Patent Description:
In general, polyamide resins have excellent mechanical characteristics, heat resistance, impact resistance, and chemical resistance, and are widely used for automobile parts, electrical parts, electronic parts, and household goods and the like. In particular, it is known that a polyamide resin to which an inorganic reinforcing material represented by glass fiber is added has significantly improved rigidity, strength, and heat resistance, and has rigidity particularly improved in proportion to the addition amount of the inorganic reinforcing material.

However, if a large amount of <NUM> to <NUM>% by mass of a reinforcing material such as glass fiber is added to the polyamide resin for the purpose of improving rigidity and strength, a molded article appearance (specular glossiness of a surface, and surface uniformity of an embossed surface and the like) is extremely deteriorated, and the commercial value of a molded article is significantly impaired. Therefore, as a method for improving the molded article appearance, it has been proposed to add an amorphous resin to a crystalline polyamide resin (Patent Documents <NUM> to <NUM>). However, by these methods, good specular glossiness of the surface, and good surface uniformity of the embossed surface cannot be obtained.

There has been known a method in which a semi-aromatic polyamide resin (MXD-<NUM>) is highly filled with nylon <NUM>, glass fiber, or mica to increase strength and rigidity (for example, Patent Document <NUM>). In this case, it is necessary to raise a mold temperature during molding to a high temperature of <NUM>, and a good molded article appearance may not be obtained even when the mold temperature is raised to a high temperature.

Therefore, a polyamide resin composition has been proposed, in which a molded article appearance is not deteriorated even when an inorganic reinforcing material such as glass fiber is added in a large amount of <NUM>% by mass or more (Patent Document <NUM>). According to the resin composition disclosed in Patent Document <NUM>, a substantially good molded article appearance can be obtained even when a mold temperature during molding is <NUM> or lower. However, there are a plurality of types of resins, and compositions of additives and pigments and the like also become complicated, which disadvantageously cause appearance defects to occur unexpectedly in molded articles and cause deteriorated weather resistance in some cases. In order to solve the problems, improvement has been required in order to provide a producing method maintaining stable quality. Reference is also made to <CIT> which relates to a method for preparing a polyamide resin composition involving polycaproamide, fillers and copper.

Therefore, the present invention provides a method for producing a polyamide resin composition which not only provides a molded article having a good appearance with a higher level when a mold temperature during molding is <NUM> or lower but also maintains stable quality of the polyamide resin composition having excellent weather resistance of the appearance of the surface of the molded article.

As a result of intensive studies to solve the above problems, the present inventors have found that a method for producing a polyamide resin composition capable of stably providing a molded article having excellent weather resistance and a highly excellent appearance (uniformity of an embossed surface) of the molded article can be provided by reviewing the polyamide resin composition described in Patent Document <NUM> in detail, blending carbon black and a copper compound in specific amounts, and particularly using the copper compound as a dispersion liquid having a specific concentration, and have completed the present invention.

A method for producing a polyamide resin composition of the present invention not only provides a molded article having a good appearance with a higher level when a mold temperature during molding is <NUM> or lower but also can stably produce a molded article having an excellent weather resistance of the appearance such as the embossed surface of the obtained molded article. Furthermore, the method has also excellent operability.

Hereinafter, the present invention will be specifically described. First, components used in the present invention will be described.

In the present invention, as for the crystallinity/non-crystallinity of a polyamide resin, when the polyamide resin is subjected to DSC measurement at a temperature rising rate of <NUM>/min in accordance with JIS K <NUM>: <NUM>, a polyamide resin showing a clear melting point peak is defined as crystallinity, and a polyamide resin not showing a clear melting point peak is defined as non-crystallinity.

Unless otherwise specified, the content (blending amount) of each component in the polyamide resin composition of the present invention is represented by an amount based on <NUM> parts by mass of the total of a crystalline polyamide resin (A) containing a polycaproamide resin as a main component, a semi-aromatic amorphous polyamide resin (B), an inorganic reinforcing material (C), and a master batch of carbon black (D).

