JP4091165B2 - Ultra high molecular weight polyethylene foam and method for producing the same - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a foam made of ultrahigh molecular weight polyethylene having a molecular weight of 300,000 or more and a method for producing the same.
[0002]
[Prior art]
A polyethylene resin having a viscosity average molecular weight of 300,000 or more (referred to as “ultra high molecular weight polyethylene” throughout this specification) is higher in abrasion resistance, self-lubrication, Resin with excellent impact properties, low temperature characteristics, chemical resistance, etc., but on the other hand, it has extremely high melt viscosity and is difficult to mold, so the molding method is limited to compression molding, etc. It is.
[0003]
The foam made of ultra-high molecular weight polyethylene resin is also expected to show the above-mentioned superior properties compared to ordinary high-density polyethylene foam, but this resin is also foamed due to its high melt viscosity. It is a difficult material.
[0004]
Conventional methods for producing ultra-high molecular weight polyethylene foams include methods such as bead foaming and extrusion foaming. Among these, in the extrusion foaming method of ultrahigh molecular weight polyethylene resin, an organic liquid plasticizer having a melting point of 150 ° C. or higher is added to the resin to lower the resin viscosity, and the resin is foamed using a physical foaming agent or a chemical foaming agent. There is a method (see Japanese Patent Application Laid-Open No. 51-70265). However, in this method, since the liquid plasticizer remains in the molded foam, the physical properties of the foam are lowered, and the excellent characteristics of the ultrahigh molecular weight polyethylene resin may be impaired. Moreover, in order to suppress this physical property fall, the process for removing and collect | recovering a liquid plasticizer from a foam is needed.
[0005]
The inventors first dissolved a non-reactive gas in a gaseous state at room temperature and normal pressure in an ultra-high molecular weight polyethylene resin under high pressure to make the resin easy to mold, kneaded and specified the molten resin. Proposed a method of obtaining an ultrahigh molecular weight polyethylene foam by extrusion foaming in the above resin temperature range (Japanese Patent Application No. 9-278859). In this method, since a non-reactive gas is used, the above-described residual plasticizer removal step is not necessary, and productivity is improved.
[0006]
[Problems to be solved by the invention]
The foam obtained by the above method retains the excellent properties of ultrahigh molecular weight polyethylene resin, such as abrasion resistance, self-lubrication, impact resistance, low temperature properties, chemical resistance, etc., and can be reduced in weight. Although it is advantageous in terms of cost, if the bubble diameter of the foamed portion becomes too large, there is a problem that the mechanical strength is lowered due to this, and application to a structural member or the like may be difficult.
[0007]
In view of the above points, an object of the present invention is to provide an ultra-high molecular weight polyethylene foam that does not require a liquid plasticizer removal step, has high productivity, and has high strength, and a method for producing the same. .
[0008]
[Means for Solving the Problems]
Ultra high molecular weight polyethylene foam according to the present invention, a viscosity-average molecular weight of 300,000 or more, the tensile strength is 20MPa or more, specific gravity 0.85 der less is, the average pore diameter is 0.1~15μm It is characterized by this.
[0009]
In addition, the method for producing an ultrahigh molecular weight polyethylene foam according to the present invention comprises dissolving a non-reactive gas in a gaseous state at room temperature and normal pressure in an ultrahigh molecular weight polyethylene resin having a viscosity average molecular weight of 300,000 or more under high pressure. In the method in which the resin is easily molded, melted and kneaded in an extruder, and then the molten resin is extruded and foamed from a mold attached to the discharge end of the extruder, the gas pressure at the time of melting the easily molded molten resin The resin is sent to the mold at the above resin pressure, and then the resin is cooled in the state where the above resin pressure is maintained. In this temperature range, extrusion is performed from the mold outlet and foaming is performed.
[0010]
First, the ultrahigh molecular weight polyethylene foam according to the present invention will be described. The resin constituting the ultrahigh molecular weight polyethylene foam according to the present invention is a polyethylene resin having a viscosity average molecular weight of 300,000 or more. A polyethylene resin having a viscosity average molecular weight of less than 300,000 does not have excellent properties such as wear resistance, self-lubricating property, impact resistance, and low-temperature properties. Does not have excellent properties.
