LATEX COMPOSITIONS AND FILMS PRODUCED THEREFROM

Abstract: Natural latex rubber compositions and films produced therefrom having reduced extractable protein levels and, as such, are less attributable to allergic sensitivities. The natural latex rubber compositions also have improved tear strength while minimizing effect on modulus. The compositions include a natural rubber latex, and stable aqueous dispersions of fumed silica.

Description: TITLE LATEX COMPOSITIONS AND FILMS PRODUCED THEREFROM BACKGROUND OF INVENTION 1. Field of Invention. The present invention relates to latex compositions and, more particularly, to natural rubber latex compositions having improved viscosity and latex films produced therefrom having reduced extractable proteins and increased tear strength. 2. Description of the Related Art. Natural rubber latex is used today in a wide variety of applications. Latex foam, for example, is used in mattresses, pillows, seat cushions and the like. Other products, to name a few, include rug and carpet backings and underlays, upholstery and fabric backings, protective clothing, paper coatings and latex based paints. The superiority of natural rubber latex relative to other rubbers and rubber-like materials is most evident, however, in the manufacture of dipped goods. Today, a variety of articles including household gloves, surgeons gloves, examination gloves, catheters and other medical goods, condoms, pacifiers, toy balloons, meteorological balloons, life jackets, football and basketball bladders, bathing caps, baby pants, squeeze toys and numerous other products are made using conventional dipping techniques such as straight dipping (with or without heat), coagulant dipping (bivalent salt or acid) and heat sensitive dipping. For example, surgeons gloves are typically made by a bivalent salt coagulant process which involves dipping an impervious pre-shaped form (such as ceramic, glass, plastic or metal) into a coagulant solution of calcium nitrate in alcohol or the like. The form is gently heated, if needed, after withdrawal to evaporate the alcohol, thereby leaving a coating of the coagulant on the form, and then dipped into a latex bath where it is held for several minutes depending on the thickness of the desired layer. The coagulating action of the solution causes a substantially smooth layer of coagulated rubber to deposit on the surface of the form. In the alternative, the form may be first dipped in a latex bath and then followed by a coagulant dip. Finally, the article is stripped from the form. Condoms and examination gloves, on the other hand, tend to utilize conventional straight dip processes. Natural rubber latex offers excellent film-forming capabilities to produce articles that are strong, flexible and resistant to sagging during normal use. As a result, natural rubber latex has been commonly used in the manufacture of polymeric glove materials, especially for those supplied to the health care industry. Notwithstanding the preferential use of natural rubber latex in glove materials for its superior properties and its effectiveness as a barrier against transmitted diseases, recent reports have shown that some users have demonstrated a hypersensitivity to the allergens found in the soluble protein of the latex. Symptoms range from mild conjunctivitis and rhinitis to severe anaphylactic shock. In addition, airborne allergens from glove powders that have adsorbed extractable proteins from the latex glove have been linked to asthma. Once sensitized to the latex protein, any future contact must be severely minimized or avoided. It is important to note that while some of the soluble proteins in the serum phase of the latex are removed during concentration, the small percentage of proteins remaining tend to concentrate during processing. For example, during the process used for dipped gloves, most of the extractable protein is located on the inner surface of the glove which is normally in contact with the wearer. As a result, several latex and/or process treatments have been developed to remove or deactivate the extractable protein level in the final product. For example, it is possible to reduce the protein level in the concentrated latex by extending the period of centrifugation. Enzymes may also be added to modify the protein by hydrolyzing the peptide bond of the proteins to produce smaller peptide units and amino acids. Similarly, chemical means may be used to de-proteinize the latex. In addition, leaching and chlorination are common techniques to reduce the amount of proteins in the final product. Alternatively, it is known that risk of allergic response may be lessened by utilizing a synthetic latex or a lining, i.e. barrier layer, of a synthetic polymer in a double-dip system. Even though one technique may be used with reasonable success, it is not uncommon to utilize more than one of the available processes to maximize the removal or deactivation of the extractable proteins. Although the above techniques will reduce the extractable protein level, each treatment technique has noted disadvantages. For example, the emulsion system of the natural rubber latex is stabilized by means of the naturally occurring soaps and proteins which concentrate at the rubber-water interface. The removal or deactivation of the protein will effect the colloidal stability of the natural rubber latex. As a result, one of the major problems associated with chemical modification or enzyme addition is the negative effect on the colloidal properties of the latex. In addition, although leaching has been used with success, the results may vary depending on the processing conditions. Similarly, if a chlorination process is utilized, it has been found that the resulting films tend to exhibit inferior mechanical and physical properties. Synthetic latexes or barrier layers of synthetic polymers tend to experience problems with delamination, incomplete coverage and are not as cost effective. In addition to the foregoing, the desired physical properties exhibited by dipped natural latex compounds will depend on the particular latex article. For example, one piece footwear rubbers are typically made of latex compounds having a high rubber content to achieve maximum resiliency and durability while providing adequate stretch. Surgeons' gloves and condoms, on the other hand, must be thin, strong, and be able to withstand sterilization processes. In addition, to meet federal requirements, a combination of good physical properties such as tensile strength and high elongation is required. Although the dipped goods produced using conventional formulations are found to have adequate tensile strength, such goods still tend to tear from small cuts and nicks. As a result, a substantial interests exists in increasing the tear strength of latex films and goods produced therefrom. Known methods to increase tear strength in latex films include the addition of fillers such as mica, rayon or cotton flock, the addition of various