The study of thermochromic offset inks

Transcription

1 The study of thermochromic offset inks Ondrej Panák University of Pardubice, Czech Republic ABSTRACT: The objective was to study the properties of thermochromic offset inks. We focused on rheological behaviour, the size determination of the pigments and printability of different papers. The rheological properties of the studied thermochromic inks are close to ideal viscous liquids. The size of the pigments was smaller than reported by the producer. The presented work is part of a joint project between the University of Pardubice (CZ) and the National Institute of Chemistry (SLO) within a KONTAKT program, which aims to support research cooperation between research centres. The project mainly involves Ph.D. students supervised by representatives: Marta Klanjšek Gunde (SLO) and Marie Kaplanova (CZ). 1 INTRODUCTION: Thermo-chromic polymer materials could be divided into several subcategories. The first could be the category of polymers thermo-chromic themselves. One of such polymer type is the Liquid crystalline polymer with helical superstructure. The incident light is reflected on the helical structure, satisfying the Bragg condition. Due to the temperature change, the structure changes as well, thus the colour change occurs. The thermochromic polymer-additive interaction is the second part of possible thermochromic polymers. Other category of thermochromic polymers are polymers doped with thermochromic additives. The mechanism is based on different absorption of the additive under changing temperature. Conjugated polymers, inorganic pigments and leucodye-developer-solvent systems are possible additives [1].The last one, leucodye-developer-solvent system, is the only one used for thermochromic offset inks, therefore it will be described in more details in the following text. Thermochromic pigment is a leucodye-developer-solvent system encapsulated in a polymer shield [1, 2, 3]. The colour change is established in the three components formulation, based on an interaction of a colour former (leuco dye) and a developer and interaction of the developer and a solvent. The colour former can be a spirolactone molecule [1, 4]. One of the possibilities is the CVL (Crystal Violet Lactone) which is colourless in the grounded lactone form. The CVL becomes coloured when the lactone ring opens (see Figure 1). It is caused by increasing the polarity or hydrogen bonding ability of the environment (developer) which increases the conjugation [4, 5]. Phenols are used as developers. They can vary in number of OH groups on one side and in length of hydrocarbon chains or short polar groups on the other side of the molecule. For example it can be a lauryl galate, bisphenol A etc. [1, 4, 6]. Solvents can be alcohols, esters, amides and acids with long chain aliphatic character. It is a relatively low melting hydrophobic compound and serves as the medium, where the colour former and developer interacts [4, 5]. Figure 1. Scheme of CVL [4] Figure 2. Capsules of thermochromic pigment 248

2 The colour change from coloured to colourless occurs upon heating the composition above its melting point. It is assumed, that the phase separation plays a major role in the process. In the melting process the colour former and the developer are dissolved in the solvent. The environment becomes relatively non-polar, so the colour former stays in the ring-closed colourless side. On cooling, the colour former and developer separate from the solution phase. The colour former experiences a more intimate contact with the developer that results in coloured composition [4]. To set proper parameters of the pigment, the well chosen developer and suitable solvent play a major role, due to the competition between developer-colour former interaction and developer-solvent interaction [4, 6, 7]. Encapsulation of leucodye-developer-solvent system is done using emulsification of thermochromic formulation in water with following interface or in-situ polymerization. The emulsification in water requires use of a surface-active compound. The compound should not react with the thermochromic formulation and in addition, it cannot interfere with the polymerisation. Size of final pigments is determined by this step, and is influenced by the concentration and type of the surface-active compound. A melamine or epoxy resin can be used in interface polymerization. The melamine resin is almost entirely insoluble in most solvents and the porosity and elasticity can be adjusted by controlling the degree of cross-linking. The epoxy resin has a higher transparency, and does not need cotensid during the encapsulation process [1, 2, 3]. Figure 2 shows an image of thermochromic pigment obtained by an electron microscope. The polymer shield is permeable and therefore the formulation of the vehicle has to take into account the specificity of the pigment. The vehicle consists of varnishes, reducers, extenders and driers. Neutral ph of such formulation is needed. Cyclohexane and other high molecular non-polar solvents are recommended. The polar solvent could interact with the pigment internal phase and thus change the thermo-chromic effect. The ph characteristic of printed substrate is also reported as a significant factor in functionality of thermochromic inks [8]. Thermochromic offset inks give pastel colours. Using these inks for security purposes requires special design of the graphics, registration of the press and use of other chemicals. It is important to establish colour specification of thermochromic and conventional inks, which are usually printed together. By heating the product the structure printed by thermochromic inks disappears, or conversely, the structure printed by the conventional inks appears [8, 9]. 2 EXPERIMENT: At this point in our experiment we had used just one set of offset thermochromic inks (red, magenta, black, producer: CTI, with activation temperature of 31 C), bought by the Slovenian partner within the framework of KONTAKT program. The curing process is the same as curing process of conventional offset inks (oxy-polymerization, and physical processes). The plan was to buy also offset thermochromic inks from another producer. As a Czech partner, we purchased three inks (red, green, blue,) with the activation temperature 35 C and a blue ink with the activation temperature 42 C. All of them were using the oxy-polymerization curing process. In addition, a UV curable ink with an activation temperature 35 C was bought. To our disappointment, all inks obtained by Czech partner had to be claimed. Very poor behaviour during milling of the ink in an IGT Printability Tester C1 was observed. Therefore, we also tried to mill the ink in an inking mechanism of a printing machine. The comparable bad behaviour was also observed in this case. Maps of the inks stayed on the cylinders even for longer milling time. Based on our claim, the distributor had to withdraw this type of ink from the market. We got new samples, but with the same unsuitable properties. This can be one of unexpected results. The buyer should be aware of the very long process of purchasing such special inks. It took over three months from placing the order and having the inks delivered. And moreover, we did not get well behaved product. If we had been a printing company, we would have failed in producing a product with thermochromic effect ordered by our customer. 249

