JP3898342B2 - Method for producing difficult-to-mold resin molded body - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing a resin molded article having no air bubbles, and more particularly to a method for producing an ultra-high molecular weight polyethylene molded article having no air bubbles having high mechanical strength.
[0002]
[Prior art]
Resins such as ultra-high molecular weight polyethylene, polytetrafluoroethylene, ultra-high polymerization degree polyvinyl chloride, and high chlorination degree polyvinyl chloride are resins that are extremely difficult to mold due to their high melt viscosity and easy decomposition. In general, it is called “difficult-to-mold resin”.
[0003]
As a conventional method for producing a molded body from such a difficult-to-mold resin having a very high melt viscosity, the following methods may be mentioned:
(1) A method of producing a plate-shaped or rod-shaped molded body by compression molding or ram extrusion molding, and molding the molded body by a cutting process such as cutting,
(2) A method in which a difficult-to-mold resin is dissolved in an organic solvent and formed into a film or a sheet by casting,
(3) As described in Japanese Examined Patent Publication No. 4-47608, the dispersion or mixture obtained by adding an organic solvent such as p-xylene or tetrachloroethane to the powder of difficult-to-mold resin is heated and melted. And then evaporating the organic solvent after shaping.
[0004]
However, the method (1) has a drawback that productivity is extremely low. In the methods (2) and (3), if the organic solvent remains inside the molded body, the physical properties of the molded body are reduced. Therefore, the molded body must be heated to evaporate the organic solvent. In order to completely evaporate the solvent, a large-scale apparatus is required, and it is necessary to evaporate for a long time, so that productivity is low. In addition, if the organic solvent is volatilized as it is in the atmosphere, there is a risk of causing pollution, so that there is a problem that the organic solvent must be recovered and the equipment cost of the recovery equipment increases.
[0005]
Therefore, a difficult-to-mold resin (especially ultra-high molecular weight polyethylene) is heated to a melting point or higher within the screw extruder without using an organic solvent, and then the mandrel rotates with the rotation of the screw. There has been proposed a method for producing a difficult-to-mold resin tube which is improved in productivity by melt extrusion from five or more tube dies (see Japanese Patent Publication No. 2-312270).
[0006]
However, the break point tensile strength of the tube manufactured by this manufacturing method is 15 MPa or more. However, in consideration of the description of the examples in the above publication, only a tube having a break point tensile strength of about 58 MPa is obtained. Therefore, the material cost cannot be reduced by reducing the thickness or the like, and it cannot be applied to a structural member that requires a considerably high mechanical strength. Therefore, in order to further improve the mechanical strength of the molded body, means for providing the molded body to a post-molding process such as stretching the molded body obtained after extrusion can be considered. Equipment is required. In addition, the molded body obtained in this case is a thin plate such as a sheet or a film, and it is extremely difficult to obtain a molded body (atypical body) having a thick pipe or a complicated shape.
[0007]
[Problems to be solved by the invention]
The present inventors have found that the mechanical strength of the molded body obtained through the melt-kneaded resin in the cross-sectionally reduced portion whose cross-sectional area is reduced in the extrusion direction is the resin channel cross-sectional area (S1) on the inlet side of the cross-sectionally reduced portion. These values are greatly influenced by the ratio (S1 / S2) of the resin flow passage cross-sectional area (S2) to the outlet side, the temperature of the resin when passing through the reduced section, the temperature of the resin extruded from the mold, etc. Obtaining the knowledge that the mechanical strength of the resulting molded product can be greatly increased by adjusting the range to a specific range, the present invention has been completed. That is, the present invention has been made to solve the above-mentioned problems, and the object thereof is to set the ratio and temperature within a predetermined range, thereby removing and recovering the organic solvent, and the obtained molding. It is an object of the present invention to provide a production method excellent in productivity of a resin molded body that does not require a post-molding step such as stretching the body and does not have bubbles having high mechanical strength.
