NL2016095B1 - Vulcanization accelerator composition for latex foam. - Google Patents

Abstract

The invention relates to a vulcanizable rubber latex foam composition comprising a rubber latex, a sulfur-containing vulcanizing agent and at least one vulcanization accelerator. The vulcanization accelerator comprises 0.50-10.0 parts per hundred based on the total weight of the foam composition of a compound which responds to formula (I) or a zinc salt of this compound : wherein R1 and R2 may be the same or different, , wherein each of R1 and R2 may be an alkyl or a cycloalkyl group having between 1 and 6 carbon atoms, or an arylalkyl group comprising 7-12 carbon atoms, wherein the vulcanization accelerator further comprises 0.01 – 10.0 parts per hundred based on the total weight of the foam composition of a thiazole or the zinc salt thereof.

Description

Vulcanization accelerator composition for latex foam.

The present invention relates to a vulcanizable rubber latex foam composition comprising a rubber latex, a sulfur-containing vulcanizing agent and at least one vulcanization accelerator, according to the preamble of the first claim.

Industrial production of rubber latex foam products nowadays generally proceeds using one of two long established processes, the so-called Dunlop and Talalay process. Rubber latex foam is usually prepared by compounding a latex of natural rubber, synthetic rubber or a mixture of natural and synthetic rubber compositions with a vulcanization system, contacting the latex composition with a blowing agent in order to produce a foam with a desired density, pouring the resulting foam in molds or any supporting substrate, setting the foam by gelling it, and subjecting the resulting gelled foam to vulcanization to obtain a foam with the desired mechanical and physical properties.

To achieve vulcanization, the latex foam composition will generally comprise a vulcanizing agent, which will usually be sulphur based, a vulcanization activator for the vulcanizing agent for example ZnO, a vulcanization accelerator and other additives such as antioxidants, dispersants. Commonly used vulcanization accelerators include zinc mercaptobenzothiazole and/or zinc diethyldithiocarbamate, but others may be used as well. In the Talalay process vulcanization is usually carried out at a moderate temperature of 110-120°C. After vulcanization has been terminated, the foam is removed from the mould, subjected to washing with water to remove any remaining reactants and reaction products, and dried. “Latex Foam Rubber” by E.W. Madge, John Wiley & Sons, New York and Mc Laren & Sons Ltd., London 1962, and “Polymer Latices : Science and Technology

Volume 3: Applications of Latices” by D.c. Blackley, Chapman & Hall, London 1997 describe the details of different foaming processes. A frequently used primary vulcanization accelerator is zinc diethyldithiocarbamate (ZDEC). ZDEC is often incorporated in rubber compositions to enable the zinc oxide present in the latex composition to assist gelation. The main function of ZDEC is however to activate cross-linking of sulfur with the rubber particles of the latex composition. While the presence of ZDEC is important to achieve a fast curing rate of the latex foam and good mechanical foam properties, increased pressure in relation to environmental and safety issues, requests urging towards a reduction or elimination of the use of ZDEC are taking ground, particularly in Europe. The Environmental Protection Encouragement Agency’s (ΕΡΕΑ) cradle-to-cradle methodology has listed ZDEC in the red category. Hence its use should no longer be continued as it is suspected to form hazardous N-nitrosamines in certain conditions.

The occurrence of hazardous N-nitrosamines in articles made of vulcanized rubber is attributed to the fact that ZDEC remains behind in the polymer rubber latex, after vulcanization has been terminated. ZDEC may be hydrolyzed to produce a secondary amine which in turn reacts with NOx, nitrites or other NOx in the environment to produce hazardous N-nitrosamines. Certain of these N-nitrosamines are carcinogenic, and a problem arises when they remain behind in the finished foam, in particular when the finished foam comes into close contact with the human body. Many countries have therefore severely restricted the maximum permissible concentration of certain N-nitrosamines in vulcanized rubber articles, in particular N-nitrosodimethylamine, N-nitrosodiethylamine, N-nitrosodi-n-butylamine and N-nitrosomethylphenylamine.

Other vulcanization accelerators exist, which do not present the problem of producing nitrosamine. Examples hereof include zinc isopropylxanthogenate, thiophosphate compounds, thiazole compounds, benzothiazolesulphenamide compounds and guanidine compounds. Zinc isopropylxanthogenate presents the problem of showing a low storage stability, an offensive smell and it gives poor vulcanization properties. Guanidine compounds are not able to provide desired vulcanization properties (Polymer Digest, 1991, 1, p65).

