Paper
Tests
Important
Parameters for Paper & Paperboard
Important
Parameters for Paper & Paperboard
All
tests are carried out at 23°C + 1°C and 50 + 2% relative
humidity which is a TAPPI standard.
Grammage
Weight per unit area expressed in g/m2. The standard
procedure is laid out in ISO 536, Tappi T 410.
Moisture
Content
The absolute moisture content, expressed as a
% of the paper/paperboard weight. The sample is generally
not conditioned while doing this test. The standard
procedures are laid out in Tappi T 412 and ISO 287.
Surface
Strength (Wax Pick)
A measure of the surface strength of the sample
or surface resistance to picking. Pick occurs due
to poor internal bonding strength, making it susceptible
to adherance to grade wax sticks (Dennison). This
test is valid only for uncoated board or paper. For
Coated stock IGT pick test is applicable.
Thickness
(Calliper)
The perpendicular distance between the two surface
of the board / paper, expressed in mm or µm, measured
with a micrometer. The standard procedures are explained
in Tappi T 411.
Water
Absorption (COBB)
The surface water absorption over 60 seconds, expressed
in g/m2, measured by Cobb Test. The procedural Standards
are explained in Tappi T 441.
Water
Absorption (WICK)
Water absorption at the edge, expressed in kg/m2,
using Wick Test. Board surface is sealed with waterproof
tape on both sides, weighed, placed in water @ 80°F
for 20 minutes and weighed again to measure the water
absorbed by wicking. It is an important test for measuring
the water absorption capacity of cupstock grade, which
is used for the manufacture of soft drink cups.
Bending
Resistance/ Stiffness
It is a measure of the resistance offered to a
bending force by a rectangular sample, expressed in
mN (milli newtons).
The instruments used are:
(i) Taber Stiffness tester
(ii) L & W tester
The standards are as per Tappi T 556.
Bursting
Strength
The maximum hydrostatic pressure required to rupture
the sample by constantly increasing the pressure applied
through a rubber diaphragm on 1.20 - inch diameter
sample. The standards are as per Tappi T 403
Compressibility
The reduction in thickness under compressive forces
or pressure. It influences the ability of paper to
change its surface contour and to conform to and make
contact with the printing plate or blanket during
printing impression. This is highly relevant in gravure
and letterpress printing. Compressibility is measured
as a ratio of roughness under to different standard
pressures in a Parker Print Surf tester
Elongation
The tensile strain developed in a test sample at maximum
tensile strength before rupture, measure as the %
increase in the length of the sample to the original
length.
Hardness
The degree to which paper will resist indentation
by some other material such as a stylus, pen or printing
plate. Hardness is measured with the help of bendtsen
smoothness tester with load on the measuring head.
Ply
Bond/ Scott Bond
The interlayer strength of the paperboard, measured
on Scott Bond Tester, expressed in J/m2. The standard
procedures are explained in Tappi T 403. In paper,
it is a measure of the internal strength of the sheet.
Resiliency
The ability of paper to recover its original thickness
and surface contour after release of the compressive
forces of printing nips.
Stiffness
(Taber)
A measure of flexural rigidity, Stiffness is the
bending moment (g-cm or mNm) required to deflect the
free and of a 1.5 in wide vertically clamped sample
15° from its center line when load is applied
50 mm away from the clamp; measured in MD & CD.
The procedural standards are explained in Tappi T
489 and ISO 2491.
Tearing
Resistance
Tearing resistance/ strengths is the ability of
the paper to withstand any tearing force when it is
subjected to. It is measure in both MD & CD, expressed
in mN (millinewtons). The procedural standards are
explained in Tappi T 414.
Tensile
Strength
The tensile force required to produce a rupture
in a strip of paperboard, measured in MD & CD, expressed
in kN/m. The procedural standards are explained in
Tappi T 404.
Brightness
It is the percentage of blue light reflected of
a sample measured at an effective wavelength of 457nm.
Measured
with to different standards - Tappi/GE and ISO. Though
there is no direct correlation, ISO brightness of
a sample is usually lower by 1-1.5 units over GE brightness.
The standards are as per Tappi T 452.
Colour
Related to perception and therefore measured or
specified in terms of colour space. A commonly used
system is the CIE L,a,b system. This is based on the
idea of colour opposites.
L
-
measure of illuminance
and varies from 100 for perfect white to 0 for perfect
black.
a
- redness to greenness.
b
- yellowness to
blueness.
Gloss
It is the specular reflection of light, which
is reflected at an equal and opposite angle. Normally
measured at 75° or 20°. Generally, gloss of unprinted
sheet/ board is measured at 75° (except for cast coated
papers). Printed and varnished surfaces are measured
at 60° angle. The standard procedures are laid out
in Tappi T 480.
Opacity
The property of a substrate to resist passage of light.
It is measured as the percentage of light absorbed
by a sheet of paper. Important in book printing where
both sides of paper are printed. The procedural standards
are explained in ISO 2471.
Whiteness
The extent that paper diffusely reflects light of
all wavelengths throughout the visible spectrum i.e
the magnitude & uniformity of spectral reflectance
measured as the percent light reflectance for the
whole wavelength range. The procedural standards are
explained in ISO 11475.
Printability
The extent to which properties of paper lends
them to the true reproduction of the original artwork.
This is influenced by the printing process and can
be evaluated in terms of - dot reproduction, dot gain,
print gloss, hue shift and print uniformity.
Print
Quality
The degree to which the appearance and other properties
of a print approach a desired result. Lot of parameters
in paper surface like roughness, gloss, ink absorption,
whiteness, brightness affect this.
