Enhancing the accessibility of starch size and cellulose to enzymes in raw cotton woven fabric by air-plasma pretreatment

Abstract

In this paper, raw cotton fabric was pretreated with non-thermal atmospheric air-plasma and the accessibility of the surface polymers of the fibers and yarns that act as respective substrates for the enzymes was evaluated. Results proved that plasma slightly destroyed and oxidized the starch size on the surface of warp yarns and partially removed the thin and perfectly hydrophobic waxy coverage of the fibers in weft yarns, creating deep “pits” with a depth of 215 nm. This latter process contributed to the exposure of cellulose and pectin located under the waxy outer layer of the elementary fibers in the weft yarns, and significantly increased the surface roughness of the fibers (from R_q of 25 to 67 nm for the raw and 180 s plasma-treated samples, respectively). Amount of the reducing sugars released during the amylase and cellulase digestion of the plasma-treated fabrics confirmed that air-plasma significantly increased the accessibility of the starch and cellulose, respectively, to the enzymes and resulted in an enhanced solubilization of both polymers. Since the plasma-treated substrates displayed significantly faster enzyme reactions, the time of enzymatic treatments can be sharply reduced.

Keywords

enzyme accessibility starch cellulose pectin desizing

Recently, a number of shorter studies and comprehensive reviews have been published regarding the effects of plasma treatment on the surface properties of natural and synthetic fibers, yarns and fabrics. By using non-thermal plasma treatment, the surface layers of the fibrous substrates can be activated and modified without alteration of the bulk characteristics. Atomic temperatures of the non-thermal atmospheric plasma are close to ambient, but electron temperatures reach values up to orders of magnitude higher. Thus, plasma with such characteristics readily interacts with solid surfaces, causing reactions that would otherwise occur only at elevated temperature.^1–4

In a raw cotton woven fabric, the warp yarns are covered by mostly a starch containing sizing agent to improve the weaving efficiency. The weft yarns, however, do not contain any size and their surface characteristics are determined solely by the constituents (such as waxes, cellulose, pectins, etc.) located in the outer surface layers of the cotton elementary fibers. Both the starch sizing agent and the non-cellulosic constituents of the fibers have to be completely removed in order to achieve a perfectly hydrophilic fabric with an appropriate whiteness and without any impurities. In the environmentally friendly enzyme-aided textile technologies amylase enzymes are widely used for the degradation and removal of the starch containing sizing agent.^5–7 For accelerating the removal of the waxy outermost layer of the cotton elementary fibers and the natural coloring matters, pectinase and glucose oxidase enzymes can be applied, respectively. Cellulase enzymes are usually used in the biofinishing of cellulosic textiles.^8–14

Plasma pretreatment can affect the removal of both sizing agent and waxy surface layer. Cold-plasma in oxygen¹⁵, oxygen/helium^16,17 and air¹⁸ atmosphere was highly effective in removing the starch size from the surface of woven fabrics, resulting in a significant increase in surface hydrophilicity. The maximum weight loss values measured in oxygen/helium and air plasma were 2.92%¹⁶ and 6%,¹⁸ respectively. The surface roughness of the size layer characterized by the root mean square roughness (rms) increased significantly (from 2.8 to 33.1 nm) with increasing the time of oxygen/helium plasma treatment (from 0 to 45 s, respectively).¹⁷ Furthermore, plasma pretreatment accelerated also the degradation of the residual starch size in the subsequently applied desizing process with NaHCO₃.¹⁷ Plasma was proved to be effective also in degradation of the waxy outer layer of cotton fibers.¹⁹ By the effective removal of the waxy materials from the fiber surface with a short plasma pretreatment, the processing time of the subsequently applied conventional chemical processes was reduced significantly.²⁰

Furthermore, plasma pretreatment can increase the efficiency of the enzyme-aided textiles processes by accelerating the enzyme action. The efficiency of pectinase enzymes in bioscouring of cotton and linen was enhanced by different plasmas (i.e. at atmospheric and low pressure, in air/argon/oxygen gases).^21–24 The synergetic effect of plasma and cellulase enzyme treatments on the physical and chemical properties of cotton fabric was also proved.²⁵ In spite of that there are a lot of benefits obtained by using plasma as a pretreatment in the enzyme-aided processes, the effects of plasma on the efficiency of the subsequently applied enzyme catalyzed reactions have not yet been fully evaluated and discussed. Furthermore, nothing is known about the accessibility of the surface polymers of the fibers and yarns that act as respective substrates for the enzymes.

