Extent of cure

One of the conveniences afforded by curing PPS is that a single uncured feedstock can give rise to an entire family of cured polymers. The flow rates, ie, the extent of cure, of the cured polymers are optimized for specific appHcations. Table 1 shows typical melt flow values of cured PPS polymers for various types of appHcations. 

An important aspect of this is the splitting of the polymer chain with thiol (eq. 5) or mercaptide ion (thiol + base catalyst). In fact, sodium sulfide or organic monothiols, eg, mercaptoethanol or decyhnercaptan, are utilized to lower the molecular weight of polysulftdes or to limit the extent of curing reactions. 

Heat resistance is iafluenced by both the type and extent of cure. The greater the strength of the chemical bonds ia the cross-link, the better is the compound s heat resistance. Peroxide cure systems, which result ia carbon—carbon bonds, result ia a range of sulfur cross-links varyiag from 1 to > 30 sulfur atoms per cross-link, and heat resistance improves as the number of more thermally stable short cross-links predominates. This is an important factor ia designing the desired cure system. 

Moisture. The presence of water in a filler is not usually beneficial. Most fillers added to adhesives have a moisture content lower than 1 wt%. Only precipitated silicas and sepiolite contain about 5-10 wt% moisture. For some applications, fillers must be completely dried to exhibit adequate performance. Moisture absorbed on the surface of fillers impacts the rate and extent of curing of rubber base adhesives. 

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. 

Adolf and Martin [15] postulated, since the near critical gels are self-similar, that a change in the extent of cure results in a mere change in scale, but the functional form of the relaxation modulus remains the same. They accounted for this change in scale by redefinition of time and by a suitable redefinition of the equilibrium modulus. The data were rescaled as G /Ge(p) and G"/Ge(p) over (oimax(p). The result is a set of master curves, one for the sol (Fig. 23a) and one for the gel (Fig. 23 b). 

Thermogravimetric analysis In thermogravimetric analysis (TGA) a sensitive balance is used to follow the weight change of the sample as a function of temperature. Its applications include the assessment of thermal stability and decomposition temperature, extent of cure in condensation polymers, composition and some information on sequence distribution in copolymers, and composition of filled polymers, among many others. 

For the measurements of extent of cure (3), the monomer mixture was melted onto a salt plate, then cured in a closed can under conditions identical to those used for the mixtures in the sol/gel experiments. The plate was removed from the oven periodically and the spectrum recorded on a Perkin-Elmer Model 267 spectrometer. 

In order to establish the effect of varying monomer structure on dynamic mechanical results, three films were cured as thin sheets under identical conditions. No significant differences appear in the Rheovibron plots (Figure 3). Thus the mechanical properties (and by inference, such properties as strength and toughness) appear to be insensitive to monomer structure. The dynamic mechanical properties should be regarded as influenced primarily by the network connectivity and extent of cure. 

In order to study the cure behavior of the PTEB system, 1JC NMR of uncured and cured PTEB in the solid state was performed using crosspolarization magic-angle spinning techniques. The results show the polymerization to be via aromatization. The extent of cure versus cure temperature was determined quantitatively. It was found that the material was almost completely cured after one hour at 215°C. As the cure goes to completion, the ability to react decreases due to the corresponding rapid increase in Tg. Chemical shifts of the resonances in the cured material are consistent with a highly crosslinked condensed aromatic network. 

As the extent of cure progresses, this reaction forms chain networks within the resin. Representative equations are... 

The emphasis of the current research is on molecular structure of oligomeric fractions leached from quality cured, industrial resins. However, the potential for applications in quality control should not be overlooked. Chromatography analysis provides positive feedback capable of molecular descriptions of extent of cure actually achieved. Oligomeric distributions coupled to kinetic reaction analysis allows for detailed estimates of crosslink architecture within the resin (7). 

Cured specimens were a transparent golden brown color. DSC indicated less than lOX residual exotherm. Following the cure cycle described above the specimens ware postcured for 1 hour at 300°C in nitrogen. The extent of cure was determined by DSC to be greater than 95% following postcure. The cure cycle and postcure described above have been used in the fabrication of all cured BCB specimens. 

Cured BCB/BMI specimens are a transparent golden brown in color, similar to what was seen for the BCB oligomer. Once the specimens have been removed from the mold they are postcured for 1 hour at 300 C in nitrogen resulting in an extent-of-cure of greater than 95X. 

Enns and Gillham [55] report values of Ex/Em, Fx/Fm, Tgo, Tgoo, and c/.g for various systems. After gelation, expressions for the glass transition temperature as a function of extent of cure and molecular weight can be found in Wisanrakkit and Gillham [59]. 

Observation of resonances became difficult with the increase of molecular weight. To overcome this problem the probe temperature was increased to 180 °C which was above the Tg of the polymer. This procedure gave reasonable spectra from which unreacted epoxide groups were detected as well as extent of cure. 

Vander Hoff and Buchler (277) found Me= 13000 for natural rubber by applying the Bueche-Mullins analysis to the data of Bristow (288). Examination of the natural rubber results published some years ago by Flory et al. (289) yields Me% 15000. The data of Mullins (290) provides a value of 10000 for 1,4 polyisoprene if the total initial modulus is used in the analysis (Mullins reported Me = 16000 by examining the effects of primary molecular weight and extent of cure on C, alone the initial modulus G is C1 + C2 and C2 0.6 Cx in the samples examined). 

The change in Tg in the later stages of cure can provide a sensitive index of the extent of cure. For example in the BADGE/DDM system 78) it was found that the final 10 % of conversion corresponded to a 70 K increase in T. ... 

Fig. 44. Dependence of the 13C Tte relaxation times (yBj = 60 kHz) for PTEGDMA on the extent of cure inner CH20 ( ), end CH20 (O), CH2 (4-) and CH3 (A) for quaternary ( ), represents Tle/10. Standard errors do not exceed the bounds of the symbols. The dots represent the compression modulus, E (MPa). V indicates the vitrification point (reprinted from ref.2481 with permission)...

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