Vinyl ester resin and process for curing same with ionizing radiation in the presence of amines

[57] ABSTRACT

The addition of about 2 to 5 weight percent of certain amines to a thermosettable mixture of certain vinyl monomers and a polymerizable vinyl ester resin reduces the dosage level of ionizing radiation required to cure the mixture.

Claims, No Drawings

3,882,003

VINYL ESTER RESIN AND PROCESS FOR CURING SAME WITH IONIZING RADIATION IN THE PRESENCE OF AMINES

H

-C-OCH₂CHCH₂This is a division of application Ser. No. 142,853, filed May 13, 1971, now U.S. Pat. No. 3,816,283.

BACKGROUND OF THE INVENTION

This invention relates to the field of ionizing radiation cure of polymerizable materials and to coatings of same and especially relates to a promoter to reduce the ionizing radiation level or dosage necessary to effect a cure of said materials.

From a commercial standpoint radiation curing offers a number of advantages over thermal catalystinitiated cures: immediate initiation of polymerization, extended pot-life of the curable materials, little temperature rise so heat sensitive substrates may be employed in coatings, better control of the polymerization reaction, superior substrate-coating bonds are produced and much higher concentrations of radicals may be produced instantaneously. However, these advantages are difficult to realize if the curable materials require high curing doses of ionizing radiation since the economics become prohibitive. Commercialization then depends on reducing the cost of the curing process by finding methods and materials to effect a cure at lower dosages.

The search for means to accelerate or promote radiation curing is evident by a number of patents relating to certain polymerizable materials. While neither the promoters or the polymerizable materials employed correspond in any way to this invention, patents representative of such efforts include U.S. Pat. Nos. 3,202,513; 3,251,759; 3,265,604; 3,352,771 and 2,979,446. Commercially it is desirable to be able to cure at dosages of no more than 2 to 3 megarads but it would be of great advantage to be able to cure at 1 megarad or even less.

SUMMARY OF THE INVENTION

According to this invention the curing dosage of ionizing radiation required to cure in an inert atmosphere a mixture of certain vinyl monomers and a polymerizable vinyl ester resin is reduced by adding to the mixture at least 0.3 weight percent of certain nitrogen containing materials.

The vinyl ester resin is prepared by reacting essentially equivalent amounts of an unsaturated monocarboxylic acid, such as methacrylic acid, with a polyepoxide of a polyhydric phenol having more than one epoxide group per molecule. The nitrogen materials include 2-oxazolines, guanidines and certain amines which are more fully described hereafter.

DESCRIPTION OF THE INVENTION

Vinyl ester resins are a newer class of thermosettable resins which may be generally defined as a reaction product of an unsaturated monocarboxylic acid with a polyepoxide resin having more than one epoxide or oxirane group per molecule. The reactants are usually combined in about equivalent proportions of acid to epoxide groups, whereby the reaction of said acid with said epoxide produces a terminal polymerizable group connected to the polymer chain by linkages. A vinyl ester resin prepared by reacting about equivalent amounts of acrylic acid with a diglycidyl ether of bisphenol A may be illustrated by the following 10 formula:

CH2«CH-C-0CH₂CHCH₂0 /Λ —c(ch_)₂15 ‘ *

OH ___ 2 ₂θ The general methods by which vinyl ester resins may be prepared are thoroughly described in the patent literature. Representative patents which describe the resins and their preparation include U.S. Pat. Nos. 3,066,112; 3,179,623; 3,256,226; 3,301,743 and ₂₅ 3,377,406.

