The mixture of fibers and water used in papermaking behaves differently than typical liquids, and additives like starch are used to make the paper strong by helping fibers and fillers stick together. The use of cationic polymer induces flocculation, which creates flocks of cellulose fibers that can be sensitive to shear force. Starch is adsorbed on cellulose and filler particles, with the amount absorbed depending on concentration and surface area. Retention of pigments and cellulose fines requires controlled flocculation of the papermaking furnish, and electrostatic attraction can be facilitated through manipulation of pH or the introduction of a cationic polyelectrolyte. Starch is used to increase bonding between fibers, and cationic starch products are used to improve paper strength. Wet-end sizing, which involves adding sizing agents to the papermaking furnish prior to sheet formation, is used to decrease water sensitivity and maintain paper dimensions.
The Wet End of the Paper Machine
The process of making paper involves a lot of physical and chemical steps, and it can be complicated. The mixture of fibers and water doesn’t behave like a typical liquid. The mixture is usually very dilute before it’s made into paper. The fibers are attracted to each other and form small clumps that need to be broken up during the process. Additives are used to help the paper fibers and fillers stick together properly and make the paper strong. One important additive is starch, which helps with the clumping of fibers and controlling the amount that sticks to the paper.
Flocculation of Cellulose Fibers and Fines
The process of adding cationic polymer to papermaking furnish to induce flocculation is affected by the stock composition, adsorption kinetics, and hydrodynamic shear. Flocculation of cellulose fibers happens quickly in turbulent flow, and the flocks are created due to charge interactions. However, they can be sensitive to shear force, leading to deflocculation and reflocculation. Polymer addition points should be selected carefully to achieve optimal paper formation with high retention. Cationic starch is used to balance flocculation and retention, promote sheet dewatering, and improve sheet formation. The degree of flocculation is influenced by the charge density, contact time, concentration of inorganic ions, and the amount of cationic starch added. Using microparticulate flocculation and retention systems can significantly improve formation, paper strength, retention, and sheet dewatering. Over-flocculation results in poor formation and weak regions between flocks, which decreases paper strength. Various methods are used to measure the evenness of mass distribution in paper, including variance of scanned light transmission and image analysis techniques.
Adsorption of Starch on Cellulose and Pigments
Cellulose fibers have a negative charge, which comes from the acidic groups on cellulose. The number of these charged groups varies for different types of papermaking fibers. The charged groups affect the swelling properties and interactions of the fibers with chemical additives like starch. Cationic starch can be adsorbed by fibers and filler particles, and the amount of starch absorbed depends on the cationic starch concentration and the specifi c surface area of the fibers. Cellulose fines have a much larger specifi c surface area than fibers, which allows them to absorb more starch. Excess cationic starch in solution can be precipitated on cellulose fibers by aluminum hydroxide. There is competition between fibers and pigments for the cationic polymer in the papermaking furnish. Cationic starch adsorbs readily on kaolin particles due to the anionic charge on their faces. Anionic starches are adsorbed at low pH due to cationic charges on the edges of kaolin particles. Adsorption on calcium carbonate depends on the adsorbent concentration and is affected by the presence of electrolytes.
Retention of Pigments and Cellulose Fines
The pigments and cellulose fines added to paper improve its brightness, opacity, density, and printability. These materials have different weights and sizes, with pigments having a higher weight and smaller size than cellulose fibers. To successfully retain these materials in the paper, controlled flocculation of the papermaking furnish is required. Small particles must be retained in the sheet despite high-speed dewatering on the paper machine, and this retention is described as first-pass and total retention. To improve retention, electrostatic attraction between fibers and particles must be facilitated. This can be achieved by manipulating the pH or increasing the ionic strength of the furnish, or by introducing a cationic polyelectrolyte. A recent development is the use of dual polymer systems for retention. Paper grades with more than 30% filler content can be produced on modern, high-speed paper machines.
Sheet Bonding by Starch
Starch is used in paper-making to increase the bonding between fibers. When the initial fiber mat is dried, water recedes from the fiber surfaces, creating contact angles that bring adjoining fibers together. This generates hydrogen bonds between fibers, but bonding only occurs in patches where the fibers are closely drawn together. Starch adsorbs on cellulose and reinforces fiber-to-fiber linkages in regions of close physical contact, increasing the relative bonded area in the paper sheet.
In the past, unmodified starch was added to papermaking furnish to increase internal and surface strength while allowing lower levels of pulp refining for improvements in sheet bulk. However, retention of starch in the process water was low. More recent practice involves the use of cationic starch products that are widely used to improve paper strength. The addition of 0.5% – 2% cationic starch is often sufficient, and further starch addition can adversely affect sheet formation. Cationic starches and other cationic polymers are used as scavengers to aggregate and precipitate electrolytes and colloidally dispersed substances that can interfere with retention processes. Highly-charged cationic starches are used in closed water systems of paper machines.
Wet-end Sizing
During the process of making paper from wood pulp, resinous substances are dissolved, making the paper hydrophilic and susceptible to losing strength when exposed to water. This can cause problems in various processes such as printing and gluing. To decrease water sensitivity and maintain paper dimensions, the paper is sized. Wet-end sizing, which is different from surface sizing, involves adding sizing agents to the papermaking furnish prior to sheet formation. Sizing agents have both hydrophobic and polar components that interact with cellulose fibers and filler pigments. The most commonly used acid sizing system uses rosin and alum, while neutral and alkaline sizing use polynuclear systems of aluminum and sizing agents such as alkenyl succinic anhydride (ASA) and alkyl ketene dimer (AKD). Cationic starch is used as a retention aid for the size. A microparticle system is another sizing strategy that enhances retention and combines effective sizing with high paper strength.
Starch Selection for Wet-end Use
Starch is a commonly used additive in papermaking for a variety of purposes, including improving paper formation, drainage, retention of fillers and cellulose fines, size retention, internal and surface strength, and reducing wastewater pollution. The most frequently used starches are corn and potato, but there are also other options like wheat, rice, and tapioca starches. Cationic starches are modified by adding chemical groups like tertiary amino, quaternary ammonium, phosphate, zwitterion, aldehyde, acetal, and graft copolymers, and their degree of substitution (DS) ranges from 0 to 3. Cationic starches are not depolymerized to maintain a high hydrated molecular volume for bridging flocculation. The addition of cationic starch at different stages of the wet-end system depends on the desired results, with an average addition level of 7-9 kg/ton. A dispersion of cationic starch and unmodified starch produces a polyelectrolyte complex that enhances hydrodynamic radius for bridging flocculation between fibers and pigments. However, the complexes can become insoluble in water and interfere with retention. In an alternative route, starch is complexed with a polyamine, poly(vinyl amine), poly(ethylene imine) or chitosan to render it cationic, which may also lead to separation over time and lower molecular weight. Dialdehyde starch is a block reactive starch used for wet-strength improvement, and it can be added by spraying onto the papermaker’s wire.