PAPER CURL, DIMENSIONAL STABILITY, 
EFFECTS OF SHRINKAGE AND XEROGRAPHIC PAPER

(Note: this website can be reached via www.PaperCurl.com)

CURL BASICS (Read more at Curl Basics v2, below )

• Curl is effect of differential dimensional changes between layers in a sheet.
• These are produced by different moisture expansions (usually from differences in fiber orientation); the release of differing amounts of internal strain; or from bending.
• Xerographic (copier) curl is usually toward the print at moistures below 5-5.5% and away from the print at higher moistures for 75gsm papers.
• Balancing amount of hydrophilic binder applied to each surface should reduce curl.
• Adjustment of jet versus wire speed can reduce the difference in wire versus felt side fiber orientation.
• A reduction in inherent moisture expansion of furnish should be helpful in reducing curl (see below).

DIMENSIONAL STABILITY (Read more at Aspects of Dimensional Stability, below)

• Inherent dimensional stability of a furnish or pulp needs to be measured on free dried sheets. If they are oriented sheets, a calculation of geometric mean expansion should be compared.
• Factors in stability include fibril angle and degree of refining, additives, and filler.
• Paper machine factors include fiber orientation, wet pressing, wet straining and sheet shrinkage.

SHEET SHRINKAGE IN DRYING

Shrinkage is perhaps the most important process effect on the mechanical properties of paper. Recently, Wahlstrom and Fellers showed that tensile stiffness, tensile strength, compressive strength and breaking strain are very closely correlate with sheet shrinkage or wet straining. A most likely explanation is that shrinkage affects micro-compressions, which in turn affects fiber properties, which affect sheet properties.  Read more at "Effects of Paper Shrinkage" and "Property Relationships", below.

XEROGRAPHIC (copy) PAPER

Free electronic articles:

Paper Physics

• Loading Curve Variables vs CME (new 9-2004, .pdf)

Loading curve data and CME were used to evaluate changes in things like bonding. Similar paper grades, such as copy paper products, seem to be produced with similar furnish and bonding qualities.  The use of only the load-elongation curve seems to suffice to make an evaluation.

• Effects of Paper Shrinkage (.pdf)

A model is discussed that increasing paper dimensions at the “reference moisture content” in manufacture or after manufacture (a) decreases coefficient of moisture expansion, (b) increases internal strain, (c) decreases strain to break and (d) increases tensile stiffness and tensile strength.  The converse is also true.  The effect of exposure to high humidity is also a subject of discussion.

• Property Relationships (.pdf)

This article is a data supplement to Predicting Modulus and Properties.

• Stiffness Index (.pdf)

This article discusses the evaluation of inherent paper stiffness by accounting for the effects of weight and caliper.  It provides a way to know whether stiffness has been improved by changes we make in composition and manufacture.   It avoids the need to make multiple runs with exactly the same basis weight, caliper and MD/CD stiffness ratio.

• Residual Strain and Reloading Modulus (.pdf)

Applied strain is used to describe residual strain.  At a given RH, strain rate, stress relaxation time, and recovery time, residual strain is approximately a linear function of the applied strain.  Test results do not vary much among machine made papers or test direction (MD or CD).

• Predicting Modulus & Properties (.pdf)

This article discusses approaches to mea­suring and predicting specific elastic modulus in a papermaking process and how to use it to predict other paper properties.

• Residual Strain From Heating (.pdf)

This presentation contains a number of graphs which show the effect of heating has on increasing residual strain significantly.  Such data could relate to many papermaking and copying processes that use elevated temperatures.

• Accelerating Creep by Heating (.pdf)

Accelerated creep from humidity cycling previously has been studied and is an established phenomenon but little or no reference has been made to heat accelerated creep. This report provides data on heat accelerated creep and puts forth an idea on how to analyze accelerated creep. When a creep sample has been rapidly heated or moistened, the elongation that is required at the test load becomes greater.  As a result the sample extends to compensate for this, a movement to equilibrium, increasing creep beyond what would normally be seen at constant RH or temperature.

• Tensile Loading Tutorial (.pdf)

It is essential to understand the principles involved in putting paper in tension.  This articles is a short tutorial on the subject.

• Other Articles (updated 11-2004)

Curl and Dimensional Stability

• Curl From One Sided Drying (new 7-05, .pdf)

This article discusses curl observed using a HOT BEND or WARM OVEN test even though there is no top to bottom unbalanced fiber orientation within the sheet thickness.  This example points out that any top vs. bottom differences found should be verified by moistening, drying and testing again.

• Paper Curl Self-Tutorial (.pdf)

This is a self study tutorial using the format: (1) a question, (2) a response (sometimes with subheadings) and (3) additional study information.

• Curl Basics v2 (.pdf)

This article reviews how curl occurs when layers within a sheet expand or contract unevenly when they originally started out at equal length.  Uneven shrinkage (or expansion) produces a bending moment because of uneven shear forces.  A model of a sheet made up of two layers of equal thickness is discussed.

• Fundamentals of Paper Curl (.pdf)

This article expands the information in the Curl Basics with more in depth treatment.

