Curable latex composition

Abstract

A curable latex composition includes a latex component selected from the group consisting of nitrile rubber, latex, natural rubber latex, and mixtures thereof; a crosslinking component comprising: a blocked isocyanate or a water-dilutable isocyanate, a metal oxide selected from the group consisting of zinc oxide, aluminum oxide, ammonium zirconium carbonate, zirconium oxide, aluminum lactate and aluminum hydroxide, and mixtures thereof; optionally, an antioxidant component; optionally, a filler; and optionally other ingredients.

Description

[0001] The present invention relates to curable latex compositions, in particular but not limited to nitrile rubber, natural rubber or other rubber compositions, and also to elastomeric products manufactured from such compositions, including latex gloves and the like.

[0002] Conventional disposable latex gloves are manufactured from natural rubber or nitrile rubber latex by a vulcanisation process using sulphur, zinc oxide and accelerators such as dithio carbamate. However, zinc oxide causes wastewater pollution and according to EU food contact regulation 10 / 2011, the specific migration limit of Zn is controlled at 5 mg / kg of food or food simulant. Zinc based accelerators are not water soluble and remain in the gloves or other products after manufacture. Accelerator residues can cause type IV allergic reactions. In addition, by-products such as nitrosamines can form which have potential carcinogenicity.

[0003] A further disadvantage of existing latex curing compositions is that, although zinc oxide provides the strength to cause cross-linking of the composition, this unstable ionic bonding has been found to lack stability in use.

[0004] It is an object of the present invention to provide a curable latex composition which is free from sulphur and accelerators. The presence of moisture in a film cross-linked by ionic bonds only can weaken the ionic bonds, leading to reduced film strength and film cracking.

[0005] According to a first aspect of the present invention, a curable latex composition comprises:

[0006] a latex component selected from the group consisting of: nitrile rubber latex, natural rubber latex, polychloroprene latex, polyisoprene latex and mixtures thereof;

[0007] a cross-linking component comprising:

[0008] a blocked isocyanate or a water reducible isocyanate;

[0009] a metal compound selected from the group consisting of: zinc oxide, aluminium oxide, zirconium ammonium carbonate, zirconium oxide, aluminium lactate, aluminium hydroxide, magnesium oxide and mixtures thereof;

[0010] an optional antioxidant component;

[0011] an optional filler; and

[0012] an optional further ingredient.

[0013] In embodiments, the curable latex composition is an aqueous composition, optionally an aqueous dispersion.

[0014] Preferably blocked isocyanates are used. Blocked isocyanates can be blocked polyisocyanates. In certain embodiments, the blocked isocyanate is not an oxime blocked isocyanate. Thus, it is preferred that blocked isocyanates are used, excluding oxime blocked isocyanates.

[0015] In this application document, blocked isocyanate or water dilutable isocyanate can be referred to simply as isocyanate.

[0016] In embodiments, the latex component is a nitrile rubber latex. Nitrile rubber latexes can be made by emulsion polymerization of acrylonitrile and butadiene. One example is disclosed in U.S. Patent 11 179908, the disclosure of which is incorporated by reference into the present specification for all purposes.

[0017] The amount of isocyanate can range from 0.01 phr to 0.6 phr, for example 0.05 phr to 0.6 phr, for example 0.3 phr to 0.6 phr, for example 0.4 phr to 0.6 phr, for example 0.1 phr to 0.4 phr, for example 0.2 phr to 0.3 phr.

[0018] The amount of metal compound can range from 0.1 phr to 1.5 phr, for example 0.4 phr to 0.8 phr.

[0019] The abbreviation phr is used to refer to parts by weight of an ingredient per 100 parts of rubber by dry weight.

[0020] Blocked isocyanates are isocyanates or polyisocyanates that have isocyanate functionality chemically blocked to control reactivity. It can be the reaction product of an isocyanate moiety, typically a diisocyanate or polyisocyanate, and a suitable blocking agent. Blocked isocyanates can be added to materials that would normally react with isocyanates, such that they do not react at ambient temperatures. They are widely used in industrial applications such as coatings, sealants, and adhesives.

[0021] The term blocked isocyanate is used in this specification to refer to a combination of an isocyanate or polyisocyanate and a blocking agent, where the blocking agent reacts reversibly with the isocyanate to reduce the reactivity of the isocyanate. The blocking reaction can be reversed by heating to a predetermined temperature to restore the reactivity of the isocyanate, allowing the composition to crosslink. See below. An alternative blocked isocyanate is a surface-inactivated isocyanate. These compounds include materials in which the surface of a solid isocyanate particle is reacted with, for example, an amine or water to form a surface of reduced activity. Such blocked isocyanates can be activated by heating, softening the solid particle and rendering it reactive.

