Process for producing rigid polyurethane foam

Abstract: A process for producing a rigid polyurethane foam by use of a decreased amount of a CFC blowing agent and an increased amount of water used as a blowing agent, maintaining a high degree of heat insulation and improving the deterioration in friability of the product. The catalyst is one of the imidazoles represented by the general formula (I): where R₁ is alkyl having 1 to 4 carbons, benzyl, dimethylaminopropyl, or hydroxyalkyl having 1 to 3 carbons; R₂ is hydrogen, alkyl having 1 to 4 carbons, allyl, or benzyl; and R₃ and R₄ are independently hydrogen, methyl, or hydroxymethyl.

Description: The present invention relates to a process for producing rigid polyurethane foam by reacting a polyol with a polyisocyanate in the presence of a catalyst, a blowing agent, a foam stabilizer, and other optional additives. More particularly, the present invention relates to a process for producing a rigid polyurethane foam in which the amount of a halogenated hydrocarbon blowing-agent is decreased by use of a larger amount of water as another blowing agent. The present invention also relates to a process for producing a rigid polyurethane foam exhibiting excellent low-temperature adhesiveness by a spray foaming method. Rigid polyurethane foams are usually produced by agitating and mixing instantaneously a polyisocyanate and a polyol containing a blowing agent (a chlorofluoro carbon, i.e. CFC which is a halogenated hydrocarbon, and water), a catalyst, a silicone foam stabilizer, and other optional additives, and foaming the mixture. Rigid polyurethanes are widely used for heat insulators in application fields requiring hot insulation or cold insulation such as building materials, boards, electric refrigerators and manufacturing plants, because of the light weight and superior heat-insulating properties. Spray foaming thereof is advantageous in applicability to in-place foaming. A rigid polyurethane foam is an urethane resin which has a structure having independent cells, namely closed cells. where a CFC compound and water used as blowing agents are enclosed in individual cells as CFC gas having low heat conductivity and carbon dioxide gas having a relatively high heat conductivity resulting from the reaction of water with an isocyanate. Accordingly, for the purpose of improving heat insulation properties, generally employed formulations have been so far such that, as the blowing agent, water is used in a minimum amount and a CFC compound is used in a larger amount: for example, 0 to 1.5 parts by weight of water, and 35 to 60 parts by weight of a CFC compound based on 100 parts by weight of the polyol. Catalysts employed generally include triethylenediamine, tetramethylhexamethylenediamine and dimethylcyclohexylamine. (see e.g. "Advances in Polyurethane Technology", Ed. J.M.Buist and H. Gudgeon, John Wiley and Sons Inc., New York 1968, 196-201 and H.Fujino et al. in "Proceedings of the SPI - 31st Annual Technical/Marketing Conference", 1988, 344) With such a formulation, useful rigid polyurethane foams have been produced industrially. As mentioned above, the heat insulation properties of conventional rigid polyurethane foams have been improved by increasing the amount of the use of CFC compounds to lower the thermal conductivity. Recently, however, the risk is indicated that the CFC compounds may destroy the ozone layer of the earth. Consequently, the regulation of use of CFCs has started throughout the world to attain no use of CFCs by the end of the present century. On the other hand, use of a conventionally large amount of the HCFC (hydrochlorofluorocarbon) alternatives for CFCs, which exhibit a low ozone destruction coefficient is not desirable from the stand point of production cost (e.g. D.S.Gubrud in "Proceedings of the SPI - 31st Annual Technical/Marketing Conference", 1988, 159). Thus it is an urgent problem to establish a technique to meet the regulation of CFCs also in the application field of rigid polyurethane foams. For this purpose, formulations are being investigated in which the amount of the CFC is decreased, and instead, the amount of water is increased as the blowing agent (e.g. J.A.Murphy and K.G. McDaniel in "32nd Annual Polyurethane Technical/Marketing Conference", 1989, 321). This poses several technical problems as follows. The use of a larger amount of water as the blowing agent generates carbon dioxide gas to fill the cell in large amount in place of the CFC gas, which lowers the CFC concentration in the cells and adversely affects the heat conductivity. The increase of the amount of water in the formulation will also cause the increase of formation of urea linkage having higher crystallinity by reaction of the isocyanate with water, in addition to the normal urethane-forming reaction of a polyol with an isocyanate. Thus the foam becomes brittle, or becomes worse in friability. The deterioration in friability is especially remarkable in the case where diphenylmethane-4,4'-diisocyanate and/or a polymeric isocyanate thereof is used as the isocyanate, and