US3342601A - Lithographic printing plate - Google Patents

Description

Sept. 19, 1967 HQULE ET AL 3,342,601

LITHOGRAPHIC PRINTING PLATE Filed Feb. 27, 1964 F IG.

l 4 SEA/SIT/VE STABILIZED 0/420 RES/Al 2 Hr0/?0PH/L/c LAYER 3 GRA/NED ALUMINUM ,A Q m m WEXPOSED STABILIZED 0/420 RES/N HYDROPH/L/C LAYER 3 GRAI/VED ALUMINUM JAMES F HOULE G/LDEN R. VA/V NORMA/V A TTO/P/VEYS United States Patent O 3,342,601 LITHOGRAPHIC PRINTING PLATE James F. Houle and Gilden R. Van Norman, Rochester,

N.Y., assiguors to Eastman Kodak Company, Rochester, N.Y., a corporation of New Jersey Filed Feb. 27, 1964, Ser. No. 347,931 14 Claims. (Cl. 96-86) ABSTRACT OF THE DISCLOSURE Light-sensitive photographic elements such as lithographic printing plates are prepared by coating a support with an aqueous composition containing a complex of an alkyl titanate and hydrogen peroxide or certain inorganic acids alone or in combination with the peroxide.

This invention relates to lithographic printing plates, more particularly to offset printing plates commonly used in the lithographic trade.

Lithographic plates are well known in the graphic arts field, particularly those having metal supports such as aluminum or zinc. In recent years aluminum has been particularly attractive for lithographic plates due to its cost, availability and light weight. However, when aluminum is 'used as a substrate for printing plates, certain inherent disadvantages must first be overcome. For instance, it has been established that the naturally occurring oxide film on aluminum is not sufiiciently hydrophilic to provide trouble-free operation on a lithographic printing press. The bare aluminum is prone to become slightly hydrophobicor oleophilic resulting in a gradual buildup of ink on the nonprinting areas. This phenomena is referred to in the trade as toning or scumming. In order to eliminate toning or scumming, a suitable, permanently hydrophilic surface which is relatively insoluble must be provided. Aluminum is also known to react with certain sensitive materials, such as diazo resins coated thereon, as indicated in US. Patent 2,714,066.

Several methods of imparting an insoluble, permanently hydrophilic surface to the aluminum substrate have been suggested in the prior art. Typical treatments include anodizing, coating the surface with a hydrophilic organic polymer, etc. For instance, a method is described in French Patent 904,255, published Oct. 31, 1945, showing chemical or electrical treatments which provide a porous layer of aluminum compounds of oxygen, fluorine or phosphorous. In addition, Clerc, in the Ilford Manual of Process Work, 1946, page 237, discloses treatment of aluminum plates with solutions containing fluosilicic acid or a fluosilicate.

These treatments, however, have not been satisfactory in producing a suitable hydrophilic coating on the aluminum adequate to provide the required ink-water differ ential for successful operation on a lithographic press. Moreover, the above treatments do not protect conventional diazo coatings from the aluminum so that the plates can be presensitized and stored over long periods of time prior to usage.

One method of providing an aluminum plate having an insoluble, hydrophilic layer which also isolates the diazo light sensitive coating from the aluminum substrate is to deposit a coating on the aluminum of a silicate containing the SiO radical.

A presensitized aluminum lithographic plate having a silicate layer thereon, has limited press life and also is subject to decreases in sensitivity during storage, especially after the plate has been exposed to the atmosphere particularly with high humidity or heat or a combination of both. Therefore, it has been desirable to provide a plate having greater press latitude, longer press life and longer shelf life than provided by the plates of the prior art.

Coating an aluminum surface with silica or similar silicate undercoating requires a smooth aluminum surface. However, it has customarily been found in lithographic printing that a grained or brushed surface provides a wider set of operating conditions on the press, particularly since a grained plate requires a less critical ink-Water balance than does the silicate coated smooth plate. Moreover, the less critical ink-water balance decreases the tendency for the ink to emulsify during multicolor printing, which normally results in a loss of image sharpness. Therefore, it has also been desirable to provide a presensitized aluminum plate having a grained or brushed surface, In order to provide an insoluble hydrophilic surface or surface coating it is desirable to eliminate the necessity of elevated temperatures and subsequent washing during the process. These steps are essential in many of the known processes. It is also desirable that the treatment be conducted from aqueous media thus eliminating toxic or flammable solvent vapors. Therefore, to achieve improved characteristics for a presensitized plate, it is desirable to obtain a plate having a grained surface, an improved method of treatment to provide a hydrophilic surface and a stabilized light sensitive coating thereon, thus giving a combination of superior printing and keeping qualities.

