As one of the most commonly used additives in the rubber industry, zinc oxide primarily serves as a vulcanization activator and reinforcing agent. It plays a uniquely important role in the vulcanization process of rubber, enhancing its mechanical properties, aging resistance, abrasion resistance, and tear resistance. Based on different production processes, zinc oxide can be divided into three main types: direct zinc oxide, indirect process zinc oxide, and hydrometallurgy of zinc. Due to differences in production processes, zinc oxide exhibits distinct characteristics in terms of particle size, purity, and specific surface area, which in turn influence its application in rubber products.

Production Process

🔸Direct Zinc Oxide: It refers to the production method where zinc-bearing ores (such as zinc concentrates) or zinc-containing waste materials undergo high-temperature treatment to generate zinc vapor, which is then converted into zinc oxide through an oxidation reaction. This production process primarily consists of the following steps:

• High-temperature heating: Zinc-bearing ore or scrap is reduced to zinc vapor at temperatures between 900 and 1000°C.
• Oxidation reaction: The zinc vapor reacts with oxygen in the air to form zinc oxide.
• Cooling and collection: The resulting zinc oxide powder is cooled, collected, and screened to obtain the final product.

🔸Indirect Process Zinc Oxide: It involves heating zinc ingots to melt them and evaporate into zinc vapor, which then burns in oxygen to form zinc oxide. This process consists of the following steps:

• Zinc Ingot Melting: Zinc ingots are melted and vaporized at high temperatures between 1300-1400°C.
• Oxidation Reaction: The vaporized zinc reacts with oxygen in the air to form zinc oxide particles.
• Collection: After cooling, the zinc oxide particles solidify into powder, which is collected through separation and screening.

🔸Hydrometallurgy of Zinc: It is a chemical method primarily involving the chemical precipitation of zinc oxide from solutions containing zinc compounds, such as zinc sulfate solutions. Here are the production steps:

• Acid Leaching: Dissolve zinc-bearing minerals or waste materials in sulfuric acid to produce a zinc sulfate solution.
• Precipitation Reaction: Add an alkaline solution to react with zinc ions in the solution, forming zinc hydroxide precipitate.
• Calcination and Dehydration: Calcine the zinc hydroxide precipitate to remove water, resulting in zinc oxide powder.

Performance Feature

🔸Direct Zinc Oxide:

• Particle Size: Direct zinc oxide features larger particles, typically ranging from 1 to 10 micrometers, resulting in relatively poorer dispersibility.
• Purity: Since direct zinc oxide utilizes ore or waste materials as raw feedstock, its purity generally ranges from 95% to 97%, containing trace impurities such as lead and iron.
• Specific Surface Area: The specific surface area is relatively low, generally ranging from 2 to 5 square meters per gram, resulting in weaker surface activity.

🔸Indirect Process Zinc Oxide:

• Particle Size: Indirect process zinc oxide features smaller, more uniform particles, typically ranging from 0.1 to 1 micrometers in diameter, making it finer than direct zinc oxide.
• Purity: Utilizing high-purity zinc ingots, indirect process zinc oxide achieves a purity of 99%-99.7%, with virtually no impurities.
• Specific Surface Area: It exhibits higher surface activity, with a specific surface area generally ranging from 4 to 10 square meters per gram, suitable for applications requiring high dispersibility.

🔸Hydrometallurgy of Zinc:

• Particle Size: The particle size can be adjusted according to control conditions during production, generally ranging from 0.02 to 0.2 micrometers.
• Purity: Due to precise impurity control during production, it achieves the highest purity, exceeding 99.9%.
• Specific Surface Area: It exhibits exceptionally high surface activity, with a specific surface area reaching up to 50 square meters per gram.

Application

🔸Direct Zinc Oxide: Direct zinc oxide, due to its low cost and moderate purity, is suitable for rubber products with less stringent performance requirements, such as:

• Tires and rubber boots: These products do not require high-purity zinc oxide, making it an ideal choice due to its low cost.
• Low-end rubber seals: Such as standard industrial rubber pipes, gaskets, etc.

🔸Indirect Process Zinc Oxide: With its high purity and excellent dispersibility, indirect process zinc oxide is suitable for high-performance rubber products such as:

• Premium tires: It enhances rubber's abrasion resistance, aging resistance, and high-temperature performance.
• Automotive and industrial seals: The high purity of indirect process zinc oxide strengthens the mechanical properties and aging resistance of rubber products, making it suitable for demanding industrial applications.
• Transparent rubber products: Due to its high purity and excellent dispersibility, indirect process zinc oxide is used in producing transparent rubber, such as medical rubber products and food-grade rubber.

🔸Hydrometallurgy of Zinc: Due to its high purity and reactivity, it is suitable for high-tech rubber products and other industrial applications:

• High-performance seals: With exceptional dispersibility and reinforcing properties, it is ideal for high-performance seals in aerospace, automotive, and other industries.
• Electronic component rubber: Its high purity meets the stringent electrical insulation requirements for rubber used in electronic products.
• Medical rubber products: In the medical field, its exceptional purity makes it suitable for medical-grade rubber products such as catheters and sealing rings.

Development Trends

As the rubber industry advances toward high performance and environmental sustainability, the application of zinc oxide continues to innovate:

🔸Functional Modification: Enhance the interfacial adhesion between zinc oxide and rubber through surface modifications such as silane modification or fatty acid modification, further improving reinforcement effects. Alternatively, develop multifunctional zinc oxides—such as those loaded with antimicrobial agents—to endow rubber products with both anti-aging and antibacterial properties, making them suitable for medical rubber and food-contact rubber applications.

🔸Quantity Reduction Substitution: To reduce costs and environmental pressure, the industry actively explores zinc oxide reduction techniques. Examples include replacing traditional micron zinc oxide with nano zinc oxide (reducing usage by 30%-50%) or blending it with other metal oxides (e.g., magnesium oxide, silicon oxide) to decrease zinc oxide consumption while maintaining vulcanization efficiency.

In the future, with technological advancements and heightened environmental requirements, the application of zinc oxide in the rubber industry will become more extensive and diverse.