Organic peroxides are widely used as crosslinking agents in rubber processing. However, different types of organic peroxides exhibit distinct characteristics and limitations in rubber crosslinking. Among these, the half-life and the types of free radicals generated during decomposition are key factors that determine whether an organic peroxide is suitable as a crosslinking agent.
For organic peroxides, the half-life must be neither too short nor too long.If the half-life is too short, scorching may occur during the mixing stage; conversely, if the half-life is too long, it may result in excessively high vulcanization temperatures or excessively long vulcanization times. Therefore, when selecting an organic peroxide as a crosslinking agent, the appropriate half-life range must be determined based on the specific application.
In addition, there are differences in the types of free radicals formed after the decomposition of organic peroxides. Alkoxy radicals are a relatively reactive type of free radical with a high efficiency in forming cross-links.Consequently, organic peroxides containing alkoxy radicals are the primary and most commonly used type of crosslinking agent. In contrast, crosslinking agents with other types of radicals are less widely used in practical applications than alkoxy peroxides.
1. Dialkyl Peroxides
Dialkyl peroxides are among the commonly used rubber crosslinking agents; they offer high crosslinking efficiency but also have certain limitations. Common dialkyl peroxides include diisopropylbenzoyl peroxide (DCP), di(tert-butyl isopropyl)benzoyl peroxide (BIPB), and 2,5-dimethyl-2,5-bis(tert-butylperoxy)hexane (Bidiwu or 101), among others.
As one of the organic peroxides with the highest annual consumption, DCP offers the advantages of high crosslinking efficiency and low cost; however, its main drawback is the strong, pungent odor of the vulcanized products. This is because acetoin, a byproduct of DCP decomposition, is highly volatile, remains in the products, and produces an irritating odor.Therefore, when selecting DCP as a crosslinking agent, careful consideration of its application context is necessary; for example, its use in the manufacture of food-contact materials should be avoided. If the use of DCP is unavoidable, the impact on the final product can be mitigated by adding other additives.
In contrast, BIPB features dual functional groups and a high content of reactive oxygen species, resulting in higher crosslinking efficiency. It also does not produce a strong odor, which is why it is referred to as “odorless DCP.” BIPB has a relatively higher vulcanization temperature than DCP; typically, a temperature approximately 5 degrees higher than that of a DCP formulation is required to achieve optimal results.However, the decomposition product of BIPB, bis(2-hydroxyisopropyl)benzene, has low solubility and may cause blooming in some rubbers; care must be taken to adjust the formulation to reduce its usage and improve the solubility of its decomposition products in the rubber.
Shuang Er Wu or 101 is primarily used for the vulcanization of silicone rubber but can also be used in the vulcanization of organic rubbers, such as EPDM and CM.Bis-2,5-dichlorobenzene is characterized by its liquid state and high volatility; care must be taken to control process fluctuations and maintain process stability as much as possible to ensure consistent crosslinking density. Since the pure product is flammable and explosive, it is not suitable for use in enclosed spaces, and safety precautions must be observed. Additionally, bis-2,5-dichlorobenzene has a low melting point, so insulation is required in winter to prevent freezing.
2. Ketone Peroxides
Compared to dialkyl peroxides, ketone peroxides are characterized by a lower crosslinking initiation temperature and are typically suitable for products requiring low-temperature vulcanization.1,1-Bis(tert-butylperoxy)-3,3,5-trimethylcyclohexane, commonly known as 3M, is one of the most common ketone peroxide crosslinking agents.
3M has a low vulcanization temperature and can undergo vulcanization at room temperature or slightly elevated temperatures, making it highly suitable for applications requiring low-temperature vulcanization. However, due to its slightly lower vulcanization efficiency, a higher dosage is required when used in saturated rubbers.In addition, 3M exhibits a certain tendency toward scorching, which can adversely affect the physical properties of the finished product. Therefore, when using 3M as a crosslinking agent, care must be taken to control the vulcanization conditions to avoid excessively high temperatures or excessively long vulcanization times.
Although 3M is not as commonly used as cross-linking agents such as DCP or BIPB, it is still widely applied in certain specialized applications, such as the manufacture of low-temperature sealing materials, insulating materials, and elastomers. Additionally, 3M can be blended with other cross-linking agents to achieve better cross-linking results.
In summary, ketone peroxides are among the commonly used rubber crosslinking agents and are typically suitable for products requiring low-temperature vulcanization. When selecting a ketone peroxide as a crosslinking agent, it is necessary to consider its characteristics, limitations, and application scenarios to meet the requirements of different materials under various conditions.It is also important to carefully control the vulcanization conditions to prevent potential scorching issues.
3. Dicarbonyl Peroxides
Compared to dialkyl peroxides, diacetyl peroxides exhibit a marked tendency toward scorching and produce less active free radicals, making them unsuitable for the processing requirements of most rubbers.Furthermore, diacetyl peroxides are susceptible to the effects of carbon black, which limits their application in certain specialized rubber products. Consequently, diacetyl peroxides are typically used only for the cross-linking of silicone rubber.
Di(2,4-dichlorobenzoyl) peroxide is one of the commonly used diacetyl peroxides, commonly known as “Shuang Er Si,” and is typically used for the hot-air vulcanization of silicone rubber and the manufacture of extruded products.Similar to other peroxides, “Shuang Er Si” also suffers from blooming, primarily due to the low solubility of its decomposition product, 2,4-dichlorobenzoic acid, in certain rubbers. To prevent blooming, the formulation can be adjusted through methods such as two-stage vulcanization or the use of anti-blooming agents.
In addition to blooming, environmental concerns also pose a limitation for PD-50S-PS. The primary issue is the potential formation of trace amounts of polychlorinated biphenyls (PCBs) during the vulcanization process. These harmful substances pose potential risks to both the environment and human health; therefore, measures must be taken to prevent pollution.For example, vulcanization conditions should be strictly controlled during production to minimize the generation of pollutants; at the same time, care must be taken in the disposal of waste materials and wastewater to reduce environmental impact.
In summary, although diacetyl peroxides exhibit a certain degree of cross-linking effectiveness, they have numerous limitations and are only suitable for specific rubber products. When using diacetyl peroxides as cross-linking agents, it is necessary to consider their characteristics, limitations, and application scenarios, and to take appropriate measures to prevent blooming and environmental issues.