Energy Efficient, Accelerator-Free, Cold 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 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 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 , 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

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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 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 , 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,

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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 chain in order for crosslinking to occur. Pure rubber can be further crosslinked this way. Suitable reactive groups can potentially be grafted onto some types of rubber , 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

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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 a metal oxide resin cure.

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McGlothlin et al. in US Patent Application 2004/0071909 make known the use of sulfur-free, free-radical-cured cis-1,4- polyisoprene for use in dip-molded medical devices. Vulcanizates made by this method are free of undesirable accelerators, can have very low odor and can be non-cytotoxic. However, physical properties of the cured films are generally lower than for traditional accelerated-sulfur vulcanizates. Elevated temperatures and oxygen free curing conditions are required. Generally, rubbers that are crosslinked exclusively through carbon- carbon bonds, as occurs in both peroxide and radiation vulcanization, have inferior tear strengths as compared with rubbers that contain sulfidic and/or polysulfidic crosslinks.

Some work has been done with the use of acrylic coagents to improve the physical properties of organic peroxide vulcanizates. These have been tried in latex formulations with some success. The coagent has the ability to react in such a way as to become the crosslink, so they could be considered resin-curing agents, but they cannot be used alone. One or more free radical generators need to be used. The free radicals can be generated, for instance, by organic peroxides. In the case of their use with organic peroxides, there will still be some breakdown products present. Elevated temperatures and oxygen free conditions are generally required.

McGlothlin, et. al in US Patent application 2004/0071909 have shown that sulfur can be used successfully as a coagent, rather than a primary curing agent in the crosslinking of latex. This method eliminates the need for accelerators and activators and can produce very high quality latex films with enhanced tear strength. This reaction requires elevated temperatures and oxygen-free curing conditions, as is the case with acrylic coagents.

Even with the above-cited methods of alternative curing of latex products, further improvements in physical properties and in the processing of latex are still desirable. With recent increases in energy costs, lower temperature and lower time curing systems can be highly advantageous. The remainder of the paper will focus on a special, rarely cited, method of curing latex, which can make a favorable impact on most of the above-cited issues and concerns.

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Polynitrile Oxides as Unique Curatives for Latex Products

To the knowledge of the present authors, there are no prior references to the use of polynitrile oxides as suitable vulcanization agents for thin walled rubber film products made from latex, other than foam products. The authors are not aware of any prior reference to the use of polynitrile oxide vulcanized latex films made by dip-molding or casting and intended for direct or indirect skin and/or tissue contact for medical use. Thus, a new opportunity is being made known to the latex dip molding industry.

Polynitrile oxides are exceptionally reactive materials, especially with respect to the double bonds of rubber materials. This reactivity is helpful in rapidly creating useful crosslinks in diene based latex rubber materials when the polynitrile oxides are used as vulcanizing agents. Not all laticies with unsaturation can be effectively cross-linked with polynitrile oxides, due to competitive side reactions with other functional groups present on some rubber polymers. This high reaction rate can also be a liability, in that it can lead to extensive prevulcanization of a latex compound prior to use. While prevulcanized latex has a long history of use in dip molding operations, using it exclusively for vulcanization generally produces articles of less than desirable physical properties. When using polynitrile oxides, it is necessary to deal with this issue in an appropriate way so as to prevent the deterioration of the desirable physical properties, while still taking advantage of the lower temperature curing conditions made possible by the use of polynitrile oxides.

There are a number of requirements and/or characteristics that a new vulcanization system would have to possess before being considered acceptable for use in dip molded medical device applications. In reviewing the suitability of polynitrile oxides for use in curing latex rubber, this paper will focus only on the commonly used diene rubber latices, as they appear to be the best candidates for vulcanization with polynitrile oxides. Thin walled natural rubber and synthetic polyisoprene film products intended for medical uses or other contact with human tissue typically have a combination of useful properties. In most cases, it is desirable to combine a relatively low 100%, 300% or 500% tensile modulus with as high as possible values for ultimate tensile strength. These properties have to be further balanced to allow for very high ultimate

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elongations, and high tear strength properties. These properties are beneficial to many currently produced latex rubber products, for example, medical gloves, condoms, anesthesia breather bags, surgical tubing, catheter balloons, dental dams and the like. This is a difficult combination to achieve, as the lower modulus usually produces a lower tensile strength material. A relatively low tensile modulus is necessary to ensure that such gloves remain comfortable during use. If the tensile modulus is too high, the user's hands may become fatigued. This is particularly problematic with gloves that are to be used for a prolonged period of time such as for a long surgical procedure. A combination of low modulus with high tensile strength is necessary to provide the desired comfort along with a very large safety margin with respect to glove failure. It is also desirable to make these products with a high degree of biocompatibility and with no nitrosamines or Type IV latex allergy generating substances.

Tear strength is a key physical property affecting the usefulness of certain thin walled latex rubber products. Baby bottle nipples and baby pacifiers benefit from high tear strength since this prevents the child's teeth from severing the nipple or pacifier during use. This property is difficult to achieve in some of the existing accelerator-free latex formulations. For catheter balloons, it is very important to combine high tear strength with high elongation to protect the balloon from bursting during use. For condoms, the combination of exceptionally high tear strength and tensile strength combined with high ultimate elongation is very desirable. For latex exercise bands, it is desirable to have very high ultimate elongation, combined with high tensile strength. Because exercise bands are often packed for travel and can impart unpleasant odors to packed clothing, it is very advantageous to produce such bands with very low odor.

