New engineering polyolefin offers clear processing opportunities.

A new family of transparent, metallocene catalyzed COCs offers a cost-effective mix of excellent optical and electrical properties, low density, and ultra-low moisture absorption.

Lenses, prefillable syringes, and moisture-barrier packaging - what these diverse applications have in common

are the benefits offered by a new family of amorphous, glass-clear polyolefins. Topas Cyclic Olefin Copolymers (COCs) give design engineers a cost-effective mix of excellent optical and electrical properties, low density, extremely low moisture absorption, strength, and stiffness. They offer processors low-shrink, low-warp resins compatible with ordinary injection molding, injection blow molding, and extrusion equipment.

The copolymers are formed from ethylene and cyclic olefins, which are aliphatic (saturated) ring-structured materials. The cyclic olefin imparts to the copolymer a degree of stiffness that radically increases the glass-transition temperature of the material and destroys crystallinity, resulting in a completely amorphous, highly transparent resin. By altering the ratio of comonomers, Hoechst has created a family of materials with heat-distortion temperatures ranging from 167 to 338 F at 66 psi. HDT is a key differentiation between Topas grades (see Table 1).

COCs have been around for some 40 years. But recent development of metallocene catalysts makes it possible for the first time to produce COCs affordably with low levels of catalyst and residual monomer. Far less costly than earlier COCs available from Japan, Topas COCs are priced from $2.50 to $2.95/lb in 10,000-lb developmental quantities. And because of their high purity, Topas resins are particularly suitable for applications in data systems and optics, pharmaceutical processing, and critical packaging.

The Topas family of COCs presently consists of five grades that have been processed by injection molding, injection blow molding, and cast film extrusion. (Blown film has also been extruded in the laboratory.) Topas 8007 has primarily been processed into films and is also seeing interest in injection molding. Topas 6013 is also used in film.

Blow molding is broadly distributed among all five grades. Injection molding has been concentrated on grades with higher glass-transition temperatures (Topas 5013, 6013, 6015, and 6017). Topas 5013, an injection molding grade with exceptionally high melt-flow rate, is designed for thin-wall parts and those that require very fine surface replication, making it suitable for optical data-storage media. Hoechst is also developing glass-filled and impact-modified grades for non-optical applications. Some colored products can also be prepared.

Topas COCs are now being supplied to selected customers in limited quantities. The material is produced at a 4-million-lb/yr plant in Japan, run by Mitsui Petrochemical, with which Hoechst is jointly developing COCs. Hoechst also has a COC pilot plant in Frankfurt, Germany. However, Hoechst is rapidly moving to fully commercial production with a planned 66-million-lb/yr facility, targeted to be built in Germany and come on stream in 1999.

GOOD MOLDABILITY

The five Topas COC polymers offer melt-flow indices from about 5 to 75 g/10 min. All can be processed readily on conventional injection molding machines with standard screws. Because of their good mold flow, Topas COCs fill complex, thin-walled parts. The highest-flow formulation easily fills walls less than 0.04 in. thick. The relative stiffness of these resins - 25% higher than that of polycarbonate - means parts can be molded in COC with thinner walls.

Cyclic olefin copolymers shrink just 0.6% to 0.7% in the mold, and their shrinkage is nearly isotropic, leading to very low or negligible warpage. Complex parts with angles and bends can be molded on balanced tools without significant warpage.

More detailed injection molding guidelines appear in the accompanying sidebar and Table 2.

BROAD USE POTENTIAL

Products made of Topas COCs have already proven successful or are in advanced testing. At present, the largest-volume COC application is cast-film extrusion. While unstretched COC cast films have an elongation at break of just 3%, biaxially oriented films made from Topas 6015 have up to 90% elongation at break. Tensile strength of oriented films is 14,500 to 21,750 psi.

Where good moisture barrier is more important than oxygen barrier, Topas COC films are candidates for replacing PVC and PVdC packaging films. Very effective moisture barrier also makes COCs suitable for prefillable syringes that were previously made almost exclusively from glass. In a joint project with a European partner, Hoechst has developed syringe bodies with 0.5-to 250-ml capacity. Lighter than glass syringes, the COC syringes have better shatter resistance. They can also be mass produced with greater precision.

LIGHT AND CLEAR

Compared with polycarbonates, acrylics, and other resins, COCs offer a better mix of optical and mechanical properties with less weight. The density of COCs is just 1.02 g/cc, 7% less than [TABULAR DATA FOR TABLE 1 OMITTED] polystyrene, 15% less than polycarbonates and acrylics, and less than half the density of glass. The weight savings in many applications can be significant. In a video camcorder, for example, a COC lens in place of glass could lighten the single heaviest camera component. For weight-conscious automotive engineers, light pipes extruded from Topas COCs can save precious ounces in place of glass.

