The cool choice: COSSMA

photo: Niederrhein University of Applied Sciences

What are the benefits and limits of cold process emulsification? How about suitable process and formulation designs or selection of ingredients? Prof Andrea Wanninger explains what is actually possible and what influence the choices have.

Cold process emulsification has become popular as a way of reducing energy, production time and costs. It is seen as a greener production method, increasing the sustainability of a cosmetic emulsion and significantly reducing the carbon footprint. Cold-cold formulations may also exhibit special textures and thus enrich the world of cosmetic products although formulators face certain issues regarding ingredients, formulation design, and methods. In this article the criteria for successful cold process emulsification are discussed.

Cold processing benefits

In traditional hot-hot processes, the oil phase and the water phase are heated separately, in some cases up to 80 – 90°C. All solid ingredients have to be molten or dissolved. Then the emulsion droplets are formed by combination of the phases and agitation or homogenisation. After that, emulsions are carefully cooled, sometimes at the rate of 0.5°C per minute ¹. The batch viscosity needs an enormous time to develop, as well as the creation of lamellar structures, and finally, heat sensitive ingredients are added.

Making laboratory work and production faster is one of the major benefits of cold processing. The heating and cooling steps are skipped and the initial viscosity can be measured faster. Significantly reducing energy and costs is the other big advantage. Heating/cooling 1 ton of water from 20°C to 70°C and back takes 120 kWh of energy². Compared to the hot-hot emulsification process about 90% of the energy and about 40% of the processing time can be saved in cold emulsification². Less energy consumption is associated with less CO₂ emissions and is thus reducing the environmental impact. In addition, the faster and low energy consuming manufacturing is much more cost effective.

Besides those effects that are predominantly important for production, development of formulations in the laboratory is more flexible. Batches can be easily split to add further ingredients and test variations. Sensitive ingredients can be incorporated at an earlier stage of the emulsification process. Adjustments to the viscosity or colour of a batch are easier¹.

Automated emulsification and analysis is also made easier and faster. A high-throughput design can lead to even more variations than the normal bench trials. And the process conditions are perfectly stable and monitored in automated processes.

Limits of cold emulsification

The cold process has clear benefits, but the selection of suitable cosmetic ingredients is limited especially when natural cosmetics are the aim, and has some preconditions.

Most ingredients have to be liquid, and solids have to be soluble in the oil phase or the aqueous phase. Some ingredients such as metal soaps or solid particles which form Pickering emulsions are dispersed in order to stabilise emulsions. Waxes, solid non-soluble emulsifiers and certain polymers, which have to be processed hot, are excluded. This limits the formulation design and also alters the rheological properties and skin feel in comparison to traditionally produced creams, lotions and sprayable emulsions. Self-emulsifying bases, wax dispersions, liquid poly-mer dispersions, surfactant-based systems, microemulsions, and emulsion concentrates can be seen as alternative ingredients.

Cold-process emulsions have to meet the standards regarding temperature stability and product performance. This can be challenging because some of the stabilising ingredients are not present. In emulsions that are produced in a hot-hot or hot-cold process, solid, wax-like ingredients are parts of stabilising lamellar gel structures in O/W emulsions or oleogels in W/O emulsions. Those structures are partly responsible for the stabilisation and skin feel of emulsions.

The texture of O/W cold emulsions is in many cases dominated by rheology modifiers such as hydrocolloids in form of polysaccharides, inorganic thickeners or polymer blends, while polyacrylates are becoming less popular.

Being aware of those alterations, cold emulsification is successfully used in skin and sun care and less often for colour products which require high temperatures, hot melting and hot-pouring processes¹. In the following paragraphs further details on cold process emulsions are given.

Process Design

In cold processes all ingredients have to be completely dissolved or dispersed in the phase they have been added to. Therefore, suitable emulsifiers are either liquid or soluble in the oil phase or the aqueous phase. Agitation has to be long enough to ensure homogeneous distribution. Stabilizers like metal soaps in W/O emulsions must be properly dispersed.

