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September 11, 2009

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Graffiti “shield” may offer hope for paint-threatened landmarks

Sept. 11, 2009
Courtesy Fraun­ho­fer IAP
and World Science staff

Graf­fiti mars many a his­tor­ic land­mark, but it can only be erased—if at all—us­ing caus­tic so­lu­tions that risk dam­ag­ing the un­der­ly­ing sur­face.

A new “breath­able” wall coat­ing, though, of­fers ef­fi­cient, all-round pro­tec­tion against at­tacks by spray­can hooli­gans, re­search­ers claim.

A graffiti-laden his­tor­ic build­ing in Ger­many. (Image cour­tesy Fraun­ho­fer IAP)

It takes sec­onds to spray on graf­fiti, but hours or weeks to re­move, es­pe­cially from the po­rous nat­u­ral stone or brick­work that char­ac­ter­ize the ma­jor­ity of his­tor­ic mon­u­ments. The paint seeps deep in­to the pores and can be­come un­re­moveable even with a pres­sure hose or pow­er­ful cleans­ing sol­vents. 

Of­ten noth­ing will do the trick short of chem­i­cals that eat away at a ven­er­a­ble wall.

Some an­ti-graf­fiti coat­ings have al­ready been on the mar­ket for sev­er­al years. They cre­ate a wa­ter-re­pel­lent seal that shuts the pores. That helps keep the paint from stick­ing to the sur­face, so that it easily comes off. 

But these coat­ings al­so seal out air and lock in mois­ture, ac­cord­ing to An­dré Laschewsky, a re­search­er with the Fraun­ho­fer In­sti­tute for Ap­plied Pol­y­mer Re­search in Pots­dam, Ger­many. That, he said, leaves the sur­face vul­ner­a­ble to mold or the de­vel­op­ment of un­sightly salt de­posits. More­o­ver, he added, these coat­ings are them­selves hard to re­move, and thus flout a prin­ci­ple dear to con­serva­t­ion­ist­s—that any changes to a his­tor­ic mon­u­ment should be re­vers­i­ble.

The trou­ble, Laschewsky said, is that a suc­cess­ful coat­ing must meet “con­flict­ing re­quire­ments.” It must­n’t seal the sur­face en­tire­ly, yet it has to keep the paint from get­ting in. It needs to re­sist weath­er­ing and wip­ing down, yet be easily re­moveable when it’s nec­es­sary.

As part of a Eu­ro­pe­an Un­ion-sponsored proj­ect, Laschewsky and col­leagues with the Pol­ish Acad­e­my of Sci­ences de­vel­oped a pol­y­mer coat­ing that they claim sat­is­fies these de­mands. Pol­y­mers are a large class of com­pounds con­sist­ing of large mo­le­cules made up of smaller re­peat­ing un­its, as in plas­tics.

The newly cre­ated pol­y­mer film seals the pores on a build­ing sur­face so as to block the un­wanted paint, Laschewsky ex­plained. At the same time, the new coat­ing has its own, much smaller “micro-pores,” which cre­ate a wa­ter-re­pel­lent bar­ri­er that al­lows air to reach the build­ing.

The coat­ing is re­moveable us­ing a salt so­lu­tion that mod­i­fies its chem­i­cal com­po­si­tion and al­lows it to be washed off, he added. In a proj­ect co­or­di­nated by the Labein Founda­t­ion, a non­prof­it re­search cen­ter in Bil­boa, Spain, and the Ger­man Fed­er­al In­sti­tute for Ma­te­ri­als Re­search and Test­ing, Laschewsky and part­ners coat­ed sam­ples of an­cient stone and brick and re­peat­edly cov­ered them with graf­fiti—which, they said, com­pletely washed off each time.

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Graffiti defaces many a historic landmark, but it can only be erased—if at all—using caustic solutions that risk damaging the underlying surface. A new “breathable” coating, though, offers efficient, all-round protection against attacks by spraycan hooligans, researchers claim. It takes seconds to spray on graffiti, but hours or weeks to remove, especially from the porous natural stone or brickwork that characterize the majority of historic monuments. The paint seeps deep into the pores and can become unremoveable even with a pressure hose or powerful cleansing solvents. Often nothing will do the trick short of chemicals that eat away at a venerable wall. Some anti-graffiti coatings have already been on the market for several years. They create a water-repellent seal that shuts the pores. That helps keep the paint from sticking to the surface, so that it easily comes off. But these coatings also seal out air and lock in moisture, according to André Laschewsky, a researcher with the Fraunhofer Institute for Applied Polymer Research in Potsdam, Germany. That, he said, leaves the surface vulnerable to mold or the development of unsightly salt deposits. Moreover, he added, these coatings are themselves hard to remove, and thus flout a principle dear to conservationists—that any changes to a historic monument should be reversible. The trouble, Laschewsky said, is that a successful coating must meet “conflicting requirements.” It mustn’t seal the surface entirely, yet it has to keep the paint from getting in. It needs to resist weathering and wiping down, yet be easily removeable when it’s necessary. As part of a European Union-sponsored project, Laschewsky and colleagues with the Polish Academy of Sciences developed a polymer coating that they claim satisfies these demands. Polymers are a large class of compounds consisting of large molecules made up of smaller repeating units, as in plastics. The newly created polymer film “seals the pores” on a building surface so as to block the unwanted paint, Laschewsky explained. At the same time, the new coating has its own, much smaller “micro-pores,” which create a water-repellent barrier that allows air to reach the building. The coating is removeable using a salt solution that modifies its chemical composition and allows it to be washed off, he added. In a project coordinated by the Labein Foundation, a nonprofit research center in Bilboa, Spain, and the German Federal Institute for Materials Research and Testing, Laschewsky and partners coated samples of ancient stone and brick and repeatedly covered them with graffiti—which, they said, completely washed off each time.