Automotive Polyurethane Composite Parts Made with Natural Fiber ...

Automotive Polyurethane Composite Parts Made with
Natural Fiber Mats and Honeycomb Cores
Terry D. Seagrave
Bayer Corporation
100 Bayer Road
Pittsburgh, PA 15205-9741
USA
Natural fiber-reinforced polyurethanes are being
used more and more in the automotive industry.
Photo 1: Mercedes S-Class Door Panel
This has not always been the case. In the past,
natural fiber-reinforced products were often
dismissed as a passing phase. Today, even sceptics
have to admit that these products have a role to
play in the automotive industry. They often surpass
other materials such as glass fiber-reinforced
materials in terms of properties and price and also
with regard to industrial hygiene. To safeguard
employees, many firms today avoid processing
glass products.
Anticipating this trend, Bayer has developed
polymer systems based on polyurethane raw
materials and, in co-operation with their subsidiary
Hennecke GmbH, have developed efficient process
engineering for manufacturing trim from a
combination of natural fiber mat and a
polyurethane system. We can now produce parts
with wall thicknesses of 1.5 – 2.0 mm - depending
on the unit weigh of the natural fiber mat used.
End products weigh from 1,250 to 1,600 g/m² with
natural fiber contents between 55 and 65 % by
weight. These end products weigh about 45 % less
than products using conventional injection molding
materials, in comparison. This potential weight
reduction is attractive to car manufacturers. Bayer's
raw materials are marketed under the trademark
Baypreg®; Hennecke offers the associated plant
engineering under the name "NafpurTec".
The natural fiber mat used nowadays consists of a
50/50 mixture of flax and sisal fibres. Other fibers
are possible to use as well. For example, hemp has
been evaluated and found to produce good
properties. The fiber products are supplied in rolls
with widths adapted to the production process. The
mat weight currently being used varies between
about 700 and 1,200 g/m², depending on the
application.

Photo 2: Natural fiber mat (Enlarged view)
The first stage in the production of trim from rolls
is the pre-conditioning of the natural fiber mat.
This stage is necessary because the moisture
content of the untreated mat varies seasonally
between about 6 and 12 %. This is unacceptable
for high-quality products, so the moisture content
has to be reduced to and kept at a low value
(preferably

mold is immediately closed. The pressing process
shapes the part itself while it cures. The reaction of
the polyurethane system is triggered by heat
supplied by the hot mold.
In contrast to other polyurethane processes,
therefore, the reinforcing material is combined with
the polyurethane system outside the mold when
natural fiber mat is processed with Baypreg. The
combined materials are inserted into the mold,
where a simple compression process carries out the
shaping and curing of the part.
Molds are generally made of metal (preferably
aluminium, or alternatively steel), for three
reasons:
1. Relatively high pressing force (about 200 t to
250 t for a door trim with an average area of
0.6 m 2 )
2. Mold temperatures of about 130 °C
3. The systems contain internal mold release
additives, which only act in metal molds.
Figure 2. shows the pressing force required to press
a flat piece of natural fiber mat having an initial
thickness of about 8 - 10 mm and weighing about
900 g/m 2 . When producing components, this piece
of mat would be compressed to a wall thickness of
about 1.7 - 1.8 mm. According to Figure 2, a
pressing force of about 12 bar, which converts to
about 75 t, is required for a door trim of average
size (~0.6 m 2 ). This pressing force is determined
on flat material and is not sufficient for producing
actual door trims, which usually have considerable
vertical regions requiring more than the pressing
force calculated for the projected area. In practice,
door trims require a pressing force of at least 150 t,
and preferably 200 t to allow a safety margin.
Figure 2: Pressing force required for pressing flat
natural fiber mat
Aluminium molds are quite adequate for normal
production. Steel would only be required if cutting
operations are incorporated in the actual pressing
mold (for example sheer edge trimming). A further
50 - 75 t pressing force may have to be added,
depending on when these cutting operations
actually take place, i.e. by how much the mat is
already compressed.
The processing of Baypreg involves mold
temperatures of about 130 °C for two particular
reasons:
1. The internal mold release additives
contained in the systems only act at mold
temperatures above about 120 °C.
External release agents can be completely
avoided during production. The in-mold
release is adjusted so that laminating
adhesives and/or other PU foams can
adhere to the finished article. Back-filling
foams (semi-rigid polyurethane foams)
can be applied by typical foaming
processes; and films, textiles, or leather

