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Data were analyzed using mixture design modelling and optimization

Update:28 Apr 2018
Summary:

The aim of this work was to study the influence of a gl […]

The aim of this work was to study the influence of a glycerol derivative with thiol groups, 3,6‐dithia‐1,8‐octanediol (DTO), on light fastness of prints overprinted with water‐based flexographic printing inks. Laboratory printing was performed on various plastic films (BOPP and PE). The effect of a small amount of DTO on printing ink colour was examined by studying the optical density of a full tone area, relative printing contrast and the colour values (CIELAB), and the total colour difference. Artificial ageing was applied in order to investigate DTO influence on colourfastness of CMY printing inks. Densitometry and spectrophotometric parameters were measured during artificial ageing by irradiation. Additionally, FTIR spectroscopy was used to observe the changes in ink film on printing bases before and after ageing. The print quality of investigated prints is widely discussed. In general, DTO addition influenced the printability of the plastic film and the optical density of a full tone area with an acceptable total colour difference. Furthermore, printing inks with added DTO exhibited higher optical density and smaller changes of during artificial ageing. Moreover, the FTIR spectra of printing ink films with added DTO after ageing did not exhibit any perceptible changes in contrast with the original printing ink.Water-based ink formulations containing wax, surfactant, and defoamer additives were prepared and printed on polyethylene film substrates. Standard test methods for adhesion, rub resistance, and gloss were done on the printed polyethylene films. Quantitative methods for the assessment and evaluation of the three print properties were developed. Image analyses were done to quantify adhesion and rub resistance.

 

Quantitative measurement was done to quantify gloss. Data were analyzed using mixture design modelling and optimization. Modelling results show that adhesion and gloss are described by special cubic model equations, while rub resistance is described by a linear model equation. Contour plots and 3D surface graphs were generated showing the response surfaces of the print properties. The effects of varying the mass fractions of wax, surfactant, and defoamer on adhesion, rub resistance, and gloss were determined. It was found that increasing wax increases rub resistance, while increasing surfactant increases gloss, and increasing defoamer increases adhesion. There is dependency found between the mass fraction of wax, surfactant and defoamer with respect to the rub resistance, adhesion and gloss. Multi-objective optimization revealed that optimum adhesion, rub resistance, and gloss is obtained by a formulation containing equal mass fractions of wax and surfactant but no defoamer.The ink industry is one of the growing markets in the world, particularly due to increasing market of printing inks for packaging applications. Currently, there are two types of printing inks based on composition: (1) solvent-based inks that are formulated using solvents an (2) water-based inks that are formulated using water as solvent. The volatile organic compounds present in solvent-based inks result in environmental issues regarding their use, which lead to the increasing demand for the more environment-friendly water-based inks.

 

Water-based printing inks have been successfully used in different printing applications, all of them involving application of the ink on various surfaces or substrates. While solvent-based inks have been successfully used on both porous substrates (e.g., paper) and non-porous substrates (e.g., plastic), water-based inks are found to be incompatible to non-porous substrates, resulting to poor substrate wetting. This poor ink-substrate adhesion due to the water’s inherently high surface tension: non-porous substrates like polyethylene films have a surface energy of 28–32 mN/m, while water has a surface energy of 73 mN/m. Substrate wetting has been a main problem in the use of water-based inks for printing in non-porous substrates, and to achieve successful substrate wetting, the surface tension of the ink must be lower than the surface energy of the substrate to be printed on.Currently, there are two ways to solve the adhesion problem of water-based inks on non-porous substrates: surface treatment (substrate modification) or changes in the ink formulation (ink modification). Surface treatments such as corona discharge treatment and plasma treatment are done to increase the surface energy of the substrate by introducing functional groups in the surface that would improve the adhesion of ink on the surface; however, the effect of the surface treatment is reversible with the dissipation of the induced increase in surface energy. Corona treatment can also lead to unwanted physical changes in the surface if it is overtreated, and can be very costly. Thus, modification of ink formulation is considered as the alternative approach done in the ink industry.Studies on the effects of the different ink components on the ink and print properties have been previously investigated, such as the addition of emulsion polymers for delivering ink properties after ink formulated; among these studies, ink additives are deemed of particular importance.

 

Additives are defined as “any substances that are added in small quantities to a coating material to improve or modify certain properties of the finished coating or of the coating material during its manufacture, storage, transport, or application”. They are generally added to ink formulations at maximum of 5% by weight, where typical amounts are about 1.5% of the total weight of the ink. However, because additives have significant effect on inks in just small additions in the formulations, they are considered by researchers as points for improvement.Several studies for ink additives and performance additives that would improve the print properties of water-based ink formulations. Three additives generally used: (1) surfactants, (2) defoamers and (3) waxes. Surfactants are added to reduce the surface tension of water-based inks to allow adhesion on nonporous substrates; however, the presence of surfactants in the formulation results to production of stable foam during mixing. To address this problem, defoamers or anti-foaming agents are also added used as additives together with the surfactants in the formulations of printing inks. On the other hand, waxes are used to impart essential surface properties such as abrasion resistance.To determine the effects of these additives, the most important print properties of the ink with modified formulation are analyzed- namely, adhesion, rub resistance, and gloss.

 

Adhesion and rub resistance are two important mechanical properties, while gloss is an optical property that is deemed important in many applications, such as in packaging. To evaluate ink performance and analyze ink formulations, modern techniques and classical chemical tests are done by manufacturers to establish quality and applicability of their products , as well as standard test methods that involve qualitative assessment. While there is a quantitative standard test method for gloss such as the ASTM Standard Test Method for Specular Gloss D523-14, adhesion and rub resistance test methods are mostly qualitative, such as the color fastness analysis on a grey scale for testing rub resistance and ASTM F2252-03 peel test. Hence, quantitative methods for evaluating adhesion and rub resistance must be developed to obtain more accurate and reliable results and to allow for performing optimization with the quantitative gloss measurements. In this study, an image-based technique is developed and employed to quantify adhesion and rub resistance, and to analyze the effects of varying the relative amounts of the three additives on the three identified print properties. After gathering and analyzing the results, a formulation that would give best print properties is identified by using optimization methods.

 

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