Foam Cellulose-based
Foam Cellulose-based polymer foams O RO Requirements, Processing and Characteristics by Florian Rapp and Anja Schneider Polymer Engineering – Foam Technologies Fraunhofer Institute for Chemical Technology ICT RO OR Fig. 1: Cellulose-ester O Pfinztal-Berghausen, Germany RO CP: R = -H; -CO-CH 2 -CH 3 O RO CAB: R = -H; -CO-CH 3 and –CO-CH 2 -CH 2 -CH 3 O OR Fig. 3: Cell morphology of cellulose-based foam particle O n Research focus In the last decade Fraunhofer ICT has focused on a broad range of technologies in Polymer Engineering where one of the research activities is in the field of polymer foaming. Usually foamed polymer products do not appear obvious in application but they can be found in many aspects of daily life, for example in insulation elements increasing the energy efficiency of a building, protecting goods as packaging elements and serving as functional elements in the automotive industry. The activities of the Foam Technologies R&D group at Fraunhofer ICT have focused for more than ten years on new materials and processes for foamed thermoplastic polymers, in both particle foams and direct foam extrusion. Today biopolymer foams play an important role of research activities. Bio-based polymers offer high potential to substitute petrochemical polymers in several applications but also need research to adapt the properties of biopolymers to meet individual requirements. The main elements of research at Fraunhofer ICT are: • Material modification regarding foaming behaviour • Processing technology (process modification and development) • Material tailoring (property modification/improvement) • Characterization of matrix materials and foams Selected materials, processes and foams based on biopolymers will be detailed in this article. Bio-Materials for foam products On the current market there is a huge range of biopolymers available. Apart from polylactide (PLA), which is well-known in many applications, there is, for example, polyvinyl alcohol (PVA), polycaprolactone (PCL), polyhydroxyalkanoate (PHA), starch-based polymers or fully or partly bio-based PE, PA or PP. Another product field is represented by cellulose-based polymers like cellulose acetate (CA), cellulose propionate (CP) and cellulose acetate butyrate (CAB). These thermoplastic cellulose moulding compounds are gained by esterification of cellulose with acetic acid, butyric acid and propionic acid (see Fig. 1). Their beneficial property profile (high impact strength, weather resistance, high level of transparency, etc.), and biodegradability of the material, offers high potential for various applications like insulation and packaging. Special emphasis at Fraunhofer ICT is placed on material development and foaming behaviour of cellulose-based polymers, and the characteristics of bio-foam parts. The modification of polymer properties depends on the application. The latest research focus deals with environmentally friendly flame-retardants, 30 bioplastics MAGAZINE [01/13] Vol. 8
Foam Fig. 2: left: particle foam extrusion line, right: tandem direct foam extrusion line which is relevant in building insulation. For this purpose the interaction between phosphorus-based and nitrogen-based products, layered silicates and graphite with cellulose-based polymers are analyzed. Cell morphology and density of bio-foams varies according to their implementation. These structures can be tailored by modifying type and content of nucleating and blowing agents. Very good results were achieved by using pentane as a blowing agent but there is also the possibility, by using nitrogen, or carbon dioxide. Processing Technology For foaming thermoplastic polymers two principal continuous processing technologies are installed at the site of Fraunhofer ICT (see Fig. 2). On the one hand particle foams can be produced in continuous extrusion line using a twin screw extruder and under water pelletizer with a throughput of 8-30 kg/h. For further processing the entire process chain with pre-expansion equipment and two steam chest moulding machines is available to investigate the sintering behavior and to produce sample parts. On the other hand semi-finished parts (boards) can be foamed using a tandem direct foam extrusion line (KraussMaffei Berstorff ZE30/KE60) with a throughput of 40-70 kg/h. Characteristics of cellulose-based foam Special attention is paid to the raw material properties by investigating thermal behaviour (DSC - differential scanning calorimetry), melt strength and elongation viscosity (Rheotens test) plus molecular weight distribution (GPC - gel-permeation chromatography). The melt strength of cellulose-ester (CAB, CP, CA) is equal or even higher than that of polystyrene, which suggests a promising foamability. To improve the flame retardant requirements, new compounds based on the non-brominated additives mentioned above are developed and tested, reducing flame height, minimizing soot production and building an intumescence layer. The characteristics of the resulting foam beads are analyzed regarding bulk density, which ranges down to 25 kg/m³ by using CAB and CP, and also cell morphology (see Fig. 3) and geometry. The moulded parts are tested for thermal conductivity as well as compression behaviour (e-modulus, compression strength). At component densities of about 40 kg/m³ thermal conductivity measurement shows values up to 35 mW/(m*K) which is close to conventional EPS. However compression behaviour shows a difference in comparison to petrochemical polymers. The e-modulus is about 4.0 MPa and compression strength amounts about 0.03 MPa for a CAB-foamed or CP-foamed part at density of 40 kg/m³. Investigations demonstrate that cellulose-based foams show equal processability to petrochemical based foams, which offers the possibility of using established processing technology. Foamed sample parts can be seen in figure 4. Outlook Part of the future research will be the optimization of material properties, especially mechanical properties, and processing technologies in order to broaden the spectrum of applications for bio-based polymer foams. One focus is to replace conventional polymers in packaging and building insulation but also in technical parts with higher material requirements. www.ict.fraunhofer.de Fig. 4: Foamed biopolymer parts bioplastics MAGAZINE [01/13] Vol. 8 31
