MXPA97002554A - Rigi polyurethane foams - Google Patents

Abstract

The present invention relates to the process for the preparation of a rigid polyurethane or urethane-modified polyisocyanurate foam by reacting a polyisocyanate composition with a polyfunctional isocyanate-reactive composition, under conditions of foaming in the presence of a mixture of physical blowing agents, each of said physical blowing agents have a boiling point in the range of -40 ° C to 80 ° C, characterized in that the mixture of the physical blowing agent comprises at least 3 components and the foam has a closed cell content greater than 85%, a core density below 28 kg / m3 and a dimensional stability expressed as the sum of the absolute values of the relative linear changes in the three dimensions after exposure to -20§ C for 14 days, below 1

Description

RIGID POLYURETHANE FOAMS DESCRIPTION OF THE INVENTION This invention relates to rigid foams ie polyurethane or urethane-modified polyisocyanate, to processes for their preparation, and to new compositions useful in said processes. Rigid foams of polyurethane and polyisoc Lanato modified with urethane are generally prepared by the reaction of the appropriate polyisocyanate and an isocyanate-reactive compound (usually a polyol) in the presence of a blowing agent. One use of such foams is as a means of thermal insulation in buildings, refrigerators and other household appliances. The industry is constantly looking for low density rigid polyurethane foams, which do not show any harmful effect on other properties of the foam such as dimensional stability. Especially low-density blown (cycle) alkane foams are difficult to obtain. preferably below 26 or even 25 kg / m3 and even more preferably below 22 kg / m3. According to another preferred embodiment of the present invention, the rigid polyurethane foam has a dimensional stability of less than 10%, preferably less than 7.5%. In general, a rigid polyurethane foam of the present invention having a core density below 20 kg / m 3 has a dimensional stability of less than 15%, a rigid polyurethane foam having a core density below 25. kg / m3 has a dimensional stability below 10%, and a polyurethane foam having a core density below 28 kg / m3, has a dimensional stability below 7.5%. The rigid polyurethane foam of the present invention is especially useful in low temperature applications (below 5 ° C). According to the present invention there is also provided a process for the preparation of such a rigid polyurethane foam, by reacting a polyisocyanate composition with a polyfunctional isocyanate-reactive composition, under the conditions of foaming in the presence of a mixture of physical blowing agents, each of the physical blowing agents has a boiling point in the range of -40 ° C to 80 ° C, preferably in the range of 15 ° C to 60 ° C. Preferably, the physical blowing agents used in the process of the present invention do not contain halogen atoms. Preferably, at least one of the physical blowing agents used in the process of the present invention have a thermal conductivity of gas at the operational temperature of foam lower than the gaseous thermal conductivity of carbon dioxide, more preferably lower than the gas conductivity of the carbon dioxide. n-pentane, more preferably less than the gaseous thermal conductivity of the cyclopentane. The operational temperature of the foam is defined as the lowest temperature at which 1 * t foam is exposed during use. For example, the operational temperature for a foam to be used in a refrigerator is -10 ° C, in a -25 ° C freezer. Preferably, the sum of the gas partial pressures of all the components of the blowing agent (including the physical blowing agents and any CO. Generated by a chemical blowing agent t as water) is at least 0.3 bar, more preferred. at least 0.45 bar and more preferably at least 0.6 bar. Preferably at the operational temperature of the foam, at most 40% of the physical blowing agent mixture is condensed, preferably at most %, Me preferably at the most 10% and most preferably at the most 2%. The mixture of the physical blowing agent for use in the process of the present invention generally comprises at least components, preferably at least 4 components, especially if the foam is intended for use at low temperatures. Preferably, at least one of the components of the physical blowing agent mixture to be used in the process of the present inventionis an alkane or a cycloalkane and / or acetone. The appropriate components for use in the present mixture of the physical blowing agent comprise 2-methylpentane, 3-methylpentane, 1-propionaldehyde, trans-1,3-pentadiene, cis-1,3-pentadiene, 1- penten-4-yne, 4-methyl-cis-2-pentenc, 4-methyl-1-pentene, l-penten-3-yne, trans-2-pentene, cis-2-pentene, methyl isocyanate, , - exadiene, 2,3-pentadiene, 1,2-pentadiene, 1,4-pentadiin, 2-methyl-1-butene, 3,3-dimethyl-1-butene, cyclopentadiene, 1,3-propyl oxide Log, dimethylacetylene, cyclopentane, cyclopentene, 1-propanal, ethyl formate, glyoxal, 2, -dimethylbutane, methyl t-butyl ether, vinyl formate, 1-pentyne, methyl isobutyl ether, methyl isopropyl ether, 3- methyl -1, 2-butadiene, 3-methyl-1-butyne, methyl N-propyl ether, 2-methyl-2-butene, eti len-imine, furan, divinyl ether, diethylamine, methylal, 1-pentene, n -pentane, methyl formate, n-propyl-amine, acrolein, isopentane, 2,2-dimet ilbutane, isopropyl mercaptan, sulfur dimethyl, tetramethylsilane, 1,2-propylene oxide, methyl acetate, 2-methyl-l-buten-3-yne, carbon disulfide, acetaldehyde, acetone, ethylmercaptan, diethyl ether, cyclohexane, methyl-cyclopentane, methyl-ethyl -ketone, sulfur dichloride, tetrachlorosilane, trichlorosilane, bromine, sulfur trioxide, 1,1-dichloro-1-fluoroethane, di-ethylchlorosilane, 1,1-difluoroethane, 2-chloropropene, n-pro-oyl chloride, t-butyl, 1,2-dichloro-l, 1,2-trifluoroethane, l, l-diclich-2,2,2-trifluoroethane, trans-1,2-dichloroethylene, cis-1,2-dichloroethylene, halothane, cyclobutane, n-hexane, diisopropyl ether, propargyl chloride, trimethylchlorosilane, 2-bromopropane, difluoroethane, ethylene oxide, ethyl chloride, perfluoropropane, methyl vinyl ether, perfluorocyclobutane, perfluorobutene, perfluorobutane, chlorine pentafluoride, ethyl fluoride , ethyl ether, cyclopropane, butadiene, cis-butene, trans-butene, methylpropene, cyclobutane, butane, methylpropane, Chlorine trifluoride, 1,1,1,2-tetrafluoroethane, 1,1-difluoroethane, chloroprene, 1,2-dibromotetrafluoroethane, methyl chloromethyl ether, 3-chloropropene, 1,1,2-trichlorotrifluoroethane, trichlorofluoromethane, dibromodifluoromethane, methyldichlorosyl anus, acetyl chloride, 1,1-dichloroethylene, 1,1-dichloroethane; isopropyl chloride, dichloromethane, bromoethane, methyl iodide, bromochlorodifluoromethane, dichlorodifluoromethane, chlorodifluoromethane, dichlorofluoromethane, bromotrifluoroethylene, chlorotrifluoroethylene, chloropentafluoroethane, dichlorotetrafluoroethane, chlorodifluoroethylene, vinyl bromide, vinyl chloride, chlorodifluoroethane, hexafluoropropyl, hexafluoroacetone.

