Kinetics of Vulcanization

A quantitative treatment of the complicated process of cross-linking in EPDMs has been proposed on the basis of four main assumptions  

The scheme summarized here is certainly a simplified representation of reality since it neglects the breaking of bonds involving primary chains and cyclization reactions. However, the overall vulcanization process can be described by two rate constants, i.e. kj and kf (Table 18), the former referring to the inactivation reactions and the latter to the reactions yielding the polymer network. 

Vulcanization kinetic curves of 1,4-HD (0.40 molA) and ENB (0.54 molA) based EPDM full line = experimraita] curve broken line = calculated curve (Ref. Conditions as in F g, 14, 

Curing formulation Polymer =100 HAF Carbon Black = 50 Naphthenic Oil = 5 ZnO = 5 S = 2 TMTD = 1 MBT = 0.5 Temp. = 145 °C. S. Table 1 ENB = 5-ethylidene-2-norbornene 1,4-HD = 1,4-hexadiene DCP = dicyclopentadiene MNB = 5-methylene-2-norbomene. Cross-linking reaction rate constant. 

Time requested to reach 90 % of TMjnax-H hest value of the torque moment. 

Termonomer Typeb Concentration (mol/1) kf (kg 1m tmin ) kr103d (min-1) TMmaxb (kgm) tje (min) l50f (min) t90g (min) a/te102 1 (min-1) 

---

The analysis of curves 1 and 2 (Figure 2.4) shows that involving the second accelerator in the composition improves the kinetics of vulcanization for the following reasons ... 

From the data of the kinetics of vulcanization (fiber content—2% for all samples) we know that the addition of the fibers, which was obtained on cobalt catalyst regardless of laundered or not it reduces the induction period and the cure time, but fiber, washed from the metal, not much increase the viscosity of the technology (Table 2 of Section 5.1, Fig. 2 of Section 5.1). As we can see from the data on swelling, samples containing fiber swell more than rubber without filler, it s indicative of a lower density of chemical bonds in the vulcanizates containing fiber. 

Vulcanization kinetic was studied. Mechanical properties of cured composites before and after various regimes of ceramization were investigated. Ceramic phase obtained after heat treatment was characterized by porosimetry and scanning electron microscopy. Morphology and mechanical properties of composites before and after ceramization depend on the type of mineral filler applied, whereas the kinetic of vulcanization is different only for composite containing surface modified montmorillonite. 

Recently a ntmber of works have been published in which the peculiarities of the structure of vulcanized latex films are described in detail. By means of a number of different methods, it was determined that these films have a microheterogeneous structure which represents the combination of a highly vulcanized polymer on the surface layer of latex globules and an unvulcanized polymer inside the globules. The relation between the topology of the vulcanizing network of latex films and their properties has been described ". The present work is devoted to an investigation of the kinetics of vulcanization process of latex films. 

The good agreement between the calculated and experimentally determined values clearly shows that our assumptions are justified. The kinetics of vulcanization of films, obtained from the solution in xylene of a blend analogous in composition to blend 7, was also investigated. From the data presented in Figure 4, one can see that the vulcanization of these films is described by 1-st order kinetics (the curve of dependence of free sulphur content on vulcanization duration becomes straight in semi-logarithmic coordinates)... 

They have studied the properties of NR-epoxidized natural rubber (ENR) blend nanocomposites also. Vulcanization kinetics of natural mbber-based nanocomposite was also smdied. The effect of different nanoclays on the properties of NR-based nanocomposite was studied. The tensile properties of different nanocomposites are shown in Figure 2.7 [33]. 

Influence of the ZnCFO contents (3,0 5,0 7,0 phr) on crosslink kinetics of the modelling unfilled rubber mixes from NBR-26 of sulfur, thiuram and peroxide vulcanization of recipe, phr NBR-26 - 100,0 sulfur - 1,5 2-mercaptobenzthiazole - 0,8 stearic acid - 1,5 tetramethylthiuramdisulfide - 3,0 peroximon F-40 - 3,0, is possible to estimate on the data of fig. 7. As it is shown, the increase of ZnCFO concentration results in increase of the maximum torque and, accordingly, crosslink degree of elastomeric compositions, decrease of optimum cure time, that, in turn, causes increase of cure rate, confirmed by counted constants of speed in the main period (k2). The analysis of vulcanizates physical-mechanical properties testifies, that with the increase of ZnCFO contents increase the tensile strength, hardness, resilience elongation at break and residual deformation at compression on 20 %. That is, ZnCFO is effective component of given vulcanization systems, as at equal-mass replacement of known zinc oxide (5,0 phr) the cure rate, the concentration of crosslink bonds are increased and general properties complex of rubber mixes and their vulcanizates is improved. 