For this invention useful polymerizable vinyl ester resins include those which are prepared by reacting an unsaturated monocarboxylic acid with a polyepoxide of a polyhydric phenol wherein the polyepoxide has an 30 epoxide equivalent weight of about 150 up to 1000 and even higher. Said polyepoxides are made by reacting at least about two moles of an epihalohydrin, such as epichlorohydrin, with one mole of a polyhydric phenol and a sufficient amount of an alkali to combine with the 35 halogen of the halohydrin. Polyhydric phenols include the familiar bisphenol A as well as p,p'dihydroxy diphenyl, p,p'-dihydroxyphenyl sulfone p,p'-dihydroxybenzophenone, p,p'-dihydroxydiphenylmethane, the various other position isomers of 40 the above polyhydric phenols, polyhydric phenolic formaldehyde condensation products (novolac resins) and the like. Mononuclear phenols such as resorcinol, catechol, hydroquinone, phloroglucinol and the like may also be employed. The polyepoxides are characterized 45 in having more than one epoxide group per molecule.

Unsaturated monocarboxylic acids useful in preparing vinyl ester resins have the formula o

n

RjOT-C-C-OH «2 where Ri may be hydrogen, chlorine, bromine or phenyl and R₂ may be hydrogen, chlorine, bromine or a lower alkyl group of 1 to 4 carbon atoms. Said acids include acrylic acid, methacrylic acid, cinnamic acid, 60 a-chloroacrylic acid and the like.

Various catalysts may be used in the preparation of vinyl ester resins. Catalysts include tertiary amines such as tris(dimethylaminomethyl)phenol, onium catalysts, triphenyl stibine and triphenyl phosphine and the like. 65 Usually hydroquinone or other like polymerization inhibitors are added to prevent polymerization during the preparation of the resin.

3,882,003

According to this invention the polymerizable materials comprise a mixture of said vinyl ester resin with certain copolymerizable vinyl monomers. Only certain monomers may be employed with this invention provided the monomers are essentially nonvolatile. By this it is meant to exclude monomers which are predominantly gases at ambient temperatures. It is obvious that if the monomer is too volatile a substantial portion of the monomer would evaporate from a film or coating before the mixture of monomer and resin could be cured. Monomers which are normally liquid at room temperature may be used even though there may be a small loss of monomer by evaporation. It is possible to operate the process of this invention under higher pressures than atmospheric pressure to minimize any loss of said normally liquid monomers.

Useful monomers meeting the above requirements include both mono- and polyunsaturated monomers. Polymerizable monounsaturated monomers include vinyl carboxylic acids such as acrylic and methacrylic acid; vinyl nitrile monomers such as acrylonitrile and methacrylonitrile; alkyl and hydroxyalkyl esters of vinyl carboxylic acids such as methyl acrylate, butyl acrylate, cyclohexyl methacrylate, hydroxyethyl methacrylate, hydroxypropyl acrylate, hydroxybutyl methacrylate and the like where the alkyl group contains from 1 to about 8 carbon atoms; vinyl amide monomers such as acrylamide, diacetone acrylamide and the like; and mixtures thereof. Aromatic monomers such as styrene are inoperative with this invention.

A variety of polyunsaturated polymerizable monomers within the above classes may be used including ethylene glycol dimethacrylate, trimethylol propane trimethacrylate, methylene bisacrylamide and other similar monomers.

The polymerizable materials of this invention are especially useful in coating various substrates such as metal, wood and the like either as a primer coating and/or a finished coating. In order to obtain the benefits of this invention at least 0.3 weight percent of the nitrogen containing material is added to the polymerizable materials. While quite large amounts of the nitrogen containing material may be added there is no advantage in doing so. Preferably the amount ranges from about 0.5 to 10 weight percent and most preferably from about 2 to 5 weight percent.

When employed as coating formulations other additives may be incorporated into the coating, for example, various inert fillers and pigments such as kaolin clay, titanium dioxide, silica, various inorganic oxides and the like. Films cast from the coating formulations may be rapidly cured by exposing them in an inert atmosphere to ionizing radiation (accelerated particulate radiation). A beam intensity of at least 50 microamperes is usually employed, but this invention is not limited thereto and lower beam intensities may be employed.

