An isocyanate-terminated prepolymer and flexible polyurethane foam prepared therefrom

DESCRIPTIVE MEMORY

resume

The present invention relates to a process for the preparation of an isocyanate-terminated prepolymer by reacting a polyisocyanate comprising 4,4'-methylene diphenyldiisocyanate and polyoxyalkylene polyol wherein the polyol has a average functionality (average number of functional groups) from 2 to 4, an equivalent hydroxyl weight of 2,200 to 3,500 and contain oxyethylene residues in an amount of 40 to 68 weight percent; and their use in a process for producing flexible polyurethane foam.

This invention relates to an isocyanate terminated prepolymer and a process for its use in the manufacture of articles and polyurethane, particularly flexible polyurethane foam.

Polyurethane foams are generally prepared by mixing under reaction conditions a substance containing isocyanate groups with a substance containing groups containing isocyanate-reactive hydrogen atoms. The isocyanate-reactive hydrogen-containing substance is generally a polyal and may be an amine or an imine terminated polyether polyol, a polyester polyol or especially a conventional polyether polyol. Isocyanate is generally an organic polyisocyanate, especially an aromatic polyisocyanate such as toluene diisocyanate or a (poly) methylene linked polyisocyanate such as methylene diphenyldiisocyanate (MDI).

In preparing flexible open cell polyurethane foam, it is advantageous to use polyisocyanates or polyisocyanate compositions which comprise a significant amount of a diisocyanate, thereby limiting crosslinking in the polymeric structure and providing desirable physical properties such as elongation. Especially desirable due to the physical properties of the resulting foam is the use of 4,4'-methylene diphenyldiisocyanate in the production of polyurethane foam.

In the case of methylene-linked diisocyanates such as methylene diphenyldiisocyanate (MDI) and especially isomer-4,4 ', processing problems may be encountered in the preparation of the foam. The 4,4-isomer is a solid at room temperature and thus, when used in a relatively pure state, requires foam preparation to be performed at elevated temperatures. In order to overcome this problem, the art of operating with mixtures of poly and / or diisocyanates comprising 4,4'-DMI is known in the art, see for example, U.S. Patent. 4,256,849. An alternative means of overcoming this processing drawback is to prepare an isocyanate-terminated prepolymer from 4,4'-DMI thereby reducing its tendency to solidify at ambient or lower temperatures.

Isocyanate terminated prepolymers are generally prepared by reacting a large molar excess of an appropriate isocyanate with an active hydrogen containing compound. Such active hydrogen containing compounds are usually difunctional or trifunctional compounds of varying molecular weights, and especially polyether polyols. Selection of the active hydrogen containing compound is made by taking into consideration the desired physical properties for the resulting foam. For example, in preparing flexible polyurethane foams known as cold curing foams it is desirable to use a polyether polyol comprising oxyethylene residues. The foams are identified as cold cure foams because when removed from the mold the need to heat the foam to accelerate polymer cure is no longer required. Such cold curing foam can generally be used for a relatively short period of time without any significant risk of deformation.

EP 22,617 discloses a process for the production of cold curable flexible polyurethane foams by reacting an organic polyisocyanate composition prepared by reacting methylene diphenyldiisocyanate with a polyoxyalkylene diol or triol containing from at least 50 percent by weight, distributed at random, of oxyethylene residues and having an equivalent hydroxyl weight of 1,000 to 2,000. While such a prepolymer essentially overcomes the processing problems associated with 4,4'-DMI and provides a foam with acceptable physical properties, it is desirable to further improve the foam properties to meet the increasingly demanding industrial needs. In particular, it is desired to improve the elongation properties while retaining all the acceptable physical properties such as the extent of cell opening within the foam.

For this purpose, the preparation of isocyanate terminated prepolymers from alternative polyoxyalkylene polyols and their subsequent use in the production of polyurethane foam has been investigated.

Surprisingly, it has now been found that in order to obtain improved foam properties, particularly elongation, it is necessary to select the polyoxyalkylene polyol taking into account both its hydroxyl equivalent weight and its oxyethylene content.

In a first aspect, this invention is an isocyanate-terminated prepolymer prepared by reaction of a polyisocyanate comprising 4,4'-methylene diphenyldiisocyanate in an amount of at least 40 weight percent with a polyoxyalkylene polyol, wherein the polyol has a functional 2 to 4 and an equivalent hydroxyl weight ranging from 2,200 to 3,500 and contain oxyethylene residues in an amount ranging from 40 to 68 weight percent.

