Elastomeric articles of manufacture are typically made by compressing a solid rubber, such as natural rubber or synthetic rubber, that contains a curative, such as sulfur, into a mold of the desired size and shape. Then, the rubber is cured (vulcanized) in the mold at an elevated temperature which thermosets the rubber permanently into the desired shape. This conventional process is relatively labor intensive in that it requires a mixing step wherein the curative and typically other rubber chemicals are blended into the rubber, a forming step wherein the rubber is compressed into the mold, a curing step wherein the rubber is cured in the mold, and a mold removal step wherein the cured rubber article is removed from the mold. This conventional process cannot be used in manufacturing intricate rubber articles where it is not possible to compress the solid rubber into a mold of the required intricate shape. An additional drawback associated with this conventional technique is that on occasion it is difficult to remove the cured rubber article from the mold and in some cases the article is damaged during its removal from the mold.
Silicone rubbers offer a significant advantage over natural rubber and conventional synthetic polydiene rubbers, such as synthetic polyisoprene rubber, polybutadiene rubber, styrene-butadiene rubber, and the like, in that they can be injection molded into shapes that can be very intricate in design. Injection molding also offers the advantage of being capable of being highly automated to significantly reduce labor requirements. Elastomeric articles made utilizing silicone rubbers can also be compounded to be visually clear which is beneficial in some applications. For instance, clear baby bottle nipples made by injection molding silicone rubber are preferred by many consumers. However, silicone rubbers are typically very expensive when compared to the cost of conventional polydiene rubber. High cost has accordingly precluded silicone rubbers from being used in many products and, of course, adds expense to other products where silicone rubbers are employed.
There is currently a demand for a low cost polymeric composition that can be injection molded to produce elastomeric articles. There is a particular demand for such a polymeric composition that is based upon a polydiene rubber, such as polyisoprene, that can be molded into clear articles.
European Patent Application No. EP 0,709,403 A1 discloses a curable composition for injection molding which comprises, as essential ingredients, (A) a saturated hydrocarbon polymer containing at least one alkenyl group capable of undergoing a hydrosilylation reaction per molecule, (B) a hardener having a molecular weight of 30,000 or lower and containing at least two hydrosilyl groups per molecule, and (C) a hydrosilylation catalyst.
U.S. Pat. No. 6,087,456 discloses a curable composition comprising: (A) an isobutylene polymer which contains per molecule, at least one alkenyl group for reacting with a hydrosilyl group; (B) a curing agent which contains at least two hydrosilyl groups per molecule; (C) a hydrosilylation catalyst; and (D) a hydrocarbon compound having 6 to 20 carbon atoms which contains per molecule, at least one alkenyl or alkynyl group reaction for reacting with a hydrosilyl group.
U.S. Pat. No. 6,183,551 discloses a curable resin composition comprising: (A) saturated hydrocarbon polymer having at least one hydroxyl or hydrolyzable group bonded to a silicone atom and is crosslinkable through the formation of a siloxane bond, in a molecule and (B) a hydrogenated oligomer of a linear α-olefin.
U.S. Pat. No. 6,320,010 notes that the main component of such curable compositions is a polymer that can have a very high viscosity depending on the main component species. In particular when an isobutylene polymer is used as the main component, it is difficult to handle the isobutylene polymer as a liquid because of its high viscosity without applying some viscosity reducing technology, although the cured products obtained are excellent in various durability characteristics and, further, have low permeability and good vibration damping properties. Thus, for utilizing such curable compositions as potting agents or coating materials, it is essential to secure fluidity by some or other viscosity reducing technology. The most generally implemented technique for viscosity reduction is the addition of a nonreactive diluent as a plasticizer, for example an oil. In that case, however, while the viscosity is reduced by the addition of the plasticizer, the mechanical strength is markedly lowered and evaporation of the plasticizer at elevated temperatures causes heating loss and volume reduction of cured products, leading to marked changes in mechanical characteristics and, further, induces blooming of the plasticizer. U.S. Pat. No. 6,320,010 further notes that reliability reduction accordingly results in cases where a plasticizer in used for viscosity reduction.
U.S. Pat. No. 6,320,010 more specifically discloses a curable composition which comprises (as essential components): (A) a saturated hydrocarbon polymer having more than one alkenyl group capable of being hydrosilylated, on average, in each molecule and having a molecular weight of 2,000 to 50,000, (B) a component comprising a compound containing more than two silicone atom-bound hydrogen atoms, on average, in each molecule, (C) a compound having, within the molecule, one alkenyl group capable of being hydrosilylated, (D) a compound having, within the molecule, at least two alkenyl groups capable of being hydrosilylated and having a molecular weight less than 2,000, and (E) a hydrosilylation catalyst.