In this paper, we focused on how the air-plasma treatment changed the raw cotton fabric surface and how these changes improved the accessibility of polymers located on the surface of warp yarns (i.e. starch) or under the surface layers of cotton fibers (i.e. cellulose) to an amylase and cellulase enzyme, respectively. Exposure of pectin by plasma treatment was also discussed.

Experimental procedure

Fabrics and enzymes

Raw cotton fabric, plain-weave, with a fabric weight per unit area of 153 g/m² and a fabric count (warp/weft) of 28/24 yarn/cm was selected for the experiments. Warp yarns of the fabric were sized by a standard starch-based sizing material. The fabric was cut to the required dimension (10 cm × 20 cm, approximately 6 g) and these samples were then conditioned at 65% relative humidity and 20℃ for 24 hours prior to the plasma treatment and any of the testing. Bleached cotton fabric (scoured with sodium hydroxide and bleached with hydrogen peroxide) with a weight per unit area of 150 g/m², plain-weave, was selected as a perfectly hydrophilic and accessible substrate with a cellulose content of nearly 100% and used exclusively in the cellulase enzyme hydrolysis without any pretreatment. Both fabrics were supplied by Pannon-Flax Linen Weaving Co., Hungary.

Two commercial enzymes, an amylase and a cellulase, were used for characterizing the accessibility of the plasma-treated fiber surface to enzymes. A commercial α-amylase (Beisol LZV) recommended for desizing of woven fabrics with starch-based sizing agent was kindly supplied by CHT Bezema AG (Switzerland) and used as received. The enzyme reaches the optimum effectiveness in the pH and temperature range between 5.4–8.0 and 65–75℃, respectively. Celluclast 1.5 L, a commercial cellulase enzyme mixture produced by Trichoderma reesei was received from Sigma-Aldrich. The enzyme exhibits maximal activity near 50℃ and at pH 4.5–5.0. All other chemicals used in this work were reagent grade.

Plasma treatment

Non-thermal plasma treatment was performed in ambient air, by a diffuse coplanar surface barrier discharge (DCSBD) type equipment (Manufacturer: Roplass s.r.o., Brno, Czech Republic). The plasma is ignited with sinusoidal high frequency, ∼10–20 kHz, high voltage with peak-to-peak values of up to 20 kV. Both sides of the cotton fabric samples were treated, applying a power of 300 W and treatment times of 30, 90 and 180 s.

Characterization of the accessibility of polymers in the fiber surface to enzymes

Surface accessibility of the raw cotton fabric to the amylase and cellulase enzymes before and after the plasma treatment was evaluated by measuring the reducing sugars liberated during the enzymatic hydrolysis of starch size and cotton cellulose, respectively. In the cellulase experiments, enzymatic hydrolysis of a bleached cotton fabric was also tested only for comparison.

The amylase enzyme was diluted with tap water to 500 fold. The untreated and plasma-treated cotton fabric samples of 0.4 g were treated with the diluted enzyme solution containing in a shaking waterbath (Medingen SWB 20) at 70℃ for 210 min with a shaking frequency of 120 rpm. The fabric to liquor ratio was 1:50. The cellulase enzyme was diluted in 0.05 M acetic acid/sodium acetate buffer (pH 5) to 250 fold. 0.6 g of each of the cotton samples were put into an Erlenmeyer flask which was filled with the prewarmed diluted enzyme solution. The fabric to liquor ratio was 1:100. The flask was placed in a shaking waterbath controlled to 50℃ and with a shaking frequency of 120 rpm for 60 min.

Dinitrosalicylic acid (DNS) color test for determining the reducing sugars liberated by the enzymes was used. The 3,5-dinitrosalicylic acid is reduced to 3-amino-5-nitrosalicylic acid, while the aldehyde groups appear to be oxidized to carboxyl groups. Absorbance of the colored reaction products can be measured at 540 nm. Concentration of the reducing sugars was determined as follows: 0.5 ml of enzyme sample solution was diluted with 1 ml of acetate buffer at pH 5, and the enzyme reaction was terminated by the addition of 3 ml DNS solution, followed by boiling for 5 min. After cooling, the absorbance was read at 540 nm. The liberated reducing sugars in glucose equivalent were estimated according to Miller.²⁶ Each reported value is the average of at least three parallel measurements.

