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Readily Processable Polyethylene/Low Ethylene EPDM Thermoplastic Vulcanizates

IP.com Disclosure Number: IPCOM000012956D
Publication Date: 2003-Jun-11
Document File: 7 page(s) / 120K

Publishing Venue

The IP.com Prior Art Database

Abstract

Commercially available thermoplastic vulcanizates (TPVs) contain EPDM rubber dispersed in a matrix of isotactic polypropylene. Typically, polyethylene has not been used as the matrix thermoplastic because less than optimum compatibility between the plastic phase and EPDM has resulted in poor melt processability. It has been found that EPDM with an appropriate ethylene content leads to improved melt processability.

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Readily Processable Polyethylene/Low Ethylene EPDM Thermoplastic Vulcanizates

Abstract

Commercially available thermoplastic vulcanizates (TPVs) contain EPDM rubber dispersed in a matrix of isotactic polypropylene.� Typically, polyethylene has not been used as the matrix thermoplastic because less than optimum compatibility between the plastic phase and EPDM has resulted in poor melt processability.� It has been found that EPDM with an appropriate ethylene content leads to improved melt processability.

Commercially available thermoplastic elastomers (TPEs) that are produced by dynamic vulcanization are typified by the thermoplastic vulcanizates (TPVs) of isotactic polypropylene crystalline plastic and EPDM rubber.� In dynamic vulcanization, a rubber curative is added to an intensively melt mixed plastic and rubber blended to selectively cure the rubber.� This process yields a molten plastic matrix filled with micron sized crosslinked rubber particles.� The melting of the crystalline plastic allows TPV thermoplastic processability while the high rubber content is responsible for TPV elastomeric behavior.� The plastic and rubber chosen should have appropriate compatibility for TPV elastomeric property and melt processability balance.� Plastic and rubber miscibility over a broad temperature range is undesirable.

For example, if the plastic and rubber were miscible, the blend would exhibit an average Tg which would be detrimental to TPV elastic properties since the crystalline plastic material of choice may have a much higher Tg than the rubber.� Selectively crosslinking the rubber phase from a miscible rubber and plastic blend would reduce thermoplastic processability due to inclusions of plastic in the crosslinked rubber particles.� Details of dynamic vulcanization have been reviewed (1).

A polyethylene (PE)/EPDM based TPV could provide better UV and thermooxidative stability than a comparable PP/EPDM based TPV.� Soft products and products with superior low temperature properties (Tg=-80ºC for PE) may be among the other advantages of PE/EPDM TPVs over PP/EPDM TPVs (Tg= 0ºC for iPP).� PE/EPDM TPVs may also have a low mold cycle time processing advantage due to the higher rate of crystallization of PE over iPP.

A serious disadvantage of PE/EPDM TPVs when compared with PP/EPDM TPVs is poor fabricability of the former materials due to unusually high product melt viscosity.� On dynamic vulcanization of a molten plastic and rubber blend there is a substantial increase in melt viscosity due to the viscous drag of the molten plastic that flows over micron sized (~5 μm) rubber particles (increased surface area of contact between plastic and rubber).� The viscosity increase observed on dynamic vulcanization of a PP/EPDM molten blend is illustrated by examples 1 and 2 of Table I.� Melt viscosity is measured using the Automatic Capillary Rheometer (ACR) where the TPV is heated at 400ºF for 4 min, after which the melt is subjected to a constant stress of...