Many products currently on the hair care market relate to a woman’s attempt to partially or completely modify her curly, frizzy and tangled hair. Hair straightening and hair relaxing formulations have been invented to achieve the desire for straight hair.
There is a degree of overlap between these two categories of products. The term hair relaxer, according to Du Yung Hsiung, is used to describe products that provide a permanent straightening effect on heavily curled hair, while hair straighteners include all the other formulas with more temporary or lower levels of straightening efficacy.1 This distinction is reflected in their different chemical compositions and use conditions.
Chemistry of Curls
The structure of hair is comprised of α-keratin chains covalently bonded together by the disulfide bonds of cystines. When some ionized thiol groups (-S-) are present in this structure, the covalent link may be involved in a reversible reaction of sulfhydryl-sulfide interchange, resulting in curls.2
If the new sulfide bond is at a lower level of physical stress or without any stress, the reaction is favored toward its direction,3 resulting in the relaxation of the initial stress and in a new, extended keratin chain structure. This reaction, if sufficiently extended, causes hair straightening. Therefore, this interchange favors the breakdown of bonds under stress and the creation of new bonds in equilibrium with the extended system. The number of ionized sulfhydryl groups in the native α-keratin structure is limited at less than one thiol of the cysteine residue for every 10 disulfide bonds of cysteine.
However, even one thiol group may progressively be involved in the exchange of many stressed disulfide bonds and be constantly replaced at each interchange. It is necessary that the weakly acidic thiol groups are in the ionized form, because only the ionic species -S- can start such reaction, while the non-ionized form SH is not reactive. This is why the reactions between disulfide groups and ionized thiols generally need to be conducted in strong alkaline conditions. To speed up the straightening process, more sulfhydryl groups must be created in the system with chemical reactions that are conducted on the disulfide bonds of the keratin chains. Usually, thiol-bearing molecules like thioglycolic acid or other reducing molecules perform such disulphide-thiol exchange reactions with the hair keratin, which lead to hair straightening.
While many cosmetic categories address the general population, the hair care industry is split into general market and ethnic market segments.4 Ethnic hair care is a separate segment of the hair care industry for good reason—excessively curly hair requires different treatments, styling techniques and product formulations than other hair textures. The need to straighten very curly hair is typical of the ethnic hair care market segment.
Hair relaxing, or lanthionization, is a chemical process whereby excessively curly hair is permanently relaxed. The term lanthionization derives from the molecule lanthionine, the monosulfide analog of cysteine that is produced during the treatment. To efficiently relax hair, strong alkaline systems are used with pH 12–13. They rely on the relaxing action of 2–4% w/w sodium, lithium or potassium hydroxide in a o/w or w/o emulsion, the alkali amount depending on the relaxing strength required. In Europe, the hydroxide maximum allowed is 4.5% in professional products and 1.2% in all other hair straighteners/relaxers. The sodium hydroxide based relaxers are called lye relaxers, while all other relaxers are termed no-lye relaxers. No-base relaxers are a subdivision of lye relaxers. They are milder to the scalp and do not require protecting it by oil or butters, while base relaxers require the application of a protective coating layer prior to the relaxer itself, creating a barrier on the skin to avoid excessive chemical aggression. In addition, there are two subdivisions of no-lye relaxers, mix and no-mix, the latter requiring the mixing of the activator into the relaxer cream.
All relaxers types: The physical form of product relaxer formulation is important for its performance, irrespective of it belonging to the lye or no-lye category. A smooth and non-pasty emulsion form is the preferred vehicle to maximize the ease of application without hindering the smoothing process. Fluid creams can envelop the entire hair shaft and perform their action efficiently during the time of application. Oils and emulsifiers in the formula need to be stable in such strong chemical conditions. For this reason, chemically non-reactive petrolatum, mineral oil or, more recently, hydrogenated polydecenes, polyisobutenes or even dimethicones are used in combination with cetyl alcohol as the emulsion stabilizer. The emulsifier choice and amount utilized in the formula are important for the relaxing performance. The emulsifier works as a wetting agent for the hair, and a good wetting of the hair shaft is necessary to speed up the relaxing action onto the keratin chains. In addition, the amount of emulsifier used should be as low as possible to get an immediate emulsion breakdown when the product is distributed onto the hair.
