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Ask the Extrusion Expert: Q&A with Allan Griff

What properties affect ESCR? Can resin really be too dry? Which cross-linking method is easier to break down? Does HDPE gas pipe contain lead? You asked the questions, and Allan, host of our Extrusion Expert webcast series and independent engineering consultant, provides his feedback while also taking great exception with Bisphenol A naysayers.

For starters, one recent question was, "How can I listen to the webcasts I've missed, Allan?" These webinars are all archived here at PlasticsToday  for a year after they are presented. The entire list is also at Allan's own website, www.griffex.com.  

And now to the more complex queries.

Q: Which properties affect ESCR (environmental stress crack resistance)?
A: You didn't specify the plastic, but I'll assume that you are referring to polyethylene, as that's where most of the work has been done. Lower melt index reflects greater molecular weight, which means better resistance to such cracking.  

The effect of density is more complicated, as higher density means more crystallinity and the effect of this depends on crystal size as well as percentage. Also, greater rigidity (modulus) may lead to more stress for a given deflection in service.

There is also an effect of branching of the molecules -- more short branches are better. Finally, the crosslinking of PE confers great resistance to stress-cracking, and is a major reason for its use.

 

Q: Assuming no additives are lost, can resin really be overdried?
A: Usually not, but too much time in a dryer is inefficient and wastes power and time.  There are two situations where overdrying may be a problem:
1) The continued exposure to heat and oxygen (in air) may create some thermal degradation, and
2) with polyamides (nylons) a little moisture is actually good as it acts as a plasticizer, improving flow through molds and dies. Too much risks hydrolytic degradation, so there is an optimum moisture level.

Q: What applications are good for low impact polymers (impact range from 1-5)
 
A: What are the units for "1 to 5?  If they are in ft-lb per inch notch in a notched Izod impact test D256, that is not a low impact range.  If it's in joules per cm, it's even better, as 1 joule/cm notch  = 1.87 ft-lb/inch.

Also important in specifying impact strength are the type of specimen (molded, extruded), its thickness, and the test temperature.

In any case, the impact strength of a plastic can often be improved by toughening additives.

Q: When thermoplastics melt, do they go back to their monomer forms?
 
A: No, they don't.  That's why they can be reused over and over again, if kept clean and not heat-abused. It takes a lot of energy to break the bonds between the carbon-carbon bonds in the chain (and sometimes carbon-oxygen and carbon-nitrogen).  
 
Sometimes there is degradation, as when a hydrogen and a chlorine break off the main chain of PVC, or when there is some oxidation (yellowing), or when a water molecule breaks apart the chain at the place where it once "lived" -- but no monomers.
 
There may be residual monomer left in the plastic after polymerization. This was important for PVC in the 1970s when the monomer, vinyl chloride, was shown to be carcinogenic, but the resin makers "cleaned up their act." Today's resins show no significant monomer emissions, and it's a non-issue now.
 
What IS an issue now is the allegation that BPA, one of the monomers for polycarbonate, is harmful in a variety of ways. The public doesn't see the difference between monomer and polymer, and thinks that there is BPA "leaching" from any PC article, notably water and milk bottles. Even members of our own industry have come to believe that PC is "bad," and have scrambled to develop and offer replacements for this "harmful" substance.  There should be no BPA left after making PC, it isn't produced when PC is melted, and if there is a residual it is a very small amount. I don't have the figures, but think that this should be the industry's first line of defense - not blindly believing the plastophobes, but, following the example of PVC, cleaning up their act and showing beyond a reasonable doubt that there is no significant BPA left in PC, nor is it re-produced when melted.
 
It is possible, with enough heat and perhaps catalysts, to break down some polymers into their original monomers, and this was proposed for PET around 1990 as "chemical recycling." It turns out that the energy needed to break the bonds is at least as much as that needed to make the polymer in the first place, so it is thus much more costly to the environment (from an energy viewpoint) than conventional recycling. When this was first popularized many people saw it as a way to make plastics look environmentally "good," even though it wasn't so good after all. Fortunately, it was also too costly in money terms to support itself compared to recycle and new manufacture, and never caught on.
 
 
Q: Which crosslinking method is easier to break down, radiation crosslinking or chemical crosslinking?

A: Are you asking about later breakdown of the links in service? It is hard to give a blanket answer, as there are many ways to crosslink by chemical or radiation means, and although polyethylene is the most common resin that is crosslinked, the process can be used with other polymers as well.