resins, or the addition of carboxylated synthetic latex to the formulation. Although the use of fillers and resins typically results in increased tear strength, such additives also produce an undesirable increase in the modulus of the resulting film. Such increase in modulus yields a stiffer film and end product which is generally less comfortable to the wearer. For example, a film having an increased modulus used in a pair of gloves tends to inhibit thumb and finger movement and promote fatigue in the hand. In addition, an increase in modulus will produce a stiffer glove and consequently result in a loss of tactile feeling by the surgeon. The use of a carboxylated synthetic latex, on the other hand, does not have such a deleterious effect on modulus but requires the use of excessive amounts of the latex to achieve the desired improvement is tear strength. Although the overall tear strength tends to improve, such improvement is typically accompanied by a different type of tear, typically referred to as a "knotty" tear (i.e. a tear having a rough edge which follows a straight or gently curved line). The use of fumed silica and aqueous dispersions of fumed silica as reinforcing agents have also been known to provide several advantages in natural latex rubber systems including improved tear strength. See for example Technical Data Sheets entitled "The Use of Cab-O-Sperse ® Dispersions in Latex Rubber Systems" (Cabot Corporation, Cab-O-Sil Division) and "Cab-O-Sil in Dipped Latex Films," (Cabot Corporation, Bulletin CRub-3, 2346/958). Although fumed silica and dispersions of same have been used with success, such use has been limited due to the high level of silica required to achieve the desired improvement and the resulting negative impact on modulus and formulation viscosity (i.e. increased viscosity) in the latex systems. As a result, a loss of thickening control is experienced during the dipping processes. A need, therefore, remains to reduce or modify the extractable protein level in latex films while maintaining colloidal stability in the latex. In addition, protein reduction alternatives that are cost effective and will aid the processability of the latex are desired. Moreover, a continuing need remains for improving the mechanical and physical properties of articles made from natural rubber latex, particularly in the area of thin films for use in gloves, such as surgeons or examination gloves, or condoms. For example, tears remain prevalent in thin films and results from a number of activities including removal of an article from its form, tears on donning and tears during contact with sharp objects during use by the wearer. Accordingly, a need remains for improved fumed silica dispersions for use in latex rubber formulations and films having increased tear strengths while minimizing the detrimental effect on modulus, elongation, tensile strength and formulation viscosity. SUMMARY OF THE INVENTION Accordingly, in one embodiment, the present invention is directed to a natural rubber latex composition including a natural rubber latex and a stable aqueous dispersion of fumed silica. The fumed silica is uniformly dispersed in the composition and present in an amount ranging between 0.5 % and 5.0% , by weight, of rubber solids. In another embodiment, the present invention is directed to a latex film including a natural rubber latex and a stable aqueous dispersion of fumed silica, wherein the fumed silica is present in an amount less than 5.0% by weight of rubber solids. The films have protein levels less than 120 μg/g. Latex films and articles produced from the natural rubber latex compositions of the present invention exhibit an extractable protein level less than that obtained when the natural latex is employed under normal processing conditions. In another embodiment, the present invention is directed to latex compositions, and films produced therefrom, comprising natural rubber latexes and stable aqueous dispersions of fumed silica having a BET surface area ranging from about 150 m /g to about 400 m 2 /g. The fumed silica is uniformly dispersed in the compositions, and is present in an amount ranging between 0.5 % and 2.5 % , by weight, of rubber solids. Latex films and goods produced from the formulations of the present invention experience an increase in tear strength without significant increase in modulus. The latex films and goods also show significant retention of tensile and elongation properties during ageing. In addition, the viscosity of the compounded latex is stabilized by the use of the aqueous dispersions of fumed silica. The present invention is also directed to a method of producing a latex film. The method includes compounding a natural rubber latex and a stable aqueous dispersion of fumed silica to form a natural rubber latex composition. The fumed silica is present in an amount ranging between 0.5 % and 5.0% , by weight, of rubber solids. A pre-shaped former is then dipped into the latex composition for a period of time sufficient to deposit a film of a desired thickness. The film is then leached, dried, and removed from the former. DETAILED DESCRIPTION OF THE INVENTION The present invention is directed to natural rubber latex compositions and films produced therefrom having reduced extractable protein levels. In addition, the colloidal integrity of the composition is maintained and films produced therefrom exhibit improved mechanical and physical properties. The natural rubber latex composition of the present invention includes a natural rubber latex and a stable aqueous dispersion of fumed silica. The fumed silica is uniformly dispersed in the composition and is present in an amount sufficient to reduce the extractable proteins to a desired level while maintaining the colloidal stability of the latex. Films produced from such compositions have extractable protein levels less than that obtained when the natural latex is employed under normal processing conditions, and typically will have extractable protein levels less than 120 μg/g (amount of protein extracted in micrograms per unit weight of the film in grams). Rubber latex naturally occurs in a wide variety of tropical plants and trees, particularly in the Hevea brasiliensis tree. The latex is obtained by tapping the tree or cutting through the outer layers of the bark in a spiral fashion. The fluid of latex droplets which flow from the tree may be captured in a suitable receptacle. Freshly tapped latex typically consists of between 30% and 40% rubber solids and approximately 15% non-rubber solids, all suspended in an ambient emulsion or serum. Typically, additives or preservatives, such as ammonia and blends thereof, are promptly incorporated into the fresh latex to prevent bacterial contamination and to stabilize the latex from a pH ranging from 6.0 to 7.0 to around a pH of 10.5. Generally, the latex is then concentrated to ensure uniformity in quality and consistency while reducing the transportation costs associated with