Once upon a time, the papermaker made the paper and the converter just cut it up and packaged it.  Sometimes the converter added a few minor finishing touches to the paper, like calendering.  More finishing processes were added, and now they make a very significant contribution to the overall quality of the product. 

In this paper, a process is a finishing process if it adds value by changing the intrinsic properties of the paper after papermaking.  Since these processes act directly upon the paper, they must be done while it is accessible in sheet form - not wound up into a roll or stacked.  On the paper machine, this could be anywhere between the crepe doctor and the reel.  However, most finishing processes would impair efficient operation of a paper machine.  In some cases, a finishing process may be included in a rewinding operation, but the most common location is on a converting line, after the parent roll unwind and before winding or folding. 

Finishing processes do so much for so little.  Since converting lines are much smaller and slower than paper machines, these processes can add value to the product at very low cost.  Also, a new process can often be added by simply installing another treatment station in an existing converting machine. 

Finishing processes are closer to the end user.  They are the last steps in the sequence of processes that build product properties.  Therefore, we will focus more upon the product properties from the user's perspective than upon the process mechanics.  This paper will introduce the concept of the "finishing curve", which the author has found to be a powerful tool for maximizing the value of the finished product, and maintaining the process on target. 

Some Common Finishing Processes

• Calendering improves the surface smoothness of the paper by pressing it between two smooth cylinders.  This is the simplest of the finishing processes, and is the only one that can be done on the paper machine without seriously affecting machine efficiency.  It is frequently a part of rewinding and converting operations. 
• Embossing creates bulk and absorbency by passing the paper between two cylinders, where one or both of the cylinders are engraved with a pattern.  This is probably the most powerful of all of the finishing processes, with the greatest potential for changing the intrinsic properties of the paper.  It is almost always done in converting. 
• Ply bonding and laminating combine two or more plies of paper into a single sheet, either mechanically or with an adhesive.  Some products are ply-bonded in a rewinder, but this is done more often in converting.  Towel laminating is a special process that is combined with embossing on a converting line. 
• Lotionizing improves softness by adding a substance to the paper surface.  Typical substances are oils, waxes, or silicones.  Application methods vary greatly, but may include contact methods, such as printing or coating, and spray methods, such as hydraulic, aerosol, mechanical, or electrostatic.  It may be done in rewinding or in converting, but rarely on a paper machine, probably due to the risk of contaminating the wet end with the lotion. 
• Printing enhances the appearance of the paper by adding decoration and color.  The two most popular methods are gravure and flexographic.  Gravure printers have been used in rewinding operations.  Flexographic printers are used on converting lines for high quality, multi-color decorating for consumer towels. 

The Intrinsic Properties of the Paper

The product properties most affected by the finishing processes include absorbency, bulk, strength, handfeel, ply count, and visual appeal.  These terms have special meanings to the tissue paper industry.  The converter who understands these properties and the mechanisms behind them is better able to tune the finishing processes to peak performance. 

The following descriptions of properties were written as an analysis of how the customer or user experiences the product.  What the product does in use is more important than what the product is.  In some cases, this viewpoint may seem strange to someone who is accustomed to focusing upon how the product is made or tested. 

• Absorbency is the ability to draw and hold a volume of liquid.  The material must first be able to pull the liquid from another surface, and then hold it against other forces such as gravity.  The liquid is held almost entirely within the empty spaces between the fibers, sometimes called pores, and very little is held inside the fibers.  Paper fibers provide surfaces that attract the liquid, especially when the liquid is water.  The fibers also provide the structural strength to hold these spaces open against capillary forces.  Due to the nature of these forces, small spaces hold small amounts tightly, and larger spaces hold larger amounts loosely.  A well designed distribution of sizes of these open spaces within the product works best.  The smaller pores contribute to wipe-dry and absorbency rate, while the larger pores provide total capacity.  The volume of liquid that is held by the product is very dependent upon the magnitude of the forces acting to remove the liquid, forces that are usually due to acceleration or gravity.  A paper towel held gently and horizontally will hold a lot more water than a paper towel that is shaken or held vertically.  Test methods vary as well, especially in how the water is drained from the sample. 
• Bulk is the capability to occupy space while resisting some amount of pressure.  More bulk makes it possible to design a larger package for the product, which adds to the perception of value in the consumer market (but not in the institutional or "away from home" market).  It is roughly equivalent to thickness, and is usually measured as the thickness of a stack of sheets under a standard load.  It is extremely sensitive to the applied pressure, and therefore the ability to resist this pressure is a key component.  Selecting an appropriate pressure for measuring bulk is difficult because the product is subjected to many different conditions.  When this pressure approximates a person's touch, bulk is closely correlated to the perception of "body".  When the test pressure equals the internal pressure that exists in a firmly wound roll of product, bulk is an excellent predictor of roll diameter.  The best correlation between bulk and absorbency is achieved when the pressure for the bulk test matches the average pressure due to capillary forces.  Determining the bulk at one pressure does not, in general, allow one to predict the bulk at significantly different pressures, especially when products are made in very different ways, as illustrated in Figure 1. 
• Strength is the ability to resist structural failure while subjected to various forces during use.  When wiping up a spill, the paper towel should not leave pieces behind.  A sneeze should not blow holes through the facial tissue.  The conditions and forces applied during use are very complex, and have a strong effect upon how well a product performs.  Most paper products are stronger when pulled in the machine direction (MD) than in the cross direction (CD), and stronger when wet than when dry.  To simplify and standardize, we normally test the paper in pure tensile mode (a uniform pull), one direction at a time, all dry or all wet, and continue pulling until it breaks.  Initially, the paper's resisting force is very low and climbs slowly as it is stretched, and then this force steadily increases to a maximum before it falls off at failure, sometimes abruptly (see Figure 2).  This maximum resisting force is reported as the tensile strength for a particular condition (dry or wet) and direction (MD or CD). 
• Handfeel, also called softness, may be the most important property of bath tissue and facial tissue.  It is also the most difficult to quantify.  It is highly subjective, with different people disagreeing about what is softer.  It is composed of at least three components:  smoothness, cushion, and drape.  Although each of these components can be measured with mechanical devices, they "add together" in a complex way to form a composite that is sometimes called overall handfeel preference.  In the author's opinion, no mechanical test or combination of mechanical tests has yet been developed that is as good as a trained human evaluator.  However, good handfeel is a required quality to be considered a premium bath or facial tissue. 
• Ply count is a product property, when considered from the user's perspective.  For the converter, it is just how the product was made.  Ply count is an attribute that is usually advertised on the package, because the user generally believes that more plies means better quality.  If the package says that it is a two-ply product, the user had better find what seems to be a two-ply product inside, regardless of how it was made.  It is very important to bond the plies properly.  If the plies are stuck together so thoroughly that they cannot be separated anywhere, then the product will be perceived as single ply.  If the ply bond is so weak that the plies separate too easily, or have already separated, then the user may believe that they were never really attached. 
• Visual appeal is a critical property of quality products.  The embossing should look purposeful, and not as though the product was damaged by a water spill.  The printed decorations should enhance the perception of quality, with clean lines and good registration between colors. 

Balancing Properties for Overall Quality

A converter's job is a balancing act.  Overall product quality requires a well balanced blend of properties.  Otherwise, the only important properties are the ones that do not meet expectations.  If a bath tissue is not soft enough, then simply making it thicker or stronger would not add value.  However, if the product has adequate or even excess bulk or strength, then there are finishing processes that can exchange some of that bulk or strength for handfeel.  The improvement in the balance of properties translates to an improvement in the overall quality and value of the product. 

Adding Value by Trading Properties

For each process, the properties that are subtracted are like currency, available to be exchanged for value.  The converter needs to know how to quantify the tradeoffs between product properties for each process.  How much handfeel can be gained by calendering at what cost in bulk?  How much absorbency can be traded for how much strength when embossing? 

In this paper, a tradeoff relationship between properties associated with a finishing process is called a Finishing Curve.  In the case of calendering, the tradeoff relationship between handfeel and bulk is a Calendering Curve.  For embossing, the tradeoff relationships between bulk and strength and between absorbency and strength are Embossing Curves.  This concept has been borrowed from papermakers who use "Technology Curves" to quantify the tradeoff effects of papermaking processes. 

Typical shapes for these Finishing Curves are shown in Figures 3 and 4.  The axes on the graphs are shown without numbers because the actual values vary greatly with different manufacturers, products, and test methods.  The procedure for determining these numbers, how to quantify a tradeoff relationship for a particular manufacturer and product, is similar for all of the processes. 

Normally, there is a single variable that is the primary process control for each process that can be continuously varied over a range.  This is usually gap or nip load for calendering and embossing, and fluid application rate (add-on) for lotionizing.  Increasing this primary variable amplifies the tradeoff between product properties.  Decreasing it diminishes the tradeoff.  An example of a primary process control variable for embossing is shown in Figure 5. 

The remaining or secondary process control variables usually affect the relationship between the primary variable and the product properties, but they rarely affect the tradeoff relationships between the properties.  This concept is best explained with an example, which is also illustrated in Figure 6. 

In an embossing nip with a steel engraved roller and a smooth rubber-covered roller, the pressure in the nip is often the primary control variable, and the hardness of the rubber cover would be considered a secondary control variable.  If nothing else changes, there is a fixed relationship between the nip pressure and the amount of bulk gained through embossing, and also a fixed relationship between nip pressure and the amount of strength lost (as in Figure 5).  However, if the hardness of the rubber cover is changed, these relationships may change considerably.  At the same pressure, less bulk will be added and less strength will be subtracted when the rubber cover is harder (see example in Figure 6).  Most of the change introduced by the harder rubber may be compensated by increasing the nip pressure. 

How to Use a Finishing Curve

As a benchmark.  A finishing curve is the best way to document the "health" of a finishing process in a specific instance.  Tracking changes in the finishing curve over time can provide advance notice of future problems, which can then be addressed during scheduled maintenance periods. 

