DRAWING MECHANISM OF POLYETHYLENE

Abstract

Uniaxial elongation of a commercial polyethylene film having the so-called a-axis orientation along the machine direction was studied in terms of the changes occuring in the crystalline orientation as revealed by the X ray method at small and wide angles and those in the orientation in the disordered region by the shrinkage of the drawn film. The results obtained are as follows: 1) The original “a-axis orientation” tends to the c-axis orientation as the draw ratio (D. R.) is increased beyond the yield point(D. R.=ca. 1.2 at room temperature) but in the intermediate stage up to ca 1.5 of D. R. the two types of orientations coexist. 2) The small angle scattering patterns are always on the meridian throughout the drawing process, implying that lamellae are piled up along the machine (or drawn) direction. 3) In the case of the elongation at room temperature, the X ray long spacing increases first with the increasing D. R., showing a maximum increase of ca. 14%, which corresponds to the macroscopic elongation at the yield point, and then drops gradually. 4) When the film was drawn at higher temperatures (80_??_130°C), a similar maximum in the long spacing appears at higher D. R. and further elongation gives rise to a rapid drop in the long spacing and then to a levelling off in the D. R. range beyond ca. 100_??_120%. This constant long spacing is dependent only on the drawing temperature; the higher the temperature, the higher the long spacing, irrespective of the original spacing, suggesting strongly the melt-recrystallization mechnism occuring in the drawing process. 5) When the film drawn at room temperature, was heated at the fixed length to a given temperature, the long spacing increases up to that of the film drawn at the corresponding high temperature. 6) The contraction of the drawn film on relaxing proceeds rather repidly (ca. within 100 min) even at the room temperature. The equilibrium contraction shows a maximum (ca. 37%) at 110% elongation for the film drawn at room temperature at the rate of 25%/min. A similar maximum, though a smaller contraction, occures at lower elongation for the film drawn at higher temperatures as measured at room temperature, suggesting the relaxation (or the coiling of the disordered tie chains) took place already at the drawing temperature. All the above results suggest that up to the yield point the elongation occurs only in the disordered region (the tie chains) and after the transitional stage where partial melting and reorganization also likely take place, the final steady state (at least locally in the microscopic neck) of the melt-recrystallization is attained, The intermediate transitional stage may cover a wider range of elongation at lower drawing temperature.