MXPA99000026A - Formulation of nitrile rubber su - Google Patents

Abstract

Nitrile rubber compositions, articles of manufacture made therefrom, and methods for making them are described. The nitrile rubber compositions of the invention are substantially free of zinc oxide, and have good strength and chemical resistance while being milder than many conventional nitrile rubbers. The invention provides articles of manufacture, including gloves, which are soft, yet strong, and can be manufactured economically.

Description

SOFT NITRILO RUBBER FORMULATION BACKGROUND OF THE INVENTION The development of modern rubber materials has made it possible to manufacture a wide range of elastomeric articles having varying strength and chemical resistance properties. A useful class of rubber compounds is the class of nitrile rubber, which is widely used to make a variety of articles of manufacture. The carboxylated nitrile, which is a terpolymer of butadiene monomers, acrylonitrile and organic acid, has at least two properties that make it useful for making elastomeric articles. These two particularities are high resistance and impermeability to certain hydrocarbon solvents and oils. The composition and curing of rubber (which is used in latex form for, e.g., immersion to provide manufacturing articles such as gloves or condoms) with other ingredients such as curing agents, accelerators and activators is generally done to optimize these properties. The level of each monomer in the polymer and the level of curing affect the resistance levels and the chemical resistance in the finished article. Polymers with higher levels of acrylonitrile tend to have better resistance to aliphatic oils and solvents, but are also stiffer than polymers that have lower levels of acrylonitrile. While the chemical nature of the monomers from which the polymer is made offers some degree of chemical resistance, when the polymer molecules are chemically crosslinked, chemical swelling, permeation and dissolution will greatly increase. The crosslinking also increases the strength and elasticity of the rubber. The carboxylated nitrile latexes can be chemically crosslinked in at least two ways: the butadiene subunits can be covalently crosslinked with sulfur / accelerator systems; and the carboxylated sites (organic acid) can be ionically crosslinked with metal oxides or salts. Sulfur crosslinks frequently result in large improvements in oil and chemical resistance. Ionic cross-links, which result, for example, the addition of zinc oxide to latex, result in a rubber having high tensile strength, puncture resistance, and abrasion resistance, as well as high elastic modulus (a measure of ^ the force required to stretch a rubber film), but low resistance to oil and chemistry. Many rubber formulations currently available usually employ a combination of the two curing mechanisms. For example, in combination with sulfur and accelerators, manufacturers of carboxylated nitrile latex frequently recommend adding 1-10 parts of zinc oxide per 100 parts of rubber. When zinc oxide is not used, the curing time required to reach an optimum curing state can be much longer and curing can be less efficient. This means that the crosslinks are longer (more sulfur atoms by crosslinking) and there may be a higher amount of sulfur than non-crosslinked polymer chains. The result can be a less effectively cured rubber that has reduced thermal resistance and less chemical resistance. However, "ionic crosslinking often increases the stiffness of an article made from rubber." This is a disadvantage for applications where a soft rubber is needed For example, surgical gloves made of soft rubbers can provide greater sensitivity of tact for the user, which is desirable to improve the "touch" of the surgeon during operations to prevent hand fatigue.

A glove of. More convenient nitrile that is easier to stretch, that is, has lower elastic modulus, can be made using a polymer that contains less acrylonitrile or crosslinking the polymer to a lesser degree. These changes, however, often compromise resistance, chemical resistance, or both, resulting in articles that are inappropriate for many applications. Consequently, a soft rubber having similar strength and chemical resistance to the more rigid rubbers is highly desirable.

