ThreeBond Technical News
Issued July 1, 201076

Light-curing adhesives compatible
with UV-LED light sources
Introduction ________________________________________________The proliferation of light-emitting diodes (LEDs) has brought the lighting industry to a majortransition point. These devices have entered use not just for special lighting applications like thoseencountered for mobile phones, liquid crystals, or in the automotive industry, but for general-purpose
domestic lighting. Driving this trend is light-emitting diode technology’s potential for addressing recent
global environmental concerns. In lighting devices, LEDs provide longer life, power savings, and lower
heat emission. Today’s eco-boom suggests these trends will gain strength. Light irradiation systems
based on light-emitting diodes are rapidly gaining a wider share in the light-curing adhesive market andhave already been introduced in various domains, including the pickup, HDD, and liquid crystal
industries.
Since its founding, ThreeBond has widely marketed adhesives for various applications, including
light-curing adhesives. Over twenty years have passed since ThreeBond marketed its first product.
During this time, ThreeBond has continued to develop light-curing adhesives that offer diverse curing
characteristics, including UV curing and visible-light curing.
This issue introduces three light sources based on light-emitting diodes and the products that can beused with these light sources: ThreeBond 3017D, ThreeBond 3017E, and ThreeBond 3017F.
(Note that ThreeBond is abbreviated “TB” in product names hereafter.)Contents
Introduction .................................................................1
1. Background............................................................2
2. Overview of UV-LED light sources.........................2
2-1 Principles of LEDs ..........................................2
2-2 Comparison of wavelength of light emitted
from UV-LED source and discharge lamp ......3
3. Applying UV-LED light sources ..............................4
3-1 Radical reactions............................................4

3-2 Use of UV-LED light sources ......................... 4
4. Products (TB3017D, TB3017E, TB3017F) ............ 5
4-1 Product features............................................. 5
4-2 Curing speed.................................................. 6
4-3 Reliability data................................................ 6
5. Overview of UV-LED system ................................. 7
Conclusion .................................................................. 81

1.

Background

Light-curing resins cure instantly when exposed
to light from a light source, a trait that has led to
growing demand in the electric market, especiallyin applications involving inks, paints, adhesives,and optical materials. Curing requires a light source.
Discharge lamps (e.g., high-pressure mercury lamps
and metal halide lamps) are conventional andwidely used light sources, but light-emitting diodes(light emission by electroluminescence and DC
voltage; LED hereafter) and their advantages overdischarge lamps have drawn attention. Since thecommercialization of GaN-based blue LEDs by
Nichia Corporation in 1993, use of LEDs as lightsources for light curing has rapidly grown. Figures
1, 2, and 3 below show examples of applications inwhich LED light sources have been introduced.
In general, special light sources in the area oflight-curing adhesives use LEDs that emit light at awavelength of 365 nm. Hereafter, we will refer to
LED light sources emitting light at a wavelength of
365 nm as UV-LED light sources.

2.

Overview of UV-LED light sources

2-1 Principles of LEDs
The basic structure of an LED is a “PN junction,”
which consists of a p-type semiconductor (asemiconductor with holes as majority carriers) andan n-type semiconductor (a semiconductor withelectrons as majority carriers). The junction iscovered by transparent resin (generally, epoxy
resin). When voltage is applied, the holes andelectrons move and collide, emitting energy. An
LED emits light when the emitted energy isconverted to light energy. This is the principle oflight emission (Figure 4). Here, the wavelength ofthe light emitted varies according to the type ofcomponent semiconductors.
p-typeLight emission
VoltagePower supply

n-typePickup market

Figure 4: Principles of light emission

Figure 1: Adhesion of lens and holder
Camera module market

Most LED products have one of two types ofstructures. One is the bullet LED, in which theframe and the LED chip are integrated andencapsulated in epoxy resin (Figure 5).
The other is the surface-mounted (SMD) LED,
which is encapsulated in silicone or epoxy resin(Figure 6). LEDs incorporate reflectors or lenses toimprove the directionality of the emitted light. Since
thermal degradation reduces an LED’s output power,
improvements in heat dissipation via heat sinks orother devices are essential to high output power.