In a second aspect, this invention is a process for preparing a polyurethane foam by intimately blending reaction condition components comprising

<img file="PT99472B_D0001.tif" />

(a) polyisocyanate composition containing an isocyanate-terminated prepolymer prepared by reacting a polyisocyanate comprising 4,4'-methylene diphenyldiisocyanate in an amount of at least 40 weight percent with a polyoxyalkylene polyol, wherein the polyol has a functionality 2 to 4 and a hydroxyl equivalent weight of 2,200 to 3,500 and contain oxyethylene residues in an amount ranging from 40 to 68 weight percent; and (b) an active hydrogen-containing composition comprising (1) a high equivalent weight of isocyanate-reactive material, (2) a blowing agent, and (3) a catalyst for promoting urethane group formation wherein component (a) ) is present in an amount providing from 0.6 to 1.3 isocyanto groups per active hydrogen atom present in component (b).

In still a third aspect, this invention is an open cell polyurethane foam having an average density of 10 to 250 kilograms per cubic meter prepared according to the above process.

The isocyanate-terminated prepolymer of this invention may be characterized as having an isocyanate content ranging from 5 to 31 weight percent, preferably 10, more preferably 15, most preferably 25, and most preferably 28 percent, plus up to 31 percent by weight.

The isocyanate-terminated prepolymer of this invention is prepared by contact under reaction conditions of a polyisocyanate with a polyoxyalkylene polyol. The polyoxyalkylene polyol used in preparing the prepolymer has an average functionality of 2 to 4, preferably 2 to 3 and most preferably has an average functionality of 3.

The term "average functionality" means the number of isocyanate-reactive sites per molecule for polyols, hydroxyl groups, capable of reacting with isocyanate groups.

Polyoxyalkylene polyol is further characterized in that it has an equivalent hydroxyl weight ranging from 2,200 to 3,500 and contains oxyethylene residues ranging from 40 to 68 weight percent.

Preferably the hydroxyl equivalent weight of the polyol is 2,300 and more preferably 2,400 and up to 3,200, more preferably up to 2,800.

The oxyethylene residue content of the polyol is preferably 55 and more preferably 58 and most preferably 60 to 65, more preferably up to 64 weight percent. An oxyethylene content of the polyol beyond these limits of variation when optionally incorporated into a polyurethane foam may not provide the desired characteristic properties of the resulting foam.

The distribution of oxyethylene residues within the polyol may consist of a block or series of oxyethylene blocks either internally or terminally within the polyoxyalkylene chain, or alternately at random along the entire polyoxyalkylene chain of the polyol, or your combinations.

The remainder of the polyoxyalkylene chain when not comprised of oxyethylene residues comprises oxypropylene, oxybutylene residues, or mixtures thereof, more especially oxypropylene residues. Such residues are obtained from the reaction of alkylene oxides, propylene oxide and butylene oxide respectively. A convenient and preferred source of oxyethylene residues is ethylene oxide. Residues from other oxides may also be present.

Methods for preparing the polyoxyalkylene polyols described above are well known in the art and generally comprise the base catalyzed addition of alkylene oxide (s), especially ethylene oxide and propylene oxide, to a di-, tri- , or tetrafunctional at elevated temperatures and pressures.

Suitable initiators include difunctional compounds such as water, ethylene glycol, propylene glycol and its higher oxyalkylene adducts, bis (4-hydroxyphenyl) -2,2-propane and phthalic anhydride glycol adducts; trifunctional compounds such as trimethylolpropane, hexanetriol and especially glycerine; and tetrafunctional compounds such as ethylenediamine and pentaerythritol.

&gt;

Preferred initiators due to rapid availability and convenient processing are water, ethylene glycol, propylene glycol, glycerine and their oxyalkylene adducts, with glycerine being especially preferred.

The polyisocyanate used to prepare the prepolymer consists essentially of a methylene diphenyldiisocyanate or a polymethylene polyphenyl polyisocyanate or mixtures thereof, wherein the polyisocyanate comprises an amount of at least 40 percent by weight of the 4,4'-methylene diphenyldiisocyanate (MDI) isomer . Preferably the polyisocyanate comprises 45, most preferably 52, and most preferably 60 to 98, most preferably up to 90 weight percent 4,4'-DMI.