SUMMARY OF THE INVENTION An object of this invention is to provide mild nitrile rubber formulations having comparable strength and chemical resistance with conventional compositions of more rigid rubber. Another object of the invention is to provide soft nitrile rubber articles, e.g., gloves, which are strong and provide good chemical resistance, while being softer and more comfortable than conventional articles. Thus, in one aspect, the invention provides a method for making a nitrile rubber composition. The method includes the steps of combining a nitrile latex base with a stabilizing agent and adjusting the pH of the nitrile latex to about 8.5-10.0 to provide a basic nitrile latex. The nitrile latex is contacted with a crosslinking agent substantially free of zinc oxide and with at least one accelerator, to form a nitrile rubber composition. This composition is substantially free of zinc oxide. In preferred embodiments, the stabilizing agent is ammonium caseinate. In preferred embodiments, the step of adjusting the pH includes adding an alkaline hydroxide to the nitrile latex. In the preferred embodiments, the crosslinking agent is sulfur or a sulfur donor compound. In preferred embodiments, the accelerator is tetramethylthiuram disulfide in combination with mercaptobenzothiazole (MBT). In the preferred embodiments, the method includes the additional step of aging the nitrile rubber composition for a period of one to three days, before the composition is formed into finished rubber articles, or other use. In a preferred embodiment, the method includes the additional step of curing the nitrile rubber composition to form a cured nitrile rubber composition. The invention further provides a cured nitrile rubber composition formed by the method described above. In a preferred embodiment, the cured nitrile rubber composition is substantially free of divalent metal oxides. The invention also provides an article of manufacture comprising a layer of the cured nitrile rubber composition formed by the method described above. In another aspect, the invention provides a cured nitrile rubber composition. The cured nitrile rubber composition includes a nitrile latex base, a stabilizing agent, a crosslinking agent substantially free of zinc oxide, and an accelerator, and is substantially free of zinc oxide. in addition, the cured nitrile rubber composition has a modulus of "elasticity of 300% less than about 77.33 kg / cm2, preferably in the range of about 28.12 kg / cm2 to about 77.33 kg / cm2." In preferred embodiments, The cured nitrile rubber composition is further characterized in that the 300% modulus is in the range of about 35.15 kg / cm2 to about 49.21 kg / cm2 In the preferred embodiments, the cured nitrile rubber is further characterized in that the tensile strength is greater than about 351.50"kg / cm2. In preferred embodiments, the rubber is substantially free of zinc. In preferred embodiments, nitrile rubber is resistant to organic solvents. In yet another aspect, the invention provides a cured nitrile rubber prepared from a nitrile latex base, a stabilizing agent, a crosslinking agent, and an accelerator, and the nitrile rubber is substantially free of zinc oxide. . The cured nitrile rubber is also characterized by having a 300% modulus of elasticity in the range of about 28.12 kg / cm2 to about 77.33 kg / cm2, and having a tensile strength greater than about 351.50 kg / cm2. In yet another aspect, the invention provides a method for making a nitrile rubber cured with sulfur, comprising the steps of combining a carboxylated nitrile latex with a stabilizing agent, adjusting the pH of the nitrile latex to about 8.5-10.0 to provide a basic nitrile latex, and contacting the basic nitrile latex with an agent of substantially free zinc oxide crosslinking selected from the group consisting of sulfur and sulfur donors, and with an accelerator, to form a nitrile rubber. The resulting nitrile rubber is substantially free of zinc oxide.

In another aspect, the invention provides a carboxylated nitrile rubber cured with sulfur, wherein the rubber is substantially free of zinc oxide, and further characterized in that the rubber has a 300% modulus of elasticity on the scale of about 28.12. kg / cm2 at approximately 77.33 kg / cm2, and in which the cured rubber has a tensile strength greater than about 351.50 kg / cm2. In another aspect, the invention provides nitrile rubber compositions made in accordance with any of the above methods In another aspect, the invention provides articles of manufacture made from any of the nitrile rubber compositions of the invention. Preferred article of manufacture is a glove In the preferred embodiments, a glove in accordance with the present invention includes a layer of a cured nitrile rubber composition of the invention, the layer having a thickness of between about 0.08 mm and about 0.76. Other features, objects and advantages of the present invention will become apparent from the following detailed description, drawings and claims.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a bar graph comparing the physical properties of the rubber material of the invention with a commercially available rubber material. Figure 2 is a bar graph comparing the chemical resistance of the rubber material of the invention with a commercially available rubber material.

