Frequently Asked Questions

How is Nitoseal MC20 mixed and applied and what pot life constraints affect the site application rate?

Nitoseal MC20 is supplied as two components in factory-proportioned containers: the polysulfide base compound and the manganese dioxide curing agent. Mixing is carried out on site by combining the two components in the correct ratio as specified on the packaging — typically the entire contents of the curing agent pack are added to the base compound container, and the mixture is stirred for 3 to 5 minutes with a low-speed mechanical paddle mixer until the two components are thoroughly combined to a uniform dark colour (the manganese dioxide curing agent gives the mixed sealant a dark brown to black colour). It is important to scrape the sides and bottom of the container during mixing to ensure all the base compound is mixed with the curing agent — unmixed base compound at the bottom of the container will remain uncured after application. The pot life of Nitoseal MC20 after mixing (the time available for application before the sealant becomes too stiff to gun-apply and tool) is temperature-dependent: at 25 degrees Celsius, typically 1 to 2 hours; at Indian summer temperatures of 35 to 40 degrees Celsius, the pot life reduces to 30 to 60 minutes. In hot summer conditions, the mixed batch size should be limited to what can be applied within the reduced pot life. Multiple small batches are better than one large batch that becomes unusable partway through application.

What primer is needed before Nitoseal MC20 application to concrete and metal joint faces?

Primer application before Nitoseal MC20 is required on most substrate types to achieve the adhesion needed for a reliable, long-lasting polysulfide sealant joint. Without primer, polysulfide sealants may adhere adequately to some substrates initially but can lose adhesion over time under the combined stresses of thermal cycling, water immersion, and UV exposure — the most common failure mode of polysulfide sealant joints in Indian climate is adhesive failure at the sealant-to-substrate interface, where the sealant peels away from one or both joint faces under movement. The correct primer for each substrate type in the Nitoseal MC20 system should be confirmed from the Fosroc technical data sheet, but the general guidance is: for concrete and masonry joint faces, a Nitoprime primer (solvent-based or water-based, as specified for the Nitoseal MC20 product variant) is applied by brush to the joint faces, allowed to dry, and the Nitoseal MC20 is applied before the primer degrades (typically within 8 to 24 hours of primer application, avoiding the use of primer applied more than 24 hours before sealant, which may require re-priming). For aluminium and steel joint faces, a metal primer appropriate for polysulfide adhesion is used. The backer rod (Nitofoam 45 or Nitofoam 40) must not be primed — the sealant bonds to the joint faces only, and if the sealant also bonds to the backer rod, the three-point bonding prevents the sealant from stretching freely under joint movement.

What is the expected service life of Nitoseal MC20 in an expansion joint of a flyover in Delhi and when should the joint be resealed?

The expected service life of Nitoseal MC20 polysulfide sealant in the expansion joints of a Delhi flyover under typical Indian traffic and climate conditions is approximately 10 to 20 years, with 15 years being a realistic median expectation for a well-installed joint. The service life is affected by: the quality of joint preparation and primer application at installation; the frequency and amplitude of the thermal movement cycling (expansion joints in flyovers in Delhi see daily temperature swings of 15 to 30 degrees Celsius in summer and annual swings from 5 to 48 degrees Celsius, driving the joint open and closed repeatedly through the service life); UV exposure (polysulfide is more UV-resistant than silicone but still gradually oxidises and loses elasticity at the surface under prolonged UV exposure); water immersion in the joint (during monsoon rainfall, joints in bridge decks may hold water for extended periods, accelerating chemical degradation of the sealant); and traffic vibration loading transmitted through the bridge deck. Inspection of bridge expansion joints should be included in the regular bridge maintenance programme every 3 to 5 years. Signs that resealing is needed include: visible cracking through the sealant bead; adhesive failure where the sealant has peeled from one or both concrete faces; extrusion of the sealant under compression (indicating loss of elasticity and permanent set); and deterioration of the sealant surface (chalking, significant hardening, or loss of the smooth flexible surface appearance).