Flexographic printing process with wet on wet capability

Abstract

The invention refers to a flexographic printing process with wet on wet capability based on controlled polymer or polymer segment precipitation that leads to gel formation of ink compounds by controlling the solubility parameter of the ink system. The mechanism to obtain a desirable wet on wet color trapping is the formation and/or presence of a gel in the applied ink film due to a controlled physicochemical mechanism of resin precipitation. This is accomplished either by controlling a change in the Hansen Solubility Parameter of the liquid in the ink through evaporation of some or all of a non-reactive and volatile solvent, or alternatively by use of a polymer comprised of two distinct and separate segments one of which is soluble in the monomer/oligomer mix and the other of which is not. The insoluble segments form a reversible gel that is broken to a liquid by shear in the application process, allowing application of a liquid ink, and is reestablished in the applied ink film in such a manner and with such strength as to allow overprinting in the wet on wet flexographic printing process.

1 . Flexographic printing process comprising the following steps:
 a) Printing on a substrate a first layer of a radiation curable ink suitable for wet on wet flexographic printing, said ink comprising a combination of one or more non-reactive polymers and optionally one or more non reactive solvents with at least one reactive monomer and/or oligomer, pigments and additives, whereby said polymer(s) are only partially soluble in said monomer(s) and/or oligomer(s) or soluble after adding non reactive solvents;   b) Bringing said printed first ink layer to a gel state, said gel ink layer having a strength sufficient to withstand subsequent printing steps;   c) Subsequently printing a second ink layer in a liquid state over at least part of said previously gelled first ink layer, said second ink layer changing into a gel layer upon printing;   d) Printing all sequential ink layers following the steps a) to c) up to the point that all colors are applied on to the substrate.   e) Curing simultaneously all the ink layers at the end of the process using EB or UV radiation.   
     
     
         2 . Flexographic printing process, according to  claim 1 , wherein a solvent is present in the ink before printing and part of all of said solvent is evaporated after printing said first ink layer to result in precipitation of said polymer and formation of said gel ink layer, said gelled ink having a hardness of at least 4 degrees of Shore 00 following ASTM D2240-05 Standard Test Method for Rubber Property. 
     
     
         3 . Flexographic printing process, according to  claim 1  wherein said ink is normally a gel and wherein said initial gel is reversibly broken by the shear forces in the application process, to provide a liquid ink for flexographic printing, that is returned to a gel state upon being applied to the substrate to be printed, said gelified ink having a hardness of at least 4 degrees of Shore 00 following the ASTM D2240-05 Standard Test Method for Rubber Property. 
     
     
         4 . Flexographic printing process, according to  claims 1  and  2 , wherein the Hansen Solubility Parameters of said ink are initially adjusted to provide a single phase liquid ink for flexographic printing and are subsequently modified in the printed ink to change said printed ink layer into a gel. 
     
     
         5 . Flexographic printing process, according to  claims 1  to  4 , wherein the final curing step at the end of the process is based on radiation to cure all ink layers, said radiation being selected from an Ultraviolet light source with emitted light in wavelengths range comprised in between 200 nm and 400 nm and electrons produced by an electron beam apparatus. 
     
     
         6 . Flexographic printing process, according to any  claims 2  to  5 , wherein said gel hardness is at least 4 Shore 00, and preferably at least 10 Shore 00. 
     
     
         7 . A flexographic printing ink, comprising a polymer and a combination of liquids mainly consisting of radiation curable monomers and/or oligomers, diluents, colorants, additives, photoinitiators, and optionally small amounts of non-reactive solvent, the components having the Hansen Solubility Parameters adjusted perfectly to generate a ink with capability to form a gel having the required physical characteristics and that is already in a liquid form in the presence of a small quantity of non reactive solvent or it is brought to a liquid state during the printing process, usually by means of mechanical or thermal action. 
     
     
         8 . A ink according to  claim 7 , wherein the polymer is a non-reactive polymers used in a percentage between 0.1% to about 10% by weight in relation to the total formulation when using a VOC free system. 
     
     
         9 . A ink according to  claim 8 , wherein when the ink has a solvent the said non-reactive polymers are used in a percentage between 1% and 15% by weight in relation to the total formulation. 
     
     
         10 . A ink according to  claim 8  wherein said non reactive polymers are selected from the group that comprises ketonics, alkyl phenolic, acrylic, methacrylic, polyamide, epoxy-polyamide, modified maleic, modified phenolic, ketone formaldehydes, polyurethane, polyesters, polyvinyl butyrals and the like. 
     
     
         11 . A ink according to  claim 7 , wherein said monomers and/or oligomers are used in a percentage between 15% to 80% (w/w) of the total ink composition. 
     
     
         12 . A ink according to  claim 11 , wherein the amount of monomer is within the range of 0% to 80% (w/w) of the total ink composition. 
     
     
         13 . A ink according to  claim 11 , wherein the amount of oligomer is within the range of 0% to 80% (w/w) of the total ink composition. 
     
