'Polyesters, Thermoplastic'. In: Encyclopedia of Polymer ...nguyen.hong.hai.free.fr/EBOOKS/SCIENCE...

504 POLYELECTROLYTES Vol. 7

POLYESTERS, THERMOPLASTIC

Introduction

Polyesters enter our lives in a most ubiquitous manner as textiles, carpets, tirecords, medical accessories, seat belts, automotive and electronic items, photo-graphic film, magnetic tape for audio and video recording, packaging materials,bottles, and so on. Their utility is illustrated by the vast range of their applications.This article describes the properties, synthesis, manufacture, and raw materialsfor the two most widely used thermoplastic polyesters: poly(ethylene terephtha-late) (PET) [25038-59-9] and poly(butylene terephthalate) (PBT) [26062-94-2]. Inorder of volume, PET comes first by virtue of its enormous market tonnage inpolyester fibers and films, as well as the resin for blow-molded bottles, containers,and food packaging.

Polyesters are linear polymeric molecules containing in-chain ester groups,formally derived by condensation of a diacid with a diol. The earliest commercialpolyesters were the alkyd resins (qv), nonlinear polymers developed for surfacecoatings shortly after World War I. It was not until the 1930s that the classicstudies of Carothers examined main-chain polyesters in a rigorous and sys-tematic fashion (1). Although he studied a wide range of polyesters, made bothfrom aliphatic diacids and diols (AA-BB) type and from ω-hydroxyacids (ABABtype), for various reasons, Carothers did not pursue polyesters derived fromaromatic diacids and alkylene diols. All his aliphatic polymers were low melting(<100◦C) and were easily soluble in common organic (dry cleaning) solvents.They had little utility as textile fibers, which largely explains why Carothersturned his attention to polyamides. The first successful synthesis of satis-factory high molecular weight poly(ethylene terephthalate), 2GT, was made inEngland in 1942, during the early days of World War II. The inventors were J. Rex

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Vol. 7 POLYESTERS, THERMOPLASTIC 505

Whinfield and W. Dickson, working at the Calico Printers’ Association (2,3). Otherpolymers pioneered by these workers included poly(1,3-propylene terephthalate),3GT, poly(1,4-butylene terephthalate), 4GT, and the polyester from ethylene gly-col and 1,2-bis(4-carboxyphenoxy)ethane, known as CPE-2G or “Fiber-O” (4). Ofthese materials, PET was selected for development as a melt-spinnable syn-thetic fiber, but commercialization was impossible until after the end of WorldWar II. Eventually, when the various national economies were back on a peace-time footing, PET polymer and fibers derived from it were put into production.The whole market-driving force for polyester at this time was in the form ofsynthetic fibers. In the United Kingdom, the new material was manufacturedby Imperial Chemical Industries Ltd. under the trade name Terylene, whileDuPont introduced it to the United States in 1953 as Dacron (see POLYESTERS,FIBERS).

In the early 1950s, additional poly(alkylene terephthalate)s were exam-ined in both United States and Europe for synthetic fiber applications. Oneof these, poly(1,4-butylene terephthalate), 4GT or PBT, had (and still has)some very attractive fiber properties. Notably, it was white and resisted photo-oxidative yellowing much better than nylon. It accepted disperse dyes muchmore readily than PET. It had excellent resilience and elastic recovery. Allthese features made it attractive for such fiber end uses as women’s wear,hosiery, and carpet fiber. However, the twin criteria of pleat-retention andcrease-resistance in apparel fabrics were commercially important at the time(it was an era when accordion pleats were highly fashionable in women’s wear)and PET was superior on these counts. Only later was it realized that PBTpolymer was highly suited to injection-molded applications. Not only was ithighly crystalline, it had a high rate of crystallization, so that parts couldbe molded in a fully crystallized state and therefore free from distortion orwarping. Unlike nylon, it had a low moisture uptake and was much more di-mensionally stable to changes in atmospheric humidity. Molding-grade PBTresins were introduced in 1970 by Celanese Corp. as Celanex and others fol-lowed.

In contrast, PET was not successful as a molding resin because of its lowrate of crystallization in a cold mold. The mold has to be heated to 130–140◦C,well above the PET glass–rubber transition temperature (Tg) to obtain adequatecrystallization rates. Satisfactory moldings were still not obtained because of un-controllable crystal morphology. During the late 1960s, fast-crystallizing gradesof PET were developed which had uniform morphology because of the presenceof specific nucleating agents (5–7). Akzo and DuPont were early entrants inthe field with their Arnite and Rynite range of PET resins; other manufactur-ers followed and PET molding resins are now widely used, particularly in theauto industry. However, the biggest single market for moldable PET today isin blow-molded bottles, which exceeds every other single end use for PET poly-mer except polyester fibers. In 1990, the annual world production of PET fibers(8) was about 9 million tons; according to Table 2 (see later) the annual pro-duction of PET bottle resin in 2000 was 1.4 million ton and growing rapidly. Itis worth noting that poly(ethylene-2,6-naphthalenedicarboxylate) (PEN) was an-other important polyester also synthesized first by ICI workers in 1948 (9) (seePOLY(ETHYLENE NAPHTHANOATE).

506 POLYESTERS, THERMOPLASTIC Vol. 7

Manufacture of Raw Materials and Monomers

PET and PBT are both made from terephthalic acid and its dimethyl ester. Tereph-thalic acid (TA) is made by air-oxidation of p-xylene [106-42-3] in acetic acid undermoderate pressure in the presence of catalysts such as divalent cobalt and man-ganese bromides (10). p-Xylene is the highest melting of three isomeric dimethyl-benzenes and is separated by fractional crystallization from the C8-aromatic frac-tion (including ethylbenzene) during petroleum refining (11). Alternatively it maybe separated by selective adsorption on a zeolite bed combined with an isomeriza-tion process (12). For PET fiber production, very pure TA is required and in theearly days, when the oxidation of p-xylene was achieved with 50% aqueous nitricacid under pressure, the process left some highly undesirable by-products such asnitroaromatics and carbazoles in the crude TA. Because of the great insolubilityof TA, it was not easily purified by recrystallization and so it was converted to itsdimethyl ester (dimethyl terephthalate, DMT), and the DMT in turn purified byredistillation under reduced pressure and a final recrystallization. PET was madeby the reaction of DMT with excess ethylene glycol in the presence of catalyststo promote ester-interchange and polymerization. When the direct air-oxidationprocess of p-xylene became the process of choice, the DMT purification route wasstill used as an interim process, even though major undesirable impurities hadbeen eliminated by the new oxidation route.

