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Monday, 28 June 2021

Aliphatic and Aromatic Polyketones – Introduction, Performance Comparison and Applications

In this blog post we review the aliphatic and aromatic Polyketones. What are the differences and what makes them special?

Introduction

Let us get started with the structure of Polyketones. The key for understanding Polyketones is the ketone group. In general, a ketone group has the structure of R2R=O, where R stands for a replacement compound. Also, ketones have a carbon-oxygen double bond (= carbonyl group). The carbonyl group is important since it leads to a high polar behavior. Carbon has a slight positive charge and the oxygen has a negative charge. The strong attraction of the carbonyl groups to one another leads to an increase in melt temperature (255°C), compared to Polyethylene (140°C).

In the case of aliphatic Polyketones, the ketone group is part of the backbone, together with carbon monoxide (copolymer) or carbon monoxide, ethylene, and propylene (terpolymer).

Figure 1: ketone unit (above) and a monomer unit of Polyketone (below) [1]

Figure 2: Polyketone made out of ethylene and carbon monoxide [1].

In the case of aromatic Polyketones, the R-rests are constituted out of phenyls, together with the carbonyl group (Figure 3). Additionally, Polyaryletherketone makes use of the diphenyl ether groups too (Figure 4). The role of the aromatic ring structure is another key for understanding the high performance properties. High macromolecule stiffness, high heat and chemical resistance are the result of having ring structures in the polymer backbone [2]. Explanation for this behaviour are the double bonds of benzene which are not statically localized, i.e. electrons move along the carbon cyclic structure, which is expressed by the ring in the structural formula.

Figure 3: diphenyl ketone - basic structure of aromatic Polyketones.

Figure 4: diphenyl ether group.

Depending how the diphenyl ether and diphenyl ketone groups are aligned in the backbone, different glass- and melt temperatures are obtained (Figure 5).

Figure 5: change in glass- and melt temperature based on the amount of ether and ketone groups in the backbone.

Property Comparison and processing

In Table 1 selected physical, thermal, and mechanical properties of aliphatic and aromatic Polyketones are shown.

Table 1: property comparison of base and reinforced Polyketones.

Performance strength of POK and PEEK

Aliphatic Polyketones are engineering polymers which compete with aliphatic Polyamides, PBT, and POM (Acetal). They have a high impact strength (impact/fatigue twice of POM), combined with excellent chemical resistance towards Automotive fluids, hydrocarbon solvents, and salts. Wear and friction properties are better than POM and barrier properties towards Gasoline and Diesel are twice better compared to PA 12. For meeting UL V-0 flame retardancy level only half of flame retardant is needed compared to other engineering polymers. Also, due to fast crystallization rate and good flow properties, short cycle times are possible. In terms of material selection, Polyketone can be used for several applications in the Automotive, Electrical/Electronics, Appliances, Industrial, and Medical market. For a deeper look into applications and properties of Polyketone, I recommend checking out our guest interview with Doug Eom from Hyosung.

Aromatic Polyketones are semi-crystalline high performance polymers (definition according UL 746B) and offer exceptional performance over a wide range of temperatures. Polyetheretherketone (PEEK) has excellent chemical resistance, low moisture absorption, and good wear and abrasion resistance. Furthermore it has excellent heat distortion properties, a low coefficient of friction, good radiation resistance, low flammability, and good resistance to hydrolysis. Also, it has a high resistance to thermal aging and it is resilient when exposed to boiling water. In case of burning, it releases very low smoke and toxic gases. Based on the aforementioned excellent properties, PEEK is used for applications in aircraft, marine, automotive medical, food processing, defense, and oil and gas industries.

Apart from the different properties between aliphatic and aromatic Polyketones, cost per kilogram material also differs between aliphatic and aromatic Polyketones with a factor of 10 as a rule of thumb.

Final remarks

Depending on your application requirements, aliphatic or aromatic Polyketones may fit your needs in terms of material selection. Due to the ongoing material shortages in the field of engineering polymers such as Polyamides, PBT and POM, I expect that Polyketones will find their way into more and more applications, and this globally.

Thanks for reading and #Findoutaboutplastics

Herwig Juster

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