Rigid polyurethane and urethane-modified polyisocyanurate foams are in general prepared by reacting a polyisocyanate with isocyanate-reactive compounds in the presence of blowing agents, surfactants, catalysts and optionally other additives. One use of such foams is as a thermal insulation medium in, for example, buildings or appliances.

In the case of urethane-modified polyisocyanurate foams a stoichiometric excess of the polyisocyanate is used.

These polyisocyanurate foams are usually made at an isocyanate index of between 150 and 500 % ; the term isocyanate index as used herein is meant to be the molar ratio of NCO-groups over reactive hydrogen atoms present in the foam formulation, given as a percentage.

Urethane-modified polyisocyanurate foams, especially the high index foams (index above 300 %), exhibit improved fire properties over polyurethane foams.

It is an object of the present invention to provide rigid polyurethane and urethane-modified polyisocyanurate foams having a combination of desirable properties.

It is a particular object of the present invention to provide rigid urethane-modified polyisocyanurate foams having improved fire properties.

According to the present invention a process for making rigid polyurethane and urethane-modified polyisocyanurate foams is provided by reacting an organic polyisocyanate composition with a polyfunctional isocyanate-reactive composition in the presence of a blowing agent, in which dielectric heating is applied to the foam-forming material.

The foams of the present invention have improved fire properties over foams of the prior art while the other physical properties are not detrimentally affected ; use of expensive fire retardants can be limited. Some physical properties such as surface cure are even improved. In terms of compression strength a more uniform set in the three dimensions is obtained which means that the foam cells are more isotropic.

Blowing agent efficiency is improved leading to lower density of the obtained foam for a given blowing agent loading. Also the density distribution within the foam is more uniform as is the cell structure. The amount of blowing agent used and the amount of catalyst used can be reduced ; thus decreasing the production cost of the

foam. In the case of blowing agents a reduction of 10 to 30 % is possible.

In the manufacturing of urethane-modified polyisocyanurate foams the amount of urethane catalyst can be substantially reduced (up to 75 % reduction) ; in some cases even totally eliminated.

For urethane-modified polyisocyanurate foams the ratio of isocyanurate groups over urethane groups (PIR/PUR ratio) increases ; a ratio of at least 1. 7, preferably at least 2 can be obtained by using the process of the present invention. The PIR/PUR ratio is determined as follows : an FTIR spectrum is taken of the fully cured foam ; the height of the isocyanurate peak is measured at a wavenumber of 1410 and the height of the urethane peak is measured at a wavenumber of 1220. The PIR/PUR ratio is a good indication of the amount of isocyanurate groups in the foam and thus also an indicator of improved fire properties.

Further the processing of high index urethane-modified polyisocyanurate foam is substantially improved.

In the case of polyurethane foams also faster demould times are obtained by using the process of the present invention ; this can advantageously be used in a process for making discontinuous panel.

The obtained foams are especially useful in making building panels where the foam is applied to one or more rigid or flexible skins.

In dielectric heating the material being treated is placed between electrodes across which is imposed a high- frequency voltage in the range of 1 to 5000 megahertz, to electrically stress the dielectric and thereby generate heat internally. Dielectric heating encompasses i. a. microwave heating (850 MHz-3 GHz) and radiofrequency heating (1-50 MHz).

A preferred embodiment of the present invention is the use of radiofrequency heating, it being more safe and efficient than microwave heating. Especially the so-called 50 Ohm radiofrequency technique (as described in "Radio Frequency Electronics"by Jon B. Hagen, Cambridge University Press (1996), ISBN 0521 55356-3 and in"Update on RF Heating", Food Manufacture (Journal), January 1994) at 13. 56 or 27. 12 MHz is preferred.

The frequency and/or power input of the dielectric heating can be modulated.

The required power will usually vary, depending upon the thickness and weight of the material and the number and strength of dipoles present in the material. The power density is preferably between 0. 5 and 8 kW per kg of foam, most preferably around 4 kW/kg foam. Similarly, the heating time will vary, generally in the order of between 1 to 60 seconds, preferably between I to 30 seconds, depending again upon the dipoles, the specific thickness and weight of a given blend and the power of the dielectric heating unit.

Preferably the foam-forming material is exposed to the dielectric heating at any time between mixing of the chemicals and the so-called gel time or string time, which is defined as the time the chemicals go from a liquid state to a stable, solid state and a foam is formed. Applying dielectric heating to the foam forming material after the gel time is not so efficient anymore.

