Thermochromic Printing Research Articles

The influence of the textile substrate color on the contrast of thermochromic prints

Thermochromic inks have attracted much attention due to their application in various fields, including the textile industry. The application of these colors in sports clothing enables the monitoring of the athlete’s physiological state through the possibility of changing the color of the material under the influence of skin temperature. Due to the printing of thermochromic inks on textile materials used for visual control, there is a need to analyze the contrast of the printed elements. Contrast plays a significant role in visually highlighting shapes and details and also facilitates the recognition of printed parts and information on textile materials. Contrast analysis of thermochromic inks on different colors of textile materials plays a role in determining the optimal combination of base colors and thermochromic inks that will result in the most noticeable color change. This analysis allows designers and manufacturers to make good decisions about choosing the colors of textile materials when printing with thermochromic inks because, in this way, a high level of aesthetics and functionality is achieved. This work aims to compare and analyze the contrast of thermochromic inks on different colors of the substrate of different types of materials, as well as to determine whether the color of the fabric on which the thermochromic ink is printed affects the color contrast when it is changed, i.e., its discoloration and return to its original state. Magenta reversible thermochromic water-based leuco dye was used to print the samples. The work used textile materials for printing with different structures and different colors. The colors were chosen to be bright and to provide a good contrast with the magenta color so that their combination attracts the viewer’s attention and can create a strong visual effect. Colorimetric and contrast analyses determined that the color of the substrate is dependent on the contrast value. In the case of polyester textile material, the white color of the substrate showed the highest contrast values. On the other hand, in the case of printed samples on a material made of a mixture of polyester and elastane, it can be concluded that the gray color of the substrate gives the highest contrast values. In conclusion, the research highlights the importance of choosing colors and materials to achieve optimal results in the design and production of textile materials with thermochromic inks.

The effect of carbonyl components of printing substrates on the durability of UV thermochromic prints

Special effects on a printed product can increase consumer’s interest in a product and therefore lead to higher revenues in consumer industries. For that purpose, graphic industry introduced various technologies and materials that would have such an effect and enhance consumer-goods interaction. One of those solutions is thermochromic printing. Although bringing added value, thermochromic inks have some challenges in application due to their higher sensitivity, especially when exposed to UV light. Therefore, the goal of this work is to determine whether amount of the UV radiation during curing of UV thermochromic prints, as well as exposure after printing would degrade the thermochromic print on various substrates. UV thermochromic prints were made on three different paper printing substrates using the screen printing method and dried in a laboratory controlled unit. Synthetic paper, recycled paper containing 100 % recycled cellulose fibers and bulky voluminous paper were used as printing substrates. During the UV curing of the ink, no photooxidation of the prints occurred. Additional exposure of the prints to UV radiation (after the ink has hardened) leads to their photooxidative degradation, i.e., a change in the initial color. The obtained colour difference (ΔE) is increasing with the increase of the irradiation amount. The highest colour difference is on the synthetic paper while prints on two other substrates are more resistant to UV light. A print on synthetic paper photooxidizes the fastest due to the presence of most carbonyl groups in it. Generated free radicals promote the instability of prints on synthetic paper. Research has proven that when using synthetic paper or substrates with similar characteristics, care should be taken to accurately determine the amount of UV energy required for curing in order to prevent photodegradation of the ink. The results also show that to explain any degradation of prints, the chemical components of the paper needs to be taken into account, which is rarely done.

