P-TMXDI POLYURETHANE ELASTOMERS WITH GOOD COMPRESSION SET PROPERTIES

Abstract: Polyurethane elastomer compositions, having a com­ pression set less than or equal to about 40% are disclosed. These polyurethane elastomer compositions comprise the reaction product of: (a) A polyurethane prepolymer having terminally reac­ tive isocyanate groups and formed as a reaction product of polymeric diol with a stoichiometric excess of p-­ tetramethylxylene diisocyanate (p-TMXDI); with (b) A curatively effective amount of a curative selected from the group consisting of: diethyltoluenediamine (DE­ TDA), 2,4,-bis(p-aminobenzyl)-aniline (BABA), and mixtures thereof; and (c) Optionally, from about 0.01% to about 2% by weight of catalyst, based on the weight of curative. These polyurethane elastomers are used in molded articles which are to be used under load as in machine rolls and wheels.

Description: Field This invention relates to cast polyurethane elastomers based on p-tetramethylxylene diisocyanate (p-TMXDI) having good compression set properties. In Journal of Cellular Plastics, Vol. 18 (1982), No. 6, page 376 - 383 m- and p-TMXDI are introduced as a new family of aliphatic diisocyanates to the polyurethane industry. Polyurethane elastomers are prepared by a reacting excess polyisocyanate with a polyol to form a liquid or low melting polyurethane prepolymer having terminally reactive isocyanate groups, which can then be reacted with a diamine curing agent to form a polyurethane elastomer. Unfortunately, some of these elastomers exhibit poor "compression set", which makes them undesirable for use in molded articles which are to be used under load as in machine rolls and wheels (see Table 6 on page 380 of the above cited reference). For such load-bearing uses it is essential for the elastomer to exhibit high dimensional stability in order to to retain its resilience and not form flat spots under extended use. This property is known in the art as "good compression set". Compression set is commonly determined by ASTM Method D395 and represents the amount of residual deformation that occurs when an elastomer sample is placed under load sufficient to reduce its height to a specified value, held under that load for a certain period of time at a constant temperature, and then released from the load. Thus, if the height before sample loading is 100 units and it is compressed under load to a height of 90 units, but recovers to a height of 95 units when the load is removed, its compression set is 50%, i.e., 50% of the deformation was permanent. Obviously, the lower the compression set value for a given elastomer, the better it is for load bearing uses. U.S. 3,997,514 discloses a process for preparing polyurethane elastomers having a compression set of 50% or less. In the process an isocyanate terminated prepolymer is prepared and vulcanized by the steps of: 1. Encapping an aromatic diisocyanate, such as 2,4-tolylene diisocyanate, with a glycol, such as poly(tetramethylene ether)glycol (M.W. about 1,000), 2. reacting the resulting intermediate with an aliphatic diisocyanate to form a prepolymer, and 3. curing the prepolymer with a suitable diamine to form a vulcanizate having improved compression set, specifically 50% or less. Examples of aromatic diisocyanates are given in the penultimate paragraph of column 1, and examples of aromatic polyamines useful as cross-linking agents are given in the penultimate paragraph of column 2. Tolylene diamines are among the examples of polyamines enumerated in this paragraph, as well as 4,4'-methylenedianiline. U.S. 3,428,610 relates to cross-linked polyurethane plastics and to a method of preparing the same. The cross-linked polyurethanes are prepared by reacting (1) one equivalent of an intermediate containing between 0.5% and 15% free NCO groups with (2) between 0.8 to 1.2 equivalents of a liquid aromatic diamine. The liquid aromatic diamine has (i) one linear alkyl substituent of 1 to 3 carbon atoms in an ortho position to one amino group and two linear alkyl substituents of one to three carbon atoms in both ortho positions to the other amino groups, or (ii) two linear alkyl substituents of one to three carbon atoms in both ortho positions of both amino groups. The patent discloses that any suitable organic polyisocyanate may be used, such as an aliphatic polyisocyanate having the formula OCN-(CH₂) n -NCO in which n = 2 to 8 and aromatic diisocyanates. Examples of the polyisocyanates are given in the paragraph bridging columns 3 and 4. Among the examples of the suitable aromatic diamines that are set forth in the first full paragraph in column 4, are 1-methyl-3,5-diethyl-2,4-diaminobenzene and 1-methyl-3,5-diethyl-2,6-diaminobenzene. (Both of these isomers are also known as diethyltoluenediamine or DETDA.) This reference, however, does not relate to the effect of the use of a particular aromatic diamine and its effect on the compression set properties of the resulting polyurethane elastomer. U.S. 4,218,543 relates to a one-step process for the production of elastomeric moldings which have a compact surface by the technique of reaction