Polyolefin compositions and blown films obtained therefrom

Details Polyolefin compositions and blown films obtained therefrom Polyolefin compositions and blown films obtained therefrom

1998-08-27

2001-04-03

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...

C525S240000, C525S323000

Reexamination Certificate

active

06211295

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 polyolefin compositions comprising: (A) a heterogeneous polyolefin composition itself comprising a fraction (A1) which is insoluble in xylene and a fraction (A2) which is soluble in xylene, and (B) a branched propylene polymer
The present invention moreover relates to a process for the preparation of blown films which have improved mechanical properties, this process comprising the use of the said compositions.
A further aspect of the present invention moreover relates to the blown films obtained from the said compositions.
The blown films sector constitutes an area of ever-increasing 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, therefore avoiding having to trim the films down to the appropriate size as is necessary in the technique of extrusion through a flat head.
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.
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 comprising:
(A) a polyolefin composition having:
(A1) a fraction which is insoluble in xylene at room temperature, comprising a substantially linear propylene (co)polymer, and
(A2) a fraction which is soluble in xylene at room temperature, comprising a substantially linear ethylene copolymer; and
(B) a propylene polymer with a branching number of less than 1, are capable of satisfying this requirement.
The said compositions are characterized in that they comprise:
(A) from 50 to 99% of a polyolefin composition having:
(A1) 35-80 parts by weight of a fraction which is insoluble in xylene at room temperature, comprising a substantially linear propylene homopolymer and/or a substantially linear 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) 20-65 parts by weight of a fraction which is soluble in xylene at room temperature, comprising a substantially linear 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; and
(B) from 1 to 50% by weight of 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 70 to 99%, more preferably from 80 to 95%, by weight relative to the total weight of (A)+(B).
The fraction (A1) which is insoluble in xylene preferably comprises a substantially linear propylene homopolymer and/or a substantially linear copolymer of propylene, preferably with ethylene, containing more than 90% by weight of units derived from propylene. The amount of the fraction (A1) is preferably between 40 and 75 parts by weight, more preferably between 50 and 70 parts by weight.
The fraction (A2) which is soluble in xylene preferably comprises a copolymer of ethylene with propylene, optionally containing smaller proportions of a diene, 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 25 and 60, more preferably between 30 and 50, 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.
A particularly preferred aspect of the invention consists of polyolefin compositions comprising (A) and (B) as described above in which the composition (A) is obtained by mixing:
(A′) a first heterogeneous polyolefin composition having:
(A′1) 75-95% by weight of a fraction which is insoluble in xylene at room temperature, comprising a substantially linear propylene homopolymer or a substantially linear copolymer of propylene with ethylene and/or another &agr;-olefin CH
2
=CHR
I
, where R
I
is an alkyl radical having from 2 to 10 carbon atoms, this fraction containing more than 85% by weight of units derived from propylene; and
(A′2) from 5 to 25% by weight of a fraction which is soluble in xylene at room temperature, comprising a substantially linear 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; and
(A″) a second heterogeneous polyolefin composition comprising:
(A″1) from 30 to 75% by weight of a fraction which is insoluble in xylene at room temperature, comprising a substantially linear propylene homopolymer or a substantially linear 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
(A″2) from 25

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