Bending Test/Bending Resistance

This test gives a measure of the ‘Stiffness’ of the paper, which is the measure of force required to bend the sample through a specified angle. Stiffness is important for paper boards, box boards, corrugated objects, and also for printing papers. This property is measured by the resistance offered to a bending force by a rectangular sample. It is generally expressed in milli Newtons (mN)

Brightness

Brightness is an optical property of paper, which measures the degree of reflectance/reflectivity of blue light at a principal wavelength of 457 nm. Taking brightness values at this wavelength indicates the quality of bleaching in the pulp of the paper. Bleaching of pulp is done to remove residual ‘lignin,’ (glue which holds the cellulosic fibers together) which if present imparts a creamish/yellowish appearance to paper. Since lignin absorbs strongly at 457 nm, brightness is measured at this wavelength. The testing standards generally followed are TAPPI T452, ISO (2469 & 2470), & ASTM methods. This property is an important selling quotient of paper as a brighter paper appears more attractive than a less bright one.

Opacity

‘Opacity’ is a fundamental optical property of paper and it is a measure of the amount of light which can be transmitted through it or absorbed by it. The more the value of opacity of a paper, lesser the light it will transmit or allow to pass through it and vice versa. Opacity is a very important property to consider in the context of printing of a variety of papers like duplex printing, stationary papers, tracer papers etc. Opacity of a paper is influenced by a number of factors and ingredients like thickness/grammage of sheet, quality and quantity of fillers, degree of bleaching of fibers, coating, paper-making processes of beating, pressing, calendering, supercalendering etc. It is measured in terms of the ratio of the percentage of light absorbed by or reflected (Diffuse Reflectance) through a sheet of paper, and this measurement is done using an ‘Opacimeter’. Two methods of measurement are there, one is the ‘Contrast ratio method’ and the other being ‘Diffuse opacity method’. The standard testing methods followed are TAPPI T425 & ISO 2471.

Contrast ratio method of Opacity measurement

This method of measurement of ‘Opacity of paper’ calculates the opacity as the ratio of reflectance (diffuse) of the paper sample when backed with a black body or sheet of paper, to the reflectance (diffuse) of the paper when backed with a white body of known reflectance.

Diffuse Opacity method for measurement of Opacity

In 'Diffuse Opacity method' the measurement of opacity is done by taking the ratio of reflectance (diffuse) of the paper specimen when backed with a black body, to the reflectance (diffuse) of the paper when backed with multiple sheets or a thick pile of the same test paper. This method is considered by many to give a better estimate of the value of opacity of a paper sample than the 'Contrast ratio method'.

Bursting Strength of Paper

Bursting strength indicates how much pressure can be tolerated by the paper before getting ruptured. This is one of the oldest tests conducted on paper to test for its strength or resistance to rupture. This property is important for those papers or paperboards which are meant to be used for packaging purposes like paper bags, corrugated boxes etc. Bursting strength is measured as the maximum hydrostatic pressure needed to burst/rupture a paper sample by constantly increasing the pressure applied uniformly across its side through a rubber diaphragm of specific dimensions. This property depends upon a number of factors like quality of fibers in terms of type, proportion/number, their method of preparation, their degree of beating and refining, and use of additives and fillers. This test is generally conducted using a ‘Mullen Tester’, measured in kPa units, and the standard followed is TAPPI T-403 (for paper) and TAPPI T-807 (for paperboard & linerboard).

Burst Index

Burst index is determined in order to facilitate the comparison of bursting strengths of papers with different grammages (g/m2), or to normalize the bursting strengths of various papers. It is obtained or calculated by dividing the bursting strength (in kPa) of the paper sample by the paper grammage (in g/m2).

Compressibility

Compressibility of a paper specimen is the reduction in its thickness on application of a compressive force/pressure. Actually, it is calculated as the decrease in surface roughness when two consecutive measurements of roughness are made without moving the test piece at two standard contact pressures specified in the standard method. In simple terms, it refers to a paper's capacity to be squeezed (upon flat surfaces) and then able to be returned to its initial state. This property is an important indicator for printability of paper. A high degree of compressibility of a paper is useful for producing a good printed image. Compressibility of paper is a function of its apparent density, degree of refinement of fibers, and the degree of calendering and supercalendering. The degree of this parameter along-with others like resiliency, hardness or softness defines a paper's 'Printing Cushion'.

