1
Flax fiber and its composites: An overview of water and moisture
absorption impact on their performance
Abdul Moudood
1
, Anisur Rahman
1
, Andreas Öchsner
2
, Mainul Islam
3
, Gaston Francucci
4
1
Griffith School of Engineering, Gold Coast Campus, Griffith University, Queensland 4222,
Australia
2
Esslingen University of Applied Sciences, Faculty of Mechanical Engineering, Kanalstrasse 33,
73728 Esslingen, Germany
3
School of Mechanical & Electrical Engineering, University of Southern Queensland, West Street,
Toowoomba, QLD 4350, Australia
4
Research Institute of Material Science and Technology (INTEMA-CONICET), National University
of Mar del Plata, J. B. Justo 4302, B7608FDQ, Mar del Plata, Argentina
Corresponding author: Gaston Francucci, e-mail:
gastonfrancucci@gmail.com
Tel: +5492236022676
Abstract
Contemporary researchers have specified that natural flax fiber is
comparable with synthetic fibers due to its unique physical and mechanical
characteristics which have been recognized for decades. Flax fiber reinforced
composites have the potential for wide usage in sport and maritime industries, and
as automotive accessories. In addition, this composite is in the development
stages for future applications in the aeronautical industry. However, designing the
flax composite parts is a challenging task due to the great variability in fiber
properties. This is caused by many factors, including the plant origin and growth
conditions, plant age, location in the stem, fibers extraction method, and the fact
that there is often a non-uniform cross-section of the fibers. Furthermore, the water
and moisture absorption tendency of the flax fibers and their composites and the
consequent detrimental effects on their mechanical performance are also major
drawbacks. Fibers may soften and swell with absorbed water molecules, which
could affect the performance of this bio-composite. Flax fibers’ moisture absorption
propensity may lead to a deterioration of the fiber-matrix interface, weakening the
interfacial strength and ultimately degrading the quality of the composite. This
review represents a brief summary of the main findings of research into flax fiber
reinforced composites, focusing on the challenges of its water and moisture
absorption behavior on their performance.
Keywords: Flax fiber composites, Moisture absorption, Mechanical properties,
Interfacial strength, Water diffusion
Introduction
Environmental awareness is a most important concern nowadays. Scientists
all over the world are enthusiastic about the research of environmentally-friendly
materials, owing to the increasing rate of greenhouse gas emissions and other
related health hazards. The various products made from plastic and metal have a
significant impact on the environment. Specifically, for composite materials, global
awareness about the environmental impact of synthetic fibers throughout their
manufacturing, usage, and end-life has greatly increased in recent years. In this
context, natural fibers and their composites are considered to be one of the major
2
alternatives and have occupied a substantial place in the field of material research
during the last few decades. They are lightweight, inexpensive and structurally
efficient materials that can replace the commonly used conventional synthetic fiber
composites in some engineering applications. Thus, the production of eco-friendly
and sustainable bio-based materials, with economic advantages, has resulted in
increased attention in developing novel natural fiber reinforced composites.
Plant fibers, a sub-section of natural fibers, are cost-effective and offer good
specific mechanical properties when compared to glass fibers.
1, 2
Monteiro et al.
3
mentioned that economic, technical, societal and environmental benefits are
achieved by using plant fibers instead of synthetic fibers for reinforcement.
Summerscales et al.
4
also reported that automobile accessories manufactured
with plant fibers are light in weight. Due to this weight reduction, vehicles require
less energy to operate. These fibers are extracted from different parts of the plant
such as from the outer part of the plant stem, i.e., the bast (e.g., jute, kenaf, hemp,
ramie, flax), leaf (e.g., sisal), fruit (e.g., coconut), and seed (e.g., cotton).
According to statistical data, the worldwide production rate of bast fibers is
higher than the other types, and they are more popular among current fiber
composite researchers.
5, 6
Further, flax is one of the most widely used natural fibers
of the bast family for composite reinforcement.
7
However, the chemical
composition and microstructure of vegetable fibers permits moisture absorption
from the environment, which causes weak bindings between the fiber and the
polymer matrix. Like other vegetable fibers, the highly hydrophilic nature of flax
fibers and the moisture sensitivity of their composites are the main
disadvantages.
8, 9
Occasionally, due to the incompatibility between hydrophilic
plant fibers and hydrophobic thermoplastic and thermoset matrices, physical and
chemical treatments on such fibers and/or matrices are needed to increase the
adhesion between them.
2, 7, 10
In particular, the overall quality of the composites depends on the properties
of the fibers, the matrix and their interface. Water and moisture absorption of plant
fibers have multiple effects, in terms of their properties, morphology, chemical
composition and dimensional stability. The composites made with fibers taken from
different relative humidity (RH) environments are expected to behave differently.
