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Home , Fine paper, Sizing

Journal of Research and Studies E-ISSN 0976-7916

Review Article : 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 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 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 . 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 give particularly good film- control coating hold-out. formation properties to the . 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 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 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. The hydrophobic • Sheet Feed Through Copiers; groups most commonly used in surface sizes are long- • Lining And Dusting In Offset Printing. chain fatty acids and phenyl groups. Long-chain fatty acids The composition of varies widely in terms of are the familiar hydrophobia in AKD wet-end sizes. They filler and fibre type and content, internal size, retention are also present in the polyurethane micro dispersions system and machine type and so it is difficult to generalize where they can rotate on the polymer backbone and about optimum conditions. A typical European fine paper orientate towards the surface. With the solution and might consist of: emulsion polymers phenyl groups derived from the styrene • A mixture of long and short fibre, sometimes copolymer introduce the hydrophobic. In the rosin-based including ; surface sizes biotic acid is used in a similar way to wet-end • Neutral or alkaline sizing with AKD or alchemy sizing. The important feature about surface sizing with succinct anhydride (ASA); synthetic polymers is that they provide 'instant' • 15%-20% ground calcium carbonate or hydrophobation with essentially 100% retention of size. precipitated calcium carbonate filler; True film-formation is developed by the copolymers • Fluorescent brightening agent (FBA) added containing carboxyl groups capable of hydrogen bonding internally and at the surface; to form films. However, a type of 'film' can also be • Starch, surface size and probably some salt introduced by a polymer distributed across the surface added at the surface. which flows in the after dryers to form a barrier to the The choice of starch used is often decided by cost and penetration of oils and grease and which also limits the availability rather than by performance. Enzyme-converted migration to the surface of materials internally in the sheet. maize starch is commonly used, although there are some In the case of coating, the objective is often to limit the examples of low cationic starch and ethoxylated starch in migration of binders or microcapsules into the sheet. use. Because a fluorescent brightening agent (FBA) is Because surface sizes are used over a wide range of end- usually included in the size-press mixture an anionic use applications it is necessary to translate physical surface size is preferred. A typical sizing specification for chemical properties such as hydrophobation and film- a plain fine paper is a Cobb(60 s) value of 20-25 g m -2 or a formation into end-use performance-related properties such HST value (80% reflectance, 1% formic acid, naphtha as printability in fine paper, coating hold-out and gullibility green ink) of 200-300 s, although examples can be found in recycled grades. of harder and softer sized sheets. The HST value is more 1. Plain fine paper sensitive to the effects of the surface treatment. The surface Plain fine paper consists of uncoated grades usually made size usually contributes a relatively small amount to the from virgin fibre. A large part of this paper segment is sizing property but can have a large effect on the multipurpose office paper or copier paper which is printability and run ability properties. This makes is produced in a range of qualities. Apart from some difficult to discuss surface sizes in conventional hydrophobicity the main requirements of the sheet are papermaking terms. appearance, especially brightness, smoothness and stiffness, The requirements and behaviour of the ink-jet systems are printability, usually on a range of printing systems, and run particularly difficult to explain in these terms. These ability through printing machines and photocopiers. These systems involve a large number of very small alkaline ink requirements have become more demanding in recent years droplets being fired very rapidly (1 droplet every 20-30 us) with the trend towards brighter and smoother paper, the at the paper sheet or 'substrate' at which point the definition increased use of ink-jet printing, the arrival of faster and quality of the image is determined. The are water- printing and copier machines, and in some cases the based but contain a range of surfactants and co solvents to introduction of recycled fibre into the sheet. As in most control the wetting, spreading and penetration into the papermaking activities the end result is usually a sheet. The inks are commonly dye-based but black pigment compromise or trade off of properties and treatments to inks are now used in some of the current printer systems. produce an acceptable end result. The aim is to produce an image with strong, bright colours Surface sizing is increasingly used in combination with (high print density), good edge acuity (low feathering, some internal sizing to achieve the exacting end-use wicking and colour bleed), even print distribution over properties required. These include: heavily printed color areas (low mottle), an ink that dries • Black Text Wicking And Feathering In Ink-Jet quickly preventing set-off to the next sheet (short dry time) Print (UP); and which does not pass through to the reverse side of the • UP Color-To-Color Bleed; sheet (low 'show-through' or 'strike-through'). Added to • UP Print Density; this, most printer systems have different ink formulations,

