WO1993002395A1 - Photocurable compositions - Google Patents

Abstract

A photocurable film-forming composite comprises a suspension or dispersion in water of a liquid or solid organic photocurable component, in which composition the aqueous phase contains particulate solid material. The composition may be used to form patterned images upon subtrates by photoimaging techniques.

Description

PHOTOCURABLE COMPOSITIONS

This invention is concerned with improvements in and relating to photocurable compositions.

^'Photocurable compositions generally comprise a photocurable material, that is a material capable of curing or polymerising upon exposure to electromagnetic radiation, typically an ethylenically unsaturated or polyethylenically unsaturated material. Where, as is most common, curing is designed to be affected by exposure to ultraviolet light, the composition will also generally comprise a photosensitizer or photoinitiator, for initiating curing or polymerisation of the photosensitive material. Photocurable compositions are widely used in the electronics industry, for example in the manufacture of printed circuit boards and the like, as so-called "photoresists", that is compositions which are applied to a substrate, exposed to appropriate radiation through a patterned mask and later "developed" (by removal of uncured portions of the material) to form a patterned image of cured material upon the substrate. Such patterned image then serves to facilitate and define the area of operation of subsequent processes such as electroplating, etching, the application of solder etc. Photoresist material may be applied to substrates in the form of dry films or in the form of liquid compositions comprising a liquid or diluent, which is later removed, by evaporation, to form a dried film of appropriate material upon the substrate.

We have, e.g. in our patent applications Nos. PCT/GB91/01046 and PCT/GB92/00451, proposed the use of liquid photocurable compositions of the above sort in which the liquid carrier comprises or consists of water. Thus, in our application No. PCT/GB91/01046, we have proposed the use of aqueous emulsions of liquid organic materials comprising a photocurable component and, in our application No. PCT/GB92/00451, we have proposed the use of aqueous dispersions of solid organic materials comprising a photocurable component.

These water-based photocurable compositions have been found to be useful. We have, however, found that when drying a layer of the aqueous composition upon the substrate there is a tendency, especially under rapid forced drying conditions, for the aqueous carrier to escape from the film in a more or less violent or uncontrollable manner. Often, drops of water may form on the film, causing cratering and pin-holing of the film. Under less extreme conditions, the film is generally coherent but may become rippled where the uneven removal of water has caused variation in film thickness. The drying process can also cause flocculation of pigments, when present, giving patchy colour quality.

It has now been found, in accordance with the present invention, that improved water-based photocurable compositions, having a reduced tendency to the formation of film effects upon drying, can be obtained by incorporating, in the aqueous phase of the composition, a finely divided particulate material.

According to the invention, therefore, there is provided a photocurable film-forming composition comprising a suspension or dispersion in water of a liquid or solid organic photocurable component, in which composition the aqueous phase contains particulate material.

A wide variety of particulate materials, both inorganic and organic, may be employed. Typically, the average particle size of the material is not more than 10 microns and may be as low as fractions of a micron. Generally, however, particles in the range 1-5 μm prove satisfactory. In the case of the particulate filler, silica, this suitably has average particle size of from 2 to 5, preferably 3 to 4, microns. The particulate material is suitably present in an amount of 0.5 to 50% by weight, based on the total weight of the composition, preferably from 3 to 30% by weight. Thus, silica is suitably present in an amount of 0.5 to 20% by weight, preferably 1 to 15% by weight based on the total weight of the composition. In general, increasing amounts of silica tend to give a more matt final deposit and the use of excessive amounts of silica, e.g. above those indicated above, tend to give a viscous, even thixotropic, product, which is not particularly suitable for application. This, however, is not the case for many of the particulate materials discussed above although kaolin and bentonite clays may have a thickening effect. In any event, it is a matter of routine trial to ascertain the particular effect of any particulate material.

As noted above, a wide variety of particulate materials may be employed. Examples of suitable inorganic materials which may be used include talc, barium sulphate, smectite clays, calcium carbonate, kaolin, aluminium trihydroxide and titanium dioxide. Examples of suitable organic materials include finely ground styrene/maleic anhydride copolymer (SMA 3000, Sartomer International Inc.) and rubber latices such as Neoprene 115 (Butadiene rubber, Dupont) and Intex 5583 (carboxylated styrene/butadiene rubber, Enichem) . Silicas which may be used include both hydrophilic and hydrophobic silicas although, in general, we have found that the former give improved results. The silicas may be coated or uncoated. Examples of suitable hydrophilic silicas include uncoated pyrogenic silicas such as "Aerosil" TS 100 (Degussa) or surface coated silicas such as "Aerosil" OK 500 (Degussa) or "Syloid" ED 30 (W.R. Grace) . Hydrophobic silicas such as "Aerosil" R 805 (Degussa) have also given good results.

