Page 16 - Handbook of Adhesion Promoters

2.1 Mechanical interlocking 9

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tively good wettability of PTFE. The nano-
composite is absorbed by PTFE substrate
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up to 1-2 μm depth. The dispersion trans-
forms into a solid form and the mechanical
interlocking occurs within the substrate
causing the nanocomposite to strongly
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adhere to PTFE.
In the high-quality zinc coatings on
steel substrates, the adhesion strength
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depended on surface roughness. Mechani-
cal interlocking, created by the polishing
Figure 2.4. Peel strength as a function of roughness process, was the main phenomenon deter-
depth. [Adapted, by permission, from Njuhovic, E; mining the adhesion of the zinc coatings to
Brau, M; Wolff-Fabris, F; Starzynski, K; Altstadt, V, the steel substrates. The optimal value of
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Composites, B66, 443-52, 2014.]
surface roughness was 0.3 μm above which
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adhesion was gradually decreased.
In another development, glass fiber-reinforced epoxy composite was sandblasted for
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2-6 seconds in order to increase adhesion of the copper coating to its surface. Figure 2.3
shows the cross-section of copper coated surface after sandblasting. Increasingly a
rougher substrate surface results from prolonged sandblasting leading to additional under-
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cuts and consequently anchor points. Figure 2.4 shows that the peel strength is a function
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of surface roughness depth which increases with the duration of the treatment. Acoustic
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emission analysis was used in combination with peel testing. For the untreated specimen
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having a very low adhesion, the acoustic emission signals could not be detected. Acoustic
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emission energy released during the peel test was a function of surface roughness. A
higher roughness depth led to a stronger adhesion force, and consequently also to a larger
total cumulative elastic energy release in
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the form of sound waves. This observation
directly proves that the mechanical inter-
locking increases adhesion because no
other treatment was performed for these
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samples.
In the study of adhesive bonding of
carbon fiber epoxy laminates, the relation-
ships were complicated by the mode of
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testing. The lap shear strength increased
with the increase in the polar component of
the surface free energy and decreased with
the surface roughness of the fibers and the
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surface area available for bonding. In the
double cantilever beam testing, the mode I
adhesive fracture energy proportionally
Figure 2.5. Calculation of the average surface rough-
ness, R , and the mean peak-to-valley height, R , values. increased with the density of summits and
a
z
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[Adapted, by permission, from Sancaktar; E, Gomatam, the mean summit curvature. Therefore, the
R, J. Adhes. Sci. Technol., 15 (1), 97–117, 2001.] main adhesion mechanisms in the single