WO2008007857A1

WO2008007857A1 - Melt-extrusion optical plastic sheet of improved dimension stability and liquid crystal device window including the same

Info

Publication number: WO2008007857A1
Application number: PCT/KR2007/002506
Authority: WO
Inventors: Jong Hun Lee; Saehan Cho; Joonhee Kim; Yong Gyun Cho
Original assignee: Lg Chem, Ltd.
Priority date: 2006-07-10
Filing date: 2007-05-25
Publication date: 2008-01-17
Also published as: KR20080005839A; KR100800971B1

Abstract

An optical plastic sheet is disclosed which includes a sheet substrate fabricated by melt-extruding an optical polymer, and hard coating layers applied to opposite surfaces of the sheet substrate to enhance the surface hardness of the sheet substrate, wherein an outer one of the hard coating layers has an anisotropic structure with respect to the inner hard coating layer to exhibit a resistance to warpage phenomenon causing the sheet to be warped in an inner surface direction of the sheet, and thus to prevent the sheet from being warped in the inner surface direction of the sheet. A liquid crystal device window attached with the optical plastic sheet is also disclosed. The optical plastic sheet suppresses warpage phenomenon of the sheet substrate fabricated in accordance with a melt extrusion method, and thus achieves a great enhancement in the dimension stability of the sheet.

Description

MELT-EXTRUSION OPTICAL PLASTIC SHEET OF IMPROVED DIMENSION STABILITY AND LIQUID CRYSTAL

DEVICE WINDOW INCLUDING THE SAME

Technical Field

[1] The present invention relates to a melt-extruded optical plastic sheet with an improved dimension stability and a liquid crystal device window including the same. In particular, the present invention relates to an optical plastic sheet which includes a sheet substrate fabricated by melt-extruding an optical polymer, and hard coating layers applied to the opposite surfaces of the sheet substrate to enhance the surface hardness of the sheet substrate, the outer one of the hard coating layers having an anisotropic structure with respect to the inner hard coating layer, to exhibit a resistance to warpage phenomenon causing the sheet to be warped in the inner surface direction of the sheet, and thus to prevent the sheet from being warped in the inner surface direction of the sheet. The present invention also relates to a liquid crystal device window to which the optical plastic sheet having the above-described structure is attached. Background Art

[2] In accordance with increased use of mobile phones, notebook computers, portable data communication terminals, etc., the use of optical plastic sheets for liquid crystal device windows in such devices has also increased. Although a glass substrate have conventionally been used for the material of liquid crystal device windows, it has drawbacks in that glass may be easily damaged by impact due to insufficient impact resistance, and there are limitations in thinness and lightness. In order to solve these problems, recently, an optical plastic sheet having superior impact resistance and capable of having a light structure has mainly been used as the material of liquid crystal device windows.

[3] For a method for manufacturing such an optical plastic sheet, there is a solution casting method, a blow molding method, or a melt extrusion method. Most of the current commercially-available products are manufactured using the melt extrusion method.

[4] The solution casting method is a batch type manufacturing method in which a resin is dissolved in a solvent to prepare a solution, the solution is applied to a caster, and is then evaporated to form a film or sheet. Although the sheet manufactured using the solution casting method has the advantages of superior dimension stability and impact strength, there is a drawback of inferior surface characteristics. Furthermore, since it is necessary to use the solvent in a large amount, there is the trouble that a large-amount solvent should be evaporated. In addition, there are environmental problems such as air pollution in the work space due to the evaporated solvent. Since this method is of a batch type, the installation investment is high, whereas the productivity is low. As a result, there is the drawback of high manufacturing costs.

[5] On the other hand, the melt extrusion method, which is another method for manufacturing an optical plastic sheet, is a method in which a sheet-shaped product in a semi-melted (semi-solidified) state is extruded by an extruder mounted with a T-die, a coat hanger die, etc., and is then cooled by cooling rolls, to continuously manufacture a sheet. This melt extrusion method has no problem associated with the trouble of the solvent removal process and environmental pollution pointed out as the drawbacks of the solution casting method. The melt extrusion method has an advantage in that it is possible to remarkably reduce the manufacturing costs because the manufacture of the product can be continuously carried out.

