CA1140492A - Rapid curing of epoxy resin coating compositions by combination of photoinitiation and controlled heat application - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A method for the high speed coating of various sub-strates utilizes an epoxy composition containing a radiation-sensitive catalyst precursor which will decompose upon exposure to electromagnetic radiation to provide a Lewis acid effective to induce cationic polymerization of the epoxy composition.
The formulation does not require any special epoxy monomeric materials or accelerators and is stable until so exposed.
Subsequent to applying the formulation to the substrate, the coating is exposed to electromagnetic radiation and a controlled temperature of 50-90°C. is maintained in the coating for a limited period of time to effect polymerization of the for-mulation to a tack free surface condition within a period of less than 30 seconds. Maintenance at the elevated temperature following initiation of irradiation may be for a period of as little as 0.5 seconds.

Description

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., 1 B~CKG~GUMD OF THE INVENTION
For a number of years the coating industry has been engaged in sub-3 stantial developmental programs in the quest for procedures which would allow 4 the coating of substrates at high production rates with the coating being cured to a tack-free condition at a speed ccmmensurate with the contiguous 6 processing steps. The industry has desired to eliminate the volatile solvents 7 required in many of the well kncwn coating processes because of potential 8 1 hazards or because of the cost of equipment to handle the evolved solvent 9 ¦ vapors. In addition, the industry has been seeking coating formulations which ¦ would produee coatings which were durable and which would permit substantial 11 ¦ additional processing of the workpieee, such as metal forming operations where 12 I the substrate is metal strip and container bodies, blanks and closures.
~ I Epoxy eoating formulations have long been recognized as affording !
14 ¦ desirable properties in the finished coating, especially the toughness to ~ I withstand further processing. Hcwever, the problem has remained to develop a 16 ¦ low cost epoxy coating formulation which ~uld ccmbi~e the desired rheologieal 17 I properties for the coatin~ application with both reasonable pot life and rapid 18 ¦ curing in the prcduetion line.
19 I In ~att United States Letters Patent No. 3,794;576 granted February 26, 1974, there are described desirable epoxy formulations whieh ccmbine the 21 I desired rheological properties with s~1itable pot life and rapid curing. me 22 ¦ formulations contain a Lewis acid precursor catalyst whic~ is decomp~sed upon 23 ¦ irradia~ion by ultraviolet light to pro~uoe rapid curing of the coating to a 24 I tack free condition. H~Yever, to achieve the desired rapid curing, the epox-~5 ¦ ide ormulati~s .herein contain at least about 15 per cent by ~eight of an ~6 ¦ epoxidic ester havi~g t~o epoxycycloalkyl groups; such esters materially in-

2~ ¦ crease the cost of the formulation as ~ pared with the more conventional 28 ¦ epoxide prepol~ er materials.

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.. 1 Since the disclosure of ~latt, a number of patents an~ publicat.ions 2 have appeared proposing various catalyst precursors for the epoxy formulations

3 which could replace the diazonium catalysts speci~ically described in the Watt ~:~ 4 Patent. ~mcng these are the onium cata.lysts disclosed in Barton Unitecl States :, 5 Letters Patent No. 4,0~0,936 granted May 23t 1978; Crivello United States Letters Patent Nos. 4,069,055 granted January 17, 1978 and No. 4,058,401 ~ 7 granted November 15, 1977. Hc~ever, the search has continued for lc~ cost i ~ formulations which would cure rapidly, i.e., formulations which ~uld not re-quire even ~c~all amounts of special components such as the epoxidic esters of the Watt Patent.
11 Although it has long been knYwn that heat will accelerate the reac-12 tion rate of polymerization following irradiatlon to effect co~plete curing, 1~ generally in accordance with the Arrhenius equation, and has long ~een sug~
14 gested to augment the irradiation treatment (see, for example, the aforemen-: 15 tioned Watt Patent at Col~n 6, lines 1-16~, such heating has also been recog-16 nized to introduoe other problems in the form of production requirements and 17 possible adverse ef~ects upcn the polymer properties since la.~ molecular 18 weight polymers may result. ~Some researchers have evaluated various factors .- 19 a~fecti.ng cure rate in such irradiated epoxide formulations and have proposed 7 20 substantially elevated temperatures to increase the cure rate while avoiding 21 ¦volatilization of the monomer (See, Crivello et al, "Triaryl Sulfor.ium Salts:
22 ¦A New Class of Photoinitiators for Cationic Polymerization", JOURNPL OF
~ ¦R~D~ArION CU~ING, Volume 5, pages 2, 10-11, January lg78).
.~ ¦ Although elevated temperatures of loo&. and above ~uld appear to .
¦be advantageous in terms of acceleration of reaction rate, such temperatures ¦ 26 ¦have been found to substantially affe~ct the quality of the polymeric coatinq .~ 2~ ¦although no monaner or volatile con~3Onent may be driven off during the pro-2~ ~cess. ~oreover, levati~l of the c ~tin to s~rl~ te eratu~es and r~intenalloe 3 ~ 3~

