Accelerators are necessary ingredients of all sulphur curing systems. They increase curing ratio and efficiency of the sulphur bonding to rubber macromolecules in form of cross-links. From technological point of view their presence in rubber compounds is very important. They significantly reduce time needed for vulcanization; they allow reducing the vulcanization temperature and sulphur content in rubber compounds. They favourably affect also properties of vulcanizates, mainly their ageing resistance. Simultaneously they reduce possibility of pre-vulcanization and reversion and also probability of sulphur blooming on surface of rubber products.
Presently exclusively organic compounds are used in function of accelerators. Most of them contain sulphur and nitrogen atoms in their molecules. According to their chemical composition they are divided into:
In technical practice the accelerators are often evaluated according to their activity in vulcanization process. According to this criterion they can be divided to:
Selecting an accelerator system is one of the most difficult problems in compounding. A number of factors must be considered while selecting an accelerator. Selection of the proper accelerator and the amount to be used for any application depends on:
Guanidine Class Accelerators
Guanidines are condensation products of aromatic amines (aniline) and carbon-disulfide with subsequent substitution of the thione functionality (>C=S) for a primary ketimine group (>C=NH). In practice the most often used is diphenyl-guanidine (DPG) and N, N’-diorthotolyl guanidine (DOTG) eventually o-tolyl-biguanidine. Guanidines are slow accelerators with short induction period and broad vulcanization plateau and require the use of zinc oxide for activation. They have alkaline character. They are suitable also into compounds containing additives of strongly acidic character and into compounds filled with SiO2. Guanidines are good choice for thick walled rubber products. The vulcanization in the presence of guanindes has relatively high crosslink density and good physical-mechanical properties; they are less resistant to thermo-oxidative ageing, because polysulfide cross-links dominate in their network. However, guanidines are not recommended for light colored goods because they cause a brown discoloration.
Thiazoles are some of the most widely used primary vulcanization accelerators. They are suitable for vulcanization of high (NR, BR, SBR) and low (IIR, EPDM) unsaturated rubbers. The most common commercial thiazoles are 2-mercaptobenzothiazole (MBT) and bis-(2-benzothiazol) disulphide (MBTS) and eventually zinc salt of 2-mercaptobenzothiazole (ZMBT). . They have improved scorch safety and allow for cure at quite high temperatures with short curing time and broad vulcanization plateau but relatively low crosslink density. From this reason the vulcanizates that are vulcanized under presence of solitary thiazoles does not have optimal strength and elastic properties. Also from this reason they are very often combined with other accelerators. All thiazole accelerators can be further boosted to increase their cure speed, they are often combined with small amounts of basic accelerators such as diphenyl guanidine (DPG), diorthotolyl guanidine (DOTG), Tetramethylthiuram monosulfide (TMTM), Tetramethylthiuram disulfide (TMTD), ZDC, etc.
Activity of Thiazole accelerators with respect to cure characteristics can be summarised as follows:
Sulfenamides are made in two ways: by the reaction of 2-mercaptobenzothiazole (MBT) with an N-chloramine and by oxidation of the appropriate amine salt of 2-mercaptobenzothiazole. Their activity in vulcanization process depends mainly on type of amine used during production of appropriate sulfenamide. The various amine functional groups of sulfenamides reflect their scorch time and cure rate differences. The scorch time is lower and increase curing rate if the used amine salt during sulfenamide preparation is highly basic in character. Recently the most frequently used sulfenamides are N-cyclohexyl-2-benzthiazol sulfenamide (CBS), N-tert-butyl-2-benzothiazol sulfenamide (TBBS) and dicyclohexyl-2-benzothiazol sulfenamide (DCBS).
Activity of sulphonamide type’s accelerator with respect to cure characteristics can be summarised as follows:
Sulfenamide accelerators are suitable also for EV and semi-EV systems. Sulfenamide accelerators are created very effective accelerating systems giving good scorch time, fast curing and reduced reversion with secondary accelerators like TMTD.
Thiurams are very speedy and also fast i.e ultra-accelerators. They are made from secondary amines and carbon disulfide. They are suitable for vulcanization of low unsaturated and into semi EV and EV vulcanization systems. Mutually they differ by character of the number of sulfur atoms in sulfur bridges (1, 2, 4, 6) and the substituents in amine groups (e.g. methyl-, ethyl-, pentamethylene-). Their stability decreases with increasing number of sulfur atoms and they are easily decomposed with sulfur creation. In practice the most commonly used of this class are tetramethyltiuram disulfide (TMTD), tetraethylthiuram disulfide (TETD) and tetramethylthiuram monosulfide (TMTM). According to their activity the above three thiuram class accelerator can be arranged as TMTD > TETD > TMTM. If TETD is substituted for TMTD, about 10% more TETD should be used. They are used in combination with other accelerators, mostly with sulfenamides and thiazoles in various times. It is also used as sulfur donors. However, the monosulfide is not strong enough to cure satisfactorily without added sulfur. Their disadvantage is generation of secondary amines in the process vulcanization. These unwanted effects are not obtained by thiurams with hindered substituents in amine groups (e.g. benzyl-, piperidyl-).
Dithiocarbamates are metallic (the most often salts of Zn and Na, but also Se, Te, Pb, Cd, Bi) or ammonium salts of dithiocarbamoic acids. They can be differentiating with respect to the cation as well as alkyl substituents in the dithiocarbamate group. They are low soluble in non-polar rubber due to their polar character, so they bloom at higher concentration. With the increase of carbon number in alkyl substituents, the solubility of dithiocarbamates in rubber molecules is also increases. They belong to ultra fast accelerators. Usually they are paired with thiazole or sulfenamide accelerators to adjust the cure rate of a stock. Some of them are soluble in water. They are used not only during vulcanization of rubber and latex compounds, but also rubber solutions. The most popular and common members of this class are the zinc dimethyl dithiocarbamates (ZDMC), zinc diethyl dithiocarbamates (ZDEC) and zinc dibutyl dithiocarbamates (ZDBC). Dithiocarbamates are infrequently used alone except in such specialty applications as spread goods (fabric covered with a rubber coating: e.g. hospital sheeting) due its powerful activities.
Xanthates are metallic predominantly zincous salts of the xanthtic acids. They belong to ultra fast accelerators. They are used mainly during vulcanization of latex systems (e.g. during foam-rubber production) and rubber solutions. Some of them are soluble in water. As they cause very fast vulcanization they are suggested for lower vulcanization temperatures application, the most often from 80 to 110 °C. Other accelerators include mainly such new types of accelerators as different triazines and sulfides or metallic and amine salts of dialkyldithiophosphoric acids. The 3-mercapto-triazine belongs to the simplest triazines, but also their disulfide and sulfenamide derivates are known. Dithiophosphates are fast accelerators. The best known from them are zinc salts of dialkyltiophosphate acids and bis-(diisopropyl) tiophosphoryl disulfide (DIPDIS). They are suggested mainly for EPDM vulcanization, but they accelerate also sulfur vulcanization of diene rubbers. Sometimes also thioureas are placed among the other accelerators; they are used during vulcanization of chloroprene rubbers.
The condensation products of aldehydes and amines are aldehydeamines. The main aldehydeamine accelerators are triotonylide tetramine (TCT), butylaldehyde aniline (BBA) and hexamethyllene tetramine (HEXA). Among this three the butylaldehyde aniline is the most common commercial vulcanization accelerator. Their accelerating effect is mainly determined by:
