Main points of PET injection and bottle blowing
The application of PET in beverage packaging has promoted the rapid development of beverage packaging industry. At the same time, the development of beverage packaging industry also provides development space for PET applications. The key to ensure the appearance and economy of PET bottle is to strictly control the PET blank and bottle blowing process. The characteristics of PET
PET is the product of the condensation of ethylene glycol and terephthalate, which is a thermoplastic polymer of saturation. The PET molecules have linear and semi-crystalline states.
The simplest procedure for producing PET is to form a monomer (esterification) of terephthalic acid and ethylene glycol, and then condensed the growth chain polymer PET. The degree of polymerization varies with temperature and pressure.
Like many plastics, PET has three state changes in the process of processing, namely, glass, high elastic and viscous flow. There are three temperature changes involved: Tg of glass temperature, Tc of crystallization temperature, melting point Tf.
The transition from the amorphous glassy to the rubber state is called vitrification, which indicates that the long chain begins to move. External heating can increase the number of molecules (link) degrees of freedom, and molecules that are solidified in glass can now move. The transformation of glass depends on the form of PET. When the characteristic viscosity (IV) is high, the freedom of the molecular chain is limited, and the Tg is higher.
With the increase of temperature, local spherizations are gradually produced, causing local molecular chains to be rearranged by intermolecular forces, which is crystallization. For PET, the maximum crystallinity is about 55 %, which is caused by the rearrangement of the aromatic ring, so the aromatic ring hinders the formation of the crystalline region.
If T < Tc, the viscosity of PET prevents the chain from moving into an orderly motion (no crystallization); T BBB 0 Tc, the thermal action interferes with the formation of amorphous regions. The melting point Tf is the temperature when all crystals disintegrate. Hydrolysis of PET and dryer
Solid PET is very easy to get wet from the air. When stored, PET will absorb moisture until it is saturated with environmental conditions. The saturation value can be as high as 0.6 %. Normally, PET is shipped at a supplier where the moisture content is less than 0.1%. In order to get the best product performance, it is necessary to reduce the moisture content to 0.004%, preferably 30ppm before melting. Resin if it contains moisture, even very low can also cause a series of reactions: when the temperature is higher than PET melting point (250 ℃), water can quickly cause polymer degradation (caused by the degradation of water chemical chain cut off), it will lower molecular weight, lower apparent viscosity and related physical properties. In fact, hydrolysis under low temperature (150 ℃) is starting to happen, but low speed, the velocity varies with temperature rise. In drying and forming conditions, the reduction of IV cannot be greater than 0.02 dl/g. The decrease of viscosity will cause the crystallization speed to increase, the opacity of the billet will be unfavourable, and the mechanical property of the bottle will decrease, and the strength and impact strength of the bottle will decrease. Thermal degradation
The influence of temperature on dry PET is complicated. It not only affects the diffusion speed of water vapor, but also affects the chemical process of drying, so it can affect the performance of the resin. Considering the potential of hydrolysis and thermal process is very necessary, as mentioned earlier, with IV drop, hydrolysis accelerated at above 150 ℃, because hot transformation process is faster than diffusion process,
Premature temperature rise in drying is unfavorable.
Also, even if most of the water vapor can be siphoned off, but too high temperature (over 180 ℃) will lead to thermal degradation and thermal oxidation () in the air drying system, in this way, the polymer chain rupture, also release a byproduct material, resulting in a decline in physical properties.
There are AA components in the by-products, and the changes in physical properties will be displayed on the flask, such as the fog crystallization, the decline of IV, the product yellowing, etc. The drying principle and basic properties of PET dryer
In a dryer with a desiccant bed, the air is first absorbed by moisture absorption desiccant, and a hot air blower presses dry hot air into the bucket. The return air is recycled through the desiccant. After being heated, the desiccant releases water vapor, which cools and absorbs moisture. Therefore, two separate gas paths must be minimized and desiccant is present.
Schematic of PET dryer system
In this closed-loop system, the drying unit is connected to the hopper by sealing pipe. The diameter of the main hopper cylinder is about 2:1, which must be insulated to ensure energy. Dry hot air to flow through the pressurized hopper and shunt core (shunt core is to protect the material and air flow), the top of the hopper closed, a single duct into the drying machine components, back on the loop filter to ensure desiccant is not contaminated. The blower drenched the air to the dryer bed, where it was dry, directly into the heating tube and finally into the hopper. At the same time, an independent fan and heater can regenerate the desiccant.
When the regenerated desiccant cools down, it is then switched to the drying system to dry the air.