Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Cellular products or processes of preparing a cellular...
Reexamination Certificate
2001-09-13
2003-03-04
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...
C521S081000, C521S083000, C521S086000, C521S098000
Reexamination Certificate
active
06528548
ABSTRACT:
TECHNICAL FIELD
This invention relates to synthetic thermoplastic resin extruded foams and a method for producing the same. More particularly, it relates to a synthetic thermoplastic resin extrusion-foamed board which has high-efficiency thermal insulating properties and suitable strength properties and is especially useful as a thermal insulation material and to a method for producing the same.
BACKGROUND ART
Having excellent thermal insulating properties, synthetic resin extruded foams are used as thermal insulating constructional materials for houses, etc. to contribute to energy saving and are expected as one of promising countermeasures against global warming.
Processes using a volatile blowing agent predominate in production of synthetic resin extruded foams. A volatile blowing agent used in the processes serves not only for foaming energy but as a plasticizer in an extrusion system. Further, a volatile blowing agent performs the function of controlling the cell size and the cell growth rate in a forming stage and, being present in the cells of a foamed body, greatly influences the performance of the foamed body, particularly the thermal conductivity which governs thermal insulating properties. Hence, selection of a volatile blowing agent has always been a primary subject of study in developing extrusion foaming techniques.
A combination of a slightly permeable blowing agent which controls the performance of foams and a easily permeable blowing agent which influences the foaming energy and the cell growth rate has often been used in synthetic resin extrusion foaming. Slightly permeable blowing agents typically include chlorofluorohydrocarbons (HCFC), such as 1,1-difluoro-1-chloroethane (hereinafter abbreviated as HCFC142b), fluorohydrocarbons (HFC), such as 1,1,1,2-tetrafluoroethane (hereinafter abbreviated as HFC134a), and hydrocarbons, such as propane and butane. Easily permeable blowing agents typically include halogenated hydrocarbons such as methyl chloride and ethyl chloride.
In recent years, environmental problems including destruction of the ozonosphere, global warring, and air and water pollution by chemical substances have been confronting us, and it has been desired to use as environmentally friendly a blowing agent as possible, It will be fulfillment of the social demand to develop synthetic resin extruded foams having high thermal insulating properties by use of a clean blowing agent such as water or inorganic gas with no, or a reduced amount of, flons, halogenated hydrocarbons and, if possible, saturated hydrocarbons.
It has been studied for a long time to utilize water, which seems typical of the cleanest blowing agents, as a blowing agent. For example, injection of water into an extruder has already been attempted as suggested in JP-A-58-176226, but every effort has failed to materialize, i,e., to be industrialized because of difficulty in dispersing water in a synthetic resin.
JP-A-3-109445, JP-A-3-27304 and JP-A-4-80240 disclose a method in which water is uniformly dispersed in a synthetic resin to improve the cell structure of foams thereby to improve thermal insulating properties. In this process SCFC142b, one of flons, is used as an example of slightly permeable blowing agents, and methyl chloride is used as an example of easily permeable blowing agents. The foam is made up mainly of cells having a diameter of 0.25 mm or smaller and cells having a diameter of 0.4 to 1 mm. In order to obtain such a cell structure, water is used in combination with a granular matter, which has hydroxyl groups on its surface layer or a water-absorbent high molecular compound.
However, because water is incompatible with synthetic resins, the amount of water that can be added is limited even when the above process is adopted, Too much water added is hardly dispersed in a synthetic resin uniformly, which can result in such phenomena as generation of pores in the foam. It has therefore been desired that water not only be used for more improvement on the cell structure but be made more effective use of as a part of a blowing agent.
On the other hand, WO99/54390 discloses powdered silicates exemplified by magnesium silicate as a water-absorbing medium. The disclosure teaches a combined use of water in the production of a cell structure having two kinds of cells (large calls and small calls) in the presence of limited kinds of gases in a limited gas ratio. As noted, the teaching is confined to application tinder specific conditions in terms of kind and ratio of gases, not covering a broad range of conditions. Further, addition of a large quantity of magnesium silicate recited in the publication tends to make it difficult to maintain a closed-cell structure and to make cells extremely smaller, resulting in insufficient teaming. For these reasons, the amount of magnesium silicate to be added has to be limited. The amount of water that can be added is also limited on account of its dispersibility in the extrusion system. Improvements have been demanded in these respects.
JP-W-8-502786 discloses a method for producing a styrene resin extruded foams by using water as part of a blowing agent, wherein the styrene resin material is made water-soluble by kneading with a low-molecular polymer, an oligomer, a hydrophobic polymer, etc. Substantially, however, the amount of water used as part of a blowing agent is limited. The teaching fails to achieve a reduction of a conventionally employed volatile blowing agent and to provide synthetic resin extruded foams with excellent thermal insulating performance.
A proposal has been awaited for producing foams with excellent thermal insulating properties by using environment friendly and inexpensive water while reducing use of a blowing agent that affects the environment.
The present invention has been accomplished in the light of the above-mentioned background art. It is an object of the present invention to provide foams having excellent thermal insulting properties by using environment friendly water while using no, or a reduced amount of, blowing agents which affect the environment causing destruction of the ozonosphere, global warming and the like, such as HCFC142b and methyl chloride.
DISCLOSURE OF THE INVENTION
The present inventors have conducted extensive investigations as to how to disperse incompatible water in a synthetic thermoplastic resin uniformly to be used as a blowing agent. They have noted water-containing silicate minerals having a characteristic crystal structure and effective in water retention. Of the silicate minerals, bentonite was found to have adequate nucleating properties to secure closed cells and not to cause insufficient foaming due to make calls extremely smaller even when added in a large quantity and also found to allow water to be added in an increased amount. That is, it was found that bentonite, when added to a synthetic thermoplastic resin, increases the amount of water that can be uniformly dispersed and held in a molten synthetic thermoplastic resin and improves the expansion ratio. Since water can be used in an increased amount, a high expansion ratio is maintained even if the amount of other blowing agents is decreased. Secondly, in producing foams whose cell structure mainly comprises cells having a cell size of 0.25 mm or smaller (hereinafter sometimes called smaller cells) and cells having a cell size of 0.3 to 1 mm (hereinafter sometimes referred to as larger cells) which are dispersed via cell walls in a sea-island structure, bentonite was found to make it easier to form the smaller cells having a cell size of 0.25 mm or smaller, which are often difficult to produce in an adequate proportion. That is, bentonite makes it possible to increase the area ratio of the smaller cells in a cross-sectional area of foams. As a result, the thermal insulating properties of the foams are further improved, and the range of the conditions for producing foams having both the smaller cells and the larger cells in a stable manner can be broadened. The present invention has been reached based on these findings.
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Hayashi Takahiro
Ito Toshiya
Tanaka Kenkichi
Tayauchi Takahiro
Armstrong Westerman & Hattori, LLP
Foelak Morton
Kaneka Corporation
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