Detailed Description of the Invention The invention provides nitrile rubber formulations useful for making articles having good strength and chemical resistance while being softer (i.e. having a lower elastic modulus) than many previously known rubber formulations. The invention also provides methods for making said nitrile rubber compositions, and articles of manufacture made thereof. The term "nitrile latex" is recognized in the art and refers to a synthetic rubber latex used in the manufacture of elastomers. The nitrile latices can be carboxylated or non-carboxylated; carboxylated nitrile latexes are preferred.

As described further below, the nitrile compositions of the invention are preferably substantially free of zinc oxide. The term "substantially free of zinc oxide", as used herein, refers to a nitrile rubber composition in which the zinc oxide is not present in an effective amount to significantly crosslink the components of the nitrile rubber , e.g., the carboxylate moieties of the carboxylated nitrile rubber. However, it will be understood by the skilled artisan that the inventive compositions are preferably substantially free of other compounds, e.g., other metal oxides, which can crosslink, and thus make rigid, the carboxylated fractions of cured rubber. Thus, even when reference is made herein to compositions that are substantially free of zinc oxide, in certain preferred embodiments, the compositions are also substantially free of other metal compounds, particularly divalent metal oxides such as lead oxide, magnesium oxide and the like, which can ionically crosslink the carboxylate fractions of rubber materials. The term "divalent metal oxide", as used herein, refers to a divalent metal oxide, that is, a metal in the +2 oxidation state. Exemplary divalent metal oxides include ZnO, PbO, BaO, MgO, CaO, and the like. In the preferred embodiments, the compositions of the invention include less than about 0.3 parts per hundred dry rubber (phr) of divalent metal oxides, more preferably less than about 0.2 phr- of divalent metal oxides, more preferably, less than about 0.1 phr of divalent metal oxides, and more preferably less than about 0.05 phr of divalent metal oxides. 1. Compositions The carboxylated nitrile latices suitable for use in the methods and compositions of the invention are known in the art, and are commercially available. While any carboxylated nitrile latex can be employed, latexes having high levels of acrylonitrile are preferred due to the increased chemical resistance of said compositions. For example, carboxylated nitrile latexes such as Perbunan N latex (KA8250 and KA8425 (Bayer Corp.) are suitable for use in the present invention Other latices are also suitable, such as Tylac 68-074 and 68-065 (Reichhold Chemical) Perbunan N A8250 latex is preferred, however, in certain embodiments, a nitrile latex with a lower level of acrylonitrile may be used, although an article made from such latex would generally have reduced resistance to hydrocarbons. , the article would also be softer than an article made from an elevated acrylonitrile latex, which is an advantage, eg, in medical gloves.In general, the use of stabilizers in the present rubber formulations may decrease the agglomeration of latex particles, reducing the formation of defects in articles, such as gloves, made from the rubber formulation, in this way, in certain modalities, the use of A stabilizer in an amount effective to prevent agglomeration is preferred. The amount of stabilizer required will vary according to factors such as the nitrile latex employed, the pH of the formulation, and other factors. The skilled artisan will be able to determine the appropriate amounts of a stabilizer. In preferred embodiments, the stabilizer is an emulsifier such as linear sodium dodecylbenzene sulfonate. Other stabilizers include sodium 2-ethylhexyl sulfate. Sodium dodecylbenzenesulfonate, when employed, is preferably present in amounts up to about 1.0 phr. The sodium 2-ethylhexylsulfate is preferably present on a scale of about 0.01 phr to about 1.0 phr, and more preferably about 0.05 phr. The amount of stabilizer used must be controlled to prevent foaming of the nitrile latex. In the preferred embodiments, a protective colloid is present as a stabilizer. A protective colloid can prevent the latex from becoming excessively viscous. Viscosity is a problem in nitrile latexes, particularly at higher pH levels, e.g., at pH greater than about 8.0. The extreme viscosity can lead to gelation of the latex, which is highly undesirable. Consequently, at high pH, a protective colloid is preferably added to the latex. however, the level of protective colloid must be carefully controlled to avoid problems such as slow curing of the nitrile rubber composition. The amount of protective colloid required depends on the characteristics of the latex such as pH, level of solids, and particle size distribution. A preferred protective colloid is ammonium caseinate. In the preferred embodiments, the ammonium caseinate is present in the scale of about 0.1 to about 1.0 phr, more preferably from about 0.2 to about 4.0 phr. Other exemplary protective colloids include plant hydrocolloids