# Effects of composition and process on the performance of thermopressed materials prepared from model and real-life biomass

Rui Ouyang, Hugo Voisin, Emilie Korbel, Laurent Chaunier, Denis Lourdin, Gael Colomines, Isabelle Capron

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Rui Ouyang, Hugo Voisin, Emilie Korbel, Laurent Chaunier, Denis Lourdin, et al.. Effects of composition and process on the performance of thermopressed materials prepared from model and real-life biomass. Industrial Crops and Products, 2026, 239, pp.122496. 10.1016/j.indcrop.2025.122496. hal-05419697

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# Effects of composition and process on the performance of thermopressed materials prepared from model and real-life biomass

Rui Ouyang<sup>a,b</sup>, Hugo Voisin<sup>a,*</sup>, Emilie Korbel<sup>c,D</sup>, Laurent Chaunier<sup>a,D</sup>, Denis Lourdin<sup>a</sup>,
Gael Colomines<sup>d,D</sup>, Isabelle Capron<sup>a</sup>

## ARTICLE INFO

### Keywords:

- Biobased materials

- Thermocompression

- Binderless materials, brewery coproducts

- Mechanical properties

- Water resistance

## ABSTRACT

There is growing interest in turning the by-products of agricultural processes into biobased and biodegradable materials. The complexity of composition and structure of biomass makes it difficult to understand the mechanisms involved in its transformation into materials, requiring model systems. In this study, solid materials were obtained from model cellulosic fibers (modulus of 5.5–5.8 GPa) and biomass from brewery co-products (modulus of 2.2–2.6 GPa) using uniaxial compression. Their properties were comparable to or exceeded those reported for conventional binderless cellulosic materials processed at higher temperatures (~200 °C), thus highlighting the efficiency of our low-temperature process. The effect of the fiber size and composition, of the processing temperature on the resulting samples mechanical properties was assessed. We show that longer initial fibers, with higher aspect ratio, led to improved flexural modulus and strength due to a higher degree of fiber entanglement, which translated into better performance for the less transformed biomass. While purely cellulosic samples displayed better mechanical properties than those prepared from biomass at a pressing temperature of 100°C, the increase to 140°C closed the gap between the two biomasses particularly in terms of water resistance due to a reduced porosity. Finally, model cellulosic polymers improved the water resistance of the cellulosic samples due to the formation of water tight matrix. This low temperature, binder-free method is promising for the green transformation of biomass coproducts.

## 1. Introduction

Biomass has shown substantial potential as a source for developing materials that can replace petroleum-based products. This renewable, abundant, and biodegradable resource offers a sustainable alternative for a large range of industrial applications. Common applications of biomass-derived materials include biodegradable packaging (Koreshkov et al., 2024), construction composite reinforcement (Bisht et al., 2020; Chen et al., 2024), antibacterial agents (Solihat et al., 2024), commercial value-added chemicals (Cheng et al., 2024) and electronic component (Lou et al., 2021, 2022), and interior components in the automotive industry, where natural fibers are incorporated to reduce reliance on synthetic materials (Oesef et al., 2024). Notably, co-products from industries such as brewing represent promising sources of biomass. Wet brewery spent grain (BSG) is the residue obtained after the mashing

stage of beer production, which involves steeping malt in hot water to extract sugars and enzymatically hydrolyze the starch. Beer production generates about 8 million tons of BSG annually in Europe and 40 million worldwide. Recovering these products is an opportunity in the current context of sustainable environmental transition. Brewer's spent grains (BSG), primarily due to their high protein content, are commonly used as animal feed and food ingredients for human consumption (Pasquet et al., 2024). However, effective valorization of these co-products faces significant logistical challenges. The high moisture content and biochemical composition of BSG (predominantly carbohydrates and proteins) make it highly perishable to chemical deterioration and microbial spoilage by fungi, yeasts and aerobic bacteria (Rossi et al., 2023), which limits transportation distances and requires non-negligible capital and operating costs (Mainardis et al., 2024), whereas smaller-scale breweries, including microbreweries and craft breweries, often lack

<footnote><sup>a</sup> INRAE, UR BIA, Nantes 44316, France</footnote>

<footnote><sup>b</sup> UMR STLO, INRAE, Institut Agro Rennes-Angers, Rennes 35000, France</footnote>

<footnote><sup>c</sup> Oniris, Nantes Université, CNRS, GEPEA, UMR, Nantes 6144, France</footnote>

<footnote><sup>d</sup> Nantes Université, Oniris, CNRS, GEPEA, UMR, Nantes 6144, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail addresses: rui.ouyang@inrae.fr (R. Ouyang), hugo.voisin@inrae.fr (H. Voisin).</footnote>
the capacity to support this logistics chain and must rely on limited local disposal methods, such as land application, bioenergy production, or limited use in animal feed. These local routes are increasingly inadequate given rising BSG volumes and the growing interest in higher-value uses; therefore, a portfolio of co-existing recovery strategies is required. In this context, the re-use of BSG for material production is an attractive alternative that has not yet been fully explored.

