Aging of membrane materials

November 17th, 2009 / By: / Continuing Education

How to keep membrane materials looking new over time.

Unlike Henry Ford, who claimed you could get his automobiles in any color as long as it was black, fabric structures do come in more than one color. However, sometimes these alternate colors are unintentional. Here are several tips on keeping things looking new over time.

“You must remember, these materials have really only been around for about 50 years,” says Georg Lind, executive vice president of the roofing business unit of Sika, Germany. “When we look at the history of the development of membrane materials used in construction, we see continuous development to increase performance and longevity in the outdoor environment and reduce costs of production, installation and maintenance. Since these things are still evolving, there hasn’t been a lot of investment in texture and color. The investment in design that has been made has been in areas such as increasing transparency or transmissivity and the development of non-stick or self-cleaning coatings.”

By the looks of it

Recently, my wife and I were driving past the new BMW Museum and the Olympiapark in Munich, on the Georg-Brauchle-Ring. As we discussed how the BMW Museum complements the Olympiapark and how the Olympic grounds (completed in 1972) still look futuristic, I spotted the weathered roof of the Eisstadion (Ice-skating stadium—now the SoccaFive Arena) near the eastern end of the park. Dr. Rosemarie Wagner, architecture professor at the Fachhochschule Munich, says this is one of the oldest membrane roofs in Munich, so I was prepared to see something showing some age, but I found the condition of this structure surprising since the tension structures in the Olympiapark are considered to be part of the architectural treasures of Germany. This prompted me to consider how membrane materials age and patinate and how aging affects a lightweight fabric structure over its life cycle.

My interest in how architectural materials age and patinate first came to me in 2007 when I visited Santiago Calatrava’s Ciudad de las Artes y las Ciencias (City of Arts and Sciences) in Valencia, Spain. From my point of view, he seems to have envisioned an optimistic sprawl of pristine glowing white forms shining in the sun, in a dry river bed in Valencia. Many of the surfaces include glossy white tile shards, so the forms appear ultra shiny. The buildings are surrounded by reflecting pools to create excitement from how the lighting plays on the surfaces of the structures and to build a sense that the structures are “places” apart from the surroundings.

Ciudad is fantastic and impressive on almost every level. However, on my first visit, as I got closer and started poking around, I realized that it is already looking worn and that the city has already had to repaint and restore things (the pools around the Opéra [opened in October 2005] were dry both times that I visited in 2007 and the Opéra building itself was undergoing some façade reconstruction.) One difficulty is that everything is white, leaving clear witness when soot from diesel vehicles and many industries around the city falls on the structures and dirty rainwater runs off of their surfaces.

It is educational to compare the appearance of the surface materials used in the Ciudad to those of some traditional materials, such as marble, copper or zinc-plated steel. These traditional materials tend to develop character as they acquire various patinas, up until their replacement points.

PVC-coated polyester compared to ETFE

With these observations in mind, let’s review some of the older membrane structures in Munich with the original architects and other interested parties. We’ll compare two structures roofed in PVC-coated polyester and review one structure roofed in ETFE. Coincidentally, all of the roof structures were fabricated by Koch Membranen of Rimsting, Germany.

The architect for the ice-skating stadium in the Olympiapark is Kurt Ackermann of Ackermann und Partner in Munich, Germany. It was roofed in white PVC-coated polyester by Koch Membranen in 1984 and was completed in 1985. I’ve visited it a few times over the past months. From a distance, there are lots of noticeable water runoff paths and large discolored areas. Moss and lichen growth on the seams, fasteners and edges are visible at closer distances. I spoke with Hans-Juergen Koch, managing director of Koch Membranen, about how this structure has weathered.

“Since PVC has a relatively high surface energy, it tends to attract dust and pollutants,” he explained. “Since the ice stadium was roofed, acrylic and flouro-top coats, with non stick or self-cleaning properties have been developed for PVC-coated polyester membranes. These coatings also help in limiting the migration of plasticizers out of the material, helping to reduce the factors leading to discoloration. That said, we have to keep in mind that this roof is 25 or 26 years old now. It has well exceeded its 15-year guaranteed service life.”

Peter Ackermann, partner at Ackermann und Partner, added, “The structural portion is composed of steel cables and wooden strips. The PVC material is just a rain cover fastened to the wooden strips and was quite inexpensive at the time. This was also a relatively new application for this material; it was originally developed to cover flat roofs. After quite a lot of debate during the design process, PVC-coated polyester was selected with the assumption that it would be replaced after its 15-year service life. It’s remarkable that the material is still weather tight, but it’s also a bit of a shame that it hasn’t been replaced, since it looks so bad now.”

