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
1999-12-10
2001-11-27
Wu, David W. (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...
C525S191000, C525S232000, C525S236000, C525S240000, C526S113000, C526S114000, C526S118000, C526S119000, C526S339000, C526S943000
Reexamination Certificate
active
06323284
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a thermoplastic composition which is a mixture of crystalline and amorphous polyolefin copolymers. This invention further relates to a process to produce such thermoplastic compositions by copolymerizing alpha-olefins and &agr;,&ohgr;-dienes using two separate catalyst systems.
BACKGROUND INFORMATION
It is well recognized that amorphous polypropylene (aPP), even at a very high molecular weight (e.g. M
w
>1000,000 g/cc) is a soft, rubbery, gel-like material which possesses very low crystallinity and therefore poor physical properties. Because of its poor mechanical strength, this material has found few practical uses compared to isotactic polypropylene (iPP), which has crystallinity and therefore better mechanical properties.
Individual polyolefins having certain characteristics are often blended together in the hopes of combining the positive attributes of the components. Typically, however, the result is a blend which displays a weighted average of the individual properties of the individual resins.
For example EP 0 527 589 discloses blends of flexible low molecular weight amorphous polypropylene with higher molecular weight isotactic polypropylene to obtain compositions with balanced mechanical strength and flexibility. These compositions show better flexibility compared to the isotactic polypropylene alone, but the elastic recovery properties are still poor.
U.S. Pat. No. 5,539,056 discloses polyolefin compositions comprising a blend of amorphous poly-alpha-olefin having a weight average molecular weight (M
w
) of at least about 150,000 and a crystalline poly-alpha-olefin having an M
w
of less than about 300,000 and less than that of the amorphous poly-alpha-olefin. These compositions were produced by polymerizing alpha-olefin in the presence of two different cyclopentadienyl transition metal compounds or by producing the polymers independently and subsequently blending them together.
EP 0 366 411 discloses a graft polymer having an EPDM backbone with polypropylene grafted thereto at one or more of the diene monomer sites through the use of a two-step process using a different Ziegler-Natta catalyst system in each step. This graft polymer is stated to be useful for improving the impact properties in blended polypropylene compositions.
Although each of the polymers described in the above references has new and interesting properties, there remains a need for new compositions offering other new and different balances of mechanical properties controllably tailored for a variety of end uses. It would be desirable to find a composition that is very strong yet having both good flexibility and elasticity characteristics. It would further be desirable to produce such a composition with a minimum of processing steps.
SUMMARY OF THE INVENTION
The present invention is broadly directed to a polyolefin polymer composition produced by copolymerizing one or more C
3
or higher alpha-olefins and one or more di-vinyl monomers in the presence of at least one stereospecific metallocene catalyst system and at least one non-stereospecific metallocene catalyst system in the same polymerization medium. The polymer composition so produced contains amorphous polymer segments and crystalline polymer segments in which at least some of the segments are crosslinked. Both the amorphous and the crystalline polymer segments are copolymers of one or more alpha-olefins and one or more monomers having at least two olefinically unsaturated bonds. Both of these unsaturated bonds are suitable for and readily incorporated into a growing polymer chain by coordination polymerization using either the stereospecific or the non-stereospecific catalysts independently such that the di-olefin is incorporated into polymer segments produced by both catalysts in the mixed catalyst system according to this invention. In a preferred embodiment these monomers having at least two olefinically unsaturated bonds are di-olefins, preferably di-vinyl monomers. Crosslinking of at least a portion of the mixture of polymer segments is accomplished during the polymerization of the composition by incorporation of a portion of the di-vinyl comonomers into two polymer segments. At least a portion of the di-vinyl monomers are polymerized into two polymer segments, thus producing a crosslink between those segments.
DETAILED DESCRIPTION OF THE INVENTION
In a preferred embodiment, the polyolefin polymer composition of this invention is a blend of polymeric segments which are crosslinked or in which at least a portion of the segments are joined chemically through carbon-to-carbon bonds. This blend includes amorphous polymer segments and crystalline polymer segments. In a preferred embodiment, the amorphous polymer segments and the crystalline polymer segments are each copolymers of one or more alpha-olefins and one or more di-olefin monomers in which both of the double bonds can be copolymerized with the one or more alpha-olefins using both the stereospecific or the non-stereospecific catalyst simultaneously.
The polymerization of both the amorphous and the crystalline polymer segments is performed in a single step. In other words, both polymerizations are carried out simultaneously with both catalysts present in the same reaction medium. Both the type and amount of the diolefin monomer are selected to produce a sufficient amount of crosslinking of polymer segments to produce the desired physical properties in the final composition. Crosslinking, for purposes of this invention, refers to the connection of two polymer segments by incorporation of each double bond of a diolefin monomer into two different polymer segments. The polymer segments so connected can be the same or different, with respect to their crystallinity. Three or more polymer segments may also be connected via incorporation of two or more diolefins in on polymer segment into two other polymer segments.
In a particularly preferred embodiment, the product produced is a blend of isotactic polypropylene segments and atactic polypropylene segments with sufficient crosslinking via diene incorporation into both types of segments to produce an improved balance of properties in the bulk composition. Polymer or polypropylene segments, as used herein, are intended to refer to copolymers containing the selected diolefin monomers as a minor constituent. The crosslinked final composition contains a mixture of linkage types via incorporation of single diolefin monomers into two separate polymer segment. These linkage types include connections between two amorphous copolymer segments, connections between two said crystalline copolymer segments, and connections between amorphous copolymer segments and crystalline copolymer segments. The presence of these crosslinked structures, produced by diene incorporation into the growing segments of the crystalline/amorphous polymer blend result in new and different physical properties versus those found in the prior art.
Monomers
A primary consideration for selection of the monomer, or combinations of monomers, is that, both crystalline and amorphous polymer segments can be formed with the proper selection of two or more different metallocene catalyst systems. It is further necessary that the level of incorporation of the diolefin monomer into the crystalline segments be limited to an amount that will not substantially alter its crystallinity. Yet another reason to limit the addition of diolefin monomer is to limit the level of crosslinking to a level such that the overall composition remains a thermoplastic.
The &agr;-olefins include linear, branched, or ring-containing C
3
to C
30
prochiral &agr;-olefins or combinations thereof capable of being polymerized by both the stereospecific and the non-stereospecific catalysts selected. Prochiral, as used herein, refers to monomers that favor the formation of isotactic or syndiotactic polymer when polymerized using the selected stereospecific catalyst(s). Some embodiments select the &agr;-olefins from C
3
to C
20
alpha olefins.
Preferred linear &agr;-olef
ExxonMobil Chemical Patents Inc.
Rabago R.
Reid Frank E
Runyan Charles E
Wu David W.
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