Heterogeneous acylation of flax fibers. Reaction kinetics and surface properties

Flax fibers composed mainly of cellulose were subjected to heterogeneous valerylation reaction. The progress of the chem. modification was assessed by transmission FTIR. The heterogeneous esterification reaction followed 1st-order kinetics, and a plateau was reached already after 30 min. The intensity of the FTIR hydroxyl absorption band (n = 3400 cm-1) did not appreciably decrease during the acylation reaction, showing that only a small fraction of the fiber hydroxyls was involved in the reaction. The degree of valerate substitution (DS) at the fiber surface (50 .ANG. thick layer) was evaluated by means of ESCA. Surface valerylation increased with reaction time and leveled off at DS around 1 after 30 min, in agreement with the FTIR data. The chem. modified fibers maintain the Cellulose I crystal structure and the original crystallinity degree up to the longest reaction time investigated (180 min). Dynamic contact angle measurements showed that surface hydrophobicity, as indicated by advancing contact angle, rapidly increased upon valerylation, reaching a plateau after about 10 min. Chem. modification does not appreciably alter fiber thermal stability (by TGA) and morphol. (by SEM).