Mixtures of the physical drying agent for use in the process of the present invention preferably contain alkanes, optionally cycloalkanes, and / or alkenes, optionally cycloalkenes, and / or acetone and / or one or more of the following hydrofluorocarbons: 1.1 , 1,3,3-pentafluoropropane (HFC 245fa), 1,1,1,2-tetrafluoroethane (HFC 134a), 1,1,1,2,2-pentafluoroethane (HFC 125), 1,1-di-fluoroethane ( HFC 152a), 1, 1, 1, 4, 4, 4-hexafluorobutane (HFC 356mffm), 1, 1, 1, 2, 3-pentafluoropropane (HFC 245eb), 1,1,2,2,3-pentafluoropropane ( HFC 245ca), 1,1,1,2,3,3-hexafluoropropane (HFC 236ea ^, 1,1,1,3,3,3-hexafluoropropane (HFC 236fa), 1,1,1,3,3- pentafluorobutane (HFC 365mfc) and / or perfluorinated alkanes and / or fluorinated ethers especially the isomers of E245 and E536. More preferred are mixtures of the physical blowing agent containing one or more (cyclo) alkanes having 5 to 6 carbon atoms. such as cyclopentane, n-pentane, isopentane, 2,2-dimet ilbutane, 2-met ilpentane, 3- methylpentane, optionally acetone and optionally hydrofluorocarbons. The relative amounts of each component to be used in the present mixture of natural blowing agent has to be determined to obtain an appropriate level of internal gas pressure (preferably above 0.3 bar) and condensation (preferably at most % at the operating temperature of the foam). In addition to the physical blowing agent mixture as described above, a chemical blowing agent that generates CO2, such as water, can be used in the process of the present invention. The total amount of the blowing agent (physical and chemical blowing agents) to be used in the process of the present invention must be determined to obtain an appropriate core density (below 28 kg / m3), and will typically be 2 to 25% by weight based on the total reaction system. Particularly preferred blowing agent mixtures for use in the process of the present invention include (the amounts are expressed as pbw on the isocyanate-reactive component): - cyclopentane (3.9), acetone (2.3), 2,2-dime "ilbutane (5.2), isopentane (10.5), n-pentane (7.1 >, 1, 1, 1,, 2-tetrafluoroethane (3.3), water (3.9! (Hereinafter referred to as MIXING 1); cyclopentane (3.9), 2,2-dimethylbutane (5.3), 2- methylpentane (3.0), 3-methylpentane (2.6), n-pentane (7.2), isopentane (10.8), water (4.0) (hereinafter referred to as as MIX 2); - cyclopentane (3.3), acetone (2.0), 2,2-dimethylbutane (4.2), isopentane (7.0), water (1.5) (hereinafter referred to as MIXTURE 3); cyclopentane (3.2), acetone (2.0), 2,2-dimethylbutane (4.2), isopentane (8.1), water (1.0't, 3-methylpentane (1.0) (hereinafter referred to as MIXTURE 4); cyclopentane ( 3.2), acetone (2.0), 2,2-dimethylbutane (4.2), isopentane (1.6), water (2.2 'i, 3-methylpentane (3.0) (hereinafter referred to as MIXTURE 5), cyclopentane (3.2) , acetone (2.0), isopentane (7.7), water (2.2) (hereinafter referred to as MIXTURE 6), cyclopentane (3.9), acetone (2.4), isopentane (9.6), n-pentane (12.0), water ( 2.0) (hereinafter referred to as MIX 7).

