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Insulating Glass Sealant: How Two-Component Secondary Sealing Works
An insulating glass unit fails quietly. There is no crack, no visible damage, just condensation between the panes a few years after installation, and by then the cause is buried inside the edge seal. The secondary sealant is the component that decides whether that happens. This guide explains what the secondary seal does, why it is usually two-component, how to read the datasheet, and which checks at the glazing line prevent the failures that show up three winters later.
What the secondary seal actually does
An insulating glass unit has two seals. The primary seal is the butyl ribbon directly against the spacer, and its job is to stop water vapour. The secondary seal sits behind it, bonding the two glass panes through the spacer and giving the unit its mechanical strength.
Three functions come out of that position. First, structural integrity: the unit must survive handling, transport, wind load and its own thermal cycling without the bond line opening. Second, a second vapour barrier: if the primary butyl is imperfect at a corner — and corners are where it usually is — the secondary seal is the backstop that keeps the dew point inside the cavity from rising. Third, edge protection: it absorbs the differential movement between the two panes as they heat and cool at different rates.
A unit with a good primary seal and a poor secondary seal passes the factory dew-point test and fails in service. That is why the secondary seal is specified by chemistry and by mechanical property, not by price per kilogram.
Why the secondary seal is two-component
One-component sealants cure by absorbing moisture from the air. Inside an IG unit there is no air to absorb — the cavity is sealed and often filled with argon. A moisture-cure product would never cure properly at depth.
Two-component chemistry solves this by putting the curing agent in a separate drum. Part A carries the polymer, part B the curing agent and accelerator; mixing them in a static mixer at the dispensing nozzle starts a reaction that does not depend on the atmosphere. The result is a controlled cure from the inside out, at a speed the line can plan around.
The practical consequences shape the whole factory workflow. The sealant must be mixed at the correct ratio or the cured rubber will be either tacky or brittle. Pot life after mixing is short, so the machine dispenses on demand rather than in batches. And the mixing equipment, not the operator, becomes the quality control point.
Comparing the three secondary seal chemistries
Three chemistries dominate insulating glass production, and each has a different failure profile.
Polysulfide was the historical standard, with good vapour resistance and moderate strength. It has largely been displaced in high-performance units by silicone and by hot-melt butyl in warm-edge designs.
Silicone offers the widest service temperature range and excellent UV and ageing resistance, which matters on exposed façades and in structural glazing where the unit edge is visible. It bonds to glass, aluminium spacer and coated surfaces without a primer in most configurations. Its weakness is higher gas permeability than butyl, which is precisely why the primary seal still exists.
Hot-melt butyl is applied warm, sets physically rather than chemically, and gives outstanding vapour resistance with no mixing. It is the standard choice for warm-edge units, but it has limited mechanical strength and poor heat resistance at high service temperatures.
For a façade in a hot or cold climate, or for a unit that is also structurally bonded, silicone is usually the specified secondary seal. For a standard warm-edge residential unit, butyl often wins on vapour performance per unit cost.
Reading the datasheet
Four numbers tell you most of what matters at the line, and a two-component neutral silicone such as BLD-6609 illustrates them.
Mix ratio is stated by volume, typically 10:1 for A:B. It is by volume, not by weight, because the dispensing machine meters volume. An operator who mixes by weight at the wrong density produces an off-ratio batch that looks correct and cures wrong.
Specific gravity, around 1.7 g/ml for a filled silicone secondary seal, converts the machine’s volumetric output into the mass used for consumption and cost calculations. It also flags inconsistency: a batch meaningfully outside the stated range suggests filler variation.
Extrusion rate describes how fast the material flows at a stated pressure, here 300 g/min. This sets the maximum line speed at a given nozzle and bead size. It is the number that decides whether a machine upgrade is needed before a product change.
Service temperature, from -50 °C to +150 °C for this grade, defines the range over which the cured sealant stays elastic. It should be compared with the actual glass edge temperature in service, which on a dark spandrel in direct sun can be far above ambient air temperature.
Checks at the glazing line
Most field failures trace back to four preventable conditions, and all four are visible within the first hour of production.
Ratio verification. Weigh a dispensed sample and compare with the theoretical mass from the mix ratio and specific gravity. Do it at the start of each shift and after any drum change.
Mix quality. Cut a cured sample and look for streaks. Incomplete mixing through a static mixer shows as glossy or dull bands and produces a sealant that never reaches full hardness.
Bead geometry. The secondary bead must fill the channel without voids, and it must not bridge the spacer face into the cavity. A void at the corner is where vapour gets in.
Adhesion to the actual substrates. Glass, spacer coating and any primer must be tested as delivered, not as described. A spacer supplier change is a substrate change and requires a fresh check, and it should trigger a new pull test rather than a review of the old one.
Frequently asked questions
How long should the secondary seal last?
It should last as long as the unit is expected to perform, which is why the sealant is chosen for ageing resistance rather than initial strength. Silicone secondary seals hold their elasticity across a very wide temperature range, which is the property that determines whether the edge seal survives twenty years of thermal cycling.
Can one-component sealant be used for IG secondary sealing?
Not for the cavity edge. A one-component moisture-cure product cannot obtain the moisture it needs inside a sealed unit and will remain uncured at depth. Two-component chemistry exists precisely for this application.
Does the mix ratio really matter that precisely?
Yes. Off-ratio material either stays tacky or cures brittle, and both conditions reduce adhesion to glass. The failure appears as edge seal delamination long after the unit has left the factory, so it is expensive to trace.
What standards apply to the finished unit?
The unit, not the sealant, is what gets certified: EN 1279 parts 2 to 5 in Europe, and ASTM E2188, E2189 and E2190 in North America. The sealant datasheet supports that certification; it does not replace it.
How should the sealant be stored before use?
Follow the drum labels: part B in particular is sensitive to moisture and temperature. Drums stored through a winter in an unheated warehouse can show crystallisation that no amount of mixing will fix.
Specification summary
Choose the secondary seal by chemistry and by the four datasheet numbers, then verify them at the line rather than at goods-in. For units that will see high edge temperatures, structural bonding or exposed edges, a two-component neutral silicone with a wide service range is the defensible choice.
The order matters when a specification is being written. Fix the chemistry first, because that decides vapour resistance and temperature range. Fix the mix ratio and specific gravity next, because those decide whether the existing dispensing machine can run the product at all. Then check extrusion rate against the line speed you actually need, and only then compare price per kilogram — a cheaper drum that forces the line to run slower is not cheaper.
Two further points belong in the project file rather than the purchase order: the drum storage conditions, and the name of the person responsible for the daily ratio check. Both are trivial to write down and both are what an auditor will ask for when a unit fails.
BLD-6609 is supplied for that duty with its mix ratio, specific gravity, extrusion rate and service temperature documented, and its technical data sheet is available on request for the project file.
Products for this application
Specification pages for the products referenced above:
