Chapter-2 LITERATURE REVIEW Polyethyleneterephthalate (PET)shodhganga.inflibnet.ac.in/bitstream/10603/8960/12/12_chapter 2.pdf · Chapter-2 LITERATURE REVIEW Polyethyleneterephthalate

Nanocomposite From Depolymerized PET Waste For Food packaging Application

5

Chapter-2

LITERATURE REVIEW

Polyethyleneterephthalate (PET)

PET is considered as one of the most important engineering polymers in the past two

decades due to rapid growth in its use. It is regarded as an excellent material for many

applications and is widely used for making liquid containers (bottles). It has excellent tensile

and impact strength, chemical resistance, clarity, process ability, color ability and reasonable

thermal stability.38

Many companies produce virgin PET globally giving it different trade

names.39-40

Commercial PET has a wide range of intrinsic viscosity [η] that varies from 0.45

to 1.2 dl g-1

with a polydispersity index generally equal to 2. The PET chain is considered to

be stiff above the glass transition temperature (Tg) unlike many other polymers. The low

flexibility of the PET chain is a result of the nature of short ethylene group and the presence

of p-phenylene group. This chain inflexibility significantly affects PET structure-related

properties such as thermal transitions.40-41

PET production process involves two different starting reactions. The first starting

reaction is an esterification reaction where terephthalic acid (TPA) reacts with ethylene

glycol (EG) at a temperature of between 240 oC and 260

oC and a pressure between 300 and

500 kPa. The second reaction is trans-esterification reaction, where dimethyl terephthalate

(DMT) is reacted with EG at 150 oC

42, 180–210

oC

39, 140–220

oC at 100 kPa

43shown in

Scheme 2.1. Trans-esterification is the much preferred process due to easier purification.

The output of both these processes is BHET. The pre-polymerization step follows in which

BHET is polymerized to a degree of polymerization (DP) of up to 30. Pre-polymerization

reactions conditions are 250–280 oC at 2–3 kPa.

39-40 The third stage is the polycondensation


6

process, where the DP is further increased to 100. Up to this stage, PET is suitable for

applications that do not require high molecular weight (MW) or [η] such as fibers and sheets.

Solid state polymerization (SSP) might be required to produce a high MW PET. SSP is used

to increase the DP to 150, and also increasing MW. SSP operating conditions are 200–240

oC at 100 kPa for 5–25 h.

43 Bottle grade PET that has an [η] of 0.7–0.81 dl g

-1 is normally

produced by SSP at 210 oC for around 15–20 h.

44-45

COOH

COOH

TPA

CH3COO

CH3COO

DMT

COOCH2CH2OH

COOCH2CH2OH

BHET

HOCH2CH

2OH

Direct esterification-H

20

HOCH2CH

2OH

Transesterification catalyst

-CH3OH

-HOCH2CH

2OH

Polycondensation

Catalyst

HOH2CH2COOC COOCH2CH2OH

130-150

PET

Scheme 2.1: PET polymerization process

Virgin PET manufacturers have tended in recent years to produce PET co-polymer;

such as isophthalic acid modified PET, rather than homopolymer PET. PET bottles are

normally made from co-polymer PET because of its lower crystallinity, improved ductility,

better process ability and better clarity.46-47


7

Virgin PET Thermal Transitions and Crystallization

Commercial PET has a melting temperature (Tm) of between 255 and 265 oC and for

more crystalline PET is 265 oC.

42 Tg of virgin PET varies between 67 and 140

oC. The

thermal transitions and crystallization of virgin PET with a focus on reversing crystallization

and melting have been analyzed by several researchers.48-49

An interesting phenomenon was

reported in which the virgin PET experiences multiple endothermic transition during thermal

analysis.48-50

It was reported that this phenomenon is attributable to morphological and

structural re-organization. As the temperature increases, better crystal structures are

achieved because of the re-organization of the less perfect crystals. Virgin PET is well

known for having very slow crystallization rate. The highest crystallization rate takes place

at 170 oC

42, or 190

oC.

41 Cooling PET rapidly from the melt to a temperature below Tg can

produce an amorphous, transparent PET. Semi-crystalline PET can be obtained by heating

the solid amorphous PET to a temperature above Tg where 30 % crystallinity can be

achieved.41

The rate of crystallization of virgin PET depends greatly on temperature and

reaches its maximum at a temperature of 150–180 oC. The rate of crystallization also

depends on other factors such as MW, the presence of nucleating agents, the degree of chain

orientation, the nature of the polymerization catalyst used in the original production of PET

and the thermal history.39

PET Applications and Processing

PET is used broadly in products such as bottles, electrical and electronic instruments,

automobile products, house-wares, lighting products, power tools, material handling

equipment and sporting goods.39

PET films and fibers are the oldest applications of PET.

