Structural adhesives toughened

Urethane structural adhesives have a morphology that is inverse to the toughened epoxy just described. The urethanes have a rubber continuous phase, with glass transition temperatures of approximately —50°C. This phase is referred to as the .soft segment . Often, a discontinuous plastic phase forms within the soft segment, and that plastic phase may even be partially crystalline. This is referred to as the hard segment . A representation of the morphology is shown in Fig. 3 [34]. 

These single-phase hybrids are very different from the two-phase toughened epoxynitrile adhesives that are discussed in Chap. 8. These two-phase adhesives have redefined structural adhesives to a great extent and have opened the door to many applications that were previously not possible because of the epoxy resin s inherent rigidity. The polymer mixtures that exist as separate phases provide significant increases in toughness but have only a small improvement in elongation at typical use levels. 

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. 

However, newer adhesives systems having moderate temperature resistance have been developed with improved toughness but without sacrificing other properties. When cured, these structural adhesives have discrete elastomeric particles embedded in the matrix. The most common toughened hybrids using this concept are acrylic and epoxy systems. The elastomer is generally a amine- or carboxyl-terminated acrylonitrile butadiene copolymer (ATBN and CTBN). 

Most conventional low-modulus adhesives and sealants, such as polysulfides, flexible epoxies, silicones, polyurethanes, and toughened acrylics, are flexible enough for use at intermediate low temperatures such as -40°C. Low-temperature properties of common structural adhesives used for applications down to -129°C are illustrated in Fig. 15.9, and the characteristics of these adhesives are summarized in Table 15.12. 

Rubber-Toughened Thermosetting Polymers in Structural Adhesives—Developments in Resins and Primers Kinloch, A. J., Ed. Applied Science Publishers London, 1980 Chapter 5, pp. 127-162. 

Acrylic structural adhesives have been modified by elastomers in order to obtain a phase-separated, toughened system. A significant contribution in this technology has been made in which acrylic adhesives were modified by the addition of chlorosulfonated polyethylene to obtain a phase-separated structural adhesive (11). Such adhesives also contain methyl methacrylate, glacial methacrylic acid, and cross-linkers such as ethylene glycol dimethacrylate [97-90-5]. The polymerization initiation system, which includes cumene hydroperoxide, IV,AT - dimethyl-p- toluidine, and saccharin, can be applied to the adherend surface as a primer, or it can be formulated as the second part of a two-part adhesive. Modification of cyanoacrylates using elastomers has also been attempted copolymers of acrylonitrile, butadiene, and styrene ethylene copolymers with methylacrylate or copolymers of methacrylates with butadiene and styrene have been used. However, because of the extreme reactivity of the monomer, modification of cyanoacrylate adhesives is very difficult and material purity is essential in order to be able to modify the cyanoacrylate without causing premature reaction. 

Ricotaff. [Ricon Resins] Anhydride/q>-oxy system structural adhesive fw polyolefins, elec, applies., epoxy toughener. 

Fig. 13.8 The T-peel resistance as a function of the crosshead speed for nitrile rubber/phenolic polyurethane liquid rubber blend toughened epoxy (upper curve) and conventional nitrile rubber-toughened epoxy structural adhesives (lower curve).

Urethanes have also been used to toughen vinyl-terminated acrylic adhesives for improved impact resistance. Thus rubber-toughened urethane acrylates [79,80], water-dispersible urethane acrylates [81], and high-temperature-performance urethane-acrylate structural adhesives have been reported [82]. Polyurethanes terminated with acrylic functionality are also used for anaerobic or radiation-cured adhesives with improved toughness [83]. 

Curing acrylic adhesives can be defined as reactive, cross-linked, high strength structural adhesives that cure by way of free radical initiation. They typically contain methacrylate monomers that are generally toughened with elastomeric polymers as part of the formulation. 

W. A. Lees, Toughened structural adhesives and their use, Int. J. Adhesion Adhesives 1(5) July (1981). 

Toughened acrylic adhesives are also classified as structural adhesives for limited exterior service environments, while polyurethane and polyesters (unsaturated) are considered as semi-structural adhesives for limited exterior service environments [3]. 

As structural adhesives, epoxies are the most widely accepted and used. They typically contain several components, the most important being the resin. To the base resin is added a variety of materials, for example hardeners, flexibilisers, tougheners and fillers. These all contribute to the properties of the resulting adhesive. Formulations may be further varied to allow for curing at either ambient or elevated temperatures. The epoxies and polyesters, together with acrylics, polyurethanes and synthetic polymer lattices will be... 

Lees, W.A. Toughened structural adhesives and their uses. International Journal of Adhesion and Adhesives, July 1981, pp. 241-7. 

In a contribution from B. F. Goodrich, Drake and Siebert extensively review the journal and patent literature since 1975 on reactive butadiene/acrylonitrile liquid and solid elastomers used in formulating epoxy structural adhesives. Areas reviewed include the preparation of elastomer-modified epoxy resins, the characterization of rubber-toughened epoxy resins, fracture mechanics and adhesive formulation and testing. 

---