Vulcanization, as the key step in rubber process，directly affects the processing and performance of rubber products. Compared with sulfur alone, the presence of small amounts of accelerator together with sulfur can significantly improve the properties of final vulcanisate. However, the present accelerators generally pose potential risks to human health and the environment，and are suffering with their poor efficiency and sole function. Therefore, developing novel green accelerator that is non-poisonous, free or low zinc oxide content added，high-effective and multifunctional, is critical for rubber industry. This review briefly introduces the development of rubber，and the recent progresses on accelerator including the ionic liquids acceleration agents, the new bis( dithiocarbimato) zinc( II) accelerators，the rare earth vulcanizing accelerator and the novel secondary accelerators. Besides,the prospect of the in-depth study of vulcanization mechanism and the development of novel vulcanization accelerator are proposed．
As early as the 15th century, when Columbus discovered the New World in America, rubber was discovered, which can produce large deformation under the action of small external forces at a certain temperature range, and can quickly recover its original shape after the withdrawal of external forces, showing excellent elasticity. However, unvulcanized rubber had the disadvantages of becoming soft in summer and hard in winter, and was accompanied by an irritating odor, so it had no practical application value for a long time.
In 1894, British chemist Weber proved that a chemical bond was formed between sulfur yellow and rubber during the vulcanization process. At present, the annual global production of rubber exceeds 17 million tons, occupying a very important position in the national economy and daily life.
It is widely used in many fields such as tires, tapes, shoes, sealing products, latex products, toys, daily necessities, and fabric coatings. Vulcanization is the process of transforming linear chain macromolecules into three-dimensional network macromolecules through chemical cross-linking, which transforms natural or synthetic rubber into valuable vulcanized rubber. The vulcanization process greatly improves the physical and chemical properties of rubber products such as high elasticity and strength. Although rubber can be vulcanized by metal oxides, peroxides, quinone oximes, amines, etc., and even some rubber can be vulcanized without vulcanizing agents (e.g., γ-ray radiation vulcanization), vulcanization is the oldest and most economical method of improving the physical and mechanical properties of rubber, using inexpensive and abundant raw materials.
The cross-linking agent dominates the market. For sulfur yellow vulcanization systems, vulcanization accelerators and activators are essential. They can reduce the amount of sulfur yellow, shorten the vulcanization time, lower the vulcanization temperature, and at the same time improve the degree of vulcanization and the physicochemical properties of the vulcanized rubber. The first accelerator, aniline, was discovered by Ornsco and gave a strong impetus to the development of rubber accelerator technology. 1918 saw the discovery of dithiocarbonate accelerators, which in turn can be oxidized to form tetramethylthiuram disulfide (TMTD). Guanidine and thiourea accelerators were also widely used in the same period, but now they are mainly used as co-promoters. At the same time, zinc oxide and stearic acid were found to activate the vulcanization reaction during the vulcanization of dithiocarbamates and tetramethylthiuram disulfide accelerators, which may react with the vulcanization accelerator to produce more reactive zinc compounds.
compound. The milestone in the history of vulcanization accelerator is the discovery of 2-mercaptobenzothiazole (MBT) type accelerator and its derivative hyposulfide accelerator by Bedford, Sebrell and others in 1930. These accelerators are inexpensive, have good performance in vulcanized rubber products and high safety in vulcanization processing, and are currently the most widely used accelerators. Vulcanization of rubber is a very complex physical and chemical reaction process, and the cross-linked structure created after vulcanization makes the vulcanized rubber insoluble in solvents, making it difficult to understand the mechanism of vulcanization reaction of rubber by common research methods. Therefore, the basic research on the mechanism of accelerator-promoted vulcanization reaction has been lagging behind the development of vulcanization technology until the 1960s, when the research on the mechanism of vulcanization reaction started slowly, but until today the mechanism of vulcanization reaction is still incomplete.
The mechanism of vulcanization reaction is still not completely clear until today. The now generally accepted mechanism of the vulcanization reaction is shown in Figure 1. First, the sulfide (sulfur yellow) reacts with the accelerator to form an active sulfur-rich compound, the exact process of which varies depending on the sulfide system. This unstable sulfur-rich compound then reacts with the allyl hydrogen on the unsaturated polydiolefin to form a cross-linking precursor .
