Effect of preswelling and ultrasound treatment on the properties of flax fibers cross-linked with polycarboxylic acids

Abstract

This paper presents the investigation of sorption and mechanical properties of single flax fibers (fiber bundle) cross-linked with polycarboxylic acids (PCA; citric acid [CA] and 1, 2, 3, 4-butanetetracarboxylic acid [BTCA]). The improvement of the chemical modification performance was carried out by pre-treating of the fibers with sodium hydroxide and ultrasound (US). Thus, the purpose of the investigations was to determine whether the sorption and mechanical properties of the cross-linked fibers would be enhanced by a preswelling and US treatment before the application of PCA. The effect of cross-linking PCA with cellulose molecules on flax fiber properties was determined by measuring and comparing the properties (fineness, tenacity, and elongation at break; breaking twist; moisture absorption; and water-retention value) of untreated and modified flax fibers. The test results indicate that treatment with PCA undoubtedly changes the tested properties: water retention values decrease while the resistance to tenacity increases. The observed changes are the result of structural alterations in fibers. Fourier-transform infrared (FT-IR) spectra confirmed that cross-linking between the carboxyl groups of acids and the hydroxyl groups of cellulose occurs due to the esterification reaction. Preswelling and US treatments have increased the accessibility of cellulose in flax fibers, and cross-linking was achieved with less fiber damage, whereby US proved to be a more successful pre-treatment.

Keywords

flax fibers citric acid 1 2 3 4-butanetetracarboxylic acid sodium hydroxide ultrasound fiber properties

In parallel with the development of techniques and interventions aimed at improving the properties of man-made fibers, the implementation possibilities of modifications of natural fibers are explored. Thus, some disadvantages of flax fibers can be reduced by a variety of physical and chemical interventions (cottonization, ultrasound [US] treatment, plasma and sol-gel treatment, treatment with polycarboxylic acids [PCA] and alkalis, acetylation, cyanoethylation, and so on). The advantage of these modifications is that fibers, particularly natural ones, maintain good physical and mechanical properties, while chemical modification improves certain other properties.^1–6 One example of chemical modification is cross-linking cellulose with PCA. The action of PCA on the cellulose leads to chemical modifications of fiber structure, whereby natural fibers so modified become useful in new textiles, in which they can compete with synthetic materials. Due to the change in the chemical structure and cross-linking of polymeric molecules in the fiber and the conditions under which the modification process is performed, bending resistance and dimensional stability of the material are increased while water retention values are decreased. In contrast to positive changes in properties, a substantial reduction of mechanical fiber properties is observed.^7–9

The primary objective of this investigation was to examine sorption and mechanical properties of single flax fibers (fiber bundle) cross-linked with PCA, such as citric acid (CA) and 1, 2, 3, 4-butanetetracarboxylic acid (BTCA) with catalyst sodium phosphate dibase (NaH₂PO₂ × 12H₂O). The presented investigation focuses on single flax fibers that excludes the impact of changes in structural parameters of complex textiles to the test properties established after the modification of PCA. A secondary objective was to determine whether the sorption and mechanical properties of the cross-linked fibers would be enhanced by a preswelling and US treatment before PCA application.

When esterification occurs between a PCA and flax cellulose, the carbonyls in the fibers exist in three forms: intermolecular ester linkage, carboxylate, and carboxyl. For PCA containing three or more carboxyl groups, all three forms of carbonyls could be present in one acid molecule. An increased intensity indicates an increase in quantity of ester groups formed in treated samples. An increase in the carbonyl band intensity ratio (ester/carboxylate) indicates an increase in the average number of ester groups formed per each acid molecule.^10–13 The effectiveness of cross-linking a PCA with flax cellulose was studied with Fourier-transform infrared (FT-IR) spectroscopy.^14,15

It is known that fiber tenacity is reduced by cross-linking. A group of authors^7,16 investigated the difference in the proportion of accessible carboxyl groups between the unmercerized cross-linked and mercerized cross-linked cotton cellulose. Their investigation results showed that a change in the fine structure of cotton cellulose due to the mercerization increases the efficiency of cross-linking with BTCA. These results were an impulse to pre-treat flax fibers with sodium hydroxide before the treatment with PCA. Exposure of cellulose fibers to US action may also cause morphological and structural changes in fibers. Ultrasound action reflects the change in sorption properties of fibers with a simultaneous increase in the resistance to the action of tensile forces.^17–22

Pre-treatments are expected to enhance the accessibility of cellulose molecules and to improve their cross-linking effect with PCA.

