Uses and Processing of Cyclic Olefin Copolymers.
When the first commercial plant for cyclic olefin co polymer (COC) starts up in Oberhausen, Germany, in the autumn of 2000, it will open new possibilities in fields as diverse as optics, packaging, medical equipment, and capacitors. This article is a review of the properties, applications, and processing parameters of Ticona's new family of amorphous engineering thermoplastics.
COC combines glass-like transparency, low density, and excellent water-vapor barrier characteristics with high heat deflection temperature (HDT) and excellent electrical properties and chemical resistance (see "Overview" box and Table 1). It is stiff, strong, and hard, and, in processing, flows well and shows little shrinkage and warpage.
The new plant will make COC in a process that first reacts ethylene and cyctopentadiene to form 2-nor-bornene (a cyclic olefin), which is then polymerized with ethylene by a metallocene catalyst. The norbornene stiffens the main chain and prevents crystallization. High norbornene levels boost stiffness, strength, glass transition temperature ([T.sub.g]) and HDT. Products from the plant will have [T.sub.g] of 80[degrees]C to 180[degrees]C (176[degrees]F to 356[degrees]F) and a HDT of 70[degrees]C to 170[degrees]C (158[degrees]F to 338[degrees]F). Melt viscosity will be adjusted by controlling molecular weight.
COC resists hydrolytic degradation and is moderately permeable to atmospheric gases. It has good resistance to aqueous acids and bases and most polar and oxygenated solvents, such as ketones, methanol, and ethanol. It should not be used with high concentrations of fats and oils or with nonpolar aliphatic or halogenated solvents, e.g., heptane, toluene, naphtha, or methylene chloride.
Processed by conventional methods, including injection molding, extrusion, and blow molding, COC can be reinforced with glass and minerals, and compounded with colorants, flame retardants, slip modifiers, ultraviolet modifiers, and other additives.
Compared with other transparent, amorphous resins, COC has low density and moisture absorption, and a high flexural modulus and HDT (Table 2). It has better stiffness than polycarbonate (PC), and its crack propagation resistance, modulus, and low elongation at break are much like that of acrylics or polystyrene (PS).
Although unstretched cast COC films have an elongation at break of just 3% to 10%, biaxially oriented films can have up to 90% elongation at break and tensile strength of 14,500 to 21,750 psi. Grades with [T.sub.g] above 100[degrees]C (212[degrees]F) have a modulus of elasticity between 450,000 and 480,000 psi, which is comparable with that of PS.
BROAD RANGE OF POTENTIAL USES
In addition to applications in optics, capacitors, packaging, medical devices, and color toner binder resin, COC has the potential to be used in housings, gears, powder coatings, foams, and filter media for air and other gases. Examples of its major uses follow.
COC is a good substitute for glass in precision optical components, especially where multiple parts are needed and other polymers cannot provide sufficient thermal or dimensional stability. These applications include lenses for copiers, printers, video cameras, projection TVs, CD drives, and computer hard drives. Easily metallized, COC also can be used in lamp reflectors, light guides, and flat panel displays, and may find application in high-capacity digital video discs.
COC offers a better mix of optical and mechanical properties than PS, PC, and acrylics (Table 2), and its transmittance at visible and near ultraviolet wavelengths is better than that of PC and PS, and almost equals that of the more temperature-sensitive and moisture-sensitive acrylics. Its stress optic coefficient is comparable with that of acrylics and much lower than that of PC. CCC is stiffer than PC and acrylics, so it holds lens shapes better under mechanical load. It also has a high Abbe number and low chromatic aberration and bireftingence.
COC films work well in capacitors because they resist dielectric breakdown and have very low dielectric loss (about 0.02 at 60 Hz, which is less than half that of PP) over a wide temperature and frequency range.
COC can replace polypropylene (PP) in thin-film capacitors. With a 11[degrees]C (20[degrees]F) higher than the melting point of PP, COC works better than PP in high-temperature capacitors. Because it has a lower dissipation factor than PP and PS, little energy loss occurs into a COC dielectric from an alternating electric field.
Its room-temperature dielectric constant is about 2.35 vs. 2.2 for PP. However, at 120[degrees]C (248[degrees]F), COC's dielectric constant is almost 20% lower (better) than that of PP. The many benefits of COC mean that a capacitor using it can be smaller and run at a cooler temperature than one using PP for such applications as AC motor starters and high-frequency semiconductor circuits.
COC has almost no extractables and provides excellent biocompatibility. Compliant with U.S. Food & Drug Administration (FDA) regulations, it has USP Class VI certification and has FDA Master File numbers for drugs and devices. In laboratory, diagnostic, and other medical applications, it offers excellent low-temperature behavior, shatter resistance, transparency, a water-vapor barrier, and the ability to undergo sterilization processes by gamma radiation, steam, and ethylene oxide. It also resists dimethyl sulfoxide and other commonly used polar solvents better than other amorphous thermoplastics.
