**3. Results**

#### *3.1. Separation between Pith and Bark*

Immediately after their harvesting, amaranth stems had to be dried (50 ◦C, 48 h) to facilitate their storage over time. Their structure was then manually studied from ten specimens. They were composed of a light pith fraction in their middle (24% ( *w*/*w*)), and a bark one, fibrous and rigid, in their periphery (76% ( *w*/*w*)) (Figure 2a).

**Figure 2.** (**a**) Photograph of the amaranth stem (cross section); (**b**) Photograph of the optimal insulation block from the BP pith fraction; (**c**) Photograph of the optimal hardboard (HB6) from extrusion-refined bark.

The structure of the amaranth stem is reminiscent of sunflower or even corn. Nevertheless, the elements of the amaranth stem are not ye<sup>t</sup> valorized in the field of renewable materials. Conversely, pith and bark fractions from sunflower stems have been the subject of recent work to transform them into low-density insulating blocks [22], or rigid [11] and semi-rigid [22] panels, respectively.

Based on the same principle as for sunflower, the amaranth stem fractions could therefore be used as basic elements for valorization in the materials sector. Indeed, the pith could be used as thermal insulation while the bark fraction could be used for the production of dense boards through hot pressing.

The large difference in density between bark and pith particles enabled their continuous mechanical separation through a two-stage process involving a grinding step of the stems, and then a suction one. On the one hand, the grinding step allowed the pith to be separated from the bark particles. On the other hand, during the suction step, the use of a vacuum cleaner resulted in the isolation of the lighter pith particles from those of bark. The suction operation was repeated three times, first on a vibrating sieve shaker and then twice on an inclined conveyor belt.

Table 1 shows the mass balance for each of the three suction stages based on 100 kg of crushed amaranth stem. For its part, Table 2 shows the evolution of the pith purity of the light fractions, sucked after the sieving step (P1) and after each passage on the conveyor belt (P2 and P3), respectively. The pith purity was measured twice by manually separating the residual bark particles from the pith ones. The pith purity was multiplied by more than four thanks to the two conveyor belt purification steps. It was greater than 90% (*w*/*w*) after the three consecutive suction steps, which was considered sufficient to produce low-density insulation blocks.

**Table 1.** Mass balance of the fractionation of 100 kg amaranth stem into bark particles and a sucked fraction containing the pith ones for the three successive suction stages, and mass of the two fractions collected at the end of the separation process.


1 At the end of the fractionation process, the bark particles collected after each suction stage were mixed together to form a single fraction of bark particles. 2 The pith fractions sucked out after stages number 1 and 2 constituted the input materials for the following stages, i.e., stages number 2 and 3, respectively. 3 The pith fraction from stage number 3 was the fraction used in this study for the manufacture of the insulating materials.


**Table 2.** Purity in pith particles of the sucked fraction after each suction stage.

#### *3.2. Chemical Composition of Pith and Bark, and Physical Characterization of Pith Particles*

Table 3 shows the chemical composition of the pith and bark, both from the amaranth stem. While the pith contains a median amount of lignocellulosic fibers (47%), but a lot of minerals and water-soluble components (19% and 31%, respectively), the bark is much richer in lignocellulosic fibers (64%), and it has much lower mineral and especially water-soluble contents. Thus, the bark is indeed the woody part (or ligneous fraction) of the stem.

**Table 3.** Chemical composition of pith and bark from amaranth stem (% of the dry matter).


In addition to these previous comments, it should be pointed out here that the sum of the chemical compounds quantified in both pith and bark was greater than 100%. This is explained by the fact that minerals in ionic form as well as some hemicelluloses, especially those with the lowest molecular weights, may be soluble in water. These chemical constituents could then be quantified twice.

