CATALYST FOR PRODUCING POLYURETHANE FOAMS COATED WITH A SKIN

Abstract: Amine catalysts useful for producing polyurethane foams coated with a skin by reaction of an organic polyisocyanate with a polyol. The catalysts comprise 4-methyl-1-(2-­ hydroxyalkyl)-piperazine, and emit no bad odor, give excel­ lent moldability and are free from deterioration of skin and discoloration of covering materials.

Description: The present invention relates to a process for producing polyurethane composite foams coated with a skin. Due to the specific amine catalysts which are used in the process, the polyurethane composite foams coated with a skin emit no bad odor, give excellent moldability, and are free from deterioration and discoloration of the skin. Polyurethane foams are usually prepared by the reaction of solution A with solution B under high speed mixing agitation. Solution A contains a polyol, blowing agents, a surfactant, a catalyst and other promotor and solution B contains the polyisocyanate. Polyurethane foams coated with a skin have a wide range of applications. Among them the most common usage is for interior materials of automobiles such as for cushion, arm rest, head rest, instrument panel and dash board. These products are usually coated with an urethane elastomer, a vinyl chloride resin or a complex of ABS and vinyl cloride resins, and frequently they are prepared as a composite foam product. Composite foams have an outer skin and are obtained by inserting the skin layer into the mold and subsequently foaming therein. On the other hand, tertiary amine catalysts, employed in the production of polyurethane foams, are partly lost by evaporation and the rest remains in the product foams. Amine compounds usually have a peculiar amine odor which emits a bad smell and imparts bad influences on the working condition or causes environmental problems. Amine compounds, if left in the foam, may bleed toward the outside of the foam and react with the vinyl chloride resin in the skin to accelerate dehydrochlorination and to discolor the skin. In other instance, halogen compounds such as fire retardants in the foam composition, react with the remaining amine compounds to produce halides which often discolor the skin. So far, the discoloration of the skin has been avoided by increasing the thickness of the skin to lessen the bleed of the amine catalyst, but it brings about a large cost disadvantage. On the other hand, a variety of amine catalysts have been proposed. For example, they include a catalyst which is reactive with the starting isocyanate (Japanese Laid-Open Patent Application No. Sho 58-17115) and a catalyst having a high molecular weight to reduce the bleeding speed (Japanese Laid-Open Patent Application No. Sho 54-20099). However, there still remained a problem that amine catalysts which deteriorated and discolored the skin to a lesser degree did not always provide foams with better physical properties. Thus, most eagerly desired was an amine catalyst which eliminates these problems. 4-Methyl-1-(2-hydroxyalkyl)piperazines have been disclosed in DE-OS 33 10 124 as catalysts for the preparation of polyurethane foams. However, there has not been described that the 4-methyl-1-(2-hydroxyalkyl)piperazines are useful catalysts to prevent deterioration and discoloration of polyurethane composite foam products coated with a skin, and further there is not found an example in which the compounds have been utilized in this field. The technical problem underlying the present invention is to provide composite polyurethane foams coated with a skin of an urethane elastomer- or a vinyl chloride resin wherein the skin is not discolored or otherwise deteriorated. The present inventors have long devoted themselves to the investigation on the catalytic action of tertiary amines. In the production of polyurethane composite foams, a tertiary amine is necessarily used to make foams with open cell structure. To overcome the serious problem of discoloration of polyurethane products, they formed four types of amine catalysts which could be used; (1) an amine catalyst of a very low boiling point; (2) an amine catalyst having a reactive group; (3) an amine catalyst of a high molecular weight; and (4) an amine catalyst of a high boiling point. In (1), a very low boiling point amine catalyst is mostly evaporated in the course of production of polyurethane foams leaving no remainder of amine catalyst. However, as mentioned above, this kind of amine compound has intolerable bad odor, deteriorates the working condition, and in an extreme case is may bring about an environmental problem. Further, a low boiling compound such as this amine catalyst may evaporate while it is kept in a pre-mixed composition solution and the reaction may not proceed at the expected reaction rate. Therefore, (1) does not provide an effective means. The remedy (3) seems a useful one, because an amine catalyst having a high molecular weight hardly bleeds and also hardly evaporates. Emitting of bad odor of the amine is no longer feared. However, for an amine catalyst of a high molecular weight, the catalytic activity per unit weight is reduced and a larger amount is needed. In addition, the amine is less miscible and some amines are solid at room