Anionic curing

Although anionic initiation can take place, its use is not widely accepted (4). Tertiary amines and to a lesser extent secondary amines can be used to cure epoxies in this way. Anionic cure usually requires a long cure cycle and the cured material has a low heat-distortion point Use of imidazole... 

Tertiary amines used for anionic curing include pyridine, 2,4,6-tris(dimethylaminomethyl)phenol, tiiell lamine and benz ddimell anime. A mechanism for curing with a tertiary amine has been described (5)... 

Secondary amines such as piperidine, diethanolamine and imidazole are better anionic curing agents than tertiaiy amines. They first undergo addition to the epoxy group by means of their active hydrogens, then act as anionic initiators by formation of a quaternary amine with the CHj group to which the epoxy oxygen is attached. 

Since the anionic cure of cyanoacrylates proceeds as a result of basic catalysis, acids should naturally be considered as stabilizers. Acids of either the Lewis or protonic types have been used successfully. 

As a result, viscosity modification, through the use of various soluble polymeric thickeners, represented the first attempt to modify the liquid properties of cyanoacrylates. Thickeners and fillers must be compatible with the cyanoacrylates and must not set off the easily triggered anionic cure. Polymers which have been used to increase the base viscosity of the monomers include polymethacrylates (the most common), poly acrylates, poly cyanoacrylates, poly vinylacetates, baked polyacrylics, poly lactic acid, cellulose nitrates and other esters such... 

The sensitivity of the cure of cyanoacrylates to various substrates, notably acidic surfaces which inhibit or slow the anionic cure, has also been a recurrent problem. This had been overcome by the use of various basic surface acti-... 

Cyanoacrylate adhesives (Super-Glues) are materials which rapidly polymerize at room temperature. The standard monomer for a cyanoacrylate adhesive is ethyl 2-cyanoacrylate [7085-85-0], which readily undergoes anionic polymerization. Very rapid cure of these materials has made them widely used in the electronics industry for speaker magnet mounting, as weU as for wire tacking and other apphcations requiring rapid assembly. Anionic polymerization of a cyanoacrylate adhesive is normally initiated by water. Therefore, atmospheric humidity or the surface moisture content must be at a certain level for polymerization to take place. These adhesives are not cross-linked as are the surface-activated acryhcs. Rather, the cyanoacrylate material is a thermoplastic, and thus, the adhesives typically have poor temperature resistance. 

Alkaline Catalysts, Resoles. Resole-type phenoHc resins are produced with a molar ratio of formaldehyde to phenol of 1.2 1 to 3.0 1. For substituted phenols, the ratio is usually 1.2 1 to 1.8 1. Common alkaline catalysts are NaOH, Ca(OH)2, and Ba(OH)2. Whereas novolak resins and strong acid catalysis result in a limited number of stmctures and properties, resoles cover a much wider spectmm. Resoles may be soHds or Hquids, water-soluble or -insoluble, alkaline or neutral, slowly curing or highly reactive. In the first step, the phenolate anion is formed by delocali2ation of the negative charge to the ortho and para positions. 

Condensation cure can also be carried out ia emulsions (200—209). In this case, the cross-linker and polydimethylsiloxanediol are emulsified usiag anionic, cationic, or nonionic surfactants ia water, and a condensation catalyst such as dibutyltin dilaurate is added. The polymer can then undergo cross-linking, forming a continuous film when the water is evaporated. 

Eor instance, exhaust appHcation is possible with cationic finishes which have an affinity for the anionic groups in polymeric materials. After appHcation, the textile is dried. Durable antistatic finishes require cross-linking of the resin. Cross-linking is usually achieved by subjecting the treated, dried material to heat curing. A catalyst is often incorporated to accelerate insolubilization. 

V,/V dipheny1ethy1enediamine. The cure mechanism probably involves an amine-catalyzed decomposition of the sulfonyl chloride group or a path of radical anions. The cross-link probably involves the HVA-2. Calcium hydroxide or other SO2 absorbers must be included for development of good mechanical properties. 

Vmulsifier Type. The manufacturers of NBR use a variety of emulsifiers (most commonly anionic) for the emulsion polymerisation of nitrile mbber. When the latex is coagulated and dried, some of the emulsifier and coagulant remains with the mbber and affects the properties attained with the mbber compound. Water resistance is one property ia particular that is dependent on the type and amount of residual emulsifier. Residual emulsifer also affects the cure properties and mold fouling characteristics of the mbber. 

Catalytic curing agents initiate resin homopolymerization, either cationic or anionic, as a consequence of using a Lewis acid or base in the curing process. The Lewis acid catalysts frequently employed are complexes of boron trifluoride with amines or ethers. 

Cyanoacrylate adhesives cure by anionic polymerization. This reaction is catalyzed by weak bases (such as water), so the adhesives are generally stabilized by the inclusion of a weak acid in the formulation. While adhesion of cyanoacrylates to bare metals and many polymers is excellent, bonding to polyolefins requires a surface modifying primer. Solutions of chlorinated polyolefin oligomers, fran-sition metal complexes, and organic bases such as tertiary amines can greatly enhance cyanoacrylate adhesion to these surfaces [72]. The solvent is a critical component of these primers, as solvent swelling of the surface facilitates inter-... 

This equation can be said to represent the condition of complete saturation of all predetermined (in relation to the periodic system) anionic and cationic valences. There are, however, numerous examples of compounds whose predetermined classic valences do not satisfy Eqn. II.4. Although these inconsistencies could, in principle, have been cured in several ways, chemists have traditionally got round the problem by maintaining the anionic valences, and leaving the adjustable cationic valences to be determined from Eqn. II.4 or equivalents thereof. It follows that Eqn. II.4 can no longer be seen as an expression having general significance for required saturation of all valences, since it now merely expresses the already invoked saturation of anionic valences. There are many cases where it is not even sufficient to manipulate the cationic valences. Therefore, the apparent symmetry of Eqn. II.4 does not represent a basic chemical principle. 

A similar example of curing inhibition In an acidic medium has been observed when moderate amounts of salicylic acid were added to epoxy-amine matrix systems. (26) Such behavior may beresponsible for the poor bond strengths observed with the NTMP-containing oxide-primer specimens. In environments containing an aggressive species (e.g.. Cl ), the anion may interact with and become incorporated into the NTMP-oxide matrix, whereby it can attack the metal surface. 

Both anionic and cationic initiation are used to cure (polymerize and crosslink) epoxy resins (Sec. 2-12d) by ROP of the oxirane end groups. Photochemical cationic initiation (Sec. 5-2a-4) is important in various coating applications (3-4c) [Decker et al., 2001]. 

Heat resistant resin compositions based on BMI/aminophenol-Epoxy blends are achieved by reacting a BMI/p-aminophenol 1 1 adduct with epoxy resin (62). Both the secondary amine and phenol functionality may react with the epoxy resin and subsequently cure through an imidazole catalyst. Imidazole catalysts promote both the epoxy/phenol reaction and the anionic maleimide crosslinking. The formation of a 1 2 BMI/aminophenol adduct, as in Fig. 20, is claimed in a patent (63). The hydroxy terminated BMI/aminophenol adduct is an advantageous curing agent for epoxy resins when high temperature performance is desired. 

For a review of ring opening of cyclopropyl anions and related reactions, see Boche Top. Cure. Chem. 1988, 146, 1-56. 

A great variety of aromatic diamines and aliphatic di- and poly-amines are used as epoxy resin curing agents, and tert-amines can act as catalysts for anionic epoxide homopolymerisation. 

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