Application of AKD wax powder in the paper industry

I. Overview of AKD Wax Powder

AKD wax powder, or alkyl ketene dimers, is a waxy, pale yellow or light yellow solid, sometimes appearing as a white to yellow paste. It is an unsaturated lactone, insoluble in water, with a melting point of approximately 51-52°C. AKD wax powder exhibits good solubility in solvents such as benzene and is stable in weak acid, weak alkali, or other penetrant environments. Its reactive ketone group is highly reactive, reacting with alcohols to form esters, with primary and tertiary amines to form amines, with acids to form anhydrides, and can also undergo esterification with the hydroxyl groups in cellulose. These unique chemical properties make AKD wax powder valuable in the papermaking industry. As a reactive, neutral papermaking sizing agent, AKD wax powder was invented as early as 1948 by Hercules Corporation in the United States. However, at that time, cationic starch and retention aids were not yet used in emulsion processes, resulting in poor emulsion stability. Furthermore, papermaking wet-end chemicals were relatively underdeveloped, leading to very low retention rates and hindering its widespread use. It wasn't until 1980 that China began using AKD sizing, and through continuous optimization, its quality and stability were significantly improved, enabling AKD to be widely used in medium-alkaline systems.

II. Advantages of AKD wax powder in paper sizing

(a) Achieving neutral sizing

Traditional paper sizing processes typically require the use of precipitants such as alum. However, AKD wax powder, as a neutral sizing agent, eliminates the need for these precipitants during the sizing process. This simplifies the sizing process and avoids problems associated with alum, such as paper aging and embrittlement. Furthermore, AKD wax powder sizing can be performed under slightly alkaline conditions, with a sizing pH of around 8.0, hence it is also known as alkaline sizing. This neutral sizing environment helps protect paper fibers and extends the paper's lifespan, making it particularly suitable for papers requiring high durability, such as archival paper and dictionary paper.

(ii) Improve the water resistance of paper

After curing, AKD wax powder imparts high water resistance to the paper surface. During the paper production process, paper treated with AKD effectively prevents moisture penetration, allowing it to maintain good physical properties and writing/printing performance even when exposed to water or humid environments. This is significant for papers that need to be used in humid environments, such as packaging paper and outdoor advertising paper. For example, in the packaging industry, using AKD-sized paper to package food and electronic products can prevent moisture damage and protect product quality.

(iii) Use of alkaline fillers

When using AKD wax powder for sizing, alkaline fillers such as calcium carbonate can be used. Compared with acidic fillers used in traditional acidic sizing processes, alkaline fillers have advantages such as wide availability, low cost, high whiteness, and fine particle size. Using alkaline fillers can improve the whiteness, opacity, and printability of paper, while also reducing paper production costs. For example, in the production of high-grade printing paper, using calcium carbonate as a filler can make the paper surface smoother and the printing effect clearer and more vibrant.

(iv) Improve paper durability

Because AKD wax powder is used in a neutral sizing environment, it avoids the damage of acidic substances to paper fibers, giving the paper excellent durability. Paper treated with AKD is less prone to yellowing, brittleness, and other signs of aging during long-term storage, thus better preserving important documents and materials. For example, using AKD sizing on archival paper ensures that archives maintain good physical properties and readability even after decades or centuries of preservation.

III. Application of AKD wax powder in different paper types

(a) Coated paper

Coated paper is the base paper for producing coated paper, and it has high requirements for surface properties and sizing effects. AKD wax powder, used as an internal sizing agent in the production of coated paper, can improve the paper's water resistance and surface strength, allowing it to better absorb coatings during subsequent coating processes and improving coating quality. At the same time, AKD sizing can also improve the printability of coated paper, making printed images clearer, more vibrant, and with higher color reproduction. For example, in high-quality printed materials such as brochures and magazines, using coated paper sized with AKD can produce exquisite printing results.

(ii) Copy paper

Copy paper needs to possess good water resistance and surface smoothness to ensure that problems such as paper jams and ink bleeding do not occur during the copying process. The application of AKD wax powder can meet these performance requirements for copy paper. Through internal or surface sizing, a uniform sizing film is formed on the surface of the copy paper, improving the paper's water resistance and surface strength. In practical use, copy paper sized with AKD produces clear copies with less blurring, meeting the needs of office and home copying.

(iii) Archival paper

The primary function of archival paper is to preserve important documents and materials for the long term; therefore, it requires extremely high durability and water resistance. AKD wax powder, as a neutral sizing agent, can be sizing under moderately alkaline conditions, effectively protecting the paper fibers and extending the paper's lifespan. Archival paper sized with AKD is less susceptible to the effects of moisture and air during long-term storage, maintaining its original properties and ensuring the safe preservation of archives.

(iv) Dictionary paper

Dictionary paper typically requires thin and lightweight paper while maintaining high strength and water resistance. The application of AKD wax powder improves the paper's water resistance and surface strength while ensuring its thinness and lightness. Dictionary paper treated with AKD sizing not only writes smoothly but is also less prone to tearing during use, meeting the requirements for dictionaries and other reference books.

(v) Writing paper

Writing paper is used for everyday writing, so it requires certain levels of ink absorbency and water resistance. AKD wax powder sizing allows writing paper to absorb ink while preventing ink seepage, keeping the paper surface clean. At the same time, improved water resistance prevents smudging caused by sweaty hands or contact with water during writing. For example, using AKD sizing on writing paper used by students in notebooks and exercise books can improve the writing experience and extend the paper's lifespan.

