Abstract
Hard surface cleaning and disinfection is critically important in environments where microbial contamination can result in the spread of food-borne illnesses and infectious diseases. Successful disinfection of a hard surface can be impeded by the presence of a soil load in the form of protein, fat or other material. Fibrous materials used as applicators for surface disinfecting solutions require careful consideration to ensure the active ingredient remains efficacious. Of equal importance are the mechanical and surface properties of the applicator including tensile strength, cleaning efficiency, and absorbency. Therefore, the successful disinfection of a hard surface can be greatly enhanced by an applicator that also effectively removes organic contaminants, is strong enough to resist structural failure, and reduces water waste when a disinfecting solution is applied. The cleaning and physical properties of nonwoven applicators in the form of disposable wipes composed of cotton, rayon, polyester, and blends thereof were evaluated with several assays. Dynamic wiping assays, surface roughness, and tensile testing were conducted to compare the physical and surface properties of the wipes. The removal of organic contaminants was found to correlate with higher wipe surface roughness that increased with cellulosic fiber content. Tensile testing demonstrated the greige cotton, which has not been subjected to processing such as scouring or bleaching, had increased wet strength while rayon wipes lost significant strength when wet. Greige cotton wipes were also produced as a sustainable product with reduced absorbency that would minimize water usage and waste. The results indicated that inclusion of greige cotton fibers in wipes offers improvements over currently available wipes without sacrificing strength or cleaning efficiency.
Introduction
Frequently contacted surfaces play a key role in the transmission of infectious pathogens, which is relevant to both foodborne illnesses and healthcare facilities. Contamination of food-contact surfaces is responsible for nearly 80% of all foodborne illnesses of both bacterial and viral origin including Escherichia coli and Listeria monocytogenes as well as several noroviruses.1–6 This is of profound consequence in healthcare settings including hospital rooms and intensive care units, where exposure to such pathogens can complicate treatment and lead to increased mortality rates.7,8 This cycle of transmission may be halted by the appropriate cleaning of such surfaces. The cleaning procedure for soiled surfaces consists of several steps including initial decontamination with soap and water to remove visible soils, rinsing and sanitization with antimicrobial solutions containing quaternary ammonium compounds, iodine or chlorine.9,10 The removal of organic debris is critical to the efficacy of the subsequent steps because such particles provide a nutritive source for pathogens, contributing to biofilm formation.11–14 Organic soils can also react with chlorine in chlorine-based disinfectants such that chlorine is sequestered and not freely available to act in a disinfecting capacity. 15 While protocols have been outlined for the sanitization process, less attention is devoted to the cloths used in this endeavor.10,16 Recently, the appropriate wipe material selection has been identified as an essential tool to reduce food contamination related illnesses and deaths.17,18 Both disposable and reusable cloths may be used; however, single-use wipes, especially nonwovens, are more popular due to the tendency of non-disposable cloths to harbor microorganisms, despite aggressive laundering.19,20
Most disposable cleaning wipes are composed of petroleum-based synthetic fibers such as polyester and polypropylene or regenerated fibers such as viscose rayon. 21 One of the major disadvantages of fully synthetic fibers including polyester, polypropylene and other thermoplastic materials is their resistance to degradation after use. 22 Polypropylene and polyester fabrics can take 20–200 years to fully decompose, respectively.23,24 These materials also contribute to microfiber pollution, a topic which is rapidly gaining momentum among increasingly eco-conscious consumers. Microplastic polymer fragments have been identified in soil, marine environments, individual organisms and even the air.25,26 Moreover, it has recently been suggested that in humans, the repeated inflammatory response elicited by microplastic irritation may contribute to cancer via DNA damage mechanisms. 27 Rayon fibers break down in as little as 2 months; however, the wood pulp used to manufacture rayon fibers is considered an non-renewable resource.28,29 Although some research has shown that cotton is incompatible for use with antimicrobial agents such as quaternary ammonium compounds due to ionic interactions, recent work has shown that these challenges are easily circumvented by the use of additive chemistries.30–32
The use of cotton fibers as a raw material for producing nonwoven textiles including wipes has increased dramatically over the past decade from less than 1% to approximately 2.4%. Based on market outlook reports, the compound annual growth rate (CAGR) of cotton fiber use in nonwovens continues to exceed the CAGR of the nonwovens industry and cotton use is predicted to increase to ~2.7% over the next several years. The increased use of cotton fibers in nonwovens is due in part to consumer demands for natural, sustainable, and biodegradable disposable products. There are multiple forms of commercially available cotton staple fibers for use in nonwovens including in order of ascending price per pound raw, mechanically cleaned raw, and scoured and bleached. Pricing on the latter two forms of cotton as well as man-made staple fibers for nonwovens is proprietary. Pricing on raw cotton fibers follows the commodity index and is currently priced lower than staple polyester fibers. Mechanically cleaned raw cotton fibers are typically priced competitively with polypropylene staple fibers, and scoured and bleached cotton fibers are typically priced competitively with specialty rayon fibers. A nonwovens manufacturer choice of raw materials for specific application require careful consideration regarding multiple factors including material properties, processing compatibility, raw material costs, and consumer response and willingness to pay possibly more for specific branding such as improved performance, natural, biodegradable, and so on.