3 First of all, the rheological properties of CTI offset inks were measured on Haake Rotoviso1 rotational rheometer, using a cone and plate measuring system. The flow curves and viscosity curves of all thermochromic inks were almost the same therefore only magenta thermochromic ink will be presented in the following text (TC mag). At each temperature, there is a limiting shear stress value. Over this limiting value the ink sample starts to flow-out from the measuring area and the shear stress values obtained cannot be evaluated. Thermochromic offset inks were compared to a conventional process magenta ink (Mag), and to an offset ink without pigment varnish (see Figure 3). It can be seen that thermochromic inks behave more like varnish than process printing ink. In addition, an analysis using the Ostwald model had been carried out. The model is based on the following equation [10]: τ = c γ p, where τ is a shear stress, γ is a shear rate, c is a flow coefficient and p is a power-law index. If the p value is lower than 1, the measured liquid is pseudoplastic, what means that the shear viscosity decreases with increasing shear rate. If the p value equal 1, it describes ideal viscous flow behaviour. By taking the logarithms of both sides of the equation a linear relationship is obtained. Form the linear regression p parameter can be easily found. The thermochromic ink seems to exhibit behaviour very close to the ideal viscous (see the p value in Table 1). From the data it can be concluded that the viscosity will be influenced by the temperature rather than by the shear rates. To describe the behaviour in the printing machine, the Casson model can be applied: (τ) 0.5 = (τ c ) 0.5 +(η c γ) 0.5, where τ is a shear stress, γ is a shear rate τ c is a Casson yield point, and η c is a Casson viscosity. Casson yield point describes the transition from the state of rest to flow. This situation occurs in ink duct, where the ink is dosed toward cylinders of an inking mechanism. As shown in Table 1, this parameter is lower for thermochromic ink and varnish when compared with process magenta ink. In practical application, it can be concluded that a smaller shear rate is needed to transfer the ink from the ink duct. Casson viscosity describes the viscosity of the liquid under shear rate limiting to infinity. Figure 3. The comparison of flow and viscosity curves Table 1. Parameters p and τc Parameter p TC [ C] mag. varnish MAG Parameter τc TC mag. varnish MAG The second objective was to measure the size distribution of the pigment particles in the ink. The images of the printed layer recorded by the electron microscope were inconvenient for this purpose. The second option was to measure a very thin layer of the ink by optical microscope. To obtain reliable data from image analysis, separated particles are needed. A small amount of ink was therefore dissolved in cyclohexane. Five images were captured for each colour. The particle size measurement was done by ImageJ from bitmapped images (see Fiure 4). An overview of the results shows Table 2. In the evaluation only particles with circularity above 0.8 were taken into account. The analysis shows that average particle size is surprisingly very small (cca 1.3 µm). The producer of the ink reports the particle size to be in interval µm, but we did not measure higher size than 3.8 µm. The black thermochromic has a slightly bigger pigments compared to magenta and red inks. 250