The polyurethane foams of the present invention are usually made at an isocyanate index of between 80 and 150

%, preferably between 100 and 120 %.

The urethane-modified polyisocyanurate foams of the present invention are usually made at an isocyanate index of between 150 and 1500 %, preferably 200 to 500 %, but even indices above 1500 % can be worked at.

The polyfunctional isocyanate-reactive compounds for use in the process of the present invention include any of those known in the art for making rigid polyurethane and urethane-modified polyisocyanurate foams. Of particular importance for the preparation of rigid foams are polyols and polyol mixtures having average hydroxyl numbers of from 300 to 1000, especially from 300 to 700 mg KOH/g, and hydroxyl functionality's of from 2 to 8, especially from 3 to 8. Suitable polyols have been fully described in the prior art and include reaction products of alkylene oxides, for example ethylene oxide and/or propylene oxide, with initiators containing from 2 to 8 active hydrogen atoms per molecule. Suitable initiators include : polyols, for example glycerol, trimethylolpropane, triethanolamine, pentaerythritol, sorbitol and sucrose ; polyamines, for example ethylene diamine, tolylene diamine, diaminodiphenylmethane and polymethylene polyphenylene polyamines ; and aminoalcohols, for example ethanolamine and diethanolamine ; and mixtures of such initiators. Other suitable polymeric polyols include polyesters obtained by the condensation of appropriate proportions of glycols and higher functionality polyols with dicarboxylic or polycarboxylic acids. Further suitable polymeric polyols include hydroxyl-terminated polythioethers, polyamides, polyesteramides, polycarbonates, polyacetals, polyolefins and polysiloxanes.

Suitable organic polyisocyanates for use in the process of the present invention include any of those known in the art for the preparation of rigid polyurethane or urethane-modified polyisocyanurate foams, and in particular the aromatic polyisocyanates such as diphenylmethane diisocyanate in the form of its 2, 4'-, 2, 2'- and 4, 4'-isomers and mixtures thereof, the mixtures of diphenylmethane diisocyanates (MDI) and oligomers thereof known in the art as"crude"or polymeric MDI (polymethylene polyphenylene polyisocyanates) having an isocyanate functionality of greater than 2, toluene diisocyanate in the form of its 2, 4- and 2, 6-isomers and mixtures thereof, 1, 5-naphthalene diisocyanate and 1, 4-diisocyanatobenzene. Other organic polyisocyanates, which may be mentioned, include the aliphatic diisocyanates such as isophorone diisocyanate, 1, 6-diisocyanatohexane and 4, 4'-diisocyanatodicyclohexylmethane. Further suitable polyisocyanates for use in the process of the present invention are those described in EP-A-0320134.

Modified polyisocyanates, such as carbodiimide or uretonimine modified polyisocyanates can also be employed.

Still other useful organic polyisocyanates are isocyanate-terminated prepolymers prepared by reacting excess organic polyisocyanate with a minor amount of an active hydrogen-containing compound.

Preferred polyisocyanates to be used in the present invention for making rigid polyurethane foams as well as rigid urethane-modified polyisocyanurate foams are the polymeric MDI's.

The quantities of the polyisocyanate composition and the polyfunctional isocyanate-reactive composition to be

reacted are such that the molar ratio of isocyanate (NCO) groups to active-hydrogen groups (OH) is generally between 80 and 1500 %, for polyurethane foam between 80 and 150 %, preferably between 100 and 120 %, and for urethane-modified polyisocyanurate foam between 150 and 1500 %, preferably between 200 and 500 %.

The process of the present invention is carried out in the presence of any of the blowing agents known in the art for the preparation of rigid polyurethane and urethane-modified polyisocyanurate foams. Such blowing agents include water or other carbon dioxide-evolving compounds, or inert low boiling compounds having a boiling point of above-70°C at atmospheric pressure.

Where water is used as blowing agent, the amount may be selected in known manner to provide foams of the desired density, typical amounts being in the range from 0. 05 to 5 % by weight based on the total reaction system.

Suitable inert blowing agents include those well known and described in the art, for example, hydrocarbons, dialkyl ethers, alkyl alkanoates, aliphatic and cycloaliphatic hydrofluorocarbons, hydrochlorofluorocarbons, chlorofluorocarbons, hydrochlorocarbons and fluorine-containing ethers.

