Flame retardant finishing for cotton using a hydroxy-functional organophosphorus oligomer
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New durable flame retardant finishing systems based on a hydroxy-functional organophosphorus oligomer (HFPO) and a bonding agent have been developed for cotton. In this research, both dimethyloldihydroxyethyleneurea (DMDHEU) and trimethylolmelamine (TMM) have been used as the bonding agents between HFPO and cotton. DMDHEU is more effective for crosslinking cotton cellulose and for crosslinking between HFPO and cotton than TMM, but the bonding formed by DMDHEU between cotton and HFPO is more durable to multiple laundry cycles than that formed by TMM. The mixture of DMDHEU and TMM were also used as a crosslinking agent for HFPO and cotton, the ratio of DMDHEU and TMM has a significant influence on the flame resistance, durability, and mechanical performance of treated cotton. An increase in the DMDHEU/(DMDHEU+TMM) ratio in the mixture increases the amount of HFPO bound to cotton, increases the laundry durability, and reduces the fabric tensile strength retention. Both DMDHEU and TMM are nitrogen providers for HFPO-based flame retardant finish system to enhance the flame resistance of cotton by means of phosphorus-nitrogen synergism. TMM was a more effective nitrogen provider than DMDHEU to improve the flame resistance of treated cotton. The mechanism of phosphorus-nitrogen synergism has been investigated by studying the thermal decomposition of treated cotton cellulose. The interaction of phosphorus-nitrogen retards the thermal decomposition of cellulose by depolymerization, and promotes the dehydration process, thus increasing the amount of solid char, and substantially improving the flame resistance of treated cotton. The extent of dehydration, not the dehydration temperature has a profound effect on the flame resistance of treated cotton. The comparison of two reactive organophosphorus flame retardant finishing systems: N-methylol dimethylphosphonopropionamide (MDPA) and HFPO has also been studied. The amount of the flame retardant agent bound to cotton is significantly higher for the HFPO system than that for the MDPA system when equal moles of the phosphorus and N-methylol group are applied to cotton. The MDPA/TMM treated fabric has a higher initial limiting oxygen index (LOI) than that of the HFPO/TMM treated fabric due to the nitrogen content in the MDPA molecule. The flame resistance of cotton is identical for both the HFPO and MDPA system when there are equal amount of phosphorus and nitrogen on the fabric. The stiffness of both HFPO/TMM and MDPA/TMM treated fabric significantly increased when a high TMM concentration was used. And MDPA/TMM treated fabric showed higher dimensional change during the laundering cycles and higher formaldehyde release level. There is a non-linear correlation between char length and LOI of flame retardant cotton. The fabric weight per unit has a significant influence on the non-linear correlation between char length and LOI.