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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.