Kinetics of cure

The mechanistic models are more representative of the resin curing kinetics because they are based on stoichiometric balances of reactants involved in the elementary reactions. As a consequence, they are much more complex than the phenomenological models, but they can better represent the kinetics of cure. The physical and mechanical behaviors of the cured resins are determined by the chemical reactions that occur dining cure. The understanding of the mechanism and kinetics of cure is one of the most important steps in evaluating the... 

Fig. 7. Kinetics of curing diglycidyl ether of resorcinol with 2.6-diaminopyridine in the stepwise temperature control3)...

Studies of rheokinetics over the whole range of polyester curing is based (as for other materials) on a dynamic method, i.e., on measurements of the time dependence of the dynamic modulus at a fixed frequency, from which the time dependence of the degree of conversion (3(t). The observed dependence P(t) for polyester resins can be analyzed by an equation of the type used for other materials. Thus the following general equation was proposed for the kinetics of curing polyester and epoxy resins 69 72... 

The correspondence between the calculated results based on the model of heat dissipation due to viscous flow and the experimental data in the decrease of the induction period at high shear rates proves that the observed effect is adequately explained by this mechanism. The effects of shearing itself on the kinetics of curing are either absent or of secondary importance. If the experimentally observed decrease in the induction period is more pronounced than predicted by the dissipative model, then it is reasonable to consider additional heat sources, for example, the exothermal effects of a reaction. Heat flux from the surroundings can also influence the kinetics of... 

As an example, let us analyze mold filling with a model polyurethane formulation. Let the kinetics of curing be described by an equation with the self-deceleration term (as was discussed above). The following values of the parameters were used U = 49.1 kJ/mol ko = 3.8xl06 = 1.1 ATmax = 25.8°C where ATmax is the maximum expected increase in temperature for adiabatic curing. 

Rheology and Other Methods of Investigating the Kinetics of Curing 241... 

Phase segregation and formation of a microheterogeneous structure, if it occurs, affects the kinetics of curing. Polymerization reactions leading to the formation of crosslinked structures are not reactions of isolated macromolecules and, therefore, cannot be considered without taking into account the morphology of the reactive system. 

Various experiments show that in some cases the kinetics of curing cannot be described by a phenomenological equation of the n-th order at any reasonable values of n [113, 114]. In a number of cases, the kinetics of curing processes is adequately described by a phenomenological equation which takes into account the self-acceleration effect [115] ... 

An identical mathematical description of the kinetics of curing of reactants different in chemical nature and that obtained on the basis of fundamentally different experimental methods allows us to assume that this apparent selfacceleration course of some rheokinetic parameters is common to the processes of formation of materials with a crosslinked structure. It should be emphasized once more that the self-acceleration" effect must not be identified with the self-catalysis of the reaction of interaction between epoxy monomers and diamines which is studied in detail on model compounds [116, 117]. For each particular curing process the self-acceleration effect is influenced by the mechanism of network formatic, namely, chemical self catalysis [118], the appearance of local inhomogeneities [120], the manifestation of gel eff t [78], parallel course of catalytic and noncatalytic reactions [68]. It is probably true that the phenomena listed above may in one form or another show up in specific processes and make their contribution into self-acceleration of a curing reaction. 

It is interesting to note that Eq. (28) is sufficiently general and describes well the kinetics of curing of fundamentally different systems with phase segregation like, for example, unsaturated polyester resin. This is shown in Fig. 32 where there are compared experimental data of Ref. [129] and curves calculated by Eq. (28). 

In the production of bonded joints, the time required for the relevant reaction kinetics of curing has to be taken into consideration. 

The nature of the vulcanizing agent and of the rubber (i.e., EPDM compounds with peroxide in this instance) is of prime importance to the kinetics of cure the activation energy and the preexponential factor play major roles, and the cure enthalpy is also of some importance in spite of its low value, whereas the order of the overall reaction is 1. Thus, because of the kinetics of cure, the time of cure varies considerably with the temperature, and thus there is a narrow temperature window through which meaningful data can be obtained. The cure enthalpy is rather low and thus does not intervene much in the process. Nevertheless, DSC experiments, which are based on measuring the enthalpy evolved, are also commonly used for determining the kinetics of cure [12]. 

The effect of the heating rate as well as the quality of the rubber-die contact was also seriously considered [24] by using a rubber compound made of 100 parts EPDM with 2 parts sulfur and 50 parts C black. The kinetics of cure was expressed by Equation 3.19 and the parameters obtained nsing Eqnation 3.21. 

Rather than look for a mwe accnrate method for obtaining data for the kinetics of cure, attention was given to evaluating the effect of the lack of accuracy in the kinetic parameters. Some studies were conducted by using calorimetry [13-15]. Thus, the effect of the order of the overall cure reaction on the rate of cure was determined, and profiles of temperature and state of cure were evaluated by varying the value of the rubber compound cured with 3% sulfur [13]. By using the same approach and the same method, the effect of a variation of the activation energy on the profiles of tanperature and of state of cure was also studied [14,15]. 

Considers the complex problems associated with heat transfer and the methods used for evaluating the kinetics of curing... 

The importance given to the kinetics of cure is so great that various techniques have been tested or even created for determining the parameters of the kinetics of cure, the cure being considered on the whole as conforming to the Arrhenius s expression. 

Kinetics of curing of a haloid containing diepoxide based on hexachlorobenzene has been studied. This oligomer (EPS-1) was cured by 4,4 -diaminodiphenylmethane (DDM) in the stochiometric ratio DDM EPS-1. Haloid containing epoxy polymer with conditional designation EPS-l/DDM has the chemical structure shown in Figure 10.1. 

Also, the kinetics of curing a haloid containing diepoxide on a basis of diphenylolpropane and hexachloroethane (HCE) were studied. 

Using commercially available software, finite element modeling and the kinetics of curing adhesives can be performed as, for example, in the curing of silicone... 

Dolbin, I. V Ligidov, M. Kh. Bejev, A. A. Kozlov, G. V. Homogeneous and non-homogeneous kinetics of curing of haloid-containing epoxy polymers. Theses of Reports of International conf Vital Problems of Modem Science . Part 3. Samara, 12-14 September 2000,26. 

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