Three Bond Technical News
Issued July 1, 1995

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Visible-Light-Curing Resin
Introduction ________________________________________________
Currently, light-curing resins are used in many forms: inks, paints, adhesives, sealants, sealing
agents, printing plates, and various resist materials. Three Bond has made great contributions to theelectric, electronic, automotive, and mechanical industries with its ultraviolet-curing adhesives, theThreeBond 3000 and 3100 Series. The development of lasers, in particular visible lasers, has led toincreasing demand for resin that is cured in visible light.
This issue introduces a technical approach to resin that is cured by visible light—that is, light with alonger wavelength than is used for conventional ultraviolet-based light-curing resin.

Contents
Introduction
1. Background of visible-light-curing resin .......................................................... 2
2. What is visible light? ........................................................................................ 2
3. Problems with ultraviolet-curing resin.............................................................. 3
4. Advantages and disadvantages of visible light................................................ 3
5. Reactive mechanism of visible-light-curing resin ............................................ 4
6. Visible-light lamp ............................................................................................. 6
7. Applying visible-light-curing resin .................................................................... 7
8. Problems with visible-light-curing resin ........................................................... 7
Conclusion ........................................................................................................... 81

1.

Background of visible-light-curing resin

While the demand for light-curing resin has been
expanding year after year, most products consist ofultraviolet-curing resin that uses ultraviolet rays as
the curing energy. Added to this on the market isvisible-light-curing resin, which cures in responseto the radiation of visible light. When did such a
resin that uses visible light as the curing energy
come onto the market? Its history dates back to thelate 1970s, when it was used as a dental resin.
Therefore, visible-light-curing resin is not a very
new development. When visible-light-curing resincame into being, light-curing technology was little
used in industrial applications. Its history as anadhesive is just short. This can be attributed to thefact that the market share of light-curing resin itself
was low in the adhesive industry.
In recent years, however, many resins that use
light as the curing energy have been developed dueto the need for a one-part, non-solvent, fast-curingconfiguration. Hence, so-called "ultraviolet-curingresin," which uses ultraviolet rays as the curing
energy, has made a fabulous debut. Due to the factthat ultraviolet-curing resin has overcome thedrawbacks of conventional resins, such as the longcuring time and two-part configuration, demand forit has grown each year.
Against such a background, ultraviolet-curingresin has found uses in inks, paints, adhesives,sealants, sealing agents, printing plates, and various
resist materials, expanding its commercial use in awide range of industries.
As the industry develops, however, industrial
materials also develop rapidly and many new
industrial materials appear. Among such materials
are those that are vulnerable to ultraviolet rays or
are hard to penetrate it. The material technology for
optical materials in particular, including special
prisms and plastic lenses, advances year after year.
Materials of these types do not pose a significant
problem for ordinary processing techniques;
however, in the field of adhesive processing using
ultraviolet-curing resin, utilizing this curing
technique for such materials is expected to bedifficult. In the bonding of these optical materials,
ultraviolet-curing resin is inferior to the adhesivesof a generation ago. Hence, curing techniques basedon active energy beams other than ultraviolet rays
have been attracting attention. The key factor in thistrend is a curing technique based on visible light.

Table 1. Classification of Electromagnetic Waves
γ-ray

X-ray

2.

Visible light

Ultraviolet rays
Far
ultraviolet
rays

Infrared ray

Near
ultraviolet
rays

What is visible light?

