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12.1 General Aspects
The proper selection of relevant analysis and test methods for raw materials, inter¬mediate or final products obviously has importance for the success of a testing program. Process and product analysis are the main areas for a large variety of very heterogeneous testing methods. Process analysis tries to define the control varia¬bles of the process that allow it to run smoothly and produce products with the necessary properties. The aim of product analysis is to define the properties that relate to the use of a specific product or material. The test results obtained should be representative for all of the paper from which the sample has been taken. Reliable, representative measuring results require
. • representative sampling as far as quality and quantity are concerned
. • adequate sample preparation
. • calibrated measuring equipment
. • careful, accurate measuring.
A straightforward process for cut-size and reel paper is described in ISO 186 (2002).
As a result of its hygroscopic properties, paper and board must be tested under standard climatic conditions and the samples must be conditioned in this climate for at least 4 hours before testing (sample conditioning). In ISO 187 (1990), the preferred climate for paper testing is specified as 23 °C and 50 % relative humidity. In the following the most relevant test methods for paper and board testing will be described.
12.2 Basic Properties
The basic properties of paper include the dimensions and the mass. In the case of sheet material such as paper, the basis weight mA is determined in accordance with ISO 536 (1995). The value mA is the ratio of the mass m to the area A of a sam¬ple.
m
A =
A
Another basic property of paper is the caliper. As paper is compressible the caliper must be defined in terms of the measuring instrument. ISO 534 (2003) specifies an instrument with two parallel planar measuring surfaces of 200 mm2 which act on the sample with a surface pressure of 100 kPa. The resulting distance between the surfaces is the caliper D of the paper.
The ratio of the basis weight to the caliper is the density of the paper. The recipro¬cal density is the specific volume.
The moisture expansion of paper, i. e., the change in dimensions with changing ambient climate, is also a basic property. According to DIN 53 130, the moisture expansion (ME) is the ratio of the change of length D L of a sample paper strip, resulting from a change in the ambient climate from 23 °C/45 % relative humidity to 23 °C/83 %relative humidity, to the original length L0. The temperature can also be 20 °C in each case.
DL
ME =
0
The moisture expansion test is standardized in ISO 8226–1 (1994) and ISO 8226–2 (1990).
12.3 Composition and Chemical Paper Testing
The fiber composition of paper is determined with a light microscope (fiber micros¬copy). Due to the varying fiber morphology, both the fiber raw material and the pulping process can be determined. Chemical dyeing methods are used for con¬trast enhancement and quantitative determination of fiber composition (ZM IV/ 55/74; TAPPI T 401 om-93, Harders-Steinhäuser, Faseratlas).
The term moisture content refers to the amount of water in the paper in equilib¬rium with a defined ambient climate. It is determined gravimetrically in accor¬dance with ISO 287 (1985). A dry sample is produced by drying the paper to constant weight at 105 °C. The moisture content is expressed in percent, based on the moist sample.
Chemical testing is used to determine the components of paper. This is of sig¬nificance in papermaking for quality control and process control. Another im¬portant field of chemical paper testing is the control of papers for the packaging of food.
12.4 Strength Properties
The entire spectrum of analytical methods used for the chemical testing of paper ranges from gravimetric methods to spectroscopy, electrochemical and enzymatic methods, as well as sensoric tests on taste and odour and microbiological tests.
Most of the testing methods to determine the composition are destructive. The components to be determined must be extracted with water or organic solvents. For example, the resin content of paper is determined gravimetrically in the extrac¬tion residue by using an organic solvent as an extracting agent.
The inorganic fillers contained in paper are determined as residue on ignition (ash content). In accordance with ISO 2144 (1997), the sample is ignited at 900 °C until no change in weight is observed. The ignition residue is then determined gravimetrically and expressed as a percentage of the original weight of the sample. Other ignition temperatures are used for chemical pulp (575 °C) and filter papers (800 °C) (DIN 54 370 (1999). Determination of CaCO3 is calculated from the differ¬ence in weight loss between 575 °C and 900 °C (CO2). X-ray microanalysis is em¬ployed for the quantitative determination of the types and amounts of fillers. Ele¬ments such as calcium, magnesium, and metals in trace amounts can be analyzed by atomic absorption spectroscopy. For the determination of organic substances in paper extracts, IR spectrophotometric and chromatographic methods are applied,
e. g., gas chromatography for volatile components and high performance liquid chromatography for thermally sensitive constituents.
