275 Hana Križova1, Viktor Tuček1, Jitka Neoralova2, Jakub Wiener1 1Technical University of Liberec, Faculty of Textile Engineering, Department of Material Engineering, Študentska 2, Liberec, Czech Republic 2National Library of the Czech Republic, Development and Research Laboratories, Sodomkova 2, Prague, Czech Republic Buffering and Antibacterial Properties of Cotton Canvas with Dolomite/ZnO-Styrene-Acrylic Complex Coating and their Comparison with Properties after the Accelerated Aging Puferske in protibakterijske lastnosti bombažne tkanine z dolomit/ZnO-stiren-akrilnim kompleksnim premazom in primerjava lastnosti po pospešenem staranju Original Scientific Article/Izvirni znanstveni članek Received/Prispelo 08-2017 • Accepted/Sprejeto 09-2017 Abstract In this study, we report evaluation of buffering and self-sterilizing coating complex obtained by dolomite and zinc oxide particles incorporation into styrene-acrylic layer applied to cotton fabric. Surface properties of the coating were evaluated by SEM, EDS and 3D optical microscopy. Antimicrobial properties were determined using a mixture of G+ and G- bacteria (Escherichia coli, Staphylococcus epidermidis and Streptococcus mutans) that was in dynamic contact with canvases for 4-100 hours. Alkalizing - buffer capacity of the surface layer supplied to the system by dolomite was tested by the addition of acetic acid. All these properties were simultaneously tested on the same canvases that have been previously exposed to the so-called hot (105 °C) and wet aging (80 °C, 65% relative humidity). The aging was provided in the climatic test room for 144 hours and the properties of canvases before and after aging were compared and evaluated. Keywords: dolomite, zinc oxide, acrylic coating, antibacterial, bicarbonate buffer Izvleček V študiji je podana ocena puferskega in samosterilizajočega učinka stirenakrilnega kompleksnega premaza z dodatkom delcev dolomita in cinkovega oksida na bombažni tkanini. Površinske lastnosti premaza so bile ovrednotene s SEM, EDS in 3D optično mikroskopijo. Protimikrobne lastnosti so bile določene z uporabo mešanice gram pozitivnih in gram negativnih bakterij (Escherichia coli, Staphylococcus epidermidis in Streptococcus mutans), ki so bile v dinamičnem stiku s tkanino od 4 do 100 ur. Bazično-pufersko zmogljivost premaza, ki so jo ustvarili z dodatkom delcev dolomita, so testirali z dodatkom ocetne kisline. Vse omenjene lastnosti so bile sočasno preizkušene na vzorcih tkanine, ki so bili pred tem izpostavljeni t. i. vročemu (pri 105 °C) in mokremu staranju (pri 80 °C, 65-odstotni relativni vlažnosti). Staranje je trajalo 144 ur v klimatiziranem prostoru. Lastnosti vzorcev so bile ovrednotene in primerjane pred staranjem in po njem. Ključne besede: dolomit, cinkov oksid, akrilni premaz, protibakterijske lastnosti, bikarbonatni pufer Corresponding author/ Korespondencna avtorica: Mgr. Ing. Bc. Hana Krfzova, Ph.D. E-mail: hana.krizova@tul.cz Tekstilec, 2017,60(4), 275-282 DOI: 10.14502/Tekstilec2017.60.275-282 276 1 Introduction Composite materials based on metal (nano)particles attract significant attention due a wide range of physical-chemical properties. These materials are not simply physical mixtures and they can be defined as complex materials with organic and inorganic components intimately mixed. Modification of coating polymeric matrices leads to novel properties such as growth prevention or adhesion reduction of microorganisms. There is a significant interest in the development of antimicrobial and durable materials for application in the food and biomedical industries. These materials are also used in the fields, such as archiving, museums, librarianship, preservation or storage, where extension of lifetime of the books and other ageing objects is a necessity. These materials should combine desirable attributes such as potent bactericidal and fungicidal efficiency, environmental safety, low toxicity, and easy, cost-effective fabrication. Carbonates are salts of carbonic acid and occur in the form of minerals and rocks commonly found in nature. Especially alkaline earth metal carbonates (Ca2+ and Mg2+) are very widely used in many productions and industrial processes in various industries like construction [1], food [2], metallurgy [3], treatment of contaminated water and soil [4, 5], and fertilizers [6]. Calcium carbonate CaCO3 (limestone) occurs in many crystalline modifications (cal-cite, aragonite, alabaster, travertine, marble). Also, magnesium carbonate MgCO3 and calcium magnesium carbonate CaMg(CO3)2 are naturally found in many modifications, varieties and with different chemical impurities (Fe, Mn, Zn, Pb and others) [7]. Dolomite is a non-toxic, inexpensive, widely available double carbonate with slightly alkaline and buffering capabilities. It crystallizes in the trigonal-rhombohedral system and depending on the admixture it is white, gray or pink. Unlike bicarbonates, carbonates, except for ammonium carbonate and alkali metal carbonates, are practically insoluble in water. Carbonates are important environmental buffering agents. They are part of