CA1048328A

CA1048328A - Photo-processable coatings

Info

Publication number: CA1048328A
Application number: CA214897A
Authority: CA
Inventors: Michael J. Canestaro; Joseph G. Cutillo; George P. Schmitt; Oscar R. Abolafia
Original assignee: International Business Machines Corp
Current assignee: International Business Machines Corp
Priority date: 1973-12-20
Filing date: 1974-11-27
Publication date: 1979-02-13
Also published as: ES433118A1; JPS5097324A; IT1030846B; CH613287A5; JPS5234935B2; FR2255629A1; GB1448643A; NL7415671A; BR7410702D0; DE2459179A1; FR2255629B1; DE2459179B2

Abstract

PHOTO-PROCESSABLE COATINGS
Abstract of the Invention A photo-processable coating is formed using (1) an epoxy prepolymer which is solid at room temperature and which has at least 2 epoxy groups per molecule, (2) an alpha, beta ethylen-ically unsaturated carboxylic acid, (3) a primary aromatic amine, and (4) a photo-initiator.

Description

8 Field of the Invention g The present invention is concerned with photosensitive coatin~s. In particular, it is concerned with photosensitive 11 coatings which may, by means of a photolytic process, result in 12 patterned or selective placement of the coating. sy means of the 13 present invention, it is possible to make a photo-processable 14 coating having a very unusual combination of properties. The coating is chemically resistant, thermally stable, mechanically 16 strong, electrically inert, and forms high strength adhesive 17 bonds to underlying substrates while retaining the ability to be 18 photo-processable. Still another unexpected advantage of the 19 present invention is that the shelf life prior to light exposure is greater than one month during storage at room temperature.
21 This contrasts to a prior art time of about six hours. Also, by 22 the present invention it is possible to obtain coating films 23 increased in thickness from the prior art value of about 1 mil to 24 greater than 5 mils. The present invention entails the reaction of an alpha, beta ethylenically unsaturated carboxylic acid with 26 an epoxy in conjunction with the reaction of a primary aromatic 27 amine with the same epoxy. The resultant product is a photo-28 sensitive polymeric material exhibiting unexpectedly outstanding 29 photolytic properties while retaining all the physical, electrical and chemical properties of a standard cured epoxy resin.

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1 A typical end use of -the present invention is as a patterned

2 protective coating for a circuit board. In the manufacture of

3 printed circuit boards it is desirable to have polymeric material

4 which can be patterned by means of a photolytic process, as is the praetice with photoresists, but which, unlike photoresists, can be 6 retained to perform one or more additional func~ional uses during 7 or after fabrication of the circuit board. Due to the unusual 8 combination of properties inherent within the products of this 9 invention, sueh funetions inelude, for example, use as a perman-ently retained eteh mask, solder mask, plating mask, dielectric 11 film, or proteetive coating.
12 Prior Art 13 Many proeesses for forming photoresists are well known in 14 the art. One sueh process is discussed, for example, in U. S.
Patent 3,469,982. That patent, however, does not teach the 16 eombination of ingredients required by the present invention.
17 U. S. Patent 3,450,613 is concerned with the formation of 18 an adhesive cement. It teaehes the use of epoxy resins in eon-19 junetion with unsaturated organie aeids. There are, however, several distinetions between this prior art patent and the 21 present invention. In particular, the prior art patent is not 22 eoneerned with, and does not show how to make, a photo-proeessable 23 eoating. Although the use of a tertiary amine as a eatalyst is ~ ;
24 shown (e.g. Example III and Example VI), the patent does not show the use as a reaetant of a primary aromatie amine hardening 26 agent whieh is required in the present invention in order to 27 obtain the unusual eombination of properties exhibited by this 28 invention. The use as reaetants of these primary aromatie 29 amines, in eonjunetion with an unsaturated organie aeid, results in a material whieh exhibits exeellent image forming eapabilities - : .