in the case where toluene diisocyanate and/or mixture thereof with the prepolymer thereof is used. The deterioration in friability causes many problems such as decrease of strength of the foam, decrease of adhesiveness between a foam material and a surface material, which is a great technical problem. In the case of spray-foamed rigid polyurethane produced by use of a conventional catalyst, such as triethylenediamine, tetramethylhexamethylenediamine, and the like, a problem is involved that adhesiveness to a plate material, particularly an iron plate or a gypsum board, becomes lower significantly when foamed at a low temperature, especially in winter season. DE-A-26 27 719 discloses a process for producing polyurethane urea foams. One of the low-density foams described was prepared from 100 parts crude MDI, 30 parts of a triol and 20 parts water and in the presence of two parts 1,2-dimethylimidazol. The foam then probably has a density in the range of 10 to 60 kg/m³. Since the primary object is to decrease the amount of CFC to be used, deterioration in heat conductivity to some extent has to be admitted unavoidably. However, the deterioration in friability is a serious problem relating to the usefulness of the products. It is the object of the present invention to provide a process for producing a rigid polyurethane foam by use of a decreased amount of a CFC blowing agent and an increased amount of water used as a blowing agent, while maintaining a high degree of heat insulation and improving the deterioration in friability of the product. This object could be achieved on the basis of the surprising finding that the use of an imidazole compound as the catalyst is significantly advantageous in improvement of heat conductivity and friability which become problems when water as the blowing agent is increased. The present invention provides a process for producing a rigid polyurethane foam having a foam density of from 10 to 60 kg/m³ by reacting a polyol having an average hydroxyl value in the range of 200-600 with a polyisocyanate in the presence of a catalyst, a blowing agent, a foam stabilizer, and other optional additives; the blowing agent comprising water in an amount of 2.0 to 10.0 parts by weight and a halogenated hydrocarbon in an amount of not more than 35 parts by weight based on 100 parts by weight of the polyol; and the catalyst being at least one of the imidazoles represented by the general formula (I): where R₁ is alkyl having 1 to 4 carbons, benzyl, dimethylaminopropyl, or hydroxyalkyl having 1 to 3 carbons; R₂ is hydrogen, alkyl having 1 to 4 carbons, allyl, or benzyl; and R₃ and R₄ are independently hydrogen, methyl, or hydroxymethyl. The present invention also provides a process for producing a rigid polyurethane foam according to a spray foaming method by reacting a polyol having an average hydroxyl value in the range of 200-800, with a polyisocyanate as mentioned above. The halogenated hydrocarbons used as the blowing agent in the present invention include known halogenated methanes and halogenated ethanes, among which preferable are CFCs such as trichloromonofluoromethane (R-11) including HCFCs such as dichlorotrifluoroethane (R-123) and dichloromonofluoroethane (R-141b). The water is used in an amount of 2.0 to 10.0 parts by weight, preferably in the range of 3.0 to 10.0 parts by weight based on 100 parts of the polyol. In a spray foaming method, the amount is 0.5 to 10.0 parts by weight, preferably in the range of 2.0 to 10.0 parts by weight. Particularly, with the amount of water used of 3.0 parts by weight or more, saving of CFC is remarkable, resulting in significant improvement of the formulation. However, with the amount of water used of 10.0 parts by weight or more, friability and heat conductivity are considerably unsatisfactory, which is disadvantageous in physical properties of the products. The CFC is used in an amount of 35 parts by weight or less, preferably in the range of from 0 to 30 parts by weight, but in spray foaming, the amount is 60 parts by weight or less, preferably 50 parts by weight or less. The ratio of water to CFC to be used cannot be limited generally, and is dependent on the intended density. The catalyst used in the present invention is an imidazole represented by the general formula (I) shown above. Among the imidazoles, particularly preferable are 1-methylimidazole, 1,2-dimethylimidazole, 1-(3-dimethylaminopropyl)imidazole, 1-n-butyl-2-methylimidazole, and 1-isobutyl-2-methylimidazole. The amine catalyst in the present invention is used usually in an amount of from 0.02 to 10 parts by weight based on 100 parts by weight of the polyol. Additionally a known cocatalyst may be suitably used if the catalyst characteristics of the present invention is not lost. The examples of the cocatalysts are tertiary amine catalysts including triethylenediamine, tetramethylhexamethylenediamine, dimethylcyclohexylamine; organic carboxylate salts of the tertiary amine catalyst with e.g. formic acid, acetic acid or octylic acid; organic tin compounds and organic lead compounds. The