Magical Thinking Won’t Help the Planet or Extruders

Fall reminds me of falling leaves, which most of us will see. Not too many extruders in deserts or on mountain tops. The leaves are made of carbon, hydrogen, oxygen, nitrogen, and a little water. They dry out, decompose, and return some of the carbon to the air as carbon dioxide. This blocks the cooling of the earth and makes the ocean warmer. No miracles, but it reminds us that plants work in both directions. Human energy use adds more carbon dioxide, and warmer oceans don't absorb it as much, so we have a future problem.

This isn't political or magical, although it has been an issue for many people who cling to the magic, and either glorify trees or consider the whole story a conspiratorial scam.

Neither extreme is the real world, and how much — numbers, probability — matters. The laws of science hold, and we can't avoid or deflect them. Fossil fuel is politics, and efficiency matters no matter where the fuel comes from. Electric vehicles need electrical energy, which needs fuel to generate. They reduce air pollution and may be more efficient, but those numbers are less popular.

In extrusion, science rules

In extrusion there is no magic, and the rules of science hold. It takes a known amount of calories to get a fixed amount of plastic from storage to melt temperature. Of the majors, PVC needs the least and PET the most. Mechanical recycling is about the same, but chemical recycling, which breaks the C-C bonds in the chains, is much worse, and I'm surprised this hasn't been exposed by now. I also would like to see energy comparisons with making new polymer.

Extruded products already save a lot of energy by lightweighting packages and vehicles, and suppressing weeds to raise crop yields (mulch). Their barrier to gases and pathogens gets them used, and the paper pushing in these markets is based on popular misinformation about plastics. 

Some areas requiring clarification:

A. Strength. Tensile, flexural, and impact — know the differences and what's needed when — and also modulus (stiffness, rigidity). All depend on resin molecule size and the orientation of the molecules in the product.

Tensile strength involves pulling apart a standard test bar, or "dog bone," at a known temperature and thickness. Yield is the point where (recoverable) elastic deformation stops and (permanent) plastic deformation starts. It is often a limit to service and, thus, may be more important than the conditions at actual failure. Looking at the test data as a graph of elongation vs. time will help to understand this.

It is also very important how the test specimen was made. Injection molding can make a material look much better, as can the use of oriented film/sheet. Test in both directions if you’re using film or sheet.

It may be useful to keep a reference material to compare and, thus, support test results.

Flexural strength is a similar test involving bending instead of pulling. 

Compression strength is different, with special uses and is much less common. 

Modulus, or stiffness, is the result of a tensile or flexural test (similar but not the same). With flex PVC, the surface hardness (durometer) may be enough to indicate stiffness and is much easier to do.

Impact strength is different: Most tests are Izod with notch; temperature and thickness matter. Molded bars may be tougher than extruded sheet.

B. Contamination and purity. This may affect physical strength (stress concentrators) or visual appeal. It’s easy to resolve by using screens (filtration) but may be at the expense of the production rate or compound cost if the screens raise the melt temperature. Looking at screened particles under a microscope can tell you a lot about their origin.

C. Opticals. These include transparency as well as haze and color, which may change if the melt is too hot for too long. Also surface gloss and friction. Yellowing in sunlight may matter, too; polymers differ and can be protected.

D. Multilayer. Co-extrusion, laminating in line vs. coating. The choice depends on available equipment and the desired thicknesses and properties. Working off scrap in a middle layer is one example.

Some properties are affected by the percentage of scrap and some by cooling rate. Blending of anything will depend on how and what is added, and on the mixing capability of the screw(s) as well as particle size and viscosity at real temperatures. Melt index may not reflect real conditions. And the use of static or dynamic mixers may take over.

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, and now in his virtual version. He wrote Plastics Extrusion Technology, the first practical extrusion book in the United States, 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 [email protected].

No live seminars planned in the near future, or maybe ever, as his virtual audiovisual seminar is even better than live, says Griff. Privacy, no travel or waiting for live dates, same PowerPoint slides but with audio explanations and a written guide. Watch at your own pace; group attendance is offered for a single price, including the right to ask questions and get thorough answers by e-mail. Call 301/758-7788 or e-mail [email protected] for more info.