polyproylene composites with natural fibres and wood - general mechnical property profiles
TRANSCRIPT
Accepted Manuscript
Polypropylene Composites with Natural Fibers and Wood - General Mechanical
Property Profiles
Lukas Sobczak, Reinhold W. Lang, Andreas Haider
PII: S0266-3538(11)00448-9
DOI: 10.1016/j.compscitech.2011.12.013
Reference: CSTE 5140
To appear in: Composites Science and Technology
Received Date: 24 October 2011
Accepted Date: 16 December 2011
Please cite this article as: Sobczak, L., Lang, R.W., Haider, A., Polypropylene Composites with Natural Fibers and
Wood - General Mechanical Property Profiles, Composites Science and Technology (2012), doi: 10.1016/
j.compscitech.2011.12.013
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Polypropylene Composites with Natural Fibers and Wood – General Mechanical Property Profiles
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Polypropylene Composites with Natural Fibers and Wood - General Mechanical Property Profiles
Lukas Sobczaka1, Reinhold W. Langb, Andreas Haidera a Competence Centre for Wood Composites and Wood Chemistry (Wood K plus), Division Wood-Polymer-Composites; St. Peter-Straße 25, 4020 Linz, Austria b Johannes Kepler University Linz, Institute for Polymer Materials and Testing; Altenberger Straße 69, 4040 Linz, Austria 1Corresponding Author [email protected] Tel.: +43 732 6911 4082 Fax: +43 732 6911 2864
Abstract
Natural Fiber Composites (NFC) and Wood Polymer Composites (WPC) based on
polypropylene (PP) have gained increasing interest over the past two decades, both
in the scientific community and in industry. Meanwhile, a large number of publications
is available, but yet the actual market penetration of such materials is rather limited.
To close the existing gap between scientific and technical knowledge, on the one
hand, and actual market applications, on the other, it is the purpose of this paper to
analyze the current state of knowledge on mechanical performance profiles of
injection molded NFCs and WPCs. As the composite properties are a result of the
constituent properties and their interactions, special attention is also given to
mechanical fiber/filler properties. Moreover, considering that NFCs and WPCs for a
variety of potential applications compete with mineral reinforced (mr; represented in
this study by talc), short glass fiber (sgf), long glass fiber (lgf) and short carbon fiber
(scf) reinforced PP, property profiles of the latter materials are included in the
analysis. To visualize the performance characteristics of the various materials in a
comparative manner, the data were compiled and illustrated in so-called Ashby plots.
Polypropylene Composites with Natural Fibers and Wood – General Mechanical Property Profiles
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Based on these comparisons, an assessment of the substitution potential of NFCs
and WPCs is finally performed, along with a discussion of still open issues, which
may help in guiding future material development and market application efforts.
Keywords: Natural Fiber Composites (NFCs), Wood (A), Short-fiber composites (A), Mechanical properties (B), Injection molding (E) Abbreviation Explanation NFC Natural Fiber Composite, meaning a fiber/matrix composite that contains natural
fibers or wood based cellulose fibers WPC Wood Polymer Composite, meaning a filler/matrix composite that contains wood
particles Mw Weight average molecular mass IS Impact strength m% mass percent mr mineral reinforced sgf short glass fiber, usually below 1 mm in length lgf long glass fiber, usually 5 – 10 mm in length scf short carbon fiber PP-(x%)y Polypropylene reinforced with y (x m% of y) unbl., bl. bleached, unbleached (Figure 4-1, Figure 4-2, Figure 4-3) u.K.p., bl.K.p. unbleached Kraft pulp, bleached Kraft pulp (Table 4-1) Ten.® Tencel®, wood based cellulose fiber (Table 4-1)
Table 0-1: Abbreviations used in the text;
1 Introduction
While polyolefins, in particular PP, have been reinforced commercially with glass
fibers and particle minerals (e.g. talc, wollastonite) for several decades, more recently
natural fibers and wood have become of engineering and commercial interest to
produce novel classes of natural fiber composites (NFC; see Table 0-1 for
abbreviations) and wood polymer composites (WPC) [1-3]. In terms of markets and
applications, it is particularly the automotive industry [4-6] and the building and
construction industry [7-9] which have expressed interest in using such materials.
Along with cost saving aspects and expected ecological benefits (e.g. improvement in
CO2-balance [10, part III;11;12]), the main motivation driving these developments is
related to the mechanical property profiles of natural fibers and wood, which indicate
a substantial reinforcement potential. Combined with the low density of natural fibers
Polypropylene Composites with Natural Fibers and Wood – General Mechanical Property Profiles
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and wood (see Table 2-1), NFCs and WPCs may result in lighter weight structures
when compared to mineral reinforced (mr), short glass fiber (sgf), long glass fiber (lgf)
and short carbon fiber (scf) reinforced materials [13-15]. In addition, NFCs and WPCs
can be processed similar to these other material classes, e.g. by injection molding
and extrusion. In fact, in terms of processing behavior, NFCs and WPCs may even
offer advantages with regard to equipment wear [13;15;16].
Despite the high industrial interest in NFCs and WPCs, and the significant scientific
efforts particularly over the past decade, no comprehensive overview exists on
mechanical property profiles of various material grades that allows for a proper
comparison among these novel PP-based materials. Such a comparison is also
lacking with materials already used commercially, such as mr, sgf, lgf and the more
novel scf composites, with which NFCs and WPCs are supposed to compete.
Hence, the overall objective of this paper is to provide a comprehensive overview of
the mechanical property profiles of NFCs and WPCs, and to compare these
properties to those achieved by existing commercial composites based on mineral,
glass and carbon fiber reinforcements. For various reasons (cost and/or
performance), NFCs and WPCs are frequently produced with high fiber/wood
content, so that special emphasis was paid to cover the natural fiber or wood content
range up to about 70 m%. This reinforcement level also corresponds to the limit of
adequate processability of high quality products by injection molding [17].
2 Materials and Methods
All of the data presented here is taken from scientific literature or from material data
sheets provided by the suppliers. To ensure a sufficient comparability of the material
Polypropylene Composites with Natural Fibers and Wood – General Mechanical Property Profiles
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property values given, only data generated utilizing injection molded specimens and
applying equivalent test procedures and conditions has been included.
Table 2-1 provides some general information on the densities and current prices for
the constituents of the various material grades included in this overview. Throughout
this paper, the term “natural fibers” refers to plant-based fibers (like jute, hemp,
kenaf, sisal, flax) and wood based cellulose fibers (Kraft pulp, unbleached cellulose,
regenerate cellulose such as Tencel®). On the other hand, the term “wood particles”
as they are used in WPCs refers to the grinded state of solid wood which lacks the
characteristic of a higher aspect ratio.
One advantage of natural fibers clearly apparent from Table 2-1 is their lower density.
While conventional reinforcements (scf, sgf, talc) exhibit a density range from 1.7 to
2.8 g/cm3, the density range for natural fibers and wood (compressed state as it
occurs in WPCs as a result of processing) is from about 1.3 to 1.6 g/cm3. The effects
of the processing steps from solid wood to wood particles, and of compression during
WPC processing, are expected to result in a corresponding alteration in mechanical
properties. Since no data for modulus and strength of wood fillers in the compressed
state are available, the comparisons performed in chapter 3 with regard to wood as
reinforcement are perhaps of limited quantitative value but were nevertheless
included to provide an overall relative picture.
The reinforcement prices range from about 0.2 �/kg for spruce at the lower end to
20.0 �/kg for short carbon fibers at the upper end. For comparison, polypropylene
typically has a price level of 1.0 to 1.4 �/kg, depending on the specific grade and the
order volume. In other words, from the reinforcements considered in this overview,
Polypropylene Composites with Natural Fibers and Wood – General Mechanical Property Profiles
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most are below the price level of the PP matrix, while in some cases, the
reinforcement prices may exceed the PP matrix price.