shipping latex in its unconcentrated form. The latex is typically concentrated to greater than 60% rubber solids by employing a variety of conventional methods, such as creaming, centrifuging, evaporating, or electrodecantation. The rubber particles of the natural rubber latex are characterized as highly polymerized molecules of cis-l :4-polyisoprene with a molecular weight in the region of one million and individual rubber particle sizes vary from 0.01 micron and 5 microns. The major non-rubber constituents are proteins and decomposition products thereof, fatty acid soaps, and a range of organic and inorganic salts. Of the non- rubber solids, proteins typically constitute between 1 % to 1.5 % of the total latex weight. About half of the protein solids are associated with the emulsion, i.e. serum phase and, as such, are partially attributed to the colloidal stability of the latex. A smaller amount is strongly associated with or bound to the surfaces of the dispersed latex particles themselves. It is generally believed that the proteins associated with allergenic reactions and sensitivities noted above are typically found in the serum phase. As noted above, when converted into a latex concentrate, some of the soluble proteins in the serum phase are removed. However, the small percentage of proteins remaining tend to concentrate during processing. For example, during the process used for dipped gloves, most of the extractable protein is located on the inner surface of the glove which is normally in contact with the wearer. It is this residual extractable protein fraction that has been a cause of wearer sensitivity and related allergies. The natural rubber latex of the present invention typically has a rubber content ranging from about 30% to about 70% by weight. In addition, the composition should have a Brookfield viscosity sufficient to yield the desired thickness of dipped forms during coagulation. In a preferred embodiment, the natural rubber latex is prevulcanized or if post-vulcanized, has undergone a moderate degree of maturation or pre-cure, typically one to five days. One method of determining the degree of cure is by measuring the swelling index (SI) of the matured latexes using for example, the linear solvent (cyclohexane) swelling method. The SI is indirectly proportional to the degree of pre-cure, I. e. the higher the SI, the lower the degree of cure. For example, the degree of pre-cure may be categorized as follows: Swelling Index (SI) Degree of Pre-Cure >2.6 Unvulcanized 2 - 2.6 Lightly vulcanized 1.8 - 2.0 Moderately vulcanized < 1.75 Fully vulcanized In addition, the natural rubber latex may be a "low protein" natural rubber latex. By "low protein" is meant that the natural rubber latex has been treated or processed using available techniques to reduce or de-activate the extractable protein levels in resulting dipped films and articles produced therefrom. Suitable additives are generally incorporated into the natural rubber latex compositions to impart a number of desired properties to the finished end product. Such additives are well known in the art and include curing, cross-linking or vulcanizing agents, vulcanization activators and accelerators, antioxidants, antidegradants, stabilizers and the like. The amount of a particular additive will vary depending on the characteristics of the latex, the rubber solids content and properties desired. As conventionally available, the natural rubber latex of the present invention is typically formulated from a prevulcanized or post-vulcanized concentrate. For example, prevulcanized compositions generally consist of ammonium preserved sulfur prevulcanized concentrate such as Revultex™ MR (manufactured by Revertex Ltd., Essex, United Kingdom), potassium hydroxide, and a suitable antioxidant such as Wingstay ® L (a registered trademark of Goodyear Tire & Rubber Co. , Akron, Ohio). A typical post-vulcanized composition includes high ammonia (HA) preserved latex such as Revultex™ HA , a suitable stabilizer such as potassium hydroxide, a surfactant such as potassium laureate, a cross-linking agent such as sulphur, an accelerator such as a zinc dibutyl dithiocarbamate or zinc oxide dispersion, and a suitable antioxidant, such as Wingstay ® L antioxidants. Other suitable prevulcanized or post-vulcanized latexes for use in the present invention are described in The Natural Rubber Formulary and Property Index (The Malaysian Rubber Producers Research Association, 1984), the disclosure of which is incorporated herein in its entirety by reference. In addition, typical low protein natural rubber latexes, such as Laptex™ prevulcanized latex (available from Revertex Malaysia Sdn. Berhad.) are also suitable. It has been found in the present invention that the addition of an aqueous dispersion of fumed silica to the natural latex rubber effectively reduces the extractable protein level in latex films and articles produced therefrom. It is important to note that, as is conventionally known and described in the prior art, fumed silica has been used in natural rubber latex to provide moderate improvement in tear strength. However, fumed silica has not been known as a means to reduce the extractable protein level in latex films and dipped articles produced therefrom. The production of fumed silica is a well-documented process which involves the hydrolysis of silicon tetrachloride vapor in a flame of hydrogen and oxygen. Molten particles of roughly spherical shapes are formed in the combustion process, the diameters of which are varied through process parameters. These molten spheres of fumed silica, typically referred to as primary particles, fuse with one another by undergoing collisions at their contact points to form branched, three dimensional chain-like aggregates. The force necessary to break aggregates is considerable and often considered irreversible because of the fusion. During cooling and collecting, the aggregates undergo further collision that may result in some mechanical entanglement to form agglomerates. Compared to the aggregates where the primary particles are fused together, agglomerates are thought to be loosely held together by Van der Waals forces and can be reversed, i.e. de-agglomerated, by proper dispersion in suitable media. The size of the primary spherical particles that comprise the fumed silica aggregates determine the surface area. The surface area of the fumed silica, as measured by the nitrogen adsorption method of S. Brunauer, P. H. Emmet, and I. Teller, J. Am. Chemical Society, Volume 60, Page 309 (1938) and commonly referred to as BET, typically ranges from about 40 m /g to about 430 m 2 /g. In the present invention, the fumed silica is preferably in a range from about 150 m 2 /g to about 385 m 2 /g and are of a high purity. High purity means that the total impurity content is typically less than 1 % and preferably less than 0.01 % (I. e. 100 ppm). The fumed silica of the present invention is uniformly dispersed in a stable aqueous medium (e.g. deionized water) using conventional methods known to those skilled in the art. By uniformly dispersed is meant that the aggregates are isolated and well distributed throughout the medium. By stable is typically meant that the aggregates will not re-agglomerate and settle out (e.g. form a hard, dense sediment). The fumed silica dispersion should have a pH between 5.0 and 10.5 and may be adjusted by the addition of a suitable base such as sodium hydroxide, potassium hydroxide, ammonia and the like. Preferably, the fumed silica dispersion of the present invention is prepared by the method described in U. S. Patent No. 5,246,624 to Miller et al., the disclosure of which is incorporated herein in its entirety by reference, has a pH ranging between 8.0 and 10.0 and has coagulation characteristics similar to that of the natural rubber latex. Although many commercially available fumed silicas are suitable, a most preferred aqueous dispersion of fumed silica is available under the name CAB-O-SPERSE ® (a registered trademark of Cabot Corporation, Boston, Massachusetts). The natural rubber latex compositions of the present invention are prepared by combining or mixing the aqueous dispersions of fumed silica, which dispersions typically range from about 10% to about 45% solids and preferably between 15 % and 30% solids, with a natural rubber latex and other desirable additives under low shear conditions (i.e. to prevent foaming) until a uniform homogeneous composition is obtained. In addition, although acceptable extractable protein levels may be achieved with the addition of such fumed silica dispersions, the natural rubber latex composition of the present invention may be added to a "low protein" natural rubber latex, as described herein, to further reduce the extractable protein level. As is conventionally known and described in the prior art, the amount of fumed silica as a function of the natural rubber latex is typically between 3.0 to 15 parts per 100 parts by weight rubber solids in the natural rubber latex and, preferably, about 5 parts fumed silica per 100 parts by weight of rubber solids (phr) in the latex. It has been found in the present invention that the loading level of the fumed silica in the latex film is critical to reducing the extractable proteins, while maintaining the colloidal stability and processability of the latex. In addition, such level is important to achieving increased tear strength without experiencing significant detrimental effects on modulus, elongation, tensile and viscosity of the composition. In addition, compositions and films produced therefrom generally experience a higher retention of modulus and tensile strength and comparable retention of elongation at break upon ageing. As a result, a preferred loading level of fumed silica has been found to range from about 0.5 part to about 5.0 parts per hundred parts, by weight, rubber solids (phr) in the natural rubber latex. At loading levels below 0.5 phr in natural rubber latex, without any additional processing or treatments, an acceptable extractable protein level as well as an improvement in tear strength is not achieved. At levels above 5.0 phr, the fumed silica has a harmful effect on the viscosity of the composition and the modulus. In a most preferred embodiment, the loading level of fumed silica has been found to be about 1.0 part to 1.5 part per 100 parts, by weight, rubber solids in the natural rubber latex. In addition, the surface area of the fumed silica has also been found to play an important role depending on whether a prevulcanized or post-vulcanized latex formulation is used. As such, a preferred fumed silica surface area is about 380 m 2 /g in a prevulcanized latex formulation and about 200 m 2 /g in a post-vulcanized latex formulation. Although not completely understood, it is noted that a critical relationship exists between the fumed silica dispersion, the loading level of fumed silica as a function of rubber solids and the use of a prevulcanized or matured post-vulcanized natural rubber latex. It is further believed that the ageing resistance in natural rubber latex formulations of the present invention is due to the high degree of hydrophilicity of the fumed silica. The moisture adsorption capacities of the fumed silica reduce the level of moisture in the latex formulation and, consequently, enhance its ageing resistance. In addition, a stable and uniformly dispersed system is essential for the fumed silica aggregates to properly form a high degree of networks. Furthermore, it is believed that the fumed silica is capable of binding and immobilizing the protein molecules. The protein molecules are adsorbed onto the surface of the highly structured silica aggregate. This adsorption is highly promoted by the polar nature of the protein molecules and the polar nature of the surface of the fumed silica aggregates. Once the proteins are bound to the silica, the size of the composite of fumed silica and proteins becomes quite large and bulky, thereby limiting diffusion and hence extraction, i.e. migration, of the protein from the latex film. Since the proteins are still present in the latex, the colloidal stability is unaffected. These proteins are further available for reinforcement of the finished film and, as a result, the performance of the latex film is not diminished. As noted above, the use of the fumed silica will also enhance the tear and puncture resistance of the articles. The latex compositions containing fumed silica dispersions as described herein can be used to produce a variety of latex goods and films having reduced extractable protein levels while maintaining or enhancing the physical and mechanical properties of the films. For example, the latex composition of the present invention is typically suited for thin film applications and may be used to produce reduced extractable protein gloves and condoms using conventional techniques such as straight dipping, coagulant dipping and heat sensitive dipping. In such applications, the films have demonstrated a significant improvement in tear strength with minimal impact on modulus and other physical properties. The present invention will be further illustrated by the following examples, which are intended to be illustrative in nature and are not to be considered as limiting the scope of the invention. EXAMPLE 1 Sample 1 of a natural rubber latex composition was prepared as a control by lightly stirring 0.3 phr (parts per hundred by weight rubber) potassium hydroxide, 1 phr 2,2'- dicyclopentylene-bis (4-methyl-6-t-butylphenol) into ammonia preserved sulfur prevulcanized natural rubber latex (Revultex MR) consisting of approximately 60% by weight natural rubber solids. Six additional compositions were similarly prepared by lightly stirring 0.3 phr (parts per hundred by weight rubber) potassium hydroxide, 0.5 phr fumed silica solids from an aqueous dispersion of fumed silica (12-17% by weight solids content stabilized with ammonia to a pH of 9.5), 