ThreeBond Technical News
Issued January 1, 2013

81

Two-Part Acrylic Elastic Epoxy that
Cures at Room Temperature
Introduction
SGAs (second generation acrylic adhesives) are two-part mixture acrylic adhesives that are
generally considered to cure quickly at room temperature for excellent productivity. They can also
increase work efficiency and save energy by eliminating the need for UV irradiation chambers, hot
air drying ovens and other curing equipment.
In this document, we will introduce a two-part mixture acrylic adhesive that boasts improved
durability and elasticity even after quickly curing at room temperature.
Hereafter, ThreeBond is abbreviated as TB.

Table of Contents
Introduction.................................................1
1. Background.............................................2
1-1 Environmental Concerns.................2
1-2 Properties Required of Adhesives...2
1-3 Sealant Adhesive Curing Methods..2
2. Acrylic Adhesive Development...............3
3. Acrylic Elastic Adhesives........................4
4. Applications............................................4

4-1 Bonding Dissimilar Materials...........4
4-2 Surface Bonding..............................5
4-3 Potting..............................................5
5. Material Properties..................................6
5-1 Curing Efficiency..............................6
5-2 Curing Efficiency..............................6
5-3 How to Use......................................7
Closing........................................................81

1.

Background

1-1 Environmental Concerns
In recent years, manufacturing industries such as automotive,
electrical and electronic manufacturers have been required
to put measures in place to prevent global warming during
production processes. Manufacturing technologies arebeing developed intensively to promote resource and energy
conservation in the ever-expanding effort to develop productsthat protect the global environment. In the automotive
industry, development continues on hybrid (HV/PHV),
electric (EV/FCV) and other environmentally friendly
vehicles in an effort to reduce carbon dioxide emissions
and convert to petroleum substitutes. When these vehicles
were first introduced on the market, they had the image ofbeing higher priced than conventional vehicles, but thanks
to initiatives lowering automotive manufacturer price,
the Ministry of Economy in Japan, Trade and Industry’s
introduction of a subsidy system to further demand andother national policies, the market share of environmentallyconscious vehicles continues to expand. In response to thisgrowing demand, automotive manufacturers must improve
both vehicle productivity and performance. Further resourcesand energy conservation efforts, such as eliminating heatgeneration, are underway within production processes inconsideration of environmental efforts.
To meet these market demands, ThreeBond has launcheda two-part acrylic elastic adhesive capable of curing atroom temperature. Mixing the main agent with a curing
agent allows it to cure quickly at room temperature without
additional curing equipment.

1-2 Properties Required of Adhesives
Various adhesives and sealants are used for a wide range ofapplications in automotive and parts manufacturing, fromareas around glass, headlights and inside seats to drive partsand cooling units in engines and transmission. In particularmaterials and parts connected to internal combustion inengines and transmission must be able to withstand heatof 150°C. To handle this, one-part silicone sealant that ismoisture curable is widely used as an adhesive sealant forthese parts.
In addition to withstanding 150°C temperatures, silicone
sealants maintain a rubbery elasticity over a wide rangeof temperatures, making them excellent for following
vibrations, resisting impact and mitigating stress createdby automobiles while driving. But while these performance
characteristics are excellent, curing speed is an issue.
Curing one-part silicone sealant in moisture requires anenvironment of 23°C and a RH of 50% over several days tocompletely cure, which is not good for productivity. Becauseof this, two-part mixture adhesives that can be cured atroom temperature for practical strength in a short amountof time have been drawing attention in the production lines
of electronic components and other places where highproductivity is required.

1-3 Sealant Adhesive Curing Methods
Sealant adhesive is divided into moisture, heat, UV andanaerobic curing as well as two-part mixture curing types.
There are plusses and minuses to each curing method, so theappropriate adhesive and sealant are chosen as required by
the application (Table 1).
In UV and heat curing adhesives, curing is performed byapplying the required energy to start the reaction. Thiselimination of restrictions on working time due to pot life in
combination with excellent curing shortens overall process
time. However, expensive curing equipment is required,making initial investment costs high, and a lot of energy
is consumed during curing. In large parts such as those
for automobiles, public works and construction, moisturecuring sealant adhesives are widely used because they canbe cured at room temperature, thus eliminating the need forcuring equipment. However, curing is more time-consuming
and is affected by changes in the usage environment, suchas temperature and humidity. Caution must be exercised
during use, because curing speed is faster in environmentswith lower temperature and humidity (Fig. 1).

