GB2036725A - Highly stable liquid carbodiimide- containing polyisocyanate compositions and polyurethane foams obtained therefrom - Google Patents

Abstract

Highly stable liquid 4,4'- diphenylmethane diisocyanate (MDI) containing MDI-carbodiimide adducts are prepared by heating MDI at a temperature of from 180 DEG C to 250 DEG C for a period of from one hour to ten hours in the presence of catalytic amounts of tris(chloromethyl) phosphine oxide and thereafter equilibrating the adduct formation. Equilibration can occur by any of several methods, e.g. by cooling the composition to a temperature of from 60 DEG C to 90 DEG C within twenty minutes, maintaining the composition at this temperature for a period of from one hour to four hours and thereafter cooling the composition to room temperature or by cooling the composition to a temperature less than 100 DEG C within twenty minutes, further cooling to room temperature, and maintaining the composition at room temperature for one week to two weeks. The resulting compositions have very low sediment contents and are particularly useful in the preparation of micro-cellular polyurethane foams by reaction with active hydrogen-containing compounds. <IMAGE>

Description

SPECIFICATION Highly stable liquid carbodiimide-containing polyisocynate compositions and polyurethane foams obtained therefrom The present invention relates to liquid polyisocyanate compositions having enhanced stability and to the use thereof in the preparation of polyurethane foams having improved physical properties. More particularly, the invention relates to liquid polyisocyanates obtained by heating at high temperatures 4,4'diphenylmethane diisocyanate (hereinafter referred to as MDI) or mixtures containing MDI in the presence of tris(chloromethyl)phosphine oxide and to polyurethane foams obtained therefrom.

The preparation of liquid carbodiimide-containing organic polyisocyanates by heating an organic polyisocyanate in the presence of catalytic amounts of tris(chloromethyl)-phosphine oxide at a temperature of up to 1 20"C is taught in U.S. Patent No.3,761,502. Disclosed in this patent is the fact that use of this catalyst provides an improvement over prior art processes since its reactivity is such that it is possible to control the carbodiimide formation thereby enabling the preparation of liquid compositions having controlled amounts of carbodiimide linkages. One of the problems encountered in following the process of this patent is that the resulting compositions are not stable and over a period of time show decreasing amounts of isocyanate content.

We have now found an improved process for the preparation of liquid carbodiimide-containing MDI employing tris(chloromethyl)phosphate oxide as the carbodiimide-promoting compound.

According to the present invention there is provided a process for the preparation of liquid carbodiimide-containing MDI having an isocyanate content of from 20 to 32 percent by weight by heating MDI at a temperature of from 180to 250"C for a period of from one hour to ten hours in the presence of tris(chloromethyl)phosphone oxide and thereafter equilibrating the adduct formation.Equilibration can occur in any of several ways, e.g. by cooling the composition to a temperature below 100 C, preferably from 60"C to 90"C within twenty minutes, maintaining the composition at this temperature for a period of from one hour to four hours and thereafter cooling the composition to room temperature or by cooling the composition to a temperature less than 1 00'C within twenty minutes, further cooling to room temperature, and maintaining the composition at room temperature for one week to two weeks. The resulting compositions are highly stable and, when employed in the preparation of polyurethane compositions, provide polyurethanes of enhanced physical properties.

In accordance with the invention, MDI or mixtures containing MDI are subjected to treatment with a catalytic amount of tris(chloromethyl)phosphine oxide fora period of from one hour to ten hours at a temperature of from 1800C to 250"C. Thereafter, the reaction mixture containing carbodiimide groups is equilibrated which, as used in the subject specification, means completing the formation of the MDI-carbodiimide adduct. This can be accomplished in several ways as set forth above. It has been determined that only by following the sequence of steps set forth above, can highly stable liquid MDI compositions be prepared.

While pure MDI is the polyisocyanate of choice in the process of the subject invention, mixtures containing MDI may also be employed. These mixtures are generally prepared by condensing aniline with formaldehyde to form a mixture of amines which is subsequently phosgenated to produce the corresponding isocyanates. These amine mixtures generally contain from 75 to 25 weight percent of diamines of which from one percent to thirty percent is the 2,4' isomer, and less than one percent to ten percent is the 2,2' isomer, and from 25 percent to 75 percent of higher functional diphenylmethane bases.

