WO2008078960A1

WO2008078960A1 - Surface protective film

Info

Publication number: WO2008078960A1
Application number: PCT/KR2007/006834
Authority: WO
Inventors: Young-Hee Kim; Seong-Hwan Kim; Jin Hur; Byung-Seob Kim; Ki-Eon Kim
Original assignee: Youl Chon Chemical Co., Ltd.
Priority date: 2006-12-26
Filing date: 2007-12-26
Publication date: 2008-07-03
Also published as: KR20080059950A; KR100857997B1

Abstract

Disclosed is a surface protective film for protecting an optical film having different surface roughness on either side. In the surface protective film of the present invention, the adhesive layer of the upper film of the surface protective film has 7 to 8 times larger adhesion than the adhesive layer of the lower film. The surface protective film of the present invention provides easy winding and unwinding and good protection from impurities or external environment, and is without the problem of the optical film being peeled off together the upper surface protective film, is required.

Description

Description SURFACE PROTECTIVE FILM

Technical Field

[1] The present invention relates to a surface protective film used as an optical protective film, wherein the upper and lower films have different adhesion.

[2]

Background Art

[3] Typically, surface protective films are attached on both sides of an optical film such as a diffusion sheet (hereinafter, "optical film"). The film attached on the upper surface of the optical film is called the upper surface protective film, and the one attached on the lower surface of the optical film is called the lower surface protective film.

[4] The surface protective film is prepared as follows. First, a lower surface protective film is attached on an optical film. The attached optical film and lower surface protective film are subject to Thomson cutting. Then, unwanted portion of the optical film is removed and an upper surface protective film is attached.

[5] The conventional surface protective film has the following problems.

[6] The lower surface protective film needs not have a large adhesion since the side of the optical film on which the lower surface protective film is adhered has little surface roughness. But, if the upper surface protective film has a small adhesion, winding and unwinding become difficult and protection from impurities or external environment becomes insufficient, since the side of the optical film on which the upper surface protective film is adhered has severe surface roughness. Accordingly, the upper surface protective film needs to have a large adhesion. However, when the upper surface protective film has too large an adhesion, the optical film may be peeled off along with the upper surface protective film.

[7] Accordingly, a surface protective film which provides easy winding and unwinding and good protection from impurities or external environment, and is without the problem of the optical film being peeled off together the upper surface protective film, is required.

[8]

Disclosure of Invention

Technical Problem

[9] The present invention has been made to solve the aforementioned problem. Accordingly, an object of the present invention is to provide a surface protective film wherein the upper and lower films have different adhesion, such that the optical film adhered to the surface protective film is easily wound and unwound, protection from external environment is possible, impurities can be removed easily, and when the upper surface protective film having a large adhesion is removed during the processing, the optical film is not being peeled off together with the upper surface protective film.

[10]

Technical Solution

[11] In order to achieve the aforesaid object, the present invention provides, in one aspect, a surface protective film comprising upper and lower surface protective films adhered above and below an optical film, wherein the adhesive layer of the upper surface protective film has an adhesion 7 to 8 times larger than the adhesion of the adhesive layer of the lower surface protective film.

[12] In an embodiment, the present invention provides a surface protective film, wherein, when the roughness of the lower surface of an optical film is 0.5 to 5 D on the average, the adhesive layer of the lower surface protective film has an adhesion in the range from 1 to 3 gf and, when the roughness of the upper surface of an optical film is 6 to 15D on the average, the adhesive layer of the upper surface protective film has an adhesion in the range from 7 to 24 gf.

[13] In another embodiment, the present invention provides a surface protective film, wherein the adhesive layer of the lower surface protective film comprises an adhesive composition comprising: 100 parts by weight of an acrylic resin as base resin; 1 to 20 parts by weight of an isocyanate-based curing agent; and 10 to 30 parts by weight of a commonly used solvent. In a more preferred embodiment, the adhesive composition further comprises an antistatic agent.

[14] In still another embodiment, the present invention provides a surface protective film, wherein the adhesive layer of the lower surface protective film comprises an adhesive composition comprising: 70 to 100 parts by weight of polydimethylsiloxane as base resin; 10 to 40 parts by weight of MQ resin; and 100 to 150 parts by weight of a commonly used solvent. In a more preferred embodiment, the adhesive composition further comprises 1 to 5 parts by weight of dimethylvinylsiloxane as anchorage agent. In another more preferred embodiment, the adhesive composition further comprises 0.5 to 5 parts by weight of polymethylhydrogensiloxane as crosslinking agent. In a further more preferred embodiment, the adhesive composition further comprises 0.5 to 2 parts by weight of tin or platinum catalyst.

