HK1018469A - Process for preparing a flexible polyurethane foam - Google Patents

Description

Process for preparing flexible polyurethane foams

The present invention relates to a process for preparing a flexible polyurethane foam and a polyisocyanate composition for preparing such a flexible foam.

The preparation of flexible polyurethane foams by reacting organic polyisocyanates with high molecular weight isocyanate-reactive compounds in the presence of blowing agents is well known. More specifically, EP-111121 discloses the preparation of flexible polyurethane foams from polyisocyanate compositions comprising semi-prepolymers. The polyisocyanate composition is prepared by reacting diphenylmethane diisocyanate with a polyol; polymethylene polyphenylene polyisocyanates (polymeric MDI) may also be used. The polyisocyanate is either completely used for preparing the semi-prepolymer or added after the semi-prepolymer is prepared. We have found that the use of polymeric MDI as suggested in EP-111121 does not provide a satisfactory combination of stability and low viscosity, particularly when polyisocyanate compositions having a relatively low NCO value, e.g.as low as 11 to 22% by weight, are used.

In EP-392788 flexible foams are prepared by reacting semi-prepolymers or prepolymers with isocyanate-reactive compositions containing a large amount of water.

In EP-269449, flexible foams are prepared by reacting polyisocyanates, polyols and water under conditions of relatively low NCO index.

Co-pending application PCT/EP95/02068 relates to a process for the preparation of flexible foams using a semi-prepolymer prepared by reacting a portion of polymeric MDI with a polyol and then adding another portion to the reaction product thus obtained. It has surprisingly been found that the polyisocyanate compositions according to the invention are stable, clear, low-viscosity liquids when a part of the polymethylene polyphenylene polyisocyanate (polymeric MDI) is used to prepare the semi-prepolymer and then another part of the polymeric MDI is added to the semi-prepolymer thus formed; as a result, the processability of the composition during use in the preparation of foams is also improved. When the polymeric MDI is used either entirely in the preparation of the semi-prepolymer or is added in its entirety after the semi-prepolymer has been prepared, the stability and/or the viscosity are adversely affected.

It has now been found that polyisocyanate compositions can contain relatively high levels of polymeric MDI, while remaining stable, to produce flexible foams of relatively low density; this lower density is achieved without significant negative impact on other physical properties of the foam.

It has now been found that the addition of a relatively large amount of polymeric MDI to the semi-prepolymer further improves the resilience and foam stability, and that the addition of such polymeric MDI to the semi-prepolymer in combination with the addition of Toluene Diisocyanate (TDI) results in foams having lower density and comfort properties such as resilience, durability and better mechanical strength.

Accordingly, the present invention relates to a process for preparing flexible polyurethane foams by reacting the following reaction systems:

1) organic polyisocyanates and

2) a polyol having an average nominal hydroxyl functionality of 2 to 3 and a number average molecular weight of 1000 to 12000; and optionally with

3) An isocyanate-reactive compound containing at least two isocyanate-reactive hydrogen atoms and having a number average molecular weight of 60 to 999; by using

4) A foaming agent; and optionally

5) A catalyst; and optionally

6) Further auxiliaries and additives known per se, characterized in that,

a) the polyisocyanate is a polyisocyanate composition having an NCO value of 11 to 24, preferably 13 to 22% by weight, which is a blend of:

a 1.50 to 95 parts by weight of an isocyanate-terminated semi-prepolymer having an NCO value of 9 to 20, preferably 11 to 18,% by weight, prepared by reacting: an excess of a polyisocyanate composition comprising 35 to 75% by weight of diphenylmethane diisocyanate and 25 to 65% by weight of polymethylene polyphenylene polyisocyanate, and a polyol having an average nominal hydroxyl functionality of 2 to 3 and a number average molecular weight of 1000 to 12000; and

