Patent GB-708718-A (GB)
Title: Improved blocking foil
Inventor: EDGE NORMAN
IPC: C04B41/45
Filed: 1950-10-02
Abstract: 708,718. Blocking foil. BRITTAINS, Ltd. Dec. 19, 1951 [Oct. 2, 1950], No. 24078/50. Class 100 (2) A blocking foil for use in printing' ceramic transfers comprises a foil such as glassine paper or regenerated cellulose foil having on one side a coating comprising a ceramic colour, a flux, and a thermoplastic binding material which can be rendered soft and adhesive by application of heat and on further heating can be volatilized without residue and without affecting the ceramic colour, the quantity of thermoplastic binding material used being just sufficient to bond each particle of ceramic colour. In the printing process the transfer paper is placed under a die and overlaid with the blocking foil, the coated surfaces of the papers being in contact, and the heated metal die under suitable pressure is quickly applied and removed to stamp out the design on the transfer paper in ceramic colour. The foil and transfer paper are then separated. The thermoplastic binding material may consist of or contain resins which possess a definite melting point such as wood rosin and hydrogenated wood rosin, or thermoplastics which have no definite melting point. The material may comprise poly-n-butyl methacrylate, polystyrene, polymethyl methacrylate or their copolymers compounded, if required, with volatile plasticizers such as the phthalates, e.g. dibutyl phthalate, or certain chlorinated diphenyls or polyphenyls. Other suitable thermoplastics can be compounded from the " Arochlor " resins (the word " Arochlor " is a Registered Trade Mark), or wood rosin, and its derivatives, or polyvinyl acetate and its copolymers, or from plasticized compositions of ethyl cellulose, or nitrocellulose. However, it is preferred to use heat-stable volatile organic thermoplastic resins such as the " Arochlors or those polymeric thermoplastics which depolymerize on heating to volatile monomers at temperatures below the maturing temperature of the ceramic colour and flux so that carbonization during firing is substantially eliminated. In one process for preparing the foil the ceramic colour and flux are ground and suspended in a solution of the thermoplastic binding material in a volatile organic solvent, and the mixture coated evenly on the foil and dried. The application may be made in one or more coats. In a second process the ground ceramic colour and flux are suspended in water and mixed with an aqueous emulsion or latex of the thermoplastic binder preferably thickened with an ashless thickening agent such as ammonium alginate or hydroxyethyl cellulose, and the mixture coated on the foil and dried. Examples of the preparation and application of the foils are given; the die temperatures being 110- 130‹ C. and the firing temperature 580-750‹ C. When papers such as cellulose acetate coated Simplex paper are used there may be difficulty in obtaining good adhesion and transfer to the printed surface due to the incompatibility of the thermoplastic binder with the paper. In such cases and when repeated overprinting is required it is advantageous to overcoat the initial coating with an aqueous emulsion or latex of the thermoplastic binder. This extra coating may be discontinuous to prevent flaking when the foil is flexed.