Metodologias para verificação da estabilidade térmica e fotoelétrica de estruturas absorvedoras de radiação UV.
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2011-12-20
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Universidade Federal de Goiás
Resumo
This work describes the thermal and photoelectric stability investigation of
commercial sunscreens benzophenone-3, octyl methoxycinnamate and octyl
salicylate. The purpose was determining thermal and photoelectric (resistance to
sunlight) stability.
For thermal stability evaluation, thermal ramp was set up in attempt to get
closer to conditions which sunscreen could be submitted in manufacture or in
people using. In this test each filter was exposed to temperature variations from 30
to 65 °C, in times and speeds of heating, under synthetic air atmosphere.
A solar simulator was used for photoelectric stability evaluation. The
measures were not performed with samples irradiated at a specific wavelength, but
exposed to all wavelengths emitted by the sun, extending from the ultraviolet to
infrared, with high reproducibility. The samples were exposed to simulated solar
light in five different times, ranging from 30 to 120 minutes. This change aims to
understand how the filter behaves with the increased exposure time.
Infrared, ultraviolet and NMR spectroscopies were used on structural
characterization of the compounds submitted to thermal and photoelectrical
evaluation.
This study has demonstrated that sunscreens evaluated are thermally stable
but photoelectrically unstable. This instability was observed by formation of minor
compounds derivative of sunscreens studied. The shift in the chemical equilibrium
producing higher concentrations of minor compounds was also observed. These
factors change the absorptive capacity of the sunscreens, causing loss of stability in
solar protection.
Octyl methoxycinnamate and benzophenone 3 have the higher molar
absorptivity (23,300 and 14,380, respectively) and presented the higher loss of
efficiency, overcoming 20% in 2 hours of exposure. On the other hand octyl
salicylate has lower molar absorptivity value (4.130) and presented lower loss of
efficiency, overcoming 10% in 2 hours of exposure.
The used methodologies have low cost and are viable, representing an
interesting tool in stability evaluation of sunscreens already established in cosmetic
market or in development. This study also emphasizes the importance of NMR
technique that enabled us to identify and quantify all structures modified without
any previous separation. The correlation between UV and NMR techniques was also
effective to demonstrate that lower losses in molar absorptivity are directly related
to lower differences on NMR spectra, as observed in octyl salicylate.
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LUZIN, R. M. Metodologias para verificação da estabilidade térmica e fotoelétrica de estruturas absorvedoras de radiação UV. 2011. 91 f. Dissertação (Mestrado em Química) - Universidade Federal de Goiás, Goiânia, 2011.