Material Density [g/cm3] Price [�/kg] Source
PP 0.90 1.0 – 1.4 [18] scf 1.70 – 1.77 10.0 – 20.0 [13;19;20] sgf 2.50 0.9 – 1.6 [13;21] Talc 2.75 0.3 – 1.0 [22] Tencel® 1.55 - [23]
Flax 1.50 0.3 – 1.3 (upper bound: pellets) [13;24, page 12] [25, page 119;26]
Hemp 1.48 0.3 – 1.0 (upper bound: pellets) [14;25, page 119;26] Jute 1.30 0.5 – 0.7 [13;25, page 119] Kenaf - 0.4 – 1.5 [14;27] Sisal 1.45 0.5 – 0.8 [13;25, page 119;27]
Spruce 0.45- 0.50 (uncompr.) ~ 1.30 (compressed) 0.2 - 0.4 [28;29]
Table 2-1: Densities and prices of Polypropylene and several conventional reinforcements plus natural fibers and wood (spruce);
For the material property data of the reinforcement constituents and the respective
PP compounds processed by injection molding (sections 3 and 4, respectively), the
“Ashby plot” [30] was chosen as means of presentation. In terms of relevant
properties, Ashby plots were generated as tensile strength vs. tensile modulus for the
reinforcement constituents, and as tensile strength vs. tensile modulus and impact
strength vs. tensile modulus for PP and its various compounds (i.e., PP composites).
Due to the lack of data covering a wider range of test conditions, for IS values room
temperature data for unnotched Charpy specimens were selected. To allow for a
comparison of the material property profiles in terms of the potential for lightweight
structural design, the Ashby plots for tensile modulus and strength are illustrated for
absolute property values but also for specific property values (i.e. absolute values
divided by the respective material density). For the latter representations, the proper
material density values were obtained from the literature when available, or were
Polypropylene Composites with Natural Fibers and Wood – General Mechanical Property Profiles
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calculated applying a simple rule-of-mixture model from the constituent volume
contents and the constituent densities in Table 2-1.
3 Fiber properties
The tensile strength vs. modulus properties of various natural fibers are compared to
those of conventional fibers for PP reinforcement (sgf/lgf, scf) in Figure 3-1 and
Figure 3-2 in terms of absolute properties and specific properties, respectively. The
data for the various fiber types and grades were taken from the references indicated
in Table 3-1 and, in the case of the natural fibers, represent dry fiber conditions.
Before depicting the fiber properties in Ashby-plots, the following aspect with regard
to the variability of fiber strength and modulus values must be pointed out. While
strength and modulus of specific grades of conventional fibers usually meet quite
narrow tolerances, natural fibers are known to vary substantially [13;31]. For
example, modulus and strength values of jute fibers may vary from about 13 - 27
GPa and from about 390 - 770 MPa, respectively. Similar variations are known for
wood (e.g., European spruce: Young’s modulus range from about 7 - 21 GPa; tensile
strength range from 20 - 250 MPa [28]). Kenaf has not yet been investigated to an
extent so that similar variations could be deduced from the literature. In any case, the
reported variability of properties of a specific natural fiber type is accounted for by
including the reported upper and lower bound values in the Ashby plots below. In
terms of absolute properties, Young’s modulus values of natural fibers and wood
range from about 7 - 70 GPa [14;28], while modulus values for conventional fibers
range from about 70 GPa (sgf/lgf [13]) to 240 GPa (scf [19]). Alternatively, the
conventional fibers exhibit significantly higher strength values, ranging from about
2,000 MPa (E-type sgf/lgf [13]) to 4,000 – 4,570 MPa (scf, S-type sgf/lgf [13]),
Polypropylene Composites with Natural Fibers and Wood – General Mechanical Property Profiles
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compared to natural fibers and wood ranging from about 20 - 1,100 MPa [13;28]. In
other words, conventional fibers significantly outperform natural fibers and wood in
terms of strength, but in terms of modulus, some natural fibers such as hemp and
kenaf nearly reach the values of at least glass fibers. Of course, when comparing just
the two groups of natural reinforcements in Figure 3-1, the fiber reinforcements are
seen to supersede the wood reinforcements both in terms of strength and modulus.
