Isothermal cure rate

Modification of the FT-IR analysis techniques to analyze coatings under isothermal cure conditions provides the data needed to determine the rate constants for each reaction. An effective method to generate the rate constants from the experimental data has been found and will be described. 

As seen in Equation 4, the concentration of free isocyanate as a function of time is dependent on both rate constants. Measuring the concentration of the isocyanate as a function of time during isothermal cure provides a convenient method to determine both rate constants. 

To obtain the cure kinetic parameters K, m, and n, cure rate and cure state must be measured simultaneously. This is most commonly accomplished by thermal analysis techniques such as DSC. In isothermal DSC testing several different isothermal cures are analyzed to develop the temperature dependence of the kinetic parameters. With the temperature dependence of the kinetic parameters known, the degree of cure can be predicted for any temperature history by integration of Equation 8.5. 

For isothermal DSC testing Equations 8.8 and 8.9 are used in data reduction to produce degree of cure and cure rate histories. With these histories, the solution of Equations 8.2 and 8.6 yields the kinetic parameters of interest. 

Fig. 11. Reduced rate curves for isothermal cure of BADGE with EDA, TMDA, and HMDA at 50 and 70 °C (From Ref. 83>, Fig. 8)...

Figure 2.34. Comparison of theoretical predictions (curve, calculated from Eq. (2.8S) according to the dissipative model of non-isothermal curing) with experimental data on the decrease of the induction period at high shear rates for phenolic-based compounds (vertical bars) and silicon-based composites at different initial temperatures 150°C (1) 170°C (2) and 190°C (3).

Figure 3.6. Changes in viscosity of polyurethane during non-isothermal curing at different scanning rates 2...

W j represents the ratio between the heat diffusion rate and the initial heat generation rate. When W[ —oo, heat diffuses at a much higher rate than it is generated. This leads to an isothermal cure at T =TW (or T = W4). When W, —> 0, the material behaves as a thermal insulator and ... 

In the Applications categories of Table 2, except for those papers identified as presenting post-cure results, all of the papers involve curing. No differentiation was made as to isothermal or ramped cure, since both types of data would be of importance to any particular resin system. The heading Cure Rate and/or Catalyst Studies includes those papers in which explicit correlations between cure temperature or catalyst concentration are presented, whereas the heading General... 

Fig. 1. Rate of isothermal cure of the DGER-mPhDA (P = 1) system at different Tcu[e. Dotted curve after a jump change of Tcure from 60 C to 90 °C...

Following the isothermal curing cycle in Step 5 the sample was cooled to ambient temperarnre and then further isothermed on a thermogravimetric analyzer for 120 minutes at 200,250,300, and 350°C in an air atmosphere with a 50 cc/min flow rate. In this experiment the plastic sample exhibited only a3.69% weight loss as determinedby TGA. 

After extended isothermal cure, the system is cooled from T,u to — 170°C, heated from —170 °C to a specified post-cure temperature, and then immediately cooled to —170 °C, all rates of change of temperature being 1.5 K/min. The temperature cycling is continued as long as is desired. The initial scan from —170 °C... 

Isothermally cured at 177 C, as determined from dynamic DSC analysis scan rate 10 C/mln. 

As may be seen in Table II, in both resins the reaction rate constants kj emd k2 appearing in eq. (5) increase with the isothermal curing temperature. In most cases, the magnitudes of kj amd k2 (only at low temperatures) are greater for the Ashland polyester resin them for the Dow vinyl ester resin. 

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