TTT isothermal cure diagram

Figure 2.8. A time-temperature-transformation (TTT) isothermal cure diagram for a thermosetting system. Adapted from Figure 1 (GilUiam, 1986).

The timc-temperature-transformation (TTT) isothermal cure diagram, as shown in Figure 5.3, was suggested by Gillham [7). Further work in this area has been carried out by the same group [8-10] and many other researchers. 

The purpose of this review is to summarize the baac features and utility of the TTT cure diagram, discuss the experimental procedures for obtaining a diagram, present experimentally-obtained diagrams for model systems, and describe recent models that have attempted to calculate the time to vitrification on isothermal polymerization. This review will concentrate on TTT diagrams of epoxy systems. 

At hi temperatures thermal degradation becomes important, and may prevent full cure from being achieved Two degradation events have been noted in relation to the TTT diagram devitrification followed by elastomer formation and vitrification followed by char formation. The devitrification event corresponds to a decrease in Tg from above to below the isothermal cure temperature the time to this event may be considered to be the lifetime of the material since it marks the limit in time for the material to support a substantial load. The second event is an elastomer-to-glass transformation, accompanied by an increase in Tg and rigidity, and is presumably due to the onset of char formation... 

The isothermal TTT cure diagram summarizes the transitions that occur on isothermal polymerization, such as gelation, vitrification and devitrification. Typically,... 

In order to construct a TTT cure diagram, a series of isothermal cures are performed at different temperatures, and the relative rigidity and logarithmic decrement are recorded vs. time. From each damped wave 1/P and A are extracted the numerical methods used have been described... 

In this Section, an experimental approach for constructing isothermal TTT cure diagrams has been described, TTT diagrams of representative epoxy systems including high Tg and rubber-modified epoxy resins have been discussed, and perturbations to the TTT cure diagram due to thermal degradation and rubber modification have been illustrated. 

Although different aspects of the isothermal TTT cure diagram have been presented in this review from an experimental point of view, this section will present some recent work that has attempted to model the cure process. Only the gelation and vitrification processes are examined, and the complicating effects of thermal degrada-... 

During isothermal polymerization below Tg, the molecular weight and T, increase, and eventually T, will equal Tjure The main purpose of this section is to discuss the calculation of the time to vitrification, where vitrification is defined to occur when Tj, equals T ure- The concepts of vitrification and the TTT cure diagram are extended to linear systems for both step growth and chain reaction mechanisms, although most of the discussion will focus on the nonlinear step growth case, of which the cure of epoxy resins is an example. 

The TTT diagram plots the cure temperature versus the time to reach different important events during isothermal cure, e.g. gelation, vitrification, iso-Tg, iso-conversion, etc. (Figure 2.29). 

The CHT diagram for the epoxy-amine system is given in Figure 2.32. The experimental points (symbols) in Figures 2.30-2.32 are data obtained with MTDSC and djmamic rheometry. The thick lines are the gelation lines, the vitrification contour (similar to the line of DF% f) and the isodiflfusion contours DF% g and DF. The thin lines display the Tg evolution as a function of time for selected isothermal (TTT) or non-isothermal cure paths (CHT). 

Fig. 3. Generalized time-temperature-transformation (TTT) cure diagram. A plot of the times to gelation and vitrification during isothermal cure versus temperature delineates the regions of four distinct states of matter liquid, gelled rubber, gelled glass, and ungelled glass. From Ref. 12.

The use of a timescale instead of a conversion one requires a previous definition of the cure schedule e.g., isothermal, constant heating rate, etc. Usually, isothermal conditions are selected to define the timescale i.e., only trajectories at constant temperature have a physical meaning. This leads to the TTT diagram. 

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