[0011]
Further, the foam according to the present invention, Ru flat Hitoshiana径0.1~15μm der. By forming these holes , the specific gravity of the foam is 0.85 or less, preferably 0.7 or less. By reducing the specific gravity in this way, the weight of the foam can be reduced. The lower limit of the specific gravity is not particularly limited, but if it is less than 0.2, it is necessary to form a very large number of holes , and molding becomes difficult . Rights Hitoshiana径occurs a reduction in mechanical strength exceeds the 15μm easily, Therefore foam hardly retain original properties resins. When the average pore diameter is less than 0.1 μm, there is almost no decrease in specific gravity and the properties as a foam are not exhibited.
[0012]
The tensile strength of the foam according to the present invention is 20 MPa or more, preferably 30 MPa or more.
[0013]
Next, the manufacturing method of the foam by this invention is demonstrated. In the method of the present invention, a non-reactive gas in a gaseous state at normal temperature and normal pressure is used as the plasticizer for the ultrahigh molecular weight polyethylene resin. Non-reactive gas, as described later, the depressurization acts as foamed agent.
[0014]
The non-reactive gas is not particularly limited as long as it is an organic or inorganic substance that is a gas at normal temperature and normal pressure and does not deteriorate the resin. For example, carbon dioxide, nitrogen, argon, neon, helium, oxygen And inorganic gases such as chlorofluorocarbon gas and organic gases such as low molecular weight hydrocarbons. These may be used alone or in combination of two or more. Inorganic gas is preferable because it has a low adverse effect on the environment and does not require gas recovery.In particular, it has high solubility in ultrahigh molecular weight polyethylene resin, has a large plasticizing effect, and is released directly into the atmosphere. Carbon dioxide is preferred because it is almost harmless and the melt viscosity of the resin is greatly reduced.
[0015]
Methods for dissolving a non-reactive gas in an ultrahigh molecular weight polyethylene resin under high pressure include a method in which the gas is dissolved in a molten resin and a method in which the gas is dissolved in a solid resin. Any of these methods may be carried out, or both may be used in combination.
[0016]
As a method of dissolving the non-reactive gas in the molten resin under high pressure, for example, using a vent type screw extruder, the vent in the middle of the cylinder is used as a gas supply port, and gas is introduced from here. And a method in which a gas is press-fitted in the vicinity of the resin inflow portion into the first extruder or the second extruder using a tandem extruder and sufficiently dissolved in the kneaded resin in the second extruder. It is done.
[0017]
Examples of the method for dissolving the non-reactive gas in the solid state resin under high pressure include the following methods;
(1) A method in which gas is dissolved in a pellet or powdered resin in a high-pressure container or the like in advance. In this method, it is preferable to supply the resin in which the gas is dissolved to the extruder as quickly as possible in order to reduce the amount of the gas dissolved in the resin that escapes into the atmosphere by diffusion.
[0018]
(2) A method in which gas is dissolved in the resin from the hopper of the extruder in the solid transport section. In the case of this method, it is preferable to provide a pressure-resistant seal structure on the screw drive shaft and the hopper so that the gas does not evaporate out of the extruder.
[0019]
The gas may be supplied directly from a gas cylinder or may be pressurized and supplied using a plunger pump or the like.
[0020]
In the present invention, when a non-reactive gas in a gaseous state at normal temperature and normal pressure is dissolved in the ultrahigh molecular weight polyethylene resin under high pressure, the ultrahigh molecular weight polyethylene resin is plasticized to increase fluidity, and the inside of the screw extruder is increased. Makes it easy to melt and knead.
[0021]
The amount of the non-reactive gas dissolved in the ultra-high molecular weight polyethylene resin is not particularly limited as long as the melt-kneading viscosity of the resin becomes a viscosity suitable for molding by gas dissolution, and is appropriately selected depending on the molecular weight of the resin and the type of gas. it can.
[0022]
Next, the resin melt-kneaded in the extruder is sent to a mold attached to the discharge end of the extruder while maintaining a resin pressure equal to or higher than the gas pressure at the time of gas dissolution. In this case, if the resin pressure is lower than the gas pressure at the time of dissolution, as shown by the broken line (b) in FIG. May blow out of the machine from the mold exit.
[0023]
Next, while the resin sent to the mold is cooled in the mold, the resin pressure is maintained at a gas pressure equal to or higher than the gas pressure at the time of gas dissolution (the crystal peak temperature at the time of temperature decrease—10 ° C.) to (temperature decrease) In the temperature range of the crystal peak temperature at the time + 5 ° C.), the resin is extruded from the mold outlet and foamed.
