Link: Lipstick formulation: not as easy as you might think | PCM
Publish date: February 2015, Personal Care Magazine

Long lasting, hydrating, matting, plumping: these are all qualities we read about in lipstick advertisements. With new slogans and the guarantee of unique quality, producers encourage us to buy new products. Oil/wax systems sound simple and uncomplicated, but what exactly does that mean for the development chemist?

The first step in formulation development is to define the qualities of the finished product and the basic features of the formulation, such as:

• Price point
• Legislation.
• Marketing claims, if we want to use special ingredients.
• Packaging
• Customer requirements, if we work for a private label service.
• Production possibilities.
• Sensorial specification.
• Physicochemical parameters

Characteristics of lipstick depend on the choice of raw materials, so if we know all the parameters mentioned above, the scale of the search area is much more limited.

Physicochemical parameters and packaging

It is very important to define the right container at the beginning of the project. On the market there are many forms of lipstick. Standard lipstick has a 12.7 mm diameter and weighs about 4 g. Jumbo sticks or slim versions of lipstick have diameters from 7 mm to 9 mm and weigh from 2 g to 3 g. That is why we need to choose the right component and try to specify basic physicochemical parameters:

• Melting point.
• Hardness.
• Viscosity – measured at high temperature, this procedure is used to avoid future problems with the filling process when the bulk is too thick even after heating.

For slim or jumbo types of lipstick, the melting point can be lower than for the standard one. The stick is protected from deformation by the container, so it will not be damaged during a normal use trial. The hardness of the lipstick can also be lower. For the standard form of lipstick the melting point should not be lower than 60˚C. It is very important to find a balance between the melting point value and hardness. If the melting point is too high and the hardness is too strong, application of lipstick is very difficult. Increasing the melting point of the bulk with high melting point waxes may decrease the firmness of lipstick. The product will be fragile and easy to break. Using dry emollients with low viscosity, like silicones, can cause the same defect. To improve firmness we can add heavier emollients with higher viscosity such as diisostearyl malate or pentaerythrityl isostearate/caprate/caprylate/adipate. We can also reduce the content of waxes with high melting points and substitute them with lower melting point waxes; ingredients like petrolatum or with consistency similar to this raw material. Another example of this raw material is lanolin and its synthetic derivatives.

Regulations

Lipstick is an oil, wax and pigment mixture in different concentrations. The most popular natural waxes are candelilla, carnauba and beeswax. The second group is mineral waxes like ozokerite, ceresine, microcrystalline and synthetic wax. To choose the right component we need to consider the legislation of the country it is destined to retail in. The German market is very strict and almost all companies are required to ensure their products comply with the OKO test regulation. This concerns in particular mineral waxes and animal origin pigments like carmine.

On the market we can buy natural products certificated by different organisations like Ecocert or Natrue. For this kind of formulation we need to choose raw materials very carefully. Using raw materials based on natural resources may hinder the ability to achieve stable formulations, and they are not easy to substitute with other ingredients.

Stability tests

The main tests to be performed on formulation are:

• Claims substantiation.
• 12 weeks stability to light and high/low temperatures.
• Physiochemical specification.
• Preservative efficacy.
• Packaging compatibility testing.

The full test is carried out under fixed temperature conditions. At the beginning to check only the stability of the recipe we can use a replacement. But to fulfill all of the examination criteria we cannot forget about the compatibility test in the target container.

An unstable formulation affects all types of lipstick. Most frequently observed instabilities are:

• Lipstick sweating.
• Undesirable crystallisation.
• Colour fading.

Lipstick sweating

To better understand the problem of lipstick sweating first we need to consider the structure of the waxes and the dependence between waxes and oils. During lipstick formulation development we create a new wax structure with trapped emollients inside.¹ Once the wax is moulded a matrix is formed, letting the oils move freely within the stick. The issue of sweating occurs when the size of this matrix is too small, not allowing the oils to move freely. Increased temperature causes the wax matrix to modify and the oil/wax equilibrium to be altered, causing the oil to migrate to the surface of the stick. In a standard formulation emollients constitute about 70%-85% of the ingredients. If the oils are not binding properly inside the wax matrix, syneresis is observed. Fine oil droplets appear on the surface of the stick, during small temperature fluctuations or during storage. The wax structure and the compatibility between the oils and waxes is the key to this problem. The polarity of the oil and waxes is another important factor; it should be as similar as possible. Otherwise we need to use co-emulsifiers to stabilise the formulation like polyethylene glycols. Esters with a low HLB value, like polyglyceryl-3 diisostearate may be the solution to the problem.

Pigment incorporation in a wax and oil mixture causes sweating to be increased because of weakening of the binding force between the wax and oil.

In order to reduce syneresis, it is necessary to incorporate heavy branched chain waxes that sterically hinder the many straight chain waxes in the formula, for example C18-36 triglyceride significantly reduces sweating. Sometimes, the best solution is to change the proportion of the waxes and oils. Practically, as the content of wax is increased, sweating is delayed and the wax structure is increased. Substitution of low viscosity value oils with higher viscosity, heavier emollients prevents sweating.

