January 14, 2022
Water, water, every where,
Nor any drop to drink.
So spoke the Ancient Mariner in Coleridge’s Rime of the Ancient Mariner (1797). He was out there in the open sea, cursed for having killed an albatross (good luck to sailors), and had a real problem. He didn’t know why we don’t drink sea water — it’s not because of plastic pollution — but as a sailor he knew to just say no. His mysterious story is still worth reading. We still have a need for magic and mystery, which amuses as well as challenges science and, thus, supports the popular image of plastic pollution, even as our synthetic science-based materials do far more good than harm.
Extrusion helps move a lot of water, of course. Irrigation helps us grow food in areas with a lot of sun but little rain (the Mariner’s situation, too). Another water-linked (and water-resistant) extruded product is filament for fish nets, now reviled for their presence as half (?) of plastic sea waste. Years ago, we would repair them, and darn socks, too. Today, nets are abandoned when torn, but maybe the current attention will lead to economical recycling, especially of nylon (polyamide), which sinks unless buoyed (PP floats). And ergonomically, too, as people push recycling even when it takes more energy to do so than to make new polymer. Reduce waste at all costs, even at high cost to the environment? I just say, “No!”
There is much rigid pipe used in water supply, usually unplasticized PVC — no phthalates involved — and some ABS, where toughness and heat resistance are needed. Polyacetal and CPVC/PVDC have some pipe uses, too, but extruders of the high-chlorine resins must use special metals in equipment that will come into contact with hot molten polymer. For round products, here are some useful extrusion tips:
Know your standards. You may be printing their numbers on the pipe itself, but compliance means enough testing to ensure that compliance, even with resin and formulation changes.
With pipe used under pressure, pay attention to screening/melt filtration, as contaminant particles can cause premature failure. There are long-term hydrostatic testing machines; if you use them, understand the curves they generate, including knees.
The distance between die exit and first cooling is important and may be adjustable. Differential thickness control around the pipe can be done mechanically, thermally, or by both.
Cooling efficiency, related to warping and sagging, may depend on sprays and/or immersion in water. Uniform sprays or a metal tube with holes at entry may help. Length of the sizing sleeve is a balance of friction vs. cooling and may control linear speed.
Vacuum over the tank water can draw the pipe out to the sleeve surface or, if applied to the sleeve, suck it directly. Remember even PVC floats when immersed, as it is empty inside. Sponge or cloth stripping of hot water in immersion tanks can also improve cooling efficiency and reduce need for refrigerated water.
I worked with polybutylene pipe many years ago — very good product, low creep, good for hot water, but costly, and replaced by PEX, cross-linked polyethylene, at least in the United States. Its makers must understand the complexities of crosslinking — different principles, heat needed, standards, how much crosslinking is wanted, testing, and recycling scrap and off-size.
Tubing is not pipe, at least to the people who make and use standards. In medical uses there has been much growth, even before COVID, in simple rounds and multilumen (more than one channel). Blood is not water, either, but it is water-based and what resists water works for blood, including sensibly formulated PVC, which gives flexible clarity, as do some silicones and modified PP. Tiny multilayer co-extrusion is done with high-precision tooling, and the traditional economics are up-ended — material x 2 = manufacturing cost; x 3 = sale. These uses are so demanding that material is no longer the primary economic driver.
Water in, so need water out. Extrusion makes drainage pipes, too – residential, agricultural, and industrial. Much of this is corrugated to get more flexibility and thinner walls, as little or no internal pressure is present.
Other water-related uses of extrusion include film/sheet for lining ponds, reservoirs, transmission ditches, and disposal pits in mining, for example. Extruded film is used in water purification and desalination. And roof gutters to keep more of us dry. And insulation of undersea cables that still carry messages over long distances. And in the construction of boats themselves (one-piece canoes and fishing boats), and water-resistant plastic-lumber components of piers and marinas.
All of these applications are good answers to those who think that all plastic is single-use, disposable, wasteful. Yes, we make waste — a condition of life — and we can reduce it if we try, especially by using less stuff and reducing our dependence on convenience, but that is another story and a very big one, too. In the meantime, extruded plastics will give us permanent and successful solutions to many of our water problems. And read Coleridge’s “Rime,” and be grateful for what we have and don’t have.
About the author
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 new audiovisual 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. No travel, no 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.
About the Author(s)
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