Mitsubishi Heavy Industries Technical Review Vol. 55 No. 2 (June 2018)
1Development of Rapid Adhesion Method for
Assembling Plastic Parts Using Near-Infrared

YUICHI YUI*1

AKIHISA OKUDA*2

MASANORI KUBOTA*3

The replacement of the conventional mechanical joint using bolts, etc., with an adhesive
joint, among methods for assembling parts, has been studied in terms of weight and cost reduction.
One of the issues in replacement is the curing time of the adhesive. To address this issue, theprinciple of the rapid heating method using near-infrared radiation1, 2 developed by Kubota
Research Associates, Inc. was verified. In addition, a simulation tool which allows the estimation of
the change of temperature of the adhesive layer by near-infrared heating was developed tofacilitate the study of the applicability of the method.

|1. Introduction
Among transportation equipment such as aircraft and automobiles, the mechanical joint
method using bolts, etc., is most frequently used to join parts, but in using this method, increased
part thickness to compensate for the reduced strength due to boring and the weight of the bolts
causes an increase in product weight. In addition, the labor cost required for a joint with bolts andthe cost of bolts themselves lead to the increase of product prices. One possible measure to addressthese disadvantages of a mechanical joint is replacement with the adhesive joint method as shownin Figure 1. Adhesives for aircraft structures which can be cured at room temperature require 24hours to generate the handling strength which allows moving on to the next process (for example,
assembly of wiring to plastic parts), and an additional one week is required for the development of
the fully-cured strength. Such a long curing time worsens the product flow time. Therefore, in orderto positively apply an adhesive joint as a joining method, it is absolutely necessary to reduce thecuring time of adhesives.

Figure 1 Example of replacement of mechanical joint to adhesive joint

As a measure for reducing the curing time on joints of plastic parts, Mitsubishi Heavy
Industries, Ltd. (MHI) focused attention on the rapid adhesive curing method (P-WaveTM/PTIRTM*method) using near-infrared radiation developed by Kubota Research Associates, Inc. and verified
its principle. As a result, it was found that the time for the handling strength generation that wasconventionally 24 hours could be within 6 minutes. Furthermore, in order to apply this method to*1*2*3

Manufacturing Technology Research Department, Research & Innovation Center
Research Manager, Manufacturing Technology Research Department, Research & Innovation Center
Kubota Research Associates, Inc. CEO MBAMitsubishi Heavy Industries Technical Review Vol. 55 No. 2 (June 2018)2

various parts/adhesives, a simulation tool which allows the estimation of the change in temperatureof the adhesive layer by near-infrared radiation was developed.
* P-WaveTM: Polychromatic Wave
PTIRTM: Pseudo-Transmission Infrared Radiation|2. P-WaveTM/PTIRTM method
One possible measure for reducing the curing time of adhesives is an increase in the reaction
rate of the adhesive by heating. But in hot air heating that is generally used, after plastic parts areheated, the adhesive is heated by heat conduction from the plastic parts, and therefore, many hours
are required for the adhesive to reach the target temperature. Even in far-infrared heating, which isused in the rapid heating of polymer resins such as adhesives, much of the energy is absorbed in theplastic parts, and the adhesive is heated by heat conduction from the plastic parts in the same way
as hot air heating, which is not effective.
The P-WaveTM/PTIRTM method is a method where near-infrared radiation which is difficultfor plastic parts to absorb are used to make the energy reach the adhesive efficiently, and PTIRTM
particles which absorb near-infrared radiation dispersed in the adhesive and emit far-infrared
radiation are used to directly and selectively heat the adhesive rapidly (Figure 2).

Figure 2 Comparison of the conventional heating method and the P-WaveTM/PTIRTM method|3. Verification of principle of P-WaveTM/PTIRTM method
In order to verify the effectiveness of the P-WaveTM/PTIRTM method, using plastic parts foraircraft and structural adhesive for aircraft (epoxy adhesive), the data on the time for the joints toattain the strength (handling strength) that allows moving on to the next process and the fully-cured
strength that can be generated when the adhesive is rapidly heated were obtained.
In order to verify the time for generating the handling strength, a test specimen using theplastic part for aircraft as shown in Figure 3 was prepared, and after near-infrared radiation wasirradiated for a predetermined time, the withstand load of the part was measured. The test
conditions are as shown in Table 1. The results indicated that if the joint is heated to about 120°C
at which the thermal effect on the base material and the plastic part is small, a sufficient handlingstrength can be obtained by 6 minutes of near-infrared radiation (Figure 4).

