Curing toughened acrylics

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. 

In recent years, the range of adhesive materials used in automotive manufacture has expanded to include polyurethanes, plastisols, phenolics, hot melts, anaerobics, cyanoacrylates, toughened acrylics and epoxies (see Structural and Hot melt adhesives). Selection criteria are based principally upon the nature of the adherends, the mechanical properties required under service conditions and application and curing characteristics. 

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." ... 

It is possible to bond PVC and GRP pipes successfully with toughened acrylics, and GRP pipes can also be bonded with cold-curing, two-part epoxies. These adhesives generally cope well with most fluids likely to be carried in such pipes. Solvent welding may also be used to bond PVC provided that the gaps are small. 

Quite contrary to popular belief - a belief founded on the use of traditional adhesives - reliable joints can be obtained from unprepared surfaces. Of course, there is no denying that the better the preparation the better the overall performance. But, providing contamination is not gross, perfectly adequate levels of performance can normally be obtained from Anaerobic Cyanoacrylate Plastisol Toughened acrylic and Toughened, heat-cured, epoxide-based adhesives. 

A toughened adhesive has small rubber-like particles dispersed throughout the adhesive which improves some of the properties. Toughened acrylics are relatively fast curing and offer high strength and toughness and have more flexibility than the common epoxies. 

Acrylic anaerobics, conventional acrylics, cyanoacrylates, the so-called second generation , toughened acrylics and adhesives having UV-activateable or UV-curing capabilities... 

Small die castings are conveniently bonded with polymethylmethacrylate dissolved in its monomer together with a peroxide initiator. Metal naphthenates are to be avoided as accelerators and hence the so-called second generation or toughened acrylics are not recommended for use with zinc. Other acrylics may be used but are slow curing after evaporation of solvent. Polyester adhesives cured with styrene monomer frequently contain some free phthalic acid as an impurity and this militates against their use. Polyurethane adhesives cured with isocyanate should behave quite satisfactorily. 

The toughened acrylics and heat cured epoxides have gained favour at the expense of the two-part epoxides whose popularity has been suppressed by their slower curing rate and the need to mix them. A summary of the major conventional characteristics of the various toughened types is given in Table 7.2. 

Steel (stainless) 2-3 Load-bearing when used with toughened acrylics and heat cured epoxides careful surface preparation may not be necessary... 

Studies of the particle—epoxy interface and particle composition have been helphil in understanding the mbber-particle formation in epoxy resins (306). Based on extensive dynamic mechanical studies of epoxy resin cure, a mechanism was proposed for the development of a heterophase morphology in mbber-modifted epoxy resins (307). Other functionalized mbbers, such as amine-terminated butadiene—acrylonitrile copolymers (308) and -butyl acrylate—acryhc acid copolymers (309), have been used for toughening epoxy resins. 

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). 

Relevant systems are mbber-toughened epoxy resins, high-impact acrylic (PMMA particles in mbber matrix, obtained by radical polymerization of 80/20 MMA/EVAc mixture), polyimide/silica hybrid materials (obtained using the sol-gel method), and a very high strength ( 60 MPa) mbbers (obtained by peroxide cure of a hydrogenated nitrile rubber/ zinc dimethacrylate system) [Inoue, 1995]. 

Room temperature cured (RTV) silicones are used as potting compounds and as in situ formed gaskets. These compositions are toughened by the addition of styrene or butyl acrylate which form minute strands of fiber in situ when they polymerize.21 Silicone sealants and coatings are being used in the NASA space shuttles.22 Paintable, fast curing-octoate-cured silicone sealants are also available. Simple tests for utility, based on a filled glass tube, have been described.22... 

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]. 

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