[0008]
[Means for Solving the Problems]
The method for producing a resin-molded product having no air bubbles according to the present invention that solves the above-described problems is a method of applying a non-reactive gas in a gaseous state at room temperature and pressure to ultrahigh molecular weight polyethylene having a viscosity average molecular weight of 300,000 or higher under high pressure. The melted and kneaded resin in the extruder is melt-kneaded in the extruder, and then the melt-kneaded resin is provided inside the mold, and the inlet side resin channel cross-sectional area (S1) and the outlet side resin channel After cooling to the melting point or lower while passing through the reduced section having a ratio (S1 / S2) to the cross-sectional area (S2) of 2 or more, the resin is further cooled within the mold (crystallization peak temperature at the time of cooling +5 C.) and cooled down to below the extrusion outlet from the mold outlet.
[0010]
The resin used in the present invention is an ultrahigh molecular weight polyethylene having a viscosity average molecular weight of 300,000 or more from the viewpoint of having excellent properties such as wear resistance, self-lubricity, and impact resistance. When polyethylene having a viscosity average molecular weight of less than 300,000 is used, excellent properties such as the above-mentioned wear resistance may not be sufficiently exhibited in the obtained molded article. In the present specification, ultrahigh molecular weight polyethylene may be simply referred to as “resin”.
[0011]
In the present invention, first, the resin is made into an easily molded state by dissolving a non-reactive gas in a gaseous state at normal temperature and normal pressure in ultrahigh molecular weight polyethylene under high pressure. Such easily molded resin is plasticized by a non-reactive gas and has reduced viscosity and improved fluidity. Therefore, as will be described later, melt extrusion (hereinafter simply referred to as “screw extrusion”) is performed from within a screw extruder or the like. (Extruding)).
[0012]
The term “non-reactive gas” used in the present specification is an organic or inorganic substance in a gaseous state at normal temperature and pressure, and does not react with ultrahigh molecular weight polyethylene, and further deteriorates this resin. A gas that does not adversely affect the resin. Such gas is not particularly limited as long as the above conditions are satisfied. For example, inorganic gas (for example, carbon dioxide, nitrogen, argon, neon, helium, oxygen), organic gas (for example, chlorofluorocarbon, low molecular weight hydrocarbon) ) And the like. Inorganic gas is preferable because it has low adverse effects on the environment and does not require gas recovery, has high solubility in resins, has a large plasticizing effect on resins, and has little harm when released directly into the atmosphere. From the viewpoint, carbon dioxide is preferable. Such a non-reactive gas may be used alone, or two or more kinds of non-reactive gases may be used in combination. In the present specification, the non-reactive gas may be simply referred to as “gas”.
[0013]
Examples of means for dissolving the non-reactive gas in the resin under high pressure include a means for dissolving the non-reactive gas in the molten resin and a means for dissolving in the solid resin. Either means may be used, or both may be used in combination.
[0014]
As a means for dissolving the non-reactive gas in the molten resin, for example, using a vent type screw, a means for mixing the non-reactive gas into the vent part from the middle of the cylinder filled with the resin, a tandem extruder Utilizing, a means of press-fitting a gas in the first extruder or in the vicinity of a resin inflow portion to the second extruder and sufficiently dissolving and kneading the gas in the resin in the second extruder, and the like can be mentioned. In this case, since the molten resin in the vicinity of the mold acts as a pressure seal material, the non-reactive gas cannot be diffused from the vicinity of the mold provided at one end of the extruder.
[0015]
As a means for dissolving in a solid resin,
(1) A means for dissolving non-reactive gas in a pellet or powder resin in a high-pressure container in advance, and (2) dissolving non-reactive gas in the resin from the pressure hopper of the molding apparatus in the solid transport section of the extruder. Means.
[0016]
In the case of the above means (1), when the resin in which the non-reactive gas is dissolved is supplied to the extruder, the non-reactive gas dissolved in the resin is prevented from escaping out of the resin due to diffusion. Therefore, it is preferable to supply as soon as possible. On the other hand, in the case of the above means (2), it is preferable that the screw drive shaft and the hopper have a pressure-resistant seal structure so that the non-reactive gas does not volatilize outside the extruder. Since ultrahigh molecular weight polyethylene has an extremely high melt viscosity, it is preferable to further dissolve the gas while plasticizing the resin by dissolving the gas in a solid state resin under high pressure.
[0017]
The non-reactive gas may be supplied directly from the gas cylinder to the extruder, or may be supplied under pressure using a plunger pump or the like.