Crushing Value

This test is a measure of the strength of aggregate, which will normally to be used in making concrete. It involves standard compression of 400kN in a mould of a known mass of aggregate of single particle size and measuring resulting percentage of crushed particles.

Resiliency

This property of paper determines the extent or degree of restoring its original dimensions and surface contours after removal of the compressing force, as in printing. 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'.

Density

Density or specific gravity of paper is its weight per unit volume, obtained by dividing the basis weight by caliper. Paper density is generally expressed in g/cm3 or kg/m3, and it indicates the degree of compactness of the paper. Generally the range of paper density lies between 0.5 to 0.8 g/cm3 or 500 to 800 kg/m3.

Thickness or Caliper

Thickness or Caliper of a paper, board and tissue is its thickness which is measured after application of a static load for a specified period of time. This measurement is vital in assessing the uniformity of paper throughout its entire length and breadth. This property can affect several basic properties of paper like strength, optical, roll quality etc., and can also affect the printability and runnability aspects while printing. Thickness is governed by a number of factors like grammage, refinement of pulp, and degree of wet pressing, calendering, and supercalendering processes. This property holds importance for filling cards, printing papers, saturating papers etc. The measurement is done using a micrometer and it measures the perpendicular distance between the surfaces of two papers shaped circular, plane, or parallel under a pressure of 1 kg/cm2. The test result is reported as μm. The standard test protocol followed is TAPPI T411. Bulking thickness is the thickness of a single sheet calculated from the thickness of several sheets.

Short Span Compression testing

‘Short Span Compression’ testing gives a measure of the compression strength of paper and especially corrugated board materials over a very short compression span. The clamping arrangement is typically designed to prevent the test piece from ‘buckling’ during the test. The standard test methods generally followed are TAPPI T826 & ISO 9895.

Ring Crush Test

'Ring Crush' testing gives a measure of the resistance offered by paper or corrugated boards formed into the shape of a cylinder, to edge-wise compressive forces. This test helps to evaluate the performance characteristics of papers or components of corrugated boxes in terms of being able to withstand damages during shipment or stacking. The test results are reported in kN/m and the standard test methods adopted are TAPPI T818 & TAPPI T822..

Grammage of paper

Grammage or ‘Basis weight’ of paper, boards or paper tissues is defined as the weight per unit area of the test sample, which has been conditioned at the following conditions: Temperature (in 0C) 23 ±1 and Relative humidity (in % RH) 50 ±2. Grammage is generally expressed as g/m2. It is an important property from the commercial point of view, as it determines the buying criteria for a particular paper product.

Folding Endurance (Double Folds)

'Folding Endurance' of a strip of paper gives a measure of the resistance offered by the strip to the application of external pressure in the form of multiple and repeated folds, before breaking. This test determines the suitability of a paper type to be used in situations where it will be subjected to repeated folding, bending, and creasing. Hence this test finds application for such papers which endure such forces or handled very frequently, like currency papers, wrapping papers, maps, pamphlets, cartons, cover papers, bond papers, ledgers etc. Currency paper has the highest folding endurance. The factors that affect this property of paper include fiber length, strength of fiber, moisture content, use of fillers etc. For studying the ageing characteristics of a paper the values of fold endurance before and after accelerated environmental ageing are made use of. Quantitatively it is measured as the number of double folds, associated with a specified load that a strip of paper, with a particular dimension, can withstand before it breaks. There are four different instrument types or designs which are used to carry out this test and the test results produced by them vary because of variations in the strip loading applied, strip folding angle, and the speed of folding. The tests using the different instruments are carried out according to separate standards, for e.g. the Schopper type tester is carried out as per TAPPI T423, and the MIT type tester is done as per TAPPI T511.

