GB1564541A - Radiation curable coatings - Google Patents

Description

(54) IMPROVEMENTS IN AND RELATING TO RADIATION CUR ABLE COATINGS (71) We, DENNISON MANUFACTURING CO., LIMITED, a Company organised under the Laws of England, of Colonial Way, Watford, Hertfordshire, 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 radiation curable, and radiation cured, coatings for providing films, inks, varnishes, overcoatings and release coatings. It also relates to methods of making the coatings and suitable prepolymer materials for preparing them. The coatings are curable by various forms of "radiation", which is the term used for electromagnetic radiation, such as ultraviolet, and also for plasma arc and electron beam bombardment.

Coatings and inks for paper, foil, film, panels, tiles and other surfaces are desired which can be easily applied, are solvent and abrasion resistant, and are tough and flexible. It is also desirable to eliminate solvents, to reduce cost, avoid pollution, and achieve improved performance. In addition, if surfaces with low adhesion can be obtained, they can be used for transfer printing, temporary cover sheets to protect pressure-sensitive adhesives, and the expeditious handling of multiple articles in machinery or the like.

Existing radiation curable coatings, while useful for many purposes, do not sufficiently satisfy the foregoing criteria. Either they have limited strength, abrasion and solvent resistance, or they require long exposure times to radiation, especially ultraviolet light. Suitable low adhesion materials heretofore have required a solvent solution, or have had to be applied as hot melts.

It is an object of the invention to mitigate one or more of the foregoing disadvantages. It is believed that it is possible to produce, by means of the invention, release surfaces which can employ a wide variety of film-forming materials and which can be readily applied without substantial solvent and quickly cured by radiation, generally in one second or less under moderate exposure.

It is also believed possible to produce, by means of the invention, radiation curable materials, and coatings resulting therefrom, which cure rapidly under moderate exposure to give films with a desirable combination of toughness, flexibility and resistance to solvents and to abrasion. Such films are useful as inks, release coatings, or protective varnishes.

According to the present invention, there is provided a liquid prepolymer for preparing a radiation curable composition, comprising progressively adding a secondary polyether polyol to a polyisocyanate thereby effecting a controlled reaction in which the polyisocyanate is at all times in excess, and further adding on ethylenically unsaturated reactive hydrogen containing compound in sufficient quantity to react with substantially all of the unreacted polyisocyanate.

Here and elsewhere in the specification the term "secondary polyether polyol" means a polyether polyol containing secondary hydroxyl groups.

In British Patent Application No. 47458/78 (Serial No. 1564543) divided from the present application there is described and claimed, a valuable new class of release materials provided by including waxy or oily materials in a radiation curable, film-forming liquid comprising a radiation curable liquid prepolymer in which the waxy material has limited compatability such that a thin layer thereof will migrate to the surface of a thin film of the liquid prior to cure. The waxy materials have good dlip or release properties, are generally lipophilic, and can comprise natural and synthetic waxes, oils, silanes, siloxanes particularly silicones and fluorocarbons. While generally non-reactive in the polymerizable liquid, they can contain reactive groups. For example, stearyl acrylate can be used with acrylate monomers and will migrate to the surface and orient with the stearyl groups toward the surface prior to reaction. The quantity of waxy material is not critical, provided enough is used to be effective, generally between about 0.5 and 10% by weight of the film-forming liquid. Undue excess should be avoided, 3% to 4% by weight is usually most preferred.

While waxes have been used with radiation curable materials to form an oxygen barrier and facilitate curing in air, they have not been used to provide release and transfer properties. The addition of a single wax component can provide high gloss coatings with release characteristics. The introduction of various mixtures of waxy materials has been found to give particularly suitable release properties.

The waxy materials above described can be incorporated in any suitable radiation curable liquid including prepolymers having ethylenic unsaturation, such as polyurethane/acrylate prepolymers, polyamides with a plurality of reactive amine groups reacted with ethylenically unsaturated polybasic carboxylic acids, acrylated epoxides, and epoxide resins catalytically polymerized with catalysts released under exposure to ionizing radiation.

Preferred radiation curable liquids for use in the present invention comprise an unsaturated prepolymer mixed with acrylic monomers and additives as further described below.

Suitable unsaturated prepolymers are unsaturated polyether-polyurethane prepolymers prepared by reacting a secondary polyether polyol with bis- or polyisocyanates and an ethylenically unsaturated reagent. In the preparation of these prepolymers a step of driving off volatile materials such as solvents can be avoided if an acrylic monomer is used as the solvent (i.e. a monomer which normally would be admixed in the succeeding formulation step). Sometimes an agent for inhibiting further polymerization (depending upon the nature of the prepolymer is present.