1 phr 2,2'-dicyclopentylene bis (4-methyl-6-t-butylphenol) into ammonia preserved sulphur prevulcanized natural rubber latex consisting of approximately 60% by weight natural rubber solids. The aqueous dispersions of fumed silica of samples 2, 4 and 6 were passed through a 5 micron bag filter (available from Bebco, Inc. of Joliet, IL) prior to adding the dispersion to the latex. The aqueous dispersions of fumed silica of samples 3, 5 and 7 were passed through a 1 micron bag filter (available from Bebco, Inc. of Joliet, IL) prior to adding the dispersion to the latex. The total solids content of the compositions were adjusted to 50% solids. The surface areas of the fumed silica varied between 160 m /g and 380 m 2 /g. Coagulant dipped films were prepared by immersing a warmed glass former into a coagulant solution consisting of 10% calcium nitrate in industrial methylated spirit and dipping into the latex mixture for 10 seconds. The latex film deposited on the form was allowed to set at room temperature for a brief period and then leached in distilled water at 50°C for 10 minutes. The film was finally dried at 70°C for approximately 30 minutes. The extractable protein level in μg/g (amount of protein extracted in micrograms per unit weight of the film in grams) was determined using a modified Lowry and Bradford Assays technique utilizing bovine serum albumin as the standard. Proteins were eluted in water, purified and concentrated by acid precipitation (combination of 5% trichloroacetic acid / 5% phosphotungstic acid). The protein was recovered as the precipitate from the elute after centrifugation. The soluble non-proteins were discarded with the supernatant. The proteins were then analyzed using colorimetric assays with a sensitivity of - 8 microgram/gram. The standard protein compared to was derived from bovine serum albumin. Table I illustrates the effect of fumed silica on the protein level of the prevulcanized latex composition. TABLE I - PREVULCANIZED LATEX FILMS t Sample Surface Area Loading Level Protein Content (m 2 /g) (phr) ( mi c rogram/gram) 1 (Control) _ 138 2 380 0.5 91 3 380 0.5 106 4 320 0.5 89 5 320 0.5 74 6 160 0.5 93 7 160 0.5 89 tAged 1 day at 100°C Table I demonstrates that the presence of fumed silica at 0.5 phr in prevulcanized latex reduced the extractable protein content of the coagulant dipped films. EXAMPLE 2 The process of Example 1 was repeated except that the loading level of the fumed silica solids from an aqueous dispersion of fumed silica (12-17% by weight solids content stabilized with ammonia to a pH of 9.5) was 1 phr. The effect of fumed silica on the protein level of the prevulcanized latex composition is shown in Table II. TABLE II - PREVULCANIZED LATEX FILMS t Sample Surface Area Loadi ing Level Protein Content (πrVg) (phr) (microgram/gram) 1 (Control) _ 138 2 380 1.0 44 3 380 1.0 30 4 320 1.0 54 5 320 1.0 68 6 160 1.0 150 7 160 1.0 64 tAged 1 day at 100°C Table II demonstrates that the presence of fumed silica at 1.0 phr in prevulcanized latex reduced the extractable protein content of the coagulant dipped films. EXAMPLE 3 The process of Example 1 was repeated except that the loading level of the fumed silica solids from an aqueous dispersion of fumed silica (12-17% by weight solids content stabilized with ammonia to a pH of 9.5) was 1.5 phr. The effect of fumed silica on the protein level of the prevulcanized latex composition is shown in Table III. TABLE III - PREVULCANIZED LATEX FILMS t Sample Surface Area Loading Level Protein Content (m-/g) (phr) ( microgram/gram) 1 (Control) _ 138 2 380 1.5 91 3 380 1.5 90 4 320 1.5 78 5 320 1.5 88 6 160 1.5 158 7 160 1.5 27 tAged 1 day at 100°C Table III demonstrates that the presence of fumed silica at 1.5 phr in prevulcanized latex reduced the extractable protein content of the coagulant dipped films. As illustrated above in Example 1-3, the results clearly show that the addition of between 0.5 phr to 1.5 phr fumed silica reduces the extractable protein level in coagulant dipped films, with significant reductions at level between 1.0 phr to 1.5 phr. In addition, the films are further expected to exhibit an increase in tear strength and enhanced retention of properties on ageing, as further illustrated in Example 7 below. Furthermore, although the viscosity of the prevulcanized latex usually increases with ageing in the presence of the fumed silica, this increase will not be significant. In fact, it is anticipated that the increase in viscosity will be considerably reduced at the higher loading levels of fumed silica. EXAMPLE 4 Seven latex compositions were prepared by lightly stirring 0.3 phr (parts per hundred by weight rubber) potassium hydroxide, 0.3 phr potassium laureate, 0.5 phr sulfur, 0.75 phr zinc dibutyl dithiocarbamate, 0.25 phr zinc oxide dispersion, 1 phr 2,2'-dicyclopentylene-bis (4-methyl-6-t-butylphenol) into ammonia preserved centrifuged post-vulcanized natural rubber latex (Revultex HA) consisting of approximately 60% by weight natural rubber solids. Except for Sample 1 (control), the compositions also contained 0.5 phr fumed silica solids from an aqueous dispersion of fumed silica (12-17% by weight solids content stabilized with ammonia to a pH of 9.5) having various surface areas. As in Example 1 , the aqueous dispersions of fumed silica of samples 2, 4 and 6 were passed through a 5 micron bag filter (available from Bebco, Inc. of Joliet, IL) prior to adding the dispersion to the latex. The aqueous dispersions of fumed silica of samples 3, 5 and 7 were passed through a 1 micron bag filter (available from Bebco, Inc. of Joliet, IL) prior to adding the dispersion to the latex. The total solids content of the compositions was adjusted to 50% solids. The latex composition was matured for 5 days at 25°C. Coagulant dipped films were prepared from the matured latex using the same procedure as in Example 1 except that the films were also post vulcanized for 10 minutes at 1 10°C. As in Example 1 , the extractable protein level was determined using a modified Lowry and Bradford Assays technique utilizing bovine serum albumin as the standard. Table IV illustrates the effect of fumed silica on the protein level of the post-vulcanized latex composition. TABLE IV MATURED POST- VULCANIZED LATEX FILMS t Sample Surface Area Loading Level Protein Content (m 2 /g) (phr) (microgram/grain) 1 (Control) _ 306 2 380 0.5 166 3 380 0.5 54 4 320 0.5 45 5 320 0.5 90 6 160 0.5 35 7 160 0.5 152 tAged 5 days at 100°C before dipping. Table IV demonstrates that the presence of fumed silica at 0.5 phr in post- vulcanized latex reduced the extractable protein content of the coagulant dipped films. EXAMPLE 5 The process of Example 4 was repeated except that the loading level of the fumed silica solids from an aqueous dispersion of fumed silica (12-17% by weight solids content stabilized with ammonia to a pH of 9.5) was 1.0 phr. The effect of fumed silica on the protein level of the post-vulcanized latex composition is shown in Table V. TABLE V MATURED POST-VULCANIZED LATEX FILMSt Sample Surface Area Loading Level Protein Content (πr/g) (phr) (microgram/gram) 1 (Control) _ 306 2 380 1.0 52 3 380 1.0 60 4 320 1.0 < 20 5 320 1.0 80 6 160 1.0 < 20 7 160 1.0 <20 Aged 5 days at 100°C before dipping. 