Long glass fiber thermoplastics - Fiber Reinforced Polymer

Long glass fiber thermoplastics
Material classification and characterisation
1. Introduction
There has been a rapid and steady growth
in the use of long fiber reinforced plastics in
recent years. This has been achieved due to
their potential of combining high performance
/ price ratios - such as reduced weight,
corrosion resistance, great design and styling
flexibility - with rapid clean processability
and the attraction of their intrinsic recyclability.
Since the early 80ties thermoplastic composites
(TC) in structural applications have
played a greater role in the automotive industry.
The combination of high volume
processing, and high end-use properties
and lower system costs, is a strong driving
force for further applications. Furthermore
their high integration potential - the consolidation
of various parts into one single component
- is rapidly being recognised as a
major advantage in comparison to traditional
materials. Typical automotive applications
are dashboard carriers, technical front-ends,
seat shells, bumber carrier, battery trays,
spare wheel dwells and – as one of the
most rapid growing markets - complete underfloor
systems.
2. New material and process developements
For a long time, the main driving force for
new thermoplastic composite applications,
was represented by Glass Mat reinforced
thermoplastics (GMT). Random in-plane
fiber reinforced GMT-sheets can be prepared
by melt impregnation of non-woven
glass mats (dry process), or by mixing
chopped fibers with polymer powder in a
fluid medium, followed by straining, drying
and consolidation (wet process). GMT
sheets were heated up in an oven system
and pressed to complex parts in a press
forming process. Although the GMT process
is an established and well known technology,
it´s role of a ´trailblazer´ for new and
innovative applications is being challenged.
Caused by the necessity of massive cost
savings - a clear demand especially from
the automotive industry - a number of alternative
materials and processes has been
developed in the last years.
Long fiber reinforced granulates (LFG), pellets
and chips (STC), prepared by wirecoating,
crosshead extrusion or several pultrusion
techniques were introduced 10 years
ago, recent developments in this area are
based on commingling and powder impregnation
techniques.
These long fiber granulates are suitable
both for the classic injection moulding process
(IM) and the injection compression
moulding process (ICM) as well as for the
extrusion compression moulding process
(ECM).
Common to all these processes is a plastification
unit in form of an injection or extrusion
screw which is the mainspring for the
process intrinsic deficit - an apparent fiber
length degradation in the finished part /1-4/.

This typical reduction of the fiber length is
irrespective of massive ameliorations in the
field of the plastification systems (screw
geometrie, mixing zones, etc.), and optimised
process parameters (screw speed,
injection speed, back pressure, etc.).
av. fiber length lm [mm]
25
20
15
10
5
0
basis length
zone 46
zone 32
zone 27
Picture 1: Typical degradation of the average
fiber length, found in ECM-process (PP-
LFG25mm, 40% glass content)
Specifically in closed mould operation – like
in the injection moulding process - the additional
fiber length reduction in the nozzle,
the gating system and the mould itself
(geometrie, radii. etc.) must be taken into
consideration to minimize fiber breaking and
nonuniform physical properties in the finished
part /1-5/.
A real breaktrough in thermoplastic composites
was the idea of combining the compounding
of a long fiber reinforced thermoplastic
material directly with the final part
production process. The reinforcing fibers
were incorporated and impregnated with the
PP-matrix directly during the compounding
and plastification process of the thermoplastic
composite (DIF-process). The leaving
out of semifinished products, like blanks
zone 16
screw zone
zone 1
extruder
head
plastificat
part
or pellets, leads to a clear benefit concerning
to the material and process costs.
The main developments were performed in
the area of extrusion compression moulding.
These process technologies could be differentiated
by different compounding methods
(single or double screw extruder), impregnation
methods (direct feeding or accessory
systems), reinforcing material basis
(chopped or endless fibers), matrix material
(PP in form of granulats, powder or water
dispersion) /6-13/. Although the development
of modified injection moulding processes
was started almost 10 years ago, a
broad industrial introduction of these systems
is expected previously for the next
years /14-18/. But the cut off of the semifinished
material step also has some drawbacks
- the part producer is not only responsible
for the final production but additionally
for the homogenisation and preparation
of the raw materials and the material
classification and quality. Furthermore is
has to be noted that an in line compounding
system needs higher investment costs and
skilful service personal.
Regarding all these developments on the
field of new materials and different processes,
the part producer is confronted with
the question which kind of process and
which kind of material is the right decision.
On the one hand he has to fulfill the demands
and requirements of his customer for
a specific part - on the other hand he has to
produce on a cost level which is accepted
by the market.

