Sealants for Building and Construction

Sealants Used for Building Joints

The building and construction industry employs multiple materials, like metals, concrete, etc. as well as many prefabricated parts, such as:

Sandwich panels
Windows and doors (made of metals, wood, PVC, etc.)
Partitions (often made of plasterboard)
Prefabricated concrete slabs for floors and exterior walls, etc.

Sealants are employed to connect and join the various parts and materials to the main structure and to themselves. They help in closing the gaps between the elements and surfaces of the construction and thus, prevent fluids and other substances from passing through surfaces and mechanical joints.

Sealants serve the following basic functions in the building and construction industry:

Filling the gap between two or more components
Providing a protective impermeable barrier, through which substances cannot pass
Maintaining their sealing properties through their expected lifetime, under the service conditions and environments for which they are specified

Also, another important requirement for sealing compounds is the high flexibility in order to tolerate movements between different materials used. These movements can happen due to:

Expansion or shrinkage because of thermal variations,
Dimensional variations due to variations of moisture content,
Deflections under loads,
Wind pressure, etc.

Various Types of Movements of the Joints and Sealants

These movements usually occur due to the different thermal coefficients of expansion of materials as shown in the table below.

Material	Coefficient of Linear Expansion (m/m-°C x 10^-6)
Clay, masonry (Brick, clay or shale)
Brick, fire clay	5 to 6
Tile, clay or shale	6.0
Tile, fire clay Material	4.5
Concrete
Gravel aggregate	10.0
Lightweight structural	8.1
Concrete, masonry
Cinder aggregate	5.6
Dense aggregate	9.4
Expanded-shale aggregate	7.7
Expanded-slag aggregate	8.3
Volcanic pumice & aggregate	7.4
Cellular concrete	11.0
Metals
Aluminum	23.8
Brass, red 230	18.6
Copper	16.5
Iron
Cast gray	10.6
Wrought	13.3
Lead, common	29.3
Monel	14.0
Stainless steel
Type 302, 304	17.0
Structural steel	11.5
Zinc	36.0
Glass, plate	8.0
Plaster
Gypsum aggregate	13.7
Plasterboard	12.0
Plastics, composites
Acrylics	80.0
Lexan® (Polycarbonate)	67.0
Flexiglas®	70.0
Polyesters, glass reinforced	18-25
PVC	59.0
Natural stones
Granite	8.0
Limestone	6.5
Marble	13.0
Basalt	9.0

Coefficients of Linear Expansion of Common Building Materials
Therefore, to achieve the desired performance & functions, it is necessary to match the most suitable sealant to the substrate materials that will be joined, i.e. one that will have adequate bonding properties and be flexible enough to tolerate anticipated movement, and so on.

Types of Construction Sealants

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.

Traditional Caulks or Putties
Sealants Based on Synthetic Polymers and Rubbers
Acrylic Sealants
Elastomeric Sealants
Impregnated Foam Sealants
Back-up Materials

Traditional Caulks or Putties

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	They contain 10 to 15% of linseed oil filled with mineral fillers (calcium carbonate). The linseed oil dries by oxidation in the air. Oxidation continues for the whole life and the product eventually becomes quite hard, brittle, after some years, and non-flexible with very little capability of movement. They were used mostly for the glazing of glass windows into wood or metal sash.
lmproved oleoresinous putties or caulks	They were based on blown soy or linseed oils, filled with calcium carbonate and fibrous talc, and some plasticizers were added to improve the plasticity (for instance fatty acids, DOP...). In the best cases, the elongation at break could reach 5% which was not enough for prefabricating techniques.

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.

