Physical property changes during cure

The mechanism for cross-linking of thermosetting resins is very complex because of the relative interaction between the chemical kinetics and the changing of the physical properties [49], and it is still not perfectly understood. The literature is ubiquitous with respect to studies of cure kinetic models for these resins. Two distinct approaches are used phenomenological (macroscopic level) [2,5,50-72] and mechanistic (microscopic level) [3,73-85]. The former is related to an overall reaction (only one reaction representing the whole process), the latter to a kinetic mechanism for each elementary reaction occurring during the process. 

It is in the technique of solidifying the mass that plastisol propellants differ so markedly from composite propellants. In composite propellants, the nonvolatile liquid is comprised of monomers or low molecular weight prepolymers. Solidification is accomplished by completion of the polymerization reactions. Much attention must be given to the degree of completion of these reactions during manufacture so as to minimize changes in physical properties as a consequence of continued slow polymerization, or so-called post-cure, following manufacture. 

In many cases the use of epoxy materials in so-called field conditions (for industrial, construction sites, etc.) demand an increase in the reaction velocity, which is usually achieved by adding accelerators. At present, the widely used accelerators include alkyl-substituted phenols, benzyl alcohol, carboxylic acids (in particular, salicylic acid), and others. A major disadvantage of these accelerators is their tendency to migrate from the cured epoxy matrix during the exploitation, which could lead to a change in the physical properties of the polymer. They also act as a plasticizer of epoxy-based polymers, and as a result reduce the polymer s chemical resistance. Thus, there is a need for new accelerator-modifiers that can provide faster curing of epoxy-amine compositions without negative side effects, and also improve the properties of the finished product. 

The silicone polymer backbone is composed of Si-O-Si bonds. This bond is very strong and stable with a bond energy of 87 Kcal/ mole. The polymer can tolerate 250°C to 300°C without decomposing.— The fully compounded silicone sealant, when cured to a rubber, can withstand 200°C for sustained periods of time with no special additives and even higher temperatures with polymer modifications and/or heat stability additives.— The Sl-O-Si molecular structure is also transparent to U.V., so silicone sealants are virtually unaffected by weather. Samples of silicone sealants used in exterior construction applications have been tested after 20 years of actual performance. These samples exhibited essentially no change in physical properties or adhesion during that time period. 

A step change of viscosity at critical conditions to infinity (3) is a convenient model for investigating the flow of a curing liquid. The simplest model employs the most important physical property of the process. In a number of papers [83-87], the dependence of the induction period t, the time during which a reactive substance retains the ability to flow, on the rate of shear y is studied. 

Hence, formation of the structure and properties of epoxy polymers during curing is determined by fundamental physical principles. This is accompanied by the change in the characteristic ratio C (molecular characteristics), although the structure of the macromolecule remains invariant. The use of the above physical principles even in the simplest version provides a correct description of the structure and properties of network polymers. 

When using Saret 633 and Saret 634 as internal adhesion promoters, it is important to remember that they do crosslink with the rubber during curing and, therefore, change the physical properties of the rubber. An example of this is shown in Table 8.16 for the Nordel EPDM formulation containing 2 phr of the metallic coagents. In this example,... 

Hsich s nonequilibrium thermodynamic fluctuation theory [51,52] directly describes the changes in physical or mechanical properties of the curing system during cure. According to this theory, the physical or mechanical properties of the resin system during cure can be expressed as... 

Although raw material costs are higher (some estimates are 18 to 20%), and some physical properties are reduced, woven fabric shortens layup times, avoids tape spread-out problems at abrupt contour changes, remains in place during cure (no distortion or fiber washout), and is frequently easier to drill or machine. 

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