RATE OF VULCANISATION

This is the ratio of the rates of vulcanisation at two temperatures 10 °C (or 18 °F) apart. For rubber vulcanisation the coefficient is approximately 2 but it varies slightly with the temperature, the type of compound and the accelerator. 

The rate of vulcanisation of a rubber compound is controllable by the choice of accelerator. The range of products offered to the rubber industry has been categorised historically into recognised classes. New developments have resulted in products that improve compound performance and which overcome dermatological problems, and do not generate nitrosamines and other extractable or volatile decomposition products. 

Accelerators, e.g. zinc oxide and fatty acids, increase the rate of vulcanisation of rubber by sulfur and they reduce the amount of sulfur required from 10% to <3%. Certain sulfur-donating accelerators, like thiuram disulfides (1) and mercaptobenzothiazole (2), will effect vulcanisation without added sulfur to yield products with greatly enhanced ageing properties.1... 

Vulcanisation of the rubber does not cease completely at the end of the press/cure cycle. As the temperature of the moulded product reduces, the rate of vulcanisation decreases, but it does still continue, even into service life. Many maturation reactions take place as the product returns to ambient temperatures and even in service some crosslinking reactions will still take place over an extended period of time. The filler, usually carbon black for most components, will form its own reinforcement structure over a period of time, thus contributing to the required product properties. Testing of the product too early can result in overstressing of the rubber in critical stress zones and this in itself can lead to premature failure of the product in service. 

Table 4 provides comparisons of the different classes of accelerators based on their rates of vulcanisation. The secondary accelerators are seldom used alone, but generally are found in combination with primary accelerators to gain faster cures. 

The study relates to the effect of a new class of donor-acceptor vulcanisation accelerators, namely phosphatised alkylamides of fatty acids, on the rate of vulcanisation of butadiene-styrene mbber and on the physical and mechanical properties of the vulcanisates. 7 refs. Articles from this journal can be requested for translation by subscribers to the Rapra produced International Polymer Science and Technology. 

A method is disclosed for increasing the rate of vulcanisation of a sulphur rubber composition comprising heating a sulphur vulcanisable rubber composition to a temperature ranging from 100-200C the rubber composition contains a sulphur vulcanisable rubber, a sulphenamide compound and a hydrated thiosulphate. Addition of the hydrated thiosulphate to a sulphur vulcanisable rubber and a sulphenamide compound significantly increases the rate of vulcanisation of the rubber. 

Hexamethylol melamine (HMM) was characterised by FTIR spectroscopy. The optimum dose of HMM in the presence of a sulphur accelerator in a NR-filled compound was determined. The cure rate and kinetics of crosslink formation indicated that HMM enhanced the first-order rate of vulcanisation in an NR compound. The results revealed good mechanical properties of an HMM-based NR carbon black-filled vulcanisate. 15 refs. 

Thiols, disulphides and sulphenamides of certain pyridazines are effective accelerators in the sulphur vulcanisation of rubber. They exhibit improved rates of vulcanisation and states of cure compared with known sulphenamide accelerators. 

Peroxides decompose when heated to produce active free radicals which ia turn react with the mbber to produce cross-links. The rate of peroxide cure is coatroUed by temperature and selection of the specific peroxide, based on half-hfe considerations (see Initiators, free-RADICAL Peroxy compounds, organic). Although some chemicals, such as bismaleimides, triaHyl isocyanurate, and diaHyl phthalate, act as coagents ia peroxide cures, they are aot vulcanisation accelerators. lastead they act to improve cross-link efftcieacy (cross-linking vs scissioa), but aot rate of cross-link formatioa. 

Accelerators are chemical compounds that iacrease the rate of cure and improve the physical properties of the compound. As a class, they are as important as the vulcanising agent itself. Without the accelerator, curing requires hours or even days to achieve acceptable levels. Aldehyde amines, thiocarbamates, thiuram sulfides, guanidines, and thiasoles are aU. classified as accelerators. By far, the most widely used are the thiasoles, represented by mercaptobensothiasole (MBT) and bensothiasyl disulfide (MBTS). 

