in biocomposites

Activities
in biocomposites
by Jörg Nickel and Ulrich Riedel
DLR – the German Aerospace Center – is the
aerospace agency of the Federal Republic of
Germany. Core competences are in aeronautics,
space flight, energy technology, and traffic
management. With 4700 employees, DLR maintains
eight sites in Köln-Porz, Berlin, Bonn, Braunschweig,
Göttingen, Lampoldshausen, Oberpfaffenhofen, and
Stuttgart and operates 31 institutes. In Braunschweig,
at the Institute of Structural Mechanics, more than
100 employees are engaged in the development of
innovative concepts for lightweight structures,
especially in the field of aerospace technology. Core
competences are applications involving fiber
composite materials. In addition to classic composites
using glass or carbon reinforcing fibers, so-called
biocomposites with their constituents derived from
renewable resources are being developed.
Biocomposites have properties similar to wellestablished glass fiber composites, but offer
additional eco-friendly recovery options.
DLR German Aerospace Center
(Deutsches Zentrum für Luft- und Raumfahrt e.V.)
Institute of Structural Mechanics
Lilienthalplatz 7
D-38108 Braunschweig, Germany
E-mail: [email protected]
URL: www.sm.bs.dlr.de
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April 2003
Aerospace technology was the original application of
fiber reinforced polymers (FRPs). Now, however,
these materials are being used in numerous other
technical fields, especially where high strength and
stiffness at a low weight are required. Their good
specific, i.e. weight-related, properties are the result
of the low densities of the applied matrix systems
(e.g. unsaturated polyesters, polyurethanes, phenolic,
or epoxy resins) and the embedded high-strength and
high-modulus fibers (from glass, aramid, or carbon).
Further benefits result from the option to tailor a
composite part to specific demands during production
by orienting the reinforcing fibers in the load
direction.
Classic FRPs, however, often cause considerable problems
in terms of reuse or recycling at the end of their lifetimes.
This is primarily because the compound consists of
miscellaneous and usually very stable fibers and matrices. A
simple landfill disposal is not an option, since increased
environmental sensitivity has resulted in tighter laws and
regulations (e.g. “Regulation for the Prevention of Packaging
Waste”, “Recycling and Waste Management Law”, 1991).
Eco-friendly alternatives are now being explored and
examined, such as the recovery of raw materials (e.g.
gasification to methanol), CO2-neutral thermal utilization, or
biodegradation.
An interesting option to meet these demands may be
provided by composite materials that are made from
completely renewable resources. These biocomposites consist
of directed or undirected natural reinforcing fibers embedded
ISSN:1369 7021 © Elsevier Science Ltd 2003
INSIGHT FEATURE
in so-called biopolymers. Economically and ecologically
acceptable manufacturing technologies are involved as well.
These have all been subjects of research at the DLR Institute
of Structural Mechanics since 1989.
Biocomposites
In biocomposites (Fig. 1), as well as in any other fiber
composite materials, the reinforcing fibers have to show a
high tensile strength and stiffness, while the embedding
matrix provides the shape of the composite structure,
transmits the shear forces between the fibers, and protects
them against radiation and aggressive media.
The selection of suitable fibers is determined by the
required values of the stiffness and tensile strength of a
composite1-3. Further criteria for the choice of suitable
reinforcing fibers are, for example, elongation at failure,
thermal stability, adhesion of fibers and matrix, dynamic and
long-term behavior, price, and processing costs.
Natural fibers can be subdivided into vegetable, animal,
and mineral fibers. All vegetable fibers (e.g. cotton, flax,
hemp, jute, etc.) are composed of cellulose, whereas fibers of
animal origin consist of proteins (e.g. hair, silk, wool).
Vegetable fibers can be generally classified as bast, leaf, or
seed-hair fibers, depending on their origin4-7.
Many natural fibers have a hollow space (lumen) resulting
in low densities, and have nodes at irregular distances that
divide the fibers into individual cells. The surface of natural
fibers is rough and uneven and provides good adhesion to the
matrix in a composite structure. When considering the
potential of natural fibers for composites and comparing the
tensile strength, elasticity, and elongation at failure with
synthetic fibers, it becomes obvious that hemp, flax, and
ramie fibers can compete with E-glass fibers (Al-B-silicate
glass5), which serve as a reference because of their great
importance in composite technology6.
For manufacturing biocomposites, the required
biopolymers and their basic constituents must also be made
predominantly of renewable resources. Similar to polymers of
petrochemical origin, biopolymers (without considering
elastomers) are subdivided into thermosets and
thermoplastics, both of which are suitable as matrix systems
for biocomposites7-9.