• Dimensional Stability Fundamentals (.pdf)

Measurements of coefficient of moisture expansion can be used to improve understanding of other properties such as elastic modulus and tensile strength and properties such as material state.

• Aspects of Dimensional Stability (.pdf)

This article contains information in condensed form about dimensional stability.  It covers many aspects of the subject.

• Measurements of Dimensional Stability (.pdf)

Plate dried handsheets have zero shrinkage and the same CME (coefficient of moisture expansion).  To determine the inherent CME of different pulps or furnishes it is necessary to use free dry handsheets.  Each pulp will have a different shrinkage and CME.

• Improving Curl Proposal (.pdf)

A reasonable and practical approaches to curl problems are used to determine the structural properties of paper and board as they relate to curl.  In end use applications curl is often more than "moisture change" curl, but include the effects of internal strain and moisture, as seen in xerographic curl.

• Fuser Curl Mechanisms (.pdf)

Observations on fuser curl of 75gsm paper are explained in some detail.

• Fuser Curl At Higher Dwell Times (.pdf new 11-2004)

This article explores some of the curl results from increased heating times not normally used in a fuser.  At normal heating times (30-50ms) of 75gsm paper in a copy machine fuser, curl is reasonably predictable when the moisture content and paper structural properties are known.  However, when heating times are increased above the normal range, curl usually changes dramatically.  The results may be useful in useful in understanding the mechanism of curl in a fuser.

• Effect of Fibril and Fiber Angle (.pdf)

A simplified calculation is used to show how fiber modulus affects paper modulus in terms of how the fibers arrangements in the paper. Since 1/CME correlates with paper modulus, the results of the calculations ought to provide information about the effect of fibril angle and fiber angle.

• Curl Terminology (.pdf)

Method(s) used to describe curl

• Hot Bend Curl Test (.pdf)

Paper strips are heated on a curved surface.  It is a good procedure for evaluating effect of wire/felt fiber orientation.

• Warm Oven Curl Test (.pdf)

Sheets are suspended in 180-200 deg. F. for 1-2 minutes, removed and hanging curl is measured.  Useful in evaluating effect of wire/felt fiber orientation.

• Xerographic Curl Guidelines (a summary, .pdf)

Various test methods reviewed are deemed important or useful in evaluation a paper’s suitability for use in copy machines.

• Procedure to Characterize CME (.pdf)

Procedures used to determine dimensional stability can easily affect the results obtained.  By following the same procedures each time results should be comparable.  This is procedure for determining CME characteristics for any method used to make the measurements.

• Curl Tests summarized (.pdf)

Short descriptions of 15 curl test methods besides the HOT BEND and WARM OVEN.  Some may find uses for particular situations.

• Other Articles (updated 11-2004)

Paper Technology

• Xerographic Paper Self Tutorial (.pdf)

This tutorial can help you understand what properties xerographic papers need to perform well in copy machines, duplicators, and laser printers.  The only difference between a laser printer and other machines is the image is placed on the photoreceptor by means of a laser.  Therefore, there is no difference between the paper requirements between laser and light lens machines with the same paper handling and image transfer equipment.

• Summary of Xerographic Paper Properties (.pdf)

There may be no single set of paper properties that are optimum for all xerographic machines (including the so called ‘laser’ printers).  This article summarizes properties and quality procedures important xerographic papers used in faster machines.

• Basis Weight Conversion Factors (.pdf)

Converts from one basis weight system to another with convenient factors.

• Thermal Properties of (Copy) Paper (.pdf)

This article should improve the understanding of thermal properties and how they could affect its use.  Although copy paper is the main thrust, the information can be used for other papers.

• Inkjet Paper Testing (.pdf)

Plain paper inkjet print quality which can be attributed to ink flow along fibers. The penetrating surface tension (PST) test method can estimate the degree of feathering. This article describes a method that uses liquids of differing surface tensions, usually mixtures of an alcohol and water. Other sizing test methods (such as KBB, Hercules and Cobb) use a single liquid, which is of limited value in comparison.

• Static Problems (.pdf)

A buildup of static in a stack of paper (sorter or output tray) can be a serous problem.  This article describes several things can contribute to a buildup of static.  Knowing these, static can be reduced or eliminated.

• Handmade Paper from Straw, etc. (A How to) (.pdf)

This article can be used to develop an interest in a career in paper and paper recycling.  Also, it can be used to enhance the knowledge of paper of students in printing or packaging.

• Twenty Questions (Xerographic Paper) (.pdf)

Questions (and answers) that may be asked about copy paper and its performance.

• Imperial System of Weights and Sizes (.pdf)

As the title suggests, there is information about the Imperial system of basis weight and sizes. The name applied to those weights and measures appointed by statute to be used throughout the United Kingdom in 1836 and today is a term loosely applied to the same traditional sizes and linked weights of the pre-metrication era.

• Internet Articles and Info (.pdf)

Links to numerous websites are listed.  Please let us know when you find broken links.

• Loose Debris Evaluation Methods (.pdf)

The article provides short descriptions of six ways to quantify loose debris.

• Other Articles (updated 11-2004)

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