SAMPE Journal - September/October 2020 - 37

Figure 5. A plug was made from polyurethane foam and inserted
inside the damaged tube, so that fresh foam could be poured directly into the tube without flowing beyond the damaged section.

of the tube, where it expanded to roughly 30x its
liquid volume. The plug prevented the liquid foam
from flowing down the tube prior to expansion and
solidification. The expanded foam core can be seen
in Figure 6. At the opening, the foam was sanded
flush with the inner dimension of the tube, creating a contoured mold surface matching the original
tube profile. It should be noted that because only
a small section of tube was filled with foam, and
because the density of the foam is low (4 pounds
per cubic foot (PCF) in this case; 2 and 6 PCF varieties are also widely available), the foam added
only negligible mass (approximately 15 grams) to
the structure.
After creating a closed cell foam mold surface,
the final step before applying a patch was surface
preparation. The recommended surface prepara-

tion for thermoset composite to thermoset composite bonding is solvent cleaning followed by a
light aluminum oxide grit blast in dry nitrogen to
expose a fresh, chemically active surface4. After
grit blasting, dust should be blown off with dry
nitrogen (rather than wiped off) to avoid surface
contamination. In this case, a suitable grit blaster
was not available, so the surfaces (foam and composite) were given a final light sanding and cleaned
with acetone and isopropanol to remove sanding
debris and any other contaminants.
The readiness of a surface for bonding can be
evaluated by the water break test, which is typically
performed with deionized water or a drop of resin. If the liquid "beads" into droplets, the surface
is contaminated with hydrophobic substances (e.g.
oil/grease). If the liquid "breaks" or spreads out and
wets the surface, then the surface is
considered ready for bonding. The
Figure 6. Polyurethane foam was poured directly into the
water break phenomenon is explained
damaged section of tube, where it expanded to fill the hole
by the force diagram in Figure 7, where
in the wall. The foam core was sanded flush with the intergSL, gLV, and gSV denote the solid-liquid,
nal tube dimensions and used as a mold surface. Because
liquid-vapor, and solid-vapor surface
the foam is closed cell, it does not absorb resin during wet
tensions, respectively, and qC is the conlay-up.
tact angle. Static equilibrium of these
forces in the horizontal direction gives
Figure 7. Illustration of surface tensions acting on a liquid droplet in equilibrium on a smooth and homogeneous substrate. Balancing these forces in the horizontal direction gives the Young
equation. If the substrate is clean, the droplet spreads and wets
the surface evenly.

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SAMPE Journal - September/October 2020

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