THERMO-KINETIC STUDIES FOR FORMATION OF CARBON MATRIX - PRECURSOR FOR REACTIVE MELT INFILTRATION
Abstract
The formation of carbon matrix precursors for reactive melt infiltration was studied by thermal analysis and thermokinetics. It was shown, that pore-forming agent (ethylene glycol) slows down the exothermal reaction of phenol formaldehyde resin curing, while the addition of catalyst (4-toluenesulfonyl chloride) makes it possible to gelate the matrix before low-molecular product evaporation. It was shown, that gelation takes place without sufficient mass loss and results in disappearance of exothermal effect of curing at 50-100 °С. The gelation time of the system, which is equal to 47 min at 60 °С, was chosen as a time necessary for structure formation due to polymerization induced phase separation. Post-curing of thermosetting matrix at 180 °С for 3 h is accompanied with removal of 67 wt.% pore-forming agent and the decreasing of material density by 32 %. The pyrolysis of cured compositions is accompanied with mass loss and chemical shrinkage. The maxima of mass loss rate take place at temperatures, which are 20-40 °C higher, than for the maxima of shrinkage rate. The regularities observed lead to the step changes of density with local increasing due to shrinkage prevailing and local decreasing due to mass loss prevailing. The final density of the pyrolysed material is 2-5 % higher than the density of initial uncured composition. The thermokinetics methods were used to propose time-temperature regime, which minimizes the local decreasing a density due to mass loss and can be used to formation of samples with uniform pore distribution. It was suggested that to obtain the matrix with developed pore structure form of the composition investigated, the reinforcing skeleton, which does not shrink at the temperatures studied, but has sufficient adhesion with pyrolysis products, should be used.
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