Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
1999-12-14
2001-07-17
Nutter, Nathan M. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C525S222000, C525S232000, C525S240000
Reexamination Certificate
active
06262177
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to polymeric blends. In particular the invention relates to methods of producing melt-processable multiphase thermoplastic compositions comprising a crystalline polyolefin resin and a cross-linked polyacrylate containing chemically functional groups.
2. Description of Related Art
Blends of plastics and elastomers are well-known in the art. Since the polymer phases of such blends are generally not compatible with each other, the resulting conventional compositions have poor physical properties. Therefore, blends of non-polar plastics and polar elastomers, such as those disclosed in U.S. Pat. Nos. 4,555,546 (granted Nov. 26, 1985 to Monsanto), and 4,782,110 (granted Nov. 1, 1988 to DuPont), normally require incorporation of a compatibilizing agent for obtaining satisfactory physical properties. The addition of such compatibilizing agents, however, adds to the expense of the blend and complicates the manufacturing process. Certain compatibilizing agents and curatives will also contribute colour to the composition.
Furthermore, when the elastomer component of blends of the above kind is uncured, the blend has high compression set and high oil swell.
An additional problem related to the multiphase thermoplastic compositions described in the above patents resides in the fact that the most desirable polar elastomers are only available in the form of large sticky chunks rather than the free-flowing pellets used for feeding conventional compounding extrusion equipment. As a result, the process for manufacturing these multiphase thermoplastic compositions uses expensive batch mixing rubber compounding equipment. Alternatively, an expensive cryogenic grinding process has to be employed to convert the sticky chunks of raw rubber to a fine powder that can be made free-flowing by the addition of a partitioning agent.
In addition to compatibilizers, crosslinking agents (other than peroxides) have also been used in attempts to crosslink the polyacrylate phase by reacting the functionality of an ethylene/acrylate copolymer with a diamine (U.S. Pat. No. 4,782,110). So far these agents appear to be rather ineffective.
In addition to the above-mentioned traditional blends, known in the art are also single free-flowing pellets consisting of melt-processible multiphase thermoplastic compositions made up of intimate homogeneous blends of polyolefins and polyacrylate elastomers. According to the teaching of U.S. Pat. No. 5,300,578, these blends can be produced by polymerizing liquid monomeric units of the acrylate component into solid elastomeric polar polymer within the pellet structure of the polyolefin component. This free radical polymerization process occurs at relatively low temperature and under low mechanical shear conditions to provide an intimate homogeneous mixture of the components in the form of a free-flowing pellet.
Although compositions of the last-mentioned kind represent a clear improvement over the prior art, tensile strength, elongation and, in particular, compression set resistance of these known blends are still unsatisfactory.
SUMMARY OF THE INVENTION
It is an object of the present invention to eliminate the problems associated with the prior art and to provide a novel process for improving the elastomeric properties of polyacrylate/polyolefin blends. In particular, it is an object of the invention to provide compounded blends with regard to excellent elastomeric properties without the use of compatibilizing additives known in the art.
It is another object of the present invention to provide thermoplastic elastomer compositions having excellent oil swell resistance and tensile strength, while at the same time the having adequate elongation, tear resistance, and compression set resistance.
These and other objects, together with the advantages thereof over known thermoplastic elastomer compositions of polyolefin resins and crosslinked polyacrylates and processes for preparing such compositions, which shall become apparent from the specification which follows, are accomplished by the invention as hereinafter described and claimed.
The present invention is based on modification of the crosslink density of the polyacrylate phase during shear compounding of the blend. Pellets of the above-mentioned kind (U.S. Pat. No. 5,300,578) as well as other conventional blends of polyacrylates prepared by radical mechanism and polyolefins contain a source of radicals, in particular of free radicals, which are subjected to termination reactions when the blends are heated and compounded in the melt phase. These reactions will decrease the number of radicals rapidly. The source of radicals is in particular constituted by residual free radicals themselves or by peroxides (substances containing peroxy groups) left in the polyacrylate phase after polymerization. The free radicals can thus be present in the polyacrylate before compounding, or they can be formed from the peroxides as a result of heating during compounding.
According to the present invention, it has now been found that by retarding the termination reactions and, thus, prolonging the life time of the radicals, an increased number of cross-links can be formed within the polyacrylate phase of the blends during compounding. As a result of the increased number of crosslinks, the mechanical properties, in particular the elastomeric properties, of the compounded blends are greatly improved.
Further, by blending crystalline polymers with elastomers and by compounding the blends as described above, permanent interfacial bonds can be generated so as to make the components more compatible with each other.
More specifically, the process according to the invention for improving the elastomeric properties of polyacrylate/polyolefin blends comprises retarding the termination reactions of free radicals from the source of free radicals during compounding in order to promote formation of crosslinks in the polyacrylate phase.
Considerable advantages are achieved by the present invention. Thus, the invention will eliminate the complications associated with handling sticky chunks of uncured elastomer, since the free-flowing feedstock contains the polar elastomer polymerized in situ within the pores of the polyolefin.
The improved properties of the present compounded blends include high tensile strength, elongation, tear resistance, flexibility fatigue resistance, compression set resistance, adequate high and low temperature properties, and high oil resistance. Excellent weatherability and absence of halogenated components are also desirable attributes of the present thermoplastic compositions.
Prior art describing thermoplastic elastomer compositions based on polypropylene continuous phase deliberately avoids the use of peroxide to crosslink the elastomer phase. This is because of the detrimental effect of free radical attack on the polypropylene polymer structure. The present invention requires the use of peroxide (free radical) crosslinking of the elastomeric phase, yet avoids any simultaneous detrimental impact on the polypropylene phase.
From an economic standpoint, it is important that the thermoplastic compositions are easily processible on thermoplastic equipment, and that scrap material of the thermoplastic compositions can be reground and reprocessed with substantially no significant change in the physical properties of the thermoplastic composition.
The invention will be described more closely with the aid of the following detailed description and with reference to a number of non-limiting working examples.
REFERENCES:
patent: 4710544 (1987-12-01), Wolfe, Jr.
patent: 5300578 (1994-04-01), Vestberg et al.
patent: 5889118 (1999-03-01), Delgado et al.
patent: 0 247 580 (1987-12-01), None
patent: 0 312 664 (1989-04-01), None
patent: 0 554 058 (1993-08-01), None
patent: WO 96/12745 (1996-05-01), None
patent: WO 96/20228 (1996-07-01), None
Bergstrom Christer
Heiskanen Tomi
Cohen & Pontani, Lieberman & Pavane
Nutter Nathan M.
Optatech Corporation
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