Many nonwoven fabrics are made by melt spinning semicrystalline fibers followed by thermal bonding using heated calendar rolls. In this work, we have studied thermal bonding of polypropylene films to simulate bonding of nonwoven fibers. We have tried to relate the thermal bond strengths with the concepts of chain dynamics via interfacial adhesion development at symmetric polymer interfaces. This requires relating the microscopic dynamics of chains with macroscopic interfacial adhesion measurements. It was found that the interfacial bond strength was proportional to the fraction of the crystals melted. This required heating the interfacial region between the polymer layers into the melting region. Bond strengths were also related to process time as t(1/2). This dependence is consistent with the literature for reptation, but is also due to the required thermal diffusion to bring the interfacial region to the bonding temperature. Finally, the bond strength is also dependent on the polymer molecular weight as 1/M-1/2, which is consistent with forming the bonds via chain reptation, provided that the bonding time is less than the reptation time. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 117: 3322-3330, 2010