THE PROBLEMS WITH URETHANE

Part 1: A Dramatization

By Nathan Zakheim

(Published orginally in the MCLA Newsletter, v. 7, n. 2, Winter, 1996)

Approximately ten years ago, it became the fashion to coat outdoor murals with a newly developed urethane emulsion coating that was touted to be a "cure" for graffiti. The main selling point of this coating was the fact that in was virtually indestructible, did not succumb to the usual hydrocarbons such as toluene, acetone, lacquer thinner, MEK etc. that are the usual solvent components of most marking pens and spray can paint. Certainly the idea of a "suit of armor" that could withstand the worst that graffiti vandals could dish out had a certain appeal to frustrated artists who had become accustomed to seeing their finely wrought work executed under physically dangerous and fumigatiously unpleasant circumstances wiped out by a few arrogant passes of a 98 cent can of fifth class hot rod primer. Since urethane cannot be dissolved in most substances known to humankind, it would seem that the final and most enduring mural protection had finally arrived courtesy of the large chemical companies that had already developed an awesome array of acrylic products (Products that included the vulnerable polymer "Rhoplex" emulsions used for mural paints as well as the harsh solvent-based acrylic monomers used for spray can paint). In the battle to the death between monomers and polymers, urethane surely seemed the shining armour in which the knights of future muraldom were destined to ride. Alas! who could have predicted the fatal and tragic flaw of mural protective hubris! As graffiti vandals gnashed their collective teeth in frustrated rage at the ineffectiveness of their impotent spray cans as a means of permanently disfiguring governmentally sanctioned public art, urethane, the kindly friend of artist and art aficionado alike began to turn in the flaky gruesome fashion of "Friendly" alien monsters in an Ed Wood film.

First, the trusting artists stood agape as the treacherous substance began to yellow in the Ultra-Violet radiance of the sun! Helplessly, they watched their vivid hues sink into ghastly dull browns and greys. Then the treacherous urethane began to separate from the mural paint itself, much as the skin molts on a snake. Those with strong nerves would wait in the heat of day and the cold of night; braving rains and frosts to gaze with horror-stricken eyes on the minute and almost imperceptible crackling of the paint as it loosened first in pin-head little bubbles, then in patches and sheets, taking the precious final glazes and color areas with it as a stain. Now, a leprous white crept over the mural composed of de-laminated urethane film.Then came the rains, bringing liquefied smog and grime, oozing beneath the leprous coating, impacting the fractured mural paint, gathering in impacted toxic globs, seething with inherent venom and petrification. Ah! the secret life of mural paint! First hot, then cold, light then dark, loose, then tight, wet, then dry whipped by rain, parched by sun, occasionally gouged by the bumper of an errant car. Using stop-frame photography like Disney dramatizations of an opening flower, they would seem to writhe! All in the thrall of Ultra-Violet light: the great killer of transparent coating film! From a microscopic view, we see the thickness of the film, following the contours of the mural paint and wall in a lumpy and ungainly way; sometimes thick, sometimes thin. The deadly rays of Solar Attack plunge through the surface of the film, only to be repelled by the surface on the mural paint side! Particles of light bouncing back and forth between the twin surfaces of the urethane film like nuclear ping-pong balls begin to smash the helpless and vulnerable molecules in their way. Accomplishing what no solvent dare attempt, they ruthlessly and indiscriminately side-swipe first electrons, then neutrons, then photons breaking down the integrity of their famous and reputed bond. Like cannon balls, crushing the fortifications of some ancient castle, cracks and fissures begin to form, and tired chemical bonds sigh and gradually release their hold. Heat and cold cause molecular tensioning within the urethane film itself; a "rigor mortis chemicus" of atrophying molecular chains. Stiff as mummified arms and legs of some ancient creature lost in the swirling mists of time, the film creaks like a mummy newly disturbed, and shrinks stiffly and rigidly across the minute voids it used to fill. If one had tiny ears and plenty of time, one could hear the crackling of the surface as these loose and brittle pin-head spots incrementally grow.

Moving back to a viewer's stance from the intense drama occurring microscopically within the coating of the mural wall, the saddened viewer simply sees the mural covered with whitish or translucent spots and blotches. He probably does not see them grow. Happily, although impossible to remove, urethane "protective" coatings gradually fall off by themselves in the fullness of time. Of course, they do not leave willingly. They take whatever paint layers with them that they can before loosening and falling away in sheets. Of course, science can step in, with wetting agents, surfactants, and bristle brushes for loosening the film. Perforating pinpricks of pattern wheels can accelerate the access of deionized water charged with loosening agents applied by trained and skillful hands.Challenged by water after being loosened by time, the coating falls away readily enough, an ignominious ending for a substance resistant to the strongest chemicals known to man!