MIX 1 and MIX 2 are especially useful for the preparation of rigid polyurethane foams of 25 kg / m3 density for use in construction, being exposed to -20 ° C. MIX 3, MIX 4, MIX 5 and MIX 6 are especially useful for the preparation of rigid polyurethane foams with a density of 27-29 kg / m3 for use in household appliances that are exposed to -5 ° C. The preferred mixture of blowing agent for use in the process of the present invention is a mixture containing cyclopentane, isopentane, n-pentane, acetone and 2,2-dimethylbutane in the following molar percentages: 13.8, 36.6, 24.7, 9.8 and 15.2 respectively. Organic polyisocyanates suitable 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 aromatic polyisocyanates such as diphenylmethane diisocyanate in the form of its 2,4'-, 2'-isomers; '- and 4,4'- and mixtures thereof, mixtures of diphenylmethane diisocyanates (MDI) and oligomers thereof known in the art as "crude" or polymeric MDI (polymethylene-polyphenylene polyethiocyanates) having a isocyanate functionality 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 that may be mentioned include aliphatic diisocyanates such as isophorone diisocyanate, 1,6-diisocyanatohexane and 4,4'-diisocyanatodicyclohexylmethane. Polyisocyanate-reactive reactive compositions, with which S can react the polyisocyanate composition to form rigid polyurethane or urethane-modified polyisocyanurate foams, include any of those known in the art for that purpose. Of particular importance for the preparation of rigid foams are polyols and blends of polyols having average hydroxyl numbers from 300 to 1000, especially from 300 to 700 mg KOH / g, and hydroxyl functionalities from 2 to 8, especially from 3 to 8. Appropriate 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, tri-ethylolpropane, triethanolamine, p pentaerythritol, sorbitol and sucrose; polyamines, for example and ilendiamine, tolylenediamine, diaminodiphenylmethane and polymethylene polyphenylene polyols and amino alcohols, for example ethanolamine and diethanolamine; and mixtures of such initiators. Other suitable polymeric polyols include polyesters (especially aromatic polyesters) obtained by the condensation of appropriate proportions of glycols and polyols of higher functionality with dicarboxylic acids or polycarboxylic acids. The additionally suitable polymer polyols include polyether-terminated polyethylenes, polyamides, polyesteramides, polycarbonates, polyacetals, polyolefins and polysiloxanes. The amounts of the polyisocyanate compositions and the polyfunctional isocyanate-reactive compositions to be reacted will depend on the nature of the rigid polyurethane foam, or the urethane-modified polyisocyanurate to be produced, and will be readily determined by those skilled in the art. . In addition to the polyisocyanate and polyfunctional isocyanate reactive compositions and the blowing agent mixture of the present invention, the reaction mixture for foam formation will commonly contain one or more other auxiliary or conventional additives to the formulations for the production of rigid polyurethane foam and urethane-modified polyisocyanate. Such functional derivatives include crosslinking agents, for example low molecular weight polyols such as triethanolamine, foam stabilizing agents or surfactants, for example, loxane-oxyalkylene copolymers, urethane catalysts, for example, seat compounds such as stannous octoate or dibutyltin dilaurate or tertiary amines such as dimethylcyclohexyl ina or triethylene diamine and fire retardants, for example halogenated alkyl phosphates such as tris-chloropropyl phosphate. According to a particular embodiment of the present invention, the process is carried out in the presence of an insoluble fluorinated compound producing foams having smaller cell sizes and improved thermal insulation. A urethane-modified polyisocyanurate foam (NCO index of 150), made in the presence of such an insoluble fluorinated compound, has also improved the friability properties.

