Get ready for PEN blow molding.

It first claimed a niche in specialty films. Fiber applications were next. NOw the curtain is rising on the main attraction - the much-heralded debut of polyethylene naphthalate (PEN) resin in stretch-blow molded containers.

PEN bottles and jars have already hit the market in Japan, and commercial introductions are expected in Europe this spring or summer. Numerous container development programs are under way in this country, but most are awaiting FDA approval.

PEN is expected to compete with glass and polycarbonate in oriented bottles and jars that require heat resistance or barrier performance not achievable with PET. A close cousin to PET, PEN offers four times higher oxygen barrier, five times better carbon dioxide barrier, and 3.5 times greater moisture barrier. PEN also has higher intrinsic uv barrier and better thermal performance: It sports a glass-transition temperature of 250F, compared with 172 F for PET. PEN also offers improved mechanical properties - 35% higher tensile strength and 50% greater flexural modulus than PET. PEN provides faster molding cycles, too.

This combination of superior properties and processability promises to make PEN the most exciting new bottle resin in a long time - once remaining regulatory and economic issues are resolved. (Suppliers hint that sheet extrusion and thermoformed rigid packaging may not be far off.)

RESIN SUPPLY IS SHAPING UP

Like PET, PEN is a polyester based on ethylene glycol. In PET, the other main ingredient is terephthalic acid (TPA). In a PEN homopolymer, it's dimethy 1-2, 6-napththalene dicarboxylate (NDC). Random copolymers can be made by reacting a mix of NDC and TPA with ethylene glycol.

Until recently, NDC was not manufactured in commercial quantities. There are now two producers in the world - Mitsubishi Gas Chemical in Japan Amoco Chemical Co. in Chicago. Amoco is just now starting up a 100-million-lb./yr NDC facility in Decatur, Ala., with full production expected in the third quarter. NDC is still costly - around $1.50/lb. Consequently, PEN resin costs close to $4/lb today. However, with larger-scale production, PEN's price is expected to fall to around $3/lb.

To help that day arrive sooner, all major polyester resin producers have stepped up their PEN development efforts. Eastman Chemical announced commercial availability of PEN homopolymer in mid-1994. The firm will increase capacity for PEN homo- and copolymers to 22 million lb/yr in the second half of this year.

Eastman has both high- and low-molecular-weight versions for use in specialty films, high-performance fibers and filaments, and molded and thermoformed parts such as medical devices. Eastman was the first to file a petition for FDA food-contact approval of PEN homopolymer back in 1988. The lack of FDA clearance is holding back introduction of PEN food and beverage containers in the U.S., but the European Union has given PEN its blessing.

The first European commercial bottle may be introduced imminently by Carters Packaging of Long Eaton, Nottingham, England. Carters' first launch is expected to be a hot-fill returnable/refillable juice bottle made of PEN copolymer from Shell Chemical.

Shell began producing PEN homopolymer and copolymer packaging resins in the U.S. last year. PEN commercial development manager Ed Sisson expects PEN copolymers to penetrate hot-fill and returnable/refillable containers in Europe. PEN copolymers are said to be hot-fillable at temperatures up to 215 F vs. 167-176 F for PET. Refillable PEN containers can be washed at up to 185 F with no shrinkage, as compared with 138 F for PET.

Shell will also introduce two PEN homopolymers for pharmaceutical containers in the U.S. Shell's PEN homopolymer and a range of PET/PEN copolymers and blends have met U.S. Pharmacopeia (USP) requirements for Class VI-121 [degrees] plastics. Last year, Shell and Amoco jointly petitioned the FDA for food-contact clearance for PEN homopolymer, copolymers, and blends with PET.

Property

Miscellaneous

Specific Gravity                      1.328
Drying Temperature                      300      F
Drying Time                             4-6     hr
Processing Melt Temperature         560-600      F
Mold Temperature                         75      F

Mechanical

Tensile Strength

 Yield                              11,300    psi
 Break                                4500    psi

Elongation

 Yield                                   8      %
 Break                                  25      %

Flexural Modulus                    350,000    psi

Flexural Yield Strength              13,500    psi

Izod Impact Strength, Notched

73 F                                    0.7     ft-lb/in.
-40 F                                   0.4     ft-lb/in.

Izod Impact, Unnotched

73 F                                   11.8     ft-lb/in.
-40 F                                   9.2     ft-lb/in.

Thermal

Heat-Deflection Temperature

66 psi                                  221      F
264                                     189      F

Optical

Haze                                       5      %

* Starting pellet I.V. (inherent viscosity) was 0.53.

