Plastic toughened plastics

Details Plastic toughened plastics Plastic toughened plastics

1999-08-13

2001-05-01

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...

Reexamination Certificate

active

06225412

ABSTRACT:

TECHNICAL FIELD
This invention relates generally to polyolefin plastics toughened by other polyolefin plastics. More specifically this invention is directed toward polypropylenes toughened with polyethylenes.
BACKGROUND
Polypropylene is a versatile and widely used thermoplastic. Many of its uses are in applications that take advantage of its excellent physical properties in molded shapes. However, while polypropylene has outstanding stiffness, it fails in a brittle mode on impact, especially at subambient temperatures. Because many applications that could take advantage of the excellent stiffness also require a tougher material, one that will fail in either brittle or brittle ductile mode, softer, more amorphous polymers have traditionally been added to polypropylene to improve polypropylene impact resistance and tensile toughness. Applications such as this include interior trim parts of vehicles, such as trucks and passenger automobiles.
To mitigate the brittle nature of polypropylene, as discussed above, softer, more amorphous polymers are often added to polypropylene, these materials have traditionally included EPR and EPDM. These ethylene &agr;-olefin co or terpolymers are most often ethylene propylene rubbers or ethylene propylene diene monomer rubber (EP or EPDM respectively). These materials are generally characterized by a heat of melting generally below 30 J/g, although ethylene &agr;-olefin copolymers including higher &agr;-olefins such as butene, hexene, octene and the like have been also been used, these materials have low crystallinity, and high extractability, and are generally in the density range 0.86 to 0.900 g/cm3 while conventional EPR or EPDM materials have densities in the range generally at or below 0.86 g/cm3.
More recently, other soft materials have been added to impact modify polypropylene, such as ethylene butene copolymers. In the publication Impact Modification of Polypropylene with Exact™ Plastomers, Society of Plastics Engineers, 1994, the author describes advantageous use of such ethylene butene polymers, with an upper density limit of 0.910 g/cm3. Of note in the data presented in this publication is that stiffness or modulus decreases fairly rapidly with increasing plastomer content. Fabricators of automotive parts often desire to have a stiff part, as well as one that is tough, as exemplified by tensile toughness. Soft or elastomeric or amorphous polymers inevitably tend to soften, or make less rigid, the polypropylene polymers that they make tougher. These softer polymers are often characterized by being totally or substantially extractable or soluble in specific solvents. An additional drawback of isotactic polypropylene/ethylene propylene polymers (iPP/EP) or iPP/EPDM blends is cost. EP or EPDM costs considerably more than polypropylene, raising the cost of the blend.
Therefore, there is an unfilled commercial need for a blend constituent with polypropylene that will maintain or improve tensile toughness especially at low temperature or even enhance the low temperature tensile toughness, while minimizing or eliminating the negative effect on stiffness, as measured by modulus.
SUMMARY
We have discovered that semicrystalline polyethylenes, specifically metallocene catalyst produced polyethylenes (mPE), generally above 0.910 in density and below 3% extractables, as determined by 21 CFR 177.1520 (d) (4) (i) (xylene), when blended with isotactic polypropylene (iPP), offer plastic toughened plastics with a surprising and unexpected balance of stiffness (as measured by modulus) and tensile toughness, both at low ambient temperatures. Traditional approaches to improving tensile toughness, such as adding blend components EP rubber, EPDM rubber, or plastomers to iPP, have improved tensile toughness, but dramatically diminished the stiffness or modulus, to an unacceptable level for many applications. Additionally, the blends of embodiments of our invention show surprising and unexpected improvements (increases) in elongation as well as the tensile toughness and modulus discussed above, as compared to Ziegler-Natta PE/iPP (Z-N PE/iPP) blends, based on what the skilled person would expect. Such a balance of desirable properties has been heretofore unattainable, and is unexpected.
As an isotactic polypropylene (iPP), most currently available iPP will be useable. Those iPPs with a relatively high Melt Flow Rate (MFR) (e.g. 1-200 dg/min) will be preferred. The iPP will be present in the blend in the range of 60-95 weight percent based on the total weight of the blend components. While Ziegler-Natta catalyst produced Z-N iPP may be used, metallocene produced iPP will have an even lower amount of amorphous or extractable material, and when blended with a mPE of low extractables, offers even better physical properties than Z-N iPP in the blends.
As an mPE, those with densities in the range of from 0.910-0.930 g/cm
3
are preferred, more preferred are densities of 0.912-0.930 g/cm
3
, even more preferred are those in the range of from 0.915-0.925 g/cm
3
, most preferred are densities in the range of from 0.915-0.920 g/cm
3
. Additionally, these mPEs will have a molecular weight distribution (MWD) as described by Mw/Mn of ≦4, a composition distribution breadth index (CDBI) of 55-90%; a crystallinity of about 40%, as may be additionally expressed by heat of melting of at least 85 J/g. These mPEs will also have substantially lower extractables than their Z-N PE analogs, especially their gas phase produced Z-N PE analogs. The mPE will be present in the blends in the range of 5-40 weight percent based on the total weight of the blend components.
Tensile toughness of these iPP/mPE blends at −10° C., as measured in MJ/m
2
, will be at least 25% greater, preferably at least 100% greater, more preferably at least 200% greater, most preferably 250% greater, than similarly proportioned iPP/Ziegler-Natta PE blends. Similarly the elongation of iPP/mPE blends will be 10% greater, preferably 20% greater, more preferably 30% greater, most preferably 40% greater than that of a similar iPP/Z-N PE blend. These improvements are realized while generally not substantially degrading the modulus or stiffness of the blends, as compared to similarly proportioned Z-N PE/iPP blends.
While not wishing to be bound by theory, we believe that these unexpected and surprising properties are due to the lower extractables levels of the blends containing one or more metallocene catalyst produced polymers. The extractables levels of the inventive iPP/mPE blends will be less than 2%, preferably less than 1.5%, more preferably less than 1%, even more preferably less than 0.75%, most preferably less than 0.5%, as measured by 21 CFR 177.1520 (d) (4) (i) (in xylene). The CDBI of a polymer is determined using the technique temperature rising elution fractionation (TREF) for isolating individual fractions of a sample of a copolymer. The technique is described in Wild, et al.,
J. Poly. Sci. Phys. Ed.
vol. 20, p 441 (1992) and U.S. Pat. No. 5,008,204, both incorporated herein by reference.


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T.C. Yu,Impact Modification of Polypropylenes with Exact Plastomers, Society of Plastics Engineers, 52ndAnnual Technical Conference, May, 1994, 1-8, Exxon Chemical Company, San Francisco, California, USA.

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