ASSEMBLY PROCESSES USING UV CURABLE PRESSURE SENSITIVE ADHESIVES (PSA) OR STAGEABLE PSA SYSTEMS

Abstract

A process for creating an assembly with a pressure sensitive adhesive includes the application of a light curable composition to a first substrate of the assembly. The light curable composition is cured with a single light exposure step to form a fully polymerized pressure sensitive adhesive. The resulting pressure sensitive adhesive is brought into contact with to a second substrate with sufficient force to bond the second substrate to the first substrate to complete the assembly with the pressure sensitive adhesive.

Claims

1. A process for creating an assembly with a pressure sensitive adhesive comprising: applying a light curable composition to a first substrate of the assembly; curing the light curable composition in only a single light exposure step to form the pressure sensitive adhesive; and contacting the pressure sensitive adhesive to a second substrate with sufficient force to bond the second substrate to the first substrate to complete the assembly with the pressure sensitive adhesive.

2. The assembly process of claim 1 further comprising degassing the light curable composition prior to said curing step.

3. The assembly process of claim 1 wherein said curing step is complete as to light curable monomer.

4. The assembly process of claim 1 further comprising shrinking the light curable composition during said curing step and prior to said contacting step.

5. The assembly process of claim 1 further comprising the light curable composition being applied to the first substrate and a light blocking bezel in contact with the first substrate.

6. The assembly process of claim 1 wherein the first substrate is a cover glass of an electronic device.

7. The assembly process of claim 1 wherein the first substrate is an electronics assembly of an electronic device.

8. The assembly process of claim 1 wherein the first substrate and the second substrate define an electronic device, the electronic device being at least one of a smart phone, a computer display, a television display, or a tablet device.

9. The assembly process of claim 1 further comprising re-working the pressure sensitive adhesive after the completion of the assembly.

10. The assembly process of claim 9 wherein said re-working step is by heating the pressure sensitive adhesive to a temperature of between 80 and 150 degrees Celsius and removing the first substrate or the second substrate from the assembly.

11. The assembly process of claim 1 wherein the light curable composition in said applying step defines a dam.

12. A light curable composition that forms a pressure sensitive adhesive upon cure comprising: a flexible oligomer of a urethane acrylate having a number molecular weight of between 100 and 100,000; a light cureable monomer; and a photoinitiator to cure said oligomer and said monomer in the presence of light.

13. The composition of claim 12 wherein said flexible oligomer is present from 5 to 50 total weight percent, and said monomer is present from is present from 5 to 50 total weight percent.

14. The composition of claim 12 further comprising an inert resin.

15. The composition of claim 13 further comprising an inert resin present from 5 to 38 total weight percent.

16. The composition of claim 15 wherein said photoinitiator is at least one of trimethylbenzoyldiphenylphosphine oxide, alpha-hydroxyketones, a benzophenone, and combinations thereof.

17. The composition of claim 12 wherein the composition is devoid of tackifiers and plasticizers.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0085] The subject matter that is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other objects, features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings wherein like reference numerals refer to like parts throughout the several views, and in which:

[0086] FIGS. 1A-1G are PRIOR ART schematics of steps of manufacturing an image display device as detailed in U.S. Patent Application Publication 2014/0069581;

[0087] FIGS. 2A-E are schematic steps of an inventive process with an inventive light curable composition applied to a transparent first substrate, illustratively including an electronic device cover glass;

[0088] FIGS. 3A-E are schematic steps of an inventive process with an inventive light curable composition applied to an opaque first substrate, illustratively including an electronic device electronic assembly;

[0089] FIGS. 4A-F are a series of photos of an inventive process with an inventive light curable composition applied as a dam, and for illustrative purposes only is shown with a transparent first substrate;

[0090] FIGS. 5A-D are a series of photos of an inventive process with an inventive light curable composition applied as a dam and including the formation of an opaque bezel, and for illustrative purposes only is shown with a transparent first substrate; and

[0091] FIGS. 6A-6F are schematic steps of an inventive process for stageable UV-PSA for device assembly.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0092] A re-workable elastomeric fast-curing with light doses ranging from 500 mJ/cm2 to 8,000 mJ/cm2adhesive is provided with ability to cure in thin bond-lines and thick bond-lines having thicknesses ranging from 25 micron-300 micron. The present invention has applications in a variety of settings where it is desirous to have structural performance with rework or repair ability and thus not structurally permanent illustratively includes electronics such as touch panels, LCD/OLED and other display technologies for cell phones, tablets, laptops, desktops and televisions.

