J. Eng. Technol. Sci., Vol. 52, No. 5, 2020, 677-690

677

Investigation of Alkaline Surface Treatment Effected on
Flax Fibre Woven Fabric with Biodegradable Polymer
Based on Mechanical Properties
A’liya Abdul Laziz1, Norkhairunnisa Mazlan1,2,3,*, Mohd Zuhri Mohd Yusoff4 &
Azmah Hanim Mohamed Ariff4
1

Institute of Advanced Technology (ITMA), Universiti Putra Malaysia, 43400 Serdang,
Malaysia
2
Department of Aerospace Engineering, Faculty of Engineering, Universiti Putra
Malaysia, 43400 Serdang, Malaysia
3
Laboratory of Biocomposite Technology, Institute of Tropical Forestry and Forest
Products (INTROP) Universiti Putra Malaysia, 43400 Serdang, Selangor
4
Mechanical and Manufacturing Engineering Department, Universiti Putra Malaysia,
43400 Serdang, Malaysia
*E-mail: norkhairunnisa@upm.edu.my

Highlights:




Composites were fabricated using thin film casting and hot-pressing.
The effects of alkaline treatment on the flexural and impact properties of flax
fibre reinforced polylactic acid (PLA) thin film were investigated.
Optical images of the treated flax fibres also showed improvement.

Abstract. Biodegradable polymers such as polylactic acid (PLA) are used
nowadays due to their degradability, durability and environmentally friendly
properties. Alkaline surface treatment of natural fibres is used to increase the
flexural properties of composites. This research investigated the flexural
properties of dry compressed PLA, wet mix PLA, untreated flax/PLA and treated
flax/PLA composites. The flax fibre was first treated with NaOH. The wet mix
PLA was prepared via solvent casting with chloroform and dried at room
temperature overnight followed by post-drying in an oven. The flax/PLA
composites were fabricated using a hot press for 6 minutes. The wet mix PLA
showed higher flexural strength compared to the dry compressed PLA. The treated
flax fibre composite showed higher flexural strength compared to the untreated
flax fibre. The flexural strength and elongation at break of the treated flax fibre
composite was increased by 4.79% and 27.76%, respectively, while the flexural
modulus decreased by 0.79% compared with the untreated flax composite. The
treated flax composite also showed an improvement in impact properties,
increasing its impact strength by about 3% and 10% at energy levels of 10 J, 15 J,
and 17.5 J compared with the untreated flax fibre. Therefore, the investigation of
the surface treatment of flax in a PLA matrix based on its mechanical properties
revealed better properties compared to untreated flax/PLA composite.
Received July 22nd, 2020, Revised August 4th, 2020, Accepted for publication August 13th, 2020.
Copyright ©2020 Published by ITB Institute for Research and Community Services, ISSN: 2337-5779,
DOI: 10.5614/j.eng.technol.sci.2020.52.5.6

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A’liya Abdul Laziz, et al.

Keywords: biodegradable polymer; fibre surface treatment; natural fibre; thin film;
flexure; low velocity impact