The catalyst employed in the process of the subject invention istris(chloromethyl)phosphine oxide. This compound is well known in the art and may be prepared in the manner set forth in U.S. 3,761,502. The catalyst is generally employed in an amount ranging from 0.001 part to 0.05 part, preferably from 0.006 to 0.012 part, by weight per 100 parts by weight of MDI. Amounts greater than 0.05 part are not useful in the subject invention because the reaction is too rapid for proper monitoring.

As mentioned above, the process of the subject invention is generally carried out under atmospheric pressure and at a temperature between 180 C and 250"C, preferably between 200"C and 230"C. It has been determined that at these temperatures the extent of reaction can be controlled to a large degree by the amount of catalyst employed. Also, at these temperatures, the catalyst causes a rapid but controlled initial reaction which is followed by a slower thermal reaction which can be readily monitored by isocyanate analysis to provide expertly controlled isocyanate contents. The resulting compositions exhibit enhanced storage stability.

The time required for carrying out the reaction may vary from one hour to ten hours, preferably two to six hours. Upon completion of the reaction, the resulting composition is preferably cooled to a temperature between 60"C and 90"C within twenty minutes. Generally, the time required to cool the composition from the reaction temperature to below 1 OO"C will be less than one minute. This is accomplished by discharging the hot reaction mixture to a cooled receiver. The total discharge time from the reactor will vary between five minutes and thirty minutes. The resulting composition is then maintained at a temperature between 60"C and 90"C for a period of from one hour to four hours and thereafter allowed to cool to room temperature.

Alternatively, the composition is cooled to a temperature below 1 00'C within twenty minutes, allowed to cool to room temperature and maintained at room temperature for one week to two weeks. Other cooling procedures employing equilibrating temperatures intermediate of the two described above are also applicable.

In a preferred embodiment of the subject invention, the liquid carbodiimide-containing polyisocyantes are employed in the preparation of polyurethane foams. The resulting foams surprisingly possess enhanced physical properties. The preparation of these foams is well known in the art and generally involves the reaction of the polyisocyanate either alone or blended with pure MDI with an active hydrogen-containing compound in the present of a chain-extending agent, a catalyst, a surfactant and a blowing agent. If a blend of polyisocyanates is employed, the compositions of the invention may comprise from 15 to 75 weight percent of the blend.

Preferred active hydrogen-containing compounds which may be employed in the preparation of the foams are compounds having from two to eight active hydrogen atoms and an equivalent weight of from 500 to 5000, preferably from 1000 to 3000. Representative compounds inciude polyalkylene polyether polyols, polyester polyols and others set forth in U.S. 3,761,502. Representative useful chain-extending agents, catalysts, surfactants and blowing agents are well known in the art as evidenced by the disclosures set forth in U.S. 3,804,782. 1,4-Butanediol may be mentioned in particular as a chain-extending agent. Further details of the preparation of these foams can be found in U.S. 3,761,502.

In the examples that follow, all parts are by weight unless otherwise indicated. The physical properties of the foams were determined in accordance with the following ASTM tests: Density - D-1622-63 Tensile Strength - D-1623-72 Elongation - D-412 Split Tear- D-470 Graves Tear - D-624 Shore "D" Hardness - D-676 Flex Recovery - D-1623-72 Flex Modulus- D-1623-72 Example I A reactor equipped with a stirrer, thermometer, condenser, inlet and outlet means and heat exchange means was heated to 500C, flushed with nitrogen and while maintaining a blanket of nitrogen, 3907 parts of molten, 4,4'-diphenylmethane diisocyanate and 0.23 part (0.006 percent) tris(chloromethyl)phosphine oxide were added thereto. The temperature was raised to 230cC while stirring the reaction mixture.The reaction mixture was maintained at 2300C until the isocyanate content decreased to about 30.6 percent (2.5 hours).

While the mixture was maintained at 230"C, the reaction mixture was transferred under nitrogen pressure to a cooled reactor such that the product was cooled to below 100 C in less than one minute during a total discharge time of about 15 minutes. The mixture was then stirred in the kettle at 800C for two hours and thereafter cooled to 250C for storage. The isocyanate content was determined to be 29.5 percent. After six months at room temperature, the isocyanate content was 29.3 percent indicating that a highly stable carbodiimide-containing liquid MDI composition was obtained.

