More Quality Control Needed

We design and manufacture power supplies that go in industrial OEM equipment used in the semiconductor industry. As a process engineer, I have to look at our operations and the problems we see with nicks and scratches in our products. Most have an aluminum sheet metal outer covering that is either painted with liquid black paint (not sure of specifics) or a black powder coating. We have a lot of manual build and move operations in our process. However, we are finding scratches everywhere, from supplied inventory to when the customer opens the box. It is my belief that we could make a better choice of coating material as far as type, sheen, etc. However, I'm not an expert in coatings. Can you guide me in the right direction?

Certainly the first step is to study how all items are handled during assembly and packaging. You need to determine at what point(s) most of the coating damage is being inflicted. Make sure that workers are aware that paint damage is a problem, and enlist their help in combating it. (Rewards such as a free pizza lunch at month's end if minimal paint scratching is achieved can work wonders.) Redesign fixtures and transfer equipment if needed to lessen damage to the finish during handling.

Since you do not indicate if the liquid paint sustains more damage than the powder paint, I can only suspect this is indeed factual. Let me suggest that a properly selected black epoxy powder coating should be quite economical. If you pick one with a hardness of 3H to 5H, it will be highly scratch resistant. Talk about this with the powder coating supplier or the company that does the powder coating for you.

Relative Humidity Matters

What effect does relative humidity (RH) have on the evaporation rate of a solvent, such as mineral spirits, assuming there is not water entrapment in the solvent due to dew point effects? I understand dew point effects and that high RH can slow evaporation due to water entrapment. But in a particular application drawing down a slow solvent in which dew point is not reached, shouldn't humidity have little or no effect on the evaporation rate? My tests have confirmed this. It would seem that temperature, film thickness and airflow would far overshadow RH effects. Isn't evaporation rate simply a function of the partial pressure of the compound?

No. The relative humidity does indeed affect the evaporation rate of a nonaqueous solvent. Simply stated, the evaporation rate of a solvent is increasingly slowed as the relative humidity rises. The complete textbook answer involves the partial pressures of the solvent, the water vapor in the air, and the molecules present in dry air. But that much theoretical detail is unnecessary. Think of evaporation this way: The more water molecules present in the air at the solvent's surface, the harder it is for the solvent molecules to escape from the liquid solvent. So, high humidity makes it more difficult for solvents to evaporate.

That said, the airflow would certainly have a much greater effect on evaporation rates, as would temperature. Higher temperatures and higher airflow rates will appreciably increase evaporation rates. In contrast, film thickness will have no significant effect on evaporation rates.

More on Friction-Reducing Coatings

I was forwarded a question about friction-reducing coatings asked by a naval architect in your column. Apparently these coatings are not familiar to these end users or to you. My company holds patents for such hydrophilic coatings, which are routinely used in the medical markets for ease of insertion and leaching of antimicrobials and drugs. In fact, we also supply the recreational boating market with a temporary "go fast" coating that is hydrophilic. We wondered if you would like some commentary on these coatings? Of particular interest were the thoughts put forward by the NPCA group that met in Virginia Beach with a large contingent of Navy and Coast Guard personnel about marine and offshore coatings. Please advise if we can help in this matter.

Thank you very much for your response with information on this topic. Many readers would certainly be interested in learning more about such coatings and hearing comments concerning their usage. I'm eager to find out the specifics of their performance.