MBTS

Mercaptobenzothiazoles. These compounds form the basis for the largest-volume organic accelerators. 2-Mercaptobenzothiazole (MBT)... 

Sulfenamides. Sulfenamides (4) are often produced by oxidising an equimolar mixture of MBT and an aliphatic amine. Alternatively, the /V-ch1oroamine can react with the sodium salt of MBT. One sulfenamide, OTOS (S), uses a thiocarbamyl functionaUty in place of the ben2othia2ole group. 

The decomposition generates amine and MBTS. SmaH quantities of MBT and benzothiazyl (Bt) polysulftdes form just before the onset of cross-linking. Sulfur disappears and large quantities of MBT are formed during the main cross-linking reaction. The development of cross-links is shown as the rheometer line (curve C) in Figure 3. 

Fig. 3. Fate of accelerator components during cure, where A is TBSI B, MBTS C, development of cross-links (rheometer) D, MBT and E, BtS Bt (13).

The inhibition chemistry has been extensively studied. As shown in equation 1, the prevulcanization inhibitor (retarder) reacts with MBT (1) before it can form polysulfides. 

As a general rule the sulfenamides exhibit faster cure rate than the thiazoles. If secondary accelerators are used, dithiocarbamates are scorchiest and give the fastest cure followed by the thiurams, then the guanidines. Figure 6 summarizes these comparisons to show a series of natural mbber (NR) recipes using either a thiazole (MBTS) or sulfenamide (TBBS) primary accelerator in combination with the various secondary accelerators (21). In this study, the initial primary accelerator levels were selected to produce nearly equivalent modulus or state of cure in the NR. 

Fig. 6. Comparison of secondary accelerators where represents Mooney scorch at 121°C and U, optimum (t90) at 153°C A, the primary accelerator is 0.75 phr MBTS and the secondary accelerator is at 0.25 phr B, 0.45 phr TBBS is the primary accelerator and there is 0.2 phr secondary accelerator.

The thiophthalimide (CTP) and sulfenamide classes of retarders differ from the organic acid types by thek abiUty to retard scorch (onset of vulcanization) without significantly affecting cure rate or performance properties. Much has been pubUshed on the mechanism of CTP retardation. It functions particularly well with sulfenamide-accelerated diene polymers, typically those used in the the industry. During the initial stages of vulcanization, sulfenamides decompose to form mercaptobenzothiazole (MBT) and an amine. The MBT formed reacts with additional sulfenamide to complete the vulcanization process. If the MBT initially formed is removed as soon as it forms, vulcanization does not occur. It is the role of CTP to remove MBT as it forms. The retardation effect is linear with CTP concentration and allows for excellent control of scorch behavior. 

Fig. 13. Relationship of nittile rubber cure systems where DCP is dicumyl peroxide MBTS, benzothiazyl disulfide ZrJDMD, zinc dimethyldithiocarbamate ...

Recipe, in parts by wt smoked sheets, 100.00 zinc oxide, 5.00 filler, as indicated nondiscoloring antioxidant, 1.00 MBTS, 1.00 TMTD, 0.10 sulfur, 2.75 stearic acid, 3.00. 

In addition to the 1 part of MBTS shown in recipe, the 20 vol HiSil 233 stock contains 0.25 parts TMTD and 0.5 parts triethan o1 amine the 30 vol stock contains 0.5 parts TMTD and 2 parts triethano1 amine and the 40 vol stock contains 0.5 parts TMTD and 3 parts triethano1 amine. 

The HiSd 233 stocks contain 2.3, 3.5, and 4.8 parts of diethylene glycol, respectively. The 30 vol stock contains 1.2 parts MBTS and 0.15 parts TMTD instead of the combination in the recipe the 20 and 40 vol stocks contain 1.5 parts MBTS and 0.1 parts TMTD. 

Cure Characteristics. Methods of natural rubber production and raw material properties vary from factory to factory and area to area. Consequentiy, the cure characteristics of natural mbber can vary, even within a particular grade. Factors such as maturation, method and pH of coagulation, preservatives, dry mbber content and viscosity-stabilizing agents, eg, hydroxylamine-neutral sulfate, influence the cure characteristics of natural mbber. Therefore the consistency of cure for different grades of mbber is determined from compounds mixed to the ACSl formulation (27). The ACSl formulation is as follows natural mbber, 100 stearic acid, 0.5 zinc oxide, 6.0 sulfur, 3.5 and 2-mercaptobenzothiazole (MBT), 0.5. 

Accelerators are chemical compounds that iacrease the rate of cure and improve the physical properties of the compound. As a class, they are as important as the vulcanising agent itself. Without the accelerator, curing requires hours or even days to achieve acceptable levels. Aldehyde amines, thiocarbamates, thiuram sulfides, guanidines, and thiasoles are aU. classified as accelerators. By far, the most widely used are the thiasoles, represented by mercaptobensothiasole (MBT) and bensothiasyl disulfide (MBTS). 

There are no known practical peroxide cure systems for the PO—AGE polymers. Apparentiy the peroxide attacks the polymer backbone at a rate that is unfavorably competitive with the cross-linking rate. A typical sulfur cure system consists of zinc oxide [1314-13-2] tetramethylthiuram mono sulfide (TMTM), 2-2-mercaptobenzothiazole [149-30-4] (MBT), and sulfur. A sulfur donor cure system is zinc oxide, di-o-tolylguanidine [97-39-2] (DOTG) and tetramethylthiuram hexasulftde. 

Fig. 2. Effect of mixture strength on exhaust gas composition (dry basis) and brake specific fuel consumption (BSFC) for an unsupercharged automotive-type engine usiag iadolene fuel, H/C = 1.86, where the ignition is tuned to achieve maximum best torque (MBT), the brake mean effective...

TBBS A/-t-ButyIbenzothiazole-2-su(phenamide MBTS Dibenzothiazole di.sulphide MBT 2-Mercaptobenzothiazole TMTD Telrainethythiutam disulphide... 

Figure 12.12 Coupled SEC-RPLC separation of Plioflex rubber stock (a) SEC (b) RPLC ti ace of fraction 1, Wingstay 100 (Eive-peak pattern is representative of diarylphenylenedi-amine isomers) (c) RPLC ti ace of fraction 2, mixed disulfide and MBTS (2,2 -thiobis (ben-zothiazole)). Obtained under the same conditions as given for Eigure 12.11. Reprinted from Journal of Chromatography, 149, E. L. Johnson et al, Coupled column chromatography employing exclusion and a reversed phase. A potential general approach to sequential analysis , pp. 571-585, copyright 1978, with permission from Elsevier Science.

Curing System S/MBTS/ZnO/ Stearic Acid BMI/MBTS DCP (40%) DCP/EGDM DCP/Sulfur DCP/HQ... 

Curing system Sulfur Sulfur/PNDA BMl/MBTS PETA/DHBP... 

Flgure 4 tanS versus temperature for NBR-PVC blends curve 1-BMI/MBTS curve 2-S/PNDA curve 3 = DHBP/ PETA. Source Ref. 21. 

Some examples of organic anionic inhibitors are sodium phosphates, thioureas and sodium MBT phosphionates (mercaptobenzothiazole). Some examples of organic cationic inhibitors are amines, amides, quaternary ammonium salts, and imidazoline. 

Azoles include the sodium and potassium salts of mercaptoben-zothiazole (MBT), benzotriazole (BZT, BTZ, BTA), and tolyltriazole (TTA). The latter two azoles are most commonly found in formulations today and are shown in Figure 10.2. 

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