The component (A) in the present invention is a crystalline polyamide resin containing a polycaproamide resin as a main component. The polycaproamide resin is obtained by polymerization of ε-caprolactam, usually referred to as nylon <NUM>. The relative viscosity (<NUM>% sulfuric acid method) of the polycaproamide resin in the present invention is preferably within the range of <NUM> to <NUM>. Particularly preferably, the relative viscosity is within the range of <NUM> to <NUM>. When the relative viscosity is within this range, the toughness and fluidity of the resin (the fluidity provides a desired molded article appearance) can be satisfied. However, it is more realistic to regulate the melt mass flow rate of the polyamide resin composition than to regulate the relative viscosity of the polycaproamide resin.

The content of the polycaproamide resin in the component (A) is preferably <NUM>% by mass or more, more preferably <NUM>% by mass or more, and still more preferably <NUM>% by mass or more. The content of the polycaproamide resin may be <NUM>% by mass. In the component (A), the crystalline polyamide resin that may be contained as a component other than the polycaproamide resin is not particularly limited, and examples thereof include polytetramethylene adipamide (polyamide <NUM>), polyhexamethylene adipamide (polyamide <NUM>), polyundecamethylene adipamide (polyamide <NUM>), polymethaxylylene adipamide (polyamide MXD6), polyparaxylylene adipamide (polyamide PXD6), polytetramethylene sebacamide (polyamide <NUM>), polyhexamethylene sebacamide (polyamide <NUM>), polydecamethylene adipamide (polyamide <NUM>), polydecamethylene sebacamide (polyamide <NUM>), polyhexamethylene dodecamide (polyamide <NUM>), polydecamethylene dodecamide (polyamide <NUM>), polyhexamethylene isophthalamide (polyamide 6I), polytetramethylene terephthalamide (polyamide 4T), polypentamethylene terephthalamide (polyamide 5T), poly-<NUM>-methylpentamethylene terephthalamide (polyamide M-5T), polyhexamethylene terephthalamide (polyamide 6T), polyhexamethylene hexahydroterephthalamide (polyamide 6T(H)), polynonamethylene terephthalamide (polyamide 9T), polydecamethylene terephthalamide (polyamide 10T), polyundecamethylene terephthalamide (polyamide 11T), polydodecamethylene terephthalamide (polyamide 12T), polylauryl lactam (polyamide <NUM>), poly-<NUM>-aminoundecanoic acid (polyamide <NUM>), and copolymers of these constituent units.

The component (B) in the present invention is a semi-aromatic amorphous polyamide resin containing an aromatic component in either a diamine component or a dicarboxylic acid component. Examples of the dicarboxylic acid include terephthalic acid, isophthalic acid, adipic acid, and sebacic acid, and examples of the diamine include tetramethylenediamine, hexamethylenediamine, metaxylylenediamine, para-xylylenediamine, undecamethylenediamine, dodecamethylenediamine, <NUM>-methylpentamethylenediamine, trimethylhexamethylenediamine, aminoethylpiperazine, and bisaminomethylcyclohexane. Among them, polyamide 6T/6I containing terephthalic acid, isophthalic acid, and hexamethylenediamine as raw materials is preferable.

The relative viscosity (<NUM>% sulfuric acid method) of the semi-aromatic amorphous polyamide resin is not particularly limited, but is preferably within the range of <NUM> to <NUM>.

The total content of the component (A), the component (B), and the master batch of carbon black (D) to be described later is <NUM> to <NUM> parts by mass, and preferably <NUM> to <NUM> parts by mass. The content of the component (B) is <NUM> to <NUM> parts by mass, and preferably <NUM> to <NUM> parts by mass. When the content of the component (B) is less than <NUM> parts by mass, a good molded article appearance with a higher level cannot be obtained. Conversely, when the content of the component (B) is more than <NUM> parts by mass, the crystallization solidification of the molded article is deteriorated, which causes mold release failure during molding or reduced hot rigidity. The content of the component (A) is not particularly limited as long as it is determined in consideration of the contents of the component (B) and the component (D), but is preferably <NUM> to <NUM> parts by mass, and more preferably <NUM> to <NUM> parts by mass.

In the present invention, the mass ratio of the component (A) and the component (B) needs to satisfy the following formula.

In the present invention, when (B)/(A) is within this range, a molded article having a good appearance with a higher level is obtained, and (B)/(A) is preferably <NUM> or more and <NUM> or less, and more preferably <NUM> or more and <NUM> or less.