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1 as a resul-t of suc}l properties as reduced ~xygen inhibition, 2 sharper walls, increased solvent resistance of pho-tolyzed areas, 3 improved adhesion, etc. The tertiary amine acid addition 4 catalyst mentioned in the prior ar-t is sufficient Eor the pur-pose of forming an adhesive cement, but does not serve to instill 6 within the sys-tem those properties necessary to make it functional 7 in those areas impac-ted by this invention. Also, an addi-tional 8 distinction between this prior art patent and the present inven-9 tion is that the prior art patent is not at all concerned with selective placement, or patterning, of a photo processable material.
11 Summar~ of the Invention 12 According to the present invention, a photosensitive protec-13 tive coating is formed by reacting an epoxy prepolymer which is 14 solid at room temperature and which has at least two epoxy groups per molecule with an alpha, beta ethylenically unsaturated car-16 boxylic acid. This reaction is carried out simultaneously with 17 or separately from the primary aromatic amine - epoxy reaction.
18 It is the use of both of these reactive components that results 19 in the unique properties displayed by this invention. Photo-sensitivity is greatly enhanced by the inclusion of a sensitizer 21 which, in one preferred variation of the present invention, is 22 a mixture of two sensitizers, as will be discussed below.
23 The epoxy resin used as the major component of the formula-24 tion should be a prepolymer which is solid at room temperature and which has at least 2 epoxy groups per molecule. It is, 26 however, preferred that the epoxy resin have at least 4 epoxy 27 groups per molecule, and it is most preferred that it have at 28 least 6. Examples of epoxy resins which meet this criteria are ~ ;
29 XD-7855 (trademark of Dow Corporation), ECN-1280 (trademark by CIBA Corporation), and ERRA-0163 (trademark of Union Carbide EN973014 -3~

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1 Corporation). The most preferred material, however~ is Epirez SU-$ (trademark o~ Celanese Corporation), an epoxy resin having 8 epoxy groups per molecule.
Many alpha, beta ethylenically unsaturated carboxyl~c acids are known and readily available commerically. Typical of this category are acrylic acid and methacrylic acid. The most preferred material is acrylic acid.
The present invention requires the use of a primary aromatic amine. Useful materials include, for example, aniline, metaphenylen-ediamine and diaminotoluene. The most preferred material is methylene dianiline.
The present invention also requires the use of a photo~nitiatoror sensitizer. Many such materials are well known to the prior art.
For the purposes of the present invention, it is preferred to use an anthraquinone type sensitizer and a halogen containing sensitizer.
Useful anthraquinone sensitizers include antraquinone itself, 2 methyl anthraquinone, and most preferably, t--butyl anthraquinone. Halogen type sensitizers include, for example, carbon tetrachlor;de, bromo-form, and most preferably carbon tetrabromide.
It has been unexpectedly found that for outstandingly good results, a combination oF a sensitizer from each type should be used. The very best results have been obtained by the simultaneous use of t-butyl anthraquinone and carbon ~etrabromide. This combination, for reasons which are not at all understood, results in a synergistic act`ivating e~fect, giving results very much better than any others.
The reaction conditions used to convert ~he epoxy material into a useful photo-processable product will of necessity vary somewhat depending upon the particular materials which have been 3~
1 selected. The optimum conditions will normally clepend on the 2 reac-tivity of epoxy, the number of epoxy groups per molecule, 3 and the rnolecular weight oE the epoxy prior -to the reaction, and the type or pri~ary aromatic amine used.
The making of a pa-tterned coating can be accomplished by 6 the 5 teps below:
7 1. Preparing a mixture of an epoxy resin, an alpha, be-ta 8 unsaturated carboxylic acid, a primar~ aroma-tic amine, with or g without a tertiary amine ca~alyst, and a suitable solvent, or alternatively adding the primary aromatic amine at a later stage 11 to the mixture.
12 2. Reacting the mixture until a proper range of epoxide-13 acid addition is achieved along with the attainment of a proper 14 range of molecular weight increase, or alternatively achieving the range of epoxide-acid addition and then carrying out the 16 primary aromatic amine addition by adding it to the part reacted 17 mixture and continuing the reaction to arrive at the proper `
18 range of molecular weight.
19 3. Adding a free monomer from a number of ethylenically unsaturated or poly-unsaturated materials to the reacted mixture.
21 This step may be omitted when the reaction leaves a sufficient 22 amount of unsaturated acid in an unreacted state, after the ~ -23 proper degree of epoxide-acid addition was attained.
24 4. Adding to the photosensitive product obtained in para-graph 2 or paragraph 3 above a combination of photolytic 26 initiators or sensitizers such as an anthraquinone and a halo-27 genated organic compound such as carbon tetrabromide.
28 5. Forming a film of the resulting mixture on a desired 29 substrate.
6. Exposing the film to ultra-violet radiation through an 31 image-bearing transparency of the desired pattern.
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1 7. Developing the e~posed film to remove the areas not ~ s-truc~ by -the ul-tra-violet light, accomplished by immersing or 3 sprayin~ ~he film with a suitable solvent.
8. IE desired, diffusing an addi-tional hardening agent into the freshly developed film from a suitable solvent. The 6 hardeninc~ agent may be selected Erom any one oE a large number 7 used to give epoxy resins particular properties. The amount of 8 hardener diffused into the film is governed by the concentration g and immersion time in the hardener solution.
9. Curing the film at elevated temperatures to the desired 11 degree of cross-linking.
12 The combination of an alpha, beta unsaturated carboxylic 13 acid and an aromatic primary amine reacted with an epoxy contain-14 ing rnolecule, impart photosensitive properties to the molecule, such that under the proper conditions of reaction, the photo-16 processability of the product is greatly enhanced compared to 17 products made from either the acid and epoxy or the amine and 18 epoxy. This is a key feature of the invention. The preferred 19 acid is acrylic acid and the preferred amine is methylene dianiline.
21 Another important element of the invention is the use of a 22 combination of two photoinitiator compounds, which give a high 23 degree of photosensitivity to the unsaturated acid-amine-epoxy 24 reaction product~ One of the initiators is selected from the anthraquinone class of sensitizers and the other from the halogen 26 containing class of sensitizer molecules. The preferred anthra 27 quinone is tertiary butyl anthraquinone, chosen because of its 28 desirable solubility characteristics and the preferred halogen 29 source is carbon tetrabromide. When used alone neither the anthraquinone, nor the halogenated class of molecules, permit ;

