Vulcanisation efficient

Benzothiazole sulphenamide accelerators are suitable for semi-efficient and efficient vulcanisation systems. 

The original crosslinking process for natural rubber, called vulcanisation, involved mixing in 2-3% of sulphur plus an accelerator. On heating to 140 °C the sulphur reacts with C=C bonds on neighbouring polyisoprene chains to form sulphur crosslinks C—(S) —C. Typically, 15% of the crosslinks are monosulphide [n = 1), 15% are disulphide and the rest are polysulphide with n > 2. The polysulphide crosslinks are partially labile, which means that they can break and reform with other broken crosslinks when the applied stresses are high. This leads to permanent creep in compressed rubber blocks. To avoid such permanent set, efficient vulcanisation systems have been developed that produce only monosulphide crosslinks. 

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). 

Conventional Vulcanisation, Semi-Efficient Vulcanisation and Efficient Vulcanisation... 

It is evident that there are trade-offs in the use of efficient vulcanisation systems. Besides the technical trade-off of improved ageing but inferior fatigue resistance, there are cost considerations - a 10% increase might be expected. 

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... 

NR (ISNR 5 grade) was modified with 10 phr each of phosphorylated cashew nut shell liquid(PCNSL) prepolymer and an aromatic oil plasticiser (spindle oil) in a typical semi-efficient vulcanisation system. Despite the lower chemical crosslink density, the PCNSL modified NR vulcanisates showed higher TS, EB, thermal stability and resistances to fatigue failure and thermooxidative decomposition, as compared with the vulcanisate containing the same dosage of spindle. 12 refs. 

An examination is made of the characteristics obtained by vulcanisation with tetrabenzylthiuram disulphide (TBzTD)/sulphenamide accelerator systems, including reversion resistance, efficient vulcanisation with sufficient scorch delay, and increased nitrosamine safety. It is shown that the combination of properties imparted by these accelerator blends results from low sulphur rank crosslinks provided by the thiuram and the scorch safety provided by the sulphenamide. The advantages of these systems over sulphur donor/sulphenamide and lower molecular weight thiuram/sulphenamide systems are illustrated by the evaluation of blends of TBzTD andN-t-butyl-2-benzothiazole-2-sulphenamide in NR tyre compounds. 13 refs. 

---