Preferred classes of blowing agents for use in the process of the present invention for making rigid polyurethane foams as well as rigid urethane-modified polyisocyanurate foams are hydrocarbons, hydrofluorocarbons and hydrochlorofluorocarbons.

Examples of preferred blowing agents include isobutane, n-pentane, isopentane, cyclopentane or mixtures thereof, 1, 1-dichloro-2-fluoroethane (HCFC 141b), 1, 1, 1-trifluoro-2-fluoroethane (HFC 134a), chlorodifluoromethane (HCFC 22), 1, 1-difluoro-3, 3, 3-trifluoropropane (HFC 245fa) and blends thereof.

The blowing agents are employed in an amount sufficient to give the resultant foam the desired bulk density which is generally in the range 15 to 70 kg/m3, preferably 20 to 50 kg/m3, most preferably 25 to 40 kg/m3.

Typical amounts of blowing agents are in the range 2 to 25 % by weight based on the total reaction system.

When a blowing agent has a boiling point at or below ambient it is maintained under pressure until mixed with the other components. Alternatively, it can be maintained at subambient temperatures until mixed with the other components.

In addition to the polyisocyanate and polyfunctional isocyanate-reactive compositions and the blowing agent, the foam-forming reaction mixture will commonly contain one or more other auxiliaries or additives conventional to formulations for the production of rigid polyurethane and urethane-modified polyisocyanurate foams. Such optional additives include crosslinking agents, for examples low molecular weight polyols such as triethanolamine, processing aids, viscosity reducers, dispersing agents, plasticizers, mould release agents,

antioxidants,-fillers (e. g. carbon black), cell size regulators such as insoluble fluorinated compounds (as described, for example, in US 4981879, US 5034424, US 4972002, EP 0508649, EP 0498628, WO 95/18176), catalysts, surfactants such as polydimethylsiloxane-polyoxyalkylene block copolymers and non-reactive and reactive fire retardants, for example halogenated alkyl phosphates such as tris chloropropyl phosphate, triethylphosphate, diethylethylphosphonate and dimethylmethylphosphonate. The use of such additives is well known to those skilled in the art.

Catalysts to be used in the present invention for making rigid polyurethane foam include those, which promote the urethane formation (the so-called urethane catalysts). Examples include aliphatic and aromatic tertiary amines and organo-metallic compounds, especially tin compounds. Examples of suitable tertiary amines include N, N-dimethylcyclohexylamine, N, N, N', N', N"-pentamethyldiethylene-triamine, N, N, N', N", N"- pentamethyldipropylene-triamine, N, N-dimethylbenzylamine, diaminobicyclooctane and 1- (2- hydroxypropyl) imidazole. Examples of suitable organo-metallic compounds include stannous octoate, dibutyltin dilaurate and lead octoate.

Catalysts to be used in the present invention for making rigid urethane-modified polyisocyanurate foams include those, which promote the isocyanurate formation (the so-called trimerisation catalysts). Examples include alkali metal or alkaline earth metal salts of carboxylic acids. The cation of the organic acid metal salt, which is preferably an alkali metal salt, advantageously is K or Na, more preferably K. Particularly preferred are C,-Cs carboxylate salts, including the sodium and potassium salts of formic, acetic, propionic and 2-ethylhexanoic acids.

Other suitable trimerisation catalysts include triazine compounds such as Polycat 41 (available from Air Products) and quaternary ammonium carboxylate salts.

Catalyst combinations can be used as well such as described in EP 228230 and GB 2288182.

In general in the production of rigid urethane-modified polyisocyanurate foam combinations of trimerisation catalysts with urethane catalysts are used. An advantageous effect of the present invention is that rigid urethane- modified polyisocyanurate foams can be prepared in the absence of urethane catalysts and with only trimerisation catalysts present.

In order to increase the polarity or dielectric constant of the foam-forming composition and hence obtain enhanced heating ability, additives such as trichloroethyl phosphate, halogenated diphenyls and dibromopropanol can be used. These substances are in fact also of use as fire retardants. Any material capable of interacting with the magnetic or electric vectors of the applied electromagnetic radiation can, in principle, be employed. Also finely divided carbon, finely divided metal powder, finely divided iron oxide, barium sulphate and barium titanate are substances which improve the heating of the medium in which they are dispersed when the medium is put under the influence of dielectric heating.

