POLYCARBONATE-BASED OPTICAL LAMINATE, MANUFACTURING METHOD THEREOF, AND COVER WINDOW USING THE SAME

Abstract

The present inventive concept relates to a transparent polycarbonate-based optical laminate, which satisfies both high surface hardness and impact resistance, making it applicable to curved designs, a manufacturing method thereof, and a cover window using the same. More specifically, the present inventive concept provides an optical laminate: including a polycarbonate/primer layer/hard coating layer, wherein (A) the hard coating layer is a cured product of a siloxane resin prepared by hydrolytic condensation of an alkoxy silane of the following Formula 1 containing a cycloaliphatic epoxy group; and (B) the primer layer is a cured product of a mixture of at least one curing resin selected from an acrylic-based resin, a polyurethane-based resin, and an epoxy-based resin, and the siloxane resin.

R.sup.1Si(OR.sup.2).sub.3[Formula 1] where R.sup.1 is a linear or branched C.sub.1-C.sub.6 alkyl group containing a cycloaliphatic epoxy group, the cycloaliphatic epoxy group being a C.sub.3-C.sub.6 cycloalkyl group that forms a fused bicycle with an epoxy group, and R.sup.2 is a linear or branched C.sub.1-C.sub.6 alkyl group.

Claims

1. An optical laminate: comprising a polycarbonate/primer layer/hard coating layer, characterized in that: (A) the hard coating layer is a cured product of a siloxane resin prepared by hydrolytic condensation of an alkoxy silane of the following Formula 1 containing a cycloaliphatic epoxy group; and (B) the primer layer is a cured product of a mixture of at least one curing resin selected from an acrylic-based resin, a polyurethane-based resin, and an epoxy-based resin, and the siloxane resin:
R.sup.1Si(OR.sup.2).sub.3[Formula 1] where R.sup.1 is a linear or branched C.sub.1-C.sub.6 alkyl group containing a cycloaliphatic epoxy group, the cycloaliphatic epoxy group being a C.sub.3-C.sub.6 cycloalkyl group that forms a fused bicycle with an epoxy group, and R.sup.2 is a linear or branched C.sub.1-C.sub.6 alkyl group.

2. The optical laminate of claim 1, wherein the primer layer and the hard coating layer chemically bond at an interface.

3. The optical laminate of claim 1, wherein the curable resin comprises at least one selected from the group consisting of an acrylic resin, an epoxy acrylic resin, a polyurethane acrylic resin, a polyether acrylic resin, a cycloaliphatic epoxy resin, and a glycidyl ether-based epoxy resin.

4. The optical laminate of claim 1, wherein R.sup.1 of the alkoxy silane is a 2-(3,4-epoxycyclohexyl)ethyl group or a 2-(3,4-epoxy-3-methylcyclohexyl)ethyl group.

5. The optical laminate of claim 1, wherein the optical laminate has an average transmittance of 90% or more in the range of 380 to 780 nm and a haze of less than 1%.

6. The optical laminate of claim 1, wherein the optical laminate has a pencil hardness of 5H-7H on the hard coating surface and a dart drop impact strength of 20-50 J according to ASTM D3763.

7. The optical laminate of claim 6, wherein the radius of curvature indicating flexibility of the optical laminate ranges from 10 mm to 25 mm.

8. The optical laminate of claim 6, wherein the optical laminate has a difference in yellowness of 0.30 or less after 18 hours of exposure to a 20W UV-B lamp positioned 20 cm away.

9. A method of manufacturing the optical laminate of claim 1, comprising the steps of: (A) preparing a polycarbonate substrate, at least one curing resin selected from an acrylic-based resin, a polyurethane-based resin, and an epoxy-based resin, and a siloxane resin prepared by hydrolytic condensation of an alkoxy silane of the following Formula 1 containing a cycloaliphatic epoxy group, respectively;
R.sup.1Si(OR.sup.2).sub.3[Formula 1] where R.sup.1 is a linear or branched C.sub.1-C.sub.6 alkyl group containing a cycloaliphatic epoxy group, the cycloaliphatic epoxy group being a C.sub.3-C.sub.6 cycloalkyl group that forms a fused bicycle with an epoxy group, and R.sup.2 is a linear or branched C.sub.1-C.sub.6 alkyl group; (B) preparing a primer composition by mixing the siloxane resin and the curable resin at a predetermined ratio; (C) forming a primer layer by coating the primer composition onto the polycarbonate substrate, followed by curing; and (D) forming a hard coating layer by coating the hydrolyzed condensate of the alkoxy silane onto the primer layer, followed by curing.

