WO2008007858A1

WO2008007858A1 - Melt-extrusion optical plastic sheet having resin layer of high molecular weight and liquid crystal device window including the same

Info

Publication number: WO2008007858A1
Application number: PCT/KR2007/002507
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: KR20080005838A; KR100829397B1

Abstract

An optical plastic sheet is disclosed which includes a sheet substrate fabricated by melt-extruding an optical polymer having a certain molecular weight, and high-strength resin layers coated over opposite surfaces of the sheet substrate to enhance mechanical properties, the high-strength resin layers having a higher molecular weight than the polymer. A liquid crystal device window attached with the optical plastic sheet is also disclosed. The optical plastic sheet has mechanical properties similar to those of a plastic sheet manufactured in accordance with a solution casting method by compensating for vulnerableness of mechanical properties such as impact resistance incurred in the optical plastic sheet manufactured in accordance with a melt extrusion method. The optical plastic sheet is also excellent in terms of manufacturing process economy and surface characteristics.

Description

MELT-EXTRUSION OPTICAL PLASTIC SHEET HAVING

RESIN LAYER OF HIGH MOLECULAR WEIGHT AND LIQUID

CRYSTAL DEVICE WINDOW INCLUDING THE SAME

Technical Field

[1] The present invention relates to a melt-extruded optical plastic sheet including a resin layer of high molecular weight 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 a polymer having a certain molecular weight, and high-strength resin layers coated over the opposite surfaces of the sheet substrate while having a higher molecular weight than the polymer so as to enhance the mechanical properties of the sheet substrate. The present invention also relates to a liquid crystal device window including the optical plastic sheet.

[2] The optical plastic sheet according to the present invention has mechanical properties similar to those of a plastic sheet manufactured in accordance with a solution casting method and is excellent in terms of economy and surface characteristics by compensating for vulnerableness of mechanical properties such as impact resistance incurred in the optical plastic sheet manufactured in accordance with a melt extrusion method. Background Art

[3] 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.

[4] 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.

[5] 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.

[6] 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.

[7] However, the melt extrusion method has problems of relatively-low mechanical properties in terms of, for example, dimension stability and impact strength. The mechanical properties of a plastic sheet are closely connected with the molecular weight of the plastic resin of the plastic sheet. In this regard, for the method for enhancing such mechanical properties, use of a polymer having a high molecular weight may be preferentially taken into consideration. In the melt extrusion method, however, there is a limitation that it is impossible to use a polymer having a high molecular weight applicable to the solution casting method, due to a torque applied to an extruder used in the melt extrusion method.

[8] For this reason, there is an increased demand for development of a plastic sheet exhibiting excellent mechanical properties while being manufactured in accordance with the melt extrusion method.

[9] To this end, the present invention proposes an optical plastic sheet in which polymer resin layers having a high molecular weight are formed over the opposite surfaces of an optical plastic substrate fabricated in accordance with a melt extrusion method, as described later.

[10] 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 No. 3463968 discloses a technique associated with a transparent conductive film for a liquid crystal display which has a gas shieldability having no dependency on temperature and moisture. In accordance with the disclosed technique, the transparent conductive film is formed by forming an organic resin layer containing a silane coupling agent on at least one surface of a transparent plastic substrate, coating a hydrolyzing solution comprising tetraalkoxy silane and one or more of metal alkoxide and phosphate ester over the organic resin layer, to form an inorganic hardening layer, laminating a thin transparent film over the organic hardening layer, and coating an ultraviolet (UV)-curing acrylate resin hardening layer over the thin transparent film, to enhance the adhering force between the inorganic hardening layer and the thin film.

[11] 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.

[12] 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 difficult to obtain desired mechanical properties such as excellent impact resistance. That is, it was confirmed that there is no technique capable of manufacturing a sheet having excellent mechanical properties while using the melt extrusion method.

Disclosure of Invention Technical Problem

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

[14] After various experiments and active research, the inventors found that, in the case of an optical plastic sheet including a sheet substrate fabricated by melt-extruding an optical polymer having a certain molecular weight, and high-strength resin layers coated over the opposite surfaces of the sheet substrate while having a higher molecular weight than the polymer, the optical plastic sheet has very excellent mechanical properties similar to an optical plastic sheet manufactured in accordance with a solution casting method. Thus, the inventors completed the present invention.

Technical Solution

[15] The present invention provides an optical plastic sheet comprising a sheet substrate fabricated by melt-extruding an optical polymer having a certain molecular weight, and high-strength resin layers coated over the opposite surfaces of the sheet substrate while having a higher molecular weight than the polymer so as to enhance mechanical properties. [16] Since the optical plastic sheet of the present invention includes the high-strength resin layers, it is possible to completely solve the problem associated with vul- nerableness of mechanical properties incurred when the optical plastic sheet is manufactured in accordance with a melt extrusion method, as mentioned above. In detail, the sheet substrate fabricated in accordance with the melt extrusion method making it impossible to fabricate the sheet substrate using a polymer having a high molecular weight is compensated to have enhanced mechanical properties, for example enhanced impact strength, by coating a polymer having a higher molecular weight than the polymer of the sheet substrate.