Other chromatographic methods are used for the determination of chlorides, nitrates, and sulfates, or for the differentiation and characterization of dyes and optical brighteners.
Apart from the analytical methods mentioned above, the sum parameters are of special importance. These include the ash content, pH (ISO 6588, 2003), electrical conductivity of aqueous extracts (ISO 6587, 1992), and the resin content of pa¬per.
Apart from the ISO standards mentioned above, other standards for chemical testing of paper are, e. g., the ZELLCHEMING, SCAN, and TAPPI test methods.
Strength Properties
Tensile strength and strain at rupture are determined as values characteristic of paper strength (ISO 1924, 1992). The tensile strength together with the sample width gives the tensile strength per unit width, measured in N m–1. Together with the sample thickness the tensile strength can be expressed in kPa. The tensile index results from tensile strength per unit width (expressed in N m–1 divided by grammage).
In paper technology, the breaking length is also of some importance as a calcu¬lated value. It is the length of a freely suspended paper strip of any constant width and thickness that just breaks at the point of suspension due to its own weight.
The breaking length L is calculated from the tensile strength force FB of the paper measured in N, the basis weight mA, the width of the strip w, and the gravita¬tional constant g = 9.81 m s–2.
B
L =
A • w • g
The bursting strength is important in the characterization of packaging papers. The bursting strength of paper is determined in accordance with ISO 2758 (2001) and that of board in accordance with ISO 2759 (2001). In the testing instrument, the free area of the sample stretched in a circular frame is exposed to increasing pressures until it ruptures.
Packing materials are also characterized by their bursting strength. For instance, set values are stipulated in DIN 55 468 Part 1 (1999) for the strength of corrugated boards, and are used for classification. The same standard specifies set values for the puncture resistance. This strength property is determined, mainly for cardboard and corrugated board, by measuring the resistance offered by a sample to the penetration of a pyramidal body (ISO 3036, 1975 and DIN 53 142–1, 2003). There is also a puncture test method available (DIN 53 142–2, 2003) which describes a linear penetration of the pyramidal body.
To determine tear resistance (Elmendorf ), a sample is torn with the help of a pendulum device starting from a predetermined cut. The work required for a tear of a given length is measured. The test is described in ISO 1974 (1990).
Z-directional strength refers to the ability of paper or board to resist tensile load¬ing in a direction perpendicular to the plane of the paper (z-direction). After ex¬ceeding the z-directional strength of the paper, a break in the paper structure occurs in the sheet but not at its surface. The z-directional strength is therefore not equivalent to the surface strength or linting tendency of the paper. Many test meth¬ods to measure z-directional strength of paper are available. Some methods have also been standardized, TAPPI UM 584, TAPPI UM 403, TAPPI UM 527, and TAPPI UM 528.
The z-directional strength (DIN 54 516, 2004) is determined by measuring the maximum force per unit sample width required to split the sample.
The crush resistance of paper under compressive stress is measured as the short span compression strength. To prevent the sample kinking, the free span length must be very small (typically 0.7 mm). The resistance measured according to DIN 54 518 gives the ultimate crushing load per unit width.
The flat crush resistance (ISO 3035, 1982) and the edge crush resistance (ISO 3037, 1996) are used to characterize the strength of corrugated board. In the for¬mer test, a circular sample of corrugated board is exposed to increasing pressure between two parallel planar plates until the corrugation collapses, and the max¬imum load is measured. In the second test, the load is applied in the direction of corrugation starting from the edge of the sample. The edge crush resistance is used in DIN 55 468 Part 1 for the classification of corrugated boards.
