EP0607312A1

EP0607312A1 - Stable melamine dispersions in polyether-polyols and polyurethane foam therefrom

Info

Publication number: EP0607312A1
Application number: EP92921915A
Authority: EP
Inventors: Olga Milovanovic; Rajinder Sood
Original assignee: Dow Chemical Co
Current assignee: Dow Chemical Co
Priority date: 1991-10-09
Filing date: 1992-09-30
Publication date: 1994-07-27
Also published as: CA2119570A1; WO1993007193A1

Abstract

Une invention a trait à la préparation de dispersions stables de mélamine dans des alcools polyvalents polymères. Des dispersions stables de particules de mélamine, présentant une granulométrie moyenne de 10 à 125 microns, en phase continue et comprenant de préférence un alcool polyvalent polymère de type acrylonitrile de styrène (SAN) et un solide de particules inorganiques doté d'une taille moyenne de particules de 5 à 30 nanomètres constituent cette invention. Ces dispersions stables de mélamine sont utiles pour la fabrication de mousse de polyuréthane souple offrant des caractéristiques de combustion modifiées.One invention relates to the preparation of stable dispersions of melamine in polymeric polyvalent alcohols. Stable dispersions of melamine particles, having an average particle size of 10 to 125 microns, in continuous phase and preferably comprising a polyvalent alcohol polymer of styrene acrylonitrile (SAN) type and a solid of inorganic particles having an average size of particles of 5 to 30 nanometers constitute this invention. These stable melamine dispersions are useful in the manufacture of flexible polyurethane foam with modified combustion characteristics.

Description

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STABLE MELAMINE DISPERSIONS IN POLYMER POLYOLS AND POLYURETHANE FOAM

THEREFROM

This invention relates to a stable dispersion of melamine in a polymer polyol an

5 the preparation of a polyurethane foam therefrom.

Polyurethane, by nature of being an organic polymer when subjected to sufficient heat in an oxygen- containing environment will burn. For many applications it is desirable to reduce or retard the burning characteristics of such polymer by incorporating a flame retardant. Commonly employed flame retardants for this purpose include phosphorus

10 and/or halogen-containing compounds such as, for example, tris(chloroethyl)phosphonate (TCEP) and dimethoxymethyl phosphonate (DMMP). As an alternative to halogen and/or phosphorus-containing flame retardants, certain nitrogen-containing compounds, notably melamine, may be used to prepare combustion-modified polyurethanes.

U.S. Patents 4,221 ,875; 4,258,141 and 4,826,844 describe the incorporation of

15 melamine into foams for imparting resistant to smoldering combustion and flaming combustion. U.S. Patent 4,892,893 discloses use of melamine, present in an amount of from 5 to 25 percent based on total weight of the flexible foam, having a mean particle size of from 25 to 500 microns, and to impart resistance to cigarette smoldering ignition of flexible polyurethane foam. According to these patents, the melamine is incorporated into the polyol

20 immediately prior to the manufacture of the foam. If not used almost immediately the melamine has a tendency to sediment out giving rise to processing problems. Melamine deposits accumulated via such sedimentation are frequently of a solid cement like nature and generally cannot be redispersed.

U.S. Patent 4,293,697 discloses a stabilized dispersion of melamine in a polyether

25 polyol and its use in preparing a flexible polyurethane foam. At least 90 percent of the melamine particles have a mean size of less than 10 microns and are stabilized as a dispersion in the polyol by the presence of silicic acids and silicates, salts of perfluorinated alkyl carboxylic acids, salts of alkyl sulfonic acids and perfluorinated alkyl sulfonic acids, polyperfluorate polyols and salts of aliphatic alcohol sulphates. Small melamine particle size is not favorable for use where a high loading of melamine in the foam is required to provide the desirable degree of flame retardation. High loadings of small particle sized melamine frequently result in poor quality or unstable foams.

