Epoxy primer/polyurethane adhesive

The (physically wetting) polyaniline primer CORRPASSIV was compared with a fast-drying two-component epoxy resin primer (Messrs. Finalin, Type 144) and a metal-reactive two-component primer with adhesion-promoting properties (Type 918). All three primers were coated with a two-component polyurethane paint (B412). CORRPASSIV, both with and without top coat, exhibited no rust formation beneath the primer. The slight undermining receded after the test, and even within the cross and scratch injuries, it was virtually impossible to detect any corrosion. 

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). 

When using two-part urethane structural adhesives, priming of the substrates is often required in order to obtain optimum bond durability, particularly when bonding metals. Primers are generally developed for use with a particular combination of adhesive and adherends. There are various types of primers for polyurethane adhesives, including one-part heat cure epoxies, two-part heat and room temperature cure epoxies, and one-part urethanes. 

For adhesive gluing, the surfaces to be joined must be adequately treated to improve adhesion. Flaming, eorona diseharge and primer are used for surface treatment. The methods for surfaee treatment are similar to those used for decorating and printing (Section 7.3). Adhesives used for bonding of PP are eontact adhesives based on polyurethanes or synthetie rubber, two-pack adhesives based on epoxy resins, polyurethanes, ete., vinyl aeetate eopolymer hot melt adhesives, pressure-sensitive adhesives and pol5mrethane eontact cements. 

Suggested adhesives include modified acrylics, epoxies, polyesters, resorcinol-formaldehyde, furane, phenol-formaldehyde, polyvinyl formal-phenolic, polyvinyl butyral, nitrile rubber-phenolic, polyisobutylene rubber, polyurethane rubber, reclaimed rubber, melamine-formaldehyde, epoxy-phenolic, and cyanoacrylates. For maximum adhesion primers should be used. Nitrile-phenoUcs give excellent bonds if cured under pressure at temperatures of 149 C. Lower-strength bonds are obtained with most rubber-based adhesives. 

Two types of paint systems were used for experiments. First system used was zinc phosphate primer (50 % Zn as compound) with micaceous iron oxide (MIO) as intermediate coat followed by polyurethane (PU) as top coat denoted as ZP. Other system used was zinc rich primer (80 % Zn as dust) with MIO as intermediate coat and PU as top coat denoted as ZR. This system is costlier with respect to ZP and considered for laboratory evaluations purpose only. ZP is designed with respect to places where test panels are deployed for exposure based on manufacturers catalogues and standards [4]. These systems possess good resistance from water permeability, weathering, abrasion and good adhesion to maintain a proper barrier to the environment. The drying mechanism of the systems is the reaction between epoxy resin and polyamide. The details of each paint system used comprising primer, intermediate and top coats with the respective layer of film thickness are shown in Table 2.2. 

The benefits of a duplex coating are achieved only if there is good, long-lasting adhesion of the paint to the zinc coating. The NMP (A(-methyl-pyrrolidone) test has been described by Van Ooij et al. (1982). The test panels are immersed in the heated solution at 60 C (Fig. 1.16) and the time before the paint lifts is measured. Figure 1.17 shows the use of this test to demonstrate how silanes increase adhesion of an automotive epoxy-polyurethane primer to galvanized steel. 

Specializing in the manufacture of adhesives, primers, varnishes and coatings based on water, epoxy, acrylic, and polyurethane, LORD Industrial Ltda.—a subsidiary of the North /tmerican LORD Corp.—began its activities in Sao Paulo, Brazil, in 1972. The following year, it transferred its productive unit to the industrial hub of Jundiai (Sao Paulo), where it remains today. Throughout this period, LORD has become a main producer in the Brazilian adhesive market, principally with regard to the fabrication of products with tow levels of heavy metals, primers for the treatment of metal surfaces and water-based adhesives. For further information, visit www tordta. com.br and www.lord.com. 

This means, therefore, that the chemistries associated with the primers could be as varied as the adhesives used with them. In aerospace applications, however, the predominant surface protection primer chemistries, for structural bonding, are those based on epoxy phenolic-, polyimide- and polyurethane-based primers are also encountered. 

---