Electrokinetic analysis of hydroentangled greige cotton–synthetic fiber blends for absorbent technologies

Abstract

Through nonwoven hydroentanglement of greige cotton blends with polyester and nylon, varying degrees of fiber surface polarity, swelling, and absorbance can be achieved. Electrokinetic properties of nonwoven blends made with Ultra Clean™ cotton (100% greige or virgin cotton) and polyester or nylon in 40:60 and 60:40 ratios demonstrated distinct differences in charge, swell, and per-cent moisture uptake capability. An electrochemical double layer analysis of charge based on a pH titration (pH 1.5–11 in 1 mM KCl) was employed to measure the relative fiber and fabric surface polarity (ζ_plateau), which ranged from −60 to −26 millivolts. A linear relationship of fiber swelling (Δζ) and per cent moisture content is apparent when greige cotton and synthetic fibers are blended. Water contact angles revealed that the cotton/synthetic fiber blends were hydrophobic (contact angle >80°) while retaining significant absorbency. The greige cotton/synthetic nonwoven materials, however, possess absorbent properties characterized by varying degrees of moisture uptake, fiber polarity, and swelling attributes similar to absorbent fluid transport materials present in the layers of incontinence products. Electrokinetic properties of the blended greige cotton/synthetic nonwovens are correlated to absorbent incontinence materials.

Keywords

Greige cotton nonwoven fabrics incontinence absorbent products electrokinetic analysis cotton-synthetic blends water contact angle

Cotton has a variety of absorbent properties which render it advantageous as a medical and hygienic textile. Nonwoven cotton has found applications in medical products including surgical gowns, swabs and drapes, gauze, disposable patient gowns, bandages, wound dressings, sheets, and bed pads.¹ In recent years, the preference to use cotton fibers in nonwoven absorbent products has increased due to its characteristic soft hand, hypoallergenic properties, absorbency, and eco-friendly cellulosic character. An increase in commercial applications of cotton nonwovens in non-implantable and hygienic products has been reflected in new product lines, renewed research and development pursuits as well as consumer preferences.²

Recent reports have outlined the production of greige (non-scoured/bleached) cotton nonwovens using a hydroentanglement (HE) process of fabric formation.³ It was demonstrated that the absorbent properties of greige cotton nonwoven fabrics could be enhanced by increasing the water pressure, i.e., the hydraulic energy that entangles the fibers.⁴ High entanglement energy during the HE process improves both absorbency and wick ability of greige cotton by partial removal of the hydrophobic constituents on the outer surface of the cotton fiber. Property enhancement results from the exposure of the inner hydrophilic cellulosic microfibrillar walls and the formation of fibrous strands and channels. It has also been reported that UltraClean cotton (greige cotton cleaned in a proprietary mechanical process that uses no chemicals, water or heat) possesses attractive absorbency and wick ability properties when made into a HE fabric.⁴

Polyester and nylon fibers are also used in a variety of nonwoven medical textiles,¹ and, when combined with cotton, increase the range of fiber surface properties – polarity, swelling, and moisture content uptake, and thus have established great utility as ‘designed’ absorbents. In recent years, the effects of blends of cellulosic and synthetic fibers have been assessed as a way of regulating fluid imbibitions.⁵ It has been proposed that improving the resiliency of the web based on the use of synthetic and cellulosic fibers (as cotton) together improves absorbency by increasing web interstitial space and modulating pore size.⁶ This approach is based on the supposition that synthetics absorb little or no fluid and preclude reduction of modulus and volume within the web. Webs containing cotton have also shown superior absorbency when compared with other cellulosic’s, such as rayon.

Hydroentangled cotton, incontinence products, and electrokinetic properties

Hydroentangled nonwovens are predominantly used in wipes, and medical textile applications.⁷ Cotton’s use in these types of absorbent products is also increasing globally. However, there is also a growing interest in the use of hydroentangled greige cotton in hygienic absorbent product materials. Previously, we have reported an electrochemical analysis of commercial incontinence products as an approach to demonstrate the role of fiber surface polarity, swelling, and water uptake in each product’s mechanism of liquid control.⁸ Here, in this study, we apply these concepts to the electrokinetic analysis of nonwovens containing greige cotton.

