Disulphide Bonds in Hair Care: Science, Damage, and Bond Repair

Quick answer: Understanding Disulphide Bonds in Hair Care is essential for maintaining hair strength and elasticity. Disulphide bonds are covalent chemical linkages formed between cysteine amino acids within keratin, providing hair fibres with their structural stability and curl pattern. While chemical treatments like bleaching, perming, and straightening intentionally or accidentally break these bonds, modern bond-building treatments can cross-link broken protein chains to mitigate damage. However, bond treatments cannot fully reverse biological hair fibre degradation or replace gentle handling.

Key takeaways

• Disulphide bonds are strong covalent bonds that give the hair shaft roughly one-third of its overall tensile strength.
• Chemical lightening (bleach), perms, and relaxers alter or break disulphide bonds, turning cystine into cysteic acid or lanthionine.
• Unlike hydrogen or salt bonds, disulphide bonds do not break when exposed to water or normal atmospheric humidity.
• Bond-building cosmetic treatments help cross-link broken protein fragments to reduce breakage, but they do not permanently restore virgin hair architecture.
• Protecting disulphide bonds requires moderating chemical processing times, lowering heat tool temperatures, and maintaining a balanced hair care routine.

Disulphide Bonds in Hair Care: what the evidence shows

Disulphide bonds represent the primary chemical foundation of hair fibre integrity. The hair shaft consists of roughly 65 to 95 percent protein by weight, predominantly alpha-keratin. Keratin contains unusually high concentrations of the sulphur-rich amino acid cysteine (around 7 to 20 percent depending on the cortex segment). When two cysteine residues oxidise, they form a covalent bond known as cystine. This disulphide cross-link binds neighbouring polypeptide chains together like the rungs of a twisted ladder.

Researchers categorise hair bonds into three main types:

1. Hydrogen bonds: Weak physical bonds broken by water and reset as hair dries. They account for temporary styling flexibility.
2. Salt (ionic) bonds: Weak physical bonds created by attraction between positive and negative amino acid side chains. They depend heavily on pH and break in excessively acidic or alkaline environments.
3. Disulphide bonds: Extremely tough covalent chemical bonds that resist physical stretching, water, and moderate heat.

Because disulphide bonds sit deep inside the hair cortex, they dictate whether hair is straight, wavy, or coily. The physical arrangement and angle at which these bonds form inside the hair follicle determine the hair's natural curvature. When intact, they prevent the hair fibre from stretching beyond its elastic limit and snapping during daily styling.

How chemical treatments break disulphide bonds

Chemical alteration of hair relies almost entirely on manipulating disulphide bonds. When you colour, perm, or relax hair, the formulation must penetrate the protective cuticle layer and enter the cortex to rearrange these structural links.

Bleaching and oxidative colouring

Lightening hair requires alkaline agents (such as ammonium hydroxide or ethanolamine) to lift the cuticle scales and allow hydrogen peroxide to oxidise natural melanin pigments. Unfortunately, hydrogen peroxide cannot target melanin exclusively. It also oxidises cystine cross-links.

During bleaching, the disulphide bond splits. Instead of reverting to stable cysteine, the sulphur atoms oxidise irreversibly into cysteic acid. Once cysteic acid forms, the bond cannot reform naturally. High concentrations of cysteic acid make hair porous, brittle, and prone to breaking when wet. Studies indicate that aggressive bleaching can destroy up to 15 to 45 percent of hair's natural disulphide cross-links.

Perming and chemical straightening

Permanent waving and relaxing work by deliberately severing disulphide bonds to alter the fibre shape.

• Permanent waving (perms): Ammonium thioglycolate acts as a reducing agent, donating hydrogen atoms to break roughly 30 percent of the disulphide bonds. Hair is wrapped around rods to shift the alignment of polypeptide chains. A neutraliser (typically dilute hydrogen peroxide) is applied to oxidise the bonds back together in their new positions. Not all bonds reform, leaving residual weakness.
• Relaxers (hydroxide-based): Sodium hydroxide, calcium hydroxide, or guanidine hydroxide break disulphide bonds through a process called lanthionisation. Hydroxide ions remove a sulphur atom from cystine, converting the disulphide linkage into a non-reducible, single-sulphur lanthionine cross-link. This creates a permanently straight fibre, but it leaves the shaft with significantly lower elasticity.

