PROCESS OF PRODUCING THERMOPLASTIC RESIN SHEET AND THE LIKE AND MOLDING SCREW THEREFOR

Abstract: This invention relates to a process of producing sheet and the like from thermoplastic resin and a screw used therefor. The resin extruder (1) includes a cylinder (3) and the screw (4). The resin is passed through a feeding section (11), a compressing section (12), a metering section (13), a mixing section (14), a shearing section (15) and a stress relaxing section (16), which are successively positioned from a resin feeding side to a resin extruding side, and a molten film-shaped resin is extruded, and this film-shaped resin is passed in the flowing direction of cooling water to be cooled.

Description: The present invention is related to an extruder screw for molding a thermoplastic resin of the type as indicated in the pre-characterizing portion of claim 1. An extruder screw of this type is known from EP-A2-186 861. In particular, said prior art reference discloses a helical screw which is provided with a resin feeding-melting section, a resin flow disturbing section and a transfer section for relaxing the resin stress. Crystalline thermoplastic resin such as polypropylene and polyethylene has been utilized in the various packaging fields and the like as sheets and containers due to its easiness in thermoforming process. Now, in producing the crystalline thermoplastic resin sheets represented by the polypropylene and the like as described above, to improve the transparency of these sheets and the like, there has been practiced that the molten resin sheet is quenched to control the crystalline state. As a quenching process of this type, water cooling is the most efficient process. However, this process has such a problem that it is difficult to cool uniformly a molten film-shaped resin extruded from an extrusion die. Then, the present applicant has proposed a process of using a multi-stage slit, through which cooling water flops down, as she process of uniform water-cooling the molten film-shaped resin (Japanese Patent Kokai (Laid-Open) No. 58-203318). However, even if the cooling conditions and the like are controlled, the cooling effect thereof is naturally limited, whereby the transparency, surface uniformity and the like of the resin sheets and the like as the molded items have not always been satisfactory. Then, as the result of the various studies carried out by the inventors of the present invention, it was found that conditions of extrusion give a great influence to a stage of the molten film-shaped resin, where the molten film-shaped resin extruded is not yet coming into contact with the cooling water, i.e. at a stage covering from feeding the material to a resin extruder to extruding of the material, and a process, in which these extruding conditions are combined with a slit water cooling method in use, was proposed (Patent Kokai (Laid-Open) No. 62-146611 and Patent Application No. 61-202146). Now, a construction of a screw is listed as one of important factors among the above-described extruding conditions. One of the commonest screws is shown in Fig. 1 for example. This screw, like a full-flighted metering screw, having a feeding section 51, a compressing section 52 and a metering section 53, which are positioned successively from a side of feeding resin to a side of extruding resin, has as its main purpose the increase in a compression ratio for satisfactory kneading of molten resin. Accordingly, although kneading of resin can be carried out relatively well, several problems have been presented because the compression ratio is high to thereby be accompanied with high shearing. More specifically, the pressure deviation at the screw tip is remarkable at the time of high speed extrusion, so that molded items of high quality, excellent in accuracy of thickness and the like cannot be obtained stably. Furthermore, heat is generated due to the high shear rate, whereby the resin temperature is raised to a high temperature beyond need, thus presenting such disadvantages that deteriorated resin leads to lowered physical properties of the molded items and take-up and sizing in the extrusion molding become difficult. Because of this, there have been proposed a screw having a construction wherein the compression ratio is made relatively small and set at 1 as the case may be to thereby control occurrence of shearing heat, another construction of a screw of two-stage extrusion, wherein a shearing section is provided at the intermediate portion of the screw, and the like. However, in either of these cases, kneading becomes unsatisfactory, with the result that ununiformity in resin temperature and dispersion failure of various additives took place, whereby it was difficult to obtain satisfactory molded items and the scope of applications was extremely limited. Particularly, with the screw of the two-stage extrusion, a shearing beyond need occurs at the intermediate shearing portion, and such problems are encountered that the resin is deteriorated due to raised resin temperature and difficulties in cooling and sizing due to the extrusion of the