Next, let's look at visible light. Table 1 shows theclassification of electromagnetic waves. Thesewaves include γ and X-rays with short wavelengths,
followed by ultraviolet rays, visible light, andinfrared ray, in that order. The table shows thatvisible light has a wavelength of 400 nm to 800 nm,
i.e., visible light has a distribution of longer
wavelengths than does ultraviolet. Generally, awider distribution of wavelengths broadens theeffective range of the wavelengths necessary forcuring, whereas visible light has only low energy,
which makes it poorly suited for curing. Table 2
shows the energy levels of electromagnetic waves.
The one that has the highest energy level is theelectronic beam, followed by ultraviolet rays,
visible light, and infrared rays, in that order. To
allow light to cause a chemical reaction, the energy
must be sufficiently high. From this standpoint, theelectronic beam is the best light source, but is rarely
used in industrial applications today. A comparisonof energy levels between ultraviolet rays and visible
light shows that that of visible light isapproximately 1/2 that of ultraviolet rays, though it
is subject to variation depending on the wavelength.
Hence, it can be said that ultraviolet rays are usefulas an energy source for curing, whereas it is not avery good light source in terms of safety. Moreover,as we will see later in this paper, visible light has
advantages that ultraviolet cannot provide.

Table 2. Energy Levels and Frequencies of Electromagnetic Waves
Electronic beam
Ultraviolet rays
Visible light
Infrared ray

2

Energy (kJ/mol)6
Up to 10300 to 600
150 to 300
8 to 60

Frequency (l/s Hz)
16
Up to 2 × 10
150.75 to 1.5 × 10
150.40 to 0.75 × 10
14
0.20 to 1.50 × 10

3.

Problems with ultraviolet-curing resin

Ultraviolet-curing resin is widely used in thefields of electric, electronic, automobiles, optics,
communications, construction, and the like, due toits one-part, non-solvent, fast-curing configuration.
This resin is cured using ultraviolet with awavelength of 200 nm to 400 nm. As its
applications expand, a number of problems with it
have attracted attention, one of which is the
emergence of materials through which ultraviolet isdifficult to penetrate.
So-called "engineering plastics," when usedoutside, have an ultraviolet-absorbing agent added
to prevent deterioration. Table 3 shows the light
penetrativity of various materials. The wavelengths
at which penetrativity was compared were 365 nm
for ultraviolet and 400 nm and 435 nm for visible
light. This table indicates that most of the materials
collectively referred to as engineering plastics do
not penetrate ultraviolet rays (or do absorb it). This
makes it difficult to bond such materials by using
ultraviolet-curing resin. In practice, however, curing
speed is accelerated by increasing the output fromthe radiator, and therefore the apparent curing time
is longer than that of the resin. While alumina used
as a filling and soda glass found in ordinary
windowpanes generally have low penetrativity, thepenetrativity of visible light is higher than that ofultraviolet rays. The special applications ofultraviolet-curing resin include mixture with afilling. When ultraviolet rays are used for curing,
however, not much filling can be used due to theshort wavelength of irradiated ultraviolet rays.
Table 3.

Light Penetrativity of Various
Materials

Material

365nm

400nm

435nm

*1

0.07%

50%

80%

Soft PVC

*2

0.3%

44%

77%

*3

0.05%

5%

10%

0.1%

13%

17%

0.5%

50%

95%

1.5%

5.2%

6.6%

48%

80%

85%

Hard PVC

ABS

*4

PBT

Polycarbonate
Alumina*6

Soda glass*7*5*1: Mitsubishi 302 (2 mm)*2: Takato Kasei (2 mm)*3: Tsutsunaka Tough Ace EAR003 (2 mm)*4: Duranex 2002 (2 mm)*5: Teijin Panlite PC-111 (2 mm)*6: Showa Denko UA-5205 30 wt% cured with
Ep-828/DYCY (2 mm)*7: Tempax (5 mm)

4.