Films are produced by biaxial orientation through heat and drawing. PET films are used in


8

photographic applications, X rays sheets and in food packaging.40

PET films are also

reported to be used in electrical and for recording tapes.42

PET is also used as an electrical

insulator. PET’s insulating properties are regarded as good due to the severe restriction of

the dipole orientation at room temperature that is well below the transition temperature.42

PET fibers are another important application of PET and are produced by forcing molten

PET through small holes in a die. Fiber strength is achieved by applying tension to align the

chains through uniaxial stretching. Virgin PET is produced at different specifications

because different application requires different properties.51

PET granules can be processed

in many ways depending on the application and the final product requirements. The main

PET processes are extrusion, injection moulding and blow moulding.

PET/Montmorillonite Clay Nanocomposites

Polymer/clay nanocomposites are synthesized via melt-intercalation, common solvent

mixing, or in situ polymerization.52

In the process of melt mixing, the layered silicate is

mixed with a molten polymer matrix. If the silicate surfaces are sufficiently compatible with

the chosen polymer, then the polymer can enter the interlayer space and form either an

intercalated or an exfoliated nanocomposite.53

Exfoliation or a high level of intercalation, is

important in producing a polymer/clay nanocomposite, with such separation of individual

clay sheets the high aspect ratios are obtained with the inorganic reinforcing materials. The

synthesis of PET clay nanocomposites has not been as successful as compared with other

polymers. Takekoshi et-al54

prepared polyester clay nanocomposites via in situ

polymerization with quaternary ammonium salt modified clay and cyclic PET oligomers.

They observed good nanoparticle dispersion and improved physical properties, such as

improved impact strength and elastic modulus. A more commercially viable approach with


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conventional polymer processing techniques is melt-mixing of the polyester with an

organicallymodified-clay.55

However, as Matayabas et al56

found, this approach has been far

less successful usually leading to poorly dispersed clay particles. This may be attributed to

the low decomposition temperature (250 °C) of the organic modifier bound to the clay

surface.

Need for PET Recycling

PET is saturated polyester of terephthalic acid and ethylene glycol. The growing

interest in PET recycling is due its widespread use mainly as jars and bottles. Since the

middle of the 1970s, first in the USA and Canada and subsequently in Western Europe,

increased quantities of PET are used for the production of soft drink bottles, and a further

increase in its application in this area is predicted. The overall world consumption of PET

currently amounts to about 13 million tons, of which 9.5 million tons is processed by the

textiles industry, 2 million tons is used in the manufacture of audio and video tapes, and 1.5

million tons is used in the manufacture of various types of packaging mainly bottles and jars.

PET is seen as a noxious material due to. its high resistance to the atmospheric and biological

agents. Ecological as well as economic considerations advocate the introduction of wide-

scale PET recycling, similar to the recycling of traditional materials such as glass, paper, or

metals.57

Classification of Polymer Recycle

Polymer recycling can be classified into four categories e.g. primary, secondary, tertiary and

quaternary recycling.


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1. Primary Recycling (Pre-Consumer Industrial Scrap)

It is the recycling of clean, uncontaminated single-type waste which remains the most

popular, as it ensures simplicity and low cost, especially when done ‘‘in-plant’’ and feeding

with scrap of controlled history.58

The recycled scrap or waste is either mixed with virgin

material to assure product quality or used as a second-grade material.59

Primary recycling of

industrial scrap produced during the manufacture of food-contact articles is not expected to

pose a hazard to the consumer.

2. Mechanical Recycling (Secondary Recycling)

In this approach, the polymer is separated from its associated contaminants and it can

be readily reprocessed into granules by conventional melt extrusion. Mechanical recycling

includes the sorting and separation of the wastes, size reduction; melt filtration and reforming

of the plastic material. The basic polymer is not altered during the process. The main

disadvantage of this type of recycling is the deterioration of product properties in every

cycle. This occurs since the molecular weight of the recycled resin is reduced due to chain-

scission reactions caused by the presence of water and trace acidic impurities. A secondary

recycling process presents some unique problems that may cause it to be inappropriate for the

production of food-contact articles, particularly if the recycler had little or no control over the

waste stream entering the recycling facility.60

3. Chemical Recycling (Tertiary Recycling)

Unlike physical recycling, chemical recycling involves transformation of polymer

chain. The polymer backbone under the recycling process is degraded into monomer units

(i.e. depolymerization) or randomly ruptured into larger chain fragments (i.e. random chain

scission) with associated formation of gaseous products. The chemical recycling is carried


11

out either by solvolysis or by pyrolysis; the former through degradation by solvents including

water and the latter through degradation by heat in absence of air or vacuum. Chemical

recycling yields monomers, petroleum liquids and gases. Monomers are purified by

distillation and drying, and used for manufacture of polymers.