a 2.26 to 50 parts by weight of polymethylene polyphenylene polyisocyanate; or 2 to 25, preferably 3 to 20 parts by weight of toluene diisocyanate and 2 to 48, preferably 5 to 35 parts by weight of diphenylmethane diisocyanate and/or polymethylene polyphenylene polyisocyanate, the sum of a1 and a2 being 100 parts by weight;

b) 25 to 120 parts by weight, preferably 35 to 100 parts by weight of polyol 2) per 100 parts by weight of organic polyisocyanate;

c) water is used as a foaming agent in an amount of 3 to 15 parts by weight, preferably 5 to 12 parts by weight, per 100 parts by weight of polyol 2); and

d) the reaction is carried out under the condition that the index is 40-130, preferably higher than 70-100.

The invention also relates to a reaction system comprising the above-mentioned components, with the proviso that the polyisocyanate is kept in a container separate from the isocyanate-reactive compound.

The invention further relates to the polyisocyanates mentioned above.

The meanings of the terms used in the description of the invention are as follows:

1) isocyanate index or NCO index or indices:

the ratio of the number of NCO groups present in the formulation divided by the number of hydrogen atoms reactive toward isocyanates, in percent, is:

in other words, the NCO-index expresses the percentage of isocyanate actually used in a formulation which corresponds to the theoretical amount of isocyanate required to react with the isocyanate-reactive hydrogen atoms used in the formulation.

It should be noted that the isocyanate index as used herein is considered from the actual foaming process involving the isocyanate component and the isocyanate-reactive component. Any isocyanate groups consumed in the formation of the semi-prepolymer or other preparation steps for the modified polyisocyanate composition, or any active hydrogen atoms which react with isocyanate to form modified polyols or polyamines, are not considered in the calculation of the isocyanate index. Only the free isocyanate groups present in the actual foaming stage and the free isocyanate-reactive hydrogen atoms (including those present in water) are taken into account in this calculation.

2) The term "isocyanate-reactive hydrogen atoms" as used herein for the calculation of the isocyanate index refers to the total number of hydroxyl and amine hydrogen atoms present in the reactive composition in the form of polyols, polyamines and/or water; that is, for the purpose of calculating the isocyanate index at the actual foaming production stage, one hydroxyl group is considered to contain one active hydrogen and one water molecule is considered to contain two active hydrogen atoms.

3) Reaction system: the sum of all components, wherein the polyisocyanate component is stored in a container separate from the isocyanate-reactive component.

4) The term "polyurethane foam" as used herein generally refers to cellular products obtained, for example, by reacting polyisocyanates with compounds containing isocyanate-reactive hydrogen atoms using blowing agents, and particularly includes cellular products obtained using water as the reactive blowing agent (involving reaction of water with isocyanate groups to form urea linkages and carbon dioxide, thereby producing polyurea-urethane foams).

5) The term "average nominal hydroxyl functionality" as used herein means the number average functionality (number of hydroxyl groups per molecule) of the polyol composition, which is assumed to be the number average functionality (number of active hydrogen atoms per molecule) of the initiator(s) used in their preparation, although in practice this number is often somewhat less due to the presence of certain unsaturated end groups.

6) The "MDI + TDI functionality" is the number average isocyanate functionality of all diphenylmethane diisocyanate, polymethylene polyphenylene polyisocyanate and toluene diisocyanate used in the preparation of the polyisocyanate composition according to the present invention, provided that the NCO groups used in the preparation of the semi-prepolymer are also taken into account in the calculations for determining this functionality.

The diphenylmethane diisocyanate (MDI) used may be selected from pure 4,4'-MDI and isomeric mixtures of 4,4' -MDI, 2, 4'-MDI and less than 10% by weight of 2, 2' -MDI, modified variants thereof comprising carbodiimide, uretonimine, isocyanurate, urethane, allophanate, urea or biuret groups. Most preferred are pure 4,4'-MDI, isomeric mixtures thereof with 2, 4' -MDI and uretonimine and/or carbodiimide modified MDI having an NCO content of at least 25% by weight, urethane modified MDI obtained by reacting excess MDI with a polyol, preferably having a molecular weight of at most 999, and having an NCO content of at least 25% by weight.