Historically, manufacturing processes for these materials have relied on traditional methods that often involve synthetic binders and chemical additives to achieve cohesion and durability (Kariuki et al., 2020). However, such binders (such as formaldehyde derivatives resins) pose environmental and health issues due to emissions and limited biodegradability. Thermocompression has emerged as an eco-friendly and cost-effective alternative, using high pressure and moderate heat to compress biomass without the need for synthetic binders. This technique exploits the natural binding properties of biomass, allowing cohesive structures to form through self-bonding mechanisms (Pintiaux et al., 2015). These sources are composed of an association of various molecules including notably cellulose, lignin, pectin and hemicelluloses. Cellulose, the most abundant polymer on Earth, is constituted of nanofibrils, it exhibits high aspect ratio, high mechanical strength (elastic modulus ≈84 GPa) (Cheng et al., 2009) and excellent thermal stability, making them ideal for applications demanding high-performance materials (Nechyporchuk et al., 2016). In addition, the thermocompression process promotes interactions between cellulose, hemicellulose, and lignin components, with lignin and hemicelluloses acting as natural binders under heat and pressure, thereby reducing or even eliminating the need for chemical additives (Antoun et al., 2024). However, traditional thermocompression methods often require high temperature, typically exceeding 180–200°C, to achieve satisfactory cohesion and mechanical performance (Hashim et al., 2011). Elevated temperatures promote the hydrolysis of hemicellulose into furan monomers and furfural. These compounds are believed to subsequently form lignin-furfural linkages or undergo self-polymerization during the pressing stage, which provides the primary self-bonding strength in binderless fiberboards (Zhang et al., 2015; Cavailles et al., 2024). These evidences support the rationale for the use of high temperatures in these processes. However, such high temperatures can increase energy consumption and may compromise the material’s environmental virtuosity.

In addition, the inherent complexity and variability of biomass make it challenging to predict material properties accurately. While studies have traditionally explored how chemical composition—such as hemicellulose and lignin—affects material properties. Recent research has broadened this understanding by showing that structural characteristics of fibers, including morphology and network, can significantly influence the strength and cohesion of lignocellulosic materials (Pintiaux et al., 2019; Lou et al., 2023; Cavailles et al., 2024). This emerging focus on fiber research highlights that both the chemical composition and the physical form of fibers are crucial to optimize the mechanical properties and durability of bio-based materials, adding another layer of complexity to understanding and controlling material behavior during processing.

This study showcases a low-tech strategy with low-temperature (max 140°C), binderless thermocompression. By comparing model cellulosic systems with BSG, we show that longer fibers significantly enhances mechanical performance through improved entanglement, while elevating temperature from 100°C to 140°C markedly improves water resistance, particularly in hemicellulose-rich BSG samples.

# 2. Material and methods

## 2.1. Material

Various types of cellulose materials were used in our study, cellulose nanofibrils (CNFs) purchased from Kadant Lamort (France) and highly pure cellulose (HPC) supplied by JRS (Germany) with three different

fiber lengths (10 µm, 200 µm and 900 µm). These HPC samples are commercial powder samples containing 99.5 % cellulose, with commercial names of ARBOCEL UFC100, ARBOCEL BWW40 and ARBOCEL B400 for fibers with an average length of 10 µm, 200 µm and 900 µm, respectively. Brewery coproduct (BSG) was provided by Oniris (France). Xyloglucan (XG) was procured from DSP Gokyo Food & Chemical Co., Ltd. (Japan) and cellulose derivatives (ethyl cellulose (EC), methyl cellulose (MC), and 2-hydroxyethyl cellulose (HEC)) were purchased at Sigma Aldrich (USA). They were stored at room temperature.

## 2.2. CNFs preparation

CNFs were in the form of a slurry with an initial water content of 95 %. To dry the CNFs, they were frozen at –18°C and subsequently subjected to lyophilization (SLK Technik) for three days. The resulting aerogel was cut into small pieces with scissors and micronized into a powder using a laboratory blender (Waring®, USA). The CNF were characterized as polydisperse micro- to nanofibers objects containing 2.1 % residual lignin.

## 2.3. BSG based material preparation

BSG used comes from the Bouffay brewery in Carquefou (France) and were essentially made up of organic barley malt. This spent grains were mechanically separated at Oniris, using a Barracuda® rotary separator (Provisur, France), into 2 fractions: one containing most of the insoluble fiber and the other containing most of the water and residual starch. In this study, only the fibrous fraction will be considered. According to the provider it contains 75 % fiber, of which 44 % is hemicellulose. The raw material was dried at 60°C for 24 h to maintain a water content of less than 5 %. Dry product was then ground into powders using a Pulverisette 14 grinder (Fritsch GmbH, Idar-Oberstein, Germany). Raw material with a size of less than 5 mm were fed from the top into the machine where the rotor disk speed was set at 14,000 rpm with 1 mm trapezoidal perforation sieve rings using a cyclone separator to improve discharge of material from the grinding chamber and limit material self-heating. The material obtained was the BSG –1 mm. A portion was kept for the study and the remainder was ground under the same conditions as above, but using a 0.5 mm trapezoidal perforation sieve rings. The material obtained was the BSG –0.5 mm. A portion was kept for the study and the remainder was ground under the same conditions as above, but using a 0.2 mm trapezoidal perforation sieve rings. The material obtained was the BSG –0.2 mm.

## 2.4. Thermocompression

Samples were pressed using a manual Carver thermopress (USA) equipped with a pressure sensor. The temperature and cooling under pressure were controlled thanks to a home-made device. For process parameters, Rossi et al. emphasized that a process temperature over 180°C was unsuitable for BSG, as the material would fail and collapse at this point. The molding pressure was set to 41 MPa with temperatures of either 100 °C or 140 °C since clear variations were observed in this range, molding time of 10 min or 60 min. At the end of the thermocompression process, a cooling water flow was used (without contacting the sample) to rapidly lower the sample temperature, which can prevent the delamination of samples (Pintiaux et al., 2019).

Prior to thermocompression, powder was conditioned at 20 °C and 59 % relative humidity (RH) for at least one week to ensure moisture equilibrium was reached. 0.7 g of powder was weighted and placed into a mold measuring 4 cm × 1 cm, resulting in final sample thicknesses between 1.2 mm and 1.5 mm and a corresponding density of 1.3–1.5 g/cm³. The powder quantity relative to the mold size was chosen to ensure a basis weight above 1750 g/m² as lower values have previously shown to impart significant macroscopic heterogeneity to the samples (Rol et al., 2020). It took approximately 10 min for the sample temperature to
reach 100 °C and around 1 h to reach 140 °C, depending on the set temperature. All samples were equilibrated for at least 2 weeks in a climatic chamber at 59 % relative humidity and 20 °C, with stable mass achieved after about 10 days.

