Extrusion basics: Plastics additives—what's in it for me (or for you)?

We know that all plastics are not alike (public opinion to the contrary), but too often we accept a supplier's product without knowing what's in it that may help or hinder our operations, and, most importantly, in what form and how much. I'm talking about additives here, not the base resins, which should be selected and specified wisely, too.

A good example is colorant. Did the supplier get a color sample before matching it, and was the type of light specified (fluorescent, LED, direct sunlight and so forth)? If gloss is important, is there a specification, and at what angle is it measured? And if transparent, remember that transmission and haze are not quite the same (see ASTM D-1003).

There is a political "angle" to all this. Some processors formulate their own compounds and know just how much of each additive is used. Others buy formulated compounds, and their suppliers understandably don't want you to know exactly how much of what is in there. The buyer, however, may need to know what's in it and how much (and sometimes what's not in it) to be sure his product is OK for food packaging, or doesn't run afoul of California's infamous Prop. 65, or doesn't put workers in any danger. If you don't know what's in it, and don't need or want to pay the cost of lab analysis, at least put property specifications on the purchased compound, such as the tensile strength/elongation/modulus of your product, or of a standard product made from the compound (beware using injection molded test specimens for extruded products).

Identifying ingredients can be a problem, too. Calcium carbonate (often abbreviated "calcium") is my favorite example, as its effects are strongly dependent on particle shape and size, surface treatment and source purity. Just saying a compound "has 15% calcium in it" isn't enough. Wood flour in WPC composites is an even more confusing mess. Some of the things to know and understand are: Longer vs shorter fibers and hardwood vs softwood, as well as the particle shape and size, and chemical treatment issues for calcium noted above.

Mixing is critical, too. One of my early awakenings was an ANTEC paper by the late George Kovach, sheet extruder and ex-president of SPE, who showed that better dispersion (smaller particles) means less colorant needed and thus lower cost. A later lesson came when I worked with carbon-HDPE conductive formulations, where too much mixing separated and encapsulated the carbon particles, requiring more carbon black and consequent higher cost and negative property changes.

The smaller the percentage of additive, the more the use of a concentrate is justified, as it's hard to get a uniform distribution of 0.25% of an additive! Sometimes money spent on continuous feeders for neat additive or concentrate can be justified by the need for less additive to guarantee proper performance. Examples of these microadditives include UV absorbers, anti-fracture processing aids, antioxidants, PET chain extenders and many pigments and dyes.

In concentrates, don't forget the resin carrier. A 10% loading in one carrier may disperse very differently than the same loading in another carrier. In general, viscosities of the concentrate and the base resin with which it's being mixed (at extrusion temperatures and shear rates) should be in proportion to the proportions.

If you know your compound intimately, you can be more confident of its performance. I was once an expert witness in a law case where the thermal stability of the resin was questioned. We discovered that the supplier had cut the antioxidant level from 0.5% to 0.3% without telling the buyers, but that didn't resolve the issue, as we still had to show that 0.3% was too little for the application. That in turn depended on the antioxidant that was used, and on the time-temperature history of the material—how long it was in the extruder and at what temperature. Even extruder screens got into the picture: If the resin degraded faster because of insufficient antioxidant, finer screens could catch smaller degraded particles, which would have promoted product failure (stress concentrators) in service.

In law cases, sometimes just such a discovery is enough to scare one side or the other to settle out of court, like almost all of the ones I'm familiar with. See the links to my two articles on "Why Do They Call Us Witnesses?" on the home page of my website.

Finally, I leave you with my favorite proverb: The optimist says the cup is half-full, the pessimist says it's half empty ... but the engineer/scientist/realist asks: How much is in the cup?

Allan Griff is a veteran extrusion engineer, starting out in tech service for a major resin supplier, and working on his own now for many years, as a consultant, expert witness in law cases, and especially as an educator via webinars and seminars, both public and in-house. He wrote the first practical extrusion book back in the 1960s as well as the Plastics Extrusion Operating Manual, updated almost every year, and available in Spanish and French as well as English. Find out more on his website, www.griffex.com, or email him at [email protected].

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