Curable Pre-Polymer Compositions, Method of Making and Method of Coating Articles Therewith

The present invention relates to the preparation of U.V. and electron beam curable compositions and to the formation of coatings from such compositions.

It is well known that many ethylenically unsaturated monomers may be polymerised upon being suitably activated. This activation may be derived from free radical initiators, by photoinitiation, by thermal initiation or by the use of U.V. or ionising radiation. These polymerisable monomers have been used as coating materials and cured in situ by one of the means referred to above. These coatings have been found to be generally unsatisfactory due to the difficulty of obtaining a complete cure of the coating. It is thought that this difficulty stems largely from the coating layer being too thin to enable a sufficiently high steady state of free radicals to be generated. A further difficulty associated with the in situ curing of the monomers referred to above is that the low viscosity of the monomer makes it difficult to form an even coating on an article and to maintain that coating until curing has taken place.

It has been proposed in U.S. Pat. Nos. 3,551,246 and 3,546,002 to form prepolymers thermally and to form U.V. curable coatings from the prepolymers. These thermally produced prepolymers have been found to be of limited use due to the dark colour generated within the prepolymer during its formation; due to the difficulty of curing the prepolymer; and due to the limited number of monomers and monomer blends which will form curable prepolymers without the precipitation of homopolymer from the mixture.

The present inventors have found that a very large number of prepolymers which are generally clear, highly viscous and water white in colour and which cure readily under U.V. radiation or an electron beam may be prepared by the use of U.V. or ionising radiation initiation.

In a process aspect, this invention relates to a process for the preparations of a prepolymer capable of being cured by ultra-violet or ionising radiation, comprising the steps of subjecting either (a) a polymerisable ethylenically unsaturated monomer, or a mixture of such monomers, or (b) a mixture of one or more such monomers and a long chain organic compound or a polymeric material to ionization radiation or, in the case of (b), to ultra-violet or ionising radiation until the monomer, or monomers, achieves a predetermined but incomplete degree of polymerisation, and removing the prepolymer so formed from the radiation. In a composition aspect, this invention relates to the prepolymers so formed.

In another aspect, this invention relates to a method of coating a substrate comprising applying to the substrate a layer of a prepolymer prepared according to the process of this invention and curing the prepolymer by subjecting it to ultra-violet or ionising radiation. In another aspect, this invention relates to a substrate having a coating thereon formed by the foregoing method according to this invention.

Any of the very large number of known polymerisable ethylenically unsaturated monomers can be used in the formation of prepolymers according to this invention. The preferred monomers are those which include a vinyl or vinylidene group. The properties of the final coating depend in part upon the monomer or monomers used to form the prepolymer and the most preferred monomers will depend upon the intended end use of the prepolymer. For coating onto paper and cardboard, as in printing inks, where a glossy, water impervious, coating is required the preferred starting materials are styrene and vinyl chloride/ethyl acrylate and vinyl chloride/methyl acrylate blends in ratios of 3:1 to 1:1. For coating onto metals such as copper, aluminium or tin plate where a very hard scratch resistant coating is required styrene, and blends of two or more of the following, styrene, butyl acrylate, methyl methacrylate vinylidene chloride, ethyl acrylate, acrylonitrile, and vinyl acetate, generally in ratios of from 1 to 3 to 3 to 1 have been found to work well. For coating onto polypropylene and other synthetic plastics materials blends of acrylic acid and alkyl acrylate in ratios of 1 to 1 to have formed coatings highly adherent to the substrate. For the coating of the surfaces of concrete structures blends of methyl methacrylate with acrylonitrile or vinyl acetate have been found to perform will in adhering to the substrate and forming a water and acid resistant coating. For the lining of concrete sewerage pipes base coatings of blends of butyl acrylate with methyl methacrylate or vinyl acetate and ter-polymers of isoprene/styrene/acrylonotrile have proved very satisfactory and have shown an ability to flow under applied stresses such that the coating will not be ruptured by cracks in the pipe. In such applications a top coat of blends of two of methyl methacrylate, vinyl acetate, acrylonitrile and styrene will give abrasion resistance and water impermeability to the coating.

