The bonding of polylactic acid (PLA) films was investigated for a broad range of temperatures and contact times above the glass transition temperature in a lap shear joint geometry using an impulse welding system. It was observed that interfacial strength was linearly dependent to the fourth root of welding time until it approached the bulk material strength. Using models based on reptation theories, the interfacial strength of lap shear welds was estimated based on thermal histories. In more detail, the activation energy for interfacial healing and self-diffusion coefficient was calculated based on shear strength measurements of samples welded with well-defined thermal histories. The parameters were then used to predict interfacial strength with varying temperature histories. This is the first work to measure the activation of energy for the interfacial welding PLA. POLYM. ENG. SCI., 2012. (C) 2012 Society of Plastics Engineers