10. The method of manufacturing the optical laminate of claim 9, wherein in step (B), the siloxane resin and the curable resin are mixed at a weight ratio of 20:80 to 80:20.

11. The method of manufacturing the optical laminate of claim 9, wherein the formation of the primer layer in step (C) is achieved by photocuring the primer composition.

12. The method of manufacturing the optical laminate of claim 11, wherein the formation of the hard coating layer in step (D) comprises the steps of: (a) photocuring the siloxane resin; and (b) heat-treating the photocured product under a relative humidity condition of 50% or more.

13. The method of manufacturing the optical laminate of claim 11 or 12, further comprising, before the step of photocuring, the step of removing the solvent by heat treatment.

14. A primer composition comprising a mixture of at least one curable resin selected from an acrylic-based resin, a polyurethane-based resin, and an epoxy-based resin, and a siloxane resin prepared by the hydrolytic condensation of the alkoxy silane of the above Formula 1.

15. A cover window for a display comprising the optical laminate of claim 1.

16. The cover window for a display of claim 15, wherein the cover window for a display is used for an automotive display.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0030] The above and other features and advantages of the present inventive concept will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which:

[0031] FIG. 1 illustrates a method of forming a laminate according to an embodiment of the present inventive concept;

[0032] FIG. 2(a) and FIG. 2(b) depicts images and a graph showing the surface hardness and impact strength of the laminate according to an embodiment of the present inventive concept;

[0033] FIG. 3 is a conceptual diagram illustrating the decrease in impact strength of the laminate due to hard coating;

[0034] FIG. 4(a) and FIG. 4(b) depicts cross-cut adhesion test result images and FTIR spectra of the laminate according to an embodiment of the present inventive concept;

[0035] FIG. 5 is a conceptual diagram illustrating the decrease in adhesion and surface hardness of the laminate due to the low reactivity of the DGEBA and the hard coating layer;

[0036] FIG. 6(a), FIG. 6(b) and FIG. 6(c) depicts FTIR and cross-cut adhesion test result images showing the increase in adhesion of PC/MFP/HC by the MFP primer layer;

[0037] FIG. 7 depicts cross-cut adhesion test result images showing the difference in adhesion according to the primer layer forming process;

[0038] FIG. 8(a), FIG. 8(b) and FIG. 8(c) depicts graphs and an image showing the surface hardness of the PC/MFP/HC laminate depending on the CEOS ratio in the MFP composition;

[0039] FIG. 9(a), FIG. 9(b) and FIG. 9(c) depicts graphs and an image showing the surface strength of the PC/MFP/HC laminate depending on the CEOS ratio in the MFP composition;

[0040] FIG. 10 is a conceptual diagram illustrating the structure of the laminate of the present inventive concept; and

[0041] FIG. 11(a), FIG. 11(b), FIG. 11(c) and FIG. 11(d) depicts graphs showing the properties of the laminate according to an embodiment of the present inventive concept.

DETAILED DESCRIPTION OF THE INVENTIVE CONCEPT

[0042] Hereinafter, the present inventive concept will be described in more detail with reference to the accompanying drawings and examples. However, these drawings and examples are provided merely as illustrations to facilitate the description of the technical idea and scope of the present inventive concept, and the technical scope of the present inventive concept is not limited or changed thereby. Based on these illustrations, it will be obvious to those skilled in the art that various modifications and changes are possible within the technical idea of the present inventive concept.

