Exothermic heat curing systems

Exothermic heat curing systems can be used when processing at room temperature by the addition to the plastic additives and/or promoters. They are used to provide the necessary heat through chemical reactions. This reaction has to be controlled so that overheating will not occur, particularly large parts where damage could occur such as voids and not meeting dimensional requirements. 

Thermal analysis by differential scanning calorimetry on as-received materials indicated that the matrix was not fully cured an exotherm was detected the first scan from room temperature to 300 °C. A 250 °C/16 hours was therefore necessary for the specimens in order to complete all the crosslinking reactions. After the postcuring, differential scanning calorimetry confirmed a fully cured system which exhibit a regular step-function increase in heat capacity at Tg. The calorimetry results are shown in Fig. 8. 

The huge, hollow mast was fabricated in five internally heated female molds. Stitched unidirectional carbon/epoxy prepreg from Cytec Engineered Materials Inc. of Wrexham Clwyd, U.K. was hand laid up to form the two back sections and the three front sections. Cure was controlled by a computer that monitored the mold heating system to prevent excessive exotherm heat. The five cured sections were bonded... 

Uniform and consistent cure kinetics. The cross-linking chemical reaction is an exothermic polymerization. The length of the cure cycle depends on the type of resin system and temperature. The degree of cure is proportional to the cross-Unked material, and can be characterized by measuring the fractional exothermic heat of the chemical reaction. The cure kinetics are usually characterized separately and may not reflect the interaction with the sizings on the fiber surfaces which may change the cure kinetics. 

Cure at elevated temperatures is normally carried out with acids or acid anhydrides. The anhydrides are preferred because they are more soluble in the resin and do not produce as much water as the acid during cure so that there is less tendency for foam formation. The advantages of this curing system, over an amine system, are a lower exotherm, less skin irritation and with the correct choice of acid a product with a higher heat distortion temperature. They do, however, produce cured products which are less resistant to alkalis. 

The applications of the unsaturated polyester resins were increased in the late 1960s by the introduction of water-extended polyesters. In these materials water is dispersed into the resin in very tiny droplets (ca 2-5 p.m diameter). Up to 90% of the system can consist of water but more commonly about equal parts of resin and water are used. The water component has two basic virtues in this system it is very cheap and because of its high specific heat it is a good heat sink for moderating cure exotherms and also giving good heat shielding properties of interest in ablation studies. 

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