The authors would like to express their sincere gratitude to R&#233;gion Occitanie (France) that funded this research through ERDF (GEOFIBNET project, grant number MP0013559).

1. Prepare the raw materials
<ol>
	<li>Use oleaginous flax shives, which are the result of a preliminary stage of mechanical extraction of the bast fibers from straw in an &#34;all fiber&#34; extraction device51. Use a vibrating sieve to remove short textile fibers that they may still contain. 
	NOTE: As the removal of these short textile fibers may be difficult, do not hesitate to repeat this sieving operation as many times as necessary. Here, the objective is to improve the flow of the ol...

The protocol outlined here describes how to process the extrusion-refining of lignocellulosic fibers before using them as mechanical reinforcement in renewable boards. Here, the twin-screw extruder used is a pilot scale machine. With screws of 53 mm in diameter (D), it is equipped with eight modules, each 4D in length, except for module 1 that has an 8D length, corresponding to a 36D total length (i.e., 1,908 mm) for the barrel. Its length is long enough to apply to the processed material the succession of several elemen...

Extrusion is a process during which a flowing material is forced through a hot die. Extrusion, therefore, permits the forming of preheated products under pressure. The first industrial single-screw extruder appeared in 1873. It was used for the manufacture of metallic continuous cables. From 1930 onwards, single-screw extrusion was adapted to the food industry to produce sausages and past. Conversely, the first twin-screw extruder has first been used for developments in the food industry. It did not appear in the field of synthetic polymers until the 1940s. For this purpose, new machines were designed, and their operation was also modeled1. A system with co-penetrating and co-rotating screws was developed, allowing mixing and extrusion to be carried out simultaneously. Since then, the extrusion technology has developed continuously via the design of new types of screws. Today, the food industry makes extensive use of twin-screw extrusion although it is more expensive than single-screw extrusion as twin-screw extrusion permits access to more elaborated material processing and final products. It is particularly used for extrusion-cooking of starchy products but also the texturing of proteins and the manufacture of pet food and fish feed.
More recently, twin-screw extrusion has seen its field of application extended to the thermo-mechano-chemical fractionation of plant matter2,3. This new concept has led to the development of real reactors capable of transforming or fractionating plant matters in a single step, up to the separate production of an extract and a raffinate by liquid/solid separation2,3,4. Work carried out at the Laboratory of Agro-industrial Chemistry (LCA) has highlighted the multiple possibilities of the twin-screw technology for the fractionation and valorization of agroresources2,3. Some of the examples are: 1) The mechanical pressing and/or &#34;green&#34; solvent extraction of vegetable oil5,6,7,8,9,10. 2) The extraction of hemicelluloses11,12, pectins13, proteins14,15, and polyphenolic extracts16. 3) The enzymatic degradation of plant cell walls for producing second-generation bioethanol17. 4) The production of biocomposite materials with protein18 or polysaccharide19 matrices. 5) The production of thermoplastic materials by mixing cereals, and bio-based polyesters20,21. 6) The production of biocomposites by compounding a thermoplastic polymer, bio-based or not, and plant fillers22,23. 7) The defibration of lignocellulosic materials for producing paper pulp13,24, and fiberboards25,26,27,28,29,30,31,32.
The twin-screw extruder is often considered as a continuous thermo-mechano-chemical (TMC) reactor. Indeed, it combines in a single step chemical, thermal, and, also, mechanical actions. The chemical one results in the possibility to inject liquid reagents in various points along the barrel. The thermal one is possible due to the thermal regulation of the barrel. Lastly, the mechanical one depends on the choice of the screw elements along the screw profile.
For the defibration of lignocellulosic materials to produce fiberboards, the most recent works have used rice straw25,28, coriander straw26,29, oleaginous flax shives27 as well as sunflower30,32 and amaranth31 barks. The current interest of lignocellulosic biomasses for such an application (i.e., mechanical reinforcement) is explained by the regular depletion of forest resources used for producing wood-based materials. Crop residues are inexpensive and may be widely available. In addition, current wood particles are mixed with petrochemical resins which can be toxic. Often accounting for more than 30% of the total cost of current commercial materials33, some resins contribute to formaldehyde emissions and reduce indoor air quality34. Research interest has shifted to the use of natural binders.
