Currently, the development of accelerators worldwide is gradually shifting toward eco-friendly, functional, and centralized approaches. From a technical perspective, there has been growing international concern in recent years over the potential toxicity of certain accelerators used in rubber processing, particularly their tendency to generate harmful nitrosamines during vulcanization. In response, numerous restrictive regulations have been introduced globally. For instance, Germany enacted legislation as early as 1982 to strictly control nitrosamine levels, while the U.S., Japan, France, and the UK have also actively pursued the development of new sulfur-curing accelerators that do not produce nitrosamines—and have progressively phased out those accelerators known to pose nitrosamine risks. As a result, environmentally friendly vulcanization accelerator systems have emerged, alongside the creation of innovative, eco-friendly product variants designed to replace materials with carcinogenic potential. From a marketing standpoint, there is an increasing emphasis on enhancing product performance through targeted application research. This includes developing functionalized masterbatches and pre-dispersed formulations of accelerator compounds. Moreover, intensifying research and innovation in compounding technologies has become a key strategy for leading global accelerator manufacturers aiming to capture and expand market share. Over the past several decades, the global rubber additives industry has undergone significant consolidation, with production and application trends increasingly converging toward a limited number of high-performance, low-impact products. Among these, the most widely used accelerators remain TBBS, CBS, and TMTD, renowned for their excellent environmental compatibility and superior performance characteristics.

II. Domestic Production and Market

China's rubber accelerator industry experienced rapid growth during the "Eighth Five-Year Plan" and the "Ninth Five-Year Plan" periods. Today, the industry not only meets domestic market demand but also exports a significant quantity of its products abroad. Key production hubs have emerged, including Zhenjiang Sopu Group, Northeast Accelerator General Factory, and Tianjin Organic Chemical Plant No. 1. As we enter the 21st century, the domestic development of rubber accelerators has accelerated even further, with a nationwide construction boom in recent years driving rapid expansion of production capacity. According to incomplete statistics, China's total annual production capacity for rubber accelerators in 2003 was approximately 120,000 tons, while actual output reached about 83,500 tons that same year. Table 4 lists the major manufacturers and their product ranges in China, while Table 5 provides data on the output of key rubber accelerator products produced domestically over recent years. Thanks to the rapid growth of China's rubber industry—and given the relatively simple synthesis processes involved—several new enterprises have entered the market in recent years, either by investing in dedicated accelerator production facilities or by developing compounded accelerator products. Notable examples include Shandong Yanggu Huatai Chemical Co., Ltd., Shandong Sanyi Fine Chemical Co., Ltd., Qingdao Huaheng Additive Factory, Shandong Huayang Xinko Chemical Co., Ltd., Jiangsu Yixing Kao Chemical Co., Ltd., Shanghai Jinghai Chemical Co., Ltd., Shanghai Changjiang Chemical Factory, Shanghai Fengxian LianGong Additive Factory, and Tianjin Labo Accelerator Factory, among others. In 2003, China's total domestic output of rubber accelerators stood at around 83,500 tons. Among these, the six primary varieties—accelerators M, DM, CBS, TBBS, NOBS, and TMTD—accounted for 88% of the overall production, reflecting stable output levels with minimal adjustments in product mix. However, a notable trend emerged: the production of NOBS, a secondary amine-based sulfenamide accelerator known to potentially generate carcinogenic nitrosamines, has been curtailed, showing a slight decline. In contrast, tertiary amine-based sulfenamide accelerators like TBBS have seen steadily increasing production, reaching 3,800 tons in 2003—a share of 15% within the sulfenamide category.

During rubber processing, certain quality issues frequently arise, disrupting the smooth flow of production operations. Given today’s high labor costs, the losses incurred during downtime waiting for materials can be substantial. However, if appropriate measures are taken to address most of these issues—especially those belonging to the same category—these problems can be effectively resolved.

Question: Below, the author shares solutions based on personal experience.

Function:

1 Sulfur is unevenly dispersed, resulting in a speckled appearance (agglomerates).

This is a problem that’s both old and new—something our predecessors should have already solved, yet here we are, revisiting it once again.

Placing the sulfur being stored directly onto cement boards is not a good practice from a moisture-control perspective—always make sure to lay it on mats or pallets instead. Before measuring, sifting the sulfur is an effective way to prevent clumping; you don’t necessarily need an extremely fine sieve—40- or 20-mesh screens will suffice, as long as they can break up any powder-like clumps in the sulfur. Additionally, when adding only small amounts of sulfur at a time, it’s best to sift and add simultaneously while working on the open mill. In such cases, even coarser mesh sizes are acceptable.

When using the traditional method, first set aside the sulfur placed on a plate, then add an equal volume of lightweight calcium carbonate or whiting powder. After thoroughly mixing the two materials, introduce them into the open mill for processing. While this approach is straightforward and convenient, there’s no better way to achieve the desired result. Here’s a brief introduction to what whiting powder is: Crushed oyster shells and seashells are piled onto outdoor cement slabs, left exposed to the elements—basking in wind, sun, and rain—for 2 to 3 years. Over time, the organic components inside naturally break down and wash away, leaving behind a pure, snow-white substance composed solely of calcium. This finely ground material isn’t just ideal for rubber applications—it also has versatile uses in other industries as well.

