KR20200144273A - Method for manufacturing back cover of mobile device - Google Patents

Abstract

The present invention relates to a method for manufacturing a cover for a mobile device, including the steps of: preparing a PC/PMMA substrate sheet; applying a coating composition for forming a hard coating layer to the surface of the PMMA layer, followed by drying; preliminarily curing the hard coating layer with UV rays at a low light dose; carrying out deposition and UV molding on the surface of the PC layer; carrying out shielding printing on the deposition and UV molding layer; carrying out press molding under heating; and irradiating UV rays at a high light dose to the preliminarily cured hard coating layer to perform perfect curing.

Description

TECHNICAL FIELD [Method for manufacturing back cover of mobile device]

The present invention relates to a method of manufacturing a cover for a mobile device, and more specifically, a cover made of a plastic material that can be applied to a cover including a curved portion mounted for the purpose of protecting, shielding, and decorating internal parts on the rear of the mobile device. It relates to a manufacturing method.

According to the recent trend, the demand for a variety of mobile devices, such as computers, PDPs, and mobile phones, which users directly carry and use, is constantly increasing, and these mobile devices are designed to attract consumers' attention with high performance in terms of functionality. Along with the diversification of products, improvements in portability and ease of use are emerging as important tasks.

In particular, in terms of design, competitors have increased the use of glass materials on the surface of mobile devices while pursuing a high-quality image along with slimming and large area of various devices and devices.

The glass is capable of expressing a beautiful design due to its high gloss and transparency, high hardness and abrasion resistance, and has a positive effect such as increasing the consumer's purchase desire by giving a luxurious image to the device. Is high, and it is not only easy to break due to impact, but there is also a problem that is contrary to the trend of pursuing weight reduction due to its weight.

For this reason, efforts are continuously being made to find alternative materials with excellent impact resistance and light weight while retaining the advantages of glass such as high gloss, transparency, hardness and abrasion resistance, and materials that emerge as a result of such efforts are transparent and resistant. It is a PC/PMMA laminated material that is a laminated composite of polycarbonate (PC), a material with excellent impact resistance, heat resistance, freedom of processing, and light weight, and polymethyl methacrylate (PMMA) with excellent hardness, transparency, and gloss.

The PC/PMMA laminated material is manufactured in the form of a coextruded sheet and is already commercially available for use in various products, but it is used in a variety of products, but it is highly likely to be scratched or exposed to friction because consumers carry it every day. On the other hand, there is a limitation in that it is difficult to obtain sufficient gloss, hardness, abrasion resistance/scratch resistance, etc. to be used as an exterior material for mobile devices that require high-sensitivity touch and surface gloss.

In order to solve this problem, research was made to supplement the hardness and abrasion resistance by forming a hard coating layer by coating a separate coating agent composition on the PMMA layer of PC/PMMA laminated material.As a result, acrylic resin, epoxy resin, urethane resin A method of forming a hard coating layer using a coating composition based on a resin, a siloxane resin, or the like is known.

However, such a hard coating layer has a very high hardness and is not softened by heat, so after forming the hard coating layer on the surface of the PMMA layer, in the case of heat compression molding, cracks occur in the hard coating layer on the curved surface, resulting in high-quality molding. Appeared to be unable to obtain.

Therefore, the method of forming the hard coating layer after heat compression molding is mainly used, but the process of forming the hard coating layer has a high defect rate, so if the hard coating layer is formed after various processing and molding at high cost, the yield is low and the manufacturing cost is low. It has an increasing problem. In particular, it is a very difficult task to uniformly form a hard coating layer on one surface after being molded into a curved shape, which is an obstacle to industrial implementation.

On the other hand, in Korean Patent Application Publication No. 2017-0003181, in manufacturing a curved transparent plastic sheet through compression molding after forming a hard coating layer on the surface of the PC/PMMA substrate sheet, the occurrence of cracks in the curved surface is minimized through preheating. How to do it is disclosed. According to this method, it is shown that cracks do not occur in the molding process while obtaining high surface hardness, but not only discloses a method of manufacturing a transparent plastic sheet that does not undergo a process such as forming a printing layer after forming a hard coating layer. , In terms of abrasion resistance, there is no disclosure of a content that is considered to have formed a hard coating layer having a level of performance suitable as a cover for a mobile device.