Figure 3

Overview of the handling strength testMitsubishi Heavy Industries Technical Review Vol. 55 No. 2 (June 2018)
3Table 1 Conditions of the verification test for the time for the generation of the handling strength
ID

Type of adhesive

Adhesion area

123

Epoxy-based, room
temperature curing-type
paste adhesive

Approximately
791 mm2

PTIR mixing ratio
0 wt%
0.04 wt%

Maximum
temperature
25°C
80°C
120°C

Duration ofnear-infrared
radiation
0 minutes*
15 minutes
6 minutes*Cured at room temperature for 24 hours

Figure 4 Verification test results of the handling strength generation time

Furthermore, to check the effect of the P-WaveTM/PTIRTM method on the fully-cured
adhesive strength, near-infrared radiation was irradiated for 15 minutes (until the temperaturereaches 120°C at maximum), and then the adhesive strength (shearing strength) after the adhesive
was completely cured at room temperature was obtained.3 The results showed that a sufficientadhesive strength of 20 MPa was obtained (Figure 5).

Figure 5

Test Conditions and results of the fully-cured strength test|4. Development of simulation tool for adhesive temperature atnear-infrared heatingBy the principle verification test shown in Section 3, the effectiveness of theP-WaveTM/PTIRTM method was proved. On the other hand, the temperature increasing property ofadhesive by near-infrared radiation depends on the near-infrared transmittance of the plastic partand the heating efficiency of the adhesive by far-infrared radiation. Therefore, the kind of materialof the adhesive and the part and the board thickness are the key parameters. As such, to determineMitsubishi Heavy Industries Technical Review Vol. 55 No. 2 (June 2018)
4TMTM

the applicability of the P-Wave /PTIR method to each product, it is important to immediatelyunderstand the heating properties when the method is applied to each product. We prepared amodel in which the near-infrared energy passes through a plastic part and far-infrared radiation,
which is absorbed in/wavelength-converted by PTIR particles dispersed in the adhesive, heat theadhesive (Figure 6) and developed the simulation tool (accuracy <6°C) (Figure 7) which allows
the determination of the behaviors of the adhesive by temperature when it is irradiated by thespecified near-infrared energy. This tool allows the calculation of the change in temperature of theadhesive layer using, as input data, the near-infrared radiation intensity, the near-infraredtransmittance of the plastic part and the temperature increasing property in near-infrared radiation
when only adhesive is targeted. For base materials, such as CFRP, which have a low heat
conductivity, the effect of the heat radiation on the base material can be ignored. For basematerials, such as aluminum, which have a high heat conductivity, a correction value must beadded to the temperature increasing property of the adhesive. In the verification of the calculation
precision of this tool, the error was +6°C in the adhesion of the GFRP part to the CFRP substrate(top in Figure 7) and the error was +3°C in the adhesion of the GFRP part to the aluminum
substrate (bottom in Figure 7). The results show that this tool has a sufficient calculation precisionand enables the estimation of the time for the generation of the handling strength when
P-WaveTM/PTIRTM is applied.

Figure 6

Adhesive layer heating model by near-infrared radiationFigure 7 Results of verification of appropriateness of the model by comparison of theexperimental values and the calculated values

Mitsubishi Heavy Industries Technical Review Vol. 55 No. 2 (June 2018)
5|5. Conclusions(1) Verification of principle of P-WaveTM/PTIRTM method- The handling strength generation time for structural adhesive for aircraft can be reduced
from 24 hours to 6 minutes.
- A reduction in adhesive strength by the use of this method was not observed.
(2) The adhesive temperature simulation tool was developed for promoting the application of thismethod to products.

References
1.
2.
3.

Kubota, M., Kubota, A., Kubota, M., & Kubota, A. T. (2006). U.S. Patent No.US20060283543 A1.
Washington, DC: U.S. Patent and Trademark Office.
Kubota, M. (2006). U.S. Patent No. US20060286362 A1. Washington, DC: U.S. Patent and Trademark
Office.
ASTM International. (2010). ASTM D1002-10 Standard Test Method for Apparent Shear Strength ofSingle-Lap-Joint Adhesively Bonded Metal Specimens by Tension Loading (Metal-to-Metal).