Colour fading

A major factor in colour fading is selection of proper pigment and its content. Excessive increasing of the pigment content may emasculate the formulation and may have influence on stability. We can choose from inorganic pigments like iron oxides and titanium dioxides or from organic like D&C Red No 7 Ca. If we use pigments in pastes (pigment dispersions, e.g. in Riccinus communis oil) adding to the formulation can be very easy if we check compatibility between the pigment solvent and the rest of the formulation. If we use powder pigments, we should consider a pre-dispersion process to make the production process as easy and cost efficient as possible.

Dispersion of pigments in a liquid medium is typically accomplished in two steps. Wetting the surface of the primary particles and agglomerates and milling the dispersion to break apart any agglomerates. The pigment wetting ability of oils needs to be qualified at the start. A well known pigment wetting agent is pentaerythrityl tetraisostearate and polyester-4.² We can also use substitutes of Riccinus communis oil like hydrogenated castor oil. For better uniformity with the rest of the ingredients of the formulation we can always use additional substances which reduce the surface tension of the mixture.

The right selection of pigments affects the photostability of the formulation. The easiest way to protect from colour bleaching is the inclusion of a UV filter like ethylhexyl methoxycinnamate. In instances where a natural origin oil is used it is necessary to harness an antioxidant, e.g. tocopherol or its mixture with ascorbyl palmitate.

There is also the option to use another ingredient which protects the formulation from oxidation and from colour fading. A well known ingredient with this effect is diethylhexyl syringylidene malonate (and) caprylic/ capric triglyceride.

A small addition of a film-forming agent like PVP/eicosene copolymer will also have a positive influence on colour fading protection.

Wax crystallisation

The crystallisation of the lipstick depends on the individual ingredients. Crystallisations on the lipstick surface most frequently contained waxes in the formula; less often active ingredients or preservatives. There is a simple way to avoid active ingredient or preservative crystallisation, namely pre-dispersion in a proper solubiliser.

The ingredients and basic material used in cosmetic formulations become the important criteria for customers when choosing a cosmetic product. Natural ingredients are preferable. Unfortunately, waxes with a natural original tend to have an unstable structure. Carnauba waxes, candelilla wax, and beeswax are able to crystalise in basic formulas. Microcrystalline wax is also in this group.

Most frequently this issue becomes a problem when incompatibility of ingredients is observed. The best way is to choose ingredients with similar polarity. As a result of the presence unsaturated fatty acids in formulations with a double bond there can be a reaction which will occur with small crystal formations on the lipstick surface. A decrease in the wax content and substitution with a gelling agent like glyceryl behenate/elcosadioate or a rheology modifier like silica or trihydroxystearin changes the thixotropy of the system and guarantees a well built structure and stable stick. Paste-like substances like lanolin can also protect the formulation.

Wax crystallisation exposes temperature fluctuations especially in lower temperature tests and in ambient temperatures after a period of time.

There is another cause of this problem: the production process. If the cooling process is too fast for the bulk production or the mold is too cold during the filling process, there may be an undesirable effect. Slowly cooling the lipstick decreases the amount of crystallisation compared to a conventional production process.3 Sometimes changing the description of the production trial can fix all these problems.

Conclusion

In conclusion, if we are planning to work on a lipstick from the beginning we should focus on marketing claims and sensorial attributes of the finished products. It makes the process much easier and we can avoid a lot of mistakes and loss of time. If the long lasting effect is the main feature and the product should stay on the lips for many hours, the best solution is to use a synthetic raw material with silicones or silicone derivative supplement. Oil/wax systems have limited potential to harness active ingredients and then it is worth considering an emulsion based formula. The content of water may reach 15%, so we have many more possibilities to use a wide range of substances and with higher content. This type of formulation is more complicated in production trials, and that is why first we also need to check our production feasibility. Scaling up the lipstick formulation is not always as easy and simple as we think.

The article shows only the fundamental steps and premises in the creating process of lipstick formulation.

As mentioned at the beginning of this article there are a lot of lipstick types available on the market; based on emulsions, with silicone content or a jelly structure and each formulation is different. To improve the sensorial properties like gloss, and long-lasting effect we need to focus on the selected example. This is a wide spectrum to consider for another publication.

Much of this article is based on experience. There is still much to be learned and it is hoped these notes will help future development of lipsticks.

References

1 Morante N. Sweating in lipsticks, June 2007 [www.specialchem4cosmetics.com].
2 Heldermann M, Burch K. Maximizing the performance of color cosmetics by multifunctional ingredients. SOFW-J 2012; 138 (3): 34-40
3 Seo SY, Lee IS, Shin HY, Choi KY, Kang SH, Ahn HJ. Observation of the sweating in lipstick by scanning electron microscopy. Int J Cosmet Sci 1999; 21 (3): 207-16.

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