HIGH-SOLID SELF-ADHESIVE AND PROCESS FOR THE PRODUCTION THEREOF

Abstract: A high-solid self-adhesive having a solid content of 65 to 75% by weight and composed of an emulsion obtained by emulsion-copolymerizing a vinyl monomer mixture contain­ ing acrylate esters as the main ingredient and further at least one hydrophilic monomer of formula (A), (B) or (C). wherein R₁ is H or COOH, R₂ is H or CH₃ and R₃ is H or CH₂OH. A process for the production of the high-solid self-­ adhesive comprising successively adding a pre-emulsion obtained by previously mixing said vinyl monomer mixture, a surfactant mixture of a nonionic surfactant and an anionic surfactant and water, to the reaction system to carry out an emulsion copolymerization in the presence of a catalyst.

Description: The present invention relates to a high-solid adhesive composition which is obtained by emulsion-copolymerizing a vinyl monomer mixture containing at least one hydrophilic monomer, and also to a process for preparing same. Methods of obtaining high-solid latexes have been disclosed in the specifications of Japanese Patent Laid-Open Nos. 84092/1978 and 141311/1981. However, these methods require two complicated stages and the solid content of the resultant latex is at most 70%. Further, no high-solid latex applicable to an adhesive composition has yet been discovered, though the use of a high-solid latex is highly desired in self-adhesive market from the viewpoint of saving energy for drying the adhesive composition during coating. GB-A-2070037 discloses pressure sensitive adhesive latexes wherein no surfactant is employed during the polymerization. Chemical Abstracts, volume 89, 1978, page 31, no. 180808p discloses acrylic polymer emulsions containing 60%, of solids. It is the object of the present invention to provide a high-solid adhesive composition and a process for preparing same. Said object is achieved by an adhesive composition which is an aqueous dispersion having an average particle size of 0.2 to 2.0 µm prepared by emulsion copolymerization of a mixture containing (1) at least 50 % by weight of at least one acrylate ester monomer and (2) 0.5 to 15 % by weight of at least one hydrophilic vinyl monomer having the formula A, B or C wherein R₁ is H or COOH, R₂ is H or CH₃ and R₃ is H or CH₂OH, and optionally (3) the balance of another vinyl monomer, in the presence of 2 to 15% by weight of a mixture of an anionic and a nonionic surfactant in a mixing ratio of 5:95 to 40:60, said emulsion copolymerisation having been conducted by adding to a reaction system comprising an appropriate amount of water and 5 to 30% by weight of a pre-emulsion comprising the vinyl monomer mixture and the surfactant, the remaining 70 to 95 % by weight of said pre-emulsion, said aqueous dispersion containing from 65 to 75 % by weight of solids, based on the weight of said aqueous dispersion. Furthermore, the present invention relates to a process for the production of said adhesive composition which comprises adding to a reaction system comprising an appropriate amount of water and 5 to 30 % by weight of a pre-emulsion comprising said vinyl monomer mixture and the surfactant, the remaining 70 to 95 % by weight of said pre-emulsion to obtain a polymer emulsion having a solid content of 65 to 75 % by weight. The hydrophilic monomers of the above formulas (A), (B) and (C) are used for the purpose of obtaining a low-viscosity latex and imparting polmerization stability required for obtaining a high-solid latex. Examples of the hydrophilic monomers include acrylic acid, methacrylic acid, fumaric acid, maleic acid, acrylamide, methacrylamide, N-methylolacrylamide, N-methylolmethacrylamide, 2-hydroxyethyl acrylate, and 2-hydroxyethyl methacrylate. The hydrophilic monomer is used in an amount of 0.5 to 15% by weight, preferably 1 to 10% by weight, based on that of the vinyl monomer mixture. When the amount is less than 0.5% by weight, no stable latex can be obtained, while when it exceeds 15% by weight, the resultant resin becomes hard and no desirable tackiness can be obtained. The vinyl monomer mixture used in the present invention is mainly composed of acrylate esters. Examples of the acrylate ester monomers include methyl acrylate, ethyl acrylate, propyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, and decyl acrylate. These monomers are used in amounts of at least 50% by weight of the total vinyl monomer mixture. Examples of other vinyl monomers which can be used include methyl methacrylate, ethyl methacrylate, propyl methacrylate, butyl methacrylate, hexyl methacrylate, octyl methacrylate, vinyl acetate, styrene, ethylene, and vinyl chloride. The high-solid adhesive composition of the present invention is one having a solid content of 65 to 75% by weight. The solid content is the value obtained by weighing about 1 g of a sample, i.e., a stock emulsion solution, in an aluminum dish, drying the sample in a hot-air dryer at 105°C, and determining the content according to the following equation: In a preferred embodiment, the high-solid adhesive composition of the present invention is produced in the following manner. 