There is thus a need to a latex foam composition, and in particular a vulcanization accelerator for the latex foam composition, with which the risk to the formation of the hazardous nitrosamine may be reduced to a minimum, or at least a vulcanization accelerator with which nitrosamine production may be seriously reduced. EP915133 discloses dip-forming vulcanizable rubber latex compositions comprising a sulfur-containing vulcanizing agent and zinc dibenzyldithiocarbamate as vulcanization accelerator, to manufacture vulcanized unsaturated nitrile rubber article without producing nitrosamine. According to EP915133, dithiocarbamic acid produces only a negligible amount of nitrosamine or secondary amines which are precursors of nitrosamine. An example of such a dithiocarbamic acid compound is zinc dibenzyldithiocarbamate.

However, the vulcanized rubber articles dip-formed from natural rubber latex using this vulcanization accelerator show cracks and have poor surface luster. According to EP915133 the occurrence of cracks may be avoided and good surface luster and vulcanization properties can be obtained with a vulcanizable dip-forming rubber latex composition which comprises an unsaturated nitrile-conjugated diene copolymer rubber latex, a sulfur-containing vulcanizer and at least one vulcanization accelerator selected from (i) dithiocarbamic acid compounds represented by the formula (1) and (ii) zinc dithiocarbamate compounds represented by the formula (2) below, and an optional thiazole compound vulcanization accelerator

In formula I and II, Ri and R2 are hydrocarbon groups having at least 6 carbon atoms which may be the same or different. In particular, Ri and R2 are an alkyl or cycloalkyl group which may have a branch, an aryl group which may have a substituent, and a benzyl group which may have one or two alkyl groups each having 1 to 5 carbon atoms in the a-carbon atom.

Specific examples of compounds according to formula (1) and (2) include dibenzyldithiocarbamic acid, di-2-ethylhexyldithiocarbamic acid), diphenyldithiocarbamic acid and dicyclohexyldithiocarbamic acid, and their zinc salts. Dibenzyldithiocarbamic acid and its zinc salt are especially preferred.

The present invention seeks to provide a latex foam composition comprising a vulcanization agent and a vulcanization accelerator with which the risk to the production of the hazardous nitrosamine may be reduced to a minimum.

The present invention in particular seeks to provide a vulcanization accelerator for use with a latex composition with which the risk to the production of the hazardous nitrosamine may be reduced to a minimum.

This is achieved according to the present invention with a vulcanizable rubber latex foam composition which shows the technical features of the characterizing portion of the first claim.

Thereto the vulcanizable rubber latex foam composition of the present invention is characterized in that the at least one vulcanization accelerator comprises 0.50-10.0 parts per hundred based on the total weight of the foam composition of a compound which responds to formula (I) or a zinc salt of the compound according to formula (I):

wherein Ri and R2 may be the same or different, but preferably are the same, wherein each of Ri and R2 may be an alkyl or a cycloalkyl group having a hydrocarbon chain containing between 1 and 6 carbon atoms, or an arylalkyl group comprising 7-12 carbon atoms, wherein the vulcanization accelerator further comprises 0.01 - 10.0 parts per hundred based on the total weight of the foam composition of a thiazole or the zinc salt thereof. A preferred thiazole or zinc salt thereof is a mercaptobenzothiazole or a zinc salt thereof, in particular 2-mercaptobenzothiazole or a zinc salt thereof.

In a preferred embodiment each of Ri and R2 or both may be an arylakylgroup having an aryl moiety comprising one or two alkyl substituents in the α-position, each of the alkyl substituents having independently of one another 1 to 5 carbon atoms.

Preferably however, the concentration of dibenzyldithiocarbamate or its zinc salt varies from 0.5-5.0 parts per hundred based on the total weight of the foam composition, preferably 1.0-4.0, more preferably 1.5-3.0 parts per hundred, in order to achieve optimum foaming and a sufficiently complete vulcanization.

The concentration of 2-mercaptobenzothiazole or its zinc salt preferably varies from 0.025 - 7.5 parts per hundred based on the total weight of the foam composition, more preferably 0.025 - 5.0, most preferably 0.5-2.5 or even 0.5-2.0 parts per hundred.

