Epoxies adhesive shear strengths

Tables 15.4 and 15.5 present the adhesion shear strength of epoxy (Epon 815C/Versamide 140)-bonded aluminum joints (anodized) and of PU (EN-4/EN-7)-bonded aluminum joints (anodized), respectively. Both types of adhesively bonded joints were primed with high MW HB PAMAM (AD-102). As is evident from the tables, significant improvements in shear strength were ob-...

Table 3.2 Adhesive shear strengths (epoxy resins)...

Fig. 19. Inlerfacial shear strengths of various fiber/matrix composites as a function of the work of adhesion as determined by IGC. 1, glass fiber-poly (ethylene) 2, carbon fiber-epoxy B 3, carbon fiber-epoxy A and 4, carbon fiber-PEEK. Redrawn from ref. [102].

Fig. I. Comparison of unprimed and eleetroprimed single lap-shear adhesive joint strengths for steel coupons bonded with imidazole-cured epoxy [43].

Flexibilized epoxy resins are important structural adhesives [69]. Liquid functionally terminated nitrile rubbers are excellent flexibilizing agents for epoxy resins. This liquid nitrile rubber can be reacted into the epoxy matrix if it contains carboxylated terminated functionalities or by adding an amine terminated rubber. The main effects produced by addition of liquid nitrile rubber in epoxy formulations is the increase in T-peel strength and in low-temperature lap shear strength, without reducing the elevated temperature lap shear. 

There has been very little work done on the development of EB-curable epoxy adhesives. When undertaking this development work the authors had two objectives. The first objective was to develop a series of adhesives for bonding aluminum-to-aluminum (Al-Al) and composite-to-composite (C-C) with lap shear strengths of 30 MPa or greater at room temperature. The second objective was that the... 

We have also looked at the lap shear strength of selected EB-ciirable epoxy adhesives. Because the adhesives being developed were being used for both aluminum-to-aluminum and composite-to-composite applications the lap shear strengths for both adherends was measured. Aluminum adherends were T2024 phosphoric acid anodized according to ASTM 3933. The composite adherends... 

Lap shear strengths (MPa) of selected EB-cured epoxy adhesives at various temperatures... 

Effects on lap shear strength of EB-cured epoxy adhesives from different surface preparations on aluminum and composite adherends... 

Allen and Alsalim 22 compared the effect of various pretreatments of stainless steel (martensitic structure) on the torsional shear strength of napkin ring joints formed with an epoxy adhesive (Redux 319 (Bonded Structures Ltd.)). They concluded that... 

Figure 8. Shear strength durability in a 57°C water immersion of epoxy/steel torsional joints with and without EME 90 (90 wt% mercaptoester unit co-polymer) coupling agent pretreatment. From ref. 6. Adhesive diglycidyl ether of bisphenol A (Epon 828) cured with a stoichiometric amount of methylene dianiline for I h at 120°C followed by 2 h at 150°C.

Both fiber-matrix interphase-sensitive mechanical tests (interlaminar shear strength, 90° flexure) and interphase-insensitive tests (0° flexure) were conducted on high volume composite samples fabricated from the same materials and in the same manner as discussed above to see if the interphase and its properties altered the composite mechanical properties and in what manner. A summary of the data is plotted as a bar graph in Fig. 7. The first set of bars represents the difference in fiber-matrix adhesion measured between the bare fibers and the sized fibers by the ITS. The composite properties plotted on the figure also show increased values for the epoxy-sized material over the bare fiber composite. 

Li and Netravali [121] modified an UHSPE fiber surface using allylamine plasma deposition to improve its adhesion to epoxy resin. The shear strength of the modified surface increased by a factor of 2-3 when measured by pull-out test using a U-shaped epoxy resin. The epoxy resin and a curing agent were mixed and molded into U-shaped form without disturbing the fiber (see Fig. 8). 

Adhesion studies of epoxy resins modified with high modulus and high glass transition temperature thermoplastics have shown adhesion can reach or even exceed that of the unmodified resin. The use of flexible polyamides or flexible epoxides resulted in shear strength increases in epoxy systems employed by Cunliffe et al. [144],polyethersulfones [18,145],polyetherimides [109,146,147], and polyetherketones [148-150]. 

FIGURE 3.9 Plot of tensile shear strength of an aluminum joint bonded with an elevated-temperature curing epoxy adhesive as a function of test temperature. 

The major problem is that the attainment of properties such as peel, flexibility, and toughness is generally accompanied by the reduction in properties such as heat resistance, chemical resistance, and shear strength. Future chapters discuss how the epoxy adhesive formulator can merge these properties. 

Epoxy adhesives have cohesive properties that significantly depend on the crosslink density. For example, torsional butt shear strength of epoxy aluminum joints has been noticed to decrease as Mc increased.20 However, flexibility and toughness are increased with increasing Mc. Figure 3.12 shows the general physical relationship between Mc and the physical state of epoxy resins. 

Polyamide cured DGEBA epoxies provide improved flexibility, moisture resistance, and adhesion over aliphatic amines alone. However, polyamide cured epoxies are generally inferior in thermal resistance and shear strength due to the reduction in crosslink density. Polyamide cured epoxies lose structural strength rapidly with increasing temperatures and... 

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