Arya Sukumar, College of Agriculture Vellayani
Polymeric foams are generally formed by the dispersion of a gas in a polymeric matrix. Polymer foams can be flexible or rigid, which depends on their cross-links and pore size. Commercial foams are often made of inorganic materials (like clay, silica etc.) or synthetic polymers (like polyurethane, polyethene) or hybrids of these. To bring down the increase of environmentally harmful emissions caused by these materials, a surge of research on bio-based materials was initiated during the last decades.
What are Bio-based polymer foams? Bio-based polymers or bio-based resins are protein/ polysaccharide-based sustainable and nontoxic materials obtained from renewable sources or formed as by-products in bio-industries. Protein nanofibrils (PNFs) are considered the best candidates for developing bio-based polymer foams, as these materials are completely biocompatible and biodegradable thus reducing landfill waste accumulation. Since PNFs are biocompatible, they are an excellent option for tissue scaffold and drug carrier preparation. Even though poor durability, higher cost, lower performance, instability in water and many more factors limit their long life application, intense studies are going on to tackle these limitations.
PNF from whey protein isolates (WPI) Recent studies have shown that PNFs with remarkable mechanical properties can be made using natural resources like whey protein isolate (WPI), egg white etc. Whey is a milk protein obtained as a by-product in cheese producing industry. PNF made from the whey protein isolate by providing ambient conditions (40g/L whey protein is treated under acidic condition (pH 2) followed by incubation at 80 °C for 24 hr) is comparatively an easy process. PNF foams are softened by the addition of glycerol, which makes the foam more ductile and flexible. PNF foams when subjected to ageing exhibits greater temperature stability than commercial-grade petroleum-based thermoplastic foams.
Ageing of PNF foams and their importance
Ageing is the process by which PNFs are incubated at 150°C for one month and this increases its mechanical properties like tensile strength and toughness to several times higher than that of unaged foam. Initially, the foams softened using glycerol (plasticizer foams) was found to be tougher than the plasticizer-free foams. During the process of ageing, the strength, stiffness and stability of the foam are found to increase. Solid-state 15N nuclear magnetic resonance (NMR) studies in aged foams have revealed that the side-chain amine groups of lysine residues were absent in aged foams. This is due to the increased number of covalent cross-links between protein chains in the aged foams. It was also noticed that unaged PNF foams have high solubility and this is because of the fewer numbers of covalent cross-links they possess. Glycerol containing PNF foams on ageing exhibited a greater shrinkage and weight loss when compared to the normal PNF foams without glycerol. The weight loss of the glycerol containing foam was also observed in glycerol containing foams, which is due to the evaporation of glycerol. However, the shrinkage of the foams did not alter the microscopic foam structure to any great extent. The glycerol-free aged PNF foam had large cells and displayed a glassy fracture surface with sharp cell wall edges, whereas the glycerol-containing PNF foam had smaller cells and the surface appeared less glassy/sharp and this microstructure difference between the PNF foams is due to the plasticizing effect of glycerol. This improves the flexibility and extensibility of the polymer. The water holding efficiency of PNF foams was determined by immersing the foams in MilliQ water for a given time. The greater water uptake was observed with the glycerol-free foams, which is mainly due to the greater porosity of these foams, than that of the glycerol-containing foams.
Great advancements have been made in recent years for developing biobased foams suiting for a range of applications. PNF foams have a wide variety of applications like for the manufacture of optoelectronic materials, biomedical applications and preparation of nanowires. Biobased foams can replace synthetic materials and can serve as a means to address specific challenges like biocompatibility, biodegradability and mechanical properties. Recent findings will enable further progress by providing more insight into biobased foams.
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Source: Ye, X., Capezza, A.J., Gowda, V., Olsson, R.T., Lendel, C. and Hedenqvist, M.S., 2021. High‐Temperature and Chemically Resistant Foams from Sustainable Nanostructured Protein. Advanced Sustainable Systems, p.2100063.https://onlinelibrary.wiley.com/doi/10.1002/adsu.202100063
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