It has to be noted that an attempt to characterized the accessibility of pectin by the action of a pectinase enzyme was also made, but because of the very low pectin content of cotton fibers, it was impossible to follow the enzyme catalyzed hydrolysis of pectin even after plasma treatment by measuring the reducing sugars as described in the previous paragraphs.

Analysis of the surface and bulk properties

Contact angle measurements were carried out at 22℃ and 55% relative humidity using a Ramé-Hart goniometer (USA) with a drop image standard software of DT-Acquire and a camera. Liquid drops of 20 µl distilled water were deposited on each fabric sample and the image of the drops deposited onto the fabric surface was captured immediately by the camera. The value reported is the average of at least five readings for each of the samples. Wettability of the plasma-treated cotton fabrics was characterized by water drop test, counting the elapsed seconds between the contact of the water drop with the fabric and the disappearance of the drop into the fabric. A burette was used to drop a single drop of water on the sample from a height of 1 cm. Ten readings were taken from different locations on the samples subsequent to the plasma treatment and the average was calculated.

X-ray photoelectron spectroscopy (XPS) was done by a Kratos XSAM 800 spectrometer using Mg Kα_1,2 radiation and fixed analyzer transmission mode (80 and 40 eV pass energies for survey and detailed spectra, respectively). The diameter of the analyzed spot was about 2 mm. The spectra were referenced to the C 1 s line (binding energy, BE = 285.0 eV) of the hydrocarbon type carbon. Data acquisition and processing were performed with the Kratos Vision 2 program. The conditions of the measurements and the spectrum elaboration were kept constant, and thus the intensities measured on parallel samples were repeatable with good precision (within ± 5%).

Some of the samples were selected for characterizing the surface morphology by a DI Nanoscope Dimension 3100 AFM (Digital Instruments Inc., USA, Santa Barbara). The fibers from weft yarns of the untreated (raw) and plasma-treated (for 180 s) fabrics were bent on a flat sample holder and fixed by glue. A series of atomic force microscopy (AFM) images in the scan size range 2–20 mm were acquired. These images are proper three-dimensional (3D) profiles of the sample surface and can be represented in a so-called 3D view, where an axonometric colored picture is calculated from the measured spatial data representing a microscopic, but perspective view of the surface. The rms roughness (the root mean square of height deviations compared to the average plane of the image in each pixel) was calculated for surface topography quantification and presented as R_q values for both samples.

Infrared spectroscopy (attentuated total reflectance Fourier transform infrared spectroscopy, ATR FT-IR) measurements of the cotton fabrics were carried out applying a Tensor 27 spectrophotometer (Bruker, Germany) with a diamond ATR cell (Bruker Platinum ATR) in the wavenumber range of 4000–400 cm^-1 using 2 cm^-1 resolution and 32 scans. The relative peak intensities were also calculated in comparison to 609 cm^-1, which can be assigned to the OH-out of plane bending in cellulose and hence it does not vary with the plasma treatment.^27,28

Efficiency of the amylase in degradation and removal of starch sizing agent was characterized by the Tegewa scale method using an iodine/potassium iodide solution.²⁹ One drop of the solution was put on the fabric after amylase treatment and gently rubbed. The starch size residue was determined visually by comparing the stained fabric swatch to the Tegewa violet scale, where 1 is dark blue and 9 has no color stain.

The effect of plasma treatment on the tensile properties of raw cotton fabric was determined by raveled strip test (1.5 cm width) on an Instron Tester Model 5566 (USA, ASTM 1682). Breaking load and elongation at rupture of the raw and plasma-treated fabrics were measured in both warp and weft directions.

Results and discussion

Effect of plasma on the surface and bulk properties of the fabric

Before discussing the effects of plasma treatment on the accessibility of enzymes to the polymers of the surface of raw cotton fabric, it has to be underlined that the warp yarns are covered with a starch-based and lubricant containing sizing agent. The thickness of a starch size film on the surface of cotton warp yarns, sized with low pressure or high pressure squeezing, was proved to be 14 and 8 µm, and the thickness of the penetration layer under the size film was 8 and 20 µm, respectively.³⁰ The weft yarns, however, do not contain any starch size; they consist only of elementary cotton fibers with a hydrophobic waxy outer layer called cuticle, which can be characterized by a thickness of about 12 nm.³¹ Cuticle is composed mainly of non-cellulosic lipophilic components such as waxes, fats and resins, and also pectin and less ordered cellulose.⁸

