High resistance to thermal decomposition in brown cotton is linked to tannins and sodium content

Authors: Nam, Sunghyun; Kim, Hee-Jin; Condon, Brian; Hinchliffe, Doug; Chang, Sechin; McCarty, Jack; Madison, Crista

Submitted to: Cellulose
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/27/2016
Publication Date: 2/12/2016
Citation: Nam, S., Kim, H.J., Condon, B.D., Hinchliffe, D.J., Chang, S., Mccarty, J.C., Madison, C.A. 2016. High resistance to thermal decomposition in brown cotton is linked to tannis and sodium content. Cellulose. 23(2):1137-1152.

Interpretive Summary: Brown cotton fiber, whose color appears when the seed pod opens to expose fibers to oxygen and sunlight, was inferior to white cotton fiber in fiber quality, i.e., shorter length, fewer twists, and lower crystallinity. However, brown cotton fibers (SA-1 and MC-BL) exhibited superior thermal properties in thermogravimetry, differential thermogravimetry, and microscale combustion calorimetry analyses. Brown cotton fibers yielded 11-23% reduction in total heat release during combustion and produced 1.2-1.4 times larger char. Washing the fibers in water and 1% NaOH solutions showed that rich natural inorganic components and condensed tannins present in brown cotton fibers are responsible for their enhanced thermal stability. Among inorganic elements, the logarithmic concentration of Na exhibited a significant correlation with heat release capacity. The condensed tannins induced strong binding with Na ions as well as exhibited the thermal resistant characteristics. These results allowed insights on the effects of inorganic components and condensed tannins, which interplay to induce the self-extinguishing property of brown cotton fibers and their potential nonwoven applications that require thermal resistance without toxic flame retardant chemicals.

Technical Abstract: Brown cotton (Gossypium hirsutum L.) fibers (SA-1 and MC-BL) studied were inferior to white cotton fiber Sure-Grow 747 (SG747) in fiber quality, i.e., shorter length, fewer twists, and lower crystallinity, but exhibited superior thermal properties in thermogravimetry (TG), differential thermogravimetry (DTG), and microscale combustion calorimetry (MCC) analyses. Brown cotton fibers yielded 11-23% reduction in total heat release during combustion and produced 1.2-1.4 times larger char. Washing the fibers in water and 1% NaOH solutions showed that rich natural inorganic components and condensed tannins present in brown cotton fibers are responsible for their thermal stability. The loss of inorganics for white cotton fiber by water wash shifted the thermal decomposition process of cellulose to high temperatures, resulting in no char yield. However, the dominant adsorption of Na ions to brown cotton fibers by 1% NaOH wash lowered the thermal decomposition temperatures and consequently decreased its heat release capacity. Among inorganic elements, the logarithmic concentration of Na exhibited a significant correlation with heat release capacity. The condensed tannins induced strong binding with Na ions as well as exhibited the thermal resistant characteristics. The limiting oxygen indices (LOI) calculated based on the MCC parameters showed the self-extinguishing property of brown cotton fiber, and its LOI value was further increased upon the adsorption of Na ions.