5 Poly(Ethylene Terephthalate) Moulding Materials

Poly(ethy1ene terephthalate) Moulding Materials 721

and a back flow valve fitted to screw injection moulding machines. Cylinder

temperatures are about 260°C and mould temperatures as high as 140°C to

promote a controlled crystallisation. Because of this high temperature it is

generally recommended that the mould be thermally insulated from the locking

mechanism and other machine parts.

An interesting feature of poly(ethy1ene terephthalate) is that according to the

moulding conditions two quite dissimilar products, one amorphous, the other

transparent, may be obtained, this being a consequence of having a Tg of about

80°C. For both types, however, there are certain common points to be

observed. As with other polymers that are slightly hygroscopic and which

require high melt temperatures the granules must be thoroughly dry, particular

care being necessary with reworked material. In addition, because of the low

melt viscosity injection, moulding screws should be fitted with back flow

valves and the barrel nozzles should have shut-off valves. Melt temperatures

are of the order of 260°C.

To produce amorphous transparent mouldings, mould temperatures should be

kept well below the T g , a temperature of less than 50°C generally being

recommended. Providing that wall thicknesses do not exceed 5-6 mm the melt

cools very rapidly and there is not enough time for significant crystallisation to

occur in the short time interval that the material is between T , and Tg. With

thicker sections it may not be possible to extract the heat out of the melt at a

sufficient rate and some crystallisation may occur. It is also important to use

grades which do not contain additives that accelerate crystallisation. Amorphous

mouldings should not be used above T g .

Crystalline, opaque, mouldings are produced by using a mould temperature of

about 130°C and grades containing crystallisation ‘accelerators’. The crystalline

grades retain their shape up to temperatures near to T , and therefore for many

applications may be used above T,.

In spite of the introduction of Arnite PETP the use of poly(ethy1ene

terephthalate) as a moulding material remained at a low level for many years. In

the 1970s it became recognised that reinforcement of the polymer with glass fibre

had an even greater influence on modulus and rigidity than with other

engineering plastics. For example, at 23°C and 50% RH the flexural modulus of

unfilled crystalline poly(ethy1ene terephthalate) is slightly less than that of a

polyacetal. On the other hand, at a glass-fibre loading of 30% the modulus of the

polyester is some 10% higher (11 000 MPa c.f. 10000 MPa). At 50% fibre

loading the modulus is as high as 15 000 MPa.

By the late 1980s it was estimated that 90% of crystalline PET moulding

materials were glass filled. Their major use was in electrical and electronic

applications. Thin, complex sections such as transformer bobbins may be formed

easily because of the ease of flow of the polymer even when fibre filled. These

materials have also been used for the housings and components for toasters,

coffee machines, industrial plugs and sockets, car heater housings and water

meter housings. Tougher grades are used for car grilles and fuel filler flaps.

Amorphous grades are used mainly for bottles.

Towards the end of the 1970s Du Pont introduced Rynite. This is a

poly(ethy1ene terephthalate) nucleated with an ionomer, containing a plasticiser

(thought to be n-pentyl glycol dibenzoate) and only available in glass-fibre-filled

form (at 30,45, and 55% filler levels). Although Tg is slightly reduced, due to the

presence of the plasticiser, to about 55-60°C the polymer is very rigid, exceeding

that of a polysulphone. It is less water sensitive than an unfilled polymer. Apart



722 Polyesters

from its poor tracking resistance, a common feature of many highly aromatic

polymers, its electrical properties are generally good whilst, as with the Amitetype materials, fire-retarding grades are now available.

In the late 1970s the benefits of biaxial stretching of poly(ethy1ene

terephthalate) were extended from sheet film to bottle manufacture. As a result

important new markets were opened. For some years the plastics industry had

made great efforts to secure part of the market for the packaging of carbonated

beverages. In the early 1970s it seemed that this hope would be fulfilled by the use

of the nitrile resins (Chapter 16) but toxicity problems largely associated with

residual acrylonitrile made this impossible. Fortunately the recognition that nitrile

resins could no longer be considered for this market coincided with the

development of techniques for bottle blowing poly(ethy1ene terephthalate). In

1978 estimates for USA consumption of poly(ethy1ene terephthalate) for bottles

were in the range 68000-86000 tonnes. By 1998 the corresponding figure was

1 430 000 tonnes. As discussed in the previous section, this involves special

polymer grades and, as was also mentioned, copolymers with isophthalic acid or

cyclohexanedimethanol are being increasingly used to improve clarity, toughness

and barrier properties. Whilst the USA market has been dominated by the

carbonated beverage market the process has been extended, particularly in Europe,

to produce bottles for other purposes such as fruit juice concentrates and sauces.

