1. Introduction
2. Materials and Methods
2.1 Materials
2.2 Pretreatment of wood chips and preparation of CTMP
2.3 Measurement of physical properties
2.4 Pitch analysis
3. Results and Discussion
3.1 Refining energy consumption and stock throughput
3.2 Fiber length and fines
3.3 Shive contents
3.4 Physical properties of CTMP
3.5 Optical properties of CTMP
3.6 Pitch comparison
4. Conclusions
1. Introduction
Chemi-thermomechanical pulping (CTMP) is a pivotal process in the pulp and paper industry, combining both mechanical and chemical treatments to produce high-yield pulp with desirable fiber properties and quality.1,2) By subjecting wood chips to a combination of chemicals, heat, and mechanical refining, CTMP enhances fiber separation and reduces the amount of energy required compared to purely mechanical pulping methods.3,4,5) This balance between mechanical and chemical treatments results in a product that is well-suited for various paper grades, including newsprint, printing, and writing papers.6,7,8)
Traditionally, CTMP production has relied heavily on homogeneous wood sources such as Pinus densiflora. This species is favored for its consistent fiber characteristics, which contribute to predictable and reliable pulp performance.9,10) However, the increasing incidence of pinewood nematode infestations poses a significant threat to these traditional wood sources. Pinewood nematode (Bursaphelenchus xylophilus) is a microscopic roundworm that causes pine wilt disease, a condition that leads to the rapid wilting and death of affected pine trees.11,12,13) This infestation has become a major concern in regions such as Korea, where it disrupts forest ecosystems and results in substantial economic losses due to decreased timber availability.14,15,16)
The pinewood nematode infestation problem calls for innovative solutions to sustain pulp production and promote sustainable forest management.17,18) One potential solution is the use of mixed wood chips sourced from clear-cut areas infested with pinewood nematodes. These mixed wood chips comprise a variety of species, including both softwoods and hardwoods, which introduce a diverse composition that could potentially influence the pulping process and the properties of the resulting pulp. The use of such mixed wood chips could help mitigate the raw material shortages caused by pine wilt disease and contribute to the overall sustainability of forest resources.
Previous studies have indicated that mixed wood chips might exhibit advantageous chemical compositions for pulping processes. For instance, they may have higher holocellulose content, which is beneficial for pulp yield and fiber quality, and lower lignin content, which reduces the energy required for refining and improves fiber separation. However, the presence of nematode-induced changes in wood structure and chemistry necessitates a thorough investigation to understand their impact on CTMP production. The chemical alterations and physical degradation caused by pinewood nematodes could potentially affect the efficiency of the pulping process and the quality of the final pulp product.19,20,21)
This study aims to characterize the CTMP process using mixed wood chips from pinewood nematode-infected forests, comparing it to traditional CTMP production using Pinus densiflora chips. By analyzing key parameters such as chemical composition, refining efficiency, fiber properties, and pulp quality, we seek to determine the viability of using mixed wood chips as an alternative raw material. The chemical composition analysis will focus on the content of holocellulose, lignin, extractives, and ash, which are critical factors influencing the pulping process and pulp properties. The refining efficiency will be assessed by measuring the energy consumption and stock throughput, providing insights into the operational advantages and cost savings associated with mixed wood chips.
Additionally, this research explores the potential benefits of utilizing wood from nematode-affected areas. One of the primary advantages is the enhanced throughput during the refining process, which could lead to increased production efficiency and reduced energy consumption. This is particularly important for pulp mills, as energy costs represent a significant portion of the overall production expenses. By utilizing mixed wood chips, mills could achieve substantial cost savings and improve their operational efficiency.
Another key aspect of this study is the evaluation of fiber properties and pulp quality. The fiber length, fines content, shive content, bulk properties, tensile strength, tear strength, and optical properties of the CTMP produced from mixed wood chips will be compared to those produced from Pinus densiflora chips. Understanding these properties is crucial for determining the suitability of mixed wood chips for various paper grades and applications.