For rubber dental dams, it is very important to combine high ultimate elongation with very high tear strength to allow for the easy placement of the rubber dam around the perimeter of the tooth without a high incidence of failure by tearing. For patient comfort and acceptance, it is also very desirable for dental dams to have low levels of odor and taste.

Historically, both natural and synthetic rubbers have been used extensively as materials for thin walled medical devices and components. The highest quality thin wall products are made via the

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dip molding process, which provides for a seamless and uniform thickness product. Also, the use of rubber in its latex format allows for the use of very high molecular weight polymers, as compared to dry rubber processing. Latex rubber can be processed without excessively breaking down the molecular weight of the rubber. However, high temperature processing of dip molded parts can cause the molecular weight to breakdown as a function of temperature and time. This generally produces a corresponding reduction of physical properties. Latex processes have an advantage in this regard over dry-rubber methods, which utilize high shear to comminute the rubber and combine it with other compounding ingredients for processing, which degrade the molecular weight. In both dry rubber and latex processing, molecular weight can break down significantly if the process exposes the rubber to a significant heat history.

In addition to molecular weight breakdown, higher temperatures in many traditional latex processes can add to the cost of the fuel needed for drying and curing of the latex. Large dip molding plants need to be used efficiently to recover their capital cost. Excessive or unnecessary drying times can contribute to inefficient utilization of the capital employed. In recent years, energy and capital equipment costs have become much more of an issue for the manufacturers of rubber goods, especially for the manufacture of examination and surgical gloves. It would be very desirable to reduce energy costs and capital equipment costs for the processing of these gloves.

From the perspective of reducing or eliminating residual chemicals that can contribute to Type IV latex allergy, it would be good to eliminate any possible vulcanization by-products. It would also be advantageous to perform accelerator-free vulcanization while achieving optimal strength of the latex article being produced. Rubber products vulcanized with a polynitrile oxide crosslinking agent incorporate an isoxazoline crosslink in the rubber material. This eliminates the need to add compounds with secondary amino groups or any other traditional accelerators in the rubber compound. Because no accelerators are needed, the use of polynitrile oxide vulcanization can provide rubber products that are optimal for contact with living tissue due to the elimination of Type IV latex allergens.

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The use of polynitrile oxides as crosslinking agents can provide dip molded rubber parts which are essentially free of reaction by-products, contain no accelerators, sulfur, activators or metal oxides, and can be processed with a lower cost, low temperature curing protocol, while maintaining the physical properties which could previously be achieved only with accelerated- sulfur vulcanizing systems. There now exists an opportunity for the latex industry to adopt the use of polynitrile oxides to address the current needs of the industry.

Polynitrile oxides (PNOs) react readily with unsaturated molecules because they participate in a 1,3-dipolar cycloaddition with a variety of multiple bond functional groups. In the reaction of a PNO with ethylenic points of unsaturation, the cycloaddition product is an isoxazoline ring. Reaction of a rubber compound with a PNO therefore provides crosslinking regions within the polymer comprised of two or more isoxazoline units, usually separated by an aromatic structure.

Historically, most polynitrile oxides are unstable and spontaneously decompose. However, some polynitrile oxides are very stable. One method of stabilization is to produce the polynitrile oxides in such a way that there are steric hindrances between the nitrile oxide groups on an aromatic ring. For example, stable polynitrile oxides can be aromatic structures wherein each polynitrile oxide functional group is located between two ortho groups of the aromatic structure. Ortho groups that provide for stable polynitrile oxides are any ortho groups that are larger than a hydrogen atom and do not react with the polynitrile oxide functionality.

As noted previously, the very rapid reaction rate of PNOs allows for crosslinking to occur at lower temperatures than with other non-accelerated vulcanization agents and without the formation of by-products (i.e., virtually all atoms of the reactants are incorporated into the rubber structure). Similarly, no catalysts are necessary, which would otherwise be left behind as unwanted substances. These clear advantages of PNO motivated the authors to study their utility as vulcanization agents for dip molded and cast latex rubber products, which had not been explored before.

However, while not studied for dip molding and cast latex products, the use of polynitrile oxides (PNOs) as low temperature crosslinking agents for various types of unsaturated rubber and other

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polymeric materials has been known for quite awhile. For example, Breslow et al. in US Patent 3390204 disclose the use of various polynitrile oxides to crosslink unsaturated polymers. Lysenko, et al. in WO 97/03034 disclose the use of a dispersion of stable polynitrile oxides useful in vulcanizing foam rubber latex materials. Specifically, the use of 2,4,6-triethylbenzene- 1, 3-dintrile oxide is cited as a useful one part room temperature crosslinking agent for latex foam rubber. Lysenko also notes the utility of 2,4,6-triethylbenzene-1,3-dintrile oxide for crosslinking various polymers to create useful one-part coatings. Interestingly, there is no mention of 2,4,6-triethylbenzene-1,3-dintrile oxide or other polynitrile oxides imparting any special physical properties to the vulcanized foam rubber articles.

Stollmaier, et al. in US Patent 6753355 also references the utility of 2,4,6-triethylbenzene-1,3- dintrile oxide for crosslinking various latex polymers for foam rubber products, including flooring, wall covering, shoe lining, and non-woven materials.