Experimental details

Pre-treatments and cross-linking

The investigation was performed on flax fiber samples obtained from fiber flax Viola (Van de Bilt Zaden, Netherlands). The plants were grown at the experimental grounds of the Faculty of Agriculture, University of Zagreb, and were processed into fibers in laboratory conditions.

Flax stems were subjected to biological maceration for 72 hours in hard hot water (32°C) under controlled conditions.²³ After retting and drying, fiber bundles were still partly bound to a wooden core from which they were separated by breaking and scutching. The next step was heckling, whereby long fibers were separated from short fibers, i.e. from tow.

Investigations were carried out on technical flax fibers. Before the modifications using PCA, the samples were exposed to sodium hydroxide and US treatment. Treatment with 5% sodium hydroxide was performed for 10 minutes at room temperature. During the alkalization, the fibers were free. The US treatment was performed in the US unit SONIS GT (US frequency 30 kHz, 400 W) for 30 minutes at 30°C in distilled water.

Afterward, the samples were impregnated with PCA using the Benz two-roll laboratory padder with a squeezing effect of 100%. After impregnation, the samples were dried at 110°C for 4 minutes and thermally treated for the period of 90 seconds at a temperature of 180°C. Intermediate drying of samples between pre-treatment and cross-linking was performed at 105°C for 24 hours.

Treatment and designations of the untreated and modified samples and composition of the impregnation bath for cross-linking flax fibers are given in Table 1.^1,24,25

Table 1.

Designation of samples and composition of the impregnation bath

Designation of samples	Types of treatment	Composition of the impregnation bath-concentration (g/l); ^1,24,25
Designation of samples	Types of treatment	CA	BTCA	catalyst
Untreated fibers	/	/
NaOH	Sodium hydroxide
US	Ultrasound
CA	Citric acid	70	/	65
NaOH + CA	Sodium hydroxide + Citric acid	70	/	65
US + CA	Ultrasound + Citric acid	70	/	65
BTCA	1, 2, 3, 4-butanetetracarboxylic acid	/	51	51
NaOH + BTCA	Sodium hydroxide + 1, 2, 3, 4-butanetetracarboxylic acid	/	51	51
US + BTCA	Ultrasound + 1, 2, 3, 4-butanetetracarboxylic acid	/	51	51

Determining the treatment efficiency

The effect of cross-linking PCA with cellulose molecules on the properties of flax fibers was determined by examining and comparing the properties of untreated and modified fibers, including fineness, tenacity and elongation at break, breaking twist, ability of moisture absorption, and water retention value (see Table 2).

Table 2.

Performed measurements and methods

Parameters	Methods	Adjustment of methods
Fineness	HRN EN ISO 1973:2008	Cogged steel clamp
Fineness	Vibroscop 400	Gauge length: 5 mm
Tenacity and elongation at break	HRN EN ISO 5079:2003	Testing speed: 3 mm/min
	Vibrodyn 400	Pre-tension: 1500 mg
		Number of measurements: 200
Breaking twist	Twister tester, tt. Branca	Gauge length: 5 mm
		Pre-tension: 2000 mg
		Number of measurements: 200
Moisture absorption capacity	ASTM D 2654—89 a:1998	Number of measurements: 3
Water retention value	ASTM D2 402-90 Centrifuge tt. Niko Železniki tip 72X	$WRV = \frac{m_{2} - m_{1}}{m_{1}} \cdot 100 (%)$ where: m², mass after centrifuging, m¹, mass before centrifuging Number of measurements: 3

Used standards and regulations are adapted to testing flax fibers. Due to the non-homogeneity of flax fibers, the number of measurements was increased and determined according to statistical indications of the degree of reliability with 95% confidence interval. All measurements of tested properties were performed on conditioned samples (48 h, 20°C, and 65% H^R).

