Wheat straw was collected from farms in Hashtgerd, Alborz province (Iran). Chemicals for pulping, fiber analyzing and determination of the chemical composition of wheat straw, i.e., mono ethanol amine (61.08 g.mol⁻¹), sodium hydroxide (40 g.mol⁻¹), anthraquinone (208.21 g.mol⁻¹), sulfuric acid (98.08 g.mol⁻¹), sodium chlorite (90.44 g.mol⁻¹), acetic acid (60.05 g.mol⁻¹), potassium permanganate (158.03 g/mol), Sodium thiosulfate (158.11 g/mol), acetone (58.08 g.mol⁻¹), potassium iodide (166 g/mol) and safranin (350.84 g.mol⁻¹), all were supplied from Sigma Aldrich (St. Louis, MO, USA). Hydrogen peroxide (35%, 34.01 g/mol) is purchased from Dr. Mojalali Company (Tehran, Iran). Cationic starch, for the measuring of kappa number, (pH = 6 in 2 W/V % solution) was obtained from Glucosan Company (Qazvin, Iran).

The wheat straw sample was cleaned and cut into 2–3 cm pieces followed by the physical separation of all impurities and non-lignocellulosic materials from the wheat straw. Pulping conditions of wheat straw can be seen in Table 2. Which was performed in a 10 lit. circulation digester (Haato, Finland). MEA at three ratios (50:50, 25:75 and 15:85) was mixed with water and a soda process (14%) was selected as control pulping. 0.05 w% anthraquinone (AQ) was used in all pulping conditions as a constant factor. Other constant factors are as follows: maximum temperature: 170°C, time to temperature: 40 min, time at temperature: 60 min and liquar/wheat straw (L/W): 8:1. After pulping, the digester was cooled down and the resulted pulp was removed from the digester and washed several times with tap water. In the next step, the shives and uncooked materials were separated from the accepted fibers followed by the disintegration of the resulted pulp in a hand-made laboratory-scale pulper. Finally, the obtained disintegrated pulp was dried to 40% consistency and kept in a polyethylene bag. Pulping yield, ash content and kappa number (T 236 om-99) [33] of the resulted pulp were determined. Fig. 1 shows the schematic image of wheat straw from farm to diaper.

A single disk laboratory refiner (L&W, Sweden) was used to refine the prepared pulp. The refining gap was set at 0.5 mm and the pulp suspension with two different consistencies (1.5% and 2.5%) that prepared according to TAPPI standard (T 240 om-02) [34], was passed through the refiner from 0 to 6 times. After each pass, a portion of the refined pulp was separated from the initial batch and its freeness was measured (SR°).

To investigate the physical, mechanical and optical properties of the papers, hand-sheets with a basis weight of 70 g/m² were manufactured according to the TAPPI standard (T 205 sp-02) [35]. For this purpose, a KCL laboratory sheet former, Lorezen & Wettre (L&W, Zurich, Sweden) was used.

A FTIR device (VECTOR33, BRUKER Co. (Germany)) was used for exploring functional groups of wheat straw and their changes after pulping with different processes. For this purpose, a small quantity of each materials (pre-dried) were mixed with potassium bromide and then spectrometry was performed on the obtained tablet.

To illustrate the morphology and external fibrillation of fibers after refining, a field emission scanning electron microscopy (FE-SEM) was used (MIRA-II TESCAN, (Czech Republic)). For the preparation of samples, a small amount of dried fabricated papers were cut and then sputter-coated with a thin layer of gold. The SEM photography was conducted in two different magnifications of 300x and 4kx, to explore the morphology of sheets and fibers, respectively.

Hand-sheets were conditioned in 23°C and 50% humidity for 24 h according to TAPPI standard (T 402 sp-03) [36]. A series of test such as physical properties, i.e., density (T 410 om-02 (grammage) [37], T 411 om-05 (Thickness) [38]), optical properties, i.e., brightness (T 452 om-02) [39] and mechanical properties, i.e., burst index (T 403 om-02) [40], tensile index (T 494 om-01) [41] and tear index (T 414 om-04) [42] were conducted on hand-sheets.

