Frequently Asked Questions

What is the difference between neutral cure and acetoxy cure silicone sealants and why is neutral cure specified for aluminium curtain walls and glazing in India?

The difference between neutral-cure and acetoxy-cure (also called acetic acid cure or acid-cure) silicone sealants is a fundamental technical distinction that is critical to the correct specification for glazing and aluminium facade applications in India — using an acetoxy-cure silicone in a glazing application can cause serious substrate damage and premature sealant failure. The curing mechanism difference is: all one-component silicone sealants cure by reaction with atmospheric moisture — the curing process releases a by-product gas or vapour as the silicone polymer chain crosslinks; in acetoxy-cure silicones, the by-product is acetic acid (vinegar odour) — a corrosive weak organic acid that is released in significant concentrations within the joint as the sealant cures; in neutral-cure silicones, the by-product is a non-corrosive material (oxime, alkoxy, or ketone, depending on the specific neutral cure chemistry) that does not attack substrates. The problems caused by acetoxy-cure silicone on facade and glazing substrates in India are: aluminium corrosion — the acetic acid released during curing attacks the aluminium oxide layer on anodised aluminium and the conversion coating on powder-coated aluminium framing, causing visible surface corrosion and staining that permanently defaces the facade; over time, the acid corrosion weakens the adhesion bond between the silicone sealant and the aluminium frame — the sealant debonds from the aluminium, allowing water ingress; structural glass — heat-strengthened and toughened glass used in building facades can undergo spontaneous fracture if acid-cure silicone is used in contact with the glass-to-glass or glass-to-frame joint — this is the stress corrosion cracking mechanism that is accelerated by acid; stainless steel fittings — acetic acid can cause crevice corrosion and pitting of stainless steel spider fittings and patch plates used in point-fixed structural glazing; electrical conductor trays and embedded metal in concrete near the joint — acetic acid vapour from acetoxy-cure sealant applied to facade joints near embedded steel can contribute to corrosion of reinforcement. Neutral-cure Nitoseal 333 releases no corrosive by-products during cure — all substrates (aluminium, glass, stone, stainless steel) are unaffected by the curing process, which is the fundamental requirement for glazing and curtain wall silicone specification.

What primer should be used with Nitoseal 333 on different facade substrates and is priming always necessary for silicone sealant adhesion?

Primer use for Fosroc Nitoseal 333 depends on the specific substrate and the service environment — on some substrates, Nitoseal 333 achieves adequate adhesion without primer in dry conditions, but primer is recommended or required on substrates that are difficult to bond in the outdoor exposure conditions typical of Indian building facades (hot sun, monsoon rain, dust). The substrate-by-substrate guidance for Nitoseal 333 priming is: float glass — most neutral-cure silicones achieve good adhesion to clean float glass without primer; however, if the glass is coated (Low-E glass, frit-coated glass, heat-reflective coated glass), the coating changes the adhesion surface and a primer specifically tested for the specific glass coating is required — confirm with the glass manufacturer or the sealant supplier; anodised aluminium — most neutral-cure silicones achieve good adhesion to freshly degreased anodised aluminium without primer; however, if the aluminium anodising is more than a few months old and has been exposed to outdoor dust and atmospheric contamination, a surface cleaner and Fosroc silicone primer is recommended; concrete and mortar — concrete and masonry substrates require priming with a silicone primer before neutral-cure silicone sealant application; the concrete surface must first be clean, dry, and laitance-free, then the primer is applied by brush and allowed to flash off (typically 15 to 30 minutes at 25 degrees) before sealant application; porous stone (granite, marble, sandstone) — requires priming; for polished stone, the primer type depends on the stone surface treatment and contamination level — a test application in a discreet area is recommended before full application; painted surfaces — in general, silicone sealants should not be applied over paint because silicone does not adhere reliably to paint and paint can lift from the substrate when the sealant is loaded in tension; the paint must be removed from the joint area before priming and sealant application on all substrates.

How wide and deep should the joints be for Nitoseal 333 application on a building facade and what is the minimum and maximum joint width for silicone facade sealant?

The joint width and depth specification for Fosroc Nitoseal 333 in facade and glazing applications follows the established joint design principles for elastomeric sealants — the joint must be large enough to allow the sealant to accommodate the expected movement without exceeding the sealant elongation capacity, but not so large that the sealant depth becomes excessive (deep joints waste material and can lead to three-point adhesion failure). The joint width-to-movement relationship for silicone facade sealant: the design rule for a +/-25 per cent movement sealant (like most silicone sealants including Nitoseal 333) is that the minimum joint width must be at least 4 times the expected joint movement amplitude — if a glazing joint is expected to move +/- 3 mm from the nominal position (6 mm total movement range due to thermal expansion of the aluminium frame), the minimum joint width is 4 x 3 mm equals 12 mm; joints designed below this minimum width will exceed the sealant movement capacity and cohesive cracking or adhesion failure will result. Joint depth: the correct sealant depth for facade joints is the same as the width for joints up to 20 mm wide (1:1 width-to-depth ratio), or half the width for joints wider than 20 mm (2:1 width-to-depth ratio); the joint depth is controlled by the use of a closed-cell polyethylene foam backer rod pressed into the joint to the correct depth before sealant application; the backer rod also provides the three-point bond prevention — silicone sealant must bond to only the two opposite joint faces (two-point bonding), not to the back face of the joint; if the sealant bonds to the back face as well (three-point bonding at the base), it cannot elongate correctly under movement and fails at much lower deformation than the rated movement capacity. Minimum joint width for facade applications: 6 mm (narrower joints are difficult to fill and tool correctly, and provide limited movement capacity); maximum joint width: 30 to 50 mm for standard sealant application (very wide joints require a large sealant volume and deep joints require a backing system — Fosroc technical should be consulted for joints wider than 30 mm).