Newtonian Fluids

A schematic of the viscosity profiles of Newtonian and non-Newtonian fluids is shown in Figure 1. Fluid (a) represents a typical viscosity of the base fluid, which might be water, |

oils or other low molecular weight solvents. The viscosity of these fluids can be modified by addition of particulates that may strictly change the viscosity index as illustrated by the higher viscosity for fluid (b). When non-interacting buoyant particles are used in these d fluids, the viscosity of the dispersion can be predicted using the Einstein relation [1].

where | and |o are viscosities of the dispersion and medium respectively and $ is the s volume fraction of the particles. Examples of such rheology-modifying substances include do

(6)

(Ol

(dl

{öl

m—

Figure 1 Schematic of flow properties of Newtonian and non-Newtonian fluids.

Lag (shear rate)

Figure 1 Schematic of flow properties of Newtonian and non-Newtonian fluids.

silica gels, fumed silica, carbon black, titanium dioxide and aluminum-magnesium-stea-rates when used at very small concentrations. Low molecular weight polymers also fit in this category and may be preferred if a smooth or fluid like formulation is desired. Their typical flow curve can also be represented by fluid (b) in Figure 1.

Non-Newtonian Fluids

Unlike Newtonian fluids, non-Newtonian fluids possess shear-rate dependent viscosities. Fluids (c), (d), and (e) in Figure 1 illustrates a range of non-Newtonian profiles observed in personal care formulations. In addition to shear-rate dependent viscosities, non-Newtonian fluids also exhibit elastic stresses when subjected to high shear rates. The usefulness of the elastic response varies with application, as will be illustrated in a later section.

The performance value of rheological additives that impart non-Newtonian characteristics to personal care formulations is demonstrated by the curve in Figure 2. On close

Figure 2 Schematic of full flow curve of non-Newtonian fluids.

Figure 2 Schematic of full flow curve of non-Newtonian fluids.

examination of this figure, it is easy to see why non-Newtonian rheology is more common in personal care formulations than Newtonian (viscous) rheology.

At low shear rates, i.e., near at rest conditions, non-Newtonian fluids exhibit high viscosities that are relatively insensitive to shear rate and characterized by zero shear viscosity. The zero shear viscosity is known to be highly sensitive to the molecular weight and concentration of the rheological additives [2]. The rates of deformation associated with this region include sedimentation and levelling forces, and one can tailor the zero shear viscosity to combat these forces. At moderate shear rates the decrease in viscosity versus shear rate helps when pouring and pumping these fluids. At high shear rates it is found that a second Newtonian plateau in viscosity is reached usually characterised by the so-called infinite viscosity. The shear forces in this area are close in magnitude to forces developed during rubbing and spraying exercises. The low viscosities exhibited by the rheological additives in this region imply low resistance to rubbing and thus a smooth sensation of the substance during its application.

Elasticity

As discussed above, non-Newtonian fluids also exhibit elastic properties, i.e., when subjected to high shear rates, non-Newtonian fluids will exhibit elastic stresses. Figure 3 illustrates the elastic functions of the non-Newtonian fluids (c), (d), and (e) from Figure 1. Note that the elastic response tends to be seen at the higher shear rates.

It is generally observed that fluids that show more shear-thinning properties tend to show more elastic response [3]. This result is well demonstrated on comparison of the viscosity profiles of fluids (c), (d), and (e) in Figure 1 with their normal stress profiles in Figure 3. The rank order of shear-thinning performance for these fluids is fluid (e)>(d) >(c). An identical rank-order of elastic performance is seen for these same fluids in Figure 3.

The desirability of the elastic response will vary with the intended use of the personal care product. In the case of toothpaste, an elastic force is needed to increase extrudate spring back during the tube filling operation in toothpaste production or while dispensing it at home. However, excessive elasticity might not be desirable, as it may make the toothpaste too stringy. High elasticity is needed to stabilize foams, for example in shaving

Figure 3 Schematic of elastic shear properties of non-Newtonian fluids.