resin heated to high temperature. Now, in order to solve these problems, the present applicant has proposed an extruder wherein the compressing section of the screw has a relatively low compression ratio, i.e. a moderate compressing section is provided (Patent Kokai (Laid-Open) No. 61-144322). A screw wherein the above-described one is further improved is proposed (Patent Kokai (Laid-Open) Nos. 62-41015 and 62-4101687). However, in the screw provided at the intermediate portion thereof with the shearing section, in order to control the shearing heat at this shearing section to a low temperature, it becomes necessary to make the lengths of the feeding section and the compressing section satisfactorily long, which is not preferable. Even if these sections are lengthened, kneading in the shearing section is not satisfactorily carried out in the case of extrusion molding at high speed. As the case may be, a large quantity of unmolten resin flows into the shearing section, whereby fluctuations in an extrusion flow rate may take place and satisfactory molded items cannot be obtained, so that the extrusion molding cannot be carried out with high productivity, at high speed and stably. An object of the present invention is to provide a screw for extrusion molding of thermoplastic resin, wherein satisfactorily uniform kneading properties, high speed and stable extruding properties and low temperature extruding properties, which have heretofore been considered difficult to make compatible, are made possible. To achieve the above-described object, the present invention contemplates in a screw for molding thermoplastic resin, wherein a first extruding section is provided which has a feeding section, a compressing section and a metering section, which are positioned successively from a resin feeding side to a resin extruding side, and a mixing section, a shearing section and a stress relaxing section are successively provided from the first extruding section to the resin extruding side, whereby satisfactory kneading is carried out while the shear heatis controlled, and stable extrusion at high speed is made possible. In the extruder used for the process according to the present invention, the aforesaid sections are constituted by a cylinder and a screw, and the screw for the extrusion is one as being a preferred single screw for use in the above-mentioned process. Further embodiments of the invention are defined in the features of the dependent claims. Fig. 1 is a side view showing the conventional common screw; Fig. 2 is a sectional side view showing a resin extruder applied to the process of producing thermoplastic resin sheets and the like according to the present invention; Figs. 3 and 4 are sectional views showing examples of modifications of the stress relaxing section of the aforesaid extruder; and Fig. 5 is a sectional view showing a slit cooling device to which to the present invention is applied. One embodiment of the present invention will hereunder be described with reference to the accompanying drawings. First, what fall under the thermoplastic resins used in the present invention are polypropylene group resins such for example as a homo polymer of propylene, a random copolymer of propylene with another α-olefin of less than 20 weight%, and a blend of these. A blend obtainable by two-stage polymerization is included in this blend. Other thermoplastic resins used in the present invention include crystalline resins such as high density polyethylene, low density polyethylene, linear low density polyethylene, polyester, polyamide, polyimide, polystyrene, polycarbonate, polyvinyl chloride, polyacrylonitrile, polyvinylidene chloride, polyphenylene oxide, polysulfone, polyphenylene sulfide and polyketone. Particularly, polypropylene group resins are used which are difficult to be molded into a sheet excellent in transparency. Further, the resin analogous to the above-described resins such for example adherent resin such as graft-polyolefin maleic anhydride, easily welding resin such as a copolymer of ethylene-vinyl acetate, and gas barrier resins such as a copolymer of ethylene-vinyl alcohol, polyamide and polyethylene terephthalate, these polymer being applied to a malti-layer. In the case of the co-extrusion multi-layer sheet, with polypropylene for example, the extruder according to the present invention is used for extruding a main component (normally, an outer layer resin) of the multi-layer sheet. Fig. 2 shows an extruder 1 used in this embodiment. This extruder 1 includes therein a cylinder 3 provided with a hopper 2 for feeding resin pellets and a screw 4 installed in this cylinder 3. In the screw 4, there are provided a first extruding section provided with a feeding section 11, a compressing section 12 and a metering section 13, which are positioned successively from a resin feeding side to a resin extruding side, and a mixing section 14, a shearing section 15 and a stress relaxing section 16, which are positioned rearwardly of the first extruding section. In the feeding section 11, the resin pellets fed through the hopper 2 are delivered, while