Thus, it is worthwhile to investigate alternative wipe materials that offer the advantages of synthetically based fabrics without the associated disadvantages. Cotton-based nonwovens are readily biodegradable and provide an attractive option to this end, but more research is needed to fully investigate the performance of cotton based nonwovens.29,33,34 Previous investigations into the comparison of cleaning efficiency between different wipes materials are limited in terms of the variation in test conditions,35,36 sample size 16 or involve the use of antimicrobial agents17,37 which may complicate the relationship between the wipe material and its cleaning capacity. Recently, the efficacy of microfiber, cotton, sponge cloth and paper towels were compared before and after reprocessing. 16 Although the microfiber cloth initially performed the best, its efficacy precipitously deteriorated after laundering, restricting its utility to single-use applications, which is not economical. 38 Cotton performed sufficiently well and is practical for both disposable and non-disposable applications. Another investigation into the bacterial removal efficiency of various cloths determined that cellulose-/cotton-based fabrics were superior in comparison to microfiber and polyester cloths. 35 Therefore, it was of interest to evaluate the cleaning capacity of various nonwovens with particular attention given to their performance as a function of their cotton content. The physical properties of a selection of nonwoven wipes of varying staple fiber compositions and blends including a rayon, polyester, mechanically cleaned greige cotton, and scoured and bleached cotton were evaluated to assess their cleaning efficacy.
Methods
Staple fibers and nonwovens production
Staple fibers used in this study for in-house nonwoven fabric production included mechanically cleaned greige cotton fibers (True Cotton, TJ Beall Company, Greenwood, MS), polyester fibers (Consolidated Fibers Inc., Charlotte, NC), and viscose rayon fibers (Consolidated Fibers). Fiber characteristics such as staple length, fineness and finish are presented in Table 1. Staple fibers were chute fed to a 101.6 cm wide textile card fitted with Cardmaster plates (Saco Lowell), followed by feeding the card web into a commercial crosslapper and needlepunch (NP) machine (Technoplants srl., Pistoia, Italy). The NP processing parameters were as previously described. 39 The NP fabrics were processed into hydroentangled (H-E) nonwoven fabrics on a 1 m wide Fleissner pilot-scale hydroentanglement system (Trützschler Nonwovens GmbH, Dülmen, Germany) running at a constant production speed of 5 m min-1. The H-E system consisted of one low pressure head for fabric wet-out and two high-pressure heads for fiber entanglement. Each strip on the pressure heads consisted of ~20 orifices per centimeter with an orifice pore size of 120 µm. The water used for the H-E fabric production was ambient temperature, which was approximately 25°C. Following H-E, the fabrics were fed directly through a gas-fired fabric drying oven (Trützschler Nonwovens GmbH) at ~170°C and wound into rolls. Eleven nonwovens wipes, shown in Table 2, were used in this study. Ten of the wipes substrates were manufactured in-house as described above. The samples selected consisted of greige cotton blends including polyester and rayon to investigate the effect of increasing the ratio of greige cotton in the sample. These wipes consisted of the following: 80% rayon/20% mechanically cleaned greige cotton, 20% rayon/80% mechanically cleaned greige cotton, 80% polyester/20% mechanically cleaned greige cotton; 20% polyester/80% mechanically cleaned greige cotton, 100% polyester, 100% rayon, and 100% mechanically cleaned greige cotton scoured and bleached. The 100% scoured and bleached mechanically cleaned cotton was produced from the mechanically cleaned greige cotton nonwoven fabric that was scoured and bleached in-house using a JFO overflow jet dyeing system (Werner Mathis AG, Oberhasli, Switzerland) as previously described. 39 All the above fabrics were produced at an energy of hydroentanglement of 4.8 MJ kg-1 calculated as previously described. 40 Three different 100% mechanically cleaned greige cotton nonwovens were also produced with energies of hydroentanglement at 6.8, 8.9 and 10.1 MJ kg-1 to determine the relationship between energy of hydroentanglement and cleaning capacity. The commercially available wipes used in the study were Berkshire 50% rayon/50% polyester hydroentangled wipes (Berkshire Corporation, Great Barrington, MA), which is commonly found in single use wipes.
