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NBR (Nitrile Butadiene Rubber) is a synthetic rubber widely used in industrial applications due to its excellent oil resistance, abrasion resistance, and heat resistance. It is commonly used in the automotive, machinery, electronics, and chemical industries. NBR rubber is particularly prevalent in rubber products such as seals, gaskets, and pipe joints. However, blue discoloration is a common problem in the production and use of NBR rubber. This phenomenon not only affects the product's appearance but may also affect its performance and market competitiveness. This article will conduct an in-depth analysis of NBR rubber from the aspects of its composition, vulcanization process, the causes of blue discoloration, and solutions.

I. Composition and Properties of NBR Rubber:
NBR rubber is a polymer material copolymerized from butadiene and acrylonitrile. Its molecular structure contains a high proportion of cyano groups (CN), which gives NBR rubber excellent oil resistance, chemical resistance, and abrasion resistance. The properties of NBR rubber are closely related to the acrylonitrile content in its composition. Generally, the higher the acrylonitrile content, the better the oil resistance, chemical resistance, and thermal stability of NBR, but its low-temperature performance and processability are relatively poor. According to different application needs, the acrylonitrile content of NBR rubber is usually controlled between 18% and 50%. As the acrylonitrile content increases, the oil resistance and chemical resistance of NBR rubber improve, but the elasticity and low-temperature performance decrease. Conversely, when the acrylonitrile content is lower, the low-temperature performance and elasticity of NBR are better, but the oil resistance and chemical resistance are worse.

II. Overview of Blue Discoloration in NBR Rubber Products:
In the production of NBR rubber, a phenomenon called "blue discoloration" often appears on the surface of the product. Blue discoloration refers to the formation of a blue or bluish-gray film or spot on the surface or near-surface area of ​​NBR rubber products. This phenomenon not only affects the appearance of rubber products but sometimes may affect their performance, such as surface friction coefficient and wear resistance. Blue discoloration usually occurs in the post-treatment process of NBR rubber vulcanization, especially in the vulcanization and later cooling stages. It mainly manifests as blue or bluish-gray spots in local areas on the surface of NBR rubber products, and this phenomenon generally intensifies over time, causing considerable inconvenience to users.

III. Causes of Blue Discoloration in NBR Rubber Products:
The causes of blue discoloration in NBR rubber products are multifaceted, involving multiple factors such as raw materials, formulations, and processes. The following are some of the main causal analyses:

1. Influence of Polymer Structure and Cyano Groups:
NBR rubber molecules contain a high proportion of cyano groups (CN), which are highly polar. When NBR rubber is exposed to high temperatures or certain environmental conditions, cyano groups may undergo chemical reactions, especially when in contact with oxygen and moisture in the air. These reactions may lead to cyano groups reacting with oxides to form blue or bluish-gray complexes, resulting in blue discoloration on the rubber surface.

2. Selection and Ratio of Vulcanizing Agents:
Vulcanizing agents play an important role in the production of NBR rubber, and the vulcanization process is a key step in obtaining the final physical properties of the rubber. However, some vulcanizing agents (especially excessive vulcanizing agents) may react with the cyano groups in NBR rubber during the vulcanization process, producing some unstable compounds that form blue complexes on the rubber surface. Blue discoloration may be more severe, especially when the temperature is too high during vulcanization or when the vulcanizing agent is improperly selected.

3. Influence of Fillers and Antioxidants:
Various fillers, such as carbon black and silica, are often added to the NBR rubber formulation to improve the rubber's abrasion resistance and UV resistance. However, the type and particle size of certain fillers and their interaction with the rubber may affect the occurrence of blue discoloration. For example, carbon black may react with cyano groups during the reaction with NBR, leading to the production of blue. In addition, the type and amount of added antioxidants, accelerators, and other auxiliaries may also affect the blue discoloration phenomenon, especially in high-temperature environments, where some antioxidants may react with cyano groups, further exacerbating the blue discoloration phenomenon.

4. Processing Technology and Temperature Control:
Temperature control during the NBR rubber processing process is also an important factor in blue discoloration. If the temperature is too high or the heating time is too long during vulcanization, it will exacerbate the oxidation reaction of cyano groups, leading to the formation of blue substances on the rubber surface. In addition, temperature changes during the cooling process may also trigger similar chemical reactions, leading to blue discoloration.

5. Influence of Environmental Factors:
Environmental conditions, especially humidity and the oxygen content in the air, also have a significant impact on the occurrence of blue discoloration. When NBR rubber products are exposed to humid or high-oxygen environments, the reaction of cyano groups with oxygen and moisture may lead to the formation of blue complexes, which is a major cause of blue discoloration.

IV. Solutions for Blue Discoloration in NBR Rubber Products:
Based on the causes of blue discoloration in NBR rubber products, this problem can be effectively solved or mitigated by optimizing the raw material formula, adjusting the processing technology, and controlling the production environment.

1. Adjusting the NBR Rubber Formula:
First, the occurrence of blue discoloration can be reduced by adjusting the NBR rubber formula. For example, reducing the acrylonitrile content, reducing the proportion of cyano groups, and reducing the possibility of their reaction with oxygen and moisture from the source. NBR materials with lower cyano group content can be selected, or other types of rubber (such as SBR, EPDM, etc.) can be added for modification to improve blue discoloration.

2. Selecting Suitable Vulcanizing Agents:
The selection of vulcanizing agents is crucial for the occurrence of blue discoloration. To reduce blue discoloration, mild vulcanizing agents or low-temperature vulcanization systems can be selected, avoiding the use of excessive vulcanizing agents. In addition, the ratio and reaction conditions of the vulcanizing agent can be optimized to reduce excessive side reactions, thereby reducing the occurrence of blue discoloration.

3. Improving Processing Technology:
Optimizing the vulcanization process is also an important means of reducing the blue bloom phenomenon. Controlling the temperature, pressure, and time during the vulcanization process, avoiding excessively high temperatures and excessively long vulcanization times, thereby reducing the oxidation reaction of the cyano group. In addition, a staged vulcanization method can be adopted, first pre-vulcanizing at a lower temperature, and then final vulcanizing at a higher temperature, thereby reducing the occurrence of the blue bloom phenomenon.

4. Adding antioxidants and stabilizers:
Adding suitable antioxidants and stabilizers is another effective method to reduce the blue bloom phenomenon. By selecting appropriate antioxidants, the oxidation reaction of the cyano group on the surface of NBR rubber can be effectively inhibited, and the formation of blue complexes can be reduced. Common antioxidants include some sulfur-containing compounds, phenolic compounds, etc., and these additives can effectively improve the antioxidant performance of rubber.

5. Control the production environment:
In addition to optimizing the process and formula, controlling the temperature and humidity in the production environment is also an important measure to reduce the blue bloom phenomenon. NBR rubber should be prevented from prolonged exposure to high-temperature and high-humidity environments, ensuring that rubber products are maintained under suitable environmental conditions during production and storage. The blue bloom phenomenon of NBR rubber products is a common quality problem that involves multiple aspects, including the chemical structure of the material, processing technology, and production environment. Through an in-depth analysis of the causes of the blue bloom phenomenon, we can take effective measures to reduce or avoid the occurrence of the blue bloom phenomenon from the source. Optimizing the formula, selecting suitable vulcanizing agents, adjusting the processing technology, and controlling the production environment can significantly reduce the blue bloom phenomenon and improve the quality and market competitiveness of NBR rubber products.

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