Method of using mask film to form relief images
A mask-forming film has a transparent layer between the imageable layer and the carrier sheet, which transparent layer has a refractive index that is lower (by at least 0.04) than that of the carrier sheet or any immediately adjacent layer between it and the carrier sheet. This lower refractive index layer modifies the path of incident radiation during mask image transfer so as to provide steeper shoulder angles in the relief image solid areas. This mask film is used to form a relief image such as in a flexographic printing plate.
1. A method of making a relief image, the method comprising:
A) forming a mask image by forming exposed and non-exposed regions in an imaged film, which film prior to imaging, comprises:
a transparent carrier sheet having thereon at least one non-silver halide thermally sensitive imageable layer and a UV-absorbing colorant disposed in the imageable layer,
an infrared radiation absorbing compound dispersed in a polymeric binder, and
a transparent layer disposed between the carrier sheet and the imageable layer, which transparent layer has a thickness of from about 0.4 μm to about 10 μm and a refractive index that is at least 0.08 lower than the refractive index of the carrier sheet or lower than the refractive index of any optional immediately adjacent layer between the carrier sheet and the transparent layer,
B) transferring the mask image in the imaged film to a radiation-sensitive element while there is complete optical contact between the mask image and the radiation-sensitive element,
C) exposing the radiation-sensitive element to curing radiation through the mask image while in complete optical contact with the radiation-sensitive element, to form an imaged element, wherein the mask image is opaque to the curing radiation, and
D) developing the imaged element to form a relief image having an average shoulder angle of at least 50°.
2. The method of claim 1 wherein the imaged film is removed from complete optical contact with the exposed radiation-sensitive element between steps C and D.
3. The method of claim 1 wherein the carrier sheet and transparent layer are removed from complete optical contact with the imaged film between steps C and D.
4. The method of claim 1 wherein the radiation-sensitive element is a UV-sensitive element that is exposed to UV radiation through the mask image to form an imaged element, wherein the mask image is opaque to UV radiation.
5. The method of claim 1 wherein the radiation-sensitive element is a flexographic printing plate precursor and the imaged element is a flexographic printing plate.
6. The method of claim 1 wherein the transferring step B is carried out by lamination.
7. The method of claim 1 wherein the transferring step B is carried out by applying a radiation-sensitive layer while in liquid form, to the imaged film, and then forming the radiation-sensitive element.
8. The method of claim 1 wherein the transparent layer comprises a fluoroelastomer and optionally an adhesion promoter.
9. The method of claim 1 wherein the film further comprises an intermediate layer between the imageable layer and the transparent layer, which intermediate layer has a thickness of from about 0.1 to about 10 μm.
10. The method of claim 9 wherein the intermediate layer comprises a poly(vinyl alcohol), poly(vinyl butyral), a cellulosic polymer, or hydrolyzed styrene maleic anhydride polymer.
11. The method of claim 1 wherein the film further comprises a barrier layer disposed between the imageable layer and the carrier sheet, the barrier layer comprising a heat-combustible polymer binder and an infrared radiation absorbing compound, which heat-combustible polymer binder is a poly(cyanoacrylate) or nitrocellulose.
12. The method of claim 1 wherein the film comprises a carrier sheet having thereon, in order:
a) a transparent layer comprising a fluoroelastomer, the transparent layer optionally including an adhesion promoter, and the transparent layer having a refractive index of at least 0.08 lower than the refractive index of the carrier sheet,
b) an intermediate layer comprising a poly(vinyl alcohol), cellulosic polymer, poly(vinyl butyral), hydrolyzed styrene maleic anhydride polymer, or a thermally resistant polymer, and having a thickness of from about 0.1 to about 10 μm, the intermediate layer optionally including a crosslinking agent, coating aid, surfactant, or release-enhancing agent,
c) a barrier layer comprising a poly(cyanoacrylate) or nitrocellulose, and an infrared radiation absorbing dye, and optionally metal oxide particles or crosslinking agents,
d) a imageable layer comprising an infrared radiation absorbing dye and a UV-absorbing colorant dispersed in a polyurethane, poly(vinyl butyral), acrylamide polymer, nitrocellulose, or polyacetal binder, and
e) an overcoat comprising a methacrylic acid copolymer and fluoropolymer particles.
13. The method of claim 12 wherein the barrier layer includes an infrared radiation absorbing compound.
14. The method of claim 12 wherein the infrared radiation absorbing compound in the imageable layer is an IR dye.
15. The method of claim 12 wherein the carrier sheet is a polyester, fluorine polyester polymer, polyethylene polypropylene, polybutadiene, polycarbonate, polyacrylate, polyvinyl chloride or copolymer thereof, or hydrolyzed or non-hydrolyzed cellulose acetate, and is from about 20 to about 200 μm thick, and optionally includes an adhesion promoter.
16. The method of claim 12 wherein the intermediate layer further comprises an adhesion promoter.