POLYURETHANE ELASTOMERS FROM HINDERED ALIPHATIC DIISOCYANATES

Abstract: A polyurethane elastomer composition resistant to deterioration on aging at elevated temperature, formed by reaction of a reaction mixture comprising aliphatic diols and/or polyols, and an aliphatic diisocyanate of the formula: or an isocyanate-terminated prepolymer derived thereform, R₁ being hydrogen or C₁-C₈ alkyl, R₂ being C₁-C₈ alkyl or substituted C₁-C₈ alkyl and X being arylene. A water reactive diisocyanate compound and, optionally a radical scavenger compound is introduced into the reaction mixture before reaction thereof.

Description: POLYURETHANE ELASTOMERS FROM HINDERED ALIPHATIC D I I SOCYANATES Hindered aliphatic diisocyanates of the formula: EMI1.1 wherein R is hydrogen or C1-Cg alkyl, R2 is C1-Cg alkyl or substituted Cl-Cg alkyl, and X represents a bridging unit, are known and described in the patent literature, as for example in Stallmann, U.S. Patent 2,723,265, and Hoover, U.S. Patent 3,290,350. Diisocyanates defined by formula (I) wherein both R1 and R2 are methyl and X is an arylene bridging group, that is, of the formula: EMI1.2 are readily prepared by the procedure described in the Hoover . These diisocyanates are useful in a wide range of polyurethane products, especially in castable and thermoplastic elastomer compositions, by application of well defined: conventional polyurethane forming techniques. Diisocyanates of the type described above may be secondary or tertiary. When they are used in the preparation of isocyanate-terminated prepolymers or in the preparation of thermoplastic polyurethanes for elastomer end-use applications, it is found that the physical properties of the resulting products exhibit marked deterioration upon aging in air at elevated temperatures and additionally exhibit discoloration. This is especially true when the tertiary diisocyanates of formula (11) are used. Moreover, it is found that this deterioration is not totally inhibited by the exclusion of air, as for example by aging the products in a nitrogen atmosphere. Since the elastomeric products obtained from these diisocyanates exhibit excellent properties initially, it is desirable to determine means to preserve these properties throughout the aging process, or at least to preserve them to an acceptable degree. it is, therefore, a principal object of the present invention to achieve stabilization of elastomeric products of the above-described type against deterioration when- exposed to elevated temperatures in air. It is another object of the invention to provide polyurethane elastomers formed from hindered aliphatic diisocyanates which exhibit excellent physical properties and resistance to discoloration. In accordance with the present invention, it has been discovered that polyurethane elastomers prepared using hindered aliphatic diisocyanates of formula (I) above exhibit improved resistance toward deterioration on aging at elevated temperatures in air when compositions containing them comprise (a) a diisocyanate compound which reacts rapidly with water and, optionally (b) a radical scavenger (antioxidant) compound. The hindered aliphatic diisocyanates of formula (I) in general react very slowly with water at temperatures up to about 600C in the absence of a catalyst. Moreover, these diisocyanates are hygroscopic and compositions prepared from them may contain up to and above 0.5 weight percent moisture. In the presence of a catalyst to promote the reaction of the isocyanate with the curative compound, water will react in competition with the curative compound at curing temperatures, which are generally in the range of about 80 to 1000C, and this may result in amine termination, with concomitant foaming, and in loss of physical properties, discoloration and oxidative instability It has been found that the addition of a suitable water-reactive diisocyanate and a suitable radical scavenger compound to the reaction mixture, before the curing reaction commences, will result in-retention of physical properties throughout the aging process and also in reduction of discoloration. Suitable water-reactive diisocyantes, readily available commercially, are those which react rapidly with water at temperatures below normal curing temperatures, and include, but are not limited to, the following: Methylenebis(phenyl isocyanate), Tolylene diisocyanate (2,4 and/or 2,6-), Phenylene di isocyanate, Xylene diisocyanate, Naphthalene diisocyanate, and the like. Methylenebis(phenyl isocyanate) is a preferred water-reactive diisocyanate The amount of water-reactive diisocyanate added to the elastomer composition will at least be sufficient to react with the moisture present in the composition before curing. In general, about 1-2: by weight of the water-reactive diisocyanate based on the total weight of the