Composite materials are popular in automotive and other applications to reduce the weight of components while enhancing durability. Bonding polymers with galvanized steel poses some problems, however, as it requires harsh chemicals and specialized equipment, and is ill-suited to mass production processes, according to researchers at the Institute of Industrial Science at the University of Tokyo. They have developed what they call a cheap and simple hybrid joining technology based on injection molding and a hot-water treatment. The technique is described in a paper published in the Journal of Manufacturing Processes.

The bonding method entails pre-treating the galvanized steel with an acid wash and dipping it in hot water. The acid wash strips the outer passive layer on the zinc coating of the steel, allowing the hot water to form rough nanoscale needle structures on the true surface, explained a press release on the university website. When a polymer is applied to the treated metal in a process called injection-molded direct joining, it fills in the tiny gaps and ridges between and within the needle structures, thereby creating very strong mechanical linkages. "We found that immersion in hot water was a simple and effective method for producing nanoscale structures on the zinc coating for the polymer to adhere to, but that prior acid-washing to remove the passive layer was a necessary step for this to occur," explained the paper’s lead author, Weiyan Chen.

The group also showed how the tensile-shear strength increased with the complexity of the nanoscale structures on the galvanized steel surface. By optimizing the hot water temperature and treatment time to achieve peak complexity in the nanoscale structuring, the team was able to significantly increase the tensile-shear strength compared with untreated metal.

“We successfully obtained strong joints of [galvanized high-strength steel] and polybutylene terephthalate (PBT) via HWT [hot-water treatment] with minimal damage to the zinc coating,” the researchers write in the paper abstract. “The effects of HWT conditions on the tensile shear strength were evaluated and the optimized strength reached 23 MPa.”

The process can be adapted for a range of hybrid joining applications in which metal and plastic parts need to be permanently bonded, said senior author Yusuke Kajihara. "Furthermore, our method does not use harsh chemicals or complicated procedures and, thus, is suited to the scale-up required for industrial applications,” added Kajihara. This work could lead to optimization of polymer-metal joining, which would be a significant asset to the manufacturing industry.