One of the main tasks in future projects
could be the qualified comparison of different
types of materials in one and the same
application, the assessment of different processes
and the efficiency of process parameters.
Therefore a basic understanding
of the general influencing factors by the designers
and the engineers is a very important
condition for a better understanding of
the performance, the advantages and the
limitations of long fiber reinforced thermoplastics.
3. Material basics of long fiber reinforced
thermoplastics
The mechanical properties of a part made of
reinforced thermoplastics are defined by the
matrix system, the fibers, the fiber content
and most of all by the orientation of the reinforcing
fibers. High performance levels
can only be obtained from a composite part
with high fiber concentrations and if the reinforcing
fibers in the final product have a
sufficiently high aspect ratio - which defines
the length to diameter relation of a single
fiber. Mainly affected on different, flow induced
fiber orientations, it is usual that a
great variation of properties could be found
over a part. The orientation and the length
of the fibers are distinctly influenced by the
production process and the variation of process
parameters. This is the reason why
different mechanical values may be found
on identical parts produced by different
technologies and why a comparison of different
materials is rather difficult although
the part geometry is the same. Even in the
same zone a difference of the mechanical
properties as a function of the traction angle
applied, may be found /19/.
It is obvious that measuring or indicating
single values – wheater it is a young´s module,
a tensile strength or a charpy impact
energy – which is often required and practiced
- is no sufficient method to handle the
complexity of these kind of materials.
3.1 Influence of the fiber orientation and
the fiber length on the mechanical properties
relative quantity [%]
9,00
8,00
7,00
6,00
5,00
4,00
3,00
2,00
1,00
0,00
Modulus [Mpa]
-90°
0°
-80° -60° -40° -20° 0° 20° 40° 60° 80°
traction angle [°]
6000
5000
4000
3000
2000
1000
Picture 2: Prediction of the tensile modulus
for a flow orientated 40%-GMT (below)
based on X-ray testing (on top) and orientation
related quantification by picture analysis
(middle)
0
90°
10 mm
10 mm
-90 -80 -70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90
traction angle [°]

���� [ - ]
Normalized Properties
With models, founded on the theory of Halpin
& Tsai it is possible to predict the stiffness
of long fiber reinforced materials with a
sufficient accuracy /20-21/. Although a
number of models for the prediction of composite
strength and impact properties exist,
there is no appropriate simulation method
for the practice /22-25/.
Recent methods are based on X-ray analysis
of test plates and cut out sections of real
parts. With powerful picture analysis programs
a complete functionality between
modulus and load angle can be plotted,
picture 2. Only a few references are handling
the relation between fiber length, orientaton
and strength. But most of the scientific
work done on this topic are more or less
theoretical and could be verified only on test
plates in an adequate state /24-26/. The
qualitative relation between the fiber length
and the mechanical properties of a glass
fiber reinforced PP is shown in picture 3.
1.0
0.8
0.6
0.4
0.2
0
0.1
modulus
strength
impact resistance
Fiber diameter d F: 10 µm
1
Fiberlength l [mm]
10 100
Picture 3: Qualitative relation between the
normalized mechanical properties and the
fiber length of a glass fiber reinforced PP
(based on /26/)
It could be determined that the stiffness of
the composite couldn´t be boosted over a
significant level by using longer fibers.
But the maximum strength and the impact
properties could be increased distinctly by
raising the fiber length. This is the reason
why the crash performance of complete
parts could be improved significantly by the
utilisation of longer fibers. Despite the fact
that these theoretical relations between fiber
length and impact performance could be
verified on principle in the reality, there is a
lack of profound work on real parts and examinations
considering the specific circumstances
of real production processes.
Moreover evident is the lack of comprehensible
and accessible data relating fiber content
and fiber length distribution with impact
properties. One of the difficulties is the correct
and reproducable measurement of fiber
length distributions in real parts.
1 mm
Picture 4: Optical determination of the fiber
length on injection moulded parts
left) short fiber PP right) long fiber PP
The existing analytical methods for the
measurement of the fiber length - like different
sieving, filtering or optical methods - are
very large-scaled, expensive and time consuming.
Furthermore these methods allows
no indication on the efficiency of the fiber

