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Diese Studie untersucht das Potenzial von Hanf als nachhaltigem Rohstoff für die Papierproduktion und konzentriert sich dabei auf seine Bast-, Hurd- und Stoppelfasern. Die Forschungsergebnisse heben die Vorteile von Hanf gegenüber traditionellen Holzfasern hervor, darunter sein schnelles Wachstum, hohe Erträge und geringere Umweltauswirkungen. Der Artikel enthält eine detaillierte Analyse der chemischen und auflösenden Eigenschaften von Hanfstoppeln und bietet spezifische Verarbeitungsempfehlungen. Es vergleicht die Leistungsfähigkeit von Hanffasern mit traditionellen Kiefernfasern und zeigt, dass Hanffasern unabhängig voneinander verwendet oder mit anderen Faserquellen vermischt werden können, um Papierprodukte mit unterschiedlichen Festigkeitseigenschaften herzustellen. Die Studie kommt zu dem Schluss, dass Hanfstoppeln eine praktikable und nachhaltige Alternative zu herkömmlichen Holzfasern darstellen und sowohl zur ökonomischen als auch zur ökologischen Dimension der Papierindustrie beitragen.
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Abstract
Hemp (Cannabis sativa L.) is widely cultivated for seed oil and fiber; however, significant portions of the plant, particularly stubble and roots, remain underutilized in the field. To address this inefficiency and explore sustainable alternatives to wood-based pulp, this study aimed to evaluate the potential of hemp stubble including stems, roots, and hurd fibers as a raw material for papermaking. Chemical composition analyses were conducted on bast and hurd fibers from different parts of the plant to assess their suitability for pulping. Distinct differences in cellulose, hemicellulose, and lignin content were identified, supporting the need for separate cooking of bast and hurd fibers, while stubble and roots were treated as whole due to their complex anatomical structure. Pulping trials showed that bast fibers produce high alpha-cellulose content and favorable optical properties, making them suitable for high-quality paper applications. Hurd fibers, due to their higher hemicellulose content, were found to be well-suited for blending with softwood kraft or recycled fibers in packaging and corrugated paper products. These results demonstrate the technical feasibility of using hemp stubble and roots in paper production. In addition to reducing agricultural waste, the use of hemp pulp supports sustainable development goals by promoting renewable raw materials and reducing dependence on wood fibers. The versatility of hemp fibers across different paper grades highlights their potential to contribute to a more circular and environmentally responsible pulp and paper industry.
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Introduction
The demand for sustainable raw materials in the paper industry has surged because of the growing environmental pressure, resource limitations, and rising global paper consumption. Traditionally, paper production has relied heavily on wood fibers (xylem), which currently dominate the pulp market. However, the increasing competition for wood resources, combined with restrictions on forest use, has driven the industry to seek alternative, renewable sources of fiber to ensure long-term sustainability. While recycling efforts help alleviate some of the pressure, shortages in raw materials remain a significant challenge for the pulp and paper industry, necessitating the exploration of other fiber sources, such as agricultural residues and annual plants (Liu et al. 2018).
Non-wood fibers, including straw, sugarcane bagasse, bamboo, kenaf, and reeds, are widely available and promising alternative materials for paper production. These plants grow rapidly and are accessible in regions with limited forest resources, offering an economically viable and ecologically friendly approach to raw material sourcing (Ashori 2006; Abd El-Sayed et al. 2020). However, nonwood fibers also have certain disadvantages, including difficulties in collection, transportation, storage, and processing. Additionally, some non-wood sources, such as rice and wheat straw, have high silica content (Atik and Ateş 2012), which complicates black liquor recovery during pulping (Han 1998; Haile et al. 2023). Despite these challenges, annual fiber plants continue to play an essential role, particularly in countries such as China and India, which account for approximately 75% of non-wood pulp production globally (Food and Agriculture Organization 2021).
