Background Art Currently, various types of boards, such as blackboard, are used in offices and classrooms for taking notes or lecturing. Normally, those types of boards are so large and heavy that it is hard to carry or move them.

When a seminar or meeting is to be held in a room without a blackboard installed therein, many people should get together to carry a blackboard into the room. Moreover, when supplementary materials need to be posted on the blackboard during a lecture or seminar, an adhesive agent such as glue or adhesive tape is required to post them on the blackboard. To make the attachment and/or detachment of materials easier, memo notes such as Post-its (by 3M) are used. With alleviated adhesiveness, Post-ite notes can be attached or detached easily. However, Post-ite notes are disposable after one-time use, and they are expensive.

Also, when a beam projector is used in a small office or room, a screen is required necessarily. However, the screen for beam projector, too, has a problem that it is so large and heavy to be moved and carried easily.

In most cases, adhesive plastic films are used for

the purpose of protecting the surfaces of a product.

Technologies disclose such protection films that are no more than adhesive resin, which is formed by blending wax- type paraffin coating solution and adhesive polyisobutylene resin, or disclose a protection film that uses a multiple layers of etylenevinyle acetate on one side of a film for writing, which is formed of a thermal plastic resin or a paper. Japanese Patent Publication No. 1984-30911 and Japanese Laid-Open Patent Publication No. 1987-103975 disclose a surface protection film that has an adhesive layer including an adhesive resin on one side of a film for writing purpose, which is formed of a thermal plastic resin, such as polyolefin resin. Also, Japanese Laid-Open Patent Publication No. 1989-129085 discloses a surface protection film using an adhesive agent formed of polystyrene block copolymer, which is a resin giving adhesiveness, and polyethylenimine having a long chain alkyl group. However, these technologies have a limitation in solving the above- mentioned problems, and they can hardly replace the various types of boards, on which people could write with a marker.

In addition, studies are performed to develop a plastic film having self-adhesiveness by generating corona- discharged static electricity or frictional force. However, the technologies have a problem in maintaining the static electricity, because the static electricity they use is generated spontaneously during the film manufacturing process. Moreover, the static electricity on the surface of the plastic film adsorbs dust and contaminants in the air, eventually making the surface seriously contaminated.

This property has obstructed long-time use and maintenance of the self-adhesive plastic film. Therefore, the plastic films of these technologies are not proper to substitute the boards, such as blackboards. U. S. Patent Publication No. 5010671 issued in April 30,1991 and International Laid-Open Patent Publication No. W096/05045 disclose a flip-chart pad and a product, both use a plastic film and

endow polypropylene resin with static electricity.

However, the generation of static electricity is what can be obtained spontaneously during the manufacturing process, and the two products adsorb dust in the air and thus contaminate their surfaces. Also, the low intensity of illumination makes the feeling of writing poor, and since the solvent of a marker infiltrates deep into the surface of the film, once written, letters are hardly erased. Even through they are erased, the surfaces of the products may be deformed. Moreover, in case where lines for assisting writing are printed on the surface, there is a problem that the lines may be erased by the frictional force between the surface of the film and the eraser.

Disclosure of Invention It is, therefore, an object of the present invention to provide a self-adhesive sheet with a smooth surface for writing which is not contaminated by a marker pen and on which what was written is erased easily, and a preparation method thereof.

Those of ordinary skill in the art of the present invention can understand the other objects and advantages of the present invention from the detailed description of the invention, claims and drawings.

After repeated researches for solving the above described problems, the present inventor (s) has developed a new self-adhesive sheet that can generate much static electricity to make the sheet self-adhesive, prevent static electricity-induced contamination on the surface, and has a low surface illumination intensity by using the matter of static electricity which is considered a problem in manufacturing conventional films, inversely.

In accordance with one aspect of the present invention, there is provided a self-adhesive plastic sheet, including: a first plastic film containing titanium dioxide and

calcium carbonate and a second plastic film containing an anti-static agent and an anti-blocking agent, wherein one side of the first plastic film is laminated with one side of the second plastic film.

