PROPERTIES OF PAPER

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 230C ± 10C and 50 + 2% relative humidity.

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Physical Properties

Basis Weight or Grammage
 
The basis weight, substance or grammage is obviously most fundamental property of paper and paperboard. 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, more efficient will be the machine operation. 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 and Density
 
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)* 1000/ Basis Weight (g/m2). 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.
 
To view density of various grades of paper go to www.paperonweb.com/density.htm

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 good 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.

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%
 
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 side.
 
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.
 

For more details on Dimensional Stability, please read Dimensional Stability Notes by Chuck Green

Typical Values
Grade MD (%) CD (%)
Carbonless Paper 0.050-0.150 0.200-0.400
Bond Paper 0.100-0.200 0.200-0.400
Coated Art Paper (under 200 g/m2) 0.090-0.150 0.150-0.350
Gasket Paper 0.400-1.000 0.500-1.100
Formation
 
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. A poorly formed sheet will exhibit more dot gain and a mottled appearance when printed
 
There is no standard method or unit to express formation. It is a relative or subjective evaluation. However when holding paper up to a light source, a well formed sheet appears uniform while a poorly formed sheet has clumps of fibers giving a cloudy look.
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  T815, the second method, which uses Horizontal Plane is withdrawn.

 

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 has 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 cut 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.
 
All strength properties are sensitive to moisture – about 1% change in a sample’s moisture content changes the compression strength with an average of 8%.
 
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 topmost layer will be top side and wire side of bottommost 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

Optical Properties

Brightness, Whiteness and Color
 
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 brightest 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 750 or 200. Generally, gloss of unprinted sheet/ board is measured at 750 (except for cast coated papers). Printed and varnished surfaces are measured at 600 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.

A paper with a relatively high opacity at 96% and above will have almost no show- through from printing on the reverse side or the sheet below. Selecting a paper with high opacity is specially important if the printing includes solid block of colors, bold type and heavy coverage.

Typical Diffuse Opacity Values
Grade Diffuse Opacity %
Newsprint (40-49 g/m2) 90-94
Stationery (50-100 g/m2) >88
Tracing Paper (60-110 g/m2) 25-40



Strength Properties

Bursting Strength
 
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
 
It is reduction in paper 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)
 
Double Fold 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 is log of double fold at base 10 (Folding Endurance = Log 10 (Double Folds)). 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, Blueprint and Record Papers. Currency paper has highest folding endurance (>10,000). 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 & T569 & 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. A paper’s resiliency depends on a number of factors, especially related to the papermaking process, like quality of fiber refining, calendering, supercalendering etc., besides other parameters like density, moisture etc. The ’ Printing Cushion’ of a paper is governed or defined by the combination of its degree of resiliency, compressibility, and hardness/softness. Resiliency is a key point of consideration in ‘ letterpress’ & ‘gravure printing’.
 
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 lumpy 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 150 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 (Strength)
 
Tearing resistance indicates the behaviour 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 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).

 
Tear Factor: TAPPI standards require that the tear factor be expressed in units of dm2 . If the tearing strength = a gf and that the basis weight = b gf/m2.  The tear factor is then
 

Tear Factor = (tearing strength)/(basis weight) = 100*a/b dm2

 
Tear Index:, like tear factor, is defined as tearing strength divided by basis weight.  The tear index, however, must be expressed in units of mN/(gf/m2).  As before, suppose that tearing strength = a gf and basis weight = b gf/m2.  The tear index is then given by


Tear Index   =   (tearing strength)/(basis weight) = 9.81*a/b mN/(g/m2)

 
 
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 paper or 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 Properties of some paper grades
Grade Tensile Strength (kN/m) Breaking  Length (KM) Stretch (%) TEA (kJ/m2)
  MD CD MD CD MD CD MD CD
Offset (107 g/m2) 5.6 3.2 5.3 3.1 2.5 4.1 14.9 15.8
Bond (75 g/m2) 3.6 2.6 4.9 3.5 1.8 4.7 6.3 13.2
Newsprint (50 g/m2) 1.8 0.9 3.7 1.8 1.1 1.4 1.8 13
 
 
Breaking Length: It is the length of paper strip, if suspended vertically from one end, would break by its own weight. Breaking length is generally used in the paper trade to characterize the inherent strength of paper.  It affords an excellent basis for comparing the strength of papers made from different furnishes and having different basis weight.