Parker in US Patent 6355826 discloses an improved method of synthesizing mesitylene dinitrile oxide. Parker cites the use of polynitrile oxides in the coating of fabrics with rubber-based coatings. Parker states that stable polynitrile oxides are desirable from the perspective of handling, as compared to unstable polynitrile oxides.

Breton, et al. in US Patent 6252009 reveals the use of polynitrile oxides for making solvent resistant thermoplastic vulcanizates. Russian Patent SU 2,042,664 demonstrates the ability of polynitrile oxides to crosslink a number of polymers that have a very low level of unsaturation. Apparently, the very high reactivity of the polynitrile oxides was advantageous, due to the scarcity of unsaturation sites for traditional vulcanization methods. The polynitrile oxide method of curing allowed for modest time and temperature conditions, inclusive of room temperature conditions. As discussed in SU 2,042,664, the use of PNOs may be very desirable for crosslinking rubber materials that have very low levels of unsaturation. In that case, the high reactivity of the polynitrile oxide compensates for what would otherwise be a very slow vulcanization process (i.e., with traditional sulfur accelerated cure packages). However, when used to crosslink highly unsaturated materials such as natural rubber and synthetic polyisoprene, the high rate of reaction

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of PNOs makes them more reactive than even the fastest of the “ultra accelerators” used in accelerated-sulfur cure systems. From the perspective of a solid rubber processor, polynitrile oxides may well be rejected for highly unsaturated rubbers, such as natural rubber, due to the poor scorch characteristics. Of course, with latex processes, it is permissible to use very rapid accelerators, such as , which would not be suitable for dry rubber. Because latex is processed at relatively low temperature, even the so-called “ultra accelerators” can be used, as even these are not too active in relatively cold latex compounds. For instance, it may take a matter of days before a latex compound is ruined due to too much unwanted pre-vulcanization at room temperature. However, with polynitrile oxides as vulcanization agents, care must be taken to prevent excessive amounts of unwanted prevulcanization immediately prior to processing. Even sub-freezing temperatures will not stop prevulcanization.

In some cases, it may be desirable not to control prevulcanization, and to just process fully prevulcanized latex. In the case of polynitrile oxide crosslinked rubber film products, if nothing is done to restrict pre-vulcanization, the resulting tensile strength properties of the product films are about 50% of what they otherwise would have been. To achieve the best physical properties, it is best to omit as much of the prevulcanization process as possible. This elimination should favorably impact the economics of latex processing, as the energy and equipment normally associated with prevulcanization is eliminated or reduced.

Method of Using Polynitrile Oxides in the Production of Latex Film Products

Thin walled, dip-molded or cast rubber film products of a natural (Hevea or Guayule) rubber or a synthetic polyisoprene rubber compound, crosslinked with a polynitrile oxide crosslinking agent have been made in the authors' laboratory trials which have superior physical properties, including improved tear and tensile strengths and ultimate elongation. The laboratory-produced films have tear strengths ranging from about 15 kN/m to about 70 kN/m, tensile strength from about 1700 psi to about 6000 psi and ultimate elongation from about 550 % to about 1200 %. The general method used consisted of preparing latex films as follows:

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(a) compounding a natural rubber or synthetic polyisoprene rubber latex so as to substantially reduce or prevent pre-vulcanization of the resulting rubber compound; (b) dip-molding or casting the rubber compound to form a rubber film product; (c) admixing the rubber compound or rubber film product with a polynitrile oxide crosslinking agent; and (d) curing the rubber compound to produce crosslinking thereof.

The pre-vulcanization of the rubber compound is reduced or prevented in accordance with one of a number of alternative techniques, for example, by reducing the temperature of the rubber latex or its ingredients prior to molding or casting, or by delaying admixture of the polynitrile oxide with the rubber compound until immediately before curing.

Our series of experiments demonstrate that with the use of polynitrile oxides, it is possible to very substantially increase the tear strength of vulcanizates even without the use of sulfur within the context of accelerator free latex formulations. These experiments further revealed that use of this vulcanization method facilitates the production of vulcanizates of both Hevea natural rubber, Guayule natural rubber and synthetic polyisoprene that exhibit minimal cell toxicity and excellent physical property profiles along with low odor and taste. It has also been noted in these experiments that these thin walled rubber film products of natural rubber or synthetic polyisoprene exhibit the superior physical properties and yet contain no components that promote nitrosamine formation.

In producing the rubber films in our experiments, it is believed that the polynitrile oxide fully reacts with the rubber polymers and becomes part of the crosslinked thin walled film product. As noted previously, film products that are cured with polynitrile oxide crosslinking agents contain rubber molecules that are bridged together with a structure containing at least two isoxazoline units.

As a specific example, the thin walled, dip-molded or cast rubber film products are crosslinked with stable polynitrile oxides and incorporate the structure shown in Figure 1.

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R3 R1 R2

O R1 4 R N

R4

R4 N R1 O

R2 R1 R3 Figure 1: Crosslink Structure

R1 represents the natural rubber or cis-1,4- polyisoprene rubber polymer chain; either R2 is methyl and R3 is hydrogen, or R3 is methyl and R2 is hydrogen. R4 is methyl or ethyl.

It is noted in our experiments that even if the polynitrile oxide were not to fully react during the initial manufacturing process, i.e., upon curing, it would fully react at room temperature shortly thereafter. Thus, it is only necessary to carry out a very rapid initial cure under mild conditions, prior to stripping of the formed film. After stripping, the remainder of the curing can occur just at room temperature. If desired, the entire cure can be rapidly completed before stripping. This complete cure could be done in a small fraction of the time that would be necessary for traditional vulcanization methods.

