Adhesion: How and Why

 

Matching to materials Types of Adhesives and their Uses Types of Sealants and their Uses
Introduction Metal Adhesives Ceramic Adhesives Introduction
Solvents Wood Adhesives Leather Adhesives Silicone Sealants
Surface Cleaning Plastic Adhesives Paper Adhesives Other Sealants
Adhesive Failure Glass Adhesives Other Adhesives  

 

Matching to materials

Introduction

A basic understanding of how adhesives work is necessary to obtain best results.

Most adhesion failures can be traced to:

This is why the selection of adhesive or sealant is so critical to success. In case of adhesive failure, observing the type of failure can help address the underlying causes: if the adhesive leaves traces on both sides of the bond (cohesive failure), the adhesive type or the curing conditions may not have been adequate. Conversely, if the adhesive leaves traces on one of the surfaces but not the other, it could have been because the adhesive is not adapted to that surface, or to unsuitable surface preparation.

To understand how adhesives work it is necessary to consider the forces that constitute the bond between the adhesive and the substrate surface and the forces that ensure that the adhesive has the necessary cohesion to hold parts together. In practice, adhesives or sealants compositions may use a combination of adhesion and cohesion forces to achieve their properties.

The other parameter to consider is the wetting properties of the adhesive.

1.    Main types of adhesion

a.    Chemical adhesion

If potentially reactive groups are present at the surface of the material, very strong covalent bonds can be formed. This is the reason why silane adhesion promoters are used in adhesives or coatings for glass or mineral surfaces rich in hydroxyl groups which chemically react with the silanes. Epoxy adhesives also contain reactive groups which can form covalent bonds with hydroxyl or acid groups on surfaces.

Ionic bonds are nearly as strong as covalent bonds and were used for example in second generation dental adhesives. The presence of organic acid groups from polyesters in an adhesive formulation will help adhesion to metals by formation of metallic salts on the surface. Unfortunately, ionic bonds are sensitive to water and for that reason these types of adhesives were abandoned in later dental adhesives generations.

The third type of chemical adhesion is based on hydrogen bonding. It is generally weaker than either covalent or ionic bonding, but can still yield excellent results. Successful bonding is based on the availability of hydroxyl groups on the interface between the adhesive and the surface and nitrogen and/or oxygen atoms. Hydrogen bonds are plentiful in animal or vegetable-based adhesives and this is why they strongly adhere to wood and paper surfaces. However, hydrogen bonds are weakened in the presence of humidity.

b.    Physical adhesion

If the surfaces to be glued together are porous, adhesive materials can fill the voids and mechanically lock them together. Some textile adhesives are based on this mechanism.

Molecules can carry electrical charges and this effect is used in dispersive adhesion when the adhesive carries a charge opposite than the substrate. They are named van der Waals forces. Since they are effective only at close distances, they are mainly put to use in self-adhesive tapes or labels. The advantage is that they can be more or less easily removed or re-used.

The merging of materials at the joint is called diffusive adhesion. It occurs when the molecules constituting both the adhesive and the substrate are soluble in each other. This type of adhesion mechanism is used to glue many plastics together. An example is a hard (unplasticized) PVC adhesive based on a solvent (often a ketone such as cyclohexanone or methyl ethyl ketone - MEK - or tetrahydrofuran - THF -) that partially swells the plastic surface, allowing the polymer molecules to diffuse in each other. This is also called a solvent weld.

2.    Surface Wetting

Successful adhesive bonding requires that the adhesive wets (or spreads on) the surfaces to be bonded. The wettability of a surface is determined by its surface energy (force per unit area) or surface tension (force per unit length). The ability of an adhesive to wet a surface depends on its surface tension and on the cohesive forces within the liquid which causes it to ball up.

Simply put, the surface tension of the liquid adhesive must be lower than the surface tension of the material.

