Prime Resins News

Why crack injection should always be performed with a hydrophilic polyurethane

Hydrophilic resins seek out water in a crack. Learn why that matters when sealing hairline cracks.

Crack Injection Demo

Hydrophilic (hydro meaning 'water', philic meaning 'has an affinity for') resins seek out water in a crack. They chase the water and absorb into the tight micro-cracks and pores of the concrete. Hydrophobic (hydro - water, phobic - fearing) resins repel water in a crack. They resist movement towards wet areas and will not absorb into tight micro cracks and pores in the concrete. All polymer repair materials get their mechanical bond in these rough areas (if the resins can penetrate them). Since hydrophilic resins seek out water, they absorb deep into the pores and micro cracks of concrete. This gives hydrophilic resins a tenacious bond to the wet concrete. Since hydrophobic resins repel water, they will not penetrate these areas. They may be forced in using high pressure, but the water will act as a bond breaker between the polyurethane foam and the concrete resulting in extremely low bond strengths.


But I have used hydrophobic materials before and they stopped the water?

There is no doubt that hydrophobic resins can be very effective at stopping water leaks. They will expand and fill up a wet crack as well as most hydrophilic resins. The failure will usually occur a few months after the job is finished. If it happens during the warranty period, the contractor will have to return to the job and fix it again. All of the profit made on the job goes out the window. The only one who benefits is the hydrophobic material supplier.



Some hydrophilic resins will absorb as much water as they come in contact with during the time of initial cure. These resins do a good job of stopping the water leaks, but usually fail if the water table drops. The water they absorbed will evaporate and cause the material to shrink, sometimes resulting in failure.

Prime Flex 900 XLV and Hydro Gel SX were designed specifically to eliminate this problem. Both of Prime Resins hydrophilic resins will only absorb as much water as they need to facilitate the reaction during initial cure. The rest of the water is rejected in the same way that hydrophobic resins reject water.



Initial cure and final cure are two separate issues. The initial cure is the time when the polyurethane resin foams up. The final cure may take up to 24 hours. It is during this time that hydrophilic resins bite into the pores and micro cracks of the concrete.


 Rigid hydrophobic resins should be used for stabilizing soil and filling voids

Polyurethanes are usually used to fill voids and stabilize soil because of their low viscosity, high expansion rates, and ability to set up under wet conditions without diluting. Hydrophobic resins repel water. When injected into loose soil or voids, they expand and displace any water that is present. The rigid hydrophobic soil stabilization materials that are on the market expand up to 1 ,000% in the soil and form a strong, impermeable mass.

Hydrophilic resins absorb water. The hydrophilic soil stabilization materials that are on the market absorb as much water as they come into contact with during initial cure. This absorption allows the resins to be extended with water and the end result is a gelatin type product that doesn't add much strength but is impermeable to water. When water tables drop, the moisture in the hydrophilic stabilization products evaporates and the products shrink. Manufacturers claim that when the water table returns to original levels, the material will re-absorb the water and regain their original size. Our testing has shown that the recovery only ranges from 60-80%. Since rigid hydrophobic materials are not affected by increases or decreases in soil moisture content, they are virtually unaffected by water table level. They retain their strength, size, and impermeability under almost all soil conditions.

Prime Flex 910 is an extremely low viscosity rigid foam that will penetrate the finest soils. The set time can be adjusted from 1 hour and 40 minutes for slow penetration down to 1 minute and 50 seconds for cutting off underground streams. Prime Flex 920 will expand up to 2,000% to fill voids behind tunnel liners, manholes, under concrete slabs, etc. The set time can be controlled using an accelerator. Prime-Flex 920 is also low enough viscosity to be used for soil stabilization. Prime Flex 940 will expand up to 2,000% to fill cracks, joints and voids with a flexible foam. Set time can be controlled using an accelerator.


As always, we are here to answer any questions you may have. Give us a call at 800-321-7212 to discuss what works best for your scenario. 

news button prev
news button next