We have found that a suitable insoluble, hydrophilic surface layer may be imparted to an aluminum sheet by treatment with aqueous solutions of certain complex titanium salts capable of reacting at or on the surface to form an insoluble layer.

One object of this invention is to provide a hydrophilic lithographic printing surface. Another object is to provide a coating composition for treating surfaces-such as metal, paper, polymeric, etc. to render a surface hydrophilic for use in lithographic printing. A further object is to provide a presensitized lithographic printing plate. A still further object is to provide a diazo sensitized, grained aluminum, lithographic printing plate. Another object is to provide a novel coating composition which adheres tenaciously to various supports including metals such as aluminum, zinc, copper, and the like, and to polymeric materials such as polyesters, polyamides, polystyrene, cellulose esters, etc., to provide a hydrophilic coating substantially free from water-soluble materials and to which can be adhered light sensitive coatings, printing inks, paints, etc.

Other objects closure.

. The above objects and other objects are attained by making a coating composition in an acid medium of a titanium compound which is substantially water soluble.

A useful source of titanium is the titanium orthoesters such as, for example, the tetraalkyl titanates. Although other titanates may .be used, the preferred tetraalkyl titanates are those having alkyl groups with 2 to 5 carbon atoms such as, for example, tetraethyl titanate, tetrapropyl titanate, tetraisopropyl titanate, tetrabutyl titanate, tetraisobutyl titanate, tetraamyl titanate, etc.

The acidic constituent which is useful for reacting with the titanate produces a water-soluble titanium complex in an acid media. Useful acidic constituents include, for example, fiuosilicic acid, hydrofluoric acid, phosphoric acid, fluoboric acid, nitric acid, etc.

The coating composition is preferably prepared by adding the alkyl titanate to the dilute aqueous acid, whereupon the alkyl titanate immediately reacts to form hydrous titanium dioxide which subsequently dissolves and/ or reacts forming a so called basic titanium salt. When fiuosilicic acid is employed to form titanyl silicofluoride, a useful molar range of fluosilicic acid to tetraalkyl titanate is from 5:1 to 1:1 moles, although our prewill be apparent from the following disferred molar range is from 2:1 to 1:1 moles. The pH of the coating compositions may be varied from 1.0 to 5.0.

A useful molar range of fluoboric acid to tetraalkyl titanate is from 7:1 to 2:1 moles.

When phosphoric acid is used as the acidic constituent, the coating composition employed is titanyl peroxy phosphate. The alkyl titanate is added to a dilute hydrogen peroxide solution. When solution is complete, phosphoric acid is then added. In a preferred composition we use 6.810.7 moles hydrogen peroxide per mole alkyl titanate and 2.3-7 moles phosphoric acid per mole alkyl titanate. It will be fully realized that the volumetric proportions may be varied and that the amount of water used is not critical but must be sufficient to provide solubility.

A useful coating composition is also obtained by preparing a pertitanic acid by dissolving the alkyl titanate in hydrogen peroxide.

Various methods of applying the coating may be used including direct roll, immersion, hopper, knife, wick, bead, spray or any method of convenience. The coating may be on one or both sides of the support.

The thickness of the titanium complex layer on an aluminum substrate is difficult to determine due to its thinness. However, it is most likely in the range of thickness of the naturally occurring oxide.

Drying occurs rapidly, but in production, driers may be used advantageously, employing heat by known means, such as hot air, infrared lamps, etc.

The light sensitive coating may be a suitably stabilized water-soluble resin containing the diazonium group which upon exposure to ultraviolet light forms an oleophilic surface or image.

The light sensitive coating may be of the general for- M is a di or trivalent metal of the groups known to stabilize the diazonium groups, e.g. cadmium, zinc, bismuth, arsenic, antimony, tin, iron, platinum, mercury.

X is an anion or mixture of anions consisting of halogen or sulfates.

