Solvent-Free Adhesive Systems

Hot-melt adhesives are 100% solids that, in the broadest sense, include all thermoplastic polymers. Polymers that are primarily used as hot-melt adhesives include ethylene-vinyl acetate copolymers (EVA), polyvinyl acetates (PVA), polyethylene (PE), amorphous polypropylene, block copolymers (thermoplastic elastomers), polyamides, and polyesters. The oldest hot-melt adhesive, which has been in use since early times, is sealing wax. In principle, glutins and glue jellies also may be regarded as hot-melt adhesives. However, modern hot-melt adhesives are primarily synthetic products. The simplest hot-melt adhesives are rosin-wax mixtures. But these products have limited strength and thermal stability. 

In general, hot-melt adhesives are solid below 80 °C. Ideally, as the temperature is increased beyond this point, the material rapidly melts to a low-viscosity fluid that can be easily applied. Upon cooling, the adhesive sets rapidly. Because these adhesives are thermoplastics, the melting/resolidification process is repeatable. Typical application temperatures of hot-melt adhesives are 150-190 °C with melt viscosities in the range of 500-3000 mPas. 

Hot-melt adhesives do not undergo a chemical reaction because they simply cool firom the melted state to form a solid film at the bond line. These adhesives are available in forms such as pellets, slabs, bars, slugs, and films that allow convenient handling by a variety of application equipment. Because hot-melt adhesives are applied in the molten state, melt viscosity is an important property. The limitations of the application equipment often influence the viscosity range that is selected for a hot-melt adhesive formulation. 

Although hot-melt adhesives consist of 100 % solids, they are rarely 100 % polymers in composition. This is due to the limited adhesion of pure thermoplastics like EVA copolymers and their lack of melt properties such as tack and wettability. The components of a hot-melt adhesive can be roughly divided into two categories polymers and diluents. Typically, diluents are waxes, plasticizers, tackifiers, stabilizers, extenders, and pigments. The functions of the diluents are  

It is desirable to have a hot-melt adhesive with high strength. But this property is generally accompanied by a high melt viscosity. A low melt viscosity is desirable for application purposes. Therefore hot-melt adhesive formulations must include diluents that decrease the polymer melt viscosity and increase the overall adhesive strength. 

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Low-performance applications snack food laminates Generally laminate structures are OPP/ink/adhesive/metallisation. The biggest challenge for EB adhesives is competition from low-cost, solvent-free adhesive systems used in this field. As converters often choose the cheapest OPP film available, the substrates keep on changing frequently. Consequently, the EB adhesive system must be insensitive to substrate changes. 

Gelled polymer for adhesives Environmentally desirable for solvent-free adhesives Sensitive colloid system... 

Electron beam-initiated modification of polymers is a relatively new technique with certain advantages over conventional processes. Absence of catalyst residue, complete control of the temperature, a solvent-free system, and a source of an enormous amount of radicals and ions are some of the reasons why this technique has gained commercial importance in recent years. The modification of polyethylene (PE) for heat-shrinkable products using this technique has been recently reported [30,31]. Such modification is expected to alter the surface properties of PE and lead to improved adhesion and dyeability. 

Traditionally, UV curable polymers have been utilized as coatings for wood and vinyl floors, but their applications have increased dramatically over the last twenty years to encompass many diverse areas, including optical fiber coatings (7), adhesives (2), disc replications (3-5), and microelectronics (6). This widespread use of UV cross-linked systems is attributed to their rapid, energy efficient curing and their solvent free, one piece formulations. Typically, UV curable systems require only a small fraction of the power normally utilized in thermally cured systems and their solvent free nature offers an environmentally safer alternative. 

Single-component epoxy adhesive formulations are the largest type of epoxy adhesives sold, with about 55 percent of the consumption, while two-component formulations account for another 44 percent of the volume. Radiation cure formulations represent the remainder of the market. Epoxy adhesives can also take many forms including solids, solvent-free liquids, solvent-borne systems, and waterborne systems. 

Hotmelt adhesives are thermoplastics that are processed free of solvents in the melted state and are therefore known as hotmelts. Since the adhesives are 100% solids, no volatile components can be released. No incineration and/or ventilation systems are required to dispose of the solvents. These adhesives are used to fill larger gaps and therefore often function as both adhesives and sealants. Foamed hotmelt adhesives comprise a special category They are capable of filling large gaps with very little material while still bonding the substrates. 

In addition to the one-component contact adhesives described above, there are also two-component systems with isocyanate compounds as hardener component as well as solvent-free formulations. 

Now that we have discussed the processes that can be used to create multilayer structures, coating, coextrusion, and lamination, the question arises, which process should be used, and when Liquid adhesive systems usually run faster than extrusion laminating and coating systems. If one is using a solvent-based system, there is the issue of whether or not any residual odors that might affect the product to be packaged remain. Water-based systems do not have solvent odors, but they are not necessarily odor-free. 

Adhesives and sealants are manufactured from a variety of polymers. Their selection and their combinations used impact solvent selection. Most solvent systems are designed to optimize the solubility of the primary polymer. Adhesives can be divided into ones which bond by chemical reaction and ones which bond due to physical processes. Chemically reactive adhesives are further divided into three more categories for those that bond through polymerization, polyaddition, or polycondensation. Physically bonding adhesives include pressure sensitive and contact adhesives, melt, or solution adhesives, and plastisols. Polymerization adhesives are composed of cyanoacrylates (no solvents), anaerobic adhesives (do not contain solvents but require primers for plastics and some metals which are solutions of copper naphthenate), UV-curable adhesives (solvent-free compositions of polyurethanes and epoxy), rubber modified adhesives (variety solvents discussed below). 

In view of the above efforts, it is surprising that the majority of recent patents on adhesives are for solvent-based systems.The new inventions include a universal primer, an adhesive composition in which solvents have been selected based on Snyder s polarity (only solvents which belong to group III are useful in adhesive for automotive applications to avoid a deleterious effect on paint), a low VOC adhesive for pipes and fittings, a solvent-containing heat-resistant adhesive based on siloxane polyimide, a water-based polyimide adhesive,and two-component solvent-free polyurethane adhesive system for use in automotive door paneling. ... 

Provided that the EPI adhesive systems are solvent free and that the emulsions used in the adhesive are VOC free , there will be no VOC emission from fully cured gluelines. 

Corhesive is a solvent-free, two-component reaction adhesive system for use with polyurethanes, plasma treated silicones and certain metals (Tables 4.4, 4,12, 4.13, and 4.14). 

One-part epoxy adhesives include solvent-free liquid resins, solutions in solvent, liquid resin pastes, fusible powders, sticks, pellets and paste, supported and unsupported films, and preformed shapes to fit a particular joint. Two-part epoxy adhesives are usually comprised of the resin and the curing agent, which are mixed just prior to use. The components may be liquids, putties, or liquid and hardener powder. They may also contain plasticizers, reactive diluents, fillers, and resinous modifiers. The processing conditions are determined by the curing agent employed. In general, two-part systems are mixed, applied within the recommended pot life (a few minutes to several hours), and cured at room temperature for up to 24 hours, or at elevated temperatures to reduce the cure time. Typical cure conditions range from 3 hours at 60°C to 20 minutes at 100 C. ... 

Development of solvent free, 100% solid adhesives Development of safer cross-linking systems for 100% solid adhesives Replacement of petroleum-derived polymers with bio-based alternatives. Examples include polylactic acid, polyhydroxyaUcanoates, elastin, and soy protein derivatives (biodegradable)... 

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