Prof. Dr. Hans-Peter Fink: From biopolymer to high-performance plastics

Biopolymers, bio-based plastics, biorefineries – these fields of work are thought to be the solution to the scarcity of fossil resources and the increase in their prices. In this regard, the area of native polymers, traditionally worked on by the Fraunhofer IAP, contin­uously increases in importance.

The focus is on the biopolymers cellulose, hemicellulose, starch and lignin, which are synthesized by nature in almost infinite amounts. Their potential for a sustainable industrial use has not been exhausted yet by far. Cellulose is the most common biopolymer on earth and in the form of pulp it is an important resource for the chemical industry. Dissolving pulp is used to make products from regen­erated cellulose (among others fibres, films, sponges, membranes) on the one hand, and multi-purpose cellulose derivatives (thermo­­plastic, fibres, cigarette filters, adhesives, additives for building materials, drilling means, hygienic products, pharmaceutical components et cetera) on the other hand. Processing the cellulose, however, is difficult as this material is infusible and insoluble in conventional solvents. This results in a need for research reaching from improving existing procedures and products to developing completely new solutions and applications.


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Another field of work are bio-based plastics. Renewable resources like lignocellulosic biomass, starch or sugar deliver their base materials (for example ethanol or lactic acid), which are then merged by chemical synthesis into long-chain molecules of the desired plastics. One example
is the bio-­based plastic polylactic acid (PLA). Its polymer molecules are synthesized from lactic acid as a bio-based raw material by means of a ring-opening polymeriza­tion and the intermediate step of Dilactid.

In extracts, this article will provide some results of the Fraunhofer IAP’s research, on the one hand, showing some new possible uses for the “old” biopolymer cellulose and on the other hand, presen­t­­ing innovative composite materials made of technical cellulose fibres (rayon) and synthetic and bio-based plastics. It will also become clear that the biggest Fraunhofer Institute in Brandenburg con­­ducts industry-oriented research in co-operation with internationally leading companies of the respective market segments.
Fibres, films and nonwovens of cellulose. Today synthetic fibres of cellulose can be made using the relatively simple and environ­­mentally friendly lyocell technol­ogy. Almost 100 per cent of the solvent N-methyl­mor­pholine-N-oxide (NMMO), which is used in the process, is recovered, which makes the “chimney-free” plant possible. The Fraunhofer IAP soon found another advantage of this technology: the heated solution of cellulose in NMMO can be processed like melt. This means that for the first time, blown films and meltblown nonwovens can be made according to the same principles as the cost-efficient thermoplastic synthetic fibres, films and nonwovens. Together with partners in the industry, the Fraunhofer IAP has developed technologies on a pilot plant scale for producing blow films and melt­­blown nonwovens of cellulose. The blown film extrusion developed in co-operation with the Belgian company Teepak can be used for the production of sausage casings, packing materials or membranes for the separation of substances. Nowadays non­­wovens are a material class with stronger than average growth and cellulose fibres are especially suitable as a raw material. The meltblown-nonwoven technology developed on a pilot plant scale by the Fraunhofer IAP together with the companies Weyerhaeuser (USA) and Reicofil (D) provides nonwovens of cellulose with excellent mechanical and adsorptive characteristics, virtually for the first time in a continuous process. But in addition to developing new technologies, the institute also is a reliable and competent partner of the traditional viscose industry in issues regarding product and process optimization. A special focus here is on new uses of high-quality products made in Germany like rayon tyre cord.

 

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Rayon-reinforced thermoplastics. The Fraunhofer IAP has shown that rayon tyre cord fibres can be used advantageously for the reinforcement of oil-based and bio-based thermo plastic materials.

In the process, the properties of the thermo­­plastics improve significantly and improved stiffness, stability and impact qualities are reached.

Due to the double pultrusion process, which was developed at the institute, it is possible to produce compounds with excel­­lent homogeneity. They can, for example, be injection moulded into various formed components. In collaboration with the rayon manufacturer Cordenka (Germany) and the European automotive supplier Faurecia, automotive interior elements made of the newly developed material on a polypropylene basis were patterned and validated. Even parts which are visible to the occupants, can be made of this material and they have a look-and-feel of high quality. As an example, the picture on the previous page shows differently coloured interior panels of this material for the doors of the VW Polo. It only seems logical to use the bioplastics, which are recently gaining in importance, instead of the oil-based matrix materials. This way you get completely bio-based and biologically degradable composites that can contribute to our being indepen­dent of petroleum and to reducing CO2 and thus to a sustainable economy. An industrially important bioplastic is poly­­lactic acid (PLA), which is made from lactic acid produced by fermentation. Its properties can be significantly improved by reinforcing it with rayon, as the graph above shows.

The notoriously brittle PLA becomes much more impact resistant due to the rayon fibres, as you can see in the unnotched (a) and notched (an) Charpy toughness. At the same time, the strength σ and stiffness E improve significantly. This is unusual: normally these two properties decrease when you add ductility modifiers that only make the material more ductile by making it softer. You get similarly advan­­tageous results if you use another promising biopolymer class called poly­­hydroxyalkanoates (PHA).


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Rayon-reinforced thermoplastics are thus a promising, bio-based alternative to tradi­­tional plastics and composites: they have advantages regarding weight, mecha­­nical properties as well as disposal and recycling.

PhotoFink2009The author studied physics in Rostock where he also did his doctorate in 1977 and obtained his habilitation in 1991. From 1975 to 1992, he worked at the Institute of Polymer Chemistry of the Academy of Sciences of the GDR. He has been first the acting director then the director of the Fraunhofer Institute for Applied Polymer Research (IAP) since 2006. Prof. Dr. Fink teaches at the University of Potsdam and at the University of Kassel.