such as sodium alginate, non-casein proteins and other water-soluble polymers such as polyvinyl alcohol. In certain embodiments, the protective colloid may be omitted. For example, the addition of water to a latex usually reduces the need for a protective colloid, even when a thinning latex often results. This thin latex may be preferred for manufacturing thin rubber articles. As described above, when zinc oxide is omitted from the previously known nitrile rubber compositions (which typically have a pH of 7.5-8.5), articles made from nitrile rubber will often suffer from low tensile strength and decreased chemical resistance purchased with nitrile rubbers containing zinc oxide. Sulfur-based curing systems are usually retarded by the presence of acidic materials, and are often more active in the presence of alkaline materials (ie, at higher pH). It has now been discovered that if the pH of the nitrile latex is raised to a scale of 8.5-9.5, more preferably around 8.9-9.2, an improved cure results. Although it is not desired to be bound by any theory, it is believed that the higher pH level of the compositions of the invention results in effective activation of the sulfur curing system (described below) and better the curing rate. The specific pH needed may vary depending on the particular nitrile latex that is being used, but can be easily determined by the skilled artisan using no more than routine experimentation. In a preferred embodiment, the latex pH is raised to at least about 8.5 by the addition of an alkali metal hydroxide. Exemplary alkali metal hydroxides include lithium, sodium and potassium hydroxides, with potassium hydroxide being preferred. However, other strong bases can also be used to raise the pH of the nitrile latex composition. In the preferred embodiments, an aqueous solution of an alkali metal hydroxide is added to the nitrile latex. The slow addition of the alkali metal hydroxide gives superior results and is preferred. The use of . Sulfur and sulfur donors (collectively, "sulfur compounds") for vulcanizing (covalently crosslinking) carboxylated nitrile latices is known in the art. A sulfur compound, optionally in combination with an accelerator (which reduces the time required for vulcanization) and / or an activator (i.e., a compound that increases the power of an accelerator), is referred to herein as a "system of cured. " Preferred curing systems for the compositions of the invention include sulfur (i.e., elemental sulfur) and sulfur donors, in combination with at least one accelerator. A variety of accelerators useful in the present invention are known in the art (see, e.g., U.S. Patent No. 5,326,828). Preferred accelerators include sodium dibutyldithiocarbamate, MBT (mercaptobenzothiazole) and TMTD (tetramethyl thiuram disulfide); MBT and TMTD are usually used in combination. Butyl zimate is effective for curing carboxylated nitrile compositions having varied content of acrylonitrile, e.g., low or high, while the combination of MBT and TMTD is generally less effective for low formulations in acrylonitrile. Butyl zimate is more effective than MBT and TMTD at lower temperatures. Consequently, in certain embodiments, butyl zimate is a preferred accelerator. However, in other preferred embodiments, e.g., if a nitrile rubber composition substantially free of zinc is desired, either sodium dibutyldithiocarbamate or the combination of MBT and TMTD is preferred as the accelerator. A nitrile rubber that is substantially free of zinc preferably does not include zinc, from any source, in an amount effective to significantly crosslink the nitrile rubber components, e.g., the carboxylate moieties of the carboxylated nitrile rubber. In certain embodiments, combinations of accelerators may be employed. The nitrile mixture can also optionally include additives commonly used in making cured latex products, including pigments, plasticizers, processing agents, coagulants and the like. A preferred pigment is titanium dioxide, which is useful when a non-transparent article is desired. It is believed that titanium dioxide does not ionically cross-link carboxylated fractions of rubber materials, and therefore, does not undesirably make the rubber materials of the invention undesirable. The plasticizers can be added to improve the "wet gel resistance" of the nitrile rubber. In the preferred embodiments, the nitrile rubber compositions of the invention are substantially free of zinc oxide. As described above, the use of zinc oxide in-1 Carboxylated nitriles are generally associated with the stiffness of cured rubber. By eliminating the use of zinc oxide, the present invention provides a nitrile rubber which is softer than many conventional nitrile rubbers, and yet has strength and solvent-resistant qualities comparable to conventional materials, as shown in Example 2 below. As the skilled artisan will observe, from the description herein, the nitrile rubber compositions of the invention are inexpensive in that they generally do not require the use of additional costly reagents. In this manner, the nitrile rubber compositions of the invention can be made at a cost comparable to that of conventional nitrile rubber formulations.