In many cases, the stabilising polymers are added to the oil phase and dispersed homogeneously. Then the aqueous phase is added and the emulsion is formed. The emulsifier has to occupy the interface and form a stable barrier against coalescence. The time required for homogenisation may be longer than for hot-hot emulsions because the interfacial tension is usually higher at low temperatures. In special types of O/W cold emulsions oil-trapping polymers are used instead of classical emulsifiers.

In some formulations for O/W emulsions with emulsifiers of lower HLB values the oil phase and the aqueous phase, have to be combined without stirring to avoid the formation of a W/O emulsion.

The lack of stability of certain rheology modifiers against shear stress is an important factor. Therefore, they may be added at a later stage of the process when the fine, homogeneous emulsion is only stirred and not further homogenised. Other thickeners require intensive shear in order to swell homogeneously and they have a tendency to form lumps. Pre-dispersions of thickeners in glycerin or in emollients are added prior to homogenisation. This may be an option to avoid aggregates.

In cold processes, temperature-sensitive ingredients and electrolytes may be added at an earlier stage than in hot-hot or hot-cold emulsification, but nevertheless their influence on the formation of stable fine droplets and the long-term stability of the cosmetic product has to be studied. The usual way to operate is to add certain actives, preservatives and the fragrance during the final steps after homogenisation.

An exception may be the microbiological protection system which can be added at an early stage in robust formulations. Equipment and materials for cold processing generally have to be extremely clean and non-contaminated. This means that ingredients, especially solid products with a large surface, have to be carefully monitored and the emulsion has to be protected in the best way possible³. It may be favorable to apply the preserving agents or multifunctionals prior to adding natural rheology modifiers to the formulation.

Formulation design

In the past years a large variety of textures and the whole viscosity range from fluid to high viscous with very light to rich skin feel have been developed. In this fast-changing industry (FMCG – Fast Moving Consumer Goods), formulators of personal care are constantly facing challenges caused by consumer trends as well as cost and time pressure. Major trends are clean beauty, skinimalism and personalisation. Cold process emulsions may be an option to create minimalised, environmentally friendly formulations and may be personalised by adding further actives.

O/W emulsions:

The choice of the emulsifiers or emulsifying blends will determine the possible viscosity range and formulation flexibility. The second crucial ingredient are the rheology modifiers. They have to be carefully evaluated in order to provide the right viscosity, texture and stability to the formulation. Combinations of polymers are favorable.

W/O emulsions:

In those emulsions the phase-volume ratio plays a major role. The viscosity of the product is very much dependent of the amount of oil phase used in comparison to the internal aqueous phase. The more water in the W/O formulation the higher the viscosity and the lighter the skin feel. Also, the oil phase viscosity directly corresponds to the product viscosity. These considerations have to be clear when developing a formulation. Stabilising electrolytes in the aqueous phase and stabilising metal soaps in the oil phase are important for the long-term stability and have to be carefully selected.

Special formulations:

In cream gels, oils are trapped in gel pores of polymers. Also, gel-in-oil emulsions are possible. Some pre-blended emulsion concentrates are on the market which are mixed with the water phase and additional oils¹. Pickering emulsions are stabilised by very small solid particles at the interface. They are also suitable for cold processes.

Ingredients for cold emulsification

There has been a lot of development regarding ingredients for cold emulsification. New types of rheology modifiers and water-based thickeners have been launched⁴.

Some of the new emulsifiers and blends that were launched in the past few years are capable of forming extremely stable sprays and low viscous products up to creams. Sprays are possible for both, O/W and W/O emulsions. Especially for W/O emulsions this is a big advantage and achievement.

In a future article, ingredients for cold emulsification are going to be featured in more detail. The variety of suitable basic ingredients, which is nevertheless still huge, will be presented and compared as an orientation for product development, focusing on emulsifiers and rheology modifiers or thickeners.

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