## 2.5. Protein content

The protein content of powders and thermocompressed samples of brewery coproduct were measured using the Dumas method with a nitrogen analyser Leco FP828 (Leco, St Joseph, Michigan, USA) and Cornerstone software. The nitrogen content was converted to protein content using a protein conversion factor (typically 5.88 for BSG).

## 2.6. Thermogravimetric analysis (TGA)

The water content on a wet basis and thermal degradation of the samples were analyzed using a TGA 2050 thermogravimetric analyzer (USA) under a nitrogen atmosphere. Data acquisition was conducted with TA Instrument Control software, and data processing was carried out using Universal Analysis 2000 software. For moisture content determination, samples were heated to 105°C at a rate of 5 °C/min and held at this temperature for 40 min under isothermal conditions until the mass loss reached a plateau which was evaluated as the water content (See Figure S1). In addition, the thermal degradation behavior of BSG was shown in Figure S2, which indicated that the thermal degradation point of BSG was approximately 200 °C. Fukuzumi et al. (2010) reported that the thermal decomposition points of original cellulose and CNF are 275 and 222 °C, respectively; the thermocompression conditions (100°C or 140 °C) used in this study thus implied no risk of the thermal degradation.

## 2.7. Dynamic mechanical thermal analysis (DMTA)

Dynamic Mechanical Thermal Analysis was performed on a DMTA MKIV (Rheometric Scientific, US). Measures were carried out on parallelepiped specimens (4 cm × 1 cm) whose thickness was precisely measured with a micrometer. The samples were analyzed at ambient temperature (20 +/- 3°C) in the dual cantilever mode on an effective sample length of 20 mm and a frequency of 1 Hz. In order to check the linear domain of deformation the storage modulus E' was measured on a strain domain from 0.005 % to 0.025 %. The modulus values were found constant over the measure range, and the E' values at 0.015 % were reported as representative of each sample and were considered as the samples' Young's modulus. For each combination of composition and processing condition, three independent fiberboards were prepared. Each fiberboard was measured individually under identical test conditions to ensure reproducibility. The reported Young's modulus for each combination represents the mean value derived from these three independent measurements.

## 2.8. Three-point flexural test (3PF)

Flexion measurements were chosen over other typical other methods (compression, traction) as it related better to the envisioned application of such materials. Mechanical tests were carried out at a constant displacement rate set at 1 mm/min, to determine the flexural modulus $E_f$, as well as the flexural strength [Pa] and strain at break [%] of the samples (experimental trials were repeated 3 times).

All sample geometry parameters were measured using a precision caliper with an accuracy of 0.01 mm. A universal testing machine (ZwickRoell S.a.r.l., Ars-Laquenexy, F-57) equipped with a 50N-range force sensor, was used to control the mechanical solicitation and acquired the force F [N] during the displacement d [m] of the loading punch until samples break, allowing the calculation of the flexural stress and strain, as following:

$$ \sigma_f = F \times \frac{3L}{2bh^2} \qquad (1) $$

and

$$ \varepsilon_f = \frac{6dh}{L^2} \qquad (2) $$

Where L is the length between the two supports (L = 0.03 m), h is the sample thickness [m], b is the sample width [m] in the present case of rectangular specimen (Chaunier et al., 2007; Jebalia et al., 2022).

The flexural modulus was characterized by the ratio of the difference in stress [Pa], $\sigma_{f2} - \sigma_{f1}$, to the corresponding difference in strain in the initial linear domain [-], $\varepsilon_{f2}(= 0.0025) - \varepsilon_{f1}(= 0.0005)$:

$$ E_f = \frac{\sigma_{f2} - \sigma_{f1}}{\varepsilon_{f2} - \varepsilon_{f1}} \qquad (3) $$

## 2.9. Scanning electron microscopy (SEM)

SEM observation were carried on a Phenom Pure G6 Desktop SEM. Sample pieces were cut to observe their surface, or their fracture profile after 3PF experiments. They were mounted on sample holder covered with conductive carbon tape. The observation was carried under low vacuum conditions with a 5 kV acceleration tension.

## 2.10. Optical microscopy

The optical microscopic pictures were taken using an upright Olympus system microscope (Model BX51) equipped with a x4 objective.

## 2.11. Granulometric measurement

The granulometry of the initial samples was characterized using a QICPIC QP0402 (Sympatec) coupled with the wet dispersion unit LIX-ELL. 50 mg of the samples powder were first soaked in 5 mL absolute ethanol, before the addition of 20 mL Milli-Q water and magnetic stirring until proper dispersion. The suspension was then characterized in the instrument. The aspect of the fibers was characterized both in size and in aspect ratio. The aspect ratio presented in this study is the shape elongation, or ellipticity, i.e. the ratio of the width over the height of a rotated minimum area bounding rectangle for the measured shape. An ellipticity value of 1 thus corresponds to a square, while lower values indicate more stretched particles

# 3. Results and discussion

We focused on three parameters in this study: the size of the cellulosic or lignocellulosic fibers before compression; the composition of the system; and the temperature used during the thermocompression process. It is worth noting that in order to ensure a process with an environmental impact as low as possible, the temperature used was much lower (in the 100-140°C range) than what is generally described in the field (180-200 °C).

