
In the rubber industry, the vulcanization process is key to transforming rubber from a plastic material into a solid with specific elasticity and strength. To improve the processing performance and vulcanization efficiency of rubber, selecting the appropriate vulcanization accelerator is crucial. The choice of vulcanization accelerator not only affects the speed of rubber vulcanization, but also directly links to the “scorch safety” of the rubber during processing (i.e., premature cross-linking caused by ozone aging, heat aging, etc.) and the final mechanical properties. This article will explore the effects of different types of vulcanization accelerators on the rubber vulcanization process, particularly their impact on “processing safety” (also known as “scorch safety”) and vulcanization rate.
1. Basic Role of Rubber Vulcanization Accelerators
Vulcanization accelerators are essential components in the rubber vulcanization system. They enhance the physical properties of rubber by catalyzing the cross-linking reaction between sulfur and rubber molecular chains. The primary function of vulcanization accelerators is to speed up the vulcanization process by facilitating the sulfur cross-linking reaction, thus achieving the desired vulcanization effect at a lower temperature.
The choice of vulcanization accelerators depends on the type of vulcanization system being used, as different accelerators can have varying effects on the vulcanization process. Based on their chemical structure and reaction mechanisms, vulcanization accelerators can be categorized into several groups, such as: mercaptan vulcanization accelerators (e.g., TMTD), disulfide vulcanization accelerators (e.g., TBBS, MBS, CBS), amine vulcanization accelerators, thiazole vulcanization accelerators (e.g., MBT, MBTS), and other specialized types of vulcanization accelerators.
2. Classification of Vulcanization Accelerators and Their Impact on Scorch Safety
2.1 Sulfenamide Accelerators
Sulfenamide vulcanization accelerators are most popular in the rubber industry. They offer good processing safety and moderate vulcanization rates, making them ideal for rubber products with various applications. A key feature of sulfenamide accelerators is their ability to effectively prevent the “scorch” phenomenon (i.e., premature cross-linking) during the vulcanization process. This type of accelerator delays the onset of the vulcanization reaction and enhances processing safety by forming an adjustable reaction intermediate.
Sulfenamide vulcanization accelerators can be categorized into several types based on their structure, including:
CBS (N-cyclohexyl-2-benzothiazole sulfenamide): This accelerator provides a shorter scorch safety during the vulcanization process but results in a faster vulcanization rate.
TBBS (N-tert-butyl-2-benzothiazole sulfenamide): TBBS has a slightly slower vulcanization rate than CBS and offers relatively longer scorch safety.
MBS (N-morpholinyl-2-benzothiazole sulfenamide): MBS has a longer scorch safety and a slower vulcanization rate, making it suitable for rubber products that require a longer processing time.
DCBS (N,N-dicyclohexyl-2-benzothiazole sulfenamide): DCBS offers the longest scorch safety and the slowest vulcanization rate, making it ideal for high-performance rubber products that require slower vulcanization.
2.2 Thiazole Accelerators
Thiazole accelerators, such as MBT (2-mercaptobenzothiazole) and MBTS (dibenzothiazyl disulfide), are also commonly used vulcanization accelerators. These accelerators typically offer faster vulcanization rates but have relatively low scorch safety. Although MBT and MBTS provide higher vulcanization rates, they are more likely to cause premature cross-linking of the rubber at high temperatures or during processing, which can affect the processability of the rubber.
MBT: Offers a higher vulcanization rate but poor scorch safety, making it suitable for fast vulcanization applications.
MBTS: Compared to MBT, MBTS offers slightly improved scorch safety, but it is still considered a relatively “impatient” vulcanization accelerator.
2.3 Thiuram and Dithiocarbamate Accelerators
Thiuram and Dithiocarbamate accelerators, such as TMTD (Tetramethylthiuram disulfide) and other disulfide vulcanization accelerators, exhibit lower scorch safety during rubber vulcanization. While this type of accelerator offers a faster vulcanization rate, its poor scorch safety can easily lead to premature cross-linking of the rubber during processing, which can negatively affect molding and subsequent processing.
3. Relationship Between Scorch Safety and Vulcanization Rate
3.1 Scorch Safety
Scorch safety refers to the ability of rubber to remain in a stable processing state for a certain period of time without undergoing premature vulcanization. The choice of vulcanization accelerator plays a crucial role in scorch safety. In actual production, selecting an accelerator with longer scorch safety ensures that the rubber does not undergo premature cross-linking during processing, preventing issues that could affect subsequent molding and vulcanization processes. Generally speaking, sulfenamide vulcanization accelerators provide longer scorch safety, with DCBS being the one with the longest safety. It is particularly suitable for rubber products with higher requirements for processing stability.
3.2 Vulcanization Rate
The vulcanization rate refers to the speed at which rubber undergoes the cross-linking reaction during heating. A faster vulcanization rate generally results in better physical properties of the rubber (such as elasticity, anti-aging resistance, etc.). However, if the vulcanization rate is too fast, it can cause difficulties in molding rubber products during production. Therefore, achieving an appropriate vulcanization rate is crucial for both the production process and the performance of the final product. The type and amount of vulcanization accelerator can significantly influence the vulcanization rate.
CBS and TBBS vulcanization accelerators are suitable for situations that require faster vulcanization rates, as they can quickly complete the vulcanization reaction.
DCBS is better for applications requiring slower vulcanization rates. This type of accelerator is typically used in rubber products that demand a longer working time or lower processing temperatures.
The Influence of Vulcanization Accelerator Selection on Rubber Properties
Different types of vulcanization accelerators not only affect the vulcanization rate and scorch safety but may also influence the final mechanical properties (such as tensile strength, tear strength, hardness, etc.) and performance characteristics (such as wear resistance, high-temperature resistance, and anti-aging properties) of the rubber. Therefore, when selecting a vulcanization accelerator, it is important to consider not only the vulcanization rate and scorch safety but also the final application requirements of the rubber product.
Vulcanization accelerators with fast vulcanization rates (such as TBBS) are suitable for production environments that require rapid prototyping and high production volumes. However, they may result in slightly lower mechanical properties of the rubber.
Vulcanization accelerators with slower vulcanization rates (such as DCBS) are more suitable for rubber products that require higher anti-aging properties and better mechanical performance, especially in the production of high-performance rubber products.
The selection of rubber vulcanization accelerators plays a crucial role in determining the processing performance, vulcanization rate, scorch safety, and final physical properties of rubber. Different types of accelerators exhibit distinct characteristics under varying production conditions. Properly choosing a vulcanization accelerator can enhance production efficiency and optimize the performance of rubber products.
In practical applications, selecting an appropriate accelerator that balances vulcanization rate and scorch safety is key to ensuring both the quality and production efficiency of rubber products.
For general-purpose rubber products, sulfenamide vulcanization accelerators are popular due to their excellent processing safety and moderate vulcanization rate. For specialized rubber products, such as high-performance rubbers or those requiring slower vulcanization rates, slow-vulcanizing accelerators like DCBS are a good choice. Ultimately, the choice of vulcanization accelerator should take into account both production needs and the comprehensive performance requirements of the rubber products.
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