Self-foaming polymers: opportunities for the next generation of personal protective equipment

Polymeric foams merge the intrinsic lightness of porous materials with low thermal and electrical conductivity as well as good energy adsorption capabilities and filtration abilities, depending on their morphology. Such com- binations explain their widespread use in many applications, including in the domain of personal protective equipment (PPE). Indeed, foams are the materials of choice to fulfill a series of essential protective functions, including: (i) insulation, (ii) dissipation, (iii) adsorption, (iv) filtration, (v) flotation and, of course, (vi) cush- ioning. Historically, foams were developed by iterative formulation works aiming at nucleating and stabilizing bubbles of gas in a polymer matrix. The foaming of polyurethanes is among the earliest – and today most mature – methodologies. Indeed, polyurethanes are obtained from isocyanate precursors that have the ability to partially decompose in gaseous CO2 in the presence of water. The gas, also referred to as the blowing agent (BA), is released concomitantly with the polymerization reaction to initiate the expansion of the growing polymer. Because the BA is primarily embedded in the molecular structure of the precursors of the polymer, this system is usually labelled as self-foaming. With the growing health and environmental awareness regarding the toxicity of isocyanates, a burgeoning number of self-foaming polymers and their precursors that circumvent the use of isocyanates are reported in the literature. They combine an interesting range of assets – from the typical ease of use of one-pack systems to the relative innocuity of their blowing gas (e.g., CO2, H2O, halogen-free alkanes) – that are very well suited to the large-scale production of foams in compliance with strict safety and environmental specifications. In this context, the present review is showcasing both historical and emerging self-foaming (pre)polymers that represent opportunities for the production of the next generation of safer and environmentally benign PPE. A special attention is dedicated to the self-foaming mechanisms – i.e., the chemical transformations of the (pre) polymers that result in the release of the blowing agent – and its interplay with the physicochemical processes resulting in the hardening of the (pre)polymers (e.g., sol-gel or rubber-glass transitions). A classification of those mechanisms – (i) thermolysis and (ii) condensation – is proposed for the first time. The properties of the resulting foams are also briefly discussed in terms of densities, cell morphology and mechanical response with the intention to guide the reader in selecting the best foaming process for the targeted polymer matrix and with a special