Due to their alveolar structure, the pith particles were light, with an estimated bulk density of only 32.2 kg/m<sup>3</sup> (Table 4). Their particle size distribution is also presented in Table 4. Considered as dust (i.e., fines), the particles smaller than 1 mm have not been kept for the manufacture of the low-density insulation blocks. The other three pith fractions generated after sieving (i.e., SP, MP and BP) were also characterized in bulk density (Table 4), and the results obtained revealed significant differences.

**Table 4.** Particle size distribution of the amaranth pith particles, and bulk density, thermal conductivity and thermal resistance in bulk at 25 ◦C of all pith particles (mix) and the SP, MP, and BP fractions generated after sieving 1.


1 Means in the same column with the same superscript letter (a–c) are not significantly different at *p* < 0.05. 2 All pith fractions. 3 Small particles (1–2 mm). 4 Medium particles (2–4 mm). 5 Big particles (>4 mm). 6 Thermal resistance for a 4 cm thick bed of pith particles. n.d., non-determined.

For its part, Table 5 proposes a distribution in weight and another one in volume for the SP, MP, and BP fractions of pith particles, in proportion to the total weight or to the total volume, respectively, of the mixture made of these three pith fractions. As a reminder, these pith fractions were the three ones used to produce the low-density insulation blocks, on their own or mixed in the right weight proportions. The results in Table 5 show relatively close weight and volume distributions, with the intermediate pith fraction (MP particles) representing for both distributions a value close to 50%, followed by the coarser fraction (BP particles) and then the fraction made up of the smallest particles (SP particles).

**Table 5.** Distribution in weight (% ( *w*/*w*)) and distribution in volume (% (*v*/*v*)) for the SP, MP, and BP fractions of pith particles, calculated in proportion to the total weight or to the total volume, respectively, of the mixture made of these three pith fractions.


Thermal conductivity measurements were also performed on each bulk pith sample using the hot wire method. The thermal conductivity values obtained are shown in Table 4. The results revealed that they were of the same order of magnitude as those of a commercial cellulose wadding used as bulk insulation for which the thermal conductivity at 25 ◦C was between 38 and 44 mW/(m.K).

#### *3.3. Low-Density Insulation Blocks from Pith*

Low-density insulation blocks were produced from pith particles through compression molding using standard conditions already implemented in a previous study [20], i.e., 9 kPa for the applied pressure, 5 min for the molding time, and 25 ◦C for the mold temperature. Once molded, all blocks were placed in a ventilated oven to evaporate the water initially added to dissolve the starchy binder. The adhesion was achieved after complete evaporation, and all the resulting blocks were cohesive enough to be machined.

Blocks having a 10% ( *w*/*w*) binder content were produced from the three sieved fractions and from the mix made of all pith particles. In addition to these four insulating materials, three other low-density blocks made from the medium pith particles (MP) were also molded using higher binder contents, i.e., 15%, 20%, and 25% ( *w*/*w*), respectively. All the characteristics of these seven low-density insulation blocks are presented in Table 6. These include density, flexural properties, thermal conductivity, and thermal resistance.

**Table 6.** Characteristics of the low-density insulation blocks produced from pith particles through compression molding (9 kPa, 5 min, 25 ◦C) 1.


1 Means in the same column with the same superscript letter (a–g) are not significantly different at *p* < 0.05. 2 Thermal resistance for a 4-cm thick low-density insulation block.

#### *3.4. Twin-Screw Extrusion-Refining of Bark*

During the twin-screw extrusion-refining pre-treatment, the amaranth bark undergoes defibring (i.e., destructuring) due to the mechanical stresses and temperature in the extruder. Three different inlet flow rates of water were tested in this study, i.e., 10 kg/h, 20 kg/h, and 40 kg/h, respectively, thus corresponding to liquid/solid ratios varying from

1.0 to 4.0. The results of the amaranth bark refining treatment in twin-screw extruder are shown in Table 7. As a first result of this treatment, the three extrudates obtained had the form of fluffy materials, with significantly reduced apparent and tapped densities compared to those of the simply ground bark particles (i.e., 147.7 ± 1.3 kg/m<sup>3</sup> and 148.5 ± 4.2 kg/m3, respectively).

