Paper Characteristics

This Section is Currently Under Construction

 

 

4. CHEMICAL AND PHYSICAL CHARACTERISTICS

4.1 OPTICAL

PAPER GLOSS

The gloss of a paper’s surface is a result of either specular reflection or specular diffusion.  If incident light rays (I) strike the paper, at an angle X to the normal (N) of the paper surface, are specularly reflected (reflected at an angle X to the normal (N)), the paper appears glossy.  If incident light rays striking the paper, at an angle normal to the plane of the paper, are diffuse reflected (reflected at angles other than X to the normal (N), the paper appears dull or matte.  The measurement of surface gloss is performed using a goniophotometer.  The goniophotometer measures the reflectance of light at various angles with an angle of incidence 75 degrees to a line normal to the paper surface.  The gloss measurement is given as a ratio of  peak reflected intensity (gloss intensity) to the average of the other measurements (diffuse intensity). Gloss is also measured by comparison of reflection intensity of the paper with a standard, such as black glass. (paper inst)

 

 

 

 

A related measurement is reflectivity.  The theory is the same, but the measurements are carried out differently.  In this case the incident light is filtered, so that the paper’s color and luminosity can be characterized by its reflectivity at various wavelengths. 

PAPER BRIGHTNESS

A paper’s brightness is measured by illuminating the specimen with a source 45 degrees to the paper’s surface and filtered at 457nm, and a photocell at 90 degrees to the paper’s surface.  The instrument is calibrated using magnesium oxide, which is referenced as 100.  The paper’s brightness therefore is a measurement of the ratio of reflected light from the paper to the reflected light from the magnesium oxide.

PAPER OPACITY

Opacity is the ability of the paper to block transmitted light.  The instrument used to measure this property, takes a measurement of the light reflected by the sample placed in front of a white standard ratioed against a measurement of the light reflected by the sample placed in front of a black standard.

PAPER COLOR

A spectrophotometer is used to measure the spectral reflectance of the paper from 380-700 nm.  This provides the manufacture with a complete record of a paper’s color.

4.2 PHYSICAL

TEAR/TENSILE STRENGTH

Tensile strength is the ability of the paper to resist tear when a tensile load (a load parallel to the paper) is applied.  The paper is placed in a standard tensile tester, where one edge of a 6cm wide test strip of paper is secured in a fixed clamp, while the other end is secured in a movable clamp.  A starting cut is applied to the paper.  Clamps are moved apart causing a stress load on the paper, until it rips and the force in kg is recorded.  Tear strength is measured perpendicular to the plane of the paper.

 

WET STRENGTH

Wet strength is the ability of the paper to retain its tensile strength when saturated with water.  Paper that can retain 15% of their dry tensile strength when wet are considered to have “good wet strength,” while papers that retain around 50% of their dry tensile strength are considered “superior.” (waldens)

FOLDING ENDURANCE

Foldability is the ability of the paper to maintain its tensile strength when folded back and forth under a tensile load.  The measurement is the number of folds before failure.  Folding endurance is usually improved with longer paper fibers, greater density, and animal sizes.

The apparatuses for measuring folding endurance are the MIT, the Kirchner, and the Schopper Fold Tester.  Because the arc distance of the three machines are different their output cannot be compared. (mead,Sindall)

 

 

   

 

BURST STRENGTH

Burst strength is related to tensile strength and fiber orientation.  It is a test of the ability of the paper to resist bursting when placed under the pressure of a diaphragm.  In the Mullen test, the specimen paper is clamped with a ring over a collapsed diaphragm, which is then filled with oil, until the paper burst.  The measurement is related as the pressure applied to the diaphragm at the moment of bursting.

 

 

PAPER POROSITY

Porosity is the ability of air to flow through to flow through the sheet of paper.  It reflects the density and spacing of the paper fibers composing the sheet.  There are two apparatuses available for making measurements, the Gurley Densometer and the Sheffeld Porosimeter.  Both apparatuses measure the amount of time it takes a given amount of air at a constant pressure to pass through the sheet.

4.3 CHEMICAL

The main chemical characteristics of the paper that are of concern to the paper manufacture are the moisture content and the pH.  The moisture content can affect the papers surface characteristics which may cause curls, ridges, shortening , or other problems that may cause end-use problems.  Another concern is the pH of the paper.  Paper with low pH levels lack longevity and can interfere with ink curing, while paper with  high pH levels can cause problems with printing press operations.

MOISTURE

The moisture content of paper is expressed as the ratio of the weight of water in the paper to the paper’s total weight.  The standard method used for this determination is by differences of masses obtained by measuring the paper’s mass, placing the paper in an 105°C oven for 1 hour, then measuring the paper’s mass again.  The difference in the two measurements is the weight of the water that was present in the paper.  (mead)

pH

pH is a measurement of the acidity of a solution based on its hydrogen ion potential.  For paper it can determine the life-span of the document as well as its printability characteristics.  Paper with low pH levels (acidic) start to deteriorate after 25 years.  Alkaline paper (high pH) can last over 200 years.(vanson)  The rosin soaps and aluminum sulfate used during the paper-making process leaves the paper acidic with pH levels around 4.5-6.0.  This lower pH level can also adversely affect the drying of inks that cure by oxidation and polymerization.  Most paper coatings are alkaline ranging between 7.5-8.0 and can render an acidic paper suitable for use with inks that require neutral or slightly alkaline substrates.  The Alkaline paper-making process produces paper that is neutral of slightly alkaline.  Lithographic printing problems with alkaline papers can occur when calcium carbonate is used as the size-coating.  The calcium carbonate can dissolve in lithographic fountain solutions, increasing the pH of the solution and causing toning, tinting, and piling problems.(Vanson)    The pH of a sheet is determined by aqueous extraction techniques.
 

 

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