Three Bond Technical News
Issued July 1, 2003

61

New Light-Curing Instant Adhesives(ThreeBond 1771E, 1773E, and 1776E)
Introduction ________________________________________________As the name suggests, instant adhesives are bonding agents that instantly fix adherends of mutual
bonding objects in several tens of seconds to several seconds. Properties such as quick curing,versatility, and ease of use have led to widespread use in home and industry. In recent years, deflation
has lowered product prices and exacerbated price competition in many product areas. Such market
conditions increase demands for cost reduction while maintaining product quality and functions withenhancing efficiency and streamlining production line. The instant bonding provided by instantadhesives is being recognized once again as a way to reduce labor requirements and improve
production speed. Light-curing technologies have advanced rapidly, too, with emerging applications inelectronics and other fields. Light-curing adhesives offer ease of use (requiring only irradiation tocure), versatility, and selective curing properties.
This issue introduces new light-curing instant adhesives that combine the advantages of the two
types of adhesives: the instant adhesion of instant adhesives and the ease of use of light-curing
adhesives.
Contents
Introduction ................................................1
1. Characteristics of instant adhesives ......2
1-1. Curing mechanism..........................2
1-2. Advantages and disadvantagesof instant adhesives ......................2
1-3. Bleaching ......................................3
2. Characteristics of light-curing instantadhesives .............................................4
2-1. Curing mechanism..........................4

2-2. Characteristics of light-curing
instant adhesives ........................... 4
2-3. Properties of light-curing instantadhesives ....................................... 5
2-4. Setting time and bonding strengthof light-curing instant adhesives..... 5
2-5. Applications of light-curing instantadhesives ..................................... 7
3. Summary ............................................. 81

1.

Characteristics of instant adhesives

1-1. Curing mechanism
The main component of instant adhesives is
2-cyanoacrylic acid ester. The molecule contains
strong electrophilic groups such as cyano andcarbonyl groups that result in nonuniform molecularelectron density distributions, with sites of higher
and lower electron density. Basic compounds suchas water or amine readily attack sites of low
electron density as nucleophiles, with anion
polymerization progressing as shown in Figure 1.
The instant bonding of instant adhesives resultsfrom this extremely sensitive anion polymerization.
The sensitive anion polymerization dominates thecuring mechanism of instant adhesives, therefore,
radical polymerization with heat or light is small
and almost negligible. Thus, trace amounts of acidic
substances are added to these adhesives to improve
preservation stability. This means that whenbonding mutual adherends, water adsorbed to thesurfaces of the adherends neutralizes the acidic
material. Curing begins when the entire adhesivecomes to alkaline region. Radical polymerization
inhibitors are generally added as well to prevent
gradual radical polymerization in preservation,although such reactions tend to be slow.
1-2.

Advantages and disadvantages of instantadhesives
Table 1 summarizes the characteristics of instantadhesives.
The quick curing property of instant adhesives
alone tends to attract attention. However, instantadhesives also happen to create powerful bonds onvirtually any material. This versatility is a powerfuladvantage, and therefore, instant adhesives are usedbroadly in homes and industries and for medical
services.
On the other hand, it is a fact that instantadhesives have disadvantages, as listed in Table 1.
Thus, bleaching effect discourages applications forwhich appearance is important. Since cured instantadhesive tends to be hard and brittle, bonded areas
have low impact, peeling, and thermal shock
resistance.
The selection of monomer types and new
additives is gradually removing such disadvantages.
For example, adding rubber to instant adhesives
improves impact resistance and peeling resistance.1)
For the bonding of optical materials andornaments, bleaching effect is probably one of mostsignificant disadvantages of instant adhesive, due todegradation of the appearance and contamination ofbonded object materials by the adhesive. The
following subsection briefly discusses the principlesof bleaching.
Table 1:

Advantages and disadvantages of
instant adhesives

Advantages

Disadvantages

1. Quick curing at room
temperature
2. One-part nonsolvent
3. High bonding strength(Shearing and tension
strength)
4. Capable of bonding awide range of materials
5. Capable of bonding
different materials
6. Highly transparent cured
objects
7. Small consumption

1. Possible bleaching
2. Inflexible cured objects
3. Low shock resistance
4. Low heat resistance(80°C)
5. Unsuitable for large gaps
or filling adhesion
6. Low water resistance
7. Highly adhesive to skinNu - (nucleophile) forms under light irradiation!

Conventional instantadhesives use water oramine as Nu- (nucleophile).