Generally the films or coatings will range in thickness from about 0.1 mil up to about 10 mils. However, depending on the accelerating voltage, thicknesses up to 250 mils or higher are feasible. The radiation curing step should be performed in an inert atmosphere. By this it is meant an atmosphere which is essentially free of oxygen since the presence of oxygen may result in an undesirable tacky surface. It is sufficient for this purpose to place a thin film of a plastic material such as a polyester (Mylar) film on the cast film or coating.

Other means may be used such as curing in a chamber containing an essentially oxygen free atmosphere such as nitrogen, helium, argon and the like.

Accelerated particulate (ionizing) radiation includes 5 particles such as electrons, protons, deuterons, other ions and the like. However, from an industrial standpoint, the cost and availability of machines limit ionizing radiation curing to accelerated electrons for the immediate future. A variety of devices are available to 10 provide accelerated electron radiation or varying voltages and beam intensities. Typical of such devices is the familiar Van de Graaff accelerator. Similar commercial accelerators utilizing various acceleration means are available from Texas Nuclear Corporation, (cascade 15 rectified system) High Voltage Engineering, (insulated core transformer system) General Electric (a resonant transformer design) and Radiation Dynamics, Inc. (radio frequency cascade rectifier system).

Nitrogen containing materials include 2-oxazolines, 20 guanidines and certain amines. The 2-oxazolines include 2-oxazoline itself, and substituted 2-oxazolines having the formula ₂₅ R'HC-N

N

R HC CR \ / 0 wherein R’ and R may be hydrogen, ethyl, methyl, phenyl and the like. R may be an alkyl, aryl, aralkyl group or hydrogen. Such substituted oxazolines include 2-methyl-2-oxazoline, 2,5-diphenyl-2-oxazoline; 2³⁵ phenyl, 5-methyl-2-oxazoline; 2-methyl, 5-phenyl-2oxazoline and the like. Also included within the term 2-oxazolines are the bis oxazolines such as 2,2'tetramethylene bis( 2-oxazoline), 2,2'-oxydiethylene bis(2-oxazoline; 2,2'-thiodiethylene bis(2-oxazoline) ⁴θ and the like. Guanidines include guanidine, tetramethyl guanidine and the like.

Amines which may be employed have the formula R]R₂R₃N wherein R, may be an alkyl or an aralkyl group, R₂ and R₃ each may be hydrogen or an alkyl ⁴5 group or R_t and R₂ together may be a cyclic alkylene radical or an oxydialkylene radical. Alkyl groups include methyl, ethyl, propyl, n-butyl, isobutyl and higher alkyl groups. Typical amines include mono-, di- and trin-butyl amine, di-isobutyl amine, triethyl amine, cyclohexylamine, benzyl amine, morpholine, piperidine and the like.

The following non-limiting examples will further illustrate the invention. All parts and percentages are by weight unless otherwise specified.

EXAMPLE 1

A vinyl ester resin was prepared by reacting methacrylic acid (31%) with a glycidyl polyether of bisphenol A having an epoxide equivalent weight (EEW) of 186-192 (D.E.R. 331). The vinyl ester resin was then mixed with n-butyl acrylate in the proportions of 2/1 and to this resin was then added 3% of various nitrogen containing compounds, based on weight of resin and ^5 monomer. A film was cast with a 7 mil draw-down bar on a Q-panel (4 X 12 x 0.03 inch) and covered with a 2 mil sheet of Mylar (polyester) film to exclude air. The coated polished steel Q panel was then passed through

3,882,003 a 2 Mev electron beam from a Van de Graaf accelerator filtered with 0.33 gm/cm² Al. A 50 microampere beam current and a conveyor speed of 3.4 cm/sec delivered a dose of 0.1 Megarad (Mrad) for each pass through the beam.

Without any accelerator the resin required 0.5 megarads (Mrad) to cure. While this dose level is already low, the addition of di-n-butyl amine reduced the dosage required to 0.1-0.2 Mrad. Similarly, when tetramethylguanidine and 2-oxazoline were employed as the accelerators the dosage was reduced to 0.2 and 0.4 Mrad, respectively. Tests made with styrene in place of the butyl acrylate failed to evidence the reduction in curing dosage.

