Recycling Polymers

Objective

The aim of this laboratory study is to familiarize with the different types of polymers that can be recycled

into new products after their intended uses. In addition to the specific polymer types, main processing

routes that are utilized during their recycling processes will be studied. Various types of used

thermoplastics will be processed into moldable melts by mechanical, chemical and thermal methods.

Theory

Polymers are among the most popular materials used in the modern world. However, their popularity is

the reason why polymers should be recycled. Instead of throwing them away, polluting the environment,

we can optimize the lifespan of certain polymers by recycling and reusing them. Polymer recycling refers

to the process of recovering waste or scrap polymer and reprocessing it into useful product. According to

the environment protection agency, less than a quarter of the thermoplastic bottles are recycled. And as

the bottles are protected from the sunlight in landfills, they do not decompose for years. Due to the fact

that most polymers are non-biodegradable, it is essential that they are recycled to reduce solid waste in

the environment.

Furthermore, manufacturing, transporting and disposing of polymers leads to greenhouse gas emissions,

which trap heat in the planet’s atmosphere. These gases are released at every stage of a product’s life

cycle, contributing to climate change. The life cycle of a PET bottle is a good example. If the bottle is

incinerated (burned), it releases carbon dioxide as a by-product, which contributes to the increase in the

rate of global warming. If the bottle is landfilled, it will slowly decompose and release methane, a

greenhouse gas that is 21 times more potent than carbon dioxide. In addition, materials that are disposed

of in landfills have to be replaced by new products manufactured from raw materials, and the extraction

of raw materials requires fossil fuel combustion, which releases more carbon dioxide into the atmosphere

and reduces a non-renewable resource. However, if the PET bottle is recycled, it will help to save energy.

According to the EPA, the current PET bottle recycling rate results in an energy savings equivalent to the

power generation of a 500MWh nuclear reactor. Recycling also leads to cleaner air. If the PET bottle

recycling rate increased by just 25 percent, we would avoid the greenhouse gas emissions equivalent to

about 500 million liters of gasoline.

A thermoplastic is a polymeric material or plastic that becomes soft and formable when heated and rigid

when cooled. It consists of chains of molecules which separate when heat is applied. This process may be

repeated a number of times without chemically altering the material. However polymers in general are

susceptible to degradation when exposed to UV light, moisture and heat. Repolymerization can revert the

changes and bind the loose monomers back. Recycled thermoplastics are usually shredded, washed,

cleaned and dried, then go back to the manufacturing process (for extrusion, injection molding, etc).

Extrusion will create continuous lengths of material, such as grass-cutting cord, while injection moulding

pushes molten thermoplastic into a metal mould to form a shape.

A thermoset is a polymeric material that undergoes irreversible chemical changes when it is cured through

heat, catalysts, or ultraviolet light. During the curing process, polymer chains are cross-linked with other

molecules. Cross-linking prevents movement of molecular chains after curing. Once cured, the structure

cannot be altered.

Table 1. Comparison of the properties of thermoplastics and thermosets

THERMOPLASTICS THERMOSETS

Advantage Disadvantage Advantage Disadvantage

High Impact Strength Typically will soften with heat

Easy to process and laminate

Often release emissions known as volatile organic compounds (VOCs)

Attractive Surface Finish

High viscosity May not need pressure or heat to form

Non-recyclable and cannot be reclaimed easily

Recyclable / Scrap is Reusable

High processing temperatures

Typically inexpensive Short workable shelf life, with some exceptions

No Emissions Typically stronger than thermoplastics

Can bond to other thermoplastics

Better suited to higher temperatures

Can be molded or shaped by reheating

Table 2. Most common thermoplastics and their typical applications

Table 3. Most common thermosets and their typical applications

Figure 1. The macrostructural and microstructural differences between thermoplastics and thermosets