4. Energy Recovery (Quaternary Recycling)

The energy content of the plastics waste can be recovered by incineration. When the

collection, sorting and separation of plastics waste are difficult or economically not viable, or

the waste is toxic and hazardous to handle, the best waste management option is incineration

to recover the chemical energy stored in plastics waste in the form of thermal energy. This is

carried out in special type of reactors called incinerators, to burn wastes in the presence of air

in a controlled manner to convert hydrocarbons of the plastic into carbon dioxide and water.

The heat produced by burning plastics in the waste in the form of superheated steam can be

utilized for generating electricity through turbine generators and the residual heat from the

waste stream for heating residential and industrial buildings. The melt residue from the

incinerator is free from toxicity hazards and may be disposed off by landfill. It should admit

that it is not possible to have zero-emission in the incineration of waste plastic. Apart from

the aforementioned methods, direct reuse of a plastic material (i.e., PET) could be considered

as a ‘‘zero-order’’ recycling technique.58

Worldwide, the main end-use of post-consumed

PET is for the production of fibers (almost 70 %), with only 4 % of PET recycled with

chemical methods.

Among the above recycling techniques, the only one acceptable according to the

principles of sustainable development is chemical recycling, since it leads to the formation of

the raw materials (monomers) from which the polymer is made.59

In this way the


12

environment is not surcharged and there is no need for extra resources (monomers) for the

production of PET.

PET Chemical-Recycling Techniques

The world’s most recyclable polymer is polyester. PET is polyester with functional

ester groups that can be cleaved by some reagents, such as water (hydrolysis), alcohols

(alcoholysis), acids (acidolysis), glycols (glycolysis), and amines (aminolysis). The recycled

PET is mostly used in the form of fibers, films, foams, sheets, bottles etc. Thus, chemical-

recycling processes for PET are divided as follows: (i) hydrolysis, (ii) glycolysis, (iii)

methanolysis and (iv) other processes. The chemical recycling of PET is discussed in

Scheme 2.2.

Scheme 2.2: Chemical-recycling processes for PET

According to the reagent used, different products are obtained. The main depolymerization

processes that have reached commercial maturity up to now are glycolysis and methanolysis.

Chemical Recycling of PET

Glycolysis

Main Product

BHET + Oligomers

Methanolysis

Main Product

DMT + EG

Hydrolysis

Main Product

TPA +EG

Other Processes

Aminolysis

Ammonolysis

Acid Alkaline Neutral


13

Nowadays there is growing interest in hydrolysis for the chemical recycling of PET, since it

is the only method with the reaction products TPA and EG, i.e. the monomer from which

PET is produced. This is associated with the trend in the new factories for PET synthesis to

produce it directly from TPA and EG, thus replacing DMT (the traditional monomer) from

the technological process. The main disadvantage of this method is the use of high

temperature (200–250 oC) and pressure (1.4–2 MPa) as well as long time needed for

complete depolymerization. Commercially, hydrolysis is not widely used to produce food-

grade recycled PET, because of the cost associated with purification of the recycled TPA.

Hydrolysis of PET can be carried out as (a) alkaline hydrolysis, (b) acid hydrolysis and (c)

neutral hydrolysis. Among previous chemical recycling methods, a recent growing interest

has been applied for the production of specialized products such as saturated, unsaturated

polyester resins, polyurethane and polymer concrete using glycolysis due to consumer needs

and cost.

Glycolysis

Another most important method in chemical processing of PET is glycolysis. This

process is used widely on a commercial scale. The glycolysis reaction is the molecular

degradation of PET polymer by glycols, in the presence of trans-esterification catalysts,

mainly metal acetates, where ester linkages are broken and replaced with hydroxyl terminals.

