Apparatus and method for producing foamed styrene resin beads

Abstract: Apparatus for producing foamed styrene resin comprising a pure water tank (10), a styrene monomer tank (11), a dispersing tank (12), a foaming agents tank (13), a cascade of a prepolymerization bath (14), a screw feeder reactor (15) and a main polymerization bath (16), wherein the styrene monomer tank (11) is connected to the prepolymerization bath (14) and the main polymerization bath (16) for supplying styrene monomer to both baths and downstream a cooler (17), a drum screen (18), a slurry storage tank (19), a centrifuge (20), a dehumidifier (21) and a dryer (22).

Description: Field of the Invention The present invention relates to an apparatus and a method for producing foamed styrene resin beads, comprising the means of carrying out the prepolymerization prior to the main polymerization, effectively cooling the resin beads in conjunction with the aging effect, facilitating the cleansing effect by using the force of movement of the resin beads, and finally removing the moisture from the resin beads prior to the drying process. The use of said apparatus with the means as set forth above in sequence results improved efficiency of the overall production line. More particularly, the invention herein relates to an apparatus and a method for producing foamed styrene resin beads with high productivity, which yields even particles with the target size, reduces the energy consumption during said production, and significantly shortens the reaction cycle as compared to that in the prior art. Background of the Invention In general, the apparatus for producing a foamed styrene resin beads consists of several tanks containing the pure water, styrene monomer, or the additives, respectively, the cooling and heating means, a bath for polymerizing said materials, the tank for washing-out the impurities attached on the particles, a centrifuge for removing the water moiety, and a hot-air dryer. The process for preparing the resin beads by use of such an apparatus can be summarized as follows: The pure water, styrene monomer and the additives supplied from the respective tanks are introduced into the bath and reacted at the pre-determined temperature, and for a predetermined time period, an amount of resin beads reacted are cooled in the polymerization bath, and then introduced into the separate tank to wash out the impurities attached on the beads. Furthermore, after washing, the water is removed out by way of a centrifuge, whilst drying the beads completely by hot-air dryer, then, finally through the step of binding the products by the desired unit volumes, the foamed styrene resin is produced. However, since if the products are not aged for a certain period of time after being produced by the internal foaming process, the physical properties of the products are degraded, hence it is essential for the products to be aged at least for 60 to 90 days in the refrigerator room, etc. Nevertheless, conventional apparatus for preparation of the foamed styrene resin beads have several disadvantages, in which there needs a considerable time period required for the reaction as well as for the cooling step because all the respective reactants are introduced in one bath in a quantity, while the warming step and the cooling step are performed, i.e. one-batch style, therefore, it is difficult to control the size of the resin particles, the size of particles having a wide distribution to the undesirable extent. Since the cleaning water containing HCI is used in the washing step, there results in contamination of the water for operation as well as achieving only an insufficient cleansing effect. In particular, since the styrene resin beads essentially needs to be aged for a long time after the preparation process in order to enhance the value of the products, the method using an apparatus of the prior art was predominantly disadvantageous with respect to the competitiveness of the products. In other words, since a considerable time period is required in the reaction and/or cooling steps as well as in the aging steps according to the prior methods, the competitiveness of the products therefrom is inevitably lowered, and it was also not effective in terms of the productivity because the sizes of the product particles are predominantly distributed within a range that the consumers do not desire. WO-A-9708231 describes a continuous process for bead-form expandable styrene polymers resulting in pearls with a narrow pearl size distribution and a low remaining styrene content, in which the styrene is first prepolymerized in bulk to a conversion of 10 to 80%. The prepolymerized styrene is then dispersed in liquid form in an aqueous phase in the presence of suspension stabilizers in a dynamical mixer, which can be a screw feeder, i. e. a simple transporting and mixing means. The prepolymerized styrene is transported through the screw feeder to a main polymerization reactor, wherein it is finally polymerized in an aqueous suspension to a residual styrene content of less than 2000 ppm. SUMMARY OF THE INVENTION Therefore, the present invention is designed to solve such problems in the prior arts, and it is an object of the invention to provide an apparatus for the preparation of foamed styrene resin beads with the higher competitiveness of the products, comprising a means for prepolymerization, a reaction means with the screw-feeder and a main reaction means, wherein the step-by-step reactions are carried out sequentially through the means, and after the reaction is ended, the products are maintained at a suitable temperature by passing through a cooling means in the swing manner and/or a washing means in the impact-shower manner, and wherein, when being