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A contribution to the fundamental understanding of Shear-Refinement of polymer melts by entanglement manipulation

Final Report Summary - SISAPEM (A contribution to the fundamental understanding of Shear-Refinement of polymer melts by entanglement manipulation)


The SISAPEM project and its ramifications (Fulbright, Ikerbasque Fellowship) resumes to this title: Reduction of Viscosity of Plastics by Entanglement Manipulation Methods to Enhance Processing.

The high viscosity of polymer melts can be reduced and the benefits preserved in pellets by application of a low frequency pulsed shear and extensional mechanical treatment that boots shear-thinning and allows “sustained orientation” to occur. Sustained-Orientation is the ability to preserve in the molded plastic for a long time and at temperatures above Tg, even in the melt, a certain amount of orientation induced by a mechanical treatment of the melt during its extrusion. The purpose of this grant was to understand the physics behind the phenomenon of “sustained-orientation”. What are the conditions of processing which make “sustained orientation” feasible? Is it universal (applicable to all polymers or to just a few)? What part of our current established understanding of polymer physics do we need to improve or change in order to understand “sustained-Orientation”? Is there a new type of equipment which we could build which would allow producing “sustained-orientation” in a laboratory? Can we modify the 1st generation of processors capable of producing sustained-orientation in polymer melts in order to drastically reduce the cost of the machines?

These questions are fundamental and shake our fundamental understanding of entanglements in polymer physics.

Executive Summary: The Marie-Curie Grant has allowed this researcher to make giant leaps forward into the understanding of the basic physics responsible for this challenging problem. Not only the incredible results of “sustained-orientation”, which had been seriously challenged for their veracity in view of the contradiction they pose to the established theories, has been validated, but also the reasons for the success or failure to preserve in a pellet the melt viscosity changes induced by Rheo-Fluidification have been elucidated. This result, overall, represents a great success for the Marie-Curie mission, although, many of the smaller objectives of the initial application could not be achieved, because of a lack of funds. For instance, the lab disentangler was not manufactured, and thus many of the answers to the question of the stability of the entanglement network after disentanglement are still pending. This report gives an overall view of several aspects of the research which

Project Description

The plastics industry is one of the major business branches in Europe, employing more than 1.9 million people who generate a turnover of more than 274 million Euros. This industry is well renowned for its continuous ability to develop and innovate, which resulted in a “plastic revolution” in the last decades, where plastic products was used in more and more high tech applications. These applications appeared just a few years ago not achievable for plastics products. At the early stages of this developments, these innovations were realized by the development of new polymers. During the last almost 50 years this changed to the development of plastic formulations combining polymers with fillers, additives and other polymers. These compounds allow to develop economically tailor made materials for a special application in a short time period.

Aiming at more and more sophisticated applications the process engineering becomes more and more important and, on the other hand, the processing window becomes closer and closer. Thus, new approaches are necessary to continue the find new applications for high-tech polymer compounds.
In the early 1980s a surprising effect in polymer processing was published in literature. The researchers found that the processing properties of a polymer melt changed when they imposed mechanical vibration on top of pure shear flow in an extruder type of processing equipment. The combination of shear-thinning and strain-softening of the melt during processing resulted in “sustained-orientation”, already defined in the first paragraph of this report. This effect was reproduced by a number of research groups around the world. Currently academia attributes a disentanglement of the usually entangled polymer chains in the melt to be the reason for this effect. Unfortunately, it cannot be described using available models describing the behavior of polymers.

The disentanglement offers exciting possibilities to the production of tailor made high tech compounds. First of all results show that it is possible to achieve high degree of homogeneity applying a low level of shear and thus at low energy consumption. Reducing the specific energy input necessary during compounding only by 0.1 kWh/kg could save more than 550 Million EUR in energy costs and 3.3 kto CO2.

Beside this it was possible to achieve extremely high degrees of filler contents in a compound which resulted in superior properties, which are required in applications that involve halogen free flame retardants (HFFR) and applications which require a high electrical conductivity (bipolar plates for fuels cells).

In some cases the disentanglement of the polymer chains could be saved in the compounds (sustained-orientation), which resulted in superior processing properties in succeeding processes, e.g. injection moulding.
Even though these effects were observed and are well documented, they are not understood, which is the reason that the development of optimized compounding processes is based on trial-and-error approach, which are usually ineffective. This is why this Marie-Curie research was awarded: the SESAME project in order to understand the rheological and molecular basis for this phenomenon.

The project is theoretical yet its immediate application aims to develop a reliable new compounding technology using a knowledge based approach. This is why we wanted to develop, in a first step, a lab compounding line (VIBOB). Using this lab disentangler fundamental, trials to investigate the fundamental principles should have been conducted. We conducted, instead, rheological experiments in the non-linear regime. The results were analyzed to generate new constitutive equations of melt submitted to shear flow, also describing the role of vibration (frequency and strain amplitude) on the viscosity in the rheometer and studying the re-entanglement post at the end of the “treatment”. The experiences gathered from this rheometer, instead of a VIBOB are used to understand the role of strain on triggering disentanglement effects, but could not be used to design and manufacture a pilot disentangler, which in our original plan would have been mounted to a standard polymer processing equipment (e.g. injection moulding machines, extrusion lines, compounding lines). As previously mentioned, the lack of funding resulted in focusing entirely on the theoretical aspect of entanglement, disentanglement and re-entanglement using the lab rheometer put at our disposal at UPPA and at the university of the Basque Country to validate the theoretical assumptions. This shift from the original plan was actually useful since we explored in more depth than originally anticipated certain aspects of calculations regarding the properties of the entanglement network and its relation to shear-thinning and strain softening.