Z may be a phenyl or substituted phenyl in which case the resin is an aldehyde condensation product of a p-diazodiphenylamine salt. In this case, Y is hydrogen.

Z and Y may also be organic radicals functionally capable of entering into an addition polymerization, polyesterification, condensation or other reaction capable of producing a relative low molecular weight resin or capable of reacting with a preformed polymer. Z and Y in this case may be the same or different, the parent diazonium compound being a substituted p-diazo aniline.

Some examples of the reactive groups are hydroxyl as in N-ethyl, -N-(B-hydroxyethyl)-anilino, 4-diazo salts capable of reacting with polymerizable groups such as acryloyl, methacryloyl, etc.

The placement of various groups on the anilino nitrogen can result in the preparation of polyesters, polyethers, polyurethanes, polyacetals, polycarbonates, etc., by direct polymerization or condensation with functionally proper preformed polymers.

In our preferred embodiment, the light sensitive coating consists of an aldehyde condensation product of a p-diazodiphenyl amine. Light sensitive aldehyde condensation products of p-diazodiphenyl amine stabilized with metal salts are well known in the art as described in U.S. Patent 2,714,066. Such resins are also available commercially, e.g. Diazo Resin No. 4 supplied by Fairmo-unt Chemical Co. This material is described as a diazonium sulfate-zinc chloride double salt. However, as stated in the prior art, aluminum reacts with the diazo resin causing decomposition. Therefore, in our preferred embodiment we use a method of preparation and stabilization of the light sensitive diazo resins such that they are not decomposed by the aluminum surface.

The presensitized, grained aluminum printing plate prepared according to our specifications, may be handled and treated in use as is customary in the art. The unexposed areas are removed and the plate lacquered, after which the plate is used on a lithographic printing press.

The sensitive layer for the lithographic plate can be exposed to visible light, ultraviolet light, infrared light, X-rays or to heat patterns in order to form differential areas for printing. It is possible to use light sensitive organic compounds such as aliphatic and aromatic esters, hydrazides and amides of napthoquinone-diazide-sulfonates, cinnamal-malonic acids, their substitution products and functional derivatives, diazonium salts of amino-diphenyl amines and their condensation products with formaldehyde, orthoand para-quinone diazides of benzals, anthracenes and heterocyclic systems, for example, quinoline, indazoles, benzimidazoles, di-phenyloxides, also diazoketones, unsaturated ketones, orthoand para-iminoquinone diazides, derivatives of alkyl-nitronaphthalenesulfonates, nitroaldehydes, acenaphthene, stilbene, azides and diazides andhigh molecular polymer diazo resins. Silver halide emulsions may also be used. Moreover, the subbing may be overcoated with a photoconductive layer having a high electrical resistance which can be charged and subsequently discharged by light or heat as in electrophotography, for example, low or high molecular weight organic photoconductors, also mixtures thereof with resins. Particularly suitable as photoconductors are oxadiazoles, imidazolones, triazole, oxazole, thiazoles, hydrozones, triazines, polyvinyl carbazole and polyvinyl oxazoles.

Useful resins may contain groups which tend to make the resins soluble in alkali, such groups include acid anhydride, carbonic acid, sulfonic acid, sulfonamide or sulfonimide groups, for example, polyvinyl polymers or mixed vinyl polymers, phthalic acid ester resins, alkyl resins, :rosins and polyacrylic acid resins.

The lithographic plates of the invention are further characterized by good adhesion and produce good removal of the unexposed areas so that they can produce a large number of copies.

The drawing shows one embodiment of our invention.

FIG. 1 shows a grained aluminum sheet 3 having thereon a hydrophilic layer produced by treatment with a titanyl salt complex 2 and having thereon a light sensitive stabilized diazo resin 1.

FIG. 2 shows the structure of FIG. 1 following exposure to light and removal of the unexposed areas of the stabilized diazo resin.

The following examples are intended to illustrate our invention but not to limit it in any way.