6/19531 PCΪYUS95/15813 ■ 19- Table V demonstrates that the presence of fumed silica at 1.0 phr in post- vulcanized latex reduced the extractable protein content of the coagulant dipped films. EXAMPLE 6 The process of Example 4 was repeated except that the loading level of the fumed silica solids from an aqueous dispersion of fumed silica (12-17% by weight solids content stabilized with ammonia to a pH of 9.5) was 1.5 phr. The effect of fumed silica on the protein level of the post-vulcanized latex composition is shown in Table VI. TABLE VI MATURED POST- VULCANIZED LATEX FILMS t Sample Surface Area Loading Level Protein Content (m 2 /g) (Phr) (microgram/gram) 1 (Control) _ 306 2 380 1.5 40 3 380 1.5 24 4 320 1.5 < 20 5 320 1.5 88 6 160 1.5 < 20 7 160 1.5 58 tAged 5 days at 100°C before dipping. Table VI demonstrates that the presence of fumed silica at 1.5 phr in post- vulcanized latex reduced the extractable protein content of the coagulant dipped films. As noted in the control sample of Examples 4-6, the extractable protein level of the post-vulcanized latex films is relatively high as compared to the prevulcanized films of Example 1-3. This is due to the denaturation of proteins during the high temperature of post-vulcanization. As clearly demonstrated in Example 4-6, the addition of fumed silica from an aqueous fumed silica dispersion markedly reduced the extractable protein level in the post-vulcanized latex films. With the exception of Samples 2 and 7 of Table IV (i.e. the 0.5 phr loading level of fumed silica), the films contained extractable protein levels less than 120 μg/g. Most of the films in Example 5 and 6, at silica loading levels of 1.0 and 1.5 phr, respectively, contained extractable levels less than 60 μg/g, with some films less than 20 μg/g (the detectable limit of the modified Lowry and Bradford Assays technique). As in Examples 1-3, in addition to experiencing a reduction in extractable proteins, the films of Examples 4-6 are also expected to demonstrate an improvement in tear strength without deleteriously effecting the properties of modulus, tensile and elongation, as further illustrated by Example 8 below. It is further expected that such properties will be retained upon ageing of the films. In addition, the viscosity of the "control" sample of the compounded latex is anticipated to significantly increase upon maturation. However, the addition of an aqueous dispersion of fumed silica to the latex will tend to diminish such increase, thereby allowing for improved control of film thickness during the dipping process. EXAMPLE 7 A first natural rubber latex formulation was prepared as the control by lightly stirring 0.3 phr (parts per hundred by weight rubber) potassium hydroxide, 1 phr 2,2'- dicyclopentylene-bis (4-methyl-6-t-butylphenol) into ammonia preserved sulfur prevulcanized natural rubber latex consisting of approximately 60% by weight natural rubber solids. Second, third and fourth formulations were similarly prepared by lightly stirring 0.3 phr (parts per hundred by weight rubber) potassium hydroxide, 1 phr fumed silica solids from an aqueous dispersion of fumed silica (12-17% by weight solids content stabilized with ammonia to a pH of 9.5), 1 phr 2,2'- dicyclopentylene-bis (4-methyl-6-t-butylphenol) into ammonia preserved sulphur prevulcanized natural rubber latex consisting of approximately 60% by weight natural rubber solids. The surface area of the fumed silica in the second, third and fourth formulations was 160 πr/g, 200 πr/g, 380 m 2 /g, respectively. Coagulant dipped films were prepared by immersing a warmed glass former into a coagulant solution consisting of 30% calcium nitrate in industrial methylated spirit and dipping into the latex mixture for 20 seconds. The latex film deposited on the form was allowed to set at room temperature for a brief period and then leached in distilled water at 50°C for 10 minutes. The film was finally dried at 70°C. Tensile, aged tensile and tear (trouser's) strength properties were determined using ISO 37 (1977), ISO 188 (1982) and ISO 34 (1979) techniques, respectively and are illustrated in Tables VII through IX. Trouser tear is used in industry to closely simulate the actual tears observed in practice. Table X shows the effect of fumed silica on the viscosity of the prevulcanized latex formulation which was determined by Brookfield LVT viscosity measurements taken from one to seven days using a No. 2 spindle at 60 rpm, 24°C. TABLE VII - PREVULCANIZED LATEX FILMS Surface Area M300I M5003 TSD EB* Tear Strength (πr/g) (MPa) (MPa) (MPa) (%) (N/mm) 1 (Control) - 1.3 2.3 26.7 870 28.0 2 160 1.3 2.5 28.2 870 29.3 3 200 1.3 2.6 29.8 870 40.0 4 380 1.4 2.8 30.8 870 42.8 tModulus at 300% Elongat ion πTensile Strength ^Modulus at 500% Elongat ion *Eloncat ion at Break TABLE VIII - AGED (2 days at 100°C) Surface Area M300t M500t TSπ EB* (πr/g) (MPa) (MPa) (MPa) 1 (Control) 0.8 1.3 (56)+* 17.1 (64) 1055 (121) 2 160 1.0 1.6 (64) 19.3 (68) 970 (111) 3 200 1.0 1.6 (61) 20.1 (67) 990 (113) 4 380 0.9 1.6 (57) 16. 1 (52) 930 (105) tModulus at 300% Elongation Strength ^Modulus at 500 % Elongation *Elongatιon at Break ** Number in brackets indicate percentage retention of properties after ageing TABLE IX - AGED (3 days at 100°C) Surface Area M300t M5003: TSπ EB* (m 2 /g) (MPa) (MPa) (MPa) (%) 1 (Control) 0.4 0.8 (34)* 5.6 (21) 1085 (124) 2 160 0.7 1.2 (48) 16.0 (56) 1070 (122) 3 200 0.5 0.9 (34) 10.4 (34) 1100 (126) 4 380 0.6 1.1 (39) 15.0 (48) 1060 (120) tModulus at 300% Elongation πTensile Strength JModulus at 500 % Elongation *Eloneatιon at Break ** Number in brackets indicate percentage retention of properties after ageing. TABLE X - VISCOSITY OF PREVULCANIZED LATEX FILMS Viscosity (cp)J Surface Area Number of Days (m 2 /g) 0 1 2 3 5 7 1 (Control) 82.0 82.0 83.5 85.0 90.0 95.0 2 160 85.0 94.5 104.0 114.5 125.0 135.5 3 200 95.0 96.0 97.0 103.5 110.0 116.5 4 380 80.0 107.0 107.0 107.0 121.0 135.00 tBrookfield, Spindle No. 2, 60 rpm. Tensile Strength is defined as the stress at the point of break. Elongation, on the hand, is the point of break during stretch. The Modulus is the ratio of stress divided by elongation or strain. Typically in thin films and goods produced therefrom, the modulus at 300% elongation is critical in determining good tactile feeling and reducing fatigue since, under normal use conditions, the film should not undergo high elongation. As a result, the modulus at 300% elongation is generally correlated with wearer comfort. As illustrated above, the results show that the addition of 1 phr of fumed silica from an aqueous fumed silica dispersion had a significant increase of the tear strength properties without deleteriously effecting the properties of modulus, tensile and