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Sorry for the confusing title, I didn't know how to describe this better. So, if we douse rubber, say, a bicycle tire or a certain type of rubber glove, with organic solvent (n-heptane), we can find n-alkanes being dissolved in the solvent. Why is this? I'm talking especially about gas chromatography to analyse the paraffinic hydrocarbons here.

I guess this is quite a trivial question but I can't figure it out. As far as I know, natural rubber consists mainly of isoprene polymer chains. But isoprene has a double bond and in GC chromatogram we can see n-alkanes being the largest peaks. And not only that, we can also find straight chain hydrocarbons in synthetic rubber (like NBR or SBR) solvent extracts. SBR has a benzene ring in its structure but still n-alkanes are the dominant finding in its solvent extract chromatogram.

This is a major subject in my bachelor's thesis but I'm an analytical chemistry major and struggling a bit with materials science in this depth. Any advice and ideas would be greatly appreciated.

To make my point clearer I present these two chromatograms:

GC-FID run of pure n-heptane (GC-FID run of pure n-heptane)

GC-FID run of n-heptane extract of bicycle tire (GC-FID run of n-heptane extract of bicycle tire)

This question still haunts me. I found this. Could that be a partial explanation for the aliphatic hydrocarbon finding?

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  • $\begingroup$ In case you're still interested in this - I'm curious what part of the tire you did the extraction of? Tires are made of multiple layers that are adhered together, and the alkanes might be from the adhesion areas. $\endgroup$ – Andrew Sep 14 '20 at 23:29
  • $\begingroup$ It was a sector of the whole tire (about 10 cm piece cut off of the length of the tire). As I have gained more experience in the field of chemistry I now strongly believe the alkanes are from paraffin wax. $\endgroup$ – nh3 Sep 21 '20 at 19:31
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Although natural rubber and synthetic rubber display a poor chemical resistance to alkanes among others, they do not dissolve in alkanes. Your GC/MS traces (I believe that so because of alkane/alkene identification) have confirmed that since they have shown n-heptane as the main peak (I believe everything else are trace amounts exhibiting below your set-up threshold).

There is a reason for your finding of straight chain hydrocarbons in synthetic rubber such as SBR in your solvent extracts. For example, SBR is the shorten form for synthetic material, styrene butadiene rubber, which is made by anionic copolymerization of styrene & butadiene. For this anionic copolymerization, frequently used initiator is a solution of lithium alkyl compound in small paraffins, for instance, n-butyllithium in hexanes. The rubber may still have few molecules of these small paraffins, which may diffuse into the n-alkane you use to extract, and hence, would show up in your GC/MS chromatogram.

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Your link almost certainly explains the alkanes. It says

commercial paraffins are mostly in the 22 to 38 range

That's exactly the range we are seeing in the chromatogram. I'd consider this the answer.

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