Industrial sealants are used to fill gaps and seal joints or openings against the intrusion of water, gases, air, dirt, or other foreign substances.  Sealants are used to ensure that the space between two mating parts, created due to dimensional variation, is void free. Mating multiple parts together magnifies this variation and results in a larger gap between parts and an increased risk of performance failure. Sealants are designed to be placed between between the mating surfaces to overcome the deficiency.  

Sealants are manufactured in several forms, including pastes and liquids, and may either be applied to the mating surfaces before they are joined or sprayed into the gap between mated parts. Unlike adhesives, many sealants remain flexible and do not lock two pieces together, and may require mechanical fasteners to maintain contact. However, some sealants can act as a sealant and an adhesive.

Sealant Material

When selecting an industrial sealant, it is important to consider a number of key specifications, including stability, bond strength, and suitability to an intended application. Choosing a sealant material has a profound effect on these specifications. The following table lists the cure method and material compatibility of seven common sealant groups.

Organic vs. Inorganic

Organic sealants use a chemical polymer backbone consisting of organic (hydrocarbon-based) polymers. Polyurethane, polysulfide, acrylic, and polyether sealants are all organic products. Organic sealants exhibit excellent initial sealing properties, including adhesion and movement, but are susceptible to degradation by UV rays found in sunlight. For example, when a cured polyurethane sealant is exposed to sunlight, it may harden and eventually crack due to normal joint movement between parts or sections of a building. Cracks in the sealant eventually lead to adhesive failure.

Inorganic sealants do not have a carbon based chemistry and are based on materials such as silicates, aluminates, and sulfates. Silicone and other inorganic sealants are ideal for use in applications that involve UV, electrical, and fire exposure.

One Component vs. Two Component Systems

Industrial sealants may be considered one component or two component systems. One component sealants consist of a single resin that hardens using surface moisture, heat, or an applied primer. Two component sealants consist of two or more resins which are mixed together before application. The second resin is often a hardener or catalyst that works to cure or crosslink the sealant.

Specific Material Composition

Industrial sealants may be comprised of various material combinations, including:

• Acrylics are derived from acrylic acid and feature excellent environmental resistance and fast curing. Acrylic polymers are produced from acrylic acids using a catalytic reaction. Cyanoacrylate, often called super glue, is a type of acrylic resin that cures instantly through a reaction with surface moisture.
• Elastomeric or rubber-based resins are very flexible and can be based on a variety of systems, including silicone, polyurethane, butyl, and isoprene.
• Epoxy resins are typically two-part resins that cure at room temperature, although some may be thermally cured. They are known for their high strength, excellent toughness, and resistance to chemical and environmental damage.
• Phenolics / formaldehyde resins are thermosetting compounds that provide strong bonds and good temperature resistance. The most durable of these resins is typically composed of phenol and formaldehyde chemicals. Phenolic resins are available in liquid, powder, and film form.
• Polymers are a diverse group of compounds that include polyamide, polyester, polypropylene, polyurethane, and vinyl. Most polymer resins require heat to cure, and some may require a catalyst or air evaporation as well. Polymers generally feature excellent strength, flexibility, and durability.
• Silicone is produced through the hydrolysis and polyermization of silanes and siloxanes. Silicone sealants typically cure at room temperature and feature excellent material compatibility.
• Wax resins are based on the original hot-melt paraffin waxes.

Material Compatibility

The most important sealant specification to consider is the type of material the sealant is designed to work with. Using a sealant on an incompatible product material can compromise its effectiveness or render it completely useless.

• Ceramics are oxides, carbides, nitrides and other non-metals with high melting points. Ceramic parts are compatible with a number of different sealant materials, including acrylic, silicone, epoxy, and polyurethane.
• Concrete and cements are used to build buildings, foundations, walls, bridges, and other structures. Common concrete sealants include liquid silicone, polyurethane foam, and silicone/foam hybrids.
• Metal parts are commonly joined using silicone sealant, which is especially compatible with iron, copper, aluminum, and steel. Polyurethane and other polymer sealants are also commonly used to seal metal joints.
• Plastics are organic, synthetic, or processed materials made of thermoplastic or thermosetting polymers. Polyurethane/silicone hybrids work best with plastic parts. Some sealant materials are incompatible with certain plastic materials. For example, polysulfide and pure polyurethane sealants may cause acrylonitrile butadiene styrene (ABS) or polyvinyl chloride (PVC) parts to harden and crack.
• Porous surfaces are covered with small orifices or openings. Sealants for this joining of porous parts typically have high viscosity or a gel-like consistency, and include elastomers, polymers, epoxies, and silicone.
• Rubber or elastomer parts are characterized by a high degree of flexibility and elasticity. Epoxy and polyurethane/epoxy hybrid sealants work well with rubber materials.
• Textile parts are often sealed with silicone products. Textile sealants are especially important to the automotive industry, and include seam sealants and other products.

Curing / Setting

Curing refers to the toughening or hardening of a sealant after it is applied to a gap or void. Depending upon the product and sealant material, a seal may take hours, days, or weeks to cure and become semi-permanent. Sealants are cured using a variety of different methods.

Anaerobic

Anaerobic sealants cure in the absence of air of oxygen.

Room Temperature

After being applied, a sealant that cures at room temperature is simply allowed to cure at ambient room conditions. These products cannot be confined to an enclosed space because they require moisture to properly cure. Room temperature sealants typically cure in 30 minutes to 4 hours, and may also be vulcanized with heat and a special agent in order to increase strength, stability, and elasticity.

Room temperature vulcanization (RTV) sealants are special silicon based products that cure at room temperature. After RTV sealants are applied, environmental moisture triggers a condensation reaction that forms crosslinks between polydimethylsiloxane chains, resulting in a strong seal.

Heat

For heat-cured sealants, cure and full adhesion is not achieved until the sealant is heated above a certain temperature. Thermally setting sealants fall into two groups:

• Thermoplastic / hot melt sealants can be repeatedly softened by heat and cured by cooling, allowing for repositioning of parts during the assembly process. Most thermoplastics melt above 180° F and then set rapidly upon cooling. Common hot melt sealants include polyethylenes, polyamides, and ethylene-vinyl.
• Thermosetting sealants are chemically are irreversably set through chemical reaction or crosslinking between chains. They have a higher heat resistance than thermoplastic sealants, but cannot be melted down and reset after curing.

Ultraviolet / Radiation

UV curing uses ultraviolet light, visible light, or electron beams instead of heat to initiate curing. Advantages of UV curing include lower energy requirements, short cure times, and improved production rates due to ease of automation. Disadvantages include the fact that an opaque filler cannot be used with UV sealants, and that at least one of the sealant substrates must be UV transparent. Due to the reliance on light, UV cure sealants may necessitate the use of a second curing method to cure the parts of the sealant shielded from the UV light. Also for this reason, the opaque surface should not have undercuts, which may generate shadows and uncured regions in the applied sealant.

References

Dow Corning - Adhesive Curing Methods

ReliablePlant - Five tips for selecting an industrial sealant

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Pittsburgh Corning