Perspectives on Supplying Attenuation Grades

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Perspectives on Supplying Attenuation Grades
of Titanium Dioxide and Zinc Oxide for
Sunscreen Applications David Schlossman, Yun
Shao, Ph.D., Pascal Delrieu, Ph.D., Kobo
Products Inc.
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Inorganic UV Filters

• Titanium dioxide (TiO2) and zinc oxide (ZnO)
  pigments are commonly used in personal care
  products to provide protection against UVA and UV
  B
• They attenuate UV light by absorption and
  scattering
• They are usually surface coated to minimize
  photo-catalytic activity.
• They are typically produced as finer crystals
  from the same feed stocks and with similar
  processes as pigmentary grades.

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Manufacturing Process

• The production of TiO2 and ZnO is a two step
  process
• 1. Purification of the raw material
• 2. Growing of crystals (primary particles) of the
  desired size
• They are two different processes for TiO2, known
  as sulfate and chloride, and three for ZnO, known
  as American, French and Wet.
• Crystals are grown at high temperatures to the
  required sizes 200 nm and above (pigmentary
  grades) and finer than 200 nm (attenuation
  grades).

Attenuation grades
Pigmentary grades
10nm 200nm
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Surface Treatment of Inorganic UV Filters
M. Kobayashi and al., Cosm Toil., Vol. 112,
No. 6, p83, 1997
Rate constant of the first order reaction of
oxydation of acetalydehyde
It is common industry practice to use surface
treated inorganic UV filters when formulating
sunscreens.
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Optical Properties - TiO2 and ZnO
Absorption TiO2 420 nm (rutile) / 390 nm
(anatase) ZnO 380 nm (3.24 eV)
Scattering Refractive Index TiO2 2.76 (
Rutile) / 2.52 (Anatase) ZnO 1.99

• Attenuation of UV light is influenced by particle
  size.
• Scattering of visible light (whitening effect) is
  influenced by particle size and the difference
  between the refractive index of the pigment and
  the surrounding media.
• Maximum scattering occurs when size equals 1/2
  the wavelength and particles are uniformly
  dispersed (Mie theory).

Wavelength
Particle size
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Particle Size and Light Scattering
Typical existing form supplied gt1 mm
UV
Visible
Size reduction
Whitening
Transparent
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Dispersions of Attenuation Grade TiO2
Comparison of Dispersions and Powders
15 nm TiO2
35 nm TiO2
Dispersion vs Powder
Dispersion vs Powder
Freeze Fracture TEM (40,000X)
Smaller primary particles are more difficult to
disperse 11/1 ratio of dispersion size to
primary particle size for 10 nm TiO2).
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TiO2 Dispersions in Cyclopentasiloxane

195 60 35 15 10 nm
195 60 35 15 10 nm
195 60 35 15 10 nm
195 60 35 15 10 nm
10nm TiO2 (110 nm dispersion particle size) makes
transparent dispersions for all skin types.
All dispersions diluted in Cyclopentasiloxane (to
20 TiO2)
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Particle Size and UV attenuation
(1) 308/360 extinction ratio indication of
UVB/UVA balance (2) 308/524 extinction ratio
indication of transparency
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Inorganic Sunscreen Formulations

Effect of Particle Size on Attenuation

Tests were performed by Consumer Product
Testing Co. - All testing on 3 people FDA
method JCIA PPD method (Rating 2-4 4 -
8 8 - 12 ).
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Conclusions

• Attenuation grade Titanium Dioxide and Zinc Oxide
  are produced using the same processes as larger
  pigmentary grades. They are usually surface
  coated to minimize their photo-catalytic
  activity.
• TiO2 and ZnO attenuate UV light according to
  their particle size
• Small primary particle sizes (10 - 15 nm) are
  necessary to produce dispersions (100 - 150 nm)
  transparent to visible light and efficient
  against UVB.
• ZnO and larger TiO2 (35 - 150 nm) are more
  efficient against UVA.
• The particle size of pigmentary grades are
  appropriate to scatter visible light, but they
  are not efficient at attenuating UV light or
  effective as sunscreens.