[6] However, the sheet manufactured in accordance with the melt extrusion method may involve warpage phenomenon that the sheet is warped in a high temperature/high moisture environment. As a result, there may be a serious problem in that the dimension stability of the sheet is relatively low.

[7] For this reason, there is an increased demand for development of a plastic sheet manufacturing method that uses the melt extrusion method to compensate for the drawbacks of the solution casting method while providing superior dimension stability.

[8] To this end, the present invention proposes an optical plastic sheet in which hard coating layers laminated on opposite surfaces of an optical plastic substrate fabricated in accordance with a melt extrusion method to provide a desired surface hardness have anisotropic structures, as described later.

[9] Meanwhile, techniques for additionally forming a polymer resin layer over an optical plastic film fabricated in accordance with a melt extrusion method have also been known. For example, Japanese Patent Unexamined Publication No. 1997-123275 and Japanese Patent No. 3040512 disclose a technique for coating an organic material layer on at least one surface of an optical film fabricated in accordance with a melt extrusion method, to cross-link the optical film, and polishing the surface of the cross-linked film by rotating rolls, thereby manufacturing a heat-resistant optical film. The disclosed technique is adapted to polish the surface of an ultraviolet-curing organic material layer for an improvement in smoothness.

[10] However, it was confirmed that, even in the case of an optical plastic sheet manufactured to have a certain thickness in accordance with the above-mentioned techniques, it is impossible to prevent warpage phenomenon. That is, it was confirmed that there is no technique capable of manufacturing a sheet involving no warpage phenomenon, and thus having improved dimension stability while using the melt extrusion method.

[11] Meanwhile, Korean Patent Unexamined Publication No. 2004-38698 discloses a film which comprises a methacryl resin layer containing rubber particles so as to have scratch-resistance, and a hardening layer formed over at least surface of the film and made of a hardening paint, and has a thickness of 100 to 1,800 D. However, the hardening layer, which functions to secure scratch-resistance, can be formed on only one surface of the film. Accordingly, it is possible to prevent warpage phenomenon from the hardening layer. This technique is different from the present invention in which upper and lower hard coating layers are formed in such a manner that they have different curing degrees, to have having anisotropic structures, and thus to prevent warpage phenomenon, as described later. Disclosure of Invention Technical Problem

[12] Therefore, the present invention has been made to solve the above problems, and other technical problems that have yet to be resolved.

[13] After various experiments and active research, the inventors found that, when hard coating layers applied to the opposite surfaces of a sheet substrate fabricated by melt- extruding an optical polymer, to enhance the surface hardness of the sheet substrate, are formed such that they have anisotropic structures by specifically configuring the hard coating layer arranged opposite to the warping direction of warpage phenomenon, it is possible to completely prevent the occurrence of the warpage phenomenon. Thus, the inventors completed the present invention.

Technical Solution

[14] The present invention provides an optical plastic sheet comprising a sheet substrate fabricated by melt-extruding an optical polymer, and hard coating layers applied to the opposite surfaces of the sheet substrate to enhance the surface hardness of the sheet substrate, wherein an outer one of the hard coating layers has an anisotropic structure with respect to the inner hard coating layer to exhibit a resistance to warpage phenomenon causing the sheet to be warped in an inner surface direction of the sheet, and thus to prevent the sheet from being warped in the inner surface direction of the sheet.

[15] The optical plastic sheet of the present invention can suppress the occurrence of warpage phenomenon and thus can achieve a great improvement in dimensional stability because the hard coating layers of the sheet have anisotropic structures. For example, in accordance with the present invention, it is possible to prevent warpage phenomenon from occurring in the sheet substrate in the vicinity of a glass transition temperature (Tg) during the manufacturing process using the melt extrusion method, and thus to achieve an improvement in the dimensional stability of the sheet by controlling the thicknesses or hardnesses of the hard coating layers such that the outer hard coating layer has a larger thickness or hardness than the inner hard coating layer.

