Guar Hydroxypropyltrimonium Chloride

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Guar hydroxypropyltrimonium chloride (GHPTC) is one of the most widely used cationic polymers in the personal care industry. According to the US Food and Drug Administration’s Voluntary Cosmetic Registration Program (VCRP), the use of GHPTC has been documented in 751 products, ranking it third behind polyquaternium-10 (1,446 reported uses) and polyquaternium-7 (1,041 reported uses).^{1, 2} GHTPC is employed primarily as a conditioning agent in shampoos and body washes. In many of these rinse-off products, it is also a critical deposition aid for the delivery of active ingredients and hydrophobic conditioning agents, e.g., silicones, to the skin and hair.

Chemistry and Manufacture

GHPTC, shown in Figure 1, is a “seminatural,” i.e., naturally derived but synthetically modified, cationic polysaccharide comprised of a nonionic guar gum backbone that is functionalized with ether-linked 2-hydroxypropyltrimethylammonium chloride groups.³ These quaternary ammonium moieties render GHPTC a non-pH-responsive cationic polyelectrolyte. The degree of substitution (DS) values for personal care grades of GHPTC typically range from 0.07 to 0.2 cationic groups per anhydro sugar unit (0.21–0.6 cationic groups per galactomannan repeat unit) corresponding to cationic charge density values of 0.4–1.1 milliequivalents of cationic charge per gram of polymer.^{4, 5–8} The cationic charge density may also be reported in terms of nitrogen content (% N), which typically ranges from 1.2–2.1% N. GHPTC usually exhibits relatively high molecular weight (MW) values ranging from 5.0 × 105 g/mol to 2.0 × 106 g/mol;⁹ however, lower molecular weight grades (0.5–2.0 × 105 g/mol) based on oxidatively and/ or enzymatically degraded GHPTC are available.^{10, 11}

Guar gum: The precursor to GHPTC is guar gum, a naturally occurring carbohydrate polymer (see Figure 1) derived from the endosperms of Cyamopsis tetragonoloba (guar) beans.^{3, 9, 12} Isolation of guar gum begins with drying of the guar bean pods, followed by removal and separation of the dried beans from the pods.^{9, 13} The cleaned beans are mechanically split, heated and sifted to separate the endosperms from the hulls and the germ of the beans. The purified endosperms, commonly referred to as “splits,” contain 80–85% guar galactomannan and may be further refined by repeating the process to yield double- or triple-purified splits. The splits may be ground to yield crude guar gum powder but more often they are left intact for ease of processing in subsequent chemical modification operations. Prior to derivatization, the guar splits may be treated with aqueous sodium hydroxide at elevated temperatures and washed with water and/ or aqueous isopropanol solutions to minimize the level of residual proteins and lipids in the guar gum.¹³

Guar gum belongs to the family of polysaccharides known as galactomannans, which are linear polymers of β-(1,4)-linked D-mannose that are substituted with α-(1,6)-linked D-galactose monomeric branches. For guar galactomannan, there is an average of one galactose unit for every 1.⁸ mannose units. Although the branches are randomly distributed along the polymer backbone, the galactomannan repeat unit for guar gum is typically drawn as two mannoses with one galactose branch.⁹ The MW values reported for guar gum range from 0.2–2.0 × 106 g/ mol.^{9, 12 , 14} The polymer is water-soluble and behaves as a slightly stiffened random coil in aqueous solution due to its β-(1,4)-linked backbone and branching.¹⁴ Its relatively high MW and chain stiffness enable guar gum to impart high viscosity to aqueous solutions at relatively low concentrations.

CHPTMAC and EPTMAC: The chlorohydrin compound 3-chloro-2- hydroxypropyltrimethylammonium chloride (CHPTMAC) and its epoxide analog 2,3-epoxypropyltrimethylammonium chloride (EPTMAC) are vital starting materials for the preparation of a wide range of cationic personal care ingredients, including GHPTC, polyquaternium-10, hydroxypropyltrimonium hydrolyzed wheat protein and dozens of other hydroxypropyltrimonium- functionalized polysaccharides and proteins.

