SUSPENSION AIDS
Figure 4: Solution of 1% commercial MFC in freshwater prior to activation (12.5x magnification).
Figure 3: A simple microscope with 12-16x magnification is useful for determining if MFC is fully ‘activated’ (ie fully dispersed).
to a hazing of the formulation resulting in phase separation or precipitation of the suspension-aid. In liquid detergents for home care,
the use of suspension aids is relatively new but is likely to increase. This is because as these formulations become more concentrated and the demand increases for using more biodegradable ingredients, many of these ‘green’ ingredients will be insoluble and, therefore, a suspension-aid is likely to be required. This is particularly true for products such as dish soap and liquid laundry detergent. For example, zeolites are a green alternative to chemical detergent- builders like phosphates, but they are completely insoluble and therefore would require a suspension-aid when used in a liquid laundry product. Unfortunately, traditional, naturally-derived suspension-aids do not generally have sufficient compatibility with these high levels of surfactants. In fact, even synthetic polymers are proving inadequate to provide suspension in these new, concentrated formulations. Current synthetic suspension-aids are either not compatible with these increased levels of surfactants or are cost-prohibitive because of the high use-levels required to give adequate suspension. Another problem with synthetic polymers is that they can increase the ‘pour’ viscosity of the product and adversely affect its flow and dispersion into water by the consumer. With these formulation
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challenges emerging in the personal and home care markets, it is clear that there is a strong need for a new suspension-aid that is naturally-derived yet also provides reliable performance, low cost-in-use, and exceptional compatibility with high surfactant and surfactant-thickened formulations. This type of polymer is now available and is known as microfibrous cellulose or ‘MFC’ for short. This polymer is produced by the fermentation of the bacterium, Acetobacter xylinum. The organism is typically grown in glucose- syrup (eg corn-syrup) based media like many other types of ‘biopolymers’ and produces cellulose that is chemically identical to plant-derived cellulose. In fact, its INCI name is ‘Cellulose’. However, though identical in chemical structure, MFC fibres are much smaller in diameter than plant-derived cellulose which leads to a far higher surface area by weight of
Figure 5: Fully activated 1% commercial MFC in water after 5 passes using a Ross Mixer MegaShear mixing head (16x magnification).
Figure 6: Picture of generic ultra soap dish (30% active surfactants) with 0.125% commercial MFC and suspended exfoliant beads. (Index of refraction was about 1.38 and the yield value was about 2.5 Pa).
PERSONAL CARE March 2012
the cellulose. For example, typical fibre diameters for plant-derived cellulose range from about 30 µm to 300 µm, whereas the fibre diameter for MFC is about 0.1 µm. This smaller fibre size means that a given weight of MFC has up to 200 times more surface area than other forms of cellulose. The increased surface area translates directly to greater efficiency in use and allows the MFC to create a very fine, three-dimensional network. It is this network structure that differentiates MFC from other cellulose forms. Being essentially insoluble, MFC is insensitive to many factors impacting water-soluble polymers such as temperature, pH, salt, oxidisers and shear. This insolubility also means that it is compatible with high levels of salts and surfactants that would cause most water- soluble polymers to precipitate. Lastly, its insolubility allows it to form a fine structure which is effective at creating a true yield stress (yield stress is a measure of the force required to initiate flow in a gel-like system). The yield stress of MFC imparts excellent stability to a wide variety of aqueous formulations. MFC’s immunity to many environmental effects is for two important reasons. First, internal reinforcement is received since one cellulose chain is bundled to the next through hydrogen bonding (this is similar to making a strong cable from many fine stands of fibre where the end result is much stronger than any of the individual fibres would be). Second, since MFC is essentially insoluble, the acids, bases, oxidisers and reducing agents cannot attack the individual linkage points as readily, whereas when polymers are fully solubilised, these linkages are much more labile to the effects of their environment. The commercial form of MFC
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