[16] Generally, the sheet substrate is very vulnerable to surface scratches. For this reason, hard coating layers are applied to the opposite surfaces of the sheet substrate to secure a desired surface hardness of the sheet, and thus to achieve an improvement in scratch resistance.

[17] Such hard coating layers may be formed using various methods such as flow coating, spin coating, dip coating, or bar coating. The process for drying the hard coating layers is very important because the process must involve a cross-linking process. The drying process may be achieved by performing a thermal curing process at a certain temperature or a photo-curing process after coating a coating solution over the substrate. In the photo-curing process, ultraviolet rays are irradiated on the coating solution.

[18] Preferably, the hard coating layers are formed by coating a coating solution containing a ultraviolet (UV)-curing material over the sheet substrate in accordance with a flow coating method, and then irradiating UV rays on the coating solution. Here, "flow coating" means that the coating solution is coated over the sheet substrate in the form of a liquid film. Also, taking into consideration the fact that the sheet substrate may be subjected to warpage phenomenon in the vicinity of the glass transition temperature (Tg), it is preferred that the UV curing method be used, rather than the thermal curing method.

[19] The hard coating layers should have a certain surface hardness. In this regard, the

UV-curing material contained in the material of the hard coating layers may be a composition including a urethane-based material, an acryl-based material, or a thermoplastic elastomer. Preferably, the UV-curing material is a composition including a urethane-based material.

[20] When the hard coating layers are excessively thin, it is impossible to obtain a desired enhancement in surface hardness due to a reduction in wear resistance. On the other hand, when the hard coating layers are excessively thick, there may be a possibility that the hard coating layers may be peeled off or cracked due to the stress thereof. Therefore, it is preferred that the hard coating layers have a thickness of 2 to 20 D. Since the thickness of each hard coating layer may vary depending on the coating method used, the coating process may be conducted several times when the thickness of the hard coating layer is insufficient.

[21] As described above, the sheet substrate fabricated in accordance with the melt extrusion method may be subjected to warpage phenomenon in the vicinity of the temperature Tg. The warpage phenomenon is a phenomenon that the sheet sequentially passing through a T-die and cooling rolls is warped in both a contact surface direction of the cooling rolls and an MD direction of the sheet.

[22] In this connection, FlG. 1 schematically illustrates a procedure in which a sheet substrate fabricated while passing through a T-die in accordance with a melt extrusion method passes through a cooling roll. For the convenience of the description, the case, in which the sheet substrate is formed with hard coating layers at opposite surfaces thereof, is illustrated.

[23] Referring to FlG. 1, the sheet substrate 110 fabricated while passing through the T- die is solidified by the cooling roll 200. Although not shown in FlG. 1, the above procedure is repeatedly conducted in an alternating manner for the opposite surfaces of the sheet substrate 110, using a plurality of cooling rolls 200.

[24] In a subsequent process, hard coating layers 120 and 130 are applied to the opposite surfaces of the sheet substrate 100, in order to increase the surface hardness of the final product. Thus, a plastic sheet 100 is manufactured.

[25] When it is assumed that the contact surface direction of the cooling roll 200 is defined as an inner surface direction, the plastic sheet 100, in particular the sheet substrate 110, is subjected to warpage phenomenon causing the sheet substrate 110 to be warped in the inner surface direction of the sheet substrate 110. That is, the phenomenon A causing the sheet substrate 110 to be warped in the contact surface direction of the cooling roll 200 and the phenomenon B causing the sheet substrate 110 to be warped in the MD direction of the sheet 100 occur simultaneously.

[26] Such warpage phenomenon is considered to be a serious problem in manufacturing a plastic sheet for an optical substrate in accordance with a melt extrusion method.

