Additives Fig.1:
Additives Fig.1: Biocoustic Module (Photo: nimbus group) Biocoustic room divider Flame retardant PLA for interior use By: Carmen Köhler Institute of Building Structures and Structural Design University of Stuttgart The prototype Biocoustic Module originated within the scope of a joint research project covering “clear lightweight construction panels from renewable raw materials as space divider elements with acoustic function” being run by the company Nimbus Group, Stuttgart, Germany together with the Institute of Building Structures and Structural Design of the University of Stuttgart. The translucent module should be able to be used as a room divider in office blocks or public buildings. To allow the use of polylactic acid (PLA) in interiors, it was modified to give the desired behaviour in case of fire, and improved thermal stability. Background As part of an increasingly intense debate about sustainable construction and resource scarcity, there is a growing demand for materials that are resource-efficient, aesthetic and versatile. In new buildings an increasingly large amount of acoustically hard materials such as concrete walls, smooth floors and large windows is used. Hence sound cannot be sufficiently absorbed. The reverberation time increases dramatically. From practical experience comes the desire for an easily movable or convertible room divider, which maintains the visual open appearance and transparency, but which is also in a position to enable an acoustically pleasant environment. The aim of the project, which was funded by DBU (Deutsche Bundesstiftung Umwelt – a German government environmental agency ), was to develop a transparent or translucent acoustic board which has a high ratio of renewable resources in its construction and which can be used as a flexible space divider, either as a movable wall or as a component of space-in-space systems. An injection moulded module was developed, which is based on two half-shells that can be joined together. The perforated surface layers, the edges of the module and connector systems are manufactured using the injection moulding process. The micro-perforation is required for realizing an acoustically effective space planning while maintaining the visual transparency. Material requirement The translucent acoustic module should be offered at a competitive price for the market. With a view to resource protection a very high proportion of components made from renewable resources is important. Therefore, the decision was made to use the biobased plastic PLA. In terms of fire behaviour the classification UL94-V0 was desired. A heat distortion temperature of 70 degree centigrade should be able to be achieved. After modification, the PLA must still have a low viscosity, because the melted polymer must flow around the numerous steel pins in the tool. In addition to the performance improvement, a certain translucency of the PLA compound should be achieved. 38 bioplastics MAGAZINE [02/12] Vol. 7
Additives Modifications and results The flame retardant triphenyl phosphate (TPP) was chosen, because it does not affect light transmission (Fig. 6 upper right). TPP is a fish toxin. It is also used as a plasticizer for cellulose acetate polymers. The experiments have shown that addition of only 7-8 % by weight is sufficient. In fire tests in line with the U.S. standard UL 94-V, the modified material always extinguished by itself within 1-3 seconds after removing the flame. Non-burning droplets emerged during the second flaming. Cotton, at a standardised distance to the specimen, did not ignite (Fig. 4). Fig.2: An example of the application of the Biocoustic Module ©Nimbus Group The softening effect of TPP lowers the softening point and thus the heat distortion temperature (Fig. 5+6) at 45-46°C. To enhance this, a nucleating agent in form of a masterbatch was added to the compound. During injection moulding the cavity temperature was increased and the cooling times were varied. The cooling time begins with the volumetric filling of the mould, and ends with the opening of the mould. The heat deflection temperature (HDT-B) increases with increasing cooling time. In experiments at a mould temperature of 100°C and a cooling time of 3 minutes a PLA compound of 7 % by wt. TPP and 3 % by wt. nucleating agents the HDT-B values were obtained that indicated an average increase of 59.7°C. If the cooling time is extended by one minute, the HDT-B improves again by 20°C (average). The individual values fluctuate strongly, since the crystallization is not completed. A subsequent tempering to minimize these fluctuations increases the HDT-B additionally (Fig. 5 t3) A variant would be the production of the mouldings with the usual mould temperature for PLA of 25°C and a conventional cooling time. Five minutes of tempering at 100°C led to an average HDT-B of 73.5 degrees. To control any tendency to warp, expensive clamping tools must be built. The shrinkage Fig. 3: Samples of different materials after flammability test burning time after each flame impingement total flaming/combustion time (10 experiments) burn-off until burning clamp after flame - and annealing time after the 2nd flame impingement t 1 t 2 t 3 t 4 t 5 Cellulose acetate + 15wt% TPP + other PLA > 40s, fire was extinguished fire was extinguished before reaching the burning clamp > 30s, fire was extinguished PLA PLA + 3wt% + 7wt% TPP potassium-diphenylsulfone-sulfonate + other + 3wt% Nanoclay + other > 35s, fire was extinguished 1 to 3s > 35s, fire was extinguished > 30s fire was extinguished before reaching the burning clamp PLA + 7wt% TPP + 2wt% Nanoclay + other 11 to 25s > 300-320s no Fig. 4: results of the flammability test (UL 94-V) yes. In other experiments fire was extinguished before reaching the burning clamp > 30s, fire was extinguished 1 to 3s > 35s, fire was extinguished > 40s combustion of the cotton yes yes no yes yes Flammability class UL94 V2 V2 V0 V2 V2 transparency before burning yes yes yes no no proportion of renewable resources [%] ~ 60 ~ 100 ~ 92,5 ~ 93 ~ 90 no bioplastics MAGAZINE [02/12] Vol. 7 39
