Introduction

The application of nanoparticles to textile materials has been the object of several studies aimed at producing finished fabrics with different performances. For example nano-Ag has been used for imparting antibacterial properties (Lee et al. 2003; Durán et al. 2007), nano-TiO2 for UV-blocking and self-cleaning properties (Xin et al. 2004; Fei et al. 2006; Qi et al. 2007) and ZnO nanoparticles for antibacterial and UV-blocking properties (Wang et al. 2004; Baglioni et al. 2003; Wang et al. 2005; Vigneshwaran et al. 2006). Inorganic UV blockers are more preferable than organic UV blockers (Riva et al. 2006; Scalia et al. 2006). In fact, zinc oxide and titanium dioxide are non-toxic and chemically stable under exposure to both high temperatures and UV. Furthermore, nanoparticles have a large surface area-to-volume ratio that results in a significant increasing of the effectiveness in blocking UV radiation when compared to bulk materials (Yadav et al. 2006).

The use of nanotechnology in the textile industry has increased rapidly. This is mainly due to the fact that conventional methods used to impart different properties to fabrics often do not lead to permanent effects, and will lose their functions after laundering or wearing. Nanoparticles can provide high durability for treated fabrics, with respect to conventional materials, because they possess large surface area and high surface energy that ensure better affinity for fabrics and lead to an increase in durability of the textile functions. Washfastness is a particular requirement for textile and it is strongly correlated with the nanoparticles adhesion to the fibers. In order to increase the washfastness, the nanoparticles can be applied by dipping the fabrics in a solution containing a specific binder (Vigneshwaran et al. 2006; Yadav et al. 2006). Washfastness can be further improved with the formation of covalent bonding between nanoparticles and the fabrics surface. In these cases the excellent UV blocking properties are still maintained after 55 home laundering (Daoud and Xin 2004; Xin et al. 2004; Lu et al. 2006).

Nanoparticle coating may affect other fabric properties like dyeing capacity, tensile strength, bursting strength, bending rigidity, air permeability (comfort) and fabric friction that play a crucial role in textile industries. Cotton fabrics treated with bulk-ZnO or nano-ZnO show different physical and mechanical properties. This reflects the improved properties of nano-sized particles with respect to conventional materials. Air permeability of the fabric is reduced when the coating process is carried out with bulk-ZnO, while it is improved when nano-ZnO is used. This basic difference has great consequences on the garment’s breathability, and eventually on the comfort of the treated fabrics. As opposed to bulk-ZnO coating, in the case of nano-ZnO treated fabric, the small size and uniform distribution of the particles results in a significant lowering of the friction (Yadav et al. 2006). Titania nanocoating slightly enhances bursting strength of treated cotton and the hand effect remains unchanged when compared to the untreated fabric. Again, this is mainly due to the nano-scaled structure of the titania layer (Xin et al. 2004).

Zinc oxide is widely used in different areas because of its unique photocatalytic, electrical, electronic, optical, dermatological, and antibacterial properties (Turkoglu and Yener 1997; Pan et al. 2001; Arnold et al. 2003; Sawai 2003; Xiong et al. 2003; Behnajady et al. 2006; Li et al. 2006; Tang et al. 2006c). For these applications, the nanoparticles need to be dispersed homogeneously in the different matrices, and a number of new synthetic strategies have been developed in order to prevent particles agglomeration, and increase the stability of ZnO nanoparticles dispersions (Guo et al. 2000; Kwon et al. 2002; Wang et al. 2002; Liufu et al. 2004; Tang et al. 2006a; Tang et al. 2006b).