In a preferred embodiment of the vulcanizable rubber latex foam composition of this invention, the molar ratio of dibenzyldithiocarbamate (ZBEC) to 2-mercaptobenzothiazole (ZMBT) or their zinc salts ranges from 1 to 3.

The use of the claimed mixture of vulcanization accelerators, permits reducing the risk to the production ofi hazardous carcinogenic nitrosamines to a minimum. The latex foam composition of this invention presents the advantage that it does not give an offensive smell, and does not produce unwanted color.

The use of the claimed mixture of vulcanization accelerators further permits to achieve the desired degree of vulcanization within a reasonably short time frame, in the temperature range conventionally used in the Talalay process, i.e. 110-120°C and at a reasonable cost. Whereas a thiocarbamate, in particular dibenzyldithiocarbamate or its zinc salt (ZBEC) is a vulcanization accelerator which is capable of providing a substantially complete vulcanization, it is relatively expensive when compared to a thiazole, in particular mercaptobenzothiazole or its zinc salt, the thiocarbamate, in particular dibenzyldithiocarbamate or its zinc salt (ZBEC) needs a relatively high temperature to be activated, and the acceleration rate provided by it is slower when compared to a thiazole, in particular when compared to mercaptobenzothiazole.

Activation of vulcanization of a rubber latex foam composition by a thiazole, in particular mercaptobenzothiazole (ZMBT) or its zinc salt may be achieved at lower temperatures, and the acceleration achieved is faster when compared to a thicocarbamate, in particular dibenzyldithiocarbamate or a zinc salt thereof. Usually however or incomplete vulcanization may be achieved when using MBT or ZMBT alone. Without wanting to be bound by this theory, the inventors assume that the presence of the thiocarbamate, in particular dibenzyldithiocarbamate or its zinc salt ensures that the onset of the activation of the vulcanization may be achieved at the relatively low vulcanization temperature of about 105-120°C, preferably 105-115°C or 110-120°C that is commonly used in the Talalay process, which is energetically favourable. Once vulcanization has been initiated at lower temperature the thiazole, in particular the mercaptobenzothiazole or its zinc salt, which normally activates at higher temperatures, gets activated as well at this lower temperature and ensures that substantially complete vulcanization may be achieved within the desirable, conventional time frame, at a suitable overall reaction rate.

The present invention further presents the advantage that the rubber latex composition of the present invention does no longer contain hazardous amounts of ZDEC, and may therefore be fully recycled at minimum, even no risk to producing unwanted of noil-recyclable side products, and is thus suitable for the production of foams according to the cradle to cradle principle. Recycling of a vulcanized rubber latex foam usually involves de-vulcanization and reducing the size of the material into material particles which are suitable to be recycled, for example processed into other compounds. Moreover, when recycling the rubber latex composition of the present invention, there is a minimum risk to deteriorating the quality of the rubber latex material. In particular upon recycling the air resistance and compression set of the recycled particles are not adversely affected, as well as the hysteresis, the dynamic fatigue hardness loss and height loss over time, the compression set and ball rebound. A rubber latex foam according to the present invention meets the ecological demands for baby products, is environmentally friendly, healthy and hygienic, it is completely free from petrochemical substances.

By using as a vulcanization accelerator a mixture of dibenzyldithiocarbamate or its zinc salt, and mercaptobenzothiazole or its zinc salt the temperature at which vulcanization is carried out may be kept relatively low, i.e. within a range of about 110-120°C, and therefore vulcanization may be achieved by using heat as a main energy source. This is unexpected as the use of dibenzyldithiocarbamate or its zinc salt (ZBEC) normally requires an activation temperature of about 135°C, which conventionally is achieved by the use of steam. Thus, the use of the claimed mixture of dibenzyldithiocarbamate or its zinc salt, and mercaptobenzothiazole or its zinc salt as vulcanization accelerator permits achieving a sufficiently complete degree of vulcanization at acceptable costs for the vulcanization accelerator and an economically feasible process in view of energy costs. The higher costs of dibenzyldithiocarbamate or its zinc salt may be compensated by using it in a mixture with the cheaper mercaptobenzothiazole or its zinc salt.

