Heat Fusible Poly(vinyl Acetate) Dispersions

Printing processes require that printing inks be both fluid and capable of subsequent ready conversion to a dry, smudge-resistant film once in place on paper or another substrate. One common type of printing ink consists primarily of a pigment and a binder which are suspended or dissolved in a volatile diluent. The binder serves the purpose of adhering the pigment to the printed substrate. The diluent must subsequently be removed by allowing it to evaporate either at room temperature, or for higher printing speeds, by heating. Large amounts of heat are needed to vaporize the ink diluent, which requires that substantial quantities of fuel be consumed. Since large volumes of air must be drawn over the drying ink film to remove the diluent vapor, frequently much heat is wasted. Further, the evaporation of the ink diluent into the open atmosphere can be a source of pollution. In many instances, it is necessary or desirable to burn off or otherwise remove the solvent vapor from the drying air before discharging it to the atmosphere. Additional fuel is consumed and special equipment must be installed for this purpose.

A second common type of printing ink consists of mixtures containing oleoresinous varnishes and/or drying oils which set by air oxidation. These inks set slowly so it is necessary to take precautions to avoid set-off (transfer) of ink between printed sheets.

Inks may also be formulated which contain drying oils or the like together with some binder and volatile diluent. In the usual printing operation, heat is applied to such an ink immediately after it is printed onto the substrate. At this stage, the diluent is driven off, reducing or minimizing problems of ink set-off. However, the ink is not fully set and rub resistance is inadequate. A subsequent drying stage is necessary in which the drying oil or the like hardens, as by oxidation and/or polymerization.

While it is necessary that a printing ink set rapidly and conveniently after printing, it is equally necessary that it not set or dry on the press. Inks containing volatile components may thicken on the press as the solvent evaporates, making it difficult or impossible to control the printing process. Air oxidizing varnishes, drying oils, and the like, may thicken or gum on the press on exposure to the atmosphere. Such inks are especially inclined to "skin over" when the press is shut down during a run. An ink which is not subject to such changes while on the printing press is said to "stay open".

Ink technologists have sought to achieve inks which set rapidly with a low level of energy input to initate setting, which do not release polluting materials to the atmosphere, and which stay open on the press, while at the same time meeting the physical and mechanical requirements of the printing process. Much attention has been directed to highly chemically reactive formulations. After printing, such inks are set by polymerization and/or crosslinking which is initiated by heat or radiation. A typical chemically reactive system contains polyfunctional acrylate esters (often in combination with other unsaturated materials), one or more ultraviolet photoinitiators, pigment and a variety of secondary components to control ink physical properties. See U.S. Pat. No. 3,804,640 and U.S. Pat. No. 3,881,942, both to Buckwalter. In normal use, such inks are printed on presses equipped with ultraviolet lamps which expose the film on the paper or other substrate immediately after printing. The inks set rapidly and with essentially no emission of polluting materials. Inks based on such highly reactive materials, however, have limited storage stability. The acrylates and other reactive materials in common use have been found to be chemically incompatible with some pigments which are desirable in printing inks. They present toxicity hazards, or are dangerous eye irritants, and are frequently skin sensitizers. When the inks are designed to be set by an ultraviolet light initiated reaction, the ultraviolet radiation itself and the high voltages necessary to power the ultraviolet lamps are additional health and safety hazards.

U.S. Pat. No. 3,024,213 to Ludlow discloses a heat drying ink vehicle based on poly(vinyl chloride) plastisols, more particularly finely divided poly(vinyl chloride) polymer dispersed in liquid plasticizers, all of which have low viscosity and solubility parameters closely matched to poly(vinyl chloride), and containing a compatible thermoplastic resinous binder to increase ink cohesion and tack. Further improvements in inks based on poly(vinyl chloride) and vinyl chloride copolymer plastisols are disclosed in U.S. Pat. No. 3,760,724 to Budzinski. These inks are set by applying heat which causes the liquid plasticizers and poly(vinyl chloride) or vinyl chloride copolymer to co-dissolve. Thus, such poly(vinyl chloride) plastisol-based inks are readily heat-settable without significant release of volatile materials to the atmosphere. These inks, however, do not have good press running characteristics. Undesirable build-up of material on the blanket of lithographic offset presses occurs when poly(vinyl chloride) plastisol inks are used. Further, both Ludlow and Budzinski require the use of at least one plasticizer in such poly(vinyl chloride) ptastisol inks. Such plasticizers may, as Ludlow discloses, damage ordinary natural rubber rolls on printing presses. It is thus necessary that ink rolls of polyurethane rubber or other plasticizer resistant compositions be used.