polyisocyanate used in the present invention may be a known organic polyisocyanate, including aromatic polyisocyanates such as toluene diisocyanate, diphenylmethane-4,4'-diisocyanate and polymeric isocyanates thereof; aliphatic isocyanates such as hexamethylene diisocyanate; alicyclic polyisocyanate such as isophorone diisocyanate; isocyanate-terminated prepolymer such as toluene diisocyanate prepolymer and diphenylmethane-4,4'-diisocyanate prepolymer, prepared by reacting the above polyisocyanate with a polyol; modified isocyanates such as carbodiimide-modified isocyanates; and mixed polyisocyanates thereof. Among them, preferable are diphenylmethane-4,4'-diisocyanate and/or polymeric isocyanate thereof (crude MDI), and mixed polyisocyanate of crude MDI with toluene diisocyanate and/or prepolymer thereof (TDI prepolymer). In the mixed polyisocyanates containing crude MDI and TDI prepolymer, those containing TDI prepolymer in an amount of not more than 70 % by weight, particularly preferably in the range of from 5 to 60 % by weight are suitably used because of satisfactory heat conductivity. In the spray foaming, crude MDI is more preferably used. The polyols used in the present invention include known tertiary-amino-containing polyols, oxyalkylated polyhydric alcohols and polyesterpolyols. The tertiary-amino-containing polyols include polyols derived by adding an alkylene oxide such as ethylene oxide and propylene oxide to ammonia, hydrazine; an aliphatic polyamine such as ethylenediamine, diethylenetriamine, and triethylenetetramine; a heterocyclic polyamine such as N-aminoethylpiperazine; an aromatic polyamine such as toluenediamine, and diphenylmethane-4,4'-diamine; an alkanolamine such as mono-, di-, and tri-ethanolamine; and mixtures thereof. The oxyalkylated polyhydric alcohols include polyols derived by adding an alkylene oxide such as ethylene oxide and propylene oxide to a polyhydric alcohol such as glycol, glycerin, bisphenol, pentaerythritol, trimethylol propane and sucrose, or mixtures thereof. The polyesterpolyols includes those derived by condensing an aliphatic polycarboxylic acid such as adipic acid, maleic acid, and fumaric acid, or an aromatic polycarboxylic acid such as phthalic acid, terephthalic acid, and isophthalic acid, with a polyhydric alcohol: and those produced from polyethylene terephthalate waste. For the rigid polyurethane foams of the present invention, the polyol used has an average hydroxyl value in the range of 200 to 600, and for spray-foamed ones, in the range from 200 to 800. For spray methods, preferable polyols include tertiary-amino-containing polyols, oxyalkylated polyhydric alcohols, and mixtures thereof. Particularly preferable are mixed systems containing 10 % by weight or more of tertiary-amino-containing polyol. The tertiary-amino-containing polyols particularly preferably used are ethylene-oxide- and/or propylene-oxide-adducts of ethylenediamine, diethylenetriamine, triethylenetetramine, N-aminoethylpiperazine, toluene diamine, and diphenylmethane-4 ,4'-diamine. The oxyalkylated polyhydric alcohols particularly preferably used are ethylene-oxide- and/or propylene-oxide-adducts of sucrose and of polyhydric alcohol mixtures mainly composed of sucrose. The useful foam stabilizers include known silicone type foam stabilizers for rigid polyurethane foams such as siloxane-oxyalkylene block copolymers and the like, and other various surfactants, usually in an amount ranging from 0.5 to 2.5 parts by weight based on 100 parts by weight of the polyol although the amount is not specially limited. Further in the present invention, other auxiliary agents may be added if necessary. For example, a flame retardant may be added, which includes reactive flame retardants such as phosphorus-containing polyols like propoxylated phosphate and propoxylated dibutyl pyrophosphate, derived by addition reaction of an alkylene oxide to a phosphorus-containing acid; phosphate tri-esters such as tricresyl phosphate; halogen-containing phosphate tri-esters such as tris-(2-chloroethyl) phosphate and tris-(chloropropyl) phosphate; halogen-containing organic compounds such as dibromopropanol, dibromoneopentylglycol, and tetrabromo- bisphenol A; and inorganic compounds such as antimony oxide, magnesium carbonate, calcium carbonate and aluminum phosphate. The amount thereof to be used is not specially limited, usually being in the range of from 4 to 20 parts by weight based on 100 parts by weight of polyol depending on the required flame resistance. Other additives which may be used include coloring agents such as pigments, fillers such as perlite, stabilizers such as antioxidants and UV-absorbers. The rigid polyurethane foam produced from the above-mentioned polyol and polyisocyanate in the present invention has a foam density in the range of from 10 to 60 kg/m³. The present invention enables improvement in friability which is deteriorated when, as the blowing agent, the amount of CFC is decreased and the amount of water is increased in rigid polyurethane foam systems, while maintaining the heat insulation