PET degradation is carried out most frequently using EG,61–67

DEG,68–70

, PG,69–71

and

DPG.66, 71

etc. Research concerning this process has been mainly conducted from the point

of view of the utilization of the products obtained; very few works have been devoted to the

description of the kinetics of glycolysis reactions.63, 65, 72

The process is conducted in a wide

range of temperatures 180–250 oC,

72–76 during a time period of 0.5–8 h. Usually 0.5 % by


14

weight of catalyst (most often zinc acetate) in relation to the PET content is added. Much

attention has been devoted to glycolysis by EG. In this system, the effect of the reaction

parameters, i.e., temperature (190–240 oC), pressure (0.1–0.6 MPa) and PET to EG ratio on

the reaction rate has been investigated.65

It has been observed that the rate of the reaction is

proportional to the square of the EG concentration at constant temperature, pressure and PET

concentration.

PET glycolyzates found application in the manufacture of unsaturated polyester

resins,69, 70, 74, 76–81

polyurethane foams,67, 71, 73, 82–85

, and polyisocyanurate foams.61, 63–66, 72, 83

One of the first method of the synthesis of unsaturated polyester resins (UPR), was

developed by Ostrysz.86

in which partial PET glycolyzed product was used. The product of

partial PET glycolysis was applied together with maleic anhydride and propylene glycol so

as to obtain unsaturated polyester. After dissolving the synthesized polyester alkyd in

styrene, UPR resin was obtained. During the following years, the Industrial Chemistry

Research Institute in Warsaw developed technology for the production of UPR with built-in

segments of oligo(ethylene terephthalate) obtained as a result of partial PET waste glycolysis

with propylene glycol at a temperature of 200 oC (240–250

oC) and a reaction time of 2 h.

68,

76 Due to difficulties in obtaining a glycolyzates with reproducible properties, a new type of

unsaturated polyester has been evolved, containing ethylene-diethylene diester obtained as a

result of PET degradation by diethylene glycol as its terephthalic part. This resin was used in

the production of polyester molding compounds.87

Recently, there is an increased interest in

the manufacture of UPR, utilizing PET waste. In one of the patents81

, PET glycolysis

products undergo a reaction with MA and subsequently a reaction with di-cyclopentadiene


15

(DCP). The polyesters obtained have wide possibilities of application, e.g., for gel coats,

casting marble, bath fixtures, car elements, etc.

Interesting research results on the synthesis and viscosity of UPR obtained by

polycondensation of PET glycolysis product and maleic anhydride was published by Kim.74

It was observed that the molecular weights of UPR increase with an increase of PET content

in the reaction system or an increase of the dicarboxylic acid/glycol ratio as well as in the

case of the application of DPG instead of propylene glycol in the same condition of

glycolysis. PET waste can be depolymerized by glycolysis to obtain oligomeric diols and

polyols, or glycolyzed into its monomeric units, (BHET) or DMT.88–90

The influence of various parameters on the kinetics of PET glycolysis by DEG,

temperature profile, catalysis and PET morphology was studied by Francis Pardal.91

The

results showed a strong influence of some experimental conditions (temperature and

catalysis) on the mixture evolution during depolymerization. The temperature study showed

a critical temperature between 210 and 220 oC which seems to be the consequence of a better

diffusion of DEG in PET, allowing easier reactions in solid phase. The initial morphology of

PET scraps does not affect the rates of reactions much, in contrast to the temperature profile,

which has a great importance. Time of PET dissolution at 220 oC is considerably shorter by

heating PET and DEG separately at 220 oC before mixing, than by heating a cold mixture of

the two reagents to 220 oC. PET fibre wastes from an industrial manufacturer was

depolymerized using excess EG in the presence of metal acetate as a transesterification

catalyst. The glycolysis reactions were carried out at the boiling point of EG under nitrogen

atmosphere up to 10 h. Influences of the reaction time, volume of EG, catalysts and their

concentrations on the yield of the glycolysis products were investigated by M. Ghaemy.92


16

The methods of glycolysis depolymerization with catalyst optimization technique

described by A.S Goje and S. Mishra reveal that it is possible to obtain almost complete and

optimal conversion of PET into value added monomeric products (EG and DMT). Optimal

reactant size is recorded as 127.5 mm. Depolymerization of PET was increased with increase

in the reaction time and temperature. Yields (%) of value added monomeric products DMT

and EG are almost equal to PET conversion. Results suggest that EG does not have a

significant role as an internal catalyst in glycolysis of PET. Depolymerization of PET was

decreased with increase in the particle size of PET. Zinc salt as well as cobalt salt show

identical results and the numerical values were greater than that of lead salt and manganese

salt during glycolysis depolymerization of PET. Zinc salt and cobalt salt appear to have

more catalytic effect on glycolysis seem to influence rates at atmospheric pressure.93

