Process for producing foamed styrene resin bead

Abstract: A process for producing foamed styrene resin bead, which comprises the steps of: (a) the first polymerization step in which 10 to 30wt% of pre-mixture comprising styrene monomer, crosslinking agent, chain transfer agent, initiator and surface active agent, 0.5 to 6.0wt% of dispersion master comprising pure water and dispersing agent, 50 to 55wt% of hot pure water and 1 to 15wt% of styrene monomer are mixed in the reactor at 50 to 70°C and then the reactor is heated to the temperature range of 80 to 100°C; (b) the second polymerization step in which, at the conversion rate of 5 to 15%, 5 to 25wt% of the pre-mixture, 1 to 15wt% of styrene monomer, 0.05 to 1.0wt% of cell controller and 0.5 to 6.0wt% of dispersion master are further added to the reactor and then polymerized; (c) the third polymerization step in which, at the conversion rate of 50 to 70%, 0.05 to 1.0wt% of cell controller and 0.5 to 6.0wt% of dispersion master are still further added to the reactor and then polymerized; (d) the fourth polymerization step in which, at the conversion rate of 80 to 90%, 0.5 to 6.0wt% of dispersion master is further added to the reactor and then polymerized; and (e) the step in which the reactor is heated to temperature of 110 to 135°C and at the conversion rate of 85 to 95%, the flame retardant, inert gas and foaming agent are added.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a process for producing foamed styrene resin bead and more particularly, to the process for producing foamed styrene resin bead characterized in that pre-mixture comprising containing styrene monomer, crosslinking agent, chain transfer agent, initiator and surface active agent, and dispersion master are mixed and polymerized to a certain conversion rate followed by polymerization with additional styrene monomer and dispersion master. The foamed styrene resin bead so prepared exhibits improvement with respect to particle size distribution, cell uniformity, foaming and other mechanical properties. Description of the Related Art The conventional process for producing a foamed styrene resin are as follows: a) one step process by which foamed styrene resin is produced continuously; and b) two step process which consists of production of styrene resin and forced introduction of foaming agent. The foamed styrene resin produced by the conventional methods has good strength at the sacrifice of foaming ability and other physical properties, good flame retardancy at the sacrifice of insulation property and other properties, or good insulation property at the sacrifice of flame retardancy, strength, and other properties. Further, according to the conventional process, the particle size of foamed styrene resin may be controlled by dispersing adjuvant and sizing agent so that the particle size distribution is too broad to control it, and the application of flame retardant with high bromine content and melting point causes non-uniformity of inter-cell and deterioration of the physical properties of the foamed styrene resin, and the reaction cycle time becomes too long. SUMMARY OF THE INVENTION To solve such problems in the prior arts, the inventors have made intensive research on a novel process for producing foamed styrene resin bead. As a result, they found a novel process performed in such a manner that pre-mixture comprising containing styrene monomer, crosslinking agent, chain transfer agent, initiator and surface active agent, and dispersion master are mixed and polymerized to a certain conversion rate followed by polymerization with additional styrene monomer and dispersion master, thereby improving uniform particle size, particle size distribution, high foaminess and mechanical properties in spite of no use of dispersing adjuvant and sizing agent. Therefore, it is an object of this invention to provide a process for producing foamed styrene resin bead with improved cell uniformity. It is another object of this invention to provide a foamed styrene resin bead prepared through the process of this invention. Description of the Drawings Fig. 1 is a graph showing particle size distribution of resin bead prepared according to this invention and conventional method. Detailed Description of the Invention This invention is characterized by a process for producing foamed styrene resin bead comprising the following steps: (a) the