5 to 30% by weight of a pre-emulsion, i.e., a mixture consisting of a vinyl monomer mixture mainly composed of acrylate esters and containing 0.5 to 15% by weight of the above-described hydrophilic monomer, a surfactant and water, is added to a reaction system containing preferably an appropriate amount of water just before polymerization. Then the remaining 70 to 95% by weight of the pre-emulsion and a catalyst are separately and successively added to the reaction system at a reaction temperature to carry out an emulsion copolymerization. The surfactant is a mixture of an anionic surfactant and a nonionic surfactant, wherein the anionic surfactant is used in an amount of 5 to 40% by weight of the nonionic surfactant. When the ratio of the surfactants is outside the range mentioned above, the stability of the polymer latex becomes poor, or the particle size becomes too small and the viscosity is increased so that it is difficult to obtain a high-solid adhesive composition. The amount of the surfactant to be added is 2 to 15% by weight, preferably 3 to 10% by weight based on the combined amount of the monomers. When the amount is less than 2% by weight, no stable polymer latex can be obtained, while when it exceeds 15% by weight, the particle size of the latex becomes too small and the viscosity is increased so that it becomes difficult to increase the solid content. Examples of the surfactants which can be used in the present invention include anionic surfactants such as various fatty acid salts, higher alcohol sulfates, alkylbenzenesulfonates, dialkyl sulfosuccinates, polyoxyethylene alkyl ether sulfates, or polyoxyethylene alkylphenol ether sulfates, and nonionic surfactants such as polyoxyethylene alkyl ethers, polyoxyethylene alkylphenol ethers, sorbitan fatty acid esters, polyoxyethylene fatty acid esters, polyoxyethylene sorbitan fatty acid esters, or polyoxyethylene/polyoxypropylene block copolymer. As the catalyst, persulfates such as potassium persulfate and ammonium persulfate are preferred and may be used in combination with various reducing agents as redox catalyst. The catalyst is used in an amount of 0.1 to 1% by weight based on that of the monomer. The emulsion copolymerization is usually carried out at a temperature of 50 to 90°C for 5 to 10 h. In the present invention, however, there is employed a pre-emulsion process comprising successively adding a pre-emulsion, i.e., a mixture of a vinyl monomer mixture containing a hydrophilic monomer, a surfactant and water to the reaction system so that the emulsion copolymerization may be carried out while successively adding the pre-emulsion and the catalyst to the reaction system. If no liquid is present at the initial stage of the polymerization in usual industrial practice, the agitating blades cannot be turned during the polymerization so that it is difficult to control the heat of polymerization and to smoothly carry out the emulsion copolymerization. The particle size of the obtained latex is from 0.2 µm to 2.0 µm to give a latex viscosity which does not cause any troubles in practical use. Usually, the viscosity of the latex is within the range of 0,5 to 30 Pa.s (500 to 30,000 cP). The resultant latex is stable and can be used as a high-solid adhesive composition having excellent sticking characteristics. Additives such as an anti-foaming agent, a thickening agent, a tackifier, or a pigment may be added to the high-solid adhesive composition of the present invention. The adhesive composition can be applied in various ways by utilizing its sticking characteristics, and is particularly effective as self-adhesive tape and sheet and also useful as tackifier and sealing agent. The following examples and comparative examples illustrate the present invention, Parts and % given below are by weight unless otherwise stated. Example 1 8 parts of water was charged in a polymerizer and 15% of a mixture consisting of 18 parts of water, 8 parts of vinyl acetate, 30 parts of butyl acrylate, 25 parts of 2-ethylhexyl acrylate, 0.6 part of methacrylamide, 3 parts of methacrylic acid, 4 parts of a nonionic surfactant (polyoxyethylene octylphenyl ether) and 0.9 part of an anionic surfactant (sodium dodecylbenzenesulfonate) was added thereto and heated. The remaining 85% of the above mixture and an aqueous potassium persulfate solution were successively added