Figure 9. Vulcanization kinetics of modeling unfilled elastomeric compositions on the basis of butyl rubber of resin vulcanization at 160°C with ZnO or ZnCFO...

Spin-spin relaxation dynamics were also used in the study of the kinetics of the vulcanization of polysulphide rubbers. The T2 values decrease with the course of the reaction and the time dependence of log (T2/T ), where corresponds to the time equal to zero, exhibits an inflection. The inflection point is attributed to gel formation, and the reaction rate constants for the two separate processes are determined from the T2 data. It was also observed that the addition of carbon black reduces T2 by a factor of 2 or 3, because vulcanization occurs both through the thiol groups and by the chemical reaction between the polymer and carbon black 37>. 

Fig. 13.28. TEM micrograph of 20 wt.% Pt-C electrocatalyst powder (Prototech). Vulcan XC-72 carbon support electrodes are now 0.2 mg cathode, 0.05 (for H2) anode. (Reprinted from M. A. Parthasarathy, S. Srinivasan, and A. J. Appleby, Electrode Kinetics of Oxygen Reduction at Carbon-Supported and Unsupported Platinum Microcrystallite/Nafion Interfaces, J. Electroanalytical Chem. 339 101-121, copyright 1992, p. 105, Fig. 2, with permission from Elsevier Science.)...

Vulcanization Kinetics of Addition-cured Silicone Rubber... 

Attempts to elucidate the complex mechanism of sulfur curing of elastomers containing conjupted unsaturations were performed by studying the vulcanization kinetics of HTI and OTI (Table 20) containing different levels of triene on the basis of the approach described in Sect. B.2.c). According to Table 20, OTI appears faster curing than HTI and HR, the latter copolymers exhibiting a simikr behavior. But the... 

FIGURE 2.4 Kinetic curves of (1) with single accelerator, (2) with accelerator system, (3) perfect curve of vulcanization. (Reprinted from O. Figovsky and D. Beilin, Building Materials Based on Advanced Polymer Matrix Review, J. Scientific Israel Technological Advantages 10, no. 3 (2008) 1-119. With permission.)... 

The change of parameters of vulcanization kinetics found in the experiment allowed more order in the composite structure, a reduction of the number of dislocations, and consequently, improvement of the mechanical characteristics of RubCon. Let us note that the optimum content of the components in the system with the additive action of accelerators for rubber of the same kind is constant for PBN it is 7 parts thiuram-D per 1 part CAPTAX. 

Diffusion coefficients are proportional to 1/M, the molecular weight of linear chains [7]. They are not well known and therefore neither is the interdigitated thickness. Hence, it is not possible to say whether the observed behavior has to be related to the interdiffusion depth, or to the number of crosslinks formed in the interfacial region or, most hkely, to both effects. These results given for the elastomer joints crosshnked by a sulfur-based vulcanizing system show that it is very difficult to separate interdiffusion and crosshnking mechanisms because the temperature influences both the chain mobihty and the kinetics of network formation. 

Bhatnagar S.K. and S. Banergie.1968. Kinetics of accelerated vulcanizates. Sulphur vulcanization accelerated with 2-mercatobenzothiazole and its Zn salts in presence of ZnO and stearic acid. Angewandte Makromol. Chemie. 3(1) 177-121. 

The nature of the vulcanizing agent and of the rubber (i.e., EPDM compounds with peroxide in this instance) is of prime importance to the kinetics of cure the activation energy and the preexponential factor play major roles, and the cure enthalpy is also of some importance in spite of its low value, whereas the order of the overall reaction is 1. Thus, because of the kinetics of cure, the time of cure varies considerably with the temperature, and thus there is a narrow temperature window through which meaningful data can be obtained. The cure enthalpy is rather low and thus does not intervene much in the process. Nevertheless, DSC experiments, which are based on measuring the enthalpy evolved, are also commonly used for determining the kinetics of cure [12]. 

---