                                Breaking Length (km) = 102*T/R or 3.658*T1/R1

                                T = Tensile Strength, kN/m
                                T1= Tensile Strength, lb/inch
                                R = Basis Weight, g/m2
                                R1= Basis Weight, lb/1000 ft2

Tensile Energy Absorption (TEA): TEA is the Tensile Energy Absorption, i.e. the amount of work required to break the sheet under tension.

Tensile Index (TI): It is the tensile strength in N/m divided by basis weight in g/m2 Grammage. TI is expressed in Nm/g

                                Tensile Index (Nm/g) = 1000*T/R or 36.87*T1/R1

                                T = Tensile Strength, kN/m
                                T1= Tensile Strength, lb/inch
                                R = Basis Weight, g/m2
                                R1= Basis Weight, lb/1000 ft2

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

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.

Zero SpanTensile Strength (Paper): This provide an idea of tensile strength of fiber and not the strength of fiber bonding. The gauge length of the tensile strength measuring instrument is set to zero so the failure is by fiber rapture only. It is measured with a strip of paper but mainly a pulp property.

 

Comparison of Individual Fiber Strength, Zero Span Tensile strength and Tensile Strength of Handsheets.
Fiber Type Average Fiber Length (mm) Tensile Strength of Handsheet (MPa) Zero-Span Tensile Strength (MPa) Fiber Strength (MPa)

 

Ratio of Zero-Span to Fiber strength
Douglas Fir (Unbeaten) 3.7 78 257 627 0.410
Southern Pine (Beaten) 3.3 83 207 590 0.351
Sweetgum (Beaten) 1.7 46 227 526 0.432
Sweetgum (Unbeaten) 1.7 8 217 665 0326
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.

 

Typical Wet Tensile Strength Values
Grade Dry Tensile Strength (g) Wet Tensile Strength (g)
Kitchen Towel  (20g/m2), 2 ply 650 200
Facial Tissue (13g/m2) 2 ply 115 35

Miscellaneous Properties

Ash Content
 
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

 

Typical Dirt Content Values
Grade %
Bleached Market Wood Pulp < 7 mm2/Kg
Newsprint from deinked pulp 100-300 ppm
Fine Paper from deinked pulp < 10 ppm
 
Electric Properties
 
Paper is used for cable wrapping, dielectric media in capacitors and other electric application. The following electric properties are important for paper to be used in electrical applications.
 
Conductance: Electrical conductivity is the ability of a material to carry the flow of an electric current (a flow of electrons). Paper is classified as bad conductor or insulator.
 
Dielectric Constant or Relative Permittivity  is the ratio of the permittivity of a substance to that of free space or vacuum. The dielectric constant of paper varies from 2 to 5.
 
Dielectric Strength:of an insulating material is the maximum electric field strength that it can withstand intrinsically without breaking down. Dielectric strength of Waxed paper is 40-60 Million Volt/m or 40-60 KV/mm. For a paper of 0.1mm thick, the dielectric strength will be 4-6KV.
pH
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 pH values.

 
Permanence & Durability
 
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.
 
The permanence and durability are two distinguish properties of paper. Permanence is basically a measure of the chemical stability of paper. Durability, on the other hand, is primarily a function of the performance of paper; it is a measure of the stability of its physical and mechanical properties . A paper that is given rough treatment over a short time should be durable, but little concern need be given to permanence. A paper that is meant to last for a century must be compounded for chemical stability. Such chemical stability can be enhanced by storing paper at a low temperature; at a constant, low relative humidity; in the dark; and in an atmosphere free of pollutants . But if the paper is also to be used--read, handled, folded, etc.--it's mechanical durability is also important . Obviously, permanence and durability are not independent of each other, but rather intimately related. This relationship is so intimate, in fact, that durability or the stability of the strength and mechanical properties of paper has become the most useful indicator of its permanence or chemical stability .
 
