D
J. Chem. Chem. Eng. 7 (2013) 626-632
DAVID
PUBLISHING
TENCEL®—A High-Performance Sustainable Cellulose
Fiber for the Construction Industry/Composites
Friedrich Suchomel* and Martin Marsche
Department of New Applications, Lenzing AG, Lenzing 4860, Austria
Received: April 24, 2013 / Accepted: May 14, 2013 / Published: July 25, 2013.
Abstract: Fibers are used in various areas for improving the performance of different materials, commonly used are synthetic fibers
and glass fibers. More and more sustainable alternatives are required to reduce energy consumption and the carbon footprint.
Traditional natural fibers (like hemp or flax) very often do not fulfill requirements for construction purposes like resistance to
elevated temperature or lacking purity. Also mechanical properties of natural fibers are influenced by factors like harvesting, kink
bands, climate and growth conditions. Lenzing AG has put a lot of efforts into developing a sustainable fiber overcoming the above
mentioned issues. The raw material for TENCEL® is wood, which is transformed into a fiber of pure cellulose in an economy
friendly process as been proven by a life cycle assessment. The properties of a composite material are highly dependent on
parameters like mechanical fiber properties, fiber diameter, quality of fiber dispersion and fiber matrix adhesion. Keeping these
properties constant throughout the whole composite part is the factor to success. The diameter as well as the mechanical properties of
TENCEL® fibers is kept within a very narrow range thanks to the unique manufacturing process. It was shown that the fiber
dispersion of TENCEL® as well as the fiber matrix adhesion is better than for natural fibers.
Key words: TENCEL®, cellulosic fiber, plastic reinforcement, concrete, sustainability.
1. Introduction
Fibers have been used in composite materials
already for centuries as technological needs demand
fibers for reinforcement and improvement of various
construction materials. In recent times these composite
materials are tailored to the specific needs, thus a wide
range of fibers are used in modern materials. Typical
examples for fibers in reinforced plastic materials are
glass fibers, whereas for construction material like as
plaster or concrete synthetic fibers are widely used
[1-3].
More and more sustainable materials are required to
reduce energy consumption and the carbon footprint.
Although there is a widespread interest in natural fibers
like hemp or flax for manufacturing composite
materials, these natural fibers often do not fulfill the
requirements for modern tailor made composite
*
Corresponding author: Friedrich Suchomel, Dr., research
fields: cellulosic fibers, fiber reinforcement and green materials.
E-mail: f.suchomel@lenzing.com.
materials [4]. The major problems of natural fibers in
this respect are the lacking purity as remaining organic
substances like lignin or hemicellulose can influence
the performance of the final composite part negatively
like the resistance to elevated temperature. The
mechanical properties are strongly dependent on the
growth conditions and the climate thus leading to
changing properties of natural fibers depending on the
time of the harvest.
Lenzing AG has developed TENCEL®—a
sustainable fiber overcoming all these issues. The raw
material for TENCEL® is wood, which is transformed
into a fiber of pure cellulose. In a LCA (life cycle
assessment) the environmental impact of Lenzing
fibers was compared to cotton, PP (polypropylene) and
PET (polyester). In this LCA 11 environmentally
relevant factors have been evaluated and it has been
shown that the environmental impact of Lenzing fibers
is the lowest of all fibers in this study [5].
Due to the manufacturing process of TENCEL® the
TENCEL®—A High-Performance Sustainable Cellulose Fiber for the Construction Industry/Composites 627
diameter is kept constant, thus mechanical properties
like tensile strength or elongation vary only very little
from one lot to the next. The various types of
TENCEL® fibers are produced in a diameter range of
9-12 µm which is at least two times lower than the
diameter of natural fibers giving at least four times the
fiber amount per weight unit thus enhancing the
reinforcement.
The fiber dispersion depends not only on the fiber
dimensions but can also be strongly influenced by the
surface. Due to the production process the surface of
TENCEL® can be modified and therefore adjusted to
give optimum fiber dispersion. Due to all these
properties the sustainable cellulosic fiber TENCEL®
plays an important role in modern tailor made
composite materials.
2. Experimental Section
Due to the production process, the fiber is obtained
as a tow which is then cut to length ranging from 34
mm to 60 mm based on the requirements for the further
processing in the textile chain. Depending on the
end-use of the fiber in the construction industry or
composite materials the length had to be further
reduced, either by cutting the fiber directly in the plant
for lengths longer than 1 mm (TENCEL® short cut) or
by using a cutting mill for the shorter length from 300
µm to 1 mm (TENCEL® FCP). The fibers used in the
following experiments were 9, 10 or 12 µm in diameter,
respectively.