In particular, there is no reason to believe that the cost can be reduced with high productivity and low defect rate while having abrasion resistance (scratch resistance) particularly required for mobile devices.

KR 10-2017-0003181 A

The present invention was made to solve the problems in the manufacturing method of the conventional cover for a mobile device described above, and while having hardness, gloss and abrasion resistance that can replace glass, a mobile that can be manufactured at high yield and low cost. It is an object of the present invention to provide a method for manufacturing a device cover.

A method of manufacturing a cover for a mobile device according to the present invention for solving the above problem,

Preparing a PC/PMMA substrate sheet in which polycarbonate (PC) and polymethyl methacrylate (PMMA) are laminated;

Applying and drying a coating composition for forming a hard coating layer on the surface of the PMMA layer of the PC/PMMA base sheet;

Temporarily curing the hard coating layer by irradiating the applied hard coating film with ultraviolet rays having a wavelength of 320 to 400 nm at a light amount of 20 to 50 mJ/cm 2;

Performing deposition and UV molding on the surface of the PC layer;

Performing shielding printing on the deposition and UV molding layer;

Heating and compression molding the PC/PMMA sheet on which the shielding printing has been made into a predetermined shape; And

It characterized in that it comprises the step of completely curing the temporary cured hard coating layer by irradiating ultraviolet rays of a wavelength of 320 ~ 400nm with a light amount of 1200mJ / ㎠ or more.

According to the method of the present invention, a curved cover for a mobile device made of a plastic material having a hard coating layer having sufficient surface hardness and abrasion resistance can be manufactured at a high yield, thereby reducing manufacturing cost.

In particular, since the process of forming the hard coating layer with high defects through two-stage curing of temporary curing and complete curing is carried out before other expensive processing processes (UV molding, printing, molding, etc.), defects in forming the hard coating layer It does not cause an increase in manufacturing cost due to

The cover for a mobile device manufactured by the method according to the present invention is inexpensive and has excellent light weight and surface gloss, transparency, and hardness like glass. In particular, steel wool under a load of 1.5 kg in abrasion resistance test using steel wool. Since it has abrasion resistance and scratch resistance of more than 300 times by using the glass, it has the effect of being able to replace the glass material used in the existing cover of mobile devices.

1 is a flow chart sequentially showing the method of the present invention;
2 is a schematic cross-sectional view of a cover for a mobile device manufactured by a method according to the present invention.

A method of manufacturing a cover for a mobile device according to the present invention provides a method of manufacturing a cover for a mobile device in the form of forming a hard coating layer on a laminated sheet of PC/PMMA instead of glass.

With reference to a flow chart showing each step of the method of the present invention shown in Fig. 1 and a cross-sectional structure of a cover for a mobile device manufactured according to the method of the present invention shown in Fig. 2, the present invention will be described in detail in each step below.

Step 1 : PC/PMMA sheet preparation step

The first step of the present invention is a step of preparing a base sheet (hereinafter, PC/PMMA base sheet) 10 having a structure in which polycarbonate (PC) and polymethyl methacrylate (PMMA) are stacked, and the PC/ The PMMA base sheet is usually produced by coextrusion of polycarbonate and polymethyl methacrylate.

Here, the polycarbonate is a plastic material having excellent impact strength, and the PC layer 11 mainly serves to secure impact strength, and polymethyl methacrylate is a plastic material having high transparency and high hardness, and the PMMA layer 12 is It mainly serves to secure surface hardness.

The PC/PMMA substrate sheet used in the present invention is a known material that is already used for exterior of electronic devices, and not only the manufacturing method is public, but also commercialization has been made.Therefore, various purchasers manufactured or marketed products with reference to known technologies It can be purchased from and used.

For example, Korean Patent Publication No. 2017-0003181 discloses a method for coextrusion of a PC/PMMA base sheet used for manufacturing a curved transparent plastic sheet, so it can be referred to, and various types such as Sumitomo Chemical and MitsubiC Chemical of Japan It can also be purchased from the manufacturer.