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.
 
Porosity
 
Since 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 Paper, 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
 
Typical Porosity Values
Grade Gurley Air Resistance (sec) Bendtsen (mls/min)
Uncoated Paper   500-1500
Coated Paper   0-10
Test Liner (186 g/m2)   25
Gasket   1-5
Blotting Paper 1 -2  
 
For more on Porosity, please visit http://www4.ncsu.edu/%7Ehubbe/TShoot/G_Poros.htm

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.
 
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
Thermal Properties of Paper
 
Thermal Conductivity of Paper:

Thermal conductivity is a measure of how easily heat passes through a particular type of material. Thermal conductivity is measured in watts per meter Celsius. Because the conductivity of materials can vary with temperature, no one single value exists for the conductivity of paper. However, under standard temperature and pressure of 25 degrees Celsius and 1 atmosphere, the thermal conductivity of paper is 0.05 watts per meter Celsius or 0.03 BTU ft/hour. Sq. ft 0F.

Specific Heat Capacity of Paper:  The specific heat capacity of a material is a measure of the amount of energy required to raise the temperature of a specific quantity of that material by a specific amount. The units of specific heat capacity are Kilojoules per Kilogram Celsius. The specific heat capacity of paper is 1.4 Kilojoules per Kilogram Celsius or 0.33 Btu/lb/0F.

 
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 @ 80oF 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.

CORRUGATED BOARD

Corrugated Boards - Bursting Strength
 
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.
Test
 
TAPPI
 
*Related ISO Method *Related ASTM
Abrasion, Taber
T 476
   
Absorption, Castor Oil
T 462   D 780
Absorption, Water Drop
T 432
  D 824
Alkaline Reserve T 553   D 4988
Ash  T 211 T 413, T 244  2144  D 586
Basis Weight (Grammage)
 T 410
 536  D 646
Brightness of Clay Fillers or Slurry
T 646
   
Brightness, Diffused T 525 2469, 3688  
Brightness Paper, Pulp, Filler or Slurry, ISO
T 571 (Withdrawn) 2469, 2470  
Brightness, TAPPI T 452 2469, 2470 D 985
Bulk or Density T 220 (Physical Testing of Pulp Hand sheets) 5270  
Burst, Mullen T 403 T 807  2758 D 774
Caliper  T 411  534, 3034  D 645
Cobb Size T 441 535 D 3285
Coefficient of Friction, Horizontal T 549 15359 D 4917
Coefficient of Friction, Slide Angle T815    
Color, Hunter, L,a,b or CIE L*,a*,b*, ISO T 527    
Color, Hunter, L,a,b or CIE L*,a*,b*, TAPPI T 524    
Dirt Count T 563   D 6101
Edge Crush, ECT T 811 3037  
Fiber analysis of paper and paperboard T 401  9184 (5 parts)  D1030
Flat Crush T 808 3035  
Fluorescent Component, TAPPI T 452 2469, 2470 D 985
Fold, MIT Fold, MIT  T 511 5626 D 2176
Fold, Schopper T 423 5626 D 643
Gloss, 200  T 653   D 523
Gloss, 750  T 480 5284 -1 D 1223
Handsheet Testing T 220 (Physical Testing of Pulp Hand sheets) 5270  
Hercules Size, 1% acid/ 80% reflectance T 530    
Hercules Size, 10% acid/ 85% reflectance T 530    
Ink Rub Resistance, Sutherland  T 830    
Machine Direction of Paper T 409    D 528
Moisture Content  T 412  287  D 644
Opacity, ISO T 519 2471  
Opacity, TAPPI or Printing T425 2471 D 589
pH Hot Extract T 435 6588 D 778
pH Cold Extract T 509 6588 D 778
pH Surface T 529    
Peel, Thermoplastic coating adhesion T 540    
Permeability (Air Perm) T 251    
Petroleum Wax in Impregnated Paper T 405    D 590
Porosity, Gurley T 460 5636-5 D 726
Ring Crush T 818 12192 D 1164
Rosin in Paper T408    
Roughness, Parker Print-Surf T 555 8791/4  
Roughness, Sheffield T 538 8791/3  
Scott Bond  T569    
STFI Short Span Compressive Strength T 826 9895  
Starch in Paper T 419   D 591
Stiffness, Gurley T 543 5628 D 6125
Stiffness, Taber T 489 or T 566 2493 D5342, D5650
Tear Resistance, Elmendorf   T 414 1974 D 689
Tensile Breaking Strength & Elongation T 404 1924/1  
Tensile  T 494, T 456 1924/2 D 828, D829
Water Vapor Transmission Rate T 448 2528  E 96, E398
Wax Pick T 459   D 2482
Water vapor transmission rate T 448 2528 E 96, E398
Wire side identification T 455   D 5039
Z-Direction Tensile T 541    