The component (C) in the present invention is an inorganic reinforcing material, and includes glass fiber (C-<NUM>), needle-shaped wollastonite (C-<NUM>), and a plate-shaped crystal inorganic reinforcing material (C-<NUM>). Examples of the plate-shaped crystal inorganic reinforcing material (C-<NUM>) include mica, talc, and unfired clay. Among them, mica and talc are preferable, and mica is more preferable. As the component (C), a fibrous inorganic reinforcing material such as whisker, carbon fiber, or ceramic fiber, or a powdery inorganic reinforcing material such as silica, alumina, kaolin, quartz, powdery glass (milled fiber), or graphite may be contained as long as the effect of the present invention is not impaired. These inorganic reinforcing materials to be used may be subjected to surface treatment such as aminosilane treatment.

As the glass fiber (C-<NUM>), general glass fiber having an average cross-sectional diameter of about <NUM> to <NUM> and a cut length of about <NUM> to <NUM> can be used. The average fiber length of the glass fiber in the molded article is reduced to about <NUM> to <NUM> in a processing step (compounding step/molding step). The content of the glass fiber (C-<NUM>) is <NUM> to <NUM> parts by mass, and preferably <NUM> to <NUM> parts by mass. If the content is less than <NUM> parts by mass, the strength and rigidity of the molded article are low. Conversely, if the content exceeds <NUM> parts by mass, it is difficult to obtain a good molded article appearance, which is not preferable.

The needle-shaped wollastonite (C-<NUM>) is wollastonite having an average cross-sectional diameter of about <NUM> to <NUM> and an average fiber length of about <NUM> to <NUM>. The content of the needle-shaped wollastonite (C-<NUM>) is <NUM> to <NUM> parts by mass, preferably <NUM> to <NUM> parts by mass, and more preferably <NUM> to <NUM> parts by mass. If the content is less than <NUM> parts by mass, the strength and rigidity of the molded article are low. Conversely, if the content exceeds <NUM> parts by mass, it is difficult to obtain a good molded article appearance, which is not preferable.

Examples of the plate-shaped crystal inorganic reinforcing material (C-<NUM>) include talc, mica, and unfired clay, and the shape thereof is a form like a fish scale. The content thereof is <NUM> to <NUM> parts by mass, preferably <NUM> to <NUM> parts by mass, and more preferably <NUM> to <NUM> parts by mass. If the content is less than <NUM> parts by mass, the strength and rigidity of the molded article are low. Conversely, if the content exceeds <NUM> parts by mass, it is difficult to obtain a good molded article appearance, which is not preferable. Among the plate-shaped crystal inorganic reinforcing materials (C-<NUM>), mica is particularly excellent in terms of strength and rigidity.

The content of the inorganic reinforcing material as the component (C) is <NUM> to <NUM> parts by mass, preferably <NUM> to <NUM> parts by mass, and more preferably <NUM> to <NUM> parts by mass. When the content is less than <NUM> parts by mass, the strength and rigidity of the molded article are low. Conversely, when the content is more than <NUM> parts by mass, a good molded article appearance cannot be obtained, and the strength is also deteriorated. When <NUM> to <NUM> parts by mass of the component (C-<NUM>), <NUM> to <NUM> parts by mass of the component (C-<NUM>), and <NUM> to <NUM> parts by mass of the component (C-<NUM>) are contained as the component (C), the strength and rigidity of the molded article are excellent, and the surface appearance (specular glossiness of the surface, and surface uniformity of the embossed surface) of the molded article is excellent.

All the components (C) exhibit a reinforcing effect in the polyamide resin composition, and among them, the glass fiber (C-<NUM>) has the highest reinforcing effect, but the warp deformation of the molded article is also large. The needle-shaped wollastonite (C-<NUM>) and the plate-shaped crystal inorganic reinforcing material (C-<NUM>) have no reinforcing effect as much as that of glass fibers, but have an aspect ratio smaller than that of glass fibers, and thus have an advantage that warpage deformation is reduced. The needle-shaped wollastonite can also contribute to the prevention of sink marks after molding, and by appropriately combining these components, a resin composition that does not cause large deformation after molding even when a reinforcing material is blended at a high concentration can be produced.

In general, a reinforced polyamide resin composition to which glass fibers and wollastonite and the like are blended at a high concentration has poor weather resistance, whereby the reinforcing material is exposed. However, weather resistance can be controlled by combining a carbon black master batch with a copper compound as described later, whereby the exposure of the reinforcing material can be prevented.

