Adhesives provide strong and durable bonds between composites or between a composite and another structural material.
In addition to sealing joints and surfaces, adhesives distribute stress more evenly.
Strong adhesive bonds require a joint designed to avoid excessive peeling stresses. This means the mating surfaces should be properly prepared and the recommended adhesive type is used.
We recommend three types of epoxy adhesives
- MMFG E-502 or E-504 kits
- Shell 828 with N40 hardener
- Dow D.E.R. 331 w/ D.E.H. 58 hardener
The E-502 and E-504 epoxy adhesive kits are available from MMFG. They are especially easy to use as the epoxy base is mixed with the hardener by volume, in equal parts.
If using the Shell 828 epoxy and V-40 hardener system or the Dow D.E.R. 331w/D.E.H. 58 hardener system, refer to the manufacturer’s mixing recommendations.
Adhesive joints are strongest when stressed only at a right angle to the joint’s plane.
In compressive or tensile shear, the joint is subjected to directional stress along its plane. Adhesive joints are not as strong under shear stresses but are adequate for most structural requirements.
When in peel, the joint is subjected to stress in a direction that falls in-between a right angle and the angle of the joint’s plane. This results in a prying or peeling effect at one edge of the joint. Adhesive joints are weakest in peel and this type of stress should be avoided wherever possible.
If peel stresses cannot be avoided, the adhesive joint must be supplemented with a mechanical fastener or tape.
Making an adhesive joint
Before composites can be bonded or glued, the surface must be properly prepared. For example, if your composite has a surface veil that creates a smooth finish, the areas to be bonded will need to be roughened for better adhesion.
- Prior to sanding, contaminated surfaces should be thoroughly cleaned by wiping with cloth dampened with a solvent such as acetone, toluol, or methyl alcohol. Ensure you do not immerse or soak the Exel Composites in solvent. Dry the area with a clean cloth
- Abrade the surface by sanding the surface using 80 grit sandpaper. For large surfaces, power sanding can be used to improve efficiency. If bonding to another composite, make sure to sand both surfaces.
- Remove any dust remaining on surface by wiping with a clean, dry brush. Do not wipe surface with solvent-dampened rag after sanding as this will cause recontamination. If this does occur, repeat steps 1-3, and lightly sand to remove any film
- Mix adhesives according to manufacturer recommendations
- Spread a thin layer of adhesive on the surfaces to be joined, making sure to cover any exposed fibers
- Clamp assembly securely while the adhesive cures
The pot life of different adhesives will vary according to the quantity mixed at one time and the ambient temperature.
We recommend that you only mix a small amount of adhesive. Small quantities of the MMFG adhesive mixed at room temperature (70°F) will be usable over a period of one hour or more, while quantities of one pint to one quart may harden in one-half hour or less. Please refer to your adhesive manufacturer’s instructions for use.
Material Safety Data Sheets (MSDS) should be closely followed when working with adhesives or cleaning solutions.
Freshly bonded joints should be held in position with clamps or weights until the adhesive cures.
The structure should not be expected to carry its design load until the adhesive joints have cured a minimum of 48 hours at 21° C / 70° F.
If an oven is available, the curing time can be reduced considerably by heating the composites moderately (90° C / 200° F) for approximately 90 minutes. Do not permit the joints to move during the curing process.
If heat curing is used, allow to cool to ambient temperature before unclamping. Lower temperatures will require longer cure time.
Joints bonded with the recommended epoxy adhesives can usually be handled with proper care after 8 hours. However, it is better to leave the clamps in place or maintain the bonding pressure on the joints for a total of 20-24 hours.
If you have used an oven to speed the cure, do not remove the clamps until cooled to room temperature.
Using mechanical fasteners
You can use self-tapping screws, bolts or rivets to hold freshly bonded joints in place. This offers the following advantages:
- No waiting for the adhesive to cure – you can proceed immediately with further fabrication
- Increased stress reliability at the joint
- Parts maintain correct alignment
Adhesive should be cleaned before it hardens. Solvents such as acetate, acetone, or methyl ethyl ketone (MEK) are suitable cleaners. Please follow the adhesive manufacturer’s recommendations.
If your configuration includes a butt joint, you can improve its overall strength, as well as its appearance. Simply use composite tape or strips of FRP mat mixed with polyester or epoxy resin. To prepare the resin and glass, use the methods recommended for adhesive joints.