Article By Dr. Philippa Cranwell Technical Content Creator at TRI Princeton
Based on a TRI Talk: 'Protecting Hair from Styling Damage', Dr Paul Cornwell, 25th October 2023
Heat has been used to style hair for millennia. However, when we heat-style our hair we can cause lasting damage. This short 101 outlines what happens inside our hair when we heat style it, how heat styling can damage our hair, what heat protection products do to prevent hair damage, and how TRI can help you support heat protection claims.
Historical evidence of heat styling
Hair styling has been undertaken by women for thousands of years, with evidence found that women in Ancient Rome used a crude curling iron, known as a calamistrum, which consisted of a hollow metal cylinder and a smaller, solid, inner cylinder. Hair was wrapped around the inner cylinder and the outer cylinder, heated on the fire, was placed on top. The use of fire for styling is, thankfully, no longer a requirement, and in modern times a range of electrical heating appliances are available including straightening irons, heated curlers and blow-dryers, all of which offer an easy and effective way to change the shape and style of your hair.
What happens when we style hair?
It is well known that if human hair is wetted with water, then held in a given shape when drying, it will tend to stay in that shape for a reasonable period. For example, straight hair can be dried with a wave or curl, and curly hair can be dried straight. Although the chemical processes occurring inside the hair during styling are complex, most hair scientists agree that removal of water from hair proteins encourages more protein–protein hydrogen-bonding, leading to a stiffening of the overall hair structure. Research suggests that, in fact, hair goes through a pseudo-glass transition as the proteins dehydrate, changing from a plasticised, more fluid state to a solid, more glassy state. It is the solid state that is needed to hold the hair in the desired shape. The influx of water into the hair from a humid environment, or if you’re really unlucky from rain, will take hair back over the pseudo-glass transition, soften the structure, and release the style-hold.
The effectiveness of styling processes depends on several factors including temperature, time and the use of styling products. Higher temperatures have been proven to give better style hold, reducing the rate of style decay and the extent of style loss. Popescu suggests that the optimum temperature for styling dry hair is 180 °C, with evidence of hair damage above this temperature. The effectiveness of the styling process also depends on the time over which the hair set in its new shape, something that is not lost on women who wear their hair in rollers all day in preparation for a night out! Finally, the use of styling polymers can help fix hair in shape for longer.
What happens when you heat hair?
When dry hair is heated to between 50 and 120 °C, the main observation is loss of water from the structure. Heating from 120 – 200 °C causes some chemical cross-linking of hair fibers and yellowing linked to protein oxidation. At temperatures over 220 °C, keratin filament denaturation and pyrolysis can occur, which can lead to visual and mechanical changes e.g. charring and roughness.
Heating wet hair creates further problems, especially if the temperature increase of the hair is rapid, as is seen, for example, with straightening irons. In this situation there further physical damage is inflicted onto the hair:
Cuticle cracking and damage are caused as the drying cuticle is stretched over a wet and expanded cortex;
Bubble fractures can be created as steam rapidly escapes the hair and bursts out of the cuticle;
Physical breakdown of structures inside the medulla and cortex can be caused by steam escaping.
A question commonly asked is, ‘at what temperature should I heat style my hair?’. Single fiber tensile tests at TRI on various commercially available straightening irons with dry hair have shown that straightening irons reaching temperatures over 200 °C tend to cause irreversible mechanical damage, Figure 2. Work by Popescu, in good agreement with this, also shows that mechanical properties in hair start to rapidly change when dry hair is heated above 180 °C. Interestingly, hair damage starts to kick-in for wet hair at 160 °C, confirming that the rapid escape of water from wet hair during heat styling is damaging. It should be noted that in our experience at TRI, the temperatures used by blow driers (<70 °C) are not high enough to cause major hair damage. Protection from damage from blow drying is, therefore, not really an issue.
Mechanisms of action of heat protection products
There are a number of approaches to protecting the hair from heat styling damage. This short section will take a look at each, giving insight into the scientific rationale for their protection claims.
1. Dissipation of heat with water
In this mode of protection, hair is treated with a water-based styling spray prior to exposure to straightening irons. The reason most commonly cited for this providing protection is that water has a very large specific heat capacity (Cp; 4181 J kg–1 K–1; for comparison the Cp of copper is 390 J kg–1 K–1), i.e. it takes 4181 J to heat 1 liter (or 1 kg) of water by 1 °C (or 1 K). This means that a significant proportion of the heat energy to which the hair is exposed is used to heat the water rather than the hair fiber, reducing the overall temperature of the hair and providing a degree of protection. However, care should be taken as it has been repeatedly shown that the combination of wet hair and heat can cause significant damage to hair fibers. The fast dissipation of water can also make nasty crackling noises!