COC weighs less and has better shatter resistance than glass, so it is a good alternative to glass in syringe bodies, serum vials, blood containers, bottles, test tubes, petri dishes, pipettes, and graduates. In multi-well (microtiter) plates, its high light transmission into the near ultraviolet, low haze (less than 1%), and low chromatic aberration allow accurate spectrophotometer readings. COC can also be used in drug-delivery systems, such as injector pens and inhalers, to extend the shelf life of moisture-sensitive medications.
The clarity, purity, and mechanical properties of COC films help improve packaging for foods, pharmaceuticals, cosmetics, personal care, and other consumer products. The low water absorption and low permeability of COC films make them ideal in situations where a moisture barrier is needed. COC can be processed as monolayer or multilayer films for use in shrink wrap, pouches, laminates, and coextruded packaging structures.
The U.S. FDA has issued a regulation for ethylene-norbornene copolymers in dry-food-contact, which amends that part of the federal Food, Drug and Cosmetic Act concerned with polyolefins (21 CFR177.1520). COC from Ticona complies fully with this act and all applicable food additive regulations for aqueous, acidic, fatty, and low-alcohol and high-alcohol foods. COC is also suitable for many food-contact applications in Europe, where the monomers used in its manufacture are listed in applicable European Union directives.
Toner Binder Resin
Low molecular weight COC resin can be efficiently milled into very small, uniform, approximately spherical particles, with a [T.sub.g] matched to the fuser temperature range of copiers and printers. COC's high clarity yields bright color in finished documents. A special COC grade has been developed that allows elimination of fuser oil in print engines, reducing both system cost and cost per copy.
Most processors use COC neat or as a transparent color by masterbatching it with colorant. If clarity is not an issue, premade color concentrates based on low-density polyethylene or linear low-density polyethylene (LDPE or LLDPE) can be used. COC can also be compounded with lubricants like amide and polyethylene waxes to aid mold release and extrusion. COC resin usually needs no drying or pretreatment before use.
COC is a versatile material that can be injection molded; extruded as films, sheets and profiles; blow molded; injection blow molded; and injection-stretch blow molded. It also thermoforms readily and can be meltblown. Although processing specifics vary with the grade of material, the equipment used, and the part made, the nature of the material requires that certain general guidelines be followed.
COC is injection molded on conventional machines with low-profile, low-compression screws. Molding typically involves moderate-to-fast screw speeds, minimal back pressure, and 10- to 120-second cycle times, depending on part thickness (Table 3). Shot size should be more than 50% the capacity of the machine. Melt temperatures are typically 110[degrees]C to 120[degrees]C (198[degrees]F to 216[degrees]F) above [T.sub.g] and usually fall between 240[degrees]C and 300[degrees]C (464[degrees]F and 572[degrees]F).
To limit residual stress, one should avoid overpacking the part and should use a mold temperature that is 10[degrees]C to 25[degrees]C (18[degrees]F to 45[degrees]F) below [T.sub.g]. Molds are best heated with an oil heating system for grades with a [T.sub.g] above 115[degrees]C (239[degrees]F). Mold surfaces should be polished smooth because COC replicates submicron surface features and picks up even minor mold imperfections.
Holding pressure should be kept at 4000 to 7000 psi, and gates should be large enough to prevent excessive shear heating or freezing off before holding pressure ends. Sprue, pinpoint, and film gates are recommended, although submarine and pill gates can be used.
COC flows well and fills complex parts with walls thinner than 0.04 inch. Parts have little or no warpage because they typically shrink isotropically in the range of 0.6%. Shrinkage can be reduced further by carefully controlling molding conditions. To ensure easy mold release, parts and tools should be designed to account for COC's high stiffness, low shrinkage, and low elongation at break. Parts should have adequate draft (2[degrees] to 5[degrees], if possible), and undercuts should be avoided. One should select open-type nozzles to ease sprue pullout and should avoid mold sprays for ejection because these can make part surfaces hazy.
COC parts emerge from the mold with smooth surfaces that need little finishing, and they should be degated when warm. If degating must be done on cold parts, one should use a band saw because snipping can cause fractures in more brittle COC grades.
Processors can add up to 10% regrind to COC melts. Although regrind retains its mechanical properties over multiple cycles, it will yellow slightly--so use of regrind is not recommended in critical applications.
When using the process of injection blow molding to produce bottles, one should follow the guidelines above and blow mold the body at 30[degrees]C (54[degrees]F) above [T.sub.g].
Extrusion creates highly transparent COC films and sheets with sparkling surfaces and good moisture barrier properties. Films can be fabricated as monolayers, coextruded structures, and multilayer laminates.
