1

5th International Seminar on Modern Polymeric Materials for Environmental

Applications, Krakow

Effects of chemical treatments on physical properties of flax fibres

J. Zhu1, J. Njuguna

1*, J. Pacacz

2, H. Abhyankar

1, K. Pielichowski

2, H. Zhu,

K. Immonen3 and A Nurmi

3.

1Centre of Automotive Technology, Canfield University, Canfield, UK

2 Cracow University of Technology, Cracow, Poland

3VTT Technical Research Centre of Finland, P.O. Box 1000,

FI-02044 VTT, Finland

Keywords: flax, chemical treatment, thermal degradation

Abstract

The hydrophobicity of flax fibres often leads to the poor fibre/hydrophobic matrix

adhesion. The flax fibres are amenable to modification so as to solve the problem.

Alkali, acetylation, silane treatment and enzymatic treatment were employed for

the flax fibre mats in author’s study. To understand how these treatments affect the

physical properties, infrared spectroscopy (FTIR) and thermal gravimetric analysis

(TGA) were conducted on flax fibres. It was found that all the treatments result in

the removal of pectin component within the primary cell wall due to the

disappearance of the characteristic peak at 1735 cm-1

. Comparing with the

untreated fibres, the onset degradation temperature of treated ones decreased,

however, the peak of the maximum decomposition rate shifted to high

temperatures, indicating a better thermal stability.

1 Introduction

Flax/tannin composites could potentially offer environmental benefits and desirable

characteristics aiming at reducing the environmental footprint of superlight electric

vehicles applications such as vehicle body panels, crash elements, side panels and

body trims [1]. The hydrophilic property of flax leads to the poor interfacial

adhesion with the nonpolar hydrophobic tannin matrix and difficulties in mixing. In

order to solve the above problems, adequate surface modifications, mainly

chemical treatments, have being investigated for many years. The hydroxyl groups

in flax fibres could be modified for hydrogen bonding with cellulose groups or to

introduce new moieties that form effective interlocks within the system.

* corresponding author. j.njuguna@cranfield.ac.uk, Tel. +44 1234 754186,

Fax: +44 1234 752473 (J. Njuguna).

2

Mercerization, acetylation, silane treatment, and other fibre pretreatments are

commonly used for flax composites as reported in the literature [2]. Alkali

treatment, also called mercerization, could possibly change chemical composition

of flax fibres due to the fact that cementing substances like lignin and

hemicellulose are removed during the mercerization process [3]. Alkali treatment

also produces fine cellulose fibres by transferring crystalline form cellulose I into

cellulose II [4]. Acetylation is a well-known esterification method originally

applied to wood cellulose to stabilize the cell walls against moisture, improving

dimensional stability and environmental degradation. The acetic anhydride reacts

with more reactive hydroxyl groups (OH) in lignin and hemicellulose, whereas the

hydroxyl groups of cellulose prevent the diffusion of reagent and result in low

extent of reaction [5]. Proper treatment of fibres with silane can increase the

interfacial adhesion to the target polymer matrices. Silane is hydrolyzed forming

reactive silanols and is then adsorbed and condensed on the fibre surface (sol-gel

process). The hydrogen bonds may be further converted into covalent bonds by

heating the treated fibres at a high temperature (see in Figure 1).

Figure 1. Grafting of silanols on flax fibre surface (redraw from Singha er al.[6]).

The fibre pretreatments are able to change not only the fibre/matrix interface but

also the thermal stability of flax fibres, greatly influencing the composite

manufacturing temperature and the associated thermal properties. Bledzki et al.[7]

reported that the degradation temperature of flax fibres increased from 319°C to

360°C after acetylation (34% acetylation). The existent of strong hydrogen bonding

between PLA matrix and flax fibres also leaded to the increase in the

decomposition temperature of composites [8]. In this paper, alkali, acetylation,

silane treatment and enzymatic treatment were to modify non-woven flax mats for

3

preparing composite laminates. The chemical change of flax fibres from

pretreatments were indicated from Fourier transformation infrared spectroscopy

(FTIR) spectrums. The effects of fibre pretreatments on thermal properties of flax

fibres were investigated through thermal gravimetric analysis (TGA).

2 Material and methods

2.1 Materials

Non-woven flax fibre mats with different surface treatments.

Table 1. Investigated flax composites and flax mats.

Sample

type

Treatment

method

Detail

T None Yes

1 5% NaOH Mercerisation

2 NaOH+BTCA Acetylation

3 NaOH+APS Silane treatment

* BTCA- Butanetetracarboxylic acid, APS- Amiopropyltriethoxysilane, Laccase-Benzenediol, Doga-dodecyl gallate.