of the foams at a better level within allowable range. The novel formulation has make it feasible to produce a rigid polyurethane foam with a decreased amount of CFC as a blowing agent, which has hitherto been regarded as being difficult. In other words, the process for producing a rigid polyurethane foam of the present invention provides a novel formulation to meet the environmental pollution problem regarding CFC compounds because of the decreased amount of CFC as a blowing agent to be used. Further, in a spray-foamed rigid polyurethane foam system, low-temperature adhesiveness which often poses the problem can be improved. The effect is remarkable particularly in a system where amount of CFC is decreased and the amount of water is increased as the blowing agents. The product has high foam hardness from the time immediately after the reaction, and is superior in curing function and foam properties. Detailed Description of the Preferred Embodiments: The present invention is described in more detail referring to examples and comparative examples without limiting it in any way. Examples 1-13, and Comparative examples 1-7 The mixing ratio of the component materials (namely these in the formulations) were as shown below. With variation of the amount of water and trichlorofluoromethane (Freon R-11®), and the catalyst, the foaming tests were conducted under a predetermined conditions. The friability and the properties of the resulting foams were measured by the methods described blow. The results of the tests are shown in Table 1 and Table 2. a. Formulation: b. Foaming conditions: Starting material liquid temperature: 20 ± 1°C Stirring speed: 6000 rpm (5 seconds) Mold: Foamed in aluminum box (dimension: 25 x 25 x 25 cm) Mold temperature: 40°C c. Measured items: The items below were measured. Reactivity: Cream time: Time lapse before start of foaming (seconds) Gel time: Time for resinification (cobwebbing) (seconds) Tack-free time: Time lapse before the surface of the foam becomes tack-free (seconds) Rise time: Time lapse before the foam stops rising (seconds) Friability: The brittleness of the foam was evaluated in the four grades below by pressing the surface of the foam with a finger 10 minutes after completion of foam formation. Remarkably brittle: × Brittle: Δ Slightly brittle: ○ Not brittle: o ○ Foam density and Thermal conductivity: The density was measured of the test piece prepared by cutting the central portion of the foam into a size of 20 x 20 x 2.5 cm. The thermal conductivity was measured of the test piece as it was by use of ANACON model 88. As is evident from Table 1 and Table 2, rigid polyurethane foams being satisfactory in thermal conductivity and friability were obtained in Examples 1-13 where the amount of the CFC was decreased and the amount of the water was increased as the blowing agent, and NMIZ, DMIZ, and DMAPIZ were used as the catalyst respectively. On the contrary, in Comparative examples 1-7, the decrease of amount of CFC was not achievable because the increase of water deteriorated the friability. Examples 14-18, and Comparative examples 8 and 9 The mixing ratio of the component materials (namely these in the formulations) were as shown below. With the catalyst varied, the foaming tests were conducted under a predetermined conditions. The properties of the resulting foams including the low-temperature adhesiveness and hardness, were measured by the methods described below. The results of the tests are shown in Table 3. a. Formulation b. Foaming conditions: Starting material liquid temperature: 20 ± 1°C Strirring speed: 6000 rpm (5 seconds) Mold: Foamed in aluminum box (dimension: 25 x 25 x 25 cm) Mold temperature: 10°C c. Measured items: The items below were measured. Reactivity, and foam density: Foaming was conducted at 20°C in a 2-liter cup, and the reactivity, and the density of the center portion was measured. Cream time: Time lapse before start of foaming (seconds) Gel time: Time for resinification (cobwebbing) (seconds) Tack-free time: Time lapse before the surface of the foam becomes tack-free (seconds) Rise time: Time lapse before the foam stop rising (seconds) Low-temperature adhesion strength: Five iron pieces of 2.5 x 10 cm (1 mm thick) were placed on a mold, and cooled to 10°C. The liquid starting material mixture was poured therein, and allowed to foam. 10 minutes later, the resulting foam was taken out from the mold. The low-temperature adhesion strength was evaluated by peeling, in the upward direction, the iron piece which was fixed with one face to a spring balance. Foam hardness: Foaming was conducted in a mold cooled to 10°C. 10 minutes later, the hardness of the foam was measured with a Shore C hardness meter. As is evident from Table 3, rigid polyurethane foams excellent in low-temperature adhesiveness and satisfactory in foam hardness were prepared in Examples 14-18 where NMIZ, DMIZ, DMAPIZ, and IBIZ respectively used as the catalyst. On the contrary, in Comparative examples 8 and 9, the resulting foams were inferior in both low-temperature adhesiveness and foam hardness.

