Multilayer thermoplastic structure

Details Multilayer thermoplastic structure Multilayer thermoplastic structure

2002-07-16

2004-03-30

Nakarani, D. S. (Department: 1773)

Stock material or miscellaneous articles

Structurally defined web or sheet

Including components having same physical characteristic in...

C264S173140, C264S173150, C428S220000, C428S515000, C428S516000, C428S517000, C428S519000, C428S520000, C428S910000

Reexamination Certificate

active

06713165

ABSTRACT:

The present invention relates to a multilayer structure based on a thermoplastic material, such as a tube or a film, which can be used in particular in the manufacture of articles for medical applications.
Articles intended for medical applications must meet not only conventional requirements, such as good mechanical strength or low cost, but also requirements—extremely strict—peculiar to this specific field of application, such as, for example, requirements relating to the biocompatibility properties of the said articles, their ability to be subjected to a sterilization treatment, their flexibility, their transparency, their weldability, their impact strength (including as regards containers filled with liquid) or the amount of extractable substances (for example, with hexane).
Until now, commercially available articles for medical use, for example infusion or blood bags, have been based on vinyl chloride polymers, for example on PVC. Although having several advantages, this type of polymer, however, has certain disadvantages, such as the need to incorporate high amounts of stabilizers in it for the purpose of improving its thermal stability or to incorporate high amounts of plasticizers in it for the purpose of obtaining sufficient flexibility. There is therefore a market demand for articles for medical applications which are devoid of chlorinated polymers.
Polyolefin-based articles of this type have already been provided. Thus, for example, International Patent Application WO 97/34951 (Sengewald) discloses multilayer polyolefin-based films which can be used in particular in medical applications.
Known copolymer-based articles exhibit, however, the frequent disadvantage of comprising a significant amount of copolymer(s) with a high content of comonomer, which certainly increases their flexibility but reduces their melting temperature, which is a disadvantage with regard to the dimensional stability of the articles during steam sterilization (121° C.), and often results in a high content of extractable constituents. In addition, the transparency of these articles is often mediocre because of the rapid crystallization of the polyolefins generally used.
Furthermore, the known articles based on propylene homopolymers often comprise polymers of low isotacticity, which certainly makes it possible to obtain good flexibility but results in a high content of extractable constituents. In addition, they often exhibit a high polydispersity, which means that their melting temperature ranges and crystallization temperature ranges widely overlap, which makes it impossible to effectively reduce the crystallinity and to obtain good transparency.
For the purpose of overcoming these disadvantages, the present invention relates to a multilayer structure based on thermoplastic polymers which is substantially devoid of vinyl chloride polymers and of plasticizers of low molecular mass and which comprises at least 3 layers:
a first layer (A) comprising at least 60% by weight of at least one polyolefin of controllable crystallinity defined as comprising at least 90% of ethylene, of propylene or of butene and as having a softening temperature of less than 121° C., the thickness of the first layer being at least 20% of the total thickness of the structure and the first layer having a modulus of elasticity of less than 350 MPa;
a second layer (B), positioned between the first layer (A) and the third layer (C), comprising at least 40% by weight of at least one polyolefin of controllable crystallinity, the second layer having an overall softening temperature of less than 121° C.;
a third layer (C) comprising at least 50% by weight of at least one polyolefin of controllable crystallinity, the thickness of the third layer .being from 5 to 30% of the total thickness of the structure and the third layer having a modulus of elasticity which is less than that of the first layer.
The term “multilayer structure” is understood to denote any multilayer semi-finished product and more particularly any film, sheet, tube or container.
An important constituent of the multilayer structure of the invention, present in each of the three layers A, B and C, is a polyolefin of controllable crystallinity. This expression denotes, in the context of the present invention, a polyolefin comprising at least 90% of ethylene, of propylene or of butene which has a softening temperature (a “Vicat point”) of less than 121° C. (in the present application, all the softening temperatures are measured according to ASTM Standard D1525). The softening temperature in question relates to the polyolefin of controllable crystallinity as present in the structure of the invention. In particular, it is possible to use a polyolefin with a softening temperature, indicated by the manufacturer, of greater than 121° C. by choosing the conditions for the manufacture of the structure so that the true softening temperature of the said polyolefin within the said structure is less than 121° C. The polyolefins of this type exhibit the distinguishing feature that their crystallinity can be easily reduced during their processing. The “polyolefins of controllable crystallinity” advantageously exhibit a relaxation time &tgr;
0
of at least 10 s (and preferably of at least 15 s) at the extrusion temperature (T
0
) and in the absence of any drawing stress.
This ready control of the crystallinity is advantageous in the content of the invention, in so far as it makes it possible to confer a reduced crystallinity on these polyolefins, provided that appropriate processing conditions are used, thus increasing the transparency and the flexibility of the whole structure. It is for this reason that the polyolefins are described as “polyolefins of controllable crystallinity”.
Several such polyolefins can be used as a mixture, whether in one or more of the layers A, B and C, the minimum amount indicated hereinabove relating, in this case, to the sum of the amounts of each of these polyolefins. An advantageous alternative form consists in using a mixture of polypropylene and of isobutene in a ratio from 1:3 to 3:1. The polyolefins of controllable crystallinity present in the layers A, B and C can be identical or different.
This polyolefin of controllable crystallinity can be a homopolymer of ethylene, of propylene or of butene. Mention may be made, as non-limiting examples of such homopolymers, of propylene homopolymers (PP) [including syndiotactic PPs (s-PP), stereo-block PPs (s-b-PP) or iso-block PPs (i-b-PP)] or poly(1-butene). The preferred homopolymers are syndiotactic, stereo-block and iso-block polypropylenes and poly(1-butene).
It is preferable for the polyolefin of controllable crystallinity to be a copolymer. It can in particular be a copolymer of several of these three monomers (ethylene, propylene, butene) comprising at least 90% of one of these monomers and less than 10% of one or each of the other two monomers. According to another alternative form, the polyolefin of controllable crystallinity is a copolymer comprising at least 90% by weight of ethylene, of propylene or of butene and less than 10% of one or more other comonomers chosen from the group composed of C
5
to C
10
hydrocarbons, carboxylic acids and their esters and carbon monoxide (examples: 1-pentene, 1-hexene, 1-octene, 4-methyl-1-pentene, vinyl acetate, methyl, ethyl or butyl acrylate, cyclohexene, norbornene, and the like). Mention may be made, as non-limiting examples of such copolymers, of low density polyethylenes (LLDPE, VLDPE) and copolymers of ethylene with other &agr;-olefins (such as 1-butene, 1-hexene, 1-octene, 4-methyl-1-pentene or 1-pentene) or with compounds such as vinyl acetate, acrylic acid or methyl, ethyl or butyl acrylate (provided that these copolymers exhibit a content of comonomer(s) of less than 10%).
It should be noted that, in the present application, in the absence of contrary indications, the term “copolymers” encompasses polymers comprising two or more comonomers.
As regards the copolymers, the amount of the minor comonomer(s) is preferably at least 1.5% by

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