Efficient vulcanisation cured

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). 

An investigation was carried out into the use of a lower amount of tetrabenzyl thiuram disulphide in NR formulations to achieve a faster cure rate without adversely affecting scorch resistance and flex related properties. Vulcanisate properties were correlated with the fine structure of the vulcanisates through a network study and vulcanisate properties at a cure temperature of 150C and cure times of 60 and 90 minutes tabulated. It was found that a combination of the above accelerator and a sulphenamide provided efficient vulcanisation with little or no loss in scorch safety and generated a crosslink... 

Purified natural rubber (PNR) is of interest because of its potentially lower toxicological effects than whole natural rubber (WNR) due particularly to the reduction in protein content. Improved dynamic mechanical properties have also been reported. Comparison of both gum and filled compounds, vulcanised using conventional cure systems (CV) and efficient vulcanisation systems (EV) prepared from PNR and WNR indicate that generally properties of the PNR are poorer than WNR. The exception is in the flex cracking resistance and the heat build up in the filled samples where PNR shows an improvement. Using the EV cure system on filled PNR gives properties almost comparable to WNR. A study of the distribution of the types of sulphur crosslink in both pNR and WNR vulcanisates indicates a more uniform distribution of monosulphidic (S), disulphidic (S2) and polysulphidic (Sx) crosslinks in the PNR samples (38.7/25/36.4 in PNR compared to 64.1/29.7/6.6 in WNR respectively). This is believed to be the reason for the better dynamic properties of PNR vulcanisates. 7 refs THAILAND... 

The chemical microstructures of cis-polyisoprene (HR) vulcanised with sulfur and N-t-butyl-2-benzothiazole sulfenamide (TBBS) accelerator were studied as a function of extent of cure and accelerator to sulfur ratio in the formulations by solid-state 13C NMR spectroscopy at 75.5 MHz [29]. Conventional (TBBS/Sulfur=0.75/2.38), semi-efficient (SEV=1.50/1.50) and efficient (EV=3.00/1.08) vulcanisation formulations were prepared, which were cured to different cure states according to the magnitude of increase in rheometer torque. The order and types of the sulfurisation products formed are constant in all the formulation systems with different accelerator to sulfur ratios. However, the amount of sulfurisation has been found to vary directly with the concentration of elemental sulfur. 

Three different techiuques for mixing a sihca and caibon black filled tyre tread compound were evaluated and the effect of surfactant-based process additives (modified zinc soaps and a zinc soap/resin combination designated Struktol HT 266) on productivity and compound quality investigated. Mixing methods studied involved the use of a separate filler addition stage for each component type and different orders of filler addition. Mixing efficiency and dispersion/filler interactions were examined and the effects of process additives on Mooney viseosity and extrusion assessed. The cure characteristics, physical properties and dynamic properties of the vulcanisates were also examined. 9 refs. 

SBR 1712 is an E-SBR extended with 37.5 phr of aromatic oil. Zinc oxide shortens the cure-time and stearic acid promotes the efficiency of the accelerator in vulcanisation. Examples of typical formulations for specific applications are given in handbooks [11]. 

This gives a very fast cure rate and is used in the continuous vulcanisation of both solid and cellular profiles, particularly in the latter to obtain a sufficiently fast cure rate for good skin formation. Disadvantages arising from the use of this material are poor processing safety and a lacrimatory effect on the operator s eyes necessitating efficient ventilation on the curing unit. 

Primary accelerators Primary accelerators are mercapto based accelerators, generally efficient and confer good processing safety to the rubber compounds, exhibiting a broad vulcanisation plateau with relatively low crosslink density. Examples are sulfenamides and thiazoles. Primary accelerators provide considerable scorch delay, medium fast cure, and good modulus development. 

The vulcanisation of natural rubber with the efficient accelerator N-cyclohexyl-2 benzothiazolesulphenamide, in the presence of zinc oxide, stearic acid, and 4,4 -dithiobismorpholine (DTBM), was studied with a disc oscillating curemeter to determine the causes of the pronounced plateaus in the cure curves observed with this and similar systems. It was determined that this behaviour was not caused by the formation of an inhibitory byproduct but was due to the occurrence of at least two distinct crosslinking reactions. DTBM was shown to act as a sulphur donor and not, as initially suspected, as an inhibitor. 14 refs. 

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