Acrylics mechanical

Fig. 31. An acrylic terpolymer designed for chemically amplified resist applications. The properties each monomer contributes to the final polymeric stmcture are for MMA, PAG solubility, low shrinkage, adhesion and mechanical, strength for TBMA acid-cataly2ed deprotection and for MMA, aqueous...

Mechanical and Thermal Properties. The first member of the acrylate series, poly(methyl acrylate), has fltde or no tack at room temperature it is a tough, mbbery, and moderately hard polymer. Poly(ethyl acrylate) is more mbberflke, considerably softer, and more extensible. Poly(butyl acrylate) is softer stiU, and much tackier. This information is quantitatively summarized in Table 2 (41). In the alkyl acrylate series, the softness increases through n-octy acrylate. As the chain length is increased beyond n-octy side-chain crystallization occurs and the materials become brittle (42) poly( -hexadecyl acrylate) is hard and waxlike at room temperature but is soft and tacky above its softening point. 

The free-radical polymerization of acrylic monomers follows a classical chain mechanism in which the chain-propagation step entails the head-to-tail growth of the polymeric free radical by attack on the double bond of the monomer. 

These monomeis were mixed with nonfluoiinated acrylates and cured conventionally, such as by free-radical mechanism. Similar monomers and their... 

Mechanical Properties Related to Polymer Structure. Methacrylates are harder polymers of higher tensile strength and lower elongation than thek acrylate counterparts because substitution of the methyl group for the a-hydrogen on the main chain restricts the freedom of rotation and motion of the polymer backbone. This is demonstrated in Table 3. 

Acrylonitrile—Butadiene—Styrene. ABS is an important commercial polymer, with numerous apphcations. In the late 1950s, ABS was produced by emulsion grafting of styrene-acrylonitrile copolymers onto polybutadiene latex particles. This method continues to be the basis for a considerable volume of ABS manufacture. More recently, ABS has also been produced by continuous mass and mass-suspension processes (237). The various products may be mechanically blended for optimizing properties and cost. Brittle SAN, toughened by SAN-grafted ethylene—propylene and acrylate mbbets, is used in outdoor apphcations. Flame retardancy of ABS is improved by chlorinated PE and other flame-retarding additives (237). 

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. 

Copolymers of acrylonitrile [107-13-1] are used in extmsion and molding appHcations. Commercially important comonomers for barrier appHcations include styrene and methyl acrylate. As the comonomer content is increased, the permeabiUties increase as shown in Figure 3. These copolymers are not moisture-sensitive. Table 7 contains descriptions of three high nitrile barrier polymers. Barex and Cycopac resins are mbber-modified to improve the mechanical properties. 

Resins are also used for permanent tooth-colored veneers on fixed prostheses, ie, crown and bridges. Compositions for this application include acryflcs, vinyl—acryflcs, and dimethacrylates, as well as silica- or quartz-microfilled composites. The resins are placed on the metallic substrates of the prostheses and cured by heat or light. These resins are inexpensive, easy to fabricate, and can be matched to the color of tooth stmcture. Acrylic facings do not chemically adhere to the metals and are retained only by curing the resin into mechanical undercuts designed into the metal substrate. They have relatively low mechanical strength and color stability, and poor abrasion and strain resistance they also deform more under the stress of mastication than porcelain veneers or facings. 

Pure polymeric acrylonitrile is not an interesting fiber and it is virtually undyeable. In order to make fibers of commercial iaterest acrylonitrile is copolymerized with other monomers such as methacrylic acid, methyl methacrylate, vinyl compounds, etc, to improve mechanical, stmctural, and dyeing properties. Eibers based on at least 85% of acrylonitrile monomer are termed acryHc fibers those containing between 35—85% acrylonitrile monomer, modacryhc fibers. The two types are in general dyed the same, although the type and number of dye sites generated by the fiber manufacturing process have an influence (see Eibers, acrylic). 

Instead of using thermal energy to trigger the hydrogen abstraction mechanism, photo-induced reactions can be also be used to successfully crosslink acrylic PSAs [74-76], In this case, photoactive compounds, such as for example those containing benzophenone, anthraquinone or triazine nuclei are compounded with the polymer or copolymerized as one of the monomers. After drying, the adhesive... 

Blocked isocyanates are particularly helpful in dual cure mechanisms. In one instance, UV light first polymerizes an acrylate polymer containing hydroxyl groups. The system also contains a malonate ester-blocked isocyanate. The one-component system is heated, which starts the polymerization of the acrylate. Higher temperatures unblock the isocyanate, permitting the cure of the urethane to proceed [15]. 

Acrylic adhesives cure by a free radical chain growth mechanism. In contrast, epoxy and urethane adhesives cure by a step growth mechanism. This has a major impact on the cure kinetics, as well as the composition of the adhesive during cure ([9], pp. 6-9). Cyanoacrylate adhesives (such as Super Glue ) also cure by chain growth, but the mechanism is ionic with initiation by surface moisture. 

The ability to tailor acrylic adhesives to fast cure times allows their use on fast, highly mechanized production lines such as those for audio speakers [150]. 

Because they are acrylic monomers, alkyl cyanoacrylate esters still require the addition of radical polymerization inhibitors, such as hydroquinone or hindered phenols, to prevent radically induced polymerization over time [3j. Since basic initiation of alkyl cyanoacrylate monomers is the predominant polymerization mechanism, large quantities of free radical inhibitors can be added, with little or no effect on adhesive performance. 

Of the commercially available EB-curable adhesives [9-12], the resins fall within one of two categories based on their curing mechanisms. The majority of EB-curable resins are based on (meth)acrylate-functionalized oligomers involving a free-radical curing mechanism. The second category is the epoxy resins that cure by a cationic mechanism. 

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