In operating the process for making rigid foams according to the invention, the known one-shot, prepolymer or semi-prepolymer techniques may be used together with conventional mixing methods and the rigid foam may be produced in the form of slabstock, mouldings, cavity fillings, sprayed foam, frothed foam or laminates with other materials such as hardboard, plasterboard, plastics, paper or metal.

It is common practice in the manufacture of rigid polyurethane and urethane-modified polyisocyanurate foams to utilise two preformulated compositions, commonly called the A-component and the B-component. Typically, the A-component contains the polyisocyanate compound and the B-component contains the isocyanate-reactive compounds together with the blowing agents, catalysts and other auxiliaries.

The foams of the present invention are advantageously, but not exclusively, used for producing laminates whereby the foam is provided on one or both sides with a facing sheet.

The laminates are advantageously made in a continuous manner by depositing the foam-forming mixture on a facing sheet being conveyed along a production line, and preferably placing another facing sheet on the deposited mixture. The facing sheets can be of a rigid (e. g. plaster-board) or flexible (e. g. aluminium foil or coated paper) nature.

The laminates can also be made in a discontinuous manner whereby the foam reaction mixture is injected into the hollow interior of a panel or into a moulding frame made up of pieces of wood that are removed after moulding is completed and re-used. Multi-daylight presses can be used to hold a stack of up to about eight pre-assembled panels. The rate of production depends upon the time taken to assemble the panels and also on the dwell-time in the press.

The method of continuously producing such laminates generally involves feeding two continuous webs of sheet material from supply rolls thereof, passing these webs through a series of operating stations disposed along a conveyor run, which serve to advance the web material from its source to a point of delivery of the finished sandwich structure. In the course of travel, the foam-forming mixture is applied to one or both of the webs, and they are then caused to converge into parallel relation with the foaming mix sandwiched between them. Provision is made for controlling the distribution of the foaming mix and causing it to adhere to the webs to produce an integrated, laminate composite which is finally cured.

In such a laminator apparatus the dielectric heating unit is preferably placed after the chemicals are mixed and dispersed on the conveyor belt and before the webs converge into parallel relation, as displayed in Figures 1 and 2.

In an installation for making discontinuous panels the dielectric heating unit is preferably integrated with the press, as presented in Figure 7.

The various aspects of this invention are illustrated, but not limited by the following examples in which the following ingredients are used : DALTOLAC XR159 : a polyether polyol of OH value 500 mg KOH/g, available from Huntsman Polyurethanes.

DALTOLAC R105 : a polyether polyol of OH value 1125 mg KOH/g, available from Huntsman Polyurethanes.

DALTOLAC R180 : a polyether polyol of OH value 440 mg KOH/g, available from Huntsman Polyurethanes.

DALTOLAC R090 : a polyether polyol of OH value 540 mg KOH/g, available from Huntsman Polyurethanes.

DALTOLAC R260 : a polyether polyol of OH value 310 mg KOH/g, available from Huntsman Polyurethanes.

Terate 2541 : a polyester polyol of OH value 240 mg KOH/g, available from KOSA.

Isoexter 4537 : a polyester polyol of OH value 356 mg KOH/g, available from Coim.

Isoexter 4565 : a polyester polyol of OH value 510 mg KOH/g, available from Coim.

Tegostab B8406 : a silicone surfactant available from Goldschmidt.

Dabco DC 193 : a surfactant available from Air Products.

L 6900 : a silicone surfactant available from Union Carbide.

Dabco K 15 : a trimerisation catalyst available from Air Products.

Polycat 5 : a urethane catalyst available from Air Products.

Catalyst LB : a trimerisation catalyst available from Huntsman Polyurethanes.

Catalyst SFB : a urethane catalyst available from Huntsman Polyurethanes.

Niax Al : a urethane catalyst available from OSi Specialities.

Polycat 43 : a trimerisation catalyst available from Air Products.

DMCHA (dimethylcyclohexylamine) : a urethane catalyst available from BASF.

Arconate 1000 : propylene carbonate available from Arcol.

TCPP (tris chloropropyl phosphate) : a fire retardant available from Clariant.

Forane 141b : HCFC 141b blowing agent available from Elf-Atochem.

SUPRASEC 2085 : a polymeric MDI available from Huntsman Polyurethanes.

