Cure rate dependence

Cure Rate Dependence on pH for the Phenolysis Lignin Resins. Cure behavior at different pH s of the resins was measured at 140°C, which is the usual hot-pressing temperature of phenolic resins. Relative rigidity change curves of LP-B at different pH s are illustrated in Figure 5. Cure advances faster as the pH of the resin increases. When the pH is 11.9, LP-B provides faster cure than the phenolic resin. A similar tendency has been found for LP-C. These findings clearly demonstrate that increasing pH of the resins improves cure rate. 

Figure 5. Cure rate dependence on pH for the phenolated steam explosion lignin-based resin LP-B.

In addition, cure time is increased five minutes for every 0.25 inches of thickness of a molding [6, 7]. In general, these rules do not apply to most polymeric systems because the phenomena of heat transfer and cure kinetics have been over-simplified. The cure rate depends on the basic polymers, curatives, cure temperature, and filler loading. The prediction of cure rate will be discussed from a new model of cure kinetics which is developed from the concept of a non-equilibrium thermodynamic fluctuation theory of chemical relaxation. 

Essentially limited as a class to co-axial mechanical assembly, retention and sealing, they also make good general purpose gasketting media. The cure rate depends upon surface activity and may require a supplementary catalyst. The family copes with the gaps of normal engineering practice. As clearances increase, the anaerobics capacity to cope well falls rapidly. The majority of materials in the family are only suitable for use in lap joints as gasketting media or to seal a gap. Only the special anaerobic materials can be considered to be true adhesives and suitable for use on unsupported lap joints. 

The water carrier is usually absorbed by at least one of the bonded surfaces though evaporation can also play some part. As water is removed, the dispersed plastic phase is forced into contact and congeals the resulting film binds the surfaces together. Cure rate depends on the speed of water extraction - ultimately, the absorbed water is lost by indirect evaporation. Direct evaporation of the emulsion to form a film is used in bonding of open weave materials but, even so, there is still a slower, subsequent release of absorbed water from the substrates. Normally, the cured adhesive film is completely thermoplastic but special acid catalysts, available for some formulations, will promote cross-links in the film to enhance environmental stability. 

It could be assumed on the basis of the results of the DSC study on kinetics of the free-radical copolymerization of unsaturated polyesterimide resins that the reactivity of the tested monomers occurs in the sequence glycerol a-allyl ether > vinyltoluene > styrene > 2-hydroxyethyl methacrylate. On the other hand, the curing rate depends on the type of polyesterimide resin and varies for the products of one- and two-step synthesis. 

Curing rate dependent on moisture in defect/woimd environment Difficult to transpose in vitro properties to some in vivo environments... 

The cure rate of a sihcone sealant is dependent on the reactivity of the cross-linker, catalyst type, catalyst level, the diffusion of moisture into the sealant, and the diffusion of the leaving group out of the sealant. For one-part sealants, moisture diffusion is the controlling step and causes a cured skin to form on the exposed sealant surface and progress inward. The diffusion of moisture is highly dependent on the temperature and relative humidity conditions. 

The choice of coagulant for breaking of the emulsion at the start of the finishing process is dependent on many factors. Salts such as calcium chloride, aluminum sulfate, and sodium chloride are often used. Frequentiy, pH and temperature must be controlled to ensure efficient coagulation. The objectives are to leave no uncoagulated latex, to produce a cmmb that can easily be dewatered, to avoid fines that could be lost, and to control the residual materials left in the product so that damage to properties is kept at a minimum. For example, if a significant amount of a hydrophilic emulsifier residue is left in the polymer, water resistance of final product suffers, and if the residue left is acidic in nature, it usually contributes to slow cure rate. 

The number of hardening agents used commercially is very large and the final choice will depend on the relative importance of economics, ease of handling, pot life, cure rates, dermatitic effects and the mechanical, chemical, thermal and electrical properties of the cured products. Since these will differ from application to application it is understandable that such a wide range of material is employed. 

Polymerization and curing rates of novolacs depend strongly on the acidity of the reaction mixture. Fig. 16 depicts the general pH dependence. Fig. 17 shows a partial structure for a hexa-cured novolac. Incorporation of amine is widely, though not universally, reported in hexa-cured novolac structures. In addition to the structure shown in Fig. 17, A, A -dibenzyl and A, A, A -tribenzylamine linkages have been reported [185-192]. The main by-products of hexa-curing conditions are water and ammonia, though formaldehyde is also produced. The structure and abundance of the amino portions of the cured polymer vary considerably with conditions. 