first polymerization step in which 10 to 30wt% of pre-mixture comprising styrene monomer, crosslinking agent, chain transfer agent, initiator and surface active agent, 0.5 to 6.0wt% of dispersion master comprising pure water and dispersing agent, 50 to 55wt% of hot pure water and 1 to 15wt% of styrene monomer are mixed in the reactor at 50 to 70 °C and then the reactor is heated to the temperature range of 80 to 100 °C; (b) the second polymerization step in which, at the conversion rate of 5 to 15%, 5 to 25wt% of the pre-mixture, 1 to 15wt% of styrene monomer, 0.05 to 1.0 wt% of cell controller and 0.5 to 6.0wt% of dispersion master are further added to the reactor and then polymerized; (c) the third polymerization step in which, at the conversion rate of 50 to 70%, 0.05 to 1.0wt% of cell controller and 0.5 to 6.0wt% of dispersion master are still further added to the reactor and then polymerized; (d) the fourth polymerization step in which, at the conversion rate of 80 to 90%, 0.5 to 6.0wt% of dispersion master is further added to the reactor and then polymerized; and (e) the step in which the reactor is heated to temperature of 110 to 135 °C and at the conversion rate of 85 to 95%, the flame retardant, inert gas and foaming agent are added. The present invention is also characterized by styrene resin bead produced by the above-mentioned process. The present invention is described in more detail as set forth hereunder: The first polymerization step First of all, pre-mixture is prepared with styrene monomer, crosslinking agent, chain transfer agent, initiator and interfacial active agent at temperature of 15 to 25°C. It is preferred that the amount of styrene monomer is 75 to 95 wt% in proportion to the total pre-mixture. The crosslinking agent, chain transfer agent, and initiator are added as additives. The crosslinking agent is preferably added by 0.01 to 2.5wt% in proportion to the total pre-mixture and is one or more selected from the group consisting of dicumylperoxide, acetylperoxide, t-allylbutylperoxide and t-allylbutylhydroperoxide. The chain transfer agent is used to increase foaming ability of resin bead, which is one or more selected from the group consisting of cyclohexane, toluene, benzene, ethylbenzene, t-allylbenzene, t-allylbutylchloride, t-allylbutylbromide and isopropylene. The amount of the chain transfer agent is preferably 0.01 to 1.0wt%; if the amount is less than 0.01wt%, the foaming ability of resin bead is never improved and in excess of 1.0wt%, the property of end-product is poorer. The initiator is used to shorten the reaction time, which is one or more selected from the group consisting of benzoylperoxide, laurylperoxide, t-butylperoxybenzoate, and 1,1-di(t-butylperoxy-3,3,5-trimethylcyclohexane). It is preferred that the amount of initiator is 0.05 to 1.0wt%; if the amount is not within 0.05 to 1.0wt%, it is difficult to control polymerization reaction. According to this invention, preferably, the both hot and cold initiator are added together. Application of both hot and cold initiators increases the initiation efficiency, so that the reaction cycle time may be shortened. It is preferred that the hot initiator is one or more selected from the group consisting of t-butylperoxybenzoate and 1,1-di(t-butylperoxy-3,3,5-trimethylcyclohexane) and the cold initiator is one or more selected from the group consisting of benzoylperoxide and laurylperoxide. In the case of the both hot and cold initiators, it is preferred that the weight ratio is 1 : 6 to 1 : 2. To endow resin bead with electrostatic screening effect, interfacial active agent is added by 0.005 to 1.0wt% in proportion to the total pre-mixture, which is one or more selected from the group consisting of glycerylmonostearate and hydroxyalkylamine. The said compositions of pre-mixture are added to pre-mixer at 15 to 25 °C and then made to pre-mixture. In the meantime, a dispersion master is prepared by following steps separated from the above preparation of pre-mixture. The pure water is heated to 50 to 70 °C in the pure water tank equipped with heater to raise the production efficiency. Dispersion master is prepared in such a manner that 65 to 95wt% of hot pure water and 5 to 35wt% of dispersing agent to the total dispersion master are added to a dispersion tank and mixed, followed by heating to 70 to 90°C. The said dispersing agent is used for the control of