water absorption by acrylic scientific study

[KENthereefnoob]

since i have been building my first custom sump, i have heard of this phenomenon of acrylic material absorbing water and expanding. i have read this many times after ding a few forum searches on the topic, one poster even suggesting that acrylic can absorb and expand 60%. i found that figure to be insane. that means that if you had an acrylic fish tank that was 10 gallons in volume, and it expanded by absorbing water, but your tank didnt crack or explode from this expansion, then after absorption, then you would have a 16 gallon acrylic tank!

maybe some of you may have come across this link before in your own research, but below is a link to an article on a formal study of water absorption by acrylic material.

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2635902/

in this study, 30 20mm x 20mm acrylic squares were submerged in water. water was determined to be absorbed by the samples, but at a rate of 11 to 30 micro grams +/- 0.55. that hardly is enough absorption to make an acrylic pane expand enough to break glass.

i believe that the expansion of acrylic, warping or breaking of any kind within someones sump may be due to other factors such as temperature changes or increases caused by lighting, and the flow of water. if someone used too thin a piece of acrylic, and placed it in a glass tank, and the flow of water in one direction was constant over time, then sure that acrylic piece could bend and break.

or maybe the light used in someones refugium was close enough to a panel, it could warp the shape and even make the acrylic piece expand.

i just dont believe that acrylic could absorb enough water to expand at a rate that could blow glass out. people experiencing this may be having this issue because the acrylic they are using is too thin, cut too wide and already bending the glass tanks they are putting them in, or using light that is heating the acrylic up in their sumps.

 

[Shawn O]

I lay a small sheet of 1/8" on top of my tank to cover the open slot at the back and it warps upward on all edges at least 1.5"

 

[RocketEngineer]

The article you linked to does not mention dimension change due to water absorption which is the cause of the sump baffle issue.

http://www.norvaplastics.com/acrylic-acrylite-ff.asp
"ACRYLITE FF sheet also absorbs water when exposed to high relative humidities resulting in an expansion of the sheet. At relative humidities of 100%, 80% and 60%, the dimensional changes are 0.3%, 0.2% and 0.1 % respectively."

So, think about that for a moment. The vendor of the material claims a 0.3% expansion of the sheet at 100% relative humidity (a.k.a. immersion). That means for a baffle 10" long, that sheet will expand 0.030" Or ~1/32".

Now, lets figure out what kind of force that would cause:

Y=(F*L)/(A*DeltaL) where:
Y=Young's Modulus (400,000psi for acrylic)
F=Force (lbs)
L= Length (baffle length = 10")
A = Area (Assume 10"X 0.25")
DeltaL = Change in length (0.030")

Rework the equation:

F = (Y*A*DeltaL)/L = 3000#s

That's right. 1.5 TONS of force are pushing on the inside surface of that pane of glass due to expansion of the acrylic.......... And its right in the middle of the glass along a very narrow region.

The moral of the story is don't mix materials when the ends are constrained.

 

[sleepydoc]

Thanks for the calcs, Rocket! I posted 6% expansion in another thread, but I'm wondering if the number I saw was actually 0.6% and I forgot the decimal point.

Many people cut the sheet narrower than the tank, but then fill the space between the baffle and the tank with silicone sealant. If you do this, you have filled the 'expansion space' with another solid material and will be transmitting the force from the baffle expansion to the tank wall via the silicone.

The force will be ameliorated somewhat, since silicone is quite a bit squishier than acrylic, and it depends on the thickness of the silicone layer, but it will still be transmitted.

Bottom line is glass is the safest option.

 

[rwb500]

The article you linked to does not mention dimension change due to water absorption which is the cause of the sump baffle issue.

exactly. end of story. I don't know how you thought you could draw any conclusions without this information.