U.S. Patent 4,644,015 discloses stabilized dispersions of melamine in standard polyetherpolyolsfor use in preparing flexible polyurethane foam. The melamine particles, having a mesh size of greater than 325 that is less than 45 microns, being stabilized by the presence of an amine compound consisting of diethanolamine, ethanolamϊne ortrihexylamine. The presence of such amine compound in the amounts required to provide a stabilized dispersion is not desirable when preparing polyurethane foam especially in the presence of water. Such amine compounds possess catalytic properties leading to system reactivities which are too quick for many commercial foam producers.

The furniture industry, for example, desires foams exhibiting enhanced load bearing properties. A convenient means of achieving the desirable load bearing characteristics of flexible foam is to use a filler or polymer polyol in its preparation. Polymer polyols are distinguishable from conventional polyether polyolsjπ that they have suspended therein a discrete organic polymer. Illustrative of polymer polyol types are those wherein the suspended organic polymer is a styrene:acrylonitrile (SAN) copolymer, a polyurea adduct(PHD), a polyisocyanate-potyamine adduct (PIPA) or an epoxy resin. In patent publication GB 2,163,762, dispersions of melamine in polyhydrazodicarbonamide and/or polyurea polymer polyols are disclosed. The publication is silent with respect to melamine particle size and storage stability of such dispersions. The preparation of flexible polyurethane foam from styrene:acrylonitrile (SAN) copolymer polyol-melamine dispersions is disclosed in U.S. Patent 4,745,133 wherein such dispersions are disclosed as containing uncrushed melamine and characterized by the absence of a stabilizing agent. The immediate processing of a polyol-containing melamine dispersed therein is not always possible and therefore it would be desirable to provide melamine/polyol formulations which are stable dispersions. For the purpose of using such dispersions in the preparation of flexible polyurethane foam having attractive physical characteristic including loading bearing properties it would be desirable to provide for a stable dispersion of melamine in a polymer polyol.

In a first aspect, this invention is a stable dispersion which comprises:

(a) a polyol or mixture thereof containing from 4 to 50 weight percent of a polymer solid;

(b) from 20 to 60 percent based on the total weight of (a) and (b), of melamine which has a mean particle size of from at least 10 to 125 microns; and

(c) from 0.2 to 5 percent, based on the total weight of (a) and (b), of an inorganic particulate solid which has an average particle size of from 5 to 30 nanometers. In a second aspect, this invention is a process for preparing a stable dispersion as described in the first aspect.

In a third aspect, this invention is a polyurethane polymer obtainable by contacting under reaction conditions an organic isocyanate with an isocyanate reactive composition which contains a stable dispersion as claimed in Claim 1 wherein the organic polyisocyanate is present in an amount to provide from 0.85 to 1.25 isocyanate groups per isocyanate reactive hydrogen atom present characterized in that the polymer has dispersed therein: from 2 to 30 percent based on total weight of polymer of melamine that has a mean particle size of from at least 10 to 125 microns; from 0.1 to 2.5 percent based on total weight of polymer of an inorganic particulate solid which has an average particle size of from 5 to 30 nanometers; and from 2 to 25 weight percent of a polymer solid.

Surprisingly, such above-described melamine polymer polyol dispersions are stable. Additionally, such stabilized dispersions can be conveniently processed to provide flexible polyurethane foam exhibiting desirable combustion-modified characteristics and overall commercially attractive physical properties.

The stable dispersion of this invention has a continuous phase comprising a polyol having suspended therein an organic polymer solid and having further dispersed therein melamine particles and an inorganic particulate solid (sometimes referred to hereinafter as IPS). By "stable" it is understood that when the dispersion is stored at room temperature, ^' without agitation, the melamine essentially remains dispersed in the continuous phase with little or no sedimentation being observed. If sedimentation should occur the dispersion of this invention may be characterized in that only a minimum of effort is required to redisperse the melamine due to the presence of the IPS. Generally little or no melamine sedimentation will occur within the first 2 weeks after preparation of the dispersion. Depending on the amount of IPS present, frequently little or no melamine sedimentation may occur in the first 12 or even first 20 weeks after preparation of the dispersion.