Summary of staple fibers used for in-house production of nonwovens roll goods converted into wipes materials.
N/A: not available.
Identification and description of each fabric sample.
Dynamic wiping assays
Stainless steel plates measured 12.7 cm x 12.7 cm and were composed of T-316/316 L 0.700 mm thick 24 gauge marine grade stainless steel (Onlinemetals.com, Seattle, WA). A M235 Martindale Abrasion Tester (SDL Atlas, Rock Hill, SC) was modified such that the clamp, padding and fabric on the lower assembly were removed and the stainless steel plate containing the organic contaminant of interest firmly affixed to the base. The upper assembly housing the cloth sample and foam padding was assembled as usual and a spindle with weights of either 9 or 12 kPa attached. The machine was set for a total of 16 movements, or one lissajous at a speed of 71.3 r/min (Figure 1).
Experimental design of dynamic wiping experiments. (a) Cross-section diagram of the Martindale apparatus indicating the relative positions of various components including fabric sample holder, weight, and stainless plate. (b) Diagram of the wipe sample holder and stainless steel plate used to simulate a contaminated hard surface. Also indicated is the lissajous wiping pattern of the sample holder.
Dynamic wiping of a simulated protein contaminant
To simulate a hydrophilic, proteinaceous hard surface contaminant, 500 µl of 5% fetal bovine serum in phosphate buffered saline (Sigma-Aldrich, St. Louis, MO, USA) was pipetted as a single spot on the center of the stainless steel plate dried at room temperature overnight. The stainless steel plates were affixed to the lower abradant holder of the Martindale instrument. The fabric samples were conditioned for 24 h at 21.1°C and 65 ± 2% relative humidity, placed in the sample holders, and the 9 kPa weights attached to the spindles. The wiping procedure was replicated 3 times for each fabric, with one control sample both dry and with the application of 1 mL of ultrapure water. For the wet trials, a spot of ultrapure water was pipetted halfway between the soil in the center of the plate and the cloth sample in the upper right hand corner. To determine the removal of fetal bovine serum, after wiping the fabric samples were placed in 5 mL of phosphate buffered saline and incubated statically at room temperature for 20 min before being placed on a rocker platform at 12 r/min for 5 min. Aliquots of 1 mL for each sample were collected for protein analysis. The samples were analyzed in 96-well microplate using the Quick Start Bradford Protein Assay (Bio-Rad, Hercules, CA, USA) with 4 technical replicates for each sample. The wet samples were diluted 1:10 in phosphate buffered saline and 100 µL of the diluted sample was added to 150 µL of Bradford dye reagent. For the dry samples, 100 µL of the undiluted sample was added to 150 µL of Bradford dye reagent. All samples were incubated for 5 min in Corning clear polystyrene 96-well flat bottom microplates (Fisher Scientific, Waltham, MA, USA) at room temperature with gentle agitation. The binding of the dye to protein in the sample was detected by measuring absorbance at 595 nm in an AccuSkan GO microplate reader (Fisher Scientific, Waltham, MA, USA). Known concentrations of bovine serum albumin were used to create a standard curve for the assay.
