WO2008054061A1

WO2008054061A1 - Method for preparation of microcellular foam with uniform foaming ratio and extruding and foaming system for the same

Info

Publication number: WO2008054061A1
Application number: PCT/KR2007/004220
Authority: WO
Inventors: Jongsung Park; Kideog Choi; Kyunggu Nam
Original assignee: LG Chem Ltd
Current assignee: LG Chem Ltd
Priority date: 2006-10-30
Filing date: 2007-09-01
Publication date: 2008-05-08
Anticipated expiration: 2009-04-30
Also published as: TW200831266A; KR20080038468A; KR100890159B1

Abstract

Disclosed herein is a method for preparing a microcellular foam, including mixing a thermoplastic polymer resin plasticized in an extruder with a foaming agent, melting the mixture and forming micro pores in the melted mixture while passing the mixture through a pressure drop zone of an extrusion die, and cooling the melted mixture formed with the micro pores while passing the melted mixture through a cooling zone of the extrusion die, and a system capable of carrying out the method, further including setting the temperature at a downstream end of the pressure drop zone and the temperature at an upstream end of the cooling zone such that there is a temperature difference of 30 to 2000C between the temperatures, to extrude the mixture in the form of a microcellular foam, and pultruding the microcellular foam emerging from the extrusion die at a predetermined foam induction ratio by a pultrusion unit, to finely control the foaming ratio and time-dependent physical properties of the microcellular foam.

Description

METHOD FOR PREPARATION OF MICROCELLULAR FOAM

WITH UNIFORM FOAMING RATIO AND EXTRUDING AND

FOAMING SYSTEM FOR THE SAME

Technical Field

[1] The present invention relates to a method for preparing an uniform microcellular foam and an extruding system for the same. More particularly, the present invention relates to a method for preparing a microcellular foam, which includes mixing a thermoplastic polymer resin plasticized in an extruder with a foaming agent, melting the mixture and forming micro pores in the melted mixture while passing the mixture through a pressure drop zone of an extrusion die, and cooling the melted mixture formed with the micro pores while passing the melted mixture through a cooling zone of the extrusion die, and further includes setting the temperature at a downstream end of the pressure drop zone and the temperature at an upstream end of the cooling zone such that there is a temperature difference of 30 to 200⁰C between the temperatures, to extrude the mixture in the form of a microcellular foam, and pultruding the microcellular foam emerging from the extrusion die at a predetermined foam induction ratio by a pultrusion unit, to finely control the foaming ratio and time-dependent physical properties of the microcellular foam. The present invention also relates to a system capable of carrying out the method. Background Art

[2] Generally, foams have many advantages of a thermal insulation property, a soundproof property, light weight, impact resistance, an electrical insulation property, optical characteristics, etc. Accordingly, such foams are widely used for various purposes in various fields associated with soundproof materials, insulating materials, cushioning materials, vibration-isolating materials, light reflection plates, light diffusion plates, etc.

[3] Such foams are prepared using a foaming process such as a method in which a polymer resin is mechanically foamed or a method in which a resin compound containing a physical foaming agent or a chemical foaming agent is extrusion-molded to prepare a foam. For the physical foaming agent, carbon dioxide, nitrogen, or hy- drofluoro carbon may be used. For the chemical foaming agent, a gas-producing organic material such as azodicarbon amide may be used.

[4] In the foaming process, it is most important to control the size, shape, and quantitative distribution of pores. The foaming process has many parameters, and has a difficulty to control conditions, as compared to other processes. In particular, the properties of a foam may considerably depend on the foaming ratio of the foam. Accordingly, it is very important to adjust the foaming ratio.

[5] Conventionally, the adjustment of the foaming ratio is achieved through adjustment of the content of a foaming agent or temperature adjustment. However, the foaming ratio adjustment is roughly achieved in the unit of a very wide deviation range, for example, a 10% range such as 10%, 20%, ... For this reason, it is impossible to finely achieve the foaming ratio adjustment in the unit of a narrow deviation range, so that there is a limitation in adjusting the properties of the foam or uniformly adjusting the foaming ratio of the foam.

[6] FIG. 1 is a schematic view illustrating an extruding and foaming system carrying out a general extruding and foaming process for preparation of a microcellular foam. FIG. 2 illustrates a conventional extrusion die including only a pressure drop zone and a temperature variation occurring in the extrusion die, through an enlarged sectional view and a graph.

[7] Referring to the drawings, the extruding and foaming process is carried out through an extruder 10, an extrusion die 20, a calibration unit 30, a pultrusion unit 40, and a cutter 50 arranged in this order in an advancing direction of an extruded product.

