Solventborne Adhesives and Sealants

Solvent based adhesives are one of the largest classes of adhesives. These adhesives are mixtures of solvents and thermoplastic or slightly crosslinked polymers such as polychloroprene, polyurethane, acrylic, silicone, and natural and synthetic rubbers (elastomers). Solvent based adhesives contain significant amounts of volatile organic compounds (VOCs), which can be harmful and irritants and are often subject to regulations across the United States, Europe or worldwide. Solvent born adhesives can be applied by roller, brush, and spray or as beads either manually or automatically. Solvent based adhesives are available with a variety of drying and bonding times to match the application method and assembly process. These adhesives typically gain bond strength when dried and, in some cases when cured (cross-linked). Because they are formulated with organic solvent, they are less sensitive than other types of adhesives to contaminates like grease and oil on the substrate to be bonded.

The solvent based adhesive can be classified as either wet or dry bonding.

Wet Bonding Adhesives
These adhesives harden and build strength when the solvent evaporates. After application of the adhesive, the substrates must be bonded while the adhesive is still liquid. Waiting for some time to evaporate some of the solvent prior to bonding (set time) will usually increase the initial strength of the adhesive bond and it also allows for quicker assembly. However, waiting to long, i.e. evaporating too much solvent, will result in poor bond performance because the adhesive will no longer wet (open time). Full bond strength is obtained after the remaining solvent is fully evaporated or absorbed by the substrate. Since the solvent has to be able to evaporate after bonding, at least one substrate has to be porous; otherwise hardening will be rather slow.

Contact Adhesives (Dry Bonding)
In this case, both substrates are coated with adhesive and the solvent is allowed to evaporate before the parts are bonded. Some contact adhesives require several hours to fully dry. The parts are then mated using only light pressure to insure intimate contact of the two adhesive films. The adhesive has to have sufficient tack to provide some bond strength. In many cases, there is no need for clamping. The full bond strength builds over time as the polymers of the two adhesive films co-mingle. Contact adhesives have usually much higher initial green strength and often higher final strength, particularly when heat cured (cross-linked). They also have better heat and creep resistance than wet bonded solvent based adhesives.

The performance of solventborne adhesives largely depends on the polymer system in the formulation. The choice of solvent and polymer type depends on the specific substrates and environmental and temperature resistance needed for the application. Sometimes oils, tackifiers and plasticizers are added to the formulation for modify the adhesive properties. Common polymers are natural rubber polychloroprene (Neoprene), urethane and polyacrylates to name only a few.

The majority of solvent based adhesives contain flammable solvents which require proper precautions for safe handling. Furthermore, most organic solvents are regulated due to environmental concerns with emissions.

The two most important reactive solventborne adhesives are acrylics and urethanes.

Reactive Solvent Based Urethane Adhesives
These adhesives consist of urethane oligomers dissolved in a solvent. The two most important building blocks besides isocyanate are polyesters and polyethers (both soft segments). The polyester based urethanes (Polyester PUR) have good chemical and solvent resistance but are attacked by basic and acid chemicals, i.e. they are not as hydrolytical stable as polyether based urethanes and tend to be more brittle at lower temperatures. They also have good resistance to UV and outdoor exposure. Compared to polyether based urethanes, they generally have better adhesion to polar materials.
Polyether based urethanes (Polyether PUR) have excellent low temperature performance, that is, they are elastic even at extremely low temperatures, and have excellent dynamic properties including high flexibility and elongation. They are also resistant to biological decay, i.e. do not degrade in soil. Both polymers are sometimes copolymerized to modify the mechanical properties. Solvent based PURs are widely used in the food packaging and laminating industry. They are sold as one-part and two-part adhesives. The most common one-part reactive PURs undergo moinsture cure. During cure the (residual) isocyanate reacts with water and carbamic acid which is unstable and decomposes to amine and carbon dioxide. The amine reacts with more isocyanate to urea.
The most common two-part urethanes are usually cured via polyol-isocyanate reaction. They usually cure much faster and also cure in dry climate. They generally have a better performance, and bond to a greater variety of substrates (i.e. non-porous) than one-part urethanes. However, they are more sensitive to contaminations (humidity), have a shorter potlife, and are usually more expensive, whereas one-part urethanes are easier to apply and are more ecconomical.

Reactive Solvent Based Acrylic Adhesives
Reactive solventborne acrylics are the most common solventborne adhesives beside urethanes. They have good aging properties and are resistant to many common media. They also are more heat and chemical resistant than their water based counterparts. However, solvent-based acrylics have usually lower shear strength and are usually more expensive than water based (emulsion/dispersion) acrylics.

The most common acrylic monomers are ethyl acrylate, butyl acrylate, and ethylhexyl acrylate. Often blends of all three monomer are copolymerized. Sometimes other ingredients are added, such as plasticizers to lower the glass transition temperature, tackifiers to increase tack (also acrylic adhesives are usually very tacky on their own), fillers to lower cost and to modify performance, and flame retardants and antioxidants to improve heat resistance. Sometimes other monomers such as acrylic acid are added to improve adhesion to metals.

Acrylic solventborne adhesives are mainly used for pressure sensitive adhesives (labels and tapes), and as construction adhesives for tile bonding and sealing (grouts) and as laminating and packaging adhesives. They are also used as architectural and industrial coatings.

Other chemistries used for solvent based adhesives include reactive modified acrylics, silicones, and epoxy-amines. However, these adhesives are usually more expensive and find fewer applications.