During the 1967–1972 period, pure TA became available in large quanti-ties (13) because of the improvements in the process and purification of TA.Harmful incomplete oxidation products in crude TA such as p-toluic acid and4-carboxybenzaldehyde (4-CBA) were eliminated by recrystallization from waterunder pressure with concomitant hydrogenation. Under these conditions, p-toluicacid is highly water-soluble and 4-CBA is hydrogenated to toluic acid (14). Theavailability of pure TA caused a major change in production methods for PET.Direct esterification (DE) processes using pure TA superseded the former ester-interchange (EI) process based on DMT. Interestingly, in recent years DMT hasmade something of a comeback, ostensibly because of the recycling of waste PET.Production of pure DMT derived from the methanolysis and glycolysis of PETwaste is carried out on a significant scale (15,16). Manufacture of TA from p-xylenehas undergone a constant process of improvement over the years and innumer-able patents have been filed on small improvements, new oxidation catalysts, etc.A very recent article describes an experimental “Green Chemistry” p-xylene oxi-dation process using hydrogen peroxide and a manganese dibromide catalyst insupercritical water as a solvent. Under these conditions, the TA remains in solu-tion (17).

Turning now to the diol components, ethylene glycol (ethane-1,2-diol) is madefrom ethylene in essence by direct air-oxidation to ethylene oxide and ring open-ing with water to give the 1,2-diol (18). Butane-1,4-diol is still made by the oldReppe process. Acetylene reacts with formaldehyde in the presence of a catalystto give 2-butyne-1,4-diol, which is hydrogenated to butanediol. The ethynylationstep depends on a special cuprous acetylide-bismuth salt catalyst, which mini-mizes side reactions (19). The hydrogenation step is best done in two stages overspecial catalysts (20). Another butanediol route starts from butadiene. In a pro-cess due to Mitsubishi Chemical Industries, butadiene reacts with acetic acid and


oxygen in the presence of a palladium catalyst to give 1,4-diacetoxy-2-butene (21),and the latter is hydrogenated over a special catalyst (22) and finally hydrolyzedto 1,4-butanediol. It is not known whether this process is in commercial use atthe time of writing. Although Reppe plants still operate, the process has been re-placed in newer plants by a route based on hydroformylation of allyl alcohol (frompropylene oxide) over a rhodium catalyst (23) to give 4-hydroxybutyraldehyde.This is reduced to butanediol using a two-stage hydrogenation route (24) tominimize side reactions. The ethynylation route is still an important one butnew plants use the Arco hydroformylation process, which was purchased byLyondell.

Polymerization Processes

Thermoplastic polyesters are step-growth polymers which need to be made tohigh molecular weight (12,000–50,000) to be useful (25). The first stage is anesterification or ester-exchange stage where the diacid or its dimethyl ester re-acts with the appropriate diol to give the bis(hydroxyalkyl)ester and some linearoligomers. Water or methanol is evolved at this stage and is removed by frac-tional distillation, often under reduced pressure at the conclusion of the cycle. ForEI, weakly basic metallic salt catalysts are used: the list is extremely long, andmany recipes are proprietary, but such salts as calcium, zinc and manganese ac-etates, tin compounds, and titanium alkoxides have been widely used (26–28).Certain EI-catalysts have the undesirable effect of promoting thermal degra-dation at high temperatures (29) encountered during the latter stages of highpolymerization. A McClafferty rearrangement of the ester unit is often deemedresponsible. To overcome this, EI-catalysts are sequestered at the end of ester-interchange by adding phosphorus compounds such as triphenyl phosphite, triph-enyl phosphate, or polyphosphoric acid in stoichiometric amounts (30). Again,such recipes are often proprietary. Titanium and tin compounds [eg Ti(OR)4 orR2SnO] act as powerful universal catalysts for both EI and polymerization re-actions and are left unchanged. They are not suitable for 2G-derived polyestersbecause of formation of a yellow color, although recipes are under developmentin Japan which claim to obviate this problem. For PBT, ester-exchange usingDMT and a titanium alkoxide catalyst is the route of choice, since butanediolcyclizes to tetrahydrofuran (THF) in the presence of acids (31). Nevertheless,the direct polycondensation of butanediol with terephthalic acid using specialreaction catalysts and conditions to minimize THF formation has been described(32–35).

Batch polymerization is usually done in an autoclave fitted with a powerfulmechanical stirrer to handle the viscous melt, under high vacuum at a temper-ature above the melting point of the final polymer. During this critical stage, itis very important to eliminate oxygen and blanket the process with inert gas,nitrogen, or argon. During the polycondensation stage, the linear oligomers andthe bis-hydroxyalkylterephthalate esters undergo a succession of ester-exchangereactions, eliminating the diol, which is removed under high vacuum, and thusincreasing the molecular weight steadily. A polymerization catalyst is needed,whether the initial process is EI-based or DE-based. As already mentioned, tin


and titanium catalysts are suitable for both ester-interchange and polymeriza-tion, but for PET, antimony trioxide is the usual polymerization catalyst (36). Itonly becomes active at high temperatures and thus can be added at the start ofthe reaction along with the other catalysts. More recently, there has been a moveaway from heavy metals like antimony, particularly in Europe, where they areviewed with increasing disfavor on environmental grounds. Even in the UnitedStates, problems can arise with heavy metal contaminants (which include anti-mony) in waste glycolysis still-bottoms. These cannot be landfilled for environmen-tal reasons and their safe disposal causes added expense. A less toxic metal wouldclearly be advantageous. However, alternatives are not universally satisfactory.Titanium alkoxides cause unacceptable yellowing of PET, apparently because ofreaction with vinyl ester chain ends. This cannot occur with PBT. Germaniumcompounds, either as the dioxide, tetraalkoxide, or glycoloxide, are good catalysts,nontoxic, and give very white polymers. However, they are usually considered tobe too expensive because of the relative scarcity of germanium. Another problemis loss of germanium by volatilization. The use of a germanium/titanium mixedcatalyst is disclosed in a patent relating to PET bottles (37). Antimony trioxideis a robust polymerization catalyst, but in PET it is susceptible to reduction toantimony metal, which can cause an undesirable gray-blue color in the polymer.

As the polymer molecular weight increases, so does the melt viscosity, and thepower input to the stirrer drive is monitored by a wattmeter so that an end pointcan be determined for each batch. When the desired melt viscosity is reached,the molten polymer is discharged through a bottom valve, often under positivepressure of the blanketing gas, and extruded as a ribbon or as strands, which arewater-quenched and chopped continuously by a set of mechanical knives. Largeamounts of PET are also made by continuous polymerization processes. PBT ismade by both batch and continuous polymerization processes (38–40).