Wide-necked jars, for coffee and for other materials, also made their appearance.

Success in bottle blowing involves first the production of a substantially

amorphous parison by injecting into a cold mould. The parison is then withdrawn

from the mould, heated (for example by infrared heaters) and subjected to a

stretch-blow process that biaxially stretches the parison, giving a thin-wall

containers of high strength and toughness combined with a low permeability to

oxygen and carbon dioxide. Further reductions in gas permeability may be

achieved using multi-layer parison extrudates. For example, in Britain PET

bottles coated with vinylidene chloride-based copolymers are used for packaging

beer. There has also been some interest in poly-m-xylylene adipamide (see

Chapter 18) and, more particularly, ethylene-vinyl alcohol copolymers as barrier

materials.

A further substantial development, although not on the scale of the bottle and

film markets, had been the use of thennoformed PET sheet for menu trays. The

high heat distortion temperature of 220°C allows these products to be used in

both traditional and microwave ovens.

In attempts to reduce the Tg of PET and hence facilitate injection moulding a

number of copolymers based on PET have been prepared. Thus a copolyester

containing 3-methylpentane-2,4-diol was found to give much slower crystallisation rates during moulding operations. The use of isophthalic acid as a partial

replacement for terephthalic acid also retards crystallinity and this has been used

commercially with 1,4-~yclohexyleneglycol instead of ethylene glycol (see

Section 25.7). The considerable success of PET for making bottles and similar

products, together with continuing demand for PET film, had led to an upsurge

in companies supplying PET materials. By 1987 nine companies were supplying

PET materials in Western Europe for injection moulding, seven for bottle

manufacture and eight for film.

As with many other plastics materials being manufactured in a large number

of countries statistics for capacity and usage are subject to considerable

uncertainty. One estimate was that in 1997 capacity for making ‘container’

grades was about 6 000 000 t.p.a. with consumption at about 4 000 000 t.p.a.



Poly(ethy1ene terephthalate) Moulding Materials 723

Other estimates placed the film and bottle market to be of a similar size in Japan

while globally the bottle market was about 20% of the total. Together with other

data this suggests that the fibre and filament market absorbs about 72% of PET

capacity, containers about 19%, film about 7% and mouldings 2%. Considerable

quantities of PET bottles are, however, recycled into fibres for use, for example,

in outdoor clothing.



25.5.1 Poly(ethy1ene naphthalate) (PEN)

As long ago as the 1940s it was known that poly(ethy1ene naphthalate) had

higher temperature resistance, higher tensile strength, higher UV resistance and

better oxygen and water barrier properties than poly(ethy1ene terephthalate).

Commercial interest only became significant when, in the late 1980s, Amoco

commenced manufacture of the precursor dimethyl-2,6-naphthalene dicarboxylate increasing their nameplate capacity to 27 000 t.p.a. in 1998. By 1989 Shell

were producing PEN in commercial quantities (Hipertuf) and by the late 1990s

they were joined by 3M, Du Pont, Eastman and ICI.



PET



PEN



Structurally the difference between PEN and PET is in the double (naphthenic)

ring of the former compared to the single (benzene) ring of the latter. This leads

to a stiffer chain so that both Tg and T , are higher for PEN than for PET (Tg is

124°C for PEN, 75°C for PET; T , is 270-273°C for PEN and 256-265°C for

PET). Although PEN crystallises at a slower rate than PET, crystallization is (as

with PET) enhanced by biaxial orientation and the barrier properties are much

superior to PET with up to a fivefold enhancement in some cases. (As with many

crystalline polymers the maximum rate of crystallisation occurs at temperatures

about midway between Tg and T , in the case of both PEN and PET). At the

present time PEN is significantly more expensive than PET partly due to the

economies of scale and partly due to the fact that the transesterification route

used with PEN is inherently more expensive than the direct acid routes now used

with PET. This has led to the availability of copolymers and of blends which have

intermediate properties.

