Polymer adhesive bonding

The preparation of the surfaces being together is often decisive in adhesion. Therefore, in the following we will briefly discuss the effects of different gas phase treatments of polyolefins (mainly plasma and corona discharge) on the adhesion for three representative groups polymer polymer or polymer adhesive bonding, metal-polymer bonding, and polymer-matrix composites. 

The fracture strength of a polymer depends on molecular weight, the chain length, and in a similar way, the fracture of a polymer-polymer adhesive bond depends on the length of the chain, which has diffused across the interface. 

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. 

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). 

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). 

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. 

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. 

The choice of initiator system depends on the polymerization temperature, which is an important factor in determining final product properties. Cold polymers are generally easier to process than hot polymers and in conventional cured mbber parts have superior properties. The hot polymers are more highly branched and have some advantages in solution appHcations such as adhesives, where the branching results in lower solution viscosity and better cohesion in the final adhesive bond. 

The adhesion of metal and ink to polymers, and the adhesion of paint and other coatings to metal, are of vital importance in several technologies. Aluminum-to-alu-minum adhesion is employed in the aircraft industry. The strength and durability of an adhesive bond are completely dependent on the manner in which the adhesive compound interacts with the surfaces to which it is supposed to adhere this, in turn, often involves pretreatment of the surfaces to render them more reactive. The nature and extent of this reactivity are functions of the chemical states of the adhering surfaces, states that can be monitored by XPS. 

As mentioned earlier, adhesive bond formation is governed by interfacial processes occurring between the adhering surfaces. These interfacial processes, as summarized by Brown [13] include (1) van der Waals or other non-covalent interactions that form bonds across the interface (2) interdiffusion of polymer chains across the interface and coupling of the interfacial chains with the bulk polymer and (3) formation of primary chemical bonds between chains or molecules at or across the interface. 

Surface analysis has made enormous contributions to the field of adhesion science. It enabled investigators to probe fundamental aspects of adhesion such as the composition of anodic oxides on metals, the surface composition of polymers that have been pretreated by etching, the nature of reactions occurring at the interface between a primer and a substrate or between a primer and an adhesive, and the orientation of molecules adsorbed onto substrates. Surface analysis has also enabled adhesion scientists to determine the mechanisms responsible for failure of adhesive bonds, especially after exposure to aggressive environments. The objective of this chapter is to review the principals of surface analysis techniques including attenuated total reflection (ATR) and reflection-absorption (RAIR) infrared spectroscopy. X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and secondary ion mass spectrometry (SIMS) and to present examples of the application of each technique to important problems in adhesion science. 

The outer surfaces of these plasma polymers are terminated with hydroxyl groups and have high surface energies. They are readily wet by adhesives and form strong and durable adhesive bonds [51]. 

The primary challenge facing adhesive bonding of metals is to obtain sufficient durability of a bonded structure. Initial bond strength in metal-polymer adhesive joints is almost invariably excellent. Challenging the application of adhesives in polymer-polymer joining, however, is the problem of obtaining a joint that is... 

These acids can be used alone or as mixtures. It is especially advantageous to use a mixture of liquid and gaseous acids. The gaseous acid will stabilize free monomer in the headspace of a container, while the liquid acid will prevent premature polymerization of the bulk monomer or adhesive. However, it is important to use only a minimum amount of acid, because excess acid will slow initiation and the formation of a strong adhesive bond. It can also accelerate the hydrolysis of the alkyl cyanoacrylate monomer to 2-cyanoacrylic acid, which inhibits the polymerization of the monomer and reduces molecular weight of the adhesive polymer. While carboxylic acids inhibit the polymerization of cyanoacrylate monomer, they do not prevent it completely [15]. Therefore, they cannot be utilized as stabilizers, but are used more for modifying the reactivity of instant adhesives. 

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