Nanopigments For Broad Spectrum Sun Protection Patricia Aikens NAFTA Technical Services
UV radiation and the skin UV absorbers Nanopigments Titanium dioxide Zinc Oxide
Electromagnetic Radiation Longer wavelength λ Shorter wavelength λ E = hν = hc/λ Means higher energy for shorter wavelength λ = wavelength ν = frequency
UVB radiation and UVA radiation are targeted by sunscreens UVC UVB UVA ll UVA l VIS 270 290 310 330 350 370 390 410 λ (nm)
SPF (Sun Protection Factor) indicates how well the sunscreen protects against UVB radiation MED = Minimum Erythemal Dose; amount of radiation that causes a sunburn Sun Protection Factor = MED with sunscreen MED without sunscreen PPD (Persistent Pigment Darkening) indicates how well the sunscreen protects against UVA radiation
Determination of Critical Wavelength
Photoaging of the skin UVA irradiation is the major cause for premature skin aging and it helps to promote skin cancer along with UVB UVA young, healthy skin physiological changes wrinkles immediate response several years... cumulated damage
AND UVA radiation penetrates deeper into the skin than UVB, Stratum Corneum 0.0 300 nm UVB UVR 350 nm UVA reaching the dermis, Epidermis where the structural components of the skin are (elastin and collagen) skin depth (mm) 0.5 Up to 50 % of UVA Dermis can reach the dermis 1.0 Increasing wavelength
Why is UVA radiation a concern? Longer wavelength = lower energy Weaker, much more required to show effects than UVB However There is much more UVA radiation in the solar spectrum reaching the earth (1000 X) UVA is 90 95% of the UV radiation and is present all day UVB is screened by glass, UVA is not
Organic Sunscreens: Chromophores absorb UV radiation C = chromophore C* absorb C emit transfer photochemistry
UV spectra of some organic sunscreens Absorbance (10 mg/l solids in ethanol) 1.40 1.20 1.00 0.80 0.60 0.40 0.20 UVB UVA Octocrylene Benzophenone-4 Benzophenone-3 OMC CN O O CH 3 O OH O OH O SO 3 H CH 3 O 0.00 290 310 330 350 370 390 O nm O H 3 CO
Inorganic Sunscreens: Nanopigments for UV Protection The nanopigments Titanium Dioxide (TiO2) and Zinc Oxide (ZnO) are highly effective for UV protection Super-fine = microfine = ultrafine = nanosized Particles are dispersed, not dissolved, in liquid Low irritation potential Sunscreen film with inorganic particles The particles can function by physically scattering and reflecting incoming UV radiation
Inorganic sunscreens also absorb UVR Electrons move within the metal oxide Goes from ground state to excited state by absorbing UV light Conduction Electron relaxes to ground state, emits lower energy radiation (such as heat) Gap e- e- Energy gap corresponds to wavelength (below which absorption occurs) Valance
Forumlation Considerations with TiO2 Absorption or transmission curves of the pigment in oil or solvent Particle properties: size, morphology (round, oval), and crystal type (rutile or anatase). Surface treatment (hydrophobic mostly or hydrophilic, one or several coatings...) Form of the product; powder or dispersion General formulation technique: Use high shear and dispersing aids to disperse metal oxide in oil phase. Emulsifiers may be added at any point Add oil phase to aqueous phase and homogenize to obtain emulsion
Pigment particles scatter, reflect, absorb and transmit UV and visible radiation Scattering of visible light creates a whitening effect on skin For particles with dia. < ½ λ, no scattering will occur. Particles < 200 nm will not scatter visible light S ( n) 2 d 6 / λ4 S = scattering coefficient n = refractive index d = particle size λ = wavelength
The size of the TiO2 particles determines their ability to scatter visible light Transparent Opaque (whitening on the skin) 2.0 1.5 1.0 0.5 0 0.25 0.5 0.75 1.0 1.25 1.5 1.75 2.0 Diameter of particle (µm)