CHPTMAC and EPTMAC are cationizing reagents that react with substrates bearing nucleophilic functional groups, such as hydroxyl (–OH) groups and amino (–NR2) groups, to yield cationic materials.¹⁵ CHPTMAC is synthesized via the base-catalyzed reaction of epichlorohydrin with trimethylammonium chloride in aqueous solution, as shown in Figure 2.¹⁶ The chlorohydrin species can be readily converted to EPTMAC by reacting that species with sodium hydroxide (NaOH), producing an equivalent of sodium chloride (NaCl) by-product that is separated from the EPTMAC.¹⁷ Most processes to produce hydroxypropyltrimonium- substituted products utilize the less expensive CHPTMAC, with the EPTMAC being generated in situ by adding an equivalent of base during the course of the reaction. Thus, reactions using CHPTMAC for cationic modification yield one equivalent of chloride salt, usually NaCl, for each cationic group added to the substrate. In applications where excess salt is undesirable or difficult to remove from the reaction product, EPTMAC is the preferred cationizing reagent.

GHTPC synthesis: GHPTC is typically synthesized by treatment of refined guar splits with CHPTMAC; a representative reaction scheme is shown in Figure 3.^{14, 18–19} A slurry of swollen guar splits is formed by dispersing purified splits in either water or mixtures of water and isopropanol. The slurry is heated and stirred as aqueous solutions of NaOH and CHPTMAC are added. In addition to converting the CHPTMAC to the more reactive EPTMAC, the NaOH also catalyzes the etherification reaction by deprotonating the guar –OH groups to form alkoxide ions, which attack the EPTMAC epoxide ring to yield the hydroxypropyltrimonium ethers. Substitution occurs mainly at the C6 positions of the galactomannan repeat units where primary –OH groups are present. At the end of the reaction time, the cationically modified guar splits are filtered and washed several times to remove unreacted cationizing reagent, NaCl and other impurities. Prior to washing and filtration, the guar polymers in the modified splits are usually crosslinked with pH-responsive crosslinking reagents, such as sodium borate at alkaline pH or glyoxal at acidic pH.^18–20 The crosslinking inhibits excessive swelling and gelation of the modified splits, enabling efficient filtration. The crosslinking step is particularly important in modern water-only processes that do not incorporate solvents, e.g., isopropanol, in the washing steps.^19–20 The washed splits are then dried and milled to yield GHPTC as a fine powder.

Properties

GHPTC is typically supplied as a free-flowing, off-white to pale yellow fine powder with a faint amine odor. GHPTC powder has a bulk density of ca. 0.75 g/cm3 and may contain 5–12% w/w water. The aqueous solution properties of GHPTC can vary tremendously depending on the grade of material in question. The high MW grades of GHPTC tend to yield high viscosity solutions (2,000–4,000 cP at 1% w/w),^{4, 6–8} yet lower MW grades are available that provide low viscosity solutions (< 100 cP at 1–10% w/w).^{5, 11} Upon dissolution, most grades of GHPTC powder produce basic solutions with pH values of ^9–11; however, buffered grades are available that dissolve to give pH neutral solutions.⁴ Solutions of GHPTC in water tend to be hazy due to the presence of residual insoluble matter, e.g., fiber, proteins and lipids, from the guar splits.³ Potential impurities in GHPTC may include the aforementioned insolubles, NaCl, residual isopropanol (if it was employed as a process solvent), 2,3-dihydroxypropyltrimethylammonium chloride (a by-product from the EPTMAC hydrolysis side reaction), and trace (ppm) levels of trimethylamine.

