Promising Applications for PEKK Powder Coatings

By Zak Eckley & Yash Adhia

For decades, polyaryletherketones (PAEKs) have offered some of the highest performance from a thermoplastic. These polymers have an outstanding combination of properties defined by excellent thermomechanical properties, chemical resistance, fire resistance, and tribological properties, making them an excellent choice for the most demanding applications.

In the family of PAEK polymers, the “backbone” of the monomers are ether groups and ketone groups which connect aryl rings. This structure is what gives PAEKs their impressive properties (Figure 1). PAEK polymers can be classified by their (K)etone/(E)ther ratio with a higher K/E ratio leading to a higher melting temperature, glass transition temperature, and overall performance. However, due to the higher melting temperature and crystallization of the high K/E ratio polymers, they can be limited by more challenging processing requirements and deployment into powder coatings (Figure 2). A solution to this is available with the polymer polyetherketoneketone (PEKK), making it especially useful for bringing PAEK properties to powder coating.

How PEKK is Different
PEKK has some of the most favorable properties for the end users in the PAEK class of polymers. It was originally developed by Dupont as a high-performance thermoplastic for the Apollo program in the 1960s. PEKK’s original success in aerospace and defense applications has since evolved into other markets including transportation, chemical processing, oil and gas, semiconductor and electrical. Figure 1 shows the typical polymer structure of PEKK material, where T and I represent terephthalic and isophthalic monomers, forming the backbone of the polymer. The T/I ratio is fine tuned to customize performance by producing a range of crystallization rates and melt temperatures, which will be discussed further below.

PEKK has a ketone/ether ratio of 2:1, giving it incredibly high strength, modulus, and thermal properties. PEKK offers a glass transition temperature (Tg) of 160-165 degrees Celsius (320-329 degrees Fahrenheit) with a relatively high modulus above the Tg. As can be seen in Figure 2, PEKK retains higher glass transition temperature while offering a range of processing capabilities as compared to other PAEK materials. These properties enable PEKK to be used in applications with high continuous use temperature, up to 250 degrees Celsius (482 degrees Fahrenheit).

PEKK is a high strength polymer: 120 to 140 MPa tensile strength, compressive strength of 140-180 MPa and a modulus of 3.6-4.4 GPa. It has excellent chemical resistance to nearly all organic solvents, and excellent resistance to acids, bases, hydrocarbons, salts, and steam, showing a retention of mechanical properties greater than 90%, after exposure.

PEKK is an inherently non-flammable polymer with excellent FST (fire, smoke, and toxicity) performance which meets the stringent performance requirements for aircraft. It is UL 94 V-0 rated without the use of additives.

Additionally, PEKK also benefits from excellent wear and friction properties and a low dielectric constant up through 220 degrees Celsius (428 degrees Fahrenheit), making it a great insulative powder coating for advanced high voltage electrical devices such as e-motors and busbars for electric vehicles.

Processability
PEKK is unique in the PAEK family as it has an additional tool to offer for end-users to tune the processability of the material, the T/I ratio. By engineering the location of the ketone on one of the aryl rings the melting temperature and crystallization rates can be controlled, which has a significant influence in powder coating applications. A higher T/I ratio will result in a polymer with a higher melting temperature and faster crystallization rate. However, as the ratio of the T/I is decreased, the melting temperature and crystallization rates also drop, allowing for higher working times during processing. A range of melting points (305-358 degrees Celsius/581-676 degrees Fahrenheit) and crystallization rates (t1/2: 2-32 minutes) can be accessed by varying the T/I ratio of monomer units without significantly altering the Tg, making PEKK a preferred material of choice for processes such as extrusion, thermoforming, 3D printing, and powder coatings.

PEKK powder coating is applied easily through conventional electrospray equipment to most metal surfaces. The pretreatment involves basic steps of degreasing and sandblasting. A primer coat is not required for PEKK-based powder coatings, owing to the strong adhesion of PEKK to metal substrates. The baking temperatures can be easily tuned between 320-380 degrees Celsius (608-716 degrees Fahrenheit) depending on the application, thus offering a wide processing window to the end user.

The user can choose to subsequently anneal the PEKK to introduce additional crystallinity in the finished surface to further enhance the finished properties of the coatings (Figure 3).

As coated, the PEKK polymer will be amorphous, which allows for a coating with more uniform coverage even for complex geometries. The amorphous nature of the coating also allows for easy application of additional coating layers to build up the required film thickness without having to worry about any buildup of residual stresses. Once the desired film thickness has been achieved, the parts can be subjected to an annealing step to introduce crystallinity. This also results in a change in the color from brown to opaque beige, giving users a convenient visual indicator on the progress of the annealing process. The rate of crystallization can be tuned depending on the shape and size of the substrate by the choice of the PEKK (a lower T/I ratio offers a lower rate of crystallization) and the annealing temperature (230-280 degrees Celsius/446-536 degrees Fahrenheit) to provide coatings with the highest performance and virtually no residual stresses. This is a unique advantage that is obtained from PEKK powder coatings in comparison with other thermoplastic powder coatings where fast, and often uncontrolled, crystallization during cooling can lead to build up of residual stresses causing defects such as craters and delamination.

PEKK powder coatings offer excellent thermal, mechanical, and chemical performance with a continuous use temperature as high as 250 degrees Celsius (482 degrees Fahrenheit). The film properties have excellent barrier and wear properties, including a Taber wear of 8-12 mg at CS-17 1kg/1000 cycles and zero blistering for salt spray of 2,000 hours; barrier properties to gases such as CO2 or H2S and chemicals including acids, bases, and organic solvents; and tribological properties including lower and controlled coefficient of friction at high temperatures and higher polarity. See Figure 4 for typical end-use properties of PEKK powder coatings.

A significant benefit of PEKK is that its unique structure allows users to work at much lower temperatures compared to other high performance thermoplastic powder coatings. For example, perfluoroalkoxy alkanes (PFA) and PEEK are common high-performance thermoplastics used in powder coating when both temperature and chemical resistance is required. However, these systems can require a primer to improve adhesion and high temperature processing (360- 400 degrees Celsius/680-752 degrees Fahrenheit) which can increase both processing and equipment costs. Other polymers such as ETFE and ECTFE offer excellent chemical resistance but have lower continuous use temperatures and still require a primer for coating.

PEKK also offers a significant price benefit in terms of economies of scale. The bulk density of PEKK is 1.27-1.29 g/cc, whereas the bulk densities of other fluoropolymers such as ECTFE, ETFE, or PFA range from 1.7-2.17 g/cc. Given a fixed film thickness, this allows for up to 40% less mass required to achieve the equivalent coating thickness compared to thermoplastic coatings.

One promising example of application of PEKK powder coatings are busbars used in electric vehicles. These parts are under stringent safety requirements and need to withstand harsh conditions which can occur during collisions. PEKK powder coatings have stable electrical properties over a very wide range of temperatures and frequencies and are well suited to provide electrical insulation for high temperature operation required in busbars and related EV mobility applications to prevent electrical break down and increase passenger safety.

The combination of PEKK’s end-use properties and easy processability also make it a very promising material of choice in applications such as pump impellers and vessels for chemical processing industries, mining semiconductor industries, and EV/mobility applications.

Zak Eckley is senior business development engineer, Kepstan for Arkema and Yash Adhia is lead engineer, PEKK at Arkema.