Nonwoven disinfecting wipes play a key role in hard surface cleaning and decontamination in household and healthcare or institutional applications. According to the recently published INDA North America Wipes Market Trends & Forecasts 2012-2017 report, North American sales of household hard surface disinfecting wipes were estimated to be $811 million at the retail level in 2012 and are forecast to increase 8% a year to reach $1.9 billion in sales by 2017. The smaller but significant market of healthcare and institutional disinfection wipes had an approximate value of $114 million in sales to end users in 2012 and is expected to grow at a rate of about 6% to a sales level of $152 million by 2017.
Hard surface wipes, by their purpose, must be multifunctional and engineered to scrub, clean and absorb contaminants while delivering one or more disinfecting chemistries onto surfaces. A balance of properties is required in wipe solutions to both promote surface wetting, cleaning and to achieve broad-spectrum kill functionality without cross-contamination. The delivery of disinfection chemistries onto surfaces, however, is the primary function of these wipes and is part of a recommended regimen or protocol for effective surface decontamination.
Despite advancements in disinfection chemistries and protocol specification, challenges remain in facilitating rapid and complete disinfection across a range of surfaces and decontamination environments. Hard surfaces are often porous and are more difficult to completely wet out with the wipe solution. Some commonly used disinfection chemistries such as quaternary ammonium salts have inherently poor wetting and emulsion characteristics. Studies indicate that disinfection can require a contact time of four to 10 minutes, which may be an inconvenient or non-practical period for some cleaning situations. Disinfectants are also subject to deactivation from surface contaminants such as blood serum, animal waste and fats and wipe components including anionic surfactants and certain fibers with charged surfaces. More specifically, negatively surface charged fibers, including viscose and cotton, have been shown to bind up to 40% of the positively charged quaternary ammonium salt therefore limiting disinfection performance and wasting valuable amounts of the wipe active.
While there are over a dozen chemical classes of biocides, the majority are cationic surfactants such as quaternary ammonium salts referred to in the industry as “quats.” One of the most common quats is alkyl-dimethyl-benzyl-ammonium chloride or ADBAC. It provides broad spectrum activity against bacteria, fungi, algae and certain viruses. Quats denature or modify the structure of the proteins of the bacterial or fungal cell, which affects the metabolic reaction within the cells and causes vital substances to leak out, causing cell death. During the inactivation of bacterial cells, the quaternary ammonium group remains intact and retains its antimicrobial ability.
Excessive binding of ADBAC and other cationic biocides by cotton and rayon has led to broad use of polyester nonwoven wipes for disinfection uses. However, nonwoven wipes containing only polyester can exhibit poor absorbency, limited surface cleaning and dirt pickup as well as non-uniform solution pickup per wipe in stacks. By contrast, cotton fiber nonwovens offer very good absorbency, solution distribution uniformity in stacks, good wet strength and unique hard surface scrubbing properties. The scrubbing and absorbency properties are attributable to the presence of convolutions and complex porous structure (see Figure 1) of cotton fibers, which lead to a high coefficient of friction and superior surface scrubbing. Cotton fibers also are cost effective and are natural, biodegradable fibers with environmental benefits.
The USDA Southern Regional Research Center in New Orleans, LA, in collaboration with Cotton Incorporated, recognized the opportunity for advantaged, cotton nonwoven disinfecting wipes if the ADBAC binding problem could be eliminated. As a result, a USDA research project was initiated in 2010 with the goals of examining the effect of wet wipe solution chemical and physical properties on the level of ADBAC adsorption on cotton nonwovens and various other benchmark fiber nonwoven webs. The contribution of fiber surface characteristics of cotton nonwovens was also studied by selecting cotton nonwovens with several stages of post-processing treatments including unbleached—also known as greige—scoured and bleached webs.
The overall research goals included: 1) identification of all the significant mechanisms impacting quat active delivery from cellulose in comparison to polyester-based nonwovens; 2) elimination of cotton binding of ADBAC through modification of conventional wet wipe formulation chemistry; 3) development of a prototype formulation using simple, EPA-friendly chemistries and a common industry quat biocide; and 4) demonstration of enabling technology for the nonwoven industry and the associated technical understanding required to leverage cotton fibers in advantaged disinfecting nonwoven wipes.