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Hemp (Cannabis sativa L.) has gained renewed attention as an alternative fiber source with significant advantages for paper production. Global cultivation of hemp peaked in 1966 but gradually declined until 1994, after which it remained relatively stable (AYDOĞAN et al. 2020). Historically, hemp has been widely used for papermaking because of its high yield, rapid growth cycle, and robust fiber properties. Hemp can produce up to 12 tons of cellulose per hectare, offering a sustainable alternative to wood with minimal environmental impact (Ahmed et al. 2022; Axelrod et al. 2023). Furthermore, hemp is suitable for chemical pulping, as it requires less energy and fewer chemicals for pulp and bleach than wood, and its core and bast fibers can be processed separately, maximising its versatility (Malachowska et al. 2015; Naithani et al. 2019).
As in other countries, hemp production has been economically analysed in Türkiye (Ceyhan et al. 2022). Hemp is generally used in two ways: oil extraction from seeds and fiber production. Both these uses have been suggested to increase competitiveness in the industry. However, the potential of hemp as a third-purpose source of paper pulp from field stubble that would otherwise remain unused has not yet been fully explored. The root system accounts for approximately 13% of the hemp plant's total aboveground biomass prior to processing (Ismagilov and Rusakov 2024). The harvestable coarse root fraction constitutes about 13% of the stem biomass. Considering practical harvesting constraints that limit recovery to this bulk root portion, we estimate the combined residual biomass (coarse roots plus stubble) represents 13–15% of the harvested stem mass. Bowyer (2001) reported that the dry yield of hemp per unit area is comparable to that of poplar wood.
Environmental benefits also underscore the appeal of hemp. Hemp cultivation typically requires fewer pesticides and herbicides, thereby reducing the ecological footprint associated with chemical input. Additionally, hemp has natural weed-suppressing properties that minimise the need for herbicides and enhance its suitability for organic farming (Mosjidis and Wehtje 2011; Pudełko et al. 2014; Shikanai and Gage 2022). Compared to wood-based papermaking, hemp requires less water and fewer chemicals, aligning well with sustainable manufacturing practices (Naithani et al. 2019). Hemp bast fibers are known for their strength, durability, and optimal length characteristics, making them ideal for a range of paper applications, from writing and printing to packaging materials (Danielewicz and Surma-Ślusarska 2010; Zule et al. 2012).
Despite the promising characteristics of hemp, research has primarily focused on the use of its bast and hurd (woody core also known as shoves, boon or shives) fibers, with limited exploration of the whole plant, including underutilised components, such as stubble and roots. Malachowska et al. (Malachowska et al. 2015) emphasized that optimized processing technologies are essential to make full use of hemp in pulp production. However, further research is needed to examine the pulping and papermaking potential of the entire hemp plant to maximise its utility and reduce waste.
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Hemp has traditionally been cultivated primarily for textile applications, meaning that the pulp industry must compete with the textile sector for high-quality fibers. The quality of hemp fibers for textile use is influenced by several factors during production and harvesting, including the plant's maturity at harvest, stem diameter and length, and the specific variety. To obtain premium textile fibers, hemp should be harvested before the onset of flowering. As the plant matures, the fibers become coarser; thus, early harvesting or utilizing only the upper portions of the stem is preferred for textile production. In contrast, coarser fibers, along with the hurd, stubble, roots, and the mature whole plant from seed production, represent valuable raw materials for pulp production.
The aim of this study was to investigate the pulping properties of all fiber containing parts of the hemp plant including stalks, stubble, and roots cultivated in Samsun. Given that hurd and root fibers are typically short, their compatibility with softwood kraft fibers was also assessed to determine their suitability in mixed pulps for packaging applications. This approach follows a common industry practice of blending short fibers with longer ones to enhance paper strength. By evaluating the potential of these underutilized agricultural residues, particularly stubble and roots, this study seeks to contribute to the development of sustainable, fiber-diverse alternatives for the paper industry.