The self-adhesive plastic sheet 110 of the present invention includes a first plastic film 101 which is adhesive and thus used for adhesion and a second plastic film 102 which is treated to be anti-static so as to prevent static electricity-induced contamination and used for the purpose of being written, the two plastic film being laminated with each other. Fig. 1 shows the lamination of the two plastic films.

In accordance with one aspect of the present invention, there is provided a method for preparing a self-adhesive sheet comprising two layers of plastic films, including the steps of: a) preparing a first plastic film by inputting titanium dioxide and calcium carbonate to a plastic resin, and then melting and cooling down the plastic resin mixture; b) generating static electricity in the first plastic film by rubbing the first plastic film with a guide roller; c) performing a corona treatment on the first plastic film, which has static electricity generated in the above, more than twice; d) preparing a second plastic film by inputting an anti-blocking agent and an anti-static agent to a plastic resin, and then melting and rapidly cooling down the plastic resin mixture; and e) laminating one side of the first plastic film, on which a corona treatment is performed, with one side of the second plastic film by performing a primer treatment.

The term"plastic sheet"appearing in the present specification specifies the plastic film of this invention that is formed in more than two layers. Therefore, the term"plastic sheet"should be understood referring to the state of more than two plastic films being laminated with each other in accordance with the present invention. The plastic film referred to in the present specification is a

conventional"plastic film"commonly used in the film industry, and prepared by using such plastic resins as polypropylene (pp) including oriented polypropylene (OPP), polyethylene (PE), polyester (PET), polyethylene terephthalate (PETP) and the like.

Brief Description of Drawings The above and other objects and features of the present invention will become apparent from the following description of the preferred embodiments given in conjunction with the accompanying drawing, in which: Fig. 1 is a diagram illustrating a plastic sheet formed of two plastic films laminated with each other in accordance with an embodiment of the present invention.

Best Mode for Carrying Out the Invention Other objects and aspects of the invention will become apparent from the following description of the embodiments with reference to the accompanying drawings, which is set forth hereinafter.

In case of wrapping films such as plastic films, corona treatment is performed thereon to make it easy to printing on the surface of the film and keep the wrapped products safely. Here, the corona treatment generates static electricity, which affects the quality of film adversely, and thus much effort is made to remove the static electricity from the films. However, focusing on the property of static electricity generated during the film preparation process, the inventor (s) of the present invention rather enhances the property of generating self- adhesiveness in a plastic film. Since the static electricity generated spontaneously during the film preparation process is not sufficient to give adhesiveness to a plastic film, a corona process is performed repeatedly.

The generated static electricity can be maintained by adding an ingredient having a foaming property to the plastic film.

A plastic sheet of the present invention has a structure two layers of plastic films are laminated with each other. A first plastic film is a part with enhanced self-adhesiveness, and a second plastic film is a part with smooth surface for writing.

Desirably, a material such as polypropylene or polyethylene resin is used for the first plastic film.

This is because a resin with a relatively low specific gravity is proper for the first plastic film, which is used for the purpose of adhesion.

The first plastic film includes titanium dioxide and calcium carbonate. These ingredients make a color with pearl gloss that improves whiteness and concealment, and they stimulate the generation of static electricity deep in the film by generating foam. Particularly, if the titanium dioxide and calcium carbonate included in the plastic film have an opening rate of 40-50%, plenty of fine openings can be formed on the surface of the plastic film. If the foam-like openings, which are generated as described above, are exposed on the surface of the plastic film, when people write on the surface of the film, the solvent of a marker pen may infiltrate into the surface to deform the surface and after predetermined time passes, the writing should not erased.

To solve the above described problems and generate and maintain static electricity more effectively, when the first plastic film is prepared, static electricity is generated by rubbing the plastic film with a guide roller and unbalancing the positive and negative charges. If more than five guide rollers are used here, static electricity can be generated more effectively, and if the guide rollers are coated with chrome, the generation of static electricity is maximized. Subsequently, corona treatment,

which polarizes the plastic film while the film is wound, is performed more than twice to generate static electricity more intensely. Desirably, the corona treatment is performed in the air by supplying voltage as high as 20- 30 kVA to the plastic film so that the plastic film could loose plenty of charges. Here, the carbonyl groups become polarized, accelerating the unbalance between positive charges and negative charges and, thus, generating static electricity more effectively. Also, when people write on the plastic film with a marker pen containing solvent or print on it, the static electricity plays the role of an adhesive agent for the writing or printing ink so that the ink could adhere on the surface of the plastic film.