[8] In order to prepare a foam, raw materials for an extruded product including a foaming agent are mixed by an agitator 11. The resultant mixture is extruded through the extruder 10 after being melted in the extruder 10. In order to form an extruded product having a certain shape, the extrusion-molding material passes through the extrusion die 20, to which an external heater 22 is mounted. As the extrusion-molding material passes through a pressure drop zone 21 defined in the extrusion die 20, it is foamed. Thus, a foamed extruded product in a high-temperature state emerges from the extrusion die 20. The foamed extruded product emerging from the extrusion die 20 in a high-temperature state then passes through the calibration unit 30, in order to cool and solidify the foamed extruded product, and thus, to maintain the profile of the foamed extruded product. The solidified product is cut into a desired length by the cutter 50 after passing through the pultrusion unit 40. Thus, foams having a desired shape can be manufactured.

[9] The extruded product, which is fed to the calibration unit 30 after emerging from the extrusion die 20 in a high-temperature state, is exposed to atmospheric pressure at a relatively high-temperature state before being sufficiently cooled. For this reason, the extruded product is post-foamed by an internal foaming pressure thereof. Furthermore, the extruded product expands not only in an extrusion direction, but also in a width direction. As a result, since the foaming ratio of the foam varies with the lapse of time, there are severe problems in that the prepared foam may not have a desired foaming ratio, and may have a non-uniform cross-sectional shape. [10] Meanwhile, the calibration unit 30 of the above-mentioned extruding/calibrating system, which functions to cool an extruded product using a coolant circulated by a coolant circulator 31, has a very long processing length of about 3 to 6m. For this reason, there is a large spatial limitation caused by the calibration unit 30. Furthermore, the extrusion rate of the extruded product is about 3 to 5min/min when it passes through the calibration unit 30. For this reason, there is a limitation in increasing the pultrusion rate of the extruded product due to the calibration process, so that a great degradation in productivity occurs.

[11] In association with preparation of a microcellular foam, a technique for adjusting the foaming ratio of a foam is disclosed in, for example, Japanese Patent No. 3199951. This patent discloses a method for preparing a thermoplastic elastomer. This method is based on the fact that an increase in foaming magnitude occurs at an increased melt ductility within a range in which the strength of the walls of cells formed in accordance with a growth of air bubbles is maintained, and includes a process for measuring, as a melt ductility, a feeding rate of a composition containing a melt ductility enhancing agent, a process for measuring a foaming magnitude of the composition, and a process for selecting a composition of the composition and/or a foaming temperature for obtaining a desired foaming magnitude, based on the measured foaming magnitude.

[12] However, the above-mentioned technique is limited to the case in which only water

(H O) is used as a foaming agent. That is, this technique cannot be applied to the case in which a polymer resin having no miscibility to water is extrusion-molded. In order to obtain a desired foaming magnitude, it is necessary to repeatedly perform a foaming magnitude adjustment several times by measuring a melt ductility of the composition while varying the contents of constituent elements in the composition. For this reason, the process for obtaining a desired foaming magnitude is very circuitous and complex. Furthermore, it is necessary to use a separate melt ductility measuring device. In addition, there is a degradation in production efficiency. It is also difficult to finely adjust the foaming temperature, and to maintain a desired temperature. Moreover, it is impossible to prevent the extruded product emerging from the extrusion die from being post-foamed before being cooled. Thus, the above-mentioned technique has many problems.

[13] Also, Japanese Patent Unexamined Publication No. 1997-057822 discloses a technique for detecting a frictional force generated between a foaming die and a resin foamed in the foaming die, and controlling a feeding rate of the resin, to prevent an extruded product from being surged due to a frictional force between an extrusion die and the extruded product, and thus, to prepare a foamed product having an uniform foaming ratio and a high dimension accuracy.

[14] However, although the above-mentioned technique can prepare a foam having a relatively-uniform foaming ratio because it can provide an uniform frictional force, there is a problem in that the foaming ratio cannot be finely controlled.

[15] Meanwhile, although having no relation to foaming ratio adjustment, Korean Patent

Unexamined Publication No. 1999-0063440 discloses a technique for preparing a foamed thermoplastic resin sheet having a large thickness and a high foaming magnitude. In accordance with this technique, an extrusion die is used which includes a foaming zone and a cooling zone. The extrusion die also includes a pressure reducing chamber which can maintain a desired pressure reduction degree by a seal member, in order to achieve additional foaming, and thus to obtain a foam having a high foaming ratio. In accordance with this technique, the spacing between a pair of walls defining the pressure reducing chamber is adjusted. Thus, it is possible to prevent air bubbles from being grown in a width direction of the sheet or in a direction perpendicular to a thickness direction of the sheet, and thus to prepare a foamed thermoplastic resin sheet having a large thickness and a high foaming magnitude.