The continuous phase of the stable dispersion comprises a polyether polyol or mixture of polyether polyols containing from 4 to 50 weight percent of a polymer solid. Preferably such polyol contains the polymer solid in an amount of from-4 to 30, more preferably from 5 to 20 percent. Polyol containing a higher solids content than this will generally be highly viscous and difficult to manage; a lower solids content normally will not provide for any noticeable effect when incorporated into a polyurethane polymer. A polyether polyol containing such solid is generally referred to as a "polymer polyol". To provide for a continuous phase having such a polymer solids content as mentioned hereinabove, advantageously the continuous phase comprises such a polymer polyol in an amount of at least 50, preferably at least 60, and more preferably at least 75 weight percent of the continuous phase. When the continuous phase is not constituted in its entirety by the polymer polyol the remaining percentage may be constituted by other conventional polyoxyalkylene polyols used in the preparation of polyurethane polymers. Suitable polyols containing an organic polymer solid for use in this present invention include SAN-, PHD- and PIPA-type polymer polyols. Processes for obtaining suitable PHD- and PIPA-type polymer polyols are disclosed in U.S. Patents 4,374,209; 3,325,421; 4,042,537 and 4,093,567. Processes for obtaining suitable SAN copolymer polyols for use in this present invention are described in U.S. Patents 3,385,351 ; 3,304,273; 3,523,093 and 3,110,695.

When using the stable dispersion for the preparation of flexible polyurethane foams it is advantageous if the polyol of the continuous phase has an average of from 2 to 4 isocyanate-reactive groups per molecule; an average hydroxyl equivalent weight of from 750 to 3000, preferably at least 1000, more preferably at least 1200; and preferablyat most 2500, more preferably at most 2000. Additionally it is advantageous forthe polyol to have a primary hydroxyl content of from at I east 10, preferably from at least 30, and more preferably from at least 50 percent of its total hydroxyl content. Use of a polyol having such characteristics generally provides for a flexible polyurethane foam with desirable physical properties. In a preferred embodiment of this invention the continuous phase comprises a SAN polymer polyol, and more preferably consists essentially of a SAN polymer polyol. Exemplary of suitable commercially available SAN polymer polyols include those sold by The Dow Chemical Company and include the products designated as VORANOL^™ CP-8020, VORANOL" CP-8010, VORANOL" CP-8030,and products designated as VORALUX^™ in conjunction with the designation codes H N200 through to H N206.

The melamine dispersed within the continuous phase is characterized in that it has a mean particle size of from 10to 125 microns. The melamine particle size is preferably at least 25, more preferably at least 40 and most preferably at least 50 microns, and preferably at most 100, more preferably at most 90 microns. Melamine having such a particle size is readily available commercially and may be obtained and used directly without the need for particle size reduction through milling, grinding or such like techniques. Use of melamine having a smaller particle size than defined provides mixtures whose viscosity is undesirably high for convenient processing and preparation of polyurethane foam. Use of melamine having an average particle size significantly greaterthan defined above is not desirable as when present in a foaming process poor quality or collapsed foam may result. The stable dispersion of the present invention comprises the melamine in an amount sufficient to provide forthe desired combustion modification of flexible polyurethane foams prepared therefrom. The dispersion comprises the melamine in an amount of from 20 to 60, preferably at least 25, and more preferably at least 30, and preferably at most 55, more preferably at most 50, and most preferably at most 35 weight percent based on total weight of melamine and polyol including polymer solids. When the melamine has a small particle size generally it will be present in a smaller amount than when having a larger particle size. The stability of the melamine dispersed in the continuous phase is provided for by the presence of an inorganic particulate solid (IPS) that has an average particle size of from 5 to 30 nanometers. Preferably, the IPS has an average particle size of from 5 to 20, and more preferably from 7 to 14 nanometers. The IPS is present in an amount of from 0.2 to 5, preferably at least 0.25, more preferably at least 0.5, and preferably at most 3.0, more preferably at most 2.0 percent based on the total weight of melamine and polyol including polymer solids present. Greater amounts of the IPS generally providing for enhanced storage stability characteristics of the dispersion and ability to redisperse the melamine should any sedimentation occur. Suitable IPS substances for use in the present invention include hydrophilic substances compatible with processes for the preparation of polyurethane polymers such as silicates or especially silicic acids and particularly fumed silicic acids. Exemplary of preferred silicic acid products for use in this present invention include substances designated as AEROSIL 200, AEROSIL 300 and AEROSIL 380 available from Degussa GmbH, Germany.

