Unbleached kraft pulp: properties, grades and applications / Articles

Unbleached kraft pulp (UKP) is a fibrous semi-finished product obtained by chemical pulping of softwood and hardwood without a subsequent bleaching stage. Unlike bleached pulp, the unbleached product retains natural lignin, which gives the material a characteristic light-brown to dark-brown colour, as well as high stiffness, strength and resistance to mechanical stress. These properties make unbleached pulp indispensable for the production of strong packaging, containerboard and technical paper grades, where appearance is not decisive, and mechanical reliability and cost efficiency come to the fore.

Chemical Nature and Physical-Mechanical Properties

The basis of unbleached pulp is cellulose fiber with a residual lignin content. The degree of delignification is assessed by the Kappa number (permanganate number), which characterizes the amount of lignin in the fiber. For unbleached softwood kraft pulp, the Kappa number typically ranges from 25 to 60 units and above, depending on the required degree of cooking and the application. For unbleached hardwood kraft pulp, Kappa numbers are generally lower. The higher the Kappa number, the more lignin remains in the fiber, which gives the material greater stiffness but may reduce formability and brightness.

Key advantages of unbleached pulp are determined by the preservation of the natural properties of the fiber:

High mechanical strength. Long softwood fibers combined with residual lignin provide high tensile, burst and tear strength. Unbleached softwood kraft pulp is the benchmark for strength among all pulp grades.
Stiffness and bulk. Residual lignin gives the fiber stiffness, which is important for containerboard and corrugated board that must withstand stacking and transportation.
Cost efficiency. Eliminating the bleaching stage reduces production costs, chemical and energy consumption, and environmental impact.
Natural colour. The brown tint of unbleached pulp is natural and does not require additional dyeing, which is important for kraft paper and kraft liner.

Classification, Grades and Standards

In Russia and CIS countries, requirements for unbleached pulp are regulated by a system of state standards that differ depending on the pulping method and wood species.

Unbleached softwood kraft pulp is produced according to GOST 11208-82 "Sulphate unbleached wood (coniferous) pulp. Specifications". The standard establishes three main grades differing in application and quality indicators:

NS-1 — for high-strength technical and packaging paper, base paper for moisture-resistant sandpaper, telephone, cartridge paper, base for paraffined paper and special paper grades. This is the strongest grade with the highest Kappa number.
NS-2 — for sack paper, light-proof paper, paper for textile spools and cones, base for adhesive tape, for smooth layers of board, boxboard, waterproof, upholstery, cushioning, footwear board and other paper and board grades.
NS-3 — for moisture-resistant paper, base paper for inner layers of decorative laminate, board for end caps of filter elements.

Unbleached softwood sulphite pulp is produced according to GOST 6501-82 and includes grades Zh-0, Zh-1, Zh-2, Zh-3, Zh-4, Zh-5, which are used for thin printing paper, high-strength packaging paper, greaseproof paper, newsprint, wallpaper, insulating and wrapping paper, as well as sanitary-hygienic paper.

In addition, unbleached hardwood kraft pulp is produced, which is used in blends with unbleached softwood pulp for the production of packaging and wrapping paper grades. Hardwood short fiber in unbleached form improves formation and surface smoothness while maintaining sufficient strength.

Production Technology

The production of unbleached pulp includes chip preparation, cooking in digesters under pressure at 160–175 °C using white liquor (a mixture of sodium hydroxide and sodium sulfide), washing, screening and cleaning. The key difference from bleached pulp production is the absence of the bleaching stage, which preserves lignin and the natural properties of the fiber. Cooking time ranges from 1.5 to 3 hours depending on the required degree of delignification (Kappa number). The finished product is dried and pressed into bales.

Modern technologies are aimed at increasing fiber yield, reducing chemical and energy consumption, and minimizing environmental impact. Unbleached pulp is considered a more environmentally friendly product compared to bleached pulp, as it does not require chlorine-containing reagents and the associated emissions.

Applications

Kraft liner and containerboard. This is the largest consumption area for unbleached pulp. Kraft liner is produced from 100% unbleached pulp and is used for flat layers of corrugated board, transport packaging, boxes and crates requiring increased strength and puncture resistance. Unbleached kraft liner is the optimal solution for packaging products where mechanical reliability and resistance to external influences are important.

Sack paper. Unbleached softwood kraft pulp is the main raw material for the production of sack paper, which is used for packaging bulk materials, construction mixtures, sugar, flour and other products. High strength and tear resistance ensure the reliability of sacks during transportation and loading.

Corrugated board and packaging. Unbleached pulp is used for the production of corrugated board, which finds application in logistics, e-commerce and consumer goods production. The growth of online trade stimulates demand for strong and inexpensive packaging, which supports unbleached pulp consumption.

Technical paper and board grades. Unbleached pulp is used for the production of moisture-resistant paper, base paper for decorative laminate, electrical insulating paper, filter materials, cushioning and footwear board. It is also used in the production of wallpaper, wrapping and insulating paper.

Electrical insulating pulp. There is a special type of unbleached softwood kraft pulp for electrical insulating applications, which is characterized by increased dielectric properties and is used in the production of cable paper and insulating materials.

Safety and Storage

Unbleached pulp in commercial form (dry bales) does not present a chemical hazard. It is a combustible material, so fire safety rules must be observed during storage. Bales should be stored in dry, well-ventilated areas, protected from direct moisture and contamination, to prevent the development of microorganisms and loss of strength properties. When working with cellulose dust generated during repulping, respirators are recommended.

Market and Trends

The global unbleached kraft pulp market demonstrates steady growth. According to industry estimates, the market size is around USD 27–29 billion, with a compound annual growth rate in the medium term estimated at 5–6%. Key demand drivers include the growth of e-commerce and logistics requiring strong packaging, tightening environmental requirements and the shift to recyclable mono-material fiber-based packaging, as well as the general trend away from plastic in packaging.

Unbleached softwood pulp occupies the dominant market share (more than half), providing maximum strength for packaging materials. Unbleached hardwood pulp demonstrates higher growth rates, as it is used in blends to improve formability and reduce cost. The largest producers and consumers are in North America, Europe, and the Asia-Pacific region.

A key trend is competition with recycled fiber (waste paper), which is used in corrugated board production to reduce costs. However, for critical packaging solutions requiring maximum strength and durability, unbleached virgin pulp remains indispensable. Producers with access to quality raw materials and modern pulping technologies occupy stable competitive positions.

Conclusion

Unbleached kraft pulp (UKP) remains a fundamental raw material for the production of strong packaging, kraft liner, sack paper and technical board. The absence of bleaching preserves the natural properties of the fiber, ensuring high mechanical strength, stiffness and cost efficiency. Understanding the grade composition according to GOST 11208-82 and GOST 6501-82, production technologies and applications is the basis for competent material selection and the organisation of efficient supplies.