From molecular bonding chemistry to field inspection — a complete guide for architects, facade engineers, and glazing contractors.
Both glass and aluminium have very different surface chemistries, yet silicone manages to create a durable, weather-resistant bridge between them. Understanding why requires looking at what each surface actually is at a molecular level.
Aluminium instantly oxidises in air, forming a thin but stable aluminium oxide (Al₂O₃) layer typically 4–10 nm thick. This oxide is covered with hydroxyl groups (–OH). Silicone primers react with these groups through condensation, forming an Al–O–Si covalent bond.
Glass surfaces (SiO₂) are rich in silanol groups (–Si–OH). When silicone contacts glass, the silicone polymer chains form siloxane bonds (Si–O–Si) — the same chemistry as the glass surface itself. This is why silicone bonds exceptionally well to glass: it is chemically related to glass.
Raw aluminium often has lubricant residues, anodising coatings, or paint. A silane-based primer deposits a mono-molecular layer that bridges between the oxide surface and the silicone, ensuring the bond is to the oxide — not to contamination. Without primer, adhesion failure (cohesive → adhesive) is almost certain within 1–2 years of UV/moisture exposure.
Si–O–Si & Al–O–Si linkages. Bond energy 450+ kJ/mol. Permanent, unaffected by water or temperature when properly formed.
Weak physical attractions between polymer chains and substrate. Important across the bulk of the contact area — strength in numbers.
Transient –OH interactions during cure. These become permanent covalent bonds after condensation reaction completes.
Surface roughness (anodised finish) creates micro-keying. Increases effective contact area by 10–40%.
An Insulating Glass Unit (IGU) consists of two or more glass panes separated by a spacer bar, with the internal cavity filled with argon or dry air. The edge seal is a dual-sealant system critical to the unit's longevity.
The first seal is a hot-applied PIB (Polyisobutylene) strip placed between the spacer and glass. Its role is purely as a vapour barrier — it has extremely low moisture vapour transmission rate (MVTR ≈ 0.2 g/m²·day). PIB does not cure; it remains thermoplastic and relies on pressure adhesion.
The secondary seal provides structural strength to the IGU assembly. It bonds the two glass panes via the spacer, resisting wind loads, thermal cycling, and handling stresses. Two main chemistry types are used:
| Property | Polysulfide (PS) | Silicone (Neutral Cure) | Winner |
|---|---|---|---|
| UV Resistance | Poor — must be sheltered | Excellent — UV stable | Silicone |
| Tensile Strength | 1.5–2.5 MPa | 0.5–1.5 MPa | Polysulfide |
| Elongation at Break | 200–400% | 400–700% | Silicone |
| Use in SSG | ❌ Not suitable | ✅ Preferred | Silicone |
| Gas Permeability | Low | Higher | Polysulfide |
| Service Temperature | –40°C to +80°C | –50°C to +150°C | Silicone |
| Compatibility with PDMS silicone | Good with primer | Direct bond | Silicone |
Structural silicone sealants come in two fundamental formulations. The choice between them affects application method, cure speed, joint geometry limits, and quality control requirements.
| Parameter | 1-Part (Moisture Cure) | 2-Part (Chemical / RTV Cure) |
|---|---|---|
| Cure Mechanism | Atmospheric moisture diffuses into joint; acetoxy, oxime or neutral by-products released | Part A (base polymer) + Part B (crosslinker/catalyst) mixed; internal chemical reaction |
| Cure Depth Limit | ~15 mm per pass (moisture cannot penetrate deeper in reasonable time) | Unlimited depth — cures throughout regardless of thickness |
| Cure Time @ 23°C/50% RH | 3–21 days for full cure (structural strength in 7–14 days) | 24–72 hours to handle strength; full cure 3–7 days |
| Application | Single cartridge, standard caulking gun — site friendly | Dual-component equipment (pneumatic pump, static mixer) — factory/shop preferred |
| Quality Control | Visual inspection; cure confirmation difficult without sampling | Snap time / butterfly test performed every drum/cartridge change for confirmation |
| Joint Depth (Bite) | Maximum 15 mm in single pass (deeper = multiple passes after partial cure) | Up to 30–50 mm in single application |
| Typical Tensile Strength | 0.8 – 1.4 MPa | 1.0 – 2.0 MPa |
| Allowable Design Stress (ETAG) | ≤ 138 kPa (20 psi) | ≤ 138 kPa (20 psi) |
| Shelf Life | 9–18 months (moisture sensitive) | 12–24 months (Part A & B separately) |
| Temperature Sensitivity during Cure | High — cure slows dramatically below 10°C | Moderate — less affected by low RH |
| Primary Use Case | Site-glazed SSG, re-glazing, repairs; weather sealing | Factory-glazed IGU secondary seal, unitised curtain wall production |
| By-products during Cure | Acetic acid (acetoxy), oximes, or alcohols (neutral) | None — clean 100% solids reaction |
| Cost | Lower (simpler formulation, less equipment) | Higher (materials + equipment cost) |
| Common Products | Dow 795, Sika 267, Ottoseal S 70 | Dow 983, Sika SG-500, Momentive SSG4000 |
The structural performance of a silicone joint comes from a combination of molecular network density, joint geometry, and substrate adhesion quality.