[15] In yet another embodiment, the present invention provides a surface protective film, wherein the adhesive layer of the upper surface protective film comprises an adhesive composition comprising: 100 parts by weight of an acrylic resin as base resin; 1 to 10 parts by weight of an isocyanate-based curing agent; and 20 to 50 parts by weight of a commonly used solvent. [16]

Advantageous Effects

[17] According to the surface protective film of the present invention wherein upper and lower films have different adhesion, the optical film adhered to the surface protective film is easily wound and unwound, protection from external environment is possible, and impurities can be removed easily. Also, when the upper surface protective film having a large adhesion is removed during the processing, the optical film is not being peeled off together with the upper surface protective film.

[18]

Brief Description of the Drawings

[19] FIG. 1 schematically illustrates the construction of a lower surface protective film according to an embodiment of the present invention.

[20] FIG. 2 schematically illustrates the construction of an optical film according to an embodiment of the present invention.

[21] FIG. 3 schematically illustrates the construction of an upper surface protective film according to an embodiment of the present invention.

[22]

Mode for the Invention

[23] Hereinafter, the surface protective film according to the present invention wherein upper and lower films have different adhesion is described in detail.

[24] FIG. 1 schematically illustrates the construction of a lower surface protective film according to an embodiment of the present invention. FIG. 2 schematically illustrates the construction of an optical film according to an embodiment of the present invention. And, FIG. 3 schematically illustrates the construction of an upper surface protective film according to an embodiment of the present invention.

[25] As illustrated in FIG. 1, a lower surface protective film comprises a base film Ia, an adhesive layer 3 a formed thereupon with a primer layer 2a optionally disposed bet ween the base film Ia and the adhesive layer 3a, and a release film 4a formed on the adhesive layer 3a. As illustrated in FIG. 2, an optical film (diffusion sheet) comprises a base film Ib, a diffusion layer 5b having a relatively large surface roughness formed on one side of the base film Ib, and an anti-blocking layer 6b having a relatively small surface roughness formed on the other side of the base film Ib. As illustrated in FIG. 3, an upper surface protective film comprises a base film Ic, an adhesive layer 3c formed below the base film Ic, a release film 4c formed below the adhesive layer 3c, and an antistatic layer 7c optionally formed on the other side of the base film Ic.

[26] According to the present invention, the adhesion of the upper surface protective film is controlled to be 7 to 8 times that of the lower surface protective film. In that case, winding and unwinding can be performed without peeling-off of the optical film along with the surface protective film. Especially, impurities formed during Thomson cutting or other processes may be removed easily due to the larger adhesion of the upper surface protective film.

[27] In upper and lower surface protective films, a larger adhesion is needed as the surface roughness of the corresponding optical film increases. Typically, the roughness of the lower surface of an optical film is 0.5 to 5 D on the average, and the roughness of the upper surface of an optical film is 6 to 15 D on the average. Considering this roughness, it is preferable that the adhesion of the upper film is controlled in the range from 7 to 24 gf, and the adhesion of the lower film is controlled in the range from 1 to 3 gf.

[28] The upper and lower surface protective films are constructed as follows.

[29] First, the lower surface protective film is described in detail.

[30] The base film of the lower surface protective film may be a commonly used one.

For example, a polyester-based resin (PET, PEN), a polyolefin-based resin (PE, OPP), a polyamide-based resin (NY), a polyimide resin (PI), etc. may be used. Considering that the base film of the lower surface protective film is half cut while the optical film is 100 % cut, during Thomson cutting, the base film must have a thickness of at least 70 D . To enhance the adhesion between the base film and the primer layer, which will be described below, corona treatment may be performed.

[31] The adhesive layer of the lower surface protective film may comprise an acryl- based adhesive or a silicon-based adhesive.

