This page contains various properties of paper, how these properties
are measured and how are they relevant to end user and/or papermaker. Under TAPPI
standard all tests are carried out at 23°C + 1°C and 50 + 2% relative humidity.
The basis weight, substance or grammage is obviously most fundamental property
of paper board. The Basis weight of paper is the weight per unit area. This can be expressed as the weight in grams per square
meter (GSM or g/M2), pounds per 1000 sq. ft. or weight in Kgs or pounds per ream (500 sheets) of a specific size.
Paper is sold by weight but the buyer is interested in area of
paper. The basis weight is what determines, how much area the buyer gets for a
given weight. e.g. if basis weight is 50 g/m2, for every 1 kg weight, the
buyer gets 20 m2. When the basis weight is expressed as ream weight, it tells
the buyers how many reams he/she getting for a given weight.
For papermaker basis weight is important from point of view of production
rate. For a given machine deckle and machine speed, the production rate per
day in MT will be = Machine Deckle (m) * Machine Speed (m/min) * Basis Weight
(g/M2) * 1440/1000000.
Papermaker always strive to get all desired properties of paper with minimum
possible basis weight.
All paper machines are designed to manufacture paper in a given basis weight
range. Tighter the range and more efficient will be the machine.
The standard procedure of measuring basis weight is laid out in TAPPI T 410, SCAN P6,
DIN53104 & ISO: BSENISO536
Typical Grammage Values
Grade
g/m2
Newsprint
40 - 50
Cigarette Tissue
22 - 25
Bond
60 -90
Paperboard
120 - 300
Accepted trade tolerance +/- 5%
Bulk
Bulk is another very important parameter of paper particularly for printers.
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.
Bulk (cubic centimeter/g) = Thickness (mm)* Basis Weight (g/m2) *
1000. Sheet bulk relates to
many other sheet properties. Decrease in bulk or in other words increase in
density makes the sheet smoother, glossier, less opaque, darker, lower in strength etc.
High bulk is desirable in absorbent papers while lower bulk is preferred for
printing papers particularly bible paper, dictionary paper etc.
Book Bulk: Book bulk is defined as the overall thickness in mm of a
given number of paper sheets. The bulking number is defined as number of
sheets required to bulk 25 mm or approximately 1". The standard procedure of measuring
book bulk is laid out in
TAPPI T 500, SCAN P7 DIN53105, ISO 534, BS: EN ISO20534
Typical Thickness Values
Grade
μm
Newsprint
60 - 80
Office/Business Paper
105 - 110
Blotting Paper (230g/m2)
540 - 590
Tracing Paper (90g/m2)
78
Label Paper (79g/m2)
63
Tissue(28g/m2)
125
Accepted trade tolerance +/- 10%
Caliper or Thickness
For a given basis weight, thickness determines how bulky or dense paper is. A
well beaten/refined pulp, short fiber pulp such as hard wood or straw pulp,
highly filled or loaded paper will show lower thickness for given basis weight. Thickness or Caliper of paper is measured with a
micrometer as the perpendicular distance between two circular, plane, parallel surfaces under a pressure of 1 kg./
CM2.
Uniform caliper is for good roll building and subsequent printing. 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.
The standard procedure for thickness measurement is explained in TAPPI T 411.
Curl
Paper curl can be defined as a systematic deviation of a sheet from a flat form. It results
from the release of stresses that are introduced into the sheet during
manufacture and subsequent use.
Paper curl has been a persistent quality issue
and is increasingly important for paper grades being subjected to high speed
printing, xerography and high precision converting processes.
There are three
basic types of curl, mechanical curl, structural curl and moisture curl.
Mechanical curl develops when one side of the paper is stretched beyond its
elastic limits. One example of this is the curl in the sheet which forms near
the centre of a roll. Structural curl is caused by two-sidedness in the sheet,
that is a difference in the level of fines, fillers, fiber area density or fiber
orientation through the sheet thickness. Moisture curl can develop when the
paper sheet is being offset printed. One side of the sheet may pick up more
moisture than the other, the higher moisture side releases the built in drying
strains and the paper will curl towards the drier.