[17] The high-strength resin layers 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.

[18] In a preferred embodiment, the high-strength resin layers may have an increased molecular weight as the coating resin coated over the opposite surfaces of the sheet substrate to form the high-strength resin layers are cured. When such a high-strength resin layer is formed on only one surface of the sheet substrate, it may be impossible to obtain sufficient mechanical properties, or warpage phenomenon may occur in the sheet substrate. Accordingly, it is preferred that such a high-strength resin layer be formed on both surfaces of the sheet substrate.

[19] For the method for curing the coated coating resin, a thermal curing method or a UV curing method may be used. Since the thermal curing method and UV curing method are known in the technical field, no detailed description will be given.

[20] 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.

[21] The polymer for the sheet substrate has a molecular weight of 50,000 to 200,000 ap- proximate 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.

[22] When the molecular weight of the polymer for the sheet substrate and the molecular weight of the polymer for the high-strength resin layer in a cured state within the above-described molecular weight ranges are expressed using a molecular weight ratio, the molecular weight ratio is preferably 120,000: 200,000 to 170,000: 250,000.

[23] When the molecular weight ratio is excessively low, it is difficult to obtain desired mechanical properties and impact strength because this molecular weight ratio means that the molecular weight of the polymer for the high-strength resin layer in a cured state approximates to the molecular weight of the polymer for the sheet substrate. On the other hand, when the molecular weight ratio is excessively high, the contact surfaces of the sheet substrate and high-strength resin layer may not be easily bonded to each other. A degradation in surface characteristics may occur. Furthermore, the curing time taken to cure the high-strength resin layer such that the high-strength resin layer has a high molecular weight is excessively long. As a result, a degradation in productivity occurs.

[24] 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.

[25] 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, and/or polymer of a low degree of polymerization.

[26] More preferably, 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 UV rays on the coating, to photo-cure the coating. A solvent such as photo-initiator may also be contained in the coating resin.

[27] 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.

[28] In a preferred embodiment, a hard coating layer may be applied to the outer surface of each high-strength resin layer, in order to enhance the surface hardness of the optical plastic sheet. Since the hard coating layer should exhibit excellent resistance to external environment, for example resistance to scratches, it should have sufficient hardness and weather resistance. 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.

[29] 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 involves a cross-linking process. Accordingly, 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.

[30] 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 deformed due to heat, it is preferred that the UV curing method be used, rather than the thermal curing method.

[31] Taking into consideration wear resistance, surface hardness, peel-off or cracking of the hard coating layer, it is preferred that the hard coating layer have a thickness of 2 to 20μm. Since the thickness of the 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.

[32] In order to provide an adhering force to the surface of the high-strength resin layer, the hard coating layer may further contain a silicon-based material.

[33] 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. [34] The present invention also provides a mobile device including the above-described liquid crystal device window. [35] 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

[36] 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:

[37] FlG. 1 is a schematic view illustrating a procedure in which a sheet substrate fabricated through a T-die in accordance with a melt extrusion method passes through cooling rolls in accordance with an embodiment of the present invention;

[38] FlG. 2 is a schematic view illustrating the cross-sectional structure of an optical plastic sheet according to an embodiment of the present invention; and

[39] FlG. 3 is a schematic view illustrating the cross-sectional structure of an optical plastic sheet according to another embodiment of the present invention. Mode for the Invention

[40] 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.

[41] FlG. 1 schematically illustrates a procedure for manufacturing an optical plastic sheet using a melt extrusion method in accordance with an embodiment of the present invention. FlG. 2 schematically illustrates the cross-sectional structures of an optical plastic sheet according to an embodiment of the present invention.

[42] As shown in the drawings, a sheet substrate 110, which is fabricated in a semi-melted

(semi-solidified) state through a T-die (not shown), is solidified by cooling rolls 210. Although not shown in FlG. 1, the solidification process is repeatedly carried out for the opposite surfaces of the sheet substrate 110 in an alternating manner, using a plurality of cooling rolls 210.

[43] In a subsequent process, high-strength resin layers 120 and 130 are laminated over the opposite surfaces of the sheet substrate 110, in order to achieve an enhancement in impact resistance. Thus, an optical plastic sheet 100 is completely manufactured. In the optical plastic sheet 100, the high-strength resin layer 120, sheet substrate 110, and high-strength resin layer 130 are laminated in this order starting from the contact surface of the optical plastic sheet 100 contacting the cooling rolls 210.

