Measurements of Coated Surface
Different printing methods have their own requirements for coated paper and paperboard. The coating amount has a big influence on the physical and optical surface properties. Physical properties are smoothness, gloss, surface strength, ink absorption, dusting/linting, piling and visual defects. Optical properties are brightness, whiteness, color shade, opacity, mottling and print unevenness. Taste and odor are important properties, especially for paper and paperboard grades used for food packages.
Coat weight measurement in the laboratory is based on the ash content (925 °C) of base paper and the ash content of coated paper. Several different technologies have been used to measure on-line coat weight. The Beta or dry weight technique meas¬ures dry weight before and after the coating operation and the coat weight is calculated by difference. Weight and moisture sensors are required at both loca¬tions.
The ash or X-ray absorption method is similar except that ash sensors are used instead of basis weight measurement. IR absorption is the newest technique to be used, as with X-ray fluorescence, latex and/or carbonate in the coating color are measured. The methods are used in differential mode, i. e., two scanners are applied.
The same basic smoothness measurements are applicable for coated paper as for base paper. As rotogravure printing is the most demanding printing method for paper surface, a special test method is used only for that purpose. The so-called “Helio-test” is performed with an IGT-tester (see Section 7.9.5). In this simulated printing test, the number of missing dots, counted visually, provides information on the printability.
Gloss describes the mirror-like property of a coated surface and is defined as the percentage of the light that is reflected from the surface at an angle equal to the angle of incidence, in comparison with a standard surface.
This is the ability of a (coated) paper surface to absorb ink during printing. If ink absorption is too slow, there may be a risk of set-off. Generally used tests are the K&N test (TAPPI Method 553), the Lorilleux Porometrique test, the Microcontour test, or the Croda test, depending on the test ink used. All the above-mentioned tests follow similar test procedures: The ink used is applied to a paper surface in a thickness of 0.1 mm; after 2 min nonabsorbed ink is wiped off. The brightness of the colored area is measured and subtracted from the original brightness of the paper to give the ink absorption value as a percentage. The test area can also be used for visual evaluation of the evenness of absorption. If the area is mottled, there is a risk of mottle during printing.
This is the internal and surface bond strengths of paper which are necessary to prevent fibers, fines, filler, or coating from being removed in printing or convert¬ing operations that involve ink and/or aqueous liquids such as in offset printing. Generally, for coated paper, the picking test and rub resistance are used. The pick¬ing test is done either by using a wax pick test (Dennison Wax/TAPPI Method UM 463) or a simulated printing by using a device from the Institute for Graphische Technik (IGT tester/TAPPI UM 591). Rub resistance describes the ability of printed paper to withstand marking, scuffing, or smudging during handling
Measurements of Coated Surface
coated material. Laboratory rub testing of dry print can be carried out with an optional number of rubs and pressure. The results are assessed visually against set standards. The rub resistance method follows TAPPI 592.
Dusting or Linting
This is an accumulation of cutter or slitter dust on the blanket around edges of the web and an accumulation of fibrous materials or/and pigment on the blanket of the print press. These phenomena influence both print quality and production efficiency. Dusting and linting are tested on a full-scale heatset press at a printing speed of 50 000 sheets h–1 (6.2 m s–1) under normal press conditions. Usually 30 000 copies of each paper have to be printed. The accumulated lint on the printing blanket from 50 % halftone and the nonprinting area is collected with a tape and weighed. The lint can be analysed with a microscope to determine the origin of the lint (small shives, fibers, ray cells fines, etc.).
This results in the accumulation of coating particles and ink on the tail-edge of the solid printing area or/and the halftone area on the blanket. The accumulation is tacky material of the same color as the ink. Piling is also tested on a full-scale heatset press at a speed of 60 000 sheets h–1 (7.4 m s–1), with a special tacky ink on the first and second units, a high water feed on the first unit to lower the surface strength and a low water feed on the fourth unit (adjusted with graphometronic; the goal is between the normal water feed and the toning level).
Further print conditions are a special layout, papers are printed to a constant density and 30 000 to 40 000 copies of each paper are printed. The degree of piling will be classified as “extreme piling” (printed amount less than 20 000 copies at good printing quality, danger of web break), “major piling” (printing amount 20 000–40 000 good copies, danger of web break), “minor piling” (printing amount up to 40 000 good copies) and “no piling” (printing amount significantly above 40 000 good copies).
As the primary objective in pigment coating is to improve the printability and appearance of the material, no visual defects should appear. Typical defects are holes, spots, blade scratches and streaks and creases. Visual defects are controlled using on-line detectors and visual checking.
Visual checking is done in two places: The machine crew take a cross sample from the web at the top of each machine reel and examine it under strong light sources with different light angles to mark the defects. In laboratories, sheets are checked visually under a light source, and defects are counted and recorded. The on-line devices measure faults like slime holes, wrinkles, creases, edge nicks, turnovers, blade stripes, etc.
