To achieve long-term non cracking of polyurethane sealant, the core is three things: selecting the right adhesive type+doing a good job in the base and joint type+standardized construction+correct maintenance. Below is a complete set of methods that can be implemented with minimal waste (single component, two-component universal).
1、 First choose the “crack resistant” polyurethane adhesive (the source is the most important)
Look directly at the parameters and compare them when buying:
Type: Polyether polyurethane (resistant to hydrolysis, good low-temperature flexibility, less prone to cracking than polyester)
Displacement capacity: ± 25%, ± 35% (higher, more crack resistant, expansion joints must be used)
Hardness: Shore A 25-45 (too soft to deform, too hard to crack)
Elongation rate: ≥ 400%, tensile strength * * ≥ 1.5MPa**
Low temperature: -40 ℃ without brittle fracture; Aging resistance: no cracks for ≥ 500 hours
Avoid pitfalls: Do not use “ordinary joint filler/hard polyurethane/asphalt adhesive” instead of special PU adhesive for expansion joints.
2、 Seam size: The width to depth ratio is incorrect, even the best glue will crack
Golden ratio: width: depth=2:1~1:1
Seam width 5-10mm → depth 5-8mm
Seam width 10-20mm → depth 8-12mm
Seam depth ＞ 12mm: foam backing strip must be plugged (to prevent three sides from sticking, falling and cracking)
Prohibited: Failure to fill deep seams with backing, narrow and deep seams, and thin adhesive layers (<3mm, prone to cracking)
3、 Inadequate surface treatment=necessary cracking (90% root cause of cracking)
Thoroughly clean
Remove dust, floating slurry, oil stains, old glue, and mold release agent
Oil stains: wipe with acetone/anhydrous alcohol, do not use water
Must be dry (most importantly)
Concrete moisture content * * ＜ 6%; Metal/glass close to 0%**
Moisture=bubbles, debonding, and later cracking
Solid and smooth
Loosening, sanding, and holes should be repaired in place first
Use primer if necessary
Smooth metal, glass, and dense stone: brushed with matching primer, significantly preventing detachment and cracking
4、 Construction key points (directly determine whether it will crack)
environment
Temperature: 5-35 ℃; Slow curing and easy cracking below 5 ℃
Humidity: 40% -85%; Too dry is prone to cracking, too wet is prone to bubbles
apply adhesive
The nozzle of the glue gun is cut diagonally, and the caliber is slightly smaller than the seam width
Continuous and uniform injection of glue, full seam, compacted, without hollowing or breakage of glue
Immediately trim the surface (scraper/curved tool), and if the surface dries, it will crack when repaired again
Two component: Accurately weigh and mix in proportion (commonly 10:1, 5:1), otherwise uneven curing and internal cracking may occur
Back lining+anti sticking
Close cell foam strips must be placed in deep joints (the diameter is slightly larger than the joint width)
Stick decorative paper on both sides of the seam, remove the edges after trimming, and ensure neat and anti adhesive edges
5、 Maintenance: Many people overlook it, leading to a high incidence of cracking in the later stages
Single component (moisture curing):
Surface drying: 2-6 hours (25 ℃/50% RH)
Initial curing: 24 hours
Complete curing: 7 days; Avoid stretching, shaking, and soaking in water for 7 days
Two components:
Operable time: 20-30 minutes
Maintenance: 7 days, the first 3 days are the most critical
6、 Common causes of cracking+quick countermeasures
Surface fine cracks: The environment is too dry, the wind is strong, and the surface dries too quickly → Spray water mist to increase humidity, avoid air vents
Edge cracking/delamination: The base surface is not clean/damp, there is no primer, and the ratio of seam width to depth is incorrect → redo the base surface+backing+primer
Deep crack/glue break in the middle: the glue is too hard, the displacement ability is not enough, and the backing is not placed → change ± 35% glue+foam strip
Low temperature cracking: ordinary glue was used, no low-temperature resistant model was selected → replaced with polyether type, -40 ℃ grade
