Phenolic Resin-Coated Insulating Paper: A Hidden
Hero in High-Voltage Applications

What Is Phenolic Resin?

Phenolic resins (PR) are synthetic polymers generally obtained by the reaction of phenol with formaldehyde. Not many transformers user even transformer manufacturer realized that some types of insulating paper used in electrical applications are coated or impregnated with phenolic resin.

What are the Role of Phenolic Resin in Paper Insulation ?

Phenolic resin is typically used to:

• Enhance thermal stability: Helps paper withstand high operating temperatures without degrading

• Improve mechanical strength: Makes the paper more durable and resistant to tearing or deformation

• Increase moisture resistance: Protects paper from absorbing water, which can reduce insulation quality.

• Provide better dielectric properties: Reduces the risk of dielectric breakdown under high voltage.

  
  

What are Potential Issues Without Phenolic Resin?

• Reduced thermal endurance, especially in high-temperature environments.

• Lower dielectric strength, which may compromise insulation performance.

• Susceptibility to moisture, leading to degradation over time.

• Mechanical weakness, especially under compression or bending.

  
  

The absence or poor distribution of Phenolic Resin can be seen when there is possibility of white spots on paper insulation (one of the root causes).

How to detect phenolic resin in insulating paper? Determining the presence of phenolic resin in insulating paper involves a combination of analytical techniques and material testing. Here are the most common methods: FTIR Spectroscopy (Non-Destructive) Solvent Extraction + Gravimetric Analysis Thermogravimetric Analysis (TGA) Differential Scanning Calorimetry (DSC) Chemical Staining or Dye Penetration

EtODS Experience in Detecting Phenolic Resin Using FTIR Spectroscopy