Detection and counting of dirt specks are done online today. With low-viscosity colors, upstand¬ing fibers in the surface cause pimples because the color climbs up these lumps of coating. Faults such as whiskers on the surface, loose fibers, coarse fibers, dust, lint, and fuzz, are usually only determined by inspection.
Brightness and Whiteness
Brightness and whiteness are highly dependent on the coating composition, pig¬ment type, coating amount and the use of optical brightening agents (see section 18.104.22.168.5.4). Fiber type and basis weight also influence optical properties. Bright¬ness measurements are taken over a range of wavelengths by using instruments conforming to ISO 2469/2470. Such instruments commonly found in use are the Elrepho 3000 (Datacolour/Zeiss) and the MacBeth White-Eye.
The American paper industry uses a standard brightness instrument that has 45° illumination and 0° viewing (TAPPI Test Method T 452).
Whiteness is a comprehensive term used to express the visual impact of near-white surfaces by means of a single value. Numerous equations have been devel¬oped to this end, and CIE (Commission Internationale de l’Eclairage) whiteness is the most common in practical use. Whiteness is also used to denote a more com¬prehensive expression of color and color shade by use of the CIE color coordinates L*, a*, and b* (or CIELAB). Three reflectance figures L*, a*, and b* are calculated from measurements using a standard D65 light source. Here the UV content of the D65 illuminant must be accurately controlled to ensure reliable assessment of surfaces containing optical brightening agents.
The coordinates a* and b* meas¬ure color. Positive figures for a* express redness, negative figures greenness, and positive figures for b* indicate yellowness, negative figures blueness. L* is a per¬centage which measures luminance on a scale where black is zero and pure white is 100 %. One can measure color coordinates as well as brightness with the same instrument, e. g., with a Hunterlab colorimeter or any other equipment. The calcu¬lations are complex and require the aid of computerized systems. Color coordi¬nates are currently also measured and controlled by fixed-point or moving on-line equipment in paper machines.
This is a measure of the amount of light transmitted through paper. When no light is transmitted, the opacity is 100 %. The measuring principle is based on compar¬ison of reflectance for a single paper sheet over a black background compared to an opaque stack of paper samples. As opacity increases with increased absorption and scattering of light, the following parameters are of prime importance for opacity: basis weight, filler content in base paper, degree of calendering, type and treatment of pulp, as well as coat weight and coating color components, especially the type of
Measurements of Coated Surface
pigment used (fine pigments increase scattering). Opacity is measured in accor¬dance with ISO 2471 and TAPPI Test Method T 425.
There are several types of mottling: first color mottling, wet repellance mottling and back-trap mottling. First color mottling results from unevenness of the phys¬ical surface structure of the coated sheet (roughness, pore structure) and from the unevenness of optical surface properties (brightness, gloss). Wet repellance mot¬tling occurs when the fountain solution layer between coating and ink interferes with ink transfer. Back-trap mottling leads to uneven ink setting in the coating layer. Mottling can be tested during normal print conditions either in a print house or research environment. For its investigation large areas (5 V 5cm2) of solid and halftone are needed.
For checking first color mottling, packing is removed from the first, third and fourth unit: no water under the cyan ink and no back-trapping after ink transfer means no water or back-trap mottling. To test wet repellance mottling, packing is removed from the first unit: if there is no water under the cyan ink then no water mottling is given. For back-trap mottling packing is re¬moved from the third and fourth units: when no back-trapping after ink transfer occurs then no back-trap mottling is given.
This means unwanted variation in density, color or gloss in print. The size and intensity of the variations are measured. Print density variations (graininess 0.5–2 mm; mottle 2–8 mm) are measured from an even black and/or cyan tone of at least 40 % and as an average of eight images with a size of 51 V 51 mm2.
Taste and Odor
When paper is used for packaging purposes, no off-taste or off-odor from the package must contaminate the product itself. Odor and taste from paper can arise from a number of sources such as wood resins in mechanical pulp or residual chemicals in pulp making, or the paper may have internal biological activity, which can also produce odorous substances. The most sensitive instrument available to measure the odor and taste of a substance is a human being. Members of trained panels assign numerical ratings and record their impression of tainting flavors or volatile odors experienced.
The test methods available are the triangle test (the test states if differences between specimens are found, DIN 10 951), the pair test (a test describing the differences between specimens, DIN 10 954) and the Robinson test (a taste test identifying how much taint a paper has given to a taste medium; chocolate very commonly being used, DIN 10 955). Beside the trained panel, gas chromatography (GC) and mass spectrometry (MS) are used to identify the origi¬nating chemical compound. The so-called “electronic nose”, an original invention from the military world, has also been utilized to detect off-odors.