2.1 Experimental Setup Concrete
In the first experiments the optimum fiber length for
the use in concrete was determined. As 6 mm and 12
mm TENCEL® fibers showed the best results in
concrete all other experiments were made with such
fibers. The experiments were all made first in a
laboratory having precise control during the
experiment and then the experiments were performed
in concrete mixing plants or directly at construction
sites to evaluate the practical implications of
TENCEL® on the product quality and the workability
of concrete. In the first series of experiments always
two types of concrete, one with TENCEL® and one
without were prepared and evaluated. In the more
advanced experiments three concrete types, one
without fibers (reference concrete), one with
TENCEL® and one with polypropylene fibers were
compared.
The properties of concrete have to be divided in two
distinct phases, the first being the fresh concrete phase
and the second the hardened concrete phase, marking
the final product of concrete. In the fresh concrete
phase the focus lies on workability of the concrete, i.e.,
how well does the concrete flow or if any problems
during the hardening stage can arise. Concrete in the
cured state is the most abundant construction material
in the world nowadays, there the focus lies mainly on
properties like compressive or tensile strength.
TENCEL® fibers are cellulosic fibers and therefore
one of the main properties of TENCEL® fibers is the
moisture management, i.e., the fast water uptake and
slower release during the curing period of the concrete.
This is the reason why the first experiments were
designed to evaluate the influence of TENCEL® on the
cohesiveness of concrete, the tendency to segregate and
bleed, also under static pressure as exerted by the loads
of the building on the foundation. This experiment is
carried out by placing the fresh concrete in a container
of known volume and applying a pressure of 3 bar for
the period of an hour collecting the filter press water
into a measuring cylinder (Fig. 1). The amount of water
is recorded every 5 min and compared to other concrete
types. This experiment was carried out with a
TENCEL® content of 0.5, 1 and 1.5 kg/m³ concrete,
equaling 0.02%, 0.04% and 0.06% per weight,
respectively. With the concentration of 1 kg fibers per
cubic meter of concrete, TENCEL® was compared
with a stabilizer commonly used for such a purpose.
Crack formation during curing of the concrete (early
crack formation) is an issue for concrete constructions,
mainly for large concrete slabs as the cracks in the
628 TENCEL®—A High-Performance Sustainable Cellulose Fiber for the Construction Industry/Composites
(a)
(b)
Fig. 2 Shrinking of concrete: (a) crack formation; (b) test
apparatus.
Fig. 1 Filter press for testing concrete stability.
concrete simplify corrosion attacks on the whole
concrete slab and thus are to be prevented. Again, the
moisture management of TENCEL® compared to PP
fibers should have a positive impact on concrete curing.
A simple, yet powerful method to evaluate the early
crack formation of concrete are so-called shrinkage
rings (Fig. 2) where the fresh concrete is filled in and
stored for 8 h while subjected to a draught of 4 m/s
(simulating a breeze) at 20 °C. After this curing time
the length of the formed cracks is measured and an
overall length is calculated as sum of the formed cracks.
The average width is calculated by measuring the crack
width at defined intervals and averaging the values.
The crack area is defined as the product of the crack
length and the average crack width.
For this experiment three different concrete types
were prepared, one without any fibers (reference
concrete), one concrete with 1.2 kg/m³ PP fibers and
one concrete with 1.2 kg/m³ TENCEL®. All three
concrete types were filled into the rings and analysed
according to the procedure described above.
Concrete always contains some water; this water
expands to water vapour when buildings like tunnels
burn and thus leading to spalling of the concrete which
in turn can cause the collapse of the whole building
when the steel reinforcement melts. To prevent these
events a common procedure in some European
countries is the addition of PP fibers to concrete as
these fibers melt during a fire leaving open pore space
where the water vapour can expand [6]. As TENCEL®
decomposes at a lower temperature than PP melts the
performance of TENCEL® as a spalling prevention
should be as good as PP fibers. Again, three kinds of
concrete are tested, a reference concrete and two
concrete types with 1.2 kg/m³ TENCEL® and PP,
respectively. Then concrete slabs measuring 30 cm ×
25 cm × 10 cm were cast containing thermo elements at
defined intervals. The slabs were put in an oven in such
a way that only one side was subjected to the fire while
the other side was at room temperature. The
temperature was increased until 1,153 °C in 4 h based
on the EN 1363-1 unless a spalling event occurred
before.
2.2 Experimental Setup Plastic Reinforcement
Following boundary conditions had to be taken into
account during the development process of TENCEL®
FCP for reinforcing thermoplastic matrices:
(1) Reinforcing the plastic material (L/D ratio);
(2) Processability on compounding machines
(dosing);
(3) Smooth surfaces of the final injection moulded
TENCEL®—A High-Performance Sustainable Cellulose Fiber for the Construction Industry/Composites 629
part (sufficient dispersion);
(4) Uniformity of mechanical and dimensional
properties of the fibers.