elastomer composition is sufficient to provide protection against reaction of the aliphatic isocyanate groups with water at curing temperatures. Additional amounts of the water-reactive diisocyanate, that is, in excess of the amount needed to react with the moisture present in the compositions, may be added to the elastomer composition prior to curing thereof, but in general provides no further advantage. Suitable radical scavengers are primary hindered phenol antioxidants of the type normally used to protect polymeric substrates against oxidative deterioration. Such compounds are readily available commercially and include, but are not limited to, the following: Tetrakis[methylene-2-(2' ,5'-di-t.butyl-4'-hydroxyphenyl)- propionateamethane--Trganox 1010, Ciba-Geigy; 2,2'-Meth ylenebis(4-ethyl-6-ttbutylphenol)--CYANO 425, American Cyanamid; 2,2'-methylenebis(4-methyl-6-t.butylphenol)-- CYANOX 2246, American Cyanamid; tris(4-t.butyl-3-hydroxy 2,6-dimethylbenzyl)-s-triazine-2,4,6-(lH, 3H, 5H)trione- CYANOX 1790, American Cyanamid; octadecyl-3-(3',5'-dit.butyl-4'-hydroxyphenyl)propionate--Irganox 1076, Ciba Geigy; thiodiethylenebis(3,5-di-t.butyl-4-hydroxy)hydro cinnamate--Irganox 1035, Ciba-Geigy; distearyl(2,6-di t.butyl-4-hydroxyphenyl) phosphate--Irganox' 1093, Ciba Geigy; tri-s(9,4-di-t.butylphenol)phosphite--Irgaphos 168, Ciba-Geigy; 1,1,3-tri(5-t.butyl-4-hydroxy-2-methylphenol)- butane--Topanol CA, ICI; and the like. The preferred radical scavenger is Irganox 1010. The amount of radical scavenger incorporated into the elastomer composition prior to curing will ordinarily range from about 0.05 to 1.0 percent by weight, based on the total weight of elastomer composition. Preferably, the radical scavenger compound will be used in an amount ranging from about 0.1 to 0.5 percent by weight, based on total weight of the elastomer composition. The water-reactive diisocyanate may be added either to the-hindered aliphatic diisocyanate or to the isocyanate-terminated prepolymer, in both instances prior to curing at elevated temperatures. The method used may vary depending on the product to be prepared, that is, whether a castable polyurethane or a thermoplastic polyurethane is to be produced. In general, castable polyurethanes are made by pre-forming an isocyanate-terminated prepolymer and then curing the prepolymer with a curative compound. In this type of system it is preferable to add the water-reactive diisocyanate to the prepolymer prior to curing. Thermoplastic elastomers may be made by reac ting essentially stoichiometric amounts of one or more diols or polyols with a diisocyanate or by reacting together essentially stoichiometric amounts of a prepolymer and one or more diols (or polyols). In general, it is prefer able to add the water-reactive diisocyanate to the prepoly mer, when used; alternatively, it may be added to the aliphatic diisocyanate before curing at an elevated temper- ature. The radical scavenger may be incorporated into the compositions at any convenient point before or during the curing step. The easiest and most convenient method is to incorporate the radical scavenger into either the prepolymer or one or more of the diol or polyol curative compounds. It is only important that the radical scavenger be incorporated in the resulting elastomeric product. The invention is illustrated by the following non-limiting examples, in which all parts and percentages are by weight, unless otherwise specified. Example I An isocyanate terminated prepolymer was pre pared by reacting 2.05 moles of a,a,a',a'-tetramethyl-p -xylylene diisocyanate with 1.0 mole of polytetramethyl ene ether glycol (mol wt 2000) at 80-820C. The resulting prepolymer contained 5.74% NCO. This was cured in a mold at 1000C using a curative consisting of 93 percent by weight of 1,4-butanediol and 7 percent by weight of trimeth ylol propane (NCO/OH = 1). After two hours in the mold, the elastomer was tested for stress-strain properties. Similar specimens were prepared and subjected to a post cure at 1000C in a circulating air oven tor 16 hours and 40 hours before testing. Test data are given below in Table I. TABLE I Cure Time (Hrs) 2 16 ¯ 40 Modulus, 100%, MPa 4.83 6.21 3.45 200%, MPa 6.21 6.90 4.14' 300Z, MPa 8.28 8.28 ¯¯ Tensile, MPa 29.0 11.7 4.83 Elongation, % 600 500 300 Hardness, Shore A 90 94 90 These data clearly illustrate the deterioration of the elastomer on aging in air at 100 C, particularly in the modulus and tensile properties. Example 2 The procedure of Example I was repeated except that the specimens were post-cured at 1000C for 16 and 40 hours under a nitrogen atmosphere. Data obtained are shown in Table II. TABLE II Cure Time (Hrs) 2 16 ¯ 40 Modulus, 100%, MPa 4.83 5.52 5.52 200%; MPa 6.21 7.59 7.59 3007., MPa 8028 10.4 9.66 Tensile, MPa 29.0 31.7 20.7 Elongation, % 