temperature. Thus, (3) does not always provide a useful means. The present inventors noted tertiary amine compounds which hold reactivity with isocyanate and the reactive functional group of which forms an amine catalyst of a higher boiling point due to formation of hydrogen bonds. In other words, they expected to solve the problem of deterioration and discoloration of the skin by the remedies (2) and (4), and their investigations were solely on the tertiary amine catalysts having these reactive groups. As a result, they found that, when a 4-methyl-1-(2-hydroxyalkyl)piperazine (hereinafter designated as MHAP) or a mixture of MHAP's and a specified amount of another tertiary amine compound was used as catalyst, the skin of the product polyurethane showed very little deterioration and discoloration and physical properties as well as moldability of the foams were excellent, and they succeeded in completion of this invention. The technical problem as mentioned above has been solved by the features of claims 1 to 4. The present invention provides a process for producing polyurethane composite foams coated with a skin which are excellent in their physical properties and moldability of foams and the skin of which is least deteriorated and discolored by the reaction of an organic polyisocyanate with a polyol in the presence of a blowing agent and other auxiliary agent, comprising using as catalyst MHAP or a mixture of MHAP with a specified amount of another tertiary amine compound. Since the catalyst used in the process of this invention contains a hydroxyl group which reacts with an isocyanate, the catalyst reacts with isocyanate and is fixed in the polyurethane foam in the process of forming the foam and therefore the catalyst does not bleed. Further, the catalyst used in the process of this invention having a high boiling point does not emit bad odor and hardly bleeds from foams. Thus, when MHAP is used as catalyst for polyurethane composite foam products coated with a skin, deterioration and discoloration of the skin can be reduced considerably. Furthermore, the catalyst used in the process of this invention makes a well-balanced progress between the blowing and the gelling reactions which compete in the urethane forming reactions. As a result, when the solution A containing the polyol and the other materials is mixed with the solution B containing the isocyanate, the resulting mixture is sufficiently fluid to flow and fill every corner of a mold, even if it is of a complex shape, showing good moldability. The polyurethane products obtained do not show any difference in their physical properties from those of previous products. Therefore, the catalyst used in the process of the present invention can be applied without making any change to the formulation. The MHAP which is used in the process of this invention may be prepared in a known process, for example by the reaction disclosed in Japanese Laid-Open Patent Application No. Sho 58-162625. The alkyl group of hydroxyalkyl of MHAP is selected from the group consisting of ethyl, n-propyl, n-butyl and iso-butyl, but preferably from ethyl and n-propyl groups. Other tertiary amine compounds compatible with MHAP which is used in the proces of this invention include known tertiary amines such as triethylamine, N,N-dimethylcyclohexylamine, N,N,N',N'-tetramethylethylenediamine, N,N,N',N'-tetramethylpropylenediamine, N,N,N',N'-tetramethylhexamethylenediamine, N,N,N',N'',N''-pentamethyldiethylenetriamine, N,N,N',N'',N''-pentamethyl-(3-aminopropyl)ethylenediamine, N,N,N',N'',N''-pentamethyldipropylenetriamine, N,N,N',N'-tetramethylguanidine, triethylenediamine, N,N'-dimethylpiperazine, N-methylmorpholine, N-ethylmorpholine, N-(N',N'-dimethylaminoethyl)morpholine, 1,2-dimethylimidazole, 3-(dimethylamino)propylimidazole, N,N-dimethylethanolamine, dimethylaminoethoxyethanol, N,N,N'-trimethylaminoethylethanolamine, 1,3-bis(N,N-dimethylamino)-2-propanol, N-methyl-N'-(2-dimethylaminoethyl)piperazine, bis(2-dimethylaminoethyl)-ether. However, triethylenediamine and/or bis(2-dimethylaminoethyl)ether are preferable. When a tertiary amine compound is used in a mixture with MHAP, the concentration of the tertiary amine is 0 to 70%, preferably 0 to 60% by weight. When triethylenediamine is used, its concentration should be 2 to 20%, preferably 4 to 15% by weight. When the tertiary amine compound is bis(2-dimethylaminoethyl)ether, 1 to 50%, preferably 2 to 40% by weight is preferred. Further, when the tertiary amine compounds are triethylenediamine and bis(2-dimethylaminoethyl)ether, concentrations preferred are 1 to 8% and 1 to 25%, respectively, by weight. If the amount of the tertiary amine compound exceeds the above-mentioned value, the amine remaining in the polyurethane product will bleed and bring about deterioration and discoloration of the skin. Further, the tertiary amine, though depending on its kind, emits a bad odor and causes serious problems for the personnel. The catalyst to be used in the process of this invention may be applied, if necessary, in the form of a solution in water or an organic solvent. Solvents to be used include water, polyols having 2 to 4 hydroxyl groups such as ethyleneglycol, diethyleneglycol, propyleneglycol, dipropyleneglycol, butanediols, 1,6-hexanediol, and