SUPRASEC DNR : a polymeric MDI available from Huntsman Polyurethanes.

SUPRASEC and DALTOLAC are trademarks of Huntsman ICI Chemicals LLC.

Properties of the obtained foams are measured according to the following methods : Density : DIN 53420 Surface friability (% weight loss) : BS 4370 P3, Method 12 Closed cell content : ASTM D2856 Compression strength : DIN 53421 : Cell size : method described in the Proceedings of the 35th Annual Polyurethanes Technical/Marketing Conference of October 9-12, 1994, page 369 under the title"The elimination of radiative heat transfer in

fine celled PU rigid foam"by G. Eeckhout and A. Cunningham : PIR/PUR ratio : the height of the PIR peak at a wavenumber of 1410 over the height of the PUR peak at a wavenumber of 1220 in a FTIR spectrum.

Thermal conductivity (lambda) : ISO 8301 Oxygen Index : ASTM D2863 * B2 test : DIN 4102 PI Cone Calorimeter : ISO 5660-1 DIMVAC : dimensional stability measured according to the test method described by Daems, Rosbotham, Franco and Singh in Proceedings of 35th Annual SPI Technical/Marketing Conference, October 1994 under the title"Factors affecting the long term dimensional stability of rigid foam for the construction industry" TGA (Thermo Gravimetric Analysis) : % weight retained at the indicated temperature ; measured using a Mettler TG50 + Mettler TCIOA/TC15TA controller, heated from 30 to 780°C at a heating rate of 20°C/min with air purge. This can be seen as an indication of the fire behavior.

EXAMPLE I Rigid foams were prepared from the ingredients listed below in Table 1.

The reference foam was made without using microwave heating while the foam according to the invention was made in an experimental microwave oven (CEM microwave systems, Type MES-1000 System, 1000 W, 2. 45 GHz) equipped with a fibre-optic thermocouple to measure temperature and a video camera mounted on the cavity door to film the foam rise. The rise profile was calculated by measuring the height frame-by-frame on different time intervals. The rise and temperature profile of the reference foam was measured on FOMAT equipment.

Physical properties of the obtained foams were measured. The results are presented in Table 1 below.

The rise profile of both foams is presented in Figure 3 and the temperature profile of both foams is presented in Figure 4.

These results show that the rise and temperature profile of foam prepared according to the process of the present invention using microwave heating is smoother than for the reference foam.

The density of the microwave-heated foam is much lower for the same level of blowing agent. The microwave- heated foam has a higher PIR/PUR ratio and shows a much better surface cure.

Other physical properties are not detrimentally affected.

Table 1 Reference Foam Invention Foam DALTOLAC XR159 pbw 100 100 Tegostab B8406 pbw 4. 5 4. 5 Catalyst LB pbw 4. 5 4. 5 DMCHA pbw 1. 8 1. 8 Water pbw 2. 7 2. 7 Forane 141b pbw 151. 8 98. 2 SUPRASEC 2085 pbw 706. 3 706. 3 Index % 500 500 Density kg/m3 28. 3 24. 6 Friability % 25 37 Closed Cell Content % 87 55 PIR/PUR ratio 1. 47 2. 38 Compression strength Elastic Modulus MPa 3. 0 2. 4 kPa 176 95 Stress at 10% Cell Size Etm 522 331 EXAMPLE 2 Rigid foams were prepared from the ingredients listed below in Table 2.

The reference foam was made without using microwave heating while the foam according to the invention was first mixed in a standard lab mixer for 6 seconds at 5000 rpm and then put in the microwave oven which was switched on at 20 seconds after the start of mixing at a set power level of 500 W and switched off when the foam was at end of rise. The catalyst and blowing agent used in the foams according to the invention was adjusted to have the same reaction profile and density as the reference foam.

Rise and temperature profile of the foams was measured in the same way as in Example 1.

Physical properties of the obtained foams were measured. The results are presented in Table 2 below.

The rise and temperature profile of the reference foam is presented in Figure 5 and the rise and temperature profile of the invention foam is presented in Figure 6.

These results show that for the same reactivity the amount of urethane catalyst can be reduced by 50 % using the

process of the present invention. For high index foams the blowing agent can be reduced by 25 % whilst still keeping the same density.

Fire properties are improved by using the process of the present invention.