EXAMPLES

Example 1: Preparation of Primer Composition

[0043] CEOS was synthesized via a hydrolytic sol-gel reaction using a conventional alkaline catalyst, as described in Adv. Mater. 2017, 29, 1700205. Specifically, 0.1 mol of ECTMS (2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane, Sigma-Aldrich), 0.15 mol of purified water, and 16 l of ammonium hydroxide were placed in a flask and stirred at 80 C. for 5 hours. The hard coating composition was prepared by mixing 2 parts by weight of aryl sulfonium hexafluoroantimonate salt (TSHFA) as a polymerization initiator and 50 parts by weight of methyl ethyl ketone as a solvent per 100 parts by weight of the prepared epoxy siloxane resin.

[0044] The primer, DGEBA (bisphenol A diglycidyl ether, Sigma-Aldrich), was dissolved in 2 times (w/v) tetrahydrofuran (THF), and the curing agent, 4,4-diaminodiphenyl sulfone (DDS), was added to the DGEBA:DDS at a molar ratio of 3:1 to prepare the coating composition.

[0045] The primer coating composition used for the formation of the primer layer according to the present inventive concept was prepared by adding the CEOS coating composition and the DGEBA coating composition, prepared as described above, in a ratio of 20-80% (w/w), and the resulting composition was named MFP-XX, where XX represents the weight percent (w %) of CEOS in the composition. For example, MFP-80 refers to a primer composition prepared by mixing CEOS 80:DGEBA 20 (w/w).

Example 2: Coating of Polycarbonate Substrate

[0046] First, a commercial polycarbonate substrate (from I-Component Co., Ltd.) was treated with ozone to activate the surface. Subsequently, the MFP or DGEBA coating composition prepared in Example 1 was used to bar-coat the substrate with a thickness of 10 m, and the solvent was evaporated by annealing at 80 C. for 1 hour. The resulting sample was then irradiated with UV light for 60 seconds using a metal halide lamp to form the primer layer.

[0047] Onto the substrate with the primer layer formed, the CEOS solution prepared in Example 1 was bar-coated to a thickness of 30 m, and the solvent was evaporated by annealing at 80 C. for 1 hour. The resulting sample was then irradiated with UV light for 60 seconds using a metal halide lamp, followed by annealing at 85 C. and 85% relative humidity (RH) for 2 hours form the CEOS hard coating layer.

[0048] The laminate composed of a PC/CEOS hard coating layer was manufactured in the same manner as described above, with the exception of omitting the formation of the primer layer.

[0049] FIG. 1 illustrates a method of forming a laminate according to this embodiment.

Example 3: Properties of Polycarbonate Laminate

(1) PC/CEOS Hard Coating Laminate (PC/HC)

[0050] The properties of the PC/CEOS hard coating laminate prepared in Example 2 were evaluated using a prior art method. The surface hardness, indicative of scratch resistance, was measured by a pencil hardness test to determine the changes in mechanical strength due to the formation of the hard coating layer, and impact strength, indicative of impact resistance, was measured by a puncture test.

[0051] The pencil hardness was measured on the surface, where the hard coating layer was formed, using pencils with hardness ranging from 9B to 9H according to the ASTM (American Society for Testing and Materials) D3363 protocol. Prior to the test, the pointed part of the pencil was flattened and smoothed. The test was conducted at a 45 angle, with a weight of 750 g, from high to low hardness according to the protocol, and scratches were initially checked with the naked eye and inspected again under an optical microscope. The puncture test was conducted using a drop tower impact tester (CEAST 8350, Instron, Norwood, MA, USA) according to the ASTM D3763 protocol. The specimens, sized at 100 mm100 mm, were fixed with a 100 N force onto a ring-shaped clamp with a diameter of 40 mm. The drop weight was released at a velocity of 4.83 m/s, corresponding to an impact energy of 60 J. The absorbed impact energy by the specimen was calculated by collecting data from the photonic sensor until the drop weight penetrated the surface at the first impact point on the specimen.

[0052] FIG. 2(a) and FIG. 2(b) depicts images and a graph showing the surface hardness and impact strength of the laminate according to this Example. As can be seen from FIG. 2(a) and FIG. 2(b), the pencil hardness of the polycarbonate substrate used in the test was only 4B; however, with the introduction of the hard coating layer, it significantly increased to 7H. On the contrary, while the impact strength of the polycarbonate substrate itself was 50 J, the impact strength of the laminate decreased significantly to 7.4 J.