[0024]
In this case, as shown by the one-dot chain line (c) in FIG. 2, if the resin pressure in the vicinity of the mold outlet is less than the gas pressure at the time of melting, bubbles with coarse pore diameters are easily formed and the appearance of the product is poor. It becomes difficult to obtain a good foam.
[0025]
In addition, when the resin is extruded at a temperature lower than (the crystal peak temperature at the time of cooling −10 ° C.), since the crystallization of the resin proceeds and the viscosity of the resin rises rapidly, almost no foaming occurs. As a result, the specific gravity does not decrease and a good foam cannot be obtained. On the contrary, when the resin is extruded at a temperature exceeding (the crystal peak temperature at the time of cooling + 5 ° C.), since the melt viscosity of the resin is low, bubbles having an average pore diameter larger than 15 μm are easily formed, resulting in a decrease in strength. There is a risk of obtaining a foam.
[0026]
In the present invention, the `` crystal peak temperature at the time of temperature decrease '' means the crystallization peak temperature when the molten resin is cooled and crystallized, and more specifically, at the time of such temperature decrease, It means the temperature at which the amount of heat generated by the resin is maximized. This temperature is measured by a differential scanning calorimeter (DSC) under atmospheric pressure. The “crystallization peak temperature” is described in detail in 9.2 of JIS K 7121 along with how to obtain it.
[0027]
When carbon dioxide is used as the non-reactive gas, the amount of carbon dioxide dissolved in the ultrahigh molecular weight polyethylene resin is preferably in the range of 1 to 30 wt%, more preferably 3 to 20 wt%.
[0028]
If the dissolved amount of carbon dioxide is less than 1 wt%, the viscosity of the ultrahigh molecular weight polyethylene resin will not be sufficiently lowered and extrusion will be difficult. If it exceeds 30 wt%, the pressure during dissolution will be increased using a large-scale facility. It may be necessary to make it extremely high, which is not preferable in terms of production efficiency.
[0029]
In order to make the dissolved amount of carbon dioxide within the above range, the pressure is preferably 0.5 to 50 MPa, more preferably 1.5 to 35 MPa.
[0030]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
[0031]
FIG. 1 is a schematic diagram illustrating an extruder that may be used in the present invention.
[0032]
As shown in FIG. 1, in this manufacturing method, first, carbon dioxide supplied from gas cylinders (10) and (11) is pressurized using a pressure pump (12) and (13), respectively, and then these high-pressure states are supplied. Carbon dioxide is supplied into the extruder (1) from gas supply ports (14) and (15) provided in the solid transport section (3) and the melt transport section (4) of the screw extruder (1). The hopper (16) has a pressure-resistant seal structure, from which ultrahigh molecular weight polyethylene resin is supplied into the rear end of the extruder (1). This resin is heated and melted by heating means (not shown) provided in the extruder (1), and is supplied from a gas supply port (14) located in the solid transport section (3) in a high-pressure state. Exposed to. Thereby, carbon dioxide is dissolved in the resin, and the viscosity of the resin is lowered.
[0033]
The resin that has advanced in the extruder (1) toward the front end by the screw (2) is completely melted by the heating means (not shown) provided in the extruder (1), and the melt transport section (4 ) And is exposed to high-pressure carbon dioxide supplied from the gas supply port (15) located at (5). This further dissolves carbon dioxide in the molten resin, further reducing the viscosity of the resin, and as a result, the ultra high molecular weight polyethylene resin is in an easily molded state. Depending on the amount of carbon dioxide dissolved, both of the two supply ports (14) and (15) may be used, or only one of them may be used.
[0034]
The easy-molded resin thus formed is sufficiently melt-kneaded with the screw (2), and then the melt-kneaded resin is attached to the extruder discharge end in a state where the resin pressure is maintained at the gas pressure or higher during melting. To the tubular mold (5).
[0035]
The pressure of the resin sent from the extruder to the mold should be such that a reduced cross-section is provided in the resin flow path of the mold, surface treatment is applied to the flow path, and the resin temperature in the mold is adjusted. By this, the gas pressure at the time of dissolution can be brought to a state higher than that.
[0036]
Next, while the resin is cooled in the tubular mold (5), the resin is maintained at a temperature equal to or higher than the gas pressure at the time of melting (the crystal peak temperature at the time of temperature decrease—10 ° C.) to (temperature decrease). A tubular foam is produced by extruding from the outlet of the tubular mold (5) in the temperature range of the crystal peak temperature at the time + 5 ° C.
