Smart packaging functions can be provided by printing functional labels onto packaging materials using inkjet printing and inks with changeable photoluminescence properties. Carbon nanoparticles are considered a perspective fluorescent component of such inks. Ink compositions based on carbon nanoparticles are developed and adapted for inkjet printing on paper packaging materials for producing smart packaging labels. The influence of technological factors of the printing process on the photoluminescence characteristics of the printed images is investigated. The main investigated factors are the concentration of carbon nanoparticles, the relative area of raster elements of a raster field of a tone image, the absorbance and surface smoothness of paper. The resulting parameters are photoluminescence intensity and color. It is found that in case of changes in surface smoothness and absorbance of paper and concentrations of carbon nanoparticles in the ink compositions, the photoluminescence intensity of a printed image changes while its photoluminescence color remains the same. To obtain the highest contrast of tone inkjet-printed images with carbon nanoparticles on papers with any absorbance, the highest concentration of carbon nanoparticles in the ink composition should be used. However, the highest contrast and the highest own photoluminescence intensity of a tone inkjet-printed image with inks with carbon nanoparticles can be achieved only on papers with the lowest absorbance. The most noticeable difference between photoluminescence intensity of printed images on papers with any absorbance can be obtained with the lower concentration of carbon nanoparticles in the ink composition (10 mg/mL). The optimum concentrations of carbon nanoparticles in the composition are determined: for papers with low absorbance—10 mg/mL, and for papers with medium and high absorbance—25 mg/mL. Analytical dependency is created for photoluminescence intensity of images printed with inkjet printing inks with carbon nanoparticles as a function of the studied technological factors. Some design solutions for photoluminescent labels are suggested.
Smart packaging is becoming more and more popular as a novel solution for customer safety. Its ability to indicate the state of a packaged product as a result of exposure to compounds emerging during food product aging allows smart packaging to supply a consumer actual and reliable information about the safety of a packaged food consumption [
Smart packaging functions through functional labels on a package surface. These elements react to the predetermined changes of a product (chemical agents that occur in a product as a result of its spoilage, defrosting, and refreezing, as discussed in several studies, for instance [
The most perspective way to apply carbon nanoparticles onto packaging material is printing. Printing techniques allow high-speed manufacture of low-cost functional labels for smart packaging on an industrial scale. The study [
Therefore, in order to enable the use of carbon nanoparticles for inkjet-printed functional labels for smart packaging, the printing inks based on carbon nanoparticles should be created and their application to packaging materials should be studied considering the influence of technological factors of the printing process on photoluminescence properties of the obtained printed images. The aim of this study is to determine the effect of process parameters of applying ink compositions containing carbon nanoparticles onto paper packaging materials by inkjet printing on the photoluminescent properties of the obtained printed images.
Nanoparticles of carbon were prepared by thermal treatment using a solid-state reaction method of a mixture of dry citric acid and thiourea with a molar ratio of the components of 1 : 3 respectfully at 200°C for 30 min [
The inkjet printing ink was created by diluting the obtained colloidal solution of carbon nanoparticles with distilled water to obtain the following concentrations of carbon nanoparticles: 5 mg/mL, 10 mg/mL, and 25 mg/mL. The initial solution of carbon nanoparticles has the viscosity compatible with inkjet printing equipment and technology; these solutions are suitable for the stable printing process on paper materials (i.e., absorbing materials).
The printed images were obtained using inkjet printing with the developed ink compositions based on carbon nanoparticles onto paper substrates. The test form was designed consisting of raster areas 2 cm × 2 cm with different relative areas of raster elements (20%, 40%, 60%, 80%, and 100%). The paper for inkjet printing was selected on the basis of its low content of optical brightness agents (OBAs) in order to avoid interference of own photoluminescence of a substrate with the photoluminescence of a printed image because intensive photoluminescence of a paper often can quench the photoluminescence of a printed image completely.
The papers have different surface smoothness and absorbance. The paper smoothness is measured by Becca and is to measure the time of passage of a certain volume of air in the vacuum chamber between the sample surface and a polished glass plate surface in certain conditions.
The influence of paper absorbance on photoluminescence properties of inkjet-printed images was also investigated. The absorbance ability of the paper is characterized by its degree of sizing. The degree of the sizing of the paper can be defined by a stroke technique. According to this technique, the standard ink is applied onto a substrate (in this case, paper) by drawing pens graded in a series of strokes of an equal length of 75 mm and a width that is growing (i.e. 0.25 mm, 0.5 mm, 0.75 mm, 1.0 mm, and 1.25 mm). The width of the stroke that has the largest width, which is not blurred on the substrate and does not pass through it at the same time, indicates the degree of sizing of the substrate in mm.
