Lip Balm Formulas

Lip balms or salves constitute a special category of protecting and soothing cosmetics. They are intended to be applied to the lip pseudo-mucosa to relieve the signs and symptoms of skin dryness and chapping. Their function and formulation are described here.

Lip Pseudo-mucosa

Lips are usually submitted to an alternating sequence of wetting and drying from the tongue and accompanying saliva. Therefore, the osmotic stress of the skin cells in the lips is continuous. This organ also is frequently in contact with foreign materials such as food, drinks and toothpastes—as well as nonresident bacteria and yeasts from kissing. It is also continuously stretched and compressed.

Lip structure is not homogeneous, progressively changing from the conformational situation near its external border, similar to normal skin, to the organization near the oral cavity, similar to the mucosa epithelium. Lip skin does not possess sweat glands. Its lower layers are permeated by blood flow and nerve endings, making lips sensitive to temperature, pressure changes and abrasion. Like other skin, lip skin is subjected to changes associated with aging. UV radiation tends to contact the lower lip more, as a perpendicular surface, which makes photoaging worse in that area.

Skin sagging due to the progressive collapse of intercellular matrix polymers induces a deepening of the folds at the two corners of the mouth, with increased possibility for microbial growth. Smoking habits induce hardening of the lips and parallel wrinkling lines that are perpendicular to the lip borders. This presents a challenge to lip care application; if the fluid percentage in a lip care product is high, the product often diffuses inside lip wrinkles or outside of the lips due to capillary forces.

Lip Balm Attributes

In addition to moisturizing the skin, lip balms must protect the lips from harsh environmental conditions such as extreme cold, wind or sun. Mild antibacterial and antifungal properties are often included in these formulas to treat Angular cheilitis, inflammatory lesions at the corner(s) of the mouth that can be caused by fungal infections, among other causes. These lesions present as deep cracks and splits that, in some cases, may painfully bleed or form ulcers and crust.

Key performance attributes of lip balms include: ease of application in a wide range of temperatures; film-forming with simple, mild pressure; physical qualities such as evenness, plasticity and elasticity of the applied layer; an emollient and soothing perception on the lips; a pleasant flavor that is palatable when licked or ingested; a long-lasting perception of protection; and a natural appearance. All these attributes must be achieved without noticeable stickiness, greasiness or excess shine, especially since lip balms are frequently used by men and children.

Without sweat glands, skin occlusion by cosmetic products does not pose a problem to the lips. This is why lips salves are frequently anhydrous products. W/O emulsions are becoming increasingly popular in lip care, as they moisturize faster with lasting adhesion to the lips. The correct combination of solid film-forming and fluid emollient ingredients results in a successful lip balm formula. This column will discuss lip balms in stick form and in anhydrous ointments separately.

Lip Balms in Stick Form

The practical advantages of lip balms in stick form reside in the ease and precision of application on the specific skin site. The stick can easily trace the lips, and the consumer is able to tell where the product is being applied and if the film amount is considered sufficient. Moreover, stick handling is easy, as the solid stick does not bleed or leak and is protected by the container. Anhydrous sticks do not require preservatives, avoiding additional risks to irritated skin. A typical example of a lip balm stick is shown in Formula 1.

Waxes: In stick lip balms, the formula structure and thickness are generally produced by waxes of animal (beeswax), vegetal (carnauba, candelilla, rice) and/or mineral (paraffin wax, microcrystalline wax) origin. Synthetic waxes, notably several behenic acid long-chain esters or triglycerides and polymethylsilsesquioxane, may also be utilized. Hydrogenated castor oil or other hydrogenated triglycerides are useful for obtaining the same structure functions.

Waxes provide most of the film-forming properties and dictate the final thickness of the applied layer. Their water-impermeable layer helps retain moisture in the lips. As waxes are sometimes considered too occlusive, they have recently been formulated into emulsions. Individually in a formula, waxes result in a hard and brittle stick. To reduce their tendency to form large crystals, resulting in poor mechanical properties, a first strategic step is to use a blend of at least three waxes. Wax blends improve the mechanical strength of sticks, lower melting points and ease formulation trials.