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1 the full range of photosensitivity to be utilized in the reaction product.
Another possible variation of our invention is the addition of an alpha, beta ethylenically unsatura-ted molecule to the unsaturated acid-amine-epoxy reaction product. It was found that when there is an insufficient amount of unsaturated acid in the reaction mixture so that there remains little or no free or non-reacted acid, the product loses a significant degree of photosensitivity. This defici-ency can be overcome simply by adding either more unsaturated acid in the original reaction mixture or preferably more monomer to the reaction product. This so-called "free monomer" may be selected from a large variety of unsaturated monomers. We have found that all the ~:
following materials are effective: polyethylene glycol d;acrylate, ethylene glycol diacrylate, ethylene glycol dimethacrylate, and acrylamide.
It has been observed that with proper formulations, the incor- .
poration of a free monomer aids dramatically in extending the shelf :
life of the invention. It has been -Found that if small amounts of an alpha, beta ethylenically unsaturated carboxylic acid and reactive ~ -~
primary aromatic amine are used in the initial -formulation, and re-actions are run to consume all these reactive components, the addition of a free monomer which does not7 at room temperature, chemically react with the epoxy resin resul~s in a very stable, long shelf li~e product.
After the reaction product has been m;xed with the photoinitiators ~:
and prepared in Film form on a suitable substrate, exposure to a light source (preferable ultra-violet light), through an image-bearing transparency will cause a latent image to be produced in the light struck portion o~ the photosensitive ,~:

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1 film. As is the case with negative wor~ing photoresists, ~he 2 portion remaining after developmen-t will be the light struck 3 areas. ~rhe -thickness of the film, the intensity and spectral range o~ the light source, and the dis-tance Erom the light source will aEfect -the duration of the exposure as with other photo-6 sensi-tive materials. However, in addition, it was Eound that 7 the material of this invention is affected by other factors as 8 well. The type of solvents used in making or applying the 9 product and the severity of drying was found to influence the duration of exposure. Non-halogenated solvents and some halo-11 genated solvents, as well as prolonged drying, increased the 12 exposure time. In order to eliminate this deficiency both 13 sensitizing agents were incorporated into this invention. This 14 was found to result in a material that was not afected by the above mechanical variables. Thus, a combination of t-butyl 16 anthraquinone and carbon tetrabromide not only improves the 17 overall sensitivity dramatically, but also resulted in a more 18 stable performing material. The development of the exposed film 19 may be performed by immersion or by spraying with any one of a number of different solvents or solvent combinations. Generally, 21 solvents which dissolve the unexposed portions of the film while 22 permitting the exposed portion to be very little affected are 23 desirable. Some solvents are of the proper strength to do this, 24 but other solvents which are too strong can be used if they are first mixed with a poor solvent. Of the preferred solvents and 26 solvent mixtures the following were found to be effective:
27 Hot trichloroethylene (60 - 70) 28 Acetone 29 Methyl ethylketone/ethylacetate ~60:40) Chloroform/methylchloroform (50:50) ~4~3;Z8 1 ~s soo~ as the film has been developed to produce a relief 2 image, a hardener may be introduced, if desired, by allowing the 3 film to be immersed in a solution of the hardener. In this way 4 the optimum properties of the epoxy product, which comprises the major portion of the film, may be attained. Two examples 6 of hardeners introduced in this fashion are as follows:
7 A 2~ soJution of triethylenetetramine in trichloroethylene 8 A 2% solution of methylene dianiline in methylethylketone/
g ethylacetate (60.40) A one to two minute immersion of the film in either solution 11 followed by a quick rinse with solvent was satisfactory. In 12 this way sotening temperatures of this film after a thermal 13 cure of 30 min. at 160C were increased from about 90C for a ~
14 film not immersed in a hardening solution, to as high as 175C. ~ ~`
~s stated above, the reaction conditions required to produce 16 the unsaturated acid-amine-epoxy product depend on the epoxy 17 resin. It was found that for best results, two parameters should 18 be controlled. They are the extent of epoxide acid reaction and 19 the molecular weight of the product arising from, primarily, the 20 epoxide-methylene dianiline addition reaction. Only the relative ~ `
21 changes in molecular weight are of interest and therefore a 22 measurement of the viscosity of the reaction mixture will suffice. ;~
23 The extent of epoxide-acid reaction is determined by titration `
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The range of values for each of these parameters have been 26 determined for "Epirez SU-8" but may be just as easily determined ~
27 for other epoxy resins. The criteria which are used to arrive~ ~ -28 at the proper range of values of the two parameters are based 29 on testing the photo-processability of the product as the para-30 meters are varied. ~oth acid addition and amine addition to the`~
31 epoxy molecule affect the photosensitivity of the product. Thus, ~ , ,., . . . . ~ .

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1 -too little o~ each will result in long ultraviolet light 2 exposures when the photoinitiators are added and the image 3 developed. Too much amine addi-tion will result in residual ~ ~aterial remaining in -the unexposed areas of the Eilm. Therefore, there is a range of values for each of these parameters which 6 will shorten the exposure time and maintain a well adhering film 7 ~f high in-tegrity and with well deflned images.
8 The sequence of adding the unsaturate~ acid and aromatic 9 amine to the epoxy material in the preparation of the reaction product ma'y be varied. Either the acid and amine may be added 11 at the start of the reaction or the acid added first, allowing 12 most of the acid-epoxy reaction to ta~e place, and then mixing 13 in the amine and allowing the amine-epoxy reaction to proceed 14 to the proper end point.
There are several methods of varying the rate of acid-16 epoxy reactions and the amine-epoxy reaction. The quantity of 17 solvent used in the reaction mixture and the temperature of the 12 reaction will affect the rate of both reactions. The use of a 19 tertiary amine catalyst will act more specifically in accelerating 20 the acid-epoxy reaction, while the quantity of unreacted acid ~;
21 remaining at any time during the course of reaction will influence 22 the extent of amine-epoxy reaction since acids catalyze the latter 23 reactions. By selecting the concentration, temperature, amount 24 of catalyst, amount of unsaturated acid and sequence of additions 25 of reactants, the preparation of the photo-processable reaction -26 product may be achieved under a variety of controls and in various 27 final forms. For example, in order to produce a product of long 28 shelf life, as little acid and amine as possible is used so that 29 no excess remains in the final product. The photo-processability of the product is then enhanced by adding one of the unsaturated 31 monomers mentioned above.

