Three Bond Technical News
Issued July 1, 200974

UV-Curing Sheet Adhesives
Introduction ________________________________________________
From its very beginnings, ThreeBond has continued to develop a wide range of liquid adhesives thathave won wide commercial acceptance.
In recent years, we have also sought to develop an adhesive based on an entirely new concept,
drawing on liquid adhesive technologies accumulated through many years of experience. The newproduct is a functional sheet-form adhesive that combines the advantages of sheet-type adhesives (e.g.,
double-sided adhesive tape) and our liquid adhesives.
This issue introduces the ThreeBond 1630 UV-curing sheet adhesive, one of the functional sheetadhesives currently being developed at ThreeBond, to which we have added ultraviolet curing
properties.

Contents
Introduction................................................................. 1
1. Background ........................................................... 2
2. What is a UV-curing sheet adhesive? ................... 2
3. ThreeBond 1630 UV-curing sheet adhesive.......... 2
3-1. Features......................................................... 23-2. Properties ...................................................... 33-3. Characteristic values ..................................... 33-4. Reliability data ................................................4
4. Using ThreeBond 1630 ..........................................54-1. Steps in bonding with ThreeBond 1630..........54-2. Recommended bonding machines.................5
5. Examples of applications .......................................6
6. Examples of advanced applications.......................7Conclusion ..................................................................811.

Background

Recent rapid progress in flat-panel technologies
for display devices, including LCD displays, had
led to the creation and practical application ofvarious new techniques, including the use ofadhesives to bond optical components—LCD
panels, touch panels, and panel overlays—toproduce flatter display devices and to improve
display qualities or to increase the shock resistanceof the display devices.
Such applications demand the following functions
and characteristics from adhesives: Increased visibility
 Characteristics that help improve device
stiffness
 Conformability to surface geometry, such as
uneven surfaces caused by printed ink High reliability (thermal, humidity, andweathering resistance)
The conventionally preferred bonding agents forsuch applications have been liquid UV-curing resins
and/or double-sided adhesive tapes designed for
optical applications. However, the former class ofmaterials is associated with various problems,including bubble inclusion, squeeze-outs fromcomponent edges, and irregular thicknesses. While
offering excellent uniformity in thickness among
optical double-sided tapes, the latter class ofmaterials cannot be made to conform to theundulations created by printed ink and also pose
reliability issues.
In response, ThreeBond developed the ThreeBond
1630 UV-curing sheet adhesive. This productresolves the problems associated with conventionalliquid UV-curing resins and optical double-sided
adhesive tape: It is free of bubble inclusions andsqueeze outs, has uniform thickness, and conforms
to adherend geometries. (ThreeBond will beabbreviated “TB” hereafter.)2.

What is a UV-curing sheet adhesive?

A UV-curing sheet adhesive is a solid sheet atroom temperature but can be fluidized by theapplication of pressure or heat, then curedcompletely within seconds when irradiated with UVlight.
Since the adhesive is a solid sheet at room
temperature, it offers highly uniform thickness andeasy handling, much like double-sided adhesive
tape. In addition, its fluid properties under pressureor heat allows it to conform, like liquid adhesives,to the surface geometry of the adherend. The end
result is a unique adhesive that combines theadvantages of double-sided adhesive tape and liquidadhesive.

2Table 1 and Figures 1 to 3 present the features ofTB1630 UV-curing sheet adhesive, liquid
UV-curing resin, and optical double-sided adhesivetape.
Table 1.

Comparison of features of variousadhesives
TB1630LiquidUV-curingresin

Opticaldouble-sidedadhesive tape



















Transparency
Uniformity of layer
thickness
Susceptibility tosqueeze-outs
Conformability tosurface geometries
Reliability

No squeeze-outs; uniform thickness of
the adhesive layer
Conformability toNo bubbles
uneven surfaces

Figure 1. TB1630
Squeeze-outs

Bubbles

Non-uniform
thickness of theadhesive layer

Figure 2. Liquid UV-curing resin
Unable to fill undulations in surface geometries

Figure 3. Optical double-sided adhesive tape3.

ThreeBond 1630 UV-curing sheetadhesive3-1. Features
TB1630 is a UV-curing sheet adhesive.
The key features of TB1630 are given below.
Features:
1. High transparency for enhanced visibility inapplications requiring use of adhesives in thelight path.
2. Its sheet form allows large surface areas to bebonded together while ensuring uniform
adhesive thickness between surfaces.
3. TB1630 is fluidized by the application ofheat/pressure to allow conformance to thesurface geometry of the adherend.
4. The flexible cured resin is highly resistant toboth shock and vibration.
5. As a reaction adhesive, TB1630 displays highresistance to temperature, humidity, andweathering stress factors.

