MXPA99005857A - Open-celled rigid polyurethane foam - Google Patents

Abstract

Disclosed is a process for preparing an open-celled rigid polyurethane foam by reacting a polyisocyanate with a polyol in the presence of a blowing agent, usually water, and a cell opening agent. The cell opening agent comprises a combination of a polyoxyalkylene polysiloxane having a cloud point of 65°C or less, and a second substance that has a critical surface free energy of less than about 23mJ/m2. Highly preferred as second substance is fine particulate poly(tetrafluorethylene) polymer.

Description

This invention relates to an open-cell polyurethane foam and a process for its preparation by reacting a polyisocyanate with a polyol in the presence of a blowing agent and an agent. of opening cells. The cell opening agent is a mixture comprising a combination of a selected polyoxyalkylene polysiloxane and a second substance having a critical surface free energy of less than about 23 mJ / m2. Cellular polymers such as, for example, polyethylene, polystyrene or polyurethane are of value in many application areas including notably thermal insulation. In this case, it is particularly advantageous that said foam exhibits attractive dimensional stability and a relatively stable thermal insulation performance. Both characteristics are determined largely by the cellular structure of the foam and by the composition of the gas (s) within the cell cavity. A thinner cell structure generally confers better insulation properties to a foam. However, as the cells become thinner, ie smaller in diameter, the compressive strength and dimensional stability of the foam often become lower. The susceptibility to poor dimensional stability is greater when the cavity of the cells contains a gas that tends to condense or alternatively to diffuse out of the cell. Any phenomenon leads to a loss of partial gas pressure within the cell, resulting in poor dimensional stability of the foam. An example of a cellular gas that has been observed to diffuse out of a cell cavity, especially polyurethane, is carbon dioxide. The use of carbon dioxide to prepare cellular polymers, and especially polyurethane, is now favored largely as a substitute for many of the commonly used physical blowing agents considered harmful to the ozone layer. To support the favored use of carbon dioxide, there is a need to develop an improved process for the preparation of cellular polymers that exhibit attractive dimensional stability and thermal insulation properties. For rigid polyurethane foam, there is a need for a process that allows the use of carbon dioxide as a blowing agent and still provides a foam having acceptable dimensional stability and thermal insulation performance. A potential solution to the problem of dimensional stability is to provide a foam with an open cell structure, that is, a structure where one or more of the cell windows are fully opened and not sealed by a membrane, allowing the free passage of air and other gases. The expanded polymer having an open cell structure can be prepared by grinding the expanded polymer after its preparation to break the cell windows. The latter means are only available for elastomeric polymers since the crushing of rigid cellular polymers could lead to permanent deformation and damage. To provide cell opening in a rigid polymer, the use of a cell-opening agent is required during the manufacturing process. Typically, said cell opening agents are high boiling liquids such as, for example, high molecular weight poly (oxyalkylene) adducts. Such methods for preparing rigid open cell polyurethane are described in several patent publications including, for example, US Patents. 5,284,882; 5,350,777; 5,318,997; 5,248,704; 3,694,385; G.B., 1102,391; G.B. 1,065,590; EP-622,388-A; EP-610,734-A; EP-547,515-A; and EP-A-188,806. However, a disadvantage associated with the use of said cell opening agents is that they generally promote the formation of polymer containing a thick cell structure and consequently unattractive physical properties including thermal insulation. Thus, there is a need to provide an alternative process for the preparation of rigid open cell foam, which can allow the formation of a foam having a fine cell structure. It has now been discovered that a combination of a polyoxyalkylene polysiloxane surfactant selected with a substance other than a polyoxyalkylene polysiloxane, which has critical surface free energy of less than about 23 mJ / m 2, can effectively function as an aperture-opening agent. cells and provide the needs expressed before. In a first