Bisphenol A monitoring during anaerobic degradation of papers with thermochromic prints in soil

Pseudoestrogene bisphenol A (BPA) can be important ingredient of thermochromic inks, increasingly used materials in thermal printing paper, security printing, advertising, design and as temperature indicators in medicine and food industry. BPA mass fraction in thermochromic inks can be up to several percent. Hence, disposal of items with thermochromic prints pose a risk of environmental pollution. In this work BPA mass fraction was monitored during anaerobic degradation of papers with thermochromic prints in soil in both matrices: papers and soil. The degradation conditions simulated deeper layers of waste at a landfill site. Six types of papers with prints of thermochromic ink containing 2% of BPA were subjected to anaerobic degradation over up to 150 days. Initial mass fractions of BPA in papers decreased form (126–460) μg/g to (<QL – 45) μg/g after 150 days. BPA amounts were reduced 10 to 50 times depending on the paper type: least for synthetic paper and most for wood-free coated. For soil analysis new HPLC-UV method was developed and validated. The method was linear from 0.75 ng/g to 0.6 μg/g of BPA in soil with correlation coefficient of 0.9994. Method precision was 4.4%, accuracy 83% and detection limit 0.9 ng/g. Expectedly, amount of BPA in soil was increasing during the experiment. Mass fractions of BPA in soil were from not detected in earlier stage of degradation to (4.9–23.2) ng/g after 150 days. Final BPA amounts in soil were similar to those found in industrial, urban and agricultural soils worldwide. Hence, BPA from papers with thermochromic prints was notably decomposed, and contaminated soil had the capacity to absorb and decompose BPA even under anaerobic conditions. After 150 days of anaerobic degradation, only up to 1.86% of BPA contained in paper prints was found in soil, whilst, on average, 4% of initial BPA remained in paper.

Biodegradation of UV Curing Thermochromic Prints with Respect to Printing Substrate

Amount of packaging waste is constantly increasing, resulting in different environmental problems. Packaging waste, mainly plastic and paper based, comprises about one-third of all municipal solid waste. Anaerobic degradation is common method of the waste management for the organic based materials. In this study, biodegradability potential of UV curable thermochromic ink, as most common sample od smart packaging, was studied. The used ink is based on polyurethane acrylate.. Results indicated that by the proper selection of materials the biodegradability aspect of printed samples can be varied. Moreover, the results point to heterogeneous degradation of prints surface. This can be confirmed by the determination of the prints’ total colour difference, which was relatively high in all cases pointing to print deterioration. Biodegradation of UV TC ink, based on polyurethane acrylate results in breaking down of the ester linkages i.e. due to the hydrolysis of the ester bonds. Within the used printing substrates, TC print on wood free uncoated paper shows the highest rate of biodegradation over 150 days of incubation, around 40%, followed by print on 100% recycled paper and wood free coated paper. The present study shows that TC prints in studied environment, are ultimately biodegradable.

Understanding the Role of Paper-Ink Interactions on the Lightfastness of Thermochromic Prints.

Thermochromic inks (TC) have received increasing attention in recent years, particularly in the design and packaging industries. Their stability and durability are crucial for their application. This study highlights the detrimental effects of UV radiation on the lightfastness and reversibility of thermochromic prints. Three commercially available TC inks with different activation temperatures and in different shades were printed on two different substrates, cellulose and polypropylene-based paper. Used inks were vegetable oil-based, mineral oil-based and UV-curable. The degradation of the TC prints was monitored using FTIR and fluorescence spectroscopy. Colorimetric properties were measured before and after exposure to UV radiation. The substrate with a phorus structure exhibited better colour stability, suggesting that the chemical composition and surface properties of the substrate play a crucial role in the overall stability of thermochromic prints. This can be explained by the ink penetration into the printing substrate. The penetration of the ink into the structure (cellulose fibres) protects the ink pigments from the negative effect of the UV radiation. Obtained results suggest that although the initial substrate may appear suitable for printing, its performance after ageing may not be optimal. In addition, the UV curable prints show better light stability than those made of mineral- and vegetable-based inks. In the field of printing technology, understanding the interplay between different printing substrates and inks is critical to achieve high-quality, long-lasting prints.

Enhancing Applicability of Reversible UV Thermochromic Offset Inks: Edge Quality Parameters and Thermochromic Printing System Modulation Transfer Function.