[0022] Blocked isocyanates and polyisocyanates that are soluble or dispersible in water or miscible with water are referred to in this specification as water-based isocyanates. Dispersions of blocked isocyanates, water-emulsifiable blocked isocyanates, water-dilutable isocyanates or surface-inactivated polyisocyanates can be used.

[0023] Examples of water-based blocked isocyanates include the following compounds.

[0024] (1) Blocked isocyanates modified to introduce hydrophilic groups that make them emulsifiable, dispersible or soluble in water and that allow the formation of storage-stable emulsions;

[0025] (2) Blocked isocyanates that are not hydrophilically modified but can be emulsified or dispersed by incorporation of water-miscible solvents and / or surfactants;

[0026] (3) Blocked isocyanates that are solid at ambient temperature and that can be dispersed as finely divided powders in water together with additional surfactants. The finely divided powders can be dispersed directly or produced in situ by a size-reduction process such as the use of a ball or bead mill;

[0027] (4) Aqueous dispersions of surface-inactivated solid isocyanates. Examples are disclosed in 4849262 and 2002193534, the disclosures of which are incorporated by reference into this specification for all purposes.

[0028] Water-dilutable isocyanates can also be used. These are hydrophilically modified isocyanate-functional materials that are emulsifiable in water. These can be less stable in aqueous media than blocked isocyanates.

[0029] Basic form of butadiene-acrylonitrile rubber (NBR) can be prepared by free radical polymerization of butadiene (BD) with acrylonitrile (AN). The reaction medium is typically water, with surfactant stabilizers added to produce an emulsion polymer. While basic NBR can be able to crosslink with sulfur, it generally does not produce the strength and elongation performance required for a barrier protective film. NBR coagulates to form a granular rubber and is used in applications requiring resistance to oil and petroleum-based fuels (e.g., hoses, seals, and fuel tanks). Major butadiene-acrylonitrile latices include Perbunan manufactured by Arlanxeo and Nipol manufactured by Zeon. The AN incorporation level in the butadiene-acrylonitrile polymer will control the level of oil resistance. Higher AN levels will generally result in a harder film, but with better resistance to oil and non-polar solvents (e.g., hexane). AN content can be classified as high (> 43%), medium (36% to 31%), and low (< 25%). x-NBR or carboxylated NBR introduces methacrylic acid (MAA) or acrylic acid (AA) into the monomer mix. The addition of carboxylic acid groups to the polymer chain allows the possibility of additional crosslinking pathways. Acid monomer levels can generally range from 3% to 10% in the polymer chain. A typical example of a carboxylated butadiene-acrylonitrile latex for use in dip operations is Synthomer Grade 6330.

[0030] Antioxidants can be included in the composition. These have the effect of protecting the properties of the polymer from the negative effects of oxidation reactions. Antioxidants can be added as a single material or as a combination including mixtures with synergists. Examples of suitable antioxidants include the butylated reaction product of p-cresol and dicyclopentadiene (Wingstay L), pentaerythritol tetrakis-(3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate), 4-((4,6-bis(octylthio)-1,3,5-triazin-2-yl)amino)-2,6-di-tert-butylphenol, 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionates octadecyl ester, 4,6-bis(octylthiomethyl)-6-methylphenol, tetramethyl dihydroquinoline polymer, didodecyl-3,3'-thiopropionate (also known as dilauryl thiodipropionate (DLTDP)), or combinations thereof. Wingstay L is also known as Antioxidant L (CAS-6861051-5).

[0031] Antioxidants can include the reaction product of p-cresol and dicyclopentadiene (Wingstay L) and didodecyl-3,3'-thiopropionate. Antioxidants can include the reaction product of p-cresol and dicyclopentadiene (Wingstay L).

[0032] Examples of accelerators that can be used are 4,6-bis(octylthiomethyl)-6-methylphenol, tris(2,4-di-tert-butylphenyl)phosphite and 3,3'-thiopropionic acid didodecylester.

[0033] Antioxidants and accelerators can be added directly or, more preferably, as an aqueous dispersion or emulsion.

[0034] The ingredients of the composition can be added separately or in combination prior to mixing with the polymer latex.

[0035] Polyisocyanates useful in the present application are those known in the art and can be selected from the group consisting of diphenylmethane diisocyanate (MDI), toluene diisocyanate (TDI), hexamethylene diisocyanate (HDI), isophorone diisocyanate (IPDI), 4-4'-diisocyanatodicyclohexylmethane (H12MDI), tetramethylxylylene diisocyanate (TMXDI), including dimers (uretdiones), trimers (isocyanurates), biurets and propyl phosphonate of the same, and adducts of diisocyanates with diols and triols.

