December 2023

Rapid curing system with near-infrared laser with CLSsystem.

Ryo Ogawa, Junji Ueyama

Introduction

Epoxy resins are a generic term for thermosetting resins with two or more epoxy groups permolecule, which exhibit excellent adhesive properties, mechanical strength, electrical insulation,
heat resistance, chemical resistance, water resistance and low shrinkage. Taking advantage ofthese characteristics, they are applied in a wide range of fields such as coatings, electrical andelectronics materials and adhesives.

On the other hand, in recent years, there are trends towards higher density, further micro-scaling

and more diversification in the fields related to information and electronics application. In the fieldof renewable energy such as photovoltaic and wind power generation, and of environmental /energy-saving such as SiC power semiconductors, high-power LEDs and electric vehicles, further

strengthening and durability of components are required. In particular, remarkable progress has
been made in the development of information and electronic equipment for in-vehicle

applications.

In these new fields, there is an abundance of demand for materials that can be adapted todifferent materials, environments, and production processes. In response to these demands, weare focusing on product development that takes advantage of our strengths. In terms oftechnology, the research and development is focused on improving the molecular structure ofresin materials and proposing new applications for bring out various physical properties. And interms of fields, research and development is being conducted with "adhesion," the mostoutstanding property of epoxy resins, as the keyword.

Adhesive technology trends

For example, in the automobile industry, technological innovations are being promoted in orderto improve fuel efficiency by reducing the weight of the vehicle and to electrify the powertrain

from the perspective of preventing global warming, as well as by utilising electronic devices in thevehicle to ensure safe and accident-free driving and to improve the interior space. For vehicle

weight reduction, the use of lighter and stronger aluminum and magnesium alloys and CFRP,
which is a composite material of carbon fiber and resin, is being considered. Therefore, joining

technology for these different materials is being studied as an alternative to welding. In addition,in order to improve the reliability of electronic equipment, new packaging materials that canprovide sufficient durability for automobile durability tests are being developed.

For adhesives that meet these market requirements, epoxy resins with excellent shear bondstrength, creep behavior, heat resistance and chemical resistance are expected to exhibit suitableproperties 1,2. In addition to basic performance requirements, there are also high productivity
requirements in the manufacturing field, such as low-temperature curing and short-time curing.

These requirements are aimed not only at productivity (time) = cost, but also at reducing theenvironmental load by saving energy through the reduction of the heating process.
©2023 ADEKA Corporation1

December 2023

1. New curing systems for epoxy resins

In general, curing of epoxy resins by amines, which are typical thermosetting resins, takes time.

This is due to the reaction mechanism and reaction rate between the epoxide and curing agent 3.

Epoxy resins and curing agents are often used as one-component types for curing at hightemperature and as two-component types mixed just before use for curing at low/ambient
temperature, mainly from the viewpoint of storage stability. In order to achieve rapid curing with
one -component type, the control of the stability control is most important technology.

We call the epoxy resin adhesives used in our laser ultra-rapid curing system the "CLS system"(Curing system of Latent hardener and Specialty epoxy resin). The CLS system enables thermoset

ultra-rapid curing adhesives using a new compounding technology that combines original epoxy
resins and latent hardeners with a highly reactive cyanate ester resins. These formulations provide
suitable stability for one-component types and rapid-cure properties can be cured by nearinfrared laser to make the curing reaction even faster.

2. Near infrared laser curing system

We have proposed a new thermal curing system that takes advantage of various excellent

characteristics of the CLS system, which is composed of epoxy resin, cyanate ester resin, and latent

hardener, such as ultra- fast curing, high heat resistance, and high adhesion. In particular, thenear-infrared laser curing system makes maximum use of the ultra-fast curing property andenables thermal curing in a short time and with low energy consumption 4. The near-infrared laser
has recently come to be utilized to its fullest extent due to the low cost of semiconductor lasers,

and is used in many fields, mainly in resin welding and bonding. In laser welding, thermoplastic
resin such as PBT (Poly Butylene Terephthalate) is used as the welding material, the material on

the laser irradiated side is light transmissive grade and the opposite side is light absorbing grade,and the welding interface is pressurized under load and adhered to the welding surface. The

welding interface is heated to above the melting point of the material under load and pressurized

to adhere to the welding interface. In such a system, the materials are limited to the same type ofthermoplastic resin that easily melts, and the processing accuracy of the bonding interface isrequired.

