Bonding in Cars

Bonding in Cars

Fast, reliable, economic – adhesives point the way forward

The future lies with bonding. As an alternative to welding and screwing for joining components, bonding is not only ideally suited for miniaturization and light-weight design, but is also highly efficient. Miniaturization makes it possible to save both material and energy. In light-weight design, bonding technology allows reliably joining a great variety of materials, to an extent that would not be possible with other techniques. This makes bonding particularly interesting for the automotive industry. 

In the automotive industry, it is increasingly important to produce in a sustainable and efficient way. However, car buyers not only set a high value on reliability, cost efficiency, and energy efficiency; most of them also want their cars to look good.

These requirements have led to the use of 15 to 18 kg of adhesive for just one single car and to 9 % of the total annual adhesive production being consumed by the automotive industry. The largest part of it is needed for the bodywork. Additional possible applications extend from the engine compartment to the car key. Bonding has long been an indispensable element of automobile production and plays a key role when it comes to light-weight design and sustainability.

Some of the many possible bonding areas in a car (Figure: DELO)

Crash-proof Bodywork

Especially in crash tests, cars with large bonding areas perform significantly better than those with welded joints. The reason is quite evident: adhesives allow joining materials without affecting or damaging them. Screws and rivets, for example, weaken components due to punctual load causing stress peaks around the holes. Adhesives, however, provide additional strength through a larger overlap between the joined components.

In addition, bonded front and rear windows increase the stability of the body and allow designing cars with low CW values, enabling significant fuel savings.

Sensors Make Cars Smart

A Hall sensor is used, among other things, for wheel rotation measurement (Figure: DELO)

Today, up to one hundred sensors are installed in a car. The trend of autonomous driving will make sensors as smart all-rounders in cars indispensable in the future. These sensors serve different purposes: safety, comfort, powertrain control.

Since the sensors are located at points where they are meant to receive and transmit information, such signals need to be reliably transmitted to control devices or evaluation electronics. To fulfill their functions and capture physical quantities such as temperature, pressure, position, or speed, the sensors must be adapted to cope with the different requirements defined by their respective positions in the car. To be specific: every sensor needs an adhesive to match its designated use.

The extraordinary features of adhesives can be illustrated using the example of Hall sensors. These sensors are the first choice when position or speed need to be captured: they measure the rotation of the wheel on the wheel hub. Adhesives used here must be highly resistant to influences of media such as oil, gas, or brake fluid. They have to produce good results in various tests such as salt spray tests, reflow tests (JEDEC) and vibration tests. Dual-curing (light/anaerobic) adhesives are also extremely well suited for production, since they guarantee fast prefixation and reliable final curing, even in shadowed areas.

360° Surround View Wanted

Rear view camera on a car (Figure: DELO)

Cameras are used for various purposes in vehicles. They serve, for example, as a collision assistant, for sign recognition, as a parking aid, or for pedestrian detection. Here again, adhesives have to meet challenging requirements. Rear view cameras, for example, are installed outside the car and are therefore exposed to various weather conditions. This requires them to be resistant to fluctuating temperatures between day and night or summer and winter, as well as to humidity depending on the weather. However, the most critical impact results from deicing salt that is whirled up from the roads during winter and may have an aging effect on joined connections.

This makes it obvious that the adhesives must have a good resistance to ensure that the bonds withstand extreme conditions. Low outgassing and low shrinkage of the adhesive are other essential factors to prevent any negative impact on the visual appearance.

Safety First – No Splintering of Displays

It is often not known that components inside the vehicle such as displays also pose a risk of injury in the event of an accident. This risk is even growing with the increasing size of navigation screens, which are now regarded as central control systems. Many manufacturers use glass for covering display panels. Glass is chosen for its pleasant feel and high quality. But the use of glass is not really safe due to its fragile nature.

With the development of a break-resistant cover glass laminate for vehicle displays, SCHOTT and DELO have contributed towards significantly increased safety in cars. The glass is bonded to the display panel over its entire surface, and the extremely soft adhesive acts as a kind of buffer between the two components.

A series of head impact tests has revealed that high-strength aluminosilicate glass in combination with special adhesives passes internationally standardized crash tests. The adhesives used are very flexible and completely transparent, and have additionally been optimized for the requirements of the automotive industry.

For example, they have been adjusted with respect to their index of refraction, thus reducing by two thirds disturbing reflections (e.g. from sunlight) and providing good color contrasts. Bonding not only improves readability, but offers even more benefits. Contamination by dust between LCD and cover glass is avoided, and there is no risk of moisture condensing behind the cover glass.

Special Bonding Process

ONSERT stud in the BMW i3 (Figure: BMW/DELO)

The so-called ONSERT technology, developed in cooperation between DELO and Böllhoff, is an alternative to welded threaded studs, which are often installed in cars for fastening cable strands, for example. Welded studs, however, always reach their limits when extremely thin sheets or non-weldable materials are used.

The ONSERT technology is a unique joining method that combines bonding and screwing. ONSERT designates a threaded stud with a plastic foot applied by injection.

The special feature of this method is the extremely short curing time. Light-curing acrylates, for example, cure within about four seconds under LED light. The resulting bond is immediately ready to bear load, providing the component in a very short time with a stable thread that can be screwed and, if necessary, unscrewed again.

Thus, ONSERTS combine the benefits of welded studs such as a high degree of automation, short cycle times, and high load-bearing capacity with the possibility of bonding on sheets of less than 0.5 mm thickness as well as on non-metallic light-weight materials. In contrast to welded elements on sheet metal, bonded elements do not become apparent on the visible side, making this method ideally suited for components with high visual requirements.

This technology has already proven itself in the case of the two electric BMWs i3 and i8 for the attachment of cables, panels, and other components.

Attractive and Modern Design Combined with Reliability

Bonding decorative elements on the dashboard, here chrome rings around ventilation slots. (Figure: Fotolia)

When buying a car, customers attach great importance to the optical design and quality of its interior. The dashboard, in particular, needs to have an attractive look and feel. That is why decorative and ornamental elements such as chrome rings around ventilation slots are fixed by bonding.In contrast to screws, adhesives are invisible and more reliable than snap hooks. Adhesives remain resistant: they easily withstand even temperature fluctuations and the associated expansion of the materials.

One-component epoxy resins are used for bonding the chrome rings that can be quickly preactivated by light. The special feature of preactivation is that the adhesive still remains liquid after a short time of exposure, allowing opaque components to be joined as well.

Conclusion

Adhesives have become indispensable for automobiles. Their positive characteristics allow using a variety of material combinations and open up new design options. The added value of adhesives is enormous: apart from their major function of bonding, they also protect against aggressive media, enable light-weight design and ensure safety. This makes bonding the joining method of the future and a guarantee of lasting success in the automotive industry.