The question we are asked most often is whether a given joint should use polysulfide or polyurethane. Both are elastic sealants with similar prices and similar appearance, but they are optimised for opposite conditions. Work through the five checks below and the answer usually falls out.
Check 1: will the joint stay under water?
This is the decisive one, and the only check that gives a clean either-or answer.
For joints permanently immersed, permanently damp, or subject to a fluctuating water table, use two-component polysulfide sealant.
The polysulfide backbone is a saturated —S—S— chain. It is chemically inert, does not hydrolyse and does not swell appreciably after cure. Reservoir, canal, water basin, sewage tank and dam face joints are all classic polysulfide duty.
Polyurethane hydrolyses slowly under permanent immersion, losing strength and adhesion year by year. One-component polyurethane is explicitly not suitable for permanently wet locations. Two-component polyurethane is more water-resistant than the one-component grade but still behind polysulfide in continuous immersion.
In short: only polysulfide tolerates permanent immersion.
Check 2: read the measured elastic recovery
Elastic recovery tells you whether the bead can still grip the concrete after the joint opens and closes. JC/T 483-2022 requires ≥80% for polysulfide; JC/T 482-2022 requires ≥70% for polyurethane.
Measured on two batches we sent for third-party testing in 2026:
| Property | 2K polysulfide | 2K polyurethane | Requirement |
|---|---|---|---|
| Elastic recovery | 81% | 85% | PS ≥80%, PU ≥70% |
| Tensile modulus at 23 °C | 0.2 MPa | 0.35 MPa | both ≤0.4 MPa |
| Skin formation time | 10 h | 5 h | both ≤24 h |
| Tooling life | 8.5 h | 3 h | PS ≥2 h, PU ≥0.5 h |
| Mass loss | 3% | 1.90% | both ≤5% |
Polyurethane posts the higher recovery figure, but its advantage is strength and cure speed, not water resistance. Compare like with like, and read each figure against its own requirement rather than against the other product.
Check 3: how much working time does the site have?
Both two-component products require on-site mixing, and mixing is where most failures start.
- Polysulfide is normally mixed A:B = 10:1 by weight. Measured tooling life is 8.5 h, so a batch mixed in the morning is still workable in the afternoon.
- Two-component polyurethane uses the same 10:1 ratio but only 3 h of tooling life. Mix it and place it promptly.
- One-component polyurethane needs no mixing at all, but the bead must not be applied too thick, otherwise the interior never sees enough atmospheric moisture to cure.
If the site has no balance, no power mixer, or a rotating workforce, choose the one-component grade and trade price for mixing reliability.
Check 4: how much movement does the joint see?
For joints with large or frequent movement (bridge expansion joints, pavement saw cuts, airport runways), specify a low-modulus grade so the bead stays soft and follows the movement. For rigid joints carrying load, specify a high-modulus grade.
Polyurethane offers more choice here: N20LM and N25LM are low modulus, N20HM and N25HM are high modulus. Polysulfide is predominantly low modulus — measured 0.2 MPa — which suits the large-movement joints typical of hydraulic structures.
Check 5: is drinking-water contact involved?
Basins, clear-water tanks and waterworks structures that touch potable water need hygiene data, not just strength. Our polysulfide and polyurethane sealants have passed GB/T 17219 hygiene assessment for drinking-water contact materials, with colour, turbidity, heavy metals and volatile phenols all within limits.
Ask for the scanned report itself rather than a verbal assurance.
Quick selection table
| Duty | Specify | Why |
|---|---|---|
| Reservoir, canal, basin, sewage tank, permanent immersion | 2K polysulfide | Hydrolysis-resistant, tolerates immersion |
| Potable water structures | 2K polysulfide with hygiene report | Passed GB/T 17219 |
| Pavement, car park, industrial floor saw cuts | 2K polyurethane | High strength, abrasion and tear resistant |
| Roofs, crack repair, hole sealing | 1K polyurethane | No mixing, fast application |
| Above 80 °C continuous, or strong acid/alkali | Neither | Use a specialist material |
Once the grade is fixed, dosage calculation, mixing accuracy and curing decide the outcome. For the dosage formula see how much sealant per metre.