BACKGROUND OF THE INVENTION
To replace the natural white paper, a polyolefin synthetic paper based on polypropylene biaxially orientated film as the intermediate substrate layer, with its surface being laminated or coated with uniaxially orientated polypropylene films comprising inorganic salt fine powder as a paper-like surface . The synthetic papers of above Japanese Patent Publications show advantages of waterproof, tear resistance, rigidity, mechanical strength and adhesion to the printing head, however, the ink drying rate in the printing process is not so ideal that they are not widely applicable in writing and printing cultural paper.
The reason is that the microvoid volume and the porosity connection degree of the synthetic paper are much less ideal than those of natural papers, resulting in that the ink drying time is quite long (4-color printing takes about 8 hours) after the printing process, it is inevitable to dry the ink on synthetic paper, also the amount of stacking is limited for preventing undesired reverse printing. As for the four-color double-sided printing, the number of printing operators is potentially needed to increase, besides, the working space to wait for drying is necessary, and because the polypropylene uniaxially orientated film structure containing inorganic salt fine powder exhibits worse ink adhesion amount than that of the natural paper, more ink should be put into the printing machine to achieve the color saturation; meanwhile, an unique printing ink at a 3-4 times expensive price than that of the natural paper printing ink is needed to prevent the intrusion of the ink solvent resulting in paper deformation and dimensional change. In other words, the main reason that the synthetic paper of the polypropylene uniaxially orientated film structure containing inorganic salt fine powder can not widely replace the natural paper products in many applications is the long printing process and cost difference as compared with the natural paper.
In addition, a surface treatment on the synthetic paper made of polypropylene, polyvinyl chloride, polystyrene or polyester film with or without orientation, although the characteristics (printability, drying property) of the resulted synthetic paper are much better than the above-mentioned synthetic paper made of polypropylene uniaxially orientated film containing inorganic salt fine powder as a paper-like surface layer, it should widely replace the cultural use of the natural paper, in fact, the manufacturing costs of this synthetic paper are quite high in various aspects, its sales in the market are not more ideal than the synthetic paper made of the polypropylene uniaxially orientated film containing inorganic salts fine powder as a paper-like surface layer, because its technology includes the use of DMF (Dimethyl formamide) solvent as a surface layer treating agent, the paper surface layer thickness after the coating is quite thick, thus the coating thickness is not less than 10 μm.
For wide application of the synthetic paper to replace quality-wisely the use of the natural paper, meanwhile also avoiding the application scope narrower due to excessively high manufacturing costs, 15 years ago before the present invention application, the present applicant also provided a polypropylene resin composition through the co-extrusion of one primary extruder and two secondary extruders into the same T-die to make 3-layer polypropylene white opaque resin-coated material sheet, and then after cooling, setting, biaxial orientation, corona treatment, the surface layer was coated a paper-like surface processing agent containing inorganic salts in oily organic solvents (chlorinated polypropylene, benzene), depending on the thickness, to obtain a printing quick-drying type biaxial orientation polypropylene pearl shining synthetic paper of thickness 25˜250 μm .
Because the microvoid volume and the porosity connection degree of the synthetic paper obtained by this invention are very close to those of natural paper products, thereby the drying time of the four-color printing ink is 10 minutes, while the drying time of the printed ink on the outer layer for avoiding the undersized reverse printing is less than one minute, so that the printing operation is entirely the same as that of natural papers, and the stacking volume control, printing plant workers increase and the waiting time for the drying operation are not necessary, besides, the ink absorber is confined to the coating layer and does not invade into the substrate layer, therefore a general natural paper printing ink can be used; meanwhile its ink printing amount is similar to that of the natural paper, the ink volume put in the printing machine is not increased. it is thus clear that the qualities of the said inventive synthetic paper are beyond those of the generally known synthetic paper, and can widely replace the natural paper in the cultural paper application for printing and writing.