Metals and glass have very high surface tensions (>290 dynes/cm) while most polymers (plastics) are characterized with low (31-47 dynes/cm) to very low surface tensions in the cases of silicone (21 dynes/cm) and Teflon™ (18 dynes/com). Given that water has a surface tension intermediate between metals and plastics (73 dynes/cm) the use of water as an adhesive solvent poses special challenges (see section on solvents). This specific issue can be addressed to some extent by addition of surfactants which lower the surface tension.

Good wetting is achieved when the surface tension of the adhesive is lower than the surface tension of the surfaces to be bonded. It explains why it is so difficult to achieve good adhesion to low surface tension materials such as polyethylene, polypropylene, silicones and polytetrafluoroethylene (Teflon™). What is sometimes required is some form of surface preparation to increase the surface chemical activity.

Solvents

With the exception of hot melts and many reactive adhesives and sealants, solvents are commonly used in formulations for the following reasons:

Solvents are also used for cleaning surfaces.

A distinction must be made between water and organic solvents.

1.    Water

Water is inexpensive, odorless, non-toxic and inflammable and is in principle the solvent of choice for adhesives and sealants.

goutte d eau small 1 230However, some water-based adhesives have their own limitations and if the performance of the adhesive is of paramount importance, the formulation may be based on an organic solvent.

For some hydrophilic polymers, especially those that are of vegetable or animal origin, water is the only solvent which can be used.

Many polymers can be made in an emulsion process enabling them to be dispersed in a water-based formulation, sometimes with the addition of small quantities of a water-soluble organic solvent. Upon drying the polymer particles coalesce, forming a highly cohesive layer bonding the parts together.

2.    Organic Solvents

Due to increasing concerns about the toxicity of certain organic solvents and the concomitant risk of exposure, the number available in consumer-level adhesive and sealant formulations has dropped in recent years. For industrial applications however, certain solvent-based adhesives are difficult to replace due to high performance expectations.

Organic solvents greatly facilitate surface cleaning operations before application of the adhesive and removal of excess adhesive.

Solvent blends are also sold to remove traces left by self-adhesive labels and tapes, and residues of polyurethane foam or cyanoacrylates from skin. Paint strippers are also effective in some cases.

The following solvents are still available in do-it-yourself stores for use by informed consumers:

Surface Cleaning

Satisfactory wetting of surfaces to be bonded can only be achieved if they are made very clean prior to bonding. The presence of any grease or oil, including sebum from contact with fingers, is detrimental to the formation of a strong bond. In the presence of dirt, dust or rust, the adhesive will attach itself to it and not to the surface of the part to be bonded.

In the case of some metals used in a humid environment such as steel, the adhesive must be chosen to prevent the formation of rust under the adhesive film.

Several methods are available to achieve a satisfactory level of surface cleaning depending on the type of surface contaminant and the quality of the bond desired.

a.    Mechanical

The surface is ground with sandpaper or with other abrasives (sand blasting). The dust formed must be thoroughly removed, ideally with hot water and detergent. Abrasion will increase the surface available to bonding. However, if the surface is too irregular the film thickness will increase, affecting the strength of the bond. Wood surfaces must be dry and plane and all debris should be removed.

b.    Physical

Certain processes capable of forming reactive sites on difficult surfaces such as polyethylene plastic are only available industrially, such as corona/plasma discharge or gas-phase fluorination.

c.    Primers

For some surfaces which are either highly porous or which require protection against rust a primer is useful. Application of the primer also requires careful cleaning.

d.    Cleaning with solvents or water

Solvents are effective degreasers (particularly chlorinated solvents which are now becoming out of fashion due to toxicity concerns).

Acetone gives good results too, but must be used with caution due to its ability to dissolve certain plastics and paints for example. As seen previously, this property can be used to "weld" plastics using acetone as a dissolvant.

Alternatively, hot water with the addition of detergent (or soap) or pressurized water can be used. Vapor degreasing is used industrially.

e.    Chemical

Certain chemicals are used industrially to etch the surface of materials, particularly metals to make them more receptive to wetting. Galvanized steel surfaces can be primed with phosphoric acid solutions while the hot sulfochromic mixture is useful for other metal surfaces to be cleaned as well as glass. Strongly alkaline solutions (15% sodium hydroxide in water) are also very effective in some cases. Due to the corrosivity of these chemicals, only trained personnel wearing adequate personal protective equipment should handle them in all safety.