Common to the design of most absorbent incontinence products are a coverstock, an acquisition layer, a distribution layer, an absorbent core, and a back sheet.⁹ The absorbent core takes up and stores fluid while the coverstock, acquisition/distribution layer, and back sheet, optimize the uptake and efficient transport and containment fluid from the body to assure the product’s performance to keep the skin dry, and avoid leakage. The composition of the absorbent core typically consists of fluff pulp, cellulose wadding, and/or a super absorbent polymer. The composition of coverstocks is typically polypropylene-based and the other layers surrounding the absorbent core vary in composition depending on the intended application and manufacturer. For example, in both adult incontinence diapers and the light incontinence pads the acquisition layer is typically cellulose fluff or rayon, whereas in the heavy incontinence pads it is often composed of polypropylene or polyester, and in moderate pads the acquisition layer is most often a cellulosic material. The distribution layer of most products when it is separated from the acquisition layer is typically cellulose, a polyester/cellulose blend or modified acrylic. The absorbent core works most efficiently in evenly absorbing and retaining moisture by virtue of the design and composition of the coverstock, and acquisition/distribution layers. Thus, the coverstock, acquisition/distribution layer, and back sheet, optimize the efficiency of the product’s performance to keep the skin dry, and avoid leakage through differences in material composition that regulate fluid transport.¹⁰ Fiber polarity, porosity, moisture uptake, and swelling, which can be measured and evaluated electrokinetically, are modulated by modification of fiber surface.

A variety of methods have been employed conventionally to characterize fiber surfaces including contact angle measurements, electron spectroscopy, and spectroscopic assessments. An approach which is useful and appropriate in assessing the fiber surface chemistry at the solid liquid interface is zeta potential (ζ potential). The ζ potential is a measure of the charge and charge density on the surface of a particle or fiber and can be used to gauge the stability of a colloidal or the ability of a material to absorb liquid. The ζ potential can be either positive or negative depending on its surface chemistry. ζ potential analysis is appropriate with absorbent materials since the fluid dynamics of the electrochemical double layer model enables measurement of functional properties similar to those that occur at the solid-liquid interface of incontinence materials.⁸ ζ potential decrease (a change in electrostatic potential at the shear plane of the electrochemical double layer) is caused by swelling of the fibers and outward movement of the aqueous shear plane where ions are in contact with the outer Helmholtz plane on the fiber surface.¹¹ Thus, a concomitant increase in swelling of the fiber occurs for most fibers as the shear plane moves out to the more diffuse layer of ions causing a decrease in ζ potential.¹²

This paper examines the electrokinetic properties of hydroentangled nonwoven materials made by blending greige cotton with synthetic fibers with a view to understanding the similarities the materials possess to the coverstock and acquisition/distribution of commercial incontinence products. We report here the preparation and electrokinetic analysis of UltraClean cotton,³ and two different ratio blends of UtraClean cotton/ polyester and UltraClean cotton/nylon, as a measure of the range of fiber and fabric surface polarity, swelling, and absorbent properties achievable with greige cotton/synthetic fiber blends.

Materials and methods

Ultra Clean cotton (UC), polyester and nylon fibers were supplied by T.J. Beall Company for producing the fiber blends. The cotton fiber characteristics were reported previously.³ The fiber length of the polyester and nylon used in the study was 1.5 inches and their fineness (linear density) was 1.5 denier (the weight in grams of 9000 m of the fiber). The fiber length and micronaire (fineness), as determined by the cotton HVI test system, of UltraClean cotton were 0.92 inch and 4.87, respectively. Although the cotton micronaire value broadly represents the fiber's fineness (linear density), it (unlike in the case of manufactured fibers) generally is not expressed in any unit. Blends of ultraclean cotton with synthetic fibers, including polyester, were prepared manually then processed on a card that delivered a consolidated web of approximately 10 g/m². The cotton/synthetic blended web was transported by a conveyor belt to a commercial cross-lapper for producing a multilap assembly, which was fed to a double-board, pre-needling machine for a light needling impact by 3-barb needles that were 9 cm in length.^13,14 The needle-punched substrates were subjected to hydroentanglement (H-E) on a 1-m wide commercial H-E system to produce the greige cotton/synthetic nonwoven fabrics. The fabric production rate was kept nominal, ranging from 5 to 25/min for different trials. The fabric was dried online in a gas-fired, hot-air chamber (containing a large perforated thru-air drum), and wound on to a paper tube at the end of the H-E line. To examine the dimensional stability of the greige and greige/synthetic blends, 1 m² sections of fabrics from each run of production were laundered using household washing machines and dryers set on ‘cotton’ cycle, and with All™ detergent, and warm water.

Zeta potential

The determination of the ζ potential was carried out with the Electro Kinetic Analyzer (Anton Paar, Ashland Va.) using the Cylindrical Cell developed for the measurement of fibrous samples. When a fiber absorbs liquid and swells, the surface charges become farther separated and the absolute value of its ζ-potential decreases. Two kinds of measurements were made on each sample: (1) swell tests to measure the rate and extent of fiber swelling (at a given pH) and (2) a pH titration in which the swelling is measured as a function of pH. The zeta potential was calculated All ζ potential measurements were made in a 1 mM KCl electrolyte.

