Abstract:
A method for producing a multicolor heat transfer decal comprises printing specially formulated thermoplastic inks successively onto a pre-release coated carrier sheet. The inks give optimum print quality using plastic printing plates and a rotary letterpress or flexographic printing process.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to multicolor heat transfer decals particularly as used to decorate plastic articles. 
     In order to produce heat transfers that give high quality images and show clear and brilliant colors, a screen process (e.g. silk screen) was originally used. The screen process enabled a printer to formulate inks that were compatible with the various plastics to be decorated since there were virtually no restrictions on the solvents that could be used to dissolve the resins that were needed to make the inks. Screen process inks are conventionally made using slow drying solvents such as white spirit, high boiling aromatics, ketones and glycol ethers. 
     To satisfy a larger volume market for heat transfers that could not be met by the comparatively slow silk screen process, various methods were devised using a gravure printing process. Gravure printing has the advantage that almost any solvent system can be used due to the solvent impervious etched metal rollers that are used to carry out gravure printing. For example, very fast evaporating solvents such as toluene, methylethyl-ketone and N-propyl acetate are used. 
     Many of the gravure heat transfer systems employ an adhesive layer or a wax to bond conventional printing inks down onto the final receptor surface or carrier film. U.S. Pat. Nos. 2,862,332 and 2,989,413 disclose examples of such a wax bonding system. The wax acts as a release system and as a bonding agent. 
     Other systems have a release layer similar to that used in the foil stamping industry. Such release layers are commonly mixtures of waxes and a high molecular weight acrylic. The inks associated with such systems are based on resins that are only soluble in very active solvents. 
     There are serious disadvantages in using either silk screen process or gravure process. The screen process is very slow and requires new screens each time a particular design is run since the screens are very susceptible to damage. With such processes, very fine color process printing beyond 60-80 lines per inch is difficult thereby seriously limiting the quality of work that can be produced. The nature of the processes preclude close register of prined material. 
     Since heat transfers are generally required in a roll or web format, screen printing machines having the ability to print on web form have been developed. However, they are very costly and require highly trained operating personnel. 
     The gravure process is ideal for producing high quality heat transfers. Thermoplastic inks printed over release coated film give the finest print quality. However, this too is a very costly process. The printing equipment is very expensive and the costs of preparing gravure cylinders is extremely high since highly skilled personnel are necessary for their manufacture. A gravure processing plant has a high capital cost further escalating the cost. 
     Alternative processes have been proposed. Various early heat transfers were printed using rubber plates on conventional flexographic equipment. However, the rubber plates give very poor quality reproduction and the inks used are not thermoplastic. Also, a secondary adhesive coating must be applied to give adhesion to the articles being decorated. The colors are very weak and tend to be transparent and have very poor resistance to abrasion. Consequently, this early process was discontinued. 
     In recent times the invention of plates made from photopolymer resins such as Dupont Cyrel® resin and or BASF Nyloprint® resin plates has resulted in a high quality plate that can give resolution and definition to letterpress printing much closer to that achieved by the gravure process. The solvent system that can be used with these &#34;plastic&#34; plates, however, is still similar to that used with the older rubber printing plates although the plastic plates will tolerate a higher percentage of more active solvent in the inks. Commonly used solvents therefore consist mainly of ethanol or isopropanol with additions in the order of 10% of propyl acetate, nitropropane and glycol ethers. 
     SUMMARY OF THE INVENTION 
     The present invention comprises the discovery of inks and their use to provide required heat transfer characteristics when printed on heat activated release coated material such as wax or resin coated polyester film or polymer coated paper e.g. polypropylene on kraft paper. The inks can be printed using the &#34;plastic&#34; printing plates and result in a method of producing heat transfers that eliminate the disadvantages of the prior art gravure or screen processes. The resultant product is a very high quality heat transfer product having a competitive cost. 
     The machines used to print the heat transfers are conventional rotary letterpress or flexographic presses which are commonly available and which have a much lower capital cost than either web fed gravure presses of web screen presses. Such presses can also be operated by their normal operators. The plastic printing plates also can be readily made using conventional processing equipment and normal personnel. The cost of such plates is only a fraction of the cost of gravure cylinders. Such plates also have the added advantage of being readily stored for use with subsequent orders. 
     In order to utilize the ease of print that the flexographic process offers and yet obtain the resistance to solvent and adhesion of more difficult to decorate surfaces, it is possible to utilize a combination gravure and plastic plate, flexographic process. For example, a gravure unit may be used for the first step to deposit a release coating. If the substrate or carrier film is already release coated (as is the normal case) another protective coating gives added abrasion or solvent resistance. Upon transference this first applied coating becomes the uppermost layer. Also, use of a gravure unit after heat transfer inks are laid down over the release coating allows the application of tie coats to give improved adhesion to difficult surfaces. In both of these gravure steps the coating would be continuous over the whole surface and not in register with the ink design elements. This would permit utilization of simple coating rollers that could be used with all and any designs. 
     Thus it is an object of the present invention to provide an improved method of printing heat transfers by means of inexpensive flexographic processes using inexpensive flexographic plates. 
     A further object of the present invention is to provide improved heat transfer ink formulations which may be used for various printing processes for the manufacture of heat transfer decals. 
     Still another object of the present invention is to provide a method of manufacture of a heat transfer from a carrier film particularly to a plastic surface wherein the transfer has high abrasion resistance and durability. 
     Another object of the present invention is to provide an inexpensive method for manufacture of heat transfers using currently available printing equipment in combination with new and specific formulations of heat transfer ink. 
     These and other objects, advantages and features of the invention will be set forth in the detailed description which follows. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The drawing comprises a single FIGURE illustrating schematically the various steps in the method of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Generally, with the present invention, a carrier film or substrate is provided for the heat transfer. Heat transfer inks are then printed on the carrier film by means of known flexographic or letter press methods. Prior to printing of the heat transfer ink on the film, a coating may be applied to the film to improve abrasion resistance of the ultimate heat transfer. The coating thus is of a type which would be released from the film by the heat transfer operation. 
     Subsequent to printing of formulated inks upon the film, it is also an optional step of the invention to apply a coating of tying material or adhesion promotion material. The application of the pre and post ink materials may be by gravure methods or by any other suitable printing method. The inks, however, are generally applied by flexographic or letterpress methods. The sequence of operations is illustrated in the drawing. 
     Preferably the plates used for the letter press or flexographic printing of the transfer inks are plastic plates which provide for high resolution printing. Typical plastic plates include those made from photopolymer resins such as the tradename products Cyrel by Dupont and Nyloprint by BASF. 
     Preferably the general composition of the printing inks is as follows: 
     Organic pigment or dye: 1-30% by weight 
     Alcohol solution acrylic resin: 3-12% by weight 
     Cellulose acetate butyrate: 0.5-5% by weight 
     Dispersion agents: 0.0-2% by weight 
     Ethanol (solvent): 25-50% by weight 
     Butyl cellosolve (solvent): 5-12% by weight 
     *Pentoxone (solvent): 0.0-3% by weight 
     2-Nitro propane (solvent): 10-25% by weight 
     N-propyl acetate (solvent): 5-15% by weight 
    
