A common and frustrating problem with polypropylene materials and especially filled polypropylene materials is surface moisture on the pellets. Polypropylene is not hygroscopic so it does not absorb moisture out of the atmosphere but if the pellets have moisture on the surface, it will cause the same problems as you would experience running wet hygroscopic material like nylon. You will experience splay and loss of physical properties.
The problem will often present as visible condensation under the liner in the Gaylord but I have also seen boxes with a lot of moisture at the bottom of the box and also with a ring of moisture about half way down the inside of the liner. It seems like this is most common with glass fiber reinforced grades.
This problem can be frustrating for processors and compounders alike and can be difficult to get rid of. In the distribution business, I have resorted to performing 100% visual inspection for condensation on glass filled polypropylene and even unfilled from certain compounders that are prone to moisture issues.
So what causes this and why does it seem to happen primarily with polypropylene?
First, you need to understand how material is turned into pellet form or pelletized. There are two primary methods used:
- Hot cut method: The molten material exits the die head from the extruder and is cut into pellets by rotating blades. The pellets are ejected into a water bath which cools the pellets and also compresses the pellets into a sphere shape (or sometimes saucer). Typically, the pellets then go into a centrifugal dryer which spins the pellets to throw off any residual moisture.
- Cold cut method: The molten material exits the die head in a strand which is then run through a trough which is filled with water. The water cools the strands which then go through a cutting head which cuts them into a cylindrical shape. Typically, there are some blowers that blow any residual water off the strands before they are pelletized.
There is no one cause or solution to moisture and condensation issues but in general it is the result of the pelletized material not being able to burn off the moisture that it picked up while being cooled during the compounding process.
The most common reason for residual surface moisture is that the material is not hot enough. It is very likely that freshly pelletized polypropylene will have some residual moisture from the cooling process. In the case of hot cut pellets, a centrifugal or spin dryer is typically used to remove the moisture. However, the centrifugal dryer relies on the material being hot enough to help burn off some of the moisture that is present. If the material is too cool, the centrifugal dryer will not be able to do its job. The temperature of the pellets is a function of the compounding line temperatures and the process water temperatures. It is important to not have the water too cool. The reason polypropylene exhibits this problem more than other materials is because it runs at cooler temperatures than most other plastic materials. It is rare to experience this type of moisture with nylon or polyester or even ABS because of the higher temperatures that they are run at.
If the pellet surface is rough and porous, it will tend to pick up more water in the cooling process. Compounding and pelletizing parameters should be adjusted to get the smoothest pellet surface that you can get. Glass fiber reinforced materials are the most problematic when it comes to moisture. Glass reinforced pellets have a very rough surface and the glass fibers can act as a wick actually drawing moisture into the pellets. In addition, the glass fibers draw the heat out of the plastic causing the pellets to cool even faster. Great care needs to be taken to avoid running wet glass fiber reinforced polypropylene. For highly loaded glass fiber polypropylene such as 40-60% you should consider post drying the material in a desiccant dryer.
While hot cut pellets tend to exhibit this problem less than cold cut pellets, there are a few special problems that hot-cut pelletizing can have. If your blades get dull, you can make pellets with small “tails” on them which can break off in the centrifugal dryer. These tails can then clog up the dryer impeding air flow which will cause you to have moisture problems. Having the process water too hot can also cause tails.
As you would expect, weather can have an effect on this problem. If the plant environment is very hot and humid, the pellets can actually come off the line at a temperature below the dew-point in the plant in which case, it is guaranteed that you will have condensation. This is similar to how a cold glass of water will sweat on a humid day. Click here for a primer on dew-points. This is a difficult problem to solve as air conditioning your plant may not be feasible. You might consider using fans to increase the air flow in the plant which may help to reduce the humidity. You may also need to increase processing and process water temperatures during periods of hot and humid weather. The other option is to heat the pellets up a little bit in a dryer before boxing the material off. The material does not need to be hot, just above the dew point.
Even after all of the previous issues have been salved, you can still have moisture issues if you seal the material up in boxes too quickly. The best practice is to leave the liners open on the boxes of polypropylene for at least 8 hours after pelletizing as an insurance policy. This can take up a bit of floor space but the hot pellets sitting in open boxes will burn off any bit of moisture that is still present. If you have to ship material in an emergency, liners should be tucked around the sides of the box and then the lid put on. Moisture can escape through the cardboard. You can also buy perforated liners which will help any residual moisture escape.
If you find that you are getting moisture at the bottom of the box or in the middle of the box, this is probably because the moisture was trapped by a closed liner and slowly drained to the bottom. You will never get this moisture out by leaving the boxes open. The material will have to be put into a dryer.
If you get material in that has visible condensation under the liners, the best thing to do is to pop the lids off as soon as possible, open up the liners and put the boxes under a radiant heater or better yet, outside on a sunny or windy day. The moisture will burn off in a few hours. For really wet material, I have set up an empty Gaylord next to the wet box that is sitting in the sun and, throughout the day, bucketed the top layer from the wet box into the empty box. It will take some time, but you can dry whole boxes this way in a pinch. You can sell this sun-dried material at a premium like sun dried tomatoes if you want (don’t try that unless the customer has a good sense of humor).
Unfortunately, there is no one cause or solution to this problem and this is the reason why so many compounders experience it. I have been in the distribution business for over 30 years now and this problem creeps up over and over again from small and large compounders alike.
I would love to hear from anyone in the compounding business that has any tips on how to get rid of this problem because I am sure that I have not covered all of the causes and solutions here.