II. Xauthate accelerators:

The most important one's are

(1) Sodium iso prophy 1 Xauthate
(2) Zinc isopropyl Xauthate and
(3) Zinc buty 1 Xauthate.

These are used in room temperature vulcanisation.

III Thiuram accelerators:

These form a group of ultra accelerators with somewhat higher processing safety than dithio Carbamates. Compounds containing thiruam accelerators are suitable for Vulcanisation accelerators of this group are:-
(1) Tetra methyl thiurum disulphids (TMTD)
(2) Tetra methy1 thiurum mono sulphide (TMTM)
When used as primary accelerator, in normal sulphur Vulcanisation, they
Impart the vulcanisates relatively high modulus, good mechanical and ageing properties.

Dosage: TMTD - 0.5 phr
TMTM - 0.5 phr.
Thiuram accelerators are used in low sulphur curing to impart good ageing properties to vulcanisates.
Dosage: TMTD - 2.5 - 3.5 phr.
TMTM - 2.5 - 3.5 phr.


Generally thiuram are used as secondary accelerators to raise rate of Vulcanisation of thiazole accelerators. It increases the scorch time and decreases time of Vulcanisation. Vulcanisate properties are also improved.

Thiuram accelerators can be used alone or in combination with other accelerators in the manufacture of transparent, white coloured, tasteless and odourless products, foot wears fabric proofing, mechanical goods. In combination with guanidines they are useful for neoprene curing.

IV. Thiazole accelarators:

Accelerators of this class are the most popular among accelerators.
a). Mercapto accelerator

Compared to dithic-Carbamates and thiuram, the mer capto accelerators have to be used in some what larger proportion and higher amount of sulphur. They also need higher temperature for Vulcanisation. Important among these are: -

1). Mercapto benz thiazole (MBT)
2). Zinc salt of mercapto benz thiazole (Zn MBT)
3). Mercapto benzo dithiazole (MBTS)

MBT and ZMBT have relatively fast on set of Vulcanisation and hence lower processing safety. When mercapto accelerators are combined with basic accelerators, dithio Carbamates or thiuram, the onset of Vulcanisation takes place faster than when they are used alone, and reduce processing safety. Secondary accelerators used with these are guanidines and HMT.

Vulcanisates with mer capto accelerators and basic secondary accelerators have a tendency to yellow. Brightening effect is obtained when HMT (hexa methylene Tetrammine) is used as secondary accelerator.

MBT or MBTS reduces blooming of TMT in sulphurless Cure with TMT and increases blooming of sulphur in normal sulphur Vulcanisation.

Mercapto accelerators are used in NE, SBR and NBR. They are inert in IR and BR unless activated with basic accelerators like HMT. In IIE compounds cured with benzo quinone diozime MBTS has a great effect as on oxidising agent.

b). Sulfonamide accelerators:

The most important are:
(1) Cyclo hexy1 benzo thiazy1 sulphenamide CBS
(2) Merpholino benzothiuazy1 sulphenamide MOR
(3) Tert-buty1 benzo thiazy1 sulphenamide TBBS
(4) Dicyclo hexy1 benzothiazy1 sulphenamide DCBS

These are delay action accelerators and scroch time increases from.
CBS-------MOR-------TBBS-------DCBS

Mechanical properties of products produced with them are superior to these from mercapto accelerators.

With sulphenamide accelerators, if the sulphur content is raised, the scorch tandency and the degree of Vulcanisation are raised and total curing time reduced. In low sulphur systems if dithio Carbamates or thiuram are added along with sulphenamides, the total quantity of accelerator can be reduced. These are use in NR, SBR, BR, IR and NBR.

Basic accelerators:

a) Butral dehyde aniline: 

This is a powerful accelerator with high tendency for scorch. So this is added last or during warming up stage of premixed compounds. This is a liquid and disperses easily in rubber. Ulcanisate produced with it have outstanding mechanical properties, especially dynamic properties. This is not used in coloured goods or products in contact with food materials. This is a good secondary accelerator, useful in NR, SBR, NBR, CR, IRand BR.

b) Hexa methylene Tetramine:

This is a slow accelerator with early on set of cure and low degree of Vulcanisation. So it is not used as the only accelerator in any system. As a secondary accelerator it is good, but less active than guanidine and butraldehyde aniline. It does not discolour and has a brightening effect on articles. For thick walled articles it gives even cure when used along with guanidines.

Synerqistic accelerators:

Synergism of accelerators in rubber compounding is the boosting affect, much greater than the additive affect of accelerators used in a system.
The advantages are:-
(a) The rate of vulcanisation can be increased resulting economic saving.
(b) The risk of blooming of accelerators can be checked.

Eg:- 3 phr TMTD blooms, but 1.5 phr TMTD and 1.5 phr TETD will not, but give the same cure effect.
Some example of synergistic accelerators:

MBTS/DPG      -   1.2: 0.4
CBS/DPG        -   1: 0.3
CBS/TMTD      -   1: 1

POPULAR ACCELERATORS

 

	Chemical Name	Short form	TEL	TradeBayer	Name	PIL
1	Zinc Diethy1 dithio carbamate	ZDC	Vulcafor ZDC	Vulkacit	LDA	Ethasan
2	Tetra methy1 thiuram disulphide	TMTD	Vulcafor TMT	Vulkacit	Thiuram	Thiurad
3	Tetra ethy1 thiuram disulphide	TET	Vulcafor TMT
4	Tetra methy1 thiuram mono sulphide	TMTM	Vulcafor TMTM			Mono thiured
5	Dipheny1 Guanidine	DPG	Vulcafor DPG
6	Dipheny1 orthotoly1guanidine	DOTG	Vulcafor DOTG	Vulkacit
7	2 Mer capto benzo thiazole	MBT	Vulcafor MBT	Vulkacit		Thiotax
8	Di benzo thiazy1 disulphide	MBTS	Vulcafor MBTS	Vulkacit	DM/C	Thifide
9	Zn Salt of mercapto benz thiazole	ZMBT	Vulcafor ZMBT	Vulkacit	ZM	Bentox
10	Cyclo hexy1 benz thiazy1 sulphenamide	CBS	Vulcafor HBS	Vulkacit	CZ	Santocure MOR
11	Benzothiazy1 2 Sulphenanorpholide	MBS	Vulcafor BSM	Vulkacit	MOZ	Santocur NES
12	Tert benty1 benzthiazy1 sulphenamide	TBBS	Vulcafor BSB	Vulkacit	NZ	Santocur
13	Dicyclo hexy1 benz thiazy1 sulphenamide	DCBS	Vulcafor DCS	Vulkacit	DZ