II. Methods Methods for composing nitrile rubber formulations are known in the art. In this way, the compositions described above can be made using conventional rubber processing equipment and techniques, thus avoiding new tools, which can be expensive. The invention thus provides inexpensive nitrile rubber compositions and articles of manufacture. As described below, certain components of the nitrile rubber can be added at any time during the compounding process, while other components are preferably added in a specific order. In general, the compounding process begins with a water-based carboxylated nitrile latex, which is usually stirred through the compounding process to ensure efficient mixing.Stabilizers, including protective colloids, are generally added before the The pH of the latex rises, the pH of the latex rises, and the curing system is added in. The pigment dispersions, if desired, can be added at any stage.After all the components have been added, the composition of Preferably it is aged for one to five days, more preferably one to three days, before "being formed toward finished items or other use. The period of aging improves the immersion qualities of the latex and results in finished articles that have fewer defects. The articles of manufacture may be made from the compositions of the invention in accordance with methods known in the art. Exemplary manufacturing methods include casting. immersion, molding and the like. Articles such as gloves, condoms and the like are often made by immersing a form towards the latex compositions, thereby preparing a latex film on the surface of the form, the residence time of the form in the composition of the composition. The latex can be selected to obtain a film of a desired thickness on the shape.The preferred shape is immersed in a coagulating solution prior to immersion in the latex composition. "A preferred coagulant solution is a calcium nitrate solution; the solution is preferably in water or an alcohol such as methanol or ethanol. The use of a coagulant can result in a cured rubber having increased strength, e.g., tensile strength. After the form has been immersed in the coagulating solution and towards the nitrile latex composition, the shape (with latex film coating thereon) is preferably immersed in a washing fluid, preferably water, to remove the residual coagulant and other (usually water-soluble) materials from the latex film. The washing time will vary according to the composition of nitrile latex and the coagulant solution used; a period of time from about twenty minutes to about forty minutes is generally appropriate. The latex film is then removed from the wash bath and dried and cured to produce a rubber article. The latex is generally dried at elevated temperature, e.g., a temperature above 382C, more preferably on a scale of about 669C to about 99SC. The time required to dry can be selected to ensure any desired level of moisture in the film before curing. The curing process usually requires somewhat higher heat than drying; Preferred temperatures for curing range from about 93SC to about 149SC. The article is cured until a desired state of cure is reached.After curing, the rubber articles may optionally be further treated according to known methods, eg, by chlorination, followed by neutralization and drying. all treatments, if any, have been completed, the articles can be separated from the forms and packaged The cured nitrile rubber formulations of the invention are softer, ie have lower elastic modulus, than many industrial nitrile rubbers conventional, but the nitrile rubbers of the invention