A method for determining reactive hydroxyl groups in natural fibers: application to ramie fiber and its modification

The hydrophilic features of natural fibers (NFs) hinder the widespread application of natural fiber/polymer composites due to the hydroxyl groups (–OH) presented in the cellulose molecule. Detecting reactive hydroxyl groups in NFs is very important for optimizing the modification process of natural fibers. This paper proposes a simple and practical methodology to measure reactive hydroxyl groups in NFs using a isocyanate group, a method we term the isocyanate group back titration (IBT) method. Application of the IBT method to ramie fiber with toluene-2,4-diisocyanate (TDI) and hexadecanol indicated that the measured value of reactive hydroxyl groups was about 150 mg KOH/g, which was less than the theoretical value of hydroxyl groups in the ramie fiber being tested. The FTIR analysis revealed that the TDI and hexadecanol were grafted onto the surface of the ramie fiber, leading the modified ramie fiber to be hydrophobic. Thus, the IBT method is also useful for modifying the surface properties of NFs and improving their compatibility with polymers, and finally leading to good mechanical properties of NF/polymer composites.