Toughened adhesives peel strength

As with other adhesives peel strengths are considerably lower than direct tensile or tensile shear values so joint design is important. A peel strength of about 4 Newtons per millimetre joint width is the best that can be achieved to date with a toughened... 

The first group, resins that are used primarily to toughen epoxy adhesive systems, is described in Chap. 8. This chapter focuses on the resinous modifiers that are used as alloy blends. Characteristics of commercially available epoxy alloy adhesives are presented in Table 7.1. Tensile shear and peel strengths that are typical of these hybrid adhesives are compared in Table 7.2. 

A variety of toughening agents have been used to modify epoxy adhesives to improve peel strength and fracture toughness without significantly affecting other properties of the epoxy base resin. Generally these modifiers can be classified into three types ... 

Table 12.14 shows a comparison of dicyandiamide cured epoxy adhesives formulated with and without a CTBN adduct. When compared to the control epoxy, the toughened formulation exhibits significantly higher peel strength and moderately higher tensile shear strength. CTBN modified epoxy adhesives are generally one-part systems, cured with dicyandiamide at elevated temperature. 

Toughened adhesive with excellent peel strength contains glass beads for bond line control... 

Figure 1. Lap-shear strength (top) and T-peel strength (bottom) of one-part epoxy adhesives (Adhesive A formulations). Key CRS, oily cold-rolled steel EGS, electrogalvanized steel CTy control (unmodified resin) T-Ay T-By and T-C refer to particle sizes of MEAc tougheners 5 to 45 pm (T-A)y 17.8 pm (T-B), and 2.3 pm (T-C) and L13y Hycar CTBN (1300x13).

Cyanoacrylates are one-part, highly polar thermoplastic polymers. The resin monomers cure in seconds when in contact with a weak base such as the moisture that is present on most surfaces. Many cyanoacrylate-adhesive formulations are commercially available, but not widely used in electronics assembly because of their poor resistance to solvents and moisture at elevated temperatures (>70 °C). Cyanoacrylates have relatively low impact and peel strengths and may be brittle unless toughened by the addition of elastomeric resins. 

Although development of toughened adhesives has been limited to the various acrylic (both anaerobic and non-anaerobic) and epoxy types, overall performance has been improved spectacularly. Both environmental resistance and peel and impact performance have been raised substantially without sacrificing shear strength. 

No account has been taken so far of the thickness of the adhesive within the joint line, but conventional wisdom, based on standard test procedures, dictates a bond line that is as thin as possible. While, to a degree, this is equally true for both the older types of conventional high-strength - though brittle - adhesives and the new toughened , non-brittle adhesives, the peel strength of the latter increases with bond thickness, as shown in Figure 2.10. And, it should be borne in mind that real joints are not usually stressed like standard test joints. 

Figure 2.10 Effect of bond line thickness on the peel strength of a toughened epoxy-based adhesive. Although not directly proportional, the effect is substantial and of considerable practical importance because bond line thickness is often difficult to control. In most cases the overall result of an increase in bond line thickness is beneficial, even though the shear strength may fall most joints fail because they cannot meet peel and cleavage overloads. Generally shear overloads are rare.

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