PROCESS FOR PRODUCING ELASTOMERIC POLYURETHANE FOAMS OF REDUCED WEIGHT

Abstract: Process for producing a high density elastic polyurethane of reduced weight having excellent resistance of bending, which employs as catalyst 4-methyl-1-(2-dimethyl­ aminoethyl)piperazine or a mixture of 4-methyl-1-(2-dimethyl­ aminoethyl)piperazine and another tertiary amine compound as co-catalyst.

Description: The present invention relates to the production of elastomeric polyurethane foams having a density of at most about 0.6 g/cm³. Particularly, the invention relates to a process for producing an elastomeric foam for use as shoe soles which is excellent in resistance to bending and of reduced weight. In recent years, polyurethane elastomers have found their increasing applications in various fields of industry such as for shoe sole materials, synthetic and artificial leather and rim products for automobiles. The demand for the material is increasing, since polyurethane elastomer is excellent in the mechanical strength and the resistance to abrasion as well as to oil. Among them, especially the high density elastic polyurethane finds use for soles because of its superiority in the resistance to bending. The elastomers used for preparing shoe soles are generally produced by the reaction of two solutions: one solution A is a homogeneous mixture of polyester polyol with a cross linking agent, a catalyst, a blowing agent and various additives; and the other solution B contains an organic polyisocyanate. As for the catalyst, various kinds of organic metal compounds and tertiary amine catalysts are proposed such as those disclosed in "Journal of Japan Rubber Society", Vol.55, No.3, p.79 (1982). But in view of the desired stability of the solution A during storage and the desired high resistance of the produced shoe soles to bending, triethylenediamine is preferred for industrial purposes. A suitable density of elastomers for shoe soles depends on whether the shoes are for men, women or for sportswear. For example, a density of 0.6 g/cm³ is suitable for men's shoes. However, less expensive production of shoes is aimed at by cutting the unit amount of materials per se and the general tendency is that lighter shoes are preferred and in order to meet with this, shoe soles of low density are also required. In order to reduce the density of shoe soles for men's shoes, for example down to about 0.6 g/cm³, some technical attempts are being made such as to increase the amount of the blowing agent or to admix certain trifunctional polyols in the formulation of raw materials. These trials, however, have been found to often result in lowering the strength of elastomer for shoe soles and, in some extreme cases, the shoe soles may absorb or leak water and the elongation property may be deteriorated. In many cases in the prior art, various properties of the elastomer for soles were sacrificed and could hardly give a low density product which could exhibit high physical properties. The object of the present invention is to provide a process for producing with ease an elastomeric polyurethane foam of a density of at most about 0.6 g/cm³ and higher quality than those of any prior art product. It has been found that the selection of the catalyst remarkably influences the reduction in weight of the elastomer for shoe soles and the present invention is essentially based on this discovery. Complex reactions proceed almost in parallel in the formation of a polyurethane foam, but basically they are competitive reactions involving a urethane forming reaction in which isocyanate reacts with a polyol to form a urethane bond and a urea forming reaction in which isocyanate reacts with water to form a urea bond and to generate carbon dioxide. Consequently, the competitive reactions can proceed in good balance to form the elastomer for shoe soles, if the best suited catalyst is selected. As a result, elastomers with very little distortion in the foam structure itself can be obtained as found by the present inventors. The present invention provides a process for producing certain polyurethane elastomers for shoe soles of reduced weight which have an excellent resistance to bending. The process of the present invention comprises the production of elastomeric polyurethane foams having a density of at most about 0.6 g/cm³ by reacting a polyester polyol having a molecular weight of 500 to 4,000 obtained from aliphatic carboxylic acids selected from malonic, succinic, adipic, pimelic and sebacic acids and an aliphatic glycol, with an aromatic diisocyanate or a prepolymer thereof, obtained by reaction of a diisocyanate with a compound having two active hydrogen atoms, and with a diol having a molecular weight of 62 to 400 in the presence of a blowing agent and an amine catalyst, characterized in that as amine