[32] The acrylic resin used as the base resin of the acryl-based adhesive may be an acrylic resin having a molecular weight ranging from about 200,000 to about 1,000,000 [e.g., butyl acrylate, ethylene-methyl methacrylate copolymer resin (EMMA), ethylene-ethyl acrylate copolymer resin (EEA), ethylene-methyl acrylate copolymer resin (EMA), etc.]. In addition to the acrylic base resin, a curing agent and a solvent are included.

[33] The curing agent may be an isocyanate-based curing agent and is preferably included in an amount from 1 to 20 parts by weight, based on 100 parts by weight of the base resin. When the curing agent is included in an amount smaller than 1 part by weight, shoving of the adhesive layer may occur due to insufficient curing. When the curing agent is included in an amount exceeding 20 parts by weight, white powders (burrs) may occur when cutting the surface protective film due to excessive curing. The solvent may be a commonly used one. For example, ethyl acetate may be used. The solvent is included in an amount from 10 to 30 parts by weight, based on 100 parts by weight of the base resin. [34] The acryl-based adhesive may further comprise an antistatic agent. The antistatic agent may be a previously known antistatic agent, such as an alkali metal salt, an organic or inorganic conductive polymer and a surfactant, or a combination thereof. The antistatic agent may be included in an amount commonly adopted in the related art. For the alkali metal salt, lithium amide (LiNH ) or lithium diethylamide ((LiN(C H ) ) is preferred in view of surface resistance and peeling static voltage.

[35] The acryl-based adhesive composition may be prepared by mixing the base resin and the solvent by stirring for 10 minutes, adding the curing agent and the antistatic agent, and further mixing by stirring for 10 minutes.

[36] The release film formed on the silicon-based adhesive layer may be a polyethylene terephthalate (PET) film coated with a siloxane-based antistatic agent having a compatibility with silicon and then silicon release-coated to prevent impurities from being attached by silicon when peeling off the release film from the silicon-based adhesive. Further, the silicon release film may be antistatic-treated. In this case, contamination by the antistatic electricity that may occur during lamination with an adherend can be minimized.

[37] Preferably, the silicon-based adhesive comprises 70 to 100 parts by weight of poly- dimethylsiloxane as base resin and 10 to 40 parts by weight of MQ resin for conferring adhesion.

[38] The MQ resin is comprised of monofunctional M units having the general formula

R SiO- and tetrafunctional Q units having the general formula SiO -, and may be represented by the following formula (1):

[39]

(1)

[40] When blending a base resin with an MQ resin, the adhesion increases as the content of the MQ resin increases. As described earlier, since the roughness of the lower surface of the diffusion sheet is 0.5 to 5 D on the average, the adhesion of the lower surface protective film is preferably adjusted in the range from 1 to 3 gf by blending 70 to lOOparts by weight of the base resin with 10 to 40parts by weight of the MQ resin.

[41] Preferably, the blending composition of the base resin and the MQ resin further comprises dimethylvinylsiloxane as anchorage agent. More preferably, a silane coupling agent is included together with the dimethylvinylsiloxane anchorage agent.

[42] The anchorage agent is added to link the adhesive layer with the primer layer and is preferably included in an amount from 1 to 5 parts by weight. When the anchorage agent is included in an amount less than 1 part by weight, a burr may occur due to insufficient linking. An amount exceeding 5 parts by weight is unnecessary and excessively added anchorage agent may act as impurities.

[43] Preferably, polymethylhydrogensiloxane is added to the blending composition of the base resin and the MQ resin as crosslinking agent in order to cure the base resin and the MQ resin. More preferably, it is included in an amount from 0.5 to 5 parts by weight. When the crosslinking agent is included in an amount less than 0.5 part by weight, it is difficult to achieve the wanted adhesion because of insufficient curing. An amount exceeding 5 parts by weight is unnecessary and excessively added crosslinking agent may act as impurities.

[44] Preferably, the blending composition of the base resin, the MQ resin, the anchorage agent and the crosslinking agent comprises tin or platinum catalyst, particularly in an amount from 0.5 to 2 parts by weight. When the catalyst is included in an amount less than 0.5 part by weight, it is difficult to achieve the wanted adhesion because of insufficient curing, as in the crosslinking agent. An amount exceeding 2 parts by weight is unnecessary and excessively added catalyst may act as impurities.

