The thermomechanical processes that occur during thermal bonding of isotactic polypropylene are discussed. Each process is described and the magnitude of each effect estimated. The results suggest that flow is important to the formation of a good bond. Flow is facilitated by line pressure and is an important means of heat transfer to the center of the bond. Conductive heat transfer is limited. Deformation heating is small, but not negligible. Solidification of polymer that has been melted and forced to flow leads to mechanical entanglement. The Clapeyron effect is invoked to suggest that only a limited amount of melting occurs under high pressures. Diffusion, estimated using Reptation Theory, does not seem to be an important process in thermal bonding. Fabric strength is limited by the thermomechanical history of polymer at the bond perimeter.
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