Optionally.when preparing the dispersions of this invention a dispersing aid may be present. When present, the dispersing aid advantageously is used in an amount of from 0.25 to 1.5, preferably from 0.5 to 1.0 percent based on total weight of melamine and continuous phase including polymer solids. Suitable dispersing aids include derivatives of fatty acids and particularly amine salts of electroneutral fatty acids. Exemplary of a commercially available and preferred dispersing aid suitable for use in this present invention is TEGODISPERS 705 available from Th. GoldschmidtAG and understood to be an amine salt of a fatty acid.

The stable dispersion described hereinabove can be prepared by contacting and mixing the desired quantity of continuous phase, melamine and IPS and optional dispersing aid. The IPS and optional dispersing aid may be present in the continuous phase prior to the addition of the melamine, added during the addition of melamine or subsequent to the addition of the melamine. For flexibility in the preparation of stable dispersions containing various amounts of melamine, advantageously the continuous phase, IPS and optional dispersing aid are premixed and then in a subsequent step mixed with the melamine. To facilitate the mixing of the components when preparing the stable dispersion it is desirable to conduct the procedure at an elevated temperature. The elevated temperature need not be greater than 60°C and preferably is from 30°C to 60°C, more preferably from 40°C to 55°C. Depending on the batch size of the preparation, components and their amounts, and mixing conditions employed, mixing times may vary from ten to five hours.

This invention also relates to a polyurethane polymer obtained by contacting and intimately mixing under reaction conditions an organic isocyanate with an isocyanate-reac ve^* composition that comprises the described stable dispersion. The isocyanate-reactive composition comprises the dispersion in a quantity sufficient to provide for an end melamine concentration within the polymer that provides for the desired modification of the combustion characteristics of the polymer. For this purpose, typically the melamine/polymer polyol dispersion will comprise at least 25, preferabl at least 35, and more preferably at least 45 percent by weight of the total isocyanate-reactive composition including dispersion present. In addition to the dispersion, the isocyanate-reactive composition may also contain other isocyanate-reactive compounds especially other polyols, including polyether polyols, polyester polyols, chain-extending agents, and same ordifferent polymer polyols optionally characterized by the absence of melamine dispersed therein. Suitable additional isocyanate- reactive compounds that may be present include polyether polyols having an average functionality of from 1.6, preferably from 1.8, more preferably from 1.9, up to 3.0; an average hydroxyl equivalent weight of from 500 to 5000, preferably from 1000 to 3000, and more preferably from 1500to 2500; and optionally a primary hydroxyl content of at least 30 and preferably at least 50 percent of its total hydroxyl content. By "average functionality" it is understood the average number of isocyanate-reactive hydrogen atoms/molecule. Exemplary of suitable commercially available polyether polyols for use this invention are those polyether polyol products designated by the trademark "VORANOL" and include VORANOL CP 4800 and VORANOL CP-6001 available from The Dow Chemical Company.

The organic polyisocyanates useful in preparing the polyurethane polymer include those containing at least 2 and preferably from 2.0 to 3.0 isocyanate groups per molecule. Suitable isocyanates include aromatic polyisocyanates, aliphatic, cycloaliphaticand heterocyclic polyisocyanates used alone or in admixture. The preferred isocyanates used in the practice of this invention are aromatic polyisocyanates and include toluene diisocyanate, especially mixtures of the 2,4 and 2,6 isomers in weight ratios of 65:35 or 80:20; and polyisocyanate mixtures comprising 2,4'- or 4,4'-methylene diphenylisocyanate; or isocyanate- -termϊnated prepolymers thereof.

