Plastic and nonmetallic article shaping or treating: processes – Pore forming in situ – By treating occluded solids
Reexamination Certificate
2000-06-20
2002-05-21
Kuhns, Allan R. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Pore forming in situ
By treating occluded solids
C264S344000
Reexamination Certificate
active
06391233
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of producing a porous product and, more specifically, to a method that is capable of producing a porous product having a porosity as high as but not less than 50% by volume and controlling the porosity thereof by the use of a conventional injection molding machine or a conventional extruder. Further, the porous product produced according to the method of the present invention may be brought into direct contact with a human body.
2. Discussion of the Related Art
One known method of producing a porous product uses a molding material formed by mixing a resin with a volatile blowing agent such as carbon dioxide gas or ammonia gas, or a decomposable blowing agent such as azodicarbondiamide or dinitrosopentamethylenetetramine. With this method, the blowing agent is volatilized or decomposed in a heat-molding process to generate gas, which in turn forms pores.
In general, the method using a blowing agent for formation of pores produces porous products of the type having closed cells and, hence, is unsuitable for the production of porous products requiring a gas permeability. Also, such a method, in general, is prone to form individually larger pores or cells and, hence, is unsuitable for the production of porous products requiring higher functional characteristics such as cushioning characteristics or sound insulating characteristics. Further, since all of the blowing agent is not necessarily expanded, it is possible that some unexpanded blowing agent remains in the resulting porous product. Such residual blowing agent not only makes it difficult to estimate porosity of a resulting porous product but it also prevents the resulting porous product from being used in an application where the porous product is brought into direct contact with a human body, for example, as an ear plug, if the blowing agent contains ammonia, dinitrosotetamine or a like substance which is harmful to a human body.
A typical method of producing a form rubber comprises admixing of a blowing agent and a coagulant with a latex, and molding the resulting mixture. On the other hand, a typical method of producing a porous product such as urethane foam composes molding of a molding material formed by mixing a prepolymer with a blowing agent such as Freon or water as well as a catalyst for a curing reaction. These methods utilize a pore-forming mechanism relying upon the generation of gas volatilized from the blowing agent and, therefore, involve problems similar to those described above.
A method of producing a porous product such as an expanded polyethylene includes irradiation of a resin with an electron beam to cause the resin to be crosslinked while expanding a blowing agent. Although such a method is capable of controlling the porosity of a porous product, the electron beam irradiation results in an increased production cost. Besides, the problem of the residual blowing agent is still left unsolved.
A less costly method of producing a porous product enabling estimation and control of the porosity of a resulting porous product has been proposed. This method comprises the steps of: adding a powdery pore-forming agent such as sodium chloride or sodium sulfide (hereinafter referred to as a “salt-type pore-forming agent”) to a resin or rubber to form a molding material; subjecting the molding material to molding to provide a molded product; and washing the resulting molded product with water to elute the salt or the pore-forming agent thereby forming pores. Such a method is called a “desalting method”.
In a desalting method, since portions from which the pore-forming agent has been eluted become pores, and since the pore-forming agent itself is not expanded or foamed in the molding process, it is required that the molding material contain the pore-forming agent in an amount corresponding to the intended porosity. Accordingly, where a porous product having a porosity of 50% is to be produced, the pore-forming agent needs to be added to the resin component so that the volume ratio of the pore-forming agent in the resulting molding material assumes 50% or more by volume.
It is, however, difficult to mold such a molding material containing 50% or more by volume of the salt-type pore-forming agent. Specifically, the salt-type pore-forming agent remains in a solid state or powdery state at a typical molding temperature of resin because of its higher melting point. For this reason, the fluidity of a molding material decreases with a higher content of the salt-type pore-forming agent, and a molding material containing 50% or more by volume of the salt-type pore-forming agent fails to exhibit a fluidity (MFR value) required for molding. In injection molding or extrusion molding, in particular, the salt-type pore-forming agent in a powdery state cannot be sufficiently extruded or injected though the hydraulic resin component in the molding material that is to be extruded through an extrusion die or to be injected into a mold. This will result in a molded product containing the pore-forming agent in a lower amount, or in a non-homogeneous molded product containing the pore-forming agent which is rich in an inner part but lean or absent in a superficial part of the product, despite the pore-forming agent being abundantly contained in the molding material. Naturally, a decrease in the content of the pore-forming agent leads to a decrease in the porosity of a molded product. Further, such a non-homogeneous molded product gives a non-homogeneous porous product which eventually has a lower porosity than desired because the pore-forming agent is not sufficiently eluted by soaking due to the presence of too small an amount of the pore-forming agent in the superficial part of the molded product.
It is conceivable to improve the fluidity of the polymer component by raising the molding temperature. Even in this case, the salt-type pore-forming agent remains in a powdery state in the molding material and, hence, the pore-forming agent is difficult to extrude or inject as compared with the polymer component when the molding material is extruded through an extrusion die or injected into a mold, thus, resulting in a molding product containing a lower amount of pore-forming agent than desired.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a method of producing a porous product which is capable of molding using a molding material containing an abundance of a pore-forming agent and controlling the porosity of an intended porous product as desired.
The inventors have studied a method which is capable of producing a porous product of which pores are individually minute and homogeneously distributed over the entirety of the porous product, and have found that the use of a pore-forming agent which assumes a solid state at room temperatures but can be melted into a liquid state at a molding temperature of a polymer component that will form the skeleton of the porous product, enables satisfactory molding without a decrease in the fluidity of a molding material as experienced in the prior art even when the amount of the pore-forming agent is increased in the molding material.
The present invention provides a method of producing a porous product which comprises the steps of: preparing a molded product by subjecting a molding material containing a polymer component and a pore-forming agent dispersed in the polymer component, the pore-forming agent assuming a solid state at a room temperature, to molding at a temperature which causes the pore-forming agent to melt; and soaking the molded product with a solvent which dissolves the pore-forming agent but fails to dissolve the polymer component, to form pores.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
First, a molding material to be used in the present invention will now be described in detail.
The molding material comprises a polymer component and a pore-forming agent dispersed in the polymer component, the pore-forming agent assuming a solid state at room temperatures.
The
Maeda Mizuho
Nemoto Masashi
Otani Kunihiko
Takagi Kazuhisa
Takamatsu Hideo
Asahi Rubber Inc.
Kuhns Allan R.
Smith Patent Office
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