[16] In accordance with the above-mentioned technique, however, a gentle temperature profile is established due to heat conduction because the foaming zone and cooling zone are in contact with each other. For this reason, the density of the foam is reduced, so that the foam may easily vary in cross-sectional shape during a subsequent pultruding process. For the adjustment of the wall spacing of the pressure reducing chamber, a separate driving unit is needed. As a result, the preparing process becomes complex. Furthermore, there is a degradation in the physical properties of the extruded product due to the additional foaming.

[17] To this end, it is highly required to provide a foam preparing technique capable of finely and uniformly controlling the foaming ratio of a foam such that the foam has a desired foaming ratio in spite of the lapse of time, and the resultant extruded product has a desired cross-sectional shape in spite of the lapse of time. Disclosure of Invention Technical Problem

[18] Therefore, the present invention has been made to solve the above problems, and other technical problems that have yet to be resolved.

[19] After active research and various repeated experiments, the inventors of the present invention found that, when a microcellular foam is extruded through an extrusion die in a microcellular foam preparing process under the condition in which the temperature at a downstream end of a pressure drop zone and the temperature at an upstream end of a cooling zone are set to have a predetermined difference, and the microcellular foam emerging from the extrusion die is pultruded through a pultrusion unit at a predetermined pultrusion ratio, it is possible to finely adjust the foaming ratio of the foam in the unit of several percentages, and to uniformly control the foaming ratio of the foam such that the foam has a desired foaming ratio in spite of the lapse of time, and the resultant extruded product has a desired cross-sectional shape in spite of the lapse of time. Technical Solution

[20] The present invention provides a method for preparing a microcellular foam, comprising mixing a thermoplastic polymer resin plasticized in an extruder with a foaming agent, melting the mixture and forming micro pores in the melted mixture while passing the mixture through a pressure drop zone of an extrusion die, and cooling the melted mixture formed with the micro pores while passing the melted mixture through a cooling zone of the extrusion die, further comprising: setting the temperature at a downstream end of the pressure drop zone and the temperature at an upstream end of the cooling zone such that there is a temperature difference of 30 to 200⁰C between the temperatures, to extrude the mixture in the form of a microcellular foam; and pultruding the microcellular foam emerging from the extrusion die at a predetermined foam induction ratio by a pultrusion unit, to finely control the foaming ratio and time-dependent physical properties of the microcellular foam.

[21] In detail, the microcellular foam preparing method according to the present invention comprises: (a) mixing a thermoplastic polymer resin plasticized in an extruder with a foaming agent, (b) melting the mixture and forming micro pores in the melted mixture while passing the mixture through a pressure drop zone of an extrusion die; (c) cooling the melted mixture formed with the micro pores while passing the melted mixture through a cooling zone of the extrusion die, and (d) pultruding the cooled microcellular foam emerging from the extrusion die at a predetermined foam induction ratio by a pultrusion unit, to finely control the foaming ratio and time-dependent physical properties of the microcellular foam.

[22] In accordance with the microcellular foam preparing method of the present invention, the foaming process, namely, the process for forming micro pores, (b), and the solidifying process for an extruded product, namely, the process for cooling the extruded product, namely, the form, (c), are completed in the extrusion die. Accordingly, the foamed product extruded in a solidified state has a dense structure at the skin portion thereof due to an abrupt temperature variation occurring when the foamed product is extruded. As a result, no expansion in a width direction occurs in the extruded product when the extruded product is exposed to atmospheric pressure. Thus, it is possible to finely adjust the foaming ratio of the foam, and to uniformly control the foaming ratio of the foam such that the foam has a desired foaming ratio in spite of the lapse of time, by controlling only the extrusion speed of the foam in the extrusion direction through a speed control for the pultrasion unit. In addition, it is possible to completely avoid a post-foaming phenomenon occurring before the extruded product emerging from the extrusion die is subjected to a cooling process in conventional cases.

[23] In accordance with the present invention, there is a temperature difference of 30 to

200⁰C between the temperature at the downstream end of the pressure drop zone and the temperature at the upstream end of the cooling zone. Preferably, there is a temperature difference is 50 to 15O⁰C.

[24] When the temperature difference is less than 3O⁰C, the growth of micro pores formed in the pressure drop zone is continued. For this reason, it is difficult for the foamed product to have a skin layer having a sufficient thickness. On the other hand, when the temperature difference is more than 200⁰C, it is difficult to execute the manufacturing process due to an abrupt solidification of the foam.

