Non-toxic rubber additives have become mainstream.

In recent years, there has been increasing international attention on the issue of certain accelerators generating harmful nitrosamines during the rubber processing. Research on the generation and impact of nitrosamine compounds has become a hot topic in the global rubber accelerator field.

Studies have shown that N-nitrosamines are generated from the reaction of secondary amines with nitrogen oxides. During the vulcanization process of rubber compounds, secondary amine-based vulcanization accelerators (such as morpholine, diisopropylamine, etc.) decompose to produce secondary amines that react with nitrogen oxides in the air or with additives to generate N-nitrosamines, which can alkylate DNA and potentially induce carcinogenic effects. In contrast, primary or tertiary amines have difficulty forming stable nitrosamines with nitrogen oxides. Many countries abroad have long classified major types of vulcanization accelerators, such as sulfonamide secondary amines, thiuram types, dithiocarbamate types, and sulfur donors, as subjects of research for potential nitrosamine generation, analyzing their mechanisms and alternatives.

In light of this, many restrictive regulations have been introduced globally. For instance, Germany enacted regulations as early as 1982 to control nitrosamine content, while the United States, Japan, France, and the United Kingdom have actively developed new types of vulcanization accelerators that do not produce nitrosamines and have subsequently ceased the use of those that do. In 1997, the Beijing Rubber Research and Design Institute analyzed and determined that no traces of morpholine residues could be detected in tires from Europe and America, confirming that some accelerators that produce nitrosamines are no longer used in these regions.

The 21st century is the era of green chemistry, and environmental protection has become particularly crucial in future chemical production. China's rubber additive industry development plan for the 11th Five-Year Plan clearly states that the focus of future work will be to adjust product structures and increase the development and use of environmentally friendly and non-toxic rubber additives.

Additionally, from the current state of the global rubber additive industry, it is now in a relatively mature period with few new varieties and technologies. With the elimination of trade barriers after China joined the WTO, foreign rubber additive companies with advantages will inevitably set new barriers to maintain their positions in the international market, with environmental barriers being their primary target. Therefore, China should follow the trend and vigorously develop and apply environmentally friendly vulcanization accelerators. It can be said that the establishment of an environmentally friendly vulcanization system has become the future development direction and key for China's rubber accelerators.

◆ The replacement of sulfonamide secondary amines with secondary amine-based vulcanization accelerators includes 2-morpholinobenzothiazole sulfenamide (NOBS), N,N'-diisopropylbenzothiazole-2-sulfenamide (DIBS), etc., with NOBS being the main one.

Currently, the new types of accelerators replacing NOBS and others are TBSS and TBSI developed by Monsanto in the United States. TBSS and TBSI have structures similar to NOBS, and they are primary amine-derived sulfenamide accelerators that do not produce N-nitrosamines. TBSS has now become the dominant accelerator variety in foreign markets, with its usage in developed countries accounting for about 35% to 45% of total accelerator consumption. China's rubber additive development plan for the 11th Five-Year Plan clearly states that by 2005, the production of TBBS in China should increase from currently less than 1,000 tons/year to around 10,000 tons/year, which will account for more than 20% of total accelerator consumption.

◆ The replacement of thiuram and dithiocarbamate accelerators.

Thiuram and dithiocarbamate accelerators are very effective in promoting vulcanization and can be used as secondary accelerators alongside sulfenamide types to shorten vulcanization time. Research has found that thiuram types such as tetramethylthiuram (TMTD) and zinc dimethyldithiocarbamate (ZDMC) can undergo N-nitrosation reactions in secondary amines. Diphenylguanidine and di-o-tolylguanidine can serve as temporary substitutes, but due to the inherent toxicity of guanidine accelerators and their slow vulcanization speed and low vulcanization modulus, they are not ideal.

Currently, dibutyl dithiophosphate is mainly used abroad to compensate for these shortcomings. In recent years, new safe thiuram accelerators such as tetrabenzylthiuram (TBZTD) and tetra-(2-ethylhexyl)thiuram (TOT-N) have been commercialized. Due to the large molecular weight and high melting point of TBZTD and TOT-N, they are difficult to decompose, thus not producing nitrosamines. Especially, TBZTD has a vulcanization speed slightly lower than TMTD, while other physical properties are almost the same as TMTD, making it a highly promising new variety of thiuram vulcanization accelerators.

◆ The replacement of sulfur donors. Currently, commonly used dithiomorpholine (DTDM) and N-oxydiethylene thioamide-N'-oxydiethylene sulfenamide (OTOS) are based on secondary amines in sulfur donor crosslinking systems. They are usually used in semi-vulcanized systems with low sulfur content because the advantages of sulfur donor crosslinking systems are good reversion characteristics and oxygen resistance. Therefore, substitutes must be selected based on these characteristics.

Generally, primary amine-containing vulcanization accelerators can be used, such as high-accelerating low-sulfur semi-effective vulcanization systems and the accelerator ZBTP. Currently, promising substitutes include hexamethylene-bis-dithiothiourea (HTS) or 1,3-bis(lemongrass) methoxybenzene (BCl) that can improve reversion and fatigue properties.

China's rubber accelerator industry must vigorously adjust product structures and develop and produce environmentally friendly vulcanization accelerators. The focus of future work should be to use TBBS and TBSI to replace the currently mainly used sulfenamide accelerators NOBS, DIBS, DCBS, etc.; to use ZBTP, TBZTD, TOT-N, etc. to replace toxic thiuram accelerators; to improve reversion and fatigue resistance, accelerators HTS or methods using BCl can be adopted; to expand the scale of TBBS and its raw material tert-butylamine production facilities and improve process technology; and to conduct research and development on TBSI, TBZTD, HTS, and BCl to achieve industrialization as soon as possible.