* “Related” does not imply “equivalence.” A “Related Standard” may be a standard for a similar property, but this should not assume identical technical content or matching results.

Go to http://www.tappi.org/content/pdf/standards/subject_index_tms.pdf for a complete list of TAPPI testing standards.

Reporting Units for Physical Testing Procedures for Paper Testing

Testing procedure1 Standard Present reporting unit2 Suggested SI metric reporting unit Conversion factors

ASTM TAPPI
Air resistance (Gurley) D726 T460 sec/100cm3 sec/100cm3  
Basis weight (grammage) D646 T410 lb/480 or 500-sheet ream g/m2 ream weight (il/500 sheets4) X 1,406
----------------------------------------------------- =
sheet area in square inches
Bulking thickness D527 T426 in. mm or µ m 1 point = 0.001 in. = 0.0254mm = 25µ m
Bursting strength D774   lb/in2 (psi) kPa, kgf/cm2 1 psi = 6.895 kPa
Caliper (thickness) D645 T411 thousandths of inch (mil) mm or µ m 1 point = 0.001 in. = 0.0254mm = 25µ m
Ply adhesion D825   gf/cm width mN/cm width gf/cm = 9.8066 mN/cm
Standard paper conditioning D685 T402      
relative humidity
Temperature
    50 ±2%
23 ±2°C
   
Stiffness   T451 mgf
Tber unit
mN
mN
1 mfg = 9.8066 x 10-3 mN
1 Taber unit = 2.03 mN
Gurley
Taber
  T451 mgf
Tber unit
mN
mN
1 mfg = 9.8066 x 10-3 mN
1 Taber unit = 2.03 mN
Tear          
Edge tearing strength
Internal tearing resistance
D827
D689
T470
T414
kgf
gf/number plies
N
mN
1 kgf = 9.8066 N
1 gf = 9.8066 mN
Tensile breaking strength: Paper and paperboard D828 T404 il/0.5 in. width or
kgf/15mm width
newton/meter width,
N/m width
kgf/15mm = 0.65378 kN/m
ibf/0.5 in. = 0.53574 kgf/15mm
Tensile energy absorption   T494 kgf * m/m2 J/m2 kgf * m/m2 = 9.8066 J/m2
Water absorption test (Cobb)   T441 g/m2 same or kg/m2  
Bulk     cm3/g same or m3/kg  
Burst factor (index)     gf/cm2
g/m2
kPa * m2/g 1 gf/cm2 = 0.098066 kPa *m2/9
   g/m2
Density     g/cm3 g/cm3  
Tear factor (index)     100 gf * m2/g mN * m2/g 100gf * m2 = 9.8066 X 102 mN * m2/g
        g

1Many physical testing units will not be changed by the conversion to metric or SI units.  Thus, such tests as brightness (%), compressibility (%), Standard Freeness (ml), opacity (%), and stretch (%) will continue to be reported in the dame units.
2Most of the suggested reporting units in SI metric terms are based on recommendations from the British Paper Makers' Association.
3Some properties will continue to be reported in present units.  Thus, such procedures as the Bendtsen air resistance and smoothness tests and the Sheffield smoothness test will continue to be reported in ml/min.
4If the given trade size is in 480 sheets, then factor shall be multiplied by 48/50 (or 0.960).

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