Page 68 - Handbook of Adhesion Promoters

4.2 Surface treatment 61

is usually made out cloth from cotton, polyester, or rayon, and typical abrasives include
aluminum oxide, diamond, emery, garnet, and silicon carbide. Sanding has to be con-
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ducted at low temperature (either speed control or water cooling). Sandblasting is the
process in which abrasive particles are blown in a form of stream towards the surface,
roughening the surface and removing contaminations. The mechanical polishing is still
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one more step to decrease surface roughness using either very fine powders or felt. The
electronic or electrolytic polishing is done by dissolution of a metal surface in an electro-
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lytic solution. Below, some examples of grinding and sandblasting are given.
Polishing down to 3 μm and grinding by 800 grits emery paper were used to prepare
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aluminum cathode material for deposition of zinc. Grinding and additional polishing
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increased the number of zinc nucleation sites.
Hand sanding increased the surface roughness of the carbon fiber/epoxy composite
increasing mechanical interlocking of adhesive and joint strength when compared with
untreated surface. 13
Sanding and plasma treatment by dielectric barrier discharge were compared regard-
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ing their influence on wood surface characteristics. Both sanding and plasma treatment
increased the surface energy of wood because of increased polar part but plasma treatment
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was superior to sanding.
The effect of sandblasting on surface properties of carbon steel was studied using
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different air pressures and particle sizes. The surface roughness and adhesive properties
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were more affected by the type of abrasive material than by variations in pressure. Glass
beads used as an abrasive material gave the highest values of surface free energy and a
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much higher value of the polar component.
The acrylic coatings were deposited on the sandblasted soda-lime glass in order to
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correct the loss of optical properties. The optical transmission drops dramatically from
91.4 to 20% after sandblasting with a mass of 200 g, while the arithmetic roughness
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increases from 0.04 to 2 μm. When a 10 μm thick acrylic coating is deposited by spray-
ing, the resultant polymeric coating reduces the surface roughness and improves the opti-
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cal transmission (to 90%). At low roughness, poor adhesion is observed and
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delamination occurs.
Sandblasting alters the surface topography of an implant, and micro-arc oxidation
provides an effective way to produce porous, adhesive, and bioactive implant coatings. 17
The treatment promoted the formation of hydroxyapatite on the oxide coatings which
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enhanced bioactivity of titanium.
The above examples show that surface roughening is not the only property changed
by mechanical processes of surface conditioning for adhesion but also surface free energy
is frequently modified.
4.2.3 PLASMA
Plasma is called the fourth state of matter because it is not a liquid, a solid, or a gas. It is a
gas containing ions and electrons. Nature occurring plasmas are lightning and auroras. In
artificial treatment units, gas or mixture of gasses are subjected to the power of radio fre-
quency generated between electrodes. Ions are created and they vibrate. The vibration is
essential for the surface cleaning abilities of plasma. Plasma glows because of emission of
UV radiation. Elevated temperature increases the efficiency of etching rate (e.g., the etch
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rate at 60 C is about four times higher than at 25 C). Plasma is used to clean and etch sur-