Polyesters are one of the largest classes of synthetic resins. They have widely varying properties depending on the composition and are of relative low cost. The polyester resin-based adhesives can be divided into two distinctive groups: saturated thermoplastic and unsaturated thermosetting resins.
Unsaturated polyester based adhesives usually involve the reaction between a dibasic acid or an anhydride and a diol. Without special catalysts, they cure only at elevated temperatures. For example, maleic anhydride and 1,2-ethanediol polymerize at temperatures between 60 °C and 220 °C. The higher the temperature, the faster the reaction. Generally, both the physical and chamical properties of the polyester based adhesives depend on the ratio and type of the acid and diol compound and on the copolymerizable monomers. For example, a higher proportion of maleic anhydride leads to a higher crosslink density and thus to greater hardness and heat resistance of the cured adhesive, whereas phthalic anhydride decreases the crosslink density and thus increases the flexural strength.
Some polyester adhesives include solvents (usually acetone or a mixture of acetone and alcohol) or reactive diluents (mainly vinyl monomers such as styrene). The reactive diluents are added to decrease the viscosity. They also act as cross-linking agents. Typical initiators are organic peroxides and hydroperoxides or hardening catalysts such as tertiary amines and cobalt salts. Extenders like silica and polymers are sometimes added as well as monomers that copolymerize with the unsaturated polyester. These ingredients are added to increase the bond strength, heat resistance, toughness, and flexibility. Polyester adhesives without extenders are slightly colored or colorless liquids. The working life ranges from several minutes to several hours.
Unsaturated polyester adhesives, when sold in cartridges, are mainly two-part systems that harden by the addition of a catalyst, usually a peroxide such as methyl ethyl ketone peroxide. Polyester adhesives are also applied to the surfaces to be bonded with a brush, a roller or by spraying. In this case, the hardener and resin of the polyester adhesive are mixed immediately before use.
The bonding conditions (temperature, pressure, and cure time) depend on the composition of the adhesive and the nature of the materials to be joined. Some polyester adhesives harden in minutes, either at elevated temperatures or by addition of a catalyst, others in several hours or days at room temperature, depending on the resins, the curative, and catalyst.
Polyester adhesives bond well to glass, many plastics, rubbers and to metal and wood. The adhesive bonds are resistant to water, gasoline, weak acids and many solvents.
Unsaturated polyester adhesives are widely used for glass fiber laminates, optical products, furniture, concrete flooring, and automobile body repair. They find many applications in the construction industry, particularly as masonry adhesives for bonding, filling and repairing natural and artificial stones.
Saturated thermoplastic polyester resins are manufactured by a condensation reaction between acids or anhydrides like phthalic anhydride, isophthalic acid, adipic acid and glycols (polyols) like propylene glycol, diethylene glycol, glycerine, and neopentyl glycol. The length of the diol chain has a major impact on the adhesive properties; with increasing diol chain length the melting point, glass transition temperature, and the degree of crystallinity decreases.
Saturated thermoplastic polyester adhesives exhibit high peel strength and are mainly used to laminate plastic films, such as polyethylene terephthalate (PET), and to bond fabrics. They offer excellent clarity and color stability, and therefore, can be used for optical equipment. This type of polyester adhesive is available in solution or as solid films. They can be used as a high performance hot melt adhesives and can be chemically crosslinked with curing agents such as isocyanates to improve thermal and chemical stability. They are often used for hotmelt adhesive applications where high(er) tensile strength and high(er) temperature resistance are needed.