Because no residual chemicals remained in the fully cured rubber films, many of the films produced were noted as being essentially free of odor and had no taste. Some taste and odor still was present in some of the natural rubber compounds due to naturally occurring chemicals found in those compounds.

The natural rubber used in our experiments was derived from two sources. The first was obtained from , which most people associate as being natural rubber. The second source

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used was from Parthenum argentatum (commonly known as "guayule" rubber). Natural rubber latex is available in several grades, including high ammonia latex, low ammonia latex, and others. All are suitable for polynitrile oxide vulcanization. One variety of latex which has received much attention is that which has had its protein content reduced by suitable means. It is sometimes referred to as DPNR, for deproteinized natural rubber latex. Polynitrile oxide vulcanization works very well for this type of latex rubber.

A number of experiments were conducted with synthetic cis-1,4-polyisoprene latex. Laticies containing both the Ziegler catalyst produced polyisoprene, and those produced via anionic polymerization were tested. Polynitrile oxide vulcanization worked exceptionally well for both types of synthetic polyisoprene latex.

In deciding upon which polynitrile oxide crosslinking agents may be suitable for use in vulcanizing latex films, it is clear that any polynitrile oxide chosen must be one that bonds the individual rubber molecules with at least two isoxazoline moieties to form a crosslink unit. The polynitrile oxide crosslinking agents for latex may be any of those previously known to work with latex foam, such as those appearing in U.S. Patent 3,390,204, U.S. Patent 6,252,009, and U.S. Patent 6,355,826. Only stable polynitrile oxides are to be considered for use in latex vulcanization applications. Two specifically suitable polynitrile oxides are 2,4,6- trimethylbenzene dinitrile oxide (PNO-A) or 2,4,6-triethylbenzene dinitrile oxide.

Compounding of the latex is similar to compounding of any other latex used for dip molding. The differences are that the sulfur, accelerator, and activators are left out of the formulation. In their place is added a dispersion or emulsion of the desired PNO. As is the case in all latex compounding, small amounts of other materials can also be included as additives or blending agents. Reinforcement agents, pigments and dyes may also be included, but are not necessary. addition is still essential to protect the dip molded or cast latex articles post- manufacture.

As previously stated, the very high level of reactivity of the polynitrile oxides should be taken into account to prevent premature reaction of the polynitrile oxide with the rubber compound,

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i.e., prior to the formation of a wet or dry gel of the rubber article. This will prevent the rubber compound from excessive pre-vulcanizing. The inhibition of pre-vulcanization can be done in a number of different ways.

One approach is to tightly control the temperature of the compounding ingredients. The temperature of the compound may be significantly reduced prior to mixing. Temperatures just above the freezing point of water would work best, with higher temperature working progressively less well. The compounded latex may then be stored at a reduced temperature (e.g., from a temperature of about 32 °F to 60 °F) prior to use. This technique is quite useful, but it only slows down the pre-vulcanization process to an extent. For rubber compounds that can be used within 60 minutes or less, from the time the polynitrile oxide is added to the rubber compound, this can be a very useful technique.

Perhaps a more practical way to deal with the prevulcanization issue is to initially compound the latex with all ingredients, except for the polynitrile oxide dispersion. Immediately prior to use, the polynitrile oxide can then be added. In a continuous system, the polynitrile oxide can be mixed in with a metering pump, for instance. Alternatively, the polynitrile oxide can be mixed directly into a small batch of otherwise fully compounded rubber, immediately prior to use. Once mixed, the polynitrile oxide is free to react, but the full volume of compounded rubber is almost immediately used up in the manufacturing process.

Another method is to allow for the constant addition of new, freshly made compounded rubber to a dip tank of relatively small internal volume, and then processing a very large number of formers very quickly. In this manner, the resonance time for the compounded rubber is very short, allowing for only a small amount of pre-vulcanization.

As indicated above, polynitrile oxides can be used in the formation of thin films with dip molding or casting techniques; however, formation of a latex into thin films and other formats can be accomplished by any conventional method, including spraying, rolling, the use of a doctor blade, or other techniques. In its fully prevulcanized form, the liquid latex can potentially be applied directly onto human skin, for instance for form-in-place gloves, dressing adhesives, etc.

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In this case, there would be no expectation of toxicity in its liquid form, if properly formulated. Current liquid latex used for skin application has many associated toxicity issues. Or course, for the most common medical and personal devices, particularly those that are hollow, such as condoms, surgical and examination gloves, and finger cots, dip molding is an especially effective and convenient means of forming the film.

The technique used for curing in the present method can be any technique for obtaining complete crosslinking of the polynitrile oxide crosslinking agent with the rubber compound. Curing can take place in a convection oven, forced convection oven, steam chamber, or molten media bath. Additionally, infrared heating or microwave heating techniques may be used, or the film product can remain at room temperature until curing is completed. Thin walled dip-molded or cast rubber film products can be cured at temperatures ranging from about 0 °F to about 350 °F. Commercial economical curing temperatures are in the range of from about 60 °F to about 212 °F. If it is a goal to reduce energy costs, it is best to use a modestly elevated temperature for a relatively short period of time. The extent of vulcanization should be sufficient to allow for the safe stripping of the formed part from the mandrel or former. Full vulcanization will then take place over the next 24 hours or so at room temperature. The level of vulcanization is determined by the amount of polynitrile oxide used in the latex compound, not by the vulcanization conditions. It is not necessary to be concerned about over cure or reversion, as is the case with accelerated- sulfur vulcanization.

