Studies of ester formation on a cellulose matrix

Abstract: In this work a model system has been developed to quantitatively study esterification of cellulosic material during drying and heating in which solid-state conditions can be said to predominate. The formation of ester cross-links in cellulosic or lignocellulosic composite materials during drying and heating is of interest for production of paper and wood composite boards, for wood modification and in giving cotton materials improved characteristics. The results support the hypothesis that direct ester formation, without anhydride intermediate, occurs between carboxylic acids of suitable structure and cellulosic hydroxyls at elevated temperatures even after such short reaction times as 5 minutes. Ester formation has been observed at temperatures ranging from 90°C to 180°C. Another important conclusion is that a hydroxy or keto group in alpha position of the carboxylic acids seems to favour ester formation. A critical parameter for ester formation is pH, and the results indicate that pH <2 strongly favours esterification reactions, whereas almost no esters are produced at pH >5-6. In the experiments, low moisture content, higher temperatures and longer reaction times also favour ester formation, with some exceptions. Another interesting finding is that three pyrrolidine structures are produced after esterification and aminolysis of 2- ketobutyric acid. One of the structures indicates that 2-ketobutyric acid is reactive in two positions and could therefore be a potential cross- linker. Esterifications of five structurally different carboxylic acids-2(R)- hydroxybutyric acid, 2-ketobutyric acid, 3-hydroxybutyric acid, hexanoic acid and veratric acid have been studied. The carboxylic acids and moisture were added to ordinary cellulosic filter paper. Samples were heated at temperatures ranging from 22°C to 180°C between 5 and 45 minutes. Quantitative analysis of ester formation, in this case between the monocarboxylic acids and the cellulose's hydroxyl groups, was performed using aminolysis/gas chromatography.