Post on 18-Apr-2018
I
STATE OF THE ART
Eco-efficient composite materials
OCTOBER 2010
Laura Laine 1 and Liva Rozite
2
1 Tampere University of Technology, Department of Material Science/Plastics and Elastomer
Technology, Kokkola unit, Finland
2 Luleå University of Technology, Division of Polymer Engineering, Sweden
II
CONTENTS
1 INTRODUCTION ................................................................................................................... 1
2 MATERIAL CANDIDATES .................................................................................................. 3
2.1 Bioresins ........................................................................................................................... 3
2.2 Biofibers ........................................................................................................................... 5
2.3 Bio core materials........................................................................................................... 11
3 PERFORMANCE IN HARSH CONDITIONS .................................................................... 13
3.1 Definition of harsh conditions ........................................................................................ 13
4 TESTING AND CHARACTERIZATION METHODS FOR DURABILITY
EVALUATION............................................................................................................................. 14
4.1 Mechanical testing.......................................................................................................... 14
4.2 Physical testing ............................................................................................................... 16
4.2.1 Moisture and water sorption testing ........................................................................ 17
5 SURFACE TREATMENT METHODS FOR BIOFIBERS ................................................. 18
5.1 Chemical treatment ........................................................................................................ 18
5.2 Physical treatment .......................................................................................................... 20
5.3 Effects on long term properties ...................................................................................... 21
6 CONCLUSIONS ................................................................................................................... 22
6.1 Best choice material candidates ..................................................................................... 22
6.2 Identification of most critical harsh conditions .............................................................. 22
6.3 Surface treatment options and recommendations........................................................... 22
REFERENCES ............................................................................................................................. 23
1
1 INTRODUCTION
This state-of-the-art is part of EU INTERREG IV A North -project called ANACOMPO
(Application of natural fiber reinforced composites in harsh environments) and contains
discussion about eco-efficient composite materials, test methods and fiber surface treatment
methods. The aim is to find commercially available biobased resins and natural fibers, make a
literature search of composite testing and fiber surface treatments and define the project
objectives based on the findings. In this report, only thermoset resins used in structural
composites are discussed, thermoplastic resins are excluded.
Increased interest in using natural fibers as reinforcement in polymer composites is due to
higher environmental awareness and the great potential of natural fibers. Natural fibers have
many advantages to be used as reinforcements in polymer composites. They are considered as
biodegradable and sustainable alternatives to other fibers. They are CO2 neutral and leave no
residues when they are incinerated. Their mechanical properties are very good and they are not
abrasive or toxic. Natural fibers have low density and low price. However, they also have
disadvantages like inhomogeneous quality and supply cycles. They have low water resistance
and dimensional stability. Natural fibers absorb moisture and lack good properties in toughness.
[1] In this report, the term „biofiber‟ stands for natural fiber.
Resins used in polymer composites are typically thermoset resins, like epoxies and
unsaturated polyesters, and thermoplastic resins, like polypropylene. Their raw material is
normally crude oil. The resin material can also be biobased and in that case it is based on
renewable resources. Biobased resin means that some amount of the raw materials is derived
from biomass, in other words renewable resources. The rest is crude oil based raw materials. The
motive to develop biobased resins is to have more environmentally friendly composite materials
and to use less crude oil.
On the market, also biodegradable resins are available. These are thermoplastics which
decompose by microorganisms into carbon dioxide and water in a short time. Biodegradable
resins, like polylactic acid, are mainly used in short lifetime applications, such as packaging. In
structural composites, biodegradation is not a main issue.
The percentage of the bio-derived content in a thermoset resin varies greatly and only a few
are nearly 100 % biobased. The biobased content varies from 12 % to 90 %, according to
manufacturers‟ own reports. The characterization of true biobased content is more difficult but
there is also a standard for defining it, ASTM D6866. In this state-of-the-art, biobased
thermosetting resins are of interest and bioresin is used as a synonym for biobased resins for
simplicity. However, the word „bioresin‟ is used in different meanings in other sources.
