Chapter 2 2.1 INTRODUCTION -...

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2.1 INTRODUCTION

The synthesis of unsaturated polyester usually involves

a reaction in bulk at elevated temperature between dibasic

acids or anhydrides and diols. Since esterification is

reversible process, the by-product water must be efficiently

removed, especially in the last stages of the reaction, where

the decrease in carboxyl group concentration is slow and the

increase in viscosity is fast. Those last stages are usually

followed under vacuum. However, in order to avoid loss of

volatile reactions an azeotropic distillation of water in the

presence of added organic solvents, such as toluene or xylene

may be used [1]. The main drawbacks of this process are

longer reaction time and the difficulty in removing the last

traces of solvent

Before going to the synthetic methods adopted in the

presence case, it is worthwhile to have a glance on some of the

important research work reported in literature pertaining to

the synthesis of polyester resins

In 1847, 15 years before Alexander Parkes introduced

Parkesine, the great Swedish chemist, Berzelius, reacted

tartaric acid with glycerol and formed are sinous mass poly

(glyceryl tartrate). Since this was the reaction product of a

polyhydric alcohol and a polybasic acid, it was a polyester

resin, although it remained a laboratory curiosity for many

years. Polyester resins can claim to be among the first of the

many synthetic resins which are now the basis of the plastics

industry.

The next landmark in the history of polyester resins was

the publication by Vorlander [2] in 1894 of the development of

the first unsaturated polyester resins, the glycerol maleates.

This was followed by the work of W. J. Smith in 1901. He

reacted phthalic anhydride with glycerol to produce poly

(glyceryl phthalate) and this led to the development of alkyd

resins by G.E.C. and B.T.H. from 1913 onwards. By modifying

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this resin with various vegetable oils an excellent base for

paints was produced having outstanding durability and the

commercial exploitation of alkyd resins for surface coating

took place over the following 20 years.

The modern history of unsaturated polyester resins

began with the filing of a patent application in 1922 by

Carleton Ellis [3] and the subsequent publication of this

patent in 1933. This covers the reaction products of dihydric

alcohols and dibasic acids and acid anhydrides for use as

lacquers. Hundreds of publications followed in the succeeding

10 years, but the important developments can be traced

through several stages. Firstly, the formation of inter -

polymers of esters of dibasic acids with vinyl compounds by

Dykstra [4] in 1934.Then followed the work by Bradley, Kropa

and Johnston who prepared polymerizable compositions based

on maleic anhydride [5-7]. This was followed by a further

publication of Ellis [8] showing the copolymerization of maleic

polyester resins with monomeric styrene in the presence of

peroxide catalyst. This patent is primarily concerned with the

preparation of lacquers and it is interesting to note that the

benzoyl peroxide catalyst is referred to as a drier. Likewise the

use of styrene is recommended to accelerate drying. This is an

important patent since it is the first time that these

compositions are suggested for use as moulding materials.

Muskat [9] then showed that phthalic anhydride can be

reacted with maleic anhydride and glycol to reduce the

tendency towards crystallization and so improve the

compatibility of the final resin with styrene. From this stage

to the present day polyester resins have not changed much in

principle. The changes which have been made mainly concern

the proportion of reactants used in the polyesterification and

the actual nature of the reactants.

No account of the history of polyester resins would be

complete, however, without mentioning the classic work of

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Carothers on the theory of condensation polymers and

polyesters [10-11] in 1929 and Kienle’s general theory of

polymer formations based on polyester studies [12-13] in 1930

and 1936.

The commercial development of unsaturated polyester

resins began in the United States of America (U.S.A.) in 1941,

when an allyl casting resin was introduced for use as a glass

substitute. In this case unsaturation was obtained by using

an unsaturated alcohol, allyl alcohol, instead of following the

more usual practice of using an unsaturated acid such as

maleic or fumaric. This was followed in 1942 by an allyl low

pressure laminating resin, allyl diglycol carbonate, which was

used for the manufacture of some of the first glass cloth

reinforced resin for aircraft.

In 1946 polyester resins were commercially available in

the U.S.A. consisting of diethylene glycol maleate and styrene

and similar resins were soon manufactured in the U.K. At

about the same time another type of polyester resin was also

made in commercial quantities in England. This was the

reaction product of methacrylic acid and phthalic anhydride

with ethylene glycol [14]. It was copolymerized with n-butyl

methacrylate and was used for some of the earliest glass fiber

reinforced plastic moulding to be made in England. With the

commercial production of maleic anhydride and styrene in

England, this type of resin has been almost entirely replaced

by polyesters of maleic anhydride and ethylene or propylene

glycol with a saturated dibasic acid such as phthalic

anhydride. These resins are mainly supplied as solutions in

monomeric styrene and they represent today the bulk of

unsaturated polyester resins used throughout the world.

Unsaturated polyester resins are one of the most

important matrix resins for commodity glass fiber reinforced

composites. They are obtained in two step process: first

unsaturated and saturated acids or anhydrides are reacted

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with diols in a polycondensation reaction; secondly the

resulting linear polyester prepolymeris dissolved in styrene

into syrup like resin. The resin is finally processed into a rigid

thermoset in a free radical co-polymerisation between styrene

and the double bonds in the polyester chain. The basic

chemistry of unsaturated polyester resins has remained very

much unchanged for the last 40 years.

The first unsaturated polyester resins of similar type as

used today were synthesized in the 1930’s. Carlton Ellis found

that unsaturated polyester prepolymers could be mixed with

styrene and copolymerized into a rigid polymer. These resins

became commercially important the next decade when they

were reinforced with glass fibers giving structural products

with high mechanical strength and low density. Today

unsaturated polyesters are one of the most important matrix

resins for composite materials [15].