Table 2 Reference Foam Invention Foam Terate 2541 pbw 100 100 Tegostab B8406 pbw 2 2 Catalyst LB pbw 6 6 DMCHA pbw 3 1. 5 Water pbw 2. 1 1. 6 Forane 141b pbw 68 51 SUPRASEC 2085 pbw 538. 3 500. 0 Index % 500 500 Microwave Power Watt 0 500 Density kg/m'31. 9 34. 5 Lambda mW/mK 21. 8 22. 2 Friability % 30. 95 29. 3 Closed Cell Content % 86.5 86. 9 PIR/PUR ratio 1. 66 2. 05 Compression strength Elastic Modulus MPa 6. 8 6. 4 kPa 211. 8 212. 0 Stress at 10 % Cell Size Um 349 306 Oxygen Index % 24. 3 24. 4 B2 Test cm 10 9

EXAMPLE 3 Rigid foams were prepared from the ingredients listed below in Table 3.

The reactivity of the foam was followed in terms of cream time (which is the time at which the foam starts to rise), string time (which is the time at which the chemicals go from a liquid state to a stable solid state and a foam is formed) and end of rise time (which is the time at which the foam doesn't rise anymore).

Physical properties of the obtained foams were measured. The results are presented in Table 3 below.

Table 3 Foam 1 Foam 2 Foam 3 Foam 4 Foam 5 Terate 2541 pbw 100 100 100 100 100 Isoexter 4537 pbw 51. 24 51. 24 Isoexter 4565 pbw 11. 80 11. 80 DALTOLAC R105 pbw 8. 73 8. 73 Catalyst LB pbw 2. 01 1. 24 6. 00 6. 00 6. 00 DMCHA pbw 1. 38 0. 93 3. 00 0. 50 0. 50 Niax Al pbw 0. 50 0. 50 Polycat 43 pbw 0. 88 0. 87 _ Water pbw 0. 88 0. 87 2. 10 1. 60 1. 60 TCPP pbw 18. 71 18. 52 Tegostab B 8406 pbw 2. 00 2. 00 2. 00 Dabco DC 193 pbw 3. 27 3. 23 HCFC 141b pbw 52. 43 51. 90 68. 00 51. 00 51. 00 SUPRASEC 2085 pbw 376. 11 371. 66 538. 44 500. 19 500. 19 Index % 230 230. 4 500 500 500 Dielectric heating None gwave None RF RF 2. 45 GHz 27. 12 MHz 27. 12 MHz 50 Ohm Power W 900 2500 1000 Power Density kJ/kg 68. 8 35. 7 28. 6 Cream time sec 17 20 9 24 String time sec 35 39 47 End of rise time sec 70 47 90 84

Density kg/m 34. 4 35 31. 9 31. 1 35. 8 Friability % 20 3 31 27 26 Closed Cell Content % 87 84 87 88 90 Compression Strength 10 % stress rise kPa 255 146 180 146 218 10 % stress perpendicular kPa 90 127 64 82 135 PIR/PUR ratio 1.13 1.75 2.47 2.1 Cell size Az llm 579 369 355 352 Bz µm 318 525 244 267 Dz µm 388 279 276 293 TGA at 300°C % 86 90 93 92 at 400°C % 67 75 76 77 at 500°C % 55 55 56 59 Lambda Initial mW/mK 22. 7 20. 2 22. 6 21. I 20. 8 5 weeks at 70°C mW/mK 28. 2 28. 1 26. 7 27. 1 B2 test cm 10 (B2) 11 (B2) 12 (B2) 8 (B2) Cone Calorimeter Flame out time sec 595 434 354 Smoke produced mi2 684 548 517 Oxygen Index % 25.3 24.2 25.7 24.8 25. 9 DIMVAC (7 days) -20°C % length-18. 2 0. 8-0. 6 70°C/40% RH % length-1. 6 0. 2 These results show that using dielectric heating in the manufacture of urethane-modified polyisocyanurate foams of varying index leads to a more uniform compression strength set, a higher PIR/PUR ratio and improved fire properties. Further the use of the trimerisation catalyst and of the urethane catalyst can be diminished.

The results also show that radiofrequency heating is more efficient than microwave heating, especially the 50 Ohm radiofrequency technique.

EXAMPLE 4 Rigid polyisocyanurate foams were prepared from the ingredients listed below in Tables 4 and 5.

Physical properties of the obtained foams were measured. The results are also presented in Tables 4 and 5 below.

