Influencia de las propiedades físico-químicas del papel prensa en la calidad de impresión
- Autores: Nuria Gómez Hernández
- Directores de la Tesis: Cristina Molleda Clara (codir. tes.), Juan Carlos Villar Gutiérrez (codir. tes.)
- Lectura: En la Universidad Politécnica de Madrid ( España ) en 2011
- Idioma: español
- Tribunal Calificador de la Tesis: Luis Jiménez Alcaide (presid.), Maria Angels Pèlach Serra (secret.), Virtudes Candela Jurado (secret.), Paulo Tavares (voc.), Joaquín Martínez Urreaga (voc.)
- Materias:
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- Resumen
This thesis studies the influence of the physical-chemical newspaper properties on print quality. Papers have been characterized in terms of optical properties (ISO brightness, Y value, opacity and L*, a* and b* coordinates), surface characteristics (roughness, topology surface and fibre formation), chemical properties (surface free energy, content and infrared spectra) and the fibre network structure (air permeability, oil absorption and pore sizes distribution by mercury intrusion porosimetry). The print quality has been measured as ink demand, print density, set-off and print through. Furthermore, an index based on the visual perception has been used to determine the observed print through.
Moreover, a novel method to study the distribution of coldset ink components on the surface and the inside of paper, based on infrared spectroscopy, has been developed. This method can be a useful tool to understand how the structural paper properties affect the distribution and setting and drying of coldset ink.
Depending on the print quality parameter assessed, different samples of commercial standard newsprints have been analyzed. The papers have been selected according to: furnish composition (virgin or deinked pulp), web forming (fourdrinier or gap), basic weight and optical and structural properties. Afterwards, they have been printed in laboratory equipments with a single colour ink.
It has been verified that the Bendtsen and the Parker Print roughness are appropriated methods to characterize the surface texture in this paper grades, whereas the parameters used to describe the paper topography, which were obtained using three-dimensional surface approaches, have not been better at explaining the paper behavior in printing process. On the other hand, the Gurley and Bendtsen permeability values have been inaccurate to evaluate the newspaper porosity. Nevertheless, the mercury intrusion porosimetry has provided detailed information about the pore size distribution of paper.
The results have showed that the use of deinked pulp as raw material, has not involved a change at printability for the assessed papers. The main differences between tested papers has been found in surface structure, particularly their roughness and surface porosity (pore diameter and void fraction), whereas all of them have similar values for the most frequent pore diameter inside the paper.
For the paper assessed, a regression study between newsprints properties and print quality has been made. This analysis has showed that the print quality has fairly well correlated with optical and structural properties. Specifically, more than 90 % of ink demand was explained by Bendtsen roughness. On the other hand the relationship between print through and opacity was also confirmed in this work. However, the surface free energy has not revealed a significant influence on paper printability.
The size of surface pores has meaningfully affected the printing colour intensity and the ink setting. A higher value of diameter that occurs most frequently on surface has resulted in a reduction in print density. Nevertheless, the presence of small pores on surface has slightly raised the trend to set-off. In a like manner, an increasing paper surface porosity has enhanced the probability of print through to appear The results of the subjective evaluation of printed samples to obtain "the observed print through", have showed that an observer always has preferred the higher luminosity (L *) newspapers and those with an even print. This means, that the observers provide an overall index about the perceived print quality, because they were not able to judge only the print through.
Fourier transform infrared spectroscopy in attenuated total reflection mode (FTIR) has allowed to measure the amount of oil and cyan pigment on printed surface, and to follow the presence of these components along the sheet thickness. The oil percentage has been evaluated as the area increment of bands between 2800 cm- 1 and 3000 cm- 1. The relative amount of cyan pigment has been determined as the area of the absorption bands at 780 cm-1, 753 cm-1 and 730 cm-1. At the same time, a suitable method to study the colour pigment penetration into the paper has been presented. This method is based on the image analysis of different paper layers, obtained along the paper thickness.
The ink oil has been mainly found at the upper 40 % of the paper thickness, whereas the pigment has been detected at the inner layers closest to the printed surface (20 %). Nevertheless, the absorbed quantity of these two components depends on the porous structure of the printed paper The study of porous structure effect on the oil and pigment absorption has revealed that the oil penetration depth is directly related to void fraction. In the same way, ink oil and pigment absorption increases with the enlargement of the mean value of surface pore diameter. The statistical results have shown that the print through depends on the depth of paper impregnated by the oil and on the amount of pigment particles absorbed to deeper layers into the paper.
In conclusion, the coldset newspaper printability can be optimized reducing its roughness, in other words, making paper with smaller porous diameter and fewer void fraction on surface. This can result in a decrease of the ink demand and a reduction of oil and pigment penetration inside the paper, which considerably improves the colour effectiveness and reduces the print through. However, the reduction of the paper roughness must be restricted until set-off problem arises.