The prepolymers, which in some cases already contain an acrylic ester reagent, if an acrylic monomer is used as a solvent and/or if an excess of acrylic hydroxyalkyl ester was used in the prepolymer preparation, may then be compounded on a mill with a further quantity of unsaturated reagent, in this case one or more acrylic or methacrylic esters (whether or not the same unsaturated reagent was used in preparing the prepolymer). Additives, as further described below, may be approximately added at this formulating stage. After milling the material is spread as a film and then irradiated briefly to cure it.

The most preferred modified polyether polyurethane prepolymer comprises reacting one isocyanato group of toluene diisocyanate (TDI) with a polyether triol, three mols TDI per mol of triol, and then reacting substantially all of the remaining isocyanato groups with an ethylenically unsaturated, reactive hydrogen containing compound copolymerizable with the acrylate monomers. TDI is particularly suitable, both 2,4 TDI and 2,6 TDI, because its two isocyanato groups have different reactivities facilitating this stepwise condensation.

Polyethers based on polypropylene oxide are the preferred triols and trimethylol propane condensed with propylene oxide with a molecular weight between about 300-4500, preferably 700-1500, is most preferred. Hydroxyl-lower alkyl acrylate or methacrylate are the preferred active hydrogen containing unsaturated compounds, where "lower" indicates a typical chain length of up to about 4. It has been discovered that such prepolymers polymerize readily and rapidly under radiation to form inks and coatings with an excellent combination of properties. Such preferred prepolymers are shown in the Examples, particularly Examples 1-9 and comprise condensation of 1 mol of triol with 3 mols TDI and substantially 3 mols of hydroxy-lower alkyl acrylate (or methacrylate).

It has been found that prepolymer materials of this invention cure extremely well with ultraviolet light and with electron beams. The films of the order of one mil or less cure readily with ultraviolet light. Thick films may require more penetrating radiation, particularly if a pigment is present, and for these plasma arc radiation, electron beam radiation, or X-rays are desirable.

The preparation of unsaturated polyether-polyurethane prepolymers for making radiation curable coatings in accordance with the invention may be carried out by reacting a polyisocyanate with a secondary polyether polyol at a temperature not exceeding 75"C. in the presence of a catalyst chosen for a low degree of promotion of side reactions. Stannous octoate is preferred as a catalyst. Stannous butyrate and stannous laurate have also been found usable. Because of the high viscosity of the reaction product, the reaction is generally carried out in a solvent. The solvent may be either a hydrocarbon such as toluene, in which case it must be removed by evaporation later, or it may be an acrylate material of the class of materials suitable for use in ink and coating formulations, in which case the solvent may be allowed to remain in the product. Examples of the latter type of acrylate monomer solvents are: trimethylolpropane triacrylate, 1 ,6-hexanediol-diacrylate, 1,3-butanediol diacrylate, neopentylglycol diacrylate, and pentaerythritol tetramethacrylate.

The solvent must be dry to avoid the production of by-products.

The reaction in the solvent is carried out in a vessel equipped with an agitator and blanketed with nitrogen. The mixture is stirred at room temperature as the polyol is added to the diisocyanate and the addition is preferably carried out at such a rate that with the available water cooling the temperature of the reaction mixture does not rise above about 50"C then after an hour or so of maintaining the reaction mixture at 50"C or just below that temperature, an ethylenically unsaturated reactive hydrogen containing compound, e.g. an alcohol or a hydroxy ester is added, preferably somewhat in excess, as for example 10% in excess, of the amount needed to react with those isocyanate groups which are left unreacted after the reaction with the polyether polyol. At this point it is preferable to switch from blanketing the reaction mixture with dry nitrogen to blanketing with dry air (oxygen is a good polymerization inhibitor). During this addition the temperature of the reaction mixture may be permitted to rise to 750C., but preferably no further. After the addition is terminated, the temperature is preferably raised to 90"C. and after an hour or so a small amount of p-methoxyphenol is preferably added to inhibit premature polymerization. When a hydrocarbon solvent is to be removed, it is best to remove the solvent under reduced pressure at a temperature as low as possible, in order to avoid premature polymerization of the prepolymer. The modified prepolymer as above prepared may then be compounded with one or more acrylate monomers esters on a roller mill. It is preferred to have trimethylolpropane triacrylate included as one of the components of this material, and of course sometimes this component is already there for having been used as a solvent in the previous step. Other acrylate monomer components for use at this stage are acrylated epoxidized soybean oil, hydroxyethyl acrylate, hydroxyethyl methacrylate, 1,4-butanediol diacrylate, neopentyl glycol diacrylate, pentaerythritol tetra-acrylate, pentaerythritol triacrylate, hexanediol diacrylate, butyl acrylate, isodecyl acrylate, octadecylacrylate, dimethyl aminoethyl methacrylate, acrylic acid, methacrylic acid, acrylamide and/or methylene bis-acrylamide. It is preferred that the radiation curable liquid contain at least one acrylate monomer which is trifunctional (ethylenically unsaturated groups) or higher to promote cross-linking in the cured film.

