Friday 10 November 2023

Design Properties for Engineers: The ABCs of Polyarylamide (PARA; MXD6)

Hello and welcome back to another post on design properties for polymer engineers. Today’s session is dedicated to the high performance polymer PolyArylAmide (PARA or MXD6 or PAMXD6). Knowing about such a polymer can be an advantage during your next polymer material selection project, especially when dealing with high strength, stiffness, and cosmetic good appearance applications.

Introduction and structure - What are Polyarylamides? 

Polyarylamides belong to the group of  semi-aromatic Polyamides and have a glass transition point (Tg) of 85°C and a melting point of 235°C, and melt temperature of 280°C. They combine several interesting properties: 

-Lowest moisture uptake among aliphatic and semi-aromatic Polyamides

-High dimensional stability enabling complex parts

-Excellent surface appearance - “best-in-class” among the Polyamides

-Outstanding stiffened and strength

-Very good flowability which allows moulding of thin walls down to 0.5 mm and also for thicker walls without sinkmarks

Figure 1 shows the chemical composition of the PARA repetition unit and Figure 2 shows the DMA curve of a 60 wt% glass-fiber reinforced PARA (in comparison to PA 6.12- GF 60 wt%). It can be seen that up to the glass transition point, high modulus values of 18 GPa can be achieved followed by a decrease to a level of 10 GPa at 130°C. Long-chain aliphatic PA 6.12 can keep the high modulus of 16 GPa until 40°C and then the decrease takes place. Furthermore, it can be seen that PARA is not a typical high heat thermoplastic, it is more a high performance plastic, combining the aforementioned key characteristics.

Figure 1: chemical structure of PolyArylAmide (PARA; MXD6).

Figure 2: DMA curve of a 60 wt% glass-fiber reinforced PARA in comparison to PA 6.12- GF 60 wt%.

Characteristic properties of PARA

Let us circle back to the low moisture uptake. Comparing the moisture absorption of PARA, PPA, and standard aliphatic Polyamides, it can be shown that 50 wt% glass fiber reinforced PARA changes only 0.32% after 24-hour water immersion (at 23°C), whereas other semi-aromatic polyamides change 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. Moulded parts in glass-fiber reinforced PARA achieve a low surface roughness value of 0.10 mu Ra  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.

The high stiffness and strength of PARA results from the fairly large molecule with its aromatic ring structures which entangle, together with glass fiber reinforcement. Table 1 compares mechanical properties of PARA to die casting metals. The mechanical performance of PARA favours it for metal replacement of applications which need high strength and excellent surface appearance. 


Table 1: Overview mechanical properties of Polyarylamide (PARA) compared to die-casting metals.

Properties PARA-GF 50 wt% PARA-GF 60 wt% AG6 (Al) AS9U3 (Al) ZAMAK (Zn) AZ91D (Mg)
Density (g/cm3) 1.64 1.77 2.7 2.9 6.6 1.83
Melting temperature (°C) 235 235 660 660 390 470
Tensile strength (MPa) 280 280 220 200 280 235
E-Modulus (GPa) 20 23 65 72 85 45
Elongation at break (%) 1.7 1.8 0.2 0.2 0.2 3.0

Design properties of PolyArylAmide (PARA)

Tensile Strength vs. Tensile Modulus
Figure 3 compares the tensile strength and tensile modulus of a PARA-GF 50wt% and a PARA-GF 60 wt% to die casting materials. If we now add the specific strength, it can be shown that it is higher up to two to three times compared to metals such as brass, zinc, and magnesium. 

Figure 3: comparison specific strength of PARA with glass fiber reinforcement to die-casting metals.

Creep Resistance at Elevated Temperatures
Figure 4 compares the creep performance of a 50 wt% glass fiber reinforced PARA to Zinc and Aluminum alloy. The data indicates that PARA deforms less than 1% after 1,000 hours (50 MPA and 50°C). PARA offers lower creep than most engineering polymers with similar reinforcement levels, as well as some metals. 

Figure 4: comparison creep resistance of glass fiber reinforced PARA to Zinc and Alumnium alloys [1].

Coefficient of Linear Thermal Expansion (CLTE) of PARA
Table 2 shows the CLTE value of a 50 wt% glass-fiber reinforced PARA, as well as metals such as steel, aluminum, brass, and zinc. 

Table 2: Coefficient of Linear Thermal Expansion (CLTE) of Polyarylamide (PARA) compared to die-casting metals.

CLTE (10-5 K-1) PARA-GF 50 wt% (flow direction) PARA-GF 50 wt% (transverse direction) Aluminium Brass Zinc Steel
/ 1.5 3.6 2.4 1.8 3 1.2

Warpage behaviour
Comparing the warping behaviour of PARA to standard Polyamides, a lower warping tendency of PARA can be found. Reason is the lower anisotropic behaviour of PARA which can be further improved with special mineral filler. 

Chemical resistance of PARA
Polyarylamides have an inherent good chemical resistance, however due to the amide group they show restrictions to some chemicals. Fast degradation takes place when exposed to powerful oxidants such as O3, and Cl2, as well as highly concentrated mineral acids such as H2SO4, and HNO3. Diluted mineral acids, acetic acid, and formic acid let to degradation at ambient temperature. Strong bases (KOH, NaOH, etc.), most organic acids, and formaldehyde lead to degradation of PARA at high temperatures.

Thermal and electrical properties of PARA
Apart from the DMA shown in Figure 2, PARA with 50 wt% glass fiber reinforcement has a heat deflection temperature (HDT; 1.8 MPa) of 230°C and an electrical Relative Thermal Index (RTI) of 130°C (at 1.5 mm). CTI of PARA-GF 50 wt% is PLC 1 class (500 V), volume resistivity is 1E+8 ohms cm  and dielectric strength is 28 kV/mm. Flame rating is HB at 0.75 mm.

Production of PARA
PARA is made by polycondensation of an aliphatic dicarboxylic acid (adipic acid) and an aliphatic diamine (1,3-Xylylendiamine).

Processing of PARA
Polyarylamides can be very well processed by injection moulding. PARA is not a typical high heat polymer, however the tool temperature during processing needs to be kept at 120°C. Advantage is that cyrstallization starts late during the moulding process, allowing to fill the cavities during the packing phase. PARA is a huge molecule, in comparison to other Polyamides, and therefore needs time to crystallize. It creates fine crystals, especially on the frozen upper and lower layer of the cavities and hides the glass fiber underneath. Therefore, a cosmetic surface with high glass fiber loading (50-60 wt%) can be achieved. Thin part moulding as well as complex part moulding is possible with PARA too. Figure 5 shows the spiral flow length of PARA at 1 mm thickness and 1000 bar injection pressure. It can be shown that PARA with glass filler loading flows as good as PPS (with glass and mineral filler), and 45% better than a standard Polyamide 6 with glass fiber reinforcement. There is also the risk of flash generation if venting and clamp force are not optimal set. Additionally, a fast way to estimate the cooling temperature of high performance polyamides is by squaring the wall thickness "w" and multiplying it by 2.5 (cooling time = 2.5 x w^2). Overall residence time in the barrel should beno longer then six minutes. 

Figure 5: spiral flow results of PARA- GF 50 wt% in comparison to PPS and PA 6. 

Applications and end markets
Applications and markets of PARA range from appliances (f.e. handle of coffee machine, oven handles, coffee drip grids, and external housing components, combining structural and cosmetic requirements), medical devices (single-use surgical instruments f.e. inserters, curettes, needle holders, forceps, retractors and extractors.), aircraft (seating components), automotive (metal to plastic shift levers, painted parts) and electrics & electronics (mobile phone, tablets, and laptop components).

Commercial grades 
Table 3 shows the major manufacturers and commercial available compounds of PARA. Most of them offer glass-fiber reinforced grades (between 30 wt%-60 wt%), mineral filled grades, UV-light resistance grades, and flame retardant grades. Also carbon fiber reinforced grades are available. 

Table 3: Main producers of PARA and commercial compounds.

Manufacturer Commercial Name
Solvay Ixef®
Mitsubishi Engineering Plastics, MGC Reny®
Akro Compounds AKROLOY®PARA
TER Plastics TEREZ®GT2

Conclusions
This high performance polyamide combines several key properties: it has a metal-like strength (tensile modulus of 22 GPa at 50% glass loading, room temperature), high dimensional stability, takes up 87% less water compared to PA 6.6 and contains high modulus levels also after moisture pick up. It has the best surface appearance among all glass-fiber reinforced polyamides (resin rich surface due to fine crystallization). In addition, its excellent flow ability (similar to PPS) and ability to realize complex part geometries makes it a favorable material candidate during your material selection. 

Further data and reading
I published several interesting posts on PARA and in this post we discuss the difference between PARA and aliphatic Polyamides:

What to do next with all this information on PARA?
Now it is time to put the PARA and its data into practice. If you have a project where PARA may be a fit, I invite you to reach out to me and I will support you in the material selection process. You can enter a request over this form here or leave me a message here. 

Thanks for reading & #findoutaboutplastics

Greetings,

Herwig Juster

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