Fosroc Nitoseal MC20
Two-component, manganese-dioxide-cured polysulphide sealant that stays elastic in moving joints exposed to water, fuel, sewage and Indian temperature swings.
What is it?
A factory-proportioned, two-part polysulphide elastomeric sealant for movement joints, cured to a tough rubber by a manganese-dioxide curing agent.
Where is it used?
Expansion and construction joints in concrete floors, pavements, retaining walls, water-treatment tanks, reservoirs and effluent-carrying structures across Delhi NCR and pan-India projects.
Why use it?
It flexes with joints that open and close daily, resists water, fuel, oils and dilute acids or alkalis, and holds its recovery for years rather than embrittling like tar or bitumen fillers.
How is it applied?
Prime the joint faces with the matching Nitoprime, fit a Nitofoam backer rod, blend the base with the curing agent for 3-5 minutes, then gun or pour the sealant and tool it off before it skins.
About Fosroc Nitoseal MC20
Nitoseal MC20 is Fosroc’s two-component polysulphide joint sealant, built for the joints that never stop moving. It arrives as a base compound and a separate manganese-dioxide curing agent in matched proportions; once the two are mixed, a chemical (oxidising) cure turns the paste into a resilient synthetic rubber that bonds to concrete, metal, stone and most masonry. Unlike bituminous or coal-tar fillers, it does not go brittle in the cold, sag in summer heat, or cold-flow out of the joint over time, which is why specifiers reach for it on structures expected to serve decades rather than seasons.
The sealant is designed for structural movement joints between roughly 6 mm and 50 mm wide, accommodating the daily and seasonal expansion-contraction cycles typical of Indian slabs, decks and tanks. It stays serviceable from about -20°C up to +80°C and resists prolonged contact with water, fuels, mineral oils, dilute acids and alkalis, and sewage. Those credentials make it a natural fit for water-retaining and effluent-handling assets, industrial floors and external pavements, where a joint that leaks or hardens quickly becomes an expensive repair. As a Fosroc authorised distributor and applicator, Space Arc Engineering supplies MC20 with the correct primer and backer-rod system and the on-site guidance to install it right the first time.
The technical insight
The performance of MC20 comes from its polysulphide backbone. Polysulphide polymers cure through an oxidation reaction rather than by drying or absorbing moisture, so the manganese-dioxide curing agent cross-links the base into a three-dimensional elastomer that is chemically stable and genuinely rubbery, not just soft. That elastomer can stretch to 100-200% of its original length and still pull back, which is what lets a two-part polysulphide sealant carry a movement accommodation factor of around 25% of the joint width in service. Because the cure is chemical and full-depth, the sealant does not depend on air contact to set, so it performs equally well as a gunned bead in a wall joint or a poured pour-grade seal in a horizontal floor or pavement joint. The trade-off is a working window: once mixed, pot life runs to roughly one to two hours at 25°C and shrinks to 30-60 minutes as temperatures climb toward 35-40°C, so batch sizes and joint lengths have to be planned around the ambient temperature on the day.