Material and methods
Materials
Hemp (Canabis sativa L.) bast fibers (Hb), hurd (Hh), and stubble, including roots (Hsr), were provided by the industrial hemp producer, Ketene Bitkisel Üretim ve Tekstil Sanayi Ticaret Company in Türkiye (www.he-po.com). Pine wood (Pinus brutia L.) was sourced from the Kanlica Forest Chief Unit in Istanbul, Türkiye. All the materials were stored under air-dried conditions prior to pulping. Stubble samples were washed before use to remove soil residues and other contaminants. The raw materials were then prepared for cooking by cutting them to the appropriate sizes; bast and stubble were cut to lengths of 5–10 mm (Danielewicz and Surma-Ślusarska 2017), while crushed hurds were used as received. Pine samples were chopped to dimensions of 25 × 25x3 mm (Fig. 1).
Fig. 1
Raw fiber material used in the study: a bast, b hurd, c stubble, and d pine Chemical analysis of samples
The literature review indicates that chemical composition varies significantly among Cannabis cultivars and is based on the growing conditions. Therefore, prior to initiating the cooking process, the chemical structure of each raw material was analysed to determine the optimal processing parameters. Preparation for chemical analysis was performed following the TAPPI T257 sp-21 standard method, with the samples ground using a Retsch SM 100 mill (Verder Group). The analyses were conducted using the following standard methods: ash content (TAPPI 211 om-22), water solubility (TAPPI T 207 cm-22), alcohol-hexane solubility (TAPPI T 204 cm-17), and 1% NaOH solubility (TAPPI T 212 om-22). Extractive-free samples were prepared specifically for lignin and cellulose analyses, with the lignin content measured using TAPPI T 222 om-21, alpha-cellulose determined using TAPPI T 203 cm-22, and holocellulose assessed using Wise’s method (Wise et al., 1946). FT-IR analyses were conducted using a Perkin Elmer 100 FT-IR spectrometer (Bea-consfield, UK) equipped with an ATR unit (Universal ATR Dia-mond Zn/Se). The FT-IR spectra were collected over the wave number range of 650–4000 cm−1.
Pulping conditions
Pulping operations were performed in a rotary batch digester with a volume of 15,500 cm3, operating at a rotation speed of 4 rpm (Karlstad, Sweden). The quantities of raw material used were 1000 g for hemp and 1500 g for pine, which were calculated on an oven-dry basis. The cooking conditions, including the chemical concentration, temperature, duration, and liquor ratio, were optimised based on multiple preliminary cookings to achieve the most effective pulping parameters. The selected conditions are detailed in Table 1, with the chemical amounts adjusted relative to the oven-dry mass of each raw material. All chemicals were of analytical grade (Merck), and the prepared white liquor was analysed according to the TAPPI T 624 cm-11. After cooking, the pulp was thoroughly washed, dewatered by squeezing, and stored in a refrigerator for subsequent papermaking. The Kappa numbers of the pulps were measured in accordance with ISO 302: 2015.
Table 1
Pulping conditions
Sample
Active alkaline
(as NaO2)
Sulphidity
(%)
Cooking temperature (°C)
Duration at cooking temperature (min)
Total duration
(min)
H-factor
Liquor: sample ratio
Hurd
%16
%20
170
60
120
992
5
Bast
%16
%20
170
60
120
989
5
Stubble
%16
%20
170
60
120
987
5
Pine
%16
%20
170
70
130
1282
4.5
Preparation of pulps, hand sheets and characterisation of papers
Pulp preparation and beating
Pulps were prepared for handsheet production by disintegrating and adjusting to a 10% (w/w) consistency (30 g oven-dry pulp) following ISO 5263–1:2004. Beating was performed in a PFI mill (Sweden) to achieve a Schopper–Riegler (SR°) degree of 45 (optimal drainage), adhering to ISO 5264–2:2011. The beating conditions were as follows:
The SR° value of each pulp was verified per ISO 5267–1:2000.
Handsheet preparation
Laboratory handsheets were fabricated from pure or blended pulps (25, 50, 75% by weight) with basis weights of 80 and 100 g/m2, using the Rapid-Köthen method (ISO 5269–2:2004).
