Adhesive fastening use
Adhesives can provide strong and durable bonds between two FRP shapes or between FRP shapes and other structural materials. In addition to sealing joints and surfaces, adhesives distribute the stress more evenly. Satisfactory bonds will be obtained if the joint is designed to avoid excessive peeling stresses. If the mating surfaces are properly prepared and if the recommended types of adhesives are used.
Several types of adhesives are recommended for use with FRP. Exel Composites recommends using three types of epoxy adhesives: MMFG E-502 or E-504 kits, Shell 828 wN40 hardener or Dow D.E.R. 331 w/ D.E.H. 58 hardener.
The E-502 and E-504 epoxy adhesive kits available from MMFG are easy to use since the epoxy base is mixed with the hardener component in equal parts (by volume). If using the 828 epoxy ad V-40 hardener system or Dow D.E.R. 331w/D.E.H. 58 hardener system, refer to the manufacturer’s recommendations for mixing.
The “pot life” of the different adhesive types will vary with the quantity mixed at one time and with the ambient temperature. It is recommended to mix only an amount of adhesive that can be consumed during the pot life of the material. 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. Material Safety Data Sheets (MSDS) should be closely followed when working with adhesives or cleaning solutions.
Joints between two structural members may be designed to carry loads by stressing the joint in pure compression or tension, in compressive or tensile shear, in peel or in some combination thereof. In pure compression or tension the joint is subjected to stresses only at right angles to the plane of the joint. Adhesive joints are strongest under these circumstances. In compressive or tensile shear the joint is subjected to stresses in a direction that is parallel to the plane of the joint. Adhesive joints are not as strong under these shear stresses but are quite adequate for most structural requirements. In peel the joint is subjected to stresses at some angle that is intermediate between the two cases mentioned above which 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 should be taped as described in a later section of this manual.
Before polyester FRP shapes can be bonded or glued, the surface must be properly prepared to insure proper adhesion.
1. Contaminated surfaces should be thoroughly cleaned by wiping with a clean cloth dampened with a solvent such as acetone, toluol, or methyl alcohol prior to sanding. Wipe dry with a clean cloth. Do not immerse or soak Exel Composites shapes in these solvents.
2. Remove surface veil left during manufacture by sanding both mating surfaces using 80 grit sandpaper. Sanding is adequate when the surfacing veil has been removed. On large surfaces, power sanding can be utilised for improved efficiency.
3. Remove any dust remaining on surface to be bonded from sanding operation by wiping with a clean dry rag or brush. Do not wipe surface with solvent dampened rag after sanding. Avoid recontamination of the surface by handling. If Surface becomes decontaminated, repeat steps 1 through 3, including a light sanding to remove any film.
4. Mix adhesives according to recommendations of manufacturer.
5. Spread a thin film of adhesive on both surfaces that are to be joined making sure to cover any glass fibers that have been exposed.
6. Appropriately clamp assembly to hold secure while the adhesive cures.
Freshly bonded joints should be held in position with clamps or weights until the adhesive cures. Joints bonded with the recommended epoxy adhesives generally can be handled with reasonable care after 8 hours. It is desirable to leave the clamps in place or maintain the bonding pressure on the joints for a total of 20 to 24 hours. If any oven is available, the curing time can be lessened considerably by heating moderately (200° F for 1½ hours). If heat curing is used, allow to cool to ambient temperature before unclamping. The structure should not be expected to carry its design load until the adhesive joints have cured a minimum of 48 hours at 70° F. Lower temperatures will require longer cure time. Do not permit the joints to move during the curing process.
Self-tapping screws, bolts or rivets can also be used to hold freshly bonded joints in place. Important advantages are available when mechanical fasteners are used:
1. No waiting for the adhesive to cure – you can proceed at once with further fabrication
2. Increased stress reliability at the joint; and
3. Correct alignment of parts can be maintained.
The adhesive should be cleaned from tools before it hardens. Solvents such as acetate, acetone, or methyl ethyl ketone (MEK) are suitable. Gasoline is not effective. Remember to use proper precautions when using flammable solvents.
In the fabrication of some items, butt joints may be present. The overall strength of the joint or connection, as well as the appearance, may be improved by taping with FRP tape or strips of FRP mat, together with polyester or epoxy resin. The resin and glass used in this process must be prepared by the methods recommended for adhesive joints.