Dryer Air Capacity to Hold Solvents

I think it's moot. Unless you have an explosion proof oven/ventilation system, you'd need to always stay below the LEL.

Or, if you really wanted to live dangerously you could stay above the UEL. (Disregard that, please)

I think you'll come to a point where you are drying/curing the materials so quickly that the coating/ink/whatever will cure and trap solvent before it can evaporate. Then you will have foam and not coating/ink/whatever.

Agreed. The only other option I can think of, is to raise the temperature up to 350-400°F and switch to powder coating.

Water based materials will tend to allow the water to continue to evaporate out through the material, (to a point). Solvent based materials will skin up and trap solvents underneath, if trying to dry too quickly.

It is an interesting question though, I've never thought about the saturation point of evaporated solvents in the surrounding air.............probably because I am also the applicator.............too high and I wouldn't survive the experience.

There are several things that we currently do in our process to help solvent removal or flash out. One is to pre-heat the substrate, which does a couple of things in the process sense we are working with PVC materials. But, more to the discussion, it allows the solvents to begin the flash out process. It also helps us to control gloss effects of heat curing 2K materials.

The solvents used are based on the compatibility of the other mixtures in the coating and their ability to release from the coating. As you can imagine this is a true balancing act in chemistry.

I still believe there is some correlation in the saturation limits of the atmosphere internal to the oven and the solvent base we use. As I have time I will work this one out as done on water based drying system. Just thought I would raise the issue here and see if there was anyone out there that might have faced this in the past as well.

It's an interesting question...............and one I don't know the answer to.

I know that with waterborne materials the saturation, (humidity), in the atmosphere is easily recognized, and has a huge effect on dry times, gloss, etc.

In theory I would imagine that the same would apply to solvent based materials and the surrounding atmosphere, but as to what effect, if any, the saturation level would have on the finish, dry time, etc., I have no clue.

One thing I do know, as a person that gets frustrated with dry times, is that there definitely is a limit to how fast something can be done with any kind of coating. I've spent most of my life pushing the envelope......................not always with great results.

Yea, I can related to your comment on dry times for sure. Need through put, dry faster = $$$.

As far as this materials goes. It is a crossed link 2K material and will chemically set on its own if left alone. But, like most us we can't just let it alone. So we push and push. Like I said, we have a smaller window of operations then the manufacture ever believed would happen and I maybe at the max in pushing this coating. But, it does raise the question on saturation points, as seen with water born materials.

The effect is similar regardless of the composition of the vapors. Higher concentration of solvent molecules above the liquid = reduced off gassing of the solvent.

So reduce the vapor pressure above the coating or increase the heat of the oven.

This still looks like trial and error testing to me. I know of no practical way to accurately calculate/predict the result. (I conformally coated thousands of circuit boards with every imaginable type of coating during my time in the aerospace industry) If you can control the coating thickness of the material you may have a chance. If you have any variation in thickness of the coating all bets are off as thicker coating translates into more solvent to "pump" out of the coating.

Yea, mill thickness can be a real problem for sure. Our WFT & DFT's run very consistent 1.2 - 1.8 per customers spec's. But, as said it could be a factor in some operations. This is on a robotic spray system using very efficient spray equipment. So not much trouble there.

I will keep plugging away at this issue for now. I have some air concentration testing scheduled in the new few weeks. I'll let you all know what I find.

This kind of work makes going work fun, don't you think!

I did really enjoy my 20 years in aerospace, although it was long ago. (think Space Shuttle electronics at the beginning of my time there)

Yes, these types of studies do make the time go by.

Good luck.

There seems to be some inconsistency in the statement of this question posed. When you say "how long before system reaches capacity to hold further solvents?" I wonder what the system design/operation is that you have.

I am familiar with web coating processes in which flammable organic solvent based polymer solutions were coated on a substrate. These were contiuous operations which were started and run to a steady state condition with air supply/extraction rates set to ensure an operation at a low fraction of the LEL, given a certain coating solution composition and feed rate. The calculation is essentially a simple material balance for the steady state condition to evaluate the explosion hazard and stay below the LEL. These units were designed to operate at no more than 10% to 15% of LEL to allow a generous safety factor. Units were always built as explosion proof. I really can't imagine a unit like this not being built to explosion proof codes.

The other aspect of the design is the effective drying rate for the coating. That property of the coating solution entered into the coordinated design of the system including oven/applicator/solution feed system in the overall concept. The parameters for this were developed on pilot coaters and scaled to the production units. The solvents you mentioned have evaporation times that are moderate, say compared to methanol and acetone solutions that we used. The length of the oven/web speed are key variables to be evaluated.

When air flows are scaled to maintain solvent vapor concentrations well below the LEL the solvent partial pressures in the dryer are consequently very low and not the controlling resistance in the overall transfer process involved in drying.