The copolymers are prepared using a mixture of dimethyl terephthalate and

dimethyl naphthalate. Published data indicates a reasonably linear relationship

between Tg and copolymer composition on the lines discussed in Section 4.2, e.g.

Tg for a 50:50 copolymer is about 100°C which is about mid-way between Tg

figures for the two homopolymers. In line with most other copolymers there is no

such linearity in the crystalline melting point (T,). As comonomer levels are

introduced T , drops from the values for both homopolymers and indeed

crystallisation only readily occurs where one of the components is dominant, Le.

80%. Thus commercial copolymers are usually classified into two types:

(a) low terephthalate (‘low tere’) copolymers which may be considered as being

effectively >80% PEN in nature;



724 Polyesters

(b) high terephthalate (‘high tere’) copolymers which may be considered as

being >80% PET in nature.

Blends are created by physically mixing two or more different resins in varying

amounts. While in theory it may be considered that the PEN and PET molecules

will be separate entities in the mix it has been reported that substantial

transesterification can occur during prolonged melting in an extruder leading to

block polymers whose block length would, presumably, decrease with melt

mixing time. Considerable development effort has been required to produce

blends of acceptable quality.

As with PET, the market for PEN is in three main areas:

(a) fibres;

(b) films;

(c) bottles and other blown containers.

While detailed discussion of the merits of PEN fibres is largely outside the scope

of this book mention may be made of the success in preliminary trials of (yacht)

sailcloths made from PEN fibre. PEN fibres have a modulus roughly 2.5X that

of PET, exhibit excellent flex life and also show very good UV resistance. It is

understood that the one yacht fitted with PEN sailcloth in the 1996 Olympics

won the gold medal in its event.

Film is said to have been the first commercial application for PEN but has only

recently become more widely available (e.g. Kaladex - ICI). The materials are

particularly interesting for electrical insulation as a consequence of their very

good heat resistance (UL continuous use ratings of 180°C (electrical) and 160°C

(mechanical); see Section 9.2.1 for explanation). Film is also being used for

purposes where heating may be involved in manufacture andlor service such as

flexible warming circuits and battery heaters, business machines with high

operating temperatures, tapes and labels and embossing films. PEN is also used

in a tape storage cartridge.

However, the greatest interest and potential for PEN is in the blown container

market. Replacing PET with PEN increases the range of materials that may be

packaged because of the higher process temperatures and lower permeability to

gases of the latter. Because of the high material cost the market for

homopolymers is largely limited to medical applications due to the sterilizability

of the material but there is also potential for use in baby foods (with hot filling

possible above 100°C) and for bottled wines and beers. The low terephthalate

copolymers, because of their high cost as well as slightly inferior properties to

the homopolymers, would also seem to have a limited market. The high

terephthalate resins would appear to have the greatest potential in that they are

less expensive and widen the end-use envelope sufficiently by allowing hot

filling to nearly 100°C. Products of interest include jams, carbonated soft drinks,

juices, cosmetics and chemical containers.

The quality of blends is strongly dependent on mixing techniques but

encouraging results have been obtained, particularly in respect of improving

barrier properties.

25.6 POLY(BUTYLENE TEREPHTHALATE)

The expiry of the original poly(ethy1ene terephthalate) patents provided the

catalysts for developments not only with poly(ethy1ene terephthalate) but also



Poly(buty1ene terephthalate) 725

with related polymers. As a consequence in the early 1970s many companies

became involved in the manufacture of poly(buty1ene terephthalate), often

abbreviated to PBT or PBTP and also known as poly(tetramethy1ene terephthalate), itself often abbreviated to PTMT.

In the USA producers included Eastman Kodak (Tenite PTMT), General

Electric Corporation of America (Valox), and American Celanese (Celanex). In

Europe major producers by the end of the decade were AKZO (Arnite PBTP),

BASF (Ultradur), Bayer (Pocan) and Ciba-Geigy (Crastin). Other producers

included ATO, Hiils, Montedison and Dynamit Nobel. With the total Western

European market at the end of the decade only about 7000 tonnes other

companies at one time involved in the market such as IC1 (Deroton) withdrew.