Figure 2: Adhesion of lens and tube
Liquid crystal (one drop fill) market

LED chipFigure 5: Bullet typeFigure 3: Temporary bonding of liquid crystal
panel

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LED chipFigure 6: Surface-mounted (SMD) typeComparison of wavelength of lightemitted from UV-LED source anddischarge lamp
Figures 7 and 8 below show the wavelength of a
UV-LED light source and a discharge lamp, respectively.
The UV-LED light source is characterized by its
monochromatic nature.
Relative intensity (%)

2-2adhesives. The reason is that the advantages in thedevice are not necessarily accompanied withadvantages in the reaction. Table 1 below shows the
wavelength dependence of energy. Energy is greaterfor shorter wavelengths. A light source with awavelength of less than 365 nm—for example, in
the range of 200 nm to 300 nm—provides several
times more energy and is a better light source forinducing reactions. For this reason, particularly withrelatively low energy at the 365 nm wavelength
provided by a UV-LED light source, improvements
in reactivity require simultaneous technicalinnovations in light-curing adhesives.
Table 1: Energies and frequencies ofelectromagnetic waves

Wavelength (mm)Relative intensity (%)

Figure 7: Wavelength of UV-LED light source

Wavelength (mm)Figure 8: Wavelength of discharge lamp

Discharge lamp

Reduced thermal
damageTemperature (°C)

The difference in wavelengths corresponds to thedifference in energy. While discharge lamps haveraised concerns regarding the effects on resinmaterials caused by high-intensity peaks in thevisible and infrared region, UV-LED light sources
are free of such concerns. Figure 9 illustrates thedifference in the measured temperature increase.

UV-LED light source

Elapsed time (sec)Figure 9: Comparison of temperature increase
The difference in the wavelength corresponds tothe difference in energy, apparently demonstrating
the clear advantage of a UV-LED light source. In
fact, this trait raises concerns when UV-LED lightsources are used for light curing. It also requires
careful design in the corresponding light-curing

Energy (kJ/mol)
6Frequency (1/s Hz)
to 2  1016

Electron beam

to 10

Ultraviolet

300 to 600

0.75 to 2  1015

Visible light

150 to 300

0.40 to 2  1015

Infrared

8 to 60

0.20 to 2  1014

Application also requires attention regarding thetransparency of the material. Table 2 below shows
the light transmission characteristics of typicalmaterials. Light-curing adhesives are used not justin applications requiring direct light irradiation, but
in applications requiring light irradiation throughmaterials—for example, when bonding twomaterials. In such cases, we cannot ignore lightabsorption, even in transparent materials. A
UV-LED light source is monochromatic at 365 nm,
requiring even greater care.
Table 2: Light transmission of various materials
365 nm405 nmRigid PVC

0.1%

50%

435 nm Remarks
80%

Mitsubishi 302 (2 mm thick)Flexible PVC

0.3%

44%

77%

Takato Kasei (2 mm thick)

ABS

0.1%

5%

10%

PBT

0.1%

13%

17%

Polycarbonate

0.5%

50%

95%

Soda glass

48%

80%

85%

Tsutsunaka Tough Ace(2 mm thick)

Duranex 2002
(2 mm thick)Teijin Panlite PC-111(2 mm thick)Tempax (5 mm thick)Note: The above materials require care when used with a
UV-LED light source.

As discussed above, using a UV-LED light source
effectively requires new designs for light-curing
adhesives based on the wavelength and energy ofthe light source and the material to be bonded. The
next section discusses various aspects affected bydifferences in light sources, using radical reactions
as an example.

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3.

Applying UV-LED light sources

3-1 Radical reactions
Radical reactions are the most common reactions
used with light-curing adhesives. Equations 1, 2,
and 3 below show typical reaction formulae. In
general, we use backbones with acrylic (ormethacrylic) groups. The main backbone structure
varies widely and includes epoxy, silicone, urethane,and polyester. The reaction begins with lightabsorption by the photo-polymerization initiator,
which generates the radical species (Equation 1).
The reaction proceeds with the radical species
initiating the polymerization reaction (Equations 2
and 3).
[Radical polymerization reaction]
Equation 1)
Equation 2)
Equation 3)
Note: PI:

Photo-polymerization
initiator
PI*: Photo-polymerization
initiator radical species
M, Mn: Monomers

Equations 4, 5, and 6 show the polymerization
termination reaction. Deactivation occurs whenoxygen combines with the photo-polymerization
initiator radical or the monomer radical andgenerates the termination reaction. Oxygen canreact with the radical species, even if the oxygen
itself is in the ground state.
[Radical termination reaction: oxygen inhibition]
Equation 4)(Termination)

Equation 5)(Termination)

Equation 6)(Termination)
Note: PI:

Photo-polymerization
initiator
I*:
Photo-polymerization
initiator radical species
M, Mn: Monomers
OxygenO2 :3-2 Use of UV-LED light sources
As discussed above, a UV-LED light source ismonochromatic. Its overall energy is relatively low.