For more details on Curl, please read
Curl
Basics by Chuck Green
The standard procedure for curl measurement are explained in TAPPI T 466 &
T520
Dimensional Stability
Cellulose fibers (main constituent of paper) swell in diameter from 15 to 20%
from dry condition to saturation point. Since most of the fiber in paper sheet
are aligned in the machine run direction, absorption and de-absorption of moisture by paper
causes the change in CD dimension. 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
fibers. It is impossible to be precise about the degree of this swelling because paper-making
fibers differ considerably in this property, and because the irregular cross-section of
fibers 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.
Dimensional stability of paper can be improved by avoiding fiber to absorb
moisture. Well sized papers have better dimensional stability.
Formation is an indicator of how uniformly the fibers and fillers are distributed in the sheet.
Formation plays an important role as most of the paper properties depend on it.
A paper is as strong as its weakest point. A poorly formed sheet will have more
weak and thin or thick spots. These will affect properties like caliper, opacity, strength etc.
Paper formation also affects the coating capabilities and printing characteristics of the paper.
There is no standard method or unit to express formation. It is a relative or
subjective evaluation.
Friction
Friction
is the resisting force that occurs between two paper or paperboard surfaces in
contact when the surfaces are brought to slide against each other. This
property is measured as a coefficient of friction, which is the ratio of the
frictional force, to a force acting perpendicular to the two surfaces.
Two components of friction can be measured, these being static and kinetic
friction. Static friction is the force resisting initial motion between the
surfaces and kinetic friction is the force resisting motion of the two
surfaces sliding against each other when already sliding at a constant
speed.
Measurement of the coefficient of friction has applications in packaging
where a high coefficient will indicate that containers such as sacks, bags
and paperboard containers will resist sliding in unit loads or on packaging
lines. This property is also important in printing papers, since a specific
coefficient of friction is needed so that individual sheets will slide over
each other, otherwise double press feeding may result.
There are two methods of measuring Co-efficient of friction of paper. One,
which uses Incline Plane is explained in TAPPI T 548 & T815, the second
method, which uses Horizontal Plane is explained in TAPPI T 549 & T816.
Typical Co-efficient of Friction Values Using
Horizontal Plane Method
Grade
Static Friction
Kinetic Friction
Office/Business Paper
0.50-0.65
0.35-0.5
Silk Coated Paper
0.45-0.55
0.30-0.45
Gloss Coated Paper
0.40-0.50
0.30-0.40
Machine and Cross Direction
In paper machine approach flow system, when stock passes through pressure
screen, the fibers are oriented lengthwise. If the stock velocity from headbox
slice is equal or less than wire speed, fibers which are already oriented
lengthwise, will align in the direction of wire run. Fiber alignment can be
altered to some extent if stock velocity is less than wire speed. So all papers
have a definite grain direction due to greater orientation of fibers in the direction of
paper machine run. 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.
The sheet which have all relevant properties same or almost same in both
direction are known as 'square sheet'.
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. E.g. 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).
There is no sure way to determine the MD or CD of a sheet but one crude
method which work is; cut a strip of about 1" wide and 2" long
paper and moist it. Put this moist sheet on a smooth surface or hand. As sheet
will dry it will curl. The direction of curl is CD as paper contract in CD more
than MD while drying.
Moisture
Almost all grade of paper has some percentage of moisture. Moisture in paper
varies from 2 - 12% depending on relative humidity, type of pulp used, degree
of refining and chemical used. Most physical properties of paper undergo change as a result of variations in moisture content. Water has the effect of
plasticizing the cellulose fiber and of relaxing and weakening the inter-fiber 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. Water comes free. Poor moisture control can adversely affect many paper properties.