For instance, the fines content and shive content can affect the smoothness and printability of the paper, while the tensile strength and tear strength are important for the durability and performance of the final product.22,23,24)
The findings from this research could have significant implications for the pulp and paper industry. By demonstrating the feasibility and advantages of using mixed wood chips from nematode-infested areas, this study provides a pathway for mills to adapt to the challenges posed by pinewood nematode infestations. It highlights the potential for mixed wood chips to enhance refining efficiency, maintain pulp quality, and support sustainable forest management practices. Moreover, this approach contributes to the broader goal of sustainable resource utilization by promoting the effective use of otherwise compromised wood resources.
The characterization of CTMP using mixed wood chips from pinewood nematode-infected forests represents a promising avenue for addressing the raw material shortages and economic impacts of pine wilt disease. This study aims to comprehensively understand the chemical, mechanical, and optical properties of the resulting pulp, offering valuable insights for the pulp and paper industry. By exploring the potential of mixed wood chips as an alternative raw material, this research supports the development of sustainable and resilient pulping practices that can adapt to the evolving challenges of forest resource management.
2. Materials and Methods
2.1 Materials
To evaluate the potential of using mixed wood chips from nematode-infested forests for CTMP production, pine chips were sourced from a clear-cut area in Jeseoksan Mountain, Geoje-si, Gyeongnam Province. Over 60% of the pine trees in this region were infected with pine wilt nematodes. After clear-cutting, the infected and healthy trees were chipped on-site using mobile chippers, resulting in a heterogeneous mixture of wood chips (Fig. 1(a)). Over 60% of the pine trees in this region were afflicted with pine wilt nematodes. Following the clear-cutting, both the infected and healthy trees were chipped on-site using portable chippers. These blended chips were subsequently utilized in the CTMP process (Fig. 1(a)). Because local loggers participated in the cutting process, the precise composition ratio of each tree species could not be ascertained.
For comparison, control pine chips (Pinus densiflora) were obtained from Jeonju Paper Co. Ltd. in Korea (Fig. 1(b)). These control chips were sourced from healthy pine trees, providing a benchmark for evaluating the properties of CTMP produced from the mixed wood chips. The chemical composition of two wood chips is shown in Table 1.
Table 1.
Chemical characteristics of wood chips
| Holocellulose (%) | Extractives (%) | Lignin (%) | Ash (%) | |
| Pine chips | 63.1±3.6 | 3.4±1.4 | 32.2±3.7 | 0.2±0.1 |
| Mixed wood chips | 74.1 ± 0.3 | 4.4 ± 0.3 | 20.9 ± 0.1 | 0.6 ± 0.1 |
2.2 Pretreatment of wood chips and preparation of CTMP
Before manufacturing CTMP, the wood chips were washed with water to remove contaminants and impurities. The washed wood chips were then impregnated at 40°C for approximately 12 hours. Subsequently, they were placed in an autoclave (DS-PAC 40, Lab House Co., Korea) and steam-treated at 90°C for 30 minutes.
For the manufacture of CTMP, 3% NaOH and Na₂SO₃, based on the oven-dried weight of the wood chips, were added to the digester with a wood-to-liquid ratio of 1:4. The wood chips in the liquor were then subjected to immersion and heat treatment in a laboratory digester (Duko, Korea) at 120°C for 60 minutes.
The pretreated and liquor-immersed wood chips were fed into a single disk refiner (KOS1, Korea) equipped with a 12-inch lightweight plate that includes both a chip-breaking zone and a defibration zone (refer to Fig. 2). The plate, made of an aluminum alloy base and stainless steel vertical bars, incorporated both a chip-breaking and a defibration zone (refer to Fig. 2).