Bleached grades of natural beeswax and dark-colored vegetal waxes should be used, as successful lip balms in stick form are generally opaque white. The best way to know if the total percentage and composition of the solid waxes in the formula, i.e., after blending with emollients, succeeds in providing a soft but mechanically resistant stick is to control the drop point of this blend. The drop-point value is influenced by a linear combination of the average melting point of the formulated blend with the viscosity of the softened mass at the same point. When the formula seems acceptable, even small changes like +/-1% of wax may strongly modify the product characteristics. A traditional Ubbelohde drop-point thermometer can be used when making lip balm sticks. Also, more recent devices use the interruption of a light beam when the first fluid drop falls from the melting orifice. In general, the drop point of stick lip balms is 60–70°C. Lower values are considered dangerous for the stability of the stick in hot climates, while higher values result in low pay-off and excessive hardness during application.

Sensory evaluation of the finished stick is necessary to correlate the drop-point value with ease of application. On average, the total amounts of wax in the formula can vary between 20–40%. In addition to technical specifications, purity levels related to heavy metals and protein materials, and the absence of suspended matters, color and odor are also important. Lip balms for atopic or allergic individuals often incorporate synthetic waxes rather than vegetal or animal waxes to reduce levels of potential allergens. To speed the wax melting process, scale and drop form waxes are preferred in the production run.

Emollients: Blending different waxes will not sufficiently produce functional stick lip balms, which require emollients, and with many oil ingredients claiming emollient properties, choosing the appropriate emollient is not easy. Waxes exhibit low polarity values; therefore, they are easily miscible with nonpolar oils such as hydrocarbons. Mineral oil, squalane, hydrogenated polydecenes and polyisobutenes represent the hydrocarbon category, which provide adequate swelling of the waxes and plasticity of the applied film. Selecting those with the appropriate average molecular weight or oil viscosity is determined experimentally in formulation trials by casting the molten stick into molds and conducting indicative sensory evaluations of the stick’s performance on the lips.

Purity levels of mineral oils used in cosmetics are defined by the general provisions of the Cosmetic Directive (Regulation (EC) 1272/2008) in that the production process of these ingredients must be transparent and carcinogenic impurities must be analyzed and removed. Hydrogenated polydecenes, polybutenes and polyisobutenes are produced at high purity levels and can be produced as food-grade ingredients.

Compatibility issues should be considered when categories of oils other than hydrocarbons are used in the formulation. The selected oils should be compatible with the wax blend both in the molten state and when the mass becomes solid. Separation of oils during the cooling phase impacts the stick’s stability. Nevertheless, polar emollients can be added to the formula with success by adopting systematic formulation trials.

The most common vegetal fluids used in lip balms are castor seed oil, shea butter and jojoba oil, and unsaturated vegetal oils such as sweet almond, macadamia and wheat germ oil, which are skin-friendly due to their unsaturated lipid content. Castor oil and the unsaturated oils must be accompanied by an adequate antioxidant system for the protection of their double bonds. As natural waxes and vegetal oils always contain some traces of iron, which promotes oxidation, sequestering agents must also be added. The oil blend helps the dissolution of waxes when heated at 75–80°C. In addition, it provides softness at room temperature and adequate plasticity, due to oils swelling the wax crystals.

Emollients are also used to address appearance. Lip balm sticks can be interpreted as an oil blend that is adequately gelled, when brought to room temperature, by a blend of waxes. The stick structure is like a sponge, created by the arranged network of liquid-softened wax crystals. In these solid crystal network structures, formulation prudence is necessary. If the emollient blend is thin, mobile and excessive when compared to the absorbing properties of the wax crystals, sweating will occur. Sweating is characterized by the surfacing of oil droplets under the combined action of dilatation and shrinking of wax crystals with the day/night cycle or just when exposed to high temperatures.

The appearance of the stick can also be altered with the crystallization of waxes in a metastable form, which frequently occurs during the cooling phase. With time and temperature, a phase transformation mechanism takes place that rearranges the crystal shapes, their interlacing and their swelling potential. To minimize this event, around 30% of emollients characterized with a pasty consistency such as shea butter, mango butters or bis-diglyceryl poly acyl adipate can be included in the formula. Thickeners such as synthetic esters, dextrin palmitate, smoked silica and glyceryl behenate can also be added to the oil phase. In addition, emollients with bulky structures such as branched chain triglycerides can also prevent the transformation. The amount of emollients in a stick can be 60–70%. Finally, albeit rarely, silicone oils can be used for their velvety feel.