Polymethylene polyphenylene polyisocyanates used in the preparation of semi-prepolymer a1) and used as or contained in polyisocyanate a2) are known per se and are polyisocyanates comprising MDI and MDI homologues thereof having an isocyanate functionality of 3 or more. These polyisocyanates, often referred to as "crude MDI" or "polymeric MDI", are prepared by the phosgenation of a mixture of polyamines obtained by the acid condensation of aniline and formaldehyde. The preparation of both polyamine mixtures and polyisocyanate mixtures is well known. The condensation of aniline with formaldehyde in the presence of strong acids such as hydrochloric acid gives a reaction product comprising diaminodiphenylmethane together with polymethylene polyphenylene polyamines of higher functionality whose precise composition varies in accordance with the aniline/formaldehyde ratio in a known manner. Polyisocyanates are prepared by phosgenation of such polyamine mixtures, and varying proportions of diamines, triamines and higher polyamines give rise to corresponding proportions of diisocyanates, triisocyanates and higher polyisocyanates. The relative proportions of diisocyanate, triisocyanate and higher functionality polyisocyanates in such crude or polymeric MDI compositions determine the average functionality of the composition, i.e., the average number of isocyanate groups per molecule. By varying the proportions of the starting materials, it is possible to vary the average functionality of the polyisocyanate composition from slightly above 2 to 3 or even higher. In practice, however, the preferred range of number average isocyanate functionality is from 2.3 to 2.8. The NCO value of the polymeric MDI mentioned is at least 30% by weight. Such compositions comprise from 30 to 65% by weight of diphenylmethane diisocyanate, the remainder being polymethylene polyphenylene polyisocyanate having a functionality of greater than 2 and by-products formed during the preparation of such polyisocyanates by phosgenation. Such products are liquid and therefore convenient to use in accordance with the present invention. The polymethylene polyphenylene polyisocyanate used as polyisocyanate 2) or contained therein may be a mixture of the polymeric or crude MDI and MDI described above, provided that the polyisocyanate having an isocyanate functionality greater than 2 comprises at least 10% by weight of the mixture.

The tolylene diisocyanates used are known per se and can be selected from all isomers and mixtures thereof, in particular from 2, 4-and 2, 6-tolylene diisocyanate and mixtures thereof.

The polyol (polyol 2) having an average nominal hydroxyl functionality of 2 to 3 and a number average molecular weight of 1000 to 12000 and the polyol used in the preparation of semi-prepolymer a1) may be selected from polyester polyols, polyester-amide polyols, polythioether polyols, polycarbonate polyols, polyacetal polyols, polyolefin polyols, polysiloxane polyols and in particular polyether polyols.

Polyether polyols which may be used include products obtained by polymerization of cyclic oxides such as ethylene oxide, 1, 2-propylene oxide, butylene oxide or tetrahydrofuran, if desired in the presence of polyfunctional initiators. Suitable initiator compounds contain a plurality of active hydrogen atoms and include water, butanediol, ethylene glycol, propylene glycol, diethylene glycol, triethylene glycol, dipropylene glycol, ethanolamine, diethanolamine, triethanolamine, toluene diamine, diethyl toluene diamine, cyclohexane dimethanol, glycerol, trimethylolpropane and 1,2, 6-hexanetriol. Mixtures of various initiators and/or cyclic oxides may also be used.