Nanocomposites

Nanocomposites are a new class of composites in which the filler dimensions are in

the nanometer (10-9

m) range.94

The concept of nanoscale reinforcement provides opportunity

for synthesis of new polymer materials with unique properties.95

Conventional composites

are based on microscale reinforcement. However, owing to the large interfacial area per unit

volume94

, dispersion of the inorganic filler at the nanometer scale have led to significant

improvements in the properties of polymer nanocomposites.95

Three types of nanocomposites can be distinguished, depending on how many

dimensions of the dispersed particles are in the nanometer range:

1. When three dimensions are in the order of nanometers, they are called isodimensional

nanoparticles, such as spherical silica nanoparticles.


17

2. When two dimensions are in the nanometer scale and the third is larger, they form

elongated strucures like nanotubes or whiskers.

3. When only one dimension is in the nanometer range, the filler is in the form of sheets

of one to a few nanometers thick, and to hundreds to thousands nanometer long.

The third type constitutes the family of composites, which can be gathered under the

name of polymer-layered crystal nanocomposites.96

Polymer-layered crystal nanocomposite materials are almost exclusively obtained by

the intercalation of the polymer (or a monomer subsequently polymerized) inside the

galleries of layered host crystals.96

Owing to the nanocomposite structure, they exhibit

concurrent improvements in several material properties, for very moderate inorganic loadings

(typically less than 6 %). Enhanced properties include improved tensile characteristics,

higher heat deflection temperature, higher barrier properties and increased flame retardancy.

At the same time, optical clarity and light weight are largely maintained. Uses for this new

class of materials can be found in aerospace, automotive, electronics and biotechnology

applications, to list only a few.97

There is a wide variety of both synthetic and natural crystalline fillers that are able,

under specific conditions, to intercalate a polymer. Amongst all the potential nanocomposite

precursors, those based on clays and layered silicates have been more widely investigated,

probably because the starting clay minerals are easily available and because their

intercalation chemistry has been studied for a long time.96

Because of its suitable layer

charge density98

, montmorillonite is nowadays the most widely used clay as nanofiller. The

model structure of montmorillonite is given in Scheme 2.3.


18

Scheme 2.3: The model structure of montmorillonite

It consists of two fused silica tetrahedral sheets sandwiching an edge-shared

octahedral sheet of either aluminum or magnesium hydroxide.

They have charge deficiencies partly due to isomorphous substitutions of Al3+

and

Si4+

in each layer, which are counter balanced by cations, principally sodium and calcium.

Water, other hydroxyl-containing groups and amines readily break these ionic bonds. Once

the sheets have been parted, many layers can be adsorbed producing a marked expansion of

the structure. Under certain conditions, separation of the platelets into primary layers can

occur giving very high aspect ratio products.99

Stacking of the layers leads to regular van der

Waals gaps called interlayers or galleries. The sum of the single layer thickness (9.6 Å) and

the interlayer represents the repeat unit of the multilayer material, so called d-spacing or

basal spacing. The d-spacing between the layers for Na-montmorillonite varies from 9.6 Å


19

for the clay in the collapsed state to approximately 20 Å when the clay is dispersed in the

water solution. The microstructure of montmorillonite can be seen in Scheme 2.4.

Scheme 2.4: The microstructure of montmorillonite

Clays are in the form of aggregates consisting of primary particles with thickness of

8-10 nm, which contains five to ten lamellae associated by interlayer ions. The grey circles

in the primary particle represent the intercalated cations (Na+, Ca

++, K

+,...). On a larger scale,

each layer can be seen as a high aspect ratio lamella about 100-200 nm in diameter and 1 nm

in thickness.98

Nanocomposite Synthesis Methods

In-Situ Polymerization Method

It is the conventional process used to synthesize thermoset-clay nanocomposites

shown in Scheme 2.5.