The use of the relatively low vulcanization temperature has the effect that drying of the foam during the processing to achieve vulcanization may be kept within desirable limits. Therewith the risk to a too high degree of drying which would involve the risk to the formation of a brittle foam, may be reduced to a minimum. Thus a vulcanized foam may be obtained with the desired quality in terms of mechanical and physical properties.

The inventors have further observed that the use of the vulcanization accelerator of this invention permits to produce latex rubber foams of which the compression set of the vulcanized foam is not adversely affected but may even be improved. In particular, the weaker foam strength provided by dibenzyldithiocarbamate or its zinc salt may be compensated and even be enhanced by the presence of the 2-mercaptobenzothiazole or its zinc salt, which tends to impart rigidity to the foam. By varying the dibenzyldithiocarbamate and mercaptobenzothiazole concentration within the ranges indicated, the foam strength may be tailored as desired. With compression set is meant a permanent loss of initial height of a foam sample after repeated application of load or pressure to the foam sample. The compression set is commonly expressed as a percentage of the original foam height.

Preferably, the molar ratio of dibenzyldithiocarbamate or its zinc salt to 2-mercaptobenzothiazole or its zinc salt ranges from 1 to 3.

The amount of the vulcanization accelerator may be varied within wide ranges, and is preferably selected such that a sufficient degree of vulcanization can be attained and the desired mechanical strength and other physical properties required for vulcanized rubber articles may be obtained. Usually the total amount of the vulcanization accelerator is 0.1 to 10 parts per hundred based the solid content in the copolymer rubber latex, preferably 2-5 parts per hundred.

Detailed description of the invention.

The vulcanizable rubber latex foam composition of this invention will usually contain the conventional components, known to the skilled person. The vulcanizable rubber latex foam composition of this invention will usually contain a rubber latex, a sulfur-containing vulcanizing agent, at least one vulcanization accelerator, a foaming or blowing agent and a gelling agent.

Many vulcanizing agents are known to the skilled person, many of them are suitable for use with an elastomeric latex and the particular curing or vulcanizing agent is not critical to the present invention. Suitable sulfur-containing vulcanizing agents for use in this invention include sulfur, sulfur donors and sulfur-containing compounds which are generally used as a sulfur-containing vulcanizing agents for polymer rubber latexes and which are well known to the skilled person. Examples of vulcanizing agents suitable for use with this invention include sulfur such as powdery sulfur, flower of sulfur, precipitated sulfur, colloidal sulfur, surface-treated sulfur and insoluble sulfur, sulfides; and sulfur-containing compounds such as sulfur chloride, sulfur dichloride, morpholine disulfide, an alkylphenol disulfide, N,N' -dithiobis(hexahydro-2H-asepinone-2), phosphorus-containing polysulfide, high-molecular-weight polysulfide, tetramethylthiuram disulfide, selenium dimethyldithiocarbamate and 2-(4'-morpholinodithio) benzothiazole, TMTD, TETD to include thiazoles, i.e., ZMBT. The concentration of the vulcanizing agent may vary within wide ranges. The amount of the sulfur-containing vulcanizing agent contained in the latex composition of this invention is not critical to the invention, but will usually vary from 1-5, preferably 1.5 to 3 parts per hundred (phr) of uncompounded latex solids, more preferably between 2 - 2.5 phr depending on the product requirements.

The latex foam composition may contain one or more vulcanization activators. Suitable examples include zinc oxide and magnesium oxide which can be used in a manner similar to the conventional sulfur vulcanization method. Active zinc oxide can be used as the zinc oxide, but is difficult to disperse. The amount of zinc oxide is suitably determined so that a sufficient vulcanization can be attained and the mechanical strength and other physical properties required for vulcanized rubber articles are obtained, and is not particularly limited. The amount of activator that may be used in the present invention is approximately 0-5 parts by weight per 100 parts by weight of uncompounded latex solids.

The vulcanization accelerator contained in the latex foam composition of this invention comprises 0.50-10.0 parts per hundred based on the total weight of the foam composition of dibenzyldithiocarbamate or its zinc salt, and 0.01 — 10.0 parts per hundred based on the total weight of the foam composition of 2-mercaptobenzothiazole or the zinc salt thereof. Although the invention does not exclude other compounds which have the effect to accelerate vulcanization, their presence is not preferred and the vulcanization accelerator used with the present invention preferably exclusively consists of dibenzyldithiocarbamate or its zinc salt and 2-mercaptobenzo-thiazole or the zinc salt thereof.

