Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Cellular products or processes of preparing a cellular...
Patent
1997-04-10
1998-04-28
Foelak, Morton
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Cellular products or processes of preparing a cellular...
521 56, 521 58, 521 59, 521 79, 521146, C08J 918, C08J 922, C08J 936
Patent
active
057445052
DESCRIPTION:
BRIEF SUMMARY
The invention relates to prefoamed polyolefin beads produced by extrusion followed by granulation.
Prefoamed polyolefin beads, in particular those based on a copolymer of propylene with small amounts of ethylene, are increasingly employed for the production of foam moldings in automotive construction, packaging, and in the leisure sector. The foam beads are usually produced by suspending fine polyolefin granules in water, impregnating them with a volatile blowing agent at superatmospheric pressure and at temperatures near to their softening temperature, releasing the pressure and thus foaming the polyolefin (see EP-A 53333). The polyolefin in the foam beads produced in this way has a specific crystal structure which, in a DSC curve, shows a double peak: a peak typical of the polyolefin and a high-temperature peak.
EP-A 588 321 describes a particularly elegant and simple process for the production of prefoamed polypropylene beads, in which polypropylene is melted in an extruder, and a blowing agent is injected into this melt and homogeneously incorporated at from 125.degree. to 250.degree. C. The homogeneous material is extruded through a round-section die, during which it expands, and is chipped to give foam granules. The polypropylene which is employed should have a melt strength of from 5 to 40 cN and a branched structure. However, it has been found that foam beads produced in this way have a very narrow processing range for production of moldings and do not fuse adequately with one another. For this reason, the moldings have relatively poor mechanical properties. In particular, the elongation at break and the tensile strength are unsatisfactory.
It is an object of the present invention to provide, starting from conventional, ie. not melt-strengthened polyolefin, by extrusion and subsequent granulation, prefoamed polyolefin beads which can be fused without difficulty to give moldings with good mechanical properties.
We have found that this object is achieved if the polyolefin in the foam beads has a crystal structure with a double peak in which the distance between the two peaks is at least 5.degree. C., preferably greater than 8.degree. C.
For the purposes of the present invention, polyolefins are crystalline olefin polymers whose X-ray crystallinity at 25.degree. C. is greater than 25%. Suitable polyolefins for the process are low-, medium- and high-density polyethylenes having a density of from 0.88 to 0.965 g/cm.sup.3, polypropylene, and ethylene and propylene copolymers comprising at least 50 mol % of ethylene or propylene units respectively. Suitable comonomers are, for example, .alpha.-alkenes with up to 12 carbon atoms, such as ethylene, propylene, butene, pentene, hexene or octene, or vinyl esters, such as vinyl acetate, or acrylates, methacrylates, maleates or fumarates of alcohols containing from 1 to 8 carbon atoms, copolymers of ethylene and acrylic acid or its derivatives, and ionomers. Mixtures of different olefin polymers can also be used.
Preference is given to ethylene-propylene copolymers and copolymers of ethylene and/or propylene with a C.sub.4 - to C.sub.8 -.alpha.-olefin.
Particularly preferred propylene copolymers are copolymers of propylene with from 0.5 to 6% by weight of ethylene or from 0.5 to 15% by weight of 1-butene. Particularly preferred ethylene copolymers are copolymers of ethylene with from 1 to 18% by weight of 1-butene, 1-hexene or 1-octene.
Particular preference is also given to copolymers of propylene with from 0.5 to 6% by weight, preferably from 1.5 to 6% by weight, of ethylene and from 0.5 to 6% by weight, preferably from 1.5 to 6% by weight of a C.sub.4 - to C.sub.8 -.alpha.-olefin, such as 1-butene, 1-pentene, 1-hexene, 1-heptene or 1-octene. The copolymers should have a torsion modulus of from 100 to 900, preferably from 150 to 750 N/mm.sup.2.
The copolymers are prepared in a known manner by copolymerization of the monomers using transition-metal catalysts. The copolymers are, in general, of linear construction, not crosslinked and not melt-strengthened. The
de Grave Isidoor
Dietzen Franz-Josef
Ehrmann Gerd
Fischer Joachim
Rieger Jens
BASF - Aktiengesellschaft
Foelak Morton
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