The polymer is then dried thoroughly and stored for subsequent process-ing. Whenever a polyester is made by melt polycondensation, a small amount ofcyclic oligomer is formed which is in equilibrium with the polymer. This can beextracted with solvents from solid polymer but when the extracted polymer isremelted, more oligomer forms until the equilibrium is reestablished. The level ofsuch oligomers is about 1.4–1.8% by weight for both PET and PBT. Thus it is im-possible to completely remove cyclic oligomers from any melt-processed polyesters.In the case of PET, the main oligomer is a cyclic trimer (41,42) while in the case ofPBT, the oligomers comprise roughly an equal mixture of cyclic dimer and trimer,together with much smaller amounts of higher oligomers (43). The presence ofsuch oligomers usually does little harm, but under certain conditions they can ex-ude to the surface. This is a problem with polyester fibers with their high surface/volume ratio. For example cyclic trimer can interfere with the fiber dyeing process.It is also a nuisance in bottle production as the trimer coats the preform injectionmolds.

It is often necessary to make polymer of much higher molecular weight thanwould be practicable in the melt, either by reason of excessively high melt viscosityor because degradation reactions would begin to overtake the rate of polymeriza-tion and limit molecular weight. For this reason, solid-state polymerization (SSP)is frequently used. Dried polymer chip of moderate molecular weight is heated ata temperature roughly 20◦C below its softening point, either in a high vacuum or


in a stream of hot inert gas in a device which agitates the solid. Typical devicesmight be a twin-cone rotary vacuum drier or a fluidized-bed unit. There are manytypes of commercial SSP units available.

One very important practical consideration with PET is that the polymerchips must be fully crystallized by careful annealing before the solid-phase poly-merization process begins. Usually the polymer has been water-quenched beforecutting and will be largely amorphous. If not precrystallized, the chips may sin-ter together on attempted solid-phase polymerization. The difficulties caused byseveral tons of polymer setting to a solid mass can well be imagined! It is possi-ble by careful annealing to raise the Tm of the PET considerably above the usualfigure (44), thus allowing the solid-phase polymerization to take place at highertemperatures and shorter reaction times. Various agitation devices (45) and poly-mer chip treatments (46) have been described to prevent sticking. An integratedcrystallizing and solid-state polymerization process has been described (47) in thepatent literature.

In the solid-state process, the volatile by-products of the polycondensationreaction (traces of water, methanol, excess diol, etc) escape by vapor diffusionthrough the solid chip and are rapidly removed from the chip surface instead ofbeing limited by viscous diffusion through a bulk melt. Esters of aliphatic diolsand aromatic diacids begin to decompose thermally even in an inert atmosphereat around 250◦C (48). In the polymer melt, local mechanical heating caused byviscous shear during agitation can cause thermal degradation. SSP eliminatesthis and the molecular weight rises within a few hours to the desired figure.

During the polymerization reaction, various by-products are formed. In thecase of PBT, the major one is THF formed by dehydration of butanediol or byinternal cyclization of the C4-ester units. This is a harmless by-product, as faras the polymerization is concerned, since it is nonreactive under polymerizationconditions and quickly volatilizes away. However, its formation is commerciallyundesirable because it constitutes an air pollution problem as a volatile organicemission, and represents the loss of valuable starting material (butanediol). PET,by contrast, has two major troublesome by-products. One is the generation of di-ethylene glycol (DEG) units in the chain by dehydration of 2-hydroxyethyl esterchain ends to form an ether link. This process cannot be entirely prevented, al-though addition of alkalis can reduce it, and it is further mitigated by restrictingthose time/temperature combinations that tend to favor DEG formation. DEGcontent is related to the change in softening point (�Tm) by the empirical relation(49)

�Tm = ( − 2.2)m(◦C)

where m is the molar % concentration of DEG.In addition to depressing melting point, DEG units adversely affect the crys-

tallinity of the polymer, reduce the strength of both fibers and oriented films,and increase the susceptibility of the polymer to chemical attack and aqueoushydrolysis.

The other major by-product is acetaldehyde, which is produced by thermaldegradation of the PET unit. Random oxygen–alkyl scission of ester units leaves avinyl ester end and a carboxyl-ended chain. The vinyl ester reacts with a polymer


end-group to form a new polymer link and expels acetaldehyde, the tautomerof vinyl alcohol (50). The vinyl ester end can also thermally polymerize to givechain-branched and cross-linked products and gel-particles, and further thermaldegradation of these polyvinyl units gives rise to colored polyenes (51,52). Al-though acetaldehyde is highly volatile, its presence is particularly objectionablein PET bottle resin used for soda bottles. Its presence may not exceed 3 ppm inthe final container if used for potable substances, as it imparts an off-taste topopular cola drinks (53). Every time PET is melted during its processing, moreacetaldehyde is generated. One reason for bottle resin undergoing a final solid-phase polymerization before stretch blow molding is to remove the last traces ofacetaldehyde.

Physical Properties of PET

The full crystal structure of PET has been established by x-ray diffraction (54–57).It forms triclinic crystals with one polymer chain per unit cell. The original cell pa-rameters were established in 1954 (54) and numerous groups have reexamined itover the years. One difficulty is determining when crystallinity is fully developed.One researcher (57) annealed PET at up to 290◦C for 2 years.

Cell parameters are a = 0.444 nm; b = 0.591 nm; c = 1.067 nm; α = 100◦;β = 117◦; γ = 112◦; density = 1.52 g/cm3

Thermochemical data depends on the degree of crystallinity in the polymer,and a very highly annealed polymer sample can have Tm = 280◦C, much higherthan the usual value of 260–265◦C (58). The heat of fusion (59) is about 140 J/g,(33.5 cal/g). The glass–rubber transition temperature (Tg) depends on both themethod of measurement and the state of the polymer. A solid chip sample asmeasured by differential scanning calorimetry (dsc) gives a value around 78◦C(60) but a highly oriented and crystalline-drawn fiber measured by the dynamicloss method will give values as high as 120◦C (61). The specific gravity of undrawnamorphous PET is 1.33, whereas crystalline-drawn fiber has a value of 1.39 (62).

As a step-growth polymer made under equilibration conditions, PET hasa molecular weight distribution very close to the theoretical value of 2.0. TheMark–Houwink equation relates the intrinsic solution viscosity [η] to the molec-ular weight as [η] = KMv

α, where Mv is the viscosity average molecular weightand K and α are the Mark–Houwink coefficients determined experimentally forindividual solvents. The usual solvents historically used for PET are 60/40 w/wphenol/tetrachlorethane (P/TCE) and 2-chlorophenol (OCP). Neither solvent sys-tem is entirely satisfactory and better results have been obtained using eitherhexafluoroisopropyl alcohol (HFIP) or a 50/50 v/v mixture of HFIP and pentafluo-rophenol (PFP) at 25◦C (63). Although expensive, these acidic fluorinated solventsreadily dissolve even highly crystalline samples of PET at moderate temperatures,thus avoiding the degradation problems commonly encountered with the older sol-vents at high temperatures. Thus the results are more likely to be reliable, sincedegradation under normal conditions is minimal. The Mark–Houwink coefficientsfor PET for a range of solvent systems are shown in Table 1.

