ACCELERATORS

ACCELERATORS: DEFINITION & CLASSIFICATION

•An accelerator is defined as the chemical added into a rubber compound to increase the speed of vulcanization and to permit vulcanization to proceed at lower temperature and with greater efficiency.

•Accelerator also Decreases the Quantity of Sulphur necessary for vulcanization thus improving 'aged' properties of the rubber vulcanizates.

•Over 150 different chemicals belonging to different classes of composition are known to function as accelerators for rubber vulcanizates of which around 50 accelerators are most commonly used by the Rubber Industry.

•The accelerators are generally classified as follows :

Chemical Composition Accelerators Vulcanisation Speed
Aldehyde Amines BA, HMT Slow
Guanidines DPG, DOTG Slow
Thiazoles ZMBT, MBT, MBTS Moderate
Thiophosphates ZDBP Semi-Fast
Sulphenamides CBS, TBBS, MBS Fast (delayed action)
Thioureas ETU, DPTU, DBTU Fast
Thiurams TMTM, TMTD, TETD, DPTT Very Fast
Dithiocarbamates ZDEC, ZDMC, ZDBC Very Fast
Xanthates ZIX, SIX Extremely Fast

SELECTION OF ACCELERATOR SYSTEM

-Before selecting an Accelerator system for the manufacture of a particular rubber product, following points have to be taken into account.

- Expected shelf life of the compound,

- Accelerators solubility in rubber (high solubility to avoid bloom and improve dispersibility),

- Various processing stages the rubber compound be required to undergo,

- Adequate processing safety for 'scorch free' processing operations,

- Faster curing rate for economical production of rubber product,

- No reversion on over cure,

- Vulcanisation method to be used (mode of heat transfer),

- Maximum vulcanisation temperature available,

- Cure cycle desired at the available vulcanisation method and temperature and requirements of vulcanizates properties,

- Effectiveness of the accelerator system over a wide range of cure temperatures and suitability for use with different polymers,

- No adverse effects on other properties (e.g. bonding, ageing, adhesion, etc.) and no adverse effects on other non-rubber components in the rubber product,

- No known health hazards upon usage as chemical / its decomposition products on cure,

- No adverse effects during end-use of the rubber product (e.g. accelerators used in the manufacture of rubber articles intended for food contact / surgical use),

- Stability of the accelerator as a chemical (e.g. problems with the use of decomposed sulphenamide accelerators),

- Easy to handle and dust suppressed physical form.

THIAZOLE CLASS ACCELERATORS

• This class of accelerators include commercially available and widely used accelerators such as Pilcure MBT, Pilcure MBTS and Pilcure ZMBT (also sodium salt of MBT finds use in few rubber and non rubber applications).

• Thiazoles are medium-fast primary accelerators with only moderate processing safety. Thiazoles are most widely used accelerators in the rubber industry for the production of wide variety of goods such as cycle tyres and tubes, footwear, beltings, hoses and other moulded and extruded goods.

• Thiazoles are activated by zinc oxide/Stearic acid combination and produce flat cure with vulcanizates having very good reversion resistance.

• Activity of Thiazole accelerators with respect to cure characteristics can be summarised as follows:

1. Scorch Safety :---Longer…. (MBT <>

2. Cure Rate :---Faster….. (ZMBT <>

3. Crosslink Density:---Higher at equal dosage….. (ZMBT <>

• Thiazoles act as retarders of cure in rubber compounds accelerated using Thiurams (TMTD/TMTM)/Dithiocarbamates (ZDC, ZDBC) or ETU class accelerators as main accelerator and also reduce bloom of Thiurams and dithiocarbamates in EV cure systems.

• All thiazole accelerators can be further boosted to increase speed of vulcanisation by using small quantities of basic accelerators such as DPG, DOTG, TMTM, TMTD, ZDC, etc

• Thiazole based accelerators can be retarded using small proportion of Pilgard PVI or other retarders like NDPA, pthalic anhydride, salicylic acid, etc.

• Addition of extra Stearic acid or partial replacement by sulphenamide accelerator also control scorch to some extent.

• Thiazoles are particularly preferred for rubber to metal bonding applications.

SULPHENAMIDE CLASS ACCELERATORS

• The sulphenamide class accelerators are most popular in the rubber industry due to the delayed action as well as faster cure rate offered by them during vulcanisation of rubber specially containing furnace blacks.

• The sulphenamide accelerators are the reaction products of 2- mercaptobenzothiazole with basic amines such as cyclohexylamine / tert-butylamine / morpholine / dicyclohexyl amine.

• Sulphenamide accelerators are usually incorporated in the rubber compound at the end of mixing cycle when temperature is above melting point of the accelerator to ensure proper dispersion.

• Generation of excess heat is avoided to prevent decomposition of sulphenamide accelerator.

