Sheet Metal 2: Aircraft Sealants: Types and Applications

Aircraft Sealants Uses

• Aircraft sealants are utilized for bonding, pressurization, sealing fuel and fumes, and preventing corrosion.

Types of Sealants

• One-part sealants come prepared from the manufacturer.
• Two-part sealants consist of part A, the accelerator compound, and part B, the base compound.
• Two-part sealants require separate packaging and are always mixed per manufacture ratios, generally by weight, and are more often used.

Sealant Manufacturers

• There exist only a few manufacturers of aircraft sealant.
• Examples of manufacturers include PPG Aerospace (PRC/Proseal), Flamemaster (Chemseal), and Advanced Chemistry and Technology (AC/MC).
• Each manufacturer has its own identification numbers for sealants, requiring cross-referencing between manufacturers.

Sealant Applications - Family / Types

• 100 is for first-generation sealants.
• 200 is for second-generation sealants.
• 300 is for third generation sealants.
• 200/300 is for fuel tank sealants.
• 600/700 is for fuselage sealants.
• 700 is for corrosion inhibitor sealants.

Sealant Classification

• Class A sealants are thin viscosity.
• Class B sealants are high viscosity.
• Class C sealants are medium viscosity.

Class A Sealant

• Class A sealant has a thin viscosity between 150-500 Poise.
• Class A sealant is designed for brush application.
• Most U.S. specifications designate this grade as Class A.
• The typical consistency is similar to cold chocolate syrup.
• Poise is the unit of dynamic viscosity.

Class B Sealant

• Class B sealants have higher viscosity, ranging from 8,000-16,000 Poise.
• Class B sealant is designed for application by extrusion via a pneumatic Semco gun.
• Class B is used for forming fillets and sealing on vertical surfaces, where a low slump/sag is required.
• Class B sealants are used most of the time.
• The typical consistency is similar to peanut butter.

Class C Sealant

• Class C sealants are medium viscosity sealants ranging from 1000-4000 Poise.
• Class C sealant is designed for application by a roller or toothed spreader.
• This grade is most often specified for fay surface sealing.
• Often referred to as Class C or Fay Surface sealants.
• The typical consistency is similar to sour cream.

Application Time

• Working life is the time during which the sealant can be applied.
• Sealant may not adhere properly after the stated working life.
• Sealant can be applied in 1/2 hour, 2 hour and 4 hour intervals.
• Tack time is when the sealant is no longer sticky.
• Cure time is the time to become completely cured.

Applying Sealant - Surface Prep

• Surfaces must be clean and free of dirt, grease, and oils.
• Surfaces should be cleaned with a solvent like MEK.
• Apply solvent using a new, lint-free cloth kept saturated.
• Dry the surface with a second clean rag to prevent redepositing contaminants.
• Rivets should be soaked in solvent to remove oils.
• Avoid touching cleaned surfaces to prevent contamination from body oils.

Applying Sealant - Mixing

• Sealant must be mixed in a proportion of 10 parts Part A to 100 parts Part B by weight.
• If mixed by volume the ratio must be 8.3:100.
• Accuracy in measurement is key to proper curing.
• Each part should be stirred separately prior to weighing.
• Part B is added to Part A and mixed until a uniform grey color is achieved.
• Avoid white or black streaks in the mixture.
• Ensure no unmixed material remains on the sides or bottom of the container.

Applying Sealant

• Avoid vigorous mixing to prevent heating the mixture, which reduces working time.
• Pre-filled tubes or sem kits have the proper proportions measured.
• Follow the instructions in the tube during mixing.
• Do not mix more sealant than can be used within the working time.
• Working time is affected by temperature and humidity.
• The specified working time is based on 77°F.
• For every 20°F increase, the working time can be cut in half.
• For every 20°F decrease, the working time doubles.

Applying Sealant - Final Application

• The mixed sealant should be applied in a thin layer to surfaces to be sealed.
• Apply sufficient sealant to slightly ooze out of the edges of the joint when pressed together.
• The joint can then be riveted together normally.
• Excess sealant that oozes out should be smoothed into a fillet around the joint, ensuring that there are no voids.
• Apply a small dab of sealant to the shop head of each rivet.
• Any excess or smeared sealant can be cleaned up with solvent.