Solvent-Based Adhesive Systems

Certain general categories are also listed, such as alloys, aromatic polymer, conductive, delayed-tack, elastomeric adhesives, anaerobic, film and tape adhesives, hot-melt adhesives, inorganic glues, microencapsulated adhesives, rubber-based adhesives, solvent-based systems, thermoplastic resin adhesives, thermosetting-resin adhesives, and water-based adhesives. 

Bonding properties of water-based contact adhesives are similar to those of solvent-based systems, but are free of flammabihty ha2ards. However, drying times are longer and the bond is sensitive to moisture. 

Compounding is quite different for the two systems. The solvent base system is dependent on magnesium oxide and a /-butylphenoHc resin in the formulation to provide specific adhesion, tack, and added strength. Neither of these materials have proven useful in latex adhesive formulations due to colloidal incompatibihty. In addition, 2inc oxide slowly reacts with carboxylated latexes and reduces their tack. Zinc oxide is an acceptable additive to anionic latex, however. Other tackifying resins, such as rosin acids and esters, must be used with anionic latexes to provide sufficient tack and open time. 

The ever increasing emphasis on reducing air pollution and on health considerations emphasized by OSHA has provided a strong impetus for adhesive manufacturers to develop water-based emulsion polymer systems to replace the organic solvent-based systems on which the industry was founded. This has been a difficult process. 

One of the newer developments in adhesives is the growing use of ultraviolet light or electron beam radiation to cure adhesives. Adhesives designed for UV- or E-beam curing are usually pressure sensitive or hot-melt systems based on acrylates, functional rubbers, or epoxidized rubbers, and use special UV or EB lamps to provide the cure. These systems can provide greatiy improved heat resistance compared to hot melts, and avoid the soivent emission problems of some of the solvent-based systems with which they compete. 

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. 

Good processing characteristics Adhesives can be supplied as contact bonding types—either in water- or solvent-based systems—hot-melt, pressure sensitive, or as reactive systems—either 100% solids content or solvent based. 

Good processing characteristics Adhesives are often used as a contact bond type— either in water- or solvent-based systems—as a hot melt, or as a pressure sensitive, in curing systems, or as heat or solvent reactivatible systems. Block copolymers provide superior pressure sensitive adhesives, but can also be compounded for contact bond adhesive or sealant applications. 

The fundamentals of pressure-sensitive hot-melt adhesives are similar to those of solvent-based systems. Most elastomers and tackifiers are suitable, although ethylene-vinyl acetate copolymers are also used and the conventional rubber types are not. Pressure-sensitive hot melts are dominated by thermoplastic rubbers, which are ideal for use in these applications. Their unique properties arise from their essentially two-phase structure, in which thermoplastic regions of styrene end blocks lock the elastomeric midsections of butadiene or isoprene at room temperature but allow the elastomer to move freely at elevated temperatures or in solvent. This gives the polymer properties that are akin to those of vulcanized rubbers at room temperature, while allowinig it to behave as a thermoplastic when heated or dissolved. This structure is illustrated in Fig. 1. 

Odor elimination is the additional benefit which has helped to drive the replacement of solvent-based systems. In packaging materials, most odors are related to the solvents used in inks, coatings and adhesives. Also, coalescing solvents from water-based systems caused odors. Elimination of solvent is a priority but solvent replacement may also change the response to the odor because solvents such as toluene and xylene smell like lubricating oils or turpentine whereas isopropanol smells more like a disinfectant. Odors stem not only from solvents but also from products of the thermal and UV degradation of other components and solvents. "... 

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. ... 

There are three general classes of adhesives. These are hot melt adhesives (HMA), pressure sensitive adhesives (PSA), and hot melt pressure sensitive adhesives (HMPSA). A HMA is a 100 % solids, thermoplastic composition which is compounded and applied molten at elevated temperatures and whose strength is obtained solely by the removal of heat. A PSA is a viscoelastic material which is permanently tacky at room temperature, such that a low force contact to a surface will cause the material to adhere instantaneously. A HMPSA is a viscoelastic material which is permanently tacky at room temperature and which is applied as a 100 % solids, thermoplastic composition in the molten state. Solvent based systems are the oldest technology. Water based systems have the advantage that they avoid flammability and generally do not give off obnoxious odors. Basic adhesive components within each of the three classes are ... 

