA

Alkaline Hydrolysis

NaOH/urea109 100% cottonb Complete dissolution of cotton

NaOH with PTC215 50/50 Complete hydrolysis of PET

Acid Hydrolysis

Superphosphoric Acid116 100% cottonb Complete dissolution

Formic Acid/HCl118 >99% cotton 22.5% glucose yield

Sulfuric Acid29 100% cottonb 73.9% glucose yield

Sulfuric Acid111 50/50 Complete separation

Sulfuric acid (two-step)110 100% cotton 80-90% glucose yield

Phosphotungstic Acid112 35/65 85% MCC and 99% polyester

Hydrothermal (HCl)121 35/65 48% cellulose and 96% polyester

Hydrothermal (Citric acid)122 50/50 95.5% cellulose and 100% polyester

Other methods

DMCA224 80/20 96.3% glucose and polyester

a) polycotton mixture indicated by x/y with x% cotton and y% polyester b) cotton purity has not been

specified by the authors in the research

50

Table 2: Commercial methods of polycotton waste separation and recovery discussed in this review.

Company name Method Polycotton

compositiona

Final products

(yield %)b

Cotton Recycling

Re:newcell226 Dissolution in 4-18% NaOH 100% cotton Low tenacity viscose

fibers

Infinited Fiber229 CCA process 100% cotton

(sometimes 90/10)

Regenerated cellulose

fibers

Blocktexx233 Mild acid hydrolysis with

hydrothermal treatment not specified

95% MCC and PET

fibers

Circ236 Hydrothermal treatment

(subcritical water)

100% cotton – 50/50

cotton/polyester

Cellulose with 98%

purity and pure TPA

SaXcell241 Adapted Lyocell method with

NMMO not specified

400-800 DP cellulose

for fiber spinning

Ioncell-F243 [mTBDH]OAc 50/50 Ioncell fibers and

degraded PET

Evrnu247 Enzymatic, acidic, or alkaline

hydrolysis; ionic liquids not specified

Regenerated cellulose

fibers and untouched

PET

Polyester Recycling

Blend Re:wind215 Depolymerization with

NaOH and PTC 50/50

Complete hydrolysis

of PET

Worn Again Technologies

LTD251

Dissolution in DMI or butyl

benzoate 40/60

39–81% regenerated

PET

Cure254 Glycolysis or methanolysis 100% polyester Regenerated PET

fibers

Unifi Inc.256 Bottle-to-fiber (not specified) 100% PET bottles Regenerated PET

fibers

Carbios257 Cutinase (enzymatic hydrolysis) 35/65 – 5/95

EG and TPA or BHET

ready for PET fiber

polymerization

Jeplan259 EG as solvent with sodium

methylate in ester exchange 100% polyester 98.1% BHET

a) polycotton mixture indicated by x/y with x% cotton and y% polyester, sometimes this has not been

specified in the paper/patent b) due to patents the yields have not been reported for most processes

4.1 Polycotton waste separation It has been shown that acid or alkaline pretreatment is a crucial step in enzymatic hydrolysis to break

down cellulose to its monomers. Despite providing better results regarding glucose recovery by

NaOH(/urea) pretreatment (Table 1), acid pretreatments outperforms that in the environmental LCAs.

Furthermore, acid and alkaline hydrolysis could be performed without the need for subsequent

enzymatic hydrolysis. Using these acids for direct hydrolysis instead of pretreatment in enzymatic

hydrolysis could be better for the environment due to easier separation of the solvents without need of

recycling the enzyme. However, the LCAs on the pretreatment step and complete enzymatic hydrolysis

process showed that the pretreatment step generates higher impact than the other steps in the process,

due to energy intensive sub-processes such as cooling, heating, neutralization and, in the case of alkaline

pretreatment, production of urea. It was shown that acidic pretreatment was less impactful than alkaline

pretreatment, but it has not been shown what the difference would be during the direct hydrolysis of

both methods, since as of yet no LCAs on these processes have been conducted.

Besides being more environmentally impactful the NaOH(/urea) treatment has other limitations.

