on the other hand , resulted in higher density bars that felt heavier and more solid . They held together very well in use , but foam evolution was slower and less copious .
It should be stressed that the pressures used were very specific to the size and shape of bars produced . The pressure required for optimum bar performance and integrity will be different for different stamps , formulations and sizes , and will best be found by trial and error .
Fibre-surfactant ratio
The amount of fibre required to create a shampoo bar or , indeed , surfactant powder depends on the water content , active matter and ultimately the rheology of the dried form . The SLES we trialled , for example , had an active content of 70 %, whereas the cocoglucoside was 52 %. Shampoos , being blends , are more complicated . It is probably best , in this case , to think of the active content as being everything that is not water .
To optimise the fibre content we must consider the oil / water absorption capacity of the fibre . The specific fibre trialled had water and oil binding capacities of 4.5g / g and 4g / g respectively . If we consider that the dried active material is , essentially , a thick sticky liquid , we would hypothesise that the maximum capacity for binding it will be around 4g / g . On this basis we can reasonably expect that we would need 70 / 4 ( 7.5 %) fibre for 70 % SLES and 52 / 4 ( 13 %) fibre for cocoglucoside .
Trials have , however , shown that the ratio of fibre required is much higher than predicted . This is most likely due to the heavy , sticky form the surfactants take when the water is removed . It might be more accurate to say that the fibres , as crystal seeds , provide a substrate for the surfactant molecules to crystallise onto ( Figure 2 ). More typical ratios are actually 58 % SLES , 42 % fibre ( 1.4-1 ) and 61 % cocoglucoside , 39 % fibre ( 1.56-1 )
Trials have shown that overloading the fibres result in a powder that is sticky and prone to clumping . In some scenarios this might be considered acceptable . Ultimately experimentation will clarify the optimum ratios required .
Testing
A number of tests were performed on the 30g prototype shampoo bars to establish long-term stability and performance characteristics . Shampoo bars , much like syndets and traditional soaps , are regularly exposed to high humidity and standing water . A climate chamber was used to compare water absorption and any corresponding changes to the bar ’ s integrity .
After two weeks there was a slight softening to the surface of the bar , consistent to what is found with current syndets and shampoo bars . The inside of the bars remained stable and dry . Ad hoc trials were also performed in a bathroom / shower room environment and results showed that the long-term stability was at least as good as traditional soaps and syndets .
Initial washing trials were completed to better understand the performance of the prototype bars . There was some concern that the high levels of fibres used would result in negative characteristics with regards to skin feel , foaming , rinse-out , etc . However , the fibres did not have a negative effect on the foam or the rinse-out and the product washed out easily . No discernible residue was left on the hair .
Once again , having achieved successful results , we decided to take things further still . Could a bottle of ‘ off the shelf ’ shampoo be converted into a bar using this technology ? Trial subjects were asked about their shampoo of choice . These were purchased and blended with fibres , dried , milled and stamped into 30g bars . Encouragingly , all ten were processed with ease .
These bars were handed to the trial subjects for evaluation . They were then asked about bar integrity in the bathroom setting : foaming , skin-feel and hair feel during and after washing . The results were very encouraging . Bar integrity was , again , considered to be good and comparable with soap , syndet and conventional shampoo bars .
Performance characteristics were also generally positive . Whilst , as would be expected , the foam took a little longer to form , it was considered generally better than conventional shampoo bars . Some subjects felt it was creamier than the liquid shampoo version , whilst others felt a slight dryness to the foam . There were some slight differences to the skin-feel versus the liquid equivalents but the final result on the hair once dried was felt to be equivalent to the liquid form .
Figure 3 – Results of trials with & without addition of fibres
46 SPECIALITY CHEMICALS MAGAZINE ESTABLISHED 1981