Solid‐State coextrusion of high‐density polyethylene. II. Effect of molecular weight and molecular weight distribution

Abstract

The recently developed technique of solid‐state coextrusion for ultradrawing semicrystalline thermoplastics has been applied in the preparation of self‐reinforced high‐density polyethylene extrudates. The extrudates consist of definite core and sheath phases composed of different molecular weights (Mw) in the range of 60,000–250,000 and different molecular weight distributions (Mw/Mn = 3.0–20). Concentric billets of two different phases were prepared for extrusion by in serting a polyethylene rod within a tubular billet of a different high‐density polyethylene followed by melting the two phases to obtain bonding between them. The billet was then split longitudinally to increase extrusion speed and extruded at 120°C, 0.23 GPa through a conical die of extrusion draw ratio 25. Extrudates of high tensile modulus (38 GPa) and strength (0.50 GPa) could be produced in a steady state process at a rate near 0.25 cm/min. The tensile properties of the extrudates from either the single or concentric billets increased with average molecular weight and were insensitive to the molecular weight distribution of the constituent phases. Thermal analysis indicated a high deformation efficiency for the sheath and core phases of the extrudates by the coextrusion technique.

Solid state extrusion of semicrystalline polymers

“…It has the advantage over other orientational methods of allowing one to obtain industrial goods with the requisite cross-section, high tensile strength, and stiffness. However the tensile properties obtained by this method are not as good as those obtained by other methods, for instance by drawing (2)(3)(4). At the same time the combination of solid state extrusion and drawing produces materials with tensile properties close to their theoretical limits (5,6).…”

“…12 Dotted lines-viscoelastic turbulence, solid lines-periodic regimes. A, , , = 8.2 (1); X , = 6.25 (2); X , = 4.9 (3).…”

Method and Mathematical Modeling of Formation of Gradually and Homogeneously Oriented Linear Polymers

“…When the desired temperature is reached, the heater, the cooler, and the active clamp are simultaneously activated. The cooler moves in the direction of motion of the heater with speed V 2 = const and V 2 < V 1 , so that the distance between it and the heater continuously increases and at any time is equal to hðtÞ (7).…”

“…There are various technical solutions for converting a polymer into an oriented state: cold or hot stretching [4,5], extruding solid polymer [4][5][6][7], rapid stretching from melts or solutions [4,8,9], and zonal stretching [10][11][12].…”

Molecular weight and molecular weight distributions in synthetic polymers

“…On the other hand, on the practical side, it is observed that performance-oriented criteria such as environmental stress crack resistance and permeability to gases also heavily depend on MWD (2). The scientific literature is filled with examples of both theoretical and applied MWD studies {e.g., see 3,4). The methods of production (e.g., catalyst type), of processing procedure (e.g., milling, foaming, casting, extruding) and the subsequent environmental exposure (e.g., moisture, radiation) all enter into the shaping of a polymer's MWD.…”