It has been shown that NaOH could be used for the dissolution of both cellulose and PET, albeit with

different temperature ranges or the addition of urea or a PTC in the different cases (for cellulose

hydrolysis or PET hydrolysis, respectively), due to their different properties. The outcome of these

51

studies implies that neither of these processes could be used effectively for the depolymerization of one

of the two components while leaving the other intact. It is, therefore, also strange that such high recovery

rates of both of the components of the polycotton have been reported. Since both components could be

hydrolyzed by NaOH, it would be expected that the other component should also be (at least partly)

hydrolyzed. Although it may be possible that this has to do with the difference in hydrolysis

mechanisms, caused by the PTC and the urea. The mechanism of PET hydrolysis required a phase

transfer of the NaOH into the organic phase in order to reach the PET. Which implies that during

cellulose treatment by NaOH/Urea the PET is left untouched in the absence of that PTC. However, it

seems that the cellulose should show signs of depolymerization during the PET hydrolysis treatment if

NaOH is present in the aqueous phase during the treatment.

Additionally, according to Pensupa et al.3, the long processing time, low solid-to-liquid ratios

during pretreatment, and needs of neutralization afterwards may hinder wide application of the alkaline

pretreatment process. These problems could be overcome by high recyclability of the reagents in spent

liquor, but should be investigated more in the future.

The combination of these limitations could also be part of the reason that development of the

Blend Re:wind process (Table 2) was discontinued in 2019 and the only companies using NaOH in their

processes focus on the recycling of pure cotton waste (e.g. Re:newcell and possibly Evrnu). Another

problem for the Blend Re:wind process was that it was shown to be comparable to viscose production

in terms of environmental impact, even though it has become clear that the viscose process is

environmentally one of the more harmful production methods of cellulose fibers.

Conversely, the acid hydrolysis methods have shown some more promise, especially when

dilute concentrations were used (e.g. phosphotungstic acid or the hydrothermal method), unfortunately

these methods have not been subjected to any LCA to find out what their environmental impact could

be. Nonetheless, the use of dilute sulfuric acid in a hydrothermal method has been shown by Blocktexx

(Table 2) to work on a pilot scale for the separation and recovery of both cellulose particles and polyester

fibers. Circ also adopted the hydrothermal method, using subcritical water instead of dilute acid as an

even more environmentally benign solvent. This method is unfortunately coupled with heating to very

high temperatures (105-190 °C during the treatment and 235-290 °C during recrystallization of PET

fibers), which might lower the environmental benefits of using the subcritical water.

NMMO has also been employed as a pretreatment step in the enzymatic hydrolysis of cellulose,

leading to 95% glucose recovery, which was mostly due to losses during enzymatic hydrolysis after the

pretreatment. Using NMMO as pretreatment seems illogical, since it has already shown the ability to

dissolve cellulose with solvent recovery rates of >99%. A property which has also been used by SaXcell

in their recycling process (Table 2). Therefore addition of another step, such as enzymatic hydrolysis,

will only provide more obstacles for an environmentally friendly process.

The other method of polycotton depolymerization included the use of the switchable

hydrophilicity solvent DMCA. In this process Yousef et al.224 reported that 96.3% of polyester and

cotton fibers could be recovered from the waste. However, no remarks were made on the composition

of these recovered products, nor on the physical properties of the recovered cotton fibers. Therefore, it

is hard to find out if this method can be used to regenerate high quality fibers. Moreover, the SHS

technique has been adapted for use in textile recycling only recently and consequently has not seen much

investigation. Hence, no improvements or drawbacks of using this methods have been found to date,

though, according to their own calculations, it is both an economically and environmentally

advantageous process. It must also be noted that most of the previously mentioned hydrolysis methods

have been used mainly for open looped recycling processes, such as SSF.124

52

Table 3: Mechanical properties of regenerated cellulose fibers produced by different processes.

