How Do Chemical Relaxers Work? | Unpacking the Chemistry

Understanding the precise chemical reactions involved in hair relaxation offers valuable insight into cosmetic science and hair care. We can explore the science behind how these products achieve their effects, breaking down the molecular changes that lead to straightened hair.

The Fundamental Structure of Hair

Hair’s strength and shape originate from its intricate protein structure. The primary protein in hair is keratin, a fibrous protein organized into complex chains.

• Keratin Chains: These long protein chains are rich in the amino acid cysteine, which contains sulfur atoms.
• Disulfide Bonds: Cysteine residues on adjacent keratin chains form strong covalent linkages known as disulfide bonds. These bonds are the primary determinants of hair’s curl pattern, elasticity, and strength. More disulfide bonds in specific arrangements lead to curlier hair.
• Peptide Bonds: These are the backbone bonds linking amino acids within each keratin chain, providing structural integrity. Relaxers do not typically break these bonds.

The hair strand itself consists of three main layers: the cuticle, cortex, and medulla. The cortex, located beneath the protective cuticle, houses the keratin proteins and their disulfide bonds, making it the primary target for chemical relaxers.

The Chemical Goal: Breaking and Reforming Bonds

The core principle of chemical hair relaxation involves breaking a significant number of these disulfide bonds, allowing the keratin chains to rearrange into a straighter configuration. Once rearranged, the hair is then typically neutralized to reform new bonds, stabilizing the straightened shape.

This process is irreversible for the treated hair, meaning the altered disulfide bonds retain their new shape. New hair growth from the scalp will, of course, retain its natural curl pattern.

Lye Relaxers: The Hydroxide Mechanism

Lye relaxers, also known as sodium hydroxide relaxers, are among the oldest and most potent forms of chemical hair straighteners. They operate through a highly alkaline chemical reaction.

Sodium Hydroxide (Lye)

Sodium hydroxide (NaOH) is a strong base. When applied to hair, the hydroxide ions (OH-) penetrate the cuticle and reach the cortex. Here, they attack the sulfur atoms within the disulfide bonds.

1. Disulfide Bond Cleavage: The hydroxide ion removes a hydrogen atom from a cysteine residue, making it susceptible to attack. The disulfide bond (R-S-S-R) is then broken.
2. Lanthionization: A unique and irreversible reaction occurs where one sulfur atom is removed, and a new, stable lanthionine bond (R-S-CH2-CH2-R) forms between two protein chains. This bond is more stable than a disulfide bond and cannot be reformed into its original state. This is why lye-relaxed hair cannot revert to its original curl pattern.

The high pH of lye relaxers (typically 12-14) causes the hair cuticle to swell significantly, allowing for deep penetration of the active ingredients. This high alkalinity also makes them highly effective but necessitates careful application to minimize scalp irritation and hair damage.

Application and Neutralization

Lye relaxers are applied to dry, unwashed hair, often with a protective base applied to the scalp. The product is smoothed onto the hair, and the processing time is carefully monitored. Over-processing can lead to severe damage.

Neutralization is a critical step. After thorough rinsing with warm water, an acidic neutralizing shampoo is applied. This shampoo serves two purposes:

• It stops the chemical reaction by lowering the hair’s pH, deactivating any remaining hydroxide ions.
• It helps to close the hair cuticle, smoothing the hair shaft and restoring some of the hair’s natural acidity.

No-Lye Relaxers: Alternatives to Sodium Hydroxide

No-lye relaxers emerged as a milder alternative, aiming to reduce the scalp irritation associated with sodium hydroxide. While often perceived as less damaging, they still operate through a similar alkaline mechanism.

Guanidine Hydroxide Systems

The most common type of no-lye relaxer uses guanidine hydroxide as its active ingredient. This is typically a two-part system, where calcium hydroxide (a base) is mixed with guanidine carbonate (a salt) immediately before use. This mixing reaction creates guanidine hydroxide.