Sealants Based on Synthetic Polymers and Rubbers

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	It is a low molecular weight polymer, which is liquid, tacky, non-drying and cheap. These polymers are often blended with fillers (calcium carbonate, talc, clays) and fatty acids. A small amount of solvent may be added to control viscosity. Sealant formulations based on polybutene are set only through drying of the solvent. They are used in construction to make non-curing sealants for curtain walls, metal to metal joints when elasticity is not important. They are also used to manufacture preformed ribbons and tapes for glazing, bedding compounds in windows. Polybutenes are frequently mixed with butyl rubber to act as a plasticizer.
Polyisobutylene (PIB)	It is a permanently tacky polymer and used only to modify other sealants such as oleoresinous or butyl rubber. PIB Sealants may be also used in bedding compounds in the glazing industry.
Butyl rubber	Butyl rubber is a copolymer of isobutylene and isoprene. It contains 2 moles percent in saturation. Butyl rubber is impermeable to gases, it has a fairly good weather and oxygen resistance. It shows some elasticity (elongation at break up to 40%, so that it may be used in joints with movements up to 15%. Formulations include: 20% butyl rubber, 5 to 10% tackifying resins such as modified or hydrogenated rosin or hydrocarbon resin are necessary to impart good adhesion to metals and glass, 50 to 60% mineral fillers (calcium carbonate, fibrous talc, clay and others), and 20 to 25% solvents such as mineral spirits and other solvents to dissolve and mix all the components and to get the required viscosity. Polybutene is often added as a plasticizer. Butyl gun grade sealants may dry and set by evaporation of the solvent and absorption of the solvent into the porous and absorbing substrates (wood, concrete) but there are also curing types which cure by some slow crosslinking after a period of time. The extruded tapes and ribbons are 100% solids so that there is no shrinking.
Butyl and polyisobutylene hot melt sealants	These are special products which are used as sealants for double (insulated) window sealing against penetration of humidity (into the space between the 2 glass panels).

Acrylic Sealants

There are 2 types of acrylic sealants:

Emulsion-based
Solvent-based

Acrylic Emulsion Sealants

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).

Solvent-based Acrylic Sealants

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

Elastomeric Sealants

The 4 chemical types of sealants which display elastomeric properties are the following:

Polysulfide sealants
Silicone sealants
Polyurethane sealants
MS Polymer sealants

These sealants may be considered as high performances sealants because they have
high capabilities of movement, service elongation from 15 to 40%.

Polysulfide Sealants

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:

HS(–C₂H₄OCH₂OC₂H₄–SS–)C₂H₄OCH₂C₂H₄–SH

Properties of Polysulfide Sealants

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

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.

Many silicone sealants used in construction are one component products,
because users do not want to mix 2 components on-site, and
there are different types of 1 component silicones

Silicone Sealants for Architectural Glazing

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.

Polyurethane Sealants

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.

MS Polymers 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

Skin over time 15 to 20 minutes.
Service elongation 25%, elongation at break 150 to 350%, elastic recovery higher than 70%.
Tensile strength 1 MPa, modulus .8 MPa.
They meet the ISO standard 11600g, class 25hm (high modulus) They have excellent adhesion on metals, plastics, wood, ceramics, without primers.
Excellent resistance to weathering, to water, they can withstand at least 15 years of service life, but we do not have a longer experience so far, except in Japan.
Although they have good adhesion to glass, they are not recommended for that because long exposure to UV would degrade this adhesion.
Shore a hardness around 40.

Expansion joints on concrete and metals.
Joints around windows and doors.
Joints on natural stones, because they do not stain these stones.
Glazing between double insulated windows and metal, PVC or wood frames.
Bonding and jointing wood parquets inside and outside (ship decks).

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Impregnated Foam Sealants

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

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.

Various types of movements of the joints and sealants

Technical Characteristics of Sealants by Use

A sealant when it is delivered in the original packaging (cartridges or sometimes drums) is a paste. This paste is applied into the gap between 2 parts of the construction, then it must be leveled, then it will dry or cure at ambient temperature and it will become plastic or elastomeric joint having the required properties: gap-filling, elasticity, adhesion to the substrates, water resistance, etc.

We will study these properties in chronological order as they appear on-site during the installation.

