Sealants Playing Important Role in Today's Construction Projects.Sealants Playing Important Role in Today's Construction Projects.
The use of sealants in construction became more widespread in the 1950s and 1960s with the increasing use of curtain wall construction. The early 1950s saw the application of the new concept of curtain wall construction in high-rise structures throughout the world. The curtain wall construction concept was based on the use of a skeleton of steel or concrete that was then wrapped in a separate envelope or non-bearing curtain wall. This concept reduced the use of the low-rise bearing wall type construction and introduced greater flexibility in design through the use of precast concrete panels, glass or metal panels, unit masonry, and combinations of these materials. Construction becomes much faster, also these structures are inherently more flexible than bearing-wall construction, but present new problems in weatherproofing.
The introduction of the curtain wall was accompanied by the introduction of elastomeric sealants that would adhere to the various surfaces and take greater movement than had existed in the older expansion joints. The old oil-based caulking compounds no longer qualified, and the new materials became “SEALANTS” rather than caulks. However, the term “SEALANT” has been too broadly used and now includes a wide assortment of weatherproofing joint materials. Sealants or caulking compounds include viscous liquids, pastes, tapes, gaskets, and even waterproofing membranes. The materials can either cure to rubber or remain in a mastic stage.
Definition
A sealant is a substance used to block the passage of fluids through the surface or joints or openings in materials, a type of mechanical seal. In construction, sealants are sometimes synonymous with caulking and also serve the purposes of blocking dust, sound, and heat transmission. Sealants may be weak or strong, flexible or rigid, permanent or temporary. Sealants are not adhesives but some have adhesive qualities and are called adhesive-sealants or structural sealants.
The main difference between adhesives and sealants is that sealants typically have lower strength and higher elongation than do adhesives. Since the main objective of a sealant is to seal assemblies and joints, sealants need to have sufficient adhesion to the substrates and resistance to environmental conditions to remain bonded over the required life of the assembly. When sealants are used between substrates having different thermal coefficients of expansion or differing elongation under stress, they need to have adequate flexibility and elongation. Sealants generally contain inert filler material and are usually formulated with an elastomer to give the required flexibility and elongation. They usually have a paste consistency to allow the filling of gaps between substrates. Low shrinkage after application is often required. Many adhesive technologies can be formulated into sealants.
Function
Sealants, despite not having great strength, convey a number of properties. They seal top structures to the substrate and are particularly effective in waterproofing processes by keeping moisture out (or in) the components in which they are used. They can provide thermal and acoustical insulation, and may serve as fire barriers. They may have electrical properties, as well. Sealants can also be used for simple smoothing or filling. They are often called upon to perform several of these functions at once.
In general, the sealant has three basic functions: It fills a gap between two or more substrates; it forms a barrier through the physical properties of the sealant itself and by adhesion to the substrate; and, it maintains sealing properties for the expected lifetime, service conditions, and environments. The sealant performs these functions by way of correct formulation to achieve specific application and performance properties. Other than adhesives, however, there are few functional alternatives to the sealing process. Soldering or welding can perhaps be used as alternatives in certain instances, depending on the substrates and the relative movement that the substrates will see in service. However, the simplicity and reliability offered by organic elastomers usually make them the clear choice for performing these functions.
Applications
• Horizontal and vertical metal-to-metal and masonry-to-masonry expansion and control joints
• Dissimilar material joints, such as metal-to-masonry or concrete-to-wood
• Joints between precast concrete façade panels
• Spandrels
• Perimeter of doors and fixed window frames
• Exposed exterior masonry control joints
• Expansion or control joints in the curtain wall
• Joints in exterior walls
• Concealed masonry-to-floor structure joints
• Repair of larger cracks
Types of Sealants
There are many types of sealants in the market and they are formulated for different uses and applications, such as exterior cladding surfaces, glazing, roofing, or interior applications. Sealants vary in their initial cost and in how long they will last, also called service life. They also vary in their flexibility at different temperatures and their ability to bond to different surfaces.
Some sealants are not compatible with other sealants or with certain building materials and will react to form unsightly and messy by-products. These reactions can be unpredictable and may impair the intended function of some building components.
Latex sealants are water-based, easy to tool, and have a limited movement capacity of up to +/- 7.5 percent. They are typically used in light building and residential construction because of their limited movement capability. Latex sealants can be painted with latex paint, and are generally less expensive and easier to clean up than the high-performance sealants. Latex sealants may be best suited to interior finish applications. Some premium latex sealants may be appropriate for exterior use (appropriate service life) and have a movement capability of +/- 25%. Where they fall short is in situations where a high movement capability is necessary, such as for high-rise buildings and moving joints. Latex is also prone to shrink, pulling away from the substrate and leaving open gaps where water can penetrate.
Acrylic sealants are solvent-based, paintable, but are more difficult to the tool. They are used more in commercial and exterior applications than latex and have very limited movement capacity. They have low shrinkage properties and hold excellent adhesion properties to any kind of building material. They don't sag even when applied in vertical joints and give a good finish and appearance due to their acrylic property. They are suitable for sealing and filling joints around aluminum, wood, and UPVC window and door frames. However the external application of this form of sealant is very much ruled by the weather, conditions must be dry and preferably warm when applying.
Butyl sealants are solvent-based, synthetic rubber materials demonstrating strong adhesion to a wide variety of substrates. They have excellent weathering characteristics but tend to be stringy and difficult to apply. They generally have limited movement accommodation. Butyls also have poor resistance to abrasion and shear forces, which limits their performance in demanding applications. Butyl sealants are sometimes used in curtain wall systems where adhesion to rubber materials is required.