thereat for any appreciable length of time presents substantial production problems when high speed processing is involved.
Accordingly, it is an object of the present invention to provide a process for coating substrates with relatively low cost epoxy prepolymer formulations which are activated by irradiation and which will produce a tack free surface condition rapidly for use on high speed production equipment.
It is also an object to provide such a process which permits the utilization of relatively low cost epoxy prepoly-mers and a wi~e range of ultraviolet sensitive catalyst pre-cursors.
Another object is to provide such a process which may be adapted to a wide variety of high speed coating lines and which does not require extensive or expensive equipment.

SUMM~RY OF THE INVENTION
It has now been found that the foregoing and related objects may be readily attained in a method in which there is applied to the substrate a fluid coating of polymerizable composition comprising at least one epoxidic prepolymer material polymerizable to a higher molecular weight at which it is tack-free and up to 5 percent by weight of a radiation-sensitive catalyst precursor which decomposes upon exposure to electromagnetic radiation to provide a Lewis acid effective to initiate ~olymerization of the epoxidic prepolymer material.
The catalyst precursor is ineffective to cure the epo~idic prepolymer material to a tack-free surface condition at ambient temperatures in a period of two minutes following exposure to radiation to effect its decomposition. The epoxidic prepolymer Inaterial contains less than about 15 percent by weight thereof of epoxidic prepolymer material having two epoxycycloalkyl groups per molecule.
The coating is exposed to electromagnetic radiation to

- 4 -effect decomposition of the catalyst precursor to thereby gener-ate a Lewis acid, and the `

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~ 92 ' l 1 COAting is maintained at a temperature of about 50-90 C. for a period of at ~ least 0.5 second and less than about 2 minutes following initiation of expc-3 sure to radiation. As a result, the polymerizable composition of the coating 4 is polymerized to a substantially tack-free surface condition in a period of less than 30 seconds follcwing the cc~bination of exposure to radiation and 6 ~aintenance of the coatirg at the prescribed temperature.
7 The preferred coating compos1tions are substantially free fro~ pre-8 1 polymer material having t~o epoxyc~cloalkyl groups per molecule and such pre-¦ polymer materials are conveniently glycidyl ethers of various aromatic alco-¦ hols. Conveniently, the propolymer is the reaction product of bis-phenol A
11 1 and epichlorohydrin.
1~ ¦ The catalyst precursor economically comprises less than 3 perc~nt by 1~ ¦ ~eight of the polymerizable oo~positic~ and is an aromatic diazonium salt or 1~ 1 an aromatic onium salt selected from the group consisting of onium salts of group Va elements, onium salts of ~roup VIa elements and halonium salts.
lG ¦ me substrate may be metallic and heating the coating to t~ desired 17 I temperature may be effected by induction heating of the metallic substrate.
18 ¦ Alternatively, conduction, infrared radiation and convection may be used to ¦ heat-any type of substrate anc3/or the coating to the desired temperature. The ~ I coating should be maintained at the prescribed t~ erature for a period of at l least about ~ seconcls, but less than about 2 minutes. Care mllst be taken to ~2 avoid overheating the coating since this may effect a reduction in the desired 23 properties of the polymer.
24 The most convenient form of radiation to effect catalyst prepolymer de~ompositon is ultraviolet radiation.
26 DETAILED DF~CRIPTION OF THE PREFERRED E~oDL~Nrs .. ~
27 As previously indicated, the coating oompositions utilized in the 28 present invention essentially cornprise a mixture of an e~oxidic prepolymer material and a radiation-sensitive catalyst precursor Other components such as pigments, dyes, fillers and diluents may be incorporated if so desired.
The epo~idic prepolymer materials which may be used herein comprise any monomeric or oligomeric material containing at least one fuctional epoxy group or oxirane ring so that they may be polymerized upon opening of the oxirane ring. In addition, polymeric epoxy materials may be empl~yed if they may be dispersed in a fluid coating composition and are capable of 10 undergoing further polymerization to produce a solid polymer coating. ~he epoxy compounds may be aliphatic, cycloaliphatic, aromatic or heterocyclic.
The epoxidic prepolymer should contain no functional groups more basic than the oxirane ring and should be a solvent for the catalyst precursor. Most desirably, the pre-polymer should contain a reasonablepercentage of epoxy compounds containing two or more epoxy groups per molecule.
mhe polymerizable material will be epoxide resins used ; either singly or in combination and will have an average epoxide value of about 0.1 -1Ø The carbon chains having the epoxy groups may include additional substituents including ethers, esters, halogens, phosphates, and the like, and the compounds may include other polymerizable functional groups such as acrylates and silicones.
Typical epoxy materials are readily available commercially, the most common being those which are the product of bis-phenol A with epicholorohydrin or those resulting from the reaction of epichlorohydrin with a phenol formaldehyde resin of relatively low molecular weight. Reference May be made to the HANDBOOK OF EPOXY RESINS by H. Lee and K. Neville (McGraw-Hill 1967) for various epoxides. In addition, the technical literature and patent literature both contain extensive discussions of various epoxidic prepolymer materials which are useful in the compositions of the present invention as will be demonstrated hereinafter.

