Surfactants aided bleaching of raw kaolin and production of delaminated nano clays via sequential milling

Industrial surfactants possessing specific functional groups are better known for obtaining stable clay dispersions. However, the effect of surfactants on the bleaching of kaolin clays during mechanical milling has not yet been studied. In this work, surfactants-assisted bleaching of kaolin clays was investigated through sequential milling (up to 3 stages) to develop an acid-free, water-based green-bleaching process. The aim of this work was to screen the surfactants that offer insitu bleaching thereby improving the brightness without any reductive bleaching using sodium dithionate, and directly produce size-controlled nano-clay particles. Kaolin clays of Indian origin were subjected to mechano-chemical bleaching via planetary milling in the presence of cationic, anionic, and polymeric surfactants in aqueous medium at neutral pH and 300 rpm for 10 min duration. The bleaching efficiency was monitored carefully by noting the color index properties. The end-products were also analyzed for the powder XRD, XRF, Near IR, color coordinates and morphology. The results confirmed that the three-stage mechanical-milling is effective in bleaching of kaolin clays when surfactants like Sodium Dodecyl Sulfate (SDS), commercially known as Sodium Lauryl Sulfate (SLS), is employed. The L* value of kaolin clay is enhanced from 83 to 90 and the b* value decreased from 12.47 to 1.98 in these surfactants. The SEM and TEM analyzes revealed that the mechano-chemical milling was also favoring the delamination of the kaolin clay booklets to produce nano-kaolin. Hence the technique is two-in-one to successfully produce optically bright, highly dispersible, IR reflective, kaolin nano-platelets through green-strategy.