Many polymeric products are filled with metals, minerals or glass and these are hard to separate from the

base thermoplastic. Usually, fillers, colorants, and modifiers are added to a base resin to adjust its

properties for achieving a certain function. For example glass fiber is added to improve strength, hydrogel

lubricant is added as a mold release, carbon is added to make the part black, etc. These additives can

change the melting temperature and even the viscosity of the melt of the base resin. If added in large

enough proportions, they can change the properties so significantly that it can be difficult to get the melt

to mix with the same resin that has no additives. In addition, it is not possible to control the concentration

of any additives in the recycled polymer which means that the properties of the recycled material will be

unpredictable. This is the reason for recycled polymers being used for products which are simpler, or are

limited to 10-20% of the make-up of the new product. Also biodegradable thermoplastics are undesirable

in recycled parts as they reduce the functionality of the new product. Therefore polymer recycling is most

easily accomplished with pure thermoplastic resins that are usually marked with a recycling number. The

different types of recyclable thermoplastics are:

PET: The full form of PET is Polyethylene

Terephthalate. It is the most general

thermoplastic polymer resin which belongs to the

polyester family. It is applicable in the fibers for

clothing, for packaging of foods and drinks. It can

be identified by observing some characteristic

properties: it is clear, tough, has barrier to

moisture and gas and softens at the temperature

of 80° C. For recycling ıt is crushed and then

shredded into small flakes which are then

reprocessed to make new PET bottles, or spun

into polyester fiber. After recycling, the recycled

PET can be used in carpets, clothing, protective packaging, tennis ball, etc. For bottles and food containers

recycled PET is usually combined with virgin PET to keep the quality high enough. Repeated use increases

the risk of leaching and bacterial growth. PET plastic is difficult to decontaminate, and proper cleaning

requires harmful chemicals. Polyethylene terephthalates may leach carcinogens.

HDPE: The full form of HDPE is High-density polyethylene. It is a

polyethylene thermoplastic made from the petroleum. When used for

pipes, it is also referred to as alkathene or polythene. Its properties are

excellent stiffness, resistance to moisture, strength, toughness,

reduced permeability to gas. It can be colored easily and softens at a

temperature of 75° C. It is commonly used in the manufacture of water,

juice and milk bottles. It is also used to make retail and trash bags for

households and business people. The recycled HDPE can be used for

manufacturing automobile parts, buckets, crates, lawn and garden products and office products.

PVC: The full form of PVC is Polyvinyl Chloride. It is third-most widely produced synthetic thermoplastic

polymer, subsequent to polyethylene and polypropylene globally. Typical PVC product contains numerous

toxins which it can leach throughout its entire life cycle. Almost all products using PVC require virgin

material for their construction; less than 1% of PVC material is recycled.

There are two kinds, namely PVC-U or Unplasticised Polyvinyl

Chloride and PVC-P or Plasticised Polyvinyl Chloride

respectively. The properties of PVC-U are tough, strong, and

soften at a temperature of 80° C whereas PVC-P is flexible and

clear. They are commonly used in the manufacture of PVC

piping and stretch films. The recycled PVC can be recycled

around seven times and has a lifespan of approximately 140

years.

C C C C C C

H H H H H H

H H H H H H

Polyethylene (PE)

Cl Cl Cl

C C C C C C

H H H

H H H H H H

Polyvinyl chloride (PVC)

LDPE: The full form of LDPE is low-density polyethylene. It is a thermoplastic which is made from the

monomer ethylene. Ease of processing, toughness, flexibility, softening at a temperature of 70° C and

resistance to moisture are few of the properties of LDPE. This thermoplastic is usually used in making

frozen food bags, flexible container lids, freezable bottles. The recycled LDPE is used for landscaping

boards, floor tiles, garbage can liners, etc. Products made using recycled LDPE are not as hard or rigid as

those made using recycled HDPE

PP: The full form of PP is Polypropylene and its

characteristic properties are toughness, waxy

surface and softening at a temperature of 140° C. It

is also lightweight, and has excellent heat-resistance

qualities. It serves as a barrier against moisture,

grease and chemicals. It is used in straws, some

yogurt containers, syrup and ketchup bottles, pill

bottles, and caps. Only about 3% of PP products are

currently being recycled into rakes, brooms, batteries and trays.