Due to the density differences of the various fiber types and wood, the specific
properties depicted in Figure 3-2 are somewhat different to those in Figure 3-1. First,
as described above (section 2), the data points for the wood type reinforcements are
of limited relevance for PP composites, as they correspond to an uncompressed
state with very low density which is not representative of the more highly compressed
state in a PP compound. Second, the specific modulus data for glass fibers are
shifted into the range of the corresponding values of natural fibers, with hemp even
exceeding the specific modulus values of glass fibers. Third, the highest values for
specific strength of natural fibers are achieved for flax, which is now approaching the
lower end of the specific strength range for glass fibers. Finally, carbon fibers, due to
their low density, still retain their superiority compared to all other fibers with regard to
specific modulus values. The lower density compared to glass fibers now also
translates into higher values for specific strength of the carbon fibers.
Table 3-1: References for the tensile property data of the various reinforcement types presented in Figure 3-1 and Figure 3-2.
Carbon Glass Tencel Flax Hemp Jute Kenaf Sisal Spruce Oak [13;19] [13] [23] [13;31] [14;31] [13] [14] [13] [28] [32]
Polypropylene Composites with Natural Fibers and Wood – General Mechanical Property Profiles
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0
500
1000
1500
2000
2500
3000
3500
4000
4500
5000
0 50 100 150 200 250
Tens
ile S
tren
gth
[MP
a]
Young's Modulus [GPa]
Carbon E-glassS-glass
Tencel® FlaxHemp JuteKenaf Sisal
European Spruce European Oak
0
500
1000
1500
2000
2500
0 50 100 150
Spe
cific
Ten
sile
Str
engt
h [k
J/kg
]
Specific Young's Modulus [MJ/kg]
Carbon E-glass
S-glass
Tencel® Flax
Hemp Jute
Sisal
European Spruce European Oak
Figure 3-1: Ashby plot presenting the absolute tensile strength versus the Young’s modulus for various fiber types.
Figure 3-2: Ashby plot presenting the specific tensile strength versus the specific Young’s modulus for various fiber types (property divided by density).
4 Composite properties
The tensile properties of various PP composites are depicted in Figure 4-1 and
Figure 4-2 in absolute and relative terms, respectively, as strength vs. modulus
diagrams. The data for the various materials were taken from the references
indicated in Table 4-1 and, in the case of the natural fiber/filler composites, represent
dry specimen conditions. In both diagrams, the property range covered by neat PP
homopolymers is included for comparison.
From the illustration of the absolute properties in Figure 4-1 it becomes apparent that
the property regions covered by conventional fiber/filler composites and by the
natural fiber/filler composites approach one another, although they are clearly
separated in Figure 3-1, which depicts the absolute fiber/filler properties. While the
general tendency of this shift can be explained by rule of mixture considerations,
several effects remain remarkable. To begin with, the property areas covered by the
NFCs and the WPCs overlap to a greater extent than is the case just for the
4.1 Tensile properties
Polypropylene Composites with Natural Fibers and Wood – General Mechanical Property Profiles
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reinforcement constituents in Figure 3-1. This is at least partly due to the fact that the
WPCs included contain up to 70 m% wood particles while the NFCs are limited to a
fiber content of about 60 m% (in terms of fiber/filler volume content the differences
are slightly larger due to differences in density; see Table 2-1). Nevertheless, while
with WPCs modulus/strength combinations of up to about 7 GPa / 55 MPa [17;33]
are obtained, for NFCs about 11 GPa / 75 MPa may be achieved [34].
When comparing the natural fiber/filler composite data to conventional PP composite
data, the following observations are made. NFCs and WPCs exist that outperform
PP-talc composites, which exhibit modulus/strength values of about 4 GPa / 35 MPa
[35], both in terms of modulus and tensile strength. Conversely, PP-sgf/lgf
composites cover a modulus/strength regime from about 5 GPa / 75 MPa [36] up to
13 GPa / 135 MPa [37], which is clearly above the property range covered by NFCs
and WPCs. Interestingly, the values achieved for PP-scf composites, both in
scientific investigations [19] and for commercial products [38;39], fall significantly
short of rule of mixture based expectations. This is particularly the case for the tensile
strength values, where PP-scf composites cover a range similar to NFCs and even
overlap with WPCs. Merely, the modulus values of PP-scf composites exceed those
of the other material classes in Figure 4-1, although in this case too, to a lesser
degree than expected based on the respective fiber modulus data.
