Role of accelerator in rubber products
During the curing process, the organic curing accelerator can greatly change the curing reaction of rubber. In the presence of the curing accelerator, the fracture activation energy of the sulfur ring is reduced. Due to the cracking of the accelerator itself, the concentration of the active center (free radical or ion) in the system is increased, the initiation of the curing chain reaction and the chain growth reaction are accelerated, and the curing reaction speed is increased. At the same time, the structure and properties of the vulcanizate are also improved.
The vulcanization reaction in the presence of accelerator may be free radical type or ionic type, or both, depending on the type of vulcanization accelerator, vulcanization condition and rubber type. The action mechanism of the most commonly used organic vulcanization accelerators is discussed below
（1） Action mechanism of thiazole accelerants
In the rubber industry, there are two kinds of accelerators containing benzothiazole groups currently used, namely thiazole accelerators and delayed sulfosamide accelerators. The vulcanization reaction involving thiazole accelerants is considered as a reaction process of free radicals. At the vulcanization temperature, the accelerants split free radicals, causing and participating in a series of vulcanization reactions.
The thiol-based benzothiazole itself can undergo reduction reaction during the curing process. When there is peroxide in the system, the peroxide will be consumed. So there should be the following reactions:
1. The vulcanization promotion of mercaptan benzothiazole (accelerator M)
The above reactions show that accelerator M can decompose into free radicals. When elemental sulfur is contained in the curing system, the following reactions will occur under the curing conditions:
In the above formula, MSH represents thiol benzothiazole, double MS · represents benzothiazole radical, MSxH represents thiol benzothiazole, which is unstable. The HS · and S9-x · radicals produced in the above reaction process can cause chain initiation and chain growth during sulfur curing. The following reactions can occur:
2. The vulcanization promotion of diphenylthiazole disulfide
Dibenzothiazole disulfide (DM) can undergo symmetrical structure splitting and free radicals splitting in the absence of ZnO,
Dissociation of asymmetric structure can also occur:
The split MS ·, MSS · and M · can participate in the vulcanization reaction of accelerator M. In addition, a large amount of accelerator M was also generated during the curing induction period, which played a role in promoting curing.
These polysulfides are very unstable and can be decomposed into two groups of active sulfur · S-S · or polysulfide · Sx ·. Both of them can make rubber molecules crosslink. The reaction mainly occurs in а- Methylene position. In addition, double bond reaction with rubber molecules can also lead to crosslinking of rubber molecules.
（2） Action mechanism of thiuram accelerator
Thiuram accelerants all contain sulfides, and rubber can be directly vulcanized with these accelerants alone. This curing system is called "sulfur-free" curing. In fact, in essence, it is also the sulfuration of sulfur. It is not accurate to call it "sulfur-free" sulfuration. These sulfur-containing compounds used as vulcanizing agents are called sulfur carriers or sulfur donors.
The most commonly used sulfur-containing compounds in the rubber industry are thiuram polysulfide and morpholine polysulfide. Due to the different structure of sulfide, the sulfur content is also different. The most commonly used in production is tetramethylthiuram disulfide (TMTD). At the curing temperature, the sulfur-containing compounds split into active sulfur and participate in the crosslinking reaction of macromolecules.
1. Chemical structure of sulfur carrier
During the curing process, sulfur-containing compounds first split into free radicals. And rubber macromolecules а- Methylene reaction, complete vulcanization according to free radical chain reaction. For example, the vulcanization reaction of tetramethylthiuram disulfide is as follows (X represents rubber macromolecular free radical):
2. Sulfuration mechanism of sulfur compounds
In sulfur-containing systems, thiurams also have a strong sulfuration promoting effect. In the process of vulcanization, the accelerator first splits into free radicals, reacts with elemental sulfur to form sulfur free radicals, and then reacts with rubber macromolecules to complete the vulcanization. For example, the reaction of tetramethylthiuram disulfide (TMTD) in the presence of elemental sulfur
3. Reaction in sulfur-containing curing system
From the above vulcanization reaction process, it can be seen that the vulcanizate structure is completely low-sulfur cross-linking or carbon-carbon cross-linking with high bond energy. The number of sulfur atoms in the cross-linking bond is only two at most, and the sulfur ring compound in the macromolecule is rarely generated. Because organic sulfur compounds are easily dispersed in rubber, the resulting cross-linked structure is also evenly distributed. There is almost no free sulfur in the rubber. It is precisely because of this relatively ideal structure of vulcanizate that it can significantly improve the heat resistance of vulcanizate.
The disadvantages of this curing system are that the curing speed is slow, the curing degree of the vulcanizate is not deep, the constant elongation strength and hardness are low, the elongation is large, and the vulcanizate is prone to sulfur spray.
4. Structure and properties of vulcanizates containing sulfur compounds (sulfur carriers)
（3） Action mechanism of accelerant containing amine group
This kind of accelerant has medium promotion effect. It is considered that its action mechanism is different from that of sulfur-containing accelerant and belongs to ionic reaction. Because the N-H bond in amine compounds is easy to undergo asymmetric splitting of electrons at curing temperature or under the induction of other substances, producing ionic groups and producing ionic chain reactions with rubber macromolecules. The mechanism of action may include the following two aspects: first, the amine containing accelerator reacts with elemental sulfur during vulcanization to produce an oxidation-reduction reaction and produce an active center that can lead to macromolecular cross-linking:
On the other hand, under the curing temperature, the negative ion group of polythioamine can split out active sulfur and participate in the crosslinking of rubber macromolecules. Namely:
This situation shows that the organic base (amine accelerator) plays a role in promoting vulcanization.
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