METHOD FOR PRODUCING A TUBE OF THERMOPLASTIC RESIN HAVING POOR MELT FLOWABILITY AND APPARATUS THEREFOR

Abstract: This invention provides a flexible tube of thermoplastic resin having poor melt flowability, the tube having an intrinsic viscosity of at least 3.5 dl/g, a tensile strength at break of 150 kg/cm², a breaking time at hot internal pressure creep of at least 200 hours, an outer diameter of 50 mmø or less, and a ratio of the outer diameter to the wall thickness of 40 or less. This tube can be prepared from a thermoplastic resin having poor melt flowability by melting the thermoplastic resin having poor melt flowability in a screw extruder (1), and melt-extruding the molten thermoplastic resin from a tube die (7) having an L/D of at least 5 provided with a mandrel (5) rotatable with the rotation of the screw (3).

Description: The present invention relates to a method and apparatus for producing flexible tubes of thermoplastic resin having poor melt flowability, such as ultra-high-molecular-weight polyethylene and fluoroplastics. Fluoroplastics and ultra-high-molecular-weight polyethylene are widely used as so-called engineering plastics because of their excellent impact resistance, abrasion resistance, self-lubrication, chemical resistance, and other properties. However, these plastics have extremely high melt viscosity and poor flowability when compared with conventional resins, so that it is very difficult to mold them by conventional extrusion molding and injection molding. Accordingly, fluoroplastics and ultra-high-molecular-weight polyethylene are generally compression molded, but continuous lengths such as pipes and rods cannot be compression molded satisfactorily. A ram extruder provided with a mandrel has been proposed in, for example, Japanese Unexamined Patent Publication (Kokai) No. 55-28896 (equivalent to US-A-4547142), as an apparatus for continuously producing pipes of ultra-high-molecular-weight polyethylene. However, as is known in the art, ram extruders have disadvantages in that the molding rate and accuracy and uniformity of section are poor. Thus, good pipes cannot be obtained. Furthermore, the molding of flexible tubes having a small outer diameter and a thin wall thickness is impossible. Although the molding rate can be increased by using a screw type extruder, neither good pipes nor flexible tubes could be molded from ultra-high-molecular-weight polyethylene. Thus, since ultra-high-molecular-weight polyethylene has extremely high melt viscosity and poor flowability as mentioned above, adjustment is needed in the uniform section and the resin is not sufficiently fused when a pipe forming die for conventional resins is used. Furthermore, sizing (i.e., regulation of the outer or inner diameter) cannot be effected by a conventional vacuum former or inner pressure control, internal stress remains after quenching, and therefore the physical properties of the molded pipes are adversely affected. On the other hand, it has been proposed in, for example, Japanese Examined Patent Publication (Kokoku) Nos. 40-6993 and 43-16859 and Japanese Unexamined Patent Publication (Kokai) No. 51-137759, that a mandrel be rotated against a die in the production of thermoplastic resin tubes. However, when this method is applied to ultra-high-molecular-weight polyethylene, the desired flexible tube of ultra-high-molecular-weight polyethylene cannot be obtained since ultra-high-molecular-weight polyethylene has an extremely high melt viscosity and poor flowability, unlike conventional polyolefins. For example, with a die head having a flow path such that molten resin partially flows in a direction opposite to the extrusion direction, as disclosed in Japanese Examined Patent Publication No. 40-6993, extrusion molding is impossible. Furthermore, when the apparatus has a crosshead, as disclosed in Japanese Examined Patent Publication (Kokoku) No. 43-16859, the molten resin separated at the crosshead cannot be fused in the die at all. When the apparatus has a circular orifice provided between the screw tip and the die, as disclosed in Japanese Unexamined Patent Publication No. 51-137759, the back pressure becomes extremely high and, therefore, extrusion moldability is poor and