Comparatively Speaking: Wetting/Eye Irritation in Trisiloxane vs. Dimethicone Copolyol

Feb 15, 2011 | Contact Author | By: Anthony J. O'Lenick Jr., Siltech LLC
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Title: Comparatively Speaking: Wetting/Eye Irritation in Trisiloxane vs. Dimethicone Copolyol
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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.

Reference
1. http://dow-answer.custhelp.com/app/answers/detail/a_id/1695/~/triton-gr-series-surfactants---wetting

 

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Table 1. Wetting of PEG-8 dimethicone as a trisiloxane vs. a dimethicone copolyol

Test Compound Molecular Weight Draves Wetting 0.1% Soln.  CMC mg/l Surface Tension at CMC Description
PEG-8 Dimethicone 633 7 sec 20 20 Trisiloxane
PEG-8 Dimethicone 855 8 sec 20 20 Dimethicone Copolyol
PEG-8 Dimethicone  1398 10 sec 20 20 Dimethicone Copolyol
PEG-8 Dimethicone  2105 18 sec 23 22 Dimethicone Copolyol
PEG-8 Dimethicone 2706 257 sec 23 22 Dimethicone Copolyol

     

 

Table 2. Eye irritation of PEG-8 dimethicone as a trisiloxane vs. a dimethicone copolyol

Test Compound Molecular Weight Eye Irritation (1 day)  Eye Irritation (7 Day) Description
PEG-8 Dimethicone 633 28 4 Trisiloxane
PEG-8 Dimethicone 855 13  2 Dimethicone Copolyol
PEG-8 Dimethicone 1398 5 2 Dimethicone Copolyol
PEG-8 Dimethicone 2105 4 0  Dimethicone Copolyol
PEG-8 Dimethicone 2706 2 0 Dimethicone Copolyol

           

 

Figure 1. Structure of a dimethicone copolyol

Figure 1. Structure of a dimethicone copolyol (PEG-8 dimethicone)

The structure of the dimethicone copolyol PEG-8 dimethicone is shown here.

Figure 2. Structure of a trisiloxane

Figure 2. Structure of a trisiloxane

The structure of a trisiloxane, a class of dimethicone copolyols, is shown here.

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