Processb Tenacity, dry/wet

(cN/tex) E modulus (GPa) Elongation (%)

Viscose76,127 22-26/10-15 5.4-11 17-25

Modala),271 31/15 13 16.5

Cuprammonium109 22.3/17.6 16 24.3

Cellulose Acetate272 23.9/27.7 3.2

CCA232 17/- 16

Superphosphoric

Acid116,272 89.4/75.8 45 6.8

NaOH (aq.)109 19.3/19 15

NaOH-urea106 10-20/- 9-21

Lyocell (NMMO)127 40-42/34-38 13.2-30 11-16

[BMIM]Cl130 45.6/39.4 11.3 13

[EMIM]Cl130 53/35.9 10.2 11

[BMIM]OAc130 44-49/- 13-16

[EMIM]OAc130 46/38.1 11

[EMIM]OAc177 24.6/21.4 3.8

[EMIM]OAc

(wet spinning)177 22.2/19.6 8

[DBN]OAc130 37.5-50.5/24.9-47.6 13.9-23.6 8.5-10.0

[DBN]OAc (Ioncell-F)196 33.6-50.7/25.7-47.5 7.4-9.8

a) Modal is a trade name for a special type of viscose with higher tenacity and wet strength compared

to regular viscose. b) References are given next to the process names.

4.2 Cellulose fiber regeneration Instead of these open loop recycling methods, closed-loop fiber regeneration has also been extensively

researched. In Table 3 an overview of the mechanical properties of spun fibers from both industrial

processes (i.e. Modal, NMMO, and viscose) and recycling methods are given. As shown here not all

processes mention the E modulus of their regenerated fibers. Additionally, there are many factors that

affect the mechanical properties, for instance initial DP, pulping type, and cellulose concentration in the

spinning solution.34 Moreover, process parameters (e.g. draw ratio, spinning temperature, spinneret

design), drying conditions, and posttreatments largely influence these numbers as well. Therefore a

straightforward comparison cannot be achieved. Despite this, Table 3 can still give a clear cut overview

of these properties.

As mentioned before, the derivatization process of cellulose has been adapted in different

methods, such as viscose, cuprammonium, cellulose acetate, and CCA. Most of these fibers show similar

mechanical properties. Modal and CCA are the biggest outliers. Modal is a viscose process that has been

modified to create slightly stronger fibers.271 While CCA, despite slightly lower properties than viscose,

has been used by the infinited fiber company to create a more environmentally benign process as was

shown by Paunonen et al.262 Unfortunately this LCA showed some limitations, especially since no data

was reported on the environmental aspects of water use and scarcity, chemical production, resource

depletion, eutrophication, and acidification. These aspects could have a major impact on the

sustainability of this process, especially since NaOH is also used in this process.

Moreover, it seems that other processes have shown better fiber properties. Fibers spun from

superphosphoric acid, for instance, show the highest tenacity (89.4 cN tex-1) and E modulus (45 GPa)

values, most likely caused by the presence of liquid crystalline structures. However, the use of

superphosphoric acid is most likely corrosive for the process equipment, which might be part of the

reason that this process has not been commercialized yet.273 In general, concentrated acids and bases

(e.g. NaOH) have not been used commercially.

53

Direct dissolution processes of cellulose have shown to regenerate fibers with higher mechanical

properties and with less environmental impacts than those of the derivatizing techniques. As mentioned

before, the Lyocell process has seen a lot of commercial success in generating cellulose fibers from

wood pulp. Additionally, the NMMO process has been adapted by SaXcell to be used on textile wastes.

The LCA by Oelerich et al.263 has shown that this process was more sustainable than sulphate pulping

processes. Nonetheless, they did not compare it to the conventional NMMO pulping method, nor did

they include the fiber production step in this LCA.

Furthermore, it was concluded that NMMO, due to the runaway reactions, has to be stabilized

and other solvents, such as ionic liquids, showed similar properties while posing less environmental

threat.264 Especially the [DBNH]OAc seems to be one of the more environmentally benign ILs capable

of dissolving cellulose.