The cross-linked precursor is then combined with other precursors. The cross-linked precursor then reacts with other carbon chains to form the initial cross-linked bond, which is also called a polysulfide bond because it contains too many sulfur atoms. Some of these polysulfide bonds undergo a series of degradation or modification reactions and eventually the cross-linked bonds are shortened to form double or single sulfur bonds with a certain distribution pattern. The tensile strength, tear strength, permanent deformation in compression and flexural properties of the rubber are also significantly improved. In most of the sulfur yellow vulcanization systems, the addition of zinc plays a very important role, either in the form of zinc oxide or zinc compounds (e.g. zinc dithiocarbamate, zinc benzothiazole) or both. Nieuwenhuizen et al. demonstrated that TMTD reacts readily with zinc oxide to form zinc dithiocarbamate (ZDMC), which is an important pathway for the formation of ZDMC in this vulcanization reaction system. Vulcanization accelerators widely used in industry can be divided into three series: (1) guanidine based accelerators, such as diphenylguanidine (DPG); (2) dithiocarbamic acid based accelerators, such as TMTD; and (3) 2-mercaptobenzothiazole based accelerators, such as MBTS. The accelerators, while improving the physical and mechanical properties of rubber, also produce carcinogenic nitrosamines, which pose serious human health and environmental risk problems. Nitrosamines are mainly formed by the reaction of secondary amines in the accelerator structure (e.g., dimethylamine (CH3 ) 2NH) with nitrogen oxides escaping from other additives in the air. Current sulfurization promoters containing secondary amines in their molecular structure include: guanidines, thiazoles, hyposulfonamides, thiurams, and diamine disulfides. This secondary amine can produce nitrosamines, which are released during the rubber mixing and vulcanization process and thus have toxic side effects on the operator. It can also be released during use, endangering the health of the user. For example, carcinogenic nitrosamines were first discovered in baby pacifiers due to the presence of nitrosamines and their precursor compounds. Epidemiological studies have found that workers involved in the production of rubber products have an above-average probability of developing cancer. With the rapid growth in global demand for rubber products, the safety and environmental concerns of rubber vulcanization accelerators are becoming more prominent, and many countries have decreed that the production and use of certain accelerators that are carcinogenic or suspected of being carcinogenic should be discontinued. About the generation of nitroso compounds, the impact of new accelerators and non-carcinogenic research has become one of the hot spots in the field of rubber vulcanization accelerators. For example, tetrabianylthiuram disulfide (TBzTD) is an efficient green accelerator synthesized by Monsanto Chemical Company, which has the characteristics of high molecular weight, high melting point, low volatility and non-decomposable, and therefore does not produce carcinogenic N-nitrosamines, and has been widely used in natural rubber butadiene rubber nitrile rubber. On the other hand, most of the widely used metal salt vulcanization accelerators are transition metal or alkaline earth metal compounds, which have the disadvantages of poor solubility, poor vulcanization stability and easy scorching in the process of use. Therefore, the development of metal salts vulcanization accelerators can prolong the scorching time and improve the performance of vulcanization promotion (such as lowering the vulcanization temperature, shortening the vulcanization time, improving the anti-return properties). Therefore, it is important to develop vulcanization accelerators that can prolong the coking time and improve the performance of vulcanization promotion (such as lowering the vulcanization temperature, shortening the vulcanization time, improving the anti-reversion properties, improving the physical and mechanical properties of vulcanized rubber), and at the same time, are non-toxic and environmentally friendly.
The development of new accelerators that are non-toxic, environmentally friendly, highly effective, non-volatile and with special functions is an important issue for the future development of the rubber industry. The key to designing new rubber vulcanization accelerator systems is to introduce substitution groups with special functions on the basis of the original vulcanization accelerator structure, modulate the metal center, dope with rare earth elements, and select new sub-promoters. At the same time, China possesses a large amount of rare earth resources, which account for more than 60% of the global production each year and are important strategic resources in modern high-tech industries and international competition. Although the research on rare earth vulcanization accelerators has just started, the rich functions of rare earths have opened up a wide range of paths for the design of efficient and multifunctional vulcanization accelerators.
With the development of computational chemistry, computational simulation studies of vulcanization accelerator systems containing transition metals and analysis of the relationship between their structure and reactivity have provided a new impetus for the study of rubber vulcanization reaction mechanisms. Combining computational chemistry techniques, we aim to reveal the mechanism of vulcanization promotion of rubber vulcanization accelerators and to construct structure-activity relationships for accelerators, such as the conformational relationship between Zn-S bond size and accelerator activity. The established structure-activity relationships can be used to predict the vulcanization-promoting activity of unknown compounds, thus providing guidance for modification of vulcanization accelerator molecules to improve vulcanization performance and opening up new ideas for the design and synthesis of new multifunctional rubber vulcanization accelerator systems.