By 1998, however, the Western European market had grown to over

90 000 t.p.a., that for the United States to about 140000 t.p.a. and that for Japan to

just over 60 000 t.p.a. There are also about a dozen USA and Westem European

manufacturers. Statistics on capacity are somewhat meaningless, as the polymer

can be made using the same plant as employed for the manufacture of the much

larger tonnage material PET. It is, however, quite clear that the market for injection

moulded PBT is very much greater than that for injection moulded PET.

A large number of grades is available, one supplier alone offering about 40,

including unreinforced, glass- and carbon-fibre reinforced, mineral filler

reinforced, impact modified, elastomer modified, flame retardant and various

combinations of the foregoing.

The polymer is produced by reacting terephthalic acid with butane-1A-diol.

Because of the longer sequence of methyl groups in the repeating unit the chains

are both more flexible and less polar than poly(ethy1ene terephthalate). This leads

to lower values for T, (ca 224°C) and Tg (22-43°C). The advantage of lower

processing temperatures is offset by lower heat distortion temperatures. At the

1.86MPa stress level in the ASTM test for deflection temperature under load

values as low as 50-65°C are obtained. Vicat softening points are also lower than

for poly(ethy1ene terephthalate) (170-180°C c.f. 261°C). Typical properties are

given in Table 25.7. As may be expected from a more hydrocarbon structure,

poly(buty1ene terephthalate) is a somewhat better electrical insulator than

poly(ethy1ene terephthalate). It also has good stability to aliphatic hydrocarbons,

alcohols and ethers but is swollen by low molecular weight esters, by ketones and

by partially halogenated hydrocarbons.

As with poly(ethy1ene terephthalate) there is particular interest in glass-fibrefilled grades. As seen from Table 25.8, the glass has a profound effect on such

properties as flexural modulus and impact strength whilst creep resistance is also

markedly improved.

About 90% of the polymer is injection moulded. Like poly(ethy1ene

terephthalate) the polymer is susceptible to hydrolysis so that the granules must

be thoroughly dried before moulding. At temperatures above 270°C the material

decomposes quite rapidly so that melt temperatures during processing are usually

in the range 240-270°C. The low Tg facilitates rapid crystallisation when cooling

in the mould, which is typically held about 50"C, and this allows short moulding

cycles. High injection speeds are generally recommended, particularly with

glass-filled grades, in order to obtain a good finish.

The use of PBT as an engineering material is more a consequence of a balance

of good properties rather than of a few outstanding ones. It does not possess the

toughness of polycarbonate, the abrasion resistance of an aliphatic polyamide,

the heat resistance of a polysulphone, polyketone or poly(pheny1ene sulphide) or



726 Polyesters

Table 25.8 Comparative properties of PET, P'M and PBT polymers

Psoperty



Specific gravity

Melting point

Glass Transition

Deflection temperature @ 1.8 MPa

Notched Izod impact

Tensile strength

Flexural modulus

Moulding shrinkage

Dielectric constant @ 1 MHz

Dissipation factor @ 1 MHz

Rockwell hardness



Units



"C

"C

"C

Jlm

MPa

GPd

mlm

M scale



PET



PTT



PBT

unfilled



PBT

30% glass

filled



1.37-1.4

265

80

65

37

61.7

3.11

0.030

3.0

0.02

106



1.35

225

45-75

59

48

59.3

2.76

0.020

3.0

0.015



1.31-1.34

228

25

65

53

56

2.34

0.020

3.1

0.02

68-85



1.52

228

25

223

85

117

7.6-8.3

0.002-0.004



-



90



,

the low water absorption of a modified PPO. As it is a polyester, there will be a

substantial number of common chemicals that will either attack it or cause

swelling, particularly at temperatures above the Tg.

However, PBT shows a good balance of properties and when it is suitably

modified by, for example, glass fibre or fire retardants, some very useful

compounds can be produced. The particular characteristics emphasised by the

suppliers include:

(1) High softening temperatures (glass-fibre-filled grades are better than

polycarbonates and modified PPOs).

(2) High rigidity, with some filled grades having a flexural modulus as high as

11 OOOMPa, a figure only exceeded by PPS amongst the engineering

thermoplastics.

(3) Good electrical insulation properties with exceptional tracking resistance for

an engineering thermoplastic and, in particular, for an aromatic polymer. In

tracking resistance most grades are generally superior to most grades of

polycarbonates, modified PPOs, PPS and the polyetherimides.

(4) Low friction and good abrasion resistance.