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/ ~ 92 1 ¦ In W.R. Watt United States Letters Patent 3,794,576, granted February ~ ¦ 26, 1974, there are described radiation-sensitive epoxidic blends containing 3 ¦ at least about 15 percent by ~eight of an epoxidic ester having at least two 4 1 epoxycycloalkyl groups per molecule in order to achieve polymerization ~nd5 I curing of the ccmposition rapidly upon exposure to ultraviolet radiation or the like. me compositions of the present invention do not require the pres-¦ enoe of such epoxy cycloalkyl esters for polymerization and curing, and ~ ¦ accordingly will contain less than 15 percent by ~ieight thereof and may be 9 ¦ totally free therefrom.
¦ Although not essential and sometimes undesirable, the polymerizable 11 ¦ composition may contain diluents to improve viscosity, and these diluents may 12 I be reactive such as those prcduced by reaction of an alcohol or a phenol with i 13 ¦ ~picholorohydrin. Exemplary of reactive diluents is the reaction product of 14 ¦ nonylphenol with epicholorohydrin. q~le amount of diluent may vary from zero I to as much as 45 percent of the ccmposition if a reactive diluent is employed 16 ¦ and is preferably less than 15 percent if nonreactive diluents such as di-¦ butylphthalate are employed.
18 ¦ For many applicationsr the composition will contain an inert pigment 19 ¦ or dye to provide a desired coloration. Generally, such pigments and dyes I will comprise less than about 40 percent by ~eight of the cGmposition. For 21 ¦ certain applications, it may be desired to include an inert filler, but such 2~ ¦ fillers may be deleterious to the desired properties for the coating and will ~3 ¦ normally comprise less than 40 percent by weight and preferably less than 15 24 ¦ peroe nt by ~ight of the polymerizable oomposition.
¦ T~ secon~ essential component of the polymerizable C~llpOSitiOIl iS
26 ¦ the radiatio~-sensitive catalyst precursor which will decompose ~xon exposure 27 to electromagnetic radiation above the visible light spectr~ so as to provide ¦
~8 ! a e~is acid whi is eEEeetive to int~ ~ polymerizeti~ oE the epoxidie pre-polymer material~ Various compounds exhibiting the desired photoinitiation characteristics have been discovered and are known to be effective at the present time, including the aro-matic diazonium salts of complex halogenides which decompose to release a halîde Lewis acid described in detail in the aforementioned Watt United States Letters Patent No. 3,794,576;
the diaryliodonium salts described by Crivello et al in JOURNAL
OE RADIATION CURING, Vol. 4, page 2 (1977); the triarylsulfonium salts described by Crivello et al in JOURNAL OF RADIATION
CURING, Vol. 5, page 2, (Janua y 1978); the aromatic iodonium and aromatic sulfoni~ complex salts speci~ically described in Barton United States Letters Patent No. 4,090,936 granted May 23, 1978; the aromatic onium salts of group Va elements des-cribed in Crivello United States Letters Patent No. 4,069,055 granted Januray 17, 1978; and the aromatic onium salts of Group VIa elemènts described in Crivello United States Letters Patent No. 4,058,401 granted November 15, 1977. Moreover, the compounds may be the bis- or tris- variants thereof. An extensive discussion of triarylsulfonium salts useful for the present invention appears in UV CURING: SCIENCE AND ^ECHNOLOGY
by S.P. Pappas, Technology Marketing Corporation, Stamford, Connecticut, at pages 5~ et_seq.
The term "Lewis acid precursor" as used herein is intended to encompass compounds which will directly generate a Lewis acid or which will indirectly generate a Lewis acid, the Lewis acid receiving an electron pair from the oxygen of the oxirane ring to open the oxirane ring and produce a cationic site for polymerization.
Exemplary of a classic Lewis acid is phorphorus pentafluoride (PF5) which will complex an electron pair, and which may be generated by a diaæonium catalyst in the following reaction:

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Ar - N = N] PF6 ~ Ar F + N2 ~ PF5 PF5 (monomer) - ~ M PF5 This mechanism is described in detail in the aforementioned Watt Patent No. 3,794,576.
Exemplary of an indirectly formed Lewis acid is the mechanism postulated by Crivello et al in "Triarylsulfonium Salts: A New Class of Photoinitiators for Cationic Polymeriæa-tion" in JOURNAL OF R~DIATION CURING, Vol 5, page 2 (January 1978). The authors postulate that the decomposition of the diaryl- and triaryl- sulfonium salts produce a 8roensted acid which in turn provides a proton which will function as the Lewis acid to accept electrons from ~he oxygen of the oxirane ring and initiate polymerization in accordance with the following ; mechanism:
6 SOlvent~HAr F + Ar. + Solvent. + HPF6 HPF6~ M(mOnomer) ~ HM PF6 Regardless of the theory o:f the action embraced, it is apparent that the photoinitiator is decomposing to generate in the reaction medium an electron acceptor acting as a Lewis acid to open the oxirane ring and thereby initiate cationic polymerization of the epoxidic prepolymer material. The reaction then proceeds as additional oxirane rings are opened until all of the monomer has been polymerized or until impurities interfere with the reaction mechanism.
Specific examples of the various classes of photo-initiators usable in the present invention are the following:
diphenyliodonium tetrafluoroborate; dit2,4-dichlorophenyl) iodonium hexafluorophosphate; diphenyliodonium hexafluorophosphate;
diphenyliodonium hexafluoroarsenate; triphenylsulfonium tetrafluoroborate; triphenylsulfonium hexafluorophosphate; tris (4-phenoxy-phenyl)sulfonium hexafluorophosphate; trifluoromethyl-diphenylsulfonium tetrafluoroborate; p-chlorobenzenediazonium ., . _ g _ hexafluorophosphate; 2,4-dichlorobenzendiazonium tetrafluoro-borate; and p-methoxybenzenediazoniom hexafluorophosphate.
The amount of the catalyst precursor may vary from as little as 0.5 precent by weight of the polymerizable com-position to as much as 5.0 precent by weight thereof and is preferably on the order of 1.0-3.0 percent~ If so desired, combinations of the catalyst precursor may be employed.
As indicated, the photoinitiator is decomposed into a Lewis acid by exposure to electromagnetic radiation. Although electron beam bombardment, X-ray radiation, and other similar forms of high energy radiation may be employed for this purpose, exposure to ultraviolet radiation has been found highly satis-factory and is desirable for commercial applications. The exposure to radiation normally required may be of extremely short duration, periods of about one-half to three seconds being normally adequate for most compositions depending upon the in-tensity of the radiation at the surface. However, for relatively thick coatings of the com~osition, it may be desirable to extend the period of exposure to four seconds or even more, to ensure adequate penetration of the radiation through the depth of the coating.
The coating must be maintained within a relatively narrow elevated temperature range for a period of 0.5-5.0 seconds following initiation of exposure to the electromagnetic radiation in order to achieve the desired rapid polymerization of the epoxidic prepolymer material to a tack-free surface condition within a period of less than 30 seconds. Although this elevated temperature range may extend from 50C. to as high as 90C., it is generally held within the range of 55-75C.
to obtain the desired rate of polymerization while avoiding adverse effects on the resulting polymer and the desired physical properties of the coating.

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The temperature of the coating may be elevated to the desired temperature range by any suitable means including in-duction heating when a metallic - lOa -A~ ~-1 ~ 9;~ ~

1 substrate is employed, conductive heating by passing or placing the coated 2 substrate over a heated element or a source of heat to heat the substrate 3 directly on its opposed surface; convection heating b~ passage of the coated 4 strip through a heated chamber; and radiation heating by exposure of the ¦ coated strip to a source of suitable radiant heat such as infrared lamps. For 6 ¦ convenience and for minimization of the equipment requirements, infrared radi-7 1 ation provided by suitable lamps is most desirably employed in conjunction with a source of~ultraviolet radiation~to produce decomposition of the cata-9 I lyst precursor.
¦ It has been found that the coating may be elevated to the de~ired 11 ¦ temperature range either before or after exposure to the source of electrc~ag-12 ¦ netic radiation. Moreover, the time period preceding or follc~ing exposure to 13 ¦ the source o~ electrcmagnetic radiation is not critical so long as there is 14 ¦ concurrently obtained activation o~ the catalyst precursor and maintenance of 1 the coating within the desired temperature range for only a limited period of 16 ¦ time, generally less than about two minutes and preferably less than about ten 17 ¦ seconds. It is likely that a protracted period of delay foll~Ying exposure to 18 the ultraviolet radiation before bringing the coating to the desired tempera-i~ 19 ture might reduce the efectiveness of the method so that desirabl~ the two steps occur within reasonably short periods of time, i.e., one minute or less.
~1 ¦ It has been found that the method of the present invention will 22 ¦ effect curing of the coating to a tack-free surface condition within a period 23 ¦ of less than 30 seconds and most generally within a period of less than 5 24 ¦ seconds foll~Ying the occurrence of both radiation exposure and maintenance ¦ within the temperature range. This is particularly significant for high speed 26 ¦ ~roduction lines where curing to a tack-free condition desirably oocurs within 27 ¦ t~ seconds or less. Full curing of the coating throughout its entire depth m y o ntinue aft r the time period descri d, particu]rr1y in the cvellt of A C~ 0 4 9 Z

