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
1999-06-29
2003-11-04
Wilson, D. R. (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...
Reexamination Certificate
active
06642316
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a blend of at least two thermoplastic components differing in their crystallinities. The blend has a heterogeneous morphology with distinct phase separated zones of different composition and crystallinity present. The resulting blend with the defined morphology shows dramatic improvements in mechanical deformation recoverability when compared to its individual unblended components. The invention also relates to improvements in the flexibility of the blend. Changes in the relative amounts or the crystallinities of the blend components or the morphology of the blend affect the recoverability and the flexibility of the blend.
The inventive blends designed for recoverability contains a dispersed phase of a greater crystallinity and a continuous phase of lesser crystallinity. The sizes of the individual domains of the dispersed phase are very small with the smallest length dimension for the dispersed phase being less than 5 &mgr;m. This phase size of the dispersed phase is maintained during processing even without crosslinking. The inventive blends designed for flexibility have a slightly wider range in morphology as the components of greater and lesser crystallinity can also be co-continuous. The components of the blend in both cases are also compatible to the extent that no compatibilizer needs to be added to attain and retain this fine morphology. Furthermore, this invention describes improving the mechanical deformation recoverability of the aforementioned blends by aging the blends and mechanically orienting the articles formed from these blends.
One of the components is a polymer comprising predominately stereospecific polypropylene, preferably isotactic polypropylene. This is the component with greater crystallinity. A second component is a copolymer of propylene and a C
2
, C
3
-C
20
&agr;-olefin, preferably ethylene. This is the component with lesser crystallinity. In the copolymer the propylene is polymerized substantially stereospecifically. The copolymer has a substantially uniform composition distribution preferably as a result of polymerization with a metallocene catalyst. Most preferably, said second component is an ethylene propylene copolymer, e.g. ethylene propylene semicrystalline elastomer.
BACKGROUND OF THE INVENTION
There is a need in the art for polymeric blends having a stereospecific polypropylene component with good tensile strength while still providing suitable mechanical recoverability (elastic recovery) and flexibility (low flexural modulus). This invention is aimed at improving the aforementioned properties of blends having a stereoregular polypropylene component, especially isotactic polypropylene. This is achieved by blending the stereoregular polypropylene component with a copolymer of propylene and a C
2
, C
3
-C
20
&agr;-olefin. This copolymer is less crystalline than the isotactic polypropylene. In the copolymer the propylene is polymerized substantially stereospecifically. Most preferably, the copolymer is an ethylene propylene copolymer, e.g., ethylene propylene thermoplastic elastomer. The copolymer has a substantially uniform composition distribution preferably as a result of polymerization with a metallocene catalyst Composition distribution is a property of copolymers indicating a statistically significant intermolecular or intramolecular difference in the composition of the polymer. Methods for measuring compositional distribution are described later.
Blends of isotactic polypropylene and ethylene propylene rubber are well known in the prior art. However, the traditional Ziegler-Natta catalysts cannot make ethylene propylene thermoplastic elastomers which simultaneously are uniform in compositional distribution, have substantially stereospecific propylene residues and have less than 35 wt. % ethylene.
U.S. Pat. No. 3,882,197 to Fritz et al. describes blends of stereoregular propylene/alpha-olefin copolymers, stereoregular propylene, and ethylene copolymer rubbers. In U.S. Pat. No. 3,888,949 Chi-Kai Shih, assigned to E I Du Pont, shows the synthesis of blend compositions containing isotactic polypropylene and copolymers of propylene and an alpha-olefin, containing between 6-20 carbon atoms, which have improved elongation and tensile strength over either the copolymer or isotactic polypropylene. Copolymers of propylene and alpha-olefin are described wherein the alpha-olefin is hexene, octene or dodecene. However, the copolymer is made with a heterogeneous titanium catalyst resulting in copolymers with non-uniform composition distribution and a broad molecular weight distribution. Non-uniform intramolecular compositional distribution is evident in U.S. Pat. No. 3,888,949 by the use of the term “block” in the description of the polymer where the copolymer is described as having “sequences of different alpha-olefin content.” Within the context of the invention described above the term sequences describes a number of olefin monomer residues linked together by chemical formed during polymerization.
In U.S. Pat. No. 4,461,872, A.C.L. Su improved on the properties of the blends described in U.S. Pat. No. 3,888,949 by using another heterogeneous catalyst system which is expected to form copolymers which have statistically significant intermolecular and intramolecular compositional differences.
In two successive publications in the journal of
Macromolecules
, 1989, V22, pages 3851-3866, J. W. Collette of E. I. Du Pont has described blends of isotactic polypropylene and partially atactic polypropylene which have desirable tensile elongation properties. However, the partially atactic propylene has a broad molecular weight distribution as shown in
FIG. 8
of the first publication. The partially atactic polypropylene is also composed of several fractions, which differ in the level of tacticity of the propylene units as shown by the differences in the solubility in different solvents. This is shown by the corresponding physical decomposition of the blend which is separated by extraction with different solvents to yield individual components of uniform solubility characteristics as shown in Table IV of the above publications.
More recently several authors have shown the formation of more refined structures of partially atactic, partially isotactic polypropylene which have elastomeric properties. It is believed that in these components each molecule consists of portions which are isotactic and therefore crystallizable while the other portions of the same polypropylene molecule are atactic and therefore amorphous. Examples of these propylene homopolymers containing different levels of isotacticity in different portions of the molecule are described by R. Waymouth in U.S. Pat. No. 5,594,080, in the article in the
Journal American Chemical Society
(1995), Vol. 117, page 11586, and in the article in the
Journal American Chemical Society
(1997), Vol. 119, page 3635, J. Chien in the journal article in the
Journal of the American Chemical Society
(1991), Vol. 113, pages 8569-8570; and S. Collins in the journal article in
Macromolecules
(1995) Vol. 28, pages 3771-3778. These articles describe a specific polymer, but do not describe the blends with a more crystalline polymer such as isotactic polypropylene.
In U.S. Pat. Nos. 3,853,969 and 3,378,606, E. G. Kontos discloses the formation of in situ blends of isotactic polypropylene and “stereo block” copolymers of propylene and another olefin of 2 to 12 carbon atoms, including ethylene and hexene. The copolymers of this invention are necessarily heterogeneous in intermolecular and intramolecular composition distribution. This is demonstrated by the synthesis procedures of these copolymers which involve sequential injection of monomer mixtures of different compositions to synthesize polymeric portions of analogously different compositions. In addition,
FIG. 1
of both patents shows that the “stereo block” character, which is intra or intermolecular compositional differences in the context of the description of the present invention, is essential to the bene
Cozewith Charles
Datta Sudhin
Periagaram Periagaram
Stachowski Edmund J.
ExxonMobil Chemical Patents Inc.
Milbank Mandi B.
Prodnuk Stephen D.
Schmidt Paige
Wilson D. R.
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