Conditioning and testing
Sheets were conditioned for 24 h (23 ± 1 °C, 50 ± 2% RH, ISO 237:1975) before testing. Key properties were evaluated as follows:
Grammage: ISO 536:2012
Thickness: ISO 534:2011 (H. E. Messmer Ltd., London)
Mechanical Strength:
Tensile index: ISO 1924–2:2008
Bursting index: ISO 2758:2014
Tearing index: ISO 1974:2012 (Elmendorf tester)
Compression strength:RCT: ISO 12192:2011
SCT: ISO 9895:2015 (Zwick-Roell 2.5 kN, Ulm)
Air permeability: ISO 5636–5:2013 (Gurley device, Troy, NY)
Optical Properties:
ISO brightness: ISO 2470–1:2009
Opacity: ISO 2471:2000
CIE L*a*b color:* ISO 5631:2009 (Elrepho, Suzhou)
For the 100 g/m2 sheets specifically, the critical structural and mechanical parameters were evaluated using the ring crushing test (RCT) and short-span compression test (SCT).
Results and discussion
Chemical composition of used raw materials
To determine the composition and proportion of bast and hurd (woody core) in the stubble and root materials, we conducted a separation process. Our analysis indicated that bast accounted for 17.07% by weight, whereas hurd accounted for 82.93%. In addition, the chemical composition of basts and hurds from stubble differs from that of stem-derived materials. Specifically, there was a decrease in the alpha-cellulose content in the bast, along with an increase in the hemicellulose and lignin content. Notably, we observed a 14.22% decrease in 1% NaOH solubility in the bast fibers, highlighting a significant change in the solubility characteristics.
The chemical properties of the raw materials used are listed in Table 2. According to the results, holocellulose remained relatively stable across all parts of the hemp plant, ranging from 80.97 to 84.16%. Previous studies indicated that the holocellulose and lignin contents of hemp bast and hurd ranged between 75.93–87.7% and 4–9.73%, 72–74.5%, and 19.47–27.4%, respectively (Gümüşkaya et al. 2007; Stevulova et al. 2014; Danielewicz and Surma-Ślusarska 2017; Naithani et al. 2019; Baptista et al. 2020; Bokhari et al. 2021; YAYLALI et al. 2022; Yimlamai et al. 2024).
Table 2
Chemical characteristics of fiber raw materials
Chemical property
Raw material
Hb
Hh
Hsr average
(bast/hurd)
Pine
Holocellulose (%)
84.16
82.82
80.97/83.17
82.79
80.96
Alpha-cellulose (%)
78.88
49.44
64.44/50.91
53.22
49.98
Hemicellulose* (%)
5.28
33.38
16.53/32.26
29.57
30.98
Lignin (%)
4.94
17.48
18.18/13.19
14.04
27.30
Hot water solubility (%)
11.89
6.13
9.40/5.58
6.23
3.44
Alcohol- hexane solubility (%)
2.25
3.55
1.94/2.55
2.45
2.00
1% NaOH solubility (%)
33.41
30.74
19.19/26.87
25.55
12.47
Ash content (%)
4.80
1.68
4.38/1.95
2.36
0.40
* Hemicellulose content was calculated as the difference between holocellulose and alpha-cellulose contents
The highest alpha-cellulose content was found in Hb (78.88%), and while the lowest is found in Hh (49.44%). Conversely, the lignin content in hemp was notably lower (4.94%) than that in other parts of the plant, which can reach 17.84% in the hurd.
Compared to wood, hemp exhibits a higher water-soluble content, primarily because of the presence of water-soluble pectin components in bast fibers, resulting in a solubility of 11.89%.
The low molecular weight plant constituents were soluble in diluted NaOH, with bast fibers showing the highest value at 33.41%, whereas the lowest content (19.9%) was observed in stubble bast fibers. The highest extractive content was found in the hurd of the stem and stubble of hemp. These results seem to be consistent with those of other studies, which found values between 0.9 and 4.23% for basts and 1.2 and 2.2% for hurds (Danielewicz and Surma-Ślusarska 2017; Naithani et al. 2019; YAYLALI et al. 2022; Yimlamai et al. 2024).