Dipl.–Ing. Wasem Ajmail, project leader for maintenance of the structures in the Olympiapark for the Stadtwerke München, GmbH (Munich City Utilities), explained why the roof hasn’t been replaced. “We’ve tested the roof of the ice stadium and it is still sound,” he said. “The performance of the material has been so good that we shouldn’t need to make a decision about replacing it until 2011. At that point we’ll reach a decision about whether to replace the roof or put an entirely new structure on that site.”

“The roof is very weathered,” says Ajmail, “but it’s just not possible to clean it without damaging the membrane material.” Dr. Wagner has an explanation for this. “UV light acts to break the polymer chains in the PVC allowing the plasticizer to escape, leading to the material becoming brittle over time. Mechanical vibration from the wind and thermal expansion and contraction combine with embrittlement to create small cracks in the surface, allowing water to wick into the fibers, leading to staining. Also, particle buildup on the surface creates areas that heat up with the sun’s energy, compounding the cracking and staining problems.”

It’s interesting to compare the ice stadium with the rhinoceros house in the Hellabrunn Zoo in Munich, which was completed in 1988. Stefan Endl of R.E. Architekten from Schaeftlarn, Germany, then working for Büro Herbert Kochta, Architekt BDA (now Kochta Architekten) of Munich, was the architect responsible for this structure. Like the ice-skating stadium, it is roofed with a PVC-coated polyester material that is not bearing any structural loading, but this membrane is dark green and is draped over a concrete shell structure.

The level of particle accretion, staining and moss and algae growth seems similar to that of the ice-skating stadium, but the fabric color and the original color choices and patination of all of the materials in the structure combine to produce a settled, organic impression not unlike that of a heavily oxidized copper roof.

When I spoke to Endl about this and showed him some recent pictures, he was surprised. “We had originally proposed that the roof be a warm grey or light blue color, but the people at the zoo insisted we go with this dark green color,” he explained. “Originally, the roof material was a glossy dark green, with light turquoise-colored tensioning buttons. It was supposed to look organic, a bit like a fly agaric mushroom emerging from the ground. I haven’t seen it in about 10 years, so I’m a bit surprised about how it has changed and that the original material is still in place. It’s interesting that it has weathered to this appearance since we developed the panel and joining geometry to reference the look of lead roofing.”

Kochta further explained, “This was a relatively new application for the PVC-coated polyester material and it required quite a lot of development and improvising to use it in this three-dimensional application. There are many things we could have done differently, but I have to say that I am quite satisfied with how it looks and how it has aged.”

The Jungle House at the Hellabrun Zoo is also from Kochta Architekten. It was finished in 1993. The roof of this structure is formed from pressurized ETFE air cushions in a framework of metal cables and channels. The surfaces are still very glossy and transparent. Up close, it shows evidence of some puncture repairs, a bit of condensation or algae growth inside the cushions where airflow doesn’t remove all the moisture, and minerals and dirt on the surfaces, but this isn’t objectionable and could probably be removed with a cleaning process.

Dipl.-Ing Architect Eduard Lehner, now managing director of DBLB Architekten + Ingenieure of Munich, worked on this during its final design stage and construction. “I’m still very satisfied with how this structure turned out,” he says. “This was one of the earlier uses of ETFE in this sort of application, but it really isn’t a surprise that it has maintained its appearance so well. The material doesn’t really patinate since it doesn’t oxidize, doesn’t change with exposure to sunlight and it has self-cleaning properties. It’s supposed to be guaranteed for 30 years, but I won’t be surprised if it lasts much longer. At the time that we developed this, one of our major concerns was whether or not the material would maintain its degree of transparency over time. I recall that Koch conducted artificial aging tests to try to determine this.”

“I’m also still very satisfied with how this structure looks. This is really the perfect application for ETFE too,” adds Kochta. “The Jungle House is really a large greenhouse and ETFE film has very high admissivity for UV light. I think the only performance and appearance risks in using this material are that algae will grow inside the cushions without proper airflow.”

To sum up, it seems that there are a few important factors to consider in designing a membrane structure that will age well. Among these are initial color choice, retention of solids on the surface and regular maintenance of the structure.

Mark Zeh, a consultant in product innovation and management, is a regular contributor to Fabric Architecture. He writes about design and architecture from Munich, Germany.

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