particle size and dispersion stability of particle, which is one or more selected from the group consisting of polyvinylpyrrolidone, tricalciumphosphate, polyvinylalcohol, cellulose derivative, calciumphosphate, and pyrrolidone. It is preferred that the amount of dispersing agent is 5 to 35wt% to the total dispersion master; if the amount is less than 5wt%, gel or cluster may be formed but in excess of 35wt%, powder or emulsified particle may be formed. Following the preparation of the pre-mixture and dispersion master, 10 to 30wt% of said pre-mixture, 0.5 to 6.0wt% of dispersion master, 1 to 15 wt% of styrene monomer and 50 to 55wt% of hot pure water are mixed at 50 to 70 °C. Then, the reactor is heated to the temperature range of 80 to 100 °C, thereby being subject to suspension-polymerization. If the reaction is carried in the outside range of said temperature, undesirably it is impossible to obtain end-product with good property. The second and third polymerization steps If the conversion rate of the first polymerization reaches 5 to 15%, 5 to 25wt% of the said pre-mixture, 1 to 15wt% of styrene monomer, 0.05 to 1.0wt% of cell controller, and 0.5 to 6.0wt% of dispersion master are added to the reactor and polymerized with stirring at a rate of 30 to 50 rpm for the control of particle size and the stability of the inter-cell of resin bead. In the third step, if the conversion rate of the second polymerization reaches 50 to 70%, 0.05 to 1.0wt% of cell controller and 0.5 to 6.0wt% of dispersion master are added and polymerized. The said cell controller is used to control uniformity of cell, which is one or more selected from the group consisting of sodium phosphate, ammonium sulfate, bromine hydrocarbon, polyethylene homopolymer. If the amount of cell controller is not within the said range, it is difficult to control uniformity of cell, so that a uniform cell cannot be obtained. According to the present invention, said dispersion master is injected in several times and it is preferred that amount of dispersion master injected is controlled at each injection. The fourth polymerization step When the conversion rate of third polymerization reaches 80 to 90%, 0.5 to 6.0wt% of dispersion master is further added for the stability of the particle distribution. The last step After the fourth polymerization is completed, the reactor is heated to 110 to 135°C. When the conversion rate reaches 85 to 95%, flame retardant, inert gas and foaming agent are added and polymerized into foamed styrene polymer with bead type. During said step, the reactor is heated to 110 to 135°C for the removal of residue, endowment of flame retardancy, cell control and impregnation of foaming agent. If less than 110 °C, it is not easy to remove residue and to control cell but in excess of 135 °C, property of end-product is degraded undesirably. Then, when the conversion rate reaches 85 to 95 %, flame retardant that is used as melted form in tank equipped with heating or pressurizing means is added. The said flame retardant is used to reduce the danger of fire of resin bead, which is one or more selected from the group consisting of hexabromocyclododecane, tetrabromobisphenol-bisallylether, tribromophenylallylether and trisdibromopropylphosphate. The amount of flame retardant is 0.05 to 1.0wt%, preferably. If the amount of flame retartant is less than 0.05wt%, the effect of flame retardancy may be reduced but if the amount more than 1.0wt%, heat resistance and other properties of resin bead may be reduced. The said inert gas such as nitrogen is injected until pressure of reactor is in the range of 2 to 4 kg/cm 2 , in order to make injection of foaming agent easier and to make cell controller and flame retardant affect resin bead uniformly. If the pressure is not within said pressure range, foaming agent, cell controller and flame retardant cannot penetrate into resin bead uniformly so that the end-product of good property cannot be obtained. Preferably, the said foaming agent is alkanes which has 3 to 6 of carbons. It is more preferred that the foaming agent is one or more selected from the group consisting of normal propane, normal butane, normal pentane, and isopentane. It is preferable that the amount of foaming agent is 5.0 to 10.0 wt%; if the amount of foaming agent is more than or less than said range, foaming ability is reduced and transformation of resin bead is