Biermann Ch. Handbook of Pulping and Papermaking
Подождите немного. Документ загружается.
174 7. PAPER AND ITS PROPERTIES
STRAIN
Fig. 7-11. The relative tensile behavior of
paper in the machine (MD) and cross machine
directions (CD). Stress is the apphed force,
strain is the sample extension (increase in
length). Stiffness = stress/strain.
The test device is shown in Fig. 7-10. The
absorption of oils occurs proportionally to the
square root of time, but aqueous liquids have an
initial wetting time before absorption
begins.
The
test is carried out using paper attached to the 25
mm (1 in.) wide circumference of a wheel 1 m
(3.3 ft) in diameter; a stationary "headbox" is
filled with a known amount of ink that is drawn
over the sheet under a light pressure.
values, since tear strength in highly dependent
upon the properties of the individual fibers (i.e.
fiber length and fiber strength. The first part of
Table 7-3 indicates the tear strength is about the
same in the MD and CD directions, although the
tensile strength varies considerably in these two
directions.
Fig. 7-11 shows a representation of the
difference in tensile strength of paper in the
machine and cross machine directions (the
direction the testing force is applied). Paper tends
to be stiffer and stronger in the machine direction
than the cross machine direction. TAPPI Standard
T 409 lists methods for determining the machine
direction of paper and paperboard.
Paper strength versus loading rate
The strength of wood and paper (and any
material) depends upon the rate of loading (how
quickly the force increases). A relatively large
load can be supported for a short period of time.
The relationship between load and loading rate is
typically logarithmic. For example, the edgewise
compressive strength of corrugated fiberboard was
found to increase about 7.5% for each tenfold
increase in loading rate. Also, the strength of an
A-flute was 40.2 lb/in. at 0.01 in./min loading
rate,
45.1 lb/in. at 0.1 in./min, and 48.1 lb/in. at
1.0 in./min (Moody and Konig, 1966). For this
reason, standard test methods for measuring paper
strength always specify loading rates.
7.6 MECHANICAL PROPERTIES OF
PAPER
Machine- and cross-direction of paper
The machine direction of paper is parallel to
the direction of travel during the manufacture of
the paper; cross direction is perpendicular to
machine direction. Paper properties such as
tensile, fold, and compression vary significantly
between these directions. This is often attributed
to fiber orientation, but often of even more
importance is the fact that in the machine direction
paper is dried under much higher restraint than in
the cross machine direction. If fiber alignment
were the principle reasons for these differences in
strength properties, one would expect to see a
large difference in MD and CD tear strength
Tensile strength
Tensile strength is measured on paper strips
20 cm (7.9 in.) long by 15-25 mm (0.6-1.0 in.)
wide using a constant rate of elongation according
to TAPPI Standard T 494. The tensile test is
shown in Fig. 7-12. The ultimate force is
reported in lb/in., kg/m, or N/m. The tensile
index (obtained by dividing the tensile strength by
the basis weight) and breaking length are alternate
means of reporting tensile strength where the basis
weight is normalized. The tensile strength of
other materials is reported in units of force/area
not force/width as is done for paper.
As with all other strength properties
dependent on the paper direction, the tensile
strength should be measured separately in the
machine and cross machine directions of paper.
MECHANICAL PROPERTIES
OF
PAPER 175
Fig. 7-12. The tensile test. The insert shows
the paper sample during a test prior to
fracture.
The tensile strength can also be measured on
pendulum type machines (Standard T 404), which
may give slightly different results. The tensile
strength of wet strength papers is measured by
Standard T 456. A zero-span tensile test can be
used to measure the strength of fibers (as opposed
to interfiber bonding) within the sheet.
Breaking length
The tensile strength of paper (or any other
material) can be reported as a hypothetical length
of paper that just supports its own weight when
supported at one end. The breaking length is
discussed in detail in Section 15.3 with examples
for many materials. It compares the tensile
strength without regard to paper thickness,
density, or width. The breaking length of most
papers varies from 2.5 to 12 km.
