Organic aerogels

Details Organic aerogels Organic aerogels

1998-02-18

1999-02-09

Foelak, Morton

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

Synthetic resins

Cellular products or processes of preparing a cellular...

521128, 521130, 521155, C08G 1832

Patent

active

058695451

DESCRIPTION:

BRIEF SUMMARY
The present invention relates to polyisocyanate based aerogels and to methods for their preparation.
Aerogels are a unique class of ultrafine cell size, low density, open-celled foams. Aerogels have continuous porosity and their microstructure with pore sizes below the free mean path of air (pore sizes in the nanometer range) is responsible for their unusual thermal properties.
Organic aerogels based on polyisocyanate chemistry are described in WO 95/03358. and optionally a higher functionality isocyanate-reactive compound in a suitable solvent and maintaining said mixture in a quiescent state for a sufficiently long period of time to form a polymeric gel. The gel so formed is then supercritically dried.
During the drying the gel shrinks substantially leading to an increase in density of the obtained aerogel.
Therefore it is an object of the present invention to provide a method for preparing polyisocyanate-based organic aerogels of lower density.
Accordingly, the present invention provides a method for preparing a polyisocyanate based organic aerogel comprising the steps of a) mixing an organic polyisocyanate, a polyfunctional isocyanate-reactive compound and an isocyanate trimerisation catalyst in a suitable solvent, b) maintaining said mixture in a quiescent state for a sufficiently long period of time to form a polymeric gel, and c) supercritically drying the obtained gel, wherein the polyfunctional isocyanate-reactive compound comprises a compound of average nominal hydroxyl functionality 2 to 8 and number average molecular weight 50 to 10000.
The polyfunctional isocyanate-reactive compound for use in the process of the present invention preferably has an average nominal hydroxyl functionality 2 to 4, more preferably 2 to 3 and number average molecular weight between 50 and 5000, more preferably between 50 and 1000. It is preferred to use linear chain polyfunctional isocyanate-reactive compounds.
Suitable polyfunctional isocyanate-reactive compounds for use in the process of the present invention include polyols, polyamines, polyesters and alkyleneoxide reaction products of these compounds, such as reaction products with ethyleneoxide, propyleneoxide or butyleneoxide. Preferred polyfunctional isocyanate-reactive compounds for use in the process of the present invention include low molecular weight, preferably linear glycols, containing from 2 to 30, preferably from 3 to 24, most preferably from 4 to 10 carbon atoms; these glycols may also contain heteroatoms such as oxygen. process of the present invention include ethyleneglycol, diethyleneglycol, triethyleneglycol, tripropyleneglycol, dipropyleneglycol, propyleneglycol and especially butanediol and hexanediol.
In the process of the present invention one of said polyfunctional isocyanate-reactive compounds is used but two or more different ones can also be used.
In one embodiment of the present invention the polyfunctional isocyanate-reactive compound is mixed with the polyisocyanate and trimerisation catalyst and optionally also a urethane or gel catalyst whereupon reaction takes place. isocyanate-reactive compound is pre-reacted with some polyisocyanate so as to form an isocyanate-terminated prepolymer which is then mixed with additional polyisocyanate and trimerisation catalyst and reacted. This prepolymer preferably has an isocyanate value of about 20 and a molecular weight of about 3000. Examples of suitable isocyanate-terminated prepolymers are described in EP-A-0582385. and pre-reacted polyfunctional isocyanate-reactive compounds (so in the form of isocyanate-terminated prepolymers) may also be used.
The polyfunctional isocyanate-reactive compound is used in the process of the present invention in such an amount that 10 to 60%, preferably 40 to 60% by weight of the NCO groups originally present are reacted therewith.
Aerogels prepared according to the process of the present invention have a lower density (due to a decrease in shrinkage, in some case more than 30%) than those prepared in the absence of a polyfunctional isocyanate-reactive compou

REFERENCES:
patent: 3574150 (1971-04-01), Jefferson et al.
patent: 5260344 (1993-11-01), Ashida et al.
patent: 5478867 (1995-12-01), Tabor

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