IV. Application Process and Precautions for AKD Wax Powder

(a) Emulsion preparation

AKD used as a paper sizing agent must be formulated as an emulsion. The particle size of the product is approximately 0.5-2 μm, and the emulsion is white. Because AKD emulsions are highly susceptible to hydrolysis, their shelf life is generally short, only 3-6 months. Strict control of process conditions is necessary during emulsion preparation to ensure the stability and uniformity of the emulsion. With advancements in synthesis and emulsification technologies, the solid content of AKD emulsions can now reach 40% or higher, and their hydrolysis and migration tendencies have been effectively limited. When preparing the emulsion, it is essential to select appropriate emulsifiers and stabilizers, and to carefully control parameters such as emulsification temperature and stirring speed to ensure the quality of the emulsion.

(II) Adhesive application process

AKD wax powder can be used as both an internal sizing agent and a surface sizing agent. When used as an internal sizing agent, AKD emulsion is typically added during pulp preparation to ensure uniform dispersion within the pulp. During this process, careful attention must be paid to the amount and timing of AKD emulsion addition to ensure its retention rate in the pulp. When used as a surface sizing agent, the AKD emulsion is applied to the paper surface using a surface sizing machine after paper forming. Surface sizing can further improve the paper's water resistance and surface properties. During the sizing process, it is also necessary to adjust sizing process parameters, such as sizing temperature and sizing pressure, according to factors such as paper type, quality requirements, and production processes.

(III) Precautions

When using AKD wax powder, it is important to pay attention to the dosage. Excessive dosage may cause powdering on the paper surface, severely affecting the printability of offset printing paper. Additionally, AKD is prone to hydrolysis at high temperatures; therefore, temperature control is crucial during paper drying and subsequent processing to prevent hydrolysis and inactivation of AKD. Furthermore, due to the short shelf life of AKD emulsions, it is essential to pay attention to the expiration date and avoid using expired emulsions, as this can negatively impact sizing performance.

V. Economic Benefits of AKD Wax Powder Application

(a) Reduce production costs

Using AKD wax powder as a sizing agent allows for the replacement of traditional acidic fillers with alkaline fillers such as calcium carbonate. These alkaline fillers are widely available and inexpensive, effectively reducing paper production costs. Simultaneously, because AKD wax powder achieves neutral sizing, it avoids problems associated with the use of precipitants such as alum, reducing paper waste and maintenance costs. For example, some large paper mills have achieved significant annual savings in raw materials and production costs by adopting the AKD sizing process.

(ii) Improve product quality and added value

The application of AKD wax powder can significantly improve paper quality, such as enhancing its water resistance, durability, and printability. This allows the paper to meet higher quality requirements, thereby increasing the added value of the product. High-quality paper is more competitive in the market and can bring greater economic benefits to enterprises. For example, some companies that produce high-grade packaging paper and printing paper have increased their product prices and expanded their market share by using AKD wax powder for sizing.

(III) Reduce environmental pollution

Traditional acid sizing processes use precipitants such as alum, generating large amounts of acidic wastewater and causing serious environmental pollution. AKD wax powder, as a neutral sizing agent, eliminates the need for these precipitants during the sizing process, reducing acidic wastewater discharge and contributing to environmental protection. Simultaneously, the use of alkaline fillers can reduce resource consumption in the papermaking process, achieving sustainable development. This not only aligns with national environmental policies but also helps companies establish a positive social image and improve their social and economic benefits.

VI. Development Trends of AKD Wax Powder in the Paper Industry

(I) Technological Innovation

With continuous technological advancements, the production and application technologies of AKD wax powder will also continue to innovate. In the future, more stable and efficient AKD emulsion preparation technologies may be developed, further improving the solid content and stability of the emulsion and extending its shelf life. Simultaneously, more advanced sizing processes will be explored to improve the retention rate and sizing effect of AKD wax powder in paper, while reducing production costs. For example, AKD wax powder may be modified using nanotechnology to enhance its dispersibility and reactivity, thereby improving sizing performance.

(II) Application Expansion

Beyond its existing applications in paper types, the use of AKD wax powder in the paper industry will continue to expand. As demands for paper performance increase, AKD wax powder may be applied to the production of more specialized papers, such as electronic packaging paper and food preservation paper. These papers have higher requirements for moisture resistance, water resistance, and antibacterial properties, and the special properties of AKD wax powder are expected to meet these needs. Simultaneously, AKD wax powder may also be used in combination with other functional additives to develop multi-functional paper products.

(III) Green and environmentally friendly

With increasingly stringent environmental regulations, AKD wax powder, as a relatively environmentally friendly sizing agent, will play an increasingly important role in the paper industry. In the future, manufacturers will place greater emphasis on the green production of AKD wax powder, reducing energy consumption and environmental pollution during the production process. Simultaneously, they will strengthen environmental monitoring and assessment during the application of AKD wax powder to ensure its environmental impact is minimized. For example, developing biodegradable AKD wax powder will further improve its environmental performance.

In conclusion, AKD wax powder has significant application value and broad development prospects in the papermaking industry. By continuously optimizing its production technology and application processes and fully leveraging its advantages, AKD wax powder will make a greater contribution to the development of the papermaking industry.