Table 1 Merits by Curing Methods
Type

Moisture-curing

Thermosetting

UV curing

Moisture-curing

Two-part mixture

Curing speed

×

△

◎

△

○

Line equipment cost
reduction

○-△

×

×

◎

◎

○-△

×

×

◎

◎

×

×

×

Energy efficiency
Adherend usability○

Curing inhibitor○

Impact of use conditions

×

Large surface adhesion

×

○-△

○-△

○-△

○

○-△

○

◎

○

○

◎: Excellent ○: Good △: Possible ×: Inadequate

2

◎

×

△

○

100

Tensile Shear Bond Strength
Development Ratio (%)

Tensile Shear Bond Strength (MPa)

2.5

2.0

1.5

1.0

0.5

23°C,50%RH

80

60

40

*Rate of 100% Cured After 24h
20

TB Acrylic

5°C,35%RH

TB Epoxy

0.0
0

2

4

Time (Days)

6

0

8

0

Fig. 1 Adhesive Development in Moisture-Curing
Sealant by Environment

Therefore, if faster curing is required without the use of
curing equipment, two-part mixture adhesives are used.

5

10Time (h)

15

20

25

Fig. 2 Two-Part Mixture Epoxy and Acrylic Resin
Curing Speeds2.

Acrylic Adhesive Development

In two-part mixture adhesives, the curing reaction begins
when the main agent and the curing agent are mixed in
the prescribed ratio. The basic categories of base resin
are epoxy, silicone and acrylic, with each demonstrating
specific attributes and physical properties (Table 2). They
are selected as needed for the application (such as structural
adhesive or elastic sealant), but acrylic resin demonstrates
the fastest curing speeds (Fig. 2).

As shown in Fig. 3-a, acrylic adhesive is a generic name foradhesives having a (meth)acrylic acid chemical structure, ora monomer or polymer of a derivative acetate thereof thatcures through radical polymerization (Fig. 3-b). Through a
combination of monomers and polymers, a variety of cured
physical properties can be achieved.

Table 2 Two-Part Mixture Base Resin Properties

H2C＝C−C−OR−

H or CH3

TB3955

Conventional
Product

Epoxy

Silicone

Curing
speed

○

○

○

△

Adhesivestrength

○

◎

◎

△

Flexibility

◎

△

×

◎

Heat
resistance

○

△

○

◎

Humidity
resistance

○

△

○

○

Chemicalresistance

○

△

○

○

＝

Base

AcrylicO

Fig. 3-a Acrylic Structural Formula→ ・I

InitialReactionI

GrowthReaction

・
I ＋M

→

M・

M・＋ M

→

MM・

MM・ ＋ nM
End
Reaction→

Pn＋2・

Pn・ ＋ Pm・ →Pn - Pm

I : Photoinitiator I・: Initiator Radical
M: Monomer
M・: Monomer Radical
P : Polymer
P・: Polymer Radical

Fig. 3-b Radical Polymerization Reaction
Mechanism3

TB3955 Features

Acrylic adhesives are divided by reaction type. Firstgeneration acrylic adhesives (FGA)*1 are composed ofacrylic monomer, a catalyst and elastomer, but no chemical
reaction occurs between monomer and elastomer during
curing. Even though the composition is almost identical,
adhesive where there is a chemical reaction between
monomer and polymer is called a second-generation acrylic
adhesive (SGA)*2. These demonstrate superior performancein various aspects, such as adhesive strength and durability.
Acrylic adhesives where polymerization is initiated byexposure to UV, electromagnetic waves or electron beams(Fig. 3-b) are classified as third generation, and these are apart of our ThreeBond 3000 product lineup. Other acrylic
adhesives include anaerobic and instant types and these
are available in the ThreeBond 1300 and 1700/7700 series,
respectively.

• Quickly cures at room temperature.
• Glass transition temperature below zero degree
that maintains rubber flexibility in a wide range oftemperatures.
• Withstands high temperature (150°C) and hightemperature and humidity (85°C, 85% RH)
environments.

4.

Applications

Here are some of the applications for our two-part acrylic
elastic epoxy that cures at room temperature.

This led ThreeBond to work towards the development of anacrylic elastic adhesive that can withstand temperatures of atleast 120°C. These products feature acrylic rubber polymer
as their base component. They boast high resistance to bothheat and oil, while also featuring a low glass transition
temperature and a silicone-like rubber elasticity that ismaintained in a wide temperature range. Until now, we haveoffered UV, heat and moisture curing resins expressly forelectrical parts in the transport market where high resistanceto heat and oil is required.

4-1 Bonding Dissimilar Materials
Advances in automotive part material development are
an important part of the technological innovation that has
made performance improvements a reality. By adopting
materials optimized for the performance of these parts andthen incorporating them into product design, performancecontinues to improve along with reductions in size andweight. Adhesives that bond materials of different propertiesare required to accompany these developments. Especiallyin materials with different coefficients of linear expansion,
expansion and contraction ratios differ in the material
itself depending on temperature changes in the usage
environment. This means that there is greater distortion
acting on the adhesive used to fix these materials, which
results in peeling and cracking. This issue occurs underchanges from the curing temperature and under severe
conditions and occurs repeatedly particularly during theheat cycle. In these cases, we need adhesives with the ability
to bond with these materials and the flexibility to adapt totheir expansion.