[25] Both the pressure drop zone and the cooling zone may be defined in a single extrusion die. Alternatively, the pressure drop zone and cooling zone may be defined in separate block type extrusion dies, respectively. In the former case, it may be possible to efficiently control the formation of the micro pores of the microcellular foam, and to efficiently achieve the formation of the skin layer. In the latter case, it is desirable to strongly clamp the extrusion dies such that the pressure at the downstream end of the pressure drop zone is maintained even in the cooling zone.

[26] Preferably, the extrusion die includes a heater, in order to prevent a temperature decrease in the vicinity of the downstream end of the pressure drop zone. The heater may be arranged inside the extrusion die. Alternatively, the heater may be arranged inside and outside the extrusion die. There is no limitation on the heater. For example, a general electrical heater may be used for the heater.

[27] Preferably, the extrusion die includes a cooler, in order to prevent a temperature increase in the vicinity of the upstream end of the extrusion die. Similarly to the heater, the cooler may be formed inside the extrusion die, or may be formed inside and outside the extrusion die. There is no limitation on the cooler. For example, a pipe line, through which a coolant flows, may be used for the cooler.

[28] If necessary, appropriate numbers of additional heaters and additional coolers may be used.

[29] The temperature at the downstream end of the pressure drop zone can be appropriately adjusted in accordance with the kind of the raw material to be extruded. Preferably, this temperature is 150 to 25O⁰C. When the temperature at the downstream end of the pressure drop zone is less than 15O⁰C, it is difficult to sufficiently form micro pores. On the other hand, when the temperature is more than 25O⁰C, there may be a degradation of the thermoplastic resin and over-foaming of the thermoplastic resin. [30] The temperature at the upstream end of the cooling zone can be appropriately adjusted in accordance with the kind of the raw material to be extruded. Preferably, this temperature is 150 to 25O⁰C. When the temperature at the downstream end of the pressure drop zone is less than 15O⁰C, it is difficult to sufficiently form micro pores. On the other hand, when the temperature is more than 25O⁰C, there may be a degradation of the raw material, namely, thermoplastic resin, and over-foaming of the thermoplastic resin.

[31] Preferably, the temperature at the upstream end of the cooling zone is maintained to be slightly higher than the melting point or softening point of the raw material to be extruded. More preferably, the temperature may be 40 to 15O⁰C. When the temperature at the upstream end of the cooling zone is less than 4O⁰C, an abrupt solidification occurs. In this case, it is difficult to execute the manufacturing process. On the other hand, when the temperature exceeds 15O⁰C, the micro pores formed in the pressure drop zone are continuously grown even in the cooling zone. For this reason, it is difficult for the foam to have a skin layer having a sufficient thickness.

[32] Preferably, the temperature variation in each of the pressure drop zone and cooling zone is within +5⁰C. More preferably, the temperature variation is within +2⁰C. When the temperature variation is beyond the range of +5⁰C, it is impossible to obtain an uniformly-extruded product. In this case, the mechanical properties of the extruded product are degraded.

[33] There is no specific limitation on the transfer speed for the melted mixture formed with micro pores. Preferably, the transfer speed is 0.5 to 20 m/min in terms of production efficiency.

[34] In a preferred embodiment of the present invention, a temperature transition zone is present between the pressure drop zone and the cooling zone. The temperature transition rate in the temperature transition zone in a process advancing direction may be 2 to 40°C/mm, based on the following Expression (1).

[35] T = (T - T )/L (1)

L H C

[36] In Expression (1), "T " represents a temperature transition rate, "T " represents the t

L H emperature at the downstream end of the pressure drop zone, "T " represents the temperature at the upstream of the cooling zone, and "L" represents the length of the temperature transition zone.

[37] Here, the "temperature transition zone" means a zone where an abrupt temperature variation occurs between the pressure drop zone and the cooling zone. This temperature transition zone functions to prevent heat exchange occurring between the pressure drop zone and the cooling zone. In the specification, the temperature transition occurring in the temperature transition zone in a process advancing direction is referred to as a "temperature transition rate". [38] When the temperature transition zone has a higher temperature transition rate, the extruded product can be solidified at a higher density. Accordingly, it is more effective for the temperature transition zone to have a higher temperature transition rate in preparing an extruded product having superior surface characteristics. When the temperature transition rate is less than 2°C/mm, it is impossible to control formation of micro pores in the cooling zone.

[39] When the length of the temperature transition zone is reduced, an abrupt temperature variation may occur. Accordingly, it is preferred that the length of the temperature transition zone be 1 to 150 mm.