FT-IR-ATR analysis

The treated samples were analyzed by FT-IR spectrometer (Perkin Elmer software Spectrum 100) using an ATR (attenuated total reflectance) detector. The spectra were recorded over the range of 4000–1000 cm⁻¹, with a resolution of 4 cm⁻¹ and per 4 scans. The spectra were normalized to the absorption band at 1730 cm⁻¹. The FT-IR analysis was used for detection of cellulose cross-linking through esterification reactions. In addition, to measure the intensity of the absorption band attributed at ester of carbonyl groups, samples were treated with a 0.1 M NaOH solution for 2 minutes at room temperature to separate the free carboxyl acid on the fibers to carboxylate anion or ester carbonyl band.

Results and discussion

In Figures 1 –6, the test results of the effect of pre-treatment on the properties of flax fibers cross-linked with PCA are presented. Along with the test results of individual properties, the coefficient of variation (CV) is also presented. Because of unevenness in the diameter of the flax fibers, a relatively high CV was established.

Figure 1.

Fineness of untreated and modified flax fibers.

Figure 6.

Water retention values of untreated and modified flax fibers.

Mechanical properties

Figure 1 shows the test results of fineness of the untreated and modified sample.

The test results indicate a small change in fiber fineness in relation to the untreated fibers. After fiber treatment with sodium hydroxide and US and after cross-linking with CA (samples NaOH, US and CA), fibers become slightly finer. In other treatments, the fineness of fibers increased as much as 4% to 12%. In the case of US and sodium hydroxide treatment, there was a partial removal of pectin in technical fibers while in other treatments the initial fineness was retained due to cross-linking.

The test results of the tenacity of untreated and modified fibers are given in Figure 2.

Figure 2.

Tenacity of untreated and modified flax fibers.

It is known that a reduction in fiber tenacity characterizes any treatment that contains a cross-linking agent in its bath. In addition, the reduction of the fiber tenacity is also probably connected with the splitting of technical fibers to the level of thinner fiber bundles and the rigidity of the cross-linked fiber structure. A negligible loss of tenacity may be explained with the characteristic irregularity of morphology and the geometry of flax fibers. It can also be observed that performed pre-treatments (preswelling and US) have influenced fiber tenacity. It can be assumed that during the pre-treatments, the fiber structure is loose and the penetration of cross-linking agents is better; hence, the distribution of cross-links is more uniform, which is the reason there is better tenacity of pre-treated fibers.

As expected, the measured results of fiber tenacity after cross-linking cellulose molecules with sample CA show that there was a significant decrease in tenacity (63%). The reason for this change is the process of cross-linking that is performed at high temperatures (180°C) and in an acidic medium (pH 2.5), which aggressively acts on cellulose and damages fibers. Sample NaOH + CA, which was treated with sodium hydroxide before cross-linking with CA, undergoes reduction of tenacity (57%) compared to the untreated sample. The reduction is less, however, compared with the sample that was cross-linked without sodium hydroxide pre-treatment. Comparison of these results (samples CA and NaOH + CA) in relation to the untreated sample shows that the preswelling treatment had an effect on relative preservation of tenacity in the process of cross-linking (sample NaOH + CA). It can be assumed that the preswelling treatment was active in terms of increasing the accessibility of hydroxyl groups and, consequently, the emergence of new cross-links in the fiber structure.

By comparing the tenacity of the samples, which before cross-linking with CA had been treated with US (sample US + CA) or sodium hydroxide (sample NaOH + CA), with the tenacity of the sample treated only with CA, it can be found out that the sample US + CA has the lowest tenacity reduction. The impact of US caused a change in the fiber structure that resulted in an increase of the tensile strengths. This phenomenon can be attributed to the following: segment of macromolecules come closer to one another due to US-induced vibrations in the fiber structure, thus forming new areas of arranged structure and decreasing internal stresses. After BTCA treatment (compared with CA treatment), the reduction in tenacity is lower, at 46%. It may be noted that US treatment enhances the cross-linking of BTCA with cellulose molecules (sample US + BTCA) and a decrease in tenacity is lower compared to the sample US + CA (45%), amounting to 38%.