[27] Accordingly, in order to prevent such warpage phenomenon from occurring, the optical plastic sheet of the present invention is manufactured to have an anisotropic structure. In a preferred embodiment of such an anisotropic structure, the hard coating layers are configured such that the outer hard coating layer has a larger thickness that the inner hard coating layer.

[28] The hard coating layers have higher warpage resistance than the sheet substrate. Accordingly, when the outer hard coating layer facing the contact surface of the cooling roll has a larger thickness that the inner hard coating layer, it exhibits high deformation resistance increasing proportional to the thickness increase. Thus, it is possible to prevent the occurrence of warpage phenomenon. The thickness of the hard coating layers can be controlled by controlling the amount of the coating solution for each hard coating layer and the number of repetition times of the coating process.

[29] When the thickness of the outer hard coating layer and the thickness of the inner hard coating layer are expressed using a thickness ratio, they have a thickness ratio of 1 : 1 to 1: 10. When the thickness ratio of the hard coating layers is excessively low, the resistance of the hard coating layers to deformation of the sheet is insufficient. In this case, it is impossible to obtain a desired warpage phenomenon preventing effect. On the other hand, when the thickness ratio is excessively high, the outer hard coating layer is unnecessarily thickened, and total sheet thickness becomes excessively large. A more preferable thickness ratio is 1: 1.3 to 1: 8.

[30] In another preferred embodiment, the hard coating layers may be configured such that the outer hard coating layer has a higher curing degree that the inner hard coating layer. The higher the curing degree, the higher the surface hardness. Accordingly, the outer hard coating layer prevents the sheet substrate from being deformed toward the inner hard coating layer having a relatively low curing degree, and thus prevents the occurrence of warpage phenomenon. The curing degree can be controlled by controlling the UV irradiation time when a UV curing method is used. When the UV irradiation time increases, the curing degree increases.

[31] Preferably, the ratio between the curing degrees of the outer and inner hard coating layers is 1: 1 to 1: 10. When the curing degree ratio of the hard coating layers is excessively low, it is impossible to sufficiently prevent the sheet substrate from being deformed. On the other hand, when the curing degree ratio is excessively high, the curing time taken to obtain an increased curing degree of the outer hard coating layer is excessively long. As a result, a degradation in productivity occurs. A more preferable ratio between the curing degrees is 1: 1.3 to 1: 8

[32] Preferably, the polymer for the sheet substrate is a polymer having optical characteristics including transparency as a basic physical property. For example, the polymer comprises a thermoplastic resin such as polymethylmethacrylate (PMMA), polycarbonate (PC), polyimide (PI), polyethersulfone (PES), polyarylate (PAR), polyethylene naphthalate (PEN), or polyethylene terephthalate (PET), a thermosetting resin such as epoxy or unsaturated polyester, acryl-butadiene-styrene (ABS), or a blend thereof which exhibits superior optical characteristics. Of course, the polymer is not limited to the above-described materials. Preferably, the polymer comprises a ho- mopolymer, copolymer, or a blend thereof selected from the group comprising PMMA, PC, PES, and ABS which exhibit superior transparency and homogeneity.

[33] In a preferred embodiment, the sheet may further comprise a high-strength resin layer having a higher molecular weight than the sheet substrate, so as to enhance mechanical properties. The high-strength resin layer is formed between the sheet substrate and each hard coating layer.

[34] The high-strength resin layer may be formed using a method in which a melted coating resin is applied, and is then solidified, a method in which a coating resin is applied in the form of a composition dissolved in a solvent, and the solvent is then removed, or a method in which a coating resin containing a monomer, oligomer, or low-molecular- weight polymer having a certain viscosity is applied, and then is cross- linked or cured. Of course, various methods other than the above-described methods may be used, and they should be interpreted as falling under the category of the present invention.

[35] The coating resin coated as described above may have an increased molecular weight, preferably using a thermal curing method or a UV curing method. Since the thermal curing method and UV curing method are known in the technical field, no detailed description will be given.