On the other hand, in the present invention, the thickness of the PC/PMMA substrate sheet used for the protective cover of the mobile device is preferably 0.64 to 1.5 mm in consideration of light weight, impact resistance, hardness, and economic efficiency. In addition, the thickness ratio of the PC layer 11 and the PMMA layer 12 in the PC/PMMA base sheet 10 is preferably 95 to 60: 5 to 40, and particularly preferably about 90:10. .

Step 2 : Hard coating layer Application and drying step of forming coating composition (S2)

When the PC/PMMA base sheet 10 is prepared, in the second step, a coating composition for forming a hard coating layer is applied to the surface of the PMMA layer 12 of the base sheet and dried to form a coating film, followed by hardening. The coating layer 20 can be formed.

PC has excellent impact resistance, but generally has weak hardness and is susceptible to scratches, so the surface of the cover exposed to the surface can be secured up to 2H by using PMMA with high hardness.

However, in order to obtain a result of 300 or more times under a 1.5 kg load of abrasion resistance test to have scratch resistance required for a cover of a mobile device, the hard coating layer 20 is again formed on the surface of the PMMA layer 12 in the present invention.

As a component of the hard coating layer 20, an acrylic resin, an epoxy resin, a urethane resin, a siloxane resin, and a mixture or copolymer resin thereof may be used.

In terms of scratch resistance and workability, a urethane acrylic resin is particularly preferably used, and the coating composition for forming the hard coating layer includes, for example, a urethane acrylate oligomer and a diluting monomer such as an acrylate or methacrylate monomer, a solvent, a photoinitiator, and It contains various additives.

The urethane acrylate oligomer includes 3 to 15 functionalities such as aliphatic trifunctional urethane acrylate, aliphatic tetrafunctional urethane acrylate, aliphatic 5-functional urethane acrylate, aliphatic 6-functional urethane acrylate, aliphatic 6-functional or higher polyfunctional urethane acrylate. The aliphatic urethane acrylate (Aliphatic trifunctional urethane acrylate) may be any one selected from or a mixture of two or more thereof.

The acrylate or methacrylate monomer for diluting is 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, trimethylolpropane triacrylate, pentaerythritol triacrylate, dipentaerythritol hexaacrylate, and Toxylated (5) pentane erythritol tetraacrylate and one or two or more monomers selected from the group consisting of acryloyl morpholine may be used.

As the photoinitiator, known photoinitiators, including representative 1-hydroxycyclohexylphenyl ketone, can be used.

In the present invention, a solvent having corrosiveness to the PMMA layer is used as a solvent. Preferably, methyl ethyl ketone (MEK), ethyl acetate (EA), and propylene glycol monomethyl ether (PGME) are mixed and used.

This is differentiated from the conventional coating composition used as a solvent such as isopropyl alcohol, and since the adhesion of the hard coating layer to the PMMA layer is increased by using such a corrosive solvent, the amount of light is reduced in the temporary curing step described later. It is also possible to secure the necessary adhesion.

As the additives, known leveling agents, antifoaming agents, stabilizers, and the like are used, and are not particularly limited.

In addition, the method of applying the coating composition is not limited, but the coating composition is applied in a flow method in which the coating composition flows from a plurality of nozzles spaced apart on the PMMA layer surface of the substrate sheet to form a uniform coating film. do.

On the other hand, since the UV molding layer and the shielding printing layer must be formed on the PC layer, the coating composition must be prevented from being applied, and for this purpose, the coating composition is applied while the protective paper is attached to the surface of the PC layer.

On the other hand, it is preferable that the thickness of the coating film is in the range of 2 to 6 μm, and when the coating film is too thick, it is difficult to obtain a temporary cured coating film which has sufficient adhesive force through temporary hardening and does not generate cracks during subsequent thermoforming. That is, for example, when the thickness of the coating film is 4 to 10 μm, in order to prevent cracks from occurring during thermoforming, temporary curing should be performed using a very low light amount of 10 mJ/cm 2 or less. It is not possible to obtain a temporary cured coating film having sufficient adhesion to allow the process to proceed.

On the other hand, if the thickness of the coating film is too thin, it is difficult to obtain sufficient hardness and scratch resistance.