Advantages and disadvantages ofvisible light

A simple comparison of energy levels shows thatthe energy of visible light is lower than that ofultraviolet rays. The long wavelength, however, has
certain advantages. Those of visible light are as
follows.
(1) High light penetrativity(2) Safe to the human body(3) Inexpensive radiation system
Advantage (1) permits the use of a filling mixed
with an adhesive and a resin for semi- to
non-transparent materials. The light penetrativity of
the resin to be cured itself improves. The
penetrativity of ultraviolet rays through a resin is
only several millimeters from the surface, whereas
visible light penetrates deeper into a material due to
its long wavelength. Thus, better thick-film curing
can be achieved (Fig. 1).
Advantage (2) is that the health of workers is not
impaired. Ultraviolet rays affects the human body.
The ultraviolet irradiator, therefore, must be
designed in consideration of the safety of workers.
Advantage (3) will become a disadvantage if ahigh-pressure mercury lamp or similar irradiation
system is used. The introduction of a system of thistype is costly. Following its introduction, therunning cost and lamp service life will pose
problems. In contrast, the visible-light-curing resin
can be cured well with a commercially available
halogen lamp, and therefore the total cost will belower than that of a mercury lamp.
Well, what are the disadvantages compared toultraviolet rays?
(1) Lower energy level than ultraviolet(2) Poor surface curing of resins
For (1), the energy level of the visible light isapproximately 1/2 that of ultraviolet rays, andtherefore the curing of visible-light-curing resin isslower than that of ultraviolet-curing resin. For
applications that require a high line speed, efforts
should be made to improve the performance of theresin or increase the output of the light source.
For (2), the low energy level of thevisible-light-curing resin, as in (1), makes it
difficult for radicals trapped by oxygen to become
an active species again.
With these advantages and disadvantages, thecuring energy of visible light has characteristics notfound in ultraviolet rays, and thus visible light isworthwhile to study for use as a curing-energy
medium.

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Ultraviolet-curing resin

Visible-light-curing resin

As the entering light is not penetrated through well, only alayer several millimeters from the surface is cured.

As the entering light is penetrated through well, the resin cures
to a greater depth.

Light

Light

Penetrating
light

Fig. 1 Light Penetration through a Resin

5.

Reactive mechanism ofvisible-light-curing resin5-1. Radical-generating mechanism
The most familiar example of photoreaction withvisible light is the photosynthesis of plants.
Photosynthesis is a "photoreaction" using sunlight.
It is a photo-oxidation-reduction reaction caused bychlorophyll and a reducer. Related technology is
widely applied in the photochemical industry.
Photo-reactive techniques using visible light range
from the silver salt photographs of centuries ago to
the high-sensitivity photopolymers of today. The
performance of photosensitive resin as a polymer
can be controlled using a blend of oligomers andmonomers in most cases. The difference from theultraviolet-curing resin lies in the type ofphoto-initiator contained. The curing mode isradical polymerization, as in the case ofultraviolet-curing resin. Fig. 2 summarizes thereactive mechanism of radical polymerization. The
mechanisms of the generation of radicals can beclassified as shown in Fig. 3. The only thing that isrequired of a technique for initiating curing(polymerization) using visible light is thedevelopment of a material that both absorbs visible
light and serves as the generating source for curable(polymerizing) active species.
Initiating reaction
Photo-polymerization
initiator

Radical generation

Growingreaction

Stopping
reactionFig. 2 Radical-Reactive Mechanism

45-2. PI-type initiator
The most popular ultraviolet-curing resin uses the
PI-type photo-initiator. The reactive mechanism ofradical generation is as follows: a single-molecule
photo-initiator cleaves and then generates two
radicals. Because little light is absorbed near thevisible-light range, the visible-light-curing resin
does not show an orthodox curing mechanism. Thus,
very few visible-light-curing resins use asingle-molecule photo-initiator such as the PI-type.
Many photo-initiators in use are the two-molecule
type, such as the PII-type.
5-3. PII-type initiator
The outstanding feature of the PII-type initiator isthat radicals are generated mainly as a result ofenergy or hydrogen migration due to light between
two molecules, which differs from the case of the PI
type. As mentioned above in this issue, curing withvisible light is applied to photopolymers in mostcases, and thus many photo-initiators consist of
combinations of dyes and radical generators (orhydrogen donors). Visible-light-curing resin, which
was developed based on dental-material technology,
normallyuses
α-diketone derivatives
asphoto-initiators. The reactive system consists of aphoto-initiator ("I" in Fig. 3) and a photo-initiation
assist ("R-H" in Fig. 3). "I," excited by theradiation of visible light, removes hydrogen from
"R-H," and results in generating radicals. Thoughhydrogen is thus removed directly, hydrogentransfer may also occur as a result of the generationof an excited compound between "I" and "R-H."
The double-molecule type causes not only a single
reaction, but also two or more reactions with
sub-reactions. Unlike the single-molecule type, thereactive mechanism of the double-molecule type
has not yet been revealed.
Shown below are the results of our investigationof the relationship between the photo-initiator andphoto-initiator assist carried out by using the
PII-type photo-initiator.