The general procedure of sample preparation, conditioning and characterization is summarized in the Fig. 1. Prior to compression, the powdered samples were kept under specific storage conditions (20°C, 59 % RH) that maintained a water content between 6 % and 10 % (See table S1). This quantity of water was enough to insure efficient heat transfer to the core of samples during thermocompressing, while not high enough to cause sample delamination after pressing as it has previously been described (Pintiaux et al., 2019; Rol et al., 2020). No effect of the variation of the moisture content of the initial material on the samples' properties could be observed, due to the low temperature employed (Pintiaux et al., 2019). This variable was thus not considered in this study.
<figure><img src="image_3.png" /><figcaption>**Fig. 1.** Sample preparation process where powder stabilized in RH are thermocompressed, the resulting plaque of (a) CNF; (b) BSG; and (c) high purity cellulose are stabilized at controlled RH before characterization.</figcaption></figure>

Typical aspects of the samples, presented in Fig. 1, were highly homogeneous, which translated into low variability of the properties of samples with the same composition and process conditions.

### 3.1. Influence of initial fibers morphology

Three highly pure cellulosic (HPC) samples varying in dimension given at 10 µm, 200 µm and 900 µm by the producer, and freeze-dried enzymatically defibrillated pulp (CNF) were characterized for their granulometry, i.e. the average length and aspect ratio (length-to-diameter ratio) (Fig. 2). The size analysis properly followed the data given by the producer. The aspect ratio followed the same trend, with the 900 µm sample definitely being the most anisotropic while the 10 µm is the less anisotropic. The CNF sample presented a slightly higher aspect ratio than the 10 µm sample; however, the results of this sample must be taken with caution as its size was smaller than the lower limit of the confidence range of the instrument. Size and aspect ratio distribution of all the samples are presented in Figure S3, and confirmed that the larger fibers also were more anisotropic.

All sources were thermo-compressed at 100 °C, under a 41 MPa pressure for 10 min, without any additional binder and without chemical modification. They all resulted in solid cellulosic samples, forming homogeneous 4 cm × 1 cm rectangle with a thickness of 1.35 ± 0.15 mm. Every sample was easy to handle, and their surface presented a smooth and glassy aspect as already reported for the production of thermo-compressed lignocellulosic samples (Mobarak et al., 1982;

Okuda and Sato, 2004; Wan Nadhari et al., 2013; Pintiaux et al., 2013; Rol et al., 2020). Their water content after equilibration was lower than the initial powders, highlighting their lower porosity. Their mechanical properties at small deformation (DMTA) and large deformation until

<figure><img src="image_4.png" /><figcaption>**Fig. 3.** Mechanical behavior up to rupture (3PF) of the cellulosic samples thermocompressed for 10 min at 100 °C at various fiber length. Average values and error bars are presented.</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>**Fig. 2.** Size (left) and shape elongation (right) distribution of the studied cellulosic samples pre-conditioned at 59 % RH for at least one week.</figcaption></figure>
rupture (3PF) were studied. (Fig. 3). At small deformation, mechanical properties did not strongly depend on the size of fibers with the Young's moduli measured between 4.7 and 5.0 GPa, and flexural moduli between 5.3 and 5.8 GPa for all HPC samples (Table 1). The modulus of the present CNF samples was significantly lower with Young's modulus of 4.4 ± 0.1 GPa and flexural modulus of 4.6 ± 0.3 GPa. These modulus values are comparable to what was described in the study of Rol et al. (2020) using a process with higher temperature (250 °C) and higher pressure (225 MPa). Slightly higher values have been described for pure CNF nanopapers (Modulus of 7 GPa) (Saito et al., 2009) but this difficulty translates into 3D bulk structure. The flexural strength, which is the highest value of stress that a sample could bear before breaking-up in the 3PF experiments, displayed larger variations between the HPC samples. Indeed, it varied between 27 ± 2 MPa for the samples with a fiber length of 10 µm, 35 ± 1 MPa for the 200 µm and 51 ± 6 MPa for the 900 µm. As all samples shared the same composition, this difference of behavior was related to a difference in structure in the compressed samples, and not to variation of the interaction strength between the fibers.

Microscopic observations of the samples, both using light microscopy and SEM, revealed highly different fracture profiles (Fig. 4). Indeed, the samples with the highest fiber length presented a very fibrous fracture aspect, with numerous fibers dangling from the solids as visible in the upper row. The SEM pictures also highlighted the fibrous aspect of the samples' core. This indicated that the entanglement of the fibers was more extensive before testing, which in turn increased the number of contact points between the fibers and improved the macroscopic mechanical behavior of the sample. This mechanical improvement aligns with the finite element analysis of Mao et al., which established a linear relationship between the elastic modulus of the fibrous network and the density of bonding between fibers. In contrast, the sample prepared from 10 µm fibers that displayed a quasi-brittle mechanical behavior showed no fibers in the fractured area (optical microscopy), and a more granular core (SEM).

In this study, it was observed that the mechanical properties of the composite materials exhibited varying dependencies on fiber length and aspect ratio. As presented in Table 1, both the Young's modulus and flexural modulus did not display a consistent trend with changes in fiber size or aspect ratio. This suggests that the optimal fiber length or aspect ratio for enhancing elastic modulus may have been surpassed. Indeed, it has previously been shown when using the Halpin-Tsai model (Halpin, 1969) to predict the modulus of nanocomposites nanopaper made from CNFs embedded in a polyvinyl alcohol matrix, that at high fiber volume fractions (above 0.9), increasing the aspect ratio did not substantially improve the elastic modulus (Benítez and Walther, 2017). However, the flexural strength showed a significant correlation with these parameters. Other reports have also typically pointed out that the reinforcing effect of fiber length on composite properties varies depending on the fibers' specific properties, with the modulus reaching a plateau earlier than the strength (Thomason and Vlug, 1997; Thomason, 2002).