injection molding, using highly reactive systems of specific polyisocyanates, polyhydroxyl compounds and specific active aromatic polyamines. This patent also relates to active hydrogen-containing blends useful in preparing elastomeric moldings, in which the blends comprise polyhydroxyl compounds, and specific active aromatic diamines. The elastomeric moldings are based on: (a) 4,4'-diisocyanatodiphenylmethane and polyisocyanates based on 4,4'-diisocyanatodiphenylmethane; (b) hydroxyl group containing compounds having a molecular weight of from 1,800 to 12,000 and preferably containing at least two primary hydroxyl groups; (c) active aromatic amines as chain-lengthening agents; (d) catalysts for the reaction between hydroxyl groups and isocyanate groups; and (e) optionally, blowing agents, auxiliary agents and/or additives known in polyurethane chemistry. Among the examples of the aromatic amines are 1-methyl-3,5-diethyl-2,4-diaminobenzene and 1-methyl-3,5-diethyl-2,6-diaminobenzene. This patent (U.S. 4,218,543) is directed towards reaction injection molding of polyurethanes where the gelation times are extremely fast (approximately 1 second or less). Based on the teachings of this patent, it would not be expected that diethyltoluenediamines (DETDA) could be utilized in polyurethane cast elastomers which require gel times of the order of several minutes. In particular, it would not be expected that good compression set properties could be obtainable with a particular polyurethane prepolymer when DETDA is used as the curing agent to produce the polyurethane elastomer. Good compression set is achievable in the known cast elastomer art, wherein, for example, toluene diisocyanate is the diisocyanate used to prepare the prepolymer, and curing is effected with methylenbis(o-chloroaniline)[MBCA, also known as MOCA® brand of methylenebis(o-chloroaniline)]. The principal drawback of this system is the requirement for use of MBCA, which has been reported to be a cancer suspect agent based on tests with laboratory animals. A welcome contribution to the art would be a polyurethane castable elastomer which has good compression set properties -- i.e., a compression set value of less than or equal to 40%--and does not require the use of MBCA. Such a contribution is provided by this invention. Summary of the Invention This invention provides a polyurethane elastomer composition, having a compression set less than or equal to about 40%, comprising the reaction product of p-TMXDI based prepolymer and an aromatic diamine selected from DETDA, 2,4-bis(p-aminobenzyl)aniline (BABA) and mixtures thereof. Detailed Description of the Invention This invention provides a polyurethane elastomer composition, having a compression set of less than or equal to 40%, comprising the reaction product of: (a) a polyurethane prepolymer having terminally reactive isocyanate groups as defined in claim 1; with between about 75 and about 110% of the stoichiometric (b) amount of a curative selected from the group consisting of DETDA, BABA and the mixtures thereof; and (c) optionally, from about 0.01% to about 2% by weight of catalyst, based on the weight of curative. Another embodiment of this invention provides a method of making a polyurethane elastomer having a compression set of less than or equal to about 40% comprising the steps of reacting the polyurethane prepolymer of (a) with a curative of (b) and optionally the catalyst of (c). In yet another embodiment of this invention, there are provided articles of manufacture made from the compositions of this invention. The chemical formula of p-TMXDI is: The p-TXMDI may also be made by methods known in the art, see for example, U.S. 4,361,518, U.S. 4,379,767, and U.S. 4,399,074, the disclosures of which are incorporated herein by reference. The term "polymeric diol" as used herein includes mixtures of compatible diols, e.g., mixtures of polyether diols. Those skilled in the art will appreciate that the term "compatible" means the diols are mutually soluble in each other so as to form a single phase. The polyether diols normally used in the preparation of polyurethane elastomers--e.g., hydroxyl-terminated--can be employed as the polymeric diols in preparing the compositions of this invention. Representative diols are polyalkylene ether glycols such as polyethylene ether glycol, polypropylene ether glycol and poly (tetramethylene ether) glycol and the polyethers prepared by the copolymerization of cyclic ethers such as ethylene oxide, propylene oxide, trimethylene oxide, and tetrahydrofuran with aliphatic diols such as ethylene glycol, 1,3-butanediol, 1,4-butanediol, diethylene glycol, dipropylene glycol, 1,2-propylene glycol, and 1,3-propylene glycol. Mixtures of polyethers, for example, a mixture of polytetramethylene ether glycol and polypropylenepolyethylene oxide ether glycol, may be used. Polymeric diols having a number average molecular weight of at least about 300 may be used. Generally, the molecular weights used range from about 300 to about 6000. A range of about 400 to about 3000 is preferred with about 1000 to about 3000 being