A Paint for wire wound Resistors
Silicone Resin based paints are recommended for coatings of wire wound resistors. These coatings have following advantages over conventional Epoxy and Vitreous Coatings.
- Curing possible at temperatures 150°C– 250°C, which are much lower than vitreous heat resistant coatings
- Readymade paint in wide range of heat resistant color pigments
- Resistance to humidity and corrosive atmosphere
- High continuous operating temperature
- Simplicity in application technique
| Property | Unit | Specification |
|---|---|---|
| Color & Appearance | -- | Bottle Green & Viscous |
| Viscosity by Ford Cup 6 mm at 27°C | Sec | 20 – 30 |
| Viscosity by BF 27°C | MPa.s | 2000 to 4000 |
| Non Volatile Content | % | 65 – 70 |
| Baking Test 180 °C/ 4 hrs | -- | OK |
Electrical Properties
- Class C Insulation
- Temperature with standing capacity up to 220 °C
| Property | Unit | Value | |
|---|---|---|---|
| Volume Resistivity | Wcm | Normal | 5.4 x 1014 |
| After Wetted | 5.4 x 1014 | ||
| Dielectric Strength | Kv/mm | Normal | 22 |
| After Wetted | 22 | ||
| Heat Resistance | °C | Normal | 220 |
Method Of Application
- The Silicone paint SP-91 is carefully formulated with heat resistant and electrical grade fillers and pigments. A special thixotropic agent is also incorporated for better coating behaviour.
- The coating is applied to the resistors by dipping, first in one direction and then in the other ( or in one direction only depending on the type of fixture used for coating ) till sufficiently thick coating has been obtained. Number of dips can be reduced by adding lesser thinner in the mixture. Too thick coating is however not suggested because of bubbles and pin holes formation in the cured coatings. Each coat is allowed to dry in air for a short time of 15 to 30 minutes before the coated resistor is transferred to oven for curing, Thixotropic coatings are best applied by vibrating either the fixture holding resistor or the bath containing compound. Automated system can be employed which gives consistent coating performance, due to controlled dipping, with drawl of resistor from bath.
Recommended Curing Schedule
- 1st dip air dry for 15 to 30 minutes oven curing 120°C /2h
- 2nd dip air dry for 15 to 30 minutes oven curing 120°C /2h
- 3rd dip (If required) air dry for 15 to 30 minutes oven curing 120oC /2h. Followed by 6-8h/220°C
- If instead of multi coats, single thick coat is to be given a carefully designed curing schedule is necessary to avoid bubbles or pin holes in the cured paint film. Generally, 80°C / 2h, 120°C / 2h and 220°C /6h is satisfactory
- The coated resistors could be given a clear silicone varnish for better glossy finish
- The resistors will pass dielectric, heat ageing and humidity tests as per the specification