page_title: Rheological weldability
section_title: The lower the η, the better the RW
hierarchical_section_title: Rheological weldability / Viscosity / The lower the η, the better the RW
context_page_description: Rheological Weldability of thermoplastics considers the materials flow characteristics in determining the weldability of the given material. The process of welding thermal plastics requires three general steps, first is surface preparation. The second step is the application of heat and pressure to create intimate contact between the components being joined and initiate inter-molecular diffusion across the joint and the third step is cooling. RW can be used to determine the effectiveness of the second step of the process for given materials.
context_section_description: 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:
   = Solid-Gas surface tension,
   = Solid-Liquid surface tension,
   = Liquid-Gas surface tension,
   = Contact angle.
  For perfectly good wetting, contact angle (θc) at equilibrium should be minimized. 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).
  Consequently, for the evaluation of weldability, viscosity of molten thermoplastics (polymer melts) have to be taken into account since welding is a rapid process. It can be said that the lower the viscosity during welding process (at welding temperature and pressure), the better the weldability.
  Recalling that viscosity (η) decreases with increasing temperature (T) and shear rate () for most polymer melts, weldability is better where temperature and shear rate (movement) are higher within all cross-section of welding region.
caption: Fig 1: An illustration of the sessile drop technique with a liquid droplet partially wetting a solid substrate at equilibrium. 
    
      
        
          
            θ
            
              C
            
          
        
      
      {\displaystyle \theta _{C}}
    
   is the contact angle, and 
    
      
        
          
            γ
            
              S
              G
            
          
           
        
      
      {\displaystyle \gamma _{SG}\ }
    
  , 
    
      
        
          
            γ
            
              L
              G
            
          
           
        
      
      {\displaystyle \gamma _{LG}\ }
    
  , 
    
      
        
          
            γ
            
              S
              L
            
          
           
        
      
      {\displaystyle \gamma _{SL}\ }
    
   represent surface tensions of the solid–gas, gas–liquid, and liquid–solid interfaces, respectively.