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ETHYLENE–NORBORNENE COPOLYMERS Introduction Copolymers made from α-olefins and cyclic olefins have been known for nearly 50 years (1,2). Ethylene–norbornene (Et–Nb) copolymers are specifically described in the patent literature as long ago as 1973 (3), with further patent filings since then (4–6). The general class of thermoplastic cyclic olefin copolymers (COCs) includes products from Mitsui Chemicals (7), Japan Synthetic Rubber (8), Nippon Zeon (9), and Ticona (10). Some of these products are manufactured by ring-opening metathesis and others by addition polymerization. Only the COCs made by Ticona, a division of Celanese AG, under the trademark Topas, are Et– Nb copolymers. These products, for which Ticona started up a 30,000-t plant in September 2000 (11), are described in this article.

Properties Ethylene–norbornene copolymers are made by the direct addition of ethylene and norbornene, using metallocene catalysts. Although regularly alternating copolymers have been made with semicrystalline characteristics, all Et–Nb products presently available are completely amorphous random copolymers, varying only in melt flow and glass-transition temperature (T g ). The bridged ring structure of norbornene makes the resins quite stiff, and their completely amorphous nature ensures high transparency. In general, the resins are stiff, strong, glass-clear, and water-white. They have low elongation at break, negligible moisture absorption and transmission, very low dielectric loss, and low density. The glass-transition Encyclopedia of Polymer Science and Technology. Copyright John Wiley & Sons, Inc. All rights reserved.

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250 Current Product Range 200

Tg, °C

150

100

50

0 0

20

40

60

80

Norbornene, mol %

Fig. 1. Effect of norbornene content on T g .

temperature increases linearly with norbornene content, as shown in Figure 1. The commercially available products range in T g from about 65◦ C to about 180◦ C. Subject to some limitations, conventional thermoplastic processing methods may generally be employed with all of these products. Because of the bulkiness of norbornene, Et–Nb copolymers remain amorphous down to very low levels of norbornene, where a clear elastomeric polymer is obtained. In the current commercial range of products, all grades on offer exhibit high rigidity, with tensile (Young’s) modulus values ranging from 2600 to 3200 MPa. Specific gravity values fall in a narrow spread from 1.00 to 1.02, and light transmission is around 92%. Typical values for a range of physical properties are shown in Table 1 for the announced commercial grades of Topas COC resins (12). These materials have excellent chemical resistance to aqueous fluids (acids and bases) and many polar solvents (alcohols and ketones), but are attacked by nonpolar solvents (hexane, toluene, oleic acid, and methylene chloride) and some oils and fats. The chemical resistance of the copolymers may be summarized as follows (13). The commercially important properties of Et–Nb copolymers include low density, high transparency and low color, high moisture barrier and low moisture absorption, low optical distortion, excellent feature replication, resistance to polar solvents, high purity, shatter resistance, good biocompatibility, extremely low dielectric loss, high temperature capability, and compatibility with polyethylenes. The resins also have the low shrinkage and warpage typical of amorphous polymers.

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Table 1. Properties of Et–Nb Copolymers, Commercial Grades of Topas Property

8007

5013

6013

6015

6017

◦

80 30 1.0 93,200 600 >1016 92

140 55 1.02 80,500 600 >1016 92

140 16 1.02 65,400 600 >1016 92

160 16 1.02 80,200 600 >1016 92

180 12 1.02 82,500 600 >1016 92

Tg , C Melt flow (260◦ C), g/10 min Density, g/mL M w (PS), g/mol H2 O absorption, % Mold shrinkage, % Tensile strength, MPaa Elongation @ break, % Tensile modulus, MPaa Charpy impact, kJ/m2b Notched Charpy impact, kJ/m2b HDTc @ 0.46 MPaa , ◦ C Dielectric constant @ 1–10 kHz Comparative tracking index, V Volume resistivity, ·cm Light transmission, % a To

convert MPa to psi, multiply by 145. convert kJ/m2 to ft·lbf/in.2 divide by 2.4. c Heat-deflection temperature. b To

FDA Drug and device master file numbers respectively are DMF 12132 and MAF 1043. All grades, save only the highest temperature one, comply with U.S.P. Class VI requirements. The FDA has issued a regulation, 21 CFR 177.1520, for Et–Nb copolymers in dry food contact (14). The 80◦ C T g grade complies with food contact requirements under conditions of use C through H, while the 140◦ C and 160◦ C T g grades comply under conditions of use A through H. A Food Contact Substance Notification, FCN 000075, became effective on August 22, 2000 for the polymers in direct food contact with all food types as films, sheets, and articles made therefrom (15). The afore mentioned properties, usually in various combinations, are the drivers for current and developing applications for Et–Nb copolymers.

Manufacturing Although, strictly speaking, Et–Nb products are copolymers of ethylene and norbornene, the manufacturing process starts with ethylene and a high purity dicyclopentadiene (DCPD) stream. The DCPD is cracked at a temperature in excess of 160◦ C to yield cyclopentadiene (CPD) (eq. (1)), which reacts with ethylene to give norbornene (bicyclo[2.2.1]-2-heptene) via Diels–Alder condensation (eq. (2)). Cyclopentadiene boils at 42◦ C, whereas norbornene melts at 47◦ C.

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The norbornene product then reacts in solution with ethylene to form the copolymer, using a zirconium metallocene procatalyst/methylaluminoxane cocatalyst system (eq. (3)).

Subsequent removal of catalyst and solvent yields a very pure polymer.

Processing Many processing methods have been employed with Et–Nb copolymers. These include injection molding, injection and coinjection blow molding, compression molding, cast film and sheet extrusion and coextrusion, blown film extrusion and coextrusion, tubing extrusion and coextrusion, extrusion compounding, film solvent casting, mono- and biaxial (tenter) orientation of film, and thermoforming of film and sheet. In co-extrusion and coinjection molding, a tie layer is normally required except for polyethylenes and high ethylene copolymers. Unless the parison is quite small, presently available polymer grades do not have sufficient melt strength for extrusion blow molding. Typical starting conditions for molding and extrusion are given in Tables 2 and 3 where the higher temperatures apply to polymers of higher T g . For processes involving a free surface, such as extrusion or blow molding, an external processing aid should be dusted on the polymer pellets to obtain the best product esthetics. For both extrusion and molding, low compression screws are recommended. A special screw design for film extrusion is shown in Figure 2 (16). In both extrusion and molding, it is important that the forming surface be maintained near the T g of the polymer grade being processed. For higher T g grades, this requires oil-heated tooling for molding and oil-heated take-off rolls for film casting. Manufacture of blown films requires a shorter tower and a lower nip than those that are used in conventional olefin processing. Post-processing assembly procedures such as lamination, machining, or diamond turning, and joining by solvent, friction, and ultrasonic bonding have all been satisfactorily demonstrated with Et–Nb copolymers. Machining lubricants should be water-based and should contain no oil. Also, relatively slow speeds and shallow cuts should be used to avoid cracking the part.

Applications Rigid and flexible packaging are currently the leading uses for Et–Nb copolymers. This category includes both film and container applications in pharmaceutical, medical and diagnostic, and food packaging end uses. Pharmaceutical blister packaging requires high moisture barrier, transparency, and thermoformability.

T1
50 C

T5
200 C

T6
210 C

T4
190 C

T3
180 C

T2
170 C

493 2D mixing section

3D shear section

6D

8D

metering section

flat compression section

5D

1,5 D

metering section

3,5 D feed section

decompression section

Fig. 2. Special screw design for film extrusion.

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Table 2. Injection-Molding Conditions for Et–Nb Copolymers Feed Zone Barrel Zone 1 Barrel Zone 2 Barrel Zone 3 Barrel Zone 4 Nozzle Melt Max barrel residence Injection pressure Hold pressure Back pressure Screw speed Injection speed Mold temperature Nozzle type Screw suckback Cushion Screw type Screw turn on inject Ram speed a To