The folding strength is an important property of both packaging and graphic papers. It can be determined either as described in ISO 5626 (1993) as the Schop¬per double fold value, or as described in ISO 5626 (1993) as the Köhler-Molin folding endurance value. The sample is subjected to specified tension in both cases. The Brecht-Wesp pressure folder functions without tension. A sharp fold in
12.6 Surface Properties
the sample is produced between two rolls and is repeated up to fifty times in the same position. The ratio of the tensile strength of the folded sample to that of the unfolded sample is the folding strength.
12.5 Load-Deformation Properties
Not only the strength properties, tensile strength and strain to rupture can be determined in the tensile strength test (ISO 1924, 1992), but also the tensile energy absorption, TEA, which is especially important for bag paper. The TEA value repre¬sents the nonelastic portion of the deformation energy and thus that portion of dissipated energy which should be high in the case of bag paper.
The resistance to flexural stress is measured as the bending stiffness under ap¬proximately pure elastic deformation, as defined in DIN 53 121. This test is usually performed with a two-point beam method. To guarantee elastic deformation, max¬imum bending angles, which depend on the span length and the sample thick¬ness, are specified. This may be circumvented by the resonance length method (ISO 5629, 1983) in which the resonance length of a free sample strip is measured. Resonance is generated via a clamp vibrating at 25 Hz.
Deformations in board and cardboard can also be produced by creasing. Testing the properties of creases requires defined production processes, e. g., DIN 55 437 Part 1. In Parts 2 and 3 of this standard, methods are described for the manual folding of the creases and for the visual evaluation of the folding or the technical evaluation of the creases with a folding-moment tester.
12.6 Surface Properties
A large number of methods are available for evaluating the topography of paper surfaces. The Bekk smoothness is determined according to ISO 5627 (2002) as follows. At a defined pressure difference, the time in seconds is which a specified amount of air requires to flow radially inwards between the paper surface and a ring-shaped glass plate and on into a vacuum chamber is measured. The smoother the surface of the paper, the longer the pressure equalization takes. The determi¬nation of roughness according to Bendtsen (ISO 8791/2–90) is related to the Bekk method for the determination of smoothness. In this case, however, the direction of the air flow is reversed and the magnitude of the air flow is measured. The air escapes under defined conditions between the measuring ring of the measuring head and the paper surface under constant overpressure. For a general description of roughness or smoothness, see ISO 8791–1.
Sometimes the topography of the paper surface is described using profile meas¬urements laser scanning. The scanning profilometer uses point sensors in con¬junction with high precision x- and y-stages to capture profiles and 3D data. The stages move the sample under the sensor and the sensor passes the captured height data to the computer for evaluation.
Another surface property of paper is the abrasion resistance. The mechanical abrasion resistance of surfaces is determined in the friction wheel process (DIN 53 109–93). In this process, the amount of abrasion which is obtained by abrading the conditioned or wet sample with an abrasion wheel of defined quality under defined conditions is measured.
12.7 Optical Properties
An object, e. g., a paper surface, is termed white when the illumination intensity and the absorption capacity of the surface are independent of the wavelength. Deviations confer a more or less pronounced color shade on the surface.
In the paper industry, a special process is used to characterize the brightness because this is one of the most important optical properties of paper. The determi¬nation of the reflectance factor (ISO brightness) is based on ISO 2470 (1999). For this test, a filter is used which has an intensity maximum at a wavelength of 457 nm. The reflectance (blue component) measured in a reflectometer under specified conditions is known as brightness. It is expressed as a percentage of the brightness of a white standard.
Another optical property of paper is its transparency, i. e., a measure of its light transmittance. It is calculated from the reflectance factors R0, Rw, and R(w), which are determined in accordance with DIN 53 147 (1993). The reflectance factor of the individual sheet on a completely black background is R0, R w is the reflectance factor of the individual sheet on a white background, and R(w) is the reflectance factor of the white base.