Figure 1: Overview of photo anion polymerization reaction mechanism21-3. Bleaching effectBleaching effect is a phenomenon in which thegas evaporated from the instant adhesive monomer
reacts with airborne moisture to form a white
powder that adsorbs to adherends, lending a cloudy
appearance, as shown in Figure 2. Bleaching effectis particularly disadvantageous for optical materials
and ornaments.
Cyanoacrylate polymer

Moisture in air(2) Using odor free, low bleaching adhesives
The monomer vapor pressure varies with the typeof ester in the 2-cyanoacrylic acid ester. Longer
ester structures result in lower vapor pressure,
impeding evaporation. Keeping the gas from
evaporating generally prevents bleaching effect.
However, large ester structures also tend to slow
setting

times

and

reducebonding

strength.

(ThreeBond 1721C)(3) Precise coating using a coater
Since bleaching effect is caused mainly by gas
vaporizing from excess monomer, using a coater tocontrol the amount of coating would resolve theproblem. This method would also increase coating
precision, reliability, and speed. However, it fails to
Adherend

Cyanoacrylate monomer

Monomer gas

address the root cause of bleaching effect. (Coater
S-II, SMF-02B)(4) Using a light-curing instant adhesive
Adding a photo polymerization initiator to anadhesive to generate anion species enables curing of
instant adhesives by exposure to light. Since excess
adhesive is cured by light, this technique avoids

Left: Bonding without bleaching
Right: Bonding with bleaching
Figure 2: Principles of bleaching and bonding
examplesGiven below are several approaches to avoiding

bleaching effect without impairing appearance anddecreasing bonding strength. However, light-curing
requires an initial investment for various equipment,
including the light source irradiator. (ThreeBond
1770 series)
Table 2 summarizes the preceding discussion. In

bleaching effect.

general, light-curing instant adhesives represent the(1) Method that forces curing with curing

best choice for preventing bleaching effect.

accelerator

The next and subsequent subsections describe

This method cures the adhesive before themonomergas

evaporates, thereby preventing

bleaching effect. However, this method still results

various details of light-curing instant adhesives.
Table 2:in an unattractive adhesive finish. Rapid curing ofWith
Low-bleaching type
accelerator TB1721C

the adhesive also creates large internal stresses,
which tends to reduce bonding strength. Lastly, theaccelerator is an organic solvent that poses
problemsfortheworkingenvironment.

Characteristics of methods forpreventing bleaching effectWithcoaterLight-curing instantadhesives

Anti-bleaching capability

Ease of use
Curing speed
Bonding strength

Nevertheless, this method offers remarkably rapid

Cosmetic aspects

curing. (ThreeBond 1796 series)

Cost effectiveness
Environmental safety32.

Characteristics of light-curing instant adhesives

2-1. Curing mechanism
As shown in Figure 1, a light-curing instantadhesive cures on irradiation by ultraviolet or
visible light, thanks to a photo polymerization
initiator contained in the adhesive that decomposesand generates anion species (Nu- (nucleophile)),which then advances anion polymerization.
Conventional light-curing resins include radical
polymerization and cation polymerization typeresins. Much technological expertise has
accumulated for radical polymerization type resinsin particular, resulting in a wide range of adhesives
based on various resin combinations.
Among these, anion types (including instantadhesives) that use anion species as polymerization
initiators remain at the stage of basic research dueto their instability and handling difficulty. Only
several studies have been reported to date. Kutal
and Yamaguchi et al. (University of Georgia, 2))reported that 2-cyanoacrylic acid esters undergophoto-induced polymerization with the addition of
metallocene derivatives such as benzoylferrocene.
However, the report addresses only aspects such as:"increased viscosity" and "gelation," with nodiscussion of preservation stability. In short, thereport is not useful in advancing practical adhesive
applications.
Our research has shown that radical generators
significantly activate the reactivity of the above
ferrocene derivative. We have succeeded indeveloping light-curing instant adhesives withexcellent light-curing properties and goodpreservation stability through concurrent use ofadhesives and radical generators.
As shown in Figure 3, light-curing instantadhesives are activated by light irradiation, whichcures excess adhesive that laps from the adherendsof bonded materials. Based on instant adhesives, theportion between the adherends cures instantly bymoisture curing, as do conventional instantadhesives.
Ultraviolet or visible light

Adherends

Curing by opticalanion polymerization

Curing as a conventional instant adhesive

Figure 3: Curing mechanism

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2-2.