Modern logo design is characterized by its ability to convey information through the use of various images and text compositions. These designs often use simple elements such as lines to capture the essence of a product. When using thermochromic inks in logo design, it is important to consider their composition and behavior, as they differ significantly from conventional printing inks. This study aimed to determine the resolution capabilities of the dry offset printing technique when using thermochromic ink, with the ultimate goal of optimizing the thermochromic ink printing process. Horizontal and vertical lines were printed using both thermochromic and conventional inks to compare the edge reproduction characteristics of the two ink types. Moreover, the impact of the type of applied ink on the share of mechanical dot gain of the print was investigated. Additionally, modulation transfer function (MTF) reproduction curves were generated for each print. Moreover, scanning electron microscopy (SEM) was conducted to investigate the surface of the substrate and prints. It was found that the quality of the printed edge produced by thermochromic inks can rival that of conventional inks. Thermochromic edges showed lower raggedness and blurriness values for horizontal lines, whereas line orientation proved to be insignificant in the case of vertical lines. MTF reproduction curves confirmed higher spatial resolution for vertical lines in the case of conventional inks, whereas they were identical for horizontal lines. The share of mechanical dot gain is not highly influenced by the ink type. SEM micrographs confirmed that the conventional ink smooths out the micro-roughness of the substrate. However, on the surface, the microcapsules of thermochromic ink (measuring 0.5-2 µm) are observable.

Assessment of Thermochromic Packaging Prints’ Resistance to UV Radiation and Various Chemical Agents

Thermochromic inks, also known as color changing inks, are becoming increasingly important for various applications that range from smart packaging, product labels, security printing, and anti-counterfeit inks to applications such as temperature-sensitive plastics and inks printed onto ceramic mugs, promotional items, and toys. These inks are also gaining more attention as part of textile decorations and can also be found in some artistic works obtained with thermochromic paints, due to their ability to change color when exposed to heat. Thermochromic inks, however, are known to be sensitive materials to the influence of UV radiation, heat fluctuations, and various chemical agents. Given the fact that prints can be found in different environmental conditions during their lifetime, in this work, thermochromic prints were exposed to the action of UV radiation and the influence of different chemical agents in order to simulate different environmental parameters. Hence, two thermochromic inks with different activation temperatures (one being cold and the other being body-heat activated), printed on two food packaging label papers that differ in their surface properties were chosen to be tested. Assessment of their resistance to specific chemical agents was performed according to the procedure described in the ISO 2836:2021 standard. Moreover, the prints were exposed to artificial aging to determine their durability when exposed to UV radiation. All tested thermochromic prints showed low resistance to liquid chemical agents as the color difference values were unacceptable in all cases. It was observed that the stability of thermochromic prints to different chemicals decreases with decreasing solvent polarity. Based on the results obtained after UV radiation, its influence in terms of color degradation is visible on both tested paper substrates, but more significant degradation was observed on the ultra-smooth label paper.

Development of Thermochromic Ink Using the Anthocyanidin-Based Red-Cabbage Extract for Anticounterfeiting Applications.

Temperature-driven colorful switching inks have been an interesting security encoding method to improve the anticounterfeiting properties of commercially available merchandise. Recently, thermochromic inks have faced many disadvantages, such as low efficiency, high cost, and low durability. In the current study, we developed self-healable ink from poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (PAMPSA) integrated with the anthocyanidin(ACY)-based red-cabbage extract in the presence of ferrous sulfate for authentication purposes. Self-healable inks have been able to guarantee durability and thermal stability. Environmentally friendly, ACY-based chromophore was extracted from Brassica oleracea L. var. Capitata (red-cabbage) to serve as a spectroscopic probe immobilized into PAMPSA. The prepared self-healable nanocomposite ink (PAMPSA-ACY) displayed temperature-induced chromism with high reversibility and thermal stability. Different self-healable nanocomposite inks of thermochromic features were prepared employing different ratios of the ACY-based red-cabbage extract. As described by Commission Internationale de L'éclairage Lab coordinates, homogeneous films were stamped on the paper surface to show a purple color (631 nm) able to switch color into red (458 nm) with the increase in temperature from 25 to 65 °C, respectively. Transmission electron microscopy, infrared spectra (FT-IR), energy-dispersive X-ray, and scanning electron microscopy were utilized to inspect the morphological behavior and chemical compositions of thermochromic prints. Both mechanical and rheological properties of ink-printed paper substrates and ink solution were also investigated. Both of antimicrobial activity and cytotoxicity study of the nanocomposite ink (PAMPSA-ACY) were also evaluated. Various industries can take the advantage of the current ink as a competent approach for anticounterfeiting purposes.