Example Il The procedure described in Example I was duplicated with the exception that the catalyst was added to the MDI at 230"C. After equilibration at 25"C, the isocyanate content was determined to be 28.6 percent. After six months at room temperature, the isocyanate content was 28.3 percent.

Examples Ill-Xl A series of experiments was carried out in a reactor equipped as described in Example I. In each run, the tris(chloromethyl)phosphine oxide was added to MDI and the temperature of the reaction mixture was maintained between 180"C and 230"C. After maintaining the reaction mixture at the reaction temperature for -a period of time, the reaction mixture was quickly cooled to below 100 C. The product was further cooled to room temperature. Details of the preparations are presented in Table I below.

TABLE I- Preparation of Carbodiimide-Modified MDI %Cat. Cat.Addn. Reaction Cooling Time,min. Final* % NCD Example in MDI Temp., C Temp., C/Time,hr. to Noted C % NCO After 6 Months III 0.006 230 230/4.3 13(65 C) 29.3 28.9 IV 0.006 230 230/4.5 < 1( < 100 C)-20(30 C) 29.8 Not detn'd.

V 0.018b 230 230/6.6 < ( < 100 C) 24.9 24.7 VI 0.006 230 230/1.7 < ( < 100 C)-12(40 C) 29.0 29.0 VII 0.006 55 230/4.0 < ( < 100 C) 28.7 28.5 VIII 0.010 180 180/6.0 10(25 C) 29.4 29.1 IX 0.012 200 200/2.0 8(27 C) 29.4 29.3 X 0.010 220 220/3.0 8(33 C) 28.8 28.7 XI 0.006 230 230/3.3 "(70-90 C)c 29.5 29.2 a. The NCO content of the product was equilibrated (reaching a stable value, final NCO, indicating the complete formation of the MDI/carbodiimide adduct) at room temperature for 1-2 weeks. To illustrate, with Example IV, the mixture was cooled to less than 100 C in less than one minute, thereafter cooled to 30 C in twenty minutes and maintained at room temperature for two weeks.

b. The catalyst was added in portions.

c. The mixture was equilibrated at 80 C for two hours.

Example XII In this Example, the importance of rapidly cooling the product to avoid excessive sediment formation is demonstrated. 4,4'-Diphenylmethane diisocyanate (3852 parts) was heated to 230"C under a slow stream of nitrogen, and 0.231 part (0.006 percent) of tris(chloromethyl)phosphine oxide was added. When the isocyanate content had decreased to about 30.5 percent, the product was cooled in ten minutes to 200"C, separated into three parts, and cooled at varying rates to 100"C, before being cooled rapidly to room temperature. After six days, the samples were analyzed for sediment (insoluble dimer) content. The results are as follows: Length of Cooling From 200"Cto 100"C, min.Percent Sediment 5 0.47 90 2.22 300 4.05 In a related experiment, 4,4'-diphenylmethane diisocyanate (210 parts) was heated to 230"C under a slow stream of nitrogen and 0.013 part (0.006 percent) of tris-(chloromethyl)phosphine oxide was added. When the isocyanate content had decreased to about 30.5 percent, the product was cooled in four minutes to 30"C and allowed to equilibrate at room temperature for one week. Analysis showed after six days, only 0.08 percent sediment.

Example Xlil 4,4'-Diphenylmethane diisocyanate (3354 parts) and 0.40 part (0.012 percent) of tris)chloromethyl)phosphine oxide was heated to 230"C, and when the NCO content had dropped to 25.6 percent (two hours), the product was transferred with nitrogen pressure to a flask stirred in an ice bath, to cool the product rapidly to 80"C. After the product was stirred at 80"C for two hours, it was cooled and analyzed. The isocyanate content was 22.2 percent. One week later (percent isocyanate 21.7), 1426 parts of the product was blended with 2674 parts of 4,4'-diphenylmethane diisocyanate to give a product identified as XllI-A, containing 29.1 percent isocyanate by analysis. After four months, the isocyanate content was unchanged.

In a similar preparation, 0.008 percent catalyst was used, and the reaction at 230"C was terminated at 28.0 percent isocyanate (three hours). After two hours at 80"C, the cooled product showed 25.7 percent isocyanate and two weeks later an isocyanate content of 25.9 percent. The product was blended with MDI to give product identified as XIII-B containing 29.1 percent isocyanate. After four months, the isocyanate content was still 29.1 percent.