dried through the dehumidifier, the temperature of the products is reduced to the lowest level as possible in order to keep the optimum physical properties of the resin, whereby being able to easily control the sizes of the resin beads as well as to yield the beads with the desired size, without any need of a separate step of aging treatment, and the productivity being increased by both saving the energy needed in the process and shortening the reaction cycle, and the products with the improved physical properties such as impact-resistance, rigidity, insulating property, foaming property, etc. being produced. BRIEF DESCRIPTION OF THE DRAWINGS In order that the invention may be clearly understood, reference is now made to the preferred embodiments illustrated hereinafter and figures, in which: Figure 1 is a schematic view of the overall construction of the apparatus for producing foamed styrene resin according to the present invention. Figure 2 is a schematic view of a polymerization bath of the apparatus for producing foamed styrene resin according to the present invention. Figure 3 is a schematic view of a cooling circulator of the apparatus for producing foamed styrene resin according to the present invention. Figure 4 is a cross-sectioned view of a cooling circulator of the apparatus for producing foamed styrene resin according to the present invention. Figure 5 is a schematic view of a cleaning drum screen of the apparatus for producing foamed styrene resin according to the present invention. DETAILED DESCRIPTION OF THE INVENTION According to claim 1 of the present invention, an apparatus for producing foamed styrene resin beads is provided, in which it comprises: a) a prepolymerization bath; b) a main polymerization bath with a heating means and a circulating water cooling means; c) a dispersing tank with heating or cooling means and with agitating means upstream and connected to the main polymerization bath; d) a pure water tank with temperature controlling means upstream and connected with the dispersing tank and the main polymerization bath; e) a styrene monomer tank upstream and connected to the prepolymerization bath and the main polymerization bath; f) a screw feeder reactor positioned between the prepolymerization bath and the main polymerization bath; g) a foaming agent tank upstream and connected to the main polymerization bath; h) a multi-bended double pipe cooling circulator downstream and connected to the main polymerization bath; i) a drum cleaning means with a motor driven drum screen and with an injection nozzle downstream and connected to the multi-bended double pipe cooling circulator; j) a slurry storage tank with agitator downstream and connected to the drum screen; k) a centrifuge and/or a dehumidifier upstream a hot-air drier. In a preferred embodiment of the present invention in the apparatus for producing foamed styrene resin beads the heating or cooling means equipped to the dispersing tank is constructed in such manner that a pipe surrounds at least five times along the interior wall of the tank, with the inlet and the outlet being positioned to the exterior thereof, and is able to heat or cool the mixtures in the tank by selectively supplying the steam or the cold water through the pipe. In another preferred embodiment of the present invention in the apparatus for producing foamed styrene resin beads the cooling circulator has a double pipe structure being able to exchange heat between the interior pipe and the exterior pipe by arranging several linear pipes to the same direction with a predetermined distance to each other, one end part of a linear pipe being connected to an end part of a rounded pipe having the same diameter, the other end part of the rounded pipe being again connected to an end part of a former linear pipe, while the other end part of a linear pipe being connected to an end part of another rounded pipe also having the same diameter, the other end part of the rounded pipe being again connected to an end part of a latter linear pipe, whereby said several linear pipes and said rounded pipes being connected for fluid communication, and the respective linear pipes finally being surrounded by the corresponding number of exterior pipes which are also connected to each other, the diameter of which are bigger than those of interior pipes. In another preferred embodiment of the present invention in the apparatus for producing foamed styrene resin beads the drum cleaning means has several screw fans, and comprises the drum screen being able to be rotated by a motor, with the guidance of a guide roller; several injecting nozzles injecting the pure water over the said drum screen; and a sealing cover and a frame containing and supporting all of them. Claim 5 of present invention further provides a method for producing foamed styrene resin beads, comprising the steps of a) heating pure water to about 70 °C in a pure water tank and supplying the water to a dispersing tank and a main polymerization bath; b) supplying a certain amount of styrene monomer from a styrene monomer tank to a prepolymerization bath and a main polymerization bath; c) supplying a foaming agent from a foaming agent tank to the main polymerization bath; d) heating the water and the additives to 70 to 90 °C and premixing them in the dispersing tank, and introducing the mixture into the main polymerization bath; e) polymerizing an amount of styrene monomer with the additives in the prepolymerization bath for the first time to the conversion ratio of 5 -10%; f) polymerizing the reactants from the prepolymerization bath in a screw feeder reactor positioned between the prepolymerization bath and the main polymerization bath for a second time to the conversion ratio of 7 -20%; g) supplying