Understanding Sealant Movement Capability (Class 25 vs 50)
Every building moves — expands, contracts, settles. If your sealant cannot move with it, the joint fails. Movement capability is the number that tells you whether a sealant can handle that movement. Here is what Class 25 and Class 50 really mean. Our 8000 structural silicone sealant is formulated for high movement-class curtain wall joints, and the 8800 structural sealant handles +-35% movement.
1. What the Class Number Means
The class number represents the percentage of joint width a sealant can stretch and compress repeatedly without failure. A Class 25 sealant handles ±25% of the joint width; a Class 50 handles ±50%. On a 10mm joint, Class 25 accommodates 2.5mm of movement, Class 50 accommodates 5mm.
2. How It Is Tested
Movement capability is measured by cycling the sealant sample under controlled extension and compression at temperature extremes, over hundreds of cycles. A sealant that survives the test at 23°C may fail at -20°C or +70°C — which is why certified grades are tested across the full service range.
3. Choosing Between 25 and 50
Wide joints and high-movement areas (large panels, thermal bridges, seismic zones) need Class 50. Narrow joints and stable substrates can work with Class 25. The rule: match the sealant’s movement capability to the joint’s expected movement — never the other way around.
4. Joint Design Comes First
Movement capability works with joint width. A correctly designed joint is 2-3 times wider than the expected movement, so the sealant never exceeds its class rating. If the joint is too narrow, even Class 50 sealant will fail. Engineering the joint is the real first step.
5. What Distributors Should Carry
Stock both classes and match them to applications: Class 25 for standard facade joints, Class 50 for high-movement projects. Verify the movement rating is certified and tested — a printed class number without test data is just marketing. Ask for the movement cycling test report.
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Related guide: Curtain Wall Sealant Selection Guide
Shandong Silicon New Material Co., Ltd — certified sealant manufacturer specializing in structural, weatherproof, MS polymer, and PV sealants.
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