My pva experiment

[Hotwire]

Before i begin i would like to make it clear i am not out to discredit any previous post or the poster, im just one of those people that if it says wet paint i have to touch to see

I took a small piece of plasterboard and spread about 3mm thick finishing/board plaster on it and scrapped a line to divide it into two and let it dry. I diluted some pva in a can at about 5 to 1, not an accurate measurement just rough. I then applied the pva to one side and let it dry till next day, i couldnt see any sort of a film on the surface nor when i rubbed it with my finger could i feel anything coming off. I dabbed a bit of water on both sides and on the pva side the water mainy sat on the surface. So the pva had formed some kind of barrier although unseen, i rubbed again with my finger to feel for any evidence of the pva reactivating but there was none. Now im no expert at tiling or plastering and i could be not seeing the obvious but i thought i would post my findings complete with a picture for others to scrutinise. Here is a picture, pva is on left.

 

[Symo]

Is the addition of a pubic hair for a strength test to be carried out at a later date?



Symo

 

[Nestor_Kelebay]

Let me also say that by my responding to the results of this experiment, people are going to think that this experiment either confirms or refutes something I said in a previous post. I'm not aware that it does or doesn't. I just want to explain to everyone how that experiment can be taken a few steps further to better investigate the suitability of using this PVA product as a tiling primer.

As for the results of the experiment as seen in the photo, the darker finishing plaster on the left hand side shows that more water was absorbed into that side (see my post re wet blue jeans). But, that could simply be that this side was exposed to more water. I can't read any further conclusions out of the photo.

But, what was the purpose in doing the experiment?

To see if PVA will prevent water from being absorbed into the plaster? Well if you just diluted it 1:5 with water, it was probably all wicked into the porous plaster, and there wasn't enough of it to form a film on top.

To see if PVA would re-emulsify if it got wet? Not sure you can tell from that experiment what's happening inside the plaster.

What I'd do now is keep applying that dilute solution of PVA to the same side as before, and letting it dry out after every application of diluted PVA.

You will find that the degree of darkening of the plaster with each applicaton of dilute PVA becomes less. Once you've applied enough PVA that it's plugged up the porosity of the plaster, then there will be no darkening of the plaster with the next application of dilute PVA because no water is absorbed into the plaster (cuz of the PVA film on top).

Then, if you want to find out whether that PVA film is waterproof, allow it to dry completely and put a drop of water on it. If it's not waterproof, then as the PVA resins re-emulsify, plastic/water interfaces are created which will scatter light and the wet spot on the film should turn white and opaque. If the PVA does not re-emulsify, then the PVA under the drop of water will remain clear and colorless.

What we have now is simply two plaster surfaces, one of which has absorbed some PVA and the other which hasn't. Both are still porous and so both will still turn dark if they absorb water.

 

[Scrit]

One thing you didn't mention was what type of PVA it was. Some PVAs are cross-linked, so that when they dry an irreversible chemical reaction occurs which makes them waterproof - other PVAs will redissolve with ease.

So inconclusive from that point of view as well

Scrit

 

[Nestor_Kelebay]

Scrit: I agree completely. There are different kinds of PVA resins just as there are different kinds of acrylic resins, just as there are different kinds of alkyd resins.

In this web site from the Government of Canada's National Research Council which funds the Institute for Research in Construction:

http://irc.nrc-cnrc.gc.ca/pubs/ctus/11_e.html

you can find the following paragraph:

"Polyvinyl acetate latex (PVA)
Two main types of PVAs are used in repair: non-re-emulsifiable and emulsifiable. Non-re-emulsifiable PVA forms a film that offers good water resistance, ultraviolet stability, and aging characteristics. Because of its compatibility with cement, it is widely used as a bonding agent and as a binder for cementitious water-based paints and waterproofing coatings. Emulsifiable PVA produces a film that can be softened and re-tackified with water. This type of latex permits the application of a film to a surface long before the subsequent application of a water-based overlay. Its use is limited to specific applications where the possible infiltration of moisture to the bond line is precluded. It is most widely used as a bonding agent for plaster, and to bond finishor base-coat gypsum, or Portland cement plaster, to interior surfaces of cured castin-place concrete."

The only thing is that when I explain that some PVA resins can crosslink so that they will no longer re-emulsify in water when they get wet, I get comments saying that this web site used to provide good sound advice to DIY'ers, but now even experienced professionals can't understand the answers posted here. Not exactly a step forward.

 

[Scrit]

Nestor

I see your point - but if you are usimng a material you should at least have some idea about what you are using. Otherwise you'll end up putting diesel into a petrol car because after all, they're both carbohydron fules, aren't they?

Scrit

 

[Nestor_Kelebay]

Sure.

But if white wood glue is made from PVA, and white wood glue re-emulsifies if it gets wet, then it's easy to understand why people would presume anything that's made out of PVA will behave the same way.

Here's the web site of a company that makes a concrete bonding agent out of PVA:

http://www.larsenproducts.com/weldcrete-uses.htm

and, obviously, it wouldn't make for a good concrete bonding agent if it re-emulsifies if it gets wet, since lots of concrete earns a living working in wet environments (like underground).

 

[empip]

Perhaps a little more 'hydraulic pressure' would amplify any porosity.

Build plasticine circular dams and fill with water on the test surfaces.

Just an aside.
We conducted such an experiment with granite engineering 'measuring surface plates' (precision measuring tables) the old granite sure absorbed the water...
(Made no effect on the 'flatness' or surface finish as checked for calibration purposes)
-

 

[Hotwire]

My main purpose in doing this experiment was to see if i could see a film formed on the plaster after using pva and by diluting the pva at 5/1 would it still stop some of the absorbtion. Now on that second point i dont know if the reason for using the pva is to just slow down the water absorbtion or to stop it completely. The pva i used was

Cromar

Admix PVA bond

Multi use admixture and adhesive used as a bonding agent for concrete, mortar and plaster. Also used as a sealant on tiling or as a wood glue.

 

[Nestor_Kelebay]

Hotwire:

I don't believe you're going to get much of a film formed with only one application of PVA diluted 5:1 with water. Substantially all of that solution is going to be absorbed into the plaster, and there won't be enough remaining on the surface to form a film. You're going to need additional coats before you get a film forming over the plaster.

Hold your piece of plaster up toward a light so that the light reflects off the surface of the plaster at a shallow angle before reaching your eye. If both sides show diffuse reflection (meaning the incident light is scattered in all directions), then the surface is still rough and no film has formed. Once a film starts to form over the surface of the plaster, you will see that the reflection becomes "specular", which means that the reflecting surface doesn't scatter the light, and that in turn, requires the surface to be smooth.

 

[masona]

Another test would be to use pva with the ratio of water on the back of the ceramic tile once dried and dip it in a half bowl of water and see what happen.

 

[Hotwire]

I think what i should have done was instead of making this a seperate post maybe i should just have made it a reply to my previous post

//www.diynot.com/forums/viewtopic.php?t=43963

I mixed the pva at 5/1 after speaking to a tech guy at unibond, if you read the post i have given a link to above everything maybe a bit clearer.