induced, finally resulting in end-product with low quality. In the last step, to shorten the reaction time and enlarge the aging effect, cold water of 5 to 7°C is injected by a certain quantity so that the foamed polymer of bead type is cooled to 15 to 20 °C. Then, the foamed polymer is washed out by drum screen for improvements of fusion and strength of foamed resin bead, dehydrated by centrifuge, and then dried by dehumidified hot air from dehumidifier to minimize the release of foaming agent and moisture content which occur in case of steam drying, by high drying-heating treatment. It is preferred that the total amount of dispersion master and cell controller is 2 to 25wt% and 0.1 to 2.0wt%, respectively. If injection time and amount of dispersion master and cell controller are not within the above range, cluster, gel, and cell non-uniformity et. al occur so that it is difficult to control particle size and to stabilize cell structure. According to this invention, foamed styrene resin bead is suspension-polymerized with pre-mixture and dispersion master. The said dispersion master is injected in several times for the control of the particle size and particle size distribution, and uniformity of cell composition. Furthermore, styrene monomer mixture is injected in two steps and the flame retardant is injected at the time that the foaming agent is added. Therefore, this invention is designed to solve the problems such as cell non-uniformity and decrease of strength in the prior art. The following specific examples are intended to be illustrative of the invention and should not be construed as limiting the scope of the invention as defined by appended claims. EXAMPLE 1 36kg of styrene monomer, 89g of dicumylperoxide, 45g of ethylbenzene, 89g of t-butylperoxybezoate, 178g of benzoylperoxide and 20g of glycerylmonostearate were mixed in the pre-mixer to prepare pre-mixture. 4.04 kg of pure water, which was pre-heated to 70°C in pure water tank and 0.71kg of tricalciumphosphate were heated to 90°C in the dispersion tank and mixed for 2 hrs so that dispersion master was prepared. From pure water tank, dispersion tank, pre-mixing tank and styrene monomer tank, respectively, 44.5kg of pure water, 1.14kg of dispersion master, 18.2kg of the pre-mixture and 2.0kg of styrene monomer were injected to polymerization bath of 60°C sequentially, heated to 90°C, and suspension-polymerized. When the conversion rate reached 10%, 44.5g of polyethylene homopolymer and 1.14kg of the third dispersion master were injected. When the reaction was almost completed, particle size was observed and the mixture consisting of 1.79g of calcium hydroxide and 30g of pure water was added. When the first reaction was completed, 1.14kg of the fourth dispersion master was injected and heated to 120°C. After 90 min, the melt of 178g of tetrabromobisphenol-bisallylether and 267g of tribromophenylallylether was injected and was filled with nitrogen gas to 2kg/cm 2 . Then, 3.56kg of pentane was injected and injection was completed after 60 min. After the conversion rate was 99.8 %, the reaction was terminated. After the resin bead was cooled to 80°C in the polymerization bath, the resin bead slurry was cooled to 15 to 20°C by cooling circulator where cold water of 5 to 7°C was injected continuously. Then, resin bead was washed out by pure water for 20 min using the washing drum screen and was injected into cooling tank at less than 15°C. Then, 45wt% of resin bead and 55wt% of water were mixed at 20rpm, in the cooling tank. After dehydrogenation and heat-drying by dehydrogenation system, foamed styrene resin bead was finally produced. EXAMPLE 2 Foamed styrene resin bead was produced according to the same procedure as described in the example 1, except the following treatment: The weight ratio of first to second pre-mixtures was 60 : 40. 