the desired product cannot be obtained. Thus heretofore flexible tubes of thermoplastic resin having poor melt flowability (such as ultra-high-molecular-weight polyethylene or fluoroplastics) have not been obtainable by extrusion molding. French Patent No. 849367 shows a method for producing a tube comprising the steps of pushing plastics material through a screw extruder and melt extruding the resin from a tube die provided with a mandrel rotatable with the rotation of the screw. Previously it has been considered that the high viscosity and poor flow properties of thermoplastics such as ultra-high-molecular-weight polyethylene "do not permit processing of this material to pipe on single or twin screw extruders with conventional pipe extrusion dies" (US-A-4547142). In accordance with the present invention, there is provided a method for producing a tube from thermoplastic resin having poor melt flowability, characterised in that the tube has an intrinsic viscosity of at least 8 dl/g (0.8m³/kg) determined at 135°C in decalin, a tensile strength at break of 150 kg/cm² (14.7 MPa) or more determined at a temperature of 23°C according to JIS (Japanese Industrial Standards) K 6771 (soft vinyl tube), a breaking time at hot internal pressure creep of at least 200 hours determined at a temperature of 80°C according to JIS K 6762 under continuous inner pressure calculated from the following equation at a hoop stress of 40 Kg/cm² (3.9 MPa) wherein P : Pressure d : Outer diameter of tube (mm) t : Wall thickness of tube (mm) f : Hoop stress = 40 Kg/cm² (3.9 MPa) an outer diameter of 50mm or less, and a ratio of the outer diameter to the wall thickness of 40 or less, the method comprising the steps of: (i) melting the thermoplastic resin having poor melt flowability in a screw extruder provided with a mandrel rotatable with the rotation of the screw; (ii) rotating the screw of the extruder and the mandrel whilst maintaining the resin at a temperature above its melting point; and (iii) melt extruding the molten thermoplastic resin from a tube die having an L/D of at least 5, wherein the L/D of the tube die is defined by the ratio of the length between the inlet portion of the tube die and the outlet portion of the tube die to the inner diameter of the outer die at the tube die outlet. In accordance with the present invention, there is further provided an apparatus for producing a tube from thermoplastic resin having poor melt flowability using the method above, comprising: (i) an extruder provided with a grooved cylinder and a screw; and (ii) a tube die composed of (a) a mandrel connected to the screw tip in such a manner that the mandrel is rotatable with the rotation of the screw and (b) an outer die, characterised by the tube die having an L/D of at least 5, a ratio S₁/S₂ of 0.5 to 3.0, and a ratio S₂/S₃ of 1.0 to 3.0, S₁ being the crosssectional area of the inlet portion of the tube die, S₂ that of an intermediate portion of the tube die, and S₃ that of the resin flow path at the outlet portion of the tube die, and also characterised by the screw having a compression ratio of between 1 and 2.5. According to the present invention flexible tube can be produced from a thermoplastic resin having poor melt flowability. The thermoplastic resins having poor melt flowability are those which have extremely high melt viscosities when compared to conventional thermoplastic resins and which are difficult to mold by conventional extruders. Examples of such thermoplastic resins are ultra-high-molecular-weight polyethylene and fluoroplastics. The ultra-high-molecular-weight polyethylenes usable in the present invention are crystalline ethylene homopolymers or crystalline ethylene-alpha-olefin copolymers having a major content of ethylene (e.g., an ethylene content of 60 to 99 mol%) having an intrinsic viscosity [η] determined at 135°C in a decalin solvent, of 3.5 dl/g (0.35 m³/kg) or more, preferably 8 to 25 dl/g (0.8-2.5 m³/kg), and a melt flow rate (MFR), determined according to ASTM D-1238(F) method, of 0.01 g/10 min or less. Examples of the alpha-olefin comonomers are those having 3 to 18 carbon atoms such as propylene, 1-butene, 1-hexene, 1-octene, and 4-methyl-1-pentene. The fluoroplastics or fluorocarbon