[40] When the length of the temperature transition zone exceeds 150 mm, the temperature transition occurring between the pressure drop zone and the cooling zone is gentle, thereby causing a slow solidification of the extruded product. In this case, the density o f the extruded product is undesirably low.

[41] Meanwhile, the case, in which both the pressure drop zone and the cooling zone are included in a single extrusion die, is more effective in that it is possible to minimize the length of the temperature transition zone.

[42] In accordance with the present invention, in the process (d) for pultruding the mi- crocellular foam at a predetermined foam induction ratio by the pultrusion unit, to finely control the foaming ratio and time-dependent physical properties of the mi- crocellular foam, it is possible to finely adjust the foam induction ratio by adjusting the linear velocity of the pultrusion unit in the unit of a fine range. Here, the "fine range" means a range corresponding to the unit of 10 (for example, the unit of 1%) or less. Accordingly, when the foam induction ratio is appropriately set, it is possible to prepare a microcellular foam having a desired foaming ratio with an allowable error range.

[43] In a preferred embodiment, the foaming ratio can be adjusted within a control range of 0 to 80%. The allowable error range of the foaming ratio in the finally-prepared foam in comparison with a desired foaming ratio may be ±0.5% or less.

[44] Thus, it is possible to accurately adjust the foaming ratio within a very wide foaming ratio adjustment range. Accordingly, the present invention can be widely used in fields in which it is necessary to finely and uniformly control the foaming ratio of a foam, for adjustment of the physical properties of the foam.

[45] Preferably, the foaming ratio variation occurring with the lapse of time is 0.1 to

1.0%. Thus, the foam prepared in accordance with the method of the present invention exhibits no or little foaming ratio variation occurring with the lapse of time. Accordingly, it is possible to maintain the cross-sectional shape of the extruded product in spite of the lapse of time. Since the foam has an uniform foaming ratio, it has very uniform physical properties. [46] There is no specific limitation on the thermoplastic polymer resin, as long as the thermoplastic polymer resin is extradable. For the thermoplastic polymer resin, polyethylene, acrylic resin, vinyl chloride resin, vinyl acetate resin, vinyl acetyl resin, polyamide resin, or celluloid resin may be used. Preferably, the thermoplastic polymer resin includes one or more polymers selected from the group consisting of acryl- butadiene-styrene (ABS) copolymer, polycarbonate (PC), polyvinyl chloride (PVC), polystyrene (PS), polymethylmethacrylate (PMMA), polyester, polypropylene, and nylon.

[47] There is no specific limitation on the foaming agent. For example, an inert gas may be used for the foaming agent. Preferably, carbon dioxide gas, nitrogen, or a mixture thereof may be used for the foaming agent. Preferably, the foaming agent is mixed in an amount of 3 to 0.1 weight % with 97 to 99.9 weight % of the thermoplastic resin. When the amount of the foaming agent is less than 0.1 weight %, foaming is insufficiently generated in the pressure drop zone. In this case, accordingly, it is impossible to form micro pores. On the other hand, when the amount of the foaming agent is more than 3 weight %, there is a residual foaming agent not dissolved in the thermoplastic resin.

[48] In particular, it is desirable for the foaming agent to be mixed with the thermoplastic resin in a super-critical state. When the foaming agent is mixed with a polymer resin in a super-critical state, it exhibits an increased miscibility to the polymer resin. In this case, accordingly, it is possible to form uniform pores in the resin, to reduce the size of the pores, and to increase the density of the pores. For the foaming agent, a foaming agent, which has already been in a super-critical state, may be used. Alternatively, the foaming agent may be changed into a super-critical state after being charged into the extruder.

[49] The present invention also provides an extruding and foaming system including an extruder for plasticizing a mixture of a thermoplastic polymer resin and a foaming agent as described above, to extrude a foam, a pultrusion unit for pultruding the extruded foam at a predetermined speed, and a cutter for cutting the extruded foam into pieces having a certain length, wherein the extruder includes an extrusion die having a pressure drop zone and a cooling zone contiguous to the pressure drop zone, the temperature at a downstream end of the pressure drop zone and the temperature at an upstream end of the cooling zone are set such that there is a temperature difference of 30 to 200⁰C between the temperatures, to enable the extrusion die to extrude a mi- crocellular foam, and the pultrusion unit has a pultrusion speed set in accordance with a foam induction ratio, to finely control the foaming ratio and time-dependent physical properties of the microcellular foam.

[50] In accordance with the above-described configuration, the extruding and foaming system according to the present invention can cool and solidify a foam while simultaneously extruding the foam. Accordingly, it is possible to achieve an enhancement in extrusion rate. It is also possible to dispense with a calibration unit or to use only a calibration unit with a minimum processing length, and thus, to minimize the spatial waste of the system. In addition, a reduction in manufacturing costs and a great enhancement in production efficiency can be achieved.