In the electrokinetic apparatus the streaming potential is measured and the zeta potential determined from the Smoluchowski equation:^15,16
$ς = \frac{d U}{dp} \frac{η χ}{s_{r} s_{o}}$
(1)
where U is the streaming potential, the potential generated when an electrolyte is forced to flow over a stationary charged surface, p the pressure, $ɛ_{r}$ and $ɛ_{0}$ the dielectric constant and the vacuum permittivity, η the viscosity and k is the conductivity of the measuring fluid. Surface conductivity of the fibrous samples was not taken into account.

pH titrations were performed over a pH range of 1.8 to 11 to ensure recording both the isoelectric point (IEP) and the plateau potential. (The IEP is the pH at which ζ = 0 and provides insights into the surface association/dissociation processes.)

Swell tests and pH titrations were carried out on hydroentangled nonwoven fabrics cotton–polyester blends, cotton–nylon blends or cotton and cotton by-products. Swell tests were performed during which the zeta potential, ζ, was measured against time. These data were subsequently fit to a first-order decay equation
$- \frac{d ς}{dt} = k (ς - ς_{\infty})$
where ζ_∞ equals ζ at infinite time and k is the decay constant. Integrating
$In (\frac{ς - ς_{\infty}}{ς_{0} - ς_{\infty}}) = - kt$
ζ₀ is the integration constant and is ζ at time = 0, i.e. the time when the fiber first contacts the electrolyte. A mathematical regression routine was developed for MathCad to evaluate swell test data for ζ₀, ζ_∞, and k. ζ₀ and ζ_∞ were then used to calculate Δζ,
$Δ ς = \frac{ς_{o} - ς_{\infty}}{ς_{0}}$
(2)

Time-dependent/swell behavior

The swell behavior of the incontinence products was measured using the Anton Paar analyzer with the cylindrical cell template. The sample was loaded into the cell, and rinsed with electrolyte solution. The experiment initiated when the electrolyte first contacted the sample. The flow rate was adjusted in the range of 60–100 ml/min by compression of the sample and the sample size. The initial pH of the sample varied between 5.5 to 6.0 to mimic the pH of urine at a 1 mM KCl concentration. The pH was adjusted in this range with 0.1 M NaOH or HCl, as needed. In any given experiment, 30–35 data points were taken.

Moisture content

The moisture content of the incontinence products was measured using a modified ASTM D629-99 or AATCC 20 A-2000 procedures. The sample was conditioned overnight in a humidity chamber at 70% and a room temperature of 23℃. The moisture measurements were taken with an Infrared Moisture Balance (Kett FD 240, manufactured by Kett Electric Laboratory in Tokyo, Japan). The balance was set for automatic wet-based moisture with a drying temperature of 110℃. Approximately a one gram sample was used for each measurement on.

Absorbency

The ‘drop test’ is an AATCC test (AATCC 79-2005; Absorbency of Bleached Textiles). A drop of water was placed on the fabric and the time elapsed until it is absorbed and the sheen disappears is measured. The absorbency time and capacity tests were conducted on the same fabric sample. The fabric was cut 3 inches wide to a length that weighs 5 grams. The fabric was rolled on itself and placed in a pre-weighed test basket of standard size and weight. The fabric and basket are placed in a water bath and the time elapsed to submersion was measured (sink time). Fabric and basket remained in the water for an additional 10 seconds then removed and allowed to drain for 10 seconds. The wet fabric and basket were weighed and the weight of the basket and dry fabric were subtracted, giving the weight of the water absorbed. The results are reported as grams H₂O absorbed/gram dry fabric.

Swell versus Per Cent Moisture Plots

The per cent moisture was plotted against the Δζ for the HE cotton and cotton blend fabrics.

Contact angle measurement

The contact angle of a water drop on nonwoven fabrics was measured using a VCA Optima XE (AST Products, Inc., Billerica, MA). Runs were performed with a 1 and 5 µl drop of distilled water syringed onto the fabric. The image of the drop was immediately captured and analyzed to yield a contact angle. Contact angles on twelve different areas were measured and their average value was presented. Contact angles were also measured as a function of time on five different areas for each fabric.

Dynamic contact angle of water on commercial nonwoven incontinence layer samples was measured using a Sigma 700 tensiometer (KSV Instruments, Ltd.). The test was based on the dynamic Wilhelmy method. Measuring principle is expressed by the following equation:
$θ = \cos^{- 1} \frac{σ}{γ_{L} \cdot P}$
where σ is wetting force (mN/m), γ_L is liquid surface tension, P is the perimeter of the sample cross-section, and θ is contact angle. Here advancing contact angle is reported.

Results and discussion

UltraClean Cotton, which is a greige cotton,³ was combined with either polyester or nylon, whereupon the blend was carded, cross lapped, and subjected to light needling prior to hydroentanglement at 50 bar wet-out water pressure and 125 bar hydroentangling water pressure. This approach to greige cotton-based nonwoven production has previously been shown to increase absorbency while retaining some of the native waxes and pectin.⁴ Depending on the hydroentangling process parameters and conditions, this approach increases hydrophobicity of the greige cotton nonwoven compared to a scoured and bleached cotton nonwoven product. Thus, the versatility of nonwoven greige cotton (to processing) merits more detailed analysis of fiber surface properties at a solid-liquid interface to evaluate its potential as an absorbent.