the water, the additives and the styrene monomer from the pure water tank, the dispersing tank and the styrene monomer tank, respectively, and the prepolymerized reactants from the screw feeder reactor to the main polymerization bath and having them react with each other at 70 to 90 °C and react again at 110 to 135 °C to the conversion ratio of 99.80%; h) cooling and ageing the reacted resin beads from the main polymerization bath in a cooling circulator at the temperature below 20 °C by the use of cold water; i) washing out the impurities attached on the surface of the resin beads in a drum cleaning means with pure water injected from an injection nozzle; j) storing the cleaned resin beads in a slurry storage tank to the temperature below 15 °C while agitating them; k) removing the water and/or the moisture from the beads using a centrifuge and/or a dehumidifier; and I) dehumidifying and/or drying the beads at the temperature below 30 °C in a hot-air drier by use of hot-air. For a better understanding of the present invention, references made to the following description and its scope will be pointed out in the appended claims. DESCRIPTION OF THE PREFERRED EMBODIMENT Figure 1 is an schematic view of the overall apparatus for producing foamed styrene resin according to the present invention. The apparatus for producing foamed styrene resin beads according to the present invention comprises: a pure water tank 10 for supplying the pure water for a dispersing tank 12 and a main polymerization bath 16; a styrene monomer tank 11 for supplying the styrene monomer; a foaming agent tank 13 for supplying the foaming agent for the main polymerization bath 16; a dispersing tank 12 for supplying the additives; a prepolymerization bath 14 for prepolymerizing the styrene monomer with the additives; a screw feeder reactor 15; a main polymerization bath 16 as a main reactor; a cooling circulator 17 for cooling and aging the resin beads; a drum screen 18 for cleansing; a slurry storage tank 19 for storing the resin beads; a centrifuge 20; and a dehumidifier 22. The said pure water tank 10 supplies the pure water for a dispersing tank 12 and a main polymerization bath 16. The pure water tank 10 is equipped with a warming means for preheating the pure water to be introduced into the main polymerization bath to 50 -70 °C, in order to reduce the time required to the reaction cycle. The styrene monomer tank 11 containing an amount of styrene monomer supplies a predetermined amount of styrene monomer sequentially for the prepolymerization bath 14 and the main polymerization bath 16. In the dispersing tank 12, the water, the dispersants, the dispersing adjutants, the sizing agents for the resin beads, etc. are mixed by the agitating means, with being heated to 70 -90 °C, and then the mixtures are introduced into the main polymerization bath 16. When the additives are directly introduced to the main polymerization bath 16, they are hardly dissolved due to being much less than the amount of the total reactants, they may be homogeneously mixed in the dispersing tank 12 and then introduced to the main polymerization bath 16, in order to achieve the dispersing effect of the additives to be distributed in the reactants. The foaming agents tank 13 supplies the foaming agents for foaming the resin to the main polymerization bath 16, and the time point to introduce the foaming agents is when the conversion rate arrives at 94% in the main polymerization reaction. The prepolymerization bath 14 having a similar structure to the main polymerization bath 16 polymerizes an amount of the styrene monomer and the various additives from the styrene monomer tank 11 at 60 °C, to achieve a conversion rate of 5 -10%, prior to the main polymerization. The screw feeder reactor 15 is a pre-reactor equipped to connect the prepolymerization bath 14 with the main polymerization bath 16, with the semi-polymerized material having a conversion rate of 5i-10% from the prepolymerization bath 14, and also supplies the semi-polymerized material having a conversion rate of 20% for the main polymerization bath 16. As seen in the Fig. 2, the main polymerization bath 16 is in the form of a sealed tank with a agitator having at least three screw fans and the heating and/or cooling means. The heating and/or cooling means of the invention consist of the polymerization bath and the jacket covering the inner wall of the polymerization bath with a spacing between therein in order to selectively supply hot or cold circulating water, with the result of heating and/or cooling the reactants in the bath by heat transfer of the interior wall of the bath. The polymerized reactants from the screw feeder reactor 15, the pure water of 50 -70 °C from the pure water tank 10, a certain amount of the styrene monomer from the styrene monomer tank 11 and the mixed additives from the dispersing tank 12 are introduced into the main polymerization bath 16. In the main polymerization bath 16, the reaction is carried out by two steps, in which the first step is carried out at 70 -90 °C and the agitating rate is depending on the size of the particles; and the second step is carried out at 110 -135 °C, in the presence of the foaming agents, under the inert gas. A conversion rate in the main polymerization bath 16 is 99.80%, and the reacted resin beads are cooled to 70 °C in its place. The cooling circulator 17 is a cooling means for refrigerating the reacted resin beads to be kept below 20 °C. For such refrigeration, as seen from the Fig 3, the cooling circulator 17 forms a structure in which a pipe having a predetermined length is bended several times, like a zig-zag shape, the inlet and the outlet being connected to the main polymerization bath 16 and the drum screen 