Adhesives chemistry, cure reactions

Adhesives based on hydrolysis resistant chemistry (i.e., adhesives D and E) show a high retention of initial properties after exposure to an aggressive corrosion environment and the failure occurs cohesively within the adhesive. The cure reactions of these adhesives involve the formation of hydrolysis resistant carbon-nitrogen bonds in reactions involving the free N-H functionality of the Versamid or Genamid hardener with the oxirane functionality of the epoxy resin that is present in the adhesive formulation ... 

In this ehapter we describe the chemistry and technology of polysulfide polymers processing and manufacture of polysulfide sealants and adhesives, including their formulations curing reactions and characterization and testing. Adhesion considerations are also discussed briefly. 

A brief description of the chemistry of these materials is described on the article entitled Alkyl-2-cyanoacrylates. Commercial adhesive products are usually based on the ethyl ester but methyl, n-butyl, allyl -methoxyethyl and -ethoxyethyl are also important. Formulated adhesives consist of essentially pure monomer with relatively small amounts of property-modifying additives. The curing reaction is anionic polymerization, initiated by traces of alkaline material present on most substrate surfaces, particularly in conjunction with low levels of surface moisture (see Chain polymerization). 

Even though such treatments were applied a long time ago only relatively recently have papers presented the direct experimental evidence for the mechanisms involved in bonding between a plasma-generated surface and various adhesives. The changed surface chemistry facilitates reaction of the adhesive with surface species during curing, to form covalent bonds with the plasma-treated interphase [155]. 

Newer silicone adhesives having solids levels up to 97% are also commercially available [109]. Instead of using silanol condensation reactions, they rely on addition chemistry between vinyl functional silicone oligomers and silicon hydride terminated silicones. This addition reaction is typically facilitated with platinum derived catalysts. This hydrosilation process can be run at reduced oven temperatures, but the finished products typically do not yield the same balance of properties as seen for condensation cure materials. 

Proper formulation of epoxy adhesives requires knowledge of the chemical reactions that lead to polymerization as well as the chemical and physical properties of both the uncured mixture and the cured material. This chapter reviews the general principles of epoxy resin chemistry including synthesis of the epoxy monomer itself and its possible polymerization reactions. 

A typical cationic uv adhesive formulation contains an epoxy resin, a cure-accelerating resin, a diluent (which may or may not be reactive), and a photoinitiator. The initiation step results in the formation of a positively charged center through which an addition polymerization reaction occurs. There is no inherent termination, which may allow a significant postcure. Once the reaction is started, it continues until all the epoxy chemistry is consumed and complete cure of the resin has been achieved. Thus, these systems have been termed living polymers. 

The most important inference is that Chemisorption is a direct response to carboxyl group concentration indicated by the XPS photopeak component at 288.7 eV. It seems likely that weak add functionality is of minor import to applications for surface treatments, while interfacial phenomena such as practical adhesion may be sensitive to small concentrations of very high site energies. Interphase modification in epoxy resins, for example, can occur by direct reaction of epoxide groups with surface carboxyls (17), or by accelerated cure chemistry near the surface (39). Carboxyl groups on carbon surfaces may interact with basic moieties in polymers such as polycarbonate or poly(ethylene)oxide (40=42), or promote interfacial crystallinity that improves impact strength and other aspects of composite performance (43, M)-... 

Acrylic adhesives cure by addition polymerization reactions. These chain reactions are initiated by the formation of free radicals that result in the adhesive curing by way of a very rapid polymer chain growth. This cure chemistry is significantly more rapid than a typical cure curve (i.e., condensation type) found in epoxy and urethane adhesives. A comparison of the cure profile of condensation (epoxy and urethane) versus addition... 

The tendency of acrylic monomers to spontaneously and quickly polymerize makes it necessary for adhesive formulators to include various types of chemical stabilizers as part of the formulation to assure good shelf stability. These stabilizers are generally complex organic compounds that have a strong ability to react with free radicals (the cure chemistry most commonly employed with acrylics). These stabilizers stop unwanted side reactions and assure good shelf life of formulated adhesive products. 

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