When preparing a polyurethane polymer according to the present invention, the isocyanate is present in an amount to provide from 0.85 to 1.25, preferably at least 0.95, more preferably at least 1.0 and most preferably at least 1.02, and preferably at most 1.15, and more preferably at most 1.05 isocyanate groups per isocyanate reactive hydrogen atom present including those of any water present.

To obtain polyurethane polymer having a reduced density it is necessary to prepare the polymer in the presence of a blowing agent. Advantageously, the blowing agent is present in an amount to provide the resulting polymer with an overall density of from 10 to 100, preferably from 15 to 80, and more preferably from 20 to 60 kg/m3. A preferred blowing agent for general use in the production of the polymer of this invention is water advantageously present in proportions of from 0.5 to 8, preferably from 1 to 6, and more preferably from 2 to 6 percent based on the total weight of the isocyanate-reactive composition. Non-reactive blowing agents can be used in conjunction with water or less preferably as a total replacement of water. These include substances which are vaporized at the temperatures produced by the exotherm of the isocyanate/reactive hydrogen reaction. The various blowing agents are well known in the art and include certain halogen- and non- halogen-substituted aliphatic or cycloaliphatic hydrocarbons having boiling points ranging from -40°Cto + 100°C including methylene chloride, volatile fluorocarbons and chlorofluorocarbons, e.g. trichlorofluoromethane, dichlorodifluoromethane and 1-chloro-2- fluoroethane and low boiling hydrocarbons, e.g. n-propane, cyclopropane, butane, isobutaπe, pentane, hexane, cyclohexane and their mixtures.

Advantageously, to promote the formation of urea and/or urethane linkages of the polymer, advantageously catalysts are present. Suitable catalysts are those known to those ^• skilled in the art of preparing polyurethane polymers and include tertiary amines and metallic compounds. Useful tertiary amines include N-alkylmorpholines, such as N-ethylmorpholine, N,N-dialkylcyclohexylamines where the alkyl groups are methyl, ethyl, propyl and butyl, trialkylamines such astriethylamine, tripropylamine, tributylamine and triamylamine, triethylenediamine, bis(2-dimethylaminoethyl) ether, N,N-dimethylaminoethyl- -N',N'-dimethylaminopropyl ether, and other tertiary amines well known in the art. UsefuB metal compounds include those of bismuth, lead, titanium, iron, antimony, uranium, cadmium, cobalt, aluminum, mercury, zinc, nickel, cerium, vanadium, copper, manganese, zirconium and tin. Tin compounds are particularly useful, examples including stannous octoate, (stannous 2-ethylhexoate) and dibutyltin dilaurate. The levels of catalyst used are conventional, typically ranging from 0.01 to 3 parts by weight per 100 parts of isocyanate-reactive composition. To promote the formation of foam of desirable quality and cell structure advantageously at least one surfactant and/or cell regulating agent is present when preparing the polyurethane. Suitable surfactants include non-silicone containing surfactants, such as poly(alkyleneoxides) and the diverse silicone surfactants, preferably those which are block copolymers of a polysiloxane and a polyoxyalkylene as described in U.S. Patent 3,629,308. Exemplary of such surfactants are the products designated as DC- 193 and Q4-3667 available from Dow Corning and TEGOSTAB B41 13 and B8681 available from Th. Goldschmidt AG. The amount of surfactants advantageously employed is from 0.1 to 3, preferably from 0.2 to 1.5 percent by total weight of the isocyanate-reactive composition.

In addition to the above-mentioned components, other components or addϊtwes which advantageously may be present when preparing polyurethane foam include fillers; pigments; antistatic agents and flame retardants, for example, tris(chloroethyl)phosphonate (TCEP) and dimethoxymethyl phosphonate (DMMP).