[0037]
As with the above, the resin pressure at the mold outlet is also provided with a reduced cross-section in the resin flow path of the mold, surface treatment is applied to the flow path, and the resin temperature in the mold is adjusted. By doing, it can be made into the state more than the gas pressure at the time of melt | dissolution.
[0038]
The product form is not limited to the tubular form, and may be a pipe, a round bar, a deformed extrudate, a sheet or the like.
[0039]
As described above, according to this production method, an ultrahigh molecular weight polyethylene resin is plasticized with a non-reactive gas, melt-kneaded, the molten resin is sent to a mold at a specific resin pressure, and then in a specific temperature range. Since it is cooled and held at a specific resin pressure, it is extruded and foamed from the mold outlet, so it is possible to reduce the specific gravity and light weight, and it has excellent properties unique to ultrahigh molecular weight polyethylene resin, such as wear resistance and self-lubrication. Further, it is possible to produce a high-strength foam that retains properties such as impact resistance, low-temperature characteristics, and chemical resistance and does not have coarse bubbles that become defects inside.
[0040]
In addition, carbon dioxide naturally dissipates from the foam, so there is no need for a device that forcibly removes it, and carbon dioxide has a significantly lower negative impact on the environment than organic substances. There is an advantage that there is no particular harm even if it is dissipated.
[0041]
【Example】
EXAMPLES Hereinafter, although an Example demonstrates this invention concretely, these Examples are illustrations and do not limit this invention.
[0042]
Example 1
Ultrahigh molecular weight polyethylene resin (“Hi-Zex Million 240M” viscosity average molecular weight 2.3 million, melting point 136 ° C., temperature drop crystallization peak temperature 118 ° C. manufactured by Mitsui Petrochemical Co., Ltd.) is extruded from a pressure-resistant seal structure hopper (16) shown in FIG. Machine (1) (single shaft, screw diameter 40 mm, L / D = 30). Carbon dioxide was used as the non-reactive gas, and this was injected from the gas supply port (14) (15) into the solid transport part (3) and the melt transport part (4) of the extruder (1) at a pressure of 15 MPa. . At this pressure, the amount of carbon dioxide dissolved in the ultrahigh molecular weight polyethylene resin was about 10 wt%. The carbon dioxide in the extruder was held in a high pressure state by the high pressure shaft sealing mechanism of the drive shaft of the screw (2), the pressure seal structure of the hopper (16), and the molten resin in the extruder. Next, in the extruder, the resin was sufficiently melt-kneaded 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.
[0043]
Subsequently, by maintaining the tip temperature of the tubular mold (5) (inlet resin channel cross section outer diameter 40 mm-inner diameter 30 mm, outlet resin channel cross section outer diameter 40 mm-inner diameter 38.5 mm) at 118 ° C, The resin pressure at the mold inlet is 40 MPa, the resin pressure near the mold outlet is 25 MPa, the temperature of the resin passing through the mold outlet is 118 ° C., and the resin is extruded from the mold outlet into a tube shape. A molecular weight polyethylene foam was prepared.
[0044]
When the tensile test (JIS K 7127 compliant temperature 23 ° C.) of the foam thus obtained was performed, the tensile strength was 45 MPa. The specific gravity was 0.52. Many bubbles with an average pore diameter of about 8 μm were observed inside.
[0045]
Example 2
By maintaining the mold tip temperature during extrusion at 110 ° C., the resin pressure at the mold inlet is 50 MPa, the resin pressure near the mold outlet is 30 MPa, and the temperature of the resin passing through the mold outlet is 110 The operation was performed under the same conditions as in Example 1 except that the temperature was changed to ° C. The resulting foam had a tensile strength of 50 MPa. The specific gravity was 0.63. Many bubbles with an average pore diameter of about 4 μm were observed inside.
[0046]
Example 3
By maintaining the mold tip temperature at the time of extrusion at 121 ° C., the resin pressure at the mold inlet is 35 MPa, the resin pressure near the mold outlet is 22 MPa, and the temperature of the resin passing through the mold outlet is 121 MPa. The operation was performed under the same conditions as in Example 1 except that the temperature was changed to ° C. The resulting foam had a tensile strength of 33 MPa. The specific gravity was 0.45. Many bubbles with an average pore diameter of about 12 μm were observed inside.