Fabric surface polarity

The moisture uptake (% MC) and the electrokinetic results including the fabric surface polarity (ζ_plateau), swelling (Δζ), rate of swelling (k), and related material density for all materials are shown in Table 1 for the cotton/synthetic blends. The zeta (ζ) potential titration shown in Figure 1 assesses the surface charge based on pH and the planar portion along the x-axis of the plot is designated as the zeta plateau value (ζ_plateau), which is a reflection of the relative hydrophilic versus hydrophobic character of the fabric. When evaluated in absorbent incontinence materials ζ_plateau is relevant to the wide pH range found in human urine. The relative fiber and fabric surface polarity for the blends, as determined from the ζ_plateau, ranges from −60 to −27 mV.⁸ The ζ_plateau was determined at a 1 mM KCl concentration. The most hydrophobic UC/synthetic blends with the exception of 100% PES (ζ_plateau = −60 mv) were the 60% UC/40% PES and 60% UC/40% NYL (ζ_plateau = −44 mV and −46 mV, respectively). On the other hand, 40% UC blends with PES or NYL do not have significantly different ζ_plateau values compared to 100% UC. This is in contrast with the material’s swelling which is increased, and the observed swelling is consistent with cellulosic/synthetic blends increasing web interstitial space and modulating pore size.⁶
Figure 1.
Streaming zeta potential titration plots, as outlined in the materials and methods section. Streaming zeta potential titrations were performed on 100% UltraClean Cotton (UC), 100% polyester, 40% UC/60% PES, 60% UC/40% PES, 40% UC/60% NYL, 60% UC/40% NYL.

Table 1.
Fiber polarity (zeta plateau), swelling (delta zeta plateau), rate of swelling, and density

Composition Moisture content (%) IEP pH Plateau potential (mV) Swell test k (min−¹) ζ₀ (mV) ζ_∞ (mv) Δζ Density (g/cm³)

100% UC 8.2 2.3 (e) −26 0.0044 −47.7 −44.5 0.065 0.147

100% PES 1.2 3.8 −60 0.0098 −60.0 −52.0 0.133 0.118

0.0130 −73.3 −58.9 0.197

60%UC/% PES 4.6 2.6 −44 0.0140 −67.3 −60.4 0.102 0.110

40% UC/60% 3.2 2.7 −27 0.0130 −75.0 −66.3 0.115 0.112

PES 0.0073 −45.7 − 40.2 0.121

60% UC/40% NYL 5.7 2.2 −46 0.0088 −42.4 −38.6 0.091 0.117

40% UC/60% 4.5 2.4 −29 0.0087 −42.6 −38.6 0.091 0.133

NYL 0.0020 −51.6 −45.8 0.112

Since the magnitude of ζ_plateau reflects the degree of hydrophobicity found in the material a similar trend would be expected with water contact angle values. A plot of the water contact angles for the blends of this study is shown in Figure 2. The trend in water contact angle magnitude of the fabrics was consistent with the ζ_plateau values that are shown in Table 1. The contact angle for the hydroentangled PES nonwoven was the greatest and thus the most hydrophobic of the samples measured. In general however, the contact angle of the blends was similar. It is interesting to compare the contact angles between the nonwoven blends of this study and water with those of absorbent layers found in incontinence products. Table 2 lists the water contact angles found in commercial coverstocks and acquisition layers. The values show generally lower water contact angles (less hydrophobicity) than are found in the blends of this study. However, this in part may be due to the higher porosity of these types of incontinence products.
Figure 2.
Water contact angles of greige cotton/synthetic nonwoven blends. Details of the determinations of the contact angle determinations are in the materials and methods section. Determinations were performed with both 1 and 5 microliter volume drops. 1 = 100% PES, 2 = 60% UC/40% PES, 3 = 40% UC/60% PES, 4 = 60% UC/40% NY, 5 = 40% UC/60% NYL.

Table 2.
Contact angles of some randomly sampled commercial converstocks and acquisition layers taken from incontinence products

Coverstock layer (heavy) 91 87

Acquisition layer (heavy) 90 62

Acquisition layer (light) 81 ND

Coverstock (bed pad) 79 ND

Swelling and moisture uptake

Figure 3 shows the inverse relationship between % moisture content and swell behavior, Δζ. The swelling of the fiber blends results in the expansion of the electrochemical shear plane formed near the fibrous surface (12) and is manifest as a decrease in the absolute value of ζ. The swelling assessments were done at the average pH of human urine in 1 mM KCl. The data presented in Figure 2 includes the complete range of cotton/synthetic blends investigated in this study. The per cent moisture versus the Δζ for all ratios of cotton to synthetic fiber reflects a correlation between the amount of moisture the material is prone to absorb and its degree of swelling.
Figure 3.
The plot shows per cent moisture uptake (%MC) versus swelling or Δζ (delta zeta) for greige cotton/synthetic blends. The mathematical derivations, and procedure or determination of the measurements are contained in the Materials and Methods section. The parallel y-axis is sink time (open white circles) from the ASTM assay.

