1. Introduction

3, The number of skin cancers diagnosed annually is growing year after year. It is well recognized that this kind of cancer is caused mainly by the negative effects of solar exposure, especially exposure to ultraviolet (UV) radiation [ 1 ]. UVB (280–320 nm) typically induces erythema and direct DNA damage via pyrimidine dimer formation, whereas UVA (320–400 nm) is associated with tanning, photo-aging and the generation of excess reactive oxygen species, which indirectly damage DNA [ 2 4 ]. For this reason, in order to reduce skin photo-damage and the carcinogenic effects of solar irradiation, the use of sunscreen products containing UV filters as an integral part of a photoprotection strategy has gained popularity [ 5 ].

An ideal sunscreen provides uniform protection against the UVB and UVA wavelength range, maintaining sensorial features that enhance the user’s experience.

In order to absorb ultraviolet radiation (UVR), an organic molecule must contain a suitable chromophore presenting conjugated π-electron systems [ 6 ]. The absorption of a UV photon results in the molecule being energized to an excited electronic state. If the absorbed energy is not dissipated as heat, the chemical bonds of the molecule may break, resulting in a degradation of the UV filter. This phenomenon can transform the UV filter to another chemical entity with less protective efficacy and, sometimes, with unknown toxicity leading to a safety reduction of the final product [ 7 ].

Most of the UV absorbers used in sunscreens are photostable under foreseeable conditions of use. Two exceptions are butyl methoxydibenzoylmethane (avobenzone) and octinoxate [ 8 ]. In particular avobenzone undergoes rapid photodegradation, when used alone, and for this reason it is often stabilized by addition of the UV filter octocrylene [ 6 ].

In the literature reports on the photostability of UV filters indicate that analyses are made in diluted solutions, however, this is not relevant to a complex matrix, like sunscreen final products, composed of water in oil or oil in water emulsions, where the photochemistry of these filters is very different from those in a diluted solution. The behavior of sunscreen final products are also not predictable from the photostability of its individual filters, so it is important to evaluate the final formulation [ 9 10 ]. In fact, sunscreen vehicles can often impact on the sun protection factor (SPF) [ 11 ]. Thus, sunscreen efficacy and safety depend on the ultraviolet filter types (organic or inorganic), their photostability, the excipients used, and the choice of packaging.

Furthermore, the lipophilic nature of filters must be taken into account, with respect to their capability to be very good solvents against the packaging material. Thus, the choice of packaging is fundamental for a good quality product.

Among the different kinds of materials used as the primary packaging, polyolefins and, especially, polyethylene (PE), are the most widely used plastics in sunscreen products. Polyethylene is classified into several categories based mostly on density and branching, with all polyethylenes being semicrystalline.

At present, many hundreds of grades of PE, most of which differ in their properties in one way or another, are available. PE also possesses good chemical stability. PE can be easily heat sealed, is tough, and has high elasticity. It has good cold resistance properties and is a good water vapor barrier. However, low-density polyethylene (LDPE) has low barrier properties to gases, aromas, and fats. With increasing density, all of the barrier properties increase, as well as the stiffness, hardness, and strength, as a result of the higher crystallinity. At the same time, there is a decrease in the impact resistance, toughness, resistance to stress cracking, cold resistance, and transparency.

LDPE has a gas permeability in the range normally expected with rubbery materials. High-density polyethylene (HDPE) has a permeability of about one-fifth that of LDPE. Polyethylene is cheap, and particularly easy to mold and fabricate. It accepts a wide range of colors, can be transparent, translucent, or opaque, has a pleasant, slightly waxy feel, can be textured or metal-coated, but is difficult to print on [ 12 ].

Furthermore, polyolefins are lipophilic materials, so they are able to retain large amounts of compounds with the same nature, like suncreens [ 13 ]. Moreover it is also necessary to take into account that these products are often stored in a very warm environment, so the entire system (formulation + package) must remain stable in these conditions [ 14 ]. Thus, the evaluation of the sunscreen formulation stability in the final packaging is important to assure the efficacy and the safety of the final product.

In view of these considerations, this work aims to study the stability of a sunscreen product, packed in a LDPE/HDPE mixture under conditions simulating the possible stress conditions encountered during their shelf life.

16,17, Butyl methoxydibenzoylmethane (avobenzone) and octocrylene were chosen as UV filters. Avobenzone (4-tert-butyl-4′-methoxydibenzoylmethane) is one of the most common UVA filters in sunscreens. The photochemical behavior of this filter has been extensively studied and its photostability has been found to be highly dependent on the polarity and proticity of the solvent. In solution, avobenzone enol forms photoisomerized keto forms, revealing a great photoinstability [ 15 18 ].

20, Octocrylene is an ester formed by the condensation of diphenylcyanoacrylic acid with 2-ethylhexanol, and is considered to belong to the family of cinnamates. The action spectrum of octocrylene (290–360 nm, peak absorption at 303 nm) covers mostly UVB wavelengths, but also short UVA wavelengths (UVAII) [ 19 ]. Usually octocrylene is considered a stable filter, and it is also able to stabilize photo-unstable filters like avobenzone, but it is expensive and difficult to incorporate into sunscreens [ 15 21 ].

In this work these filters were used both in unencapsulated and encapsulated forms in sol-gel silica glass from commercially-available aqueous dispersions [ 22 ].

The encapsulation of organic UV filters should permit the increase of their UV stability and to be able to prevent both interactions between the substances and the skin and those between filters and the external environment (e.g., with the package).

Oil/water (O/W) emulsions were then prepared with both forms of UV filters maintaining the same ratio between the two filters and choosing excipients consistent with the commercial product.

In order to evaluate system stability, analyses were carried out both on packaged and unpackaged formulations. Containers underwent tensile testing, colorimetric assessment, and extractable testing. Sunscreen formulations were characterized in terms of pH, organoleptic properties, rheological behavior, and filter content. Moreover, a further two non-destructive techniques, including near-infrared spectroscopy (NIR) and the multiple light scattering technique, were used in order to determine the stability of the formulations.