It is an important feature of the compositions of the present invention that the particulate materials should be present in the aqueous, carrier, phase of the composition rather than in the organic photocurable component. To this end, the particulate material may be added to the aqueous phase before or after formation of the dispersion or suspension of the organic photocurable material therein.

As will be appreciated, a wide variety of organic ^■ photocurable systems may be used in the compositions of the invention. Thus, for example, in the case of a composition in accordance with the invention taking the form of an aqueous emulsion of a liquid organic component, one general class of photocurable system comprises a curable reactive material (generally oligomeric or polymeric in nature) together with an initiator component which on exposure to the appropriate radiation, reacts with the curable component thereby to cross-link it or cure it. A particular embodiment of this sort of photocurable system comprises polyvinyl alcohol as curable component together with a diazo initiator.

Another general class of photocurable system, which is also particularly suitable for use for compositions taking the form of suspensions or dispersions of solid powdered photocurable material, comprises an ethylenically unsaturated curable material (again generally oligomeric or polymeric in nature) together with a photoinitiator which, on exposure to the appropriate radiation, gives rise to free radicals which initiate polymerisation of the double bonds" in the curable component. Examples of photocurable materials which may be used in this second general class of photocurable systems include include multifunctional acrylate oligomers such as pentaerythritol triacrylate, trimethylolpropane triacrylate and ethylene glycol polyacrylate. Other photosensitive materials are-those which may be obtained by reacting multifunctional isocyanate compounds with ethylenically unsaturated compounds containing a group containing an active hydrogen atom such as a hydroxyl group or carboxylic acid group. Examples of suitable isocyanates include hexamethylene di-isocyanate, tolylene di-isocyanate or isophorone di-isocyanate, or dimers or trimers formed therefrom. Suitable unsaturated compounds containing active hydrogen include, for example, hydroxyl-ethyl acrylate, hydroxyethyl methacrylate, acrylic acid and methacrylic acid. A further class of UV sensitive curable materials are those formed by the reaction of poly-epoxy compounds^* (so-called "epoxy resins") with ethylenically unsaturated acids such as acrylic acid or methacrylic acid; which reaction products may simply be termed "epoxy acrylates". The epoxy compound may be a simple glycidyl ether such as ethylene glycol diglycidyl ether or phenyl glycidyl ether; or a bis-phenol A/epichlorohydrin adduct such as those sold under the trade name "EPIKOTE". Further epoxy resins which may be employed are epoxy novolak resins (including epoxy phenyl novolak and epoxy cresol novolak resins) such as those sold under the trade name "QUATREX" . The "epoxy acrylates" derived from such resins are particularly suitable for use as components in photoresists for preparing solder masks.

In order to render the epoxy acrylate material soluble or developable by aqueous alkaline solutions, the epoxy acrylate resin may be reacted with one or more dicarboxylic acid anhydrides (serving to introduce free carboxyl groups into the final epoxy acrylate) . Suitable dicarboxylic acid anhydrides for this purpose includes succinic, itaconic, maleic and phthalic anhydrides. A wide variety of photoinitiators are known for use in photocurable systems and examples of these include ant raquinones such 2-ethyl-anthraquinone, 2-methyl- anthraquinone and 1-chloro-anthraquinone; thioxanthones such as 2,4-dimethyl-thioxanthones, 2,4-diethyl- thioxanthones and 2-chloro-thioxanthones; ketals such as benzyl-dimethyl ketal and acetophenone-dimethyl-ketyl, benzophenones, and benzoin and ethers thereof. These photoinitiators can be alone or in admixture and may also be used together with photopolymerization accelerators such as benzoic acid type accelerators or tertiary amine accelerators.

In addition to the photocurable system, the photocurable material may also contain other components such as pigments, rheological additives (flow aids and degassing agents) and thermal curing agents and surfactants. Surfactants or thickening agents may serve to assist in suspending the particulate material in the composition. Suitable surfactants include nonionic surfactants such as those sold under the trade names Synperonic PE/F 108, Pluronic 6800 and Disponil 23. Suitable thickeners include acrylic polymers such as Acrysol RM 8 (Rohm S_ Haas) and FX1010 (Servo Delden B.V.) . Photocurable compositions in accordance with the invention may be employed simply as photocurable coating materials or, as indicated above, find particular application as photoresist materials for use in the electronics industry, for example in the formation or production of printed circuit boards or the like. Thus, the photocurable compositions of the invention are particularly suitable for use in a method of forming a patterned image upon a substrate which method comprises the steps of:

(a) forming a layer of the liquid photocurable composition upon the substrate;

(b) drying the layer of photocurable composition on the substrate;

(c) imagewise exposing the dried layer to electromagnetic radiation to cure portions of the layer exposed to the radiation; and

(d) "developing" the exposed layer by removing unexposed portions of the layer.