The term "insoluble" as used herein with respect to the insoluble fluorinated compound, is defined as showing a solubility in either the isocyanate-reactive composition or in the polyisocyanate composition with which it is mixed, less than 500 ppm in weight at 25 ° C and at atmospheric pressure. The insoluble fluorinated compounds for use in the process of the invention include any of those described in U.S. Patent No. 4,981,879, U.S. Patent No. 5,034,424, U.S. Patent No. 4,972,002, U.S. Patent Applications Nos. 0508649 and 0498628 and PCT Patent Application No. 95/18176. It is preferred to use an insoluble, substantially fluorinated or perfluorinated compound, having a boiling point of at least? 0 ° C at atmospheric pressure, and preferably at least 40 ° C and more preferably at least 60 ° C, or 80 ° co 100 ° C. The term "substantially fluorinated" as used herein with reference to the substantially fluorinated, insoluble compound used in the process of the invention, should be understood to encompass those compounds in which at least 50% of the hydrogen atoms of the compound do not Fluorinated are replaced by fluorine. Suitable compounds include substantially fluorinated or perfluorinated hydrocarbons, substantially fluorinated or perfluorinated ethers, substantially fluorinated or perfluorinated tertiary amines, substantially fluorinated or perfluorinated amino ethers, and substantially fluorinated or perfluorinated sulphonates. Suitable examples of substantially fluorinated or perfluorinated hydrocarbons are those containing from 1 to 15 carbon atoms., which may be either cyclic or acyclic, or aromatic or aliphatic and either saturated or unsaturated, such as the following substantially fluorinated or perfluorinated compounds namely methane, ethane, propane, butane, pentane, hexane, heptane, octane , nonane, decane, cyclobutane, cyclooct, cyclohexane, cyclopentane, cycloheptane, norbornadiene, decalin, dimethylcyclobutane, methylcic lohexane, 1-methyldecalin, phenanthrene, dimethylcyclobutane, and isomers thereof. Particular mention may be made of various isomers of the perfluoropentane and the perfluorohexane, such as perfluoro-n-pentane and perfluoro-n-hexane and of hexamers and trimers of hexafluoropropene such as perfluoro (4-methylpent-2-ene). Certain insoluble fluorinated compounds, suitable for use in the process of the present invention can themselves act as blowing agents under the conditions pertaining to the foaming reaction, particularly where their boiling point is lower than the temperature of exotherm achieved by the reaction mixture. For the avoidance of doubt, such materials may, partially or completely, fulfill the function of the blowing agent in addition to that of the fluorinated insoluble compound. The amount of the insoluble fluorinated compound used in the process of the present invention is in the range of 0.5 to 10%, preferably 0.1 to 5%, more preferably 0.6 to 2.3% by weight based on the total composition for the formation of foam. The insoluble fluorinated compound will usually be incorporated into the foam-forming reaction mixture in the form of an emulsion or preferably a microemulsion in one of the main components, ie in the isocyanate-reactive compound and / or the polyisocyanate component. Such emulsions or microemulsions can be prepared using conventional techniques and appropriate emulsifying agents. Suitable emulsifying agents for the preparation of stable emulsions or microemulsions of liquid fluorinated compounds in organic polyisocyanate compounds and / or isocose reagents include the surfactants chosen from the group of nonionic, ionic (anionic or cationic) and amphoteric surfactants. Preferred eurfactants are silicone surfactants, fluorosurfactants and / or alkoxylated alkanes. The amount of emulsifying agent used is between 0.02 and 5 pbw per 100 pbw of foam forming reaction system, and between 0.05 and 10 pbw per 100 pbw of the polyisocyanate or polyol composition. In operation, the process for the production of rigid foams, according to the invention, can be used. The known techniques of a single shot, of prepolymer or semi-prepolymer, together with the conventional methods of mixing, and the rigid foam can be produced in the form of plate, moldings, cavity fillings, sprayed foam, foamed foam or Laminates with other materials such as chipboard, plasterboard, plastics, paper or metal. It is convenient in many applications to provide the components of polyurethane production in premixed formulations based on each of the primary components of polyisocyanate and isocyanate reagent. In particular, many reaction systems employ a reactive polyisocyanate composition which contains the major derivatives such as the blowing agent and the catalyst, in addition to the polyisocyanate-reactive component or components. Therefore, the present invention also provides a polyisocyanate composition or a polyisocyanate-reactive composition comprising a mixture of physical blowing agent, as specified above. The various aspects of this invention are illustrated, but are not limited by the following examples, in which the following ingredients are used: The polyol A which is a PET-type polyester polyol containing about 30% recycled polyethylene terephthalate, having an approximate functionality of 2.3 and a hydroxyl value of 350 g KOH / g. Tagostab B 8408 is a silicone surfactant available from Goldschmidt. Tagostab B 8546 is an eilicone surfactant available from Golschmidt. PET is triethyl phosphate. DMEA is N, -dimethylaminoethanol. NIAX Al is an amine catalyst available from Air Products. Dabco T 45 is an amine catalyst available from Air Products. SUPRASEC 2085 is a polyisocyanate available from Imperial Chemical Industries PLC. DALTOLAC XR 159 is a polyether polyol available from Imperial Chemical Industries PLC. DALTOLAC XR 144 is a polyether polyol available from Imperial Chemical Industries PLC. DALTOLAC R 130 is a polyether polyol available from Imperial Chemical Industries PLC .. DALTOLAC P744 is a polyester polyol available from Imperial Chemical Inudstries PLC. DALTOREZ XP 805 is a polyester polyol available from Imperial Chemical Industries PLC.

