Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Patent
1987-10-06
1989-08-29
Seccuro, Carman J.
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
525240, 525197, C08L 2326, C08L 2318, C08L 2310, C08L 2304
Patent
active
048618349
DESCRIPTION:
BRIEF SUMMARY
It is known to polymerize ethylene under very high pressure and at high temperature in the presence of a free-radical initiator. In this manner a polymer is obtained which is known by the name of free-radical low density polyethylene. Regardless of its melt flow index, this polymer generally exhibits acceptable stretchability during the preparation of tubular web and mediocre tear strength. On the other hand, it enables good stability of the bubble to be obtained during extrusion blowing and it possesses excellent processability; the processability is defined as, on the one hand, the absence of premature fracture of the extrudate ("melt fracture"), and on the other hand, the power consumed during the extrusion.
Substantial improvements to the stretchability and the tear strength have been brought about by preparing copolymers of ethylene and at least one .alpha.-olefin having from 3 to 12 carbon atoms, such as propylene and 1-butene, in the presence of a catalytic system of the Ziegler type. However, during their extrusion blowing, there is found to be some instability of the bubble, a spontaneous tendency for the holes to enlarge in cases where the bubble perforates, and a tendency to premature fracture of the extrudate.
Moreover, modified copolymers of ethylene and at least one .alpha.-olefin having from 3 to 12 carbon atoms, comprising from 0.5 to 10 mol% of units derived from the said .alpha.-olefin, are known from the document FR-A-2,519,007, these copolymers being characterized in that their measured limiting viscosity is between 1.5 and 10 times their limiting viscosity calculated from the distribution of the molecular weights. A process for preparing these modified copolymers consists in bringing the copolymer into contact with 0.01 to 1 millimole per kilogram of copolymer of at least one free-radical initiator at a temperature of 220.degree. to 320.degree. C. for a period of 5 to 200 seconds, in a processing machine such as an extruder.
The modified copolymers obtained by this process are completely soluble and extractable by the usual solvents for polyolefins, such as, in particular, trichlorobenzene and xylene. However, it has been found that the films prepared from these modified copolymers by extrusion blowing have slight defects which can only be detected by very sophisticated analytical means. Thus, study in polarized light shows up some surface defects due to the presence of gelled microparticles which it was impossible to detect by a simple test of solubility and extraction by the usual solvents. It has been found, moreover, that the use of amounts of free-radical initiators greater than 1 millimole per kilogram of copolymer leads to the presence of gels (that is to say, particles which are not extractable by the usual solvents), in an amount such that the modified copolymers obtained are either impossible to make use of by the technique of extrusion blowing or capable of being processed to films which possess defects which are incompatible with the use to which the films are put.
It has now been found possible to obtain modified polymers with improved properties, leading, in particular, to films not showing surface defects after study in polarized light, by bringing the polymer and the free-radical initiator into contact under the conditions of the present invention.
The subject of the present invention is compositions containing:
(a) from 90 to 99.8% by weight of at least one modified polymer of ethylene and, where appropriate, at least one .alpha.-olefin having from 3 to 12 carbon atoms, the said polymer containing at least 80 mol% of units derived from ethylene, and
(b) from 0.2 to 10% by weight of at least one polymer phase comprising at least one polymer containing at least 90 mol% of units derived from at least one .alpha.-olefin having from 3 to 12 carbon atoms, and at most 10 mol% of units derived from ethylene, the said compositions having a measured limiting viscosity of between 1.5 and 100 times their limiting viscosity calculated from the distribution of the molecular weights
REFERENCES:
patent: 4460750 (1984-07-01), Thiersoult et al.
patent: 4486579 (1984-12-01), Machon et al.
patent: 4535125 (1985-08-01), McCullough, Jr.
patent: 4612155 (1986-09-01), Wong et al.
patent: 4678834 (1987-07-01), Bovin et al.
Audureau Joel
Piecuch Edouard
Seccuro Carman J.
Societe Chimique Des Charbonnages S.A.
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