The starting material may include additionally long chain organic molecules such as natural drying oils e.g. linseed oil, shellac, cellulose acetate and natural resins e.g. gum rosin, or polymeric materials such as polyamides, polyesters including alkyd resins, polycarbonates, epoxy resins, polyvinylalcohols, polyvinyl chlorides, polysiloxanes and polysilanes either with or without solvents. These additional materials may be used with single monomers or with mixtures of monomers. Styrene/Gum resin and Styrene/alkyd resin blends have been found to be advantageous in the formation of printing inks while butyl acrylate/linseed oil prepolymers are useful in forming base coats in concrete sewerage pipes. Blends of linseed oil, alkyd resins and gum rosin with one or more of the following monomers viz. .alpha. methyl styrene, styrene and butyl acrylate have formed coatings on metal which exhibit excellent scratch resistance. Mixtures including long chain organic molecules or polymers are preferably prepolymerised using U.V. radiation.

The monomers referred to above, when used alone, in mixtures or in blends with long chain compounds or polymers are particularly preferred however the present invention is not limited to the use of these monomers. A large number of other monomers may also be used to form curable prepolymers and these include diallyl phthalate, vinyl ethyl ether, 2-hexyl ethyl acrylate, diallyl isophthalate, allyl glycidyl ether, acrylamide, alkylamine, allylacrylte, vinylpyridine, maleic anhydride, acrylic acid and divinyl benzene. These monomers may be used alone, together in mixtures or, if the prepolymer is to be cured by U.V. radiation, together with long chain molecules or polymers.

The formation of the prepolymer is brought about by U.V. or ionising radiation. U.V. and .gamma.-radiation are particularly preferred due both to the relative ease of radiation and the good results achieved however other ionising radiation such as x rays or electron beams may be used. The selection of the source does not appear to be critical however the present inventors have found a 130 w U.V. Wotan lamp set 13 cm. from the centre of a glass reaction vessel to be satisfactory, similarly for .gamma. radiation doses of from 0.01 to 5 M rads frm a spent fuel rod source have proven satisfactory. For any given starting material and any given source simple experimentation will be required to find the optimum duration of irradiation. The duration will also depend upon the intended end use of the prepolymer. In general the radiation will be continued until the prepolymer will have a suitable viscosity once it has cooled to ambient temperature after the irradiation. What is a suitable viscosity will largely depend upon how the prepolymer is to be coated onto the substrate and the nature of the substrate itself. If the prepolymer is to be sprayed on or if the substrate is to be dipped into the prepolymer then the prepolymer should not be unduly viscous. If the prepolymer is to be brushed, rolled or spread with a scraper onto the substrate then the prepolymer is preferably very viscous. Absorbant substrates such as wood and paper require viscous prepolymers so that the prepolymer will not be absorbed into the substrate before it can be cured. As a rough guide the present inventors used a magnetic stirrer of conventonal type to stir the monomer mixture and found that for most applications it was satisfactory to remove the prepolymer from the radiation when the viscosity of the prepolymer was such that the stirrer no longer rotated. It should be stressed that the prepolymer must be removed from the radiation while it still has a capacity for chain propogation i.e. before polymerisation has been completed.

The formation may be carried out at room temperature or may be carried out at an elevated temperature. The application of heat is possible within the limits of the boiling point of the monomer or monomers used to form the prepolymer and their tendency upon heating to form solid homopolymers rather than liquid prepolymers. Heating has been found to be particularly advantageous where the prepolymer is formed by U.V. initiation as the increased temperature allows a shorter period of U.V. initiation resulting in less colouration of the prepolymer due to oxidation. The high energies available with commercial .gamma. ray sources render temperature effects with .gamma. ray initiation negligible.