Characteristics of light-curing instantadhesives
Light-curing instant adhesives combine both lightcuring and moisture curing. This combination
reduces bleaching effect and improves surface
curing and filling bonding properties, and vastly
expands applications.
Table 3 compares the characteristics oflight-curing instant adhesives, conventional instantadhesives, and typical light-curing resin.
Table 3:Characteristics of light-curing instantadhesives
Instant
adhesives

Light-curing instantadhesives

Light-curingresins

Not required

Not required

Required

Bonding of light-blocking materials
Curing of excess portions
Surface curing property
BleachingTemporary retaining jigs

(1) Good bonding properties for light-blocking
materials
Although transparent materials pose no problems,ordinary light-curing resins fail to cure when used
to bond light-blocking materials, as shown in Figure
3, since the materials block light. In contrast,
light-curing instant adhesives cure instantly via
moisture curing in the gaps between adherends.
Light-curing instant adhesives are thus associatedwith lower incidence of curing failures thanlight-curing resins due to insufficient lightirradiation, even for transparent adherends.
(2) Good surface curing of excess adhesives
prevents bleaching effect
While conventional instant adhesives cureinstantly in the gaps between adherends, excess
adhesive remaining as a liquid cures slowly,
reacting with airborne moisture. This behavior isdue to an imbalance between the acid stabilizer andthe alkaline initiator, with too much stabilizer forthe amount of moisture. Under these conditions, themonomer gas evaporates readily and promotes
bleaching effect. In contrast, light-curing instantadhesives and light-curing resins require only lightto cure.
It should be noted that oxygen inhibits the curingof light-curing resins that cure by radical
polymerization, resulting in poor surface curing
characteristics.
(3) No fixing jig required at bonding2-3.

While light-curing resins require fixing jigs until
exposed to light, most light-curing instant adhesives
have the advantage of adhering when applied to theadherends, eliminating the need for fixing jigs. This
extremely useful characteristic reduces labor costs
and increases speed of production lines.

Properties of light-curing instantadhesives

Table 4 shows the properties for ThreeBond
1771E, 1773E, and 1776E.

MonomerTable 4: Properties of light-curing instant adhesives
Items

Unit

Color and appearance-

Transparent yellow liquid

Main component-

2-cyanoacrylic acid ester

Viscosity

mPa s

2

150

1000

Linear expansion coefficient (0 - 100 )10 6/

81-103

75-99

70-99

123

117

Ultraviolet curing

kJ/m

10

10

10

Visible light curing

2

kJ/m

0.4

0.2

0.2

kV/mm

27.0

Polymerm

Volume resistivity
Surface resistivity

Dielectric
tangent

2-4.

TB1776E

124

Dielectric breakdown voltage

Dielectric
constant

TB1773E

2

Glass transition temperature Tg
Standard curingconditions

TB1771E

26.0

25.8
13

5.4 1013

1.2 1013

5.9 1012

1.2 1013

5.4 1013

5.9 10

1 MHz

-

3.27

2.69

2.90

1 MHz

-

4.05

3.34

3.61

1 MHz

-

0.0511

0.05290.0555

1 MHz-

0.0522

0.05340.0548

Setting time and bonding strength oflight-curing instant adhesives
Light-curing instant adhesives offer good bonding
properties for various substrates, similar toconventional instant adhesives. Table 5 shows anexample of the setting time and bonding strength oflight-curing instant adhesives. The tension shearing
adhesive strength in the table is assessed in atension shear test after applying adhesives underconditions of 25°C and 50% humidity, followed by
24 hours of curing. Except for materials (such as
metals) that are stronger than the adhesives
themselves, most adherends exhibit material break(i.e., the material breaks before the adherends due tothe strength of the bond).
Not shown in Table 5 is another advantage oflight-curing instant adhesives, this being that
nonpolar or highly crystalline resins, generally

regarded as difficult to bond, can bond strongly. For
example, in combination with a primer (ThreeBond
1797), light-curing instant adhesives create strong
bonds when used with polyethylene, polypropylene,
polyacetal, soft polyvynyl-chlorides including large
amounts of plasticizer, silicone rubber, and EPDM
rubber.
Figure 4 shows the curing speed for moisturecuring and light curing. As previously discussed,
conventional instant adhesives are moisture-curing
type. However, moisture-curing adhesives do not
cure fully when initially applied to adherends.
Although bonding strength increases rapidly at first,
to complete curing generally it requires 12 to 24
hours at room temperature.
Light irradiation allows a bond to attain final
strength in approximately 10 seconds. Thus, theoptimal curing method is to temporarily fix the5adherends by moisture curing, then to curecompletely by light irradiation.
Instant adhesives generally function also asreactive "solvents" capable of dissolving thesurfaces of certain plastic materials, including
acrylic, polycarbonate, and ABS. Table 5 shows thatthe setting times for acrylic and polycarbonate are

relatively slow. This is due the mutual solution of
instant adhesive and the plastic in the bonding area.
Since they cure instantly, light-curing instantadhesives can prevent the dissolution of plastic
surfaces in such cases.

