Thermosets exotherm control

Perry and Lee [28,29] offer an enhancement of QPA, based upon use of dual heat flux sensors and additional thermocouples in autoclave curing. This enhancement entails determining heat transfer properties during the cure, then using these properties in conjunction with PID regulatory control of autoclave temperature. Using the additional sensors, Perry and Lee employ an on-line Damkohler number in lieu of the second time-derivative of temperature to avoid exothermic thermal runaway within the prepreg stack thermoset resin. The Damkohler number is defined as ... 

The proper balance between viscosity and gas evolution can be controlled by a number of factors such as a suitable type and concentration of catalyst and surfactant, the presence of a nucleating agent (not always necessary) (17,18) and control of reaction temperature (or exotherm). Additional factors that must be considered are the use of a suitable chemical blowing agent, which is especially important for the production of thermoplastic foams, and the formation of oligomers (prepolymers) which exhibit higher viscosities than monomers in the preparation of thermoset foams (e.g. polyurethane foams). 

The final state of a phenolic resin varies dramatically from thermoplastic to thermoset and from solid to liquid, and includes solutions and dispersions. With a hulk process, resole resins, in neat or concentrated form, must be produced in small hatches ( 2-10 m ) in order to maintain control of the reaction and obtain a uniform product. On the other hand, if the product contains a large amount of water, such as liquid plywood adhesives, large reactors (20 m ) can be used. Melt-stable products such as novolaks can be prepared in large batches (20-40 m ) if the exotherms can be controlled. Some reactors are reportedly as large as 60 m (Ref. 9, p. 83). 

Fillers, through their essentially volumetric and thermal inertness at typical thermoset resin-processing temperatures, offer a valuable means of exotherm and shrinkage control. 

Manufacture of components from thermosetting resins or from rubbers involves not only shaping the material, but also carrying out an exothermic chemical reaction. Proper control of temperature is required in order to obtain an acceptable rate of reaction without overheating and causing unwanted reactions which lead to thermal runaway. It may therefore be necessary to... 

A suitable method of process control is illustrated in Figure 12.4. These steps control the process, most of which are self-explanatory. As thermosets undergo exothermic reactions on curing, the temperature rise on curing could be monitored, as an indication of the degree of cure and the time after which a component can be taken from the mould. 

DSC analysis is the most common method used to assess the degree of cure [3] of thermosets. In fact Quality Control Procedures usually require that no exothermal peak associated with the cure process is present. 

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