[51] In the extruding and foaming system according to the present invention, the temperature at the downstream end of the pressure drop zone and the temperature at the upstream end of the cooling zone are set such that there is a temperature difference of 30 to 200⁰C between the temperatures, as described above. Due to such an abrupt temperature difference, the skin of the extruded product can be stably solidified in a dense state. As a result, there is no deformation of the extruded product even when there is an unsolidified portion at the central portion of the extruded product. Accordingly, it is unnecessary for the extruded product to pass through a calibration unit.

[52] In a preferred embodiment, a calibration unit having a short processing length may be selectively arranged between the extruder and the pultrusion unit.

[53] That is, when it is necessary to additionally cool and solidify the extruded product solidified in the cooling zone, it is possible to selectively configure the system such that the extruded product passes through a calibration unit. The calibration unit is configured to have a relatively short processing length. For example, the calibration unit may have a processing length of 3 m or less that is shorter than the processing length of a conventional calibration unit, namely, 3 to 6 m. Preferably, the calibration unit has a processing length of 2 m or less.

[54] The present invention also provides a microcellular thermoplastic resin foam prepared in accordance with the above-described method.

[55] A preferred example of the microcellular foam according to the present invention may have a structure including a skin layer thicker than that of a general microcellular foam, and a core layer formed with micro pores, as disclosed in Korean Patent Application No. 2005-115637. The content of this application is incorporated in the specification, for reference.

[56] Accordingly, the microcellular foam according to the present invention can be used for interior and exterior materials for construction or optical reflection plates for display devices. In particular, the microcellular foam is suitable for interior and exterior materials for construction. More particularly, the microcellular foam is suitable for interior and exterior materials for construction such as soundproof materials, insulating materials, construction materials, lightweight structural materials, packaging materials, electrical insulating materials, cushioning materials, or vibration- isolating materials. [57] Hereinafter, the present invention will be described in more detail with reference to the drawings. Of course, the present invention is not limited to the following description.

[58] FIG. 3 is a schematic view illustrating an extruding and foaming system carrying out an extruding and foaming process for preparation of a microcellular foam according to according to an embodiment of the present invention. FIG. 4 illustrates an extrusion die included in the extruding and foaming system according to the embodiment of the present invention and a temperature variation occurring in the extrusion die, through an enlarged sectional view and a graph.

[59] As shown in FIGS. 3 and 4, in accordance with the method of the present invention, an extrusion process is executed through an extruding system in which an extruder 100, an extrusion die 200, a pultrusion unit 400, and a cutter 500 arranged in this order in an advancing direction of an extruded product.

[60] In detail, a thermoplastic resin is mixed with a foaming agent by an agitator 110, and the resultant mixture is then plasticized while passing through the extruder 100. The resultant plasticized material is extruded in a melted state. The extruded product is fed to the extrusion die 200, so as to have a certain shape. When the extruded product passes through a pressure drop zone 205 of the extrusion die 200 mounted with an inner heater 220 via a nozzle 210, micro pores are formed in the extruded product. The extruded product is discharged out of the extrusion die 200 after being solidified while sequentially passing through a temperature transition zone 270 and a cooling zone 260, which are included in the extrusion die 200. The temperature transition zone 270 has an abrupt temperature transition rate. The cooling zone 260 is internally mounted with a cooler 230. After passing through the pultrusion unit 400 at a certain pultrusion speed, the extruded product is cut into a desired length by the cutter 500. Thus, desired extruded products are prepared. The pultrusion speed of the pultrusion unit 400 is set in accordance with a desired foam induction ratio.

[61] Since the extruded product emerges from the extrusion die 200 in a cooled and solidified state, and the temperature at the downstream end of the pressure drop zone and the temperature at the upstream end of the cooling zone are set such that there is a temperature difference of 30 to 200⁰C between the temperatures, a foam having a dense skin structure is formed in accordance with an abrupt temperature transition rate as the extruded product passes through the temperature transition zone 270. Accordingly, even when the extruded product is exposed to atmospheric pressure, there is no expansion in a width direction at the extruded product. As a result, the extruding system can be controlled to have a desired foam induction ratio in accordance with adjustment of the pultrusion speed of the pultrusion unit 400. Thus, it is possible to finely adjust the foaming ratio of the foam, and to uniformly control the foaming ratio of the foam such that the foam has a desired foaming ratio in spite of the lapse of time.