18, respectively, in order to keep a sufficient distance for the beads to flow for a desired time. In addition, as seen from the Fig 4, an another pipes through which the coolant, e.g. the cold water flows is positioned concentrically around the said pipe, that is, in the shape of double piping. Thus, during the period that the resin beads flows along the interior pipe, there can be a heat exchange with the coolant, to loss the heat, resulting in the desired temperature of the resin beads kept. The resin beads pass through the cooling circulator 17 for a certain time period while keeping the temperature below 20 °C, therefore, the interior cell of the resin beads becomes even in sizes, with completely excluding additional aging period. As seen from Fig. 5, the said drum screen 18 as a washing means for the purpose of enhancing the melt-adhesive property and providing a smooth surface, has several screw fans, and comprises a cylindrical drum rotated by the driving motor; an injecting nozzle positioned over the drum; a sealing type cover and a frame covering and supporting the above elements. In the drum screen 18, the impurities are removed from the surface of the resin beads by cleansing the cooled beads from cooling circulator with the pure water injected from the nozzle. Since the resin beads proceeds along the route provided by the screw with impacting each other, and are subjected with the washing force of the pure water, the method of the invention can achieve much higher cleansing effects compared to the prior art, in which the beads are stirred in the cleansing liquid. And the washing process can continuously proceed, so that the continuation and automation of the process may be possible. The slurry storage tank 19 as a cooling bath keeps the cleaned resin beads below the temperature of 15 °C with agitating. The centrifuge 20 is a means for dehumidifying as much moisture contained in the resin beads as possible, prior to the drying step. In particular, the apparatus according to the invention are equipped with an additional dehumidifying system 21 which dehumidifies the dehumidified resin beads once more, and by using the system 21, the foaming agent is prevented from leaking out from the resin beads with drying temperature below 30 °C. The dehumidifier/dryer 22 is a means for completely excluding the moisture from the resin beads, resulting in the end products having a desired particle size. Hereinafter, according to the invention, the process for preparation of the foamed styrene resin beads is described in more detail. The process for preparation of the foamed styrene resin beads according to the invention include adding various additives depending on the reaction steps, such as the pre-reaction step, the screw feeder reaction step and the main reaction step, the main reaction step being divided into two stage reaction depending on the temperature. The composition in the prepolymerization step comprises styrene monomer, the flame-retardant, the flame-retardant adjuvant, and the cross-linking agents to be polymerized at a conversion rate of 5-10%, and with an aromatic hydrocarbon chain-transfer agents added in the screw feeder reactor, the polymerization proceeds at a conversion rate of 7-20%, and then the mixtures are introduced into the main polymerization bath. The amount of the pure water introduced for the first time into the main polymerization bath is the same as the styrene monomer. The pure water introduced into the main polymerization bath is preheated to 50-70 °C by heating tank in order to shorten the reaction cycle. Also, the water, the dispersing agents, the dispersing-adjuvant, and the sizing agents are added with heating to 70-90 °C. The mixture is introduced and an amount of the styrene monomer is also introduced into the main polymerization bath. In addition, the polymerized reactants from the screw feeder reactor are introduced into the main polymerization bath with increasing the temperature to 70-90 °C for 30 to 60 minutes. The reaction in the main polymerization bath consists of two stages, the first stage being carried out between 70-90 °C, the second stage being carried out between 110-135 °C, and when the temperature is increased from the first stage to the second stage, the foaming agents and the inert gas is introduced at the same time with the desired pressure. The inert gas serves to permeate the foaming agent, etc. into the resin beads. After reaction, the resin beads are passed through the cooling circulator to reduce the reaction cycles. The resin beads passed through the cooling circulator is washed with the pure water for 20-30 minutes via a drum screen in order to improve melt-adhesive, impact-resistance, strength, etc. of the beads, and then agitated under the temperature below 15 °C. In particular, the apparatus according to the invention is equipped with the additional dehumidifying system 21 which dehumidifies the dehumidified resin beads once more, and by using the system 21, the foaming agent is prevented from leaking out from the resin beads with drying temperature below 30 °C. The beads produced through the above steps of the invention do not need an additional aging step, and have a stable storage property without a leak-out of the foaming agents, a good foaming property, etc. As described above, the invention provides various advantages, as follows: that is, the method of the invention can achieve much higher cleansing effects compared to the prior art; the washing process can be continuously proceeded, so that the continuation and automation of the process may be possible; the beads do not need an additional aging step, and have a stable storage property without a leak-out of the foaming agents, a good foaming property, etc.