Fastness Improvement of a Crystalline Liquid Thermochromic Print on Cotton Fabric by the Application of Silica Nanoparticles from Rice-Husk

Most commercially available thermochromic dyes are not resistant to washing and rubbing when applied to textile materials. This is due to their low affinity for fibre. The addition of silica extracted from rice husk ash using the sol-gel method was performed to improve colour fastness and maintain the stability of thermochromic dyes printed on cotton fabrics. The rice husks used in this study were derived from the Baroma rice variety with silica content in ash and silica gel of 97.05% and 99.20%. The morphological structures and particle sizes of the silica obtained were analysed using a scanning electron microscope (SEM) and particle size analyser (PSA). The particle sizes of the silica product, thermochromic dye and silica-dye mixture were 53.64–60.66 nm, 2.603 nm and 5.827 nm, respectively. The printing process of silica: the dye mixture was applied to cotton fabric in a ratio of 1:1. Fluid of silica: the dye showed good stability until the seventh day of observation. Colour fastness to washing assessed using a staining scale was better with the addition of silica than without silica, i.e. 3–4 in the first washing and 3 in the third washing. Similarly, fastness to rubbing was also better with the addition of silica, i.e. 3–4 dry rubbings and 3 wet rubbings. Moreover, the combination of silica, binder, PDMS and dye (in a ratio of 1:1:1:1) gave the best colour fastness to washing and rubbing.

Description of Thermochromic Offset Prints Morphologies Depending on Printing Substrate

In this study, the influence of physio-chemical properties of four printing substrates, fibrous papers (filter, bulky, recycled), and polymer film (synthetic paper) on the size of the thermochromic (TC) prints microcapsules was investigated. Results indicate that interaction between thermochromic ink and printing substrate determine the print morphology, i.e., porosity and roughness of printing substrate affect dimensions of TC microcapsules of resulting prints, while ink adhesion affects dimensional changes during heating–cooling cycle. Atomic force microscopy (AFM) analysis showed that microcapsules of the prints, on the surface of the smoothest synthetic paper, possess the smallest diameter and height, while, on the most porous and the most roughened surface of F paper, the microcapsules of the prints possess the highest diameter and height. By increasing the temperature to 40 °C, the biggest changes in the shape of the microcapsules (increase in height and decrease in diameter) were obtained using the surface of the hydrophilic filter paper. While using the recycled paper surface, the situation is opposite; the height and diameter of the microcapsules are reduced, and the microcapsules penetrate deeper into the paper structure (due to optimum adhesion). On the bulky paper surfaces, which are more hydrophobic than recycled paper (higher interfacial tension), the increase of temperature does not cause any significant changes in the shape and position of the microcapsules. The same behavior is observed using hydrophobic non-porous synthetic paper.

