Sphagnum moss as a functional reinforcement agent in castor oil-based biopolyurethane composites

(1) Sphagnum mosses are an abundant source of structural molecules, similar to lignocelluloses in rooted plants, having properties advantageous in biopolyurethane composites: their hydrophobicity when dry facilitates dispersion into biopolyols; hydroxyl groups provide cross-linking with the polyurethane (PU) matrix; and easy grindability enables small particle size, down to 10 μm with low energy consumption and a large interface for isocyanate to react with. (2) The present study introduces a novel use of Sphagnum moss as a reinforcement filler incorporated into castor oil. Two moss grades, a slightly and a moderately humified moss (having hydroxyl, equivalent to KOH, of 255 and 359 mg g-1, respectively), were pulverised and dispersed into castor oil (KOH equivalent 160 mg g 1) in mass proportions of 0–28 %. Mixtures were reacted with polymeric diphenylmethane diisocyanate to produce reinforced biopolyurethane (bioPU) composites. (3) The mechanical properties tested according to ISO 527 improved similarly with both fillers. With increasing filler content, tensile strength increased monotonically up to 450 % and Young’s modulus up to 350 % while the strain at break point (at around 90 %) remained almost unchanged over the whole range of filler contents. The shift of the ‘'tan delta' peak’ (energy damping coefficient during deformation) toward higher temperatures with increasing filler content indicated strong cross-linking between the filler and the PU matrix, while the width of the peaks was narrow and remained virtually unchanged showing the filler to be homogeneously dispersed within the matrix. (4) Overall, Sphagnum filler may provide a straightforward and cost-effective reinforcement for (bio)polyurethanes.