By ingredient · Sub-chapter 02
The structure of a hair shaft, what breaks it, and what restores it. Protein treatments and bond builders are different tools — here is when each applies.
94 how-to's · Updated 30 April 2026 · Avg. 5 min per piece · Edited by Nelly · Beauty & Style Director
Editor's note
Protein and bond builders both claim to strengthen hair, and both do — but they work on entirely different parts of its architecture. Protein treatments deposit hydrolysed proteins onto and into the hair shaft, reinforcing the cortex from the outside in. Bond builders work at a molecular level, reconnecting disulphide bonds that chemical processes — colour, bleach, relaxers, perms — partially or fully break. Confusing the two leads to over-proteinated hair that feels brittle, or expensive bond treatment on hair that just needed moisture.
Protein Treatments
Hair is approximately 95% keratin — a fibrous structural protein. Protein treatments deposit hydrolysed proteins (broken into smaller chains) onto the hair shaft. The smallest fragments penetrate the cortex; larger ones coat the cuticle. The result is a temporary reinforcement of the hair's mechanical structure. Over-proteinated hair feels hard, stiff, and snaps rather than stretching. The rule: protein for damaged, porous, or chemically treated hair. Healthy, low-porosity hair needs it rarely if at all.
Bond Builders
Hair's cortex is cross-linked by disulphide bonds — sulphur-sulphur connections between adjacent keratin chains. Chemical processes break these bonds as part of their mechanism. Bond builders — Olaplex's bis-aminopropyl diglycol dimaleate being the landmark chemistry — work by reconnecting broken disulphide bonds during or after chemical processing. They do not add protein to the shaft; they restore the shaft's internal cross-link network. The distinction is structural and cannot be substituted.
Other ingredients sub-chapters
Everything we've published on protein and bonds
- The protein–moisture balance: a practical guide
- Over-proteinated hair: how to know, how to fix
- Bond builders: the chemistry, finally explained
- Hydrolysed keratin vs silk amino acids
- Hair porosity and protein: the connection
- Stacking protein and bond treatment correctly