The absolute moisture content is 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, SCAN P4
Typical Moisture Values
Grade
%
Newsprint
7.5 - 9.5
Office/Business Paper
4 -4.5
Marketing Wood Pulp
10
Printing Paper
6 -7
Tissue
2 - 7
Accepted trade tolerance +/- 10%
Smoothness
It is most important parameter for printer. Smoothness is concerned with the
surface contour of paper. It is the flatness of the surface under testing
conditions which considers roughness, levelness, 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 is 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.
Smoothness (Bekk Method): This test is an indirect measure of paper
smoothness when it is under moderate pressure( 100 kPa). The standards test procedure is described in TAPPI T
479.
Roughness (Sheffield Method): This test is an indirect measure of paper
smoothness or roughness. It is a measurement of air flow between the specimen
(backed by flat glass on the bottom side) and two pressurized, concentric
annular lands that are impressed in to the sample from top. The standards test procedure is described in TAPPI T
538.
Roughness (Print-surf Method): Very similar to Sheffield methods. The standards test procedure is described in TAPPI T
555.
Typical Smoothness Values
Grade
Parker Print Surf (μm)
Bendtsen (mls/min)
Newsprint (40 - 49g/m2)
2.6-4.5
80-140
Stationery (45-135g/m2)
0.8-2.6
50-300
Business Papers (80g/m2)
100-300
Test Liner (186 g/m2)
1750
Temperature and Humidity: Conditioning of Paper
As explained above it is important to control the moisture content of paper and
keep it stable during converting operation. To keep moisture content constant,
it is important that paper is conditioned. Conditioning of paper is also of
important 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.
Cellulose fibers 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.
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
manufacturing is called the wire side. The other side is top side. Before a thin
layer of fibers deposit on machine wire, fines and fillers drain out hence wire
side has less fines and fillers compared to top side. Certain properties such as
smoothness, texture and ink absorbency differ between wire and felt side and it is customary to measure these properties on both
sides. This difference of properties on two sides of paper is known as
two-sidedness. Highly filled or loaded or paper made from short fiber pulp will
show higher two-sidedness.
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.
Wire side and top side described above are in reference to single ply paper.
In case of multi-ply paper/board, every ply will have wire side and top side.
The top side of top most layer will be top side and wire side of bottom most
layer is wire side of multi-ply board. Different type of fibers, fillers and
chemicals are used in different layers for techno-economical reasons.
The standards
procedure is described in TAPPI T 455
Brightness may or may not add much value to the 'useful' properties of the paper
but it is the most important selling feature. It is a bragging right every paper
manufacturer want to have that he/she produces most bright paper.
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 arbitrarily defined, but carefully
standardized, 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.
Brightness is measured with two different standards - TAPPI/GE and ISO. Though there is
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 is related to perception and therefore measured or specified in terms of color space.
A commonly used system is the CIE L,a,b system. This is based on the idea of color opposites.
L - measure of luminance and varies from 100 for perfect white to 0 for perfect black.
a - redness to greenness.
b - yellowness to blueness.
Whiteness is the extent to which 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
for the measurement of whiteness are explained in ISO 11475.
Typical Brightness Values
Grade
% ISO
Newsprint
62-65
Fully Bleached Pulp
90
Office/Business Paper
80-95
Bond
70-92
Coated Paper
85-90
American Forest & Paper Association (AFPA)
Brightness Quality Levels
Level
% TAPPI
Premium
88.0 & above
No. 1
85.0 - 87.9
No. 2
83.0 - 84.9
No. 3
79.0 - 82.9
No. 4
73.0 - 78.9
No. 5
72.9 & Below
Color
The quality of light given off by a sheet as described by its hue (tint),
saturation (strength), and value (darkness or lightness). A whiter sheet
reflects equal amounts of red, green, and blue light - the entire visual
spectrum. While most balanced white sheets have a slightly yellowish cast,
most people will perceive a sheet with a slightly blue tint to be whiter.
Fluorescence
Fluorescence measures the
amount of fluorescent whitening agent present in the paper. Optical brightening
agent absorbs UV light and re-emits it as visible blue light. Under lighting
with a UV component this makes the paper appear more blue and brighter. All high
white grades have high levels of optical brightener. Less than 5 fluorescence
indicates very little optical brightener is present.