Active ingredients: Actives in lip balms provide long-lasting restorative, soothing and protective properties. These properties are generally achieved through the use of oil-soluble vitamins A and E, soy sterols or tocotrienols from palm oil. Alfa-bisabolol and glycyrrhetinic acid are also common soothing ingredients, together with shea butter and avocado oil or its nonsaponifiable fraction. Marigold oil extract is commonly used for its anti-irritant and repairing benefits.

Several types of oil-insoluble actives such as micronized allantoin, vitamin H, orotic acid and zinc oxide are frequently added in suspension for their skin-soothing and healing properties, which take place upon contact with humidity on the lip surface. In this case, care should be taken during the molding process to avoid settling of these powders. Panthenol, which has the same separation problems, was used in the past for its re-epithelializing action but it is better to use its ester, panthenyl triacetate.

Sunscreens are also often incorporated into lip balms. Physical filters like ultrafine zinc oxide and titanium dioxide are used but can impart a white trace on lips. Generally, these physical filters are used for special applications like lip balms for winter sports. In other cases, the allowed organic filters are used, generally as a combination of UVA and UVB sunscreens. Oil-soluble filters such as cinnamates leave behind a film that is shiny. In general, actives in lip balms vary between 1–10% according to the formula requirements.

Wetting ingredients: For a long- lasting stay on the keratinized, hydrophilic lip surface, a lip balm formula should contain some polar, or wetting, ingredients. This is easily accomplished with low HLB emulsifiers or hydrophilic esters such as lactates or monoglycerides, which create a lasting link due to physical attraction between the humid skin of the lips and the applied film. Humidity is also slowly incorporated into the film by the emulsifier, making the wear of the applied amount more comfortable. An example of such a hydrophilic ester is polyglyceryl-3-di-isostearate. When used at high amounts, i.e., up to 10%, emulsifiers with HLB values around 8 allow the incorporation of water into the formula to produce a w/o solid emulsion lip balm. Glyceryl monostearate can also provide a certain affinity for the lip surface. Evaporation of the emulsified water can be avoided by adding hydrophilic polymers, e.g., hyaluronic acid, adequate amounts of hydrotropes such as glycerin or betaine, and salts such as magnesium chloride.

Antioxidants: Antioxidants are generally necessary in this type of formula for the protection of unstable oils and to shield against the decomposition of flavor molecules, which are repeatedly submitted to high temperature cycles. The complex formed by citric acid as an iron chelator, lecithin and ascorbyl palmitate is a traditional blend used as an antioxidant but more modern types are also effective. Recently, a lipophilic chelating agent^a has been introduced that stabilizes lipogels. Vegetal antioxidants like rosemary and green tea extracts are also utilized, and rosemary exhibits preservation activity.

Antibacterial agents: Antibacterial agents are not necessary in solid anhydrous products, as microorganisms cannot develop without water and spores cannot be killed by bactericides. In some cases, preservatives are added to avoid bacterial development on the stick surface due to saliva and skin cell residues. However, unless the stick balm formula is conducive to such development, traditional preservatives should not be used so as to prevent any accumulation in the body.

Flavor: Flavor is a requirement for making the stick pleasant to the consumer while hiding the odor of base materials. The right flavor acts as a communication ingredient, suggesting healing and soothing perceptions. In general, vanilla, violet, honey, “candy” and mint notes are adopted in amounts ranging from 0.5–1.0%. Sweeteners such as saccharin and fructose can also be added to complete the pleasant profile of the product and avoid any rejection when tasting it with the tip of the tongue.

Controls

The most common quality controls for lip balm formulas are: drop point, sensory evaluations during application, and temperature stability at 45°C and 4°C. Controls should be carried out with the sticks in the final containers to check compatibility and verify if the sticks are tightly held by the small cup on the bottom of the packaging. To prepare samples for drop point measurement, the sticks should be melted rather than cut. This is because during molding in cool molds, the external part of the sticks is enriched in high melting point ingredients while the inside has a larger amount of low melting point ingredients.

Practical molding experiments into the final shape allow the manufacturer to understand the time necessary for cooling and identify difficulties in extraction. Differential scanning calorimetry can be used to understand thermal behavior of the formula, where transition absorption or release of heat during progressive heating of the sample can be seen. Phase changes, glass transitions, emulsified water evaporation, decomposition of delicate ingredients and the solidification/melting process can be seen and used for formulation and quality control.

