In June 2018, the redesigned Crown of Toyota Motor hit the market.
The brand long coveted as the “dream car” announced its evolutionary full-fledged connected car fleet that would open up the new era of automobiles.
Dubbed as the sedan for drivers, the 2018 model Crown accentuates its symbolic new design.
The model really comes down to the feeling of the driver with astounding maneuverability, featuring a platform with the low-positioned powertrain to allow a lower center of gravity based on Toyota New Global Architecture (TNGA).
One of the technologies behind this sedan that delivers a blend of performance and agility is the structural adhesive.
Aisin Chemical, the only group company specializing in chemical products, is responsible for the adhesive being used in the new Crown.
Structural adhesives, mainly composed of epoxy resin, cure under heat and can join steel sheets. These compounds offer the highest load-bearing capabilities, e.g., adhesive strength and durability.
The stiffness of the body is a fundamental requirement for any car.
Using steel plates that are thicker or harder is a common method to improve the stiffness of a vehicle body.
However, the thicker the material, the heavier the vehicle, and the harder the material, the less design flexibility.
Structural adhesives used together with conventional spot/laser welding provide better stiffness without the increase in weight.
“They are not designed to oust welding technology from automotive applications. A car body can be made more rigid if structural adhesives are used effectively along with welding,” said the head of Chemical Engineering.
But, there is a story of years before in his comment that has been widely recognized in the industry.
The first use of structural adhesive in wheel arches dates back more than 10 years ago.
The application of adhesives made the end of the arch flange shorter, adding sleekness to vehicle designs.
Aisin Chemical showcased its structural adhesive at one of the trade shows for the group companies. In the booth were two types of steel plate assemblies -- one spot-welded and the other bonded with the structural adhesive the company developed -- to show the difference in stiffness between the two.
The straightforward demonstration was well-received by the visiting auto manufacturers. Most, however, said,
“The structural adhesive certainly has advantages. But, I don't think its durable and aging-proof enough to be used in auto parts.”
Spot welding physically joints parts together. In contrast, adhesives use chemical reactions and many thought adhesive-bonded parts were less stiff than spot-welded parts.
Because there is no chance of bonded interfaces being exposed to sunlight, few external factors, such as UV, that affect the performance of the adhesive agent after curing exist. If used together with welding, it is durable enough to meet the durability requirements of assemblies.
What was needed was evidence that would demonstrate the physical chemistry properties of structural adhesives, e.g., long life without degradation and durability under harsh conditions, but the project members did not have that in hand.
Meanwhile, amid the worldwide trend towards lower CO2 emissions, automobiles were required to be lighter.
Structural adhesives contribute to reduced weight while providing high strength and stiffness for car bodies. A tailwind that broadened the range of applications started to blow from the green movement.
High-performing structural adhesive Felco 7000, with enhanced bonding strength, hit the market in 2012.
More than a decade had passed since the cars that came with wheel arches bonded with structural adhesive first ran the road. These years saw the adhesives become a reliable choice for auto manufacturers, prove their effectiveness in bond strength and improve rigidity.
Felco 7000’s being selected for the Lexus LS600 was an important engineering milestone.
It was not used in large volume but brought excitement to the chemical engineers at Aisin Chemical.
“The Lexus pamphlet touched upon Felco 7000 as the structural adhesive used for the new model. That is not what happens often. We still have the pamphlet here,” said the chief of Industrial Chemical Engineering.