Thiazole Class

This category of accelerators includes widely used commercial accelerators such as MBT, MBTS, and ZMBT (NaMBT, the sodium salt of MBT, is also used in some latex goods manufacturing). Thiazoles are primary accelerators with a medium-fast curing rate and moderate processing safety. They are extensively employed in the rubber industry for the production of various goods like cycle tires, footwear, beltings, hoses, and molded/extruded products.

Thiazole accelerators are activated by a combination of Zinc oxide and Stearic acid, resulting in a flat cure and vulcanizates with excellent resistance to reversion. The activity of thiazole accelerators can be summarized as follows:

Scorch Safety: Thiazoles offer longer scorch safety compared to (MBT < MBTS < ZMBT).
Cure Rate: Thiazoles provide faster cure rates, with (ZMBT < MBTS < MBT).
Crosslink Density: Thiazoles exhibit higher crosslink density at equal dosage, with (ZMBT < MBT < MBTS).

Thiazoles act as cure retarders when used in rubber compounds accelerated by Thiurams (TMTD/TMTM)/Dithiocarbamates (ZDC, ZDBC), or ETU class accelerators. They also help reduce bloom of Thiurams and dithiocarbamates in EV cure systems.

To further increase the speed of vulcanization, small amounts of basic accelerators like DPG, DOTG, TMTM, TMTD, ZDC, etc., can be added to thiazole accelerators. Conversely, thiazole accelerators can be retarded by incorporating small proportions of retarders such as Pilgard PVI, NDPA, phthalic anhydride, salicylic acid, etc. Additionally, adding extra Stearic acid or partially replacing it with sulphenamide accelerators can help control scorching to some extent.

Thiazole accelerators are particularly preferred for their versatility, wide application range, and effectiveness in rubber compounding processes.

Sulfenamide Class

The sulfenamide class of accelerators, including CBS, TBBS, MBS, DCBS, and others, is widely utilized in the tire industry due to their delayed action and accelerated curing rate when vulcanizing rubber compounds containing furnace blacks.

Sulfenamide accelerators are produced through the reaction of 2-Mercaptobenzothiazole with basic amines like Cyclohexylamine, Tert-Butylamine, Morpholine, Dicyclohexylamine, and more. These accelerators offer a broad range of crosslink densities, depending on the type and dosage used, resulting in a flat and reversion-resistant curing process. Increasing the amount of sulfenamide accelerator improves scorch delay, cure rate, and overall state of cure.

To further boost the cure rate (at the expense of scorch safety), basic accelerators such as DPG, DOTG, TMTM, TMTD, etc., can be added alongside sulfenamide accelerators. Retarding the sulfenamide accelerators can be effectively achieved by incorporating a small proportion of Pilgard PVI (CTP) without affecting their cure rate.

Vulcanizates obtained with sulfenamide accelerators possess a distinctive “aminic” odor and exhibit higher stress-strain properties, as well as enhanced resilience and flex-fatigue resistance compared to Thiazoles.

Sulfenamide accelerators exhibit rapid decomposition in the presence of steam, making them advantageous for the production of open steam-cured rubber products that require a faster onset of cure for better shape retention. However, this advantage is lost in the case of hot air-cured products.

When incorporating sulfenamide accelerators into rubber compounds, it is recommended to add them at the end of the mixing cycle when the temperature is above the melting point of the accelerator to ensure proper dispersion. Excessive heat generation should be avoided to prevent the decomposition of sulfenamide accelerators. If the addition of sulfenamide accelerator is delayed, it is advisable to utilize a sulfenamide rubber master batch.

The activity of sulfenamide accelerators can be summarized as follows:

• Scorch safety: Longer duration (CBS < TBBS < MOR < DCBS)
• Cure rate: Faster rate of curing (DCBS < MOR < CBS < TBBS)
• Crosslink density: Higher at equal dosage (DCBS < MOR < CBS < TBBS)

Sulfenamide accelerators have limited storage stability, and their degradation rate is greatly influenced by storage conditions such as humidity and heat. Hence, it is crucial to store them in a cool and dry environment (below 30°C & 60% RH), away from acids, acidic substances, and fumes to prevent rapid degradation. It is advisable to strictly adhere to the first-in, first-out principle when using sulfenamide accelerators.