The present invention provides nonvolatile, heat-set pastes. It has been discovered that pastes of particulate, glassy thermoplastic poly(vinyl acetate) type polymers dispersed in a soft resinous phase can be prepared which heat-set by the fusion of the poly(vinyl acetate) binder. When pigmented or otherwise colored, these formulations have utility as printing inks. When uncolored, they can be used as clear over-print coatings. In addition, these fusible pastes have utility as specialty coatings and adhesives and can be fabricated into self-supporting shapes.

According to this invention, nonvolatile, heat-set vehicles for printing inks can be made by dispersing particulate, poly(vinyl acetate) type polymer binders in a soft resinous phase. More particularly, the printing ink vehicles of this invention contain particulate, poly(vinyl acetate) type polymers dispersed in a tackifying, cohesion-increasing material which may be a resin or mixture of resins whose softening point is either at or below room temperature, or has been lowered by dilution with an oil.

The binder materials used in this invention comprise particulate, glassy thermoplastic poly(vinyl acetate) type polymers with a molecular weight above about 50,000.

While the particle size of the binder is not critical, for lithographic or letterpress inks particles about 0.05 to about 2.0 microns in cross-section are preferred, with particles of about 0.1 to 0.5 micron in cross-section being most preferable. Larger particles may give poor uniformity and press performance. In screen printing, much heavier ink films are printed, so much larger particle sizes may be tolerated. The limit in particle size for inks used in screen printing is fixed by the mesh size of the printing screen and the thickness of the ink film desired. For use with coarse mesh screens and heavy ink films, particle sizes as high as about 50 microns can be used.

The poly(vinyl acetate) type polymers used in the pastes of this invention are based on vinyl acetate, either alone or in combination with one or more comonomers, with at least 60% of the monomers being vinyl acetate. Comonomers with the vinyl acetate monomer may include one or more addition polymerizable compounds such as other vinyl esters, including vinyl formate, vinyl propionate, vinyl stearate and vinyl benzoate; the saturated esters of ethylenically unsaturated carboxylic acids, including acrylate-type monomers, which class is here taken to include acrylates, methacrylates, acrylonitrile and methacrylonitrile, including methyl methacrylate, ethyl methacrylate, butyl methacrylate, isobutyl methacrylate, methyl acrylate, ethyl acrylate, butyl acrylate, and the like; and vinyl and vinylidene halides, including vinyl chloride and vinylidene chloride.

Other comonomers may be included if they do not interfere with the operation of the invention. It may be desirable to include certain additional polymer components to facilitate the preparation of particles of appropriate size. Such monomers may include ethylenically unsaturated carboxylic acids which may be mono- or polycarboxylic acids such as acrylic acid, methacrylic acid, itaconic acid, fumaric acid, maleic acid and anhydride, citraconic acid and anhydride and other such acids.

Thermoplastic poly(vinyl acetate) type polymer particles of a size useful in this invention can be conveniently obtained by evaporating the diluent from emulsion or dispersion polymerized systems, as described in Dispersion Polymerization In Organic Media, edited by K. E. J. Barrett (John Wiley & Sons, N.Y., 1975). For materials with glass transition temperatures below about 50.degree. C., it may be desirable to add some portion of the soft resin phase of the paste to the polymer dispersion before evaporating the polymerization diluent. Alternatively, particles may be prepared by grinding or milling the polymers, by controlled precipitation from solution. In many cases, a particle size classification process may be necessary to obtain the optimum size of the material.

