The science between this popular ingredient commonly found in sun-protective hair products.
PQ-59 has been found to mitigate damage caused by ultraviolet radiation and produces excellent results by preserving hair’s natural hydrophobicity, maintaining fiber tensile strength, and reducing combing forces. The synergistic combination of both substantive conditioning polymeric properties and UV-absorbing properties has the potential to create a uniquely effective molecule.
Many ingredients and products carry bold claims, but it can be difficult to determine true efficacy of the material. Fortunately the manufacturers of this polymer have done a large amount of scientific testing to evaluate the performance of PQ-59. It is important to keep in mind that with a vested economic interest in the success of the polymer, they are not unbiased, and thus the procedures, data and conclusions should be examined closely and with some degree of skepticism.
A variety of quantitative testing techniques were used to determine the ability of PQ-59 to provide protection from harmful UV radiation. Hair was treated with a 2% polymer solution in SLES (sodium lauryl ether sulfate) and run against untreated hair and hair treated with small molecule uv-absorbers. All hair was tested before and after a dose of exposure to UV radiation equivalent to eighteen days.
The claims were substantiated by the following results. Dynamic contact angle experiments showed a (statistically significant) higher degree of retained hydrophibicity on the surface of the hair compared to hair treated with a small molecule. Measurement of tryptophan levels at the surface via fluorescence spectroscopy revealed higher levels of retained tryptophan in treated hair versus untreated hair. Mechanical testing showed a five percent increase in tensile strength for irradiated hairs that had been treated with PQ-59 compared to untreated hairs. Scanning electron microscopy imaging of the cuticle showed a smoother, more intact surface for hairs treated with the polymer. Finally, spectrocolorimetric evaluation demonstrated increased color retention in fibers treated with the polymer. Third party visual inspection and comparison of treated versus untreated tresses also concluded that PQ-59 improved color retention after prolonged UV exposure.
The data certainly seem to support that this polymer can effectively mitigate damage caused to hair by exposure to ultraviolet radiation. It also does not appear to be necessary to use large quantities of the polymer, as 2% seems to provide significant benefit, even in a rinse-off product. It would be beneficial to see data run by an objective third party, but oftentimes, finished goods manufacturers do not make their data available to the public.
Notes for Curly Hair or No-Shampooers
Curly hair is especially susceptible to anything that could cause damage to the protein structure of the hair. For this reason, protecting it from environmental damage is essential, which includes limiting ultraviolet radiation exposure. PQ-59 seems to be an ingredient that can be quite useful for this purpose and is currently being incorporated into products with increasing frequency.
Polyquaternium-59 is extremely soluble in water, alcohol and glycerin. It is also very soluble in surfactant-containing mixtures, such as SLES, SLS, ALS and presumably cocamidopropyl betaine (although no specific data were found regarding the betaine). It also has a relatively small molecular size relative to other polymers used in hair care applications. So, while the positively charged polymer is substantive to the surface of the hair via electrostatic interactions with the negative charges on the cuticle, it seems probable that removal should be fairly simple via several different mechanisms, depending upon the preference of the user. No specific experiments studying the potential for buildup of these polymers on hair was found though, so as always, try it yourself, and see if you like it.