Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Cellular products or processes of preparing a cellular...
Patent
1998-05-15
2000-02-22
Kuhns, Allan R.
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Cellular products or processes of preparing a cellular...
264 50, 264 53, 521142, 525240, C08J 916, C08J 922
Patent
active
060281219
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to pre-expanded noncrosslinked polyethylene resin beads which are packed into a mold having a desired shape and expansion-molded with heating with a heating medium such as steam to give a foamed molding for use mainly as a cushioning material, heat insulator, float, etc. The present invention also relates to a process for producing the beads and a process for producing a molding from the same.
BACKGROUND OF THE INVENTION
In recent years, pre-expanded beads of not a crosslinked polyethylene resin but a noncrosslinked polyethylene resin are being developed because of advantages thereof that some steps including the step of crosslinking are unnecessary and recycling is possible (see, for example, JP-B-60-10047, JP-B-60-10048, JP-A-6-316645, JP-A-6-157803, etc.). (The terms "JP-B" and "JP-A" as used herein mean an "examined Japanese patent publication" and an "unexamined published Japanese patent application", respectively.) These techniques are intended to eliminate the drawback in expansion molding using a polyethylene resin as a feedstock resin wherein when the resin has not been modified by crosslinking, pre-expanded beads of satisfactory quality cannot be obtained therefrom and a foamed molding suitable for practical use cannot hence be obtained, and to enable pre-expanded beads and foamed moldings of satisfactory quality to be obtained through the foaming of a noncrosslinked polyethylene resin.
Difficulties in the foaming of polyethylene resins are attributable to the fact that a considerable proportion of the foaming agent (inorganic gas) infiltrated into beads escapes during heating for foaming, thereby making it difficult to obtain expanded beads having an expected cell structure and expansion ratio, and that the resins have a large temperature dependence of melt viscosity, thereby making it difficult to regulate the melt viscosity so as to be suitable for foaming. In this connection, the temperature range in which noncrosslinked high-pressure-process low-density polyethylene resins can retain a melt viscosity suitable for foaming is said to be 1.degree. C. at the most. Because of the difficult in controling the temperature, optimal foaming is difficult. For example, JP-A-6-316645 discloses pre-expanded beads obtained using as a base resin a noncrosslinked ethylene resin that has a specific melt viscosity range so as to impart excellent in-mold moldability to the pre-expanded beads, and also discloses, as an example of the base resin having such specific viscosity properties, a mixed resin comprising a linear low-density polyethylene having a resin density of from 0.920 to 0.940 g/cm.sup.3 and a high-density polyethylene having a resin density of 0.940 g/cm.sup.3 or higher. Further, JP-A-6-157803 discloses noncrosslinked pre-expanded beads made of a mixed resin, as a base resin, comprising from 20 to 85 wt % low-density polyethylene, from 0 to 45 wt % linear low-density polyethylene, and from 0 to 40 wt % high-density polyethylene and having a resin density of from 0.920 to 0.940 g/cm.sup.3 and also discloses a process for producing the same, in order to provide molded foams equal to molded foams of crosslinked polyethylene resins in flexibility, toughness, and durability in compression strain and to provide pre-expanded beads of a noncrosslinked polyethylene resin which are excellent in the ability to expand during in-mold molding and in fusibility.
There is a recent trend in the market wherein foamed moldings are having more complicated three-dimensional shapes. For example, FIG. 1 shows an example of foamed moldings of complicated shapes which are highly desired in the market; the example shown is a set of cushioning foamed moldings for use in the assembly packaging of liquid-crystal modules. FIG. 1 (A) and (B) show foamed moldings of the same shape, which are put together in such a manner that a liquid-crystal module (D) is sandwiched therebetween and are then put in a corrugated fiberboard container (C). The frame 1 and the partitions 2 in FIG. 1
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Nakayama Yutaka
Takeda Noboru
Asahi Kasei Kogyo Kabushiki Kaisha
Kuhns Allan R.
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