     The following examples give various examples of combinations of carriers, printing inks, tie down materials and release materials which may be used in the printing process. 
     EXAMPLE 1 
     A roll of polyester film 0.001&#34; in thickness was precoated with a release coating as follows: 
     *Elvacite 2041--15% 
     Toluene--40% 
     Methyl ethyl ketone--35% 
    
     This coating is applied by gravure or metering bar process to give a dry coating weight of 0.2 grams per square meter. 
     Inks can be printed over this layer. The inks typically have the following formulations: 
     
         ______________________________________                        Percentage(1) BLUE INK     Supplier    by Weight______________________________________Phthalocyanene blue pigment            Dupont      7.70 ± 3.00Orasol solvent soluble dye            Ciba Geigy  4.20 ± 2.00Titanium dioxide Dupont      1.40 ± 1.00.sup.1 Elvacite 2013            Dupont      9.23 ± 3.00Cellulose acetate butyrate            Eastman Kodak                        2.28 ± 1.50.sup.2 Span 80   I.C.I       .04 ± .02.sup.3 G. 3300   I.C.I.      .04 ± .02Ethanol                      34.17 ± 5.00Pentoxone        Shell Chemical                        .71 ± 1.50Butyl cellosolve             8.02 ± 2.50.sup.4 Paraplex G. 60            Rohm and Haas                        3.49 ± 1.752. Nitro propane             19.42 ± 5.00N-propyl acetate             9.30 ± 5.00                        100.00%______________________________________ .sup.1 Elvacite 2013  a low molecular weight methyl/n butyl methacrylate copolymer resin .sup.2 Span 80  sorbitan mono oleate, wetting agent .sup.3 G. 3300  ionic surfactant  ackyl aryl sulphonate, dispersant .sup.4 Paraplex G. 60  a polymeric plasticizer 
    
     
         ______________________________________                        Percentage(2) YELLOW INK   Supplier    by Weight______________________________________Dalamar yellow pigment            Dupont      4.99 ± 5.00Orasol yellow 2 gln. dye            Ciba Geigy  3.00 ± 2.00Titanium dioxide Dupont      1.46 ± 1.00Elvacite 2013    Dupont      10.62 ± 3.00Cellulose acetate butyrate   2.64 ± 1.50Span 80 (sorbitan oclate)            I.C.I.      .18 ± .10G. 3300 (Alkyl arylsulphonate)      I.C.I.      .18 ± .10Ethanol                      34.46 ± 5.00Pentoxone (4-Methoxy4-Methyl Pentanone-2)            Shell Chemical                        1.93 ± 1.50Butyl cellosolve             5.49 ± 2.50N-propyl acetate             12.70 ± 5.00Paraplex G. 60(polymeric plasticizer)            Rohm and Haas                        2.40 ± 1.752. Nitro propane             19.95 ± 5.00                        100.00%______________________________________ 
    
