How Kraft Liner Board Has Changed the World of Packaging? – Manufacturing and Industrial Insights to the Sustainable New Force

This industry insight talks about how kraft liner board has changed the world of packaging and especially their essentialness for secondary packaging, and for food and dairy industries.

What is Coated Kraft Paperboard?

Coated kraft paperboard or kraft liner refers to long-fiber kraft paper made from North American pine, with a coated surface and a yellow reverse side, similar to regular kraft paper. Coated kraft liner combines typically the high strength and toughness of kraft paper with printing performance that parallels closely to that of whiteboard cardboard or paper.

Kraftliners

The most highlighted feature of coated kraft liner, particularly the American-made variants (such as those by Sumurfit Kappa), is its water resistance that allows maintaining exceptionable strength even under freezing and refrigeration conditions. Some more advanced variants manufactured by European suppliers also provide waterproof features, despite some differences of liner made in the two regions.

With regard to the structure, the coated kraft liner is a kind of paper made with unbleached chemical pulp as the base layer, a middle layer composed of unbleached chemical pulp (or a blend of unbleached chemical pulp and CTMP – Chemithermomechanical Pulp), and a surface layer of bleached chemical pulp. The final product is coated with kaolin clay to enhance its smoothness and print quality.

Coated Kraft Liner is Widely in Food, Dairy, Drinks and Beverage Industries for Multipacking

The coated kraft liner plays a critical role in multipack packaging, especially in the food, dairy, beverage, and other industries requiring durable, visually appealing, and water-resistant packaging solutions. However, its reliance on a few global suppliers highlights a pressing need for innovation and localized alternatives.

Multipack packaging, also known as cluster packaging, involves bundling multiple products together to form an independent sales unit. This packaging method offers several benefits for product sales:

• Increased Consumer Appeal: Large, visually striking displays on shelves capture consumers’ attention and stimulate purchase interest.
• Simplified Price Management: Reducing the price of individual items often creates irreversible consumer expectations, making it difficult to revert to standard pricing. However, promotional pricing applied to multipacks helps protect the unit price of individual products and maintains the overall pricing system.
• Enhanced Brand Image: Well-designed multipack packaging can elevate a brand’s image. Unlike supermarket POP posters, multipack packaging delivers brand messages directly to consumers’ homes, acting as a powerful promotional medium, much like branded shopping bags.

Manufacturing Process of Coated Kraft Line

Coated kraft linerboard, a specialized type of packaging paper, is widely recognized for its superior strength, printability, and moisture resistance. It is extensively used in the packaging of products requiring high durability, such as beverages, dairy products, and frozen foods. The manufacturing process of coated kraft linerboard is a sophisticated procedure involving several precise steps, each designed to ensure the paper meets the rigorous standards required for its applications. Below is a detailed explanation of the production process as outlined in the patented method.

Pulp Preparation

The production of coated kraft linerboard begins with the selection of high-quality wood pulp. The pulp is carefully chosen and blended to create the desired characteristics in the final product. The layers of the paper are composed of different types of pulp:

• Surface Layer: This layer is made from bleached chemical pulp, with a specific blend of hardwood and softwood fibers. The proportion of bleached hardwood pulp ranges from 20% to 80%, while the bleached softwood pulp accounts for the remaining 20% to 80%. The bleached chemical pulp ensures a high degree of whiteness and printability on the paper’s surface.
• Core Layer: The core layer consists of a mixed pulp composition, including unbleached wood pulp (30% to 70%) and mechanical wood pulp (30% to 70%). This layer provides the necessary strength and bulk to the paper, supporting the surface and bottom layers.
• Bottom Layer: This layer is composed entirely of unbleached wood pulp, which ensures a solid foundation and contributes to the paper’s overall strength and structure.

Kraft Pulping – Credit: TAPPI

The combination of these various pulps in the multi-layered structure of the paper allows the final product to maintain an optimal balance between strength, flexibility, and printability.

Sheet Formation

Once the pulp is properly prepared, it is evenly distributed onto a forming wire, where it undergoes a process known as sheet formation. In this stage, approximately 90% of the water is removed from the pulp, causing the paper to begin taking shape. The consistency of the pulp for each layer is meticulously controlled to ensure uniformity:

• The surface layer pulp concentration is maintained between 1.0% and 5.0%.
• The core layer pulp concentration is controlled between 2.0% and 6.0%.
• The bottom layer pulp concentration remains between 2.0% and 5.0%.

This careful control of the pulp concentrations ensures that the paper forms consistently, without irregularities that could affect its final properties.

Pressing and Dehydration

The next critical step in the production process involves the removal of additional moisture through mechanical pressing. This step enhances the paper’s strength and smoothness by applying pressure to remove as much water as possible, while simultaneously improving the fiber bonding within the sheet. The pressing process consists of several stages, each applying specific pressures to achieve the desired properties:

• First Press: Pressures between 0.5 MPa and 7.5 MPa are applied to eliminate moisture and compact the paper.
• Second Press (Shoe Press): A higher pressure of 2.5 MPa to 8.0 MPa is used to further remove moisture and enhance the fiber strength.
• Third Press: This final pressing stage applies a pressure of 0 MPa to 5.5 MPa to refine the paper’s smoothness and ensure consistent thickness across the sheet.

Through this multi-stage pressing process, the paper’s strength and surface smoothness are significantly improved, preparing it for the next steps.

