Structure-properties relationships of cellular materials from biobased polyurethane foams

The polyurethanes (PU) are a very versatile family of materials mainly obtained by combinations of polyols and polyisocyanates. Based on their annual worldwide production of around 20 million tons and a global market of $50 billion (2016), PUs rank 6th among all polymers. Through their value chain, PUs involve different players: (i) the chemists producing most of PUs raw materials, (ii) the PUs producers from the raw materials, (iii) the compounders/assemblers who formulate PUs into their final products, and finally (iv) the end-users. Due to the multiplicity of their structures, PUs can be used in various forms and applications. Cellular materials are the largest part of this market (more than 60 %) with segments including the furniture, automotive, bedding, insulation, building or construction markets. Two main types of foam can be fabricated: (i) flexible with open cells, stress and tensile properties, e.g., furniture or bedding, and (ii) rigid with closed cells, low thermal conductivity, low density and high dimensional stability mainly for thermal insulation, e.g., building industries. The formulation step significantly influences the microstructure or morphology of these cellular materials and impacts the final foam properties. Even if some partially biobased compounds (polyols) can be used, commercial PU cellular materials are till now mainly based on fossil resources. However, future materials will combine high performance with low environmental impact in order to fulfill societal expectations. In this way, new biobased compounds combining different fields such as biotech, chemistry, science and materials engineering are more and more used in complex formulations for renewable foams, leading to specific renewable macromolecular architectures. This review aims to highlight the main biobased components (polyols, polyisocyanates and additives) used in formulations for PU foams, in relation to the corresponding fabrications, morphologies and properties. The main renewable sources come from (mono and poly)sugars, oleo-chemistry, polyphenols (lignins, tannins ...), or different compounds from white biotech processes from agro-wastes ... The impact of these different components on material performances is discussed more particularly for rigid polyurethane foams. The structure-property relationships are analyzed with a scope on cellular morphology, mechanical, thermal properties, fire resistance, and insulation behavior. Finally, an analysis focus on future perspectives on biobased PU foams is conducted.