Meaning
Mixing of macromolecular segments across an interface creates a cohesive bond between two thermoplastic surfaces. This polymer chain interdiffusion occurs when the material is heated above its glass transition or melting temperature, allowing the chains to move and entangle. It is the fundamental mechanism behind heat sealing, ultrasonic welding, laser welding and solvent bonding.
Entanglement Density
Strength of the resulting weld depends on the number of chains that cross the boundary and become entwined with the opposite side. If the polymer chain interdiffusion is incomplete, the joint will fail under low stress. Deep entanglements provide a bond that can be as strong as the bulk material itself.
Thermal Activation
Heat provides the energy necessary for the long chain molecules to overcome their local constraints and migrate. During the welding process, the time and temperature must be sufficient to facilitate polymer chain interdiffusion without causing thermal degradation of the resin. Insufficient heat leads to a cold weld that lacks structural integrity.
Molten states provide the highest rate of movement, while solid state diffusion is too slow for commercial production speeds.
Weld Strength
Testing the joint through tensile or peel methods reveals the quality of the molecular mixing. Successful polymer chain interdiffusion results in a failure that occurs in the base material rather than at the interface. This indicates that the molecular bridge is fully formed and capable of transferring loads across the original boundary.