catalyst 4-methyl-1-(2-dimethylaminoethyl) piperazine, if desired together with another tertiary amine compound as co-catalyst is used. 4-Methyl-1-(2-dimethylaminoethyl)piperazine is abbreviated hereinafter as TMNAEP. TMNAEP per se is a well known catalyst as disclosed in U.S. Patent 2,949,431 and "Journal of Applied Polymer Science" Vol.4, No.11, p.207 (1960) and in British Patent 1,365,015. These prior arts teach that TMNAEP generally acts as a catalyst in the polyurethane formation reaction, and may be employed as catalyst for flexible foams, and for epoxyurethane denatured resins having shock resisting properties, but they do not even hint at using TMNAEP for high density elastic polyurethane, particularly for reducing the weight of elastomers for shoe soles. GB-A-1 587 046 describes foamed polyurethane elastomers obtained by reacting a compound having at least two Zerewitinoff-active hydrogen atoms of molecular weight of from 400 to 10 000 having pending phosphorate or sulfonate groups, an organic polyisocyanate, a chain extender of molecular weight between 32 and 400 and fillers, blowing agents and catalysts. EP-A-0 137 321 describes in example 4 the reaction of a polyether polyol with crude TD1, common additives such as surfactant and blowing agent using a catalyst which is a mixture of an organic carboxylic acid salt of a tertiary amine and 1-methyl-4-(2-dimethylaminoethyl) piperazine. The catalyst as used in this invention is superior in balancing the urethane and urea formation reactions in the production of elastomers for shoe soles. More particularly, the so-called tack-free time is very close to the rise time. The former is the time at which the formed foam loses the tacking on the surface when the urethane formation reaction of polyurethane has proceeded by far, and the latter is the time at which the foam is no longer observed to further expand at the completion of the urea forming reaction. The tack-free time indicates the time in which hardening is almost completed, so that the rise time close to the tack-free time means that the hardened foam does not continue to expand any more and the elastomer obtained has little distortion in the structure. In other words, when previous catalysts are employed and products are removed from their metallic mould, the products often suffer from so-called shaping problems such as 'shrinking' and 'waving', and therefore it has been difficult to prepare a lower density elastomer. When the catalysts as used in this invention are employed, elastomers of reduced density of at most about 0.6 g/cm³ can be prepared without making any change in the formulation and thus the necessary amount of the raw materials can be reduced. In addition, excellent equipment and devices can be used without applying any substantial modification. The catalysts as used in this invention have PKa values relatively smaller than those of most tertiary amine catalysts. Therefore, in the solution A which contains a mixture of raw material polyester polyol as stated above, cross linking agent which is a diol having a molecular weight of 62 to 400, blowing agent and other necessary additives, the polyester polyol is not hydrolyzed and, as a result, the solution A has good stability during storage. Furthermore, the catalysts as used in this invention do not accelerate the hydrolysis even if the elastomers shaped as shoe soles are exposed to high temperatures and high humidities, and therefore, the physical properties of the elastomer are hardly deteriorated by hydrolysis. The catalysts as used in the present invention are best miscible with the raw materials of elastomer for shoe soles and therefore exhibit high performance in the hardening stage of operation. As a result, the best property obtained is the high resistance to bending which is the most important property of elastomers for shoe soles. With the catalysts as used in this invention, the elastomers for shoe soles, even if they are made less dense, have a resistance to bending which is no less inferior to that of conventional elastomers. On the other hand, when catalysts other than those of this invention are employed in preparing elastomers for shoe soles of low density, the elastomers obtained are not satisfactory in practical usage on account of their much lower resistance to bending. In producing high density polyurethanes, especially elastomers for shoe soles, production of elastomers of low density with the shaping property improved and the resistance to bending not deteriorated has only been realized for the first time by the present invention. In addition, the catalysts as used in this invention, due to the low vapor pressure, generate a less offensive smell, which otherwise causes difficulties in the production of elastomers for shoe soles. Thus, the invention serves to