Application
Adhesives for bonded windscreen replacement
Bonded glazing is structural. The bead holds the glass in a crash, seals against water at 120 km/h and has to reach drive-away strength before the customer finishes paying. Three things decide whether it does: the adhesive, the bead, and the surface.
What the adhesive actually has to do
A windscreen adhesive is asked for two contradictory things: stay flexible enough to absorb body flex and vibration, and be strong enough that the glass does not separate in a rollover. Polyurethane squares that circle better than silicone because it bonds to glass, paint and metal without a primer in the newer formulations, and keeps its adhesion through thermal cycling.
The number that matters most on the data sheet is not tensile strength. It is elongation at break, because the bead lives in shear and peel, not in pure tension. Our three windscreen grades run 320% to 450% elongation with tensile from 3.5 to 6.5 MPa — the stronger grade is also the more flexible one, which is not a coincidence.
Primerless does not mean preparation-free
Second- and third-generation PU sealants bond a range of substrates without a primer coat. That removes a step and a solvent, and it removes the primer as an excuse when a bead lifts. What it does not remove is contamination: silicone residue from a previous bead, mould release agent on a plastic trim, or finger grease on the frit will defeat any adhesive.
The practical rule: clean with the activator, let it flash off, and do not touch the frit afterwards. K-901 activator is designed for that step; where the substrate is unknown or the vehicle has had a previous silicone repair, use the K-801 primer as well and test a coupon first.
Bead geometry and positioning time
A triangular bead about 8–10 mm wide and 10–12 mm high is the usual starting point for a passenger car; the exact profile comes from the vehicle maker, not from the adhesive supplier. What the adhesive controls is how long you have to seat the glass and how fast it reaches handling strength.
KA530F is the fast grade: 15 minutes skin time, 4 mm/day cure and enough green strength for two-hour positioning. KA518 and KA520 are the general-purpose and high-strength grades at 30 and 40 minutes skin time. In a cold bay, cure rate falls before skin time does — that is the failure mode behind "the glass slid overnight", and it is a temperature problem, not a batch problem.
The three failures that come back
Water leak at the top corner: usually a bead that was not continuous at the corner, or glass seated before the activator flashed off. Replacing the adhesive will not fix it twice.
Bead separation at the frit: contamination or a frit that was not cured. Check with a peel test on the removed glass — if the adhesive comes away clean, the bond failed at the interface, not in the adhesive.
Glass cracked at the edge: the bead was too hard or too tall for the gap, and road shock transferred straight into the glass. A softer grade or a thinner bead is the fix, not a stronger adhesive.
Models for this job
KA518
Polyurethane Windshield Adhesive
General-purpose moisture-curing adhesive for bonded windscreens, 310 ml cartridges.
3.5 tensile · 320 elongation
KA520
High-Strength Windscreen Adhesive
Higher-modulus grade for bus and coach glazing.
4.5 tensile · 450 elongation
KA530F
Fast-Drying Glass Adhesive
Two-hour positioning at 23 °C with a 6.5 MPa cured tensile strength.
6.5 tensile · 450 elongation
Typical values from the catalogue. For a specific vehicle, follow the car maker's bead profile and cure-time specification over ours.
Questions that come up
Can I drive the car two hours after fitting?
Only with a fast-positioning grade such as KA530F, and only at the temperature it was tested at. Below about 15 °C the cure slows enough that the two-hour figure no longer holds — keep the vehicle longer or apply heat.
Do I need a primer with these adhesives?
The catalogue grades are primerless on clean glass, paint and metal. Use the activator every time; add K-801 primer where the substrate is unknown, previously repaired with silicone, or painted with an aftermarket coating.
Which grade for a bus or coach?
KA520, the high-strength grade: larger glass, higher wind load and a body that flexes more. Its 4.5 MPa tensile and 450% elongation are there for that load case.
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