[0053] Polycarbonate is known for its low chemical resistance. Therefore, it was determined whether the solvent contained in the CEOS coating solution affects the impact resistance. To this end, a mixed solution excluding ECTMS from the solution for the preparation of CEOS in Example 1 was prepared and applied to the polycarbonate substrate. After annealing at 80 C. for 1 hour to evaporate the solvent, the impact strength was measured. As a result, it was observed that the solvent treatment alone led to the formation of cracks on the surface, resulting in a significant decrease in impact strength to 30 J (not shown). From this, it can be interpreted that the surface of the substrate is partially dissolved by the solvent, causing crystallization, and during the hard coating process, the siloxane particles constituting the hard coating layer penetrated into the interior of the substrate surface, exacerbating the formation of cracks and the reduction in impact strength. (See FIG. 3).

(2) PC/DGEBA/HC Laminate

[0054] With the use of a primer during the formation of the PC/HC laminate, the primer layer acts as a barrier to prevent the introduction of siloxane particles into the interior of the polycarbonate substrate. Therefore, the PC/primer/HC laminates were formed and their properties were evaluated. DGEBA was selected as the primer, which is widely used in the construction of multilayer laminates due to its excellent adhesion to various substrates and chemical resistance to most solvents.

[0055] As can be seen from FIG. 2(a) and FIG. 2(b), the formation of the DGEBA primer layer significantly increased the impact strength of the laminate from 7.8 J to 41 J, demonstrating the effectiveness of the above-mentioned strategy; however, the pencil hardness decreased from 7H to 5H.

[0056] Therefore, in order to determine whether the decrease in pencil hardness was due to low adhesion, a cross-cut adhesion test was performed. The cross-cut adhesion test was conducted using a cross-cut tester (Elcometer 104, UK) according to the guideline of ASTM D3359-17. The cuts were made at 1 mm interval using a cutter to create a grid pattern on the coated surface, and an adhesive tape was applied and then removed. The results were examined under a microscope and shown in FIG. 4(a). The adhesion is classified as follows; 5BNo impact; 4BEffect area<5%; 3BEffect area 5-15%; 2BEffect area 15-35%; 1BEffect area 35-65%; and OBEffect area>35%. The adhesion of the PC/HC laminate according to the cross-cut adhesion test was 4B, while the adhesion of the PC/DGEBBA/HC laminate deteriorated to 3B.

[0057] Referring to the Fourier-transform infrared spectroscopy (FTIR, Bruker Vertex 70v) spectra of PC/DGEBA shown in FIG. 4(b), the underlying cause of the decrease in surface hardness and adhesion can be inferred. The epoxy peaks at 915 and 883 cm.sup.1 observed in the PC/DGEBA sample before the formation of the CEOS hard coating layer significantly decreased after photocuring, compared to the samples where the solvent was removed by annealing after coating. This indicates a high level of epoxy conversion in the primer, resulting in reduced epoxy reactivity with the hard coating layer. The decreased reactivity with the hard coating layer is interpreted to cause a decrease in adhesion and surface hardness (refer to FIG. 5).

(3) PC/MFP/HC Laminate

[0058] In order to address the issue of decreased adhesion and surface hardness typically observed in laminates using conventional epoxy primers, a laminate was manufactured using MFP, a new mixture of epoxy resin and CEOS, as a primer, and its properties were evaluated.

[0059] First, it was determined by FTIR whether the aggregation or precipitation occurred during the mixing of DGEBA and CEOS for the preparation of the MFP primer. Visual observation indicated that the DGEBA solution and the CEOS solution were could be mixed uniformly to for a homogeneous mixture. In the FTIR spectra of the MFP solution shown in FIG. 6(a), the peaks corresponding to the epoxy groups at 915 and 883 cm.sup.1 observed in DGEBA and CEOS, respectively, were detected, and the intensity of these peaks depended on the composition ratio. Additionally, the peaks at 1000-1200 cm.sup.1 indicating the presence of siloxane were observed in the MFP solution, similar to those observed in the CEOS solution. These results demonstrate that DGEBA and CEOS form a homogeneous mixture without unwanted reactions.