Acrylic monomers useful in this invention may include acrylic or methacrylic acids, preferably esters thereof, and preferably esters thereof condensed with polyols and polyamines to form polyacrylates, that is monomers having two or more, and preferably three or more unsaturated acrylate groups.

It is also preferred to introduce at this stage a small to moderate amount of a photoinitiator for coatings to be cured by UV or plasma arc radiation (i.e., by actinic light generally).

Coatings to be cured by electron beam or X-ray radiation do not generally require a photoinitiator. The following are suitable photoinitiators benzil, benzoin, benzoin alkyl ethers, acyloin derivatives in general, benzophenone, acetophenone and Michler's ketone.

Other compounds useful as photoinitiators for this purpose are those listed in Table 5-13, page 132, Molecular Photochemistry by N.J. Turro (W. Benhamin, Inc., 1967). Since acrylate monomers are essentially transparent to UV, sufficient photosensitizer should be used to permit rapid polymerization under moderate irradiation, preferably under 1 second. From about 0.5 to about 20% by weight can be used with about 10% by weight of coating solution being preferred.

To provide a release coating or oxygen barrier there may be introduced at this stage generally between one half of 1% and a few percent of wax, e.g., between about 0.5% and about 10% by weight, either paraffin wax, ester wax, a fluorcarbon wax or some other waxy material such as a higher alkyl alcohol or acid or oleamide, or a waxy silicon-containing material such as a siloxane (including polysiloxanes) particularly silicone or silane or a mixture of some of these. Even a small quantity of such waxy material tends to migrate to the surface of the coatings and provides a release type surface with characteristics similar to those of fluorocarbon polymers or silicones.

When the monomers added at this stage are such as to produce a mixture of low viscosity, the result is a coating material very useful as an overprint varnish. The coating can be cured by an exposure to ultraviolet radiation from a medium pressure mercury lamp for from 0.1 second to several seconds to form a hard glossy coating. When the monomers added are such as to produce a heavy oil, the resulting material is useful for coating rigid panels, such as paperboard or veneer panels, with a film having a thickness of one to several mils. In this case ultraviolet light may be insufficient to provide quick curing and higher energy radiation can be used, such as electron beam radiation on X-rays. In the absence of air, an exposure to about 2-6 Mrad is sufficient for curing with electron beam radiation. In the presence of air, 10-20 Mrad or more are sometimes needed.

For the manufacture of inks, pigments such as Lithol rubine pigment, molybdate orange, chrome yellow, phthalocyanine blue, carbon black or dyes are mixed in at the same time as most of the acrylic monomers. Here also it is necessary to add 5-20% by weight of a photoinitiator to inks intended to be curable by UV or plasma arc radiation. The resulting ink may be printed onto paper, paperboard, plastics film, metal or other stock, and the printing can be rapidly cured by exposure for a fraction of a second to ultraviolet light from a low pressure mercury lamp. An abrasion resistant and solvent resistant pnntmg Is thereby produced, which does not require an overprint varnish.

It has further been found advantageous to include in the radiation curable liquid solution an essentially linear polymer which is soluble therein, preferably one having a molecular weight of about 4,000 to the limit of solubility, and more preferably between about 10,000 and 20,000. Such addition has been found to improve the physical properties of the cured films by reinforcement and to limit shrinkage. This is especially useful with acrylate monomers which may have relative high shrinkage on curing and which are highly cross-linked.