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In terms of reaction formulae, this means that thetermination reaction indicated by Equations 4, 5,
and 6 occurs readily, because the termination
reaction caused by oxygen inhibition and thepolymerization reaction are competing reactions.
Lower energy increases oxygen reactions. In otherwords, we must be aware that a UV-LED lightsource requires consideration of oxygen inhibition
to an extent we might not expect with conventional
light sources. To use a UV-LED light source
effectively, we must design the light-curing
adhesives with an eye to preventing the reaction
termination and efficiently curing the resin.
Manufacturing problems stemming from thereaction termination include adhesion of foreign
objects caused by lowered surface curing, increased
outgassing, and increased water absorption. All
these phenomena result in product defects andrequire care. Reaction terminations that generate thephenomena occur readily with flexible resins, since
reactions in flexible resins tend to be slow, withpoor surface curing even when using conventional
light sources. Any change in light sources will
degrade curing still further.
In sum, developing flexible light-curing resinscompatible with UV-LED light sources poses
numerous challenges for conventional technologies.
Based on its early start, ThreeBond has alreadybrought products to market. The next andsubsequent pages introduce our product line offlexible light-curing adhesives compatible with
UV-LED light sources.

4.

Products(TB3017D, TB3017E, TB3017F)

4-1 Product features(1) Curable with a UV-LED light source.
(2) Flexible; excellent surface curing performance(3) Excellent bonding strength to COP(4) Excellent environmental durability(5) Low halogen products with total chlorine andtotal bromine, each less than 900 ppm andtogether less than 1,500 ppm
Tables 3, 4, and 5 below give the physicalproperties.

ThreeBond has developed the TB3017D,
TB3017E, and TB3017F mainly for bonding lenses
and holders in the electronics market, including
optical pickup and camera modules. The productfeatures are listed to the right. The most prominent
feature of these flexible light-curing acrylic
adhesives, which tend to be sensitive to oxygen
inhibition, is smooth curing with UV-LED lightsources. Compared to conventional adhesives, theproducts offer improved bonding strength tocycloolefin polymers (COP), a difficult-to-bondmaterial. They also meet halogen control
requirements for today’s markets.

Table 3: General properties
Test item

Unit

TB3017D

TB3017E

TB3017F

Test method

Features-Low viscosity

High viscosity

High strength-Appearance

-

White

White

White

3TS-201-1

Viscosity

Pas

13

25

7.5

3TS-210-10Specific gravity

-

0.93

0.93

0.93

3TS-213-02

Remarks

Shear velocity: 20s−1

Table 4: Characteristic values
Test item

Unit

TB3017D

TB3017E

TB3017F

Test method

Remarks

Hardness

Type-A

41/40

35/33

58/59

3TS-215-01

30kJ/m2

Water absorption%

0.2/0.3

0.4/0.6

0.3/0.4

3TS-233-03

30kJ/m2

Cure shrinkage%

6.4/6.0

6.5/6.2

6.6/6.3

3TS-228-01

30kJ/m2

Thick film curingmm

2.5/2.6

3.0/3.3

2.6/2.9

3TS-222-01

30kJ/m2Pa
DMA

9.1  106/7.1  106 6.9  106/5.9  106

1.1  107/1.2  107

C-54/-52-55/-52

2.0

C-18/-16-26/-23

13/11

Note:

E', 25C, 30kJ/m2
3TS-501-04

E'' peak, 30kJ/m2
tan  peak, 30kJ/m2

A or B indicated identifies the curing conditions as below.
A: Discharge lamp (30 kJ/m2, dominant wavelength 365 nm)
B: UV-LED light source (30 kJ/m2)

Table 5: Tensile shear bonding strength
Test item

Adherend

Unit

ZnDc/glass
Tensileshear
bonding
strength

COP/COP (Zeonex®)
COP/ZnDc

MPa

TB3017D

TB3017E

TB3017F

4.3/4.3

3.0/3.2

3.4/3.2

2.6/2.7

2.8/3.0

2.8/3.0

3.0/2.7

3.0/3.2

3.8/3.9

COP/LCP (Vectra® E130i)

1.0/0.9

1.0/1.1

2.0/1.9

COP/PPS (Tosoh Susteel® GS40)

1.2/1.2

1.2/1.2

2.4/2.2

Note:

Test method

3TS-301-13

A or B indicated identifies the curing conditions as below.
A: Discharge lamp (30 kJ/m2, dominant wavelength 365 nm)
B: UV-LED light source (30 kJ/m2)

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4-3 Reliability data
This product line not only offers sufficient curing
speed with UV-LED light sources; they offer better
performance than conventional adhesives withrespect to durability under various environmental
conditions.
Figures 12 and 13 below show the results ofdurability tests.