Burst strength
The burst strength, which is also called the
Mullen or pop strength, measures the amount of
hydrostatic pressure (which increases at a specified
rate) required to rupture a piece of paper. The
Mullen tester is shown in Fig. 7-13; inserts show
the test area. The paper is ruptured by a small,
circular (30.5 mm, 1.20 in. diameter), rubber
diaphragm pushed against the paper by glycerin.
The maximum pressure before rupture is measured
in psi or kPa. The burst test is highly correlated
to the tensile strength. TAPPI Standard T 403
describes the test for paper, T 807 for paperboard
and linerboard, and T 810 for corrugated and solid
fiberboard. The burst may also be reported in
units of force divided by basis weight to give the
burst index, burst factor, or burst ratio.
index
=
factor =
ratio
=
kPa
g/m^
g/cm^
psi
lb/ream
U.S.
Federal regulation Rule 41, item 222
requires the burst strength and basis weight of
corrugating medium and linerboard to be reported
on most corrugated boxes used in the U.S. A
recent modification effective since January, 1991
allows edge crush strength to be used instead
(Kroeschell, 1991). Many people think that the
linerboard STFI compression test is a much better
indication of the final box strength than the burst
test in most applications.
176 7. PAPER AND ITS PROPERTIES
Fig. 7-13. The mechanical (top) and electronic burst testers. The top insert shows a sample
breaking during the test. The bottom insert shows the sample test area.
MECHANICAL PROPERTIES OF PAPER 177
Fig. 7-14. The STFI compression strength tester. The insert shows the sam}i>ie test area with
moisture meter. The moisture content correction is calculated automatically.
Edgewise compression strength, STFI
The short span compression (edgewise
compression) strength of paper gives values
similar to those of ring crush tests (described
below). The results can be correlated with tensile
and burst strengths of paper. It is very important
in predicting how a corrugated box will perform
with a particular linerboard. This test is becoming
the standard for container board papers. The
instrument is shown in Fig. 7-14 and may contain
a moisture meter (TAPPI Standard T 826.)
Strength determination by sound velocity
A recent development (Fig 7-15) is the use of
devices that measure the sound velocity in paper.
Ultrasound (frequencies above that which humans
can here) is used. Physics tells us that the speed
of propagation (c) of a transverse wave is a
function of the mass per unit length (/i) and the
tension or stiffness (5) of the material for a
perfectly flexible string. The relationship is:
c =
(S/IJL)
^ (for a transverse wave)
178 7. PAPER AND ITS PROPERTIES
MD 3.75 km/s max
* CD
^2.03
• km/s
Cross Length Ratio - CD/MD
av.
Cross Length Ratio - MAX/MIN
Peak Value - MIN MD
:^P^
av.
maximum = L86
average = L77
minimum = L68
maximum = L84
average = L78
minimum = L69
maximum = 3.811 km/s
average =
3.766
km/s
minimum =
3.680
km/s
maximum =
2.230
km/s
average = 2.128 km/s
minimimi = 2.016 km/s
Fig. 7-15. Device to automatically measure the speed of sound in paper as a function of angle at
a point or position
across the
reel
with
printouts at one position (top)
and across
the reel (bottom).
179
The speed of sound in paper is incorrectly
attributed only to fiber alignment. In fact, the
stiffness is also a function of drying under tension
and the properties of the individual fibers
themselves, which also depends upon whether they
were dried under tension or not. Fig. 7-15 shows
a device (with some sample printouts) used to
measure sound velocity in paper. The device
automatically scans across a sample of paper
previously cut from the width of the reel. Time
will determine the usefulness of this method.
Ring crush, concora, other container board tests
A variety of ring and other crush tests are
used on paperboards. The test apparatus for
measuring the strength of samples is shown in
Fig. 7-16, using concora flat crush (Tappi
Standard T 808.) Most standard paper tests such
as tensile, burst, and tear do not relate well to how
corrugating medium will perform in actual boxes,
so medium is fluted in the laboratory and tested to
determine its corrugated board properties.
