Effects of ultraviolet radiation combined with IR radiation

[kingofthehill]

First of all Hello! I'm new here!

 

Now for what I hope is a reasonably basic query....

 

I am trying to devise a test to evaluate detrimental effects of UV radiation, primarily on rubber and plastic components. There are many standards that I have found covering this sort of thing but I am having difficulty finding a suitable one because my understanding of UV radiation is limited. I don't have much time to research the subject extensively so here I am!

 

Originally, I had a test planned in that requires the use of full spectrum lamps that replicate the full light spectrum emmited by the sun. This test obviously has an element of IR radiation that causes considerable heating effects on the product. However, I am primarily concerned about the effects of UV radiation on rubber and plastic components so have been considering finding a more straight forward test that may be cheaper (i.e. static UV lamp instead of motorised full spectrum lamps replicating diurnal cycle).

 

My question....Is the actinic effect of UV radiation worsened if the temperature of the item that is subjected to the radiation is higher as a result of IR radiation?

 

I have searched the net but have not found a basic article that may help understand this. If anybody could point me in the right direction for good information on the subject, that would be appreciated greatly.

 

TIA

Kingofthehill

Edited June 8, 2017 by kingofthehill

[DrP]

I don't think the IR will effect the rubber short of warming it. The UV can obviously cause scission of chains or even crosslinking in many plastics... it depends on what plastic it is and what functional groups are on the polymer as to whether they crosslink, break or unzip. I don't think IR hurts many plastics either beyond warming.

 

So, I could be wrong but, unless you have some exotic functional groups or some weird polymer backbone I don't think the IR will harm the material beyond heating. It is usually the UV that can cause chemical changes in these types of polymers and I don't think temperature has any effect on the number of chemical actions per dose of radiation.

 

(Sorry if that's not much help - no-one else has chimed in, so I thought I'd share my take on it - I couldn't find anything on the net either from a brief search so I shared what I know - Please let us know if you find out more!)

[kingofthehill]

Thanks for that!

 

The only thing I could find that suggested that reaction to UV light was affected by temperature was a school science experiment that examines the effects of temperature on the duration of colour change using photochromic plastic beads. At 0°C, when exposed to sunlight, beads took in excess of 1 minute to change colour. At 60°C, colour change took less than 10 seconds.

 

It's an ultra basic experiment but it seems to me that this answers my question? Photochemical reaction occurs at a faster rate at higher temperature in this particular example. Or is this something other than photochemical?

 

Thoughts?

Edited June 8, 2017 by kingofthehill

[DrP]

SPECULATION:

The colour change happens after the quanta of UV has hit the sample I think... that well might be faster at a higher temperature. The breaking of bonds in chain scission due to incident UV light happens instantly I think... so would not be effected by temperature. With crosslinking, however, the incident UV light creates a radical on the chain or functional group somewhere along the polymer which then reacts with nearby chains to cross link. As this is a chemical reaction I would not be surprised if the speed of this reaction is influenced by temperature... even if only very slightly. I would guess (as there is nothing reported about it from what we can tell) that the effect would be very slight.

[kingofthehill]

This is frustrating. I really need a feeling of certainty about this before I plump for a particular test method. I haven't got that warm fuzzy feeling at the moment.

[DrP]

yea - sorry I can't be of more help. I guess that the detrimental effects of UV on rubber come from the scission rather than the crosslinking type reactions.As I suggested above I doubt this will be speed up by a slight increase in temperature.

 

How would you test for the number of chemical incidents per rad dose between the 2 samples? You can have one hot and one cold rubber sheet exposed to the same amount of UV dosage.... but how to test the sheet after to tell number of actions that took place for the dose you applied? Solubility change? Integrity/strength? I don't think that the number of incidents will change between the 2 samples anyway... it has to be down to the exposure dosage, not the temperature imo.... but the resultant chemical reactions that take place AFTER the UV has cause radicals to occur could well be faster at higher temperature... I would assume this because most other chemical reactions are faster at higher temperature.

[swansont]

I think you need to recognize that the heating of the material and exposure to IR are not necessarily the same thing. The heating could be from conduction or convection as well as radiation, and in most cases the reaction will not depend on the method of heating. It's purely a matter of temperature, rather than method.

 

There is a small chance you could see damage because the IR excites the material and then a UV photon has enough energy to interact (but the UV photon alone had insufficient energy to e.g. ionize on its own) but unless you are specifically worried about such interactions, speaking about IR is a bit of an unintentional red herring

[DrP]

To sum up - I would assume that the number of incidents per dosage will be the same regardless of temperature. It is the speed of chemical reactions AFTER the formation of radicals caused by the incident UV that will be sped up by a temperature increase.

 

(Sorry - cross post with swasont).

Edited June 8, 2017 by DrP

[kingofthehill]

I think you need to recognize that the heating of the material and exposure to IR are not necessarily the same thing. The heating could be from conduction or convection as well as radiation, and in most cases the reaction will not depend on the method of heating. It's purely a matter of temperature, rather than method.

 

This particular test specimen is an electrical item that will be energised for the duration of the test. So the resultant response temperature of the materials would be combined effects of heat emitted from electrical components and IR. In this respect, the temperature could be quite severe (simulation of hottest dessert conditions) which is why I have mentioned IR. It will be impossible to determine what the resultant temperature will be without first testing with IR. Subsequent tests could perhaps be set up without IR when peak temperature has previously been obtained from the IR test.

To sum up - I would assume that the number of incidents per dosage will be the same regardless of temperature. It is the speed of chemical reactions AFTER the formation of radicals caused by the incident UV that will be sped up by a temperature increase.

 

(Sorry - cross post with swasont).

 

Some of the materials being evaluate are Nitrile rubber, Polyamide, TPU and FKM (fluorocarbon). I have no idea if radicals would be present in these materials as a result of UV exposure. Test duration is 10 days exposure.

I should also add that I will be evaluating hardness changes in rubber parts (embrittlement). Various functional tests will be carried out to evaluate presence of structural defects (cracks, delamination, electrical defects, failure of bond lines etc.), as well as visual examination (surface coatings, crazing, blistering, fading etc.).

[DrP]

The UV breaks bonds in and along the polymer which causes radicals... which then react (crosslinking reactions, unzipping or chain scissions depending on the chemistry of the polymer). As I said - I am fairly confident that the number of radicals produces will be constant regardless of temperature. The reactions taking place due to these radicals are what might be effected by temperature, but I would guess this would be slight.

 

Sample density could effect the amount of incident UV that gets absorbed and forms radicals. A dense block will likely absorb more of the UV than a thin sheet (where a lot will pass straight through). As long as all samples are identical sizes, thickness and density it should be comparable.

 

My guess is that temperature (unless hundreds or 1000C or something silly) will not effect the level of degradation in the samples over the 10 days beyond what will be seen at room temperature due to the UV anyway. Please let us know the results - I would like to know how the results turn out for personal interest.

 

Regards,

 

P.

[kingofthehill]

Thank you all for your comments