1

Introduction

Several applications that use composites reinforced by natural fibres have
attracted the attention of the composites community in the current year. Due to
synthetic fibres requiring high expenses, natural fibres have been studied as
potential reinforcement in composites applications [1,2]. The properties of
natural fibre composites have been considered for a wide range of natural fibres,
including flax, jute, hemp, banana, bamboo, pineapple, agave and rubber wood
in several different studies. In order to achieve better reinforcement of polymer
matrix composites it is important to relate their characteristics to the properties
of the natural fibre, such as low density, low cost and biodegrability [3,4].
There are many different types of natural fibres with good mechanical properties,
one of which is flax fibre. Murali [5] found that the specific tensile strength of
flax fibre is comparable to that of glass fibre. Many forms and configurations of
flax fibre are used in applications such as mats, roving, fabrics and
monofilaments. There is a variety of manufacturing techniques to produce these
forms of fibres. Using flax fibres normally has relatively low cost compared to
using glass fibres. There are negative effects of using glass fibres, as reported by
Bos [6], where these are suspected of causing lung cancer, whereas there is no
such problem with natural fibres. The process of burning flax fibres with few slag
left gives the thermal recycling of flax fibres a great advantage over glass fibres.
Thus, it makes flax a suitable fibre to replace conventional glass fibre as
reinforcement of composite materials [7-10].
A natural fibre like flax has unique properties that are fundamentally different
from glass and other synthetic fibres. As a known natural fibre, flax is majorly
influenced by its hydrophilicity. This characteristic leads to poor matrix adhesion
and high water absorption due to the presence of free hydroxyl and polar groups
in the fibre itself [11,12]. To improve the interface properties and adhesion to the
polymer matrix, a number of different chemical treatments can be used, such as
alkaline treatment, silane treatment, benzoylation, acetylation and enzymatic
treatment [13-16]. Waxy substances are chemical substances in the fibre. The
ratio of cellulose will be increased by the removal of these substances by chemical
treatment of the fibre. The mechanical properties of the fibre are improved while
the water absorption is decreasesd by using these treatments [11,13]. Huner found
that sodium hydroxide (NaOH) treatment improved the mechanical properties of
an epoxy matrix composite [17]. Van de Weyenberg found that NaOH has high
swelling compared to potassium hydroxide (KOH) or lithium hydroxide (LiOH)
[18]. Alkali treatment can give an increase of up to 30% in the longitudinal

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679

properties due to the removal of pectins [19]. Thomas concluded that tensile
testing of the fibres showed improvement in strength by increasing the soaking
time duration up to 2 hours.
Polylactic acid (PLA) is derived from 100% renewable resources of a naturally
occurring organic acid, i.e. lactic acid. PLA is well known as a biodegradable,
highly versatile aliphatic polyester [18]. PLA melts at low temperature, in the
range of 180 to 220 °C, with a glass transition temperature of 60 to 65°C.
Moreover, the formation of non-harmful and non-toxic compounds from PLA
and its composites results from simple hydrolysis of the ester backbones, which
can degrade easily, making PLA degradable in nature. Recycling of PLA to its
monomer can be done by using hydrolysis or thermal depolymerisation. PLA also
provides easy access to a wide range of materials that can easily be processed by
injection moulding, casting, extrusion and spinning. Thus, PLA is an ideal
polymeric material for different practical applications all over the world based on
its biocompatibility, complete biodegradability and the non-toxic nature of its
degradation products [20].
In this study, two types of PLA composite were investigated, including two types
of flax fibre: treated with alkaline and untreated. The two types of PLA used were
pellets and thin film layers. Both types of PLA were tested in neat form without
any reinforcement for flexural analysis. Laminates of treated and untreated flax
fibre were fabricated with the PLA thin film composite. The prepared composites
of both types of flax fibre were subjected to low-velocity impact testing with a
drop weight machine. The effects of alkaline treatment on the flax fibre were then
observed with an optical microscope.

2

Materials and Experiments

2.1

Materials

In this study, woven flax fibre was used as received from Mecha Solve
Engineering. The polylactide acid thermoplastic was purchased from Innovative
Pultrusion Sdn. Bhd. NaoH was used to treat the flax fibre and chloroform was
used as solvent. Both chemicals were purchased from R&M Chemicals.

2.2

Composites Production

2.2.1

Fibre Surface Treatment

Woven flax fibre was used as reinforcement, while PLA was used as the matrix
in the composite. The woven flax fibre material was cut into dimensions of 260
x 260 mm per piece as mould size. The fibre then underwent surface treatment

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A’liya Abdul Laziz, et al.

by using alkaline treatment via an NaOH solution. The fibre was treated by
soaking in a 2% NaOH solution for 2 hours.
After treatment, the fibres were washed with distilled water until the pH became
natural. Then, the fibres were dried at room temperature for 48 hours. The dried
fibres underwent further drying in an oven at temperature 80 °C for 24 hours to
remove excess water.

Figure 1 Example of the composite lay-up.