However, the static electricity generated by the corona treatment adsorbs dust and contaminants in the air and makes the surface of the film dirty, which goes against the object of the present invention. To solve this problem, an anti-blocking agent and anti-static agent are inputted to the second plastic film. The anti-blocking agent and anti-static agent that can be used here have no restriction in their kinds and doses, as long as the desired effects could be obtained. For example, silica can be used as an anti-blocking agent, and as for the anti-static agent, various ionic or non-ionic alkyl amines, alkyl amides, multivalent alcohol esters can be used.

Since the second plastic film is for writing purpose, it is desirable to use a film with a low frictional coefficient. Table 1 below shows frictional coefficients of films.

Table 1 Frictional PE Film PET Film OPP Film Coefficient Static F. C. (us) 0. 7-0. 8 0. 25-0. 30 0.63 Dynamic F. C. (pk) 0. 5-0. 6 0. 20-0. 25 0. 55

According to Table 1, it can be seen that PET film has the most smooth surface and good to write on. Also, since it does not adsorb solvent very well, it is suitable for erasing job, too. Therefore, it is desirable to use PE resin for the second plastic film.

The anti-blocking and anti-static agents contained in the second plastic film prevent'deformation of the surface of the second plastic film, resulting from the infiltration of the solvent, and prevent the generation of static electricity on the surface so that the plastic film should not get dirty easily. Also, they provide nice feeling of writing by making the surface smooth, and provide easy and nice erasing performance. Besides, their gloss effect makes the white color of the surface clear.

Particularly, if an inorganic solid ingredient such as silica is used as an anti-blocking agent and the particle size is minimized no more than 3 pm uniformly, the illumination intensity of the surface can be minimized.

Also, to minimize the crystal size of the surface of the second plastic film, it is desirable to cool down the plastic film rapidly, when the film needs to be cooled down.

A plastic sheet of the present invention is prepared by laminating the first plastic film with the second plastic film, produced as described above, through a primer treatment. The lamination machine used in this process is conventional machine, such as DRY-LAMI.

As described above, the first plastic film is used for adhesion, and the second plastic film is used for writing.

The present invention may further include a step of printing a predetermined pattern on at least any one of the to-be-laminated sides of the first plastic film and the second plastic film. Since the printed side is not exposed onto the surface, the printed pattern is not made blur even after repeated erasing performance and maintained permanently. The printing is performed just as normal plastic film printing process, and the speed of a printing

roller is controlled properly based on the kind and characteristics of the used printing ink.

It is also possible to include an additional step of maturing the plastic sheet, which is primer treated, for a predetermined time at a predetermined temperature. Here, this maturing step is not the conventional maturing process for stabilizing the characteristics of the resin in the film preparation process, but a process for controlling the curling of the plastic sheet.

The plastic sheet of the present invention has two different plastic films laminated with each other, and the two plastic films have different physical characteristics.

So, it is not easy to maintain a flat surface. The different characteristics between the two plastic films can cause curling on the surface of the plastic films, when the temperature is changed. Since the curling should not be generated on the second plastic film, which is a part for writing, it is recommendable to mature the plastic films for a sufficient time at a high temperature so that the curling should be generated on the plastic film for adhesion. For example, when oriented polypropylene (OPP) is used for the first plastic film and PET is used for the second plastic film, it is desirable to mature the two plastic films at 90°C for more than 48 hours.

Also, it is possible to include a step for generating static electricity in the plastic sheet, which is laminated through a primer treatment or matured, by performing rubbing and corona treatment thereon, additionally. Here, the rubbing process is performed by using a roller with a line speed of more than 200 RPM to obtain stronger rubbing effect, and the corona treatment may be performed more than twice.

Herein, the technology of the present invention will be described more in detail with reference to embodiments.

Following embodiments should be understood not to limit the scope and concept of the present invention, and it is

apparent to those of ordinary skill in the art of the present invention that various substitution, modification and change can be performed within the scope and concept of the present invention.