resins are fluorine containing resins such as tetrafluoroethylene resin, tetrafluoroethylene-hexafluoropropylene copolymer resin, and trifluorochloroethylene resin. According to the method of the present invention, flexible tubes of ultra-high-molecular-weight polyethylene are prepared from the above-mentioned ultra-high-molecular-weight polyethylene and they have an intrinsic viscosity [η], of 3.5 dl/g (0.35 m³/kg) or more, preferably 8 to 25 dl/g (0.8-2.5 m³/kg), determined at 135°C in decalin, a melt flow rate (MFR) of 0.01 g/10 min or less, determined according to ASTM D-1238(F) method, a tensile strength (TS) at break of 150 kg/cm² (14.7 MPa) or more, preferably 200 kg/cm² (19.6 MPa) or more, and more preferably 250 kg/cm² (24.5 MPa) or more, determined according to JIS (Japanese Industrial Standards) K 6771, a breaking time at hot internal pressure creep of 200 hours or more, preferably 500 hours or more, and mere preferably 1000 hours or more, determined at 80°C under hoop stress of 40 kg/cm² (3.9 MPa); such flexible tubes have an other diameter of 50 mm or less, preferably 1 to 20 mm , and a ratio of the outer diameter to the wall thickness of 40 or less, preferably 2.0 to 20. The tubes have an indefinite length. Continuously screw melt extruded flexible tubes having the above properties are the subject of European Divisional Application No. 88106886.0 (published as EP-A-0288091). When the intrinsic viscosity [η] is less than 3.5 dl/g (0.35m³/kg), torsion is caused due to the co-rotation of the resin and the mandrel during the extrusion moulding and non-uniformity in wall section is likely to be caused due to deflection of the mandrel when the L/D is large. Thus, the moldability is poor and the physical properties of the resultant molded articles are degraded (e.g., poor abrasion resistance and large friction coefficient). When the present method is applied to thermoplastic resins having a low melt viscosity such as polypropylene, polyamide, and polyacetal, the moldability is poor as in the above-mentioned case of polyethylene having an intrinsic viscosity [η] of less than 3.5 dl/g (0.35 m³/kg). According to the present invention, a tube of thermoplastic resin can be produced by melting thermoplastic resin having poor melt flowability in a screw extruder, preferably a screw extruder provided with a grooved cylinder (or barrel), and then melt-extruding the molten thermoplastic resin from a tube die having an L/D of 5 or more, preferably 10 or more, and more preferably 30 to 70, and having a mandrel rotatable with the rotation of the screw. When a tube die having an L/D of less than 5 is used, the molten thermoplastic resin having poor melt flowability is not completely uniformly fused before being extruded from the die, since the molten thermoplastic resin is a rubber-like mass unlike the molten product of conventional resins and, therefore, only tubes having both poor appearance and poor mechanical strength can be obtained. On the other hand, although there is no upper limit to the L/D of the tube die, the practically preferable upper limit of the L/D is 70 or less. Furthermore, the L/D of the tube die is correlated to the productivity. That is, the production rate is increased with increase in L/D. The mandrel of the tube die used in the present invention must be rotated with the rotation of the screw of the extruder. There is no limitation to the rotation speed of the mandrel, which is not necessarily the same as that of the screw, in the present invention. The object of the present invention may be accomplished if the mandrel is rotated. The preferred rotation speed of the mandrel is about 5 to 100 rpm, more preferably about 10 to 50 rpm. When the mandrel is not rotated, the mandrel becomes eccentric so that a tube having a uniform thickness cannot be obtained. The mandrel can be attached to the screw tip in any conventional manner. For example, the mandrel may be directly fixed, screwed, fitted, or inserted into the screw tip or may be attached by means of an appropriate joint. However, it is convenient to directly screw the mandrel to the screw tip, from the standpoint of disassenbling and cleaning the tube die and changing the inner diameter size, because in