GB1598967A

GB1598967A - Process for the production of polyurethane coatings

Info

Publication number: GB1598967A
Application number: GB20050/78A
Authority: GB
Original assignee: Bayer AG
Current assignee: Bayer AG
Priority date: 1977-05-24
Filing date: 1978-05-17
Publication date: 1981-09-30
Also published as: IT7849475A0; DE2723293C2; BE867406A; FR2392091B1; FR2392091A1; DE2723293A1; IT1105408B

Description

(54) PROCESS FOR THE PRODUCTION OF POLYURETHANE COATINGS (71) We, BAYER AKTIENGESELLSCHAFT, a body corporate organised under the laws of Germany, of 509 Leverkusen, Germany; do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a new process for the production of coatings by coating substrates of various types with new coating compounds which contain little or no solvent and which contain, as binder, a mixture which reacts to form a polyurethane.

Coating compounds which contain, as binders, mixtures which react to form polyurethanes are known (see e.g. Wagner/Sarx, Lackkunstharze, 5th Edition, Carl Hanser Verlag, Munich, 1971, pages 153-173).

On pages 169-170 of the above-mentioned publication there are also described solvent-free systems by means of which coatings of any thickness may be produced in a single operation. Solvent-free coating compounds have become particularly interesting in the context of the greater importance attached to environmental hygiene. The manufacture of solvent-free coatings requires the use of low viscosity starting materials. For the polyisocyanate component, these substances are available, for example, in the form of diphenylmethane diisocyanate derivatives, but difficulties arise in the choice of the polyol component. Although the hydroxyl group-containing polyester polyols available give rise to products having good mechanical properties after they have been cross-linked with polyisocyanates, there are distinct shortcomings in the resistance to hydrolysis. Moreover, hydroxyl group-containing polyester polyols are highly viscous on account of the chemical constitution thereof. This property is a disadvantage for the manufacture of solvent-free lacquers. The known hydroxyl group-containing polyether polyols, e.g. those based on ethylene oxide and/or propylene oxide. have more favourable viscosity characteristics and are more resistant to hydrolysis, but the polyurethanes obtained from them are inferior to polyester urethanes in the mechanical properties thereof. In addition, the known polyether polyols are unsuitable for the reaction with polyisocyanates in coating compounds because of the hydrophilic properties thereof which give rise to a tendency for the formation of bubbles and the coatings produced from them also have a low resistance to weathering.

It was therefore an object of the present invention to provide a new process for the production of coatings which could be carried out using solvent-free or low solvent coating compounds in which the binders were free from the disadvantages mentioned above.

This problem could be solved according to the present invention by using, as binder, a reaction mixture which reacted to form a polyurethane, in which reaction mixture the polyol component consists of a dispersion of a polyisocyanate polyaddition product in an organic polyhydroxyl compound.

The present invention therefore relates to a process for the production of a coating by coating a substrate with a solvent-free or low solvent (as hereinafter defined) coating composition which contains, as binder, a mixture of a polyisocyanate component and a polyol component which reacts to form a polyurethane, characterised in that the polyol component used is a dispersion of a polyisocyanate poly-addition product in an organic polyhydroxyl compound.

As used herein, the term "low solvent content" means that the coating composition has a maximum solvent content of 40%, by weight, based on the total composition.

The present compositions generally have a solids content of from 60 to 100%, preferably from 70 to 100%, by weight.

The dispersions which are an essential feature of the present invention are stable dispersions generally having a solids content of from 5 to 60%, by weight, preferably, from 10 to 30%, by weight, of polyisocyanate polyaddition product and, in particular, polyureas and/or polyhydrazodicarbonamides in organic polyhydroxyl compounds, such as those described, for example, in German Auslegeschrift No. 1,260,142, German Offenlegungsschrift No. 2,513,815 or German Offenlegungsschrift Nos. 2,550,833; 2,550,862; 2,550,796; 2,550,797; or 2,550,860.