For the rubber compound containing a high proportion of hard clay, it remains unclear why sulfur is unevenly dispersed. Despite numerous attempts to address this issue, none have yet proven effective. To resolve the problem, we mixed the masterbatch—prepared by combining the rubber with an equal weight of hard clay—as thoroughly as possible using a kneader. This mixture was then further processed in the company’s custom-built 40-liter kneader (operating at approximately 30 rpm/min) for about 1 hour. Remarkably, no visible sulfur particles were detected in the final masterbatch—a result that could be described as a lucky success.

The homemade sulfur masterbatch is intended for the company’s own use. However, if all the rubber within the company were to switch entirely to masterbatch, the volume required would become excessive and the variety too extensive—necessitating specialized mixing equipment dedicated solely to processing the masterbatch. This approach would be both time-consuming and labor-intensive. Therefore, aside from the currently trouble-free compounded rubber and certain compounds that can tolerate a small amount of sulfur particles, sulfur masterbatch is now reserved only for rubber formulations where it’s absolutely essential. We use a masterbatch containing 100 parts by weight of rubber and 50 parts by weight of sulfur, meaning the masterbatch itself already includes one-third sulfur—effectively tripling the standard sulfur content added during compounding.

To prepare the masterbatch rubber, we initially used NR, but as soon as winter arrived, the rubber became hard and difficult to cut into small pieces. Based on our experience, we switched to blending 50 parts of NR with 50 parts of BR, which effectively solved the issue.

Sulfur-based masterbatch is now available on the market and is incredibly convenient to use. While it does come with a slight increase in cost, you can decide whether to adopt it based on its cost-effectiveness.

Additionally, there is a resinous form of sulfur known as polysulfide (manufactured by Toyo Chemical), which appears as yellow, brittle chunks. This sulfur is melted on an open mill before being dispersed into the rubber, making it commonly used in rubber products such as rubber balls.

When the rubber compound, after being processed in the internal mixer, is discharged onto the calender, its temperature can easily reach over 140°C. If sulfur is added to the hot rubber mixture at this stage, some of the sulfur will melt and become liquid within the rubber matrix. While it’s perfectly fine for sulfur to disperse uniformly in the rubber as a liquid, the moment the rubber cools down on the calender due to the cooling process, the molten sulfur will solidify into lumpy chunks. These clumps then get incorporated into the rubber compound, forming sulfur particles roughly the size of match heads. As a result, these sulfur lumps fail to disperse further and remain trapped within the rubber in their small, chunky form—unaffected even by subsequent re-mixing or reprocessing. Therefore, it’s best to wait until the rubber has cooled down to below 70°C before adding the sulfur.

Sulfur powder sold on the market comes in two grades: 200 mesh and 300 mesh. The 300-mesh particles are finer, leading many people to believe they disperse more easily—and thus prefer using this grade. However, whether this is truly the case still needs to be proven through experimentation.

In our experiment, we subjected 100 parts by mass of NR to plastication—preferably using sheet rubber or SM RSL—and then added 3 parts by mass of sulfur to the plasticated compound. Next, we fed this compound into the laboratory-scale open mill, adjusting the roll gap to its minimum setting. When we tried to peel off the rubber sheet wrapped around the front roll, it shrank unpredictably, resulting in a sheet that lacked a uniform appearance. To address this issue, we temporarily stopped the mill and carefully placed a roughly 10 cm x 10 cm piece of cellophane over the rubber sheet on the front roll before peeling it off. This simple step effectively prevented shrinkage, allowing us to prepare microscope slides suitable for analysis. Using a microscope with magnification around 400x, we were able to clearly observe individual sulfur particles. Given the cost-effectiveness of 300-mesh sulfur, its advantages are self-evident. However, the authors of this study conclude that, for most applications involving rubber products, switching to 200-mesh sulfur is sufficient to meet performance requirements.

More than 30 years ago, the raw material for rubber vulcanization—sulfur powder—was derived from sulfur mined directly from sulfur mines, collectively referred to as mineral sulfur. Today, however, nearly all the sulfur used is recovered sulfur obtained as a byproduct of petroleum refining. Mineral sulfur crystals are large and relatively soft, making them easy to crush. In contrast, recovered sulfur features smaller, harder crystals, which makes it significantly more challenging to grind into powder. Because recovered sulfur tends to clump together easily when turned into powder, trace amounts of anti-caking agents are added to prevent this issue. Additionally, there’s also a specialized form of sulfur—known as coated sulfur—that includes a 3% blend of processing oil, specifically designed for tire manufacturers.

It is well known that sulfur has poor dispersibility in NBR, making it less noticeable when producing black-colored products. However, when manufacturing lighter-colored rubber items, unsightly dark-brown spots can appear. While this may seem like an old issue, sulfur masterbatch is now available on the market. If the problem persists despite these solutions, you can try the following methods.

Place the NBR into the open mill, add sulfur and mix thoroughly—this will cause the roller temperature to rise as the sulfur dissolves into the rubber. Next, sheet out the resulting compound and allow it to cool. The following day, reprocess the material on the open mill, adding various compounding agents according to standard procedures. Finally, incorporate the vulcanization accelerator and blend uniformly. Using this method effectively eliminates any spotting or unevenness in the final product.