Water is an invaluable tool in achieving reliable contamination control and appropriate surface quality. Sometimes, however, water itself is a contaminant.
For critical components or parts, water may be the best approach for removing other soils. Outgassing of lower boiling organic solvents is a concern; however, higher boiling solvents can, like water, be trapped in blind holes or absorbed into plastics or other synthetics.
The consequences of contamination by either water or solvents include corrosion, gradual breakdown of the product during use, and degradation of coatings. If the coating failure is primarily aesthetic, the main consequence will be customer dissatisfaction. If the coating is required for critical surface characteristics, as in biomedical implantables, even if the product is used in an environment which is itself primarily water, residual underlying moisture at the coating stage can shorten product lifetime.
Water can also interact with other fluids found in the completed product. Water can, for example, interact with the flotation fluids used in some gyroscopes. Residual water can be a problem in other areas as well; moisture retained in motion picture film has been associated with the breakdown of master copies of classic movies.
This doesn’t necessarily mean that all water must be removed from all products. In some cases, as in lyophilization, the quantity of water must be controlled, not necessarily minimized to below any imaginable detection limit.
Water can contaminate and degrade organic solvents and refrigerants. Certain esters, when combined with water, can break down to acids and alcohols. In addition to the more obvious changes in solubility characteristics, the breakdown products may have a less desirable toxicological profile than the original solvent. This may impact solvent accessibility. For example, one of the issues involving de-listing of t-butyl acetate as a volatile organic compound involves questions by a California regulatory agency concerning the toxicity of a breakdown product of t-butyl acetate, t-butyl alcohol. Classic chlorinated solvents such as perchloroethylene and methylene chloride can break down in the presence of water to form acids.
Acidic solvents are particularly damaging where there is extensive exposure of the liquid to metal surfaces, as in liquid/vapor phase cleaning systems. It is not unknown for costly cleaning systems to be ruined by acidic solvent.
In the case of classic chlorinated solvents, stabilizers are added to prevent solvent breakdown. Water and acid content can be monitored by relatively straighforward tests conducted in the manufacturing plant. With more complex blends or azeotropes, there is the potential for solvent modification if one or more components partitions to the aqueous phase. In general, exposure of the solvent to water can be limited by using appropriate equipment and process design. With some of the more esoteric solvents, such as parachlorobenzotrifluoride (PCBTF), water may be present as a result of the manufacturing processes. The best approach is likely to be to work with a reliable supplier and obtain solvent certification data indicating water content of the batch in question.
The question of controlling and detecting water contamination in a product is more complex. Often the emphasis is placed on presumptive water removal rather than detection. Water may be removed by physical drying methods or displaced by organic compounds which are themselves presumed to be more readily evaporated. Sometimes vacuum drying for hours or even days is used, with the presumption that all residual organic solvents and water are removed. This presumption may be backed up by product performance data or accelerated stress testing of the product itself. Potential problems with this approach include reproducibility (particularly when the assembly or surface preparation approach changes) and documentation. In some cases, surfaces treated to remove moisture may be stored at a temperature lower than the dew point temperature, thus causing condensation and “contamination” of the surface.
Over the next few months, we’ll cover some approaches to detection of water including the Kauri-Butanol method, Standard Gravimetric method, electrical impedence, near infrared, and millimeter wave (MMW) techniques. We’ll also address the strengths and weaknesses of these techniques and discuss some of the applications in which these detection methods are used.