The datasheet says: "After polymerization, the block sometimes turns to a light pink or pink/red color. This seems to be quite normal and should not be a cause for concern. "
Indeed, after UV-polymerization the blocks usually are pink and going to be colorless in a few weeks.
But after thermal polymerization (without UV irradiation) they always are totally colorless. The sample preparation (freeze substitution, infiltration) was the same. Which reaction could turn the blocks pink/red?
Good question. I've wondered about this and have never gotten an answer I'm convinced by. The answer I've heard by two labs who do a lot of HM20 work is that the HM20 get "activated" by UV and not heat; with free radicals generated causing the color change. Exactly how this color change then dissipates over time within the polymerised plastic is left with a shrug and a bit of hand waving.
Dear colleagues,
Sorry for the late answer. I've heard "free radicals" as an possible explanation too. But it is just the mechanism of acrylate and methacrylate resins polymerization - benzoyl peroxide which is used for thermal polymerization gives free radicals too. Acetone and other reagents except resin initiator were the same for both blocks.
Cheers,
Denis
I've not used acetone for HM20 embedding. I've used methanol after freeze substitution and -20 polymerisation or ethanol at 4 degree C for dehydration and polymerisation. Both of these result in pink colored blocks that eventually fade to clear.
Thanks Brent. I used only acetone (Merck). The choose of the solvent is an interesting question itself.
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