Arguably the hottest topic (pardon the pun) throughout the hair care world in the past five years has been heat protection. This new consumer proposition is likely in response to an increased incidence in the use of heat styling devices, which has accompanied fashion trends toward very straight hair styles. In addition to the creation of desired new styles, straightening irons produce a number of other short-term hair benefits. Freshly heat-straightened hair feels soft and smooth; possessing a high level of shine; frizz is minimized1 and the newly styled hair moves in an attractive, flowing motion. Only later, when these benefits wear off, do the cumulative effects of this rather harsh process materialize—as the hair can become rough, fragile and unruly.
The damaging nature of this process appears to be recognized by consumers; yet for many, the benefits overwhelmingly outweigh the negatives—and the onus for their actions is passed on to product manufacturers in terms of demands for products, which will “protect” the hair during this practice.
The Science of Heat Styling
Heat styling makes use of what is commonly termed the water-set process. In short, water is a plasticizer for hair and accordingly its removal creates additional internal structuring, which is often sufficient to anchor temporary styles. Ideally these styles would last until the hair is re-wetted at the next washing; but in reality, induced changes progressively relax as hair gradually re-adsorbs water from the atmosphere to a level commensurate with the relative humidity of the environment.2
While it may seem that straightening or curling irons need only to attain, or slightly surpass, the boiling point of water to induce these transformations, heat styling devices typically employ considerably higher temperatures, frequently reaching as high as 230°C (≈450°F). It can easily be demonstrated that increased efficacy accompanies higher iron temperatures. Figure 1 shows the shape of ringlets that were created by employing progressively higher curling iron temperatures.
Clearly, tighter curls can be created using higher temperatures—although all conditions are considerably above water’s boiling point. A seemingly logical explanation involves the enhancement in water’s evaporation rate, which would accompany higher iron temperatures.
Differential Scanning Calorimetry (DSC)
Insights into the nature of events occurring during heat treatment can be obtained using Differential Scanning Calorimetry (DSC). Thermodynamics teaches that every chemical or physical transformation is accompanied by a transfer of energy. This energy may be adsorbed in an endothermic event or released during an exothermic process. DSC provides a means of observing and quantifying such events. Figure 2 shows a sample DSC thermogram for hair, which was obtained using a 5°C/min temperature program.
The most prominent feature is a broad downward (endothermic) peak, beginning slightly above 100°C and reaching a maximum drop at around 150°C. This represents the evaporation of water from the hair. As mentioned earlier, water evaporates at 100°C, but these experiments are performed in sealed sample pans, wherein evaporating water builds up in the head space and somewhat retards further evaporation in accordance with Le Chatelier’s Principle. As a result, this transition occurs somewhat above where one may suspect.
The next observable event is a sharp endothermic peak at approximately 230°C, which represents thermal denaturation of hair’s alpha-helical protein structure. Finally, a further peak at 240°C signifies the decomposition temperature of hair protein—a process which is less scientifically detected by the development of an especially objectionable odor in the laboratory. This experiment illustrates why the temperatures employed in commercial heat-styling appliances generally top-out at 230°C.
Thermal Imaging Cameras
The heat settings on commercial devices are termed closed-face temperatures by their manufacturers. These represent the reading obtained when a thermocouple is placed directly between the device’s plates. Yet, the temperature attained by the hair itself is more important to the hair scientist. In an attempt to gain insight, our team used a thermal imaging camera to approximate the temperature of hair during heat straightening. Figure 3 shows an image captured from such an experiment, wherein hair tresses were straightened using an iron with a 230°C closed-face temperature.
Testing consistently showed the hair often attaining temperatures ~15°C less than the iron setting. For example, heat straightening with an iron set at 230°C would produce a maximum hair temperature of approximately 215°C. In short, even when using the highest appliance settings, it would appear that hair temperatures remain appreciably below that which would be necessary for decomposition. Therefore, in theory at least, it may be suspected that such temperatures are not especially worrisome.
Perhaps the most straightforward approach for assessing hair damage involves single-fiber tensile testing experiments.3 The mechanical properties of hair fibers are a direct consequence of their complex internal make-up, and therefore any decline in hair strength is indicative of disruptions to this structure. Figure 4 shows wet-state break stress results from experiments, which investigated the effect of both straightening iron temperatures and the number of heat passes on hair strength. Undoubtedly, the tensile properties of hair can be considerably compromised by these treatments—with higher iron temperatures producing the most damage. Again, these sizable effects arise even though hair apparently remains well below the decomposition temperature.
When heat-treating hair tresses at higher temperatures, it is common to observe volatiles emitted from hair. Consumers often describe this as the hair “smoking.” Such emissions can be theorized as simply water—yet this occurrence persists even in pre-conditioned hair at very low humidity and where the moisture content is minimal. On the other hand, there is no characteristic noxious odor to indicate thermal decomposition.