Polypropylene Composites with Natural Fibers and Wood – General Mechanical Property Profiles
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0
10
20
30
40
50
60
70
80
90
100
110
120
130
140
150
0 2 4 6 8 10 12 14 16 18 20 22 24
Tens
ile S
tren
gth
[MP
a]
Young's Modulus [GPa]
neat PP
Carbon202740
Glass20 sgf30 sgf40 sgf50 lgf
Talc2040
Flax253060
40 Hemp
Jute405060
Kenaf20405060
30 Sisal
unbl. Kraft pulp3045
40 bl. Kraft pulp
33 Tencel®
Wood particles3040506070
Figure 4-1: Ashby plot presenting the tensile
strength versus the Young’s modulus of various PP compounds. The numbers in the legend give the fiber/filler content in [m%].
0
10
20
30
40
50
60
70
80
90
100
110
120
0 2 4 6 8 10 12 14 16 18 20 22 24
Spe
cific
Ten
sile
Str
engt
h [k
J/kg
]
Specific Young's Modulus [MJ/kg]
neat PP
Carbon202740
Glass20 sgf30 sgf40 sgf50 lgf
Talc2040
Flax253060
40 Hemp
Jute405060
Kenaf20405060
30 Sisal
unbl. Kraft pulp3045
40 bl. Kraft pulp
33 Tencel®
Wood particles3040506070
Figure 4-2: Ashby plot presenting the specific
tensile strength versus the specific Young’s modulus of various PP compounds (property divided by density).
Filler Content
[m%] Sources Filler Content
[m%] Sources Filler Content
[m%] Sources
scf 20 [39] Flax 25 [40] u.K.p. 30 [41] 27 [19] 30 [42-45] 45 [41] 40 [19;38] 60 [46] b.K.p. 40 [47] sgf 20 [36] Hemp 40 [47-49] Ten.® 33 [23] 30 [50;51] Jute 40 [34] Wood 30 [52;53] 40 [35] 50 [34;54] 40 [53;55;56] lgf 50 [37] 60 [34] 50 [17;33;53] Talc 20 [57;58] Kenaf 20 [59] 60 [17;33] 40 [35;60] 40 [59;61] 70 [17;33] 50 [35;61] 60 [59] Sisal 30 [62;63]
Table 4-1: References for the mechanical property data of the various compounds presented in Figure 4-1, Figure 4-2 and Figure 4-3; neat PP data is taken from [64;65].
Polypropylene Composites with Natural Fibers and Wood – General Mechanical Property Profiles
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The specific modulus/strength diagram in Figure 4-2 reveals similar tendencies as
discussed above for the absolute property ranges of the various material types.
Reflecting the density differences, the NFCs and WPCs now appear further improved
compared to PP/talc composites, and the upper bound strength ratio between PP/lgf
and NFCs is now reduced from about 1.8:1 in terms of absolute strength to 1.5:1 in
terms of specific strength. In terms of specific modulus, the upper bound values of
these two material classes now even approach one another. Similar ratios were
found by Wambua et al. in a comparison of glass fiber and natural fiber reinforced PP
composites (i.e. 40 m% fiber content, prepared by a film stacking method) [66].
Furthermore, the differences in the upper bound values of specific modulus data
between PP-scf and PP-sgf are slightly enhanced.
The impact properties of the various material classes are illustrated and compared in
Figure 4-3 as unnotched Charpy values vs. Young’s modulus values. Most
remarkably is the specific NFC grade utilizing the commercial cellulose fiber Tencel®.
It is the only reinforcement type that allows for significant improvements in modulus
without sacrificing the impact properties when compared to the lower bound range of
neat PP which represents grades with low weight average molecular mass, (Mw) [23].
However, higher Mw PPs reach unnotched Charpy impact strength (IS) values
reaching 100 kJ/m2 and even higher, up to a point where unnotched specimens do
no longer break upon the impact [67]. Apart from the Tencel® reinforced PPs,
unnotched impact strength values of all other PP composites are reduced compared
to neat PP, with PP-sgf exhibiting the least reductions, followed by PP-talc
composites and NFCs, with WPCs revealing the most significant reductions. A study
by Wambua et al. on compression molded composites prepared by a film stacking
4.2 Impact properties
Polypropylene Composites with Natural Fibers and Wood – General Mechanical Property Profiles
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method (cited already above for tensile properties) also supports the data reviewed
here for injection molded composites in terms of material ranking and property ratios.
