Building & Construction
Building & Construction Biobased plastics for exterior facades Fig. 1-3: PLA/Lignin/Cellulose composite before and after 18 months of natural weathering, Lignin/Cellulose composite (3mm plate) after 18 months of natural weathering (Photo: C. Köhler) Fig. 5: Tension rod made from PLA and modified PLA before and after natural and artificial weathering (see table) (Photo: C. Köhler) While plastics are already indispensable materials for the production of pipes, seals, vapour barriers or insulation in the construction industry, this type of material is also increasingly used for interior and exterior cladding. This may be due, on one hand, to novel digitised design and production methods which allow for flowing shapes with free geometries and that can often only be realized with plastic materials. On the other hand, additional advantages of this material (such as its resistance to corrosion, its versatility for creative applications, and its low thermal conductivity or light weight that compares favorably to other materials such as glass) lead one to a decision for this type of building material. Using bio-based thermoplastic materials in the abovementioned applications could therefore become a resourceefficient alternative in the future. These materials can be freely shaped and recycled and can combine such properties with the ecological advantages of materials made from renewable resources such as wood. To be able to supply all types of buildings, building components made from bioplastic materials need to be of low flammability (DIN EN 13501-1 B or C). Materials of regular flammability can be used in buildings with a maximum height of 7 meters and with no more than two units, which have to be smaller than 400 m². Exterior facades should be able to withstand threshold temperatures of -20°C and +80°C [1]. Furthermore, the building’s stability has to be warranted for a time frame that is economically feasible; in the case of exterior cladding, this means a time span of 30 years. Composite materials from lignin and cellulose fibres, which are the main components of wood, are less suited for exterior application. Both the results of three years of natural weathering in accordance with DIN EN ISO 877 (Fig. 1-3), as well as the results of artificial weathering in accordance with EN ISO 4892, show inadequate resistance to weathering. UV radiation degrades lignin. This results in a water-soluble decomposition product which is washed away by rain. What remains is whitish cellulose that exhibits a grayish layer when it is colonized by micro-organisms [2]. The material 12 bioplastics MAGAZINE [04/13] Vol. 8
Building & Construction by Carmen Köhler Institute of Building Structures and Structural Design (ITKE) University of Stuttgart, Germany Fig. 6: Booth at the Hannover Expo 2013 (Photo: M.R. Hammer/ ITKE) is no longer stable (Fig. 3). The absorption of water also minimizes the stability of the composite. Furthermore, a 96 hour immersion in water in accordance with EN ISO 62 shows that composites containing lignin and cellulose are not suitable for long term exterior applications. The water absorption rate is 37%. The test object (4mm) breaks. A composite of PLA and wood fibres (in approximately equal parts) shows a water absorption rate of 25 % under the same conditions, which results in the material tearing. PLA, however, absorbs only 1% moisture. The 18 months process of natural weathering of PLA, PLA-blends, and PHB does not result in yellowing. The surface becomes slightly more matte in appearance. Tensile strength at yield and elongation at yield do not indicate any loss of mechanical properties after 18 months of natural weathering (see table). In separate trials, PLA and a nucleating agent were compounded with an ecologically harmless flameretardant containing phosphate. Fire class UL 94-V0 was achieved by adding 10 % by weight (to the 4mm test object). Fire tests with building elements have yet to be conducted. Heat-related shape retention (ISO 75-2 B) of the polyactide, that has been modified for exterior use, is at a mean value of 79.2°C. Two % by weight of nucleating agent was added. PET-G, which is also used in exterior facades, exhibits HDT-B of approx. 70-72°C. Tensile Elongation N° in strength test requirements at yield figure 5 at yield [%] [N/mm²] 1 PLA 60 2,5 PLA after 240h immersion in water DIN EN ISO 62 60 2,5 PLA after 18month natural weathering DIN EN ISO 877 61 2,7 2 PLA after 855h artificial weathering DIN EN ISO 4892-3, process A, cycle 3 15 0,7 3 4 5 PLA after 1024h artificial weathering - behind window glass PLA + 10wt% phosphate containing flame retardant + 2 wt% nucleating agent ( NA) PLA + 10wt% Ph FR + 2wt % NA after 240h immersion in water PLA + 10wt% Ph FR + 2wt % NA after 18month natural weathering PLA + 10wt% Ph FR + 2wt % NA after 1024h artificial weathering PLA + 10wt% Ph FR + 2wt % NA after 340h artificial weathering - behind window glass test requirement tensile test: ISO 527 initial load speed initial load DIN EN ISO 4892-3, process B, cycle 5 16 0,9 46,5 3 DIN EN ISO 62 46,5 3 DIN EN ISO 877 46 2,6 DIN EN ISO 4892-3, process A, cycle 3 DIN EN ISO 4892-3, process B, cycle 5 2 N/mm² 10 mm/min testing speed 5 mm/min 15 0,7 48 2,6 Fig. 4 Tensile strength at yield and elongation at yield before and after weathering and immersion in water of PLA and modified PLA (Source: C. Köhler ) Fig. 7: Moulded components made from bio-based materials by Tecnaro (Photo: Bauer Thermoforming) After 1024 hours of artificial ageing (EN ISO 4982-3), which corresponds to a simulation of twelve months, elongation at yield is reduced from 46.5 N/mm² to 15 N/mm². Tensile strength at yield declines from 3 to 0.7 %. It is interesting, however, that the loss of properties after artificial weathering compares to that of pure polylactide (PLA). The ecologically harmless, phosphate based flame-retardant does not seem to influence weathering. Environmental simulations often take place under more extreme conditions in order to accelerate ageing. bioplastics MAGAZINE [04/13] Vol. 8 13
