Induction period, vulcanization

To assist in control of the onset of vulcanization, a retarder or prevulcanization iuhibiter (PVI) is used. Retardation of the onset of cure does not mean that the rate of cure is slowed, in fact cure rate may actually be increased. Rather, there is an induction period prior to cure. 

In all the above methods, it is necessary to cure specimens of test samples for each of a series of curing times and then perform the desired test on the vulcanizate. However, in the test for continuous measurement of vulcanization complete information could be obtained with saving in time. The mooney viscometer test approaches this objective. However a weakness of the mooney viscometer test is that the test is completed before a measurable modulus value after the scorch point has been obtained. This is because the test sample is destroyed after the induction period is passed due to tearing by continuous rotation of the rotor whether small or large. To overcome this deficiency and to provide a total cure curve for the entire vulcanization cycle, a series of instruments called cure meters was developed. In each of these instruments the stiffness or modulus of the compound was chosen as parameters for vulcanization continuously. The Vulkameter developed by Bayers, Germany was the first of the cure meters developed. 

An increase in the induction period of vulcanization (AB) when the structure is formed ... 

The rapid reaction between atomic oxygen and polymer films is discussed. This typical interface reaction is considerably influenced by the structure of the polymer. All polymers immediately react with atomic oxygen there is no evidence of even short induction periods or autocatalysis. Most readily attacked are highly branched polymers such as polypropylene and polymers with ether links for example, polyformaldehyde. Perfluorinated polymers, rubbers vulcanized with sulphur, and highly aromatic polymers are most resistant (Fig. 22). Oxidation of polymers by atomic oxygen occurs only at or near the surface of the polymer. For this reason the elucidation of the reaction kinetics and mechanism is very difficult. The conventional physico-chemical methods, UV and IR spectroscopy, are in this case inadequate. 

The sulfur vulcanization mechanism has also not been completely established. The reaction is not accelerated by peroxides consequently, it is presumably of ionic character. An induction period is usually observed (Figure 37-1). Two different kinds of cross-linking reaction appear to occur in vulcanization. One of these is accompanied by chain degradation produced by... 

This figure also illustrates the three main regions of rubber vulcanization. The first regime is the induction period, or scorch delay, during which accelerator complex formation occurs. The second time period is the cure period, in which the network or sulfurization structures are formed. The network structures can include crosslinks, cyclics, main chain modification, isomerization, etc. The third regime is the overcure, or reversion regime. 

Ultrasonic devulcanization also alters revulcanization kinetics of rubbers. It was shown [93] that the revulcanization process of devulcanized SBR was essentially different from that of the virgin SBR. The induction period is shorter or absent for re vulcanization of the devulcanized SBR. This is also true for other unhlled and carbon-black-filled rubbers such as GRT, SBR, NR, EPDM, and BR cured by sulfur-containing curative systems, but not for silicone rubber cured by peroxide. It was suggested that a decrease or disappearance of the induction period in the case of the sulfur-cured rubbers is due to an interaction between the rubber molecules chemically modified in the course of devulcanization and unmodified rubber molecules, resulting in crosshnking. It was shown that approximately 85% of the accelerator remained in the ultrasonically devulcanized SBR rubber [93]. 

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. 

The vulcanization curves at 170 C for the model terpolymer EA/MAA/AGE catalyzed by three different quaternary ammonium salts are shown in Figure 1. The cure rate constants, K2, of various onium salts are given in Table 4, assuming that crosslink formation is a first order reaction after an induction period. High crosslinking rates were achieved when onium salts were used as catalysts. 

Several other sulphur donors have been investigated. In one study (Ascroft et ai, 1969) tetramethyl thiuram disulphide, tetraisopropyl thiuram disulphide, dimorpholyl disulphide, diisopropyl xanthogen disulphide and diisopropyl thiophosphoryl disulphide were compared alone and in blends. Mixtures of the dimorpholyl and the thiophosphoryl disulphides are characterized by long induction periods followed by high rates of vulcanization. The dimorpholyl disulphide, also known as N,N -dithiobismorpholine is available commercially as a vulcanizing agent. 

As is apparent from Table 3, the molar ratio COOH/epoxy (xj) significantly influences the maximal modulus and optimal vulcanization. The rate constant of vulcanization is influenced to a substantially lesser degree by this factor and the induction period is not influenced at all. The MR of COOH/epoxy is also the decisive factor which influences the complex of physical properties... 

Sodium hexamethylene-l,6-bisthiosulflde dihydrate, when added to the vulcanization system, breaks down and inserts a hexamethylene-1,6-dithiyl group within a disulfide or polysulfide crosslink. This is termed a hybrid crosslink. During extended vulcanization periods or accumulated heat history due to product service, polysulfidic-hexamethylene crosslinks shorten to produce thermally stable elastic monosulfidic crosslinks. At levels up to 2.0 phr, there is little effect on compound induction or scorch times, nor on other compound mechanical properties (Rubber Chemicals, 1998). 

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