Claims: An apparatus for producing foamed styrene resin beads, comprising: a) a prepolymerization bath (14); b) a main polymerization bath (16) with a heating means and a circulating water cooling means; c) a dispersing tank (12) with heating or cooling means and with agitating means upstream and connected to the main polymerization bath (16); d) a pure water tank (10) with temperature controlling means upstream and connected with the dispersing tank (12) and the main polymerization bath (16); e) a styrene monomer tank (11) upstream and connected to the prepolymerization bath (14) and the main polymerization bath (16); f) a screw feeder reactor (15) positioned between the prepolymerization bath (14) and the main polymerization bath (16); g) a foaming agent tank (13) upstream and connected to the main polymerization bath (16); h) a multi-bended double pipe cooling circulator (17) downstream and connected to the main polymerization bath (16); i) a drum cleaning means (18) with a motor driven drum screen and with an injection nozzle downstream and connected to the multi-bended double pipe cooling circulator (17); j) a slurry storage tank (19) with agitator downstream and connected to the drum screen (18); k) a centrifuge (20) and/or a dehumidifier (21) upstream a hot-air drier (22). The apparatus for producing foamed styrene resin beads according to claim 1, wherein the heating or cooling means equipped to the dispersing tank (12) is constructed in such manner that a pipe surrounds at least five times along the interior wall of the tank, with the inlet and the outlet being positioned to the exterior thereof, and is able to heat or cool the mixtures in the tank by selectively supplying the steam or the cold water through the pipe. The apparatus for producing foamed styrene resin beads according to claim 1, wherein the cooling circulator (17) has a double pipe structure being able to exchange heat between the interior pipe and the exterior pipe by arranging several linear pipes to the same direction with a predetermined distance to each other, one end part of a linear pipe being connected to an end part of a rounded pipe having the same diameter, the other end part of the rounded pipe being again connected to an end part of a former linear pipe, while the other end part of a linear pipe being connected to an end part of another rounded pipe also having the same diameter, the other end part of the rounded pipe being again connected to an end part of a latter linear pipe, whereby said several linear pipes and said rounded pipes being connected for fluid communication, and the respective linear pipes finally being surrounded by the corresponding number of exterior pipes which are also connected to each other, the diameter of which are bigger than those of interior pipes. The apparatus for producing foamed styrene resin beads according to claim 1, wherein the drum cleaning means (18) has several screw fans, and comprises the drum screen being able to be rotated by a motor, with the guidance of a guide roller; several injecting nozzles injecting the pure water over the said drum screen; and a sealing cover and a frame containing and supporting all of them. A method for producing foamed styrene resin beads, comprising the steps of a) heating pure water to about 70 °C in a pure water tank (10) and supplying the water to a dispersing tank (12) and a main polymerization bath (16); b) supplying a certain amount of styrene monomer from a styrene monomer tank (11) to a prepolymerization bath (14) and a main polymerization bath (16); c) supplying a foaming agent from a foaming agent tank (13) to the main polymerization bath (16); d) heating the water and the additives to 70 to 90 °C and premixing them in the dispersing tank (12), and introducing the mixture into the main polymerization bath (16); e) polymerizing an amount of styrene monomer with the additives in the prepolymerization bath (14) for the first time to the conversion ratio of 5 -10%; f) polymerizing the reactants from the prepolymerization bath (14) in a screw feeder reactor (15) positioned between the prepolymerization bath (14) and the main polymerization bath (16) for a second time to the conversion ratio of 7 -20%; g) supplying the water, the additives and the styrene monomer from the pure water tank (10), the dispersing tank (12) and the styrene monomer tank (11), respectively, and the prepolymerized reactants from the screw feeder reactor (15) to the main polymerization bath (16) and having them react with each other at 70 to 90 °C and react again at 110 to 135 °C to the conversion ratio of 99.80%; h) cooling and ageing the reacted resin beads from the main polymerization bath (16) in a cooling circulator (17) at the temperature below 20 °C by the use of cold water; i) washing out the impurities attached on the surface of the resin beads in a drum cleaning means (18) with pure water injected from an injection nozzle; j) storing the cleaned resin beads in a slurry storage tank (19) to the temperature below 15 °C while agitating them; k) removing the water and/or the moisture from the beads using a centrifuge (20) and/or a dehumidifier (21); and I) dehumidifying and/or drying the beads at the temperature below 30 °C in a hot-air drier (22) by use of hot-air.

Inventors: HWANG GYU EOK, HWANG, GYU EOK
Applicants: SHINHO PETROCHEMICAL CO LTD, SHINHO PETROCHEMICAL CO. LTD.
Application year: 1998
Main classification: B29C44
IPC codes: B29C44, B01J19, C08J9