What Adhesion and Compatibility Tests Are Needed for Structural Glazing Silicone?

In a structurally glazed facade, a bead of silicone is the only thing holding the glass to the building.

No pressure plate, no cover cap, no mechanical capture. Wind load transfers from the pane into the frame entirely through the adhesive bond, and that bond has to perform for the life of the building across every temperature cycle, every movement, and every contaminant it meets.

Which is why the testing is not optional, and why it takes longer than most programmes allow for. A sealant that bonds perfectly to one coated glass product can fail on a nominally similar one from another supplier, because the coating chemistry is different. The only way to know is to test the actual materials the project will use.

This article covers which tests, why each one exists, and when to start. PRANCE supplies curtain wall systems including structurally glazed configurations.

Why Project-Specific Testing Is Required？

Generic product data tells you the sealant works. It does not tell you the sealant works on your glass.

Four variables change the answer, and all of them are project-specific:

Glass coating. Low-E coatings vary in composition between manufacturers and between product generations. The sealant bonds to the coating, not to the glass beneath it.

Frame finish. Powder coat, PVDF, and anodized surfaces present different chemistries, and formulations differ between coating suppliers and even between production batches.

Adjacent materials. Setting blocks, spacers, gaskets, backer rod, insulating glass secondary sealant, and any tape in the glazing pocket all sit in contact with or near the structural silicone.

Site conditions. Temperature and humidity during application affect cure, and surface contamination affects bond.

Any one of these can cause a failure that generic data would never predict. Surface condition and its effect on bond is the single most common root cause when bonds do fail.

Structural Silicone Adhesion Testing

The core requirement, verifying that the sealant actually sticks to the specific substrates.

ASTM C794 Peel Adhesion

The workhorse test. A bead of sealant is applied to a sample of the actual substrate with a fabric strip embedded, cured, then peeled back at a defined angle while the force is recorded.

What matters as much as the force value is the failure mode:

• Cohesive failure, where the sealant tears within itself leaving material on the substrate, is the desired result

• Adhesive failure, where the sealant peels cleanly away from the substrate, is a failure regardless of the recorded force

A result showing high peel strength with adhesive failure is still a failure, and this distinction gets missed by people reading only the number.

ASTM C1135 Tensile Adhesion

Measures tensile strength and modulus of the cured sealant joint between two substrates, using a standard H-bar specimen pulled to failure.

This produces the design values the engineer uses to calculate bite dimension. The structural silicone joint is sized from these numbers together with the design wind pressure and the pane dimensions.

ASTM C1184 Specification

The specification standard for structural silicone sealants themselves, covering the properties a product must meet to be classified as a structural glazing sealant. It qualifies the product rather than the project assembly.

Structural Silicone Compatibility Testing

Different question from adhesion. Compatibility asks whether the materials in contact with the sealant degrade it, stain it, or interfere with cure.

ASTM C1087 Compatibility

Samples of every accessory material that will contact or sit adjacent to the structural silicone are placed against the cured sealant and exposed to accelerated conditions, usually ultraviolet light through glass.

The assessment looks for discolouration, softening, loss of adhesion, or any visible interaction.

Materials to submit:

• Setting blocks and edge blocks

• Glazing gaskets and spacer tape

• Insulating glass secondary sealant, which is in direct contact at the unit edge

• Weather sealant, where a different product is used for the weather joint

• Backer rod

• Any tape used in the glazing pocket

• Cleaning solvents and primers proposed for use

The insulating glass secondary sealant matters particularly. Polysulfide and polyurethane secondary seals are not always compatible with structural silicone, which is why structurally glazed insulating units normally use a silicone secondary seal.

ASTM C1248 Staining

Tests whether the sealant stains porous substrates. Relevant where the facade incorporates stone, concrete, or other porous materials adjacent to the glazing, and not usually required on an all-glass-and-aluminum assembly.

Glazing Sealant Substrate Testing: What to Submit？

The testing is only valid if the samples are genuinely representative, and this is where submissions go wrong.

Send to the sealant manufacturer:

• Actual glass samples with the specified coating, from the production run where possible

• Actual frame extrusion samples with the specified finish, from the finishing batch

• Every accessory material listed above, as supplied

• The proposed cleaning solvent and primer

• Project details including design wind pressure, pane sizes, and expected service temperature range

What comes back is a project-specific approval letter from the sealant manufacturer, confirming the materials tested, the results, the recommended surface preparation, whether a primer is required, and any limitations.