DALTOLAC R 105 is a polyether polyol available from Imperial Chemical Industries PLC. Tegostab B 8423 is a silicone surfactant available from Goldschmidt. RS 201 is a silicone surfactant available from Union Carbide (Air Products). RS 210 is a silicone surfactant available from Union Carbide (Air Products). SR 234 is a silicone surfactant available from Osi. PFO is perfluoro (4-methylpent-2-ene) The SFC catalyst is an amine catalyst available from Imperial Chemical Industries PLC. SUPPASEC DNR is a polyisocyanate available from Imperial Chemical Industries PLC.

SUPRASEC, DALTOLAC and DALTOREZ are trademarks of Imperial Chemical Industries PLC.

EXAMPLE 1 Rigid polyurethane construction type foams were prepared, starting from the formulations indicated in Table 1 as follows.

The polyol blends were mixed and vigorously stirred (by manual stirring or mixing with propellant) until a uniform homogeneous mixture or a uniform white emulsion was obtained This mixture was then mixed with the polyisocyanate using the standard foam equipment. The initial temperature of the chemicals was 23 ° C ± 2 ° | C. The foams were allowed to cure for at least 16 hours at room temperature before measuring the physical properties. From the foams obtained, the density of free elevation was measured, the dimensional stability after 1 day at -20 ° C and after 14 days at -20 ° C and the thermal conductivity (initial, then He 1 week at 70 ° C and after 3 weeks at 70 ° C). The results are indicated in Table 1 below. These results show that for the foams A and B according to the present invention, in addition to improvements in dimensional stability, the thermal conductivity improved substantially compared to the Reference Foam. twenty-one TABLE 1 EXAMPLE 3 Rigid polyurethane foams of the device type were prepared, starting from the formulations indicated in Table 3 along the lines described in example 1. From the foams obtained, the content of closed cells (CCC), the dimensional stability after 1 day at -20 ° C and after 14 days at -20 ° C and thermal conductivity (initial, after 1 week at 70 ° C, 3 weeks at 70 ° C and 5 weeks at 70 ° C ). The results are indicated in Table 3 below.

TABLE 3 EXAMPLE 5 Rigid polyurethane foams of the construction type were prepared starting from the formulations indicated in table 5, along the lines described in example 1. From the foams obtained, density, isotropic cell size, friability (according to BS 4370 standard) and lambda value (initial and after 1 week, 2, 3, 4 and 5 weeks). The results are indicated in table 5 below. These results show that the foams of the present invention, which also incorporate an insoluble fluorinated compound, have finer cells, improved thermal insulation properties and improved friability compared to such foams that do not contain an insoluble fluorinated compound.

