PROPERTIES OF PAPER

PROPERTIES OF PAPER

2.7.1    Physical Properties of Paper

            Most of important properties of paper are physical rather than chemical. This section will be limited to such physical properties as grammage, thickness and density (Casey, 1981b).

2.7.1.1 Grammage

            Weight is the most common specification made on paper. Because paper is used in sheet form and area is more important than volume, weight of paper is expressed per unit of area rather than unit of volume as in the case with most other materials. Weight of paper is expressed as grams per square meter in the metric system and is termed grammage (Casey, 1981b).

2.7.1.2 Thickness

            Thickness, or caliper, of paper is measured with a micrometer as the perpendicular distance between two circular, plane, parallel surfaces approximately 16mm (0.62 in.) in diameter. Thickness affects nearly every physical, optical, and electrical property of paper (Casey, 1981b).

2.7.1.3 Density

            The density of paper is probably most important fundamental paper property. Density is related to porosity, rigidity, hardness, and strength of the paper; in fact, density influences every optical and physical property except sheet weight. Density is expressed as grams per cubic centimeter ( and is calculated by dividing the weight in grams per square meter by the thickness in micrometers (g/ /  (Casey, 1981b).

2.7.2    Mechanical Properties of Paper

2.7.2.1 Tensile strength

            Tensile strength is determined by measuring the force required to break a narrow strip of paper where both the length of the strip and the rate of loading are closely specified (Smook, 1992). Tensile strength more nearly approaches a fundamental measurement than other conventional strength measurements made on paper (Casey, 1981b).

The tensile strength of paper is determined by the combined effect of the following factors at which first is the strength of the individual fibers of the stock furnish. Second is the average length of fiber. Third is the inherent bonding ability of the fiber surface both in terms of bonded area and of strength per unit of bonded area and lastly is the structure and formation of the sheet (Britt, 1970).

2.7.2.2 Bursting strength

            Bursting strength is determined by clamping a paper sample over a rubber diaphragm through which pressure is applied at a gradually increasing rate, and noting the pressure at rupture (Smook, 1992). It is a complex function of tensile strength and stretch. The stress exerted in the test is largely one of tension resulting from a pressure differential across the sheet at the time of rupture. The strain is approximately equal in all directions, thus building up unequal stresses in the paper (Casey, 1981b).

            The primary function of bursting test is to indicate the resistance of a paper to rupture in use. It is quick and easy to perform and one test is sufficient for both directions of machine-made paper if compared to the average tensile strength which requires testing in both principal directions of the paper (Britt, 1970).

The factors which will affect bursting strength of paper are the amount of interfiber bonding, individual fiber strength, ratio of machine and cross-direction strength and by the stretch of paper (Britt, 1970).

2.7.2.3 Tearing resistance

            Tearing resistance normally determined with the Elmendorf apparatus which uses a falling pendulum to continue a tear in the paper sample when the force is applied perpendicular to the plane of the sheet; the loss of energy (as measured by the height of swing of the pendulum) is related to the force required to continue the tear (Smook, 1992).

            Tearing resistance of paper will be influenced by the physical properties of the fiber. Fiber length and interfiber bonding are both important factors in tearing strength (Britt, 1970).

2.7.2.4 Folding endurance

            Folding endurance is measured by the number of folds sustained before rupture occurs when a sample is flexed through a specified angle under controlled tension (Smook, 1992). Folding endurance is a measure of the strength and flexibility of paper. It is not a measure of foldability, which is the ability to fold a heavy paper or paperboard without cracking (Casey, 1981b).

            Folding endurance has a double significance. The test subjected to papers such as envelope, wrapping, and cover stock indicates performance. Besides that, it is also a sensitive measure of influences tending to degrade fiber. So, it is sue to indicate aging of paper (Britt, 1970).

  

2.7.3    Optical Properties of Paper

2.7.3.1 Brightness

            Brightness is measured as the reflectance value (relative to a magnesium oxide standard) in the blue region of the visible spectrum (specifically, at a wavelength of 457 nm) (Smook, 1992). Reflectance in the blue portion of the spectrum was originally chosen for brightness because, of all the single-valued measures of whiteness potential of the final product, blue reflectance is at once the most sensitive and relevant (Britt, 1970).       

Brightness is not whiteness, nor is it a colorimetric measurement. However it can provides values for measurement of the maximum whiteness that can be achieved with proper tinting (Britt, 1970).

2.7.3.2 Opacity

            Opacity is determined by the amount of light transmitted by the paper. If all the light is transmitted and none is reflected or adsorbed, the opacity will be zero. If no light is transmitted and all of it is reflected or adsorbed, the opacity will be 100 percent. Most papers fall between the two extremes (Casey, 1981b).

Although opacity can be determined by the amount of light transmitted, it is usually determined by what is known as contrast ratio. This is the ratio of the diffuse reflectance from a single sheet of paper backed by a white body. Opacity is an important property of printing, bond, and writing papers and it usually a part of the specification of these papers (Casey, 1981b).