Hot-Melt Adhesives
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Hot-Melt Adhesives C.W. Paul Abstract Hot-melt adhesives facilitate fast production processes because the adhesives set simply by cooling. Formulations contain polymers to provide strength and hot tack (resistance to separation while adhesive is hot), and tackifiers and/or oils to dilute the polymer entanglement network, adjust the glass-transition temperature, lower the viscosity, and improve wet-out (molecular contact of the adhesive with the substrate over the entire bonding area). Some adhesives also contain waxes to speed setting, lower viscosity, and improve heat resistance. Obtaining adequate strength and heat resistance from nonreactive hot melts requires that some component of the hot melt separate out into a dispersed but interconnected hard-phase network upon cooling. The hard phases are commonly either glassy styrene domains (for adhesives based on styrenic block copolymers) or organic crystallites (for adhesives based on waxes, olefinic copolymers, or ethylene copolymers). This article will describe first the material properties relevant to the processing and performance of hot-melt adhesives, then the chemistry and function of the specific raw materials used in hot melts, and will conclude with illustrative application examples and corresponding formulations. Keywords: block copolymers, ethylene copolymers, hot-melt adhesives, labeling, packaging, pressure-sensitive adhesives (PSAs), tackifiers, waxes.
Of course, the adhesive must be able to wet-out the substrates and preferably will exhibit high specific adhesion. Most hot melts contain aliphatic ingredients and thus exhibit good wet-out on most surfaces. However, obtaining high adhesion on plastic surfaces requires some level of treatment, which is usually accomplished by corona treatment of the plastic. A corona is an electrical discharge that causes the plastic surface to become oxidized. Surface energies as high as 5.0 102 N/m can be obtained on polyolefins by this process. While surface treatments decay over time due to diffusion of high surface energy groups or adsorption of contaminants from the air, as long as the surface energy remains above 3.6 102 N/m adhesion ordinarily remains adequate. Creep resistance, particularly at elevated temperatures, can be difficult to achieve with thermoplastic hot melts. Most adhesives will creep under minimal stress at far below the melting or softening point of their hard-phase physical cross-links. Obtaining adequate creep resistance at the usage temperature for a given application while minimizing viscosity at the application temperature is the constant struggle of the adhesive formulator.
Raw Materials Introduction For many bonding applications, a variety of adhesives perform adequately. Hotmelt adhesives are normally chosen for applications in which process speed is critical. Since hot melts have no carrier vehicle (solvent or water) and thicken rapidly as they cool, they are limited in their ability to (1) penetrate low-porosity substrates or to wet-out rough surfaces (i.e., achieve molecular c
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