S. Murakami et al., Comparative study of structure and tensile properties of melt-pressed and extruded-blown films of high-density polyethylene, J MACR S PH, B39(5-6), 2000, pp. 645-655
Bulk high-density polyethylene (HDPE) films crystallized from isotropic mel
ts show a spherulite structure; the tensile stress-strain (S-S) curves of s
uch melt-pressed (M-P) films show a characteristic shape caused by necking.
On the other hand, extruded-blown (E-B) HDPE films show an anisotropic str
ucture in which lamellae are stacked normal to the extrusion direction. The
S-S curves of E-B films showed various shapes for various elongation direc
tions relative to the lamellar orientation direction. When drawn perpendicu
lar to the extrusion direction (parallel to the lamellar orientation direct
ion), the S-S curve showed typical necking behavior resulting from unfoldin
g from one part of the lamellae. When drawn diagonal to the extrusion direc
tion (diagonal to the lamellar orientation direction), the yield stress was
smaller due to slippage between the lamellae. When drawn parallel to the e
xtrusion direction (perpendicular to the lamellar orientation direction), t
he S-S curve was smooth, but it was shown that micronecking occurred repeat
edly in various parts of the specimen. The characteristic shape of the S-S
curve of the M-P films was reconstructed as the sum of all S-S curves of th
e E-B films in various directions. From a structural viewpoint, the deforma
tion behavior of the bulk film, which has a spherulite structure, can be si
mulated at least qualitatively by the deformation behavior of anisotropic E
-B films in various directions.