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EBB 427 Application and Technology ofEngineering Polymers (Second Half)

Dr. Hazizan Md Akil

School of Materials and Mineral Resources EngineeringEngineering Campus, USM.

THERMOSETTING POLYMERS:Processing, Application and Future

Direction

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Polyester resin

The most important esters are those derived fromcarboxylic acids

Polyesters are defined as polymers containing recurring

CO-O- groups in the main chain A large number of polyesters is commercially available:

Unsaturated polyesters

Poly(allyl ester)s

PET

etc

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Unsaturated Polyesters resin

Linear polyester containing aliphatic unsaturation whichprovides sites for subsequent cross-linking

First became available in USA, 1946

Prepared from diethylene glycol and maleic anhydride

and could be cross-linked by reaction of styrene The polymers was of interest for the preparation of glass-

fibre laminate by technique which did not involve highpressure

The commercial production of GFR polyester established1949 mainly for large structures such as boat hulls,sports car bodies and roofing.

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1950, Bulk Moulding Compounds were developed 1960, Sheet Moulding Compounds were developed.

During the past few years the use of polyester mouldingcompositions has grown significantly, particularly in the

transportation field

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Linear unsaturated polyesters are prepared commerciallyby the reaction of a saturated diolwith a mixture of anunsaturated dibasic acidand a modifying dibasic acid(or corresponding anhydrides).

In principle, unsaturation desired in a polyester can bederived from either an unsaturated diol or an unsaturatedacid

For economic reason, unsaturated acid is invariablypreferred

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Unsaturated acid provides sites for subsequent cross-linking

The function of the modifying acid is to reduce thenumber of reactive unsaturated sites along the polymerand hence to reduce the cross-linking intensity andbrittleness of the final product.

Some acids and anhydrides which are used to modifypolyesters are, in fact, unsaturated but the double bondsare not sufficiently reactive to represent sites for

subsequent cross-linking

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DiolsPropylene glycol

Is the diol most widely used for manufacture of linearunsaturated polyesters

It is prepared by the hydration of propylene oxide

Commonly, the reaction is carried out without catalyst atabout 200C and 2 MPa

Propylene glycol is isolated by distillation under reduced

pressure.

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Preparation of propylene glycol

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Propylene glycol is the preferred diol because it formspolyester which are compatible with styrene and whichshow little tendency to crystallise and readily available at

low cost.

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Other diols

Diols other than propylene glycol are utilised to a lesserextent

Typical examples of other diols are:Diethylene glycol

Neopentylene glycol

Ethylene glycol

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Unsaturated acids and anhydrides

Typical examples are:

Maleic anhydride

Phthalic anhydride

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Maleic anhydride

Most important unsaturated component used inmanufacture of linear unsaturated polyesters

Mostly obtained by the oxidation of benzene

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Maleic anhydride

The reaction is carried out in the vapour phase bypassing a mixture of benzene and excess of air over a

vanadium pentoxide catalyst at 350C - 450C The effluent is cooled and most of the maleic anhydride

is condensed

The non-condensed material passes to a scrubber where

the remaining anhydride is hydrolysed to maleic acid

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The maleic anhydride solution is then passes toevaporators for concentration and dehydration

High purity anhydride is ontained by distillation underreduced pressure

Because of rise in price of benzene, increasing amountsof maleic anhydride are being obtained from cheaper n-

butane

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Reaction:

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Maleic anhydride is a white crystalline solid, m.p. 52C-53C

Maleic anhydride is preferred to maleic acid since it ismore reactive and give rise to less water on esterifation

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Other acids(Fumaric acid)

Trans-isomer of maleic acid is sometimes preferred tomaleic anhydride as it is less corrosive and gives lighter-coloured

Chloromaleic acid and chlorofumaric acid may be used inthe production of self-extinguishing resins

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Modifying acids and anhydrides

Phthalatic anhydride

The most important modifying component used in

manufacture of linear unsaturated polyester

The anhydride is generally obtained by the oxidation of o-xylene

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Production of Phthalatic anhydride

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The reaction is carried out in the vapour phase by

passing a mixture of o-xylene and air over a catalyst suchas vanadium pentoxide supported on silica and promotedwith titanium dioxide at about 400C

The exit gas is cooled and the phthalic anhidride is

collected and purified by distillation under reducedpressure

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Older processes based on the air-oxidation of

naphthalene (obtained from coal) are now of minorimportance

Phthalic anhydride is a white crystalline solid mp. 131C

Phthalic anhidride gives polyester which are compatible

with styrene and the crosslinked products are hard andrigid.