thereto at about 70°C over a period of 5 h to carry out the emulsion polymerization. The solid concentration of the resultant latex was 71.5% and the viscosity thereof was 7 Pa.s (7000 cP). Example 2 6.5 parts of water was charged in a polymerizer and 10% of a mixture consisting of 20 parts of water, 6 parts of vinyl acetate, 55 parts of butyl acrylate, 1.5 parts of 2-hydroxyethyl methacrylate, 2.5 parts of methacrylic acid, 3.5 parts of a nonionic surfactant (polyoxyethylene lauryl ether) and 0.8 part of an anionic surfactant (sodium laurylsulfate) was added thereto and heated. The remaining 90% of the above mixture and an aqueous ammonium persulfate solution were successively added thereto at about 75°C over a period of 4 h to carry out the emulsion copolymerization. The solid concentration of the resultant latex was 68.5% and the viscosity thereof was 12 Pa.s (12000 cP). Example 3 5 parts of water was charged in a polymerizer and 20% of a mixture consisting of 18 parts of water, 8 parts of ethyl methacrylate, 58 parts of 2-ethylhexyl acrylate, 0.8 part of acrylamide, 2 parts of methacrylic acid, 3.5 parts of a nonionic surfactant (polyoxyethylene nonylphenyl ether) and 0.7 part of an anionic surfactant (sodium polyoxyethylene alkylphenyl ether sulfate) was added thereto and heated. The remaining 80% of the above mixture and an aqueous ammonium persulfate solution were successively added thereto at about 75°C over a period of 5 h to carry out the emulsion copolymerization. The solid concentration of the resultant a's latex was 73% and the viscosity thereof was 7 Pa.s (7000 cP). Comparative Example 1 The same starting materials as those of Example 1 were used in the same amounts as those of Example 1, and the polymerization procedure was repeated with the following modification of the conditions. 26 parts of water and the whole amounts of the nonionic and anionic surfactants were charged in the polymerizer and 15% of the monomer mixture was added thereto and heated. The remaining 85% of the above monomer mixture and the aqueous potassium persulfate solution were successively added thereto at about 70°C over a period of 5 h to carry out the emulsion copolymerization. The solid concentration of the resultant latex was 70.5% and the viscosity thereof was 50 Pa.s (50000 cP). Comparative Example 2 The procedure of Example 3 was repeated except that 2.8 parts of methyl methacrylate was used in place of acrylamide and methacrylic acid to carry out the emulsion copolymerization. The solid concentration of the resultant latex was 71% and the viscosity thereof was 30 Pa.s (30000 cP). The latex was very unstable. Each of the latexes obtained in the above Examples and Comparative Examples was applied to a release paper in an amount to give an areal weight of about 25 g/m² (on a dry basis), dried at 105°C for 3 min and transferred onto a polyester film of 25 µm in thickness. The adhesive strength, the tack and the holding power of the resultant self-adhesive tape were evaluated. The results are shown in Table 1.

Claims: An adhesive composition which is an aqueous dispersion having an average particle size of 0.2 to 2.0 µm prepared by emulsion copolymerization of a mixture containing (1) at least 50 % by weight of at least one acrylate ester monomer and (2) 0.5 to 15 % by weight of at least one hydrophilic vinyl monomer having the formula A, B or C wherein R₁ is H or COOH, R₂ is H or CH₃ and R₃ is H or CH₂OH, and optionally (3) the balance of another vinyl monomer, in the presence of 2 to 15% by weight of a mixture of an anionic and a nonionic surfactant in a mixing ratio of 5:95 to 40:60, said emulsion copolymerisation having been conducted by adding to a reaction system comprising an appropriate amount of water and 5 to 30% by weight of a pre-emulsion comprising the vinyl monomer mixture and the surfactant, the remaining 70 to 95 % by weight of said pre-emulsion, said aqueous dispersion containing from 65 to 75 % by weight of solids, based on the weight of said aqueous dispersion. The composition of claim 1 wherein the viscosity of the emulsion is 0.5 to 30 Pa.s (500 to 30,000 cP). A process for the production of an adhesive composition by emulsion-copolymerizing a mixture according to claim 1, which comprises adding to a reaction system comprising an appropriate amount of water and 5 to 30 % by weight of a pre-emulsion comprising said vinyl monomer mixture and the surfactant, the remaining 70 to 95 % by weight of said pre-emulsion to obtain a polymer emulsion having a solid content of 65 to 75 % by weight.

Inventors: ASANO, TOSHIO, FUROMOTO, MITSURU
Applicants: DAICEL CHEMICAL INDUSTRIES, LTD.
Application year: 1984
Main classification: C08F220
IPC codes: C08F220, C08F20, C09J4, C08F2, C09J133