Properties
of Paper
Basis
Weight (GSM)
Brightness,
Whiteness and Colour
Bulk
Dimensional
Stability
Folding
Endurance (Double Folds)
Formation
Gloss
Machine
and Cross Direction
Moisture
Opacity
Porosity
Sizing
/ Cobb
Smoothness
Stiffness
Stretch
(Elongation)
Tearing
Resistance
Temperature
and Humidity: Conditioning of Paper
Thickness
Wax
Pick No. (Surface Strength)
Wire
side and Felt side
Basis
Weight (GSM)
The weight or substance per unit area is obviously
fundamental in paper and paper board products. The
Basis weight of paper is the weight per unit area.
This can be expressed as the weight in grams per square
metre (GSM or g/M2), pounds per 1000 sq. ft. or weight
in kgs or pounds per ream (500 sheets) of a specific
size. REAM WEIGHT is a common term to signify the
weight of a lot or batch of paper. Control of basis
weight is important as all other properties are affected.
Variations in moisture content in paper affects the
grammage.
Brightness, Whiteness and Colour
Brightness is defined as the percentage reflectance
of blue light only at a wavelength of 457 nm. Whiteness
refers to the extent that paper diffusely reflects
light of all wave lengths throughout the visible spectrum.
Whiteness is an appearance term. Colour is an aesthetic
value. Colour may appear different when viewed under
a different light source. Brightness is an arbitrarily
defined, but carefully standardised, blue reflectance
that is used throughout the pulp and paper industry
for the control of mill processes and in certain types
of research and development programs. Brightness is
not whiteness. However, the brightness values of the
pulps and pigments going into the paper provide an
excellent measure of the maximum whiteness that can
be achieved with proper tinting. The colour of paper,
like of other materials, depends in a complicated
way on the characteristics of the observer and a number
of physical factors such as the spectral energy distribution
of the illuminant, the geometry of illuminating and
viewing, the nature and extent of the surround and
the optical characteristics of the paper itself.
Bulk
Bulk is a term used to indicate volume or thickness
in relation to weight. It is the reciprocal of density
(weight per unit volume). It is calculated from caliper
and basis weight. Sheet bulk relates to all other
sheet properties. Decrease the bulk or in other words
increase the density, and the sheet gets smoother,
glossier, less opaque, darker, lower in strength etc.
Dimensional
Stability
An important consequence of the absorption and de-absorption
of moisture by paper is the change in dimension that
usually accompanies changes in moisture content. Such
changes in dimension may seriously affect register
in printing processes and interfere with the use of
such items as tabulating cards. Uneven dimensional
changes cause undesirable cockling and curling. Dimensional
changes in paper originate in the swelling and contraction
of the individual fibres. It has been observed that
cellulosic fibres swell in diameter from 15 to 20%
in passing from the dry condition to the fibre saturation
point. It is impossible to be precise about the degree
of this swelling because paper-making fibres differ
considerably in this property, and because the irregular
cross-section of fibres creates difficulty in defining
diameter. Change that occurs in the dimensions of
paper with variation in the moisture content is an
important consideration in the use of paper. All papers
expand with increased moisture content and contract
with decreased moisture content, but the rate and
extent of changes vary with different papers.
Folding
Endurance (Double Folds)
Folding endurance is the paper's capability of withstanding
multiple folds before it breaks. It is defined as
the number of double folds that a strip of 15 mm wide
and 100 mm length can withstand under a specified
load before it breaks. It is important for printing
grades where the paper is subjected to multiple folds
like in books, maps, or pamphlets. Fold test is also
important for carton, box boards, ammonia print paper,
and cover paper etc. Folding endurance is a requirement
in Bond, Ledger, Currency, Map, Blue Print and Record
Papers.
Formation
Formation is an indicator of how the fibres and fillers
are distributed in the sheet. Formation plays an important
role as most of the paper properties depend on it.
Paper that is poorly formed will have weak, thin spots
and thick spots. These will affect properties like
caliper, opacity, strength etc. Paper formation also
affects the coating capabilities and printing characteristics
of the paper.
Gloss
It is the specularly and diffusely reflected light
component measurement against a known standard. Gloss
is important for printing such things as magazine
advertisements. The level of gloss desired is very
dependent on the end use of the paper. Gloss and smoothness
are different properties and are not dependent on
each other.
Machine
and Cross Direction
Paper has a definite grain direction due to greater
orientation of fibres in the direction of travel of
the paper machine. This grain direction is known as
machine direction. The cross direction is the direction
of paper at right angles to the machine direction.
Some of the properties vary with the MD and CD and
hence the values are reported in both the directions.
While sheeting the paper, machine and cross direction
are to be kept in mind and the sheet cutting to be
done to suit the end use requirements. Examples: 1.
All printing papers are to be cut in long grain (The
biggest dimension in the grain direction). 2. Book
papers fold better and the book stays open better
if the sheets are out so that the machine direction
runs up and down the pages. 3. Wrap around labels
for metal cans and bottles are to be cut with the
machine direction vertical to obtain greater flexibility
about the can. Long grain and Short grain : The sheet
is in long grain if the larger dimension is parallel
to grain (MD) direction. The sheet is said to be in
short grain if the larger dimension is parallel to
cross direction (CD).
Moisture
Most physical properties of paper undergo change as
a result of variations in moisture content. Water
has the effect of plasticising the cellulose fibre
and of relaxing and weakening the interfibre bonding.
The electrical resistance and the dielectric constant
of paper both vary with moisture content. The absorption
and reflectance of certain bands of infrared and microwave
radiation by paper are affected by its moisture content.
