Plastic and nonmetallic article shaping or treating: processes – Pore forming in situ – By mechanically introducing gas into material
Reexamination Certificate
1999-07-30
2002-01-01
Kuhns, Allan R. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Pore forming in situ
By mechanically introducing gas into material
Reexamination Certificate
active
06334970
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for producing a foamed article by which a foamed article having a high expansion ratio, having uniform fine cells in a high closed cell ratio and capable of being recycled, which is utilizable as heat insulating materials for construction, cushioning materials for wrapping, cushioning floats for ships, floats for sports and leisure and the like, can be obtained readily and safely using a clean inorganic gas as a foaming agent, and to a foamed article obtained by the method.
2. Description of the Related Art
Hitherto, as a method for producing a foamed thermoplastic resin article, there have been known a chemical foaming method in which a heat-decomposable foaming agent is mixed with a thermoplastic resin by kneading, and foaming is performed by heating over the decomposition temperature of the foaming agent, and a gas foaming method in which an organic gas having a boiling point not higher than the softening point of a resin, such as butane, pentane and dichlorodifluoromethane (Freon R-12), or a volatile liquid is supplied into the molten resin under pressure, and then the mixture is discharged to a lower pressure to be foamed.
Among the above-mentioned conventional foaming methods, in the case of using a chemical foaming agent, a foamed article having uniform fine closed cells can be obtained. The foamed article, however, contains decomposition residues of the foaming agent to raise problems of discoloring, generation of smell, deterioration in physical properties, etc.
On the other hand, in the case of the gas foaming method, when a foaming agent is a low boiling organic solvent such as butane and pentane, there is a risk of fire and explosion in the manufacturing process. When the foaming agent is chlorofluorocarbon compounds, there can readily be obtained a foamed article with a high expansion ratio having uniform fine closed cells without any risk of explosion, but there are environmental problems such as destroying the ozone layer. Thus the use of such chlorofluorocarbon compounds is proceeding toward a complete removal.
As methods for solving the foregoing problems, for example, a method in which an inorganic gas, such as carbon dioxide, nitrogen and the air, is mixed with a polypropylene resin, and the mixture is then extruded to foam, as described in Japanese Laid-open Patent Publication No. 60 (1985)-31538.
SUMMARY OF THE INVENTION
However, in the above-mentioned method, especially when intending to obtain a foamed article by discharging from a circular die having a tubular channel, an undesirable apparent defect of many streaks was often observed. When an organic solvent is used, such an apparent defect is not observed, or is slightly observed, and it does not damage commercial values of products. On the contrary, the apparent defect, which seems to occur inherently when the inorganic gas is used, seriously damages the commercial values of the products.
The present invention intends to provide a method by which a foamed thermoplastic resin article not having any apparent defect observed in the aforementioned prior art can be obtained using a inorganic carbon dioxide gas as a foaming agent.
The present inventors have intensively studied to improve the above-mentioned streak-like defect by a production method. As a result, their finding that a foamed article with good appearance can be obtained by taking the following action has accomplished the present invention.
That is, the present invention provides;
1. A method for producing a foamed resin article comprising a melting step of feeding a thermoplastic resin into an extruder and heating it to melt, a kneading step of mixing an inorganic gas with the thermoplastic resin in the extruder to produce a kneaded material, and a foaming step of discharging the kneaded material from a mold having a tubular channel mounted to the end of the extruder to form the foamed article,
wherein extrusion molding is carried out within the range where a numeric X, which is defined by the following expression:
X=
100
P+
3.5
H
where a resin feeding pressure of the thermoplastic resin from the extruder to the mold is represented by H (kgf/cm
2
) and an expansion ratio is represented by P, and the resin feeding pressure H satisfy the following expressions simultaneously:
780≦
X≦
4000,
and
80≦
H≦
1000
2. The method for producing a foamed resin article according to above 1, wherein a numeric Y, which is defined by the following expression:
Y=
100
P+
3.5(
H
−10
G
)
where the amount of the inorganic gas supplied to 100 g of the thermoplastic resin is represented by G g, satisfies the following expression:
760≦
Y≦
4000
3. The method for producing a foamed resin article according to above 1 or 2, wherein the shape of the tip (lip) of the mold having a tubular channel satisfies the following expressions simultaneously:
0.1≦
L≦
1;
0.1≦tan &thgr;≦6;
and
5≦
T≦
500
where a lip gap, a taper angle, and a taper land are represented by L (mm), &thgr;, and T (mm), respectively.
4. The method of producing a foamed resin article according to any one of above 1 to 3, wherein, as for the shape of the tip (lip) of the mold having a tubular channel, an angle &agr; which is formed by the center axis of the core of the lip and the taper land satisfies the following expression:
0.1≦tan &agr;≦6
5. The method for producing a foamed resin article according to any one of above 1 to 4, wherein the inorganic gas is carbon dioxide.
6. The method for producing a foamed resin article according to any one of above 1 to 5, wherein the thermoplastic resin is a propylene-based polymer.
7. The method for producing a foamed resin article according to above 6, wherein the propylene-based polymer is a polymer which is obtained by continuous production comprising polymerizing, in the first stage, monomers mainly composed of propylene to produce a crystalline propylene-based polymer (A) having a limiting viscosity of 5 dl/g or more and successively polymerizing, in the second stage, monomers mainly composed of propylene to produce a crystalline propylene-based polymer (B) having a limiting viscosity of less than 3 dl/g and which is composed of the crystalline propylene-based polymer (A) and the crystalline propylene-based polymer (B) wherein a content of the polymer (A) is in the range of 0.05% by weight or more and less than 35% by weight, a limiting viscosity of the whole polymer is less than 3 dl/g, and Mw/Mn is less than 10.
8. The method for producing a foamed resin article according to claim
7
, wherein the propylene-based polymer satisfies the following expression:
W
A
≧400×EXP(−0.6[&eegr;
A
])
where [&eegr;
A
] (dl/g) is the limiting viscosity of the crystalline propylene-based polymer (A) and W
A
(% by weight) is the crystalline propylene-based polymer (A) content in the whole polymer.
9. The method for producing a foamed resin article according to above 7 or 8, wherein, the propylene-based polymer is composed of the crystalline propylene-based polymer (A) and the crystalline propylene-based polymer (B), each of them being a propylene homopolymer, a random copolymer of propylene and 10% by weight or less of ethylene, a random copolymer of propylene and 30% by weight or less of butene, or a random terpolymer of propylene, 10% by weight or less of ethylene and 30% by weight or less of butene.
10. The method for producing a foamed resin article according to any one of above 7 to 9, wherein, as for the propylene-based polymer, the crystalline propylene-based polymer (A) has a limiting viscosity [&eegr;
A
] of 7 dl/g or more.
11. The method for producing a foamed resin article according to any one of above 7 to 10, wherein, as for the propylene-based polymer, the crystalline propylene-based polymer (A) contains ethylene, as a comonomer, in a proportion of 1% by weight or more and 10% by weight or less.
12. The method for producing a foamed resin article according
Kuroda Ryuma
Nagamatsu Tatsuhiro
Kuhns Allan R.
Sumitomo Chemical Company Limited
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