Wheat straw has been identified as a potential source of polyols by a team at Spain's University of Cordoba. Working in collaboration with Chile’s Advanced Polymers Research Centre, researchers managed to transform it into polyols for polyurethanes.
Nearly 740 million metric tons of this agricultural waste is produced worldwide every year, researchers said, but there is no well-defined use for the cheap material. Although there are potential applications such as bioethanol production, it is usually discarded.
An alternative strategy would be to convert the biomass into liquid fuels without the usual gasification step. While thermochemical conversion mechanisms such as this have potential, the Cordoba group said that to date the ideal has been neglected because the process is complex and expensive, and therefore difficult to scale up.
Recent advances have led to increased yields of products such as polyols by using organic solvents at atmospheric pressure and low temperature. The method used by the Cordoba team used glycerol as solvent, and catalytic amounts of sulfuric acid. The resulting polyol had a hydroxyl number of 604 mg KOH/g. This compares to about 200 for petrochemical-derived polyols, and 408 for castor oil.
"[The straw-derived polyol] does not offer complete hardness and dryness once exposed to air," according to chemist Esther Rincon. This is necessary for the proper formation of foam, she said, and is in contrast with the behavior of castor oil polyols, which fail to cross-link completely because of an insufficiency of double bonds in the molecules.
The polyols were then used to make PU foam, using different proportions of straw-derived polyols and castor oil. It worked with both TDI and MDI, but increasing the proportion of straw polyol above 50 percent did not give a stable foam. The team suggested the optimal mix was 40 percent straw polyol and proportion for the straw polyol was 40 percent, and 60 percent castor oil. The characteristics of the resulting foam resembled those of foam made using petroleum-derived polyols.
‘We were able to obtain very desirable parameters in the manufacturing of foam, converting 96% of the wheat used with an almost maximum performance,’ Rincon said. An additional benefit was an increase in biodegradability.
The team now plan to use the PU foams in plant nurseries to help with plant growth. ‘Instead of watering the plant, and with the aim of dealing with drought problems and preventing overwatering, we would inject the water into the foam so that the plant can consume it as needed,’ she said.
The work has been published in the journal Polymers.