Journal of Engineering Research and Studies E-ISSN 0976-7916 Review Article PRINTING PAPERS: SIZING AND ITS ROLE Manisha S. Deshpande Address for Correspondence M.E. (Printing) Third Semester, Pune University E Mail [email protected] Guided by- Prof R. G. Kaduskar, P. V. G’s College of Engineering and Technology, Pune ABSTRACT This document gives information about water resistant property of paper i.e. sizing. It gives details of sizing materials and requirement of properties for different printing methods. KEYWORDS Sizing, Printing Methods, Balanced sizing I. INTRODUCTION conventional practice to add the sizing agent to the paper It is considered that printing; has fostered communication furnish and thereafter add a precipitating agent which aids and propagated the knowledge, has done more to develop in setting the size of the papermaking fibres. The civilization. It has been so only because of use of paper. precipitating agents used are water-soluble polyvalent The forerunner of paper (c. 2600 BC) was papyrus, a metal salts, such as alum or aluminum chloride. Use of the material made out of the papyrus plant, which grows in aforesaid combination of sizing agent and precipitating Africa. The actual invention of paper produced from plant agent can be accomplished in conventional papermaking fibres such as bamboo fibres or cambric grass dates back to machines without the aid of any special apparatus. about AD 105; recent literature refers to paper production Moreover, the materials used are of low cost and are actually dating much further back. Tsai Lun from China is generally quite effective in imparting to paper the ability to accredited as the inventor. Now a day it is widely used. But resist penetration by liquids. However, there are certain different printing methods require different properties of significant limitations and disadvantages associated with paper. One of such important property is sizing. 4 their use. II. FUNCTIONS OF SIZING For example, in some instances the water resistance Sizing refers to imparting some degree of resistance to the attainable with these agents is inadequate. Furthermore, in absorption or penetration of liquids, especially water. using these sizing agents at the concentrations customarily Sizing of paper is a very old and well-established art and a employed serious problems can arise with regard to wide variety of materials have been sized. As is well formation of translucent "spots" in the paper and there may known, the sizing agent may be applied to the fibres during also be problems associated with accumulation of the the papermaking operation, in which case the process is sizing agent at the press rolls of the papermaking machine called internal sizing (also known as beater sizing or or with clogging of pipes or other equipment by the sizing engine sizing), or it may be applied to the surface of the agent. paper after web formation, in which case it is called Al 2(SO 4)3 + 6H 2O = 2Al(OH) 3 + 3H 2SO 4 external or surface sizing. 2. External sizing /surface sizing 1 Applied to surface Most of the end-use properties that the size includes one sizing this implies that the resistance is achieved by or several of the following: applying chemicals to the surface of the paper or board • Increased Hydrophobic; after it has been formed rather than adding chemicals to the • Film-Formation or Barrier Properties; wet pulp. In modern technology, surface sizing also • Increased Surface Strength; involves conferring properties besides hydrophobicity. • Modified Frictional Properties; Parameters loosely referred to as printability, gluability • Modified Optical Properties; and runnability may also be important in some • Reduced Sheet Porosity. applications. A better expression than surface size in these III. TYPES OF SIZING circumstances is surface modifier or surface enhancer. 1. Internal Sizing 1 - A particularly important class of There has been a drive towards higher specification paper sizing agents for internal sizing are the fatty acid sizing grades. In some cases the paper properties required would agents, i.e. sodium, potassium or ammonium salts of long- be difficult or impossible to achieve without application of chain saturated fatty acids. In employing these agents, it is chemicals at the surface. Increased production of coated JERS/Vol.II/ Issue II/April-June, 2011/17-21 Journal of Engineering Research and Studies E-ISSN 0976-7916 grades has also encouraged surface sizing which is used to added. Acetylated starches give particularly good film- control coating hold-out. formation properties to the starch. The advantage of Increased emphasis on environmental issues and the need cationic starches is that they form ionic bonds with the to clean up the 'wet-end' system have also promoted anionic fibres and fillers and so remain with the fibre modern surface sizing. A cleaner wet end allows better during repulsing. This has been reported to result in at least control of the papermaking process and makes substantial a 50% reduction in biological oxygen demand (BOD). financial savings possible on large, fast machines. The