GB2073760A - Cationically Polymerizable Radiation Curable Compositions - Google Patents

Abstract

There are disclosed compositions comprising a compound having at least one cyclic vinyl ether moiety and a catalyst capable of forming an acid when exposed to radiation e.g. ultraviolet light. The compositions cure via a cationic polymerization mechanism when exposed to the radiation. Particularly useful compounds are the derivatives of 3,4- dihydropyran-2-methanol.

Description

SPECIFICATION Cationically Polymerizable Radiation Curable Compositions Coatings play a useful role in the manufacture of a great many articles which find wide use in nearly all facets of contemporary life. Until recently, nearly all coatings were applied with the employment of a hydrocarbon based vehicle which evaporated leaving the dried coating on the article which was to be coated. This system met with increasing disfavor as the cost of energy needed to drive off the solvent at the rate required by industry increased, as the price of the organic solvent itself increased and as the deieterious environmental effects of the evaporated solvent became better understood. Systems aimed at solvent recovery to reduce pollution and conserve solvent have generally proven to be energy intensive and expensive.

In response, those skilled in the art have devised a class of coatings termed radiation curable coatings. These curing systems have displayed high desirability because they are relatively free of volatile solvent and cure quickly at low temperatures.

Among the most successful of the radiation curable coatings are those which are curable by ultraviolet light. The success is attributable in part to the lower capital investment required for equipment to cure these coatings. These ultraviolet light curable coatings, termed photocurable coatings, typically employ a solution of a photoinitiator in a reactive coating liquid. The liquid approaches a pollution-free system as almost all of the liquid is converted to cured coating with little or no solvent emission upon the brief exposure to ultraviolet light. Among the most successful photocurable coating systems have been the acrylate based systems because of the toughness of the cured surface.

One of the disadvantages of photocurable systems is the need to employ expensive and possibly harmful photoinitiators. Another disadvantage is the frequent requirement that the curing process be conducted in an inert atmosphere because of the inhibiting effect of oxygen.

Responding to these problems, those skilled in the art have devised photocurable coatings which cure through a mechanism termed cationic polymerization. In these systems the starting materials are mixed with catalysts which form acids when exposed to ultraviolet light; the starting materials are therefore polymerized via cationic catalysis which involves the formation and propagation of carbonium ions to generate the polymer molecules. This cationic polymerization mechanism is essentially limited to those monomers with electron-releasing substituents.

Epoxy resins have been shown to be suitable starting materials for photocure via cationic polymerization as disclosed in U.S. 3,794,576, for example. However, the cured coatings using these starting materials have not demonstrated strong reistance to wear.

A photocurable coating composition formulated with a compound which can be cured via cationic polymerization and when cured displays increased toughness over such compositions as are currently known would be highly desirable.

It has now been found that photocurable coatings can be prepared by use of compounds containing a cyclic vinyl ether moiety. These photocurable coatings can be cured through cationic polymerization thereby avoiding the requirements of conventional photocurable coatings of expensive photoinitiators and an inert atmosphere. These novel photocurable coatings cure to a surface which exhibits markedly increased toughness over the cured surface of the heretofore known photocurable coatings curable via cationic polymerization. A further advantage of the novel photocurable coatings of this invention is that they can be made from starting materials which are relatively inexpensive.

The compositions of this invention are radiation curable coatings formulated with compounds of the formula:

wherein R is hydrogen or methyl and X is

wherein n has a value of from 1 to 10 and wherein R' is the residue of a carboxylic acid and can be hydrogen, alkyl, substituted with any substituent which does not unduly interfere with the polymerization or unsubstituted, linear or branched, containing up to 20 carbon atoms, such as methyl, ethyl, isopropyl, decyl, eicosyl and the like, and aryl, substituted with any substituent which does not unduly interfere with the polymerization or unsubstituted, having 6 to 10 ring carbon atoms, such as phenyl, naphthyl, benzyl, phenethyl, and the like.

Under acidic conditions these cyclic vinyl ethers undergo facile cationic polymerization when exposed to ultraviolet light to give hard, tack free, wear resistant films.

Among the many compounds which contain the cyclic vinyl ether moiety and are therefore useful in the compositions of this invention are the reaction products of the Diels-Alder reaction of acrolein and the reaction products of acrnlein with alkene or with vinyl ethers and the subsequent derivatives.

Another useful compound in the compositions of this invention is the Tischenko reaction product of 3,4-dihydro-2H-2-formylpyran as prepared, for example, in Example 2 of U.S. 2,537,921. This compound being 3,4-hydropyran-2-methyl(3,4-dihydropyra n-2-carboxylate).