The presteamed chips were refined at a rotational speed of 1500 rpm until desired pulp fiber characteristics were achieved. Stock throughput was determined by measuring the amount of stock discharged from the refiner. Pulp production efficiency was compared with power consumption during the refining process. Following refining, a small bundle of wood fibers (shives) was removed using a Sommerville Screen (DM-850, Daeil Machinery Co., Korea). The screen was equipped with a slot screen of 0.15 mm width and 45 mm length. The percentage of shives remaining on the slot screen was calculated using the following formula:
To achieve a target freeness of approximately 150 mL CSF, as determined by ISO 5264-1, the shive-free pulps were further beaten using a valley beater (DM-822, Daeil Machinery Co., Korea).25)
2.3 Measurement of physical properties
To determine mean fiber length and fines content of CTMP fibers, an FQA-360 fiber quality analyzer (Optest Equipment Inc., Canada) was employed. For the evaluation of physical and optical properties, handsheets with a basis weight of 60 g/m2 were prepared using a laboratory handsheet former. Tensile strength and tear strength were measured according to ISO 1924-1 and ISO 1974, respectively. Brightness and opacity were assessed using an Elepho Spectrophotometer (Lorentzen & Wettre, Sweden).26,27)
2.4 Pitch analysis
To analyze the pitch content in CTMP, the quantitative method described by Nam et al. (2015) was adopted.28) Sudan IV dye (CI-26105, Tokyo Chemical Ind. Co. Ltd., Japan) was used to selectively stain the hydrophobic pitch present in the CTMP (refer to Fig. 3). Images of the stained pitch particles were captured using a stereomicroscope (Olympus BX51TF, Japan) equipped with a ×4 objective lens. Image analysis software (Axiovision 4, Carl Zeiss Vision, Germany) was then employed to quantify the pitch content based on the captured images.
3. Results and Discussion
3.1 Refining energy consumption and stock throughput
The increased energy requirement for refining the pine chips can be attributed to the inherent properties of the wood. Pine chips typically have a denser and more resinous structure compared to the mixed chips, which consist of various wood species. This variability in species composition among the mixed chips can influence refining energy consumption and stock throughput during the refining process. Fig. 4 compares the refining energy consumption and stock throughput of CTMP prepared from the pine chips and the mixed chips sourced from clear-cut forests in the nematode-infested area.
The higher resistance of pine chips necessitated greater energy input to achieve the desired level of fiber separation and pulp quality. In contrast, the mixed chips, due to their varied species composition, might exhibit lower average resistance to mechanical processing, thereby requiring less energy. The lower stock throughput observed with the pine chips under identical refining energy conditions further supports the notion of increased processing difficulty. The reduced throughput implies that the refining process is less efficient with the pine chips, potentially due to the need for more aggressive mechanical actions to break down the fibers adequately. These findings highlight the challenges associated with using the pine chips for CTMP production. The higher energy consumption and reduced throughput could impact the overall cost-effectiveness and efficiency of the pulp manufacturing process. In contrast, the mixed chips, benefiting from the diverse properties of different species, may offer a more balanced and energy-efficient refining process.29)
Therefore, optimizing the refining conditions and exploring potential pre-treatment methods to reduce the energy requirements and improve throughput when using pine chips could be beneficial for industrial applications. Additionally, investigating the specific contributions of different species in the mixed chips could provide insights into optimizing the blend for enhanced refining efficiency. Further research is recommended to explore these optimization strategies and their potential impact on the quality and economic viability of CTMP produced from both pine and mixed chips.
3.2 Fiber length and fines
Fig. 5 compares the mean fiber length and fines content of CTMP prepared from the pine chips and the mixed chips sourced from clear-cut forests in the nematode-infested area. Before refining, the mixed chips exhibited a shorter mean fiber length compared to the pine chips. However, after refining, the CTMP produced from the mixed chips showed a slightly longer mean fiber length than that produced from the pine chips. Additionally, the refining process resulted in the generation of more fines from the pine chips compared to the mixed chips.
The observed changes in fiber length and fines contents during the refining process can be attributed to the differing properties of pine and mixed chips. Initially, the shorter mean fiber length of the mixed chips may be due to the diverse species composition, which includes species with inherently shorter fibers. In contrast, the pine chips, being more uniform, have longer fibers before refining. The observed increase in fiber length of CTMP produced from the mixed chips after refining indicates that the refining process may have been more effective in maintaining or even extending the fiber length of the mixed species compared to pine chips alone. This could be due to the varied mechanical properties of the mixed species, which might collectively respond better to the mechanical refining process, resulting in less fiber shortening.
On the other hand, the pine chips generated a slightly higher amount of fines during refining, with fines content for CTMP from pine chips measured at 28%, compared to 26% for CTMP from mixed chips. This marginal increase in fines content can be explained by the more resinous and dense structure of pine wood, which may lead to a greater degree of fiber breakage when subjected to the mechanical forces of refining. Although the difference in fines content is not substantial, it still indicates that pine chips are somewhat more prone to fiber damage.