elongation. More specifically, the latex formulation containing 1 phr fumed silica from the fumed silica dispersion demonstrated a higher retention of elongation at break after ageing as compared to the control sample, as shown in Tables VIII and IX. For extended ageing of 3 days at 100°C, the retention of modulus at 500% elongation and tensile strength was superior for the fumed silica containing films. In addition, the tensile strength is desirably increased with increasing surface area. Furthermore, the modulus at 300% elongation is not affected. It is important to recognize that the use of fumed silica dispersions in the natural latex rubber formulations of the prior art also demonstrated improvement in tear strength. However, such improvement was also accompanied by a significant increase in modulus, sometimes up to 3x. As noted earlier, this increase in modulus is undesirable especially in glove applications where an increase in modulus in known to reduce the wearer's comfort, reduce the tactile feeling and promote fatigue. Table X illustrates that the overall effect on the viscosity was small, particularly for Sample #4 consisting of the 380 m /g fumed silica. The viscosity the formulated latex will control the thickness of a coagulated film. Depending on the variations in viscosity, manufacturers of dipped products will need to make adjustments in the dip or dwell time and the coagulant salt concentration to control the film thickness and ensure consistency. As a result, a relatively minimal or small impact on viscosity, as shown in the present example, is highly desirable. EXAMPLE 8 Four latex formulations were prepared by lightly stirring 0.3 phr (parts per hundred by weight rubber) potassium hydroxide, 0.3 phr potassium laureate, 0.5 phr sulfur, 0.75 phr zinc dibutyl dithiocarbamate, 0.25 phr zinc oxide dispersion, 1 phr 2,2'-dicyclopentylene-bis (4-methyl-6-t-butylphenol) into ammonia preserved centrifuged natural rubber latex consisting of approximately 60% by weight natural rubber solids. The second, third and fourth formulations also contained 1 phr fumed silica solids from an aqueous dispersion of fumed silica (12-17% by weight solids content stabilized with ammonia to a pH of 9.5) having surface areas of 160 m 2 /g, 200 m 2 /g, 380 m 2 /g, respectively. The latex formulation was matured for 3 days at 25°C. Coagulant dipped films were prepared from the matured latex using the same procedure as in Example 7 except that films were also post vulcanized for 10 minutes at 110°C. As in Example 7, tensile, aged tensile and tear (trouser's) strength were determined using ISO 37 (1977), ISO 188 (1982) and ISO 34 (1979) techniques, respectively and are illustrated in Tables XI through XIII. The effect of the fumed silica on the viscosity of the matured post-vulcanized latex formulation was determined by Brookfield LVT viscosity measurements taken from one to seven days using a No. 2 spindle at 60 rpm, 24 °C, as shown in Table XIV. TABLE XI MATURED POST- VULCANIZED LATEX FILMS Surface SI M300t M500Ϊ TSπ EB* Tear Strength Area (πr/g) (MPa) (MPa) (MPa) (%) (N/mm) 1 (Control) - 1.82 1.0 2.4 30.7 890 25.2 2 160 1.91 1.1 2.5 30.5 910 24.5 3 200 1.86 1.1 2.7 30.4 890 40.8 4 380 1.91 1.2 2.1 29.4 920 23.3 tModulus at 300% Elongation Strength tModulus at 500% Elongation *Elongatιon at Break TABLE XII - AGED (2 days at 100°C) Surface Area M300t M500| TSD EB* (πr/g) (MPa) (MPa) (MPa) ( %) 1 (Control) 0.9 (90) 1.6 (67)** 23.8 (77) 1000 (112) 2 160 1.0 (91 ) 1.6 (64) 20.9 (68) 990 (108) 3 200 0.8 (73) 1.6 (59) 21.7 (71) 1000 (112) 4 380 0.9 (75) 1.5 (71) 22.5 (76) 990 (107) tModulus at 300% Elongation αTensilt ; Strength tModulus at 500% Elongation ♦ Elongation at Break ** Number in brackets indicate percentage retention of properties after ageing. TABLE XIII - AGED (3 days at 100°C) Surface Area M300t M500J TSπ EB* (m 2 /g) (MPa) (MPa) (MPa) (%) 1 (Control) 0.9 1.4 (58) 20.0 (62) 1050 (110) 2 160 1.0 (90) 1.5 (60) 18.7 (61) 1020 (112) 3 200 0.8 (91) 1.4 (52) 15.9 (52) 980 (110) 4 380 0.9 (75) 1.3 (108) 16.3 (55.4) 1000 (109) tModulus at 300% Elongation ^Tensile Strength tModulus at 500% Elongation ♦ Elongation at Break ** Number in brackets indicate percentage retention of properties after ageing. TABLE XIV VISCOSITY OF MATURED POST VULCANIZED LATEX FILMS Viscosity (cp)t Surface Area Number of Days (m 2 /g) 0 3 4 1 (Control) - 40.0 147.0 222.0 238.5 255.0 2 160 75.0 77.0 142.0 155.0 168.0 3 200 60.0 65.0 100.0 100.0 100.0 4 380 75.0 85.0 125.0 137.5 150.0 JBrookfield, Spindle No. 2, 60 rpm. As illustrated in Example 8, the addition of 1 part of fumed silica from an aqueous fumed silica dispersion markedly improved the tear strength of post- vulcanized latex films prepared from a matured latex formulation. As in Example 7, the improvement in tear strength results without deleteriously effecting the properties of modulus, tensile and elongation. In addition, the retention of these properties are improved upon ageing, as shown in Tables XII and XIII. Finally, as noted in Table XIV, the addition of fumed silica appears to act as a stabilizer to the latex formulation by preventing an unduly high increase in viscosity, as normally occurs upon maturation. As noted from the examples, the latex compositions of the present invention produce dipped latex films and goods produced therefrom with reduced extractable protein levels and physical and mechanical properties equal or superior to those obtained with similar dipped goods containing no silica. In addition, the use of the fumed silica at the loading levels described herein yield latex compositions with improved colloidal stability and viscosity control while films produced therefrom experience an increase in tear strength. The films and goods further exhibit a higher retention of tensile and elongation properties upon ageing. As a result, the films and goods produced by the latex compositions of the present invention benefit from the significant reduction in extractable proteins, while maintaining or enhancing the tear strength, without increasing the thickness of the final product and consequently, loss of sensitivity to the wearer. It is further understood that the present invention is not limited to the particular embodiments shown and described herein, but that various changes and modifications may be made without departing from the scope and spirit of the invention. For example, it is contemplated that precipitated silicas meeting the requirements of the present invention would also be suitable. What is claimed is:

Claims: CLAIMS 1. A natural rubber latex composition, comprising: a natural rubber latex and a stable aqueous dispersion of fumed silica, characterized in that said fumed silica is uniformly dispersed in said composition and is present in an amount ranging between 0.5% and 5.0% , by weight, of rubber solids, said fumed silica having a minimal impact on composition viscosity and acting as a viscosity stabilized, wherein films produced from said composition have protein levels of less than 120 μg/g. 2. The composition of claim 1 wherein said fumed silica is present in an amount ranging between 0.5% and 2.5 % , by weight, of rubber solids. 3. The composition of claim 1 wherein said natural rubber latex has a total rubber solids content between about 30% and about 70% . 4. The composition of claim 1 wherein said fumed silica has a BET surface area ranging from about 50 m 2 /g to about 400 m 2 /g, and is present in amount between 1 % and 1.5%, by weight, of latex rubber solids. 5. The composition of claim 1 wherein said stable dispersion of fumed silica has a pH ranging from about 5.0 to 10.5, and has coagulation characteristics similar to natural rubber latex. 6. The composition of claim 1 wherein said stable dispersion of fumed silica has between 10% and 45 % , by weight, fumed silica solids. 7. The composition of claim 6 wherein said stable dispersion of fumed silica has between 15 % and 30% , by weight, fumed silica solids. 8. The composition of claim 1 wherein said composition further comprises at least one additive selected from the group consisting of: curing agents, cross- linking agents, vulcanization activators, vulcanization accelerators, antioxidants, stabilizers and antidegradants. 9. The composition of claim 1 wherein said natural rubber latex is pre¬ vulcanized. 10. The composition of claim 9 wherein said pre-vulcanized latex is a low protein pre-vulcanized latex. 11. The composition of claim 9 or 10 wherein said fumed silica has a BET surface area of about 380 m 2 /g and is present in an amount of about 1 % by weight of latex rubber solids. 12. The composition of claim 1 wherein said natural rubber latex is post- vulcanized. 13. The composition of claim 12 wherein said post-vulcanized natural rubber latex is a low protein post-vulcanized latex. 14. The composition of claim 12 or 13 wherein said post-vulcanized latex has a linear swelling index between about 1.8 and about 2.0. 15. The composition of claim 14 wherein said fumed silica has a BET surface area of 200 πr/g and is present in an amount of about 1 % , by weight, of latex rubber solids. 16. A latex film, comprising: a natural rubber latex and a stable aqueous dispersion of fumed silica, characterized in that said fumed silica is uniformly dispersed in said composition and is present in an amount less than 5.0%, by weight, of rubber solids, wherein said film has a protein level of less than 120 μg/g. 17. The latex film of claim 16 wherein said fumed silica is present in an amount ranging between 0.5% and 2.5%, by weight, of rubber solids. 18. The latex film of claim 16 wherein said natural rubber latex has a total rubber solids content between about 30% and about 70%. 19. The latex film of claim 16 wherein said fumed silica has a BET surface area ranging from about 50 m 2 /g to about 400 m 2 /g and is present in amount between about 1 % and 1.5% , by weight, of latex rubber solids. 20. The latex film of claim 16 wherein said stable dispersion of fumed silica has a pH ranging from about 5.0 to 10.5 and has coagulation characteristics similar to natural rubber latex. 21. The latex film of claim 20 wherein said stable dispersion of fumed silica has a pH between 8.0 and 10.0. 22. The latex film of claim 16 wherein said stable dispersion of fumed silica has between 10% and 45% , by weight, fumed silica solids. 23. The latex film of claim 22 wherein said stable dispersion of fumed silica has between 15% and 30%, by weight, fumed silica solids. 24. The latex film of claim 16 wherein said natural rubber latex is pre¬ vulcanized. 25. The latex film of claim 24 wherein said pre-vulcanized latex is a low protein pre-vulcanized latex. 26. The latex film of claim 25 wherein said fumed silica has a BET surface area of about 380 m 2 /g and is present in an amount of about 1 % by weight of latex rubber solids. 27. The latex film of claim 16 wherein said natural rubber latex is post- vulcanized. 28. The latex film of claim 27 wherein said post-vulcanized natural rubber latex is a low protein post-vulcanized latex. 29. The latex film of claim 28 wherein said post-vulcanized latex has a linear swelling index between about 1.8 and about 2.0. 30. The latex film of claim 29 wherein said fumed silica has a BET surface area of 200 m 2 /g and is present in an amount of about 1 % by weight of latex rubber solids. 31. The latex film of claim 16 wherein said film is an article selected from the group consisting of: surgeons' gloves, examination gloves, condoms, catheters, and balloons. 32. A method for producing a latex film which comprises the steps of: compounding a natural rubber latex and a stable aqueous dispersion of fumed silica to form a natural rubber latex composition, wherein said fumed silica is present in an amount ranging between 0.5 % and 5.0% by weight of rubber solids; dipping a pre-shaped former into the latex composition for a period of time sufficient to deposit a film of a desired thickness; leaching and drying said film; and removing said film from the former. 33. The method of claim 32 wherein said natural rubber latex has a total rubber solids content of 30% and about 70% . 34. The method of claim 32 wherein said fumed silica has a BET surface area ranging from about 50 m 2 /g to about 400 m 2 /g and is present in amount between about 1 % and 1.5%, by weight, of latex rubber solids. 35. The method of claim 32 wherein said dispersion of fumed silica has a pH ranging from about 5.0 to 10.5 and has coagulation characteristics similar to natural rubber latex. 36. The method of claim 35 wherein said dispersion of fumed silica has a pH between 8.0 and 10.0. 37. The method of claim 32 wherein said dispersion of fumed silica has between 10% and 45 % , by weight, fumed silica solids. 38. The method of claim 37 wherein said stable dispersion has between 15 % and 30% , by weight, fumed silica solids. 39. The method of claim 32 wherein said natural rubber latex is pre¬ vulcanized. 40. The method of claim 39 wherein said pre-vulcanized latex is a low protein pre-vulcanized latex. 41. The method of claim 32 wherein said natural rubber latex is post- vulcanized. 42. The method of claim 41 wherein said post-vulcanized natural rubber latex is a low protein post-vulcanized latex. 43. The method of claim 32 wherein said composition further comprises at least one additive selected from the group consisting of: curing agents, cross-linking agents, vulcanization activators, vulcanization accelerators, antioxidants, stabilizers and antidegradants. 44. The method of claim 32 which further comprises the steps of: immersing the pre-shaped former into a coagulant solution prior to dipping. 45. The method of claim 32 which further comprises the steps of: post-curing the film prior to removing the film from the former.

Inventors: ANAND JOGINDER N, ANAND, JOGINDER, N.
Applicants: CABOT CORP, CABOT CORPORATION
Application year: 1995
Main classification: C08K3
IPC codes: A61B19, A61F6, A61L29, C08C1, C08J5, C08K3, C08L7