The environmentally- friendly combination of new materials and the demonstration of new engineering and architectural possibilities is the declared goal of a project at the Department of Conceptual and Structural Design – Composite Structures at the Technical University (TU) Berlin, Germany. An experimental pavilion with a novel composite façade, designed by 20 students, aims to improve the durability of its timber structures by combining the timber frame with upcycled compressed polyurethane (PU) panels which are protected by a 3mm thick sprayed PU coating. Members of the EcoCommercial Building programme, like Bayer MaterialScience together with its partners Puren and AB-Polymerchemie, are contributing financial assistance and material solutions to the research and teaching project.
The small experimental building, a white, shiny container with 5 x 7m of floor space and a 3m ceiling now stands at the very centre of the Technology and Innovation Park in Berlin-Wedding, Germany. The individual modules of the pavilion consist of pinewood frames with oriented strand board (OSB) walling. The container’s four doors each can be rotated 360° to make the green inside walls visible on the outside, and to signal from a distance when the pavilion is open. The side is outfitted with vertical, transparent slats constructed of twin-wall polycarbonate (PC) sheets made of Makrolon from Bayer MaterialScience.
Hybrid façade system
Conventional wooden plates can be used outdoors only in usage class 2. This refers to construction parts which are installed outdoors and must not be weathered. Plywood, for example, can be used in usage class 3, but direct weathering must be prevented by a plastic protection panel. Otherwise, chemical wood preservatives have to be applied to guarantee sufficient durability.
The composite façade of the pavilion combines the timber frame with upcycled compressed polyurethane (PU) panels which are protected by a 3mm thick sprayed PU coating. The compacted PU panels are upcycled from chippings of the PU foam production. They are resistant to weathering, do not rot in water and are not susceptible to moisture. As they are vapour-permeable, they are well suited to be used as part of a timber structure, especially along the joints of large-sized plates which are always a weak point in conventional timber panel constructions. The PU panels do not require any chemical wood preservatives, especially if they are combined with 3mm sprayed PU coating.
PU press plates
PU press plates are manufactured by glue-pressing shredded waste material left over from the manufacture of PU insulating foam. This is similar to conventional chipboard manufacture, where a homogeneous material is formed. Methylene diphenyl diisocyanate (MDI) polyisocyanates in combination with water are used as a binder. With a bulk density around400kg/m³, the plate thickness before pressing is approximately four or five times the subsequent thickness. The plates are manufactured with different thicknesses and usual densities between 300 and 850kg/m³, depending on their application. The thicknesses vary between 10 to 80mm in gradation of 2 to 5mm.
The PU plates can be processed like conventional wooden plates but are resistant to moisture like a plastic material which makes it suitable for façade applications. The material is non-ageing and rot-proof and can therefore be used in usage class 3 of DIN 1052:2008-12 with direct weathering without applying any wood preservative. Hitherto, press plates on a PU base have been used mainly for furniture construction, as core material for sandwich elements. In this project they are used as non-load-bearing outer planking for the façades.
The heat transfer coefficient with ? between 0.06 and 0.10W/(mK) is particularly important in façade structures. The plates act as insulating material and thus reduce the heat loss. As yet, the upcycled plates have not been used in load-bearing structures. Prof Dr-Ing Volker Schmid sees one reason in the fact that the mechanical material properties have only been determined according to the regulations governing insulating material. “These tests are of course not adequate for a reliable evaluation of the load bearing capacity. As part of the design of the experimental pavilion, small-scale pretests, like pull-through tests for screws, were first of all carried out to give an initial indication of the load bearing characteristics of this new material.”
Sprayed PU thick-film coating
The PU plates are covered with the 2 to 3mm thick PU coating as are the joints between the individual panels. Beforehand, the edges of the plates are bevelled and the resulting grooves are filled with a permanently elastic material. Due to the all-over-coating with its gap-bridging capacity, a level surface with a uniform surface can be achieved. According to Schmid, no subsequent treatment or reinforcement of the joint is necessary.
The easy applicability of the PU coating permits the reliable sealing of complex geometrical shapes, edges and protrusions by spraying. The unusually high elasticity of the PU coating together with the good adhesion on the press plate is said to enable new and more complex geometries with this prototype and more versatility in timber constructions. The development in modern wood construction with its design of ever more complex shapes already favours the application of PU coatings on timber structures.
One example of the new combination of timber with a PU coating can be seen in the Metropol Parasol at the Plaza de Encarnacion in the centre of Seville, Spain. The waffle-shaped building is claimed to be one of the largest and most innovative bonded timber-constructions with a PU coating. The project was designed by the architect at JMH, Berlin, together with the engineers from Arup, Berlin and Project Manager Prof Dr-Ing Volker Schmid.
One reason for the increased number of applications of sprayed PU coating on timber is the reduced crack width in modern wooden building components, such as, for example, like oriented strand boards, plywood and veneer laminated wood plates. The pavilion uses the experience gained in the course of these projects, but Schmid sees the real novelty of the new façade structure of the pavilion in the application of PU coating in combination with upcycled PU pressed plates.
Schmid summarises the current state of play: “The example can offer new opportunities in the construction of hybrid façades using wood and upcycled PU press plates. The combination of these materials leads to an increased durability and in the long term to an economical and lasting façade construction. The small-scale tests showed that the pressed plates can be used not only as a space enclosing planking but indicated that the PU plates may also be used for load-bearing functions.”
“Over the next two years,” he says, “monitoring of the prototype will be carried out in order to investigate the long-time performance of this new and sustainable façade system. The pressing of chippings from the PU foam production is costly, but a cost benefit comparison shows that economic efficiency is achieved for applications in outdoor weathering.”
Unlimited recycling, however, is not possible, since the material is damaged upon repeated processing. Impurities from coatings and adhesives complicate the material recycling, so that further research on the separation of coating and press plate will need to be carried out.
Researching under practical conditions
For the students at the Technical University (TU) Berlin the pavilion is their own on-campus event location and café. The building simultaneously gives researchers an opportunity to test and evaluate the innovative façade system under practical conditions. Shear and pull-out tests conducted by the students themselves at the Institute of Civil Engineering’s testing centre helped to compile initial information on the load-bearing capacity of the PU compressed panels and the purenit/wood screw connections. The results ultimately will provide some indication of the durability of the new façade design.
Just recently, the programme has been expanded yet again and raised to a more active level as “ECB 2.0”. In addition to working out energy-efficient building solutions as before, the programme will now also establish a development network that will serves as a platform, bringing together material manufacturers into direct contact with developers.