Generally, sealants are classified according to:
Their chemical types, such as polyurethanes, polysulfides, silicones, acrylics, etc.
Their elasticity, such as the caulks (which cannot withstand deformation), the plastomeric sealants and the elastomeric sealants,
Their form, such as those supplied in cartridges which are extruded on-site, the preformed sealants (supplied as dry tapes, ribbons, or extruded shapes), or the hot-melt sealants.
Let study each class separately.
Earlier (before 1950s), joints between different materials, such as between glass, metals, wood, concrete, etc. were filled with some traditional caulks, based on:
Oleoresins, such as linseed oil, or
Bitumen and tar in civil engineering work.
These formulations could only withstand a few percent elongation at break, and moreover they had a bad resistance to weathering.
| Material | Properties |
| Linseed oil putties |
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| lmproved oleoresinous putties or caulks |
|
Bitumen based formulations - ln civil engineering applications, the gaps between parts or works may be quite high, so high performances polymers would be too expensive to fill large volumes. Also, civil engineering people were accustomed to use bitumen and tar.
Therefore, many applications still use bitumen or tar sealants, but the formulations have been often improved, starting in the seventies, by adding rubbers, styrenic polymers such as SBS, or polyurethanes, in small amounts. The pure bitumen or tar compounds may only withstand a few percent elongation at break, and the best-modified formulation may go up to 10 -15% and the movement capabilities in service are only 20 - 25% of the elongation at break to be safe.
The fast development of prefabricated parts in the construction and development of new synthetic polymers resulted in the disappearance of these caulks from the market in the years 1950 to 1975.
Synthetic polymers allow to manufacture high performances sealants with very high elasticity and long durability and could be "tailor-made" to any specific requirement through adequate formulation. Some of the polymer classes are discussed in the table below.
| Material | Properties |
| Polybutene |
|
| Polyisobutylene (PIB) |
|
| Butyl rubber |
|
| Butyl and polyisobutylene hot melt sealants |
|
There are 2 types of acrylic sealants:
Emulsion-based
Solvent-based
They display good adhesion to the absorbing materials such as wood, concrete, plaster, and they also have fairly good adhesion to metals and glass, although not as good as silicones on glass.
They are only plastomeric, with a maximum movement capability in service of 10 to 15%.
Dry solids vary from 80 to 85% so that they show 10 to 20% shrinkage during drying through the evaporation of the water that they contain.
They have fair to good weather resistance because they are sensitive to water. One can expect 15 years of durability for outside use.
They have very good resistance to UV and discoloration, and may be formulated in a large variety of colors in order to match the colors or the materials (brown like wood, white for plastic windows or tiles, grey like concrete or aluminum like the windows).
Acrylic solvent-based sealants have outstanding adhesion to many materials, such as concrete, aluminum, steel, wood etc. They have excellent weather resistance, resist to UV and staining.
Acrylic solvent-based sealants are only plastomeric, their movement capability is only 10% for long-range service outdoor. They are generally used for joints, such as:
Curtain walls joints, exterior sidings,
Masonry prefabricated panels,
Metal to concrete joints such as joints between metal windows and concrete,
Wood to concrete joints (between wood windows and concrete).
In these sealants, the base polymer is usually an 80% solids acrylic dissolution which accounts for 50% of the total weight of the formula. There is also some 50% of fillers (mainly calcium carbonate, plus some pyrogenated silica, magnesium silicate and/or talc or clay), a small amount of plasticizer may be added such as DOP, DBP, pine oil may be added as a filler dispersant, and some solvent is added in order to adjust viscosity.
The maximum solids content is usually 85% so that there is some shrinking during drying, therefore it is necessary to start with an elastomeric acrylic polymer and to add some plasticizer so that the shrinkage will not bring too much stress at the interface between the sealant and the materials to be jointed.
Common Additives Used in Acrylic Sealants
Fillers reinforce and increase the volume of the sealant and lower the cost. Common fillers used are calcium carbonate, clays, barium sulfate and fumed silica. Fumed silica, a thixotropic filler, reduces the sag and improves gunnability.