These results show that using the process of the present invention leads to foams of lower density (although the amount of blowing agent is reduced), better fire resistance and a more uniform cell structure. Also the amount of catalyst can be reduced.

Table 4 Foam 6 Foam 7 Foam 8 Foam 9 Foam 10 Foam 11 DALTOLAC R180 pbw 100 100 100 100 100 100 Catalyst LB pbw 0. 5 8. 00 26. 20 0. 5 8. 00 26. 20 DMCHA pbw 5. 00 1. 60 3. 60 2. 50 0. 80 1. 80 Tegostab B8406 pbw 2. 00 4. 60 9. 20 2. 00 4. 60 9. 20 Water pbw 1. 50 3. 40 6. 15 1. 50 3. 40 6. 15 HCFC 141b pbw 25. 00 48. 00 85. 00 20. 00 38. 40 68. 00 SUPRASEC 2085 pbw 199. 00 638. 44 1371. 70 199. 00 638. 44 1371. 70 Index % 150 350 500 150 350 500 Microwave None None None 200 W 200 W 200 W 2. 45 MHz 40 sec 40 sec 60 sec Density kg/m3 35.0 31.4 30.6 32.9 30.4 25.9 Friability % 16.0 43.1 64.5 24.9 81.1 99.0 Closed Cell Content % 91.5 92.9 90.7 81.2 89.3 82.2 Compression Strength 10 % Stress Rise kPa 253. 4 188. 0 160. 6 143. 3 114. 2 74. 6 10 % Stress kPa 87.1 82.4 60.0 119.4 80.5 41. 1 Perpendicular PIR/PUR 0.8 1 1.12 1.58 1.67 2.36 Cell Size Az µm 449 519 375 442 Bz Fm 341 487 303 364 Dz µm 374 497 325 388 TGA At 300°C % 81 89 89 87 92 93 At 400°C % 56 71 75 59 75 80 At 500°C % 41 49 51 47 57 60 Lambda Initial mW/mK 22. 6 22.7 23.4 22.2 23.3 27.31 5 weeks at 70°C mW/mK 28. 8 29. 4 29. 9 29. 8 31.1 34.0 B2 test cm 20 (B3) 15 (B3) 13 (B2) 20 (B3) 14 (B2) 11 (B2) Oxygen Index % 20. 0 23. 8 25. 1 19. 8 23. 3 24. 7 Table 5 Foam 12 Foam 13 Foam 14 Foam 15 Foam 16 Foam 17 Terate 2541 pbw 100 100 100 100 100 100 Catalyst LB pbw 0. 91 7. 95 5. 95 0. 60 2. 50 6. 30 DMCHA pbw 0.34 1.06 3.01 0.10 0.25 0. 75 Tegostab B8406 pbw 0.64 1.48 2.01 2.00 2.00 2. 00 Water pbw 0. 68 1. 59 2. 08 1. 00 1. 30 1. 60 HCFC 141b pbw 17. 07 50. 35 67. 76 25. 00 32. 50 57. 00 SUPRASEC 2085 pbw 107. 82 367. 21 535. 63 152. 39 300. 65 503. 62 Index % 150 350 500 200 350 500 Microwave 2. 45 MHz None None None 200 W 100 W 300 W 40 sec 40 sec 40 sec Density kg/m3 40.2 29.4 31.9 41.0 31.5 30.5 Friability % 0.5 17.3 31.0 0.2 23.8 36.1 Closed Cell Content % 86. 6 83. 7 86. 8 64. 7 85. 7 88. 5 Compression Strength 10 % Stress Rise kPa 158. 9 163. 4 180. 2 112. 3 174. 7 130. 7 10 % Stress kPa 53.4 58.7 64.0 72.4 112.0 86.0 Perpendicular PIR/PUR 0. 78 1. 63 1. 75 0. 89 1. 75 2. 41 Cell Size Az llm 290 562 355 407 354 429 Bz µm 229 393 244 313 337 362 Dz Um 247 442 276 341 343 383 TGA At 300°C % 82 88 90 88 98 93 At 400°C % 58 72 75 67 76 78 At 500°C % 43 50 55 55 62 60 Lambda Initial mW/mK 21. 4 22. 6 22. 6 19. 2 21. 2 21. 7 5 weeks at 70°C mW/mK 26. 7 27. 7 28. 1 27. 6 28. 7 B2 test cm 15 (B3) 11 (B2) 11 (B2) 15 (B3) 11 (B2) 10 (B2) Oxygen Index % 22. 4 25. 2 25. 7 23. 6 25. 2 25. 3

EXAMPLE 5 Rigid polyisocyanurate foams were prepared from the ingredients listed below in Table 6.