The continuous soft resinous phase of the paste is comprised of a tackifying, cohesion-increasing material which may be a resin or mixture of resins with a softening point at or below room temperature, or which has been diluted with an oil to suitably lower the softening point. The term "resin" is here restricted to amorphous organic solids, semi-solids and viscous liquids with room temperature viscosities above about 2000 centipoise. Such resins are commonly polymeric, but number average molecular weight must be under about 5000. Typical classes of resins which may be used are hydrocarbon resins, including naturally occurring coal tar and other fossil resins, synthetic aliphatic and aromatic hydrocarbon resins prepared from petroleum- or coal-derived feedstocks, and resins based on pure hydrocarbon monomers and combination of monomers, such as styrene and alkyl-substituted styrenes, indene and dicyclopentadiene; terpene resins; rosin and rosin esters such as hydrogenated methyl ester of rosin and phenolic modified pentaerythritol ester of rosin; ketone formaldehyde thermoplastic resins; condensed ketone thermoplastic resins; and resins based on acrylate esters and alkyl-substituted acrylate esters; and transesterification products of alcohols with dimethyl terephthalate process residues.

Diluent oils may be aliphatic, aromatic or naphthenic in character, and may contain polar functionality, but must be compatible with and dissolve in the resin system. Typically, they have solubility parameters between about 7.0 and 8.5. They must be substantially nonvolatile at the temperature at which the ink is set, typically 150.degree. C. or higher. It is important to choose diluents which do not rapidly solvate the particulate thermoplastic phase at room temperature. This may be accomplished by avoiding those diluent oils which closely match the solubility parameter of the particulate thermoplastic. Typically, the solubility parameter midpoint for poly(vinyl acetate) with respect to weakly hydrogen-bonded solvents such as hydrocarbon oils or chlorinated hydrocarbons is at or about 9.4. Poly(vinyl acetate) is insoluble in strongly hydrogen-bonding solvents. Liquid plasticizers of the sort typically used to prepare poly(vinyl chloride) plastisols (commonly referred to as "primary plasticizers") such as those disclosed by Ludlow in U.S. Pat. No. 3,024,213, have solubility parameters between about 8.5 and 11 and are not used in the pastes of this invention.

There is no such restriction on the solubility parameter of the resin itself. The rate of solvation is slow enough that even inks containing resins which closely match the solubility parameter of the particulate thermoplastic may be prepared and run on a press. In extreme cases, solvation may proceed at a rate that would limit certain combinations of resin and thermoplastic polymers to applications where inks are prepared as needed, and are not stored longer than a day or so. Printing inks are commonly prepared as needed in larger printing plants. Under such circumstances, the chemical composition of the particulate thermoplastic polymers and the resin may be essentially identical, the materials differing only in physical form and molecular weight.

The poly(vinyl acetate) polymer and soft resin of this invention should be used in proportions such that the ratio of polymer to resin will preferably be between about 1:6 and about 1:1. Also, the ratio of soft resin to diluent oil may range from these cases in which the formulation is oil-free (i.e., where the soft resin is naturally thin enough to achieve the desired rheology in the final fusible paste formulation) to about a 1:15 ratio of resin to oil.

In addition to the above-described essential ingredients, when the pastes are used as printing inks surface active agents, pigment dispersing aids, waxes, slip aids, tack modifiers and the like may be included.

The primary mechanism by which the pastes of this invention set appears to be the fusion of the particulate thermoplastic poly(vinyl acetate) binder to form a continuous film on heating the plate, in which the soft resinous phase is largely dissolved and/or entrapped. In some circumstances, it may be desirable for the paste film to harden further. This can be achieved by choosing for the soft resinous phase, either in whole or part, materials capable of undergoing polymerization or cross-linking during the thermal fusion of the particulate thermoplastic material or subsequent to it. In such cases, it may be appropriate to include suitable catalysts, driers, accelerators, or other additives which are necessary or desirable to initiate or promote the desired polymerization or cross-linking reaction. Such subsequent hardening of the thermally set paste film may be achieved by including one or more unsaturated alkyd resins as part of the resinous phase, such as alkyds based on phthalic, isophthalic, adipic, azelaic, sebacic, terephthalic, hydrophthalic, maleic, fumaric, and benzoic acids and anhydrides modified with unsaturated fatty acids; rosin modified alkyds; phenolic modified alkyds; epoxy alkyds; polyamide alkyds; isocyanate alkyds; and styrene or substituted styrene alkyds, such as styrene and acrylonitrile modified alkyds. The unsaturated isophthalic alkyds are preferred.

