POLYISOCYANATE BASED AEROGELS

Abstract: Process for preparing polyisocyanate based organic aerogels comprising the steps of a) mixing an organic polyisocyanate and a carbodiimide catalyst and optionally a polyfunctional isocyanate-reactive compound in a 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.

Description: DESCRIPTION POLYISOCYANATE BASED AEROGELS The present invention relates to organic aerogels and more specifically to polyisocyanate based aerogels and to methods for their preparation. Polyisocyanate based rigid foams such as polyurethane and polyisocyanurate foams are well known in the art and are used as a thermal insulation medium as, for example, in the construction of refrigerated storage devices. These foams are in general prepared by reacting the appropriate polyisocyanate and polyol in the presence of a blowing agent. The thermal insulating properties of rigid foams are dependent upon a number of factors including, for closed cell rigid foams, the cell size and the thermal conductivity of the contents of the cells (i.e. of the blowing agents used in the preparation of the foams). A class of materials which has been widely used as blowing agent in the production of rigid polyurethane and polyisocyanurate foams is the fully halogenated chlorofluorocarbons, and in particular trichlorofluoromethane (CFC-11). The exceptionally low thermal conductivity of these blowing agents, and in particular of CFC- 11, has enabled the preparation of rigid foams having very effective insulation properties. Recent concern over the potential of chlorofluorocarbons to cause depletion of ozone in the atmosphere has led to an urgent need to develop systems in which chlorofluorocarbon blowing agents are replaced by alternative materials which are environmentally acceptable and which also produce foams having the necessary properties for the many applications in which they are used. Such alternative blowing agents proposed in the prior art include hydrochlorofluorocarbons, hydrofluorocarbons and (cyclo)alkanes. Although these materials are environmentally more acceptable than chlorofluorocarbons (their ozone depletion potential (ODP) being less or even zero) they are inferior in thermal insulation. In view of strong demands on energy consumption and environmental legislation alternative polyisocyanate based insulation materials are being investigated. Evacuated insulation panels filled with open-celled rigid polyurethane foam is one of these options. The use of vacuum will eliminate the gas conductivity from the samples thus reducing the thermal conductivity of the sample. However the thermal conductivity of such an evacuated insulation panel rapidly increases with internal pressure increase with lapse of time owing to i.a. gases such as air and water vapor diffusing gradually inside the panel. Aerogels are known for their super insulation properties which are due to the elimination of any contribution from the gas phase. These materials are environmental friendly since they are air filled. Moreover they are not subject to ageing. 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. Traditional aerogels are inorganic (for example, silica, alumina or zirconia aerogels) made via the hydrolysis and condensation of metal alkoxides. Silica aerogels have been developed as superinsulating material, for example, for double pane windows. Organic aerogels would be expected to have an even lower thermal conductivity and, thus, provide less heat loss in insulating applications. Recently, organic aerogels have been developed. US Patents Nos 4997804 and 4873218 describe resorcinol-formaldehyde aerogels. US Patents Nos 5086085 and 5081163 describe melamine-formaldehyde aerogels. Organic aerogels based on polyisocyanates are described in WO 95/03358, WO 96/36654, WO 96/37539, WO 98/44028 and WO 98/44013 and are based on polyisocyanurate chemistry. It is an object of the present invention to provide organic aerogels based on the polycarbodiimide chemistry, optionally in combination with polyisocyanurate chemistry and methods for their preparation. Accordingly, the present invention provides a polyisocyanate based organic aerogel. The aerogel according to the present invention has pore sizes less than or equal to 100 nm. Generally the pore sizes are in the range 5 to 100 nm and more generally in the range 5 to 50 nm. Pore sizes can be determined visually by transmission electron microscopy (TEM) or by Brunauer-Emmet-Teller nitrogen adsorption (BET) or Mercury porosimetry. The density of the aerogel of the present invention is generally in the range 100 to 900 kg/m 3 and more specifically in the range 200 to 700 kg/m 3 . Densities can be measured by weighing and determining the displaced volume of