The amount of water present in a sheet of paper is
usually expressed as a percent. The amount of water
plays an important role in calendaring, printing and
converting process. Moisture control is also significant
to the economic aspect of paper making. Poor moisture
control can adversely affect many paper properties.
Opacity
Opacity is the measure of how much light is kept away
from passing through a sheet. A perfectly opaque paper
is the one that is absolutely impervious to the passage
of all visible light. It is the ratio of diffused
reflectance and the reflectance of single sheet backed
by a black body. Opacity is important in Printing
Papers, Book Papers, etc.
Porosity
Because paper is composed of a randomly felted layer
of fibre, it follows that the structure has a varying
degree of porosity. Thus, the ability of fluids, both
liquid and gaseous, to penetrate the structure of
paper becomes a property that is both highly significant
to the use of paper. Paper is a highly porous material
and contains as such as 70% air. Porosity is a highly
critical factor in Printing Papers Laminating Paper,
Filter Paper, Cigarette Paper. Bag Paper, Antitarnish
Paper and Label Paper. Porosity is the measurement
of the total connecting air voids, both vertical and
horizontal, that exists in a sheet. Porosity of sheet
is an indication of absorptivity or the ability of
the sheets to accept ink or water. Porosity can also
be a factor in a vacuum feeding operation on a printing
press.
Sizing
/ Cobb
Because paper is composed of a randomly felted layer
of fibre, it's structure has a varying degree of porosity.
Thus, the ability of fluids, both liquid and gaseous,
to penetrate the structure of paper becomes a property
that is both highly significant to the use of paper.
The need to limit the spreading of ink resulted in
"sizing" the paper with gelatinous vegetable materials
which had the effect of sealing or filling the surface
pores. Later, the term "sizing" was applied to the
treatment of paper stock prior to the formation of
the sheet, with water-repellent materials such as
rosin or wax. Resistance towards the penetration of
aqueous solution / water is measured by Sizing or
Cobb values.
Smoothness
Smoothness is concerned with the surface contour of
paper. It is the flatness of the surface under testing
conditions which considers roughness, liveliness,
and compressibility. In most of the uses of paper,
the character of the surface is of great importance.
It is common to say that paper has a "smooth" or a
"rough" texture. The terms "finish" and "pattern"
are frequently used in describing the contour or appearance
of paper surfaces. Smoothness in important for writing,
where it affects the ease of travel of the pen over
the paper surface. Finish is important in bag paper
as it is related to the tendency of the bag to slide
when stacked. Smoothness of the paper will often determine
whether or not it can be successfully printed. Smoothness
also gives eye appeal as a rough paper is unattractive.
Stiffness
Stiffness is the measure of force required to bend
a paper through a specified angle. Stiffness is an
important property for box boards, corrugating medium
and to certain extent for printing papers also. A
limpy and flimsy paper can cause feeding and delivery
problems in larger sheet presses. A sheet that is
too stiff will cause problems in copier machines where
it must traverse over, under, and around feed rollers.
Bond papers also require certain stiffness to be flat
in typewriters etc.
Stretch
(Elongation)
Stretch is the amount of distortion which paper undergoes
under tensile stress. Stretchlelongation is usually
expressed, as percent stretch to rupture. Stretch
can be related to the paper's ability to conform and
maintain conformance to a particular contour, e.g.
Copier paper, multicolor offset printing papers, liquids
packing cartons base papers etc. It is an important
property in sack kraft papers which are used for cement
bags etc. Stretch is higher in cross direction than
machine direction.
Tearing
Resistance
Tearing resistance indicates the behaviour of paper
in various end use situations; such as evaluating
web runnability, controlling the quality of newsprint
and characterising the toughness of packaging papers
where the ability to absorb shocks is essential. fibre
length and interfibre bonding are both important factors
in tearing strength. The fact that longer fibres improve
tear strength is well recognised. The explanation
is straight forward; longer fibres tend to distribute
the stress over a greater area, over more fibres and
more bonds, while short fibres allow the stress to
be concentrated in a smaller area.
Temperature
and Humidity: Conditioning of Paper
Conditioning of paper is also of importance in many
printing and converting operations. In addition to
the effect of moisture content on physical properties,
it also determines the build up of static of the paper
sheet subjected to pressure and to friction. The tendency
for paper to develop static becomes greater with increasing
dryness. Cellulosic fibres are hygroscopic i.e. they
are capable of absorbing water from the surrounding
atmosphere. The amount of absorbed water depends on
the humidity and the temperature of the air in contact
with the paper. Hence, changes in temperature and
humidity, even slight changes, can often affect the
test results. So, it is necessary to maintain standard
conditions of humidity and temperature for conditioning.
Thickness
Thickness or Caliper of paper is measured with a micrometre
as the perpendicular distance between two circular,
plane, parallel surfaces under a pressure of 1 kg./
CM2. Caliper is a critical measurement
of uniformity. Variations in caliper, can affect several
basic properties including strength, optical and roll
quality. Thickness is important in filling cards,
printing papers, condenser paper, saturating papers
etc.
Wax
Pick No. (Surface Strength)
This indicates the surface strength of the paper.
This test is important for all uncoated printing papers.
Wire
side and Felt side
Also referred as wire side and top side. The side
which is in contact with the paper machine wire during
paper manufacture is called the wire side. The other
side is top side. Certain properties differ between
wire and felt side and it is customary to measure
these properties on both the sides. In case of paper
to be printed on one side only, best results are obtained
by printing on felt side. Postage stamps are printed
on wire side and then gummed on felt side, where the
smoothness is helpful for attaining an even application.