Adhesive Failure

Observation and analysis of the causes of adhesive failure can help avoid repetitive mistakes such as inadequate surface preparation or adhesive selection.

The first step is to identify where the adhesive has failed:

failure cohesive failure adhesive failure both failure structure
Cohesive failure Adhesive failure Adhesive and Cohesive failure Structural failure

 

The design of the joint should also be considered. How are the forces leading to failure being applied?

Environmental conditions should also be considered: presence of light (especially UV light), of humidity, elevated service temperatures, and very cold conditions leading to brittleness.

 

Types of Adhesives and their Uses

Metal Adhesives

Glues and sealants are often marketed as suitable for many materials. While it potentially avoids having to store many different types of adhesives in anticipation of future do-it-yourself jobs, we should be aware that certain types will generally perform better than others. This can be critical for certain jobs.

Metals pose special challenges because their surfaces are easily contaminated by grease, soil or rust. Excellent surface preparation is therefore essential (see section on surface cleaning).

The best types of adhesives for metals are generally reactive. Due to their reactivity, precautions (gloves, glasses) are necessary to prevent contact with skin or eyes. See section on health, safety and environment.

1. Epoxy Adhesives

Epoxy adhesives are made of two components (the epoxy resin and the curative) that must be thoroughly mixed before use. When small quantities are sufficient, it is more convenient to purchase the two components in two syringes that are pressed at the same time, dispensing them in the correct ratio.

Because the chemical reaction between the two components starts as soon as they are mixed, the pot life (time to double the viscosity of the adhesive) or open time (time during which a serviceable bond can be made) is limited. In general, the faster the adhesive, the shorter the pot life.

After setting, full adhesion then develops over time.

Commercial products are available in several versions according to their setting speed. The following table will give an indication of their curing rates at 20°C/70°F.

Type of Epoxy Adhesive

Fast

Normal

Slow

Pot Life (± setting time) @ 20°C/70°F

90 seconds

5 minutes

10 hours

Fully Cured Time

4 hours

4 hours

24 hours

 

Heating the assembled parts in an oven up to 100°C/212 °F will shorten the fully cured time and will generally result in improved adhesion.

Once the two components are thoroughly mixed in equal amounts (the generally clear resins become opaque), each part must be coated with a thin layer of adhesive and then pressed together. Any method (tape, rubber band, pincers, etc.) can be used to maintain the parts together until the adhesive is fully cured.

Excess adhesive can be wiped off with organic solvent (acetone, white spirit).

2. Methacrylate Adhesives

Methacrylate adhesives are often used in the automotive industry due to their ability to bond metal to plastic. They are sometimes called "second generation acrylic adhesives". They are particularly suitable for objects made of copper, wood, glass, aluminium, stone and some plastics.

Methacrylate adhesives are available in two versions:

An adhesive bond is achieved after a few minutes and a permanent bond after a few hours.

Methacrylate adhesives are more forgiving if the metal parts are not thoroughly degreased. In addition, they are generally better at filling the gap between parts.

3. Cyanoacrylate Adhesives ("Superglues")

Cyanoacrylate Adhesives are very fast as they set in a few seconds in the absence of air when parts are brought together.

They work best with small parts. Surfaces must be perfectly plane and clean.

Remember that cyanoacrylate adhesives stick nearly instantly to skin, so wear latex gloves when using them.

Wood Adhesives 

 Different types of glues are used depending whether the wood is to be kept indoors or outdoors.

1. Adhesives for Indoor Use

a. Vinyl and acrylic adhesives

Vinyl adhesives are dispersions of polyvinyl acetate in water. They have a milky appearance but are colorless when dry. Faster-drying formulations are available.

They are mainly applied on woodwork to be used indoors, in the absence of humidity.