Temperature and Humidity During Application

Construction sealants are applied on-site, at various temperature, according to the climate and the time of the year. Most sealants will not cure properly if the outside temperature is too cold (less than 5 to 10°C) and they will dry or set to fast if the temperature is too high (more than 40°C). Thus, the worker should respect the manufacturer's instructions for working conditions.

PUR sealants are the only ones that would accept some humidity on the surface of /or inside the substrate because PUR reacts with humidity. For other sealants, this humidity will be detrimental because it will prevent adhesion.

Viscosity, Non-sagging Properties or Slump Resistance

The wall sealants should be non-sagging because as they are applied on walls they should stay in place without any deformation or flow or sagging. European standard EN 27390 or ISO 7390 provides a test method for the resistance to vertical sagging, and stipulate that it should be less than 3 mm in this specific test.

Floor sealants must flow into the joints, but just enough to fill the joint, because anyway they will be forced into the joint by the worker with the proper tool.

On the other hand, the sealant must be easily extrudable from the hand cartridges, by using a handgun or sometimes an air-powered gun.

Sealants are heavy, thick pastes, and therefore their viscosity (usually in the range 80000 to 400 000 m.Pas) is not meaningful to the end-user.

Therefore, the sealants manufacturers use a test in order to measure the flow rate: ASTM C 603 standard measure this by extruding 200 grams of sealant through a 5 mm hole under 3 bars pressure, at various temperatures.

Mode and Time of Setting/Curing

Most of the modern sealants used in construction are now one component sealants that will set and cure by a chemical reaction with humidity in the air. This is the case of silicone, PUR and MS polymers sealants. This reaction progresses at a speed of 1 mm inside the mass of the sealant in a few hours, and it will thus take 1 to several days to get a full cure in the whole thickness of the joint. These sealants are of the elastomer (rubber-like) type.

Some sealants are plastic polymers which set by drying only, such as acrylic water-based sealants, or the older oleoresinous, or rubber/solvent-based sealants. Here the drying comes from the evaporation of the water or the solvent, so that the surface of the sealant will be dry to the touch after 30 to 60 minutes, and then the drying will progress slowly into the depth of the joint.

In construction, 2 components sealants may be used, but very seldom, (PUR or silicones or thiokols) because they are not convenient to use on-site. They cure faster than the one-component sealants. They have a limited "pot life", which is the maximum time the worker can wait between mixing and application.

The past oleoresins or bitumen sealants had 100% solids and they would remain plastic until they would become oxidized by aging in the air and become harder. Then they would eventually crack.

Preformed putty sealants are plastic polymeric dry products, based on butyl or oleoresins, 100% solids, shaped by the manufacturers as ribbons, cords or ropes, from 5 to 15 mm diameters. They do not cure or dry, they always remain plastic, and have only a fair resistance to aging due to their formulation.

The last type is the preformed rubber gaskets which are pressed also between the parts to be sealed: this is often used to install window glass panels into windows frames. We will not study these gaskets here, because these are not sealants.

Cross Section and Width of the Sealant

Some sealants are elastomeric and will accept wide variations of the joint width, some are only plastic and do not withstand wide movements.

Therefore, in order to accommodate movements of the joint, it is advisable to have wide joints.

Depth of the Sealant

The depth of the sealant should be always less than its width in order to minimize the stresses resulting from the deformation of the surface of the sealant.

The following rules are used:

Minimum dimensions are 5 x 5 mm,
For width from 5 to 12 mm, the depth should be slightly less than width,
For width from 12 to 25 mm, the depth should be 8 to 12 mm,
For width higher than 25 mm, the depth should be 12 to 18 mm according to the chemical type of joint and should be preferably half of the width.

The depth of joints is adjusted using a backup material, which is usually a foam strip inserted and compressed between the two lips of the joint.

Consumption

It depends on the cross-section of the joint and the specific gravity

"Dry to the Touch" Time

We have explained above that after application the sealant will dry or cure on the surface after a certain time and it will become dry to the touch : this may take from 20 min to 1 or 2 hours depending on the type of sealant, mode of curing, temperature and humidity.

