Solvents in Composite Repair

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Questions and Answers

Why is compressed air not recommended for drying surfaces?

Because it is too slow a process

Because it may contain oil that can degrade the repair's durability (correct)

Because it is not effective in removing moisture

Because it can cause overheating of the surface

What is the main purpose of hand abrasion or grit blasting in surface preparation?

To remove moisture from the surface

To apply a subsequent treatment to the surface

To roughen the surface for the adhesive to 'key' into it

To generate a fresh, chemically active surface (correct)

What should be used to dry surfaces before bonding?

Compressed air

Hot air from a handheld hot air gun or an external hot air blower

A recirculating air oven

All of the above except compressed air (correct)

Why is it important to apply subsequent treatments soon after hand abrasion or grit blasting?

To prevent unwanted contamination or oxide formation Signup and view all the answers

What is the consequence of inadequate drying of the surface before bonding?

Micro-voiding in adhesive joints Signup and view all the answers

What should be done after water break testing?

Heat the surface to a temperature specified in the repair procedure or SRM Signup and view all the answers

What is the primary purpose of using Methyl Ethyl Ketone (MEK) in composite component repair?

Removing dust, grease, and mould release agents Signup and view all the answers

What is a critical requirement for solvents used in repair surface preparation?

They must produce no residue on the surface Signup and view all the answers

Why should chlorinated solvents not be used on titanium alloys?

They may cause stress corrosion cracking Signup and view all the answers

What type of wipe cloths should be used for wiping the surface with solvents?

Clean, lint-free white facial tissues or approved aircraft wipes Signup and view all the answers

Why must wipe cloths be changed frequently during solvent degreasing?

To prevent the transfer of contamination over the surface Signup and view all the answers

What is the final step in the solvent wiping process?

Performing a final solvent wipe with a new cloth Signup and view all the answers

What is the primary purpose of a hot patch bonding machine?

To perform two functions: applying atmospheric pressure and heat Signup and view all the answers

What is the purpose of a temperature recording unit in hot bonding units?

To record the temperatures of the curing process Signup and view all the answers

Why is it crucial to control the temperature during the curing process?

To achieve effective bonding results Signup and view all the answers

What is the purpose of a thermocouple in the curing process?

To sense the temperature of the part Signup and view all the answers

What is the purpose of the controller in hot patch bonding equipment?

To control the temperature at a constant set point and specific rate of change Signup and view all the answers

Why is it necessary to include permanent records of the cure cycle in the aircraft log book?

To maintain a record of the repair Signup and view all the answers

What is the purpose of applying a special UV-blocking dark paint to a component?

To reduce the degradation of resins by UV radiation Signup and view all the answers

What is the primary benefit of applying a white or light-coloured paint finish to fibreglass-reinforced plastics?

To reduce temperature rise through heat soakage Signup and view all the answers

What is necessary for polyurethane paint to provide effective protection against erosion?

A waterproof undercoat Signup and view all the answers

What is the effect of UV radiation on cured epoxy?

It causes the resin matrix to become more brittle and prone to cracking Signup and view all the answers

Why is it recommended to keep an aircraft out of the sun while it is on the ground?

To reduce the effect of heat soakage Signup and view all the answers

What is the benefit of using an opaque paint with a UV barrier to protect composites?

It protects against UV radiation Signup and view all the answers

What is the primary purpose of the water break test?

To evaluate the surface condition of prepared metals and composites Signup and view all the answers

What happens if the surface fails the water break test?

Solvent degreasing must be repeated before continuing with the preparation process Signup and view all the answers

What is the consequence of inadequate drying prior to application of adhesives?

Serious degradation of adhesive strength due to micro-void formation in the adhesive Signup and view all the answers

How is the water break test performed?

By spraying, pouring or squirting distilled water onto the surface Signup and view all the answers

What should be observed during the drying process after water break testing?

Water breaks Signup and view all the answers

Why is solvent degreasing repeated if contamination is detected on the final wipes?

To ensure the surface is properly cleaned by solvent action Signup and view all the answers

Study Notes

Solvent Properties

Solvents used for repair surface preparation must meet specific requirements, including:
- Readily dissolve surface contaminants
- Have a rapid evaporation rate to prevent re-contamination
- Produce no residue on the surface
- Have no deleterious effect on the material being cleaned
- Be of a guaranteed high grade of purity
Chlorinated solvents should not be used on titanium alloys to prevent stress corrosion cracking

Solvents Used

Methyl ethyl ketone (MEK) is used for removing dust, grease, and mould release agents from composite components
Acetone is used for general clean-up of tools and equipment and to clean composite parts after sanding as a pre-bond prep

Solvent Wipe Cloths

Only clean, lint-free white facial tissues or approved aircraft wipes should be used for wiping the surface with solvents
Wipe cloths must be changed frequently during solvent degreasing to prevent re-contamination
The frequency of changing cloths increases as the degreasing process progresses

Water Break Test

The water break test is used to evaluate the surface condition of prepared metals and composites
The test assesses the surface tension of water placed on the surface
The test is performed after solvent degreasing and prior to generating a fresh, chemically active surface
If the surface is properly clean, the water break test should pass; if not, solvent degreasing must be repeated

Drying Procedures

Drying is performed after water break testing and before application of adhesives
Inadequate drying can lead to serious degradation of adhesive strength and durability
Drying methods include:
- Using hot air from a handheld hot air gun or an external hot air blower fitted with air filters
- Placing the item in a recirculating air oven
Compressed air must not be used, as it can degrade the repair’s durability

Exposing Fresh Material

Following solvent degreasing, water break testing, and drying, surfaces are prepared for adhesive bonding by:
- Hand abrasion or grit blasting to generate a fresh, chemically active surface
- Applying subsequent treatment as soon as possible to prevent unwanted contamination or oxide formation
The procedure must be thorough, and the exposure time during this step is critical to the bonding process

Hot Patch Bonding Equipment

Hot patch bonding equipment applies atmospheric pressure and heat to the repair area
Heat blankets are used to control the temperature, and the rate of temperature rise and fall is critical
The controller allows for slow temperature rise, holds the temperature constant, and then allows a slow decline
Recording the temperatures of the curing process is essential, and permanent records must be kept in the aircraft log book

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Description

Learn about the use of solvents like Methyl Ethyl Ketone (MEK) and Acetone in cleaning and preparing composite components for repair. Understand their properties and engineering requirements.