The polymers according to this invention can be prepared by any of the methods known in the art, including prepolymer and quasi-prepolymer methods though preferred are one-shot procedures. Typical of a suitable manufacturing procedure is that as disclosed in U-S. Patent 3,874,988. Other suitable manufacturing procedures are such as described in, for example, "Polyurethanes Handbook" by Gϋnter Oertel Hanser Publishes Munich ISBN 0-02- 948920-2 (1985). The polyurethane polymer obtained according to this invention comprises the melamine in an amount sufficient to provide the polymer with the desired modification of the combustion characteristics of the polymer. Typically the polyurethane polymer contains melamine particles from 2 to 30 and more typically from 5 to 25 percent of total polymer weight including melamine. These percentages are lower than those of the melamine content of the stable dispersion due to dilution by other reactive components such as the polyisocyanate employed to prepare the polyurethane polymer. Accordingly the polyurethane polymer will also contain the organic polymer solid and IPS in correspondingly loweramounts. The IPS typically being present in an amount of from 0.1 to 2.5, more typically from 0.2 to 1.5 percent of total polymer weight including melamine present. The organic polymer solid typically being present in an amount of from 2 to 25, and more typically from 2 to 15 percent of total polymer weight.

The polyurethane polymers, particularly foams, prepared from the dispersion of this invention are useful in the preparation of articles such as upholstery materials, packing materials and insulation for sound or heat, and automotive applications such as, for example, head rests and steering wheels.

In the following examples, all amounts are given as parts by weight. The examples according to the invention are illustrative, but not limitative. The materials used in the examples are identified as follows:

Melamine A melamine, average particle size 90 microns, supplied by DSM; Melamine B melamine, average particle size 10 microns, supplied by DSM;

Continuous phase Polyol A SAN copolymer polyol (10% solids), VORANOL" CP-8020 supplied by The Dow Chemical Company;

Polyol B SAN copolymer polyol (15% solids), VORALUX" HN 202 supplied by The

Dow Chemical Company; Inorganic Fumed silicic acid, AEROSIL 200, particulate solid supplied by Degussa; with an (IPS) average particle size of 12 nanometers;

Dispersing Aid TEGODISPER 705 supplied by Th. GoldschmidtAG.;

Thermolin 101 a halogen/phosphorus flame retardant additive supplied by Olin S.A.;

Catalyst a combination of DABC033LV supplied by Air Products; NIAXA-1 supplied by Union Carbide Corporation; and dibutyltin dilaurate, weight ratio 28:12:60 respectively;

Surfactant TEGOSTAB B8681 supplied by Th; Goldschmidt AG.;

TDl-80 Toluene diisocyanate (2,4-/2,6-

-isomer ratio 80:20). Reported properties of foams as obtained are determined according to the following procedures; tensile strength and elongation - DIN 53571 ; compression load deflection (CLD)- DIN 53577; and for indentation load deflection (ILD)- DIN 53576. Flame retardant, hereinafter F.R., performance of foam is observed according to Crib 5 as specified in 5 BS 5852, part 2. The reported flame retardancy performance is representative of that to be obtained with such foam but should not be considered as reflecting the performance under actual fire conditions. Example 1

The following dispersions are prepared with components as indicated in Table 1.

10 Melamine is added to the continuous phase, the polyol, in the presence of the inorganic particulate solid and optional dispersing aid as indicated. The mixture is mechanically stirred until a temperature of 50°C is reached and then allowed to cool to 25°C before measuring the viscosity. Stability ofthe resulting dispersion is monitored byobserving the time elapsed until , _ the first visual observation of sedimentation is noted. Longer time periods being considered indicative of better storage stability characteristics.

From the data presented in Table 1 it is determined that the presence of the IPS provides for the stability of the dispersion. Comparison of Dispersion 6 with Dispersion 1 indicates that the observation of enhanced stability is not merely an artifact of increasing ₀ vi scosity of the system .

5

0

5 Table 1

i

H o

* Not an example of this invention

** Deposited Melamine not readily redispersible

Example 2

A stable dispersion of melamine in a PHD polymer polyol is prepared, which in the absence of an inorganic particulate solid is also observed not to possess desirable stability characteristics. A similar lack of dispersion stability is also observed for melamine dispersed in a PIPA polymer polyol when an IPS is absent.

Table 2

10

15

20

25

It can also be seen from this example that use of a PHD polymer polyol provides dispersions which have an undesirably higher viscosity than like dispersions prepared from a SAN polymer polyol of comparable solids content, see Example 1, Dispersion 2 and Dispersion A.