At the same time, in the joining field, there is a growing demand for joining technology fordissimilar materials. For joining dissimilar materials, intermediate materials play a major role, andvarious developments have been made 5. In order to meet these requirements, we have developed

an adhesive system that can be cured by near-infrared lasers.

Near-infrared laser heating can heat materials instantaneously. To take advantage of this andmake the process more productive, it is necessary to cure the adhesive in a shorter time. On theother hand, with conventional epoxy adhesives, although the curing reaction starts upon heating,a long period of heating (=laser irradiation) is required to obtain sufficient performance. Therefore,
even when general thermosetting adhesives are heated with a laser, a long period of laser

irradiation is required to complete the curing process. With long time near-infrared laser

irradiation, the advantage of short curing times is lost. Furthermore, due to thermal damagearound the irradiated area, the advantage of using the near-infrared laser is lost. Therefore, we©2023 ADEKA Corporation2

December 2023
designed a special adhesive that applies the ultra-fast curing property of the CLS system toshorten the curing process using the near-infrared laser (Figure 1).

When the adhesive layer isirradiated with near-infrared laser,
the photo-thermal exchangematerials heats up instantly andlocally adhesive layer. At the sametime, CLS adhesive can be cured.

Figure 1 Laser Adhesive System.
The features of this system are "low energy load", "high-speed processing" and "minimization ofthermal damage" (Figure 12). Compared to the conventional heating method using a furnace, theheating method using a near-infrared laser beam requires power only when necessary, thus

reducing energy consumption. In addition, the time required in the curing oven, which was about

one hour in the past, is no longer necessary because the adhesive can be cured by laser irradiation

in a few seconds. Furthermore, since only the irradiated area of the near-infrared laser can beefficiently heated, the entire part can be cured without heating, thereby minimizing thermal
damage to the part.

We are currently developing the practical application of the near-infrared laser adhesive system.

In addition to adhesion of electronic components and protection of terminals, we are proposing
the possibility of application to assembly such as casing and hole filling (Figure 2). In addition toADEKA REMYLOP CLS-1132 (described below), we are also developing other adhesives for use inthis system by adjusting the curing start temperature and various properties to suit the application.
In the curing process, we have proposed various innovations in the irradiation of near-infraredlaser light. For example, in addition to bonding using near-infrared laser- transparent materials
such as plastic and glass, direct irradiation to adhesives and/or heating of the substrate surface

by irradiation to the base material are proposed as heating methods. In addition to spot irradiation,
the use of a near-infrared laser with a micro-diameter of several hundred μm enables
rearrangement irradiation using diffractive optical elements and remote high-speed irradiation
using galvanometer lenses, which are proposed as new heating methods.
The important points in realizing a laser curing system are as follows 6.
(1) Adhesives for laser curing systems(2) Laser irradiation process©2023 ADEKA Corporation3

December 2023

Figure 2 Laser gluing system application examples.
The fast curing adhesive process using a laser curing system not only enables reduced time forthe adhesive process, but also offers new possibilities for the adhesive process itself (Figure 3).

For example, the in-line process using laser/short-time curing enables shorter production times,a smaller facility area and a higher level of quality control compared to the conventional thermal

curing process using batch-type ovens. In addition, as local heating is applied only to thenecessary areas and no standby heating power is required, as in conventional ovens, a significant
reduction in energy consumption is possible.

In-line process
Figure 3 Benefits of laser and/or fast curing system

2.1 Adhesives for Near-Infrared Laser Curing System

In the development of fast curing adhesives by laser irradiation, several issues had to beovercome. The largest challenge of these was the solution to the internal shrinkage stresses

associated with the cross-linking reaction that occurs instantly in the adhesive during short-timecuring.

Internal shrinkage stress occurs when the distance between monomer molecules shrinks due tothe bonds formed by the cross-linking reaction. If this internal shrinkage stress remains inside theadhesive or at the adhesive interface, it can lead to material cracking and a loss of interface
adhesive strength. For excellent adhesive properties, it is important to release the internal
shrinkage stresses generated by the curing reaction.

