**5. Foaming of biocomposites**

Biocomposites used in the construction and automotive sector are frequently called "artificial wood" because their many properties and appearance are like wood (Matuana et al., 1998; Migneault et al., 2008; Bledzki et al., 2008). Unfortunately, the density of biocomposites, even if markedly lower than that of glass fiber reinforced composites, is still twice as high as the natural wood density. That drawback can be reduced by foaming of biocomposites that are lighter and feel more like real wood (Rodrigue et al., 2006; Guo et al., 2004, Bledzki & Faruk, 2006, Kozlowski et al., 2010). The earliest known foamed and woodfilled thermoplastics were based on polystyrene (PS) - this amorphous polymer is a perfect bubble catcher. Wood flour itself has been proved as an efficient nucleating filler in polyethylene foamed with azodicarbonamide (Rodrigue et al. 2006). As far as length of natural fibers is concerned, short fibers (75-125 μm) are favorable for foaming, since they do not disturb the cell growth process, like do the long fibers (4-25 mm).

Fig. 30. Cross section of foamed PP filled with 30 wt.% of wood flour - injection molded (left) and extruded profile (right)

Selection of the polymer matrix is very important for properties of biocomposites. Because cellulose fibers are polar, the hydrophobic matrices (like polyolefines) need addition of adhesion promoters in order to facilitate regular fiber distribution and efficient stress transfer across the composite during deformation in a molten state and during/after solidification.

Cellular biocomposites can be manufactured both by the extrusion or injection molding technology, however the extrusion foaming provides better results (Fig. 30), as it allows for a more precise process control.

Lightweight Plastic Materials 315

The study on foaming of biocomposites confirmed the findings formulated for non-filled polymers. High drop in the melt pressure between that in a die and ambient is favourable for manufacturing foams of fine, regular cells (Fig. 31), however if it is too high, that causes

In general foaming is more difficult due to the high melt viscosity of biocomposites and low melt strength, however the results of research reported in recent years for foaming of wood composites with the chemical or physical blowing agents are promising. The technology of extrusion foaming seems to be fully controled and the profiles manufactured looks like wood outside (Fig. 33) and in a cross-section (Fig. 34). The possibility of foaming composites filled with cellulose fibers make them ideal candidates for the low weight and thermal

a cell damage and foam collapse (Fig. 32).

insulating engineering materials in all transport modes (Fig. 35).

Fig. 33. Extrusion foaming of PP filled with 30 wt.% of wood flour

Fig. 34. Cross-section of PP/woof flour composite

Fig. 31. Dependence of foam morphology on melt pressure for LDPE/wood flour composite

Fig. 32. Dependence of foam morphology on melt pressure for PP/wood flour composite

Fig. 31. Dependence of foam morphology on melt pressure for LDPE/wood flour composite

Fig. 32. Dependence of foam morphology on melt pressure for PP/wood flour composite

The study on foaming of biocomposites confirmed the findings formulated for non-filled polymers. High drop in the melt pressure between that in a die and ambient is favourable for manufacturing foams of fine, regular cells (Fig. 31), however if it is too high, that causes a cell damage and foam collapse (Fig. 32).

In general foaming is more difficult due to the high melt viscosity of biocomposites and low melt strength, however the results of research reported in recent years for foaming of wood composites with the chemical or physical blowing agents are promising. The technology of extrusion foaming seems to be fully controled and the profiles manufactured looks like wood outside (Fig. 33) and in a cross-section (Fig. 34). The possibility of foaming composites filled with cellulose fibers make them ideal candidates for the low weight and thermal insulating engineering materials in all transport modes (Fig. 35).