aspect, this invention relates to a process for preparing an open cell rigid polyurethane foam by reacting an organic polyisocyanate with a polyol in the presence of a blowing agent and a cell opening agent, wherein said cell opening agent is a composition comprising: a) a polyoxyalkylene polysiloxane substance having a cloud point of 65 ° C or less; and b) a substance having a critical surface free energy of less than about 23 mJ / m2 and which, when it is a solid, has an average particle size of about 20 microns or less and, when it is a liquid, has a boiling point greater than the maximum temperature found in the process to prepare the polymer. In a second aspect, this invention relates to a rigid polyurethane foam prepared according to the aforementioned process, wherein said foam has an open cell content of at least 70 percent. In a third aspect, this invention relates to a composition suitable for use as a cell opening agent when preparing rigid open cell polyurethane foam comprising: a) a polyoxyalkylene polysiloxane having a lower haze point temperature than about 65 ° C; and b) a substance having a critical surface free energy of less than about 23 mJ / m2 and which, when it is a solid, has an average particle size of about 20 microns or less and, when it is a liquid, has a boiling point greater than the maximum temperature found in the process to prepare the polymer, where (a) and (b) are present in a ratio of parts by weight of 8: 0.1 to 1: 8. In a fourth aspect, this invention relates to a polyisocyanate composition suitable for use when preparing open cell rigid polyurethane foam which, based on its total weight, comprises: i) from 99.9 to 90 weight percent of an organic polyisocyanate; and ii) from 0.1 to 10 weight percent of a cell opening agent having a polyoxyalkylene polysiloxane (a) having a cloud point temperature of less than about 65 ° C and is free of any functional group reactive with isocyanate and a substance (b) which has a critical surface free energy of less than about 23 mJ / m2 and which, when it is a solid, has an average particle size of about 20 microns or less and, when it is a liquid, has a higher boiling point that the maximum temperature found in the process to prepare the polymer, where (a) and (b) are present in a ratio of parts by weight from 8: 0.1 to 1: 8.

In a fifth aspect, this invention relates to a polyol composition suitable for use when preparing open cell rigid polyurethane foam, which, based on its total weight, comprises: a) from 99.9 to 90 weight percent of a polyol; and b) from 0.1 to 10 weight percent of a cell opening agent having a polyoxyalkylene polysiloxane (a) having a turbidity point temperature of less than about 65 ° C and a substance (b) having a critical surface free energy of less than about 23 mJ / m2 and which, when a solid, has a particle size average of about 20 microns or less and, when it is a liquid, it has a boiling point greater than the maximum temperature found in the process to prepare the polymer, where (a) and (b) are present in a ratio of parts in weight of 8: 0.1 to 1: 8. The rigid open cell polyurethane foam obtained according to the process described herein, can be characterized as having an open cell content of at least 70, preferably at least 80 and even more preferably at least 90. percent by weight of the total cell content. The foam advantageously has an overall free lift density of at least 25 kg / m3, or alternatively a molded density of at least 30 kg / m2.

Rigid open cell polyurethane is obtained by reacting an organic polyisocyanate with a polyol in the presence of a blowing agent and a selected cell opening agent. The cell opening agent is a composition comprising a first component, a polyoxyalkylene polysiloxane selected; and as a second component, a substance having a critical surface free energy of less than about 23 mJ / m2 and which, when it is a solid, has an average particle size of about 20 microns or less and, when it is a liquid, has a Boiling point greater than the maximum temperature found in the process to prepare the polymer. The polyoxyalkylene polysiloxane component has a cloud point of less than about 65 ° C, preferably less than about 60 ° C and more preferably less than about 50 ° C, when viewed as the 4 weight percent solution in water. In a preferred embodiment, the polyoxyalkylene polysiloxane component is free of any isocyanate-reactive functionality, notably the polyoxyalkylene polysiloxane is free of any hydroxyl functionality. The absence of such functionality provides isocyanate compatibility. Examples of commercially