According to the information disclosed in the documents cited above, such dispersions may be obtained by reacting polyisocyanates with polyhydroxyl compounds and, preferably, polyamines having primary and/or secondary amino groups and/or hydrazines and/or hydrazides in organic polyhydroxyl compounds used as reaction medium. Moreover, in the case of the less preferred use of polyhydroxyl compounds as reactants, the polyhydroxyl compounds used as reactants exclusively have primary hydroxyl groups while the polyhydroxyl compounds used as reaction medium preferably have secondary hydroxyl groups in order to obtain a selective reaction.

It is particularly preferred to use dispersions of the type described in German Offenlegungsschrift No. 2,513,815 and in German Offenlegungsschrift Nos. 2,550,833 and 2,550,862 in polyhydroxyl polyethers having molecular weights of from 200 to 16,000, preferably from 500 to 12,00 or in mixtures of such polyhydroxypolyethers with up to 10%, by weight, based on the total quantity of polyhydroxyl compounds, of low molecular weight polyhydroxyl compounds having molecular weight of from 62 to 200. Similar dispersions in castor oil as continuous phase are also suitable for the process according to the present invention.

Among the particularly preferred dispersions in polyhydroxyl polyethers those in which the polyether component has a hydroxyl functionality of from 2.5 to 3.5 and a hydroxyl number of from 25 to 60 and which have a dispersed solids content of from 10 to 30%. by weight, and a low viscosity below 3500 cP at 25"C are particularly suitable. The dispersed solids content consists most preferably of reaction products of diisocyanates of the type exemplified below, in particular 2,4- and 2,6-diisocyanatotoluene or 4,4'diisocyanatodiphenylmethane, with approximately equivalent quantities of hydrazine or hydrazine hydrate. A typical method of preparing such dispersions is described in Example 1.

If desired, the dispersions mentioned above may be diluted with other compounds which have isocyanate-reactive groups and are monofunctional or polyfunctional ln the isocyanat. polyaddition reaction before the process according to the present invention is carried out.

Masked chain-lengthening agents, i.e. in particular compounds which are inert towards isocyanate groups, at room temperatures but release compounds having isocyanate reactive groups in the presence of moisture may also be added. However, the additional (masked) reactant for polyisocyanates which may be added to the dispersions according to the present invention, of which examples have been given, must preferably be limited to such a quantity that the resulting polyol component preferably still contains at least 5%, by weight, more preferably at least 10%, by weight, based on the polyol component, of pol > ddition products of the type exemplified.

The inclusion of the above-mentioned masked chain-lengthening agents is preferred for many fields of applications of the process according to the present invention.

Compounds containing isocyanate-reactive groups suitable for use as diluents for the dispersion include, for example, low molecular weight monohydric or polyhydric alcohols having molecular weights of from 32 to 200, e.g. methanol, ethanol. i-propanol, ethylene glycol, diethylene glycol, ethylene glycol monoethyl ether, propylene glycols. butylene glycols, trimethylolpropane or glycerol. Higher molecular weight polyhydroxyl compounds, such as, in particular, castor oil or polyhydroxypolyethers of the type already used as continuous phase for the preparation of the dispersions may also be used as "diluents" containing isocyanate-reactive groups. The polyhydroxy-polyesters or polyhydroxypolycarbonates having molecular weights of from 200 to 5000 known in polyurethane chemistry may also be used, but are less advantageous.

Compounds which may be used as so-called "masked chain-lengthening agents" include, for example, the oxazolidines described in US Patent Nos. 3,661,923; 3,864,335 and 3,743,626 and in German Offenlegungsschrift No. 2,446,438, aldimines and ketimines described in US Patent Nos. 3,420,800; 3,567,692; 3,267,078 and 3,715,338 and enamines of the type described in German Offenlegungsschrift No. 2,152,247.