Antioxidants may be assessed in a variety of ways. For screening and for fundamental studies the induction period and rate of oxidation of petroleum fractions with and without antioxidants present provide useful model systems. Since the effect of oxidation differs from polymer to polymer it is important to evaluate the efficacy of the antioxidant with respect to some property seriously affected by oxidation. Thus for polyethylene it is common to study changes in flow properties and in power factor in polypropylene, flow properties and tendency to embrittlement in natural rubber vulcanisates, changes in tensile strength and tear strength. 

Increase the rate of the cross-linking action with sulfur considerably allow for lower sulfur content to achieve optimum vulcanisate properties. Organic accelerators (e.g. thiuram, dithiocarbamate, etc.) are of major importance. In some cases it is necessary to retard the onset of vulcanisation to assure sufficient processing safety. The antioxidant 2-mercaptobenzimidazole (MBI) acts as a retarder for most accelerators. 

An accelerator which permits processing of rubber compounds to be carried out with less risk of scorching but which does not slow down the rate of cure at normal vulcanisation temperatures. Demoulding... 

Oxidative ageing of rubbers is limited by the rate of diffusion of oxygen into the rubber product and is usually confined to the outer 3 mm. Antioxidants are used to protect rubbers from the effects of thermal oxidation and the vast majority of compounds will contain one or more. Peroxide vulcanisates are usually protected with dihydroquinolines. Other antioxidants react adversely with the peroxide inhibiting the crosslinking reaction. 

The solubility of wax in vulcanised rubbers is low (of the order of 0.5% for NR) but enough wax has to be added to a rubber compound to ensure that once the compound has been vulcanised and the rubber cools, the rate of migrational movement of the wax from the rubber mass to the surface of the rubber is rapid. Dependant upon the application, the addition level of wax can be up to about 10 phr. Migration of the wax to the rubber surface will also carry other ingredients such as antioxidants, antiozonants and other materials (e.g., vulcanisation residuals), to enhance the surface protection. 

Systems are available which use molten salt as the curing medium. These systems can be either unpressurised or pressurised, and can have a salt recirculation system fitted. The advantage of this method of vulcanisation is the high temperatures which can be achieved with consequent greater product throughput rates. 

If a fully compounded thermosetting rubber is subjected to a plasticity measurement at a high enough temperature and for long enough, it will cure and, consequently, there is not always a clear distinction between a plasticity test and a test for scorch or rate of cure. For example, the Mooney viscometer is used to measure scorch, i.e. the onset of vulcanisation, and an oscillating disc rheometer will measure the plasticity of the compound before the onset of cure as well as the increase in stiffness as curing takes place. 

Tests for scorch and rate of cure should be distinguished from tests for degree of cure or optimum cure measured on the vulcanised material. The latter type of test estimates degree of cure by measuring the physical properties of test pieces vulcanised for various times, tensile properties, swelling and set measurements being the parameters most commonly used. 

Rubbers are also being extruded, in a not essentially different way from plastics. Cooling of the extrusion cylinder is necessary to prevent premature vulcanisation as a result of the heat developed by internal friction. The extruder is fed by ribbons, obtained from milled sheets. End products are hoses, profiles, and cable mantles. On-line vulcanisation can be achieved by passing the extrudate through a steam channel, while the rate of extrusion is adjusted to the rate of curing. For this purpose high-rate vulcanisation recipes have been developed. Steam temperatures of about 200 °C are being applied (15 bars pressure). Treads for motorcar tyres are also extruded they are wrapped round the pre-formed carcass and then formed and vulcanised in a press. 

Recently, it was shown using FT-IR that the decrease of vulcanisation rate and final crosslink density of sulfur vulcanised NR upon increasing silica content may be related to increased absorption of zinc stearate onto the silica surface [69]. 

Similarly, considering the swelling effect of vulcanised poly(butadiene), induced by a good solvent, the relaxation rate has been shown to obey the simple equation ... 

