PEEK Film Processing Guide

Polyetheretherketone (PEEK) is a linear aromatic polymer material and the most intensively studied and widely commercialized representative of the polyaryletherketone (PAEK) family.

PEEK is a model of comprehensive performance among special engineering plastics, and its status is comparable to the top of the pyramid of the plastic industry. Its outstanding properties include excellent mechanical properties, heat resistance, flame retardancy, wear resistance, corrosion resistance, hydrolysis resistance, peeling resistance and biocompatibility.

PEEK film is another important application form of PEEK besides being used as engineering plastics and fibers.

1. Basic properties of PEEK film

PEEK is a semi-crystalline polymer, and the transparency and physical properties of the film are closely related to the polymer molecular chain structure.

PEEK film is opaque brown in a highly crystalline state and transparent light brown in an amorphous to low crystalline state.

The density of PEEK film is 1.26~1.30g/cm³ (amorphous density is 1.26g/cm³), the glass transition temperature is 145°C, the cold crystallization peak temperature is 174°C, and the melting point is 341°C.

Typical Victrex PEEK film properties

2. Factors affecting the performance of PEEK film

Heat treatment

The PEEK film with a transmittance of 84.6% and a thickness of 95μm was heat treated at 250°C for 20 minutes. The figure below shows the wide-angle XRD spectra of the PEEK film before and after heat treatment.

PEEK belongs to the orthorhombic system. Before heat treatment, the film has a low degree of crystallinity. After heat treatment, the film shows four main diffraction peaks at 18.7°, 20.6°, 22.6° and 28.6°, corresponding to the 110, 111, 200, 211 and 202 directions, respectively, and the peaks in the 211 and 202 directions overlap.

The experimental data are consistent with the literature reports. Since PEEK is a semi-crystalline polymer, the amorphous region helps to obtain a higher diffraction peak background.

Tensile orientation

The figure below shows the SEM photo of the cross-sectional structure of the PEEK film.

The roller temperature of the PEEK film prepared by the extrusion casting method is higher than the glass transition temperature of PEEK. Since the casting roller has a certain speed, the film will experience a certain amount of stretching in the longitudinal direction.

From the figure, it can be observed that the longitudinal cross-section of the film shows high orientation of the polymer molecular chain along the stretching direction.

Film crystallinity

The temperature and speed of the casting roller directly affect the structure, morphology and thickness of the film. When the film thickness is the same, the higher the crystallinity, the lower the transmittance.

The haze of the film can also reflect the color state of the film. The higher the crystallinity, the higher the haze value. When the crystallinity is less than 5%, the film shows high transparency.

3. Processing technology and equipment of PEEK film

Preparation technology of PEEK film

Melt extrusion casting method: PEEK film is prepared by melt extrusion casting through a T-shaped die. The die temperature must be above 400°C. The extruder screw can adopt the screw structure for PA or PP film. In order to make the PEEK film have excellent properties such as high tensile strength, it can be stretched in one direction or two directions.

Blown film method: PEEK film can also be prepared by extrusion blow molding, which can obtain thinner films, but it is difficult to obtain wide films.

Screw and barrel

Recommended screw parameters:

1. Aspect ratio L/D: 24:1~30:1

2. Feed section: >8D

3. Compression ratio CR: 2~3

Recommended screw type

Barrel heating

The barrel heating coil must be able to maintain 400°C. It is recommended to use at least four independent heating zones, each with independent thermocouples and proportional integral differential (PID) controllers for precise temperature control. The temperature should be controlled within ±2°C.

The typical barrel temperature setting is shown in the figure below. The specific setting should refer to the processing parameters of the specific specifications.

Barrel Capacity and Residence Time

The size and output of the extruder should be matched, and ideally, the residence time is less than 30 minutes.

Running at low screw speeds (<10rpm) will result in longer residence times during the extrusion process and increase the chance of gel degradation. All internal surfaces should be free of "dead corners" and cleaned and polished before extrusion begins.

Screens and Perforated Plates

The perforated plate in the die helps to create back pressure in the screw and stop the rotation of the melt. The hole size should be proportional to the extruder size and designed to eliminate as many dead corners as possible. The holes should be chamfered to enhance flow.

Typically, a filter group is placed in front of the filter to remove any residual impurities in the material. If the filter screen is too fine, it will produce excessive shear and pressure drop in the melt.

Connecting the die and die

Dead corners/low flow can cause localized degradation, resulting in discoloration of the material or black spots in the melt. Therefore, the flow through the die, die and die should be streamlined, the cross-sectional changes should be gradual, and the joints must be aligned without gaps. The system can be simulated to ensure there are no low flow areas.

It is recommended to use individually controlled heaters for each part (die head, flange, die head) and to apply insulation materials whenever possible.

Slot dies are commonly used to process natural PEEK sheets, and controlling the temperature of the die lip is critical for good surface finish and dimensional control.

Thin sheets (<500μm) can be produced in semi-crystalline or amorphous forms by controlling the temperature of the roller. Roller temperatures below Tg will produce amorphous transparent films, while temperatures above 170°C will produce opaque semi-crystalline films. Thicker films can crystallize under their own retained heat.

