What’s Happening at Missouri S&T:

Formulation: There is still time to register for the “Introduction to Formulation¿? short course on May 16-20. If you are new to formulation or your job requires that you have a basic understanding of the basics of formulation or you have to trouble shoot a coating, then this course is for you. “Basic Composition of Coatings¿? also has a couple of spots available yet, March 14-18.

Fall Short Course Schedule Set:
Basic Composition of Coatings September 12-16, 2005
Introduction to Formulation October 10-15, 2005

Addendum to Volume 2 Issue 1:

Glyn Raeber contacted us with an excellent point which readers might not know about.

When the tint is shaken and poured into the dispenser, air is entrained. The density of the tint containing foam will be incorrect until the tint looses the air. Since the tint is dispensed by volume, the air will result in erroneous tints. The tint dispensers should be filled at night at closing. This will allow maximum time for the air to work itself out of the tint. The tint dispensers are designed to use the bottom portion of the tint first and since they are stirred, the foam is going to rise up the tint column. This approach minimizes the effect of the foam on tints especially if the dispenser contains some tint when filled. Most dispensers are designed to hold more than one quart of pigment and thus if it is filled before becoming empty the upper tint containing foam will not have a significant effect on the density of the lower, next to dispense, portion of the tint.

Technical Insights on Coatings Science

Degradation of Organic Coatings Caused by Exposure to Natural Weathering
By Katherine Durham and Michael R. Van De Mark

Exposure to ultraviolet radiation, moisture, ozone, and temperature fluctuations can lead to the degradation of polymeric coatings. This deterioration is generally accomplished through oxidation, hydrolysis, and mechanical stress.

Exposure to UV radiation results in the photoinitiated but internally propagated oxidation of polymers. UV radiation induces bond cleavage in polymers to form free radicals, which undergo chain reactions with oxygen to form hydroperoxides and peroxides. The reaction of oxygen with the free radical on a carbon is a very fast process. These products of the polymer degradation are unstable and readily dissociate when exposed to sunlight and moderate heat to form alkoxy and hydroxy radicals. These radicals then react with the intact polymer to form more carbon based radicals on the polymer, thus continuing the oxidative cycle.

Some of the defenses against photoinitiated oxidation are: UV absorbers excited state quenchers, and antioxidants. Absorbers and quenchers work by converting UV energy into thermal energy, thus returning the stabilizer to its ground state and warming the coating. The absorbers and quenchers do not eliminate the absorption of UV by the polymer, but they compete for the photons. The higher the concentration of UV absorber the more photons are absorbed by it rather than the resin. It should be noted that the UV absorber is generally not a binder and thus reduces the coatings performance mechanically. UV absorbers are a relatively costly additive. Alternatives to UV absorbers are pigments. Pigments can also absorb UV radiation. Titanium dioxide is a white pigment which very strongly absorbs UV. Uncoated TiO2 can also speed up the photodegredation of the organic binder in coatings by interacting with water and oxygen while in its photoexcited state to oxidize the resin.1 Virtually all TiO2 is coated with silica, alumina or a similar thin layer to prevent this damage. Since the TiO2 absorbs the light but cannot use the energy, the excited state decays to ground state without causing damage. In contrast to absorbers and quenchers, antioxidants undergo chemical reactions with the products of oxidation in order to render them harmless. There are two classes of antioxidants: preventive and chain-breaking.1 Preventive antioxidants include the group of peroxide decomposers, which reduce hydroperoxides into alcohols. Chain-breaking antioxidants react directly with the radicals to end the chain propagation.

Exposure to ozone is another cause of the oxidative degradation of polymers. Although ozone is only present in the atmosphere in extremely low concentrations, between zero and seven parts per hundred million, it can cause extensive damage to unprotected polymers.2 Ozone attacks double bonds to oxidize polymers, producing peroxides, ketones, aldehydes and acids. When dissolved in water, ozone decomposes into reactive hydroxyl and superoxide radicals.3 As with oxidation caused by UV exposure, the oxidative effects of ozone can be combated with antioxidants.

The presence of moisture can cause chalking, blistering, frosting and cracking of a coating. Blistering results from water accumulating in the wood under the coating layer, although this is more of a problem with oil than with latex based paints. Frosting in latex paint occurs when water and carbon dioxide permeate the coating and dissolve calcium carbonate, which diffuses out to the surface and forms a chalky deposit. Because wood swells with an increase in humidity, a moist environment results in the cracking and peeling of many coatings.

Moist conditions also lead to the degradation of polymers through hydrolysis. Hydrolysis is facilitated by a low or high pH environment, such as one caused by acid rain or residual catalysts left over from a curing process. Alkaline pH, which often exists over zinc, galvanized steel, or concrete, also promotes hydrolysis. A polymer’s tendency to hydrolyze decreases as it becomes less soluble in water. Also, the addition of steric hindrance can be used to reduce the tendency of a certain group, such as an ester, to hydrolyze. Esters, amides, urethane and urea groups are all hydrolysable.

Temperature fluctuations also play a part in the deterioration of coatings. When there is a rapid change in the temperature, the different expansion coefficients of the polymer and the material that it coats result in the microcracking of the coating. These cracks will then become the sites for oxidation and hydrolysis reactions, further deteriorating the coating. Certain additives can also migrate as a function of temperature and temperature gradients. Plasticizers are well known to migrate and potentially leave the coating if they are warm and there is air movement. This can result in a more brittle film and, eventually, cracking.

References:
1.Zeno W. Wicks, Jr., Frank N. Jones, S. Peter Pappas. Organic Coatings: Science and Technology, 2nd ed. SPE Monograph Series. New York: Wiley-Interscience, 1999.

2.F. Findik, R. Yilmaz, and T. Koksal. (2004). Investigation of mechanical and physical properties of several industrial rubbers. Materials & Design, 25(4), 269-276.

3.Y.S. Song, F. Al-Taher, and G. Sadler. (2003). Migration of volatile degradation products into ozonated water from plastic packaging materials. Food Additives and Contaminants, 20(10), 985-994.

Is there a topic you would like discussed? Contact us by e-mail at coatings@mst.edu.