Estimating minimum required dwell time for the heat sealing of talc containing polypropylene/low‐density polyethylene packaging films

Dwell time is one of the main parameters influencing heat seal quality and process efficiency in flexible packaging. In this study, the rheological behaviour of polypropylene (PP)/low-density polyethylene (PE-LD)-based compound films containing different ratios of talc was examined. The minimum dwell time required to supply a sufficient sealing was estimated based on heat conduction time and reptation time. Reptation is a rheological concept used to described the behaviour of a polymer melt, and the reptation time is the time required for a molecule to escape from its surrounded entangled structure. The results showed that increased levels of talc are associated with elevated complex viscosities, storage and loss modulus as well as an increased seal strength. The estimated minimum required dwell time at the seal initiation temperature (SIT) dropped sharply with the initial introduction of 10 wt% talc. However, increasing the talc ratio to 30 wt% did not create any major change in the estimated time value. Also, observed T-peel strength values for 0.1 and 0.5 s dwell time at SIT confirmed the predicted minimum required dwell time values. On the other hand, when the seal temperature is increased to the higher end of the operation window, time estimations did not match with the observed seal strengths. Here, the melting already starts and the molecules diffuse up to a certain distance before the interface temperature reaches the desired level. Recalculation of the average diffusion distance by including this pre-melting situation can present a better approach for the temperatures above SIT. When the molecules diffused around halfway along the sealant thickness observed, seal strengths reached a sufficient level (2.5 N/25 mm) for both SIT and the highest end of the operation window. In conclusion, it has been revealed that knowing the reptation time and the required time for the heat conduction during the sealing process can help to predict the minimum required dwell time to achieve quality sealing, especially at low sealing temperatures. Additional surface characterizations also helped to elaborate on the influence of changing insoluble filler ratio on the seal initiation properties. It has been showed that the added talc increases the surface free energy and supplies easier wetting at the seal interface. Therefore, adding talc or a similar insoluble filler can help to make the heat sealing process more efficient.