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.
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.
Based on the feedback of injection moulders, level of crystallinity of POM and PA66 are lower than the presented values. With a proper processing, one can reach with POM-C around 70%, around 5% more with POM-H and for PA66 33% in crystallinity.
Thank you for reading and #findoutaboutplastics
Best regards,
Herwig Juster
Literature
[1] https://www.hanser-elibrary.com/doi/book/10.3139/9783446443532
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