**Table 7.** Inlet flow rate of water, liquid/solid ratio, outlet flow rate of the filtrate, content of watersoluble components in the filtrate, extraction yield in water-soluble components, and apparent and tapped densities of the extrudate after drying for the three twin-screw extrusion-refining experiments applied to the amaranth bark particles (10 kg/h for their inlet flow rate) 1.


1 Means in the same line with the same superscript letter (a–c) are not significantly different at *p* < 0.05. 2 The difference between the cumulative inlet flow rate (bark particles plus water) and the cumulative outlet one (extrudate plus filtrate) can be explained by the loss of part of the added water by evaporation, at the level of the filtration module and at the outlet of the barrel.

When the quantity of water increased, this facilitated the transport of the solid material whose residence time in the twin-screw extruder decreased. Thus, the extrudate obtained has undergone less shearing, and its fiber length was therefore better preserved. This was previously evidenced in the case of rice straw [27], and the same was true in this study. Nonetheless, the values obtained for both apparent and tapped densities of the extrusionrefined fibers remained in the same order of magnitude for the three tested liquid/solid ratios.

Looking at the content in water-solubles remaining in the extrudate, the values obtained were all three significantly different. With a higher liquid/solid ratio, the flow rate of the filtrate collected at the penultimate module increased, also bringing with it more water-soluble components initially present in the bark. The extrudate obtained from the higher liquid/solid ratio (i.e., 4.0) was therefore more depleted in water-soluble components. From the extrusion-refining data in Table 7, the flow rate of water-soluble compounds extracted from the bark and contained in the filtrate could be calculated. These extracted water-soluble compounds were then expressed as a percentage of the incoming water-solubles in bark, for the three liquid/solid ratios tested (i.e., 1.0, 2.0 and 4.0). The results were 11.7%, 32.9%, and 54.2%, respectively. It was therefore well observed that the extraction yield in water-soluble components increased with the amount of water added.

To conclude, although three liquid/solid ratios have been tested in the twin-screw extruder, only the extrudate produced using the liquid/solid ratio of 4.0 has been used in the production of hardboards through hot pressing. Indeed, due to the higher amount of water added during the thermo-mechanical defibration of the amaranth bark particles in the twin-screw extruder, the length of the fiber bundles was better preserved.

#### *3.5. Hardboards from Bark*

Hardboards were produced through hot pressing from ground (GB) or extrusionrefined (ERB) amaranth bark particles. The molding conditions used were as follows:

200 ◦C for the mold temperature, 20 MPa for the applied pressure, and 5 min for the molding time. In particular, the 200 ◦C mold temperature was used as optimal temperature to ensure an efficient mobilization of the internal binders inside bark, i.e., free sugars, hemicelluloses and lignins [13,30]. Thanks to the adhesive ability of these chemicals, cohesive hardboards were obtained without exogenous binder.

An improvement in the molding process consisted in adding to the bark particles an amaranth-based binder (AB). This exogenous binder had the form of grinded and deoiled amaranth seeds, and it was the starch and to a lesser extent, proteins contained in AB that have given it its aptitude for adhesion.

All the characteristics of the six hardboards produced (HB1 to HB6) are presented in Table 8. These include density, flexural properties and water resistance. In addition, these characteristics were also compared in Table 8 with those of two commercial woodbased materials, i.e., MDF and chipboard (CH).

**Table 8.** Density, flexural properties, and water resistance of hardboards (HB) produced from ground and extrusionrefined barks (GB and ERB, respectively) through hot pressing (200 ◦C mold temperature, 20 MPa applied pressure, and 5 min molding time) 1, and comparison with the properties of two commercial wood-based materials (i.e., MDF and chipboard (CH)).


Means in the same column with the same superscript letter (a–d) are not significantly different at *p* < 0.05.

