Monday, July 08, 2013

Solvents and polymers

Working with polymers at the lab bench is usually quite different than working with them at a larger scale (pilot plant, production, etc.). Benchtop versions of blown film lines, injection molders, extrusion lines...don't exist. Small extruders exist, but they don't have mass-loss feeders, custom screw elements and a whole host of other features that their big-boy versions have.

And so at the benchtop we cheat. We use solvents to dissolve the polymers, which then allows us to easily stir in additives or other materials. We eventually have to get the solvent out, but that is usually pretty easy. Depending on the volatility of the solvent, a hood can work just fine. In other cases, an oven or a hot plate will do nicely. In short, the solvents make our lives much easier.

In contrast, solvents at a large scale are generally avoided. Not only is flammability a concern (which then leads to higher capital equipment costs for the electrical equipment to make it non-sparking) but also the solvents need to be either recovered (more capital equipment) or oxidized (more capital equipment) which leads to CO2 emissions. But at the most basic level, the solvents are an added cost. If your material is coming into the plant as 67% solvent, that means that for every 3 pounds of it you buy, you are only getting 1 pound of final product material. (A fellow engineer assures me that the same results are true if the metric system is used, but I remain suspicious!) As a result, pilot plants and production equipment are usually solvent free.

This differences in processing options with the size of the operation can lead to real nightmares in scaling up products. You can dissolve a newly-developed polyolefin in hot decalin and add in all the antioxidants you want, but you can't find out if it will make a nice blown film until you go to the pilot line. Even a cast film of the polyolefin will not provide meaningful samples as they would be completely lacking in the strength-inducing orientation that the blown-film line provides.

One exception to this in the past has been coating operations. When I used to work for an unnamed Minnesota-based Mining and Manufacturing company making pressure-sensitive adhesive products, many of the tapes were made by coating solvent dispersed polymers onto the tape backings and then drying off the solvent in a belt-fed oven. I suspect that most of those solvent-based systems are long gone, replaced either by water-based substitutes or hot-melt based systems, which was my contribution to the cause back in the day. But as we continue to move more-and-more into a solvent-free production world, even this exception will disappear. The bottom line is that as time goes on, it is becoming increasingly difficult for benchtop operations to mimic production results. Surprising, isn't it?

1 comment:

tea-earlgray-hot said...

Coming from the other direction, how far have advances in modelling allowed us to bridge the gap? Together with microanalytical tools like AFM we can do a lot more, a lot faster, with relatively scant data from the benchtop. I would argue those tools are still somewhat underutilized today, but they've made a big impact.

I'd say the biggest challenges in scale up are still things like reactor fouling, continuous flow, etc, but I'd defer to your experience.