7.2kg of pure water and 0.8kg of polyvinylpyrrolidone were injected, heated to 90°C and mixed at 40 rpm for 3hrs so that dispersion master was produced. When the conversion rate of the first reaction reached 85 %, 2.2g of tincarboxylate was added. When the reaction conversion rate reached 90%, 1.79g of calcium hydroxide and 30g of pure water were added and mixed, and foamed styrene resin bead was finally produced. EXAMPLE 3 Foamed styrene resin bead was produced according to the same procedure as described in the example 1, except the following treatments: The weight ratio of first to second pre-mixtures was 70 : 30 and chain transfer agent was not used. 2.23kg of pure water and 223.0g of polyvinylalcohol were injected into dispersion tank, heated to 80 °C and mixed at 50rpm for 1 hr so that dispersion master was produced. When the conversion rate of the first reaction reached 85%, 2.2g of tincarboxylate was added. When the reaction conversion rate reached 90%, 0.9g of ammonium laurylsulfate and 30g of pure water were added and mixed, and foamed styrene resin bead was finally produced. COMPARATIVE EXAMPLE Foamed styrene resin bead was produced according to the same procedure as described in the example 1, except following treatments: Pure water of room temperature, dispersion master, styrene monomer, initiator and flame retardant were injected together before polymerization, of which amounts were the same as example 1, respectively and cell controller in the process was added at once, finally preparing foamed styrene resin bead. TEST The properties of resin bead produced according to example 1 to 3 and comparative example were tested and the results is shown in Table 1. Category Example 1 Example 2 Example 3 Comparative Example Expansion ratio (times) Immediately after production 84 83 84 70 After aging for 90 days 80 79 81 63 Strength (kg/cm 2 ) Compression Strength 1.6 1.4 1.5 1.0 Flexural Strength 3.3 2.8 2.7 2.2 The expansion ratio of the expanded styrene resin bead is measured with a beaker (1L) for measuring volume designed to measure prepuffs. The prepuffs is produced by expanding with steam pressure 0.3kg/cm2 for 50sec by the pressure batch expander (Shinyoung Machine Co., Korea), followed by drying in fluidized bed dryer. The compression and flexural strengths are measured based on KSM-3808 with the tensile-strength tester (Instron Co., US), after resin beads prepared in examples and comparative example was molded in the mold (Shinyoung Machine Co., Korea) of 55 x 60 x 90 cm As shown in figure 1, in case of example 1, the particle size of resin bead 98.7% is within 0.8 to 1.2mm and reaction cycle is shortened by more than 5 hrs as compared to that in the prior arts. In the present invention, the expansion ration of expanded resin bead is improved and blocking effect of expanded resin bead seldom occurred (0.05 %). The surface shrinkage of expanded resin bead scarcely be observed and both internal and external cell structure is uniform. The molded product shows superior fusion property, smooth and gloss surface and lower humidity. Further, after aging for 90days, the expansion ratio and fusion property of the molded product are seldom decreased as compared to the resin bead immediately after production. The expansion ratio and the fusion property of the resin bead in prior art is notably decreased. As mentioned above, the foamed styrene resin bead prepared according to this invention is prepared in such a manner that unlike the conventional process, styrene monomer, dispersing agent and cell controller is separately added to polymerization mixture in several times, thereby making the control of the particle size and particle size distribution easier. The foamed styrene resin bead prepared in accordance with this invention shows excellent the expansion ratio, fusion property, cell uniformity and mechanical property. Further, since the release of foaming agent is prevented so that the conservative force, fusion property at molding, flame retardancy and moisture resistance are improved.

Claims: A process for producing foamed styrene resin bead, which comprises the steps of: (a) the first polymerization step in which 10 to 30wt% of pre-mixture comprising styrene monomer, crosslinking agent, chain transfer agent, initiator and surface active agent, 0.5 to 6.0wt% of dispersion master comprising pure water and dispersing agent, 50 to 55wt% of hot pure water and 1 to 15wt% of styrene monomer are mixed in the reactor at 50 to 70 °C and then the reactor is heated to the temperature range of 80 to 100 °C; (b) the second polymerization step in which, at the conversion rate of 5 to 15%, 5 to 25wt% of the pre-mixture, 1 to 15wt% of styrene monomer, 0.05 to 1.0 wt% of cell controller and 0.5 to 6.0wt% of dispersion master are further added to the reactor and then polymerized; (c) the third polymerization step in which, at the conversion rate of 50 to 70%, 0.05 to 1.0wt% of cell controller and 