Thus, for a comparable fiber content, glass mat reinforced PPs also exhibit about
twice the unnotched Charpy IS of natural fiber mat reinforced PPs [66].
Numerous studies exist on factors controlling the impact strength of NFCs and
WPCs. As to the influence of the particle size, it is clear that this parameter may play
an important role, however no clear and unambiguous tendencies can be deducted
from the published literature as yet [52;68-71]. Conversely, for unnotched specimens,
IS is usually improved by enhanced coupling [45;55;71;72], whereas for notched
specimens, the improvement is often not so significant, with even reductions in
impact values resulting from increased coupling having been reported [56;68;73-77].
Furthermore, it is well proven that IS of NFCs and WPCs can be enhanced
significantly by rubber toughening of the PP matrix, however in all cases at the cost
of modulus reductions [55;78-80].
Nevertheless, despite all the efforts to study impact properties of NFCs and WPCs,
no study is available that unambiguously explains the cause of the poor impact
performance of these materials, when compared to neat PP and PP-sgf and PP-talc
composites. Moreover, there is not a sufficient database for a comprehensive
comparison of impact properties for the material grades of interest to this paper.
Particularly, a more detailed analysis of the effects of notches on impact strength and
the influence of test temperature, to deduce brittle-ductile transitions, is lacking. In
this context, it is known from preliminary investigations that notched impact properties
of PP-sgf composites and PP composites with Tencel® fibers may even exceed the
values obtained with neat PP [23;36;50;51]. Clearly, more work is needed to study
and perhaps optimize the impact behavior of NFCs and WPCs.
Polypropylene Composites with Natural Fibers and Wood – General Mechanical Property Profiles
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Figure 4-3: Ashby plot presenting the unnotched Charpy impact strength vs. the Young’s modulus of various PP compounds. The numbers in the legend give the fiber/filler content in [m%].
5 Open Issues
As modern PP-NFCs and PP-WPCs still represent rather novel classes of materials,
and despite the fact that quite a lot is known on the properties and on the behavior of
these materials, it is also not surprising that there are a number of issues that are as
yet unresolved and thus deserve further attention. These open issues include
aspects related to material property and performance profiles, to the processing
behavior and adequate processing conditions, and – last but not least – aspects in
relation to the ecological impact and life cycle assessment.
With regard to material and performance related issues, a particular problem in the
case of polyolefin-based NFCs and WPCs is the inherent incompatibility between the
non-polar hydrocarbon matrix and the usually hydrophilic ligno-cellulosic
reinforcements. While numerous scientific studies in recent years aimed at improving
the interfacial adhesion in these materials, a detailed understanding of the
mechanisms of bonding affecting and improving the fracture properties and the
failure behavior is still lacking [14;81-84].
0
10
20
30
40
50
60
0 2 4 6 8 10
IS C
har
py
unn
otch
ed [k
J/m
2 ]
Young's Modulus [GPa]
neat PP
Glass20 sgf30 sgf
Talc2040
30 Flax
unbl. Kraft pulp3045
33 Tencel®
Wood particles3040506070
Polypropylene Composites with Natural Fibers and Wood – General Mechanical Property Profiles
page 14 of 22
Another aspect of practical relevance that needs further study is related to the effects
of moisture on the material properties and performance profiles of polyolefinic NFCs
and WPCs. For example, for WPCs with high filler levels (70 – 87 m%), Svoboda [10,
page 145 ff.] found that the tensile properties (strength and modulus) are reduced by
up to 55 % (70 m% wood) when the material has absorbed more than 10 % moisture
upon water immersion. Viksne et al. report significant reductions (up to 30%) in
flexural properties for PP-based WPCs with 50 m% filler content upon three water
absorption/desorption cycles [85]. Similar effects on tensile properties were found by
Arbelaiz et al. [46] for PP reinforced with 20 – 60 m% flax fibers. These studies
provide a good indication of the property reductions to be expected by moisture
absorption, and numerous other reports on the effects of moisture on the mechanical
behavior exist [71;86-100]. And yet, considering the pronounced sensitivity to
moisture uptake of these PP based NFCs and WPCs, further investigations are
needed to deduce guidelines for component design and performance for applications
under typical climatic conditions.
