Unlocking Advanced Surface Treatments: Introduction to Sol-gel Technology in Aerospace

The sol-gel process is a method for producing solid materials from small molecules. This involves transitioning a solution (sol) into a solid gel phase, forming a network or particles. This method can produce coatings, fibers and powders with properties like high thermal stability, corrosion resistance, and specific electrical or optical characteristics.

Sol-gel process is used different industries:

• Aeronautical
• Aerospatial
• Optical
• Micro-electronics
• Clothes

Sol-gel technology serves various applications:

• Functional coatings:
  • Adhesion promotion,
  • Paint adhesion,
  • Against corrosion,
  • Hydrophobic,
  • UV resistant,
  • Scratch resistant,
  • Fungicidal,
  • Acts as a coupling agent,
  • Resistant to chemical attacks,
  • Surface structuring,
  • Etc.
• Encapsulation
• Nanomaterials

In aerospace applications, sol-gel process main application is adhesion promoter in a paint system that protects against corrosion. Sol-gel solutions are compliant with the regulatory evolutions (REACh, RoHS, etc.). 

Rivet Rash:

Rivet rash is the selective loss of paint from rivet heads on aircraft which is a common and undesirable occurrence that affects the aesthetic appearance of the airplane. This condition happens due to the exposure of the fuselage to environmental factors that cause paint degradation.

Sol-Gel Application:

The sol-gel process is a non-chromated conversion coating used to improve paint adhesion on aluminum alloys. This method ensures enhanced protection and paint durability on aircraft surfaces.

SOCOGEL is a sol-gel product widely used in the aerospace industry. It is a type of sol-gel coating specifically designed for the protection of plane structures especially for the rivets rash prevention. It offers excellent adhesion and reinforces the corrosion resistance properties of the global system.

The main characteristics for sol-gel products are:

• Environmental-friendly: this chrome-free solution is beneficial for compliance with environmental regulations (REACh) and is Low-VOC
• Adhesion Promoter before bonding or painting (for example with MANKIEWICZ painting system): SOCOGEL enhances the adhesion of primers and paints to metallic surfaces, ensuring a more durable and long-lasting coating, crucial for aerospace components exposed to rough environments.
• Corrosion Resistance: as improving a long lasting coating , it helps protecting airplanes from corrosive environments, prolonging the lifespan of components.
• Lightweight: sol-gel are exceptionally lightweight, which is beneficial for plane efficiency and fuel economy.
• Industry Qualification: SOCOGEL is qualified by major OEMs including AIRBUS and BOEING.

SOCOGEL can be applied by 3 different methods:

• By spraying (as a mist not coating), the covering power is 30-40 m2/L
• By wiping with the SOCOSAT PPA60 wipes, the covering power is 50-60 m2/L
• By Touch-up application (pen application). Check out the preparation of the adhesion promoter pen

Before applying paint:

• Drying time before primer application
• 60 minutes at room temperature
• Or 15 minutes at 60 °C (use of an oven to speed up the drying step)

1. Precursor Solution: The starting material.
2. Hydrolysis and Condensation: These reactions transform the precursor solution into a sol (a colloidal suspension).
3. Gelation: The sol undergoes gelation, forming a gel (a 3D solid network in liquid phase).
4. Drying Process: solvent evaporation in order to concentrate the network.
5. Final Product: The resulting dried sol-gel (coating in a very thin layer, powder, etc.) is the final product.

SOCOMORE sol-gel called SOCOGEL enhances paint adhesion.
The interface between two layers is less than 1µm.

Based on epoxysilane, SOCOGEL enhances adhesion properties with epoxy.

• Enhanced adhesion and corrosion resistance. It replaces chrome VI adhesion promoter as alodine and wash primer.
• Environmental benefits: focus on the technology's compliance with environmental regulations like REACH, its role in reducing VOC emissions, and its non-reliance on chromium.

Environmental, Health and Safety  aspects:

• Operational efficiencies: process efficiencies such as reduced application time and the elimination of post-application rinsing.

Rivet rash is the selective loss of paint from rivet heads on aircraft, which detracts from the decorative appearance in visible areas like the nose and entry doors. Airlines dislike this because it makes planes look older within just six months of delivery.

Airlines and MRO paintshop need to use SOCOGEL:

• To replace chromated pre-treatment : Alodine & Washprimer
• To reduce rivet rash
• To optimise the process: time & cost saving

This system was implemented Mankiewicz in the US many years ago and, meanwhile, has successfully replaced alternative systems now being one of the main stack-up in the US market. Since incorporation of this system in the Aisbus´s SRM/CML we can observe a huge demand in the European market which we expect to continue to grow in the coming months.