After the coating composition is applied, it is dried to form a coating layer.

Drying is carried out for 2 to 4 minutes at a temperature of 50 to 70°C. If the drying temperature is too high, abrasion resistance becomes poor, which is not preferable.

Step 3 : Hard coating layer Temporary hardening Step (S3)

After the coating composition is applied and dried, the hard coating layer 20 is temporarily cured by irradiation with ultraviolet rays (UV). Through this temporary curing, the coating composition has adhesion and hardness enough to undergo a subsequent processing process, but since complete curing does not proceed, it is possible to prevent the occurrence of cracks in the curved portion even when thermoforming through a compression molding machine. do.

At this time, the temporary curing is performed by irradiating an amount of light of 50 mJ/cm 2 or less using UV of a long wavelength of 320 to 400 nm. Preferably, in the case of a coating film having a thickness of 2 to 6 μm, when irradiation with a light amount of 20 to 50 mJ/cm 2, temporary curing is achieved at a level where adhesion is good and cracks do not occur during thermoforming. When the amount of light is lower, adhesion is poor.

In the case of irradiation with a higher energy amount of light, the curing proceeds excessively and cracks may occur during subsequent thermoforming.

When the thickness of the coating film is thicker, in order not to generate cracks during thermoforming, UV radiation of lower energy should be irradiated, but in this case, sufficient adhesion of the hard coating layer to the PMMA layer 12 cannot be obtained.

Step 4 : Deposition and UV molding step (S4)

According to the present invention, after the application of the coating composition and the temporary curing step of the hard coating layer, deposition and UV molding are performed on the surface of the PC layer 11.

In the evaporation and UV molding process, the thickness of the UV molding layer 30 is 10 to 15 μm, and a light amount of 600 to 800 mJ/cm 2 is irradiated using UV of a medium wavelength of 280 to 320 nm. In this process, since a medium-wavelength UV having a low transmittance is used, it does not affect the hard coating layer 20 applied to the surface of the PC layer 12 on the opposite side, and additional curing does not proceed.

Step 5 : Shield printing step (S5)

In the next step, shielding printing is performed on the upper portion of the UV molding layer 30 to shield the inside of the cover and give various designs. Since the substrate sheet made of PC and PMMA is a material having excellent transparency, the printed pattern passes through the PMMA layer 12 and the hard coating layer 20 and is transparent to the outside.

The printing process may be performed with reference to a conventionally known technique, and is not particularly limited. Preferably, after printing so that the thickness of the printing layer 40 is 10 to 15 μm, it is sufficiently dried at a temperature of 60 to 70° C. for 80 to 120 minutes.

Step 6 : Heating Compression molding Step (S6)

In the sixth step, the PC/PMMA sheet formed up to the printed layer 40 is molded in accordance with the cover shape of the mobile device, and is thermoformed in a compression molding machine having a mold having a desired shape.

During molding, the mold temperature is about 120 to 150°C, and the molding time is preferably 20 to 60 seconds. After molding, morphological stability is imparted through cooling and aging.

According to the method of the present invention, the hardness of the hard coating layer 20 is not high because it is only temporarily cured, and no cracks are generated in the hard coating layer even when heat compression molding is performed, so that there is a high risk of defects caused by heat compression molding. low.

Step 7 : Of the hard coating layer Complete curing step (S7)

Lastly, UV is irradiated to the hard coating layer 20 of the cover molded body for a mobile device that has been thermoformed so that the hard coating layer coating film is completely cured.

Preferably, a long wavelength of 320 to 400 nm is used to irradiate an amount of light of 1200 mJ/cm 2 or more, so that the resin of the coating composition is completely cured, and has a hardness of 4H to 7H, and under a load of 1.5 kg using BONSTAR STEEL WOOL. A fully cured hard coating layer 20 having a performance of 300 or more times in a wear resistance test is formed.

Example

Hereinafter, the present invention will be further described through Examples and Comparative Examples.

