Natural rubber latex has been used in the manufacturing dipped latex products such as rubber medical gloves for many years. The natural rubber latex medical gloves so produced have excellent elastic properties and superior barrier performance against the transmission of blood-bourn pathogens. The manufacturing of the natural rubber gloves involves a vulcanizing process whereby free sulfur, which acts as a vulcanizing agent and chemicals which act as accelerators in the vulcanization process, are added to natural rubber latex. More specifically in rubber glove manufacturing, a mold or former in the shape of a hand is dipped, in one or several times, into the emulsion of natural rubber latex compounded with the addition of sulfur and accelerators, until a rubber film of the desired thickness is deposited. The rubber glove having the desired thickness is then dried and vulcanized at elevated temperature. The vulcanization process is essential to impart highly elastic properties to natural rubber film. The resulted rubber glove product made from the natural rubber latex has favorable barrier performance, mechanical properties and physical properties.
The natural rubber latex contains less than 5% of non-rubber such as proteins, lipids and trace elements. It is also recognized that the increase use of natural rubber latex medical gloves in the hospital settings has resulted in certain users developing Type I hypersensitivity. This immediate type I hypersensitivity in users of natural rubber latex gloves is caused by the residual extractable latex proteins present in natural rubber gloves. The immediate hypersensitivity, which usually occurs in less than two hours after contact, is an allergic response mediated by IgE (an antibody found in the circulation). On the skin, this can present as hives that migrate beyond the point of contact with latex. Systemic allergic symptoms can include itching eyes, swelling of lips or tongue, breathlessness, dizziness, abdominal pain, nausea, hypotension, and very rarely anaphylactic shock. People who have developed sensitization caused by natural rubber latex proteins are advised to avoid further contact with natural rubber latex and products made from natural rubber latex.
Synthetic latices such as nitrile latex, carboxylated nitrile latex, polychloroprene latex, or polybutadiene latex do not contain proteins. It is therefore recommended that those people who have developed protein allergy should use only synthetic latex gloves that are made from either nitrile latex, carboxylated nitrile latex, polychloroprene latex, or polybutadiene latex. Some of the gloves made from these synthetic latices are reported to have equivalent or better physical properties compared to natural rubber latex gloves but the barrier performance of the synthetic latex gloves are not as good as those of natural rubber latex gloves.
The manufacturing of synthetic rubber medical gloves adopts almost the same process used in the manufacture of natural rubber latex medical gloves.
Using the same procedure, a thin film of desired thickness of the synthetic elastomeric latex compound is deposited on a mold or former of the hand shape, dried, and vulcanized, whereby the resultant thin elastomeric rubber glove product has the desired mechanical and physical properties. For the purpose, a variety of synthetic elastomers gloves have been developed and introduced in the market. Carboxylaated nitrile rubber is the most common material used in making synthetic rubber gloves.
Manufacturing of rubber gloves made from natural rubber latex and synthetic latex very often use an accelerated sulfur vulcanization system whereby chemicals such as dithiocarbamate, tetramethyl thiuram disulfide (TMTD) and mercaptobenzothiazole (MBT) are used as accelerators in the sulfur-based vulcanization process. These accelerators, as the name implies, are used to enhance the rate of vulcanization. Without the use of accelerators, the vulcanization process using sulfur alone will be very slow requiring several hours at high temperature of 140 C.
Extensive use of these accelerators in the rubber gloves manufacturing industry has created another health-related issue. These accelerators can give rise to the delay Type IV hypersensitivity such as allergic contact dermatitis. The delayed type IV hypersensitivity usually occurs 24-72 hours after contact. The reaction is usually localized rashes, redness of the skin, sometimes with, cracking and blistering of the skin, usually on the wrists or hands.
Switching from natural rubber latex gloves to nitrile latex gloves to avoid Type I hypersensitivity may therefore induce the occurrence of Type IV hypersensitivity if the nitrile gloves are found to have excessive residual accelerators. The industry is therefore looking for a synthetic glove manufacturing process without the use of sulfur vulcanization system and thus eliminating the use of accelerators. In the case of carboxylated nitrile latex, crosslinking without the use of sulfur and accelerators will therefore involve a unique, non-conventional process requiring the zinc ions, present as zinc oxides for example, to participate in the ionic as well as the covalent bond crosslinking mechanism. This technology is urgently needed in spite of its difficulty. This is the gist of the current invention.
Depicted in U.S. Pat. No. 5,014,362 (as Patent Document 1 in this specification) is a carboxylated nitrile rubber which is subjected to the vulcanization process with the help of zinc oxide and sulfur.