Concora is an acronym from the COMainer
CO/?poration of /4merica. There is a concora
fluting machine for forming corrugated samples in
the laboratory. There are concora ring and crush
tests that are performed on corrugating medium
that is fluted in the laboratory.
The ring crush correlates well with the STFI
edgewise compressive strength. The procedure is
given in TAPPI Standard T 818 without rigid
support and T 822 with rigid support.
Stiffness
The Taber stifftiess (Fig. 7-17) measures the
bending moment of a vertically clamped 38 nmi
(1.5 in.) wide paper specimen at 15° from its
center line (TAPPI Standard T 489.) The test has
been commonly used with other materials besides
paper products. Taber stiffness is important for
papers used in wrapping, structural uses, and
printing. TAPPI Standard T 453 describes the
Gurley tester for measuring stiflhess.
Of historical interest is the flexing resistance
of paper and lightweight paperboard, which is
measured by the Clark stiffness test (TAPPI
Standard T 451). It is a measure of how paper
resists bending when handled and is particularly
applicable to newsprint.
Fig. 7-16. The crush tester (top) and a flat
crush test in progress (insert).
180 7. PAPER AND ITS PROPERTIES
Fold endurance
The fold endurance (TAPPI Standard T 511)
is a measure of the number of double folds a
piece of paper 15 mm wide will endure before its
tensile strength falls below a standard value,
usually 1 kg^ . It measures the strength and
flexibility of paper. Because only a very small
area, perhaps 15 mm^ is measured, there is a large
variation between measurements from the same
piece of paper. The MIT folding endurance is
shown in Fig. 7-18. The Schopper method is
described in Standard T 423. Since heat is
developed during the test, and since the folding
endurance depends heavily on the paper moisture
content (Table 7-4), a fan (supplied in newer
machines) should be used to direct air at the
testing surface during the test.
weight loss with time is measured. The abrasion
test apparatus is shown in Fig. 7-19.
Tear
The internal tear resistance measures the
energy required to propagate an initial tear through
several sheets of paper for a fixed distance. The
value is reported in g-cm/sheet. The difference in
potential energy of a pendulum before and after
the test is measured by an angle along the
pendulum. Fig. 7-20 shows the Elmendorf dtvicc
(TAPPI Standard T 414). Edge tear is measured
by the Finch method, Standard T 470; highly
directional papers are measured as described in
Standard T 496. The edge tearing resistance of
paper by the edge-tear stirrup method is described
in Standard T 470.
Abrasion tester
The abrasion test is conducted by rubbing the
surface of paper with a rotating wheel. The
Internal bond strength of paperboard, z-direction
tensile, Scott bond test
Plate 27 shows the Scott bond test for
Fig. 7-17. The mechanical (left) and electronic Taber stiffness testers.
MECHANICAL PROPERTIES OF PAPER 181
Fig. 7-18. The MIT fold tester. The msert is a
side view of the test sample. Several samples
are clamped at the top and tested one at a time.
measuring the tensile strength in the z direction
(TAPPI Standard T 541.), the direction of the
paper thickness. The stress is applied by pulling
the top and bottom surfaces of paper in opposite
directions. This is accompUshed by applying
double sided, pressure-sensitive adhesive tape to
both sides of a 1 in. by 1 in. square test specimen.
The specimen is pressed between two platens to
bond the tape. The platens are then pulled apart
while the energy required to cause the specimen to
fail is monitored.
Coefficient of static friction
The coefficient of static friction is important
for many packaging papers. Sufficient friction in
these papers is important to keep boxes from
sliding off of each other during shipping. It is
measured by the inclined plane method (Fig. 7-21)
The abrasion tester.
and reported as the tangent of the angle required
for like surfaces placed crosswise to allow sliding
to commence. TAPPI Standards include T 503
(sack
papers),
T 548 (writing and printing papers),
T 542 (packaging papers), and T 815 (corrugated
and solid fiberboard.) The horizontal plane
method (for example, TAPPI Standard T 816)
gives identical results. The friction of paper can
be increased during production by surface
treatment at the calender stack.
