Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymers from only ethylenic monomers or processes of...
Patent
1994-10-07
1998-06-30
Wu, David W.
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Polymers from only ethylenic monomers or processes of...
5261242, C08F 1002, C08F11002
Patent
active
057735476
DESCRIPTION:
BRIEF SUMMARY
The invention relates to an ethylene polymer having an intrinsic viscosity of at least 4 dl/g and a method for the preparation thereof. An ethylene polymer of this type is disclosed in WO 87/03288. The ethylene polymer described in this publication can be stretched at temperatures below its melting point. To this end, the polymer is compressed to form a film, which can be stretched to at least 15 times its size at temperatures of 60.degree.-145.degree. C. The stretched articles have a tensile strength of greater than 15 g/den (1.3 GPa) and a modulus of elasticity of greater than 500 g/den (45 GPa). The ethylene polymer is prepared through polymerisation of ethylene in the presence of a vanadium-containing catalyst at temperatures of, for example, -40.degree. C. to 20.degree. C. and pressures of less than 0.2 MPa absolute pressure. The polymerisation is carried out in the presence of a homogeneous vanadium catalyst or a vanadium catalyst applied to a support.
If the ethylene polymers according to WO 87/03288 are prepared using a homogeneous vanadium catalyst, the shape of the polymer obtained is irregular. The polymer has the macroscopic form of fragments and threads having dimensions of the order of one centimeter. Ethylene polymer in such a form is not or barely processable on an industrial scale.
If the ethylene polymers according to WO 87/03288 are prepared using a heterogeneous vanadium catalyst, such as a vanadium catalyst on a silica support (see the examples in WO 87/03288), the amount of catalyst residues in the polymer is high. In the examples this is 56-2310 ppm of vanadium. Such high contents of catalyst residues are undesirable. The amount of catalyst residues is calculated in a known manner from the polymer yield and the amount of active catalyst component (vanadium or titanium) which is supplied to the reactor.
According to WO 87/03288, comparative experiment F, titanium-containing catalysts having a high catalytic activity give a polymer which is not readily stretchable. The maximum degree of stretching is 5 at a stretching temperature of 100.degree. C.
According to the invention there is provided an ethylene polymer which has a maximum draw ratio of at least 20, is pulverulent, has a bulk density of at most 300 kg/m.sup.3 and has an amount of catalyst residues of less than 50 ppm.
It has been found that some catalysts, under specific conditions during polymerisation, lead to a highly stretchable ethylene polymer, specifically when the polymerisation of ethylene or of a mixture of ethylene and not more than 5 mol% higher olefins is carried out at a temperature of at most 80.degree. C. and an ethylene pressure of at most 0.2 MPa in the presence of a transition metal catalyst having a specific surface area of at most 150 m.sup.2 /g. Preferably, the specific surface area is at most 50 m.sup.2 /g and more particularly at most 10 m.sup.2 /g. The specific surface area is determined with the aid of ASTM standard D 3663-78. Pressures are indicated as absolute pressure in Pa.
The catalysts which are used according to the invention are transition metal catalysts known under the name Ziegler catalysts. Catalysts of this type can be prepared in a known manner. The following may be mentioned as transition metals which can occur in the Ziegler catalysts: vanadium, titanium, zirconium, hafnium and chromium. Preferably, the catalyst according to the invention is a titanium-containing catalyst having a high catalytic activity. Preferably, the catalysts which are used according to the invention have a yield of more than 800 g of polyethylene/g of catalyst under polymerisation conditions such that the ethylene pressure is 0.07 MPa and the polymerisation temperature 60.degree. C. Preferably, the catalysts are applied to a support which contains magnesium, more specifically magnesiumchloride. Preferably, the Mg/Ti molar ratio in such a catalyst is at least 5 and in particular at least 10. The following may be mentioned as examples of very suitable catalysts: UM-1.sup.R from Toho Titanium and Lynx 715.sup.R from
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Nooyen Godefridus A. H.
Oostra Hendrikus
DSM N.V.
Wu David W.
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