Gloss
It is the specularly and diffusely reflected light component measurement against a known standard.
Gloss is important for magazine advertisements printing . 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.
Gloss 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.
Typical Gloss Values
Grade
Gloss at 750
Uncoated Printing Paper
4-6
Matt Coated
10-30
Silk Coated
25-50
Art Coated
65-86
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. The opacity of paper
is influenced by thickness, amount and kind of filler, degree of bleaching and
coating etc.
Opacity 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 and TAPPI T425.
Bursting strength tells how much pressure paper can tolerate before rupture.
It is important for bag paper.
Bursting strength is measured as the maximum hydrostatic pressure required to rupture the sample by constantly increasing the
pressure applied through a rubber diaphragm on 1.20 - inch diameter (30.5 mm) sample. The standards
procedure is described in TAPPI T 403.
Bursting strength depends on basis weight of paper. To normalized the
bursting strength for various paper, bursting strength is reported
as
Burst Index = Bursting Strength (kPa)/ Grammage (g/m2)
or
Burst Factor = Bursting Strength ( g/cm2)/ Grammage
(g/m2) or
Burst Ratio = Bursting Strength ( lb/inch2)/ Basis
Weight (lbs/ream)
Typical Bursting Strength Values
Grade
KPa
Coated Paper (130 g/m2)
200-300
Coated Paper (250 g/m2)
300-650
Bond Office/Business Paper (100 g/m2)
250-300
Carbonless Paper (50-60 g/m2)
150-200
Bleached Kraft (60 g/m2)
210-260
Test Liner (186 g/m2)
250-475
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
two different standard pressures in a Parker Print Surf tester.
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. Folding endurance has been useful in
measuring the deterioration of paper upon aging. 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. High folding endurance is a requirement in Bond, Ledger, Currency, Map,
Blue Print and Record Papers. Currency paper has highest folding endurance
(>2000). Long and flexible fibers provide high folding endurance.
The procedural standards for measuring Folding Endurance using MIT tester are explained in TAPPI T
511.
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 Internal
Bond Strength of paper or paperboard (also known as Ply Bond Strength or Z
Directional Strength) is the ability of the product to resist splitting when a
tensile load is applied through the paper’s thickness i.e. in the Z direction of
the sheet.
The internal
bond strength is often determined on high tack coated Fine papers, offset papers
and for multiply papers (e.g. top liner of carton board or abrasive paper used
to form belts in grinding machines). One particular application is determining
the ply bond strength of “Peelable”
The interlayer strength of the paperboard, measured on Scott Bond Tester, expressed in J/m2. The
standard procedures are explained in TAPPI T 403 & T833 & SCAN P80. In paper, it is a measure of the internal strength of the sheet.
Typical Scott Bond Values
Grade
J/M2
Cover Paper
125-230
Offset Paper
240-290
Xerographic Paper
220-400
Coated Cover Paper
200-315
Coated Text
240-365
Resiliency
The ability of paper to recover its original thickness and surface contour after release of the compressive forces of printing nips.
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.
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.
Droop
Rigidity CD: Droop rigidity measures the stiffness of
the paper or board, more often applied to lighter weight grades.
CD refers to cross direction, and MD to machine direction, Droop
rigidity is higher in the machine direction. The higher the
value the stiffer the paper.
Droop Rigidity MD: Droop rigidity measures the
stiffness of the paper or board, more often applied to lighter
weight grades. CD refers to cross direction, and MD to machine
direction, Droop rigidity is higher in the machine direction.
The higher the value the stiffer the paper.
Bending Resistance/ Stiffness (Lorentzen & Wettre): It is a measure of the resistance offered to a bending force by a rectangular sample, expressed in mN
(milli Newtons). The standards are as per Tappi T 556.
