Fosroc Nitoseal MB77 formerly Plastic 77
A hard-grade, hot-poured rubber-bitumen sealant for low-movement horizontal joints in factory floors, pavements and water-excluding slabs, engineered to resist grit and trafficking.
What is it?
A hot-poured, hard-grade rubber-bitumen joint sealant (formerly branded Plastic 77) for sealing low-movement horizontal and gently inclined joints in concrete and asphalt.
Where is it used?
Factory and warehouse floor slabs, pavements, hardstandings, podiums and water-retaining or water-excluding structures where joints are closely spaced and subject to grit and traffic.
Why use it?
It is an economical, abrasion-resistant compound that bonds well to concrete and asphalt and resists the ingress of dirt and grit that would otherwise jam closely spaced trafficked joints.
How is it applied?
Melted in an oil-jacketed heater to 180-190 C and poured into clean, dry, Primer B2-primed joints over a bond-breaker/backer rod; not a gun-grade or cold-applied sealant.
About Fosroc Nitoseal MB77 formerly Plastic 77
Fosroc Nitoseal MB77, sold for many years as Plastic 77, is a hot-applied rubber-bitumen joint sealant designed for horizontal and lightly inclined joints in concrete and asphalt. It belongs to Fosroc’s hot-pour family alongside Nitoseal MBN2 and the polymer-modified Nitoseal MB333, and it occupies the hard-grade, low-movement end of that range. Where modern gun-grade elastomeric sealants such as polyurethanes and polysulphides accommodate substantial cyclic movement, MB77 is built for a different job: keeping closely spaced floor and pavement joints sealed against water, dirt and abrasive grit under foot and wheeled traffic, at an attractive cost per metre. The compound is supplied as a solid block and is melted on site in a thermostatically controlled, oil-jacketed heating vessel fitted with a stirrer and thermometer, then poured into the prepared joint. It develops good adhesion to concrete, asphalt, stone and brick paving once the substrate has been primed with Fosroc Primer B2. For Indian contractors and applicators, MB77 remains a practical choice for industrial flooring contraction joints, dock and yard slabs, and similar surfaces where movement is modest but resistance to traffic and contamination is the priority. Because it is a thermoplastic bitumen product it softens on reheating, so it is deliberately not specified for floors with under-floor heating or for areas exposed to plant heat or steam cleaning. Space Arc Engineering, as an authorised Fosroc distributor and applicator serving Delhi NCR and project sites Pan-India, supplies MB77 with the matching Primer B2 and on-site guidance on joint design, heating and pouring so the sealant performs as intended.
The technical insight
MB77 is a thermoplastic rubber-bitumen blend rather than a curing elastomer, which is the single most important fact governing how it is specified. It has no chemical cure and no published movement accommodation factor in the way ISO 11600 or ASTM C920 sealants do; its sealing performance comes from cohesive flow and adhesion, so it suits low-movement joints only. The hard grade trades extensibility for traffic and grit resistance, the reverse of softer hot-pours such as MBN2. Correct pouring temperature of 180-190 C is critical: too cool and the compound will not wet the joint faces or self-level; overheating degrades the binder. Adhesion is entirely dependent on a clean, dry joint primed with Primer B2 and allowed to reach touch-dry (typically 1-4 hours) before pouring. One specification trap worth flagging is fuel resistance: MB77 does not meet the fuel-resistant requirements of BS 2499, so it should not be used where spillage of fuels, oils or aggressive solvents is expected; a fuel-resistant grade should be specified instead.
Where it is used
- Contraction and construction joints in industrial and warehouse floor slabs where joints are closely spaced
- Horizontal and inclined pavement joints up to a slope of 1 in 20
- Hardstandings, loading yards, container yards and dock aprons subject to wheeled traffic and grit
- Water-retaining and water-excluding floor slabs and reservoirs (non-fuel exposure)
- Resealing of existing trafficked concrete and asphalt joints during refurbishment
- External paving joints in concrete, stone and brick where a poured, traffic-resistant seal is required
Key advantages
- Economical sealing of closely spaced horizontal joints at a low cost per linear metre
- Hard grade resists abrasion, dirt and ingress of grit associated with trafficked pavements
- Good adhesion to concrete, asphalt, stone and brick when used with Primer B2
- Poured application self-levels into the joint, suiting long runs and closely spaced joints
- Bonds to both concrete and asphalt, useful at transition joints between the two
- Part of the established Fosroc Nitoseal hot-pour range with a compatible primer and companion grades
Limitations & considerations
- Low-movement only – not an elastomeric sealant and unsuitable for high-movement expansion joints
- Horizontal and lightly inclined joints only (up to 1 in 20); not for vertical or overhead joints
- Softens on reheating – not for under-floor heating, plant heat or steam-cleaned areas
- Does not meet the fuel-resistant requirements of BS 2499; avoid fuel, oil and solvent exposure
- Requires hot application with an oil-jacketed heater, stirrer and thermometer – more plant and skill than cold sealants
- Joint depth must not exceed 50 mm (25 mm recommended), needing a backer rod or bond-breaker in deeper joints
- Hot pouring carries burn and fume hazards demanding trained applicators and PPE
Selection & best practice
Design joints for low movement and detail width-to-depth carefully: keep poured depth to a maximum of 50 mm and ideally around 25 mm, using a closed-cell backer rod or bond-breaker tape so the sealant bonds only to the two joint faces and is not three-sided. The joint must be clean, dry and free of laitance, oil and old sealant; prime concrete, asphalt, stone or brick faces with Fosroc Primer B2 and allow it to reach touch-dry (about 1-4 hours) before pouring. Melt MB77 in an oil-jacketed heating vessel with stirrer and thermometer, cutting the block into small pieces and adding gradually while stirring continuously, and pour only when the compound is at 180-190 C, taking extra care in cold weather. Confine MB77 to low-movement horizontal or lightly inclined joints; where significant cyclic movement, vertical joints, or fuel/chemical exposure is expected, specify an elastomeric or fuel-resistant alternative instead.