Despite the earlier mentioned advantages of ILs in the recycling of cellulose waste, several

limitations for industrialization can be pointed out. For industrial use, any selected IL has to match

specific economic and ecological criteria in order to create a sustainable process. These criteria include:

cellulose dissolution capability, recyclability (>99.5%), low amount of side reactions, and lowest

possible toxicity.274,275

The first of these criteria has been met by both the imidazolium-based and superbase-based ILs.

Although, imidazolium-based ILs containing chloride anions are limited in their practical application

due to high temperature requirements for cellulose dissolution.276

Nevertheless, the second criterion, solvent recyclability, is already forming a limitation,

especially for the imidazolium based ILs. In theory several recycling strategies can be applied to

recovers ILs such as adsorption and liquid-liquid extraction. Nevertheless, the ILs capable of dissolving

cellulose possess a hydrophilic nature limiting the separation efficiency.277,278 Another, more common

way to separate solvent-water systems is evaporation. Rosenau et al.75 have shown that this recovers

NMMO in the lyocell process with yields of >99%. Several studies have reported that high recoveries

of ILs were also possible with this method.185,279,280 Comparably, pervaporation of ILs could deliver high

recovery rates, however this method requires low viscosity solutions, elevated temperatures (100 °C),

as well as prolonged treatment times (5 h), making it industrially unviable for regeneration of ILs.281,282

A final method is crystallization, but a preliminary study showed that extremely low temperatures (-75

°C) were needed for coherent separation.283

On the other hand, Parviainen et al.195 showed that, in the Ioncell-F process, [DBNH]OAC could

be recycled from aqueous media with a recovery rate of 95.6 wt%. Unfortunately, it was also reported

that during the recovery of [DBNH]OAC from diluted aqueous solutions by thermal treatment, the IL

undergoes a hydrolysis reaction.190 This reaction is initiated by a nucleophile (i.e. water) with the

unsaturated nitrogenous group of the amidine-based IL, which produces 1-(3-aminopropyl)-2-

pyrrolidonium acetate ([APPH]OAc) as an undesirable side product. While this hydrolysis reaction is

reversible, an irreversible acetylation reaction of the primary amine may occur under more severe

circumstances, which produces 1-(3-acetamidopropyl)-2-pyrrolidone (APPAc).190,195

The third criterion, low amount of side reactions, has also not been met for imidazolium-based

ILs, since they are not inert. It has been shown that imidazolium-based ILs (e.g. [BMIM]Cl or

[EMIM]Cl) decompose over time or with increasing temperature.284,285 This degradation mainly

involves dealkylation yielding 1-alkylimidazoles and its dimers.286,287 The reaction rate of this

dealkylation depends on the nucleophilicity and size of the anion as well as the alkyl chain length.

Liebner et al.142 have shown with a thermal degradation study that freshly purified imidazolium salts

with chloride or acetate counterions contained 0.001-0.01% degradation products after heating at 200

°C for 24 h.288 These small amounts of byproduct could significantly influence the dissolution of

cellulose, due to the high basicity of the ILs. Moreover, these compounds cannot be removed by

evaporation, subsequently resulting in accumulation upon multiple stages of recycling. Furthermore,

carboxylate imidazolium-based ILs (e.g. [Emim]OAc) have shown to react with cellulose in two ways.

Firstly, the cation can react with the cellulose forming new cationic species.284,289 Secondly, cellulose

degrades over time in these ILs, especially at temperatures higher than 90 °C.60,177,180,284 The formation

of this degradation and these side reactions cannot be entirely prevented. However the degradation rate

can be reduced by lowering the operating temperature.