[0036] An exemplary water-based isocyanate blocking agent is 3,5-dimethylpyrazole (3,5-DMP). Other blocking agents include butanone oxime, also known as methyl ethyl ketone oxime [MEKO], ε-caprolactam [s-Cap], diethyl malonate (DEM), or phenolics, including, for example, alkyl phenols, cardanol, vanillin, and amines, such as diisopropylamine. Other blocking agents include triazoles, ethylimidazole, 3,5-di-tert-butylpyrazole, and butylbenzylamine.

[0037] Blocked polyisocyanates can be prepared by known methods or are commercially available. They can be solvent-based, 100% active (i.e., not water or solvent based), or water-based, i.e., aqueous emulsions or dispersions of blocked isocyanates.

[0038] Examples of commercially available water-based blocked polyisocyanates include the Trixene Aqua series from Lanxess; Imprafix, Impranil, Baybond and Bayhydur series from Covestro; Aqualink X from Aquaspersions. These materials are hydrophilically modified isocyanates based on hexamethylene diisocyanate (HDI) isocyanurates, biurets of HDI or isophorone diisocyanate (IPDI) isocyanurates. These can be hydrophilically modified by reacting one of the isocyanate moieties with a hydroxyl functional carboxylic acid, and blocking the remaining moieties with 3,5-DMP or other blocking agents such as 8-hexanolactam, with the carboxylic acid moiety neutralized with, for example, an amine such as triethylamine. Alternatively, the carboxylic acid can be replaced by a low molecular weight polyethylene oxide polymer of less than 10 monomer units.

[0039] Another example is an adduct of IPDI and a low molecular weight triol, where the isocyanate moieties are capped with bisulfite, simultaneously blocking and making the material water emulsifiable, such as Synthapret BAP from Tanatex.

[0040] Another example of a water-based blocked isocyanate is Kagebond D50 from Isochem Kautchuck. This includes MDI blocked with epsilon-caprolactam dispersed in water.

[0041] Examples of solvent-based blocked isocyanates that can be emulsified with additional surfactants are: Trixene BI 7982 from Lanxess. This is an HDI trimer blocked with 3,5-DMP in a methoxy propanol solvent; Trixene BI 7960 from Lanxess, an HDI biuret blocked with 3,5-DMP in a methoxy propanol solvent; Tolonate D2 from Vencorex. This is an HDI trimer blocked with MEKO in an aromatic hydrocarbon solvent, Desmodur PL350 from Covestro, which is an HDI trimer blocked with DMP in a methoxy propyl acetate / aromatic hydrocarbon solvent, Desmodur BL3175 from Covestro, which is an HDI trimer blocked with MEKO in an aromatic hydrocarbon solvent.

[0042] Examples of dispersible solid blocked isocyanates are as follows: Vestagon B1400 from Evonik, a linear IPDI prepolymer blocked with epsilon-caprolactam.

[0043] Examples of aqueous dispersions of surface-inactivated polyisocyanates are: AquaLink D-HT from Aquaspersions, which is IPDI trimer inactivated with a polyamine and dispersed in water; Aqualink U-HT from Aquaspersions, which is TDI dimer inactivated with a polyamine and dispersed in water.

[0044] One or more fillers can be included in the composition, such as calcium carbonate, clay, aluminosilicate, or silica. Amounts of 10 to 100 phr can be used.

[0045] The composition according to the present invention provides many advantages compared to previously disclosed compositions. The composition and products made from the composition can be free of crosslinking agents such as sulfur and can have a reduced content of metal oxides such as zinc oxide, ammonium, zirconyl carbonate, zirconium oxide, aluminum lactate, and aluminum hydroxide. The strength of the composition can be improved. The chemical permeability of the composition can be improved. The level of swelling before break can be improved. Without wishing to be bound by theory, it is believed that the covalent bonds formed by the isocyanate crosslinking agent of the present invention are stronger than the ionic bonds obtained using zinc oxide crosslinking.

[0046] The composition of the present invention can be used to make latex articles, such as disposable gloves, for example medical, surgical, or laboratory gloves, condoms, balloons, pacifiers, and other articles typically made from elastomeric rubber compositions.

[0047] Accordingly, the present invention envisages a method of making a cured latex article, comprising the steps of: preparing a curable latex composition according to the present invention;

[0048] applying the composition to a forming mold to form a coated forming mold;

[0049] heating the coated forming mold to cure the composition to form an article on the forming mold, and removing the article from the forming mold.