Specifications
| Product type | Hot-poured rubber-bitumen joint sealant (hard grade), formerly Plastic 77 / Pliastic 77 |
| Cure / set mechanism | Thermoplastic – sets on cooling; no chemical cure |
| Joint orientation | Horizontal and inclined joints up to a slope of 1 in 20 |
| Pouring temperature | 180 C to 190 C |
| Heating method | Oil-jacketed heating vessel with stirrer and thermometer; add cut pieces gradually, stir continuously |
| Primer | Fosroc Primer B2 (touch-dry approx. 1-4 hours before sealing) |
| Max joint width – trafficked | Up to 30 mm |
| Max joint width – non-trafficked / horizontal | Up to 65 mm |
| Joint depth | Maximum 50 mm; 25 mm recommended for most uses (use backer rod / bond-breaker) |
| Suitable substrates | Concrete, asphalt, stone and brick paving |
| Colour | Black |
| Pack size | 15 kg (solid block) |
| Fuel resistance | Does NOT meet the fuel-resistant requirements of BS 2499:1993 |
| Temperature limitation | Softens on heating – not for under-floor heating, plant heat or steam-cleaning areas |
Industry use cases
Industrial & warehouse flooring
Sealing closely spaced contraction joints in heavy-duty factory and warehouse floor slabs where forklift and pallet-truck traffic demands a grit-resistant, abrasion-tolerant joint seal.
Ports, logistics & hardstandings
Container yards, dock aprons, loading bays and lorry hardstandings where horizontal joints carry constant wheeled traffic and must resist dirt ingress at low cost per metre.
Roads & external pavements
Horizontal and lightly inclined concrete or asphalt pavement joints, including transition joints between concrete and asphalt, on non-fuel-exposed surfaces.
Water-excluding structures
Floor slabs of reservoirs, tanks and basements where low-movement horizontal joints need a watertight, poured bitumen seal (excluding fuel or chemical exposure).
Refurbishment & resealing
Routing out and resealing failed or contaminated joints in existing trafficked concrete and asphalt floors and pavements during maintenance campaigns.
Frequently asked questions
Is Nitoseal MB77 the same as the old Plastic 77?
What temperature should MB77 be poured at?
Does MB77 need a primer?
What joint widths and depths can MB77 seal?
Can MB77 be used for expansion joints or vertical joints?
Is MB77 resistant to fuel and oil spillage?
Can Space Arc supply and apply MB77 in India?
How we position Fosroc Nitoseal MB77 formerly Plastic 77
Within the Fosroc range, Nitoseal MB77 is the economical hard-grade, low-movement hot-pour for trafficked horizontal joints – the rebranded Plastic 77. Its closest siblings are Nitoseal MBN2 (a companion hot-pour grade) and the polymer-modified Nitoseal MB333 for more demanding hot-pour applications. It is deliberately not an elastomeric sealant: where joints genuinely move, specify a gun-grade Fosroc Nitoseal polyurethane/hybrid or a Thioflex 600 two-part polysulphide for general movement and construction joints, and Colpor 200PF for cold-applied, trafficked expansion joints. For fuel or chemical exposure, choose a fuel-resistant grade rather than MB77. Comparable products from other manufacturers include Sika’s hot-applied and Sikaflex pavement-joint sealants, MBCC/Master Builders Solutions MasterSeal joint sealants, and STP (Pidilite) bituminous and polyurethane joint sealants. The honest position: MB77 wins on cost and grit/traffic resistance for closely spaced low-movement floor and pavement joints, but it must not be over-specified onto moving, vertical or fuel-exposed joints where a curing elastomeric or fuel-resistant sealant is the correct engineering choice.
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