In a previous "Comparatively Speaking" column, Tony O'Lenick explained the differences between trisiloxane and dimethicone copolyol in terms of surface tension. Here he explains the differences in terms of wetting and eye irritation.
Dimethicone copolyol compounds, referred to in the INCI nomenclature as PEG/PPG dimethicone, are divided into two classes, one of which has “D” units. Figure 1 shows the structure of PEG-8 dimethicone, a dimethicone copolyol, with “a” units substituted for "D" units. The second class of dimethicone copolyol compounds, i.e. trisiloxanes, do not have "D" units (see Figure 2).
While trisiloxanes are good wetting agents, they demonstrate problems with stability at high and low pH values. In this case, the quest for the formulator is to find a silicone that provides the desired stability.
The ability of a surfactant to wet substrates rapidly is a key performance property in many applications. The Draves Wetting Test (AATCC Standard Test 17) is a widely used laboratory procedure for ranking the relative wetting efficiencies of surfactants. This test is a timed determination of the wetting of a cotton skein by dilute surfactant solutions. Shorter wetting times or lower wetting concentrations are indicative of better wetting efficiencies.1
Table 1 compares PEG-8 dimethicone (a trisiloxane) to a series of increasing molecular weight dimethicone copolyols. PEG-8 dimethicone is considered a good wetting agent (7 sec); as the molecular weight increases to 1398, the wetting remains good. Once the molecular weight increases to 2708, the wetting is far less efficient. The formulator should note this when choosing a wetting agent.
Table 2 compares the same trisiloxane to a series of increasing molecular weight dimethicone copolyol compounds at 10% active in water for eye irritation. It shows that trisiloxanes are as irritating as sodium lauryl sulfate but as the molecular weight increases, the irritation level drops dramatically.