The patterned layers or resists obtained by the above route may be used, as indicated above, as lithographic resists plating resists, etch resists or solder masks.

The liquid photocurable composition may be applied to the substrate by any suitable coating method such as, for example, by screen printing, curtain coating or, electrostatic spraying. The thickness of .the resist layer will, of course, depend upon the intended end use but, in general, thicknesses of the order of 20 - 80 μm are generally suitable.

In order that the invention may be well understood the following Examples are given by way of illustration only.

Example 1

A clear varnish formulation (suitable as a clear cover varnish for metallic materials) was prepared by mixing the following materials:-

% Urethane acrylate resin 68%

(Craynor 934, Cray Valley Products) Trimethylol propane triacrylate 24%

Photoinitiator (Irgacure 651) 8%

The viscous fluid produced was then emulsified using a suitable emulsificant (ICI speciality chemicals Atlas G1350) and distilled water:-

Photosensitive Lacquer (see above) 67% Atlas G1350 8%

Distilled Water 25% The materials were mixed together on a Silverson high speed stirrer to yield a stable white coloured emulsion. To this fluid was then added 8% (by weight of total formulation) of an organically coated silica, ED 30 (W.R. Grace) . This material was dispersed in the emulsion by low speed stirring, using a laboratory paddle stirrer. The lacquer was then coated by draw-down bar onto an uncoated, bright brass, substrate. The coated sample was then force dried in a fan driven oven at 120°C, beforebeing given a final UV cure under a high pressure mercury vapour lamp of 120 w/ins power input. The film produced was tack free and free from blemishes or pin-holes.

Example 2

An ink formulation suitable as a photoimageable solder resist was prepared by first mixing the following materials:-

Carboxylated epoxy novolak acrylate resin 67.4%

Propylene glycol methylether acetate 19.4% Photoinitiator (I)

(Quantacure ITX, Ward Blenkinsop) 0.8% Photoinitiator (II)

(Irgacure 907, Ciba Geigy) 6.4%

Pigment (Phthalocyanin green) 1.0%

Aerosil TS100 (Degussa) 5.0% [ Prepared by reaction of 1 equivalent of acrylic acid with 1 equivalent of an epoxy novolak resin

(Quatrex 2410, Dow Chemicals) followed by reaction of the acrylated resin with sufficient of an equimolar mixture of maleic anhydride and tetrahydrophthalic anhydride to give an acid value of 65 mg KOH/g. ] .

The viscous green ink produced was then emulsified by the addition of a suitable emulsificant, followed by deionised water, to give an emulsion of suitable viscosity for screen printing, as follows:-

Resist ink (see above) 72%

Atlas G1350 (Emulsificant) 8%

Deionised water 20%

To this emulsion was added 5% w/w of total formulation of Aerosil TS100 (a pyrogenic silica, particle size nominally 1-9 μ) supplied by Degussa. This material was then printed onto an IPC test printed circuit using a 22T mesh silk screen) . A sample of the ink without silica was also printed thuswise for comparison. Both sample boards were dried at 100°C for half an hour in a forced air convection oven. The quality of the two boards on removal from the oven was markedly different. The film produced without silica in the continuous phase was uneven and bubbled, the resist with silica in the aqueous phase however was of a high quality with no blemishes or discontinuities in it. Example 3

A material suitable for use as a lithographic etch resist was prepared by first mixing the following materials in a twin screw extruder: -

(2) Solid carboxylated epoxy cresol novolak acrylate 90.3'

Photoinitiator I (Quantacure ITX,

Ward Blenkinsop) 2.4%

Pigment (methylene blue) 3.3%

Flow aid (modaflow 2, Monsanto Inc.) 3.0%

[ (2)Prepared by reacting 1 equivalent of acrylic acid with 1 equivalent of a cresol novolak epoxy resin

(Quatrex 3710, Dow Chemicals) in toluene. The acrylated resin was carboxylated using an equimolar mixture of maleic anhydride and tetrahydrophthalic anhydride and tetrahydrophthalic anhydride to give an acid value of 70 mgKOH/g. The toluene was distilled off under vacuum to give an opaque straw-coloured resin of m.p. 90-95°C] .

The solid cake produced was then milled to give a powder with a particle size substantially below 5 μm.