water by immersion. The surface area of the aerogel of the present invention is generally in the range 10 to 800 m 2 /g and more specifically in the range 10 to 500 m 2 /g. Surface areas can be determined by BET and by mercury porosimetry. The advantage of the aerogels of the present invention based on polycarbodiimide chemistry compared to aerogels of the prior art based on polyisocyanurate chemistry is that the surface area of the present materials is smaller. As a consequence less volatiles and less water will be absorbed by these carbodiimide based aerogels. Therefore the deterioration of the thermal insulation due to water absorption can be limited. In addition to that, the surface area and the rigidity of the aerogels of the present invention can be tailored by combining carbodiimide chemistry with polyisocyanurate chemistry. It is known that polycarbodiimides, particularly aromatic polycarbodiimides, have a high level of heat resistance, and as such polycarbodiimide based aerogels may be used for high temperature applications. The organic polyisocyanate based aerogels of the present invention are prepared by mixing a polyisocyanate and a carbodiimide catalyst or a mixture of a carbodiimide catalyst and a ~ trimerisation catalyst and optionally a polyfunctional 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. Polyisocyanates for use in the present method include aliphatic, cycloaliphatic, araliphatic and aromatic polyisocyanates known in the literature for use generally in the production of polycarbodiimide materials. Of particular importance are aromatic polyisocyanates such as tolylene and diphenylmethane diisocyanate in the well known pure, modified and crude forms, in particular diphenylmethane diisocyanate (MDI) in the form of its 2,4'-, 2,2'- and 4,4'-isomers (pure MDI) and mixtures thereof known in the art as "crude" or polymeric MDI (polymethylene polyphenylene polyisocyanates) having an isocyanate functionality of greater than 2 and the so-called MDI variants (MDI modified by the introduction of urethane, allophanate, urea, biuret, carbodiimide, uretonimine or isocyanurate residues). The polyisocyanate is used in amounts ranging from 1 to 25 % by weight, preferably from 3 to 20 % and more preferably from 5-18 % by weight based on the total reaction mixture. The isocyanate-reactive compound contains an OH, COOH, NH 2 or NHR group, preferably an OH group. Suitable compounds include polyether polyols, polyester polyols, hydroxyl functional acrylic resins, phenolic resins, hydroxyl functional polyester resins. As carbodiimide linkage forming catalysts, cyclic phosphorous compounds, especially oxides of phospholenes and phosphorous nitrogen heterocyclic compounds are described to have high reactivity. Carbodiimide catalysts for use in the present method include phospholene oxide containing catalysts as disclosed in US Patent No 3,657,161. It is preferred to use 1 -methyl phospholene oxide, 3 -methyl- 1-phenyl phospholene oxide, 1- phenyl phospholene oxide, 3 -methyl- 1 -benzyl phospholene oxide, 3 -methyl- 1 -ethyl phospholene oxide, l-phenyl-3-(4-methyl-3 -pentyl) phospholene oxide and l-phenyl-3- chlorophospholene oxide. It is most preferred to use 1 -methyl phospholene oxide or 1-phenyl phospholene oxide. Trimerisation catalysts for use in the present method include any isocyanate trimerisation catalyst known in the art such as quaternary ammonium hydroxides, alkali metal and alkaline earth metal hydroxides, alkoxides and carboxylates, for example potassium acetate and potassium 2-ethylhexoate, certain tertiary amines and non-basic metal carboxylates, for example lead octoate, and symmetrical triazine derivatives. Especially the triazine derivatives are preferred. Specific preferred trimerisation catalysts for use in the present method are Polycat 41 available from Abbott Laboratories, and DABCO TMR, TMR-2 and TMR-4 available from Air Products. The ratio carbodiimide catalyst/trimerisation catalyst is preferably between 0.4 and 2.5, more preferably between 0.5 and 2. When isocyanate-reactive compounds are added to the reaction mixture, suitable catalysts will be included in the catalyst package. Most widely used and preferred catalysts are the tertiary amine catalysts and the organotin catalysts. Examples of the tertiary amine catalysts include, for example, triethylenediamine, N- methyl-morpholine, bis(dimethylaminoethyl)ether, dimethylcyclohexylamine, dimethylbenzylamine. Examples of organotin catalysts include dimethyltin dilaurate, dibutyltin dilaurate, dioctyltin dilaurate and stannous octoate. The polyisocyanate/total catalyst weight ratio varies between 10 and 300, preferably between 20 and 200 and most preferably between 25 and 100. The solvent to be used