Among all the technological factors which can be controlled during the printing process, the following factors were considered to have the most significant influence on photoluminescence properties of the obtained images; so in this study, they varied as follows: (1) the content of printing ink varied depending on the concentration of an initial carbon nanoparticle solution in the ink, namely, 5 mg/mL, 10 mg/mL, and 25 mg/mL; (2) the relative area of raster elements was the following: 20%, 40%, 60%, 80%, and 100%; (3) the absorbance of the paper was the following: relatively low for paper A (with the degree of sizing of 2.0 mm), medium for paper B (with the degree of sizing of 1.25 mm), and the highest for paper K (with the degree of sizing of 0.75 mm); (4) the surface smoothness of the papers was the following: the highest for paper A (125 sec), medium for paper B (95 sec), and the lowest for paper K (65 sec).
The main investigated parameters of photoluminescence of the printed images obtained using the inks with carbon nanoparticles are photoluminescence intensity and photoluminescence color. The photoluminescence color is defined by the interrelation of the positions and peak heights of photoluminescence bands.
An inkjet printer Epson Stylus SX 4300 with 600 dpi resolution was used to apply the developed inkjet printing ink onto paper substrates. Optical density spectra were recorded with a spectrophotometer Analytik Jena Specord 210. Photoluminescence spectra were recorded with a photoluminescence spectrometer Perkin-Elmer LS55. Transmission electron microscopy (TEM) data were obtained with a Selmi PEM-125К (Ukraine) with an accelerating voltage of 100 kV.
According to TEM microphotographs (see Figure
TEM photograph of the obtained carbon nanoparticles.
The edge of the absorption band of the resulting solution of carbon nanoparticles is at 450 nm (Figure
The absorption spectrum of a solution of carbon nanoparticles with the concentrations of 50 mg/mL (1) and 5 mg/mL (2).
As shown in Figure
The photoluminescence excitation spectrum (1) and photoluminescence spectrum (2) of the solution of carbon nanoparticles;
Besides, as can be seen from Figure
Figure
The photo of the obtained inkjet-printed images under UV light and daylight for three types of papers which differ by their absorbance: (a) paper A, (b) paper B, and (c) paper K. Concentration of carbon nanoparticles in the ink; 25 mg/mL. Currency detector Pro 7 Intellect Technology (UV lamp output is 7 W).
Figure
The photoluminescence spectra of raster fields with a relative area of raster elements: 0 (photoluminescence of paper), 10, 20, …, 100% for paper A.
Figure
The dependencies of photoluminescence intensity of inkjet-printed images at
As shown in Figure
As shown in Figure
Contrast of reproduction of gradations with photoluminescent printing inks for papers A, B, and K at the concentration of carbon nanoparticles in the ink compositions of 5 mg/mL, 10 mg/mL, and 25 mg/mL.
Since own photoluminescence of the paper causes a significant impact on the photoluminescence intensity of the printed images, the photoluminescence intensities of papers were subtracted from the photoluminescence intensities of the printed images. The resulting data are presented in Figure
The dependencies of intensities of own photoluminescence (photoluminescence without paper) of the inkjet-printed images at
As shown in Figure
For a more detailed study of this phenomenon, there were built dependencies of own photoluminescence intensity of the printed images on the concentration of carbon nanoparticles for the set of relative areas of raster elements of raster fields (20, 50, 70, and 100%) for each of the studied papers alone (Figure
The dependencies of the own photoluminescence intensity of the printed images at
Thus, it was found that for papers with low absorbance (paper A), the optimum concentration of carbon nanoparticles in the ink composition is 10 mg/mL. The use of larger concentrations is not recommended because with the increase of the concentration of carbon nanoparticles in the composition up to 25 mg/mL at the low relative areas of raster elements (20%, 50%), own photoluminescence intensity of printed images decreases by 44.1% and 33.7%, respectively. At the high relative areas of raster elements (70%, 100%), it reduces slightly (by 5.7%) and remains unchanged accordingly (Figure
For papers with average absorbance (paper B), the optimum concentration of carbon nanoparticles in the ink composition is 25 mg/mL, since the use of high concentrations of carbon nanoparticles in the ink composition allows to significantly increase the own photoluminescence intensity of printed images at high relative areas of raster elements (70%, 100%) by 85.9% and 58.0%, respectively. However, the own photoluminescence intensity of printed images at low relative areas of raster elements (20%, 50%) rises less significantly (by 47.1%) or remains unchanged, respectively (Figure
For papers with high absorbance (paper K), the optimum concentration of carbon nanoparticles in the ink composition is 25 mg/mL, since the use of high concentrations of carbon nanoparticles in the ink composition can drastically increase the own photoluminescence intensity of printed images at high relative areas of raster elements (70%, 100%) by 128.1% and 217.9%, respectively. However, the own photoluminescence intensity of printed images at low relative areas of raster elements (20%, 50%) slightly reduces (by 51%) or increases less significantly (by 52.6%) (Figure
A full factorial experiment was conducted, and a mathematical model was created to obtain the analytical dependence of photoluminescence intensity of printed images
The equation of a mathematical model in normalized coordinates is
The equation of a mathematical model in nonnormalized (native) coordinates is
To sum up, the effects were studied of technological factors of the inkjet printing technique on photoluminescence properties of printed images with carbon nanoparticles. Based on the calculated analytical dependence (in normalized coordinates) of photoluminescence intensity of printed images on the concentration of carbon nanoparticles, the relative area of raster elements of raster fields, and the surface smoothness of paper, it was stipulated that
The surface smoothness of the paper has the greatest influence on the photoluminescence intensity of inkjet-printed images with carbon nanoparticles in the ink composition. The relative area of raster elements of raster fields has less influence on the photoluminescence intensity of inkjet-printed images with carbon nanoparticles in the ink composition. The concentration of carbon nanoparticles in the ink composition has relatively the least impact on the photoluminescence intensity of inkjet-printed images. However, this is due to the fact that it is recommended to choose the average concentration of the photoluminescent component (carbon nanoparticles) in the ink composition (10 mg/mL) because higher concentration leads to an insignificant increase in the photoluminescence intensity of the resulting printed image.
