Acrylic Absorbs Water?

[bfessler]

I had a response to my frag tank build on reefs.org that has me nervous.

Wetworx101 suggests that acrylic sheets siliconed to a glass tank will absorb water and expand and either crack the glass or cause a separation of one of the outside panels.

I have heard of many people making sumps from glass tanks using acrylic for baffles. Has anyone heard of this causing a separation or break in the glass?

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.

Anyone with similar experiences or knowledge of how much acrylic can expand when submerged in water please share your experience.

Thanks,

[Phurst]

LOL, BS pure and simple. Acrylic is hydrophobic and will not absorb water. However, acrylic siliconed to glass simply doesn;t bond as well as glass siliconed to glass, and may let go somewhere down the road.

[Iron E]

LOL, BS pure and simple.

Got a + 1 on that!

[Phurst]

Oh, I should add though, acrylic expands and contracts more than glass with temperature change. You should n't see any significant temperature swings in your tank if you are doing your job But it would be wise to leave some expansion room between the acrylic and glass just to be safe.

[bfessler]

Thanks Phurst,

That's what I thought but when I told the guy that he posted this...

"So why doesn't acrylic absorb water? Because you say so? Want to put some money on that?

Most acrylics, and for that matter, most plastics & rubbers, are porous and expand underwater, many will suck up about 1% of their weight in water in just 24 hours. If you weigh a piece of acrylic before and after soaking it in water for say... a few months, you might be shocked how much heavier it gets. There are some acrylics that have their surfaces 'sealed'... scratch resistant and chemical resistant ones... they are almost impossible to bond with weld-on because of this. But the rest... cast, extruded, etc... they suck up some water. In fact, there have been patents and research devoted to the formulation of acrylics sheets that are 'low moisture absorbing'. FWIW, acrylic is MADE from water.

This is why acrylics aren't approved for use around food by the FDA. PE is used instead because its 'safe'. This special “safe” quality comes from the fact that PE is a CLOSED CELL material. That means it doesn’t absorb any water. And because it doesn’t absorb any water, it floats (unlike acrylic which will quickly sink to the bottom). That’s why polyethylene is also commonly used to make kayaks and other things that float as well as tarps and pool covers.

I suppose you dont believe that CO2 cant pass through regular airline hose either... "

I looked up Acrylic Properties and found that the absorption of water by acrylic after 24 hours is .03% which is pretty small if you ask me but I am no chemist. Anyway it's good to have some confirmation of what I suspected.

I cut the panels about 1/8" short and siliconed them in place. I figured the silicone would give sufficiently to make up for any expansion of the acrylic due to heat. I also thoroughly cleaned the acrylic with MEK as recommended by the supplier and the bond seems sound enough. The tank has been up about 6 weeks and I haven't seen any problems.

[Iron E]

I would think that with the water in constant contact with the glass/plastic, temp variations would have to be EXTREME to have that affect.

But leaving a little room as a general practice certainly couldn't hurt.

[Iron E]

I looked up Acrylic Properties and found that the absorption of water by acrylic after 24 hours is .03% which is pretty small if you ask me but I am no chemist. Anyway it's good to have some confirmation of what I suspected. I cut the panels about 1/8" short and siliconed them in place.

By those figures, a 12" piece of acrylic could expand a bit less than 1/32" or 9mm. If you left 1/8" you will have more than enough room. I still think that , even if acrylic did absorb water, it would not have the force to push glass apart.

[SaltyDawg]

9mm is like 5/16"

[Iron E]

D'oh!

Somebody slap me next time I open my mouth! I'm outta' here! I bow to the greater wisdom!

How embarraskin'!

Tom

JFTR 12"x .03 = .36" = 9/25" = 9.114mm

[saxman]

salty beat me to it!

FWIW, i've used acrylic baffles with glass tanx for YEARS with no problems. i do think that lots of peeps inadvertantly stress the glass tank by making the baffles just a BIT too snug, and that's where most problems lie.

[Phurst]

I agree with Greg.

Even if it does absorb a MINUSCULE amount of water, that's not going to blow your tank.

[bfessler]

This is the link to the Acralite Technical Support Site and a chart for weight and dimensional change of acrylic submerged in water.

http://cyro.custhelp.com/cgi-bin/cyr...i=&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".

[V]

very interesting subject guys, quite a hard one when screwing down the nuts & bolt of it.

I looked up Acrylic Properties and found that the absorption of water by acrylic after 24 hours is .03% which is pretty small if you ask me but I am no chemist. Anyway it's good to have some confirmation of what I suspected.

that's a manufacturers guideline based on avg temp of 23deg c & only after 24hrs Problem is, there's only so much space between atoms in a sheet of acrylic (minus protective coating of course). I haven't found anyone whom can produce that constant absorption value yet.


torsional flex can be thrown into the cauldron of a pressure equation vs manufacture specifications. However we're really not talking bout that are we.

That means it doesn’t absorb any water. And because it doesn’t absorb any water, it floats (unlike acrylic which will quickly sink to the bottom).

Leaving its water absorbing ability aside for a min, I believe the above statement isn't entirely correct in telling the whole story to begin with. Yes the added mass of water is a factor, however Its got to start with the original value of density. Density of the water, density of the product.

"simple rule of thumb" water for the sake augment, weights 1gram per cubic per centimeter @ 4deg C (Density=mass/ volume) This is the avg value give or take as temp & atmosphere can effect this value as well.

so lets say avg water is 1.00g/cm3

If acrylic is 1.18g/cm3, then its not rocket science (hey even i can work that out), it heavier & will sink.


i'll anyone else can shed some light on this, you have my interest

[V]

bah, snail mail post of mine , its windy as here so my wireless internet is on the blink. I'll check your link bud

[Iron E]

I could be wrong, (Hell, I've done it before!) but I just can't see how, even if the plastic had the capacity to absorb 50% water by weight or volume, it would be subjected to the pressure required to expand the plastic enough to break glass.

If you fit it too tight to begin with or subject it to some freak hot/cold incident, I can see that breaking the glass.

Tom

[V]

i think you summed it up ironman, real smug fitments, or major temp fluctuations