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ELASTOMERIC TRIBLOCK COPOLYMER
COMPOSITIONS AND ARTICLES MADE
BACKGROUND OF THE INVENTION
This invention is related to the composition of an elastomeric triblock copolymer, and, additionally, to the use of such an elastomeric copolymer to prepare dip-formed articles.
Thin-walled, extensible articles such as gloves and condoms have long been made from natural rubber. In normal production, such articles are formed from natural rubber latex, a naturally occurring emulsion of rubber and water, with added stabilizing agents and vulcanizing chemicals. A form of the appropriate shape, previously coated with a coagulating solution in some cases, is dipped into the latex mixture once or several times to build up a layer of the desired thickness. The water is allowed to evaporate, leaving a solid rubber film. The film must be vulcanized to provide adequate mechanical and physical properties.
Natural rubber has many advantages in these applications, being strong and highly elastic and having good "tactility" or feeling to the user. The good tactility is believed to be a result of its low deformation stress at 10-500 percent elongations and high elastic recovery from these elongations. Natural rubber has the shortcoming that the resulting rubber film devices are sometimes prone to having "pinholes" therethrough, often on the order of micrometers in diameter. The pinholes are probably due to minute impurities in the natural latex which are difficult to filter out, and to the fact that the process converts a stable emulsion to a film by coagulating chemicals or heat. Natural rubber is also susceptible to rapid attack by ozone which causes scission cracking, as well as to oxidative attack during storage which causes cracking and destroys the physical integrity of the product. Even with these problems, for many applications the pinholes do not prevent the rubber article from having utility, and latex gloves and condoms are widely used. However, the presence of the pinholes is unacceptable in other applications, as for example where there is any possibility of the presence of the AIDS virus. Natural rubber is also not hypoallergenic due to the residual surfactants, vulcanizing agents, stabilizing agents, antioxidants, or protein materials in the rubber. Persons who are particularly susceptible to irritation or sensitization, or use the rubber products for extended periods of time may experience allergic reactions.
Various types of synthetic elastomeric polymer products have been developed for use in thin articles produced by dip forming. Synthetic rubber compositions can be dissolved in solvents to form a true solution, so that pinholes are much less likely to be present. Available synthetic rubber compositions have various other shortcomings, including unacceptable tactility. While each may meet some of the requirements, the known synthetic compositions do not have the required combination of strength, tactility, resistance to environmental damage, and hypoallergenicity required for many products such as examination and surgical gloves and condoms. Several synthetic elastomeric systems have provided strength and tactility, but none has solved the problems related to environmental damage and hypoallergenicity required for surgical gloves and condoms.
There is therefore a need for an improved elastomeric material for use in thin, dip-formed articles such as examination or surgical gloves and condoms. Such a material must have the required properties of strength 5 and elastic elongation. It also must be pinhole free when the article is formed and used, resistant to immediate environmental or damage occurring during storage or use, and hypoallergenic. The present invention fulfills this need, and further provides related advantages.
10 SUMMARY OF THE INVENTION
The present invention provides an elastomeric solution for dipping articles, an elastomeric composition, a process for preparing such dipped articles, and the arti
15 cles themselves. The elastomeric blends have a combination of good strength and elasticity, together with a tactility comparable to natural rubber as shown by low deformation stress at 50-500% elongation and highly elastic recovery. Thin goods formed of the elastomers
20 avoid the problem of pinholes, or, alternatively stated, have a high degree of impermeability. The elastomeric composition is not prone to scission cracking upon exposure to ozone or cracking upon aging, and is well suited for use in thin goods formed by dipping, such as
25 gloves and condoms.
In accordance with the invention, an elastomer liquid solution consists essentially of a block copolymer component comprising at least two S-EB-S triblock copolymers having different solution viscosity/copolymer
30 concentration values; a plasticizer in an amount sufficient to provide tactility in dip formed products made from the composition; and a solvent in an amount sufficient to form a stable solution of the block copolymer component and the plasticizer and to permit dip form
35 ing of products from the liquid solution. Preferably, three S-EB-S block copolymers form the block copolymer component, the plasticizer is mineral oil, and the plasticizer is present in an amount sufficient to reduce the deformation stress of the solid formed elastomer to
40 less than about 5.5 MPa (millions of Pascals) at 500 percent elongation.
The liquid elastomer solution that is used to form thin solid articles has three principal ingredients, the block copolymers, the plasticizer, and the solvent. The block
45 copolymers are of the styrene-ethylene/butylene-styrene or S-EB-S type. Block copolymers of this family are inherently strong and of high modulus at intermediate elongations, leading to a rather poor tactility in the final product. A plasticizer, preferably mineral oil, is
50 added to the S-EB-S block copolymer to improve its tactility and elasticity, and reduce its modulus at intermediate elongations to improve its feel and tactility. An optimal balance of these properties, comparable to those of natural rubber, is achieved by using at least
55 two, and preferably three, S-EB-S block copolymers of differing solution viscosity/copolymer concentration relationships in a solvent, in specific relative amounts. The solvent dissolves the block copolymers and the plasticizer to form a stable, true solution, rather than a
60 mixture or an emulsion. Dipped articles exhibit a greatly reduced tendency to have pinholes when formed from a solution rather than a mixture. The amount of solvent must be sufficient to permit preparation of product by dipping procedures. 65 In accordance with the processing aspect of the invention, a process for the preparation of elastomer articles comprises the steps of furnishing a liquid solution consisting essentially of at least two different S-EB-S
block copolymers having different solution viscosity/copolymer concentration values, a sufficient amount of an oil to provide tactility in the dip molded products, and a sufficient amount of a solvent to permit dip molding of products: dipping a form into the solution and 5 withdrawing the form from the solution; and evaporating the solvent from the film on the form, leaving a coherent extensible film on the form. The elastomeric block copolymers become physically crosslinked by the time the solid film forms, and no subsequent curing or 10 vulcanizing is necessary.