STEP 1: Preparation of lst Plastic Film A first plastic film is prepared by inputting CaCO3 and TiO2 to a core layer of oriented polypropylene (OPP) and melting them, and drawing and cooling down the mixture.

Here, the contained quantity of CaCO3 and TiO2 is about 5-10 wt% of the total raw materials. To generate static electricity, more than five guide rollers are used and the speed of the guide rollers is maintained more than 200m/min.

Subsequently, a corona treatment is performed on the plastic film more than twice. at 1.5 kVA, and the temperature of the surface of the film is more than 40°C.

Here, the cubical resistance value of the surface of the first plastic film is maintained to be 1016 Q-m.

STEP 2: Preparation of 2nd Plastic Film A second plastic film is prepared by inputting 2, 000-3, 000 PPM of silica and 5, 000-7, 000 PPM of alkyl amine to polyester as an anti-blocking agent and anti- static agent, melting them and then cooling down the mixture to 40°C. Here, the particle size of the anti- blocking agent is controlled to be less than 3 um to minimize the illumination intensity of the surface of the second plastic film.

STEP 3: Lamination of 1st and 2nd Plastic Films The first plastic film prepared in the first step and the second plastic film prepared in the second step are laminated with each other with lamination machine, such as

DRY-LAMI, which is a brand name. Here, due to the thermal characteristics and disparity of the two plastic films, curling occurs between the first and second plastic films.

To solve this problem, the plastic sheet is re-wound in reverse and then matured at 90°C for 48 hours.

Experimental Example 1: Comparison of Surface Contamination Following experiment is performed with respect to a flip chart pad (product"E") of Every company, U. S. , an adhesive film (product"N") of Nordenia company, Germany, and the plastic sheet of the present invention to compare the level of contamination.

The products"E"and"N"and the plastic sheet of the present invention are adhered on an identical surface under controlled temperature and humidity of a laboratory, and then after 30 days, their state of contamination and the state of contamination after physical removal of the contaminants are examined. The result is shown in Table 2 below.

<Table 2> After Removing After 30 Days Contaminants Physically Not erased very well, Product"E"Contaminated a lot much contaminants still remaining. Not erased very well, Product"N"Contaminated a lot much contaminants still remaining. Plastic sheet of Contaminants erased very Rarely contaminated or present well almost as clean as contaminated a little invention initial state As shown from the above table, the products"E"and

"N"adsorb the dust in the air considerably, so the surfaces of the two products become very dirty. However, the plastic sheet of the present invention is rarely contaminated and maintains the surface as clean as the initial state, or contaminated a little. Also, when the same level of physical force is applied to the plastic films to remove the contamination from their surfaces, the contaminants of the products"E"and"N"are not erased very well and their surfaces are hardly restored to the initial states. However, the contaminants on the plastic sheet of the present invention are erased easily when the same force is applied, and the clean surface is maintained.

This result shows that the problem of contamination on the surface of a plastic film, which is caused by the generation of static electricity intended for self- adhesiveness, is solved.

Experimental Example 2: Comparison of Adhesiveness The static electricity test is performed with respect to the products"E"and"F"and the plastic sheet of the present invention. The static electricity of the products "E"and"F"are 1013 Q m, both sides alike. In case of the plastic sheet of the present invention, the static electricity of the writing side is 101l Q m with a half life of one second, and the static electricity of the adhesive side, the static electricity is 1016 2-m and it has no half life. This result shows that the plastic sheet of the present invention has stronger static electricity in its adhesive side than the conventional products. This signifies that the plastic sheet of the present invention has a superior adhesiveness, and since the writing side of the plastic sheet generates less static electricity, it can make the surface clean and less contaminated.

As described above, the self-adhesive plastic sheet of the present invention can adhere to a wall or blackboard

without an additional adhesive agent. Despite the excellent adhesiveness owing to the adhesive static electricity, the plastic sheet is not contaminated easily.

Also, the smooth surface provides excellent feeling of writing and comfortable erasing performance. In addition, it has an advantage that it can maintain a printed pattern for a long time.

While the present invention has been described with respect to certain preferred embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the scope of the invention as defined in the following claims.