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1 Specific ~Eormulatlons which have been pxepared under ? speciEic conditions will be given in -the Examples below. However, 3 some typical ranges of rnaterial concen~ra-tions and end-poin-t parameter values using an epoxy resin with B epoxy groups per molecule are as follows: (The expression "phr" means parts by 6 weight per hundred parts of epoxy resin.) 7 Solvent 30 - 200 phr 8 Acrylic Acid 2 - 16 phr g Methylene dianiline 0.2 - 1.5 phr ~-~
Tertiary amine catalyst 0.05 - 0.5 phr 11 Temperature 50 - 110C
12 Reaction Time 2 hrs. - 18 days 13 Acid consumption in the 14 reaction product 1.5 - 4.5 phr Unsaturated Monomer 3 - 15 phr 16 Tertiary butyl anthraquinone 1.5 - 2.5 phr ~ ~, 17 Carbon tetrabromide 1 - 5 phr 18 The use of additives which are normally used in epoxy tech- -19 nology can generally be employed with photo-processable coatings ~;
of this invention. Obviously, additives which drastically 21 interfere with either the light adsorption of the coating or 22 with the photoresists must be avoided. When additives are used 23 it would be preferable to add them after the reaction product 24 has been prepared in order to avoid any possible deleterious interactions with the reaction mechanisms.
26 Substrates over which the photo-processable coating is 27 spread may be any metal, plastic, glass or any other material 28 over which epoxy materials are normally spread.
29 The range of light exposure time and intensity for a typical ;
photo-processable coating may be ascertained from the following 31 typical conditions:

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l a. One mil ~hiek film, 30 inches from a high pressure 2 short arc mercury lamp manufactured by OSRAM G~sH, Germany, 3 Model Hso, 500 watts for a period of 20 sees. to 120 secs.
4 b. 2 mil thick film, same conditions as above for a per:iod of 30 to 160 secs.
6 c. 5 mil thick film, same conditions AS above for a 7 period oE 60 to 240 secs.
8 The following Examples serve to illustrate preEerred embodi-g ments of this invention and are not in-tended to limit the teachings as set forth herein.

12 100 grams of "Epirez SU-8" epoxy resin was dissolved in 13 200 g of chloroform. 16 grams of acrylic acid were added and 14 the mixture allowed to reaet at 50C for 12 days. At this point, 15 the aeid consumption was 1.8 phr. 1.3 grams methylene dianiline - ' 16 was added and the temperature maintained at 50C for 16 hours.
17 The viscosity was found to be 150 centistokes.
18 To the above reaction produet solution was added 100 g 19 ethyleneglycol monomethylether and 2 phr t-butylanthraquinone.
The mix was eoated over a eopper substrate, dried for 1 hour 21 at 50C to give a 1.0 mil thiek coating and exposed through a 22 step wedge 30 inehes from the 500 watt lamp mentioned ahove.
23 The exposure time which produced an acceptable image after 24 development in triehloroethylene for 2 minutes was noted as the "minimum expose time." For this mix~ "minimum expose time" was 26 noted as 420 sec.
27 Shelf life was determined by allowing the dried fllm to 28 remain at room temperature in the absenee of light. Under such 29 eonditions, a eross-llnking reaetion slowly oeeurs until condi-tions exist such that after image development in triehloroethylene, " ' "

1 yelled material remains in those areas that should be soluble.
2 The amount of time from -the coating dry opera-tion up to the -time 3 -that residual gel material is noticed is called -the ma-terial's 4 "Shelf l,lEe." For this form-llation, a shelf life of 24 hours was observed.
6 SoEtening point in~ormation was acquired for the coating 7 after a 30 minute cure at 160C. The softening point (Ts) was ~`
8 found to be 90C~
g EXAMPLE II
To the reaction product solution prepared in the first 11 paragraph of Example I was added 5 phr carbon tetrabromide. ~;
12 The minimum expose time was found to be360sec. ~-13 The shelf life was found to be 24 hours. ;~
14 The softening point after a 30 minute cure at 160C was 90C.
EXAMPLE III
16 To the reaction product solution prepared in the first 17 paragraph of Example I was added 2 phr t-butylanthraquinone and 18 5 phr carbon tetrabromide.
19 The minimum expose time was found to be 60 sec.
The shelf life was found to be 24 hours.
21 The softening point after a 30 minute cure at 160C was 90C. ;

23 Samples treated as in Sample 3 were immersed immediately 24 after development into a 2% solution of triethylenetetramine in trichloroethylene for 2 minutes, followed by a clean trichloro-26 ethylene rinse. After a 30 minute cure at 160C, the softening 27 point was measured and found to be 175C.

