How to choose vulcanization accelerator?
1. Concept and function of accelerator
The vulcanization accelerator is called accelerator for short. Adding a small amount of accelerator into the rubber compound can greatly promote the reaction between the rubber and vulcanizing agent (cross-linking agent). Any compound that can accelerate the curing reaction speed, shorten the curing time, reduce the curing reaction temperature, reduce the amount of vulcanizing agent, improve the production capacity or improve the physical and mechanical properties of vulcanized rubber is called vulcanizing accelerator.
1. Shorten the curing time, reduce the amount of sulfur and reduce the curing temperature;
2. Improve the scorching performance of rubber compound;
3. Make the vulcanization of rubber compound smooth and improve the product performance;
3. Improve the appearance quality of rubber products.
2、 How to choose accelerator?
1. Find the purpose and effect of the product;
2. Select the accelerant suitable for the product based on various factors;
3. The selected accelerant is tested and its effect is analyzed;
4. Select stable and reliable accelerant supply.
3、 14 considerations when selecting accelerant
1. Rubber type
Different rubbers have different performance characteristics, and different vulcanization systems are used. When selecting different types of accelerators for different vulcanization systems, the matching between them should be considered.
2. Accelerator quality
The quality of accelerator is one of the problems considered by the formulation engineer. In the current market, there are still many phenomena of using fake and inferior products, so we should be careful when selecting them, otherwise it will directly affect the vulcanization or anti-aging properties of rubber.
3. Effect on the properties of compound and vulcanizate
Accelerators participate in the formation of rubber spatial network structure during vulcanization, which has a direct relationship with the structure of vulcanizates, that is, its physical and mechanical properties and aging properties. The accelerator has softening effect on the compound, which increases the plasticity of the compound and changes the processing performance of the compound.
4. Scorching performance
The accelerator has a decisive effect on the scorching time of the rubber compound (i.e., the duration before the start of thermal vulcanization of the rubber compound). Therefore, the accelerator selected should have a considerable scorching time of the rubber compound, which has a significant effect on the operational safety of the rubber compound in the process of mixing, rolling, extrusion or injection, the fluidity of the rubber compound at the initial stage of vulcanization and the adhesion to the fabric. Especially in recent years, in order to improve the production efficiency, the processing conditions are becoming more and more strict, and the requirements for the scorch resistance of the rubber compound are also becoming higher and higher. However, too long scorching time will lead to an increase in the total curing time, which is not only detrimental to the improvement of labor efficiency, but also has a negative impact on the deformation of the molded vulcanized products during the curing process.
The scorching resistance of accelerant is directly related to its curing critical temperature (i.e. the temperature at which the accelerant acts on the curing process), at which the activity of accelerant is not significant; The accelerator is activated above this temperature to give full play to its vulcanization effect. Unfortunately, most accelerants have not yet measured this temperature. When several accelerants are used together, the activity of some accelerants is inhibited, and some accelerants will have greater activity even below their critical temperature.
5. Vulcanization flatness
The accelerator should make the rubber compound have a broad vulcanization flat curve during the suitable vulcanization period. In other words, the properties of the vulcanizate should not show obvious changes during this period, which is particularly important for rubber thick products and solid products. Rubber is a poor conductor of heat. The surface and internal heating conditions of vulcanized rubber are not consistent. Wide vulcanization flatness is the guarantee to avoid excessive sulfur and make all parts of the product vulcanized evenly.
Accelerator with poor dispersion performance in rubber compound not only needs a long mixing time, increases equipment power consumption, but also tends to cause uneven vulcanization of products due to uneven mixing. Generally, the low melting point accelerator is easy to melt and diffuse in the rubber compound, so it has good dispersion; For high melting point accelerant, powder products with small particle size are used; For some promoters that are difficult to disperse, they can be used as master compound.