Important product areas for unsaturated polyesters are

marine, automotive, electric and electronic, building,

construction, sport and leisure, domestic and sanitary

appliances, furniture as well as military applications. A

special use is in gel coats, which are used as coloured and

protecting surface coatings in composites. Unsaturated

polyester resins are very versatile as the processing into a

composite product can be done using several techniques: hand

lay-up and spray lay-up lamination, casting, compression

molding, pultrusion, resin transfer moulding (RTM), vacuum

infusion and filament winding [16]

A. Abdeen and co-authors [17] synthesized polyester

resin taking different proportions of phthalic anhydride and

maleic anhydride for esterification with propylene glycol.

Polyester based on unsaturated diols was prepared by

V.Stelian and co-workers [18] by trans- esterification of di -

ethyladipate and diols: cis-2-utene -1, 4-diol and trans-2-

butene -1, 4-diol having molecular weight 500-2000. A.

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Michiaki and coworkers [19] synthesized polyester by heating

propylene glycol, phthalic anhydride, maleic anhydride and

fumaric acid in molar ratio 1.1:0.5:0.25:0.25 at 205ºC to acid

value 60 and vacuumed to 5 torr for 2 hrs to acid value18.

Dicyclopentadiene, maleic anhydride and propylene glycol

were heated at 210ºC for 10 hrs by K.Akria and coauthors

[20]. Maleic anhydride, phthalic anhydride, polyethylene glycol

and (C6H5O)3P were heated at 160ºC for 5 hrs by Dabholkar

and coworkers [21] to give polyester having acid number 45.

The synthesis of unsaturated polyesters involving two step

reactions based on phthalic anhydride, maleic anhydride and

propylene glycol was studied by Korbar et.al. [22]. The first

step involves a reaction of an anhydride with glycol producing

monoester followed by a step growth polymerization reaction

producing polymer. Resin having light colour was obtained by

Frietag et.al [23] on heating dicyclopentadiene, maleic

anhydride, water and 25% hydrophosphorus acid at 130ºC and

then reacting with diethylene glycol at 210ºC. X. Yansheng

[24] prepared polyester using maleic anhydride, phthalic

anhydride and diethylene glycol. O.Yasuhiro [25] synthesized

polyester by heating a mixture of terephthalic acid, propylene

glycol, neopentyl glycol and ethylene glycol at 230ºC adding

triphenyl phosphate and maleic anhydride at 220ºC and

continued the reaction for 5 hrs at 215ºC.

2.2 CROSS-LINKING OF UNSATURATED POLYESTER RESIN

In order to get a rigid, structural material the

unsaturated polyester resin is cross- linked into a rigid

thermoset in a free radical copolymerization between the

styrene monomer and the polyester double bonds originating

from the unsaturated dicarboxylic acid. The copolymerisation

is initiated by peroxides activated by a redox reaction

thermally or with cobalt salts. During the crosslinking the

resin undergoes gelation, which is a dramatic physical

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change. The viscosity increases rapidly, the resin becomes

elastic and begins to behave as a rubber. The extent of

reaction, at which an infinite molecular network starts to

form, is called the gel point and the time to achieve it is the

gel time. The chemical reaction continues in the gel state and

more polyesters are linked to the network. Each polyester will

finally be linked to each other at several points in the network

and one gigantic molecule is formed. The cross – linking

reaction is a highly exothermic reaction and the temperature

can increase up to 100-200°C, depending on the resin

composition, laminate thickness and the initiator system. The

crosslinking reaction is not complete, however. Even when

the final solid state is achieved there will be unreacted

styrene monomers and double bonds left. This residual

reactivity can be removed by post -curing simply by heating at

a temperature above the glass transition temperature of the

cross- linked unsaturated polyester. This process of network

formation is often named in the literature as curing and the

degree of cure is taken as the cross - linking density.

2.3 METHODS FOR DETERMINATION OF CROSSLINKING

The crosslinking reaction is a very important stage in

the processing of unsaturated polyester into a composite

product. In order to achieve good product quality the cross-

linking reaction should occur in a controllable manner. It is

also necessary to carefully follow the reaction and to check

the degree of cure after the completion of the processing.

There are several techniques available to characterize the

crosslinking of an unsaturated polyester resin. They can be

divided into methods based on the changes in physical

properties of the resin and methods based on the changes in

chemical properties of the resin [26].

Physical properties, which change during cure, include

shear and torsional modulus, hardness, dielectric constant

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and viscosity. Viscosity measurements are used to follow the

earlier stages of the cross - linking and are particularly useful

for the determination of the processability of the resin. Both

steady shearing flow measurements [27] and oscillatory

shearing flow measurements [28] have been used. The

industry uses several empirical techniques for cure

monitoring, for example the cure can be followed with a stop-

watch while mixing the resin in a beaker or by measuring the

maximum temperature generated by the crosslinking

reaction with a thermo element embedded in the resin.

Hardness is a common empirical method to determine the

degree of cure in cured laminates, for example by using the

Barcol impressometer. Dielectric cure monitoring is based on

the measurement of changes in dielectric properties of the

resin and can be used both for the liquid and solid states of

the resin.

Among the chemical techniques, differential scanning

calorimetry (DSC) is the most important. DSC involves the

measurement of the heat of reaction (exothermal heat) which

is liberated in the curing reaction of the thermoset. The

technique can be used for the simulation of the curing process

in a composite. The analysis is based on the assumptions that

the exothermal reactions monitored are those of the curing

reaction and that the heat generation is directly proportional

to the rate of cure. Several other factors (heating rate, sample

preparation, atmosphere, resin properties and thermal

history) also affect the data analysis.