have good strength and chemical resistance.Thus, in one aspect, the invention provides a nitrile rubber material which, when cured, has a 300% modulus of less than about 77.33 kg / cm2 (kilograms per square centimeter) In preferred embodiments, the cured nitrile rubber material has a 300% "modulus on the scale of from 28.12 kg / cm2 to approximately 77.33 kg / cm2, more preferably, from 35.15 kg / cm2 to approximately 49.21 kg / cm2. In preferred embodiments, the cured nitrile rubber material has a tensile strength greater than about 351.50 kg / cm2, more preferably greater than about 386.65 kg / cm2. In preferred embodiments, the cured nitrile rubber material has a chemical resistance characterized by an increase of less than about 7% in area after 24 hours of exposure to hexane at room temperature, more preferably less than about 5% of increase in area, and still more preferably less than about 3% increase in area, and especially preferably around less than about one percent increase in area after 24 hours of exposure to hexane. A cured nitrate rubber material having less than about 7% increase in area after 24 hour exposure to hexane is referred to herein as "chemically resistant". In certain preferred embodiments, the cured nitrile rubber material has a chemical resistance characterized in that there is no complete permeation of carbon tetrachloride after seven hours of exposure, as measured in accordance with the method of ASTM F739. In the preferred embodiments, the cured nitrile rubber material is substantially impermeable to water, ie, liquid water and water vapor. Gloves manufactured in accordance with the methods of this invention preferably have a layer of a cured nitrile rubber of the invention having a thickness of between about 0.08 mm and about 0.76 m. Thin gloves provide better user feel, which is desirable, eg, for surgical gloves. Accordingly, surgical gloves preferably include a layer of a cured nitrile rubber of the invention having a thickness between about 0.08 mm and 0.28 mm, more preferably between about 0.13 mm and 0.25 mm. Thicker gloves provide better resistance and chemical resistance, which is desirable, eg, for laboratory or industrial gloves. Thus, laboratory or industrial gloves preferably include a layer of a cured nitrile rubber of the invention having a thickness between about 0.25 mm and 0.76 mm, more preferably between about 0.38 mm and 0.71 mm. Of course, the skilled artisan will observe that the cured nitrile rubber layer of these gloves (or other articles) can be thicker than 0.76 mm, if greater protection is desired. The improved soft qualities of the nitrile compositions of the invention make possible gloves that are thicker than conventional gloves, but which are not unduly stiff or uncomfortable. The invention also contemplates composite articles comprising a layer of a cured nitrile rubber of the invention. In this way, for example, a layer of a cured nitrile rubber of the invention may adhere to a layer of another material, eg, a different rubber or plastic, to form a composite article. These composite articles, in some embodiments, may provide qualities such as increased chemical resistance, puncture resistance, or tensile strength as compared to conventional articles. In another aspect, the invention provides a nitrile rubber composition substantially free of zinc having low elastic modulus but high tensile strength and chemical resistance. In this regard, it is preferable to use an accelerator that does not contain zinc. Suitable zinc-free accelerators include sodium dibutyldithiocarbamate or MBT and TMTD, as described above.