available polyoxyalkylene polysiloxane substances that can be used in the process of this invention include Silicone F-318 available from Shin-etsu Chemical Company Ltd., Surfactant 6164 available from OSI, and the aforementioned products available from Th. Goldschmídt. AG, which are understood to be substances compatible with the isocyanate. Product Turbidity Point Reported TEGOSTAB B 1048 37 ° C TEGOSTAB B 1903 50 ° C TEGOSTAB B 8407 56 ° C Polyoxyalkylene polysiloxane substances known to have a hydroxyl portion on the polyoxyalkylene chain include TEGOSTAB B 8408 (cloud point, 81 ° C) Th. Goldschmidt; D-193 (turbidity point 87 ° C) available from Dow Corning; and Siiicone F-305, F-308, F-335, and F-338 available from Shin-etsu Chemical Company Ltd, are not suitable for use in the practice of the present invention. Other polyoxyalkylene polysiloxane substances having a cloud point greater than 65 ° C include, for example, TEGOSTAB B 8427 (turbidity point 71 ° C). As mentioned, the second component of the cell opening agent is a substance other than the polyoxyalkylene polysiloxane which is essentially chemically inert under the conditions for preparing the cellular polymer and having a critical surface free energy of less than about 23, preferably less than about 20 and more preferably less than about 18.5 mJ / m2. An explanation of the critical surface free energy is provided on page 425 and subsequent pages of the 1983 publication, "CRC Handbook of Solubility Parameters and other Cohesion Parameters" by A. F. M. Barton published by CRC Press I nc.; I SBN 0-8493-3295-8. In the case of liquid substances, it should be appreciated that the critical free surface energy depends on the temperature, since the temperature generally increases as the critical surface free energy decreases. Accordingly, liquids suitable for use as the cell-opening agent are those which upon exposure to an elevated process temperature have a critical surface free energy of less than about 23 mJ / m 2 at the elevated temperature and preferably those having a surface free energy. critical less than approximately 23 mJ / m2 at room temperature. When it is a solid, the substance has a particle size commensurate with the thickness of the membranes that occupy the window regions of the cell. Normally, the average particle size is less than about 20, preferably less than about 15, more preferably less than 10, and even more preferably less than about 3 microns. With an average particle size of about 10 microns, the distribution advantageously is such that at least 90 percent of the particles are 10 microns or less and with an average particle size of about 3 microns or less, then less about 90 percent of the particles are less than about 6 microns and at least 10 percent of the particles are less than 1 micron. Such sizes and particle distribution can be determined by conventional shearless laser techniques, using equipment such as the Malvern Laser Diffraction Analyzer. It is further advantageous if the particle has a specific surface area of at least 3, preferably at least 4.5 and more preferably at least 6.5 m2 / g determined by the absorption of krypton. Examples of suitable solid particulate agents include fluorinated polymers comprising poly (trifluoroethylene) with a critical surface energy of 22 mJ / m2; poly (hexafluoropropylene), 16.2 mJ / m2; poly (1,1-dihydro-perfluoro-octyl) methacrylate, 10.6 mJ / m2; and especially poly (tetrafluoroethylene), 18.5 mJ / m2. The particulate PTFE suitable for use in this invention is commercially available and includes products designed by the FLUOROGLI DE brand available from ICI such as FL1710 and FL1200 and products available from Dupont under the trademark TEFLON including TEFLON MP 1 100, TEFLON MP 1200 , MP 1200 and MP 1500. When it is a liquid, the substance advantageously has a boiling point at atmospheric pressure which is higher than the maximum process temperature encountered during the preparation of the cellular polymer. If the liquid agent has a boiling point significantly lower than the maximum temperature of the process, it will function predominantly as a blowing agent. By "significantly less" is meant a boiling point that is at least 10 ° C below the maximum process temperature. The normal process temperatures for an extrusion process of a thermoplastic polymer are at least 100 ° C. Alternatively, said process can be a reactive molding process such as that used in the preparation of the thermosetting polymer, notably polyurethane, where a temperature in excess of 100 ° C can often be found. Accordingly, suitable