In classical models, such as the Cox shear lag model (Cox, 1952) and

the Cox-Krenchel model (Yan et al., 2023), typically assuming a linear relationship between the modulus and strength and a composite to the reinforcing fiber length, they are insufficient to properly describe the purely fibrous materials of this study and overestimate the mechanical properties (Mao et al., 2017). Actually, for thermopressed materials made from pure cellulose fibers, the individual fibers can be considered as the reinforcing 'fibers' in the traditional composite sense, while the hydrogen-bond network formed between the fibers acts as the matrix. This network is the primary factor influencing elastic properties such as Young's modulus and flexural modulus, particularly when fiber orientation is not controlled in our conditions. In contrast to the elastic regime dominated by this molecular-scale bonding, the response during inelastic deformation is governed by more macroscopic structural features, such as fiber length and aspect ratio, which enable mechanisms like fiber pull-out and entanglement. As a consequence, the different effect of the fiber length over the modulus and strength can be understood by considering that as the materials described in this study exhibit elastic-plastic behavior, the influence of fiber length becomes more pronounced in the plastic deformation region. While modulus measurements were conducted within the elastic region (strain between 0.05 % and 0.25 %), flexural strength assessments typically occur in the plastic region (strain exceeding 0.5 %). There, accurately predicting flexural strength requires to take into account mechanisms such as plastic displacement zones at fiber ends, interfacial slip, and fracture mechanisms (Lafranche et al., 2017). Indeed, Zhuo et al. (2025) have also reported that the mechanical properties of composites are significantly influenced by fiber aspect ratio, that when the fiber aspect ratio was below the critical length (e.g., L/D = 5), the composite exhibited inferior mechanical performance due to interfacial debonding and pull-out. However, when the aspect ratio increased to 20, both the strength and ductility of the composite improve markedly.

Therefore, even if Young's modulus and flexural modulus are not significantly affected by further increases in fiber length beyond the critical length, the flexural strength can be further improved by the increased tendency of fibers to fracture rather than pull-out. The crack bridging capacity of fiber of higher aspect ratios, as well as their entanglement, is for instance clearly visible on the optical and electronic microscopic images of the samples and their fracture (Fig. 4). Such entanglement at the fracture also relates well to the higher strain at break of the materials prepared from the samples with longer fibers.

Both mechanical behavior and structural features were thus in very good agreement with the samples geometric analysis (Fig. 2). The longest fibers (HPC – 900 µm) with the highest aspect ratio displayed by far the highest strength and strain at break. In contrast, the shorter HPC – 10 µm presented decreased flexural strength and flexibility with a strain at break of 0.45 % as the only one showing a brittle fracture behavior (Fig. 3).

In order to compare with a cellulosic rich product issued from biomass, a sample issued from waste of brewery, rich in cellulose was analyzed. After drying, the BSG were characterized in composition, with a protein content of 12.5 % that did not change after thermocompression. The grinding and sieving through three different mesh sizes led to three fiber populations with different sizes and aspect ratio (Fig. 5). Similarly, to the cellulosic samples, the samples with the higher fibers size were also the most anisotropic, as the larger fibers generally presented the highest aspect ratio (Figure S4).

Samples prepared from BSG were also successfully thermocompressed into homogeneous and easy to handle samples using a compression temperature of 140°C. They displayed lower mechanical properties than the pure cellulosic samples (HPC) but in the same order of magnitude, with Young's modulus around 2.6–2.7 GPa and flexural modulus around 2.2–2.7 GPa regardless of fiber length. A recent study utilized Brewer's Spent Grains to produce particleboard at 150–180°C and 2.1 MPa pressure. However, their flexural modulus ranged only from 0.46 to 0.77 GPa, and even with added chemical binders, it reached just 1.09 GPa (Rossi et al., 2023). This may be related to its

<table><caption>**Table 1**

Mechanical properties at low deformation (DMTA) and up to rupture (3PF) of cellulosic samples thermocompressed for 10 min at 100°C (59 % RH).</caption><thead><tr><th>Fiber size</th><th>Young's modulus (GPa) -DMTA-</th><th>Flexural modulus (GPa)</th><th>Flexural strength (MPa)</th><th>Strain at break (%)</th></tr></thead><tbody><tr><td>10 µm</td><td>4.68 ± 0.12</td><td>5.83 ± 0.15</td><td>27.04 ± 2.18</td><td>0.45 ± 0.04</td></tr><tr><td>200 µm</td><td>4.98 ± 0.12</td><td>5.35 ± 0.16</td><td>35.25 ± 0.87</td><td>1.07 ± 0.02</td></tr><tr><td>900 µm</td><td>5.01 ± 0.25</td><td>5.52 ± 0.14</td><td>50.88 ± 6.03</td><td>2.71 ± 0.22</td></tr><tr><td>CNF</td><td>4.40 ± 0.08</td><td>4.59 ± 0.36</td><td>25.99 ± 1.55</td><td>0.94 ± 0.04</td></tr></tbody></table>
<figure><img src="image_6.png" /><figcaption>Fig. 4. Optical (upper row) and scanning electronic (lower row) microscopic image of the fractured samples after 3PF testing. Bridging fibers are highlighted with arrows.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>Fig. 5. Size (left) and shape elongation (right) distribution of the studied BSG populations (59 % RH).</figcaption></figure>

relatively low process pressure and excessive particle size. In our study, the water content of BSG samples was very similar to the cellulosic samples, indicating that it was not responsible for the decrease of the modulus. Only the flexural strength was found to vary with the length

between 14.7 and 18.5 MPa (Fig. 6 and Table 2). The lower properties of the material compared to HPC probably arose both from a difference of composition and fiber morphology. Similar variation of the mechanical properties with the fiber size were observed albeit less strongly, with a slight increase of the modulus from 2.2 to 2.6 GPa and of the strength from 14 to 18 MPa when the fiber size was increased from 200 to 500 µm. However, no further increase in strength was observed when the fiber size was increased to 1 mm, suggesting a threshold value for the fiber length and aspect ratio above which the benefits of larger and longer fiber size may decrease. The fibers sieved over a 500 µm or

<figure><img src="image_8.png" /><figcaption>Fig. 6. 3PF measurements of the BSG based samples thermocompressed for 10 min at 140 °C, with variation of the fiber size.</figcaption></figure>

<caption>Table 2</caption>

Mechanical properties at low deformation (DMTA) and up to rupture (3PF) of BSG based samples thermocompressed for 10 min at 140 °C, with variation of the fiber size (59 % RH).