coupling and the interphase properties of a
fiber, pictures 4 and 5.
amount of fibers [%]
20
18
16
14
12
10
8
6
4
2
0
0-1
1-2
2-3
3-4
4-5
5-6
6-7
Picture 5: Typical fiber length distribution in
an injection moulded part (PP-LFG 15mm,
30% glass content)
4. A new practical approach for the characterisation
of orientation and fiber
length on the mechanical properties
Due to the absence of easy, practise oriented
test methods for the characterisation
and determination of the fiber length distribution
in a part, large-scale and cost intensive
component tests are commonly used.
Based on this unsatisfying situation a systematic
investigation of the relations between
different materials, processes and
geometries was performed /27/. With the
use of a typical press moulded GMT part
for representation, it is shown that with a
simple test procedure the relation between
fiber orientation, stiffness and strength could
be determined. By cutting specimen out of
the complete part surface, the whole variety
of flow induced fiber orientations in the part
can be considered. After the flexural specimen
is tested in a standardized bending test
(ISO178), the influence of the fiber orienta-
7-8
8-9
fiber length [mm]
9-10
10-11
11-12
12-13
13-14
14-15
tion can be visualized by plotting the flexural
strength and the related stiffness in one
diagram. A linear connection between the
maximum flexural strength and the flexural
modulus could be found, picture 6.
Under the assumption that the fiber length
distribution in a mat based GMT is quite
narrow, this outcome points out that in this
case the fiber orientation is the main factor.
Well oriented fibers lead to higher stiffness
and higher strength values, specimen containing
fibers oriented in a perpendicular
way to the load shows poor values.
Flexural strength [Mpa]
200
175
150
125
100
75
50
25
0
GMT 30
1500 2500 3500 4500 5500 6500 7500 8500
Flexural Modulus [MPa]
GMT 40
Picture 6: Linear relation between the
maximum flexural strength and the flexural
modulus (30% and 40% mat reinforced
GMT)
Whereas this outcome is a rather simple
and a well-known phenomenon in the daily
testing practise of long fiber reinforced materials,
the acquired experience of these
experiments is building the root for a connection
between the static and the impact
behaviour.
The next step was just a slight extension of
the commonly used charpy impact test.
Cut out and prepare of specimen
(15�80�thickness, accord. ISO 3167) in dif-

ferent positions of the part, at least about 15
samples with different flow orientations. The
specimen were tested in a bending trial
(ISO178) without rupture within linear elasticity
(max 2 mm deflection, 23°C) and the
flexural modulus is measured. After that the
same specimen were used for the charpy
impact test (ISO 179eU) and the impact
strength is measured. The two valuescharpy
impact energy and flexural modulus -
are drawn in one plot, picture 7.
Represented for GMT with three different
fiber percentages (20, 30 and 40%) it is obvious
that a linear relation between the
charpy impact energy and the stiffness exists.
Charpy Impact [KJ/m²]
70
65
60
55
50
45
40
35
30
25
20
15
10
GMT 20
GMT 30
1500 2500 3500 4500 5500 6500 7500 8500
Flexural Modulus [MPa]
GMT 40
Picture 7: Linear relation between the
charpy impact energy and the flexural
modulus of 20, 30% and 40% mat reinforced
GMT materials
The most interesting outcome is the slope of
the charpy / modulus-ratio, which is nearly
identical for the three different fiber contents.
It was shown that the flexural modulus
the flexural strength and the charpy impact
energy are direct related to the fiber orientation
in the part.
The real benefit of this new testing approach
could be displayed by the investigation of
different processes and materials. Represented
in picture 8 is the flexural strength to
modulus behaviour of specimen from parts
with 30% glass fiber content, but produced
with three different processing technologies.
Flexural strength [Mpa]
175
150
125
100
75
50
25
0
all Materials: PP - 30 % GF
2000 3000 4000 5000 6000 7000
Flexural Modulus [MPa]
LFG - ICM
LFG - IM
SFG - IM
Picture 8: Influence of the fiber length on the
linear relation between the maximum flexural
strength and the flexural modulus (30%
glass reinforced PP materials)
The specimen made with long glass fiber
granulates in an injection compression
moulding process (LFG-ICM) – which diminish
the fiber breakage - exhibit the highest
strength and stiffness values. The
specimen cut out of parts produced with the
identical granulate type, but in a common
injection moulding process (LFT-IM), are
showing mediocre values and at least the
specimen from parts injected with normal
short fiber granulates (SFG-IM) are manifesting
the lowest stiffness and strength
values. This outcome points out that – beneath
the fiber orientation – a second influence
is taking effect – the fiber length, here
as the result of different production tech-

