Graded cured pultrusion profiles for increasing composite adhesion to SMC

Reducing vehicle weight plays a key role in rail vehicle construction. Lightweight construction is particularly important in the design of passenger cars, as the aim is to achieve the highest possible payload within the specified permissible wheel loads. At the same time, comfort, safety, and fire protection must not be compromised, nor must the cost-effectiveness of the solutions be called into question.

The PulPro-SMC project contributes to resolving this conflict of objectives by enabling the project partners to further develop and combine innovative lightweight construction technologies in a targeted manner: 

• Carbon fiber-reinforced profiles are manufactured using pultrusion technology.
• TowPreg technology enables the production of load path-compatible preforms.
• Carbon short fiber-reinforced sheet molding compound (SMC) is used to press profiles and load path-compatible preforms into a node structure.

This node structure is used a total of 42 times in a representative car body, resulting in a weight saving of 144 kg compared to conventional metal construction. The weight reduction achieved enables either a higher payload or lower energy consumption over millions of kilometers driven.

A major challenge in combining pultrusion, TowPreg, and SMC technologies is that cured thermoset components can usually only be joined together with the aid of additional joining materials, such as adhesives or screws. However, this is too complex and cost-intensive for large-scale series production in rail vehicle construction. The aim of the PulPro-SMC project is therefore to develop a process route that enables the material-locking connection of profiles, TowPreg and SMC without pre-treatment or additional auxiliary materials.

The innovative core of the project lies in the development of a novel pultrusion technology: the production of partially cured profile structures on the surface. Such a gradation (i.e., different degrees of curing across the cross-section) means that the profile is only 70% cured on its surface, while otherwise it has a significantly higher degree of curing of up to 98%. Due to the partial curing, the profile still has reactive groups that can react with the thermosetting materials used in the subsequent TowPreg and SMC processes at the material level, resulting in covalent bonds between the profile, TowPreg, and SMC. 

Gradation can be achieved during the pultrusion process by adjusting the temperatures in the mold and in the core so that a solid profile is pultruded, but the resulting fiber-plastic composite (FKV) has not yet fully reacted. The effect of the gradation was verified using DSC. Initial pressing tests confirmed the innovative approach, with knot structures being successfully pressed.