1. Field of the Invention
The present invention relates to embossed process paper for transfer which, when producing a dry type of synthetic leather, is used as a medium for extracting the gloss, embossment, handling, etc. of the surface thereof, said synthetic leather having been widely used for a variety of articles such as shoes, bags, clothes, furniture and golf bags as well as for interior finish and interior trimming, and method for making such paper.
2. Statement of the Prior Art
In general, a dry type of synthetic leather products have been produced in the following manner. Process paper having a releasable resin laminated on the surface of backing paper is first provided. Then, a solution composed mainly of such a synthetic resin as vinyl chloride or urethane resin is coated and pre-dried on the releasable resin of the process paper. Subsequently, a backing material (fabric)--such as a staple fiber fabric, a mixed fabric of staple with polyester fibers, a knit fabric of nylon tricot, etc. and an nonwoven fabric of a mixture of staple fibers with polyester or polyamide resin--is pressure-laminated on the pre-dried product through an adhesive, followed by heating and drying. Finally, the process paper is released from the end product.
The embossing on the surface of such a dry type of synthetic leather may be achieved by a direct process wherein the synthetic leather is manually crumpled, embossed or stretched, or an indirect process wherein process paper pre-embossed to have an embossed pattern is used to transfer that embossed pattern on the synthetic leather. The latter indirect process now prevails, because it is superior to the former direct process in terms of the texture homogeneity, gloss variation, handling and other factors of the finished synthetic leather product.
The production of such process paper having an embossed pattern on the surface has so far been carried out in two steps. At the first step, a releasable resin is laminated on the surface of backing paper to make a flat type of process paper in which the back side of the backing paper and the surface of the releasable resin are both flat. Then, at the second step, this flat type of process paper is passed between an embossing roll and a rubber roll, or guided between a pair of a male-female type of embossing rolls designed to mesh with each other, just as would be in the case of gears, for moisture conditioning, heating and embossing (patterning). At this second step, the latter indirect process is superior to the former direct process in terms of the depth and clearness of the embossments. However, both the techniques are subject to some unavoidable limitation as to the depth and homogeneity of the embossments, because the flat type of process paper made of the releasable, already solidified resin and the backing paper is further processed by an additional embossing machine. In other words, the depth of the embossment imparted on the flat type of process paper by conventional techniques is up to about 60%, as expressed in terms of the effective transfer percentage with respect to the depth of the embossment provided on the embossing roll. In this regard, see Japanese Patent Laid-Open No. 58-25926, page 4; entitled "Method and apparatus for carrying out surface transfer on releasable coats on supports as well as release sheets". A variation in this effective transfer percentage is largely attributable to gloss variations, satinizing and lack of handling of synthetic leather. With the conventional techniques, it is also impossible to emboss only the releasable resin laminated on the backing paper; the backing paper is unavoidably embossed. Thus, the resulting embossed process paper tends to occasion "paper-making defects" such as fish eyes, bubbles, water streaks, sagging, ribs, satinizing and contamination, which otherwise come to appear in the form of "embossing defects" such as tensional wrinkles and wavy edges. As a result, it is impossible to obtain any uniform synthetic leather surface, when a solution composed mainly of vinyl chloride or urethane resin is coated on the releasable resin of the embossed process paper. In addition, the backing paper is deprived of the embossment by heating while the process paper is passed through the steps of heating and drying that solution, and this in turn causes deprivation of the embossment from the releasable resin, reducing the number of recycling of the process paper. Moreover, due to the fact that the backing paper is embossed, the backing paper-forming fibers are torn off during embossing through an embossing machine, causing a drop in the strength of backing paper and reducing the capability of the backing paper to be recycled. As the releasable resin to be laminated on the surface of the backing paper, polypropylene may be used alone. However, the polypropylene, if extruded at a temperature enough high to enable lamination, suffers a considerable variation in extrusion and tends to surge. For this reason, there is no choice but to incorporate in that resin a high-pressure low-density polyethylene in an amount of usually about 10-20%, although the resulting product is softer in a solidified state than that obtained with the sole use of polypropylene. This leads to another defect of the conventional process paper that it suffers premature thermal degradation at the steps of heating and drying a solution composed mainly of a synthetic resin and coated on the releasable resin.
The present invention seeks to provide embossed process paper which is substantially free from the above-mentioned defects of the prior art process paper and is less expensive and superior in performance to the prior art product, and a method for making such embossed process paper under conditions rationalized in view of industrial and economical considerations.