[45] When the lower surface protective film comprises a silicon-based adhesive, it is preferred to form a primer layer. The primer layer prevents the adhesive layer from being separated from the base film due to moisture and, thereby, prevents the adhesive from migrating to the diffusion sheet. Further, it prevents breaking and shoving during Thomson cutting and prevents the blocking during rewinding of the tape.

[46] The primer layer comprises a base resin, a crosslinking agent, a catalyst and a solvent. Particularly, it is preferred that a silane coupling agent is used as base resin, and polyorganosiloxane is used as crosslinking agent. The silane coupling agent may be, for example, aminopropyltrimethoxysilane or glycidoxypropyltrimethoxysilane. More preferably, the base resin has a vinyl group. And, more preferably, polyorganosiloxane having an Si-H group is used as the crosslinking agent. The silane coupling agent having a vinyl group has an Si-(CH=CH ) structure. When reacting with polyorganosiloxane having an Si-H group in the presence of a catalyst, it forms an -Si-CH -CH -Si- structure. The vinyl group of the base resin functions as a bridge between the base film with the adhesive layer and, thus, is advantageous in attaining superior solvent resistance and durability, uniform coating thickness, and superior adhesion and processability.

[47] The silane coupling agent used as the base resin is solid, and the content of the solid base resin based on the total weight of the solid base resin and the solvent may be adjusted by varying the amount of the solvent based on the total weight of the solid base resin and the solvent. [48] The catalyst is a commonly used one, and either tin or platinum may be used, depending on the reaction type of the primer. Tin is preferred because it is less expensive. The solvent is also a commonly used one and, for example, toluene, MEK, etc. may be used.

[49] The viscosity can be controlled by adjusting the content of the solid base resin in the primer composition based on the total weight of the solid base resin and the solvent, which enables the control of sufficient and uniform coating thickness, thereby resulting in superior durability, particularly solvent resistance, and superior adhesion and processability, together with sufficient and uniform coating thickness.

[50] Specifically, when the content of the solid base resin is less than 5 wt% based on the total weight of the solid base resin and the solvent, the viscosity becomes lower than 10 cps. In this case, it is difficult to attain a sufficient and uniform coating thickness. When the content of the solid base resin is in the range from 5 wt% to 20 wt% based on the total weight of the solid base resin and the solvent, the viscosity ranges from 10 cps to 300 cps. In this range, superior solvent resistance and durability, and a sufficient and sufficient and uniform coating thickness can be attained. When the content of the solid base resin exceeds 20 wt% based on the total weight of the solid base resin and the solvent, the viscosity exceeds 300 cps. In this case, the process of coating itself may become difficult because of excessive viscosity.

[51] For example, the content of the solid base resin based on the total weight of the solid base resin and the solvent may be adjusted to 5-20 wt% as follows. The content of the solid base resin is adjusted to 20 wt% by using 20 parts by weight of the solid base resin and 80 parts by weight of the solvent. Then, the solvent is further added while keeping the amount of the solid base resin constant in order to adjust the content of the solid base resin to 5-20 wt%. As such, the viscosity can be adjusted to 10-300 cps.

[52] Preferably, the primer composition comprising 20 parts by weight of the base resin and 80 to 380 parts by weight of the solvent further comprises 1 to 10 parts by weight of a crosslinking agent and 1 to 5 parts by weight of a catalyst to in order to attain improved processability and adhesion.

[53] When the crosslinking agent is included in an amount less than 1 part by weight, binding force decreases because of insufficient crosslinking, coating is not performed well, and a burr may occur. When the crosslinking agent is included in an amount more than 10 parts by weight, processability may be deteriorated. Although a larger amount of crosslinking agent leads to the improvement of binding force and adhesion, the crosslinking agent may act as impunities, if the content is excessive, and deteriorate processability in large excess. Therefore, the crosslinking agent is preferably included in an amount from 1 to 10 parts by weight. [54] When the catalyst is included in an amount less than 1 part by weight, a wanted effect is not attained. That is, as in the cros slinking agent, when the catalyst is included in an amount less than 1 part by weight, the binding force between the curing agent and the base resin decreases, thereby resulting in poor coating property and burr generation. Although a larger amount of catalyst leads to the improvement of binding force, the binding force is not improved further when the amount of the catalyst exceeds 5 parts by weight. In excess of 5 parts by weight, the catalyst may act as impunities and deteriorate processability.