Dynamic wiping of a hydrophobic residue
A 50 ± 2 mg paraffin wax bead (Tyco Healthcare/Kendall, Hampshire, UK) was placed on the center of the stainless steel plate and the plate placed in an oven at 23.9°C for 10 min. The melted paraffin was formed into a 25 cm2 square in the center of the plate. The paraffin was allowed to dry on the plate at room temperature for 30 min before being affixed to the lower abradant holder of the Martindale. The fabric samples, which were conditioned for 24 h at 21.1°C and 65 ± 2% relative humidity, were placed in the sample holders and 12 kPa weights attached to the spindles to simulate the average force used in wiping applications. Four replicates were taken for each fabric sample. The results of the wax removal assay were quantitated gravimetrically using an analytical scale to the thousandth place.
Statistical analyses
Statistical analyses of the hard surface protein and wax removal measurements for the various fabrics were performed using a one-way analysis of variance (ANOVA) with multiple comparison testing equating all other wipes to the commercial wipe composed of 50% rayon/50% polyester. The p-values were corrected for multiple comparison testing using Dunnett’s test and significant differences were determined at the 95% confidence interval (adjusted p-value = 0.05).41,42 The correlation of the fabric surface roughness with hard surface cleaning efficiency was determined by Pearson correlation coefficient at the 95% confidence interval (p-value = 0.05). All statistical analyses were performed using GraphPad Prism 7 for Windows Version 7.02 (GraphPad Software, La Jolla, CA).
Surface roughness
The surface roughness of each fabric was determined using a Hirox KH-8700 digital microscope (Hirox-USA, Inc., Hackensack, NJ) with a model MXB-2500REZ triple objective turret zoom lens. Three-dimensional images of each fabric surface were obtained at 35x magnification which allowed for surface roughness measurements from an area of approximately 52 mm2. A total of 14 measurements were obtained from each image with 7 in cross-direction and 7 in machine-direction equidistant from each other. The measurements were used to calculate the surface roughness described by JIS B 0601 (2001) as arithmetical mean roughness (Ra), maximum height roughness (Rz) and 10 points mean roughness (Rzjis) as shown in equations (1) to (3). 43 For each fabric, five distinct areas were measured for a total of 70 measurements for each fabric
where ℓ is the length, Yp is the highest point, and Yv is the lowest point
Nonwovens fabric testing
Standardized testing was carried out by the Textile Technology Center (Belmont, NC, USA) using an Instron model #5566 (Instron Corp., Norwood, MA) according to ASTM D3776 (fabric weight), ASTM D5034 (grab strength), ASTM D5035 (wet and dry strip strength), and ASTM D2261 (tongue tear) at 21°C and 65% ± 2% relative humidity in compliance with the standard conditions stated in ASTM D 1776/D1776M-16.44–48 The thicknesses of the fabrics were measured using a Hanetek Precision Thickness Gauge Model FT3 (Rhopoint Instruments, East Sussex, UK) and reported as the mean of five measurements taken at different locations on each of the wipes materials. The absorbency of the fabrics were determined according to the protocol described in AATCC Test Method 79-2014. 49
Water contact angle
Static contact angles for each fabric sample were measured using a VCE Optima XE (AST Products, Billerica, MA) Contact Angle Measurement System. For each sample, either a 1 or 5 µL drop of deionized water was applied directly to the fabric, the droplet image immediately captured and the contact angle measured. Contact angle measurements were taken for both 1 and 5 µL volumes with 10 repetitions for each.
Results and discussion
Dynamic wiping
The affinity of a contaminant for a surface is largely a function of their respective surface energies, which are determined by the dispersive and polar components of each entity. Stainless steel surfaces, which are commonly found in the food service industry, are considered to have higher surface energies and possess a unique affinity for both hydrophilic and hydrophobic residues such as proteins and fatty acids. 50 In order to remove such contaminants, the adhesive interactions between the surface and the residue must be abolished. 51 The reduction of these adhesive forces may be induced by several means including soil solubilization, emulsification, micelle formation, or mechanical action. 50 While any combination of these mechanisms may be effective, it was of particular interest to determine the effect of cloth fiber composition in the disruption of these adhesive forces. Toward this end, two types of surface contaminants were used separately in dynamic wiping studies to mimic deposits found on food contact and medical environment surfaces. Fetal bovine serum was used to mimic an organic, protein contaminant and paraffin was used as a hydrophobic, waxy residue.