Example 1.Titanyl silicofluaride A sheet of brush-grained aluminum having a thickness of 5 mils is immersed in a solution of titanium complex prepared by agitating vigorously a solution of 750 ml. of distilled Water in 50 ml. of 31.9% fiuosilicic acid, while 10 ml. of tetraisopropyl titanate is added rapidly in a fine stream. The aluminum is then passed between two mechanically driven inch diameter rubber rolls under sufficient pressure to remove the excess solution. A very thin film of solution is thus obtained which dries rapidly at room temperature. Similar coatings are obtained on other supports including polymeric materials, paper, metal, etc.

Example 2.Titanyl peroxyphosphate To 740 ml. of Water is added 10 ml. (18.3 g.) of percent phosphoric acid (0.1588 mol) and 30 ml. (33 g.) of 28 percent hydrogen peroxide (0.262 mol). Then with good agitation 10 ml. (9.55 g.) of tetraisopropyl titanate (0.0336 mol) is added. The result is an orange-yellow solution which is ready for coating. This solution is coated as in Example 1 to give a thin coating on the surface of the support.

Example 3.-Preparatin of litany! fluoborate coating solution To 750 ml. water is added 14.7 g. (0.0672 mole) of 40 percent fluoboric acid with suificient stirring to mix. To this solution 10.0 ml. (9.55 g., 0.0336 mole) of tetraisopropyl titanate is added with good agitation. The ortho ester hydrolyzes immediately forming a white percipitate which gradually redissolves forming a clear solution. This solution is then coated on a grained aluminum sheet, dried, and sensitized with a dilute aqueous solution of diazo resin as in Example 5. When properly exposed and processed, as in Example 6, the titanyl fluoborate solution produces a suitable hydrophilic printing surface. Similar coatings are obtained on the supports of Example 1.

Example 4 .-Preparati0n 0 f a pertizanic acid coating composition Tetraisopropyl titanate is added to a dilute aqueous solution of hydrogen peroxide in the same manner as the previous example. The white hydrolysis product rapidly turns yellow and gradually dissolves to produce a clear yellow solution. A particularly useful composition for application as in Example 1 contains 0.045 mole per liter of titanium and 0.307 mole per liter of hydrogen peroxide. The hydrophilic character of the surface obtained is enhanced by drying at an elevated temperature. Similar coatings are obtained on the supports of Example 1.

Example 5 .Diaz0 resin A diazo resin is prepared by mixing 34 grams p-diazodiphenyl amine sulfate-zinc chloride double salt with 135 grams concentrated sulfuric acid which has been cooled to a temperature of C. The mixture is agitated while the temperature is maintained at less than 3C. while 3.25 grams paraformaldehyde, and 6.05 grams anhydrous cadmium chloride are added. The viscosity increases over a period of. approximately one-half hour and the mixture becomes homogeneous. The mixture is then poured over 150 grams of ice and stirred until the ice melts, the yellow precipitate is filtered, redissolved in water and reprecipitated into two liters of isopropyl alcohol, filtered and dried under a Vacuum in the dark. Twenty-eight grams are obtained. This resin is a stabilized diazo resin.

Example 6 The treated, grained aluminum of Examples 1 and 2 is then sensitized by immersing in a dilute 2%, aqueous solution of diazo resin, passed between two rubber rolls to remove the excess and dried. When exposed at approxi mately 24 inches through a negative with a high intensity 220 volt carbon arc in a NuAr-c Printer, the exposed portions are hardened.

The unexposed portions of the diazo resin are removed by washing with water. The image is then lacquered with a commercially available image lacquer.

The plate is then put on the lithographic printing press and produces 90,000 satisfactory copies. Presensitized plates prepared according to this example are found to have a shelf life for more than two years.