Hoechst Celanese Corp. has also filed a petition with FDA for food-contact uses of PEN, reports Lewis Morison, commercial director of PET packaging resins. Hoechst has been developing a broad range of PEN homopolymers, copolymers, and blends for films, fibers, and rigid packaging. Pilot quantities have been produced for potential applications such as beverage bottles and hot-fill containers. Commercial quantifies will become available later this year.

ICI Americas has been producing PEN resin at Fayetteville, N.C., since late 1992, using it to make biaxialty oriented film in Scotland. PEN development manager Paul Wright says ICI is working on rigid containers at its Wilton R&D center in England. The focus is on PET/PEN blends and copolymers. ICI has yet to make any commitment to produce PEN resin commercially for rigid packaging.

A small commercial PEN market has emerged in Japan over the last year or so. For example, Teijin Ltd. produces at least 2.2 million lb/yr of PEN, 90% of which is used to make magnetic recording tape. Japanese polyester suppliers have been selling PEN mainly in the form of low-PEN/high-PET copolymer. Current commercial containers include bottles for mouthwash, a fresh-juice lemonade drink, and multi-vitamins. The latter two applications require PEN's uv barrier.

LOW PEN CONTENT CUTS COST

"Most people will be interested in using PEN as a blend or copolymer because the cost will he lower," says ICI's Wright. "For example, a copolymer with 8% NDC will be priced close to what customers are willing to pay."

Amoco global marketing manager Jeff Toth expects the market to be dominated initially by both low-level copolymers (5-8% PEN) and high-level copolymers (92-96% PEN), as well as PET blends with PEN contents from 1015% up to 50%.

Suppliers see three especially promising initial domestic applications for PEN-containing materials:

* Juice bottles (1.5-liter) that require hot-fill temperatures up 212 F.

* Wide-mouth, hot-fill jars for salsa, spaghetti sauce, and jellies, which are heated above 212 F.

* Retortable packaging (e.g., for baby foods) requiring deformation resistance at 265-280 F and high pressure for an extended time.

Other applications include single-serving carbonated soft-drink bottles, which needs a higher gas barrier than that of PET. Returnable/refillable bottles for pasteurized juices, mineral water, and even beer are expected to succeed - primarily outside the U.S. Industrial, cosmetics, and pharmaceutical applications also look promising.

PROCESSING DIFFERENCES

PEN homopolymer has both a higher [T.sub.g] and a higher melting point (518 F) than PET (478 F). PEN homopolymer thus requires higher processing temperatures. However, the material can be modified to match PETs melting temperature without significantly affecting the [T.sub.g] or heat resistance.

Like PET, PEN strain-hardens readily in stretch-blow molding. During injection molding, however, PEN crystallizes much more slowly, allowing thicker preform designs to be molded. The higher [T.sub.g] means the hot resin "sets up" at a higher temperature, allowing the part to be demolded hotter and sooner.

Shell and other researchers conclude that containers can be made with PEN copolymers and blends on existing PET stretch-blow molding machinery. However, modified preform and/or blow mold designs are required to achieve optimized containers.

There are key processing differences between PEN random copolymers and blends. "PET/PEN copolymers will hold onto their properties no matter what is done to them, whereas blends will not yield as good a bottle with existing PET processes," explains Shell R&D chemist Doug Callander. "With copolymers, you have to modify some process conditions, but no machine modifications are required. With both copolymers and blends, optimized preforms are required to achieve optimal performance."

PET/PEN random copolymers behave similarly to PET during stretching and blowing. According to Amoco's Toth, low-level copolymers containing up to about 15% NDC and high-level copolymers containing about 85% or more NDC fall within the region of semi-crystallinity in Fig. 2. Materials in this region undergo strain-hardening during stretching and blowing, which produces enhanced physical properties. Copolymers in the middle range - around 1596-85% NDC - remain amorphous, making stretch-blow molding less effective in producing strong, lightweight containers.

