Bleached hardwood kraft pulp (BHKP): properties and applications / Articles

Bleached hardwood kraft pulp (BHKP) is a product of kraft pulping and subsequent bleaching of hardwood species — birch, aspen, eucalyptus, poplar, beech and oak. It is one of the two key market pulp grades globally (along with softwood BSKP/NBSK). Due to its short and uniform fiber, BHKP provides paper and board with exceptional smoothness, opacity, formation uniformity and high absorbency. While softwood pulp is responsible for strength, hardwood pulp delivers surface quality, printing properties and softness of the final product.

Note: The request mentions the abbreviation BSKP, but historically and technically BSKP (Bleached Softwood Kraft Pulp) refers to bleached softwood pulp. The correct abbreviation for bleached hardwood pulp is BHKP (Bleached Hardwood Kraft Pulp).

Chemical Nature and Physical-Mechanical Properties

The basis of BHKP is cellulose fiber derived from hardwood. The key difference from softwood pulp is fiber length. The average fiber length of hardwood kraft pulp ranges from 0.8 to 1.5 mm (on average about 1 mm), whereas for softwood it is in the range of 2–4 mm. The short fiber creates a dense and smooth sheet structure, which is critical for high-quality printing and the softness of hygiene products.

Key quality indicators for bleached hardwood kraft pulp:

Brightness: not less than 88–89% ISO for top grades.
Fiber length: 0.8–1.5 mm depending on species (birch, eucalyptus).
Dirt count: minimal, ensuring the purity of the final product.
Viscosity: characterizes the degree of polymerization and mechanical strength.
Opacity: high, important for printing papers and board.

BHKP imparts to finished products:

Smoothness and softness — due to short fiber and high sheet formation ability.
Opacity — thanks to good fiber coverage, preventing show-through in printing.
Bulk — important for board and tissue.
Good absorbency — critical for hygiene products.

Classification, Grades and Standards

In Russia and CIS countries, requirements for bleached sulphate pulp from a mixture of hardwood species are established by GOST 28172-89 "Sulphate bleached pulp from a mixture of hardwood species. Specifications". The standard provides for grades LS-0, LS-1, LS-2, LS-3 and LS-4, which differ in brightness, mechanical strength and application. Products are supplied in bales weighing 200–250 kg, wrapped in kraft paper.

In international practice, BHKP is classified by the region of origin of raw materials:

NBHK (Northern Bleached Hardwood Kraft) — from northern species (birch).
SBHK (Southern Bleached Hardwood Kraft) — from southern species (eucalyptus).

Production Technology

The production of BHKP is similar to that of softwood kraft pulp and includes the following stages: chip preparation, kraft pulping in an alkaline solution (white liquor) at high temperature and pressure, washing, bleaching (mainly using ECF — Elemental Chlorine Free technology), drying and pressing into bales. Modern technologies are aimed at minimizing environmental impact and improving quality stability.

Applications

Bleached hardwood kraft pulp (BHKP) is used in segments requiring smoothness, opacity, softness and good formation.

Hygiene products (tissue). This is one of the largest consumption areas for BHKP. Short fiber provides softness, bulk and high absorbency for toilet paper, paper towels, napkins and facial tissues.

Printing and office paper. BHKP is a key component for the production of high-quality office paper (A4), coated and uncoated paper, as well as digital printing paper. It provides a smooth surface, good opacity and uniform coating application.

Board and packaging. BHKP is used in the production of folding boxboard (FBB), liquid packaging board (LPB), white top kraft liner (WKL) and other packaging boards where surface smoothness, stiffness and opacity are important.

Specialty papers. BHKP is used for label, thermal, glassine papers, as well as release liners.

Furnish blends. In paper production, BHKP is often used in a blend with softwood pulp (BSKP/NBSK) in a ratio from 30:70 to 70:30 to achieve an optimal balance between strength and surface quality.

Safety and Storage

Bleached hardwood kraft 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 properties. When working with cellulose dust, respirators are recommended.

Market and Trends

The global market for bleached hardwood kraft pulp (BHKP) is a large and dynamic segment of the forest products industry. Key producers are the countries of South America (Brazil, Uruguay, Chile), where eucalyptus is grown, as well as Northern Europe (Finland, Sweden) and Russia, where birch is used. The main demand drivers are the growth in consumption of hygiene products, the development of the packaging industry and the recovery of demand for printing paper in certain regions. BHKP is an exchange-traded commodity; hardwood pulp futures are traded on the Shanghai Futures Exchange, making pricing transparent.

Key trends: transition to totally chlorine-free bleaching technologies, tightening environmental requirements, digitalization of logistics and an increasing share of certified pulp from responsibly managed forests.

Conclusion

Bleached hardwood kraft pulp (BHKP) remains an indispensable raw material for the production of soft hygiene products, quality printing paper and packaging board. The unique properties of short hardwood fiber — smoothness, opacity, softness and good formation — make it a critically important component in modern paper furnishes. Selecting the pulp grade with optimal brightness and properties, as well as organizing stable supplies, requires an individual approach that takes into account the specific requirements of paper production.