Polymer Processing under Microwaves

Over the last decades, microwave heating has experienced a great development and reached various domains of application, especially in material processing. In the field of polymers, this unusual source of energy showed important advantages arising from the direct microwave/matter interaction. Indeed, microwave heating allows regio-, chemio-, and stereo-selectivity, faster chemical reactions, and higher yields even in solvent-free processes. Thus, this heating mode provides a good alternative to the conventional heating by reducing time and energy consumption, hence reducing the costs and ecological impact of polymer chemistry and processing. This review states some achievements in the use of microwaves as energy source during the synthesis and transformation of polymers. Both in-solution and free-solvent processes are described at different scales, with comparison between microwave and conventional heating.

1. Introduction
Three main challenges arise from polymer science. (i) The first one consists of improving the properties of the material: According to the use the polymer is designed to, it must present adequate properties from different points of view (such as mechanical, thermal, rheological, optical, and chemical properties). (ii) The second one is the optimization of the processes involved in polymer chemistry or transformation: In order to achieve gains in time and cost, the processing parameters have to be tailored, depending on the polymer properties, to meet the goals of the process in terms of quality, productivity, and economy. (iii) The third one is compliance with the ecological requirements: Since the environmental issues are major scientific and societal challenges, both the polymer material and the process should allow a limitation of nonenvironmentally friendly substances emitted and reduce the energy consumption during the polymer synthesis or transformation.

Heating is a key parameter in both polymer chemistry and processing; the heating method can play an essential role. In that purpose, microwave (MW) heating seems to be a promising solution to fulfill the three points mentioned above, as it presents a particular heating with special features.

MW heating consists of exposing a material to electromagnetic radiation. The MW frequencies are between 300 MHz and 300 GHz, corresponding to wavelengths between 1 m and 1 mm. The use of these waves for industrial, medical, and scientific applications is regulated to avoid interferences with radars and radio communication; the 2.45 GHz frequency is the most commonly used. Like other electromagnetic fields, microwaves can interact with matter. According to their dielectric or magnetic properties, some materials are able to interact with microwaves and consequently rise in temperature.

Indeed, it is well-established that MW heating comes from the motion of charges (dipoles/ions) in the material; it is called volumetric microwave heating [1]. This mode of heating was studied by polymer scientists and has proven its efficiency. In many cases, the particularities of microwave heating offer many advantages: time and energy savings during polymer processing; fast heating rates leading to faster chemical reactions with higher yields; and chemical selectivity and easy achievement of free-solvent reactions. However, there are some constraining aspects of MW heating, such as thermal runaway, heterogeneous heating, and difficulties in temperature measurements.

Many reviews have been published during about the use of MW heating in the field of polymers during the last decade; dealing with polymer synthesis [2], modification [3], degradation [4], and curing [5], the current review deals with the use of MWs during polymer processing. The theoretical and practical characteristics of MW heating in the field of polymers are first discussed, with a comparison between the conventional and MW heating based on sample reactions. Some applications at the industrial scale are highlighted.