2
In Chapter 2, bioresins, biofibers and bio core materials are reviewed. In Chapter 3, the
definition of harsh conditions is discussed. Characterization methods for durability evaluation
and literature values are presented in Chapter 4. Surface treatment methods for natural fibers are
discussed in Chapter 5 and conclusions are made in Chapter 6.
3
2 MATERIAL CANDIDATES
Possible material candidates in this project are environmentally friendly composite materials that
have potential to be used in structural applications and in difficult environmental conditions. The
aim is that both resins and reinforcements are to some extent made from biobased raw materials
or constituents with the ideal target a composite made from 100 % biobased raw materials.
Concerning reinforcements, natural fibers have shown very promising mechanical properties.
Biobased resins are quite comparable with crude-oil resins, except the price is higher and
therefore the usage of biobased resins is growing slowly. In this chapter, commercial bioresins,
biofibers and bio core materials are discussed.
2.1 Bioresins
The term „bioresin‟ means that a resin is composed of raw materials, which are wholly or partly
derived from renewable resources. Bioresin can be a thermoplastic, a thermoset or a
biodegradable plastic. Here, only thermoset bioresins are discussed. A thermoset polymer is
highly cross-linked and is cured by using a curing agent and heat or heat and pressure, and the
result is a material with high strength, modulus and durability [2].
In a review article [2], thermosetting bioresins are divided into phenolics, epoxies,
polyurethanes, polyesters and other resins. The article contains detailed information on raw
materials used in different resins, including chemical formulas. To produce bioresins, commonly
used renewable raw materials are plant oils (for instance soybean oil, castor oil, pine oil),
polysaccharides (cellulose, starch) and proteins. Currently, petroleum-based raw materials are
added to renewable raw materials to make bioresins.
In Table 1, current manufacturers for commercial thermosetting bioresins are presented with
information about raw materials, chemical formula or manufacturing method and application
areas. As can be seen, used raw materials are diverse, including soybean oil, epoxidised pine oil
waste, castor oil, furfuryl alcohol and lactid acid. In this group, biobased content varies from
18% to 50-90%. Three of them are unsaturated polyester based, one is an epoxy and others are
varied.
4
Table 1 Commercially available bioresins, their raw materials and applications.
Manufacturer (trade name)
Raw
materials Chemical formula Applications Sour
ce Ashland e.g.
ENVIREZ®
1807
Unsaturated
polyester Soybean oil Bio 18%
e.g. tractor panels [3]
Amroy Europe
Oy EpoBioXTM
Natural
phenols
distilled from
forest industry
waste stream,
eg. epoxidised
pine oil waste Bio 50-90%
kayaks, boats, tent
poles, glues,
electrical cars
[4]
DSM Palapreg
ECO P 55-01 Unsaturated
polyester Bio 55%
Not available SMC/BMC
applications [5]
TransFurans
Chemicals
bvba BioRez™
furfuryl resin
Furfuryl
alcohol based
resins from
biomass
Furfural (FF) is produced from
hemicellulosic agricultural wastes and
source for FA production via catalytic
hydrogenation.
Varied applications [6]
JVS-Polymers
Ltd. LAIT-X /
POLLITTM
Lactic-acid
based Aliphatic hydroxyl acids, such as lactic
acid, are polymerized into poly(lactic
acid) by direct polycondensation with
a use of suitable catalyst
composites,
impregnated
products, coatings
and biomedical
applications
[7]
Bioresin Bioresin
castor oil Not available Automotive,
marine [8]
Reichhold ENVIROLITE
™ Unsaturated
polyester Soya oil Bio 25%
Not available SMC/BMC and
pultrusion
applications
[9]
Cognis Tribest acrylate
functional
resin system
derived from
soya oil
[10]
University of
Borås Soybean oil: Methacrylated,
Methacrylic
anhydride
modified,
Acetic
anhydride
modified
Eg. Methacrylated soybean oil
O
O
O
O
O
O
2 3
3 3
7
O
O O
OH
O
OH
O
O
OH
5
2.2 Biofibers
Natural fibers can be divided in groups of bast, leaf, seed, fruit, wood and grasses. Flax and
hemp fibers belong to bast fibers. [11] Here, the inspection is limited mainly to flax and hemp.
