Polyurethane (PU)

The width-to-depth ratio of a sealant joint is critical to ensure proper elasticity and long-term performance. Incorrect joint geometry is one of the most common reasons for early sealant failure such as cracking, debonding, or cohesive tear.

General Rule (SpecX Standard)

For most elastomeric sealants (PU, polysulphide, hybrid, silicone), maintain a Width : Depth ratio of approximately 2 : 1 once joint width exceeds 10 mm.

Why the 2 : 1 Ratio Matters

• Ensures the sealant stretches and compresses evenly under cyclic movement.
• Prevents excessive internal stress on the sealant body.
• Reduces risk of adhesive failure at joint edges.
• Maintains optimum elasticity across the joint cycle.

Other Key Application Notes

• Always provide a bond‑breaker tape behind the sealant when no backer rod is used to avoid three‑side adhesion.
• Use closed‑cell backer rods to control depth and support tooling.
• Tool the surface into a smooth concave profile to distribute stress evenly.
• Ensure all sealant edges are primed and clean before application.

• Clean and dry the joint thoroughly.
• Apply primer if recommended by the system.
• Insert a closed-cell backer rod to control depth and maintain geometry.
• Gun-apply PU sealant and tool to a neat concave finish.
• Allow full curing, typically 24–48 hours.

• Elongation: ≥ 400% (ASTM D412)
• Adhesion: ≥ 1.0 N/mm (ASTM C794)
• Shore A Hardness: 25–40 (ASTM D2240)
• Movement capability: ±25% (ASTM C719)
• UV resistance: No cracking or chalking (ASTM G154)