It is also possible to prepare a foam having a desired cross-sectional shape in spite of the lapse of time. [62] When it is necessary to additionally cool and solidify the extruded product, a calibration unit 300 may be arranged between the extrusion die 200 and the pultrusion unit 400. In this case, the extruded product emerging from the cooling zone 260 passes through the calibration unit 300. The calibration unit 300 preferably has a short processing length of about 2 m or less. [63] Accordingly, it is possible to avoid a spatial waste of the extruding system and to reduce the manufacturing costs. Since the processing length is reduced, the extrusion speed increases. As a result, the productivity can be highly enhanced. [64] Meanwhile, a coolant circulator 310 is connected to the cooling zone 260 of the extrusion die 200 and/or the calibration unit 300. Accordingly, in accordance with circulation of a coolant, the extruded product can be cooled and solidified.

Brief Description of the Drawings [65] The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: [66] FIG. 1 is a schematic view illustrating an extruding and foaming system carrying out a general extruding and foaming process for preparation of a microcellular foam; [67] FIG. 2 illustrates a conventional extrusion die including only a pressure drop zone and a temperature variation occurring in the extrusion die, through an enlarged sectional view and a graph; [68] FIG. 3 is a schematic view illustrating an extruding and foaming system carrying out an extruding and foaming process for preparation of a microcellular foam according to according to an embodiment of the present invention; and [69] FIG. 4 illustrates an extrusion die including a pressure drop zone, a temperature transition zone, and a cooling zone in accordance with the present invention and a temperature variation occurring in the extrusion die, through an enlarged sectional view and a graph.

Mode for the Invention

[70] Hereinafter, the present invention will be described through examples only for illustrative purpose, but the scope of the present invention is not limited to those examples. [71]

[72] [Examples 1 to 4]

[73] An extrusion apparatus for extrusion-molding a thermoplastic resin was prepared by mounting an extrusion die having an integral structure constituted by a temperature- controllable high-temperature die as a foaming zone, a temperature transition zone, and a low-temperature die as a cooling zone to a bi-axial extruder, together with an adapter. In this case, the high-temperature die had a length of 125 mm, the temperature transition zone had a length of 27 mm, and the low-temperature die had a length of 40 mm.

[74] 98 weight parts of a hard polyvinyl chloride (PVC) compound (manufactured by LG Chemical Co., Ltd.) was supplied to the extruder, to completely plasticize the PVC. Thereafter, 2 weight parts of nitrogen was supplied to a barrel of the extruder, using a high-pressure pump. Thus, a single-phase mixture was molded to manufacture a PVC sheet having a thickness of 2 mm and a width of 100 mm.

[75] The conditions of the extruder were set to vary the temperature of the barrel in the order of 19O⁰C - 18O⁰C - 175⁰C. Also, the adapter was maintained at a temperature of 13O⁰C. The conditions of the high-temperature die, temperature transition zone, and low-temperature die are described in the following Table 1.

[76] [77] [Comparative Example 1] [78] A PVC sheet was manufactured in the same manner as Example 1, except that a conventional extrusion die, which only includes a high-temperature die without including an extrusion die having an integral structure constituted by a temperature-controllable high-temperature die as a foaming zone, a temperature transition zone, and a low- temperature die as a cooling zone, and a cooling process was carried out using a calibration unit having a length of about 4m. The conditions of the high-temperature die are described in the following Table 1.

[79] [80] [Experimental Example 1] [81] The extrusion speed in the PVC sheet extruding process was measured for the above- described Examples 1 to 4 and Comparative Example 1. The results of the measurement are described in the Table 1.

[82] <Table 1> [83]

[84] The foaming ratio of the PVC sheet extruding process exhibited with the lapse of time was measured for the above-described Examples 1 to 4 and Comparative Example 1. The results of the measurement are described in the following Table 2.

[85] <Table 2> [86]

[87] Referring to Table 2, it can be seen that the error between the foam induction ratio and the actual foaming ratio in each PVC sheet of Examples 1 to 4 is considerably lower than that of Comparative Example 1. It can also be seen that the foaming ratio variation exhibited with the lapse of time in each PVC sheet of Examples 1 to 4 having a foam induction ratio of 0.0 to 73.2% is considerably lower than that of Comparative Example 1 having a foam induction ratio of 21.1%.

[88] Thus, it can be seen that it is possible to stably prepare an extruded foam having a desired foaming ratio within a wide foaming ratio range in accordance with the present invention.

[89] Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.