<table><thead><tr><th>Fiber size</th><th>Young's modulus (GPa) DMTA</th><th>Flexural modulus (GPa)</th><th>Flexural strength (MPa)</th><th>Strain at break (%)</th></tr></thead><tbody><tr><td>200 µm</td><td>2.6 ± 0.2</td><td>2.2 ± 0.2</td><td>14.7 ± 0.3</td><td>0.81<br>± 0.002</td></tr><tr><td>500 µm</td><td>2.7 ± 0.1</td><td>2.7 ± 0.2</td><td>17.8 ± 5.2</td><td>0.81<br>± 0.126</td></tr><tr><td>1000 µm</td><td>2.7 ± 0.2</td><td>2.6 ± 0.1</td><td>18.5 ± 0.4</td><td>0.90<br>± 0.031</td></tr></tbody></table>
1000 µm mesh size may result in comparable levels of fibers entangle- ments. Consequently, there was no additional improvement in the in- ternal fiber network to further improve the flexural strength of the material.

## 3.2. Effect of process temperature on the material properties

Numerous studies have investigated the influence of thermocom- pression conditions on the mechanical properties of cellulose-based materials, with temperature and time identified as two of the most critical parameters in practical applications that were well reviewed by Pintiaux et al. (2015). However, due to the variability in raw materials, reaching a consensus on optimal conditions remains challenging. In our experiments, visually homogeneous cellulosic samples could be ob- tained at 100°C and 140°C. Furthermore, increasing the temperature did not significantly enhance the Young's modulus of the cellulosic samples conditioned at 59 %RH (~4.5 GPa) or flexural strength (~27 MPa) of CNF materials (Table 3). This is consistent with the results of Rol et al. (2020) who found that increasing temperature from 200 to 250 °C did not significantly improve the mechanical properties of the CNF material. Vaca-Medina et al. (2013) also reported limited effect of a process temperature increase from 25°C to 160°C on pure cellulosic samples.

When considering the effect of water on biobased materials, its softening action is well-known due to its plasticizing action on hygro- scopic biopolymers such as polysaccharides. The thermocompressed samples conditioned in higher relative humidity conditions were thus expected to present decreased mechanical properties. Hence, we esti- mated the water resistance of the various samples by comparing their properties when conditioned at 59 %RH and 75 %RH (Table 3). Water resistance was thus defined as the retention of Young's modulus when exposed to moisture (LM). To quantify this, we measured the Young's modulus of thermocompressed samples at 59 % RH and 75% RH and calculated their reduction as follows:

$$LM = \left(1 - \frac{E_{f75\%}}{E_{f59\%}}\right) \times 100 \ (\%) \qquad (4)$$

Where $E_{f59\%}$ and $E_{f75\%}$ are the Young's modulus of the sample at 59 % RH and 75 % RH at 20°C.

Temperature had a greater impact on this parameter, as the loss of Young's modulus (LM) was 25.3 ± 2.4 % for samples prepared at 100°C compared to 15.1 ± 0.6 % for samples prepared at 140°C.

In the case of the BSG, soft processing conditions (100°C for 10 min) did not lead to a homogeneous material (Fig. 8b), and the samples were grossly prone to shedding little fragments due to the lack of inter-fiber cohesion, which resulted low mechanical properties (Young's modulus at 0.78 GPa). Exposition to higher RH (75%) led to excessive swelling and disruption of the samples, preventing their characterization. The necessity to use higher temperature compared to the purely cellulosic samples probably arose from a difference of composition. Indeed, the

<table><caption>**Table 3**
properties of material thermocompressed at various temperatures for 10 min.</caption><thead><tr><th>Type of biomass</th><th>Process conditions</th><th>Young's modulus at 59 %RH (GPa)</th><th>Flexural strength at 59 % RH (MPa)</th><th>Young's modulus at 75 %RH (GPa)</th><th>LM (%)</th></tr></thead><tbody><tr><td rowspan="2">CNF</td><td>100°C – 10 min</td><td>4.4 ± 0.1</td><td>26.0 ± 1.6</td><td>3.3 ± 0.3</td><td>25.3 ± 2.4</td></tr><tr><td>140°C – 10 min</td><td>4.6 ± 0.1</td><td>28.4 ± 5.2</td><td>3.9 ± 0.2</td><td>15.1 ± 0.6</td></tr><tr><td rowspan="3">BSG</td><td>100°C – 10 min</td><td>0.8 ± 0.1</td><td>N.A.</td><td>N.A.</td><td>N.A.</td></tr><tr><td>100°C – 60 min</td><td>2.4 ± 0.2</td><td>7.2 ± 0.7</td><td>1.5 ± 0.1</td><td>37.6 ± 3.7</td></tr><tr><td>140°C – 10 min</td><td>2.7 ± 0.1</td><td>14.7 ± 0.3</td><td>2.2 ± 0.1</td><td>20.3 ± 1.3</td></tr></tbody></table>

presence of other biopolymers such as hemicellulose and lignin in particular may require higher temperature to properly mobilize all fibers into a homogeneous material due to a higher rigidity; the fiber size also play a role. Suzuki *et al.* (1998) have reported that 125°C was not enough to produce a solid and cohesive product if the initial biomass was not steam treated, which tends to fibrillate the fibers. This also explains that the purely cellulosic samples could be obtained at low temperature as the starting material was already fibrillated.