29 100 g of "Epirez SU-8" was dissolved in 200 grams of chloro-form. To this was added 0.8 grams methylenedianiline and allowed .. . . . .
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1 to dissolve, 16 grams of acrylic acid was added. The mixture was heated at 50C for 11 days. Upon addition of 2 phr t-butyl anthra-quinone and 5 phr carbon tetrabromide, the minimum expose time was found to be 60 seconds, and shelf life was 48 hours.
EX~MPLE VI
9.6 kilograms of "Epirez SU-8" was dissolved in 19.2 kilograms of chloroform in a 12 gallon reactor. To this was added 76.S grams of methylene dianiline and allowed to dissolve. Then 1.536 kilograms of acrylic acid was added. The mixture was heated with slow stirring, over a period of 11 days at 50C, to obtain the product. The acid `
consumption of the product was 2.1 phr and the viscosity was 170 centi-stokes. With 2 phr t-butylanthraquinone and 5 phr carbon tetrabromide, the minimum expose time was 60 seconds measured as in Example 1. Shelf life was found to be 48 hours.
After dilution with cellosolve* acetate, 12" by 15" circuit panels were spray coated on both sides to a dry film thickness of 2.5 to 3.0 mil. The panels were exposed through an image which would allow photo-polymerization to occur in all areas except over the plated-through-hole land areas, wherein the material would be removed during development.
Development was accomplished by spraying the exposed panels for 3 minutes with hot trichloroethylene (65C). Immediately after development, the panels were immersed in a solution of 2% triethylene tetramine for 1 minute and given a rinse with hot trichloroethylene. The panels were subjected to a 30 minute cure at 160C after development and hardener application. The cured panels were put through a solder wave process at 480F after fluxing, without any deleterious effects.
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2 The material produced by the method of Example Vl was 3 applied to glass cloth-resin laminates whlch were prepared for electroless copper deposi-tion. After exposing, developing, hardening and curin~ a circuit pattern on -the coated laminates, 6 by ollowing the procedure of Example VI, the laminates were 7 processed through the electroless copper bath by being kept 8 immersed in the bath over a 24 hour period. A satisfactory 9 circuit was deposited without deleterious effects to the photo-processable coating.

12 100 grams of "Epirez SU-8" was dissolved in 150 grams of 13 chloroform. 0.4 grams of methylene dianiline and 16 grams of 14 acrylic acid were added to the solution and the reaction carried out at 60C for 5 days to obtain the product. Acid consumption 16 was 2.1 phr and viscosity 442 centistokes. To the product 2 phr 17 t-butyl anthraquinone and 5 phr carbon tetrabromide were added.
18 The product was then diluted with cellosolve acetate and spray 19 coated over copper foil to a thickness of 2.5 mils (after drying for 1 hour at 50C). The minimum expose of this product measured 21 as in Example I, was 60 secs. Line resolution, with a resolw-22 tion pattern, as the image, after development in a mixture of 23 methyl ethylketone/ethylacetate ~60:40 by volume) for 90 secs.
24 was 2 mil. wide lines with 2 mil spacings. Shelf life was 48 hours.

