SILICONES 31
Selective silicones in ethanol-based sanitisers
n Raymond Cen PhD - BRB North America, US
Hand cleansing and sanitisation are key steps to prevent diseases caused by contact-transmission of microbials. With the COVID-19 global pandemic, various hand sanitisation products are in high demand. The dominant products are ethanol-based hand sanitisers. They are widely used to fight the pandemic, in liquid or gel forms. However, ethanol-based hand sanitisers irritate and defat skin and have sticky after- feel when not optimised. Alcohol topical usage has been controversial.1-2
Glycerin
and propylene glycol (PG) can be added into a hand sanitiser to partially mitigate the dryness but impart unpleasant tackiness hand after-feels. Such tackiness is much more pronounced in gel form. The unmet consumer needs prompted the development of an improved hand sanitiser gel system for superior hand after-feel. A variety of silicones are selected with
dominantly silicone copolyols, to replace glycerin and/or PG, in combination with carbomer 940/ triisopropanolamine (Carbomer/TIPA) primary gelling system, to form aquo-alcohol based hand sanitising gels. As arbitrary addition of silicones followed by trial-and-error practice is never the most efficient where phase boundary may exist, this leads to the approach to build the formulation system “ground up” in order to achieve the end results while learning phase information at same time.
Seeking clear effective base gels Carbomers are poly(acrylic acid) which can be neutralised to form effective aqueous gels at extremely low concentration. Although simple inorganic bases can be used for neutralisation, organic bases frequently deliver a wider clarity range when ethanol or solvents are used. As Carbomer 940 is widely available, it has been selected for this study. Other carbomers can easily replace it for similar results. Among different organic bases, Triisopropanolamine (TIPA) is selected to neutralise Carbomer 940 in search for clear hand sanitiser gels. Other organic bases like Diethanolamine (DEA) is also a pretty good choice.9 Carbomer 940 is a fluffy white powder
November 2020 Abstract
Isotropic crystal-clear phases are formed by incorporation of PEG-12 Dimethicone, PEG/PPG-18/18 Dimethicone and PEG-10 Dimethicone, in defined areas on different phase diagrams of carbomer 940/ triisopropanolamine/ water/ ethanol/ silicone copolyols. Other selected silicone ingredients were also screened to check for formulation clarity and sensory benefits. At the end formulation space and several unique combinations of copolyol/gum blends are proposed to yield optimised performance. The leading hand sanitisers thus developed have no need to use glycerin or propylene glycol. They provide smooth, moisturising, non-tacky, silky hand after-feel. This technology contributes to world consumer needs in seeking better hand sanitisers to combat COVID-19 and other contact-transmission diseases.
Figure 1: A sequential dilution of SDA 40B with A gel (from left to right): (1) 10% A as a thin fluid; (2) 20% A as fluid; (3)30% A as a shear thinning flowable clear gel; (4) 33.6% shear thinning flowable translucent gel; (5) 40% to 95% A as a thick translucent gels; (6)97.5% A as haze clear gel; (7)100% A as a clear gel.
and TIPA a chunky solid at room temperature. To study the equivalent point, these ingredients are diluted into 1% and 10% respectively in water then titrated each other. In both cases it shows 1: 0.775 (w/w) ratio with end pH from 6.0. pH 7 has not been explored to leave room for production adjustment as needed. At this ratio mother loads of 1% and 10% clear pre-gels called gel A and gel B are being prepared for subsequent study. Pre-gel is a great approach to save time and material because forming a homogeneous thick gel is most time consuming while gel dilution is relatively easy. Both gel A and B are highly viscoelastic ring gels. A sturdy mechanic stirrer is recommended to provide rigorous stirring.
Due to the gelatin-like texture of the pre-gels a homogeniser is not an option in the pre-gel making process. However, a homogeniser is the fastest tool for gel dilution. In this study, only a few seconds is required to complete gel dilution at high rpm. The phase behaviours of gel A and B in
mixing with ethanol are highly dependent on the quality and type of the commercial product. For example, a cloudy gel region was observed from (33.3 % gel A: 66.7% SDA 40B) to (97.5% gel A: 2.5% SDA 40B) when SDA 40B is used as diluent, as shown in Figure 1. However, when alcohol changed to a
food grade 100% ethanol, the above cloudy region disappeared to give a complete
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