Some of commercial natural fiber and biofiber reinforcement producers are presented in Table 2.
Natural fibers for composite applications are available typically in the form of yarns and fabrics,
which can be woven or non-woven. Some manufacturers provide fibers as preimpregnated to
different resins or plastics. As shown, flax providers are easier to find than hemp providers.
Table 2 Manufacturers of natural fibers and natural fiber reinforcements.
Manufacturer Fiber Webpage Libeco-Lagae, Belgium flax www.libeco.be Engtex AB, Sweden flax www.engtex.se Composites Evolution Ltd, UK flax www.compositesevolution.com Group Depestele, France flax e.g. www.lbn-lin.com Lineo NV, Belgium flax www.lineo.eu Safilin, France/Poland flax www.safilin.fr Fimalin, France flax www.fimalin.com Stemergy: Renewable fibre
technologies, Canada hemp www.hempline.com
HP Johannesson Trading AB,
Sweden jute www.hpjtrading.se
CORDENKA GmbH, Germany regenerated
cellulose www.cordenka.com
The natural fiber has a very complex multi-scale internal structure: it consists of elements
with different size scales. The main constituents involved in composition of plant fibers are
polymers themselves: cellulose, hemicellulose, lignin and pectin (see Table 3). [12]
Figure 1 shows a schematic picture of composition and built of a flax stem (see also the
micrograph in Figure 2). Technical fibers (approximately 1 m long) are isolated from the flax
plant and consist of elementary fibers with length generally between 2 and 5 cm, and diameters
between 19 and 25 μm. The elementary fibers are glued together by a pectin interface. These
fibers have polyhedron shape (see Figure 3) with 5 to 7 sides to improve the packing in the
technical fiber. [12]
6
Table 3 The chemical composition of different natural fibers. [13]
Fiber Type Cellulose, % Hemicellulose, % Lignin, % Pectin, %
Flax 81 14 3 4
Hemp 74 18 4 1
Wood ~ 46 ~ 27 ~ 27 -
Sisal 73 13 11 2
Figure 1 Composition and built of flax stem. [14]
7
Figure 2 Micrograph showing multi-scale structure of the flax stem. (Constructed from
individual images published in [15]).
Figure 3 SEM image of elementary flax fiber. [46]
Elementary fibers are single plant cells and the most common chemical material in plant cell
walls is cellulose (C6H10O5)n. The chemical structure of cellulose monomer is showed in Figure
4. Most of the elementary fibers consist of oriented, highly crystalline cellulose fibrils and
amorphous hemicellulose. The crystalline cellulose fibrils in the cell wall are oriented at an angle
±10º with the fiber axis and give the fiber its high tensile strength (see Figure 5). [12]
8
Figure 4 The chemical structure of cellulose monomer. [12]
Figure 5 Different layers in cell wall. Middle lamella (ML), Primary wall (P), Secondary
wall (S), Lumena (L).
Natural fibers have many properties that make them interesting as reinforcement in
composites. These fibers are renewable, recyclable, have high specific properties and good
isolation properties. Also these fibers have low bulk cost and low weight. However, natural
fibers have several disadvantages compared with synthetic fibers. The main drawback is quality
variations, depending on growth conditions, processing and other reasons, which may also
influence the price of fibers. Natural fibers are sensitive to moisture and have lover durability.
9
The maximal processing temperature is lower than for synthetic fibers. Table 4 shows some
properties for the most popular natural fibers, E-glass and wood fibers.