The ash content was more than double in the tissues forming the outer surface of the plant, whereas the ash ratios determined by Baptista were relatively similar. Considering the ash content of the hemp raw materials relative to those reported in previous studies, the hemp bast and hurd ash contents ranged from 1.5–4.82% and 0.8–2.5%, respectively (Stevulova et al. 2014; Danielewicz and Surma-Ślusarska 2017; Naithani et al. 2019; Baptista et al. 2020; Bokhari et al. 2021; YAYLALI et al. 2022; Yimlamai et al. 2024). The small differences in the chemical properties of the hemp raw materials compared to those reported in the literature may be due to the origin of the raw material, climatic conditions, and growth conditions.
Figure 2 shows the FTIR spectra of P, Hb, Hh, and Hsr pulp. The positions of bands identified in the FTIR spectra and their corresponding functional group assignments are presented in Table 3, along with the literature reference. FTIR spectra indicate typical fingerprints of cellulose at 897, 1030, 1060, 1106, 1160, and 1203 cm−1. A broad and intense absorption band observed at around 3300 cm−1 in all spectra is attributed to O–H stretching vibrations of hydroxyl groups in the cellulose. The band observed at around 2900 cm−1 corresponds to asymmetric and symmetric stretching vibrations of aliphatic –CH and –CH2 groups. The characteristic band of lignin-specific aromatic skeletal vibrations in the 1598 cm−1 region is prominent in the P, Hh, and Hsr samples. In contrast, this band’s absorbance intensity is reduced in the Hb spectrum, which is in agreement with the kappa number results. Overall, the FTIR spectra of all samples displayed similar characteristic absorbance bands; however, the band attributed to the aromatic C-H out-of-plane bending vibration related to lignin at approximately 813 cm−1 was distinctly observed only in the P sample.
According to the crystallinity index calculations from FTIR data, no significant difference was observed, as shown in Fig. 3. Samples containing hurd (Hsr and Hh) exhibit the highest Total Crystallinity Index (TCI, A1372/A2900) values. Meanwhile, the Lateral Order Index (LOI, A1430/A897) of pine and Hb samples is approximately 20% higher than that of the hurd-containing samples. The A3400/A1320 ratio, which reflects hydrogen bonding interactions, correlates with cellulose crystallinity and water retention. This water retention indicator may be associated with the beating properties of the samples, as hurd-containing samples were subjected to moderate beating conditions. Additionally, hydrogen bonds contribute to inter-fiber connections in the paper structure. Microphotographs of hemp, and Calabrian pine fibers are presented in Fig. 4.
Microscope slides were prepared from the obtained pulps to analyze fiber dimensions and morphologies. Overall, the results were consistent with those reported in previous studies (Danielewicz and Surma-Ślusarska 2017), as also supported by the fiber morphology of the pulps presented in Table 4. The felting power was measured at 74.03, a value typical of coniferous species. The highest felting power, 109, was recorded for Hb, while the lowest, 36, was also observed for Hb, suggesting variation within this sample group. The elasticity coefficient ranged from 50 to 70 across all samples, indicating a medium-density fiber type. The Runkel ratio for all pulps was below one, which is generally favourable for papermaking. Notably, the highest Runkel ratio was found in the root sample, likely due to the need for thicker cell walls in this region to support the structural load of the plant. The Mühlsteph ratio values were relatively similar among all samples. The rigidity coefficient was higher in Hb and Hsr samples, aligning with their lower bursting resistance, as reflected in the mechanical strength data presented below. The F-factor, which reflects both fiber length and wall thickness, was highest in Hb at 53.31, followed by pine, Hsr, and Hh, respectively—consistent with expectations based on their anatomical characteristics.
