Prime Practices

Hydrophilic VS Hydrophobic

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Chemical grouts are a cost-effective solution for many leak sealing, void filling and soil stabilization projects. Choosing the right material and installing it correctly are critical to the long-term success.


Chemical grouts fall into two classes: hydrophilic grouts that have an affinity for water and seek it out and hydrophobic grouts that repel water and tend to resist moving toward wet areas. This means that they are better suited for some applications than others.

Hydrophilic Grouts

Because hydrophilics seek out water in a crack, they absorb into tight cracks and pores of the concrete or masonry and have a tenacious bond to wet concrete. They should be used to stop leaks via crack or joint injectionand are a very effective long-term solution. Polyurethanes are most common, but acrylates are an option as well.

Hydrophilic soil stabilization material absorbs as much water as it comes into contact with during initial cure. This absorption allows the resin to be extended with water. The end result is a gelatin type product that doesn’t add much strength, but is impermeable to water. When the water table drops, moisture in the hydrophilic stabilization product evaporates and the product shrinks. Our testing has shown that re-absorption recovery only ranges from 60-80%. This means that a hydrophilic resin for soil stabilization is best suited for soils that remain moist most of the time such as places with high groundwater. That said, hydrophobic materials are usually a better choice for soil stabilization.

Hydrophobic Grouts

A hydrophobic grout will not bond tightly with concrete. This is an example of a crack injected with a hydrophobic material (not a Prime Resins product). You can see that the cured hydrophobic material easily peels off; this material did not seal the leak. Later injection with Prime Flex 900 XLV sealed the crack.

Hydrophobics repel water in a crack, and the water will act as a bond breaker between the polyurethane foam and the concrete, resulting in low bond strengths. Their hydrophobic nature means they will not be diluted and their reaction is not affected by the presence of water. For these reasons, hydrophobics are not well suited for crack injection, but they are an excellent choice for curtain grouting, sealing gushing leaks, compression sealing, void filling and soil stabilization.

Hydrophobic polyurethane is usually used to fill voids and stabilize soil because of its low viscosity, high expansion rate, and ability to set up under wet conditions without diluting.

When injected into loose soil or voids, it expands to form a rigid foam that displaces any water present. The material expands up to 1000% in soil and forms a strong, impermeable mass by binding the soil particles together. Because rigid hydrophobic material is not affected by fluctuation in the moisture content of the soil, it is virtually unaffected by water table level. It retains its strength, size and impermeability under almost all soil conditions. This makes it a popular choice for curtain grouting manholes and seawalls.

Why is bond important?

Concrete expands and contracts with changes in temperature. As temperatures rise, concrete expands, and cracks tighten up. When temperatures fall, concrete contracts and cracks open up. Polyurethane foams are designed to seal leaking cracks. However, if the material does not bond to wet concrete, it is destined to eventually fail during these thermal cycles, particularly during cold weather. The cracks will open up and leave a gap between the concrete and the foam allowing water to seep back through. This is why hydrophilics are better suited for sealing leaks with crack injection and why hydrophobics work well for curtain grouting.

  Hydrophilic   Hydrophobic
Behavior Seeks water Repels water, reaction and cured structure unaffected by water
Reaction   Reacts with water Needs a catalyst
Cured state  Flexible foam or gel Rigid foam
Best applications  Crack injection of leaks
Stopping active infiltrations
Sealing cracks subject to movement or vibration
Soil stabilization of moist soils
Permeation grouting (acrylate)
Curtain grouting to seal leaks
Filling voids
Soil stabilization
Compaction grouting
Permeation grouting
Stopping gushing leaks
Prime Resins product Prime Flex 900 XLV
Prime Flex 900 MV
Prime Flex Hydro Gel SX
AR 800
AR 870
Prime Flex 910
Prime Flex 920
Prime Flex 940
Prime Flex 985