Imperfections Formed During Paper Production
In a perfect world, paper would
have no imperfections at all whenever it is made. Reality,
however, paints a different picture. One of the paper machine
design goals is to reduce these nonuniformities. These imperfections
have numerous causes, including the fact that pulp is usually
constituted from several sources, filler agents are not uniformly
distributed, and each individual fiber is of variable size.
Some of paper's desired properties, however, rely on the existence
of these nonuniformities. There are four types of nonuniformities
associated with the physical properties of paper(See diagram
Paper imperfections within a paper grade are sometimes what
makes that a paper grade unique, as pointed out above. Imperfections
and inconsistencies are sometimes part of the uniqueness of
a piece of paper, but it is these 4 types of defects described
above that must be kept to a minimum for high quality paper
to be achieved.
- Machine and Cross Direction Anisotropy--Anisotropy
means that there exists a difference between properties
measured in the machine direction(i.e. the direction parallel
to which the paper travels in a paper machine) and those
same properties measured in the cross direction(i.e. direction
perpendicular across the width of the forming paper web).
can easily discern a paper's machine and cross direction
in the following manner: cut about a 5 cm diameter circle
from a piece paper and float this piece on top of some water.
The paper will curl up and the long axis of this curl is
the machine direction. The cross direction lies perpendicular
to this axis. The difference in properties along the two
respective directions is a consequence of the uneven distribution
of fibers oriented in the machine direction and those oriented
in the cross direction. The orientation distribution ratio
of the paper fibers changes somewhat as the forming paper
web is pulled down the line in a paper machine. The direction
that most of the fibers lie in is mostly in the machine
direction("Wire Side" fibers in Fig 1), since these fibers
lie slightly askew of the machine direction. If plotted
by angles of orientation against number of fibers lying
at those angles on a polar diagram, the shape that appears
is dumbbell or hourglass shaped(enlarged ends with a somewhat
constricted middle) lying on the central diagonal running
through the origin of the graph(See Fig. 2). The differential
angle of the general fiber orientation that is swept out
from the machine direction(MD in Fig.2) is called the fiber
orientation angle(Angle Θ[Theta] in Fig. 2). This
angle, in conjunction with another angle known as the TSO
angle, is partly responsible for paper properties as well
as how well paper is run through a paper machine. The factors
that have a major influence on fiber orientation lie at
the "wet-end" of the paper machine, where headbox consistency,
water drainage rates, fiber length, angle at which the water
jets spray, and wire tension, among others. There are several
ways to measure fiber orientation:
The way the fibers are oriented affects many of paper's
physical properties including:
- Ultrasound -- Sensitive wet end and downstream
- Polarized Laser Light -- Measurement of angles
accurate within ± 0.5°. Lasers are pulsed on both the
top and bottom of the forming sheet. Resultant scattering
pattern is indicative only of the orientation of the
- Transmitted Light -- Sensitive only to the
orientation of wet-end fibers.
Paper that has fairly equal numbers of fibers oriented in
the machine and square direction results in what is known
as square paper. The squareness of paper is expressed
as its squareness value, and this value is different
for each paper grade.
- Paper Machine Runnability(Lack of resistance
of paper when run through a a paper machine)
- Hygroexpansion(Expansion due to the absorption
It is possible for fibers to be oriented through the Z-dimension,
or thickness, of the paper, but it is overwhelmingly more
probable that the fibers lie in the two-dimensional plane
of the paper itself.
- Machine and Cross Direction Variation--
As paper stock flows out of the headbox onto the forming
table, its grammage, or weight in grams per square meter, is not uniformly
distributed along the machine and cross sections. These
variations are known as grammage variations and are
responsible for moisture variations in the finished paper
product. Failure to control these properties results in
the paper being dried or coated unevenly and also may cause
paper to cockle(wrinkle). Uniform grammage in the
machine direction is partly due to factors such as stock
flow and consistency, headbox, and performance of the retention
aid system, among others.
- Uniform Paper Web Formation--During
paper web formation, the paper fibers aggregate into patches,
or flocs(See Fig. 1). In rare cases, these flocs
are so extremely localized that they actually leave holes
in the paper(called pinholes in the paper industry).
Observation of paper web formation(and the quality thereof),
can be achieved by holding paper up to a light source(this
is called observing the look-through). Paper with
flocs that are readily observable is called wild
or cloudy paper. Every paper grade has flocs, but
they are kept to a minimum and fiber distribution is kept
at an optimal uniform level. Uniform fiber distribution(and
keeping flocs at a minimal level) is very important for
all paper grades, as it is linked to several desirable paper
There are many ways that formation is quantified in the
- Smooth ink absorption in printing papers.
- Uniform surface coating for magazines, catalog,
and similar paper grades.
- Tear resistance for all paper grades. As a
chain is only as strong as its weakest link, so paper
is at its thinnest point. This property also facilitates
the manufacture of paper grades produced in fast paper
- As stated above, varying degrees of translucence can
be found by shining light through the paper, or variances
in grammage can be found by firing beta(ß) particles
or X-rays through the paper.
- Full sheet imaging, a new technology which provides
100% of a sheet to be seen at once, instead of the usual
1% that is normally seen with conventional equipment.
- Profilometry, where a sensitive pencil-like
tip is run over the paper's surface.
- Testing of tensile strength as compared to a handsheet(i.e.
handmade sheet) made from the same material.
is a property difference between the two sides of paper.
Sometimes this can be caused by a difference in textures
between the wire side and the top side of the paper. This
effect can be reduced by using twin-wire paper machines,
as they allow water drainage off both sides of the paper