( 5 ) Good impact strength at low temperatures and excellent creep rupture

strength.

( 6 ) Low water absorption and good chemical resistance, including resistance to

stress cracking.

(7) Good dimensional stability, partially as a consequence of the low water

absorption but also because of a low coefficient of thermal expansion.

(8) Capability of compounding to give UL94 V-0 flammability ratings.

(9) Good mouldability, with easy flow and rapid setting.



It should, however, be noted that good flame retardancy is only achieved with the

use of flame retardant additives and that some of the best of these, such as the

brominated diphenyls and brominated diphenyl ethers, are restricted in their use

in some countries.

PBT has tended to replace polyamides in a number of precision parts due to its

better dimensional stability.



Poly(buty1ene terephthalate) 727

Poly(buty1ene terephthalate) finds use as an engineering material on account of

its dimensional stability, particularly in water, and its resistance to hydrocarbon

oils without showing stress-cracking. The stiffness of glass-filled grades is also

of some importance. Typical applications include pump housings, impellers,

bearing bushings, gear wheels and in measuring equipment.

As with poly(ethy1ene terephthalate) PBT-based copolymers have been

introduced to overcome some of the deficiencies of the homopolymer. For

example, the rather low notched impact strength of unreinforced grades has been

overcome by partial replacement of the terephthalic acid with a longer chain

aliphatic dicarboxylic acid. Improved toughness has also been obtained by

grafting about 5% of ethylene and vinyl acetate onto the polyester backbone.

There has also been active interest in blends of PBT with other polymers.

These include blends with PMMA and polyether-ester rubbers and blends with

a silicone/polycarbonate block copolymer.

Blends of PBT with polycarbonates have been widely used for car bumpers.

Interest in PBT/PET blends and PBT/ASA has arisen because of the good surface

finish possible even with glass-reinforced grades. Copolyesters based on PBT but

with some longer chain diol or acid are also now produced.

In the late 1990s it has been been estimated that in Western Europe the market

share was:

Electronics/electrical applications

Automotive applications

Household goods

Other



34%

40%

9%

17%



Amongst the diverse uses in the electrical/electronics field are coil formers,

miniature circuit breakers, picture-tube mountings, edge connectors and

telephone distribution boxes.

In the automotive sector PBT compounds are widely used for small interior

mouldings such as ashtrays, foot pedals, door handles and safety belt

components, whilst external uses include windscreen wipe holders and exterior

mirror housings. There has also been extensive use of PBT/PC blends for

bumpers but these have more recently tended to be replaced by polypropylene.

To counter this development at least one manufacturer has used ABS as an

impact modifier to produce a more competitive material.

Business machinery applications include keys for keyboards, typewriter

ribbon guides, plug and socket connectors and optical cable sheathing.

In household applications PBT has found use not just because of its high heat

distortion temperature, rigidity, very good electrical insulation properties and

dimensional stability but also because of the resistance of the material to many

liquids and chemicals encountered in the home. These include detergents and

cleaners, oils and fats, fruit and vegetable juices, beverages, many foodstuffs and

spices. Established applications include oven door handles, component parts of

coffee makers and deep friers, electric iron housings, styling hair brush

components and heated hair curlers.

Miscellaneous uses include textile bobbins, guns for hot melt adhesives and

bilge pump housings. These materials are normally found in reinforced form. In

addition to glass fibres, other fillers such as glass beads, talc and mica are used

in conjunction with coupling agents.

Carbon-fibre-filled grades exhibit interesting tribological properties and useful

antistatic behaviour.



728 Polyesters

Blends with polybutadiene rubber as the disperse phase have improved

toughness but show only moderate thermal aging. Newer grades have been

achieved by impact modification using cross-linked acrylic materials. Although

these show better heat aging, they do not have such good impact properties as the

polybutadiene-modified grades.

25.7



POLY(TR1METHYLENE TEREPHTHALATE)



Although poly(trimethy1ene terephthalate) has been known for many years it was

only introduced by Shell in the late 1990s as a consequence of a breakthrough in

the synthesis of the monomer 1,3-propane diol which enabled the polymer to be

produced at costs suitable for commercialisation. The polymer itself is prepared

by melt condensation of the diol with terephthalic acid.