[55] Typically, the coating of the primer layer is performed by gravure coating, but bar coating, reverse coating, spray coating, etc. are possible. Here, drying is performed at 130 to 150 ⁰C for about 1 minute.

[56] The silicon adhesive layer is coated on the primer-treated base film by gravure coating, bar coating, reverse coating, spray coating, etc. to a thickness of 5 to 100 D. Drying is performed at 130 to 150 ⁰C for 1 minute. The viscosity is adjusted to 500 to 5000 cps. In order to attain the aforesaid coating thickness, the viscosity has to be at least 500 cps. A viscosity exceeding 5000 cps is unnecessary and is undesirable in cost.

[57] The release film is formed to a thickness of normally 5 to 10 D to protect the silicon- based adhesive layer. For the release film, it is preferable to use an inactivated film. It is because the adhesive included in the adhesive layer may migrate at the contact of the release film when the release film is activated. Accordingly, it is preferable to use an inactivated release film, for example non-corona treated PET film or non-corona treated biaxial oriented polypropylene (OPP) film.

[58] Next, the upper surface protective film is described in detail.

[59] Like in the lower surface protective film, in the base film of the upper surface protective film, for example, a polyester-based resin (PET, PEN), a polyolefin-based resin (PE, OPP), a poly amide-based resin (NY), a polyimide resin (PI), etc. may be used. The base film is formed to a thickness of 5 to 200 D such that the adhesive layer can be formed thereupon.

[60] The adhesive layer of the upper surface protective film may comprise an acryl- based adhesive or a silicon-based adhesive. An acryl-based adhesive composition is preferred because the silicon-based adhesive is expensive.

[61] The acryl-based adhesive composition is prepared by blending 100 parts by weight of an acrylic resin as the base resin, 1 to 10 parts by weight of an isocyanate-based curing agent and 20 to 50 parts by weight of a solvent such as ethyl acetate (EA).

[62] The adhesion of the adhesive layer is controlled by the blending proportion of the acrylic resin and the curing agent. Within the aforesaid content range, the adhesion decreases as the amount of the curing agent increases. As described earlier, since the roughness of the upper surface of the optical film is 6 to 15 D on the average, the adhesion of the lower surface protective film is preferably adjusted in the range from 7 to 24 gf. When the curing agent is included in an amount less than 1 part by weight, a wanted adhesion cannot be achieved. An amount exceeding 10 parts by weight is unnecessary and may cause a problem due to excessive adhesion. The solvent is included in an amount from 20 to 50 parts by weight, based on 100 parts by weight of the acryl- based adhesive.

[63] The acryl-based adhesive layer is coated on the base film by gravure coating, bar coating, reverse coating, spray coating, etc. to a thickness of 5 to 50 D . Drying is performed for example at 100 to 120 ⁰C for 30 seconds to 2 minutes.

[64] The release film of the upper surface protective film may be, for example, a silicon- coated PET film. The antistatic layer 7c illustrated in FIG. 3 may comprise 5 to 20 wt% of a conductive polymer (PEDOT/PSS), 10 to 20 wt% of 2-propanol and 50 to 80 wt% of water, and may be coated by gravure coating, micro gravure coating, reverse coating, kiss-roll coating, etc. Drying is performed at 100 to 120 ⁰C for 30 seconds to 2 minutes.

[65] If necessary, a color may be provided to the upper surface protective film for easier distinction from the lower surface protective film. For example, by printing with ink between the base film and the adhesive layer of the upper surface protective film, a color may be provided to the upper surface protective film.

[66] Hereinafter, the present invention will be described in detail with reference to examples. However, these examples are for the illustrative purpose only, and the invention is not limited by the examples.

[67] [Measurement of average surface roughness of optical film] [68] Diffusion sheets having the largest roughness (diffusion sheet A) and the smallest surface roughness (diffusion sheet B) were purchased from Kimoto (Japan), and the surface roughness was measured for 3 times.