Additionally, proteins are another extensively reported factor that may influence inter-fiber cohesion and cellulosic material properties (Frihart and Birkeland, 2014; Evon et al., 2015; Guo and Wang, 2016). Proteins possess polar groups capable of interacting with the polar groups in plant cellulose, which can enhance fiber cohesion (Nguyen et al., 2018). However, proteins are sensitive to temperature and can undergo degradation at elevated temperatures, potentially diminishing their binding ability (Uitterhaegen et al., 2017). Despite this sensitivity, the protein content remained stable under the applied conditions in our study. The protein content of the BSG powder and the thermocom- pressed samples, both reserved at 20°C and 59 % RH, was constant at 12.5 %. This indicates that the processing temperature was insufficient to cause significant protein elimination, thus preserving their potential contribution to cohesion.

The higher thermocompression temperature or the longer compres- sion time generally resulted in a more compact material with a Young's modulus of 2.7 ± 0.1 GPa. In addition, temperature plays a significant role in both water resistance and strength in our study. The flexural strength of the samples compressed at 140°C was observed to be 2–3 times that of the samples compressed at 100°C. This increase was noted to be from 7.2 ± 0.7 MPa (100°C for 60 min) to 14.7 ± 0.3 MPa (140°C for 10 min) and 21.3 ± 1.8 MPa (140°C for 60 min) (Fig. 7). The water related loss of modulus was also decreased from 37.6 % to 20.3 % by increasing the pressing temperature from 100° to 140°C.

Pintiaux et al. (2015) reported that higher temperatures reduce voids in the structure by increasing fiber deformability, which promotes greater fiber contact and facilitates the formation of a stronger H-bond network. SEM observations (Fig. 8) confirmed a more compact structure in BSG samples processed at higher temperatures. The samples ther- mopressed at 140°C exhibited surface characteristics that were closer to those of CNFs materials, showing fewer visible surface voids and the large fibers more tightly integrated in a homogenous matrix. This compactness explained the significant effect of temperature on the sample mechanical properties. Higher temperature processing could thus effectively limit the absorption and internal diffusion of moisture, preserving the mechanical properties at high relative humidity.

Interestingly, the effect of temperature was clearer on BSG samples than on HPC. This difference likely stemmed from the composition, with hemicelluloses playing a key role. Hemicelluloses, which are abundant in brewery coproducts as they account for 44 % of the fibers, are known for their amorphous structure and greater thermal deformability compared to cellulose. Under heat, hemicelluloses can soften and

<figure><img src="image_9.png" /><figcaption>**Fig. 7.** Properties of 200 µm BSG based material at various process conditions.</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>Fig. 8. Electronic and optical microscopy of CNF based sample processed at 100°C (a), and BSG based samples processed at 100°C (b) and 140°C (c).</figcaption></figure>

promote better fiber mobility, compaction, and internal bonding (Kumar et al., 2018) These characteristics suggest that hemicelluloses may significantly influence the mechanical properties of the material. Subsequently, we conducted comparative analyses of the effect of the addition of natural and artificial soluble polysaccharides on the material properties and structure to further understand the mechanism by which it forms a network with cellulose fibers.

### 3.3. Effect of composition on the material properties: addition of a cellulosic linker

We systematically studied the effects of the addition of different polysaccharides at increasing weight fraction on the samples mechanical properties when combined with CNFs. Specifically, the introduction of xyloglucan (XG), a type of natural hemicellulose, and other esters of cellulose (thereafter called cellulosic derivatives): methyl cellulose (MC), ethyl cellulose (EC), and hydroxyethyl cellulose (HEC), affected both the Young's modulus and strength of the materials at 59 % RH (Fig. 9).

As the mass fraction of polymeric matrix content increased, we

<figure><img src="image_11.png" /><figcaption>Fig. 9. Mechanical properties of thermocompressed CNFs samples mixed with polysaccharides (xyloglucan, methyl cellulose, ethyl cellulose and hydroxyethyl cellulose) at different mass fractions of binding polymer and conditioned at 59 % RH.</figcaption></figure>

observed a general trend of declining Young's modulus across all samples. For instance, the Young's modulus for MC- and EC-containing samples decreased from around 4.4 GPa at 5 wt% to 3.3 GPa at 30 wt % binding polymer content, demonstrating the weakening effect of cellulosic derivatives on CNFs materials. This was probably due to the replacement of the hydroxyl groups on the cellulose backbone that are essential to form hydrogen bonds inter chains links. As a result, these derivatives could reduce the mechanical strength and integrity of materials by limiting efficient fiber-fiber interaction, thus weakening the overall network.

On the other hand, the Young's modulus of the materials prepared with xyloglucan remained relatively stable at 4.4 GPa, the same as the pure CNFs. The flexural strength of XG showed an increase with higher hemicellulose content, from 25.4 ± 2.0 MPa at 5 wt% XG to 32.4 ± 1.4 MPa at 30 wt% XG, suggesting that natural hemicellulose types such as XG, not only maintained a stable Young modulus but also improved the flexural strength of the material, similarly to what has been described for nanopapers made from holo-cellulose nanofibers compared to pure cellulose nanofibers (Galland et al., 2015). The values of the moduli of these mixed samples are the same range than previously described CNF-containing composites (moduli of 2–6 GPa for a starch matrix (Takagi and Asano, 2008), 4–6 GPa for PLA (Wang and Drzal, 2012), or 1–4 GPa for a regenerated cellulose matrix (Hai et al., 2017)); however, the CNF content in these materials (between 2 % and 30 %) is higher than described in this study which complicates the comparison.