[0093] This application describes an inventive processes referred to herein as Pressure Sensitive Adhesive Optically Clear Adhesive (PSA OCA) Assembly.

[0094] Embodiments of the PSA OCA process first applies a liquid adhesive to either side of the assembly and then exposes it to UV and/or visible light. This causes the formation of a fully cured PSA film on the device or on the back side of the cover glass. The lamination of the two pieces takes place with immediate fixture because of the PSA nature of the cured adhesive.

[0095] Should rework be required the thin wire method may be used. In an alternative, advantageous method, the assembly can be heated to a moderate temperature, of approximately—80-100° C. At these temperatures the peel strength of the PSA is greatly reduced and the assembly can easily be peeled apart, thereby facilitating the re-working of the pressure sensitive adhesive after the completion of an assembly. Thus, a re-working of an assembly may be by heating the pressure sensitive adhesive to a temperature of between 80 and 100 degrees Celsius and removing a first substrate or a second substrate from the assembly, as will be shown in the examples and figures below.

[0096] Embodiments of the inventive light curable composition that forms a pressure sensitive adhesive upon cure may include: a flexible oligomer of at least one of: an aliphatic urethane acrylate having a number molecular weight of between 100 and 100,000, and combinations thereof; a monofunctional oligomer of at least one of a monofunctional urethane acrylate, having a number molecular weight of between 100 and 50,000, and combinations thereof; a diluting monomer; and a photoinitator to cure the monofunctional oligomer and the diluting monomer in the presence of light. In specific embodiments the flexible oligomer may be present from 5 to 18 total weight percent, and the monofunctional oligomer may be present from 5 to 18 total weight percent, and the diluting monomer may be present from 5 to 18 total weight percent. The light curable composition that forms a pressure sensitive adhesive upon cure may further have a modified saturated polyester resin and an inert resin, where the inert resin may have a modified saturated polyester resin present from 5 to 38 total weight percent. In a specific embodiment the photoinitiator is at least one of trimethylbenzoyldiphenylphosphine oxide, alpha-hydroxyketones, a benzophenone, and combinations thereof.

[0097] The advantages of the PSA OCA Assembly process are as follows. [0098] Only one light exposure is required, reducing process step(s). [0099] The light exposure completely cures all areas of the adhesive prior to assembly; [0100] This significantly reduces strain on the final assembly because all of the shrinkage occurs before lamination. [0101] There are no shadowed areas. Adhesive under the bezel will be as resistant to leaching and delamination as the bulk of the bonded area. [0102] In a preferred method of the invention the UV PSA adhesive used does not have extractable tackifiers or plasticizers. That is, a plurality of all components of the formulation polymerize into the final adhesive matrix.

[0103] Referring now to the figures, FIGS. 2A-2E and 3A-3E illustrate the application of the UV

[0104] PSA adhesive to two different sides of an assembly. FIGS. 2A-E are schematic steps of an inventive process with an inventive light curable composition 24 illustratively including Liquid Optically Clear Adhesives (LOCA) applied to a transparent first substrate, illustratively including an electronic device cover glass 20 and over a light blocking bezel 22 as shown in FIG. 2B, where the light blocking bezel 22 is applied along the perimeter of the cover glass 20 (FIG. 2A). In FIG. 2C the light curable composition 24 is cured in a single light exposure step to form the pressure sensitive adhesive 24′ (FIG. 2D), where the light exposure is illustrated as arrows 28. The curing light may be ultraviolet or visible light. The fully cured pressure sensitive adhesive 24′ is contacted to a second substrate 26 in FIG. 2E with sufficient force to bond the second substrate 26 to the first substrate 20 to complete the assembly with the pressure sensitive adhesive 24′. In a specific embodiment the second substrate 26 may be a display base, and the partial assembly of FIG. 2D may be flipped when joined to the second substrate.

[0105] In the embodiment shown in FIGS. 3A-3E the first substrate is the display base 26 with the light blocking bezel 22 applied along the perimeter of the display base 26 as shown in FIG. 3A. In FIG. 3B the inventive light curable composition 24 illustratively including Liquid Optically Clear Adhesives (LOCA) is applied to the display base 26. It is noted that in a specific embodiment the first substrate may be an electronics assembly of an electronic device. In FIG. 3C the light curable composition 24 is cured in a single light exposure step to form the pressure sensitive adhesive 24′ (FIG. 3D), where the light exposure is illustrated as arrows 28. The curing light may be ultraviolet or visible light. In FIG. 3E a laminate layer 30 is applied to the pressure sensitive adhesive 24′.

