PEFTEC - Abstract

Abstract Information :

Linear low density polyethylene (LLDPE) is the most widely used for flexible packaging application. Due to its higher stiffness, toughness and thermal properties, LLDPE is preferred for blown film applications. The microstructure and properties of LLDPE are primarily influenced by parameters such as, comonomer type, comonomer content and its distribution, molecular weight and molecular weight distribution. Metallocene catalysed LLDPE (mLLDPE) have salient parameters like narrow molecular weight distribution, uniform comonomer content and distribution across polymer chains. Films of mLLDPE have superior mechanical strength, heat sealing and optical properties compared to films of LLDPE based on Ziegler-Natta catalyst. However, in practice for blown film applications, LLDPE is often blended with minor amounts of low density polyethylene (LDPE) for ease of processing.

In this study, several LLDPEs with comparable macro parameters such as density and melt flow rate were evaluated using thermos-rheological tools. From rheological measurements, it was demonstrated that LLDPE was miscible with LDPE up to 30 wt.% . The rheological behaviour of the mLLDPE-LDPE blends was strongly dictated by LDPE content. Furthermore, rheological response, especially, the zero shear viscosity and shear thinning index, were found to be sensitive to subtle microstructure differences in the LLDPE and LDPE.

mLLDPE/LDPE blends were subjected to successive self-nucleation and annealing protocol in a differential scanning calorimeter showed multi-peaked melting curves corresponding to all pristine LLDPEs and LDPE. This indicated chemical composition distribution across the molecular weight of these polymers. The thicker lamellae of LDPE were found to co-crystallise with homo-polyethylene-like fraction of LLDPE. The thinner lamellae of LLDPE containing relatively higher amount of comonomer were largely independent of LDPE crystallisation.

Even though the macro parameters of the LLDPEs were comparable, the subtle differences in the microstructure manifested by comonomer type and distribution affected sealing performance of the LLDPE-LDPE blends. It was demonstrated that it was not necessarily sufficient to achieve sealing at lower temperature by only having relatively higher comonomer content in LLDPE matrix. In LLDPE matrix, the comonomer distribution that favoured formation of thinner lamellae was found to be more critical for heat sealing to occur.