being heated, to the compressing section 12 by a propulsive force of the screw. Normally, the feeding section 11 is the one having a given channel depth of a compression ratio 1. Incidentally, grooves may be formed on the inner wall of the cylinder under the hopper throat. With this arrangement, a resin extrusion flow rate can be increased. In the compressing section 12, normally, the channel depth of the screw is made progressively smaller, whereby the resin is compressed while being fused and plasticated, and the compression ratio is 2 - 4 and preferably 2 - 3. If the compression ratio exceeds 4, then, the shearing stress in the terminating end portion of the compressing section 12 is increased, whereby raised resin temperature, break-up phenomenon and the like take place unfavorably. Next, the metering section 13 normally has a given channel depth and, when fusion is incomplete in the compressing section, fusion and plastication are facilitated therein. The metering section according to the present invention is relatively larger in the channel depth than an ordinary one, the depth being 3 - 15mm. In the mixing section 14, the plasticated resin delivered from the metering section 13 is mixed, and the mixing section 14 has functions of dividing and breaking up an unmolten resin layer (solid bed) in the case of extrusion molding at high speed in particular. Here, the types of the mixing section 14 are not necessarily limited, and include a gear type, a dulmage screw, a reversing screw with cuts, a multi-pin type and the like. The mixing section 14 has the main functions of dividing and breaking up the unmolten resin layer. In the mixing section 14, the kneading action due to high shearing is relatively low and abnormal heating and the like of the resin due to the shearing force do not occur. Especially, the gear type mixing section has such many advantageous characteristic features that the construction is simplified, working can be easily made, the solid bed can be reliably divided and broken up and the shearing action is low. Incidentally, as the type of the gear type mixing section, a spur gear is satisfactory, and, there is no particular limit on the type and any type such as a helical gear may be adopted only if the resin is divided when the resin is passed through the mixing section 14. Examples of specific numerical values in the case where the spur gear type mixing section 14 is adopted will hereunder be described. The number of crests of the gear, the thickness of the gear, a clearance between the crest and the cylinder 3 and the like are different depending upon the type of the thermoplastic resin, molding conditions, the size of the screw (diameter=D) and the like. However, generally, the clearance between the crest of the gear and the cylinder 3 is set to 0.1 - 3mm, and preferably 0.2 - 2.5mm. It is disadvantageous that, when the clearance is less than 0.1mm, the shearing action becomes high, and, when the clearance exceeds 3mm, the dividing and break-up actions become disadvantageously weak. As for the thickness of the gear, it is sufficient only if the thickness stays within a scope of (0.1 - 1)D and the thickness is less than 0.5D from the viewpoint of the dividing action. Furthermore, the number of the gears may be plural. The resin mixed in the mixing section 14 is passed through a pressure releasing section 17 of a cylindrical shape provided between the mixing section 14 and the shearing section 15 as necessary, and delivered to the shearing section 15 has functions of uniformly fusing the molten resin and making the resin temperature uniform by shearing for a relatively short period of time. Here, as the shearing section 15, a torpedo, a sluice ring, a Maddok, a Torester and the like are adopted. There is no particular limit imposed on adoption of these members, only if obstruction in the resin flow takes place due to the a small clearance between the shearing portion 15 and the cylinder 3, whereby the shearing becomes high. However, in general, the torpedo is favorably used from the viewpoints of simplified construction and good workability. Here, as for the torpedo, the length thereof may be 0.05 - 2 times the diameter (D) of the screw and preferably 0.1 - 1 time. The clearance between the torpedo and the cylinder 3 may be 0.3 - 6mm and preferably 0.5 - 4mm. However, as these values, the most suitable ones may be selected in accordance with the combination of the size of screw, the length of torpedo and the clearance and the type of resin, etc. These numerical values need not necessarily be adhered to. The pressure releasing section 17, in addition to its function as a connection in the construction between the mixing section 14 and the shearing section 15, has a function of facilitating the passage of resin through the shearing section because, if the resin passed through the mixing section 14 is immediately delivered to the shearing section 15, then, the passage of resin through the shearing section 15 tends to be difficult. As described above in detail, the extruder 1 according to the present invention features