the carbon dioxide/bicarbonate/carbonate buffer, which, above all, serve the world's oceans to neutralize and counterbalance acidic influences in the environment. This buffering system is essential in many biological processes such as maintaining a constant blood pH [8] and internal environment within homeostasis [1]: CO2 + H2O^H2CO3^HCO3- + H+ (1). Buffering and Antibacterial Properties of Cotton Canvas with Dolomite/ZnO-Styrene-Acrylic Complex Coating and their Comparison with Properties after the Accelerated Aging Zinc oxide ZnO at low concentration is non-toxic to human and displays good biocompatibility to human cells. ZnO showed a wide range of antibacterial activities of different microorganisms, including both Gram-negative and Gram-positive bacteria [9]. It was also confirmed that reinforcement of ZnO in the biocomposites introduces their antibacterial activity [10]. 2 Experimental part 2.1 Preparation of samples Cotton fabric (Royal, PK) was used for preparation of coated canvas (area weight 156 g/m2, thickness 0.29 mm, plain weave, fineness of warp yarn 50 tex, fineness of weft yarn 40 tex, warp density 270 ends/10 cm (68.6 ends per inch), weft density 230 picks/10 cm (58.4 pics per inch), pre-dyed with the Direct Green 28 (CI No. 14155, azodye, concentration 0.5%), coated with starch on the reverse side, width 50 cm. Acronal S 996 S (BASF SE, GE) - aqueous polymer dispersion based on ester of acrylic acid and styrene (viscosity of 2 Ns/m2) was used as a basic coating paste. All following additives were thoroughly mixed into this basic matrix: laboratory-prepared cellulose dispersion to increase strength [11, 12], granulated natural pink dolomite (Forestina, CZ), powdered zinc oxide declared as nanoparticles (Bochemie, CZ), two ben-zisothiazole pigments: Aquacolors Red (temperature resistance 160-180 °C and light fastness 5-6, pH 7.5; Sioen, BE), and Aquapaste Green 856 (temperature resistance to 200 °C and light fastness 8, pH 8; Orgapaste, FR). The resulting composition of the paste was as follows: - 340 g of cellulose dispersion (8% of dry matter), - 54 g of dolomite, - 72 g of ZnO pulver, - 162 g of water, - 1800 g of styrene-acrylic basis, - 40 g of thickener AXILAT 44 Bulk (Momentive Specialty Chemicals, CZ), - 5.4 g of red pigment (0.3%) and - 64.8 g of green pigment (3.6%). The 2-layers coating was performed on the Coate-ma device (Coatema Coating Machinery GmbH, DE): 1st passage was done at speed of 2.13 m/min, temperature of 130 °C, and downforce of 150 N, 2nd passage at speed of 3.55 m/min, temperature of Tekstilec, 2017,60(4), 275-282 130 °C, and downforce of 200 N. The coated canvas had an area weight of 220 g/m2 and a thickness of 0.33 mm. Part of the canvas was subjected to the artificial accelerated aging. According to standards [13, 14] for accelerated aging of paper and board by dry heat treatment or moist heat treatment, part of the canvas was subjected to the so-called hot aging (at 105 °C for 144 hours), another part of the canvas was subjected to the so-called wet aging (at 80 °C and 65% of relative humidity for 144 hours). Artificial aging was carried out in the WK3 climatic test room with a capacity of about 350 liters (Weiss Umwelttechnik GmbH, Reiskirchen, GE). Wet aging imitates weathering and weather influences or storage in wet and warm environment, while hot aging imitates dry tropics or other long-term high temperature stress. It is difficult to imitate experimentally the influence of time. The relation between accelerated and natural aging represents a serious problem. The changes monitored upon accelerated aging procedures must be correctly extrapolated to the ambient conditions. Nevertheless, in accordance with data in the literature, it is stated that three days of accelerated aging of paper and other cellulosic materials at 105 °C correspond to 25 years of natural aging [15]. 2.2 Characterization of coating surfaces The particle size used as additives and the appearance of the coated surface were scanned by optical methods. Particle size of ZnO and dolomite was measured using the particle size analyzer and laser scattering device (HORIBA LA-920, JP). The surface and cross-section of canvas were monitored using SEM scanning electron microscope VEGA TS 5130 (TESCAN, CZ) and the 3D digital multifunction microscope HIROX RH 2000 with MXB 2500REZ lens and diffuse adapter (HIROX, JP). This optical method also characterized the canvas surface roughness, Rz. In accordance with standard JIS [16], Rz is the sum of the average absolute value of then height (five highest peaks) and the average absolute value of the depth (five lowest valleys from the average line of the roughness curve) on the weft yarn, in microns. Ten-spot average roughness was chosen at zoom 500-x. Energy-dispersive X-ray spectroscopy (EDS), when backscattered electrons at a sampling depth of 1-2 microns characterize the chemical composition of investigated surface, is part of SEM device. EDS was chosen as a method of Buffering and Antibacterial Properties of Cotton Canvas with 277 Dolomite/ZnO-Styrene-Acrylic Complex Coating and their Comparison with Properties after the Accelerated Aging chemical characterization of the coating surface and percentage quantification of the monitored metals (Zn, Ti, Ca, Mg). 