Gelling Agents

Hermans [20] suggested that the name gel should be given to systems that display the following features: 1) coherent, two-component systems formed by a solid substance finely dispersed or dissolved in a liquid phase; 2) exhibit solid-like behaviour under the action of mechanical forces; 3) both the dispersed component and the solvent should extend continuously throughout the whole system, each phase being interconnected.

Rheological characterization divides gels into two major classes, strong and weak gels. Strong gels possess the canonical features of true gels. They manifest typical behaviour of viscoelastic solids and rupture beyond a certain deformation value rather than flow. Weak gels resemble strong gels at low deformation rates but their three dimensional networks get progressively broken down at higher deformation rates and they flow as a dispersed system. Physical gels produced by these rheological substances are best described by their viscoelastic properties. Using dynamic oscillatory experiments, the elastic and viscous components of gels can be quantified by G', the elastic modulus which is a measure of energy storage and G", the loss modulus, a measure of energy dissipation at a given deformation. Physical gels will typically show G' to be much higher than G" when measured as a function of frequency. The slope of G' values as a function of frequency best differentiates strong gels from weak gels. Strong gels exhibits a nearly flat G' profile as opposed to weak gels that show a more positive slope [21].

There are several polysaccharides used in personal care formulations that can undergo gelation as a function of ionic strength, pH, and heat treatment. Gelatine, agar, pectins, alginates, and kappa carrageenans will undergo gelation to yield strong gels. Solid air fresheners are a good example of the type of strong gel character achievable with polymers such as carrageenans.

Salts of crosslinked polyacrylic acid, iota-carrageenan, and cellulose ethers, will also form gels and are used in personal care formulations that exploit weak gel properties. They are highly useful in skin creams, shaving gels, hair styling gels, and gel toothpaste formulations.

Literature and formulation ingredients in commercial creams and lotions suggest that a popular approach to providing both emulsification and stabilization is through a three-dimensional surfactant/cosurfactant network. Rheological characterization of commercial creams and lotions, performed using oscillation tests on a controlled stress rheometer, are shown in Figure 8. These results demonstrate the range of rheologies available on combination of different polymeric stabilizers with these surfactant structures [22].

Figure 8 Viscoelastic properties of commercial creams and lotions at 25°C.

The surfactant-gel network system provides a yield stress, a high degree of elasticity, shear-thinning behavior, and time-dependent structure build-up (thixotropy). These rheo-logical attributes are very important in the consumer's perception of skin feel during lotion application and rub-in.

The primary component of these liquid crystalline gel network systems is a co-surfactant. Cosurfactants are water-insoluble fatty amphiphiles that are too lipophilic to promote o/w emulsions. Cosurfactants combined with a small fraction of a water-soluble surfactant having a high hydrophilic-lipophilic balance (HLB), produce swollen lamellar-gel networks after thermal processing and cooling.

A physical gel forming rheological additive, such as a cellulose ether, cross-linked polyacrylate, clay, or xanthan gum is added to improve temperature stability and modify the rheology of these systems.

A plot of elastic modulus, G' as a function of imposed stress for commercial creams containing xanthan gum (XG), crosslinked sodium polyacrylate (carbomer), Acrylate/ C10-30 alkyl acrylate crosspolymer and polyacrylamide/silica is presented in Figure 8. The elastic modulus, G', at low stresses is a measure of the gel rigidity of the sample. These results serve to distinguish the more solid-like creams, with G' values > 1000 Pa at low shear, from the more liquid-like lotions, with G' values < 1000 Pa at low shear.

Other materials that can form weak gels when given the appropriate mechanical treatment are silica gels and fumed silica. These materials are sometimes used in combination with other polymers to yield weak gels. They are used in toothpaste where it serves a dual role as an abrasive and a rheology modifier. The thickening silicas are the only inorganic products used extensively to structure toothpaste. They provide a good thickening effect and high thixotropic behaviour, but they lack the ability to bind water in the lean solvent slurry. As a result, they are unsuitable for syneresis control. Therefore, a water-soluble organic binder is necessary to modify the toothpaste rheology and to prevent water separation. Carboxymethylcellulose and carrageenans are often combined with silica for this purpose.

Due to the broad performance criteria that personal care products have to meet, most formulators find it necessary to use a mixture of rheological additives to achieve desired properties in final formulations. Mixtures of materials can bring significant synergy in desired properties. In conclusion, rheological additives significantly influence the mechanical, textural, stability, and ultimately the quality of personal care products.

11 Habits To Make or Break For Soft Flawless Skin

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