These "masked chain-lengthening agents" optionally used in the process according to the present invention may be added in such quantities that the polyol component contains from 5 to 50 parts, by weight, preferably from 10 to 40 parts, by weight, of the masked chain-lengthening agent per 100 parts, by weight, of the dispersion according to the present invention. It should be noted that, in the polyol component which contains the above-mentioned masked chain-lengthening agent in addition to the dispersions which are an essential feature of the present invention and optionally other diluents of the type mentioned above, the quantity of dispersed polyaddition product, based on the total quantity of polyol component, should preferably not fall below 5%, by weight, more preferably, particularly if substantial quantities of masked chain-lengthening agent are added, not below 10% by weight. The parts, by weight, and percentages, by weight, given above do not include any auxiliary agents and additives used, such as solvents, pigments or fillers.

By "isocyanate-reactive groups" are meant all primary or secondary amino groups, hydroxyl groups and semicarbazide end groups (the latter particularly in the disperse phase) present in the polyol component, but not the urethane and urea groups which are relatively inactive towards isocyanate groups particularly those present in the disperse phase.

Any organic isocyanates may be used in the process according to the present invention, e.g. aliphatic, cycloaliphatic, aromatic and heterocyclic polyisocyanates, such as those described by W. Siefken in Justus Liebigs Annalen der Chemie, 562, pages 75 to 136. The following are examples: ethylene diisocyanate, tetramethylene-1,4-diisocyanate, hex amethylene-l .6-diisocyanate, dodecane-1 , 12-diisocyanate, cyclobutane-1 ,3-diisocyanate, cyclohexane-1.3- and -1,4- diisocyanate and mixtures of these isomers, 1-isocyanato-3,3,5- trimethyl-5-isocyanatomethylcyclohexane (German Auslegesschrift No. 1,202,785), hexahydrotolylene-2,4- and -2,6-diisocyanate and mixtures of these isomers, hexadrophenylene-1,3- and/or 1,4-diisocyanate, perhydrodiphenylmethane-2,4'- and/or 4,4'diisocyanate, phenylene-1,3- and -1,4-diisocyanate tolylene-2,4- and -2,6-diisocyanate and mixtures of these isomers, diphenylmethane-2,4'- and/or -4,4'-diisocyanate, naphthylene1 ,5-diisocyanate, triphenylmethane-4,4' ,4"-triisocyanate, polyphenol-polymethylene polyisocyanates which may be obtained by aniline/formaldehyde condensation followed by phosgenation, e.g. those described in British Patent Nos. 874,430 and 848,671, perchlorinated aryl polyisocyanates, e.g. those described in German Auslegeschrift No. 1,157,601, polyisocyanates containing carbodiimide groups as described in German Patent No.

1,092,007, diisocyanates of the type described in US Patent No. 3,492,330, polyisocyanates containing allophanate groups as described, for example, in British Patent No. 994,890, Belgian Patent No. 761,626 and published Dutch Patent Application No. 7,102,524, polyisocyanates containing isocyanurate groups, e.g. as described in German Patent Nos.

1,022,7891,222,067 and 1,027,394 and in German Offenlegungsschrift Nos. 1,929,034 and 2,004,048, polyisocyanates containing urethane groups as described, for example, in Belgian Patent No. 752,261 or in US Patent No. 3,394,164, polyisocyanates containing acylated urea groups according to German Patent No. 1,230,778, polyisocyanates containing biuret groups, e.g. as described in German Patent No. 1,101,394, in British Patent No. 889,050 and in French Patent No. 7,017,514, polyisocyanates prepared by telomerisation reactions, e.g. as described in Belgian Patent No. 723,640, polyisocyanates containing ester groups, e.g. as described in British Patent Nos. 956,474 and 1,072,956, in US Patent No. 3,567,763 and in German Patent No. 1,231,688 and reaction products of the above-mentioned isocyanates with acetals according to German Patent No. 1,072,385.

The preferred polyisocyanates are those corresponding to the following general formula: Q(NCO)n wherein Q represents at least two substituted or unsubstituted phenyl groups linked through methylene bridges and n represents an integer of from 2 to 4, as well as the prepolymers prepared from these polyisocyanates, which prepolymers contain free isocyanate groups and are obtained by partial reaction with hydroxyl polyethers having molecular weight of from 200 to 3000 of the type known in polyurethane chemistry which are based on ethylene oxide and/or propylene oxide.

