Coates PTC 100 ink
By Dr W.J. Thatcher. Special Projects Manager

The resistance of the final component which is manufactured is important. However it should be recognised that this resistance can be affected by many things. The print thickness will affect the resistivity. For this reason it is vital that the thickness is controlled during printing. Thickness will be affected by screen mesh size, squeegee hardness and angle, snap offprint speed, print pressure, and room temperature which affects the ink viscosity. The screen should always be left flooded between prints to prevent drying out and blocking. From the formulators viewpoint the more stable PTC inks utilise in general resins which are more difficult to dissolve. Coates¹ policy prohibits the use of solvents classified risk R43 and above. This, while giving the manufacturer and the end user a greater degree of safety, limits the solvent choice open to the formulator .As a consequence we have to use a solvent which may dry more quickly than some other products. While this may have a small effect on the screen stability, it offers immediate benefit in faster drying and much more rapid stability of the ink¹s resistivity after drying, The PTC effect of the ink is directly related to the resistivity as can be seen in graph 2. The PTC ratio is defined as the resistance at a given temperature divided by the resistance at room temperature for a given ink. As can be seen this ratio varies almost linearly.

The ability of a PTC ink to recover from being taken to a high temperature is known as its relaxation time. The relaxation time i.e. the time to recover to the original resistance value, is dependent upon both the resin used in the ink and the resistivity. The more conductive the ink the quicker or shorter the relaxation time and the more stable the ink. The graph 3 illustrates the relaxation time of PTC100 compared with a competitors product. The PTC 100 is the lower of the 2 lines. It can be seen immediately how very much more stable this ink is.

The operating limits of PTC inks are set by two main factors. The temperature which is required and the breakdown point of the PTC ink when the electrical current reaches saturation point, i.e. the ink can no longer carry a higher current.

The temperature is controlled by the resistivity of the ink. Clearly a thicker coating will be less resistive and hence be capable of carrying a higher current. At a constant voltage of (say) 12 volts the maximum power - amps x volts =watts, will be higher.

At the same time because the resistance is lower the PTC ratio will also be lower, and so the cut off effect will be reduced. Most car wing mirror manufacturers are now aware of this and prefer the stability of a more conductive ink to the erratic and sudden changes brought about by high PTC ratios and long relaxation times.

The graphs opposite, 4 and 5, illustrate the temperature/time curve and maximum temperature/watts per sq. cm. curve for PTC100. The ink resistance has been selected and the ink has been formulated to operate to maximum stability at 12 volts Inks can be selected in this manner for any operating voltage for example up to 240 volts but IT IS VITAL that inks are not used beyond their performance limits. For this reason the customer and supplier must always work together to ensure specification meets performance.

Home | Contents | Carbon Inks | Carbon Inks-pcb | ACME | PTC 100 | Conductive | SunCare | Distributors