Zinc oxide is actually one of the best biofriendly absorbers of UV radiation that mainly come on Earth from the Sun through the atmospheric ozone layer (Sato and Ikeya 2004). UV radiation is responsible for the generation of free radical species (Bednarska et al. 2000; Takeshita et al. 2006) that are supposed to participate in the development of various pathologies such as cancer, ageing, Alzheimer’s disease, inflammatory disorders, and so on (Liebler 2006). In order to decrease the health risks due to overexposure to UV radiation, the World Health Organization (WHO) has also recommended the use of loose-fitting, full-length clothes with a high protection factor (Algaba et al. 2007). However most sportwear and light clothing commonly worn in the summertime do not guarantee an efficient shield against UV. The protection provided by fabrics depends on several parameters: fiber type, color, the presence of UV absorbers and additives, the porosity, thickness, mass per unit surface, other finishing processes and laundering, and the wearing conditions (stretched, dry or wet) (Gamblicher et al. 2002; Wang et al. 2005; Algaba et al. 2007).

The present work addresses the synthesis and characterization of ZnO nanoparticles obtained through a homogeneous phase reaction between zinc chloride and sodium hydroxide at high temperature. In order to evaluate the effects of the experimental conditions on the particle size and morphology, the temperature (90 or 150 °C) and the reaction medium (water or 1,2-ethanediol) were changed. The particles were then characterized, by evaluating their chemical composition through FTIR spectroscopy, their crystallinity through X-ray diffractometry, their shape and size via TEM microscopy, and the specific surface area.

ZnO nanoparticles were then applied to cotton and wool fabrics in order to evaluate the sunscreen activity in the treated textiles through UV spectrophotometry. The thermal behavior of the fabrics was assessed through thermogravimetric analysis (DTGA), and the mechanical resistance was evaluated through tensile strength and elongation tests.

Experimental

Synthesis

ZnCl2 (min. 98%), NaOH (pellet min. 99%), 1,2-ethanediol (min. 99.5%), and 2-propanol (min. 99.5%) were purchased from Merck (Darmstadt, Germany). All products were used as received. Water was purified by a Milli-RO6 Plus Millipore Organex system (Resistance > 18 MΩ cm).

Zinc oxide nanoparticles were synthesized following a procedure reported elsewhere (Moroni et al. 2005). The synthesis was carried out at a high degree of supersaturation, in order to achieve a nucleation rate much greater than the growth rate (Ambrosi et al. 2001). ZnCl2 (5.5 g) was dissolved in 200 mL of water at 90 °C in an oil bath. 16 mL of 5 M NaOH aqueous solution were added dropwise to the zinc chloride solution, with a gentle stirring over a period of 10 min at 90 °C. The particles were separated from the supernatant dispersion by sedimentation. The supernatant solution was discarded, and the remaining suspension washed five times with distilled water to lower the concentration of NaCl below 10−6 M. Each time, the dilution ratio between the concentrated suspension and washing solution was about 1:10. The complete removal of NaCl from the suspension was checked with a solution of AgNO3. The purified particles were then peptized with 2-propanol in an ultrasonic bath for 10 min at room temperature. The peptization process is necessary to disrupt the microagglomerates and release the nanounits of zinc oxide (Perez-Maqueda et al. 1998; Salvadori and Dei 2001). The particles were then collected by centrifugation at 6,000 rpm for 15 min. The washing procedure was repeated three times. Thermal treatment of the particles at 250 °C for 5 h lead to the formation of ZnO. The synthesis in 1,2-ethanediol (ED) was carried out in the same way, but at 150 °C.

Fabric treatments

White wool and cotton fabrics, kindly provided by Grado Zero Espace (Florence, Italy), were used as received. The mass per unit surface was 146 g/m2 for cotton and 339 g/m2 for wool.

The fabric samples were conditioned at constant relative humidity (33%) and temperature (20 °C). The wool and cotton samples (10 cm × 10 cm) were soaked for 10 min in a 2-propanol dispersion of ZnO nanoparticles (5% w/w), under gentle magnetic stirring. The clothes were then squeezed to remove the excess dispersion, and dried in a oven at 130 °C for 15 min at atmospheric pressure (dry heat). In order to evaluate the nanoparticles adhesion to the textile fibers, the treated fabrics were washed five times according to a standard method (UNI EN ISO 26330:1996). A model A1 Wascator Electrolux automatic laundry machine (internal drum diameter 51.5 cm, internal drum depth 33.5 cm, heating capacity 5.4 kW) was used, and the washing cycles were performed at 30 °C, with an ECE phosphate reference detergent (B) without optical brighteners. The drying step was carried out on a horizontal flat surface. The fabric specimen were tested before and after the washing cycles via TEM and UV spectrophotometry.

