The DSC ( differential scanning calorimetry ) technique was used to observe the thermal behavior of the hair. This technique measures the thermal energy provided to a material to perform physical transformations, such as vaporization heat, melting heat and transitions of phase in materials.
In summary, the DSC technique evaluates the thermodynamic processes that happen when the hair sample undergoes a constant heat flow and relates these processes to structural changes that happen in the hair fiber. Its main objective in use is to evaluate how the hair responds to high temperature and how the thermal protection products can effectively stop the hair from suffering big alterations with the use of hair straighteners or straightening processes.
Typical curve of DSC for a hair sample in which four different processes can be observed.
DSC curve for virgin hair swatches of different ethnic groups and animal keratin
The first event, endothermic, relates to dehydration or loss of water from the hair fiber;
The second event, also endothermic, is related to the melting temperature of the crystal material present in the hair or denaturation of keratin;
The third event, endothermic, is related to the thermal decomposition of the rest of the hair matrix;
The fourth event, exothermic, is related to the energy involved in the elimination of the carbonaceous material resulting from the third event.
Thus, quantifying the enthalpy of the second event, it is possible to verify how much the product protects the hair from thermal events, such as the use of hair straightener and/or driers. Since keratin presents around 85% of this material, the use of this equipment damages, irreversibly, the material present in the hair.
Therefore, when submitted to a thermal process and after analyzed by DSC, the hair that presents more enthalpy is also the one that has more amount of keratin remaining from the thermal process applied.
In addition, it is possible, through analysis of enthalpy of the first event, to quantify the amount of water present in the fiber. This analysis, literally know as moisturization of the hair fiber, is commonly mistaken with the hydration felt by the consumers after the use of a certain product. However, many companies still use the result of the enthalpy of water vaporization for the support of the claim of hydration and, therefore, we also use this analysis for this function, but we always ponder its non-correlation with the perception of hydration felt by the consumer.
Another curious thing from this test is related to the fact of the transition observed in the curves, in which the test performed with the crucible (or sample holder) open, being observed at lower temperatures.
During the study conduction, the burning of the crystal material releases a big amount of sulfur that damages the interior of the equipment used. To stop that from happening, the test is performed with a closed crucible, so as to prevent the sulfur from escaping and damaging the equipment. However, when closing the samples holders, the internal pressure increases and the transitions will happen at lower temperatures. The same manner that happens in the cooking of foods in pressure cookers, the process is accelerated. That is, it happens faster.
Some companies use this fact to talk about thermal protection at lower temperatures, since the transition happens before the 200ºC with the closed crucible. However, in real situations, it is not possible to observe the degradation of the fiber at temperatures lower than 200ºC and this form of analysis ends up being a way of taking advantage of the non-knowledge of the clients to produce a bigger portfolio of tests and claims. Because the temperature, itself, is not the main parameter upon analyzing the thermal protection, but the amount of thermal energy provided to the fiber and it is better represented by the enthalpy of alpha-beta keratin transition.