[0047]
Comparative Example 1
By maintaining the mold tip temperature during extrusion at 126 ° C., the resin pressure at the mold inlet is 30 MPa, the resin pressure near the mold outlet is 18 MPa, and the temperature of the resin passing through the mold outlet is 126. The operation was performed under the same conditions as in Example 1 except that the temperature was changed to ° C. The tensile strength of the obtained foam was 15 MPa. The specific gravity was 0.13. Many bubbles with an average pore diameter of about 200 μm were observed inside.
[0048]
Comparative Example 2
By maintaining the mold tip temperature at the time of extrusion at 100 ° C., the resin pressure at the mold inlet is 55 MPa, the resin pressure near the mold outlet is 35 MPa, and the temperature of the resin passing through the mold outlet is 100 The operation was performed under the same conditions as in Example 1 except that the temperature was changed to ° C. The specific gravity of the extrudate was 0.89, which was hardly foamed.
[0049]
Comparative Example 3
By setting the outlet resin flow path cross section of the mold to an outer diameter of 40 mm and an inner diameter of 37 mm, and maintaining the mold tip temperature at the time of extrusion at 120 ° C., the resin pressure at the mold inlet is 30 MPa, and the mold outlet is in the vicinity of the mold outlet. The operation was performed under the same conditions as in Example 1 except that the resin pressure was 6 MPa and the temperature of the resin passing through the mold outlet was 120 ° C. A lot of very rough bubbles appeared, the appearance was poor, and a good foam was not obtained.
[0050]
【The invention's effect】
The foam comprising the ultra-high molecular weight polyethylene resin according to claim 1 retains properties such as abrasion resistance, self-lubricating property, impact resistance, low temperature property, and chemical resistance, which are excellent properties of the resin. In addition, since the inside contains bubbles having fine pore diameters that do not substantially become defects, it is possible to maintain both weight reduction and high mechanical strength due to a decrease in specific gravity.
[0051]
According to the method for producing an ultrahigh molecular weight polyethylene foam of claim 2, the resin is plasticized and melted by dissolving a non-reactive gas in a gaseous state at normal temperature and normal pressure in an ultrahigh molecular weight polyethylene resin under high pressure. while enabling kneading extrusion, non-reactive gas also acts as a foaming agent, an ultra-high molecular weight polyethylene foam fine pore pore size can be easily manufactured. Therefore, an organic solvent removal and recovery process is not required, and an ultrahigh molecular weight polyethylene foam having high productivity and high strength can be obtained.
[0052]
Further, in the method according to claim 3, since carbon dioxide is used as the non-reactive gas, the solubility of the gas in the ultrahigh molecular weight polyethylene resin is high, and the melt viscosity of the resin can be greatly reduced. It can be improved further.
[Brief description of the drawings]
FIG. 1 is a schematic view showing an extruder and a mold that can be used in the present invention.
FIG. 2 is a graph showing a resin pressure pattern in an extruder and its mold.
[Explanation of symbols]
In FIG.
1: Extruder 2: Screw 3: Solid transport unit 4: Melt transport unit 5: Mold 10: Gas cylinder 11: Gas cylinder 12: Pressure pump 13: Pressure pump 14: Gas supply port 15: Gas supply port 16: Hopper
Solid line a: Example of resin pressure pattern in the present invention Broken line b and alternate long and short dash line c: Example of resin pressure pattern outside the scope of the present invention

Claims (3)

A viscosity-average molecular weight of 300,000 or more and a tensile strength of not less than 20 MPa, Ri der specific gravity of 0.85 or less, an average pore diameter is characterized by a 0.1~15μm ultra high molecular weight polyethylene foam .   A non-reactive gas in a gaseous state at normal temperature and normal pressure is dissolved in an ultrahigh molecular weight polyethylene resin having a viscosity average molecular weight of 300,000 or more under high pressure to make the resin easy to mold, and then melt-kneaded in an extruder, In the method of extruding and foaming the molten resin from a mold attached to the discharge end of the extruder, the molten resin in an easily molded state is sent to the mold at a resin pressure equal to or higher than the gas pressure at the time of melting, and then the resin pressure is maintained. In this state, the resin is cooled and extruded from the mold outlet in the temperature range of (crystallization peak temperature at the time of falling temperature −10 ° C.) to (crystallization peak temperature at the time of cooling + 5 ° C.). A method for producing an ultrahigh molecular weight polyethylene foam.   The method for producing an ultrahigh molecular weight polyethylene foam according to claim 2, wherein the non-reactive gas is carbon dioxide.

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