ThreeBond Technical News
Issued July 1, 2016

88

Light-Induced Color Change TechnologyIntroduction
Design is an important part of being interested the customer’s attention in the handheld devices
and electronics that fill our lives. Light curing resins are widely used in the manufacturing ofthese because they shorten manufacturing tact time and do not require a heat source. However,in a world where design is important, resins that change color as they cure have not yet come into
heavy usage as transparency issues make them inferior in molded products and solvent resins. AtThreeBond, we have been developing light curing resins with an emphasis on design in recentyears, and within these projects, we have also looked into light-induced color changing technology
as well as how to overcome weaknesses in color-changing light curing resins.
In this issue, we would like to introduce this light-induced color changing technology, its
possibilities and its application.

Table of Contents

Introduction.................................................1
1. Pigment and Dye.....................................2
2. Instances of Deep Coloration in Light
Curing Resin............................................2
3-1. Leuco Dye............................................23-2. Light Induced Color Change
Technology in Leuco Dye.....................34-1. Light Curing Technology in thisTechnology...........................................34-2. Wavelength Control via the Light
Source (2-Step Cure)...........................34-3. Delay of Color Development Using
an Amine Compound...........................5
5. ThreeBond 3020.....................................6
6. Future Issues...........................................6
Closing........................................................811.

Pigment and Dye

Some kind of coloring agent must be used to make a resinchange color, and these are broadly divided into pigments
and dyes.
Pigments exhibit powerful opacity with a very clear tone.
For example, inorganic compounds such as carbon black,iron oxide, titanium-oxide, etc., where black pigment
exhibits the most powerful light blocking effects. Black
pigment is well-known for its carbon black transmissivity as
it absorbs the light of the entire visible range that the humaneyes is capable of seeing.
Dye, on the other hand, is less opaque than pigment, witha tone that gives a sense of depth. Dyes are characterized
by the way they absorb specific wavelengths of individualmolecules. However, it is difficult to make a substance witha complicated structure that will absorb the entire visible
light range equally with only a single type of molecule.
Therefore, black dyes are often composed of several different
color compounds rather than a single black compound.1)

2.

Instances of Deep Coloration in Light
Curing Resin

There are several cases where light curing resin is madeto produce a black color for the purpose of blocking light.
However, when even a small amount of jet black high-carbon
black pigment is added, curing deteriorates considerable, so
the compounds base light curing resin characteristics are
not displayed at all.2)

Unfortunately, the small amount of black dye used insituations such as the one described above is insufficient for
producing a jet black color. As a result, this is not suitablefor use in thin films because in order to achieve jet black, thefilm thickness must be increased.
In both these cases, we can see there is always a tradeoff
between how jet black the color is and the curability of
deep areas. To achieve the desired results considering both,
we must make a dual curing resin that combines curing
mechanisms that react due to applied heat as well as via
moisture in the air same as a caulking agent. We focusedon leuco dye as the material that would solve these coloring
issues.

3-1. Leuco Dye

The basic structure of leuco dye is shown in Figure 1. In
leuco dye, the lactone ring inside the molecule reacts withacid to open the ring and release color. But when a basemakes contact with the ring, it closes and the color fades. In
other words, tone can be tuned on and off using acids andbases.4)
By taking advantage of these features, leuco dye is usedin heat and pressure-sensitive paper5), and in recent years
has also be used in the rewriting6) technology used on ICcards. Various tones are combined with a developer and a
discoloration temperature adjuster to form an ink whosecolor disappears in frictional heat. This is used in manyfamiliar products, such as stationary.7)

Because dyes contain a mixture of various tints, they can be
designed to allow a specific wavelength to pass, such as 365
nm. These dyes already exist on the market, so if they canbe designed in line with the specific wavelengths emitted inhigh-pressure mercury lamps, such as 365 nm or 405 nm,light curing with a deep color is possible.3)

RRNROR

R

N+O

R+H+O

R-H+OH
COO

Color fades(closed ring)

Color develops(open ring)