[0060] Subsequently, the absorbance of FTIR spectra was observed at each stage of forming the MFP primer layer on the polycarbonate substrate and is shown in FIG. 6(b). The peaks corresponding to the epoxy groups observed after the annealing process for solvent removal after bar coating of the MFP primer layer indicate that only a portion of the epoxy groups are converted after photocuring, and most of the epoxy group are converted due to the reaction after additional heat-moisture treatment. Therefore, it is anticipated that the formation of the hard coating layer after photocuring will result in a laminate with excellent adhesion and surface hardness due to the reaction between the epoxy groups remaining in the primer layer and the epoxy groups in the hard coating layer. As can be seen from FIG. 6(c), the PC/MFP/HC laminate actually exhibited excellent adhesion of 5B in the cross-cut adhesion test.

[0061] It can be inferred from the above results that the formation of the primer layer would affect the adhesion. Therefore, the differences in adhesion according to the formation of the primer layer were investigated. As predicted in FIG. 6(b), when only the photocuring of the primer layer was formed followed by the formation of the hard coating layer, excellent adhesion was observed; however, when additional heat curing was performed under moist heat conditions after the photocuring, followed by the formation of the hard coating layer, the adhesion was rather decreased (FIG. 7).

[0062] Therefore, the surface hardness and impact strength of the PC/MFP/CEOS laminate according to the CEOS ratio in the MFP composition were measured by the same method as in the above Example, and the results are shown in FIGS. 8 and 9, respectively. As a Comparative Example, in FIGS. 8 and 9, the CEOS ratio of 0 represents the PC/DGEBA/HC laminate using DGEBA as the primer, and the CEOS ratio of 100 represents the PC/HC laminate without a primer layer.

[0063] FIG. 8(a) depicts images showing the surface hardness according to the composition of the primer layer, FIG. 8(b) is a graph showing this, and FIG. 8(c) is an image showing the test process according to the ASTM D3363 protocol. The pencil hardness of the laminate ranged from 5H to 7H, depending on the composition of the primer layer. As the CEOS ratio in the primer layer increased, the pencil hardness also increased, and when the CEOS ratio exceeded 60%, pencil hardness remained constant at 7H.

[0064] FIG. 9(a) and FIG. 9(b) depict graphs showing the impact strength according to the composition of the primer layer, and FIG. 9(c) is an image of the laminate after the experiment. The impact strength of the laminate decreased as the CEOS ratio increased, and in the case of MFP-20, the pencil hardness significantly increased to 5H, compared to 4B for the polycarbonate substrate, while the impact strength reached a level comparable to that of the polycarbonate substrate. Therefore, by adjusting the ratio of CEOS in the MFP depending on the properties required for specific applications, it is possible to provide a laminate with a desired level of impact strength and surface hardness.

[0065] The laminate of the present inventive concept can exhibit high surface hardness and impact strength, and the reason for this can be explained as the siloxane bonds with the epoxy resin in the primer layer and the interfacial bonding with the hard coating layer exhibits excellent adhesion, as shown in FIG. 10.

[0066] In addition to the surface hardness and impact strength, the optical properties are also required for cover windows, and when applied to curved displays, the cover windows require flexibility to allow curved shaping. Polycarbonate has the property of yellowing when exposed to light, and thus the durability due to UV stability is also an important property. Accordingly, each property was evaluated, the results are shown in FIG. 11(a), FIG. 11(b), FIG. 11(c) and FIG. 11(d), and the measured properties are summarized in Table 1.

[0067] The optical properties were evaluated by the average transmittance of 380-780 nm measured from the UV-Vis spectrum within the range of 300-800 nm (FIG. 11(a)) and the haze value measured from a haze meter according to ASTM D1003. Transmittance and haze are important factors in delivering information cleanly and clearly. The polycarbonate substrate itself exhibited a high transmittance of 90.54% and a low haze of 0.23, indicating almost no light scattering. The introduction of the hard coating layer or the primer layer/hard coating layer did not significantly affect the optical transmittance, and the haze value slightly increased, but remained very low, making it suitable for cover windows.