4. Secondary processing of PEEK film

PEEK film can be processed by a variety of secondary processing methods, giving engineers the flexibility to take advantage of all the high performance of PEEK.

Coating and metallization

Using conventional coaters, a variety of materials can be coated on PEEK film, including silicone or acrylic adhesives, B-stage heat-activated adhesives, hard coatings, and printable label topcoats. Surface energy modified PEEK film enhances adhesion and supports more coating options.

PEEK film supports a variety of metallization processes, including vacuum deposition, sputtering, chemical deposition, and direct thermal bonding of metal foils. PEEK film can also be bonded to many metals (including aluminum, copper, and stainless steel) without the use of adhesives.

Thermal Lamination and Thermoforming

As a thermoplastic material, PEEK film can be bonded to a variety of substrates such as metals, fabrics, and other reinforced and unreinforced polymer films by batch thermal lamination or "roll-to-roll" continuous thermal lamination processes.

When designing custom parts, PEEK film can be processed into parts of various shapes using a thermoforming process.

Temperature and time are key processing factors in the thermoforming process, which determine the crystallinity of the film and affect the performance and dimensional stability of the film. Different thickness films require different processing temperatures and times, refer to the table below.

Annealing of films

It is important to note that if a part is not fully crystallized during the molding process, the dimensions of the part may change during the subsequent heating process.

Post-processing annealing steps should be performed as part of the part process development to determine that the part has achieved satisfactory dimensional stability.

Die-cutting and stamping

PEEK films can be die-cut and stamped to design gaskets, liners and components of different shapes and sizes.

Although water-conducting laser cutting processes have proven to be very effective for PEEK films, most processors still use mechanical processing methods.

Welding

PEEK films can be welded to a variety of substrates, whether using heat, ultrasound or laser.

When welding, the film is heated to the melting point and appropriate pressure is applied to bring the molten polymer surface into close contact with the substrate to complete the weld.

Heat sealing

The film is heated to the melting point and pressure is applied to bring the molten polymer into close contact, which can easily achieve sealing.

The typical heat sealing process for films is heating to 340~350°C, sealing pressure of 4 bar for 2.5 seconds, and cooling to below Tg under pressure.

Ultrasonic welding

Ultrasonic welding uses high-frequency sound energy to increase the temperature of the PEEK film to melt it. The surfaces to be joined are brought together under pressure and then exposed to ultrasonic vibrations to melt the polymer surfaces in close contact.

Welding is performed using a frequency of 10~40kHz with a cycle time of <1 second. This process is particularly suitable for films >100μm.

Laser welding

The working principle of laser welding is to generate an intense beam of electromagnetic radiation, which causes the molecules in the film to resonate, causing local heating and melting of the material to form a bond.

Laser welding can be achieved using CO2, YAG or diode lasers, and is not suitable for thicker opaque films.

V. Application scenarios of PEEK film

PEEK film is mainly divided into two categories, namely high crystalline PEEK film and low crystalline PEEK film. The surface morphology of the film can be divided into three different effects: glossy/glossy, glossy/matte and matte/matte. Special types also include carbon fiber reinforced and mineral reinforced films to meet the application requirements of different customers on different products.

Consumer Electronics Speaker Diaphragms

Early applications for PEEK film were planar transducers, compression driver edge suspensions, and high-end speakers. Mineral-filled PEEK diaphragms provide both high damping and better acoustic signature matching with woofers.

Most headphone drivers are one-piece diaphragms of PET polyester, approximately 25μm thick. On top of the thin film diaphragm are two very thin layers of PEEK film laminated to the damping layer. In the high-end headphone diaphragm market, flagship products from Sennheiser (HD-800 and HD-820), Bayer (T1), and AKG (K701 and K702 Variomotion) all feature laminated PEEK diaphragms.

PEEK has become the product of choice for use in more than 4 billion smartphones because of its near-infinite fatigue life and consistent extrusion thickness, which avoids rocking modes at elevated excursions.

Other markets where PEEK could be considered include laptops/tablets, car audio, TV/soundbars, smart speakers, headphones, and true wireless earbuds.

Supercapacitor, power battery insulation layer

PEEK film has excellent moisture resistance, high temperature resistance and chemical resistance, which can reduce the loss of comprehensive performance. The thin and tough film can withstand high power density and optimize the manufacturing process.

5G antenna substrate, FCCL board

The dielectric constant of PEEK film is very stable and low in a wide temperature and frequency range, so it is often used in high-performance composite materials (laminates composited with glass cloth, carbon fiber reinforced plates, etc.), FPC circuit boards, temperature regulating films, heat-resistant electrical insulation tapes, switch and sensor barrier films, etc.

Aerospace insulation

PEEK film is used in thermal insulation and sound insulation systems in aerospace, which helps improve thermal management and improve fire, smoke and poison (FST) performance. Compared with PVF film, the weight can be reduced by up to 60%.