Example 1-4 and Comparative example 1~7

A PC/PMMA substrate sheet purchased from Sumitomo, Japan was used, and urethane acrylate oligomer, acrylate and methacrylate monomers, methyl ethyl ketone (MEK), ethyl acetate (EA), and propylene glycol monomethyl ether (PGME) Prepare a coating composition for forming a hard coating layer containing a solvent, a photoinitiator, and an additive, and uniformly apply it to the surface of the PMMA layer in two types of thicknesses of 2 to 6 μm and 4 to 10 μm in a flow manner. After drying at temperature for 3 minutes, temporary curing of the hard coating layer was performed while changing the amount of light as shown in Table 1 using UV having a wavelength of 380 nm. After the temporary curing, the state of the coating film was observed to determine the adhesion state of the coating film.

After provisional curing, the deposition, UV molding and shielding printing steps were performed according to a conventional method, and then thermoforming was performed to observe whether or not cracks were generated.

division Film thickness Light intensity
(mJ/㎠) During thermoforming
Whether cracks occur Attachment
(JIS-K5400) X-CUT Comparative Example 1 2~6㎛ 100~150 Cracking ○ Comparative Example 2 50~80 Cracking ○ Example 1 20-50 No crack ○ Comparative Example 3 below 10 No crack × Comparative Example 4 4~10㎛ 100~150 Cracking ○ Comparative Example 5 50~80 Cracking ○ Comparative Example 6 20-50 Cracking ○ Comparative Example 7 below 10 No crack ×

As shown in Table 1, when the thickness of the coating film is 4 to 10 μm, it is possible to obtain a molded product that does not generate cracks during thermoforming only in the case of a very low light amount of 10 mJ/cm 2 or less in the temporary curing step. However, in this case, it was found that it was difficult to form a durable hard coating layer due to poor adhesion to the base sheet.

In the case of forming a coating film thickness of 2 to 6 μm, even when the amount of ultraviolet light during temporary curing was 20 to 50 mJ/cm 2, cracks did not occur during thermoforming, and adhesion to the base sheet was also found to be good.

Wear resistance test

On the other hand, in the same manner as in Example 1, but in the drying step, a sample obtained by drying while changing the drying time to 50, 60, 70, 80°C as shown in Table 2 below was used, and the complete curing step was performed. Then, a fully cured hard coating layer was formed on the surface of the PMMA layer, which was subjected to a wear resistance test.

division Drying temperature and time Wear resistance test
(BONSTAR STEEL WOOL, 1.5kg load, over 300) Example 1 60℃*3min ○ Example 2 50℃*3min ○ Example 3 70℃*3min ○ Example 4 80℃*3min ×

As shown in Table 2, in the case of drying at a relatively low temperature of 50 to 70°C, abrasion resistance was excellent at more than 300 times under a load of 1.5 kg, and it was found to have scratch resistance necessary for a cover of a mobile device. , When drying at 80° C. where the drying temperature is somewhat high, the abrasion resistance was lowered, and it was found that the abrasion resistance was not more than 300 times under a load of 1.5 kg.

10: PC/PMMA substrate sheet
11: PC floor
12: PMMA layer
20: hard coating layer
30: UV molding layer
40: printed layer

Claims (2)

Preparing a PC/PMMA substrate sheet in which polycarbonate (PC) and polymethyl methacrylate (PMMA) are laminated;
Applying and drying a coating composition for forming a hard coating layer on the surface of the PMMA layer of the PC/PMMA base sheet;
Temporarily curing the hard coating layer by irradiating the applied hard coating film with ultraviolet rays having a wavelength of 320 to 400 nm at a light amount of 20 to 50 mJ/cm 2;
Performing deposition and UV molding on the surface of the PC layer;
Performing shielding printing on the deposition and UV molding layer;
Heating and compression molding the PC/PMMA sheet on which the shielding printing has been made into a predetermined shape; And
A method of manufacturing a cover for a mobile device, comprising the step of irradiating the temporary cured hard coating layer with ultraviolet rays having a wavelength of 320 to 400 nm at a light amount of 1200 mJ/cm 2 or more to completely cure. The method of claim 1, wherein the coating composition of the second step is one of methyl ethyl ketone (MEK), ethyl acetate (EA), and propylene glycol monomethyl ether (PGME) or a mixture of two or more thereof as a solvent. A method of manufacturing a cover for a mobile device, characterized in that.

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