From Experiment to Final Print: Understanding Self-Regulation PTC Heaters

Thibaut Soulestin, PhD ; Lead Application Engineer at Henkel Printed Electronics; [email protected]

Henkel Adhesive Technologies has developed a large material portfolio of conductive inks and coatings suitable for printed electronics technology. Our portfolio offers material solutions ideal for various smart surface technologies, including self-regulating foil heaters. Self- regulating foil heaters are enabled by Henkel’s Positive Temperature Coefficient (PTC) inks in combination with silver and dielectric inks. Understanding the origin of the PTC effect, typical characterizations, and basic design rules enable our customers to reveal the full potential of this technology.

1 Introduction to Henkel Positive Temperature Coefficient (PTC) carbon inks

Different types of conductive polymer composites (CPC) exhibit positive temperature coefficient (PTC) properties. They have been extensively studied and some are commercially available. A vast majority is obtained by compounding a polymer binder with conductive fillers, mostly carbon-based.

The increase in resistance of the conductive networks during heating is caused by the thermal expansion of the polymer and the change in the distance between the conductive fillers. The PTC effect usually occurs during the phase transition of the polymer matrix, the glass transition, or the melting. After the maximum PTC effect, if the temperature keeps rising, a negative temperature coefficient (NTC) effect can be observed due to the re-aggregation of the conductive particles.

Henkel carbon PTC inks are like other screen-printable carbon inks and are based on three main components:

(i) carbon particles for the electrical conductivity,

(ii) a polymer binder for the mechanical properties and adhesion to the substrate, and

(iii) a solvent.

The dry ink layer is then obtained after drying and solvent evaporation.

Henkel developed a specific and patented technology using micronized wax particles to introduce the PTC effect. A fine wax powder is added as a fourth main component to the ink. The resistance will increase exponentially near the wax melting point, resulting in a high PTC ratio and a well-controlled self-regulation temperature. By finely controlling the melting point and the size of the wax particles, and thus the particles’ volume expansion, the PTC effect can be finely tuned to match customer requirements.

Table 1 overviews the commercially available and under-development Henkel PTC ink range. Low voltage inks are formulated to self-regulate at voltages below 50 V. High voltage inks, identified by HV in the name, can be used for voltages above 50 V. Low and high voltage inks differ mostly by their sheet resistance. A non-conductive ink, NCI, is also available for each self- regulation temperature, allowing the printer to adjust the sheet resistance.

Table 1. Henkel PTC ink range.

2 Typical Example of 9V, 60 °C, self-regulating demo heater

2.1 Layout

Figure 2 shows the exploded view for a typical PTC heater. The polyester substrate is an industry standard. A first layer of highly conductive silver ink tracks is printed, typically with LOCTITE ECI 1010. Two main areas can be identified: the busbars and the fingers. On the sides, the busbars carry the current and must be designed according to the maximum current peak to avoid local heating. The finer silver fingers give the interdigit structure and allow having all PTC elements in parallel. Then, the PTC ink, from LOCTITE ECI 8000 series is printed on top of the silver tracks as numerous independent elements. On top of the conductive inks, an insulating layer is mandatory. This layer can either be a printable dielectric or a laminated foil. As PTC inks are living materials with a variable resistance, the influence of the insulating layer should be closely monitored. It should not deteriorate the ink properties, such as PTC ratio and long-term reliability.

2.2 Typical Voltage Sweep Characterization

Voltage sweep curves are characteristic of one self-regulation in a specific condition. Taping a heater on different surfaces, like a metallic plate or a foam, will give different results. For characterization purposes, it is interesting to characterize the heater on an insulating foam to expose potential limitations like hotspots or printing inhomogeneities.

Figure 3 shows the resistance and average surface temperature change with increasing applied voltage. Three typical zones of a self-regulating PTC heater are nicely exemplified.

• Zone I, Heating-Up. Temperature increases with the increasing voltage thanks to the Joule effect until reaching the onset of self-regulation.
• Zone II, Self-regulation. With the increasing voltage, resistance increases thanks to the volume expansion of the wax particles.
• Zone III, Overruled PTC effect. The voltage is too high. The resistance cannot increase anymore. Temperature rises above the self-regulation, reaching the melting point of the wax. Melting of the wax brings the carbon particles closer and the resistance drop. Runaway may occur.

Applying 5, 8, or 10 V to the heater will give the same self-regulation temperature based on this voltage sweep. This is due to the sharp PTC effect around 60 °C. The higher the PTC ratio, the larger the self-regulation plateau. High PTC ratio of Henkel inks enables rapid heating than to high initial power.

3 Heater Initial Guidelines

3.1 Requirement Definition

To start your self-regulating PTC heater project, you must have some crucial primary information:

• The driving voltage that will power your heater. It will be closely related to the spacing between two silver fingers and the resistance of one PTC unit. The higher the voltage, the higher should be the resistance of the PTC unit. It is possible either by increasing the distance between the silver fingers or increasing the sheet resistance of the PTC ink.
• The initial heating power. To heat an object, you need a heat source with a higher temperature than the object and sufficient power to heat this object. If you have a low power hot heater, then the object will heat-up very slowly. Conversely, you may overrule the PTC effect if your heater is too powerful. Knowing the driving voltage and the initial heating power will allow the calculation of the resistance of the heater, ?? = ??2???.Voltage and resistance define the initial in-rush current ?? = ?????. Silver busbars must be designed according to the in-rush current.
• The self-regulation temperature to choose the adapted PTC ink.
• The heater dimension or available area.

It will also be important to identify the heater integration and heat diffusion behavior early in the project as it strongly impacts the heater design and layout.

3.2 Basic Design Rules and Calculations

4 Accelerating Customer On-Boarding

Developing self-regulating heaters requires a large range of expertise such as material, printing, circuit design, heat transfer, and integration. Following a methodology based on years of feedback enables a progressive learning curve and the identification of key parameters.

Figure 4 gives an overview of this methodology. If specific expertise is needed, Henkel team can bring external partners to the table for the success of customer projects.

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