When not comprised of 4,4'-MDI, the remainder of the pilisicianate used in the reaction to prepare the prepolymer may comprise 2,4'-MDI, 2,2'-MDI, polymethylene polyphenyl polyisocyanates or mixtures thereof. When present, advantageously 2,4'-MDI constitutes no more than 50 weight percent of the isocyanate used to prepare the prepolymer and polymethylene polyphenyl polyisocyanate no more than 50 weight percent.

In a preferred embodiment of this invention, the polyoxyalkylene polyol, preferably a triol, is reacted with a polyisocyanate consisting essentially of 4,4'-MDI, 2,4'-MDI and polymethylene polyphenyl polyisocyanate wherein the weight percent ratio between 4,4'-MDI and 2,4'-MDI based on the total weight of polyisocyanate present ranges from 40:40 to 90: 2, preferably from 50:30 to 75:10, more preferably from 50: 25 and 65; 12 percent, the remainder being polyphenyl polymethylene polyisocyanate.

I

Exemplary of such a preferred polyisocyanate for use in the prepolymer preparation is that obtained by mixing 4,4'-MDI and 2,4'-MDI in a 70:30 weight ratio with a methylene diphenyldiisocyanate composition such as, for example, example,

TM

VORANATE M220 supplied by Dow Chemical Company, and containing polymethylene polyphenyl polyisocyanate (approximately 57.5 weight percent), 4,4'-MDI (40 weight percent) and 2,4'-MDI (2.5 weight percent). cent by weight). The mix combination 70:30, TM

4,4 '- / 2,4'-MDI with VORONATE M220 in a ratio of for example

60:40 by weight provides a polyisocyanate suitable for use in prepolymer production consisting essentially of 4,4'-MDI (58 percent), 2,4 '(19 percent) and polymethylene polyphenyl polyisocyanate (23 percent). Such a prepolymer obtained from the preferred polyisocyanate is also suitable for immediate use in the preparation of a flexible polyurethane foam without, for example, any subsequent mixing with other polyisocyanates.

In a less preferred embodiment, the polyoxyalkylene polyol may be reacted with a polyisocyanate consisting essentially of 4,4'-MDI and 2,4'-MDI to provide a prepolymer which if desired may be mixed with other isocyanates including diphenyldiisocyanate. of raw methylene.

In preparing the prepolymer the relative amount of polyoxyalkylene polyol to polyisocyanate is such that it provides a resulting prepolymer having the above isocyanate content. In preparing the prepolymer the polyoxyalkylene polyol may optionally also be used in combination with other isocyanate-reactive substances conventionally used in the preparation of isocyanate-terminated prepolymers. When used in combination with other conventional isocyanate-reactive substances the polyoxyalkylene polyol described above is present in an amount ranging from 1 to 99 percent by weight of the entire isocyanate-reactive mixture used to prepare the isocyanate-terminated prepolymer. Preferably the polyoxyalkylene polyol is present in an amount of at least 10 and more preferably at least 20 weight percent. Such conventional substances include for example ethylene glycol, porpylene glycol, and their higher molecular weight adducts such as those obtained by reaction with C oxide._O to C. alkylene, and which are not within the scope of the foregoing description of the polyoxyalkylene polyol. Exemplary of high molecular weight adducts are polyoxypropylene glycols having a molecular weight ranging from 134 to 2,000 such as, for example, those designated as VORANOL * P400, VORANOL * P1010 and VORANOL * P2000 all provided by The Dow Chemical Company. Other conventional isocyanate-reactive substances also include primer oxyalkylene adducts such as those indicated for the polyoxyalkylene glycol described above.

The prepolymer may be prepared using conditions that have been described in prior art for such preparations. Typically, the prepolymer may be prepared by controlled addition of the polyol to the isocyanate while continuously mixing. The rate of addition of the polyol to the isocyanate is advantageously such that, if necessary with heating or cooling, the reaction temperature is maintained at 45 ° C to 90 ° C, preferably 60 ° C to 80 ° C.

In the second aspect of this invention there is provided a process for the preparation of polyurethane foam by intimately mixing under reaction conditions comprising&gt;

(a) the prepolymer as described above optionally as part of the polyisocyanate composition; and (b) an active hydrogen-containing composition comprising (1) a high equivalent weight isocyanate reactive material, (2) a blowing agent, and (3) a catalyst for promoting the formation of urethane groups wherein component (a) ) is present in an amount providing from 0.111 isocyanate groups per active hydrogen atom present in component (b).