As the Δζ value (x-axis) of the material increases there is a concomitant decrease in moisture uptake (% MC). These two properties (moisture uptake and swelling) drive fluid transport in incontinence products. The inverse relationship between moisture uptake capability and fiber swelling of the blends is useful in directing fluid absorption and transport, which occur between the skin and top sheet, and between the top sheet and acquisition/distribution layer.

The linear correlation of swelling and water uptake shown in Figure 3 is consistent with absorbency properties as shown for the cotton synthetic fiber blends in Table 3. As seen in Figure 3, a similar linear correlation exists for the blends between the ASTM sink time measurement (an indirect measurement of the materials swelling properties) and per cent moisture uptake. As shown in Table 3 the large difference in sink times between the greige and bleached cotton also reflect the influence of the fiber surface waxes and pectin. Swelling and rate of swelling in the greige cotton is noticeably increased through blending UltraClean Cotton with polyester and nylon. The relationship of moisture uptake and swelling is consistent with reports on the use of hydrophobic fibers to improve the wet resiliency of hydrophilic/hydrophobic fiber blends.^5,6 The moisture uptake was the highest in the 100% UltraClean Cotton and the lowest in 100% polyester. It is notable that 60% UC blended with 40% PES or NYL results in swelling of the material increasing up to two-fold. The absorption capacity (Table 3) was also highest in these blends. Thus, the hydrophobicity of the outer greige cotton fiber begins to contribute to the overall swelling of the material at a level between 40 and 60 percent greige cotton present in the blend. The increased swelling due to increasing the ratio of total greige cotton may also be a result of an additive contribution of waxes and pectin from the greige cotton, which are expected to contribute hydrophobicity at the fiber surface. In addition, the similarity of the isoelectric points (IEPs) among the UltraClean Cotton (UCC) samples is consistent with the composition of the samples having cotton or cellulose.
Table 3.
Results of ASTM-based absorbency studies

Fiber blend Weight (g/m²⁾ AATCC drop test (sec.) Drop test syn. Urine (seconds) ASTM 1117 sink time (sec) ASTM absorbency capacity (g H₂O per g of fabric)

100% UC 54.6 <1 4.8 40.8 4.97

100% PES 68.6 >60 >60 >300 0.14

60%UC/ 40%PES 75.4 >60 23.2 113.8 7.05

40%UC/ 60%PES 66.6 >60 19.4 241.0 7.97

60%UC/ 40%NYL 55.5 <1 8.3 57.1 7.04

40%UC/ 60%NYL 79.5 25.3 23.3 162.8 6.62

100% UC-SB 54.6 <1 24.1 2.62 6.35

Fiber codes: UC = UltraClean, PES = Polyester Staple, PA = Nylon, UC-SB = UltraClean sample scoured and bleached. Synthetic urine was prepared as outlined by Mayrovitz and Sims.¹⁷

Cotton/synthetic blends: bound versus free water and structural considerations

The linear relationship of fabric swelling and moisture uptake properties and the respective absorbent properties of the blends in this study prompt consideration of the role of bound versus free water in the swelling of the UCC and synthetic blended fabrics. As discussed previously cellulose blends have been shown to increase web interstitial space and modulate pore size.^5,6 This property would account for some degree of swelling observed in this study as polyester and nylon have a higher elastic modulus than cellulose. Thus it would be expected that the energy to release the strain from microstructural elements of dried and collapsed polyester and nylon in the presence of water are less than for cotton, and the formation of channels and pores occurs more freely than for cotton, resulting in the observed greater swelling in the UCC/synthetic blends than for 100% UCC. However, the binding of water to cotton also plays a role in the swelling of the blended fabrics. The microstructure of cotton fibers and pores allow penetration of water, and in the case of the greige cotton nonwovens studied here, water accessible sites of cellulose are formed from cotton contact with the high pressure water jets of the nonwoven process allowing the primary cell wall of the fiber to be exposed as cellulose-bound water during the hydroentanglement process.⁴ The hydroentanglement process promotes disruption of the fiber cuticle that retains some wax and pectin while exposing cellulose fibrils and microfibrils to water penetration.