2.2 Characterisation

FTIR. FTIR spectroscopy was carried out with a shimadzu FTIR 8400S to

characterize all the sample to identify the chemical change of flax fibres. The scan

number is 64 per minute with the band range from 600 to 4000 cm-1

.

TGA. The thermal analysis of untreated and treated flax fibres, and tannin resins

were carried out by a TGA STA 449 instrument supplied by NEZSCH at Cracow

University of Poland. Around 20mmg of each specimen was heat from room

temperature to 600°C at the heating rate of 10°C/min. Air condition was applied

throughout the experiments.

3 Results and discussion

3.1 Effects of surface treatments on fibre chemical composition

To confirm and exam the efficiency of the selected treatment, the investigation of

the unmodified fibres is of importance for further comparison. Garside and Wyeth

[9] has introduced the use of Fourie transform infrared (FTIR) with attenuated total

reflectance (ATR) spectroscopy technique to identify and distinguish cellulosic

fibres (e.g. flax, hemp, jute etc.) as an alternative simply and highly reproducible

way to micro-spectroscopy. In terms of chemical compositions published in the

literature, the flax fibre mainly comprises polysaccharides including cellulose

(64.1%), hemicellulose (16.7%), pectin (1.8%) and lignin (2.0%), with minor

4

components such as bound water, waxes, and other inorganic materials. The fibrills

of cellulose in the crystalline form are deposited within the hemicellulose and

lignin acting as an amorphous phenolic polymer matrix. Based on the reference

data, the important characteristic band assignments for the investigated flax

fibres are summarised in

Table 1.

Table 1. Infrared band assignments for flax fibres.

Position/cm-

1

Assignment Belonged chemicals

3335 v (OH) free -

2850 v (CH2) symmetrical Organic compounds

1735 v (C=O) ester Pectin

1635 Adsorbed water All the chemicals

1595 v (C=C) aromatic Lignin

1505 v (C=C) aromatic Lignin

1155 v (C-C) ring breathing Largely from cellulose

1105 v (C-O-C) glycosidic Cellulose

Figure 2. FTIR spectrum of unmodified and treated flax fibres.

There is no big difference for the major chemical constituents between unmodified

and 5% NaOH mercerised flax fibres due to the similarity of the two spectrums as

shown in Figure 2. However, the peak around 1735 cm-1

standing for the existence

of pectin as mentioned before has disappeared in the FTIR spectra of flax fibres

modified with 5% NaOH. Apparently, the absence of this peak is attributed to the

removal of the pectin component within the primary cell wall. The peak at 1505

cm-1

was too weak to be observed by eye, indicating that the lignin was almost

0

700120017002200270032003700

Unmodified5% NaOH

Ab

sorb

ance

Wavenumber (cm-1)

5

removed after alkali treatment. As a result, the presence of absorbed water mainly

contributes to the peak shifting to 1582 cm-1

, a lower frequency compared to 1632

cm-1

for the IR spectra of unmodified flax fibres.

The most significant change in the FTIR spectra before and after NaOH-BTCA

treatment was observed in the range of 1500-1700cm-1

.The appearance of the peak

around 1730 cm-1

does not arise from the C=O bonds of pectin removed together

with other ingredients in the primary cell during the NaOH treatment but actually is

contributed from the ester bonds introduced by BTCA reacting with OH groups.

The peak (1635 cm-1

) representing the absorbed water in cellulosic crystalline

shifted towards the low-frequency side (1563 cm-1

) along with a new partially-

overlapped peak at 1540 cm-1

. The shift of the peak position is probably correlated

with the carboxylate carbonyl via NaOH treatment.

Efficiency of silane treatment, such as aminopropyl triethoxy siloxane (APS), is

reported higher higher for the alkaline treated fibre than for the untreated fibre

because more reactive site can be generated for silane reaction [6]. The combined

NaOH treatment in NaOH-APS type removed the waxy epidermal tissues, lignins,

adhesive pectins and hemicellulose. It can account for the disappearance of peaks

at 1735 cm-1

and 1505 cm-1

assigned to C=O in pectin and aromatic C=C in lignin,

respectively. In addition, the well-defined absorption band at around 690 cm-1

is

obtained due to the symmetric stretching of –Si-O-Si- structure on the APS treated

fibres derived. The increment of the peak intensity at 1000 cm-1

and 982 cm-1

is

possibly attributed to the presence of Si-O-C bonds derived from the silane

chemical treatment. .