600 600 700 Hardness, Shore A 90 91 90 Comparison of the data in Table II with the data shown in Table I clearly illustrates that aging of the elastomers in air results in deterioration while aging in nitrogen atmosphere does not, thereby indicating an oxidative process. Example 3 The procedure of Example 1 was followed except that 2% by weight of MDI--methylenebis(pgenylisocyanate)- was added to the prepolymer befote curing. The prepolymer contained 6.34 % NCO. Cures were made at NCO/OH ratios of 1.0 and 0.95. The elastomers were then post-cured in air for 16 hours and 40 hours. Data are given in Table III. TABLE III Cure Time (Hrs) 2 16 40 Stoichiometry 0.95/1.0 0.95/1.0 .95/1.0 Modulus, 100, % MPa 5.52/5.52 5.52/6.21 6.21/6.9 200 %, MPa 6.21/6.9 6.9/7.59 6.9/- 300 %, MPa 8.28/8.97 8.97/9.66 --/- Tensile, MPa 28 28.3/30.4 26.9/18.6 8.28/8.28 Elongation, % 590/610 630/640 320/205 Hardness, Shore A 90/90 90/92 93/94 Example 4 The procedure of Example 3 was followed except that the-prepolymer was cured with a mixture of 93% by weight of 1,4-butanediol and 7% by weight of aminopropanediol. Data are shown in Table IV. TABLE IV Cure Time (Hrs) 2 16 40 Stoichiometry 0.95/1.0 0.95/1.0 0.95/1.0 Modulus, 100%, MPa 5.52/5.52 5.52/6.21 6.21/6.9 200%, MPa 6.21/6.21 6.9/7.59 6.9/7.59 300%, MPa 8.28/8.28 9.66/9.66 8.28/8.28 Testile, MPa 24.2/28.3 24.2/16.6 8.97/8.28 Elongation, % 640/580 620/590 340/320 Hardness, Shore A 90/90 90/92 92/94 Example 5 A prepolymer was prepared as described in Example 1. The prepolymer was then divided into three portions: to one portion (A) was added 2% by weight of MDI; to a second portion (B) was added 2% by weight of MDI and 0.5 percent by weight of an antioxidant, Irganox 1010; and a third portion (C) was a control. All of the prepolymers were cured for 90 minutes at 1000C using the curative of Example 4. Stress-Strain data were obtained as described in Examples 1-4. Data are given in Table V. TABLE V Sample A B C Cure Time (Hrs) 2 16 40 2 16 40 2 16 40 Modulus, 100%, MPa 6.42 7.84 7.38 6.71 8.07 8.25 7.21 7.19 300%, MPa 9.42 11.67 10.10 9.46 11.92 12.16 10.14 - Tensile, MPa 23.86 21.12 11.55 24.38 18.12 17.36 19.27 9.84 5.41 Elongation, % 688 596 423 658 639 600 579 384 74 Hardness, Shore A 93 93 92 94 94 94 94 93 93

Claims: WHAT IS CLAIMED IS: 1. A polyurethane elastomer composition resistant to deterioration on aging at elevated temperature, comprising a water-reactive diisocyanate compound. 2. A polyurethane elastomer composition according to Claim 1, further comprising a radical scavenger compound. 3. A polyurethane elastomer composition according to Claim 2, wherein said radical scavenger compound is a hindered phenol compound. 4. A polyurethane elastomer composition according to Claim 1 wherein said water reactive diisocyanate compound is selected from the group consisting of Qethylenebis(phen- ylisocyanate); 2,4-tolylene diisocyanate; 2,6-tolylene diisocyanate; phenylene diisocyanate; xylene diisocyanate; and naphthalene diisocyanate. 5. A polyurethane elastomer comprising a reaction product formed at temperature of about 600 to about 1000C of a moisture-containing reaction mixture comprising (a) an aliphatic diol or polyol, (b) an aliphatic diisocyanate of the formula: EMI13.1 or an isocyanate-terminated prepolymer derived therefrom, wherein R1 is hydrogen or C1-Cg alkyl, R2 is a C1-C8 alkyl or substituted C1-Cg alkyl, and X is an arylene bridging group, (c) a catalyst for reaction between said aliphatic diol or polyol and said aliphatic diisocyanate or isocyanate-terminated prepolymer which is catalytically effective in a temperature range of about 60 to about 100 C, and (d) a water-reactive diisocyanate, in an amount sufficient to react with substantially all water present in said reaction mixture. 6. A polyurethane elastomer according to Claim 5 further comprising from about 0.05 to 1.0 percent by weight, based on the total weight of said elastomer, of a hindered phenol radical scavenger compound. 7. A polyurethane elastomer according to Claim 5 wherein said aliphatic diisocyanate is a compound of the formula: EMI14.1 8. A polyurethane elastomer according to Claim 5, wherein said isocyanate-terminated prepolymer is a product of reaction of one or more polyols with a stoichiometric excess of said aliphatic diisocyanate. 9. A polyurethane elastomer according to Claim 5, comprising a reaction product of at least one member of the group consisting of aliphatic diols and polyols and an essentially stoichiometric amount of said aliphatic diisocyanate. 10. A polyurethane elastomer according to Claim 5, wherein said water-reactive diisocyanate is methylenebis (phenyl isocyanate).

Inventors: BEZWADA, RAO SRINIVASA
Applicants: AMERICAN CYANAMID COMPANY
Application year: 1983
Main classification: C08G18
IPC codes: C08L75, C08G18, C08K5