The wood surface must be plane, sandpapered and dust-free. Oily or waxed wood must be degreased with a cloth impregnated with solvent.

A thin coating of glue is applied with a brush on both sides of the parts and then brought together. A clamp is necessary to keep the parts together until the glue is fully dried.

Any excess can be wiped off with a damp cloth provided it is done before setting.

Other vinyl adhesives (also called synthetic latex emulsions) are used for gluing carpets and plastic tiles. Vinyl adhesives work best with PVC but acrylic latexes are more polyvalent.

Polyvinyl acetate glues formulated for use on wood bind molecularly and mechanically to the substrate without a film, and required intimate contact between the surfaces to be glued. In restauration work, surfaces are never in perfect contact and when pressure is applied permanent damage can result.

b. Old adhesives of animal origin

Historically, synthetic glues have substituted old glues of animal origin mainly because they were easier to apply and of their adhesive properties. Today these glues are extensively used in woodworking and in the high-volume manufacturing of furniture. However, in cabinet-making and especially in the restauration of antique furniture, after a few years of use, synthetic glues have shown their weaknesses:

The glue used in woodworking during the 18th century before the advent of synthetic glues was made from the sturgeon’s swim bladder (also called fish maw or air bladder). The swim bladder contracts and expands permanently during the fish lifetime, which means that the constituting collagen fibers are very flexible. The glue which is derived from it forms a flexible joint that never hardens completely.

Faced with the rarefaction of swim bladder glue, cabinet makers have developed a blend of bone glue and of sinew glue. As these glues do not adhere very well to metal, the grip can be improved by wiping garlic on the surface and by scarring the surface with a knife.

For less demanding wood gluing applications, hide glue (made from rabbit skin) gives satisfactory results. The glue is supplied as powder or as plaques, the former usually containing fewer impurities. In that form it can be stored indefinitely. Before use, plaques or powder are dissolved in 90% water at 60°C and the glue is maintained at that temperature in a water bath (avoid boiling water).

Hide glue has other uses such as wood surface sizing in the manufacturing of icons, for gilding (framing) and in a mixture with gesso for the preparation of canvas for painting.

2. Other Adhesives for Outdoor Use

a. Polyurethane Glues

Polyurethane-based wood adhesives react with the humidity present even in dry wood and form very strong bonds, even with other materials such as polystyrene insulating foam, fiberglass cloth and concrete or masonry.

These glues are generally colored meaning that any excess should be removed before they set.

b. Neoprene (polychloroprene) Contact Adhesives

Contact adhesives are used primarily for gluing laminates on wood (or particle boards, Masonite, cork, etc.). They are sold in either liquid or gel formulations, the latter being particularly useful for vertical surfaces.

Contact adhesives are applied on both surfaces with a notched trowel and are left to dry a few minutes until no longer sticky to the finger. Position carefully the two surfaces, separating them temporarily with cardboard strips. Remove the strips one by one pressing the surfaces together. Place a piece of flat wood on top and hit it with a wooden hammer all over the surface. Alternatively, use clamps when appropriate while being careful about not leaving marks.

There are three types of neoprene glues:

 

c. "Marine" Adhesives

These adhesives are based on two components (a liquid and a formaldehyde/resorcinol resin in powder form) and are formulated to resist to water immersion (soft and seawater). Care should be used during use to prevent skin contact as the glue is an irritant. It is also highly flammable. Excess can be wiped off before setting with a mixture of alcohol and water. After setting, any excess can only be removed by sanding.

 

d. Hotmelts

Hotmelts are sold as sticks that are inserted in special electrically heated "guns". By pulling a trigger, the viscous liquid adhesive is extruded on one of the surfaces to attach and then sets in less than one minute by cooling.

DIY hotmelts are not the strongest wood adhesives, but are convenient to use, for example in modeling. The absence of solvents is also an advantage.