Preferred polyisocyanates also include the reaction products of tolylene-2,4-diisocyanate (or mixtures thereof with the 2,6-isomer) with polyols, such as trimethylol propane, butane-1,3-diol, diethylene glycol and polypropylene or polyethylene glycol polyethers having a molecular weight of from 200 to 3000 which reaction products have been substantially freed from excess diisocyanates by distillation.

To carry out the process according to the present invention, the polyol component is preferably mixed with the polyisocyanate component in proportions corresponding to an equivalent ratio of isocyanate groups to all groups which are isocyanate-reactive in accordance with the definition given above, including any isocyanate-reactive groups formed from the masked chain-lengthening agents optionally used of from 0.5:1 to 2.5:1, preferably from 0.8:1 to 1.5:1. The mechanical properties of the products finally obtained may easily be adjusted by suitable choice of the equivalent proportions, as well as by suitable choice of the starting components exemplified above. The degree of cross-linking and hence the hardness of the sheet products obtained may also be adjusted by suitable choice of the functionality of the individual components.

Before the individual components are mixed as mentioned above, the auxiliary agents and additives commonly used in the chemistry of polyurethane lacquers may be added.

It should be particularly mentioned here that the reaction mixtures used according to the present invention may easily be applied by the known coating techniques without the use of any solvent or with only small quantities of solvent. The solvents which are optionally used are at most used m quantities resulting in from 60 to 100%, preferably from 70 to 100%, by weight, compositions, i.e. solids contents within these ranges.

Any of the solvents normally used for polyurethane lacquers are suitable, e.g. ethyl acetate, butyl acetate, methyl ethyl ketone, methyl isobutyl ketone, ethylene glycol monoethyl ether acetate, toluene, xylene or mineral spirit. Mixtures of such solvents may, of course, also be used.

Other auxiliary agents and additives optionally used in the process according to the present invention include, for example, plasticizers, such as tricresylphosphate or dioctyl phthalate, pigments and fillers, such as titanium dioxide, barium sulphate, chalk or carbon black; catalysts such as N,N-dimethylbenzylamine, lead octoate or dibutyl tin dilaurate; levelling agents; thickeners and, optionally, stabilizers, such as Ionol ("Ionol" is a Registered Trade Mark). Any moisture in the fillers and pigments may first be removed by drying them or by adding water-absorbing substances, such as molecular sieve zeolites.

The process according to the present invention may be carried out by any method normally used for coating. The products may be applied, for example, by casting, by application using doctor wipers or rollers, by spread coating or by spraying using airless spray machines, by the heat spraying process or by conventional spraying, using the conventional apparatus. The process according to the present invention is suitable for coating various substrates e.g. metals, concrete, asbestos, cement, asphalt or plastics, such as PVC, polystyrene or polyurethane. The process according to the present invention is not limited to the production of thin lacquer films, but is also particularly suitable for the manufacture of comparatively thick coatings of the type required, for example for resilient ground surfaces for sports purposes. If elastic fillers, such as rubber granulates, are also included, the process according to the present invention is also suitable for the manufacture of resilient athletics tracks.

The process of the present invention is illustrated by the following Examples, wherein all the percentage figures quoted are used on a weight basis unless othewise indicated.

Example 1 A polyether of propylene oxide which has been started on trimethylolpropane and tipped with ethylene oxide (proportion, by weight, of propylene oxide to ethylene oxide 83:17) and has a hydroxyl number of 34 is used for preparing a dispersion polyol.