2.4 CHEMICAL COMPOSITION

The basic chemistry of linear unsaturated polyesters is

rather simple [29-32]. A mixture of unsaturated and saturated

dicarboxylic acid is reacted with diols in a melt

polycondensation. Monofunctional alcohols and acids are also

used in some formulations to tailor the properties. The most

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traditional composition is maleic anhydride, phthalic

anhydride and 1, 2-propanediol, which are cheap raw

materials. Other common raw materials are fumaric acid,

isophthalic acid, terephthalic acid, adipic acid, ethylene

glycol, diethylene glycol, dipropylene glycol, neopentyl glycol

and bisphenol -A.

The properties of the final product can be varied almost

endlessly by changing the composition of the unsaturated

polyester using these raw materials. Generally aromatic

groups improve the hardness and the stiffness while aliphatic

chain components increase the flexibility. The most common

raw materials and their influence on end-use properties are

listed in Table -2.1[33].

COMPOUND PROPERTIES

Maleic anhydride •Branched polyesters compared to those

from fumaric acid

•Lower degree of unsaturation compared to

those from fumaric acid

fumaric acid •Increased reactivity of polyesters

Phthalic

anhydride

•Polyesters with low molecular masses

•Improved hardness and stiffness in cured

resins

•Good compatibility with styrene

Terephthalic acid •Improved hardness and stiffness in cured

resins

•Increased heat deflection point

Isophthalic acid •Higher molecular masses

•Excellent physical and chemical

properties

•Improved hardness and stiffness in cured

resins

Adipic acid •More flexible chains

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•Soft products

•Increased toughness

•Reduced water and weathering resistance

Ethylene glycol •Reduced solubility of unsaturated

polyester in the vinyl monomer

•Increased rigidity

Propylene glycol

(1,2-propanediol )

•Good compatibility with styrene

Diethylene glycol •More flexible chains

•Soft products

•Increased toughness

•Reduced water resistance

Neopentyl glycol •Good corrosion, UV, water , and chemical

resistance

Hydrogenated

Bisphenol -A

•Good corrosion, water, and chemical

resistance

Table-2.1 Common components used in unsaturated

polyester resins and their influence on the properties of

the final product [33].

The unsaturated polyester prepolymer is finally blended

with styrene to a reactive resin solution, in which form the

resin is sent to the end user. The styrene acts both as a cross-

linking agent and as a viscosity reducer so that the resin can

be processed. In conventional unsaturated polyesters the

styrene content varies between 35 and 45 wt%.

The adjustment of the molar ratio of the unsaturated

dicarboxylic acid and the saturated dicarboxylic acid (maleic

anhydride respective phthalic anhydride) is an important

method to tailor the properties of the resin. This ratio controls

the reactivity of the unsaturated polyester and also the cross –

linking density of the final network. If the saturated

dicarboxylic acid is used in a molar excess the reactive

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unsaturated bonds will be distributed in the polyester chain

sparsely and the reactivity of the unsaturated polyester will be

lower. If the unsaturated dicarboxylic acid is used in a molar

excess the reactive double bonds will be distributed much

more densely and the reactivity will be higher, as there will be

a larger number of reactive sites in each polyester chain. This

will also give a much denser network in the cured resin, which

will result in a brittle material with poor mechanical

properties.

For this reason commercial unsaturated polyesters are

usually formulated using an excess of saturated acid. The

unsaturated polyester has typically a molecular mass between

1000 and 5000. The molecular mass is regulated by the diol /

dicarboxylic acid ratio, according to the principles of

Carothers [34]. Usually the diol is in excess, as the used diols

are liquids, while the dicarboxylic acids and anhydrides are

solids. An excess of solid reactants can cause a problem in the

form of sublimation of the reactants during polycondensation.

A high molecular mass will give a higher hardness, tensile and

flexural strength of the final cured material. If the molecular

mass is too low, the mechanical properties of the cured resin

will be poor. A too high molecular mass increases the viscosity

of the resin solution, which will cause problems with the

processing of the resin. Air entrapment in the laminate, poor

wetting of the reinforcement, long mould fill ing times and

processing times are typical practical problems due to the

resin viscosity.

2.5 EXPERIMENTAL

The work reported in the present chapter deals with the

preparation of unsaturated polyesters by melt poly-

condensation process.

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2.5.1 Materials

Phthalic anhydride, maleic anhydride, propylene glycol

and adipic acid were obtained from Chiti –Chem Corp. Ltd,

Vadodara. All the chemicals used were of analytical grade.

2.5.2 Synthesis Of Unsaturated Polyester Resin

Almost all commercial production of unsaturated

polyesters is done by the melt polycondensation of

unsaturated and saturated acids or anhydrides with glycols.

No solvents are used and the formed water is continuously

removed, in order to force the esterification reaction towards

completion. The condensation temperature is typically

between 170-2300C. At the end of the condensation, vacuum

is often applied in order to remove remaining water from the

viscous melt. The total reaction time can be from 8 to 25 hrs,

and the reaction is followed by acid number titrations and

viscosity measurements.

