PEX compatibility with spray-on urethane chemistry

[DaveBP]

My understanding of oxygen-barrier PEX for hydronic heating water is that there are two broad categories. The type with a thin layer of aluminum embedded in the PEX to block the oxygen transfer to the water inside and the other using a layer of some oxygen impermeable polymer. This barrier layer is on the outside of the tubing, I believe. Gives it a shiny look from the examples I've seen.

Lot of folks doing admirable work insulating their buried lines from outside boilers to buildings. I like the look of all that insulation on lines that are so exposed to heat loss. Much better looking to me than tubing wrapped in thin miracle layers inside ADS drain pipe.

A question that pops into my mind once in a while is whether anyone has done research (maybe even checking with PEX tubing manufacturers) on the compatibility of whatever that external O2 barrier is and the chemistry of the various urethanes while they are curing. Urethanes (and other resins for that matter) are pretty inert after they are cured but are their components so innocuous before they are cured?

If anyone has looked into this can you make a short report for us idly curious souls?

 

[tom in maine]

Not much bonds well to polyethylene. It is a pretty inert material.
That being said, the oxygen migration barrier is not PE.
I have sprayed a lot of foam on PEX (with and without an O2 barrier) and never have seen any deterioration.
I know that I have had this discussion with some manufacturers but can't remember who anymore.
They said it was not an issue, but since I can't tell you who anymore, take it with a grain of salt.

 

[coolidge]

I dont have a report but as a general rule of thumb i try and spray a thin1" coat on first. And so on. There is a great amout of exothermic heat that can be achieved if you put an excess amount on therefore damaging the pex. Hope thats helpful.

 

[Fred61]

Keeping the heat down is a good idea. Urethane is 100% solids so there are no solvents to attack the surface of the pex.

 

[mole]

My understanding of oxygen-barrier PEX for hydronic heating water is that there are two broad categories. The type with a thin layer of aluminum embedded in the PEX to block the oxygen transfer to the water inside and the other using a layer of some oxygen impermeable polymer. This barrier layer is on the outside of the tubing, I believe. Gives it a shiny look from the examples I've seen.

Lot of folks doing admirable work insulating their buried lines from outside boilers to buildings. I like the look of all that insulation on lines that are so exposed to heat loss. Much better looking to me than tubing wrapped in thin miracle layers inside ADS drain pipe.

A question that pops into my mind once in a while is whether anyone has done research (maybe even checking with PEX tubing manufacturers) on the compatibility of whatever that external O2 barrier is and the chemistry of the various urethanes while they are curing. Urethanes (and other resins for that matter) are pretty inert after they are cured but are their components so innocuous before they are cured?

If anyone has looked into this can you make a short report for us idly curious souls?

Dave, Its an interesting question. I haven't looked into it with pex or urethane suppliers, but there is actually a reasonable chance for reaction of the urethane withe the oxygen barrier.

The oxygen barrier is typically EVOH (ethylene vinyl alcohol). EVOH posesses very low gas permeability toward a wide range of gases , including oxygen. The EVOH contains pendant secondary alcohol groups which are reactive with Isocyanates

The part "A" of a two part urethane is usually an MDI (methylenediphenylisocyante) based prepolymer containing a large amount of free, unreacted MDI. This is very reactive stuff. It will react with a wide variety of chemicals and functional groups, including water, amines, alcohols and more. It would not react with polyethylene though.

Part "B" of the two part urethane foam is usually a "polyol". Polyols are a very broad class of polymers, but the types used here are usually what are called "primary alcohol terminated".

When the two parts are combined, the primary alcohols react with the free isocyanate groups. It's a very exothermic reaction, generating a lot of heat. The heat volatilizes a low boiling liquid blowing agent, which gets entrapped as bubbles, causing the mass to to foam as it forms a cured urethane polymer. The primary alcohol groups in part "B", are a lot more reactive than the secondary alcohols of the EVOH. That doesn't mean the secondary alcohols can't react though. The system may be so reactive that the isocyante reacts with whichever alcohol it runs into first. Also, if the ratio of "A" to "B" was high by a little, that would tend to leave some unreacted isocyanate which will have more time to react with secondary alcohols, before reacting with atmospheric moisture.

The big question is DOES IT MATTER? That question is probably best left to the EVOH producers, who are likely the most knowledgeable of how changes to this polymer affect it's gas permeability.

JR

 

[DaveBP]

Mole, your description of the chemistry is exactly what I was concerned about. Except that organic chemistry is where I dropped out of university...

I suppose that any reactions with that EVOH that might happen would have produced some horror stories by now if it were a really bad interaction. Tom has been doing this stuff long enough that he might have seen some puzzling results by now. If the urethane components don't hurt the PE itself, the worst you would end up with is plain old non-barrier PEX.

For all I know, any new compounds formed may be even better oxygen barriers. Still, "closed cell" urethane makes me think less air will get into the system with the tubing surrounded by it than exposed to air.

 

[mole]

Yeah, Exactly - worst case scenario is likely an increase of oxygen permeability offset by the presence of about 2-3" of closed cell foam, which, the oxygen would have to slowly diffuse through before reaching the surfce of the pex. The fact that the lines are buried in the ground reduces oxygen exposure further. I agree that we'd have seen the horror stories by now.

 

[heaterman]

The company I use has been spraying pex basically since there was pex. When I started using them I quizzed 'em pretty carefully about this very subject. They have had no reported problems with O2 infiltration or degradation of the pex itself in over 15 years. I have also inquired of a couple manufacturers (Viega and Rehau) regarding the barrier and have been told there should be no issue at all. That doesn't however speak to any and all brands of pex so everyone should perform due diligence on whatever product or brand they happen to be using. There are definitely variations in the barrier used by different companies.

 

[in hot water]

If in doubt, use a pex tube with an aluminum O2 barrier. I like the Viega Fosta Pex best as they take a full size pex, wrap the aluminum, then add a protective layer over the aluminum. Ends up being the thickest walled pex tube around.

I'd also put the tube inside a PVC pipe, then foam it in the trench. This assures 100% waterproof, and allows the tube to be removes, in the rare, un-likely event you have a meltdown

4" S&D PVC runs a bit over a buck a foot, extremely cheap insurance.

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