0.5 to 6.0wt% of dispersion master are still further added to the reactor and then polymerized; (d) the fourth polymerization step in which, at the conversion rate of 80 to 90%, 0.5 to 6.0wt% of dispersion master is further added to the reactor and then polymerized; and (e) the step in which the reactor is heated to temperature of 110 to 135 °C and at the conversion rate of 85 to 95%, the flame retardant, inert gas and foaming agent are added. The process for producing foamed styrene resin bead as claimed in claiml, wherein said pre-mixture comprises 75 to 95wt% of styrene monomer, 0.01 to 2.5wt% of crosslinking agent, 0.01 to 1.0wt% of chain transfer agent, 0.05 to 1.0wt% of initiator and 0.005 to 1.0wt% of surface active agent. The process for producing foamed styrene resin bead as claimed in claim 1 and claim 2, wherein said crosslinking agent is one or more selected from the group consisting of dicumylperoxide, acetylperoxide, t-allylbutylperoxide, and t-allylbutylhydroperoxide. The process for producing foamed styrene resin bead as claimed in claim 1 and claim 2, wherein said chain transfer agent is one or more selected from the group consisting of cyclohexane, toluene, benzene, ethylbenzene, t-allylbenzene, t-allylbutylchloride, t-allylbutylbromide and isopropylbenzene. The process for producing foamed styrene resin bead as claimed in claim 1 and claim 2, wherein said initiator is one or more selected from the group consisting of benzoylperoxide, laurylperoxide, t-butylperoxybenzoate, and 1,1-di(t-butylperoxy-3,3,5-trimethylcyclohexane). The process for producing foamed styrene resin bead as claimed in claim 1 and claim 2, wherein said initiator comprises hot initiator and cold initiator of which the weight ratio is 1 : 6 to 1 : 2. The process for producing foamed styrene resin bead as claimed in claim 6, wherein said hot initiator is one or more selected from the group consisting of t-butylperoxybenzoate and 1,1-di(t-butylperoxy-3,3,5-trimethylcyclohexane) The process for producing foamed styrene resin bead as claimed in claim 6, wherein said the cold initiator is one or more selected from the group consisting of benzoylperoxide and laurylperoxide. The process for producing foamed styrene resin bead as claimed in claim 4, wherein said surface active agent is one or more selected from the group consisting of glycerylmonostearate and hydroxyalkylamine. The process for producing foamed styrene resin bead as claimed in claim 1, wherein said dispersion master comprises 65 to 95wt% of pure water, 5 to 35wt% of dispersing agent which is one or more selected from the group consisting of polyvinylidone, tricalciumphosphate, polyvinylalcohol, cellulose derivative, calciumphosphate and pyrrolidone. The process for producing foamed styrene resin bead as claimed in claim 1 and claim 7, wherein the injection number of said dispersion master can be changed according to dispersion stability. The process for producing foamed styrene resin bead as claimed in claim 1, wherein said cell controller is one or more selected from the group consisting of sodiumphosphate, ammoniumsulphate, hexabromocyclododecane, and polyethylene wax. The process for producing foamed styrene resin bead as claimed in claim 1, wherein said polymer comprises crosslinking agent, chain transfer agent, and surface active agent and flame retardant and foaming agent is added to said polymer. The process for producing foamed styrene resin bead as claimed in claim 4, wherein 0.05 to 1.0wt% of said flame retardant is one or more selected from the group consisting of bisallylether, tribromophenylallylether, and trisdibromopropylphosphate. The process for producing foamed styrene resin bead as claimed in claim 1, wherein 5.0 to 10.0wt% of said foaming agent is one or more selected from the group consisting of alkanes with 3 to 6 carbons. The process for producing foamed styrene resin bead as claimed in claim 1, wherein said inert gas is injected until the pressure of reactor is in the range of 2 to 4 kg/cm 2 . The process for producing foamed styrene resin bead as claimed in claim 1, the cooling washing, dehydrating, and hot drying steps follows step (d). A foamed styrene resin bead prepared by the process in claim 1.

Inventors: CHANG YOUNG BAE, CHANG, YOUNG BAE
Applicants: CHANG YOUNG BAE, CHANG, YOUNG BAE
Application year: 1999
Main classification: C08J9
IPC codes: C08J9, C08F257, C08F12