Azeotropic polycondensation in the presence of organic

solvents such as xylene or toluene can also be used. The

reaction takes place at lower temperatures and it is possible

to avoid losses of volatile reactants. The drawbacks are longer

reaction times and environmental problems with solvent

removing and recycling. Polyester resins were prepared in the

present work by the technique reported by B.Parkyn [35].A

mixture of Propylene glycol (PG), Phthalic anhydride, mailic

anhydride, p- Toluene sulfonic acid (PTSA) and Xylene as

distillating solvent was charged in a four - neck reaction

kettle equipped with stirrer, thermometer, nitrogen-gas

introducing tube, Dean & Stark apparatus and

watercondenser. The mixture was mechanically stirred and

heated at 140-2000C under nitrogen gas stream and

esterification was carried out while removing water formed by

the reaction from the reaction system, continues heating at

140-2000C until an acid number of 25-30 were reached. The

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Xylene was completely distilled out and reaction product was

allowed to cool.

The details about the molar ratio of acids/anhydrides to

glycols, styrene monomer and reaction temperature for the

synthesis of the unsaturated polyester resins (UPR) are as

shown in the Table -2.2(a) & Table-2.2(b)

Reactants Amount

Phthalic anhydride (PA) 33.36 gms

Maleic anhydride (MA) 22.08 gms

Propylene glycol (PG) 44.56 gms

Molar ratio = MA : PA : PG 0.5: 0.5: 1.3

Styrene as reactive diluent 35%

Reaction temperature 140-200oC

Table-2.2(a) Composition and reaction temperature for unsaturated polyester resin (UPR-S1)

Table-2.2(b) Composition for unsaturated polyester resin

(UPR-S1)

SAMPLE: S1

MONOMERS MOLECULAR

Wt. gm/mole

MOLES

gm

WEIGHT

gm

Wt. OF 100%

BASIS IN gm

P.A 148.12 0.5 74.06 33.36

M.A 98.06 0.5 49.03 22.08

P.G 76.10 1.3 98.93 44.56(43.01 ml)

TOTAL Wt. - - 222.02 99.99

* PTSA = 2%/100gm = 2 .0 gm

* Weigh = Molecular Wt. x Moles * ρ = m/v

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Scheme -2.1: Reaction mechanism for UPR-S1

2.5.3 Adipic Acid Modified Unsaturated Polyester Resin

A mixture of Propylene glycol (PG), Phthalic anhydride,

mailic anhydride, adipic acid,p- Toluene sulfonic acid (PTSA)

and Xylene as distillating solvent was charged in a four - neck

reaction kettle equipped with stirrer, thermometer, nitrogen-

gas introducing tube, Dean & Stark apparatus and

watercondenser. The mixture was mechanically stirred and

heated at 140-2000C under nitrogen gas stream and

esterification was carried out while removing water formed by

the reaction from the reaction system, continues heating at

140-2000C until an acid number of 25-30 were reached. The

Xylene was completely distilled out and reaction product was

allowed to cool.

The details about the molar ratio of acids/anhydrides to

glycols, styrene monomer and reaction temperature for the

synthesis of the adipic acid modified unsaturated polyester

resins (AAMUPR) are as shown in the Table -2.2(c) to Table-

2.2(l)

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Reactants Amount


Adipic acid (AA) 6.59 gms



Molar ratio = PA :AA: MA: PG 0.4:0.1:0.5:1.3



Table-2.2(c) Composition and reaction temperature for

adipic acid modified unsaturated polyester resin (AAMUPR-

S11)

Table-2.2(d) Composition for adipic acid modified

unsaturated polyester resin (AAMUPR-S11)

SAMPLE: S11

MONOMERS MOLECULAR

Wt. gm/mole

MOLES

gm

WEIGHT

gm

Wt. OF 100%

BASIS IN gm

P.A 148.12 0.4 59.25 26.71

A.A 146.12 0.1 14.61 6.59

M.A 98.06 0.5 49.03 22.10

P.G 76.10 1.3 98.93 44.60(43.05 ml)

TOTAL Wt. - - 221.82 99.99

* PTSA = 2%/100gm = 2 .0 gm


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Reactants Amount








Table-2.2(e) Composition and reaction temperature for

adipic acid modified unsaturated polyester resin (UPR-S12)

SAMPLE: S12

MONOMERS MOLECULAR

Wt. gm/mole

MOLES

gm

WEIGHT

gm

Wt. OF 100%

BASIS IN gm

P.A 148.12 0.3 44.44 20.05

A.A 146.12 0.2 29.23 13.19

M.A 98.06 0.5 49.03 22.12

P.G 76.10 1.3 98.93 44.64(43.09ml)

TOTAL Wt. - - 221.63 99.99

* PTSA = 2%/100gm = 2 .0 gm


Table-2.2(f) Composition for adipic acid modified


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Reactants Amount








Table-2.2(g) Composition and reaction temperature for


S13)

Table-2.2(h) Composition for adipic acid modified


SAMPLE: S13

MONOMERS MOLECULAR

Wt. gm/mole

MOLES

gm

WEIGHT

gm

Wt. ON 100%

BASIS IN gm

P.A 148.12 0.2 29.63 13.38

A.A 146.12 0.3 43.84 19.80

M.A 98.06 0.5 49.03 22.12

P.G 76.10 1.3 98.93 44.68(43.13ml)

TOTAL Wt. - - 221.42 99.98

* PTSA = 2%/100gm = 2 .0 gm


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Reactants Amount








Table-2.2(i) Composition and reaction temperature for


S14)

SAMPLE: S14

MONOMERS MOLECULAR

Wt. gm/mole

MOLES

gm

WEIGHT

gm

Wt. OF 100%

BASIS IN gm

P.A 148.12 0.1 14.81 6.69

A.A 146.12 0.4 58.46 26.43

M.A 98.06 0.5 49.03 22.16

P.G 76.10 1.3 98.93 44.72(43.17ml)