Plasticizers, such as phthalates, dibenzoates, propylene glycol alkyl phenyl ether, etc. increase flexibility and elongation, and reduce the glass transition temperature which improves low-temperature flexibility.
Dispersing aids improve the incorporation of fillers and improve also the viscosity and package stability (if there are no dispersing aids, the fillers will slowly absorb the polymer at its surface and consequently the viscosity will increase during the shelf life). Low molecular weight polycarboxylic acid salts may be used as dispersing agents.
Silanes may be used also to improve the adhesion to impervious substrates such as metals and glass. The acrylic sealants which contain small proportions of silanes are often called siliconized acrylics.
» Get Inspired to Formulate Acrylic Sealants using Starting Point Formulations
The 4 chemical types of sealants which display elastomeric properties are the following:
These sealants have been developed in the sixties in the USA by THIOKOL Corporation, and they were the first elastomeric sealants. They are based on polymers with -SH end groups with an average molecular weight of 4000.
One such example is THIOKOL LP® 32 which has the following formula:
Curing - Curing proceeds by converting the -SH termination into disulfide bonds. This is achieved by oxidizing agents like peroxides, PbO2 and MnO2. It is accelerated by an alkaline environment.
One component polysulfide has limited package stability. A dry to the touch skin will form after 30 minutes to 1 hour at 20°C and 50 to 60% RH, and then the cure will progress into the depth of the sealant at a speed which depends on the thickness of the joint, the temperature and the humidity of ambient air. The cure of polysulfide is slow: it takes one week to reach 50% of ultimate strength. Shrinkage after cure is negligible.
Hardness – Depending on the formulation, hardness may vary from Shore A 20, equal to soft rubber, for vertical joints such as curtain walls, to 50, (hard rubber hardness) with heavily filled formulations, for floor and concrete joints or aircraft runways, where the joints must resist penetration and traffic.
Solvent, fuel and oil resistance – They have excellent resistance, this is the reason why polysulfides have been used widely and are still used for airport runways joints.
Water resistance and weathering - Polysulfide sealants have excellent resistance to water, oxidation, sunlight and weathering. They maintain excellent adhesion after UV and water exposure. A durability of 20 years outside in normal conditions may be expected. Polysulfides are waterproof to water vapor so that they are used for double insulated windows for the exterior seal.
Modulus, Ultimate elongation, Service elongation - Most Polysulfide have high modulus and fairly high elongation at break (100 to 200%). Because modulus is high, these sealants will develop high stresses when elongated, so the recommendation is to use polysulfide only at 15 to 25% service elongation. They have poor puncture resistance.
Creep and stress relaxation - Creep test is a recording of elongation versus time at a constant load. Figure 1 shows a typical creep curve for polysulfide sealants. We can see that the behavior of polysulfides is partly elastic and partly viscous or plastic, and after unloading there is an irreversible deformation resulting from the plastic creep. Elastic recovery is only 60 to 80%.
Application of Polysulfide Sealants: Because they are not 100% elastic and their prices are fairly high, polysulfide sealants are less and less used, and have been replaced by silicones and polyurethanes. However, some jobs still use it:
In Construction: floor joints between concrete and/or metal elements, expansion joints, curtain walls joints, joints between prefabricated panels (concrete panels…), double insulated windows.
In Civil engineering: joints between concrete slabs in airport runways, joints in concrete bridges.
» Explore All Polysulfide Polymers Suitable for Sealants!
Silicone sealants are based on polydiorgano siloxanes polymers, which have the following general formula:
For example, PDMS:
Two main types of silicone sealant are following:
One component silicone sealant is formulated by mixing and reacting in anhydrous conditions the silanol-functional polysiloxane with an excess of hydrolyzable trifunctional silane RSiX3 as shown here under
When the sealant is extruded, the atmospheric moisture reacts with the hydrolyzable groups, and the silanol condenses. This reaction continues until a 3-dimensional network is formed. The by-products which result from the cure may be acetic acid (which gives a typical smell), oximes, amides, alcohols.