Similarly, drying oils or semi-drying oils such as linseed oil, tung oil, oiticica oil, dehydrated castor oil, soya oil, safflower oil, fish oil, tall oil, or the like may be used as hardening agents in the resinous phase. These oils may be used in the natural state or may be bodied to increase viscosity by heating or other partial polymerization.

When such oxidation drying materials are included in the paste, suitable driers or catalysts which promote oxidative cross-linking and polymerization may be included, typically salts or complexes of metals capable of existing in more than one valence state, such as vanadium oxyacetylacetonate, vanadium oxysulfate, vanadium oxy-1,1,1-trifluoroacetylacetone, vanadium oxy-1-phenylacetylacetonate, ferric acetylacetonate-benzoin, manganese octoate, lead naphthenate, cobaltic acetylacetonate, titanyl acetylacetonate, cobaltous naphthenate, cobaltous 2-ethylhexanoate, cobaltous stearate, cobaltic stearate, cobaltous acetylacetonate, manganous stearate, manganic stearate, manganous acetylacetonate, manganic acetylacetonate, manganese naphthenate, zirconium acetylacetonate, vanadyl naphthenate, ferrous sulfate, ferrous pyrophosphate, ferrous sulfide, the ferrous complex of ethylenedinitrolotetraacetic acid, ferrous o-phenanthroline, ferrous ferrocyanide, ferrous acetylacetonate and the corresponding nickel, copper, mercury and chromium compounds.

The use of a colorant is not critical to the invention. The fusible paste dispersions can be used unpigmented as a clear overprint varnish. When a colorant is desired, it may be a pigment or dye. The colorant normally may be most conveniently dispersed in the continuous soft resinous phase of the ink. However, it may be desirable in some circumstances to include the colorant within the particulate thermoplastic material.

When used as printing inks, the pastes of this invention may be printed by any of the printing processes which require a paste type ink. Common printing processes in which paste inks are used include letterpress, lithography, and screen printing. Printing may be sheet-fed or web-fed, and may be on paper, board, fabric, metal, glass, plastic, wood, leather, rubber, or other substrates. Variations in ink rheology and tack properties are required for optimum performance for any chosen printing process and substrate. Inks based on the pastes of this invention can be suitably formulated to meet these specific requirements by appropriate choice and relative concentration of materials used in the resinous phase. This will be readily apparent to those skilled in printing ink formulation.

In addition to printing, pastes may be applied to substrates by roll-coating, doctoring, screening, dipping, or the like, especially for the preparation of speciality coatings or for use as heat-set thermoplastic adhesives. Self-supporting shapes may be prepared by cavity molding, dip molding, rotational molding, slush molding or the like. Variations in place rheology and tack properties are required for optimum performance for any chosen fabrication process. The pastes can be formulated to meet these requirements by appropriate choice and relative concentration of materials used in the resinous phase. This will be readily apparent to those skilled in the art of materials fabrication.

After printing, coating, or forming, the pastes of this invention may be set by heating to at least about 75.degree. C. and preferably to at least about 150.degree. C. to fuse the particulate, thermoplastic polymer. This may be done in a conventional forced-air drying oven, in common use in the printing industry. However, since no air flow is necessary to remove volatile material, it is frequently advantageous to heat the paste film by irradiation, typically in the infrared spectral region. At 150.degree. C., the paste may be fused by heating for from about 0.01 second to about 2 minutes, longer times being required for those pastes which do not contain pigments than for those which do contain pigment. Also, the duration of the heating may vary depending upon the thickness of the paste coating, substrate, or purpose for which the paste is used (e.g., adhesive, molding material, etc.).

Finely divided poly(vinyl acetate) is prepared as follows. A solution is prepared which consists of 223 parts n-heptane, 148 parts n-hexane, 59.3 parts vinyl acetate, (VAc) 0.8 grams azobisisobutyronitrile (AIBN) and 18.4 parts of poly(lauryl methacrylate-co-glycidyl methacrylate) in which 80% of the glycidyl groups had been reacted with methacrylic acid (mole ratio of LMA to GMA=19). The solution is heated to 85.degree. C. for 45 minutes and then a solution of 0.8 part AIBN in 337.8 parts VAc is added over 3 hours while the mixture is held at 80.degree.-85.degree. C. The dispersion that results has a solid content of 52.4%, and a particle size of 0.47 micron. The PVAc in the dispersion has a molecular weight of 225,000 and a glass transition temperature of 37.degree. C.

