Kinetics curing reaction

An interesting trend is the increasing use of in situ NMR to study polymerization kinetics, curing reactions, or to determine the comonomer reactivity ratios (127). High-pressurCy high resolution NMR has been employed to study polymer/solvent interactions in poly(l,l-hydroperfluorooctyl acrylate) and its copolymer with styrene (128). 

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. 

Kinetics and mechanism of curing reactions using ylides as a new curing agent. 

The kinetics of resole cure reactions monitored via FTIR suggest that a diffusion mechanism dominates below 140°C. The cure above 140°C exhibits a homogeneous first-order reaction rate. The activation energy of the cure reaction was -"-49.6 kJ/mole.66... 

Kinetically controlled epoxy curing reactions, 10 423 Kinetic barriers, 11 529 Kinetic friction, 15 224 Kinetic incompatibility, in acrylonitrile copolymerization, 11 203 Kinetic measurements, 14 607-629. See also Very fast kinetics combined methods for unstable reagents, 14 621... 

The curing sequence and kinetics of this adhesive system prevent the NTMP from Inhibiting the reactive amines until the curing reaction Is well under way. In contrast, the epoxypolyamide primer contains free amino groups at room temperature and may be inhibited by the electrophilic NTMP species prior to curing. (Equation 3)... 

In order to quantify diffiisional effects on curing reactions, kinetic models are proposed in the literature [7,54,88,95,99,127-133]. Special techniques, such as dielectric permittivity, dielectric loss factor, ionic conductivity, and dipole relaxation time, are employed because spectroscopic techniques (e.g., FT i.r. or n.m.r.) are ineffective because of the insolubility of the reaction mixture at high conversions. A simple model, Equation 2.23, is presented by Chem and Poehlein [3], where a diffiisional factor,//, is introduced in the phenomenological equation, Equation 2.1. 

The techniques to be described here cannot differentiate the multiple reactions that take place during the curing reaction instead, only an overall kinetics is measured [32,142]. The most commonly used techniques will be described briefly. 

Kinetic models determine the minimum time required to cure the resin (i.e., guarantee sufficient physical and mechanical properties). They also determine the heat of reaction of the resin for use by heat transfer models and the degree of crosslinking for use in viscosity submodels. The exothermic cure reaction for the transformation of the epoxy resin to the cured matrix polymer can be expressed as ... 

If the curing reaction occurs at adiabatic or isothermal conditions above the glass transition temperature of the ultimately cured polymer, the reaction kinetics is adequately described by the adopted mechanism 5> 16,l7 69 7i However, at temperatures substantially below the final glass transition temperature, the reaction rate at a certain... 

In Chapter 2 the DSC technique is discussed in terms of instruments, experimental methods, and ways of analysing the kinetic data. Chapter 3 provides a brief summary of epoxy resin curing reactions. Results of studies on the application of DSC to the cure of epoxy resins are reviewed and discussed in Chapter 4. These results are concerned with the use of carboxylic acid anhydrides, primary and secondary amines, dicyanodiamide, and imidazoles as curing agents. 

Although the simple rate expressions, Eqs. (2-6) and (2-9), may serve as first approximations they are inadequate for the complete description of the kinetics of many epoxy resin curing reactions. Complex parallel or sequential reactions requiring more than one rate constant may be involved. For example these reactions are often auto-catalytic in nature and the rate may become diffusion-controlled as the viscosity of the system increases. If processes of differing heat of reaction are involved, then the deconvolution of the DSC data is difficult and may require information from other analytical techniques. Some approaches to the interpretation of data using more complex kinetic models are discussed in Chapter 4. 

In general the amine-epoxy resin curing reactions show complex kinetics typified by an initial acceleration due to autocatalysis, while the later post-gelation stages may exhibit retardation as the mechanism becomes diffusion-controlled. However some workers 72 80) have found that over a limited range of conversion the kinetic data may be described by the simple models of Eq. (2-6) or (2-9). 

The results of the DSC studies on DICY-cured systems are summarised in Table 5. The general comments at the end of Section 4.2, on the complexity of the kinetics apply also to dicyanodiamide cure. In this case the systems are even more complex because the dicyanodiamide is initially present as a dispersion of crystalline particles, so that the onset of the curing reaction is usually coincident with the onset of dissolution of the dicyanodiamide. 

Although this copolymerization has found application as a curing reaction for technically important epoxy resins, gel formation and difficulties connected with the evaluation of chemical analysis data have often led to different interpretations of the results. Attention has often been concentrated on kinetic problems of the copolymerization, but its mechanism has not yet been solved satisfactorily. 

Figure 1 shows kinetic curves of the DGER-mPhDA (P = 1) cure reaction at different T.ure. From these curves, the existence of the conversion at which the reaction stops due to diffusion control, adlf, can be seen. The measurement of the total reaction heat Q at different Tcure and its comparison with the estimated value based on the specific heat of epoxy ring opening give the values of adif for any T 16). 

It has been shown in Section 3.3 that the initiated polyetherification should be described by the kinetic theory and that a simplification is possible, if groups of independent reactivity in polyfunctional monomers participate in the curing reaction. The results of the treatment of an ideal postetherification following the epoxyamine addition are summarized below. 

A generalized kinetic model of cure is developed from the aspect of relaxation phenomena. The model not only can predict modulus and viscosity during the cure cycle under isothermal and non-isothermal cure conditions, but also takes into account filler effects on cure behavior. The increase of carbon black filler loading tends to accelerate the cure reaction and also broadens the relaxation spectrum. The presence of filler reduces the activation energy of viscous flow, but has little effect on the activation energy of the cure reaction. 

The model described in Eq.(l) not only can predict the cure behavior measured by standard curometers, but also can explain filler effects on the cure reaction. The model enables one to predict scorch time and cure time of elastomers at various filler loadings and cure temperatures. In the following discussion, this kinetic model of cure will be extended to explain and predict the modulus or viscosity of elastomers/thermosets during... 

---