Adhesives bonding

Adhesives available in many forms liquids, emulsions, gels, pastes, films, tapes, powder, rods and granules. 

Curing mechanisms heat, pressure, time, chemical catalyst, UV light, vulcanization or reactivation, or a combination of these. 

Various additives catalysts, hardeners, accelerators and inhibitors to alter curing characteristics, silver metal flakes for electrical conduction and aluminum oxide to improve thermal conduction. 

Adhesives can be applied manually or automatically by brushing, spreading, spraying, roll coating, placed using a backing strip or dispensed from a nozzle. 

Natural animal (beeswax, casein), vegetable (gum, wax, dextrin, starch) and mineral- (amber, paraffin, asphalt) based glues. Commonly low strength applications such as paper, cardboard (packaging) and wood. 

An adhesive is a substance made principally from TP and TS plastics (also vegetable, animal by-products, silicates, etc.) which applied, as an intermediate is capable of holding material together by surface attachment. Mechanism of adhesion (adherence) is the phenomenon in which interfacial forces hold siu feces together. Adhesion may be by molecular attraction, mechanical, electrostatic, or solvent depending upon whether it results from interlocking action, from the attraction of electrical charges, from valence forces, or solvent action, respectively. 

Advances in the use of TP and TS plastic adhesives have made possible the adhesive bonding of RP structural and nonstructural parts in appliances, automobile, aircraft, medical devices, and so on. Adhesives with strengths higher than some metals are used (epoxy, etc.). The wealth of adhesive technologies that are available could make adhesive selection a task if one does use the proper approach such as determining specifically what performance requirements are needed (as with any selection procedure). The best adhesive for an application will depend on processing considerations and meeting the performance requirements. Tables 5.23 and 5.24 provide information on types and use of adhesives. 

Bond properties Standard epoxies Two-part polyurethane Methcrylates (Plexus) Silicones Cyano- acrylates 

Shear strength Poor-fair Good-excellent Excellent Poor-fair Excellent 

Flexible composite textile substrates are formed by either coating textile substrates with a continuous polymer layer or lamination of two or more textile substrates together by use of an adhesive polymer layer. The polymer coating can be applied neat, from solution, as an emulsion, or in the form of a film or thin foanr Inflexible fiber-polymer composite substrates are formed by imbedding fibers, fiber webs, fiber tows, or fabries in a stiff polymer matrix. Inflexible composites have found extensive use in engineering and aerospace applications particularly where high performance properties are important. Owing to their inflexibility, they lose any properties characteristic of textile substrates. 

In order to get a good bond between the fabric substrate and the polymer coating in laminated and coated fabrics it is essential to get a good adhesive bond between the textile substrate and the coating or adhesive. This is particularly important since the substrate and polymer may have very different stretch and recovery properties. Adhesive failure under stress is the most probable cause of failure in these materials. In coated fabrics, elastomeric properties are necessary to achieve a serviceable coated substrate. 

In coated substrates, the polymer coating is metered and spread evenly onto the fabric surface as a controlled viscosity solution, aqueous emulsion, or hot melt polymer followed by drying and curing if needed. A urethane foam can be bonded to a textile substrate, by carefully melting the foam surface with a flame and joining the melted foam surface to the tex-ti le substrate under pressure followed cool ing. Laminated fabrics are bonded under similar conditions except that two fabric faces are brought together and bonded by the polymer layer used. 

Polybutylene Terephthalate (PBT) Solvents recommended by General Electric for their VALOX thermoplastic polyester are (11)  

The solvent is brushed on the mating surface and dried under pressure. These solvents are toxic and should be applied only in areas of positive ventilation. 

Polyetherimide (ULTEM ) Methylene chloride, with or without a 1-5% solution of ULTEM , is recommended. Moderate pressures of 100-600 psi for 1-5 minutes are required (12). 

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Munns, G.A. Georgiou, Non-destructive testing methods for adhesively bonded joint nspection - a review, INSIGHT, Vol 37, No 12, Dec 1995, pp 941-952... 

When the scanning of the adhesive bonded joint between the shells on the leading edge is complete, the rotor blade is rotated 180° and another special designed Y-module is applied for inspection of the trailing edge of the rotor blade in set-up 3, illustrated on figure 5. 

Evans and Ritchie, 1997] Evans, E., and Ritchie, K. Dynamic strength of molecular adhesion bonds. Biophys. J. 72 (1997) 1541-1555... 

Plywood requirements—includes wood species used, synthetic repair requirements, veneer grades, veneer layers and thicknesses, panel grades with respect to end-use, adhesive bond requirements, panel constmetion and workmanship, scarf and finger-jointed panels, dimensional tolerances, moisture content, and packaging and loading... 

The bulk physical properties of the polymers of the 2-cyanoacryhc esters appear in Table 2. AH of these polymers are soluble in /V-methy1pyrro1idinone, /V,/V-dimethy1foTm amide, and nitromethane. The adhesive bonding properties of typical formulated adhesives are Hsted in Table 3. 

Table 3. Adhesive Bond Properties of 2-Cyanoacrylic Esters with Metals and Various Polymeric Materials...