liquid agents include those substances which advantageously have an atmospheric boiling point of at least 100 ° C, preferably of at least 130 ° C and still more preferably of at least 150 ° C and still more preferably of at least 175 ° C and wherein said substances are insoluble or only poorly soluble in the polymer or precursors thereof. Suitable liquid agents include polyfluoro- and especially perfluorocarbon organic compounds which advantageously have an average molecular weight of at least 350 and preferably at least 400. Illustrative of suitable liquid agents include the organic fluorinated compounds marketed by 3M under the trademark commercial FLUOR INERT including substances identified as FC-104, FC-75, FC-40, FC-43, FC-70, FC-5312 and FC-71, and substances marketed by Rhone-Poulenc under the trademark FLUTEC, including substances identified as PP3, PP6, PP7, PP10 , PP11, PP24 and PP25. Said liquid agents have a critical free surface energy of 9 to 16 mJ / m2 at room temperature. In a highly preferred embodiment of this invention, it is preferred to use a first component of the cell opening agent composition, a polyoxyalkylene polysiloxane component that has no isocyanate-reactive functionality and also has a turbidity point of less than 65 °. C; and as a second component, a solid particulate substance as described above. In the process of this invention, the polyoxyalkylene polysiloxane is present in an amount of 1 to 8, preferably 2 to 6 and more preferably 2 to 5 parts per 100 parts by weight of polyol, or alternatively by weight of polyisocyanate, if it is introduced by means of the polyisocyanate component. In the process of this invention, the substance having a critical surface free energy of less than about 23 mJ / m2 is present in an amount of 0.1 to 8, preferably 0.5 to 5 and more preferably 2.5 to 5 parts per 100 parts in weight of polyol, or polyisocyanate. In a preferred embodiment, the cell opening agent contains the polyoxyalkylene polysiloxane in an amount of 2 to 6 parts per 100 parts by weight; and the substance having a critical surface free energy of less than about 23 mJ / m2 in an amount of 2.5 to 5 parts per 100 parts by weight of polyol, or polyisocyanate. In the present invention, it should be appreciated that the cell opening agent can be provided for the process as a suitable composition comprising as main components the polyoxyalkylene polysiloxane (I) in combination with the substance (II) having a critical surface free energy less than approximately 23 mJ / m2. Advantageously, the composition comprises (I) and (II) in a ratio of parts by weight of 8: 0.1 to 1: 8, preferably of 6: 0.5 to 5: 2. In addition to its main components, said composition may also comprise water. Alternatively, the cell opening agent can be provided to the process by pre-mixing the major components with polyisocyanate to give a polyisocyanate composition, or pre-mixing with a polyol to give a polyol composition. The polyisocyanate or polyols composition advantageously comprises the cell opening agent in an amount of 0.1 to 10, preferably 2 to 8 percent, based on the total weight of the composition, with the proportion of polysiloxane modified with polyether and the substance having a critical surface free energy of less than about 23 mJ / m2 having the weight ratio as described hereinabove. The polyisocyanates or polyols are as described below. Suitable polyisocyanates include aromatic, aliphatic and cycloaliphatic polyisocyanates or combinations thereof. A crude polyisocyanate can also be used in the practice of this invention, such as the crude toluene diisocyanate obtained by the phosgenation of a mixture of toluene diamines or the crude diphenylmethane diisocyanate obtained by the phosgenation of methylene diphenylamine. raw Preferred polyisocyanates are aromatic polyisocyanates such as those described in the U.A. 3,215,652. Especially preferred are polyphenyl polyisocyanates connected with methylene and mixtures thereof with crude diphenyl methane di-isocyanate, due to their ability to crosslink the polyurethane. Suitable polyols include those which are conventionally used in the preparation of rigid polyurethane foam and which normally have an average hydroxyl equivalent weight of 50 to 700, preferably 70 to 500, more preferably 70 to 300. Additionally, said polyols generally they will contain from 2 to 8, preferably from 3 to 8, and more preferably from 3 to 6 hydroxyl groups per molecule. Examples of suitable polyols are polyether polyols as described more fully in the U.S. Patent. 