Where it is used
- Expansion and contraction joints in concrete floors and industrial pavements
- Movement joints in water-retaining structures – reservoirs, tanks and clarifiers
- Effluent and sewage-carrying channels where dilute chemical exposure is expected
- Construction joints in retaining walls and basement structures
- Horizontal deck and podium joints using the pour-grade approach
- External hardstanding, service yards and vehicular pavement joints
- Joints subject to occasional contact with fuels, oils and mild chemicals
- Perimeter and abutment joints between concrete elements
Key advantages
- Cures to a resilient rubber that recovers after repeated joint movement
- Accommodates joint movement of the order of 25% of joint width
- Resists water, fuels, oils, dilute acids and alkalis, and sewage
- Stays serviceable from about -20°C to +80°C without embrittling or sagging
- Avoids the cold-flow and thermoplastic failings of tar and bitumen fillers
- Bonds to concrete, masonry, stone and metal when correctly primed
- Available for both gun-grade vertical work and pour-grade horizontal joints
- Factory-proportioned kits remove on-site guesswork from the mix ratio
Limitations & considerations
- It is a mixed-on-site two-part system, so pot life is limited – only 30-60 minutes in hot weather
- Joint faces must be primed with the correct Nitoprime; skipping the primer risks adhesion failure
- Backer rod must be closed-cell Nitofoam and must NOT be primed, or bond-breaking is compromised
- Not intended as a heavy-duty trafficked-surface sealant where an epoxy-modified polyurethane would be specified
- Polysulphide chemistry can be affected by continuous UV and some solvents; check exposure before specifying
- Joint width and depth must fall within the recommended range and depth-to-width ratio to perform as designed
Selection & best practice
Success with MC20 is mostly about preparation and timing. Clean the joint back to sound, dry, dust-free concrete and mask the arrises to keep lines crisp. Prime the joint faces with the matching Nitoprime (a solvent- or water-based primer for concrete and masonry, or a metal primer on aluminium and steel) and let it develop for the specified 8-24 hours before sealing. Insert a Nitofoam closed-cell backer rod to set the correct sealant depth and act as a bond breaker – never prime the backer rod. Add the full curing-agent tin to the base and mix for 3-5 minutes with a slow-speed mechanical paddle until the colour is uniform, then work only what can be placed within the pot life. Gun or pour the sealant, tool it firmly against both faces to wet out the primer, and protect the joint from traffic and water until it has cured. In hot weather, mix smaller batches and seal in the cooler part of the day.
Specifications
| Chemistry | Two-component polysulphide elastomeric sealant |
| Cure mechanism | Oxidising cure via manganese-dioxide curing agent |
| Grades | Gun grade (vertical) and pour grade (horizontal) |
| Joint width range | Approx. 6-50 mm (with backer rod) |
| Movement accommodation factor | Approx. 25% of joint width |
| Elongation at break | 100-200% or greater |
| Service temperature | -20°C to +80°C |
| Pot life | ~1-2 hrs at 25°C; 30-60 min at 35-40°C |
| Chemical resistance | Water, fuels, oils, dilute acids/alkalis, sewage |
Industry use cases
Water & Wastewater
Sealing movement joints in reservoirs, treatment tanks and effluent channels where the seal faces standing water and dilute chemical attack.
Industrial Flooring
Flexible sealing of expansion joints in factory and warehouse floors that see thermal movement and occasional oil or fuel spillage.
Roads & Pavements
Pour-grade sealing of horizontal joints in external hardstandings, service yards and concrete pavements exposed to weather and light traffic.
Commercial & Institutional
Construction and expansion joints in basements, retaining walls and podium decks where long-term watertightness matters.
Infrastructure
Durable joint sealing on concrete structures needing decades of service without the embrittlement of tar-based fillers.
Frequently asked questions
Is Nitoseal MC20 a polysulphide or a polyurethane sealant?
What size joints can it seal?
How much movement can the joint take?
Do I need a primer, and which one?
How long do I have after mixing?
Can I use it on floors and pavements as well as walls?
How we position Fosroc Nitoseal MC20
Nitoseal MC20 is the workhorse two-part polysulphide for moving joints in water-retaining and industrial concrete. Where a project calls for a higher-performance polysulphide to BS 4254 for demanding vertical and horizontal joints, Fosroc Thioflex 600 is the natural step up. For non-sag or trafficked pavement joints needing polyurethane toughness, the Fosroc Nitoseal PU range (such as PU220, a pitch-modified two-part PU) is the better fit, while modern hybrid silyl-modified polyether options like Nitoseal MS600 suit UV-exposed facade and weather-seal joints. The joint system is completed with Fosroc Nitoprime primers and Nitofoam backer rod. Across our other brands, Space Arc can also offer Sika and STP polysulphide and PU sealants where a client has a preferred manufacturer or an approved-vendor list to match.
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