On the other hand, when the composites are made with dry fibers, these can also
absorb moisture in various conditions of humidity. Furthermore, if the composites
are immersed under water, their properties are likely to be degraded by absorbing
water. Therefore, the effect of RH and water or moisture absorption on the
properties of flax fibers and their composites are the main points of interest of this
review paper.
Natural and synthetic fibers
Natural fibers are very popular reinforcements used for manufacturing
composites for a wide variety of engineering applications. These fibers are
gradually occupying the place of synthetic fibers in some applications due to a
sustainability viewpoint. The composites made with synthetic fibers, such as
carbon or glass fibers, are known as high-performance composites. Several
studies
11-13
reported about the recycling problems of the synthetic fiber composites
and how environmental hazards were caused by these fibers. On the other hand,
researchers
11,14-16
indicated that natural fibers, especially plant fibers, are a
3
possible replacement for synthetic fibers. The extraction and production process
of the plant fibers are almost free of contaminants. Furthermore, all residues
coming from the leftovers during fiber processing are non-toxic and non-
hazardous. In a study, Joshi et al.
17
stated that composites made with natural fibers
are environmentally better than the synthetic fiber composites, in most cases, in
terms of the performance indicators. The cultivation of natural fibers is dependent
on solar energy and the fiber extraction and production process require only a
small amount of fossil fuel energy. However, the synthetic fiber production
processes require a massive amount of fossil fuel which causes an obvious
environmental problem. In addition to the environmental advantages of natural
fibers over synthetic fibers, Wambua et al.
18
concluded that the mechanical
properties of the natural fiber composites are fairly comparable to those of glass
fiber composites. Furthermore, few specific properties of the natural fiber
composites have been reported to be superior to the glass fiber composites.
Nevertheless, natural plant fibers are also regarded as the best choice for
commercial purposes.
19
Classification of natural fiber
According to the origin, natural fibers may be classified as plant fibers,
mineral fibers and animal fibers. The plant or vegetable fibers are often termed as
cellulosic fibers. These fibers can be further sub-divided into non-wood natural
fibers and wood natural fibers. Non-wood natural fibers can be divided into bast,
leaf, seed, straw and grass fibers. Among all natural fibers, plant fibers are the
ones generally used in the composite industry as reinforcement. A common natural
fiber classification is shown in Figure 1.
Figure 1. Classification of natural fiber.
Flax natural fiber-an important bast fiber
Flax fiber is considered as the most important member of the bast family for
composite reinforcement due to its unique properties.
7
The bast fibers are
collected from the fibrous bundles which are situated in the inner bark of a plant
stem. The inherent high strength and stiffness of flax fiber and low elongation to
failure are the important characteristics of this fiber that make it particularly
interesting in composite research. Flax (Linum usitatissimum) is typically grown in
a moderate climate region. Charlet et al.
20
reported that flax plants are cultivated
widely in Western Europe where the daily temperature is below 30°C generally.
However, flax is also grown in Southern Europe, Argentina, India, China, and
Canada. Flax fibers are not continuous fibers as compared to the synthetic fibers,
but they have a structure similar to that of composites and are hierarchically
organized. Their macroscopic properties arise from their micro and nano-structural
level. Flax is an important industrial fiber that has been used since ancient time.
More than 30,000 years ago, prehistoric hunters were using twisted wild flax fibers
for making cords for hafting stone tools, weaving baskets, or sewing garments.
21
4
Structure and composition of flax fiber
A flax stem has the constituents of bark, phloem, xylem and a void at the
center. The fibers are located as fiber bundles in the outer surface of the plant
stem as shown in Figure 2. The flax plants can grow to heights of 80 to 150 cm in
less than 110 days since the plants are fast growing by nature. The bundles
(technical fibers) are between 60 and 140 cm long and their diameter ranges from
40 to 80 μm. A flax stem contains 20-50 bundles in its cross section. Each bundle
consists of 10-40 spindle-shaped single (elementary) fibers of 1-12 cm long and
15-30 μm in diameter.
22
Charlet et al.
23
reported that the elementary fiber
diameters are different if taken from the bottom, the middle and the top part of the
flax stems. The mean fiber diameter was found to decrease from the bottom to the
top of the stems.
Figure 2. Structure of the flax fiber: (a) cross section of flax plant stem and
position of the bundles of elementary fibers and technical fibers after extraction,
(b) SEM image of a technical fiber with its constituting elementary fibers
(reproduced with permission from
24
).
Figure 3. An elementary fiber structure (reproduced with permission
from
29
).
The elementary fiber (Figure 3) denotes a single cell in the flax plant. Each
elementary fiber is composed of concentric cell walls which are different from each
other in terms of thickness and the arrangement of their constituents. Each cell
wall consists of what is known as a primary (outer) and a secondary cell wall.