Fig. 1 Leuco Dye Structure and Reversible Reaction

2R

UV light
Before exposure

After exposure

Colorless

ColoredBasic composition(Meth) acrylic resin
Photoinitiator

Curing reaction

Photoacid generator
Leuco dyeColor reaction

Utilize the difference inreaction rate

Fig. 2 Curing and Color Development Mechanisms in Previous Leuco Dye Technology

3-2. Light Induced Color Change
Technology in Leuco Dye

Applying a low energy light source such as a blacklight toleuco dye can be used for both light curing and light-induced
color change.8)
The basic composition is that it is a liquid before light
exposure and made of (meth) acrylic resin, photoradical
polymerization initiator, leuco dye and photoacid generator.
The curing reaction progresses through photoradical
polymerization just like in general light curing resins.
The color develops when the leuco dye contacts the acidgenerated by the photoacid generator. The point to note inthis procedure is that by utilizing the difference in reaction
rates, inhibition of curing caused by color development canbe reduced.
Additionally, tone depth is adjusted through the acidconcentration. The color is still inferior to pigment, but asopposed to conventional dye, sufficiently deep color is still
possible, even for thin film. This makes it easier to achievea jet black color while ensuring that deep areas are cured.
The previous technology mentioned above has beendeveloped for a limited number of manufacturing purposes,
but due to the time required in exposure to light, it is not
yet appropriate for adoption in designed parts for electronic
appliances and other mass production.
Now, however, we have found a way to produce color
development marinating the short curing time that lightcuring resins excel in, which we will introduce below.

4-1. Light Curing Technology in thisTechnology

The following two methods were used to improve both deeparea curing and curing speed.
1. Wavelength control via the light source (2-step cure)
2. Delay of color development using an amine compound

4-2. Wavelength Control via the Light
Source (2-Step Cure)

Figure 3 is an image of the mechanism for curing performed
via light exposure in two steps. The first step involves acuring wavelength of 400 nm or more, and the second stepis where color is developed at a wavelength of 370 nm orless. This procedure keeps curing and color development
separate to maintain adhesive effectiveness.
Although the basic composition of this technology is thesame as the previous technology mentioned above, the focus
here is the difference in the spectrum of absorption between
the photoradical polymerization initiator and the photoacid
generator.
This difference in the spectrum of absorption between thephotoradical polymerization initiator and the photoacid
generator is shown in Figure 4. As shown in Figure 4, there
is a range where the absorbed spectrums of the two do not
overlap (shaded area). We verified that by keeping in thisrange, we could cure without color developing.

3

1st STEP: Light exposure
at 400 nm or more

2nd STEP: Light exposure
at 370 nm or less

1st STEP:
Curing2nd STEP:
Color development

Beforeexposure

ColoredColorlessFig. 3 Wavelength Control via Light Source (2-Step Cure) Mechanism

We exposed the adhesive to LED light at both 365 nm and405 nm wavelengths. Light transmittance and reactivity
for 365 nm exposure versus time is indicated in Figure 5and Figure 6, respectively. Light transmittance decreases
immediately after exposure at 365 nm, whereas it maintains
its initial value at 405 nm. Additionally, Figure 6 shows thatthe reaction reaches equilibrium more quickly and is more
highly reactive at 405 nm than at 365 nm.

Photoinitiator A

Light absorption (a.u.)

Photoacid generator A
Acetonitrile solution

Figure 7 shows a picture of cured material. These results
show that exposure at 405 nm results in complete curing ofeven the deepest areas without color developing, while at365 nm curing was hindered due to color obstructing lighttransmission.

250

300

350

400

450

100

500

90Wavelength (nm)Fig. 4 Spectrum of Absorption for the
Photoradical Polymerization Initiator and
the Photoacid Generator

Reaction Rate (%)

8050
Light transmittance at 365 nm (%)

Exposure wavelength: 365 nm
40Exposure wavelength: 405 nm

6050
40Exposure wavelength: 365 nm

30Exposure wavelength: 405 nm

2030
Illuminance: 100 mW/cm2
Film thickness: 1.0 mm

Illuminance: 100 mW/cm2
Film thickness: 1.0 mm

1002001020304050

Time exposed (s)10Fig. 6 Reactivity versus Exposure Time

001020304050

Time exposed (s)Fig. 5 Changes in Light Transmittance versus
Time at 365 nm Exposure Wavelengths

4706060

Even after curing is completed at 405 nm, exposure atwavelengths of 370 nm or less produces the same color
change as in other previous technologies.

4-3. Delay of Color Development Using anAmine Compound

Figure 8 is an image of the curing mechanism. We haveadded an amine compound to the basic composition of theprevious technology. Amine compounds are sometimes used
to improve stability of photo-cationic polymerized storage9).
This is because the amine compound traps the acid releasedby the photoacid generator to act as a polymerization
inhibitor. We took advantage of this in our technology,
creating a design which uses the amine compound to trapacid released by the photoacid generator during exposure
to a single 365 nm wavelength. Doing this slows the start of365 nm only405 nm only

365 nm only

Fig. 7 Resin Appearance Before and After Exposure to LightUV-LED 365 nm
Before exposure

After exposure

Radical polymerization starts
Time lag

Colorization reaction starts

Basic Composition(Meth) Acrylic Resin
Photo-radical Initiator

Curing Reaction

Photoacid Generator
Leuco Dye

Colorization Reaction

Amine Compound

Colorization RetardantFig. 8 Color Retardant Mechanism Using Amine Compounds

5the reaction that develops color, creating a lag between thestart of curing and that start of color development so that thecuring of deep areas is improved.
Light transmittance over time at exposure to 365 nm isshown in Figure 9, and reactivity is shown in Figure 10.
Light transmittance immediately weakens after exposure inproducts that do not contain the amine compound, while itis maintained for several seconds after exposure when thecompound is added. The reaction rate in the figure indicatesadding the amine compound to produce a time lag speeds up
curing and also produces higher reactivity.
50

Light transmittance at 365 nm (%)

No amine compound
40

Amine compound added30

Illuminance: 100 mW/cm
Film thickness: 1.0 mm

20ThreeBond 3020

6.