TOTAL Wt. - - 221.23 100.00

* PTSA = 2%/100gm = 2 .0 gm

* Weight = Molecular Wt. x Moles * ρ = m/v

Table-2.2(j) Composition for adipic acid modified


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Reactants Amount


Maleic anhydride (MA) 22.18gms

Propylene glycol (PG) 44.76gms

Molar ratio = MA : AA : PG 0.5: 0.5: 1.3



Table-2.2(k) Composition and reaction temperature for


S15)

SAMPLE: S15

MONOMERS MOLECULAR

Wt. gm/mole

MOLES

gm

WEIGHT

gm

Wt. OF 100%

BASIS IN gm

P.A 148.12 0.0 - -

A.A 146.12 0.5 73.07 33.06

M.A 98.06 0.5 49.3 22.18

P.G 76.10 1.3 98.93 44.76(43.21ml)

TOTAL Wt. - - 221.3 99.99

* PTSA = 2%/100gm = 2 .0 gm

* Weight = Molecular Wt. x Moles * ρ = m/v

Table-2.2(l) Composition for adipic acid modified

unsaturated polyester resin (AMMUPR-S15)

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Scheme -2.2: Reaction mechanism for AAMUPR-S11

Scheme -2.3: Reaction mechanism for AAMUPR-S15

2.6 CHARACTERIZATION OF RESINS

All the unsaturated polyester resins were characterized

by various techniques as shown below:

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1. Functional Group Analysis

2. Viscometric Study

3. Infrared Spectroscopy

4. Molecular Weight Determinations

5. Thermal Study

2.6.1 Functional Group Analysis

2.6.1(a) Acid Value

Acid value of unsaturated polyester resin was determined

according to the process reported by Mantell [36].

About one gram (exactly weighed) of the sample was

dissolved in 50ml of acetone in 250 ml conical flask. After

standing for a few minutes, the solution was titrated with

0.1N alcoholic potassium hydroxide solution, using

phenolphthalein as an indicator. Blank determination was

made at the same time without a sample. The acid value is

reported as the number of milligram of potassium hydroxide

required to neutralize one gram of resin sample.

Calculation:

Acid value = 56.1 (A - B) x N

W

Where, A = Consumption of KOH solution by the sample (ml)

B = Consumption of KOH solution by blank reading

(ml)

N = Normality of KOH solution

W = Weight of sample (gm)

Acid value indicates the amount of unreacted acid and

free acid group present in the resin. Acid value of unsaturated

polyester resins are reported in Table: 2.3 & Table: 2.3 (a)

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2.6.1(b) Iodine Value

Unsaturation in the polyester resin was determined in

terms of Iodine value by the standard method reported in the

literature [37].

About one gram (exactly weighed) of the sample was

dissolved in 50 ml chloroform in a 250 ml Iodine flask. After

adding 25 ml Wij’s reagent, the flask was stoppered and

allowed to stand in the dark for 30 minutes with occasional

shaking. At the end of the reaction period, 20 ml of the

potassium iodide solution (10%) and 100 ml of water were

added and the contents immediately titrated against 0.1 N

sodium thiosulphate solutions with vigorous shaking to

ensure the extract ion of all the iodine from the organic layer

until the liquid becomes light yellow. 2% starch solution (∼2

ml) was added and the titration was completed till the end

point reaches blue to colour less.

The blank determination of 25 ml of the iodine

monochloride solution (Wij ’s reagent) was carried out in

similar fashion omitting the sample. Iodine value was

calculated using the formula:

Iodine Value = 12.69 (V1 - V2) x N

W

Where,

V1 = Vol. (ml) of thiosulphate solution used for blank

reading.

V2 =Vol. (ml) of thiosulphate solution used for sample

reading.

N = Normality of sodium thiosulphate solution

W = Weight (gm) of the resin sample.

Iodine value indicates the number of unsaturation

present in the unsaturated polyester resin. Iodine value of

unsaturated polyester resins are reported in Table -2.3

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Sample

No.

Monomer

used

Molar ratio Acid

value

Iodine

value

UPR-S1 MA: PA: PG 0.5: 0.5:1.3 29.8 28.6

Table-2.3 Acid value and Iodine value of unsaturated

polyester resins.

Sample No. Monomer used Molar ratio Acid

value

AAMUPR-S11 PA :AA: MA: PG 0.4:0.1:0.5:1.3 29.3




AAMUPR-S15 MA : AA : PG 0.5: 0.5: 1.3 28.3

Table-2.3(a) Acid value of unsaturated polyester resins.

2.6.2 Viscometric Study

One of the characteristics of a polymer sample is that it

yields a solution whose viscosity is relatively greater than that

of pure solvent. Staudinger drew attention to the usefulness of

solution viscosity as a means of characterizing polymers [38-

41]. When long chain polymer molecules are caused to move

about in solution they produce frictional effect which show

increase in viscosity. The relation between solution viscosity

and molecular mass is empirical. Relationship between

solution viscosity and molecular mass is well familiar.

Viscosity measurements, requiring relative simple apparatus,

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provide a rapid and convenient molecular characterization of a

polymer. For most linear polymers, the “intrinsic viscosity or

limiting viscosity number denoted by (η) is related to

molecular weight by the Mark-Houwnik equation.

(η) = KMa ………………………….. 2.1

Where ‘K’ and ‘a’ are constants for a given

polymer/solvent system, that can be found by absolute

molecular weight determination methods (e g. From light

scattering or osmotic pressure) and (η) is equal to (ηsp/c)C-0.