2. Alteration of heat transfer to the fibers
For this approach, hair is either treated with: (a) a product that promotes cohesive forces between hair fibers, causing them to aggregate, or bundle; or, (b) treated with a product that inhibits, directly, heat transfer to the hair fiber.
It has been suggested that when in a bundle, the hair fibers in the middle are more protected from heat compared with those on the outside; this can be thought of as the inverse to a huddle of penguins in the Antarctic, i.e. bundling hairs keeps them cooler. In support of this it has also been shown that temperature rises more quickly in single hair fibers than in a bundle when blow drying.
Treatment with a polymer is reported to give some protection to hair. This is likely to be due to the polymer providing a physical barrier (albeit very thin) between the hair fiber and styling iron. Two pieces of research go into more detail in relation to this, the first from a team at International Specialty Products looking at the impact of heat styling polymers on Asian hair, and one presented at the IFSCC 2022 by Dow. However, more research is needed in this area to draw definitive conclusions.
3. Surface conditioning effects
Here, conditioning agents are used to ensure that the hair feels soft and healthy after styling. In this case, it is not always entirely clear if the products themselves are providing any protection from the heat treatment or are offsetting any damage by the straightening irons, for example by mitigating the effects of cuticle damage.
Supporting heat protection claims at TRI
Ironically, the most useful technique for supporting heat protection claims is the thermal analysis technique, DSC. DSC involves the heating of hair samples and recording the thermal transitions that take place as the sample temperatures increase. At TRI we measure both the temperature and the magnitude of the keratin denaturation transition in wet hair fibers, which gives a precise measure of the structural integrity of the proteins inside the hair.
In heat protection studies we measure the changes in the keratin denaturation peak because of heat styling damage, and the effects of heat protection treatments. Figure 3 shows typical data where it can be seen that the keratin denaturation peak (Td) reduces after flat ironing, indicating hair damage. It can also be seen that the damage is reduced by treatment with a heat protection spray, although hair is still damaged when compared with non-heated hair.
Our wealth of experience at TRI has shown DSC to be a very sensitive technique that can detect small levels of heat protection. For this reason, it is often our method of choice for clients. However, the drawback of DSC is that any effects seen using this method do not easily translate to consumer-perceptible levels of hair damage, and that the degree of change in the keratin denaturation transition does not, for example, translate directly to hair breakage. DSC, therefore, whilst very sensitive, can only give a yes/no answer: either a treatment significantly reduces heat styling damage or it does not.
Automated repeated grooming (hair breakage) measurements, in contrast to DSC, are less sensitive to subtle changes in heat damage, but do relate more obviously to the consumer perception of hair damage and can be used for quantitative claims. In these experiments dry hair tresses are repeatedly brushed and the broken hairs collected and counted. Figure 4 shows typical data collected for a heat protection experiment, where it is clear that the number of hairs broken by brushing increases after using straightening irons, and that an applied treatment reduces hair breakage significantly.
At TRI we believe that repeated grooming is especially good at showing reduced breakage when the treatment products have a surface conditioning effect that reduces both friction and tangling. This technique is also useful because it can be used for consumer-relevant quantitative claims around hair breakage (e.g. X% less breakage from heat styling). The disadvantages of repeated grooming, however, are that it is less sensitive to subtle changes than DSC and that surface conditioning effects can sometimes mask heat protection in the fiber. Finally, care should be taken to ensure that any heat protection treatment being tested with repeated grooming does not increase hair friction or tangling, from incorporated styling polymers, for example. In this instance, these treatments would not be suitable for this test.
Heat styling is a very effective method of reversibly changing your hair style and relies upon hair undergoing a pseudo glass-transition to a more solid state.
Although using a hair dryer does not lead to detectable damage, the use of straightening irons, particularly when hair is wet, can lead to significant damage.
The use of a heat protection spray goes some way towards damage mitigation, with a number of modes of action suggested. However, further research in this area, particularly in relation to the use of heat protecting polymers, is needed.
The two main techniques for supporting claims in relation to heat protection and heat damage are differential scanning calorimetry (DSC) and automated repeated grooming (hair breakage).