Charpy Impact [KJ/m²]
nologies. As described in the literature the
stiffness and the strength is directly related
to the fiber length. The efficiency of the fiber
length could be directly evaluated by the
slope of the strength / stiffness-ratio, which
is raising significantly towards longer fibers.
How easily and comprehensibly the method
could be used to draw clear distinctions
between the described process technologies
for long fiber reinforced thermoplastics
is shown in picture 9. When the charpy impact
values of the different specimen is
plotted over their related stiffness the three
processes could be distinguished very well.
The increase of the fiber length leads to a
significant increase of the charpy / modulusratio.
35
30
25
20
15
10
5
0
all Materials: PP - 30 % GF LFG - ICM
LFG - IM
SFG - IM
2000 3000 4000 5000 6000 7000
Flexural Modulus [MPa]
Picture 9: Influence of the fiber length on the
linear relation between the charpy impact
energy and the flexural modulus (30% glass
reinforced PP materials)
Whereas the slope of the charpy / modulusratio
for the injection moulded short fiber PP
is almost zero, a distinct difference could be
noticed for the LFG-IM and the LFG-ICM
process. With increasing fiber length you will
increase the charpy / modulus-ratio as
shown.
5. Outlook
With a rather simple mechanical test procedure
fundamental relations between the
fiber length and the fiber orientation of long
glass fiber reinforced composites with polypropylene
matrix can be determined and
visualized. With a slight extension of the
charpy impact test – which is in any case
part of the normal testing program for thermoplastic
composite parts – it is possible to
obtain a lot more information and correlations
about the material performance, as if
common single point testing methods were
used.
With this practise oriented measuring
method, the producer of TC-parts is enabled
to optimise process parameters, to compare
different kinds of materials and to classify
different kinds of production processes.
6. References
1. Schweizer, R.A. : Glass fiber length degradation
in thermoplastics processing, Proc.
36 th Ann. SPI Conf., Session 9A, p 1-4
(1981)
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Thermoplasten, Kunststoffe 79
(1989) 7, p 624-630
3. Sanschagrin, B. ; Ehrhardt, P. ; Fisa, B. :
Fiber length degradation of long glass fiber
reinforced Polypropylene during injection
molding, Proc. 46 th Ann. SPI Conf., Session
9A, p 1-10 (1991)
4. Wolf, H.-J. : Faserverkürzung beim Verarbeiten
langfasergefüllter Thermoplaste,
Kunststoffe 83 (1993) 1, p 69-72
5. Hafellner, R., Pichler, M. ; Wörndle, R. ;
Steinbichler, G. ; Egger, P. : Lange Fasern
spritzgießen, Kunststoffe 90 (2000) , p 44-48

6. Hawley, R. C. : Extruder Apparatus and
Process for Compounding Thermoplastic
Resin and Fibers, U.S. Patent Number
5,165,941 (4 Nov. 1992), to Composite
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glasfaserverstärktem PP im Naß-
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http://www.dieffenbacher.de/e/news/artikel/in
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98-101
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Compoundieren, Kunststoffe 88 (1998)
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effects on modulus of GMT and other inhomogeneous
composites, J. of thermoplastic
comp. mat., Vol. 5 April (1992), p 105-114
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p732; (1969)
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