The latex composition of the present invention may also comprise a gelling agent. Suitable gelling agents are well known to the skilled person. An example of a suitable gelling agent is Zn(NH4)n C12. The amount of gelling agent used may vary within some ranges, and is chosen such that it is sufficient to gel the latex upon removal of a sufficient amount of ammonia; preferably, approximately 0.5-2 parts by weight per 100 parts by weight of uncompounded latex solids. Examples of other suitable gelling agents include polyethers, low molecular weight glycols, silicone polyethers, alkali metal silicofluorides, ammonium or amine salts of carboxylic acids in the presence of a divalent metal ion, preferably zinc.

According to the need, further additives can be incorporated in foam composition of the present invention for imparting desired properties to vulcanized rubber articles, which include, for example, reinforcing materials such as carbon black, silica and talc, fillers such as calcium carbonate and clay, plasticizers, anti-aging agents, flame retardants, antioxidants and natural fibers.

The latex composition suitable for use with the present invention may comprise either a natural or a synthetic rubber or a mixture thereof. Natural rubber mainly consists of elastomeric polyisoprene, with minor impurities of other organic compounds, and water. It has a large stretch ratio, a high resilience and is waterproof. Other latex compositions suitable for use with the present invention include synthetic latex dispersions. Synthetic rubber may be made by the polymerization of a variety of petroleum-based precursors or monomers. The most prevalent synthetic rubbers are styrene-butadiene rubber (SBR) derived from the copolymerization of styrene and 1,3-butadiene. Other synthetic rubbers are prepared from isoprene (2-methyl-1,3-butadiene), chloroprene (2-chloro- 1,3-butadiene), and isobutylene (methylpropene) with a small percentage of isoprene for cross-linking. Other latex compositions suitable for use with the present invention include cis-polyisoprene (IR), Styrene butadiene (SBR), Cis polybutadiene (BR), or Butyl rubber (HR) (and variants chlorobutyl (CIIIR), or bromobutyl (BrIIR) rubber, liquid polybutene (PB); liquid Natural Rubber; liquid cis polyisoprene; liquid cis polybutadiene; liquid styrene butadiene; or any suitable combination of the afore-mentioned latex rubber compositions. Still other latex compositions suitable for use with this invention include elastomeric latex dispersions, for example those based on acrylonitrile, chloroprene, isoprene, butadiene-styrene, butadiene-acrylonitrile, polyacrylonitrile, polyisoprene, polystyrene, polyvinylidene chloride, polyvinyl chloride, polyvinyl acetate, polymethyl methacrylate, co-polymers of the monomers of these resinous polymers, resinous copolymers of these monomers with other copolymerizable monomers, such as C4 -CIO conjugated dienes and blends of two or more of the afore-mentioned materials. The solids content of the latex composition may vary within wide ranges, but is preferably at least 15% by weight total solids before compounding; preferably, about 40%-75% by weight total solids.

Anionic, cationic or non-ionic surfactants can be chosen as a foaming agent, depending on the process requirements whereby anionic surfactants are preferred. Preferred anionic surfactants are fatty acid soaps, fatty alcohol sulfonates and alkylaryl or aralkyl sulfonates, succinates and amido sulfosuccinates. Particularly preferred are alkali metal and ammonium salts of fatty acids and rosin acids and combinations thereof, most preferred are alkali metal salts of fatty acids and rosin acids and combinations thereof.

The order in which the foregoing components of the latex composition of the present invention are mixed with each other is not critical. However, it is preferred to prepare a first blend by mixing the latex, gelling agent and optionally the enhancers, surfactants, pigments, antioxidants, thickeners, dispersants and the like. These ingredients can be blended together in a conventional manner, such as using a planetary mixer. A second blend which comprises the vulcanization accelerator, the vulcanizing agent and optionally the vulcanization activator may be prepared and mixed with the first blend to form the finished vulcanizable latex foam composition.

The thus obtained vulcanizable latex foam composition can be processed by any process known to the person skilled in the art for making foamed articles. The thus obtained vulcanizable latex foam composition is particularly suitable for use with the Talalay process for producing foamed articles.

