Rheological weldability
Rheological weldability (RW) is proposed as a criterion to determine reliably weldability of thermoplastics. In a practical manner, RW of thermoplastics is assessed through their rheological properties: viscosity (η) and activation energy (Ea). According to RW criterion, the lower the viscosity (η) during welding process and the lower the absolute value of activation energy (|Ea|), the better the weldability.
Regarding sessile drop technique, wetting is characterized by degree of interfacial contact and quantified via contact angle (θc) of a liquid on a solid surface at equilibrium, as shown in Fig. 1. Interrelation between contact angle and surface tensions at equilibrium is given by the Young equation:
where
For perfectly good wetting, contact angle (θc) at equilibrium should be a minimum. However, it is valid only at equilibrium, and rate of the equilibrium depends on the balance between driving force of wetting and viscosity of the liquid. In the case of polymer melts, viscosity can be very high and it may take a long time to reach the equilibrium contact angle (dynamic contact angle is likely higher than the contact angle at equilibrium).
...
Wikipedia