The component (D) in the present invention is a carbon black master batch, and it is preferable that LD-PE (low density polyethylene) or AS resin (acrylonitrile-styrene copolymer) which is compatible with a polyamide resin is used as a base resin, and <NUM> to <NUM>% by mass of carbon black is contained in the master batch. By using these master batches, not only the dispersibility of carbon black is excellent and the effect of excellent working environmental properties is obtained, but also the effect of suppressing the floating and exposure of glass fibers and other inorganic reinforcing materials is high, and the effect of improving the weather resistance of the molded article appearance is exhibited. The content of the carbon black master batch is <NUM> to <NUM> parts by mass, and preferably <NUM> to <NUM> parts by mass. The content of the carbon black is preferably within the range of <NUM> to <NUM> parts by mass, and more preferably within the range of <NUM> to <NUM> parts by mass. When the content of the carbon black is more than <NUM> parts by mass, mechanical physical properties may be deteriorated.

The component (E) in the present invention is a copper compound, and is not particularly limited as long as it is a compound containing copper, and examples thereof include copper halide (copper iodide, cuprous bromide, cupric bromide, cuprous chloride, and the like), copper acetate, copper propionate, copper benzoate, copper adipate, copper terephthalate, copper isophthalate, copper salicylate, copper nicotinate and copper stearate, and copper complex salts in which a chelating agent such as ethylenediamine or ethylenediaminetetraacetic acid is coordinated to copper. One type of these may be used alone, or two or more thereof may be used in combination.

Among the copper compounds listed above, the copper compound is preferably copper halide, more preferably one or more selected from the group consisting of copper iodide, cuprous bromide, cupric bromide, and cuprous chloride, still more preferably one or more selected from the group consisting of copper iodide, cuprous bromide, and cupric bromide, and particularly preferably cupric bromide. By using the copper compound, a polyamide resin composition having excellent weather resistance can be obtained.

The content of the component (E) is <NUM> to <NUM> parts by mass, and preferably <NUM> to <NUM> parts by mass. If <NUM> parts by mass or more of the component (E) is added, the risks of metal corrosiveness and discoloration increase.

When the copper compound (E) is uniformly mixed in the resin composition, the copper compound (E) needs to be used as a dispersion liquid having a concentration of <NUM>% by mass to <NUM>% by mass. When the copper compound is used as the dispersion liquid having a concentration of <NUM>% by mass or less, the copper compound can be uniformly kneaded in the resin composition. The concentration is preferably <NUM>% by mass to <NUM>% by mass. When the concentration is less than <NUM>% by mass, a dispersion medium (for example, water) increases in order to add a necessary amount of the copper compound (E), which may cause problems in terms of operability, such as an increase in the amount of vapor generated from the dispersion medium during kneading, and adhesion of a raw material component to a hopper or the like of a kneader stand due to the influence of the dispersion medium. The dispersion liquid of the copper compound (E) may be a solution in which the copper compound (E) is completely dissolved. The dispersion liquid of the copper compound (E) is preferably an aqueous solution of the copper compound (E).

Furthermore, the polyamide resin composition of the present invention is required to have a melt mass flow rate of <NUM>/<NUM> or more and less than <NUM>/<NUM> at a moisture content of <NUM>% (<NUM>% by mass) or less, and a temperature-lowering crystallization temperature (TC2) measured with a differential scanning calorimeter (DSC) of <NUM> ≤ (TC2) ≤ <NUM>.

The melt mass flow rate (MFR) is a value measured at <NUM> under a load of <NUM> in accordance with JIS K <NUM>-<NUM>: <NUM>. The temperature-lowering crystallization temperature (TC2) is a peak temperature obtained in measurement in which a differential scanning calorimeter (DSC) is used to raise the temperature to <NUM> at a temperature rising rate of <NUM>/min under a nitrogen stream, retain the temperature for <NUM> at the temperature, and lower the temperature to <NUM> at a rate of <NUM>/min.