Typical Bending Values
Grade
Bending Moment
Stiffness (mNm)
Resonance Length
Stiffness (mNm)
MD
CD
MD
CD
Coated Paper (135 g/m2)
65
45
1043
721
Office/Business Paper (80 g/m2)
39
17
493
160
Carbonless Paper (46 g/m2)
7.5
3.3
76
34
Stretch (Elongation)
Stretch is the amount of distortion which paper undergoes under tensile stress.
Stretch elongation 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.
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. The procedural standards are explained in TAPPI
T 494
Surface Strength (Wax Pick No.)
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
adherence to grade wax sticks (Dennison). This test is valid only for uncoated board or paper. For Coated stock IGT pick test is applicable.
Acceptable pick level for uncoated papers =>Wax #6
Acceptable pick level for Gloss papers =>Wax #11
IGT is a measurement of the surface strength of the paper. A tacky ink is applied to
sample of the paper at an increasing speed. As the speed increases the peeling
force applied to the paper also increases and the speed at which the fibers
begin to be pulled from the sheet is recorded as the IGT. A high IGT (>300)
indicates a strong surface strength suitable for demanding offset applications.
Tearing Resistance
Tearing resistance indicates the behavior of paper in various end use situations; such as evaluating web
runnability, controlling the quality of newsprint and characterizing the toughness of packaging papers where the ability to absorb shocks is essential.
fiber length and inter-fiber bonding are both important factors in tearing strength. The fact that longer
fibers improve tear strength is well recognized. The explanation is straight forward; longer
fibers tend to distribute the stress over a more fibers and more bonds, while short
fibers allow the stress to be concentrated in a smaller area.
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
(mili Newton).
The procedural standards are explained in TAPPI T 414, ISO 1974 & SCAN P11
Typical Tear Resistance Values
Grade
Elmendorf Tear mN
Coating Base (80g/m2)
500 - 700
Bond (100g/m2)
700
Office/Business Paper (80g/m2)
500 - 600
Test Liner (186g/m2)
1800
Tensile Strength
The tensile force required to produce a rupture in a strip of paperboard, measured in MD & CD, expressed in
kN/m. Tensile strength is indicative of fiber strength, fiber bonding
and fiber length. Tensile strength can be used as a potential indicator of
resistance to web breaking during printing or converting. The procedural standards are explained in TAPPI
T 494.
Tensile Energy Absorption (TEA):
TEA is the Tensile Energy
Absorption, i.e. the amount of work required to break the sheet under tension.
Z Direction Tensile Strength: Or internal bond strength provides an
indication of strength of board in relation to glue bonding at carton side
seams and possible Delamination on scoring, or use of high tack coating. The procedural standards are explained in TAPPI T
541.
Typical Tensile Index Values
Grade
MD (Nm/g)
CD (Nm/g)
Newsprint (40 - 49g/m2)
45 -60
-
Stationery (50-100 g/m2)
40-70
20-40
Tracing Paper (60-110 g/m2)
70
40
Test Liner (186 g/m2)
175
80
Wet Strength
Some grades of paper such as tea bag paper, coffee filter paper etc. come in
contact with water in use. So these paper have to be strong enough to
withstand tear, rupture or falling apart when saturated with water. To impart
wet strength, paper are treated chemically.
The residue left after complete combustion of paper at high temperature. It
is generally expressed as percent of original test sample and represents
filler content in the paper. As it is ash content is not important property of
paper but in some grade of papers such as filter papers are ash free and other
such as cigarette tissue have certain level of filler to control cigarette
burning rate.
The ash content measurement procedural standards are explained in TAPPI T 413,
SCAN P5, ISO 1762.
Typical Ash Content Values
Grade
%
Market Wood Pulp
0.3 - 0.5
Newsprint
0 - 12
LWC
30 - 50
Fine Paper
0 - 35
Dirt Content
The paper
may have number of dirt specks or contraries. These specks can be any unwanted
foreign particle that is visible to the eye such as bark, undigested wood (shives),
pitch, rust, plastic, slime etc. For pulp, paper and board the number or area
covered by such specks on both surfaces and sometimes in the body of the
material, can be estimated in either reflected or transmitted light.
