Cyclohexanedimethanol Polyesters.pdf

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CYCLOHEXANEDIMETHANOL

POLYESTERS

Introduction

1,4-Cyclohexanedimethanol (CHDM) is an important commercial cycloaliphatic

diol that is used in a variety of important commercial polyesters. It is produced

by the catalytic hydrogenation of dimethyl terephthalate (DMT) (1). CHDM can

exist as the cis or trans isomer; however, most commercial high volume produc-

tion CHDM is a mixture of isomers with an approximately 70/30 trans/cis ratio.

This isomer ratio plays an important role in determining the ﬁnal properties of

polyesters containing CHDM by inﬂuencing the ability of chains to pack efﬁciently.

Structure 1 , trans (axial, axial) enchainment, and structure 2 , cis enchainment,

schematically illustrate the differences in these structures. Most CHDM contain-

ing polyesters are based on the 70/30 (trans/cis) mixture and the stereochemistry

of CHDM is maintained during the polycondensation.

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The amount of CHDM incorporation as a comonomer also has a large effect on

the crystalline nature of the backbone since it is so dissimilar in size and shape to

other common diols (2). Poly(ethylene terephthalate) (PET) containing low levels

of CHDM (less than 5 mol%) has enjoyed widespread use in stretch blow molded

carbonated soft drink containers, where the CHDM modiﬁes strain hardening

during stretching, which therefore serves to widen the processing window. When

incorporated into a terephthalic acid (TPA) based polyester at higher levels, the

absolute level of CHDM can regulate whether the polyester is a high melting

crystalline material or a tough clear amorphous composition. In general, when

CHDM is copolymerized with other aliphatic diols with common diacids such as

TPA, it increases the T g of the backbone as well as rendering the backbone more

resistant to hydrolysis. The ﬂexible character of the CHDM unit is also known to

impart enhanced impact and toughness to polyesters. The origin of these effects

has been studied and a recent report correlates these properties to secondary

relaxation modes arising from the CHDM structure (3).

The most important commercial polyesters containing CHDM are based

on TPA and ethylene glycol (EG). Isophthalic acid (IPA) can also be used

as a comonomer with TPA. The polyester of CHDM with TPA is poly(1,4-

cyclohexylenedimethylene terephthalate) (PCT), which melts at 300 ◦ C. When this

structure is modiﬁed with an acid such as IPA, the abbreviation PCTA (acid-

modiﬁed PCT) is used. PCT polyesters with glycol modiﬁcation of up to 50 mol%

EG are abbreviated as PCTGs. Likewise, PET polyesters with up to 50% CHDM

content are called PETGs. The abbreviations of PETG, PCTG, and PCTA are gen-

erally reserved for compositions that contain enough of the comonomer to render

the backbone slow to crystallize or amorphous. This article discusses the chem-

ical characteristics and properties of the important commercial crystalline and

amorphous examples of polyesters based on CHDM.

Crystalline Polymers Based on CHDM

The primary crystalline polymer based on CHDM is the terephthalate, PCT. PCT

was originally developed for ﬁber applications but has since found wider utility

as a reinforced polymer for injection molding and (when copolymerized with a

small amount of IPA) as a material for crystallized food packaging trays. The key

property of PCT, which sets it apart from other thermoplastic polyesters in these

latter applications, is melting point.

When made with the normal 70/30 trans/cis CHDM isomer ratio, the melting

point of PCT is about 290 ◦ C. The melting point varies substantially with isomer

ratio, however, as shown in Table 1 (4).

Table 1. Effect of trans/cis Isomer

Ratio on Melting Point of PCT

% trans

T m , ◦ C

100

315

290

275

250

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For comparison, the melting point of poly(butylene terephthalate) (PBT) is

225 ◦ C and that of PET is in the range of 250–260 ◦ C.

Crystallization of PCT is relatively rapid, but because of its higher T g (90 ◦ C)

the maximum rate of crystallization occurs at a higher temperature than is typical

of other crystalline polymers such as PET ( T g about 70 ◦ C) or PBT ( T g about 35 ◦ C).