Where the specific viscosity (ηsp) is given by

ηsp = ηs – ηo = (ηs /ηo ) – 1 ……………….. 2.2 ηo

Where ‘K’ and ‘a’ are the viscosity of the solvent and polymer

solution respectively. ηs/η0 is called as relative viscosity and

(ηsp/c) is called a reduced viscosity.

A clean and dried suspended type Ubbelhode viscometer

was placed in temperature bath maintained at 35 ± 0.1ºC. The

solvent, chloroform (6ml) was carefully introduced into the

viscometer. The viscometer containing solvent was allowed to

attain the bath temperature. The efflux time was noted

following the normal technique. Three independent readings

were noted and the average flow time (t, sec) was noted.

The process was repeated using 6 ml of the solution of

the given polymer (2.5 gm/100ml) also in chloroform to

measure the efflux time (t, sec). The solution was diluted by

adding 2 ml of the solvent into the viscometer. A slow stream

of air was passed through the solution to ensure uniform

mixing. The efflux time of the diluted solution was measured

after 2 min, during which time, the solution in the viscometer

attained the bath temperature.

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Subsequent dilutions were made by successive addition

of 2 ml of solvent. The efflux time for each of these solutions

was measured. The quantities, specific viscosity [ηsp = (t –to) /

to] and reduced viscosity (ηsp/c) were calculated and plots of

reduced viscosity against concentration of polymer (g/dl) were

made. The plot of (ηsp/c) Vs C for UPR -S1 is presented in Fig.

2.1. The intrinsic viscosity data are furnished in Table 2.4.

Figure 2.1 Plot of Reduce Viscosity Vs Concentration (g/dl) for UPR-S1

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Figure 2.2: Plot of Voltage reading (mV) Vs Concentration

(gm/kg) for UPR-S1

Sr. No. Sample Intrinsic viscosity

[ηηηη]

1 UPR-S1 0.053

Table-2.4 intrinsic viscosity of unsaturated polyester

resins

2.6.3 Infrared Spectroscopy

Infrared (IR) spectral study of polymers provides useful

information about their structural features as it gives the idea

about the nature of functional groups and the skeletal

structure of the polymers. In suitable cases, the IR spectral

study has been proved useful in understanding the course of a

polymerization reaction [42]. There are several reports about

the IR spectral study of the polymers in the detection of

crystallinity in the sample and the estimation of stereo

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regularity in the polymer chains [43]. IR spectroscopy is

widely used for structural elucidation of the polymers [42].

A Nicolet Impact 400D FT- IR Spectrophotometer was

employed for the measurements. The spectrum was run by

applying resin sample on KBr cellcovering the range of

frequencies from 400-4000 cm -1. The IR spectra of

unsaturated polyester resins are given in Fig 2.3. All the IR

spectra are similar in general shape, however some variations

in characteristic absorption peaks are observed. The

important bands observed and their possible assignments are

described in Table-2.5.

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Figure- 2.3 Infrared spectra of UPR-1

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No. Group IR Characteristics

(cm -1)

IR for UPR-

S1

A Esters

α,β-unsaturated

>C=O

stretching

1730 – 1715 1725.00

B C-H Stretching in

aromatic ∼3030 3000.20

C C-C multiple bond

stretching in Alkene 1620−1680 1652

D C-C multiple bend

stretching in aromatic ∼1600 1600.16

E Hydrocarbon

Alkane –CH2- 1445-1485 1464.35

F Alcohols

O-H bond stretching 1260-1350 1290.30

G C-H stretching in

aromatic ring (O-

disubstituted)

735-770 745.88

Table-2.5. Anticipated IR spectral features

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Figure- 2.14 Infrared spectra of AAMUPR-1

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No. Group IR Characteristics

(cm -1)

IR for UPR-

S1

A Esters

α,β-unsaturated

>C=O stretching

1730 – 1715 1727.00

B C-H Stretching in

aromatic ∼3030 3001.20

C C-C multiple bond

stretching in Alkene 1620−1680 1655

D C-C multiple bend

stretching in aromatic ∼1600 1600

E Hydrocarbon

Alkane –CH2- 1445-1485 1450.30

F Alcohols

O-H bond stretching 1260-1350 1295.20

G C-H stretching in

aromatic ring (O-

disubstituted)

735-770 750.84

Table-2.9. Anticipated IR spectral features

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2.6.4 Molecular Weight Determination By Gel Permeation

Chromatography (GPC)

Gel permeation chromatography is one of the most

powerful and versatile analytical technique available for

understanding and predicting polymer performance. It is the

only proven technique for characterizing a polymer by its

complete molecular weight distribution. Gel permeation

chromatography can determine several important parameters.

These includes Z-average molecular weight, weight average

molecular weight, number average molecular weight, viscosity

average molecular weight and the most fundamental

characteristics of a polymer – its molecular weight

distribution. Several workers have found out the molecular

weight and molecular weight distribution of various polymer

systems by using GPC [44-45].

Gel permeation chromatography was generated using

Waters GPC system. The HPLC grade Tetrahydrofuran (THF)

was used as a mobile phase at a flow rate of 1.0 ml/min. The

GPC system was equipped with two ultrastyragel columns

packed with styrene - DVB copolymers of 103 and 106 A0

porosity connected in series to cover an exclusion limit of 200

to 10 × 106. A Waters 410 RI detector was used with internal

temperature of 350C for peak detection. The GPC system was

calibrated with eight different polystyrene standards having

molecular weight ranging from 2000 to 2.5 × 106. The number

average (Mn) of unsaturated polyester resin sample is 1739,

weight average (Mw) is 4087 and polydispersity (Mw/Mn) is

2.3503. Gel permeation chromatography of unsaturated

polyester resin is shown in figure -2.4.