The foregoing aspects of the invention are illustrated in detail in the following examples wherein all parts are by weight. Examples 1 to 9 and 15 illustrate the preparation of various prepolymers in accordance with the invention, and Examples 10 to 14 illustrate various coatings using the prepolymers of the previous Examples. Examples 16 to 25 illustrate the advantageous inclusion of linear polymers in the radiation curable coatings.

Example 1 522 parts 2,4-toluene diisocyanate, 500 parts dried toluene and 0.5 parts stannous octoate are placed in a vessel equipped with an agitator and blanketed with nitrogen. With stirring, a solution of 735 parts poly p ropylene oxide) triol (Dow Voranol (Registered Trade Mark), CP700, Hydroxyl No.229; 0.02 % H20) in 750 parts dried toluene is added at such a rate that the temperature of the reaction mixture does not exceed 50"C. The reaction mixture is stirred at 50"C for 1 hour. Then the nitrogen blanket is replaced by a dry air blanket and 383 parts hydroxyethyl acrylate (10 % excess , mixed with 100 parts of dried toluene, are added at such a rate that the temperature of the mixture does not rise above 75"C. After the addition is completed, the reaction mixture is kept stirring at 90"C for 1 hour. 1 part p-methoxy phenol (polymerization inhibitor) is added and the solvent is removed under reduced pressure at or below 30"C. 1650 parts of a clear, colourless, resinous oil, containing a trace of toluene solvent are obtained. Analysis of this material shows 0.06% by weight isocyanate content.

Example 2 522 parts 2.4-toluene diisocyanate. 500 parts dried toluene and 0.5 part stannous octoate are placed in a vessel equipped with an agitator and blanketed with nitrogen. With stirring, a solution of 735 parts poly (propylene oxide) triol (Dow Voranol CP700; Hydroxyl No. 229; 0.02% H2O) in 750 parts dried toluene is added at such a rate that the temperature of the reaction mixture does not exceed 50"C. The reaction mixture is stirred at 50"C. for 1 hour.

Then 191.4 parts allyl alcohol (10% excess) are added at such a rate that the temperature of the mixture does not rise above 85"C. After the addition is completed, the reaction mixture is kept stirring at 90"C. for 1 hour. 1 part p-methoxy phenol (polymerization inhibitor) is added and the solvent is removed under reduced pressure at or below 30"C. 1440 parts of a clear, colourless, resinous oil, containing a trace of toluene, are obtained. Analysis of the resulting material shows 0.04% by weight isocyanate content.

Example 3 522 parts 2,4-toluene diisocyanate 780 parts trimethylol propane triacrylate, and 0.5 parts stannous octoate are placed in a vessel equipped with an agitator and blanketed with nitrogen. With stirring, 735 parts of poly (propylene oxide) triol (Dow Voranol CP700; hydroxyl No.229; 0.02%H2O) are added at such a rate that the temperature of the reaction mixture does not rise above 50"C. The reaction mixture is kept at this temperature for an additional 1 hour. Then the nitrogen blanket is replaced by a dry air blanket and 383 parts hydroxyethyl acrylate (10% excess) are added at such a rate that the temperature of the reaction mixture does not exceed 75"C. The reaction mixture is stirred for an additional 1 hour at 90"C. Then 1 part p-methoxy phenol is added and the reaction mixture is allowed to cool to room temperature. The product, 1605 parts of unsaturated prepolymer in 780 parts of trimethylolpropane triacrylate and 35 parts of hydroxyethyl acrylate, is a clear, colourless, viscous oil. Analysis of the resulting material shows 0.06% by weight isocyanate content.

Example 4 522 parts 2,4-toluene diisocyanate, 780 parts trimethylolpropane triacrylate, and 0.5 part stannous octoate are placed in a vessel equipped with an agitator and blanketed with nitrogen. With stirring, 735 parts of poly (propylene oxide) triol (Dow Voranol CP700; Hydroxyl No.229; 0.02%H20) are added at such a rate that the temperature of the reaction mixture does not rise above 50"C. The reaction mixture is kept at this temperature for an additional 1 hour. Then 191.4 parts allyl alcohol (10% excess) are added at such a rate that the temperature of the reaction mixture does not exceed 75 C. The reaction mixture is stirred for an additional 1 hour at 90"C. Then 1 part p-methoxy phenol is added and the reaction mixture is allowed to cool to room temperature. The product consists of 1413 parts unsaturated prepolymer in 780 parts trimethylolpropane triacrylate and 17.4 parts allyl alcohol. It is a clear, colourless, viscous oil. Analysis of the resulting material shows 0.03% by weight isocyanate content.