Table 4
Fiber morphology of pulps
Hb
Hh
Hsr
Pine
Fiber length (mm)
2.14
0.52
0.69
2.30
Width (µm)
19.63
14.41
14.46
31.11
Wall thickness (µm)
4.02
2.82
3.12
4.94
Felting power (Fiber slenderness) ratio
109.10
36.07
47.76
74.03
Elasticity Coefficient
59.07
60.89
56.80
68.27
Runkel ratio
0.69
0.64
0.76
0.46
Mühlsteph Classification
65.11
62.92
67.74
53.39
Rigidity coefficient
20.47
19.55
21.60
15.86
F-factor
53.31
18.44
22.11
46.66
The pulping results are presented in Table 5. The highest yield was observed in bast fibers, which had the highest alpha cellulose content, whereas the lowest yield was found in hurd, which contained the most hemicellulose. Naithani et al. (Naithani et al. 2019) demonstrate that it is possible to produce high-yield (59%) kraft pulp from hurd; however, the high lignin content (14.5%) renders it unsuitable for subsequent bleaching. The Kappa number results were aligned with the lignin content of the raw materials. The optical properties of the samples were also affected by the amount of lignin and its derivatives in the pulp. Hb fibers exhibited the highest brightness and lightness, while pine pulp showed the lowest values. Pine pulp, which had the highest lignin content, as indicated by its kappa number, also showed the highest CIE a* and b* values, resulting in a more saturated color appearance. The yield and optical properties of the stubble pulp were found to be between those of bast and hurd, which was expected, as the stubble consists of both Hb and Hh fibers.
Table 5
Cooking results and optical properties of pulp from relevant raw materials
Raw material
Cooking yield (%)
Kappa No
ISO Brightness
Opacity
CIE L*
CIE a*
CIE b*
Hb
64.94
5.24
33.52
97.10
72.67
4.21
14.67
Hh
43.15
20.89
32.86
98.05
72.92
4.58
15.30
Hsr
48.35
19.11
31.77
97.74
71.36
4.21
14.92
Pine
50.15
32.22
17.97
96.72
61.03
7.53
20.69
Figure 5 illustrates the optical properties of the papers from the blended furnish. The brightness of the papers decreased with the amount of pine in the furnish. The variation in opacity was relatively low compared to the brightness properties because the difference between pine and other pups was small. The CIE L* color properties were parallel to the brightness of the samples (Fig. 6a). The amount of pine pulp in furnish increased the saturation of color, expressed as a* and b* values, which correlated with the lignin amount (kappa number) in the pulp.
Fig. 5
Brightness a and opacity b properties of the paper derived from blended pulp
As shown in Fig. 7, the Hb fibers exhibited significantly lower density values than the other fibers analysed. The densities of the Hrs, Hh, and pine fibers were relatively similar, with only minor variations. The Gurley air permeability results showed a direct correlation with density, with the highest air resistance found in papers containing pine-Hh and pine-Hsr fiber mixtures. Pine fibers, which are longer and more flexible than Hb fibers, combine with shorter Hh fibers and reduce the air permeability by effectively filling the voids in the paper matrix. The samples with 50% Hb fibers did not exceed an air permeability of 88 s, indicating a lower density-related resistance. The relationship between the paper density and air permeability is well established, with a high correlation of 0.86, as illustrated in Fig. 8.
Fig. 7
Density a and Gurley air permeability b properties of paper samples
Figure 9a shows the tearing properties of the paper sample. The lowest tearing strength (6.78 mN·m2/g) was observed in samples with a high proportion of Hh fibers, with similarly low values found in samples containing Hrs fibers. This suggests that increasing the proportion of short and thin Hh fibers reduces the tearing strength of the paper. In the other hand, combinations of Pine and Hb fibers improved the tearing strength and stability, making these blends more suitable for applications requiring enhanced durability. Previous studies have reported low tear resistance for both Hh and Hb fibers, which researchers attribute to their low hemicellulose content and reduced susceptibility to internal fibrillation (Danielewicz and Surma-Ślusarska 2017). However, our findings indicate that the reduction in tearing resistance for Hb fibers was not as pronounced, potentially due to differences in Hb fiber dimensions influencing the results. In another study, the tear resistance (4.18 mN·m2/g) of hemp pulp was lower than that in this study (TUTUŞ et al. 2020). A possible reason for this may be the higher alkali and sulfidity rates in the kraft cooking conditions in that study. Meanwhile, Ateş et al. (2015) reported an exceptionally high tearing index value of 26.61 mN·m2/g, which was likely due to the sample cutting length of 40–50 mm.
