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Monday, September 25, 2017

Smoothness (Roughness) in the Paper Industry

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Terminology:
The terms “smoothness” and “roughness” are generally well understood as far as a dictionary meaning goes; however, the use of the terms in test methods is sometimes confusing. In the title of a test method, the term “smoothness” is used when an increasing number is correlates with a smoother surface measurement. An example of this would be the Bekk method, where a smoother surface requires more time for a given volume of air to leak across the surface. Since the reporting for Bekk smoothness is in units of time (seconds), a surface that measures 500 Bekk seconds is smoother than a surface that measures 200 Bekk seconds. If the Bekk instrument was originally configured to report in Bekk flow, which is the reciprocal of Bekk time, then the test methods would categorize it as a Bekk roughness tester.

The two most common instruments that directly report airflow across the surface are the Sheffield and Bendtsen methods. A rougher surface causes higher airflow; therefore these instruments are designated as roughness testers. The Parker Print Surf method is also a roughness tester; however, the reporting (in microns roughness) is a function of the cube root of the measured air flow. In the last few decades, the zeitgeist has been to accurately name the test methods in accordance with the function and not to continue using the manufacturer’s earlier assigned name, if it was not technically correct.

The Papermaking Process:

There are several manufacturing processes that shape a continuous web and wind the product on a roll. When metallic materials are plastically deformed through a series of roll nips, the end product is quite uniform, due to the malleable properties of the materials. The modulus of elasticity is quite high for metallic materials, as compared to that of paper. Metallic materials leave a roll nip substantially the same thickness as the roll gap, with a surface finish somewhat equal to the roll finish.

By comparison, paper is extremely compressible. There are numerous voids in paper, including the presence of air within the fibers, which resemble small capillary tubes. In the calendering process, the nips are loaded to a certain nip pressure, “pli”, or pounds per linear inch. The resulting distance between the mating roll surfaces is primarily a function of the nip loading, the compressibility of the paper and the deformation of the roll surfaces. The mechanical action in the nip imparts a smoother surface on the paper, and there is a decrease in the thickness of the web after it is calendered. Further, there is a difference in the stacking height of such calendered papers, due to both the thickness reduction, and the way in which rough surfaces stack together. The properties that affect stacking height are surface roughness, compressibility, and stack
loading.

When building a reel, the highest priority is to wind a uniform roll, as a roll with ridges and valleys will give a perceived value of poor quality, and also there can be runnability issues with such rolls.

The process control systems in use today have a strong history of development around basis weight, moisture, and caliper (thickness) control. Basis weight and thickness variations can cause the calendering action to be different across the web. When a web has reasonably uniform basis weight, but has caliper uniformity problems, a process control system can make very small, but effective adjustments to the calender stack to build a level roll. Such adjustments may affect the smoothness profile, but generally there are no on-line sensors that provide such feedback. When a CD strip from a reel is run through a profiling smoothness tester, there are generally regions of high and low smoothness values that show up at the same places, reel strip after reel strip. When the smoothness values fall within the accepted limits, there is generally no concern in fixing the problem. The astute production manager will observe such trends, and take action before the measurements reach upper or lower control limits.

Formation:
There is an interrelationship among roughness, porosity, and optical formation measurements. Those regions that are calendered the heaviest will be smoother, denser, and will have lower opacity than the adjacent regions that are calendered lightly. Local variations in opacity will show up as having poor formation on optical formation testers. A rough surface will absorb more ink, and that same rough surface will be more porous, as it received less calendering action. The more porous region will also absorb more ink into its interstices. When printers correlate poor printing with poor formation, perhaps it is the roughness and porosity variations that are the culprits, and the formation tester is one additional piece of test equipment that verifies the root of the problem. These formation problems occur on a small scale, smaller than what a basis weight process control system will discriminate. Formation problems are perhaps quite detectable by analyzing the standard deviation of porosity and roughness measurements within a small region. The roughness and porosity variations caused by fiber flocs are much smaller than the test area of either the roughness or porosity measuring heads. There can be other reasons for high standard deviations, such as non-uniform sizing, which may also be caused by the localized absorbency properties in the region of fiber flocs.