TABLE ? EXAMPLE 6 Rigid polyurethane foams of the construction type were prepared, starting from the formulations indicated in table 6 along the lines described in example 1. From the foams obtained density and friability were measured. The results are indicated in Table 6 below.

TABLE 6

Claims (24)

1. The process for the preparation of a polyurethane or urethane-modified polyisocyanurate foam by reacting a polyisocyanate composition with a polyfunctional isocyanate-reactive composition, under foaming conditions in the presence of a mixture physical blowing agents, each of said physical blowing agents have a boiling point in the range of -40 ° C to 80 ° C, characterized in that the mixture of the physical blowing agent comprises at least 3 components and the foam has a content of closed cells greater than 85%, a core density below 28 kg / m3 and a dimensional stability expressed as the sum of the absolute values of the relative linear changes in the three dimensions after exposure to -20 ° C per 14 days, below 15%.

2. The process according to claim 1, wherein the foam has a core density of less than 27 kg / m3.

3. The process according to claim 2, wherein the foam has a core density less than 25 kg / m34.

The process according to any of the preceding claims, wherein the foam has a dimensional stability expressed as the sum of the absolute values of the relative linear ties in the three dimensions, after exposure to -20 ° C for 14 days, less than 10%

5. The process according to claim 4, wherein the state has a dimensional stability expressed as the sum of the absolute values of the relative linear changes in the three dimensions, after the exposure at -20 ° C for 14 days, lower to 7.5%.

6. The process according to any of the preceding claims, wherein each of the physical blowing agents has a boiling point in the range of 15 ° C to 60 ° C.

7. The process according to any of the preceding claims, wherein none of the physical blowing agents contains a halogen atom.

8. The process according to any of the preceding claims, wherein at least one of the physical blowing agents has a thermal conductivity of gas at the operational temperature of the foam, lower than the gaseous thermal conductivity of the carbon dioxide to that temperature.

9. The process according to claim 8, wherein at least one of the physical blowing agents has a gaseous thermal conductivity at the operational temperature of the foam lower than the gaseous thermal conductivity of the n-pentane at that temperature.

10. The process according to claim 9, wherein at least one of the physical blowing agents has a gaseous thermal conductivity at the operational temperature of the foam lower than the gaseous thermal conductivity of the cyclopentane at that temperature.

11. The process according to any of the preceding claims, wherein the sum of the gas partial pressures of all the components of the blowing agent is at least 0.3 bar.

12. The process according to claim 11, wherein the sum of the partial gas pressures of all the components of the blowing agent is at least 0.45 bar.

13. The process according to claim 12, wherein the sum of the partial gas pressures of all the components of the blowing agent is at least 0.6 bar.

14. The process according to any of the preceding claims, wherein at most 40% of the physical blowing agent mixture is condensed at the operational temperature of the foam.

15. The process according to claim 14, wherein at most 20% of the physical blowing agent mixture is condensed at the operational temperature of the foam.

16. The process according to claim 15, wherein at most 10% of the mixture of the physical blowing agent is condensed at the operational temperature of the foam.

17. The process according to claim 16, wherein at most 2% of the physical blowing agent mixture is condensed at the operational temperature of the foam.

18. The process according to any of the preceding claims, wherein at least one of the components of the physical blowing agent mixture is an alkane or a cycloalkane.

19. The process according to any of the preceding claims, wherein at least one of the components of the physical blowing agent mixture is an acetone.

20. The process according to claim 18 or 19, wherein the mixture of the physical blowing agent contains one or more (cyclo) alkanes having 5 to 6 carbon atoms, optionally acetone and optionally hydrofluorocarbon (s).

21. The process according to any of the preceding claims, wherein the reaction is carried out in the presence of water.

22. The process according to any of the preceding claims, wherein the reaction is carried out in the presence of an insoluble fluorinated compound.

23. The process according to claim 22, wherein the non-soluble fluorinated compound is a substantially fluorinated or perfluorinated hydrocarbon.

24. The process according to claim 23, wherein the insoluble fluorinated compound is perfluoropentane or perfluorohexane or perfluoro (4-methylpent-2-ene). ABSTRACT The rigid polyurethane foam having a closed cell content greater than 85%, a core density of less than 32 kg / m 3 and a dimensional stability expressed as the sum of the values of the absolute relative linear changes; in the three dimensions after exposure to -20 ° C for 14 days, less than 15%.