The equivalent high-weight isocyanate-reactive material present in component (b) is a compound containing from 2 to 8, preferably from 2 to 4, isocyanate-reactive hydrogen atoms per molecule and having an average equivalent weight ranging from 500 and 3,000 and preferably between 800 and 2,500. Typical materials of equivalent high weight isocyanate reactive include polyamines, polyester polyols and especially polyether polyols. The polyether polyol may be a polyoxypropylene polyol or a poly (oxypropylene oxyethylene) polyol or a mixture thereof. Such polyether polyols are well known in the art and many are commercially available. Poly (oxypropylene oxyethylene) polyol includes oxyethylene coated with polyoxypropylene polyols and other randomized or block products obtained by reacting ethylene and propylene oxide with active hydrogen containing initiators.

Appropriately high equivalent weight isocyanate reactive material for use in the preparation of polyurethane foams is comprised of polyether polyols, generally diols and triols having an average hydroxyl equivalent weight ranging from 500 to 3,000 and preferably from 800 to 2,5000.

Examples of such polyether polyols are those sold by TM

The Dow Chemical Company under the trademark VORANOL e. TM TM TM include, for example VORANOL 4711, VORANOL 6001, VORANOL

TM. . . . . .

3322 and VORANOL 1421. Other suitable polyols include PHD, PIPA or SAN type copolymer polyols such as, for example,

VORANOLTM

CP-8020.

The blowing agent contained in component (b) is present in an amount providing the resulting polyurethane foam with a total density ranging from 10 to 250 kilograms per cubic meter, preferably 15, more preferably 25, and with more preferably 30 and preferably up to 100, more preferably up to 80 kilograms per cubic meter.

In a preferred embodiment of this invention the blowing agent comprises water. The water reacts with isocyanate moieties leading to carbon dioxide generation, the carbon dioxide source thus generated functioning as a blowing agent providing a cellular structure to the polyurethane.

Typically, the amount of water present providing sufficient blowing capacity to give foams of the desired densities ranges from 1.0 to 7 parts by weight per 100 parts by weight of component (b), based on total weight. of all constituents present in component (b). Advantageously, such amounts of water are preferably 2.5, more preferably 3.5 to 6, more preferably up to 5.5 parts by weight.

The amount of blowing agent required to prepare foams of the desired density may be provided entirely by the use of water. However, in addition to water, small amounts of physical blowing agents may be present when necessary to achieve the desired density. Exemplary of such physical blowing agents which may be present include, for example, methylene chloride, trichlorofluoromethane, didlorotrifluoromethane, chlorodifluoroethane;

hydrocarbons such as pentane and hexane; and entrained gases such as nitrogen and carbon dioxide from the air.

Suitable catalysts that may be used in the process of this invention for promoting urethane grouping include tertiary amines and organometallic compounds especially tin compounds. Exemplary of suitable catalysts are tertiary amine compounds including N, N-dimethylcyclohexylamine, N, N-dimethylbenzylamine, N, β-dimethanolamine, bis (dimethylaminoethyl) ether, 1,4-diazobicyclo [2.2.2] octane; are tin compounds including stannous octoate and dibutyltin dilaurate. Combinations of amine and / or tin compounds as catalysts may be advantageously used in the process.

In the process of this invention component (a) is present in an amount sufficient to provide from 0.6 to 1.3 isocyanate groups per isocyanate-reactive hydrogen atom present in both components (b). Preferably, the amount of component (a) is such that it provides from 0.8 to 1.15, more preferably up to 1.05 isocyanate groups per active hydrogen atom. The amount of isocyanate present may also be expressed in terms of an isocyanate reaction index. In this case, an isocyanate reaction index of 100 corresponds to a ratio of 1 between the isocyanate group and the isocyanate-reactive hydrogen atom.

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Other conventional components which optionally, but advantageously are present in the process include surfactants such as, for example, siloxane-oxyalkylene copolymers such as products sold under the trademark Tegostab for Th. Goldschmidt including B-4113 and B-4690; chain extenders such as, for example, ethylene glycol, 1,4-butanediol, diethanolamine, diisopropanolamine and polyamine; fillers; fire retardants, for example melamine; pigments and dyes.

The foaming reaction components may be mixed together in any convenient manner, for example using any mixing equipment described in the prior art for this purpose. If desired non-reactive components may be premixed in order to reduce the number of component streams that require intimate mixing with each other. It is generally advantageous to use a two-stream system in which one stream normally comprises prepolymer and optionally any additional polyisocyanate, while the second stream typically comprises all other components of the reaction mixture and which essentially do not interact prior to mixing with the isocyanate stream. .

