Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Cellular products or processes of preparing a cellular...
Reexamination Certificate
2003-06-04
2004-09-28
Foelak, Morton (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Cellular products or processes of preparing a cellular...
C521S056000, C521S060000
Reexamination Certificate
active
06797734
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a polypropylene resin pre-expanded particle and an in-mold expanded article thereof. More specifically, the present invention relates to a polypropylene resin pre-expanded particle which can suitably be used for preparing an in-mold expanded article of polypropylene resin used in cushioning packaging material, returnable containers, insulating material or bumper core of an automobile, and an in-mold expanded article made of the particles.
BACKGROUND ART
Conventionally, a method of preparing pre-expanded particles by dispersing polypropylene resin particles in an aqueous dispersion medium along with a foaming agent, raising the temperature, making the pressure and temperature constant to impregnate the foaming agent in polyolefin resin particles and then discharging into low pressure atmosphere, is known. Regarding the foaming agent, disclosed are the methods of using a volatile organic foaming agent such as propane or butane (JP-B-56-1344) and using inorganic gas such as carbon dioxide, nitrogen and air (JP-B-4-64332, JP-B-4-64334).
However, volatile organic foaming agents are expensive and so costs will rise. Also, volatile organic foaming agents such as propane and butane have the effect of placticizing polyolefin resin and though high expansion ratio can be obtained, due to the placticizing effect, there are problems such as difficulty in controlling the expansion ratio and crystal structure of the pre-expanded particle.
When using inorganic gas such as carbon dioxide, nitrogen and air, because the inorganic gas is difficult to be impregnated into the polyolefin resin, usually impregnation must be conducted at a high pressure of approximately 3 to 6 MPa. As a result, the impregnating vessel for impregnating the foaming agent into the polyolefin resin must have high pressure resistant properties and so there is the problem of high facility costs.
As a method for economically preparing polyolefin resin pre-expanded particles which can suitably be used for preparing an in-mold expanded article and solves the above problems, a method of using the water used for the dispersion medium as a foaming agent has been suggested.
A method using water as a foaming agent which has been suggested is the method of preparing crystalline polyolefin polymer expanded particles, which comprises dispersing crystalline polyolefin polymer particles containing 10 to 70% by weight of an inorganic filler into water which is the dispersion medium in a sealed vessel, impregnating the water which is the dispersion medium under pressure which is higher than the saturated vapor pressure of the dispersion liquid and temperature which is at most the melting point of the crystalline polyolefin polymer particles, while maintaining a high pressure range which is under the temperature conditions in which crystallization of the polymer particles progresses, and then discharging the dispersion liquid in a low pressure range (JP-B-49-2183). However, because the pre-expanded particles obtained in this way contain a great deal of inorganic filler, cell diameter is extremely small, open cell ratio tends to become high and fusion, surface appearance and mechanical properties such as compressive strength when made into an in-mold expanded article are not sufficient.
Another method which has been suggested is the method of preparing propylene random copolymer resin expanded particles which comprises dispersing propylene-ethylene random copolymer resin particles containing 1 to 12% by weight of ethylene into water in a sealed vessel, then introducing inorganic gas to make the pressure within the vessel at least 5 kg/cm
2
G, heating to a temperature which is between the melting point of the copolymer resin particles and 25° C. higher than the melting point and discharging the dispersion liquid in a low pressure atmosphere (Japanese Patent No. 1880374). However, according to this method, in order to impregnate with water, high temperature and high pressure conditions must be maintained for a long period and productivity is extremely low. Also, the fluctuation in expansion ratio of the obtained pre-expanded particles is not of a satisfactory level.
Another method which has been suggested is the method of preparing polyolefin resin pre-expanded particles, which comprises dispersing polyolefin resin particles containing a hydrophilic polymer and an inorganic filler into water in a sealed vessel, then heating to at least the softening temperature of the resin particles to make water containing polyolefin resin particles, and then discharging the dispersion liquid in a low pressure atmosphere (for example JP-A-9-838048, JP-A-10-306179, JP-A-11-106576). By this method, polyolefin resin pre-expanded particles having a high expansion ratio can be obtained under low pressure in the vessel, compared to the case of using inorganic gas such as carbon dioxide, nitrogen and air as a foaming agent. In addition, maintenance of high temperature and high pressure for a long period is unnecessary and economical production is possible. As for the obtained polyolefin resin pre-expanded particles, fluctuation in expansion ratio and cell diameter is small and also, fusion and surface appearance of the in-mold expanded article are favorable.
However, regarding fluctuation in expansion ratio and cell diameter, the level of demand has become higher than in the past. Even in the method of preparing polyolefin resin pre-expanded particles by using polyolefin resin particles containing a hydrophilic polymer and an inorganic filler and using water for the dispersion medium as a foaming agent, cases in which fluctuation in expansion ratio and cell diameter do not satisfy the level of demand are arising and further improvement is desired.
When fluctuation in expansion ratio is large, there is the problem of large fluctuation in weight when made into an in-mold expanded article. In recent years, quality standards for products have become more severe and in order to cut down on the man hour for weight examination of the in-mold expanded article, pre-expanded particles having smaller fluctuation in expansion ratio than in the past are in demand.
In addition, further improvement is in demand as fluctuation in cell diameter causes unevenness in color and damages in appearance. In the case of an in-mold expanded article colored by including pigment or dye, particularly in the case of an in-mold expanded article colored in black, unevenness in color is more noticeable than in an in-mold expanded article which is uncolored and white and therefore demand for improvement in fluctuation of cell diameter is strong.
DISCLOSURE OF INVENTION
The object of the present invention is to provide pre-expanded articles having little fluctuation in cell diameter and expansion ratio and decreased unevenness in color and weight fluctuation when made into an in-mold expanded article in the case of preparing polypropylene resin pre-expanded particles by using water as a foaming agent.
As a result of devoted research to achieve this goal, it was found that the above objective could be achieved by using as a base resin, a polypropylene resin composition prepared by adding a compound having a specific triazine skeleton to polypropylene resin and the present invention was completed.
The present invention relates to a polypropylene resin pre-expanded particle comprising as a base resin a polypropylene resin composition containing polypropylene resin (A) and a compound having a triazine skeleton and molecular weight of at most 300 per triazine skeleton unit (B).
The above polypropylene resin pre-expanded particle may also comprise hydrophilic polymer (C).
The above polypropylene resin pre-expanded particle preferably contains 0.005 to 6 parts by weight of the compound (B) having a triazine skeleton and molecular weight of at most 300 per triazine skeleton unit based on 100 parts by weight of polypropylene resin (A).
The above polypropylene resin pre-expanded particle preferably contains 0.01 to 20 parts by weight of the hydrophilic polyme
Goda Takayuki
Iwamoto Tomonori
Brinks Hofer Gilson & Lione
Kaneka Corporation
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