PET is a strong, stiff polymer. High tenacity tire-cord fibers with a tensilestrength (tenacity) of up to 0.9 N/tex (10 g/den) and a tensile modulus of 11 N/tex


Table 1. Mark–Houwink Coefficients for PETin Various Solvents at 25◦C

Solvent K, 10− 4 dL/g α

OCP 6.31 0.658P/TCE 7.44 0.648HFIP 5.20 0.723PFP/HFIP 4.50 0.705

(125 g/den) are readily obtained. Normal (ie apparel) textile fibers usually havetenacity around 0.5 N/tex (5.6 g/den) with a modulus of 7 N/tex (80 g/den). Thetensile modulus of PET is significantly higher than that of either PBT (4GT) orPPT (3GT) polyester fibers.

Applications of PET

The applications of PET are almost too numerous to mention and new uses appearconstantly. Historically, PET was first commercialized in the early 1950s as afiber-forming polymer. Staple (noncontinuous) fiber for blending with wool (qv)and cotton (qv) for increased fabric performance was the first product and assuch was highly successful. Later, continuous filament yarn, frequently bulkedmechanically, for knitwear and especially for women’s apparel was introduced. Thehigh Tg and low moisture regain (0.4%) gave excellent wash-wear fabric propertieswith significantly reduced tendencies to wrinkle. Industrial high strength fiberswere then developed, notably tire cords for automobile tires. These were usuallymade from high intrinsic viscosity (IV) polymer (ca 0.75) and spun as continuousfilament yarn. When automobile seat belts became the norm, they were also madefrom polyester because, unlike nylon, it had a high modulus and low tendency tostretch. The whole topic of polyester fibers is a very large one and for a morecomprehensive treatment, see Ref. 64 and Polyesters, Fibers.

Another market where PET proved of great value was in the form of polyesterfilms, which were made by extrusion through a slit die, followed by a biaxialdrawing process. They are used for photographic film backing and for magneticaudiotape and videotape substrates. Another market is drafting film. In all ofthese outlets the high strength and high modulus of PET, particularly for biaxiallydrawn films, give the desired dimensional stability and low stretch. For furtherdetails of film fabrication and processing, see Ref. 65 and films, manufacture; films,orientation.

One of the largest uses of PET resin and certainly the most dramatic ingrowth during the last 20 years is the stretch blow-molded PET bottle; annualconsumption runs into billions of units in United States alone with correspondingrapid expansion on a worldwide scale. The advantages of a thermoplastic bottleare self-evident: it is light-weight, shatterproof, and recyclable. The early marketdriver was weight, which reduced transportation costs compared with glass duringa severe energy shortage. Improved product safety associated with shatter-proofplastic bottles was another factor: recycling did not become a factor untillater.


The major technical problem facing any thermoplastic bottle manufactureris the permeability of the plastic bottle wall to oxygen and carbon dioxide, whichaffect the shelf-life of the contents. The average 2-L soda bottle maintains aninternal pressure of CO2 of ca 520 kPa (75 psi). To stop the product going “flat,”carbon dioxide pressure must be retained during storage before unsealing forseveral weeks. Likewise, oxygen must not diffuse in through the bottle walls tooxidize the contents, spoiling the flavor of the product. PET is semipermeableto such gases. At one time, attempts were made to coat PET with impermeablelayers but safety, cost, and recycling difficulties defeated them, although morerecent processes appear promising.

The basic enabling invention in 1973 was the stretch blow-molded PET bot-tle process (66,67). In this process, the polymer bottle wall is subjected to a rapidbiaxial drawing, which greatly increases the molecular orientation of the bottlewall. Not only does this increase the mechanical strength of the bottle but it alsoreduces the permeability of the walls (68). Blow-molding thermoplastic hollow ar-ticles is a highly specialized process and for more details the reader should consulta specialist publication (69). However, the process will be outlined so that the rea-sons for certain polymer properties will become apparent. A typical process is thetwo-stage blow-molding process. A bottle “preform” is molded by a conventionalinjection-molding process with thick walls and the neck with screw-cap threadsin place. This preform is amorphous and clear. Multiple cavity dies are used toincrease productivity. In the second stage, the preforms are heated in a mold to acarefully controlled temperature above the glass–rubber transition temperature,typically to 90–100◦C. The inside of the mold cavity is the size and shape of thefinished bottle. The preform is subjected to a combined axial and radial stretchingprocess. A hollow metal mandrel passes into the preform and partially elongatesit in the axial direction while dry air at about 345–690 kPa (50–100 psi) blows thewalls of the softened preform outward to fill the mold, giving radial stretching.The mold opens to allow the bottle to cool. This combined process results in bothradial and axial drawing of the bottle walls, which causes strain-induced crys-tallization and gives a container superior strength, clarity, and no environmentalstress-cracking.

The usual 2-L bottle preforms weigh about 50 g and the final blown bottlehas a wall thickness of about 0.38 mm (0.015) in. (70). In view of the enormousnumber of bottles produced annually, the processes are constantly being modifiedto raise throughput rates and are all highly automated. Several large machinerymanufacturers (eg Cincinnati-Milacron, Sidel) specialize in building the complexblow-molding equipment. Typical cycle times for the actual blow molding are 3–6 s. This process had one serious drawback in the early days—it gave a bottlewith a hemispherical base, which is clearly not capable of standing upright. Aflat base could not be fully oriented, so gases would permeate out through it. Forseveral years, PET soda-bottles were fitted with separate flat-bottomed basecaps,usually molded from high density polyethylene (HDPE) and secured with a hot-melt adhesive. This meant extra cost because of extra material and processingsteps and interfered with recycling. The invention by the Continental Group ofthe so-called petaloid base bottles with a five- or six-lobed pattern base was amajor advance: it made possible a one-piece, fully biaxially oriented bottle whichcould stand up by itself (71).


PET Bottle Resin. Stretch blow-molding is a mechanically severe opera-tion with very high deformation rates. The consumer wants a glass-clear bottle:any opacity caused by strain-induced crystallization is highly undesirable. PETbottle resins are usually made to high molecular weight (Mn = 22,000–40,000; IV0.75–0.90 dL/g) so as to withstand the severe blow-molding operation. Such anintrinsic viscosity is too high for melt-polymerization, and solid-stage polymer-ization is required. The process has been reviewed (72). PET bottle resin is madein a continuous melt-polymerization plant either by direct esterification usingTA/glycol, or by EI using purified DMT, often recovered from recycled PET bot-tles. The process to be described is a direct esterification process. Firstly, the acidand glycol are thoroughly mixed to a paste and catalyst and stabilizers are added.The paste is pumped to the esterifiers where water is driven off. The molten mix-ture of low polymer and oligomers passes through various polymerization stages:a prepolymerizer, an intermediate polymerizer, and finally the melt arrives at thehigh polymerizer or finishing stage where the final IV is about 0.65 dL/g. Duringthe various successive stages, the melt grows increasingly viscous and high vac-uum is applied at the finisher to complete the reaction. The agitators used in thepolymerizers are designed to disengage volatiles without high shear-rates and toobviate dead spots where polymer melt could stagnate and undergo degradation.A series of DuPont patents (73–75) describes a process whereby no vacuum isneeded to build molecular weight to the desired level. By use of specially designedmechanical agitators and an ingenious reactor design, which continually gener-ates thin films of polymer melt, a brisk stream of inert gas (nitrogen or CO2) issufficient to cause efficient disengagement of the excess glycol and thus drive thereaction to the desired IV. The entrained glycol can be recovered from the exit gasstream and the inert gas recycled. Clearly, the absence of vacuum equipment re-duces capital costs and, it is claimed, the process can be operated both batchwiseand continuously and is suitable for several commercial polyesters including PET,PEN, and PBT.