in the method according to the present invention should be a solvent for the monomeric polyisocyanate as well as for the reacted polyisocyanate. Preferably the solubility parameter of the solvent should be below 20 (MPa) 1 2 . Further the critical pressure and critical temperature of the solvent should be as low as possible so as to simplify the critical drying step. Suitable solvents for use in the method according to the present invention include hydrocarbons, alicyclic ethers, dialkyl ethers, alkyl alkanoates, aliphatic and cycloaliphatic hydrofluorocarbons, hydrochlorofluorocarbons, chlorofluorocarbons, hydrochlorocarbons and fluorine-containing ethers. Mixtures of such compounds can also be used. Suitable hydrocarbon solvents include lower aliphatic or cyclic hydrocarbons such as n- pentane, isopentane, cyclopentane, neopentane, hexane and cyclohexane. Suitable alicyclic ethers which may be used as a solvent include tetrahydrofuran, dioxane, tetrahydropyran and solvent mixtures thereof. Of these ethers, tetrahydrofuran (THF) is preferred. Suitable dialkyl ethers to be used as solvent include compounds having from 2 to 6 carbon atoms. As examples of suitable ethers there may be mentioned dimethyl ether, methyl ethyl ether, diethyl ether, methyl propyl ether, methyl isopropyl ether, ethyl propyl ether, ethyl isopropyl ether, dipropyl ether, propyl isopropyl ether, diisopropyl ether, methyl butyl ether, methyl isobutyl ether, methyl t-butyl ether, ethyl butyl ether, ethyl isobutyl ether and ethyl t-butyl ether. Suitable alkyl alkanoates which may be used as solvent include methyl formate, methyl acetate, ethyl formate and ethyl acetate. Suitable hydrofluorocarbons which may be used as solvent include lower hydro fluoroalkanes, for example difluoromethane, 1 ,2-difluoroethane, 1,1,1,4,4,4- hexafluorobutane, pentafluoroethane, 1,1,1,2-tetrafluoroethane and 1,1,2,2- tetrafluoroethane. Suitable hydrochlorofluorocarbons which may be used as solvent include chlorodifluoromethane, l,l-dichloro-2,2,2-trifluoroethane, 1,1 -dichloro- 1-fluoroethane, 1- chloro- 1 , 1 -difluoroethane, 1 -chloro-2-fluoroethane and 1,1,1 ,2-tetrafluoro-2-chloroethane. Suitable chlorofluorocarbons which may be used as solvent include trichlorofluoromethane, dichlorodifluoromethane, trichlorotrifluoroethane and tetrafluorodichloroethane. Suitable hydrochlorocarbons which may be used as solvent include 1 - and 2-chloropropane and dichloromethane. Suitable fluorine-containing ethers which may be used as solvent include bis- (trifluoromethyl) ether, trifluoromethyl difluoromethyl ether, methyl fluoromethyl ether, methyl trifluoromethyl ether, bis-(difluoromethyl) ether, fluoromethyl difluoromethyl ether, methyl difluoromethyl ether, bis-(fluoromethyl) ether, 2,2,2-trifluoroethyl difluoromethyl ether, pentafluoroethyl trifluoromethyl ether, pentafluoroethyl difluoromethyl ether, 1,1,2,2-tetrafluoroethyl difluoromethyl ether, 1,2,2,2-tetrafluoroethyl fluoromethyl ether, 1 ,2,2-trifluoroethyl difluoromethyl ether, 1,1-difluoroethyl methyl " ether, 1, 1, 1,3,3, 3-hexafluoroprop-2-yl fluoromethyl ether. Preferred solvents for use in the method according to the present invention are dichloromethane, tetrahydrofuran, trichlorofluoromethane (CFC 11), chlorodifluoromethane (HCFC 22), l,l,l-trifluoro-2-fluoroethane (HFC 134a), 1,1- dichloro- 1-fluoroethane (HCFC 141b) and mixtures thereof such as HCFC 141b/CFC 1 1 mixtures. Another suitable solvent is liquid carbondioxide (CO 2 ). The polyisocyanate, the catalyst and the solvent are mixed by simply shaking the reaction vessel or by slowly stirring the mixture. Mixing can be done at room temperature or at somewhat higher temperatures. In case of low boiling solvents (boiling point below room temperature), for example HCFC 22, the solvent is added to a pressure vessel containing the polyisocyanate and the catalyst, under its own vapour pressure. Thereafter the mixture is left standing for a certain period of time to form a polymeric gel. Temperatures in the range of from about 20°C to about 45°C may be employed, a temperature of about 10°C below the boiling point of the solvent used being preferred. In the case of low boiling solvents such as HCFC 22 the pressure in the closed vessel is maintained at its saturated vapour pressure. Although the mixture gels within a few hours, it has been found to be necessary to cure the gels for a minimum of 24 hours so as to obtain a solid gel before they could be easily handled in subsequent processing. The small cell sizes of the obtained gels necessitates supercritical drying. Large capillary forces at the liquid-vapor interface cause the gel to shrink or crack if the solvent is removed by evaporation. In the case of supercritical drying