This blue powder was then high speed stirred into a solution of Atlas G1350 surfactant to give the following formulation. Powder Resist (as above) 40% Atlas G1350 8% Distilled water 52%

To the stable suspension of solid particles in water was added 10% w/w (total formulation) of Aerosil R805 hydrophobic silica by low speed stirring on a Silverson high shear stirrer. This material was then draw down bar coated onto plain aluminium substrate. At the same time a similar piece was prepared using a suspension without added silica. Both were force dried at 110°C in a fan assisted convection oven. After 30 minutes the samples were removed from the oven and examined for film quality. The material without silica had an uneven, bubbled finish which rendered it unsuitable for use as a lithographic etch resist, where an even thickness of coating is of paramount importance. The material with added silica showed excellent film quality and performed well as an etch resist.

Example 4

A green pigmented ink, suitable for use as a photoimageable solder mask, was prepared by mixing the following ingredients: Carboxylated epoxy novolak resin 82.0% (as used in Example 2)

Photoinitiators 5.0% (Irgacure 90 and Quantacure ITX)

Triglycidyl isocyanurate triacrylate 8.0%

Di-trimethylolpropane tetraacrylate 0.5%

Pigment (Unisperse green, Ciba Geigy) 3.7%

The resultant viscous green liquid photosensitive lacquer was then emulsified using a Silverson high speed stirrer with the following further components in the following proportions:

Photosensitive lacquer 50.0%

Surfactant (Synperonic PE 108) 0.8%

Poly(vinyl pyrrolidone) 2.7%

Distilled water 26.0%

Thickening agent (Acrysol RM 8) 4.6%

15.9% by weight of barium sulphate (blanc fixe) was stirred into one batch of the emulsion using a Silvenson stirrer. Another batch of the emulsion was retained as a comparative sample.

Each of the two emulsions were coated, by means of a draw-down bar, onto samples of scrubbed copper laminate. The samples were force dried for 30 minutes at 100°C in a fan drier oven and then inspected. The ink containing the barium sulphate produced a continuous glossy film, free from craters and blemishes. The ink net containing any barium sulphate produced a glossy film which was badly marked by craters and ripples.

Example 5

An ink suitable for the preparation of a lithographic printing plate was prepared by first mixing the following materials together:-

Carboxylated epoxy cresol novolak 89.0% Resin

Photoinitators (Irgacure 900) 2.5%

(Irgacure 900) 1.5%

(Quantacure ITX) 1.0%

Pigment (methylene blue) 1.5%'

Di-trimethylolpropane triacrylate 4.5% ^•

[ (3)Prepared as m Example 2 but using Quatrex

3410 in place of Quatrex 2410] .

The ingredients were mixed together using a Diaf high speed dispersion stirrer to give a deep blue viscous ink. The ink was then emulsified, in a colloid mill, with the following components in the following amounts. Ink 49.0%

Sufactant (Atlas G1350) 8.0%

Distilled water 43.0%

3.5% by weight of calcium carbonate (Calafort S) was dispersed in the emulsion, using a Silverson high speed stirrer, to give a stable, lighter blue, emulsion.

Example 5

The procedure of Example 5 was repeated using 5% by weight of finely ground styrene/maleic anhydride copolymer (SMA 3000) in place of the calcium carbonate.

Example 7

The procedure of Example 5 was repeated using 8% by weight of aluminium trihydroxide (SF 11) in place of the calcium carbonate and stirring this into the emulsion using a Diaf high shear dispersing stirrer.

Example 8

The procedure of Example 5 was repeated except that 5% by weight of neoprene rubber was used in place of the calcium carbonate. The rubber was introduced as a latex which was mixed with the emulsion by means of a low speed paddle stirrer. Each of the inks prepared in Examples 5-8, together with comparative samples not containing any particulate additive, were draw down coated onto bright aluminium plate and then force dried in a fan assisted oven for 15 minutes at 100°C. All the inks containing the particulate additive gave unblemished films but those not containing the additive were uneven and cratered.

Claims

CLAIM:

1. A photocurable film-forming composition comprising a suspension or dispersion in water of a liquid or solid organic photocurable component, in which composition the aqueous phase contains particulate solid material.

2 . A method of forming a patterned image upon a substrate which comprises the steps of:

(a) forming a layer of a liquid photocurable composition as claimed in claim 1 upon the substrate;

(b) drying the layer of photocurable composition on the substrate;

(c) imagewise exposing the dried layer to electromagnetic radiation to cure portions of the layer exposed to the radiation; and

(d) developing the exposed layer by removing unexposed portions of the layer.