that way the mandrel is easily attached to and removed from the screw tip. There is no specific limitation on the extrusion temperature of the thermoplastic resin used in the present invention, as long as the extrusion temperature is substantially higher than the melting point of the thermoplastic resin but is lower than the decomposition temperature thereof. For example, the above-mentioned ultra-high-molecular-weight polyethylene is generally extruded at a temperature of at least the melting point thereof but, preferably, 350°C or less. When the extrusion temperature is less than the melting point of the thermoplastic resin, the resin is likely to clog the die, possibly causing the destruction of the apparatus. In the case of the above-mentioned ultra-high-molecular-weight polyethylene, the preferred temperature conditions are an extruder temperature of 230°C to 330°C, a temperature between the inlet and the intermediate portion of the tube die of 180°C to 310°C, and a temperature between the intermediate portion and the outlet of the tube die of 136°C to 170°C, since this prevents land melt fractures from occurring in the tube die. The method of the present invention can produce so-called flexible tubes having excellent flexibility as well as rigid tubes and pipes having a larger diameter. One example of apparatus suitable for use in the production of tubes, including flexible tubes, from thermoplastic resins having poor melt flowability will be explained in detail with reference to the accompanying drawing. The illustrated apparatus for producing tubes from thermoplastic resins having poor melt flowability comprises (i) an extruder 1 provided with a grooved cylinder 2 and a screw 3 having a compression ratio of between 1 and 2.5, preferably 1.3 to 1.8; and (ii) a tube die composed of a mandrel 5(connected to the screw tip 4 in such a manner that the mandrel is rotatable with the rotation of the screw) and an outer die 6, the tube die having an L/D of at least 5, preferably at least 10, and more preferably 30 to 60, a ratio S₁/S₂ of the crosssectional area S₁ of the inlet portion 71 of the tube die 7 to the crosssectional area S₂ of the intermediate portion 72 of the tube die 7 of 0.5 to 3.0, preferably 1.0 to 2.0, and a ratio S₂/S₃ of the crosssectional area S₂ of the intermediate portion of the tube die 7 to the crosssectional area S₃ of the resin flow path at the outlet portion of the tube die 7 of 1.0 to 3.0, preferably 1.1 to 2.0. The function of the groove portions 21 of the grooved cylinder 2 is to ensure a stable feed of the powder particles of thermoplastic resin such as ultra-high-molecular-weight polyethylene to the compression zone 22. As mentioned above, the compression ratio of the screw 3 should be within the range of between 1 and 2.5. When the compression ratio of the screw 3 is less than 1.0, the contact stress of the resin against the cylinder wall is small, so that the extrusion rate becomes unstable and the resultant extrusion molded articles will have a poor appearance due to surging or insufficient deaeration. Contrary to this, when the compression ratio of the screw 3 is more than 2.5, the resin temperature is abnormally increased due to clogging and the generation of frictional heat in the compression zone, so that the physical properties (e.g., friction coefficient and abrasion resistance) of the extrusion molded articles are impaired due to the decrease in the molecular weight of the resin caused by thermal decomposition. The mandrel 5 is screwed into the tip of the screw 3. As mentioned above, the L/D of the tube die should be 5 or more. If the L/D of the tube die is less than 5, good flexible tubes cannot be obtained because the molten resin is not completely fused. The L/D of the tube die is defined by the ratio of the length between the inlet portion 71 of the tube die and the outlet portion 73 of the tube die to the inner diameter of the outer die 6 at the tube die outlet 73. Also, as mentioned above, the ratio S₁/S₂ should be within the range of 0.5 to 3.0; when the ratio S₁/S₂ is less than 0.5, the space in the die is not sufficiently filled along the configuration of the die by the molten resin- although the die is sufficiently filled when