It will certainly depend on the amount of chemical agent used and the processing temperature, as well as the type, intensity and exposure time for the radiation.

Also, there are different agents of breakdown that may have varying effects - excess heat, UV light, oxidation or other chemical exposure.

The companies that make the processing equipment and the crosslinking agents should be able to provide more information.  
I'm sorry I couldn't give you a more specific answer, but hope that my comments are useful.

Q:  How is the ductility of HDPE increased? Through the extrusion process or via a downstream process?

A: By ductility, I think you mean the amount the plastics can be deformed before it actually fails. Ductility is related to the difference in elongation after yield but before break in a tensile test. The rate of testing is important; behavior in a fast test like impact strength is different from the 2"/minute tensile test, and for some applications like pipe, the behavior over a period of months or years is important.
 
The main factor affecting ductility is the choice of resin itself. In general, the higher the molecular weight, the tougher the material, but different HDPE resins will behave differently, and true comparison must take into account the service conditions such as the rate of loading noted above, as well as the temperature. Most additives will decrease ductility, but there may be exceptions. If the chains are cross-linked, it may help.  

Extrusion conditions may have an effect if the product is strongly oriented and cooled fast, which will freeze-in stresses and line up the molecules more in one direction than the other.  

If a product shows low ductility or a classic "brittle" failure, it may be a case of contamination rather than the resin's fault.  Test several specimens at the appropriate rate, as not all will be contaminated in the same way.  If all specimens test poorly, it is more likely a resin problem.

 

Q: The company I used to work for was making yellow polyethylene gas pipe. Is it true that pipe used lead as an additive for making the pipe flexible so it could be coiled? If so, what would be the by-products of burning this plastic and should you burn it inside a building full of people? If this burning of plastic, to remove dies from the machine and clean them, caused injury and death to the workers, how would Plastic Today perceive this? Is this a common practice with-in the industry or would this one factory be an isolated incidental practice?

I would think that if this practice was isolated to just a few, that the rest would be up in arms and not condone this. It would be a major blight on the safety record of the industry and this publication should address this safety issue of open burning of plastic, by which I hope is only a small number of manufactures.

A: I spent a lot of time working with gas pipe around 1985-2000. Somebody has confused PE with PVC. Rigid PVC was tried for gas distribution pipe back in the 1970s but HDPE won that race and has been the resin of choice for the last 25+ years.

The yellow pipe you refer to is HDPE, colored yellow so anyone who digs will know that it has gas inside of it, not water. It is only used as buried, and is thus far superior to iron/steel which corrode in the earth, as well as foul inside from acid content of the gas.

PVC is also used for pipe, and lead stabilizers (not metallic lead) can be used with rigid PVC formulations; it is in combined form so it is not so easy to get into anyone's system (Salt = sodium chloride) doesn't emit either sodium or chlorine.) Even so, fear of lead and availability of competitive materials have reduced its use.

More misunderstandings: lead is a very old pipe material on its own - the Romans had it 2000+ years ago - and it was indeed flexible (bendable) which was an asset compared to iron. But the lead compounds used as PVC stabilizers (at around 2%) wouldn't add flexibility - someone is now confusing the rigid PVC pipe compounds with plasticized PVC, which can be coiled and which may contain lead stabilizer, or they are further confusing the lead with phthalate plasticizers, which are indeed added to make the product flexible. No plasticized PVC has even been used for gas pipe, except for the case of someone who used garden hose to steal gas from a neighbor's service line, which then leaked and blew up, which is how we know about this case.

Another: PVC doesn't support combustion, and burning isn't a very good idea - even for other plastics that do burn - and least of all inside a building. There must still be people who will burn out tools to clean them, as it is quick and effective, but it also may harm the tool, and there are cleaning devices that do the same with vapor control. It is more likely a last resort or quick trick with PE; however, PVC is more easily dissolved and cleaning can be done this way with proper safeguards.

Very little plastic is burned in factories - it is usually capable of remelting and reuse, and is worth too much to lose it. As a factory trainer in extrusion, I discourage burning, but am more concerned from a safety point of view with hazards of electricity, heat, heavy components, cutters and shears, spills of water or pellets on the floor, failure to wear protective equipment, and tired workers who are working 12- and 16-hour shifts, or who may do 8 hours after too little rest.

[ 更新时间:2011-10-18 ]
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