Analysis of dart impact resistance of low-density polyethylene and linear low-density polyethylene blown films via an improved instrumented impact test method

An improved instrumented impact test method is used to correlate the impact resistance of low-density polyethylene and linear low-density polyethylene blown films to their solid-state deformation process during impact test. The two materials, low-density polyethylene and linear low-density polyethylene, showed different deformation behaviors under the same impact testing conditions. The region before the peak force in the load versus deformation plot for low-density polyethylene was linear, whereas for linear low-density polyethylene two different slopes were observed. The average peak force values of linear low-density polyethylene film is about 14% greater than that of low-density polyethylene film. Furthermore, the measured deflection at the peak force is about 50% higher in the case of linear low-density polyethylene film. In addition, the energy to peak force of linear low-density polyethylene is more than twice higher than that of low-density polyethylene. In the region after the peak point, the low-density polyethylene exhibited another peak before failing and no such peak was observed for linear low-density polyethylene. Tensile test results with low strain rate of deformation have also been compared. The tensile stress-strain in machine direction (MD) and transverse direction (TD) direction play an important role during the biaxial deformation during the impact test. The crystallographic deformations were explained using the scanning electron microscopy (SEM) and differential scanning calorimetry (DSC) techniques for lamellar structure and orientation. The low-density polyethylene film has developed columnar clusters of lamellae, which are aligned parallel to the TD direction. On the other hand, the linear low-density polyethylene film structure shows more randomly orientated and thicker lamellae than that of the low-density polyethylene film. The result of this structure is a more balanced toughness in both MD and TD directions.