Light fastness of liquid crystal-based thermochromic printing inks

Thermochromic inks are known for their poor stability against UV light, appearing differently in leuco dye-based and liquid crystal-based ink types. The leuco dye-based inks lose the colour contrast between the coloured and discoloured states but the temperature at which the colour changes stays the same. These inks retain thermochromic functionality, i.e. the relation between colour and temperature, as long as the colour contrast between both states remains visible. Liquid crystal-based inks reflect a narrow band of light with wavelengths equal to the pitch length of the helical structure formed in the chiral-nematic mesophase. While the length of this helix depends on temperature, the wavelength of the reflected light shifts across the spectrum. This unique thermochromic effect is also called the colour play effect and is used for visual determination of temperature. However, the helical structure is highly sensitive to UV light which changes the temperature-related colours; consequently, such an ink loses its thermochromic functionality. Almost complete deterioration of the colour play effect occurs on unprotected samples in less than 60 days of exposure to natural indoor daylight and after 10 h under the artificial daylight. UV-protective foil or UV filter protects the ink against detrimental effects of UV light and keeps its functionality even after 10 h of exposure but the colour play effect is shown at lower temperatures.

Production of thermochromic microcapsulated inks for smart packaging and examination of printability properties

PurposeStimulant-sensitive materials exhibit physical or chemical reversible changes in their properties as a result of environmental variables. One of these materials is thermochromic materials. Materials with thermochromic sensitivity change their color with heat exchange. For this reason, it can be used in many different fields such as security inks. Such substances decompose rapidly by being affected by weather conditions. Furthermore, the particle sizes are larger than normal pigments, and therefore, it is difficult to stabilize thermochromic dyes. Because of all these adverse conditions, thermochromic colorants must be protected before use in the ink. This protection is planned to be provided by the microcapsulation technique. The purpose of this study is to determine the thermochromic printing inks that can be stored stably by microcapsulation technique, to protect it from environmental conditions and the determination of printability parameters.Design/methodology/approachIn this study, capsules with a core material of thermochromic dyeing with polyurea formaldehyde (PUF) or poly-phenolmelamine formaldehyde (PMF) shell were synthesized at appropriate pH and temperature using the appropriate solvent and mixing speed. The chemical structure and dimensions of the obtained capsules were examined by ATR-FTIR and scanning electron microscopy, respectively. The produced thermochromic microcapsules were mixed with alkyd resin and mineral oil and screen printing ink was obtained. Printability tests such as surface morphology, color, gloss and light fastness were applied.FindingsAs a result, it was determined that PMF is not a suitable encapsulation technique for thermochromic dyes under suitable conditions and eliminates thermochromic property by providing heat stability. It was found that PUF microcapsulation can be used in thermochromic dyestuff encapsulation and does not lose the thermochromic property. It has also been found that PUF microcapsules increase the lightfastness and stability of thermochromic dye ink.Originality/valueThis study provides experimental research on the encapsulation of a thermochromic dye and its use in ink.

Migration of pseudoestrogen bisphenol A from various types of paper with thermochromic prints to artificial sweat solutions

Thermochromic inks, materials that change color at a certain temperature, are increasingly used on papers and other materials in the areas of design, commercials, and security printing. Pseudo-estrogen bisphenol A (BPA) may be one of their main compounds present in mass fractions of up to several percent. In this work, the mass fractions of BPA in thermochromic prints on seven types of paper were determined. Migration of BPA from the surface of the thermochromic print to artificial sweat solutions was investigated as well. Total amount of BPA in papers with thermochromic prints was determined by an HPLC-UV method with ultrasonic-assisted extraction in methanol developed and validated in this work. Total amount of BPA, which ranged from 0.126 to 0.778 mg/g, was compared with the amounts extracted under the same conditions in two artificial human sweat solutions, which differed in chemical composition, ionic strength, and pH-value. Mass fractions of BPA extracted with artificial sweat solutions were from 0.047 to 0.175 mg/g with respect to the mass of the paper. On average, the mass fraction of BPA was four times less when extracted with artificial sweat solutions than the maximal amount extracted with methanol. The amounts of extracted BPA raise a concern of health risk through dermal exposure to BPA from thermochromic prints on paper.