The binding of water to cotton can be further characterized as being present in three different states including: (1) strongly bound or non-freezing water, (2) anisotropically constrained or perturbed water, and (3) unperturbed water, or water undergoing isotropic motion.^18,19 There are considerations to take note of in terms of water binding and fiber structure, from a crystalline to fibrillar state. Crystalline cellulose (crystallites of 36 chains) has been characterized as low water-binding.^20–22 However, ordered microfibrillar cellulose, which is composed of cellulose crystallites, possesses surface hydroxyls presenting accessible water binding sites where penetrating water forms a monolayer (termed non-freezing water) at a level of (0.1 g/g cotton).²³ The presence of a strongly bound monolayer of water on cotton has been consistent historically with thermal calculations (90 cal/g) approximating that of the heat of fusion for ice, and validating the hydrogen bonding forces to cellulose.^21–25 From this state further water sorption then assumes the character of capillary condensation and has been characterized as free water i.e. perturbed and unperturbed water. A recent paper by Taylor et al., which used NMR relaxation times to characterize water binding, indicated that cotton moisture, which was set at 22℃ with a relative humidity of 33%, was present in multiple monolayer’s (in the range 14–17), as summarized in the above three listed energetic states of water in cotton.¹⁸ Thus, based on numerous past studies that characterize the role of water binding to moisture in cotton the results of this study suggest that the 100% UCC which possesses 8% moisture content probably has most of its water strongly bound as non-freezing water under ambient conditions. As shown in Table 1 and Figure 2, the presence of UCC also tends to increase the moisture content of the cellulose/synthetic blends, but decreases the observed swelling properties of the blends. Thus, it is likely that most of the water associated with swelling of the UCC/synthetic blends is free unperturbed water. This is likely as well as it has previously been shown that the density of interfacial water (strongly bound water) on cellulose is increased when it is perturbed. This property improves the wettability of the UCC/synthetic blends and may be seen as contributing synergistically to the swelling properties observed, i.e. as the cotton is wettable, so is it absorbent.

Increased swelling of the fabric correlates with a more negative zeta potential. With swelling of the fabric’s fibers the outer layer of diffusive ions in the electrochemical double layer constitutes the unperturbed or isotropic water discussed above since it is predicted to be greater than 6 nm from the surface of the fiber based on monolayer dimensions.^18,21,22 Some reflection of the material topography may be derived through a composite picture of the fabrics net swelling based on electrokinetic properties of delta zeta value, zeta plateau, and IEP. For example increased cellulose crystallinity should yield a higher IEP, a less negative zeta plateau value, and less swelling (Δζ) since there is less ion exchange due to crystal packing and less water of hydration.^21,22 Based on this principle, it can be inferred that a more amorphous cellulosic material will tend to result in (1) a more negative zeta plateau, due to more exposure of negative charge on the cellulosic surface, (2) increased mobility of open channels and pores in the material also suffered by an increased elastic modulus as seen in this study with cellulose/synthetic blends, and (3) a higher free energy of water adsorption by the cotton fiber. However, though others have correlated porosity with electrokinetic properties we have not observed that to be the case here.^26,27 In a separate study of porosity on these cellulose/synthetic blends where air permeability/porosity was measured, increased porosity did not correlate with electrokinetic properties.

On the other hand, a study by Ribitsch and Stana-Kleinscheck have correlated removal of non-cellulosic wax with a more negative zeta plateau,¹¹ and thus relates a more negative zeta plateau value to the increased exposure of negatively charged groups. In this study wax and pectin were principally retained on the cotton fiber yet a negative zeta plateau was comparable to values reported in the analysis of material with complete cotton fiber wax removal.¹¹ This may suggest an increased hydrophobicity due to retained fiber wax and more amorphous cellulose leading to increased interfibrillar channels and pores, which would be expected to arise from the breakage of fiber during hydroentanglement.⁴ Thus the increased amount of fiber wax could explain why there is an apparent increase in hydrophobicity observed with the 60% UC/40% PES over the 40%UC/60% PES, and the additional wax may enhance polyester/cotton associations in the web. However both charge and degrees of structural order affect the electrokinetic results of a material, and there have been few studies that closely compare the relative contributions of these on electrokinetic parameters in singular materials.