Generally, a flexible component such as rubber filler is added to add toughness, but this does

not provide good relaxation of stress that occurs in a few seconds, as is the case with laser curingsystems, because of the long time required for stress relaxation. To solve this problem, we©2023 ADEKA Corporation4December 2023
thoroughly reviewed the chemical structure of the adhesive components. As a result, we had

succeeded in instantaneously relaxing the stress generated by instantaneous curing by adding astress relaxation function to the chemical structure of the adhesive's curing component itself.

This paper introduces the adhesives that have achieved physical properties suitable for various

joints for the practical application of laser curing systems. Typical properties of ADEKA REMYLOP
CLS-1132, the standard, are shown in Table 1. The cured material has a linear expansion coefficientof 25ppm (<Tg), which is close to that of various engineering plastics and solders, contributing todurability. In terms of adhesion, excellent adhesion can be obtained specially to engineering
plastic substrates such as PBT and PPS, which have been considered difficult to bond (Figure 4).

In addition, a lineup of adhesives with various properties is currently available for various
applications, such as bonding materials with large differences in linear expansion coefficients andbonding brittle materials such as glass (Table 1).

Figure 4 ADEKA REMYLOP CLS-1132: Adhesion test results.
Table 1 Typical properties of adhesives for laser adhesive systems
GradeViscosity / 25 ºC
Pa･s / 20rpm
Filler Contentwt%

Halogen freeStorage modulus(<Tg) GPa

Glass transition temperature(DMA) ºC

Coefficient of linear expansion(<Tg) ppm

CLS-1132

High Tg & Low
CTE

Low elasticity

Ultra-low modulus

30

30

20

755

70－－

Possible

Possible

Possible

Possible

614

1

0.1

120

14050

30

25

1763

75

2.2 Curing by laser irradiation

In the laser irradiation curing process, the light-thermal conversion component of the adhesive

rapidly heats the adhesive layer by near-infrared laser irradiation. When the heating temperaturerises to the reaction-active temperature of the adhesive, the adhesive layer is heated and cured
together with the curing reaction heat of the adhesive itself.

If the laser irradiation energy is not sufficient, the material cannot reach the reaction-active

temperature and the curing reaction does not proceed. On the other hand, if the laser irradiation©2023 ADEKA Corporation5December 2023
energy is too strong, the material becomes overheated, resulting in partial thermal decompositionof the material and failure to achieve the desired physical properties. Due to this points, the correct
parameter setup is important for and homogenous curing result. For laser adhesive systems

utilizing the CLS system, the following points are used as the standard conditions for laser curing.
a) Laser shape: Top-hat laser 3 to 15 W/4 φb) Energy required for curing: 0.24 W/mm2 × 3 to 10 sec.
These conditions are examples of the energy required for curing adhesives, so various

adjustments will be necessary when applying the laser to actual parts. Furthermore, the laser

irradiation equipment is not limited to mechanical irradiation position adjustment using an X-Ystage, robot arm, etc., as used in laser soldering, etc., but also includes irradiation using

galvanometer lenses, which are often used in laser marking, etc., and various jigs introduced inlaser resin welding, etc. Various jigs can also be applied, and it is important to select the equipmentthat best suits the purpose.

In combination with a system that can measure the temperature of the irradiated area

simultaneously with laser irradiation, it would be possible to track the irradiation profile andcontrol the bonding quality with high accuracy.

2.3 Bonding and curing methods

As a general joining method, an adhesive is applied between the adherends, the adherends areirradiated with a laser from the near-infrared laser-transparent side, and the adhesive layer iscured. It is also possible to cure the adhesive by direct laser irradiation of the adhesive, rather thanby laser irradiation through a laser-transparent material. This laser adhesive system has thefollowing features: it can be applied to joining dissimilar materials, which is difficult withconventional laser welding; it can be applied to joining materials that do not thermally melt; andit does not require precised surface treatment or pressurized adhesion of the joining interface.

It is also possible to cure the adhesive by direct laser irradiation of the adhesive, rather than bylaser irradiation through a laser-transparent material. By optimizing the laser irradiation

conditions, direct irradiation to the adhesive can be performed, enabling a wide range ofapplications such as temporary fixing where only the adhesive fillet is cured and processes where
coating and curing are performed at the same time.