The statistical analysis was conducted using SPSS statistics software (Version 16, Armonk, NY, USA). Univariate analysis was done for analyzing the data. After observing a significant difference between the levels, Duncan’s test was used to group the means at α-value ≥ 0.05 confidence level. Each treatment was tested with five samples.

The chemical compositions of unprocessed wheat straw are summarized in Table 3. As seen, wheat straw with a high percentage of holocellulose and an approximate low amount of lignin is an excellent candidate for the pulp and paper industries. The only limitation is its high ash content, which is expected to be largely removed from the pulp during pulping and bleeping stages.

Medium fiber length is one of the most important characteristics of fluff pulp, and fiber refining keeps many of the critical functional properties of fluff pulp (as discussed below) at an ideal level. Fiber refining should not lead to fiber length reduction. The fiber length measurements of wheat straw and refined and unrefined MEA pulp (50:50) are shown in Fig. 2. As seen, there is no significant difference between the fiber length of wheat straw and unrefined pulp, while a sharp decrease was observed in the fiber length of refined pulps. The fiber length measured in this study is almost in agreement with the results reported by other researchers [43,44].

The total yield, kappa number, and ash content of different pulps are presented in Fig. 3. As shown, the total yield, kappa number, and ash content in soda pulp are much less than in the MEA pulp in different concentrations. Results showed that the MEA pulps, total yield, kappa number, and ash content were enhanced as the MEA concentrations decreased. It is worth mentioning that the ash content in soda, MEA 50:50, MEA 25:75, and MEA 15:85 compared to the pure wheat straw decreased by 59.04%, 28.35%, 24.15%, and 16.68%, respectively. In addition, more carbohydrates were retained in the MEA pulp than in the soda pulp. Therefore, the total yield can be expected to be higher in MEA pulp. As the percentage of MEA decreased (from 50% to 15%), the kappa number increased due to the reduction of lignin-attacking substance, which means that more lignin is preserved in the pulp.

FE-SEM images of soda and MEA 50:50 pulps in different refining intensities at low magnifications are given in Fig. 4. As can be seen, the fabricated paper’s texture becomes denser in both soda and MEA pulps as the refining intensity increases. However, this increase in density in MEA pulp is more intense than that of soda pulp. A significant number of vessels can be seen in the pulps resulting from the soda process, while the number of vessels is much less in the MEA process. This is probably due to the softening of the pulp by amine groups of MEA, which leads to thin-walled cells like vessels that are easily crushed by the refiner blades.

Fig. 5 illustrates the FE-SEM images of soda and MEA 50:50 pulps in different refining intensities at high magnifications. As can be seen, with the increase of refining intensity, the fiber surfaces become strongly fibrillated (external fibrillation). At the same level of refining, MEA pulp demonstrated better fibrillation than soda pulp.

FE-SEM images of MEA 50:50 pulp refined with different consistencies are exhibited in Fig. 6. As seen, pulp refined with 2.5% consistency (Fig. 6b) is much denser than that refined with 1.5% consistencey (Fig. 6a). Moreover, an extraordinary level of external fibrillation could be observed in pulp refined with 2.5% consistency (Fig. 6d), while in pulp refined with 1.5% consistency (Fig. 6c), the fibrillation level is much lower.

FTIR spectroscopy of wheat straw, soda and MEA 50:50 pulps is presented in Fig. 7. The peak around 3400 cm⁻¹ is related to the stretching vibration of the hydroxyl groups in cellulose fibers [45]. The peak near 2900 cm⁻¹ is attributed to C-H stretching vibration [46]. The stretching vibration of carbonyl groups (C=O) in the lignin structure can be seen in 1740 cm⁻¹ [47]. This peak, which associated to the unprocessed wheat straw, has been removed after pulping in both soda and MEA pulps. The peaks around 1600 cm⁻¹ and 1422 cm⁻¹ are attributed to the asymmetric and symmetric stretching vibration [45,48] of carboxyl groups, respectively, which is probably related to the structure of xylene in hemicellulose of wheat straw. The intensity of these peaks in soda and MEA pulps has slightly decreased compared to the pure wheat straw. The peak in 1510 cm⁻¹ is related to the aromatic C-H vibration [49]. This peak is also greatly reduced in soda pulp and completely eliminated in MEA pulp. The decrease in the intensity of this peak in soda pulp is related to the degradation of lignin and its removal from wheat straw, and its removal in MEA pulp is related to the amination reaction of the aromatic part of lignin [49]. The peak in 1044 cm⁻¹ associated to the C-O stretching vibration, which represents the structure of polysaccharides [4]. It is necessary to explain that in the MEA pulp, a new peak was formed in 1375 cm⁻¹, related to the C-N bond, which is caused by the formation of the amine group on the remaining lignin in this pulp [49].