4 The CTI inks were printed onto four different papers: a matt paper, a glossy paper, a label paper and a metallic paper. The laboratory prints were carried out by IGT Printability Tester C1. The printing speed was 0.3 m/s. Printing force for gloss and metallic paper was set to 250 N, for matt and label paper to 400 N, for the black ink and label paper 300 N. A range of prints with increasing amounts of ink had been done for each paper and ink combination. The colour intensity increases with amount of transferred ink but according to producer: maximum colour might develop a slightly mottled appearance [11]. Figure 4. Image of TC mag. pigments obtained by optical microscope (left)and its bitmapped variant (right) Figure 5. Scanned sample 6 (left) and 15 (right) of TC mag. on glossy paper The samples were scanned with resolution of dpi (Figure 5) to obtain some results about uniformity from simple image analysis. At first, RGB values were transferred to L*a*b* colour space and distribution of lightness and chroma values were evaluated. Standard deviation skewness and kurtosis of scanned images were compared. The values do not differ very much and we could not conclude which sample (amount of transferred ink) should be chosen. Therefore a method suggested by Tadeja Muck [12] was applied. The non-uniformity factor is computed as the UX LX, where UX is the mean value of the interval determined by maximum and median of the data set, and LX is the mean value of the interval determined by minimum and median of the data set. In Table 3 there are non-uniformity parameters computed using chroma and lightness parameters and intensity of gray scale image. It seems that simple evaluation of gray scale image gives some reasonable values for evaluation. From a subjective point of view, the sample no. 6 was chosen as the optimal amount of ink transfer, where mottling still did not appear. There is also the highest difference in non-uniformity factor between sample no. 7 and sample no. 6. However, for more reliable evaluation a more sophisticated model should be used. Table 2. Pigment size [µm] Black Mag. Red Mean STD Min Max range Table 3. Non-uniformity factors of magenta prints on a glossy paper Sample Ink transfer [mg] Chroma Ligthness Grayscale CONCLUSIONS AND FUTURE WORK: The special formulation of thermochromic inks led us to study their rheological properties, in comparison with conventional process ink and also offset varnish. Thermochromic inks behave very similarly to ideal viscous liquids. The studied inks are expected to behave well in the inking mechanism of a printing machine. But other analysis is needed. The Casson model can be replaced by a more representative model. For this purpose, the measurements with higher resolution (smaller intervals) of shear rates are needed. The method of 251

5 acquisition of particle size distribution seems to be sufficient. Some can argue that the threshold image does need to accurately represent real particle sizes due to the low focal depth of recorded images. It is planned to obtain images on a more accurate optical microscope to compare results for size distribution. However, it seems that the particle size is smaller than reported by the producer. In evaluation of optimal ink transfer there is still a subjective aspect involved. For more objective evaluation a more sophisticated method is needed. It is expected, that the eye resolution should be included. Therefore more study on available method mottling evaluation has to be done. These results are just a small part of the research we are doing now. It is expected that thermochromic inks from different producers will have different particle size, thus also different rheological behaviour. At the moment, we are preparing samples for investigation of the mechanical and optical properties of offset thermochromic inks. We would like to know what impact a specific substrate and the protecting layer (varnish) has on the colour hysteresis curve. The ink printed onto four substrates will be covered by a conventional offset varnish and by the UV curable varnish. The stability improvement is expected by adding those protecting layers. In an application of thermochromic inks in security printing, an image consisting of fine lines was reported [8]. Another plan for further investigation is an examination of the print quality of thermochromic inks by printing a specially designed test chart. REFERENCES: [1] Seeboth A. and Lötzch D. (2008). Thermochromic phenomena in polymers. Smithers Rapra Technology Limited. United Kingdom. [2] Seeboth A. et all(2007). Thermochromic polymer materials. Chinese Journal of Polymer Science. Vol 25 (No 2): [3] Small L. D. Et all (1999). Thermocromic ink formulations, nail lacquer and methods of use. US patent no [4] Burkinshaw S. M. et all(1998). Reversibility thermochromic systems based on ph-sensitive spirolactone-derived functional dyes. Journal of Material Chemistry. Vol [5] MacLaren D. C. White M. A. (2003). Dye-developer interactions in the crystal violet lacton-lauryl gallate binary system: implications for thermochromism. Journal of Material Chemistry. Vol [6] MacLaren D. C. White M. A. (2005). Design rules for reversible thermochromic mixtures. Journal of Material Chemistry. Vol [7] MacLaren D. C. White M. A. (2003). Competition between dye-developer and solvent-developer interactions in reversible thermochromic system. Journal of Material Chemistry. Vol [8] Phillips G. K. (2000). Combining thermochromics and conventional inks to deter document fraud. Proceeding of SPIE. Vol [9] Johansson L. (2006). Creation of dynamic printed images. Ph. D. Thesis, Norrköping universitet [10] Mezher T. G. (2002). The rheology handbook. Vincentz Verlg. Germany [11]CTI. ( ): Technical notes: thermochromic offset ink. Offset%20-%20Technical%20Sheets/Technical%20Notes%20on%20Thermochromic%20Offset.pdf [12] Muck T. Hladnik A. Stanić M. (2009). Analysis of Print Quality with ImageJ, 4th Symposium about Novelties in Graphics (4. simpozija o novostih v grafiki). Faculty of Natural Sciences and Engineering. Ljubljana. Slovenia 252