Hello and welcome to a new post. Today we discuss the crystallization behavior of PolyArylAmide (PARA; PA MXD6), a semi-crystalline high performance Polyamide. Learn the ABCs of PARA here and the differences between PA and PARA here.
PARA combines excellent surface finish with high glass fiber loadings, outstanding stiffness (over 38 GPa possible), outstanding strength (>300 MPa possible), low creep, excellent flow (thin walls), and a CLTE which is similar to that of metal.
Crystallinity is Key
PARA's performance heavily relies on the level of crystallinity achieved during processing. Higher crystallinity leads to improved physical properties, dimensional stability, and high-temperature performance.
Semi-crystalline materials like PARA require temperatures above their glass transition temperature (Tg = 85°C for PARA) to crystallize effectively. Mould temperature directly influences the material's temperature during solidification. Studies indicate that a mould temperature around 120°C is necessary, especially for thin-walled parts, to maximize crystallinity. Figure 1 shows the relative crystallinity of PARA (PA MXD6) as a function of mould temperature [1]. The measured part had a thickness of 3 mm and samples from the core and from the skin where checked for crystallinity.
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| Figure 1: Relative crystallinity of PARA (PA MXD6) as a function of mould temperature [1]. |
DSC analysis as an effective tool to check crystallinity of semi-crystalline polymers
Differential Scanning Calorimetry (DSC) can assess crystallinity in an effective way. An exothermic peak in the DSC curve around 80-120°C signifies incomplete crystallization during injection moulding. Figure 2 shows a DSC curve of a fully crystallized and incomplete crystallized PARA. In the area of the Tg (85°C), an exothermal peak is visible, indicating that the tool temperature during moulding was too low. Sometimes lower tool temperatures are applied to decrease cycle time or to decrease shrinkage and warpage effects. However, this comes at the cost that the part which uses a high performance polymer, will not perform like a high performance polymer.
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| Figure 2: Comparing a DSC curve of a fully crystallized and incomplete crystallized PARA [1]. |
The maximum crystallinity of PARA can vary depending on factors like processing conditions, specific grade of PARA, and additives. PARA typically exhibits crystallinity levels ranging from 40% to 60%.
What are factors influencing crystallinity in general?
- Moulding temperature: As discussed, high mould temperatures (> 120°C) are crucial for achieving higher crystallinity.
- Cooling rate: Slower cooling rates generally allow for more complete crystallization.
- Nucleating agents: Adding nucleating agents can significantly enhance crystallinity rates.
- Increased water absorption: Higher amorphous content leads to increased water absorption, affecting dimensional stability.
- Post-Crystallization and distortion: Incomplete crystallization can lead to post-crystallization after moulding, causing part distortion.
- Higher creep: Parts moulded at lower temperatures exhibit increased creep behavior.
- Poor surface finish: Irregular surfaces and fiber appearance can result from insufficient mold temperature.
- Ejection issues: Very low mould temperatures can lead to extremely low shrinkage, hindering part ejection.
Melt temperature and residence time are important too
Apart from applying the correct mould temperature with PARA, checking the melt temperature and residence time is important too. Too high melt temperatures (>285°C) in combination with long residence time can lead to thermo-degradation. This is not only affecting the base polymer, in our case PA-MXD6), but also the processing additives and other additives. Aim for a residence time of 5 minutes, with a maximum residence time of PARA of 10 minutes.
If you are not sure about the residence time, you can easily estimate it with this calculator.
The upper DSC curve in Figure 3 shows again a fully crystallized PARA with two melting peaks. The first, larger melting peak belongs to the PARA base resin and the second smaller belongs to an additive which is used in the formulation (most probably a processing aid). The lower DSC curve shows PARA when exposed to high melt temperatures (>285°C), combined with too long residence times (>10 minutes). The second smaller melting peak disappeared indicating that termo-degradation did take place. This in turn has an influence on the final mechanical properties of the moulded PARA part. Also, it can happen that the additive melting peak is overlapping with the main melting peak of the PARA, making it harder to detect it.
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| Figure 3: Thermo-degradation of additives in the PARA compound formulation caused by too high melt temperature and residence time. |
Conclusion
High mould temperatures (>120°C) are crucial for processing PARA (PA MXD6) compounds. They ensure optimal crystallinity, leading to superior mechanical properties, dimensional stability, and overall part performance. Also, check from time to time the mould surface tempature to ensure that there is a match between set temperature vs. real temperature.
I hope this blog article has been helpful. If you have any questions, please feel free to leave me a message.
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Literature:
[1] https://www.syensqo.com/en/brands/ixef-para/documents
[2] https://www.findoutaboutplastics.com/2023/11/design-properties-for-engineers-abcs-of.html