20

Scheme 2.5: In-Situ polymerization method

First, the organoclay is swollen in the monomer. This step requires a certain amount

of time, which depends on the polarity of the monomer molecules, the surface treatment of

the organoclay and the swelling temperature, then the reaction is initiated. For thermoset

such as epoxies or unsaturated polyesters, a curing agent or a peroxide, respectively, is added

to initiate the polymerization. For thermoplastics, the polymerization can be initiated either

by the addition of a curing agent or by an increase of temperature. During the swelling

phase, the high surface energy of the clay attracts polar monomer molecules so that they

diffuse between the clay layers. When a certain equilibrium is reached the diffusion stops and

the clay is swollen in the monomer to a certain extent. When the polymerization is initiated,

the monomer starts to react with the curing agent. Polymer-clay nanocomposites based on

epoxy, unsaturated polyester, polyurethanes and polyethylene terephthalate have been

synthesized by this method.98


21

The “Melt Intercalation” Process

This process is shown in shown in the scheme 2.6. The layered silicate is mixed with

the polymer matrix in the molten state. Under these conditions and if the layer surfaces are

sufficiently compatible with the chosen polymer, the polymer can crawl into the interlayer

space and form either an intercalated or an exfoliated nanocomposite. In this technique, no

solvent is required.96

Polypropylene nanocomposites are generally synthesized by this

method shown in Scheme 2.6.

Scheme 2.6: Melt intercalation method

The “Solution Intercalation”

The layered silicate is exfoliated into single layers using a solvent in which the

polymer (or a prepolymer in case of insoluble polymers such as polyimide) is soluble. It is

well known that such layered silicates, owing to the weak forces that stack the layers together

can be easily dispersed in an adequate solvent. The polymer then adsorbs onto the

delaminated sheets, and when the solvent is evaporated (or the mixture precipitated), the


22

sheets reassemble, sandwiching the polymer to form, in the best case, an ordered multilayer

structure96

shown in Scheme 2.7.

Scheme 2.7: The “solution intercalation” method

Nanocomposite Structures

The key to the performance of nano-clays is how well they can be dispersed into a

polymer matrix. Types of dispersion that can be achieved need to be considered. Three

types of structures can be recognized99

in Scheme 2.8.

1. Conventional Composite:

Silicate tactoids exist in their original aggregated state with no intercalation of the

polymer matrix into the galleries. For this case, the particles act as microscale fillers.95

2. Intercalated Nanocomposite:

A single (or sometimes more than one) extended polymer chain is intercalated

between the silicate layers resulting in a well ordered multilayer morphology built up with

alternating polymeric and inorganic layers.96

3. Exfoliated/Delaminated Nanocomposite:


23

Extensive polymer penetration, resulting in disorder and eventual delamination of the

silicate layer, produces exfoliated structures consisting of inidividual 1 nm thick silicate

layers suspended in the polymer matrix.100

Scheme 2.8: Nanocomposite structures

The fully dispersed form is most useful for the majority of commercial applications,

and is the one that is normally aimed for, although conventional processing methods often

give mixed structures.99

Unsaturated Polyesters and Their Application Areas

Unsaturated polyesters (UP) are linear polycondensation products based on

unsaturated and saturated acids/anhydrides and diols or oxides dissolved in unsaturated vinyl

monomers. They comprise a versatile family of thermosetting materials, which are generally

pale yellow oligomers with a low degree of polymerization. Depending on the chemical

composition and the molecular weight (1200-3000 g/mol), these oligomers may be viscous,

liquids or brittle solids. The unsaturation in the backbone provides sites for reaction with

vinyl monomers using free-radical initiators, leading to the formation of a three-dimensional


24

network. The solutions of unsaturated polyesters and vinyl monomers which are reactive

diluents are known as UP resins.101

UP resins either in the form of pure resin or in the compounded form with fillers find

a wide range of applications. Their main applications are in the construction industry (non-

reinforced or glass-fiber-reinforced products), automotive industry, and industrial wood and

furniture finishing. Some of the important products based on UP resins are cast items such as

pearl buttons, knife and umbrella handles, and encapsulated electronic assemblies. Polyester

compounds have been formulated for the manufacture of bathroom fixtures. Floor tiles have

been manufactured by mixing UP resin with fillers such as limestone, silica, china clay, and

more. The applications of UP resins in a variety of areas such as transportation, electrical

appliances, and building and construction are largely due to development of bulk and sheet

molding compounds using glass fibers.101

Synthesis, Curing and Casting of Unsaturated Polyester Resins

The polyester backbone is composed of three basic types of structural units Scheme

2.9. They are synthesized by the polycondensation reaction of a saturated acid or anhydride,

an unsaturated acid or anhydride and a glycol. In the case of the general-purpose polyester,

these components usually consist of phthalic acid, maleic acid and propylene glycol.

