Equilibrium cure system

Serves as a reversion resistor in equilibrium cure systems. 

As reviewed earlier, a semi-EV system is a compromise designed to produce, in structural terms, a vulcanizate containing a balance of monosulfidic and polysulfidic crosslinks at a defined optimum cure state. If polysulfidic crosslinks are to persist over extended periods, new ones must be created to replace those lost through reversion. With use of normal accelerations systems, there is limited opportunity for such events. Maintenance of a polysulfidic network through the curing process thus dictates utilization of a dual-cure system both of which are independent of each other. This is the principle of the equilibrium cure (EC) system. Here, bis(3-triethoxysilylpropyl)tetrasulfane (TESPT) is added as a slow sulfur donor [38] (Fig. 7). 

This reaction is extremely slow in the absence of catalysts but can be serious in the presence of either acid or base catalysts. Polymers should therefore be free of ionic polymerization catalyst residues and their vulcanizates free from acidic residues of curing systems. It has been found that in a closed system in which water is present the depolymerization proceeds until an equilibrium molecular weight is reached which is inversely proportional to the water vapour pressure. 

As Rosenberg emphasizes the apphcation of the approach developed is based on the assumption that phase separation proceeds at equihbrium conditions. For curing systems this condition is imreahstic because of the simultaneous proceeding of both the chemical and physical processes. The criterion of the equilibrium of the process is determined by the ratio of the rate of the chemical reaction rate and mutual diffusion coefficient at which the process may be considered as equihbrium. In his work Rosenberg also proposes models which describe the nucleation and growth of particles of the dispersed phase. These models accoimt for the effects of the reaction rate and nonuniform space distribution of components in the comse of phase separation. The models allow the final morphology of the system to be described... 

Therefore, the spontaneous formation of the equilibrium supramolecular system structure, which provides its kinetic curing stability, and does not depend on the prehistory of the sample, occurs after the completion of swelling, within the exposure period 0contact with the liquid phase. During this period, local molecule concentration redistribution and spontaneous supramolecular structure elements formation (cybotactic groupings and associates) occur in the macroscopic sample volume. The characteristics of these elements (lifetime, correlation radius, strength of physical bonds) differ from the similar indices or coefficients in the initial oligomer and in the polymer-oligomer film, which is in contact with liquid. 

If water-soluble substances are introduced when compounding the rubber, moisture will eventually diffuse to the water-soluble particles and form solution droplets within the rubber matrix. The difference in the vapour pressure of the internal solution and the vapour pressure of any external water phase then becomes the driving force for further infusion of water. When the rubber matrix is vulcanised, the elastic modulus of the rubber network exerts pressure on the internal solution, and when this pressure equals the osmotic pressure of the internal solution, equilibrium is established and absorption ceases.Consequently, when compounding for minimum water absorption it is important to minimise the introduction of electrolytic substances into the compound and to select the grade of butyl rubber and curing system which will give the highest elastic modulus consistent with other performance needs. 

Addition of bis-(3-triethoxysilylpropyl)-tetrasulphide plus accelerator and sulphur can counter loss of crosslinking. Accelerator systems which respond to this antireversion agent are the thiazoles and the sulphenamides. Thiurams do not respond. For cure state equilibrium to be maintained the proportions of the three constituents (sulphur, accelerator and antireversion agent) are adjusted to give a constant modulus. 

At low cure times, only Ale and A2c polysulfidic structures (50 ppm) are observed. At longer cure times, Ale and A2c polysulfidic structures reduce in sulfur rank to monosulfide (45 ppm), and Blc (58 ppm). Bit (64 ppm) and Clc (45 ppm) polysulfidic structures are observed. A small amount of ds-to-trans isomerisation was observed, which increased with sulfur content. The reversion reactions of TBSI-accelerated systems result in a lower degree of sulfurisation as opposed to TBBS-accelerated samples. Based on the equilibrium swelling measurements, TBSI is found to be a less efficient accelerator than TBBS. 

Solvents can also be used to reduce the surface tension of the adhesive formulation. The surface tensions of common solvents are shown in Table 3.5. Of course, when using solvents, one needs to make sure that they evaporate from the bond line before cure. Solvent solutions do not change the equilibrium surface energetics of the system. They only provide lower viscosity so that wetting is established at a faster rate. 

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. 

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