Furthermore, by applying the curing process by direct irradiation, it is also possible to propose

bonding by curing in the depth direction. As an example, Figure 5 shows depth curing of anadhesive by laser irradiation. Although a thickness of several tens of micrometers is sufficient for

the adhesive to function as an adhesive, it can be cured to a depth of 5 mm or more by continuous
laser irradiation. This application of the new adhesive shows the possibility of bonding.

For example, by using a laser adhesive system instead of screws, not only the number of screwparts can be reduced, but it is also possible to bond shapes that were not possible withconventional materials, such as an octopus shape for more secure bonding.

©2023 ADEKA Corporation

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December 2023

Figure 5 Results of deep curing test by laser irradiation.

2.4 Approaches to Practical Application

ADEKA has opened an in-house open laboratory for the practical application of laser adhesivesystems. The lab is equipped with various laser irradiation devices and a range of evaluation andanalysis equipment, enabling curing tests to be carried out on test substrates and actualcomponents.

An example of the experimental apparatus for the near-infrared laser adhesion system is shown

below (Figure 6). This apparatus is equipped with a LED laser irradiator capable of irradiating lasersof 3 to 75 W/φ4, a camera for observing the irradiated area, and thermography for measuring theamount of heat, as equipment for examining the laser adhesive system. In addition, the equipment
is equipped with interlocks and other devices to provide a safe working environment for handlingnear-infrared lasers. The irradiation lens installed in the equipment has a work distance of 200
mm to the focusing area, and all of the irradiation areas on the large 200 mm × 200 mm stage
are equipped with arms that can move the irradiation areas, making it possible to perform laser

irradiation tests using parts of various shapes. Furthermore, space is secured to install variousdevices, making it possible to conduct various evaluations of laser curing behavior at the sametime.

The experimental equipment area is also equipped with evaluation testing equipment that is

important when conducting experiments. These include surface treatment equipment foradherends such as UV treatment and plasma treatment for pretreatment processes, automaticdispensers and jet dispensers for adhesive application processes, thermal analyzers for curing
evaluation, various thermal analyzers for evaluation after adhesive curing, adhesive strength

measurement equipment such as die bond testers, and other equipment for post- curing andpost-adhesive evaluation. In addition, we are equipped with various observation microscopes toobserve the state of the adhesive after curing and after adhesive strength evaluation.

We have prepared an environment that allows us to carry out everything from pretreatment andcuring to evaluation and examination, and we are verifying the practical application of the laser
adhesive system.

©2023 ADEKA Corporation

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Laser irradiation conditions
Wavelength

940 nmDiameter

φ4 mm

Laser power

3 to 75 WScanning speed

1 to 10 mm/sec.

Figure 6 Example of experimental setup for laser adhesive system.

Conclusion

As an example of application of the CLS system, which has a new and unconventional curing

behavior, we have succeeded in establishing a rapid curing system using a near-infrared laser. We

expect that this laser curing system, a curing technology that reduces the energy required forcuring, will contribute to manufacturing as a curing process with overwhelmingly low CO2emissions. In addition, with the increasing demand for quality-critical manufacturing, it will bepossible to apply the process as an adhesive curing process that enables new quality control
different from the conventional one.

We expect that various joining technologies utilizing the CLS system will create new value andcontribute to the development of society.

©2023 ADEKA Corporation8

December 2023

Bibliography

1. Osamu Hara, ThreeBond Technical News, December 20, 1991

( https://www.threebond.co.jp/technical/technicalnews/pdf/tech36.pdf )2. Junichiro Matsuoka，(1989) Surface Technology，40，11，1199
3. Takashi, K., (1985) Kobunshi Ronbunshu, 42 (9), 577-583
4. ADEKA corporation home page

( Japanese : https://www.adeka.co.jp/develop/laboratory/polymer/laser/ )( English : https://www.adeka.co.jp/en/develop/laboratory/polymer/laser/ )5. Satoshi Matsumoto, Ph.（2017）Plastic Age，63，51
6. Sugiura, A., Suzuki, K., Ina, O., Kato, K., (2009) Denso Technical Review 14

https://www.denso.com/jp/ja/-/media/global/business/innovation/review/14/14-doc-13ja.pdf?la=ja-jp&rev=049d4dac9d8a456593a9a67bc06c03d0&hash=BD6D55993B87A631CFCD0D3075495
29E©2023 ADEKA Corporation9