Achieving adhesion values between PP and TPE plastic materials sufficient to withstand the transfer of tensile and shear forces across the contact surface depends primarily on:
- the chemical-physical affinity of the materials, i.e. the "miscibility" (polarity, melting point, surface tension).
- the technology and process parameters.
- the geometry of the piece and the contact surface.
The junction between the two materials is the result of the sum of the adhesion and cohesion forces.
By adhesion forces we mean the electrostatic interactions (van der Waals forces) that are generated at the contact surface level.
However, if the chemical affinity of the materials used is high, a cohesive bond is created (for example "hydrogen bridge" iterations). If the bond between plastic materials is predominantly of an adhesive type, there will then be a breakage at the contact surface, if instead a cohesive type interaction prevails, a tensile stress will cause the tearing of one of the two materials.
If the molding conditions allow contact of the materials in the molten state, there will then be an interpenetration of the polymer chains which will give rise to the iterations necessary for the formation of a stable bond.
The thickness of this interdiffusion zone is normally extremely thin due to the rapid cooling of the materials.
Nonetheless, the bond created has a rather high resistance.
