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Lexikon
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A • B • C • D • E • F • G • H • I • J • K • L • M
N • O • P • Q • R • S • T • U • V • W • X • Y • Z |
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Surface Strength
The surface strength of paper is its ability to resist a force which would otherwise pull out fibres or fibre bundles from the surface of an uncoated paper, or pigments or coating particles from the surface of a coated paper. A high surface strength prevents, for example, linting problems in the offset printing process, which can cause runability problems at the printer, adversely affecting the printing result. In the case of uncoated paper, the surface strength can be measured by gluing wax sticks onto the paper surface and then removing them to see whether fibre bundles are pulled out of the paper (Dennison Test). With coated paper, the Prüfbau dry pick test or an IGT test is usually used.
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Surface Roughness
The surface roughness influences the printability of paper. Depending on the use to which the final product is to be put, the paper surface can be influenced by mechanical treatment (calendering) or by the application of a coating layer on one or both surfaces. The pictures below show the topography of uncoated and coated paper under the microscope. The surface of the uncoated paper is rougher compared to the coated paper. This can be seen in the more marked “valley and mountain” structure of the paper. The surface roughness of uncoated paper is often specified in Bendtsen units (ml/min) by measuring the air flow between the paper surface and a measuring head. A greater flow indicates a rougher surface. Uncoated paper can have a surface roughness in the range of 50–500. The surface roughness of coated paper is often specified using the Parker Print Surface (PPS) method and is expressed in micrometers. The major difference between the PPS method and the other air leak method lies in the close simulation of the printing process conditions while testing the sheet. Matt coated papers usually have a surface roughness of about 2.5 µm to 5 µm, while glossy coated papers have a surface roughness of less than 1 µm.
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Surface Properties
Coated fine paper can have very different surface properties, depending on the coating technology as well as the coating formulations used. The paper surface can have a matt, silk or glossy appearance.
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Scattering
Scattered or diffusely reflected light falls on the paper surface, penetrates to a certain depth in the paper and is reflected from the boundary surface of particles such as fibres or fillers. Finally it is reflected back to the surface.
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