Vulcanisation reaction

Sulfur reacts very slowly with rubber, and so is compounded with rubber in the presence of accelerators and activators. Typical accelerators are thia-zoles and a typical activator is a mixture of zinc oxide and a fatty acid. The chemistry of the vulcanisation reactions is complicated, but generates a three-dimensional network in which rubber molecules are connected by short chains of sulfur atoms, with an average of about five atoms in each chain. 

A compounding ingredient which speeds up the vulcanisation reaction, enabling it to take place in a shorter time or at a lower temperature or both the physical properties of the vulcanisate and its resistance to ageing are also improved. 

The amount of sulphur which has not combined with the rubber during the vulcanisation reaction. The determination of the free sulphur content is widely used in assessing the state of cure of a rubber product. The addition of the free sulphur curve to the modulus and tensile/time curves of the particular compound enables the state of cure of the vulcanised product to be estimated. 

A curing agent which splits off active sulphur during the vulcanisation reaction. One example is di-morpholinodisulphide (Sulfasan R). 

Calcium silicate produced by precipitation is a fine powder with particle sizes down to 1 uni. It is a reinforcing filler with a reactivity greater than aluminium silicate. It requires the use of additional accelerator as it slightly retards the vulcanisation reaction. 

When properly dispersed there will be no difference in the effects of the two sulphur types on the course of the vulcanisation reaction. 

French and European Union legislation relating to the control of emissions in the workplace is examined, with particular reference to the rubber processing industry. Air sampling and analytical procedures used in the detection of nitrosamines formed in vulcanisation reactions are reviewed. 2 refs. 

A plastic foam is a heterogeneous blend of a polymer with a gas. The gas cells are between 1 mm and 0.1 mm. Foams are made from thermoplasts, thermosets and rubbers. In all these cases the foam structure is generated in the fluid condition with thermoplasts it is fixed by solidification, with thermosets and rubbers by the curing or vulcanisation reaction. 

Curing reactions Vulcanisation reactions Isothermal ageing Product stability Thermal degradation Identification of processing aids Plasticisers Mould lubricants Blowing agents Antioxidants Flame retardants Safety concerns... 

There is wide variety of vulcanisation agents and methods available for crosslinking rubber materials including peroxide, radiation, urethane, amine-boranes, and sulfur compounds [20]. Because of its superior mechanical and elastic properties, ease in use, and low cost, sulfur vulcanisation is the most widely used. Although vulcanisation with sulfur alone is not practical compared to the accelerated sulfur vulcanisation in terms of the slower cure rate and inferior physical properties of the end products, many fundamental aspects can be learned from such a simply formulated vulcanisation system. The use of sulfur alone to cure NR is typically inefficient, i.e., requiring 45-55 sulfur atoms per crosslink [21], and tends to produce a large portion of intramolecular (cyclic) crosslinks. However, such ineffective crosslink structures are of interest in the understanding of complex nature of vulcanisation reactions. 

Side reactions including cis-to-trans isomerisation and sulfidic cyclisation are observed along with the formation of crosslinks in the BR cured with sulfur alone. In the sulfur-donor vulcanisation of BR, cis-to-trans isomerisation is the predominant feature of the vulcanisation reaction sequence and seems to obey first-order kinetics with respect to the concentration of accelerator. 

This method is potentially the most versatile, since it can be carried out in a variety of ways and adds relatively little to the cost of the final polymer. The nitroso-ene reaction in natural rubber already referred to is one example of such a modification. However, in order to avoid the necessity to gear the modification process to the vulcanisation reaction, we have concentrated in our own work on antioxidant adduct formation to the rubber double bonds either by vinyl grafting or by the Kharasch thiol addition reaction... 

Table 3. Mechanochemical binding of MADA to Rubbers, before and after vulcanisation. (Reaction at 70 C).

Some more specific polymer chemistry applications for TG-FTIR are solvent and water retention, curing and vulcanisation reactions, isothermal ageing, product stability, identification of base polymer type and additives (plasticisers, mould lubricants, blowing agents, antioxidants, flame retardants, processing aids, etc.) and safety concerns (processing, product safety, product liability, fire hazards) [357]. A wide variety of polymers and elastomers has been studied by TG-FTIR [353,358,359]. The potential applications of an integrated TG-FTIR system were discussed by various authors [346,357]. 

Guzman and co-workers [27] investigated whether it is possible to use waste tyre crumb as a replacement for zinc oxide as an activator in the sulfur vulcanisation of natural rubber (NR). They used the unsaturated organic compound squalene as a model compound for NR in their work, and followed the course of the vulcanisation reaction using the analytical technique high-performance liquid chromatography. The results confirmed that waste rubber crumb was an alternative to zinc oxide as an activator in the curing of NR compounds by sulfur-based cure systems. 

Results are presented of a study in which the sulphur vulcanisation of polyisoprene was carried out at 150C and gas chromatography/mass spectrometry and HPLC were used to determine the chemical nature of intermediates and accelerator residues formed during the vulcanisation reaction. 15 refs. 

Studies on the vulcanisation of a black and oil filled styrene-butadiene rubber compound accelerated by a number of different sulphenamide and sulphenimide compounds were made using a conventional curemeter operated to normal ASTM standards. The vulcanisation reactions were also studied using different modelling software, CODESSA software for deriving quantitative structure-property relationships and MOPAC software for semiempirical molecular orbital calculations which together yielded excellent correlation to onset of cure and maximum cure... 

The methodology used in a study of nitrosamine formation in vulcanisation processes accelerated by tetraethylthiuram disulphide is described. This involved the characterisation of nitrosable species and the mechanisms of their formation, and studies of the influence of vulcanisation reactions and curing systems on the formation of nitrosable species. The results of the study are summarised. 

BIS(DIISOPROPYL)TfflOPHOSPHORYL DISULPHIDE IN CIS-1,4-POLYISOPRENE VULCANISATION REACTIONS. I. AS A SULPHUR DONOR... 

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