The flooding in plastic-critical Texas reminded me that water-resistant plastic products—boats, tarps, bags of all kinds—are saving a lot of property and even lives, but in extrusion, as in Houston and Florida, water is often an enemy.
This happens in one of two ways (or both):
Samuel Taylor Coleridge's Ancient Mariner, who said, "water, water everywhere, nor any drop to drink," is illustrated here by Gustave Doré. Note how the mast beam looks like an extruder screw.
Cosmetic—water absorbed by the resin will boil to steam as it leaves the die, and make bubbles, pits or dotted lines on the product surface. It gets well above 100° C (212° F) inside the extruder, but pressures are too high to permit boiling and the water stays in liquid form until its pressure drops as it passes through the die lips.
As a rough rule, anything above 0.1% of water (a 16-oz water bottle in a Gaylord) will make visible marks, but formulation, die exit speed and drawdown will have effects, as well.
Many polymers (such as ABS and PMMA) absorb more than this from normal atmosphere; others (PS) are borderline enough to show at high humidity; and some (PE, PP, PVC) absorb no moisture at all.
This doesn’t let them off the hook, however, as they may contain additives such as white or black colorants and fillers like wood or calcium carbonate. Or, the polymers may have gotten wet from being stored outdoors in the rain or indoors under a leaky roof, or even from a cold-water pipe over an open hopper, on which warm-air water is condensing.
Chemical reaction —some polymers are condensation polymers, notably polyesters (such as PET), polycarbonates and the nylon (polyamide) family. These are all made by reacting an -H from one monomer with an –OH from another, to make H-OH = H₂O = water.* The water is driven off in the manufacturing process, the two loose ends bond together and the resulting polymer is indeed water-resistant. However, if it gets hot enough, as when melted in an extruder, any water molecules around want to go back where they came from, and thus break the chain formed when they originally reacted. The amount of water that can seriously damage the chains is far less than that needed to avoid visible (cosmetic) marks—it’s more like a spoonful, rather than a 16-oz bottle, of water in a Gaylord.
So what can we do? The two most common ways of avoiding these problems are predrying and venting. With predrying, the feed is heated with hot air in its hopper above the extruder, or in a similar vessel nearby, and fed to the extruder continuously. The hopper has to be big enough to allow enough drying time for the material being dried. The feed is therefore hot though still solid, and extrusion conditions are not the same as with room-temperature feed.
With venting, the barrel has a hole in it—in single-screws, about 70% downstream of the feed; in twins, the location varies. Screw designs are different from unvented systems, with deep channel(s) at the vented location to