Particularly useful polyether polyols include polyoxypropylene diols and triols, and polyethylene oxide-polyoxypropylene diols and triols obtained by the simultaneous or sequential addition of ethylene oxide or propylene oxide to di-or trifunctional initiators, as fully described in the prior art. There may be mentioned random copolymers having an ethylene oxide content of from 10 to 80%, block copolymers having an ethylene oxide content of up to 50%, based on the total weight of the alkylene oxide units, in particular those in which at least part of the ethylene oxide groups are located at the end of the polymer chain. Mixtures of the diols or triols may be particularly useful. Minor amounts of polyethylene oxide diols or triols may also be used; the amount used is generally less than 20% by weight of the amount of polyol 2).

Polyester polyols which may be used include hydroxyl-terminated reaction products of polyols, such as ethylene glycol, propylene glycol, diethylene glycol, 1, 4-butanediol, neopentyl glycol, 1, 6-hexanediol, cyclohexanedimethanol, glycerol, trimethylolpropane, or polyether polyols or mixtures of such polyols, with polycarboxylic acids, in particular dicarboxylic acids or their ester-forming derivatives; polycarboxylic acids and derivatives thereof are, for example, succinic, glutaric and adipic acids or their dimethyl esters, sebacic acid, phthalic anhydride, tetrachlorophthalic anhydride or dimethyl terephthalate or mixtures thereof. Polyesters obtained by the polymerization of lactones, such as caprolactone, in addition to polyols or hydroxycarboxylic acids, such as hydroxycaproic acid, may also be used.

The polyester-amides may be obtained by adding an amino alcohol, such as ethanolamine, to the polyesterification mixture.

Polythioether polyols which may be used include products obtained by condensation of thioether diols alone or together with other diols, alkylene oxides, dicarboxylic acids, formaldehyde, amino alcohols or aminocarboxylic acids.

Polycarbonate polyols which may be used include products obtained by reacting diols such as 1, 3-propanediol, 1, 4-butanediol, 1, 6-hexanediol, diethylene glycol or tetraethylene glycol with diaryl carbonates such as diphenyl carbonate or with phosgene.

Polyacetal polyols which may be used include products prepared by reacting glycols such as diethylene glycol, triethylene glycol or hexanediol with formaldehyde. Suitable polyacetals can also be prepared by polymerizing cyclic acetals.

Suitable polyolefin polyols include hydroxy-terminated butadiene homo-and copolymers; suitable silicone polyols include polydimethylsiloxane diols and triols.

Other polyols which may be used as polyol 2) and/or in the preparation of semi-prepolymer a1) include dispersions or solutions of polyaddition or polycondensation polymers in polyols of the type mentioned above. Such modified polyols, often referred to as "polymer polyols", have been fully described in the prior art and include: products obtained by the in situ polymerisation of one or more vinyl monomers, such as styrene and/or acrylonitrile, in a polymeric polyol, such as a polyether polyol, or by the in situ reaction of a polyisocyanate with an amino-and/or hydroxy-functional compound, such as triethanolamine, in a polymeric polyol. Polyoxyalkylene polyols in which 5 to 50% by weight of the polymer is dispersed are particularly preferred. Preferably, the polymer dispersed therein has a particle size of less than 50 microns.

The number average molecular weight of the polyol 2) and the polyol used for preparing the semi-prepolymer a1) is preferably 1000 to 8000, most preferably 1500 to 7000, and the hydroxyl value thereof is preferably 15 to 200, most preferably 20 to 100.

Most preferred are polyethylene oxide-polypropylene oxide polyols having a number average molecular weight of 2000 to 7000, an average nominal functionality of 2 to 3 and an ethylene oxide content of 10 to 25% by weight, and preferably containing ethylene oxide groups at the end of the polymer chain.

Over the last few years, several processes have been proposed for the preparation of polyether polyols having a low degree of unsaturation. Due to the advances made in this regard, it has now become possible to use higher molecular weight polyether polyols because many polyols can now be prepared with acceptable levels of unsaturation. According to the invention, polyols having a low degree of unsaturation may also be used. In particular, such high molecular weight polyols having a low degree of unsaturation can be used to prepare flexible foams having high ball rebound values.

