Curing kinetics

DTA has also been used in polymer pyrolysis kinetics studies [14, 15]. 

In the second stage, diffusion process dominates the network formation as the reactants mobility is greatly reduced by polymerized networks. The incorporation of nano-alumina particles to the epoxy-amine system has an accelerating effect on the curing reaction. In this case, both physical interaction and chemical interaction at the nanoparticle surface are possible. 

All these characteristics are attributed to the catalytic effect of the fillers [71]. 

The curing behavior of the DGEBA-DDM system is summarized in Table 9.1. 

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Cure kinetics of thermosets are usually deterrnined by dsc (63,64). However, for phenohc resins, the information is limited to the early stages of the cure because of the volatiles associated with the process. For pressurized dsc ceUs, the upper limit on temperature is ca 170°C. Differential scanning calorimetry is also used to measure the kinetics and reaction enthalpies of hquid resins in coatings, adhesives, laminations, and foam. Software packages that interpret dsc scans in terms of the cure kinetics are supphed by instmment manufacturers. 

Dynamic mechanical analysis provides a useful technique to study the cure kinetics and high temperature mechanical properties of phenoHc resins. The volatile components of the resin do not affect the scan or limit the temperature range of the experiment. However, uncured samples must be... 

Acrylic adhesives cure by a free radical chain growth mechanism. In contrast, epoxy and urethane adhesives cure by a step growth mechanism. This has a major impact on the cure kinetics, as well as the composition of the adhesive during cure ([9], pp. 6-9). Cyanoacrylate adhesives (such as Super Glue ) also cure by chain growth, but the mechanism is ionic with initiation by surface moisture. 

R. W. Biemath and D. S. Soane, Cure Kinetics of Epoxy Cresol Novolac Encap-sulant for Microelectronic Packaging, in Contemporary Topic in Polymer Science. Advances in New Material. Vol. 7, J. S. Salamone and J. S. Riffle (Eds.), Plenum, New York, 1992, pp. 103-160. 

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. 

The cure kinetics will depend on the initial isocyanate to hydroxy ratio and on the humidity. Assuming that the concentration of water in the coating is constant during cure, it is possible to define the following parameter which determines the effect of humidity on cure ... 

In order to understand the effect of each process variable, a fundamental understanding of the heat transfer and polymer curing kinetics is needed. A systematic experimental approach to optimize the process would be expensive and time consuming. This motivated the authors to use a mathematical model of the filament winding process to optimize processing conditions. 

This paper will discuss the formulation of the simulator for the filament winding process which describes the temperature and extent of cure in a cross-section of a composite part. The model consists of two parts the kinetic model to predict the curing kinetics of the polymeric system and the heat transfer model which incorporates the kinetic model. A Galerkin finite element code was written to solve the specially and time dependent system. The program was implemented on a microcomputer to minimize computer costs. 

Crosslinking of many polymers occurs through a complex combination of consecutive and parallel reactions. For those cases in which the chemistry is well understood it is possible to define the general reaction scheme and thus derive the appropriate differential equations describing the cure kinetics. Analytical solutions have been found for some of these systems of differential equations permitting accurate experimental determination of the individual rate constants. 

Isothermal differential scanning calorimetry (DSC) measurements were carried out to investigate the curing kinetics [85]. Conversion vs time curves of DGEBPA-PACP systems prepared with 1 wt % of catalyst and without catalyst at identical curing temperature are overlaid in Fig. 31. 

Size Exclusion Chromatography Analysis of Epoxy Resin Cure Kinetics... 

The isothermal cure kinetics of a series of TGMDA/DDS resin formulations were investigated over the temperature range 121°-187°C. Figure 4 illustrates data obtained for the resin TGMDA/DDS(25%) at 177°C. During the early stage of cure prior to the onset of... 

Coatings based on these different crosslinkers have substantially different cure kinetics, network structure, and durability. Formation and degradation of crosslink structure in urethane and melamine crossllnked coatings are compared in this paper. Key differences in cure chemistry and kinetics which result differences in coating performance are identified. The chemistries of network structure degradation on exposure to UV light and water are discussed in terms of their effect on ultimate durability. 

Conversion of the mixed system to the B-stage cure condition to yield Isothermal cure kinetics. 

Cure Kinetics. The cure kinetics of the mixed system to the B-stage were determined by the method outlined by Senich, MacKnight and Schneider (7) for two epoxy resins cured with dicyandianide by dynamic spring analysis (DSA). Senich et al. (7) used the elapsed time to the loss peak maximum of tan delta as a measure of the rate of the reaction at each temperature and for each frequency. The slope of an Arrhenius plot of In (tmax) vs. 1/T was then used to determine the activation energy. 

The criteria by which these resins are evaluated include thermal analysis, cure kinetics and rheological studies of the uncured resin. Mechanical properties including hot/wet sample testing and thermal analysis are then obtained from cured neat resin speciswns. The results from these tests run on the neat resin will give some indication of the suitability of that resin for use as a composite matrix material and future studies to be conducted. 

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