Lignocellulosic biomass is mainly composed of cellulose and hemicelluloses, forming a heterogeneous complex. Hemicelluloses are impregnated with layers of lignins that form a three-dimensional network around these complexes. The use of lignocellulosic biomass for the manufacture of fiberboards generally requires a defibration pre-treatment. For this, it is necessary to break down the lignins that protect cellulose and hemicelluloses. Mechanical, thermal, and chemical35 or even enzymatic36,37,38 pre-treatments must be applied. These steps also increase self-adhesion of fibers, which can promote the production of binderless boards27 even if an exogenous binder is most often added.
The primary purpose of pre-treatments is to improve the particle size profile of micrometric fibers. A simple grinding offers the possibility to reduce the fiber size27,39,40. Inexpensive, it contributes to increase the fiber specific surface. The components of the inner cell wall become more accessible and the mechanical properties of the obtained panels are improved. The efficiency of defibration is significantly increased when a thermo-mechanical pulp is produced, e.g., by digestion plus defibration41, from different pulping processes42 or by steam explosion43,44,45,46,47. More recently, LCA has developed an original pre-treatment of lignocellulosic fibers using twin-screw extrusion25,26,27,28,29,30,31,32. After TMC defibration, the extruder also enables the homogeneous dispersion of a natural binder inside fibers. The resulting premix is ready to be hot pressed into fiberboards.
During the defibration of rice straw, twin-screw extrusion was compared to a digestion plus defibration process25. The extrusion method revealed a significantly reduced cost, i.e., nine times lower than the pulping one. Furthermore, the amount of added water is reduced (1.0 max liquid/solid ratio instead of 4.0 min with the pulping method), and a clear increase in the average aspect ratio of refined fibers (21.2-22.6 instead of 16.3-17.9) is observed as well. These fibers present highly improved mechanical strengthening capability. This was demonstrated for rice straw-based fiberboards, in which pure non deteriorated lignin (e.g., Biolignin) was used as a binder (up to 50 MPa for bending strength and 24% for thickness swelling after 24 h immersion in water)28.
The interest of TMC defibration in twin-screw extruder has also been confirmed with coriander straw26. The aspect ratio of refined fibers varies from 22.9-26.5 instead of only 4.5 for simply ground fibers. 100% coriander-based fiberboards were obtained by adding to the extrusion-refined straws a cake from the seed as protein binder (40% in mass). Their flexural strength (up to 29 MPa) and especially their resistance to water (up to 24% thickness swelling) were significantly improved compared to panels made from simply crushed straw. Moreover, these panels do not emit formaldehyde and, as a consequence, they are more environmentally and human-health friendly than medium-density fiberboard (MDF) and chipboard29 classically found in the market.
Similarly, panels entirely based on amaranth31 and sunflower32, combining extrusion-refined fibers from bark as reinforcement and seed cake as a protein binder, were successfully produced. They showed flexural strengths of 35 MPa and 36 MPa, respectively. However, their water resistance was found to be lower: 71% and 87%, respectively, for thickness swelling. Self-bonded panels based on extrusion-refined shives from oleaginous flax straw can also be obtained27. In this case, it is the ligneous fraction, released during the twin-screw TMC defibration, that contributes to the self-bonding. However, hardboards obtained show a lower mechanical strength (only 12 MPa flexural strength), and very high thickness swelling (127%).
All the extruded fiber-based panels presented above can find industrial applications and are, therefore, sustainable alternatives to current commercial wood-based materials. According to the International Organization for Standardization (ISO) requirements48,49,50, their specific applications will depend on their mechanical and water sensitivity characteristics.
In this paper, the procedure to extrude and refine lignocellulosic fibers before using them as mechanical reinforcement in renewable boards is described in detail. As a reminder, this process reduces the amount of water to be added in comparison to traditional pulping methodologies, and it is also less energy consuming25. The same twin-screw machine can also be used for adding a natural binder to fibers.
More specifically, a detailed outline for conducting the twin-screw extrusion-refining of shives from oleaginous flax (Linum usitatissimum L.) straw is presented. The straw used in this study was commercially obtained. It was from the Everest variety, and the plants were cultivated in the South West part of France in 2018. In the same extruder pass, a plasticized linseed cake (used as exogenous binder) can also be added in the middle of the barrel, and then mixed intimately to the refined shives along the second half of the screw profile. A homogeneous mixture having the form of a fluffy material is collected at the machine outlet. The one-step TMC operation is conducted using a pilot scale machine. Our goal is to provide a detailed procedure for the operators to conduct properly the extrusion-refining of shives, and then the cake addition. Following this operation, the obtained premix is ready for subsequent manufacture of 100% oleaginous flax-based hardboards using hot pressing.

