Tertiary aromatic amine accelerators

It has been reported that tertiary aromatic amines can be encapsulated in formaldehyde-crosslinked microbeads, and used in an adhesive [54]. These microbeads burst when the joint is mated, and cure is accelerated. 

It is made by dimerizing cyanamide in basic aqueous solution, and is a colorless solid melting at 208°C. Dicyandiamide is soluble in polar solvents, but at room temperature is insoluble in bisphenol A epoxy resins. It can be made into a very fine powder and milled into epoxy resins to form stable dispersions. Because the dicy is insoluble in the epoxy, the only possible reaction sites are at the particle surfaces. Although some reaction certainly occurs over a short time, the adhesives easily can have a useful shelf life of six months. On heating to about 150°C, the dicyandiamide becomes soluble in the epoxy resin, and the adhesive polymerizes rapidly. Cure can be accelerated by incorporation of tertiary aromatic amines or substituted ureas. 

The reactivity of the amine as a polymerization accelerator depends upon the a+ value of the meta- or para-substituent of the amine, where a+ is the electrophilic substituent parameter previously described and tabulated (23). When the kinetic rate constant (or the reciprocal of the polymerization time with amine and peroxide initial concentrations held constant) is plotted against the a+ value on a semi-logarithmic plot (Figures 1 and 2), an inverted "V" shaped curve results. The kinetic data for Figure 1 were taken from a published article describing the polymerization of methyl methacrylate in the presence of BP and various tertiary aromatic amines as shown in Figure 2 ( 5). ... 

One unusual and surprising characteristic of tertiary aromatic amines is that in addition to acting as accelerators, the same compounds in low concentration in the presence of oxygen and an initiator may act as inhibitors of polymerization (41, 42). This behavior has also been attributed to the ability of the amine to engage in charge-transfer complex formation. Oxygen also inhibits radical polymerization and results in uncured films at the surface of dental sealants (42). 

Accelerators. To allow the degradation of the peroxides at room temperature, the activation energy must be reduced. This is done by adding accelerators. Hydroperoxides are cleaved by heavy metal salts and acyl peroxides by tertiary aromatic amines. Up to 2% of the latter are added by the manufacturers to resins for fillers. These are known as amine-accelerated UP resins. It is common to add 0.02-0.05% cobalt (calculated as cobalt metal) in the form of cobalt naphthenate or cobalt octoate dissolved in aromatics (not white spirit) to the systems hardened with hydroperoxides. The accelerator should only be added shortly before application for reasons of storage stability and drift. 

Promoters. Substances such as acetyl acetone, ethyl acetoacetate, amides of ace-toacetic acid [2.107], acetyl cyclopentanone [2.108] or tertiary aromatic amines have an accelerating effect on the curing reaction initiated by the hydroperoxide/cobalt octoate. Ethyl acetoacetate is most frequently used. The amount added is 1-3%, calculated on the resin supply form. 

Humphreys, of the Loctite Corporation, reported on the chemistry of accelerators for curing anaerobic adhesives. He showed that the reaction between N,N-dimethylaniline derivatives and cumene hydroperoxide is relatively slow even at lOO C. He concluded that the accelerated polymerization of anaerobic adhesives of ambient temperatures caused by cure systems containing combinations of tertiary aromatic amines, hydroperoxides, and sulfonimides does not result from the hydroperoxide-amine reaction. 

Tertiary aromatic amines hydroperoxides and sulfonimides are important components of many of the common anaerobic adhesive cure systems. While various formulative aspects of these compounds are well understood, a detailed explanation for their dramatic effect on the rate of polymerization of the adhesive has been lacking. Our approach to the problem has been to study the chemistry of the isolated components of this cure system under well defined conditions and to apply the results to understanding the mechanism by which these compounds accelerate the polymerization of anaerobic adhesives. Herein, we report some of the results of our studies of the reactions of N,N-dimethylaniline derivatives, which are typical amines used in anaerobic formulations, with cumene hydroperoxide (CHP). Connections will be made between the chemistry of the isolated systems and that which occurs in anaerobic formulations, both during storage and cure. 