The thermoplastic biopolymers that have been developed
primarily for the packaging industry do not have the material
properties to meet the matrix system requirements for fiber
Fig. 1 Components of biocomposites.
composite materials. In particular, the overly high breaking
elongation and high processing viscosity are disadvantageous
for this intended usage. As a result, there has been
considerable need for development in the area of thermosets
from renewable resources. In contrast with thermoplastics,
thermosetting materials cannot be plastically softened by
heating because the polymer chains are cross-linked by
intermolecular bonding. Thermosets are usually supplied as
partially-polymerized or monomer-polymer mixtures. Crosslinking is achieved during fabrication using chemicals, heat, or
radiation in a process known as curing or vulcanization. For
the development of naturally-based thermosets, suitable
starting substances can be provided by maleinated
triglycerides, epoxidized vegetable oils, polyoles, and
aminated fats. Petrochemical reagents are still needed to
cross-link these monomers and to create and integrate stable
molecule sequences. Among these substances, isocyanates,
amines, polyoles, and polycarboxylic acids are preferred. The
aim, of course, is to maximize the proportion of renewable
resources used while retaining acceptable material properties.
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The first polyurethanes on the market were composed of
petrochemically-based 55% isocyanate and 45% natural
polyolefin, as renewable resources were not yet an issue.
Research is now, however, concentrating on the development
of an isocyanate from a biological source. Vegetable oil
epoxy acrylates and vegetable oil epoxy resins are also being
developed. Various combinations have been examined in
order to find suitable solutions. In addition, different fillers,
e.g. organic substances (starch, etc.), and inorganic
substances (calcium carbonates, magnesium oxides,
aluminum phosphates, etc.), have been tested. The latter play
an important role as stabilizers or flame retardants.
As the result of a research project with Cognis
Deutschland GmbH, three oleo-chemically-based
thermosetting resin systems (acrylate basis) have been
optimized and qualified for numerous technical applications.
Furthermore, they can be easily adapted to customers’
requirements. Curing temperatures10,11 vary from 70°C to
160°C, covering a wide range of curing times between
30 minutes and 30 seconds.
At the DLR Institute of Structural Mechanics, biopolymers
are tested as matrices for biocomposites10-12. In addition,
their potential for application with new manufacturing
techniques is being examined (e.g. new resin injection
procedures such as the differential pressure resin transfer
molding (DP-RTM) and single line injection molding (SLI)
technologies, which were developed at the DLR Institute of
Structural Mechanics)13.
Manufacturing techniques for the production of
biocomposites are being developed, analyzed, and optimized
based on commonly used procedures in composite
technology. These include press molding, hand lay-up,
filament winding, and pultrusion14, which are
being tested with only slight modifications for the
manufacture of components. Unidirectional (UD) fiber
reinforced laminates or noncrimp fabrics have to be applied
in order to take advantage of the anisotropy of the fiber
reinforced composites. Nonwovens are used as
reinforcements to meet minor requirements in terms of
mechanical properties.
The concept of structural materials made of renewable
resources shows the potential of this new class of materials.
In a R&D project carried out in cooperation with Schuberth
Helme GmbH (funded by the Ministry of Food, Agriculture,
and Forestry of Lower Saxony), the goal was to develop an
industrial safety helmet (Fig. 2) made of a minimum of 85%
renewable resources. An appropriate fiber/matrix system
based on the concept of biocomposite materials was
developed to meet the German Industrial Standard
requirements for industrial safety helmets, DIN EN 397.
While working on the material aspect of the problem, a
manufacturing process with appropriate productivity and
quality had to be developed for series production of the
helmet bowls. Test results showed that requirements for the
helmet bowls were met or even surpassed with the new
material. Because of the optimized lay-up sequence, as well
as making use of the lightweight construction potential of
the natural fibers, it was possible to achieve a reduction in
weight of 5-10% and considerably improve comfort for the
wearer.
Interior panelings for rail vehicles
Panelings for air columns were developed for the LIRex
(Light Innovative Regional express, which was launched at
the Innotrans in Berlin) concept study by Alstom, with funds
from the Lower Saxony Ministry of Food, Agriculture, and
Forestry. The columns form connection elements between
two of the train’s side windows.
In pursuit of applications
Developmental work at the DLR began in 1989 and emphasis
has been placed on selected industrial collaborations, which
have led to marketable or completely new products.
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April 2003
Fig. 2 Industrial safety helmet.