Surface strength, wax picking
The surface strength of paper is measured by
the wax pick test (TAPPI Standard T 459) shown
in Plate 28. Calibrated waxes of various adhesive
powers are used to quantify the surface strength of
paper. The waxes are melted and pushed against
the paper while they cool. The wax stick is then
removed from the paper. Waxes with high
adhesive powers will pull the surface fibers away
from the paper.
Other tests
There are numerous other tests of paper
covered in the Tappi Standards and other
182 7. PAPER AND ITS PROPERTIES
f^j^
Fig. 7-20. The mechanical (left) and electronic tear testers. The top insert shows the initial sample
cut. The bottom insert shows the sample tearing during the test.
standards. Only some of the more common tests
have been discussed in this chapter.
7.7 CHEMICAL ANALYSIS OF PAPER
Many of the chemical analysis tests that are
applied to paper can be applied to pulp. Section
6.3 and others regarding chemical analysis of
pulps should be consulted for additional
information.
represents the ash content. It is a measure of filler
content in papers containing clay, calcium
carbonate, or titanium dioxide fillers and soda
content in brown
papers.
Several modifications of
this method are used. The ash content (TAPPI
Standard T 413) is determined by weighing the
residue after igniting the paper at 900 ± 25°C.
Standard T 244 describes the ash content after an
acid extraction to remove the metal salts, leaving
mostly silica and silicates.
Ash content, silica
The amount of mineral residue left after the
complete combustion of organic material
Content of selected elements
Many TAPPI Standards are available to
measure elemental content of paper. T 266
CHEMICAL ANALYSIS OF PAPER 183
Fig. 7-21. The coefficient of friction (inclined plane test). The
bottom picture shows the weight starting to move down the paper.
describes the determination of sodium, calcium,
copper, iron, and manganese content of pulp and
paper by atomic absorption spectroscopy. Acid
soluble iron (which excludes iron that is an
integral part of fillers or silicates that probably
would not interfere with dyes, etc.) is measured
according to TAPPI Standard T 434. Sodium is of
interest from the point of view of chemical
recovery, while iron and copper may cause
discoloring and interfere with dyes. Silica and
silicates are determined by the wet ash method (T
245).
Cadmiimi and zinc fillers are used in highly
opaque pigments and measured according to T
438.
Organic nitrogen is determined by the
Kjeldahl method (T 418).
SEM with elemental analysis
Scanning electron
microscopy (SEM) with energy
dispersive x-ray analysis
(EDAX) is a very useful tool
for qualitative and semi-
quantitative analysis of elements
with atomic numbers of 13 or
higher (aluminum and heavier)
on the surface of paper.
Electrons have very little
penetrating power, so only the
surface is characterized. It is
very useful for looking at
element distributions on the
surface or through the thickness
of paper if cross-sections are
made.
When elements are
bombarded with electrons, they
give off X-rays at frequencies
(measured in energy units of
electron volts) characteristic to
the element. With proper
calibration, the number of X-
rays given off show the relative
amount of that material.
Fig. 7-22 shows an
analysis of a 3M Post-it^^ note
from 1987 (before most mills
went to alkaline papermaking).
The base sheet contains
aluminum (probably from rosin
sizing and as a general retention
aid) silicon, and small amounts of magnesium
(probably this paper has pulp bleached with
hydrogen peroxide where magnesium and silicates
were used to protect the fiber.) The coated sheet
contains these elements in addition to zinc and
chloride (possibly from a zinc oxide filler.) The
adhesive on top of the coating does not alter the
spectrum except to attenuate the signal, since the
adhesive contains only light elements (C, H, and
O) and absorbs some of the electrons before they
can reach other elements beneath them.
This method has numerous important
widespread applications to scale residues,
corrosion products, and sludge analysis. The
method is simple, fast, and reliable.