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Emulsions are used in metal working when cooling and lubrication are called for. The following review of factors governing the choice of emulsifier will, it is hoped, guide you through the jungle of different types.
In a previous article we studied HLB value and the importance of VGC in formulation. The present article deals with the response of different types of oil. Naphthenic-base oil needs less emulsifier than paraffin-base oil to give a stable emulsion.
A modern metalworking fluid can have more than 15 different components. These can be divided into the following groups:
To make this mixture soluble in water, a properly balanced emulsifier is needed. All additives interact with the emulsion system, and so the recipe has to be tailored to the exact requirements defined by oneself or the customer.
An emulsion consists of two or more insoluble phases. If the external phase is water and the internal one oil, the emulsion is called an oil in water (O/W). Milk, for example, is a typical O/W emulsion containing about 3.5% fat in a water phase, and butter is a W/O emulsion with about 20% water in the fat phase. The principle of the emulsifier is to have a polar tail for the water phase and a non-polar one for the fat phase. Through enrichment in the boundary zone between the phases, the emulsifier forms a transition between them.
Two kinds of emulsifiers are normally used in industry: ionic system and non-ionic systems. Mixtures of ionic and non-ionic systems occur. Co-emulsifiers and coupling agents are used for stabilising the system. The non-ionic emulsifiers cover the entire scale of HLB values and can be used in all products. Unfortunately they cost more than the ionic emulsifiers, which are the commonest type. There are great variations, and in deciding a recipe one has to choose between many different grades.
This type of emulsifier is used mostly in W/O systems and for balancing the ionic emulsifier in O/W formulas. On account of its stability and pH value, this is the type commonly used in metalworking oils.
There are two sorts of ionic emulsifier: anionic and cationic. The commonest types are anionic, soap and sulphonates being the commonest of all. Sulphonates and petroleum sulphonates have various molecular weights. A large proportion of sulphonates are alkyl sulphonates, by-products of alkyl benzene production. Sulphonated alkyl benzenes are the dominant product used for cleaning agents. The high pH values give protection against corrosion but they also cause environmental problems.
A coupling agent is soluble in oil and augments the solubility of the inner phase.
Alcohol is soluble in water and reduces the solubility of the outer phase. The solubility of the oil makes a difference to formulation. A highly soluble naphthenic-base oil needs less emulsifier than a paraffin-base oil. A more refined naphthenic-base oil also needs less emulsifier than one that is less refined. The charts show a stability test performed on four different emulsifiers. The difference in response between oils can be seen quite clearly. The smaller the layer of "cream" which settles on top of the mixture, the greater its stability will be. A naphthenic-base oil can give a stable emulsion with the addition of much less emulsifier than a paraffin-base oil. This can be seen best from diagrams 1-4.
|Emulsifier in the
|Base oil concentrate||150 SUS p-base
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