The process of the invention and the isocyanate terminated prepolymer are particularly useful in the preparation of polyurethane foams especially cold curing foams. These foams are suitable for many fields of application including padding, sound and vibration damping and padding.

The use of prepolymers in this invention in the preparation of polyurethane foams provides foams having a better degree of elongation compared to foams obtained from prepolymers known in the reaction art.

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The invention is illustrated by the following Examples in which all parts and percentages are by weight unless otherwise indicated.

The prepolymers are prepared according to the process described below and evaluated by preparing from them flexible polyurethane foams using the polyol formulation indicated below. Polyurethane foam is prepared by intimately mixing the prepolymer isocyanate with the polyol formulation using a high pressure foam production machine.

Polyol Formulation

100 parts ι

polyoxypropylene oxyethylene (15% term) - glycerine-initiated polyol; molecular weight 6000

3.75 pieces

0.8 part

0.15 part triethylenediamine water (33% in dipropylene glycol)

NIAX-A1 Catalyst Converter provided by Union Carbide Co.

0.8 part

Surfactant, Tegostab B4113 provided by Th Goldschmidt Ag.

The physical properties of the resulting molded foams obtained by mixing the reaction components under high pressure conditions and introducing the reaction mass into a mold with thermostat turned to 50 ° C are reported. Test procedures used to measure physical properties are referred to tensile strength and elongation - DIN 53571; compression load strain (CLD) - DIN 53577; and for Deflection with Dummy Load (ILD) - DIN 53576.

Prepolymer 1

An isocyanate-terminated prepolymer having an isocyanate content of 29 weight percent is prepared by reacting 11.1 parts of polyoxyethylene-oxypropylene triol (hydroxyl equivalent weight 2463; 61 percent randomly distributed oxyethylene content) with 100 parts of a mixture of isocyanates consisting of 4,4'-MDI (58 percent), 2,4'-MDI (19 percent) and polymethylene polyphenyl polyisocyanate (23 percent).

The polyol is added to a mixture of isocyanates while mixing continuously at a rate such that it maintains a reaction temperature between 65 ° C and 70 ° C. When the total amount of polyol is added the resulting reaction mixture is stirred at the same temperature for an additional hour to ensure completion of the reaction.

The properties of the foams obtained by reacting the prepolymer thus prepared with the identified polyol formulation are set forth below.

η

<td>index Reaction Isocyanate</td><td>Foam 1 90</td><td>Foam 2 100</td><td>Foam 3 110</td>

<td>Density rising freely (kg / m³)</td><td> 36</td><td> 36</td><td> /</td>

<td>Density shaped (kg / m³)</td><td> 44,4</td><td> 44,9</td><td> 45,2</td>

<td>Stretching (%)</td><td> 143</td><td> 133</td><td> 119</td>

<td>Resistance Tensile (kPa)</td><td> 117</td><td> 138</td><td> 181</td>

<td>CLD (kPa)</td><td> 3,56</td><td> 4,44</td><td> 6,43</td>

<td>ILD (N)</td><td> 150</td><td> 195</td><td> 260</td>

&gt;

Prepolymer 2

An isocyanate-terminated prepolymer having an isocyanate content of 29 percent by weight is prepared according to the same process as that of Prepolymer 1 by reacting 11.0 parts of a polyoxyethylene oxypropylene triol (hydroxyl equivalent weight 2270; of 59 percent oxyethylene at random) with 100 parts of an isocyanate mixture consisting of 4,4'-MDI (58 percent), 2,4'-MDI (19 percent) and polymethylene polyphenyl polyisocyanate (23 Percent).

The properties of the foams obtained by reacting the prepolymer thus obtained with the identified polyol formulation are set forth below.

index

Reaction

Isocyanate

<td>Foam 4</td><td>Foam 5</td><td>Foam 6</td>

<td> 90</td><td> 100</td><td> 110</td>

Molded Density (kg / m³)

Stretching (")

44,7 44,4 44,5

130 125 118

Resistance

Tensil (kPa)

134 152 180

CLD (kPa)

4,34 5,41 6,44

Comparative Prepolymer A

A comparative prepolymer having an isocyanate content of 29 percent is prepared by the same process as that used for Prepolymer 1 by reacting 10.8 parts of a polyoxyethylene oxypropylene triol (hydroxyl equivalent weight 1670; oxyethylene content 72). randomly distributed) with 100 parts of a mixture of isocyanates consisting of 4,4'-MDI (58 percent), 2,4'-MDI (19 percent) and polymethylene polyphenyl polyisocyanate (23 percent) .