The present invention therefore also relates to a method for producing a vulcanized, foamed product, wherein a vulcanizable rubber latex foam composition as described above or a composition according to the appending claims is subjected to vulcanization. The invention in particular relates to a method for producing a vulcanized, foamed product wherein a vulcanizable rubber latex foam composition according to any one of claims 1-8 is poured into a mould, let to foam to a desired density, after which the foam structure is stabilized in a desired shape by cooling the mould, followed by heating the mould to a temperature of 105-120°C to cause gelling and vulcanization.

According to a preferred embodiment of this method, the vulcanizable latex foam composition is pre-foamed to produce a foam with a desired density, and then poured into a mould with a desired shape, which will usually be similar to the shape of the article that is to be produced from the foam. After the mould has been closed, it is evacuated to allow the latex foam composition to foam to the finally desired density. Thereby, the foam will completely fill the mould. Thereafter, the foam structure is stabilized or fixed by cooling the mould to approximately -30°C. In a next step, carbon dioxide is supplied to the mould to pressurize the mould, and lower the pH of the frozen foamed latex compound to achieve coagulation and gelling. Usually this is carried out at a temperature of -30°C. The foam may than be vulcanized at 110°C-120°C. After vulcanization has been achieved to a desired degree, the vulcanized latex foam article is removed from the mould, optionally washed to remove any remaining surfactants and other unwanted products and dried.

The present invention also relates to a vulcanized rubber latex foam obtained by foaming a vulcanizable rubber latex foam composition as described above to a desired density, followed by cooling (-30°C) to stabilize and coagulating (gelling) the foam structure in a desired shape and by re-heating the mould to a temperature of 105-120°C to cause vulcanization of the vulcanizable foamed latex composition. In a preferred embodiment, the vulcanized rubber latex foam has a compression set of 1.0-2.0 %, in particular a compression set at 50% compression, 72 hours at 23°C, height loss in 24 hours.

The vulcanized latex foam of the present invention may be used for a wide variety of applications for example for the production of mattresses, pillows, neck rests, toppers, shock absorbers, shaped parts of shoes, shoe inside soles, garments padding, protectors for sportswear, athletic implements, bike saddles, motorbike saddles, furniture upholstery material, bumpers, automotive dashboards and carpets. The present invention therefor also relates to an article selected from a mattress, pillow, neck rest, topper, shock absorber, shaped part of a shoe, shoe inside sole, garment padding, protector for sportswear, athletic implement, bike saddle, motorbike saddle, furniture upholstery, bumper, automotive dashboard and carpet comprising a vulcanized rubber latex foam as described above and in the claims, or a vulcanized rubber latex foam obtained by the method described above and in the claims, or a foam composition as described above or in the claims.

The present invention also relates to a method for recycling a vulcanized rubber latex foam described above and in the claims, or a vulcanized rubber latex foam obtained by the method described above and in the claims, or a vulcanized rubber latex foam obtained by vulcanization of the foam composition described above and in the claims, wherein the vulcanized rubber is subjected to devulcanization and the size of the foam material is reduced to provide material particles which are suitable to be recycled, in particular to be processed into other compounds. The size of the foam particles to be recycled may vary within wide ranges, depending on the nature of the intended use. The size of the foam particles to be recycled may for example vary from a few micrometer to one or a few centimeter. The foam particles may for example be recycled and incorporated in the above-described latex foam composition to form vulcanized latex foam products. The foam particles may for example be recycled and incorporated in a rubber latex foam used for producing mattresses, pillows, neck rests, toppers, shock absorbers, shaped parts of shoes, shoe inside soles, garments padding, protectors for sportswear, athletic implements, bike saddles, motorbike saddles, furniture upholstery material, bumpers, automotive dashboards and carpets as described above.

The invention is further illustrated in the appending examples and comparative experiments.

In the examples below, use is made of a vulcanization system (Weserland VS), supplied by Weserland GmbH.

The vulcanization system comprises sulfur, zinc oxide, antioxidant, ZMBT as secondary accelerator, and ZBEC as a primary accelerator. Another example is from Synthomer, this vulcanization system (Synthomer VS) comprises sulfur, zinc oxide, ZMBT as secondary accelerator, and ZBEC as a primary accelerator plus other necessary additives to stabilize the dispersion. Typical properties of Weserland and Synthomer vulcanization systems are shown in Table 1 below.

