Dr. Sichel's notes on Dr. Pocius's tutorial on adhesives

• All the action of adhesion takes place at the first few atomic layers of the surface.
• The strength of an adhesive bond is not the same thing as adhesion. Even modest adhesion can give a stong bond.
• Kevlar 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.
• Adhesives 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.
• Why 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.)
• Grit 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.
• We 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.
• There 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.
• There 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.
• Why did some of our epoxies fail to work in cold water (0^o C) but worked fine in 20^o C water? The epoxy will not gel if the temperature is too low. The reaction will stop and go no further. Above T_g, the glass transition temperature, the polymer molecules are moving around and participating in reactions.
• What 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.