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bfessler 
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 Topic: Acrylic Absorbs Water?
Posted: September 22 2009 at 8:59am |
Has anyone ever heard of acrylic absorbing water and expanding to the point that it breaks or separates the glass panels of a sump? I have seen several posts about using acrylic to make the baffles in glass sumps but never heard about a danger of seam separation or cracking the glass panels of the tank being used.
On reefs.org I received this comment regarding my frag tank's filter system in which I used acrylic to create the filter section.
 by wetworx101 » Mon Sep 21, 2009 9:41 pm
Beware, that acrylic will absorb up to 60% of its weight in water from being in the tank like that. As it does over months, it will expand, and if it doesnt have anywhere to expand (like if its siliconed between two opposite glass panels), it will either crack the glass, or it will actually push the glass panels apart with enough force to sheer the silicone.
I have built a few tanks, around the same size as yours, with internal acrylic baffles like that (the whole 'enclosed' concept just like yours). Every single one of them blew out... either by cracking or the silicone being pushed apart. I even tried leaving a spacer, more 'expansion space' etc... still ended up with a leaky tank every time. One time, I even pre-soaked it for 3 months so that it wouldnt need to expand any more... right? Oops... when I took down the tank once and let it dry out, a few months in dry storage resulted in the acrylic shrinking and pulling on the side panel so hard it cracked.
When you put acrylic baffles/walls in a glass tank, you need to leave some sort of expansion space. You can silicone the acrylic in at an angle to the glass. You can have a bow or angles in the acrylic, etc. This way, as the acrylic expands/contracts... it can do so without applying force on the glass.
VP of the Wisconsin Reef Society
Anyone ever heard of this or had a panel break using acrylic baffles?
Edited by bfessler - September 22 2009 at 9:03am
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Burt
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snoyce
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Posted: September 22 2009 at 9:17am |
I have heard this, but I think the 60% figure is a little extreme.
I put 1/4 inch thick acrylic panels in my 20 long sump and left about an 1/8" gap and siliconed them in. its been 2 years and I have not had a problem. It does not seem to have expanded enough to notice any bow in the glass tank. I even had the sump dried out for a few weeks when switching tanks and everything seemed fine.
I think acrylic probably does expand a very minumally in water over time, but that if you did not really wedge the baffle in tight, and left a small gap it will not harm the tank, at least that has been my experiance.
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Scott Noyce
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TriggerHappy
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Posted: September 22 2009 at 9:47am |
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Great post...I've never even thought of that! I also wonder if cast is different from extruded...maybe he was using extruded?
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MadReefer
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Posted: September 22 2009 at 11:08am |
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For some reason I thought it was because of heat. I've always left a little space because of the possibility.
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snoyce
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Posted: September 22 2009 at 12:14pm |
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hmm maybe extruded expands more, I know I used Cast, Cryo Industries Acrylilite GP Cell Cast sheet, from regional supply
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Scott Noyce
90G reef ready AGA display
basment 20G sump, 29G fuge
4x54 watt T5 retro SLR
Octopus NW150
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bfessler
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Posted: September 22 2009 at 12:40pm |
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This is the same product I use. I couldn't find any information on the amount of water it absorbs after 3 months but at 24 hours its only .03%. I haven't been able to find any indication of dimensional instability either. It seems like if this was a serious problem there would be more information available and more people having tank failure using acrylic.
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Burt
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Posted: September 22 2009 at 1:40pm |
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Acrylic does absorb water, I have seen many supme that have been made of glass with acrylic dividers crack the glass due to this, I can't say how much the acrylic is absorbing but i know it does, This is also a problem when you have a acrylic sump or tank that is left out in the sun after having water in it the acrylic shrinks to fast and will weaken the acrylic.
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thefu
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Posted: September 22 2009 at 1:52pm |
Wow, this is a great piece of info to know!
Interesting that this must also mean that in acrylic tanks that the acrylic must all absorb fairly equally or there would be a failure.
Again, who would think something that seems as solid as acrylic could actually absorb water.
Is there a difference for cast or acrylic? How about different thicknesses? Willthe water only penetrate so far? And, you would think if it were porous enough to absorb the water, that water would eventually find its way through to the other side.
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bfessler
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Posted: September 23 2009 at 12:35am |
OK. I've been trying to find some accurate data regarding the adsorbtion of water by acrylic and its dimensional change. Here is a link to the technical support site of Acralite for a chart with the manufacturers testing on the subject. http://cyro.custhelp.com/cgi-bin/cyro.cfg/php/enduser/std_adp.php?p_faqid=126&p_created=978729897&p_sid=-_J1fDIj&p_accessibility=0&p_redirect=&p_lva=&p_sp=cF9zcmNoPTEmcF9zb3J0X2J5PWRmbHQ6MSZwX2dyaWRzb3J0PSZwX3Jvd19jbnQ9NTEsNTEmcF9wcm9kcz00JnBfY2F0cz0wJnBfcHY9MS40JnBfY3Y9JnBfc2VhcmNoX3R5cGU9YW5zd2Vycy5zZWFyY2hfbmwmcF9wYWdlPTEmcF9zZWFyY2hfdGV4dD13YXRlciBhZHNvcmJ0aW9u&p_li=&p_topview=1
In the 24 Hour submersion test the weight gain was .2% soluble mater lost 0% water adsorbed .2% and dimensional change .2%. The chart also gives data as to weight gain at 7, 14,21,35 and 48 days but unfortunately it did not give the other factors. At 48 days the weight change was 1.1% and if the correlation between weight gain and dimensional change are linear this would translate to a dimensional change of 1.1% as well but I cannot verify this by the provided chart but if this is true than the dimensional change in 10" would be about 1/10". Any chemists out there that can shed some light on this?
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Burt
An equal opportunity reefer,
I support all hobbyists and organizations involved in Marine Aquarium Keeping.
[email protected]
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