The comparison of the impact values of the studied technological factors on the luminescence intensity of the printed labels was conducted in order to determine which technological factors are the most effective to be varied to increase or decrease the output luminescence intensity of the printed labels.
Surface smoothness of the paper has the greatest effect on the photoluminescence intensity due to the fact that the surface of a more rough paper is less covered with ink, ink is more absorbed between fibers of the paper. At the same time, in irradiation with UV radiation of the image on a smooth paper, it physically receives more UV light directed vertically at the image, while the roughness of paper causes some optical quenching in case of irradiation of a label with UV light as well as fluorescent emission returning to an observer.
The relative area of a raster element also causes the influence on the luminescence intensity, since it expresses the physical amount of ink (and hence the amount of luminescent substance) distributed in the area of a printed image. However, when comparing the impact values of the above technological factors, it was experimentally found that surface smoothness of the paper has a relatively greater impact value in percentage terms than the relative area of a raster element.
When comparing the impact values of the abovementioned technological factors, it has been experimentally found that the concentration of carbon nanoparticles has a relatively small influence on the luminescence intensity of the samples in percentage terms than the surface smoothness of paper and the area of the raster element of the raster field. However, this influence is still significant and must be taken into account (see Figures
In addition to providing advanced functionality to food packaging, printed photoluminescent labels can perform a finishing function. Combining printed with ink photoluminescent ink elements of an image with the elements of those printed by traditional printing inks, several options of design solutions can be reproduced:
An invisible in daylight image printed solely with photoluminescent inks (see Figure A combined image—partly visible in daylight image containing the elements reproduced with photoluminescent inks and visible only when irradiated with UV light
A variation of the latest option is a smart packaging label with the standard for comparison. Such label is divided into two parts, one of which is in contact with a packaged product and changes its optical characteristics in response to changes in the state of a packaged product. The second part is not in contact with the product and do not change its optical characteristics and serves as a benchmark for comparison.
In this research, there are investigated parameters of the process of inkjet printing with ink compositions containing carbon nanoparticles on photoluminescence characteristics of the obtained printed images. It was found that the position of photoluminescence peak of the printed images does not change for the studied paper types and concentrations of carbon nanoparticles. In this case, only the photoluminescence peak height changes. It means that the photoluminescence intensity of these cases changes while the photoluminescence color remains the same. The highest contrast of tone images on papers with any absorbance can be obtained using the highest studied concentration of carbon nanoparticles (25 mg/mL) in the ink composition. At any concentration of carbon nanoparticles in the ink composition (5 mg/mL, 10 mg/mL, or 25 mg/mL), the highest contrast of tone image can be achieved by the use of papers with the lowest absorbance. Papers with the lowest absorbance also allow obtaining the highest own photoluminescence intensity of printed impressions (excluding photoluminescence of paper). The lower the concentration of carbon nanoparticles in the ink composition is, the more noticeable the difference between photoluminescence intensities of printed images on paper with low absorbance and papers with higher absorbance. The optimum concentrations of carbon nanoparticles in the composition are determined: for papers with low absorbance—10 mg/mL, and for papers with medium and high absorbance—25 mg/mL. Analytical dependence is calculated for the photoluminescence intensity of images printed with inkjet printing inks with carbon nanoparticles as a function of the concentration of carbon nanoparticles, the relative area of raster elements of a raster field, and the surface smoothness of paper. A few design solutions for photoluminescent labels are suggested for them to perform not only informative but also finishing features, combined with the function of protection against forgery. This is achieved by combining parts of an image printed visible in daylight inks and inks with carbon nanoparticles which are visible only under UV light.
The data are available on request. Olha Hrytsenko should be contacted via email at
The authors declare that there is no conflict of interest regarding the publication of this paper.
Publications are based on the research provided by the grant support of the State Fund for Fundamental Research of Ukraine (Project N F64/10-2016).