Degradation of ABS 2

[crispy55]

I'm dealing with an ABS part that has lost significant tensile strength over time. The parts have been stored in plastic bags (not hermetically sealed), which are stored in cartons. The parts have been stored for around 2 years in a warehouse, where the temperature is generally controlled. Since the parts are not exposed to UV light, what would be the source(s) of degradation in the confined environment?

Thanks in advance.

 

[TVP]

Any chemical exposure? ABS has generally poor resistance to a number of organic solvents, etc. Have you had any analysis performed such as FTIR, DSC, etc.?

 

[crispy55]

No chemical exposure. The only exposure has been ambient temperature over a 2+ year period.

No analytical work has been done. We could run FTIR, but what specifically would we be looking for? I presume we'd need a control or baseline.

 

[TVP]

crispy55,

Yes you would need a baseline or control sample to compare against the suspect sample using FTIR. This may be able to detect whether or not the ABS has been contaminated, has been exposed to a solvent or other chemical prior to packaging, etc.

Check out the following links for more information on Environmental Stress Cracking (ESC):

http://www.immnet.com/articles?article=401

http://www.umgabs.co.jp/en/solution/trouble/t_12.htm

Michael Sepe's column The Materials Analyst in Injection Molding Magazine is probably the best single resource I have ever seen with respect to failure analysis of polymer components. He illustrates the various analytical methods like FTIR, DSC, TGA, etc., and shows how to use them to solve problems such as this. Molded-in stresses may be sufficient in the presence of a chemical to cause ESC of ABS. THe following link will allow you to search the archives of IMM:

http://www.immnet.com/search

enter Environmental Stress Cracking
or residual stress
or ABS

 

[crispy55]

Thanks. The references are excellent resources.

 

[Compositepro]

A possible cause is simple oxidation in air. Most polymers contain antioxidants and stabilizers to increase life. Most oxidation occurs during high temperature processing to make the part but will occur at room temperature. If, during processing the polymer degradtion was started the oxydation will continue at room temperature. It is called auto-catalytic oxidation. It involves formation of peroxides and free-radical with air. Peroxides are also used to make ABS from monomer. There are also many contaminants that will catalyze degradation of polymers. I've been surprised to learn how unstable most polymers are over a period of years. Just think how many plastic parts you encounter that are over 20 years old. Of course most of that can be attributed to just wearing out or environmental exposures, but even a dark closet at room temp. causes environmental degradation due to oxygen in the air.

 

[crispy55]

CompositePro - thanks for the comments.
There has been some discussion how to study shelf-life of these items in the warehouse. The proposal is to accelerate aging with UV exposure or temperature. My understanding is the degradation mechanism in UV exposure is different than that of oxidation, as you mentioned above. Would you agree? Does temperature accelerate oxidiation, or is another degradation mechanism involved there, too?

 

[IRstuff]

Temperature should accelerate oxidation processes. The catch with accelerated testing is that you ideally should characterize the failure mode to determine its acceleration factor for temperature. The Arrhenius equation is used to model the accelertation of temperature using an activation energy.

It's a bit of a science project, but you ideally need to run life test at two different temperatures. The relative acceleration of failures between those temperature allows you to determine the activation energy. You can then use that energy to calculate the room temperature life based on accelerated temperature testing.

TTFN

 

[Compositepro]

The ultimate test for shelf life would be mechanical testing vs. time. But for your type of part shelf life should be much longer than service life. If you are trying to determine how to make a better product you need to find the cause of your problem and fix it. There are ways to determine if it is oxidative degradation but these techniques are complicated and require an expert with experience. You could store your products in freezers or under nitrogen but that is unlikely to be feasible. And then your customer will see the problem in two years or less. The test for oxidation is oxygen aborption of a polymer sample in cc of O2 per gram of polymer versus time. The temperature must usually be a lot higher than room temp. to observe a measureable rate. Tests at several temperatures can be extrapolated to room temp.

 

[BillPSU]

You need to know the exact material that the part was made from. What colorants, fillers, reinforcement, plasticizers, base material before anyone really give you an answer. The best source is the injection house/resin supplier. Ambient temperature what does that mean? North Dakota -25F, Texas 110F

 

[crispy55]

BillPSU

Ambient is controlled in the warehouse, between 65F-75F. Regarding the material, we know the manufacturers material designation.

 

[btrueblood]

You might want to look at your plastic bags - they may have been treated with antistatic coatings or plasticizers that have bad effects on the ABS.

 

[BillPSU]

Talk to the resin supplier and the manufacturer of the part. Plastics are process sensitive and sometime humidity sensitive. One of the additives could be causing you problems or some part of the processing may also be causing you problems. The resin supplier can probably tell you what is wrong.