PET bottles are known for us, which belongs to the semi-crystal. It is combined by many sections of “modular” with the following chemical structure for each group.
Its inside structure looks like a chain with good flexibility. Compared with the metal, the array structure of metal, it looks like a big square combined by the array of many small squares due to its molecule structure difference.
Its 3D structure is also a stable structure without flexibility. However, the array of PET chain is not a stable one. It is in a plane form among the chain arrays with weaker binding strength. That is why the plastic is easier for extension compared with the metal.
The material for PET is in a non-crystal structure. In short, it possesses the structure of liquid but in a form of solid from its appearance, where the glass, mayonnaise and PET belongs to this type. When the molecule is lower than the glass transition temperature (Tg), the molecule chain shall freeze and lead to the hardening.
If the temperature is between glass transition temperature (Tg) and melting point (Tm), the molecule chain shall defreeze with the increase of flexibility. That is why it is necessary to bring the temperature of PET bottle preform up to this range while in bottle blowing.
Currently the plastic could be divided into two categories in the market:
thermosetting polymer and thermoplastic.
It means that it could not be softened by heating after shaping. This kind of plastic could only be molded once. It could not be collected for recovery like the epoxy resin
It is the plastics possessing the property of softening by heating, solidified by cooling and softening again, etc., which is a material for recovery like PET.
The method for crystallization of PET could be in 3
Crystallization from natural cooling, extension for crystallization and nuclei crystallization.
Crystallization from Natural Cooling (homogenized crystallization):
When PET is in the states of liquid, its molecule chain is not in an array. When it is cooled down, the chain shall start to be in the states of array and parallel accumulation as well as gradually forming as a spherical crystal, which could be started after the temperature cooling down to below Tm (melting point).
The speed for crystalline could be at the tope when the temperature is at 180oC and shall gradually stop afterwards. When the temperature is lower than Tg, the crystallizationwill bestopped. The structure of all molecule chains shall be formed. It is generally called semi-crystalline because it is mingled with crystalline and non-crystalline in the structure for PET.
Here is the structure diagram of the speed for crystallization and molecule at each point of temperature. Its main purpose is to indicate the formation of crystalline when it is lower than the melting point as well as the freezing of molecule structure is lower than the states ofglass transition temperature.
PET crystalline looks like in white because of its light refraction on the crystalline.
Is there any relationship between the crystalline under natural cooling states and blow molding machine manufacturer?
If we gradually cool down (natural cooling) PET from the liquid, it shall start to crystalline, which shows the states of crystalline under the states of solidtoo. Why the PET bottles we are using are still transparent? It is in relation to the quick cooling. Let’ssee what kind of reaction it will be for the molecule structure of PET during the process of ejection.
Step 1: Ejection
During this phase, the material is in the states of liquid because the temperature is higher than Tm. There is too much thermal energy on the molecule chain and very active. It is not possible to crystalline. The molecule structure at this point of temperature belongs to the states of non-crystalline.
Step 2: Quick Cooling
After the material ejection, the temperature of PET preform shall be cooled down in short time from 283oC to 80oC. The molecule reaches the temperature of the glass states because it does not have enough time for reaction. Its molecule chain keeps the states of non-crystalline in transparent when it is in a solid states.
The reason for non-crystalline PET preform keeping transparent is that the light could go through the structure of non-crystalline.
The crystallization of PET is because that molecule chain starts to array while in cooling down. Whether it is possible to make it in crystallization through the impact from outside? The answer is Yes. When the PET is pulled by the external force, its molecule chain shall be pulled straight. This is also a kind of crystallization.
This kind of crystalline through extension is usually the phenomenon in the textile industry. PET could also be used as the material for clothes, which is also in the states of non-crystalline for liquid through the extrusion and wire drawing so that the molecule chain structure could be in a forced array to become the crystalline. When PET is in wire drawing, the whole wire is a crystalline.
In the PET blow molding machine manufacturer, our PET is also extended during the bottle blowing but not to such extent. The bottles keep the states of transparency. Generally speaking, this kind of method for crystalline is not suitable for applying on the plastic package.
Nuclei Crystallization (heterogeneity crystalline)
This kind of method
for crystallization is different from the spherical crystals. Normally, it is
applied on the thermal bottles. It is basically the pure PET material mixed
with some catalyst to become a kind of mixed PET preform, which is also what we
mentioned thermal material or hot preform. The pure PET preform could achieve
the quickest crystalline speed at 1800oC.
But the PET preform is mingled with catalyst starts to grow the crystalline by using the nuclei of catalyst as the center at the temperature of 1100oC.
This kind of crystallization is somewhat different from the pure PET crystalline. The pure PET crystalline is in 3D spherical. Inside the academic circle, it is classified in the homogeneity crystalline. However, the crystalline for catalyst is in 2D plane form. Its crystalline property is classified as the heterogeneity crystalline.
Let us take the above figure as an example. Bottle Perform A is not mingled with catalyst. Even after the natural cooling, the growth of crystalline is still limited but each “impurity” could be the source of growing crystalline after the mingling of catalyst (Bottle Perform B). There shall be relatively more opportunity for growing during the natural cooling.
It is necessary to use the mold heater while in the bottle blowing so that the hot oil could heat up the mold to 180-900oC. During the bottle blowing, the nuclei of catalyst starts to crystalline.
The circle with dotted line in the figure below indicates the temperature for two kinds of crystalline growth:
This area is a 3D crystalline in spherical crystals with the temperature range for the quickest growth speed (180oC)
This is a 2D crystalline nuclei with the temperature range for the quickest growth speed (110oC).
There are 3 kinds of catalysts, respectively talc, sodium benzoate as well as ionomer (Ion, Na+), of which sodium benzoate is the most commonly used.
PET bottles possess the property of better tenacity, light in weight, air-proof, resistant to the acid and alkaline, etc., which is a commonly used as the container for the water, soda water, fruit juice, carbonate drinks, etc. But the PET bottles are of low resistance to the heat. It could not be used for holding the liquid of hot temperature. It is also necessary to avoid placing under the burning sun as well as the hot temperature environment like the car without air conditioner, etc.
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