Now, we are pleased to introduce ThreeBond 3955 (hereafter
abbreviated to TB3955), our two-part mixture adhesive thatcures at room temperature (Table 2, 4).

Motor magnet affixing is one example of this kind of bondingof dissimilar materials. Most of the synchronous motors
used in HV (PHV), EV and other environmentally friendly

3.

Acrylic Elastic Adhesives

Until now, a wide variety of acrylic adhesive products
have been developed to meet a wide range of applications.
These products are now widely used, and demand continues
to grow. However, they could not be used in automobileinternal combustion and other environments requiring
resistance to high heat and flexibility.

Table 3 Acrylic Elastic Adhesive Product Lineup

4

Curing Method

Moisture-curing

Thermosetting

UV curing*Curing Conditions(23°C, 55% RH)
× 7 days

150°C, 90min

45kJ/m2

Product name

TB1158

TB1156B

TB3013Q

Remarks

Features

Unit

Testing Method

Measured Value

Hardness—

3TS-2B00-004

A20

A6

A32

Durometer A

Elongation rate%

3TS-4190-001

300

275

195

—

Tensile strengthMPa

3TS-4190-001

1.8

1.2

2.7

—

Tensile shear bondstrengthMPa

3TS-4100-013*, 023**

1.8 (Al/Al)**

1.5 (Al/Al)**

4.1 (Fe/glass)*—

Glass transition
temperature°C

3TS-4730-001

-40.0

-40.5

-31.0

DMA method 1Hz,E” peak*1*2

First generation acrylic adhesive
Second generation acrylic adhesiveautomobiles are PM*3 motors, which use permanent magnets
in the rotor. Among permanent magnets, neodymium
magnets produce the largest amount of energy and are highly
magnetic, making their adoption ideal for high capacity,
low size motors. In addition to use in electric and hybrid
automobiles, PM motors are also the main electric motor
used for EPS*4 and other systems that use small motors. An
epoxy, anaerobic and two-part (acrylic) adhesive that curesat room temperature are used to affix the permanent magnet
to the stator. In high torque-generating medium and large
electric motors that produce high temperatures, a one-partthermosetting epoxy adhesive with a high glass transition
temperature and high heat resistance is used. The issue withthis is that when temperature changes, such as in the postcuring cooling process or in the heat cycle, resin contraction
and the difference in the coefficient of linear expansion
between the stator and the permanent magnet cause thepeeling as well as cracking in the magnet. Additionally,
quick room temperature curing is also required to improve
productivity by reducing heat curing equipment andprocesses to better handle mass production of small motors.
TB3955 has a low glass transition temperature and is highly
flexible after curing, making it excellent for applicationin areas with large amounts of expansion and contraction
where stress mitigation is required. It is also ideal for
affixing motor magnets because it cures quickly at room
temperature, eliminating the need for curing equipment toimprove productivity.

4-2 Surface Bonding
Two-part mixture adhesives are effective for bonding large
areas. Generally, when UV or heat cannot be applied to anadherent, adhesives that cure at room temperature are used.
However, because moisture-curing types require contact
with air, completely curing large adherents all the way tothe center becomes problematic, which negatively impacts
both reliability and productivity. Therefore, two- part
mixture adhesives that uniformly bond large surfaces andcure quickly are used.
Looking at parts for electric vehicles, the battery, which
requires laminated cells, is one such example of this kindof application. There are two main types of battery cell
shape, round and long, which are developed by various
manufacturers. When installing either shape of cell into
an automobile, several cells are formed into a pack thenassembled as a module (Fig. 4).

*3*4

Permanent magnet
Electronic power steering

Fig. 4 Battery

Cell

Pack

ModuleAdhesive is used between the cells to affix them to eachother for laminating and packing. Though thermosetting
adhesives are effective for bonding large areas, they cannot
be used in this particular application because heat cannot
be applied to the cells. For this reason, two-part mixture
adhesive that cures at room temperature is used to bondcell surfaces. This adhesive must also have the flexibility toadapt to vibrations incurred after installation, making twopart mixture acrylic elastic adhesive the ideal material forbonding the various parts used in battery assembly.

4-3 Potting
Elastic adhesives are used in potting to affix and protect
parts in electronic assemblies. The rubbery elasticity of thepotted adhesive protects assembly parts and absorbs impacts
to the assembly in which they are installed to reduce the riskof detachment which can lead to product failure. For manyof these applications, one and two-part moisture-curable
silicon and modified silicone elastic adhesive cut with lowmolecular siloxane are used. These adhesives maintain an
excellent rubbery elasticity when hardened and can be cured
at room temperature so that boards need not be inserted
into curing ovens after assembly. However, as mentioned
previously, even two-part mixture silicone and modified
silicone resins take considerable time to fully cure, so thistime must be further reduced. Productivity is improved inthis regard by using a two-part mixture acrylic resin, which
can be cured at room temperature more quickly. Potting
agents must also be moisture resistant to better protect
electrical and electronic parts, so TB3955 is the optimaladhesive for affixing and protecting assembly parts as it
is moisture resistant and maintains flexibility when cured(Fig. 7).