A number of acrylic resins are used for bonding cloth, plastics, leather, and, in some cases, metal foils. The acryhc monomers most commonly used in adhesives are ethyl acrylate, methyl acrylate, methacryhc acid, acrylic acid, acrylamide, and acrylonitrile. The polymers or copolymers are soluble in common organic solvents and can be supplied in much the same manner as other solvent-based systems. In addition, the polymers are soluble in the monomers. When a catalyst is added, monomers polymerize, thus providing good bonding to glass and to plastic surfaces of similar composition (e.g., polymethylmethacrylate). ... 

When a solvent-based system is applied to a porous or permeable substrate, the solvent may pass out of the bond throngh the adherends. When the adherends are relatively impermeable, a common practice is to coat both mating surfaces and allow them to become snbstantially dry before bringing them into contact while the adhesive stUl retains some degree of Tack. This technique is familiar to many through its application to the repair of punctures in bicycle tyres or the lamination of decorative ( melamine ) laminates to various substrates. 

Polyurethane adhesives can be conveniently divided into two types - solvent-free and solvent-based systems. 

These are usually solvent-based systems of low solids content (ca. 10%), chemically similar to the adhesive to be used. Often a primer is a simple solution of its equivalent adhesive, without insoluble fillers, and so on. 

Solvent-based systems may be considered the origin for any trend line in adhesive development. They represent the oldest technology and a wide range of chemistries. Solvent-based adhesives have been valued through the years for providing fast drying conditions and good adhesive properties, especially durability. 

Because of its broad activity against mould producing fungi it has been recommended to use trans-1.2-bis(/i-propylsulphonyl) ethene as a paint and adhesive film fungicide (Taylor, 1965). Its applicability is, however, limited to solvent based systems, as the a.i. is not stable in aqueous alkaline media. 

With the advantages and disadvantages of these systems it is still going to be difficult to convince customers that the reliability of the bonded products using waterborne adhesives will be as good as for products produced with solvent-based systems. 

In Figure 8, the viscoelastic properties of natural rubber latex (Hartex 103 from Firestone Co.) and milled smoked sheet natural rubber were examined. Both natural rubbers have tan 8 peak maximum temperature at -58°C. However, the latex has higher room-temperature modulus than the milled natural rubber. Natural rubber latex based pressure-sensitive adhesives offer an advantage over solvent-based systems (milled smoked sheet) because of the molecular-weight difference between the two systems. The high-molecular-weight portion of natural rubber is insoluble in solvent and therefore cannot be used in solvent-based adhesives. Natural... 

Adhesives for Laminated Films. Laminated films are currently produced by two techniques lamination and extrusion. In lamination, the low-viscosity adhesive solution is applied by rolls to one of the films in a thickness of a few micrometers and dried. The two films are then combined in the laminator. The adhesives used are either two-component or one-component, moisture-cross-linking polyurethane adhesives, depending on requirements (type of film, sterilization resistance, sealing seam strength, etc.). Besides the traditional solvent-based systems developments have been made in the direction of solvent-free systems (high-solids products) and of polyurethane emulsion-based systems. 

These adhesives consist of a polymerizable liquid matrix and large volume fractions -of electrically insulating thermally conductive filler. Typical matrix materials are epoxies, silicones, urethanes, and acrylates, although solvent-based systems, hot-melt adhesives, and pressure-sensitive adhesive tapes are also available. Aluminum oxide, boron nitride, zinc oxide, and increasingly aluminum nitride are used as fillers for these types of adhesives. The filler loading can be as high as 70 - 80 wt %, and the fillers raise the thermal conductivity of the base matrix from 0.17-0.3 up to about... 

Many health and safety issues with adhesives have been mitigated with the movement away from solvent-based systems. Some activators and primers are still solvent-based, but have been changed from chlorinated solvents to safer ones such as alcohols, acetone and hydrocarbons. Some reactive acrylic activators are used undiluted or dissolved in monomers. 

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