A carboxylated nitrile rubber typically comprises acrylonitrile, butadiene, and organic acid segments of various composition ratio. It is possible to produce covalent bond crosslinking in the sub segment of butadiene using sulfur and accelerators. Also, the vulcanization of the carboxylated acrylonitrile part (organic acid) can be effected by ionic bonds using metal oxides such as zinc oxide or other metal salts.
The crosslink of covalent bonds with the help of sulfur can significantly improve the durability of rubber in contact with oils and chemicals. In addition, the addition of zinc oxide encourages the production of the ionic bonds of zinc ions. The ionic crosslinking with the help of zinc ions will increase the tensile strength, the force at break and the abrasive resistance as well as the elastic modulus of the rubber film.
In case that the crosslinking mechanism depends simply on the ionic bonds, the resistance to oils and chemicals will be declined resulting in the rubber products having lower quality reliability.
It is now taught in common that the crosslinking of carboxylated nitrile rubber products such as gloves is effectively implemented by a combination of the covalent bond crosslinking with the help of sulfur and accelerators and the ionic crosslinking with the help of metal oxide such as zinc oxide or metal salts.
The use of accelerators in the vulcanization process, however, will create health related issue of delayed Type-IV hypersensitivity.
It is widely known that the strength of rubber is improved by adding zinc dimethacrylate and/or basic zinc methacrylate to the rubber in order to promote the polymerization with organic peroxide.
More specifically, the mixing of poly-butadiene with methacrylic acid and then with zinc oxide produces a composition having improved abrasive resistance (See Patent Document 2 of JP Patent Laid-open Publication No. 53-125139 and Patent Document 3 of JP Patent Laid-open Publication No. 52-121653). The adding of non-polymeric carboxylic acid to a mixture of diene rubber, methacrylic acid, zinc oxide, and organic peroxide produces a blend which is improved in the tensile strength (See Patent Document 4 of JP Patent Laid-open Publication No. 53-85842). The use of methacrylic acid, zinc oxide, and peroxide enables the crosslinking in NBR but without the presence of the ionic bonds.
A soft nitrile rubber product is provided which is as high in the tensile strength and the resistance to chemicals when the crosslinking is carried out with the help of zinc oxide or a sulfur curing accelerator. The resulting rubber product is softer than any conventional like product (See Patent Document 6 of JP Patent No. 3517246 or JP Patent Indication No. 2000-503292). The vulcanizing accelerator is tetramethyl-thiuram-disulfide combined with mercaptobenzothiazole (MBT). The resultant soft nitrile rubber which is a product of the relevant reaction will hence contain sulfur.
The carboxylated nitrile rubber which is a copolymer of acrylonitrile, butadiene, and unsaturated carboxylic acid allows the ionic bonds to be produced in the carboxy group with zinc ions. However, it is difficult to produce covalent bond-crosslinking in a compound using zinc ions. It is hence essential to use a minimum dosage of sulfur to provide covalent crosslinking. More particularly, a method of manufacturing a glove product is depicted in JP Patent Publication No. 2002-527632 (Patent Document 7 in this specification) where the crosslinking is achieved by adding 1 to 3 phr of sulfur and 0.5 phr of multivalent metal oxide to carboxylated nitrile rubber which is a copolymer of acrylonitrile, butadiene, and unsaturated carboxylic acid.
In U.S. Pat. No. 6,673,871B2 (Patent Document 8 in this specification), an elastomer product such as a glove product is disclosed where the crosslinking agent is a metal oxide such as zinc oxide without the use of conventional sulfur and accelerators. The elastomer used is a polybutadiene latex which is not carboxylated and the product can be vulcanized at temperatures less than 100 C. and more particularly at temperatures less than 85 C. A corresponding Japanese patent, JP Patent Laid-open Publication No. 2004-526063 (Patent Document 8) is disclosed where the crosslinking in the synthetic polymer described previously is carried out without the use of accelerators. More particularly, it claims from a series of examinations to feature a step of forming an elastomer film which contains no sulfur where the vulcanization process is carried out at a temperature of not higher than 85° C. with the use of a metal oxide such as zinc oxide for conducting the sulfur substitution. It is however found that the vulcanization consisting mainly of a metal oxide is feasible only with some practical difficulties.
Non-patent Document 1 (“Cross-linking in carboxylated nitrile rubber dipped films” by Andrew Kells and Bob Grobes, LATEX 24-25, January 2006, Frankfurt, Germany) is appended where it is shown that to obtain carboxylated nitrile latex gloves with acceptable durable properties, a small amount of sulfur with the use of vulcanizing accelerators such as type tetramethyl thiuram (TMTD), 2,2′-dithio-bis(benzothiazole)-(MBTS), N-cyclohexylbenzothiazole-2-sulfinamide (CBS), and zinc diethyldithiocarbamate (ZDEC) are required in addition to zinc oxide. It is apparent that it is difficult to produce durable carboxylated nitrile latex gloves without the use of sulfur and sulfur-based accelerators. While the attempt for manufacturing a glove product from the self-crosslinked material is disclosed, the action of self-crosslinking as well as the process needed to produce the desired gloves was not explained in the technical paper. It is true that the technology for implementing the self-crosslinking latex falls short of its acceptable goal.