[0050] Suitable forming molds for latex articles are known in the art. Typically, the forming mold provides the shape and configuration of the latex article.

[0051] The present invention also envisages a latex article formed by the method of making. Furthermore, the present invention envisages a latex article comprising the cured latex composition of the present invention.

[0052] The latex article can be a glove, a condom, a pacifier, or a pacifier balloon.

[0053] Unless otherwise specified, percentages and other amounts mentioned in the present specification are by weight and selected from any range referenced to total 100%.

[0054] The application is further described by way of examples, but without any limiting nature.

[0055] Example 1 (latex compounding)

[0056] The pH of the introduced latex was measured. A solution of the latex stabilizer sodium dodecylbenzenesulfonate (0.3 phr) was added and the mixture was stirred for 15 minutes. The pH adjuster potassium hydroxide was added until the latex reached a pH of 10.

[0057] The following ingredients were used. The abbreviation phr refers to parts of the ingredient per 100 parts of rubber by dry weight.

[0058]

[0059]

[0060] The blocked isocyanate (38% Aqualink X 0.3 phr) was diluted with water to a 1 : 1 ratio and added to the pH adjusted latex. The mixture was stirred for 5 to 10 minutes. The zinc oxide (0.70 phr) was diluted with water in a 1 : 1 weight ratio. The diluted zinc oxide dispersion was added to the pH adjusted latex. The mixture was stirred for 5 to 10 minutes. The antioxidant Wingstay L (0.2 phr) was diluted with water in a 1 : 1 ratio and added to the antioxidant dispersion and pH adjusted latex mixture. The latex was further diluted to provide a glove with the desired thickness. The pH of the final compounded latex was checked and adjusted to a pH of 10 or higher using a potassium hydroxide solution if necessary. The latex was stirred using a low shear stirrer at a speed of 80 rpm for 18 to 24 hours to cure. The compounded latex was passed through a 200 mesh screen to remove any solid particles.

[0061] Example 1 b

[0062] The composition having the following ingredients was used following the procedure described in Example la.

[0063] Chemicals phr 45% latex (6330 from Xiameter) 100 25% sodium dodecylbenzenesulfonate 0.3 10% KOH 1.7 38% Aqualink U-HT 0.5 60% zinc oxide 0.7 50% antioxidant 0.2

[0064] The antioxidant was 0.2 phr Wingstay L (Antioxidant L).

[0065] Example 2 (dip method)

[0066] The glove shaped forming mold was dipped into the calcium nitrate solution coagulant at a temperature of 55°C to 60°C. The coated forming mold was dried and kept at a temperature of 60°C to 65°C. The coated forming mold was dipped into the latex composition prepared according to Example 1 to obtain the desired weight and thickness of the coating. The latex coated forming mold was passed through a coagulum oven at 60°C and then to a hemming station. The coated forming mold was dipped into a pre-leach tank at 60°C for 1 minute and then placed in a curing oven at a temperature of 120°C for 30 to 40 minutes. After curing, the glove on the forming mold was dipped into a post-leach bath at a temperature of 60°C for 1 to 2 minutes. After leaching, the glove on the forming mold was subjected to a surface treatment process, if required. The dried glove on the forming mold was then dried in an oven at a temperature of 80°C to 90°C. After drying, the glove was stripped from the forming mold, tumbled and conditioned for 16 hours before being subjected to physical testing.

[0067] The following parameters were used.

[0068]

[0069] The processing conditions can be adjusted according to the production line equipment and the material being processed.

[0070] Example 3 (durability testing)

[0071] The durability test was performed using a conventional method.

[0072] The following results were obtained.

[0073]

[0074]

[0075]

[0076] [COMPARATIVE EXAMPLE]

[0077]

[0078] The accelerator package used was a proprietary blend of sulfur, zinc oxide and zinc based accelerators.

[0079] The latex used was 6330 supplied by Xiameter.

[0080] The antioxidant was a blend of the butylated reaction product of p-cresol and dicyclopentadiene (Wingstay L) and didodecyl 3,3'-thiopropionate.

[0081] No defects were observed in samples A to E during visual inspection. However, sample F exhibited a break during the test after being immersed for three hours. On the other hand, unlike sample F, the membrane compounded with Aqualink alone did not show any membrane breakage during the experiment.

[0082] The tensile strength of samples D, E and F were compared.

[0083]

[0084] From the results obtained, it is evident that sample D did not deteriorate. Sample F deteriorated during the test. Furthermore, the tensile strength of sample D was comparable to that of the traditionally cured sample E.