As ZBEC is known to activate at high temperatures (>100°C) and proceeds in a much slower speed as compared to the ultra fast ZDEC and ZMBT accelerators, the use of secondary accelerator ZMBT was increased to be able to have curing in a typical Talalay operating curing temperatures of 110 - 120°C in 10 minutes.

The ingredients ofi the foam composition as listed in table 2, 3, 4 and 5 of respectively example 1 and 2 below, comprising the latex, soap, vulcanization system and other additives were compounded in a mixing tank and allowed to mature for 2 hours. The resulting compound were then frothed to the required density and poured in a mold to fill about 20 - 30% of the mold's volume. The mold was then closed and the frothed compound was allowed to expand inside the mold by applying vacuum in the system. Once vacuum was complete, the latex was flash frozen to -30°C to set the foam inside the mold and to avoid settling of the latex. Carbon dioxide was introduced in the mold to effect gelling of the foam thereby setting completely its structure. The frozen foam was then thawed by slowly heating the foam to 15°C and then 3.0°C and the foam was finally vulcanized at 110 - 120°C for 8-15 minutes. The resulting foam was then withdrawn from the mold, washed, dried, and post-vulcanized prior to final quality check.

Typical formulations to illustrate the invention are shown in the following examples. EXAMPLE 1.

In Table 2 above, Synthetic Latex, e.g., Lipolan F2420, is a cold polymerized styrene-butadiene latex, commercially available from Synthomer Deutschland GmbH, Marl, Germany; Natural Latex is a high ammonia or low ammonia (HA/LA) natural latex from Southeast Asia; Thickener is a viscosity modifier ingredient prepared by mixing Tylose NS 299 KG4 (SE Tylose GmbH & Co. KG, Wiesbaden, Germany) and sodium dodecyl sulphate (commercially available as Serdet NL 30 from Elementis Specialties Netherlands B.V., Delden, Netherlands) with water; Weserland VS is a complete vulcanization system with sulphur, zinc oxide, antioxidant, stabilizers, and accelerators ZBEC and ZMBT, commercially available from Weserland GmbH, Hannover, Germany; Synthomer VS is a complete vulcanization system with sulphur, zinc oxide, antioxidant, stabilizers, and accelerators ZBEC and ZMBT, commercially available from Synthomer, Oss, The Netherlands; Antioxidant (Wingstay L) is a anti oxidant system commercially available from Synthomer, Oss, The Netherlands; and Soap is an ammonium ricinoleate type commercially available from Christeyns NV, Gent, Belgium.

In the above “phr” means parts per hundred.

All materials were mixed and the compound was allowed to mature for about 2 hours. The matured compound was then processed as described in the United States patent to Talalay No. 2,432,353 wherein the compound is frothed and poured in the mould and then frozen immediately to avoid drainage of the foam. The foam was then coagulated with an acid gas, in this case, a carbon dioxide gas and then the coagulated foam is then vulcanized at 110°C-120°C.

The resulting foam has good physical and mechanical properties and has comparable properties as that of a product with normal vulcanization systems. The mechanical properties of the foam are summarized in table 6 below. EXAMPLE 2.

Natural latex is of high ammonia stabilized type from Southeast Asia; potassium oleate is a soap stabilizer commercially available from Christeyns NV, Gent, Belgium; Slipol U57 is another natural latex stabilizer commercially available from Weserland GmbH, Hannover, Germany; Weserland VS is a complete vulcanization system with elemental sulphur, zinc oxide, antioxidant, stabilizers, and accelerators ZBEC and ZMBT also commercially available from Weserland GmbH, Hannover, Germany; Synthomer VS is a complete vulcanization system with sulphur, zinc oxide, stabilizers, and accelerators ZBEC and ZMBT, commercially available from Synthomer, Oss, The Netherlands; Anti-oxidant (Wingstay L) is a anti oxidant system commercially available from Synthomer, Oss, The Netherlands; thickener is again a viscosity modifier prepared by mixing Tylose NS 299 KG4 (SE Tylose GmbH & Co. KG, Wiesbaden, Germany) and sodium dodecyl sulphate (commercially available as Serdet NL 30 from Elementis Specialties Netherlands B.V., Delden, Netherlands) with water.

The materials in Table 3 are mixed and the resulting compound was allowed to mature for about 2 hours and then foam was prepared according to the procedure above. The resulting products give good physical and mechanical properties.