When the melt mass flow rate is less than <NUM>/<NUM>, a good molded article appearance is not obtained. In order to obtain a polyamide resin composition having a melt mass flow rate of <NUM>/<NUM> or more, if a crystalline polyamide resin having a relative viscosity of <NUM> or more, which is usually used, is used, the range of the melt mass flow rate may not be reached (less than <NUM>/<NUM>). Therefore, it is preferable to use a crystalline polyamide resin having an ultra-low viscosity (relative viscosity: <NUM> to <NUM>) or to adopt a formulation in which a molecular chain-cleaving agent for the polyamide resin is added during compounding processing, or the like. As the molecular chain-cleaving agent (also referred to as viscosity-decreasing agent) for the polyamide resin, aliphatic dicarboxylic acids and aromatic dicarboxylic acids and the like are effective. Specific examples thereof include, but not particularly limited to, oxalic acid, malonic acid, succinic acid, adipic acid, azelaic acid, sebacic acid, phthalic acid, and terephthalic acid. When the molecular chain-cleaving agent is added (contained), and the addition amount thereof is around <NUM> to <NUM> parts by mass with respect to <NUM> parts by mass of the total of the components (A), (B), (C), and (D) of the present invention, the melt mass flow rate of the composition of the present invention is <NUM>/<NUM> or more. However, the effect of the molecular chain-cleaving agent varies depending on compounding processing conditions, and as a matter of course, as a processing temperature is higher, or as a polymer retention time at the time of compounding is longer, the effect is more excellent. Usually, a compounding processing temperature is within the range of <NUM> to <NUM>, and a polymer retention time at the time of compounding is generally within <NUM> to <NUM> seconds.

When the melt viscosity of the resin is low, drawdown or difficult weighing may occur at the time of injection molding. When the melt mass flow rate is <NUM>/<NUM> or more, a molding condition width at the time of injection molding may be narrowed.

Furthermore, when the temperature-lowering crystallization temperature (TC2) does not satisfy <NUM> ≤ (TC2) ≤ <NUM>, a good molded article appearance with a higher level is not obtained due to the crystallization rate of the polyamide resin composition.

To the polyamide resin composition of the present invention, a heat resistance stabilizer, an antioxidant, an ultraviolet absorber, a light stabilizer, a plasticizer, a lubricant, a crystal nucleating agent, a release agent, an anti-static agent, a flame retardant, a pigment, a dye, or another type of polymer or the like can also be added as necessary. In the polyamide resin composition of the present invention, the total of the components (A), (B), (C), (D), and (E) preferably accounts for <NUM>% by mass or more, and more preferably <NUM>% by mass or more.

The method for producing the polyamide resin composition of the present invention requires a single screw or twin screw extruder, a kneading machine, and a kneader and the like, but from the viewpoint of productivity, a twin screw extruder is preferable. Screw arrangement is not particularly limited, but a kneading zone is preferably provided in order to more uniformly disperse components. As a specific method, a crystalline polyamide resin (A) containing a polycaproamide resin as a main component, a semi-aromatic amorphous polyamide resin (B), a master batch of carbon black (D), a dispersion liquid of a copper compound (E), and other optional components are preblended with a blender, and the preblended product is charged from a hopper into a uniaxial or biaxial extruder. Then, in a state where at least a part of the components (A), (B), and (E) are melted, glass fiber (C-<NUM>), needle-shaped wollastonite (C-<NUM>), and a plate-shaped crystal inorganic reinforcing material (C-<NUM>) as an inorganic reinforcing material (C) are charged into the molten mixture by a side feed method, and melt-kneaded. Then, the melt-kneaded product is discharged into a strand shape, cooled, and cut.

The liquid component (dispersion medium) containing the component (E) can suppress gradual separation and segregation of the component (A), the component (B), and the component (D) with a very weak adhesive force. Therefore, as the degree of difference in the shape, apparent specific gravity, and friction coefficient and the like of the pellets of each component is larger, the effect of the present invention is exhibited.

The polyamide resin composition produced by the present invention has the following excellent weather resistance because it is produced with the composition as described above. That is, a color difference ΔE after a weather resistance test (in accordance with JIS K-<NUM>-<NUM>) using a xenon weather meter is <NUM> or less, more preferably <NUM> or less, still more preferably <NUM> or less, and particularly preferably <NUM> or less. Details of the weather resistance test depend on a procedure described in Examples described later. When the color difference ΔE is equal to or less than the above value, the polyamide resin composition can withstand outdoor use exposed to rainfall.

Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to these Examples at all.

Characteristics and physical property values shown in the following Examples and Comparative Examples were measured by the following test methods.

