The demands on release agents vary according to external circumstances such as the material the form is made of and the air temperature. In many countries there are also special requirements concerning the environmental and health impact of release agents.
Concrete's distinctive behaviour determines the release mechanism. When damp concrete comes into contact with organic acids, a soap-like film is formed. It is this film that acts as a lubricant and prevents the concrete sticking in the form, and, for this reason, release agents for concrete often contain fatty acids. To achieve the desired effect, it is important that the amount of fatty acid and the acid value are within clearly defined limits. This can be achieved by dissolving the fatty acids in a mineral oil.
As well as preventing the concrete sticking in the form the release agent must itself stick firmly to the surface of the form, so that it does not run off before the concrete is poured in. This is particularly important when it comes to metal moulds, when it can be difficult to get a release agent to remain in place, especially if the form is exposed to strong vibrations. Placing concrete in the mould also puts a mechanical strain on the release agent.
The usual way of applying the mixture is to spray it around the surfaces of the mould, which means the viscosity must be fairly low. If the product is to be used in cold climates it must also be fluid in the winter.
Fatty acids added
A functioning release agent is created by using an oil as a base and adding various substances to produce the right properties.
The desired acid value is obtained by adding fatty acids, and, to get the right stickiness, wetting agents are added that reduce the oil’s surface tension.
Other additives are needed for a range of functions, including preventing air holes forming in the concrete and counteracting corrosion.
All these technical requirements on the carrier oil would be easy to satisfy if there were no environmental or health aspects to take into account. If that was the case, diesel oil could be used – as it is in some countries.
Diesel oil has the right solubility properties, although these vary and can never be guaranteed. It can also cause discolouring and lead to the formation of air holes in the concrete.
The biggest problem with diesel oil, however, is that it is refined to a low level and has high contents of substances that are harmful to the health, such as carcinogenic polyaromatic hydrocarbons. Using a toxic oil means exposing construction workers to substances that are hazardous to their health in a way that is incompatible with even moderate requirements for a good working environment.
In addition, traces of these substances could remain in the finished concrete and be harmful to those living or working in buildings made of concrete, and small quantities of oil could also find their way into the ground and watercourses.
Right for the environment
The oils that are suitable from the environmental viewpoint are highly refined naphthenic mineral oils, highly refined paraffinic mineral oils and vegetable oils.
How highly refined the oils should be depends on the level of regard that is being given to the environment. Some manufacturers use technical white oils, while others are satisfied with non-labelled oils, ie those not classified as toxic.
In some places it has long been the custom to equate environmentally friendly products with those that are biologically degradable. However, this standpoint is being increasingly questioned, as life cycle assessments indicate that the production of vegetable oils involves an environmental load that counteracts the advantages of the biological degradability (see Naphthenics Magazine number 2/01). It is possible, therefore, to claim that a highly refined mineral oil is equivalent to a vegetable oil from the environmental point of view.
Biological degradability has another downside: poor stability. A vegetable oil can be subject to microbial degradation during storage, so it quite simply goes rancid. This could also happen with traces of oil that might be left on the concrete, leading to an unpleasant smell.
Release agents have to fulfil the following requirements:
- Good solubility of substances such as fatty acids, wetting agents and other additives.
- Good low temperature properties.
- Low viscosity.
- Good environmental characteristics.
Good at low temperatures
We have addressed point one by making a comparative study of how naphthenic and paraffinic oils dissolve some of the most common additives (see below). Vegetable oils are different: there is no point in studying how they dissolve fatty acids since they already contain more fatty acids than is necessary.
The difference in low temperature properties of naphthenic and paraffinic oils is well studied and documented. Naphthenic oil’s superiority in this respect is one reason why naphthenic oils dominate among transformer oils.
The properties of vegetable oils at low temperatures depend on their composition. If they contain a high level of unsaturated fatty acids they can maintain a low viscosity at low temperatures. This can, however, mean that their acid value is too high, so solvents have to be added to reduce the viscosity – something that can make the vegetable oil more expensive and less attractive from the environmental standpoint.
When it comes to viscosity, it is possible to get both naphthenic and paraffinic oil at the desired viscosity, but it can be more difficult with vegetable oils (see low temperature properties).
Comparative solubility test
A naphthenic and a paraffinic oil were compared in respect of their ability to dissolve an emulsifier, an anti-corrosion agent and an acrylate that protects against the formation of air holes. These additives all commonly occur in release agents.
The emulsifier was added to both the oils at room temperature and stirred for five minutes. Initially it seemed the emulsifier dissolved in both the oils. After a while the naphthenic oil became less transparent. The paraffinic oil at first glance appeared totally transparent. However, it emerged that this was because the emulsifier, which was the same colour as the oil, had totally separated from the oil.
The anti-corrosion agent was added to the oils at room temperature and mixed. In the paraffinic oil the additive did not dissolve at all at room temperature and, when warmed to 60degC, a milky solution was formed. In the naphthenic oil a totally transparent solution formed at room temperature.
Five percent of the acrylate that protects against the formation of air holes was added to the oils and stirred. The acrylate dissolved easily in both oils, but the naphthenic oil dissolved the acrylate much faster, which means that it can dissolve larger amounts of the additive.
In the comparison between naphthenic and paraffinic oils it is clear that naphthenic oils are more technically suitable in every category. Vegetable oils indicate certain difficulties when it comes to viscosity and properties at low temperatures, but this is, in part, connected to their fatty acid content – something that must be added to the mineral oils.
Altering the vegetable oil's properties by increasing the level of refining also leads to problems, since a reduction of the acid value can result in higher viscosity.
An interesting way of solving these problems could be to combine a naphthenic oil with a vegetable oil. This could result in a mixture with the right amount of fatty acids from the vegetable oil, the right viscosity and low temperature properties from the naphthenic oil, and good environmental characteristics from both.
Chemical Engineer and sales
representative, Nynas Naphthenics Norden
|Technical properties of the tested oils
|Density (15°C kg/dm3)
|Flash point PM (°C)
|Pour point (°C)
|Viscosity, 40°C mm2/s (cSt)
|Viscosity, 100°C mm2/s (cSt)
|Hydrocarbon type analysis