THERMAL SHRINKAGES OF HOT-PRESSED POLYETHYLENE SINGLE-GRYSTAL MATS DURING ANNEALING

Abstract

According to the hot compression ratio (CR), the polyethylene (PE) single-crystal mats are known to have the fine structures1); the intra-lamellar slip or chain tilting type for the lower stage of CR, and the fiber texture type for the higher stage of CR. In this study, we observed with a micrometer the changes in the external dimensions, length l, width w and thickness d, of hot-pressed PE mats upon annealing in free-length at temperatures between 100°C and 130°C. A coefficient of shrinkage is defined by Sx=Δx/x, where x denotes l, w or d.Results were interpreted in terms of the structural changes during the annealing, which has been discussed in a previous paper2):(1) Sl for the mats compressed with low CR shows a maximum annealing temperature at Ta=120°C, this has been ascribed to the rotation of lamellar fragments vertically to the direction of deformation axis. While for the mats compressed with high CR, Sl increased monotonically with increasing Ta up to 130°C.(2) The value of Sd reaches a maximum at Ta=110°C and 120°C for the low CR and the high CR mats, respectively. Above these temperatures Sd decreases as a result of the lamellar thickening and the growth of crystallite size.(3) With increasing Ta, the values of Sw are influenced by the increase in the degree of (100) planar orientation, hence the value becomes negative.(4) The bulk contruction Sv=Sl+Sd+Sw has a maximum value at Ta=110-120°C, whereas Sv for the low CR mats is larger than that of the high CR ones. The decrease in Sv above 120°C is brought on by the fact that the expanded unit cell dimensions decrease to their original sizes during the annealing at elevated temperatures.