 

[KENthereefnoob]

I never said that acrylic couldnt expand by absorbing water, I should mention that I found this link after a previous post I made in which I was building my first sump. I mentioned that I left 1/8" gap between my acrylic baffles and the glass walls on the tank, and filled it with silicone. I had read many forum posts in which many posters had made claims that acrylic could expand by as much as 60%, I found that to be very unlikely.

http://www.utahreefs.com/forum/forum_posts.asp?TID=35326

I have no doubt that the acrylic panels could expand over time, I was mostly worried by thermal expansion, and not water absorption at first until I heard of water absorption. I found that link, and in it it does not state volumetric expansion, but a hypothesis can be made that a 20 x 20 mm piece of acrylic absorbing just 33 micrograms of water could not expand at the rate many people have claimed.

I find it fully possible for acrylic at such a thin thickness (0.220") pressed against glass could put up that much force, much like pressing the end of a flathead screwdriver could deliver force, there are many much more deceiving materials that can put out that kind of force if made to, what I am saying is that I find it hard to believe that an 11.75" piece of acrylic could expand over a quarter inch and actually make contact with the glass panes of the tank. If 20 x 20mm of acrylic is absorbing just 33 micrograms of water, I don't see how 33 micrograms of water could make it expand as much as 60%.

I'm no rocket engineer, but I am am engineer, and often times people with ask me what happened to some electrical device they own. I often ask them if they checked their fuses. My point to that is that Many times when something goes wrong they are looking for some grand answer, when often times the problem is fairly simple. I think a lot of people who have had the experience of their custom tanks busting go onto the net for answers and read about how acrylic absorbs water and expands and think that's where they went wrong, but it often times is something simpler like the baffles being butted up against the glass to tight making the glass bow, or using panes of acrylic that are too thin, or a really hot light for their plants and live rock. I'm not saying that acrylic doesn't expand, I'm saying that I don't see it expanding at the rates I've read many people posting. With enough room, and control over variables such as temperature and water flow, I don't see why using acrylic baffles in a glass tank would be a bad idea. I'm sure many custom sumps have been built this way to great success if done properly.

Here is a link to my build where this discussion originally took place, I'm not saying I have all of the answers, in fact in my build thread I am asking for people to poke holes in my design so that I can improve upon it. I feel that more heads are better than one in most endeavors, so please don't rush to judgement as if I'm telling everyone what's what. I'm just trying to start discussion and build the best sump I can. I think it is a little bit narrow minded and smug to simply say that it should he end of story on a discussion questioning whether 33 micrograms of water can make a piece of 20mm x 20mm acrylic expand at rates of 60%.

 

[KENthereefnoob]

My sump build thread:

http://reefcentral.com/forums/showthread.php?t=2499218

 

[RocketEngineer]

I have no idea where that huge number is coming from in the link. I only use trusted sources and a single post on the internet does not qualify. :headwally: Also, the article you link has no dimensional influence information. How do you or I know whether that small amount of water absorption has a large or small influence on the physical dimensions of the sample? The answer is you don't because they were never measured pre- and post- submersion. I will instead rely on the material spec sheet which says 0.3% dimensional change. That to me is a much more reliable source of information.

Nobody who understands the numbers is saying the acrylic expands 0.25". What we are saying is that it expands 0.030" which in turn could place a huge amount of stress on the tank. The baffles being butted against the glass wouldn't be a problem if the acrylic didn't expand. But since it does expand in the presence of water, being tight to the glass is a huge problem. If a baffle flexes/bows then yeah, its too thin. A baffle that breaks a glass tank is anything but thin.

As an example of why acrylic baffles in a glass tank are a bad idea, let me show you what happens when they are built incorrectly: Sump Busted.

The point we are all trying to make isn't that its impossible to do successfully, its that installing acrylic baffles into a glass tank is running a risk. It is a risk because folks do make the baffles very tight and because acrylic does expand just slightly when it goes from being in air to being underwater. That expansion is small at 0.3% but over the width of a sump it is enough to introduce huge forces onto the glass.

