Pva Versus Primers | Always Use A Primer When Tiling, And Not Pva

[DHTiling]

Re: P.v.a. Versus Primers......

Tilers Primers stop a reaction with gypsum and cement based adhesives.( Ettringnite )..PVA won't do that.....stick with tile primers...can't see whats hard to understand here.....

 

[grumpygrouter]

Re: P.v.a. Versus Primers......

Sounds to me like we need to recruit a Chartered Chemist if we start getting techinical posts like this more regularly!

 

[pt44]

Re: P.v.a. Versus Primers......

Tilers Primers stop a reaction with gypsum and cement based adhesives.( Ettringnite )..PVA won't do that.....stick with tile primers...can't see whats hard to understand here.....

? Well - sorry, but I find it hard to understand!

I thought the discussion here was based on the fact that PVA when it dries does not turn into a stable product. i.e. when you re-add water to it, it becomes a liquid again - and is therefore not stable. (it turns milky white and returns to its previous state). I thought the whole reason for using SBR and Primers was that they work with a chemical action, turning into a different substance on drying (like plaster and cement do), therefore once dry, they are stable and cannot be reactivated by adding water. They are therefore a better choice for "priming" a surface.

I'm not sure how "Ettringnite" comes into it - as you say for gypsim and cement based adhesives. As for example, what if I need to prime some WBP (waterproof ply)? There is no gypsum involved - so I merely need to prime the timber. Therefore - I repeat my previous question - as to what is Waterproof PVA doing exactly? Surely it changes its state and becomes stable after it dries. If it doesn't - and its still able to change its state when water is added - then its claim at being waterproof is surely not valid. Hence why I ask the question. (in fact it was my client that asked - and I am seeking the answer).

I'm not knocking primers - I'm sure they are great - but would simply like to know the answer to the question I asked - so that I can correctly answer my client with an informed answer.

Paul

 

[DHTiling]

Re: P.v.a. Versus Primers......

Paul have a read of this post by sWe...it might also help you further.............and just because a product says it is waterproof doesn't mean it is.....a common phrase used by some diy products...

Re: sWe's guides
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Cement, Gypsum, Anhydrite, Ettringite Crystals, and PVA; A Discourse


Introduction

There have been many debates on these subjects, and my intention is to, in a reasonably easily understood manner, explain the "why" of it all. I'm not an expert on this, but I have read up on it enough to form an opinion, and I wish to pass on my findings.

This post is based on a pair I did very recently, in a thread pertaining to these matters.


A Brief Summary

If you do not want to read the whole post, here follows a summary of what I'm going to explain.

• PVA is not suitable in any tiling related application.
• Cementious materials and anydrite (or gypsum) materials are not compatible, and must be completely separated by, for example, a primer.

There. Now onto the main part of this discourse.


PVA and Cementious Materials

I hadn't even heard of using PVA for anything tiling related before I came on here. Thus, I read up on it, and here follows my findings:

PVA stands for polyvinyl acetate, and it is a rubbery synthetic polymer. It is commonly emulsified in water and used as glue. Many know it simply as "wood glue", or "carpenter's glue".

Cementious materials, such as many tile adhesives and grouts, or other materials which contain cement, such as concrete, are alkaline. Simplified, that means they have a high pH.

Alkali slowly attacks polyvinyl acetate, forming acetic acid, which has a low pH. Cement doesn't dry per se; it cures through hydration, which means it binds the water you mix it with chemically. This causes the pH of the substance to rise dramatically. Introducing an acid negates that process to some extent, preventing the cement or conrete from binding all the water it needs to harden properly.

It is hydrolysis which gives cement and concrete products strength, and holds them together. Without this process, it would merely be the powder you started with.

The acetic acid which is formed when cement and PVA comes into contact, either through mixing them, or "priming" with PVA, will continually free the water bound in the cement, and that will weaken the bond and/or integrity of the material. The effect is accelerated if the material is subjected to moisture, which is more or less always the case.

PVA isn't water resistant. It becomes slightly live when exposed to moisture, and this in combination with the exposure to alkali, accelerates the forming of acetic acid. PVA which is marketed as "water resistant" or "exterior grade", has additives which makes them water resistant, but they're not alkali-resistant.


Anhydrite, gypsum, and cement

Anhydrite products are mainly composed of calcium sulfate, and gypsum products are mainly composed of calcium sulfate dihydrate. When anhydrite is exposed to water, it forms gypsum. In other words, it hydrates. Essentially, it cures, but not to the same extent as cement.

Gypsum always has a proportion on anhydrite crystals left in it.

Cement has a proportion of calcium aluminate. Calcium aluminate reacts with calcium sulfate, which is the main component of anhydrite products, and which is present in gypsum. The reaction forms hexacalcium aluminate trisulfate hydration; in other words, ettringite crystals. These expand, and force away anything which is fixed onto where they form.

As I've previously explained, cement cures, which means it binds water through hydrolysis. That means water is always present in cement. If anhydrite is put into direct contact with cement, there will be a reaction. The reaction won't be as severe with gypsum, as it's already hydrated most of the anhydrite (the dihydrate part), but there is still some present.