Bond Type	Bond Mechanism	What Breaks It?	Reversibility	Impact on Hair
Disulphide Bond	Covalent (sulphur-to-sulphur)	Bleach, perms, relaxers, extreme heat (>220°C)	Partially reparable via synthetic cross-linkers	Tensile strength, curl pattern, fracture resistance
Salt (Ionic) Bond	Electrostatic (pH-dependent)	Extreme pH shifts (below 3.5 or above 8.0)	Completely reversible when pH normalises	Elasticity, temporary fibre rigidity
Hydrogen Bond	Physical (dipole attraction)	Water, sweat, ambient humidity	Completely reversible upon drying and cooling	Temporary shape, blowout memory, styling flexibility

What the evidence does not show about bond builders

In recent years, bond-building products have transformed salon services and home routines. These treatments use active ingredients (such as bis-aminopropyl diglycol dimaleate, maleic acid, or plant-derived cross-linking complexes) designed to bridge the gap between broken cysteine groups.

While the scientific literature confirms that bond builders reduce mechanical breakage during bleaching, there are key misconceptions:

• They do not regenerate virgin hair: Bond repair formulations deposit cross-linking molecules that link free thiol groups. They do not rebuild the exact native biological architecture of virgin keratin.
• They cannot prevent all damage: Adding a bond-building additive to bleach does not make hair invulnerable. Leaving bleach on the hair too long will still cause extensive structural degradation.
• They do not reconnect cysteic acid: Once sulphur has fully oxidised into cysteic acid, standard bond builders cannot reverse that chemical state. They primarily work on lingering, partially cleaved thiol groups or provide secondary matrix reinforcement.

Recognising compromised disulphide bonds

When disulphide bonds degrade, the structural symptoms are immediate and observable. Because the cortex loses its internal scaffolding, the fibre behaves differently under tension:

• Spongy texture when wet: Hair feels mushy, slimy, or gummy after washing because water floods the damaged, porous cortex.
• Loss of elasticity: Healthy hair stretches up to 30 percent of its length when wet and returns to its original shape. Hair with broken disulphide bonds either stretches excessively without bouncing back, or snaps under minimal tension.
• Curl pattern distortion: Natural curls or waves become limp, loose, or irregularly straight at the mid-lengths and ends.
• High porosity: Damaged hair absorbs water instantly, takes hours to dry, and loses moisture just as quickly, resulting in chronic frizz.
• Fibre breakage: The hair snaps during light detangling or styling, leaving short, split, white-tipped fragments.

Practical strategies for protecting disulphide bonds

Preserving disulphide bonds requires a combined approach of chemical moderation, mechanical care, and functional topicals.

1. Moderate chemical processing

• Extend processing intervals: Space highlighting or bleaching sessions at least 8 to 12 weeks apart to avoid overlapping previously lightened sections.
• Select lower peroxide volumes: Opt for 10-volume or 20-volume developer where possible. High-volume developers (30 or 40 volume) accelerate melanin breakdown at the cost of catastrophic cystine oxidation.
• Monitor processing time strictly: Never leave alkaline lighteners or perming chemicals on the hair longer than the manufacturer guidelines.

2. Guard against extreme heat

High heat degrades hair proteins through thermal decomposition. Temperatures above 220°C cause keratin to denature, transforming helical proteins into disordered beta-sheet structures and degrading disulphide linkages.

• Keep heated styling tools set below 185°C.
• Always apply a protective cosmetic barrier formulated to buffer thermal transfer.

3. Maintain balanced topical hair care

Healthy hair requires a balance between internal cortex strength and external lipid protection.

• Use bond-strengthening treatments containing multifunctional amino acids and dicarboxylic acids once weekly or fortnightly.
• Incorporate gentle cleansing formulations that respect the scalp and cuticle. For example, Watermans offers products formulated with natural hydrolysed proteins, biotin, and rosemary that support a balanced scalp environment and help reinforce the surface of fragile hair strands.
• Follow chemical services with acidic conditioning rinses (pH 3.5 to 5.0) to flatten the cuticle scales, which helps seal moisture inside and shields internal cortex bonds from environmental wear.

