Cured thermosets, transitions

The resin processed at 200°C reaches 100% cure because the glass transition temperature of fully cured epoxy is 190°C, less than the processing temperature. On the other hand, the sample processed at 180°C reaches 97% cure and the one processed at 160°C only reaches 87% cure. Figures 2.26 and 2.27 also illustrate how the curing reaction is accelerated as the processing temperature is increased. The curing reaction of thermally cured thermoset... 

Since the equipment to measure the temperature-induced dimension variation employs an LVDT displacement sensor, the requirement of a solid sample is mandatory. The effects of post-cure reaction on the measurement, instead, need much more detail and discussion. For a partially cured thermosetting resin, a post-cure reaction is expected when the actual temperature rises above the corresponding glass-transition temperature, Tg. Above this temperature, the volume variation due to post-cure reactions is superimposed upon thermal expansion (18). For this reason, TMA is a technique for measuring the CTE of the partially cured sample that is suitable only within the glassy region (T < Tg), while for a fiilly cured sample useful information can also be obtained for the CTE in the rubbery... 

The more common techniques used to analyze thermosets and composites are thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC), which can determine the thermal properties and also the best conditions for application of the materials. These techniques can be apphed to polymers to determine their specific heat, degree of polymerization, flanunability, degradation, cure, glass transition temperature (Tg), and other characteristics. Analysis of composites should consider the behavior of each component, including the matrix, reinforcement, plasticizers and fire retardants. 

Figure 2.69. Typical DSC scan of a partially cured thermoset. The glass transition appears as an endothermic shift in the heat flow over a temperature interval. The residual heat appears as an exothermic peak following the glass transition.

Figure 2.110. Nonisothermal experiment (l°C/min underlying heating rate l°C/60s modulation, two consecutive heatings) on the 30-min Devcon glue, showing initial glass transition (at T, o), heat capacity increase at Tgo [ACp(T g,o)], reaction enthalpy (AHj) and glass transition of fully cured thermoset (at Tg ), heat capacity increase at Tg [AC ,(rg )], and reaction heat capacity (ArCp) (Swier, unpublished results).

Most of the epoxies used to prepare conductive adhesives are those described in the previous section because they allow the production of systems with high glass transition temperatures and good thermal resistance. However, the cured thermoset is rigid and brittle material that develop significant thermal stresses when applied to large-area substrates. The... 

For thermosetting resins, the effect of nanoconfinement on the glass transition temperature of thin films has only been examined by one research group [8]. In that work, the glass transition temperature of the microtome-sliced epoxy thin film is found to decrease with film thickness, and the Tg is depressed by 7.8°C for a 40 mu thin film [8]. On the other hand, no studies have examined the effect of curing thermosetting resins under nanoscale constraint, although considerable efforts have... 

None of the above described ring opening polymerization methods has, as yet, proved useful for the formation of polysilazane preceramic polymers. However, Si-N bond cleavage and reformation, as it occurs in reaction (13), is probably responsible in part for the curing or thermoset step in transition metal catalyzed dehydrocoupling polymerization of hydridosilazanes (31), as described below. 

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