Know everything about Paper -4
(Written by : N.R. Jayaraman)
6. Tear resistance:
While the Tensile strength is meant to determine the point at which the paper will crack or burst, the Tear resistance is another important property for paper that is taken into account while manufacturing bond papers, cover papers and wrapping papers. Tearing resistance indicates the ability of the paper to withstand tearing force when it is subjected to. When a paper gets torn at the edges, how fast it can be torn in either directions will determine the tear resistance. The test will demonstrate whether the paper will get easily torn or strong enough to withstand the wear and tear during various usages. If the paper gets easily torn (once the edge starts tearing) then it can be considered to be a weak paper. Therefore the paper is subjected to tear resistance test in both ways i.e Machine direction and Cross direction and average worked out to determine this factor. The fiber in the paper determines the strength and therefore fibers floating in the machine direction will be more stronger, at the same time more suitable to use on jobs where creasing or scoring operations in addition to folding operations are involved. However for wrappers, covers and other such usages like packaging papers, the paper needs to be stronger in both ways. in order to ensure that the paper remains stronger in both ways, the pulp travel path wire is shaken in regular intervals to ensure that the fiber float direction is made both vertical and horizontal.
The paper must have fairly a smooth and even surface to take on the print or writing on it. The smoothness is derived both by a process called sizing and super calendering. The term sizing though related with Paper making process can not be delinked with paper as the coating used in sizing influences the smoothness along with porosity of the paper . Good sized paper will have higher strength. Similarly in order to smoothen the surface of the paper the finished paper is fed in between two highly polished rollers. Therefore smoothness is an important factor in the paper.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.
Stiffness is the measure of force required to bend a paper at a specified angle and is an important property for box boards and to some extent for printing papers also. A flimsy paper without certain amount of stiffness can cause feeding and delivery problems in larger sheet run presses. At the same time on printing machines too stiff a paper may not cause feeding problem, they can cause problems in copier machines where it must traverse over, under, and around feed rollers.
9. Dimensional Stability:
Most physical properties of paper undergo changes as a result of atmospheric variations due to the presence of moisture in the air. Water has the effect of plasticizing the cellulose fiber and weakening of the inter fiber bonding. The electrical resistance ie static electricity (artificial sticking of paper) vary with moisture content. 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. Poor moisture control can adversely affect many paper properties. Ability of paper to retain their exact size, which is called dimensionally stable paper is an important factor while printing multi color jobs. The paper should retain the dimensions even when affected by changes in moisture content or humidity in the press room atmosphere, during the printing .
Cotton fibers which is the main constituent of the paper, swell or shrink in diameter during extreme weather conditions and variation in the moisture content in the air. Since most of the fiber in the sheet are aligned in the machine run direction, absorption and de -absorption of moisture by paper causes the change in cross direction as the paper fibers when absorb water expand primarily in width, but only slightly in length. Uneven dimensional changes cause undesirable cocking and curling.
10. Folding endurance:
Fold endurance i.e ability to withstand multiple folds without getting tear will show the durability of paper when repeatedly folded under constant load. Therefore the paper used for printing special documents such as bank notes and currencies need to have sufficient folding endurance. This character is important to those documents which are quite frequently handled such as Currencies and Bank notes, Maps, Archival documents, Wrapping papers etc so that they do not deteriorate faster due to repeated bending, folding, and creasing etc. In short we may say that the Folding endurance is important for carton, box boards, ammonia print paper, and cover paper etc. It is important factor for printing grades where the paper is subjected to multiple folds like in books, maps, or pamphlets. Folding endurance is a requirement in Bond, Ledger, Currency, Map, Blue Print and Record Papers.
......To be continued