[0018]
The amount and pressure of the non-reactive gas dissolved in the resin and the degree of decrease in the melt viscosity of the resin after dissolution depend on the type and molecular weight of the resin and non-reactive gas, but carbon dioxide is used as the non-reactive gas. In this case, the amount of carbon dioxide dissolved in the resin is preferably in the range of 1% by weight to 30% by weight, and more preferably in the range of 3% by weight to 20% by weight. When the amount of carbon dioxide dissolved in the resin is less than 1% by weight, the viscosity of the resin is not sufficiently lowered and the fluidity is lacking. On the other hand, when trying to increase the amount of carbon dioxide dissolved in the resin to an amount exceeding 30% by weight, it may be necessary to extremely increase the pressure during dissolution using a large-scale facility, which is inappropriate. .
[0019]
When carbon dioxide is used as the non-reactive gas, the carbon dioxide pressure is preferably about 0.2 MPa or more and about 50 MPa or less in order to keep the amount of carbon dioxide dissolved in the resin within the above range. More preferably, the pressure is about 0.6 MPa or more and about 35 MPa or less.
[0020]
Next, in the present invention, the resin obtained by melting and kneading the non-reactive gas as described above is divided into a resin flow passage cross-sectional area (S1) on the inlet side and a resin flow passage cross-sectional area (S2) on the outlet side. The ratio (S1 / S2) is 2 or more, and cooling is performed to the melting point or less while passing through the reduced cross section provided in the mold.
[0021]
When the ratio (S1 / S2) is smaller than 2, a molded article having a sufficiently high mechanical strength cannot be obtained. A preferred ratio (S1 / S2) is 3 or more and 60 or less, and more preferably 3 or more and 40 or less. When the ratio (S1 / S2) exceeds 60, a very large pressure is applied to the resin passing through the reduced cross-section portion, and the resin may not be extruded.
[0022]
In addition, when the temperature of the resin at the exit of the reduced cross section after passing the resin through the reduced cross section exceeds the melting point, there is a disadvantage that the effect of increasing the mechanical strength of the obtained molded body is small.
[0023]
The temperature when the resin passes through the reduced cross-section inside the mold is preferably (crystallization peak temperature at the time of cooling-20 ° C) or more (melting point + 50 ° C), and (crystallization peak temperature at the time of cooling) or more. A range of (melting point + 30 ° C.) or less is more preferable. If the temperature is less than (crystallization peak temperature at lower temperature −20 ° C.), since the resin is in a considerably hardened state, the extrusion pressure required when the resin passes through the cross-section reduced portion becomes high, The resin may not be extruded. On the other hand, in the case of (melting point + 50 ° C.), the cooling of the resin at the cross-sectional reduced portion is insufficient, and the resin may not be pushed out from the outlet of the cross-sectional reduced portion below the melting point. As used herein, the term “crystallization peak temperature during cooling” refers to the crystallization peak temperature when the molten resin is cooled and crystallized, and more specifically, This means the temperature at which the amount of heat generated by the resin at the time of cooling is maximized, and is described in detail in 9.2 of JIS K 7121 along with how to obtain it. The “crystallization peak temperature when the temperature is lowered” can be measured by a differential scanning calorimeter (DSC) under atmospheric pressure.
[0024]
The mold (5) used in the present invention is provided with a mandrel (51) and a mold body (52) when obtaining a tube-shaped molded body. As shown in FIG. 2A, the mandrel (51) is provided. In the vicinity of the central portion of the mold, the diameter is increased in the extrusion direction, whereby a cross-sectional reduced portion (6) is formed in the mold (5). 2A, the mold body (52) may also have an increased diameter in the extrusion direction. On the other hand, in the case of obtaining a solid molded body such as a rod shape, as shown in FIG. 2B, the mold body (52) is reduced in diameter in the extrusion direction, thereby forming the cross-sectionally reduced portion (6). The form of the formed body finally obtained by passing through the mold outlet is not limited to the solid rod shape and the hollow tube shape, and a molded body (atypical body) having a complicated shape can also be obtained.