[36] The polymer for the sheet substrate has a molecular weight of 50,000 to 200,000 approximate to a limit for melt extrusion. Any polymer may be used for the polymer of the high-strength resin layer, as long as the polymer has, in a cured state, a higher molecular weight than the polymer for the sheet substrate. However, it is preferred that the polymer for the high-strength resin layer have a molecular weight of 150,000 or more which is in the range larger than the molecular weight of the polymer for the sheet substrate, because the attraction between molecules increases at a higher molecular weight.

[37] Any resin may be used for the coating resin for the high-strength resin layer, as long as the resin exhibits superior properties in terms of impact resistance, thin film formability, and flexibility. For example, urethane acrylate or polyester acrylate may be used for the coating resin.

[38] In particular, urethane-based resin has a UV-curing functional group, and can function to increase the elongation of the resin after being cured, and thus to reduce external impact. Accordingly, in a preferred embodiment, the coating resin may contain a urethane-based urethane acryl monomer, oligomer, or polymer of a low degree of polymerization.

[39] In a preferred embodiment, the high-strength resin layer may be formed by coating a urethane acryl-based oligomer having the degree of polymerization of 500 to 5,000 over the opposite surfaces of the sheet substrate, and irradiating UV rays on the coating, to photo-cure the coating. A solvent such as photo-initiator may also be contained in the coating resin.

[40] In the optical plastic sheet according to the present invention, the thickness of the sheet substrate may vary depending on the application of the optical plastic sheet. Preferably, the thickness of the sheet substrate is 500 to 1,500 D. When the sheet substrate has a thickness of 500 to 1,500 D, the high-strength resin layer may have a thickness of 1 to 300 D. When the optical plastic sheet and high-strength resin layer are excessively thin, it is difficult to expect an improvement in mechanical properties such as shieldability against certain materials such as liquid crystal and impact resistance. On the other hand, when the optical plastic sheet and high-strength resin layer are excessively thick, a degradation in workability and an increase in the thickness of the sheet may occur. [41] In order to provide an adhering force to the sheet substrate, the hard coating layers may further contain a silicon-based material. [42] The present invention also provides a liquid crystal device window manufactured using the above-described optical plastic sheet. The liquid crystal device window according to the present invention may have various structures depending on the type of the device. Accordingly, there is no particular limitation on the structure of the liquid crystal device window. [43] The present invention also provides a mobile device including the above-described liquid crystal device window. [44] Representative examples of the mobile device include a mobile phone, a notebook computer, a portable data communication terminal, etc. Since detailed structures and manufacturing methods of these devices are known in the technical field, no detailed description thereof will be given.

Brief Description of the Drawings [45] The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: [46] FlG. 1 is a schematic view illustrating warpage phenomenon occurring during a process of fabricating an optical plastic sheet in accordance with a melt extrusion method; [47] FIGS. 2 and 3 are schematic views illustrating the cross-sectional structures of optical plastic sheets according to different embodiments of the present invention; and [48] FIGS. 4 and 5 are schematic views illustrating the cross-sectional structures of optical plastic sheets according to embodiments of the present invention different from the above-described embodiments.

Mode for the Invention [49] Hereinafter, the present invention will be described in more detail with reference to the drawings associated with the embodiments of the present invention. Of course, the present invention is not limited to the following description. [50] FIGS. 2 and 3 schematically illustrate the cross-sectional structures of optical plastic sheets according to different embodiments of the present invention, respectively. [51] As shown in FlG. 2 or 3, the optical plastic sheet 300 or 301 has a structure in which hard coating layers 320 and 330 or 321 and 331 are laminated over opposite surfaces of a sheet substrate 310 or 311. That is, the inner hard coating layer 320 or 321, sheet substrate 310 or 311, and outer hard coating layer 330 or 331 are laminated in this order starting from the contact surface of the optical plastic sheet 300 or 301 contacting a cooling roll (not shown). [52] For the sheet substrate 310 or 311 polymethylmethacrylate (PMMA) exhibiting superior optical transparency and homogeneity is generally used. The sheet substrate

310 or 311 is fabricated in accordance with a melt extrusion method. For this reason, when the sheet substrate 301 or 311 is fabricated in accordance with the melt extrusion method, there is a limitation in melt-extruding a polymer having a high molecular weight due to a torque applied to the extruder.