2.2.2

Fabrication of Composite

The wet mix PLA was cast by mixing PLA in pellets with chloroform at a ratio
of 1:5. The solution was then stirred using a mechanical stirrer for 30 minutes
with a speed of 500-700 rpm. For the first 15 minutes, heat at 60 ℃ was applied
to the solution. After 30 minutes of stirring, the solution was poured into a steel
mould and dried in an oven at 55 ℃ for 2 hours and 30 minutes. On the other
hand, the dry compressed PLA without chloroform was prepared by hot pressing
of the PLA in pellet form.
Laminate fabrication of flax with the wet mix PLA composite was done using the
hot press method. The flax fibre, wet mix PLA and dry compressed PLA were
pressed by using a manual hot press machine at 20 tonnes pressure and a
temperature of 190 ℃. The laminate specimens consisted of six layers of wet mix
PLA and five layers of flax in the middle, as shown in Figure 1. After 5 minutes
of pre-heating, hot-pressing was done for 1 minute with 5 times bumping. Mild
steel plates with dimensions of 260 x 260 x 32 mm were used in the hot-pressing
process as the mould. The dry press PLA, wet mix PLA, untreated flax/PLA and
treated flax/PLA composite are herein represented by ‘P’, ‘T’, ‘UF’ and ‘TF’.

2.3

Characterizations

2.3.1

Flexural Test

The flexural properties of the composites were determined using three-point
flexural bending test according to the ASTM D-790 standard using a Universal

Investigation of Alkaline Surface Treatment Effected..

681

Testing Machine (Instron, Model 3365). Ten specimens were tested for each
sample at room temperature with a crosshead speed of 2.0 mm/min and a gauge
length (support span) of 60 mm. The specimens had a rectangular shape of 125 x
12.7 x 3.2 mm per piece.

2.3.2

Low Velocity Impact Test

The impact strength of the composites was determined by using a low-velocity
impact machine (Imatek Ltd, type 8000D, model D5000, Knebworth, UK), also
known as a drop test machine. The drop test was performed according to the
ASTM D7136 standard, with dimensions of 150 x 100 x 3.2 mm per piece. Three
specimens were tested for each sample of untreated flax and treated flax for three
different energy loadings, i.e. 10 J, 15 J, and 17 J. The impact energy of the
specimens was tested and calculated according to Eq. (1):
F=mgh

(1)

where F is the force (kN), m is mass (g), g is gravity (m/s), and h is the height (m)
of the striker before release to impact the specimen, depending on the value of
the impact energy as stated in Table 1.
Table 1

Value of impact energy and striker height.

Impact Energy (J)
10
15
17.5

2.3.3

Height of Striker (m)
0.2
0.3
0.35

Morphology Analysis

Changes in the surface morphology of the treated and untreated flax fibre were
observed under an optical microscope using an Olympus Optical Microscope
(OM). The image surface was captured at 50x magnification.

3

Results and Discussion

3.1

Observations on PLA Thin Film Casting Method

Initially, different drying temperatures were investigated to identify PLA thin
film that produced low trapped bubbles. The thin film of wet mix PLA in Figure
2(a) was dried in an oven at a temperature of 80 °C. The surface produced many
bubbles after drying. The thin film in Figure 2(b) was dried in an oven at a
temperature of 55 °C, producing a smooth surface and easier handling. As
reported by Mohan, bubbles form during the first thirty minutes of drying due to
the large amount of chloroform trapped in the sample when the solidified casting

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A’liya Abdul Laziz, et al.

is done too fast [21]. Therefore, a low drying temperature of 55 °C was used to
slow down the solidification process and mitigate bubble formation.

(a)

(b)

Figure 2 PLA thin film after drying at different temperatures: (a) 80 °C, (b)
55 °C.

3.2

Flexural Properties of the Composites

A flexural mechanical test was performed up to initial cracking until the failure
of the matrix of the polymer composite to analyse the elastic behavior of the fibre
and the matrix. Table 2 shows selected samples of the dry press PLA, wet mix
PLA, untreated flax/PLA and treated flax/PLA composites after flexural testing.
It can be clearly seen that the dry compressed PLA samples were broken, while
the wet mix PLA was only bent at certain angles. The untreated flax/PLA
composite was clearly delaminated before failure, which indicates early failure
due to the limited bending angle when load is applied to the composite. However,
the treated/PLA composite bent only slightly. Figure 3 shows the flexural strength
of the dry compressed PLA, wet mix PLA, untreated flax and treated flax under
loading. A great difference was recorded for the flexural properties, where the
wet mix PLA and TF had much greater values than the dry press PLA and UF.
From the results, the flexural strength of the wet mix PLA increased by 9%
compared to the dry press PLA in pellet form. The NaOH surface treatment of
the flax fibre improved the flexural strength, i.e. an increase of 11 MPa or 5%
compared to the untreated flax composite.
The elongation at break of the treated flax is shown in Figure 5, which shows the
flexural extension properties of all composites. The tendency to delaminate of
composites that contain flax fibres may strongly depend on the stacking sequence
selected, as observed in [22]. Therefore, the configuration presented is not
specific, while the results are not definite.

