Sunday, 7 March 2021

Rule of Thumb in Polymer Processing: Crystallinity of Thermoplastics

 


Hello and welcome to a new rule of thumb blog post. Today we discuss how processing conditions are impacting crystallinity and as a consequence the performance of plastics parts.

Crystallinity only plays a role with semi-crystalline polymers such as polyamides and polyolefins. High-density polyethylene can achieve crystallinity levels of 85% and ranges among the polymers with the highest crystallinity. Allover, for most semi-crystalline polymers crystallinity ranges below 50%.

Mechanism of crystallinity

Main drivers for crystallinity are time and temperature. Formation of crystallinity starts below the melting point and stops below the glass transition area. As long the material is above the glass transition point, molecule mobility is given to form regions of crystallinity within the amorphous regions. Therefore, the most effective temperature window is below the melting point and above the glass transition point. Crystal formation and growth varies for each semi-crystalline polymer and there is an optimum temperature for growth. Slowest growth is achieved just below the melting point.

What to take care during processing (injection moulding)

In general, the faster the crystals form and with them the material modulus, the faster the part demoulding can take place. For optimizing the cycle time, moulders tend to lower the mould temperature, which in fact is counterproductive. Selecting the optimal mould temperature will result in high yield quality parts. Table 1 shows the crystallinity of selected polymers with their tool temperatures.

Table 1: Overview crystallinity and mould temperatures of selected semi-crystalline polymers


Advantages of high crystallinity

The high crystallinity results in high strength, stiffness, higher chemical resistance and a higher resistance towards environmental stress cracking (ESC). Furthermore, the modulus retention of unfilled semi-crystalline polymers above the glass transition temperature is higher compared to amorphous polymers. Unreinforced PBT has a modulus of 2340 MPa at room temperature and at 100°C the modulus still achieves levels of 330 MPa. Another example are polyamides: the water uptake occurs mainly in the amorphous regions. The higher the crystallinity, the lower are the amorphous regions with water uptake possibilities. As a result part dimensions will be kept more accurate.

Conclusions

Polymers such as PE with a glass transition below room temperature can handle colder mould temperatures better than engineering polymers which have glass transitions above room temperature. In the latter case, colder mould temperate will result in lower crystallinity and problems with part performance. 

Thanks for reading and #findoutaboutplastics

Best regards,

Herwig Juster

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Literature

[1] https://www.hanser-elibrary.com/doi/book/10.3139/9783446443532

[2] https://www.ptonline.com/articles/materials-a-processors-most-important-job-part-2


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