Claims: A process for producing a rigid polyurethane foam having a foam density of from 10 to 60 kg/m³ by reacting a polyol having an average hydroxyl value in the range of 200-600 with a polyisocyanate in the presence of a catalyst, a blowing agent, a foam stabilizer and other optional additives; said blowing agent comprising water in an amount of 2.0 - 10.0 parts by weight and a halogenated hydrocarbon in an amount of not more than 35 parts by weight based on 100 parts by weight of the polyol-; and said catalyst being at least one of the imidazoles represented by the general formula (I): where R₁ is alkyl having 1 to 4 carbons, benzyl, dimethylaminopropyl, or hydroxyalkyl having 1 to 3 carbons; R₂ is hydrogen, alkyl having 1 to 4 carbons, allyl, or benzyl; and R₃ and R₄ are independently hydrogen, methyl, or hydroxymethyl. A process for producing a spray-type rigid polyurethane foam by mixing and foaming a polyol having an average hydroxyl value in the range of 200-800 and a polyisocyanate in the presence of a catalyst, a blowing agent, a foam stabilizer and other optional additives according to a spray method; said blowing agent comprising water in an amount of 0.5 - 10.0 parts by weight and a halogenated hydrocarbon in an amount of not more than 60 parts by weight based on 100 parts by weight of the polyol: and said catalyst being at least one of the imidazoles represented by the general formula (I): where R₁ is alkyl having 1 to 4 carbons, benzyl, dimethylaminopropyl, or hydroxyalkyl having 1 to 3 carbons; R₂ is hydrogen, alkyl having 1 to 4 carbons, allyl, or benzyl; and R₃ and R₄ are independently hydrogen, methyl, or hydroxymethyl. The process of claim 1, wherein the blowing agent comprises 3.0 parts by weight or more of water and 30 parts by weight or less of the halogenated hydrocarbon based on 100 parts by weight of the polyol. The process of claim 2, wherein the blowing agent comprises 2.0 parts by weight or more of water and 50 parts by weight or less of the halogenated hydrocarbon based on 100 parts by weight of the polyol. The process of claim 1, wherein the polyisocyanate is diphenylmethane-4,4'-diisocyanate and/or a polymeric isocyanate thereof; or a mixed polyisocyanate comprising diphenylmethane-4-4'-diisocyanate and/or a polymeric isocyanate thereof and toluene diisocyanate and/or prepolymer thereof. The process of claim 5, wherein the concentration of toluene diisocyanate prepolymer in the mixed polyisocyanate is not more than 70 % by weight. The process of claim 2, wherein the polyisocyanate is diphenylmethane-4,4'-diisocyanate and/or a polymeric isocyanate thereof. The process of claim 1 or claim 2, wherein the imidazole compound is at least one selected from 1-methylimidazole, 1,2-dimethylimidazole, 1-(3-dimethylaminopropyl)imidazole, 1-n-butyl-2-methylimidazole, and 1-isobutyl-2-methylimidazole.

Inventors: ARAI SHOJI, ISHIDA MASAKI, OKUZONO SHUICHI, TAMANO YUTAKA, YOSHIMURA HIROYUKI, ARAI, SHOJI, ISHIDA, MASAKI, OKUZONO, SHUICHI, TAMANO, YUTAKA, YOSHIMURA, HIROYUKI
Applicants: TOSOH CORP, TOSOH CORPORATION
Application year: 1990
Main classification: C08G18
IPC codes: C08G18