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POLY(ETHYLENE NAPHTHALATE) (PEN) Introduction Poly(ethylene naphthalate) (PEN) is a thermoplastic polyester consisting of ethylene naphthalate repeat units:

PEN is the polycondensation product of either dimethyl-2,6naphthalenedicarboxylate (2,6-NDC) or naphthalate dicarboxylic acid (2,6-NDA) with ethylene glycol. To date 2,6-NDC is the preferred feedstock in view of the fact that it can be produced more economically in high purity form relative to purified 2,6-NDA. Other naphthalate-based polyesters include the butanediol-based analogue, poly(butylene naphthalate) (PBN), as well as copolymers and blends of PET [poly(ethylene terephthalate)] with PEN. Naphthalate polyesters exhibit physical and mechanical properties generally superior to their terephthalate analogues made from terephthalic acid (TA) or dimethyl terephthalate (DMT), such as PET or poly(butylene terephthalate) (PBT). PEN polymerization was first reported by ICI in 1948 (1). Its superior thermal, mechanical, and physical properties were recognized early on. However, in the 40 years that followed, PEN did not find significant commercialization or application development, due primarily to the lack of commerical availablity of the key monomer, 2,6-NDC. Although studies on PEN film and fiber production and applications were begun by Teijin in the 1960s, it was not until the 1970s that 2,6-NDC became available in sufficient quantities for the first PEN films and fibers to be produced on a semitechnical scale. High value speciality videotapes were found to benefit from the use of PEN film in Japan during the 1980s. In the early 1990s Amoco Chemical Co. announced their plan to invest in a world-scale facility for production of 2,6-NDC in Decatur, Illinois. As a result of the promise of larger scale and more economic raw material supply, plus greater interest from the end market, PEN films were launched commercially in the early 1990s. Since then, the start-up of commercial production at Amoco’s (now BP) 30,000 tons/year 2,6-NDC facility has significantly aided the economics of PEN production. This has driven increasing use of PEN in a wide range of applications including films, fibers, and rigid packaging. An approximate percent breakdown of PEN usage worldwide per end-use segment for the year 2002 is provided in Table 1. At this time, commercial producers of PEN homopolymer and/or copolymer resins are Teijin Ltd. (Japan), M&G (USA), 3M (USA), KoSa (Europe), Toyobo (Japan), Kolon (Korea), MCC (Japan), Kimex (Mexico), and Shinkong (Taiwan). Encyclopedia of Polymer Science and Technology. Copyright John Wiley & Sons, Inc. All rights reserved.

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Table 1. Major PEN Market Segments and Approximate Percent Breakdown by End-Use Segmenta Approximate consumption, %

End Use Film

70

Rigid plastic packaging Fiber

15

a Data

15

Examples Imaging, recording tape, displays, memory storage, insulators, capacitors Beer returnable/refillable bottles, pharmaceutical and cosmetic bottles Tirecord, hose reinforcement, sailcloth

from BP Amoco Chemical Co.

Polymer Properties Naphthalate polyesters, such as PEN and PBN, derive their performance improvement from the double ring structure of the naphthalene group. Incorporation of this double ring structure in the polymer chain increases thermal, chemical, mechanical, and barrier performance versus polymers based on single aromatic rings. Table 2 illustrates the wide variety of property improvements exhibited by PEN versus PET. Partial substitution of terephthalate-based polymers with the naphthalate group, in a copolymer or blend form, can provide enhanced properties that are

Table 2. Comparative Properties of PEN vs PETa Property T g ,◦ C Shrinkage, % Dry at 150◦ C Wet at 100◦ c Mechanical continuous use temperature, ◦ C UV absorbance at 360 nm, % Radiation resistance, MGyb Tenacity retention, 45 min. at 150◦ C, % Young’s modulus, MPac Tensile strength, MPac Oligomer extraction, mg/m2 Hydrolysis resistance, h d Permeability, mn2mol·m ·s·GPa CO2 O2 e Water vapor transmission, n mmol·m 2 ·s a Source:

Teijin Ltd., Japan. convert MGy to Mrad, multiply by 100. c Convert MPa to Psi multiply by 145. d To convert n mol·m to cm3 ·mm , multiply by 0.2. m2 ·s·GPa m2 ·day·atm b To

e To

convert

n mol·m m2 ·s

to

g·mm , m2 ·day

multiply by 1.55.

