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Coated Inorganic Filters for Elegant and Protective Sunscreens

Contact Author Howard Epstein, Judy Zhuang, My Trinh Luu and Qinyun Peng, EMD Performance Materials Corp., Philadelphia, USA
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Sunscreen regulations differ around the world. In the European Union, it is permissible to formulate with inorganic and organic sunscreen combinations, which achieve high SPFs and provide adequate UVA protection. In the United States, however, some combinations are not permitted. For example, the organic UV filter butyl methoxydibenzoylmethane (avobenzone) currently cannot be combined with the inorganic filter titanium dioxide due to instability concerns. New coatings have been developed to stabilize such combinations, and these are used in Europe; however, the U.S. Food and Drug Administration’s (FDA) Final Sunscreen Monograph1 has not yet been revised in light of new coatings and improved stability.

This monograph1 challenges formulators to develop inorganic-based products that achieve both high SPF, i.e. > 30, and broad-spectrum protection. This is difficult to achieve while also providing exceptional formulation elegance, because high SPF and UVA ratings are related to the absorbance of UVA and UVB sunscreens and the calculation necessary to measure UVA protection. Higher UVA protection becomes a challenge with higher levels of protection due to the UVA/UV ratio. Simply put, since the SPF value is in the denominator, higher SPFs tend to reduce the UVA value.

Sunburn is primarily caused by UVB, although both UVB and UVA are associated with skin cancer and premature skin aging. In relation, in the United States, a broad-spectrum claim on a sunscreen label indicates the product provides substantial protection against both UVA and UVB—i.e., it must meet a critical wavelength value of 370 nm.2 Currently, few sunscreens (~10%) meet this requirement; therefore, a certain percentage of the total protection must be against UVA. Under the new FDA regulations, this broad-spectrum protection claim, along with the SPF level, should appear on the front of the product label.2, 3 Higher SPF values indicate higher levels of overall protection and currently, the maximum permitted claim is 50+. By contrast, any sunscreen not labeled as broad-spectrum, or having SPF values between 2 and 14, has only been shown to help prevent sunburn.3

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Inorganic sunscreens typically are formulated with various particle sizes of micronized titanium dioxide and zinc oxide to both optimize their total UV protection and provide elegant aesthetics. However, some consumers prefer inorganics for their more “natural” or mineral claim, although these filters can also exhibit a pasty consistency and undesirable opacity when applied to skin. This paper describes formulating strategies using micronized titanium dioxide with different coatings to optimize its dispersion, protective benefits and ease of processing, while enhancing elegance. The following examples compare the SPFs, critical wavelengths (CW) and formulation aesthetics of various sunscreens. Formulations were sent to an independent facility for SPF testing and CW measurements, per the current in vitro FDA Sunscreen Monograph. Aesthetics were determined by an internal panel of evaluators. Note that the benefits described are based on the specific coatings and varieties of titanium dioxide selected as well as their particle size; particles were measured by centrifugal particle sizing (CPS) and x-ray diffraction (XRD).

Titanium Dioxide and Coatings

Two different crystalline forms of titanium dioxide are used in sunscreens: rutile and anatase, which come in different shapes, i.e., “needles,” spherical or lanceolate (longer than wide). The primary particle size used for sunscreens ranges from 20–100 nm. Nanoparticles are generally known to stick together to form agglomerates that may be larger than 100 nm,4 and particles larger than 100 nm give a whitening effect when applied to skin. Titanium dioxides with smaller particle sizes, referred to as UV attenuation grades, tend to be more transparent on skin. Further information regarding the safety and composition of nano-sized titanium dioxides used in sunscreens may be found in the Scientific Committee on Consumer Safety’s “Opinion on Titanium Dioxide (nano form, COLIPA, number S75).”4, 5

Beyond differences in aesthetics, the degree of UVB versus UVA protection provided is related to the particle size of the titanium dioxide. In general, a smaller particle size, e.g., 15-20 nm, gives better UVB protection, while a larger particle size, e.g., 70-80 nm, provides better UVA protection. The key is to find the right mix of particle sizes to create the highest SPF and UVA protection. Good application on the skin, and minimal agglomeration in the product are also factors.