no surface tension is exerted across the pores, and the dry aerogel retains the original morphology of the gel. Supercritical drying of the obtained aerogels of the present invention involves placing the solvent-filled gel in a temperature-controlled pressure vessel and bringing the vessel to a pressure above the critical pressure of the solvent (for example by filling with nitrogen gas). Nt that point the vessel is then heated above the critical temperature of the solvent. After a few hours the pressure is slowly released from the vessel while keeping a constant temperature. Nt atmospheric pressure and after a cool down period the aerogel is removed from the vessel. The foams are higher in density than their theoretical values because of shrinkage during the drying step. Before the supercritical drying step the gel may be exchanged into a solvent more suitable for supercritical drying, for example liquid carbondioxide. An advantage of the aerogels and their preparation method according to the present invention is that such a solvent exchange step is not necessary. The known organic and inorganic aerogels being based on aqueous systems always need at least one solvent exchange step before they can be supercritically dried. The so obtained aerogels are translucent, yellow-orange in color and showed an openly porous structure with cell sizes less than 100 nm. The aerogel or aerogel like materials obtained using the current invention can be applied in a number of different application areas. The obtained material can either be in a monolith block form which can be subsequently machined or in the form of granules, regular or irregular particles and fine powders. The exact nature of the end physical form will be selected to best suit the needs for the final application area. The aerogel can be used in its pure formed or mixed with other additives such as opacifiers, anti-static agents, colorants, pigments, carbon blacks, lubricants. The aerogel can also be bounded together by mixing it with various other polymers which can be polymerised or with adhesives. Of particular interest is the binding of the polyisocyanate aerogels with isocyanate based binder compositions. As a number of potential applications can be named as a non-exhaustive list: The use of these materials in a sealed environment under slightly evacuated or fully evacuated conditions. The partly evacuated systems may be re-filled with gasses to provide a low thermal conductivity or/and improved structural strength. These systems are most commonly used for their excellent thermal and/or acoustic insulation properties. Typical application areas may be, for instance, vacuum panels in appliances such as refrigerators, freezers and water heaters, insulation of solar heating systems, reefers and road transport vehicles, panels in construction industry or clad on wall applications. Another application area is the use of these aerogel like structure as insulation material for space vehicles in which the excellent thermal insulation is automatically derived due to the vacuum in space. Panels made from these materials may be used as chock and particle absorbers in space vehicles to capture solar dust or other interstellar particles. Another application area is in the field of high energy physics where aerogels can be used to capture and visualise high energy particles as, for instance, in Cherenkov type detectors. The current described materials can also be used in pipe-in-pipe insulation, oil well insulation, district heating gas insulation and latent heat storage devices. Another application area is in the insulation of cryogenic tanks, storage and pipes. The combined properties of high thermal resistance and sound deadening properties may be useful in the aviation industry or in marine applications as filler material between inner and outer hull. Particularly in the air plane industry the excellent insulation in combination with the low weight may be advantageous. Another application is the use as carrier materials or controlled release agents in cosmetic, medical or agricultural applications as, for instance, for controlled release and distribution of fertilisers, pesticides and herbicides. Due to their high internal surface these materials may be employed as (selective) absorbers, filters or filter membranes and catalyst supports. One particular application is the use of these materials in desalination applications. Their particular acoustical aspects may be employed in sound insulating and sound deadening applications as, for instance, in flat panel speaker systems or as impedance matchers. These structures may be used as selective probes or probe carriers. A subgroup of these structures having translucent properties can be used