Thiurams Class

The thiuram class comprises accelerators like TMTM, TMTD, TETD, TBzTD, and DPTT. Thiurams are highly efficient accelerators used in the vulcanization of NR, SBR, BR, NBR, and other highly unsaturated rubbers. They are particularly favored as primary accelerators for low-unsaturation rubbers like butyl (IIR) and EPDM, which undergo sulfur curing. In combination with thiazole/sulfenamide accelerators, thiurams are commonly used as secondary accelerators at levels of 0.05 – 0.4 phr. This combination enables faster curing rates and higher crosslink density, albeit with a trade-off in scorch safety and reduced heat buildup compared to DPG/DOTG activated sulphenamide cures.

Thiurams demonstrate superior processing safety compared to dithiocarbamate accelerators. When used as vulcanizing agents (2.5 – 3.0 phr) in the absence of sulfur, thiurams contribute to vulcanizates with a high degree of cure, resulting in enhanced tensile strength, modulus, reduced elongation at break, improved rebound resilience at elevated temperatures, and lower tear resistance.

In Polychloroprene compounds, thiurams are employed alongside guanidine to ensure optimal processing safety. When combined with dithiocarbamates and xanthates, thiurams exhibit a retarding effect on vulcanization without altering the overall curing rate.

One notable advantage of thiuram accelerators is that they do not cause discoloration in the vulcanizates, and the resulting products are tasteless.

The activity of various thiuram accelerators can be summarized as follows in terms of cure characteristics:

1. Scorch Safety: TMTD exhibits the longest scorch safety, followed by TETD, and then TMTM.
2. Cure Rate: TMTM, TETD, and TMTD all possess similar fast curing rates.
3. Crosslink Density at Equal Dosage: TMTM, TETD, and TMTD provide comparable higher crosslink densities.

Dithiocarbamate Class

The dithiocarbamate class comprises accelerators such as ZDMC, ZDEC, ZDBC, ZBzDC, and others. Dithiocarbamate accelerators are extensively used as ultra-fast accelerators in NR latex-based compounds. They also find applications as primary or secondary accelerators in most dry rubber-based sulfur-cured compounds. Activation of dithiocarbamate accelerators requires Zinc oxide and Stearic acid, resulting in rapid vulcanization.

Dithiocarbamates exhibit very low scorch safety, faster cure rates, and higher crosslink density. This enables rubber products to be vulcanized quickly at relatively low temperatures (115 – 120°C). However, compounds accelerated with dithiocarbamates have a narrow plateau, which means over-cure and reversion can occur rapidly. In the case of low-unsaturation rubbers like EPDM and IIR, dithiocarbamates can be used as secondary accelerators (at higher dosages) in conjunction with thiuram class accelerators as primary accelerators.

Dithiocarbamate accelerators have limited solubility in rubber compounds, leading to excess quantities blooming on the surface of the vulcanizates. Despite this, dithiocarbamates are non-staining and non-discoloring, even when exposed to light, making them suitable for manufacturing transparent goods.

The activities of various dithiocarbamate accelerators in dry rubber compounds, regarding cure characteristics, can be summarized as follows:

1. Scorch Safety: ZDMC exhibits the longest scorch safety, followed by ZDEC, and then ZDBC.
2. Cure Rate: ZDBC, ZDEC, and ZMDC all have similar fast cure rates.
3. Crosslink Density at Equal Dosage: ZDBC, ZDEC, and ZDMC provide comparable higher crosslink densities.

Guanidines

The primary guanidine accelerators employed are DPG and DOTG. The chemical structures of these accelerators are shown in figure:                                                                                                                     

Guanidines exhibit a slow vulcanization rate but are considered relatively safe accelerators for processing. They are not commonly used as primary accelerators due to their slow curing properties, although this characteristic can be advantageous when curing large-sectioned articles. Their main application is as secondary accelerators in NR or SBR compounds, accelerated with thiazole or sulphenamide. DPG is also utilized in specialty rubbers such as CR and polysulphide rubber, where it acts as a peptizer. In silica-containing compounds, DPG is employed as a cure activator, effectively adsorbing onto the acidic silica surface and preventing cure system deactivation. Additionally, DPG aids in the compatibility of the silica/elastomer mix, thereby enhancing dispersion and processability.