Production

Once weighed, the waxes should be added into the heated melting vessel with the oils that are stable at high temperatures. At around 80°C, when melted sufficiently as to avoid problems with mixing, antioxidants and sequestering agents are added with slow mixing. The remaining oils can then be blended in, followed by the oil-soluble actives. The liquid is filtered through a metallic sieve to eliminate all solid impurities. Known solid ingredients should be dispersed into the fluid mass. Flavor and sweetener are added last.

When a tinted lip balm is desired, pigments typically are not added as pure powders. They are separately milled in a ball mill together with a wetting agent such as castor oil until their dimensions are below 15–30 μm. The entrapped air is easily released under slow mixing; therefore, this process is usually not conducted under a vacuum. And as long as it is kept low, humidity does not influence the outcome of the product.

If not immediately cast into molds, the melted stick formula is poured into large metallic or plastic containers to solidify into cake form, which can be accelerated by refrigeration. To transition the mass into the melting vessel, it must be cut due to its large weight and dimensions. Since the composition is a crystallized solid and not homogenous, i.e., fractional crystallization, it should be cut vertically from the center out while maintaining equal parts of central, side and intermediate solids in each piece. The solid parts are then fed into the melting vessel and cast in molds or inserted into automatic molding machinery. To compensate for flavor losses due to continuous heating and repeated melting of the scraps, some flavor can be added to the melting vessel.

Lipogel Lip Balms

The formulation principle of lipogel lip balms (see Formula 2, from HallStar)¹ is similar to that of sticks but the amount of wax materials is much lower. This does not mean that the formulation strategy is lighter because lipogel viscosity is dependent on the cooling phase procedure to determine the swelling of waxes and the dimensions of their crystals. Obtaining a thixotropic behavior in lipogel lip balms is not easy given the small amount of available oil-soluble thickeners (see previously mentioned thickeners of the oil phase). Without the constraints of a mechanically resistant solid stick, blending the ingredients and actives into the formulation is easier. Lower total amounts of waxes allow for a larger portion of polar oils, which enhances performance.

These products, like stick balms, are filled at high temperatures. Conversely, there no solid materials are produced during filling that need to be recycled into the melting vessel. For this reason, heating cycles are short, and product stability is not strongly influenced by oxidation.

Long-term thermal behavior and crystallization phenomena happen slowly and should be carefully followed in the final tubes, the typical packing for these balms. The tube opening should be appropriate to the viscosity of the product. Some oil separation over time is practically unavoidable in lipogel lip balms, but silica dimethyl silylate can counteract this inconvenience.

Emulsions in Lip Balms

When water, hydrotropes, preservatives and water-soluble active principles are required in a formula, a premix emulsion with a lipophilic emulsifier that is solid at room temperature can be used. This may be combined with the aqueous phase at 75°C and allowed to solidify. The proportion of water:emulsifier would be around 50:50. This is then added to the molten formula of oils and waxes just before casting. To maintain the standard production process and the stability of the stick, the maximum final water content should not exceed 10%.

Innovation

Some manufacturers have innovated lip balms by adding makeup properties or additional aesthetics to them. Examples include lip balms with lipstick properties such as pigment or pearl,^{b, c} or those with lip gloss features such as large amounts of different branched chain esters^d. Likewise, lip stains^e and primers have been introduced^f with moisturizing benefits, and one recent balm launch combines sensorial attributes such creamy application and velvety after feel, with both light and rich textures. This balm also addresses chapped winter lips with a blend of soothing vegetal oils and hydrogenated triglycerides in a silicone wax structure^g.

In terms of actives, an extract of mustard sprouts^h has demonstrated lip-plumping activity due to increased blood microcirculation. Lip-plumping is also claimed from an extract of rose moss^j, and a blend of soybean glycerides and shea butter^k claims to moisturize, heal fine lines and soothe chapped lips. Finally, calcium hydroxyapatite in micronized form has demonstrated skin elasticity improvements in aged skin due to its capability to slowly release skin-strengthening calcium and phosphate ions. It also shows an enhancement in sunscreen effects.^{2, 3}

References

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1. Skin Care Formulary, Cosmet & Toil 125(1) 65 (2010)
   
2. SK Jain and NK Jain, Multiparticulate carriers for sun-screening agents, Int J Cosmet Sci 32 89–98 (2010)
   
3. L Rigano et al, A new physical sunscreen (in Ital), Cosmetic Technology 12(6) 17–21 (2009)