Improved properties of high-density polyethylene by integrating high content of bio-fillers based on green nanolignin for applications in plastic industry

Journal Pre-proof Improved properties of high-density polyethylene by integrating high content of bio-fillers based on green nanolignin for applications in plastic industry Olalla. Sanchez-Sobrado , Alexander F. Tiniakos , Rebeca. Abalde , Marisol. Rivas , Alexios. Grigoropoulos , Angeliki. Nikolaou , Alexandros. Zoikis-Karathanasis , Ioanna. Deligkiozi , Ricardo. Losada PII: S2666-6820(25)00130-6 DOI: https://doi.org/10.1016/j.jcomc.2025.100688 Reference: JCOMC 100688 To appear in: Composites Part C: Open Access Received date: 5 September 2025 Revised date: 28 November 2025 Accepted date: 13 December 2025 Please cite this article as: Olalla. Sanchez-Sobrado , Alexander F. Tiniakos , Rebeca. Abalde , Marisol. Rivas , Alexios. Grigoropoulos , Angeliki. Nikolaou , Alexandros. Zoikis-Karathanasis , Ioanna. Deligkiozi , Ricardo. Losada , Improved properties of high-density polyethylene by integrating high content of bio-fillers based on green nanolignin for applications in plastic industry, Composites Part C: Open Access (2025), doi: https://doi.org/10.1016/j.jcomc.2025.100688 This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. ©2025 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) 1 Highlights  The study examines polyethylene composites with high amounts of two modified nanolignins as green, bio-based fillers.  The developed methodology achieves good dispersion of high lignin loadings.  Adding nanolignin improves mechanical, thermal, and antioxidant properties.  Tests at 15–35% filler show strong mechanical gains, up to 60% at 35%. 2 Improved properties of high-density polyethylene by integrating high content of bio-fillers based on green nanolignin for applications in plastic industry Olalla. Sanchez-Sobrado*1, Alexander F. Tiniakos2, Rebeca. Abalde1, Marisol. Rivas1, Alexios. Grigoropoulos2, Angeliki. Nikolaou2, Alexandros. Zoikis-Karathanasis2, Ioanna. Deligkiozi3, Ricardo. Losada1 (1) AIMEN Technology Center, 36400, O Porriño, Pontevedra, Spain (2) Creative Nano PC, 43 Tatoiou, Metamorfosi, 14451, Athens, Greece (3) AXIA Innovation GmbH, Fritz-Hommel-Weg 4, 80805 München, Germany *Corresponding author: [email protected] Abstract A study on the processability and physical and mechanical properties of different thermoplastic-based green nanocomposite materials for applications in injection processes is presented: commercial high-density polyethylene acting as the polymeric matrix and high contents of two types of modified nanolignins serving as bio-based fillers. It is a novel procedure to obtain good dispersibility of high loadings of bio-fillers in polyethylene which is difficult to achieve according to literature. The inclusion of green fillers brings property enhancements that are normally seen with expensive nanofillers like carbon nanotubes or graphene: improvement of mechanical properties while keeping good thermal stability and antioxidant properties has been obtained. To compare the effect of the inclusion of the two different nanofillers at 15, 25 and 35 % wt loadings on the characteristics of polyethylene, physical, mechanical, and morphological tests were performed. Injection pellets were fabricated and used to fabricate coupons to evaluate the mechanical features by performing both flexural and tensile tests. Remarkable enhancements (up to 60% with respect to the neat polymer) of certain mechanical properties were observed for composites with 35% wt loading of nanofillers. Moreover, investigating the effect of the bio-based nanolignins inclusion at high loadings on both crystallization and melting temperature suggested the robust thermal stability of the presented composites. As nanolignin is a renewable filler-type, it will bring properties like low-cost, low toxicity and partially replace petroleum-based stabilizers or other additives. Polyethylene with nanolignin becomes a “partially bio-based” high-performance material without changing the polymer backbone. Keywords 3 High density polyethylene (HDPE), nanolignin, nanocomposite, bio-filler, rheology, mechanical properties, injection molding 1. Introduction The integration of biomass derived components such as processed lignin or nanolignin into blends of high-density polyethylene (HDPE) represents a forceful advancement in sustainable materials science nowadays. The global demand in bio-based alternative materials market was valued at approximately 1.32 billion EUR in 2024 and is projected to grow at a CAGR of 4.54% from 2025 to 2030. This growth is driven by regulatory pressures and the adoption of new Circular Business Models (CBMs), especially in Europe [1]. Despite its abundance, only a small portion of lignin is currently valorized into high-value applications. From a freedom-to-operate perspective, the patent landscape for lignin-polymer composites is still in an emerging phase. This combination of robust market growth and needs along with a strong regulatory alignment is expected to position nanolignin-reinforced HDPE as a strategic material for future research and industrial driven applications. From another perspective the broad use of polymers and plastics in different industrial sectors has not stopped growing in the last century [2]. The adaptability of these materials to many different fabrication processes is key to understanding this growth potential. Considering this, numerous methodologies to modify physical and chemical properties of polymers have been successfully developed and applied. The preparation of polymeric compounds by reinforcing them with fillers and nanofillers is one of the most versatile strategies to provide plastic materials with new properties [3], [4] or to reduce, if convenient, their presence in formulations of new advanced composite materials. Several different aspects need to be considered when a nanofiller based polymeric composite is proposed. Under optimized loads of reinforcements, polymeric compounds may present improved mechanical properties like mechanical strength [5], stiffness [6] and toughness [7] since they provide reinforcement at the nanometer scale [8]. Also enhanced thermal stability can be achieved under controlled addition of