[44] For the sheet substrate 110 polymethylmethacrylate (PMMA) exhibiting superior optical transparency and homogeneity is generally used. As described above, when a sheet is fabricated using such a polymer in accordance with a melt extrusion method, it is necessary to take into consideration the torque applied to an extruder by the melt. For this reason, it is impossible to increase the molecular weight of the polymer to a certain level.

[45] On the other hand, in accordance with the present invention, a coating resin containing a polymer having a low degree of polymerization (for example, urethane acryl-based monomer, oligomer, etc.) is coated over one or both surfaces of the sheet substrate 110, and is then UV-cured. Thus, high-strength resin layers 120 and 130 having a higher molecular weight than the sheet substrate 110 can be formed over the sheet substrate 110. The high-strength resin layers 120 and 130 can compensate for the mechanical properties of the sheet substrate 110 which has a limitation in molecular weight due to the manufacture thereof according to the melt extrusion method. Desirably, the high-strength resin layers 120 and 130 can also minimize distortion phenomenon generally occurring in the sheet substrate 110, so called "warpage phenomenon".

[46] FIG. 3 schematically illustrates the cross-sectional structure of an optical plastic sheet according to another embodiment of the present invention.

[47] As shown in FIG. 3, the optical plastic sheet 101 has a structure in which high- strength resin layers 120 and 130 are laminated over the opposite surfaces of a sheet substrate 110, and hard coating layers 140 and 150 are laminated over the outer surfaces of the high-strength resin layers 120 and 130. That is, the hard coating layer 140, high-strength coating layer 120, sheet substrate 110, high-strength resin layer 130, and hard coating layer 150 are laminated in this order starting from one surface of the optical plastic sheet 101.

[48] Since the hard coating layers 140 and 150 are arranged as outermost layers, the optical plastic sheet 101 exhibits very high surface hardness. The hard coating layers 140 and 150 should have an excellent barrier effect to protect the liquid crystal device from gas including air and moisture, excellent formability, and sufficient tensile strength and weather resistance to stably protect the liquid crystal device from external environment. In this regard, it is preferred that the hard coating layers be made of a urethane-based resin. The hard coating layers may contain a small amount of a silicon- based material in order to achieve an enhancement in the adhering force to the high- strength resin layers 120 and 130.

[49] 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

[50] As apparent from the above description, the present invention provides an optical plastic sheet including a sheet substrate fabricated by melt-extruding an optical polymer having a certain molecular weight, and high-strength resin layers coated over the opposite surfaces of the sheet substrate while having a higher molecular weight than the polymer. The high-strength resin layers compensates for the vulnerableness of the mechanical properties of the sheet possibly caused by the manufacture of the sheet according to the melt extrusion method. Thus, the optical plastic sheet has very excellent mechanical properties similar to an optical plastic sheet manufactured in accordance with a solution casting method.

Claims

[1] An optical plastic sheet comprising: a sheet substrate fabricated by melt-extruding an optical polymer having a certain molecular weight; and high-strength resin layers coated over opposite surfaces of the sheet substrate to enhance mechanical properties, the high-strength resin layers having a higher molecular weight than the polymer.

[2] The optical plastic sheet according to claim 1, wherein the high-strength resin layers are formed by coating a coating resin over the opposite surfaces of the sheet substrate, and curing the coated coating resin such that the coating resin has an increased molecular weight.

[3] The optical plastic sheet according to claim 2, wherein the curing is achieved by a thermal curing process or an ultraviolet (UV) curing process.

[4] The optical plastic sheet according to claim 1, wherein the polymer for the sheet substrate is a polymer having optical characteristics including transparency as a basic physical property.

[5] The optical plastic sheet according to claim 4, 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).

[6] The optical plastic sheet according to claim 1, 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.

[7] The optical plastic sheet according to claim 1, wherein the molecular weight of the polymer for the sheet substrate and the molecular weight of the polymer for the high-strength resin layer in a cured state have a molecular weight ratio of 140,000: 200,000 to 170,000: 250,000.

[8] The optical plastic sheet according to claim 2, wherein the coating resin for the high-strength resin layers contains a urethane-based urethane acryl monomer, oligomer, and/or polymer of a low degree of polymerization.

[9] The optical plastic sheet according to claim 8, wherein the high-strength resin layers are 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.

[10] The optical plastic sheet according to claim 1, 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.

[11] The optical plastic sheet according to claim 1, further comprising: a hard coating layer formed over an outer surface of each high-strength resin layer to enhance surface hardness.

[12] The optical plastic sheet according to claim 11, wherein the hard coating layer comprises an urethane-based resin.

[13] The optical plastic sheet according to claim 12, wherein the hard coating layer further comprises a silicon-based material to provide an adhering force to the sheet substrate. [14] A liquid crystal device window comprising a plastic sheet according to any one of claims 1 to 13. [15] A mobile device comprising a liquid crystal device window according to claim

14.