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Ask the Extrusion Expert: Your plastics and processing questions, answered

Allan Griff, independent consulting engineer and plastics extrusion expert, has agreed to extend his relationship with PlasticsToday. Already the host of our well-received Extrusion Expert webinar series, Allan now also authors this column. The formula is simple: readers send questions; Allan chooses the best or most intriguing and answers them.

PlasticsToday Staff

October 26, 2011

7 Min Read
Ask the Extrusion Expert: Your plastics and processing questions, answered

Our aim is that this column becomes the leading source of extrusion and plastics information on the web. Have a plastics extrusion question for Allan? Send it to him at [email protected]. We will not publish your name or company name unless you expressly ask us to do so, but also will not accept anonymous questions as this opens the opportunity for a supplier to send a "baiting" question for which the answer might highlight his products.

If you would like to watch the taped versions of Allan's previous webinars, simply register for them here; you also can download the slides.

To the Q&A:

Q: To control color (i.e. yellowness index), I add a little drop of water in nylon compounding. Does this also have an effect like an additional plasticizer?

A: Yes, it does work that way, reducing the viscosity and thus generating less frictional heat and/or allowing lower melt temperature. Either one of these will reduce the yellowing (as will some additives, too). It is important to run fast enough that the water doesn't have time to react with the chains (hydrolyze) and reduce strength. How fast to run, and how much water, would depend on the material itself as well as the equipment.

This can be managed on a production line by testing moisture content of feed and yellowness and strength/viscosity of the product. Keep in mind, too, that the feed may vary in moisture content based on its history immediately before feeding, and the way of adding the water also matters (spray, drops, soak a few pellets and mix in like a concentrate).

Q: Why isn't the plastic industry doing a better job marketing itself to an ever-growing environmentally conscious society, and debunking a lot of the misinformation being used as facts by certain groups?
A: That is a big question as different people have different answers. I'll give you mine, but keep in mind that there are many others.
I think our industry people have accepted some of the "sacred cows" of the general public as needed to protect the environment: recycling, degradables, less carbon footprint, and the demonizing of certain plastics such as PVC and polycarbonate (and most recently and most virulently: plastic bags). And the consumer products makers and sellers, led by companies like Procter and Gamble and Wal-Mart, are scrambling to do what they think the public wants - which is also reflected in legislation based on what the legislators think the public wants (a price of democracy).
It is hard to challenge these concepts, even with science. The public doesn't know science and is understandably afraid of it, even as we reap the benefits of its advances. Science is amoral and neutral, and challenges the supernatural and mystical in all its forms. Our defenses are often too "technical," and worse yet, are sometimes connected to money/costs, which make the detractors think that the industry (read: big chemical companies) is sacrificing our health for their profits.
Some of this may be true, but what underlies much of this behavior is reluctance (even fear?) to deal with the issue of changing our consumption patterns - not only "doing the same with less" (as in some Chevron ads) but also doing less with even less. It is not easy, just as quitting smoking is not so easy for those addicted to it. It means more responsible consumption, rather than a childish desire to have what we want when we want it, and let Mommy and Daddy (Government and Industry or vice versa) clean up the mess.

And it means doing/having less now so the next generations don't have even less, as in the need to recognize global warming as real. I don't care who caused it; what matters is if we have any power to slow it down or reverse it.

Q: Is there a dryer available for nylon to control the amount of moisture?

A: All dryers "control" the amount of moisture by controlling the airflow, its temperature, and (for dehumidifying dryers) the moisture content of the circulating air.

The user still has to adjust those settings to give the desired result, which means some testing of the material as fed to the dryer and as it leaves the dryer. Remember that "nylons" are really a family of related but not identical polymers, and some absorb more moisture than others.

 Also, some processes (such as injection molding) will benefit more from the plasticizing effect than others. There are also many drying principles, such as hot air, vacuum, freezing, and infrared heat. If you already have drying machinery, find the people in the manufacturer's company who understand this, and also talk with the technical department of your resin suppliers.

Q: Is there a shop floor test to determine the ability of a profile extruded from recycled PS to resist breaking/cracking when joined with angled nails approx .3mm thick?
A: My practical side says to make a holder than can grab a length of profile at both ends, and drive a nail through it to see if it cracks. It doesn't have to be a 0.3mm nail for the test; try different sizes and surfaces. Temperature will affect results, so make sure the specimen temperature is always the same - that means either leaving samples for a while in a temperature-controlled room, or (for speed) just dunking them into a temperature-controlled bath for a few minutes. It depends on the profile, but try after 1 minute, 2 minutes, etc., until you see no difference.
If you are buying the profiles as extruded lengths, you have the time to test properly. If you are extruding yourselves, testing right at the cutter may be a problem unless profile temperature is always the same. An interesting idea might be to put the test pieces in ice water, which will always be at a controlled 32 F. Cracking will be worse at such cold temps, but maybe that's what you want to show - that the product is OK at 32 F.
As you are using recyclate, you will be subject to variations due to different contents of rubber and/or other additives, unless you really know the source and it's a consistent one. Also, recyclate inevitably has more particle contamination than virgin, but if you do multiple tests, that contamination can wash out, as the contamination will not be the same in each of 5 specimens. If there is much "scatter" in the results, it may be contamination. If they all test the same, it's the plastic.

Q: Why do we have "hazy and contaminated" nylon in a vacuum-chamber metalizing operation?
A: I have not worked closely with vacuum-metalizing molded parts, but would suspect they need thorough drying to prevent moisture being drawn to the surface and out via the vacuum (outgassing). Maybe the water vapor gets in the way of the aluminum atoms that are striking the part surface. Makers of metalizing equipment should know this.

If it only happens with nucleated nylon, maybe that process affects moisture absorption, or maybe the nucleant itself is organic and can be affected by the vacuum. A nucleated product has smaller crystal size, which in turn may affect absorption and rate of its outgassing. I wonder why the nylon is nucleated at all.

Q: I am curious as to what causes plastics behavior towards fire. For instance, polypropylene "runs away" from flame, whereas PE will burn.

A: Polypropylene burns, too, but unlike PE it burns like a candle, without flaming drip, while PE will burn with such dripping. I'm not sure why, but suspect it has to do with relative heat resistance; PP's is higher.

Some other plastics, however, won't support combustion as they contain enough oxygen (or in the case of PVC, chlorine) to discourage burning of the remaining carbon and hydrogen.

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