Additives which can be used to modify prepolymer formation and/or curing of the prepolymer, include sensitisers, solvents, chain transfer agents and inhibitors.

U.V. initiation of prepolymer formation is generally enhanced by the presence of from 1 to 20% by weight of a conventional photo-sensitizer such as benzoin ethers, uranyl nitrates, benzophenone or Michler's ketone. This photosensitiser is preferably added to the monomers before the prepolymer formation however additional photosensitiser may be added prior to the U.V. cure of the prepolymer coating. It appears to be desirable to avoid photosensitiser additions if the prepolymer is to be cured using an electron beam.

Solvents are preferably avoided in the present invention for economic and environmental reasons however for spraying some prepolymers it is desirable to dissolve the prepolymer in a compatible solvent. Solvents may also be added to maintain one phase systems with otherwise incompatible monomers.

Chain transfer agents of conventional types such as carbon tetrachloride or carbon tetrabromide may be added to the prepolymer to enhance the curing of the prepolymer under U.V. or electron beam initiation. Chain transfer agents are particularly desirable when the prepolymer is pigmented with a U.V. opaque pigment.

Conventional pigments may be added to the system either at the prepolymer formation stage or at the curing stage by conventional means. Dyes may also be added provided that they are not destroyed by the curing process.

If necessary conventional inhibitors may be added to the prepolymer to prevent change in the prepolymer during storage. Inhibitors such as methyl ethyl hydroquinone may be used in amounts of up to 2000p.p.m.

The prepolymers formed by the process according to the present invention may be applied to any suitable substrate such as paper, cardboard and other cellulosic materials such as wood, particle board and the like; metals such as copper, aluminium, tin plate and mild steel; concrete products such as building blocks, pipes and concrete buildings; synthetic plastics such as polypropylene; natural polymers such as leather; and glass.

The coating of the substrate may be achieved by dipping, spraying, brushing, rolling, spreading with a blade or by any other suitable means such as by a printing press.

The curing of the prepolymers is achieved either using U.V. or ionising radiation, the ionising radiation preferably being an electron beam. The time of cure is proportional to the incident radiation and accordingly where rapid cure is desired a high intensity source is preferred. The present inventors used a 6 inch 200 w/inch Hanovia U.V. lamp at a distance of 30 cm from the substrate to achieve curing times of from 1 to 50 minutes; subsequent use of a 36 inch lamp of 20 w/inch fitted with a reflector and at a shorter distance from the lamp showed cure times approximately 300 times faster. Electron beam irradiations of the order of 1 M rad or more were generally necessary to achieve surface or through cure of prepolymer coatings. If electron beam curing is to be used it is desirable to carry out the cure under nitrogen to avoid oxygen at the coating surface acting as a radical scavenger and reducing the rate of cure of the prepolymer.

Hereinafter given by way of example only are preferred examples of the present invention and comparative examples showing the advantages of the present invention over the prior art.

The following examples show use of prepolymers prepared by the .gamma. radiation of the monomers listed. The prepolymers were cured on the substrates listed using a U.V. lamp of 200 watts/inch at a distance of 30 cm from the sample. The solvent nominated was used solely to enable the prepolymer to be sprayed on to the substrate using a low viscosity spray gun.

The following examples show the use of prepolymers prepared by U.V. radiation of the monomers listed. The prepolymers were cured on the substrates listed in the manner described for the previous examples. The solvents used were solely to enable the prepolymer to be sprayed onto the substrate using a low viscosity spray gun.

13 hour irradiation produced a clear viscous prepolymer. The viscosity did not change much after an initial viscosity increase and so the preparation time is arbitrary.

1% Divinyl Benzene added, 391/4 hours irradiation.

64% by weight of a prepolymer formed from a 1:1 mixture of acrylanitrile and methyl methacrylate by U.V. initiation was mixed with;

14% by weight trimethylal propane triacrylate

6% by weight benzaphenone

2% by weight michelers ketone

14% by weight phthalocyanine blue.

