Common problems and solutions of PET bottles in the use of hot filling lines
Because PET is hygroscopic, placing PET (including slices, preforms and bottles) in the air will absorb moisture from the air, and the longer it is placed, the more water it will absorb. The moisture content in PET will directly affect its performance. For hot-filled bottles, it will affect the heat-resistant temperature of the hot-filled bottle. The higher the moisture content, the lower the heat resistance temperature of the bottle. Generally speaking, for hot-filled bottles, from the production of preforms to the filling of beverages, the placement time is recommended:
Bottle storage period: >1L can be used within two weeks, <1L can be used within three weeks; however, more and more manufacturers have recently used lightweight bottles and connected production, that is, blowing and filling, and the bottle storage period is within 6 hours. The bottle that is blown and filled can be filled with hot liquid at 95 °C, and the bottle that is stored for more than 24 hours after blowing can only be filled with hot liquid at 88 °C.
The material and storage conditions of the bottle (room temperature, relative humidity, and storage time) will affect the technical indicators of the hot-filled bottle, that is, the production of the bottle should be based on the above different materials, storage conditions, customer requirements, etc., accordingly. Adjust the process and technical parameters of blowing. Under normal humidity conditions, PET will undergo a hydrolysis reaction during melt plasticization. High humidity levels often lead to immediate reactions, resulting in molecular chain scission, degradation, and molecular weight reduction (ie, IV reduction). The mechanical properties of PET are related to the intrinsic viscosity IV, and the lower the IV, the worse the mechanical properties of PET.
The average relative humidity in Jiangnan and coastal areas is 85% throughout the year. In some areas, the relative humidity can be as high as 90% in spring and summer. In a high humidity environment, PET will absorb moisture and reach the maximum saturated humidity.
The higher the moisture content, the greater the drop in the IV value of the PET. When the water content of a certain type of PET is 0.01%, its intrinsic viscosity is 0.73, and when the water content is 0.02%, its intrinsic viscosity becomes 0.63. At 180°C, the intrinsic viscosity decreased by 0.10 due to the reduction of drying time by 3/4 hour.
The longer the drying time, the lower the moisture in the PET raw material, but excessive drying will also cause the PET to degrade. When heated to 180°C, for the raw material with a maximum initial moisture content of 0.3%, the moisture content drops to 0.14%; drying for 4 hours can obtain a moisture content of 0.004%, which is the upper limit for the preform to control the moisture content. The moisture in the molecule of the bottle mouth will accelerate the crystallization of PET, and the moisture in the molecule of the bottle body will affect the arrangement of the molecular chain.