Reaction of the PI-type
photo-initiator

Cleaves molecules,
generating radicals
Reaction of the PII-type
photo-initiator

Excites to the singlet

Crosses from the singlet
to the triplet

Generates an excitedcompoundRemoves hydrogen, andgenerates radicals

Fig. 3

Classification of Radical-Generation
Mechanisms

Glass transition point (°C)

For the photo-initiator assist, most chemicalcompounds can be used, provided that they arehydrogen donors. As can be seen from feedback in
the field of dental-material technology, most of suchcompounds are amine in general. The type,chemical structure, and amount of added aminevary with the type of α-diketone derivative of thephoto-initiator and the added amount ofphoto-initiator. Fig. 4 shows the effect of added
amounts of amine. The diagrams show the
relationship between the glass transition point andthe added amount of amine. These diagrams
indicate that, as the added amount of amine
increases, the glass transition point lowers. From
this, it can be seen that the photo-initiator assist acts
as a curing accelerator, whereas the excessive
addition of it may inhibit curing. Moreover, theadded amount of amine significantly affects otherphysical properties of resin. Fig. 5 shows therelationship between the heated percent decrease in
the resin after curing, and the added amount ofamine. As is clear from the diagram, the excess
portion of amine only evaporates during heating.

Added amount of amine (wt%)

Heated percent decrease (wt%)

Fig. 4 Relationship between the Added Amount of Amine and the Glass Transition Point

Added amount of amine (wt%)

Fig. 5 Relationship between the Added Amount of Amine and the Heated Percent Decrease56.

Visible-light lampAstheabsorption
wavelengthsofphoto-polymerization initiators range from 400 nm
to 500 nm, any light source can be used forvisible-light-curing resin. Though each lamp has its
own advantages and disadvantages, the optimal one
can be chosen in accordance with the applications.
Table 4 lists the advantages and disadvantages ofvarious light sources. Details are shown below.
(1) Halogen lamp
This is the most commonly used light source, andis outstandingly low in price. As the light source forvisible-light-curing resin, dental composite resin
has shown proven results. Many irradiators for thisapplication are on the market. This light source is
not only inexpensive, but it is also readily available.
The drawback is that the lamp cannot be produced
in a large size. In addition, there are limitations on
increasing the output. Thus, the halogen lamp is
more suitable for handy- and spot-type irradiators
than for large-area radiation.
(2) Xenon lamp
The wavelength distribution of a xenon lamp issimilar to that of sunlight, and it can produce a highoutput. Thus, the xenon lamp is considered best
suited for industrial irradiators. The majority of the
wavelengths are in the range of visible light, andthus it is hoped that the lamp becomes increasingly
used as a radiator in this field.
(3) Metal halide lamp
Most metal halide lamps have been in use as alight source for ultraviolet-curing resin. Withchanges in the type and composition of metals
sealed in a lamp, the lamp can easily be converted
into a visible-light lamp by using current technology.
Thus, for metal halide lamps, it is hoped that aradiator with a low price, high output, and longlamp service life will be developed.
(4) Incandescent lamp
Though this is most inferior in performance as alight source, it is cheap and easy to obtain. Theseadvantages allow this lamp to be included in thelistings of light sources, along with the halogen
lamp. As the output is low, however, it is difficult to
introduce the lamp in industrial use. In addition, theservice life is rather short.
(5) Sunlight
This is a light source that exists in nature, and cantherefore be used for adhesion outside. As theamount of irradiance cannot be controlled, thephysical properties after curing vary.