Physical properties of the obtained foams were measured. The results are also presented in Table 6 below.

Table 6 Foam 18 Foam 19 Terate 2541 pbw 100 100 Dabco K 15 pbw 3. 20 4. 00 Polycat 5 pbw 0. 60 0. 00 Tegostab B8406 pbw 2. 50 2. 50 Water pbw 0. 50 0. 50 HCFC 141b pbw 30. 00 26. 50 SUPRASEC 2085 pbw 176.0 177. 6 Index % 255 255 Microwave Power W 0 300 Input time sec 0 15 Time on sec 10 Cream Time sec 7 String Time sec 20 20 End of rise time sec 49 37 Density kg/m3 29.9 32.4 Friability % 23.1 24.7 Closed Cell Content % 92. 6 92. 8 Compression strength 10% stress rise kPa 181. 6 195. 5 10% stress perpendicular kPa 84.1 105. 6 Cell Size Az µm 534 360 Bz µm 284 251 Dz µm 350 283

TGA 300°C % 89 89 400°C % 65 66 500°C % 46 45 Lambda Initial mW/mK 21. 3 21. 0 5 weeks at 70°C mW/mK 26. 9 27. 4 B2 test cm 11 (B2) 11 (B2) Oxygen Index% 24. 1 23. 9 DIMVAC -20°C/7 days % length-5. 2-0. 6 40°C/100% RH/7 days % length-1. 2-1. 3 These results show that by using the process of the present invention urethane-modified polyisocyanurate foams can be made without any urethane catalyst being present in the foam formulation.

EXAMPLE 6 Rigid polyurethane foams were prepared from the ingredients listed below in Table 7.

Physical properties of the obtained foams were measured. The results are also presented in Table 7 below.

These results show that using the claimed process can substantially reduce the amount of catalyst.

Further a smoother rise profile is obtained ; at string time the foam has obtained 93% of its height compared to only 79 % for foams of the prior art.

EXAMPLE 7 Rigid polyurethane foams were prepared from the ingredients listed below in Table 8.

Physical properties of the obtained foams were measured. The results are also presented in Table 8 below.

Table 7 Foam 20 Foam 21 DALTOLAC R180 pbw 64.90 64. 90 DALTOLAC R090 pbw 7. 00 7. 00 DALTOLAC R260 pbw 13. 00 13. 00 L 6900 pbw 1. 10 1. 10 Niax A I pbw 0. 10 0. 00 Catalyst SFB pbw 3. 00 0. 75 DMCHA pbw 0. 30 0. 00 Arconate 1000 pbw 8. 00 8. 00 water pbw 2. 60 2. 60 HCFC 141b pbw 13. 00 13. 00 SUPRASEC DNR pbw 145. 34 145. 27 Index % 113. 5 113. 5 Cream Time sec 19 28 String Time sec 98 92 End of Rise Time sec 161 132 % Height at String % 79 93 Microwave 2. 45 GHz Power Watt 0 400 Input Time sec 0 30 Start Time sec 0 15 Density kg/m330. 1 37. 0 Table 8 Foam 22 Foam 23 Terate 2541 pbw 100 100 Dabco K 15 pbw 0. 00 3. 50 DMCHA pbw 3. 00 0. 00 Catalyst LB pbw 6. 00 0. 00 Tegostab B8406 pbw 2. 00 2. 00 Water pbw 2. 10 2. 10 HCFC 141b pbw 68. 00 50. 00 SUPRASEC 2085 pbw 538. 4 471. 2 Index % 500 500 Microwave Power 330 Input time sec 0 40 Time on sec 0 15 Cream Time sec 5 30 String Time sec 54 54 End of rise time sec 95 77 Density kg/m 31. 1 29. 1 Closed Cell Content % 89. 9 88. 9 Compression strength 10% stress rise kPa 159. 2 122. 3 10% stress perpendicular kPa 117. 7 92. 3 Lambda Initial mW/mK 20. 8 21. 3 B2 test cm 10 (B2) 13 (B2) Oxygen Index % 25. 1 25. 2