Comparisons of cotton/synthetic blends with absorbent products

In Figure 4, the fabric swelling/moisture uptake and the surface polarity (ζ_plateau) values of incontinence coverstock and acquisition/distribution layers compare favorably with the cotton/synthetic blends of this study. However, the densities of the commercial absorbent layers are considerably less than the blends with the exception of an acquisition layer for a moderate incontinence product where the density is very similar to 100% UC,⁸ i.e. the 100% UC matches the swelling capacity, fiber polarity, moisture uptake, and as previously shown the density of a commercial moderate incontinence acquisition layer. In addition the similarity of the isoelectric points (IEPs) among the UltraClean Cotton samples is consistent with the composition of the samples having cotton or cellulose.
Figure 4.
A comparison of swelling and fabric polarity properties of absorbent incontinence product coverstock, acquisition and distribution layers: (a) swelling per moisture uptake (Δζ/%MC);(b) zeta plateau (ζ _plateau ). Absorbent product layers are aligned in sequence in keeping with the design (see Figure 4) coverstock/acquisition/distribution in A–F: (A) coverstock and acquisition/distribution layer of heavy incontinence product; (B) coverstock, acquisition, and distribution layer of heavy incontinence product; (C) coverstock, acquisition, and distribution layer of moderate incontinence product; (D) coverstock, acquisition/distribution layer of moderate incontinence product; (E) coverstock, acquisition/distribution layer of light incontinence product; F) coverstock, acquisition, distribution layer of incontinence bed pad product; G–I are layers of absorbent incontinence prototypes: (G) 3-layer sandwich design to promote wicking: 60%UC/40%NYL, 100% UC, 60%UC/40%NYL; (H&I) coverstock/ADL moderate incontinence (H) 40%UC/60%PES & 100% UC, (I) 60%UC/40%PES & 100% PES.

The percent moisture content (%MC) and swelling (Δζ), which range from 1.2 – 8.2% and, 0.065 – 0.121 , respectively, are characteristic of some absorbent materials found in incontinence products (Figure 4).⁸ The rate of swelling (k (min^-1) as derived from equation 2 and shown in Table 1) is more rapid for the polyester-containing materials. The fiber polarity (ζ_plateau) for the cotton/synthetic blends is more hydrophobic with the two blends that contained 60% UC, which is consistent with the material swelling properties discussed above.

The fiber polarity (ζ_plateau), moisture uptake (%MC) and swelling (Δζ) found in the cotton/synthetic blends have some similarity to those found in coverstock and acquisition layers from commercial products. Electrokinetic results for commercial absorbent products contrasted in Figure 4 demonstrate some of the similarity of swelling capacity and fabric surface polarity with the cotton nonwoven materials of this paper. The electrokinetic properties for the coverstock and acquisition/distribution layers have previously been related to fluid transport from uptake of urine by the coverstock and transport within the layers of absorbent products (11). In Table 3 the results of the AATCC Drop Test using artificial urine showed that 100% UC wicks urine more rapidly than UC that was scoured and bleached for purposes of comparison. The wicking time of the Ultra Clean/Polyester blends was also decreased significantly with artificial urine versus distilled water. Figure 4 demonstrates the comparison in terms of swelling capacity per moisture uptake and fabric surface polarity (ζ_plateau). Comparison of the swelling capacity of coverstocks and ADLs utilized for varying levels of incontinence show a 2–4 fold difference in swelling (Δζ). For example, the distribution layer (layer directly in contact with the absorbent core) of a heavy incontinence product has a 4-fold greater swelling capacity (Δζ), than the distribution layer in a light incontinence liner. This increased swelling found in a heavy incontinence product allows for a high volume and even fluid distribution required to prevent gel blocking and rewet in the diaper. The percent moisture uptake and swelling values for the coverstock of a heavy incontinence product is within the range of those found in this study as seen with the 40%UC/60% PES and 100% UC.

Table 4 outlines some putative applications of the blends as absorbent incontinence layers. The pairing of two different greige cotton materials in Table 4 as coverstock/ADL is based on a fiber polarity and swelling relationship that might be conducive to fluid transport as diagramed in Figure 5 where polar gradients are shown between a coverstock and acquisition/distribution layer are diagrammed. On the other hand, the percent moisture uptake and Δζ values of the light incontinence liners exhibit a pattern where the coverstock, contact and distribution layers are similar and thus interchangeable among products.⁸ The relationship of swelling and moisture uptake is based on a ‘sandwich-like’ polar gradient design: in light incontinence products a hydrophilic/hydrophobic/hydrophilic; coverstock/acquisition/distribution layer charge arrangement exists. This pronounced amphiphilic gradient promotes rapid wicking and fluid uptake into an acquisition layer that swells rapidly. As shown in Figure 4a, three types of coverstock, acquisition, and distribution layers fall within the range of moisture uptake and swelling seen for light incontinence liners.
Figure 5.
Diagram of absorbent layers including coverstock, acquisition, and distribution layers, and the range of ζ_plateau values that are indicative of the polar gradient that has been identified between the layers⁸.

Table 4.
Cotton/Synthetic Blends listed as per fiber surface properties that are applicable to areas of incontinence product layers

Potential incontinence prototype Coverstock/ADL Coverstock Acquisition layer Distribution layer

Heavy 40%UC/60% PES and 100% UC NR NR NR

Moderate 60%UC:40%PES/ 100% UC NR NR NR

Light 60% UC/40% NYL 60% UC/40% NYL 100% UC 40%NYL/60%NYL

As discussed above, the fiber polarity of moderate absorbent liners aligns with some of the materials of this study and especially for the 100% UC. However, swelling and high hydrophilic character are sometimes a trade-off, i.e., there may be less swelling of an absorbent layer with increased hydrophilicity. On the other hand as shown in Figure 4a the 100% UC compares favorably in its swelling and percent moisture uptake with the acquisition layers of moderate absorbent incontinence liners.

