Organic peroxides have a wide range of applications as crosslinkers in rubber processing. However, different types of organic peroxides have different characteristics and limitations in rubber crosslinking. Among them, the half-life and the type of free radicals generated by decomposition are important factors affecting the suitability of organic peroxides as cross-linking agents.
For organic peroxides, the half-life should be neither too short nor too long. If the half-life is too short, there will be burning problems in the mixing stage; while the half-life is too long, it will lead to too high a vulcanization temperature or too long vulcanization time. Therefore, when choosing organic peroxides as crosslinking agents, it is necessary to determine the range of half-life according to the specific application.
In addition, there are differences in the types of radicals formed upon decomposition of organic peroxides. Alkoxy radicals are one of the more reactive types of radicals and have a high efficiency in forming cross-linking bonds. Therefore, organic peroxides with alkoxy radicals are the predominant and most commonly used type of crosslinker. In contrast, other radical types of crosslinkers are not as commonly used in practice as alkoxy peroxides.
1. Dialkyl peroxide
Dialkyl peroxides are one of the commonly used rubber cross-linking agents with high cross-linking efficiency but with some limitations. Common dialkyl peroxides include diisopropyl peroxide (DCP), bis(tert-butylperoxyisopropyl)benzene (BIPB), and 2,5-Dimethyl-2,5-di(tert.butylperoxy)hexane (bisdifive or 101).
DCP, as one of the most consumed organic peroxides in a year, has the advantages of high cross-linking efficiency and low price, but its main disadvantage is the big and pungent odor of the products after vulcanization. This is because the acetophenone produced by the decomposition of DCP is highly volatile and will remain in the products and produce irritating odor. Therefore, when choosing DCP as a cross-linking agent, it is necessary to carefully consider its application scenarios, such as avoiding the use in the manufacture of food contact materials. If DCP must be used, its effect on products can be reduced by adding other additives.
In contrast, BIPB has the characteristics of bifunctional groups, high active oxygen content, higher cross-linking efficiency, and does not produce strong odor, so it is called "odorless DCP".The vulcanization temperature of BIPB is relatively high compared with that of DCP, and it usually requires a temperature about 5 degrees higher than that of DCP formulations in order to achieve the best results. However, the decomposition product of BIPB, bis(2-hydroxyisopropyl)benzene, is less soluble and will form a spray frost in some rubbers, so it is necessary to pay attention to adjusting the formula to reduce the amount of use and to improve the solubility of its decomposition product in the rubber.
Bis-25 or 101 is mainly used in the vulcanization of silicone rubber, and can also be used in the vulcanization of organic rubber, such as EPDM, CM and so on. Bis(2+5) has liquid state, strong volatility and other characteristics, need to pay attention to control process fluctuations, try to keep the process stable to ensure the stability of the crosslinking density. Due to its pure product is flammable and explosive, it is not suitable for operation in confined space, need to pay attention to safety issues. At the same time, the melting point of bis(2,5) is low, so we need to pay attention to heat preservation in winter to avoid freezing.
2. Peroxide peptide
Compared to dialkyl peroxides, ketal peroxides are characterized by a lower temperature of initiation of crosslinking and are often used in products requiring low temperature vulcanization. 1,1-Di(tert.butylperoxy)-3,3,5-trimethylcyclohexane, commonly known as 3M, is one of the most common ketal peroxide crosslinkers.
3M has a low vulcanization temperature and can be vulcanized at both room temperature and microtemperature, making it ideal for situations where low-temperature vulcanization is required. However, due to its slightly lower vulcanization efficiency, it is necessary to increase the dosage when used in saturated rubber. In addition, 3M has a certain tendency to scorch, which may adversely affect the physical properties of the products. Therefore, when using 3M as a cross-linking agent, it is necessary to pay attention to the control of vulcanization conditions, in order to avoid too high a temperature or too long a vulcanization time.
Although 3M is not a commonly used cross-linking agent like DCP or BIPB, it is still widely used in some special application scenarios, such as the manufacture of low-temperature sealing materials, insulating materials, and elastomers. Meanwhile, 3M can also be mixed with other crosslinking agents to achieve better crosslinking effect.
In conclusion, ketal peroxides are one of the commonly used rubber cross-linking agents, which are usually applied in products requiring low temperature vulcanization. When choosing ketal peroxides as crosslinkers, it is necessary to consider their characteristics, limitations and application scenarios to meet the needs of different materials under different conditions. Attention also needs to be paid to controlling the vulcanization conditions to avoid possible scorching problems.
3. Diacyl peroxides
Compared with dialkyl peroxides, diacyl peroxides have an obvious tendency to scorch, and at the same time produce weaker radical activity, so it is difficult to meet the processing needs of most rubbers. In addition, diacyl peroxides are susceptible to carbon black, which limits their application in some special rubber products. Therefore, diacyl peroxides are usually used only for crosslinking of silicone rubber.
Di(2,4-dichlorobenzoyl) peroxide is one of the commonly used diacyl peroxides, commonly known as bis(2,4-dichlorophenyl) peroxide, and is commonly used in the hot air vulcanization of silicone rubber and in the manufacture of extruded molded products. Similar to other peroxides, bis(2,4-dichlorobenzoic acid) suffers from frosting problems, mainly due to its decomposition product, 2,4-dichlorobenzoic acid, which is less soluble in some rubbers. To prevent frosting problems, formulations can be adjusted by two-stage vulcanization or the use of anti-frosting agents.
In addition to spray frosting, environmental issues are also a limiting factor for bis-2-four. The main problem is the formation of traces of polychlorinated biphenyls, or PCBs, during the vulcanization process.These hazardous substances are potentially risky to both the environment and human health, and measures need to be taken to avoid contamination. For example, vulcanization conditions should be strictly controlled during the production process to reduce the generation of pollutants; and attention should be paid to the disposal of waste materials and wastewater to reduce the impact on the environment.
In conclusion, although diacyl peroxides have a certain cross-linking effect, they are more limited and are only suitable for use in specific rubber products. When using diacyl peroxides as cross-linking agents, it is necessary to consider their characteristics, limitations and application scenarios, and to take appropriate measures to avoid frosting and environmental problems.
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