First experiments where set up to evaluate the
reinforcing potential of the fibers in polypropylene
parts. Fibers taken for this trials have been TENCEL®
FCP with a diameter of 10 µm and a length of 400 µm
(L/D = 40), TENCEL® FCP with 35 µm and a length
of 350 µm (L/D = 10) and for reference purposes a
commercial available pulp length of 400 µm (average
diameter 20 µm).
In the next step the dosing behavior of the fibers was
evaluated as the production process of thermoplastic
compound materials requires quick adding of the fiber
material. For this purpose a hopper equipped with an
agitator was used. The hopper was filled with the
different fibers and the flow behavior of the fiber was
observed visually.
The dispersion of the fibers in the plastic compounds
was measured using a 3D computer tomography system
(RayScan
250E)
as
this
method
allowed
a
non-destructive analysis of the plastic compounds
showing the fiber distribution in the compound in detail.
3. Results and Discussion
3.1 Results Concrete
The results from the filter press (cohesiveness) are
summarized in Fig. 3 as obtained from concretes mixed
in the lab as well as from large scale trials carried out in
concrete mixing plants. As the filter press tests
simulates the pressure of the surrounding areas on the
concrete filled into holes for piling purposes, the less
water is collected, the better the performance of the
concrete in question. The loss of water in the concrete
also leads to removal of the fine aggregates and thus
can lead to a reduced stability of the concrete. The
improved cohesiveness in comparison to reference
concrete can be clearly seen as more water is collected
from the reference concrete than from the concrete
containing TENCEL®. The performance of
TENCEL® is similar to a conventional stabilizer
indicating that the sustainable TENCEL® fibers can be
used as a replacement for a chemical stabilizer
normally used in the concrete industry.
In Fig. 4 the effect of TENCEL® on the early crack
formation is clearly visible. Although the crack length
45
One of the most important topics when using fiber
is
the
behavior
under
various
environmental conditions like rain or UV radiation. For
that reason the specimens where tested according to EN
ISO 4892-2 where the reinforced plastic parts are
40
water amount [l/m³]
reinforcement
35
Reference
30
25
stabiliser
(2,4 kg/m³)
20
TENCEL
(1 kg/m³)
15
10
5
exposed for 1,500 h to UV radiation and humidity
0
0
10
20
simulating changing weather conditions. Two matrix
materials have been selected for this test. One was
polypropylene, the other one was polylactic acid. For
30
40
120%
reference concrete
PP
100%
TENCEL® FCP were prepared. In order to obtain also
inspection mechanical tests like tensile strength,
TENCEL® 6mm
crack values
PLA was also subjected to this test. In addition to visual
80%
60%
40%
impact strength, flexural strength and impact strength
20%
notched were performed as these tests are typically
0%
performed for the determination of the performance of
plastic parts.
60
Fig. 3 Filter press results—TENCEL® showing same
performance as conventional stabilizer.
both matrix materials compounds containing 30%
the behavior of the pure matrix material, virgin PP and
50
time [min]
crack length
crack width
crack area
Fig. 4 Crack formation of concrete—comparison between
PP and TENCEL®.
630 TENCEL®—A High-Performance Sustainable Cellulose Fiber for the Construction Industry/Composites
is the same for TENCEL® and PP, the crack width
using TENCEL® is considerably reduced compared to
PP fibers. The early crack formation is a common
problem in concrete technology, mainly when building
large floors (e.g., for industrial purposes). As these lab
results are promising large concrete floors have been
made using TENCEL®. The first results showed a low
crack formation and an improved surface quality.
Further studies will carried out during the lifetime of
the floor as often cracks form after some time (1-2
years).
(a)
In Fig. 5a the spalling effect of concrete exhibited to
fire can be clearly seen, while in Fig. 5b the positive
effect of TENCEL® is evident. The results from the
fire experiments show that TENCEL® performs as
well as PP fibers commonly used in concrete for
spalling prevention. Further studies with other fire
curves like the RWS (Rijkswaterstaat) curve are
planned as this fire curve simulates the worst case of an
oil containing truck which starts to burn inside a tunnel
where the temperature rises to 1,140 °C in only 5 min
and the maximum temperatures reach 1,340 °C.
3.2 Results Plastic Reinforcement
(b)
Fig. 5 (a) Spalling of conventional concrete after fire
testing; (b) No spalling of concrete with TENCEL(R)® after
fire test.
In Fig. 6 the influence of the L/D-ratio on the tensile
15 µm lead to fibers longer than 500 µm resulting in
100
50
80
45
40
difficulties dosing such fibers. The main problem of
35
longer fibers could be observed as bridging behaviour in
30
the hopper. In addition longer fibers lead to a lower bulk
60
40
FCP 35/350
It can be clearly seen that fiber diameters higher than
55
pulp
diameter of the fibers in question is depicted.