Drying and Surface Sizing

Following the pressing process, the paper is subjected to a controlled drying process to remove the remaining moisture. This is achieved by passing the paper through heated drying cylinders. The temperature at each stage of drying is carefully regulated to ensure optimal moisture removal:

• First Drying Stage: 60°C to 100°C.
• Second Drying Stage: 80°C to 120°C.
• Third Drying Stage: 90°C to 180°C.
• Fourth Drying Stage: 50°C to 90°C.
• Fifth Drying Stage: 0°C to 50°C.

As the moisture content is reduced, the paper becomes more rigid, and its physical properties begin to stabilize. In this stage, a starch-based surface sizing solution is applied. This sizing improves the paper’s resistance to moisture and enhances its printability. The starch solution is applied at a dry weight of 1-3 g/m², further enhancing the paper’s functionality and performance.

Calendering (Hard Pressing)

Calendering is a process that involves passing the paper through a series of rollers to achieve the desired surface smoothness and uniformity. The paper undergoes a hard calendering process, which applies high pressure to further refine its texture. This stage not only increases the paper’s smoothness but also ensures that the thickness is uniform across the entire sheet. The pressure used during calendering typically ranges from 20 kN/m to 70 kN/m, depending on the specific finish required.

Calendaring in Cardboard Manufacturing Processes – Credit: The Magical Engineering

Coating

In the coating stage, a fine coating mixture is applied to the surface of the paper. The coating consists of a combination of finely ground particles such as calcium carbonate, kaolin clay, and latex, which work together to improve the paper’s opacity, whiteness, and surface gloss. The coating is applied at a controlled weight of 15-30 g/m², and it serves several key purposes:

• It enhances the paper’s opacity and brightness.
• It improves surface smoothness and gloss, making the paper more suitable for high-quality printing.
• It optimizes the paper’s ink absorption, ensuring that printed images are sharp and vibrant.

Soft Calendering

After the coating is applied, the paper undergoes a second calendering process known as soft calendering. This step further enhances the paper’s surface smoothness and fine texture, improving its overall appearance. The pressure applied during soft calendering typically ranges from 40 kN/m to 70 kN/m.

Quality Control and Defect Detection

Throughout the production process, rigorous quality control measures are employed to ensure that the paper meets the highest standards. A sophisticated imaging system is used to detect and identify any surface defects or irregularities. This ensures that only the highest-quality paper moves forward to the next stages of the process.

Winding, Rewinding, and Cutting

Once the paper has undergone all necessary treatments, it is wound into large rolls. These rolls are then rewound and cut according to the customer’s specific requirements. The paper is subsequently ready for further processing and use in packaging applications.

Sustainability of Coated Kraft Line and Corporate Social Responsibility & Environmental Social Governance

The development of food-grade coated kraft liner boards represents a significant step forward in packaging innovation. By combining high-performance characteristics with environmentally conscious materials, these liners meet both the functional and sustainability demands of today’s food industry, aligning with the goals of both manufacturers and consumers who seek to make more responsible choices.

Sustainability of Coated Kraft Liner

Coated kraft liner is an increasingly popular choice in sustainable packaging due to its eco-friendly features. Made primarily from renewable wood fibers, it offers a biodegradable and recyclable alternative to plastic-based packaging. The manufacturing process of coated kraft liner also prioritizes sustainability by utilizing efficient production methods, reducing waste, and minimizing the consumption of water and energy. Additionally, the strength and durability of coated kraft liner reduce the need for excessive packaging, leading to lower material usage and less waste. Its ability to withstand various environmental conditions further enhances its sustainability by offering long-lasting protection for products, reducing the frequency of replacements and disposal.

In the context of packaging, the sustainability of coated kraft liner is further bolstered by certifications such as FSC (Forest Stewardship Council) and PEFC (Program for the Endorsement of Forest Certification), which guarantee that the materials used are sourced responsibly and come from forests that are managed in an environmentally and socially beneficial way. By promoting the circular economy, coated kraft liner supports recycling and reduces landfill waste, contributing to a more sustainable packaging ecosystem.

Corporate Social Responsibility (CSR) & Environmental, Social, Governance (ESG)

Coated kraft liner plays a pivotal role in advancing Corporate Social Responsibility (CSR) and Environmental, Social, and Governance (ESG) initiatives for businesses that use it. CSR involves ethical considerations such as the welfare of employees, community engagement, and environmental responsibility, and coated kraft liner aligns with these values by supporting ethical sourcing and production. By opting for sustainably sourced materials and ensuring minimal environmental impact, companies enhance their reputation and show a commitment to long-term social and environmental goals.

From an ESG perspective, coated kraft liner addresses all three pillars: environmental, social, and governance. Environmentally, it reduces reliance on fossil-based materials, lowers carbon footprints, and supports the circular economy through recyclability. Socially, it helps companies meet the growing consumer demand for more sustainable packaging options, enhancing brand reputation and consumer trust. Governance-wise, the transparency of the sourcing process and the use of certified materials ensure that businesses can demonstrate their commitment to sustainable practices, meeting the rising expectations of stakeholders, investors, and regulatory bodies.

As companies increasingly embrace sustainability in their operations, coated kraft liner stands as a crucial element of their packaging strategies, aligning their business practices with the broader goals of sustainability, ethical responsibility, and good governance.