Using two mixers having spiked stirrers arranged in series (volume of mixing chamber 1.5 litres or 0.5 litres, stirrer speed 1500 revs/min), 800 g per minute of the polyether, 169 g per minute of a mixture of 80% of 2,4- and 20% of 2,6-tolylene diisocyanate and 49 g per minute of hydrazine hydrate are continuously introduced at room temperature. The three components are directly fed into the mixing zone of the first stirrer vessel separately from each other. The polyether is delivered from its storage tank by a gear wheel pump, while the two low viscosity liquid components are delivered from separate storage vessels by means of piston feed pumps. Polyaddition takes place in the stirrer vessel as an exothermic reaction. The reaction temperature is adjusted to from 100 to 105"C by cooling the stirrer vessel. After a residence time of about 2 minutes, an almost completely reacted white dispersion leaves the second stirrer vessel. The dispersion is transferred to a dwell vessel where it is maintained at from 80 to 100"C with stirring. The water from the hydrazine hydrate is then distilled off under vacuum at 100"C. A white, stable, finely divided 20% dispersion having an OH number of 27, a viscosity of 2100 cP/25 C and a pH of 8.2 is obtained.

Example 2 A composition according to the present invention is prepared by mixing 100 parts, by weight, of the dispersion polyol prepared according to Example 1 with 80 parts, by weight, of 4,4'-diphenylmethane diisocyanate (isocyanate content 31.5 % by weight) and 1.5 parts, by weight, of N,N-dimethylbenzylamine as catalyst. The mixture has a pot-life of 50 minutes. It is poured out to form a layer 1 mm in thickness on glass plates. After a hardening time of 24 hours, the films obtained are assessed as follows: Appearance: opaque Surface: smooth, non-tacky and glossy.

Examination of the physical properties produce the following results: Tear resistance according to DIN 53 455 in MPa 3.0 Elongation on tearing according to DIN 53 455 in % 74.0 Tear propagation resistance according to DIN 53 515 in KN/m 6.0 Shore A hardness according to DIN 53 505 80.0 Loss of hardness after 10 days storage in water at 700C 15% Weight increase after 10 days storage in water at 700C 9% Example 3 (Comparison Example) The same polyether as in Example 1, but without the dispersed polyurea is used for preparing a polyurethane coating. The quantity of diisocyanate used is slightly increased in order to provide the same NCO/OH ratio as in Example 2. 500 g of a polyether of propylene oxide and ethylene oxide started on trimethylolpropane as indicated in Example 1 are mixed with 45 parts, by weight, of diphenylmethane diisocyanate (isocyanate content 31.5% by weight) and 1.5 parts, by weight, of N,N-dimethylbenzylamine and the mixture is poured out on glass plates. The time available for processing is one hour. After a hardening time of 24 hours, the films are assessed as follows: Appearance: cloudy with streaks Surface: very tacky, disfigured by blisters.

Owing to these properties, it was not possible to examine the physical properties.

Example 4 A mixture is prepared by adding 200 parts, by weight, of benzyl butyl phthalate to 300 parts, by weight, of a 10% dispersion of a polyurea of tolylene diisocyanate and hydrazine in a polyester of adipic acid and diethylene glycol which has a hydroxyl content of 1.7% (preparation analogous to Example 1).

340 parts of calcium carbonate as filler and 35 parts, by weight of iron oxide as pigment are added to this mixture. 35 parts, by weight, of a molecular sieve zeolite are added to absorb the moisture. 2 parts, by weight, of N,N-dimethylbenzylamine are added to the mixture to catalyse the reaction.

The mixture is homogenised by means of rollers.

50 parts, by weight, of 4,4'-diphenylmethane diisocyanate are added to produce an elastic coating. The time available for processing is 50 minutes. Films cast on glass plates are assessed as follows: Surface: glossy, non-tacky Mechanical properties: Shore hardness A according to DIN 53 505 70 Tear resistance according DIN 53 455 in MPa 2.9 Tear propagation resistance according to DIN 53 515 in KN/m 11.8 Elongation on tearing according to DIN 53 455 in % 220 Resistance to weathering (measured in sunshine weather o-meter type XW) after 500 hours exposure onset of chalking Example 5 (Comparison Experiment) The procedure described in Example 4 is employed, using the same polyester, but without any dispersed constitutents, and the same additives; an elastic coating is produced.

