energy efficient, accelerator-free, cold vulcanization of

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Energy Efficient, Accelerator-Free, Cold Vulcanization of Latex Articles

Mark W. McGlothlin, Whitney A. Williams, and Scott W. H errick

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

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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 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,

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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 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

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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 dithiocarbamates, 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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R4

R4

R4

N

O

O

N

R1

R3

R1

R1

R2

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 Hevea brasiliensis, 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.

Antioxidant 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

17

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.

18

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 polyethylene

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.

19

Tab

le 1

: Com

paris

on o

f Syn

thet

ic P

olyi

sopr

ene,

Hev

ea N

atur

al R

ubbe

r an

d G

uayu

le N

atur

al R

ubbe

r La

tices

C

ured

with

PN

O-A

and

Dic

umyl

Per

oxide

Cro

sslin

king

Age

nts

with

Var

ious

Rei

nfor

cing

Age

nts

E

xam

ples

1-3

PN

O-A

No

SBR

, No

Sulf

ur

Com

p. P

rep.

A-C

Per

oxid

e

No

SBR

, No

Sulf

ur

Exa

mpl

e 4-

5

PN

O-A

5 ph

r SB

R

Com

p. P

rep.

D-E

Per

oxid

e

5 ph

r SB

R

Exa

mpl

e 6

PN

O-A

5 ph

r SB

R 0

.4

phr

Sulf

ur

Com

p. P

rep.

F

Per

oxid

e

5 ph

r SB

R 0

.4

phr

Sulf

ur

Ingr

edie

nt P

arts

by

Wei

ght

(Dry

)

Par

ts b

y W

eigh

t

(Dry

)

Par

ts b

y W

eigh

t

(Dry

)

Par

ts b

y W

eigh

t

(Dry

)

Par

ts b

y

Wei

ght (

Dry

)

Par

ts b

y W

eigh

t

(Dry

)

Syn

thet

ic P

olyi

sopr

ene

Late

x

100

- -

100

- -

100

- 10

0 -

100

100

Hev

ea N

atur

al R

ubbe

r La

tex

-

100

- -

100

- -

100

- 10

0 -

-

Gua

yule

Nat

ura

l Rub

ber

Late

x

- -

100

- -

100

- -

- -

- -

PN

O-A

Dis

pers

ion

1.2

1.4

1.4

- -

- 1.

2 1.

4 -

- 1.

2 -

Dic

umyl

Per

oxid

e E

mul

sion

-

- -

1.2

1.4

1.4

- -

1.2

1.4

- 1.

2

Sty

rene

But

adie

ne R

ubbe

r

Late

x (S

BR

)

- -

- -

- -

5 5

5 5

5 5

Sul

fur

Dis

pers

ion

- -

- -

- -

- -

- -

0.4

0.4

Sur

fact

ant

0.5

Aqu

eous

Sili

ca D

ispe

rsio

n 2

Ant

ioxi

dant

Dis

pers

ion

2

Dei

oniz

ed W

ater

D

ilute

to 4

5% s

olid

s

20

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

(psi)

4912 3337

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.

21

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


(psi)

4857 3893



(%)

907 718


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.

22

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


(psi)

4030 3252

Increase in Tensile Strength 24%


(%)

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.

23


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


(psi)

4911 3402



(%)

1124 801


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).

24

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


(psi)

5463 4141



(%)

820 666


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.

25


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


(psi)

4666 4361

Increase in Tensile Strength

(Ex. 3 vs. Comp. Prep. C)

7.0% -


(%)

1081 1084


Increase in Tear Strength

(Ex. 3 vs. Comp. Prep. C)

118.8% -

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.

26

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

3337

4912

3893

4857

3252

4030

0

1000

2000

3000

4000

5000

6000

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

Guyule (PNO-A)

Ten

sile

Str

eng

th (

psi

)

27

Figure 3: Percent Elongation Comparison of Synthetic Polyisoprene (SPIL), Hevea Natural Rubber (NRL) and Guayule Latices Cured with PNO-A and Dicumyl Peroxide


791

1105

718

907833

1149

0

200

400

600

800

1000

1200

1400

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

Guyule (PNO-A)

Per

cen

t E

lon

gat

ion

(%

)

Figure 4: Tear Strength Comparison of Synthetic Polyisoprene (SPIL), Hevea Natural Rubber (NRL) and Guayule Latices Cured with PNO-A and Dicumyl Peroxide


11.2

33.2

13

32.8

0

5

10

15

20

25

30

35

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

Tea

r S

tren

gth

(kN

/m)

28

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

3402

4911

4141

5463

0

1000

2000

3000

4000

5000

6000


Ten

sile

Str

eng

th (

psi

)

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

801

1124

666

820

0

200

400

600

800

1000

1200


Per

cen

t E

lon

gat

ion

(%

)

29

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

0

5

10

15

20

25

30

35

40

45


Tea

r S

tren

gth

(kN

/m)

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

4361

4666

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

SPIL (Peroxide) SPIL (PNO-A)

Ten

sile

Str

eng

th (

psi

)

30

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

1084 1081

0

200

400

600

800

1000

1200


Per

cen

t E

lon

gat

ion

(%

)

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

18.1

39.6

0

5

10

15

20

25

30

35

40

45


Tea

r S

tren

gth

(kN

/m)

31

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

Tensile Strength

(PSI)

Ultimate

Elongation

(%)

Tear

Strength

(kN/m)

Example 1 66 91 164 257 4911 1124 36.2

Comp. Prep. G (Films

prepared after 24 hours

standing at room

temperature)

62 92 168 270 2179 995 18.3

32

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.

33

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

Latex Aged in Ice Bath

Modulus Values

(PSI)

Time Interval after

compounding

(hours)

50 100 300 500 Ultimate

Tensile Strength

(PSI)

Ultimate

Percent

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

compounding

(hours)

50 100 300 500 Ultimate

Tensile Strength

(PSI)

Ultimate

Percent

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

34

Figure 11

Potlife Study of MDNO Cured Films

0

1000

2000

3000

4000

5000

6000

0 10 20 30Time (Hours)

Tens

ile S

tren

gth

(PS

I)

Ice Bath Aging

Room Temp. Aging

Baseline Start Value

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.

35

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 in3. 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.

36

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

37

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 Neoprene Vulcanization” (Industrial and Engineering

Chemistry, Vol 47, No. 5 pages 1090 – 1094, May, 1955).

energy efficient, accelerator-free, cold vulcanization of

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