Common Chemistries for Liquid Adhesives

Epoxy Adhesive

Epoxies are one of the oldest synthetic chemistries for adhesives, dating back to the 1940s. The long history and substantial number of reactive chemistries available make epoxies one of the most diverse adhesive chemistries. Formulations are available for low and high temperatures, flexible and rigid, toughened and brittle, etc. Epoxy adhesives are most commonly used for structural applications where there is a very high load and/or small bond area. Common in aerospace, defense and industrial transportation, epoxy chemistry is also used when there are temperature or chemical exposure requirements.

Primary Advantages

Very high ultimate strength

Excellent temperature and environmental resistance

Long shelf life at room temperature

Primary Disadvantages

Slow to react

Require very clean substrates

Higher cost in highly engineered formulations

Epoxy reactions are step-wise polymerizations, meaning that for every reactive group “A”, there must be a reactive group “B” that it can react with. For two-component systems, A and B are in separate sides and mixed through a nozzle. For one-component systems, one of the components is activated using heat that allows the reaction to proceed.

Urethane Adhesive

Urethanes are most well known in other formats such as foam, synthetic rubber or coatings. Urethanes also make great resins for adhesives, and the adhesives have many of the same properties: flexibility, energy absorption and durability, to name a few.

Urethane adhesives are most common in industries such as construction that require bonds to traditional materials (e.g. wood, brick, concrete). However, their unique flexibility and energy absorption properties have allowed highly engineered urethane adhesives to find a fit in many industrial applications such as transportation.

Primary Advantages

Flexibility when cured

High peel strength

Adhesion to most traditional materials, highly gap filling

Primary Disadvantages

Slow to react

Poor adhesion to glass and metal without priming

Limited shelf life, limited moisture/humidity effects

Similar to epoxy chemistry, urethane reactions are step-wise polymerizations, requiring a reactive group “A” and “B”. For two-component systems, A and B are in separate sides and mixed through a nozzle. For one-component systems, the other reactive group comes from ambient moisture (H2O), curing the adhesive from the outside in.

Acrylic Adhesive

Cyanoacrylates (one example of the acrylic chemistry family) were discovered during World War II while searching for a plastic material to use in weapons. The technology was originally overlooked because it stuck to everything during processing! Since then, acrylic chemistry has advanced tremendously to include two-component, light curing, and many other forms of industrial adhesives.

Acrylic liquid adhesives are best known for their rapid cure speed. Some acrylic adhesives are capable of reaching 1000 psi in lap shear strength within a minute. This process speed with high ultimate strength makes acrylics suitable for processes that require fast throughput, such as electronics.

Primary Advantages

Very fast cure rates

Ability to bond to the widest variety of materials

Least sensitive to surface preparation

Primary Disadvantages

Lower resistance to many harsh environments compared to epoxy or urethane chemistries

Sensitive to storage conditions

High strength products are often brittle, requiring the addition of tougheners

Two-component acrylic reactions are called “radical polymerization”. One of the components contains the “initiator” that allows the reaction to begin; once initiated, the polymerization process occurs very rapidly. One-component adhesives rely on ambient moisture (H2O) or UV light to initiate the reaction. Acrylics may also be emulsified in water and used as a sprayable or coatable adhesive, often used for large surface lamination bonds.

Silicone Adhesive

Liquid silicones have a very low surface tension, meaning that they will readily wet out many surfaces – even those with very low surface energy like PTFE. It’s no wonder silicone caulk adheres quite well to nearly every surface in a home from kitchen to bathroom.

Silicone liquid adhesives are best known as “sealants” used broadly in many industries. However, their ability to bond a wide range of materials and tolerate high temperatures and chemical exposure suits them for many industrial bonding applications. They are relatively low cost, commonly used in building and construction. Two-component silicones have very high temperature resistance, with many appliance or solar applications.

Primary Advantages

Silicones are LSE materials themselves, relatively resistant to mold and fungi

High temperature resistance

Flexibility to use as a sealant

Primary Disadvantages

Low strength

Added oils may leach out over time

Tendency to “migrate” around manufacturing locations, often causing contamination issues for neighboring bonding work cells

Silicone chemistries have reaction mechanisms very similar to urethanes, but their inorganic nature (silicon is the backbone, not carbon) means the bonds formed have greater resistance to high temperatures. To aid in processing, oils are often added to improve flow and wet out, especially for one-component systems. The leaching of these oils may cause aesthetic issues over the life of the adhesive.

Rubber Adhesive

Natural rubber has been used for adhesives since before the industrial revolution. To this day, most of the natural rubber used for adhesive formulations is “smoked” to eliminate fungi or bacteria that can adversely affect the bond over time. (The chemistry of this “smoking” is actually similar to the chemistry of smoking meats to preserve them.)

While many of the rubbers used for adhesives are naturally derived (such as from the Hevea rubber plant), “rubber” may also refer to synthetic materials such as polychloroprene (Neoprene) or various block co-polymers (e.g. SBR). Their ability to be “tackified” makes them attractive, low-cost solutions for large surface lamination bonds or bonds requiring immediate handling strength and lower ultimate strength.

Primary Advantages

Immediate handling strength

Ability to bond many surfaces, including LSE materials

Solvent or water carrier allows easy application to large surfaces

Primary Disadvantages

Low strength

Lower resistance to environmental conditions (e.g. UV, temperature)

Adhesion to LSE materials requires the use of solvents in the formulation

Natural rubber (poly cis-isoprene) is mechanically worked to provide lower molecular weight polymers that can be readily dissolved or dispersed in a solvent. Synthetic polymers (such as styrene-isoprene block copolymers) may also be used. Tackifiers such as pinene (from pine sap, among other sources) are added to give the adhesive additional tackiness allowing it to be used as a PSA.

As they have for centuries, the chemistries for liquid adhesives grow every day. New reaction mechanisms, new fillers and additives, and new manufacturing processes guarantee that new liquid adhesives will continue to be developed.

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