The Printing Process:
One of the most important reasons for measuring and controlling surface smoothness is for print quality. For the contacting-type printing processes, the ink film will transfer to a paper surface upon physical contact. When the voids in the paper surface are deep enough prevent such contact, ink will not transfer to the low spots, and non-uniform ink transfer causes poor print quality. When the ink film is adjusted to achieve satisfactory print density on the rough areas of a web, the same ink film may be too heavy to achieve optimum print quality on the smoother portions, perhaps causing mottle and other problems.

Xerography Processes:
There are many reasons why the manufacturers of photocopy machines have target ranges for Sheffield roughness. A xerographic machine needs optimum paper surface properties for reliable sheet feeding, image transfer, and image fix. The fix level decreases as the Sheffield roughness increases, as it affects toner adhesion. Print density loss is observed as roughness increases. There also can be image problems with papers that are too smooth. Toner particles can be flattened and appear as larger dots, thus increasing the perception of the background. Rougher papers produce less background. With regards to paper handling, smoother papers are less stiff for a given basis weight. Smoother papers increase “electrostatic tacking” in the image transfer process. The coefficient of friction decreases with increasing roughness, a factor that is important in sheet transfer operations. The Sheffield roughness properties are carefully specified for the
electrostatic copier printing process.

Inkjet printing:
Similar to the photocopy machines, inkjet printers have paper handling requirements. The method in which a single sheet is transferred from the supply stack generally relies upon the friction differences in paper-to-rubber versus paper-to-paper in a stack. In recent years, there has been development work on optimizing 2-sided surface roughness for ink jet printers. The printing surface was manufactured to be smooth for image quality and the back side was rough in order to facilitate paper feeding and also to avoid excessive contact with a freshly-printed surface as printed sheets are successively stacked in the printer tray.

The Parker Print Surf test:
The Parker Print Surf tester was one of the first roughness testers that featured both quick test results and a digital display. Since it had those desired features, it showed up in applications where test conditions had no relevance to the nip loading for which the PPS was designed. The PPS loading was designed to replicate the conditions of offset, gravure, and letterpress printing processes; where the operator could select 0.5 mPa, 1.0 mPa or 2.0 mPa loading. The theory was that paper under test should be subjected to the same compression loads found in a printing process.

The Sheffield test:
The xerography and inkjet processes are common examples of processes where the PPS tester does not replicate the loading factors. The Sheffield test subjects the paper to loading pressures of 0.09 mPa at zero Sheffield units, and 0.154 mPa at 400 Sheffield units. The reason for the nonconstant loading is related to the design of the air system, and the “hovercraft effect” of the variable pressure between the measurement lands. When the PPS instrument is set to measure at the lowest loading pressure, it is still about 4 to 5 times higher than the Sheffield loading.

With the introduction of digital Sheffield testers in the late 1980’s, the Sheffield method maintained
its prominence for these grades, at least in the USA. There are many regions of the world where the Bendtsen test is used; however, the correlation between Bendtsen and Sheffield for these grades is excellent. There are many grades where the Sheffield method gives more uniform results than Bendtsen; those grades being the higher basis weight and stiffer grades, where the Bendtsen deadweight is not heavy enough to fully flatten the sheet under test.

Some of the other reasons for testing roughness are related to converting processes. The die-cut sheet feed in an envelope machine requires only one sheet at a time to be picked up and transferred, whereas multiple sheets will cause paper jams. The coefficient of friction between plies has a high correlation to Sheffield measurements.

There are some applications where metallized films are applied to the surface of paper. The reflectance properties of the film can expose wire marks on the base sheet. This is another example where the gentle loading force of Sheffield test better replicates end use properties of the paper, as compared to the PPS test.

Many plastic films are packaged in reams, like paper, for use in a photocopier to produce overhead projector transparencies. When the surfaces of the films are extremely smooth, there are static forces and cohesive forces that interfere with single sheet feeding. The manufacturers of such films generally create rough surfaces that enable an air film to exist between sheets. It iscommon to use Sheffield test results to control the process that generates the rough surface. Again, the PPS test would have measuring head loading that is excessive for this test.

When selecting a test instrument for paper, it is important to understand the relationship between the end-use of the product and the physical test parameters of the instrument. A further requirement is to use a test where process control settings on a paper machine (or plastic web processing equipment) can be modified to optimize the final product for its intended end use. The old adage was “If you can’t control it, why measure it?” In today’s marketplace, the customer will be able to find a supplier who makes the product he wants.