In line with the common observation of condensation or rearrangement

polymers containing an odd number of methylene groups in the repeat unit, T ,

is less than that for PET and PBT but as shown in Figure 25.14 is only very

slightly less than for PBT.

In most respects PTT can be considered as intermediate in properties between

PBT and PET but does appear to possess two special properties:



(1) The repeat length in the triclinic polymer crystals (75.3 nm) is significantly

less than for PBT (86.3nm) and PET (99.5nm). This has been claimed to

make the crystal more spring-like in the long axis resulting in enhanced

resilience and wear resistance in carpet fibres to a level approaching that of

polyamide fibres.

(2) Glass-filled grades have a higher flexural modulus than corresponding PBT

and PET materials (For 30%w/w glass-filled PTT the modulus is quoted as

10.35 GPa, for PBT 7.60 GPa and for PET (at the slightly lower glass content

of 28%) 8.97GPa.

As is common for crystalline thermoplastics the deflection temperature of

unfilled grades is similar to Tg (quoted as being in the range 45-70°C) while for

glass-filled grades it is much closer to the T , of 225°C.

25.8 POLY-( 1,4-CYCLOHEXYLENEDIMETHYLEIWTEREPHTHALATE)

(PCT)

Replacement of ethylene glycol with 1,4-~yclohexyleneglycol (also known as

cyclohexane dimethanol) gives a polymer with a regular structure but a

somewhat stiffer chain than PET. Such a semicrystalline polymer has a T , of

about 289°C compared with about 250°C for PET. In turn this gives, for the

commercial 30% glass-filled grade (Valox 973 1 -General Electric), a Vicat B50

softening point of 270°C and a deflection temperature under load at 0.45 MPa of

275°C (both about 50°C higher than PET). Perhaps surprisingly the deflection

temperature under 1.8 MPa load at 200°C is very similar to that for a typical PET

compound.

More to be expected of a more hydrocarbon polymer than PET is a somewhat

lower water absorption, typically about 70% that of PET. With appropriate flame

retardants, grades can have a UL V-0 rating at 0.8 mm thickness.



Poly-(I ,4-cyclohexylenedimethyleneterephthalate)

(PCT) 129

Since PCT is sensitive to hydrolysis at the high moulding temperatures, it must

be thoroughly dried before moulding for 4-6h at 65-70°C in a desiccant

dehumidifying drier. Typical melt temperatures for moulding are 295-3 lO"C,

and mould temperatures can range from 65 to 135"C, although for the important

circuit board components temperatures of 95-1 20°C are used to reduce postmoulding shrinkage and optimise surface finish.

PCT may be used for the production of electronic and automotive components

such as circuit board components, connectors, switches and relays, and alternator

armatures and pressure sensors. The main application has been in the fabrication

of surface-mount connectors that can withstand infrared reflow soldering

operations.



25.8.1



Poly-(1,4-Cyclohexylenedimethyleneterephthalate-co-isophthalate)



In 1972 Eastman Kodak introduced a copolymer produced by reacting

1,4-~yclohexyleneglycol with a mixture of isophthalic and terephthalic acids.

Thus the polymer contains 1,4-cyclohexanedimethyloxy units (I), terephthalic

acid units (11) and isophthalic acid units (111) (Figure 25.20).



The copolyester was first marketed as Tenite Polyterephthalate 7DR0 but is

now sold as Kodar PETG.

Being irregular in structure the polymer is amorphous and gives products of

high clarity. In spite of the presence of the heterocyclic ring the deflection

temperature under load is as low as that of the poly(buty1ene terephthalates) and

is also slightly softer. Some typical properties are given in Table 25.9.

Early interest in the material centred round the ability of the polymer to be

thermoformed at draw ratios as high as 4:l without blushing or embrittlement.

Because of its good melt strength the material performs well during extrusion

blow-moulding whilst the low moulding shrinkage facilitates injection moulding.

Table 25.9 Typical properties of the copolyester Kodar PETG

Property

Specific gravity

Transparency (film)

Deflection temperature

(at 1.86 MPa stress)

Yield strength

Elongation at break

Flexural modulus

Rockwell hardness

Impact strength (unnotched)



Units



Value



-



1.2



%

"C



70-80

68



MPa



51.3

210

2100

108

no break



%



MPa

R-scale



730 Polyesters

A similar product is Kodar PETG 6703 in which one acid (terephthalic acid) is

reacted with a mixture of glycols (ethylene glycol and 1,4-~yclohexylene

glycol).