The latest results on processing various random copolymers on standard PET machinery and molds were due to be presented last month by Shell and Carters Packaging at the Bev-Pack Americas '95 conference in Tarpon Springs,. Fla. Recent Bev-Pack conferences in the U.S., Europe, and Asia (sponsored by Directions 21, Inc.) have been the major source of public information on progress toward PEN commercialization.

BLINDS ARE DIFFERENT

There's a consensus among polyester resin producers that PET/PEN blends are still something of a "black art." As they are not a homogeneous material, there is more uncertainly about their processing behavior. The main conclusion of blend trials to date is that processing at the preform molding stage is critical to making blends work.

Peculiarities of PEN/PET blends were presented by Shell's Sisson at Bev-Pack Americas '94 in Tarpon Springs last April. PEN and PET are intrinsically immiscible. But with high-shear mixing at residence times of 1-10 min, the two polymers will undergo a chemical reaction (transesterification), yielding a clear extrudate. Such residence times are generally not possible with conventional extrusion equipment and are undesirable in injection molding, Sisson noted. Long mixing cycles also imply loss of intrinsic viscosity (I.V.) and increased acetaldehyde (AA) generation.

Researchers at ICI's U.K. labs say PEN/PET blends may be compatibilized to some extent (which could speed up processing) and AA scavengers can be added. The key advantage of blends, they say, is to permit formulating PEN/PET ratios of 15:85 to 85:15, which correspond to the amorphous range of the copolymer spectrum [ILLUSTRATION FOR FIGURE 2 OMITTED]. In that range, copolymers give relatively poor barrier and heat resistance. Blends, however, retain their crystalline properties.

At Bev-Pack Asia '94 in Hong Kong last September, Amoco's Toth gave a joint presentation with Husky Injection Molding Systems and Sidel, the French stretch-blow machine builder. Toth described how a wide range of PET/PEN blends were injection molded in a two-cavity, 160-ton Husky preform system. The 59-g, 1.5-liter juice-bottle preforms were subsequently molded into bottles by Sidel. The tests yielded clear preforms and aesthetically acceptable bottles with improved barrier properties relative to straight PET. But the processing conditions required to obtain a clear preform caused long cycle times and resin degradation, as evidenced by high AA levels and reduced I.V.

Results of subsequent work with blends were being compiled at press time by Amoco, Husky, and Sidel for this year's Bev-Pak Americas meeting. A key focus of the latest work is a new experimental mixing screw, according to Husky R&D engineer Laura Martin.

Typical PET screws are low-shear, non-mixing types designed for gentle melting and low resin degradation, making them unsuited to mixing blends. The new screw was modified in the transition and metering zones to achieve mixing at low shear by forcing the material back and forth within the flight channels. A secondary mixing section between the transition and metering zones provides dispersive mixing of solid agglomerates.

The latest trials used blends of 85% PET and 15% PEN copolymer (containing 8% terephthalate in order to lower the melting point to match that of PET). Preliminary data show that the new screw mixes very gently and that very clear preforms can be obtained. "We anticipate that final results on samples will show lower AA and less of a drop in I.V. - in other words, within normal PET specifications," Martin reports.

She says a factorial-design experiment was used in these trials to determine the effects of backpressure, melt residence time, and other process parameters on transesterification, AA, I.V., and other barrier properties. The two most significant factors were found to be temperature and residence time. "Here is where we have come out of black art into science. We now understand how processing parameters affect preform and bottle properties, and we have established the groundwork for successful scale-up," says Martin.

Meanwhile, Husky has built a mold for a new preform design of Sidel's. Trials were to get under way this month. Martin notes that the optimum stretch ratio for PEN is different from that for PET: "For PEN, you need more longitudinal stretch and less hoop stretch so the preform is shorter and fatter." Next on the agenda for Husky is scale-up to commercial multi-cavity tooling.

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Directions 21, Inc., Sarasota, Fla. Eastman Chemical Co., Kingsport, Tenn. Hoechst Celanese Corp., Spartanburg, S.C. Husky Injection Molding Systems, Ltd., Bolton, Ont. ICI Americas, Wilmington, Del. Shell Chemical Co., Akron, Ohio Sidel Inc., Doraville, Ga.