O

O

O + O +HO

OH

O

O

HOO

O

O

O

CH3

OH

O

O CH3 O

X Y

CH3

Scheme 2.9: Synthesis reaction of UP from maleic anhydride, o-phthalic anhydride and

propylene glycol


25

The unsaturated acid provides the sites for the cross-linking, the saturated acid determines the

degree of spacing or concentration of the unsaturated acid molecules along the polyester

chains and the glycol, of course, provides the means for esterification and for bridging the

acids to form a prepolymer.102

The obtained unsaturated prepolymer is then diluted in

styrene and the resin solution is cross-linked by the addition of a peroxide initiator.

Unsaturated polyesters are versatile, since a large amount of different glycols and unsaturated

as well as saturated acids can be used to design different polymers. The composition of the

prepolymer and the curing process can be tailored for a specific purpose.103

The curing of

unsaturated polyester resins can be described as a free-radical copolymerization reaction

Scheme 3.0. As there are several double bonds per polyester molecule, the polymerization

proceeds with extensive crosslinking and results in a three-dimensional network.104

Hydroquinone is widely used in commercial resins to provide stability during the

dissolution of the hot prepolymer in styrene. The addition polymerization reaction between

the unsaturated polyester and the styrene monomer is initiated by free-radical catalysts

usually benzoyl peroxide (BPO) or methyl ethyl ketone peroxide (MEKP), which can be

dissociated by heat or redox metal activators like cobalt naphthanate into peroxy and

hydroperoxy free radicals. This catalyst system is temperature sensitive and does not

function effectively at temperatures below 10 oC, but at temperatures over 35

oC, the

generation of free radicals can be too prolific, giving rise to incomplete cross-linking

formation.105

.


26

C CH CH C O CH2 CH2 O

O O

( )n +RO + CH CH2

Polyester Peroxide initiator Crosslinking agent

Styrene


O O

( )n

O R

.

Free radical formed

from double bond

Attatchment of free radical

from initiator

Initiation step

.


O O

( )n

OR

.

CH CH2.New free radical

Bond formed between polymer & crosslinking agent

( )

Bridging step( )

.

.


O O

( )n

OR

.

CH CH2

.

.


O O

( )n. .

Bond between crosslinker & second polymer

Site for other crosslinks

Crosslinked polymers( )

Scheme 3.0: Schematic view of crosslinking of unsaturated polyester

In the casting process, a liquid material is poured into a mold and allowed to solidify

by chemical (e.g., polymerization) means, resulting in a rigid object that generally reproduces

the mold cavity detail with great fidelity. A large number of resins are available and a

variety of molds and casting methods are used in casting processes. Therefore, the choice of


27

material, mold type and casting technique is determined by the particular application. In

casting processes, the resin is added with an appropriate amount of hardener, catalyst, or

accelerator, mixed manually or mechanically and then poured into a mold, which is normally

coated with a mold-release agent. Air is removed if necessary, and the resin is allowed to

solidify. The casting process is relatively slow and employs comparatively cheap equipment.

To facilitate the removal of the cast part from the mold, mold releasing agents such as high

melting waxes, silicone oil, greases and some film-forming agents are used to coat the mold.

Among other considerations, the choice of mold-release agents is based upon the absence of

interaction between the resin system and the release agent.106

Sorption and Diffusion Studies of Polymer

Sorption and diffusion of the solvents in and through polymers have been widely

investigated from both theoretical as well as experimental point of view.107–109

The swelling

technique is a commonly used method to determine various coefficients such as diffusion,

sorption, and permeability coefficient.110-114

In swelling experiments, the polymer of known

dimension is dispersed in a solvent and the solvent mass uptake versus time is recorded and

the data is used to calculate the various coefficients. These coefficients give an idea about

the use of polymers in various applications such as membranes, ion-exchangers, controlled

release systems, packaging, microchip manufacturing, etc. Sorption kinetics in polymers

exhibit a variety of deviations from normal Fickian behavior, attributable to (a) slow viscous

relaxations of the swelling polymer or (b) differential swelling stresses generated by the

constraints imposed on local swelling during sorption. Several models have been proposed

for the study of swelling behavior of the polymers.115-120

In situ study by FTIR- is also


28

carried out by many researchers for the prediction the sorption behavior of the polymers.118-

120

Polymer nanocomposites are the future of the global industries.121

The polymer nano-

composites are prepared by dispersing a nano-filler into the polymer.120-123

These platelets

are then distributed into a polymer matrix creating multiple parallel layers, which force flow

of gases and liquids through the polymer in a torturous path, forming a complex barrier.124

Different types of fillers are utilized for the preparation of nano-composites. Amongst these,

the most common is a nano-clay called montmorillonite - a layered smectite clay.125-130

Additional nano-fillers include carbon nano-tubes, graphite platelets, carbon nano-fibers,

etc.128,131-133

The barrier properties of the nano-composites are supposed to increase due to

nano-clay loading.124,134

These properties of saturated and unsaturated polyester synthesized

from glycolyzed PET and their nanocomposites are studied in the subsequent chapters.

