FORMULATING FOR MILDNESS 113
isethionates, the carboxylate group is ‘masked’ (Fig 4b) by esterifying coconut fatty acid with isethionate. Both sulfonate surfactants (lauryl sulfo succinates and cocoyl isethionates) are carboxylic acid esters and, therefore, are readily biodegradable by nature´s enzymes. The excellent mildness of both of them is proven by tests.6
Composition of solid cleansing bars: syndets versus soaps In soaps the counterions are a key parameter for the preparation of solid bars. Potassium soaps are soft soaps and usually used for liquid products whereas sodium soaps are a suitable base for solid bars (curd soaps). The background is that for steric reasons the crystallisation of potassium soaps is hindered. Besides this physical prerequisite to prepare a solid soap bar, there are some other important factors. Apart from good cleansing and foaming properties, consumers have a certain expectation regarding the quality of solid cleansing products. The product should be without cracks after using and not getting mushy after contact with water. It also should bring benefits like additional caring effects or nice scent and colour. To match the customer’s needs, other ingredients than surfactants have to be added to the final product. Concerning syndet bars, binders are required to prevent ‘cracking’ whereas plasticisers are added to reduce the viscosity of the syndet mass during the manufacturing process. Fillers like starch are added to give the syndet bars ‘structure’. As emollients, natural waxes can be used. For an attractive sensual perception colouring and perfuming agents are added. A special challenge is the mushing (sloughing) tendency particularly of syndet bars after contact with water. To reduce the water uptake and mushing of syndet bars effectively, Zschimmer & Schwarz developed Altriform S (INCI: Aluminum Triformate) more than 30 years ago.8
The challenge regarding soap bars is the cracking tendency which does not occur
a O O N b O O
N H
O Figure 7: Amino acid-based surfactants: Sodium acyl sarcosinate (a) and sodium acyl glutamate (b). April 2019 Na+ OH O-
Amino acid surfactants – carboxylate-based alternatives to soaps? Up to now, we described two ways to overcome the drawbacks of using soaps: To replace them partly (combo bars) or completely (syndet bars) by sulfonate or sulfate surfactants. Another way is to maintain the carboxylate groups of soaps but to use tricks to make them water-
PERSONAL CARE EUROPE O- Na+
Figure 6: Zschimmer & Schwarz raw material for producing solid cleansing bars.
with syndet bars. To the best of our knowledge the ‘cracking-problem’ of soaps is still not solved satisfactorily. More information about ingredients beyond surfactants and the manufacturing process of cleansing bars can be found in references.6, 8, 9
Aesthetic aspects Hill and Moadell10
describe solid cleansing
bars as a heterogenous mixture of different surfactant phases (liquid crystals, solid crystals). As a consequence, incident light is strongly scattered so that solid cleansing bars generally are not transparent. The reason for light scattering are inhomogeneities in the refractive indices of the different phases. The trick to get a transparent cleansing bar is to ‘equalise’ the refractive indices throughout the complete mixture. This can be achieved to a sufficient extent by adding solvents like propylene glycol or glycerol to the cleansing bar raw material. Additionally, by this measure the dimensions of the dispersed solid crystals decrease. Both leads to very attractive transparent cleansing bars (Fig 5).
Tailor-made products Zschimmer & Schwarz produces speciality surfactants including well-balanced combo
and syndet bar raw materials (pellets and noodles, Fig 6): The Zetesap-Types. By simply adding colourants and perfumes, solid cleansing bars can be prepared easily. We developed a unique manufacturing process which ensures a highly homogenous raw material for the preparation of solid cleansing bars. Zetesap (now referred to as ‘Z’) C 11 (surfactants: sodium stearate, sodium laureth sulfate, sodium laurate) is a basic material for visually attractive transparent combo bars (Fig 5). It contains propylene glycol and glycerol. In addition, the product can be handled easily by melting at about 70°C. It is therefore suitable for customers who do not have the mechanical equipment to produce ‘usual’ syndet bars. The final product can easily be poured in molds of different shapes. There are no limits in creativity concerning colour, shape and perfume. As Z C 11 contains soaps, the formulator has to keep in mind that the pH-value can only be adjusted in a certain range.
Zschimmer & Schwarz additionally offers various tailor-made syndet bar raw materials for any kind of needs. The surfactants in Z 813 P are disodium lauryl sulfo succinate and sodium coco sulfate. Z 900 has the same surfactant basis as Z 813 P but offers further caring properties due to the addition of natural waxes (glyceryl stearate and lauryl PCA, a derivative of the amino acid glutamic acid). Z 5165 is a sulfate-free syndet bar. It is based on a combination of disodium lauryl sulfo succinate and sodium cocoyl isethionate. All Zetesap-Types have very good lathering properties and are very mild. They are easily perfumable, colourable and processable. The pH of a 10% aqueous solution of this raw material is adjusted to be 6.5. To lower the pH in aqueous solutions to 5.0 – 5.5 organic acids like citric acid can be added to the final wash piece.
Solid cleansing bars contain water. The neutral to slightly acidic pH-value of syndet bars offers an additional advantage regarding the scent. Some perfumes used in soaps are unstable in an alkaline environment. Due to a degradation they start smelling unpleasantly after a while. Perfumed syndet bars manufactured with Zschimmer & Schwarz raw material, however, smell unchanged even more than 30 years after their production.
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