## **4. Discussion**

1

#### *4.1. Physical Characterization of Pith Particles*

Once isolated, pith particles were sieved, resulting in the next particle size distribution: 18% for small particles (SP) (from 1 to 2 mm), 42% for medium particles (MP) (from 2 to 4 mm), and 25% for big particles (BP) (>4 mm) (Table 4). Fines (<1 mm) represented 15% (*w*/*w*) but they were not used in the present study. All pith particles revealed an alveolar structure, and this resulted in low bulk densities, e.g., only 30 kg/m<sup>3</sup> for the smaller particle size (SP).

Despite the fact that small pith particles had fewer inter-particle voids, due to a better arrangemen<sup>t</sup> (i.e., stacking) of the particles in relation to each other, it was these smaller particles (SP) that revealed the lowest bulk density value (Table 4). This could be explained by the fact that the larger ones (BP) and, to a lesser extent, the medium ones (MP) still contained some long fibers from the bark fraction after the sieving step of the pith particles. For the BP fraction, the decrease in bulk density that should have been observed due to more inter-particle voids was therefore largely compensated by the presence of these denser bark fibers, which contributed to increase the density of the BP pith fraction. Conversely, the smaller particles were too small to contain residual bark fibers after sieving.

As seen previously with bulk density, the thermal conductivity at 25 ◦C of the bulk pith therefore also increased with the particle size, with the residual bark particles present in the BP fraction being denser and above all more conductive than the pith particles (Table 4).

The amaranth pith thus appears as a promising bulk raw material for the thermal insulation of buildings, as previously observed for sunflower pith [20,22]. This makes it possible to position the pith or one of its sieved fractions, especially the smaller one (i.e., the SP fraction) for which the thermal conductivity and thermal resistance are significantly

different from the two others, in the building insulation market. For example, it could be blown into the attic of houses or used as a filling for the interior partitions. However, it remains to be seen how the pith particles will behave over time. If the pith is compacted, it could become denser over the years and its thermal conductivity will increase. In the same way, for long-term use, it will also be necessary to judge the pith's behaviour to fire and its ability to resist fungi.

For future work, the coating of the pith particles by a hydrophobing agen<sup>t</sup> (e.g., hydrogenated oils, vegetable oil derivatives, etc.) and a fireproofing product would improve their water and fire resistances, respectively. In the same way, glycerol esters could be a bio-based solution favourable to render the pith particles more resistant to microbial growth when coated at their surface [22].

#### *4.2. Low-Density Insulation Blocks from Pith*

Cohesive low-density insulation blocks were produced through compression molding thanks to the addition of the starchy binder. A progressive increase in the density of the blocks made of MP particles was observed with the binder content (Table 6). In parallel, the bending performance of the blocks improved since a larger binder quantity allowed the pith particles to be better impregnated with the starchy glue. Logically, the overall cohesion of the block was thus progressively improved. However, this densification resulted in a lower internal porosity inside the blocks, which then had a higher and higher thermal conductivity and therefore a lower and lower thermal resistance.

As the MP-based blocks remained machinable even with only 10% (*w*/*w*) binder, this content was retained for the other pith fractions (Figure 3) since it was expected to give to the insulating blocks a better thermal insulation performance. For such binder content added, the densest and most conductive insulating block was the one made from the mix of pith particles, which could be explained by the fact that this fraction still contained about 9% (*w*/*w*) of bark impurities (Table 2). For the blocks made from the sieved fractions, the values obtained for the density and thermal conductivity were significantly different. A significant reduction in the density and thermal conductivity was observed with increasing particle size when comparing the materials made from the SP and MP fractions, respectively. The most likely reason for such a result was the presence of more inter-particle voids in the case of the block made from the medium size particles. Conversely, and as it was already observed for the bulk pith particles, a significant increase in the density and, to a lesser extent, in thermal conductivity was obtained for the block made from the bigger particles. This was evidently due to the substantial amount of dense bark impurities remaining inside the BP fraction after sieving.

