Plastic Deterioration

[llamallama]

All,

I am relatively new to the industrial arena, being a civil/environmental most of my career. Therefore some of the questions I have about plastics and polymers may be very elementary, I apologize in advance. I am currently working for a food processing company that uses batch retorts in its process. These retorts process product on injection molded plastic trays (the product is pre-packaged prior to processing). We have noticed a residue on the packaging after processing, and I am attributing it to the plastic in the trays breaking down. The exterior of the trays appears softer than when placed in service and a chalky material is coating the trays.

This is occurring at about six months after being placed in service. I do not know the exact material, but am working on an answer. Coincidental to the deterioration, the condensate in the retort appears to be going more acidic. The retorts run at about 265-280F, and the batch time varies from 1-1.5 hours. The runs are about 16 hours four days a week. The residue left on the product is becoming very problematic.

Would symptoms of material deterioration include the higher pH and chalking? Also, is there a cost effective material that would prevent this, or at least extend the length of time before the deterioration occurs?

Thanks

 

[Compositepro]

No one can give a reasonable answer without knowing what polymer your trays are made from. The conditions you describe are considered severe for most polymers. All polymers will degrade over time. Even stainless steel will not last forever.

 

[ornerynorsk]

Have you ruled out residue coming form the food product itself? Have you had it tested for its composition? Do you have some metal trays that can be run through your process to confirm or deny it? AS Compositpro mentioned, that is a fairly high temp and long duration for most plastics.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[rickfischer51]

As mentioned above, what kind of plastic?

An interesting experiment would be to take a few virgin trays and subject them to FT-IR analysis. hAve the lab save the data trace from each tray. Then run the trays through the proccess, retrieve them, and repeat the FT-IR process. The lab can subtract the second trace from the first. The difference would be whatever leached out.

 

[rickfischer51]

Also, some plastics produce acids as a byproduct of hydrolytic degredation. Is the condensate water vapor? If so, this may be the problem. If the material is undergoing hydrolysis, it's molecular weight will decrease. Have a lab run a melt flow test on virgin and processed trays and compare the results. An increase in melt flow rate means lower molecular weight, lower viscosity and higher flow. Also, the material will loose ductility. Take a piece and bend it.

 

[llamallama]

All,

Thanks for the replies. I am still working on getting information on the material. Yes, the condensate is water vapor. I should have mentioned that this is a steam retort, so the trays are subject to steam heating in a pressure vessel. I am following through with some recommended testing, but am starting to conclude this is a case of the wrong material for this application. Is there a plastic material that would hold up in this environment, or would I be better to go to stainless trays?

Thanks

 

[Compositepro]

I would go stainless unless you are dealing with acids. What is the retort made of? It will last longer than any polymer and is probably cheaper and more available than exotic polymer trays.

 

[llamallama]

Thanks again for the replies.

I finally got the material composition from the MFG. 69% polypropylene resin, 30% talc filler, and 1% elastomeric additive. Does this seem appropriate for a steam retort? We are checking on the stainless option, but have considerable investment in the plastic trays.

 

[ornerynorsk]

I think you just defined the problem. You may have to process outside of the tray and place the product in the tray once it's cooled somewhat. Your processing temp and duration is very high for PP, you're very near the melting point. The chalky material you're describing might be talc filler coming out of solution, but a lab could confirm or deny that for you, provided you've already eliminated food product residue as a possibility.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[llamallama]

ornerynorsk,

Yes, I do believe the residue is talc. We are confirming that. The trays are needed to hold the product while in the retort, but also for transport, so any tray would be start to finish. Would you expect breakdown of the PP with steam? Others here are looking for boiler chemicals as the culprit, but I am becoming convinced that the process itself is degrading the trays (steam retort) without any help from steam additives.

Thanks again, this has been very helpful.

 

[ornerynorsk]

I think that amount of heat, regardless if the source is steam, IR, electric, etc, is too high for PP with that duration of time.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[rickfischer51]

First, what do you mean by breakdown? To me breakdown means chemical degradation, ie chain scission of the PP molecule. This will show up as a reduction of molecular weight, and result in a loss of ductility and impact resistance. Or are you referring to what appears to be a loss of talc?

I do not believe the combination of temperature and moisture is the problem here. As far as I know, PP does not hydrolyze. There needs to be a double bonded pendant oxygen molecule, like with nylon, polyester or polycarbonate, to be an initiation site for the hydrolysis to take place. PP is hydrophobic: it repels water. Water and PP simply do not react.

PP is prone to oxidation at elevated temperatures. The higher the temp, the faster it oxidizes. If follows an Arrhenius relationship. There is a rule of thumb that is loosely based on Arrhenius that says for every 10C increase in temp, the rate of degredation would double. The test here would be a melt flow test. As the degredation increases, the molecular weight and viscosity go down, and the melt flow value goes up. Any good polymer testing lab can do this. Send him virgin and used trays and compare the numbers. Oxidation could cause the softening you reported.

16 hrs per day times 4 days a week times 24 weeks is 1536 hrs at 265-280F (130C+). That strikes me as pretty severe for a commodity grade thermoplastic. Assuming the 10 degree rule, thats 1536 hrs * 2^11 or 3,145,728 hours at 20C, or 3641 years. You are essentially conducting an accelerated degredation by oxidation test here. The 10 degree rule is very approximate, but if its off by 100 times, you are still putting 36 years of degredation into 6 mos. Thats pretty nasty!

Take new and old trays han hav TGA run. That test will burn off the organics and leave the talc as an ash residue. If you are loosing a lot of talc, you may see it.

 

[chicopee]

Remind me not to buy food from your company. The FDA may have a say about your process if you are located in the USA.

 

[TomDOT]

Talc is easy to confirm by FTIR. The spectrum is distinctive.

Examples:

http://tera.chem.ut.ee/IR_spectra/index.php?option=com_content&view=article&id=126&Itemid=93

http://www.gly.uga.edu/Schroeder/11Schroeder.pdf

 

[Demon3]

A chalky appearance is often microcracks in the surface due to the heat degrading your polymer. Very likely for PP. Look at PP garden chairs degraded by the sun for an example. The filler may contribute. PP is not great under heat for prolonged periods and the stabilizers make all the difference. Labs that can anlyze the plastic and work out your problem include PlasticExpert.com and Stabilization-Technologies.com

Chris

Chris DeArmitt - PhD FRSC

Plastics & Materials Consulting

http://www.phantomplastics.com

Plastic Training Seminars

http://www.plastictraining.com