Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymers from only ethylenic monomers or processes of...
Reexamination Certificate
1999-11-10
2001-01-09
Wu, David W. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Polymers from only ethylenic monomers or processes of...
C525S240000
Reexamination Certificate
active
06172172
ABSTRACT:
The present invention relates to polyolefin compositions which are particularly suitable for use in the preparation of blown films with improved mechanical properties. In particular, the present invention relates to a polyolefin composition obtained by mixing:
(A) a heterogeneous polyolefin composition itself comprising a fraction (A1) which is insoluble in xylene and a fraction (A2) which is soluble in xylene;
(B) a polyolefin composition comprising a low density ethylene copolymer (B1) and a propylene copolymer (B2) of relatively high insolubility in xylene; and, optionally,
(C) a branched propylene polymer.
The blown films sector constitutes an area of everincreasing importance in the field of polyolefin films. The reason for this is that the films obtained by blowing have a tubular shape which makes them particularly advantageous in the production of bags for a wide variety of uses (bags for urban refuse, bags used in the storage of industrial materials, for frozen foods, carrier bags, etc.) as the tubular structure enables the number of welding joints required for formation of the bag to be reduced when compared with the use of flat films, with consequent simplification of the process. Moreover, the versatility of the blown-film technique makes it possible, simply by varying the air-insufflation parameters, to obtain tubular films of various sizes.
The polyolefin material mainly used in the production of blown films is polyethylene, understood as LDPE, LLDPE or mixtures thereof, since they are endowed with properties in the molten state which enable films to be obtained with a high level of production efficiency, and in a very wide thickness range, without compromising the stability of the bubble.
In contrast with these excellent properties of processability, polyethylene-based films have mechanical properties that are not as good, thereby making it necessary in certain particular applications such as, for example, bags used to store industrial materials, to use quite thick (180-200&mgr;) tubular films with a consequent increase in costs. This is because lower thicknesses would not allow the bags to be filled with materials at temperatures above room temperature without undergoing deformations which would compromise the stability of stacks made of bags placed one on top of another. The same problem of stability may be encountered in storage during summer, when room temperatures may be high enough to lead to deformation of the bags.
The use in these applications of blown films obtained from polypropylene-based polymers, on the other hand, is particularly difficult given the poor processability properties of polypropylene which give rise to frequent tearing of the bubble or, in any case, to excessive orientation of the film, resulting in an impact strength which is so low as to render it unusable in certain applications.
It is therefore seen that there is a need for a polyolefin material which at the same time is of good processability on blown-film production lines and is capable of providing films with mechanical properties that are such as to allow a reduction of thickness when compared with polyethylene-based films.
It has now been found, surprisingly, that polyolefin compositions obtained by mixing:
(A) a polyolefin composition having:
(A1) a fraction which is insoluble in xylene at room temperature, comprising a propylene polymer, and
(A2) a fraction which is soluble in xylene at room temperature, comprising an ethylene copolymer; and
(B) a polyolefin composition comprising:
(B1) an LLDPE; and
(B2) a propylene copolymer of defined insolubility characteristics in xylene; and, optionally
(C) a propylene polymer with a branching number of less than 1, provide blown films with improved mechanical properties when compared with polyethylene-based films, the films provided conserving, at the same time, good processability.
A subject of the present invention thus consists of a polyolefin composition obtained by mixing: from 50 to 95% by weight of a heterogeneous polyolefin composition (A) having:
(A1) from 20 to 90 parts by weight of a fraction which is insoluble in xylene at room temperature, comprising a propylene homopolymer and/or a copolymer of propylene with ethylene and/or another &agr;-olefin CH
2
═CHR
I
, where R
I
is an alkyl radical having 2-10 carbon atoms, this fraction containing more than 85% by weight of units derived from propylene; and
(A2) from 5 to 80 parts by weight of a fraction which is soluble in xylene at room temperature, comprising an amorphous copolymer of ethylene with propylene and/or another &agr;-olefin CH
2
═CHR
I
, where R
I
has the meaning described above, this fraction optionally containing smaller proportions of a diene and containing more than 15% by weight of units derived from ethylene;
from 5 to 50% by weight of a polyolefin composition (B) comprising:
(B1) from 80 to 110 parts by weight of a copolymer of ethylene with one or more &agr;-olefins CH
2
═CHR, where R is an alkyl radical having from 1 to 10 carbon atoms, the said ethylene copolymer having a density of between 0.88 and 0.945 g/cm
3
, a melt index of from 0.1 to 10 g/10′ and containing up to 20 mol % of &agr;-olefin CH
2
═CHR;
(B2) from 5 to 30 parts by weight of a copolymer of propylene with ethylene and/or one or more &agr;-olefins CH
2
═CHR
I
, where R
I
is a hydrocarbon radical having from 2 to 10 carbon atoms, the said propylene copolymer containing from 60 to 98% by weight of propylene and having an insolubility in xylene of greater than 70%; and, optionally from 1 to 30% by weight of a component (C) comprising a propylene polymer with a branching number of less than 1.
The heterogeneous polyolefin composition (A) may conveniently be prepared by sequential polymerization, working in at least two stages; in a first stage, a crystalline propylene homopolymer and/or copolymer with an isotactic index of greater than 85 is prepared, and, in a second stage, mixtures of ethylene, propylene and/or an &agr;-olefin CH
2
═CHR
I
are polymerized to give a predominantly amorphous copolymer. The composition (A) is preferably present in amounts ranging from 60 to 90%, more preferably from 75 to 85%, by weight relative to the total of (A)+(B).
The fraction (A1) which is insoluble in xylene preferably comprises a propylene homopolymer and/or a copolymer of propylene, preferably with ethylene and/or another &agr;-olefin CH
2
═CHR
I
, where R
I
has the meaning described above, this fraction containing more than 90% by weight of units derived from propylene. The amount of fraction (A1) is preferably between 30 and 85 parts by weight, more preferably between 45 and 70 parts by weight.
The fraction (A2) which is soluble in xylene preferably comprises a copolymer of ethylene with propylene, this fraction optionally containing smaller proportions of a diene and containing more than 20% by weight of units derived from ethylene and preferably having an intrinsic viscosity of between 1.5 and 4 dl/g. The amount of fraction (A2) which is soluble in xylene is preferably between 10 and 70, more preferably between 10 and 40, parts by weight.
Examples of heterogeneous polyolefin compositions and methods for preparing them are described in U.S. Pat. No. 4,521,566, EP-A-400,333 and EP-A-472,946, the description of which is incorporated herein by way of reference.
The copolymer (B1) is preferably present in amounts of between 80 and 95 parts by weight and has a density preferably of between 0.89 and 0.94 g/cm
3
. These values are more preferably between 0.90 and 0.935.
The melt index (determined according to ASTM method D-1238, condition E) of the copolymer (B1) has values generally of between 0.1 and 10 g/10 minutes, preferably of between 0.2 and 3 g/10 minutes, more preferably of between 0.2 and 1 g/10 minutes.
The &agr;-olefin CH
2
═CHR may be chosen, for example, from propylene, 1-butene, 1-hexene, 1-octene and 4-methyl-1-pentene; 1-butene or 1-hexene is preferably used. In the preparation of the component (B1), the olefins CH
2
═CHR may als
Burgin Emanuele
Cometto Claudio
Perdomi Gianni
Bryan Cave LLP
Montell Technology Company BV
Rabago R.
Wu David W.
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