UV absorption properties

The UV-screen properties of the treated fabrics were investigated by absorption spectroscopy using a UV–Vis spectrophotometer (Perkin-Elmer Lambda 35, equipped with a 60-mm integrating sphere). The blank reference was air. The UV profiles of the untreated samples were compared to the spectra collected from the same fabrics treated with ZnO nanoparticles, and the effectiveness in shielding UV radiation was evaluated by measuring the UV absorption, transmission and reflection. Each measurement is the average of four scans obtained by rotating the sample by 90°. The transmission data were used to calculate the UPF (ultraviolet protection factor) and the percent UV transmission, according to the following equations (Gamblicher et al. 2001, 2002):

$$ {\text{UPF}} = \frac{{\int\limits_{\lambda _1 }^{\lambda _2 } {{\text{E}}\left( \lambda \right) \cdot {\text{S}}\left( \lambda \right)\,{\text{d}}\lambda } }} {{\int\limits_{\lambda _1 }^{\lambda _2 } {{\text{E}}\left( \lambda \right) \cdot {\text{S}}\left( \lambda \right) \cdot {\text{T}}\left( \lambda \right)\,{\text{d}}\lambda } }} $$
(1)
$$ {\text{percent UV transmission}} = \frac{{\sum\limits_{\lambda _1 }^{\lambda _2 } {{\text{T}}\left( \lambda \right)} }} {{\left( {\lambda _2 - \lambda _1 } \right)}}. $$
(2)

E(λ) is the relative erythemal spectral effectiveness, S(λ) is the solar spectral irradiance in W m−2 nm−1, and T(λ) is the spectral transmission of the specimen obtained from UV spectrophotometric experiments. The values of E(λ) and S(λ) were obtained from the National Oceanic and Atmospheric Administration database (NOAA). The UPF value was calculated for UV-A in the range 315–400 nm, and for UV-B in between 295 and 315 nm. The percent UV transmission, obtained from Eq. 2, was determined for UV-A and UV-B radiation from the transmission spectra of the fabric samples.

Physical and physico-chemical characterization

Physical tests (tensile strength and elongation) of the treated fabrics were performed according to the standard methods (UNI EN ISO 13934-1:1999).

The chemical composition of the synthesized materials was checked by FTIR spectroscopy with a Biorad FTS-40 spectrometer. The crystallinity was determined by XRD using a Bruker D8 Advance X-rays Diffractometer equipped with a Cu Kα (λ = 1.54 Å) source (applied voltage 40 kV, current 40 mA). About 0.5 g of the dried particles were deposited as a randomly oriented powder onto a Plexiglass sample container, and the XRD patterns were recorded at angles between 20° and 80°, with a scan rate of 1.5°/min.

The crystallite domain diameters (D) were obtained from XRD peaks according to the Scherrer’s equation (Jenkins and Snyder 1996):

$$ {\text{D}} = \frac{{0.89 \cdot \lambda }} {{\Delta {\text{W}} \cdot \cos \uptheta}} $$
(3)

where λ is the wavelength of the incident X-ray beam (1.54 Å for the Cu Kα), θ is the Bragg’s diffraction angle, and ΔW the width of the X-ray pattern line at half peak-height in radians.

The shape and size of the particles were obtained through TEM, using a Philips EM201C apparatus operating at 80 kV. The samples for TEM measurements were placed on carbon-coated copper grids (supplied by Agar Scientific Ltd, U.K.). The samples for TEM measurements were prepared from very diluted dispersions of the particles in 2-propanol. Surface area measurements were determined from BET on a Coulter SA 3100 surface area analyzer, under N2 flow.