1 thicker coating deposits since the tack-free surface condition permits handl-ing and further processing of the coated substrate.
3 The substrates which may be utilized in the present invention include 4 metallic substrates such as metal strip, formed container bodies, an~ the like, synthetic resin substrates such as polypropylene, polyvinyl chloride 6 strip and container bodies; fibrous substrates such as nonwoven materials 7 formed from natural fibers, synthetic fibers or mixtures of natural fibers and 8 1 synthetic fibers woven fabrics of natural and synthetic fibers, and mixtures 9 ¦ thereof; and laminates o the various oregoing materials. In additionj cera-¦ mic substrates such as glass may also be employed.
11 ¦ The method of coating will normally depend upon the nature and shape 12 ¦ of the substrate and the preceding and ~oLlowing production steps. Knife 13 ¦ coating, gravure coating, spray coating, dipping and the like are all useful, ~4 ¦ depending upon the particular product involved.
¦ The me~hods of the present invention are particularly applicable to - 16 ¦ various processes wherein durable coatings are desired for either aesthetic or 17 ¦ protective purposes. They find particular advantage in the field of graphic 18 ¦ arts because o~ the resistance of the coating to solvents and chemicals as 19 1 well as to abrasion, because of the excellent adhesion to various surfaces ~0 I including metals and because of the ability to withstand drawing and forming operations. E'or example, metal strip and container blanks, bodies and clos-22 ¦ ures may be coated and then formed without rupturing the continuity of the ~3 ¦ coating. With some ~onmetallic substrates such as synthetic resins, it may be 2~ 1 desirable to apply a primer to improve adhesion.
¦ Illustrative of the various aspects of the present invention are the ~6 ¦ foll~ing speci~ic examples wherein alI examples are parts by t~ight unless 27 otlr ise 1Idi~ted.

-l2 A~ 1 11~0492 ~X~MPIE ONE
2 A coating formulation is prepared having the follGwing composition:
3 ~ç~ Parts __ 4 Bis-phenol A/epichlorohydrin epoxy prepolymer material (sold by ~ow Chemical under the designation DER 332) 100.0 6 N-butanol 10.0 7 Fluorocarbon surfactant (sold by Minnesota Mining ~ Manu-8 ¦ facturing under the designation FC-430) 0.5 9 ¦ Photoinitiator solution (33% by weight p~nethoxybenzene lO ¦ diazonium hexafluorophosphate in sulfolane) 4.0 11 1 114.5 12 ¦ Electrolytic tinplate is used as the substrate and is coated with a 13 ¦ No. ~ draw bar to provide a coating of about 0.36 mil thickn~ss. Specimens of 14 ¦ the coated substrate are placed upon a heated stage held at a constant temper-¦ ature until temperature equilibrium is obtained throughout the specimen and Vl~ ¦ then the coating is exposed to ultraviolet radiation e6~ e~ ~ a 360 watt 17 I General Electric UA3 mercury arc lamp for a period of 4 seconds at a distance 18 ¦ of 4 1/2 inches.
19 ¦ The time period for the surface of the coating to become tack--free is : ~n noted and, at least one hour after exposure of the coating to the ultraviolet ~1 ¦ radiati~n, the coating is subjected to two separate tests to determine thQ
2~ ¦ mechanical properties of the polymer prcduced thereby.
~3 ¦ In one test, a paper tissue saturated with methylethylketone ~MEK) is 2~ ¦ wrapped arolmd the index finger and rubbed across the coating us;ng m~derate ~5 ¦ fin~er pressure; one complete stroke back and forth is considered "one rub".
2~ ¦ ~ny break or erosion of t~e surface of the coating is considered to represel~t 27 1 a failure.