For this 10% (*w*/*w*) binder content, the flexural performance of the insulating blocks made from the sieved fractions revealed significant differences, and it increased with the size of the pith particles used. Indeed, the larger the particles were, the smaller was their cumulative surface area. Thus, with the pith particles from the BP fraction, more starch-based adhesive was present on their surface, which resulted in a better bonding of the particles to each other and therefore better mechanical performance in bending of the corresponding block. The flexural strength of the block originating from the mix of all the pith particles was also significantly different and logically median, i.e., situated between those of the MP and BP-based blocks.

For the optimal 10% (*w*/*w*) binder content, the best compromise between flexural and heat insulation properties was obtained using big particles (>4 mm) (Table 6). Light and insulating, this optimal BP-based block (Figures 2b and 3b) especially preserved very good machinability, and it could thus be positioned at all levels of the buildings, e.g., walls, floors, roofs, etc. However, its thermal conductivity at 25 ◦C (56 mW/(m.K)) was higher to those of sunflower-based insulation panels [20,22] and especially commercial expanded polystyrene [20]: 38–41 mW/(m.K) and only 32 mW/(m.K), respectively. These results made it a slightly less efficient insulator than the two other materials mentioned above. Indeed, based on the thermal resistance of these three types of insulating materials, defined as the ratio of the material thickness to its thermal conductivity, the thickness of the amaranth-based optimal block should be increased by 37–47% or by up to 75%, respectively, to reach the same thermal insulating capacity as sunflower-based low-density blocks or polystyrene.

**Figure 3.** (**a**) Photograph of the low-density insulation block made from the MP sieved fraction of pith particles; (**b**) Photograph of the low-density insulation block made from the BP sieved fraction of pith particles; (**c**) Photograph of the low-density insulation block made from the SP sieved fraction of pith particles; (**d**) Photograph of the low-density insulation block made from the mix of the SP, MP, and BP pith particles. All the insulation blocks have a 10% (*w*/*w*) starchy binder content.

However, these results are only preliminary, and some improvements are still possible. On the one hand, a better purification of the BP pith particles, which would then contain a lower proportion of bark impurities, should lead to a reduction in the thermal conductivity of the obtained insulating block. On the other hand, an adaptation of the compression molding conditions should also improve the insulating capacity of the amaranth-based block. For future work, its water vapor permeability will also have to be determined. As for the low-density blocks made from sunflower pith [20,22], it should be quite high, which could also make this insulating material a promising water regulator.

As for the pith particles used in bulk, additional developments will also be required to improve the durability of the optimal low-density insulation block before proposing it to the market. A reduction in its water sensitivity could be achieved thanks to the replacement of the starchy binder by another polysaccharide binder with physical curing, e.g., alginates and especially *Citrus* pectins and chitosan [50]. It could be also achieved thanks to its coating by hydrophobing agents even if its ability as a water regulator may be partially impaired after such post-treatment. The addition of a fireproofing product to the formulation would also render the amaranth-based block more resistant to fire. Lastly, glycerol esters appear as a promising renewable solution to protect it against microbial growth [22].

#### *4.3. Hardboards from Bark*

The high lignocellulose content for bark (43.6% cellulose, 17.6% hemicelluloses, and 20.3% lignins) contributed to its potential for producing hardboards (HB), i.e., dense fiberboards, through hot pressing. Cohesive hardboards were obtained with no external binder added (HB1 and HB4) (Table 8), due to the adhesive ability of some chemicals inside bark (i.e., free sugars, hemicelluloses, and lignins). Both HB1 and HB4 boards could thus be considered as promising binderless fiberboards.

Especially, the HB4 hardboard obtained from a previously extrusion-refined bark (ERB), using water at a 4 liquid/solid ratio, was much more mechanically resistant (+50% for flexural strength) than that from ground bark (GB). This was already observed for coriander straw [13], and for shives from oleaginous flax [30]. The extrusion-refining pretreatment contributed to a much more favorable fiber morphology (large increase in their mean aspect ratio), just as to an efficient separation between cellulose, hemicelluloses, and lignins inside the extruded material, thus facilitating the mobilization of internal binders during hot pressing.

