Toughened epoxy-based adhesives

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.

The ECV 3 is probably one of the most innovative cars ever built. A significant and unique feature is its aluminium monocoque body which relies entirely on a toughened epoxy-based adhesive for its structural integrity. (Photograph courtesy BL Technology Ltd adhesive supplied by Permabond Adhesives Ltd.)... 

The durability of adhesives for the structural bonding of metals is well documented and reference [1] is one of many papers on this subject. Much of the data that exist are based on lap shears using toughened epoxy-based adhesives and epoxies are widely used for the bonding of steel or aluminium parts. 

Fig. 7.3. Electron micrograph revealing the structure of a toughened epoxy-based adhesive the regular, evenly distributed rubbery phase is clearly seen.

Over the past several decades, significant advances have been made in developing epoxy-based adhesives having improved performance over these early adhesive systems. These improvements were made possible by (1) the incorporation of toughening additives into epoxy resin formulations and (2) the use of multifunctional epoxy resins primarily for high-temperature applications. These innovations are discussed in later chapters. 

Toughened adhesives acrylic or epoxy-based adhesives cured by a number of methods and can withstand high shock loads and high loads in large structures. 

Toughened adhesives. Toughened adhesives contain a dispersed, physically separate, though chemically attached, resilient rubbery phase. The toughened concept, in the modem sense, has so far only been successfully applied to acrylic and epoxy based adhesives. Since their introduction, some ten or more years ago, these adhesives have been shown to be very durable and to possess an outstanding resistance to shock loading. 

These adhesives are generally based on blends of solid epoxy resins with resole-type phenolic resin. The epoxy resin component is often not the predominant component in the blend, depending on the end properties required. Phenolics are compatible with epoxy resins and will react through the phenolic hydroxyl group. The amount of phenolic resin used is generally much greater than that required to crosslink with the epoxy, so one can debate whether (1) the epoxy toughens the phenolic adhesive or (2) the phenolic increases the heat resistance of the epoxy. 

Novel, toughened one-component epoxy structural adhesives based on epoxy-terminated polyurethane prepolymer incorporating an oxolidone structure were developed to provide improved toughness, fracture resistance and adhesive properties with good chemical and moisture resistance.21 The hybrid resin cures with a standard latent curing agent/accelerator. 

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 ... 

Adhesive KH-225 is a paste structural adhesive based on CTBN-toughened epoxy resin. It can be cured with EMI at a temperature such as 60 to 80 C under contact pressure. 

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. 

These structural composites, often epoxy-based may be bonded readily with a range of adhesives preferably the toughened types. 

The modification of epoxy structural adhesives by the addition of flexibilizing and/or toughening agents has greatly increased the utility of these products. Most structural adhesives based on DGEBA or cycloaliphatic resins cure to give products which are very strong but brittle. Such adhesives perform well when loaded in tensile and shear, but poorly when... 

Functionalized, liquid polybutadiene derivatives have also been developed as hybrid flexiblizers for epoxy resins. Carboxyl-terminated butadiene/acrylonitrile polymers, butadiene homopolymers, and maleic anhydride-amino acid grafted butadiene homopolymers have been used as flexibilizers to impart good low-temperature strength and water resistance to DGEBA-based epoxy adhesives. An epoxy system toughened by polybutadiene with maleic anhydride is claimed to provide a hydrophobic backbone, low viscosity, softness, and high tensile strength and adhesion (Table 7.10). 

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