Toughened adhesives

The homopolymers, which are formed from alkyl cyanoacrylate monomers, are inherently brittle. For applications which require a toughened adhesive, rubbers or elastomers can be added to improve toughness, without a substantial loss of adhesion. The rubbers and elastomers which have been used for toughening, include ethylene/acrylate copolymers, acrylonitrile/butadiene/styrene (ABS) copolymers, and methacrylate/butadiene/styrene (MBS) copolymers. In general, the toughening agents are incorporated into the adhesive at 5-20 wt.% of the monomer. 

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

Hysol EA 9696 Loctite-Henkel 1 h at 121°C Moisture-resistant, toughened adhesive service temperature to 121 °C... 

According to Kinloch, the addition of 1-8 wt.% of nanosilica particles with an average diameter of around 20 nm to a typical rubber-toughened adhesive, based... 

A second critical formulation area is the use of selective, reactive tougheners in combination with thermoset polymers, which results in a highly toughened adhesive that does not sacrifice mechanical properties. These avenues are far superior to the traditional method of softening the polymer matrix with thermoplastic materials that will degrade mechanical properties. Further development in this area continues. 

Lap shear tests are close to peel tests however, this method is more quantitative in the base. Two sheets are bonded to each other by toughened adhesives (acrylic) in the test configuration and tensile tester applies load at a rate of usually 1 mm/min [96]. The lap shear specimen represents the most utilized geometry for studies of adhesive bonding because specimens as single- or double-lap shears (Fig. 8.13a, b) are easy for the measmement [97]. 

Fig. 2.3. Schematic representation of failure mechanisms in rubber toughened adhesives, (a) Shear yielding, b) Crazing (not necessarily applicable to epoxies).

The nylon-toughened adhesive, AF-44, presents a special problem. The nylon-toughening material which gives this adhesive its exceptional properties is hydrophilic and its "toughening" is likely due to the presence of intermolecular hydrogen bonds. In the presence of... 

The use of adhesives/sealants in both the industrial and consumer spheres has increased dramatically in the past 20 years. In the industrial segment, both reactive and nonreactive systems are used in a wide variety of applications. The increasing use of reactive systems has, however, tended to overshadow that of the nonreactive systems. Most prominent amongst the reactive systems favored in industrial applications are anaerobic sealants (methacrylate ester based), instant adhesives (alkyl cyanoacrylate ester based), acrylic (toughened) adhesives, epoxy resin adhesives, polyurethane/isocyanate-based adhesives, silicone adhesives/sealants, and phenolic resin adhesives. 

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. 

It has been shown that in comparison with spot welds, rivets and a variety of selfpiercing mechanical fastenings, toughened structural adhesives may yield single lap shear joints of superior energy absorption when used to bond aluminium alloy sheet the findings were confirmed in tests on full-size bonded vehicles (see Toughened adhesives, Structural adhesives). 

In contrast to polyethylene and EVAs, Epoxide adhesives are thermosets and are much stiffer and less ductile. The adhesion fracture energy (Gc, see Fracture mechanics), between both unmodified and rubber-toughened epoxies (see Toughened adhesives) and several metals has generally been found to be much higher when microfibrous surfaces were involved (Table 2) ... 

Some important Structural adhesives are two-phase materials composed of dispersed rubbery particles in a glassy matrix (see Toughened adhesives). Optimum performance depends upon achieving the correct phase structure, which is achieved only by careful attention to the procedure for mixing, applying and curing the adhesive. 

Information on particular types of adhesive may be found in articles cited above and in the following entries Hot melt adhesives. Selection of adhesives. Silicone adhesives addition cure. Silicone adhesives condensation cure. Toughened adhesives. 

Figure 2 shows fracture surfaces for bonds between zinc and rubber-toughened epoxy resin (see Toughened adhesives). In Fig. 2(a), there is a region of cohesive failure within the resin the sites of bubbles, which may have initiated the fracture, can be seen. In Fig. 2(b), a piece of resin is seen adhering to what appears to be the bare zinc substrate. 

Other articles of relevance are Toughened adhesives and Toughened epoxide adhesives. In Structural adhesives, properties of epoxide and Polyurethane adhesives are compared with those of toughened acrylics. 

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