ASTM C 2377 - 84 provides a test for measuring tack-free time of sealants and caulking compounds.

Shrinking

When the sealants are cured by chemical reactions and have 100% solids, they do not show any shrinkage on curing.

But other sealants which dry by evaporation of water or solvents and have much less than 100% solids will show some shrinking during drying because the departure of volatile compounds will cause a reduction of the volume.

The ASTM standard C 733 may be used for measuring shrinkage.

Physical and Mechanical Characteristics of Sealants

Adhesion to the Substrates

The adhesion of the sealants to various substrates depends on the sealant type and substrates.

PUR sealants have very good adhesion to many different materials: metals, concrete, cement, wood, glass, plastics such as PVC.
With silicones, primers may be necessary to get good adhesion to some metals and plastics, adhesion to glass is always excellent. Silanes primers are used.

Sealants manufacturers should indicate clearly in their technical data sheets the adhesion of their sealants on the various materials used in construction and civil engineering, with and without primers.

Sealants have good adhesion to different materials

Refer to the chemical types section to get detailed information on the adhesion of various types of sealants on various substrates.

Test Methods to Measure Adhesion

When the sealant is stressed during the increase of the width of the joint, if the sealant has a high modulus, the bonds to the lips of the joint are submitted to high tensile stresses, and this may break the bond. Therefore, standard test methods have been developed to measure the adhesion on substrates under tensile stress. Let us mention for instance the European standards:

ISO 9046 or EN 29046: measurement of adhesion and cohesion at a constant temperature,
ISO 9047 and EN 85 519: measurement of adhesion and cohesion at variable temperature.

This tensile test must also be performed after water immersion and artificial weathering (for instance with an equipment called weather-o-meter in which several cycles are realized: water spray at different temperatures, UV light, drying and again water spray…).

Let us mention here again some ISO and US standards:

ISO 10591, Determination of tensile properties after water immersion,
ISO 10590, Determination of tensile properties at maintained extension after immersion in water,
ASTM C 1135 Determination of tensile adhesion properties of structural sealants.

The tensile test may be realized until the bond failure (ISO standard 28339), and it is agreed that the sealant should only be stressed up to 25% of this stress at failure, but we will see that ISO standard 11600 has set specific requirements and classification of the sealants, according to maximum service elongation.

Modulus of Elasticity or Tensile Modulus

Figure below shows typical curves of stress versus deformation.

Curves stress/strain for different chemical types of chemical sealants
(test specimen steel or Aluminum 25 x 9.5 mm joint thickness 1.4 mm tested in shear)

Usually the elasticity modulus is defined as the stress measured at 50 or 100% elongation. Modulus is measured by ISO standard 8339: Determination of tensile properties. Modulus gives a very useful indication of the stresses which act on the lips of the joint when it is elongated.

To reduce these stresses, it is recommended to use low modulus sealants, such as the low modulus silicone shown in the figure.

In the ISO standard, DIS 11600 elastomeric sealants are classified according to their secant tensile modulus, besides other specifications, as we will discuss hereunder.

		Classes							Test Method
Properties		25LM	25HM	20LM	20HM	12.5E	12.5P	7.5
Elastic recovery, %		≥70	≥70	≥60	≥60	≥40	-	-	ISO 7389
Tensile properties
Secant Tensile Modulus	at 23°C, MPa	≤0.4	>0.4	≤0.4	>0.4	-	-	-	ISO 8339
	at 20°C, MPa	≤0.6	>0.6	≤0.6	>0.6	-	-	-
	at an extension of, %	200	200	160	160	-	-	-
Elongation at break, %		-	-	-	-	-	≥200	≥120	ISO 8339
Adhesion/cohesion properties	at variable temperature	nf	nf	nf	nf	nf	-	-	ISO 90547
Adhesion/cohesion properties	at constant temperature	-	-	-	-	-	nf	nf	ISO 90546
Tensile properties at maintained extension		nf	nf	nf	nf	nf	-	-	ISO 8340
Tensile properties at maintained extension after water immersion		nf	nf	nf	nf	nf	-	-	ISO 10590
Tensile properties after water immersion Elongation at break, %		-	-	-	-	-	≥100	≥20	ISO 10591
Loss of volume, %		≤10	≤10	≤10	≤10	≤25	≤25	≤25	ISO 10563