AFM images of the fibers derived from the weft yarns of untreated and plasma-treated (for 180 s) fabrics in Figure 1 show that plasma created deep ‘pits’ with a depth of 215 nm. Significantly larger pits with depths of 1000–2000 nm were created on the surface of bleached flax fibers by oxygen plasma.³² Compared to the surface roughness of the untreated raw fiber (Figure 1, left; R_q = 25 nm), the plasma-treated (Figure 1, right) fiber surface has a significantly rougher structure with an R_q value of 67 nm. We assume that the applied plasma treatment can only etch and attenuate the relatively thick (i.e. 8–20 µm) sizing layer on the warp yarns, but it can contribute to the localized ablation of the waxy outer layer of the elementary fibers in the weft yarns, making the fiber surface extremely rough.

Figure 1.

Atomic force microscopy (AFM) images of the elementary fibers of weft yarns derived from raw (left) and air-plasma-treated cotton with exposure time of 180 s (right). Image scale: 5 µm in x and y directions and 500 nm (left) and 1000 nm (right) in z direction.

Changes in the waxy surface layer of the elementary fibers in the weft yarns were also proved by ATR FT-IR (Figure 2). Since the penetration depth of light is typically of the order of a few microns (ca. 0.5–3 µm), the results provided by FT-IR ATR were used successfully for characterization of the plasma-induced changes in constituents of the surface of elementary fibers in weft yarns. Extra peaks in the spectrum of the raw fabric at 2916 and 2850 cm^-1, associated with the asymmetric and the symmetric stretching of methylene groups in long alkyl chains, indicate the presence of waxy materials in the fiber surface.³³ For plasma-treated samples, however, the decrease in relative intensities of the peak at 2850 cm^-1 (Table 1) and the absence of the peak at 2916 cm^-1 (Figure 2) was observed, indicating a partial removal of the waxy outer layer of the elementary fibers in weft yarns by plasma treatment.

Figure 2.

ATR FT-IR spectra of the raw and air-plasma-treated (P) cotton fabrics with exposure time of 30 s, 90 s and 180 s.

Table 1.

Relative intensities for IR absorption bands, O/C atomic ratio and results of deconvolution of the C 1 s peak by XPS for the untreated (raw) and air-plasma-treated cotton fabrics as a function of time of plasma treatment

	Relative intensities at^a				C1 %	C2 %	C3 %
Fabric samples	2850	1730	898	O/C	285.0 eV	286.7 eV	288.3 eV
		(cm^-1)			C−C, C–H	C−O	C = O, O−C−O
Raw	1.01	0.25	0.67	0.25	76	18	6
Plasma-treated – 30 s	0.97	0.31	0.70	n.d.^b	n.d.	n.d.	n.d.
Plasma-treated – 90 s	0.98	0.37	0.72	0.54	57	25	18
Plasma-treated – 180 s	0.95	0.41	0.73	0.82	45	38	17

Compared to 609 cm^-1.

n.d. – not determined.

For characterizing the chemical changes of the fabric surface occurred by plasma treatment, XPS was used and a spot with a diameter of about 2 mm and with a sampling depth of 10 nm was analyzed. The samples selected for this study included the untreated raw cotton and the plasma-treated fabrics with exposure times of 90 and 180 s. The O/C ratio found for raw cotton fabric was 0.25 (Table 1), which was higher than that expected for a purely waxy surface (O/C ≈ 0.11)^34,35 and lower than that measured for the starch containing sizing agent (O/C ≈ 0.34)¹⁷. A comparison of the O/C values suggested that the surfaces analyzed by XPS were composed of the starch and lubricant containing sizing agent on the warp yarn, and of the waxy cuticle of the elementary fibers in the weft yarns. They are consistent with the high percentage of carbon in C–C and C–H type (76%), as well as in C–O type (18%) bonds in the surface layers.