During cure, crosslinking reactions create a 3D polymer network. Higher crosslink density = higher modulus but lower elongation. Structural silicones are formulated for a balance: stiff enough to transfer loads, flexible enough to accommodate thermal movement (±15% joint movement capability).
True structural performance requires cohesive failure (failure within the silicone bulk) rather than adhesive failure (failure at the interface). Cohesive failure confirms the substrate bond is stronger than the material itself — the design goal.
Silicone tears through its own bulk. Substrate surface retains silicone residue. Bond is confirmed stronger than material. Design is structurally sound.
Clean peeling at the silicone–substrate interface. No silicone residue on substrate. Indicates surface contamination, missed primer, or incompatible substrate. Reglazing required.
The ETAG 002 / ASTM C1401 design rules limit allowable design tensile stress to 138 kPa (20 psi) regardless of the silicone's higher tested strength. This safety factor accounts for long-term creep, UV degradation, thermal cycling, and workmanship variability. The joint aspect ratio (Bite W ÷ Thickness T) must stay between 1 and 3.
Glass weight exerts a permanent static shear stress on the silicone. Most manufacturers allow 11,000–14,000 Pa for dead load shear. Glass must be supported on setting blocks; silicone alone should not carry dead load in standard SSG systems — only in specially designed full-structural systems.
Before any structural silicone is applied on a project, a series of tests must be conducted to verify that the silicone, primers, substrates, and adjacent materials are chemically compatible. This is non-negotiable per ETAG 002, ASTM C1401, and most manufacturer warranties.
Many materials commonly used in facades — EPDM gaskets, foam tapes, PVC sealants, bitumen, certain coatings — contain plasticisers, oils, solvents, or reactive chemicals that migrate into silicone during or after cure. This can cause:
| Test | Standard | Method | Pass Criterion | Duration |
|---|---|---|---|---|
| Adhesion (Peel) | ASTM C794 / ISO 8510 | 180° peel test on substrate at 50 mm/min | ≥ 80% cohesive failure | 7 days cure + 7 days immersion |
| Adhesion-in-Peel (with aging) | ASTM C794 | Specimens aged in water at 70°C for 7 days, then peel | No loss of cohesive failure after water immersion | 21 days total |
| Staining Test | ASTM C510 / EOTA TR012 | Sealant applied to stone/masonry and assessed visually after UV exposure | No visible staining beyond 3 mm from edge | 7 + 28 days |
| Compatibility (Migration) | EOTA TR012 / ASTM C1087 | Adjacent material placed in contact with silicone in curing state; cure quality assessed | Full cure within expected snap time; no inhibition | 48–72 hrs |
| Tensile Adhesion | ASTM C1135 / EN ISO 8339 | H-bar specimens pulled at 50 mm/min (12×12×50 mm joint) | ≥ 0.14 MPa; ≥ 75% cohesive failure | 21 days cure |
| Modulus of Elasticity | ASTM C1135 | H-bar tensile at 12.5% and 25% elongation | Per manufacturer spec (low mod preferred) | 21 days cure |
| Accelerated UV / Weathering | ASTM G154 / ISO 4892 | Xenon arc or UV-B lamp, 1000–2000 hours | ΔE colour < 5; tensile strength retention > 70% | 6–10 weeks |
| Thermal Cycling | EOTA TR010 | –20°C to +70°C × 100 cycles, joint under 12.5% tension | No cracking, delamination or > 10% strength loss | 4–5 weeks |
| Substrate | Preparation | Primer | Special Notes |
|---|---|---|---|
| Float Glass (clear) | IPA wipe × 2 | Usually not required with neutral silicone | Tin side vs air side — test both |
| Coated Glass (Low-E, frit) | IPA wipe, consult coating supplier | Often required | Some coatings incompatible — must test |
| Mill-finish Aluminium | MEK or IPA clean | Silane primer (e.g. AP-133) | Remove mill oil completely |
| Anodised Aluminium | IPA clean | Required (porous surface) | Primer penetrates anodising micro-pores |
| PVDF-coated Al | MEK clean | Specific primer per brand | Test each coating brand separately |
| Stainless Steel | IPA clean | Required | Ensure passive oxide layer intact |
| Structural Foam Tape | N/A | N/A | Run compatibility test before use as backer |
The butterfly test (also called the snap time test or bead fold test) is the primary in-shop quality control check performed before every shift and every drum/pail change of 2-part structural silicone. It verifies that Part A and Part B are mixing correctly and that the material will cure within specification.
