Spray deposition of adhesive polydopamine and tannic acid films for the metallization of plastics: Optimization via design of experiments

Thin polymerized films of natural catechols such as polydopamine (PDA) and tannic acid (TA) have been successfully introduced as adhesion promoters for the eco-friendly metallization of polymers. The present paper introduces the deposition of ultrathin adhesive PDA and TA films by a simple air-spraying technique. Compared to the established bath application, spraying provides a higher versatility and resource efficiency, especially for the coating of three-dimensional parts, but requires precise adjustment of deposition conditions to obtain the desired properties. In the study at hand, PDA and TA films were sprayed with an air gun onto polyethylene surfaces which were subsequently coated with nickel by electroless metallization. The use of Design of Experiment allowed an efficient simultaneous evaluation of numerous influencing factors, such as additives to accelerate the polymerization reaction, the aging of the reaction solution, the position of the sample in the spraying jet, the duration and number of spraying cycles, and various post-treatments. Since detailed surface analysis of the ultrathin sprayed films is very challenging, the influence of the spraying parameters on the adhesive properties of the PDA or TA films was evaluated basing on the quality and adhesion of the subsequently deposited nickel layer. The statistical analysis of the data identified the best settings for the spraying of PDA and TA films. The adhesive films deposited under these conditions provided a very good metallization quality of the PE plates with an adhesive strength of up to 3.6 N/mm² and required a similar preparation time as the bath process. • Bio-inspired adhesion promoters enable green principles in plastics metallization. • The resource-efficiency of their deposition was improved by air-spray deposition. • Design of Experiment was employed to optimize the spraying parameters. • Spraying under optimal conditions ensured superb metal layer quality and adhesion.