Table 4 Properties of different fibers. [12,16,17]
Fibers Density
(g/cm3)
Modulus
(GPa)
Strenght
(MPa)
Strain
(%)
Diameter
(μm)
Specific
modulus
Specific
Strenght
E-glass 2.54 72 3530 1.8-3.2 10 28.2 1390
Wood 1.54 30-40 400-800 - 20-40 ~ 25 ~ 390
Flax 1.4-1.5 50-70 500-900 1.5-2.4 10-30 ~ 41 ~ 480
Hemp 1.48 30-60 300-800 1.1-2 10-50 ~ 30 ~ 370
Sisal 1.45 9-20 510-700 2.2-2.9 10-40 ~ 10 ~ 420
Cordenka 1.5 ~20 700-800 13-15 12.5 ~ 13 ~ 470
The process to make flax or hemp plant into a reinforcement fiber has several stages. Fibers
are in fiber bundles, so first the bundle is broken down by retting. Retting is done by spreading
the crop in the fields and leaving there for 3-8 weeks or immersing the crop in water for 1-2
weeks. At present, also many industrial retting methods are being developed, like enzyme
retting. When the retting has broken down the bundle into single fibers, fibers are dried and
mechanically separated from the straw by stripping and combing. [18]
Natural fibers have been used in lines, ropes and other one dimensional products, textiles,
canvas, and papers. As natural fiber mechanical properties can be competitive to glass fibers,
they can substitute glass fibers in composites. However the adhesion between natural fibers and
polymer is a problem. Adhesion can be improved by fiber treatment or by the use of more
compatible polymers (for example bio-based resins). Also the length of natural fiber and
orientation is an issue for using them in the composites.
10
Figure 6 The viscose process. [19]
One of the latest revelations in the type of reinforcement with plant origin is regenerated
cellulose fibers. These fibers, similar to flax and hemp fibers, have high cellulose content, but
unlike flax and hemp they are manmade. “Terms regenerated cellulose, rayon, and viscose rayon
tend to be used interchangeably” [19]. The flow diagram for the viscose process is given in
Figure 6.
First wood pulp is dissolved in caustic soda. Then steeping is performed - for a specified
period of time composition is shredded and allowed to age. The period of aging determines the
viscosity of the viscose. The longer is the ageing time the higher will be the viscosity of solution.
Afterwards aged pulp is treated with carbon disulphide to form a yellow-colored cellulose
xanthate, which is dissolved in caustic soda. This is the starting stage of viscose formation [20].
11
Although these are manmade fibers, they are made out of the natural polymer directly on
contrary to the fibers made out materials with fossil origin. These fibers are continuous and it is
easy to arrange them into fabrics with stable orientation and geometry. At the moment, the main
application for these fibers is in textile industry. However, currently there are many on-going
studies dealing with the evaluation of these fibers as perspective reinforcement for bio-based
polymer composites.
Figure 7 Stress-strain curves of CA (Cordenka EHM), CB (Cordenka 1840), CC (Enka
Viscose), CD (Cordenka 700), CE (Alternative cellulose) and CF (Lyocell) regenerated
cellulose fibers and steam exploded flax and field retted hemp fibers. [21]
There are several types of regenerated cellulose fibers. Typical stress-strain curves for
different fibers are presented in Figure 7. One of the regenerated fibers considered for this
particular project are Cordenka fibers. These fibers are produced by the German company,
Cordenka GmbH, and the main application is for reinforcing rubber in car tires.
2.3 Bio core materials
The most used natural core material in composites is balsa wood. Commonly balsa is used as a
plate which consists of balsa pieces cut to the same thickness. Balsa has good strength properties.
[22] For example, 3A Composites (Switzerland) manufactures balsa for sandwich structures with
12
trade name BALTEK® (more information: www.corematerials.3acomposites.com/baltek-
balsa.html).
In this project, also other biobased core materials are interesting. For instance, Saarpella Oy
(Finland) produces non-woven flax felt as an insulation material, which could be used as core
material. Bioresin (Brazil) has derived polyurethane foam, BIOFOAM, from castor oil.
Interesting is also the possibility to use the bioepoxy of Amroy Europe Oy (Finland) as foam.
In literature, Burgueño et al. [23] studied load-bearing natural fiber composite cellular beams
and plates made of hemp or flax fibers and unsaturated polyester. Dweib et al. [24] used
acrylated epoxidized soybean oil (AESO) resin and flax mats to produce biocomposite sandwich
beams for structural applications. Faruk et al. [25] has made a review of microcellular foamed
wood-plastic composites.
13
3 PERFORMANCE IN HARSH CONDITIONS
Reinforced composites are normally used in applications, where good mechanical and other
properties are required, such as technical and structural applications. They can also be exposed to
difficult or extreme environment conditions in their applications. These conditions can affect the
properties of composites and decrease their performance. Therefore, studies in difficult
conditions are relevant and aid to choose a right application and predict the performance of
composite. In this chapter, the definition of harsh conditions of biobased composites is discussed.