glycerine. Among them, preferable are water, ethyleneglycol, dipropyleneglycol and 1,4-butanediol. Water is a favorable solvent because it acts to lower the freezing point of the catalyst. In case water is applied, the mixing rate should be not more than 20%, preferably not more than 15%. When water is present in an amount exceeding 20%, the melting point of the catalyst is not lowered and, on the contrary, a formulation which is almost free from water does not work properly. On the other hand, the amount of organic solvents to be applied is not particularly restricted, but usually in the range from 5 to 70% by weight. A larger amount of an organic solvent makes too low a concentration of amine in the catalyst or influences significantly the physical properties of the product. The catalyst, that is MHAP or a mixture of MHAP with a tertiary amine compound as promotor, is usually employed in the form of a mixture with a solvent which is selected depending on purpose. But, in the production of urethane foams, MHAP and tertiary amine may be added separately to the pre-mixed polyol solution. For example, MHAP and an organic solvent which is added on purpose is mixed under a nitrogen atmosphere, and a tertiary amine as promotor is then added to the mixture. The catalyst thus prepared is used in the production of the polyurethane composite foams. The amount of catalyst is 0.01 to 10 parts, preferably 0.05 to 5 parts, to 100 parts of polyol. Less than 0.01 part of the catalyst gives a low catalytic activity and requires a long time before completion of reaction. On the other hand, an amount of more than 10 parts gives unfavorable effects to the physical properties of polyurethane foams, especially the compression strength (ILD) is decreased. Polyisocyanates from which polyurethane foams can be produced with the catalyst which is used in the process of this invention may be the known polyisocyanates including, for example, toluene diisocyanate, diphenyl methane-4,4'-diisocyanate, polymerized isocyanates, aliphatic polyisocyanates and those prepolymers containing free isocyanate groups which are produced by the reaction of the mentioned compounds with polyols. Polyols to be used in this invention include the known polyesterpolyols, polyetherpolyols, polymeric polyols and mixtures thereof. Known polyesterpolyols usually include those compounds which are derived from dibasic acids and polyhydric alcohols. Known polyols concerned are polyether polyols and amine polyols. Polyether polyols are obtained from polyhydric alcohols such as glycol, glycerine, pentaerythritol, trimethylolpropane and saccharose by the addition reaction with ethylene oxide or propylene oxide. Amine polyols are produced by the addition reaction of an amine compound such as ammonia, an aliphatic and aromatic amine with ethylene oxide or propylene oxide. Known polymeric polyols include those which are produced from corresponding polyether polyols and ethylenic unsaturated monomers such as butadiene, acrylonitrile, and styrene in the presence of a radical polymerization catalyst. In this invention, a cross-linking agent or a chain-extending agent may be added if necessary. The cross linking agent and the chain-extending agent mentioned include polyhydric alcohols of a low molecular weight such as ethyleneglycol, 1,4-butanediol, glycerine and polyamine polyols of a low molecular weight such as diethanol amine, triethanol amine and polyamines such as ethylenediamine, xylylenediamine and methylene-bis-ortho-chloroaniline. If necessary in this invention, a blowing agent may be used which is selected from the group consisting of halogenated methanes such as freon 11, freon 12, methylene chloride and water. Further, surfactants such as organic silicon compounds, coloring agents, flame retardants and other known additives may be added if necessary. The kind and amount of the additives should be in accordance with ordinary usages without going far beyond known procedures. The method of producing polyurethanes in which the catalyst is applied is a previously known one. It is applicable to the one shot method and also to the prepolymer method. The process of the invention for producing polyurethane composite foams is particularly useful with molded polyurethane coated with skin, but it is also applied with success to producing foams by the hot mold, semi-hot-mold, HR mold and the RIM method. When the skin is made from a vinyl chloride resin, polyurethane foams with skin are produced by the so-called vacuum mold and slush mold processes, and this invention is applied to either of the processes. If the skin is made from a urethane elastomer, the integral skin urethane foam process is the best known one and the molding of the foam has recently been carried out almost solely by the RIM mold process, to which the present invention is also applied with success. Polyurethane foams coated with a skin have been used for a variety of industrial products. These products were mostly black colored and therefore discoloration of the foams has not been a serious problem. However, it is a recent trend to color industrial products and many of them are colored brightly. In this connection, no useful amine catalysts were known. Now, the problem is solved for the