Returning to the original process description, at the exit from the high poly-merizer, the molten polymer is extruded into a water-bath and continuously dicedto small chips about 2.5–3.5 mm across. The chips are dried and passed to thecrystallizers where they are annealed in the solid state above Tg, gradually ris-ing to a temperature close to the point of maximum crystallization rate (approx.40–160◦C). They are slowly agitated to prevent them from sintering together asthey move to the solid-state polymerizer. During the crystallization stage, the chipdensity increases from 1.33 to about 1.37 g/cm3. Finally the chips pass into thesolid-phasing towers. Here they descend slowly under gravity in a plug-flow modethrough a long hot zone under a countercurrent flow of inert gas to sweep awaythe volatile by-products. The speed of descent is controlled so that an optimumtime/temperature profile is maintained for the desired final IV. The scale of oper-ation is impressive: typical melt-continuous polymerizers run at 10 tons/h, whichimplies a capacity of approximately 70 million kg/year; a large manufacturer mayhave several such units and each one provides enough polymer for well over onebillion 2-L soda bottles. Newer plant capacities are even more remarkable: a newplant in South Carolina has a capacity of 182 million kg/year in a single line.

The number-average molecular weight for typical bottle resin is between24,000 and 31,000 Da/molecule. One of the most objectionable by-products of


PET polymerization is acetaldehyde, which affects the taste of cola drinks atconcentrations as low as 60 ppb. The specification for acetaldehyde in the fi-nal product must not exceed 3 µg of acetaldehyde per liter of headspace. Thebulk of the acetaldehyde produced in the polymerization process is removed dur-ing the final solid-phase polymerization stage. Since blow molding is carried outwell below 200◦C, only minute amounts of fresh aldehyde are formed by thermaldegradation at the last stage. Originally, bottle preforms weighed 60–70 g butthis has now been reduced to about 50 g. The lighter weight bottles have thin-ner walls so that during biaxial drawing, excessive strain-crystallization (opac-ity) becomes a problem. Most manufacturers (76,77) have introduced copolymersof PET containing minor amounts (2–5 mol%) of such comonomers as isoph-thalic acid (PET-A) or cyclohexanedimethanol (PET-G) to reduce the polymermelting point by about 4–12◦C with a lower tendency to crystallize. Polyestershydrolyze very rapidly at 280◦C in the melt. Rigorous polymer-drying to a chipmoisture content below 50 ppm moisture is necessary before the injection mold-ing of bottle preforms. (PET stored under ambient conditions can easily have amoisture content of 2000 ppm). Melt-dyed polymers using FDA-approved dyesare used to mold preforms for colored bottles where the customer requiresthem.

In recent years, more and more potables have been packed in PET bottles—one of the most rapidly growing areas, first in Europe but now in the United Statesas well, is bottled drinking water, both noncarbonated and name brand spa wa-ters. Interestingly, for such potables, acetaldehyde must be even more stringentlyexcluded than for cola drinks. A major market prize would be a bottle sufficientlyimpermeable to oxygen for packaging beer and wine without spoiling the flavor.This is a much more difficult task than packaging carbonated soft drinks. Nu-merous designs and bottle coatings have been explored but recycling must notbe impaired. Plasma-coating of PET bottles with inorganic materials such as sil-ica and glass to bring about a fivefold reduction in oxygen permeability has beendisclosed (78,79). Sidel Corp. (80) has gained FDA approval for a process whichplasma-coats amorphous carbon on the insides of 600-mL PET beer bottles at arate of 10,000 bottles/h. The bottles, which are amber-colored, are said to be fullyrecyclable.

Other PET Packaging Products. In addition to blow-molded bottles, apackaging market has appeared in recent years for other types of PET polymer.One is amorphous PET (APET) which uses a comonomer (isophthalic acid is one)to minimize crystallinity. Such polymers are used in clear bubble and blister packsfor a variety of pharmaceutical products and medical devices (81). Food packagingis also a rapidly growing market for APET and clear heat-sealable thermoformedpackages are widely used. By use of blowing agents, foamed PET with a spe-cific gravity of as little as 0.05 g/cm3 can be made. Markets include baking traysand the very light foams can be used in food packaging. PET bottle resin is thefeedstock and the foams can be recycled. One ingenious process uses the hydro-carbon heptane at 249◦C as a PET blowing agent (82). These products are largelyspin-offs from the vast and rapidly growing bottle resin process and provide newopportunities and markets for resin manufacturers. PET is regarded as a premiumpackaging material and its recyclability is both a market and an environmentaladvantage (83).


Table 2. World Nonfiber Polyester Resin Consumption Patterns1999–2000, a,b 103 t

PET

Country or region Bottles Film and sheet Molded Total PBT

USA and Canada 1299 136 423c 2132 671389 148 454c 2317 71

Western Europe 1211 79d NA 1520 1161352 83d NA 1703 129

Japan 370 242 20 632 119380 250 20 650 130

China NA NA NA NA 36NA NA NA NA 37

aRef. 84.bThe values in bold are for the year 2000.cCompounded resin.dDoes not include recording film.

PET Molding Resins. It is difficult to establish accurately the world mar-ket size for PET as a molding resin. The total tonnage is small compared with thevast amounts used for fibers and bottle resin. However Table 2 shows estimatesfor 1999–2000 consumption of nonfiber PET and PBT resin, broken down intoend uses in several major world markets. The bulk of the U.S. market for PETmolding resin is the auto industry. Its Tm and heat distortion temperature arehigher than the corresponding values for PBT, and its low moisture uptake anddimensional stability with respect to changes in humidity make it superior tonylon. Both PET and PBT engineering resins have good resistance to chemicalsand being crystalline, do not suffer from the solvent stress-cracking problems thatplague amorphous materials such as polycarbonate. Polyesters are only attackedby powerful acidic or phenolic solvents, hot strong aqueous alkali, and certainbases such as hydrazine.

