Durable Wash-Resistant Antimicrobial Treatment of Knitted Fabrics

Nowadays, antimicrobial textile is one of the fastest growing areas of functional textiles. And it has become extremely important these days, when faced with this difficult global COVID-19 problem. Taking into account that textile products such as face masks must be frequently washed, antimicrobial finishing has to be durable, wash-resistant, and nontoxic. Most synthetic fibers, due to their hydrophobicity, are more resistant to microorganism growth than natural fibers. Natural fibers and various fiber combinations are prone to microorganism growth and bacteria colonization due to the textile surface characteristics. With increasing demand from consumers, the aim is to protect the fibers as well as the human body from microbial and bacterial growth [1]. The application of antimicrobial treatment to textiles can prevent bacterial growth. A suitable temperature, moisture, and receptive surface provide the perfect condition for the growth of microorganisms on textile that is in contact with the human skin. The growth of microorganisms on textiles inflicts a range of unwanted impacts not only on the textile itself, but also on the wearer: these include unpleasant odors, discoloration in the fabric, reduction of fabric mechanical strength, and so on [2].

Antimicrobial treatment is one of the most important finishes of the functional textile. There are several aspects to the antimicrobial treatment of textile. The first aspect is to protect the textile material from damage caused by microorganisms; the other aspect is protection of the user from growth of pathogenic microorganisms. Almost all antimicrobial agents used for finishing of commercial textiles, for example, silver, zinc oxide, triclosan, copper compounds, and so on, are biocides. Due to their importance, the number of different antimicrobial agents for textile application has dramatically increased. These antimicrobial agents differ in their chemical structure, effectiveness, method of application, and influence on humans, the environment, as well as on cost [2,3,4]. First, the antimicrobial treatment should be effective against a broad spectrum of bacterial and fungal species, but at the same time not cause toxicity, allergy, or irritation to the user. Second, the finishing should be durable to washing, dry cleaning, and hot pressing, and this is the greatest challenge as textile products are subjected to repeated washing during their lifetime. Also, the finishing should not have a negative effect on the quality or appearance of the textile. And finally, the finishing should be compatible with other textile chemical processes such as dyeing or printing, be cost-effective, and not harm the environment [2].

The antimicrobial agents can be applied to the textile substrate by exhaust, pad-dry-cure, coating, spray, and foam techniques. Antimicrobial agents are usually applied at the finishing stages of textile production, while in some cases biocide can be incorporated into synthetic fibers during extrusion [5,6,7,8,9]. A large number of studies were conducted to investigate the resistance of a varied spectrum of antimicrobial materials on textile. Many researchers have analyzed the antimicrobial activity after antimicrobial treatment of fabrics made of cotton, wool, polyamide, polyester yarns, and their various combinations [10,11,12,13]. It was found that antimicrobial-treated knits with the same fiber composition and very similar total linear density of yarns but with different number of folded yarns in the structure have different antimicrobial activities, because a higher number of folded yarns in the knitted loop has a larger surface area and, therefore, a larger antimicrobial acting area [14]. The antimicrobial activity of the plaited knits, in which yarns of different raw material are distributed on opposite surfaces, depends on the construction of yarns, too [14,15,16].

The application of antimicrobial finish has to prevent bacterial growth on the textile, however, it may change the fabrics or products physical and/or aesthetic properties, like the texture or surface characteristics such as density or porosity, especially if wet treatment processes are used. It is well known that the surface porosity and loop density in knitted fabrics have a direct influence on the air permeability of the fabric. Air permeability is considered to be one of the most important features of clothing comfort which ensures physiological comfort [15,16,17,18].

The aim of the present work was to determine the influence of antimicrobial textile finishing using “Si Bactericidal” on durability (wash-resistance) of the antimicrobial activity and on changes of structural parameters and air permeability of the treated knitted fabrics.

Seven variants of knitted fabrics were produced for antimicrobial treatment and investigated in this research. All the experimental samples were knitted in a rib 1x1 pattern on the same flat 10E gauge double-needle-bed weft knitting machine CMS530 (Stoll, Germany). The samples were knitted using yarns of different raw materials: pure woollen, cotton and acrylic yarns, and blended wool/acrylic yarns (with three different percentages of the wool and acrylic fibers in the yarn) and cotton/acrylic blended yarns. Structure parameters of the knits were analyzed according to the Standard LST EN 14971:2006. The main structural parameters of these knitted fabrics are presented in Table 1.