A variety of methods can be used to measure particle size, and the measurement technique used can result in different particle size measurements for the same material. For this reason, particle size indications also should include the method by which they were measured. For the following discussion, Table 1 indicates the particle sizes for the titanium dioxide sunscreen materials used in the described prototypes, as measured by CPS and XRD.

Formulating an elegant, high-SPF, broad-spectrum sunscreen requires selecting ingredients carefully to promote the sunscreen active’s stability and uniform application to skin. In addition, the other ingredients must interact minimally with the sunscreen, and it must easily and uniformly be incorporated into the formulation. This is where coatings and coating processes come in, which are just as important yet perhaps less obvious. For example, if excessive agglomeration occurs with titanium dioxide coating, the sunscreen product will perform poorly and exhibit limited transparency.

The type of coating used permits a variety of formulation and processing options. For example, a glycerin coating can be processed in a water phase during compounding, whereas a methicone coating can be formulated in an oil phase. Also, the appropriate coating can be formulated in a w/si or w/o emulsion to achieve water-resistance, as well as high UVB and UVA protection. Beyond these key points, the type of coating can permit claims for natural filters, or alumina- or silicone-free as desired by particular manufacturers.

Various methods can be used to deposit coatings. Two important steps in the coating process are calcination and sintering. Calcination involves heating the compound to drive off chemically bound water and promote the crystalline transformation of the titanium. Sintering is the process of forming a solid mass of material using heat and/or pressure. The atoms in a sintered material diffuse across the boundaries of particles, fusing the particles together. This sintering process can be used to control the particle size and shape, which as noted, affect levels of UVA and UVB protection.6 Controlling the compatibility between the particles and the matrix is key to “tune” the microstructure of the final material.

SPF/CW and Feel vs. Coating and Dispersion

To achieve a high SPF and CW for UVA protection, it is important to ensure good dispersion of the coated titanium dioxide. To accomplish this, the emulsifying system, thickeners and stabilizers must be considered, and again, agglomeration must be minimized. Polar and non-polar oils may be used with inorganic filters, and dispersing aids help to minimize microfine pigments, which tend to agglomerate. Such aids include cetyl dimethicone, polyhydroxy stearic acid and polyglycerol-3 polyricinoleate. Coating titanium dioxide with alumina and triethoxy caprylysilane makes it formulating-friendly so that it disperses easily in various oils and emollients, and its small surface area permits quick and easy wetting of the material.

The effects of changes in coatings and phase placement on SPF and CW were compared using three test formulas. The first sunscreen, a w/o emulsion, used microfine titanium dioxide coated with alumina and simethicone at 8.0% w/w in the water phase, and incorporated microfine titanium dioxide coated with alumina and triethoxycaprylsilane at 6.0% w/w in the oil phase (see Formula 1a). A variation of this formula incorporated 4.0% w/w of the same coated titanium dioxide in both the oil phase and water phase. This surface coating and the particle size impart unique properties; i.e., the coating has a high degree of transparency when the filter is employed even at levels as high as 10% to 12%, and it disperses easily in both the oil and water phases, aiding long-term stability. In the case of pigmented sunscreen formulations, the coating material also has excellent compatibility with zinc and iron oxides due to its isoelectric point at pH 8.8.

The isoelectric point of pure titanium dioxide is 3.8, and 9.0 for zinc oxide. This isoelectric point changes with different coatings. With alumina, it goes to about 7.0 and with silica, it shifts to 2.1. Therefore, to maintain the stability of sunscreens with pigments, the formulator typically adds polymeric electrolytes to alter the surface charge; salts, magnesium sulfate or sodium cloride and glycols aid in freeze-thaw stability. This consideration is import for emulsions, not nonaqueous products.

A third sunscreen also used microfine titanium dioxide, but was coated instead with triethoxycaprysilane and alumina. This sunscreen contains a larger particle size (see Table 1), making it appropriate for UVA efficiency. This filter was incorporated only in the oil phase (see Formula 1b). Note that the nature of a coating can also help the formulator to select the appropriate emulsifier.

