perspectives on supplying attenuation grades of titanium dioxide and zinc oxide

www.koboproducts.com

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.


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.


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


* Rate constant of the first order reaction of oxydation of acetalydehyde

Pigment type (primary particle size)

Treatment Rate *

None 4.76

2% Methicone < 0.01

Alumina 0.13

3% Lecithin 0.033

None 1.83

3% Methicone < 0.01

Pigmentary TiO2

(410 nm)

Attenuation grade TiO2

(30 - 50 nm)

Attenuation grade ZnO (15 - 35 nm)

M. Kobayashi and al., Cosm & Toil., Vol. 112, No. 6, p83, 1997

Surface Treatment of Inorganic UV Filters

It is common industry practice to use surface treated inorganic UV filters when formulating sunscreens.

Surface Treatment


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

Optical Properties - TiO2 and ZnO

• 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


Particle Size and Light Scattering

UV

Visible

Size reduction

Whitening Transparent

Typical existing form supplied >1

m


Dispersions of Attenuation Grade TiO2

15 nm TiO2 35 nm TiO2

Dispersion vs Powder

*Freeze Fracture TEM (40,000X)

Dispersion vs Powder

Comparison of Dispersions and Powders

Smaller primary particles are more difficult to disperse :

11/1 ratio of dispersion size to primary particle size for 10 nm

TiO2).


10nm TiO2 (110 nm dispersion particle size) makes transparent dispersions for all skin types.

TiO2 Dispersions in Cyclopentasiloxane

195 60 35 15 10nm

195 60 35 15 10nm

195 60 35 15 10nm

195 60 35 15 10nm

All dispersions diluted in Cyclopentasiloxane (to 20% TiO2)


PigmentPrimary particle

sizeDispersion

particle sizeUVB / UVA

balance (1) Transparency (2)

10 nm 100 - 120 nm 7.0 - 8.0 70 - 90

15 nm 120 - 150 nm 3.4 - 5.5 11 - 55

20 nm 130 - 160 nm 2.0 9

35 nm 150 - 180 nm 1.5 - 2.0 7 - 17

150 nm > 250 nm 1.1 2.6

20 nm 130 - 180 nm 1.0 - 1.1 30 - 37

60-100 nm 180 - 250 nm 1.1 6.3

120 nm > 250 nm 0.9 2.9

ZnO

TiO2

(1) 308/360 extinction ratio : indication of UVB/UVA balance(2) 308/524 extinction ratio : indication of transparency

Particle Size and UV 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: +++).

Inorganic Sunscreen Formulations

FormulaKSL-026 (O/W)

KSL-027 (O/W)

KSL-043 (W/O)

KSL-016 (W/O)

Pigment TiO2 TiO2 ZnO ZnO

PPS (nm) 15 35 15 - 25 25 - 35

PS in disp. (nm) 125 154 130 228

Active (%) 10.29 10.29 13.80 14.97

SPF** 50 28.4 25.4 14

PA*** 4.50 6.75 4.75 7.50

SPF/ Active% 4.85 2.41 1.84 0.93

SPF/ PA 11.10 4.21 5.35 1.87

Effect of Particle Size on Attenuation


• 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.

Conclusions

perspectives on supplying attenuation grades of titanium dioxide and zinc oxide

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