[0068] Flexibility was measured by bending the laminate into a U-shape around a cylindrical bar with a diameter of 10 mm to 50 mm and measuring the minimum radius of curvature ( of the corresponding cylinder diameter) at which the coating layer can be restored to its original state without breaking. Flexibility is a property that affects the production of curved or panoramic displays. The polycarbonate substrate exhibited excellent flexibility with a radius of curvature of 5 mm, while the PC/HC laminate was very vulnerable to bending as the substrate broke even at a radius of curvature of 25 mm. On the contrary, with the introduction of the primer layer, flexibility was greatly improved, demonstrating a radius of curvature of 10 mm.

[0069] Polycarbonate is known to have low UV stability, leading to the formation of radicals by UV exposure, followed by the reaction with polymer chains, resulting in decreased molecular weight and yellowing. UV stability refers to weather resistance for performance and visibility, and it is crucial, especially in display cover windows exposed to direct sunlight, such as automotive displays. UV stability was evaluated by exposing the laminate to a 20 W UV-B lamp, positioned 20 cm away according to the ISO 4892-3 test method, for 18 hours, and then measuring the difference in yellowness according to the ASTM D1925 method. FIG. 11(c) shows the differences in yellowness over the duration of UV exposure, and FIG. 11(d) shows the UV-VIS spectra after 18 hours of exposure.

[0070] The difference in yellowness (YI) of the polycarbonate substrate was 0.47, indicating a significant change in color. However, with the formation of the siloxane hard coating layer on the PC substrate, YI decreased significantly to 0.16, and the laminate PC/DGEBA/HC with an epoxy primer layer also showed a slightly reduced YI value of 0.32. The PC/MFP/HC laminate demonstrate an intermediate improvement, with a YI value more than more than 50% lower than that of polycarbonate.

TABLE-US-00001 TABLE 1 PC/MFP/HC PC/DGEBA/ PC PC/HC HC Transmittance (%) 90.54 90.61 90.71 90.64 90.38 90.51 90.51 Haze (%) 0.23 0.51 0.61 0.68 0.58 0.63 0.72 Pencil hardness 4B 7H 7H 7H 6H 5H 5H Radius of curvature 5 Break 50 20 20 20 10 (mm) Adhesion 4B 5B 5B 5B 4B 3B Impact strength (J) 50.425 7.461 25.186 37.798 43.002 47.873 41.135 UV stability (YI) 0.47 0.16 0.22 0.31

[0071] Laminate were manufactured using MFP as a primer, a mixture of acrylic-based resin and CEOS, instead of epoxy resin, and their properties were evaluated.

[0072] For this purpose, instead of epoxy resin, MFP was manufactured by mixing polyurethane acrylic resin 3052 (ThreeBond) or acrylic resin 3042B (ThreeBond) with CEOS at a weight ratio of 40:60. Using this, PC/MFP/HC laminates were manufactured and their properties were evaluated Table 2 shows the results. Like the epoxy resins, the acrylic-based resins such as polyurethane acrylic resin and acrylic resin also exhibited a decrease in pencil hardness in the PC/primer/HC laminates compared to the PC/HC laminates, and the cause for this was interpreted to be due to a decrease in adhesion. On the contrary, in the case of the PC/MFP/HC laminate, it maintained the pencil hardness equivalent to the PC/HC laminate, while preserving the optical properties.

TABLE-US-00002 TABLE 2 Transmittance Structure (%) Haze (%) Pencil hardness Adhesion 3052 PC/HC 90.68 0.51 7H 4B (Polyurethane PC/3053/HC 90.47 0.60 4H 3B acrylate) PC/MFP/HC 90.70 0.58 7H 5B 3042B PC/HC 90.68 0.51 7H 4B (Acrylate) PC/3052B/HC 90.51 0.59 5H 4B PC/MFP/HC 90.74 0.63 7H 5B

[0073] While the inventive concept has been shown and described with reference to certain preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the inventive concept as defined by the appended claims. Therefore, the scope of the inventive concept is defined not by the detailed description of the inventive concept but by the appended claims, and all differences within the scope will be construed as being included in the present inventive concept.