Toughened acrylic adhesives

The use of rubbers (low glass transition polymers) to toughen acrylic adhesives... 

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. 

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. 

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]. 

In the mid-seventies, spurred by developments from the DuPont Corporation, a new kind of two-component acrylic adhesive system was introduced to the joining industry. The DuPont technology has been widely licensed and is now referred to by numerous terms such as "second-generation acrylics", "reactive adhesives", "modified acrylics", "toughened acrylics", etc. These adhesives seemed to answer the need for improvements over some of the major disadvantages found in anaerobic structural adhesives, namely those of cost, the need for scrupulous surface preparation, and the ability to bond plastic surfaces. 

Acrylic adhesives are applied to the substrates to be bonded as a liquid mixture of unreacted methacrylate (or acrylate) monomers, polymers to thicken or toughen, reactive resins for strength, adhesion promoters and polymerization agents. Polymerization or curing is... 

The versatility of acryhc adhesives comes from the large number of different monomers and resins available, which give flexible and tough or hard and rigid adhesives when cured, depending on the potential Glass transition temperature or on the functionality of the monomers (see Toughened acrylic adhesives). 

The second means of transforming a liquid adhesive entirely into a solid without the loss of a solvent or dispersion medium is to produce solidification by a chemical change rather than a physical one. Such reactive adhesives may be single-part materials that generally require heating or exposure to electron beam or UV or visible radiation (see Radiation-cured adhesives) to perform the reaction, and which may be solids (that must be melted before application), liquids or pastes. The alternative two-part systems require the reactants to be stored separately and mixed only shortly before application. The former class is exemplified by the fusible, but ultimately reactive, epoxide film adhesives and the latter by the two-pack Epoxide adhesives and Polyurethane adhesives and by the Toughened acrylic adhesives that cure by a free-radical Chain polymerization mechanism. 

Aspects of this subject are also dealt with in other articles, notably Acrylic adhesives. Durability - fundamentals. Joint design general. Joint design cylindrical joints. Joint design strength and fracture perspectives. In the article on Toughened acrylic adhesives, some properties are compared with those of Epoxide adhesives and anaerobic adhesives. 

Reactive acrylic adhesives generally consist of a solution of a toughening rubber (chlorosulphonated polyethylene) in a partly polymerized mixture of monomers this is mainly methylmethacrylate but ethane diol dimethacrylate is added as a cross-linking agent. The remaining monomer is polymerized in a free radical chain polymerization redox initiation involves an organic peroxide and a tertiary amine. Acrylic cements consist of a partly polymerized acrylic monomer containing an initiator. Cure is established by the thermal or UV decomposition of the initiator (see Radiation-cured adhesives). 

Properties of structural polyurethanes are compared with those of Epoxide adhesives and Toughened acrylic adhesives in the article on Structural adhesives. 

Structural adhesives are normally categorized or subdivided into subclasses based on the resin chemistries used in their formulation, which can be in the form of solids, liquids, pastes or films. The most important categories of structural adhesives are the thermoset cured adhesives based on phenolic, epoxy and polyurethane or acrylic resins and include phenolic adhesives. Epoxide adhesives, Toughened epoxide adhesives, Polyurethane adhesives, Acrylic adhesives and Toughened acrylic adhesives. Several of the categories can be further subdivided into both one- and two-component adhesives. The one-component structural adhesives, which can be liquids, pastes or solids (films), usually require the inclusion of added energy for activation or to effect cure in the... 

Toughened acrylic adhesives are structural adhesives and comprise mono-functional methacrylate monomers containing a dissolved rubber polymer added as a toughener, cure... 

The cure accelerator (a reducing agent) can be applied to the substrate as an activator, that is, as a solution in an organic solvent in the activator type toughened acrylic adhesives or can be contained in a second adhesive component (two-part adhesive). The most widely used reducing agent is the crude product, often described in patent literature as the condensation product, of aniline and butyraldehyde, the active component of which is N-phenyl 2-propyl 3,5-diethyl, 2 dihydropyridine (II). The aromatic amine are also... 

A significant recent innovation in the formulation of toughened acrylics is the ability to bond polyolefins (polyethylene and polypropylene) without any substrate pre-treattnents. This beneficial aspect of the toughened acrylic adhesives performance is attributed mainly to newly developed and patented cure technologies and provides an unique performance feature that cannot currently be matched with any other adhesive type." ... 

Table 2. Comparative benefits of industrial adhesives (toughened acrylics, anaerobics, epoxies)...

Other articles of relevance are Toughened acrylic adhesives. Epoxide adhesives. Structural adhesives and Acrylic adhesives. 

Toughened acrylic adhesives B KNEAESEY Setting mechanism applications advantages and disadvantages... 

Plate 15 The magnetic ferrites of this electric motor are bonded to the external case with a rubber toughened acrylic adhesive. This assembly technique avoids the problems of dimensions and distorted magnetic flux incurred when other methods are used. Material Permabond F241. 

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