KA518 vs KA520 vs KA530F: choosing a windscreen adhesive

Three adhesives, one job: hold the glass in a crash, keep the water out, and let the car leave before the customer runs out of patience. They differ by less than you would think on the shelf and a lot in the bay.

The numbers side by side

	KA518	KA520	KA530F
Tensile strength	3.5 MPa	4.5 MPa	6.5 MPa
Elongation at break	320%	450%	450%
Skin time	30 min	40 min	15 min
Shore A	65	60	58
Cure rate	3.5 mm/day	3.2 mm/day	4.0 mm/day

A pattern worth noticing: the strongest grade is also the softest and the fastest. That is not a coincidence — a higher-modulus network with more crosslinking reaches strength faster, and the formulation compensates with plasticiser to keep elongation.

Question 1: how long can the vehicle stay in the bay?

This decides more installations than any strength figure. KA530F skins in 15 minutes and cures at 4 mm/day, which is what makes a two-hour positioning claim possible at 23 °C. KA518 gives you 30 minutes to tool the bead; KA520 gives 40.

Below about 15 °C, every grade slows down — but the fast grade has further to fall, so the two-hour promise is a warm-weather promise. If your bay is unheated in winter, either heat the glass or keep the car overnight.

Question 2: what is the glass doing?

A passenger car windscreen mostly carries wind load and its own weight. 3.5 MPa with 320% elongation (KA518) is ample.

A bus or coach windscreen is a different structure: larger area, more body flex, higher wind pressure. KA520’s 4.5 MPa and 450% elongation exist for that load case, and its 40-minute skin time suits a slower, larger installation.

Question 3: is the surface really primerless?

All three are primerless on clean glass, paint and metal. Primerless removes a step and a solvent — it does not remove contamination. Silicone residue from an earlier repair, mould release on trim, or grease from a hand on the frit will defeat any of them.

Clean with the activator, let it flash off, and do not touch the frit afterwards. Where the substrate is unknown or previously sealed with silicone, add primer and run a coupon test.

Question 4: what does the failure look like if it goes wrong?

• Leak at a corner — the bead broke continuity, or the glass was seated before the activator flashed off.
• Separation at the frit with clean adhesive — interface failure, not adhesive failure. Preparation, not product.
• Crack from the edge — bead too tall or too hard for the gap; road shock went into the glass.

Only the third is a product-selection problem, and it points to a softer grade rather than a stronger one.

A decision table for the bay

The four questions collapse into one table when the variables are fixed.

Bay situation	Grade	Why
Passenger cars, vehicle can stay 4–6 h, heated bay	KA518	Ample strength, 30 min to tool, cheapest per vehicle
Passenger cars, throughput pressure, winter work	KA530F	15 min skin, 4 mm/day, two-hour positioning at 23 °C
Buses, coaches, large glass, unheated depot	KA520	40 min skin for a slower seat, 4.5 MPa, 450% elongation
Mixed fleet, one grade only	KA520	The compromise grade: enough strength for a car, enough skin time for a coach
Unknown or coated substrates	Any grade + primer	Chemistry does not fix a surface problem

The “one grade only” row is the one most distributors should read twice. A mixed fleet served from one cartridge is better served by KA520 than by the fastest grade, because the failure mode of too-fast skinning (a bead tooled after it has skinned) is worse than the cost of an extra 25 minutes of bay time.

What the grade does not change

Buyers spend their attention on the strength figures and then lose the job on the parts that no grade fixes:

• Bead geometry. A 6 mm bead where the drawing says 10 mm is a different structure, whatever the tensile strength.
• Glass temperature. A cold glass slows the cure of every grade; the fast one just has further to fall.
• Surface preparation. Activator, flash-off, no touching the frit.
• Drive-away discipline. The cure time on the data sheet assumes 23 °C and 50% RH. A workshop at 12 °C should double it, not round it down.

The grade is one line in a specification that has five. The other four cost nothing to get right and are where the comebacks come from.

How to compare quotations

Two suppliers quoting “a windscreen adhesive” are not quoting the same thing unless the quotation states:

1. Tensile strength, elongation and skin time, each with a test method.
2. Cure rate and the bead section it was measured on.
3. Whether the primerless claim covers glass frit and OEM paint.
4. Cartridge size and cartons per pallet.
5. Shelf life at 30 °C, not at 20 °C.

A quotation missing items 2 and 5 is a catalogue extract, not a specification. Ask for the batch test record on the first order — it is the only document that tells you what the factory actually made, rather than what it can make.

What this means for buyers

1. Specify by skin time and cure rate first, strength second — the bay’s throughput is the constraint.
2. Keep one fast grade and one general grade in stock rather than three mid-range ones.
3. Buy the activator with the adhesive; a cartridge without surface prep is half a system.
4. Ask for the batch record on tensile and elongation, not just the catalogue range.