Chemicals have the potential to interact with the product, and not always in a desirable manner. Chemicals are essential to the manufacturing process. We have to strike a balance between the desirable function of the process fluid or cleaning agent and the potential for damage to the product being fabricated.
Sources of materials incompatibility
Because chemicals are ubiquitous in manufacturing and in the surrounding environment, it is important to have an eagle eye and an open mind. Cleaning agents are always a likely culprit, but other process fluids can inadvertently damage the surface. Co-processed product can result in galvanic interaction, particularly in aqueous processes. Airborne molecular contamination (AMC) can be a source not only of thin film contamination but also of surface changes.
Consider the impact of chemical reactivity. Even if the fluid itself does not damage the product, breakdown products may be problematic. One classic example is the breakdown of halogenated solvents to form acids that can damage both product and processing equipment. Acetone is miscible with water. In one instance, while no problems were reported at ambient temperature, when acetone was used in a specially-designed low-flashpoint system so that it could be heated, the acetone/water blend reacted with magnesium.1
Often, cleaning agents are evaluated solely on the basis of compatibility tables provided by the chemical vendor or are, at most, tested by immersion of one material of construction at a time in the cleaning agent under consideration. Compatibility tables can provide indications of what might or might not be suitable.
Tables and single material ambient immersion might be sufficient to exclude a chemical from consideration. However, the same conditions that promote effective cleaning, such as temperature, time of exposure, and cleaning force, can also exacerbate materials compatibility issues. Therefore, proposed processes for critical cleaning must be tested using dynamic compatibility studies prior to being adopted for routine use. This involves evaluating the sub-assembly or product in the proposed process itself. In a companion article, we expand on this concept.2
What does incompatibility look like?
Materials compatibility problems are diverse and ubiquitous. In grossly obvious compatibility problems, the part may actually dissolve in the chemical in question. The surface may change in color or texture. In other instances, there may be dimensional changes, discoloration of the process fluid, or changes in weight. The significance of an observed problem depends on the application.
Compatibility problems that are readily and immediately obvious, while vexing, are not nearly as worrisome as the problems that lead to field failure down the road. If, for example, the adherence of a coating degrades after a change in the cleaning process (hopefully, this happens during initial testing rather than in production!), the problem might be readily resolved by modifying the cleaning process. What if there is loss of adherence after months or even years?
What is significant?
Weight gain of a given material of construction is sometimes treated as a compatibility problem—and the problem may or may not be considered significant. For example, weight gain due to solvent adsorption may be reversible. It can sometimes be dealt with by adding a bakeout; such bakeout effectively enhances the drying step. Drying or bakeout must be adequate. This may require testing, for example, by headspace gas chromatography. Since there is no such thing as zero residue, it is up the manufacturer to determine the level of bakeout that is adequate. Outgassing of solvent is very likely to be unacceptable when the product will be used in a sealed system. Outgassing is virtually certain to be unacceptable with implantable medical devices, where the solvent would be considered to be a leachable residue.
What does the surface look like?
Fear of changing the surface means that process development and process change are often fraught with angst. This may lead to dependence on a sub-optimal process. While an over-emphasis on lab testing can devour scarce company resources, judicious use of surface characterization can provide confidence and short-cut failure analysis times.
The right surface analysis can clarify if there is a residue problem, a materials compatibility problem, or a combination of the two. Therefore, particularly with high-value product, we suggest both extractive and in-situ surface analysis as a benchmarking tool. Selecting the best methods of analysis requires skill, logic, and experience. It is usually desirable to look at the physical attributes of the surface. The right testing may also involve elemental analysis and speciation. It may involve determining the pattern of residue relative to surface topography. Such studies can be costly, so it is a good idea to develop a testing strategy. However, if a new process is being developed, benchmarking the new process against a current, acceptable process at the surface level can help to avoid future problems and to more quickly resolve problems that may occur.
Negative or positive interaction
Taking a holistic view of materials compatibility, compatibility problems involve a chemical or a mixture of chemicals as well as a manufacturing process, a manufacturing environment, and/or a storage environment. These factors change either the surface or the bulk material of a product in a manner that is detrimental to product performance.
Probably all manufacturing processes have the potential to modify surfaces or product. If the effects are positive, and if we are aware of those positive effects, the process may be purposely added to the manufacturing protocol. Sometimes, however, desirable incompatibility or desirable surface modification may occur without awareness. The concept of beneficial contamination, where a surface “contaminant” results in improved product performance, has been recognized for some time. One notable example, for instance, is in the improved performance of stents.3 Therefore, it is strongly suspected by many that analogous desirable surface materials compatibility results in a superior, modified surface.
References
1. B. Kanegsberg, “Cost-Effective Cleaning for Quality Thermal Spray Coating,” Presentation, International Thermal Spray Association, Orlando, Fla. April 17, 2009.
2. B. Kanegsberg and E. Kanegsberg, “Compatibility,” Clean Source-The BFK Solutions Newsletter,” July 2013.
http://bfksolutions.com/compatibility
3. B. Kanegsberg and M. Chala, “Beneficial Contamination,” A2C2 Magazine, January 2002.
Barbara Kanegsberg and Ed Kanegsberg (the Cleaning Lady and the Rocket Scientist) are experienced consultants and educators in critical and precision cleaning, surface preparation, and contamination control. Their diverse projects include medical device manufacturing, microelectronics, optics, and aerospace. Contact: [email protected]