Examples XIV-LX A number of microcellular foams was prepared employing the carbodiimide-modified MDI compositions of the previous examples along with various other ingredients. All of the formulations utilized an NCI/OH index of 105. In addition to the ingredients presented in Table II, each formulation comprised 0.5 part of triethylene diamine and 0.02 part of dibutyltin dilaurate. The following abbreviations are employed in Table II: Polyol A - a 6800 molecular weight polyol prepared by capping with ethylene oxide a propylene oxide adduct of trimethylolpropane, said polyol having an oxyethylene content of 15 percent by weight of the polyol.

Polyol B - a 4000 molecular weight polyol prepared by capping with ethylene oxide a propylene oxide adduct of propylene g lycol, said polyol having an oxyethylene content of 20 percent by weight of the polyol.

l-143L and M-CD - two pure MDI modified isocyanates having a free isocyanate content of 29-30 percent, known commercially as Isonate-143L (Upjohn) and Mondur-CD (Mobay).

TABLE II - Microcellular Foam Preparation and Properties Example XV XVI XVII XVIII XIX XX XXI XXII Polyol A, parts 100 100 100 100 - - - Polyol B, parts - - - - 100 100 100 100 1,4-butanediol, parts 21.5 21.5 26.5 26.5 21.5 21.5 26.5 26.5 Composition of Ex. III I-143L III I-143L III I-143L III I-143L Properties Density, pcf. 60.9 61.3 63.5 62.0 61.9 62.3 63.0 61.9 Tensile strength, psi 1,840 1,750 2,480 2,200 2,140 1,980 2,170 2,130 Elongation, % 70 80 70 70 140 150 80 70 Split Tear, pi. 90 77 120 100 158 144 190 172 Graves Tear, pi. 250 223 380 360 370 368 440 440 Shore "D",-Instantaneous 48 51 57 58 52 51 55 58 -5 seconds 44 42 51 52 44 44 48 50 Heat Sag 0.16 0.40 0.06 0.18 0.16 0.36 0.10 0.18 Flex Recovery 7/3 9/5 10/5 14/8 9/4 11/6 11/6 13/8 Flexural Modulus-20 F 30,747 36,225 56,622 67,392 45,988 29,077 62,198 62,914 72 F 14,711 14,678 27,820 27,875 17,412 15,868 25,978 30,223 150 F 10,542 8,988 18,874 18,071 11,001 9,690 17,570 18,026 Ratio-20 F/150 F 2.92 4.03 3.00 3.73 4.18 3.00 3.50 3.49 Example XXIII XXIV XXV XXVI XXVII XXVIII XXIX XXX Polyol A, parts 100 100 100 100 - - - Polyol B, parts - - - - 100 100 100 100 1,4-butanediol, parts 21.5 21.5 26.5 26.5 21.5 21.5 26.5 26.5 Composition of Ex. V I-143L V I-143L V I-143L V I-143L Properties Density, pcf. 60.2 61.3 61.4 62.0 75.5 62.3 63.0 61.8 Tensile strength, psi 1873.3 1740 2450 2200 1763.3 1980 2217 2130 Elongation, % 70 80 50 70 70 150 60 70 Split Tear, pi. 76 77 120 100 82 144 135 172 Graves Tear, pi. 180 223 288 360 185 368 326 440 Shore "D",-Instantaneous 51 51 55 58 47 51 58 58 -5 seconds 47 42 49 52 40 44 51 50 Heat Sag 0.20 0.40 0.14 0.18 0.18 0.36 0.18 0.18 Flexural Modulus 72 F 12,135 14,678 33,044 27,875 16,438 15,868 31,254 30,223 Example XXXI XXXII XXXIII XXXVIV XXXV XXXVI Polyol A, parts 100 100 100 - - Polyol B, parts - - - 100 100 100 1,4-butanediol, parts 24.5 24.5 24.5 24.5 24.5 24.5 Composition of Ex.VI VI VI VI VI VI Properties Density, pcf. 62.9 61.8 61.4 61.8 60.7 61.4 Tensile strength, psi 2,510 2,613 2,313 2,373 2,517 2,300 