A practical technique to manufacture biaxially oriented high-density polyethylene film via precise control of amorphous region

Biaxially oriented high-density polyethylene (BOHDPE) films are urgently required to address the pollution issue of traditional plastic films, which remains a great challenge worldwide. This study prepared four HDPE/liquid paraffin (LP) precursor sheets with LP content ranging from 1 % to 3 %. The condensed structures were characterized by positron annihilation lifetime spectroscopy and dynamic mechanical analysis tests. It is found that the diluent LP does not change the crystalline structure of PE and exists directly in the amorphous region between lamellar crystals, leading to the reduction of free volume pores fraction from 4 % to 2.2 %. Furthermore, LP activates the beta relaxation of stress transmitter (ST) in the amorphous region of HDPE. Then, the stress-strain behavior was conducted systematically, showing that the minor addition of LP improves the stretchability of HDPE substantially. LP can act as ST to initiate homogenous yielding and accelerate the subsequent disentanglement of ST at higher strain. Additionally, we discovered that cavitation is suppressed completely as LP content reaches 2 %, proving that cavitation is triggered by the coalescence of adjacent free volume pores rather than the enlargement of individual free volume pores. Finally, biaxial stretching was conducted, showing that the maximal draw ratio of pristine HDPE is only 3 x 3, while that of PE@3 %LP is 7 x 7. Accordingly, BOHDPE films with superior performances were successfully produced, providing a practical solution to solve the problem of plastic pollution via the "Reduce, Reuse and Recycle" strategy.