Experimental Examples

The following is a sampling of some very specific laboratory experiments we conducted, which were useful in characterizing the attributes of polynitrile oxides as vulcanization agents for latex films.

Materials:

(1) Latices

Synthetic cis-1, 4-polyisoprene latex containing approximately 60% solids

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Natural Rubber Latex (NRL) 61.3% solids

(2) Crosslinking Agents

Polynitrile Oxide

30% active 2,4,6-trimethylbenzene dinitrile oxide (PNO-A) aqueous dispersion, prepared by Apex Medical Technologies, Inc. using conventional dispersion techniques, which included the use of a common surfactant and dispersing agent.

Organic Peroxide

Dicumyl Peroxide Dispersion: A master batch of 37% active dicumyl peroxide dispersion was prepared by conventional methods, resulting in a dispersion in which the dicumyl peroxide was uniformly dispersed.

Sulfur Dispersion

A 68% active commercially available dispersion of sulfur was used.

Zinc Oxide Dispersion

A 62% active commercially available zinc oxide dispersion was also used

Surfactant

A 30% total solids Sodium Alkyl Sulfate solution was used.

Reinforcing Agents

A 52% total solids 77% styrene content Styrene Butadiene Rubber Latex (SBR) was prepared by standard procedure

20% (by weight) aqueous dispersion of fumed silica.

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Antioxidant

The antioxidant consisted of a dispersion of 4-[[4,6-Bis(octylthio)-s-trianzin-2-yl]amino]-2,6-di- butylphenol

Coagulant Solution

The coagulant consisted of an aqueous solution of 20% solids calcium nitrate and 0.5% of Nonyl Phenol 9 Mole Ethoxylate

Preparation of Test Films:

Ingredients for each of the following formulations were weighed into a 500 mL bottle and mixed thoroughly. The formulation was filtered into a polyethylene graduated cylinder and all bubbles were removed in preparation for dip-molding

The respective test films were dip-molded on 32 mm OD glass mandrels without a maturation period for the compounded latex. The mandrels were pre-heated in a 150 ° F oven, then dipped into the coagulant solution at a speed of 0.8 inches per second and lifted out at a speed of 0.2 inches per second. The coagulant coated mandrels were dried for 5 minutes in a 150 ° F oven. The mandrels were thereafter dipped into the latex at a speed of 0.8 inches per second and lifted out at an exit speed of 0.2 inches per second. The mandrels were allowed to dwell in the latex for 15 seconds.

Once dipped, the films were dried for 5 minutes in a 150 ° F oven. The films were leached in a 140º F water bath for 3 minutes. The resulting films were then dried in a 150 ° F oven for 60 minutes. The peroxide cured films were additionally cured for 9 minutes in a 350º F salt bath.

The films were rinsed, stripped with powder and readied for tear testing per ASTM D624, and for tensile testing per ASTM D3492.

The following test films prepared in the following examples were formed by dip molding as described above.

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Table 1: Comparison of Synthetic Polyisoprene, Hevea Natural Rubber and Guayule Natural Rubber Latices Cured with PNO-A and Dicumyl Peroxide Crosslinking Agents with Various Reinforcing Agents

Examples 1-3 Comp. Prep. A-C Example 4-5 Comp. Prep. D-E Example 6 Comp. Prep. F PNO-A Peroxide PNO-A Peroxide PNO-A Peroxide No SBR, No Sulfur No SBR, No Sulfur 5 phr SBR 5 phr SBR 5 phr SBR 0.4 5 phr SBR 0.4 phr Sulfur phr Sulfur Ingredient Parts by Weight Parts by Weight Parts by Weight Parts by Weight Parts by Parts by Weight (Dry) (Dry) (Dry) (Dry) Weight (Dry) (Dry) Synthetic Polyisoprene Latex 100 - - 100 - - 100 - 100 - 100 100 Hevea Natural Rubber Latex - 100 - - 100 - - 100 - 100 - - Guayule Natural Rubber Latex - - 100 - - 100 ------PNO-A Dispersion 1.2 1.4 1.4 - - - 1.2 1.4 - - 1.2 - Dicumyl Peroxide Emulsion - - - 1.2 1.4 1.4 - - 1.2 1.4 - 1.2 Styrene Butadiene Rubber ------5 5 5 5 5 5 Latex (SBR) Sulfur Dispersion ------0.4 0.4 Surfactant 0.5 Aqueous Silica Dispersion 2 Antioxidant Dispersion 2 Deionized Water Dilute to 45% solids

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Table 2: Comparison of Synthetic Polyisoprene Latex Films with PNO-A (Example 1) and Dicumyl Peroxide (Comp. Prep. A) Crosslinking Agents with no SBR and no Sulfur

Property Example 1 Comp. Prep. A

Tensile Modulus

50% 56 48

100% 84 77

300% 152 165

500% 238 335

Ultimate Tensile Strength 4912 3337 (psi)

Increase in Tensile Strength 47.2% -

Ultimate Percent Elongation 1105 791 (%)

Tear Strength (kN/m) 33.2 11.2

Increase in Tear Strength 196% -

It is apparent in Example 1, that utilizing PNO-A as the crosslinking agent provides for superior tensile and tear properties as compared with Comp. Prep. A (utilizing a peroxide crosslinking agent, but otherwise identical), even with the SBR reinforcing agent omitted. These experiments show the limited contribution that the SBR makes to increased tear strength, while the PNO-A crosslinker significantly increases the tensile strength and tear strength.