Example 1 A carboxylated nitrile rubber formulation was prepared as follows, using the amounts of the components shown in Table 1. All amounts are given in parts per hundred dry rubber (phr).

Table 1 Latex Perbunan N KA8250 100 phr Casemonium Ammonium 0.25 phr Potassium Hydroxide 1.0 phr Sulfur 1.0 phr Butyl Zymato 0.50 phr Titanium Dioxide 1.0 phr Phthalocyanine Blue 0.10 phr 2-Sodium Ethylhexyl Sulfate 0.05 phr Zinc Oxide 0 phr Notes: phr = parts per 100 parts of rubber Zylate butyl = zinc dibutyldithiocarbamate.

First, the appropriate amount of latex (Perbunan N late KA8250, Bayer Corp.) was weighed into a mixer vessel. The latex was slipped to remove contamination, including pieces of coagulated rubber, and an agitator was placed in the container; the latex was stirred through the compounding process. The level of agitation was sufficient to disperse the other ingredients rapidly as they were added, a small amount of defoamer can be added if necessary. The stabilizers were then added to the latex. In this example, sodium 2-ethylhexyl sulfate was used to improve the appearance and quality of the articles made from the nitrile rubber to prevent backing off the latex after immersion. Ammonium caseinate, a protective colloid, was added to prevent the excessive viscosity of latex and to control the rate at which the chemicals in the curing system affect the rubber particles in the latex. The protective colloid was mixed in the latex for 20-30 minutes before proceeding. Next, an eight percent solution of potassium hydroxide in water was added slowly to raise the pH of the formulation to approximately 8.9-9.2. The sulfur / butyl zimate curing system used in this example is effective at lower pH levels than curing systems with other accelerators. Subsequently, water-based dispersions of sulfur, butyl zylate (sulfur curing components), and titanium dioxide and phthalocyanine blue (pigments) were added and mixed to the latex, Zinc oxide was not used. The formulation was allowed to mix for at least 60 minutes, was re-added, and then aged for 1-5 days before immersion to form finished articles.

Example 2 Gloves were made from the nitrile latex prepared in Example 1 above, by the following procedure. Ceramic glove shapes were cleaned with conventional surfactant solutions before use. The clean forms were preheated in an oven at a temperature in the range of 77SC to 104SC. The forms were then immersed in a coagulant solution of calcium nitrate, followed by immersion in the latex compound described in Example 1. The thickness of the glove was determined, at least in part, by the length of the latex immersion period. The forms coated with latex are then submerged in water for 20-40 minutes to remove residual calcium nitrate and other water-soluble materials. The forms were then heated in an oven for a sufficient time to dry the gloves, and further heated to cure the rubber; he Drying was generally performed at about 71-99 ° C, and curing was performed at. around 116-1492C. Finally, the finished gloves separated from the shapes. A sample was cut from a glove produced by the above method, and various tests were performed. The results of the tests are illustrated graphically in Figure 1 (the scale is thousands of pounds per square inch). as shown in Figure 1, the glove of the invention ("Soft Nitrile") is softer (ie, has a lower elastic method) than a commercially available nitrile glove ("Pdn Nitrile," a 0.36 mm thick glove available from North Safety Products product code LA142G). The glove of the invention is slightly stronger than the commercially available glove, as shown by the greater resistance to tension of the present glove. The chemical resistance of the glove produced as described above, as measured in accordance with ASTM method F739, is illustrated graphically in Figure 2. The present material ("Soft Nitrile") provides greater resistance to chemicals than the material of control, a commercially available nitrile latex rubber (a 0.38 mm thick glove available from North Safety Products, product code LA153G). The material of the invention provides improved levels of permeation resistance to all tested chemicals, compared to the control nitrile rubber formulation. The cured nitrile rubber of the invention provides excellent resistance to complete permeation with carbon tetrachloride, with which no permeation was seen even after seven hours. The present nitrile rubber material also exhibited an increase in area (swelling) of less than 1% after 24 hour exposure to hexane at room temperature. In this manner, the nitrile rubber of the invention provides improved softness, while maintaining chemical resistance and resistance comparable to, or superior to, a conventional nitrile rubber. The present nitrile rubber is also free of zinc oxide, and, if a zinc-free accelerator is employed, it can be made substantially free of zinc. In certain embodiments, the present nitrile rubber compositions are substantially free of divalent metal ions. The contents of all publications cited through this specification are hereby incorporated by reference in their entirety.

Equivalents Those experienced in the field will recognize, or be able to ensure the use of no more than routine experimentation, numerous equivalents to the specific procedures described herein. It is considered that said equivalents are within the scope of this invention and are covered by the following claims.

