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Cracks in Plexi

  • 1.  Cracks in Plexi

    Posted 08-29-2022 16:13
    I am curious to know if anyone has any thoughts about mysterious cracks that are starting to form on our plexi vitrines.  Basically, over the past few months we have been noticing a significant amount of our vitrines starting to form small cracks where 2 sheets of plexi connect.  (It is possible this has been going on for a long time, but for some reason it is just becoming more obvious to us now).  

    Here is what we know:

    1)  The cracking pattern resembles icicles hanging from a roofline- very small (most about 1/8"-1/4" long, all spaced out about 1/8" from one another)

    2)  Typically, it begins as a small isolated cluster of cracks somewhere, which then eventually spreads out to a wider area along the plexi connection seam. 

    3)  They seem to "just start appearing"- they don't seem to need a strong physical force hitting them to start the process.  

    4)  We use one mixture of deionized water and isopropyl alcohol for all glass cleaning in the museum.  It used to be 50/50, but we bumped it up to 75% isopropyl after reopening post-covid because we heard a 70% alcohol level is needed to kill Covid.

    Based on those 4 points, does anyone have any thoughts or suggestions?  I would appreciate hearing any thoughts you might have.

    Thanks!
    -Todd


  • 2.  RE: Cracks in Plexi

    Posted 08-31-2022 08:39
    Hi all,

    It is almost certainly the alcohol that is causing the cracking.  The reason that it forms at the edges first is that those areas of the acrylic are heat polished or laser cut, which makes the material more vulnerable to the alcohol.  If you would like to learn more about how damage to acrylic can occur when exposed to alcohol, then you might want to check out these videos.
    Travis





  • 3.  RE: Cracks in Plexi

    Posted 08-31-2022 08:39
    Following, as we have the same issues with some of our vitrines.  I thought possibly it was just age, but interested in following the responses.

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    Vicki Starnes
    Director
    Jefferson GA
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  • 4.  RE: Cracks in Plexi

    Posted 09-01-2022 13:59
    After reading these responses, I'm now feeling some real concern about the plexiglass in our building. When I first started working at our museum, I was told to always use alcohol to clean the plexiglass because the ammonia-based cleaners discolor it. During the pandemic, we went from a 30% concentration to 75% to improve its sanitizing properties. This was especially important in our model railroad room, where plexiglass surrounding our large model railroad has to be cleaned daily because of handprints.

    It hasn't cracked yet, but it sounds like it's a real risk if we keep using alcohol. I would love to know what you're using to clean that is safe, sanitizing, and won't be a danger to the little ones who touch the plexiglass and then put their hands in their mouths.


    ------------------------------
    Diana Staresinic-Deane
    Executive Director
    Old Depot Museum/Franklin County Historical Society
    Ottawa KS
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    Original Message


  • 5.  RE: Cracks in Plexi

    Posted 08-31-2022 08:39
    What you are seeing is a well known phenomenon of Plexi called stress crazing. If it continues, it can leads to cracks. Stress can be preexisting in the plastic from manufacture or caused by the joining process either by the solvent used in the adhesive or by heat generated from the adhesive curing. Mechanical action like sanding the edges of the Plexi can also cause stress to build up in the plastic. Crazing is the result of the uneven release of the stress. Crazing can happen immediately from the processes listed above or at a later time usually from a solvent. Most solvents including isopropanol (and all alcohols) will cause stress crazing in Plexi. As Plexi ages, it also becomes more susceptible to damage from solvents. I am so curious to know if others have seen this damage to Plexi cases due to Covid cleaning protocols.

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    Susan Costello
    Conservator of Objects and Sculpture
    Harvard Art Museums
    Cambridge, MA
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  • 6.  RE: Cracks in Plexi

    Posted 08-31-2022 08:39
    Tod,

    What you are likely witnessing is "crazing".  Most frequently the result of the previous heat-treating. At the edges where they flame polish the acrylic to make the edges nice and smooth, localized stresses are introduced into the acrylic and since the treatment is not followed by annealing, the stresses remain and present as cracks.

    Not much can be done (economically) to remedy the problem once it occurs.

    Regards,

    George






  • 7.  RE: Cracks in Plexi

    Posted 08-31-2022 08:39
    Hi Todd,

    It's my understanding that alcohol will damage plexiglass and will cause crazing.  We've always just used Brillianize or some other acrylic cleaner to clean our cases and vitrines.  I know there was a lot of discussions on the list-servs during Covid as to how to sanitize plexiglass.  Someone can correct me if I'm wrong, but I think it was recommended to clean plexiglass with soap and water and not alcohol to sanitize.


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    Julie Blood
    Collections and Exhibits Manager
    Lodi CA
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  • 8.  RE: Cracks in Plexi

    Posted 08-31-2022 08:39
    Here are some pictures to give a better idea of what is happening.  When viewed at a the right angle, you can see a "chatter type" pattern that runs along the plexi joint.  I am not sure if what ever caused that pattern in the manufacturing process is connected to the cracking that starts eventually.

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    Todd Holmberg
    Collections Maintenance Technician
    Minneapolis Institute of Art
    Minneapolis MN
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  • 9.  RE: Cracks in Plexi

    Posted 09-01-2022 09:11

    Thank you so much for these responses- they are very informative and helpful.

    I am curious to know if it is typical for museums to have just one master glass cleaner solution (and if so, what is that), or is it common for people to keep multiple solutions for different types of glass and "glass-like materials". 

    I am more focused right now on finding a solution for just simply "cleaning the glass" vs solutions that are Covid sanitizers as well.   Ideally, it would be something we could mix in-house.  We have access to deionized water.

    Thanks again everyone for your feedback.



    ------------------------------
    Todd Holmberg
    Collections Maintenance Technician
    Minneapolis Institute of Art
    Minneapolis MN
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    Original Message


  • 10.  RE: Cracks in Plexi

    Posted 09-01-2022 14:00
    When Covid protocols had us disinfecting surfaces I contacted Arkema Inc., the registered maker of Plexiglas, for recommendations. For general cleaning they recommended just soap and water:
    "Wash Plexiglas® sheet with a mild soap or detergent and a lukewarm water solution. Use a clean soft cloth or sponge and as much solution as possible. Rinse well. Dry by blotting with a damp cloth or chamois."

    They could not comment on specific procedures to disinfect surfaces but did provide two information sheets that are helpful. One has general cleaning recommendations for different contaminants and the other gives a relative rating of various chemicals on Plexiglas plus some common commercial products that can and can not be used under normal conditions (see attached).

    If your institution wants to continue disinfecting surfaces (some do) I was told that as long as your plexi is not stressed (e.g. curved) and is under ambient conditions, ammonia and chlorine based products are fine for relatively short durations like would occur during cleaning/disinfecting, but alcohol based products should be avoided.  

    Cheers!

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    Rhonda Brewer
    Museum Curator
    Pecos National Historical Park
    Pecos, New Mexico
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    Original Message


  • 11.  RE: Cracks in Plexi

    Posted 09-05-2022 10:16

    If the concern is also about viruses, though, surfactants (soaps and detergents) do a very good job of disabling and removing viruses (and some bacteria as well).

    https://healthmatters.nyp.org/how-does-handwashing-with-soap-kill-the-coronavirus/
    https://www.qub.ac.uk/coronavirus/analysis-commentary/how-soap-kills-covid-19-virus/



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    Alyssa Opishinski
    Graduate Student
    University of Rhode Island
    Kingston RI
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    Original Message


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