In the unfilled state, PET is not a good molding resin and all commercialgrades are filled with either chopped glass strand (typically 3–4 mm long) ormineral fillers (usually mica) or a mixture of the two. Various proprietary nucle-ating agents added are often sodium salts of various organic carboxylic acids (seeRef. 5). Some manufacturers supply fire-retardant (FR) polymer grades as well.Such formulations often involve a synergistic mixture of an aryl halide with anti-mony oxide. One difficulty with flame-retardant PET polymer is that recipes whichcontain antimony trioxide can suffer severe polymer degradation at molding tem-peratures around 280–290◦C. As has been discussed earlier, antimony trioxide isa polymerization catalyst and can also act as prodegradant at high temperatures.This is a less serious problem with FR grades of PBT because of lower processingtemperatures (240–250◦C). To mitigate the effect on PET, pentavalent antimonycompounds such as sodium antimonate is used in some FR formulations. The halo-genated species is often a ring-brominated polystyrene (85–90). These materials


are approved for Underwriters Laboratory V-0 classification and are nonfugitiveand less likely to be environmentally harmful than are small molecule additiveslike decabromodiphenyl oxide. In Europe there has recently been a move to elimi-nate organohalogen compounds from FR formulations for environmental reasons.As a result, organophosphorus derivatives such as triarylphosphine oxides arebeing introduced (91).

Since the early 1990s in the United States, there has been pressure on man-ufacturers from the automotive industry, led notably by the Ford Motor Co., to useat least 25 wt% of post consumer recycled (PCR) material in their resins ratherthan 100% virgin polymer. In the case of PET, this is readily achievable, becauseof the large volume of recovered PET bottle polymer chip now available in theUnited States and in Europe. A recent article (82) states that during 1998–1999,273,000 t of PET waste was recycled in the United States. Some suppliers use100% recycled polymer in their compounded PET resins. The PCR recovery pro-cess and subsequent melt-compounding and reformulation reduce the initial IVof the original resin from around 0.75–0.85 dL/g to values around 0.62–0.65 dL/g,but this is still in the range of virgin melt-polymerized resin (as opposed to SSPresin). In the automobile industry, components are often pigmented black, so thatdifferences in feedstock PCR chip color are minimized.

Initially, PET moldings were used in small components, typically electricalconnectors and covers for fuses, etc (see Figs. 1 and 2). This is still the case, even

Fig. 1. Electrical automotive connectors molded from CelanexR PBT, Grote & Hartmann(Germany). Courtesy of Ticona, A business of Celanese AG.


Fig. 2. Appliance timer housings molded from CelanexR PBT, Mallory (USA). Courtesyof Ticona, A business of Celanese AG.

Fig. 3. Healthcare inhaler for asthma patients molded from CelanexR PBT, Astra Tur-bohaler(tm). Courtesy of Ticona, A business of Celanese AG.

more so with the growing complexity of the modern automobile with its numerouson-board electronics, but recently the trend has been to use PET moldings moreand more for non-load-bearing structural parts, such as radiator grille supportsand headlamp mountings (Fig. 4). Glass/mineral filled PET moldings do not have


Fig. 4. Automotive, electroplatable grill-opening retainer molded from ImpetR PET, Mer-cury Sable. Courtesy of Ticona, A business of Celanese AG.

a smooth enough surface for exterior body-parts. However, they are very suit-able for internal structural components. This trend is part of the drive to reduceautomobile weight to improve gas-mileage or boost the range and performanceof electric cars. Some moldings are dimensionally quite large, weighing well over5 kg per shot. Improvements in mold design and better understanding of melt-flowbehavior in molds, brought about by increasing use of computer-aided design andflow-simulation programs, have helped to make these large moldings possible on aroutine production basis. Figures 5 and 6 show an experimental concept car wherethe bulk of the body/chassis pan is molded in a PET formulation. As stated, PET

Fig. 5. Chrysler composite concept vehicle (CCV) with large body parts molded fromImpetR PET, Daimler Chrysler. Courtesy of Ticona, A business of Celanese AG.


Fig. 6. Large body parts for Chrysler composite concept vehicle (CCV) molded from Im-petR PET, Daimler Chrysler. Courtesy of Ticona, A business of Celanese AG.

does not crystallize well in the unoriented state even in a hot mold unless nucle-ating agents and/or plasticizers (qv) are added. Commercial PET molding-gradepolymers are nearly always filled. Typical compounded polymer properties areshown in Table 3.

Table 3. Typical Properties of PET Molding Resinsa

Glass

ASTM (mineralProperty method 30% 45% 35% filler)

Specific gravity D792 1.58 1.70 1.60 1.60Tensile strength, MPab D638 166 197 97 103Elongation at break, % D638 2.0 2.0 2.2 2.1Flexural strength at 5%, MPab D790 245 310 148 152Flexural modulus, GPac D790 9.66 14.5 9.66 9.66Notched Izod, J/md D256 80.1 107 58.7 58.7Heat deflection D648 224 229 202 216Temperature at 1.82 MPab, ◦CFlammabilitye UL-94 HB HB HB HBDielectric strength, V/25 µm D1495.2 mm 565 540 500 4501.6 mm 904 631 550 5750.8 mm 975 951 810 860Volume resistivity at 23◦C, 50% rh, �·cm D257 3.0 × 1015 1.0 1.0Dielectric constant ε, H2 D150103 3.2 3.5 3.8 3.8105 3.1 3.4 3.6 3.7aRef. 92.bTo convert MPa to psi, multiply by 145.cTo convert GPa to psi, multiply by 145,000.dTo convert J/m to ft·lbf/in., divide by 53.38.eHB = Brinell hardness.


Economic Aspects of PET

The total world market for nonfiber PET polymer is still growing because of thedemand for bottle resin. This grew worldwide from 1993 to 2000 at about 15%/year,although overproduction led to a falling-off recently. A published article (93) statedthat the global consumption of PET grew at 19% during 1992 to a world total of1,719,000 ton (3.79 billion lbs). Adding up the bottle resin figures from Table 1given an estimated total of 3,121,000 ton, a 181% increase in 7 years. The worldtotal devoted to beverage bottles is astonishing, but it is significant that Europehad by the year 2000 almost caught up with the United States in total bottle resintonnage. There are interesting regional variations, notably the strong demand forpackaging for mineral waters in Europe and the large hot-fill market in Japan.Looking ahead, the future growth of PET looks to be assured, its balance of prop-erties and ability to be recycled being extremely favorable factors. New outlets forPET packaging are constantly appearing.