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Figure:2.4 Gel permeation chromatography (GPC) of

polyester resin

2.6.5 Thermal Study Of Unsaturated Polyester Resin

Proper knowledge of the thermal stability of polymers is

essential for their appropriate applications [46-48]. The

thermal behaviour of polymers with reference to their thermal

stability, kinetic parameters such as energy of activation,

order of reaction, therefore is of paramount importance.

In most of processing techniques of thermosetting

polymers, curing process is involved in which the polymeric or

oligomeric polyfunctional reactive groups are transformed into

a cross linked macromolecular three dimensional structure.

The thermal methods of characterization are very useful to

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understand curing reactions and working temperature range

of thermosetting polymers.

The most frequently used thermal methods are

thermogravimetric analysis (TGA), differential thermal

analysis (DTA) and differential scanning calorimetry (DSC).

Literature survey regarding the use of DSC techniques to

understand the curing reaction, thermal stability and

properties of cured resin is thought worth to cite here.

S.V.Pusatcioglue et.al. [49] investigated the curing of a

thermoset polyester using isothermal and dynamic techniques

of differential scanning calorimetry and proposed a kinetic

model that can be utilized to obtain the rates of heat

generation and the extent of cureat different cure

temperatures. T. R. Cuadrado [50] studied the curing kinetics

of general purpose unsaturated polyester with styrene using

benzoyl peroxide as an initiator and found that at low

temperature range (70- 90°C) the curing rate attained a

maximum showing a first order decay and at high temperature

(100-160°C) a second order kinetics.

The effect of particulate fillers on both, the rheological

properties during cure and curing kinetics of unsaturated

polyester resin has been investigated by K.M. Lem and C.D.

Han [51]. They combined rheological and DSC measurement to

obtain a correlation between viscosity and the degree of cure

during isothermal curing operation. Y. S. Yang and L. James

[52] used differential scanning calorimetry and infrared

spectroscopy for measurement of polymerization kinetics.

P.M.K. Lam et. al. [53] studied the curing kinetics of

orthophthalic polyester, isophthalic polyesters and vinyl ester

resin and compared them on the basis of the kinetic

parameters including conversion, cure rate, reaction rate

constant, heat of cure reaction, half life and Arrhenius

parameters.

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Y. S. Yang et. al. [54] have studied the reaction kinetics

of styrene -unsaturated polyester resins at curing

temperatures (120° -160°) and pressures. Several authors [55-

58] has studied the solidification process of unsaturated

polyester resin by differential scanning calorimetry.

N. Haffane et. al. [59] and J. B. Abdelouahab et.al . [60]

have determined the kinetic parameters for the heat generated

by the overall cure reaction of unsaturated polyester resin

from the heat – flux temperature curves obtained by DSC with

samples around 180mg and a heating rate of 1°C / min.

M. Saminathan et.al. [61] studied the thermal properties

and phase behaviour of the polyester by combination of DSC

and optical polarized microscope (PLM).

S.H.Mansour et.al. [62] studied the thermal behaviour of

styrenated polyester using differential scanning calorimetry .

M. Nagata and M.Nakae [63] studied the curing and thermal

stability of thermotropic polyesters and copolyesters based on

terephthalic acid, 3- (4-hydroxy phenyl) propionic acid and

glycols by differential scanning calorimetry and

thermogravimetric analysis.

X. Ramis et.al. [64] studied the curing process of

polyurethane -unsaturated polyester IPN by differential

scanning calorimetry. The present study deals with the

thermal analysis of unsaturated polyester resins. Differential

scanning calorimetry (DSC) accomplished the thermal analysis

of unsaturated polyester resins. The curing kinetic parameters

of unsaturated polyester resins were analyzed by DSC.

2.6.5(a) Differential Scanning Calorimetry

The synthesized polyester resins were cured by DSC

technique using benzoyl peroxide as catalyst.

The TA Instruments, USA model 5000/2920 diferential

scanning calorimetry was used to obtain the data on the

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exothermic curing reactions. The sample weights used for the

investigation were in the range of 8 - 10mg.

The sample contained in a metal pan and a reference

(the empty pan) are placed on raised plat forms formed in a

thermo electric disc, which serves as the primary mean of heat

transfer to the sample and reference from a temperature

programmed furnace (i.e. heating block). Traditionally, the

temperature of the furnace is raised (from room temperature

to 500°C) in a linear fashion at a heating rate 10°C/min, while

the resultant differential heat flow to the sample and reference

is monitored by thermocouples fixed to the underside of the

disc platforms . These thermocouples are connected in series

and the differential heat flow is measured using the thermal

equivalent of Ohm's law.

The DSC scans of resins are analyzed by the method

proposed by Borchardt and Daniels [65].

The Borchardt and Daniels method, employed with

microcomputer, is described in the literature [66]. This

method assumes that the reaction follows the n th order

kinetics, i.e. it obeys the relationship described below in

equation 2.3.

dα/ dt = K ( T ) (1 – α)n ……………….. 2.3

Where, α= Fractional Conversion

K (T ) = Specific rate constant at temp. T (sec -1)

n = Reaction order

The method also assumes that the temperature

dependence of the reaction rate follows the Arrhenius

expression as described below in equation 2.4.