EXAMPLE 2

When the resin of the previous example was mixed with 2-hydroxyethyl acrylate (50/50), the mixture required 0.3 Mrad to cure. Even at this low dosage the addition of 3% by weight of dibutyl amine reduce the dosage to cure to 0.15 Mrads.

EXAMPLE 3

Another vinyl ester resin made by reacting methacrylic acid (32.0%) with a mixture of D.E.R. 331 (17.7%) and an epoxy novolac (50.3%) having an EEW of 175-182 was mixed with n-butyl acrylate (2/1) and tested as in Example 1. Without an accelerator 0.5-0.6 Mrad was required to cure the mixture. The addition of 3% of di-n-butyl amine, 2-oxazoline or tetramethylguanidine reduced the dosage to cure to 0.2, 0.4 and 0.3 Mrad, respectively.

EXAMPLE 4

The resin of Example 3 was mixed with 2-hydroxyethyl-acrylate 50/50 and the curing dosage was reduced from 0.3 to 0.15 Mrad.

EXAMPLE 5

A further series of tests were run with the vinyl ester resin of Example 1 mixed with different monomers and employing various accelerating materials of the invention. The curing dose in Mrad for each of the various combinations employing the procedure of Example 1 is shown below.

Curing Dose, Mrad

Resin of Example 1 mixed with n-butyl cellosolve hydroxypropyl acrylate acrylate acrylate*

Additive (3%) (2/1) (2/1) < l/l)

<td> None</td><td> 0.5-0.6</td><td> 0.5-0.6</td><td> 0.3 Mrad</td>

<td></td><td> Mrad</td><td> Mrad</td><td></td>

<td> di-n-butylamine</td><td> 0.2</td><td> 0.2</td><td> 0.1-0.15</td>

<td> triethylaminc</td><td> 0.2-0.3</td><td> 0.2</td><td> 0.1-0.15</td>

<td> piperidine</td><td> 0.2</td><td> 0.2</td><td> 0.1-0.15</td>

<td> morpholine</td><td> Ο.2-Ο.3</td><td> 0.2-0.3</td><td> 0.15-0.2</td>

<td> benzylamine</td><td> 0.3</td><td> 0.3</td><td> 0.2</td>

<td> tetramethyl-</td><td></td><td></td><td></td>

<td> guanidine</td><td> 0.2-0.3</td><td> 0.2</td><td> 0.15-0.2</td>

<td> 2,2’-tetramethyl-</td><td></td><td></td><td></td>

<td> ene bis(2-oxazo-</td><td></td><td></td><td></td>

<td> line)</td><td> 0.3-0.4</td><td> 0.3</td><td> 0.2-0.25</td>

•25 microump beam current

EXAMPLE 6

Similar to Example 5 another series of tests were made with the resin of Example 3 with the following results.

Curing Dose, Mrad

Resin of Example 3 mixed with n-butyl acrylate hydroxyethyl acrylate*

Additive (60/40) (60/40)

<td> None</td><td> 0.5-0.6</td><td> 0.25-0.3</td>

<td> di-n-butyl amine</td><td> 0.2</td><td> 0.1-0.15</td>

<td> triethylaminc</td><td> 0.2</td><td> 0.1-0.15</td>

<td> piperidine</td><td> 0.2</td><td> 0.1-0.15</td>

<td> morpholine</td><td> 0.2-0.3</td><td> 0.15-0.2</td>

<td> benzylamine</td><td> 0.3</td><td> 0.1-0.15</td>

<td> 2,2'-tetramethylene bis(2-oxazoline)</td><td> 0.4</td><td> 0.15</td>

<td> tetramethylguanidine</td><td> 0.3</td><td> 0.1-0.15</td>

*25 microamp beam current

It will be understood that the present invention is not limited to the specific materials, steps and other specific details described above but may embody various modifications insofar as they are defined in the following claims.