INSIGHT FEATURE
The guidelines for DB AG (Deutsche Bahn
Aktiengesellschaft) German Railroad, Inc. vehicles, according
to TL 918413 internal standard, were also maintained. It
should be noted here that the DIN 5510-1 German Industrial
Standard for fire protection has considerable significance and
is very high. The biocomposite had to be equipped with
halogen-free flame retardants and attained the highest levels
of fire-protection (class S4), smoke development (class SR2),
and fluidity (class ST2).
In addition, biocomposite seat panelings (Fig. 3) have been
developed fulfilling similar requirements and resulting in
weight savings of 30% compared with glass fiber reinforced
materials. In a pilot scheme, five Hamburger Hochbahn trains
have been equipped with these panels as a first series
application and are now being tested in long-term usage.
Hand-friendly image products
The hand-friendly image product (Fig. 4), or h.i.p., is the
result of a collaboration between the DLR Institute of
Structural Mechanics and the Burg Giebichenstein School of
Art and Design in Halle, Germany. It consists of two
geometrically identical mold parts made by hot pressing.
Between these two mold halves is a snap fastener to hold a
CD-ROM, which contains audio-visual information about the
material itself. The housing is tactile and designed to be
‘hand-friendly’. In addition to its function as a CD-holder,
the h.i.p. product is exemplary of any type of ‘housing’,
such as those for cellular phones, portable CD or MP3
players, personal digital assistants (PDA), etc. Beyond its
function as a ‘design product’, the material provides an
interesting approach to a multitude of technical applications.
An observer is able to experience the material with all of
their senses, understand it, and the direct and indirect
information it contains will, it is hoped, inspire the observer
to find new applications for the material or to take a new
look at its properties.
The textile and graphic structures integrated during the
manufacturing process directly highlight the multitude of
possibilities surface design has to offer. Since it is a natural
material, signs of wear and tear and the formation of patina
have deliberately been taken into account. These types of
effects generally increase the personal value of leather and
other natural products. With technical products, however, a
perfect finish that looks new for as long as possible is usually
only acceptable. This fact, in turn, provides new approaches
for product ideas.
Fig. 3 Seat paneling element.
Fig. 4 Hand-friendly image product.
April 2003
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INSIGHT FEATURE
After completion of the project in February 2002, a small
series of 120 samples of the h.i.p. product and 1000
CD-ROMs were manufactured and released at the Hanover
Industrial Fair 2002 as a give away for interested visitors and
potential customers. The initial response was very positive,
but the next step is to continue realizing these insights and
to apply them to specific products.
The DLR’s experiences of its R&D projects have shown that
weight-related mechanical properties enable biocomposites
to be used in applications that are still dominated by glass
fiber reinforced plastics. At this time, however, limitations
must be accepted in applications where there are extreme
environmental conditions. The main target groups are,
therefore, interior paneling elements in automobiles and rail
vehicles, and the furniture, sports, and leisure industries.
Products are not yet available on the market, however, it can
be expected that paneling elements for rail vehicles will go
into series production this year and additional products will
follow. Further research activities must be focused on both
manufacturing technologies and the optimization of
components. It is expected that after the introduction of the
first biocomposite products, market acceptance will
considerably increase and new markets will open up.
it is extremely difficult to replace even some of the materials
currently being used.
In regard to the increase in retraction obligations
(especially the recent European Union guidelines for old cars),
the drive for using biocomposites in market products is
significant. Political specifications are helpful on the one hand
but, on the other hand, producers are increasingly taking life
cycle costs into account. The conclusion is that researchers
must be actively involved in changing the boundary
conditions on all levels to help make these new materials
successful. In addition to this issue, new applications for
biocomposites such as in the furniture industry, and in other
components for outdoor usage have to be identified.
Furthermore, cost-effective manufacturing technologies have
to be developed in order to greatly increase marketing
chances.
Knowing that material substitution is very difficult, new
ways of launching products onto the market are having to be
found. Beyond the description of materials with physical
parameters and economical performance figures, it is
important for the product determination to include other
characteristics such as haptics, optics, etc. The motivation
behind the close cooperation of the DLR with industrial
designers is the long-term aim of creating new products from
biocomposites. MT
Perspectives
Acknowledgments
Considering the results of the DLR’s R&D projects, it becomes
evident how challenging it is to meet all the requirements in
order to launch new products onto the market. At this point,
The support of the various projects by Lower Saxony governmental authorities and by the
Agency for Renewable Resources (FNR) is gratefully acknowledged. In addition the
authors wish to express their gratitude to the DLR biocomposite research group for
creative assistance.
Conclusions
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April 2003
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