The dry wiping technique, with no added solvents, surfactants, or antimicrobial agents, relies purely on the mechanical force. Figure 2(a) shows that greige cotton 6.8 MJ kg-1 removed the most protein and is statistically different from the 50% rayon/50% polyester commercial wipe. Other samples which performed comparatively to the commercial wipe include rayon, greige cotton 10.1 MJ kg-1, scoured and bleached cotton, 20% rayon/80% greige cotton, and 20% polyester/80% greige cotton. Greige cotton 8.9 MJ kg-1, 80% rayon/20% greige cotton, 80% polyester/20% greige cotton, and polyester were significantly different from the commercial wipe, with polyester removing only a negligible amount of protein compared to the others. In most cases, the cleaning capacity of the individual fabrics is inversely proportional to their polyester content. The polyester fibers used in the production of our nonwoven wipe sample is composed of smooth, uniform, and semi-crystalline fibers which generate inadequate frictional forces to overcome the adhesion of the dried protein residue to the stainless steel plate. It may be noted that although many synthetic fibers, such as polyester, may receive some degree of chemical finishing, the finish status of the fibers we used was not disclosed. When cotton is incorporated, such as the case with the polyester/greige cotton blends, the amount of protein removed increases with the proportion of cotton. For instance, including 20% greige cotton into a polyester cloth increases the amount of protein removed nearly fourfold.
Results of dynamic wiping experiments and surface roughness correlation. Four replicates were produced for each experimental trial. (a) Hard surface contaminated with fetal bovine serum wiped dry. (b) Hard surface contaminated with fetal bovine serum wiped after application of distilled water to the surface. (c) Hard surface contaminated with paraffin wax wiped dry. Surface roughness of the various samples calculated as (d) arithmetical mean Ra, (e) maximum height Rz, and (f) ten points Rzjis. In (a), (b), and (c) Pearson correlation coefficients and p-values for each roughness measurement are displayed in the lower right of the graphs. 1: commercial wipe 50% rayon/50% polyester, 2: rayon, 3: polyester, 4: Greige Cotton 6.8 MJ kg-1, 5: Greige Cotton 8.9 MJ kg-1, 6: Greige Cotton 10.1 MJ kg-1, 7: scoured and bleached cotton, 8: 80% polyester/20% greige cotton, 9: 20% polyester/80% greige cotton, 10: 80% rayon/20% greige cotton, 11: 20% rayon/80 greige cotton. Data points that are shaded gray were statistically different than the commercial wipe as determined by one-way ANOVA with multiple comparison testing.
The effects of soil solubilization are introduced when a solvent, such as water, is incorporated into the system. Since cleaning solvents are usually aqueous, the hydrophobicity of the wipe material can play a crucial role in cleaning performance. In this case, the organic load is hydrophilic, and possesses a greater affinity for water than for the stainless steel. One might assume that this action alone would lead to soil removal. However, the physiochemical and surface properties of the wipe material must also be taken into consideration. Since the ultimate goal of any cleaning endeavor is to transfer a contaminant from the soiled surface to the fibers of the wipe material, when aqueous solutions are employed, water absorption by the wipe material may also play a role in wet wiping efficiency through the concomitant absorption of water solubilized matter. Wet wiping trials (Figure 2(b)) were carried out in an identical manner to the dry wiping trials with the addition of 1 mL of ultra-pure water applied just prior to wiping. In the wet wiping trials, the 80% rayon/20% greige cotton sample removed the most protein, but was not significantly different from the 50% rayon/50% polyester commercial wipe, rayon, greige cotton 8.9 MJ kg-1, or scoured and bleached cotton. Similar to the dry wiping trials, polyester and the polyester/greige cotton blends removed less protein than the other samples. This behavior is rationalized by the non-abrasive nature of its fibers as well as the inherent lipophilicity of polyester. 52 In conjunction with the smoothness of the fibers, the addition of a lipophobic solvent exacerbates the poor performance of polyester.