Elongation, % 153 100 117 227 143 190 Split Tear, pi. 115.7 88.7 78.3 183.7 143.0 151.0 Graves Tear, pi. 292.3 246.7 225.0 372.7 332.7 346.0 Shore "D",-Instantaneous 48 53 49 49 51 48 -5 seconds 43 51 46 43 46 43 Heat Sag, 250 F 0.43 0.33 0.40 0.35 0.28 0.44 Flex Recovery 12/9 15/11 13/10 12/9 14/9 12/8 Flexural Modulus-20 F 40,800 43,700 40,600 43,000 40,300 40,000 72 F 15,236 22,852 14,978 16,064 20,054 14,107 158 F 8,370 14,000 8,630 8,070 11,280 7,270 Ratio-20 F/1588F 4.2 3.1 4.7 5.3 3.6 5.6 Example XXXVII XXXVIII XXXIX XL XLI XLII XLIII XLIV XLV XLVI XLVII XLVIII Polyol A, parts 100 100 100 100 100 Polyol B, parts - - - - 1,4-butanediol, parts 21.5 21.5 21.5 26.5 26.5 100 - - - - - Compoisition of Ex.IV I-143L M-CD IV I-143L - 100 100 100 100 100 100 26.5 21.5 21.5 21.5 26.5 26.5 26.5 Properties M-CD IV I-143L M-CD IV I-143L M-CD Density, pcf. 65.2 63.8 64.8 65.8 64.2 61.8 64.7 63.9 64.6 64.0 63.2 63.3 Tensile strength, psi. 2,100 2,270 2,080 2,750 2,380 2,240 2,200 2,170 2,150 2,600 2,480 2,550 Elongation, % 60 70 90 70 50 40 250 110 170 250 70 130 Split Tear, pi. 97 79 87 107 100 97 274 130 152 333 140 172 Graves Tear, pi. 325 312 327 411 394 388 510 400 431 600 441 479 Shore "D",-Instantaneous 55 54 53 62 60 54 54 57 51 54 58 54 -5 seconds 48 49 44 54 52 50 43 47 44 48 53 50 Heat Sag 0.56 0.68 0.58 0.28 0.54 0.32 0.34 0.34 0.37 0.16 0.36 0.32 Flex Recovery 12/8 12/9 11/7 13/8 16/11 15/11 12/7 12/9 12/7 12/8 14/9 13/9 Flexural Modulus -20 F 46,262 49,423 35,244 70,958 71,534 59,848 38,725 51,895 41,704 66,331 76,086 60,881 72 F 20,471 22,343 16,824 33,949 35,302 28,594 18,339 23,414 18,232 29,584 34,466 31,415 150 F 13,993 15,294 11,425 25,906 22,650 25,163 11,706 15,325 11,325 21,372 23,096 20,175 Ratio -20 F/150 F 3.3 3.2 3.1 2.7 3.2 2.4 3.3 3.4 3.7 3.1 3.3 3.0 Example XLIX L LI LII LIII LIV LV LVI LVII LVIII LIX LX Polyol A, parts 100 100 100 100 100 100 - - - - - Polyol B, parts - - - - - - 100 100 100 100 100 100 1,4-butanediol, parts 21.5 21.5 21.5 26.5 26.5 26.5 21.5 21.5 21.5 26.5 26.5 26.5 Composition of Ex. XIII-A VIII-B I-143L XIII-A VIII-B I-143L XIII-A VIII-B I-143L XIII-A VIII-B I-143L Properties Density, pcf. 52.0 53.3 52.0 47.0 50.7 55.9 53.4 49.5 50.2 50.3 50.2 Tensile strength, psi. 1,490 1,330 1,310 1,600 1,420 1,220 1,400 1,520 1,320 1,440 1,640 1,300 Elongation, % 110 80 70 90 70 60 140 100 130 140 190 130 Split Tear, pi. 83 74 66 92 62 73 120 102 111 158 192 109 Graves Tear, pi. 184 179 160 206 152 151 226 226 213 249 279 208 Shore "D",-Instantaneous 37 48 39 45 40 42 36 39 33 34 41 35 -5 seconds 34 45 35 46 40 42 36 39 33 34 41 35 Heat Sag 0.05 0.22 0.18 0.19 0.17 0.26 0.07 0.13 0.27 0.12 0.15 0.24 Flex Recovery 9/5 9/4 10/6 11/8 13/10 16/11 10/5 8/4 7/3 11/8 12/8 13/10 Flexural Modulus -20 F 15,625 20,672 23,283 40,500 37,400 34,800 19,553 21,499 27,483 31,200 37,500 35,500 72 F 7,928 8,640 10,224 18,800 17,600 15,000 7,903 10,956 7,963 13,600 15,300 12,400 158 F 5,732 5,954 5,804 13,200 11,700 8,800 5,720 7,359 4,724 9,600 9,300 6,800 Ratio -20 F/158 F 2.7 3.5 4.0 3.1 3.2 3.9 3.4 2.9 5.8 3.3 4.0 5.2