The elastomer of the invention is most advantageously used to manufacture thin protective articles such as gloves or condoms by dip forming. These products are substantially free of pinholes and are highly 15 impermeable to small particles, viruses, bacteria, and the like. The integrity of the article is maintained even after storage and/or exposure in ultraviolet light, ozone, and oxidizing conditions.
Other features and advantages of the invention will 20 be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS 25
FIG. 1 is a graph depicting deformation stress as a function of elongation for several block copolymers;
FIG. 2 is a graph depicting deformation stress as a function of elongation for natural latex and for triblock 30 copolymer and plasticizer blends;
FIG. 3 is a graph depicting the ultimate tensile stress and tactility properties as a function of the elastomeric composition and plasticizer content;
FIG. 4 is a graph depicting the deformation stress at 35 500 percent elongation as a function of the elastomeric composition and plasticizer content;
FIG. 5 is a perspective view of a glove; and
FIG. 6 is a perspective view of a condom.
DETAILED DESCRIPTION OF THE 40
In accordance with a preferred aspect of the invention, an elastomer liquid solution consists essentially of a block copolymer component comprising at least two 45 S-EB-S block copolymers, each block copolymer having from about 25 to about 35 percent by weight of polystyrene blocks, the total mass of S-EB-S block copolymers having from about 40 to about 60 percent by weight of a first S-EB-S block copolymer with a 50 solution viscosity of about 6500 cps at 25 percent by weight of copolymer in toluene at 77" F., and from about 60 to about 40 percent by weight of a second S-EB-S block copolymer with a solution viscosity of about 2000 cps in toluene at 10 percent weight of poly- 55 mer in toluene at 77° F.; a plasticizer in an amount of from about 30 to about 65 parts by weight of the total mass of the S-EB-S block copolymer component; and an organic solvent for the block copolymer component and the plasticizer. 60
Even more preferably, an elastomer liquid solution consists essentially of a block copolymer component comprising three S-EB-S block copolymers, each block copolymer having from about 25 to about 35 percent by weight of polystyrene blocks, the total mass of S-EB-S 65 block copolymers having from about 40 to about 60 percent by weight of a first S-EB-S block copolymer having a solution viscosity of about 6500 cps at 25 per
cent by weight of copolymer in toluene at 77° F., from about 15 to about 59 percent by weight of a second S-EB-S block copolymer having a solution viscosity of about 2000 cps in toluene at 10 percent weight of polymer in toluene at 77° F., and from about 1 to about 40 percent by weight of a third S-EB-S block copolymer having a solution viscosity of about 1600 cps in toluene at 25 percent weight of polymer in toluene at 77° F.; a plasticizer in an amount of from about 30 to about 65 parts by weight of the total mass of the S-EB-S block copolymer component; and an organic solvent for the block copolymer component and the plasticizer.
The polymer base for the elastomer is a styrene-ethylene-butylene-styrene (S-EB-S) block copolymer formed from an ethylene-butylene copolymer central block and styrene end blocks. The polystyrene end blocks typically constitute about 25-35 percent by weight of the total molecule. The total molecular weight of the copolymer is typically from about 50,000 to about 300,000. The different copolymers have different solution viscosity/copolymer concentration relationships in a selected solvent such as toluene under selected conditions.
The S-EB-S block copolymer is to be distinguished from other block copolymers that have sometimes been used in synthetic rubber compositions, such as styreneisoprene-styrene (S-I-S) and styrene-polybutydiene-styrene (S-B-S) block copolymers. It has been known to make thin rubberlike articles from S-I-S and S-B-S block copolymers, see for example, U.S. Pat. No. 3,933,723. The use of these block copolymers eliminates the need for vulcanization of the articles, but the articles are subject to oxidation and ozone damage.
The use of an S-EB-S block copolymer, as distinct from other types, is critical to the success of the present invention, for two reasons. First, elastomers based upon the S-EB-S block elastomeric triblock copolymers are resistant to attack by ozone, or by oxidative conditions, while S-I-S and S-B-S elastomers suffer from rapid cracking, when exposed to ozone, and cracking or hardening under oxidative conditions. Both of the latter copolymers are thus subject to failure even when protected by specific additives such as antiozonants or antioxidants which are deleterious in medical and other applications. The use of such special additives is undesirable, as they may cause allergic reactions in some persons. The present elastomeric composition is hypoallergenic and may be contacted with the skin of the user for extended periods of time. Additionally, in spite of the special additives, scission cracking can lead to premature failure by ozone cracking of the articles made from S-I-S and S-B-S compositions, particularly when the articles are stored in a folded condition and then stretched before and during use.
Second, the mechanical properties of the S-EB-S triblock copolymers can be modified to provide the desirable combination of tensile strength, elasticity, and tactility that is required in some applications. S-EB-S elastomeric triblock copolymers have higher tensile strength, lower elastic elongation, and higher stress at 50-500 percent elongation than the S-I-S and S-B-S triblock elastomers. By themselves, individual S-EB-S block copolymers are unacceptable for the applications described herein owing to their unacceptably high stress at low elongations and resulting insufficient tactility. With appropriate combination of at least two, and preferably three, S-EB-S elastomers and a suitable plasticizer such as a nonaromatic napthenic mineral oil, the