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Table 3: Comparison of Hevea Natural Rubber Latex Films with PNO-A (Example 2) and Dicumyl Peroxide (Comp. Prep. B) Crosslinking Agents with no SBR and no Sulfur

Property Example 2 Comp. Prep. B

Tensile Modulus

50% 56 56

100% 82 91

300% 158 212

500% 471 604

Ultimate Tensile Strength 4857 3893 (psi)

Increase in Tensile Strength 24.8% -

Ultimate Percent Elongation 907 718 (%)

Tear Strength (kN/m) 32.8 13.0

Increase in Tear Strength 152.3% -

The films of Example 2, utilizing PNO-A as the crosslinking agent but free of accelerator, exhibit superior tensile and tear properties as compared with Comp. Prep. B (utilizing a peroxide as crosslinking agent, but otherwise identical). Once again, even with the increase of tensile and tear properties, the tensile modulus values remained low.

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Table 4: Comparison of Guayule Natural Rubber Latex Films with PNO-A (Example 3) and Dicumyl Peroxide (Comp. Prep. C) Crosslinking Agents with no SBR and no Sulfur

Property Example 3 Comp. Prep. C

Tensile Modulus

50% 41 41

100% 58 61

300% 93 129

500% 172 225

Ultimate Tensile Strength 4030 3252 (psi)

Increase in Tensile Strength 24%

Ultimate Percent Elongation 1149 833 (%)

The films of Example 3, utilizing PNO-A as the crosslinking agent but free of accelerator, exhibit superior tensile and tear properties as compared with Comp. Prep. C (utilizing a peroxide as crosslinking agent, but otherwise identical). Once again, even with the increase of tensile and tear properties, the tensile modulus values remained low.

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Table 5: Comparison of Synthetic Polyisoprene Latex Films with PNO-A (Example 4) and Dicumyl Peroxide (Comp. Prep. D) Crosslinking Agents with 5 phr SBR

Property Example 4 Comp. Prep. D

Tensile Modulus

50% 66 58

100% 91 92

300% 164 199

500% 257 396

Ultimate Tensile Strength 4911 3402 (psi)

Increase in Tensile Strength 44.36% -

Ultimate Percent Elongation 1124 801 (%)

Tear Strength (kN/m) 36.2 12.7

Increase in Tear Strength 185% -

As will be apparent from the preceding tabulation, Example 4 (utilizing PNO-A as the crosslinking agent with a conventional SBR reinforcing agent but free of accelerator) exhibits superior tensile and tear properties as compared with Comp. Prep. D (utilizing a peroxide crosslinking agent, but otherwise identical).

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Table 6: Comparison of Hevea Natural Rubber Latex Films with PNO-A (Example 5) and Dicumyl Peroxide (Comp. Prep. E) Crosslinking Agents with 5 phr SBR

Property Example 5 Comp. Prep. E

Tensile Modulus

50% 74 70

100% 112 112

300% 273 304

500% 1084 1235

Ultimate Tensile Strength 5463 4141 (psi)

Increase in Tensile Strength 31.92% -

Ultimate Percent Elongation 820 666 (%)

Tear Strength (kN/m) 41.1 13.1

Increase in Tear Strength 213.7% -

The films of Example 5, utilizing Hevea Natural Rubber and an PNO-A crosslinking agent with a conventional SBR reinforcing agent but free of accelerator, exhibit superior tensile and tear properties as compared with Comp. Prep. E, which utilize Hevea Natural Rubber and a peroxide crosslinking agent, but otherwise identical. Again, even with the increase of tensile and tear properties, the tensile modulus values remained low.

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Table 7: Comparison of Synthetic Polyisoprene Latex Films with PNO-A (Example 6) and Dicumyl Peroxide (Comp. Prep. F) Crosslinking Agents with 5 phr SBR and 0.4 phr Sulfur

Property Example 6 Comp. Prep. F

Tensile Modulus

50% 65 55

100% 96 80

300% 176 150

500% 291 245

Ultimate Tensile Strength 4666 4361 (psi)

Increase in Tensile Strength 7.0% - (Ex. 3 vs. Comp. Prep. C)

Ultimate Percent Elongation 1081 1084 (%)

Tear Strength (kN/m) 39.6 18.1

Increase in Tear Strength 118.8% - (Ex. 3 vs. Comp. Prep. C)

Example 6 shows that films vulcanized with PNO-A and sulfur as the crosslinking agents with a conventional SBR reinforcing agent but free of accelerator, exhibit modestly higher tensile and substantially higher tear strength as compared with Comp. Prep. F, which utilized peroxide and sulfur as crosslinking agents, but was otherwise identical. Even with the increase of tensile and tear properties, the tensile modulus values remained low. This is very useful when viewed in the context of making medical gloves. 25

Figures 2 through 10 below compare the physical properties of the synthetic polyisoprene, Hevea natural rubber and guayule latices cured with PNO-A and dicumyl peroxide with various reinforcing agents.