Optical properties of deinked pulp containing thermochromic offset inks

This paper evaluates the efficiency of thermochromic offset prints recycling using deinking flotation method. Since offset thermochromic inks differ in their composition from conventional offset inks, the aim of this research was to determine how effectively they can be recycled. In the experimental part of the work, the laboratory printing of two thermochromic inks was conducted. For both types of inks, white uncoated paper was used as a substrate. After accelerated aging, the prints were mixed in equal proportions and then recycled in laboratory conditions, by means of deinking flotation. During the recycling process, handsheets and filter pads were prepared from the pulp obtained before and after deinking flotation. Handsheets and filter pads were also prepared from unprinted paper that was used as printing substrate, in order to obtain reference samples. The deinking flotation efficiency was evaluated by calculating the flotation loss, ash elimination, pulp’s brightness and whiteness increase and determination of residual ink area. The obtained results indicate that thermochromic prints are poorly deinkable. A small increase in deinked pulp’s brightness and whiteness (approx. 7% and 11%) is noticed. The image analysis shows that flotation deinking managed to eliminate around 40% of the total dirt area of thermochromic ink.

The influence of printing substrate on colorimetric properties of thermochromic offset prints

This paper investigates the colorimetric properties of reversible thermochromic offset printing ink on different paper substrates. Reversible thermochromic (TC) printing ink is printed by offset printing on four different printing substrates. Colorimetric measurements are performed by measuring diffuse reflection by determining uniform measurable physical size in the CIELAB colour space of the examined sample at temperatures ranging from 35°C to 75°C during heating and at the same temperatures up to 20°C. As the colour of the TC prints depends on the temperature, which results in colour hysteresis, it is noted that the colour hysteresis is affected by the printing substrate. The activation temperature given by the producer (63°C), is above the obtained temperatures, for print on filter paper 61,3°C, for print on synthetic paper 61,5°C, for print on bulky paper 60,9°C, with the highest deviation in the case of print on recycled paper (59,4°C). The results of total colour difference of samples measured between heating and cooling at the temperature well below the final chromic temperature and the CIEDE2000 total colour difference between the samples in the heated and cooled state as a function of temperature showed that the highest colour difference was obtained in TC print on the filter paper.

Ink tack stability readings of the offset thermochromic inks

PurposeThis paper aims to analyse the tack stability measurement readings of thermochromic offset inks. For this purpose, three reversible leuco-dye formed thermochromic inks were used to evaluate their tack. The used inks differ in their activation temperature, colour, drying mechanism, viscosity and chemical content. Thermochromic offset inks differ from conventional inks in formulation and size of colourants due to the presence of the microcapsules.Design/methodology/approachPrinting inks in offset lithography must have sufficient tack so they can be transported from a reservoir onto the substrate. The ink transfer takes place through the ink splitting by inking rollers, printing plate and blanket transport. An IGT Tackoscope device was used to produce the dynamic readings of ink tack as a function of distribution and splitting time. The data can be used for prediction of the ink press stability under controlled settings, such as temperature.FindingsThis research explains the temperature and velocity influence on the tack stability measurement readings of thermochromic inks. It also gives an insight of the chemical compounds that dictate the tack measuring reading results.Originality/valueDevelopment of new printing inks, such as thermochromic printing inks, may affect the printing of different products. The research was conducted due to a lack of available data for the tack of thermochromic inks.

Printed thermochromic displays

Thermochromic displays, which were evaluated in this study, combine printed electronics with the thermochromism phenomenon. Conductive lines printed on the reverse side and thermochromic printing ink printed on the front side of cardboard packaging form a thermochromic display that gives cardboard packaging additional value. Displays were printed on different printing materials, and thermochromic printing ink was deposited in one and two layers. In addition, half of the samples were varnished. The influence of the printing material, the thickness of the thermochromic printing ink layer, the varnish, the high temperature, and light fastness on the display's operability were all evaluated. It was clearly shown that the choice of printing material plays a crucial role in the display's operability. Moreover, high temperature and light fastness also have a significant influence, although the impact is negligible when the display is used at room temperature.