The final criterion, lowest possible toxicity, might also form an issue. The evaluation of the

hazardous potential of ILs for humans and the environment is very important before commercialization

54

can occur.290 Just like their properties, the toxicity of ILs is strongly dependent on their cations and

anions and no general statements such as ‘green’ or ‘toxic’ can be used to classify all ILs.275 Although,

some general remarks can be made: one of the most important properties of ILs is their high liquid range

and ability to withstand temperatures up to 500 K.291 This stability makes them poorly chemically

degradable. Furthermore, they are non-decomposable by microorganisms and persistent in the aquatic

environment.292 A wide range of ILs have been investigated towards their antimicrobial activity on

microorganisms.293 Latała et al.294 reported that in the case of imidazolium-based ILs, the main

toxicological effect is due to the cation, where (cyto)toxicity increases accordingly to the alkyl chain

length up to a certain point. This increase is caused by the ability of the longer chains to more easily

embed and disrupt cell membranes.295–297 In addition, cations with shorter alkyl chains display weaker

hydrophobic interactions with phospholipid membranes and therefore incorporate only slightly to induce

milder forms of disorder.296 Conversely, the anion’s contributions to the toxicity is minimal, except for

fluorinated anions, which show higher toxicity due to their hydration to fluoride hydrates in aqueous

solutions.298 This hydration could lead to decomposition towards HF, which could be released during

acidic reactions. However, the corrosive nature of imidazolium halides toward metal processing

equipment is also a potential limitation.47,132

Moreover, as mentioned before, recovered polyester fibers were still contaminated with residual

cellulose and required further purification steps. This results in a higher environmental impact as well

as higher processing costs.194

Therefore, as mentioned in the commercialization process of Ioncell-F, Elsayed et al.246

proposed a third generation of guanidine-based ILs (i.e. [mTBDH]OAc), which could be thermally

recovered in the lyocell process without losing its dissolution power. DBU and DBN have shown to be

substantially unstable in water due to a fast relief of ring strain that produces primary amines from the

amidines.299 The sp2-hybridized carbon of guanidine based mTBD possesses a higher electron density

and therefore protects the base more against nucleophilic reactions with water.

When all of these criteria are met by one of the ILs, a cradle-to-gate LCA should be conducted

in order to find out if that specific IL is sustainable enough to replace NMMO in the lyocell process. It

appears that guanidine-based ILs could be the most interesting solvents for the effective separation and

regeneration of polycotton waste.

4.3 Limitations Despite the variety of methods discussed in this research, it should be taken into account that the scope

was limited to polycotton blends, even though a lot of clothing also includes other materials in their

blends such as elastane, wool, and nylon. Although the blends usually contain only a small fraction of

one of these other components, it does increase the difficulty and environmental impacts of the

processes. Both Navone et al.5 and Quartinello et al.298 have recently reported that wool/polyester/cotton

blends could be separated and recycled, but much more research is necessary.

Additionally, the commercialized processes have been investigated through patents and released

data from some institutes working in collaboration with companies as seen in Table 2. Despite giving

some descriptions of the methods used, these documents do not provide enough information on the

outcomes of the methods, such as intermediate and final products, yields or characterization of the

untouched component, which limits the usefulness of these sources.

Furthermore, other key steps in the recycling process have also not been mentioned such as

bleaching, dying, or color retainment, as well as compound degradation by laundering during the use

phase of the textile. These steps and their impacts have been described by other authors.301–304

Other limitations can be found in the LCAs. Sandin and Peters25 reviewed a lot of the LCAs on

textile recycling. In their review they mention two knowledge gaps commonly found in LCAs, which

can be applied on the LCAs reviewed here. Firstly, more detailed studies are needed, including updated

and publicly available primary inventory data. Sometimes rough estimates on impacts from certain

process steps are made, resulting in relatively uncertain results. These estimates are also often used as

input in other studies, thereby reproducing these uncertainties. Secondly, there is a lack of studies

exploring the limits of reuse and recycling, for instance by integrating multiple recycling loops or

combination of both reuse and recycling.

