Sugar Derivative
An unsaturated uronic acid forms when the side groups of hemicellulose are chemically modified during alkaline pulping. In the kraft process, hexenuronic acid is generated from 4-O-methylglucuronic acid groups on xylan chains. This chemical group remains bound to the pulp fibers until specific acid or bleaching stages remove it.
Bleaching Consumption
The presence of this compound in the pulp increases the consumption of expensive bleaching chemicals like chlorine dioxide and ozone. Because hexenuronic acid reacts readily with these oxidizing agents, it consumes them without actually contributing to the removal of residual lignin. This increases chemical costs and generates more organochlorine byproducts in the mill effluent.
Running an acid hydrolysis stage before the main bleaching sequence selectively destroys these groups, which optimizes chemical efficiency and reduces the environmental impact.
Paper Yellowing
Retention of these groups in the finished pulp degrades the brightness and stability of the paper over time. When hexenuronic acid is exposed to heat or light, it degrades into colored compounds that cause the paper to yellow. This compound also binds metal ions, which can catalyze oxidation reactions that weaken the paper sheet.
Removing it during the bleaching process is essential for high quality printing papers that must remain white and durable for long term archiving.