that the mixing section 14 of low shearing is especially provided before the shearing section 15, whereby the solid bed is divided and broken up, so that the unmolten resin of a large quantity can be prevented from being transferred to the shearing section 15. Accordingly, the lengths of the feeding section 11, compressing section 12 and metering section 13 may be optionally determined in consideration of the type of resin and the like. However, according to the present invention, the provision of the mixing section 14 makes it possible to carry out the high speed molding with stability in extrusion without making the length of the screw to the metering section 13 relatively long. Subsequently, the resin uniformly kneaded in the shearing section 15 is extruded into the stress relaxing section 16. In this case, the shearing stress due to the shearing remains, whereby, if the resin is extrusion-molded from a die as it is, then the surface characteristics of the resin is lowered due to a sharp stress relieving at the die outlet, so that sheets and the like excellent in the transparency, glossiness and the like cannot be produced. Accordingly, in the extruder 1 according to the present invention, the resin passed through the shearing section 15 is extruded from the die after the remaining shearing stress is relaxed by the stress relaxing section 16 continuous to the shearing section 15. In this case, as the stress relaxing section 16, any extruding screw commonly used may be adopted. Most commonly, a screw having the metering effect is used. In an illustrated example, a releasing section 18 having a deep channel is formed at a position close to the shearing section 15, and a rear stage of this releasing section has a given channel depth which is smaller in depth than in the former stage. The compression ratio of the screw in the region of the stress relaxing section 16 may be 3 - 0.5 and preferably 2.5 - 0.8. Particularly, to produce the sheets satisfactory in transparency from the crystalline resin such as polypropylene by quenching, it is preferable that a moderate compression type screw having a compression ratio of less than 1.5 is used to relax the remaining stress in the molten resin. More specifically, in the common cases, new kneading is not necessary in this stress relaxing section 16, however, in order to disperse and make uniform an additive, a pigment agent and the like, a mixing section may be provided at the forward end portion of the stress relaxing section 16. In this stress relaxing section 16, metering of the molten resin (adjusting the discharge flow rate) is carried out, the shearing stress remaining in the molten resin is relaxed, and further, making the cooling of the molten resin uniform is performed. The length of this stress relaxing section 16 differs depending on the construction, however, a length for satisfactorily achieving the relaxing, i.e. 4 - 20 times the diameter D of the screw is normally needed. The main purpose of this stress relaxing section 16 is to remove the remaining shearing stress and the stress relaxing section 16 has a construction in which high kneading and high shearing do not take place. Additionally, the construction of a portion from the stress relaxing section 16 to the die may be the one shown in Figs. 3 and 4 for example. The stress relaxing section shown in Fig. 3 is formed by a combination of the screw 4 and a non-screw portion 20 consisting of the cylinder 3 only. Further, as shown in Fig. 4, the provision of a gear pump 21 after the stress relaxing section or in the intermediate portion makes it possible to control the extrusion to a constant flow rate. In this specification, description has been given to the most common single thread screw and constant pitch screw. However, a multiple-flighted screw, a partial multiple flighted screw, a variable pitch screw and the like may be adopted, and, as far as the effects of the present invention are retained, there is no limit imposed on these types of screws. The compression ratio means "(the channel depth at the resin flow-in end)/(the channel depth at the resin extruding end)" when the pitch of the screw is constant in each of the sections. In the extruder 1, the ratio L/D between the total length L and the diameter D of the screw 4 is set at 15 - 40 and preferably 20 - 34. A groove in the axial direction may be formed on the inner wall of the cylinder 3 of the feeding section 11 (particularly, on a lines portion), so that the resin pellets can be smoothly fed. In the above-described extruder 1, the resin fed into the cylinder 3 is passed through the feeding section 11, compressing section 12, metering section 13, mixing section 14, shearing section 15 and stress relaxing section 16, and extruded from the extrusion die. Incidentally, in the extruder 1 according to the present invention, it is more preferable to provide pressure releasing sections 17 and 18 on the respective resin extruding sides of the mixing section 14 and the shearing section 15. Furthermore, in the extruder 1 according to the present invention, as the shearing section 