risk Chemical treatment of the starch increases its cost and so of penalties charged on high levels of chemical oxygen use of chemically-modified starches must usually be demand (COD) in the mill effluent can provide a strong Justified by giving added value to the paper, improving incentive to increase the amount of chemical treatment at paper machine run ability or reducing effluent loadings in the paper surface where retention of chemicals applied is the backwater. Although starch is by far the most common essentially 100%. additive at the size press, surface size can be applied The formulation applied at the size press will usually without starch or with an alternative co-agent, such as consist of a mixture of chemicals. These can be classified polyvinyl alcohol, sodium alginates, or carboxy-methyl according to their function: cellulose. These materials are chosen for their good film- • transport medium; forming properties which bond together fibres at the • surface strengthening agents; surface and fill holes in the paper structure. Another • specialty chemicals; important property is their viscosity-modifying effect • surface sizes or hydrophobing agents. which influences the pick-up of the size-press mixture. The The surface strengthening agent commonly used is starch. surface size can also be applied as part of a coating mix This is the most abundant material present in the which then effectively becomes the carrier. This may be a formulation, and the starch solution acts as the transport convenient way to add the size in cases where there is no medium or carrier for the surface size. Nevertheless, it separate size press available provided the surface size is should be stressed that the type, viscosity, temperature, and compatible with the coating mix, but generally the size is pH of the starch can all affect the amount of surface size less effective when used in this way because it tends to be applied, where it ends up in the sheet, and how it performs. within the coating layer and less available for influencing The main sources of starch are potato (farina), maize (corn), the surface properties. Specialty chemicals can consist of a wheat, and tapioca derived from cassava, which is an wide range of chemicals for specific end-use applications. important source in hotter regions of the world. The starch Typically they include materials such as dyes, fluorescent is usually a by-product and so its price and purity depend brightening agents (FBAs) and security chemicals. on many factors such as agricultural policy and extraction The surface sizes or hydrophobing agents can be divided process. Recently in Europe there has been a reduction in conveniently into the following categories: the availability of potato starch and so maize starch is most • Alkyl Ketene Dimmer (AKD); commonly used. However, wheat starch often offers a • Rosin-Based Products; cheaper alternative but contains some protein impurities • Synthetic Polymeric Materials; (residual gluten) which can affect foaming properties and • Others viscosity control. Of these the synthetic polymeric materials are the most Before it can be used at the size press the native or pearl important in modern surface sizing. They have been starch must be converted to modify the viscosity properties developed specifically for application at the surface and are of the starch solution. The starch may be reconverted by able to produce novel properties in the sheet. the starch supplier but most mills prefer to make their own IV. PRINTING PROPERTIES REQUIREMENT conversion to save costs. The conversion involves breaking The size press provides the last opportunity to modify the bonds in the large starch molecule and can be done by an chemical properties of paper, or prepare it for coating. The enzyme, thermo mechanical or thermo chemical process. properties to be delivered by the surface size vary greatly The conversion may be a continuous or batch process and and depend on the end use for which the paper and board is selection will depend on cost, viscosity, and solids content intended. In some cases the precise physical and chemical requirements. Whatever starch and conversion process is properties required are not well defined, and the end-use chosen it is important to maintain consistent size-press property is specified only by empirical performance tests starch solution properties for efficient application of such as ink flotation or dust generation after a prolonged surface size. The starches may also be modified chemically period. by reacting with other chemical groups onto the converted As the surface size is normally used with a carrier, starch molecule. The most common derivative starches are the properties of the surface size are not discrete and ethylated, acetylated, oxidized or have cationic groups interactions occur. Two of the more important properties JERS/Vol.II/ Issue II/April-June, 2011/17-21 Journal of Engineering Research and Studies E-ISSN 0976-7916 for which the surface size is responsible are • Toner Adhesion For Copiers And Laser Printers; hydrophobation and barrier formation.