Heat styling reshapes hair rather than coating it, and the change happens inside the strand. Understanding which bonds are involved explains both why a style holds and why it eventually stops holding.
Hair is held in shape by two kinds of bond
The protein chains inside a hair are connected by hydrogen bonds, which are weak and numerous, and by disulphide bonds, which are strong and far fewer in number.
Hydrogen bonds break easily with water or heat and reform as the strand cools or dries. They are what a blow-dry or a curling iron is actually manipulating.
Disulphide bonds hold the permanent shape and require chemistry to alter, which is what a relaxer or a perm is doing. Heat alone is not supposed to touch them.
Why a style sets as it cools
Heat and moisture together break hydrogen bonds and leave the chains free to move. While the hair is hot and pliable it can be pulled into a new configuration.
The bonds reform as temperature drops, locking the strand into whatever shape it is holding at that moment. This is why releasing a curl before it has cooled loses most of it.
The set is temporary because hydrogen bonds respond to ambient moisture too. Humidity supplies water that breaks them again, and the hair returns towards its natural pattern.
Where damage begins
Above a certain temperature the effects stop being reversible. Proteins begin to denature, meaning their folded structure unravels and does not return to its original arrangement.
The temperature at which this starts is lower than most styling tools reach at their maximum settings, which is why tools with adjustable heat exist at all.
Water inside the strand adds a separate problem. Applying high heat to wet hair can turn that water to steam faster than it can escape, forcing the cuticle open.
What the cuticle loses first
The outer cuticle is a layer of overlapping scales that lies flat on healthy hair and reflects light evenly, which is the physical basis of shine.
Repeated heat lifts and eventually fractures those scales. Light scatters instead of reflecting, so the hair looks dull regardless of what is applied to it.
A lifted cuticle also loses moisture faster and tangles more, so damage compounds through the mechanical friction of brushing and washing.
What heat protectants can and cannot do
Heat protectants form a film that slows the rate at which the strand reaches the tool's temperature, spreading the energy more evenly across the surface.
They reduce the peak temperature the interior experiences but do not eliminate it. A protectant does not make a very hot tool safe, it makes a moderate one safer.
Because the damage is cumulative and structural, it cannot be reversed by conditioning. Products can smooth and coat a damaged strand, but the protein change itself is permanent.