General Terms & Information about Paper
The Basis weight of paper is the weight per unit area. This can be expressed as the weight in grams per square metre (Gsm or g/m2) or pounds per 1000 squares ft.
TEMPERATURE & HUMIDITY : CONDITIONING OF PAPER
Paper is hygroscopic in nature 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.
Resistance towards the penetration of aqueous solution/water is measured by sizing or cobb values.
The cobb size tester measures the weight, in grams, of water absorbed in a given length of time, normally one minute (GSM/minute).
Formation is an indicator of how the fibers and fillers were distributed in the sheet.
Bulk is the volume of the paper per unit weight and is expressed as cc/gm. It is the reciprocal of density (weight per unit volume). It is calculated from caliper and basis weight.
Bulk = caliper in microns/Basis weight in GSM.
Stiffness is the measure of force required to bend a paper through a specified angle.
FOLDING ENDURANCE ( DOUBLE FOLDS)
Folding endurance is the paper’s capability of withstanding multiple folds before it breaks.
Fold test is also used as an indicator for aging properties of paper as it is a sensitive measure of influence tending to degrade fiber.
It is the amount of work done in tearing the paper through a fixed distance after an initial out. Tearing resistance is more in cross direction than machine director.
BURST STRENGTH/BURST FACTOR/BURST INDEX
The Bursting strength is the resistance to puncture by a blunt object. The results are reported in pounds per square inch or kgs. Per square centimeter or kilo pascals required to take the sample to rupture. Bursting strength indicates a composite of strength and toughness. Hence it is useful in testing for bag, wrapping papers and box boards where the paper is subjected to a stress similar to that exerted in the bursting tests.
Burst factor/burst Index are the terms used to indicate the strength of the paper irrespective of the basis weight of the sheet i.e. ratio of bursting strength in gms/cm2 or K. Pa to GSM.
Burst Index : Bursting Strength in K.Pa/ GSM.
TEARING STRENGTH/TEAR FACTOR/TEAR INDEX
Tear is the determination of the average gram force/milli Newton required to tear a single sheet of paper through a specified distance after an initial tear has been introduced.
TENSILE STRENGTH/BREAKING LENGTH/TENSILE INDEX
Tensile strength is a measure of the force per unit width required to break a specimen in either the machine or cross machine direction.
It is the strength (load) in Kgs. that a paper strip of 15mm in width and 180mm in length can withstand before it breaks. Tensile strength is more in MD than CD.
The crust test is used to determine the compression strength of a material. The crush test also detects poor adhesive penetration, spotty adhesion and carton board with low board strength.
The addition of material either to the stock (internal sizing) or to the surface or to the surface of a paper or board (surface sizing), generally in order to increase its surface strength and its resistance to the penetration and spreading of aqueous liquids, for example writing ink.
Operation carried out by means of a calendar on the at least partially dried paper or board, with the aim of improving the finish.
The Overall width of the wet web as it leaves the forming zone. It indicates total width of paper coming out of paper machine.
TWO-LAYER PAPER OR BOARD
Paper or board consisting of two furnish layers combined together during manufacture, while still moist, without the use of adhesive.
MACHINE-FINISHED (M.F.) PAPER OR BOARD
Paper or board treated mechanically on the making machine to improve the smoothness and uniformity of appearance on both sides.
MACHINE-GLAZED (M.G.) PAPER OR BOARD
Paper or board which has had one side made smooth and glossy by drying in contact with a heated, polished metal cylinder which forms part of the drying section of the machine. The other ide of the paper or board remains relatively rough.
SUM USEFUL CALCULATION
1. REAM WT (kg) = Length (inches) x Breadth (inches) x Gram weight (gsm) / 3100
2. To calculate gram weight (GSM)
GRAM WEIGHT (gsm) = Weight (Kgs) x 3100 / Length (inches) x Breadth (inches)
3. LENGTH OF PAPER (metres) = Weight of Reel (Kg) x 100000 / Reel Width (cm) x gsm
4. SUBSTANCE (gsm) = Weight of Reel (Kg) x 20000 / Length (cms) x Width (cms)
5. NO OF SHEETS = Weight of Reel (Kg) x 10000000 / Substance (gsm) x Width (cm) x Length (cm)
6. BURST FACTOR = Bursting strength in kg/cm2 x 1000 / Gram Weight (gsm)
Paper has a definite grain direction due to greater orientation of fibers in the direction of travel of the paper machine. This grain direction is known as machine direction. The cross direction is the direction of paper at right angles to the machine direction.
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).
DETERMINATION OF GRAIN DIRECTION OF THE SHEET
A: Short grain (SG): 76 x 102 cm
B: Long grain (LG): 51 x 76 cm
They are usually referred to as the "inner" liner which faces the package contents and the "outer" liner which is exposed to the surrounding environment with the fluting sandwiched in between.