Industrial Applicability

[90] As apparent from the above description, in accordance with the method of the present invention, the extrusion die is configured to have a predetermined processing condition such that a microcellular foam is extruded, and the extruded microcellular foam is pultruded at a predetermined foam induction ratio by the pultrusion unit. Accordingly, it is possible to finely adjust the foaming ratio of the microcellular foam and the physical properties of the microcellular foam exhibited with the lapse of time, and to maintain a desired cross-sectional shape of the microcellular foam in spite of the lapse of time. Since the system of the present invention may selectively include a calibration unit having a short processing length, it is possible to avoid a spatial waste of the extruding system and to reduce the manufacturing costs. As a result, the productivity can be highly enhanced.

Claims

[1] A method for preparing a microcellular foam, comprising mixing a thermoplastic polymer resin plasticized in an extruder with a foaming agent, melting the mixture and forming micro pores in the melted mixture while passing the mixture through a pressure drop zone of an extrusion die, and cooling the melted mixture formed with the micro pores while passing the melted mixture through a cooling zone of the extrusion die, further comprising: setting a temperature at a downstream end of the pressure drop zone and a temperature at an upstream end of the cooling zone such that there is a temperature difference of 30 to 200⁰C between the temperatures, to extrude the mixture in the form of a microcellular foam; and pultruding the microcellular foam emerging from the extrusion die at a predetermined foam induction ratio by a pultrusion unit, to finely control a foaming ratio and time-dependent physical properties of the microcellular foam.

[2] The method according to claim 1, wherein the extrusion die includes a heater for preventing a temperature decrease in the vicinity of the downstream end of the pressure drop zone.

[3] The method according to claim 1, wherein the extrusion die includes a cooler for preventing a temperature increase in the vicinity of the upstream end of the extrusion die.

[4] The method according to claim 1, wherein the temperature at the downstream end of the pressure drop zone is 150 to 25O⁰C.

[5] The method according to claim 1, wherein the temperature at the upstream end of the cooling zone is 40 to 15O⁰C.

[6] The method according to claim 1, wherein each of the pressure drop zone and the cooling zone exhibits a temperature variation ranging within +5⁰C.

[7] The method according to claim 1, wherein the melted mixture formed with the micro pores transfers at a transfer speed of 0.5 to 20 m/min.

[8] The method according to claim 1, wherein the extrusion die includes a temperature transition zone present between the pressure drop zone and the cooling zone, and the temperature transition zone exhibits a temperature transition rate of 2 to 40°C/mm in a process advancing direction, the temperature transition rate being calculated based on the following Expression (1): T = (T - T )/L (1)

L H C in which, "T " represents the temperature transition rate, "T " represents the temperature at the downstream end of the pressure drop zone, "T " represents the temperature at the upstream of the cooling zone, and "L" represents a length of the temperature transition zone.

[9] The method according to claim 8, wherein the length of the temperature transition zone is 1 to 150 mm.

[10] The method according to claim 1, wherein the control range of the foaming ratio in the step of controlling the foaming ratio is within a range of 0 to 80%.

[11] The method according to claim 1, wherein the foaming ratio in the step of controlling the foaming ratio is controlled to have a foaming ratio error range of ±0.5% or less.

[12] The method according to claim 1, wherein the foaming ratio of the microcellular foam varies with the lapse of time within a range of 0.1 to 1.0%.

[13] The method according to claim 1, wherein the thermoplastic polymer resin comprises one or more polymers selected from the group consisting of acryl- butadiene-styrene (ABS) copolymer, polycarbonate (PC), polyvinyl chloride (PVC), polystyrene (PS), polymethylmethacrylate (PMMA), polyester, polypropylene, and nylon.

[14] An extruding and foaming system comprising an extruder for plasticizing a mixture of a thermoplastic polymer resin and a foaming agent, to extrude a foam, a pultrusion unit for pultruding the extruded foam at a predetermined speed, and a cutter for cutting the extruded foam into pieces having a certain length, wherein: the extruder includes an extrusion die having a pressure drop zone and a cooling zone contiguous to the pressure drop zone, and a temperature at a downstream end of the pressure drop zone and a temperature at an upstream end of the cooling zone are set such that there is a temperature difference of 30 to 200⁰C between the temperatures, to enable the extrusion die to extrude a microcellular foam; and the pultrusion unit has a pultrusion speed set in accordance with a foam induction ratio, to finely control a foaming ratio and time-dependent physical properties of the microcellular foam.

[15] The extruding and foaming system according to claim 14, further comprising: a calibration unit arranged between the extruder and the pultrusion unit, the calibrating unit having a short processing length.

[16] The extruding and foaming system according to claim 15, wherein the processing length of the calibration unit is 2 m or less.

[17] A microcellular thermoplastic resin foam prepared by a method according to any one of claims 1 to 13.