Figure 3 Schematic of elastic shear properties of non-Newtonian fluids.

creams, as it provides strength to film at the air/liquid interface in the matrix of bubbles. In the case of creams and lotions, a short texture with less elasticity may be desired.

Examples of substances that impart viscosity as well as elasticity to a fluid are cellulose ethers, xanthan gum, and crosslinked polyacrylic acids. Clays can impart viscosity without elasticity. In the following section, some of the many rheological additives available to personal care formulators will be highlighted. As will be seen, a variety of additives are available in the marketplace that allow formulators to create a range of viscosities and elasticities in the final product.

Interacting particulates such as smectic, hydrophilic and organoclays represent one class of materials used in personal care products that can impart non-Newtonian characteristics to formulations. At a very low concentration, they are known to impart significant viscosity enhancement to the base fluid without any significant elasticity. They typically exhibit a flow curve similar to fluid (C) in Figure 1. It is well-documented [4,5] that these materials cause gelling if used at higher concentrations.

In the case of polymers, their zero shear viscosity, shear-thinning, and elasticity characteristics are a function of their structural characteristics. The rigidity of the polymer, its weight average molecular weight, polydispersity, and degree of branching each play a part in determining these properties.

Water-soluble cellulose ether derivatives such as carboxymethylcellulose (CMC), hydroxyethylcellulose (HEC), hydroxypropyl cellulose, and methylcellulose impart pseudoplastic or shear-thinning rheology to formulations [6a,b]. This characteristic makes these polymers attractive candidates as thickening agents in personal care products.

For instance, this flow characteristic enables a product to pour as a rich, viscous solution from the container, yet be easily applied to a substrate like hair, as its viscosity reduces with shear. These polymers tend to impart high viscosities at low shear. They exhibit moderate shear-thinning behavior, but possess little elasticity at a moderate range of deformation rates, similar to the rheology profile of fluid (d) in Figure 1.

Some of the applications where these polymers are used include shampoos, conditioners, hair spray, and hair-styling gels, toothpastes, and denture adhesives.

This pseudoplastic rheology is particularly beneficial in surfactant-based haircare formulations like shampoos where cellulose ethers can be used to reduce or eliminate inorganic salt added for thickening [7]. Cellulosic thickeners can be used to achieve viscosities higher than possible with salt or even salt combined with alkanolamide. In many cases, even the alkanolamide can be replaced by the cellulose ether [8].

For example, incorporation of 1% hydroxyethylcellulose into a TEA-lauryl sulfate luxury shampoo increased the formulation viscosity from a Brookfield viscosity of 460 cps to a gel with a viscosity of 5300 cps [9].

Additional benefits can also be realised on incorporation of cellulose ethers into formulations. Unlike salt, cellulose ethers do not influence surfactant cloud points, and they can be used to viscosify surfactant systems that are difficult to thicken, such as imida-zolidine-derived amphoterics, sulfosuccinates, and highly ethoxylated alkyl ether sulfates [10].

In other haircare applications, such as conditioning hair rinses, addition of a low level of hydroxyethylcellulose polymeric thickener can significantly increase finished product viscosity and improve shelf stability [11].

Cellulose ethers in general have this effect on product viscosity and shelf stability. Methylhydroxypropylcellulose effectively thickens sodium laureth sulfate; a surfactant commonly used in surfactant-based haircare formulations, yielding solutions with excel-

Figure 4 Flow properties of 1% cellulosic ether solutions at 25°C.

lent high temperature freeze/thaw stability. Cellulosics achieve this enhanced shelf stability by maintaining the viscosity of the formulation at room temperature, and during freeze/ thaw cycling.

A typical rheological profile for two commercially available cellulose ether products, CMC and HEC, are shown in Figure 4. Note that the flow profiles for these materials resemble the profiles for fluids (d) and (e) in Figure 1.

11 Habits To Make or Break For Soft Flawless Skin

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