Epoxy stoichiometric ratio

Stoichiometric combustion air requirement, 72 322t Stoichiometric concentration, 27 840 Stoichiometric organic synthesis, metal carbonyls in, 76 72 Stoichiometric parameters, in reactor technology, 27 337-338 Stoichiometric ratios, epoxy/curing agent, 70 418-420... 

From the variation of Tg and sub-Tg with frequency, transition maps of tn (frequency) vs. 1/T were made from which the activation energy could be determined. Table IV shows that the activation energy (Tg) of the fully cured resin at the stoichiometric ratio agrees fairly well with that reported by Senich et al. (7) for two amine-cured epoxy resins. 

Table IV. Post C-Stage Curing of Neat Resin at the Stoichiometric Ratio of Epoxy to Amine...

Epoxy Curing Agent Stoichiometric Ratio (wt.%) Technique State of Stress Measurement Frequency Range, Hz Me (g/mole) Ref. 

Alteration of this epoxy structure is the result of the fact that the epoxy molecules are both reacting and diffusing at the same time. This process forms a concentration gradient with a high epoxy monomer concentration at the surface which gradually reduces to the bulk concentration away from the surface. The properties of an epoxy with an excess of resin can be quite different from the stoichiometric amount. Figure 2, for example, illustrated the alteration of cured epoxy mechanical properties with epoxy/amine ratio. Excess epoxy or less than the stoichiometric amount of amine produces a brittle material if the mixture is cured in the same manner as the stoichiometric amount (Fig. 2). The stoichiometric sample has the lowest modulus while excess amine produces increased brittleness. The potential for variation in local properties within the epoxy due to the presence of a 200 nm or less layer must be considered. 

The epoxy-hydroxyl reaction (etherification) modifies the initial stoichiometric ratio based on epoxy to amino hydrogen groups. 

The stoichiometric ratio of amine/epoxy equivalents is given by... 

Figure 3.13 shows the variation of the gel conversion of the limiting reactant as a function of the stoichiometric ratio. For r > 3, no gel is formed and the polymer remains in the liquid state after complete reaction of epoxy groups. If the amount of epoxy monomer necessary to obtain a stoichiometric system is added in a second step, polymerization restarts, leading to gelation and the formation of a network. The two-step polymerization is the basis of several commercial thermosetting polymers. 

The structure of precursors, the number of functional groups per precursor molecule, and the reaction path leading to the final network all play important roles in the final structure of the polymer network. Some thermosets can be considered homogeneous ideal networks relative to a reference state. It is usually the case when networks are prepared by step copolymerization of two monomers (epoxy-diamine or triol-diisocyanate reactions) at the stoichiometric ratio and at full conversion. 

All the above observations seem to justify Porter s approach (Eq. 11.11)), according to which the Poisson s ratio should depend only on the cumulative loss tangent. It was found that the unrelaxed Poisson s ratio determined from ultrasound (5 MHz) propagation rate, for 12 of amine-crosslinked epoxy stoichiometric networks, displays only small variations (Av < 0.01), in spite of the relatively large variations of the cohesive energy density (0.59 < CED <0.66 GPa) and the crosslink density (2.0 5.9 mol kg 1)-... 

In the case of the adhesive supplier or the manufacturing engineer who finds it necessary to internally formulate an epoxy adhesive, stoichiometric ratios will need to be determined so that one can find a safe mixing ratio to start formulating and to understand the implications of various reactive proportions. 

Calculate the stoichiometric ratio of amine to use with the epoxy resin. 

The chemical structures of important amines for curing epoxy resins in adhesive systems are identified in Fig. 5.1. Diethylenetriamine (DETA), triethylenetetramine (TETA), ra-aminoethylpiperazine (AEP), diethylaminopropylamine (DEAPA), ra-phenylenediamine (MPDA), and diaminodiphenyl sulfone (DDS) are the most commonly used members of this class. They are all primary amines. They give room or elevated temperature cure at near stoichiometric ratios. Ethylenediamine is too reactive to be used in most practical adhesive formulations. Polyoxypropyleneamines (amine-terminated polypropylene glycols) impart superior flexibility and adhesion. 

Reversion is usually much faster in flexible materials because water permeates them more easily. Hydrolysis has been seen in certain epoxy, polyurethane, and cyanoacrylate adhesives. The reversion rate also depends on the type and amount of catalyst used in the formulations and the degree of crosslinking. Best hydrolytic properties are obtained when the proper stoichiometric ratio of base material to catalyst is used. 

In principle, three methods are available for determination of this ratio (1) reaction kinetics preferably on model monoamine-monoepoxide systans by monitoring the time change in the concentration of epoxy or amino groups > ), (2) chromatographic determination of reactants and products of the reaction of a monoamine or diamine with monoepoxide for excess amine over the stoichiometric ratio (3) critical conversion at the gel point or preferably determination of the so-called critical molar ratio necessary for gel formation at 100% reaction of epoxide i5-i8,s9) -pjjg theoretical dependence of the critical conversion at, in a stoichiometric mixture of diamine and diepoxide and of the critical molar ratio is shown in Figs. 5 and 6. 

These epoxy values were used to determine the stoichiometric ratios of curing agent. 

A very recent study based on FTIR analysis of the isothermal co-reaction between tetrafunctional epoxy and cyanate ester resins at different stoichiometric ratios and temperatures substantiates some of these findings. Rheological char-... 

The condensation of l-chloro-3-diazopiopanone (57) with aldehydes provides epoxy diazo ketones (equation 18). Treatment of a methanolic solution of (57) and benzaldehyde, in a stoichiometric ratio, with aqueous sodium hydroxide furnishes l-diazo-3,4-epoxy-4-phenyl-2-butanone (59a) in 69% yield the rrans-epoxide is obtained stereoselectively, analogous to the Darzens condensation of benzaldehyde and chloroacetone. The reaction is reported to proceed to give also the diazo ketones (59b-59e), and the epoxides obtained are exclusively of the trans configuration. ... 

After surface treatment, all substrates were stored less than 2 hours in an air-conditioned room (20 2°C and 50 5%r.h.), before polymer application. The epoxy prepolymer used was diglycidyl ether of bisphenol A (MW=348 g mol , DGEBA DER 332 from Dow Chemical). The curing agents were either IPDA from Fluka or DETA from Aldrich. Assuming a functionality of 4 for IPDA, 5 for DETA, and 2 for the epoxy monomer, the stoichiometric ratio aje used was equal to 1 (exceptions are mentioned). 

Finally, the variation of the glass transition temperature for both systems (DGEBA-IPDA or DGEBA-DETA) versus the stoichiometric ratio (a/e) is reported (e.g., see Fig. 7.6) for either pure or modified materials. Usually, as the functionalities of epoxy and amine monomer were well defined, mixing materials at the stoichiometric ratio of 1 led to the formation of the most crosslinked network having the highest glass transition temperature. From Fig. 7.6 it can be... 

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