Before reaction

Tensile shear bonding
strength (MPa)

4-2 Curing speed
Curing speed is generally measured by changes in
the characteristic peaks of the functional groups
observed in FT-IR. As shown in Figure 10, the peak
shapes differ before and after the reaction. Here,changes appear in the double-bond peaks of thevinyl group near 1610 cm−1 to 1630 cm−1. The
peaks decrease as the reaction progresses. Thisdecrease is used to check the reaction percentage.
By monitoring changes in reaction percentage
over time, we can also measure reaction percentage
per unit time, or the reaction rate. As shown in
Figure 11, the product line is also compatible with
UV-LED light sources with respect to curing speed;these products provide even faster curing.
After reaction

CyclesFigure 12: Heat cycles (−40°C to 80°C, 30 min.
each)
Tensile shear bonding strength after
predetermined duration(Adherend: COP/COP)Note:

The resin is cured with a UV-LED light source (200mW  15 sec.)

Tensile shear bonding
strength (MPa)Figure 10: Decrease in peaks associated withreaction

Reaction
percentage (%)

Typical flexible UV resin

Time (h)Elapsed time (s)Figure 13: Humidity (60°C  95% RH)
Tensile shear bonding strength after
predetermined duration(Adherend: COP/COP)

Figure 11: Reaction rate measurement by FT-IR
Note:

The resin is cured with a UV-LED light source (200mW  15 sec.)

Note:

The resin is cured with a UV-LED light source (200mW  15 sec.)

Table 6: Tensile shear bonding strength for different UV-LED illuminance
Test item
Tensile shear
bonding
strengthAdherend
COP/COP(Zeonex®)

Unit
MPa

TB3017D

TB3017E

TB3017F

2.6

2.9

2.9

2.7

3.0

2.9

2.7

3.0

3.0

Test method
3TS-301-13

Note: The resin is cured with a UV-LED light source (200 mW  15 sec.)

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5.

Overview of UV-LED system

This section discusses overall differences insystem configuration, beyond the difference in thelight sources discussed above. (Data provided by
Omron Corporation)
Table 7 below compares the two light source
systems. The table shows that there are manydifferences among light source systems. We will
discuss the details of major items later. UV-LED
light source systems also have other advantages
with respect to the environment, such as
mercury-free construction and reduced power
consumption.
Table 7: Comparison of light sources
UV-LED light source
Illuminance

94kW/m2

Discharge lamp
2

Approx.40kW/m

Life40,000 hours

Approx. 3,000 hours

Space saving

Yes

No

Light source
response

ON/OFF control
possible

Continuous lighting

Stress to adherend

Small

Large

No mercury

Contains mercury

Approx. 53 W

Approx. 250 W

Environmental
concerns
Power consumption[Space savings and branching efficiency]
The two light source systems differ significantly
in one particular respect: whether the light-emitting
section is inside or outside the system main body.
With discharge lamp systems, the light-emitting
section is positioned within the main body,requiring a relatively large space. With UV-LED
light source systems, the light-emitting section is at
the cable end, outside the main body. The latter
configuration allows space-saving designs.
Conventional
lamp system

1/8
size

1
branch2
34branches branches branches

UV-LED
light source
system

100%

100%

100%

100%

Discharge
lamp
system

100%

85%

66%

57%

Figure 15: Comparison of output after branching[Proposal for reducing damage to adherends]
A UV-LED light source can reduce the heatgenerated during extended uses. This feature
resolves problems with targets potentially subject todeformation or shrinkage caused by heat.
(See Figure 9.)[Proposal for reducing operating costs]
The higher stability of the light source output
eliminates the need for continuous lighting. The
system provides longer life and can significantly cut
operating costs. Products with considerably reduced
initial operating cost are also available.
40 fold

120,000 hours(Approx. 41 years)

13 fold
40,000 hours(Approx. 4.5 years)

3,000 hours(Approx. 0.34 years)

Figure 14: Volume of light source
Further, branching in UV-LED light source
systems does not reduce output, for the same
reason: the light source section is not inside themain body. With discharge lamp systems, whichhave the light source inside the main body, output
falls at every branching, and final illuminance also
falls due to use of long fiber. UV-LED light source
systems improve productivity by allowing controlof large-scale branching.

Discharge lamp
(Continuous
lighting)

LED lamp(Continuous
lighting)

LED lamp(ON/OFF
controlled)

Figure 16: Comparison of operation life
Note: Omron ZUV Series

7Conclusion
The demand for spot curing systems based on UV-LED light sources is growing annually in the areas of camera
modules, HDDs, and pickup systems. Area curing systems are also scheduled for marketing in liquid
crystal-related applications.
ThreeBond has continued to provide adhesives suitable for customer systems, and it will continue to strive toserve as an effective partner by developing light-curing adhesives that meet changing needs.
<References>
Omron Corporation
 Catalog: Introduction to UV Curing
 Catalog: UV Curing System ZUV Series
 Catalog: Applications
Junya Iwasawa
Electric Development Division
Development Department
Headquarters and R&D Laboratory
ThreeBond Co., Ltd.

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

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