Extrusion basics: Troubleshooting? It’s OK to monkey with the resin

First, my monthly reminder: There are no toxic plastics, none of them. Our materials are safe and do far more good for the environment than harm. When you meet anyone who thinks otherwise, try to find out why they want to believe the contrary so fiercely. You may be surprised and maybe even sympathetic. If you ask for statistics, toxicity data and their sources, you probably won’t get any, or they will be selective or come from questionable sources. Anything you think is really significant, please let me know.

MonkeyNow, I can talk about extrusion. I’ve done a good deal of extrusion troubleshooting over the years, and found that many people want to believe that the equipment is the problem. If we can just change the screw or the screen pack or maybe do something to improve cooling, the problem(s) will go away. That sometimes happens, of course, but I’m a chemical engineer and my “grad school” was my job in tech service for a resin supplier, so I also look to materials for solutions, especially because that’s where most of the money goes, too.

One thing we can do is blend resins with each other. We often do this, anyway, if we work our trim and scrap into the product, but sometimes it makes sense to blend two or more different resins. For example, we might blend some regular LDPE with LLDPE to raise the speed we can run without melt fracture. Proportions will depend on needed properties and relative resin costs. 

Dissimilar resins sometimes blend, too. ABS may be used to toughen PVC, and SEBS resins are typically sold as additives to other resins.

With all additives, mixing is important, and even if the resins are compatible, their viscosity differences have to be considered. My rough rule is to use viscosities in proportion to the proportions—that is, a minor component should be less viscous than the major component, so that the minor component can flow more freely and, thus, be dispersed more evenly. We can use melt index to represent viscosity, but melt index is tested much more slowly and usually at different temperatures than in actual production. The ideal is to have viscosity data at mixing temperatures and shear rates—the data are easy to get but rarely used. 

With polyethylene (PE), blending different densities can be done. Density is important, as even the third decimal place (0.922 versus 0.925, for example) may have a major effect on rigidity (modulus) and lesser effects on barrier and physical strength. This assumes a similar type of PE, as linear lows and regulars at the same densities will still be different. The proportion-to-the-proportions rule applies here, too.  Remember that density also depends on cooling rate, and the same resin(s) may give different densities if cooled differently. 

Whatever the polymer, it pays to specify density and melt index: Make sure that what you are buying meets that specification (it’s best to specify a range rather than a single value). And be prepared to test both incoming materials and finished product. Test often, with multiples on each sample. When you know what good is, you’ll know what fishy is. Density columns are easy to use, but anyone with a cup and a scale can use Archimedes’ method of water displacement. 

With most polymers other than PE, density variation usually implies additives such as fillers, which increase it, or foam bubbles that decrease it.

Among such non-resin additives, you may want to add some extra thermal stability. That means antioxidants in PE, PP and the styrenics, and stabilizers with the PVC family. These are available in concentrate form, either standard or made to order, and earn their keep by allowing hotter melt and, thus, maybe faster production, and by discouraging degradation in long runs or with less-than-streamlined head/die passages.

Residual stability in scrap/trim is a sneaky factor, as a material may run OK as virgin, but may discolor or weaken when the scrap is added, because some of the stabilizer was depleted in the first extrusion.

Testing thermal stability can be done with a torque rheometer (PVC), by oven discoloration with or without vacuum, and by chemical and spectral tests, too. How do we know how much of what to add? There are entire books written on this subject, but suppliers of the additives can help, as well as us resin-maker tech service people, if you can find the right ones.

If the product is to be used or stored outdoors, UV absorbers may be needed. Carbon black at 2% or more is the best and cheapest light stabilizer, but if you don’t want a black product there are many chemical absorbers available. How much to use is a cost problem as well as a technical one; there are accelerated UV exposure tests, plus experience and tradition. The non-black color matters, too: White pigments offer some UV protection, and light-colored products stay cooler in direct sunlight, which slows down the degradation reaction.

With UV protection or any other surface concern (including color), there is often an option of coating or co-extruding an active top layer over an unprotected interior. This saves additive cost, but may add equipment cost and complicate the process. Scrap and trim can be worked into the underlayers—it’s the simplest solution but wastes some additive. If used in the exposed layer, more additive may be needed.

Another group that is usually added at the hopper are additives that migrate to the surface. If they are included in the virgin material, this not only increases cost, but makes it harder to adjust additive content, and the material will feed more slowly per RPM if it is slippery.

There are dozens of other additives for special uses, such as rodent and even shark repellent, animal medicines (yes, the resins are nontoxic), and fillers and reinforcements of all kinds. Be careful with calcium carbonate, which is available in a large variety of particle sizes and surface treatment—all calcium is not the same!

Conclusion? It’s a plus on any extrusion line to have equipment in place to feed ingredients uniformly and continuously, as well as people on board who know how to use materials to improve the product, reduce costs, gain and regain sales, and solve field problems.

 

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 e-mail him at algriff@griffex.com.

Griff will present live seminars in Houston on Dec. 15, Toronto on Jan. 11 and Chicago on Jan. 23. If you can’t attend these live events, he offers a Virtual Seminar, which can be seen at any time, any place. E-mail Griff at the address listed above for more information.

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