Preceding blogs have discussed acidic and caustic based cleaning chemistry. There are also chemistries, however, which are neither acidic or caustic but, rather, rely on other more mechanical than chemical mechanisms to affect cleaning. In a capsule, these mechanisms are wetting and emulsification. The reader should recognize that ingredients that promote these removal mechanisms are frequently present in formulations that are fundamentally acidic and caustic in nature but that they can also be effective without the benefit of either an acid or caustic underlying formulation.
Wetting refers to the ability of a liquid to be attracted to or to adhere to a surface. In general, ingredients that facilitate wetting reduce the surface tension of a liquid. In addition, they may make the cleaning formulation have an affinity to be be highly attracted to a particular surface like, for example, aluminum. Wetting agents do two things – first, their ability to reduce surface tension allows cleaning solution to penetrate into areas where surface tension might otherwise impede access. This facilitates cleaning surface features with finely detailed geometry. Low surface tension also allows liquids to surround and penetrate between small particles and substrates which facilitates separation of the particles from the substrate. This is especially true in the case of ultrasonic cleaning. Once the particles are free of the substrate, they can be rinsed away. Secondly, wetting agents may exhibit what are called surface active properties. In simple terms, the wetting agents are formulated to be highly attracted to a surface. If the cleaning solution is more highly attracted to the substrate than the contaminant (oil, for example), the contaminant looses the “tug-of-war” and is displaced by the cleaning solution to be rinsed away. The wetting properties of a cleaning solution benefit the removal of particles as well as other contaminants.
Emulsifying agents allow suspension contaminants that would otherwise not mix with water. In essence, emulsifiers contain complex molecules called “micelles.” One end of each micelle is hydrophilic (water loving) while the other end is hydrophobic (resisting bonding with water but able to bond with hydrocarbon or other specific hydrophobic liquids). Acting as intermediaries, micelles allow water and oil to co-exist as a mixture (as opposed to a solution as described in an earlier blog). By allowing oil (or other contaminants) to mix with water, emulsifiers promote the removal of otherwise water-resistant contaminants.
As an extension of the use of emulsifiers, they can also be used to make cleaning formulations that exhibit the cleaning properties of two discreet cleaning agents. For example, mixtures of water based chemistry and a solvent such as kerosene can be used as an emulsion to remove a mix of contaminants that couldn’t be removed by either component alone. Emulsifiers allow the co-existence of the water based component and the solvent based component in a uniform mixture.
Note – Most coolants used in machining operations are emulsions of water and oil in one form or another combined with emulsifying agents. Coolants are frequently used effectively for in-process cleaning of parts during the manufacturing process.
As stated at the beginning of this series of blogs, their intent is not to be an exhaustive study of cleaning chemistry but, rather, a general overall view of some of the basic concepts involved. The formulation of cleaning chemistries is a science of its own which is constantly evolving to meet new requirements and hardware capabilities.
– FJF –