Adhesive epoxide

HDGS) upon exposure to a humid environment. As deseribed above, substrates were prepared by applying a phosphate eonversion eoating and then a ehromate rinse to HDGS. Lap joints were prepared from substrates having dimensions of 110 X 20 X 1.2 mm using a eommereial epoxide adhesive with a bond line thiekness of 250 (i.m. The joints were exposed to 95% RH at 35°C for 12 months and then pulled to failure. 

To determine the wet bond strength coated panels were immersed in distilled water for 1500 h, removed and discs 25.4 mm in diameter stamped from them. The surfaces were wiped with a dry tissue and bonded between two cylindrical test pieces using a polyamide cured epoxide adhesive and immediately placed in a sealed container at 100% RH for the adhesive to cure. After 16 h the specimens were broken on an Instron Universal Test Machine with minimum delay. Recovered values were measured after the panels had dried out at room temperature and humidity for 7 days. Clearly, it is unlikely that the values reported represent the minimum bond strengths, as some drying out is almost inevitable, but the values are directly comparable. 

As it had been shown that silanes were effective as pretreatments for a variety of coatings and particularly so when used as additives, selected silanes were examined as pretreatments and additives in conjunction with a two pack polyamide cured epoxide adhesive (Epikote 828/Versamid 115, 1/1) and a structural polyurethane adhesive based on diphenylmethanediisocyanate and a polyester resin. 

Reference to Table 12 will show the data for the structural polyurethane adhesive, from which it can be seen that the pattern for the epoxide adhesive is almost repeated, with all the silanes used as pretreatments producing marked... 

Comparison of silanes as pretreatments and additives for a structural epoxide adhesive Butt tensile, grit-blasted metal, 2 wt%as pretreatment and additive... 

The effect of silanes on the bond strength of two epoxide adhesives to glass is shown in Table 13. The test mode was that of Sandwich Butt Tensile in which a glass disc was bonded between two aluminium specimens in the form of a sandwich, and the resulting composite specimen broken on an Instron Universal Test Machine. Clearly in such a composite test specimen, alignment is important and there are many potential sites and modes of failure. 

The effect of storage at 100% RH at 21°C on stainless steel butt tensile/ structural epoxide adhesive joints is shown in Fig. 3. AAMS, GPMS and MPS were examined as pretreatments and the choice of stainless steel as the substrate... 

Figure 3. Effect of 100% RH at 2I°C on the bond strength of a stainless steel joint—polyamide cored epoxide adhesive.

Epoxides. Two-part epoxide adhesives were introduced soon after the second World War and are the type used most commonly for structural work. They have been investigated extensively and very wide ranges of resins, modifiers, curing agents, and other additives have been suggested for them (a complete summary is beyond the scope of the present chapter). 

Epoxide adhesives are based on precursors containing the three-membered, oxygen-containing oxirane group ... 

Chlorinated vinyl acrylic dispersions (for epoxide adhesives)... 

Quantitative confirmation of this outcome was obtained by shear tests, for which specimens were prepared both with and without the primer, a steel strip being bonded over the grit by an epoxide adhesive and the test carried out after curing of the second adhesive. Adhesion strengths recorded for the grit and adhesive were as shown in Table VII. 

Dannenberg, and May, Epoxide Adhesives, in Treatise on Adhesion and Adhesives. 

The strength of these systems is typically adequate for most structural applications. However, the deleterious effect moisture has on the bonding between the metal and adhesive must be taken into account if epoxy resins are to be used for bonding steel in moist or humid environments. This places a severe limitation on the number of structural bonding applications where epoxide adhesives could be used. 

Fig. 4.22. Condition of the bonded interface of unstressed double lap joints after 2 years natural exposure. (Joints constructed with gritblasted steel adherends united by 5 types of epoxide adhesive, and cured at 20 C).

A flexibilised cold-curing epoxide adhesive containing an aromatic amine hardener is likely to be the most successful, particularly if... 

Table 2 compares acrylic adhesives to two other important classes of adhesives used widely in industrial assembly operations Epoxide adhesives and Cyanoacrylate adhesives. The speed of bonding and ease of use of cyanoacrylates makes them ideal where... 

Other articles of relevance are Cyanoacrylate adhesives, Epoxide adhesives and Anaerobic adhesives. 

The second means of transforming a liquid adhesive entirely into a solid without the loss of a solvent or dispersion medium is to produce solidification by a chemical change rather than a physical one. Such reactive adhesives may be single-part materials that generally require heating or exposure to electron beam or UV or visible radiation (see Radiation-cured adhesives) to perform the reaction, and which may be solids (that must be melted before application), liquids or pastes. The alternative two-part systems require the reactants to be stored separately and mixed only shortly before application. The former class is exemplified by the fusible, but ultimately reactive, epoxide film adhesives and the latter by the two-pack Epoxide adhesives and Polyurethane adhesives and by the Toughened acrylic adhesives that cure by a free-radical Chain polymerization mechanism. 

Aspects of this subject are also dealt with in other articles, notably Acrylic adhesives. Durability - fundamentals. Joint design general. Joint design cylindrical joints. Joint design strength and fracture perspectives. In the article on Toughened acrylic adhesives, some properties are compared with those of Epoxide adhesives and anaerobic adhesives. 

When mixed with acid anhydrides or primary or secondary amines, epoxide adhesives cure by step polymerization. However, when the hardener is a tertiary amine or a Lewis acid such as BF3 (Acids), cure is by ring opening ionic chain polymerization. 

Many of the entries in this book dealing with the science of a(fliesion and Theories of adhesion are relevant to composition materials in general. There are a number of articles that discuss specific aspects of composite materials that come within the scope of this book. Toughened adhesives, particnlarly Epoxide adhesives and Toughened acryiics, consist of polymers with a rubbery phase dispersed as small spheres within a more glassy matrix. Appropriate adhesion between the phases is crucial for effective toughening. 

Sometimes, these organic layers will be removed, or at least reduced, by a Pre-treatment, but on many occasions, the substrates will be used as received . With suitable selection of adhesives, paints and printing inks, good adhesion can often be achieved with these as-received surfaces. For example, oily steel can be bonded satisfactorily with some Acrylic adhesives, hot curing Epoxide adhesives and Solvent-based adhesives see Selection of adhesives. 

The effect of deliberately contaminating substrates has been studied. Brewis spread a layer of petroleum (c. 2 p,m thick) over HDPE, which had been etched with chromic acid. The effect on joint strength is shown in Table 1. The fact that most of the beneficial effect of the pre-treatment is retained indicates that most, if not all, the petroleum jelly has been absorbed by the epoxide adhesive. 

Table I. The effect of a layer of petroleum jelly (c. 2 p,m thick) on the lap shear strength of HDPE bonded with epoxide adhesive ...

Often, structurally bonded metals exhibit apparent interfacial failure when viewed either optically or at higher magnifications using electron microscopy. However, when viewed on the atomic scale, for example, using XPS, it is frequently found that mechanical loading will induce failure of the boundary polymer immediately adjacent to the metal surface. With structural Epoxide adhesives, recent studies have shown that this boundary polymer may have chemical and mechanical properties radically different from the bulk adhesive material and which are not incorporated within current models. 

Epoxide adhesives are used to bond a large range of components their versatility means that the two adherends being bonded do not have to be of the same composition. 

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