Energy Efficient, Accelerator-Free, Cold Vulcanization of Latex Articles Mark W. McGlothlin, Whitney A. Williams, and Scott W. Herrick Apex Medical Technologies, Inc. Abstract The rapidly increasing cost of energy needed to produce high-volume dip-molded or cast latex products, such as medical gloves and other dip-molded or cast rubber film products of natural rubber or synthetic polyisoprene, is of concern to the latex industry. The use of rubber accelerators, the formation or presence of Type IV latex allergens, nitrosamine formation, and excessive energy usage are still of concern to the industry. The method of latex film vulcanization discussed in this paper reveals how a specific class of curing agents can effectively address these issues. These curing agents, known as polynitrile oxides, can rapidly vulcanize latex films at only modestly elevated temperatures, or even at room temperature or below. It is only necessary to dry the latex films on the production line, eliminating or greatly reducing the need for an in process heated curing step. This can save much energy and can increase production capacity. No post-stripping cure is needed, as full cure occurs at room temperature over time without concern for under-curing or over-curing. The cured articles are clear, free of sulfur, activators, accelerators, nitrosamines, nitrosatables, and odors. As compared to alternative accelerator-free methods, they have superior physical properties, including improved tear and tensile strengths and ultimate elongation. Tear strength of up to 70 kN/m, tensile strengths of up to about 6000 psi, and ultimate elongations in the range of about 550 % to about 1200 % have been achieved. It is not necessary or desirable to prevulcanize. It is also not necessary to use any specialized production equipment. Purpose This paper will first present a historical perspective with respect to solid rubber and latex vulcanization methods. Then, the emphasis will provide insight into the favorable aspects of 1 vulcanizing latex with a certain class of resin curatives, which have the potential to leave behind no residual chemicals that contribute to Type IV latex allergy. The particular classes of resin cure agents which will be discussed in this paper are polynitrile oxides, and are identified in this paper as having especially beneficial properties with regard to latex processing. They can be used to produce latex articles with very biocompatible properties and can contribute to the goal of eliminating Type IV latex allergy and nitrosamines. Historical Background of Vulcanization Options for Latex The forming of useful vulcanized articles, such as medical gloves, finger cots, condoms, balloons, etc. from natural rubber latex dates back many decades. During this time period, a number of methods of vulcanizing latex have been put into industrial use. The most common method employed on a commercial basis today is that of accelerated-sulfur vulcanization. Vulcanization with sulfur has traditionally been performed in the presence of vulcanization accelerators, such as dithiocarbamate and thiuram accelerators, because non- accelerated sulfur vulcanization typically leads to poor physical properties and poor aging stability. The added use of a metal oxide, such as zinc oxide, with sulfur improves matters, but still does not lead to adequate physical properties for most uses. However, these substances, and their breakdown products, can contribute to adverse reactions in individuals with whom the resulting rubber articles may come into contact. The reaction is commonly referred to as a Type IV allergy, which is mediated by T cells, generally occurs within six to 48 hours of contact with the rubber article, and is localized in the area of the skin where contact is made. Secondary amine-containing accelerators are also referred to as nitrosatable amines since they can produce nitrosamines, which have been identified as potential human carcinogens. Many attempts have been made to introduce accelerator-free vulcanization systems to the latex industry to address some of these concerns. Perhaps the next most prevalent accelerator-free vulcanization systems are those using metal oxides as crosslinkers. These are common for the curing of polychloroprene articles and nitrile articles. They can be used without the use of accelerators or sulfur. Other methods include radiation prevulcanization of natural rubber, 2 organic peroxide and/or hydroperoxide prevulcanization of natural rubber, and peroxide postvulcanization of synthetic polyisoprene and natural rubber. With the exception of the pure prevulcanization systems, all of these known methods use significantly elevated temperatures to achieve an adequate level of vulcanization. Some work has been done to