This increase in fines content, even though modest, could potentially impact the quality of the pulp by slightly increasing the fines content, which may affect the strength and other properties of the final paper product. It suggests that while both types of chips undergo significant fiber shortening and fines generation during refining, the pine chips might require more careful handling or specific refining conditions to minimize fiber damage and optimize pulp quality. Therefore, it is essential to optimize the refining process to minimize fines generation and preserve fiber length, especially when using the pine chips. Exploring pretreatment methods or adjusting refining parameters could help achieve better outcomes.30,31,32) Additionally, understanding the individual contributions of different species in the mixed chips could provide valuable insights for optimizing the blend to achieve desirable fiber characteristics and refining efficiency. Further research in these areas is recommended to enhance the quality and economic viability of CTMP produced from both pine and mixed chips.
3.3 Shive contents
Fig. 6 shows that there is no difference in shive contents for CTMP prepared from the pine chips and the mixed chips sourced from clear-cut forests in the nematode- infested area. This observation suggests that both types of chips exhibited similar levels of incomplete fiber separation despite their differing physical properties and responses to the refining process. Shives are coarse, unrefined fragments of wood that remain after the refining process and can negatively impact the quality of the pulp by creating defects in the final paper product.33,34) The similar shive content between CTMP from pine chips and mixed chips indicates that the refining process was equally effective (or ineffective) in breaking down the larger wood fragments in both types of chips. The lack of difference in shive contents could be influenced by the use of chemicals such as sodium hydroxide (NaOH) and sodium sulfite (Na2SO3) in the CTMP process. These chemicals help to soften lignin and hemicelluloses in the wood, facilitating mechanical fiber separation.35,36,37)
NaOH acts as a strong alkaline agent that swells fibers and breaks down lignin, potentially reducing shive content by minimizing the likelihood of coarse, unrefined fragments. Similarly, Na2SO3 sulfonates lignin, making it more soluble and aiding in fiber separation.38,39,40) The combined effect of these chemicals can lead to more efficient fiber breakdown, thus potentially explaining the lack of difference in shive contents between the pine and the mixed chips.
3.4 Physical properties of CTMP
The bulk properties of CTMP prepared from the pine chips and the mixed chips sourced from clear-cut forests in nematode-infested areas were analyzed and compared. The results, illustrated in Fig. 7, indicate a slight difference in the bulk of CTMP derived from these two sources. Specifically, the CTMP produced from the mixed chips exhibited approximately 6% greater bulk compared to that from the pine chips. The observed increase in bulk for CTMP prepared from the mixed chips compared to the pine chips can be attributed to several factors inherent to the mixed chips sourced from nematode-infested areas.
Nematode infestation may induce stress responses in trees, potentially altering the chemical composition and physical structure of wood fibers, resulting in a more voluminous and less densely packed pulp. Mixed chips, comprising various species and different proportions of hardwood and softwood, contribute to a diverse range of fiber morphologies, enhancing the bulk properties of the resulting CTMP. In contrast, pine chips, being more homogeneous, produce a more uniform and denser pulp.
Additionally, clear-cutting practices in nematode-infested areas may include a wider age range of trees, including younger, less dense wood with higher porosity and lower lignin content, contributing to increased pulp bulk.
The increased bulk of CTMP from the mixed chips has practical implications for its end-use applications, enhancing the absorbency and softness of paper products and making it suitable for products like tissue and towels.41) However, it may also impact the strength properties of the paper.
The analysis of tensile strength for CTMP prepared from the pine chips and the mixed chips sourced from clear-cut forests in nematode-infested areas reveals that the CTMP from the mixed chips exhibits higher tensile strength compared to that from the pine chips, as shown in Fig. 8. This increase in tensile strength can be attributed to the diverse fiber characteristics present in the mixed chips, which include various wood species and potentially a mix of hardwood and softwood fibers. These mixed fibers likely provide superior inter-fiber bonding and structural integrity, resulting in stronger pulp.42)
Moreover, the nematode infestation and clear-cutting practices may include younger wood with different fiber properties, such as higher flexibility and better bonding capabilities. This variation in fiber morphology and chemical composition can enhance the tensile strength of CTMP produced from mixed chips. The higher tensile strength of this CTMP has practical implications, particularly for applications requiring durable and strong paper products. Therefore, selecting mixed chips as a raw material can be advantageous for producing high-strength pulp, highlighting the importance of understanding and utilizing the properties of different wood sources in pulp and paper manufacturing.