The typically trivial hurdle of formula stabilization is, in this case, more complicated than usual. The high amount of oil phase required for efficient scalp protection and the ionic strength of highly alkaline solutions make stabilization difficult. In addition, the emulsifier works as a wetting agent for the hair. For these two reasons, the stabilizer might be used in formulae at higher concentrations than those strictly necessary for the emulsion stabilization. Generally, propoxylated and ethoxylated stearyl chains are used to alter the solubility and the flow characteristics of the emulsion to enhance stability.
Vegetal butters are used to decrease the degreasing action of all relaxers. A recent hair relaxer launch has incorporated hair fortifying ceramides, which work throughout the straightening process to repair breaks.5 In general, relaxers break the disulfide bonds, eliminate them from the keratin structure and hydrolyze the peptide bonds of keratin.6 The hair is softened by a general disruption of all the chemical bonds and keratin is degraded.
Hydroxyl ions raise the hair cuticle and break salt bridges and other ionic bonds. New lanthionine cross linkages form between the protein chains. Breaking and reforming bonds permanently rearranges the protein structure of curly hair. Severe damage is caused during the relaxing process; therefore, reconstructive products are used to minimize the damaging effect. Proteins, quaternium salts or polymers are used for repair reasons. Of course, thorough elimination of the relaxer trace after the period of application and the use of an acidifying shampoo are suitable steps for quenching any residual reaction on the hair.
Processing/mixing time of emulsions is critical. Though over processing may result in a stable emulsion, it will not break and will be irritating to the scalp with no oil barrier coating.
Ideal viscosity is in the range of 150,000–250,000 mPa•s, and lower value viscosities will run onto the scalp and burn unprotected areas. The viscosity can be tuned by varying the hydrotrope and emulsifier amounts. Wetting agents can speed up the relaxing process, but mineral oil, proteins, quaternium and conditioning agents can also influence it.7 Lye relaxers: As noted, to limit the aggressiveness of these alkaline systems on the scalp, the use of protective oils on the scalp, the ears and the forehead prior to the relaxer application is recommended. These are called base relaxers.8 Formula 1 is the basic formula of a typical lye relaxer with possible ingredient alternatives.
No-lye relaxers: In the attempt to reduce the damage to the skin surface caused by lye relaxers, the less strong alkali guanidine hydroxide has been adopted. Guanidine hydroxide is unstable and is, therefore, formed in situ by combining two products. This category of no-lye relaxers, as mentioned, is referred to as mix relaxers. One formula contains guanidine carbonate while the other is an emulsion with calcium hydroxide, emulsifier, oils and thickeners.9 By double exchange, guanidine hydroxide is formed, while calcium carbonate precipitates in fine crystals, driving the equilibrium toward complete reaction.
No-lye relaxers usually contain 5% calcium hydroxide instead of sodium hydroxide, and are mixed with an aqueous thickened solution of guanidine carbonate at about 25%—depending on the required relaxing strength just before use. Formula 2 is one example of a no-lye relaxer that has been modified.10
No-lye mix relaxers require additional consumer preparation, with the main relaxer cream requiring the addition of a thickened activator. The quest for some conditioning effect after relaxing is provided by the alkali-stable quaternary compound, while the emollient/protective activity is performed by vegetal oils/modified waxes. It is not clear how long these molecules would stand the hydrolysis reaction induced by calcium hydroxide at such high pH. For the same reason of high alkalinity formulae, preservatives are not required.
No-lye, no-mix relaxers that are based on lithium and calcium hydroxide also exist. They represent an attempt to simplify the relaxer preparation and to reduce their chemical aggressiveness by removing sodium hydroxide, i.e., no-lye. The alkalinity is still high in no-lye, no-mix formulations; therefore, the formulator must carefully combine different alkali, suitable vehicles and anti-irritant ingredients—reducing the potential mistake of improper mixing by the consumer and offering a simplified application procedure. In general, calcium ions exhibit lower skin aggression than other alkalines.
