Thin walled elastic polyurethane articles

Details Thin walled elastic polyurethane articles Thin walled elastic polyurethane articles

1999-03-10

2002-05-21

Pyon, Harold (Department: 1772)

Apparel

Hand or arm coverings

Gloves

C428S036800, C428S036900, C524S591000, C604S349000

Reexamination Certificate

active

06389602

ABSTRACT:

This invention relates to thin walled polyurethane elastic articles and to a method of making them.
Many devices for biomedical applications require the use of soft thin walled elastic articles. Examples of such articles are gloves for surgical operations and examinations under clean room conditions, and balloons for catheters and condoms. Natural rubber from latex is currently the material of choice for these types of applications due to its outstanding balance of mechanical properties. Typical properties are high elongation at break (800-900%), low modulus [a modulus at 100% elongation of 0.7-0.9 MPa], acceptable tensile strength (20-35 MPa) and a low degree of creep. Natural rubber does, however, have the drawback that the presence therein of proteins and other undesirable compounds, such as vulcanisation accelerator residues, can lead to human allergic reactions if these compounds are leached from the rubber network during use. Another potential hazard is the formation of nitrosamines which are suspect carcinogens. It would therefore be beneficial to replace natural rubber with a synthetic elastomer for the fabrication of articles for biomedical applications.
Polyurethanes have been used to fabricate thin walled elastic articles, usually by dip coating from organic solvent based solutions. However, these solutions are usually quite viscous, due to the high molecular weight of the polymers, and this often presents problems with processing. The molecular weight of a polyurethane elastomer is a key parameter to performance, particularly in regard to the physical properties of the material which will be poor if a suitably high molecular weight is not attained.
Quite apart from these general problems of using polyurethanes to fabricate thin walled elastic articles, there are further difficulties in using them as a replacement for natural rubber. Whilst, in general, polyurethane films can be made of good tensile strength (e.g. 30 to 60 MPa) and moderate elongation (450-650%), these known materials are much harder than natural rubber and will normally have a significantly higher modulus at 100% extension (hereinafter “S100”), for example of at least 2.2 and often much higher. Attempts to make softer polyurethanes have resulted in a lower modulus, but tensile strength and elongation at break have also been reduced and, most importantly, there has been an unacceptable loss in elasticity. Thus, it has not been possible to date to provide a polyurethane dipped film whose physical properties have been close to those of natural rubber.
European patent application 0741152 describes aqueous polyurethane dispersions based on polyether polyols of low unsaturation or monol content. The use of such polyols having an unsaturation of less than 0.02 meq/g polyol and preferably less than 0.01 meq/g polyol, is described as providing polyurethane dispersions of better properties. For example, substitution of the low unsaturation polyols for conventional polyols is shown to give films (cast on glass plates) having higher tensile strength, higher 100% and 300% moduli, and lower ultimate elongation.
European patent 0781791 also describes the use of low unsaturation copolymer polyols for making polyurethane elastomers. The unsaturation is 0.06 meq KOH/g or less, and the polyols are polyoxyalkylene polyether block copolymers. A principle use of the polyurethanes is for sealants and adhesives.
We have now found that by using certain low unsaturation polyols in a limited range of polyurethane polymers, it is possible to make soft thin-walled elastic polyurethane articles with physical properties very similar to such articles made of natural rubber.
In one aspect, the invention provides a soft, thin-walled elastic article made of a linear polyurethane having physical properties close to those of natural rubber wherein the article is made of a linear polyurethane which comprises an &agr;,&ohgr;-dihydroxy polyol selected from poly(propylene glycol)s, said polyol containing no more than 0.01 milliequivalents unsaturation per gram; an aliphatic diisocyanate; and a chain extender; said polyurethane having a ratio of hard:soft segments from 20:80 to 40:60, a number average molecular weight (Mn) of from 90 to 150 kg/mole and a ratio of average molecular weight (Mw) to Mn of from 1.2 to 2.2; and wherein the said film has an S100 of less than 2.0 MPa, an elongation at break of at least 800% and a tensile strength of above 15 MPa.
In another aspect, the invention provides a method of making a soft, thin walled glove or condom of a linear polyurethane have properties close to those of natural rubber, which comprises dipping a former in an organic solution or aqueous dispersion of a linear polyurethane which comprises &agr;,&ohgr;-dihydroxy polyol selected from poly(propylene glycol)s, said polyol containing no more than 0.01 milliequivalents unsaturation per gram; an aliphatic diisocyanate; and a chain extender; said polyurethane having a ratio of hard:soft segments from 20:80 to 40:60; a number-average molecular weight (Mn) of from 90 to 150 kg/mole and a ratio of average molecular weight (Mw) to Mn of from 1.2 to 2.2; drying the coated former and removing the glove or condom therefrom.
As used herein, the term “polyurethane” includes “polyurethane-urea”.
The polyurethanes of the invention are linear and are made from one or more isocyanates, one or more polyols and one or more chain extenders. The polyols are polyether polyols based on propylene glycol and thus are &agr;, &ohgr;-dihydroxy polyols. They preferably have a molecular weight of from 400 to 12000 daltons.
The polyurethanes of the invention can be made either by the so-called “one-shot” bulk polymerisation method, or by chain extending prepolymers.
The general method of preparation of the novel polyurethanes is conventional and, as such, will be well known to those skilled in that art. In general, we prefer to use the prepolymer method because it provides good control over hard/soft segment proportions and over product quality.
In order to achieve good physical properties in polyurethanes, high molecular weights are needed. It has been recognised in the art that &agr;,&ohgr;-hydroxy polyols sometimes contain monohydroxy terminated species (called “monols”) as impurities. These monols have only one hydroxy terminal and prevent the formation of high molecular weight products. The occurrence of monols in polyols can be reduced by using certain organometallic catalysts in the preparation, so that the polyol has only about 0.02 milliequivalents/g of unsaturation, but even this low level is not without effect. According to a feature of the present invention, we use &agr;,&ohgr;-hydroxy polyols which contain no more than about 0.01, and most preferably no more than about 0.007, milliequivalents unsaturation per gram. Materials of this specification are available commercially. For example, Lyondell Chemical Co. (USA) supply “Acclaim” polyols which are said to have a very low level of monol impurity. These “Acclaim” polyols are for use in making high performance cast polyurethane elastomers to meet requirements not met by conventional rubbers and plastics, e.g. to provide high performance flexibility and toughness. Their utility in the present invention, in contributing to the production of polyurethanes closely matching natural rubber, is quite different from their proposed use for cast polyurethanes and, indeed, it is surprising that they are useful for the quite different purpose of the present invention.
Aliphatic diisocyanates are used to make the polyurethanes of the invention, since aromatic diisocyanates tend to give products of too high stiffness and creep to match natural rubber. Among the aliphatic diisocyanates which can be used is 4,4′-methylenebis(cyclohexyl isocyanate) (HMDI), available commercially as Desmodur W from Bayer. Others include isophorone diisocyanate (3-isocyanatomethyl-3,5,5-trimethylcyclohexyl isocyanate IPDI), available from Veba; hexamethylene diisocyanate (HDI) available from Bayer; cyclohexane-1,4-diisocyanate (C

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