Fig. 8 shows the effect of the pulping process and refining intensity on the freeness of pulps. As seen, the highest and lowest freeness are related to MEA 50:50 and MEA 15:85, respectively. The freeness was also enhanced as the refining intensity increased. An increase in water absorption characteristics of pulps is plausible by increasing the percentage of MEA and consequently decreasing the kappa number due to the removal of more lignin. For this reason, as the MEA increased, the freeness increased. Some researchers have reported that in pulps with lower kappa number, the beatability index (time needed to reach SR° freeness 30) increases [50]. However, although the soda pulp in this study had a lower kappa number than MEA 50:50, its freeness was less. This is probably due to the effect of the remaining hydrophilic amine groups on the remaining lignin (FTIR section) in MEA pulp, which caused a slight increase in the freeness compared to soda pulp. By increasing refining intensity, more fibrillation of the fiber surfaces led to an enhancement in freeness because the fiber surfaces also contain hydroxyl groups and can absorb water. Freeness is a very important factor in examining pulp and paper properties [51]. This quantity also seems very important in optimizing fibers for use as fluff pulp. The pulp with a low freeness usually represents little bonding, resulting in easy fiber separation in the hammer mill, but the final pulp may break in the production lines due to the lack of stiffness and tensile strength, leading to the stoppage of the production line. On the contrary, the high freeness of the pulp leads to strong bonding between fibers, making the final pulp difficult to dry-defiberized in a hammer mill. In this situation, undefiberized fibers (knots) can be abundantly seen in the pulp [7].

The effect of pulping types and refining intensity on the density of fabricated papers is demonstrated in Fig. 9. As shown, the highest density is related to MEA 50:50. As the intensity of refining increased the density of papers enhanced. As mentioned, the density of fluff pulp is one of the most important characteristics of this product. The density of fluff pulp depends on many factors, such as the intensity of refining [22,52] and as a result, the freeness [52], the coarseness of the fibers (specific volume of fluff pulp) [21], etc. Like freeness (discussed earlier), density also determines the type of dry-defibration of the pulp in the hammer mill. Pulps with a low density lead to the production of low strength pulp which may break in the production line. On the contrary, pulps with a high density leads to the production of very stiff pulp which spends a lot of energy to separate its fibers in the hammer mill [53]. An increase in the density of papers, with increasing intensity of refining, can be related to more fibrillation of the fibers and their better entanglement [52]. The greater density of MEA 50:50 can be probably related to the greater flexibility of this pulp due to the presence of amine groups on the fiber surfaces compared to other pulps. As mentioned, the density of the papers is strongly related to their fiber coarseness. Unfortunately, there is no research focusing on the relationship between the density of the paper and the fiber’s coarseness to estimate the optimal density suitable for fluff use. However by exploring the sources, a range between 0.5 to 0.6 g/cm³ can be considered for it [7,9] (0.5 g/cm³ for papers fabricated with low fiber coarseness and 0.6 g/cm³ for papers made with high fiber coarseness). MEA 50:50 pulp with a density of about 0.35 g/cm³ has the closest density to the optimum level (0.5 g/cm³, because wheat straw has a low fiber coarseness). Its lack of density can also be compensated by mixing this pulp with softwood pulps that have fibers with higher coarseness.

Fig. 10 shows the effect of the pulping process and refining intensity on the brightness of the hand sheets. As seen, the highest and lowest brightness were related to soda and MEA 15:85, respectively. An upward trend was observed in the brightness of the pulps as refining intensity increased. The higher brightness of soda pulp compared to other pulps is related to its lower kappa number [54]. By examining the sources, it can be seen that pulps with less than 2.5% remaining lignin (i.e., kappa of about 16 or less) are bleached more easily and with fewer chemicals than pulps with higher kappa numbers [20]. Therefore, soda pulp with a kappa of 11.6 and an initial brightness of about 32, as well as MEA 50:50 with a kappa of 12.6 and an initial brightness of about 27, will be bleached much easier than other pulps (MEA 25:75 and 15:85).

