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Radiation Resistance of New Polymers and Blends

03/07/2013 5:10 PM

My lab needs many types of wire and cable that stand up to ionizing radiation well. Due to changing legislation creating increasing demand for LSZHFR (low smoke, zero halogen, fire retardant) cables, existing polymers used in making wire and cable are being either phased out or severely modified. PVC with its phthalate plasticizers is just one example. According to a SpecialChem post, there are about 500 new plasticizers invented annually. Not all of them make it to market. Add to that several antioxidant and other additives to stop degradation of the polymer with time and mild radiation. And there are a large number of new polymers to go with these additives. And polymers can be blended. A vast number of combinations is thus possible. I cannot get any ionizing-radiation resistance figures for any of these combinations because all the manufacturers I have talked to are very secretive about their recipes and they all say "we don't test for that". Why not? Seems very strange, especially since they need to use antioxidants. Is the industry really that hard up for testing budget? Or is the industry just holding its cards very close to its chest, as it were?

I would like to assign a Radiation Index to each new polymer formula used in manufacturing wire and cables. RI is now a European standard where the RI of a material is the base-10 log of the number of Grays of ionizing radiation it takes to reduce the elongation-at-break to 50% of its virgin-material value. Thus it is a measure of how polymers get brittle with increasing radiation dose.

A lot of the new items in the SpecialChem database don't even have a listing for radiation resistance (ie., excellent, good, fair, or poor), whereas the older polymers do.

Can anyone comment on this situation, and is anyone out there doing this work or does anyone have any RI numbers (or even approximations) for us? CERN and NASA have published data on existing polymers for the past 50 years or so, but I got an email from someone at CERN saying they weren't doing it any more.

Thanks for your help! ....wiremanager.

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#1

Re: Radiation Resistance of New Polymers and Blends

03/07/2013 5:56 PM

They probably don't test for it because plastics don't do well in radiation.

Breaks the bonds, as you know.

Parylene® comes to mind, but not very practical.

Maybe silicone?

Not much help, I know.

Good luck.

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#2
In reply to #1

Re: Radiation Resistance of New Polymers and Blends

03/07/2013 6:18 PM

Well some polymers do very badly and some polymers do very well. Take Kapton and PEEK for instance. Both are excellent in radiation. The range of Radiation Indexes for polymers that I know about goes from about 8 to about 3. That is five orders of magnitude. Suppose I have a cable with an RI of 3, say an old-fashioned teflon. And at the radiation level where it is located it lasts one year. If I were to replace that with a cable with and RI of 8, say Kapton, it would last 100,000 years. Not bad. So a cable's RI is very important to designs of reactors and accelerators and other radiation devices. This is what I face every day and it's getting downright impossible with modern changing blends. Manufacturers, to their credit, are pretty free with their samples. But even if I were to test a sample, and that takes a better portion of a year here, there is no guarantee the manufacturer won't change the secret recipe tomorrow. .... wiremanager.

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#3

Re: Radiation Resistance of New Polymers and Blends

03/07/2013 9:58 PM

Part of the problem is that the new polymers are just too new. Teflon was originally considered fine in radiation because it played a pivotal part in many early nuclear devices. (Until these devices went bang, there really was not a lot of radiation produced. By that time the Teflon did its job.) The new polymers have not been exposed to controlled radiation levels for a long enough period of time for their survivability and failure modes to be quantified in this harsh environment. Teflon coaxial cabling worked fine in many high radiation field circuits, until the cable was moved. The Teflon would turn to a dust with similar electrical characteristics. Once the cable was moved the center conductor would now invariably short out to the shield.

I'm still investigating the chemistry why halogens are used in many plastic insulators and why they produce such problems in high radiation environments. My working hypothesis is that the halogen at the end of a polymer hydrocarbon chain bind the end of the chain so tightly that the breakdown ionization energies become higher. Thus insulation could be thinner for the same voltage. However, breakdown from ionizing radiation could break the halogen bond. The halogen ion would then often combine with a hydrogen to form a halogen acid that could now pit or corrode the conductor.

As I said, this is just my temporary working hypothesis. Even if I stumbled into an accurate analysis of the problem, it unfortunately does not give you or me an easy method to discern which polymers should work well in radiation and which ones shouldn't.

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#4

Re: Radiation Resistance of New Polymers and Blends

03/07/2013 10:40 PM

If there is enough interest in this, you could turn it into an entrepreneurial opportunity. Maybe give Survey Monkey a go?

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#5

Re: Radiation Resistance of New Polymers and Blends

03/08/2013 3:53 AM

Is this a candidate for a university research programme, perhaps?

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#6
In reply to #5

Re: Radiation Resistance of New Polymers and Blends

03/08/2013 9:59 AM

Getting cooperation from the many suppliers of these materials might be tricky, but it sounds like a fine research project for some grad students.

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#7
In reply to #6

Re: Radiation Resistance of New Polymers and Blends

03/08/2013 1:05 PM

My lab is attached to a university. We thought of these ideas. Trouble is industry has better facilities. We can do thermal neutron analysis to determine the elements used in the polymers and if it were crystalline we could determine the molecular construction with x-ray diffraction. These polymers are not necessarily crystaline. And we can use our protons to test for damage (the test takes months) but we can't do straight gammas or betas. We can do mixed radiation tests but they also take months.