For centerlining.  A finishing curve can be used as a process control tool.  It can show the normal operating range as well as the limits of the process. 

As a diagnostic.  The finishing curve can help identify the source of an operating problem when it is compared to a condition in which the system was running well.  If the problem is in the base paper, the finishing curve will clearly indicate this as well. 

For consistency.  The finishing curve can be used to improve consistency or standardization in product made from different machines and at different sites. 

For comparing "apples and oranges".  The finishing curve provides a common basis to evaluate and compare dissimilar processes and process options, combinations of processes, or different base sheets. 

For process understanding.  The finishing curve shows causality between process, base paper, and product.  It shows what is possible with the process in its current state. 

A Detailed Example:  Using a Finishing Curve to Solve a Problem

Creating a finishing curve the first time requires more effort than later ones, because several decisions need to be made at the beginning.  Also, the first curve establishes many things about the tradeoff relationship between the properties that are fairly stable, and therefore may not need to be reconfirmed every time.  Less data will be needed in future curves.  With experience, there are many shortcuts to be found. 

The procedure for creating a quantified finishing curve is similar for all of the finishing processes.  Using a specific example with a fictitious scenario will make it possible to explain this in greater detail. 

Scenario:  The product is a single-ply, heavily embossed bath tissue for the consumer market.  The problem is that the bulk has recently fallen below acceptable levels, causing "mushy" or loosely wound rolls of product.  The base paper has been recently upgraded, and is therefore a suspect, even though the "upgrade" has produced higher bulk from the paper machine.  Fortunately, an embossing curve has been found that was created before the base paper was changed. 

Solution plan:  Create a new embossing curve with the new base paper, and compare it with the old one.  In both embossing curves, the tradeoff is between bulk and CD dry tensile strength (CDT).  The primary process control variable is embossing penetration, which is the distance that one steel engraved roller has penetrated or enmeshed into the other.  More penetration means more embossing, more bulk gain, and more CDT loss. 

Step 1.  Select and document the base paper to be used in this test.  It is best to locate a very average and uniform parent roll.  Record complete identification of the parent roll.  All critical properties of the base paper should be tested and recorded.  Always test basis weight, tensile strength in both directions, and bulk. 

Step 2.  All extraneous variability should be minimized.  If cross direction variability is a problem, either in the base paper or in any of the subsequent processes, then all testing should be done at the same CD positions in the parent roll and in the product.  For embossing, a position which is halfway between the center of the sheet and either edge works very well.  If MD variability through the parent roll is a problem or is suspected, then the base paper should be tested more frequently as the parent roll is consumed.  Other processes in the converting line can mask the effects of the embossing process.  These may be eliminated in this particular case by collecting the embossed paper directly out of the embossing nip at slow speed. 

Step 3.  Decide what levels of embossing should be run and tested.  Since an older embossing curve already exists in this case, three or four levels of embossing should be adequate.  These embossing conditions may be called A, B, C, and D for labeling purposes.  The test range should encompass the normal operating range, and it should also extend beyond the normal range in both directions, even if that would result in unacceptable product. 

Step 4.  Adjust the embosser to the first penetration level and run enough paper through the nip for testing.  Label the sample with the embossing condition number and test for the following properties:  basis weight, bulk, and dry tensile strength in both directions (MDT and CDT).  Repeat with every embossing penetration level.  After all samples of base paper and embossed paper have been collected, the converting machine may be returned to production. 

Step 5.  About product property testing.  The procedures for measuring the properties of the base paper and of the embossed paper should be exactly the same.  The bulk test should include a low pressure bulk test, because that will correlate much better with how firm a roll of product can be made. 

Step 6.  Collect the data, preferably in a spreadsheet, and make a printed copy.  The data should include the base paper properties, treated as if it were just another embossing variant.  An example is shown in Figure 7. 

Step 7.  Graph the data, as shown in Figure 8, and make a printed copy.  Make sure that the scales of both axes (bulk and CDT) for the new graph match the axes on the old graph.  Include the base paper data as part of the curve. 

Step 8.  Compare the new embossing curve with the old one.  In the example shown in Figure 9, the new curve appears to be simply shifted to the left (less strength) and up (more bulk), relative to the old one.  The shift almost matches the difference in base paper properties.  However, we see that the new base paper does not have enough strength to allow the embossing to work as well.  Since such a very large portion of the final product bulk comes from the embossing, the increase in base paper bulk is too small to make up for this loss.  This is an example where an increase in the bulk of the base paper does not translate into an increase in the bulk of the final product. 

FIGURES

Figure 1:  How Bulk Is Affected by Applied Pressure

Figure 2:  Tensile Test Curve

Figure 3:  Finishing Curve for Calendering

Figure 4:  Finishing Curve for Embossing

Figure 5:  Example of a Primary Process Control Variable

Process = Embossing
Primary control = Nip line load (pli)

Figure 6:  Effects of a Secondary Process Variable

Process = Embossing
Secondary variable = Rubber hardness

Figure 7:  Test Data for Example Problem

Figure 8:  Finishing Curve for Example Problem

Figure 9:  Comparing Curves for Example Problem