Response Surface Methodology

Response surface methodology (RSM) is a collection of mathematical and statistical

techniques for empirical model building. By careful design of experiments, the objective is

to optimize a response (output variable), which is influenced by several independent variables

(input variables). An experiment is a series of tests, called runs, in which changes are made

in the input variables in order to identify the reasons for changes in the output response.

Response surface-based methods aim to develop approximate functions that are

surrogates for long running computer codes.135,136

Techniques for constructing response

surfaces in reliability problems can be classified in two broad categories. In methods

developed from statistical sampling theory, factorial designs and regression analyses are used

to fit response surfaces. This approach has been used for studying soil structure interaction


29

problems137,138

static nonlinear structures139,140

and to obtain statistics of response for

nonlinear oscillators.141

As the design of experiments is centered around the mean and is

independent of the limit surface geometry, the fitted response surface may not always

conform to the true failure surface, especially, when it is at a great distance from the mean.

Alternative methods, which however, by pass some of the mathematical requirements of

response surfaces, obtain satisfactory results by incorporating reliability concepts for fitting

the response surface in the vicinity of the design point. These methods have been widely

reported in the literature for assessing the reliability of a variety of linear/nonlinear,

static/dynamic problems.142-149

It has been shown, that the failure probability estimates are

highly sensitive to the algorithm parameters.150

Moreover, it is implicitly assumed that the

contribution to the failure probability arises only from a single design point. This leads to

erroneous estimates, when there are multiple design points or multiple regions that make

significant contributions to failure probability.151

Techniques Used for the Characterization of Nanocomposites

Generally, the structure of nanocomposites has typically been established using

WAXRD analysis and transmission electron microscopic (TEM) observation. Due to its

easiness and availability WAXRD is most commonly used to probe the nanocomposite

structure 152-156

and occasionally to study the kinetics of the polymer melt intercalation.157

By monitoring the position, shape and intensity of the basal reflections from the distributed

silicate layers, the nanocomposite structure (intercalated or exfoliated) may be identified.

For example, in an exfoliated nanocomposite, the extensive layer separation associated with

the delamination of the original silicate layers in the polymer matrix results in the eventual

disappearance of any coherent X-ray diffraction from the distributed silicate layers. On the


30

other hand, for intercalated nanocomposites, the finite layer expansion associated with the

polymer intercalation results in the appearance of a new basal reflection corresponding to the

larger gallery height.

Although WAXRD offers a convenient method to determine the interlayer spacing of

the silicate layers in the original layered silicates and in the intercalated nanocomposites

(within 1–4 nm), little can be said about the spatial distribution of the silicate layers or any

structural non-homogeneities in nanocomposites. Additionally, some layered silicates

initially do not exhibit well-defined basal reflections. Thus, peak broadening and intensity

decreases are very difficult to study systematically. Therefore, conclusions concerning the

mechanism of nanocomposites formation and their structure based solely on WAXRD

patterns are only tentative. On the other hand, TEM allows a qualitative understanding of the

internal structure, spatial distribution of the various phases, and views of the defect structure

through direct visualization. The WAXRD patterns and corresponding TEM images of three

different types of nanocomposites are presented in Figure 2.1. Both TEM and WAXRD are

essential tools 158

for evaluating nanocomposite structure. However, TEM is time-intensive,

and only gives qualitative information on the sample as a whole, while low-angle peaks in

WAXRD allow quantification of changes in layer spacing. Typically, when layer spacing

exceed 6–7 nm in intercalated nanocomposites or when the layers become relatively

disordered in exfoliated nanocomposites, associated WAXRD features weaken to the point of

not being useful. However, recent simultaneous small angle X-ray scattering (SAXS) and

WAXRD studies yielded quantitative characterization of nanostructure and crystallite

structure in N6 based nanocomposites.159


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Figure 2.1: (a) WAXRD patterns and (b) TEM images of three different types of

nanocomposites.

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