Fig. 9
Strength properties of paper samples: a tearing index, b bursting index, c tensile index, d modulus of elasticity, and e extension
Figure 9b shows that samples composed entirely of pine exhibit the highest bursting index (6.81 kPa·m2/g), highlighting the significant contribution of pine fibers to the paper’s bursting strength. Conversely, Hb fibers yield the lowest bursting index values, both when used alone (3.3 kPa·m2/g) and in combination with Hh or pine, indicating their limited contribution to the paper strength. The bursting strength reported by Ateş et al. (2015) was 2.59 kPa·m2/g, a value notably lower than that observed in this study. Hh fibers demonstrate a moderate effect on bursting strength, suggesting they provide a balance between structural integrity and other material properties. The bursting index obtained in this study closely aligns with the findings reported by Tutuş et al. (2020) at 5.32 kPa·m2/g.
The tensile strength properties presented in Fig. 9c show trends similar observed for the bursting strength. Samples composed entirely of pine displayed a high tensile index (81.3 N·m/g), highlighting the substantial contribution of pine fibers to the tensile strength. In comparison, Hb fibers have the lowest tensile index (43.47 N·m/g), indicating a significant reduction in tensile strength and making them the least effective component in this context. Similar findings were reported by Danielewicz (Danielewicz and Surma-Ślusarska 2017), who observed that the tensile index for hemp stalks and hurds could reach approximately 90 N·m/g, while the tensile index for bast fibers was notably lower. Ateş et al. (2015) also reported a lower tensile index of 30.51 N·m/g for Hb fibers, marking is as the lowest amont the agricultural residues studied.
In this study, the tensile index for Hh fibers was determined to be 94.39 N·m/g, which is slightly lower than the value reported by Tutuş et al. (2020) at 111.8 N·m/g (converted from a breaking length of 11.4 km). These results suggest that Hb fibers are more suitable for applications where tensile strength is not a critical factor. Conversely, combinations of Hh and pine fibers prove to be optimal for applications requiring high tensile strength, with Hh being the most effective component for enhancing this property, followed by pine.
The lowest elasticity modulus were observed for Hb fibers 3.32 GPa (Fig. 9d), which is significantly higher the range observed by Axelrod et al. (Axelrod et al. 2023) of 1.9–2.1 GPa. A similar effect was observed in the elasticity modulus values, where the samples containing pine fibers remained at a relatively constant level, and the elasticity modulus of the samples containing soybean fibers decreased as the soybean content increased (Yilmaz et al. 2015).
When the elongation rates during the tensile tests were examined, pine samples exhibited the highest extension percentage (2.28%). The elongation of all other fibers was less than 1.5%, Showing a marked decrease. Pine fibers enhance flexibility, whereas hemp fibers decrease extension, making them suitable for applications that require rigidity. As the content of Hh, Hrs, and Hb fibers increased, the flexibility of the paper decreased.
If this data relates to material strength, then pine-Hb combinations are optimal for applications requiring strength and stability, while Hh-based materials may be better suited for applications where less rigidity is required. Naithani investigated the potential of using hemp pulp for tissue paper and observed improvements in tensile index, burst resistance, and softness of tissue handsheets when compared to solely kraft hardwood pulp handsheets without adversely impacting water absorption (Naithani et al. 2019).
The RCT, CMT30, and SCT analyses used to evaluate corrugated board papers were performed on 100 g/m2 prepared samples, and the results are shown in Fig. 10. The highest RCT value of 18.17 kN·m/g was observed for the 25% pine 75% Hh sample. The highest values were determined for this blend for all samples, except for the samples containing Hb. The corrugated medium CMT30 strength test was similar to RCT; the highest values were determined for the 25% pine 75% Hsr and Hh samples, with a more obvious difference.
When examine the results in terms of CMT30, no one of the samples do not meet the CEPI criteria (CEPI 2022) of ≥ 1.9 N·m2/g for semichemical 2 fluting grades. Only samples containing 75% Hh and Hsr was higher than ≥ 1.3 N·m2/g that is the recommendation for recycled medium grade 3.
