During the vulcanization process, organic vulcanization accelerators can greatly change the vulcanization reaction of rubber. In the presence of the vulcanization accelerator, the activation energy of the sulphur ring is reduced. Due to the cleavage of the accelerator itself, the concentration of active centers (free radicals or ions) in the system is increased, which accelerates the initiation of the vulcanization chain reaction and the chain growth reaction. , Improve the vulcanization reaction speed, at the same time, also improve the structure and performance of the vulcanizate.
The vulcanization reaction in the presence of accelerators, depending on the type of vulcanization accelerator, vulcanization conditions, and rubber types, the reaction may be of free radical type or ion type, or both. Next, we will discuss the mechanism of action of the most commonly used organic vulcanization accelerators
(1) The mechanism of action of thiazole accelerators
In the rubber industry, there are currently two types of accelerators containing benzothiazole groups, namely thiazole accelerators and delayed-acting sulfenamide accelerators. The vulcanization reaction that the thiazole accelerator participates in is considered to be a reaction process of free radicals. At the vulcanization temperature, the accelerator cleaves free radicals to cause and participate in a series of vulcanization reactions.
The mercaptan benzothiazole itself can undergo a reduction reaction during the vulcanization process. When peroxide is present in the system, it will consume the peroxide. So the response should be as follows:
1. The vulcanization promotion effect of mercaptan benzothiazole (accelerator M)
The above reaction shows that the accelerator M can decompose free radicals. When the vulcanization system contains elemental sulfur, the following reactions occur under vulcanization conditions:
In the above formula, MSH represents thiol benzothiazole, double MS· represents benzothiazole radical, MSxH represents polythiol benzothiazole, which is unstable. The HS· and S9-x· radicals generated in the above reaction process can cause chain initiation and chain growth during sulfur vulcanization. Can have the following reactions:
2. The vulcanization promotion effect of dibenzothiazole disulfide
Dibenzothiazole disulfide (DM) under the vulcanization conditions without the presence of ZnO can split the symmetric structure, splitting free radicals,
Splitting of asymmetric structures can also occur:
The split MS·, MSS· and M· can participate in the vulcanization reaction such as accelerator M. In addition, a large amount of accelerator M is also produced during the vulcanization induction period, which plays a role in accelerating vulcanization.
These polysulfides are very unstable and can be decomposed into double active sulfur·S-S· or polysulfide·Sx·. All of them can cross-link rubber molecules. The reaction mainly occurs at the а-methine position. In addition, the reaction with the double bond of the rubber molecule can also lead to the cross-linking of the rubber molecule.
(2) The mechanism of action of thiuram accelerators
Thiuram accelerators all contain vulcanizates, and these accelerators alone can directly vulcanize rubber. This vulcanization system is called "sulfur-free" vulcanization. In fact, it is also the vulcanization effect of sulfur in its essence, and it is not accurate to call it "sulfur-free" vulcanization. These sulfur-containing compounds used as vulcanizing agents are called sulfur carriers or sulfur donors.
The most commonly used sulfur compound vulcanizing agents in the rubber industry are thiuram polysulfide and morpholine polysulfide. Because of the different sulfide structure. The amount of sulfur contained is also different. The most commonly used in production is tetramethylthiuram disulfide (TMTD). At the vulcanization temperature, the sulfur-containing compound cracks into active sulfur, which participates in the cross-linking reaction of macromolecules.
1. The chemical structure of the sulfur carrier
During the vulcanization process, sulfur-containing compounds are first split into free radicals. Then it reacts with the а-methine group of the rubber macromolecule, and completes the vulcanization effect according to the free radical chain reaction. For example, the vulcanization reaction of tetramethylthiuram disulfide is as follows (X represents the free radical of rubber macromolecules):
2. The vulcanization mechanism of sulphur-containing compounds alone
In sulfur-containing systems, thiurams also have a strong vulcanization acceleration effect. In the vulcanization process, the accelerator first splits into free radicals, and reacts with elemental sulfur to generate sulfur free radicals. Then it reacts with the rubber macromolecule to complete the vulcanization. Such as the reaction of tetramethylthiuram disulfide (TMTD) in the presence of elemental sulfur
3. Reaction in sulfur-containing vulcanization system
It can be seen from the above vulcanization reaction process that the structure of the vulcanizate is completely low-sulfur cross-linking or carbon-carbon cross-linking with higher bond energy. The number of sulfur atoms in the cross-linking bond is only two at most, and there are very few Generate sulfur ring compounds within the macromolecule. Since the organic sulfur-containing compounds are easier to disperse in the rubber, the resulting cross-linked structure distribution is also more uniform. There is almost no free sulfur in the rubber. It is precisely because of this ideal vulcanized rubber structure that the heat resistance of the vulcanized rubber can be significantly improved.
The disadvantage of this kind of vulcanization system is that the vulcanization speed is slow, the degree of vulcanization of the vulcanized rubber is not deep, the tensile strength and hardness are low, the elongation is large, and the vulcanized rubber is prone to vulcanization.
4. The structure and properties of sulfur-containing compound (sulfur carrier) vulcanizates
(3) The mechanism of action of amine-containing accelerators
This type of accelerator has a moderate promoting effect. According to research, it has a different mechanism of action from sulfur-containing accelerators and is an ionic reaction. Because the N—H bond in amine compounds is prone to asymmetric splitting of electrons at the vulcanization temperature or under the induction of other substances, generating ionic groups, and generating ionic chain reactions with rubber macromolecules. The mechanism of action may have the following two aspects: One is that the amine-containing accelerator interacts with elemental sulfur during vulcanization, and an oxidation-reduction reaction occurs, resulting in an active center that can lead to cross-linking of macromolecules:
The other is that at the vulcanization temperature, the anion group of polythiamine can split into active sulfur, which participates in the cross-linking of rubber macromolecules. which is:
This situation shows that organic bases (amine accelerators) play a role in vulcanization acceleration.