Example 7 .Preparation of a preferred diazo resin sensitizer An 800 ml. beaker is equipped with a mechanically driven propeller type stirrer and external cooling is provided. To the reaction vessel is added 540 g. (5.35 moles) of 96 percent sulfuric acid followed by portionwise addition of 138 g. (0.377 mole) of Sensitizer DP (para-diazodiphenylamine sulfate-zinc chloride double salt, obtained from Fairmount Chemical Co.) with stirring at 25 to 35 C. until solution is complete. The mixture is then cooled and maintained at 0 C. during the gradual addition of 100 ml. (0.592 mole) of 40 percent fluoboric acid followed by the addition of 13.0 g. (0.432 mole) paraformaldehyde with vigorous agitation to provide rapid and uniform dispersion of the paraformaldehyde. An additional ml. (0.592 mole) 40- percent fluoboric acid is then added. The resultant mixture is now deep red in color whereas prior to the initial addition of fluoboric acid the solution was brownish in color. The reactant mixture is stirred and maintained at 0 C. for a period of one hour and ten minutes whereupon it is poured with good agitation into 800 g. ice containing 210 ml. (1.24 moles) of 4% fluoboric acid. The precipitate which forms is then filtered and collected. The moist cake is then dissolved in 700 ml. dimethylformamide and refiltered. The clear filtrate is then agitated during the addition of 220 g. (0.965 mole) of cadmium chloride (CdCl -2 /2 H O) dissolved in 250 ml. water. The precipitate thus formed is filtered and collected. The moistcake is then added to 2 liters of ispropyl alcohol and stirred vigorously to produce a small uniform particle size which is filtered, collected and reslurried in the same manner" as above in 1 liter of diethyl ether. The slurry is then filtered, collected and dried under vacuum at room temperature, care being taken to break up the moist cake to provide uniform drying. The yield of dry resin is 100 g. in the form of a light orange yellow powder.

Example 8. Prepa1rali0*n of a diazo resin sensitizer II In a 1500 ml. glass beaker was placed 1040 g. of 96 percent sulfuric acid. The beaker was suspended and equipped with a variable speed stirrer. The stirring acid was cooled below 0 C. with a Dry Ice-acetone bath.

A 272 g. portion of Fairmount DP salt (Fairmount Chemical Co.) and26 g. of paraformaldehyde were mixed together thoroughly in the dry state and then added in portions to the acid, keeping the temperature below 5 C. The addition took 10 minutes. Stirring was continued for 75 minutes, keeping the temperature below 0 C.

The acid solution was poured over a mixture of 2200 g. of crushed ice and a solution of 1 lb. of cadmium chloride in 600 ml. of water with good agitation. A 3 liter portion of isopropyl alcohol was added and the resultant slurry filtered. The precipitate was slurried in 4 liters of water and 3 moles (624 g.) of barium chloride added. The resultant precipitate was filtered off and subsequently extracted twice with three liter portions of 0 .75 M HF solution and a third time with 2 liters of distilled water. The filtrates were combined and a solution of 1 lb. of cadmium chloride in 400 ml. added. Then 8 liters of isopropyl alcohol were added and the slurry filtered. The product was dried under vacuum at room temperature. The yield was 286 g.

Example 9 When the stabilized diazo coatings of Examples 6-8 are coated onto untreated grained aluminum sheet, they are found to have a shelf life for more than two years indicated stability of the resin layer without an undercoating. When the unstabilized zinc chloride double salt of the pdiazodiphenyl amine sulfate is coated over the treated aluminum plate, the shelf life is approximately three months.

It will be appreciated that while aluminum is described generically, that various aluminum alloys are intended to be Within the scope of our invention as well as metal laminates.

The solutions used to remove the unhardened areas of the diazo resin are not critical. Water may be used, but aqueous gum solutions known in the art are also useful. It will be appreciated that other lacquers available commercially may also be used.

The diazo resins which may be used in our invention are prepared, stabilized and used in the dark or under yellow light. Therefore, when these resins are coated on the treated plate, the coating operation is carried out under appropriate safelight conditions.

7 Example The treated grained aluminum of Examples 1 and 2 is sensitized by coating with light sensitive polymers using the whirler coating technique. Kodak Photo Resist yields a clean appearing image. When inked, 50 press copies are obtained on the lithographic press. Kodak Metal Etch Resist is developed by tray development followed by water wash-off and compressed air dried down to yield a robust, clean appearing image. The image is resistant to prolonged rubbing. Light sensitive compositions of the type described in U.S. Patent 2,948,610 are swab developed with 0.5% sodium carbonate to yield adherent, ink-receptive images. Azoplate Positop developed with Positop developer yields adherent robust images which show good press life. Light sensitive polycarbonate resins of the type described in US. Patent 3,043,802 are tray developed to yield clean, ink-receptive images. A negative working wash-off light sensitive silver halide emulsion is transferred to the treated surface from the strippable support. After activation, the untanned gelatin is washed off.