At this point, please do more research. You link one study and one random post and are using those as your stick to beat home you point. Considering the first is unrelated to the topic at hand and the second is unsubstantiated by any facts, you would do well to find real values from reliable sources. Go look at the material spec sheets from acrylic vendors (aka the link I posted). Look at the information on supplier websites. As an example McMaster-Carr lists water absorption for various plastics and lists 0.2-0.4% for cast acrylic which aligns with the sheet the vendor lists. Only with proper references will your comments be given the serious consideration they deserve but what you've provided here comes up well short of the mark. Sorry. :sad1:

 

[nvladik]

Would also like to highlight that if this was a true case, all our skimmers would be ****ed. They are submerged somewhere between 6" and 9", if that expansion was true once removed for cleaning they would all look very very odd.

 

[KENthereefnoob]

Rocketengineer

I dont think you get what I am trying to say or question here. One random link? I was not going to post every link I read where I saw some guy make a crazy claim. I posted the most extreme claim I came across in an example of why I believe too many people are making the assumption that acrylic expansion is the reason why their sumps bust, when the answer in their failure may be based on poor design, or another simpler variable that wasn't accounted for.

Again, I am a believer in acrylic expanding by the absorption of water, but not by the claims I have read by so many on the internet. You are trying to drive home a point to an argument that is not in contention. Nobody said that your figures are wrong, or your equations don't make sense but that wild claims about acrylic expansion are just one of an infinite amount of topical feeding frenzies on the net.

I believe you are arguing with yourself, maybe us lesser beings than you have a hard time wrapping our heads around why our acrylic tanks and protein skimmers don't bust at the seams after years of use, by the rate of 60, 30, or even 10%. I am just posing questions I had and a link I had found to a study that suggests that acrylic doesn't absorb water at the high rates one would imagine based on such rates claimed on the internet.

I work in the silicon valley, I have met many people that have to let everyone know what they do, or how smart they are, they are brilliant minds, they explain heavy answers to simple questions but are often too wrapped up in their own answers to really reciprocate with others. You can continue to post your answer and having the other guy call game and say its the end of the story.

Again, I'm not saying acrylic wont expand, I am positive glass on glass is the best way to go, I am just saying that I don't think acrylic expands at such high rates claimed on many message boards. I am confident everything you wrote out into equations is dead on, but nobody is questioning that. I think with proper planning, glass on acrylic should work, and I will see for myself if it fails ir not, or whether my acrylic panes will swell 6%, or 10 or 50 or whatever.

Anyway, ill post my experiences with this glass on acrylic sump, hopefully they turn out good. For anyone who would actually like to see it, please post your thoughts, concerns or any potential flaws you see in it if you care to do so, it will really help me in making it a better sump/refugium.

 

[RocketEngineer]

How do you make it better? Easily. Just use the correct material. Glass in a glass tank, acrylic in an acrylic tank. Its not that hard.:hammer:

Lets come at this from a different direction. I've built two glass sumps for my systems. The glass baffles ran about $40 total for four baffles from a local glass shop each time. Looking online, a 24"X 48" sheet of acrylic costs $60. SO, I saved $20 AND made a sump with no risk of failure.

What's the advantage of acrylic baffles in a glass tank again? What did I miss?

 

[smedlin]

RocketEngineer's first post made my head hurt (and I'm a software engineer).

This is actually a good discussion though.

I'm getting ready to do my own sump, and I see a lot of acrylic baffles in glass tanks.

I guess I'll take a gander at glass baffles.

 

[sleepydoc]

Rocketengineer

I dont think you get what I am trying to say or question here....

Again, I am a believer in acrylic expanding by the absorption of water, but not by the claims I have read by so many on the internet. ...

Actually the links you posted all refer to absorption of water by weight, not expansion, two very different phenomena.

Whether you intended to do so or not, your post(s) came across as questioning the validity and accuracy of claims of acrylic expansion and its issues related to glass tank failure. Citing articles on absorption rather than expansion added to that appearance.

As I said in my post on your other thread there are many reports of glass sump failures with acrylic baffles. These are, of course, all anecdotal, and you will never find a study on the issue, but given the fact that acrylic expands in water and the fact that outward pressure on a glass tank can lead to failure, it is not at all unreasonable to conclude that the acrylic baffles may have contributed.

Having built a sump with both acrylic and glass baffles, I can say that acrylic baffles are no better than glass, are more expensive and have potential drawbacks, hence my recommendation to stick with glass.