Cellulose nanofibre-poly(lactic acid) microcellular foams exhibiting high tensile toughness

We report here the morphology and tensile properties of polylactic acid-cellulose nanofibre (PLA-CNF) microcellular nanocomposites. Two types of CNF were used for the nanocomposite preparation, native and surface acetylated CNF (ac-CNF). Samples were foamed in a mould to enable tensile testing. The effect of the mould use on the foam morphology was first assessed by comparison with free foamed samples. We found that the mould affected the cell growth stage of the foaming process in neat PLA foam while its effect was less important in nanocomposites. Stiffening and strengthening effect of CNF was greatly enhanced by foaming when compared to their solid counterparts. The most notable change in tensile properties was however the large increase in strain at break resulting in the high tensile toughness of microcellular PLA-CNF nanocomposites. Strain at break increased up to 7.5 times in neat PLA and up to 31.5 times in the foam containing 3% of CNF. Surface acetylation of CNF significantly affected the properties of foams with 9% of CNF loading: while foams with ac-CNF were stiffer, foams with native CNF exhibited higher strain at break and so higher overall toughness.