3.1 Definition of harsh conditions
Biobased composites can be used in applications, where the conditions are challenging to
composites, in other words harsh conditions. In marine industry for instance, composites are
exposed to water and in sea areas to saline water. Also humidity can sometimes be very high and
have influence on composites. Biofibers tend to absorb moisture. In a report [26], harsh weather
conditions are said to contain humidity, elevated temperature, UV radiation and rain. Low
temperatures can be added to the list, because outdoor temperature in the Nordic countries is
several degrees below zero in winter. UV radiation from sunlight is at its highest in summer.
Rarely these conditions are present separately and thus they have a combined effect on
composites. During this project, the effect of these conditions on biobased composites is going to
be studied.
In literature, Mehta et al. [26] tested hemp reinforced UPE composite samples in an
accelerated weatherometer. They used the measurement cycle of 48 cycles of UV treatment (340
nm) at 60 °C for 2.5 h followed by water spray for 30 min and condensation at 45 °C for 24 h.
They repeated it 12 times with total duration 2016 h. Weight loss was less than 2% for untreated
hemp composite and 1.25% for acrylonitrile treated hemp composite, surface roughness
increased with exposure time and color changed. Storage modulus decreased only slightly
(values 6-8 GPa) and Tg decreased less than 3°C.
14
4 TESTING AND CHARACTERIZATION METHODS
FOR DURABILITY EVALUATION
Testing and characterization methods in order to evaluate the durability of composites are
divided into mechanical tests, physical tests and chemical tests. Mechanical testing is the most
used test method of these. Mechanical tests provide information on strength, compression,
impact, bending and shear properties. In many applications, the strength properties of composite
are of interest. Physical tests give knowledge of thermal, absorption and structural properties.
Chemical tests give insight into the chemical properties or structure of composites. In this
chapter, numerical data from various studies are collected mainly in tables to show their
performance.
4.1 Mechanical testing
Typical mechanical tests for composites are tensile, flexural, impact and fatigue tests. In
structural applications, composites are required to carry load and good tensile properties are
demanded. In Table 5, data from various studies are collected for flax reinforced composites and
hemp reinforced composites.
The composite structure of test samples varies and that has effect on their properties.
Reinforcements are in the form of mats, woven fabrics, fibers or non-woven mats. Also the resin
is varied: epoxies, polyesters, soybean oil resin and acrylated epoxidized resins. Mechanical test
results differ quite lot, probably partly due to the composite structures. For instance, tensile
strength for flax reinforced composites is between 15 to 90 MPa. In this project, one purpose is
to study, if biofibers are suitable for being used in applications, where good strength properties
are required.
15
Table 5 Test data from the literature for flax and hemp reinforced composites. X means
that tests are done but figure is not found.
Fiber/ Matrix
Processing Tensile
strength Tensile
modulus Flexural Fatigu
e Impact
strength Refere
nce
Flax /
Soybean
oil resin
(MMSO)
(60:40)
Compression
moulding, air
laid / woven
fabric
(5kN) Strength
90 MPa, Mod. 5 GPa
(Charpy) 24-29
kJ/m2
[27]
Flax /
Vinylester
or
Modified
acrylic
resin
RTM, mats (25kN, 2
mm/min) ≈ 70-90
MPa
≈ 7.5-9.5
GPa [28]
Flax /
Acrylated
epoxidized
soy oil
(AESO)
RTM, fiber (5mm/min) ≈ 15-30 MPa
≈ 3.2-4.7
GPa Strength 42- 64
MPa, Mod. 2.7 - 4.2
GPa
[29]
Flax / EP yarns X [30]
Flax / EP Laminates, non-woven
mat
(1mm/min) ≈ 47 MPa
(untreated) ≈ 60-75 MPa
(treated)
≈ 4.5-4.8
GPa [31]
[32]
Hemp /
Acrylated
epoxidized
soy oil
(AESO)
RTM, mats 35 MPa 4.4 GPa Strength 35.7 ±
5.9 MPa / 51.3 ±
2.7MPa, Mod. 2.6 ± 0.2
GPa / 2.7 ± 0.2
GPa
[29]
Hemp /
UPE Compression
moulding,
non-woven
mat
≈ 70 MPa
(AN treated
hemp)
≈ 8 GPa
(AN
treated)
X (notched
Izod) 26
J/m
[33]
Hemp /
UPE
RTM, mat
Strength 22 ± 3
MPa / (Fungal
treated) 26 ± 3.5
MPa, Mod. 3.3 ± 0.3
GPa / (Fungal
treated) 3.7 ±
0.5 GPa
[34]
16
4.2 Physical testing
Data from physical tests, including thermal analysis, are collected in Table 6. Typical physical
tests are water absorption, swelling and moisture content. They are discussed in chapter 4.2.1.