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Other acids and anhydrides Modifying components other than phthalic anhydride are

frequently used in the preparation of unsaturated

polyesters in order to impart special properties to the finalproduct

Adipic and sebacic acids are employed to give flexiblematerials and isophthalic acid is used for tough products

with higher heat distortion temperatures

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Other acids and anhydrides The use of endo-methylenetetrahydrophthalic anhydride,

the dies-Alder reaction product of cyclopentadiene and

maleic anhydride leads to a substantial improvement inheat resistance

Flame resistance materials are obtained by the use ofchlorinated acids and anhydrides, e.g.

tetrachlorophthalic anhydride and chlorendic acid

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Cross-linking monomers It is possible to cross-link unsaturated linear polyester

chains directly one to another, however, reaction is slowand low degree of cross-linking is achieved.

These limitations are overcome by the introduction of amaterial which forms bridges between the chains.

The materials most commonly used to cross-linkunsaturated linear polyesters in this way are vinylmonomers

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Cross-linking monomers The addition of a liquid vinyl monomer to the polymer

also leads to a reduction in viscosity and this facilitatesthe impregnation of glass-fibre in the preparation of

laminates. Styrene is the most widely used cross-linking monomer,

being preferred because of its compatibility, low viscosity,ease of use and low price.

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Cross-linking monomers

Other materials are sometimes employed when specialproperties are required.

For example, methyl methacrylate is used, often inconjunction with styrene, for the preparation oftranslucent sheeting.

Diallyl phthalate and triallyl cyanurate (solid) is used forheat resistance products.

Partially polymerized diallyl phthalate (solid) is used asthe cross-linking agent in moulding powders (the so-

called alkyd polyester moulding powders) based onlinear unsaturated polyester

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Preparation of linear unsaturated polyester

Linear unsaturated polyesters are prepared batch-wiseby heating a mixture of the appropriate acidic andhydroxy components in a reactor jacketed for heating andcooling and fitted for distillation.

A typical formulation for general-purpose material might

be as follows:

Propylene glycol 100 parts by weight

Maleic anhydride 72 parts by weight

Phthalic anhydride 54 parts by weight

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The molar ration of the ingredients shown above is

1.2:0.67:0.33

The excess of glycol is to allow for loss during thereaction and to restrict the molecular weight of thepolymer.

The mixture is heated at 150C-200C for 6-16 hours andwater is continuously distilled from the reactor.

Sometimes xylene is added to the reaction mixture toassist in the removal of water by azeotropic distillationand sometimes catalyst such as p-toulenesulphonic acidis added to reduce the reaction time.

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In order to prevent discolouration, the reaction is carriedout in an inert atmosphere of either carbon dioxide ornitrogen.

Heating is continued until the average molecular weightof the polyester reaches about 1000-2000.

The polymer is then cooled to about 90C and pumpedinto a blending tank containing vinyl monomer to whichhas been added an inhibitor such as hydroquinone.

In a general purpose material, the weight of styrene used

is about half that of the polymer

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The blend (which is commonly referred to as polyester

resin) is then allowed to cool to room temperature.

The reaction between hydroxy-compound and anhydrideproceeds in two distinct steps.

In the first step, esterification of the anhydride occurs toform a free acid group which is then esterified in thesecond step.

Typical reaction;

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The first step proceeds more rapidly than the secondsince the anhydride group is more reactive than the freeacid group.

Clearly, a diol and an anhydride may interact through asequence of reactions of the foregoing type to yield alinear polyester.

Thus a segment of the polyester obtained from propyleneglycol, maleic anhydride and phthalic anhydride mighthave the following structure:

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Appreciable cis-trans isomerization generally occursduring the polyesterification of unsaturated dibasic acidsand anhydrides.

Such isomerization is particularly marked with maleicanhydride, which becomes incorporated into the polymerchains mostly from fumarate groups.

The extent of isomerization is governed by severalfactors including the structure of diol, the reactionconditions, the catalyst used (if any) and the molecularweight of the polymer

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The polymerzation of maleic anhydride with propyleneglycol gives almost entirely poly(propylene fumarate) butin most other cases the final polymer contains 70-90%fumarate groups and 10-30% maleic groups.

It is fortunate that maleated-fumarate isomerization doesoccur because the fumarate group shows much greaterreactivity towards vinyl monomers than the maleatedgroup; hence subsequent cross-linking proceeds more

readily.

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Cross-linking of linear unsaturated polyester

ebb 427 (2) hazizan

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