30

35

*. Example 3

Polyurethane foam is prepared from Dispersions 4 and 10. The formulation used to prepare the foam and the physical properties ofthe resulting foam is given in Table 3. Comparative Foam A is prepared from similar components but wherein the melamine is mixed directly in the isocyanate-reactive composition and used immediately in the preparation of a polyurethane foam. Processing and physical properties of the resulting foams are seen to be essentially equivalent with the exception of results form the F.R. testing. Comparative foam A is observed to have a significantly inferior F.R. performance compared to Foam 1. A further _n advantage observed when preparing polyurethane foam with the stable dispersion of this invention by a continuous foaming process is the enhanced reproducibility/consistency ofthe F.R. performance ofthe foam. During the continuous preparation of foam, the concentration of melamine in the foam can vary depending on whether the foam sample considered is produced at an early or late stage ofthe continuous foaming process. The variance in _C. melamine concentration can be observed through poor reproducibility of the F.R. performance observed for different samples ofthe foam. Non or poor stability of the dispersed melamine is considered to result in a non uniform distribution of melamine.

Table 3

Claims

CLAIMS:

1. A stable dispersion which comprises:

(a) a polyol or mixture thereof containing from 4 to 50 weight percent of a polymer sol id; (b) from 20 to 60 percent based on the total weight of (a) and (b), of melamine which has a mean particle size of from 10 to 125 microns; and

(c) from 0.2 to 5 oercent, based on the total weight of (a) and (b), of an inorganic particulate solid which has an average particle size of from 5 to 30 nanometers.

2. A dispersion as claimed in Claim 1 wherein the polymer solid comprises a SAN-, PHD-, or PIPA- polymer

3. A dispersion as claimed in Claim 1 wherein the melamine has a mean particle size of from 25 to 100 microns.

4. A dispersion as claimed in Claim 1 wherein the inorganic particulate solid comprises a silicate, a silicic acid or a fumed silicic acid.

5. A dispersion as claimed in any one of Claims 1 to 4 wherein the inorganic particulate solid isa fumed silicic acid that has an ave^'rage particle size of from 5 to 20 nanometers.

6. A dispersion as claimed in any one of Claims 1 to 4 wherein the inorganic particulate solid is present in an amount of from 0.25 to 3.0 weight percent.

7. A dispersion as claimed in Claim 1 which comprises:

(a) a polyol containing from 5 to 20 weight percent of a SAN polymer solid;

(b) from 20 to 60 percent, based on the total weight of (a) and (b) present, of melamine which has a mean particle size of from 25 to 100 microns; and

(c) from 0.25 to 3.0 percent, based on the total weight of (a) and (b) present, of an inorganic particulate solid which is a fumed silicic acid that has an average particle size of from 5 to 20 nanometers.

8. A dispersion as claimed in any one ofthe preceding claims further comprising a dispersing aid.

9. A process for preparing a stable dispersion which comprises contacting;

(b) melamine having a mean particle size of from at least 10 and up to 125 microns; and

(c) an inorganic particulate solid which has an average particle size of from 5 to 30 nanometers, characterized in that (b) is present in an amount of from 20 to 60 percent based on totaf wetght of (a) and (b); and (c) is present in an amount of from 0.2 to 5 percent based on total weight of (a) and (b).

10. A polyurethane polymer obtainable by contacting under reaction conditions an organic isocyanate with an isocyanate reactive composition which contaiπsa stable dispersion as claimed in Claim 1 wherein the organic polyisocyanate is present in an amount to provide from 0.85 to 1.25 isocyanate groups per isocyanate reactive hydrogen atom present characterized in that the polymer has dispersed therein: from 2 to 30 percent based on the total weight of polymer of melamine that has a mean particle size of from at least 10 and up to 125 microns; from 0.1 to 2.5 percent based on the total weight of polymer of an inorganic particulate solid which has an average particle size of from 5 to 30 nanometers; and from 2 to 25 weight percent of a polymer solid.