Adhesives primers, surface treatment

In developing criteria for the ranking of adhesive formulations or adherend surface treatments or primers, it is necessary to distinguish between two different situations. In one case (contact adhesion), a true interface is believed to exist across which intermolecular forces are engaged, while in the other, an interphase is formed by diffusive interpenetration or interdigitation between the adhesive and the adherend (diffusion interphase adhesion). Even in the case of contact adhesion, more often than not, an mi vphase of macroscopic thickness forms on... 

Primers are required to be resistant to all of the same fluids and environments as the adhesive, and are in addition expected to be compatible with secondary finishes such as corrosion and fluid resistant primers applied to cured bond assemblies. The most commonly used primers for 250°F cured epoxy adhesives also have active corrosion inhibitors themselves to combat corrosion at bondlines. This last requirement is somewhat dated, evolving from the severe corrosion and delamination problems experienced before U.S. airframe manufacturers adopted durable surface treatments. 

The application of surface treatments to mbbers should produce improved wettability, creation of polar moieties able to react with the adhesive, cracks and heterogeneities should be formed to facilitate the mechanical interlocking with the adhesive, and an efficient removal of antiadherend moieties (zinc stearate, paraffin wax, and processing oils) have to be reached. Several types of surface preparation involving solvent wiping, mechanical and chemical treatments, and primers have been proposed to improve the adhesion of vulcanized SBR soles. However, chlorination with solutions of trichloroisocyanuric acid (TCI) in different solvents is by far the most common surface preparation for mbbers. 

Adhesion of fluorosilicone compounds requires surface treatment. For particularly difficult surfaces plasma treatment is necessary. Flowever, for most common applications, satisfactory bonding is achieved by using a specialized primer.80... 

Unlike substrate surface treatments, primers always add a new organic layer to the surface and two new interfaces to the joint structure. Most primers are developed for specific adhesives, and many are developed for specific adhesive/substrate combinations. 

Primers can be used to protect both treated metal and nonmetal substrates after surface treatment. The use of a primer as a shop protectant may increase production costs, but it may also provide enhanced and more consistent adhesive strength. The use of a primer greatly increases production flexibility in bonding operations. Usually primer application can be incorporated as the final step in the surface preparation process. The primer is applied as soon as possible after surface preparation and usually no more than a few hours later. The actual application of the adhesive may then be delayed significantly. 

As with metallic substrates, primers may be used to protect treated nonmetallic substrates. After surface treatment, a high-energy substrate has an active surface that will readily adsorb atmospheric contamination. The primer protects the treated surface until the time when the adhesive or sealant is applied. Primers are especially useful in this way for the protection of polymeric parts that are treated by flame or corona discharge. Primers also find benefit on polymeric substrates in that their solvents will soften the surface, and some of the primer resin will diffuse into the bulk of the substrate, thereby increasing the adhesive strength by molecular diffusion. 

Sometimes primers can take the place of surface treatments. Two examples are with porous substrates and with certain plastic substrates. With weak porous substrates, such as wood, cement, or porous stone, the primer can be formulated to penetrate and bind weakly adhering material to provide a new, tightly anchored surface for the adhesive. Chlorinated polyolefin primers will increase the adhesion of coatings and adhesives to polypropylene and to thermoplastic olefins. The chlorine atoms in the outer surface of the primer increase surface energy and enhance adhesion of adhesives, sealants, and paints. 

To obtain a usable adhesive bond with polyolefins, the surface must be treated. A number of surface preparation methods, including flame, chemical, plasma, and primer treatments, are in use. Figure 16.4 illustrates the epoxy adhesive strength improvements that can be made by using various prebond surface treatments to change the critical surface tension of polyethylene. 

After the adequacy of the surface treatment has been determined, precautions must be taken to ensure that the substrates are kept clean and dry until bonding. The adhesive or primer should be applied to the treated surface as quickly as possible. 

Surface Treatment of Polytetrafluoroethylene Primer, Adhesive, Corrosion Inhibiting, -67 to 200°F Primer, Adhesive, Corrosion-Inhibiting, for High Durability Structural Adhesive Bonding... 

The interphase region is complex, and its composition is usually unknown. Primers or surface treatments often are used to improve control of the interphase and provide increased adhesion, durability, and resistance to aggressive environments. The chemistry of primers and surface treatments is as varied as the chemistry of the adhesives, but they will not be considered further in this chapter. 

Table 28.4 Surface Treatment Effect of TMS by Succeeding Plasmas on Adhesion of Spray Paint Primer [Spraylat EWDY048 (primer D)] to TMS Plasma-Coated [7A](Ace/0) Aluminum Panels ...

Table 31.1 Adhesion Test Results of Chromated Spray Primers [Deft 44-GN-36 (A) and Courtauld 519X303 (G)] to A1 Alloys Prepared with Chemical Cleanings and Plasma Surface Treatments...

Primers are pigmented coatings that are applied to new surfaces or to old cleaned surfaces, prior to the application of undercoats or top coats. Its main functions are to achieve adequate adhesion to the substrate and to provide good intercoat adhesion for subsequent coats. They are specifically formulated for particular substrates such as wood, metals, concrete, and other masonry surfaces. Concrete and other masonry surfaces are alkaline and often require special surface treatments. For etching and neutralization of these alkaline surfaces, hydrochloric or phosphoric acid washing is usually done. 

Wafer surface priming. A surface treatment with a primer drives off moisture and improves adhesion of a photoresist. Typically, a wafer is heated in the presence of chemical vapor of the primer. Hexa-methyldisilazane seems to be one of the best primers. 

Choice of an appropiate surface treatment and a suitable primer are important because adhesion to the substrates presents difficulties [11.37]. Primers based on modified alkyd resins or two-pack epoxy-resins for derusted ferrous metals mainly contain zinc phosphate and zinc OKide as corrosion protection pigments. Nonferrous metals are first washed with an ammoniacal wetting agent before applying the primer that contains a binder based on synthetic resins (e.g., PVC copolymers, chlorinated rubber) which ensure good adhesion to the substrate. The same primer must be used on zinc or galvanized surfaces because the use of alkyd resins causes embrittlement [11.38] The primed surfaces are largely topcoated with alkyd resin systems. 

Primers are also used in conjunction with a surface treatment either to improve adhesive performance or to increase production flexibility in a bonding operation. Isocyanates have been used for over 50 years as primers on substrates such as rubber. 

Chem. Descrip. Reactive epoxy-functional siloxane oligomer in water Uses Adhesion promoter between inorg. materials (glass, minerals, metals) and org. polymers (thermosets, thermoplastics, elastomers) surface treatment aid for mins. reinforcement aid for glass fibers primer for glass and metals surface modifier for org. materials additive for water-borne polymers... 

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