improve the working environment. In the process of this invention, an organic solvent may be added, if necessary, to the catalyst of TMNAEP or a mixture thereof with other tertiary amine compounds as co-catalyst, to produce less distorted high density elastic polyurethanes of lower density, especially elastomers for shoe soles. TMNAEP as used in this invention can be prepared by permethylation from N-aminoethylpiperazine in a known method such as the Leuckart-Wallach reaction disclosed in U.S. Patent 4,026,840 and by the reductive methylation reaction which is described in Japanese Patent Publication (Appl. No. 58-073,007). The tertiary amine compounds which can be used as co-catalyst in combination with TMNAEP according to the present invention include the following popular tertiary amine compounds: triethylamin N,N-dimethylcyclohexylamine, N,N,N',N'-tetra-methylethylenediamine, 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"-penta-methyldipropylenetriamine, 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'-trimethylaminoethanolamine, 1,3-bis(N,N-dimethylamino)-2-propanol, N-methyl-N'-(2-hydroxyethyl)piperazine, bis(2-dimethylaminoethyl)ether. However, triethylenediamine is the most preferred among them all. If a tertiary amine compound is employed in a mixture with TMNAEP, the concentration of the tertiary amine compound should be in the range from 0 to 95% by weight, but preferably from 5 to 80% by weight. If the concentration exceeds the above value, the concentration of TMNAEP becomes too low to achieve the object of this invention. An organic solvent may be added, if necessary, to a mixture of TMNAEP with a tertiary amine compound. The organic solvent is selected from the group of polyols having 2 OH groups in the molecule, such as ethyleneglycol, diethyleneglycol, propyleneglycol, dipropyleneglycol, butanediols and hexanediol. But among them, ethyleneglycol, diethyleneglycol and 1,4-butanediol are preferred. These solvents may be used without any particular restriction to the amount of application usually in the range from 5 to 70% by weight. Too large amounts of organic solvent should be avoided because the concentration of amine in the catalyst becomes too low or the organic solvent might have a detrimental effect on the quality of the products that cannot be neglected. The mixing procedure of components of the catalyst composition takes place as follows. In case an organic solvent is needed to make up a catalyst, the solvent is mixed to TMNAEP under a nitrogen atmosphere, then a tertiary amine compound as co-catalyst, such as triethylenediamine, is added. Otherwise TMNAEP and the tertiary amine compound may be mixed separately to the original foaming solution. When the catalyst as used in this invention is applied in the production of high density elastic polyurethane, particularly of elastomers for shoe soles, the catalyst may be used in an amount of 0.05 to 10 parts, preferably 0.1 to 5 parts, per 100 parts of polyol. This catalyst may be used in parallel with commonly known organic tin compounds. The components for the polyesterpolyol are aliphatic carboxylic acids selected from malonic, succinic, adipic, pimelic and sebacic acids, and aliphatic glycols such as ethyleneglycol, butyleneglycol, and propyleneglycol. The polyesterpolyols employed in this invention are polymers prepared by known methods from a combination of one or more of these acids and glycols, or a mixture of these polyesters. The molecular weight of these polyesterpolyols should be 500 - 4,000, preferably 800 - 2,200. In the same way, the diisocyanates as used in this invention are aromatic diisocyanates and prepolymers thereof. Further, diisocyanates and/or diisocyanate prepolymers may be those semiprepolymers which can be prepared by a commonly known method by the reaction of a diisocyanate with a compound having two active hydrogen atoms reactive with an isocyanate. These diisocyanates, prepolymers and semiprepolymers may be those to which some acid ingredients or other stabilizers are admixed beforehand to suppress deterioration of properties. Cross linking agents include diols having a molecular weight of 62 - 400 such as ethyleneglycol, 1-4-butanediol, propyleneglycol, diethyleneglycol, dipropyleneglycol, and hexanediol, but the most preferred are ethyleneglycol, 1,4-butanediol and diethyleneglycol. Usable blowing agents include water and those low boiling aliphatic halogen compounds of common use such as trichloromonofluoromethane, and methylenechloride, either alone or a combination thereof. But the most preferred is water when it is used alone. Furthermore, various kinds of additives may be used in this invention. They include, for example, cell stabilizers and surfactants to provide the products with a tiny and uniform structure and, if necessary, a variety