Figure 2: Tensile Strength Comparison of Synthetic Polyisoprene (SPIL), Hevea Natural Rubber (NRL) and Guayule Latices Cured with PNO-A and Dicumyl Peroxide Crosslinking Agents with No SBR and No Sulfur

6000

4912 4857 5000

4030 3893 4000 3337 3252

3000

2000 Tensile Strength Tensile (psi) Strength

1000

0 SPIL (Peroxide) SPIL (PNO-A) NRL (Peroxide) NRL (PNO-A) Guayule Guyule (PNO-A) (Peroxide)

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Figure 3: Percent Elongation Comparison of Synthetic Polyisoprene (SPIL), Hevea Natural Rubber (NRL) and Guayule Latices Cured with PNO-A and Dicumyl Peroxide Crosslinking Agents with No SBR and No Sulfur

1400

1200 1149 1105

1000 907 833 791 800 718

600

Percent ElongationPercent (%) 400

200

0 SPIL (Peroxide) SPIL (PNO-A) NRL (Peroxide) NRL (PNO-A) Guayule Guyule (PNO-A) (Peroxide)

Figure 4: Tear Strength Comparison of Synthetic Polyisoprene (SPIL), Hevea Natural Rubber (NRL) and Guayule Latices Cured with PNO-A and Dicumyl Peroxide Crosslinking Agents with No SBR and No Sulfur

35 33.2 32.8

30

25

20

15 13 11.2 Tear Strength (kN/m) Strength Tear 10

5

0 SPIL (Peroxide) SPIL (PNO-A) NRL (Peroxide) NRL (PNO-A)

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Figure 5: Tensile Strength Comparison of Synthetic Polyisoprene (SPIL) and Hevea Natural Rubber (NRL) Latices Cured with PNO-A and Dicumyl Peroxide Crosslinking Agents with 0.5 phr SBR

6000 5463

4911 5000

4141 4000 3402

3000

2000 Tensile (psi) Strength

1000

0 SPIL (Peroxide) SPIL (PNO-A) NRL (Peroxide) NRL (PNO-A)

Figure 6: Percent Elongation Comparison of Synthetic Polyisoprene (SPIL) and Hevea Natural Rubber (NRL) Latices Cured with PNO-A and Dicumyl Peroxide Crosslinking Agents with 0.5 phr SBR

1200 1124

1000

801 820 800 666

600

400 Percent ElongationPercent (%)

200

0 SPIL (Peroxide) SPIL (PNO-A) NRL (Peroxide) NRL (PNO-A)

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Figure 7: Tear Strength Comparison of Synthetic Polyisoprene (SPIL), Hevea Natural Rubber (NRL) and Guayule Latices Cured with PNO-A and Dicumyl Peroxide Crosslinking Agents with 0.5 phr SBR

45

40

35

30

25

20

15 Tear Strength (kN/m) Strength Tear

10

5

0 SPIL (Peroxide) SPIL (PNO-A) NRL (Peroxide) NRL (PNO-A)

Figure 8: Tensile Strength Comparison of Synthetic Polyisoprene (SPIL) Cured with PNO-A and Dicumyl Peroxide Crosslinking Agents with 0.5 phr SBR and 0.4 phr Sulfur 5000 4666

4500 4361

4000

3500

3000

2500

2000 Tensile Strength Tensile (psi) Strength 1500

1000

500

0 SPIL (Peroxide) SPIL (PNO-A)

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Figure 9: Percent Elongation Comparison of Synthetic Polyisoprene (SPIL) Cured with PNO-A and Dicumyl Peroxide Crosslinking Agents with 0.5 phr SBR and 0.4 phr Sulfur

1200 1084 1081

1000

800

600

400 Percent ElongationPercent (%)

200

0 SPIL (Peroxide) SPIL (PNO-A)

Figure 10: Tear Strength Comparison of Synthetic Polyisoprene (SPIL) Cured with PNO-A and Dicumyl Peroxide Crosslinking Agents with 0.5 phr SBR with 0.4 phr Sulfur

45 39.6 40

35

30

25

20 18.1

15 Tear Strength (kN/m) Strength Tear

10

5

0 SPIL (Peroxide) SPIL (PNO-A)

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As can be seen from Figures 2 through 10, it is clear that polynitrile oxides can be used to produce both natural rubber and synthetic polyisoprene dip-molded film products, which have superior tensile strength, tear strength, and elongation properties.

EXAMPLE 7

Influence of a Maturation Period on Physical Properties of Film Product of Example 1

This example illustrates test films prepared from the aqueous latex of Example 4, but with a maturation period of 24 hours at room temperature prior to formation of the films. A comparison of the tensile properties of the test films prepared with (Comp. Prep. G) and without (Example 4) a maturation period are shown in Table 8.

Table 8: Demonstration of Undesirable Pre-vulcanization from Maturation

Modulus Values (PSI) 50% 100% 300% 500% Ultimate Ultimate Tear Tensile Strength Elongation Strength (PSI) (%) (kN/m) Example 1 66 91 164 257 4911 1124 36.2 Comp. Prep. G (Films 62 92 168 270 2179 995 18.3 prepared after 24 hours standing at room temperature)

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EXAMPLE 8

Dental Dams Formed by Casting Techniques

Dental dams were formed from the compounds used in Examples 4 and Comp. Prep. G. Rather than being dip-molded, as in Example 4, the dental dams were formed by casting the compounded latex onto thin flat sheets of stainless steel. The dental dams were vulcanized by essentially the same method as that of Example 4. The resulting dental dams were compared for taste and odor. The rubber dam made with the formulation of Comparative Preparation G had an odor and a detectable taste. The rubber dam made from the formulation of Example 4 did not have any detectable taste or odor.