15 thereof, instead of the torpedo provided on the side of the screw 4, a ring valve combined with the cylinder wall, i.e. a so-called shearing force adjustable ring valve may be adopted. In this case, the resin pellets supplied are delivered under pressure to the compressing section 12, while being heated in the feeding section 11. In the compressing section 12, the resin pellets are fused and plasticated, and delivered under pressure to the metering section 13. In the metering section 13, fusion and plastication of the resin are facilitated, in the mixing section 14, the unmolten resin layer (solid bed) is divided and broken up, the unmolten resin of a large quantity (30% or more) is prevented from being transferred to the shearing section 15, the resin is melted further uniformly in the shearing section 15, and the resin temperature is made uniform. Subsequently, the molten resin thus made uniform is delivered under pressure to the stress relaxing section 16, where the remaining shearing stress in the resin is relaxed and the resin is cooled and made uniform, and thereafter, extruded from the die as a molten film-shaped resin. Fig. 5 shows a slit cooling device for cooling the molten film-shaped resin extruded from the extruder 1. This slit cooling device corresponds to the device of the aforesaid proposed (Patent Application Kokai (laid-open) No. 62-146611) and is positioned downwardly of a die 30 provided with the extruder 1 for realizing the aforesaid extrusion. This slit cooling device includes: a top stage water tank 33 having a slit 31, through which the cooling water flows down, and porous flow regulating members 32 for regulating the flow of cooling water; a bottom stage water tank 36 positioned at a stage downwardly of the top stage water tank 33 and having a slit 35 positioned immediately beneath the slit 31; a water tank 37 positioned further downwardly of this bottom stage water tank 36; clamping rolls 39 and 40 for clamping the extruded molten film-shaped resin 38 under pressure to maintain the running speed of the molten film-shaped resin to a constant value; rolls 41 and 42 for taking up the film-shaped resin 48 passed through the water tank 37; a group of rolls 43 for heat-treating the film-shaped resin 48 in a predetermined manner; and a pump 45 for circulating the cooling water, which has flowed down through the water tank 37 to be cooled, through the top stage water tank 33 and the bottom stage water tank 36. In the slit cooling device, the molten film-shaped resin 38 extruded from the extrusion die 30 is introduced into the slits 31 and 35 of the top state water tank 33 and the bottom stage water tank 36, through which the cooling water flows down, passed through the cooling water in the water tank 37 through the clamping rolls 39 and 40, taken up by the tank-up rolls 41 and 42, and thereafter, heat-treated in the predetermined manner by the group of rolls 43 as necessary, to thereby complete the molding of the resin sheet. In this case, the flow-down speed of the cooling water from the slits 31 and 35 is made larger than the running speed of the film-shaped resin 48, whereby the quantity of the cooling water being in contact with the film-shaped resin 48 is increased, so that the cooling effect can be improved. Incidentally, it is preferable that the cooling water should be prevented from stagnating at an inlet of the slit 31 when the cooling water flows into the top stage slit 31. For this purpose, the water level in the top stage water tank 33 is needed to be held low. Here, if the stagnation occurs, then, ununiform cooling of the molten resin film-shaped material 38 takes place due to the rise in the temperature of the cooling water, so that the sheets and the like being low in haze on the outer surface cannot be obtained. A specific example based on the process of producing the thermoplastic resin sheets and the like according to the present invention will hereunder be described. EXAMPLE The raw material resin was homopolypropylene (density: 0.91/cm², MI(Melt-Index): 8g/10min, melting point: 170°C, manufactured by Idemitsu Petrochemical Co., Ltd., trade name: Idemitsu Polypro F-700N). This resin was extruded from a T-die into a film shape by use of a extruder having the following screw under the conditions of resin temperature of 250°C and die lip temperature of 280°C. Subsequently, the molten film-shaped resin was introduced into the slit cooling device shown in Fig. 5 (the top stage slit 31 had a height of 50mm and a width of 2.5mm, the top stage water tank 33 had a water level of 5mm, the water temperature thereof was 5°C, the flow regulating members 32 were used which had four stages each consisting of doubled metal screens of 60mesh, the bottom stage slit 35 had a height of 10mm and a width of 5mm, the bottom stage water tank 36 had a water level of 10mm and the water temperature thereof was 5°C), where the molten film-shaped resin was quenched, and the film-shaped resin was molded by the clamping rolls 39 and 40, whereby a polypropylene sheet having a thickness of 0.2mm was obtained. Further, this sheet was