Tuesday 11 April 2023

Polyarylamide vs Polyamide (PARA vs PA): What are the Major Differences Between PARA and PA (Polymer Material Selection Tip)?

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 or MXD6) 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. Table 2 shows the dimensional change after moisture absorption of PARA, PPA, and standard PA. It can be shown that 50 wt% glass fiber reinforced PARA changes only 0.32% after 24-hour water immersion (at 23°C), where else other semi-aromatic polyamides changed twice and standard polyamides four times as much compared to the PARA value. Apart from the low water uptake, PARA offers the best surface among all Polyamides due to its fine crystallization 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 crystallization allows during injection moulding to apply longer effective packing pressure and prevent sink marks. 

Table 1: property comparison of PARA, PA 6, PA 6.6 and PBT.

Table 2: dimensional change after moisture absorption of PARA, PPA, and PA after 24 hours at 23°C, ISO 62 test
Another major difference between PARA and standard Polyamides is the surface finish. Moulded parts in glass-fiber reinforced PARA achieve a low surface roughness value of 0.10 mu Ra  (Figure 1) and standard Polyamides are around 0.25 mu. Mechanically polished steel has the equivalent surface roughness value as PARA. Among the Polyamides (aliphatic and semi-aromatic), PARA has the lowest surface roughness value. Reason is the fine crystallization of PARA in the surface layers of a moulded part. 

Figure 1: surface roughness of glass-fiber re-inforced PARA and standard Polyamides

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 2. 

Among the high heat plastics, PPA and also PPS play an important role; PPAs based on 6T/6I/66 have a Tg of 123°C, a 6T/6I has even a Tg of 133°C which makes them useful for high heat applications. 

PARA on the other hand has a Tg of 85°C and although you need for moulding tool temperatures of 120°C (for optimal crystallization), it is not a typical high heat plastic.

However, due to this small difference in the main chain, it has a high Young's modulus and makes it a perfect metal replacement material where optimal surface aspects are needed too. Here an example of a metal replacement for a children buggy where high strength, and stiffness as well as outstanding surface finish is needed. 

Figure 2: 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 fulfil 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. 

In conclusion, if you need a combination of low moisture uptake, high dimensional stability (complex parts), excellent surface, together with outstanding stiffness & strength, and very good flowability than PARA is the material of choice. 

Thank you for reading and #findoutaboutplastics


Herwig Juster

Interested to talk with me about your polymer material selection, sustainability, and part design needs - here you can contact me 

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[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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