‘Melt fracture’ or extrudate distortions

Abstract

Molten polymers, when forced or extruded through dies at high rates, will show extrudate distortions. Such phenomena have been described since the advent of thermoplastic polymeric materials. Over the years, a very large number of publications has been devoted to this subject in efforts to define the nature and origin of these extrudate distortions, commonly referred to as ‘melt fracture’ [1]. Many observations have been made regarding the varying degrees in which extrudate distortions can manifest themselves [2]. Visual observations range from smooth-glossy extrudates (no distortions), over extrudates showing degrees of surface irregularities defined as matt, loss of gloss, ‘orange peel’, ‘’shark skin’ , wavy, screw thread, to extrudates which are volumetrically distorted as a consequence of their oscillating emergence from the die (often referred to as’ spurt’) or their very irregular,‘chaotic’ shape. For most polymers, such a succession of extrudate distortions can be observed with increasing flow rates. Certainly, the common terminology, melt fracture, has a too narrow connotation to capture all observed phenomena as described above. It only suggests that the melt emerging from the die fractures. Therefore, the term ‘extrudate distortion’ seems more appropriate to refer to all these phenomena. The long-standing academic and industrial interest in this subject may be associated with several driving forces. One, which poses a continuing intellectual challenge, is related to the elusiveness of finding the origin of extrudate distortions in relation to the molecular composition of the polymer. The complexity of the issue is related to the many melt processing variables (e.g. temperature, die geometry and alloy) and polymer-type variables (e.g. molecular weight, molecular weight distribution and branching) which need to be considered. The effect of each variable on extrudate distortion is difficult to determine experimentally. Often contradictory findings are reported. Another driving force is the evolution to faster melt processing equipment and a shift to higher flow rates. Accordingly, the practical barrier to extrudate distortion, which is considered to limit industrial melt processing operations, needs to be shifted. Existing solutions to alleviate or reduce extrudate distortion such as the use of extra additives or changing die geometry all tend to have a direct (negative) impact on the economics of the industrial practice of melt processing.Keywordsmelt fractureshark skinlubrication instabilitiesextrudate distortionslipprocessing