[53] The hard coating layers 320 and 330 or 31 and 331 are formed to secure a desired surface hardness of the sheet because the sheet substrate 310 or 311 is vulnerable to surface scratches. The hard coating layers 320 and 330 or 321 and 331 may be formed by coating a coating solution containing a ultraviolet (UV)-curing material over the sheet substrate 310 or 311 in accordance with a flow coating method, and then irradiating UV rays on the coating solution to UV-cure the coating solution. Generally, the UV-curing material is a urethane-based material. The UV-curing material may contain a small amount of a silicon-based material in order to achieve an enhancement in the adhering force to the sheet substrate 310 or 311.

[54] In particular, in order to prevent the occurrence of warpage phenomenon at the sheet substrate 310 or 311 fabricated in accordance with the melt extrusion method, the hard coating layers 320 and 330 are configured such that the outer hard coating layer 330 has a larger thickness than the inner hard coating layer 320. Alternatively, the hard coating layers 321 and 331 are configured such that the outer hard coating layer 331 has a higher curing degree than the inner hard coating layer 321. The outer hard coating layer 330 or 331 having a relatively large thickness or a relatively high curing degree generates high resistance when the sheet substrate 310 or 311 warps in the inner surface direction, thereby preventing the occurrence of warpage phenomenon.

[55] FIGS. 4 and 5 schematically illustrate the cross-sectional structures of optical plastic sheets according to embodiments of the present invention different from the above- described embodiments.

[56] As shown in FIG. 4 or 5, the optical plastic sheet 302 or 303 has a structure in which high-strength resin layers 342 and 352 or 343 and 353 are laminated over opposite surfaces of a sheet substrate 312 or 313. In this case, hard coating layers 322 and 332 or 323 and 333 are laminated over the opposite surfaces of the sheet substrate 310 or

311 or over the outer surfaces of the high-strength resin layers 342 and 352 or 343 and 353. That is, the inner hard coating layer 322 or 323, high-strength resin layer 342 or 343, sheet substrate 312 or 313, high-strength resin layer 352 and 353, and outer hard coating layer 330 or 331 are laminated in this order starting from the contact surface of the optical plastic sheet 332 or 333 contacting the cooling roll. Although not shown, only one of the high-strength resin layers 342 and 352 or 343 and 353 may be selectively laminated over the associated surface of the sheet substrate 312 or 313.

[57] In order to compensate for the mechanical properties, for example impact resistance, of the sheet substrate 312 or 313 fabricated in accordance with the melt extrusion method, the high-strength resin layers 342 and 352 or 343 and 353 should have a larger molecular weight than the sheet substrate 312 or 313.

[58] In order to configure the high-strength resin layer 342 and 352 or 343 and 353 to have a high molecular weight, a coating resin containing a urethane acrylate monomer or oligomer may be used. In this case, the coating resin is coated over the sheet substrate 312 or 313, and is then polymerized in accordance with a UV curing process.

[59] Similarly to the case of FlG. 2 or 3, it is possible to prevent the occurrence of warpage phenomenon causing the sheet substrates 312 or 313 to be warped in the inner surface direction in the case of FlG. 4 or 5 because the outer hard coating layer 332 or 333 is configured to have a larger thickness or higher curing degree than the inner hard coating layer 322 or 323.

[60] Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims. Industrial Applicability

[61] As apparent from the above description, the optical plastic sheet according to the present invention can suppress the occurrence of warpage phenomenon, and thus can achieve a great improvement in dimensional stability by forming hard coating layers applied to the opposite surfaces of a sheet substrate fabricated in accordance with melt extrusion of an optical polymer, so as to increase the surface hardness of the sheet substrate such that the hard coating layers have anisotropic structures.