4,394,491. Illustrative polyether polyols include those commercially available under the trademark VORANOL and include VORANOL 202, VORANOL 360, VORANOL 370, VORANOL 446, VORANOL 490, VORANOL 575, VORANOL 640, VORANOL 800 and sold by The Dow Chemical Company. Other preferred polyols include alkylene oxide derivatives of Mannich condensate as taught, for example, in the Patents of E. U.A. 3,297,597; 4, 137, 265 and 4383, 102, and polyether polyols initiated with aminoalkylpiperazine as described in the Patents of E.U.A. 4,704,410 and 4,704,411. When a cellular polymer according to this invention is prepared, a blowing agent is present. The blowing agent is present in an amount to provide the cellular polymer with the desired overall density. The blowing agent used comprises carbon dioxide, advantageously generating in situ by the reaction of water with polyisocyanate, optionally in combination with a physical blowing agent. The carbon dioxide can also be chemically obtained by other means including the amine / carbon dioxide complexes such as those described in the Patents of E.U.A. 4,735,970 and 4,500,656 to be used as a blowing agent. Suitable physical blowing agents include, for example, volatile (cyclic) alkanes such as (cyclo) pentane, (cyclo) hexane or halogen-containing substances such as (per) fluorocarbons and hydrogen-containing chlorofluorocarbon compounds examples of which they include dichlorofluoromethane, chlorodifluoromethane, dichlorotrifluoroethane, chlorotetrafluoroethane, trifluoroethane, tetrafluoroethane, dichlorofluoroethane, chlorodifluoroethane, fluoroethane, perfluoropentane and perfluorohexane. The selection of a physical blowing agent is not a critical aspect of this invention, since the resulting foam product essentially has open cells, will not be found in the foam and will not influence physical properties such as, for example, insulation performance. . When it is desired to improve processing properties such as, for example, flow, then advantageously a physical blowing agent may be present. Suitable physical blowing agents that have been found effective for this purpose include lower alkanes such as, for example, pentane, in a highly preferred embodiment of this invention, the blowing agent consists essentially of water. Typically for this purpose, the amount of water present is 0.5 to 15, preferably 2.0, more preferably 3.0 and preferably up to 10, more preferably up to 8 parts per 100 parts by weight polyol. When a physical blowing agent is present, usually the amount is from 0.5 to 10, preferably from 1 to 5 parts per 100 parts by weight of polyol. Optionally, other ingredients may be present when the polyurethane foam is prepared. Among these other ingredients are the catalysts, surfactants, dyes, antioxidants, reinforcing agents, fillers, antistatic agents and flame retardants. Suitable flame retardants include phosphorus-containing substances such as tris (chloroalkyl) phosphate and tris-alkyl phosphates, for example, triethyl phosphate; and nitrogen-containing substances such as melamine carbonate or guanidine. One or more catalysts for the reaction of the active hydrogen-containing compound with the polyisocyanate are advantageously present. Suitable catalysts include tertiary amine compounds and organometallic compounds. Exemplary tertiary amine catalysts include triethylenediamine, N-methylmorpho- lin, pentamethyldiethylenetrineamine, tetramethylethylenediamine, 1-methyl-4-dimethylaminoethylpiperazine, 3-methoxy-N-dimethylpropylamine, N-ethylmorpholine, diethylethanolamine, N-coco morphine, N, N -dimeti I- N ', N' -dimeti I iso propi I propi lendiamina, N, N-diethyl-3-d-ethylaminopropylamine and dimethylbenzylamine. Examples of organometallic catalysts include organomercury, organolead, or organoferric and organotin catalysts, with organotin catalysts being preferred among these. Suitable tin catalysts include stannous chloride, tin salts of carboxylic acids such as di-2-ethyl-dibutyltin hexanoate, as well as other organometallic compounds such as those described in the US Pat. 2, 846, 408. Herein, a catalyst for the trimerization of polyisocyanates and poly-isocyanurate polymer formation, such as an alkali metal alkoxide, alkali metal carboxylate or quaternary amine compound can also be optionally employed. When employed, the amount of catalyst used is sufficient to increase the rate of polymerization reaction. The precise amounts should be determined experimentally, but generally vary from 0.01 to 3.0 parts by weight per 1000 parts of polyol depending on the type and activity of the catalyst. The amount of polyisocyanate