Fig. 9 compares the tear strength of CTMP prepared from the pine chips and the mixed chips sourced from clear-cut forests in nematode-infested areas. The results indicated that while the tear strength of CTMP from the mixed chips was slightly higher than that from the pine chips, the difference was not statistically significant. This suggests that the tear strength of CTMP is largely comparable regardless of the raw materials used.
As shown in Fig. 5, the mean fiber length of CTMP from the mixed chips was slightly longer than that from the pine chips. Since tear strength is greatly influenced by fiber length, the marginally longer fibers in the mixed chips may contribute to the observed slight increase in tear strength.43,44) However, the lack of a significant difference in tear strength implies that the fiber lengths in both sources are sufficiently similar. This indicates that while mixed chips may offer some advantages in bulk and tensile strength due to their diverse fiber characteristics, their impact on tear strength is minimal. Therefore, both pine and mixed chips can be effectively used to produce CTMP with adequate tear resistance, highlighting the importance of considering fiber length and other processing conditions in pulp production.
3.5 Optical properties of CTMP
Fig. 10 displays the optical properties of CTMP prepared from the pine chips and the mixed chips sourced from clear-cut forests in nematode-infested areas. The results showed no significant difference in optical properties between CTMP derived from the pine and mixed chips. This indicates that the types of wood chips used, whether homogeneous pine or heterogeneous mixed species, does not substantially affect the optical characteristics of the resulting pulp.
The similarity in optical properties, such as brightness and opacity, suggests that the chemical composition and processing conditions of the CTMP are consistent across both types of raw materials. This uniformity is advantageous for the pulp and paper industry, as it implies that either the pine chips or the mixed chips can be used interchangeably without affecting the visual quality of the final paper products. Consequently, TMP mills have the flexibility to utilize available raw materials without compromising the optical performance of CTMP. This finding underscores the reliability of the CTMP process in delivering consistent optical properties, irrespective of the differences in wood chip sources.
3.6 Pitch comparison
Fig. 11 displays the mean pitch number and area of CTMP prepared from the pine chips and the mixed chips sourced from clear-cut forests in nematode-infested areas. The results indicate that CTMP from the mixed chips had a lower number and smaller area of pitches per unit area compared to CTMP from the pine chips. This difference suggests that the mixed chips may contain fewer resinous components or that the processing conditions for mixed chips are more effective in reducing pitch content.
The lower pitch number and smaller pitch area in CTMP from mixed chips can be advantageous for several reasons. Reduced pitch content can enhance the quality and cleanliness of the pulp, leading to fewer issues during papermaking, such as pitch deposits on equipment and finished products. Additionally, lower pitch levels can improve the overall efficiency of the pulp and paper production process by minimizing downtime and maintenance requirements related to pitch problems.45,46,47)
These findings indicate that utilizing mixed chips from nematode-infested areas can result in CTMP with better pitch characteristics than using the pine chips alone. This advantage, combined with the observed improvements in bulk and tensile strength, makes mixed chips a valuable raw material for CTMP production.
4. Conclusions
The study compared CTMP produced from pine chips and mixed chips sourced from nematode-infested areas in Korea. Results indicated that pine chips required higher refining energy and exhibited lower stock throughput, suggesting greater processing challenges. CTMP derived from mixed chips demonstrated superior fiber length, lower fines content, improved bulk, and higher tensile strength, making it well-suited for durable paper applications. While tear strength and optical properties were comparable between both sources, CTMP from mixed chips exhibited a lower pitch content, leading to improved pulp cleanliness and processing efficiency.
Overall, mixed chips from nematode-infested mountains in Korea offer significant advantages in terms of energy efficiency, fiber quality, and pitch content. This makes them valuable raw materials for CTMP production, contributing to the economic and environmental sustainability of the pulp and paper industry in Korea. The findings highlight the potential of utilizing mixed chips as a sustainable alternative to traditional pine chips in the production of high-quality pulp and paper products.