The function of hair straighteners is similar to products that add permanent waves, but work in an opposite way—reducing the wave or curl of hair through chemical reactions, with or without the influence of high temperatures. Hair straighteners with ammonium thioglycolate bases (8% thioglycolic acid) are similar in their formulation and mechanism of action to permanent wave formulae. The only significant difference in formulation is that they are usually in the form of thick creams to help hold the hair straight during treatment.11 The pH of these hair straighteners is 7.5–9.5. The adoption of glyceryl thioglycolate, a milder agent than ionic thioglycolates, has been ceased because of its sensitizing action. Usually, the formation of sulfur bonds among the reduced keratin molecules following the treatment is obtained by a mild rinse with diluted hydrogen peroxide at no more than 5–10 %w/w. Sulfur containing compounds like cysteamine, cysteine and cystine can replace thioglycolate for a milder but long-lasting (3–4 months) action. For even softer effects, the action of hydrolyzed proteins or other polymeric film-forming agents is used. Formula 3 is a formulation by Croda that utilizes the softening effects of hydrolyzed proteins. Polymers exhibit smoothing effects that last for a short time, while proteins that are substantive to the hair surface may last 2–3 months.
In Formula 4, the straightening effect is provided by the alkalinity.12 Urea helps swell the keratin fibers. The vehicle is an o/w emulsion with ethoxylated emulsifiers (alcohols, fatty acids) and solid stabilizers/structuring agents (cetyl alcohol and glyceryl stearate). Again, the long-term stability of glyceryl stearate and urea at high pH should be carefully verified.
An example of humidity-resistant smoothing and straightening cream intended for curly/wavy hair without the use of harsh chemicals or heavy oils is provided in Formula 5. This product is a leave-on formula, where a styling polymer provides the straightening effect. This is evident from the importance given to the fixative hydrophilic polymer, as it would have no meaning in a rinse-off formula.
The light emollient effect is due to the volatile silicone and di-isopropyl adipate combination. This is a stabilized suspension rather than a standard emulsion, where the polyacrylate 14 and carbomer provide enough thickening and a high enough yield point that oil phase droplets, thickened by the solid fatty alcohols, remain in stable suspension. Panthenol is added as an additional hair conditioner.
Safety Issues in Straightening
The 1.7–1.8% aqueous solution of formaldehyde was a breakthrough efficient hair straightener but has been eliminated due to safety concerns regarding the high amount of formaldehyde released during application. Methylene glycol, the hydrated form of formaldehyde, poses a similar safety issue, with formaldehyde still being released when heated. Formaldehyde is a component of all existing water-soluble aldehydes, and its hydration does not result in a new compound but rather a chemical species in dynamic equilibrium within an aqueous solution. This can be likened to carbon dioxide being identified as carbonic acid in sparkling water.
In Europe, the use of formaldehyde solutions is allowed up to 0.2% as a preservative, at 5% in nail strengtheners and at 0.1% in oral care, regardless of whether it is in a simple formaldehyde, formaldehyde and water combination or in a polymeric form. When formaldehyde-releasing antibacterial agents are used, products must be labeled as “contains formaldehyde” when its content in the product exceeds 0.025%. The warnings are necessary when formaldehyde is a carcinogen, a strong irritant and a sensitizing molecule. Event 0.2% solutions can release formaldehyde that exceed 0.08 ppm in the air, the World Health Organization’s indoor air quality guideline for short-term exposure. Therefore, the use of concentrated formaldehyde solution in hair straightening is prohibited in the EU. In the U.S., the Cosmetic Ingredient Review has declared that the use of formaldehyde and methylene glycol in hair straightening is forbidden.
A silicone conditioning agenta was recently launched that reduces the damage to hair induced by flat ironing while enhancing straightening due to fiber alignment. In addition, a newly developed blend of the amino acid cystine and proteinsb provides a straightening effect; high molecular weight protein hydrolyzed (100 kDa avg.) rich in cysteine and cystine residues claims to have a conditioning effect, especially in ethnic hair straighteners and relaxers. Moreover, urea and sulfite are added—the first disrupting some covalent bonds in the hair while the second breaks the disulfide bridges. The hair is therefore straightened, and new bonds are reset in the new conformation.
Protein-based straighteners are already present on the market. An at-home spray was developedc by Boca Cosmetics Group containing hydrolyzed keratin that claims to reduce application time, replenish the hair with keratin, and achieve the same straightening effects as salon treatments.