ISO/DIS 11600 Requirements for Construction Sealants
*Maximum 25 % change of volume (upon cure) for water-based latex sealants
For detailed specification of test conditions see ISO/DIS 11600

Failure mode:nf: no failure (all three specimens pass the test) A specimen has failed the test if the sum of the adhesive and cohesive failures exceed 5% of the sealant/substrate interfacial area (600 mm2). There is an ongoing discussion in ISO TC 59/SC8 on how to define failure criteria. It is likely that for the cyclic movement tests (ISO 9046 and ISO 9047), the 5% value will serve as a limit for the failure after the first movement cycle. Specimens that have passed the first movement cycle are considered as having failed the test, if the sum of additional adhesive or cohesive failures in subsequent movement cycles exceeds 100%.

Elastic Recovery and Plastic Flow

When the stresses which caused elongation are withdrawn, the sealant may come back to its initial width (full recovery) or may display only partial recovery. This is called elastic recovery and it is measured with the ISO 7389 and NF EN 27389 (July 1991): Sealants, determining elastic recovery.

ISO 7389 and ASTM C 736-82 test standards can be used for determining elastic recovery and measuring extension recovery and adhesion of latex sealing compounds after artificial weathering.

Good elastomeric sealants like silicones and PUR return almost completely to their original dimension. On the other hand, the plastic sealants (such as butyl, acrylic…) do not return to this original dimension at it is shown in the figure and they show some plastic flow and residual deformation.

Typical plastic flow curve

ISO standard 11600 considers that sealants are elastomeric if their elastic recovery, according to standard ISO 7389 is higher than 60%. Usually, to measure the relaxation of stresses, the sealant is elongated by 25 or 50%, then the test specimen is maintained at this elongation and the stresses are measured against time, which gives the curve as shown in the figure below.

Stress relaxation typical curve for a sealant

Elongation at Break

The elastomeric sealants such as silicones may withstand very high elongation at break up to 400 to 500%. Thus, the elongation at break is used, in ISO 11600, only to differentiate the various plastic sealants. Elongation at break is measured according to ISO 8339.

Maximum Service Elongation

This is the service elongation that a given sealant may withstand for a long-term exposure outside, by taking into account a safety factor for the weathering/aging effects. The European and ISO standard 11600 have defined up to 7 classes of construction sealants, according to the maximum service elongation, and also according to 9 other properties, those which we studied above.

Resistance to Compression

This test evaluates the behavior of the sealant under compression: it should not flow out of the joint during compression. The curve "deformation against compression stress" is plotted.

ISO 11432 is used to measure the compression properties.

Hardness and Resistance to Indentation and Tearing

This is important for the floor sealants which should withstand traffic. ASTM C 661 is used to measure hardness with a durometer according to Shore A or D hardness.

Resistance to Heat, Cold and Temperatures Cycles

The exterior sealants must withstand variations of temperature, according to the climates and countries where they are installed. Heat, rain and sunlight may degrade the sealants through oxidation, exudation of low molecular weight content, extraction of additives such as plasticizers, etc. in these cases the sealant will harden, degrade and eventually crack.

Several standards have been designed to measures the effects of these agents:

French standard NF P 85-512 measure the diffusion of some constituents of the sealant,
ASTM C 793-80 Test for effects of accelerated weathering of elastomeric joint sealants,
ISO 10563: Determination of change of weight and volume,
ASTM C 765-84, test for low-temperature flexibility of preformed sealing tapes, etc.

