Comparative study of the influence of blow-up ratio in the properties of HDPE and LDPE extruded blow films

The purpose of this study is to evaluate the influence of the polyolefin type and the blow-up ratio (BUR) on the thermal physical properties of films produced in a blown film extrusion process. It was used low density polyethylene (LDPE) and high density polyethylene (HDPE) processed in a blown film extrusion line at three BUR of: 1.5:1; 2.5:1 and 3.5:1. The results of physical characterization on these films demonstrated that the increase in the BUR shows a higher thickness variation in the perimeter of tubular films and, therefore, optical properties variation. The coefficient of friction (COF) of the films was smaller for HDPE and did not have significant variation with the increase in the BUR applied. Regarding mechanical properties, HDPE films showed higher tensile strength and lower strain values in the yield point, in both directions. The largest BUR showed higher similarity of the tensions and strains between the machine direction (MD) and transverse direction (TD). The ideal BUR observed for films processed with LDPE is 2.5:1, for HDPE this number is higher and close to 3.5:1. The HDPE films showed more opacities and less brightness when compared to LDPE films. The results prove well known values in the literature, however, there was an innovation introduced in this study which is the use of the portable digital thermo graphic technique that allows the instantaneous visualization (on-line) of the temperature variation in the bubble perimeter in the frost line (crystallization or solidification area of the film).

LDPE; HDPE; LLDPE; blow-up ratio; frost line; blown film; thermographic