The cured polymers are hard, clear, and glassy thermoplastic resins with high tensile strengths. The polymers, because of their highly polar stmcture, exhibit excellent adhesion to a wide variety of substrate combinations. They tend to be somewhat britde and have only low to moderate impact and peel strengths. The addition of fillers such as poly (methyl methacrylate) (PMMA) reduces the brittleness somewhat. Newer formulations are now available that contain dissolved elastomeric materials of various types. These mbber-modifted products have been found to offer adhesive bonds of considerably improved toughness (3,4). 

The interphase is the volume of material ia which the properties of one substance gradually change iato the properties of another. The iaterphase is useful for describiag the properties of an adhesive bond. The interface contained within the iaterphase, is the plane of contact between the surface of one material and the surface of another. Except ia certain special cases, the iaterface is imaginary. It is useful ia describiag surface eaergetics. 

Fig. 2. Illustrations of forces to which adhesive bonds are subjected, (a) A standard lap shear specimen where the black area shows the adhesive. The adherends are usually 25 mm wide and the lap area is 312.5 mm. The arrows show the direction of the normal apphcation of load, (b) A peel test where the loading configuration, shown by the arrows, is for a 180° peel test, (c) A double cantilever beam test specimen used in the evaluation of the resistance to crack propagation of an adhesive. The normal application of load is shown by the arrows. This load is appHed by a tensile testing machine or other...

The principal type of shear test specimen used in the industry, the lap shear specimen, is 2.54 cm wide and has a 3.23-cm overlap bonded by the adhesive. Adherends are chosen according to the industry aluminum for aerospace, steel for automotive, and wood for constmction appHcations. Adhesive joints made in this fashion are tested to failure in a tensile testing machine. The temperature of test, as weU as the rate of extension, are specified. Results are presented in units of pressure, where the area of the adhesive bond is considered to be the area over which the force is appHed. Although the 3.23-cm ... 

Peel tests are accompHshed using many different geometries. In the simplest peel test, the T-peel test, the adherends are identical in size, shape, and thickness. Adherends are attached at thek ends to a tensile testing machine and then separated in a "T" fashion. The temperature of the test, as well as the rate of adherend separation, is specified. The force requked to open the adhesive bond is measured and the results are reported in terms of newtons per meter (pounds per inch, ppi). There are many other peel test configurations, each dependent upon the adhesive appHcation. Such tests are well described in the ASTM hterature. 

Emulsion Adhesives. The most widely used emulsion-based adhesive is that based upon poly(vinyl acetate)—poly(vinyl alcohol) copolymers formed by free-radical polymerization in an emulsion system. Poly(vinyl alcohol) is typically formed by hydrolysis of the poly(vinyl acetate). The properties of the emulsion are derived from the polymer employed in the polymerization as weU as from the system used to emulsify the polymer in water. The emulsion is stabilized by a combination of a surfactant plus a coUoid protection system. The protective coUoids are similar to those used paint (qv) to stabilize latex. For poly(vinyl acetate), the protective coUoids are isolated from natural gums and ceUulosic resins (carboxymethylceUulose or hydroxyethjdceUulose). The hydroHzed polymer may also be used. The physical properties of the poly(vinyl acetate) polymer can be modified by changing the co-monomer used in the polymerization. Any material which is free-radically active and participates in an emulsion polymerization can be employed. Plasticizers (qv), tackifiers, viscosity modifiers, solvents (added to coalesce the emulsion particles), fillers, humectants, and other materials are often added to the adhesive to meet specifications for the intended appHcation. Because the presence of foam in the bond line could decrease performance of the adhesion joint, agents that control the amount of air entrapped in an adhesive bond must be added. Biocides are also necessary many of the materials that are used to stabilize poly(vinyl acetate) emulsions are natural products. Poly(vinyl acetate) adhesives known as "white glue" or "carpenter s glue" are available under a number of different trade names. AppHcations are found mosdy in the area of adhesion to paper and wood (see Vinyl polymers). 

Bonding. Surface treatment, such as chemical etch, corona, or flame treatments, is required for adhesive bonding of Tefzel. Polyester and epoxy compounds are suitable adhesives. 

C. A. L. Westerdahl and co- oSk.cx.s Activated Gas Plasma SuTpace Treatment of Polymers for Adhesive Bonding, Part III, Technical Report 4279, Picatinny Arsenal, Dover, N.J., 1972. 

Some flexible packaging is fabricated by converters into bags and pouches. Bag material is either small monolayer or large multiwall with paper as a principal substrate. Pouches are small and made from laminations. Bags usually contain a heat-sealed or adhesive-bonded seam mnning the length of the unit and a cross-seam bonded in the same fashion. 

The method used to apply and dry a fixative affects the degree of stiffness and hold it imparts to the hairstyle. If appHed and then manipulated with a comb, bmsh, or fingers as the polymer forms its film, the adhesive bonds between the hair are broken, the film coating the hair is broken, and the end result is a soft feel with Htde set retention. If the fixative is allowed to dry undisturbed, then the result is a firmer feel and better hold. 

LARC-TPI is a linear thermoplastic PI which can be processed ia the imide form to produce large-area, void-free adhesive bonds. Mitsui Toatsu Chemicals, Inc., has obtained Hcense to produce this product commercially for appHcations such as adhesives, films, mol ding compounds, etc. These are thermooxidatively stable and show essentially no loss ia weight at 300°C ia air. Weight loss does not exceed 2—3% after isothermal aging ia air at 300°C for 550 h. 

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