Viscoelasticity is the property of a material, which responds to deformation as a viscous and as an elastic material. A viscous material, e.g. honey, is capable of resisting tension and shear forces and responds linearly to the external deformation, while an elastic material, after removal of the external load, immediately returns to its starting position. Viscoelastic materials have elements of both listed types of responses, the viscous and elastic. It is a time-dependent response to tension. Elasticity is usually a consequence of the ability of intermolecular bonds to expand along the crystalline structure of the polymer, while the viscosity is the result of scattered atoms or molecules within the amorphous stage of the polymer. The purpose of the Ph.D. thesis ”Viscoelastic properties of graphic papers” was based on an examination of the following: a) the impact of the fiber composition, b) technological parameters of manufacturing, c) structural characteristics, d) physical-mechanical properties and e) the interdependence of "conventional" and "new" analytical methods in investigation of the viscoelastic properties of the paper. The survey included classical and new methods of investigation of paper, which was divided into five sections, i.e. basic physical, physical-mechanical, surface and structural-chemical composition, uniformity of the paper, and the deformation under load composition and viscoelastic properties. The 12 test samples (V1V12), were distinguished according to material composition (primary or secondary cellulose fibers), technological production (produced on three paper machines) and finishing (coating, calendering), whereas for all of the 12 samples the printing technique was common, i.e. web printing of roll on roll/sheet. Application of secondary cellulose fibers for newspapers, V1V3, and graphic papers, V4V6, is a long-established practice, while manufacturers still solely use virgin cellulose fibers in production of specialty papers such as papers for flexible packaging, V7V9, or label papers, V10V12. The results of the survey have shown that papers produced from secondary, V1V6, and primary, V7V12, cellulose fibers are substantially similar, which overall suggests the possibility of reducing the proportion of the primary cellulose fibers and replacing them in part with appropriate deinked pulp, i.e. secondary cellulose fibers. In spite of the weakened physical-mechanical properties, they are still sufficiently "strong" to withstand the stresses which occur during the wet and drying part of the paper machine, coating, calendering, winding, rewinding, and a web printing process.
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