Example 5 504 parts 1,6-hexamethylene diisocyanate, 750 parts trimethylolpropane triacrylate, and 0.5 parts stannous octoate are placed in a vessel equipped with an agitator and blanketed with nitrogen. With stirring, 735 parts of poly (propylene oxide) triol (Dow Voranol CP700; Hydroxyl 229; 0.02%H20) are added at such a rate that the temperature of the reaction mixture does not rise above 50"C. The reaction mixture then is kept at 60"C. for 2 hours with stirring. Following this, 409.2 parts 5-norbornene-2-methanol (10% excess) are added at such a rate that the temperature of the reaction mixture does not rise above 75"C. After the addition is completed, the temperature is raised to 90"C. and the reaction mixture is stirred at this temperature for an additional 2 hours. Then the reaction mixture is allowed to cool to room temperature. The product consists of 1611 parts unsaturated prepolymer in 700 parts trimethylolpropane triacrylate and 37.2 parts 5-norbornene-2-methanol. It is a clear, colourless, viscous oil. Analysis of the resulting material shows 0.08% by weight isocyanate content.

Example 6 750 parts p,p'-diphenylmethane diisocyanate, 1000 parts toluene, and 0.5 part stannous octoate are placed in a vessel equipped with an agitator and blanketed with nitrogen. The mixture is stirred at room temperature and 258 parts poly (propylene oxide) triol (Voranol (Registered Trade Mark) CP260; Hydroxyl No. 653; 0.02% H2O) are added at such a rate that the temperature of the reaction mixture does not rise above 50"C. The reaction mixture is kept at this temperature for an additional 1 hour. Then the nitrogen blanket is replaced by a dry air blanket and 383 parts of hydroxy-ethyl acrylate (10% excess) are added at such a rate that the temperature of the reaction mixture does not exceed 75"C. After the addition is terminated, the temperature of the reaction mixture is raised to 90"C. for 1 hour. After adding 1.0 part p-methoxy phenol, the reaction mixture is allowed to cool to room temperature and the solvent is removed under reduced pressure at or below 30"C. The liquid product, 1398 parts, is a clear off-white resin. Analysis of the resulting material shows 0.01 % by weight isocyanate content.

Example 7 1746 parts dimeryl diisocyanate, 500 parts dried toluene, and 1.0 part stannous octoate are placed in a vessel equipped with an agitator and blanketed with nitrogen. With stirring, 258 parts poly (propylene oxide) triol (Dow Voranol CP260; Hydroxyl No. 653; 0.02%H20) are added at such a rate that the temperature of the reaction mixture remains below 50"C. After the addition is completed, the reaction mixture is kept at 60"C. for 2 hours with stirring. Then 191.4 parts of allyl alcohol (10% excess) are added at such a rate that the temperature of the reaction mixture does not exceed 75"C. After the addition is completed the temperature is raised to 900 C. and the reaction mixture is kept stirring at this temperature for 2 hours. Then 1.0 part p-methoxy phenol is added, and the solvent is removed under reduced pressure.

2185 parts clear, colourless heavy oil is obtained as the product. Analysis of the resulting material shows 0.08% by weight isocyanate content.

Example 8 774 parts 4,4'-dicyclohexylmethane diisocyanate, 1000 parts trimethylolpropane triacrylate, and 0.5 part stannous octoate are placed in a vessel equipped with an agitator and blanketed with nitrogen. The mixture is stirred at room temperature and 735 parts of poly (propylene oxide) triol (Dow Voranol CP700; Hydroxyl No. 229; 0.02% H2O) are added at such a rate that the temperature does not exceed 50"C. After the addition is completed, the reaction mixture is kept stirring at 50"C. for an additional 1 hour. Then the nitrogen blanket is replaced by a dry air blanket and 383 parts hydroxyethyl acrylate (1 0 % excess) are added at such a rate that the temperature of the reaction mixture does not rise above 75"C. Following this step the reaction mixture is stirred for 2 hours at 90 C. Then 1.0 part p-methoxyphenol is added and the reaction mixture is allowed to cool to room temperature. The product, a clear, colourless, viscous oil, consists of a mixture of 1857 parts unsaturated prepolymer, 1000 parts trimethylolpropane triacrylate, and 35 parts hydroxyethyl acrylate. Analysis of the resulting material shows 0.06% by weight isocyanate content.