Plastic Adhesives

One of the challenges in applying adhesives on plastics is the identification of the type of plastic.
Certain plastics cannot be glued by conventional techniques:

On the other hand other plastics are easily glued with the right types of adhesives:

a. Hard plastics such as unplasticized PVC (drains, window frames), polyamides (Nylon™, Rilsan™ in electrical components) and Polystyrene (electronics enclosures, insulating foam).
The best results are obtained with polyurethane or epoxy glues. . DIY polystyrene glue is polystyrene in a volatile solvent. It is particularly good at gluing polystyrene plastic such as insulation foam or small-scale models. The glue is transparent and dries quickly. Excess of glue can be wiped off with a dissolvent.
Cellulosic glue is mainly used in DIY applications for repairing porcelain or glass decorative objects, or for modelling. The bonds are not very strong, but the short drying time makes it useful for some difficult jobs. It's a contact adhesive meaning the two surfaces to be attached must be coated with glue and partially dried before being brought together. A permanent bond is formed after 24 hours. It is semi-transparent and can work in high humidity or warm conditions.

b. Plasticized PVC (also called vinyl: beach balls, rainwear, seat coverings, electrical cable insulation)
Use glue especially formulated for plasticized PVC. For leaks repairs, the glue is supplied with small patches of transparent vinyl.

Plastics can be affixed to each other using a welding technique ("solvent weld"): an organic solvent is contacted with each surface until it is softened and the surfaces pressed on each other. This technique is beyond the scope of this website.

Glass Adhesives

1. Light-curing adhesives

The natural transparency of glass can be taken advantage of by using a specially formulated type of reactive adhesive that is sensitive to light (especially UV). The setting can be fast especially in the absence of air, when the parts are brought together.

These adhesives can also be used to bond glass to hard plastic.

2. Cyanoacrylate adhesives

Cyanoacrylate adhesives must be specially formulated for glass. Otherwise most standard cyanoacrylate glues do not bond well with smooth glass, and are mainly used for quick and temporary bonds.

Much care should be applied during use as these adhesives are generally quite highly toxic.

 

Ceramic Adhesives

 Ceramics is a general term comprising:

The types of jobs are generally different since tiles are destined to be affixed to walls or to floors while pottery occasionally needs to be repaired.


1. Tiles

Tiles are glued to perfectly flat and dust-free floors or walls with specially formulated tile adhesive. This is supplied either as a powder to be mixed with water, or as ready mixes in pails. Some tile adhesives are formulated for outdoor use.

The cement used to fill spaces between tiles is called grout. It is available in various colors, either as a powder or a ready-mix for convenience.

The tile adhesive is applied with a notched trowel allowing some movement to adjust the position of the tile on the support. When wet, the adhesive has enough tack to prevent the tile from moving or falling from a wall.

It is generally advisable to apply a grout sealant, preferably as a spray, to prevent humidity from penetrating.


2. Pottery and Porcelain

Unless the pottery is destined to a museum, where the practice is to let the repairs be apparent, repairs should be as conspicuous as possible.

Special formulations are used depending on the type of repair needed:

Leather Adhesives

For leather the job required most often is to stick a loose sole. Contact adhesives or organic solvent-free special leather adhesives are used for that purpose. They are flexible and well suited for shoe jobs.

Contact adhesives are formulated with Neoprene (chloroprene) dissolved in an organic solvent.

First, clean the surfaces to be glued with methylated spirit (methanol, denatured alcohol). Coat both surfaces with adhesive and let dry to the touch. Press firmly together both parts. While the adhesion strength is quite high at that point, it is advisable to clamp the parts together for a few hours. 

Paper Adhesives

The main non-professional uses for paper adhesives are very diverse and call for different technical solutions:


1. Wallpaper Adhesives

Wallpaper adhesives are made from methylcellulose flakes, mixed with water into a paste. They are supplied in various grades according to the weight of the paper to be affixed. Manufacturers also recommend higher concentrations of dry adhesive for heavier paper.

The adhesive is also available ready-made commercially.

According to Wikipedia, wallpaper pastes have a typical shear thinning viscosity and a high wet adhesive tack. These properties are needed to slow down the penetration of the adhesive into the paper and wall, and to give a slow bonding speed which allows enough wallpaper hanger time to line up the wallpaper correctly on the wall.

