Chemical apparatus and process disinfecting – deodorizing – preser – Chemical reactor – Organic polymerization
Reexamination Certificate
1998-04-23
2004-02-10
Johnson, Jerry D. (Department: 1764)
Chemical apparatus and process disinfecting, deodorizing, preser
Chemical reactor
Organic polymerization
C422S224000, C422S228000, C425S00400R, C425S00400R, C264S045800, C264S050000, C264S051000, C264S053000
Reexamination Certificate
active
06689325
ABSTRACT:
FIELD OF THE INVENTION
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 preferebly 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.
BACKGROUND OF THE INVENTION
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
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 utilized, 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 compared with spontaneously formed water/CO
2
adducts.
Combinations of physically dissolved carbon dioxide and other physical or chemical blowing agents which boil at low temperature, such as water or chlorinated hydrocarbons, are also proposed, in accordance with EP-A 89796.
None of these proposals has led to industrially applicable solutions for the use of CO
2
, physically dissolved under pressure, as a blowing agent for the production of polyurethane foams.
The tests underlying the present invention were based on the concept that the conditions under which the polyurethane reactive mixture containing CO
2
under pressure is expanded have an essential effect on the formation of the foam.
According to U.S. Pat. No. 3,184,419, the pressurised reactive mixture containing carbon dioxide which emerges from the mixing device is evidently suddenly expanded through a valve. According to EP-A 145 250, expansion should take place gradually, it being possible for the gradual reduction in pressure to take place while the reactive mixture containing carbon dioxide flows through a pipe. In this case release of some of the gas while still inside the pipe is not necessarily regarded as a disadvantage because this process may assist the formation of bubble nuclei. Within the context of the tests underlying the present invention, it has however been shown that such a premature, i.e. spontaneous and non-induced, formation of bubble nuclei is rather disadvantageous for the pore structure of the foam because such a spontaneous formation of bubble nuclei generally results in a foam which not only has a very variable pore structure, but which also has large holes and voids.
On the basis of these and other observations, the following requirements were placed on the development of a process for preparing foam materials made from two-component reactive plastics using carbon dioxide which is physically dissolved under pressure as a blowing agent:
1. The formation of bubble nuclei in the liquid two-component reactive mixture must be controlled in such a way that every bubble nucleus produced can participate in an identical manner in the release or carbon dioxide from the reactive mixture, so that as uniform pores as possible are produced.
2. When inducing bubble nuclei, the number of bubble nuclei produced should already correspond to the number of pores in the hardened expanded plastic material.
3. The controlled, induced production of bubble nuclei must take place at that moment in time at which the liquid reactive mixture changes from a state which is not saturated with dissolved carbon dioxide to a state in which it is supersaturated with carbon dioxide, i.e. at that moment in time at which it expands from a pressure above the saturation pressure of the dissolved carbon dioxide to a pressure which is below the saturation pressure of the dissolved carbon dioxide.
4. Immediately after the formation of nuclei, the greatest possible degree of supersaturation of dissolved carbon dioxide should be produced in the reactive mixture. i.e. expansion from a pressure above the equilibrium pressure of the dissolved carbon dioxide to ambient pressure should take place as fully instantaneously as possible.
It was found that the requirements mentioned above can be satisfied when the reactive mixture which contains carbon dioxide dissolved under pressure is expanded with the production of high rates of shear in the reactive mixture.
Accordingly, the present invention provides a process for producing foam materials from two-component reactive plastics using carbon dioxide as a blowing agent by mixing at least one of the reactive components with carbon dioxide under pressure, mixing the reaction components while maintaining a pressure which is greater than that of the saturation pressure of the carbon dioxide in the mixture, expanding the reactive mixture containing carbon dioxide and hardening, characterized in that expansion is performed suddenly with the production
Althausen Ferdinand
Ebeling Wilfried
Eiben Robert
Raffel Reiner
Bayer Aktiengesellschaft
Cheung Noland J.
Doroshenk Alexa A.
Gil Joseph C.
Johnson Jerry D.
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