can be laminated without treating the
surface of the components.
2. Molding times can be markedly reduced
owing to the high temperature of the mold
in combination with carefully selected
catalysis. Today, molding times of 45
seconds are feasible, and we aim to
reduce them to 30 seconds.
Once the mold has been opened, the components
can be released automatically (Figure 1, item 7)
and placed on a cooling rack. After cooling, they
are conveyed to subsequent fabrication stages such
as cutting and lamination.
With a PU content of < 45 % by weight, based on
the end product, the molded components can still
be laminated under vacuum. Vacuum holes do not
generally have to be drilled into the components
under these conditions. Fasteners (such as glass
fiber-reinforced polyamide) can be inserted and
pressed directly in the mold.
Another use of this natural fiber mat technology
incorporates the use of paper honeycomb. The
paper honeycomb used as a core or spacer to create
a sandwich panel composite. The paper
honeycomb provides the physical property feature
of exceptionally high stiffness at a very low
composite part weight.
The manufacturing process is practically identical
to the natural fiber mat process described earlier. In
this case, a paper honeycomb core separates two
outer layers of natural fiber mat. This entire
composite is handled by the gripper system for the
subsequent spraying process. Since the honeycomb
core acts as a thermal insulator during the curing
step in the mold, the cure times are somewhat
longer than that of natural fiber mat alone, as
described previously. The pressing force for this
particular process is much lower, since the paper
honeycomb crushes easily. 30t to 75 t is typically
adequate for interior trim parts.
The Baypreg/sandwich composite construction is
becoming quite popular for sunshade, spare tire
covers, and load floor applications. In Europe, this
particular composite technology is being used for
BMW, DCX, and Audi, to name a few.
Photo 3 shows the Audi 4 spare tire cover, which
uses Baypreg sandwich composite construction.
Photo 3: Audi A4 Spare Tire Cover
Photo 4 shows a cut away view of the honeycomb
sandwich composite. Notice that this shows also
the incorporation of metal brackets integrated into
the design.

Photo 4: Sandwich composite with integrated
metal piece
The paper honeycomb can be used in various
thicknesses for a specified application. While the
Baypreg manufacturing process is not generally
changed, the molded part’s physical properties do.
Table 1 shows some comparison of properties with
and without paper honeycomb, and at different
paper honeycomb thicknesses. Note that the
different values reported in the Composite
Thickness category are a result of using the mat
alone, or in combination with a paper honeycomb
core. Note also that the paper honeycomb core
sandwich properties are exceptionally good at
composite densities below 0.5 g/cc.
Conclusions
The combination of natural fiber mat, honeycomb
cores, and suitable polyurethane systems, produced
by a simple and efficient process, has resulted in a
progressive alternative material for top-quality,
lightweight components, which represent a good
value for the money. The process to produce the
molded parts is similar in practically all aspects of
mass production. This fact lends itself to
multiplicity of parts utilizing the same NafpurTec
processing equipment.
The first car to be launched with door trims made
of natural fibre-reinforced Baypreg was the
Mercedes-Benz S-class (internal code W 220).
Other vehicles and other European manufacturers
have followed follow suit with sunshades, spare
tire covers, and door trim panels production.
Indications are that the North American market has
begun to embrace this technology for themselves.
Table 1: Mechanical properties of Baypreg with
Composite
Construction*
Natural Fiber
Mat, g/m 2
Composite
Thickness, mm
Composite
Density, g/cc
Weight/Unit
Area, g/m 2
Flexural
Modulus, MPa
*Legend:
Natural fiber and Honeycomb Sandwich
components
1. 2. 3.
800 525 525
1.6 8.0 12.0
0.75 0.49 0.40
1200 3920 4800
3000 2700 2990
No. 1=Natural mat only
Nos. 2 & 3= Natural mat with 8 & 12 mm paper
honeycomb core, respectively

BIOGRAPHY
Terry D. Seagrave
Terry D. Seagrave is the Manager for the
Automotive SRIM Business Team at Bayer
Corporation. He has held several positions with
increasing responsibility during his ten years at
Bayer. These positions include Sr. Technical
Marketing Representative for Automotive SRIM,
Development Chemist and Technical Service
Representative for Automotive RIM. He holds a
B.S. Degree in Chemistry from Taylor
University.