Thermal analysis methods can be thought as physical testing. In the table, the numerical values
of tests from differential scanning calorimetry (DSC), thermogravimetry (TG), dynamic
mechanical analysis (DMA) and scanning electron microscope (SEM) are used to give the scale
to physical properties.
In addition to the table, Gulati et al. [35] used inverse gas chromatography (IGC) to
investigate acid-base characteristics of hemp fibers. They found that highest improvements in
acid-base interactions between fiber and matrix are correlated with actual improvement in the
mechanical properties of RTM manufactured hemp reinforced unsaturated polyester composites.
Table 6 Physical test data from the literature.
Fiber/ Matrix
Processing DSC TG DMA SEM Refere
nce
Flax/Soyb
ean oil
resin
(60:40)
Compression
moulding, air
laid / woven
fabric
Relative
low
thermal
stability
(≈370°C)
Shorter fiber
pullout with
styrene
[27]
Flax /
Acrylated
epoxidize
d soybean
oil
(AESO)
Vacuum
RTM /
infusion, mat
(different
kinds of vol-
%)
(3-point
bending) E‟ ≈ 1500 -
2000 MPa E‟‟ ≈ 200 -260
MPa Tg ≈ 57 – 70°C
[32]
Hemp /
UPE Compression
moulding,
non-woven
mat
Tg = 95°C (max
tan δ) Smaller degree of
fiber pullout (AN
treated)
[33]
Hemp /
Acrylated
epoxidize
d soybean
oil AESO
Vacuum
RTM /
infusion, mat
(3-point
bending) E‟ ≈ 2200 MPa E‟‟ ≈ 270 MPa Tg ≈ 65°C
[32]
Hemp /
Water-
based
acrylic
thermoset
(20-100°C)
heat
capacity of
cured resin
1.98-2.7 J g
-1 K
-1 /
of hemp
2.2-3.4 J g
-1 K
-1
[36]
17
In Table 6, when comparing DMA results, hemp reinforced AESO composite has better
strength than flax reinforced according to higher storage modulus, E‟, value. The larger E‟, the
better strength composite has. Glass transition temperature, Tg, of flax and hemp reinforced
AESO composites can be somewhat lower than it could be due to an incomplete curing process.
Otherwise, the values seem to be moderate. Typically, biofibers are thought to have a limited
processing temperature at approximately 200 °C, which must be taken into account when
choosing a processing method.
4.2.1 Moisture and water sorption testing
Moisture and water sorption tests are important in order to study the water resistance of biofiber
reinforced composites. Natural fibers are commonly known as to easily absorb moisture. In
Table 7, rather different values for water sorption are collected from literature. Water uptake
varies from 2 % to 18 % increase in composite weight. The reason for variation can be found in
different matrices and surface treatments. Moisture absorption for hemp is found to be about 1 to
3 weight-%. Fiber surface treatments can decrease water and moisture absorption.
Table 7 Moisture and water test data from the literature.
