Phenolic adhesives shear properties

Plywood panels produced in Europe for building purposes use urea-formaldehyde (UF), melamine-urea-formaldehyde (MUF) and phenol-formaldehyde (PF) adhesives. Standards requirements are not focused on direct properties of these adhesives but properties of the end-products (closely linked to adhesive properties and process parameters). Nevertheless, EN 314-1 (2005) [1] defines a test method for the evaluation of the bonding quality of plywood panels, but also blockboards, laminated boards and laminated veneer lumbers (LVL), by shear testing before and after ageing treatments. Relevant requirements are specified in EN 314-2 (1993) [5] which also defines the ageing treatments required for the intended service class of the tested panel as shown in Table 1. 

Epoxy-phenolic adhesives are exceptional in that they have good adhesive properties at both elevated and low temperatures. Vinyl-phenolic adhesives maintain fair shear and peel strength at -423 °F, but strength decreases with decreasing temperature. Nitrile-phenolic adhesives do not have high strength at low service temperatures, because of rigidity. 

Examples of commercial nitrile-phenolic adhesives include Henkel s Plastilock and 3M s Scotch-Weld AF film adhesives. Features of these adhesives include fuel and chemical resistance and retention of strength after aging. Some of the properties of two grades of Plastilock which retain flex-ibiUty and shear strength in a service temperature range of — 55°C to 132°C can be found in Tables 5.1 and 5.2. Examples of performance data for Scotch-Weld are presented in Table 5.3. 

The degree of tensile strength improvement is often in the 50 to 100 percent range. The effect of various fillers and loading ratios on the strength properties of epoxy adhesive formulation is indicated in Fig. 9.9. The effect of different fillers loaded at a constant 100 pph is indicated in Table 9.12 for shear strength on phenolic laminate and aluminum substrates. 

Various types of polymeric modifiers have been researched as possible candidates to impart impact resistance of epoxy resins. Sometimes, properties other than impact resistance are to be modified. For example, for adhesive applications, the main focus is usually on improving shear and peel strength. In these cases, epoxies blended with elastomeric nitrile rubbers (Hycar CTBN, B.F. Goodrich), phenolics, nylons (soluble types such as DuPont Zytel-61), and pol5nirethanes are commonly used. For impact modification, there are several approaches, as discussed below. 

Peel resistance. The straight epoxies cured by either polyamines (amides) or anhydride are poor in peel. Polyvinyl formal-phenol formaldehyde (P/F) and butyral-P/F are moderate but the peel varies with the P/F content. As the P/F content increases, room temperature lap-shear and peel strength decrease but these properties improve at elevated temperatures. The epoxy-polyamides (epoxy-nylon) such as FM1000 (Cyanamid Corp.) give extremely good peel resistance under dry conditions. Hockney (1970) quotes 511N for an adhesive of this type with a strip 25 4 mm wide, peeled at 90°. The peeling... 

Adhesives recommended include epoxies, vinyl alcohol—vinyl acetate copolymer, polyvinyl alkyl ether, polyacrylate (carboxylic), polyurethane (two-part), epoxy-phenolics, silicones, and cyanoacrylates. The high-strength thermoset and alloy adhesives are rarely justified for bonding lead. Even when other properties recommend these adhesives, the designer should check to see whether some low-cost or easier-to-use adhesive is also suitable. An exception is teme (lead-coated steel). This is a much stronger metal than lead, and lap-shear strengths exceeding 2.1 MPa are reported for adhesive joints with teme.° ... 

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