Claims

[I] An optical plastic sheet comprising a sheet substrate fabricated by melt-extruding an optical polymer, and hard coating layers applied to opposite surfaces of the sheet substrate to enhance the surface hardness of the sheet substrate, wherein an outer one of the hard coating layers has an anisotropic structure with respect to the inner hard coating layer to exhibit a resistance to warpage phenomenon causing the sheet to be warped in an inner surface direction of the sheet, and thus to prevent the sheet from being warped in the inner surface direction of the sheet.

[2] The optical plastic sheet according to claim 1, wherein the hard coating layers are formed by coating a coating solution containing an ultraviolet (UV)-curing material in accordance with a flow coating method, and irradiating UV rays on the coated coating solution. [3] The optical plastic sheet according to claim 2, wherein the UV-curing material comprises a urethane-based material. [4] The optical plastic sheet according to claim 1, wherein the hard coating layers have a thickness of 2 to 20 D. [5] The optical plastic sheet according to claim 1, wherein the outer hard coating layer has a larger thickness than the inner hard coating layer so as to obtain the anisotropic structure. [6] The optical plastic sheet according to claim 5, wherein the outer and inner hard coating layers have a thickness ratio of 1: 1 to 1: 10. [7] The optical plastic sheet according to claim 1, wherein the outer hard coating layer has a higher curing degree than the inner hard coating layer so as to obtain the anisotropic structure. [8] The optical plastic sheet according to claim 7, wherein the outer and inner hard coating layers have a curing degree ratio of 1: 1 to 1: 10. [9] The optical plastic sheet according to claim 1, wherein the polymer for the sheet substrate comprises a polymer having optical characteristics including transparency as a basic physical property. [10] The optical plastic sheet according to claim 9, wherein the polymer comprises a homopolymer, a copolymer, or a blend thereof selected from the group comprising polymethylmethacrylate (PMMA), polycarbonate (PC), polyethersulfone (PES), and acryl-butadiene-styrene (ABS).

[II] The optical plastic sheet according to claim 1, further comprising: a high-strength resin layer formed between the sheet substrate and each hard coating layer to enhance mechanical properties, the high-strength resin layer having a higher molecular weight than the sheet substrate. [12] The optical plastic sheet according to claim 11, wherein the high-strength resin layer is formed by coating a coating resin over the opposite surfaces of the sheet substrate, and thermally curing or ultraviolet (UV)-curing the coated coating resin such that the coating resin has an increased molecular weight.

[13] The optical plastic sheet according to claim 11, wherein the polymer for the sheet substrate has a molecular weight of 50,000 to 200,000 approximate to a limit for melt extrusion, and the polymer for the high-strength resin layer has, in a cured state, a molecular weight of 150,000 or more which is in the range larger than the molecular weight of the polymer for the sheet substrate.

[14] The optical plastic sheet according to claim 11, wherein the coating resin for the high-strength resin layer comprises a urethane acryl monomer, oligomer, or polymer of a low degree of polymerization.

[15] The optical plastic sheet according to claim 14, wherein the high-strength resin layer is formed by coating a urethane acryl-based oligomer having the degree of polymerization of 500 to 5,000 over the opposite surfaces of the sheet substrate, and irradiating ultraviolet (UV) rays on the coating, to cure the coating.

[16] The optical plastic sheet according to claim 11, wherein the sheet substrate has a thickness of 500 to 1,500 D, and the high-strength resin layer has a thickness of 1 to 300 D.

[17] The optical plastic sheet according to claim 16, wherein the hard coating layers further contain a silicon-based material to provide an adhering force to the sheet substrate.

[18] A liquid crystal device window comprising a plastic sheet according to any one of claims 1 to 17.

[19] A mobile device comprising a liquid crystal device window according to claim

18.