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What are plastic extrusion crystals? 4 ways to teach you to easily control the crystallinity and crystallization quality of plastics!

2023-04-20 Page view : 19 views

The composition of plastics is a collection of many linear, elongated polymer compounds. The degree of crystallisation (crystallinity) is measured by the degree of regular arrangement of the molecules, and the degree of crystallisation can be measured by the reflection of x-rays. The structure of organic compounds is complex, and the structure of plastics is even more complex, and the structure of molecular chains (linear, bulbous, folded, spiral, etc.) varies a lot, so their structure also varies greatly depending on the forming conditions.

A plastic with a high degree of crystallinity is a crystalline plastic and the gravitational forces between molecules interact easily, making it a strong plastic. In order to crystallise and align properly, the volume becomes smaller and the shrinkage and thermal expansion of the mould becomes greater. Therefore, the higher the crystallinity, the less transparent it is, but the stronger it is.

Crystalline plastics have a distinct melting point (Tm), and when solid the molecules are regularly arranged, stronger and stronger in tension. When melted, the volume changes greatly, and after curing, it is easier to shrink, the internal stress is not easily released, the finished product is not transparent, the heat dissipation in forming is slow, the shrinkage after the production of cold moulds is larger, and the shrinkage after the production of hot moulds is smaller.
In contrast to crystalline plastics, there is another type of non-crystalline plastic, which has no obvious melting point, the molecules are irregularly arranged when solid, the specific volume does not change much when melting, it is not easy to shrink after curing, the finished product is transparent, the higher the temperature of the material the more yellow it is, and the heat is dissipated quickly during moulding.

The characteristics of crystalline plastics are as follows:

1. Temperature control method
a. the lower the melting temperature, the lower the melting temperature, the more conducive to homogeneous nucleation nucleus formation, increasing the crystal growth point, that can improve the crystallinity, but also can make the crystal size reduction. Therefore, in the specific processing process to ensure the premise of plasticisation, the melting temperature is slightly lower, which is beneficial to crystallisation.
b. cooling temperature cooling temperature on the crystallization degree and crystallization quality has the greatest impact, is the most effective party to control crystallization slow cooling, can make the plastic in the crystallization area stay longer, so that the crystallization degree is higher, but slow cooling but easy to produce coarse ball crystal, the toughness is not conducive to rigidity and hardness. Rapid cooling, on the one hand, allows the plastic to pass rapidly through the crystallisation zone, thus reducing the degree of crystallisation; on the other hand, due to the short crystal growth time, it also makes the crystallisation size finer, which is conducive to the improvement of transparency and toughness. In practice, whether slow or fast cooling is adopted depends on the product performance needs. If the transparency of the product is high, it needs to be cooled quickly; if the rigidity and hardness of the product are high, it needs to be cooled slowly.

2. Nucleating agent control method
The nucleating agent is mainly added to promote heterogeneous nucleation, increase the crystal growth point, make the crystallinity increase, and make the crystal particles finer, so as to improve the impact strength, yield strength and gloss.
There are three types of nucleating agents: inorganic, organic and polymeric:
a. inorganic nucleating agent inorganic nucleating agent to talc, including: CaCO3, mica, inorganic pigments, etc.. These nucleating agents have an impact on the transparency of plastics, and therefore should be limited in the amount of transparent products.
b. organic nucleating agents organic nucleating agents are: sodium, magnesium, aluminium, titanium and other metal aromatic carboxylates, organic phosphates, sorbic acid enzymes, etc.
c. organic polymer nucleating agent organic polymer nucleating agent for some high melting point of the polymer, such as vinyl cycloalkanes can be only in PP, etc..
It is worth noting that recently it has been found that nucleating agents can not only make the crystal size fine, but also determine the specific type of crystal.

3. Stretching control method
Stretching of plastic film and sheet products that have been crystallised can cause the crystals to break down and form smaller crystals, and form a series of crystals along the stretching direction, which can improve the toughness of the products and substantially increase the tensile strength, gloss, hardness, barrier capacity and other properties. Stretching method that can change the plastic crystalline quality, can also improve its crystallinity.

4. Heat treatment control method
Heat treatment on the one hand can further promote the crystallization and increase the crystallinity; on the other hand can improve the crystallization quality, so that the crystallization defects left behind by the hasty crystallization can be fully repaired.
Heat treatment can also make the crystallization of different types of mutual transformation. Such as PP products containing β crystalline, heat treatment above the melting point will all melt, recrystallization, will be transformed into α crystalline, while the proposed hexagonal crystalline in 70 C above heat treatment that can be transformed into α crystalline.

Take PA6 as an example, after heat treatment of its products, its various properties change as follows:
a. tensile strength in the heat treatment temperature of 120’180 ℃ and holding time of 10’120min, tensile strength with the increase in treatment temperature and holding time and the extension of the increase, the maximum change can reach about 10%.
b. impact strength in the holding time of 4h, treatment temperature from 120 ℃ to 140 ℃, impact strength decreased by nearly 60%. However, after the temperature exceeded 140°C, the decline was smooth. At a temperature of 180, when the holding time was extended from 10min to 30min, the impact strength also decreased by 60%. After the holding time exceeds 30min, the decline is flat.
c. hardness in a certain range, with the heat treatment temperature and the extension of the holding time, the hardness has slowly increased, the highest increase of about 10%.
d. crystallinity heat treatment can promote secondary crystallization, and thus can improve the crystallinity. In the insulation 4h premise with the heat treatment temperature increases, crystallinity rising; start slightly faster, more than 140 ℃, a little slower. In the heat treatment temperature of 180 ℃ premise, with the extension of the insulation time, the beginning of the crystallinity of the same; but insulation when more than 120 min, the crystallinity of the rapid increase.

How to determine the crystallinity in plastics?
There are three common methods: density method, XRD diffraction method and DSC method.
The DSC method is more commonly used and more accurate, followed by the XRD method, which, in addition to measuring crystallinity, can also determine the content of each crystal type according to the spectrum; the density method has the greatest error and is generally only used to compare the relative crystallinity of the same substance.
In addition to these three methods, there is also an infrared spectroscopy method, which is based on a simple calculation by spectrogram software and is rarely used.

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