The test results of the elongation at the break of untreated and modified fibers are given in Figure 3.

Figure 3.

Elongation at break of untreated and modified flax fibers.

The elongations at break for all cross-linked samples, regardless of pre-treatment, were lower in relation to the initial samples (samples: untreated, NaOH, and US). These smaller elongations at break values are connected with the occurrence of crosslink.

The test results of the breaking twist of untreated and modified fibers are given in Figure 4.

Figure 4.

Breaking twist of untreated and modified flax fibers.

The breaking twist may be defined as the number of twists or turns that are required to rupture a fiber. It measures the resistance to shear. When fibers are able to withstand a larger number of turns before rupture, they are said to be less brittle and more flexible. The number of twists is correlated with the length, uniformity, strength, fineness, and structure of fibers.

As shown in Figure 4, the breaking twist level decreased for all modified samples, which means that the impact of all treatments was detrimental to the structure and consequently to the mechanical properties of fibers. There are several reasons for the reduction of resistance to shear, where the following two are most important. First, the fiber damage occurs due to aggressive cross-linking conditions, making the fibers weaker. Second, the cross-linking causes more brittle fibers, which are less able to withstand more twists before rupture. After cross-linking with CA, the breaking twist–level decreased by approximately 74%. The samples cross-linked with BTCA show similar results.

Sorption properties

Figure 5 gives the mean values of three parallel measurement series of moisture absorption capacity of the samples before and after the performed modifications.

Figure 5.

Moisture absorption capacity of untreated and modified flax fibers.

Moisture absorption capacity only slightly changed, probably due to the unreacted hydroxyl groups of PCA. Therefore, the test results indicate that the flax fibers retained their characteristic positive sorption properties. After performed treatments, the fibers continue absorbing water vapor excellently.

Figure 6 shows the test results of the water retention value of the untreated and modified sample.

All chemically modified samples had a reduction of water retention value after the centrifuging operation. Fiber swelling was reduced up to 50%, which indicates the success of the modification process. Since fiber swelling can be correlated with the behavior of finished textile products in use, it may be assumed that an advanced product can be made from the flax fibers modified this way, where the fibers will retain their excellent sorption properties, and because of a reduced swelling ability, they will not be prone to deformation (creasing).

Our results show preswelling and US treatments have influenced to the sorption and mechanical properties of the cross-linked fibers by having a positive effect on mechanical properties and no effect on sorption properties of the cross-linked fibers.

FT-IR-ATR analysis of the changes of cellulose functional groups induced by pre-treatments and PCA

In order to determine the chemical changes of functional groups of flax fibers after their modifications, the spectral analysis of untreated and treated flax samples using pre-treatments and PCA (CA and BTCA) as a cross-linking agent was performed. The FT-IR spectra of studied fibers are illustrated in Figures 8 and 9, while the spectrum of untreated flax fibers is presented in Figure 7.

Figure 7.

FT-IR–ATR spectra of untreated flax fibers.

Figure 8.

FT-IR–ATR spectra of flax fibers treated with (a) CA, (b) US and CA, and (c) NaOH and CA.

Figure 9.

FT-IR–ATR spectra of treated flax fibers with (a) BTCA, (b) US and BTCA, and (c) NaOH and BTCA.

Similarly to all lignocellulosic fibers, flax presents typical signals of a cellulose and lignin combination. The spectrum has a broad peak at 3290.85 cm⁻¹ typical for hydroxyl groups of cellulose (O-H). It is also observed a small peak at 1733.83 cm⁻¹ corresponding to the carbonyl groups (C = O) due to the presence of acetyl ester and carbonyl aldehyde groups on hemicellulose and lignin. The peak at 1631.85 cm⁻¹ apparently does not belong to the fiber; however, its presence has been associated with absorbed water and is commonly observed on lignocellulosic spectra.²⁶

The FT-IR–AR spectra confirm the cellulose cross-linking through esterification reactions, as shown in Figures 8 and 9.

