The main sources of starch for papermaking are corn, potato, waxy maize, wheat, and tapioca. Starches come in powder form or slightly aggregated pearl starch. In the paper industry, unmodified (native) starch is rarely used except for laminates and the corrugating process. Most starches are modified through hydrolysis, oxidation, or derivatization.
Starches that are thinned out by acid-hydrolysis (Acid-hydrolyzed starches/ acid-treated starches) are made by breaking them down with acid. However, because they don’t keep their thickness very well, they can’t be used much in paper mills. They are mainly used for surface sizing at the size press or calender stack.
Oxidized starches are made by reacting starch with sodium hypochlorite or peroxide. They come in a variety of viscosities and are commonly used in papermaking as a low-cost surface sizing agent or coating binder. Bleached starches are a type of oxidized starch with low carboxyl content, and they require heat or high shear to disperse. They are used only for surface sizing. However, the use of oxidized starches has decreased due to their poor dispersing properties, which can affect pigment retention and increase the TTS content of paper mill discharges. Oxidation and depolymerization of starch can be done at the paper mill before using it in papermaking.
Starch ethers are made by mixing starch with a substance containing unsaturated molecules, then adding acid to adjust the thickness. Common substances used include ethylene oxide, epichlorohydrin, and propylene oxide. The reaction used to create starch ethers can happen in a water-based mixture or using a gas/solid process to prevent side reactions and meet environmental regulations. Starch ethers come in different thicknesses and are often used for coating and surface sizing. Starch esters, on the other hand, are created by mixing starch with acetic acid anhydride or acid chlorides. They are less commonly used in papermaking because they can break down at high pH levels.
Anionic starches are made by reacting starch with phosphoric acid or alkali metal phosphates or by adding carboxymethyl groups. This process is mainly used to add amphoteric properties to cationic corn starch, which is used in the wet end of the paper machine. Anionic starches with carboxymethyl substitution are used as thickeners in coating colors or as binders in coatings for specialty paper grades. Oxidized starches are already anionic but do not thicken, while potato starch has enough natural anionic charge to be amphoteric after cationization.
Grafted starches are made by combining ethylenically unsaturated monomers with starch through a process called free radical copolymerization. The resulting product is a mix of copolymer, homopolymer, and unreacted starch. Grafted starch has been used as a surface size on the metered size press, but it hasn’t been successful as a sole coating binder because paper mills prefer to blend starch and synthetic binders. The use of grafted starch is also restricted because the product must be free of unreacted monomer.
Hydrophobic starches are created through different reactions, such as mixing with octenyl-substituted succinic acid anhydride. They can work as a type of surface agent that can interact with latex in a coating mixture. This can impact the way the mixture behaves under different conditions. Despite their potential, hydrophobic starches are not commonly used in papermaking.
Aldehyde starches are created by using periodic acid/periodate ions to turn certain hydroxyl groups into aldehyde groups. Dialdehyde starch can then react with cellulose to make bonds that increase the strength of wet tissue and sanitary products.
Waxy maize starch comes from a specific type of corn that has only one type of starch molecule. This starch has high molecular weight and is naturally resistant to retrogradation. Cationic waxy starch is added to papermaking to help with pigment retention and bonding.
High-amylose corn starch is a type of starch that comes from corn with high amylose content. Amylose can be separated from regular starch using butanol or specific salt solutions. However, it’s expensive to produce and not easily dispersed, so it’s not widely used in papermaking.
Dextrins are made by heating starch with acid. They come in different thicknesses and colors and are mostly used in paper converting like making envelopes and laminating. In Europe, a thin dextrin is used as a coating binder for TCF paper.
Starch fibers and films can be made by extrusion or spraying alkaline starch into a concentrated solution of ammonium sulfate. These can be used as bonding agents in paper, but they are not commonly used.
Pregelatinized starches are made by drying a starch dispersion on a heated roll or with hot air. They can be rewetted with surfactants. Using pregelatinized starch makes it easier to prepare coating colors, but it’s not commonly used because it’s more expensive and difficult to make it uniform.
Mechanically-modified starches are made by using extrusion. This process can offer more options to modify starch, but it’s not commonly used in paper mills.
The paper industry’s recent developments have affected starch supply. By implementing statistical process control, there is now more awareness that the use of high-quality materials is crucial for producing high-quality paper. As a result, there is a greater emphasis on ISO guidelines for starch supplies and rigorous testing to maintain uniform quality.
In the past, the paper industry preferred to buy unmodified starch and modify it at the mill. However, the difference in products was sometimes greater than when supplied by the manufacturer. Now, modified starches with specific properties are preferred. Due to the growth in paper recycling, there is a shift towards using starch as a coating binder instead of synthetic materials.
Starch can be combined with synthetic monomers through emulsion copolymerization to create new products that could potentially be used for flocculation, sizing, modified rheological characteristics, bonding to a range of substrates, film formation, and effluent treatment. However, in order for these products to be viable for use in specific paper grades, it will be necessary to remove any hazardous residues and obtain approval from the Food and Drug Administration (FDA).
To introduce new starch products, significant technical services are necessary to adapt them to closed paper machine wet-end systems, deinked pulp, and high-speed paper coating.