These are concentric cylinders with a small open channel in the middle called a
lumen. The primary cell wall can be up to 0.2 µm thick and the lumen can be as
small as 1.5% of the fiber cross-section. The secondary cell wall contains three
sub-layers S
1
, S
2
, S
3
.
25, 26
The single flax fibers have been shown to possess
different shapes within cross-sections along the fiber axis, which some researchers
approximated to hexagonal or pentagonal cross-sections.
27
However, the fibers
vary in their non-uniform geometrical shapes along the axis. Owing to these
irregularities in the thickness of the cell walls, the fibers vary greatly in strength.
26
The main constituents of flax fibers are cellulose, hemicellulose, lignin and
pectin. A small percentage of wax, oil and structural water are also found.
9, 28
Both
primary and secondary cell walls are composed of cellulosic materials. Cellulose
fibrils (diameter between 0.1 μm and 0.3 μm) are surrounded by concentric
lamella, composed of about 2% pectin and 15% hemicellulose, which contribute to
the thermal degradation and water uptake behavior of the fibers.
30
The secondary
cell wall is the major part of the fiber diameter and S
2
layer is its dominating
constituent. This layer consists of highly crystalline cellulose microfibrils bounded
by lignin and hemicellulose. The microfibrils in the S
2
layer follow a spiral pattern
at an angle of 5-10° along the fiber axis, which explains the stiffness and strength
of the fiber in the axial direction. The middle lamella is considered to be the matrix
which bonds the cell together.
26
Bos et al.
26
described the technical fibers, which
are extracted by partially separating the fiber bundles in the flax plant and can be
as long as the stem length (approximately 1 m). The technical fibers (i.e., the
bundles of elementary fibers) consist of 10-40 elementary fibers in the cross-
5
section. The elementary fibers overlap for a considerable length and are glued
together by an interphase known as a middle lamella, consisting mainly of pectin
and hemicellulose which is a mixture of lower molecular weight branched
polysaccharides. Table 1 illustrates the composition and mechanical properties of
flax and other bast fibers.
Table 1. Bast fibers: compositions, physical and mechanical properties.
1, 31-33
Factors affecting the properties of flax fibers
Flax is investigated at the elementary and technical fiber level. The great
variability reported for flax fiber properties (tensile strength and modulus of
elasticity, among others) is a consequence of many factors, including plant origin
and growth conditions, plant age, location in the stem and a non-uniform cross-
section of all the fibers. Due to this inherent variability, a Weibull distribution
function was used to describe the tensile strength of the flax fibers.
34-37
Charlet et al.
23
found that mechanical properties of flax fibers were influenced
by the location in the stem. Flax fibers located at the bottom of the stem display
the poorest mechanical properties, while the fibers located in the middle are the
ones that show the best mechanical performance. The biochemical analysis
confirmed that both cellulose and non-cellulosic polymers are to be found
prolifically in most extensive contents of the middle fibers. Cellulose is considered
as the equivalent reinforcing material of a composite structure, whereas non-
cellulosic materials are the matrix constituent that supports the exchange of load
from one microfibril to another. Bos et al.
26
reported that the technical fiber strength
decreases when the clamping length increases because of the similar composite-
like structure of this fiber. They performed tensile tests to determine the strength
of elementary and technical flax fibers and found that elementary flax fibers
showed a considerably higher strength than technical fibers of the same length
due to a bundling effect. During testing of the technical fiber bundle, it was found
that all elementary fibers are not firmly bonded with the matrix constituents, which
happens especially in the secondary cell wall region. As a result, less efficient
stress transfer was found in the tensile tests, producing reduced strength as
compared to elementary fibers. These results are quite consistent with Bensadoun
et al.
24
Fiber extraction methods also influence the mechanical properties of the flax
fibers. Bos et al.
26
revealed that the tensile strength of the fibers is dependent on
the isolation procedure, with manually isolated fibers being stronger than
mechanically isolated ones. The mechanical processes of fiber extraction were
found to induce kink bands in the fibers, thus reducing their tensile strength.
However, they noted that the scatter in strength is much larger for the elementary
fibers isolated by hand than for the standard mechanically isolated ones. They
claimed that the mechanical fiber processing methods generate a number of large
defects, which reduces the scatter in the fiber strength, although the fibers show a
lower mean strength. In a different study
38
of elementary flax fiber tensile tests, it
was found that fibers separated by enzyme treatment may receive less damage
than mechanical processes. Zeng et al.
39
introduced a new method of fiber
extraction from 35% aqueous ammonia pre-treated flax stems, comparing this with
a standard extraction process. They found both tensile and flexural properties of