All the experiments were carried out in the standard atmosphere according to Standard ISO 139, that is, 20 ± 2°C temperature and 65 ± 4% humidity.

All knitted samples were treated by “Si Bactericidal Textile Finish,” a technology proposed by the Portuguese company, Smart Inovation, Lda. This technology has advantages such as easy application and nontoxicity. It does not use heavy metals, silver, or other toxic substances and is eco-friendly. The active antimicrobial agent is Benzalkonium chloride (BKC). BKC was covered on the textile materials using the method of wet impregnation. The main conditions of the antimicrobial treatment were as follows: aqueous solution of 30°C was prepared by inserting the components in the bath in the following order and ratio: 41 ml/l of “Si Bactericidal” and 7 ml/l of auxiliary Smart Fix, pH of the bath at 6–6.5, the absorption rate at 70%, and duration of the treatment at 15 min. After that, drying for 15 min at 120°C was applied.

After the antimicrobial treatment, the samples were washed 50 times, and after each washing and drying cycle their structural parameters, antimicrobial activity, and permeability to air were measured.

The washing procedure was performed according to the standard ISO 6330:2012. The samples were washed for 15 ± 0.5 min in 40 ± 2 °C temperature washing solution with 3 g/l washing powder concentration. After the main washing, the samples were rinsed thrice in 20 ± 2 °C temperature. The duration of each rinse was 1 ± 0.1 min. The rinsed samples were spin-dried (frequency of revolution 1,000 min⁻¹) for 1 ± 0.1 min and dried for 24 h on a smooth surface in the standard atmosphere conditions.

The antimicrobial efficiency of samples was tested with the Gram-negative E. coli (KMY1T) and the Gram-positive S. aureus (ATCC25923) bacteria. The qualitative evaluation of antimicrobial efficiency of the treated knitted samples was carried out in accordance with EN ISO 20645:2004 (Agar diffusion plate test). The plates were incubated for 18 h at 37°C and afterwards the width of the inhibition zone was calculated as follows: (1)H=D−d2;H = {{D - d} \over 2}; where H is the inhibition zone in mm, D is the total diameter of sample and inhibition zone in mm, and d is the diameter of sample in millimeters.

Three tests of antimicrobial activity were performed for each sample variant. The evaluation criteria of the antimicrobial effect are presented in Table 2.

The air permeability test of the knitted fabrics was conducted according to Standard EN ISO 9237:1997, using a head area of 5 cm² and pressure difference of 100 Pa. 10 tests were performed for each sample variant. The air permeability was calculated according to the following equation: (2)R=DA⋅167;R = {D \over A} \cdot 167; where R is the air permeability in dm³/(m²s); D is the average of air flow rate, dm³/min; and A is the operative area of the sample equal to 5 cm².

After the antimicrobial treatment of seven variants of knits with different raw composition by using the easy applicable method of wet impregnation in aqueous solution with “Si Bactericidal” and repeated washing and drying cycles, it was found that this treatment gives very good antimicrobial activity against the Gram-negative E. coli (KMY1T) and the Gram-positive S. aureus (ATCC25923) bacteria as well as very good resistance of this activity to washing. Good antimicrobial activity with the inhibition zone against E. coli bacteria remains after at least 10 washing cycles and against S. aureus bacteria after at least 20 washing cycles of all investigated raw compositions of knits: 100% woollen, 100% acrylic, different percentages of blended woollen/acrylic (70%/30%, 50%/50%, 30%/70%, respectively), 100% cotton, and 50% cotton/50% acrylic. And even after 30 washing cycles, the treated fabrics have some limited antimicrobial activity. It is a very promising antimicrobial treatment of textiles, especially taking into account its possible compatibility with other finishing processes (dyeing, printing, laundering, etc.), the application to different raw materials, and very good durability during washing.

The antimicrobial treatment does not significantly change dimensions nor permeability to air of the treated knitted fabrics, while the obtained changes were in the ranges of error, with the only exception for air permeability of cotton and cotton-blended fabrics. This is a very positive conclusion because the treatment used to obtain additional functionality cannot worsen the other properties of the fabric.