Assessment of the lacquer films: Surface: glossy, slightly tacky Mechanical properties: Shore A according to DIN 53505 60 Tear resistance according to DIN 53 455 in MPa 2.8 Tear propagation resistance according to DIN 53 515 in KN/m 11.75 Elongation on tearing according to DIN 53 455 220 Resistance to weathering (measurement in sunshine weather o-meter Type XW) After 300 hours: onset of chalking after 500 hours: severe chalking and signs of degradation.

The laminate according to the present invention (Example 4) has minor advantages in its physical properties, but major advantages in its resistance to weathering.

Example 6 1000 parts, by weight, of 4,4-diphenylmethane diisocyanate were mixed with 800 pal-ts, by weight, of a branched polypropylene glycol ether having an OH content of 1.7%. The mixture obtained was maintained at 80"C for 12 hours. At the end of this time, it had a viscosity of 4800 mPas and an isocyanate content of 15.1 %.

1000 parts, by weight, of the dispersion polyol described in Example 1 were added to 160 parts of this isocyanate prepolymer. After the addition of 0.1% of dibutyl tin dilaurate as catalyst, it was found to have a processing life of 40 minutes. Films cast on glass plates were assessed as follows: Surface: Glossy, tack-free and free from bubbles Physical properties: Shore hardness A according to DIN 53 505 72 Tear resistance according to DIN 53 455 in MPa 3.4 Elongation on tearing according to DIN 53 455 in % 132 Tear propagation resistance according to DIN 53 515 in KN/m 6.2 Loss of hardness after 10 days' storage in water at 70"C in % 15 Weight increase after 10 days' storage in water at 700C in % 8.0 Example 7 100 parts, by weight, of the dispersion polyol prepared according to Example 1 are mixed with 19 parts, by weight, of a 75 % solution of a reaction product obtained by reacting excess tolylene diisocyanate with trimethylol propane in ethyl acetate (isocyanate content: 13.5%). 0.1 part, by weight, of dibutyl tin dilaurate is added as catalyst. Films cast on glass plates have a glossy, tack-free surface without bubbles when hardened.

Example 8 This Example describes the preparation and properties of some solvent containing white lacquers having a high solids content for lacquering steel and demonstrates with the aid of Comparison Experiments the considerable advantages of the process according to the present invention.

The following reactants are used: Substance (A): Linear polyether of propylene oxide which has been started on bisphenol A and has a hydroxyl number of 200 Substance (B): The polyether used as substance (A) contain ing 20%, by weight, of a polyurea of hydrazine and tolylene diisocyanate; hydroxyl number 158.

Substance (C): Castor oil, hydroxyl number 210.

Substance (D): Castor oil in which 30%, by weight, of a polyurea of hydrazine and tolylene diisocyan ate are dispersed; hydroxyl number 168.

Subtance (E): The dispersion polyol described in Example 1.

OH number 27.

Substance (F): Bisaldimine corresponding to the following formula:

Substance (G): Urethane-bisoxalodine corresponding to the following formula:

Substance (H): A polyisocyanate prepared by reacting butane-1,3-diol and trimethylolpropane (molar ratio 3:7) with excess tolylene 2,4-diisocyanate and removing the free tolylene diisocyanate by distillation.

The product is dissolved as a 75 % solution in ethyl acetate.

Isocyanate content of the solution: 13.0%, by weight.

Substance (I): A polyisocyanate prepared by reacting a polypropylene glycol ether having an average molecular weight of 1000 trimethylolpropane (molar ratio 1:1) and excess tolylene-2,4-diisocyanate and removing the free tolylene diisocyanate by distillation. Dissolved at a concentration of 80% in a 1:1 mixture of ethylene glycol acetate and butyl acetate.

NCO content of the solution: 10.5 %, by weight.

The various reactants were combined with the addition of solvents, pigments, catalysts and additives to prepare the formulations of the coating compositions. The compositions of these lacquers are shown in Table I and the properties of the lacquer films in Table II.