27 To 400 grams of "Epirez SU-8" was added 120 grams of chloro-28 benzene. The mixture was heated 110C and after a period of 29 one half hour a clear solution was obtained. While the solution was kept at llO~C lO grams of acrylic acid and 1.2 grams of EN9730:L4 -15-' . ' ' , 32~ :
1 methylene dianiline were added. The reaction produc-t was 2 obtained in 3.5 hours of heating. To portions of the product 3 ~ere added 3 phr o~ carhon tetrabromide and 2 phr of t-butyl 4 anthraquinone and various types of unsaturated monomers and the sensitivity determined as in Example 1. Table 1 gives the 6 results.
7 Table 1 Minimum 8 Free Monomer Quantity, phr Expose, Sec.
9 Control 0 600 Polyethylene glycol diacryla-te 5 40 10 Acrylic Acid 15 60 Pentaerythritol tetracrylate 10 60 11 Ethylene glycol methacrylate10 40 ~crylamide* 5 40 * Acrylamide was added and the mixture heated for 30 minutes at 13 60C to dissolve.
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EXAMPLE X
16 400 grams of "Epirez ~U-8" was dissolved in 100 grams of 17 chloroform. 2.4 grams of 4,4' - methylene dianiline was added to 18 the solution and dissolved. 9.0 grams of acrylic acid (98.41%
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19 assay) was added. The mixture was brought to 60C and maintained over a period of time.
21 At various times the mixture was sampled to determine acid 22 cGnsumption, viscosity, and sensitivity. The sensitivity was 23 determined by adding 2 phr tertiary butyl anthraquinone, 3 phr 24 carbon tetrabromide, and 10 phr of unsaturated monomer, poly-ethylene glycol diacrylate. The mix was coated over a copper 26 substrate, dried for 1 hour at 50C, to give a 1.5 mil thick 27 coating and exposed through a step wedge 30 inches from the 28 500 watt lamp mentioned above. The time which producad an 29 acceptable image after development in hot trichloroethylene for 2 minutes was noted as the minimum expose time. Table 2, below, : ~.

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1 gives the results obtained at the different times until the ~ -2 reactions were judged to be completed.
3 Table 2 4 Reactloll Time at 60C, ~Iours 6 Acid Consumption, phr 0.9 1.2 1.4 1.6 Viscosity, Centistokes 1290 1420 1690 2075 7 Minimum Expose Time 160 lO0 60 30 ~ -g EXAMPLE XI
The makerials and conditions given in Example 1, were re-11 peated except that a tertiary amine catalyst benzyl dimethyl 12 amine was added to the reaction mixture at a level of 0.2 phr.
13 After 42 hours of reaction the minimum expose time was found to 14 be 35 seconds and the acid consumption was 2.2 phr. The shelf life of the formulation as a coatlng was greater than 1 month 16 at room temperature.

18 To 100 grams of "Epire~ SU-8" was dissolved in 100 g chloro-19 form. To thls solution was added 8 g acrylic acid and 0.15 g benzyl dimethylamine. The mixture was heated at 65C for 16 21 hours whereùpon 1 gram of methylene dianiline dissolved in 50 22 grams of chloroform was added. The total mixture was then heated 23 at 65~C for 4 hours. To this composition was added 1.5 phr 24 t-butylanthraquinone, 3.0 phr carbon tetrabromide, and 5 phr polyethyleneglycoldiacrylate. Minimum expose time was observed 26 ko be 120 sec, with a shelf life of 2 weeks.
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Claims

The embodiments of the invention on which an exclusive property or privilege is claimed are defined as follows:

1. A process for preparing a photo-processable protective coat-ing comprising reacting on a substrate: (1) an epoxy prepolymer which is solid at room temperature and which has at least 2 epoxy groups per molecule, (2) an alpha, beta ethylenically unsaturated carboxylic acid, (3) a primary aromatic amine, and (4) a photo-initiator.

2. A process as claimed in Claim 1 wherein the epoxy prepoly-mer has about 8 epoxy groups per molecule.

3. A process as claimed in Claim 1 wherein the carboxylic acid is acrylic acid.

4. A process as claimed in claim 1, claim 2 or claim 3 where-in the photo-initiator is a mixture of an anthraquinone compound and a halogen containing compound.

5. A process as claimed in claim 1, claim ? or claim 3 where-in the photo-initiator is a mixture of t-butylanthraquinone and carbon-tetrabromide.

6. A process as claimed in claim 1, claim 2 or claim 3 where-in the primary aromatic amine is methylene dianiline.

7. A process as claimed in claim 1, claim 2 or claim 3 where-in from about 2 to about 16 parts by weight of unsaturated acid are reacted with 100 parts by weight of epoxy prepolymer.

8. A process as claimed in claim 1, claim 2 or claim 3 where-in from about 0.2 to about 1.5 parts by weight of primary aromatic amine are reacted with 100 parts by weight of epoxy prepolymer.

9. A process as claimed in claim 1, claim 2 or claim 3 where-in from about 1 to 7 parts by weight of photo-initiator is used for 100 parts by weight of epoxy prepolymer.