Process and device for producing foam using carbon dioxide disso

Details Process and device for producing foam using carbon dioxide disso Process and device for producing foam using carbon dioxide disso

1998-11-19

2000-11-14

Cooney, Jr., John M.

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Cellular products or processes of preparing a cellular...

425 4C, 521130, 521137, C08G 1804

Patent

active

06147133&

DESCRIPTION:

BRIEF SUMMARY
FIELD OF INVENTION

The present invention relates to a process and a device for the production of foams by means of carbon dioxide dissolved under pressure as a foaming agent, wherein the composition to be foamed is mixed under pressure with what is preferably liquid carbon dioxide and is subsequently depressurised with the formation of foam. Liquid starting materials for plastics, which harden to form the foamed plastic due to an addition polymerisation or condensation polymerisation reaction which sets in after foaming, are used in particular as the foamable compositions. This invention relates in particular to polyurethane foamed materials.


BACKGROUND OF INVENTION

In the production of polyurethane foamed material, at least one of the reactive components (a polyisocyanate and compounds containing hydrogen atoms which are reactive towards isocyanates, particularly polyols) is treated with a liquid or gaseous foaming agent, mixed thereafter with the another component, and the mixture obtained is introduced either batch-wise into a mould or continuously on to a conveyor belt, where the mixture foams and hardens.
A series of processes for the production of foam have found wide application in the technology. Firstly, liquids which volatilise at low temperature are used, such as low molecular weight chlorofluorocarbons, methylene chloride, pentane, etc., which volatilise from the reactive mixture whilst the latter is still liquid and form small bubbles (physical production of foam). In addition, air can be beaten into the reactive mixture or into one of the components (mechanical production of foam), and finally water is added as a foaming agent to the polyol component for polyurethane foams; after mixing with the isocyanate component, the water releases carbon dioxide as a foaming gas by reaction with the isocyanate (chemical production of foam).
For reasons of environmental compatibility and industrial hygiene, and due to the comparatively high solubility of liquid carbon dioxide in the polyol component, liquid carbon dioxide has already often been proposed as a foaming agent (GB-A-803 771, U.S. Pat. No. 3,184,419), but these prior proposals have hitherto not become part of the technology, apparently. due to the difficulties of producing uniform foams during the requisite depressurisation of the reactive mixture from pressures between 10 and 20 bar. In this connection, the problem is firstly that the carbon dioxide volatilises relatively suddenly immediately after depressurisation, so that a very considerable increase in volume occurs of the reactive mixture, by a factor of about 10 for example, which is difficult to control. Secondly, the reactive mixture tends to produce distortions in the evolution of the carbon dioxide, which may occur at 3 to 6 bar below the equilibrium vapour pressure of CO.sub.2 at the temperature concerned, so that this results in sudden explosion-like evolutions of carbon dioxide, with the consequence that large bubbles or voids are included in the foamed material.
In order to produce a uniform foam structure, it is also known that finely-divided air or nitrogen bubbles can be introduced into the liquid reactive mixture; these serve as bubble seeds, so that any local supersaturation of physically dissolved or chemically produced foaming agent is prevented.
When physically dissolved carbon dioxide is used as a foaming agent, however, the problem exists that the release of carbon dioxide occurs over periods of time in which the addition polymerisation reaction scarcely progresses, so that the froth obtained after the release of the carbon dioxide is still very susceptible to stresses due to shearing forces. Shearing forces acting on the froth result in the destruction of foam bubbles, so that larger foam bubbles are formed and a non-uniform foam structure results. This is the situation in particular when the foam bubbles reach a diameter at which the shape of the bubble departs from a spherical form, i.e. when the volume of bubbles in the froth occupies a spatial proportion which i

REFERENCES:
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patent: 5665287 (1997-09-01), Fiorentini et al.
patent: 5789457 (1998-08-01), Eiben et al.
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