Claims (21)

1. - A method for making a nitrile rubber composition, comprising the steps of: combining a nitrile latex with a stabilizing agent, adjusting the pH of the nitrile latex to about 8.5-10.0 to provide a basic nitrile latex, and contacting the basic nitrile latex with a substantial zinc oxide-free crosslinking agent and with at least one accelerator to form a nitrile rubber composition, wherein the nitrile rubber composition is substantially free of zinc oxide.

2. A method according to claim 1, wherein the stabilizing agent is selected from the group consisting of ammonium caseinate, sodium alginate and polyvinyl alcohol.

3. - A method according to claim 1, wherein the step of adjusting the pH includes adding an alkaline hydroxide to the nitrile latex.

4. A method according to claim 1, wherein the crosslinking agent is a sulfur compound.

5. A method according to claim 1, wherein the at least one accelerator comprises tetramethyl thiuram disulfide and mercaptobenzothiazole.

6. A method according to claim 1, comprising the additional step of aging the nitrile rubber composition for a period of one to three days before use.

7. - A method according to claim 1, which comprises the additional step of curing the nitrile rubber composition to form a cured nitrile rubber composition.

8. A cured nitrile rubber composition formed by the method according to claim 7.

9. A cured nitrile rubber composition formed by the method according to claim 7, wherein the nitrile rubber composition is substantially free of divalent metal oxides.

10. An article of manufacture comprising a layer of the cured nitrile rubber composition according to claim 8.

11. A cured nitrile rubber composition comprising: a nitrile rubber base, a stabilizing agent a crosslinking agent substantially free of zinc oxide, and an accelerator, wherein the nitrile rubber composition is substantially free of divalent metal oxides, and has a modulus of elasticity 3005 of less than about 77.33 kg / cm2.

12. A cured nitrile rubber composition according to claim 11, further characterized in that the 300% modulus is on the scale of about 35.15 kg / cm2 to about 49.21 kg / cm2.

13. A cured nitrile rubber composition according to claim 11, further characterized by being substantially free of zinc.

14. A cured nitrile rubber composition according to claim 11, further characterized in that the nitrile rubber composition is resistant to organic solvents.

15. An article of manufacture comprising a layer of the cured nitrile rubber composition according to claim 11.

16. An article according to claim 15, wherein the article is a glove.

17. An article according to claim 15, wherein the layer of the cured nitrile rubber composition has a thickness of about 0.08 mm to about 0.76 mm.

18. A cured nitrile rubber prepared from a nitrile latex base, a stabilizing agent, a crosslinking agent substantially free of zinc oxide, and an accelerator, wherein the nitrile rubber is substantially free of rust. zinc, and further characterized in that the cured nitrile rubber has a 300% modulus of elasticity in the scale of about 28.12 kg / cm2 to about 77.33 kg / cm2, and that the cured rubber has a higher tensile strength of approximately 351.50 kg / cm2. 19.- A method to make a nitrile rubber cured with sulfur, comprising the steps of: combining a carboxylated nitrile latex with a stabilizing agent, adjusting the pH of the nitrile latex to about 8.5-10.0 to provide a basic nitrile latex, and contacting the basic nitrile latex with a crosslinking agent selected from the group consisting of sulfur and sulfur donors, and with an accelerator, to form a nitrile rubber, wherein the nitrile rubber cured with sulfur is substantially free of divalent metal oxides. 20. A carboxylated nitrile rubber cured with sulfur, wherein the rubber is substantially free of divalent metal oxides, and further characterized in that the rubber has a 300% modulus of elasticity on the scale of about 28.12 kg / cm2 at about 77.33 kg / cm2, and in that the cured rubber has a tensile strength greater than about 351.50 kg / cm2. 21. A cured nitrile rubber characterized in that the rubber has a 300% modulus of elasticity in the scale of about 28.12 kg / cm2 to about 77.33 kg / cm2, and a tensile strength greater than about 351.50 kg / cm2, and further characterized in that the rubber is resistant to organic solvents.