Hydrophilic or hydrophobic: the argument nobody settles

Why there is an argument at all

Both materials are MDI-based polyurethanes that cure on contact with water. That is where the similarity stops.

In a hydrophobic resin the water is a reactant and nothing more. It triggers the cure, the reaction releases carbon dioxide which blows a closed-cell foam, and the finished polymer contains no water at all. In a hydrophilic resin, built on a polyethylene-oxide-rich polyol backbone, water is both the curing agent and a permanent component of the cured material. The polymer holds water inside its structure the way a sponge does.

That single structural difference produces two entirely opposite behaviours, and each side of the argument is describing a real advantage. Hydrophilic resin actively seeks water out and wets out damp concrete, so it chases the leak path into micro-cracks, capillaries and damp pores that a hydrophobic foam will bridge straight over, and it bonds tenaciously to a wet substrate. Hydrophobic resin does not absorb water, so it does not shrink or swell as the wall wets and dries, and it does not care how many seasons pass.

The reason nobody settles it is that both are true. There is no chemistry that both bonds to permanently wet concrete by holding water and stays dimensionally stable when the water leaves. You are choosing which property matters more on this particular crack, in this particular building, in this particular climate.

What the two words mean, without the chemistry

Hydrophobic means water-fearing. The resin uses water to set but pushes it away rather than absorbing it. Think of it as a plastic plug that happens to be triggered by moisture. It sets into a closed-cell foam, it fills the crack, and after that the water in the wall is nothing to do with it.

Hydrophilic means water-loving. The resin seeks water out and takes it in, and keeps it. Think of it as a sponge that sets. That is what lets it chase a leak into places a foam cannot reach and stick to concrete that is already wet, and it is also what makes it vulnerable when the water disappears.

A useful physical image: leave a hydrophobic plug in a wall that dries out for six months and you come back to the same plug. Leave a hydrophilic plug in the same wall and you come back to something smaller than you left.

The two chemistries side by side

The one question that decides it

Will this crack ever dry out permanently?

Not "is it wet today", because both materials handle a wet crack. The question is what this crack does across a full year, and across the years after that. Everything else is secondary.

Work through it like this.

• If the answer is no, never, this crack is permanently wet, hydrophilic is defensible and often better. A planter box, a tank wall, a lift pit sitting in groundwater, a basement wall below the water table. The bond to permanently damp concrete is worth having, and the dimensional stability you would gain from hydrophobic is stability you will never need.
• If the answer is yes, it dries out every winter and wets up every summer, choose hydrophobic. That describes most of South East Queensland above the water table, and it means you are asking the seal to survive a complete drying cycle every single year for the life of the repair.
• If the answer is yes, permanently, because the building is being dewatered or the drainage is being fixed at the same time, choose hydrophobic. Hydrophilic in a crack that is about to be dried out for good is a repair with a known expiry.
• If the honest answer is that nobody knows, choose hydrophobic. The two mistakes are not symmetrical. Guess wrong with hydrophobic and the seal may not chase into the very finest capillaries. Guess wrong with hydrophilic and you get a seal that shrinks, loses its key, and may not recover even when the water comes back.
• If the crack moves, neither foam is the primary answer. Small seasonal movement suits a flexible hydrophilic grade or an acrylate gel. Real, continuing structural movement is not a waterproofing problem at all until an engineer has established why it is moving.

Desiccation shrinkage: the failure mode that actually matters

This is the reason the question above is the question. Hydrophilic polyurethane holds water inside the cured polymer. If the crack dries out permanently, that water evaporates and the material shrinks. The leak returns, usually at the sealed line, usually at the start of the next wet season, and usually a year or more after everybody has stopped thinking about the job.

It is not a manufacturing defect and it is not bad workmanship. It is the direct cost of the property that makes the material good. The same water affinity that lets it chase a leak into a capillary and grip damp concrete is what makes it dependent on that water staying there.

What makes it worth arguing about is that the failure is delayed and seasonal, so it does not get attributed correctly. A repair that holds through the first wet season, sits dry all winter and weeps again the following February reads to most people as a repair that simply did not last. The pattern is the diagnosis: sealed line, dry winter, wet again next summer, repeat.

The re-swell argument, and what it is worth

The counter-argument from the hydrophilic camp is that shrinkage does not matter because the material re-swells when the water returns, commonly quoted at two to four times. As theory that is sound and the swelling is real: these materials genuinely do take water back up and expand.

