Thermoplastic elastomer compositions

Details Thermoplastic elastomer compositions Thermoplastic elastomer compositions

2001-10-16

2003-11-11

Yoon, Tae H. (Department: 1714)

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

Synthetic resins

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

C524S491000, C524S523000, C524S525000, C525S222000, C525S232000

Reexamination Certificate

active

06646056

ABSTRACT:

FIELD OF INVENTION
This invention relates to thermoplastic elastomer compositions including blends of propylene and ethylene polymers for extrusion, calendering, blow molding, thermoforming, and foam processing, and articles made therefrom.
BACKGROUND OF THE INVENTION
There is a need for recyclable materials that can be used as alternatives to polyvinyl chloride for the fabrication of articles. Polyvinyl chloride, often used with a plasticizer, can be formed into a rubbery, thin sheet for use as a skin layer over a rigid or soft substrate. Due to the combination of the tactile feel (softness) and the melt strength during processing, plasticized polyvinyl chloride can be a very desirable material. Polyvinyl chloride, however, is not easily recyclable or melt blendable with non-polar polymers, which has limited the utility of polyvinyl chloride to applications where recyclability is not desired. Recyclable materials with processing characteristics similar to polyvinyl chloride, such as high melt strength, are being actively sought.
Olefinic polymers, as a class of materials, offer the capability to be recycled with very little loss of physical properties due to the high level of hydrocarbon saturation. In order to achieve a soft tactile feel similar to cured animal leather or polyvinyl chloride sheets in a recyclable product, several thermoplastic polyolefin technologies have been developed.
Olefinic thermoplastic elastomers including thermoplastic olefin blends (TPO), thermoplastic polymer alloy compositions, and dynamically vulcanized thermoplastic elastomers have been explored for such applications.
A thermoplastic elastomer (TPE) is a material that exhibits rubber-like characteristics, yet may be melt processed with most thermoplastic processing equipment, such as by extrusion. The rubber-like characteristics typically desired are high extensibility, mechanical recovery, resiliency, and low temperature ductility. An olefinic thermoplastic elastomer includes primarily polymers manufactured by the polymerization of at least 50 mole percent olefinic monomers, such as ethylene, propylene, butylene, iso-butylene, alpha-olefins, olefinic dienes, and the like.
Physical blends of thermoplastic polyolefins are commercially available as recyclable alternatives to plasticized polyvinyl chloride. One such material, DEXFLEX® E280, commercially available for thin sheet extrusion from Solvay Engineered Polymers of Auburn Hills, Mich., is prepared by melt blending polypropylene with high molecular weight ethylene-propylene rubbers. This and other similar materials are often referred to as flexible thermoplastic olefins (f-TPO). The advantages relative to polyvinyl chloride are low temperature ductility, weatherability, higher temperature service, and comparable cost per volume. The family of most melt-blended f-TPO products, however, tends to have a lower melt strength for high temperature processing, e.g., high speed sheet extrusion, calendering, thermoforming, blow molding, and foaming.
A polymer blend that includes an irradiated partially crystalline polyolefin with high melt strength and a non-irradiated polyolefin is disclosed in U.S. Pat. No. 5,508,318. This composition exhibits many desirable characteristics for extruded thin sheets, but has the disadvantage of higher cost due to the electron beam irradiation process and the subsequent number of melt blending steps required to achieve the desired material by incorporation of other raw materials and ingredients.
One family of thermoplastic polymer alloy compositions can be prepared from blends of polypropylene, ethylene copolymer ionomer resin, ethylene glycidyl acrylate or methacrylate copolymer, and uncrosslinked ethylene propylene rubber, such as are disclosed in U.S. Pat. No. 5,206,294. The reaction of the epoxide group with the acrylic acid group creates a partially crosslinked network that results in a material with improved melt strength and desirable physical properties. A product similar to this is available commercially as DEXFLEX® E250 from Solvay Engineered Polymers of Auburn Hills, Mich. This technology tends to be more expensive due to the specialty ethylene-based copolymers that are produced with a high pressure reaction process. Also, these materials tend to exhibit an undesirable high surface gloss when extruded in sheets, which gloss requires additional processing to be removed.
Thermoplastic elastomers called dynamically vulcanized alloys (DVAs) can be prepared through the process of dynamic vulcanization, such as that described in U.S. Pat. Nos. 3,758,643 and 3,806,558. Using this process, an elastomer can be crosslinked during melt mixing with a rigid thermoplastic polyolefin to yield a material that is melt processable, yet exhibits characteristics similar to thermoset elastomers. Compositions obtained with this process are micro-gel dispersions of cured elastomer in an uncured matrix of thermoplastic polymer. Commercial olefinic thermoplastic elastomer materials that use this technology of dynamic vulcanization are well known and are disclosed in U.S. Pat. Nos. 4,130,535 and 4,311,628. The materials disclosed in these patents are commercially known as SANTOPRENE® and utilize a phenolic resin to crosslink the olefin elastomer phase. The SANTOPRENE® materials are melt processable and can be extruded into profiles such as sheets. They also tend to exhibit high melt strength, but have very little ductility and draw, which reduces the utility of the material technology for processing applications such as thermoforming, blow molding, and foaming.
The use of organic peroxide to crosslink the elastomer phase in an olefinic-based DVA is well known to those of ordinary skill in the art. For example, U.S. Pat. No. 3,758,643 discloses that peroxide 2,5-bis(t-butylperoxy)-2,5-dimethylhexane at a concentration of 0.05 to 0.4 weight percent is useful for crosslinking the elastomer phase in the olefinic DVA. The use of peroxide alone, however, can be detrimental to the high molecular weight polypropylene due to the beta-scission that occurs and results in a very low molecular weight for the thermoplastic phase. The consequences of this degradation include lower melt strength and poor solid-state mechanical properties.
U.S. Pat. No. 4,454,092 discloses a process for the single-step manufacture of an olefinic-based DVA in which the elastomer is crosslinked with organic peroxide at a concentration of 0.3 weight percent. To minimize the adverse consequences of organic peroxide upon the thermoplastic polypropylene, the free radical crosslinking aid, divinyl benzene, is used as a co-agent at a concentration of 0.5 weight percent. The relatively high organic peroxide content disclosed here tends to cause significant chain scission of the polypropylene, thereby leading to lower viscosity (or higher melt flow rate) and a resulting loss in melt strength properties.
International Patent application No. WO 98/32795 discloses that a thermoplastic elastomer can be prepared from a blend of ethylene-octene elastomer and polypropylene when rheologically modified with organic peroxide at a concentration of 0.15 to 1 weight percent. These materials exhibit improved melt strength and contain less than 10 weight percent of non-extractable gel content as measured with a 12-hour boiling reflux extraction with xylene. The absence of significant gel formation shows that the material has been modified without any crosslinking of the elastomer to improve the melt strength. The use of peroxide at this high concentration, however, has been found to cause detrimental deterioration of the molecular weight of the polypropylenic polymer.
U.S. Pat. No. 5,569,717 and Graebling et al.,
Journal of Applied Polymer Science,
Vol 66, pp. 809-819, 1997, disclose that a multifunctional co-agent, or monomer, can be used to modify the rheology of polypropylene-containing materials via peroxide initiation. The preferred compositions contain 10 to 25 weight percent polyethylene with a density greater than 0.92 g/cm
3
, more than 0.5 weight percent of trimethylolpropane t

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