Have a question about plastics extrusion or the science of plastics? Send it along to Allan Griff, independent consulting engineer and plastics extrusion expert. Already the host of our well-received Extrusion Expert webinar series, Allan now also authors this "Ask the Extrusion Expert" column. The formula is simple: you send questions and Allan answers them. Read on for questions from your processing peers and for Allan's answers.  

Have a plastics extrusion or plastics 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.

To the Q&A:

Q: Can sulphur be used in the manufacture of polypropylene? If so, to what purpose?

A: There are many sulfur-containing compounds that serve as stabilizers / antioxidants, notably for PVC, but some may work for PP. See the 1969 article on this subject, http://onlinelibrary.wiley.com/doi/10.1002/app.1969.070130701/abstract

I don't know of any sulfur compounds that are used as catalysts; these are usually compounds of metals like titanium or zirconium, but I am not an expert in that field, so there may be something new I don't know about.

Elemental sulfur is not very good for PP - in fact, it has been studied as a degradation promoter to assist in recycling operations.

Q: Why is there such a difference in molecular weight (viscosity) of material if the polymer chain length can be controlled?

A: That is because different processes and applications require different viscosities. The biggest difference is between injection molding - when we want as much flow as possible consistent with strength needs - and extrusion, where we want more self-supporting melt strength. As a result, molding resins have lower viscosities than extrusion resins, with the exception of extrusion-coating, where we want the low viscosity to soak into the (usually) fibrous paper substrate, and we don't need the self-support because the melt is always supported by the paper.

Even within a process, there are differences. In extrusion, lower viscosities may run faster, or run cooler, or use less power, and higher viscosities usually mean more strength, and certainly less sag in thermoforming sheet. In molding, some molds are easier to fill than others, so we might choose a higher viscosity there to optimize strength.

Q: Can the concentration of antioxidants in a plastic be accurately measured to determine if changes are occurring with successive heat cycles?

A: I think it can be, but you'd have to talk to antioxidant suppliers to be sure. However, the antioxidant isn't the only variable, and it might be better to make sample batches with repeated regrinding and test them for stability. How to test is an interesting problem. There are oxygen uptake tests which measure the amount of oxygen that the material can absorb. One of my fellow PlasticsToday webinar presenters, Mike Sepe, is a consultant in this field, and may be able to help you. He lives in Arizona, and his email is [email protected].

If you have clear or white products, yellowing might be a good measure. However, if your product is black, you can't use this method.

I had this problem in the 1980s when a client was accused of making weaker HDPE gas pipe because it reused its regrind. The color wasn't black, but it was orange, which made yellowing just as difficult to follow. Instead, I used a torque rheometer (Haake or Brabender) and ran a sample of ground pipe for an hour in the chamber of this machine, held at extrusion melt temperature. As suspected, we found that the torque actually increased slowly for the first half-hour, implying a cross-linking and strengthening reaction, but then the torque began to fall off, implying that the antioxidant was all used up and the breakdown rate was now faster than the cross-linking rate.

Q: Will ABS suffer the same chain breakdown from moisture as nylon or polycarbonate?

A: No, ABS is an addition polymer much like polyethylene. No water is driven off in its polymerization (as with PET and the other condensation polymers) so there is no place the water can return to.

Q: What does the POE molecule look like?

A: By POE I think you mean PolyOxyEthylene, which is also known as PEO (PolyEthylene Oxide) and PEG (PolyEthyleneGlycol).

It is a simple thermoplastic polyether (oxygen in the chain), with the repeating unit of [-CH2-CH2-O-]. It is a water-absorbing slippery polymer. I worked with it maybe 20 years ago as an extruded strip attached to disposable razors to lubricate the blade.

If by POE you mean polyolefin elastomers (I've heard this use), that could be any of several polymers and copolymers, and you'd have to be more specific to get a correct molecular diagram.

Q: Can you briefly explain amorphous and semi-crystalline plastics?

A: In amorphous polymers the molecules are all tangled up as one mass, and no part is any different than any other. Light can pass through unbent, and thus these polymers are usually transparent.

In semi-crystalline polymers, the molecules line up in certain areas, but not in others. Think of an aerial picture of any city -- Corvallis is fine -- link is

http://maps.google.com/maps?q=portland+or+map&hl=en&rlz=1Q1WDIA_enUS390US426&um=1&ie=UTF-8&hq=&hnear=0x54950b0b7da97427:0x1c36b9e6f6d18591,Portland,+OR&gl=us&ei=-IBlTtPLCqvPiAKd_NCqCg&sa=X&oi=geocode_result&ct=title&resnum=1&ved=0CB4Q8gEwAA

where there are areas with streets on a grid, and other areas where there are parks, golf courses, lakes, etc. The grid streets are the crystalline portions and the other areas are the amorphous portions.

That is a brief explanation; the polymer scientists can explain why only a portion of the molecules are lined up, and how the branches (number and length) have something to do with this.