The ZnO-treated fabrics were analyzed through scanning electron microscopy (SEM), using a Stereoscan S360 Oxford–Cambridge. The samples were previously coated with a thin layer of gold deposited by sputtering under vacuum.

Thermogravimetric analysis was performed with an SDT 2960, series Q600 apparatus (TA Instruments, Milan, Italy). The temperature range was between 25 and 500 °C, with a scan rate of 20 °C/min. All runs were performed with a nitrogen flux of 100 mL/min.

Results and discussion

Synthesis and characterization of ZnO nanoparticles

The results indicate that the experimental conditions greatly affect the morphology and size of the particles, prepared with the different procedures (see Table 1). In fact, increasing the reaction temperature from 90 °C in water up to 150 °C in 1,2-ethanediol results in a significant lowering of the nanoparticles size and of their agglomeration number N, calculated as (Shvalagin et al. 2007):

$$ {\text{N}} = \frac{4} {3}\pi {\text{R}}^3 {\text{N}}_{\text{A}} \frac{\rho } {{\text{M}}} $$
(4)

where NA, R, ρ, and M are the Avogadro number, the radius of the nanoparticles, the density (5.47 × 106 g/m3) and the molecular weight (81.408 g mol−1) of zinc oxide, respectively. The values of N are reported in Table 1.

Table 1 Reaction conditions, crystallite diameter calculated according to Scherrer (Eq. 3), particles diameter (obtained by averaging more than 400 particles from TEM micrographs), specific surface area (As), and agglomeration number N (from Eq. 4) for the different samples
Full size table

Figure 1 shows the FTIR spectra of the synthesized nanomaterials. The spectrum of the material obtained from synthesis 1 (in water at 90 °C; Fig. 1a) clearly shows the Zn-O absorption band near 430 cm−1. The peaks at 3,450 and 2,350 cm−1 indicate the presence of –OH and C=O residues, probably due to atmospheric moisture and CO2 respectively. The same spectrum was obtained from the nanomaterial produced via synthesis 2 (in 1,2-ethanediol at 150 °C, see Fig. 1b).

Fig. 1
figure 1

FTIR spectrum of ZnO nanoparticles obtained from synthesis 1 (a: full line) and synthesis 2 (b: dotted line)

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TEM images of the ZnO nanoparticles are shown in Fig. 2. Nanoparticles are nearly spherical and quite monodisperse. However, there are some larger aggregates in the sample obtained from synthesis 1 (Fig. 2a), because of the high surface energy of ZnO nanoparticles that results in aggregation, especially when the synthesis is carried out in an aqueous medium. Particles obtained from synthesis 2 are more monodisperse and isolated than the particles obtained from synthesis 1. This result indicates a larger peptization yield for synthesis 2 (Fig. 2b) respect to synthesis 1, and it is confirmed by comparing the particles size distribution obtained from TEM micrographs (Fig. 3). The mean diameter of the particles in the dry powder was measured by averaging the size of a large number of particles (>400) from the analysis of TEM micrographs. The particles diameter ranges between 12 and 38 nm for the material obtained according to synthesis 1 (Fig. 3a), and between 5 and 15 nm in the case of synthesis 2 (Fig. 3b). Figure 2b shows some halos surrounding the particles due to the retention of some diol, that remains adsorbed on the ZnO nanoparticles, and produces the tiny peaks at 2,850–2,920 cm−1 in the FTIR spectrum (see Fig. 1b).