     
         ______________________________________                        Percentage(3) WHITE INK    Supplier    by Weight______________________________________Titanium dioxide Dupont      29.88 ± 7.50Elvacite 2013 (MethylButyl Methacrylate)            Dupont      9.50 ± 3.00Cellulose acetate butyrate   2.29 ± 1.502. nitro propane             13.40 ± 5.00Butyl cellosolve             5.24 ± 2.50Ethanol                      28.72 ± 5.00N-propyl acetate             7.05 ± 5.00Span 80          I.C.I.      .06 ± .10G. 3300          I.C.I.      .06 ± .10Pentoxone        Shell Chemical                        1.51 ± 1.50Paraplex G.60    Rohm and Haas                        2.29 ± 1.75                        100.00%______________________________________ 
    
     The inks were printed using Dupont Cyrel photopolymer plates. On the application of heat and pressure the inks transferred cleanly and easily to an article made of high impact polystyrene giving a decoration with excellent abrasion resistance. 
     EXAMPLE 2 
     A coating consisting of Ouricury wax dissolved as a 5% solution in trichlorethylene was applied to 0.00075 polyester carrier film using a wire wound metering bar and inks as described in Example 1 above were printed using B.A.S.F. Nyloprint photopolymer or plastic plates. 
     The resultant print transferred easily and cleanly on application of heat and pressure to a plastic article made of polycarbonate resin. 
     EXAMPLE 3 
     A coating of Ouricury wax on polyester film as described in Example 2 was overcoated with the following abrasion resistant lacquer: 
     
         ______________________________________     Supplier    Percentage by Weight______________________________________.sup.1 V.Y.H.H.       Bakelite      3.30 ±.sup.2 Acryloid A.101(30% solution)       Rohm and Haas 13.00 ±.sup.3 Chlorowax 40       Diamond Shamrock                     2.70 ±Methyl ethyl ketone       40.00 ±Toluene                   41.00 ±                     100.00%______________________________________ .sup.1 V.Y.H.H.  medium molecular weight vinyl chloridevinyl acetate copolymer .sup.2 Acryloid A.101  high molecular weight methyl methacrylate .sup.3 Chlorowax 40  chlorinated paraffin wax 
    
     The coating was applied by wire wound metering bar giving a dry coating weight of 2-5 grams per square meter. 
     The duplex coated film was then printed on the flexographic press using Cyrel plates with the inks as described in Example 1. 
     The resultant multi-layer system, when applied by use of heat and pressure to polystyrene, resulted in a hard, abrasion and alcohol resistant decoration suitable for decorating cosmetic containers. 
     EXAMPLE 4 
     An overall key or tie coating was applied by using a plain Cyrel (Dupont) plate. The coating had the following formulation and was applied over previously inked film as described in above examples: 
     
         ______________________________________                    Percentage          Supplier  by Weight______________________________________CP. 515-2Chlorinated polyolefin            Eastman Kodak                         39.00 ± 7.50Elvax 40 (10% in heptane)Ethylene vinyl acetate            Dupont       18.96 ± 5.00Aerosil 300 (Silica filler)            Degussa      4.37 ± 7.00Heptane (solvent)             37.67 ± 10.00                        100.00%______________________________________ 
    
     This system gave good adhesion to polyethene and polypropylene when transferred using heat and pressure. 
     EXAMPLE 5 
     Inks based on the following formulations were printed from Cyrel® (Dupont) plastic printing plates onto release coated polyester film. 
     
         ______________________________________                Percentage       Supplier By Weight  Range______________________________________.sup.1 Krumbhaar Resin 1717         Lawter(50% solution)         Chemical Co.                    30.00      20-30%Precipitated             21.00       5-25%chalk (pigment).sup.2 Versamid 750         General Mills                    15.00      10-20%(40% solution)Dye                       4.00      0-7%N-propyl acetate         11.00       5-15%(solvent)Nitrocellulose(1/2 sel grade)         Hercules   15.00      2-7%Di-octyl phthalateplasticizer               2.00      0.5-3.5%Titanium dioxide          1.80      0-7%Dispersion agents         0.20      0-2%                     100.00%______________________________________ .sup.1 Krumbhaar Resin 1717  condensed ketone resin .sup.2 Versamid 750  polyamide resin 
    
     On application of heat and pressure these inks without any further tie coat or adhesion promoter will adhere to low and high density polyethene and polypropylene. 
     In the examples, various ranges of constituents have been set forth. Additionally, alternative components have been identified. 
     While there has been set forth various preferred embodiments of the method and formulations associated with the present invention, it is to be understood, however, that various changes can be made while not departing from the spirit and scope of the invention. Thus, the invention is to be limited only by the following claims and their equivalents.