in the window insulation for day lighting applications. Another application area is the carbonisation of the aerogel or aerogel like structures to form a carbon aerogel which can be used as electrode or in high energy density storage devices (batteries) or as capacitor. One particular application area for the carbodiimide based aerogels is in the pipe and pipe- in-pipe insulation where its high temperature stability is combined with the excellent heat insulation capabilities. Particularly the off-shore and oil well insulation can benefit from this application. The present invention is illustrated but not limited by the following examples. Example 1 A known amount of polymeric MDI (SUPRNSEC DΝR available from Huntsman Polyurethanes; SUPRNSEC is a trademark of Huntsman ICI Chemicals LLC) was weighed into an open cup. THF (tetrahydrofuran from Aldrich ) was added subsequently and the mixture was stirred to obtain a homogeneous blend (10 wt % SUPRNSEC DΝR). Catalyst l-methylphospholene oxide (MPO) (available from Hoechst NG) was added by means of a microliter syringe (weight ratio polyisocyanate/catalyst 50) and after stirring for 10 seconds with a spatula the cure time was started. The obtained gel was supercritically dried. The gel was transferred in vials and pressurised at room temperature using nitrogen gas. After attaining a pressure of 70 bar the temperature of the oven which contained the vessel was increased to a temperature of 250°C. The gel was continuously flushed for two hours with nitrogen keeping pressure and temperature constant. The pressure was slowly released from the vessel until atmospheric conditions were reached while keeping a constant temperature. After the cool down period (15 minutes) the dried aerogel was removed from the vessel. The obtained gel was transparent and had a white-yellow colour. Density of the obtained gel was 350 kg/m 3 . Brunauer-Emmet-Teller nitrogen adsorption (BET) measurements on the aerogel revealed a specific area of 99.5 m 2 /g and a pore diameter of 7.2 nm. Example 2 Following the procedure as described in example 1 above a series of gels were made at 10 wt % SUPRASEC DΝR with varying catalyst package. Table 1 shows how the surface area of the gels can be tailored by varying the catalyst composition. (CR = weight ratio polyisocyanate to catalyst). (TMR is a quaternary ammonium salt used as trimerisation catalyst). Table 1 : Influence of the catalyst package on the surface area of the resulting aerogels. * the CR is defined as weight ratio polyisocyanate to total catalyst concentration ; between brackets is indicated the ratio of MPO/TMR catalysts

Claims: CLAIMS 1. Process for preparing polyisocyanate based organic aerogels comprising the steps of a) mixing an organic polyisocyanate and a carbodiimide catalyst and optionally a polyfunctional isocyanate-reactive compound in a 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. 2. Process according to claim 1 wherein the carbodiimide catalyst is a phospholene oxide. 3. Process according to claim 2 wherein the carbodiimide catalyst is 1 -methyl phospholene oxide or 1-phenyl phospholene oxide. 4. Process according to any one of the preceding claims wherein a trimerisation catalyst is also mixed with the other ingredients in step a). 5. Process according to claim 4 wherein said trimerisation catalyst is selected from the group consisting of quaternary ammonium hydroxides, alkali metal and alkaline earth metal hydroxides, alkoxides and carboxylates and triazine derivatives. 6. Process according to claim 4 or 5 wherein the ratio carbodiimide catalyst/trimerisation catalyst is between 0.4 and 2.5. 7. Process according to any one of the preceding claims wherein the ratio polyisocyanate/total catalyst varies between 10 and 300. 8. Process according to any one of the preceding claims wherein the organic polyisocyanate is polymethylene polyphenylene polyisocyanate. 9. Process according to any one of the preceding claims wherein the organic polyisocyanate is used in amounts ranging from 1 to 25 % by weight based on the total reaction mixture. 10. Process according to any one of the preceding claims wherein the solvent is tetrahydrofuran. 11. Organic aerogels based on polycarbodiimide chemistry. 12. Aerogel according to claim 11 having pore sizes less than or equal to 100 nm, a density in the range 100 to 900 kg/m 3 and a surface area in the range 10 to 800 m 2 /g. 13. Use of an aerogel as defined in claim 11 or 12 as pipe insulation material.

Inventors: WALRAEDT SASKIA RACHEL, WALRAEDT, SASKIA, RACHEL
Applicants: HUNTSMAN ICI CHEM LLC, WALRAEDT SASKIA RACHEL, HUNTSMAN ICI CHEMICALS LLC, WALRAEDT, SASKIA, RACHEL
Application year: 1999
Main classification: C08G18
IPC codes: C08G18, C08G101, C08J9