Foam A *

Foam B *

Foam C * Index

Reaction

Isocyanate

100

110

Freely rising density (kg / m³)

Molded Density (kg / m³)

Stretching (%)

Resistance

Tensil (kPa)

CLD (kPa)

ILD (N) /

45,7 45,6

120 115

122 140

3,81 4,77

150 190

45,5

159

5,89

240 * not an example of this invention

Comparative Prepolymer B

A comparative prepolymer having an isocyanate content of 29 weight percent is prepared by the same procedure as that used for Prepolymer 1 by reacting 11.1 parts of a polyoxyethylene oxypropylene triol (hydroxyl equivalent weight 2670; of randomly distributed 70 percent oxyethylene with 100 parts of a mixture of 4,4'-MDI isocyanates (58 percent), 2,4'-MDI (19 percent) and polymethylene polyphenyl polyisocyanate (23 percent) ).

Foam D *

Foam E * index

Reaction

Isocyanate

100

Density

47,8

46.8 molded

<img file="PT99472B_D0002.tif" />

Stretching (%)

117

103

Resistance

Tensil (kPa)

118

143

CLD (kPa)

4,12

5.08 * Not an example of this invention

Foams 1 to 6 exhibit the increased elongation performance that can be obtained with the prepolymers of this invention compared to comparative prepolymers.

Prepolymer 3

An isocyanate-terminated prepolymer having an isocyanate content of 29 weight percent is prepared according to the procedure indicated for Prepolymer 1, this example only, 11.1 parts of a polyoxyethylene oxyalkylene triol (equivalent weight hydroxy content 2635; 61 percent oxyethylene content equally distributed as an inner oxyethylene block and terminal oxyethylene block) is reacted with the same mixture of polysocyanates.

Comparative Prepolymer C

A comparative prepolymer having an isocyanate content of 29 weight percent is prepared by the same process as that used for Prepolymer 1 by reacting 10.9 parts of a polyoxyethylene oxypropylene triol (hydroxyl equivalent weight 1954; oxyethylene content 64 percent, randomly distributed) with 100 parts of an isocyanate mixture consisting of 4,4'-MDI (58 percent), 2,4'-MDI (19 percent) and polymethylene polyphenyl polyisocyanate (23 percent) ).

The following table shows the physical properties of foams obtained by reacting prepolymers 1, 3 and C with the polyol formulation presented hereinbefore. The foams are prepared in this case using a low pressure mixing / dispersing unit.

The data obtained clearly indicate the advantageous stretching performance that can be obtained with the prepolymers of this invention.

<td rowspan="2">Prepolymer index Reaction Isocyanate</td><td colspan="2"> 1</td><td colspan="5"> 3</td><td colspan="2">ç*</td>

<td> 90</td><td> 100</td><td> 110</td><td> 90</td><td> 100</td><td> 110</td><td> 90</td><td> 100</td><td> 110</td>

<td colspan="3">Density 36 37 rising freely (kg / m³)</td><td> 36</td><td> 36</td><td> 37</td><td> 36</td><td> 37</td><td> 37</td><td> 39</td>

<td>Density shaped (kg / m³)</td><td> 48</td><td> 48</td><td> 48</td><td> 47</td><td> 49</td><td> 48</td><td> 49</td><td> 49</td><td> 49</td>

<td>Stretching (%)</td><td> 138</td><td> 145</td><td> 118</td><td> 147</td><td> 142</td><td> 132</td><td> 126</td><td> 119</td><td> 108</td>

<td>Resistance</td><td> 130</td><td> 178</td><td> 200</td><td> 150</td><td> 188</td><td> 206</td><td> 146</td><td> 174</td><td> 227</td>

Tensil (kPa) * Not an example of this invention

Prepolymer 4

An isocyanate terminated prepolymer having an isocyanate content of 27.5 weight percent is prepared according to the general procedure as set forth for Prepolymer 1. In this example 11.1 parts of a polyol blend comprising 24 weight percent of a polyoxyethylene oxyalkylene triol (hydroxyl equivalent weight: 61% randomly distributed oxyethylene content) and 76 weight percent of a polyoxy glycol propylene having a molecular weight of 1,000, are reacted with 89 parts of a mixture of polysocyanates. The polyisocyanate mixture consists of 4,4'-MDI (61%), 2,4'-MDI (9%) and polymethylene polyphenyl polyisocyanate (30%).