Example 9 348 parts 2,4-toluene diisocyanate, 500 parts dried toluene, and 0.5 part stannous octoate are placed in a vessel equipped with an agitator and blanketed with nitrogen. Then 407 parts poly (propylene oxide) diol (Dow Voranol P400; Hydroxyl No.275; 0.02%H20) are added at such a rate that the temperature of the reaction mixture does not rise above 50"C. The reaction mixture is stirred at this temperature for an additional hour. Following this step, 286 parts hydroxyethyl methacrylate (10% excess) are added at such a rate that the temperature of the reaction mixture is raised to 90"C. and the stirring continued for an additional 1 hour.

At the end of this period 1.0 part p-methoxy phenol is added and the solvent is removed under reduced pressure. A clear, colourless, viscous oil is obtained as the product. Analysis of the resulting material shows 0.02% by weight isocyanate content.

Example 10 - UV Curable Overprint Varnish 100 parts of the product of Example 1, 235 parts trimethylolpropane triacrylate, 30 parts hydroxyethyl acrylate, 3 parts stearyl acrylate and 25 parts benzoin isobutyl ether are mixed well on a roller mill. A clear, colourless homogeneous light oil is obtained as the product.

Films of 0.4 mil thickness are applied with a wire wound coating rod onto paper, aluminium foil, vinyl coated aluminium foil, polyester coated Mylar (Registered Trade Mark), and steel.

The coated substrates are exposed for 1/ 10 second to the UV radiation given off by a medium pressure mercury vapour lamp (Hanovia (Registered Trade Mark) 200 W/inch) at a distance of 5" from the lamp. This distance coincides with the second focal point created by the elliptical reflector.

After this exposure all samples are cured to hard, glossy coatings with a pencil hardness of at least 2H and a rub-resistance of at least 40 rubs, using methyl ethyl ketone as the solvent.

Example 11 - UV Curable Overprint Varnish 20 parts reaction product from Example 3, 8 parts trimethylolpropane triacrylate, 12 parts 1.4-butanediol diacrylate, 10 parts acrylated epoxidized soybean oil (Union Carbide Co.

ActomerX-70), 10 parts acrylic acid, 0.4 part stearyl acrylate and 2.75 parts benzoin isobutyl ether are mixed on a roller mill. A homogeneous, clear, colourless light oil is obtained. Films are applied and cured as shown in Example 10. The cured films have a pencil hardness of HB, are insoluble in solvents and rub resistant.

Example 12 - Electron Beam Curable Coating 30 parts of reaction product from Example 3, 20 parts trimethylolpropane triacrylate and 10 parts neopentyl glycol diacrylate are mixed on a roller mill. A clear, colorless heavy oil is obtained as the product. Steel and wood panels are coated with this composition with a 1 mil film knife. The coated test panels are exposed to 6 Mrad electron beam radiation under exclusion of air. After the irradiation, the coatings are clear, colorless, and glass-like. They have a pencil hardness of 4H, show excellent adhesion to metal and wood and are unaffected by solvent.

Example 12 - Electron Beam Curable Coating 20 parts reaction product from Example 3, 10 parts trimethlolpropane triacrylate, 10 parts reaction product from Example 9, 10 parts acrylated epoxidized soybean oil (Union Carbi H20) are added at such a rate that the temperature of the reaction mixture does not rise above 50"C. The reaction mixture is stirred at this temperature for an additional one hour.

Following this step, 510 parts 2-hydroxyethyl acrylate (10% excess) are added at such a rate that the reaction mixture starts to reflux gently. The stirring is continued at reflux for an additional one hour. At the end of this period 2.0 parts p-methoxyphenol is added and the solvent is removed under reduced pressure. A clear, colourless, viscous oil is obtained as the product.

Examples 16 to 25 illustrate coatings and films which are radiation curable and which have good film strength, good adhesion, good abrasion and solvent resistance, and which can also be cast to reproduce with good fidelity the surface over which they are cast. They incorporate, in the radiation curable films as herein disclosed, other essentially linear polymers of generally higher molecular weight which are soluble in the radiation-curable liquid composition. They should preferably have a molecular weight of at least about 4,000 up to the limit of solublility, and more preferably about 10,000 to 20,000. These polymers add bulk, strength, and control shrinkage in the cured composite films. The added polymers are pre-polymerized and generally do not react during radiation curing, although they can advantageously include reactive groups, such as ethylenic unsaturation, for example, as end groups, which will co-polymerize during curing. By inclusion of a wax or wax-like material of limited compatibility as herein disclosed, and which migrate to the surface, an oxygen barrier is provided which permits the films to be cured in the presence of oxygen, e.g., in air, and which provide films with release properties.