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683

Table 2 PLA Composites after flexural testing of dry press PLA, wet mix PLA,
untreated flax/PLA and treated flax/PLA composites.
Sample

PLA in pellet form

PLA in thin film form

Untreated flax fibre

Treated flax fibre

Polylactic
acid
(PLA)

Sample

Flax/PLA
composit
e

Flexural Strength (MPa)

140
120
100
80
60
40
20
0
P, T

UF, TF
Samples
P, UF

T, TF

Figure 3 Flexural strength of different PLA process and flax/PLA composites.

The flexural modulus showed a similar trend as shown in Figure 4. The flexural
modulus of the wet mix PLA and the treated flax/PLA composite were reduced

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A’liya Abdul Laziz, et al.

slightly, by 0.32 and 0.09 GPa, i.e. with 4.18% and 0.79%, respectively. The
decrease in flexural modulus may be related to the high strain ratio of the wet mix
PLA and the alkaline surface treatment, which affects their flexural behavior.
Polymer materials generally have no regular structure in the polymer chains,
which are loose. Therefore, plastics have high ductility. For example, when force
is applied to them, they are able to stretch out due to the amorphous phase when
being composed. This can be compared to the PLA thin film results. Liu states
that lamellae crystals will form during polymer crystallization when the polymer
chains fold into layers and stacking. A rigid structure is created during this
process [23].

Flexural Modulus (GPa)

7
6
5
4
3
2
1
0
P, T

UF, TF
Samples
P, UF

T, TF

Figure 4 Flexural modulus of different PLA process and flax/PLA composites.

Figure 5 shows higher flexural strain displayed by the wet mix PLA, with an
increase by 10% and about 50% compared to the dry press PLA, while the treated
flax/PLA composite also showed an improvement by 2% and about 20%
compared to the untreated flax/PLA composite. The alkaline treatment improved
and enhanced the flexural strength and elongation at break as revealed by the
flexural mechanical testing results compared to the untreated flax/PLA
composite, as shown in Figures 3 and 5.
This enhancement of the flexural properties can be explained by better adhesion
of the fibres after the alkaline surface treatment, which removes waxy substances
from the fibres [11]. As reported by Yan, the alkaline treatment of flax composite
increased the flexural strength by 16.1%, respectively, while in this study the
flexural strength was increased by 5%. The removal of impurities from the fibre
surface enhances the flexural properties due to the increased amount of cellulose.

Elongation at break (%)

Investigation of Alkaline Surface Treatment Effected..

685

16
14
12
10
8
6
4
2
0
P, T

UF, TF

Samples
P, UF,

T, TF

Figure 5 Flexural modulus of different PLA process and flax/PLA composites.

Table 3 shows optical images of the untreated and treated flax fibres. The treated
flax fibres had a rough surface, while the untreated flax fibre surface was
smoother. This is due to the result of chemical treatment using NaOH, which
makes the fibre surface rougher and creates physical interlocking with the
polymer [24]. Shibata has reported that fibrillation improves the interfacial region
and that the contact surface area with the matrix is more effective. According to
Reddy, the inner fibre structure, stiffness and density decreased after the removal
of wax, lignin and hemicellulose from the fibres after NaOH surface treatment.
The rough surface of the treated flax fibres increases the adhesion between the
matrix and the fibres.
Table 3 Optical images of untreated flax fibre and treated flax fibre at 50x
magnification.
Magnification

Untreated flax fibre

Treated flax fibre

50X

Fewer gaps in the interfacial region lead to better adhesion between the matrix
and the fibres. Thus, the area of the fibres for the wetting process is also increased.
After treatment with NaOH, the fibre bundle become loose due partial removal