7. Frost spraying
The frost spraying of the mixed rubber will cause poor adhesion in the next process and reduce the appearance quality of the product, so it should be avoided. Accelerator frosting is related to the compatibility of accelerator with rubber. Generally, nonpolar EPDM rubber is easy to frost, while polar nitrile rubber and neoprene rubber are difficult to frost. In accelerants, especially accelerants TMTD and ZDMC have poor compatibility with rubber, which may cause frosting. The solubility of accelerants ZDEC and ZDBC with large alkyl group with rubber is better than that of TMTD and ZDMC, and the frosting phenomenon is reduced.
8. Contamination and coloration
Some accelerants stain rubber products due to their different colors, and some discolor products after chemical action or sunlight exposure. These accelerants cannot be used in white or bright color products. Some accelerants have obvious odor and bitter taste, and some accelerants can also pollute other substances through contact and migration.
9. Environmental protection
At present, most of the accelerants used are non-toxic or extremely toxic, but there are still some others with toxicity. These accelerants cannot be used in medical and health products and rubber products in contact with food. The use of toxic or highly toxic accelerants should be protected during operation.
The development trend of accelerants tends to be environmentally friendly and functional. In recent years, the international community has paid more attention to the harmful nitrosamine accelerants that are easy to produce in the rubber processing process. Most of the environmentally-friendly new accelerants have been developed from developed countries such as Europe and the United States. In recent years, China has gradually attached importance to the development and production of environmentally-friendly accelerants and pre-dispersions. The representatives of environmentally-friendly accelerants include ZBEC, TBzTD, TIBTD, ZBPD, ZDTP, NS, TBSI, MTT, etc.
10. Water solubility
For the production of latex products, it is convenient to select water-soluble accelerant, among which the representative products are ZDEC, ZDBC, ZMBT, TMTD, MBT, etc
11. Combination of accelerator
Each accelerator has its own characteristics. In order to meet the requirements that the rubber compound has good technological performance in the process of operation and that the vulcanizate has excellent physical and chemical properties, several accelerators can be used together to learn from each other.
The properties of rubber are different due to the characteristics of accelerant - accelerant is divided into main accelerant and auxiliary accelerant. The main accelerator, also known as the first accelerator, generally uses acidic accelerator (called type A) or neutral accelerator (called type N). Most of them are thiazole and thiuram, and the order of scorching time of thiazole is: M<dm<mz<sulphonamide. Thiuram type main accelerator is only used for film products or model products with very short curing time. Dithiocarbamate is the main accelerator, which is often used in latex products, ethylene-propylene rubber, butyl rubber and other low-unsaturated rubber.
The side accelerant, also known as the second or third accelerant, generally uses alkaline accelerant (called type B), mainly guanidine (such as DPG, DOTG, etc.). Accelerator H can also be used to prolong the scorching time, and the amount of auxiliary accelerator varies with the type of main accelerator. The hyposulphonamide accelerant itself is a compound accelerant. At the curing temperature, the accelerant M and amine compounds will be decomposed to form the AB compound system. Therefore, it is generally not necessary to choose a side accelerator. Of course, a small amount of accelerant DPG or thiuram type accelerant can also be used according to the process requirements.
Factors such as interaction with other rubber additives and raw materials for rubber should also be considered when selecting accelerator.
When selecting accelerant, the difficulty of purchasing the product in the market should be taken into account, and the impact on production should be paid attention to when using a large amount of accelerant that is not common or difficult to purchase.
The purchase price of accelerant often seen in the market should also be considered. The use cost of accelerant is one of the issues that should be considered by the formulation engineer.
It is easy to find a formula for common rubber products, but for special or technical rubber products, formula design consumes a lot of energy and financial resources. Designing a rubber formula and keeping it secret is crucial for the development of enterprises. The confidentiality of the formula must involve the confidentiality of the selected accelerator products, which is also one of the important issues considered by the formula engineer and even the company head.
Accelerator m can be used as lubricant and adsorbent. It also has special applications in electrochemistry. For example, adding a small amount of accelerator m to the electrode material can change the electrode performance, which is of great significance for the development of electrochemistry.
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