One hundred grams of this dispersion is mixed with 52.4 grams of the hydrogenated methyl ester of rosin (Hercolyn D) and the volatile hydrocarbons are removed by distillation under reduced pressure. The resulting dispersion of PVAc in resin is mixed with a dispersion of carbon black in the hydrogenated methyl ester of rosin plus ink varnish.

The final composition is:

The ink is printed on paper on a lithographic offset press. The printed sheets are passed at about 45 meters/minute under a 100 watt/cm infrared lamp with a peak output of about 1 micron (model 5193-5 line heater manufactured by Research Incorporated, Minneapolis, Minnesota, operated at 230 volts) at a distance of 2.5 cm from the lamp reflector edge which results in the rapid setting of the ink film. After setting, the ink is dry and resistant to smudging or smearing.

An ink is prepared as in Example 1 with the following formulation:

The ink is printed and set as in Example 1. After setting, the ink is dry and resistant to smudging or smearing.

An ink is prepared as in Example 1. The final formulation is:

The ink is printed and set as in Example 1. After setting, the ink is dry and resistant to smudging and smearing.

An ink is prepared as in Example 1 with the following formulation:

The ink is printed and set as in Example 1. After setting, the ink is dry and resistant to smudging and smearing.

An ink was prepared as in Example 1 with the following formulation:

The ink is printed and set as in Example 1. After setting, the ink is dry and resistant to smearing and smudging.

An ink is prepared as in Example 1 having the following formulation:

The ink is printed and set as in Example 1. After setting, the ink is dry and resistant to smudging and smearing.

An ink is prepared as in Example 1 with the following formulation:

The ink is printed and set as described in Example 1. After setting, the ink is dry and resistant to smudging and smearing.

An ink is prepared as in Example 1 having the following formulation:

The ink is screen printed onto glass and set by oven heating for 2 minutes at 135.degree. C. The resulting film is dry and scratch resistant.

The ink of Example 1 is screen printed onto cotton fabric. The ink is set by IR radiation as in Example 1. The cotton fabric is laundered in an automatic washing machine with a detergent with no loss of image.

A paste is prepared as in Example 1 with the following formulation:

The paste is printed as an overprint varnish on a lithographic offset press. The printed sheets are heated to 200.degree. C. for 2 minutes in a forced air oven. After setting, the coating is dry and smudge resistant.

A paste is prepared as in Example 1 with the following formulation:

The paste is roll-coated onto paper and heated by passing under an IR lamp, which results in rapid setting of the paste film. After setting, the coating is dry and resistant to smearing.

A paste is prepared as in Example 1 with the following formulation:

The paste is blade-coated onto a Bonderite 100 treated cold rolled steel panel. The coated panel is heated for five minutes in a forced-air oven at 175.degree. C. On cooling, the resulting coating is dry and rub-resistant.

The paste of Example 12 is doctored onto two pieces of bleached poplin. The coated fabrics are pressed together with an iron at 120.degree. C. for under one minute. On cooling, a strong fabric-to-fabric bond results.

The paste of Example 12 is pressed into a metal mold which had previously been coated with a mold release. The filled mold is placed in an oven at 200.degree. C. for 10 minutes. On cooling, the resulting molded object is removed from the mold and has accurately replicated the design of the mold cavity.

An ink is prepared as in Example 1 with the following formulation:

The ink is printed on a lithographic press and on a Van der Cook proofing press and prints are set by passing under an IR lamp as described in Example 1. After setting, the ink is dry and smudge-resistant.

A fusible paste ink is prepared with the following composition:

The ink is prepared by (1) dissolving the phenolicmodified pentaerythritol ester of rosin in the hydrogenated methyl ester of rosin; (2) dispersing the polystyrene in about two-thirds of the above solution on a three-roll mill; (3) dispersing the carbon black in the remaining one-third of the resin solution on a three-roll mill; and (4) blending the two dispersions together with a light mixing pass on a three-roll mill.

The ink is printed on a Little Joe press. Prints are set by placing in an oven at 150.degree. C. for 90 seconds, in an oven at 200.degree. C. for 30 seconds and on a curved hot plate at 288.degree. C. for 3 seconds.

A paste is prepared as in Example 16 with the omission of the carbon black. The paste is used as an overprint varnish by printing on a sheet-fed lithographic press. The resulting clear coating is set by placing in an oven at 150.degree. C. for 90 seconds.

Finely divided vinyl acetate-ethyl acetate copolymer is prepared exactly as described in Example 1 except that in place of the vinyl acetate there is used an 85:15 mixture of vinyl acetate and ethyl acrylate. The resulting dispersion contains 51.4% solids, and has a particle size of 0.45 micron. The polymer has a reduced specific viscosity of 1.32 determined in chloroform solution at 25.degree. C., which indicates a molecular weight above 100,000, and has a second order transition temperature of 19.degree. C.

To 100 parts of the above dispersion are added 51.4 parts of the hydrogenated methyl ester of rosin. After evaporation of volatile diluents, the resulting dispersion is mixed with a dispersion of carbon black in the hydrogenated methyl ester of rosin and ink varnish to give a composition identical to that of Example 1 except for substitution of the polymer component.

The ink is printed and set as in Example 1. After setting, the ink is dry and resistant to smudging or smearing.

Finely divided vinyl acetate-ethyl acetate copolymer is prepared exactly as described in Example 1 except that in place of the 59.3 parts vinyl acetate there is used a 70:30 mixture of vinyl acetate and ethyl acrylate and the post-heating time at 85.degree. C. is three hours. The resulting dispersion contains 50.8% solids and has a particle size of 0.44 micron. The polymer has a reduced specific viscosity of 1.24 which corresponds to a molecular weight above 100,000 and has a second order transition temperature of 6.degree. C.

To 100 parts of the above dispersion is added 300 parts of the hydrogenated methyl ester of rosin in which is dispersed 50 parts of carbon black. After evaporation of volatile diluents, the resulting ink is printed and set as in Example 1. After setting, the ink is dry and resistant to smudging and smearing.

Finely divided vinyl acetate-methyl methacrylate copolymer is prepared exactly as described in Example 1 except that in place of the vinyl acetate there is used a 75:25 mixture of vinyl acetate and methyl methacrylate and the heating temperature is maintained in the range of 75.degree. to 85.degree. C. After the postheating period, 60 parts of unreacted vinyl acetate is removed by evaporation with about 180 parts of the hydrocarbon solvent so that the ratio of the monomers in the polymer is about 68:32. The resulting dispersion has a solids content of 52.4% and the particle size of 0.41 micron. The reduced specific viscosity of the polymer is 1.54 which corresponds to a molecular weight of approximately 217,000. The polymer has a glass transition temperature of 38.degree. C.

To 100 parts of the above dispersion is added 200 parts of the hydrogenated methyl ester of rosin in which is dispersed 25 parts of carbon black. After evaporation of the volatile diluents, the resulting ink is printed and set as in Example 1. After setting, the ink is dry and resistant to smudging and smearing.

Finely divided vinyl acetate-ethyl acrylate copolymer is prepared exactly as described in Example 1 except that in place of vinyl acetate there is used a 60:40 mixture of vinyl acetate and ethyl acrylate. The postheating time is extended to 2-1/2 hours at 80.degree. C. followed by distillation to remove 66.5 parts of a mixture of unreacted monomer and solvent. The resulting dispersion contains 53.6% solids and has a particle size of 0.43 micron. The polymer has a reduced specific viscosity of 1.24 which indicates a molecular weight above 100,000 and has a glass transition temperature of 0.degree. C.

100 parts of this dispersion is mixed with 52.4 parts of the hydrogenated methyl ester of rosin and the volatile hydrocarbons removed by distillation under reduced pressure. The resulting dispersion is mixed with a dispersion of carbon black in the hydrogenated methyl ester of rosin plus ink varnish.

The final composition is:

The ink is printed on paper on a lithographic offset press as described in Example 1. After setting, the ink is dry and resistant to smudging or smearing.