<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>Analogue durometer</td>
			<td>Bareiss</td>
			<td>HP Shore</td>
			<td>Device used for determining the Shore D surface hardness of fiberboards</td>
		</tr>
		<tr>
			<td>Ash furnace</td>
			<td>Nabetherm</td>
			<td>Controller B 180</td>
			<td>Furnace used for the mineral content determinations</td>
		</tr>
		<tr>
			<td>Belt dryer</td>
			<td>Clextral</td>
			<td>Evolum 600</td>
			<td>Belt dryer used for the continuous drying of extrudates at the exit of the twin-screw extruder</td>
		</tr>
		<tr>
			<td>Cold extraction unit</td>
			<td>FOSS</td>
			<td>FT 121 Fibertec</td>
			<td>Cold extractor used for determining the fiber content inside solid materials</td>
		</tr>
		<tr>
			<td>Densitometer</td>
			<td>MA.TEC</td>
			<td>Densi-Tap IG/4</td>
			<td>Device used for determining apparent and tapped densities of extrudates once dried</td>
		</tr>
		<tr>
			<td>Double-helix mixer</td>
			<td>Electra</td>
			<td>MH 400</td>
			<td>Mixer used for preparing the solid mixture made of the raw shives and the plasticized linseed cake for producing board number 12</td>
		</tr>
		<tr>
			<td>Fiber morphology analyzer</td>
			<td>Techpap</td>
			<td>MorFi Compact</td>
			<td>Analyzer used for determining the morphological characteristics of extrusion-refined shives</td>
		</tr>
		<tr>
			<td>Gravimetric belt feeder</td>
			<td>Coperion K-Tron</td>
			<td>SWB-300-N</td>
			<td>Feeder used for the quantification of the oleaginous flax shives</td>
		</tr>
		<tr>
			<td>Gravimetric screw feeder</td>
			<td>Coperion K-Tron</td>
			<td>K-ML-KT20</td>
			<td>Feeder used for the quantification of the plasticized linseed cake</td>
		</tr>
		<tr>
			<td>Hammer mill</td>
			<td>Electra</td>
			<td>BC P</td>
			<td>Crusher used for the grinding of granules made of plasticized linseed cake</td>
		</tr>
		<tr>
			<td>Heated hydraulic press</td>
			<td>Pinette Emidecau Industries</td>
			<td>PEI 400-t</td>
			<td>Hydraulic press used for molding the fiberboards through hot pressing</td>
		</tr>
		<tr>
			<td>Hot extraction unit</td>
			<td>FOSS</td>
			<td>FT 122 Fibertec</td>
			<td>Hot extractor used for determining the water-soluble and fiber contents inside solid materials</td>
		</tr>
		<tr>
			<td>Image analysis software</td>
			<td>National Institutes of Health</td>
			<td>ImageJ</td>
			<td>Software used for determining the morphological characteristics of raw shives</td>
		</tr>
		<tr>
			<td>Oleaginous flax straw</td>
			<td>Ovalie Innovation</td>
			<td>N/A</td>
			<td>Raw material supplied for the experimental work</td>
		</tr>
		<tr>
			<td>Piston pump</td>
			<td>Clextral DKM</td>
			<td>Super MD-PP-63</td>
			<td>Pump used for the water quantification and injection</td>
		</tr>
		<tr>
			<td>Scanner</td>
			<td>Toshiba</td>
			<td>e-Studio 257</td>
			<td>Scanner used for taking an image of raw shives in gray level</td>
		</tr>
		<tr>
			<td>Side feeder</td>
			<td>Clextral</td>
			<td>E36</td>
			<td>Feeder used to force the introduction of the plasticized linseed cake inside the barrel (at the level of module 5) for configuration (b)</td>
		</tr>
		<tr>
			<td>Thermogravimetric analyzer</td>
			<td>Shimadzu</td>
			<td>TGA-50</td>
			<td>Analyzer used for conducting the thermogravimetric analysis of the solids being processed</td>
		</tr>
		<tr>
			<td>Twin-screw extruder</td>
			<td>Clextral</td>
			<td>Evolum HT 53</td>
			<td>Co-rotating and co-penetrating pilot scale twin-screw extruder having a 36D total length (D is the screw diameter, i.e., 53 mm)</td>
		</tr>
		<tr>
			<td>Universal oven</td>
			<td>Memmert</td>
			<td>UN30</td>
			<td>Oven used for the moisture content determinations</td>
		</tr>
		<tr>
			<td>Universal testing machine</td>
			<td>Instron</td>
			<td>33R4204</td>
			<td>Testing machine used for determining the bending properties of fiberboards</td>
		</tr>
		<tr>
			<td>Ventilated oven</td>
			<td>France Etuves</td>
			<td>XL2520</td>
			<td>Oven used for the discontinuous drying of extrudates at the exit of the twin-screw extruder</td>
		</tr>
		<tr>
			<td>Vibrating sieve shaker</td>
			<td>RITEC</td>
			<td>RITEC 600</td>
			<td>Sieve shaker used for the sieving of the plasticized linseed cake</td>
		</tr>
		<tr>
			<td>Vibrating sieve shaker</td>
			<td>RITEC</td>
			<td>RITEC 1800</td>
			<td>Sieve shaker used for removing short bast fibers entrapped inside the oleaginous flax shives</td>
		</tr>
	</tbody>
</table>