• Mechanism: Guanidine hydroxide, like sodium hydroxide, is a strong base. Its hydroxide ions attack and break disulfide bonds through a process that also leads to lanthionization.
• pH Level: While still highly alkaline (typically pH 9-11), it is generally less caustic than sodium hydroxide, which can translate to less immediate scalp burning for some individuals.
• Calcium Buildup: A notable side effect of guanidine hydroxide relaxers is the potential for calcium buildup on the hair shaft. This can lead to dryness, brittleness, and a dull appearance over time. Chelating shampoos are often recommended to address this.

Other Hydroxide-Based Systems

Other no-lye relaxers employ different hydroxide compounds, such as lithium hydroxide or potassium hydroxide. These also function by generating hydroxide ions to break disulfide bonds and induce lanthionization, similar to sodium hydroxide, but with varying degrees of alkalinity and potential side effects.

The Reduction-Oxidation (Thio) Method

While hydroxide relaxers dominate the market for permanent straightening, another chemical approach uses the reduction-oxidation (redox) method, primarily with ammonium thioglycolate. This method is common in permanent waving (perms) but can also be adapted for straightening.

Ammonium Thioglycolate

Ammonium thioglycolate is an alkaline reducing agent. Its mechanism differs fundamentally from hydroxide relaxers:

1. Reduction: The thioglycolate ion attacks the disulfide bonds, breaking them by adding hydrogen atoms. This is a reduction reaction. The hair’s natural curl is temporarily relaxed as the keratin chains can move freely.
2. Reshaping: The hair is then physically straightened and held in the desired configuration.
3. Oxidation (Neutralization): A neutralizer, typically hydrogen peroxide, is applied. This is an oxidation reaction that re-forms new disulfide bonds in the hair’s new, straightened shape.

The key distinction is that thioglycolate systems break and then re-form disulfide bonds, whereas hydroxide systems break disulfide bonds and form irreversible lanthionine bonds. This means that, theoretically, hair treated with thioglycolate could, under certain conditions, revert more towards its original state if the new bonds are not properly set or are later broken.

The Stages of Chemical Relaxation

The application of chemical relaxers follows a structured process to achieve straightening while managing potential damage.

Pre-Treatment and Application

Before applying the relaxer, the scalp is often protected with a petroleum-based product to create a barrier against the caustic chemicals. A strand test may be performed on a small section of hair to determine the appropriate processing time and assess hair’s tolerance to the chemical. The U.S. Food and Drug Administration provides guidance on cosmetic product safety, including hair relaxers.

The relaxer cream is then applied quickly and evenly, typically starting from the back of the head and moving towards the front, avoiding previously relaxed hair if performing a touch-up.

Processing Time

The processing time is crucial and depends on hair type, texture, desired straightness, and the relaxer’s strength. Fine or chemically treated hair processes faster than coarse, virgin hair. The product is left on just long enough to break the necessary disulfide bonds without causing excessive damage. Overlapping relaxer onto already relaxed hair or leaving it on too long can lead to severe breakage and scalp burns. The National Institutes of Health conducts research related to hair health and chemical exposures.

Rinsing and Neutralization

Once the processing time is complete, the hair is thoroughly rinsed with warm water to remove all traces of the relaxer cream. This rinsing is critical to prevent further chemical action. Following rinsing, a neutralizing shampoo is applied. This shampoo is typically acidic, which serves to halt the chemical reaction and restore the hair’s pH balance, helping to close the cuticle and reduce swelling.

Understanding Hair Damage and Aftercare

Chemical relaxation, by its nature, involves significant chemical alteration to the hair’s structure. This process can lead to various forms of damage.

• Protein Loss: The strong alkaline environment can degrade some keratin proteins, weakening the hair structure.
• Cuticle Damage: The lifting and swelling of the cuticle during processing can make the hair more porous and susceptible to moisture loss and external damage.
• Reduced Elasticity: The permanent alteration of disulfide bonds reduces the hair’s natural elasticity, making it more prone to breakage when stretched.

Proper aftercare is essential to maintain the health of relaxed hair. This includes regular deep conditioning treatments to replenish moisture, protein treatments to strengthen the hair, and minimizing heat styling. Avoiding harsh shampoos and styling products also helps preserve hair integrity.