heat-treated by use of the group of rolls 43 at 130°C. In the manufacture of this sheet, when the maximum extrusion flow rate whose wall thickness accuracy in the flowing direction of the obtained sheet remained within a range of ± 5% was searched, it was 116 kg/h at the screw revolution of 100r.p.m., and 93.1kg/h at 80r.p.m. The hazes of the obtained sheet were 4.1% and 3.7% at the respective rotary speeds described above, and the glossinesses were 124% and 129%, respectively. COMPARATIVE EXAMPLE 1 In the screw used in the Example, except that no mixing section was provided and the length of the metering section was set to 365mm, a sheet was molded in accordance with the Example. The maximum stable extrusion flow rate was 84.5kg/h at 80 r.p.m. COMPARATIVE EXAMPLE 2 In the screw used in the Example, except that no metering section was provided, the length of the feeding section was set to 500mm and the length of the compressing section was set to 690mm, a sheet was molded in accordance with the Example. The maximum stable extrusion flow rate was 79.1kg/h at 80r.p.m. From the above-described specific Example and Comparative Examples, it is understood that, in producing a polypropylene sheet excellent in the transparency, when the extruder according to the present invention is used, the sheet can be molded at high speed, and, in comparison at the same rotary speeds, the extrusion flow rate of the extruder according to the present invention is more than 10% higher than others and excellent from the viewpoint of the energy consumption. EXAMPLES 2, 3, & COMPARATIVE EXAMPLES 3, 4 By use of the screw having the shape shown in Table 1 (the diameter is 50mm, the ratio between the total length L and the diameter D is L/D=26) and a die having a diameter of 25mm or a T-die of 500mm × 1.5mm, polypropylene (trade name: Idemitsu Polypro F-200S, MI(Melt-Index): 2g/10min, manufactured by Idemitsu Petrochemical Co., Ltd.) was extrusion molded under the conditions of temperatures 250°C of the both cylinder and the die, and the molding properties and the extruded products were evaluated. The results are shown in Table 2. As apparent from Table 2, when comparison is made at the same r.p.m., the Example of the present invention is superior in deviations in pressure at the die portion and the resin temperature to others at the die outlet. Furthermore, in the Example, the stably extrusible extrusion flow rate is improved to a great extent. As has been described hereinabove, by adoption of the extruder capable of carrying out the optimal extrusion and the specific water cooling method, in the manufacture of the crystalline thermoplastic resin such as polypropylene, which has heretofore been difficult to manufacture, sheets excellent in transparency has been produced. Moreover, even if the extrusion flow rate is increased 30 - 40% higher than the conventional process, sheets being satisfactory in the sheet thickness accuracy in the flowing direction can be produced, thus offering the advantages of that the extruding power can be reduced, the productivity is improved and the sheets can be manufactured inexpensively. Moreover, improved kneading properties, low temperature extrusion molding and stable high-speed extrusion molding which have been considered incompatible in the past, can be achieved.

Claims: A screw (4) for molding a thermoplastic resin comprising: a first extruding stage including; a feeding section (11), a compression section (12), and a metering section (13) provided successively from where said thermoplastic resin is introduced into said screw, characterized in that said screw also has a mixing section (14) in which at least one spur gear is provided; and in that said screw (4) additionally has a shearing section (15) and a stress relaxing section (16) provided successively after said first extruding stage. A screw for molding thermoplastic resin as set forth in claim 1, wherein said shearing section (15) is of a torpedo type. A screw for molding thermoplasic resin as set forth in claim 2, wherein the compression ratio of said compressing section (12) is 2 - 4. A screw for molding thermoplastic resin as set forth in claim 3, wherein the compression ratio of said compressing section (12) is 2 - 3. A screw for molding thermoplastic resin as set forth in claim 1, wherein a cylindrical pressure releasing section (17) is interposed between said mixing section (14) and said shearing section (15). A screw for molding thermoplastic resin as set forth in claim 1, the compression ratio of said stress relaxing section (16) is 3 - 0.5. A screw for molding thermoplastic resin as set forth in claim 6, wherein the compression ratio of said stress relaxing section (16) is 2.5 - 0.8. A screw for molding thermoplastic resin as set forth in claim 1, wherein the length of said stress relaxing section (16) is 4 - 20 times the diameter of said screw (4).

Inventors: FUNAKI, AKIRA IDEMITSU PETROCHEMICAL CO. LTD., TAKUBO, TOYOKAZU IDEMITSU PETROCHEMICAL CO. LTD.
Applicants: IDEMITSU PETROCHEMICAL CO. LTD.
Application year: 1988
Main classification: B29C47
IPC codes: B29C47