Lacquer formulations 1 and 3 prepared according to the known art are shown for comparison and are more particularly comparable to the formulations of experiments 2 and 4.

To explain how comparison was carried out, all lacquers were equally highly pigmented and adjusted to a solids content of approximately 80%, by weight. The lacquers were then sprayed on untreated, stainless steel sheets to form films having a thickness of about 80 m when dry.

TABLE I Lacquer formulations (given in parts) Components 1 2 3 4 5 6 7 Substance (A) 29.0 - - - - - Substance (B) - 36.2 - - 70.0 40.0 40.0 Substance (C) - - 23.6 - - Substance (D) - - - 33.7 - - Substance (E) - - - - 30.0 - Substance (F) - - - - 8.0 14.0 Substance (G) - - - - - Substance (H) - - 32.2 32.3 69.0 - Substance (I) 42.3 42.4 - - - 68.5 70.0 Levelling agent 10% in ethyl glycol acetate (a polymer of ethylhexylacrylate) 0.7 0.8 0.6 0.7 1.8 1.4 1.5 Diazobicyclooctane 10% in ethyl acetate 2.5 2.8 2.2 2.3 6.1 3.9 4.0 TiO2 (rutile type) 31.9 34.8 24.6 29.0 75.9 50.5 55.2 Butyl acetate/xylene 1:1 11.8 13.5 7.0 10.7 31.8 20.0 21.4 TABLE II Testing and properties of lacquer formulations Test method 1 2 3 4 5 6 7 Pot-life (time available for processing) ca.10 h ca.8 h ca.2 h ca.2 h ca.8 h ca.2 h ca.6 h Sand drying according to DIN 53 150 (drying stage 1) room temperature (120 m when wet) ca. 12 h ca.7 h ca.12 h ca.1 h ca.5 h ca. h ca. 1h Pendulum hardness according to DIN 43 157 130' - 80' + 16 h - 25 C 70' 120' 50' 138' 110' 130' 145' Elasticity according to DIN 53 156 (thickness of layer ca. 80 m) 9 mm 8 mm 8 mm 8 mm 9 mm 8 mm 8 mm adherence according to DIN 53 151 2 0 1 0 0 0 0 Exposure to moisture in salt spray test according to DIN 53 167 migration under the surface after a test time of 24 hours 4 mm < 1 mm 5 mm 1 mm 2 mm < 1 mm < 1 mm To test the suitability of the coating compositions according to the present invention for use in the field of protection against corrosion, the lacquered metal sheets were kept for one hour under the conditions of the work-room after the 240 hours' salt spray test. An adhesive tape was then applied with firm pressure and torn off with a vigorous pull.

Results: The lacquer films obtained from products 1 and 3 were removed in broad patches, but lacquers 2, 4, 5, 6 and 7 remained completely intact.

These results show the advantages of lacquers produced by the process according to the present invention.

They combine a comparable pot-life with more rapid surface drying, greater hardness, excellent adherence and resistance to moisture. This make

Claims

WHAT WE CLAIM IS:

1. A process for the production of a coating which comprises coating a substrate with a coating composition which is solvent free or has a low solvent content (as hereinbefore defined), said composition comprising, as binder, a polyurethane- producing mixture of a polyisocyanate component and a polyol component, the polyol component being a dispersion of a polyisocyanate polyaddition product in an organic polyhydroxyl compound.

2. A process as claimed in claim 1 in which the said binder additionally comprises a masked chain-lengthening agent which is inert towards isocyanate groups at room temperature and from which an isocyanate-reactive chain-lengthening agent is formed under the action of moisture.

3. A process as claimed in claim 1 substantially as herein described.

4. A process as claimed in claim 1 substantially as herein described with reference to any one of the Examples.

5. A coating when produced by a process as claimed in any of claims 1 to 4.

6. A coating composition which comprises, as binder, a mixture as defined in claim 1.

7. A composition as claimed in claim 6 substantially as herein described.