In the application of large underwater pelletizers in the plastics processing industry, pellet oxidation is one of the key factors affecting product quality. Oxidation leads to yellowing of the pellet surface, decreased performance, and even degradation, directly impacting the stability of downstream applications. To mitigate this phenomenon, a multi-dimensional protective system needs to be built, encompassing equipment design, process control, and environmental management.
A fully enclosed water system is the fundamental design for inhibiting oxidation. Traditional open water systems easily lead to contact between water and air, increasing dissolved oxygen levels and accelerating oxidation reactions on the pellet surface. A fully enclosed system, through circulating water and degassing devices, can effectively reduce the dissolved oxygen concentration in the water, reducing oxidizing agents. For example, some high-end equipment incorporates a nitrogen protection device in the water tank, introducing a small amount of nitrogen into the water to further displace dissolved oxygen and form an inert protective layer. Furthermore, the inner walls of the water pipes are polished to reduce water flow resistance and eddies, preventing electrostatic adsorption of impurities caused by friction between pellets and the pipe walls, indirectly reducing the risk of oxidation.
Precise water temperature control is crucial for inhibiting oxidation. Excessively high water temperatures accelerate the volatilization of residual additives on the particle surface and promote hydrolysis reactions; excessively low water temperatures may result in incomplete solidification of the particle surface, prolonging contact time with water and increasing the probability of oxidation. In practice, a water temperature range must be set according to the material characteristics. For example, a low-temperature granulation process is used for heat-sensitive materials, with a multi-stage cooling device to gradually reduce the water temperature and avoid stress cracking caused by sudden temperature changes. Simultaneously, water temperature uniformity needs to be achieved by optimizing water flow distribution, such as using spiral guide plates or distributed nozzles, to ensure consistent heat exchange efficiency between the particles and water, reducing localized overheating or undercooling areas.
The compatibility between the pelletizer and the die directly affects the particle forming quality, thus influencing oxidation risk. If the pelletizer wears down, resulting in uneven cutting force, burrs or debris are easily generated on the particle surface. These irregular structures increase the contact area with water, accelerating oxidation. Therefore, the sharpness of the pelletizer and the smoothness of the die holes need to be checked regularly. High-hardness alloy materials should be used to extend the life of the pelletizer, and the inner wall of the die holes should be repaired using laser cladding technology. Furthermore, the gap between the pelletizer and the die needs precise adjustment. An excessively large gap will cause pellet tailing, while a gap that is too small may trigger metal friction and sparks, both of which exacerbate oxidation.
Water quality management is a hidden key to oxidation control. Impurities in the water, such as metal ions, microorganisms, or organic matter, can act as catalysts to accelerate the oxidation reaction. Therefore, multi-stage filtration and ion exchange treatment of circulating water are necessary, such as using activated carbon filters to adsorb organic matter and reverse osmosis devices to remove metal ions. Simultaneously, the conductivity and pH of the water should be regularly monitored to ensure water quality stability. Some companies also add antioxidants or slow-release agents to form a protective film in the water, further inhibiting the oxidation process.
Material pretreatment plays a crucial role in inhibiting oxidation. If the moisture or volatile matter content in the raw materials is too high, bubbles or pores are easily generated during granulation, and these defects can become entry points for oxidation. Therefore, it is necessary to reduce the moisture content of the material using a dryer or vacuum degassing device, and to ensure uniform plasticization of the material and reduce internal stress through the design of the shearing and compression sections of the extruder screw. Furthermore, the selection of additives must also consider antioxidant properties. For example, adding hindered phenolic or phosphite antioxidants can improve the oxidation resistance of the pellets from the source.
Strict adherence to operating procedures is the last line of defense in oxidation control. For example, when shutting down, the material feed valve must be closed first, and the pelletizer should only be shut down after no material is extruded from the die head to prevent residual material from oxidizing due to prolonged contact with water at high temperatures. Before restarting, the water system must be thoroughly cleaned to prevent impurities from remaining. During production, the appearance of the pellets must be checked regularly; if yellowing or brittleness is found, process parameters must be adjusted immediately.
Oxidation control of large underwater pelletizers needs to be integrated throughout the entire process, including equipment design, process optimization, water quality management, material pretreatment, and operating procedures. Through a fully enclosed water system, precise temperature control, blade and die matching, water purification, material pretreatment, and standardized operation, pellet oxidation can be significantly reduced, improving product quality and stability.