S₁/S₂ is over 0.5 but still less than 1.0, because the intermediate portion of the die is set at a relatively high temperature and also because back pressure is exerted from the tip of the die. Contrary to this, when the ratio S₁/S₂ is more than 3.0, the extrusion of the desired tubes becomes difficult due to the excessive increase in back pressure. Furthermore, as mentioned above, the ratio S₂/S₃ should be within the range of 1.0 to 3.0. When the ratio S₂/S₃ is less than 1.0, the molten resin is not completely fused. Contrary to this, when the ratio S₂/S₃ is more than 3.0, the desired extrusion molding of tubes becomes difficult due to an excessive increase in the resin pressure. As mentioned above, the crosssectional area of the flow path of the tube die becomes basically narrower in the direction of the outlet of the tube die. That is, a so-called tapered die is generally used. However, the tip portion 74 of the tube die is preferably of constant flow bath crosssectional area (i.e., so-called straight) so that high dimensional accuracy can be retained. The L/D of the straight portion is preferably from 5 to 10. In addition, the tip 51 of the mandrel 5 preferably projects externally from the outer die 6 because possible jetting of the extruded molten resin is preferably corrected. Flexible tubes (e.g. of ultra-high-molecular-weight polyethylene) produced by the method of the present invention can have excellent flexibility, small friction coefficient, good pressure resistant strength, good appearance, internal face self-lubricating properties, no hygroscopicity, and excellent food sanitation properties, without impairing the inherent abrasion resistance, impact strength, and stress-breaking resistance. Accordingly, such tubes are suitable for use as transportation pipes for foods and chemicals, cable conduits, utility pipes or various parts of apparatuses by utilizing the above-mentioned characteristics. Furthermore, according to the tube extruding method and apparatus according to the present invention, desired tubes having a substantially uniform wall thickness can be stably produced from the above-mentioned thermoplastic resins having poor melt flowability (e.g. ultra-high-molecular-weight polyethylene). EXAMPLES The present invention will now be further illustrated by, but is by no means limited to, the following Examples. Example 1 and 2 and Comparative Example 1 Flexible tubes of ultra-high-molecular-weight polyethylene were produced using an apparatus as illustrated in Fig. 1. The specifications of the apparatus used are as follows. Screw: Outer diameter = 20 mm⊘ Effective screw length (L/D) = 22 Flight pitch = 12 mm (constant) Compression ratio = 1.8 Adaptor length = 90 mm Tube die: Length = 240 mm L/D = 40 Inner diameter of outer die at die outlet = 6 mm⊘ Outer diameter of mandrel at die outlet = 5 mm⊘ S₁/S₂ = 1.52 S₂/S₃ = 1.17 The particulars of the polyethylene resins used in Examples 1 and 2 and comparative Example 1 are as follows: Hizex® Million 240 M (Example 1): Powered ultra-hih-molecular-weight polyethylene resin available from Mitsui Petrochemical Industries, Ltd having an intrinsic viscosity η of 17 dl/g (1.7 m³/kg) an MFR of less than 0.01g /10 min, a melting point of 136°C, and a bulk density of 0.45 g/cm³ (4.5x10² kg/m³). Hizex® Million 220 M (Example 2): Powdered ultra-high-molecular-weight polyethylene resin available from Mitsui Petrochemical Industries, Ltd. having an intrinsic viscosity [η] of 17 dl/g (1.7 m³/kg), an MFR of less than 0.01/10 min, a melting point of 115°C, and a bulk density of 0.45 g/cm² (4.5x10² kg/m³). Conventional polyethylene (Comparative Example 1): Polyethylene having an intrinsic viscosity of 2.6 dl/g (0.26 m³/kg), an MFR of 0.11 g/10 min, a density of 0.956 g/cc (9.56x10² kg/m³), and a melting point of 131°C. The above-mentioned resins were extruded from the above-specified extruder under the following conditions: Setting temperature Extruder = 270°C Adaptor (AD) = 250°C Die base (D₁) = 180°C Die tip (D₂) = 145°C Screw revolution speed = 25 rpm The extruded tubes were hauled off at a rate of 1 m/min by haul-off equipment. Thus, flexible tubes having an outer diameter of 5 mm⊘, an inner diameter of 4 mm⊘, and a ratio of the