[0085] The results for sample D remained within the specifications of the ASTM standard, with a minimum tensile strength of 14 MPa (ASTM D 6319).

[0086] Samples A and B with Aqualink X alone showed that the presence of blocked isocyanate helped to formulate a stronger membrane that did not break during the immersion process. In contrast, sample F did break.

[0087] The addition of antioxidant to this combination of blocked isocyanate and zinc oxide led to excellent durability, showing that the antioxidant also helped to improve the membrane strength.

[0088] Example 4 (further testing)

[0089] Gloves were formed by a single dip process generally following the method of Example 2 using the latex composition of Example lb having a total solids content of 15%.

[0090] The gloves were subjected to standard durability tests using standard methods compared to controls formed by traditional sulphur curing methods. The following results were obtained.

[0091]

[0092] Aged Aged Aged Thickness (mm) 0.058 0.052 Tensile strength (MPa) average 30.01 32.88 Elongation at break (%) 667 500 M300 (MPa) 3.76 7.64 M500 (MPa) 8.60 31.71 Breaking force (N) 6.54 6.79

[0093] The physical properties of the gloves of the present application were comparable to those of the control gloves. However, there were improvements compared to the control gloves, for example, the unaged and aged gloves of the present application had better elongation at break compared to the control gloves. It is also noteworthy that the elongation at break of the gloves of the present application improved after ageing, whereas the elongation at break of the control gloves decreased after ageing.

[0094] The aged control gloves only just met the requirements for elongation at break.

[0095] Cytotoxicity results

[0096]

[0097]

[0098] The compositions of the present application were tested for toxicity. The controls exhibited worse toxicity results. The control compositions needed to be diluted to a 1 :32 ratio to be non-toxic. Whereas the compositions of the present application only needed to be diluted to a 1 :8 ratio to be non-toxic. This demonstrates that the present application provides a solution to the toxicity problems faced by compositions cured by prior art sulfur-based curing methods.

[0099] Chemical permeability results

[0100] Chemical permeation testing was performed by standard methods. The performance of the present application was superior to the control samples with sulfur curing. The following results were obtained.

[0101]

[0102] In particular, the present application has better chemical permeation than the sulfur cured control for heptane and formaldehyde.

Claims

1. A curable latex composition comprising: Latex components, selected from nitrile rubber, latex, natural rubber latex and mixtures thereof; Crosslinking components, including: Blocked isocyanates or water-dilutable isocyanates, Metal oxides selected from the group consisting of: zinc oxide, aluminum oxide, ammonium zirconium carbonate, zirconium oxide, aluminum lactate, and aluminum hydroxide, and mixtures thereof; Optional antioxidant components; Optional fillers; and Other optional ingredients.

2. The composition according to claim 1, wherein the amount of said blocked isocyanate or water-dilutable isocyanate is in the range of 0.01 phr to 0.6 phr.

3. The composition according to claim 2, wherein the amount of the isocyanate is in the range of 0.1 phr to 0.4 phr or 0.3 phr to 0.6 phr.

4. The composition according to claim 3, wherein the amount of isocyanate is in the range of 0.2 phr to 0.3 phr.

5. The composition according to any one of the prior claims, wherein the amount of said metal compound is in the range of 0.1 phr to 1.5 phr.

6. The composition according to claim 5, wherein the amount of said metal compound is in the range of 0.4 phr to 0.8 phr.

7. The composition according to any one of the prior claims, wherein the isocyanate is a polyisocyanate.

8. The composition according to any one of the prior claims, wherein the metal compound is selected from zinc oxide, aluminum oxide, zirconium oxide, magnesium oxide, and mixtures thereof.

9. The composition according to any one of the prior claims, wherein the antioxidant is Wingstay L.

10. The composition according to any one of the prior claims, wherein the isocyanate is emulsified or dispersed.

11. The composition according to any one of the prior claims, comprising an antioxidant and a synergist.

12. The composition according to claim 11, wherein the synergist is selected from: 4,6-bis(octylthiomethyl)-6-methylphenol, tris(2,4-di-tert-butylphenyl) phosphite and 3,3'-didodecyl thiopropionate.

13. A method for manufacturing a cured latex product, comprising the following steps: Prepare a curable latex composition according to any one of the prior claims; The composition is applied to a molding die to form a coated molding die; The coated molding die is heated to cure the composition, thereby forming an article on the molding die, and the article is removed from the molding die.

14. A latex article comprising a cured latex composition according to any one of claims 1 to 12.

15. The latex article of claim 14, wherein the article is a glove, a condom, a pacifier, or a pacifier balloon.

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