Fig. 2
figure 2

TEM micrographs on the materials obtained from synthesis 1 (a) and synthesis 2 (b) after three peptizations

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Fig. 3
figure 3

Mean particle size and particle distribution of nanocrystalline zinc oxide obtained from (a) synthesis 1 and (b) synthesis 2 (Table 1)

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Figure 4 shows the XRD spectra of the ZnO nanomaterials. The spectra show well-defined peaks typical of ZnO in the crystal structure of zincite, according to the Joint Committee on Powder Diffraction Standard (JCPDS) card number 36-1451. This indicates crystallinity of the synthesized solids. Traditionally, the broadening of the peaks in the XRD patterns of solids is attributed to particle size effects (Jenkins and Snyder 1996). The mean crystallite size of a powder sample was estimated from the full width at half-maximum (FWHM) of the diffraction peak according to the Scherrer equation. An interesting feature is that the peaks from synthesis 2 (Fig. 4b) are broader than the peaks from synthesis 1 (Fig. 4a). This result suggests that the particles obtained from synthesis 2 are smaller than the particles obtained from synthesis 1, as confirmed by TEM micrographs (Fig. 2), and reflects the effects due to the experimental conditions on the nucleation and growth of the crystal nuclei. The particles size calculated on the basis of Scherrer’s equation agrees quite well with the value obtained through TEM micrographs (Table 1).

Fig. 4
figure 4

XRD patterns of the material obtained from (a) synthesis 1 and (b) synthesis 2 (Table 1)

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The specific surface areas obtained from BET experiments are 24.8 m2/g in the case of synthesis 1, and 60.1 m2/g in the case of synthesis 2, respectively (see Table 1). These data, agree very well with the expected values calculated from the particle diameters, within the experimental error, and confirm the effects induced by the experimental conditions on the nucleation and growth rates of the crystal nuclei.

Thermal analysis

Thermogravimetric analysis profiles are shown in Fig. 5. Untreated cotton (Fig. 5a) shows a consistent loss of weight above 300 °C with a maximum pyrolysis temperature at about 375 °C. Untreated wool show a small weight loss between 25 and 100 °C due to the release of moisture, and a consistent loss of weight above 200 °C, with a maximum pyrolysis temperature at about 328 °C (Fig. 5b). The treatment of cotton and wool fabrics with nanosized ZnO does not modify significantly their thermal stability and maximum pyrolysis temperature.

Fig. 5
figure 5

First derivative weight loss as a function of temperature. (a) (●) untreated cotton, (■) cotton + ZnO synthesis 1, (◆) cotton + ZnO synthesis 2. (b) (◆) untreated wool, (▼) wool + ZnO synthesis 1, and (◯) wool + ZnO synthesis 2

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Electron microscopy

The surfaces of the treated fabrics were observed by SEM microscopy. In Fig. 6, SEM micrographs show the nanoscaled ZnO particles on cotton (a) and wool (b) samples. The nanoparticles are well dispersed on the fiber surface in both cases, although some aggregated nanoparticles are still visible. The particles size plays a primary role in determining their adhesion to the fibers: it is reasonable to expect that the largest particle agglomerates will be easily removed from the fiber surface, while the smaller particles will penetrate deeper and adhere strongly into the fabric matrix. SEM images (Fig. 6c, d) confirm that the large agglomerates are removed from the textile surface after washing. Instead, although the nanoparticles are not covalently grafted to the fabric materials, preliminary gravimetric essays show that more than 50% of their initial amount remains bound to the fibers surface after washing.

Fig. 6
figure 6

SEM images of ZnO nanoparticles from synthesis 1 on: cotton (a) and wool (b) before washing, cotton (c) and wool (d) after washing

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Sunscreen activity of fabrics

The solar UV radiation is actually composed of UV-A (400–315 nm), UV-B (315–290 nm) and UV-C (290–200 nm). These radiations are present in natural terrestrial sunlight in different amounts, due to the filtering activity of the upper atmosphere, and to local conditions (latitude, altitude, clouds, etc.). UV-C and most of UV-B are filtered by the ozone layer. UV spectra were performed on the untreated and treated fabrics, by measuring the absorbance, transmission and reflection (see Fig. 7). Untreated cotton does not absorb UV radiation (Fig. 7a) while untreated natural wool strongly absorbs in the UV region between 200 and 300 nm (Fig. 7b).