Future Issues

ThreeBond has adopted the technology described above
and released ThreeBond 3020. ThreeBond 3020 is asingle-liquid acrylic resin that cures through exposure tolight at 200 to 420 nm. Table 1 shows values indicating its
various characteristics. Before exposure to light, it is a paleyellow, but turns black once exposed to 370 nm or less. For
ThreeBond 3020, we recommend the afore-mentioned 2
Step Cure as this ensures better curing of deep areas than
exposure to a single wavelength.

In addition, light-curing resins that use leuco dyes containphotoacid generators so the existence of incredibly powerfulacids must be taken into consideration. This makes themdifficult to develop and apply for electrodes, wiring areas
and other places where acid is not compatible.

100102030405060

Time exposed (s)Fig. 9 Light Transmittance at 365 nm versus
Exposure Time
100
9080
Reaction Rate (%)

5.

All dyes, including leuco dyes, may fade over time10).
Measures must be put in place to protect against
environmental weak points, such as high temperature above
the base resin’s glass transition point, high humidity orexpose light.

2070605040No amine compound

30

Amine compound added

20Illuminance: 100 mW/cm2
Film thickness: 1.0 mm

10001020304050Time exposed (s)Fig. 10 Reactivity versus Exposure Time

6The length of this time lag depends on the amount of aminecompound added, its classification and functional group,
and is also a factor of adjusting the level of black and deeparea curability.

60

Moving forward, these issues must be addressed and solved.

Table 1 ThreeBond 3020 Characteristic Values
FeaturesUnit

2 Step Cure*1

Test conditions

Remark(s)External appearance(liquid)

Single 365 nm
wavelength exposure*2—

Pale yellow

Pale yellow

3TS-2100-020—

Viscosity (25°C)

mPa·s

1210

1210

3TS-2F00-007

Shear velocity 76.6 s-1

BlackBlack

3TS-2100-023

—mm

2.0

0.9

3TS-3160-001—Hardness

—

D65

D60

3TS-2B00-010

Type-D

Tensile shear bondstrengthMPa

7.4 (material failure)

7.5 (material failure)

3TS-4100-013

Glass/GlassLight transmittance

%

1.2

1.1

3TS-2940-003

Film thickness: 150 μm,
wavelength: 550 nm

External appearance(cured)Thick film curingperformance*1 Curing conditions: LED (405 nm) 100 mW/cm2×30s + LED (365 nm) 500 mW/cm2×30s*2 Curing conditions: LED (365 nm) 500 mW/cm2×30s

7Closing

This has been an introduction on light-induced color change technology that utilizes leuco dye.
This technology has finally been developed to the point where it can be practically applied. The future is certain to bringincreasingly demanding requirements, so we at ThreeBond will continue to focus our efforts on product and technology
development in line with market trends in order to ensure the contribution our products make to the industry.
<References>
1) Nishi, Hisao. Chemistry of Pigment, Kyoritsu Shuppan, pg. 16, 1985
2) Failure, Inhibiting Factors and Countermeasures in Light Curing, Technical Information Institute Co., Ltd., pg. 319, 2003
3) Japanese patent: 2015-63643
4) Nishi, Hisao; Kitahara, Kiyoshi, Chemistry of Pigment, 2nd Ed., Kyoritsu Shuppan, 60, 1992
5) Japanese patent: S51(1976)-44706
6) Japanese patent: 2012-51354
7) Japanese patent: 2005-88308
8) Japanese patent: H11(1999)-60962
9) Nakashima, Atsushi; Sasa, Nobumasa; Okubo, Kimihiko; Arai, Takeo. Konica Minolta Technology Report, Vol. 3, 200610) M. Hida, Journal of Synthetic Organic Chemistry, 27, 1178-1194, 1969

ThreeBond Co., Ltd. R&D Headquarters
Development Division 2, Electric Development Division 2
Yuko Tomita
Naoya Otsuki
Katsunori Ito

ThreeBond Co., Ltd.
84-3-3 Minamiosawa, Hachioji,Tokyo 192-0398
Tel: +81-42-670-5333