Claims (20)

1. A highly-stable liquid 4,4'-diphenylmethane diisocyanate composition containing a diphenylmethane diisocyanate-carbodiimide adduct having an isocyanate content of from 20 to 32 percent by weight prepared by heating 4,4'-diphenylmethane diisocyanate at a temperature of from 1 80"C to 250"C for a period of from one hour to ten hours in the presence of a catalytic amount of tris(chloromethyl)phosphine oxide and thereafter equilibrating the adduct formation.

2. A process for the preparation of a liquid polyisocyanate composition having an isocyanate content of from 20 to 32 percent by weight which comprises heating 4,4'-diphenylmethane diisocyanate at a temperature of from 1 80"C to 250"C for a period of from one hour to ten hours in the presence of a catalytic amount of tris(chloromethyl)phosphine oxide and thereafter equilibrating the adduct formation.

3. A process as claimed in claim 2 wherein the adduct formation is equilibrated by cooling the composition to a temperature of from 60"C to 90"C within twenty minutes, maintaining the composition at this temperature for one to four hours and thereafter cooling it to room temperature.

4. A process as claimed in claim 3 wherein the adduct formation is equilibrated by cooling to a temperature of about 80"C within ten minutes and maintaining it at that temperature for about two hours.

5. A process as claimed in claim 2 wherein the adduct formation is equilibrated by cooling the composition to a temperature less than 1 00"C within twenty minutes, further cooling to room temperature and maintaining the composition at room temperature for one week to two weeks.

6. A process as claimed in any of claims 2 to 5 wherein the reaction temperature is from 200"C to 230"C.

7. A process as claimed in any of claims 2 to 6 wherein the reaction time is from two to six hours.

8. A process as claimed in any of claims 2 to 7 wherein from 0.001 to 0.05 part by weight of tris(chloromethyl)phosphine oxide is used per 100 parts by weight of 4,4'-diphenylmethane diisocyanate.

9. A process as claimed in any of claims 2 to 8 wherein the 4,4'-diphenylmethane diisocyanate is used in the form of a crude mixture containing it and obtained by condensing aniline with formaldehyde to form a mixture of amines which is subsequently phosgenated to form a mixture of isocyanates.

10. A process for the preparation of a liquid 4,4'-diphenylmethane diisocyanate composition containing a diphenylmethane diisocyanate-carbodiimide adduct carried out substantially as described in any of the foregoing Examples I to XI or XIII.

11. A highly stable liquid 4,4'-diphenylmethane diisocyanate composition as claimed in claim 1 when prepared by a process as claimed in any of claims 2 to 10.

12. A highly stable liquid polyisocyanate composition comprising from 15 to 75 weight percent of a composition as claimed in claim 1 or 11 and 85 to 25 weight percent of 4,4'-diphenylmethane diisocyanate.

13. A polyurethane foam made from a composition as claimed in claim 1, 11 or 12 and an active hydrogen-containing compound.

14. A polyurethane foam prepared by the reaction in the presence of a blowing agent of an organic compound having from two to eight active hydrogen atoms and an equivalent weight of from 500 to 5000 with a liquid 4,4'-diphenylmethane diisocyanate composition as claimed in claim 1, 11 or 12.

15. Afoam as claimed in claim 14wherein the compound having active hydrogen atoms is a polyalkylene polyether polyol.

16. Afoam as claimed in claim 14 or 15 prepared in the presence of a surfactant.

17. Afoam as claimed in any of claims 14to 16 prepared in the presence of a urethane catalyst.

18. A foam as claimed in any of claims 14 to 17 prepared in the presence of a chain-extending agent.

19. A foam as claimed in claim 18 wherein the chain-extending agent is 1,4-butanediol.

20. A polyurethane foam as claimed in any of claims 13 to 19 and substantially as described in any of the foregoing Examples XIV to LX.