| Abstract: | Failure criteria of the heat‐sealed part of oriented polypropylene (OPP) and cast polypropylene (CPP) heat seals made by an impulse type heat‐sealing machine were investigated. Circular notches and precracks were introduced to direct failure to specific areas such as inside the seal, at the border, or at the unsealed part. The notched strength as a function of heat‐sealing temperature revealed that the seals were stronger in the transverse direction (TD) as compared with the machine direction (MD). Tensile failure that occurred inside the heat seal is more sensitive to sealing temperature, whereas that at the unsealed part is immune. The stress intensity factor (K1) is generally higher along the TD. At high sealing temperatures, i.e. above 150°C, orientation in the OPP film is relaxed and this results in consistently low mechanical properties at this temperature range. This morphology was revealed by studies of crystalline orientation state and FTIR imaging. POLYM. ENG. SCI., 46:205–214, 2006. © 2005 Society of Plastics Engineers |