The isocyanate-terminated semi-prepolymer a1 is prepared by first mixing diphenylmethane diisocyanate with polymethylene polyphenylene polyisocyanate, and subsequently adding the polyol and allowing the mixture to react. The reaction is carried out at 60-100 ℃ and generally without the use of a catalyst. The relative amounts of polyisocyanate and polyol can be readily calculated by those skilled in the art based on the desired NCO value of the semi-prepolymer, the NCO value of the polyisocyanate mixture used and the OH value of the polyol. After the above reaction is completed, the polyisocyanate a2 may be added in any order and then mixed. The polyisocyanate composition according to the invention has an "MDI + TDI functionality" of from 2.05 to 2.35, preferably from 2.05 to 2.30.

Chain extenders and crosslinkers (isocyanate-reactive compounds 3)) which may optionally be used are selected from amines or polyols containing from 2 to 8, preferably from 2 to 4, amino and/or hydroxyl groups, such as ethanolamine, diethanolamine, triethanolamine, ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, dipropylene glycol, butanediol, glycerol, trimethylolpropane, pentaerythritol, sorbitol, sucrose, polyethylene glycols having a molecular weight of up to 999, toluene diamine, diethyl toluene diamine, cyclohexane diamine, phenylene diamine, diphenylmethane diamine, alkylated diphenylmethane diamines and ethylene diamine.

If used, the chain extenders and crosslinkers are used in amounts of up to 25, preferably up to 10 parts by weight per 100 parts by weight of polyol 2).

Among the auxiliaries and additives which may be used are, in particular, catalysts which promote the formation of urea and urethane, such as tertiary amines and tin compounds, and also surfactants, stabilizers, flame retardants, fillers and antioxidants.

The flexible polyurethane foam is produced by mixing the components 1) to 6) together and then allowing the mixture to foam. Preferably, components 2) to 6) are premixed and subsequently combined with the polyisocyanate. The relative amounts of the polyisocyanates on the one hand and the components 2) to 6) on the other hand are determined according to the desired index and can be readily calculated by the person skilled in the art.

The process can be used to make slabstock or molded flexible foams. The foam plastic has a density of 15-80 kg/cubic meter, and can be used as a buffer material in furniture, an automobile seat cushion and various cushions.

The present invention will now be illustrated by reference to the following examples.

Example 1

A semi-prepolymer was prepared as follows: 1) 9.1 parts by weight of diphenylmethane diisocyanate comprising 85% by weight of 4,4 '-diphenylmethane diisocyanate and 15% by weight of 2, 4' -diphenylmethane diisocyanate, and 5.5 parts by weight of a polymethylene polyphenylene polyisocyanate having an NCO value of 30.7% by weight and a number average isocyanate functionality of 2.7, and about 38% by weight of diisocyanate were mixed together, 2) 19.2 parts by weight of a polyethylene oxide-polypropylene oxide polyol having a nominal functionality of 3, a number average molecular weight of 6000 and an ethylene oxide content of 15% by weight (total endcapping) were added to the mixture, followed by mixing, and finally, 3) the mixture was allowed to react at 85 ℃ for 4 hours.

The NCO value of the semi-prepolymer thus obtained was 12.9% by weight, 11.6 parts by weight of diphenylmethane diisocyanate comprising 80% by weight of 4,4 '-diphenylmethane diisocyanate and 20% by weight of 2, 4' -diphenylmethane diisocyanate, and 6.6 parts by weight of the above polymethylene polyphenylene polyisocyanate were added thereto, and then mixed.

The composition obtained is the isocyanate composition according to the invention; having an NCO value of 19.8% by weight and a viscosity at 25 ℃ of 950 mPas; the composition remained clear and stable (stability was determined by visual inspection, i.e. the composition was judged to be stable when no solid particles and turbidity were observed when visually) for 2 weeks at 0 ℃ and room temperature, with an "MDI + TDI functionality of 2.20.