Example 16 The following ingredients are mixed well, all parts being by weight.

11.4 parts unsaturated polyether-polyurethane prepolymer, prepared from 1 mole polyether triol, 3 moles 2,4 toluenediisocyanate, and 3 moles 2-hydroxyethyl acrylate, according to Example 1 above 17.35 parts trimethylol propane triacrylate 10.65 parts neopentyl glycol diacrylate 2.4 parts acrylated epoxidized soybean oil (ACTOMER X-70, Union Carbide Corp.) 3.95 parts 2-hydroxyethyl acrylate 0.45 parts cyclododecane, dissolved in 2.3 parts methylene chloride 50.0 parts methyl methacrylate copolymer, 40% solids in a mixture of toluene and isopropanol (ELVACITE (Registered Trade Mark) 6014, DuPont).

After mixing well, the solvents are removed at room temperature in the dark, under reduced pressure, and in the presence of copper shaving as polymerization inhibitor. Thereafter, the following ingredients are added.

6.0 parts benzoin isobutyl ether (photoinitiator) 2.15 parts N-beta (aminoethyl)-gamma-aminopropyl trimethoxy silane (Silane A-1120, Union Carbide Corp.), (a limited-compatibility waxy material). The resulting liquid composition can be coated as hereinbefore described. It can also be cast, e.g. over an optical reflective pattern on a steel surface, with or without release agent, rapidly cured with radiation and stripped to provide a self-supporting film containing on one surface a good replica of the optical pattern.

Examples 17 to 25 Examples 17 to 25 below illustrate other additional polymers which can be substituted for ELVACITE 6104 in Example 16 to provide, by the same procedure, good radiation curable casting films and release films or coatings.

TABLE I Example No. Parts Ingredients 17 17.0 G-CURE 869 S, General Mills Chemicals Inc., a hydroxy functional acrylic copolymer, 60% solids in a mixture of xylene and cellosolve (Registered Trade Mark) acetate.

18 10.0 PM Polymer (Phillips Petroleum Co., a mercaptan terminated hydrocarbon polymer 19 42.0 Bakelite (Registered Trade Mark) T-24-g (Union Carbide Corp., a vinyl alcohol-vinyl acetate co polymer, 25% solids in toluene).

20 40.0 Bakelite Vinyl Butyral XYLS-4 (Union Carbide Corp., 25% solids).

21 10.0 Polyketone Resin 250 (Union Carbide Corp., dissolved in 50 parts toluene).

22 17.0 Spencer-Kellogg DV-2230, an aliphatic polyurethane lacquer, 30% solids in a mixture of tetrahydrofuran and iso propanol.

23 10.0 Castomer (Registered Trade Mark) P-0010 (Witco Chemical Co., an unsaturated polyurethane dissolved in 50 parts methylene chloride.

24 20.0 Acryloid AT-71 (Rohm & Haas Co., a polyacrylate with carboxyl function ality, 50 solids).

25 20.0 Acryloid AT-63 (Rohm & Haas Co., a polyacrylate with hydroxyl functionality, 50% solids in xylene).

WHAT WE CLAIM IS: 1. A method of producing a liquid prepolymer for preparing a radiation curable composition. comprising progressively adding a secondary polyether polyol to a polyisocyanate thereby effecting a controlled reaction in which the polyisocyanate is at all times in excess, and further adding an ethylenically unsaturated reactive hydrogen containing compound in sufficient quantity to react with substantially all of the unreacted polyisocyanate.

2. A method as claimed in Claim 1 further comprising the addition of an acrylic ester monomer copolymerizable by radiation with the said prepolymer to provide coating viscosity.

3. A method as claimed in Claim 1 or Claim 2 further comprising the addition of a photoinitiator for promoting rapid cure under exposure to radiation.

4. A method as claimed in Claim 3 in which; the polysocyanate is a diisocyanate, the secondary polyether polyol is a polyether triol and the ethylenically unsaturated reactive hydrogen containing compound is an unsaturated alcohol selected from allyl alcohol, 5-norbornene 2-methanol and hydroxy lower alkyl esters of acrylic or methacrylic acid; and the acrylic ester monomer is selected from trimethylolpropane triacrylate, hydroxyethyl acrylate, neopentyl glycol diacrylate, acrylated epoxidised soybean oil, hexanediol diacrylate, 1,4 butanediol diacrylate. pentaerythritol tetraacrylate, pentaerythritol triacrylate,butyl acrylate, isodecylacrylate, octadecyl acrylate and mixtures of two or more of these and the photoinitiator is a benzoin lower alkyl ether and/or acetophenone and/or benzophenone and or Michler's ketone or any of these photoinitiators together with a coinitiator comprising a hydroxyalkyltertiaryamine.