55

Another limitation in the LCAs is that different research groups base their information on

geographical data, for instance when considering the energy grid of the country where the research was

performed. In their LCA on enzymatic cotton hydrolysis Subramanian et al.266 concluded that the most

impactful part of the depolymerization of cotton were pretreatments and the melt-spinning steps, due to

their contributions to non-renewable energy use, as the electricity grids of Hong Kong and Taiwan were

used, which are mostly based on fossil fuels. In this way it is very difficult to compare different LCAs

to each other. Other limitations can be found in the processes to which the new technologies are

compared. For instance, the LCAs on the Blend Re:wind process and the infinited fiber process

compared the new technologies to both virgin cotton and viscose, despite these two processes being the

most environmentally harmful industrialized textile fabrication processes in many impact categories.267

Besides that, Maciel et al.305 mention in their review on LCAs of ILs that impact factors on

human toxicity and ecotoxicity were not included, due to scarcity of characterization factors of ILs in

these characterization factors. It was also reported by Santos et al.306 that most LCAs on industrial

processes focus on the impact categories related to GHG emissions and their effects on ecosystems,

while the effects of raw material extraction and the GHG emissions’ effect on human health is not

considered as often.

With these limitations taken into account, the LCAs can still give a general overview of the

different processes and could be useful for policymakers and buildup of databases on sustainability.

56

5 Concluding remarks and future outlook This literature thesis discussed several potential polycotton textile waste chemical recycling methods

and their environmental impacts. In order to achieve full recycling, either open- or closed-loop, the

blended materials needed to be separated. Four different categories of separation in chemical recycling

were distinguished: depolymerization of either PET or cotton while leaving the other compound intact,

dissolution of polyester in a suitable solvent, and dissolution of cellulose in a suitable solvent which

leaves PET to be extracted by filtration.

Depolymerization of cotton could be achieved through three different hydrolysis methods.

These methods include acid, alkaline, and enzymatic hydrolysis, of which enzymatic hydrolysis is

usually performed with either an acid or alkaline pretreatment to improve the results. Some of the

hydrolysis methods could only handle pure cotton waste, while others were able to hydrolyze cotton

from WBFs with conservation of PET during the process. Furthermore, it was found that the efficiency

of enzymatic hydrolysis was highly dependent on multiple factors such as pretreatment temperature,

pH, substrate loading, enzyme dosages and structural features of the substrate. Hydrothermal treatments

with dilute acid could increase the hydrolysis rate and reduce corrosiveness of the process. Some of

these processes could be used in open loop recycling while others could be used for regeneration of

fibers as well. Of those closed loop recycling methods, acid hydrolysis and hydrothermal treatment have

also been applied on the commercial scale, while alkaline hydrolysis was not due to the higher

environmental impact and corrosive nature.

Moreover, research in PET depolymerization of waste textiles has shown that PET could be

depolymerized to its monomers. Methods used in this depolymerization included alkaline treatment with

addition of a PTC, enzymatic hydrolysis, or the use of switchable hydrophilicity solvents. Especially the

last method could be interesting since it was found that this method could lead to both economic and

environmental benefits, despite the research still being in its early stages. Additionally, it was shown

that the alkaline treatment was commercialized and has similar environmental impacts to the viscose

process, despite not being optimized yet.

Finally, the direct dissolution of cellulose has shown great potential, with the adaption of the

commercial lyocell process to be used for the regeneration of polycotton waste fibers. This could be

achieved through the use of NMMO and ILs. ILs were considered as an alternative to NMMO, due to

runaway reactions of the NMMO. Despite imidazolium-based ILs showing early promise with good

dissolution potential and solvent recovery, some of the ILs used in the process posed environmental

hazards, derivatization problems, and highly viscose solutions, inhibiting their industrial potential. A

second group, superbase-based ILs, seemed to resolve most of these problems. However when looked

at the LCA, the limitations of this group became apparent, especially in the solvent recovery and high

environmental impact of precursor synthesis. A third group, guanidine-based ILs, was recently

discovered as even more environmentally benign. These ILs have also been used in a pilot plant for the

Ioncell-F process.

In the coming years more research in the life cycle and economic benefits of the guanidine-

based ILs should be performed. More generally, the implementation of LCA should be part of all

commercialized processes during pilot stages. Moreover, even though the Ioncell-F process shows the

most promise, the other methods should not be neglected in further research. A combination of reuse,

open loop and closed loop recycling should be implemented to utilize textile waste most efficiently and

the hydrolysis methods could play a large role in that.

57

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