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In the second test, the needle of a thermal mechanical analyzer is placed upon the surface of the coating and the tem-perature elevated until the needle penetrates the coating.
The temperature of penetration is indicative of the degree of-cross linking and the quality of the polymeric coating.
The results of the series of tests on the afore-mentioned specimens are set forth in the following table:

Tack-free Time, Softening Cure Temp., C. _conds MEK Rubs Point, C
*60+ 13 66 1 *100+ 66 1 *100-~ 63 110 1 4 ~2 130 no cure 1 5 * Maximum time of observation and maximum number of rubs, respectively.
As can be seen from the foregoing test dat~, the coating will polymerize to a tack-free surface state within 5 seconds after irradiation at 50C. but curing and cross linking below the surface of the coating apparently requires a more extended period based upon the low resistance to surface abrasion with ~EK. At 60C., the cure rate of the surface is still further materially improved and the resistance to surface abrasion is significantly benefitted. Within the optimum temperature range of 65-80C., the surface properties are very greatly improved. However, if the temperature increases above ~ - 14 -4~2 about 85C, deterioration in properties is observable, both on the ~EK rub test and on the sQftening point test. At 130 C, curing fails to take place. This is indicative of the criti-cality of the elevated temperature range at which the coating is maintained following irradiation.
EXAMPLE TWO
To evaluate the effect of substituting different epoxy monomers, a series of test formulations II-VI is pre-pared and tested under conditions simulating a relatively high speed production line. The substrates again comprise panels with electrolytic tinplate and the formulations are applied with a No. 5 wire rod to provide a coating equal to 95 lbs.
per thousand square feet of su~strate surface. The coated face of the panels is preheated to 150F. by infrared radia-tion as determined by a thermal paint indicator and then irradiated with two 200 watt/inch ultraviolet lamps while the panels are being advanced at a line speed of 110 feet per minute. The time for the surface of the coating to become tack-free is noted.
Component Parts FORMULATION II

Low molecular weight bis-phenol A-based epoxy resin (sold under the trademark ARALDITE 6004 by 63.7 Ciba-Geigy) Aliphatic diglycidyl ether (sold under the designation RD 2 by Ciba-Geigy~ 27.2 n-butanol 3.8 Silicone resin flow agent (sold under the mark SR 82 by General Electric) 1.9 Photoinitiator solution (33~ by weight of p-methoxybenzenediazonium hexafluorophosphate 3.4 solution in sulfolane~
la0.0 Time to tack-free surface 5 seconds L9~

FORMULATION III
Low molecular weight bis-phenol A-based epxoy resin (sold by Ciba-Geigy under the designa- 47.6 tion ARALDITE 6004~
Tetrafunctional aromatic epoxy resin (sold by Shell Chemicals under the designation 17.2 EPON 1031~
Aliphatic diglycidyl ether (sold by Ciba-Geigy under the designation RD-2~ 15.2 Aliphatic mono~lycidyl ether ~sold by Procter & Gamble under the designation EPOXIDE-7~ 15.2 Silicone resin flow agent ~sold by General Electric under the designation SR-82~ 1.9 Photoinitiator solution ~33~ by weight p-methoxybenzenediazonium hexafluorophosphate 2.9 in sulfolane) 100. 0 Time to tack-free surface 2 seconds FO~MULATION IV
Monomeric diglycidyl ether of bis-phenol A
(sold by Celanese Corporation under the 62.9 designation JD-508~
Cresyl glycidyl ether (sold by Celanese Corporation under the designation 28.6 EPIREZ 5011) n-butanol 3.8 20 Silicone resin flow agent (sold by General Electric under the designation SR-82) 1.9 Photoinitiator solution (33% ~y wei~ht p-me~hoxybenzenediazonium hexafluorophosphate 2.0 in sulfolane~
100. 0 Time to tack-free sur~ace - 5 seconds 4~2 FORMUL'ATIaN V
Monomeric diglycidyl ether of ~is-phenol A
(sold by Celanese Corporation under the 31.3 designation JD-508~

Epoxidized linseed oil ~sold by Viking Chemical Co. under the designation VIKO~LEX 7190~ 62.5 Silicone resin flo~ agent ~sold by General Electric under the desi~nation SR-82~ 2.3 Photoinitiator solution (33% by weight p-methoxybenzenediazonium hexafluorophosphate 3.9 in sulfolane) 100.~

Time to tack-free surface 3 seconds FORMULATION VI

Monomeric diglycidyl ether a~ bis-phenol A
(sold by Celanese Corporation under the 53.6 designation JD-508) Aliphatic diglycidyl ether (.sold by Ciba-Geigy under the designation RD-2) 26.8 Cumphenyl glycidyl ether tsold by Kenrich Chemical Company;under.~.the designation CPE) 14.3 Silicone resin flow agent tSold hy General Electric under the designation SR-82~ 1.9 Photoinitiator solution (33% by weight p-methoxybenzenediazonium hexafluorophosphate 3.4 in sulfolane~
100.O
20 Time to tack-free surface 2 seconds EXAMPLE THREE