120
FCP 10/400
and the required fiber length taking the average
Tensile strength in N/mm²
the relationship between the desired L/D-ratio of 40
no. of fibers
60
no. of fibers in millions/g
Tensile strength of compound (20%)
strength of the plastic compound can be seen. In Fig. 7
20
-
5
10
15
20
25
30
35
40
45
50
55
60
Diameter in µm
density and therefore to a lower dosing speed and thus a
Fig. 6 Influence number of fibers on reinforcement.
lower production capacity of the plastic compounds. As
seen in Fig. 8. In Fig. 9 the impact of a lacking fiber
distribution on the plastic part can be clearly seen. On
been produced showing no surface effects while the
part on the right side contains a standard natural fiber
(coarser than TENCEL®) exhibiting a lower
dispersion and forming agglomerates which tend to
for production needs the fiber length should be as low as
possible, only fibers having diameters lower than 15 µm
are of interest showing a sufficient L/D-ratio and a
reasonable production capacity.
The excellent fiber dispersion of TENCEL® can be
TENCEL®—A High-Performance Sustainable Cellulose Fiber for the Construction Industry/Composites 631
Target: L/D ≥ 1/30 for optimal reinforcement
L/D = 40
2500
fiber length [µm]
2000
glass fiber,
TENCEL FCP
1500
flax, hemp,
Pulp
1000
500
0
0
10
20
30
40
50
60
fiber diameter [µm]
Fig. 7 L/D-ratio and resulting fiber length for different
fiber types.
Fig. 8 3D CT pictures of a test bar; 20% TENCEL® FCP
in PP.
break through the outer layer of the polymer. In the
magnification this effect is clearly visible.
All these factors together make TENCEL® an
excellent reinforcing fiber as can be seen in Fig. 10
where several mechanical properties are compared
between pure PP and PP reinforced with TENCEL®.
After the weathering tests the reference specimen
made from pure PP showed nearly no change in the
Young modulus, however the tensile strength was
reduced by 50% of the initial value. Referring to the
reinforced samples it can be stated that the modulus
behaved in the same way. Concerning the tensile
strength a reduction of approximately 13% was
observed. As this reduction in tensile strength is also
observed for pure PP parts after the weathering tests, it
can be deduced that TENCEL® itself is not degraded
during the weathering. On the opposite, using
TENCEL® as a reinforcement of PP parts leads to an
improved performance of such parts even after
weathering.
4. Conclusions
The results obtained with TENCEL® in such
different areas of applications like reinforcing plastic
composites and construction materials like concrete or
plaster show that TENCEL® can play an important
role in the ever increasing demand for optimized
Fig. 9
Left: part made from TENCEL®, right: part made
from conventional natural fiber.
Tensile modulus
requirements for reinforcing plastic compounds or
pure PP
improving the quality of construction materials but as a
PP + 30% FCP
sustainable and natural fiber has an excellent position
250%
200%
HDT-A
150%
100%
Tensile
strength
50%
for manufacturing sustainable goods.
TENCEL® is under constant development for
further improving modern composite materials like
0%
exploring the potential of incorporating active
Flexural
modulus
Impact strength
notched
materials. TENCEL® fulfills not only the technical
Flexural
strength
Fig. 10 Comparison mechanical properties between pure
PP (basis 100%) and PP reinforced with TENCEL®.
ingredients or new surface modifications to enhance
the properties of modern composite materials.
Acknowledgments
The authors thank Mr. Salaberger from FH Wels for
making the CT images of the plastic parts.
632 TENCEL®—A High-Performance Sustainable Cellulose Fiber for the Construction Industry/Composites
References
[1]
[2]
[3]
Balaguru, P. Contribution of Fibers to Crack Reduction of
Cement Composites during the Initial and Final Settling
Period. ACI Material Journal 1994, 91, 280-288.
Eichhorn, S. J. Regenerated Cellulose Reinforced Plastics;
Kluwer Academic Publishers: Norwell, Mass., 2004; pp
287-303.
Xanthos, M. Functional Fillers for Plastics; Wiley-Vch,
2010.
[4]
[5]
[6]
Fuqua, M. A.; Huo, S.; Ulven, C. A. Natural Fiber
Reinforced Composites. Polymer Reviews 2012, 52,
259-320.
Shen, L.; Patel, M. K. LCA Single Score Analysis of
Man-Made Cellulose Fibres. Lenzinger Berichte 2010, 88,
60-66.
Zeiml, M.; Leithner, D.; Lackner, R.; Mang, H. A. How
Do Polypropylene Fibers Improve the Spalling Behavior
of in-Situ Concrete? Cement and Concrete Research 2006,
36, 929-942.