Topas COCs are exceptionally clear at visible-light wavelengths. They have 92% light transmittance, better than polycarbonate and polystyrene and nearly equal to more temperature- and moisture-sensitive acrylics. COCs' higher heat resistance makes them suitable for lenses in projection tv sets or large computer displays, where the heat of the lamps could cause acrylic lenses to soften and deform. Even at high temperatures, Topas COCs lose little of their creep resistance.

Water-white cyclic olefin copolymers make optical parts with less than 1% haze and low chromatic aberration. The new resins can be used, for example, in the multi-well plates of diagnostic equipment where clarity is important for accurate spectrophotometer readings. Thanks to their low birefringence, COCs have also been evaluated for high-density optical disks.

With a flex modulus of 500,000 psi, Topas COCs are stiffer than polycarbonate and acrylic and can hold accurate lens shapes better despite mechanical loads. Water absorption is typically less than 0.01%, so copier or printer lenses molded in Topas COCs will not swell with increases in ambient humidity. The new resins have a tensile strength near that of polycarbonate. They are also relatively hard, with 89 Shore D hardness.

TABLE 2 - INJECTION MOLDING CONDITIONS FOR TOPAS 6015

Barrel Temp., F

Feed                                     [less than]176
Zone #1                                         446-518
Zone #2                                         473-554
Zone #3                                         500-572
Zone #4                                         500-572
Nozzle                                          518-572
Mold Temp., F                                   230-300
Injection Pressure, psi                     7000-16,000
Holding Pressure, psi                         4000-7000
Backpressure, psi            [less than or equal to]100
Screw Suckback                          None or minimal
Cushion                                           Small
Screw Type                        Low-compression best;
                                    G-P also acceptable
Screw Speed                                        Fast
Screw Turn on Inject                                 No
Ram Speed                              Moderate to fast

CHEMICAL/ELECTRICAL USE

Topas COCs have good chemical resistance in contact with aqueous and oxygenated organic chemicals including hydrochloric and sulfuric acids, caustic soda, methanol, and ethanol. They are not suitable for use with high concentrations of fats, oils, and aliphatic solvents such as hexane and toluene. The new resins have already received an FDA Drug Master File number, and several grades meet U.S.P. Class 6 requirements. Hoechst is pursuing a new FDA regulation under 21 CFR.

The low polarity and ionic content of cyclic olefin copolymers gives them low dielectric losses. Their electrical properties remain constant over a wide temperature and frequency range. COCs can replace polycarbonate and polypropylene in thin-film capacitors. The new resins offer Tg's up to 20 [degrees] F higher than the melting point of PP.

Tips on Injection Molding COCs

Drying: Drying is generally not necessary, although some drying may be required if the material has become wet through improper storage.

Melt temperature: Molding temperatures of different COC grades generally follow the differences in heat-deflection temperatures. Suitable melt temperatures for Topas COCs are 140-250 [degrees] F above the Tg of the grade being processed and usually at the high end of that range.

Screw design: A low-profile, low-compression screw is preferred, due to the amorphous nature of the melt.

Screw speed: Generally, moderate to fast (50-100 rpm) is recommended. Backpressure: Minimal backpressure is recommended.

Nozzle: Because the material has very low shrinkage (typically 0.6-0.7%), an open-type nozzle is recommended for easy pull-out of the sprue.

Injection speed and pressure: COCs should be molded with medium to high injection rates of 2 to 6 in./sec. Injection pressure should be 7000 to 16,000 psi.

Holding pressure: Holding pressure should be reduced in stages to 40007000 psi. An excessively long holding-pressure phase or high holding pressure produces unacceptable internal stresses in molded parts. They increase sensitivity to stress cracking. Virtually birefringence-free moldings are obtained if a high injection rate is combined with relatively low holding pressure, short holding time, and high mold temperature.

Mold temperature: A maximum of 50-70 [degrees] F below the Tg will minimize molded-in stresses and give good optical clarity. Mold temperature above 95 F is recommended for grades of Topas COC with high HDT.

Hot runners: Topas COCs are run on hot-runner systems without significant problems. Indirectly heated systems with free-flow nozzles are preferred.

Gating and mold design: Despite the good flow of Topas COCs, gates should be large enough to prevent excessive shear heating or freeze-off before the end of holding pressure. Sprue, pinpoint, and film gates are recommended. Parts can be molded with submarine and pin gates and require no additional finishing. To facilitate mold ejection, undercuts in the part or runner system should be minimized.

In designing the part and the tool, the relatively low elongation at break of COC must be taken into account. For mold ejection, the draft angle should therefore be as large as possible - between 2 [degrees] and 5 [degrees].

Cycle times: Molding cycles are competitive with those of polycarbonate.

Regrind: COCs are very stable in multiple regrind cycles for non-optical applications, although recommended regrind levels have not been developed.

Dr. Thomas Weller, Marketing Europe, and Detlef B. Schulz, Diplom-Ingenieur, are with Hoechst AG, Hoechst Technical Polymers in Frankfurt, Germany. Dr. Donal McNally, Topas Marketing Americas, is with Hoechst Technical Polymers, Summit, N.J.