A flexible foam was prepared in a mold as follows: pouring into the mould a mixture comprising the following ingredients: 52 parts by weight of the above-mentioned isocyanate composition according to the invention and 48.1 parts by weight of an isocyanate-reactive composition (index 74) comprising 38.5 parts by weight of a polyol having a nominal functionality of 3, a number average molecular weight of 4800 and an ethylene oxide content of 15% by weight (total end capping), 2.5 parts by weight of water, 6 parts by weight of a polyethylene oxide polyol having a nominal functionality of 2 and a number average molecular weight of 600, 0.1 part by weight of D33 LV (catalyst from air products), 0.05 part by weight of dimethylaminopropylamine, 0.3 part by weight of triethanolamine, 0.3 part by weight of DC 5043 (surfactant from DOW Corning) and 0.35 part by weight of dimethylethanolamine. The mixture was allowed to react and foam. The foams obtained were soft foams having a free rise density of 32 kg/m (ISO845), a resilience of 58% (JIS B1501), a compression set at dry state/50% humidity of 7/8(ISO1856), a crushing hardness (25%) of 120N (ISO 2439), an elongation of 106% (ISO1798) and a core density of 38 kg/m (ISO 845).

Example 2

The above experiment was repeated with the same components except that 2.5 parts by weight of TDI was added to the polyisocyanate composition prepared in example 1 and an index of 74 was used.

The obtained flexible foam had a free rise density of 30 kg/m (ISO845), a spring back of 60% (JIS B1501), a compression set at dry state/50% humidity of 7/9(ISO 1856), a crushing hardness (25%) of 90N (ISO 2439), an elongation of 120% (ISO1798) and a core density of 38 kg/m (ISO 845).

Claims (9)

1. A process for producing a flexible polyurethane foam by reacting:

1) organic polyisocyanates and

2) a polyol having an average nominal hydroxyl functionality of 2 to 3 and a number average molecular weight of 1000 to 12000; and optionally with

3) An isocyanate-reactive compound containing at least two isocyanate-reactive hydrogen atoms and having a number average molecular weight of 60 to 999; by using

4) A foaming agent; and optionally

5) A catalyst; and optionally

6) Further auxiliaries and additives known per se, characterized in that,

a) the polyisocyanate is a polyisocyanate composition having an NCO value of 11 to 24% by weight, which is a blend of:

a 1.50 to 95 parts by weight of a semi-prepolymer terminated with isocyanate having an NCO value of 9 to 20% by weight, which is prepared by reacting: an excess of a polyisocyanate composition comprising 35 to 75% by weight of diphenylmethane diisocyanate and 25 to 65% by weight of polymethylene polyphenylene polyisocyanate, and a polyol having an average nominal hydroxyl functionality of 2 to 3 and a number average molecular weight of 1000 to 12000; and

a 2.26 to 50 parts by weight of polymethylene polyphenylene polyisocyanate; or 2 to 25 parts by weight of toluene diisocyanate and 2 to 48 parts by weight of diphenylmethane diisocyanate and/or polymethylene polyphenylene polyisocyanate, the sum of a1 and a2 being 100 parts by weight;

b) 25 to 120 parts by weight of polyol 2 per 100 parts by weight of organic polyisocyanate);

c) water is used as a foaming agent, and the amount of the water is 3-15 parts by weight per 100 parts by weight of the polyol 2); and

d) the reaction is carried out under the condition that the index is 40-130.

2. The process according to claim 1, wherein the organic polyisocyanate has an MDI + TDI functionality of from 2.05 to 2.35.

3. A process according to claims 1 and 2, wherein the organic polyisocyanate has an NCO value of 13 to 22% by weight, the semi-prepolymer has an NCO value of 11 to 18% by weight, the amount of polyol 2) is 35 to 100 parts by weight per 100 parts by weight of organic polyisocyanate, the amount of water used is 5 to 12 parts by weight per 100 parts by weight of polyol 2), the index of the system is greater than 70 to 100, and the organic polyisocyanate has an MDI + TDI functionality of 2.05 to 2.30.