Formula 1a achieved a CW of 376 and SPF of 60.9, while Formula 1b showed a CW of 378 and SPF of 62.6. These formulations also were evaluated for water-resistance, as established under the FDA Sunscreen Monograph. Water-resistant formulations tend to feel heavy; however, the right emulsion system with a correctly coated filter can feel elegant while still providing optimal SPF and UVA protection. Results indicated both formulas supported a water-resistant claim after 80 min of immersion. In addition, Formula 1a maintained an SPF of 53 and Formula 1b, SPF 54.5. Although there was little difference in SPF between the two, an internal panel of evaluators agreed that Formula 1b had an improved feel and spread more easily on skin, compared with the Formula 1a variation. This demonstrates that the feel and texture of a formulation can be adjusted by adding the appropriate coated sunscreen in both the oil and water phases.

Transparency vs. Coating

The transparencies of Formulas 1a-b on skin types I, III and V are shown in Figure 1, Figure 2 and Figure 3, as well as another w/o test formula (Formula 2) and a w/si formula (Formula 3). The additional w/o test formula incorporated micronized titanium dioxide coated with silica and glycerin in the water phase, and silica and methicone in the oil phase. The SPF and CW were evaluated by the same third party under the FDA protocol; results indicated a CW of 370 and SPF of 57.5—once again, achieving a high SPF and broad-spectrum protection.

The internal panel also found the aesthetics of this formulation to be good, with a high degree of transparency on skin, as shown in Figure 1. This is likely due to the ~9 nm particle size of the titanium dioxide. To aid in dispersion, the titanium dioxide is rendered hydrophilic by the silica and glycerin coating in the water phase (see Table 1), and hydrophobic by the silica and methicone coating in the oil phase. This material also meets NaTrue requirements7 for a green or mineral sunscreen, providing the nature-compliant claim. In some countries, it also is permitted to be formulated with butyl methoxydibenzoylmethane (avobenzone).

As noted, a w/si formulation also was evaluated for SPF and broad-spectrum protection (see Formula 3), for comparison’s sake. Here, a total of 15% sunscreen was incorporated, again based on titanium dioxide coated with triethoxycaprylylsilane and alumina. In this example, however, two titanium dioxides of different particle sizes but having the same coating were used (see Table 1); i.e., one 50 nm, and the other 20 nm. This formulation strategy achieved a high SPF value of 59.2 and CW of 378 nm—well within range for a broad-spectrum claim. Carrying the chemically bonded and hydrophobic silane, the smaller-particle titanium dioxide was more easily dispersed in non-polar oils and very stable. Besides producing a high SPF and broad-spectrum protection, the w/si formulation also provided strong water-resistance, maintaining an SPF of 54.5 after 80 min immersion. Thus, the same coating on particles of different sizes can make a difference. In addition, this calls to question if, and how much, homogenation should be used during manufacturing.

Processing vs. Coating

Processing is another aspect of formulating where coatings can assist. The appropriate surface treatment paired with the most appropriate vehicle will ensure minimal aggregation and yield enhanced transparency. In addition, it will provide a homogeneous dispersion, which is easier to manufacture in large-scale production. This ensures good stability, resulting in a long product shelf-life. In addition, it can eliminate the need for dispersing agents in the formulation.

As an illustration, another o/w emulsion (not shown) was developed using 7.50% titanium dioxide coated with alumina and manganese dioxide in the water phase, and 6.00% titanium dioxide coated with alumina and stearic acid in the oil phase. The latter sunscreen could be added to the oil phase with light to moderate agitation, requiring low energy input for processing. This sunscreen also had little influence on the viscosity of the emulsion even at levels as high as 25%. Furthermore, the stearic acid component of the coating is vegetal-derived, meeting NaTrue standards for green or mineral sunscreens.

The water-phase titanium dioxide, coated with alumina and manganese oxide, is dispersible in water, oil or silicone, giving the formulator a large degree of flexibility with respect to processing and equipment limitations; a typical use level is 2–15%. Furthermore, the manganese oxide coating is a highly effective radical scavenger, and the filter also meets NaTrue requirements for natural, green or mineral sunscreens.7

Taken together, this formula provided an SPF of 51.5 and a CW of 376. The internal aesthetic panel also rated it highly for rub-in and appearance on skin. Both sunscreens provided good UVB and UVA absorbance.

