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Evaluation of Sun Care Products

    thewiki Editorial

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    Parameters generally tested by cosmetic companies to evaluate the quality of their sunscreens include spray characteristics;

    • Aerosol can leakage
    • Actuation force
    • Pressure test for aerosol products
    • Spreadability
    • Extrudability
    • Texture
    • The firmness of lotions, creams, or gels
    • Stick hardness
    • Stick softening
    • Melting point
    • Stick pay-off and glide
    • Preservative efficacy
    • Viscosity
    • pH

    The most important spray characteristics include spray rate, spray pattern, and droplet size.

    Spray rate, otherwise known as aerosol discharge rate, is usually determined by measuring the weight loss over time. This determination measures the amount of the product in weight before the measurement; this value is the initial weight. Then, the product is discharged at a constant rate using a standard apparatus for a given period. By reweighing the container, the change in weight per time is the discharge rate, which then can be expressed as grams per second.

    Spray pattern refers to the area of the sunscreen spray that hits the skin. This parameter can be determined by using alcohol-sensitive paper (as aerosol sunscreens are alcohol-based). As the droplets are deposited on the paper, they initiate a color change (staining). The shape and diameter of the stain provide information on the spray pattern. Normally, a circle should be seen on a piece of paper. Other shapes such as a hollow cone or flat stream sprayed in any direction are not acceptable.

    Sunscreen spray droplets are formed upon expansion and rapid evaporation of the propellant after expelling the product from the can. The droplet size depends on the pressure within the can, the type of actuator, and the physical properties of the formulation. The particle size distribution produced during atomization relates directly to the rate of drying and the likely formation of actual droplets on the skin after application. Generally, the laser diffraction technique is used to determine both the droplet size and the droplet size distribution of aerosol sprays. It is based on the principle that particles (droplets) scatter light. The intensity of scattered light can be measured as a function of the angle, which can be then used to yield size distribution.

    Aerosol Can Leakage: Leakage testing is a basic requirement for all aerosol containers. One of the general testing methods is the immersion method. It is performed by completely immersing the filled aerosol can in cold/hot water and examining whether air bubbles are formed or not. This testing is typically done under conditions that could cause a can to erupt, presenting safety risks to the operator of the equipment.

    The USP describes a method where the weight of filled aerosol cans should be measured and recorded. The method applies only to topical aerosols, that are fitted with continuous valves. The weight should be measured within 3 days, and leakage can be calculated using a formula.

    Actuation Force for Aerosol Cans: Actuation force testing allows measurement of the force required to release sunscreens from aerosol cans. Testing of the actuation force is important to see whether users will be able to get the product out of the container by pressing the actuator. Therefore, this test provides an imitative test and helps assess any issues related to the use of the product.

    Pressure Tes: Pressure in aerosol cans can be tested in several ways. An option is to subject containers to hot water. The duration of the test and the temperature should be such that the internal pressure reaches that which would be reached at 55-degree celsius. If the contents are sensitive to heat, the temperature of the bath must be set between 20 degrees Celsius and 30 degrees Celsius, and one container in 2000 must be tested at a higher temperature. No leakage or permanent deformation of a container may occur during the test.

    Stick Hardness Test: The hardness of sunscreen sticks depends on the type and ratio of the ingredients. Waxes provide luster, adequate firmness, and appropriate molding properties for the sticks and also significantly influence the final texture of the product. Waxes, however, differ in hardness. Measuring the hardness of a stick by carrying the composition and wax container is a useful tool in quality control for assessing whether a wax of interest is suitable for producing the desired properties.

    The generally used test is the penetration test, adopted from the ASTM Standard Method of Test.

    The penetrometer contains a metal needle probe that penetrates the product. methods used may vary; they either determine the distance of penetration when applying a specified force over 5 seconds, or they measure the force of penetration at a defined deformation distance of 5 mm. The test can monitor the hardness over a range of temperatures that the stick may be subjected to during transportation and storage. In addition to determining the stick hardness, this test may indicate the presence of unwanted trapped air bubbles or a "grainy" texture.

    Melting Points of Sticks: Determination of the melting point is important as it is an indication of the limit of safe storage. This is mainly and significantly determined by the ratio and types of waxes used. The melting point is the temperature at which a solid phase converts to a liquid phase at 1 atm of pressure. It is usually determined by the capillary tube method in which a small sample of the substance is placed into the melting point capillary to a depth of about 0.04 in. (1 mm). The capillary is then placed into the capillary apparatus and the sample is gradually heated. the temperature at which the liquid is first seen in the lower end od the melting point range.

    Softening Points of Sticks: A sunscreen stick should withstand a range of conditions to which it will be subjected in the consumer's handling. It should be resistant to varying temperatures and just as easy to apply in hot as in cold weather.

    Softening point is usually determined by the ring and the ball method. In this process, a stick is fitted into a ring or support orifice, and the extra mass above and below the ring/orifice is removed with a sharp blade, so practically, a tablet of the stick remains inside the ring/orifice. This is placed in a refrigerator for a few minutes, for example, 10 minutes, and then the ring is fastened onto a stand. A breaker containing 500 ml water at room temperature is placed on a hot plate having a magnetic stirrer. A steel ball is delicately placed on the stick tablet. The bar with its support is then started and the temperature is monitored using a thermometer. The temperature at which the mass and still balls are loosened and fall to the bottom of the beaker is the softening point.

    Pay-off and Glide: Pay-off refers to the weight of a product transferred to a surface, for example, the skin upon application. Glide refers to the easiness of moving the stick over a surface. It can be characterized by measuring the friction as the stick is moved on a surface. Both properties depend on the wax/oil ratio of the formulation and are also a relationship to the hardness of the product.

    Harder and more rigid sticks have a lower level of pay-off, which is unfavorable from a container perspective. Inadequately, soft products (containing more oils) may, however, soften at lower temperatures and lose their shape. The equipment used for this test usually contains a piece of paper and an arm for fixing the sticks. The stick is moved on the surface of the paper at a constant speed with standard pressure, mimicking the force applied by consumers. The mass in grams transferred to the paper i.e., the pay-off is measured afterward. Additionally, the friction coefficient is also measured during the movement, which refers to the glide.