EXAMPLES 9 – 10

Polyisoprene Condoms Formed from Pre-Cooled Rubber Compound

Two batches of synthetic polyisoprene latex were compounded as described in Example 4. Multiple sets of latex condoms were prepared by the same method as that of Example 4 at time intervals of .75 hours, 7.5 hours, 24.5 hours, and 31.5 hours, all in relation to initial time (t = 0) corresponding to formulation time.

During the time intervals between the film preparation of the respective film products, one batch of latex was stored at room temperature and the second was stored in a bath of ice and water at approximately 0 ºC. Once dipped, the films were processed in the same manner as Example 4.

This technique was utilized to produce films for tensile testing. Tables 9 and 10 and Figure 2 show the results of physical property testing, per ASTM D3492, as a function of preparation time. As may be seen, placing the formulated latex into an ice bath prior to use is an effective way to slow down the pre-vulcanization of the compounded latex. It is clear that pre-vulcanization occurs in the liquid latex by noting the continually increasing 100% modulus value of the test specimens cut from the resulting condoms, which is an indicator of overall cure levels. However, the tensile strengths of the test films from the condoms drop over time, due to the undesirable nature of pre-vulcanization.

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Table 9: Physical Property Data for Rubber Compounded at 0 ºC (Example 9)

Latex Aged in Ice Bath Modulus Values (PSI) Time Interval after 50 100 300 500 Ultimate Ultimate compounding Tensile Strength Percent (hours) (PSI) Elongation (%) 0.75 (Baseline) 58 84 155 241 4961 1168 7.5 60 85 155 247 3739 1106 24.5 58 85 152 239 3334 1109 31.5 57 83 153 241 2939 1086

Table 10: Physical Property Data for Rubber Compounded at 25 ºC (Example 10)

Latex Aged at Room Temperature Modulus Values (PSI) Time Interval after 50 100 300 500 Ultimate Ultimate compounding Tensile Strength Percent (hours) (PSI) Elongation (%) 0.75 (Baseline) 58 84 155 241 4961 1168 7.5 59 86 161 256 3312 1078 24.5 64 91 164 264 2112 987 31.5 67 99 176 276 2107 991

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Figure 11

Potlife Study of MDNO Cured Films 6000 Ice Bath Aging 5000 Room Temp. Aging Baseline Start Value 4000

3000

2000

Tensile Strength (PSI) Strength Tensile 1000

0 0 10 20 30 Time (Hours)

Example 11 Hypothetical Latex Gloves Formed by Continuous Dipping

Synthetic polyisoprene latex is prepared by the same method as that of Example 4 for a continuous glove dipping operation. As glove formers are processed through the dipping tank, a measurable quantity of latex is removed. Newly compounded latex is continually added into the dipping tank in quantities that closely or nearly equal the amount of latex that is being removed on the glove dipping formers.

This technique keeps the volume of latex in the dipping tank nearly constant, while continually replacing aging latex. In doing so, the latex in the dipping tank is refreshed at a given rate, keeping the residence time for the compounded latex very short, allowing for only a small amount of pre- vulcanization.

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The glove made in this example is a standard size 6-½ latex surgical glove that has a weight of about 12.5 grams. The volume of latex, at 45% total solids content (TSC), used to produce this glove is 30 mL.

In the example, 30 mL of latex is added to the dipping tank for each dipped glove former that is removed from the tank. Once dipped, the glove former proceeds along the glove-dipping machine, which carries out the essential latex article processing steps described in Example 4. At the end of the line, the glove is stripped from the former and the resulting glove is fully vulcanized with excellent properties.

A dipping tank that can accommodate 25 glove formers at any given time has dimensions of 20” wide by 20” long with a height of 12”. This tank has a filled volume of 4800 in 3. Given this volume, each former dipped removes 0.0375% of the total volume. At this rate, one tank volume worth of latex is used for every 2667 glove formers dipped. 100 glove formers are dipped per minute resulting in one tank volume being added every 27 minutes. Accordingly, for every 2.3 tank volumes removed and replenished, only 10% of the original volume of latex remains. This translates into 90% of the original latex being removed every 62 minutes. This method prevents the latex from reaching an unacceptable state of pre-vulcanization.

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As shown in Table 11, the average age of the latex in the tank reaches steady state conditions after about 248 minutes (4 hours), and remains indefinitely at an average age of 37.89 minutes.

Table 11: Average Latex Age in Dipping Tank

Time (minutes) Average Age (minutes) 0 0.00 62 34.10 124 37.51 186 37.85 248 37.89 310 37.89 372 37.89 434 37.89 496 37.89 558 37.89 620 37.89

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References :

1. THE VULCANIZATION OF RUBBER WITH PHENOL FORMALDEHYDE DERIVATIVES. I Author: Van Der Meer, S. Rubb. Chem. Tech., Volume 18, 1945, pp.853-873

2. THE VULCANIZATION OF RUBBER WITH PHENOL-FORMALDEHYDE DERIVATIVES. INAPPLICABILITY OF THE CHROMANE THEORY Author: Van Der Meer, S. Rubb. Chem. Tech, Volume 20, 1947, pp. 173-181

3. Peter Kovacic, “Bisalkylation Theory of Vulcanization” (Industrial and Engineering Chemistry, Vol 47, No. 5 pages 1090 – 1094, May, 1955).

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