2.3 Buffering test 25 cm2 of each canvas including the reference sample (cotton fabric coated with only acrylic coating without additives) was immersed in 50 ml of purified deionized water for 2 days at 22 ± 2 °C to achieve an equilibrium of water with both CO2 from the atmosphere and carbonate containing dolomite. After 48 hours, the pH of the water in each beaker was measured. Then 0.5 ml of 27 mM acetic acid was added to each beaker. All pH changes were continuously potentiometrically measured at various time intervals up to 24 hours after application of acetic acid using glass silver chloride reference electrode. 2.4 Microbiological test A mixture of G+ and G- bacteria containing E. coli, Staphylococcus epidermidis and Streptococcus mutans was prepared using a liquid culture medium Trypto-Soya Broth (TSB) (Biovendor, CZ) at 37 °C for 24 hours. Bacterial suspension of concentration 3 • 105 CFU/ml was prepared by dilution with distilled water. Thirty ml of this suspension was dosed into plastic tubes, samples of all four canvas were added (each with an area of 32 cm2), i.e. canvas without aging, canvas after hot aging, canvas after wet aging, and comparative sample (canvas without addition of additives). One tube with bacterial suspension was left without the addition of canvas. All samples inserted into the bacterial suspension in tubes were shaken on a shaker at speed of 150 rpm to achieve dynamic contact of the bacterial suspension with the canvas surface. After 4, 48 and 72 hours, 1 ml of suspension was taken from each tube, it was spotted on the surface of the agar in Petri dishes (TSA-Tryptone soya agar, Biovendor, CZ). These samples were cultivated at 37 °C for 24 hours and then bacterial colonies were counted. 3 Results Results of particle size measurement using device HORIBA LA-920 (Figure 1 and Figure 2) show that the particle size ranged between 0.5 and 80 microns Tekstilec, 2017,60(4), 275-282 278 Buffering and Antibacterial Properties of Cotton Canvas with Dolomite/ZnO-Styrene-Acrylic Complex Coating and their Comparison with Properties after the Accelerated Aging Figure 1: Distribution of dolomite particle size Figure 2: Distribution of ZnO particle size for both dolomite and zinc oxide. While the mean of the dolomite particle size was 15.7 ^m and the median of 13 |im, the powder ZnO declared by the producer as nanoparticles had a mean particle size of 25 |im (median of 24 ^m) due their easy agglomeration when used in a powdered state without a dispersant. Unfortunately, the shear forces used in the standard mechanical mixing of the coating composition under laboratory conditions were not sufficient to scatter ZnO agglomerates into nanoparticle sizes. As can be seen from the surface (Figure 3) and the cross-section (Figure 4) SEM images of the canvas, this particle size also reflected in a relatively high surface roughness. The Rz value was calculated as Tekstilec, 2017,60(4), 275-282 10 microns using 3D microscope at a magnification of 500x (Figure 5). Figure 3: SEM image of canvas surface ■ Figure 4: SEM image of canvas cross-section Buffering and Antibacterial Properties of Cotton Canvas with 279 Dolomite/ZnO-Styrene-Acrylic Complex Coating and their Comparison with Properties after the Accelerated Aging The EDS spectra of the canvas surface were measured by SEM at the values of the middle vacuum (3-150 Pa) and was taken from two locations (Figure 6): 1 - spot particle point (in place of surface irregularity in the coating), 2 - area (element mapping). EDS Layered Image 1 V % > Sp«tryfn i r 0 ' * Spectrum 2 [Ti 5 • j* V f * V flE , f Figure 6: Two sites of EDS analysis (spot Spectrum 1 and small area Spectrum 2) and calcium-zinc occurrence mapping on the canvas surface As shown in Figure 7 and Figure 8, for example, the zinc was detected in an amount of 31.1 wt.% (spot spectrum 1) or also 1.4 wt.% (area spectrum 2, bottom of Figure 8). The total average content of selected elements from the mapping of canvas surface is shown in Table 1. Figure 5: Surface roughness of canvas (3D HIROX, 500x) Rate at which ionising elements are detected by a radiological measurement instrument [cps/eV] Figure 7: EDS spectra of the canvas surface - spot spectrum 1 Tekstilec, 2017,60(4), 275-282 280 Buffering and Antibacterial Properties of Cotton Canvas with Dolomite/ZnO-Styrene-Acrylic Complex Coating and their Comparison with Properties after the Accelerated Aging While zinc oxide was used in an amount of 2.8 wt.% of wet coating ratio, in the dry surface layer, zinc only represents approximately 0.7 wt.%. The dolomite, which originally represented an amount of 2.1% from wet coating paste, was detected in the dry surface as calcium and magnesium in a total weight of 0.2 wt.%. Due to the weight ratio in the dolomite, where Ca and Mg represent about one third of (anhydrous) carbonate, a real weight of dolomite in the coating may be assumed to be about 0.6 wt.%.