The practical problem is geometry, not chemistry. A seal in a crack works because the plug is keyed into the irregular faces of the crack over the full depth of the element. Once the material has shrunk it has pulled away from those faces. When it re-swells it expands into whatever space is now available, which is not the same thing as re-seating itself against the crack faces under the pressure it originally cured against. In field terms, shrunk foam can lose its key and never fully recover the seal it had.

So treat the re-swell figure as a genuine property of the material that is being asked to carry more weight than it can. It is a reason hydrophilic is forgiving of a short dry spell. It is not a reason to put hydrophilic in a crack that goes bone dry for five months a year.

This is also where the honest version of the industry disagreement lands. Neither camp is fabricating anything. One camp emphasises a real bonding advantage, the other emphasises a real stability advantage, and the re-swell claim is where the marketing gets ahead of what a shrunk plug in a real crack actually does.

Where hydrophilic genuinely wins

There are situations where hydrophilic is not a compromise, it is the right call, and refusing to use it out of habit is its own kind of error.

• Planter boxes and podium garden beds. The substrate stays permanently wet, which is exactly the condition the chemistry needs. Note that injection here is usually an interim measure regardless of resin, because the real answer is normally a failed membrane that needs replacing.
• Water tank and pool shell walls, where the structure is wet by design and the water is not going anywhere.
• Basements and lift pits below the water table, where the groundwater level does not seasonally disappear.
• Fine cracks and porous zones that a hydrophobic foam will bridge over rather than penetrate. Hydrophilic chases into capillaries a foam cannot enter.
• Damp weeping rather than gushing. Hydrophilic can be washed out or diluted if it is injected into fast-moving water before it gels, so it is the wrong tool for a running leak but the right one for a slow bead.
• Re-injectable joint hose systems, where the seal is expected to be topped up over the life of the structure.
• Structures with small seasonal movement where you want a seal that re-swells and re-closes rather than one that stays rigid.

Where hydrophobic genuinely wins

And the mirror image. These are the situations where hydrophobic is not just the safe pick, it is the correct one.

• Active, running or gushing leaks under head. A fast accelerator dose gives a gel time down around 20 seconds, which is what stops the resin simply washing away down the crack before it sets.
• High-flow cold joints and construction joints, which are where most of the water in a basement or car park actually comes from.
• Tie-bolt and form-tie holes, which are straight pipes through the wall and need a plug that will not change dimension.
• Any crack that dries out seasonally, which above the water table in South East Queensland is most of them.
• Lift pit walls and slab-to-wall junctions with visible flow, where the priority is killing the water fast.
• Anywhere the seal has to survive the wall drying out completely in summer without anybody going back to look at it.

Using both on the same job, which is common and correct

The choice is not always either-or, and on the most common leak path in an SEQ basement it usually is not.

On a slab-to-wall kicker joint with real flow, the sequence that works is hydrophobic polyurethane first, with a fast accelerator dose, to kill the flow and get the joint under control. Then a flexible hydrophilic or an acrylate gel to seal the residual weeping that a foam bridged over rather than penetrated. Two materials, two jobs, one joint.

The same logic applies to a single gushing point in an otherwise manageable crack. Kill the gusher first with a very fast hydrophobic gel, or a hydraulic cement plug or oakum packing where the flow is heavy enough to warrant it, then return to the sequence and inject the crack methodically from the bottom up.

A quote that specifies two resin families for one element is not confusion or upselling. It is usually a sign that somebody has actually rated the flow before pricing the job.

When neither polyurethane is the right material

Specify microfine cement by d95 in microns rather than by the word microfine, because the definitions differ between bodies: ISRM uses d95 under 16 microns, ACI Committee 552 uses a maximum diameter under 15 microns, and the Portland Cement Association uses under 10 microns.

How to read this on a quote you have been given

Most injection quotes in this market say "polyurethane injection" and stop, which tells you nothing about the decision that most determines whether the repair survives five years. These are the things worth asking, and none of them are unreasonable.

