What is the paper sizing process in a kraft paper making machine?

The paper sizing process is a crucial step in the production of kraft paper, and understanding its intricacies is essential for those involved in the industry. As a supplier of Kraft Paper Making Machine, I've had the privilege of witnessing firsthand the significance of this process in creating high-quality kraft paper. In this blog, we'll delve into what the paper sizing process entails in a kraft paper making machine.

Understanding Kraft Paper

Before we dive into the sizing process, it's important to understand what kraft paper is. Kraft paper is known for its strength and durability, which is primarily due to the kraft pulping process used to make it. This process involves treating wood chips with a mixture of sodium hydroxide and sodium sulfide, which breaks down the lignin in the wood, leaving behind long, strong cellulose fibers. These fibers give kraft paper its characteristic toughness and tear resistance.

The Purpose of Paper Sizing

Paper sizing is a process that involves applying a sizing agent to the paper to alter its properties. The main purposes of sizing in kraft paper production are to improve the paper's resistance to liquids, especially water and ink, and to enhance its surface properties. By doing so, the paper becomes more suitable for various printing and packaging applications. For example, in packaging, sized kraft paper can better withstand moisture, preventing the contents from getting wet and damaged. In printing, it ensures that the ink does not bleed or spread, resulting in sharp and clear images.

Types of Sizing Agents

There are two main types of sizing agents used in kraft paper production: internal sizing agents and surface sizing agents.

Internal Sizing Agents

Internal sizing agents are added to the pulp during the papermaking process. They are typically hydrophobic substances that bond to the cellulose fibers in the pulp. Common internal sizing agents include rosin, alkyl ketene dimer (AKD), and alkenyl succinic anhydride (ASA).

Rosin sizing has been used for a long time in the paper industry. It works by reacting with alum in the pulp to form a water - resistant layer on the fibers. However, rosin sizing is more suitable for acidic papermaking conditions.

AKD and ASA are more modern internal sizing agents. They are effective in neutral and alkaline papermaking environments, which are becoming more popular due to their environmental benefits and the ability to use calcium carbonate as a filler. AKD reacts with the hydroxyl groups on the cellulose fibers to form a covalent bond, creating a hydrophobic surface. ASA also reacts with the fibers but has a faster reaction rate compared to AKD.

Surface Sizing Agents

Surface sizing agents are applied to the surface of the paper after it has been formed. They can further improve the paper's surface properties and liquid resistance. Common surface sizing agents include starch, polyvinyl alcohol (PVA), and styrene - acrylate copolymers.

Starch is a widely used surface sizing agent. It can improve the paper's strength, smoothness, and printability. PVA is known for its excellent film - forming properties, which can provide a high level of water resistance. Styrene - acrylate copolymers can enhance the paper's surface strength and resistance to ink penetration.

The Sizing Process in a Kraft Paper Making Machine

Pulp Preparation

The first step in the papermaking process, including sizing, starts with pulp preparation. The wood chips are cooked and refined to obtain the pulp. During this stage, if internal sizing agents are used, they are added to the pulp in the appropriate proportion. The pulp is then mixed thoroughly to ensure that the sizing agent is evenly distributed among the fibers. The consistency of the pulp, which is the ratio of dry pulp to water, also affects the effectiveness of the sizing. A proper pulp consistency is necessary for the sizing agent to interact with the fibers effectively.

Paper Formation

After the pulp is prepared, it is fed onto the wire section of the kraft paper making machine. Here, the water in the pulp is drained through the wire, and the fibers start to form a web. The forming process is crucial as it determines the initial structure of the paper. The presence of internal sizing agents at this stage helps in starting to build the water - resistant properties of the paper.

Pressing

In the pressing section, the wet paper web is passed through a series of press rolls. The main purpose of pressing is to remove more water from the paper and to increase its density. During pressing, the internal sizing agents continue to bond with the fibers under the pressure. The pressure also helps in consolidating the paper structure, which can enhance the effectiveness of the sizing.

Drying

The paper then enters the drying section, where it is dried using steam - heated cylinders or infrared dryers. The heat in the drying process is important for several reasons. For internal sizing agents like AKD and ASA, the heat activates the chemical reactions between the sizing agent and the cellulose fibers. It also helps in evaporating the remaining water in the paper. The temperature and drying rate need to be carefully controlled. If the drying is too fast, the sizing agent may not have enough time to react properly. If it is too slow, it can lead to inefficiencies in the production process.

Surface Sizing

If surface sizing is required, it is usually done after the paper has been dried to a certain extent. The paper passes through a size press, where a solution of the surface sizing agent is applied to one or both sides of the paper. The size press can be a vertical or horizontal type. In a vertical size press, the paper passes between two rolls, and the sizing solution is applied to the rolls, which then transfer it to the paper. In a horizontal size press, the paper is immersed in a bath of the sizing solution.

After the surface sizing, the paper goes through another drying stage to remove the water from the sizing solution and to set the surface sizing agent on the paper surface. This second drying stage is also important for ensuring that the surface sizing agent forms a uniform and durable layer on the paper.

Quality Control in the Sizing Process

Quality control is an integral part of the paper sizing process. Various tests are conducted to ensure that the sized kraft paper meets the required standards.

Cobb Test

The Cobb test is used to measure the water absorbency of the paper. A circular area of the paper is exposed to water for a specified time, and the amount of water absorbed is measured. A lower Cobb value indicates better water resistance, which means the sizing process has been more effective.

Printing Tests

Printing tests are also carried out to evaluate the printability of the sized paper. These tests can include measuring the ink density, dot gain, and print contrast. If the sizing is done correctly, the ink should adhere well to the paper surface, and the printed images should be sharp and clear.

Physical Property Tests

Physical property tests such as tensile strength, tear strength, and burst strength are also important. Sizing can affect these properties, and the results of these tests can help determine if the sizing process has any negative impact on the paper's mechanical strength.

The Role of the Kraft Paper Making Machine

A well - designed kraft paper making machine plays a crucial role in the paper sizing process. The machine should be able to accurately control the addition of sizing agents, the pulp consistency, the forming, pressing, drying, and surface sizing operations.

For example, the dosing system for the sizing agents needs to be precise to ensure that the correct amount of sizing agent is added. The drying cylinders in the machine should be able to maintain a consistent temperature to activate the sizing reactions properly. The size press should be able to apply the surface sizing agent evenly across the paper surface.

As a supplier of Kraft Paper Making Machine, we ensure that our machines are equipped with advanced control systems and high - quality components to optimize the paper sizing process. Our machines are designed to be flexible, allowing for adjustments in the sizing process according to different production requirements.

Conclusion

The paper sizing process in a kraft paper making machine is a complex but essential part of producing high - quality kraft paper. By understanding the types of sizing agents, the steps involved in the process, and the importance of quality control, manufacturers can produce kraft paper that meets the diverse needs of the market.

If you are in the market for a reliable Kraft Paper Making Machine or are interested in learning more about the paper sizing process, we encourage you to contact us for more information and to discuss your specific requirements. Our team of experts is ready to assist you in making the best choices for your papermaking operations. We also offer Corrugated Paper Machine for those involved in the corrugated paper production. Let's work together to achieve high - quality paper production.

References

• "Handbook of Pulp and Paper Technology" by G. A. Smook
• "Papermaking: The Principles of Paper Stock Preparation and Paper Machine Operation" by J. D. Casey