The number of specks of each area are expressed either as mm2/Kg for
pulp or mm2/m2 for paper
The pH value of paper can show residual acidic/alkaline chemicals in pulp,
or atmospheric pollutants (e.g. SO2) in
valuable paper archives.
The pH value of paper
can be determined by:
- Disintegrating the paper in hot
distilled water and determining the pH of the extract.
- Disintegrating the paper in cold
distilled water and determining the pH of the extract.
- Directly using a wet electrode on
the paper surface.
These 3 methods measure different
solutions and so give different
Permanence
Permanence is degree to which paper resists deterioration over time.
Permanent paper can resist large chemical and physical
changes over and extended time (several hundred years). These paper are generally acid-free with alkaline reserve
and a reasonably high initial strength. Paper containing pure cellulose
fiber are more permanent. Permanency is desirable in currency, bond and record
papers.
Pin Holes
Imperfections in paper which appear as minute holes upon
looking through the sheet. They originate from foreign
particles, which are pressed through the sheet. Absence of pin hole in
electrical grade papers is very important.
Because paper is composed of a randomly felted layer of fiber, 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 much as 70% air. Porosity is a highly critical factor in
Printing Papers Laminating Paper, Filter Paper, Cigarette Paper. Bag Paper,
Anti-tarnish 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.
Air Resistance (Gurely Method): It is the resistance to the passage
of air, offered by the paper structure, when a pressure difference exist
between two sides of paper. It is measured as the time for a given volume of
air to flow through a specimen under specified conditions. Air resistance is
indirect indicator of degree of beating, compaction of fibers and type and
amount of fillers.
The Gurely Method is explained in TAPPI T 460 and TAPPI T 536 for low and high
air resistance respectively.
Air Resistance ( Sheffield Method): is explained in TAPPI T 547
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.
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.
Sizing / Cobb
Because paper is composed of a randomly felted layer of fiber, 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.
The surface water absorption over 60 seconds, expressed in g/m2, measured by Cobb Test. The procedural Standards are explained in
TAPPI T 441.
Typical Cobb Values
Grade
g/m2
Bond
24-30
Office/Business Paper
22-26
Test Liner (186 g/m2)
100
Unsized
50+
Carbonless Base
18-22
Water Absorption (EDGE 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.
The combined tensile strength and stretch of a material as measured by the
ability of the material to resist rupture when pressure is applied under
specified conditions to one of its sides by an instrument used for testing the
property. Testing for the bursting strength of paper is a very common procedure,
although its value in determining the potential permanence or durability of
paper is suspect.
Corrugated Boards - Ring Crushed Test
Ring Crush is a traditional test of linerboard and corrugating medium strength.
Ring crush measures compression resistance, and this compression strength is
considered to relate to the eventual compression strength of combined board made
from the component. Linerboard called high strength or high performance
linerboard is board that is able to achieve a specified minimum ring crush at
basis weights that are lower than traditional basis weights.
Corrugated Boards - Concora Crush Test
The Concora Crush Tester performs a series of tests to determine the rigidity
and crush resistance of corrugated material. It is used in conjunction with the
Concora Liner Tester. The first test measures the flat crushing resistance of a
laboratory-fluted corrugated material. The second test determines the edgewise
strength, parallel to the flutes, of a short column of single-, double-, or
triple-wall corrugated board. The third test evaluates the ability of corrugated
material to contribute to the compression strength of a corrugated box by
measuring the edgewise compression strength of a laboratory-fluted strip of
corrugated material in a direction parallel to the fluted tips.
Corrugated Boards - Flat Crush Strength
The flat crush test is a measure of the resistance of the flutes in corrugated board
to a crushing force applied to the surface of the board under prescribed
conditions. Flat crush is a measure of the flute rigidity of corrugated board. A
high flat crush value indicates a combination of good flute formation and at
least adequate strength medium. Low flat crush may indicate a number of
ctionluding low strength medium, leaning flutes and crushed flutes.