(A) KRAFT LINER
Kraft liner is made with mainly virgin fibre. The percentage of virgin to recycled material will vary between paper manufacturers, but generally, one would consider a Kraft liner to be one with at least 55% virgin material.
This paper will consist of mainly long fibres. The long fibres will give the board rigidity and strength, while the shorter fibre will give the paper a smoother surface for printability.
(B) TEST LINER
Test Liner is usually made entirely of recycled fibre and is usually weaker than Kraft Liner.
Recycled paper, however, is significantly cheaper than Kraft Liner – making it a cost effective choice for most corrugated cartons.
(C) MULTI - LAYERED LINERS
Recent developments in papermaking technology have given manufacturers the ability to form the paper in layers, each with it's own characteristics of strength, moisture resistance, smoothness and colour.
(D) WHITE LINER
White liner is a multi layered paper that has a white outer facing made of bleached paper fibres. The base paper onto which the white layer is laid may be either Kraft or Test liner leading to their naming as White Top Test Liner (WTTL) and White Top Kraft Liner (WTKL).
(A) MEDIUM (FLUTING) PAPERS
The fluting (or medium) in corrugated board is the wavy paper between the liners. Its “wavy” shape provides high compression resistance and strength characteristics to the corrugated board – making it the secret ingredient in the strength corrugated board.
While liners generally have higher strength and moisture resistance, fluting has to be more pliable so that they can be formed into the characteristic wave shape. At the same time, however, the fluting cannot be too absorbent – otherwise it will soften and fail under humid conditions.
(B) HIGH PERFORMANCE FLUTING PAPER
High performance fluting is a relatively new development. These papers have been specifically manufactured to provide increased compression strength, reduced moisture absorption and consistent performance. Like high performance liners, the purpose of high performance fluting is to reduce the weight of packaging, without negatively impacting its strength.
(C) SEMI-CHEMICAL FLUTING PAPER
Semi-chemical fluting is used where hygienic considerations prohibit contact of the packaged goods with possible contaminants in recycled paper. This applies in packages which are in direct contact with products like food or other sensitive.
Tearing Resistance of Paper
Tearing resistance is primarily a property of the inner structure of the sheet, but surface treatment sometimes affects the values to a marked degree. Moisture content of the paper has a large effect on its tearing resistance so test specimens must be carefully conditioned. Higher moisture content and higher relative humidity of the test environment actually increase the tearing resistance.
The tearing resistance is the force necessary to propagate a tear through a paper sheet; this force is measured in units of gram‑force. The principal method used for evaluating the tearing resistance is the Elmendorf tear test (named after the inventor of the standard instrument). The instrument operates on the principle of determining the work done in tearing the paper for a predetermined distance by measuring the loss of potential energy from a pendulum. The desired tearing resistance (force) is obtained by dividing the tearing work by the distance over which the tearing force acts (work = force ´ distance).
The tearing resistance of paper and paperboard grades varies over a very wide range, from fairly small values in tissue grades to very great values in paperboard. To deal with the problem of having to cover this wide range with one instrument, two different testing procedures have been developed and codified in TAPPI Test Method T 414 and International Standard ISO 1974.
TAPPI Test Method T 414 is based largely on the standard instrument of capacity 1600 gf. Of the two tear testers (Lorentzen & Wettre) available and standing side by side in our paper testing laboratory, it is the one on the right-hand side that should be used with T 414. The main disadvantage of method T 414 is that the number of plies to be torn at a time varies from one grade to another and becomes unreasonably large with tissue paper.
By contrast, International Standard ISO 1974 prescribes that always four plies be torn at a time. (ISO stands for “International Organization for Standardization”.) This requirement necessitates the use of several tear testers of different capacity. Accordingly, both tear testers will be used in this procedure.
The tear test has great significance in the requirements of paper and paperboard that are to be subjected to tearing forces in conversion or in actual use. This would include utility papers such as bags, wrapping stock, building papers, map paper, paper used in children's books, and others. 0ften, onverters rely more heavily on tearing resistance than any other single property. Tearing resistance is an empirical property often used as a control test in manufacturing. It is sometimes interpreted to reflect the general nature of the fiber present in the paper as well as the beating or refining treatment to which it has been subjected. This is because the test is somewhat dependent upon fiber length and the bonding strength between the fibers. Generally speaking, a dense sheet gives a lower tear than a bulky one of the same material and weight; and long fibers offer greater resistance to tear than short ones.
Occasionally, the tearing resistance is misused by people who wrongly assume it to be a measure of runnability of the paper sheet either on the paper machine or in a web‑fed printing operation. Tearing resistance has nothing whatever to do with runnability. To obtain a measure of runnability, an entirely different test must be made, and this has been termed the "in‑plane tearing test" (see Van den Akker, Wink, and Van Eperen, TAPPI 50