Cosmetic product quality is judged by the sensory experience for the consumer through texture, visual, appearance and also odour.
The physical characteristics are often the most important, with the way a product feels to the touch being the critical factor in customer acceptance. This must be combined with the expected performance, meeting the manufacturer’s claims, in a retail environment where some products are very expensive.
Many products are applied to the skin and the texture characteristics during application are as influential as the subsequent feel. Chemists in the cosmetics, personal care and toiletries sector are constantly challenged to meet consistent physical texture expectations, delivered with a range of colours and scent options.
Solid or semi-solid cosmetics and personal care products are generally of the following types:
Creams (crèmes), lotions and gels - containing emulsions, emollients, hydrocolloids, lipids and polyols – are viscous semi-solids such as moisturisers, sun-protection/tanning lotions, make-up removal creams, shampoos, bath or shower gels, shaving creams, waxes and so-called “liquid” soaps. These products are often required to have some firmness, but spread and flow easily, requiring consideration of their rheological characteristics.
Skin crèmes have the conflicting texture requirement of being smooth, rich and creamy in addition to being light and non-greasy or slippery. The desired lack of oiliness in products containing oils, intended to moisturise and hydrate, is a typical formulation objective.
Solid products come in the form of cohesive blocks, for example bar soaps, lipsticks and similar lip balms, plus solid block deodorant/antiperspirants and eye/lip liners in a pencil form.
These items are expected to wear and deform in the course of their use, but in terms of their application, are required to be hard and maintain this structural integrity without breakage, flaking or crumbling.
Compacted or loose powders are solids in a particulate state. These cosmetics are drier by nature and are applied to the skin by brushes, sponges or similar applicators.
Make-up foundation and eye shadow preparations must flow in a controlled, consistent manner, resisting clumping or caking.
These products require reliable application in formulations that contain a wide variety of colour-producing raw ingredients.
Texture test methods for cosmetics products are similar to those suitable for food types which have a comparable physical structure. Firmness, flow characteristics, consistency and stickiness are evaluated for the semi-solid, viscous products, while hardness and break resistance tests are valuable to the analysis of solid products. Powders are measured for flow and spread, in associated with visual inspection for clumping.
The fixtures and procedures for are primarily designed to simulate the handling and application of the product, usually manipulation with the fingers - which is replicated with an appropriately shaped probe. In the cosmetics industry, the manner in which a product is dispensed from its packaging is also important. The flow properties of semi-solid samples are tested by extrusion methods – using texture testing accessories - in association with testing the effort to squeeze or pump the product from its container.
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Compression, using either cylindrical or spherical probe fixtures enables textural properties to be evaluated such as firmness, cohesiveness and fracturability.
Used for semi-solids, which can be tested for their behaviour in isolation, using extrusion fixtures, or their interaction with packaging for dispenser design. Forward or back extrusion can assess the characteristics of flow, consistency, adhesiveness and spreadability, influenced by ingredient viscosity.
Small cylinders, needles and cones are used to penetrate into a solid sample’s surface to test the products strength. A ball probe can test the firmness and thickness of a semi-solid.
Tests relating to the performance of products that undergo stresses during use, can be influential in the formulation and hence have an impact upon texture. A lipstick bullet will experience bending forces during application, as will other solid cosmetics items.
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The adhesive force measured of the return, tension, stroke of the test is an indication of the stickiness of the product. This may be an undesirable attribute of a cream required to be silky to the touch.
Many cosmetics products include gelling agents in their formulations. Gel industry standard tests for the raw ingredient or the final product, which is in the form of a gel, may be performed to evaluate the strength.