The effect of pulping process and refining intensity on the burst and tensile indices of the hand-sheets is shown in Fig. 11. As shown, the highest and lowest burst and tensile indices are related to soda and MEA 15:85, respectively. On the other hand, the both burst and tensile indices of the hand-sheets enhanced as the refining intensity increased. Burst strength is one of the tests, based on its results, it is possible to find out how the final pulp is dry-defiberized in the hammer mill. According to the sources, pulps with burst index between 1.8 and 2.5 Kpa.m²/g are well dry-defiberized in a hammer mill [5,9,14]. Pulps with lower burst index should be refined to reach the optimum burst strength, but pulps with higher burst strength should be mixed with the other pulps or a de-bonding agent [6] should be used in their manufacturing process to reduce their burst strength. An increase in the burst index with increasing of the intensity of refining, is related to the external fibrillation of fibers, which leads to the bonding improvement [55]. A higher burst strength of soda pulp is related to its lower kappa number. Although soda pulp represented lower density and freeness than MEA 50:50, but the lower kappa number of soda pulp has a greater effect on increasing its burst index than the lower density and freeness on reducing the burst index. Based on the obtained results, even though MEA pulps with concentrations of 25:75 and 15:85, represented a burst index closer to the optimal level required for fluff pulp (1.8–2.5 Kpa.m²/g), but since they are difficult to bleach, they are not good pulps from this point of view. Soda pulp also has the highest burst index. But the MEA 50:50 pulp can be bleached and has a burst index closer to the optimal level, and its excess burst index can be adjusted to the optimal level using a little de-bonding agent or combining it with unrefined softwood pulps. It can be expected that the tensile strength of manufactured papers will increase with the increase of refining intensity and the increase of hydrogen bonds between fibers [51]. The higher tensile index of soda pulp is also related to its lower kappa number. The tensile index is important from the point of view that it shows the strength of the paper roll during the production of baby diapers and other hygienic products.

The effect of the pulping process and refining intensity on the tear index of the fabricated papers is shown in Fig. 12. As can be seen, the highest tear index is related to the soda process. An initial tear index increase was observed as the refining intensity increased, but with further increases in refining intensity, the tear index of the hand sheets decreased. Among the mechanical properties, the tear index is strongly affected by the fiber length [56]. Since the length of the fibers in the pulps decreased with the increase of the refining intensity, it can be expected that the tear index also decreases with the increase of the refining intensity.

The effects of the pulp’s consistency during refining on the SR° freeness and density of the fabricated pulp are shown in Fig. 13. Based on this figure, as pulp consistency increased during refining, both the freeness and density of the fabricated pulps increased by 23.53% and 0.26%, respectively. In order to be able to optimize the properties of the fabricated pulp, and since the pulps’ consistency during refining greatly affects the final pulp’s properties [55], the refining operation was carried out at two different consistencies (1.5% and 2.5%). The increase in the freeness and density of the papers due to the increase in consistency during refining can be related to the fibrillation of the pulp fibers [55]. More fibrillation of fibers makes the fibrillated surfaces, which mainly contain hydroxyl groups, able to absorb more water. Moreover, the fibrillation of the fiber surfaces causes more fiber entanglement during the formation of the network and causes the paper to be less thick at a constant weight, which leads to an increase in density [52].

Fig. 14 shows the effect of pulp consistency during refining on the brightness, burst, tensile, and tear indices of the fabricated pulp. A slight decrease was observed in the brightness when the consistency of pulps increased. On the other hand, the burst index increased by 14.15% as the pulp consistency increased during refining. This increase in burst index is related to more fiber fibrillation (refer to the FE-SEM images) due to refining with higher consistency [55]. The increase in burst index is a negative factor here because it is far from the optimal level required for fluff pulp (1.8–2.5 KPa.m²/g). Additionally, as the consistency of the pulps increased, the tensile index increased by 13.74%, while the tear index decreased by 9.74%.