Morphology of Super-fine TiO2 Uncoated TiO2 Oval, 10 x 50 nm TiO2 coated with methicone Oval, 10 x 50 nm TiO2 coated with triethoxycaprylyl silane Round, 20 nm
Anatase and Rutile are different crystal stuctures of TiO2 with different angles and cleavages Anatase : tetragonal crystal system; with a steeper pyramid than rutile, 2 pyramids share one common corner (1 oxygen ) More whitening Rutile : more common than anatase, also a tetragonal crystal structure, but 2 pyramids share a common edge (2 oxygens) rather than just a corner Less whitening rutile Ti Ti anatase
Different coatings on TiO2 affect absorbance values 100 7% super-fine TiO2 in silicone oil 90 Transmittance (%) 80 70 Coating A Coating B Coating C 60 400 450 500 550 600 650 700 750 800 Wavel engt h (nm)
15% super-fine TiO2 in petrolatum, 0.5 mg / cm2 2,4 2,2 2,0 1,8 1,6 1,4 1,2 1,0 0,8 0,6 0,4 0,2 Absorption Coatings for TiO2 TiO2 A TiO2 B - Uvinul TiO 2 T - Li te TM SF T - Lite TM SF - S -A -B -C -D -E 0 290 300 310 320 330 340 350 360 370 380 390 400 [nm]
Super-fine TiO2 Particle morphology Absorption in vitro in paraffin at 15% pigment load Quality of dispersion in paraffin 1% weight SPF in vivo COLIPA Quality of dispersion in formulation Oval highest 20 Oval medium 9 Round high 9
The type of coating can affect the viscosity of the formulation TiO2 (and) aluminum hydroxide (and) dimethicone /methicone copolymer TiO2 (and) aluminum hydroxide (and) dimethicone /methicone copolyme (and) silica TiO2 (and) aluminum stearate (and) aluminum hydroxide TiO2 (and) alumina (and) dimethicone 100 µm 100 µm 100 µm 100 µm 9,000 mpas 26,000 mpas 20,000 mpas 22,600 mpas Formulation containing 5 % titanium dioxide Magnification x 200
UV Spectra of super-fine zinc oxide and titanium dioxide Abs; (5% dispersion in petrolatum) 2.50 2.00 1.50 1.00 0.50 0.00 UVC UVB UVA 250 270 290 310 330 350 370 390 nm ZnO TiO2
Formulating With Microfine Zinc Oxide Can be used coated or uncoated Uncoated ZnO is dispersed in water Hydrophobically coated ZnO is readily dispersed in the oil phase ZnO is a skin protectant ZnO and TiO2 can be formulated together to give a high SPF inorganic sunscreen The key to successful formulations is the use of high shear to break up agglomerates
ph of TiO 2 and ZnO TiO 2 : acidic (IEP, ph 5) ZnO: basic (IEP, ph 8,2) basic surface isoelectric ph TiO 2 acidic surface 7 8.2 Z-COTE
ph behavior of zinc oxide in water In water, self regulating buffer- ph remains constant 8 7,7 4% Z-Cote dispersed with Ultra Turrax 13 500 rpm 5min in deionised water ph measurement under stirring 7,4 ph 7,1 6,8 6,5 0 200 400 600 800 1000 1200 time of measurement in min
Traditionally, ZnO is incompatible with carbomer thickeners Example; ZnO (and) triethoxycaprylyl silane Without polyacrylic acids With polyacrylic acids ZnO ZnO Hydrophobic coating unperturbed ph constant Hydrophobic coating perturbed ph increase Non-acrylate thickeners such as polysaccharides and silicates are suitable for hydrophobic coatings
From Problem to Solution: Need to prevent zinc oxide from interacting with water phase and carboxylic acids A less hydrophobic coating is the key to carbomer compatibility Example; ZnO (and) diphenyl methicone ZnO Carbomer thickeners can be used with the hydrophilic coating: emulsion stability with no ph drift
Electron microscopy of microfine zinc oxide ZnO (and) triethoxycapryl silane ZnO (and) dicapryl methicone
Conclusion Nanopigments provide transparent broad-spectrum UV protection for the skin. They function by both physically blocking and chemically absorbing UV radiation Polymeric coatings on the pigments make them hydrophobic for easy dispersion in oil The coatings also prevent photoreaction in the case of TiO2 The choice of coating has a direct effect on the formulation, especially the compatibility with acrylate thickeners.