Extruders for COC should have relatively shallow-flight screws with an L/D ratio of at least 25 and relatively low compression values of 1:1.8 to 1:2.0. Screws with higher compression can be used but may need external lubricants to prevent friction-induced fisheyes. Some processors dust external lubricants onto COC pellets even for low compression ratios to reduce feed zone friction and aid film clarity.
Pellets should be melted at low friction in the metering section with melt temperatures reaching about 110[degrees]C to 120[degrees]C (198[degrees]F to 216[degrees]F) above [T.sub.g]by the end of the screw. Main casting rolls should be held about 10[degrees]C (18[degrees]F) below [T.sub.g]because COC can stick if the rolls are too hot or become brittle if they are too cold.
Extruded COC films can be either biaxially stretched for use in capacitors, monoaxially stretched for use in shrink films, or used in multilayer structures. Biaxial orientation on a polyester-type frame [using 3x3 stretching at 20[degrees]C to 30[degrees]C (36[degrees]F to 54[degrees]F) above [T.sub.g]] greatly improves the mechanical properties of COC films as thin as 7m. For instance, this can increase the modulus of elasticity by 1.5 times, tensile strength by 2.5 times, and elongation at break by 20 times.
Multilayer COC films are made by coextrusion or lamination. In the case of LDPE or LLDPE, no tie layers are needed. For other multilayer materials, functionalized polyolefin tie layers can be used in coextrusion with COC to
hold the structure together. The COC layer gives such films a good moisture barrier and enhances the mechanical properties.
Multilayer films using a central COC barrier have proven especially effective in thermoformed push-through packaging. A 12-mil film used in aspirin blister packs (with a COC core, adhesive or tie layer, and PP external layer) creates more uniform blisters than those using polyvinylidene-chloride-coated polyvinyl chloride (PVC) films. The COC structure draws more uniformly during thermoforming, so corners on blisters in the COC structure thin out less than those in the PVC structure. Use of COC multilayer films also avoids the use of halogen-based materials.
Cast and blown films containing polyethylene (PE)-COC blends can be used in flexible packaging for food and consumer items. Adding 10% to 25% COC to FE films gives them better strength, stiffness, and tear propagation resistance, in addition to lower friction and less blocking. COC boosts film stiffness depending on how much is present, greatly improving the effectiveness of stand-up pouches and allowing manufacturers to make a thinner film at the same stiffness.
COC also increases film heat seal initiation strength and tack force, so pouches can be processed faster. The use of COC in FE blends also enhances the moisture barrier and does not significantly change haziness or other film properties. Blends of COC and FE can be made directly on the film line because the two are highly miscible.
When blowing films containing significant COC layers, one may find a short tower height to be beneficial. The air used should be warmed so that the polymer is more workable as it goes through the collapsing nip.
COC can be finished in many ways. Its films, for instance, can be metallized on both sides at once without pretreatment. Vacuum-deposited metals, such as aluminum, chromium, or silver, adhere well to COC for use in mirrors, reflectors, and capacitors.
COC parts can be joined using polyurethane adhesives. Manufacturers can also form solvent bonds using 15% solutions of COC in cyclohexane or heptane. Few materials stick to COC surfaces because they are nearly inert. Before printing or coating a COC part, one should modify the surface to allow a bond to form through the use of corona or plasma treatments.
When cutting, drilling, or otherwise modifying COC parts, one should keep the applied forces low to avoid shattering, i.e., one should make conservative cuts at relatively slow speed when machining COC. Machine surfaces and tools should be oil free to avoid chemical stress cracking; only water-based cutting lubricants should be used.
The availability of commercial quantities of cyclic olefin copolymers opens many performance options to those who work with plastics, especially in the optical, medical, and packaging fields. COC can be used in highly transparent precision lenses for many consumer and industrial products. In the medical, laboratory, and diagnostic sectors, its typical applications include syringes, vials, microtiter plates, and drug delivery systems. Packaging made with COC films, blends, and laminates offers a high moisture barrier and other properties for foods, pharmaceuticals, and cosmetics. The known end uses for this advanced polymer are just the beginning. As designers, processors, and end users work with COC in the years ahead, the extent of applications for it should grow dramatically.
OVERVIEW OF COC PROPERTIES
* High dimensional stability
* Low density
* Glass-like transparency; low chromatic aberration and birefringence
* Extremely low water absorption and permeability
* High heat deflection temperature
* Resists acids, bases, and polar solvents
* High stiffness, modulus, and surface hardness
* Low elongation at break
* High purity, i.e., very low level of extractables
* Excellent insulator with extremely low dielectric loss
* Good metallization characteristics
* Broad range of available glass transition temperatures
* Replicates submicron surface features
* Good processability, including low shrinkage and high flow
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|Comment:||Uses and Processing of Cyclic Olefin Copolymers.|
|Author:||Lamonte, Ronald R.; McNally, Donal|
|Date:||Jun 1, 2000|
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