Stiffness

Stiffness of paper refers to the ability of a test sample to withstand a bending force. It is determined as the force required to bend a paper specimen through a specified angle. Stiffness is generally expressed in two forms: (1) Flexural Stiffness: It refers to a paper’s resistance to an externally applied bending force. This is also known as ‘Rigidity’ (2) Handling Stiffness: It refers to the ability of a paper to support its own weight. Factors affecting stiffness of paper include the type of fibers used, degree of fiber refining, grammage of paper, thickness, and the bulk. Stiffness is an important consideration for some papers, like box boards, corrugated boxes, bond papers, and printing papers. The measurement can be made using different types of instruments employing different conditions. For example, Clarke stiffness measures the stiffness in terms of the free length (in cm) of a paper strip which bends under its own weight. This is measured in accordance with TAPPI T451..

Smoothness/Roughness

This property of paper is a measure of the evenness or contour of a paper’s surface. The contour or appearance of a paper’s surface can be described by terms like “finish” & “pattern”. Smoothness & Roughness are inversely related, and depend on a number of factors involved in paper-manufacturing process like degree of fiber refining, extent of wet pressing, extent of calendering, supercalendering, use of coatings, and the desired paper finish. Smoothness is an important parameter in writing, printing characteristics of paper, aesthetic value, and also important for paper bags. The smoothness of paper is generally measured using ‘Air Flow measurement instruments’. These instruments measure on the principle that the volume of air voids between a paper and a plane surface (like glass) is proportional to the smoothness or roughness of the paper and that the rate of airflow between these surfaces is proportional to the volume of air voids present in paper. Some of the instrument types are Bekk, Bendtsen, Sheffield, and Print-surf smoothness testers. Depending upon the instrument used for testing, the standard protocol of testing varies.

Tear Strength/Resistance

This property gives a measure of the resistance of a sheet of paper to a tearing force that it is subjected to. It is measured in both machine direction & cross direction, and is expressed as mN. Two types of instruments are used, Elmendorf & Trouser tear instruments. Tearing resistance depends on the degree of fiber refining and the quality & quantity of fillers used. This property assumes importance in many applications involving cover papers, wrapping, bond papers, & envelope papers. Standard test methods followed are TAPPI T414, TAPPI T470, and ISO 1974.

Tensile Strength

It is the force required to produce a rupture in a strip of paperboard, and measured in both machine direction & cross direction, i.e. parallel and opposite to grain direction. It is expressed in kN/m, and gives an indication of the ability of a paper to withstand stretching forces. Hence, this property is an important consideration in those cases where a paper has to be used in high-stretching conditions. Tensile strength is indicative of fiber strength, fiber bonding, and fiber length. Standard test method followed is TAPPI T494.

Water Absorbency & Cobb Test

'Water Absorbency' is a measure of the amount of water absorbed by the wetted surface of a paper specimen and board materials. Water absorbency is reported in g/m2

Ash Testing

Ash testing of paper gives an estimate of the inorganic fillers and coating contents (like Calcium Carbonate, China clay, and Titanium dioxide) used in the paper manufacturing processes. This parameter is determined by complete combustion of the organic matter at a specified temperature and then calculating the result on the basis of oven-dried sample weight. Though the importance of this property of paper is limited, but it assumes importance in cases of ash-less filter papers as of Whatman’s. The measuring standards are TAPPI T413 & ISO 1762.

pH

pH is an estimate of the hydrogen ion concentration, and hence it refers to the paper being acidic or basic. pH is equal to the negative logarithm of the hydrogen ion concentration. A paper with a pH less than 7 is acidic; if pH is more than 7 is alkaline, and in case it is 7 is neutral. This property has a bearing on the ‘Permanence’ of paper. The pH of a paper can be determined in a number of ways like: (1) Determining the pH of paper extract obtained by disintegration in hot distilled water. (2) Determining the pH of paper extract obtained by disintegration in cold distilled water. (3) Determining the pH by directly using a wet electrode on the surface of the test paper. These three modes give different test results on the same paper specimen.