BF3: amine catalysts

In this Sect, we describe the starting material impurities and their effect on the processing and cure reactions of TGDDM-DDS epoxies. The cure reactions are characterized by differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FTIR) studies. The BF3 amine catalysts used to accelerate the cure of TGDDM-DDS epoxies are characterized by nuclear magnetic resonance (NMR) spectroscopy studies. 

The impurities in the BF3 amine catalysts are highly variable and are discussed in detail in the following Sect. 3.3. 

The two most common BF3 amine catalysts used commercially to cure epoxies are boron trifluoride monoethylamine, BF3 NH2C2H5, and boron trifluoride piperidine, BF3 NHCsHi0, complexes. Such complexes are latent catalysts at room temperature but enhance epoxide group reactivity at higher temperatures. 

In C-fiber-TGDDM-DDS prepregs that do not contain a BF3 amine catalyst one exotherm peak associated with the cure reactions has been observed by DSC 20 22). However, prepregs that contain BF3 amine catalyst have been reported to exhibit 2 or 3 additional DSC peaks at lower temperatures, which have been associated with the catalyzed cure reactions 20 23). 

In this Sect, we report systematic DSC studies of (i) the constituents of boron tri-fluoride monoethylamine (BF3 NH2C2H5) catalyzed TGDDM-DDS epoxies and their mixtures (ii) the nature of the catalyzed cure reactions and (iii) the environmental sensitivity of the BF3 NH2CzH5 catalyst. DSC studies are also reported on the cure reaction characteristics and environmental sensitivity of commercial prepregs that contain BF3 amine catalysts. 

Happe, J. A., Morgan, R. J., Walkup, C. M. H, 19F and, lB Nuclear Magnetic Resonance Characterization of BF3 Amine Catalysts Used in the Cure of C Fiber-Epoxy Prepregs, Polymer (In press)... 

DGEBA epoxy resin (EPON 828, Resolution Performance Products) Ground calcium carbonate (ExCal W3, Excalibar Minerals Inc.) Fumed silica (Cab-O-Sil TS-720, Cabot Corp.) BF3 amine catalyst (Leecure 8-239B, Leepoxy Plastics, Inc.) 50 48 2 1.5... 

Starting formulation for an epoxy adhesive cured with BF3 amine catalyst... 

Scognamillo et al studied the cationic curing of a triepoxy using latent BF3-amine catalysts. 

The commercially available TGDDM, DDS and BF3 amine components all contain impurities, some of which may act as catalysts towards the cure reactions. 

The cure reactions, the viscosity-time-temperature profile, the processing conditions, the resultant epoxy chemical and physical structure, and the mechanical response of a C-fiber/TGDDM-DDS cured epoxy composite are modified by the presence of a BF3-amine complex catalyst within the prepreg. These factors also will be modified... 

In practice, epoxy-amine reactions in carbon fiber prepregs, and epoxyphenol reactions in molding compounds, are often accelerated by the addition of a Lewis acid (typically a BF3 - amine complex) or a Lewis base (often a tertiary amine), as catalysts. ... 

C and 200°C. Catalysts such as aluminum hydrosilicate, phosphoric acid, sulfuric acid, BF3, tertiary amine, or sodium alkyl sulfate are used to increase the reaction rate see Eq. (64). 

The reaction of diazo compounds with amines is similar to 10-15. The acidity of amines is not great enough for the reaction to proceed without a catalyst, but BF3, which converts the amine to the F3B-NHR2 complex, enables the reaction to take place. Cuprous cyanide can also be used as a catalyst. The most common substrate is diazomethane, in which case this is a method for the methylation of amines. Ammonia has been used as the amine but, as in the case of 10-44, mixtures of primary, secondary, and tertiary amines are obtained. Primary aliphatic amines give mixtures of secondary and tertiary amines. Secondary amines give successful alkylation. Primary aromatic amines also give the reaction, but diaryl or arylalkyl-amines react very poorly. 

The initial product is a salt of hydrazinesulfonic acid, which is converted to the hydrazine by acid treatment. Diazonium salts can also be reduced to arenes (14-24). N-Nitrosoamines can be denitrosated to secondary amines by a number of reducing agents, including H2 and a catalyst, BF3—THF—NaHC03, and NaBH4— TiCU, as well as by hydrolysis. ... 

Besides 1-15 to 1-17, several other formylation methods are known.302 In one of these, dichloromethyl methyl ether formylates aromatic rings with Friedel-Crafts catalysts.303 ArCHClOMe is probably an intermediate. Orthoformates have also been used.304 In another method, aromatic rings are formylated with formyl fluoride HCOF and BF3.305 Unlike formyl chloride, formyl fluoride is stable enough for this purpose. This reaction was successful for benzene, alkylbenzenes, PhCl, PhBr, and naphthalene. Phenols can be regioselectively formylated in the ortho position in high yields by treatment with two equivalents of paraformaldehyde in aprotic solvents in the presence of SnCL and a tertiary amine.306 Phenols... 

When compared to aliphatic amines, aromatic amines generally have reduced exotherm and reactivity. Elevated temperatures are required to achieve optimum properties. In certain cases aromatic amines can be cured at room temperature with catalysts such as phenols, BF3 complexes, and anhydrides. 

There is direct evidence, from ir and nmr spectra, that the fert-butyl cation is quantitatively formed when ferf-butyl chloride reacts with AICI3 in anhydrous liquid HCl. In the case of alkenes, Markovnikov s rule (p. 1019) is followed. Carbocation formation is particularly easy from some reagents, because of the stability of the cations. Triphenylmethyl chloride and 1-chloroadamantane alkylate activated aromatic rings (e.g., phenols, amines) with no catalyst or solvent. Ions as stable as this are less reactive than other carbocations and often attack only active substrates. The tropylium ion, for example, alkylates anisole, but not benzene. It was noted on p. 476 that relatively stable vinylic cations can be generated from certain vinylic compounds. These have been used to introduce vinylic groups into aryl substrates. Lewis acids, such as BF3 or AIEta, can also be used to alkylation of aromatic rings with alkene units. 

Nitrones, C=N" (R)=0, are generated by the oxidation of N-hydroxyl secondary amines with 5% aq. NaOCl. ° Secondary amines, such as dibenzylamine, can be converted to the corresponding nitrone by heating with cumyl hydroperoxide in the presence of a titanium catalyst. Imines are oxidized to amides with mcpba and BF3 OEt2. ° ... 

Combined use of BF3-OEt2 and AcOH was found to be effective for allylation of aldimines. The catalyst system is also effective for the three-component synthesis of homoallylic amines starting from aldehyde, amine, and allylgermane (Scheme 11.11) [24]. 

Boron trifluoride forms a range of complexes with ethers, nitriles and amines. It is commercially available as the adduct Et20 BF3 (12.11). Being a liquid at 298 K, it is a convenient means of handling BF3 which has many applications as a catalyst in organic reactions, e.g. in Friedel-Crafts alkylations and acylations. 

Fluorides. PF5 is easily prepared by the interaction of PC15 with CaF2 at 300-400°. It is a very strong Lewis acid and forms complexes with amines, ethers and other bases as well as with F" in which phosphorus becomes 6-coordinate. However, these organic complexes are less stable than those of BF3 and are rapidly decomposed by water and alcohols. Like BF3, PF5 is a good catalyst, especially for ionic polymerization. 

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