[0106] In specific embodiments the light curable composition may be degassed prior to the curing step. In specific embodiments the light curable composition may be shrunk during the curing step and prior to the contacting step. It is noted that the first substrate and the second substrate define an electronic device, the electronic device being at least one of a smart phone, a computer display, a television display, or a tablet device.

[0107] A further inventive use of UV PSA materials are as damming materials as shown in

[0108] FIGS. 4A-4F for a transparent first substrate and in FIGS. 5A-5D that includes the formation of an opaque bezel with a transparent first substrate. As shown in FIGS. 4A and FIGS. 5A and 5B a liquid material may be dispensed around the perimeter of an area to be bonded by LOCA and then cured by light (FIGS. 4B and 5C) to a PSA bead of desired height as shown in FIG. 4C. The interior of this dammed area may be filled with liquid adhesive as shown in FIG. 4D and a cover glass laid down such that the PSA bead immediately bonds and seals the area as shown in FIGS. 4E and 4F, which forces entrapped air out ahead of the laminated glass. Once assembled the PSA perimeter acts as a fixturing mechanism so that the piece can be safely handled, examined and moved through the light exposure that cures the interior LOCA. This process may be used to replace time consuming taping methods currently in use.

[0109] An advantageous variant is to use a UV PSA that has been pigmented to an opaque color, illustratively including black or white as best shown in FIG. 5B, and use that as both a dam and as a replacement for the light blocking bezel currently printed at the edge of all displays.

[0110] Another advantageous use of UV PSA materials in device assembly is by the preparation of “Stageable UV PSA” 34 products as shown in FIGS. 6A-6F. The UV cure creates a PSA matrix 34′ that also has material triggered by a secondary cure mechanism illustratively including time or heat that, after a delay, polymerizes to a co-polymer or interpenetrating network (IPN) 34″ as shown in FIG. 6F. The immediate preparation of a PSA material conveys the above described advantages for a period of time and then the bond “stages” or cures further to a permanent, more structural adhesive. The LOCA adhesive in this way would become a structural component of the fully assembled device.

[0111] The advantages of a permanent bond also would be for devices operating at higher temperatures, more extreme environmental conditions and more physically demanding situations.

[0112] The time delay to the staging reaction can be varied as can the method of the secondary cure. For example both cures can be initiated by the light exposure with the staging reaction lagging behind the PSA formation. This reaction system is described herein.

[0113] Also other staging reactions activated by heat, moisture, two component catalyst activation, and the like are possible. Non-limiting illustrative chemistries for staging may include epoxies, urethanes, silicones, N-methylols and other materials known in the art.

[0114] Technical Description

[0115] Table 1 is a listing of ingredients by weight and function of an embodiment of the inventive UV PSA.

TABLE-US-00001 TABLE 1 Ingredient list for a UV PSA #1 for specific embodiment and range of ingredient values provided as parentheticals. UV PSA #1. Ingredient % by Weight Proposed Function aliphatic urethane acrylate  7 (5-50%) Flexible oligomer monofunctional urethane  8 (0-15%) Mono-functional acrylate oligomer oligomer modified saturated 27 (0-30%) Resin to adjust modulus polyester resin urethane acrylate monomer 27 (5-50%) Light curable monomer inert resin  9 (0-25%) Resin to adjust viscosity ethoxyethoxy ethylacrylate 15 (0-25%) Flexible monomer mixture of  6 (1-10%) Photoinitiator blend trimethylbenzoyldiphenylphosphine oxide, alpha-hydroxyketones and benzophenone derivatives

EXAMPLES

[0116] These examples demonstrate the processes to be claimed in this patent filing. It should be remarked that other additions and modifications as known in the art are also expected to be covered. Several in particular are light and heat stabilizers to maintain clarity of the cured adhesives, adhesion promoters, flow control additives, pigments and dyes and various blends of photoinitiators.