Another prototype formula (not shown) incorporated titanium dioxide coated with silica in its water phase. This sunscreen is compatible with acrylate thickeners and could also be incorporated in the oil phase, which again, provides formulation flexibility. In this o/w emulsion, a titanium dioxide coated with alumina and triethoxy caprylylsilane was also included the oil phase, and the formula achieved a CW of 385—well above the required 370 nm for a broad-spectrum claim. In addition, it provided an SPF of 28. The latter sunscreen complies with NaTrue standards as a green or mineral sunscreen, and it is a strong UVB as well as UVA absorber.

Conclusions

The SPF and CW values of the example formulations show it is possible to formulate high-SPF, broad-spectrum sunscreens in o/w, w/o and w/si systems, thanks to various coatings. Also, excellent aesthetics were achieved using different phase-placement strategies, which again is made possible by the different coatings. While other factors can impact the SPF, UVA values, aesthetics and stability of sunscreen formulations, the key point addressed here what can be achieved using various coated inorganic sunscreens in a variety of emulsion types. These options provide the formulator the ability to create a variety of sunscreens to meeting consumer needs.

References

  1. www.fda.gov/downloads/Drugs/DevelopmentApprovalProcess/DevelopmentResources/Over-the-CounterOTCDrugs/StatusofOTCRulemakings/ucm090244.pdf (Accessed Feb 25, 2015)
  2. www.fda.gov/forconsumers/consumerupdates/ucm258416.htm (Accessed Feb 25, 2015)
  3. Code of Federal Regulations, Title 21, Volume 5, 21CFR 352.10, revised (Apr 1, 2012)
  4. http://ec.europa.eu/health/scientific_committees/consumer_safety/index_en.htm (Accessed Feb 25, 2015)
  5. A Aimable and P Bowen, Nanopowder metrology and nanoparticle size measurement—Toward the development and testing of protocols, Processing and Application of Ceramics 4(3) 157-66 (2010)
  6. B Faure, G Salazar-Alvarez, V Ahniyaz, G Berriozabal, YR De Miguel and L Bergström, Dispersion and surface functionalization of oxide nanoparticles for transparent photocatalytic and UV-protecting coatings and sunscreens, Sci and Tech of Adv Mat 14 1-23 (2013)
  7. www.natrue.org/certification (Accessed Mar 6, 2015)
 

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Table 1. Particle Sizes for the Inorganic Test Sunscreens

Table 1. Particle Sizes for the Inorganic Test Sunscreens

Particle sizes for the titanium dioxide sunscreen materials used in the described prototypes, as measured by CPS and XRD

Figure 1. Transparencies of formulas on type III skin

Figure 1. Transparencies of formulas on type III skin

Formula 1a-b, Formula 2 and Formula 3 on type III skin; test area = 3 x 3 cm; 0.02-g product sample (a) and test data per chromameter assessments (b)

Figure 2. Transparencies of formulas on type V skin

Figure 2. Transparencies of formulas on type V skin

Formula 1a-b, Formula 2 and Formula 3 on type V skin; test area = 3 x 3 cm; 0.02-g product sample (a) and test data per chromameter assessments (b)

Figure 3. Transparencies of formulas on type I skin

Figure 3. Transparencies of formulas on type I skin

Formula 1a-b, Formula 2 and Formula 3 on type I skin; test area = 3 x 3 cm; (a) and test data per chromameter assessments (b)

Formula 1. O/W Dispersion Test Formulas

Formula 1. O/W Dispersion Test Formulas

The effects of changes in coatings and phase placement on SPF and CW were compared using three test formulas.

Formula 2. W/O Transparency Test Formula

Formula 2.

The additional w/o test formula incorporated micronized titanium dioxide coated with silica and glycerin in the water phase, and silica and methicone in the oil phase. The SPF and CW were evaluated by the same third party under the FDA protocol; results indicated a CW of 370 and SPF of 57.5— once again, achieving a high SPF and broad-spectrum protection.

Formula 3. W/Si Transparency Test Formula

Formula 3. W/Si Transparency Test Formula

This formulation strategy achieved a high SPF value of 59.2 and CW of 378 nm—well within range for a broad-spectrum claim. Carrying the chemically bonded and hydrophobic silane, the smaller-particle titanium dioxide was more easily dispersed in non-polar oils and very stable.

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