Greige cotton 6.8 MJ kg-1 was also observed to be less efficacious in wet wiping soil removal trials than the 8.9 and 10.1 MJ kg-1 greige cotton fabrics. This observation is likely explained by the key difference among these samples; namely, the energy of hydroentanglement used in their production. The greige cotton 6.8 MJ kg-1 was subjected to lower water jet pressures and thus, experienced a lower energy of hydroentanglement compared to the 8.9 and 10.1 MJ kg-1 samples. As a result of these varying energies of hydroentanglement, low molecular weight hydrocarbons are removed from the fibers. 40 Thus, increasing the pressure of the water jets increases the removal of lipophilic contaminants and increases the hydrophilic character of the cotton fabric structure, explaining the propensity of the more hydrophilic 8.9 and 10.1 MJ kg-1 cotton fabrics for removing protein under wet conditions, compared to the more hydrophobic 6.8 MJ kg-1 cotton fabric.
In order to investigate the activity of these cloths toward the wiping of an unctuous, hydrophobic soil, paraffin wax was melted onto stainless steel plates. In these trials (Figure 2(c)), the greige cotton 10.1 MJ kg-1 performed the best, removing nearly 9 mg of paraffin wax. The success of the greige cotton 10.1 MJ kg-1 is likely attributed to the increased fibrillation generated by the increased energy of hydroentanglement, providing sufficient friction to disrupt the affinity of the paraffin wax for the stainless steel plate. The increased fiber surface area which occurs with fibrillation may also increase the amount of soil that is able to be removed. The greige cotton 6.8 MJ kg-1, 20% polyester/80% greige cotton, and polyester were significantly different from the commercial wipe in that the greige cotton 6.8 MJ kg-1 and the 20% polyester/80% greige cotton performed better than the commercial wipe, and the polyester sample performed worse than the commercial wipe. Since mechanical force applied via the fabric sample is the primary means used to disrupt the adhesion between the wax and the stainless steel, the effects of polarity are minimized in this case. Therefore, as observed previously in both of the protein wiping experiments, polyester performs the poorest, in this case, due to the smoothness of its fibers and non-abrasive properties of its fabric structure.
Surface roughness and surface morphology
The surface roughness of each fabric sample (Figure 2(d) to (f)) was assessed using a 3-D digital microscope. Statistically, most of the samples were comparable in terms of surface roughness, with the exception of scoured and bleached cotton, 20% greige cotton/80% polyester blend and polyester. The scoured and bleached cotton sample is expected to be smoother than the other cotton samples since it has been scoured to remove impurities. In the polyester/greige cotton blends, it is evident that as the percentage of polyester increases, the roughness decreases. This observation may be a result of the twist, or gyre, of the fibers. Polyester fibers are highly uniform, smooth fibers with minimal twisting whereas cotton and rayon fibers have prominent twists, perhaps leading to increased surface roughness. In the rayon/greige cotton blends, increasing the percentage of greige cotton from 20% to 80% results in an increase in roughness. Among the greige cotton samples made at different pressures, the highest pressure 10.1 MJ kg-1 cotton was the roughest, which is most likely a result of fibrillation generated by the increased pressure from the water jets. Interestingly, the roughness measurements follow similar trends as the dynamic wiping data. The strongest correlation between roughness and dynamic wiping is seen in the paraffin trials in Figure 2(a) (Ra = 0.846, p = 0.001; Rz = 0.827, p0.002; Rzjis = 0.819, p = 0.002). Such an observation is rationalized by the tackiness of the paraffin wax, which results in increased friction and thus, increased removal for the roughest surfaces. The dry wiping for the protein residue in Figure 2(b) were also in good agreement with the roughness data (Ra = 0.709, p = 0.015; Rz = 0.702, p = 0.016; Rzjis = 0.704; p = 0.16). However, the wet wiping trials for the protein residue showed a slightly weaker correlation (Ra = 0.680, p = 0.021; Rz = 0.676, p = 0.022; Rzjis = 0.662, p = 0.026), which is easily explained by the additional variables introduced by water.
