Flexible materials with PTC effect

Self-regulating heating systems are just one of many possible applications for polymer based positive temperature coefficient (PTC) thermistors. To create an electrically conductive polymer it is necessary to incorporate a conductive filler into the polymer matrix with a filler concentration around the percolation threshold. The specific property of PTC materials is the sharp rise of electrical resistance at an adjustable temperature. The steady state temperature depends on the selected polymer matrix, the applied voltage, the dimensions of the component or the electrodes. Besides this, the PTC effect is reversible. The materials available on the market often have an insufficient flexibility and a poor adhesion to the overmoulded electrodes. In order to solve these issues, we need to focus on the development of flexible materials with PTC effect.

In the category of carbon-based fillers carbon black is the most popular one. Its price-performance ratio is unreachable for other members of this group like graphite, carbon nanotubes or graphene. Further examples for conductive fillers are silver, copper, nickel and many more.

The addition of an electrically conductive filler into a polymer matrix does not automatically provide electrical conductivity. The filler concentration has to reach the percolation threshold.

The composition of the polymer matrix also plays an important role with regard to the performance of electrically conductive polymers. The addition of different kinds of polymers to form a polymer blend can influence the location and distribution of the carbon black particles in the polymer matrix. In addition, the mechanical properties (i.e. hardness, tensile strength, flexibility), the rheological properties (i.e. flowability), the adhesion to other polymers or inorganic materials or the media and temperature resistance can be influenced through the setup of the polymer matrix.

Polymeric PTC materials are thermally sensitive resistors whose resistance increases sharply at a defined temperature. The graph of the electrical resistance versus temperature of a polymeric PTC material is schematically illustrated in figure 2. At low temperatures the carbon black network passes the electric current. With increasing temperature the polymer matrix starts to expand and the carbon black network gets separated. Less electrically conductive paths are available and the electrical resistance increases sharply (PTC effect). At high temperatures the polymeric PTC material behaves like an electric insulator. When temperature drops, the polymer matrix contracts and the carbon black network is formed again ln the end the PTC effect is a reversible phenomenon.

By applying voltage to the electrodes current flows through the system which leads to a rising temperature of the PTC material. Simultaneously the electrical resistance of the PTC material increases therefore the current passing through the material is decreasing. Finally the system reaches an equilibrium where it will stabilize at a defined temperature (steady state temperature). This specific temperature can be influenced varying the polymer matrix, by the applied voltage and/or the distance of the electrodes. The big of heating and self-regulation in one device which can be efficiently produced by an injection  order to make a statement about the reproducibility of the developed PTC compounds a climatic change test was performed. A test plate of VEL07116738 was produced. The test plate was stored in a refrigerator at -20 °C and actively heated by applying 12 V every four hours. No noticeably changes in the steady state temperature appeared after 50 cycles. Since the electrodes were not protected against humidity cycling to be realized with properly sealed proto type process.

It has been shown that self-regulating heating systems can be effectively produced by over moulding electrodes with a thermo plastic PTC compound. Such compounds can be formed from a thermoplastic matrix comprising a switching material and a matrix material such as HDPE, PP ( PA, or PBT and conductive carbon black. Since housings for water-based liquids are often based on HDPE, this matrix material was chosen as basis for a heating system with steady state temperatures between 50 °C and 90 °C  at applied voltages be has been shown that self-regulating heating systems can be effectively produced by electrodes with a thermo plastic PTC compound. 

The advantages of a chemical inert material with the effective of a PTC compound allow the construction of housings with integrated heater in a very efficient way. Future work will concentrate on optimization of adhesion to the electrodes for the moulding process and applying electrodes 。 A recent new project is a PTC compound development which has sufficient permeability for radar radio waves, in order to construct a housing  between 8 V and 32 V. Future work will concentrate on optimization of adhesion to the electrodes for the moulding process and applying electrodes onto  thermoplastic PTC materials.

Source: TPE Magazine