Waxes are a class of chemical compounds that are at room temperature moldable to hard. They usually melt above 45°C giving a low viscous liquid without decomposition.
When cooling they re-solidify but remain chemically unchanged. For waxes solubility and consistency are a function of the temperature. Of course they are insoluble in water but soluble in organic, nonpolar solvents. Most waxes are buffable by slightly rubbing and give a remarkable surface brilliance. They are inflammable and therefore the perfect ingredient for candles.
Natural waxes are typically esters of fatty acids and long chain alcohols while synthetic waxes are long-chain hydrocarbons lacking functional groups. Besides the free wax acids and alcohols, natural waxes usually also contain phytosterols and resins and have a very variable, complex composition which makes them individual. Hydrogenated oils are also sometimes called wax as they seem to fit into the definition, but they are more like butters and have different properties such as no oil binding capacity or a low melting point and therefore no good impact on heat resistance.
Natural waxes are produced by bio-organisms to protect themselves against mechanical stress, loss of moisture or molding, UV radiation and parasites. The gas- and water-permeability of wax films is essential, otherwise the organism cannot breathe anymore. Natural waxes are truly sustainable and readily biodegradable.
Characteristics of waxes
To identify a wax, the hardness gets determined which is described as penetration. It can be measured via needle for hard waxes or cone for softer ones which gets pressed into the wax. The extent of penetration shows the hardness. Carnauba wax as one of the hardest natural waxes has a penetration of 0 1/10 mm at 25 °C. The higher the number the softer the wax is.
The crystallisation of a wax is counterproductive to the gloss of an oleogel which is dependent on the refractive index. Waxes with high crystallinity have large crystals when congealed and reduce the glossiness of a formulation or surface. Due to the crystals, waxes cannot form transparent systems. Theoretically wax crystals do not grow over time. Only butters, hydrogenated oils, fats and solid triglycerides change over time their crystal size. In unfavourable combinations this can cause over time visible crystals on the surface of cosmetic products which is called ‘blooming’. But also waxes of low quality containing a higher amount of impurities which can function as a matrix for excessive crystal growth. Furthermore it also happens that the purchasing department thinks ‘wax is wax’ and changes the source for cost saving. Unfortunately if the required quality has not been strictly defined before, a lower quality might pop up and all of a sudden a stable formulation shows issues.
The importance of melting point
The production temperature has a huge influence on the final result. Waxes need to be heated by approx. 20 °C over their melting point to ensure that all crystals are entirely molten and the particles move freely. If this high temperature is not possible to realize, a lower melting wax with similar properties has to be used. For example carnauba wax is very high melting (MP 80°C-86°C) and would need to be heated to least to 105°C, this is for several formulations not feasible. But sunflower seed wax with a melting range of 74°C-80°C having similar characteristics might just work.
Fast melting and melting at high temperatures lead to harder or higher viscous oleogels. A long heating time and repeated melting reduce hardness or viscosity. In case the bulk congeals completely and gets re-heated, it is crucial that it is heated again over the cloud point to break the crystalline structure again. Fast cooling causes smaller crystals and leads to harder oleogels, but can also result in extreme shrinkage and cracks. The cooling effect is faster in the outer zone, therefore a discrepancy in viscosity/ hardness between outer and inner zone can occur. For most oleogels, a slow cooling at room temperature results into the most stable formulations but is not practiced as a lot of space and time would be necessary.
A good example is an anhydrous hair wax formulation that has been made in the laboratory which had a good hardness and an excellent stability. This type of hot poured formulation is usually filled at 70°C -80°C in a jar and allowed to stand for congealing at room temperature. Then first production in large scale takes place and of course the thousands of little jars make their way through the cooling tunnel. Afterwards they are all cracked in the middle. The production procedure cannot be changed, so the formulator has to modify now the formulation to reduce shrinkage and improve flexibility. This is a lot of work and might never give the exact same product that was originally designed in the laboratory. Therefore it is much better to cool prototypes for congealing there as well to be aware of what might happen.
Besides the temperature, also the conditions in production are essential. Agitation of oleogels below the cloud point destroys the crystalline structure and reduces significantly final hardness/ viscosity. The cloud point marks the change from the transparent melt with loose or no crystals to the opaque crystal matrix. Therefore clear instructions at which temperature stirring is still possible should be made. Reheating of bulks (especially polyethylene based ones) that have been stirred almost until congealed show a lower hardness but are much creamier and have a better heat resistance. Additionally, entrapped air bubbles get released. The filling temperature and behaviour also has a crucial impact on hardness/ viscosity and on final stability. Oleogels from production are usually softer than from the laboratory due to the longer heating/ production times. Variations in the production procedure lead to very different results! For example simple oil-wax blends poured at 35°C are usually instable semi-liquids but poured at 80 °C form stable, homogenous oleogels.
The crystallinity changes significantly when the pure wax is combined with other ingredients. This also affects the DSC (Differential Scanning Calorimetry) curve which shows the difference in the amount of heat required to increase/ reduce the temperature of a wax sample characterising its melting and congealing behaviour. The DSC curve and the melting/ congealing point of a complex formulation cannot be deduced by the data of the contained waxes. The melting and congealing points are not so relevant for the stability but important for the manufacturing instructions.
It is highly recommended to know the processes in production exactly and to mimic them in the laboratory as well. This is the only way of getting the same final results. A lot of formulations works perfectly when made in the lab, but as soon as it is produced in large scale they are much softer or show signs of instability. There are many issues that are actually not caused by a bad formulation composition but by a variation of manufacturing parameters.