Gelation and vulcanization

Hess, W. Vilgis, T. A. Winter, H. H. Dynamical critical behavior during chemical gelation and vulcanization. Macromolecules, 1988, 21(8), 2536-2542. 

Polymerization, gelation and vulcanization have been discovered as new subjects in statistical mechanics. They have been embedded in the theory of critical phenomena and statistical mechanics of polymers in general (see, for example, refs. 3,4 and 5), and many new discoveries have been made. The new developments in theoretical physics of these subjects initiated by the classical... 

Another realization of this transition is crosslinking of preformed polymer. In this case one starts with a polymer melt or a concentrated polymer solution, whereas ( -functional crosslinks have been added. This process is called vulcanization. It turns out already in the classical theory that both models, gelation and vulcanization, belong to the same universal class of liquid-solid transitions, but there are a few differences, to be reported within this section. 

Various versions of the classical theory of gelation and vulcanization are summarized in Flory s book, which have been developed by Flory and Stockmayer. The simplest version of the theory starts with ( -functional units like (1, where

branched molecule, schematically or alternatively, one has a (/>-functional object like (2) and a bifunctional unit B—B which connects to a part of the final macromolecule like (3). 

Similar studies have been made by Boots et In this study the authors compare step reaction (gelation) and vulcanization from preformed polymers by computer simulation, and one can see directly the difference of the structure and the gel point. 

Critical behaviour in gelation and vulcanization of polymers can be discussed in the context of fractals " and we want to discuss this in more detail, because these ideas have been developed very recently. 

Ideally theory should predict the macroscopic behaviour from the gelation and vulcanization process. This is only possible for extremely simplified models. Because of lack of such a theory we choose another way in this section. We first discuss the problems of rubbers and networks from a theoretician s point of view and then briefly come back to the problem of formation. 

It is also clearly desirable that the double bond should not become involved in polymerization reactions leading to gelation and cross-linking during the polymer manufacture. Additional requirements are that the diene should not seriously influence catalyst reactivity, should react efficiently during vulcanization and should be efficiently converted (or easily recycled, or both) during polymerization. 

The static theories of equilibrium gelation (percolation theory) and vulcanization (Flory-Stockmayer theory) are well known. But many of the interesting experiments on polymer sols and gels concern dynamics viscoelasticity), which are less universal... 

The consideration of microgels as a new class of polymers has been put forward mainly by Funke who has developed a great deal of expertise on these systems. Microgels will be defined as crosslinked polymers roughly of spherical structure with dimensions of the order of the size of ordinary polymer molecules, branched polymers or lattice animals. Thus microgels are microscopic networks. Their physical properties depend on crosslink density, their connectivity and presence of solvent, etc. The formation of these objects is therefore a microgelation problem , and we may use the usual theories of gelation or vulcanization. 

Dibutyltin diacetate, dilaurate, and di-(2-ethylhexanoate) are used as homogeneous catalysts for room-temperature-vulcanizing (RTV) silicones. The dialkyltin compounds bring about the cross-linking of the oligomeric siloxanes, to produce flexible, silicone rubbers having a host of different uses, such as electrical insulators and dental-impression molds. Recent work has also shown (560) that various dibutyltin dicar-boxylates catalyze both the hydrolysis and gelation of ethyl silicate under neutral conditions. 

Time-cure superposition is valid for materials which do not change their relaxation exponent during the transition. This might be satisfied for chemical gelation of small and intermediate size molecules. However, it does not apply to macromolecular systems as Mours and Winter [70] showed on vulcanizing polybutadienes. 

In contrast, chemical gelation involves formation of covalent bonds and always results in a strong gel. There are three main chemical gelation processes condensation, vulcanization, and addition polymerization. 

Elastomer-modified epoxy resin systems with more complexity to their preparation scheme have been demonstrated. Two examples suffice. Shelley and Clarke (9 ) instruct that a vulcanization procedure can be successfully employed to Improve elevated temperature properties in the cured resin mass. This step occurs subsequent to the esterification regime. It can be practiced with impunity at low rubber contents (7.5-10%) without gelation or Indeed very much viscosity Increase. Peroxides appear to be preferred over sulfur/sulfur donor systems. Table VII displays an example of this procedure with a solid DGEBA resin. 

As stated in Section 2.4, the condensation reaction may be the basis for the mechanism of stepwise polymerization. As long as the functionality equals 2 (di-acid and di-alcohol in polyester or di-amine in polyamide), only linear chains are obtained. However, once polyfunctionality prevails (appearance of tri-alcohol like glycerol, or tri-acid), reactive branches are formed that may interact and lead to a three-dimensional structure, called cross-linked (gelation). This is the basis for thermosetting polymers on one hand, or for stabilizing the elastomeric chain on the other hand (replacing vulcanization). 

Set n. (1) To convert an adhesive into a fixed or hardened state by chemical or physical action, such as condensation, polymerization, oxidation, vulcanization, gelation, hydration, or evaporation of volatile constituents. (2) Condition of a paint or varnish film when it has dried to a point where, for all practical purposes, it ceases to flow. See cure and dry. 

Early theoretical approaches to the gel-formation [1-4] as the Flory-Stockmayer theory do not take into account several aspects which naturally occur as the individual molecules grow to form the gel, such as cyclic bond formation, excluded volume effects and steric hinderance. The Flory-Stockmayer theory assumes that in the gelation process each bond between two individual monomeric, oligomeric or polymeric molecules is formed randomly. Thus this theory assumes point-like monomers. This apparently is not the case when already existing macromolecules are crosslinked, i.e., in vulcanization reactions as well as in copolymerization reactions of macromolecules with the functionality /> 3 with bifunctional monomers. 

Unlike vulcanizing latex adhesives, which have a storage life measured in weeks, vulcanizing solution adhesives have a limited pot life and are stored as two-component mixtures. The two parts are mixed immediately before use and the solution will then have a workable life of perhaps 2-4 hours before gelation occurs. It is usual to formulate the two parts with the sulfur... 

In this chapter we emphasize the theoretical aspects of gelation, vulcanization, rubber formation, rubbers and related problems of microgels. There has been much progress in the statistical mechanics of networks and network formation during recent years and new theoretical concepts have been developed. 

The second section of the chapter is devoted to the rubbery state of the material. If the crosslinking and gelation is extended until all molecules have eventually reacted, a rubber is formed. The rubber is a true solid by having a reagent added which joins each chain to a neighbour, in vulcanization for example. Obviously the number of crosslinks formed determines most properties of the material. Sometimes the number of such links will be a few per chain, but most of the applications are of highly crosslinked material. Alternatively irradiation by y- or X-rays, or by electrons will crosslink the chains as well, but this process will not be discussed here. 

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