Thermoset cure processes temperatures

The final physical properties of thermoset polymers depend primarily on the network structure that is developed during cure. Development of improved thermosets has been hampered by the lack of quantitative relationships between polymer variables and final physical properties. The development of a mathematical relationship between formulation and final cure properties is a formidable task requiring detailed characterization of the polymer components, an understanding of the cure chemistry and a model of the cure kinetics, determination of cure process variables (air temperature, heat transfer etc.), a relationship between cure chemistry and network structure, and the existence of a network structure parameter that correlates with physical properties. The lack of availability of easy-to-use network structure models which are applicable to the complex crosslinking systems typical of "real-world" thermosets makes it difficult to develop such correlations. 

Consist of a range of chemicals which promote cross-linking can initiate cure by catalysing ( catalysts , hardeners, initiators), speed up and control cure (activators, promoters) or perform the opposite function (inhibitors) producing thermosetting compounds and specialised thermoplastics (e.g. peroxides in polyesters, or amines in epoxy formulations). The right choice of a cure system is dependent on process, process temperature, application and type of resin. 

Thermoplastic polymers can be heated and cooled reversibly with no change to their chemical structure. Thermosets are processed or cured by a chemical reaction which is irreversible they can be softened by heating but do not return to their uncured state. The polymer type will dictate whether the compound is completely amorphous or partly crystalline at the operating temperature, and its intrinsic resistance to chemicals, mechanical stress and electrical stress. Degradation of the basic polymer, and, in particular, rupture of the main polymer chain or backbone, is the principal cause of reduction of tensile strength. 

Frequency dependent complex impedance measurements made over many decades of frequency provide a sensitive and convenient means for monitoring the cure process in thermosets and thermoplastics [1-4]. They are of particular importance for quality control monitoring of cure in complex resin systems because the measurement of dielectric relaxation is one of only a few instrumental techniques available for studying molecular properties in both the liquid and solid states. Furthermore, It is one of the few experimental techniques available for studying the poljfmerization process of going from a monomeric liquid of varying viscosity to a crosslinked. Insoluble, high temperature solid. 

Thermochemical submodel The thermochemical submodel provides temperature, viscosity, degree of cure (for thermosets), crystallinity (for thermoplastics), and the time required to complete the cure process. 

Polymer thick films also perform conductor, resistor, and dielectric functions, but here the polymeric resins remain an integral part after curing. Owing to the relatively low (120—165°C) processing temperatures, both plastic and ceramic substrates can be used, leading to overall low costs in materials and fabrication. A common conductive composition for flexible membrane switches in touch keyboards uses fine silver particles in a thermoplastic or thermoset polymeric binder. 

SiLK resin is a solution of low molecular weight, aromatic, thermosetting polymer. The polymer s molecular weight and solution concentration were tuned to enable precise and convenient deposition by spin coating, a technique universally used by the industry for the deposition of photoresist materials. After deposition on a wafer, the polymer is thermally cured to an insoluble film that has a high glass transition temperature. The polymer has good mechanical properties at process temperatures, which is required for the application, and it is also resistant to process chemicals. 

Through the analysis of the particular selected examples it was shown that it is possible to get a good description of temperature and conversion profiles generated during the cure of a thermosetting polymer. Thermal and mass balances, with adequate initial and boundary conditions, may always be stated for a particular process. These balances, together with constitutive equations for the cure kinetics and reliable values of the necessary parameters, can be solved numerically to simulate the cure process. 

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... 

The behavior of curing thermosetting resins can be represented with the generalized time-temperature-transformation (TTT) cure diagram developed by Enns and Gillham[7] it can be used to relate the material properties of thermosets as a function of time and the processing temperature as shown in Fig. 2.28. 

As was mentioned above (Sect. 4), for all considered polymers prepared at Tciire < T , their experimental glass transition temperature T p is close to their Tcure. The thermosetting reaction becomes quenched by vitrification, and for a new reinitiation of the cure process the polymer is to be softened by an increase of Tcure. Experimentally, in all cases, two consecutive processes take place after a sudden increase of T r (1) the softening of the polymer followed by (2) the next step of cure up to a new txdir (Fig, 25). 

Optimization of the cure cycle is inextricably linked to the design of processes for specific applications. Typically, this involves a clear understanding of the influence of temperature and time used to cure the thermosets. This is key to predicting resin flow and the degree of cure. Consequently, much attention has been placed on the curing process/morphology/ structure relationship of thermosets. " ... 

These studies pointed out that a comprehensive model of thermoset cure for stress calculations must account for a large number of processing influences and material properties. Mass transfer (22.), chemical kinetics, network structure formation, and material property development are essential ingredients. Induced strains must be accurately calculated, as must stress relaxation. Properties dependencies on temperature are significant and must be accounted for, as must the inter-relationship between reaction kinetics and diffusion. 

---