Conditioning Polymers

Conditioning polymers are effective skin and hair modifying agents because they are designed to deposit, bind, adhere to surfaces, or be absorbed by the protein-containing elements of hair and skin. In the personal care industry, the term "substantivity" describes a material's adhesion to these surfaces. However, for a material to be an effective conditioner, it is not enough that it merely adheres to skin and hair—it must also possess certain qualities that the consumer considers beneficial for these surfaces. In fact, conditioning effect is among the most important characteristics that consumers seek when purchasing personal care products. This article concerns a type of synthetic conditioning polymer, the cationic polymer.

Synthetic Conditioning Polymers

Table 1 shows the structure and INCI nomenclature of many cosmetically useful synthetic, cationic, conditioning polymers. These structures are distinguished by the nature of the cationic moiety that characterizes each cationic polymer class. It is evident that only a few monomers carrying cationic charge are currently used in the industry. Most of these polymers are products of addition polymerization reactions employing unique acrylate or vinyl monomers. The structure of these polymers is determined by the core activity of the monomers. Various terms, many of them confusing, are used in patent and open literature to describe the "amount" of cationic charge on the cationic polymer. Table 2 lists three of the most common terms. It is important to distinguish cationic nitrogen from other nonionic nitrogen forms in these polymers. For example, both Kjeldahl and combustion analysis, which can report both cationic nitrogen and other nitrogen combinations, can be applied to Polyquaternium-7. However, only cationic nitrogen is important in these charge calculations. It is also important to be aware that some nitrogen-containing monomers are cationic if the formulation pH is low enough to protonate the nitrogen group. As an example, Polyquaternium-16 consists of a quaternary vinyl imidazol monomer bonded to the polymer backbone through a tertiary vinyl amine. While quaternary nitrogen is almost always positively charged, tertiary nitrogen may or may not be positively charged depending on the pH of the formulation. When calculating the charge on this polymer, the formulator should consider that the tertiary nitrogen can contribute to the polymer's cationic nature. Cationic substitution (CS) is a dimensionless descriptor that tells the formulator something about the moles of cationic groups per mole of polymer. Charge density is calculated from CS and is defined as milliequivalents per gram (mEq/g). It expresses the amount of cationic charge per gram of polymer. A polymer with charge density between 3-6 is highly charged. Poly(ethylenimine) is reported to have a charge density of approximately 20 at pH 4.5. Polymers with charge density below 1.0 are lightly cationic. Few studies have been conducted on the deposition of synthetic polymers onto proteinaceous substrates. The most successful methods for measuring this are radioactive labeling and electrokinetic measurements.

Table 1. Structure, Chemical Name and INCI Nomenclature for Various Synthetic, Cationic, Conditioning Polymers

Radioactive labeling is quite effective, but the use of radioactive polymers is not recommended in most research institutions. Signal suppression effects from amyl and primary nitrogens prevent various spectroscopic methods that include hair and skin polypeptide proteins. Any technical method used to measure deposition must be able to exclude results from these proteinaceous nitrogens.

Despite these quantitative difficulties, there remains little question that cationic polymers have a strong effect on skin and hair. More subjective analyses show that the deposition of cationic polymers on hair and skin provides improvements in the structure and appearance of these surfaces. However, these techniques cannot, for example, distinguish the conditioning effect of one cationic polymer from that of a low molecular weight cationic conditioning agent. Careful control experiments are required to isolate effects solely to the polymer. The cationic charge on a polymer significantly affects its behavior when surfactants are included in formulations. Cationic polymers (members of a broader polymer class called polyelectrolytes) generally interact strongly with anionic surfactants, weakly cationic surfactants, and unpredictably with nonionic and amphoteric surfactants.

Anionic surfactant binds to cationic polymers at concentrations well below the critical micelle concentration (CMC).

The low surfactant concentration at which the polymer and surfactant begin to interact is known as the critical aggregation concentration. As anionic surfactant concentration increases, most of the cationic regions of the polymer become complexed with the surfactant. Increasing charge forces polymer chains to extend and causes the chain to coil. Additionally, developing surfactant micelles promote interpolymer cross-linking. This, together with increasing chain coiling, causes an increase in viscosity. This effect depends on the polymer concentration, which should be in the concentrated regime. With further surfactant addition, neutralization of available cationic charge occurs, and the polymer and surfactant form a complex phase known as a coacervate. Excessively, the polymer/surfactant complex can become water-insoluble and precipitate. These precipitates are called "polyelectrolyte complexes" (PEC). The development of a surfactant/polymer coacervate or PEC depends on other factors, such as the chain length of the surfactant hydrophobes and the presence of electrolytes like salt. The presence of a polymer/surfactant coacervate is thought to be important for the deposition of the polymer onto the anionic surfaces of hair and skin. When a cationic polymer is left on hair as a coacervate, subsequent rinsing has been shown to remove the anionic surfactant more rapidly than the bound polymer. In these electrokinetic measurements, the overall charge of the hair gradually becomes more cationic. Also, it should be kept in mind that when a concentrated anionic surfactant solution containing a dissolved cationic polymer is diluted with water, a coacervate can form. This is the typical manner of application to hair and skin during washing of conditioner shampoos. Deposition occurs during the rinse cycle and is appropriately termed "deposition by dilution."

Polyelectrolyte complexes have found less practical application in personal care because they are essentially water-insoluble.

However, they are still extremely useful, such as in drug delivery vehicles. Recently, for example, a PEC formed between polymers such as Polyquaternium-6 and retinoic acid (Vitamin D), a potent skin-peeling agent, has been shown to help stabilize the unstable vitamin, extend its shelf life, and enhance its potential. Polymer and surfactant concentration are linear in the appearance of coacervate charge neutrality. At surfactant concentrations exceeding those needed to neutralize the cationic polymer, the coacervate disappears and the surfactant generally, though not always, dissolves the polymer. Polymers with higher charge density are more difficult to dissolve, and polyelectrolyte complexes typically do not redissolve. The behavior of cationic polymers in the presence of highly concentrated surfactant solutions, where micelles are no longer spherical but can exist as rod-like and branched structures, has not been adequately addressed. In such a strongly anionic environment, the rheological effect of cationic polymers is probably minimal. Sources 1. Kuhn T. Structure of Scientific Revolutions, 2nd ed. Chicago: University Press, 1970. 2. Doi Y. Microbial Polyesters. New York: VCH Publishers, 1990. 3. Burdick DL, Leffler WL. Petrochemicals in Nontechnical Language, 2nd ed. Tulsa, OK: PennWell Publishing, 1990. 4. Volk H, Friedrich RE. Polyacrylamide. in: Davidson RL, ed. Handbook of Water-Soluble Gums and Resins. New York: McGraw-Hill, 1980: Chapter 16, 1-19.     Ömer Arif Kural Foreign Trade Manager Cosmer Kimya A.Ş.