Hydrophobic vs Hydrophilic Polyurethane Grout: How to Choose

TL;DR

These two PU grouts look similar in the drum but behave fundamentally differently after injection. Choosing wrong wastes material and risks repeat leakage. Use this guide to match the chemistry to the site condition.

Quick answer

Active running water → Hydrophobic. Long-term seepage in a flexing joint → Hydrophilic. Many real jobs need both, applied in sequence.

Side-by-side comparison

Criterion	Hydrophobic (oil-based)	Hydrophilic (water-based)
Cured structure	Rigid closed-cell foam	Flexible elastic gel
Volumetric expansion	8–15×	Absorbs water ~200% by weight
Reaction time	Seconds	1–3 minutes
Best leakage type	Active flowing water	Slow seepage, capillary moisture
Substrate movement	Rigid — fails under deformation	Elastic — accommodates joint movement
Cold-weather tolerance	Better	Slower reaction below 5°C
Typical cost per kg	Lower	Higher
Common uses	Tunnel leak stoppage, pipe-through-wall	Movement joints, long-term seepage zones

How hydrophobic polyurethane works

Hydrophobic (oil-based) PU prepolymer repels water but reacts with it. When it contacts water in a crack, the reaction releases CO2 — this is what creates the rapid, visible foaming on site. The expanded foam is rigid, closed-cell, and acts as a physical plug with structural support against hydrostatic pressure.

Reacts within seconds of water contact
Forms a rigid closed-cell foam structure
8–15× volumetric expansion fills cavities
Strong physical plug — stops moving water immediately
Chemical-corrosion resistant once cured

For current technical specs, see Hydrophobic Polyurethane Grout.

How hydrophilic polyurethane works

Hydrophilic (water-based) PU prepolymer is water-attracting. Instead of foaming aggressively, it absorbs water and cross-links into a flexible elastomeric gel. The cured material can absorb additional water during service — making it self-healing under wet/dry cycles and ideal for joints that move.

Absorbs water rather than just reacting violently with it
Cures into an elastic, flexible gel
Accommodates substrate deformation and joint movement
Self-healing behavior under wet/dry cycling
Long-term performance in capillary-moisture environments

See Hydrophilic Polyurethane Grout for current specs.

Decision tree

Is the leakage active running water? → Use hydrophobic to stop it first.
Is the crack a movement joint or in a structure that flexes? → Use hydrophilic for long-term performance.
Is the crack in a load-bearing structural member that you also need to reinforce? → Stop water with PU first, then inject epoxy for structural repair.
Are you doing a continuous water-stop curtain (e.g., behind a tunnel lining)? → Consider acrylate grout for large-area continuous coverage.

Common mistake

Using hydrophobic PU on a joint that's still moving will eventually crack the rigid foam and re-leak. If the substrate moves, you need an elastic gel — not a brittle plug.

Combined-system jobs

Real engineering sites often need both. A common sequence: inject hydrophobic PU first to stop the active leak, then come back and inject hydrophilic PU into the same area as a redundant secondary seal that handles long-term moisture and structural movement. This belt-and-braces approach is standard practice on metro tunnel and basement repair projects.

Frequently asked questions

Can hydrophobic and hydrophilic PU be mixed in the same injection?+

They should not be co-injected through the same packer at the same time. They can be applied in sequence — hydrophobic first to stop active flow, hydrophilic afterward as a redundant elastic seal.

Which expands more?+

Hydrophobic typically achieves 8–15× volumetric expansion through CO2 release. Hydrophilic absorbs water up to 200% by weight but does not foam to the same volumetric expansion — it gels rather than foams.

Which performs better in cold weather?+

Hydrophobic generally tolerates lower temperatures better. Hydrophilic reaction slows significantly below 5°C and may need pre-warming of the material.

What's the cost difference?+

Hydrophobic is typically less expensive per kg. But hydrophilic's flexibility often makes it more cost-effective over the project lifetime by avoiding re-injection of moving joints.

Can hydrophilic grout be used as the only sealant?+

Yes, where there's no active running water — long-term seepage zones, movement joints, capillary-moisture areas. For active flowing leaks, you generally need hydrophobic to stop the water before any elastic system can perform reliably.