Examination of the fabric samples at 35x magnification showed a striking uniformity in fabric construction, even between the commercial sample (Figure 3(a)) and the in-house generated samples (Figure 3(b) to (j)). At this magnification, the difference between the higher pressure 10.1 MJ kg-1 cotton (Figure 3(e)) and the lower pressure 6.8 MJ kg-1 cotton (Figure 3(d)) is evident in the larger apertures and overall rougher appearance of the higher pressure cotton. Polyester fibers are easily identified by their glossy, lustrous appearance compared to the dullness of the cotton fibers in the polyester/greige cotton blends (Figure 3(g) and (h)), in agreement with the data obtained from the dynamic wiping assays. Overall, it is clear that increasing the amount of cotton results in an increase in surface roughness and cleaning capacity, regardless of the hydrophobicity of the residue.
Digital microscope images of fabric samples at 35x and 200x magnification; (a) commercial wipe 50% rayon/50% polyester, (b) rayon, (c) polyester, (d) Greige Cotton 6.8 MJ kg-1, (e) Greige Cotton 10.1 MJ kg-1, (f) scoured and bleached cotton, (g) 20% polyester/80% greige cotton, (h) 20% polyester/80% greige cotton, (i) 80% rayon/20% greige cotton and (j) 20% rayon/80% greige cotton. In each panel, the 35x image on the left the black scale bar = 2000 µm, and in the 200x image on the right the scale bar = 500 µm.
Physical and mechanical properties of the nonwoven fabrics
The results of standardized textile testing including weight, thickness, absorbency, and tensile strength are shown in Tables 3 and 4. Strength is a key determinant of a fabric’s success as a wipe material, since it the wipe material will experience some degree of force during the wiping process. Among the 100% cotton samples, the greige cotton 8.9 MJ kg-1 has the highest breaking strength for both the grab and tongue tear tests. The 100% polyester and the polyester/greige cotton blends are the strongest materials for the grab and tongue tear tests. For the 100% rayon and rayon/greige cotton blends, the 80% rayon/20% greige cotton has the highest strength for the grab test. In the tongue tear test, the inverse ratio of 20% rayon/80% greige cotton is the strongest. Interestingly, for both the tongue tear and grab tests, the 50% rayon/50% polyester sample is nearly 50% stronger in the machine direction than in the cross direction. This anisotropic behavior is explained by the fiber orientation ratios used during carding, meaning that more fibers are orientated in the machine direction, sharing the load exerted upon them. Therefore, the fabric is stronger in that direction. 53
Results of standardized textile testing to determine physical and mechanical characteristics of the various nonwoven fabrics.
MD: machine direction; CD: cross direction; SD: standard deviation.
Physical characteristics of nonwoven materials; SD in parenthesis where applicable.
Since most wipes are used wet, the absorbency and wet strength of a cleaning cloth is important. Generally, greige cotton gains strength when wet, except in the case where the energy of hydroentanglement is maximized, due to weakened fibers which cannot endure the same forces as their lower energy of hydroentanglement counterparts. 31 The breaking force for greige cotton 6.8 MJ kg-1 increases by 0.823% in the machine and 9.15% in the cross direction when wet. In greige cotton 8.9 MJ kg-1, the strength increases even more to 21.4% in the machine and 29.8% in the cross direction. For the highest energy, mechanically cleaned 10.1 MJ kg-1, the sample loses strength in the wet strip test by 25.5% in the machine direction and 21.1% in the cross direction. The greige cotton 8.9 MJ kg-1 sample, which has the greatest wet strength increase, when scoured and bleached, loses strength. The loss of wet strength in the scoured and bleached sample is explained by the observation that scouring and bleaching removes protective waxes and other components from the cotton fibers, making them more susceptible to water penetration, weakening the hydrogen bonds which impart strength. This observation is supported by the results of the absorbency drop test, presented in Table 4. Greige cotton 8.9 MJ kg-1, which has an absorbency time of 8.2 s and a contact angle measurement of 80°, when scoured and bleached, has an absorbency time of 0 s and thus, no measurable contact angle.
