Sichel's notes on Dr. Pocius's tutorial on adhesives
the action of adhesion takes place at the first few atomic
layers of the surface.
strength of an adhesive bond is not the same thing as adhesion.
Even modest adhesion can give a stong bond.
is in the nylon family of polymers. The Kevlar additive
in Bio-Fix 911 is added to bulk up the mixture so that the
epoxy liquid is easier to apply. It probably does not give
any added strength to the cured epoxy. We will find out
if its strength is different from the other epoxies as soon
as we have Prof. Ucci's strength tests for the various epoxies.
adhere well to aluminum because of the nanoscale fingers
of aluminum oxide on the surface of aluminum. We should
notice if our epoxy tests reveal higher bond strength to
aluminum surfaces than to steel surfaces. The reason is
probably the nanoscale fingers (dendrites) of Al2O3.
Good adhesion to oak wood is due to the fibrous structure
of cellulose (wood fiber). The adhesive has a microstructure
to grab on to.
did our original 5 epoxies not adhere to rocks underwater?
The reason may be the fine scale structure of the rock surfaces.
Although the rocks appear rough to our eyes, they may be
smooth on a nanoscale. (A nanometer is 10-9 meter.
One nanometer is about the size of 10 hydrogen atoms lined
up side by side.)
blasting (sand blasting), chemical etching, and plasma etching
are helpful to prepare an adherend for bonding. Sandpapering
is bad because the abraided channels are too large for capillary
action of the adhesive and, in fact, form air pockets that
prevent the adhesive from reaching the surface.
should try RTV silicone adhesive because RTV silicone lets
water percolate through it. Therefore, the thin layer of
water on the surface of the rocks will not prevent the RTV
silicone from adhering to the surface. It is unusual for
adhesives to make a good bond underwater, because the thin
layer of water on the adherend prevents the adhesive from
making a bond. It will be interesting to see if any of our
underwater tests on metals give strong bond strengths! Dr.
Pocius predicts they will not.
are chemical coupling agents that retard attack of water
on adhesive bonds. Tri-methyl...silane (?) is such a coupling
agent. It is used in fiberglass construction of boats. The
glass fibers are coated with the coupling agent before being
formed into boat hulls. If not for the coupling agent, fiberglass
boat hulls would not last a single season in the water.
are adhesives on the market that adhere to surfaces even
if they are contaminated with oil. They are very useful
in the automotive industry because cars are put together
on the assembly line where the parts are still contaminated
with oil from the previous step in the manufacturing process.
did some of our epoxies fail to work in cold water (0o
C) but worked fine in 20o C water? The epoxy
will not gel if the temperature is too low. The reaction
will stop and go no further. Above Tg, the glass
transition temperature, the polymer molecules are moving
around and participating in reactions.
should we use as a pigment in clear epoxies? Dr. Pocius
recommends TiO2. It is white and will show up
well under Alvin's floodlights at the ocean floor. Make
sure we get pigment grade TiO2 when we order
it from a chemical supply house.