Bogor, Indonesia – Acacia mangium is extensively grown in plantation forests in Indonesia, However, the uses for the bark waste from pulp and paper making are limited, not least because of its black coloration. This bark is a rich source of tannins, and these complex flavonoid compounds might be used as bio-based polyols to make polyurethanes.
A range of tannin-based polyurethanes have been developed by scientists in Indonesia. The aim was to create more environmentally friendly resins that could be used to modify ramie fibres. Ramie is derived from the plant Boehmeria nivea, a member of the nettle family that is native to east Asia, and widely used in industrial textiles.
While ramie fibres are more environmentally friendly than synthetic fibres, they have a number of disadvantages, including a relatively high water absorption, affecting their mechanical properties. They also have relatively low fire retardancy, and impregnating them with PU may also increase heat resistance.
Both isocyanate-based and isocyanate-free versions of the tannin-based PU were created, with the tannin extracted from acacia bark. The NIPU was made using this tannin extract as a bipolyol, plus dimethyl carbonate and hexamethylene diamine, while the conventional PU was created using polymeric diphenylmethane diisocyanate with the tannin extract.
Two forms of ramie were used in the subsequent studies: a natural fibre, and a pretreated version that had been soaked in hydrogen peroxide. Both were impregnated with the PU resins in a vacuum chamber, and both resin types produced urethane groups. The NIPU gave lower viscosity and cohesion, and the untreated fibres were more thermally stable than the treated ones.
In all cases, however, the resins improved the properties of the fibres. The scientists suggested that the impregnated fibres could have applications as functional materials. Further research into formulations that have even better thermal stability and mechanical strength would be useful, they said, as would developing a better pretreatment to give softer ramie fibres.
The work has been published in the journal Polymers.