The wallpaper adhesive is applied to the wallpaper to let the moisture of the adhesive soak into and penetrate the paper. The paper will thus expand before hanging rather than on the wall, which would cause vertical bubbles in each panel of wallpaper as the adhesive dried from the edges inwards. When using nonwoven or glassfiber wallpaper—paper that does not expand—the adhesive is applied to the wall instead.

Wallpaper is also sold pre-treated with dry adhesive. Before use, the wallpaper is soaked in a water trowel.

To facilitate later removal of the wallpaper, it is advisable to protect plastered walls with primer paint.

2. All Purpose Office Adhesives

Originally, paper adhesives used in offices and schools were made with gum arabic. Gum arabic is very effective but unfortunately not impervious to humidity. Gum arabic had been progressively displaced by rubber cements in organic solvents. Concerns about glue-sniffing have now led to a nearly complete replacement with stick glues and water-based polymer dispersions. Stick adhesives, first marketed by Henkel in 1969 appear to be based on poly(N-vinyl2-pyrrolidone) and are claimed to be solventless and non-toxic.

Aerosol applied glues are extremely convenient for large paper or cardboard surfaces, avoiding the need for a brush. However, the propellants are highly flammable and their use is being increasingly controlled due to regulations about Volatile Organic Compounds (VOCs) which allegedly contribute to the formation of tropospheric ozone.

 

Other Adhesives

Natural latex adhesives

Glues sold as "natural" latex adhesives are in fact a complex formulation containing many other ingredients. One manufacturer discloses the composition as a mixture of natural latex, cooked linseed oil, water, orange skin extract (a mix of terpenes), rosemary oil (also a mix of mainly terpenes), milk casein, clay and a thiazole preservative. They should be used indoor only to glue carpeting, fabric, paper, jute or cardboard. It becomes very hard when dry. Any surplus should be softened first with a hydrocarbon (White Spirit) and then washed off with soapy water or terebenthine. Users must be aware of the strong residual odour which may fade off over time, and the possible difficulty in removing carpets or linoleum later on without damaging the underlying flooring.

 

Types of Sealants and their Uses

Introduction

Sealants are a category of adhesives which are mainly used for the following reasons:

Sealants typically have higher elasticity and lower adhesion properties than adhesives.

The job consisting of sealing surface imperfections for decorative or interior finishing purposes is called caulking.

Because the materials to be sealed may have very different dilatation coefficients sealants must have sufficient elasticity to compensate for dimension changes, as well as having good adhesion to surfaces. Interior walls of new buildings which have not yet stabilized on their foundations may form small cracks or crevices.

Selection of the appropriate sealant is therefore dependent on the job to be done and in particular an estimation of the movement that may take place after the sealant is dry or fully polymerized.

Construction sealants are classified according to the maximum movement or dimensional change they can support before failure, as shown in the following table. For example, a sealant accommodating a 10% dimensional change can fill a one-inch (25mm) gap which may enlarge to 1.1" (27.5mm) without breaking.

Sealant Categories and Elasticity

Sealant Categories

Maximum Dimensional Change, %

Examples
 

0

Painter’s putty (chalk and linseed oil), polyester

Plastic

5-12.5

Acrylic dispersions, silicone-acrylics, bituminous sealants

Elastic

15-33

Silicone, polyurethane, polysulfide

 

With the development of double glazing replacing single-pane windows, painter’s putty made from linseed oil and chalk is nearly no longer used. Once dry, it became hard and unable to accommodate dimensional changes.

Filled polyester is made from a plasticizer and a powder that is mixed before use according to the density desired. The resulting paste is used to repair car or boat bodywork. After setting (usually ½ hour depending on the ambient temperature) excess must be sanded, polished and the surface painted.

Plastic sealants flow easily when a force is applied but tend to break if the force is applied repeatedly.

