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This question was posted in Electronics. Something is really odd about it! What could possibly be causing this "melting" reaction between a common flexible covering (vinyl, apparently) on test lead clips and the plastic there?

a couple of breadboards had been lying in a box with a rubbered alligator wire sandwiched between. They've been there for maybe a month untouched. No other chemical or electricity has been present and only ambient heat (hot summer but 30°C tops here in Sweden). I'm pretty shocked that the plastic of these two different styles of breadboards both got so messed up simply by touching this rubber:

Common alligator clip and common electronics breadboard

This is a very common alligator clip and very common electronics breadboard, often used together. I'm wondering whether the asker is telling us everything.

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    $\begingroup$ Although I understand that you are likely not to have that information, it would be interesting to know which polymers are involved here :) $\endgroup$ – The_Vinz Aug 25 '18 at 0:36
  • $\begingroup$ @The_Vinz Indeed! I was hoping that a chemist here would ask that question there. $\endgroup$ – Mike Waters Aug 25 '18 at 1:38
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    $\begingroup$ Check on the breadboard, there should be a recycling symbol somewhere on its back. Google for the number in the recycling symbol, it tells you the polymer class it's made of, and report here. $\endgroup$ – Karl Aug 25 '18 at 8:01
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It is probably a consequence of the plasticiser not the polymer

It is hard to be sure what has happened here without checking the details of which polymers are involved which the picture can't make clear. But we can make some intelligent guesses based on commonly used polymers.

First: the cable isn't made from natural rubber. Most likely it is made from plasticised PVC. This is a good and cheap cable material with good insulating properties and doesn't decay like rubber from interacting with atmospheric oxygen. This involves a lot of plasticiser, usually something like a phthalate. The cable might contain 10-40% of plasticiser by weight.

Depending on the exact polymer formulation and the specific plasticiser, the plasticiser can migrate into other materials if they are in contact for long periods of time. When I used to work in a lab, for example, the cheap PVC tuning we used in low pressure gas lines would sometimes accumulate a thick liquid in the lower parts of the tubing loops if used for years: this liquid was plasticiser that had migrated out of the PVC over years of use. This migration is worse when other volatiles are in contact with the polymer.

This may be what is happening in this case. In warm conditions the plasticiser in the PVC might migrate and damage other plastics. It isn't possible to tell from the picture what the breadboards are made from but ABS is most likely in a good quality breadboard, though acrylic, polystyrene or polycarbonate are all possible. All of these polymers can be affected by the migration of plasticiser from PVC as this study shows (though it depends on the specific plasticiser). Good choice of materials for the board and plasticiser can minimise this problem but cheap breadboards and cables are likely to have worse problems.

In short, this is the likely problem here.

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