[69] Surface roughness measurement results for upper surface and lower surface of diffusion sheet A is given in Table 1. [70] Table 1

[71] Surface roughness measurement results for upper surface and lower surface of diffusion sheet B is given in Table 2. [72] Table 2

[73] As can be seen Tables 1 and 2, the roughness of the lower surface was in the range from 0.5 to 5 D , and the roughness of the upper surface was in the range from 6 to 15 D

[74] [Construction of upper and lower surface protective films]

[75] A lower surface protective film was prepared by forming an adhesive layer on a

75-D thick base film (PET) and then forming a release film thereupon. The adhesive composition for the adhesive layer of the lower film was a silicon-based adhesive composition comprising:

[76] 80 parts by weight of polydimethylsiloxane (7226, Dow Corning) as base resin;

[77] 20 parts by weight of MQ resin (7657, Dow Corning);

[78] 2 parts by weight of a dimethylvinylsiloxane anchorage agent (9250, Dow

Corning);

[79] 0.5 part by weight of a polymethylhydrogensiloxane curing agent (7367, Dow

Corning);

[80] 0.5part by weight of platinum catalyst; and

[81] 100 parts by weight of toluene solvent.

[82] An upper surface protective film was prepared by forming an antistatic layer on the upper surface of a 25-D thick base film (PET), and forming an acryl-based adhesive layer on the lower surface and then forming a release film thereupon.

[83] The antistatic coating solution comprised:

[84] 15 wt% of PEDOT/PSS as conductive polymer;

[85] 15 wt% of 2-propanol; and

[86] 70 wt% of water.

[87] The adhesive composition of the acryl-based adhesive layer comprised:

[88] 100 parts by weight of an acrylic resin (BPS6024, Toyo Ink) as base resin;

[89] 5 parts by weight of an isocyanate-based curing agent (BXX5134, Toyo Ink); and

[90] 30 parts by weight of ethyl acetate solvent.

[91] The diffusion sheets A and B (Kimoto, Japan) were adhered on the lower surface protective film, and Thomson cutting was carried out. The coating amount of the adhesive layer of the lower surface protective film was adjusted such that the adhesion of each adhesive layer of the lower surface protective film adhered to the diffusion sheets A and B was 1 gf and 3 gf, respectively.

[92] On the other side of the diffusion sheets A and B, the upper surface protective film was adhered by applying a pressure of 2 kgf. The coating amount of the adhesive layer of the upper surface protective film was adjusted such that the adhesive layer of the surface protective film had different adhesion (1 to 30 gf). [93] [Measurement of adhesion] [94] Surface adhesion measurement results for the upper and lower films of the diffusion sheet B are given in Tables 3 and 4.

[95] The lower surface protective film could be adhered even when the adhesion was as low as 1 gf, because the lower surface of the diffusion sheet has a small roughness (0.5 to 5 D on the average). In contrast, an adhesion of at least 7 gf was required as for the upper surface of the diffusion sheet, because it had a large surface roughness (6 to 15 D on the average).

[96] Table 3

[97] Table 4

[98] Surface adhesion measurement results for the upper and lower films of the diffusion sheet A are given in Tables 5 and 6. [99] Table 5

[100] Table 6

[101] [Peeling, winding/unwinding and impurity removal test] [102] Three days later, the upper and lower surface protective films of the surface protective films of Examples and Comparative Example were peeled at a peeling speed of 300 mm/min and a peeling angle of 180°. Measurement was made for 10 times. When the diffusion sheet was peeled off along with the upper surface protective film in at least one of the 10 runs, a grade of "x" was given to denote poor peeling property. When the diffusion sheet was not peeled off along with the upper surface protective film in any of the 10 runs, a grade of "O" was given to denote superior peeling property.

[103] It was observed whether the upper surface protective film was peeled off during winding and unwinding. Experiment was performed for 10 times. When the upper surface protective film was peeled off at least one of the 10 runs, a grade of "x" was given to denote poor winding/unwinding property. When the upper surface protective film was not peeled off in any of the 10 runs, a grade of "O" was given to denote superior winding/unwinding property.

[104] After peeling-off of the upper surface protective film, the surface was observed with eyes. It was observed whether the white powders (burrs) occurred during Thomson cutting and impurities from the outside had been removed. Experiment was performed for 10 times. When impurities or burrs remained in at least one of the 10 runs, a grade of "x" was given to denote poor impurity removal property. When no impurities or burrs remained in any of the 10 runs, a grade of "O" was given to denote superior impurity removal property.

[105] Results are given in Table 7. [106] Table 7

[107] In Comparative Examples 7-10, 15 and 16, in which the adhesion of the upper surface protective film was larger than 8 times that of the lower surface protective film, the diffusion sheet was peeled off along with the upper surface protective film.

