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I have seen videos and discussions about alcohol cracking acrylic, and waterblock manufacturers reminding users to not use alcohol in cooling systems with plexiglass acrylic, but why is this so?

The question proposed as duplicate only asked for the effect, but here I already know the effect. I am asking for the reason for the effect, which is not answered in the answer to the 'duplicate' question.

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  • $\begingroup$ possible duplicate of Ethyl alcohols use on polycarbonate and acrylics $\endgroup$
    – Mithoron
    Apr 9, 2015 at 19:03
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    $\begingroup$ I don't think this is a duplicate. The other question didn't ask about mechanism. There was no "why" component in the earlier question but there is here. $\endgroup$
    – Curt F.
    Apr 9, 2015 at 20:46
  • $\begingroup$ Thanks Mithoron for finding a related question. Thanks Curt for helping me to explain. Yup I am asking for why it happens, not what will happen when alcohol is applied to plexi. $\endgroup$ Apr 10, 2015 at 10:55

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If a solvent causes an amorphous polymer to craze, this is because the polymer is principally soluble in it. It diffuses into the surface, but without instantly dissolving it, as the polymer chains are still stuck in the glassy network of the substrate. The polymer swells (like a fruit gum in water), but the volume increase can only happen perpendicular to the surface.

The polymer chains thereby end up oriented towards the surface, weakening the structure. When the solvent dries off again, the chains adhere to each other again, but cannot reentangle in time. The surface has become thicker now, but as it is still the same amount of polymer, the additional volume is made up by the cracks and crazes appearing in it.

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    $\begingroup$ I believe the name for this phenomenon is environmental stress cracking. We blew more than one acrylic apparatus (thankfully not terribly expensive!) by cleaning with ethanol before we realized what was going on. $\endgroup$
    – hBy2Py
    Dec 18, 2015 at 12:17
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According to chemical resistance of plexiglass acrylic alcohol is not as aggressive as you mention with plexiglass acrylic. But, we have to signal that acrylics is a big family of polymers (thermoplastics or thermosets) derived from acrylic acid, methacrylic acid or other related compounds. Plexiglass is only a member of this family.

Any way, in response to your question why alcohol might cause severe crazing and cracking? It's simply because alcohol, as a good solvent, has the ability to attack the surface of acrylics and dissolve the polymer chains.

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  • $\begingroup$ Yes, acrylics are a wide family, but the second part of this explanation is so oversimplified it become plainly wrong. $\endgroup$
    – Karl
    Mar 1, 2017 at 7:14
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The given explanation cannot be correct, because I've watched (and listened to) an acrylic glass cracking and splitting and developing leaks after pouring in a splash of bourbon. No drying out is involved. It may be simply swelling of the plastic as ethanol penetrates; this could cause cracks if the bulk of the plastic is too rigid to stretch, and can't accommodate the swelling.

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    $\begingroup$ An experimental observation deserves positive consideration, not a downvote, although this answer could have been worded a little more gently: "Perhaps the above answer is not the whole explanation." I've seen the effect with polystyrene and some solvents, but never with acrylics - I must be more careful. I wonder if the acrylics that fail are substandard in some way. $\endgroup$ Dec 20, 2019 at 14:33
  • $\begingroup$ Definitively makes some sense. Also the solvent would relieve orientation, leading to strong swelling on one direction, and very little or none in the other. $\endgroup$
    – Karl
    Dec 20, 2019 at 17:56
  • $\begingroup$ @JamesGaidis I wouldnt say "substandard". Its just another type, which might be cheaper, or have some other redeeming properties. $\endgroup$
    – Karl
    Dec 20, 2019 at 17:57
  • $\begingroup$ Indeed. I don't think drying has anything to do with the process. I submerged a plexiglass character display in pure IPA and cracks started forming right away. When I took it out, the surface felt like rubber. After a short while it became rock hard again $\endgroup$ Apr 18, 2020 at 12:33
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A factor I haven't seen mentioned is which processes the acrylic part was subjected to during manufacturing. If the acrylic was flame polished during post processing (common in PC water cooling parts to achieve desired aesthetics), it can significantly weaken the bonds near the surface of the material and even cause microscopic crazing to occur.

When alcohol or a similar solvent is then applied, it will rapidly dissolve the weakened bonds and cause crazing, seep into the cracks, and dissolve the polymer within the cracks. The polymer will then swell, move upwards in the channel, and potentially crack the material upon drying. This is the process described in @Karl's answer.

Anecdotally, I have used near-pure isopropyl alcohol to clean the residue off many lasercut acrylic sheets without issue, and have never encountered crazing or cracking over the lifetime of those parts. However, many people report immediate cracking when cleaning milled acrylic water blocks with isopropyl alcohol. It seems likely that flame polishing is largely to blame for the part's poor alcohol resistance.

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