Protection film for base sheet

Provided is a protection film for a base sheet that excellently transmits ultraviolet rays through itself, that does not easily make any reactions with antistatic agent, and that has an appropriate level of adhesive strength. The protection film for a base sheet is the one that is used for the base sheet of a lens sheet. In this protection film, the lens sheet has its lens surface formed of a UV-curable resin layer; the protection film has provided on its one-surface side an adhesive agent layer; and the transmittance of ultraviolet rays of which wave length is 320 nm transmitting through the protection film is 82% or more.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will hereafter be explained on the basis of the embodiment illustrated in the appended drawings. FIG. 1 is a sectional view illustrating a protection film F that has been adhered onto each of the surfaces of the base sheet 1 . The protection film F comprises a synthetic resin film 2 and an adhesive agent layer 3 that has been formed on one surface side of the synthetic resin film 2 . As the synthetic resin material, from the standpoint of its light transmission property being excellent, polyethylene, vinyl chloride copolymer, polypropylene, or a kneaded material of polyethylene and polypropylene is suitably used. As the adhesive agent constituting the adhesive agent layer 3 there is usually used by selection one of olefin-based, synthetic-rubber-based, and acrylic adhesive agents. The base sheet 1 is the one that has been obtained by passing an acrylic resin, which has been heated at a high temperature to a molten state, through between two metal rolls so that it may be rolled while it is being cooled and by molding it into a plate-like acrylic plate. On the base sheet 1 , after its being molded, on one surface thereof, an antistatic agent layer 4 is formed through the use of spray coating. As the antistatic agent forming the antistatic agent layer 4 , for example, there is used a cationic surface active agent. The reason for coating the antistatic agent onto one surface of the base sheet 1 is for the purpose of, after the base sheet 1 is in stock or has become a product, preventing dirt, dust, etc. in the atmospheric air from adhereing to the surface of the base sheet 1 due to the static electricity. There is a case where the base sheet 1 is stocked for a long period of time until it is used for molding the lens sheet. Accordingly, during that moment, the protection film F needs to hold an adhesive strength that is to such an extent as it cannot easily be exfoliated from the base sheet 1 . From this point of view, the adhesive strength of the protection film F, preferably, is 0. 2N/25 mm or more, further preferably 0.4N/mm or more. Also, during that moment, the adhesive agent layer 3 of the protection film F and the antistatic agent layer 4 coated on the surface of the base sheet 1 are contacted with each other at all times in a planar state. Accordingly, it is necessary to examine a combination between the adhesive agent and the antistatic agent so that these two agents may not make a chemical reaction within a long period of time and so that the antistatic effect may persist. Here, as the antistatic agent for the base sheet 1 , there is widely used a cationic surface active agent. Accordingly, it is necessary to select an adhesive agent that makes no chemical reaction with that cationic surface active agent. While, as stated previously, usually, the adhesive agent is used by being selected from among olefin-based, synthetic-rubber-based, or acrylic adhesive agents, here from the viewpoint of its chemical reactivity with the cationic antistatic agent being very low, synthetic-rubber-based adhesive agent, further preferably olefin-based adhesive agent is recommended for use. The inventor of this patent application coated the cationic antistatic agent onto the base sheet 1 in an amount corresponding to 0.29 g/m 2 . Thereafter, the protection film F equipped with the olefin-based adhesive agent layer was pasted onto the resulting base sheet 1 . The resulting base sheet 1 was kept in custody for one year. Then the surface resistance value was measured using a surface ohmmeter (the model type: SM-8210 or SME-8310) that is made by Tokyo Denpa Co., Ltd. As a result, a value of 6.07×10 10 &OHgr; was obtained. Usually, it is said that the surface resistance value that is the limit imposed upon the antistatic agent performance is 10 12 &OHgr;. That obtained figure is the one that is sufficiently smaller than this value. Accordingly, the performance of the antistatic agent that is obtained from the above-described coating is judged as being at a fully satisfactory level. In advance of the use for a UV curable resin molding process, one of the protection films F pasted on both surfaces of the base sheet 1 is exfoliated. At this time, it is preferable that the adhesive strength of the protection film F be set to be 3. 5N/25 mm or less, further preferably 2. 