ABSTRACT
Synopsis A comparative study was conducted on the influence of widely used humectants [glycerol, propylene glycol (PG), polyethylene glycols 400 and 600 (PEG 400 and PEG 600)] on the physical characteristics of solidified sodium stearate-based sticks (SSSS). - Sticks prepared with PG or PEG 600 generally tended to be harder than the corresponding ones containing glycerol or PEG 400; - PG lowered the penetrability of the sticks while PEG 400 tended to increase it. Glycerol or PEG 600 produced sticks with intermediate penetrability values; - all investigated sticks showed practically acceptable softening points; - PG-based sticks showed the lowest yield by abrasion values while PEG 400-based ones showed the highest such values. Glycerol or PEG 600 produced sticks with intermediate behaviour; - sticks prepared with PG generally showed the highest tendency to dry by evaporation. There was a clear tendency towards reduction in evaporation with all the considered humectants, proportional to humectant concentration increase; - sticks formulated with PEGs showed considerably longer disintegration times than those formulated with glycerol or PG; - all sticks showed a plastic thixotropic flow behaviour; - static and dynamic yield value-humectant concentration curves exhibited an almost identical behaviour with all investigated systems; - generally speaking, SSSS with PG showed the highest dynamic and static yield values while those formulated with PEG 400 showed the lowest such values; the presence of PEG 400 or glycerol resulted in sticks with intermediate behaviour in this respect; - PG produced sticks showing the highest thixotropic breakdown by shear; glycerol came next in this respect. PEG-based sticks showed strong fluctuations in their thixotropic breakdown-humectant concentration behaviour; - PG gave sticks with the highest plastic viscosity; they, indeed, had a tough texture. Glycerol or PEG 600-based sticks had intermediate plastic viscosities and acceptable spreading characteristics. PEG 400-based sticks had the lowest plastic viscosity and best spreading characteristics; - almost all sticks investigated proved to be stable after 18 months storage as far as.
