1. Field of the Invention
The present invention relates to a film-forming T die for low viscosity resin. The T die of the present invention is of particular value in the manufacture of pressure-sensitive tapes using solventless adhesives.
2. State of the Art
Existing bonding agents and adhesives for paper, cloth, plastic film, and other adhesive tapes are largely the solvent type, and, for the most part, roll coater equipment is being used to apply the adhesives. As a rule, organic solvents are inflammable, there are such problems as generation of toxic gases, and a demerit of application using solvent-type adhesives is the considerable expense required for solvent recovery equipment and environmental improvement equipment. Solventless adhesives (hot melt) with a base of low melting viscosity synthetic resin of low molecular weight have lately been put to practical use, and are replacing solvent-type adhesives. However, with low viscosity synthetic resins, there are technical problems with application and film formation. T dies for film-forming synthetic resin heretofore in use, such as the hangercoat type and the fishtail type, were originally made for film formation of widely-used, general purpose resin (PVC, PE, PP, etc.). Because the general purpose resins have a considerably high viscosity even when they melt, it is possible to form a film by means of high pressure extrusion using extruders, but because the solventless adhesives have extremely low viscosity when melting, there are difficulties in carrying out thin-film application or film formation using conventional dies.
Comparing fluidity of heated low viscosity resins used for solventless adhesives with general purpose resins, as shown in FIG. 1, there is such a difference that they can't be compared using, for example, Melt Flow Rate or Melt Index, and both resins must be treated as if they have totally different properties when melting. If conventional T dies for general purpose resin are used to melt and extrude low viscosity resins, the result will be as shown in FIG. 2. Because the fluidity of the low viscosity resin extruded from the die is too high, and because resistance within the die is very low, almost all of the resin is extruded from the center part of the T die, as shown in the drawing, and a uniform film can not be formed. Industrially, the desired width of the thin film is 1000 mm or more, and with existing extrusion T die methods there was no possibility of obtaining a wide film of uniform thickness.