JERS/Vol.II/ Issue II/April-June, 2011/17-21

Journal of Engineering Research and Studies E-ISSN 0976-7916

printer head architecture and color management software times across a toner printed area and either measuring the systems. It is not surprising then that measurements of mass of toner removed or the decrease in print density of Cobb, HST and air porosity of the sheet have failed to the printed area. 3 predict UP performance. The print-paper interactions are The requirements of offset printing must also be satisfied. obviously complex and not fully understood which is why A concern of offset printing is the build-up of dust from a large amount of research and development is in progress the paper sheet on the printing blanket. There seems to be to understand them better and improve print quality in the no satisfactory test method available to measure and existing and new printer systems which appear on the predict this behaviour other than to print a large number of market each year. Application of starch at the surface sheets until the build up of dust makes it necessary to stop usually contributes towards increasing the print density but the printer and clean the blanket. Styrene acryl ate is not sufficient to give optimum print definition. A surface emulsion polymers and polyurethane micro dispersion size such as a styrene acryl ate emulsion added typically at surface sizes have both been reported to reduce dusting and 1-5 kg t _1 of 25%-35% solids emulsion on paper improves lining. the print quality by modifying the paper-wetting, spreading 3. and specialties 3 As the name suggests, and absorption properties. These are related to the surface specialties include a wide range of diverse paper types free energy and contact angle of the sheet. Some of the which often form niche market segments. One common equipment recently introduced to measure dynamic feature of these grades is the use of a surface size to form a changes in penetration, absorption and contact angle offer barrier on one side of the sheet. Its function may be to stop better information about the ink-jet behaviour since they the penetration of a compound of the coating mix from can be used with ink-jet inks and are able to measure passing through the sheet, as in the case of the changes in properties over very short time periods. microcapsules applied in the coating mix in the production Future trends are expected to include faster ink-jet printing of ; or to stop glue passing to the systems for the small and home office user. This will mean reverse side as in the case of one-side coated labels. that a short drying time will become more critical. Many Surface sizing is the preferred treatment because the size is attempts have been made to include a small amount (@ 3-5 placed at the surface where it is required and the sizing g m -2) of pigment in the size-press mixture to increase the effect is developed 'instantly'; unlike AKD size which absorption capacity at the surface and increase the drying requires a time to cure. This is important for on-line rate. However, care must be taken not to reduce print coating since the coating treatment then follows directly density by this treatment. after the surface size. In the example of high solids coating 2. Multipurpose office paper must be able to perform well mixes the sizing is required prior to coating to prevent the on a range of impact and non-impact printing systems. It is water from the coating mix penetrating into the sheet and not sufficient for the paper to give well UP quality if it is preventing an even application of coating on the surface. unable to pass well through a photocopier or has poor toner The choice of surface size depends very much on the adhesion properties. Toners are dry powders consisting of coating treatment and the end-use application. If very hard mainly resin and pigment (carbon black), although there sizing (Cobb(60 s) < 10 g m~ 2) is required, a polyurethane are small amounts of specialty materials which are unique surface size can be used without risk of the dusting to the particular printer system. Black toners usually problems associated with AKD applied at the surface; this contain a styrene-acrylic polymer, and so a styrene acryl can also present a high concentration of hydrophobic ate surface size, because of its similar chemical properties, groups at the surface. The poly-urethanes are also flexible usually gives improved toner adhesion properties. Three polymers, which allows them to resist 'cracking' or types of test are commonly used to assess toner adhesion. 'breaking' during hard calendaring. They are also resistant One test is referred to as a 'tape-peel' test in which the to extremes of pH once applied on the sheet, which is paper sample is printed with a toner ink (either copier or' important in some post-treatments such as development of laser printer), an adhesive tape is applied to the printed diazole papers under highly alkaline conditions. area and then peeled off under controlled conditions. The One drawback of the highly hydrophobic surface of the ratio of the print density after tape peel to the original print polyurethanes may be that it reduces the adhesion of glues density gives the retained print density (RPD); the higher and adhesives to the sheet. The styrene acrylates are less the RPD the better the toner adhesion. In another test the hydrophobic and have good gluing properties. Addition paper sample is printed and then folded to simulate a letter levels of surface size vary greatly in this market segment. being inserted into an envelope. The width of the crack If a particular specification has to be met then a higher along the print gives an indication of the toner adhesion: surface size addition level may be justified in a specialty the greater the width the lower the toner adhesion. The market segment. third test involves rubbing a paper surface a number of