Elastic sealants are also able to change their shape when a force is applied, but like rubbers can regain their original shape when the force is released. As long as the dimensional changes remain within certain limits, they can support repeated movements indefinitively.

Some sealants are formulated to combine both elastic and plastic properties.

The ISO 11600 (1993) standard defines the classes to which construction sealants belong. In principle, the class is printed on the label as a code. G-type sealants are dedicated to window glazing, while F-type sealants are for opaque elements. The letter is followed by a number characterizing the sealant’s aptitude to recover a deformation. There are four subclasses: 7.5, 12.5, 20 and 25 each corresponding to the percent deformation relative to initial dimensions. Finally, Class 20 and 25 sealants are divided in two subclasses according to the secant elastic modulus which can be either high (HM subclass) or low (LM subclass). In simple terms, a high modulus (HM) sealant is stiffer. This means that high modulus sealants are preferred when strength is important, such as in structural glazing, while low modulus sealants allowing them to expand with minimal force. The following table lists the main uses for HM, LM and medium modulus sealants.

Typical Sealants Modulus According to Use

Modulus

Main Use

High (HM)

Window glazing

Medium

Weathersealing

Low (LM)

Best for porous substrates and high joint movement locations; composite cladding

 

Another important property for sealants is whether they are suitable for outdoor or indoor conditions.

As with adhesives good surface preparation is the key to success: they must be dry, dust-free and degreased. Cracks or crevices must first be opened in a V shape with a special tool. With masking tape applied on the edges of the area to be sealed, the result can be aesthetically pleasing even for non-experts.

Deep crevices should first be fit with a backing material such as a backer rod sized at least 25% larger than the joint.

As with adhesives many chemistries are used to formulate sealants with different properties and cost according to needs. The next chapters describe the most important of them.

 

Silicone Sealants

Silicone sealants are usually considered premium products because of their durability, flexibility and resistance to weathering. There are two main types of silicone sealants according to their curing method. Each type can be formulated to be transparent or colored, and may contain a biocide to prevent black mold from forming on the surface in humid conditions.

Silicone sealants are reactive in that they cure in the presence of humidity naturally present in air. Depending on their curing systems, they may emit an odor (vinegar or alcohol) for a few days until the reactions are complete. The curing is in two stages: the first stage lasts a few minutes until the sealant surface is tack-free; a few weeks may be necessary to fully develop properties.

Some manufacturers add extenders to silicone sealants to lower their cost. This is not a problem in most cases as the cured sealant will slightly contract leaving a curved surface. In some cases the extenders may stain surfaces, such as marble. For natural stone, it is preferable to specify a non-staining sealant.

The following table provides a summary of the main properties and uses of the two main types of silicone sealants, according to their curing system.

Silicone Sealants Curing Systems (Room Temperature Vulcanizable – one component or RTV-1)

Type of Cure

Cure Time

Adhesion

Incompatibilities

Other Properties

Acid (acetoxy)

Fast

Good

Metal (corrosion)

Vinegar odor during curing

Neutral (oxime, alkoxy or benzamide)

Slower

Lower

None (use non-staining grades for natural stone)

Excellent oil and temperature resistance, no vinegar odor during curing

 

Silicones are usually difficult to paint, especially with water-based paints.

Other Sealants

There are many other types of sealants which are suitable for less demanding uses where other considerations such as cost are important. They often compete in the same markets. The best-known are listed in the following table.

Non-silicone Sealants
Sealant Type

Main Properties/Uses

Butyl

Low gas permeability; excellent thermal and electrical insulant; used in construction, appliances, car manufacturing and rubber roof repair and maintenance. Available as tapes or in cartridges.

Asphalt (bitumen)

Resist water, oil and gasoline; used for driveways

Polyurethane

Gap-filling; foam has insulating properties

Polysulphides

Fuel resistance; mainly used in double glazed windows and airport construction

Hybrid polymers

High strength but less durable than silicones, non-staining; mainly used in transportation

Acrylic

Water-based acrylic dispersions; paintable; mainly used in construction when flexibility and water resistance are less important