As seen in Table 1, the O/C ratios for the plasma-treated samples were significantly higher (for 90 s: 0.54; for 180 s: 0.82) than for that of the raw cotton (0.25). Upon plasma treatment, both components C2 (286.7 eV, carbon in C–OH and C–O–C) and C3 (288.3 eV, carbon in C = O and O–C–O) increased (Figure 3 and Table 1). It is remarkable that the surface O/C atomic ratio of the fabric treated with plasma for 180 s is 0.82, which was almost equal to 0.83, which was calculated to the pure cellulose.³⁶ The increase of the surface concentration of C atoms bonded to oxygen was compensated mainly by the decrease of the C1 component (285.0 eV, pertaining to carbon C–C and C–H bonds). The significant increase of the O/C ratio and of the concentrations of the various oxidized states of carbon (mainly C–O, but also C = O and/or O–C–O) suggests the introduction of some hydrophilic groups to the surfaces by plasma-aided bond scission and oxidation of the starch molecules in the sizing agent, and the exposure of cellulose and other polymers (such as pectin, O/C ratio: 0.94; unpublished data) to the surface of elementary fibers in the weft yarns by the partial degradation of the waxy materials upon plasma treatment. Furthermore, the oxidation of the latter polymers (i.e. cellulose and pectin) can also occur. An increase in O/C ratio was also detected in the low-pressure oxygen plasma treatment of cellophane foils, which was attributed to the decomposition of polymer chains and the oxidation reactions.³⁷ While the increase in aldehyde groups in the sample treated with plasma for 480 s indicated a partial decomposition of the cellulose macromolecules, the increased number of carboxylic and/or carboxylate groups revealed oxidation reactions of the C–OH groups.

Figure 3.

Deconvolution of the C 1 s peak. Untreated raw cotton (top); air-plasma-treated cotton with exposure time of 90 s (middle) and 180 s (bottom).

Exposure of cellulose and pectin in the elementary fibers of weft yarns was confirmed by ATR FT-IR (Figure 2). Significant changes were observed in the ester region at ∼1730 cm^-1 as well as at 898 cm^-1. The former is related to the pectin molecule, since approximately 70% of carboxyl groups of galacturonic acid units in pectin are esterified with a methoxyl group, and the latter is associated with the C₁–O–C₄ β-glycosidic bonds present in cellulose molecule.^8,25 Results in Table 1 proved that the relative intensities increased significantly by plasma treatments (at 1730 cm^-1: 0.25 versus 0.31, 0.37 and 0.41; at 898 cm^-1: 0.67 versus 0.70, 0.72, 0.73; for raw versus plasma-treated fabrics with exposure time of 30 s, 90 s and 180 s, respectively). This means that the partial degradation and removal of the waxy outer layer of cotton, which occurred during the applied air-plasma treatment, exposed the pectin and cellulose constituents and made them more detectable on the fiber surface by ATR FT-IR.

Both degradation of the waxy outer layer of the elementary fibers and introduction of some hydrophilic groups to the surfaces significantly modified the absorbency of raw cotton fabric and provided a hydrophilic fabric surface, which can be characterized by a short wetting time (a few seconds only) and a low water contact angle (less than 30°), when the plasma treatment was carried out for 180 s (Table 2).

Table 2.

Values of wetting time, water contact angle and tensile properties of the untreated (raw) and air-plasma-treated cotton fabrics as a function of time of plasma treatment

	Water wetting	Water contact	Breaking load (N)		Elongation (%)
Fabric samples	Time (s)	Angle (°)	Warp	Weft	Warp	Weft
Raw	> 180	106 ± 6	113 ± 7	105 ± 12	14.5 ± 0.3	14.7 ± 0.7
Plasma-treated – 30 s	143 ± 19	82 ± 4	111 ± 11	111 ± 10	13.4 ± 0.4	15.1 ± 0.9
Plasma-treated – 90 s	17 ± 7	61 ± 3	111 ± 5	112 ± 4	13.9 ± 0.4	14.3 ± 0.5
Plasma-treated – 180 s	5 ± 4	29 ± 6	116 ± 10	117 ± 11	13.8 ± 0.9	13.9 ± 0.5

Since these changes in the surface, to an extent, could affect tensile properties of the fabrics, an investigation was conducted to determine the breaking load and elongation of the fabrics. Since any shrinkage and alteration in appearance of the fabrics did not occur during the DCSBD plasma treatment (namely, there was no change at all in the fabric count by plasma), the dimensional changes did not bias the data derived from the tensile testing. Thus, it can be concluded from the results in Table 2, that the applied plasma treatments did not cause significant change in tensile properties of the raw cotton fabric. None of the data in Table 2 reveal fabric degradation.