When breakage is mistaken for other conditions

It is critical to distinguish between broken disulphide bonds and medical hair shedding. Broken bonds cause hair breakage (fibre fracture along the shaft), which leaves short pieces of hair without a white bulb.

In contrast, hair shedding (such as telogen effluvium) or alopecia (such as androgenetic alopecia or alopecia areata) stems from the hair follicle in the scalp. If you notice uniform thinning across your crown, circular bald patches, or hairs falling out from the root with a small white bulb attached, the root cause is follicular rather than structural fibre damage. Bond treatments will not resolve scalp-level shedding or genetic thinning.

Frequently asked questions about Disulphide Bonds in Hair Care

What are disulphide bonds in hair care?

Disulphide bonds are strong covalent chemical bonds formed between sulphur atoms in cysteine amino acids. In hair care, they provide the structural backbone of keratin, giving the hair shaft its tensile strength, mechanical resilience, and natural wave pattern. When broken by chemical services like bleaching or perming, hair becomes fragile, overly stretchy, and prone to breaking.

Can broken disulphide bonds ever be permanently fixed?

Broken disulphide bonds cannot be permanently re-fused back to their original virgin state. While bond-building treatments can bridge broken chemical groups and significantly improve hair strength, these bonds are not identical to native biological bonds and can gradually wear away with repeated washing, heat, and physical stress.

How does bleach damage disulphide bonds?

Bleach uses alkaline agents to lift the hair cuticle and hydrogen peroxide to oxidise melanin pigments. In the process, the peroxide inadvertently oxidises cystine (disulphide) bonds, splitting them and creating cysteic acid. Because cysteic acid cannot reform natural bonds, the hair cortex loses structural strength and becomes porous and weak.

Do bond builders replace deep conditioning treatments?

No. Bond builders work deep in the cortex to link broken protein chains, while conditioning treatments deposit lipids, fatty alcohols, and emollients on the outer cuticle. Healthy hair needs both internal structural integrity (from intact or cross-linked bonds) and surface lubrication to prevent friction, tangles, and dehydration.

Can heat styling tools break disulphide bonds?

Yes. Excessive heat, particularly above 220°C, causes thermal degradation of keratin proteins. This denatures the hair's structural matrix, oxidises cystine, and weakens disulphide bonds. Regular use of flat irons at high temperatures without heat protection leads to rapid mechanical breakage and loss of natural curl definition.

Safeguarding hair strength and integrity

Achieving healthy, resilient hair requires a fundamental appreciation of Disulphide Bonds in Hair Care. These microscopic sulphur linkages are the unsung heroes of hair strength, defining curl structure and shielding hair fibres from daily mechanical stress. While modern cross-linking treatments offer an impressive safety net for colour-treated hair, they are not an invitation to over-process. By respecting the chemical limits of your hair, moderating heat exposure, and incorporating balanced, protein-supportive care, you can maintain strong, flexible, and vibrant hair from the inside out.

Sources and references

• Robbins, C. R. (2012). Chemical and Physical Behavior of Human Hair (5th ed.). Springer Science & Business Media.
• Cruz, C. F., et al. (2016). "Human hair characteristics: A review." International Journal of Cosmetic Science, 38(3), 227-235.
• Popescu, C., & Wortmann, F. J. (2010). "The hair protein chemistry and its role in cosmetic science." Cosmetic Science and Technology: Theoretical Principles and Applications, 327-340.
• Dawber, R. (1996). "Hair: its structure and response to cosmetic preparations." Clinics in Dermatology, 14(1), 105-112.
• Federal Institute for Risk Assessment (BfR) / Scientific Committee on Consumer Safety (SCCS). (2018). Opinion on the safety of cosmetic ingredients: Hair relaxers and chemical waves.

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Find these ingredients in Watermans products

• Biotin: you will find this ingredient in our Hair Growth Shampoo.