Bonding at the push of a button

The adhesive effect of the adhesive capsules can be tested directly using the demonstrator.

Bonding joins components over large surface areas, brings together different materials and enables lightweight designs. In industrial practice, however, applying adhesive is often an additional process step: adhesives have to be dosed, applied and cured.

Direct handling of reactive adhesive components can also place additional demands on occupational safety, process management and training. Depending on the material system, temperature, substrate and load, the adhesive effect of conventional adhesive tapes may also change.

A new approach based on microencapsulation is intended to simplify these steps. At Fraunhofer IAP, researchers are developing microscopic capsules that contain the components of a two-component adhesive as part of the Fraunhofer Cluster of Excellence Programmable Materials CPM.

"You can think of the microcapsules as many tiny reservoirs for adhesive," says Dr. Christian Neumann, scientist at Fraunhofer IAP. Each capsule contains one of the two components. As long as the capsules remain intact, the system remains inactive. When pressure is applied, the capsules burst. The components come into contact and the adhesive crosslinks. This creates a strong bond precisely at the contact surface. "A major advantage is that this crosslinking takes place at room temperature. Additional heating or further curing steps are therefore not required," says Neumann.

The technical challenge lies in the encapsulation itself: two-component adhesives are reactive materials and can react with the chemicals used during production of the capsule shell. However, the researchers can adjust the capsule chemistry with great precision. As a result, the adhesive components remain active, are reliably enclosed, can be stored and processed, and open in a targeted manner during pressing.

Key factors that make handling the adhesive safer

"For industrial applications, it is crucial that the adhesive is initially safely enclosed," says Neumann. "This means employees come into less direct contact with reactive components, as activation only takes place during the joining step."

For industrial bonding processes, the approach combines two advantages. First, encapsulation reduces open handling of adhesives: the reactive components are released in the bond line only during joining. This can simplify handling and support occupational safety during processing.

Second, the researchers are focusing on isocyanate-free adhesive systems based on acrylates or epoxies. This makes the capsule and carrier system interesting for applications in which companies want to avoid using adhesives containing isocyanates.

How the adhesive fabric is used

In the next step, the microcapsules are to be applied to sheet-like carrier materials that can be processed like an interlayer material. Suitable options include textile carriers, fibre scrims or other sheet-like materials. "This turns the adhesive into a manageable material: it can be positioned in the component and only develops its effect during the joining step," explains Neumann.

Possible fields of application include processes in which components need to be bonded or joined over large areas, in a controlled manner and without open adhesive application. In the automotive industry, for example, battery stacks are a possible use case. Other fields of application include mechanical engineering, electronics manufacturing and microstructured components with fine channels, where conventional dosing and application methods are technically complex, difficult to access or economically unattractive.

To develop these adhesive textiles, Fraunhofer IAP combines its expertise in microencapsulation with application-oriented testing. The strength of the resulting bonds is being investigated together with the Fraunhofer Institute for Machine Tools and Forming Technology IWU and forms the basis for assessing suitable fields of application with partners in a targeted way.

Ready for practical testing

The approach is now to be transferred to concrete applications. To this end, Fraunhofer IAP is looking for partners from industry and research who can contribute their own components, carrier materials or assembly processes. Companies that want to simplify sheet-like joining processes, make the handling of adhesives safer or integrate the bonding process more effectively into existing assembly workflows are particularly relevant.

"Together, we can examine whether the capsule and carrier system can be adapted to specific requirements and what quantities of material are needed for application-oriented testing," says Neumann.

If a partner contributes a suitable use case, Fraunhofer IAP will tailor the capsule and carrier system specifically to it. At the synthesis pilot plant of the Fraunhofer Pilot Plant Center for Polymer Synthesis and Processing PAZ at Fraunhofer IAP, microcapsules can also be produced on a scale of up to several tonnes. This means sufficient quantities of material are available to test the technology under application-oriented conditions.