Other aspects not sufficiently addressed so far are related to expected improvements
in noise and vibration damping of PP based NFCs and WPCs [6;101-103]. In this
context, advantages of NFCs and WPCs over conventional polymer composites are
frequently argued, however, quantitative data and information is rather scarce. Thus,
to our knowledge no article exists which compares the acoustic properties of, for
instance, NFC or WPC based automotive interior panels with PP-talc based panels.
On the other hand, NFCs and WPCs are used in several applications with proven
positive results in terms of acoustic performance (e.g. automotive interior [101],
musical instruments [104] loudspeaker-boxes etc. [105]).
Polypropylene Composites with Natural Fibers and Wood – General Mechanical Property Profiles
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As has been addressed above, and completing the list of open issues related to NFC
and WPC properties, natural fibers and wood are known to exhibit substantial lot-to-
lot variations in their properties depending on plant growth and harvesting conditions
[12;106, page 8]. Moreover, although several studies exist [107-111], emission and
odor problems that may potentially arise particularly in indoor building and automotive
interior applications deserve further attention and investigations.
Turning to open issues related to processing behavior and conditions of NFCs and
WPCs, reduced equipment wear is often argued to be advantageous compared to
PP-sgf/lgf or PP-mr [13;15;16]. While it seems likely that natural fibers or wood,
containing mostly cellulose and lignin, should cause less abrasion on the surfaces of
processing equipment than hard and sharp-edged glass fibers, for example, no
published study exists supporting and quantifying this assertion. In addition, more
precise pre-conditioning and processing conditions need to be defined to account for
the hydrophilicity of natural fibers/fillers, and for the degradation sensitivity of NFC or
WPC compounds when being processed at elevated temperatures under
simultaneous mechanical shear [112-114].
Another processing-related problem that arises when natural fibers are used as
reinforcements is their inaptitude to metering via usual dosing scales. Traditionally,
such fibers are supplied as bales or staple fibers, and are thus not free flowing.
Basically, there are two ways to overcome this drawback: First, by cutting or milling
the fibers down, until a sufficient ease of flow is achieved. Second, pelletizing of the
fibers is an option. Both of those technical solutions increase costs. Furthermore, the
first approach potentially leads to a reduction of composite properties by a reduced
fiber length. The second approach, on the other hand, raises the issue of re-
dispersion of the fibers during the compounding step and poses a problem when the
Polypropylene Composites with Natural Fibers and Wood – General Mechanical Property Profiles
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pellets are well consolidated. A method which avoids those issues is the long fiber
granulate (LFG) process developed by the Thuringian Institute of Textile and Plastics
Research (TITK) [115]. It is a pull-drill treatment by which strands of fibers are coated
with a thermoplastic matrix. Thus, granules containing long fibers (of the length of the
granule) can be produced. Despite appearing like a promising alternative, to our
knowledge the process has not yet been implemented on industrial scale.
There are several other topics and problems that require attention when applying
NFCs and WPCs in technical products. These include the long-term performance of
these materials and products, both in terms of mechanical properties but also
concerning visual appearance (e.g. color change). In fact, quite a few studies are
available on specific effects of weathering on optical and mechanical properties, and
thus on the long-term stability of these materials. The most obvious consequence of
weathering on WPCs is whitening resulting from lignin degradation [116-120]. Of
course, weathering also affects mechanical properties, usually leading to significant
reductions [93;116;121;122]. However, analogous to the effect of moisture
absorption, it is difficult here too to provide general guidelines for component design
and performance for applications under typical long-term weathering conditions.
To complete the most important requirements as to material properties and product
performance profiles, flame retardance is a prime prerequisite in certain applications.