Example 1

[0117] The formulation of Table 1 was coated onto clear Mylar film with a draw down bar at a nominal thickness of 3-mil. The coated Mylar film was exposed for 45 seconds to a doped Mercury bulb flood lamp at an intensity of 20 mW/cm2 measured with a 365 nm radiometer. The resultant total dose was measured to be 900 mJ/cm2.

[0118] The full cure dose can be calculated by integrating an FT-IR absorption peak area (X) at from 790 to 830 cm.sup.−1 above a baseline of an FT-IR measurement chart about a resin composition layer before UV irradiation and an absorption peak area (Y) at from 790 to 830 cm.sup.−1 above a baseline of an FT-IR measurement chart about the resin composition layer after a range of UV doses. The 100% cure dose is when the resultant plot of peak area versus mJ/cm2 has substantially stopped changing, that is the tangent point of the curve as it levels off

[0119] The resultant film was clear and tacky without significant removal from the Mylar when pressed with a finger, which is a good initial determinant of a PSA product. The PSA film was laminated to a 1-inch wide steel lap with a roller and approximately one pound of pressure. The next day the peel strength was measured at 4 pounds per lineal inch (pli).

[0120] The coated Mylar material was used to simulate an actual cell phone display assembly process using 2 inch by 3 inch glass slides as seen in the photo stream of FIGS. 4A-4F.

Example 2

[0121] To simulate the processes as described for FIG. 2 and in FIG. 3, a 3-mil nominal coating of liquid PSA #1 was applied to a glass slide using a draw down bar. The liquid was exposed to a dose of 900 mJ/cm2 and then carefully laminated to another glass slide. An immediate fixture of the two slides was achieved. After 2 hours the slides could not be separated without breakage of the glass. However when heated to 100° C. the pieces could be peeled apart thereby providing for re-workability of assemblies.

Example 3

[0122] The damming process was demonstrated as in the photo stream of FIGS. 4A-4F using PSA#1 and a conventional LOCA material, CTECH 20-187-3A. A bead was drawn using the PSA, exposed to 900 mJ/cm2 which creates a PSA dam with two exit holes at one end. The liquid 20-187-3A was used to fill the interior and then the matching slide slowly lowered driving the liquid in front towards the exit holes. The entrapped air escaped and a clear bond line resulted. This was exposed to 1500 mJ/cm2 creating the resultant final assembly.

[0123] A further variant as shown in FIGS. 5A-5D was created by adding black pigment to PSA#1. When used as a PSA dam it created an opaque bead after UV exposure that retained its PSA properties.

[0124] Table 2 is a listing of ingredients by weight and function of an embodiment of the inventive stageable UV PSA.

TABLE-US-00002 TABLE 2 Ingredient list for a stageable UV PSA #2. Stageable UV PSA #2. Ingredient % by Weight Proposed Function UV PSA#1 64.6 (25-90%) UV PSA UCB UVA1500 22.0 (5-40%) Cycloaliphatic di-epoxide 4-Hydroxybutyl acrylate  8.6 (0-20%) Crosslinking additive iodonium, (4-  4.3 (0.5-7%) Photoacid generator methylphenyl)[4-(2-   methylpropyl)phenyl]-,   hexafluorophosphate(1-)   isopropylthioxanthone  0.5 (0-5%) Visible light sensitizer

Example 4

[0125] A sample of Formulation #2 as listed in Table 2 was coated on to Mylar and cured with 1000 mJ/cm2 exposure. The resultant solid film was a tacky PSA product that accepted a steel lap readily and immediately held the assembly when lifted vertically. This shows that the PSA property of the original formulation was maintained even with the introduction of a significant amount of material that had not immediately reacted.

[0126] By the next day after ambient storage the open film was tack free and firm and the bond line when peeled apart was also tack free and the adhesive remained on the steel surface. That is, it had transferred from the original surface to the new steel substrate.

[0127] Both of these observations prove that the light initiated epoxy reaction was delayed and cured or “staged” at a later time to a non-PSA, permanent adhesive.

[0128] This material would suitably demonstrate the process as shown in FIG. 6.

[0129] Patent documents and publications mentioned in the specification are indicative of the levels of those skilled in the art to which the invention pertains. These documents and publications are incorporated herein by reference to the same extent as if each individual document or publication was specifically and individually incorporated herein by reference.

[0130] The foregoing description is illustrative of particular embodiments of the invention, but is not meant to be a limitation upon the practice thereof. The following claims, including all equivalents thereof, are intended to define the scope of the invention.