<NUM>) Evaluation of production stability: The color difference ΔE in the evaluation of the weather resistance test in the above <NUM>) was measured using the resin composition immediately after the start of melt-kneading, the resin composition after <NUM> from the start of melt-kneading, and the resin composition after <NUM> hour from the start of melt-kneading. Determination was made in accordance with the following criteria.

Raw materials used in Examples and Comparative Examples are as follows.

When an aqueous solution of a copper compound was charged, an addition method A was described in Table <NUM>, and when the copper compound was directly charged as it was, an addition method B was described in Table <NUM>. In the addition method A, an aqueous solution of cupric bromide as the copper compound was used. The concentration of the aqueous solution of the copper compound was adjusted to the concentration described in Table <NUM>.

Raw materials other than the inorganic reinforcing material were mixed in advance so as to have compositions shown in Table <NUM>, and the mixture was charged from a hopper part of a twin screw extruder. Each reinforcing material was charged from a side feeder of the twin screw extruder. Compounding was performed at a cylinder temperature of the twin screw extruder of <NUM> and a screw rotation speed of <NUM> rpm to produce pellets. The obtained pellets were dried with a hot air dryer until the moisture content was <NUM>% or less, and then various characteristics were evaluated.

From Table <NUM>, test pieces of Examples <NUM> to <NUM> had a small color difference ΔE before and after the weather resistance test, a highly excellent surface appearance (uniformity of the embossed surface), weather resistance capable of maintaining an excellent surface appearance even after the weather resistance test, and excellent productivity. Meanwhile, in test pieces of Comparative Examples <NUM> and <NUM>, the surface appearance (specular glossinesss) was not satisfactory. The surface appearance could not be maintained after the weather resistance test, and the productivity was also poor. In a test piece of Comparative Example <NUM>, the surface appearance (specular glossiness) was slightly poor. The surface appearance (uniformity of the embossed surface) could not be maintained after the weather resistance test, and the production stability was also slightly poor. In a test piece of Comparative Example <NUM>, the surface appearance (specular glossiness) was not satisfactory. The surface appearance (uniformity of the embossed surface) could not be maintained after the weather resistance test, and the production stability was also poor. In Comparative Example <NUM>, the aqueous solution concentration was <NUM>% by mass or more, whereby the operability had no problem, but the aqueous solution concentration was high, whereby the dispersion in the raw material did not proceed, which caused a problem in production stability. In Comparative Example <NUM>, the aqueous solution concentration was less than <NUM>% by mass. The resin adhered and remained in the hopper, and a large amount of gas was generated from the vent port.

Claim 1:
A method for producing a polyamide resin composition containing a crystalline polyamide resin (A) containing a polycaproamide resin as a main component, a semi-aromatic amorphous polyamide resin (B), an inorganic reinforcing material (C), a master batch of carbon black (D), and a copper compound (E),
wherein:
the polyamide resin composition contains glass fiber (C-<NUM>), needle-shaped wollastonite (C-<NUM>), and a plate-shaped crystal inorganic reinforcing material (C-<NUM>) as the inorganic reinforcing material (C);
a melt mass flow rate (MFR) of the polyamide resin composition at a moisture content of <NUM>% or less is <NUM>/<NUM> or more and less than <NUM>/<NUM>, the MFR being measured according to the description;
a temperature-lowering crystallization temperature (TC2) of the polyamide resin composition, the temperature-lowering crystallization temperature (TC2) being measured with a differential scanning calorimeter (DSC) according to the description is <NUM> or higher and <NUM> or lower;
a mass ratio of (A) to (B) satisfies <NUM> < (B)/(A) ≤ <NUM>;
when the total of (A), (B), (C), and (D) is <NUM> parts by mass, the content of (E) is <NUM> to <NUM> parts by mass;
the content of each of the components satisfies the following formulae: <MAT> <MAT> <MAT> <MAT> <MAT> <MAT>and <MAT>
a dispersion liquid of the copper compound (E) has a concentration of <NUM>% by mass to <NUM>% by mass; and
the crystalline polyamide resin (A) containing a polycaproamide resin as a main component, the semi-aromatic amorphous polyamide resin (B), the master batch of carbon black (D), and the dispersion liquid of the copper compound (E) are mixed in advance, followed by charging the mixture is into a hopper part of an extruder, and charging the inorganic reinforcing material (C) into the extruder by a side feed method.