Rheological weldability (RW) 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.
Rheology Rheology is the study of material flow as well as how a material deforms under an applied force. Rheological properties are typically applied to non-Newtonian fluids but can also be applied to soft solids such as thermoplastics at elevated temperatures experienced during the welding process. The material properties associated with the rheological behavior include viscosity, elasticity, plasticity, viscoelasticity, and the material's activation energy as a function of temperature.
Rheological properties To understand the rheological properties of a material it is also important to recognize the stress strain relationship for that material at varying temperatures. This relationship is attained through experimental measurement of the resultant deformation as a function of an applied force.
Influences of microstructure and composition A material's rheological behavior is influenced by a combination of the material's microstructure, its composition, the temperature and pressure acting on the material at a given time. The rheological and viscoelastic properties of a polymer melt are sensitive to the material's molecular structure, including molecular weight distribution and effects of branching. As a result, rheology can be used to develop relationships between differing material combinations.
Determining microstructure Melt rheology has shown to be an accurate method in determining the polymer's molecular structure. This is beneficial in determining weld compatibility between materials; as materials with drastically different flow characteristics will be more difficult to join compared to those with more closely matched viscosity and melting temperature properties. This information can also be used to help determine weld parameters for the given welding process to be used.
Viscosity The lower the η, the better the RW 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:
γ S G = γ S L + γ L G cos θ c , {\displaystyle \gamma _{SG}\ =\gamma _{SL}+\gamma _{LG}\cos {\theta _{c}},}
Where:
γ S G {\displaystyle \gamma _{SG}} = Solid-Gas surface tension,
γ S L {\displaystyle \gamma _{SL}} = Solid-Liquid surface tension,
γ L G {\displaystyle \gamma _{LG}} = Liquid-Gas surface tension,
θ c {\displaystyle \theta _{c}} = 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 ( γ ˙ {\displaystyle {\dot {\gamma }}} ) for most polymer melts, weldability is better where temperature and shear rate (movement) are higher within the entire cross-section of the welding region.
Elasticity The lower a material's elasticity, the better the RW Elasticity is best described by stretching a rubber band. As one pulls on the rubber band it stretches and when the pulling force is lessened and finally removed the rubber band returns to its original length. Similarly, when a force or load is applied to most materials the material deforms and as long as the force has not exceeded the material's yield strength the material will return to its original shape when the force or load is removed. The material property associated with a material's elasticity is called Young's modulus and the relationship between the amount of deformation for a given load is described by Hooke's law.
σ = E ( L − L o ) / L o {\displaystyle \sigma =E\left(L-Lo\right)/Lo}
Where σ {\displaystyle \sigma } , or the stress experienced by the material and equals the change in length divided by the original length multiplied by the material's elasticity or Yong's modulus "E".
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