That letter is normally a submittal requirement and frequently a warranty condition. Laboratory evidence for the assembly covers the parallel performance testing on the system as a whole.

Programme Implications

The part most often underestimated.

Testing typically takes several weeks from receipt of samples, and that assumes the samples arrive promptly and complete. Add time for sourcing representative samples, which requires the glass and finish to be settled, and the sequence looks like this:

• Glass and finish specification confirmed

• Samples obtained from actual production

• Samples shipped to sealant manufacturer

• Testing carried out

• Approval letter issued

• Submittal reviewed and approved

Starting that chain late pushes glazing production, which pushes erection. Starting it early costs nothing.

A practical note: if the glass supplier or the finish changes after testing, the testing is void. Any substitution requires retesting, which is why value engineering on coated glass late in a project is more disruptive than it appears.

Field Quality Control

Laboratory approval covers the materials. Site work still has to execute properly, and structural glazing failures are more often workmanship than material.

Surface preparation. Two-cloth cleaning method, with solvent applied with one clean cloth and wiped dry with a second before evaporation. Contaminated or reused cloths defeat the process.

Primer application where the approval letter requires it, within the stated time window before sealant application.

Joint dimensions. Bite and glueline thickness checked against the engineered values. A joint too thin fails structurally, and one too thick cures poorly.

Cure conditions. Temperature and humidity within the manufacturer's range, with adequate cure time before the units are handled or loaded.

Snap test and hand pull test. Destructive sampling on a defined frequency, pulling a cured bead to confirm cohesive failure.

Deglazing test. Periodic removal of a sample unit to verify bond across the full joint.

Records of these checks belong in the handover documentation. Bonding curved and faceted assemblies adds further complexity, since cold-bent glass exerts continuous force on the bond rather than only wind-driven load.

Common Causes of Bond Failure

Five, in rough order of frequency.

Inadequate surface cleaning. Residual release agent, dust, or finger oils on the bonding surface.

Incompatible accessory material. Usually a setting block, tape, or secondary sealant substituted on site without checking.

Wrong primer or no primer where the approval required one.

Cure disturbed. Units handled or loaded before the sealant reached adequate strength.

Substrate substitution. A different coated glass or a different finish batch than the one tested.

Firestop sealants and their own approvals follow a similar logic at the slab edge, where a different family of products carries its own compatibility requirements.

Installation guidance covering site procedures is published alongside our system documentation.

Frequently Asked Questions

1. Why does structural silicone need project-specific testing?

Because the sealant bonds to the coating and finish rather than to glass and aluminum in the abstract. Coating chemistry varies between manufacturers and product generations, and finish formulations vary by supplier and batch, so only testing the actual materials confirms the bond.

2. What is the difference between adhesion and compatibility testing?

Adhesion testing confirms the sealant sticks to the substrates and fails cohesively rather than peeling away. Compatibility testing confirms that adjacent materials such as setting blocks, gaskets, and secondary sealants do not degrade, stain, or interfere with the structural silicone.

3. What failure mode should an adhesion test show?

Cohesive failure, where the sealant tears within itself and leaves material bonded to the substrate. Adhesive failure, where the sealant peels cleanly away, is a failure regardless of the recorded force value, and that distinction is frequently overlooked.

4. How long does structural glazing sealant testing take?

Typically several weeks from receipt of samples, plus the time needed to obtain representative samples from actual production. The chain starts with the glass and finish being confirmed, so starting it late delays glazing production and erection.

5. What happens if the glass or finish changes after testing?

The testing is void and must be repeated. Any substitution of coated glass, frame finish, or accessory material invalidates the approval, which makes late value engineering on these items considerably more disruptive than it first appears.

Getting the Testing Done Properly

Confirm the glass coating and frame finish early, because everything downstream waits on them. Send genuinely representative samples together with every accessory material that will sit in the glazing pocket. Read the failure mode rather than only the force value. Then hold the site team to the surface preparation and cure requirements the approval letter sets out.

The bond is the structure in a structurally glazed facade. It deserves the same rigour as any other load path.

PRANCE manufactures structurally glazed and captured curtain wall systems with sealant compatibility coordination, documented adhesion testing, and field quality control procedures. Send your glazing and finish specification and request sealant testing support.