3.2 Effects of surface treatments on fibre thermal degradation

TGA analysis was undertaken for untreated NaOH, NaOH-BTCA and NaOH-APS

treated flax fibres so as to investigate the effects of different treatments on thermal

degradation. For comparison purpose, the weight loss and decomposition rate as a

function of temperature are shown in Figure 3 (TG) and Figure 4 (DTG,

respectively. The quantitative information of TGA analysis results are summarised

in TG curves can to some extent reflect the difference and similarity of thermal

properties influenced by surface treatments. Origin, NaOH and NaOH-BTCA,

NaOH-APS flax fibres have the similar water content around 5.5% to 6%,

indicating no significant hydrophobic enhancement of flax fibres after

modifications due to the polar OH groups of cellulose and other polar introduced

ingredients. The onset of thermal degradation of these flax fibres was almost the

same, however the weight loss period after 350°C differed quite a lot. Obviously,

NaOH-BTCA exhibited the best thermal stability due to the slight weight loss

between 300-500°C. The residue content of 35% may be due to the improved

thermal stability or too much inorganic material in the selected fibres. The non-

6

cellulose decomposition period of the NaOH only treated flax fibres was shifted to

a higher temperature of 500°C than unmodified and NaOH-APS types (410°C).

Table 2.

This decomposition profile of untreated flax fibres shows strong agreement of the

typical TGA-air of flax fibres found in the publications [10]. According to this TG

curves, it is obviously that some weight loss of around 5.47% occurred up to 200°C

due to the evaporation of absorbed water of flax fibres upon heating. Then the

biggest weight loss of 65.94% occurred in the approximate temperature range of

250°C to 380°C, followed by the second weight loss of 24.99%, which terminates

at 420°C. After that, a weight loss of 3.28% took place till 600°C and left the

residual content of 0.32wt%. The main decomposition temperature was obtained at

305°C, which is similar to the temperature of 319°C observed in literature [11].

The degradation kinetics of flax fibres is complicated due to their inhomogeneous

constituents, and can be better visualised by DTG curves. The first DTG peak is

broad and below 100°C as a result of water removal. The second DTG peak is

caused by cellulose degradation while the third DTG peak of around the 417°C is

largely attributed to the degradation of non-cellulose components. One thing worth

noting is that the cellulose degradation still continued after the first peak until

500°C since cellulose suffered different degradation reactions depending on

temperatures. Generally, in the temperature from 300°C to 500°C, cellulose is

firstly converted into active form to yield phenol, followed by the ring scission at

approximately 340°C to form anhydrosugars like levoglucosan with further

hydration, and the formation of polycyclic aromatic compounds (350°C), mainly

constituting the ash content in the 400°-500°C temperature range [12]. After the

third DTG peak, most of the decomposition products integrated into ash, and

further oxidised with temperature and left around 0.3% inorganic residue after

600°C.

Figure 3. TGA curves of investigated flax fibres.

0

50

100

0 100 200 300 400 500 600

UntreatedNaOH

7

Figure 4. DTG curves of investigated flax fibres.

TG curves can to some extent reflect the difference and similarity of thermal

properties influenced by surface treatments. Origin, NaOH and NaOH-BTCA,

NaOH-APS flax fibres have the similar water content around 5.5% to 6%,

indicating no significant hydrophobic enhancement of flax fibres after

modifications due to the polar OH groups of cellulose and other polar introduced

ingredients. The onset of thermal degradation of these flax fibres was almost the

same, however the weight loss period after 350°C differed quite a lot. Obviously,

NaOH-BTCA exhibited the best thermal stability due to the slight weight loss

between 300-500°C. The residue content of 35% may be due to the improved

thermal stability or too much inorganic material in the selected fibres. The non-

cellulose decomposition period of the NaOH only treated flax fibres was shifted to

a higher temperature of 500°C than unmodified and NaOH-APS types (410°C).

Table 2. Summary of TGA results for treated and untreated flax fibres.

Flax

type

Moisture

(wt%)

Main decomposition

temperature (°C)

DTG peaks (°C)

1st 2nd 3rd

Untreated 5.47 305.5 50.2 336.5 417.4

NaOH 5.88 250.3 59.6 293.0 528.1

NaOH-BTCA 5.83 258.5 78.2 301.5 451.3

NaOH-APS 6.49 261.8 48.3 299.9 421.4

Compared with TG curves, the effects on thermal stability were more clearly

obtained from DTG curves. The first DTG peak is associated with water

evaporation on heating. The higher peak temperature of NaOH-BTCA sample

around 80°C possibly indicates that BTCA treatment can lead to stronger hydrogen

bonding to water. The second peak related to cellulose degradation decreased from

336°C to the temperature range of 290-300°C for all the treated samples. This may

0

10

20

30

0 100 200 300 400 500 600

Untreated

NaOH

NaOH-BTCA

NaOH-APS

8

correlate to the increase of amorphous cellulose which the degradation begined

with. Another noticeable change is the decrease of the maximum decomposition

rate at 1st peak. This is attributed to that pectin, and part of lignin and

hemicellulose on the flax fibre surface was removed, leading to an improvement of

thermal stability. Many researchers [12-14] have reported the positive influence of

alkali treatment on thermal degradation of natural fibres. NaOH-BTCA flax sample

shows a much smaller 2nd peak, indicating that BTCA significantly improves the

thermal stability of non-cellulose components attributable to 2nd peak. In addition,

the maximum decomposition temperature of the 2nd peak of pure NaOH treated

flax fibres was around 100°C bigger than that of untreated sample. The pure

mercerisation shows better effect than NaOH-BTCA and NaOH-APS to enhance

the thermal stability of non-cellulose constituents in flax fibres.

4 Conclusions

The effect of fibre treatments on chemical change and the associated thermal

properties of flax fibres were investigated through adequate characterizations and

analysis. It was proved that the pectin and lignin was removed during the NaOH

treatment due to the disappearance of their characteristic IR band at 1735 cm-1

and

1505 cm-1

, respectively. For other treated samples, the FTIR results also indicate

the presence of cross-linked BTCA (ester bond) and the introduced APS (Si-O-Si).

Due to the chemical composition change, the thermal properties of flax fibres

differed after treatments. All the treated samples displayed a much lower

decomposition rate at the initial degradation step. Additionally, NaOH-BTCA has

been found to offer the best improvement of thermal stability of the non-cellulose

part of flax fibres.

Acknowledgement

The authors are thankful the ECHOSELL (project no. 265838) and Cracow

University of Technology (project No. UMO-2011/01/M/ST8/06834) for the

financial and technical support for this research.

References

[1] J Zhu H Abhyankar E Nassiopoulos, J.Njuguna (2012), IOP Conference

Series: Materials Science and Engineering, vol. 40, no. 1, pp. 012030.

[2] Van de Weyenberg, I., Ivens, J., De Coster, A., Kino, B., Baetens, E. and

Verpoest, I. (2003), Composites Science and Technology, vol. 63, no. 9, pp.

1241-1246.

[3] Bledzki, A. K., Fink, H. -. and Specht, K. (2004), Journal of Applied

Polymer Science, vol. 93, no. 5, pp. 2150-2156.

9

[4] Van de Weyenberg, I., Chi Truong, T., Vangrimde, B. and Verpoest, I.

(2006), Composites Part A: Applied Science and Manufacturing, vol. 37, no.

9, pp. 1368-1376.

[5] John, M. J. and Anandjiwala, R. D. (2008), Polymer Composites, vol. 29, no.

2, pp. 187-207.

[6] Singha, A. S. and Rana, A. K. (2012), " Journal of Polymers and the

Environment, , pp. 1-10.

[7] Bledzki, A. K., Mamun, A. A., Lucka-Gabor, M. and Gutowski, V. S. (2008),

Express Polymer Letters, vol. 2, no. 6, pp. 413-422.

[8] Qua, E. H., Hornsby, P. R., Sharma, H. S. S., Lyons, G. and Mccall, R. D.

(2009), Journal of Applied Polymer Science, vol. 113, no. 4, pp. 2238-2247.

[9] Garside, P. and Wyeth, P. (2004), Studies in Conservation, vol. 48, no. 4, pp.

269-275.

[10] Gassan, J. and Bledzki, A. K. (2001), Journal of Applied Polymer Science,

vol. 82, no. 6, pp. 1417-1422.

[11] Hänninen, T., Thygesen, A., Mehmood, S., Madsen, B. and Hughes, M.

(2012), Industrial Crops and Products, vol. 39, no. 3, pp. 7-11.

[12] Van De Velde, K. and Kiekens, P. (2002), Journal of Applied Polymer

Science, vol. 83, no. 12, pp. 2634-2643.

[13] Zhou, L. M., Yeung, K. W., Yuen, C. W. M. and Zhou, X. (2005), Journal

of the Textile Institute, vol. 96, no. 4, pp. 213-220.

[14] Xie, Y., Hill, C. A. S., Xiao, Z., Militz, H. and Mai, C. (2010), "

Composites Part A: Applied Science and Manufacturing, vol. 41, no. 7, pp.

806-819.