Amine-epoxy curing reactions

Amine-epoxy curing reactions, Arrhenius relationships. 

The nature of the cure reactions in these epoxies can be confirmed by monitoring the epoxide consumption via near infra-red spectroscopy for a series of epoxide-amine mixtures containing a range of amine contents. A plot of % epoxide consumption vs. amine concentration for DGEBA-T403 epoxies is illustrated in Fig. 2. This plot confirms that the DGEBA-T403 epoxy system forms exclusively from epoxide-amine addition reactions, because (i) 100% epoxide consumption is attained at the stoichiometric amine concentration associated with exclusive epoxide-amine addition cure reactions and (ii) extrapolation of this plot to zero amine content indicates there is no epoxide consumption i.e. there are no epoxide homopolymerization reactions. 

Characterization of the epoxy cure reactions ensures that a composite can be fabricated and the epoxy is fully cured, assuming that the epoxide and amine starting components are initially homogeneously mixed. 

The importance of deviations from the structure of an ideal network due to stoichiometric imbalance or incomplete reaction was recognized in amine-epoxy curing by Bell who developed semi-empirical corrections. Their applicability was, however, limited to rather small deviations from the perfect state. The degree of approximation has never been tested against the complete theory. 

The functionalization of CNTs through plasma treatment represents a novel and easy approach to scale up towards industrial applications. In more recent works, there were many attempts to fluorinate CNT sidewalls in such manner. The CF4 plasma treatment of SWNT sidewalls was demonstrated to enhance the reactivity of tubes with aliphatic amines.The cure reaction of diglycidyl ether of bisphenol A-based epoxy resin (DGEBA), when reacted with butylamine molecules (BAMs) anchored on to the plasma treated fluorinated SWNTs, was reported. The advantage of this method was that the functionalization could be achieved through a simple approach, which is widely used in thin film technologies. As covalently modified CNTs with fluorine groups offer the opportunity for chemical interactions with the amine systems, it was recently demonstrated that this... 

Epoxies can be synthesized fiom halohydrins and also fiom alkenes. Epoxies react readily with active hydrogen compounds such as alcohols or amines and are frequently cured with diamitKxliphenyisulfone. Most epoxy curing reactions occur by a cationic mechanism. 

Polyamine and organic acid anhydride hardeners serve as co-reactive hardeners, which become incorporated into the epoxy resin, whereas tertiary amines, such as 2,4,6-tris-(dimethylaminomethyl)phenol (tris-DMP) and N,N-dimethylbenzylamine, imidazoles, and boron trihalide amine complexes are catalyst-type curing agents, which may not be chemicaUy bound to the resin molecules during epoxy curing reactions (Guin and Work 1995 Muskopf and McCoUister 1987). 

The two-part epoxy adhesive, readily available in hardware stores or other consumer outlets, comes in two tubes. One tube contains the epoxy resin, the other contains an amine hardener. Common diamine room temperature epoxy curing agents are materials such as the polyamides, available under the trade name Versamid. These polyamides are the reaction products of dimer acids and aUphatic diamines such as diethylenetriamine [111-40-0] ... 

Figure 1. Three main epoxy - amine curing reactions.

Aromatic amines formed from the reduction of azo colorants in toy products were analysed by means of HPLC-PDA [703], Drews et al. [704] have applied HPLC/ELSD and UV/VIS detection for quantifying SFE and ASE extracts of butyl stearate finish on various commercial yarns. From the calibrated ELSD response the total extract (finish and polyester trimer) is obtained and from the UV/VIS response the trimer only. Representative SFE-ELSD/UV finish analysis data compare satisfactorily to their corresponding SFE gravimetric weight recovery results. GC, HPLC and SEC are also used for characterisation of low-MW compounds (e.g. curing agents, plasticisers, by-products of curing reactions) in epoxy resin adhesives. 

Recently, FTIR spectroscopy studies have been reported which support the above observations. Moacanin et al 3) concluded that two reactions dominate the TC3fDA/DDS cure epoxy-primary amine addition is the principal reaction occurring during the early stage of cure followed by the epoxy-hydroxyl addition reaction. Indeed they find that the rate of epoxy-hydroxyl addition is at least an order of magnitude slower than for the epoxy-primary amine reaction at 177 C. Furthermore, Morgan et al (4) report that the epoxysecondary amine addition and epoxy-epoxy homopolymerization reactions also occur at 177°C but at rates that are approximately 10 and 200 times slower, respectively, than the epoxy-primary amine react ion. 

Other possible reactions, such as homopolymerization (epoxide+epoxide) and epox-ide+hydroxyl group (in the latter stages of cure), can be neglected when the ratio of epoxide to amine is stoichiometric and in the absence of catalyst or accelerator [194], For TGDDM/DDS resins, the homopolymerization reaction may be neglected at cure temperature below 180°C [84], At temperatures between 177°C and 300°C, dehydration and/or network oxidation occur, which results in formation of ether cross-linkings with loss of water. Decomposition of the epoxy-OH cure reaction can also take place, which results in propenal... 

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. 

Tertiary amines catalyze the homopolymerization of epoxy resins in the presence of hydroxyl groups, a condition which generally exists since most commercial resins contain varying amounts of hydroxyl functionality (B-68MI11501). The efficiency of the catalyst depends on its basicity and steric requirements (B-67MI11501) in the way already discussed for amine-catalyzed isocyanate reactions. A number of heterocyclic amines have been used as catalytic curatives pyridine, pyrazine, iV,A-dimethylpiperazine, (V-methylmorpholine and DABCO. Mild heat is usually required to achieve optimum performance which, however, is limited due to the low molecular weight polymers obtained by this type of cure. 

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 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 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. 

In Chapter 2 the DSC technique is discussed in terms of instruments, experimental methods, and ways of analysing the kinetic data. Chapter 3 provides a brief summary of epoxy resin curing reactions. Results of studies on the application of DSC to the cure of epoxy resins are reviewed and discussed in Chapter 4. These results are concerned with the use of carboxylic acid anhydrides, primary and secondary amines, dicyanodiamide, and imidazoles as curing agents. 

In general the amine-epoxy resin curing reactions show complex kinetics typified by an initial acceleration due to autocatalysis, while the later post-gelation stages may exhibit retardation as the mechanism becomes diffusion-controlled. However some workers 72 80) have found that over a limited range of conversion the kinetic data may be described by the simple models of Eq. (2-6) or (2-9). 

After the amines, acid anhydrides constitute the next most commonly used reagents for curing epoxy monomers. The epoxy-acid reaction proceeds through a stepwise mechanism (Sec. 2.2.4) while the reaction of epoxides with cyclic anhydrides, initiated by Lewis bases, proceeds through a chain-wise polymerization, comprising initiation, propagation, and termination or chain transfer steps. Some of the postulated reactions are shown in Table 2.25 (Matejka et al., 1985b Mauri et al., 1997). 

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