All industry has to do is contract a few hours from the food-irradiation industry for each polymer or blend invented. It is extremely simple and routine, but it does cost a few dollars. I would bet that such tests ARE being secretly done. In molecular science this data would be too interesting to ignore. I absolutely know that radiation is used to MAKE some polymers. Some polymers require radiation to form and harden. So the equipment exists somewhere. The fact that no one publishes the destruction results and the manufacturers more or less tell inquisitive people like me to 'go away', as it were, speaks volumes. Oh sure, they will offer free samples but that simply causes a delay while their in-house research goes on. Besides, offering free samples is good for business. They know that people can just buy a sample anyway. It really reminds me of the tooth-brush industry, always trying to get one little step ahead of the competition by some new gimmick.

You can tell that I think there is something sinister going on here. I have a theory and some evidence, but let us see if someone can guess it. I will give you a clue -- 'green'. ;-)

If I am right, I would like to know to what degree. Stonewalling from industry has been making my job a living you-know-what lately.

Once you get that one figured out, then we can talk about neutrons and what is going on in the fire-retardant-cable-jacket world and why.

I think this discussion deserves a wider audience. Does anyone think this discussion should be cross-posted to another CR4 group?

...wiremanager

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#8
In reply to #7

Re: Radiation Resistance of New Polymers and Blends

03/09/2013 2:05 PM

'....I have a theory and some evidence, but let us see if someone can guess it. I will give you a clue -- 'green'.....'

.

Okay, I'll play.

.

You suspect ionizing radiation degrades polymers made of fluorine, carbon, and sometimes chlorine in such a way as to off-gas chlorofluorocarbons.

.

Am I close?

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#9
In reply to #7

Re: Radiation Resistance of New Polymers and Blends

03/09/2013 3:37 PM

'...And we can use our protons to test for damage (the test takes months) but we can't do straight gammas or betas. We can do mixed radiation tests but they also take months....'

.

If months of exposure is needed, you might want to run many different samples at the same time. The fact that the test takes months shouldn't really be a factor, unless you are running up against a deadline you haven't mentioned...

.

It will be important to test the polymers as they are used, especially as wire insulation.

The effects on a material of being exposed to ionizing radiation can vary significantly depending on what the material is in contact or close proximity to.

.

Insulators wrapped in a beryllium copper RF shield or surrounding silver or tin plated copper, or in contact with things as common place as aluminum, water and nitrogen could behave significantly different for radiation of a given type and energy.

.

Given the wide variety of possible reactions of commonly used metal conductors when exposed to the same radiation, it seems it would be best to test each insulator with a wide variety of commonly used conductors....

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#10
In reply to #9

Re: Radiation Resistance of New Polymers and Blends

03/09/2013 7:19 PM

These last two comments raise good points. The first is right on the edge of my theory. In fact, the release of chlorine or hydrogen chloride doing damage is something we have seen. Take a stainless steel pipe and pipe hanger separated by a tight PVC bushing to ease vibration and prevent metal-to-metal contact. Put that in a gamma and beta environment to several megarads over 30 years. You will have pin-holes etched into the pipe because the evolved HCl could not vent to atmosphere. The key is that radiation can make AND break down polymers. Now please speculate one logical step further. You might do some google research on polymer irradiation and find an approximately 140-page PDF with some info on the subject. From this and other nearby articles in the google search the reason for the secrecy will become apparent, I believe. Again, these articles make it abundantly clear that the manufacturers have an excellent, precise knowledge of how radiation affects such polymers. Can you specify the reason for the secrecy? I found my first clue from a proposal from a 2003 conference just when the environmental movement was spreading its wings. - wiremanager

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#11
In reply to #9

Re: Radiation Resistance of New Polymers and Blends

03/10/2013 12:28 AM

"If months of exposure is needed, you might want to run many different samples at the same time. The fact that the test takes months shouldn't really be a factor, unless you are running up against a deadline you haven't mentioned..."

Yes. This is exactly the case. We can do about a dozen samples a year. And, the deadline problem is very obvious. We have to do wiring with products that are available which changes every year but last year's tests are NO GOOD on secret changeable recipes. I need the manufacturer to specify the Radiation Index and the elements in the cable that is currently stocked

Jacket composition changes with time to meet fire codes. For instance, if you have thermal neutrons, you don't want any antimony. You may be surprised at the amount they add! They often use it as a Fire Retardant because it works very well to release sequesteted water vapour which retards the flame, and antimony holds onto halogens thus helping prevent toxic gas release. However, antimony becomes easily radioactive in a neutron field. Better choices are aluminum or magnesium or boron. - wiremanager

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#12
In reply to #11

Re: Radiation Resistance of New Polymers and Blends

03/10/2013 3:04 AM

To know much of anything at all with even the slightest degree of certainty about the effects of radiation on various insulated wires will require far more than 12 samples a year.

It might be possible to expose extensive numbers of samples to high energy gammas at a spent fuel pool. A survey when positioning the samples, maybe a couple during and one at completion would give a pretty good idea of level of exposure. Low energy gammas could also be completed at spent fuel pools, just putting the samples a little more distant.

.

Commercial power plants, research reactors, and the naval reactors would be a good place to expose large numbers of samples to a strong neutron flux. The Navy may be very interested in this type of research.

.

The NRC will need to view this research favorably for this to have hope of success. That makes it a good place to start looking. They might be very interested in furthering this research as well.....but then again, I don't have their ear so that is just a guess. You might find a bureaucratic nightmare. Either way, this is going to end up at their doorstep eventually, might as well try to partner early.

If you do manage to get the NRC to agree on some level, you might find a little more cooperation from companies that sell wire to those markets...maybe.

I think it is worth a shot.

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