twin-screw extrusion process to produce renewable fiberboards

A versatile twin-screw extrusion process to provide an efficient thermo-mechano-chemical pre-treatment on lignocellulosic biomass before using it as source of mechanical reinforcement in fully bio-based fiberboards was developed. Various lignocellulosic crop by-products have already been successfully pre-treated through this process, e.g., cereal straws (especially rice), coriander straw, shives from oleaginous flax straw, and bark of both amaranth and sunflower stems.
The extrusion process results in a marked increase in the average fiber aspect ratio, leading to improved mechanical properties of fiberboards. The twin-screw extruder can also be fitted with a filtration module at the end of the barrel. The continuous extraction of various chemicals (e.g., free sugars, hemicelluloses, volatiles from essential oil fractions, etc.) from the lignocellulosic substrate, and the fiber refining can, therefore, be performed simultaneously.
The extruder can also be used for its mixing ability: a natural binder (e.g., Organosolv lignins, protein-based oilcakes, starch, etc.) can be added to the refined fibers at the end of the screw profile. The obtained premix is ready to be molded through hot pressing, with the natural binder contributing to fiberboard cohesion. Such a combined process in a single extruder pass improves the production time, production cost, and may lead to reduction in plant production size. Because all the operations are performed in a single step, fiber morphology is better preserved, thanks to a reduced residence time of the material inside the extruder, resulting in enhanced material performances. Such one-step extrusion operation may be at the origin of a valuable industrial process intensification.
Compared to commercial wood-based materials, these fully bio-based fiberboards do not emit any formaldehyde, and they could find various applications, e.g., intermediate containers, furniture, domestic flooring, shelving, general construction, etc.

During the fiber refining of oleaginous flax shives using configuration (step 3.1.1), water was deliberately added at a liquid/solid ratio equal to 1.0. According to previous works25,26,27, such a liquid/solid ratio better preserves the length of the refined fibers at the twin-screw extruder outlet than lower ratios, which simultaneously contributes to an increase in their average aspect ratio. Furthermore, the amount of water a...

Watch this Scientific Journal Video about Twin-Screw Extrusion Process to Produce Renewable Fiberboards at JoVE.com

Twin-Screw Extrusion Process to Produce Renewable Fiberboards

A versatile twin-screw extrusion process to provide an efficient thermo-mechano-chemical pretreatment on lignocellulosic biomass was developed, which leads to an increased average fiber aspect ratio. A natural binder can also be added continuously after fiber refining, leading to bio-based fiberboards with improved mechanical properties after hot pressing of the obtained extruded material.

A versatile twin-screw extrusion process to provide an efficient thermo-mechano-chemical pre-treatment on lignocellulosic biomass before using it as ...