A light sensitive silver halide emulsion comprising the first gelatin layer containing an oxidizing developer and a second layer containing light sensitive silver halide is coated on the treated support. A satisfactory lithographic plate is obtained after activation and inking of the surface.

The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention as described hereinabove and as defined in the appended claims.

We claim:

1. A presensitized planographic printing plate comprising a support having on at least one side a hydrophilic surface provided by coating with an aqueous solution consisting esesntially of a complex of an alkyl titanate and a solubilizing amount of at least one solubilizing component selected from the class consisting of fluosilicic acid, hydrofluoric acid, fluobo-ric acid, hydrogen peroxide and a mixture of hydrogen peroxide and phosphoric acid, and having thereon a light-sensitive coating.

2. A plate according to claim 1 in which the light-sensitive material is a di-azo resin.

3. A plate according to claim 1 in which the light-sensitive material is a silver halide emulsion.

4. A plate according to claim 1 in which the lightsensitive coating contains a light-sensitive polymeric material.

5. A plate according to claim 1 in which the light-sensitive material is an azido resin.

6. A plate suitable for use in planographic printing and related uses comprising a support having on the surface a hydrophilic layer formed in situ from an aqueous coating composition consisting essentially of a complex of an alkyl titanate and at least one solubilizing compound selected from the class consisting of fluosilicic acid, hydrofluoric acid, fluoboric acid, hydrogen peroxide and a mixture of hydrogen peroxide and phosphoric acid.

7. A plate according to claim 6 in which the support is metal.

8. A plate according to claim 6 in which the support is aluminum.

9. A plate according to claim 6 in which the support is an organic polymeric material.

10. A plate according to claim 6 in which the coating composition consists essentially of a complex of an alkyl titanate and at least a solubilizing amount of fluosilicic acid.

11. A plate according to claim 6 in which the coating composition consists essentially of a complex of an alkyl titanate and at least a solubilizing amount of hydrofluoric acid.

12. A plate according to claim 6 in which the coating composition consists essentially of a complex of an alkyl titanate and at least a solubilizing amount of fluoboric acid.

13. A plate according to claim 6 in which the coating composition consists essentially of a complex of an alkyl titanate and at least a solubilizing amount of hydrogen peroxide.

14. A plate according to claim 1 in which the coating composition consists essentially of a complex of an alkyl titanate and at least a solubilizing amount of hydrogen peroxide and phosphoric acid.

References Cited UNITED STATES PATENTS 2,825,697 3/195 8 Carroll et al 148-6.27 X 2,950,174 8/1960 Lagally.

2,983,221 5/1961 Dalton et al. 9633 X 3,017,282 1/1962 Brill 106-299 X 3,061,483 10/1962 Coles 148-6.14 3,073,723 1/1963 Deal et al 9633 X 3,110,596 11/1963 Heiss et al 9633 X 3,160,506 12/1964 OConnor et al 9633 3,161,521 12/1964 Cohn et al 9633 X 3,231,376 1/1966 Sorkin et al. 9633 3,281,243 10/1966 Sorkin et al. 9633 X NORMAN G. TORCHIN, Primary Examiner.

C. BOWERS, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,342,601 September 19, 1967 James F. Houle et a1.

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 8, line 30, for the claim reference numeral "1" read 6 Signed and sealed this 29th day o f October 1968.

(SEAL) Attest:

EDWARD J. BRENNER Commissioner of Patents Edward M. Fletcher, Jr.

Attesting Officer

Claims (1)

1. A PRESENSITIZED PLANOGRAPHIC PRINTING PLATE COMPRISING A SUPPORT HAVING ON AT LEAST ONE SIDE A HYDROPHILIC SURFACE PROVIDED BY COATING WITH AN AQUEOUS SOLUTION CONSISTING ESESNTIALLY OF A COMPLEX OF AN ALKYL TITANATE AND A SOLUBILIZING AMOUNT OF AT LEAST ONE SOLUBILIZING COMPONENT SELECTED FROM THE CLAS CONSISTING OF FLUOSILICIC ACID, HYDROFLUORIC ACID, FLUOBORIC ACID, HYDROGEN PEROXIDE AND A MIXTURE OF HYDROGEN PEROXIDE AND PHOSPHORIC ACID, AND HAVING THEREON A LIGHT-SENSITIVE COATING.

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