Curing process typical profile

In this chapter, the evolution of conversion and temperature profiles during typical cure processes is discussed. This is useful for analyzing the possibility of attaining the maximum conversion, avoiding undesired high... 

Figure 6.2 Curing cycle temperature-time profile for typical graphite-epoxy composite in a vacuum bag autoclave process. Autoclave pressure is applied during the 135°C (275°F) hold...

The standard process cycle for polymer matrix composites is a two-step cure cycle, as seen in Figure 8.1. In such cycles the temperature of the material is increased from room temperature to the first dwell temperature and this temperature is held constant for the first dwell period ( 1 hour). Afterward, the temperature is increased again to the second dwell temperature and held constant for the second dwell period (2-8 hours). After the second dwell, the part is cooled down to room temperature at a constant rate. Because there are two dwell periods, this type of cure cycle is referred to as a two-step cure cycle. The purpose of the first dwell is to allow gases (e.g., entrapped air, water vapor, or volatiles) to escape and to allow the matrix to flow, which leads to compaction of the part. Thus, the viscosity of the matrix must be low during the first dwell. Typical viscosity versus temperature profiles of polymer matrices show that as the temperature is increased, the viscosity of the polymer decreases until a minimum viscosity is reached. As the temperature is increased further, the polymer begins to cure rapidly and the viscosity increases dramatically. The first dwell temperature must be chosen judiciously so that the viscosity of the resin is low while the cure is kept to a minimum. 

The typical application situation is where a new material or structure is being implemented. The question to be resolved is what thermal cure profile (time, temperature, pressure) should be specified. The current approach has been to expend much time and material producing simulated panels for validation. This process is neither rigorous... 

In a typical formulation, an ethylene-n-butyl-acrylate-carbon monoxide (60/30/10) terpolymer (60 wt%) is melt compounded with plasticized PVC (40 wt%) in a twin-screw extruder and the ethylene terpolymer dispersion cured in situ during the mixing by catalytic amounts of a suitable peroxide (0.3%) and a bismaleimide crosslink promoter (0.2%). The extruded pellets of the elastomeric blend can be used in conventional melt fabrication processes such as profile extrusion, extrusion coating, milling and calendering of sheets, injection and/or compression molding. 

The first objective was an in-depth study of the process of cure in the sample between the MDR dies, by considering not only heat transfer through the sample but also the heat generated by the overall cure reaction. Three various EPDM samples were selected with different percentages of peroxide ranging from 2 to 19%, associated with a cure enthalpy between 14 and 51 J/g. Another purpose was to evaluate the state of cure in the sample at various times, and thus profiles of temperature and of state of cure developed through the thickness of the rubber can be obtained at various times, especially associated with typical values of the torque (5, 50 and 95% of maximum torque). Comparison could be made between the values of the state of cure and the value of the torque. 

Without use of accelerators, external heat is required, making a system suitable for mechanical processes, such as hot press molding and continuous impregnation of sheet and profile. Temperatures in the range 120-160C (248-320F) are used to cure in a short cycle accelerators offer no advantage as the rate of cure depends on thermal decomposition of the peroxide, and typical cycle times are 1-10 min. 

Numerical results reported (2) on a typical TGDDM-DDS matrix laminate, assuming that the prepregs are suddenly expose to die cure temperature, are diown in Fig. 24 (a,b,c) as me variation of die tenqierature, decree of reaction and viscosity as a function of the processing time, bodi on the dam and on the core of the laminate. Input data of die full model are givmi in Table 9 (2). Due to the contribution of die thermal conductivity of the fibm the tenqierature at the center of die laminate nqiidly reaches the external inqiosed temperature and increases as a con uence of die imbalance between the rate of heat generation and the thermal diffiisivity of the composite (Fig. 24a). When these two quantities are comparable, the temperature profile reaches a maximum. 

Polymeric Conductive Formulations. The conductive systems used in polymerie formulations inelude C, Ag, Ni, Cu, and various mixtures. Table 8.5 illustrates typical sheet resistivities for various metal powders and flakes. Silver flakes produce the most conductive system, since silver oxide remains conductive and exhibits slow oxidation kinetics, resulting in wide processing latitude with respect to curing cycles. Base metals oxidize in air and, therefore, require very limited curing profiles. As seen in Table 8.3, polymeric conductors have reasonable corrosion resistance and can be used for organic die attachment applications. However, wire bonding and solderability are typically poor for these systems. 

Continuously cured profiles are based on very fast curing compounds, a cure of Imin at 200 °C being typical so that thiourea acceleration is necessary, with DETU (Iphr) most commonly used. Frequently the magnesia is reduced from the standard 4 phr to 2 phr to obtain a faster onset of cure. The processing safety of these compounds is obviously limited and requires careful control of all processing steps to limit the heat history. The compounds should be two stage mixed with the accelerators added shortly before extrusion. 

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