4. A reaction system comprising:

1) an organic polyisocyanate;

2) a polyol having an average nominal hydroxyl functionality of 2 to 3 and a number average molecular weight of 1000 to 12000; and optionally

3) An isocyanate-reactive compound containing at least two isocyanate-reactive hydrogen atoms and having a number average molecular weight of 60 to 999;

4) a foaming agent; and optionally

5) A catalyst; and optionally

6) Further auxiliaries and additives known per se, characterized in that,

a) the polyisocyanate is a polyisocyanate composition having an NCO value of 11 to 24% by weight, which is a blend of:

a 1.50 to 95 parts by weight of a semi-prepolymer terminated with isocyanate having an NCO value of 9 to 20% by weight, which is prepared by reacting: an excess of a polyisocyanate composition comprising 35 to 75% by weight of diphenylmethane diisocyanate and 25 to 65% by weight of polymethylene polyphenylene polyisocyanate, and a polyol having an average nominal hydroxyl functionality of 2 to 3 and a number average molecular weight of 1000 to 12000; and

a 2.26 to 50 parts by weight of polymethylene polyphenylene polyisocyanate; or 2 to 25 parts by weight of toluene diisocyanate and 2 to 48 parts by weight of diphenylmethane diisocyanate and/or polymethylene polyphenylene polyisocyanate, the sum of a1 and a2 being 100 parts by weight;

b) 25 to 120 parts by weight of polyol 2 per 100 parts by weight of organic polyisocyanate);

c) water is used as a foaming agent, and the amount of the water is 3-15 parts by weight per 100 parts by weight of the polyol 2); and

d) the polyisocyanate 1) is present in a quantity relative to the other components such that, when mixed together, the index is from 40 to 130; with the proviso that the polyisocyanate is stored in a separate container from the isocyanate-reactive compound.

5. A reaction system according to claim 4, wherein the organic polyisocyanate has an MDI + TDI functionality of from 2.05 to 2.35.

6. A reaction system as claimed in claims 4 and 5, wherein the organic polyisocyanate has an NCO value of 13 to 22% by weight, the semi-prepolymer has an NCO value of 11 to 18% by weight, the amount of polyol 2) is 35 to 100 parts by weight per 100 parts by weight of organic polyisocyanate, the amount of water used is 5 to 12 parts by weight per 100 parts by weight of polyol 2), the index of the system is greater than 70 to 100, and the organic polyisocyanate has an MDI + TDI functionality of 2.05 to 2.30.

7. An organic polyisocyanate composition having an NCO value of 11 to 24% by weight and which is a blend of:

a 1.50 to 95 parts by weight of a semi-prepolymer terminated with isocyanate having an NCO value of 9 to 20% by weight, which is prepared by reacting: an excess of a polyisocyanate composition comprising 35 to 75% by weight of diphenylmethane diisocyanate and 25 to 65% by weight of polymethylene polyphenylene polyisocyanate, and a polyol having an average nominal hydroxyl functionality of 2 to 3 and a number average molecular weight of 1000 to 12000; and

a 2.26 to 50 parts by weight of polymethylene polyphenylene polyisocyanate; or 2 to 25 parts by weight of toluene diisocyanate and 2 to 48 parts by weight of diphenylmethane diisocyanate and/or polymethylene polyphenylene polyisocyanate, the sum of a1 and a2 being 100 parts by weight;

8. the composition according to claim 7, wherein the composition has an MDI + TDI functionality of from 2.05 to 2.35.

9. A composition according to claims 7 and 8, wherein the NCO value of the composition is from 13 to 22% by weight, the NCO value of the semi-prepolymer is from 11 to 18% by weight and the MDI + TDI functionality of the composition is from 2.05 to 2.30.