[0025]
Finally, the resin that has passed through the reduced cross-sectional portion as described above is cooled to below the resin (crystallization peak temperature at the time of cooling + 5 ° C.) or less inside the mold and extruded from the mold outlet. When extrusion molding is performed from the die outlet at a temperature exceeding (the crystallization peak temperature at the time of cooling + 5 ° C.), since the gas dissolved in the resin is foamed, only a molded body having bubbles that become defects inside. Such a molded body does not have the desired mechanical strength.
[0026]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
FIG. 1 is a schematic view of a molding apparatus provided with an extruder (1), a mold (5) and the like and used in the present invention.
As shown in FIG. 1, in the method according to the present invention, first, carbon dioxide supplied from a gas cylinder (10) (11) is pressurized using a pressure pump (12) (13), and then this high-pressure state is obtained. Carbon dioxide is supplied into the extruder (1) from gas supply ports (14) and (15) provided in the extruder (1). The hopper (16) has a pressure resistant structure, from which resin is fed into the extruder (1) to the extruder (1). The resin is provided in the extruder (1) by proceeding in the right direction of the drawing in the solid transport part (3) in the extruder (1) by the screw (2) provided in the extruder (1). While being heated and melted by the heating means (not shown), it is exposed to high-pressure carbon dioxide supplied from the gas supply port (14) provided in the solid transport section (3). Thereby, carbon dioxide is dissolved in the resin, the viscosity of the resin is lowered, and the resin is easily molded.
[0027]
Further, the resin that has advanced toward the right in the drawing by the screw (2) is completely melted by the heating means (not shown) provided in the extruder (1), and is provided in the liquid material transport section (4). It is exposed to high-pressure carbon dioxide supplied from the gas supply port (15). This further dissolves carbon dioxide in the molten resin, further reducing the viscosity of the resin. Thereby, resin will be in an easily-molded state. Depending on the amount of gas dissolved in the resin, two gas supply ports (14) and (15) may be used as described above, or only one gas supply port may be used.
[0028]
The easily molded resin is sufficiently melt-kneaded by the screw (2), and then the melt-kneaded resin is passed through a mold (5) disposed on the outlet side of the extruder (1). Inside the mold (5), a mandrel (51) connected to the tip of the screw (2) of the extruder (1) is provided, and the vicinity of the center of the mandrel (51) is expanded in the extrusion direction. Thereby, the cross-sectionally reduced portion (6) is formed by making the mold (5) into a tubular shape. The ratio (S1 / S2) of the resin channel cross-sectional area (S1) on the inlet side (61) and the resin channel cross-sectional area (S2) on the outlet side (62) of the cross-section reduced portion (6) thus formed. ) Is 2 or more. The melted and kneaded resin is allowed to pass through such a cross-sectionally reduced portion (6), and then cooled to the melting point or lower, and further cooled to the resin (crystallization peak temperature during cooling + 5 ° C.) or lower in the mold (5). Then, a resin molded body having no tube-like bubbles is obtained by extruding from the outlet of the mold (5).
[0029]
According to this manufacturing method, since the resin is plasticized by dissolving carbon dioxide in the resin, the resin passes through the reduced section even if it is cooled to the melting point or less by the reduced section (6). Thus, the extrusion pressure required for the extrusion does not become high, so that the pressure resistance of the extruder is insufficient, torque is not easily generated, and extrusion is not difficult. On the other hand, when a non-reactive gas such as carbon dioxide is not dissolved in the resin (for example, in the case of the method described in Japanese Patent Publication No. 2-312270), the resin starts to harden when cooled to the vicinity of the melting point. The extrusion pressure required when the resin passes through the cross-sectionally reduced portion increases, resulting in insufficient pressure resistance of the extruder, torque over, and the like, making extrusion difficult.
[0030]
Further, the cross-sectional reduced portion (6) in which the ratio (S1 / S2) of the resin flow path cross-sectional area (S1) on the inlet side (61) to the resin flow path cross-sectional area (S2) on the outlet side (62) is 2 or more. By passing the melt-kneaded resin through and cooling it to the melting point or lower, a molded article having very high mechanical strength and no bubbles can be obtained. That is, since the cross-sectionally reduced portion (6) having a ratio (S1 / S2) of 2 or more has a structure in which the resin flow path is narrowed in the extrusion direction, the resin molding without bubbles obtained thereby. The mechanical strength of the body can be greatly increased.
