Waterborne adhesives are very popular and are conceived as an environmentally friendly and economically viable alternative to solvent-based adhesives. One of the major advantages of waterborne adhesives is the absence of volatile organic compounds (VOCs). Furthermore, water can be used to clean excess adhesive off of mated parts and to clean the equipment. These adhesives are usually considered very environmental friendly.
For this class of adhesives a wide variety of polymers are used (usually thermoplastics and/or elastomers), which are either dispersed or dissolved in the continuous aqueous phase.
Waterborne adhesives are well suited to high-speed application and assembly (bonding). They are usually wet bonding and, therefore, at least one of the substrates has to be water permeable to allow water to escape from the bondline. This explains why these adhesive are widely used for bonding wood, paper, textiles, leather and other porous substrates. In the case of emulsions solid fillers are sometimes added to lower cost and to create pastes. Such adhesives can be used for tile and wall panel bonding and sealing (grouts).
Water based adhesives can be divided into two major classes:
Water Based Solution Adhesives
These adhesives harden and build strength when the water evaporates or when it is absorbed by the substrates. After application of the adhesive, the substrates must be bonded while the adhesive is still wet.
Waiting for some time to evaporate some of the water prior to bonding (set time) will usually increases the initial strength of the adhesive bond and it also allows for quicker assembly. These adhesives are
formulated with natural and synthetic polymers that readily dissolve in water. They harden by evaporation of water or by absorption of water into the porous substrate. They are mainly used in the paper industry
for bonding paper and paper products and for moistenable adhesives like those used on stamps, envelopes, and labels.
Water Based Polymer Dispersions/Emulsions
Water based dispersion adhesives are formulated with a larger number of polymers including vinyl acetate polymers (PVA) and copolymers (PVAC), ethylene vinyl acetate (EVA),
polyacrylates, natural rubber latex,
and synthetic elastomers such as styrene-butadiene rubber (SBR) and polyurethane (PUR). Like latex paint, these adhesives are heterogeneous mixtures consisting of a dispersed liquid or solid polymer phase in
an aqueous phase. They can be applied by brush, roller, spray, or in beads. The wet adhesive can be easily removed with water. However, dried adhesive is usually no longer water dispersible and requires either
solvents and/or mechanical cleaning.
Similar to solvent based adhesives, emulsion/dispersion adhesives can be classified as either wet or dry bonding:
Wet Bonding Adhesives
These adhesives harden and build strength when the water evaporates. After application of the adhesive, the substrates must be bonded while the adhesive is still wet. Waiting for some time to evaporate some
of the water 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 water, will
result in poor bond performance because the adhesive will no longer wet. Full bond strength is obtained after the remaining water is fully evaporated or absorbed by the substrate. Since the water
has to be able to evaporate after bonding, at least one substrate has to be porous; otherwise hardening will be rather slow. These adhesives are widely used in the paper, packaging and furniture industry.
Contact Adhesives
As with solvent based contact adhesives, both substrates are coated with adhesive and the water is allowed to evaporate before the parts are bonded. Some contact adhesives require up to 24 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. Contact adhesives have much higher initial strength and generally also have higher final strength. They also have better heat and creep resistance than wet bonded water-based adhesives
Water-based emulsion and dispersion adhesives contain water soluble additives such as surfactants, emulsifiers, and protective colloids that stabilize the emulsion or dispersion. Other ingredients that are sometimes added include defoamers, biocides, pigments, flame retardants, anti-static and wetting agents. To achieve good bond strength no residual moisture should remain in the adhesive.
Water dispersed adhesives based on natural and synthetic rubber and vinyl acetate copolymers provide flexible bond lines and improved adhesion to plastic, metal, rubber and leather compared to vinyl acetate wood adhesives. Water-based dispersion adhesives can be applied by brush, roller, in beads or by spraying. Cleaning of dried adhesive usually requires use of organic solvent and sometimes abrading. Waterborne adhesives are well suited for automated assembly of wide area laminates consisting of water permeable substrates such as film or foil to paper and wood veneer to wood or wood composite.
Types of Waterborn Adhesives
Waterborn adhesives can be classified as follows
Starch/Dextrin Adhesives are water-based adhesives made from starch or modified starch. They are commonly used for paper applications like bookbinding.
Protein/Casein Adhesives are derived from proteins found in animal milk. They have fast set time, and are widley used in the food packaging industry.
Vinyl Acetate Adhesives are made from emulsion polymers like vinyl and ethylene vinyl acetate. They bond well to cellulose-containing substates. Although these emulsions are bright white mixtures they produce a clear and flexible bond when dry, making them ideal for bonding wood, paper, and cardboard. They are knows as white glue and carpenters glue and are probably the best known water based adhesives. They provide rigid to semi-rigid bonds and are utilized extensively.
Rubber Latex Adhesives are made from emulsified elastomers (rubbers). Depending on the specific formulation, these adhesives form either a solid rigid bond or more flexible bond that is used in applications like stamp and envelope production and for binding fabrics and wood.
Polyacrylate Emulsions have good aging properties and resistance to many media. However, solvent-based acrylics are superior. They are mainly used as pressure sensitive adhesives for labels and tapes, as construction adhesives for tile bonding and sealing and as laminating adhesives. The most common acrylic monomers are ethyl acrylate, butyl acrylate, and ethylhexyl acrylate. Often blends of all three monomer are copolymerized.
Polyurethane Emulsions and Dispersions have good aging properties and resistance to many media. They have high elasticity and good adhesion strength. However, solvent-based (reactive) urethanes are usually superior. Both are widely used in the packaging and shoe-making industry.
Other polymers used in waterborne adhesives include polyvinylidene chloride (used as a laminating adhesives for food packaging), styrene-butadiene copolymers (laminating adhesive), and polychloroprene. The latter has high elasticity and good adhesion to many substrates.