of pigments and dyestuffs for coloring the products and in some cases flame-retarding agents. The species and the amount of these additives are not limited insofar as they remain within the formality and usage of the common knowledge. Actual examples of the present invention are illustrated below in the comparative description of reference examples, but this invention is not restricted to the examples. Examples 1, 2 Free foaming was carried out with the aid of the catalyst as used in this invention using the following formulation. As a result thereof, very close values were obtained for the tack-free time and the rise time. The elastomers formed were of little distortion in the cellular structure of the foam. The elastomers were less dense, more elastic and tenacious. Conditions of foaming Temperature of solutions A and B 40°C Speed of agitation 5,000 r.p.m. Foaming vessel (Polyethylene beaker) 500 cc Explanation CT (Cream time) Time interval between mixing of the raw material solutions and initiation of the urea formation reaction in which the urea bonds are formed and carbon dioxide is evolved. TFT(Tack-free time) Time to the point where hardening of the polyurethane surface proceeds in an elastomer formation reaction and the tacking is no more felt by fingers. Usually, it corresponds to the rate of the urethane formation reaction. RT (Rise time) Time to the point where the urea formation reaction has almost completed and the foam height has reached the maximum. ρ (Foam density) Reference Example 1 Free foaming was carried out using the same process as in Example 1, except that TEDA-L30B was employed as catalyst. As a result, the tack-free time obtained was different by far from the rise time on account of the reaction rate. The elastomer produced had a high foam density and a remarkably distorted foam-cell structure. Examples 3, 4 Mechanical foaming was carried out using the same formulation as in Examples 1 and 2. More particularly, a solution A consisting of polyesterpolyol, water, silicone foam adjusting solution, cross linking agent and catalyst was stirred and kept at 40°C. On the other hand, a solution B of semiprepolymer was also kept at 40°C. The two solutions A and B were delivered into an aluminum mold with the index adjusted to be 100. The aluminum mold was 200 mm × 200 mm × 10 mm large, and kept warmed at 40°C by means of a plane heater. 5 and a half minutes after the solutions were mixed, a foam was obtained from the mold. The density of the foam was estimated from the weight (in grams) divided by the volume (in cc), and hardness was measured with a Shore C hardmeter immediately and 3 days after the demolding at 20°C. The tensile strength and the elongation property were measured according to JIS-K6301. In order to estimate the De Martia flexing, a test piece of a dimension 150 mm × 25 mm × 10 mm was cut off and the crack developing speed was measured according to ASTM D430. Namely, a crack of 2 mm was cut on the test piece and the piece was repeatedly bent with the use of the De Martia test machine, to obtain the resistance to bending of the product from the number of bendings and the speed of crack development. The results are shown in Table 2. Reference Examples 2, 3 Using the same formulation as in Reference Example 1, and following the same procedure as in Examples 3 and 4, foams of different densities were produced. Physical properties of these foams are shown in Table 2.

Claims: Process for producing elastomeric polyurethane foams having a density of at most about 0.6 g/cm³ by reacting a polyester polyol having a molecular weight of 500 to 4,000 obtained from aliphatic carboxylic acids selected from malonic, succinic, adipic, pimelic and sebacic acids and an aliphatic glycol, with an aromatic diisocyanate or a prepolymer thereof, obtained by reaction of a diisocyanate with a compound having two active hydrogen atoms, and with a diol having a molecular weight of 62 to 400 in the presence of a blowing agent and an amine catalyst, characterized in that as amine catalyst 4-methyl-1-(2-dimethylaminoethyl) piperazine, if desired together with another tertiary amine compound as co-catalyst is used. Process according to Claim 1, wherein the polyester polyol is a polyester synthesized from adipic acid, ethylene glycol, diethylene glycol, 1,4-butanediole and/or their mixture. Process according to Claim 1 or 2, in which 20 to 95% of 4-methyl-1-(2-dimethylaminoethyl) piperazine is used as the amine catalyst together with 5 - 80 weight % of triethylenediamine as the co-catalyst. The use of the high-density elastomeric polyurethane foams according to Claims 1-3 for preparing shoe soles.

Inventors: ARAI, SHOJI, HASEGAWA, MASAZUMI, TAMANO, YUTAKA
Applicants: TOSOH CORPORATION
Application year: 1985
Main classification: C08G18
IPC codes: C08G18