PEN value

PET value

122

80

0.6 1 160 17 11 99 5200 60 0.8 200

1.3 5 105 1 2 45 3900 45 20 50

12 3 0.129

61 12 0.452

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Fig. 1. Melting point of PET/PEN copolymers and blends. Blend. Data from BP Amoco Chemcial Co.

Copolymer,

typically intermediate between the respective homopolymers. A description of key thermal, physical, and mechanical properties is provided below. Thermal Properties of Naphthalate-Containing Polyesters. Naphthalate homopolymers melt at higher temperatures vs terephthalate homopolymers. Table 3 compares the T m of PEN and PBN vs PET and PBT respectively. In random copolymers, where both TA and NDC are present, the melting point decreases as NDC partially substitutes TA (eg, starting from PET) or when TA partially substitutes NDC (eg, starting from PEN). This melting point depression effect is caused by a disruption in the regular crystalline pattern by the minor component (Fig. 1). Figure 1 shows that the melting point of a PEN copolymer modified with 8 mol% DMT (PENT-8) equals the melting point of a PET homopolymer. Use of the lower melting PENT-8 copolymer in preparing PET/PEN blends is preferred over use of a PEN homopolymer. Not only are lower melt temperatures required in the blending process, which reduces the risk of PET degradation, but there is a better viscosity match between the naphthalate and terephthalate polymers which improves mixing and miscibility. Within the range of approximately 10–90 mol% NDC, PET/PEN random copolymers (PETN) remain essentially amorphous. On the other hand, PET/PEN blends are capable of crystallizing and exhibiting a melting point at any T/N ratio, which depends not only on this ratio but also on the degree of randomness or Table 3. Melting Point of Polyester Homopolymersa Homopolymer PET PEN PBT PBN a Data

Tm , ◦ C 250 268 223 242

from BP Amoco Chemical Co.

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Fig. 2. Glass transition of PET/PEN copolymers and blends. Data from BP Amoco Chemical Co.

transesterification present. The dotted line in Figure 1 provides a cursory representation of this behavior. Naphthalate homopolymers have a glass-transition temperature significantly higher than the corresponding terephthalate homopolymers (Table 4). Unlike T m , the behavior of T g vs NDC content is linear and follows the mixing rule (Fig. 2). This linear relationship offers many opportunities for enhancing the thermal resistance of PET by simply blending-in varying amounts of naphthalate. Practical applications include hot-filled, pasteurized, or washable containers, and outdoor glazing. An increase in T g also affords improved long-term aging behavior or reduced embrittlement of amorphous parts. It enables a reduction in cycle time during injection molding (eg, of bottle preforms), since the parts can be released hotter. A 20% faster cycle time has been achieved in the case of PEN preforms vs PET. Finally, the higher T g of PEN contributes greatly to the reduction of thermal shrinkage, which is of value in tire cord and other industrial fiber applications as well as films (see Table 8). Crystallization. As in the case of PET, PEN can be quenched to amorphous glass. Its amorphous density, 1.325 g/cm3 [BP Amoco Chemical Co.], is slightly less than that of PET (1.333) (2). Crystallization from the glass or from the melt at low temperatures (0.9) solid-stated PEN resin is used. High IV PEN resin is available from suppliers such as M&G and Teijin. In addition to rubber reinforcement (for tires, belts, and hoses) applications for PEN fibers include ropes and cordage, and sailcloth. Tire Reinforcement. PEN’s superior thermal and mechanical properties make it a natural candidate for the reinforcement of radial passenger and light truck tire carcasses. These superior properties are summarized in Table 11. PEN yarn has higher tenacity and improved modulus vs PET, nylon, and rayon. Its thermal properties, eg, dry heat shrinkage, are also superior. This lower thermal shrinkage is a significant benefit in tire cord manufacture because of the heat-setting treatment and preshrinkage relaxation required to convert yarn to twisted cord. Compared to PET, the lower thermal shrinkage of PEN enables less preshrinkage treatment, which results in a final cord that has improved tenacity and modulus.

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Table 11. Physical Properties of High Strength Industrial Yarns Used in Reinforcement

Density Tenacity, N/tex Tensile modulus, N/texe Tensile elongation, % Boiling water shrinkage,% Dry heat shrinkage (1 min at 177◦ C)

PEN

PETa

Nylonb

Rayonc

Aramidd

1.36 0.9f 32g 8h 1g 4g

1.39 0.8 9.7 14 5 8

1.14 0.8 4.4 15 8 10

1.52 0.44 11 12 Decomposes 6

1.44 2 49 4