     
         14 . A ink according to  claim 7 , wherein said oligomers are formed by radiation curable materials used in ink, paint and coating formulations, and comprise one or more of Polyester acrylates, low viscosity Epoxy acrylates, Acrylic acrylates, and Polyurethane acrylates, Amine Acrilate, Epoxidized Soybean Oil Acrylate and the like. 
     
     
         15 . A ink according to  claim 7 , wherein the monomers used in the inks are selected from radiation curable materials used in ink, paint and coating formulations, and comprise one or more acrylic esters, selected from following group: Trimethylolpropane triacrylate (TMPTA), 1,6-Hexanediol diacrylate (HDDA), Tripropylene Glycol Diacrylate (TRPGDA), Ethoxylated (3) Trimethylolpropane Triacrylate (TMP3EOTA), Ethoxylated (6) Trimethylolpropane Triacrylate (TMP6EOTA), Ethoxylated (9) Trimethylolpropane Triacrylate (TMP9EOTA), Propoxylated (6) Trimethylolpropane Triacrylate (TMP6POTA), Propoxylated (3) Glyceryl Triacrylate (G3POTA), Di Trimethylolpropane Triacrylate (DTMPTA), Dipropylene Glycol Diacrylate DPGDA, Ethoxylated (5) Pentaerythritol Tetraacrylate (PPTTA), Propoxylated (2) Neopentyl Glycol Diacrylate (NPG2PODA), Ethoxylated (2) 1,6 Hexanediol Diacrylate (HD2EODA); cycloaliphatic epoxide, one or more polyfunctional cycloaliphatic epoxides and multifunctional vinyl ethers and the like. 
     
     
         16 . A ink according to  claim 7 , wherein said non-reactive solvents are selected from the group that comprises glycol ethers and glycol esters, as well as alcohols, ketones, aromatic and aliphatic hydrocarbons, esters, and any other solvents that can be combined with radiation curable materials in a very low level and promote the control of the Hansen Solubility Parameter in order to produce a liquid medium capable of dissolving the chosen polymer or given segments thereof. 
     
     
         17 . A ink according to  claims 7  and  16 , wherein the non-reactive solvent is used in a percentage between 1% and 15% by weight in relation to the total formulation. 
     
     
         18 . A ink according to  claim 8 , wherein the gellant curable or not curable is present in an amount of from about 1% to about 50% by total weight of the ink if the ink is VOLATILE ORGANIC COMPOUNDS free. 
     
     
         19 . A ink according to  claim 8 , wherein the organic gellant is also applicable in UV cationically curable system wherein the curable functional groups include epoxy, vinyl ether, allyl, styrene and other vinyl benzene derivatives, or oxetane groups, cationically curable monomers and oligomers. 
     
     
         20 . A flexographic printing ink based on phase change, comprising a non curable material and a radiation curable medium comprising monomers and/or oligomers in which said gellant material is partially soluble, said ink being VOLATILE ORGANIC COMPOUNDS free. 
     
     
         21 . An ink, according to  claim 20 , wherein the ink has a hardness of at least 4 Shore 00 of Hardness following the ASTM D2240-05 Standard Test Method for Rubber Property. 
     
     
         22 . An ink, according to  claim 20 , wherein said curable medium contains at least one ethylenically unsaturated compound comprising monomers, oligomers or their combinations in an amount of from about 10% to 70% by total weight of the ink. 
     
     
         23 . An ink according to  claim 18 , wherein the gellant is formed by a block polymer partially insoluble in said radiation curable medium at temperatures of about 15° C. to about 35° C. 
     
     
         24 . An ink, according to  claim 21 , wherein said curable medium composition comprises one or more of polyester acrylates, epoxy acrylates, acrylic acrylates, and polyurethane acrylates, Trimethylolpropane Triacrylate, Tripropyleneglycol Diacrylate, 1,6-Hexanediol Diacrylate, n-Vinyl Pyrrolidone, n-Methyl Pyrrolidone (NMP), isobornyl (meth)acrylate, 2-phenoxyethyl acrylate and the like. 
     
     
         25 . A ink according to  claim 8 , wherein the ink contain diluents, colorants, stabilizer additives, leveling additives, dispersing additives and/or synergist additives. 
     
     
         26 . A flexographic printing machine comprising means for heating or subjecting to shear forces a gelled ink according to any  claims 18  to  24  that is suitable to be changed by agitation or heating application from a gel state into a fluid ink with less than 4000 and preferably less than 2500 cps in order to be applicable by the ink system present in the flexographic printing machine. 
     
     
         27 . A flexographic machine, according to  claim 26 , wherein said machine comprising an ink transfer means including anilox rolls and printing plates, further comprising cleaning means to remove excess ink applied over the anilox rolls in such way as to leave the ink only inside of the anilox roll cells, like a blade applied in gravure or conventional flexographic system, with or without enclosed chamber.