A fundamental step in the production of timber panels is the attachment of the planking to the wooden ribs. When joining two materials by nailing, stapling or screwing, the force transmission is only punctual, i.e. at specific points. Furthermore, the material is damaged by the penetration of the fastener. Bonding allows the force to be transmitted over a wide area without weakening the material. Conventional adhesives have not been able to establish themselves on the market as large hot presses and application facilities are required for the processing procedures. Moreover, the processing time is limited, as the joining must take place within the pot life of the adhesives.
Through the utilization of switchable adhesives, the application of the adhesive and the joining of the components can be carried out independently of each other. The basic feasibility has already been described in science - we are researching the details and developing a product which is ready for application. Our goal is a polyurethane adhesive whose adhesive effect can be switched on and off in a temperature-controlled manner. We aim to achieve this through a special structure of the adhesive and the integration of chemical functions. Below 60 °C, these functions form chemical bonds with each other and the adhesive is cross-linked. Above 120 °C, these bonds dissolve and the adhesive melts. This process can be repeated several times.
In order to selectively introduce the heat necessary for switching the adhesive, we will incorporate special metal particles, so-called susceptors, into the adhesive. By applying an electromagnetic alternating field, these particles can be heated within seconds. The heat is transferred to the surrounding adhesive. The functional principle is comparable to an induction hob.
The application and heating of the adhesive during prefabrication in the factory or on the construction site should be as simple as possible. In collaboration with our project partners, we are therefore evaluating various methods. Investigations into fire retardancy are also an integral part of the project.
Social relevance
Buildings made from wood and other renewable raw materials are good for the climate and consume few finite resources. Prefabricated houses can contribute towards increasing the proportion of renewable raw materials in the construction industry. Through switchable adhesives, we are creating a possibility which enables them to be planned more flexibly and produced more quickly and more economically. This would enable, for example, a rapid response to the demand for additional living space in metropolitan areas. The reversible adhesive effect also allows simple dismantling of the buildings. Entire wall elements could be reused and would not have to be produced from scratch.
In the future, reversible bonding technology could also play a major role in other areas of life. For ecological reasons, lightweight components made from bio-based, high-performance hybrid materials are increasingly being used, for example as fiber-composite plastics in vehicle construction. It is conceivable that the switchability could be integrated directly into the material. This would create completely new recycling opportunities.
Economic advantages
In the construction and housing industry, there is growing pressure from an economic, social and environmental perspective to build sustainably and, at the same time, economically. Switchable adhesives can enhance the economic attractiveness of the construction of prefabricated houses. Such buildings can not only be produced in a time-optimized and cost-efficient manner; they can also be flexibly adapted to the requirements of the customer. With forward planning, it would be possible, for example, to add or remove walls as needed during the period of use. The automotive and furniture industries could also benefit from the flexible design possibilities as well as the good recycling capabilities. For adhesive manufacturers, a new business field is being opened up.
Fraunhofer Institute for Wood Research 