The foregoing data may be also be applied to other conventional lattices containing fillers, and other known modifying materials as long as these said materials can comply to EPEAs criteria for toxicity.

Claims (16)

CONCLUSIONS. A method of manufacturing a vulcanized foamed article, wherein a vulcanizable rubber latex skew composition comprising a rubber latex, a sulfur-containing vulcanizing agent and at least one vulcanizing accelerator is subjected to foaming, gelling and vulcanizing, characterized in that the at least one vulcanizing accelerator 0.50-10.0 parts per hundred parts based on the total weight of the foam composition comprises of a compound corresponding to formula (I) or a zinc salt of this compound: wherein R 1 and R 2 may be the same or different, but are preferably the same, each of R 1 and R 2 being an alkyl or cycloalkyl group having a hydrocarbon chain containing between 1 and 6 carbon atoms or an arylalkyl group containing 7-12 carbon atoms, at preferably an arylalkyl group with an aryl radical with one or two alkyl groups each having 1 to 5 carbon atoms in the a-position, the vulcanization accelerator further comprising 0.01-10.0 parts per hundred based on the total weight of the foam composition of a thiazole or zinc salt thereof, wherein the molar ratio of dibenzyldithiocarbamate to 2-mercaptobenzothiazole or the zinc salts ranges from 1 to 3, and wherein vulcanization is carried out at a temperature of 105-120 ° C, preferably 105-115 ° C. A method according to claim 1, wherein the vulcanized foamed object is selected from a mattress, cushion, neck support, topper, shock absorber, bicycle saddle, motorcycle saddle and furniture cover. A method according to claim 1 or 2, wherein the compound of formula (I) is dibenzyldithiocarbamate. A method according to any one of the preceding claims, wherein the concentration of dibenzyldithiocarbamate or the zinc salt varies from 0.5 to 5.0 parts per hundred based on the total weight of the foam composition, preferably from 1.0 to 4.0, with more preferably 1.5-3 parts per hundred. A method according to any one of the preceding claims, wherein the thiazole compound is a 2-mercaptobenzothiazole or the zinc salt thereof. A method according to claim 5, wherein the concentration of the 2-mercaptobenzothiazole or its zinc salt ranges from 0.025-7.5 parts per hundred based on the total weight of the foam composition, more preferably 0.025-5.0, with most preferably 0.5-2.5 parts per hundred. A method according to any one of the preceding claims, wherein the total amount of vulcanization accelerator ranges from 0.51 to 20.0 parts per hundred based on the total weight of the foam composition. A method according to any one of the preceding claims, wherein the composition further comprises a sulfur-containing vulcanizing agent. A method according to any one of the preceding claims, wherein the composition further comprises a vulcanization activator, in particular zinc oxide or magnesium oxide. A method according to any one of the preceding claims, wherein the latex is a natural rubber latex or a synthetic rubber latex or mixture of a natural rubber latex and synthetic rubber latex. A method according to any of claims 1-10, wherein the vulcanizable rubber latex foam composition is poured into a mold, foamed to a desired density, after which the foam structure is stabilized in a desired shape by cooling the mold to cause gelling, followed by heating the mold at a temperature of 105-115 ° C to effect vulcanization. A method according to any one of the preceding claims, wherein the vulcanized foamed article has a compression set of 1.0-2.0%. A vulcanized rubber latex foam obtained by foaming a vulcanizable rubber latex foam obtained by the method of any one of claims 1 to 12 to a desired density, followed by cooling to stabilize the foam structure to a desired shape and cause gelation and by re-gelation heating the mold to a temperature of 105-120 ° C, preferably 105-115 ° C to effect vulcanization of the vulcanizable foamed latex composition. A vulcanized rubber latex foam according to claim 11, with a compression set of 1.0-2.0%. A method of recycling a vulcanized rubber latex foam obtained with the method according to any of claims 1-12, or a vulcanized rubber latex foam according to claim 14, wherein the vulcanized rubber latex is subjected to the vulcanization and the size of the foam material is reduced to provide material particles suitable for recycling, in particular for processing into other compounds. An item selected from a mattress, cushion, neck support, topper, bicycle saddle, motorcycle saddle, furniture cover, comprising recycled vulcanized rubber latex foam obtained with the method of claim 15.