Enzymatic hydrolysis of starch size on the yarn surface

An amylase enzyme was chosen for characterizing quantitatively first, the effect of plasma on the hydrolysis rate of starch size, second, the changes in the amount of starch occurred by plasma treatment and third, the efficiency of the enzymatic desizing process. For raw and plasma-treated cotton fabrics, Figure 4 shows the concentration of reducing sugars liberated from the starch size as a function of time of enzyme hydrolysis.

Figure 4.

Reducing sugars released from the starch size in raw (filled squares, solid line) and air-plasma (P) treated (for 30, 90 and 180 s) cotton fabrics (closed symbols) by 10% of amylase enzyme as a function of time of hydrolysis.

It is obvious that in the first 10 minutes the amylase was much more efficient in solubilization of starch from the plasma-treated fabrics (closed symbols) than from the untreated one (▪). We remember that hydrophilicity and surface roughness of the plasma-treated cotton fabrics is significantly higher (Table 2 and Figure 1, respectively) than those of the raw fabric, which are presumably the main reasons of the increased and fast solubilization from the plasma-treated fabrics. Starch in raw cotton, however, had a lower accessibility to amylase, which could be explained by the compact and hydrophobic surface of warp yarns.

Results also reveal that concentration of reducing sugars released from the plasma-treated fabrics increased rapidly at first, then levelled off after 60 min in the range of 0.34 to 0.47 g/l. It is noteworthy that almost the same level of reducing sugar concentrations was reached from the plasma-treated fabrics and the values indicated only marginal differences among the fabrics treated with plasma for different time. For raw cotton, however, the concentration of reducing sugars increased gradually and levels off after about 120 min at about 0.51 g/l. Thus, the maximal amount of reducing sugars released from the plasma-treated fabrics was slightly lower than that from the raw cotton.

Furthermore, the iodine solution test (Tegewa) proved that amylase treatment for 210 min removed perfectly the total amount of starch from all fabrics (Figure 5). Since the maximum reducing sugar data (in Figure 4) are directly proportional to the amount of starch on the fabrics, it is obvious that plasma treatment slightly degraded the starch sizing agent on the surface of warp yarns. Notwithstanding that we could not detect any considerable change in the weigh per unit area of the raw fabric upon plasma treatment, the results from amylase hydrolysis proved a partial (10–20%) removal of starch by plasma etching.

Figure 5.

The Tegewa violet scale ratings for raw and air-plasma-treated (P) fabrics (for 30, 90 and 180 s) as a function of time of amylase treatment.

Highly efficient hydrolysis of the lower amount of starch on the plasma-treated fabrics resulted in an increased desizing effect, which can lead to a significant shortening of the desizing process. The plasma-treated fabrics showed the commercially acceptable violet scale rating of 5 with a hydrolysis time of 10 minutes. For the untreated fabric, however, a longer enzyme treatment (60 minutes) was required to obtain acceptable desizing effect (Figure 5). Thus, the increase hydrophilicity and accessibility of the plasma etched surface and the more efficient hydrolysis of the residual starch resulted in an increased desizing effect, which can lead to a significant shortening of the desizing process (from 60 to 10 minutes).

Enzymatic hydrolysis of cellulose in the fiber surface

Plasma-aided degradation and removal of the waxy cuticle of the cotton fibers in the weft yarns can contribute to the exposure of cellulose. For detecting the presence of cellulose and characterizing its accessibility, a cellulase enzyme was used as an analytical tool and the amount of reducing sugars released from the raw and plasma-treated cotton fabrics was measured and compared to that released from the bleached cotton. It can be considered as a highly accessible and pure cellulose sample.

In the plot of Figure 6 the concentration of reducing sugars released from different fabrics is shown as a function of time of cellulase hydrolysis. In raw cotton, the elementary fibers in the weft yarns are perfectly hydrophobic, because the waxy materials evenly cover the cotton fiber, hindering the hydrolysis of cellulose by a cellulase enzyme. More than 27 minutes (intersection of line on abscissa) were needed for releasing reducing sugars in detectable amount. Owing to the fairly intact structure, cotton fibers in raw cotton fabric have the lowest accessibility to the cellulase enzyme, since the waxy materials prevent the access of enzyme to cellulose.

Figure 6.

Reducing sugars released from raw, air-plasma (P) treated (for 30, 90 and 180 s) raw and only bleached cotton fabrics by cellulase enzyme as a function of time of hydrolysis.