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A’liya Abdul Laziz, et al.

of some of the lignin during the treatment. In addition, more wetting areas are
exposed on the fibre surface to create a good contact with the matrix [25]. On top
of that, the fibre structure is disrupted, hence reducing the void content in the
fibres to reveal a compressed cellular structure after the alkaline surface treatment
[26].
Figure 6 shows the impact properties based on the strength of the untreated/PLA
and the treated flax/PLA composites at impact energy levels of 10 J, 15 J, and
17.5 J. As shown in Figure 6, there is a similar trend of increased impact strength
of the UF and TF composites for all energy levels.
At an energy level of 10 J and 15 J, the TF impact strength increased with 0.63 J
and 0.86 J, or 3.2% and 3% for both the UF and TF composites, while at 17.5 J
the increase was about 3.22 J, or 11% compared to the untreated flax.

Impact Strength (J)

18
16
14
12
10
8
6
4
2
0
10J

15J
Sample
UF TF

17.5J

Figure 6 Impact properties of untreated and treated flax fibre based on strength
with different energy loadings.

Table 4 provides more information about the impact test at three different energy
levels (10 J, 15 J, and 17.5 J) for the untreated flax/PLA and the treated flax/PLA
composites. The maximum force of UF and TF is slightly different, where TF had
higher force compared to UF for the composite impacted at energy 15 J and 17.5
J. However, at an impact energy of 10 J the force of TF was lower than that of
UF. As for the impact energy, UF impacted from a height of 0.3 and 0.35 m
resulted in penetration of the flax fibre laminates, which did not occur with the
TF laminates. This shows that the alkaline treatment improved the impact
properties of the flax fibre.

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687

Table 4 Average maximum load (kN) and non-elastic energy during impact at
different energy levels.
Energy (J)

Laminate
UF
TF
UF
TF
UF
TF

10
15
17.5

Max load
(kN)
1.105
0.965
0.943
1.5
0.94
1.26

Absorbed energy
(J)
9.465
10.095
13.68667
14.55
13.25
16.47

Max displacement
(mm)
13.275
14.075
28.42667
17.29
36.47
17.85

Table 5 Image of untreated and treated flax fibre after low velocity impact at
different energy levels.
Energy (J)

View

Front
10

Back

Front
15
Back

Front
17.5
Back

Untreated Flax fibre

Treated Flax fibre

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A’liya Abdul Laziz, et al.

Table 5 shows a front and back view of the untreated and the treated flax fibres
after low-velocity impact at energy levels of 10 J, 15 J, and 17.5 J. Different
lamination occurred due to the effect of the impact, producing damage in the
composite. Extensive tearing from the impact energy was noticed in all the
untreated flax composites. A comparison of all impact energy levels shows that
the tearing of UF was more extensive compared to that of TF.
The extension of crack tearing increased with increasing the impact energy. For
an impact energy level of 10 J the composites did not tend to be fractured away
from the TF composite, while at 15 J UF the composites were broken. Increasing
the impact energy to 17.5 J, the UF crack tearing extended until the end of the
composite and the sample was damaged. The entire sample showed pine-tree
cracking damage, where the crack propagation became larger at the back of the
impacted area compared to the impacted area.

4

Conclusion

In this study, the effects on the flexural and impact properties of dry press PLA,
wet mix PLA and flax/PLA composites were investigated and evaluated. The
results showed that the wet mix PLA and treated flax/PLA composites had higher
flexural strength and elongation at break compared to the dry press PLA and
untreated flax/PLA composites. Optical images of the treated flax fibres showed
an improvement of the surface condition compared to the untreated fibres. The
impact strength for the treated flax/PLA composite was improved, where the
strength of the laminates increased, especially during impact at energy levels of
15 J and 17.5 J. Therefore, combining wet mix PLA and treated flax woven fibres
produces better mechanical properties.

Acknowledgments
The authors would like to thank the Universiti Putra Malaysia (UPM)
acknowledge the financial support provided by the Centre of High Excellence
(HiCoE), Institute of Tropical Forestry and Forest Products (INTROP) (6369108)
to fund this research study.

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