HEAT SEALING OF LLDPE - RELATIONSHIPS TO MELTING AND INTERDIFFUSION

The effect of heat sealing variables (platen temperature and dwell tim e) on seal strength of a linear low-density polyethylene (LLDPE) was e xamined. In order to characterize the development of interfacial stren gth, blown films were heat-sealed for times from 1 to 100,000 s, much longer than the typical sealing times of less than 1 s. The seal tempe rature ranged from 100 to 130 degrees C. From the differential scannin g calorimetry thermogram, the LLDPE was determined to be completely me lted at 130 degrees C. Therefore, the films ranged from partially to f ully melted when they were heat-sealed. The seal strength was measured in the T-peel configuration, and the peel fracture surfaces were exam ined in the scanning electron microscope. A temperature of 115 degrees C or higher was required to form a good seal. The strong effect of se al temperature was related to the heterogeneous composition of the LLD PE studied. At 115 degrees C, the lower-molecular-weight, more highly branched chains easily diffused across the interface. Crystallization upon cooling produced connections across the interface. However, becau se these chains represented a small fraction of the crystallinity and the molecular weight was low, they contributed much less than the full peel strength. Conversely, chains with less branching represented the main fraction of crystallinity (anchors for tie chains) and the highe st molecular weights (more entanglements). Only at temperatures at whi ch the higher-molecular-weight, less branched chains began to melt and diffuse across the interface could high peel strengths be achieved. ( C) 1998 John Wiley & Sons, Inc.