Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Cellular products or processes of preparing a cellular...
Reexamination Certificate
2001-08-31
2002-07-02
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...
C521S143000
Reexamination Certificate
active
06414047
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a polyolefin-based crosslinked foam and a polyolefin resin composition to be used therefor. More particularly, the present invention relates to a polyolefin-based crosslinked foam usable in automobile interior, building heat insulating material, industrial material and furniture. Even more particularly, the present invention relates to a high-expanded crosslinked foam having excellent flexibility, toughness, heat resistance and compressibility, a smooth surface and uniform cells.
BACKGROUND OF THE INVENTION
A polyethylene-based crosslinked foam is excellent in flexibility, lightness, heat insulation, sound insulation, etc., and is worked in products having various shapes which are used widely used in arts such as building heat insulation, cushion, automobile interior, packaging material and bath mat.
As methods for the preparation of crosslinked foam product of polyolefin resin there have heretofore been known various methods. For example, a block foam comprising a polyethylene resin is prepared by kneading a polyethylene with a peroxide and a decomposable foaming agent under the conditions that the two components cannot be decomposed to prepare a crosslinkable foamable resin composition, and then subjecting the crosslinkable foamable resin composition to (a) one-shot press-molding method which comprises packing the crosslinkable foamable resin composition in a mold in a press, heating the crosslinkable foamable resin composition under pressure, and then removing the pressure to produce a foam or (b) two-shot press-molding method consisting of a first step which comprises heating the crosslinkable foamable resin composition in a sealed mold under pressure to cause the foaming agent to be partially decomposed, and then removing the pressure to obtain an intermediate foam and a second step which comprises heating the intermediate foam at atmospheric pressure to cause the residual foaming agent to be decomposed. In accordance with the former one-shot press-molding method, the polymer expands momentarily when pressure is removed and thus can be easily deformed. Thus, the one-shot press-molding method is used to form a foam having an expansion ratio of about 10. On the other hand, the two-shot press-molding method requires a longer production cycle and hence a higher production cost than the one-shot press-molding method but is advantageous in that a high-expanded foam having an expansion ratio of about 30 can be obtained.
Also are known a method which comprises mixing a polyolefin resin with a thermally decomposable foaming agent, forming the mixture into a sheet, and then irradiating the sheet with ionizing radiations so that it is crosslinked and heated to cause crosslinked foaming and a method which comprises mixing a polyolefin resin with a thermally decomposable foaming agent and an organic peroxide having a lower decomposition temperature than the foaming agent, forming the mixture into a sheet, heating the sheet so that the organic peroxide is decomposed and crosslinked, and then allowing the foaming agent to be decomposed to cause crosslinked foaming. The polyolefin-based foam prepared by the foregoing method is excellent in flexibility and heat insulation and thus can be used as various cushioning materials or heat insulating materials. As the polyolefin resin to be used in these foaming methods there is mainly used a low density polyethylene which can be easily predetermined in gel fraction and foaming conditions (hereinafter referred to as “LDPE”) prepared by a high pressure radical polymerization process. However, conventional LDPE has a wide distribution of molecular weight, making it difficult to invariably produce a crosslinked foam having a constant quality. Further, the resulting foam has too low a strength to freely control its rigidity.
On the other hand, in order to facilitate the control of degree of crosslinking and hence obtain a foam having improved toughness and flexibility, a method has been practiced involving the blending of LDPE with ethylene-&agr;-olefin copolymer (hereinafter referred to as “LLDPE”).
Further, JP-A-7-188442 (The term “JP-A” as used herein means an “unexamined published Japanese patent application”) proposes the use of LLDPE, as a foam, having &agr;-olefin uniformly present as a copolymerizing component in molecular chain and having a narrow distribution of molecular weight obtained in the presence of a metallocene compound as a polymerization catalyst.
However, the crosslinked foam prepared using LDPE is disadvantageous in that it not only exhibits low break strength but also cannot be freely predetermined in its flexibility.
The crosslinked foaming with LLDPE for the purpose of providing the foam with higher toughness and flexibility is disadvantageous in that it has too high a gel fraction to effect two-shot press molding, causing troubles in working and hence providing only a foam having a low expansion ration. This foaming method is also disadvantageous in that the range of foaming is very narrow, causing troubles in working.
The method involving blending of LDPE with LLDPE makes it possible to improve the controllability of degree of crosslinking and toughness and flexibility, but the improving effect is not satisfactory because this method requires that the added amount of LDPE be not smaller than 50%.
The method proposed in JP-A-7-188442 is disadvantageous in that the gel fraction, which is a problem with LLDPE obtained in the presence of conventional Ziegler catalyst, is too high, giving a low foaming ratio that makes it impossible to solve the problems that two-shot press-molding indispensable for high ratio molding cannot be effected and the range of foaming is thus very narrow.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to overcome these problems, i.e., provide a crosslinked foam excellent in rigidity, heat resistance, etc., which can be used in the art such as building material and automobile interior.
As a result of extensive studies to overcome the above problems, it has been found that a foam having specific properties exhibits high flexibility, toughness, dimensional stability, heat resistance, compressibility and expansion ratio and has a smooth surface and uniform cells in addition to the excellent properties of conventional polyethylene-based crosslinked foam and thus can provide a polyolefin-based crosslinked foam which can be used for automobile interior or as building material requiring heat resistance and rigidity.
The polyolefin-based crosslinked foam according to the invention is a polyolefin-based crosslinked foam having the following characteristics (a) to (c) obtained by the crosslinked foaming of a foaming resin composition comprising a polyolefin resin and a foaming agent:
(a) Gel fraction represented by the residue obtained by extracting with p-xylene at 120° C. for 24 hours, and then immediately drying at 80° C. and 10 mmHg for 24 hours is from 10% to 90%;
(b) Degree of swelling represented by the following equation (1) is 30 or less:
Degree of
Swelling
=
Residue (g) shortly after 24 hours of
Extraction with p-xylene at 120° C.
Residue (g) obtained by extracting with
p-xylene at 120° C. for 24 hours, and then
immediately drying at 80° C. and 10 mmHg
for 24 hours
(
1
)
and;
(c) When glass transition point determined from the temperature dependence of dynamic viscoelasticity is represented by the peak of &dgr; determined by the measurement of dynamic viscoelasticity at a frequency of 10 Hz, there is observed no peak or one peak, if any, and the ratio &agr; of storage modulus at 0° C. (E′
0
) to storage modulus at 100° C. (E′
100
) (&agr;=log E′
0
/E′
100
) is from 10 to 30.
It has also been found that the desired foam can be obtained by using a polyolefin resin composition comprising 100 parts by weight of a polyolefin resin and from 5 to 50 parts by weight of a low molecular polyolefin wax, particularly a polyol
Foelak Morton
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Tosoh Corporation
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