outer diameter/the wall thickness of 10 were obtained. The physical properties of the resultant flexible tubes were determined according to the following methods. The results were as shown in Table 1. Density: ASTM D1505 Outer diameter and wall thickness of product: measured by Nikon MEASURESCOPE MODEL II, Nikon LINEAR PULSE SCALE, and Nikon VM-8A. The data were an average of 5 measurements (n=5) and the standard deviations were calculated (i.e., Average value/Standard deviation in Table 1) Tensile test: Tensile strength at break (TS) was determined at a temperature of 23°C according to JIS (Japanese Industrial Standards) K 6771 (soft vinyl tube) Hydraulic destructive creep: Burst pressure was determined by hydraulic pressure according to JIS K 6771 Hot internal pressure creep: The tube breaking time was determined at a temperature of 80°C according to JIS K 6762 under continuous inner pressure calculated from the following equation at a hoop stress of 40 kg/cm² (3`9 MPa) wherein P: Pressure d: Outer diameter of tube(mm) t: Wall thickness of tube(mm) f: Hoop stress 40 kg/cm² (3.9 MPa) Examples 3 and 4 Flexible tubes of the ultra-high-molecular-weight polyethylene were produced in the same manner as in Examples 1 and 2, except that the particulars of the apparatus were changed as follows: Outer diameter of mandrel = 4 mm⊘ Extruding temperature (C₂) = 300°C Extruding temperature (C₃) = 300°C Haul-off rate = 2.3 m/min Thus, flexible tubes having an outer diameter of 3 mm⊘, an inner diameter of 2 mm⊘, and a ratio of the outer diameter/the wall thickness of 6 were produced. The physical properties of the resultant tubes were as shown in Table 1.

Claims: A method for producing a tube from thermoplastic resin having poor melt flowability, characterised in that tube has an intrinsic viscosity of at least 8 dl/g (0.8³/kg) determined at 135°C in decalin, a tensile strength at break of 150 kg/cm² (14.7 MPa)or more determined at a temperature of 23°C according to JIS (Japanese Industrial Standards) K 6771 (soft vinyl tube), a breaking time at hot internal pressure creep of at least 200 hours determined at a temperature of 80°C according to JIS K 6762 under continuous inner pressure calculate fro the following equation at a hoop stress of 40 Kg/cm² (3.9 MPa) wherein P : Pressure d : Outer diameter of tube (mm) t : Wall thickness of tube (mm) f : Hoop stress = 40 Kg/cm² (3.9 MPa) an outer diameter of 50mm⊘ or less, and a ratio of the outer diameter to the wall thickness of 40 or less, method comprising the steps of: (i) melting the thermoplastic resin having poor it flowability in a screw extruder (1) provided with a mandrel (5) rotatable with the rotation of the screw (3); (ii) rotating the screw of the extruder and the mandrel whilst maintaining the resin at a temperature above its melting point; and (iii) melt extruding the molten thermoplastic resin from a tube die (7) having an L/D of at least 5, wherein the L/D of the tube die is defined by the ratio of the length between the inlet portion (71) of the tube die and the outlet portion (73) of the tube die to the inner diameter of the outer die (6) at the tube die outlet (73). A method as claimed in claim 1 wherein the screw extruder is provided with a grooved cylinder (2). An apparatus for producing a tube from thermoplastic resin having poor melt flowability by the method of claim 1, comprising: (i) an extruder (1) provided with a grooved cylinder (2) and a screw (3); and (ii) a tube die (7) composed of (a) a mandrel (5) connected to the screw tip (4) in such a manner that the mandrel is rotatable with the rotation of the screw and (b) an outer die (6), characterised by the tube die having an L/D of at least 5, a ratio S₁/S₂ of 0.5 to 3.0, and a ratio S₂/S₃ of 1.0 to 3.0, S₁ being the crosssectional area of the inlet portion of the tube die, S₂ that of an intermediate portion of the tube die, and S₃ that of the resin flow path at the outlet portion of the tube die, and also characterised by the screw having a compression ratio of between 1 and 2.5.

Inventors: SHIRAKI, TAKESHI, YOSHIMURA, YOSHIHIRO
Applicants: MITSUI PETROCHEMICAL INDUSTRIES, LTD.
Application year: 1985
Main classification: F16L11
IPC codes: F16L11, B29C47, F16N11, B29K23