[108] In Comparative Examples 1-6 and 11-14, in which the adhesion of the upper surface protective film was smaller than 7 times that of the lower surface protective film, winding and unwinding operations were not easy because the adhesion of the upper surface protective film to the roughness of the upper surface of the diffusion sheet was weak. Further, the burrs occurred during Thomson cutting and impurities from external environment were not removed effectively. [109] In contrast, in Examples 1-6, in which the adhesion of the upper surface protective film was 7 to 8 times larger than that of the lower surface protective film, the diffusion sheet was not peeled off along with the upper surface protective film, and only the impurities were removed effectively.

[110] As described, the present invention is advantageous in that the impurities generated during the processing or transfer of the diffusion sheet can be effectively removed without the need of an additional process, by controlling the adhesion of the upper and lower surface protective films.

[I l l]

Industrial Applicability

[112] The surface protective film of the present invention in which the upper and lower films have controlled adhesion is advantageous in that the optical film to which the surface protective film is attached can be easily wound and unwound and effectively protected from external environment, and the impurities generated during the processing can be easily removed. Also, when the upper surface protective film having a large adhesion is removed during the processing,the optical film is not peeled off along with the upper surface protective film.

[113]

[114] While the present invention has been described with an emphasis upon preferred embodiments, it will be obvious to those skilled in the art that variations in the preferred embodiments may be used and that it is intended that the invention may be practiced otherwise than as specifically described herein. Accordingly, the present invention includes all modifications encompassed within the spirit and scope of the invention as defined by the claims that follow.

[115]

Claims

[1] A surface protective film comprising upper and lower surface protective films adhered above and below an optical film, wherein the adhesive layer of the upper surface protective film has an adhesion 7 to 8 times larger than the adhesion of the adhesive layer of the lower surface protective film.

[2] The surface protective film according to claim 1, wherein, when the roughness of the lower surface of an optical film is 0.5 to 5 D on the average, the adhesive layer of the lower surface protective film has an adhesion in the range from 1 to 3 gf and, when the roughness of the upper surface of an optical film ranges is 6 to 15 D on the average, the adhesive layer of the upper surface protective film has an adhesion in the range from 7 to 24 gf.

[3] The surface protective film according to claim 1, wherein the adhesive layer of the lower surface protective film comprises an adhesive composition comprising: 100 parts by weight of an acrylic resin as base resin; 1 to 20 parts by weight of an isocyanate-based curing agent; and 10 to 30 parts by weight of a commonly used solvent.

[4] The surface protective film according to claim 3, wherein the adhesive composition further comprises an antistatic agent.

[5] The surface protective film according to claim 1, wherein the adhesive layer of the lower surface protective film comprises an adhesive composition comprising: 70 to 100 parts by weight of polydimethylsiloxane as base resin; 10 to 40 parts by weight of MQ resin; and 100 to 150 parts by weight of a commonly used solvent.

[6] The surface protective film according to claim 5, wherein the adhesive composition further comprises 1 to 5 parts by weight of dimethylvinylsiloxane as anchorage agent.

[7] The surface protective film according to claim 5, wherein the adhesive composition further comprises 0.5 to 5 parts by weight of polymethylhydro- gensiloxane as cros slinking agent.

[8] The surface protective film according to claim 7, wherein the adhesive composition further comprises 0.5 to 2 parts by weight of tin or platinum catalyst.

[9] The surface protective film according to any of claims 5 to 8, wherein the lower surface protective film comprises: a base film; a primer layer formed on the base film; and an adhesive layer formed on the primer layer.

[10] The surface protective film according to claim 9, wherein the primer layer is a coating layer of a primer composition comprising 20 parts by weight of a solid base resin as silane coupling agent, 1 to 10 parts by weight of poly- organosiloxane as crosslinking agent, 1 to 5 parts by weight of a commonly used catalyst, and 80 to 380 parts by weight of a commonly used solvent, and the primer composition has a viscosity in the range from 10 to 300 cps.

[11] The surface protective film according to claim 1, wherein the adhesive layer of the upper surface protective film comprises an adhesive composition comprising: 100 parts by weight of an acrylic resin as base resin; 1 to 10 parts by weight of an isocyanate-based curing agent; and 20 to 50 parts by weight of a commonly used solvent.