Mineral Fillers: ‘Hamburger Helper’ for plastics
I'll declare my position up front: I don't care for Hamburger Helper. For those of you who haven't experienced this delicacy, Hamburger Helper is a soy-based additive that, when added to ground beef, extends the culinary range of hamburger...
I‘ll declare my position up front: I don’t care for Hamburger Helper. For those of you who haven’t experienced this delicacy, Hamburger Helper is a soy-based additive that, when added to ground beef, extends the culinary range of hamburger and adds flavour.
Polymers have their own versions, stretching expensive resin and adding the plastics equivalent of flavour, namely enhanced physical, chemical and electrical properties. Fillers can be as exotic as carbon-based nanocomposites or as common as dirt. And many popular fillers, like kaolin clay, mica and talc essentially are dirt, mined in ton quantities and priced to match, at least by the standards of our industry. They’re also old as dirt: wood flour was a popular filler in phenolic themosets like Bakelite long before thermoplastics took hold in the ’30s and ’40s.
For pure fillers, cost is everything, which is why the really big volume applications like PVC construction profiles and piping use ground calcium carbonate extensively. You and me know it as limestone, but like many fillers, it can do nice things for your products. Take that limestone, for example, then add a step by dissolving the calcium carbonate, then precipitating it back out of the solution and the resulting filler now adds additional impact strength.
Mineral fillers are orders of magnitude cheaper than engineered additive compounds. Composites players in the aerospace sector are familiar with 300-bucks-a-pound chemistry. For compounders selling resin for garden furniture, it’s more like $300 a ton.
But are they fillers or impact modifying additives? With mineral fillers, it’s hard to say; one way of thinking about it is to consider why it is in your compound. If it is needed for the part’s physical, electrical, or chemical properties, it is an additive. Or do you think of it as a substance that doesn’t degrade the part’s properties, and is cheaper than the resin it’s compounded in? Then it’s a filler.
Depending on what you’re processing, that ground rock can add substantially to resin properties. Wollastonite in polyamides adds dimensional stability. So does surface-treated kaolin clay. Alumina trihydrate in polyethylene provides flame retardancy and translucency. Mica or talc in polypropylene improves flexural strength.
Great, so why can’t processors order a dump truck load of ground rock and just toss it in the hopper? For several reasons, the most important being the need to disperse the filler uniformly in the resin matrix. Local concentrations of filler will perform like the bulk properties of the filler itself, rather than a composite. That’s because of the way that fillers enhance bulk properties. Mica is a typical example. Ground mica forms tiny platelets that have the twin virtues of flexibility (by sliding readily relative to each other) and of having a large surface area compared to their volume. Basically, there’s more for the surrounding resin to hold on to.
Clumping, however (the lab coat people prefer “agglomeration”) results in regions where mica platelets slide against each other instead of sliding between the polymer chains. It also can have nasty side effects on part appearance as well as its physical properties. To solve this problem at the press, you would need at least a precision metering weigh blender, and a special screw, if not a different extruder/press entirely.
The next wave in filler/additives is nanocomposites and while conventional minerals aren’t going away for the foreseeable future, nanocomposites are a hot topic of interest. I’ll dig deeper into the really small next month.