address the goal of using a crosslinking agent that readily incorporates itself into the crosslinked rubber network, without the need to leave behind residual chemicals. This is clearly a worthy goal, but it has been hard to achieve. In the solid rubber industry, some work has been done with regard to "resin curing" of rubber articles. Resin cure systems have the advantage in that the curative agent does become part of the cross-linked network. One apparently commercially viable method for solid rubber is that of phenol formaldehyde systems [1,2]. Unfortunately, this method does leave behind residual chemicals and requires very high temperatures for vulcanization to occur. Another resin curing method utilized diisocyanate. Such curing systems for rubber have been proposed but require that a suitable reactive group be present on the polymer chain in order for crosslinking to occur. Pure polyurethane rubber can be further crosslinked this way. Suitable reactive groups can potentially be grafted onto some types of rubber polymers, which could allow for a resin cure with diisocyantes. This could be an area for further investigation by the latex industry. The authors are not aware of any latex system using this sort of curing system at this time. As shown in US Patent 6753355, in the case of latex foam rubber, both epoxy silanes and polynitrile oxides have been investigated with some success. In the case of epoxy silanes, as in the case of diisocyantes, a suitable functional group needs to be present on the polymer chain. Carboxylation is the most common method to functionalize the polymer. With respect to nitrile oxides, it is only necessary to have some minor level of unsaturation present. The common laticies of natural rubber and synthetic polyisoprene have multiple points of unsaturation, and thus are good candidates for polynitrile oxide crosslinking. As previously noted, metal oxides are likely the best known and most utilized class of chemicals used to crosslink latex rubber without the use of accelerators. They are very commonly used in both nitrile gloves and in polychloroprene gloves. If used in unique ways, they can do away with the use of sulfur accelerators. There are, however, issues with respect to the physical properties of such gloves in the event that only metal oxides are used for curing. Nevertheless, some 3 medical gloves made from polychloroprene or nitrile latex are vulcanized by treatment with divalent and trivalent metal oxides, etc. The most common metal oxide curative appears to be zinc oxide. The use of zinc oxide provides for a commercially viable method of producing synthetic latex gloves. Polychloroprenes are grouped into two classes, sulfur-modified types and non-sulfur-modified types. US Patent 4018750 shows that sulfur-modified chloroprene requires only a metal oxide such as magnesium oxide, zinc oxide or lead oxide for vulcanization, whereas with non-sulfur-modified chloroprene, special vulcanization accelerators have to be used in addition to the metal oxides. However, US patent 6706816 makes evident that with a carboxylic acid modification of the polymer, it is possible to vulcanize with just the metal oxide. The combination of zinc oxide and magnesium oxide has gained the most widespread acceptance as the preferred formulation for vulcanizing chloroprene. Zinc oxide is used as the crosslinking agent while magnesium oxide is used as the chlorine acceptor. Zinc oxide allows for immediate vulcanization, but if used alone produces crosslinks that are inadequate. Magnesium oxide leads to safer processing, but if used alone leads to slow vulcanization and a lower degree of vulcanization. Magnesium oxide and zinc oxide, when used together, produce a synergistic vulcanizing effect resulting in a balanced combination of cure time and degree of vulcanization. The bis-alkylation theory of chloroprene vulcanization is most widely accepted. The theory proposes that the cross-linking of chloroprene takes place at the sites on the polymer chain where there are tertiary allylic chlorine atoms formed by 1,2 polymerization of the chloroprene monomer. This accounts for about 1.5% of the total chlorine in the chloroprene. The metal oxide, in most cases zinc oxide, initiates the curing process by reacting with the chlorine present to form zinc chloride, which is a catalyst for the alkylation. The zinc chloride cross-links via bis- alkylation at the reactive tertiary allylic chlorine sites of the polymer chains [3]. Grafting of a carboxylic acid group onto synthetic polyisoprene (and presumably natural rubber) can be achieved with some level of complexity. US Patent 3887527 details a method to graft a carboxylic acid group onto the polymer by using malaeic anhydride. Conceivably, this could allow for