Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Cellular products or processes of preparing a cellular...
Patent
1997-05-21
1998-11-24
Foelak, Morton
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Cellular products or processes of preparing a cellular...
521155, C08J 900, C08G 1800
Patent
active
058407782
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a process and a device for producing foams using carbon dioxide dissolved under pressure as blowing agent, wherein the material to be expanded is mixed under pressure with preferably liquid carbon dioxide and is then expanded with the formation of foam. Liquid starting materials for plastics are preferably used as materials which can be expanded and which can be hardened by means of a polyaddition or polycondensation reaction, which is initiated after expansion, to give foamed plastics. The invention relates in particular to polyurethane foams.
When producing polyurethane foams, at least one of the reactive components (polyisocyanate and compounds which contain isocyanate-reactive hydrogen atoms, in particular polyols) is mixed with a liquid or gaseous blowing agent, then mixed with the other component and the mixture obtained is fed either batchwise into a mould or continuously onto a conveyer belt, where the mixture expands and hardens.
There are a number of processes with widespread application in industry for producing foam materials. On the one hand, liquids which evaporate at low temperature, such as low molecular weight chlorinated hydrocarbons, methylene chloride, pentane etc. are used, which evaporate out of the still liquid reaction mixture and form small bubbles (physical foam production). Furthermore, air can be beaten into the reactive mixture or into one of the components (mechanical foam production) and finally water may be added to the polyol component as a blowing agent during polyurethane foam production, which releases carbon dioxide as the foam-producing gas after being mixed with the isocyanate component due to reaction with the isocyanate (chemical foam production).
For reasons of environmental compatibility and occupational hygiene and because of the comparatively high solubility of liquid carbon dioxide in the polyol component, liquid carbon dioxide has already been proposed as a blowing agent on several occasions (GB-A 803 771, U.S. Pat. No. 3 184 419). However, carbon dioxide has not hitherto been used in industry, evidently due to the difficulties involved in producing uniform foams when the pressure in the reactive mixture is, as required, reduced from pressures of between 10 and 20 bar. The problem is that on the one hand the carbon dioxide evaporates relatively suddenly immediately after reducing the pressure, so there is a very large increase in the volume of the reaction mixture, by a factor of for example about 10, which is difficult to control and on the other hand there tends to be time-lags--which may be 3 to 6 bar below the equilibrium vapour pressure of CO.sub.2 at the particular temperature--in the release of carbon dioxide from the reactive mixture, so that sudden, explosive releases of carbon dioxide occur with the result that large blisters or voids are included in the expanded material.
According to DE-A 2 628 785, the introduction of air into the polyol component, before carbon dioxide is dissolved therein, has already been suggested, evidently to provide nuclei for the release of carbon dioxide.
According to EP-A 145 250, the ability of carbon dioxide to form adducts with water and other low molecular weight liquids is utilised, in order to produce the delayed release of carbon dioxide from the reactive mixture so that foam formation due to the release of carbon dioxide begins only after the pressure has been reduced in the reactive mixture. After destruction of the adduct, the water is then available as an additional chemical blowing agent. However, the controllability of foam formation in a large scale process is not really improved by this procedure because both the formation of the adduct and its decomposition are extremely labile in the presence of the other conditions prevailing in the reactive mixture, except when using adducts prepared in a separate step, which are also described in the said reference, with the aid of low molecular weight tertiary amines which apparently have a considerably extended decomposition time as
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Althausen Ferdinand
Ebeling Wilfried
Eiben Robert
Raffel Reiner
Bayer Aktiengesellschaft
Cheung Noland J.
Foelak Morton
Gil Joseph C.
Hennecke GmbH
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