most preferred. Any polyether having a number average molecular weight within the range specified above may be used. However, in general, polyethers are used having a number average molecular weight within the range of from about 445 to about 2100. Products within this range are commercially available. Preferably, the number average molecular weight is within the range of from about 800 to about 2000. A stoichiometric excess, as stated above, of p-TMXDI is reacted with polymeric diol to form the prepolymer of (a). Generally, the ratio of equivalents of -NCO groups in the p-TMXDI to -OH groups in the polymeric diol is from about 2:1 to about 5:1 with about 2.5:1 to about 4.5:1 being preferred and about 3.0:1 to about 3.5:1 being most preferred. The prepolymers from which the polyurethanes are prepared have an isocyanate content (% NCO) of at least about 2 wt %, based on the weight of the prepolymer. Usually the isocyanate content is within the range of from about 2.0 to about 9.0 wt %, with about 4.0 to about 7.5 wt % being preferred, and from about 4.5 to about 7.0 wt % being more preferred. The % NCO is related to the hardness of the final cured elastomer, and may vary depending on the hardness desired. The curative that is reacted with the prepolymer of (a) is selected from the group consisting of DETDA, BABA and mixtures thereof. DETDA usually refers to the two isomers 1-methyl-3,5-diethyl-2,4-diaminobenzene and 1-methyl-3,5-diethyl-2,6-diaminobenzene. A mixture reportedly containing 76% by weight of the 2,4-isomer and 21% by weight of the 2,6-isomer is available commercially from Ethyl Corp. (The balance of the product reportedly comprises other alkylated m-phenylenediamines.) Any combination of the 2,4- and 2,6-isomers in admixture or the individual isomers themselves may prove useful. However, since separation of the isomers is difficult, due to their very similar physical properties, it is economically advantageous to use DETDA which is commercially available. A mixture of DETDA and BABA may be used with the amount of each in the mixture varying according to the properties of the elastomer desired. However, there is no added advantage in combining the two. Both DETDA and BABA result in elastomers having good compression set properties, however, DETDA is preferred. This is because, for polymers having the same isocyanate content, using DETDA appears to result in elastomers having higher values for properties such as tensile strength, elongation, and tear strength. Preferably, about 75% to about 110% of the stoichiometric amount of the curative is used. Unlike known analogous toluene diisocyanate based prepolymers cured with known curatives such as MBCA, the stoichiometric ratio (equivalents of amine in the curative/equivalents of isocyanate in the prepolymer) is not highly critical. However, for convenience, about 85% to about 95% is a preferred stoichiometric ratio. Polyurethanes are prepared by well-known methods whereby one or more of the aforementioned polymeric diols is reacted with a stoichiometric excess of the aforementioned p-TMXDI. The resulting reaction product (prepolymer) is then further reacted with the above described curative. During prepolymer preparations, benzoyl chloride may optionally be added in amounts from about 0.005 to about 0.1 percent by weight based on the weight of the prepolymer. Those skilled in the art will appreciate that the benzoyl chloride may be employed to keep the prepolymer slightly acidic as may be necessary or desirable in a given application for reasons of processing or obtaining specific prepolymer characteristics. Optionally a conventional polyurethane catalyst may be used in the curing operation. Catalysts which may be used include but are not limited to mono- and dicarboxylic acids and their metal salts, such as glutaric acid, pimelic acid, azelaic acid, propionic acid, valeric acid, caproic acid, caprylic acid, myristic acid, palmitic acid, stearic acid, stannous stearate, chromium naphthenate, molybdenum naphthenate, lead naphthenate, dibutyltin dilaurate, dibutyltin diacetate, dimethyltin dilaurate and the like. Also useful are tertiary amines; such as triethylene diamine, dimethyl oleylamine, triethylene diamine, N-ethylmorpholine, and the like; and anhydrides of carboxylic acids, such as stearic anhydride, phthalic anhydride, and the like. The catalyst preferably should either be soluble in the polymer composition or compatible therewith at curing temperatures. The preferred catalysts for the reaction because of ready availability and low cost, are the mono- and dicarboxylic acids. They are normally used in an amount of from about 0.01% to about 2% by weight, based on the weight of curatives, with about 0.1% to about 1% by weight being preferred. The following examples are provided for the purposes of illustration only. The examples should not be construed as limiting the invention in any way. Prepolymer Preparation In Example 1 a polyether prepolymer is made by mixing: (a) 1000g (1 equivalent) of TERACOL® 2000 brand of a 2000 number average molecular weight poly(tetramethylene ether)glycol from