Thermochromic ink–paper interactions and their role in biodegradation of UV curable prints

In this study, biodegradability aspects of UV thermochromic leuco dye print on three different paper materials (synthetic, recycled, and bulky) were studied using the soil burial test under anaerobic conditions. Biodegradation of UV curable thermochromic prints were evaluated for changes by visual examination, microbial growth assay, weight loss measurements, Fourier transform infrared spectroscopy, scanning electron microscopy and colorimetric measurements. Results showed a better absorption of ink into the bulky paper structure, followed by recycled paper. Synthetic paper is not absorbent. The ink binder altogether with classic pigment and microcapsules penetrates into the structure of bulky paper. Recycled paper absorbs ink binder mostly with classic pigment. Better adsorption of binder into the bulky paper structure results in thinner layer of ink binder on the paper surface. On non-absorbent synthetic paper, microcapsules are covered with thicker layer of ink binder. In the case of bulky paper, the highest rate of biodegradation was observed, resulting in a higher number of bacteria, higher weight loss, higher changes in colour (destruction of almost all microcapsules) and almost complete reduction in thermochromic effect of the prints. Results show that microcapsules, which penetrate into the structure of bulky paper, are promoting the rate of bulky paper biodegradation. The opposite behaviour was noticed for the penetration of classic pigment into the structure of recycled paper, which resulted in remarkable reduction of recycled paper biodegradation rate. The thicker layer of ink binder (containing classic pigment) on the surface of microcapsules on recycled paper resulted in slower rate of microcapsules degradation and smaller colour change of the print. The thickest layer of ink binder and classic pigment on synthetic paper surface causes the slowest rate of biodegradation of print.

Photo-oxidation stability of microcapsules in thermochromic prints

In this paper, photochemical stability of two thermochromic prints was investigated: vegetable oil based offset and UV curing screen printing ink. The obtained preliminary results can be used for further detailed examination of prints stability. It is well known that thermochromic printing inks are very unstabile when exsposed to UV irradiance and this is why they are mainly used for applications that are not directly exposed to sunlight. The results of the study show the heterogeneous nature of photooxidative degradation of thermochromic prints, and the opposite behaviour of photooxidation can be noticed comparing examined prints. Microcapsules in the UV curable screen print by fixation with polar polymer binder can create a new products stable to photoxidation. For this reason, the areas where the microcapsules and binder are bonded together are stable. Degraded only areas where binder is not related to microcapsules. Microcapsules in offset print do not have the ability to create new stabile forms due to smaller polarity and different chemical composition of the offset oxidized binder. In the offset print, the microcapsules are the least photooxidative stable and also cause lower photooxidative stability of the binder in contact with them. Cavities are formed in the areas where microcapsules are in contact with the binder, while the areas in which the binder is not in contact with microcapsules are not degraded.

Solvents interactions with thermochromic print

In this study, the interactions between different solvents (benzene, acetone, cyclohexanone, various alcohols and water) and thermochromic printing ink were investigated. Thermochromic printing ink was printed on metal surface. Components of thermochromic printing inks are polymeric microcapsules and classic yellow offset printing ink. Below its activation temperature, dye and developer within the microcapsules form a blue coloured complex. Therefore, thermochromic print is green. By heating above the activation temperature, blue colour of the complex turns into the leuco dye colourless state and the green colour of the prints turns into the yellow colour of the classic offset pigment. The results of the interaction with various solvents show that the thermochromic print is stable in all tested solvents except in ethanol, acetone and cyclohexanone. In ethanol, the green colour of the print becomes yellow. SEM analysis shows that microcapsules are dissolved. In acetone and cyclohexanone, the green colour of the print turns into blue, and the microcapsules become significantly more visible. Thus, the yellow pigment interacts with examined ketones. Based on the obtained interactions it can be concluded that the microcapsules have more polar nature than the classical pigment particles. Solvent-thermocromic print interactions were analysed using Hansen solubility parameters that rank the solvents based on their estimated interaction capabilities.