Table 5: Setting time and bonding strength of light-curing instant adhesives
Tension shearing adhesive strength (MPa)Setting time (seconds)

Adherends

TB1771E

TB1773E

TB1776E

TB1771E

TB1773E

TB1776E

Polycarbonate

12

15

20

11.0 (*)

8.9 (*)

9.1 (*)

ABS

2

3

7

7.4 (*)

6.7 (*)

7.0 (*)

6-nylon
PET

5

7

15

7.1 (*)

7.1 (*)

7.2 (*)

2

7

10

12.2 (*)

12.9 (*)

12.8 (*)

Acrylic

15

25

45

5.8 (*)

8.4 (*)

8.5 (*)

NBR

2

2

2

0.5 (*)

0.5 (*)

0.5 (*)

Paper (woody paper)

5

15

30

- (*)

- (*)

- (*)

Iron

3

5

10

15.1

15.9

16.8

Stainless steel

3

7

15

11.9

11.3

12.4

Aluminum

3

10

20

10.6

11.2

11.4

Copper

3

7

20

11.5

13.5

11.5

Brass

3

5

15

9.2

10.3

10.9

Test method: 3TS-220-04 (Setting time)(*) indicates material breaking.

3TS-301-11 (Tension shearing adhesive strength) [Tension speed 10 mm/min]

TB1773E<<Moisture curing>>

Tension shearing adhesive strength (MPa)Tension shearing adhesive strength (MPa)TB1771E

12
10
8
6
4
2
0
0

6

12

18

24

<<Light curing>>
12
10
8
6
4
2
0
0

Time (h)TB1776E

6

12

18

24

Time (h)2(Note) The light irradiation condition is 3.5 kJ/m with a 4-kW high-pressure mercury-vapor lamp.
Moisture curing is performed in shade at 25°C and 50% humidity.

Figure 4: Comparison of curing speed for moisture curing and light curing (Adherends: polycarbonate)

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2-5.

Applications of light-curing instantadhesives
Light-curing instant adhesives offer a vast range
of applications, including electric and electronic
components, fields in which conventional instantadhesives are not used, and precision components.
They are used not just to create surface bonding
with various adherends, but for applications
involving sealing, potting, and fixing. Additionally,
since light-curing instant adhesives have theadvantage of curing at lower illumination thantypical light-curing resins, that is, since they curefaster, they should improve productivity in fast
production lines. Major applications are listed
below.
(1) Electric, electronic, optical, and generalcomponents requiring non-bleaching properties(2) Fixing and bonding of supports around opticallenses(3) Fixing and bonding of various light-blocking
adherends(4) Reinforcement of IC attachment(5) Bonding of fitting sections such as printer heads
and bearings(6) Bonding of the fitting section of a drum motorand main shaft(7) Fixing and bonding of ornamental materials(8) Assembly of hypodermic needles and syringes(9) Creation of Braille characters and graphics

Advancing miniaturization and driving to reduceweight has resulted in single components having
extremely fine structures. Even the portions that
could conventionally be irradiated with sufficient
light, now the irradiation often causes problems. In
such cases, light-curing instant adhesives that use
both light and moisture curing are effective inproviding reliable curing and bonding.
With increasing miniaturization, printed circuitboards use less solder. But since bonding strengthmust be maintained, light-curing instant adhesives
are used to reinforce solder.
For
metal
platedplastic
components,
conventional light-curing or anaerobic resins
sometimes fail to bond the adherends. In some cases,
light-curing instant adhesives are used, due to theireven broader bonding capabilities for a wide rangeof materials.
Components such as hypodermic needles andsyringes have direct effects upon the human body.
Reliable complete bonding and curing is critical intheir assembly. Light-curing instant adhesives arealso used for Braille characters and graphics, since
the surface curing of light-curing resins is relatively
poor due to oxygen-induced curing inhibition, as
already discussed, and touching incompletely cured
portions with hand may result in skin irritations.
Light-curing instant adhesives are not affected bycuring inhibition with oxygen. It can be said a better
adhesive.

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3.

Summary

In view of recent trends toward international green technologies, the effectiveness and benefits of light-curing
technology (including reduced use of organic solvents) in the areas of environment and energy are being
recognized once again. Adding light curing properties overcomes the disadvantages of instant adhesives. The
range of potential applications for light-curing adhesives continues to expand rapidly.
We plan to pursue the commercialization of shock-resistant and highly peel-resistant products, with the goal of
overcoming the remaining disadvantages of the materials discussed, thereby exploiting new applications and newmarkets.
Masakazu Motoki
Industrial Materials Development Division
Development Department
Research Laboratory
Three Bond Co., Ltd.

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

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