Most white papers and paperboards, and therefore also secondary fiber materi¬als, currently contain an optical brightener. Brightness measurements for such materials depend on the relative proportion of UV radiation in the illuminant used for the determination. The standard test methods for ISO brightness have not defined the standard illuminant for use in the determination. The relative amount of UV has also not been defined. As a result, widely different R457 reflectance factors exist for the same kind of fluorescent material. The problem has recently been solved. A revised ISO method, ISO 2470, states that the UV radiation of the illumination must correspond to the relative amount of UV in the standard illumi¬nant C when measuring fluorescent objects.
The opacity is a measure of light-tightness. It is defined in ISO 2471 (1998) as the ratio of the reflectance factor R0 to the reflectance factor R' . Both reflectance factors are determined in accordance to DIN 53 145 (2000). R0 is measured as the reflectance factor of an individual sheet on a completely black background and R' as the reflectance factor of an “infinitely” thick stack of the same paper.
12.9 Behavior towards Liquids
12.8 Printing Properties
The printing properties of paper result from complex interactions between print¬ing ink, printing process, and paper. Practice-oriented printability tests must be performed to evaluate these properties. Test printers are also suitable for this pur¬pose. Instruments for offset printing, gravure printing, and flexographic printing can be used with standard printing inks under laboratory conditions to test the dry pick resistance or the wet pick resistance of papers. Missing dots, mottling and ink penetration properties can also be tested. Conversely, the behavior of various print¬ing inks towards standard papers can also be evaluated.
Using an image analyzer for testing printed image and printing process has increased during the last decade. This is due to the improved capability of the analyzers and because the importance of the quality of the printed image has increased. The image analyzer and its use in paper testing has already been men¬tioned earlier in connection with paper tests. The image analyzer is used to meas¬ure the uniformity of paper (print mottle, number of missing dots, and properties of dot), surface properties of paper (fiber rising, picking, and contact angle meas¬urement), and many other properties of paper, i. e., width of cracking at the fold.
Furthermore, defined proof copies produced with the test printers can be used to test full ink coverage, color density, color gloss, shade, abrasion resistance, stack¬ing ability, and contact yellowing.
More reliable results obviously come from tests that closely simulate the actual printing process. That is why tests developed for full-scale printing machines give the best predictions of the actual printability properties of a paper [5].
12.9 Behavior towards Liquids
The behavior of liquids towards paper is characterized by the processes of wetting and penetration. In both cases, the characteristic physical property is the surface tension. This value can be measured directly and tensiometrically in the case of liquids and indirectly, via the contact angle of test liquid droplets, in the case of solids such as paper. A liquid wets the surface of paper only if its surface tension is lower than that of the paper. The same holds for the wetting of the capillary walls upon penetration of liquids into the capillaries of the paper.
If the wetting and the penetrating capacity of liquids are to be changed, the surface tension of the paper must also be changed. This is achieved, for instance, by sizing the paper, a process which must fulfill the requirements regarding printability with aqueous inks. According to DIN 53 126 (2001), paper is considered to be printable if a standard ink line drawn with an adjusted drawing pen has neither run nor penetrated into the paper after 24 h.
The water absorption WA (Cobb) (ISO 535, 1991) refers to the amount of water that is absorbed by a certain area of paper on one-sided contact for a specified exposure time. The time of exposure to water must be chosen such that a sufficient amount of water enters into the fiber matrix, but does not penetrate to the opposite side of the sample.
In the determination of grease permeability (DIN 53 116), red-colored palm kernel oil is used as the testing agent. The passage of fat through the sample under specified conditions is then evaluated.
12.10 Exclusion of Gases and Vapors
As a rule, papers have only a limited ability to exclude gases and vapors. Of partic¬ular importance are the air permeability (e. g., for filtration properties) and the water-vapor permeability.
There are two standardized test methods available for the determination of the mean air permeability: the Bendtsen method (ISO 5636-3, 1992) and the Schopper method (ISO 5636-2, 1984).
A gravimetric method for the determination of water-vapor permeability is de¬scribed in ISO 12 572 (2001). This method is suitable for building materials. For foils, laminated paper and board also DIN 53 122-1 (2001) is recommended.
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