Rubber blends with cure rate mismatch is a burning issue for elastomer sandwich products. For example, in a conveyor belt composite structure there is always a combination of two to three special purpose rubbers and, depending on the rubber composition, the curatives are different. Hence, those composite rubber formulations need special processing and formulation to avoid a gross dissimilarity in their cure rate. Recent research in this area indicated that the modification of one or more rubbers with the same cure sites would be a possible solution. Thus, chlorosulfonated polyethylene (CSP) rubber was modified in laboratory scale with 10 wt% of 93% active meta-phenylene bismaleimide (BMI) and 0.5 wt% of dimethyl-di-(/ r/-butyl-peroxy) hexane (catalyst). Mixing was carried out in an oil heated Banbury-type mixer at 150-160°C. The addition of a catalyst was very critical. After 2 min high-shear dispersive melt mix-... 

The crosslinking reactions are illustrated in Reaction 1.8, and they demonstrate that, in principle, only a trace of curing agent is necessary to bring about cure of epoxy resins. Selection of curing agent depends on various considerations, such as cost, ease of handling, pot life, cure rates, and the mechanical, electrical, or thermal properties required in the final resin. 

Unlike a plastic blend where the properties largely depend on the properties of the individual component and the compatibUizer used, those of a rubber blend depend on the solubility and diffusivity of the curatives, reaction rates, scorch time, etc. Figure 11.16 gives relative cure rate and scorch time for a number of accelerators. Hence, in designing a rubber blend, aU these parameters have to be taken into consideration in order to obtain good properties along with good processability. 

Minimizing the cycle time in filament wound composites can be critical to the economic success of the process. The process parameters that influence the cycle time are winding speed, molding temperature and polymer formulation. To optimize the process, a finite element analysis (FEA) was used to characterize the effect of each process parameter on the cycle time. The FEA simultaneously solved equations of mass and energy which were coupled through the temperature and conversion dependent reaction rate. The rate expression accounting for polymer cure rate was derived from a mechanistic kinetic model. 

A synthetic rubber made by copolymerising isobutylene with 1% to 3% of isoprene, depending on the degree of unsaturation required. The percentage of isoprene determines the cure rate of the compound, the higher the isoprene content the faster the cure. 

Prognosis predominantly depends on age and stage patients older than 65 to 70 years are 50% as likely to be cured as younger patients. Patients with limited stage disease (stages I to II) have a 90% to 95% cure rate, whereas those with advanced disease (stages III to IV) have a 65% to 75% cure rate. 

It is fundamental, therefore, to control the rate of heat absorption and temperature variations during the cure cycle [33,34]. The cure cycle depends on the part geometry, thermal... 

In a dynamic and cross-linkable system, such as the curing of a thermoset that contains a thermoplastic, the phase separation is more complicated than nonreaction system. The phase separation is controlled by the competing effects of thermodynamics and kinetics of phase separation and cure rate of thermoset resin (i.e. time dependent viscosity of the system). 

To obtain the cure kinetic parameters K, m, and n, cure rate and cure state must be measured simultaneously. This is most commonly accomplished by thermal analysis techniques such as DSC. In isothermal DSC testing several different isothermal cures are analyzed to develop the temperature dependence of the kinetic parameters. With the temperature dependence of the kinetic parameters known, the degree of cure can be predicted for any temperature history by integration of Equation 8.5. 

The reaction of epoxides with carboxylic acids in the presence of ammonium perchlorate can be very slow. The search for a catalyst to accelerate the cure rate for a particular formulation can be particularly rewarding. The slowness of the reaction can lead to a disadvantage of the system if not carefully investigated. If cure is stopped before all of the epoxide groups have been consumed, cure will continue at a rate which depends on storage temperature. The epoxide can continue to react with carboxyl groups rapidly at elevated temperatures, or very, very slowly at ambient temperature, to yield highly crosslinked propellant systems with low strain capability. 

Estimated value of the cure rate and temperature dependence of the cure rate. 

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