E.I. du Pont de Nemours & Co., Inc.; with (b) 390 g (3.2 equivalents) of p-TMXDI; in the presence of (c) 0.01% by weight, based on the weight of (a + b), of dibutlytin dilaurate (M & T catalyst T-12, a trademark of M & T Chemicals, Inc.) at 85°C for 4 hours. The isocyanate capped prepolymer had a NCO content of 6.2%. The polyether prepolymers of Examples 2-15 were prepared following the procedure of Example 1. Table I gives the amount of TERACOL® 2000 brand of poly(tetramethylene ether)glycol and p-TMXDI used to prepare the prepolymers. The catalyst used was the same as that used for Example 1. In Table I, "eq" is the abbreviation for "equivalent." Prepolymer Preparation NCO CONTENT, % TERACOL® 2000 p-TMXDI 6.6 1000g (1 eq) 422g (3.46 eq) 5.9 1500g (1.5 eq) 578g (4.74 eq) 5.5 1500g (1.5 eq) 549g (4.5 eq) 5.1 1500g (1.5 eq) 520g (4.26 eq) 5.2 2000g (2 eq) 693g (5.68 eq) Curing For examples 1-15, curing of the prepolymer was done by mixing a stoichiometric amount of the curative, usually 90% of the stoichiometric amount, unless indicated otherwise, with the prepolymer at 100°C and then pouring into a mold at 100°C and pressing to form a 2.03mm (0.08") thick sheet or 12.7mm (0.5") thick buttons. After demolding, usually after 30-60 minutes, the sheets and buttons were post-cured in an oven for another 16 hours at 100°C. The pot life (at 100°C) for the compositions of this invention generally falls within about 2 to about 10 minutes, depending on how much, if any, catalyst is used. This range is within the acceptable limits for commercial applications. In Tables II - V, the physical properties listed were determined by the following test procedures: Examples 1 - 6 Table II, Examples 1-6, reports the results of physical tests performed on the compositions of this invention. The-DETDA used in Examples 1-5 was obtained from Ethyl Corp. The compression set values of DETDA-cured and BABA-cured polymers were all below 40%. Comparative Examples 7-10 Table III, Comparatives Examples 7-10, reports the results of physical tests performed on compositions not of this invention--i.e., compositions that were cured with a curative other than DETDA or BABA. The compression set values of these compositions were all above 40%. In Table III, the following abbreviations are used: MDA = 4,4'-methylenedianiline MDPA = methylenebis(dipropylaniline) (available from Ethyl Corp.) BDO = 1,4-butanediol Examples 11-15 Examples 11-15 (Table IV) show the surprising insensitivity of the favorable results for the compositions of this invention, in particular the compression set, to stoichiometry. The procedures of Example 1 was followed using a prepolymer from p-TMXDI and polytetramethylene ether glycol having 5.2% NCO, with DETDA as the curative, and the stoichiometric ratio was varied between 75% and 110%. In all cases, attractive physical properties were found for the cured elastomer. The compression set was always well below 40%.

Claims: A polyurethane elastomer composition, having a compression set less than or equal to 40 %, comprising the reaction product of: (a) a polyurethane prepolymer having terminally reactive isocyanate groups and an isocyanate content of 2.0 to 9.0 wt.-%, based on the weight of the prepolymer, and formed as a reaction product of a polymeric diol selected from polyether diols with a stoichiometric excess of p-tetramethylxylene diisocyanate; with (b) between 75 and 110 % of the stoichiometric amount of a curative selected from the group consisting of diethyltoluenediamine, 2,4-bis(p-aminobenzyl)aniline, and mixtures thereof; and (c) optionally, from 0.01 % to 2 % by weight of catalyst, based on the weight of curative. A polyurethane elastomer composition according to Claim 1, wherein said optionally employed catalyst is selected from the group consisting of mono- and dicarboxylic acids, metal salts of mono- and dicarboxylic acids, carboxylic acid anhydrides and tertiary amines. An elastomer of any preceding claim wherein the ratio of equivalents of p-tetra-methylxylene diisocyanate to polymeric diol is 2 : 1 to 5 : 1, or 2.5 : 1 to 4.5 : 1, or 3.0 : 1 to 3.5 : 1. An elastomer of any preceding claim wherein said prepolymer has an isocyanate content of 4.5 to 7.0 wt.-%, based on the weight of the prepolymer. A method of making a polyurethane elastomer having a compression set of less than or equal to 40 % comprising the steps of reacting (a) a polyurethane prepolymer having terminally reactive isocyanate groups and an isocyanate content of 2.0 to 9.0 wt.-%, based on the weight of the prepolymer, and formed as a reaction product of a polymeric diol selected from polyether diols with a stoichiometric excess of p-tetramethylxylene diisocyanate; with (b) between 75 and 110 % of the stoichiometric amount of a curative selected from the group consisting of diethyltoluenediamine, 2,4-bis(p-aminobenzyl )aniline, and mixtures thereof; and (c) optionally, from 0.01 % to 2 % by weight of catalyst, based on the weight of curative.

Inventors: CHANG, EUGENE YUE CHIEH
Applicants: AMERICAN CYANAMID COMPANY
Application year: 1985
Main classification: C08G18
IPC codes: C08G18