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Crack growth in PE using surface active environments (Igepal, Arkopal)

Hi everyone,

I am conducting research on slow crack growth (SCG) in polyethylene (PE). Currently I am trying to find out how surface active environments (e.g. solutions of Igepal, Arkopal) promote SCG:

For very slow crack growth rates it is generally reported that the surfactant flows into the craze zone (I agree with that), diffuses into the amorphous regions of the fibrils and acts plasticizing. As a result the polymer molecules entangle more easily and SCG is promoted; the surfactant molecules are often compared with a lubricant.

Furthermore it has been observed that the craze-zone gets larger and the crack tip more blunt. This should actually slow down crack growth. As a result I don't believe that the plasticizing effect is the whole story and that there are also other mechanism caused by surfactants that promote SCG.

Further evidence for my assumption are that the above theory can not explain the following:

=> A higher concentrated solution of surfactant results in faster crack growth rates, but conducting tests in 100 % surfactant slows down crack growth, although more "lubricant" is available.

=> Surfactants with a higher molar mass cause higher crack growth rates; the explanation that larger molecules are less absorbed by the fibrils and consequently the crack tip is less blunt got me all confused, because it is in disagreement with the theory that the surfactant acts as a lubricant.

My Questions:

Does anybody believe (against the general believe) that surfactants cause chemical degradation in the fibrils?

Is it reasonable to assume that the specific surface area of the craze fibrils is large enough that the drop in interfacial tension produced by wetting of the fibrils with Igepal CO-630 can account for the drop in effective yield stress. Thus promoting the growth of craze zones and the breakdown of fibrils?

Has anybody heard of other theories why surfactant promote SCG and can recommend further literature?

Any help is greatly appreciated!

Regards

Markus

Reply to
Markus
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Sorry about this goofy reply. I am using Google as a newsreader and it doesn't allow me to insert my comments in the appropriate spots of your post. So you're going to repeat yourself!

You said: "For very slow crack growth rates it is generally reported that the surfactant flows into the craze zone (I agree with that), diffuses into the amorphous regions of the fibrils and acts plasticizing. As a result the

polymer molecules entangle more easily and SCG is promoted; the surfactant molecules are often compared with a lubricant."

I think you mean "disentangle", not "entangle". When the amorphous regions (tie molecules, cili, loops) pull out of the crystalline areas, the materials ability to withstand stress decreases, leading to crazing and cracking.

You said:"A higher concentrated solution of surfactant results in faster crack growth rates, but conducting tests in 100 % surfactant slows down crack

growth, although more "lubricant" is available."

Perhaps water plays a critical role in arranging the surfactants' heads and tails? I.e., are micelles necessary?

You said:"Does anybody believe (against the general believe) that surfactants cause chemical degradation in the fibrils?"

I certainly don't believe it. Alkanes are one of the most unreactive materials that there is. It would also be possible to monitor/observe the reaction that would be occuring.

John Aspen Research, -

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"Turning Questions into Answers"

Opinions expressed herein are my own and may not represent those of my employer.

Reply to
john.spevacek

FYI,

There are two methods to reply on Google Groups. The one which you know is the 'Reply' button at the bottom of each post. The one which you want is to click the 'Show Options' link at the top of the post, and then hit 'Reply' in the dropdown task bar which emerges. This will get you a text box with fully quoted and attributed references... Google Groups needs to fix that reply button at the bottom. It messes up posts for anyone not using Google Groups to look at Usenet.

Also FYI,

You owe me a proposal and quote on my formaldehyde emission project ;-)

Regards R. David Zopf Bomar Specialties

Reply to
atomweaver

messes

Kewl. Now I can top post ;)

It's coming. I'm still trying to catch up after going to ANTEC.

John Aspen Research, -

formatting link
"Turning Questions into Answers"

Opinions expressed herein are my own and may not represent those of my employer.

Reply to
john.spevacek

Hallo John,

thanks for the reply!

This might be a possibility and has been mentioned by some authors. Reconsidering it, I can imagine that the micelles struktures might influence the sorption of the surfactant at the surface of the fibrils. But I don't think that the micelles alter the rate of diffusion within the fibrills.

I have worked a lot with Arkopal N 110. (nonyl-phenol ethoxylate =>

containing 11 ehtylenoxide groups). It is reported that this type of detergents suffer oxidation of the hydroxyl-group at the end of the ethoxylate chain into acid within the a few days (C-O-H => COOH). Afterwards the ethoxylate chain is split into smaller units and these become oxidized into formic acid and other products. Consequently the ethoxylate chain becomes shorter (In my experiments I was able to confirm the shortening of the ethoxylate chain.).

Due to this reactions I wondered if during the degradation of the Arkopal molecules, radicals could be formed that might trigger the degradation of the material in the fibrils. Thus enhance crack growth.

Best Regards

Markus Polymer Competence Center Leoben -

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Reply to
Markus

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