Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Cellular products or processes of preparing a cellular...
Reexamination Certificate
1998-09-25
2003-07-22
Reddick, Judy M. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Cellular products or processes of preparing a cellular...
C521S056000, C521S058000, C521S060000, C524S425000, C524S427000, C524S451000, C524S506000, C524S515000
Reexamination Certificate
active
06596782
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a water-containing polyolefin resin composition, pre-expanded particles made thereof, a process for preparing the pre-expanded particles and a cellular molded article. More particularly, the present invention relates to polyolefin resin pre-expanded particles which can be suitably used, for example, as a raw material for in-mold foamed articles and a process for the preparation thereof, a water-containing polyolefin resin composition which can be suitably used for preparing the pre-expanded particles, and a cellular molded article obtained from the pre-expanded particles.
BACKGROUND ART
In general, polyolefin pre-expanded particles are prepared by a method in which polyolefin resin particles containing a volatile organic blowing agent are dispersed in an aqueous medium, in a vessel and heated to a temperature not lower than the softening point of the resin while maintaining a pressure in the vessel at vapor pressure of the blowing agent or above, and the dispersion is released into an atmosphere of lower pressure than the pressure in the vessel, thereby expanding the particles (see Japanese Patent Publication Kokai No. 52-77174). As the volatile organic blowing agent, hydrocarbons such as propane, butane, pentane, trichlorofluoromethane, dichlorodifluoromethane and the like are used. However, since this method uses the volatile blowing agent, it has the defect of higher cost. In addition, the use of butane and fluorohydrocarbons is not preferable from an environmental point of view.
Therefore, there has been recently a need for development of a method which can prepare polyolefin resin pre-expanded particles having desired physical properties without using volatile blowing agents which have been required.
Japanese Patent Publication Kokai No. 61-4738 discloses a method in which polypropylene resin particles containing 0.05 to 2% by weight of an inorganic material such as aluminium hydroxide, calcium carbonate or the like, are used in the preparation of pre-expanded particles from polypropylene resin particles containing a volatile blowing agent or inorganic gas.
However, this method has problems from a viewpoint of environment and cost when a volatile blowing agent is used. Also, since carbonic acid gas among the inorganic gases is responsible for warming the earth, it is preferred not to use. Further, in case that gases other than carbonic acid gas among inorganic gases, such as nitrogen gas and air are used, nitrogen, air and the like insufficiently permeate into polyolefin polymer particles and have a rapid rate of releasing from the polyolefin polymer and, therefore, the method has the problem that an expanded article is difficult to be obtained and, even if obtained, at best 2 to 9 of expansion ratio is attained.
As one example of the methods for preparing polyolefin resin pre-expanded particles without the use of a volatile blowing agent, there has been proposed a method by holding a dispersion of crystalline polyolefin polymer particles containing 10 to 70% by weight of a filler based on the polymer in a disperse medium in a high pressure area under conditions of a pressure which is not lower than saturated vapor pressure of this dispersion and a temperature which is not higher than the melting point of the crystalline polymer and at which crystallization of the crystalline polymer proceeds, thereby permeating the disperse medium of the dispersion into the polymer particles by means of volumetric shrinkage associated with crystallization of the crystalline polymer, then injecting the thus obtained dispersion of expandable crystalline polymer particles from the high pressure area into a lower pressure area which is under conditions of a pressure not higher than the saturated vapor pressure of the dispersion and a temperature not higher than the melting point of the crystalline polymer, to give pre-expanded particles (Japanese Patent Publication Kokai No. 49-2183).
However, in this method, a filler must be included therein in an amount of 10 to 70% by weight based on the crystalline polyolefin polymer, and a molded article obtained by expanding such pre-expanded particles inevitably has lowered physical properties such as flexibility, cushioning characteristics and the like because it contains a large amount of the filler and, therefore, it can not be used for utility where higher level properties are required. In addition, this method has the problem that the filler prevents the particles from fusing together upon in-mold molding.
Further, there has been proposed a method for obtaining pre-expanded particles having an expansion ratio of not less than 5 without the use of a volatile blowing agent, by using as resin particles a propylene-ethylene random copolymer having an ethylene content of 1 to 12% by weight, applying a pressure to a dispersion by increasing the pressure in a sealed vessel to or above 5 kg/cm2G with an inorganic gas such as nitrogen or the like and, thereafter, maintaining the temperature of the dispersion at a temperature between the melting point of the resin particle and a temperature higher than the melting point by 25° C. (Japanese Patent Publication Kokai No. 60-221440).
However, when trying to obtain pre-expanded particles having a high expansion ratio by this method, it is required that the ethylene content in the propylene-ethylene random copolymer resin particle must be not less than 4% by weight and the temperature to be maintained is not lower than 160° C. or the time to maintain is not less than 10 hours. Pre-expanded particles obtained under such conditions are insufficient in mechanical strength and heat-resistance originally required for a polypropylene resin cellular molded article. In addition, when prepared at such a temperature to be held and such a time, not only agglomeration of particles is easy to occur but also productivity is low and, thus being not economical.
In general, in a case of polyolefin resin pre-expanded particles used for in-mold foaming, if the cell diameter is too small, a molded article obtained by molding the pre-expanded particles in a mold has the defects that it tends to have a large shrinkage rate and has a distorted shape (usually called sink or warpage in the art) thus decreasing the commercial value of molded article and adversely affecting the production techniques.
As pre-expanded particles from which a molded article having a small shrinkage rate after the in-mold foaming and having no distorted shape can be obtained, there has been proposed polyolef in resin pre-expanded particles comprising as a base resin a polyolefin resin into which a polymer having a carboxyl group is incorporated in an amount of 0.1 to 10 parts by weight based on 100 parts by weight of the polyolefin resin, and having a cell diameter of 200 to 500 &mgr;m (Japanese Patent Publication Kokai No. 62-115042). The polyolefin resin pre-expanded particles have, indeed, superior effects such that the shrinkage rate of a molded article after in-mold foaming is small and the shape of the molded article is not distorted. However, the use of a blowing agent is required for the preparation of the polyolefin resin pre-expanded particles and, therefore, the use of a blowing agent such as a volatile hydrocarbon or an inorganic gas raises problems as mentioned above.
Thus, an object of the present invention is to provide a method for preparing polyolef in resin pre-expanded particles having desired physical properties without the use of a volatile blowing agent such as hydrocarbon or an inorganic gas such as carbonic acid gas.
Another object of the present invention is to provide a polyolefin resin composition which can be suitably used for the preparation of polyolefin resin pre-expanded particles without the use of a volatile blowing agent.
A further object of the present invention is to provide polyolefin resin pre-expanded particles suitable as a raw material for in-mold foamed articles, without the use of a blowing agent which is not preferable from a viewpoint of environment and cost, such a
Akamatsu Naruhiko
Mihayashi Tsuyoshi
Mogami Kenji
Nakamura Kyoichi
Kaneka Corporation
Leydig , Voit & Mayer, Ltd.
Reddick Judy M.
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