[0031]
Furthermore, when extrusion molding is performed from the outlet of the mold (5) under the condition of (crystallization temperature at the time of cooling + 5 ° C) or less, the elongational viscosity of the resin is very large at or below the temperature of the crystallization (temperature of crystallization + 5 ° C). Therefore, it is possible to suppress the foaming of carbon dioxide in the resin, whereby a molded article having high mechanical strength that does not have bubbles that become defects inside can be produced.
[0032]
In addition, since carbon dioxide is naturally diffused into the atmosphere from the obtained molded body, an apparatus for artificially removing and collecting carbon dioxide is not required. Furthermore, carbon dioxide has an extremely low adverse effect on the environment as compared with organic substances, and has the advantage that there is no particular harm even if it is naturally diffused in the air.
[0033]
【Example】
Hereinafter, the present invention will be described specifically by way of examples. However, the following examples are used only for the purpose of illustration and should not be used for the purpose of limitation.
Example 1
Ultrahigh molecular weight polyethylene ("Hi-Zex Million 240M" manufactured by Mitsui Petrochemical Co., Ltd., viscosity average molecular weight 2,300,000, melting point 136 ° C, crystallization peak temperature 118 ° C during cooling) is a pressure-resistant hopper ( 16) to a single screw extruder (1, screw diameter 40 mm, L / D = 30). Carbon dioxide is used as a non-reactive gas, which is supplied from the gas supply port (14) (15) to the solid transport part (3) and the liquid transport part (4) of the extruder (1) at 150 kg / cm ² (14. 7 MPa). The amount of carbon dioxide dissolved in ultrahigh molecular weight polyethylene under this pressure was about 10% by weight. At this time, the extruder (1) uses a high-pressure shaft sealing mechanism for the screw drive shaft, a pressure-resistant hopper structure, and a molten ultra-high molecular weight polyethylene in the vicinity of the extruder to bring carbon dioxide in the extruder (1) into a high-pressure state. Held on. Next, the resin supplied to the extruder (1) was sufficiently melt-kneaded inside under the conditions of an extrusion rate of 2 kg / h, a screw rotation speed of 10 rpm, and a barrel set temperature of 200 ° C.
[0034]
Next, the melt-kneaded resin was passed through a mold (5). The reduced section (6) in the mold (5) is S1 / S2 = 4.5, tubular, inlet side (61) resin channel outer diameter 40 mm, inner diameter 30 mm, outlet side (62) Resin channel outer diameter is 40mm and inner diameter is 38mm. By keeping the temperature of the mandrel (51) and mold body (52) excluding the vicinity of the tip at 125 ° C, the resin at 160 ° C on the inlet side (61) While passing through the reduced section (6), the outlet side (62) is cooled to 125 ° C, and the same diameter shape (outer diameter 40 mm, inner diameter 38 mm) as the outlet side (62) of the reduced section (6). By keeping the temperature of the die tip held at 105 ° C., the resin was cooled to 105 ° C. and extruded from the die tip to produce a tube-shaped ultrahigh molecular weight polyethylene molded body. The pressure applied to the extruder near the tip of the extruder at this time was 40 MPa.
When the obtained molded product was subjected to a tensile test (based on JIS K 7127, carried out at a temperature of 23 ° C.), the tensile strength was 80 MPa.
[0035]
(Comparative Example 1)
Except for not dissolving carbon dioxide, extrusion was performed under the same conditions as in Example 1. However, the pressure applied to the extruder near the tip of the extruder exceeded 100 MPa, which is the limit of the pressure resistance of the extruder, and the extrusion was performed. It became impossible.
[0036]
(Comparative Example 2)
By maintaining the temperature of the mandrel (51) and the mold body (52) excluding the vicinity of the tip at 140 ° C., the resin on the inlet side (61) is allowed to pass 175 ° C. resin through the reduced section (6). Extrusion was carried out under the same conditions as in Example 1 except that the mixture was cooled to 140 ° C. in 62) to produce a tube-shaped ultrahigh molecular weight polyethylene molded body. The pressure applied to the extruder near the tip of the extruder at this time was 30 MPa. When the obtained molded product was subjected to a tensile test, the tensile strength was 40 MPa.
