Multimodal ethylene, alpha-olefin and diene polymers,...

Details Multimodal ethylene, alpha-olefin and diene polymers,... Multimodal ethylene, alpha-olefin and diene polymers,...

2003-02-07

2004-02-03

Teskin, Fred (Department: 1713)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Mixing of two or more solid polymers; mixing of solid...

C525S053000, C525S211000, C524S554000, C524S570000, C526S065000, C526S282000, C526S336000, C526S339000

Reexamination Certificate

active

06686419

ABSTRACT:

FIELD
This invention relates generally to multimodal ethylene, alpha-olefin and diene polymer compositions, processes for making them and to devices comprising such compositions. These compositions have been found to have an improved balance of processability, resilience and durability.
BACKGROUND
Elastomers useful in applications requiring high resilience have in general a high elasticity in the medium to high frequency range (typically between 5 and 600 rad/sec) which results in low loss tangent (tan &dgr;), at temperatures ranging from approximately room temperature to over 100° C. Natural rubber is an elastomer of this type.
Ethylene elastomers contain sufficient copolymerised alpha-olefin to produce amorphous to semi-crystalline compounds of density generally below 0.9. In the class of ethylene based elastomers, such as EPDM elastomers, polymers of very high molecular weight (as measured by their Mooney viscosity) are usually required for high resilience applications. Although high molecular weight EPDM elastomers are commercially available, their inherently very high viscosity (Mooney viscosity in general greater than 200) creates difficulties in processability which requires them to be produced with the addition of extender oil to reduce their apparent viscosity. Very high molecular weight polymers are not only difficult to separate from their polymerization solvent without inducing significant macromolecular chain breakage, but they are difficult to mix and compound. The addition of extender oil to the elastomer eases processability in terms of production process recovery, elastomer mixing and compounding.
The required level of extender oil depends on the molecular weight of the elastomer, but is usually sufficient to reduce the apparent viscosity to a Mooney viscosity of about 100 or below. Typical extender oil level is in general in the range of from 10 to 150 phr. Commercially available very high molecular weight EPDMs which would be useful in high resilience applications contain from about 50 to about 125 phr extender oil.
Problems arise from the use of extender oil however. Elastomers extended with oil are limited in compounding latitude since they already contain a level and/or type of oil which may be undesirable in the compound recipe and restrict further compounding options. Furthermore, the extender oil simply acts as a processing aid and does not participate in the formation of a tight cross-link network upon vulcanization, which reduces the elasticity of the vulcanized compounds.
The elasticity of ethylene based elastomers can be increased by introducing long chain branches into their structure. This can be achieved using certain acidic transition metal catalysts, or through the use of dienes having two polymerizable double bonds such as norbornadiene, dicyclopentadiene, vinyl norbornene or alpha-omega dienes. However, a high level of branching is required for significantly improved elasticity, and such branching can result in polymers that are almost impossible to process without the use of extender oil as described above. Moreover, the presence of long chain branching adversely affects physical properties such as tear resistance, tensile strength and elongation.
Another method of increasing processability is to produce bimodal elastomers having a major fraction of lower molecular weight polymer (Mooney viscosity less than about 100) and a minor fraction of high molecular weight polymer (Mooney viscosity greater than 120), as in ExxonMobil Chemical's Vistalon™ Bimodal EPDM grades. However, these polymers tend to have an elasticity which is too low.
Among the more demanding applications for processable, highly elastic applications are vibration damping devices. Vibration damping devices are used to absorb vibrational energy generated by machines such as automobile, jet and other engines, air conditioners, vehicle exhaust systems and other dynamic devices that generate significant vibration during operation. Unless a damping device is used, this vibration is directly transmitted to support and surrounding structures. The result can range from annoying to destructive. The damping device should maintain its performance under a wide range of temperature and other environmental conditions and for a relatively long period of time.
Examples of vibration damping devices include formed shapes, mountings, harnesses, rings, bushings and belts used to isolate vibrators from what would otherwise be vibrated. While natural rubber is often used in vibration damping applications because it provides high resiliency and desirable physical properties like tear resistance, natural rubber does not last long under conditions of extreme heat (>120° C.) or ozone concentration such as is found in many automotive applications.
Despite years of research in this area there is still a need for economically produced synthetic materials that have an improved balance of processability, resilience and durability.
U.S. Pat. No. 3,884,993 describes the preparation of the blends via a parallel reactor process using traditional Ziegler-Natta catalysts. These blends have a maximum Mooney viscosity of 53 (1+8@121° C.) which corresponds to a Mooney viscosity of about 57 when measured at 1+4@125° C. even where the amount of high molecular weight fraction reaches approximately 58 weight percent. The low molecular weight fraction has an Mn below 25,000.
WO00/26296 discloses an ethylene-alpha olefin elastomeric composition made by a series reactor operation in which the high molecular weight component has a Mooney not exceeding 120 and is present in an amount no greater than 50 weight percent.
U.S. Pat. No. 4,078,131 describes blends of EPDM having components with differing average propylene contents. A wide range and variation of properties in the components is disclosed, but there is no disclosure of a blend comprising 50 weight percent or more of a high molecular weight fraction having a Mooney of over 120.
SUMMARY
In the invention a multiple reactor process may be used to produce an elastomer of high Mooney viscosity in one reactor, while a second reactor in series or parallel produces an elastomer of low Mooney viscosity which acts as an internal plasticizer for the high Mooney viscosity polymer component thereby producing an elastomer having an overall Mooney viscosity low enough to enable easy processing while maintaining normal compounding operations. The composition is solid, that is to say not fluid at room temperature, and substantially free of solvent. The composition can be formed into bales or pellets for subsequent processing, compounding and mastication.
In a polymer product aspect of the invention there is provided a multimodal elastomer including a fraction of high Mooney viscosity and a fraction of low Mooney viscosity which acts as an internal plasticizer for the high Mooney viscosity polymer fraction thereby producing an elastomer having a Mooney viscosity low enough to enable easy processing while maintaining normal compounding operations. The elastomeric nature of the internal plasticizing component allows it to participate in a cross-linked network upon vulcanization. Some forms of the invention may provide a polymer providing high resiliency and durability even under exposure to prolonged high temperature conditions.
In an elastomer compound aspect of the invention there is provided a formulation containing a polymer of the invention; conventional fillers, cross-linking and stabilizing additives, plasticizing oil and optionally other compounding ingredients. Compared to recipes containing high Mooney viscosity polymers extended with oil, the total amount of plasticizing oil in the recipe can be reduced.
In one embodiment the invention is a solid multimodal polymer composition comprising units derived from ethylene, alpha-olefin and diene, said polymer composition having a tan (&dgr;) of 0.5 or less when measured at 125° C. and at a frequency of 10.4 rad/sec, and an overall Mooney viscosity of at least 60 (1+4@125° C.); said polymer

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