Another corrugated medium paper strength criterion, SCT values, showed that while Hh and Hsr samples showed good performance, the values of samples containing Hb were relatively low. All samples containing less than 50% Hb have SCT performance comparable to Semi chemical fluting papers where the requirements is SCT index equal or higher than ≥ 21 N·m/g and ≥ 17 N·m/g for grade 1 and grade 2 respectively. The SCT performance of Hb fibers equals recycled medium 2 paper grades with ≥ 15.0 N·m/g.
As observed in all three analyses of samples related to corrugated paper grades, the Hh and Hsr samples meet the CEPPI criteria for semichemical fluting medium and can therefore be successfully used for corrugated medium paper. Moreover, the addition of long fibers results in the highest strength levels. These findings indicate that incorporating Hh and Hsr pulps can significantly improve the performance of recycled fluting paper grades.
Considering the physical resistance properties, it was revealed that hemp can be used as a raw material for paper. As suggested by Malachowska et al. (2015), before industrial application, appropriate technology and optimal conditions should be determined. This study investigated the potential of different parts of hemp pulp for the production of packaging paper and corrugated medium for cardboard.
The influence of fiber morphology on the properties of corrugated paper can be summarized as follows: Felting power and F-factor appear to affect the SCT index positively, benefiting from long and slender fibers. In contrast, lower Runkel ratios and rigidity coefficients are associated with improved compressive strength, as reflected in higher SCT, RCT and CMT₃₀ values.
Conclusion
This study demonstrates the viability of utilizing hemp stubble an agricultural by-product typically left in the field as a potential raw material for paper pulp, supporting a third major use for hemp beyond seed oil and fiber production. By assessing the chemical and pulping properties of hemp stubble, the research provides specific processing recommendations, such as the need to cook bast and hurd fibers separately due to their distinct chemical compositions, whereas the stubble, particularly the root, should be processed as a whole because of its complex anatomical structure.
The results clearly revealed that each fiber type contributes uniquely to the final paper strength. In terms of tensile strength, hemp hurd pulp exhibited the highest performance (approximately 94 N·m/g), outperforming pine (81 N·m/g) and more than doubling that of bast pulp (43 N·m/g). Similarly, bursting strength was greatest in the paper made solely from pine (6.81 kPa·m2/g), whereas bast pulp showed the lowest (3.3 kPa·m2/g), with hurd pulp yielding intermediate values (~ 5 kPa·m2/g). Regarding tearing resistance, which is closely related to fiber length, bast pulp showed superior performance (up to ~ 12 mN·m2/g), while hurd-rich samples had the lowest values (around 6–7 mN·m2/g), highlighting the reinforcing role of long bast fibers.
In packaging applications where compression strength is critical (SCT and RCT), hurd and stubble fibers clearly outperformed bast. The SCT results demonstrated that samples containing less than 50% bast achieved SCT values above 21 N·m/g, meeting the industry criteria for semi-chemical fluting. In contrast, pure bast pulp only reached ~ 15 N·m/g. Likewise, the highest RCT value (18.17 kN·m/g) was recorded in the sample containing 75% hurd and 25% pine, far exceeding values from other combinations.
Pulping trials revealed that hemp fibers derived from stubble are versatile and can be used independently or blended with other fiber sources, such as recycled paper or chemical pulp, to produce paper products with varied strength properties. The inclusion of hemp root fibers increased thickness, compressive strength (CMT30 index – 1.50 N·m2/g), and opacity, owing to their dense and compact morphology. Bast fibers enhanced surface strength thanks to their long and robust structure, while hurds provided moderate reinforcement. This adaptability positions hemp as a promising material for a range of applications, from high-grade writing paper to durable corrugated cardboard, addressing the growing demand for sustainable materials in the paper industry.
The recyclability of hemp paper further enhances its environmental value, as it can endure more recycling cycles than wood-based paper due to the durability of hemp fibers. Additionally, hemp’s capacity for significant carbon sequestration during growth offers another environmental advantage, aligning with sustainable development goals.
In conclusion, leveraging hemp stubble as an alternative raw material for paper production not only reduces agricultural waste but also provides a sustainable, high-performance alternative to traditional wood fibers, contributing positively to both the economic and ecological dimensions of the paper industry.
Competing Interests
The authors declare no competing interests.
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