present when the polyurethane foam is prepared is such that it provides 0.6 to 3.0 isocyanate groups per reactive isocyanate atom present in the polyole (s) and any amount of water that may be present. Preferably the amount of isocyanate is such that it provides 0.7, more preferably 0.8 and preferably up to 2, more preferably up to 1.6 and even more preferably up to 1.05 isocyanate groups per reactive isocyanate atom. The polyurethane foam of this invention is of value to the construction industry and the appliance industry where its open cell characteristic gives utility in the preparation of vacuum panels. The invention is illustrated by way of the examples given hereinafter, which should not be considered as limiting the scope of the invention. Unless indicated otherwise, all amounts given are parts by weight. The raw materials used in the examples are presented in more detail below. DMCHA dimethylcyclohexylamina PM D ETA pentamethyldiethylenetriam ina VO RANATE 229 a crude polymethylene polyphenylene isocyanate with an isocyanate content of about 31, available from The Dow Chemical Company. Polyol A: A formulated polyol containing the following components (amounts in parts by weight): 30 VORANOL RN 41 1, an oxypropylene adduct of sucrose / glycerin with hydroxyl number 41 1, available from The Dow Chemical Company; 42.6 Polyol 585, an experimental oxypropylene-oxyethylene adduct of a phenol / formaldehyde adduct of hydroxyl number 1 96 and average functionality of 3.3; 7.5 Poly (oxyethylene) glycol, molecular weight 200; 7.5 Poly (oxyethylene) glycol, molecular weight 400; 7.5 VORANOL 1421, an adduct of oxypropylene-oxyethylene diethylenediamine with hydroxyl number 35, available from The Dow Chemical Company; 18. 8 VORANOL RA, 640 an oxypropylene-oxyethylene diethylene diamine adduct with hydroxyl number 640, available from The Dow Chemical Company; Polyol B: A formulated polyol containing the following components (amounts in parts by weight): 46.7 VORANOL RN 411 an oxypropylene adduct of sucrose / glycerin with hydroxyl number 411, available from The Dow Chemical Company; 66.5 Polyol 585, an experimental oxypropylene-oxyethylene adduct of a phenol / formaldehyde resin with hydroxyl number 196 and average functionality of 3.3; 11.7 Poly (oxyethylene) glycol, molecular weight 200; 11.7 Poly (oxyethylene) glycol, molecular weight 400; 11.7 VORANOL 1421, an adduct of oxypropylene-oxyethylene glycerin with hydroxyl number 35, available from The Dow Chemical Company; 29.3 VORANOL RA640, an oxypropylene adduct of ethylenediamine with hydroxyl number 640, for The Dow Chemical Company. Polyol C: A formulated polyol containing the following components (amounts in parts by weight): 17.2 VORANOL RN 411, an oxypropylene adduct of sucrose / glycerin with hydroxyl number 411, available from The Dow Chemical Company; . 1 Polyol 585, an experimental oxypropylene-oxyethylene adduct of a phenol / formaldehyde resin with a hydroxyl number 196 and average functionality of 3.3; 4.3 Poly (oxyethylene) glycol, molecular weight 200; 4.3 Poly (oxyethylene) glycol, molecular weight 400; 34.5 VORANOL 1055, an oxypropylene glycerin adduct with hydroxyl number 168, available from The Dow Chemical Company; 4.3 VORANOL 1421, an oxypropylene-oxyethylene adduct of glycerin with hydroxyl number 35, available from The Dow Chemical Company; 10.7 VORANOL RA 640, an oxypropylene adduct of ethylenediamine with hydroxyl number 640, available from The Dow Chemical Company; 21.5 VORANOL RN 482, an oxypropylene adduct of sorbitol with hydroxyl number 470, available from The Dow Chemical Company. Polyol D: A formulated polyol containing the following components (amounts in parts by weight): 9.5 VORANOL RN 411, an oxypropylene adduct of sucrose / glycerin with hydroxyl number 411, available from The Dow Chemical Company; 19.3 Polyol 585, an experimental oxypropylene-oxyethylene adduct of a phenol / formaldehyde resin with a hydroxyl number 196 and average functionality of 3.3; 2.4 Poly (oxyethylene) glycol, molecular weight 200; 2. 