A related glass-reinforced grade (Ektar PCTG) has also been offered.

25.9



HIGHLY AROMATIC LINEAR POLYESTERS



It has already been shown (e.g. Chapters 20 and 21) that the insertion of a

p-phenylene into the main chain of a linear polymer increased the chain stiffness

and raised the heat distortion temperature. In many instances it also improved the

resistance to thermal degradation. One of the first polymers to exploit this

concept commercially was poly(ethy1ene terephthalate) but it was developed

more with the polycarbonates, polysulphone, poly(pheny1ene sulphides) and

aromatic polyketones.

During the period of development of these materials work proceeded on heatresistant polyesters. It was found, for example, that reaction of resorcinol with

terephthalyl chloride gave a polymer that showed no signs of melting below

500°C (Figure 25.21).



OH



+



ClOC *COCl



- +

-



Figure 25.21



The polyester made by reacting hydroquinone with terephthalic acid also

melted above 500°C. That from bis-phenol A and 4,4’-(2,2-butylidene)dibenzoic

acid is said to be stable in nitrogen to above 400°C.

In the 1960s the Carborundum company introduced the polymer Ekonol

P-3000. This was the polymer of p-hydroxybenzoic acid (I) (Figure 25.22), in

practice produced by the self-ester exchange of its phenyl ester to prevent

decarboxylation. A blend with PTFE,

Ekonol T-4000, was also produced.

A number of related copolymers were also introduced. Ekkcel C-1000

contained the units (I), (11) and (111) whilst Ekkcel 1-2000 contained the units (I),

(IV) and (V) (Figure 25.22).



73 1



Highly Aromatic Linear Polyesters



The homopolymer (I) (Figure 25.22) had an average molecular weight of

8-12000. It is insoluble in dilute acids and bases and all solvents ‘up to their

boiling points’, The polymer also has a high level of thermal stability. The weight

loss after 200 h at 260°C is 1% and at 400°C it is 1% per hour. The limiting

oxygen index is about 37%. Some typical properties are given in Table 25.10.

The homopolymer is difficult to fabricate and has been shaped by hammering

(like a metal), impact moulding and pressure sintering at 420°C and 35MPa

pressure. The copolymers are somewhat easier to fabricate. The difficulty in

fabrication has severely limited the development of these polymers.

Table 25.10 Typical properties of commercial polyhydroxybenzoate and related copolymers

PI-opei-q



Units



Copolymers



Homopolymer

(Ekonol P-3000)



(Ekkcel



(Ekkcel

c-1000)

Specific gravity

Deflection temperature under

load (1.86 MPa)

Tensile strength

at 23°C

at 260°C

Flexural modulus

at 23°C

at 260°C

Water absorption (24 h)

Dielectric constant at 1 kHz



1-2000)



-



1.45



1.35



1.40



“C



>550



370



413



70

21



99

21



3200

880

0.04

3.68



4900

1410

0.025

3.16



MPa

MPa

MPa

MPa



510



%



0.02

3.28



-



With a somewhat lower level of heat resistance but with many properties that

make them of interest as engineering materials alongside the polycarbonates,

polysulphones, poly(pheny1ene sulphides) and polyketones are the so-called

polyarylates which are defined as polyester from his-phenols and dicarboxylic

acids.

One such material is the copolymer first marketed by the Japanese company

Unitika in 1974 as U-Polymer and more recently by the Belgian company Solvay

as Arylef and Union Carbide as Ardel. (Around 1986 the Union Carbide interest

in Ardel, as well as in polysulphones, was taken over by Amoco.) Similar

polyarylates have since been marketed by Hooker (Durel), Bayer (APE) and

DuPont (Arylon). This is a copolyester of terephthalic acid, isophthalic acid and

bis-phenol A in the ratio 1 : 1:2 (Figure 25.23).

The use of the two isomeric acids yields an irregular chain inhibiting

crystallisation. This has two consequences:

(1) The absence of a T , allows the material to be processed at much lower

temperatures than would be possible with a crystalline homopolymer using

only one of the acids.

(2) Unfilled polymer is transparent.



The high concentration of aromatic rings nevertheless assures a high T g ,

variously quoted as being between 173 and 194°C. As with other polymers of