Every housebuilder wants complete safety against seepage, leakage and cracks in his house. Looking for such a commitment, he visits the manufacturers of construction chemicals and gets confused by the vast range of products available to him. A systematic approach to these materials is, therefore, required to avoid such confusion. Let us, therefore, peep into the world of construction chemicals:
There was a time when everything — be it water proofing or sealing a gap or repairing a crack or filling a joint — was dependent on only one thing called Bitumen.
Bitumen had its drawbacks as it couldn’t attain cent per cent water proofing or sealing and cracked on hardening. But there was no alternative. Now, polymer-based products have been invented and these are much more effective than bitumen. These polymer-based products have invaded the construction sector on a large scale. These chemicals not only fulfil the purpose for which these are used but also enhance the life, service and durability of the structure wherever used.
Sealants are used to seal the gaps or the cracks or the joints. In addition, these are also used in the glazing joints and sealing of porous surfaces to non-porous surfaces. Sealants further are of different types, based on the type of chemical used in their manufacture. These can thus be classified as polysulphide sealants, acrylic sealants, silicon sealants and polyurethane (PU) sealants.
Sealants used in filling of gaps, joints and cracks must be shrink free. This is the most important property. Otherwise these will lose their purpose. Sealants should also have excellent bonding and adhesion properties so that these don’t come off when subjected to the sun or the rain, heat or cold temperature.
After application and on drying, these should form a permanent elastic rubber like seal to serve their purpose. These should therefore be UV resistant and should offer long-term durability.
Further, these shouldn’t resist the paint material applied over them after their application to the affected area.
Polysulphide sealants are supplied in two parts. One part is called Base material and the other part is called Accelerator.
The base material consists of polysulphide polymer liquid. It is white in colour while the accelerator is black in colour. On mixing, obviously, these produce a grey coloured material that has to be used within 2 hours of mixing the two parts. The mix ratio of the two materials has to be exactly as specified by the manufacturer for best results. Weight wise, most of the weight used shall be of base material while the accelerator weight should be small in comparison. Polysulphide sealants weigh almost 1.5 times the water and demand curing time to be fully effective. Temperature plays an important part in their getting cured at a fast rate.
Uses: Polysulphide sealants are almost shrinkage free and resist water and sun attack effectively. These have good adhesion properties and form a tough rubber seal on curing and drying. Therefore, these are capable of accommodating movements without cracking.
Polysulphide sealants are, therefore, ideal for sealing of moving joints in concrete slabs, columns and retaining walls etc. These are also suitable for repair of cracks, pipe joints, air conditioning duct joints and filling of expansion and contraction joints.
Depending upon the use, whether to be applied by pouring or by use of a gun, manufacturers supply them in pour grade and gun grade. These can be used to fill joints of even 50 mm width. These are used in cans for work of large magnitude.
Silicon sealants are the most commonly used sealants. These are transparent and take 10 to 20 minutes to form the sealing skin over the joint or gap. Their movement accommodation is not as high as in case of polysulphide sealants. These have a good resistance to high temperature. Apart from being available in transparent colour, these are produced in white and black colour also as certain situations demand these colours of the sealant. These sealants offer long term durability and provide the 'apply and forget' facility to the user.
Uses: Silicon sealants are mostly used in filling the gaps around the door and window frames and for fixing glass into aluminium or wooden frames. Their application is most easy as these are available in ready to use form.
Mostly, these are supplied in cartridges of 240 to 250 ml which can be provided with a nozzle to draw the sealant out.
Silicon sealants can fill the gaps and make them waterproof. On drying, these sealants get converted to flexible rubber like seals, have high durability and stay in place for a long time.
To make use, the sealant tubes, sealed at the top, pierced or cut to create a pin hole there. A nozzle supplied with the tube is fixed on top of it and the sealant tube is ready for application. For right and uniform application of sealants, an iron gun is used in which the tube can be easily fitted. A little pressure on the gun trigger extracts the sealant from the tube along the gap. After filling the gap with silicone sealant, the sealant is finished with a pallet knife available with the applicator. This knife draws out any air bubbles in the sealant layer.
As Silicon sealants are commonly used in houses, one may prefer to have one such sealant gun permanently and when required, a sealant tube can be purchased from the market for use.
These sealant tubes have short expiry period and should, therefore, be purchased afresh instead of storing them. After a few trials, even a house owner can use these sealants by observing the precautions mentioned on the sealant cartridge. For a 5 mm deep and 5 mm wide gap, a 300 ml sealant tube will fill up about 10 meters length of gap. This formula can be used to work out the number of tubes required for the gap filling work.
Like silicon sealants, acrylic sealants are also single component elastomeric acrylic sealants. These are white in colour. These convert into a sealing skin in about 20 minutes after their application. Their movement accommodation is lesser than silicon sealants and polysulphide sealants. These have low shrinkage property and hold excellent adhesion property to any kind of building material. These don’t sag even when applied in vertical joints and give good finish and appearance due to their acrylic property and white colour. Another advantage of them is that these accept any paint material readily when painted over them.
Uses: Acrylic sealants are best suited for gaps between window sills and for gaps in wooden or metallic window frames. These are supplied in ready to use tubes and thus are very convenient to use. These are suitable for both internal as well as external application and receive acrylic emulsion painting very well. These are supplied in 200 gm plastic tubes and can be readily applied. These sealants are well suited to fill joints of 4 mm to even 25 mm width. On drying, these become highly durable.
Polyurethane (PU) sealants
Polyurethane sealants are supplied in single component as well as in two parts. Single components are toothpaste like materials while for two component sealants, the mix ratio is similar to polysulphide sealants i.e. Base part has more weight than accelerator part. PU sealants consist of PU polymers, pigments and additives required to seal joints in buildings. PU sealants have wide application and in their case, the joint width to depth ratio is to be checked as per design given by the PU sealant manufacturer.
Polyurethane sealants are shrinkage free
These are UV resistant, become tough after application and can tolerate 25% movement like polysulphide sealants. These, too, have excellent adhesion properties to all materials.
Uses: Polyurethane sealants find wide use in high rise buildings, building construction including plumbing and sanitary ware joints and expansion and contraction joints. While some of them are ideal for horizontal application like in floors, some products are ideal for vertical and even overhead joint filling. Their selection should therefore be made as per area of application.
— The writer is HOD and Chief, Civil Engineering Department in a Punjab PSU
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