3-2. Properties
Table 2 presents the properties of TB1630. Fig. 4 illustrates the product structure.
Table 2. Properties of TB1630
UnitTB1630

Test method

Appearance

Colorless, transparent

3TS-201-1

Notes

Adhesive layer thicknessm30

3TS-261-01

Flow initiation temperatureC65

3TS-209-01Rheometer tan =1

Loss on heating%

3.0 or less

3TS-216-1

120C  30 min

Roll core
Substrate film 38 m
Adhesive layer 30 m
Separator film
25 mUnwinding direction

Figure 4. Product structure

3-3. Characteristic values
Table 3 presents the characteristic values ofTB1630.
TB1630 is designed to have nearly the samerefractive index as glass. It provides hightransmission in the visible light range—99% or
better—when used on glass adherends. This makes
it ideal for optical assembly applications.

Table 3. Characteristic values of TB1630Tested items
Refractive index
Tensileshear
strengthUnit

TB1630

Test method



1.51

3TS-536-01

glass/glass
acrylic/glass

5.1
MPa

polycarbonate/glass

Elastic modulus (25C)
Visible light transmission

Notes

4.0

3TS-301-11

3.5
Pa%

1.9  106
99.94 to99.74

3TS-501-04

DMA loading in tensile mode (1 Hz)

500 to 800 nm

glass/TB1630/glass sandwich
structure

UV/Visspectrophotometer

Reference: glass (1 sheet)

Curing condition: Integral radiation of 30 kJ/m2 (measured at 365 nm wavelength)3high-temperature and high-humidity tests. Spectral
transmittance remained virtually unchanged, evenafter 1,000 hours of simulated weathering as logged
on a weather meter. These results point toremarkable resistance to environmental factors.

Tensile shear strength (MPa)Tensile shear strength (MPa)3-4. Reliability data
Figures 5 to 8 present the results of reliability
testing.
We confirmed high reliability with respect toadhesive strength in both high-temperature andglass/glass
glass/polycarbonate

glass/glass
glass/polycarbonate

Time (hours)

Time (hours)

Tensile shear strength (MPa)Figure 5. Change in tensile shear strength
when subjected to conditions of60C  95 RH

Figure 6. Change in tensile shear strength
when subjected to conditions of85C  85 RH

glass/glassglass/polycarbonate

Time (hours)

Transmission ()Figure 7. Change in tensile shear strength when subjected to temperature of 100°C

Wavelength (nm)Figure 8. Change in spectral transmittance when subjected to simulated weathering (no reference)4

4.

Using ThreeBond 1630

4-1. Steps in bonding with ThreeBond 1630
The three main steps in bonding with the TB1630
consist of the transfer, bonding, and curingprocesses. Figure 9 presents the steps that make up
the bonding process.
1. Transfer process [Figure 9, (1), (2), & (3)]The transfer process for TB1630 involves thefollowing two steps: in step (1) peel off theseparator film on the light-peeling side. In step (2)use a roll laminator to apply heat and pressure toincrease contact between the sheet adhesive and theadherend and to ensure a secure transfer.

(1)

2. Bonding process [Figure 9, (4) & (5)]After the transfer is successfully completed, instep (4) peel off the remaining substrate film on thereverse side of TB1630 and press onto the otheradherend. In step (5), the adherends placed incontact are pressed together under heat to create thebond. Ideally, this step should be performed in a
vacuum to ensure a bubble-free adhesive layer.
Recommended adhesion devices include a
vacuum press and vacuum laminator. These will bediscussed further below.
3. Curing process [Figure 9, (6)]Cure TB1630 by irradiating with UV light. The
curing conditions is 30 kJ/m2 (measured at 365 nm).

(2)

Peel separator film

(6)(3)

Pass through hot roller(5)

In vacuum

UV irradiation

Apply heat/pressure

Curing process

Bonding process

Transfer complete(4)

Peel substrate film

Figure 9. Step-by-step bonding with the TB1630

4-2. Recommended bonding machines
We recommend the following equipment for theabove processes:
1. Roll laminator
This device performs pressure-bonding bypassing the adherend elements through rotating
rubber rollers.
Ideal for pressure-bonding films and thin flat
plates, the roll laminator is used to transfer TB1630
to the adherend and to bond films. The process canbe performed continuously from roll to roll toenhance productivity.
We recommend using heat roll laminators tomaximize contact between the adhesive andadherend.
Figure 10 presents a schematic diagram of a rolllaminator unit.