We showed on Fig. 7 that the temperature used in the process had a strong effect on the water resistance of the BSG samples, due to the presence of lignin and hemicelluloses. The effect of the process temperature on the water resistance of samples containing 10 wt% of soluble binding polysaccharides was similarly assessed by studying the loss of Young's modulus when the samples were conditioned at 75 % RH compared to 59 % RH (Fig. 10). The water content at equilibrium of samples thermocompressed at 140°C was also systematically lower than samples thermocompressed at 100°C, validating the lower porosity and water accessibility (5.0 % against 6.0 % for MC samples, 4.5 % against 8.0 % for XG sample, 4.5 % against 6.8 % for EC sample and 4.5 % against 6.0 % for HEC sample).

Both XG and EC showed great improvement of the water resistance using a higher temperature process (140°C) with a loss of modulus of only 7.3 ± 0.2 % and 8.5 ± 0.3 % respectively (Fig. 10). EC is the most hydrophobic polymer presented here, and its glass transition temperature ($T_g$) is estimated at ca. 128–130°C (Lai et al., 2010). It is thus reasonable to assume that the temperature increases above $T_g$ led to a more efficient filling of the pores and coating of the CNF, EC being moreover a water-insoluble polymer. The behavior of XG-containing samples was more surprising as its $T_g$ is estimated at 250°C (Mabasa Bergström et al., 2012). This suggested specific interactions between the two naturally occurring polymers, as numerous studies have shown that the strong adsorption of XG onto cellulose could enhance fiber

<figure><img src="image_12.png" /><figcaption>Fig. 10. Loss of Young's modulus of CNF based samples containing 10 wt% of various polysaccharides preconditioned between 59 %RH and 75 %RH, molded at 100°C (blue) and 140°C (orange).</figcaption></figure>
interaction (Doineau et al., 2020, 2021; He et al., 2024). Additionally, the increased temperature (140°C) could lead to higher amount of H-bonds through dehydration and hence formation of stronger network bonding (Rol et al., 2020). This combination of reduced porosity and enhanced bonding explains the improved water resistance observed in BSG when processed at higher temperatures. It is also possible that lignin played an important role, as while its glass transition has been estimated in the 120–180°C range (Henrik-Klemens et al., 2024), it was shown that when at 8 wt% moisture content, similar to the conditions of this study (See Table S1), the glass transition of wheat straw lignin decreased to 53–63°C (Stelte et al., 2012).

Both MC and HEC present a glass transition around 120°C (Nadour et al., 2017), explaining the slightly better water resistance when processed at 140°C due to the decrease of porosity. However, their intrinsic hydrophilicity limited this improvement, highlighting the importance of specific interaction (XG) or hydrophobicity (EC) for the choice of the matrix polymer in the preparation of water-resistant biobased materials.

## 4. Conclusion

We showed that it is possible to process cellulose-based composites by thermo-compression without the use of resins or binders, and a compression temperature much below the usual temperatures. Cellulose combined with various polysaccharides used as model systems displayed behaviors similar to that of brewery co-products. The length of the fibers in all tested materials was identified as a determining parameter to insure a sufficient entanglement which strongly improved maximum flexural strength. We further highlighted that compression at 140 °C insured a structure cohesive enough to decrease the softening due to water diffusion in presence of hemicellulose or hemicellulose-like polymer that allowed them to build a tighter material under pressure. While the BSG samples were weaker than purely cellulosic ones, it is worth noting that the biomass here was not refined. BSG was not subject to other pre-treatment than a simple mechanical grinding, no chemical additive was added and the thermocompression conditions were relatively mild (short time, low temperature, low pressure). This highlights the potential of this process for simple and virtuous valorization of such biomass.

Future works will focus on the inclusion of lignin in model samples to assess its influence in particular on the materials water susceptibility as lignin is considered hydrophobic and responsible for the water resistance of plant tissues.

### CRediT authorship contribution statement

**Rui Oyang:** Writing – review & editing, Writing – original draft, Investigation, Formal analysis, Data curation. **Hugo Voisin:** Writing – review & editing, Writing – original draft, Supervision, Methodology, Investigation. **Emilie Korbel:** Writing – review & editing, Resources, Data curation, Conceptualization. **Laurent Chaunier:** Writing – review & editing, Methodology, Formal analysis, Conceptualization. **Isabelle Capron:** Writing – review & editing, Supervision, Project administration, Methodology, Conceptualization. **Denis Lourdin:** Writing – review & editing, Methodology, Conceptualization. **Gael Colomines:** Writing – review & editing, Resources.

### Funding

This work was supported by Région Pays de la Loire and The French Agency for Ecological Transition (ADEME) through Bioloop project (n° 2022_07880).

### Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence

the work reported in this paper.

### Acknowledgements

The authors gratefully acknowledge the Pays de la Loire Region, the European Regional Development Fund (ERDF), and Nantes Metropole for their financial support of the MAPE platform. We would like to further thank the members of the ISD team of the BIA Unit (INRAE Nantes), in particular Camille JONCHERE, for setting up the device for bending tests, Julien LAUNAY (GEPEA, IUT Nantes) for the preparation of the samples, Julie ATTIA (BIA, INRAE Nantes) for preliminary experiments on BSG thermocompression and Pierre SARTELET (BIA, INRAE Nantes) for the granulometry measurements.

## Appendix A. Supporting information

Supplementary data associated with this article can be found in the online version at doi:10.1016/j.indcrop.2025.122496.

### Data availability

Data will be made available on request.

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