UPRs are composed not only of UPs and a crosslinking monomer, usually styrene they contain, moreover, initiators (hardeners), curing promoters (accelerators) and polymerization inhibitors, hi the systems with benzoyl peroxide as the initiator, tertiary aromatic amines, e.g. N,N-dimethylaniline or Ar,M-dimethyl-p-toluidine, are applied as the promoters. Some amino-glycols were built into the UP molecules, thus increasing the reactivity [170]. The best results were achieved when using 3,6-diaza-3,6-diphenyloctane-l,8-diol (Scheme 25) ... 

When formulated into one-component adhesive systems, the product is stable when stored for 6 months to 1 year at room temperature. It will then cure when exposed to 145 to 160°C for about 30 to 60 min. Since the reaction rate is relatively slow at lower temperatures, the addition of 0.2 to 1 percent benzyldimethylamine (BDM A) or other tertiary amine accelerators is common to reduce cure times or cure temperatures. Other common accelerators are imidazoles, substituted urea, and modified aromatic amine. 

Combined tertiary/secondary amine curatives, prepared from ter-tiary/primary diamines and epoxy resins, have been used as accelerators for primary aromatic amine cures of DGEBA-type epoxy resins. For example, diethylaminopropylamine/DGEBA resin adducts have been used in m-phenylenediamine-cured epoxy resin systems to provide relatively low temperature cures (50-160°C). The cured resin systems have good toughness and heat distortion temperatures near 130°C. 

Important Initiators and accelerators unsaturations, aromatic carbonyl compounds (deoxyanisoin, dibenzocycloheptadienone, flavone, 4-methoxybenzophe-none, 10-thioxanthone), hydrogen bound to tertiary carbon at branching points, aromatic amines, groups formed on oxidation (hydroperoxides, carbonyi, carboxyi, hydroxyi) substituted benzophenones, compiexes with ground-state oxygen, quinones (anthraquinone, 2-chioroanthraquinone, 2-tert-butyi-athraquinone, 1-methoxyanthraquinone, 2-ethyianthraquinone, 2-methyianthraquinone), transition metai compounds (Ni < Zn < Fe < Co), ferrocene derivatives, titanium dioxide (anatase), ferric stearate, poiynuciear aromatic compounds (anthracene, phenanthrene, pyrene, naphthaiene ... 

Both aromatic and aliphatic amines react with butadiene to give tertiary octadienylamines 143 [51]. Amines with higher basicity show higher reactivity, and electron-donating substituents on aniline have an accelerating effect [68],... 

Unreactive compounds which can coordinate reversibly with the catalytic complex in competition with monomer, such as aromatic hydrocarbons or pyridine and other tertiary amines, may accelerate the rate in small amounts [106] but in high concentrations have a retarding effect. These effects have been interpreted in terms of site blocking and activation and, in the case of heterogeneous catalysts, by increasing the number of active sites. For a soluble catalyst the scheme... 

Di-n-butyltin catalysts are being used in the preparation of polyurethane foams. Most polyurethane foams utilize aromatic isocyanates such as toluene diisocyanate (TDI) or diphenylmethane diisocyanate (MDI) as the isocyanate, and a polyester or polyether polyols as the coreactant. Tertiary amine catalysts are used to accelerate the reaction with water and formation of the carbon dioxide blowing agent. To achieve a controlled rate of reaction with the polyol, an organotin catalyst can be used. Polyurethane foams are not only applied in place, but are also cast in a factory as slabstocks. These foam slabs are then cut for use in car seats, mattresses, or home furnishings. DBTDL is an excellent catalyst in high resiliency slabstock foams. DBTDL shows an excellent reaction profile for this application replacement for DBTDL in such an end-use is difficult and requires a substantial reformulation of the foam. 

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