But why adhesives?

The reason is simple: Unlike screws, adhesives are transparent, while being more reliable than snap hooks. Adhesives easily withstand temperature fluctuations and material warpage that may occur as a result. Furthermore, as proven by tests, they provide added safety. Head-on crash tests have revealed that clipped decorative elements may break or splinter in the event of an accident, thus presenting a potential risk of occupant injury. However, if these elements are attached by bonding, this risk can be minimized, with trim pieces remaining in place during impact.

An emerging application among higher-end vehicles is crystal switchgear, found on components such as gear shifts and infotainment controls. This application particularly highlights adhesive requirements such as transparent appearance and yellowing resistance, important in all decorative interior applications but especially with optically clear substrate materials like crystal.

Adhesives vs. adhesive tapes

As an alternative, pressure-sensitive adhesive tapes may also be used for fixing decorative elements in the car. These have the advantage of fast processing since they adhere immediately and thus reach their final strength without any further steps. They are primarily used for bonding brand emblems onto parts such as the steering wheel or key fob. As these tapes are small and invariable, it makes sense to use them for this kind of application. Using these tapes for more complex geometries such as display bezels, however, may not make as much sense. If tight enough at the corners, which many tapes are not, they would be used for die-cut frames, which produce high quantities of waste. Either case makes it difficult to bond interior panels using this method.

Using light for faster bonding

Decorative elements are often made of opaque materials, such as dark plastics or chrome. This makes pure light curing impossible. But, of course, the automotive industry seeks to achieve high production numbers at short cycle times, which is why special resins are used in this case. Many DELO adhesives feature preactivation, a curing mechanism that allows light-curing adhesives to reach final curing when bonding even opaque components. 

The way this special feature works is that the adhesive is still liquid following a brief irradiation time known as “open time,” when its adhesive properties are “activated,” allowing opaque components to be joined as well. During this period, components can still be adjusted. After open time, the adhesive cures within minutes without any external light influence. 

To summarize, this process takes place in the following order:

• Adhesive dispensing
• Open time
• Joining of components

One-component adhesives also have the advantage of being dispensed and processed more easily than two-component adhesives. DELO resins are particularly suitable for bonding materials such as PC, PPS, or chrome-plated surfaces. 

The many materials used in automotive interiors and their environmental influences can prove to be demanding for adhesives that are used for their assembly. For one, temperature fluctuations and changing media influences call for higher-performance properties. And while reliability is crucial, flexibility also matters since excess heat may cause embrittlement. Modified acrylates like DELO PHOTOBOND LA are particularly suitable in this case and allow joining of plastics such as PC/ABS.

Another consideration is that these adhesives’ glass transition temperature lies at -65 °C. This means that they remain flexible in temperatures typically demanded for interior applications, ranging from -40 °C to +100 °C. Adhesives can also be fluorescent, which is very useful for dispensing control in production lines.

Conclusion

When bonding decorative elements of a car’s interior, it is important that the adhesives cure quickly—even in fully automated production processes—and that high throughput can be achieved. Therefore, rapid final curing and the ability to be dispensed in very small quantities are essential. The properties of the adhesives presented here can be adjusted to the specific needs of different application points. In contrast to other joining methods, they are flexible and reliable enough to maintain the appearance of a car’s decorative elements throughout its lifetime.

Are you looking for a tape alternative to bond decorative interior elements in your auto manufacturing process? Find out more about the possibilities of liquid pressure-sensitive adhesives in our white paper. Or take advantage of our free project consultation to discuss your specific application.

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About DELO: DELO is a leading provider of high-tech adhesives, offering solutions for automotive, electronics, and semiconductor industries for over 25 years. Our innovative technologies set industry standards. Companies like Bosch, Huawei, and Siemens trust DELO for superior adhesive technologies.