coloration of polyurethane composite foams coated with a skin. At the same time, the amine catalyst which has been used against the worker's will because of its bad odor could be replaced to improve working conditions. In addition, it is now possible to reduce the thickness of the skin which has been made necessarily thick to minimize the trouble of discoloration. This is also effective to reduce the manufacturing costs. Preparation of the catalyst Catalysts used in the following Examples, each named by a capital letter, were prepared from tertiary amine(s) and solvent(s), as specified in Table 1, in a 500 ml 4-necked flask provided with a stirrer in a nitrogen atmosphere. Examples 1, 2, 3 and 4: Using the catalysts O, P, Q and T specified in Table 1, foaming tests were carried out with the following formulation. Results are shown in Table 2. Remarks: Polyol I; Product from Sumitomo-Bayer Urethane Co. Trade name, Sumiphen 3063 OH value, 28 mg KOH/g Polyol II; Product from Sumitomo-Bayer Urethane Co. Trade name, S.B.U. Polyol 0480 OH value, 550 mg KOH/g Isocyanate I; Product from Sumitomo-Bayer Urethane Co. Trade name, Sumidure 44v20 NCO group content 31% with crude MDI (2) Foaming test conditions Temperature of the starting material solution, 20 ± 1°C Speed of agitation, 5000 rpm. (for 5 min.) Temperature of the mold, 35 ∼ 38°C (3) Items of measurement 1) Density of foam A predetermined amount of solution was poured in a 2 liter beaker, and the foaming reaction was completed. A 7 cm x 7 cm x 6 cm portion was cut at the center of the foam and submitted to the measurement. 2) Hardness of foam Using the above-mentioned foam sample, compressive strength of the foam was measured when the foam was compressed by 40%. 3) Flow properties A certain amount of a mixed solution was injected from the bottom of an aluminum mold of 50 cm length, 12 cm width and 0.5 cm thickness which was placed on a 25° slant, the foaming reaction was completed. The distance of flow of the produced foam was measured. 4) Discoloration test A test piece for the discoloration test was prepared by pouring the starting material for the polyurethane onto a vinyl chloride resin (GR-20 from Plastech Co.) which was then foamed. The piece obtained was heated in an oven at 80°C and the discoloration in 400 hr. estimated with a color meter was expressed by the color difference (ΔE). 5) Odor With the catalysts employed in the foaming tests, the odor of the catalysts was expressed by the following symbols: 1 : Very weak 2 : Weak 3 *: Strong 4 **: Very strong Comparison Examples 1 and 2: Foaming tests were conducted in the same manner as in Example 1, except that the catalysts used were TEDA-L33 (a 33.3% solution triethylenediamine in dipropyleneglycol, supplied from Toyo Soda Industry Co.,Ltd.) and TEA (triethylamine). The results are shown in Table 2. Examples 5 and 6: Using catalysts R and S appearing in Table 1, the foaming test was conducted with the following formulation. The results are shown in Table 3. Remarks: Polyol III; Product of San'yo Chemical Co. Trade name, FA-703 OH value, 33 mg KOH/g Polyol IV; Product of San'yo Chemical Co. Trade name, FA-728 OH value, 28 mg KOH/g Silicone I; Product of Toray Silicone Co. Trade name, SRX-274C Silicone surfactant Isocyanate II; Product of Nippon Polyurethane Co. Consisting of T-81/MR-200=80/20 (TDI and crude MDI blended) (2) Foaming test conditions Temperature of the starting material solution, 20 ± 1°C Stirring speed, 6000 rpm (5 sec.) Mold temperature, 50 ± 2°C (3) Items of measurement 1) Reaction profiles Foaming reaction was performed in a 2 liter polyethylene beaker and CT, GT and RT were estimated. 2) Discoloration test Foam prepared in a 2 liter polyethylene beaker by foaming was coated with a gray vinyl chloride resin, and the change of color was observed in 400 hours during which period the object was kept at 80°C. Comparison Examples 3 and 4: The same foaming tests as in Example 4, except that the catalysts used were TEDA-L33 and TMHMDA (N,N,N',N'-tetramethylhexamethylenediamine), were conducted. The results are shown in Table 3.

Claims: Process for producing polyurethane composite foams coated with a skin of an urethane elastomer- or a vinyl chloride resin in a mold which comprises inserting the skin layer into the mold and reacting in the mold an organic polyisocyanate with a polyol in the presence of a blowing agent and a 4-methyl-1-(2-hydroxyalkyl)-piperazine wherein the alkyl group is ethyl, n-propyl, n-butyl or iso-butyl as catalyst in an amount of 0.01 to 10 parts per 100 parts of the polyol. Process according to claim 1, wherein the catalyst is 4-methyl-1-(2-hydroxyethyl)-piperazine or 4-methyl-1-(2-hydroxy-n-propyl)-piperazine. Process according to claim 1 or 2, wherein another tertiary amine is added as a promoter in an amount of up to 70 % by weight of the mixture of catalyst and promoter. Process according to claim 3, wherein the promoter is triethylenediamine and/or bis-(2-dimethylaminoethyl)-ether and the amount is 1 to 8 % and 1 to 25 % by weight, respectively, of the mixture of catalyst and promoter.

Inventors: ARAI, SHOJI, HASEGAWA, MASAZUMI, MIZOE, SHINYA
Applicants: TOSOH CORPORATION
Application year: 1986
Main classification: C08G18
IPC codes: C08G18, C08J9, C08G101