A dramatic increase in the rate of cellulose hydrolysis in the plasma-treated fabrics was observed. By increasing the fiber surface roughness and partial removal of the waxy outer layer of the fiber with plasma, the cellulose became more accessible and more open to contact with a cellulase enzyme. After an initial short period (about 10–15 min) of slow sugar liberation, the results were described by straight lines. The slopes of the lines, applicable over the latter period of time were 0.0024, 0.0024 and 0.0020 g/lċmin for the plasma-treated fabrics for 30, 90 and 180 min, respectively, indicating that reducing sugars were liberated at comparable rates by cellulase enzymes from the fabrics treated with plasma for different times. It has to be noted that the slope of line for raw cotton was significantly lower (0.0008 g/lċmin).

For comparison, Figure 6 also shows the reducing sugar data for cellulase hydrolysis of the bleached cotton, which can be considered as pure (100%) cellulose with perfect accessibility. The slope of the line was 0.0045 g/l min, indicating a much faster hydrolytic degradation of cellulose into sugars. On the perfectly accessible cellulose surface of the bleached cotton (obtained after removal of the cuticle and primary wall by alkaline scouring and bleaching), the cellulase enzyme digested gradually the secondary wall, i.e. the main body of the fiber.

It has to be noted, that theoretically the reducing sugars liberated from the cotton fabrics by a cellulase enzyme can be derived not only from the fibers in weft yarns, as discussed above, but also from the warp yarns. Since the warp yarns are covered by a thick starch sizing layer, which can only be removed by a long hydrolysis (for 60 min and 120 min for plasma-treated and raw fabric, respectively) with an amylase enzyme (Figure 4), it is likely that under the circumstances of the cellulase catalyzed hydrolysis, the starch size still remained on the warp yarns. Thus, it is unlikely that the accessibility of the elementary fibers inside the warp yarns to the cellulase enzyme is high enough to enable the hydrolysis of cellulose.

Conclusions

In this paper, we focused on how the air-plasma treatment changed the surface of raw cotton fabric, and how these changes affected the enzyme reactions of cotton. Results proved that plasma slightly destroyed and oxidized the starch size on the surface of warp yarns and partially removed the thin and perfectly hydrophobic waxy coverage of the cotton fibers in the weft yarns, resulting in a more hydrophilic fabric with a significantly shorter wetting time and lower water contact angle. Plasma etching of the surface was accompanied by a creation of deep ‘pits’ with a depth of 215 nm, which contributed to both the partial oxidation and removal of starch size from the warp yarns and the exposure of polymers such as cellulose and pectin located under the waxy outer layer of the fibers in weft yarns. Oxidation of the OH groups present in the cellulose and pectin molecules to aldehyde and carboxylic groups also occurred during the process. Furthermore, the rms roughness of the untreated elementary cotton fibers (R_q = 25 nm, which is similar to that previously reported in the literature³⁸) in the weft yarns of the fabric increased significantly to R_q = 67 nm by a 180 s plasma treatment. The fabrics’ breaking load and elongation as well as dimensional stability, however, seemed to be unaffected by the applied cold plasma treatment.

An amylase and cellulase digestion of the plasma-treated cotton fabrics confirmed that air-plasma significantly increased the accessibility of polymers (i.e. respective substrates) in the fibers surface to the enzymes, resulting in an enhanced solubilization of both starch and cellulose, respectively. Reducing sugars liberated from the starch-based size by amylase treatment proved that about 10–20% of starch size in the raw cotton was removed by the air-plasma treatment. The increase in hydrophilicity and accessibility of the plasma etched surface and the more efficient hydrolysis of residual starch resulted in an increased desizing effect, which can lead to a significant shortening of the desizing process (from 60 to 10 minutes).

Raw cotton fibers in the weft yarns of the fabric had a low accessibility to cellulase, which is due to the hindering effect of the waxy outer layer. Almost 30 minutes were needed for the enzyme molecules to penetrate through the discontinuities of the wax to inner fiber structures and attack cellulose. By plasma-aided degradation and removal of the waxy layer, the accessibility of cellulose in the primary and secondary cell walls to cellulase enzyme improved significantly and a dramatic increase in the rate of cellulose hydrolysis was observed. Since the plasma-treated substrates displayed significantly faster enzyme reactions, the enzymatic treatment time can be sharply reduced.

Footnotes

Declaration of Conflicting Interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This research was supported by the Hungarian National Science Foundation (grant number OTKA K82044).

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