One significant trend in recent years has been the complete disappearance ofsome well-known major producers of PET polymer and the emergence of new ones.In the late 1990s, several major chemical companies sold off commodity chemicaland polymer businesses to concentrate on other more profitable market areassuch as pharmaceuticals and life-sciences. In 1998–1999, Hoechst Celanese splitinto Celanese AG and Aventis, a pharmaceutical company. It sold its large PETinterests to the Koch-Saba group, now called Kosa, and headquartered in Houston,Tex. Celanese’s remaining engineering polymer interests were taken over by itssubsidiary Ticona. During 1998–2000, Shell gave up its polymer business and soldsome 300,000 ton of PET capacity to the Mossi & Ghisolfi Group of Milan, Italy(94). ICI sold its PET plants in Europe to DuPont, and Dow in 1996 became amajor PET manufacturer by acquiring 270,000 ton of plant capacity. Dow is alsoin the process of buying Union Carbide. Allied-Signal became Honeywell and werethen almost acquired by GE until (2001) the European Union prevented the saleon antitrust grounds. Table 4 shows major world manufacturers of PET resin inlate 1998, a situation that has now changed again with the removal of Shell andHoechst Celanese as PET producers.

Table 4. Major World PET Producersa

Manufacturer Nameplate capacity, 103 t

Eastman 1556Kosa 840DuPont 758Shell 750Nan Ya 540Wellman 490Rhodia-Ster 280Hoechst-Celanese 250Sunkyong 240Far Eastern 200Total 5904aRef. 83.


Commercial PET Engineering Resins. The first company to introducenucleated PET molding resins was Akzo Plastics BV with their Arnite® range,later acquired by DSM NV. DuPont introduced their Rynite® fast-crystallizingmaterials in 1978, followed by other manufacturers. The present NorthAmerican market is still dominated by DuPont, but there are other notablesuppliers like Honeywell (Allied-Signal) with Petra® and Ticona with Impet®.Eastman also markets PET injection-molding grades under the tradenameThermx. In Europe the situation with PET is confined primarily to two suppli-ers, DSM NV and DuPont, while in Japan the main suppliers are Teijin, Toray,and Toyobo. The market prices for PET molding resins at the time of writing in theUnited States are $2.90–3.15/kg for 30% glass-filled PET, $3.26–3.41/kg for 55%glass-filled PET, and $3.23–3.45/kg for 30% glass-filled FR grade PET (95). Thesecompare with a current commodity price for PET bottle resin of $1.36–1.50/kg (seeENGINEERING THERMOPLASTICS).

Safety and Environmental Factors

PET. PET polymer is safe and poses no threat to animals or humans. PETfibers have been in use for nearly 50 years and PET has U.S. Food and Drug Ad-ministration (FDA) approval for use as a food-packaging material. PET fibers havebeen used in internal arterial prostheses. The only significant hazard in handlingPET resins is the dust hazard associated with mineral or glass fillers during chipgrinding or compounding operations. Appropriate protective equipment must beworn. All extruders or machinery handling molten polymer should be properlyventilated to remove harmful fumes from the decomposition of molten polymer.Molten PET can cause serious contact thermal burns: it has a high heat capacityand sticks to the flesh. Adequate protection must always be worn when handlinghot polymer.

PBT. PBT resins are not harmful or hazardous when handled at room tem-perature under normal conditions according to their Materials Safety Data Sheets.No problem with contact with the pellets has been encountered under normal con-ditions. Glass fines can however cause skin irritation, and if glass-filled resins arebeing ground or reground, due precautions must be taken. Inhalation of dustmust be guarded against, as is true for grinding any glass-filled resin. During themolding, the temperature must not exceed 520◦F and never over 550◦F as decom-position with the evolution of harmful vapors can occur. As with all thermoplastics,adequate ventilation must be provided around injection-molding machines.

PBT Molding Resins

Poly(butylene terephthalate) is historically the oldest of the crystalline ther-moplastic polyester molding resins, having been introduced by Celanese Corp.in 1970 under the trade name Celanex. General Electric Co. then brought outtheir own version Valox®, and today there are numerous suppliers includingBASF, Bayer AG, and DSM. Celanese have recently become Ticona. In Japan,


the major manufacturers are Polyplastics, Toray, Teijin, and Mitsubishi (seeENGINEERING THERMOPLASTICS). As already explained, unlike PET, PBT has theability to crystallize very rapidly (like nylon-6,6) even in a cold mold. It gives tough,distortion-free moldings without special additives or nucleants. Although the un-modified polymer has very good flow properties, and is used in electrical connec-tors and fiber-optical cable buffer tubes, it performs even better if reinforced withinorganic fillers, notably ∼3 mm (1/8 in) chopped glass fiber. Additional mineralfillers are incorporated for special applications where high heat-deflection temper-ature and stiffness are important. These mineral fillers include mica, talc, wollas-tonite, and even barium sulfate, the latter for special applications such as counter-tops.

The filled grades of PBT are tougher, stiffer, and stronger materials andthey have improved notched Izod impact strength, since unfilled PBT is notch-sensitive (96,97). Even when unfilled, the plastic has good strength, rigidity andtoughness, low creep, minimal moisture absorbance, and does not undergo di-mensional changes with changes in humidity. It is characterized by excellentelectrical and dielectric properties and a high surface finish. It has found wideacceptance in a variety of end uses where precision molding and a high qualityfinish are required. Typical are the electrical and electronic markets where it iswidely used in such parts as connectors, plugs, switches, typewriter and computerkeyboard components, printed circuit boards, and small electric motor compo-nents. PBT is widely used in the automotive industry for electrical componentssuch as distributor caps, coil-formers, rotors, windshield wiper arms, headlightmountings, and other fittings. In the auto market, “under the hood” componentshave to maintain their dimensional stability at elevated temperatures as wellas resist various automotive fluids. Other uses for PBT are home appliances,such as food mixers, hair dryers, coffee makers, toasters, and camera parts. PBTis used in industrial machinery, for example in molded conveyor-belt links andin medical devices such as nasal sprays and nebulizers. An appreciable quan-tity is used in polymer alloys and blends with other polymers. PBT is marketedin both standard and flame-retardant grades, the latter being essential in theUnited States to meet Underwriters’ Laboratory 94V-0 standards in thin-walledsections.

Properties of PBT

Physical Properties. Unlike PET, the polymer PBT exists in two poly-morphs, the α and β forms, which have distinctly different crystal structures. Thetwo forms are interconvertible under mechanical stress (98–100). Both crystalforms are triclinic and the crystal parameters are shown in Table 5.

The change in the two forms mainly involves the c lattice dimension, whichlengthens from 1.174 to 1.300 nm. It is believed that the relaxed α-form has agauche–trans-gauche conformation of the three C C bonds in the C4-glycol unit,and the extended β-form exists in an all-trans form (62). The melting point of PBTis 222–224◦C, depending on the degree of crystallization and annealing conditions.The heat of fusion is about 140 J/g (101) and the Tg is usually quoted at about 45◦C,although this depends on the physical nature of the sample (102). PBT crystallizes


Table 5. Crystal Parameters for the Two Forms of PBT

Cell parameter α form (unstretched) β form (stretched)

a, nm 0.482 0.469b, nm 0.593 0.580c, nm 1.174 (1.165)a 1.300α, degrees 100 (98.9)a 102β, degrees 115.5 (116.6)a 120.5γ , degrees 111 105Volume, nm3 0.260 0.267density, g/cc 1.41 1.37aResults given in Ref. 100.