Laboratory experiments were conducted using a matrix of 50-70 gsm hydroentangled nonwovens containing unbleached cotton, bleached cotton, rayon, polyester and cotton/polyester blends from webs prepared on site at the Southern Regional Research Center. The nonwoven samples were immersed in an aqueous bath of ADBAC biocide containing commercial wet wipe formulation ingredients. ADBAC was removed using a depletion method and any adsorption of the ADBAC on the surface of the fibers was measured via UV-vis spectroscopy. Pectin content was assessed using a Ruthenium Red stain and high-resolution camera image analysis. Solution experimental variables of concentration, time, liquor ratio, pH, temperature, electrolyte concentration, substrate pretreatment and co-formulation with other surfactants, small quats and alcohols were evaluated.
Adsorption of ADBAC on cotton nonwovens can be attributed to electrostatic interactions, dispersion forces and hydrophobic interactions. Preliminary findings showed that rayon and unbleached cotton adsorbed more ADBAC than bleached cotton, while there was negligible adsorption in polyester nonwoven webs. An increase in the adsorption of ADBAC with an increased level of both unbleached and bleached cotton was observed in various cotton nonwovens blended with polyester fibers as shown in Figure 2.
This trend combined with other observations indicates the binding mechanism of ADBAC on cotton surfaces is electrostatic interactions. Adsorption of ADBAC in cotton was also found to be dependent on the amount of pectin present and to a lesser degree on the presence of natural waxes. Wet wipe formulations containing nonionic surfactants, small quats, electrolytes and ethanol showed an optimal reduction in ADBAC adsorption on cellulosic fabrics comparable to the polyester control. Figure 3 shows this reduction in ADBAC retention in unbleached and bleached cotton webs versus a polyester substrate with the unmodified (control) and several modified disinfection solutions.
Nonionic surfactants and small quats had the highest impact on the observed reduction. At a constant surfactant concentration, the liquor ratio, pH, temperature and electrolyte concentration in the solution were found to have a second level effect on ADBAC binding.
The following figures show the level of ADBAC exhausted as a function of the concentration of the co-formulated nonionic surfactant, polyethylene-oxide 12 (Figure 4) and small quat, tetramethyl ammonium chloride (Figure 5).
With these results, optimized biocidal solutions were then scaled up to match commercial formulations. The solutions were added at a wet pickup of about 1.5 times the weight of 8” x 12” substrate samples and packaged in airtight bags for external lab testing of biocide stability and efficacy. The optimized formulae were shown to inhibit excessive adsorption over 21 days of storage. The USDA and Cotton Incorporated have teamed up and are working with an external lab to quantify the performance of the laboratory nonwoven samples using industry standard testing procedures (ASTM E2362-09).
In summary, the modified USDA wet wipe disinfection formulations containing a common ADBAC biocide were shown to not bind the quat in a 100% hydroentangled bleached cotton fiber nonwoven wipe. Adsorption of the ADBAC was controlled by varying the chemical properties of the prototype wet wipe surfactant solution. Added electrolyte and co-formulation with nonionic surfactants and small quats was shown to be the most effective formulation.
Further formulation optimization specific to application and industry company formulation preferences, and EPA approval are important next steps. Nonionic surfactants are commonly used in quat-based disinfection formulations and can aid in surface tension reduction and micelle delivery and absorption to the cell walls of microbes. Care is needed, however, to avoid quat activity reduction from nonionic surfactants related to the micellular formation.
The USDA research team, under the direction of Dr. Brian Condon, has demonstrated the mechanism of cationic biocide binding in cotton-based disinfection wipes and developed a functional prototype formulation. The formulation was targeted to be as close as possible to conventionally used commercial formulations with EPA-friendly, simple chemistry modifications. Work is underway to further quantify wipe disinfection performance in a laboratory setting through the teamwork of the USDA and Cotton Incorporated. Dr. Condon is interested in supporting further refinements to the technology required for commercialization with an industry partner.
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