Durability

Resistance to water - Of course, all the modern polymers which are used for sealants have good resistance to water for outside exposure to rain. But water may penetrate between the sealant and the substrate and if this substrate is cement the alkaline conditions plus water may degrade the adhesion of the sealant. The user should ask the sealant manufacturer about its resistance in such conditions, which primers should be used.

We have indicated above the standards which are used for measuring adhesion/cohesion after water immersion. ASTM C 1247 may be used to measure the durability of sealants exposed to continuous immersion in liquids.

Resistance to weathering - ASTM C 793-80 provides a test for measuring the effects of accelerated weathering on elastomeric sealants.

Resistance to sunlight, UV - The old sealants and putties such as oleoresins and the butyl sealants have poor resistance to sunlight, UV and outside aging. They oxidize in the air, become brittle and eventually crack.

Modern sealants (PUR, silicone, thiokol) all have a long-term resistance to outside exposure.

ISO standard 11431 and ASTM C 718-83 provide test methods to measure adhesion and cohesion after exposure to light, through the glass. ASTM standard C 718 also allows to measure the UV resistance of the sealants

Mold growth resistance - Sealants must have protection against mold growth included in the formulation.

Resistance to hot-cold cycles - These alternated cycles can damage the sealant after a number of cycles. Refer to the same standards which have been mentioned above. To summarize let us say that durability during outside exposure may be estimated approximately by combining some of the above tests. The best sealants may last up to 40 years on outside exposure, or even more but we have not yet such a longer experience.

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Design of the Joints Key Considerations

Whatever be the movements, the sealant must withstand it without failure, and therefore it must be elastic as we have seen in properties above. Therefore, the design of the joints and the selection of sealant type in order to meet these movements requirements is very important.

Usually, the architect, the designer or the contractor prescribe 2 or 3 starting and basic requirements:

The dimensions and shapes of the building and its components: frame, panels, prefabricated panels, floors, partitions, doors, windows, etc.
The types of materials to be used: poured in place or prefabricated concrete, masonry or metal or wood structures, concrete or metal floors, metal, PVC or wood windows and doors, bricks or plasterboard partitions, etc.
The shapes of the joints which may be square or rectangular sections, or a different section in order to adapt it to the shapes of the construction components and the contact surfaces.

Starting from these requirements, the joints contractor must:

Compute the maximum expected joint movements,
Select the type of sealant which will be able to withstand such movements,
Design and compute the joint dimensions so that the sealant will not be overly stressed and strained.

These 3 tasks go together because the width of the joint depends
on the expected movements and also on the elasticity of the selected sealant.

Depth of Joints

The maximum stresses are located at the junction between the substrates and the sealant, and at these junctions, stresses may be 2 to 4 times higher than deeper into the sealant. Also, it is very important to note that a thin sealant strip will give much lower stresses than a thick sealant.

Therefore, there is a rule which states that the thickness or depth of the sealant should not exceed 50 to 70% of its width.

The general rules regarding the depth of joints are following:

Minimum dimensions of joints are 5 x 5 mm,
For joint width from 5 to 12 mm, the depth should always be less than width,
For width from 12 to 25 mm, the depth should be around 12 mm,
For a width higher than 25 mm, the depth should be preferably less than half of the width.

A back up material (e.g. foam) is used in order to control the joint depth.

The sealant should stick only on the 2 substrates and not on the bottom of the joint so that it will change freely its shape. If it would stick on three sides, this would increase the stress and it would tear. Therefore, a release tape should be installed prior to extruding the sealant, as indicated in the figure.

Sealant with or without release back-up tape

The sealant should stick only to the 2 sides of the joint:
a) No back-up tape: When the joint enlarges, the sealant will tear off
b) With a release back-up tape: The sealant strip may change freely of shape; less stresses and no risk of tearing.

It is also important to note that there are many rules according to the kinds of jobs, the countries and the techniques which one should also consider while designing joints.

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