Rolling of polymeric materials with side constraints

It is often observed that the plastic deformation of crystalline polymers, especially on drawing, causes significant amount of cavitation. Cavitation can be suppressed in the process of rolling due to compressive component. A m innovative method of obtaining highly oriented polymeric materials is by unidirectional rolling in a channel formed on the circumference of one roll with another roll, having the thickness matching the width of the channel. The side-walls of the channel on one roll constitute the side constraints while the other roll works like a plunger in channel-die. Rolling with side constraints is cavity-free and is advantageous over conventional rolling, channel-die compression or solid state extrusion since it gives the possibi lity of obtaining relatively thick and infinitely long highly-oriented shap es or profiles in a fast continuous manner. The resulting profiles can have considerably high cross-section area. The design of the rolling machine is described. Few examples of rolling with the rate 4.23 m min(-1) of isotact ic polypropylene (iPP) and high density polyethylene (HDPE) shapes are pres ented. Tensile strength of the rods rolled to the compression ratio 5.4-6.6 with final cross-section of 10-12 mm approached 200 MPa for both polymers. The oriented rods of iPP and HDPE demonstrated high and sharp texture prod uced in HDPE by the activity of (100) [001], (010) [001] and (100) [010] cr ystallographic slips while in iPP (010) [001], (110) [001] and (100) [001] slip systems were active. In addition to those slips twinning modes were ac tive on unloading. Appropriate combination of rolling rate, temperature, in itial thickness of rolled bar, as well as molecular weight of the polymer w ill apparently lead to rods with higher strength. (C) 2001 Elsevier Science B.V. All rights reserved.