Thus, if you want to tile onto such products, you will need to separate them entirely. This is best done with a products which seals, and which is also water resistant, such as acryllic dispersions.
Even if you use water resistant "PVA", the separation will deteriorate with time, due to the chemical reaction between the cement, which is alkaline, and the polyvinyl acetate. If the bond of the cement onto the substrate hasn't already been compromised because of that, the formation of ettringite crystals will very likely cause complete debonding.

Rapidly curing cements may have some gypsum added when manufactured. It accelerates the curing, but does not affect the integrity of the product, because it's present in such small quantites, and during the early stages of curing.


Final Notes

PVA is not suitable as a primer, sealer, impregnator, or admix. The uses of PVA may be many, but they do not include anything tiling related. Use proper manufacturer approved primers and additives instead. Using PVA will likely cause liability issues when problems arise, and that is bound to cost alot more than buying proper materials to begin with.

 

[graham31]

Re: P.v.a. Versus Primers......

It's like when they say that something is water resistant doesn't mean it's water proof

beside water proof PVA will still only sit on the top of the substrate meaning the tiles are still only bonding to a thin layer of PVA.Primers impregnate the substrate meaning the tiles are bonding onto the substrate.

I hope i have this right

 

[pt44]

Re: P.v.a. Versus Primers......

Well, as I need to tile onto some Waterproof Ply this week, I called Weber/Howtex to see what there advice was for a primer. They informed me that unless the surface is dusty, then I don't need to use anything, as the Adhesive I am using - Weber Thick Bed - has enough polymer in it to do the job. Only suggested priming it if it were dusty due to building works etc. with an Acrylic Primer PR360 from their range.

Interesting stuff. Very informative forum as usual. This has changed my construction methods to my benefit, as using WBP is a far nicer material to work with when boxing in toilets/sinks etc. and turning them into cupboards. Their edges are far easier to finish that wedi board or hardibacker and they are easier to screw down near to the edge, without the edges breaking away.

Paul

 

[pt44]

Re: P.v.a. Versus Primers......

It's like when they say that something is water resistant doesn't mean it's water proof

beside water proof PVA will still only sit on the top of the substrate meaning the tiles are still only bonding to a thin layer of PVA.Primers impregnate the substrate meaning the tiles are bonding onto the substrate.

I hope i have this right

If you do - then you have given the best explanation so far. Its waterproof in itself, but it is acting as a barrier which is possibly not adhering to either surface very well.

I'm still a little confused as to why its used so extensively in the building trade though? Perhaps because of ignorance on the subject and costs. But from what I can understand of this thread, PVA shouldn't even be suitable for rendering or any kind of building work where the substrate might get wet later on.

Another thought just occured to me. PVA when you use it as a glue - its sticks to the substrate so well that it is the wood that gives way when you try and separate the two pieces. If it has not penetrated the wood/substrate at all, then that should not happen, the bond should break first. But it does not. Contradicts the theory of how much the PVA sinks into the surface. Not trying to cause a fight - just wondering how that is?

Paul

 

[grumpygrouter]

Re: P.v.a. Versus Primers......

Well, as I need to tile onto some Waterproof Ply this week, I called Weber/Howtex to see what there advice was for a primer. They informed me that unless the surface is dusty, then I don't need to use anything, as the Adhesive I am using - Weber Thick Bed - has enough polymer in it to do the job. Only suggested priming it if it were dusty due to building works etc. with an Acrylic Primer PR360 from their range.

Interesting stuff. Very informative forum as usual. This has changed my construction methods to my benefit, as using WBP is a far nicer material to work with when boxing in toilets/sinks etc. and turning them into cupboards. Their edges are far easier to finish that wedi board or hardibacker and they are easier to screw down near to the edge, without the edges breaking away.

Paul

Weber Tiling Solutions manual advise sealing the back and edges of of the ply with Weber AD250.

 

[AllurePTS]

Re: P.v.a. Versus Primers......

Hi guys .....what would you suggest doing with a ply substrate that the builders have already coated in PVA ???

Regards, Keith :mad2:

 

[White Room]

Re: P.v.a. Versus Primers......

I take it your going to tile onto the ply, Is that floor, wall, shower area a few more details would helpful
:huh2:

 

[AllurePTS]

Re: P.v.a. Versus Primers......

Hi Prestige ..it's a wall ....well, actually a full boxout for a wall hung toilet .....thinking about it now, I suspect there will be pva on the plaster walls too :mad2:

 

[White Room]

Re: P.v.a. Versus Primers......

What sort of tiles are you putting on to the walls

 

[AllurePTS]

Re: P.v.a. Versus Primers......

White ceramic, about A4 size (you can tell I'm a newbie huh !)

 

[White Room]

Re: P.v.a. Versus Primers......

I would rough it up with some course decoraters paper, Don't know what addy your using, Powdered would be better though

 

[AllurePTS]

Re: P.v.a. Versus Primers......

It's actually a job for a friend ... I'm sure its ready mixed he bought ! .....I'll rub it and scratch it up with a blade first then ...cheers