If the butterfly test fails, all units glazed since the last passing test must be quarantined and re-tested (peel adhesion on witness samples). Material in the line must be purged for a minimum of 3× the static mixer volume before the next test. Document all actions in the QC log.
Silicone cures by condensation crosslinking (1-part moisture cure or 2-part RTV). Here is what happens at the molecular scale:
Linear poly(dimethylsiloxane) chains with –OCH₃ or –OH terminal groups. MW ~15,000–50,000 g/mol. Viscous liquid.
–Si–OCH₃ + H₂O → –Si–OH + CH₃OH. Silanol groups generated on chain ends & crosslinker. Moisture-triggered (1-part) or catalyst-triggered (2-part).
–Si–OH + HO–Si– → –Si–O–Si– + H₂O. New siloxane bonds form, linking chains. Network begins to grow and viscosity rises sharply.
Critical crosslink density reached. Material transitions from viscous liquid to elastic gel. This is the "snap" in the butterfly test. Tack-free surface forms.
3D network fully developed. All reactive groups consumed. Shore A hardness stabilises. Design tensile and shear strength achieved. No further significant property change.
| Parameter | Value | Effect on Properties |
|---|---|---|
| PDMS backbone MW (uncured) | 15,000–50,000 g/mol | Higher MW → better elongation but slower cure |
| Crosslinker MW (e.g. MTMS) | 136 g/mol | Lower MW = more crosslink sites per gram |
| MW between crosslinks (Mc) | 3,000–10,000 g/mol | Lower Mc = stiffer, higher modulus |
| Network strand density (ν) | 0.1–1.0 mol/L | Higher ν = stronger but less flexible |
| Filler (fumed silica) MW | ~60 g/mol per SiO₂ unit | Reinforces network; increases tensile 3–5× |
| Si–O bond dissociation energy | 452 kJ/mol | Thermal stability to >200°C |
| C–C bond (organic polymer) | 347 kJ/mol | Silicone more UV/heat stable than organics |
The Si–O–Si backbone has a bond angle of 143° (vs 111° for C–C), giving silicone exceptional conformational flexibility at low temperatures. The high bond energy (452 kJ/mol) and the large Si–O bond length (1.64 Å) make silicone resistant to UV photodegradation at wavelengths down to 280 nm. Combined with the non-polar methyl groups that resist water absorption, silicone is the only polymer suitable for 50+ year structural glazing applications.
Deglazing is the controlled removal of glass bonded with structural silicone from a facade or IGU assembly. It requires careful methodology to avoid glass breakage, substrate damage, and personal injury.
Deglazing from height requires scaffold or elevated work platform. Full PPE: safety glasses, cut-resistant gloves (EN 388 Level 4+), hard hat, and fall arrest equipment. Two-person operation minimum for panels > 0.5 m².
Bi-metal or diamond blade for cutting cured silicone close to the glass edge. Most controlled for thin joints.
Hook-bladed lame knife or razor scraper for final separation and silicone removal from frames.
Mechanical glass lifters rated for the panel weight × 3 safety factor. Used to support glass during cutting.
Apply duct tape or protection film grid to glass face before cutting to contain potential breakage.
After deglazing, a thorough visual and tactile inspection of the removed silicone and substrates reveals the bond quality and failure mode. Here is a field guide:
Photograph every deglazed unit showing the failure mode in cross-section with a scale reference. Mark cohesive vs adhesive failure areas as a percentage. This data feeds back into the root-cause analysis and any warranty or insurance claims.
Based on ETAG 002 / ASTM C1401 methodology. Calculates minimum structural bite width and joint thickness for a rectangular glass panel under wind load and dead load, using allowable stress design.
This calculator provides preliminary estimates for guidance only, based on simplified ETAG 002 / ASTM C1401 methodology. Results must be verified by a qualified facade engineer before use in construction. Always consult the silicone manufacturer's technical data and project-specific structural calculations.