Page 365 - Handbook of Adhesives and Sealants
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Adhesive Composition and Formulation  321


            in the adhesive formulation is dependent on the chemistry of the spe-
            cific reaction involved. By over or under using a polyamide hardener,
            the resulting formulation will have more or less of the characteristics
            of the hardener, but a usable adhesive system will generally result.
            For example, 10–25% by weight additional polyamide hardener in a
            two part epoxy formulation will result in a system with greater flex-
            ibility and peel strength but lower tensile strength and environmental
            resistance due to the flexible nature of the polyamide molecule. Less
            hardener will provide higher shear strength and environmental resis-
            tance but poorer peel strength. It must be noted, however, that certain
            hardeners (e.g., amines, acids, and anhydrides) do have critical mixing
            requirements, and slight deviations from the manufacturers’ mixing
            instructions could significantly affect the adhesive system.
              Catalysts remain unchanged in the curing reaction, causing the pri-
            mary resin to crosslink and solidify. Acids, bases, salts, sulfur com-
            pounds and peroxides are commonly used. Only small quantities are
            usually required to influence curing. Unlike hardeners, the amount of
            catalyst used is critical, and poor bond strengths can result when res-
            ins are over or under catalyzed.
              An accelerator, inhibitor,or retarder is sometimes incorporated into
            an adhesive formulation to accelerate or de-accelerate the curing rate.
            These are critical components that control the curing rate, storage life,
            and working life of the adhesive formulation.
              Solvents are sometimes needed to disperse the adhesive to a consis-
            tency that is more easily applied such as by brush or spray. Solvents
            are also used to aid in formulating the adhesive by reducing the vis-
            cosity of the base resin so that additions of other components and
            uniform mixing may be more easily achieved. Solvents used with syn-
            thetic resins and elastomers are generally organic in nature, and often
            a mixture of different solvents is required to achieve the desired prop-
            erties. Polar solvents are required with polar resins; nonpolar solvents
            with nonpolar resins. When solvents are used in the adhesive for-
            mulation, they must be completely evaporated prior to cure. Otherwise
            bubbles could form in the bond-line causing a weak joint. The sub-
            strate must also be tested so that the chosen solvents do not attack
            or degrade it.
              Water is sometimes used as a solvent for water soluble resins. Cer-
            tain adhesives are also available as a water based emulsion or latex
            formulation. In the early 1970s, during the time of the petroleum cri-
            sis, water based adhesives were thought of as a possible replacement
            for solvent based adhesives systems. However, water based adhesives
            never met these lofty expectations primarily because of the time and
            energy required to remove water from the bond-line, the corrosion that
            the water causes in drying ovens, and the poor moisture resistance of
            cured water based adhesives.
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