[0037]
(Comparative Example 3)
Extrusion was carried out under the same conditions as in Example 1 except that the resin was cooled to 125 ° C and extruded from the die tip into a tube by keeping the temperature at the die tip at 125 ° C. When a molecular weight polyethylene molded body was produced, the resin extruded from the mold (5) foamed and became an ultrahigh molecular weight polyethylene foam. In addition, the pressure applied to an extruder in the vicinity of the front-end | tip of an extruder at this time was 20 MPa, When the tensile test of the obtained foam was done, tensile strength was 15 MPa.
[0038]
(Comparative Example 4)
By maintaining the temperature of the mandrel (51) and the mold body (52) excluding the vicinity of the tip at 140 ° C. without dissolving carbon dioxide, the resin at 175 ° C. is reduced at the cross-section reduced portion (6) on the inlet side (61). Is cooled to 140 ° C. on the outlet side (62) while passing through, and the mold tip temperature is kept at 140 ° C., so that the resin is kept at 140 ° C. and extruded from the outlet of the die (5). Except that, extrusion was performed under the same conditions as in Example 1 to produce a tubular ultrahigh molecular weight polyethylene molded body. The pressure applied to the extruder near the tip of the extruder at this time was 60 MPa. When the obtained molded product was subjected to a tensile test, the tensile strength was 50 MPa.
[0039]
(Example 2)
Instead of the cross-sectional reduction part (6) of Example 1, the cross-sectional reduction part is S1 / S2 = 9, tubular, inlet side (61) resin channel outer diameter 40 mm, inner diameter 30 mm, outlet side (62 Extrusion was performed under the same conditions as in Example 1 except that the outer diameter of the resin flow path was 40 mm and the inner diameter was 39 mm, thereby producing a tubular ultrahigh molecular weight polyethylene molded body. At this time, the pressure applied to the extruder near the tip of the extruder was 70 MPa. When the obtained molded product was subjected to a tensile test, the tensile strength was 105 MPa.
[0040]
【The invention's effect】
Since the method for producing a resin molded body having no air bubbles according to the present invention is configured as described above, there is no need to remove and recover the organic solvent, high productivity, wear resistance, self-lubricating property, It is possible to obtain a resin molded article having excellent properties such as impact resistance and low temperature characteristics and having no mechanical bubbles with high mechanical strength.
In addition, when carbon dioxide is used as the non-reactive gas as in claim 2, it has a low adverse effect on the environment and does not require removal / recovery of the gas, but also has a solubility in ultrahigh molecular weight polyethylene. It is high, and since the plastic viscosity is great because the melt viscosity of the resin is greatly reduced, the moldability of the resin can be further improved.
[Brief description of the drawings]
FIG. 1 is a schematic view of a molding apparatus equipped with an extruder (1), a mold (5) and the like and used in the present invention.
FIG. 2 is a cross-sectional view of a mold (5) that can be used in the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Extruder 2 ... Screw 3 ... Solid conveyance part 4 ... Liquid substance conveyance part 5 ... Mold 51 ... Mandrel 52 ... Mold main body 6 ... Cross-section reduction part 61 ... Entrance side of cross-section reduction part 62 ... Exit of cross-section reduction part Side 10 ... Gas cylinder 11 ... Gas cylinder 12 ... Pressure pump 13 ... Pressure pump 14 ... Gas supply port 15 ... Gas supply port 16 ... Hopper

Claims (2)

A non-reactive gas in a gaseous state at normal temperature and normal pressure was dissolved in an ultrahigh molecular weight polyethylene having a viscosity average molecular weight of 300,000 or more under high pressure, and melted and kneaded in an extruder, and then melt kneaded. Cross section reduction in which resin is provided inside the mold and the ratio (S1 / S2) of the resin flow passage cross-sectional area (S1) on the inlet side to the resin flow passage cross-sectional area (S2) on the outlet side is 2 or more Of the resin molded body having no air bubbles, which is cooled to below the melting point while passing through the part , and further cooled to below the resin (crystallization peak temperature at the time of cooling + 5 ° C.) within the mold and extruded from the mold outlet. Production method. The method for producing a resin molded body having no bubbles according to claim 1, wherein the non-reactive gas is carbon dioxide.

Images