Fiber/ Matrix
Processing Water sorption Moisture
absorption Refere
nce
Flax /
Acrylated
epoxidized
soy oil
(AESO)
RTM, fiber (24h) ↑ 2.3 - 4.1 wt% (7 weeks) ↑ 10.4 - 12.4 wt%
[29]
Hemp /
UPE Compression
moulding, mat (30°C, 90% RH)
↑0.7 wt% (untreated) ↑0.3 wt% (AN treated)
[26]
Hemp /
UPE RTM, randomly
oriented mat
(different kinds of
vol-%)
(23°C, saturation
200 days) ↑ ≈2 – 4 wt%
(23°C, 94% RH,
200 days) ↑ ≈1.5 – 2.7 wt% No saturation
[37]
Flax / EP Hemp / EP
(816 h) ↑17.2% (flax) ↑18.4% (hemp)
[38]
18
5 SURFACE TREATMENT METHODS FOR BIOFIBERS
Natural fibers have typically poor adhesion and wettability with resin materials because biofibers
are hydrophilic in nature and resins are hydrophobic. If the reinforcement is not properly adhered
to the matrix, it does not add the strength of the composite. Therefore, different surface treatment
methods for biofibers have been invented and tested to improve the adhesion between biofibers
and polymer matrices. Treatments naturally raise the price of end products.
Generally, treatment methods can be divided into chemical treatments and physical
treatments. Chemical treatment can be defined as a chemical reaction between some reactive
constituents of chemical reagent and biofiber to form a covalent bond [39]. Physical treatment
methods do not change the chemical structure of biofibers, only the surface properties. In this
chapter, the literature search results of treatment methods are presented and their effect on the
long term properties of composites is discussed.
5.1 Chemical treatment
Common chemical treatments are dewaxing, mercerization, bleaching, cyanoethylation, silane
treatment, benzoylation, peroxide treatment, isocyanate treatment, acrylation, acetylation, latex
coating and steam-explosion. [39] John et al. [40] also adds the condensation of coupling agents
onto the cellulose surface.
Many studies have been made to investigate the effect of different surface treatments on the
properties of fibers. Good review articles [39-41] are found in the scientific literature. For
example, possible silanes to be used in silane treatment are various and Xie et al. [42] have made
a table with references about “Silanes used for the natural fiber/polymer composites”.
The main things from the literature articles are collected into Table 8 and Table 9. The
articles that studied only fibers [42,43] are left out of the tables. Surface treatments for flax and
for hemp are separated into different tables.
19
Table 8 Effect of different chemical treatments on hemp reinforced composite properties.
Fiber/ Matrix
Chemical
treatment Conditions Processing Effect on
composite
properties +
Effect on
composite
properties -
Referenc
e
Hemp/
EP Mercerizati
on 22% NaOH, 60
min, 10°C Filament
winding,
UD
Flex. strength ≈ 45% Flex.mod.
of elast. ≈ 100%
[44]
Hemp/
UPE Alkalizatio
n /
Acetylation
6% NaOH for
48h, room
temp. / glacial
acetic acid for
1h, room temp.
RTM,
contains
also glass
fiber mats
Higher
improvement with
alkali in flex.
mod. and strength
[35]
Hemp/
UPE Fungal -
modificatio
n
0.5% glucose
and 0.1% yeast, in rotary shaker
for 4, 6 or 8
days
RTM, mat Flex. strength
21%, flex. mod.
12 % (improved
interfacial
adhesion)
[34]
Hemp/
UPE Acrylonitril
e grafting 3% acrylonitrile,
0.5% dicumyl
peroxide, 96.5%
ethanol, 15 min
Compressio
n
moulding,
non-woven
mat
Tensile strength
80%, tensile mod.
25%, elast.mod.
30%, impact
strength 50%,
storage mod. &
loss mod.
increased
[33]
Hemp/
UPE Alkali /
Silane
treatment
Alkali: 2%
NaOH, 23°C,
1h, dried Silane: 1% 3-
aminopropyltriet
hoxysilane, 30
min, dried
RTM,
randomly
oriented
mat
Treatment
not
significantly
improve the
water
resistance
[37]
Hemp/
UPE Esterificati
on Methacrylic
anhydride +
pyridine: 100°C,
48h, dried / Pyridine
(same)
RTM, non-
woven mat Better interfacial
adhesion Higher flex. mod.,
flex. stress at
break no change
except in mode of
failure
Lower
impact
strength,
Low
toughness Pyridine: no
effects on
properties
[45]
20
Table 9 Effect of different chemical treatments on flax reinforced composite properties.