52. Vacuum press
The vacuum press laminator applies heat andpressure in a vacuum via an air cylinder. Figure 11
presents a schematic diagram of a vacuum pressunit.
The vacuum press unit enables high-pressurepressing under vacuum conditions. It is ideal for
applications such as bonding two highly parallel
rigid bodies or bonding films onto surfaces.
3. Diaphragm vacuum laminator
The vacuum laminator consists of two chambers,
an upper chamber and a lower chamber, separated
by a rubber diaphragm. Both chambers areevacuated. Air is then allowed to flow into theupper chamber. The pressure difference createdbetween the chambers inflates the rubber diaphragm,
which presses the elements to be laminated against
each other. Figure 12 illustrates the vacuum
laminator mechanism. A hot plate is placed in thelower chamber, which makes it possible to apply
both heat and pressure simultaneously.
Since the vacuum laminator uses a rubber
diaphragm for pressing, the diaphragm conforms tothe surface geometry of the work element andapplies pressure evenly over the entire surface. The
rubber diaphragm allows effective pressure
application even at low pressures, making thedevice ideal for bonding delicate elements. Figure
13 is a photo of a vacuum laminator unit.

upper chamber
Evacuation

lower chamber(1) Work set(2) Evacuating the upperchamberOpen (press)

Evacuation(3) Evacuating the lowerchamber(4) Open the upper chamber(pressing process)Figure 12. Vacuum laminator operating
mechanism

Figure 13. Vacuum laminator

5.

Figure 10. Roll laminator

Air cylinder

Vacuumchamber

Figure 11. Vacuum press

6

Examples of applications

In addition to enhanced visibility, TB1630
produces an adhesive layer free of air bubbles andsqueeze-outs with high conformity to irregularities
in the adherend surface—characteristics beyond thereach of conventional liquid UV-curing resins or
optical double-sided adhesive tapes when eitherclass of products is used alone.
TB1630 is ideal for areal adhesion onto display
modules that require highly transparent display
surfaces for visibility.
[Main applications](1) Bonding of panel overlays* to LCD modules(2) Bonding of touch panels to LCD modules(3) Bonding of panel overlays to touch panels(4) Bonding glass/glass surfaces(5) Bonding optical films(6) Other optical assembly applications

* Panel overlays are protective panels for displays
such as LCDs (as shown in Figure 14), typicallycomposed of materials such as glass or plastic(acrylic resins, polycarbonates, etc.).

Panel overlay

Touch panel

LCD module

Figure 14. Example of applications: LCD
module/touch panel and touch
panel/panel overlay

6.

Examples of advanced applications
Bonding materials with different
thermal expansion coefficients

In recent years, manufacturers have begun
exploring switching from glass to plastics to makecell phone panel overlays. Candidate materials
include acrylic resins or polycarbonates. However,such changes pose certain technical issues.
 Heating generates gas from the plastic andcreates bubbles, creating cosmetic defects.
 The difference in coefficients of thermal
expansion between the glass LCD module andthe plastic panel overlay generates distortion
when heat is applied, creating display problems.
Additionally, warping of the plastic material
can cause the panel overlay to peel off.

2. Transfer process
Perform the standard transfer process.
3. Bonding process
When heat is applied during the bonding process,the difference in thermal expansion coefficients
between the materials generates distortions in thematerials. Pressing adherend elements when they
are distorted increases the likelihood of display
problems.
To avoid this, bonding should be performed in avacuum at normal temperatures. Bonding at normal
temperatures will reduce distortion caused bydifferences in thermal expansion characteristics.
4. Post-process
Bonding at normal temperatures will produce
inadequate adhesion to the adherend. The adherend
elements must be subjected to a thermal treatment(aging) after bonding to achieve the necessary
degree of wetness.
The yield rate of the manufacturing process canbe improved by removing the small amounts of air
bubbles remaining by applying heat and pressure in
an autoclave.
Applying these steps when necessary on a case bycase basis should eliminate air bubbles and display
problems when bonding plastic panel overlays toLCD modules using TB1630

Glass panel
overlay

Baking (degassing)Transfer

Transfer

Vacuum bonding (heating)

Vacuum bonding (normal temperature)

We propose the following procedure to resolvethese technical issues. Figure 15 presents a flow
chart for the recommended procedure.
1. Preparatory process
The air bubbles produced when bonding plastics
is due to the gases present in the plastic material.
The plastic material to be used must be baked(heated) before processing to degas the material andsuppress bubble generation during bonding.

Plastic panel
overlay

Aging (thermal treatment)

Autoclave

Curing (UV irradiation)

Curing (UV irradiation)

Figure 15. Flowchart of the bonding process

7Conclusion
Functional sheet adhesives are revolutionary adhesives that combine the advantages of liquid UV-curing resins
and optical double-sided adhesive tapes. ThreeBond is currently developing a full range of sheet adhesive
products in addition to the UV-curing sheet adhesive introduced in this issue, including heat-cured products andproducts offering other functions.
We intend to continue in our efforts to develop sheet adhesives offering new functions to provide customers
with an ever broadening lineup of products.
Takashi Nemoto
Yoshiaki Koga
Functional Material Development Department
Product Development Division
R&D Group
ThreeBond Co., Ltd.

1456 Hazama-cho, Hachioji-shi, Tokyo 193-8533, Japan
Tel: 81-426-61-1333

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