5

Material Properties

5

Tensile Shear Bond Strength (MPa)5.

Here are some of the properties of TB3955, our two-partacrylic elastic epoxy that cures at room temperature.

5-1 Curing Efficiency
Previous two-part room-temperature curing adhesives
had excellent reaction speed, but this was also problematic
because it meant a short pot life that reduced working time.
Therefore, we optimized pot life for TB3955 in considerationof workability and productivity (Fig. 5). However, we also
ensured that once the curing reaction starts, it accelerates
rapidly to quickly reach practical strength. When cured
at a 25°C room temperature, the adhesive has a pot life ofaround five minutes, then develops to a strength of 2 MPaor more in 60 minutes and is almost completely cured after
12 hours (Fig. 6).

4

3

2

1

*Curing Conditions: 25°C, Fe/Fe
0

0

30

60

90

120

Curing Time (min)Tensile Shear Bond Strength (MPa)10

5-2 Curing Efficiency
Once cured, TB3955 has a low glass transition temperature
for a rubbery elasticity that allows it to be used in a wide
range of temperatures (Fig. 4).
100

Shear Bond Strength Retention (%)

Curing Time 0 to 120min

80

Curing Time 0 to 24h
8

6

4

2

*Curing Conditions: 25°C, Fe/Fe
0

60

0

6

12

Curing Time (h)18Fig. 6 TB3955 Adhesive Development

40

Furthermore, it does not weaken and maintains performanceeven in high temperature environments of 150°C and highhumidity and temperature environments of 85°C and 85%
RH (Fig. 7).

20*Curing Conditions: 25°C, Fe/Fe
005Time (min)10

15

Fig. 5 TB3955 Adhesion Pot Life
Table 4 TB3955 Cured Characteristics
Features

Unit

Measured Value

Testing Method

Remark(s)Hardness—

A65

3TS-2B00-004

Durometer A

Elongation rate％130

3TS-4190-001

No. 3 dumbbell

Tensile strength

MPa

5.2

3TS-4190-001

No. 3 dumbbell

Tensile shear bond strength

MPa

Rate of contraction when curing％

0.9

3TS-Z365-001

φ20

Glass transition temperature

°C

-45

3TS-4730-001

DMA method 1Hz, E” peak

Coefficient oflinear expansion
Curing conditions: 25°C×24h

6

24

α1
α2

ppm/°C

6.6
6.2

66.0
221.5

3TS-4100-013

3TS-4740-001

Fe / Fe (SPCC,SD)
SUS/ SUS (SUS-304)−100 to −70°C
0 to 50°C

5-3 How to Use

In TB3955, the curing reaction begins when the main
agent and the curing agent are mixed in a 1:1 ratio. This
product was commercially released in twin cartridges sothat it can be mixed in a static mixer then applied directly
to the adherent (Fig. 8). Workability is excellent because
the required amount can be mixed in the appropriate ratio
without weighing.

100

Hardness (A)

80
60
40
20
0

0

250

500

750

1000

Time (h)
120°C

150°C

85°C,85%RH

Tensile strength (MPa)

10
8
6
4
2
0

Fig. 8  TB3955 Specifications
0

250

500

750

1000

Time (h)
120°C

150°C

85°C,85%RH

500

750

Elongation rate (%)

200
160
120
80
40
0

0

250

1000

Time (h)Tensile Shear Bond Strength [Fe] (MPa)

120°C

150°C

85°C,85%RH

500

750

25
20
15
10
5
0

0

250

1000

Time (h)
120°C

150°C

Fig. 7 TB3955 Durability

85°C,85%RH

7

Closing

TB3995, our new two-part acrylic elastic adhesive which cures at room temperature, boasts durability, flexibility and curing
speeds unprecedented in previous two-part mixture adhesives. It is perfect for a wide range of fields and application development,from transportation to electronics and even public works material markets.
At ThreeBond, we are constantly striving to further our technological development in order to create products that meet anever-widening range of customer needs.
<References>
ThreeBond Technical News No. 44, 72
ThreeBond Co., Ltd. R&D Headquarters
Development Division, Transportation Development Department,
Kenji Takahashi
Masao Inoue
Nao Arita

ThreeBond Co., Ltd.
8

4-3-3 Minamiosawa, Hachioji,
Tokyo 192-0398
Tel: +81-42-670-5333