Fig. 33. Extrusion foaming of PP filled with 30 wt.% of wood flour

Fig. 34. Cross-section of PP/woof flour composite

Lightweight Plastic Materials 317

Ellison, G. C.; McNaught, R. & Eddleston E. P. (2004). *The use of natural fibres in nonwoven* 

Ministry of Agriculture Fisheries and Food, London, United Kingdom Guo, G.; Rizvi, G. M.; Park, C. B. & Lin, W. S. (2004). Critical Processing Temperature in

http://www.akzonobel.com/expancel/application\_areas/benefits/porous\_structure/inde

http://www.sulzerchemtech.com/en/portaldata/11/Resources//brochures/mrt/Optifoa

Huang, H.-X.; Wang, J.-K. & Sun, X.-H. (2008). Improving of Cell Structure of Microcellular

Institute of Materials, Minerals and Mining. (2004). Lightweight Platic Panels Manufactured

Khorasani, M. M.; Ghaffarian, S. R.; Babaie, A. & Mohammadi N. (2010). Foaming Behavior

Klesov, A. A. (2007). *Wood-plastic composites*, Wiley, ISBN 978-0470148914, Hoboken, USA Kozlowski M. & Kozlowska, A. (2005). Biocomposites from waste plastics, *Proceedings of Global Plastics Environmental Conference 2005*, Atlanta, USA, February 23-25, 2005 Kozłowski, M.; Kozłowska, A. & Frąckowiak, S. (2010). Materiały polimerowe o strukturze

Lee, J. W. S. & Park C. B. (2006). Use of Nitrogen as a Blowing Agent for the Production of

Lee S.T. (2000). Foam Nucleation in Gas-Dispersed Polymeric Systems, In: *Foam Extrusion.* 

Lee, S. T.; Park, C. B. & Ramesh, N.S. (Eds.). (2006). *Polymeric Foams: Science and Technology*,

Li, G.; Wang, J.; Park, C.B. & Simha R. (2007). Measurement of Gas Solubility in

Matuana, L. M.; Park, C. B. & Balatinecz J. J. (1998). Cell Morphology and Property

Fine-Celled High-Density Polyethylene Foams. *Macromolecular Materials and* 

Linear/Branched PP Melts. *Journal of Polymer Science Part B-Polymer Physics.* Vol.45,

Relationships of Microcellular Foamed PVC/Wood-Fiber Composites. *Polymer*

komórkowej. *Polimery*. Vol.55, No.10, pp. 726-739, ISSN 0032-2725

*Principles and Practice*, Lee, S.T. (Ed.), CRC Press, Boca Raton, USA

Mapleston, P. (2002). Modern Plastics Worldwide, November, p.31, ISSN 0026-8275

*Engineering and Science*. Vol.38, No.11, pp. 1862-1872, ISSN 1548-2634

*Engineering*. Vol.291, No.10, pp. 1233-1244, ISSN 1438-7492

Foams Based on Polypropylene/High-density Polyethylene Blends. *Journal of* 

from Recycled Plastics - Panels for Commercial Vehicles. *Materials World*, Vol. 12,

and Cellular Structure of Microcellular HDPE Nanocomposites Prepared by a High Temperature Process. *Journal of Cellular Plastics*, Vol.46, No.2, pp. 173-190, ISSN

*Applied Polymer Science*. Vol.91, No.1, pp. 621-629, ISSN 0021-8995

http://www.adeka-palmarole.com

http://www.azflex.pl/izolacje/producent/armacell.php http://www.cinpres.net/index.php/cinpres-technologies/igm http://www.mazda.com/mazdaspirit/env/engine/mold.html http://www.specialchem4polymers.com/resources/articles

No. 6, pp. 25-27, ISSN: 0967-8638

CRC Press, Boca Raton, USA

No. 17, pp. 2497-2508, ISSN 0887-6266

http://www.trexel.com/injection-molding-solutions/index.php

*Cellular Plastics*, Vol.44, No.1, pp. 69-85, ISSN 1530-7999

x.aspx

m\_e.pdf

1530-7999

*structures for applications as automotive component substrates,* R&D Report NF0309,

Manufacture of Fine-Celled Plastic/Wood-fiber Composite Foams. *Journal of* 

Fig. 35. Properties of PP, PP biocomposite and their foams