Another group of compounds which are useful in the compositions of this invention are the alkylene oxide adducts of 3,4-hydropyran-2-methanol of the general formula

where R and n are as hereinbefore defined. This class of compounds is prepared by the ethoxylation or propoxylation of 3,4-dihydropyran-2-methanol with a base catalyst such as potassium or sodium metals. The catalyst concentration can be from 0.1 weight percent to 0.4 weight percent, preferably from 0.2 weight percent to 0.3 weight percent based on the weight of the final product. The reaction can be carried out at a temperature of from 750C to 1 500C, preferably from 1 000C to 1 200C. If, desired, the ethoxylated or propoxylated 3,4-dihydropyran-2-methanol can be linked by multifunctional dissocyanates, such as, for'example, the toluene dissocyanates.

Still another group of compounds which can be used in the compositions of this invention are the esters of at least one organic carboxylic acid and 3,4-dihydropyran-2-methanol corresponding to the formula

wherein R and R' are as hereinbefore defined. These compounds are prepared by conventional esterification or transesterification procedures with a suitable catalyst and can contain substituents in the molecule provided they do not unduly interfere with the reaction. These procedures and catalysts are well known to those skilled in the art and require no further elaboration. In the transesterification, the lower alkyl esters or organic acids are preferred sources of the acid moiety.

The photocurable coatings of this invention also contain a catalytically effect amount of a catalyst sufficient to catalyze the reaction. This amount can vary from 0.1 weight percent to 10 weight percent, preferably from 0.5 weight percent to 5 weight percent, based on the weight of the cyclic vinyl ether moiety containing compound. The catalysts are those which form an acid when exposed to ultraviolet radiation such as the aryldiazonium salts, the aryliodionium salts and the arylsulfonium salts.

Illustrative of such catalysts one can name p-methoxybenzenediazonium hexafluorophosphate, benzenediazonium tetrafluoroborats, toluenediazonium tetrafluoro arsenate, diphenyliodionium hexafluoroarsenate, benzenesulfonium hexafluorophosphate, toluenesulfonium hexachloroantimonate, and the like.

The catalyst can be introduced directly but preferably it is introduced in solution with a suitable solvent such as sulfolane or the aromatic hydrocarbons such as xylene.

The cyclic vinyl ether moiety containing compound and the catalyst can be combined under any practicable conditions of temperature, preferably from 1 OOC to 400C.

The photocurable compounds of this invention can be used per se as coatings or in admixture with conventional solvents, pigments, fillers and other additives. They can be applied by conventional means including, spray, curtain, dip, pad, rollcoating and brushing procedures. They can be applied to any acceptable substrate such as wood, metal, glass, fabric, paper, fiber or plastic that is in any shape.

The photocurable coating compositions of this invention are cured by exposure to particulate or nonparticulate radiation. The exposure time will vary and will depend upon a number of factors such as the coatings components and the film thickness, as well as the type and intensity of radiation used, but is generally quite short, generally less than 30 seconds and usually less than 10 seconds.

The radiation curable coatings of this invention find wide use in many coating applications. The novel coatings of this invention, because of their cationic polymerization curing mechanism are advantageous over conventional photocurable coatings in that neither expensive photoinitiators nor an inert curing atmosphere need to employed. Furthermore the novel coatings of this invention cure to a surface having better wear characteristics than cured films of many of the heretofore known cationically polymerizable photocurable coatings. It was completely unexpected and unobvious that this would be so.Nether the fact that coatings containing compounds having at least one cyclic vinyl ether moiety could be cured when exposed to ultraviolet light via cationic polymerization nor the fact that the cured films of such coatings would display distinct advantages over heretofore available cationically polymerized photocured films could have been predicted.

The following examples serve to further illustrate the invention. In these examples the following definitions apply to the tests employed and to the values reported.

Acetone resistance is a measure of the resistance of the cured film to attack by acetone and is reported in the number of double rubs or cycles of acetone soaked material required to remove one hall of a film from the test area. The test is performed by stroking the film with an acetone soaked cheesecloth until that amount of film coating is removed. The number of cycles required to remove this amount of coating is a measure of the coating solvent resistance.

Pencil hardness is a measure of film hardness. The adhesion and cohesive strength of the film also influences pencil hardness. Pencils of known lead hardness are shaped to a cylindrical point with a flat tip. The pencils are manually pushed into the coating surface at a 450 angle. Pencil hardness is recorded as the hardest pencil which does not cut the coating.

Reverse impact measures the ability of a given film to resist rupture from a falling weight. A Gardner Impact Tester using a eight pound dart is used to test the films cast and cured on a steel panel. The dart is raised to a given height in inches and dropped on to the reverse side of a coated metal panel. The inches times pounds, designated inch-pound, absorbed by the film without rupturing is a measure of the reverse-impact resistance of the film.

Example 1 A coating composition was produced by uniformly blending 20 grams of 3,4-dihydropyran-2 methyl (3,4-dihydropyran-2-carboxylate) and 0.8 grams of a 25 percent solution of p methoxybenzenediazonium hexafluorophosphate in sulfolane. The mixture was coated on steel panels with a No. 40 wire wound rod. The coatings were cured by passing through an ultraviolet photocuring unit which delivered an ultraviolet flux density of about 2500 watts per square foot; the path length was 2 feet and the conveyor belt speed was 40 feet per minute, giving an exposure time to the ultraviolet radiation of about 3 seconds. A clear tack free film about 1.5 mils in thickness was thus obtained. The cured film was evaluated and the results were as follows: Acetone Resistance > 100 Pencil Hardness 3H Reverse Impact < 5 in-lbs.