Cotton/nylon-containing blends

The 60% UC/40%NYL and 40% UC/60% NYL also exhibited characteristic moisture uptake and swelling similar to light incontinence coverstock material. It is noteworthy that although nylon is not found in most wearable incontinence materials it has been used and studied for its low friction applications in materials where lowering skin friction at the skin-clothing interface is desired,²⁷ i.e. sportswear and decubitus prevention bedding, clothing and dressings.^29,30 The higher contact angles for the nylon blends, which indicate a more hydrophobic surface, also suggest a potentially lower friction surface which has recently been reported in cotton textiles.³¹

Conclusion

This paper has focused on the application of an electrokinetic approach to characterize the fiber surface properties of greige cotton/synthetic blends. Greige cotton cleaned to make UltraClean cotton, blended with either polyester or nylon and formed into hydroentangled nonwoven fabric demonstrated absorbent and swelling properties while having significant hydrophobicity. Thus the use of cleaned greige cotton with retention of some of the waxes and pectin native to the cotton fiber, which are removed during traditional scouring and bleaching of cotton, in combination with aqueous-surface exposed cellulose from the hydroentanglement process confers varying amphiphilic properties to the cotton fiber. When combined with synthetic fibers, a higher swelling more hydrophobic material is produced. As discussed in this paper some of the fiber surface polarity found with UltraClean Cotton and its synthetic blends are similar to those found in the layers of commercial incontinence materials that surround an absorbent core. Though this study has focused on the corollaries of the relative fiber and fabric surface properties between several types of cotton/synthetic blends and commercial incontinence layers, there are potentially other nonwoven applications including wipes, dressings and blood contacting materials which share similar profiles of absorbency and material surface properties. These studies show the potential to blend greige cotton with polyester and nylon and manipulate properties to control absorbency, swelling, charge, and polarity. This study has focused on some of the general characteristics of nonwovens that can be obtained with UltraClean greige cotton and blends. However, the extent to which the quantities of waxes and pectin can be retained on the fibers and manipulated has not yet been exhaustively studied. The ability to control levels of pectin and waxes on the surface of the cotton fiber by tailoring the hydroentanglement processes through pressure and/or water jet modifications would undoubtedly yield a wide spectrum of potentially attractive nonwoven products. This will be the subject of future studies.

Composition	Moisture content (%)	IEP pH	Plateau potential (mV)	Swell test k (min−¹)	ζ₀ (mV)	ζ_∞ (mv)	Δζ	Density (g/cm³)
100% UC	8.2	2.3 (e)	−26	0.0044	−47.7	−44.5	0.065	0.147
100% PES	1.2	3.8	−60	0.0098	−60.0	−52.0	0.133	0.118
0.0130	−73.3	−58.9	0.197
60%UC/% PES	4.6	2.6	−44	0.0140	−67.3	−60.4	0.102	0.110
40% UC/60%	3.2	2.7	−27	0.0130	−75.0	−66.3	0.115	0.112
PES	0.0073	−45.7	− 40.2	0.121
60% UC/40% NYL	5.7	2.2	−46	0.0088	−42.4	−38.6	0.091	0.117
40% UC/60%	4.5	2.4	−29	0.0087	−42.6	−38.6	0.091	0.133
NYL	0.0020	−51.6	−45.8	0.112

Fiber blend	Weight (g/m²⁾	AATCC drop test (sec.)	Drop test syn. Urine (seconds)	ASTM 1117 sink time (sec)	ASTM absorbency capacity (g H₂O per g of fabric)
100% UC	54.6	<1	4.8	40.8	4.97
100% PES	68.6	>60	>60	>300	0.14
60%UC/ 40%PES	75.4	>60	23.2	113.8	7.05
40%UC/ 60%PES	66.6	>60	19.4	241.0	7.97
60%UC/ 40%NYL	55.5	<1	8.3	57.1	7.04
40%UC/ 60%NYL	79.5	25.3	23.3	162.8	6.62
100% UC-SB	54.6	<1	24.1	2.62	6.35

Potential incontinence prototype	Coverstock/ADL	Coverstock	Acquisition layer	Distribution layer
Heavy	40%UC/60% PES and 100% UC	NR	NR	NR
Moderate	60%UC:40%PES/ 100% UC	NR	NR	NR
Light	60% UC/40% NYL	60% UC/40% NYL 100% UC	40%NYL/60%NYL

Footnotes

Funding

This work was supported by the Agricultural Research Service of the USDA.

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Coverstock layer (heavy)	91	87
Acquisition layer (heavy)	90	62
Acquisition layer (light)	81	ND
Coverstock (bed pad)	79	ND