3N/25 mm or less so that that exfoliation operation may become easy to perform. The base sheet 1 whose protection film F of the one-surface side having no antistatic agent coated thereon has been exfoliated is used in the UV-curable resin molding process. FIG. 2 is a sectional view illustrating the relationship that exists among a stamper (die), the UV-curable resin layer, the base sheet 1 , and the protection film F in the UV-curable resin molding process. In the resin molding process, first, a liquid-state UV-curable resin is dropped onto the stamper that is a molding die. Next, the base sheet 1 is lidded over the upper surface of the UV-curable resin layer and is pressed against it over the entire surface of it. Through the execution of that process, the under side of the liquid-state UV-curable resin layer is cohered to the upper surface of the stamper that has formed thereon a reverse concavities/convexities configuration of the lens sheet. Resultantly, a normal concavities/convexities configuration of the lens sheet is transferred onto the underside of that resin layer. On the other hand, the upper surface of the liquid-state UV-curable resin layer coheres to the base sheet 1 of the surface side that the protection film F has been exfoliated. Thereby, the both elements are adhered to and integrated with each other. Further, as illustrated in FIG. 3 , from above the protection film F and base sheet 1 , ultraviolet rays are radiated so as to transmit through this film F and sheet 1 to thereby cure the UV-curable resin. With the use of that method, a sheet-like lens such as a Fresnel lens sheet is manufactured. As will be apparent from FIG. 3 as well, in order that a sufficient amount of ultraviolet rays may arrive at the UV-curable resin layer, the protection film F needs to have the property of excellently transmitting ultraviolet rays. According to the present inventor's experiments, for example, a polyethylene film has more excellent transmission property of ultraviolet rays when compared with a polypropylene-based or vinyl-based film. Provided, however, that, for example, an ordinary polyethylene film that is used for agricultural use purposes, in many cases, has mixed therewith an ultraviolet ray absorption agent. Therefore, for the protection film F of the present invention, it is preferable to select and use a polyethylene film having no ultraviolet ray absorption agent mixed therewith. Concretely, a polyethylene film that has a figure of 75% or more, or further preferably 82% or more as the transmittance of ultraviolet rays the wavelength of that is for example 320 nm is recommended to be used. Incidentally, in the above-described embodiment, as the lens sheet, there has been taken up an example regarding a Fresnel lens sheet. However, the present invention is not limited thereto. Namely, the present invention can of course be also applied to the protection film for the base sheet that is used for other lens sheets such as a lenticular lens sheet, a fly-eye lens sheet, or a linear Fresnel lens sheet, too. The present invention is not limited to the above-described embodiments and permits changes or modifications to be suitably made without running counter to the subject matter or idea of the invention that is readable from the claims and the entire specification. The protection films for the base sheets that have been changed or modified as such are also included in the technical scope of the present invention. According to the following protection film for the base sheet, the following advantages can be obtained. Namely, that protection film that as explained above is the one used for the base sheet for the lens sheet, the lens sheet having its lens surface formed of a UV-curable resin layer, the one-surface side of that protection film having provided thereon an adhesive agent layer, the transmittance of the ultraviolet rays of which wave length is 320 nm transmitting through the protection film is 82% or more. The advantages are that even when radiating ultraviolet rays onto the UV-curable resin layer so as to pass through the protection film, most part of the ultraviolet rays transmit through the protection film to reach the UV-curable resin layer, with the result that it is possible to sufficiently cure the UV-curable resin. Also, if it is arranged that the adhesive strength of the protection film for the base sheet be 0. 4 to 2. 3N/25 mm, because the adhesive strength of the protection film falls within a predetermined range, there is no possibility that when the base sheet is being stocked the protection film will unexpectedly be exfoliated off. Also, in case peeling the film off the base sheet immediately before executing the resin-molding process, it is possible to perform reasonable peel-off of the film. Also, if in case having pasted the protection film for the base sheet from the adhesive agent layer side onto the surface of the base sheet having coated thereon the antistatic agent including a cationic surface active agent it has been arranged that the surface resistivity after the lapse of one year of the base sheet be maintained at a value of 10 12 &OHgr;/&square; or less, the antistatic performance of the base sheet is maintained as is over a long period of time. This prevents the occurrence of an accident wherein the dust in the atmospheric air adheresa to the base sheet while the same is being stocked. If in such a construction it is further arranged that the adhesive agent be the one that contains polyolefin-based resin components, even in case the surface of the base sheet has coated thereon a cationic antistatic agent, there is no possibility that the adhesive agent and the antistatic agent will make any chemical reactions with each other. Thereby, the antistatic performance can be maintained over a long period of time. Further, if in the above-described modes of the present invention it is arranged that the material of the base sheet be acrylic compounds, each of the above-described modes of the present invention can be applied to the protection film of the acrylic-made base sheets.