The bulky in-chain CHDM moiety results in several other important differ-

ences between PCT and crystallizable polyesters such as PET. The amorphous

density is signiﬁcantly lower, 1.195 g/cm 3 for PCT compared to 1.334 g/cm 3 for

PET. PCT also exhibits a strong sub- T g molecular relaxation, which results in a

relatively low modulus at room temperature (155 MPa vs 240 MPa for PET) and

improved toughness in the amorphous state. As an example of the latter phe-

nomenon, the notched Izod impact strength of amorphous PCT is greater than

1000 J/m, while that of amorphous PET is less than 100 J/m.

The CHDM isomer ratio also has an effect on gas barrier properties, with

better barrier properties resulting from higher cis levels. Because of the higher

local free volume contributed by the CHDM structure, the diffusivity through

PCT is generally higher than that through denser structures like PET (5). How-

ever, the magnitude of this effect is strongly dependent on the isomer ratio, as

shown in Table 2. A recent patent (6) discloses that PCT with a 93/7 cis/trans

ratio containing 50% 2,6-naphthalene dicarboxylic acid has a permeability of

2.68 (cm 3

mil)/(100 in. 2

24 h

Table 2. Effect of cis Isomer Content

on Gas Barrier Properties of PCT

Oxygen permeability,

% cis CHDM

nmol/(m · s · GPa) a

a To convert nmol/(m

GPa) to (cm 3

mil)/

(100 in. 2

24 h

atm), divide by 2.

atm).

Preparation of PCT is best accomplished from DMT using standard trans-

esteriﬁcation catalysts such as titanium compounds. Because of the high melting

point of the polymer, ﬁnal polyesteriﬁcation temperatures must be high (greater

than 300 ◦ C at typical commercial trans/cis ratios) (7). PCT prepared in the melt

phase can be crystallized and then solid phase polymerized to obtain even higher

molecular weights.

If PCT is modiﬁed with relatively high levels of comonomer, substantially

amorphous materials result (as described later). However, it is possible to main-

tain crystallinity at lower levels of modiﬁcation. For example, replacing up to

about 10 mol% of the terephthalate units with isophthalate results in a polymer

with reasonable crystallization rates and ultimate degrees of crystallinity.

One might expect that replacing the terephthalate unit in PCT with naph-

thalene dicarboxylate would make an interesting high temperature polymer.

This polymer cannot be successfully prepared, however, because its melting

point is above its degradation temperature. Reduction of the melting point by

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copolymerization (for example either with terephthalic acid or isophthalic acid)

eliminates this problem (8).

Crystalline polyesters from CHDM and aliphatic diacids are possible, but

generally are of little interest because of low melting points and low glass-

transition temperatures. Cyclic aliphatic diacids offer some potentially attrac-

tive possibilities since the melting points are not so depressed. For example, the

polyester of CHDM with the high trans isomer of 1,4-cyclohexane dicarboxylic

acid has a melting point similar to that of PBT (7).

Processing of Crystalline PCT-Based Polymers. Melt processing of

the high-melting PCT-type polymers must be done carefully, owing to a relatively

small window between the melting point and the temperature at which degra-

dation rates become signiﬁcant. The degradation is both thermal and hydrolytic

in nature. While it may be argued that the PCT structure is inherently more

hydrolytically stable than other polyesters such as PET, the higher processing

temperature compensates by accelerating the rate. Thus drying of the polymer or

formulation before processing is recommended. Dessicant drying at temperatures

up to about 125 ◦ C is commonly used.

Degradation results in color formation, loss of molecular weight, and deteri-

oration of critical mechanical properties such as toughness.

Injection Molding Applications. PCT forms the basis of a family of re-

inforced, crystalline plastics for injection molding. As mentioned previously, the

high melting point of the polymer is a key property, as this results in high heat-

deﬂection temperatures (HDT) in glass-ﬁber-reinforced formulations. Good tough-

ness, ﬂow into the mold, and rapid crystallization are also important attributes

in these applications.

