Improvement of metal to polymer adhesion in  overmolding processes

Aumento dell’adesione metallo – polimero

In the manufacturing field, the adhesion between two chemically different materials is a topic to be faced on a daily basis. If the adhesion must take place in the absence of an adhesive, as in the case of injection overmolding (the injection of a thermoplastic polymer at melting temperature onto a substrate, polymeric or metallic, positioned inside a mold or moving inside an extrusion die) when molding plastics, the two materials must have good chemical compatibility at the interface, otherwise there will be little chance of them adhering to each other. Not only the molding parameters of the polymer to be over-injected and the temperature of the substrate, but also the chemistry at the interface between the two materials plays a fundamental role in the success of the bonding. Often, in fact, temperature and pressure are not enough to make two materials of different nature adhere. 

The plasma can help the chemical modification of the surface which must receive the polymer to be overprinted, so as to optimize the adhesion, guaranteeing a chemical anchoring, without the addition of further material which could frustrate the geometry and tolerances of the final product, or increasingly limited primers as they are less and less green. Plasma in vacuum solves the lack of adhesion, but for processes to be included in the production line, or for substrates with a continuous profile, the long waiting times for vacuum to form must be avoided: plasma at atmospheric pressure is the most suitable.

 Within the regional project Joinplasm and in collaboration with VBN company, Nadir has developed a tubular geometry atmospheric plasma device (Tubular Plasma) for the treatment and chemical functionalization of profiles, evaluating the bonding between thermoplastic polyurethane and stainless steel. Steel pipes have been pre-treated by flowing inside the Tubular Plasma, depositing a nano-coating on the entire external surface of the steel pipe. Downstream of the plasma, an extrusion line over-injected a TPU sheath a few millimeters thick. The choice of the chemical precursor to be used within the plasma was fundamental: a siloxane monomer suitably chosen that easily binds to metal substrates, while the functional groups contained in it contribute to the bond with the thermoplastic polyurethane chains, so as to obtain an interface cohesive. The image alongside is emblematic: a sample of TPU over-injected to a steel plate is in traction between the jaws of the strain gauge, the cohesion at the interface between the two materials is evident. The load versus displacement graph shows that there is no adhesion without plasma treatment. Simple plasma activation increases the surface energy of the steel, promoting adhesion. Thanks to the plasma functionalization of the steel, cohesion is obtained between the two materials. The plasma process avoids the use of expensive, toxic and environmentally incompatible primers, resulting in an economic, safe and eco-sustainable process.

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