Part 2: A Case Study

THE FOLLOWING IS A TRUE ACCOUNT AND IS MEANT AS A GUIDE FOR THOSE VICTIMIZED BY THE DECEPTIVE PROMISES OF URETHANE:

We were brought to inspect reports of deterioration of a coating on a public mural that shall remain unnamed. The report of unpeeling paint led us to discover that the urethane coating had finally begun to detach itself from the painted mural surface in larger and larger areas. As our previous inspection report indicated, the de-lamination began as sporadic, and having severely oxidized (which included yellowing as well as a tendency toward opacity) obscured the vivid colors beneath the areas where the yellowed urethane coating was lifted from the painted surface, but not yet flaked away from the surface. The underlying brilliant colors would then appear as pastel due to the semi-opacity of the urethane coating.Now that larger areas have delaminated, we can see that there is a largely intact field of color under the exfoliated urethane sheets, but that the pigment is now covered with very severe discoloring and thick layers of road grime that has sifted into the "pockets" formed by the somewhat loosened coating. "Road grime" consists of the following: Smog and auto emission residue. Airborne dust particles. Powder from brake lining wear as well as powdered tire rubber. Organic matter from de-composing tree leaves, grass, paper etc. Spores and molds that begin to take root in the organic matter.The road grime has built up a film over the exposed paint layer and in some cases has caked into a highly pigmented crust of pollutants.As the urethane began to split from the wall, the surface of the mural paint has tended to adhere to it, and thus the paint layer has been split to some degree or another. No doubt the tight covering of urethane has also created some sort of micro-environment that has promoted the break-down of the mural paint by facilitating water vapor to form from moisture that has leeched in from the reverse side of the concrete wall that supports the mural (The sun heating the front of the mural both draws the moisture to the urethane covered surface, but also causes it to become vapor that can then attack the water soluble elements in the mural paint itself). If the urethane coating were not there, the vapor would pass through the mural paint and be evaporated into the atmosphere causing little or no damage to the paint film. It is also likely that the paint layer did not have enough binder at the outset. We draw this conclusion due to the fact that the "skin" (surface layer) of the mural paint became fused to the urethane coating, and when the coating began to peel away, it took a very thin layer of the mural paint surface with it leaving a layer of underbound pigment that tends to be friable and powder. This would indicate that the paint used did not contain a sufficient amount of binder to fully saturate the pigment of the paint. Another explanation would be that the microenvironment created under the urethane coating caused the binder to crystalize and for the water soluble elements to break down, causing the paint to appear friable.

NOTE: If good quality paints are used, with acrylic gel or medium added to the tube or jar paints, the mural paint film thus formed becomes very "plastic" or "rubbery" and resists the tendency to powder. Of course, the acrylic medium can itself crystalize, and begin to form microscopic "crumbs" that give the impression of underbound pigments.

Such crumbling of the acrylic layer can be immediately corrected by spraying the affected area with mist coats of xylene. For even better results, a 10% solution of Acryloid B-66 or B-72 diluted in xylene can be applied via spray to the surface of the crumbling mural surface in gradual layers until full saturation is achieved. This not only consolidates the crystallized crumbs of acrylic into a flexible plastic film once again, but fully restores the original depth of color that invariably takes on a pastel aspect during the crumbling process. Refraction of light makes the broken surface of the pigment layer appear pastel, just as ceramic glazes tend to appear "white" until fired.

PROPOSED TREATMENTS

Obtain a source of hot water (Buckets, drums, water-blasting unit with heater, gas fired burner on which pots of water can be heated, "igloo coolers" filled with boiling water etc.) Mix the water to be applied with Shaklee Basic-H. (1/16 tsp per gallon of water). Spray a generous application of this solution onto the entire mural being careful not to brush or scrub any area at first. Use small "pattern wheels" (rowers) to perforate the blistering areas of urethane.
Use small hand held sprayers and a sponge to thoroughly wet areas that are covered with caked on black road grime. Carefully wipe the areas of grime away with the sponge. Stop immediately if there is excessive pigment loss.
Go to the areas that are fully blistered and carefully scrub them with a bristle scrub brush (a vegetable scrub brush works well) while continually spraying the water mixture onto the surface. You will find that some areas will clean easily, while others stubbornly resist. Once the easy areas are clean, spray more solution at an angle to the resistant edges and wait for a while. (if they begin to loosen or curl back, then scrub the new areas carefully with the brush. If they do not loosen, add more pinholes with the wheel and repeat the process. If they cannot be loosened, then leave them alone. Rinse the wall with flowing, clear water. (A pressure system with a reservoir would be useful. The pressure should be set at no more than 75 psi.)

NOTE: The end result, if the coating removal is incomplete, will have a unsightly blotchy appearance almost identical to a badly peeling sunburn. The newly revealed mural surface will be bright and colorful, and the areas still covered by a stubborn layer of urethane, will be several shades darker. This contrast may not be so very noticeable to the traffic driving by.

FINAL PROCEDURES FOR CONSOLIDATION AND PROTECTIVE COATINGS

Allow the surface to dry for at least one day. This means a HOT day. With temperatures below 60 degrees F. several drying days should be allowed
Spray Acryloid B-72 dissolved in xylene onto the newly cleaned areas. Overspray onto the urethane is O.K. if not excessive. When the Acryloid B-72 is set (2-4 hours), spray a coating of Soluvar onto the surface as an intermediate protective reversible varnish. Soluvar, which dissolves in petroleum naphtha, can act as a graffiti barrier, since it can be removed from under the spray-can graffiti. It is not a good substitute for sacrificial wax, however, as the spray graffiti sometimes penetrates the Soluvar layer and attacks the mural surface. Wait for six months or a year to repeat the above process on any areas of urethane coating that may remain. A third treatment should not be required as the accelerated disintegration of the urethane will probably be complete by that time. Apply the sacrificial spray wax to the areas that are clear of the urethane. It is not desirable for the remaining urethane coated areas to be treated with sacrificial wax. It is advisable to mask such areas with visqueen when the wax is being applied.