Hello and welcome to a new post. During material selection you may realize that high strength, stiffness, excellent surface properties, low water uptake and good processing is needed which cannot be fulfilled with the selected aliphatic Polyamide, then Polyarylamide (PARA) may be an excellent way forward.
Differences between PARA and PA
Table 1 compares the properties of PARA, PA 6.6, and PA 6 with a 50 wt% glass fiber loading, together with a PBT having a 30 wt% glass fiber reinforcement. PARA takes up 87% less water compared to PA 6.6 and contains high modulus levels also after moisture pick up. Apart from the low water uptake, PARA offers the best surface among all Polyamides due to its fine crystallisation in the surface regions. This makes it a good choice for coating or painting applications. The high modulus comes from the fact that PARA is a fairly large molecule with its aromatic ring structures which entangle. Furthermore, this kind of crystallisation allows during injection moulding to apply longer effective packing pressure and prevent sink marks.
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| Table 1: property comparsion of PARA, PA 6, PA 6.6 and PBT. |
Differences between PARA and PPA
If we compare PARA to PPA (Polyphthalamide) we see one major difference: the location of the aromatic rest with the double bonds. PARA is build up via polycondensation of an aliphatic dicarboxylic acid (adipic acid) and an aliphatic diamine with aromatic ring (1,3-xylylenediamine; MXD)
-PPA: aromatic rest is coupled to C=O
-PARA: aliphatic rest is coupled to C=O
This difference is shown in Figure 1.
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| Figure 1: differences in chemical structures of PARA and PPA. |
Pros of PARA
-low water uptake (<1,5%)
-high dimensional stability which enables complex parts
-high stiffness and strength (metal replacement)
-very good flow properties (like PPS) and thick parts without sink marks are possible
-PARA has low thermal expansion and it is similar to glass making PARA a good candidate for automotive interior applications. Polymers such PC/ABS and PPS do not have such low thermal expansion.
-crystallisation in injection mould takes place slowly with a fine crystal structure. This has the advantage of filling the part even in the packing phase and having a part with outstanding surface appearance although it has a high glass fiber loading (up to 60 wt%).
-thin wall moulding down to 0.5 mm is possible too
Cons of PARA
-it is not a typical high temperature polymer such as PPS and PPA, however for many metal replacement and aliphatic Polyamide temperature resistance levels are sufficient. PARA with 60 wt% glass reinforcement can compete with a PPS - 65 wt% glass and mineral filling up to 150°C as shown here.
-it has not the best UV resistance capabilities, however with proper additives is possible to fullfil certain Automotive UV standards.
-price range is between PA 6.6 and PPA.
Application examples:
-Metal replacement with PARA for healthcare applications
-Other application fields include air vents in car interior and buggy parts.
Thank you for reading and #findoutaboutplastics
Greetings,
Herwig Juster
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Literature:
[1] https://www.solvay.com/en/brands/ixef-para/properties
[2] https://www.findoutaboutplastics.com/2020/02/metal-replacement-with-polyarylamide.html
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