• In case sulphenamide accelerator is to be added at a later stage; addition in the form of sulphenamide rubber master batch is recommended.

• These accelerators provide wide range of crosslink densities depending on type and dosage of accelerator used and exhibit flat reversion resistant cure.

• Progressive increase in dosage of sulphenamide accelerator shows improvement in scorch delay, cure rate and state of cure.

• Sulphenamide accelerators can be boosted by using DPG, DOTG, TMTM, TMTD type basic accelerators for further increasing the cure rate (but at the expense of scorch safety)

• Vulcanizates of sulphenamide accelerators have a typical 'aminic' odour and exhibit higher stress-strain properties along with better resilience and flex-fatigue resistance as compared to Thiazoles.

• PVI (CTP) is a very effective prevulcanisation (scorch) inhibitor for Sulphenamide / activated Sulphenamide cure systems without compromise on the rate of cure . The improvement in scorch safety is directly proportional to the dosages of and minimum dosages are required for obtaining desired scorch safety. Excess dosage of PVI is avoided as same may cause undesired delay for onset of cure besides affecting the state of cure to some extent.

• Sulphenamide accelerators decompose rapidly in the presence of steam and hence are preferred for manufacture of open steam cured rubber products requiring faster onset of cure for better shape retention. However, this advantage is lost in the case of hot air cured products.

• Activity of Sulphenamide accelerators can be summarised as follows:

1. Scorch safety :---Longer (CBS <>

2. Cure Rate :---Faster (DCBS <>

3. Crosslink density :---Higher at equal dosage (DCBS <>

THIURAM CLASS ACCELERATORS

• Thiuram class includes accelerators such as TMTM, TMTD, TETD and DPTT.

• Thiurams are ultra-fast accelerators for NR, SBR, BR, NBR and other highly unsaturated rubbers and the most preferred primary accelerator for sulphur cured low-unsaturation content rubbers like butyl (IIR) and EPDM.

• Thiurams exhibit higher processing safety and late onset of cure compared to dithiocarbamate accelerators.

• Thiurams are widely used as secondary accelerator generally at 0.05 - 0.4 phr with thiazole / sulphenamide class accelerators to achieve faster curing rate, higher crosslink density with a compromise on scorch safety and Vulcanizate exhibits lower heat build up compared to DPG/DOTG activated sulphenamide cures.

• Thiurams, when used as vulcanising agents (2.5 - 3.0 phr) in the absence of sulphur, vulcanizates attain high degree of state of cure exhibiting high tensile strength, high modulus, lower elongation at break, higher rebound resilience at elevated temperatures and lower tear resistance.

• Thiurams are also used along with Guanidines in Polychloroprene compounds to achieve good processing safety.

• In combination with Dithiocarbamates and Xanthates, Thiurams have retarding effect without changing the rate of vulcanisation.

• Thiuram accelerators do not discolour the vulcanizates & the products are tasteless.

• Activity of various thiuram accelerators with respect to cure characteristics can be summarised as follows :

1. Scorch Safety :---Longer (TMTD <>:--- Faster (TMTM = TETD = TMTD) 3. Crosslink density at equal dosage :--- Higher (TMTM = TETD = TMTD)

DITHIOCARBAMATE CLASS ACCELERATORS

• Dithiocarbamate class includes accelerators such as ZDBC, ZDEC, ZDMC, etc.

• Dithiocarbamate accelerators are widely used as ultra fast accelerator for NR latex based compounds and also finds applications as primary or secondary accelerators in most dry rubber based sulphur cured compounds.

• The dithiocarbamate class accelerators require Zinc oxide and Stearic acid for activation and produce rapid vulcanisation.

• Dithiocarbamate s exhibit very low scorch safety, faster cure rate and higher crosslink density and rubber products can be vulcanised in a short time at low temperature (115 - 120°C).

• Compounds accelerated with dithiocarbamates have a very narrow plateau hence reversion due to over cure can take place very rapidly.

•

Low unsaturation content rubbers such as EPDM and IIR can be cured using dithiocarbamate as a secondary accelerator (at relatively higher dosage) along with Thiazoles and thiuram class accelerators.

• Compounds accelerated with dithiocarbamates have a very narrow plateau hence reversion due to over cure can take place very rapidly.

• Dithiocarbamate accelerators have limited solubility in rubber compounds and hence excess quantity tends to bloom on the surface of the vulcanizates.

• Dithiocarbamates are non staining and non discolouring even on exposure to light and are suitable for the manufacture of transparent goods.

• Activities of various dithiocarbamate accelerators with respect to cure characteristics in dry rubber compounds can be summarised as follows:

1. Scorch Safety :--- Longer (ZDMC <>Faster (ZDBC = ZDEC = ZMDC) 3. Crosslink density at equal dosage :---Higher ( ZDBC = ZDEC = ZDMC)