The ink was printed onto paper and cured by one pass at the rate of 85 feet/minute beneath a 200 w/inch U.V. lamp at a distance of 7 cm from the paper. The ink cured to a hard dry coating.

The following examples show the advantages accruing from the use of U.V. initiation as against thermal initiation. The U.V. initiation preferably takes place at an elevated temperature to give short prepolymerisation times, and clear viscous prepolymers which are relatively light in colour.

a. U.V. at 25.degree. C produces in 13 hours a slightly viscous clear one phase prepolymer.

b. Thermal at 110.degree. C produces in 2 hours a clear one phase very viscous prepolymer.

c. Thermal and U.V. at 110.degree. C produces in 11/4 hours a clear very viscous prepolymer.

a. Thermal at 110.degree. C produces in 21/2 hours a clear very viscous prepolymer.

b. Thermal and U.V. at 110.degree. C produces in 11/4 hours a clear very viscous prepolymer.

a. Thermal at 110.degree. C produces in 121/2 hours a yellow slightly viscous one phase prepolymer

b. Thermal and U.V. at 110.degree. C produces in 13 hours a yellow viscous prepolymer.

a. Thermal at 110.degree. C produces in 101/2 hours a clear yellow slightly viscous prepolymer.

b. Thermal and U.V. at 110.degree. C produces in 101/2 hours a clear yellow very viscous prepolymer.

a. Thermal at 110.degree. C produces in 151/2 hours a slightly viscous dark brown one phase prepolymer.

b. Thermal and U.V. at 110.degree. C produces in 16 hours a light brown one phase very viscous prepolymer.

a. U.V. at 25.degree. C produces in 41/2 hours a slightly yellow slightly opaque one phase viscous prepolymer

b. Thermal at 60.degree. C produces in 40 hours a clear yellow one phase viscous prepolymer

c. Thermal and U.V. at 60.degree. C produces in 21/4 hours a clear yellow one phase viscous prepolymer.

a. U.V. at 25.degree. C produces in 23/4 hours a clear very viscous prepolymer.

b. Thermal at 80.degree. C produces in 5 hours a slightly yellow viscous one phase prepolymer

c. U.V. and Thermal at 80.degree. C produces in 11/2 hours a clear viscous one phase prepolymer.

In addition to the monomers referred to above the following monomers have been found to be useful in the present invention;

Allyl Acrylate

Allylamine

Acrolein

Allyl Isophthalyl Cyanate

Cyclo - hexene

Bis Phenol A Bis 2 Hydroxy Propyl Methacrylate

Ethoxylated Bis Phenol A Diacrylate/Ethoxylated Bis

Phenol A Demethacrylate/Diallyl Fumorate

Cyclo Hexyl Methacrylate

Halogenated Olefines such as 2, 3 Dichloro 1, 3

Butadiene

2 Ethyl Hexyl Acrylate

1, 6 Hexane Diol Diacrylate

Hydroxy Ethyl Acrylate

Hydroxy Ethyl Methacrylate

Diethyleneglycol Diacrylate

Diethylene Glycol Dimethacrylate

Dimethyl Amino Ethyl Acrylate

Dimethyl Amino Ethyl Methacrylate

Isapropenyl Acetate

Cetyl Vinyl Ether

Dodecyl Vinyl Ether

Methylene Bis Acrylamide

Maleice Anlhydrid

N-methylol Acrylamide

Methacrylic Acid

Neo Pentyl Glycol Diacrylate

Pentaerythritol Triacrylate

Pentacrythritol Tetracrylate

Trimethylol Propane Triacrylate

Halogenated Styrenes

Glycidyl Methacrylate

The following example illustrates the use of an electron beam to cure a prepolymer according to the present invention.

64. A 1:1 acrylonitrile/styrene prepolymer prepared using .gamma.-rays initiation was coated onto a clear copper shut. The coating was subjected to six passes beneath a 400,000 e.V. electron beam under a nitrogen atmosphere. A hard, clear, scratch resistant film was produced.