Table 6. Mark–Houwink Parameters for PBT

Solvent K, dL/g α

P/TCE (at 30◦C) 1.17 × 10− 4 0.87OCP (at 25◦C) 6.62 × 10− 5 0.915

very readily from the melt and it is difficult to obtain a truly amorphous sample.Its crystallization kinetics are thus not easy to determine (103). The density of theannealed crystalline unfilled polymer is 1.33 g/cm3 whereas the amorphous mate-rial has a value of 1.26 g/cm3 (104). Like PET, PBT is made to various molecularweights, the Mn values being in the 20,000–50,000 range. Intrinsic viscosities areusually measured in o-chlorophenol (OCP) or a phenol/tetrachlorethane mixture.The Mark–Houwink parameters are shown in Table 6 (105,106).

Flame-retardant grades of PBT usually consist of synergistic mixtures ofantimony trioxide with various halogenated (brominated) aromatic compounds.A typical recipe for PBT might be 10 wt% decabromodiphenyl oxide and 5 wt%antimony oxide. Recently the trend has been to use polymeric or oligomeric bromi-nated additives. A typical additive is an end-capped polycarbonate derived fromtetrabromobisphenol-A [94334-64-2]; another is a mixture of epoxy oligomers de-rived from the diglycidyl ether of tetrabromobisphenol-A [68928-70-1]. The bromi-nated polystyrenes (loc. cit.) have only limited usefulness in PBT as they have alow melt compatibility (107).

Chemical Properties. PBT is highly crystalline and does not suffer fromsolvent stress corrosion cracking as do amorphous materials. It is resistant atroom temperature to most common chemicals and solvents, lubricants, greases,and automotive fluids. Ketones will attack it at elevated temperatures. Partsmade from PBT are dishwasher-safe, but will not withstand repeated steam au-toclaving. PBT is attacked by aqueous alkali and other strong bases and by di-lute acids, particularly at elevated temperatures. PBT has very good resistanceto weathering, and black pigmented grades with uv-stabilizers have excellentoutdoor stability. Like all polyesters, PBT is susceptible to hydrolytic attack bymoisture in the melt. Injection-molding screw temperatures are usually about250◦C and IV drop is rapid unless the polymer chip is dried to below 50 ppmmoisture content and kept dry. Inadequate drying is the cause of most molding


Table 7. Mechanical Properties of PBTa

30% glass

Unfilled, grade, General Flame HighProperty low mol. wt. purpose retardant impact

Specific gravity 1.31 1.54 1.66 1.53Tensile strength, MPab 57 135 135 97Tensile modulus, GPac 2.5 9.7 11.7 8.3Elongation, % 5 2 1.5 3.1Flexural strength, MPab 85.5 193 193 152Flexural modulus, GPac 2.5 8.3 10.3 6.9Notched Izod, J/md 37.4 90.7 69.4 160Unnotched Izod, J/md 1228 240 214 641HDT at 1.82 MPab, ◦C 51 206 208 191Volume resistivity, �·cm 1015 1016 5 × 1015 4 × 1014

Dielectric strength, V/25 µm 420 560 490 500Dielectric constant ε, 100 Hz 3.2 3.7 3.9 4.3Flammability UL94, at 0.8 mm HB HB V0 HBaRef. 108.bTo convert MPa to psi, multiply by 145.cTo convert GPa to psi, multiply by 145,000.dTo convert J/m to ft·lbf/in., divide by 53.38.

problems. As the PBT melt is quite fluid, drooling from nozzles is sometimes aproblem.

Mechanical Properties. Properties of typical grades of PBT, either as un-filled neat resin or glass-fiber filled, and FR grades are set out in Table 7. Thistable also includes impact-modified grades which incorporate dispersions of elas-tomeric particles inside the semicrystalline polyester matrix. These dispersionsact as effective toughening agents which greatly improve impact properties. Themechanisms are not fully understood in all cases. The subject has been discussedin detail (109), and the particular case of impact-modified polyesters such as PBThas also been discussed (110,111).

Economic Aspects of PBT

According to information published between 1999 and 2000 the market for PBTin the United States and Canada increased from 67,000 to 71,000 t (147.6–156.6million lbs), as shown in Table 2. The European and Japanese markets also uselarge amounts of PBT. The main outlets are electrical, electronic, and automotivecomponents. The major manufacturers of PBT in North America, Europe, andJapan are shown in Table 8.

A more detailed analysis of end uses for PBT in North America, Europe, andJapan are shown in Table 9.

The future prospects for economic growth for PBT look good. It is still displac-ing thermosets from some markets and its versatility, excellent flow properties,and ease of molding will assure it a prominent place in the world for years to


Table 8. Principal World Manufacturers of PBT According to Region

USA Europe Japan

BASF – ULTRADUR® Bakelite AG General ElectricDuPont – CRASTIN® BASF Mitsubishi Eng. PlasticsGeneral Electric – VALOX® Bayer AG Polyplasticsa

Ticona – CELANEX® DSM NV Teijina

DuPont TorayGeneral Electric Toyobo

aThese two companies merged in 2000 to form Wintech which will produce both PET andPBT molding resins.

Table 9. End Uses of PBT for 1999–2000, ton

Market 1999 2000

United States and Canadaa

Appliances 7,020 7,392Consumer/recreation 2,267 2,267Electrical/electronic 14,286 15,420Industrial 7,165 7,392Transportation 33,559 35,827Others 2,494 2,721TOTAL 66,971 71,019Western Europeb

Appliances 11,000 12,500Consumer/recreation 9,000 9,900Electrical/electronic 36,700 39,700Industrial 8,700 9,800Transportation 43,300 48,200Other 7,300 8,900TOTAL 116,000 129,000Japanc

Automotive and vehicle 30,000 33,000Electrical/electronic 38,000 44,000Other 19,000 19,000Total domestic 87,000 96,000Exports 32,000 34,000TOTAL 119,000 130,000aRef. 112.bRef. 113.cRef. 114.

come. Quoted prices (Nov. 1995) for PBT resins were $3.61–3.85/kg for unfilledresin, $3.74–4.18/kg for 30% glass-filled FR grades, and $4.29–4.51/kg for highimpact grades (115).

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Vol. 7 POLYIMIDES 529

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ANTHONY J. EAST

Consultant

POLYETHERETHERKETONES (PEEK). See ENGINEERING

THERMOPLASTICS.

POLYETHYLENE. See ETHYLENE POLYMERS.

POLY(ETHYLENE-NORBORNENE). See ETHYLENE-NORBORNENE

COPOLYMERS.

POLY(3-HYDROXYALKANOATES). See Volume 3.

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