Applications and Formulation

GHPTC is a popular conditioning agent for skin and hair due to its outstanding substantivity from rinse-off systems. Its use in leave-on systems is rare, but has been formulated into solutions for wipes, hair styling products and leave-in conditioners. While GHPTC is used mainly in liquid cleansers, cleansing bars have been developed incorporating it for conditioning and improvement of mildness.²¹ High MW, high charge density GHPTC is particularly efficient at forming polymer-surfactant coacervates with anionic surfactants for dilution deposition from shampoos.²² It is the conditioning polymer of choice for deposition of silicones and the antidandruff active zinc pyrithione.²³ The stiffness of the guar backbone also enables high MW GHPTC to help build viscosity in formulations when it is employed at concentrations above the critical overlap concentration, i.e., the concentration required for the onset of intermolecular entanglements. High MW grades are generally the most efficient at building viscosity regardless of charge density. Although many water-soluble polymers can be used to improve lather aesthetics in cleansers, GHPTC is particularly effective in this regard as it thickens foam bubble lamella and viscosifies the fluid in the lamella to provide stable, creamy, lubricious foams with high moduli for superior sensory profiles.

GHPTC is typically employed in formulations at levels ranging from 0.1–1.0% w/w depending on the desired level of conditioning and/or viscosity enhancement. For maximum substantivity to skin and hair, high charge density grades of GHPTC are most desirable; for maximum deposition, GHPTC having both high MW and high charge density is utilized. This ingredient is known to build up on hair, however, so it should be used judiciously in hair care formulas. Lower MW grades of GHPTC are often cited as having less potential for buildup, compared to their high MW analogs.^{5, 11}

Achieving clear formulations with GHPTC can be challenging and typically requires pairing the correct grade of GHPTC with a robust surfactant system containing a high level of anionic surfactant, such as sodium laureth sulfate, sodium lauryl sulfate and/or cocamidopropyl betaine, to completely solubilize the GHPTC-surfactant complex coacervates.

Incorporation of GHPTC powders into formulas is usually achieved by adding the polymer “up front” before mixing in surfactants and other ingredients. It is recommended to slowly add the powder to the aqueous phase with moderate agitation to ensure good dispersion. Alternatively, the GHPTC can be premixed with a nonsolvent, e.g., glycerin, then added to the aqueous phase under agitation. Except for the grades designated as self-hydrating, most GHPTC powders are treated with pH-responsive crosslinking reagents to prevent rapid, uncontrolled hydration during initial dispersion, which could lead to lumping and the formation of “fish eyes.” Thus, once a uniform dispersion of the powder is achieved, it is usually necessary to adjust the pH of the dispersion to break the crosslinks, allowing the polymer to fully hydrate and dissolve. For borate-treated GHPTC, dissolution is achieved by lowering the solution pH to 3.5–5 with an acid, such as citric acid, whereas for glyoxal-treated GHPTC, the solution pH must be raised to ≥ 7 to enable dissolution.

The order of ingredient addition is also important when formulating with GHPTC. Once the GHPTC is fully dissolved, it is recommended to first add the amphoteric and/or nonionic surfactants to the batch before adding the anionic surfactants to prevent the formation of insoluble complexes. GHPTC may form insoluble complexes with anionic polymers, such as polyacrylate rheology modifiers, so compounding processes should be designed to ensure that the addition of any anionic polymers to GHPTC solutions is preceded by the addition of surfactants. In situations where the anionic polymers must be added up front, a side phase of GHPTC solution in water can be prepared and added to the batch after the surfactants have been added.

References

1. Compilation of ingredients used In cosmetics in the United States, First Edn, JE Bailey, ed, Personal Care Products Council, Washington, DC USA (2011)
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3. JV Gruber, Polysaccharide-based polymer in cosmetics, ch 8 in Principles of Polymer Science & Technology in Cosmetics and Personal Care, ED Goddard and JV Gruber, eds, Marcel Dekker Inc.: New York USA (1999) pp 325–389
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