In a recent patent, the minimization of skin and hair damage during a hair relaxing treatment is achieved through a blend of dried herbs added to the alkaline base at a pH higher than 11.13 The blend is comprised of Apocynum cannabinum root, Urtica dioica, Rosmarinus officinalis leaf, Arctium lappa root, Betula alba, Rosa canina fruit, Chondrus crispus, Tussilago farfara flower, Prunus serotina bark, Taraxacum officinale, Sambucus nigra, Equisetum hiemale and Echinacea purpurea. The alkali swells the plant cellulose, and soothing active components are released during treatment.
The authors demonstrated that the break load of the relaxed hair using this treatment is the same for untreated shampooed hair. When reducing irritation with herbs, it should be noted that Zanthoxylum alatum extract was shown to reduce irritation and decrease skin sensitivity following hair dyeing.14
A professional straightenerd developed by the Macadamia Natural Oil Company uses a blend of macadamia oil, quinoa seeds, Gingko biloba and other ingredients to envelop hair strands for a smooth finish in hair straightening. Vegetable oils and butters create an almost insoluble barrier for aqueous fluids, even alkalines, to prevent the deep penetration inside the hair. They also soften the hair and supply protective and anti-irritant vegetal sterols to the skin. Macadamia oil, it should be noted, is also being rediscovered and utilized for its high content of vitamin E.
To create new methods for hair relaxing, Nippon Fine Chemicals Co. disclosed a two-product relaxer kit (see Formula 6) and a method for processing smoother hair.15 The kit is shown to quickly improve hair smoothness and elasticity. The first of the two products contains trialkoxyalkylsilane, an inorganic and an organic acid, a lower alcohol, and a polyalkylene-modified silicone. The second product contains a basic substance and a lower alcohol. After mixing, the blend is heated and applied to the hair to form a coating membrane.
Finally, BASF introduced a blend of potassium silicates, caprylyl/capryl glucoside and glycerin that is said to be designed for ethnic hair caree. Introduced into a hair relaxer, it straightens hair while protecting and strengthening.
- DY Hsiung, Hair Straightening, chapter 59 in the Chemistry and Manufacture of Cosmetics, Second Ed, Volume IV, MG De Navarre, ed, (1975) pp 1157–1158
- RW Burley, Volume D, Physics of wool and other visco-elastic fibres, in Proceedings of the International Wool Textile Research Conference, Australia, Wool Textile Research Laboratories: Australia (1955) p 88
- M Feughelman, Morphology and properties of hair, chapter 1 in Hair and Hair care (Cosmetic Science and Technology Series), Vol 17, DH Johnson, ed, Marcel Dekker Inc., New York, USA (1997) p 84.
- AN Syed, Ethnic hair care products, chapter 9 in Hair and Hair care (Cosmetic Science and Technology Series), Vol 17, Marcel Dekker, New York, USA (1997) pp 235–245
- For ethnic and hard-to-style hair, Parfum Cosmétiques Actualités 197 14 (2007)
- J Jachowicz, Hair damage and attempts to its repair, J Soc Cosmet Chem 38 263–286 (1987)
- S Vermeulen, A Banham and GJ Brooks, Ethnic hair care, Cosm & Toil 117(11) 69–78 (2002)
- P Obukowho, Designing emulsions for relaxer activities, Cosmet & Toil 121(5) 69–74 (2006)
- C Zviak and A Sabbagh, Permanent waving and hair straightening, Chap 6 in The Science of Hair Care, Marcel Dekker, New York, USA 207 (1986)
- Hair care formulary, Cosm & Toil 123(6) 84 (2008)
- RR Wickett, Changing the shape of hair, Chap 5 in Practical Modern Hair Science, T Evans and RR Wickett, eds, Allured Books, Carol Stream, IL USA (5) 181 (2012)
- Gattefossé brochure on Emulium (2010)
- US Pat 8,025,909 B2, Use of herbs in hair relaxation, D Jarvis and L Kelly, assigned to Roux Laboratories Inc. (Sep 27, 2011)
- G Guglielmini and A Cristoni, A Lotion containing Zanthoxylum alatum extract reduces skin irritation, IFSCC-Scancos Conference Proceedings (2001)
- C Fox, Natural hair dyes, self-tanning solutions and other topics: Literature findings, Cosm & Toil, 124(11) 28 (2009)