Spectra shown in Figure 8 reveal the presence of absorption bands at 1727.06 cm⁻¹, 1728.68 cm⁻¹, and 1731.85 cm⁻¹ attributed to the carbonyl group of the ester and/or carboxyl groups. The intensity of these absorption bands could indicate the amount of generated ester linkages between the hydroxyl groups of the cellulose and the carboxyl groups of the carboxylic acid in such modified cellulose. Additionally, the intensity of the absorption band at 1727.06 cm⁻¹ of samples treated with CA was smaller in relation to the another two treated samples (US + CA and NaOH + CA). The presence of absorption bands at 1581.03 cm⁻¹ (sample CA), 1578.96 cm⁻¹ (sample US + CA), and 1576.12 cm⁻¹ (sample NaOH + CA) was attributed to the carboxylate anion.

The band at 1729.98 cm⁻¹, 1727.47 cm⁻¹, and 1724.35 cm⁻¹ was attributed to the ester of carbonyl group, which could confirm the covalent bond between the hydroxyl groups of cellulose and carboxylic groups of PCA. The intensity of the absorption band at these values was higher for the sample BTCA. The presence of absorption bands at 1572.21 cm⁻¹ (sample BTCA), 1569.78 cm⁻¹ (sample US + BTCA), and 1568.48 cm⁻¹ (sample NaOH + BTCA) was attributed to the carboxylate anion.

As shown in Figures 8 and 9, it is obvious that preswelling and US treatment additionally improves the reactivity of the hydroxyl groups and therefore esterification of flax cellulose with PCA.

Conclusion

A review of the literature revealed that the process of cross-linking cellulose molecules with PCA is mainly carried out on cotton fabrics. There are also a small number of studies devoted to the modification of individual fibers, especially flax fibers. With the aim of excluding the impact of structural parameters of yarn or fabric on the effect of cross-linking cellulose molecules with PCA, the impact of cross-linking PCA on the sorption and mechanical properties of individual flax fibers was investigated. In order to improve the effect of cross-linking, sodium hydroxide and US pre-treatment were performed prior to a chemical modification.

Based on the results of testing the properties of untreated and modified flax fibers, the following can be concluded:

Fiber fineness compared with untreated fibers slightly changed.

After cross-linking, an expected reduction in tensile tenacity occurred. The presented investigation is carried on single a flax fiber, which excludes the impact of changes in structural parameters of complex textiles to the test properties established after the modification of PCA. As the results show, the loss of tenacity exceeds 40%, and it can be assumed that the aggressive action of cross-linking with PCA (tenacity loss due to cross-linking) and decreased flexibility of the fibrillar system is far greater than the one mentioned in the literature.²⁴ Since cross-linking was performed on fibers, a real insight into the changes in the fibers was obtained.

Pre-treatment has increased the accessibility of cellulose in flax fibers, and cross-linking was achieved with less fiber damage, whereby US proved to be a more successful pre-treatment. The comparison of the samples tenacity of US + CA and NaOH + CA with sample CA shows that the US + CA sample has the lowest tenacity reduction. In addition, US treatment enhances the cross-linking of BTCA with cellulose molecules (sample US + BTCA) and the decrease of tenacity is lower compared with the US + CA sample.

During chemical modification, BTCA acid proved to be a more successful modifier. Compared with CA treatment, the reduction of tenacity after BTCA treatment is lower without and with pre-treatments.

Fiber damage confirms a reduction in the fiber’s resistance to twisting.

The water retention value decreased in all of the samples after cross-linking when compared with untreated ones. Owing to a reduced swelling ability, the resistance of flax fibers to creasing increased.

Preswelling and US treatments additionally improved the accessibility of the hydroxyl groups and therefore the esterification of flax cellulose with PCA, which was confirmed with FT-IR spectra.

Footnotes

Funding

The paper presented here is a part of broader research within the scientific project ‘High performance textile materials and added-value fibers’ (code: 117-1171419-1415) conducted with the support of the Ministry of Science, Education and Sports of the Republic of Croatia.

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