4 Poly (oxyethylene) glycol, molecular weight 400; 1 9 VORANOL 1055, an oxypropylene glycerin adduct with hydroxyl number 168, available from The Dow Chemical Company; 2.4 VORANOL 1421, an adduct of oxypropylene-oxyethylene glycerin with hydroxyl number 35, available from The Dow Chemical Company; 5.9 VORANOL RA 640, an oxypropylene adduct of ethylenediamine with hydroxyl number 640, available from The Dow Chemical Company; 1 1 .8 VORANOL RN 482, an oxypropylene adduct of sorbitol with hydroxyl number 470, available from The Dow Chemical Company. Tenosurfactant I TEGOSTAB B 1048, from Th. Goldschmidt, which has a stiffness point of 37 ° C and is understood to be free of isocyanate-reactive functional groups. Tensoactive Agent I I TEGOSTAB B8408, from Th. Goldschmidt, which has a turbidity point of 81 ° C and is understood to contain hydroxyl functionality and is therefore comparative. TEFLON MP 1 1 00: Poly (tetrafluoroethylene) in particles available from Dupont. Example 1 Open cell rigid polyurethane foam was prepared according to the formulation given in Table 1. Where physical properties were reported, they are observed according to the following test procedures: compression hardness, D I N 53421; and open / closed cell content according to ASTM D 2856. Table I The foams 1 and 2 exhibited a significantly higher open cell content than the comparative foams A to E and emphasize the importance of selecting the correct polyoxyalkylene polysiloxane. With reference to Foam E, it was clearly demonstrated that it was only the combination of selected polysiloxane with the low critical free surface energy substance that is provided for the effective formation of an open cell foam. In our studies, the particulate poly (tetrafluoroethylene) polymer has been completely replaced by another fine particulate material such as aluminum oxide, AEROSI L R202, available from Degussa AG, when polyurethane foam is prepared. In this case, open cell contents of between 20 and 30 percent were observed. Combinations of particulate aluminum oxide and particulate poly (tetrafluoroethylene) were found effective only in the production of rigid open cell polyurethane foam when combined with the required type of polyoxyalkylene polysiloxane. In further studies, surfactant I I has been replaced by surfactant lll (comparative), TEGOSTAB B8427, available from Th. Goldshmidt, having a cloud point of 71 ° C and which is understood to have hydroxyl functionality. The rigid polyurethane foam prepared in the presence of 1.7 parts of Surface Active Agent 11 and 4.4 parts of TEFLON MP1 100 was found to have an open cell content of 64 percent. Comparison of foam data associated with comparative surfactants, suggested that the characteristic aspect of the polyoxyalkylene polysiloxane cloud point was important. Example 2 This example demonstrated the variance in amount of polyoxyalkylene polysiloxane and low critical surface ey substance that can be employed when preparing open cell polyurethane foam. The formulation details and physical properties of the resulting foam are reported in Table II. Table I I

Claims (9)

1. REVIVAL DICATIONS 1. A process for preparing an open cell rigid polyurethane foam by reacting an organic polyisocyanate with a polyol in the presence of a blowing agent and a cell opening agent, whereby said cell opening agent is a composition that comprises: a) a polyoxyalkylene polysiloxane substance having a cloud point of 65 ° C or less; and b) a substance having a critical surface free energy of less than 23 mJ / m2 and which, when it is a solid, has an average particle size of 20 microns or less and, when it is a liquid, it has a boiling point greater than the maximum temperature found in the process to prepare the polymer. The process of claim 1, wherein the substance is solid and comprises poly (trifluoroethylene), poly (terafluoroethylene), poly (hexafluoropropylene) or poly (1,1-dihydro-perfluoro-octyl) methacrylate. 3. The process of claim 2, wherein the solid substance is poly (tetrafluoroethylene). 4. The process of claim 3, wherein the poly (tetrafluoroethylene) has an average particle size of 10 microns or less. The process of claim 1, wherein the substance is a liquid having a boiling point at atmospheric pressure of at least 130 ° C and is an organic polyfluoro- or perfluorocarbon compound with a molecular weight greater than 350, which is insoluble or sparingly soluble in the polyisocyanate or polyol. The process of claim 1, wherein the polyoxyalkylene portion of the polyoxyalkylene polysiloxane is free of any functional group reactive with isocyanate. 7. The process of claim 1, wherein the polyoxyalkylene polysiloxane is present in an amount of 1 to 8 parts per 100 parts of polyol. 8. The process of claim 1, wherein the substance having a critical surface free energy of less than about 23 mJ / m2 is present in an amount of 0. 1 to 8 parts per 1 00 parts of polyol. 