K (T) = Z . e –E / RT …………………2.4

Where,

Z = Pre exponential factor (sec -1)

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R = Gas constant (8.314 J/ mole. ºK)

E = Activation energy (J/ mole)

T = Absolute temperature (°K)

The kinetic parameters such as activation energy,

reaction order, heat of reaction and log Z, from DSC

thermogram are evaluated using Borchardt –Daniels method

and are represented in Table 2.6. The corresponding DSC

scans of resins are depicted in Fig.2.5. Thermal parameters

like the temperature at which the curing starts (T i), the peak

exotherm temperature (Tp), temperature of completion of

curing (T f ), the curing range of temperature (T i - T f ) and time

required for curing i.e. cure time are estimated from the DSC

scans. The results corresponding to these thermal parameters

are represented in Table 2.7.

Sample Activation

Energy

(kJ/mole)

logZ (min -1) Reaction

Order n

Heat of

reaction

(J/gm) UPR-S1 171.4 22.90 1.46 40.7

Table: 2.6 Kinetic parameters

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Figure: 2.5 DSC Scan of UPR-S1

Sample T i

ºC

Tp

ºC

T f

ºC

Cure

range

(T i -T f )

ºC

Total

cure

time

(min)

Actual

cure

time

(min)

UPR-S1 100.40 121.90 143.50 100.40-

143.50

11.9 4.4

Table: 2.7 Thermal parameters

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Figure: 2.6 to 2.13 TGA & DTA Scan of UPR-S1 to S15

Figure: 2.6: TGA Scan of UPR-S1

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Figure: 2.7: TGA Scan of AAMUPR-S11

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Chapter 2

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Figure: 2.10: DTA Scan of UPR-S1

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Figure: 2.11: DTA Scan of AAMUPR-S11

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Chapter 2

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Sample % Weight loss at various Temperature (OC) from TGA

100 200 300 400 500 600 700 800

S1 0.38 7.68 47.51 86.66 88.54 89.24 90.42 92.22

S11 0.59 5.63 26.31 88.82 90.45 91.26 92.93 95.70

S12 0.59 4.53 24.04 89.05 90.70 91.39 92.71 94.36

S13 0.59 3.42 21.76 89.27 90.96 91.59 92.79 94.45

S14 0.70 3.08 17.62 89.96 91.70 92.40 93.56 95.55

S15 0.88 2.75 13.50 90.63 92.59 93.41 94.51 96.54

Table: 2.8 TGA data

2.7 Results and Discussion

Acid value and Iodine value of the unsaturated polyester

resins are mentioned in Table 2.3 & Acid value of the AAMUPR

are mentioned in Table 2.3(a) indicates the amount of -COOH

functional groups present in unsaturated polyester resins, as

well as the extent of polycondensation. While the iodine value

indicates the number of unsaturation present in unsaturated

polyester resin. The initial acid value from the beginning of

the reaction (456) decreases to 25-30 indicating good extent of

conversion of anhydrides during the process and thus

polyesters are expected to have low to medium molecular

weight.

Intrinsic viscosity of unsaturated polyester resins are

presented in Table 2.4. The results show that the intrinsic

viscosity of unsaturated polyester resins is relatively low

indicating low to medium molecular weight of the resins.

The IR spectra of unsaturated polyester resins are as

shown in Fig. 2.3. The data regarding the IR spectral

characteristics presented in Table 2.9 reveals that small

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S.P.University 112

variations in the location of the peaks due to absorptions by

functional groups like –CH2, C=C, and C=O and are observed

depending upon the structure of the diols/ anhydrides/acids .

For UPR-S1 , a strong absorption band at 745.88 cm -1 can

be arributed to –C-H stretching in aromatic ring (O-

disubstituted). Spectrum absorption bend at 1725.00 cm -1 &

3000.20 cm -1 confirms the presence of α,β-unsaturated >C=O

bond in ester linkage & C-H stretching in aromatic

respectively. Absorption peak appearing at 1652 cm -1 &

1600.16 cm -1 was C-C multiple bond stretching in alkene &

C-C multiple bend stretching in aromatic. Alkane –CH2- & O-H

bond stretching was confirmed by the presence of bend at

1464.35 cm -1 and 1290.30 cm -1 respectively.

For AAMUPR, a strong absorption band at 750.84 cm -1

can be arributed to –C-H stretching in aromatic ring (O-

disubstituted). Spectrum absorption bend at 1727.00 cm -1 &

3001.20 cm -1 confirms the presence of α,β-unsaturated >C=O

bond in ester linkage & C-H stretching in aromatic

respectively. Absorption peak appearing at 1655 cm -1 & 1600

cm -1 was C-C multiple bond stretching in alkene & C-C

multiple bend stretching in aromatic. Alkane –CH2- & O-H

bond stretching was confirmed by the presence of bend at

1450.30 cm -1 and 1295.20 cm -1 respectively.

The results of DSC study (Table 2.6 and 2.7) shows that

the curing reaction starts at or above 100°C and gets

completed almost around 142°C. This clearly indicates that

the unsaturated polyester resins have good induction period

under this condition and therefore it can be expected that at

room temperature the unsaturated polyester resins would

have a very long induction period which is needed for better

safe storage stability.

The curing range is of 43°C to 44°C which indicates that

after initiation of curing actual curing gets completed within

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S.P.University 113

4.4 min which is also very good indication of processing time

for which unsaturated polyester resins are to be kept under

compression during compression moulding process. The

kinetic parameters indicate the heat of reaction of UPR-S1

(40.7 J/gms).

The TGA data in table 2.8 indicates that the thermal

stability increases when phthalic anhydride is replaced by

adipic acid up to 300oC temperature. AAMUPR-S15 (without

phthalic anhydride) is thermally more stable then UPR-S1

(without adipic acid).After 300oC temperature, thermal

stability is approximately similar for UPR & AAMUPR.

Chapter 2

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