Non-patent Document 2 (“Tailored synthetic dipping latices: New approach for thin soft and strong gloves and for accelerator-free dipping” by Dr. Soren Buzs, LATEX 23-24, January 2008, Madrid, Spain) is appended teaching one promising direction of the technology that the alternative crosslinking process in NBR latex includes a direct covalent crosslinking by functional reactive groups (R) and the ionic crosslinks produced with the help of zinc oxide with the carboxyl groups of the NBR latex. More specifically, the method of vulcanization comprises substantially direct covalent bonding of polymer chains for R bonds and ionic bonding with the help of the carboxy group and zinc oxide. Under laboratory condition, the vulcanization temperature can be reduced from 120° C. to 85° C. The direction of the technology states that the R bonds derived from the covalent bonds linked directly from polymer chains are featured in addition to the ionic bonds bridging with the help of the carboxy group and zinc. Unfortunately, the approach fails to clarify what is the nature of the functional group R which acts in the covalent crosslinking and the actual method of constructing the same.
Disclosed in JP Patent Indication No. 2008-512626 (Patent Document 9) is a polymer latex manufactured by the radical emulsion polymerization where the soft phase part has a unit structure based on a group of separately conjugated diene, ethylene-form unsaturated mono-carboxylic acid, ethylene-form unsaturated dicarboxylic acid, and their anhydride, mono-ester and mono-amide, (meta)acrylonitrile, styrene, substituted styrene, □-methylstyrene, alkylester having 1 to 10 carbons in (meta)acrylic acid, amide in (meta)acrylic acid, N-methylol amide group, ethylene-form unsaturated compound containing their ester derivative and ether derivative, and their mixture while the hard phase part has a unit structure of monomer selected separately from a group of ethylene-form unsaturated mono-carboxylic acid, unsaturated dicarboxylic acid, and their anhydride, mono-ester and mono-amide, (meta)acrylonitrile, styrene, substituted styrene, □-methylstyrene, ester of C1 to C4 in (meta)acrylic acid, amide in (meta)acrylic acid, and their mixture.
It is presumed in that invention that the crosslinking is implemented by the covalent bonds in the soft phase part. In particular, metal components such as zinc ions are not used.
Disclosed in JP Patent Laid-open Publication No. 2008-545814 (Patent Document 10) is an elastomer product manufactured by the steps of (a) preparing a carboxylated nitrile butadiene rubber composition which comprises 0.25 to 1.5 parts of zinc oxide for 100 parts of dry rubber, alkali for having greater than 8.5 of the pH level, a stabilizing agent, and one or more accelerators selected from a group of guanidine, dithiocarbamate, and thiazol compound, if desired, (b) dipping a former in the carboxylated nitrile butadiene rubber composition, and (c) curing the carboxylated nitrile butadiene rubber product.
Crosslinking is done with the help of zinc oxide. For permitting the product to have a desired degree of the resistance to chemicals, crosslinking accelerators are used. The vulcanizing accelerator used is a dithiocarbamates. For improving the resistance to chemicals, a mixture of the dithiocarbamate accelerator, diphenyl guanidine, and zinc mercaptobenzothiazole used to ensure higher effects. This disclosure employs crosslinking of rubber using sulfur and accelerators, the gloves produced will have the problems of Type IV hypersensitivity.
Disclosed in the specification of U.S. Pat. No. 7,005,478 (Patent Document 11) is an elastomer product of which the elastomer having carboxy groups is produced by the reaction with (a) a carboxylic acid or its derivatives, (b) a bivalent or trivalent metal contained compound, and (c) an amine or amino compound, and (d) a neutralizing agent for neutralizing at least a part of the carboxyl group in a base polymer. During the reaction, none of the accelerators, thiuram, and carbamate is used. The base polymer may be selected from natural latex rubber, synthetic latex polymer (e.g., acrylonitrile), or butadiene rubber such as synthetic butadiene rubber, and carboxylated butadiene rubber. Also, carboxylated acrylonitrile latex is not used. This reaction requires essentially (c) the amine or amino compounds. The amine group or amino group is used to solubilize the divalent or trivalent metal salts that can then react with the carboxylic derivatives to form ionic bonds. The complex solubilization process will disturb the stable reaction of ionic crosslink.