Polysulfide sealant is a high-performance sealant that is based on a synthetic polysulfide polymer or rubber. Polysulfide Sealants are particularly water and chemical-resistant. They are designed to seal joints subject to the movement where a tough, flexible, watertight seal is required. They offer the best option for expansion joints. Typical applications include swimming pools, fountains, cooling towers, fuel and chemical storage tanks, wastewater treatment, and petrochemical plants. Polysulfide sealants often require a primer. They tend to be relatively expensive.
Silicone sealants are used in a wide variety of building applications because of their strong performance characteristics: UV resistance, temperature resistance, highest movement capability, generally longer service life, and continued flexibility over time. Silicone sealants can have a strong odor and take considerable time to fully cure. They can be used structurally in glass assemblies. The cost for silicone sealants is in the high range. Pure silicone sealants are not paintable. They are easily vandalized and tend to collect dirt.
Polyurethane sealants have first-rate resistance to water, permanent elasticity under all climate conditions, good movement capability, and excellent adhesion to many substrates. Unlike silicone sealants, they can be painted. They can be stiff and more difficult to apply and tool than silicone and cannot be used in structural glass assemblies. Polyurethane sealants find wide use in high-rise buildings, building construction including plumbing and sanitary ware joints and expansion and contraction joints.
MS Polymer sealants are “Hybrids”, they have chains (silyl) that modify both silicone and polyurethane sealants (MS stands for silyl-modified), combining some of the strengths of each. Their chemical profiles are better because they are solvent and isocyanate free. MS Polymer sealants have great durability, flexibility, excellent adhesion to non-porous surfaces, will bond to damp surfaces, and are paintable. They are best used as an adhesive or sealant on surfaces that experience frequent vibration or strain. They are also ideal for gluing panels, skirting boards, windows, mirrors, and insulation materials.
Intumescent sealants are designed for fire retarding constructions and generally have acoustic properties. They are suitable for indoor joints where a fire retardant seal is required to prevent the passage of smoke and vapors. Roof and Gutter sealants are rubberized sealants specially developed for repairing leaking gutters, downpipes, and flashings. They can also be used for bedding roof sheets and felts.
Sealant Properties
More joint leakage is caused by using the wrong sealant than is caused by specifying the right sealant badly. When selecting a sealant, consider the properties that most impact the specific application at hand. Key sealant properties to evaluate include:
Consistency: Joint sealants come in two consistencies: non-sag and pourable. Vertical joints require non-sag sealants so the sealant will not run down out of the joint. Horizontal joints can use either non-sag or pourable sealants, but the pourable variety will yield better-looking results with less effort—the sealant is poured into the joint and levels itself under gravity.
Polyurethane sealants are usually available in both consistencies. Acrylic latex, butyl, solvent-based acrylic, and silicone are non-sag only.
Durability: The expected service life of a sealant under ideal conditions may not be the same as the actual field lifespan, especially if the sealant was misapplied or incompatible with the substrate. Generally speaking, silicones have the longest service life, estimated at 20 years or more, while some acrylics and butyls were last a little more than five years.
Exposure resistance: The best exterior-grade sealants perform well in response to sun, temperature extremes, and moisture. Measures of exposure resistance include flexibility at low temperatures, freeze-thaw resistance, UV stability, and susceptibility to heat aging. Acrylic latex is not suitable for exterior exposure unless it is not subject to rain or freezing temperatures. Silicone sealants are eminently suitable for exterior exposure from a durability point of view, but they tend to “pick up” atmospheric dust.
Hardness: The harder a sealant is, the greater its resistance to traffic and vandalism. However, as hardness increases, flexibility decreases, so the trick is to find the right balance of damage resistance and movement capability for a given situation.
Movement Capability: This is the gauge of how many extensions and compression the sealant can withstand without either pulling away from the sides of the joint or failing in the body of the sealant. It is measured as a plus/minus percentage of the joint width at the time of installation. Movement capability over 7.5 percent rules out latex and butyl sealants. Movement capability over 25 percent is available but not universal in polyurethane and silicone sealants. Movement capability of over 50 percent is rare.
Modulus: Short for “modulus of elasticity,” modulus refers to sealant stress at a given elongation. Low-modulus sealants usually have high movement capability, and vice versa, although this is not always the case. Low-modulus sealants are generally used for delicate substrates, for which it is desirable to have low stress at the joint edge. High-modulus sealants are best used for static, non-moving joints because they exert a very high force on the substrate when stretched. Medium-modulus sealants are general-purpose products that balance stress at the adhesion surface with stiffness of the sealant.
Adhesion: Suitability for a specific substrate usually comes down to adhesion. Minimum adhesion is usually taken for granted, but there are variations—most of which cannot easily be quantified. Polyurethanes generally have the best adhesion, followed by silicones, then butyl and acrylic.
Staining: The components of some sealants may leach into porous substrates, particularly natural stone, leaving a visible stain. To evaluate compatibility with the substrate, even sealant rated as non-staining should be tested in an unobtrusive area before use.
Ease of application: Curing characteristics and tool ability are the two major factors affecting a sealant's ease of application. Toolability refers to the ease of achieving a smooth surface of correct geometry. Curing properties vary widely, from fast curing sealants to those, such as polyisobutylene, that are designed to remain uncured
Current Issue
12-2025