JERS/Vol.II/ Issue II/April-June, 2011/17-21

Journal of Engineering Research and Studies E-ISSN 0976-7916

4 Recycled grades 2. .Akpabio U.D.,Roberts J.C.’ Alkylketene dimer sizing’ These include paper and board made from recycled fibre. Tappi J. 3. Treatment on wetting dynamics on ink jet,by Tappi By far the largest market segment is the test-liner and Press fluting segment where mechanical fibre, typically old 4. Handbook of Print Media, Heidelberg publication corrugated containers, are recycled without de-inking or an 2001e (Offset printing) extensive cleaning procedure. This means the board is 5. The paper Chemistry’ Second Edition by J.C. Roberts, BLACKIE ACADEMIC AND PROFESSIONAL often produced under conditions contaminated by a high 6. Materials in Printing Processes By L.C. Young, Focal concentration of anionic trash. In extreme cases internal Press mLondon. sizing may be completely ineffective. The properties needed in this segment usually include: • long-term hydrophobicity (low Cobb(30 min)); • good gullibility of hot melts, cold adhesives or tapes; • Satisfactory printability on some surfaces. Because of the prevailing anionic conditions cationic surface sizes are usually used because of their excellent sizing efficiency. Cationic styrene-acryl ate emulsion products are popular, often used with alum or polyaluminium chloride. Recycled grades may also include recycled writing and printing papers in which the fibres are a mixture of chemical and mechanical types and some de- inking may have been performed. A particular characteristic of this grade is the high level of fines in the stock and the variable amount and type of filler present. Again, internal sizing may be difficult to achieve and the variable composition of the stock makes it difficult to control. However, the main concern is often dusting at the surface and wicking of print into the fibres because of the fines and broken fibres present. In this case a surface size is essential to improve the surface properties and a styrene acryl ate emulsion or styrene-malefic anhydride solution polymer may be used. V. BALANCED SIZING The final properties of the paper and board are influenced by the treatment given at the wet end, the surface and in post-treatments. Although surface sizing gives essentially 100% retention the effect of the surface treatment still depends on the properties of the base paper entering the size press. Where the end-use application of the paper requires good printability, good base-paper properties are essential to give a good finished product. Surface sizing should not be regarded as a miracle cure or a way of covering up deficiencies at the wet end. The most effective treatment usually involves a good balance between the wet end and the surface treatment. Combined internal and surface sizing is recommended to produce optimum paper properties in a clean and cost- effective system. 6 REFERENCES 1. Mini-Encyclopedia of Papermaking Wet-End Chemistry Additives and Ingredients, their Composition, Functions, Strategies for Use, Martin hubbe, Associate Professor of Wood and Paper Science, N C State University, [email protected] .

JERS/Vol.II/ Issue II/April-June, 2011/17-21