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The drying degree of PET plastic directly affects the quality of bottle blowing machine

Solid PET is easy to absorb moisture from the air. During storage, PET will absorb moisture until it is saturated with environmental conditions. The saturation value can be up to 0.6% by weight. Generally, when PET is shipped from the supplier, its moisture content is less than 0.1% by weight. In order to obtain enough good product performance, it is necessary to reduce the water content to 0.004%, which is 30 ppm before melting.

If there is water in pet resin, even if it is very low, it will cause a series of reactions:
When the temperature is higher than the melting point of PET (about 250 ℃), water will quickly cause polymer degradation (chemical chain is cut off due to water degradation), which will reduce molecular weight, apparent viscosity and related physical properties. In fact, hydrolysis begins at a lower temperature (such as 150 ℃), but its speed is lower, and its speed increases with the temperature. Under drying and molding conditions, the reduction of I Ⅳ cannot be greater than 0.02dl/g. If the viscosity is too large, it will lead to an increase in the crystallization rate, which is detrimental to the transparency of the preform, and lead to a decline in the mechanical properties of the finished bottle blown by the bottle blowing machine, as well as a decline in the bearing strength and impact strength.

The effect of temperature on drying PET is very complex. It not only affects the diffusion rate of water vapor, but also affects the chemical process during drying, so it will affect the performance of the finished resin. It is necessary to consider the potential hydrolysis and thermal process. As mentioned earlier, with the decline of Ⅳ, the hydrolysis rate will be accelerated when it is above 150 ℃, because the thermal transformation process is faster than the diffusion process, and it is unfavorable to increase the temperature prematurely during drying.

Similarly, even though most of the water vapor can be extracted, too high temperature (such as higher than 180 ℃) will lead to thermal degradation and thermal oxidation (in the air drying system). In this way, the polymer chain will break, and the by-product substances will also be released, resulting in the decline of physical properties.

There are AA components in the by-products, and changes in physical properties will be shown on the preform, such as foggy crystallization, decline in IV, and yellowing of the product.

Due to the strong hygroscopicity of PET, drying treatment before processing is necessary. The recommended drying condition is 120~165 ℃, 4~6 hours drying treatment. In case of sheet extrusion line, the moisture content after drying shall be less than 200ppm; In case of bottle embryo production line, the moisture content after drying shall be less than 50 ppm; Conventional spinning is less than 50ppm, and high-speed spinning is less than 35ppm.
Generally, the higher the drying temperature, the higher the drying rate; The shorter the drying time, the lower the moisture content of raw materials after drying. The drying temperature of stirred crystallizer is usually between 120~160 ℃. The dehumidification and drying temperature is 160~180 ℃. The crystallization time is 1-2 hours, and the crystallization time is inversely proportional to the pre-crystallization temperature. The drying time is 4~6 hours to make the water content of PET close to or reach the equilibrium water content; Drying time is related to drying temperature, vacuum degree and moisture content of dry air.