Fig. 7
figure 7

Absorption spectra (a, b), transmission spectra (c, d) and reflection spectra (e, f) of the fabric samples. (●) untreated cotton, (■) cotton + ZnO synthesis 1, (▴) cotton + ZnO synthesis 2, (◆) untreated wool, (▼) wool + ZnO synthesis 1, and (◯) wool + ZnO synthesis 2

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The application of nanosized ZnO particles on the cotton fabric increases the absorption of UV light over the entire investigated UV spectrum. Higher values of UV absorbance were obtained when ZnO nanoparticles from synthesis 2 were applied on cotton (Fig. 7a). Similar results were obtained for the treated wool samples. The application of nanosized ZnO on wool fabric increases UV light absorbance in the region between 300 and 400 nm if compared to the untreated wool, in both cases (Fig. 7b). The results imply that the effectiveness in shielding UV radiation is due to the UV absorption capacity of ZnO nanoparticles on the fabrics surface. UV transmission and reflection spectra confirm these conclusions, as the ZnO-treated cotton and wool samples do not transmit or reflect the radiation over the entire UV spectrum (Fig. 7c, d, e and f).

The values of the ultraviolet protection factor (UPF) and the percent of UV transmission for UV-A and UV-B ranges were calculated according to Eqs. 1 and 2, respectively, and are listed in Table 2. The data reflect the higher protection against UV radiation provided by the ZnO-treated fabrics, particularly for the cotton samples loaded with zinc oxide nanoparticles synthesized according to procedure 2, while in the case of wool the nanoparticles produced with the two different methodologies gave comparable results. Although the calculated UPF are significantly lower than the standard values required for classifying the clothing as “excellent” in UV-shielding, however these results confirm the protection against UV radiation produced by the treatment with nanosized ZnO on the fabrics. This effect can be enhanced by:

  1. (1)

    selecting a more compact fabric,

  2. (2)

    selecting another kind of textile material (polyester, viscose, etc.),

  3. (3)

    using a different procedure for curing the fabric with the ZnO dispersion,

  4. (4)

    spreading an appropriate filming additive on the fabric in order to increase the amount of zinc oxide embedded into the textile structure.

Table 2 UPF (Eq. 1) and percent UV transmission (Eq. 2) for the different samples for UV-A (315–400 nm) and UV-B (295–315 nm) radiation
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Mechanical properties

The application of ZnO nanoparticles on cotton increases the mechanical strength of the fabric, especially in the warp direction, and decrease their elongation if compared to the untreated cotton (see Table 3). Probably the treated fabric is stiffer than the untreated specimen, and this results in a higher maximum force and lower elasticity. In the case of wool, the application of ZnO nanoparticles increases the maximum force, both in the warp and weft direction. However, the elongation of treated wool samples is higher than that of the untreated specimen (see Table 3). This effect could be related to the peculiar behavior of wool fabrics in the presence of aqueous environments (shrinkage), and particularly in alkaline conditions, that lower its resistance and deteriorate its structural properties.

Table 3 Tensile strength (N) and elongation (%) of the different samples
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Conclusions

We report the synthesis of zinc oxide nanoparticles through a homogeneous phase reaction starting from zinc chloride and sodium hydroxide at high temperature, in water or in 1,2-ethanediol. The reaction in 1,2-ethanediol at 150 °C results in the formation of smaller nanoparticles with respect to the reaction carried out in water at 90 °C. In both cases, the nanoparticles appear to be nearly spherical and with a quite narrow size range. Nanoparticles were analyzed through electron microscopy, X-ray diffraction, FTIR, and specific surface area experiments. The homogeneous phase reaction processes offer a valid alternative for an industrial-scale production of ZnO nanoparticles for many applications.

The peculiar performance of ZnO nanoparticles as UV-absorbers, can be efficiently transferred to fabric materials through the application of ZnO nanoparticles on the surface of cotton and wool fabrics. The UV tests indicate a significant increment of the UV absorbing activity in the ZnO-treated fabrics. Such result can be exploited for the protection of the body against solar radiation and for other technological applications.

Further studies are currently being carried out in order to enhance the UV-shielding activity of ZnO-loaded textiles.