5. A method as claimed in any of Claims 1 to 4 further comprising the addition of a polymer to act as a strengthening and shrinking control agent.

6. A method of producing a liquid prepolymer substantially as described herein with reference to any of Examples 1 to 9.

7. A liquid prepolymer when produced bv a method as described in any of Claims 1 to 6.

8. A prepolymer substantially as described herein with reference to any one of Examples 1 to 9.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (9)

1. **WARNING** start of CLMS field may overlap end of DESC **.

   TABLE I Example No. Parts Ingredients

   17 17.0 G-CURE 869 S, General Mills Chemicals Inc., a hydroxy functional acrylic copolymer, 60% solids in a mixture of xylene and cellosolve (Registered Trade Mark) acetate.

   18 10.0 PM Polymer (Phillips Petroleum Co., a mercaptan terminated hydrocarbon polymer

   19 42.0 Bakelite (Registered Trade Mark) T-24-g (Union Carbide Corp., a vinyl alcohol-vinyl acetate co polymer, 25% solids in toluene).

   20 40.0 Bakelite Vinyl Butyral XYLS-4 (Union Carbide Corp., 25% solids).

   21 10.0 Polyketone Resin 250 (Union Carbide Corp., dissolved in 50 parts toluene).

   22 17.0 Spencer-Kellogg DV-2230, an aliphatic polyurethane lacquer, 30% solids in a mixture of tetrahydrofuran and iso propanol.

   23 10.0 Castomer (Registered Trade Mark) P-0010 (Witco Chemical Co., an unsaturated polyurethane dissolved in 50 parts methylene chloride.

   24 20.0 Acryloid AT-71 (Rohm & Haas Co., a polyacrylate with carboxyl function ality, 50 solids).

   25 20.0 Acryloid AT-63 (Rohm & Haas Co., a polyacrylate with hydroxyl functionality, 50% solids in xylene).

   WHAT WE CLAIM IS: 1. A method of producing a liquid prepolymer for preparing a radiation curable composition. comprising progressively adding a secondary polyether polyol to a polyisocyanate thereby effecting a controlled reaction in which the polyisocyanate is at all times in excess, and further adding an ethylenically unsaturated reactive hydrogen containing compound in sufficient quantity to react with substantially all of the unreacted polyisocyanate.
2. 2. A method as claimed in Claim 1 further comprising the addition of an acrylic ester monomer copolymerizable by radiation with the said prepolymer to provide coating viscosity.
3. 3. A method as claimed in Claim 1 or Claim 2 further comprising the addition of a photoinitiator for promoting rapid cure under exposure to radiation.
4. 4. A method as claimed in Claim 3 in which; the polysocyanate is a diisocyanate, the secondary polyether polyol is a polyether triol and the ethylenically unsaturated reactive hydrogen containing compound is an unsaturated alcohol selected from allyl alcohol, 5-norbornene 2-methanol and hydroxy lower alkyl esters of acrylic or methacrylic acid; and the acrylic ester monomer is selected from trimethylolpropane triacrylate, hydroxyethyl acrylate, neopentyl glycol diacrylate, acrylated epoxidised soybean oil, hexanediol diacrylate, 1,4 butanediol diacrylate. pentaerythritol tetraacrylate, pentaerythritol triacrylate,butyl acrylate, isodecylacrylate, octadecyl acrylate and mixtures of two or more of these and the photoinitiator is a benzoin lower alkyl ether and/or acetophenone and/or benzophenone and or Michler's ketone or any of these photoinitiators together with a coinitiator comprising a hydroxyalkyltertiaryamine.
5. 5. A method as claimed in any of Claims 1 to 4 further comprising the addition of a polymer to act as a strengthening and shrinking control agent.
6. 6. A method of producing a liquid prepolymer substantially as described herein with reference to any of Examples 1 to 9.
7. 7. A liquid prepolymer when produced bv a method as described in any of Claims 1 to 6.
8. 8. A prepolymer substantially as described herein with reference to any one of Examples 1 to
9. 9.