The commonly held 240°C decomposition temperature for hair specifically refers to its protein content, which represents around 90% of the hair structure; the remaining 10% consists of lipid materials. Moreover, the smaller organic molecules comprising the lipid structures will possess considerably lower decomposition and volatilization temperatures. Therefore, it is theorized that this lipid structure of hair and its components are considerably disrupted by these temperatures—even if the proteins are not. This volatilization or decomposition of lipid species should be detectable by DSC experiments; although it is entirely possible these events are masked beneath the large, broad water evaporation peak (see Figure 2).
In support of this presumption, our team used gas chromatography, coupled with mass spectrometry (GC-MS), to perform headspace analysis after hair samples were heated to differing temperatures in sealed vials. The results showed the presence of a large number of volatile species, even when using temperatures considerably below hair’s conventional decomposition temperature. The identification of these materials is ongoing, but evidently the atmosphere is rich in composition.
A dramatic and startling manifestation of heat damage can be observed when performing repetitive grooming experiments to study hair breakage. This common method was discussed previously4 and involves periodically counting the number of broken fibers generated as a function of prolonged automated grooming. Figure 5 shows results from a study intended to show the effects of heat straightening on hair breakage.
All testing involved grooming 10 replicate hair tresses through 1,000 brush strokes, at a controlled humidity of 60%. The first set of data shows the number of broken fibers for a control set of tresses, which received no heat treatment. Based on results in Figure 4, it would be predicted that heat straightening would leave the hair markedly weaker and more prone to breakage. Yet, the second set of data in Figure 5 shows considerably less breakage in hair, which was heat-straightened with 100 passes of a 230°C iron.
This seemingly unusual occurrence can be explained by recognizing the short-term benefits of heat straightening: with fibers in a highly aligned state, the hair feels smooth, soft and manageable. This alignment produces lower grooming forces, less friction, reduced snagging and tangling, and diminished fatiguing forces—in short, benefits traditionally obtained through the use of conditioning products.
Only after this short-term benefit wore off did the true effect of the heat treatment materialize. The third set of data in Figure 5 shows the marked increase in breakage arising after the hair reverted to something approximating its initial shape. This reversion process was facilitated by exposing the hair to 90% relative humidity for 4 hr. Past work with this method showed the extent of breakage in these experiments is strongly influenced by environmental conditions.5 Therefore, it is necessary to allow sufficient time (over night) for the hair to re-equilibrate at 60% RH before performing further testing.
Finally the fourth set of data in Figure 5 shows the effect of incorporating a conventional, commercially available hair conditioner into this heat-straightening process. In this case, hair tresses were heat-straightened with 90 passes of the iron in the manner described earlier. The hair was then treated with a conditioning product, dried and 10 additional iron passes were applied, bringing the total to 100. This straightened hair was also allowed to revert at an elevated humidity before identical re-equilibration and testing.
The results shown in Figure 5 illustrate the sizable increase in breakage, which can occur as a direct result of the heat-straightening process. However, they also demonstrate the capability for conditioning products to protect this fragile hair from breaking—such that this condition is not realized by the consumer.
Heat styling appliances utilize alarmingly high temperatures, which can have considerable adverse effects on the structure and properties of hair. Nonetheless, for many, the short-term benefits produced by these products outweigh the concerns, and frequent use is still common. Accordingly, there is the demand for products helping to “protect” the hair during such treatments.
A common theme in the present series of articles is the idea that the cosmetics industry is often muddled with the consumer language and scientific language behind the same idea not necessarily matching. From a scientific positioning, the concept of heat protection may imply a literal definition of somehow moderating hair temperatures or minimizing the structural damage that arises under these conditions. Yet, this would seem an unlikely occurrence given the extreme temperatures involved and the ultra-thin layers of surface deposits left behind by conventional product treatments.
Clearly, consumers do not possess the means to technically characterize their hair’s condition—assessment is instead achieved through a combination of their own observations. Declining tactile properties and an increased tendency for breakage are likely high on this list. Therefore, the ability to prevent these symptoms from occurring is viewed as protection from the consumer’s perspective.
The lubrication provided by conditioning products is able to mask degrading surface properties arising from all manners of insults. At the same time, hair manageability is greatly improved because combing and brushing are considerably less arduous. This same benefit also leads to sizable benefits in terms of reducing breakage—even though the hair can be in a considerably weakened state.
Technical evidence suggests that heat treatments can be highly damaging to hair. Indeed, there is evidence to suggest the application of high temperatures associated with curling and straightening irons may be near the top of the list of insults hair must endure. Nonetheless, use of good conditioning products can prevent the realization of degrading hair properties by leaving the hair manageable, with a healthy feel and minimized tendency for breakage.
- TA Evans, Defining and controlling frizz, Cosm & Toil 46(4) (May 2015)
- TA Evans, Measuring the water content of hair, Cosm & Toil 129(2) 64-69 (Mar 2014)
- TA Evans, Measuring hair strength, part 1: Stress-strain curves, Cosm & Toil 128(8) 590-594 (Sep 2013)
- TA Evans, Measuring hair strength, part 2: Fiber breakage, Cosm & Toil 128(12) 854-859 (Dec 2013)
- TA Evans, Hair breakage, in TA Evans and RR Wickett, eds, Practical Modern Hair Science, Alluredbooks, Carol Stream, IL USA (2012)