PS: Polystyrene is a synthetic aromatic polymer which is made from the monomer styrene. It can be either

solid or foamed. One more kind of PS is PS-E. The PS is amorphous, glassy polymer which is generally rigid.

It is inexpensive, lightweight and easily-formed plastic with a wide variety of uses. It is most often used to

make disposable styrofoam drinking cups, take-out food containers, egg cartons, plastic picnic cutlery,

foam packaging and the foam chips used to fill shipping boxes to protect the contents. Because

polystyrene is structurally weak and ultra-lightweight, it breaks up easily and is dispersed readily

throughout the natural environment. It may leach styrene, a possible human carcinogen, into food

products (especially when heated in a microwave)

Other: The last category was designed for polycarbonate (PC) and all other thermoplastics, so reuse and

recycling protocols are not standardized within this category. Number 7 polymers are used to make baby

bottles, sippy cups, water cooler bottles and car parts. BPA (a carcinogen) is found in polycarbonate plastic

food containers often marked on the bottom with the

letters PC by the recycling label #7. Some polycarbonate

water bottles are marketed as ‘non-leaching’ for minimizing

plastic taste or odor, however there is still a possibility that

trace amounts of BPA will migrate from these containers,

particularly if used to heat liquids. A new generation of

compostable polymers, polylactic acid that is made from

bio-based polymers like corn starch, is being developed to

replace polycarbonates. These compostable polymers have

the initials PLA on the bottom near the recycling symbol.

H H

H H H H

Polypropylene (PP)

C C C C C C

CH3

H H

CH3

CH3

H

Once collected, these recyclable thermoplastics go through a series of steps within the recycling process:

Sorting: Because there are many different kinds of thermoplastics, they are sorted into ‘like’ families of resin. The first and main sort is using the code (#1-#7) on the item to separate it into families (#1-PET, #2-HDPE, #3-PVC, #4-LDPE, #5-PP, #6-PS, #7-Other, see Figure 2). Once this process has been done, it gets compressed into bales. These bales are then sent to a secondary processor which does more sorting or resin separation depending on the material provided.

Figure 2. Recycling codes for the most common thermoplastics

Mechanical or chemical processing: The secondary processors focus on the resin comparability and quality to make sure all of the material will be compatible in the manufacturing process. After a complete separation, the flakes or chunks are then washed with detergents to remove the remaining contamination. Then they are either mechanically processed or chemically processed. Mechanical processing of polymers consists of the material be shredded and then either converted into smaller flakes or processed into pellets depending on the manufacturing process that will use them. The flakes or pellets are created as raw materials for a specific production process so processors might create multiple streams for manufacturers. Polymers are extruded, blow molded or injection molded. Because of this categorization, there are different product streams being made. The plastic flakes are subjected to moderate heat to dry and to subsequently melt. They can be melted down and molded into a new shape or they can be melted down and processed into pellets. The melting process is done under regulated temperatures to prevent degradation of the polymers.

Chemical recycling is similar except that it uses chemical processes to denature the degraded polymer back into the base monomers for reprocessing. The new material can then be treated as virgin inputs into the recycling process. Regaining the calorific value of plastic by burning it as fuel is an alternative to recycling. Thermal processing is a relatively new process due to the fact that the technology has evolved to make this process net energy positive. Until recently, it took more energy to crack the plastic back into fuel then it generated. The resultant fuel product varies based on the process and needs to be further refined to be useful in most cases. There are some processes that produce a product which can be directly substituted for heating oil without the need for processing. While this technology holds promise, it is very expensive to build and the fuel product sales are directly competing with virgin petroleum so the long term economic viability on a more mainstream scale is still being studied.

Experimental study:

Obtain various recyclable thermoplastic products with the recycling codes printed on them. Subject

them to mechanical and chemical processing by first cleaning, shredding and then melting above their

melting temperatures for mechanical processing and dissolving them in acidic or basic solutions. Mold

and shape the molten polymers to a new product.

Experimental apparatus:

250 ml beakers

Scissors

Lab oven

Distilled water

100 ml 0.1 M HCl solution

100 ml 0.1 M NaOH solution