To evaluate the efficacy of the method of the present invention with different catalysts, a series of different test formulations VII-X are prepared and specimens are coated and subjects to the same conditions as set forth in Example Two. Again, the time for the surface of the coating to cure to a tack-free condition is no~ed.

z Component FORMULATION VII Parts Aliphatic diglycidyl ether (sold by Ciba-Geigy under the designation RD-2) 44.4 Monomeric diglycidyl ether of bis-phenol A
tsold by Celanese Corporation under the 44.4 designation JD-5Q8¦
Silicone resin flow agent (sold by General Electric under the designation SR-82) 1.9 Photoinitiator ~sold by 3M Company under the designation FC-503~ 9.3 lQO.O
10 Time to tack-free surface 2 seconds FORMULATION VIII
Aliphatic diglycidyl ether (sold by Ciba-Geigy under the designation RD-2) 47.1 Monomeric diglycidal ether of bis-phenol A
(sold by Celanese Corporation under the 47.1 designation JD-5Q8~
Silicone resin flow agent (sold by General Electric under the designation SR-82) 1.
Photoinitiator (sold by General Electric Company under the designation UV Cat-14) 3.9 100. 0 ' ; Time to tack-free surface 2 seconds FORMULATION IX

Aliphatic diglycidyl ether (sold b~ Ciba-Geigy under the designat~on R~-2~ 46.2 Monomeric diglycidyl ether of bis-phenol A
(sold by Celanese Corporation under the 46.2 designation JD-508~

Silicone resin flo~ agent Csold by General Electric under th.e dèsignation SR-82~ 1.9 Bis-4tdiphenylsulfonio~phenyl sulfide bis-hexa~luorophosphate ~Technical Grade) 5~.7 10~. ~
Time to tack-free surface 2 seconds lQ FORMULATION X

Aliphatic diglycidyl ether (sold by Ciba-Geigy under the designation RD-2~ 47~2 Monomeric diglycidyl ether of bis-phenol A
(sold by Celanese Corporation under the ~7.2 designation JD-508~

Silicone resin flo~ agent ~sold by General Electric under the designation SR-82~ 2.0 Photoinitiator solution (33% by weight p-methoxybenzenediazonium hexafluorophosphate 3.6 in sulfolane1 100.0 Time to tack-free surface 2 seconds Thus, it can be seen from the above test data that 2Q the method of the present invention is applicable to composi-tions using diazonium catalyst precursors which are believed to function to generate directly Lewis acids as well as those using the sulfonium catalyst precursors which are postulated to generate first Broensted acids and then an active hydrogen ion as the Lewis acid.
EXA~IPLE F~UR
To evaluate the effectiveness of the method of the present inVention with coatings of different thicknesses, a standard formulation is p~epared aS follows:

Component Parts Monomeric di~l~cidyl ether of bis-phenol A
(sold by Celanese ~orporation under the 47.2 designation JD-5Q8~

Aliphatic diglycidyl ether (sold b~ Ciba-Geigy under the designation RD-2~ 47.2 Silicone resin flow agent Csold hy General Electric under the designation SR-82~2.0 Photoinitiator solution ~33% by weight p-methoxybenzenediazonium hexafluorophosphate 3.6 in sulfolane~ 100.0 Test panels are prepared and treated substantially in accordance with the procedure described in Example Two except that varying amounts of the coating formulation are applied to provide different depths o~ coatings. The results of the tests are set forth in the following table.
Applied Film W2eight, Tack-free time, sec Mg. per 4 in. -The first three specimens are found to be cured through immediately, but the last specimen continues to cure slowly over the next several minutes until cured through.

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1 ¦ EXAMPLE FIVE
¦ To evaluate the effect of first effecting photoinitiation of the cat-3 ¦ alyst precursor and of interposing tLme delays before ~ringin~ the coating to 4 ¦ the desired temperature of 65~., a æries of test panels are prepared usiny ¦ the follcwing coating formulation:
6 Component Parts 7 ¦ Low molecular weight bis-phenol A-based epox~ resin (sold 8 ¦ by Ciba-Geigy under the designation A~ALDITE 6004) 56.8 9 ¦ Aliphatic diglycidyl ether (sol~ by Ciba-Gei~y under 10 ¦ the designation gD-2) 37.9 11 I Silico~e rsin flGw agent (sold by General Electric l~ ¦ under the designation SR-82) 1.9 13 ¦ Photoinitiator solution (33~ by weight p-methoxybenzene 14 ¦ diazonium hexafluoroposphate in solEolane) 3.4 15 1 lO0.0 16 ¦ Follcwing coating, the panels are placed on a belt moving at a line 17 1 speed of llO feet per munute ~nder a pair of 200 watt/inch ultraviolet la~s 18 ¦ to irradiate the coatin~. Following irradiation, the panels are then heated 19 ¦ to 65C. after varying time delays, and the time to cure to a tack free sur-~0 1 faoe condition is again noted. l'he results are set forth in the following ~l ¦ table.
TLme Delay, Tack-Free 23 ¦ Seconds Time, Sec.