For comparative purposes a cationically polymerizable photocurable coating based on a blend of 20 grams of (3,4-epoxycyclohexoyl)methyl 3,4-epoxycyclohexanecarboxylate and 0.8 grams of a 25 percent solution of p-methoxybenzenediazonium hexafluorophosphate in sulfolane was produced and coated on a steel panel and cured using a procedure similar to that described above. A clear, tack free film, of about 2.6 mils in thickness was obtained. The cured film was evaluated and the results were as follows: Acetone Resistance 8 Pencil Hardness B Reverse Impact < 5 in-lbs.

The results establish the superior toughness of the films of photocurable coatings of this invention over films of the heretofore available cationically polymerizable epoxide based photocurable coatings.

Example 2 There were charged to a 1 liter 4 neck flask 49.8 grams of 3,4-dihydropyran-2-methanol and 0.4 gram of potassium metal. The reactor was purged with dry nitrogen during the addition of the potassium metal. Thereafter, 111.8 grams of ethylene oxide was fed into the reactor over a 4-1/2 hour period while the reaction temperature was maintained at from 11 00C to 1 200C. The unreacted ethylene oxide was then removed under reduced pressure at from 700C to 800C using a Rotovac evaporator. The ethylene oxide adduct of 3,4-dihyropyran-2-methanol produced weighed 168.4 grams and had an average of 6 ethyleneoxy units in the molecule.

There were charged to a dry 500 ml, 4 neck, round bottom flask 40 grams of the above adduct, 0.42 gram of stannous octoate as catalyst and 50 grams of 3,4-dihydropyran-2-methyl (3,4 dihydropyran-2-carboxyiate). Thereafter there was added 1 8.8 grams of toluene diisocyanate over a 35 minute period while the temperature was kept below 400C, while the mixture was stirred and purged with nitrogen. Then, while stirring at room temperature for an additional 2-1/2 hours, 50 grams more of 3,4-dihydropyran-2-methyl (3,4-dihydropyran-2-carboxylate) were added to the reaction mixture to give a final product having a total solids content of 37 percent.

Twenty grams of this 37 percent solids composition was combined with 17 grams of 3,4dihydropyran-2-methyl (3,4-dihydropyran-2-carboxylate) and 1.1 grams of a catalyst of a 25 percent solution of p-methoxybenzenediazonium hexaflorophosphate in sulfolane to produce a photocurable coating. Films of about 1 mil in thickness of this coating were applied on steel panels and cured, by passing through an ultraviolet photocuring unit which delivered a flux density of about 2500 watts per square foot; the path length was 6 feet and the conveyor belt speed was 50 feet per minute, giving a exposure time to the ultraviolet radiation of about 7.2 seconds. The hard, tack free cured film was evaluated and the results were as follows: Acetone Resistance > 100 Pencil Hardness 4H Reverse Impact 10 in-lbs.

Claims (9)

Claims

1. A radiation curable composition comprising (I) a compound of the formula

wherein R is hydrogen or methyl and X is

and wherein R' is selected from the group of hyrogen, alkyl, substituted or unsubstituted, linear or branched, containing up to 20 carbon atoms, aryl, substituted or unsubstituted containing 6 to 10 ring carbon atoms, and n has a value of from 1 to 10; and (II) a catalytically effective amount of a catalyst sufficient to cure said composition, said catalyst chosen from the group comprising the aryldiazonium salts, the aryliodionium salts and the arylsulfonium salts which are capable of forming an acid when exposed to radiation.

2. A composition as claimed in claim 1, wherein said components (I), is 3,4-dihydropyran-2methyl (3,4-dihydropyran-2-carboxylate).

3. A composition as claimed in claim 1, wherein said component (I) has the formula

wherein R is hydrogen or methyl and n has a value of from 1 to 10.

4. A composition as claimed in claim 1, wherein said component (I) has the formula

wherein R is hydrogen or methyl and R' is hydrogen, alkyl, linear or branched, containing up to 20 carbon atoms, phenyl, or naphthyl.

5. A composition as claimed in any one of the preceding claims, wherein said catalyst is pmethoxybenzenediazonium hexafluorophosphate.

6. A composition as claimed in any one of the preceding claims, wherein said catalyst is present in a concentration of from 0.1 weight percent to 10 weight percent based on the weight of said component (I).

7. A radiation curable composition as claimed in claim 1, substantially as hereinbefore descri.bed in Example 1 or Example 2.

8. A coating comprising a composition as claimed in any one of the preceding claims after radiation curing.

9. A coating as claimed in claim 8 substantially as hereinbefore described in Example 1 or Example 2.