Fiber/ Matrix
Chemical
treatment Conditions Processing Effect on
composite
properties +
Effect on
composite
properties -
Refere
nce
Flax/Acryl
ated
epoxidized
soybean oil
+ styrene
Lignin
treatment Aqueous
NaOH solution
of kraft lignin
VARTM,
mat Tensile and flexural
properties and fiber
wettability
improved
[46]
Flax/EP Ethylene
diamine
tetraacetic
acid
NaOH, 60 °C,
3 h Laminates, non-woven
mat
Tensile strength ≈ 50%
[31]
Flax/EP Alkalizatio
n NaOH 1% -
3%, 20 min,
room
temperature
Autoclave,
UD mat (Improved interface
quality) Increased
longitudinal and
transverse
flex.mod. and
strength
(fiber
strength
decreased)
[47]
Flax/EP Alkali / Silane / Isocyanate
NaOH 1% / 3-
aminopropyltri
ethoxysilane / Phenyl
isocyanate
Autoclave, UD mat /
Random
non-woven
mat
Tensile strength
17%, tensile mod.
25% No change in
impact toughness
[48]
As can be seen in the tables, the surface treatments influence mainly positively to the
properties of natural fiber reinforced thermoset polymer composites. The comparison of results is
quite challenging, because the results depend on fiber content, composite manufacturing method
and test methods. In generally, in many studies tensile strength, tensile modulus, flexural
strength and flexural modulus increased because of the surface treatment. Mercerization, also
called alkalization, seems to be the most used treatment for flaw and hemp fibers.
5.2 Physical treatment
Typical physical treatment methods are stretching, calendaring, thermotreatment, production of
hybrid yarns, corona and cold plasma. Also other interesting methods have been tested. Gouanvé
et al. [49] studied the effect of autoclave treatment and helium cold plasma treatment on the
water sorption of flax fiber (no matrix). The moisture resistance increased after autoclave
treatment, but the plasma treatment had no effect on the resistance. Hepworth et al. [50] used a
urea treatment to penetrate epoxy resin into the cell walls of flax fibers. Treated fiber reinforced
composites had higher tensile modulus (15 GPa) than untreated fiber composites (11.5 GPa)
although strength was not affected (118 MPa).
21
5.3 Effects on long term properties
Composites are typically used in applications, whose time of use is long, from years to decades.
Hence, the effect of surface treatments on the long term properties of composites should be
studied. The question, how biofiber reinforced composites maintain their properties during a long
period, is interesting. For instance, water and fungus can cause harm to biofibers within time.
Many studies cover short or relatively short term property changes. Changes in long term
properties are one of the main objectives to be explored in this project.
22
6 CONCLUSIONS
The purpose of this state-of-the-art is to give insight into eco-efficient composite materials and
make conclusions that operate as guidelines to further actions in the ANACOMPO project.
Materials that are chosen to be good material candidates tested in this project are discussed,
likewise the recommended surface treatment to be used. The most critical harsh conditions
concerning the use of biocomposites are considered.
6.1 Best choice material candidates
A few from commercially available bioresins are selected for further characterization. The
bioepoxy, EpoBioXTM
, is a good material candidate because epoxies typically have very good
properties and this bioepoxy also has a large bio content percent. Epoxies perform presumably
well in harsh conditions. It is also produced in Finland and hence quite near to all project
partners. ENVIREZ® unsaturated polyester bioresins can also be purchased from Finland and
ENVIREZ® resins have been in the market for almost a decade, so they are worth to take as good
candidates. Also the unsaturated polyester bioresin, Palapreg ECO P55-01, from DSM has quite
high bio content percent, a half of the raw materials. These four resins are selected for further
tests.
6.2 Identification of most critical harsh conditions
Data from harsh condition testing is quite limited available. Probably a good way is to start tests
in all typical condition areas that are known to be difficult. UV radiation, water, humidity and
temperature and also the combined effect of those are commonly of interest.
6.3 Surface treatment options and recommendations
In tables 8 and 9, the most commonly used surface treatment is chemical treatment with sodium
hydroxide. This treatment is quite easy and inexpensive.
23
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