~6 1 20 ` 2 Il ' ~~1,_ ' ` ~ 45~

1 ¦ It can be seen from the foregoing results that the sequence of ¦ irradiation and temperature elevation can be reversed and that the time frame 3 ¦ between irradiaticn and temperature elevation is not critical within reason-4 ¦ able limits for production equipment.
¦ From the foregoing detailed specification and examples, it is appar-6 1 ent that significant advantages in accelerated cure rate to a tack-free sur-7 ¦ face condition can be readily attained without excessive loss in polymer prop-8 ¦ erties by a closely controlled and limited exposure to a 1aw elevated tempera-9 ¦ ture. As a result, relatively lcw cost epoxide formulations may be used in ¦ high speed coating applications where a tack free surface condition must be 11 ¦ attained rapidly to permit further processing. Where the coatings are of nor-12 1 mal thickness, curing throughout occurs substantially instantaneously; with 1~ ¦ relatively thick coatings, curing below the surface may continue as the coated ~ workpiece undergoes further processing.

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Claims (14)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLU-SIVE PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In a method for coating substrates with an epoxy coating material to develop a tack-free surface condition in relatively short periods of time, the steps comprising: A.
applying to a substrate a fluid coating of a polymerizable composition comprising at least one epoxidic prepolymer material polymerizable to a higher molecular weight at which it is tack-free and up to 5 percent by weight of said polymerizable composition of a radiation-sensitive catalyst precursor which decomposes upon exposure to electromagnetic radiation to pro-vide a Lewis acid effective to initiate polymerization of said epoxidic prepolymer material, said catalyst precursor being ineffective to cure said epoxidic prepolymer material to a tack-free surface condition at ambient temperatues in a period of two minutes following exposure to radiation to effect de-composition thereof, said epoxidic prepolymer material contain-ing less than about 15 percent by weight thereof of epoxidic prepolymer material having two epoxycycloalkyl groups per molecule; B. exposing said coating to said electromagnetic radiation to effect decomposition of said catalyst precursor and thereby generate a Lewis acid; and C. maintaining said coating at a temperature of about 50-90°C for a period of at least about 0.5 second and less than about 2 minutes following initiation of said exposure to radiation to effect polymeriza-tion to a substantially tack-free surface condition in a period of less than 30 seconds following the combination of said steps of radiation exposure and temperature maintenance.

2. The method in accordance with Claim 1 wherein said epoxidic pre-polymer material is substantially free from epoxidic prepolymer material hav-ing two epoxycycloalkyl groups.

3. The method in accordance with Claim 1 wherein said epoxidic pre-polymer material is a glycidyl ether of an aromatic alcohol.

4. The method in accordance with Claim 1 wherein said epoxidic pre-polymer material is the reaction product of bis-phenol A and epichlorohydrin.

5. The method in accordance with Claim 1 wherein the amount of said catalyst precursor is not more than 3 percent of said polymerizable composi-tion.

6. The method in accordance with Claim 1 wherein said catalyst precur-sor is an aromatic onium salt selected from the group consisting of onium salts of group Va elements, onium salts of group VIa elements, and halonium salts.

7. The method in accordance with Claim 1 wherein said catalyst precur-sor is an aromatic diazonium salt.

8. The method in accordance with Claim 1 wherein said substrate is metallic and heating of said coating to said temperature is effected by induc-tion heating of said substrate.

9. The method in accordance with Claim 1 wherein the heating of said coating to said temperature is effected by conduction of heat through said substrate.

10. The method in accordance with Claim 1 wherein the heating of said coating to said temperature is effected by infrared radiation directed upon said coating.

11. The method in accordance with Claim 1 wherein said electromagnetic radiation comprises ultraviolet radiation.

12. The method in accordance with Claim 1 wherein said polymerization of said coating to a substantially tack-free surface condition occurs in a period of less than five seconds.

13. The method in accordance with Claim 1 wherein said temperature at which said coating is maintained is within the range of 55-75°C.

14. The method in accordance with Claim 1 wherein said coating is maintained at said temperature for a period of less than about ten seconds following said initiation of said exposure to said radiation.