Table 4. Comparison of Various Light Sources
Light source

Halogen lamp

Xenon lamp

6

Advantages

Low cost; compact
High output; wavelengths primarily inthe range of visible light

Disadvantages

Short life; intermediate output

High cost

Metal halide lamp

Compact; long service life

High cost

Incandescent lamp

Low cost; compact

Short life; low output

Sunlight

No equipment required

Control impossible

7.

Applying visible-light-curing resin

Visible-light-curing resin has characteristics thatare not found in ultraviolet-curing resin, and suchcharacteristics can be applied to various purposes ofuse. As described earlier in this issue, in addition tobonding and sealing materials that do not penetrate
ultraviolet, visible-light-curing resin can be used asa resin for the pouring mold, sealing, and coating.
Listed below are the major possible applications
and applied fields.
(1) Optical communications
Bonding couplers, optical splitter, optical
couplers, light-emitting devices, photo-detectors,
and the like for optical communications.
(2) Optical pickups
Precision-bonding optical lenses, mirrors, prisms,
photodiodes, and the like for optics.
(3) Medical
Fastening and bonding injector needles in place.
(4) Decoration
Bonding ultraviolet-cut glass, colored glass, andglass with a large surface area for interior
decoration.
The curing system can use not only a generic
irradiation lamp, but also a laser as the light source.
Recently, 3D modeling by using a CAD-controlledlaser beam has been attracting a great deal ofattention. In the future, resins that cure by using avisible-light laser such as a relatively inexpensive
semiconductor laser may be developed.

8.

Problems with visible-light-curingresin

Though visible-light-curing resin is a new type oflight-curing resin that compensates for the
drawbacks of ultraviolet-curing resin, it poses anumber of problems involving the use of visible
light.
(1) Need for shielding
As visible-light-curing resin cures under theradiation of visible light (including sunlight), it
must be shielded from light as much as possible
during handling. While curing using interior lights
is slow, exposure to light for an prolonged period
significantly reduces the storage stability of theresin.
(2) Skin-irritation
As the main component of visible-light-curingresin is a special acrylic resin, it is lightly tointermediately skin-irritation, as in the case ofgeneric ultraviolet-curing resin. While working,workers should wear protective gloves.
(3) Low reactivity
As
PII-type
photo-initiatorisused,polymerization is inhibited by oxygen during thegeneration of radicals, causing an inductive phaseduring the reaction. Currently, this problem isgenerally solved with the aid of an irradiator, but
the inferior surface curing should be improved.
(4) Resin coloration
As photo-initiator excited by visible light has anabsorbing range in the visible-light range (a rangeof wavelengths that can be sensed by the humaneye), the resin is colored in advance (some resins
are colorless). In spite of this coloring, however,some resins discolor due to the conjugated bondwithin the molecule being cut after thephoto-initiator generates radicals.

7

Conclusion ________________________________________________
Thus far, I have described technical matters involving visible-light-curing resin. Compared withultraviolet-curing resin, visible-light-curing resin has a short history and few results in the industry. However, thisis simply due to the technology for visible-light-curing resin having not yet been as well established as that for
ultraviolet-curing resin, despite the fact that the former is an extension of the latter.
With the development of the industry, the advantages of light-curing resin will be improved in terms of bothproductivity and ease of handling. Under such circumstances, I believe that we will be able to makevisible-light-curing resin available to you in the not-so-distant future.
Katsuhiko Kishi
Second Research Section, Research Department
Three Bond Co., Ltd.

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

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