Cure kinetics defined

The cure kinetics will depend on the initial isocyanate to hydroxy ratio and on the humidity. Assuming that the concentration of water in the coating is constant during cure, it is possible to define the following parameter which determines the effect of humidity on cure ... 

Crosslinking of many polymers occurs through a complex combination of consecutive and parallel reactions. For those cases in which the chemistry is well understood it is possible to define the general reaction scheme and thus derive the appropriate differential equations describing the cure kinetics. Analytical solutions have been found for some of these systems of differential equations permitting accurate experimental determination of the individual rate constants. 

The cure kinetics is calculated from Eq. (9.21) outside the vitrification region (T > Tg). For T < Tg, the reaction rate is (arbitrarily) neglected. The density of the composite is defined as... 

One valuable approach to quantify the effects of diffusion (or mobility restrictions) on the cure kinetics is via direct estimation of a diffusion factor, DF. The latter is defined as the ratio of the experimentally measured conversion rate (dx/d/)obs over the predicted conversion rate at the same reaction conversion x in the absence of mobility restrictions (dx/d/)kin ... 

Differences in Network Structure. Network formation depends on the kinetics of the various crosslinking reactions and on the number of functional groups on the polymer and crosslinker (32). Polymers and crosslinkers with low functionality are less efficient at building network structure than those with high functionality. Miller and Macosko (32) have derived a network structure theory which has been adapted to calculate "elastically effective" crosslink densities (4-6.8.9). This parameter has been found to correlate well with physical measures of cure < 6.8). There is a range of crosslink densities for which acceptable physical properties are obtained. The range of bake conditions which yield crosslink densities within this range define a cure window (8. 9). 

Curing of epoxy thermosets requires a knowledge of the chemical kinetics and the crosslinking reactions. This information is necessary to optimize the cure cycle. The parameters that define the cure cycle ultimately determine the crosslink density and the final physical properties of the polymer. In addition to temperature, these parameters include the rate of temperature increase, the number of stages in the cure, the hold temperature at each stage, the pressure at which cure takes place, and the time allotted for the cure cycle. These parameters are usually determined empirically. Once the kinetics are understood and the actual chemistry behind the cure is established, these cure cycle parameters can be chosen based on the desired end properties. Usually the cure cycle seeks to establish a certain degree of cure that is in line with the expected final properties. 

There are various ways to determine the parameters of the kinetics of the cure reaction. The parameters to consider are the cure enthalpy, which is the heat evolved from the overall cnre reaction the order of the reaction which is concerned with the concentration of active agent remaining free during the reaction at time t and the two parameters that allow defining of the effect of the temperature on the rate of the reaction, that is, the energy of activation and a constant that depends only on the compound, by following the Arrhenius expression. Thus, by considering aU these parameters, the cure of rubbers is considered a simple reaction. 

We have to understand that if the heat flux defined by Equation 3.5 at the rubber sensor is of interest, because it provides the intensity and shape of the cure exotherm, the other taking place in the air, expressed by Equation 3.6, is characterized by the loss in heat for the cure reaction. From a first approach, based on a logical consideration, this loss in heat affects essentially the values of the enthalpy of cure which is reduced somewhat. The shape of the exotherm, which gives rise to the kinetics of the reaction, should not be affected as in Equation 3.6, the loss in heat is proportional to the difference in temperatures, in the same way as for the gradient of temperature shown in Equation 3.5. The loss in heat could follow kinetics similar to that observed on the sensor of the calorimeter. 

Attempts of the authors [5-7] to linearize the dependence of (1-Q) on t for the system EPS-4/DDM by the Eqs. (87) and (88) of Chapter 1 were not successful. Therefore the following assumption was made [6], The Eq. (87) of Chapter 1 describes low-molecular substances reaction kinetics at large density fluctuations in EucUdean space with dimension d, which is equal to 3 in the considered case. If we assume that the fractal clusters (microgels) formation with dimension D defines reaction curing course in a fractal space with dimension D, the dimension d in the Eq. (87) of Chapter 1 should be replaced by D. The dependence of In (1-Q) on corresponding to the Eq. (87) of Chapter 1 with the indicated re-... 

The previously reported method of kinetics data analysis for DSC data is not readily adaptable to raw data obtained in the integral form of fractional conversion or cure obtained from DMA, TGA or spectroscopic methods such as FT-IR. The basic assumption of the kinetics analysis for DMA is that the change in relative modulus at a given time and temperature during the dynamic temperature scan, divided by the change in relative modulus exhibited by the fully cured system at the same temperature, is proportional to the extent of cure at that point of the reaction. This is then used as the fractional degree of cure in the calculations. This normalization of raw DMA data to fractional degree of cure F(t,T) is defined as... 

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