9. The process of claim 8, wherein the substance having a critical surface free energy less than about 23 mJ / m2 is present in an amount of 0.5 to 5 parts per 1000 parts of polyol. The process of claim 1, wherein the cell opening agent contains: a) the polyoxyalkylene polysiloxane in an amount of 1 to 8 parts per 1 00 parts of polyol, with said polysiloxane having a point temperature of turbidity less than about 65 ° C and being free of any functional group reactive with isocyanate; and b) the substance having a critical surface free energy of less than about 23 mJ / m2 in an amount of 0. 5 to 8 parts of polyol, with said substance being a polymer of poly (trifluoroethylene), poly (tetrafluoroethylene), poly (hexafluoropropylene) or poly (1,1-dihydro-perfluoro-octyl methacrylate). eleven . The process of claim 1, wherein the organic polyisocyanate comprises polymethylene polyphenylene polyisocyanate. The process of claim 1, wherein the polyol has a molecular weight of at least 60 and contains two or more hydrogen atoms / molecules of active isocyanate. The process of claim 12, wherein the polyol is a polyether or polyester polymer. The process of claim 1, wherein the blowing agent consists substantially of water. The process of claim 14, which comprises reacting a polyisocyanate, a polyphenylene polymethylene isocyanate, with a polyether polyol or polyether containing two or more hydrogen atoms / molecules of active isocyanate, in the presence of a cell opening agent which contains: a) the polyoxyalkylene polysiloxane in an amount of 1 to 8 parts per 1 00 parts of polyol, with said polysiloxane having a turbidity point temperature less than about 65 ° C and being free of any functional group reactive with isocyanate; and b) the substance having a critical surface free energy of less than about 23 mJ / m2 in an amount of 0.5 to 8 parts of polyol, with said substance being a polymer of poly (trifluoroethylene), poly (tetrafluoroethylene), poly (hexafluoropropylene) or poly (1,1-dihydro-perfluoro-octyl methacrylate). 16. A rigid polyurethane foam prepared according to the process of claim 1, wherein said foam has an open cell content of at least 70 percent. 17. A composition suitable for use as a cell-opening agent when preparing rigid open-cell polyurethane foam comprises: a) a polyoxyalkylene polysiloxane having a turbidity point temperature less than about 65 ° C; and b) a substance having a critical surface free energy of less than about 23 mJ / m2 and which, when it is a solid, has an average particle size of about 20 microns or less and, when it is a liquid, has a boiling point greater than the maximum temperature found in the process to prepare the polymer, where (a) and (b) are present in a ratio of parts by weight of 8: 0.1 to 1: 8. 18. A polyisocyanate composition suitable for use when preparing open cell rigid polyurethane foam which, based on its total weight, comprises: i) from 99.9 to 90 weight percent of an organic polyisocyanate; and ii) from 0. 1 to 10 weight percent of a cell opening agent having a polyoxyalkylene polysiloxane (a) which has a temperature point of your strenght of less than about 65 ° C and is free of any isocyanate-reactive functional group and a substance (b) that it has a critical surface free energy of less than about 23 mJ / m2 and that, when it is a solid, it has an average particle size of about 20 microns or less and, when it is a liquid, it has an ebu It is higher than the maximum temperature found in the process to prepare the polymer, where (a) and (b) are present in a weight ratio of 8: 0. 1 to 1: 8. 9. A polyol composition suitable for use when preparing open cell rigid polyurethane foam, based on its total weight, comprises: a) from 99.9 to 90 weight percent of a polyol; and b) from 0.1 to 10 percent by weight of a cell-opening agent having a polyoxyalkylene polysiloxane (a) having a haze point temperature of less than about 65 ° C and a substance (b) that has a critical surface free energy of less than about 23 mJ / m2 and that, when it is a solid, it has an average particle size of about 20 microns or less and, when it is a liquid, it has a boiling point greater than maximum temperature found in the process to prepare the polymer, where (a) and (b) are present in a ratio of parts by weight from 8: 0.1 to 1: 8. RESU MEN A process for preparing an open cell rigid polyurethane foam by reacting a polyisocyanate with a polyol in the presence of a blowing agent, usually water and a cell-opening agent, is described. The cell opening agent comprises a combination of a polyoxyalkylene polysiloxane having a cloud point of 65 ° C or less and a second substance having a critical surface free energy of less than about 23 mJ / m2. The poly (tetrafluoroethylene) polymer in fine particles is highly preferred as a second substance.