This twin-screw extrusion technology can be used to conduct continuously and efficiently the pretreatment of lignocellulosic biomasses prior to fiberboard molding. The homogeneous thermo-mechanical pulps are produced at low cost and using a reduced volume of water. And the versatility of extrusion enables the addition of a natural binder in a single pass.Using combined temperature moisture content is a key for a successful twin-screw extrusion of lignocellulosic fibers. So you always start with an excess of water, then you decrease the amounts until getting the desired texture. As the process is really sensitive we have to rely on all of the senses.So the video adds sight and hearing to the paper, smell and touch in the near future. Demonstrating the procedure will Laurent Labonne, an engineer in my laboratory. Laurent will be assisted by Saif Ullah Khan, a PhD student in the lab.Begin by using half clamps to connect the twin-screw extruder modules in the correct configuration for fiber defibration only. Position the water inlet pipe laterally at the end of module two to connect the piston pump to the machine, and check the screw element type, length, pitch, and staggering angle. Insert the screw elements along the two splined shafts in pairs, taking care that the threads of each inserted screw element are always perfectly aligned with the previously assembled elements.Once the entire screw profile has been assembled, manually screw the screw points at the ends of the two shafts. Completely close the barrel of the machine and use a torque wrench to tighten the two screw points to the tightening torque recommended by the manufacturer. With the barrel of the machine partially reopened and the shafts retracted into the barrel over a distance of approximately one dimension, turn the screws at a maximum of 25 revolutions per minute to ensure that the entire screw profile is correctly fitted.When both shafts have been entirely trapped inside the barrel, use half clamps to secure the barrel to the machine and use a level tester to confirm that the barrel is perfectly horizontal. For twin-screw extrusion treatment, enter the set temperatures of each of the modules and start the temperature control of the barrel. When the temperature has stabilized, turn the screws at a maximum of 50 revolutions per minute and gently feed the twin screw extruder with water at a flow rate of five kilograms per hour.Wait about 30 seconds for water to come out of the end of the barrel before introducing the oleaginous flax shives into module one at a three kilogram per hour flow rate. Wait about one minute for the solid to begin coming out of the extruder before gradually increasing the speed of the screws, water, and shive flow rates in at least three success steps until the desired set points have been reached. Allow the machine to stabilize for 10 to 15 minutes until the electrical current consumed by the engine over time varies no more than 5%from the 125 amp average value.For the addition of natural binder, once the machine has stabilized begin introducing the plasticized linseed cake at a rate of 0.5 kilograms per hour. Then, increase the flow rate in at least three successive steps up to the desired set point. Once the electrical current consumed by the twin-screw extruder motor is perfectly stable, make sure that the temperature profile measured along the barrel conforms to the set values given by the operator, and start sampling the extruded shives at the outlet.At the end of production, switch off the two solid dosing units and the piston pump, and empty the machine while gradually reducing the speed of the rotation of the screws to 50 revolutions per minute. When no additional sampling is coming out of the barrel end, use water to clean the inside of the barrel of the twin-screw extruder while the screws are still rotating, until the solid residues disappear completely at the outlet of the barrel. Then, stop the rotation of the screws and switch off the heating control of the machine.Immediately after the twin-screw extrusion process, dry the extrudates with hot air flow to a humidity between 3%and 4%When the extrudate has dried, position the material in a preheated mold and preheat the material. After three minutes, apply the appropriate amount of pressure to the material and mold the material for 150 seconds at the 200 degrees Celsius already set up. Once the press is open, demold the panel and check its cohesion.Following the extrusion refining pre-treatment, the chemical composition of the extrusion refined fibers can be determined. In the absence of liquid extract generation during the extrusion refining pretreatment, no significant difference in chemical composition was observed between the raw shives and the extruded ones. In terms of appearance, extrusion refined fibers have a fluffy material form, indicating that the extrusion process, in particular the high shear rate applied, contributes to a modification of the flax shive structure.This is confirmed by the lower apparent and tapped densities of the extruded shives compared to the values obtained with the raw shives. Morphological analysis of the fibers also confirms this observation as a very significant increase in their aspect ratio, using a fiber morphology analysis device. Boards made from extruded fibers alone, without the addition of plasticized linseed cake as an external binder, are not only all three cohesive, but above all present significantly improved usage properties compared to boards obtained by hot pressing of the raw shives.Overall, the addition of plasticized linseed cake to the extrusion refined fibers increases the flexural properties of the fiber boards obtained. Indeed, with the addition of 25%of this binder the obtained board has a flexural strength of 10.6 mega pascals, instead of only 3.6. For process intensification, the fiber refining and the binder addition can be conducted in a single pass.This process will adapt to all types of raw materials.

Twin-Screw Extrusion Process to Produce Renewable Fiberboards

Abstract