Know everything about Paper -5
(Written by : N.R. Jayaraman)
The Paper is tested for different qualities as explained below:
1) The Thickness
2) The weight
3) Water absorption of paper
Oil absorbency of paper
1) The brightness of paper
2) The opacity of paper
3) Smoothness or Gloss
PRINTING PROPERTIES :
1) Printability of paper- Oil absorbency
2) The print quality of paper
STRENGTH PROPERTIES :
1) The tearing resistance
2) The Elongation
3) Bursting strength
5) Folding endurance
The Paper is tested in different ways some of which are briefly explained below:
- The Thickness of the paper is measured by a micrometer or thickness gauge. The paper thickness will be measured in several places and average worked out. It is expressed in microns.
- The GSM is nothing but grams per square inch. One meter square paper is weighed and the GSM calculated. It is expressed in GSM.
- Bursting strength is measured and indicated in kPa which is Kilopaskal which is again a metric unit. The Paper sample will be placed in a equipment and forced air will be applied. The amount of air needed to burst the paper will be the bursting strength.
- Opacity is the degree of light that travel through the paper. Opacity is determined by a ratio of reflectance measurements which is the measure (percent) of the amount of light passing through a sheet of paper. Several instruments and equipments are available in the market to test the opacity. The general principle is that the paper with a black object kept at the back is illuminated from the top side at a particular angle and the reflected light measured. Similarly the same paper with a white background is measured without altering any of the setting. The difference between the two is compared to work out the opacity of the paper.
- The Brightness of the paper is measured with a special equipment which has the scale from 0 to 100. The brightness of the paper will influence the print quality. Paper brightness is defined by the percentage of light that it reflects. Paper with a higher brightness allows colors to stand out, while lower-brightness paper is easier for reading or extended viewing.
- Tearing resistance indicates the ability of the paper to withstand tearing force. When a paper gets torn at the edges, how fast it can be torn in either directions will determine the tear resistance. One of the most commonly used instrument to measure the tear test is the Elmendorf Tear Tester which measures the force perpendicular to the plane of the paper required to tear multiple plies through a specified distance after the tear has been started. All the tests are carried out in specified conditions with specified size of samples. Four ply test samples are clamped to the clamper provided on the machine and with the integral knife handle fitted on the same machine the initial cut is performed. Since the machine is pre programmed, the extent to which the initial cut is to be made etc will be uniform. Then the pendulum will be released to perform the tearing. Then the readings of both the force required for initial cut and the tear with pendulum apparatus will be computed and result obtained.
- The Tensile strength is measured by a Tensile tester by applying constant rate of elongation to the test strip of specified width and length. The tensile strength of paper is measured by applying a tensile force parallel to the plane of the paper. The tensile strength of paper being defined as the breaking force per standard width, will depend on basic weight of the paper. Tensile strength is usually quoted in kN/m.
- The Water absorbency called Cobb test of the paper is measured by an equipment. The blank paper of a particular size will be first weighed. Then the same paper will be clamped on a special equipment and water poured over it and kept for specified time. Then the excess water will be removed, paper blotted with blotting paper and then weighed to determine the water absorbency of paper.
- The Folding endurance is tested by an equipment on which a specified length and width of paper will be clamped and folded many times in both ways. The number of double folds that are required to make a test piece break under standardized conditions will indicate this factor through a formula.