It is well-documented that rayon loses strength when wet, which our results also indicate. 54 In the greige cotton/rayon blends, where the majority component is rayon, the fabric loses strength when wet. However, when the majority component is greige cotton, the inverse is true. This phenomenon may be explained by the cellulose allomorphs present in each fabric sample. Although both greige cotton and rayon are both cellulosic materials, greige cotton contains cellulose I, which is more crystalline, restricting water access to amorphous regions. Regenerated fibers such as rayon are composed of cellulose II, which forms hydrates in both its amorphous and crystalline regions. When hydrates form, intermolecular bonds are pushed apart and weakened. 55 For this reason, rayon exhibits decreased wet strength compared to greige cotton, which exhibits increased wet strength due to low water penetration leading to increased load sharing between fibers. Polyester also loses strength when wet, but only in the machine direction. Similarly to the rayon/greige cotton blends, the wet strength of the polyester/greige cotton blends increases with the proportion of greige cotton. The results of the wet and dry strip tests suggest that greige cotton is a suitable material for wet cleaning applications since it is not only effective, but also maintains structural integrity.
Contact angle (Figures 4 and 5) and absorbency drop test results (Table 3) show that the presence of rayon contributes to the hydrophilic character of the wipe sample. For instance, samples such as 50% rayon/50% polyester, rayon, and 80% rayon/20% greige cotton are immediately absorbed in the drop test and form no contact angles with water. However, the 100% polyester and 20% rayon/80% greige cotton samples have higher contact angles of 137° and 89°, respectively, and show decreased absorbency in the drop test compared to the samples where rayon is the majority component. This behavior supports the idea that rayon experiences decreased wet strength because the water molecules are able to form hydrates within the structure, weakening the fabric.
Images of the water contact angles of selected samples (3) Polyester (4) Greige Cotton 6.8 MJ kg-1 (5) Greige Cotton 8.9 MJ kg-1 (8) 80% polyester/20% greige cotton (9) 20% polyester/80% greige cotton (11) 20% rayon/80% greige cotton. Other samples (1) 50% rayon/50% polyester (2) rayon (6) Greige Cotton 10.1 MJ kg-1 (7) Scoured and bleached cotton (10) 80% rayon/20% greige cotton not shown were immediately wetted by the water droplet.
Water contact angles (1 and 5 µL) of selected samples (1) polyester (2) Greige Cotton 6.8 MJ kg-1 (3) Greige Cotton 8.9 MJ kg-1 (4) 80% polyester/20% greige cotton (5) 20% polyester/80% greige cotton (6) 20% ayon/80% greige cotton. Other samples not shown were immediately wetted by the water droplet.
Another trend which is evident is the effect of energy of hydroentanglement on the greige cotton nonwovens. As the energy of hydroentanglement increases, the contact angles decrease. For example, Greige Cotton 6.8 MJ kg-1 has the highest contact angle of all of the greige cotton samples at 139°, while Greige Cotton 8.9 MJ kg-1has a contact angle of 80° and the contact angle of Greige Cotton 10.1 MJ kg-1cannot be measured since the drop is immediately absorbed. A similar trend is evident in the absorbency tests where Greige Cotton 10.1 MJ kg-1takes greater 2.6 s to absorb the water droplet, Greige Cotton 8.9 MJ kg-1 takes slightly longer at 8.2 s and Greige Cotton 6.8 MJ kg-1 takes over 60 s, demonstrating that, like the contact angle, absorbency is inversely proportional to the energy of hydroentanglement.
Conclusion
Effective removal of contaminants from food-contact and healthcare environment surfaces is a critical factor in sanitizing techniques and the selection of an appropriate applicator can determine the success or failure of this endeavor. The majority of wipes materials used for this purpose are composed of synthetic fibers such as polyester or rayon. The cleaning capacity of greige cotton, and its polyester and rayon blends as well as scoured and bleached cotton was investigated to identify a more environmentally friendly, cost-effective alternative. Dynamic wiping studies showed that increasing the proportion of cotton increased the removal of both hydrophobic and hydrophilic residues from stainless steel surfaces, while the inverse was true for polyester. Greige cotton also shows excellent wet strength, a key feature of successful wipe materials. These results suggest that greige cotton is an improvement over synthetically derived wipes materials in terms of cleaning capacity.
Footnotes
Declaration of conflicting interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The author(s) received no financial support for the research, authorship, and/or publication of this article.