• Which resin family, hydrophobic or hydrophilic? If the quote does not say, ask, and get the answer in writing on the quote rather than over the phone.
• Which product, and does its technical data sheet declare conformity with EN 1504-5? That is a product performance specification rather than an Australian code, but nearly every serious injection resin manufacturer declares against it, and the series classifies injection products by function: F for force-transmitting, D for ductile filling, S for swelling.
• If the answer is hydrophilic, ask directly: does this crack dry out, and what happens to the seal if it does? A good contractor will already have an answer and it will not be defensive.
• If the answer is hydrophobic on a planter box, a tank wall or anything permanently saturated, ask why, because the usual reasoning has been inverted.
• If a resin volume has been estimated, ask what expansion figure it was based on. If somebody has calculated off a free-rise figure of 20 or 30 times, the volume is wrong. Confined expansion in a real crack is closer to 3 to 8 times.
• Ask for the safety data sheet for whatever is being used. You are entitled to it, and on a strata or commercial job the building manager should be asking as a matter of routine.

Questions we actually get asked

My quote just says polyurethane injection. How do I find out which one they mean?

Ask, and ask for the answer to be added to the quote. There is no way to tell from the phrase alone, and there is no default in the industry, which is the whole point of the disagreement. Get the product name as well as the family, because that lets you look up the technical data sheet yourself and see the expansion figures, the gel time range and whether conformity with EN 1504-5 is declared. A contractor who cannot tell you which resin family they intend to use has not made the decision that most determines whether the repair lasts.

The manufacturer's rep told me hydrophilic is always the right choice. Is he wrong?

He is describing a real advantage and presenting it as a universal rule, which it is not. Hydrophilic bonds better to damp concrete and chases into fine cracks that a foam bridges over, and both of those are genuine. What the position leaves out is what happens when the crack dries out permanently, which is that the entrapped water evaporates, the material shrinks, and the leak returns. If the crack in question never dries out, the rep's advice is sound. If it dries out every winter, it is not. The rep also happens to sell hydrophilic, exactly as the rep on the other side of the argument sells hydrophobic. Neither is lying. Both are selling.

Does hydrophilic resin really re-swell after it has dried out?

It genuinely does take water back up and expand, commonly quoted at two to four times. The problem is not whether it swells, it is whether swelling restores a seal. The plug worked because it was keyed into the irregular faces of the crack under the pressure it cured against. Once it has shrunk it has pulled away from those faces, and expanding again into the available space is not the same as re-seating against them. In practice shrunk foam can lose its key and never fully recover. Treat re-swell as a reason the material tolerates a short dry spell, not as a reason to put it in a crack that goes bone dry for five months a year.

Is one more expensive than the other?

Not by enough to drive the decision. On a typical crack injection job the resin is a modest fraction of the cost compared with labour, access, exposure and making good, which is why the indicative band for accessible polyurethane crack injection sits at roughly $150 to $450 per linear metre regardless of which family is specified. If a contractor is steering you toward one chemistry on price grounds, the more likely explanation is what is already in the truck. That is not automatically wrong, but it is worth knowing that it is the reason.

Can anybody tell which resin was used on a repair somebody else did?

Not reliably from the outside. Cured hydrophobic foam is a closed-cell material that stays the same size wet or dry, and cured hydrophilic is spongier and holds water, but distinguishing them means getting at the cured resin inside the crack, which means coring or excavating a packer. That is destructive and rarely worth it. The practical answer is whatever the original job left behind on paper: if the quote named the resin family and the product, or the contractor can tell you, that is your answer. If nothing was ever written down, the resin question is one of several things about that job that cannot now be established.

What about acrylate gel? Is that a third option or a different job entirely?

Mostly a different job, with a genuine overlap. Acrylate gel is about as thin as water, commonly 10 to 50 mPa.s, so it permeates soil and hairline cracks that no polyurethane will enter, and cured it is highly elastic with elongation often in the hundreds of per cent. That makes it the correct material for curtain injection into the ground behind a wall, for honeycombed concrete, and for cracks below the reach of a foam. It has zero structural strength, it must stay damp because several formulations shrink irreversibly if they dry out completely, and it needs a correctly calibrated two-component pump. So it is a real third option for damp fine cracks in permanently wet structures, and the only sensible option for curtain work, but it is not a substitute for hydrophobic foam on a running leak.

Where to next

• polyurethane crack injection in Brisbane and the Gold Coast
• find out which resin your crack needs
• when injection lasts and when it does not
• leaking planter boxes and podium garden beds