Two components silicones are used only for architectural glazing because this glazing is made in the factory to obtain preglazed windows and panels.
These sealants are 2 components products with neutral cure, which have:
Very good adhesion to glass and metals,
Tensile strength up to 1 MPa,
Excellent tear resistance,
Moderate elongation at break (100 to 160%),
Shore A hardness ranging from 35 to 45,
Excellent resistance to ozone, UV, aging, heat (service temperature from -40° to +150°C).
The sealing operation can only be made in the factory prior to installation on-site, in order to guarantee excellent bonds for maximum safety.
Silicone sealants are the most successful sealants since the seventies because they display a combination of many excellent and important features, such as:
Excellent resistance to water, chemicals, weathering, aging, heat, temperature cycles (heat and cold), and consequently excellent durability up to 40 years.
Modulus may be low or higher according to the formulations, elongation at break is very high, up to 500%, so that the service elongation may reach 25 to 50% which are the best values achievable for all sealants.
Price is now very moderate because they are produced in very large quantities.
There are 2 types of polyurethane sealants:
The single-component sealants which are terminated by isocyanate groups -NCO and react with the ambient humidity,
The 2 components sealants where part A is a polymer with -NCO terminal groups and Part B a polymer with hydroxyl -OH terminal groups, these 2 groups reacting together in several well-known modes and reactions.
By varying polymer composition, NCO/OH ratio, catalyst, a wide range of products and properties may be obtained.
General Properties of Polyurethane Sealants
All PU sealants have:
Good elongation at break: 250 to 600%,
Low to high modulus: 0.25 to 1 MPa
Excellent elastic recovery higher than 90%
Excellent abrasion resistance and tear strength, their resistance to indentation makes them the best sealants for floor joints,
Service elongation range from 12 to 25% according to formulations
Excellent adhesion to a wide variety of substrates: concrete, metals (preferably with a primer), wood, PVC
Fair resistance to water (some formulation may be sensitive to hydrolysis), excellent aging resistance, a 20 years durability can be achieved or expected
The only drawbacks include:
Slow cure (skin over time 5 to 20 minutes at 20°C and 50% RH, complete cure after 2 to 7 days at a speed of 2 mm/day)
Resistance to UV is only fair
Moderate resistance to chemicals, oils, solvents, acids and alkalis, and moderate resistance to hydrolysis
Some Uses of Polyurethane Sealants in Construction
Pourable sealant for floor joints
One component sealant for curtain walls joints
One component sealant for concrete prefabricated panels
Other utilization for one component PU sealants are: installation of wood and metal windows into the masonry, sealing roofs, expansion joints in masonry.
» Related Read: A Comprehensive Guide to Polyurethane Resins (PU) for Adhesives & Sealants
These are relatively new products. These are polyethers terminated with silyl groups. Most of these sealants are one component that is cured by reaction with ambient air humidity. They cure at a speed of 3 mm/day, faster than one component PU. Their key properties and applications are listed below.
| Properties | Applications |
|
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These are polyurethane polyester foam strips which are impregnated with various sealing tacky compounds (butyl, PIB...) in order to have a sealing tape which must be compressed between the parts to be sealed.
It is used for sealing of prefabricated concrete panels, curtain walls, installation of windows (wood, aluminum, or PVC), wood panels.
Back-up materials are usually foam strips round or rectangular sections which are inserted at the bottom of the joints, before application of the sealant. This has 2 purposes:
To control the depth of the sealant in the joint
To provide support for the sealant in horizontal joints
The sealant should not stick to the backup material and the solvents of the sealant should not affect the backup material.
Back-up materials are usually polyurethane or polyethylene foams, sometimes neoprene foams, and other materials.
Foams may be open cells or closed cells: the choice between these two depends on the type of sealant used and the job site conditions. Users will refer to the sealant supplier for advice.