An additional improvement in the bending properties was obtained by adding to the bark grinded and deoiled amaranth seeds. Here, because of its richness in proteins and especially starch, this AB amaranth-based binder was used as a natural external binder, and its binding ability was evidenced for the ground bark, just as for the extrusion-refined one. In addition, especially for the ERB fibrous reinforcement, the more the amaranth-based binder added, the more the flexural strength and the more the elastic modulus. The best flexural properties were obtained when adding 20% AB to ERB (Table 8). Such amount of AB added allowed the ERB fibers to stick well together.

From the point of view of sensitivity to water of the hardboards produced, using the extrusion-refined bark divided the thickness swelling and water absorption values by up to 2 for a panel without exogenous binder, and by up to 1.5 for a panel hot pressed with 20% amaranth-based binder added (Table 8). The interest of the preliminary defibration of the amaranth bark in a twin-screw extruder regarding the water resistance of the obtained panels is thus indisputable.

The optimal hardboard (HB6) (Figure 2) has a 36 MPa flexural strength, which is significantly different from all the other strength values in the present study. This value is also 23% higher than that of another fiberboard recently developed from coriander (29 MPa) [13]. It is also much higher than the flexural strengths of two commercial woodbased materials tested in a previous study [29], i.e., MDF and chipboard: 73% and up to 251%, respectively (Table 8). HB6 board is thus undoubtedly a viable, sustainable alternative for replacing current building wood-based materials like plywood, chipboard, OSB, MDF, etc. According to the NF EN 312 standard [51], it is already usable for interior fittings (including furniture) in a dry environment (P2 type board). In addition to furniture, its possible uses in buildings include interior partitions, underlays for floors, panels for suspended ceilings, etc. However, its fire resistance will have to be characterized before using it inside houses. The addition of a flame retardant to the formulation prior to hot pressing may be considered in the event of poor fire resistance.

The optimal board nevertheless reveals a greater sensitivity to water after 24 h immersion than commercial MDF or chipboard type panels (Table 8). With much improved water resistance (10% max thickness swelling), it could also be used as a P7 type board, i.e., a board working under high stress, used in humid environment. For future work, multiple solutions could be investigated to improve the water resistance. Firstly, a reduction in thickness swelling could be attained thanks to a heat post-treatment. This was already evidenced for two types of renewable fiberboards, i.e., one made of coriander straw as reinforcement and press cake from seeds as binder [13], and another made of oleaginous flax shives as reinforcement and plasticized linseed cake as binder [31]. At the end of this post-treatment, an increase in the flexural strength was even observed at the same time as the reduction in thickness swelling for these two innovative materials.

In order to meet the same objective of increasing water resistance, other treatments may be tested after hot pressing, e.g., coating, chemical, or steam treatment [30]. In particular, the application of a coating based on vegetable oils or their derivatives could be a promising renewable solution to improve the stability in the board dimension after soaking in water. Among these bio-sourced additives, drying oils such as linseed oil or hydrogenated oils that have the property of being solid at room temperature appear to be good candidates.

For future industrial process intensification, one single extruder pass would allow the continuous production of a premix ready to be molded through hot pressing, and associating the refined fibers from amaranth bark as mechanical reinforcement, the amaranth-based binder and even water-repellent and/or flame-retardant additives [31]. Indeed, in addition to its ability evidenced in this study to refine plant fibers, the twin-screw extrusion technology is also known for its particularly intense mixing ability [25,26]. From the same machine, the amaranth bark could thus be extrusion-refined in the presence of water in the first half of the screw profile. Then, the deoiled amaranth seeds and any additives could be added in its middle thanks to a side feeder. Finally, the second half of the screw profile could be used for the intimate mixing of all these components with each other. Using the as-described combined process, the premix would be produced at a reduced cost before being transformed into hardboards.