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. 

The 13C NMR examination of CBS cured SBR was performed on unfilled, silica filled, and carbon black filled samples [41], Several different SBR samples with respect to styrene content and cis, trans, and vinyl BR content were used. The unfilled SBR samples gave 3 major peaks that appeared at 51.0, 50.2, and 49.3 in a spectrum similar to BR vulcanisation. Unfortunately, peaks below 45 ppm are obscured by the different main chain structural peaks of SBR. A difference was seen in the rate of formation of these peaks in filled samples with silica inhibit the vulcanisation rate compared to carbon black filled samples. 

The rate of diffusion in a given elastomer is found to be related chiefly to the size of the liquid/gases molecule. It is observed that the presence of polar group or methyl group in the polymer molecules reduces the permeability to a given liquid/gas. Therefore butyl, Neoprene and nitrile, along with ebonite, have a low value of permeability when compared with natural soft rubber vulcanisates. 

ISO 6179. Rubber, vulcanised—Rubber sheets and rubber coated fabrics Determination of transmission rate of volatile liquids (gravimetric technique). 1989. 

The choice of vulcanisation system for the rubber can have a dramatic effect on adhesion. Typically sulphur cured rubbers are easier to bond to than sulphur-free or peroxide cured rubbers. This is believed to be due to the interaction of sulphur with key curative materials in the adhesive. The more sulphur that is present, the more interactions that are available, and hence the better the chance of getting good adhesion. SEV (semiefficient vulcanisation) and EV (efficient vulcanisation) cure packages are typically more difficult to bond because of their lower free sulphur contents. EV refers to cure systems which give predominantly monosulphidic or disulphidic crosslinks whereas conventional sulphur cure systems produce mostly polysulphidic crosslinks. SEV systems fall somewhere between EV and conventional systems in the type of crosslinks produced. Vulcanisation proceeds at different rates and with different efficiencies in different types of polymers, so the amount of sulphur needed to produce an EV cure system will also vary. For example, in NR, an EV system will generally contain between 0.4 and 0.8 phr of sulphur, while in NBR the sulphur level will generally be less than 0.3 phr of free elemental sulphur. 

In sulphur cured rubbers, accelerators are generally used to reduce the dependency on sulphur in order to achieve more efficient vulcanisation, to improve heat and flex resistance due to the presence of more monosulphidic crosslinks, and to increase the cure rate of the rubber and improve production capacity. Two accelerators which have been shown to enhance bondability of rubbers are 2-mercaptobenzothiazole (MBT) and mercaptobenzothiazole disulphide (MBTS). An accelerator which is known to negatively impact on adhesion is tetramethyl thiuram disulphide (TMTD). 

The composition of the vulcanisation systems and the dynamics of vulcanisation can affect the bond strength significantly [6]. The flow behaviour of the stocks is controlled by the vulcanisation rate. During vulcanisation gradual transition from plastic flow... 

It is known that the effects of fixing are more prevalent under conditions of high humidity and high ambient temperatures. The rate of the chemical force generation is strongly influenced by the increase in ambient temperature which causes generation of chemical bonds from the nitrile surface of the rubber to the metal surface. The effects of humidity and the presence of bloomed materials on the vulcanisate surface were also investigated [46]. These blooms were created artificially and wiped from solution... 

Kodama et al [805] have analysed antioxidants in vulcanised SBR compounds using heat-desorption by means of a double-shot pyrolyser followed by GC-MS analysis. Schulten et al [675] performed TPPy-FIMS experiments using direct introduction. Rubber vulcanisates (BR, NR, SBR) were heated without any pretreatment from 50°C to 750°C in high vacuum with a heating rate of 1.2°C s mass range recorded 50-1500 Da. Figure 2.41 shows the thermogram for some pyrolysis products for BR. The counts (in arbitrary units) for the total ion current, butadiene (m/z 54), butadiene dimer (m/z 108), mercaptobenzothiazole (MBT) (m/z 167),... 

---