As PP-based NFCs and WPCs per se are not improved over neat PP and in certain
properties are even inferior [123;124], large amounts (usually 10 – 30 m%) of flame
retardant additives must be employed to achieve significant improvements [125-127].
Of course, utilizing such high amounts of additives may have pronounced effects on
the processing behavior and the remaining property profile of these materials
[124;128;129], which must be accounted for in specific applications.
Polypropylene Composites with Natural Fibers and Wood – General Mechanical Property Profiles
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Finally, and despite the fact that ecological considerations are frequently used as
selling argument [7;130], only limited information is available on validated eco-
balances and life cycle assessment (LCA) data of PP based NFCs and WPCs.
Svoboda provides a performance oriented assessment comparing several materials
based on renewable resources, including wood and highly filled WPCs, and
conventional materials like PE, PP, PVC and aluminum, among others [10, page 177
ff.]. Depending on the conditions applied, WPCs may or may not be favorable in
terms of ecological performance. Furthermore, Michaud et al. performed LCA studies
on highly filled PE-based WPCs [131]. Also, hardly any LCA data exist on NFCs, and
the available data as to the actual ecological performance are ambiguous
[11;132;133]. Hence, as for the other issues mentioned above, further studies are
needed to unambiguously provide information on the ecological performance of
particularly polyolefin and PP based NFCs ad WPCs compared to other materials.
6 Summary and Conclusions
As to the fiber properties, conventional fibers such as short/long glass fibers (sgf/lgf)
and short carbon fibers (scf) exhibit significantly higher strength values than even the
best natural fibers (factor 2 – 4). In terms of modulus, some natural fibers, like, e.g.,
hemp and kenaf, show values similar to sgf/lgf, with scf modulus values superseding
these fibers by a factor of 3. Due to the lower density of natural fibers compared to
glass fibers, the specific properties of natural fibers, on the one hand, and sgf/lgf, on
the other, shift closer together. Particularly remarkable is that hemp even supersedes
sgf/lgf in the specific modulus, and flax approaches sgf in specific strength.
For the resulting PP composites, in terms of absolute properties, the picture is largely
similar, with some remarkable exceptions. First, the relative difference between NFCs
Polypropylene Composites with Natural Fibers and Wood – General Mechanical Property Profiles
page 18 of 22
and WPCs, on the one hand, and PP-sgf/lgf on the other, is reduced due to rule of
mixture based effects. Second, WPCs supersede PP-talc composites, both in
modulus and strength, while NFCs largely overlap with the PP-sgf/lgf range for
modulus and approach its lower end for strength. In terms of specific properties the
position of NFCs and WPCs relative to the conventional composites (except for PP-
scf) is again somewhat improved, due to the aforementioned density differences.
The perhaps most significant drawback of NFCs and WPCs, compared to the
conventional PP composites, is related to their lower IS. While NFCs at least partly
overlap with the PP-talc range, all WPCs exhibit inferior impact behavior. It should be
pointed out, however, that one NFC grade, i.e. PP-Tencel®, performs remarkably
well, being the only PP composite presented which retains the (unnotched Charpy) IS
level of neat PP, thus even exceeding PP-sgf composites.
Overall, NFCs may substitute PP-sgf composites when some reduction in strength is
accepted. WPCs, on the other hand, may replace PP-talc composites, in applications
where impact strength is not critical. Reflecting on the current state of knowledge and
technology in the field of NFCs and WPCs, there are several open issues and
aspects yet to be addressed, These include the effects of temperature and moisture
uptake on mechanical properties and processing behavior. Moreover, there is a large
variability in properties of natural fibers and wood, depending on growth and
harvesting conditions. Also, for a number of advantages usually associated with
NFCs and WPCs (i.e. reduced abrasiveness in processing, improved noise damping
behavior, improved overall ecobalance compared to conventional composites) there
is a need for quantitative and reliable data in support of these reputed benefits.
Finally, based on constituent property considerations, the performance potential of
natural fiber and wood composites at this stage may not yet be fully exploited. Hence,
Polypropylene Composites with Natural Fibers and Wood – General Mechanical Property Profiles
page 19 of 22
further research is warranted on elucidating structure-property relationships for these
materials to overcome current weaknesses (e.g. impact strength).
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