Formulations for injection molding typically contain 30–40 wt% glass ﬁber

or a mixture of glass ﬁber and mineral ﬁller. Stabilization packages to improve

processing stability and additives to enhance crystallization rate are also incorpo-

rated. These products may or may not be ﬂame-retarded, depending on the applica-

tion area, with HDTs ranging from about 250 ◦ C to about 260 ◦ C. This level of heat

resistance makes PCT-based plastics suitable for high temperature applications

such as electronic connectors, where high soldering temperatures are encountered.

Typical competitive materials in this market include poly(phenylene sulﬁde), with

an HDT of about 260 ◦ C, and high temperature polyamides, with HDTs in the range

of 270–280 ◦ C. Other properties of a typical 30% glass-reinforced, ﬂame-retarded

grade of PCT are shown in Table 3.

Table 3. Properties of 30% Glass-Reinforced

Flame-Retarded PCT

Speciﬁc gravity

1.63

Tensile strength, MPa a

120

Tensile elongation, %

Flexural modulus, MPa a

9600

Notched Izod impact, J/m b

HDT, 1.82 MPa, ◦ C

255

UL Subject 94 ﬂammability

a To convert MPa to psi, multiply by 145.

b To convert J/m to ft

lbf/in., divide by 53.38.

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Flame-retarded grades are widely used for various computer connectors and

circuit board components. Representative applications are edge card connectors,

grid arrays, and memory modules. Non-ﬂame-retarded grades ﬁnd use in auto-

motive (under the hood) applications, typically connectors and related parts. As

higher temperature soldering techniques become more common in the automotive

industry, the use of high temperature plastics such as PCT is expected to increase.

It is also possible to formulate unreinforced PCT with crystallization aids and

tougheners to provide a material similar in some respects to supertough nylon.

PCT provides advantages in dimensional stability and lower moisture sensitivity

compared to the polyamide-based products.

Extrusion Applications. A well-established application for extruded

unreinforced PCT (copolymerized with isophthalate) is in the preparation of crys-

tallized, thermoformed trays for foods. Crystallized PET is widely used for this

application, but where higher temperature performance is needed the PCT-based

polymer may be chosen. Such trays are formed from extruded sheet, using a

hot mold to promote crystallization. Isophthalate-modiﬁed PCT polymers are ap-

proved by the Food and Drug Administration for high temperature food contact

use.

The good hydrolytic stability of PCT-based polymers leads to applications

for monoﬁlament in paper machine belts. Monoﬁlament is extruded from high

molecular weight polymer, drawn, and crystallized, then woven into a screen.

Such belts are found in the drying section of paper machines, where there is a

combination of high moisture and high temperature. Because of their hydrolytic

stability, PCT-based polymers provide much longer service life in this application

than PET-based.

Amorphous Polyesters, CHDM-Modiﬁed PET and PCT

The size and shape of CHDM render it as an effective diol for modifying crys-

tallinity in polyesters. The decrease in crystallization rates for low levels of CHDM

incorporation into PET is shown in Figure 1, where the half-times (measured

by dsc) for crystallization vs temperature in the melt are plotted. When copoly-

merized with EG and TPA, the middle composition ranges, from approximately

70/30 CHDM/EG to 20/80 CHDM/EG, have very slow crystallization rates and do

not show crystallization peaks when scanned by dsc at 20 ◦ C/min and thus are

amorphous polyesters. These compositions can be processed into clear, transpar-

ent sheets and molded articles.

As the composition is varied from a PETG composition to a PCTG compo-

sition, several general property trends are noted. These are illustrated in the

schematic in Figure 2.

Increasing CHDM level leads to an increase in toughness, as measured by

impact and increasing elongation-to-break values, a decrease in tensile modulus,

and increases in T g and HDT. A comparison of the physical and mechanical prop-

erties for a PETG composition of approximately 70 EG/30 CHDM and a PCTG

composition of approximately 60 CHDM/40 EG is shown in Table 4.

Preparation of Amorphous PETGs and PCTGs. In general, amor-

phous PETG and PCTG polyesters can be made by standard melt-phase polycon-

densation processes, starting either with DMT or TPA. One signiﬁcant difference

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