nanofillers by increasing heat resistance which leads to materials more suitable for applications requiring elevated temperature performance [9]. On the other hand, polymeric based composite materials can also become multifunctional when carefully selecting nanoparticles to be integrated. Hence, increased electrical conductivity [10], enhanced optical properties [11], better wear resistance [12] or lower permeability [13] are examples of material properties and functionalities that can be conveniently tuned by developing polymeric compounds integrating nanofillers. There are also relevant research lines like the one presented by Junchao Ren et al. developing a novel approach to create nanocomposite films entirely from biomass-derived materials exhibiting 4 promising properties such as mechanical strength and environmental sustainability [14] and an in-plane thermal conductivity and electromagnetic interference (EMI) shielding effectiveness [15]. Other works present biobased polymers like PLA to investigate the incorporation of carbon quantum dots (CQDs) into composite and develop nanocellulose based films with enhanced degradation, thermal, and mechanical properties since the addition of CQDs improves the dispersion of cellulose nanofibers within the PLA matrix [16]. It is important that all advantages brought by nano compounding science to materials can be obtained while keeping lightness of polymeric structures which is crucial in many of the most important industrial sectors like aeronautics of wind energy [17]. Beyond these advancements, current standards and regulations demand green transition to new ecological alternative materials, thus reducing the use of nonenvironmentally friendly traditional materials. To this extent, in the last decades numerous works dedicated to the development of fibers and nanoparticles from biobased sources have been published [18], [19]. A wide range of potential advantages are gained. Not only do these types of components contribute importantly to reduced environmental footprints by being derived from renewable, biodegradable, or non-toxic sources but also, they bring significant improvements concerning low toxicity, recyclability, reduced emissions while matching regulatory compliance and market trends developed in the last years. One of the polymeric matrices that has shown better processability performance and ability to integrate functional bio-fillers is HDPE. Recent published works show relevant advancements. A. Qingfa Zhang et al. obtained HDPE based composites reinforced with biochar derived from agricultural waste improving mechanical strength, elasticity, and thermal stability [20]. On the other hand, considering the use of bio-fillers influence in processability and properties of HDPE, works have been published concerning the use of natural fibers, especially in those obtained from wood. The study of Muhamad Shafiq Sukiman et al.[21] shows results concerning the potential use of wood–plastic composites manufacturing formwork structures through thermoforming, also by integrating wood based particles Demagna Koffi al.[22] obtained a reduction of the specific heat capacity of HDPE indicating that wood fillers act as partial thermal insulators and also Fouad Erchiqui et al.[23] published a very important paper which demonstrates that composites with moderate wood content (≤ 40%) offer a balanced compromise between stiffness and thermoformability of HDPE, whereas higher loadings lead to poor deformation and excessive internal stresses. But also, in another work [24], the same author proves that the inclusion of the wood flour content in HDPE composites makes the material more elastic, reducing its viscoelastic deformability. In another interesting work by K.K. Agbozouhoue et al. [25] the use of ZnO to treat fibers effectively prevented fungal growth, demonstrating antifungal properties in PE based fibers composites whereas M. Tazi et al. [26] conveniently investigated the impact of softwood filler 5 content on the biodegradability and morphological properties of wood– polyethylene composites. As well the reduction of the damage and durability of plant fiber-reinforced HDPE are key topics in the recent studies. In their paper, Alencar Bravo et al. [27] evaluate the fatigue life and thermomechanical behavior of novel green and bio-composite gears reinforced with birch fibers, comparing them to high-performance thermoplastics. The same author presents a comprehensive model for assessing fatigue life and damage modes of plastic gears [28]. Demagna Koffi et al. [29] investigated the impact of cooling holes on the thermal and mechanical performance of plastic gear teeth. In the presented work, two distinct types of nanolignin were dispersed into HDPE. The inclusion of this type of particles within this polymer means an interesting research route in materials science since it combines the convenience of renewable resources with the high-performance of HDPE and applications in biodegradable plastics [30], [31]. Lignin, which is derived from a complex polymer in plant cell walls, is a promising additive used to improve the sustainability of polymers when compounding together [32], [33]. In the case of HDPE, several advances can be obtained while maintaining this improved sustainability like enhanced UV resistance and better chemical barriers [34]. Nevertheless, some important challenges and drawbacks are yet to be overcome. On the one hand developing a homogeneous dispersion of lignin nanofillers within the polymeric matrix during industrial processes like extrusion could be a challenge due to the natural hydrophobicity of polyethylene and the polar nature of lignin. An effective adjustment of process parameters such as temperature or processing time must be considered to reduce the possibility of particles aggregations formation. Whereas the effect of nanolignin on the mechanical properties of polyethylene has not been widely reported yet, there are some studies reporting increases in flexural strength when lignin is chemically modified that are very interesting. Laura Dehne et al. [35] reported an enhancement in the flexural strength up to +30% vs unmodified-lignin blends based in Polyethylene with 50 wt% loadings of lignin. Furthermore, there are works emphasizing the importance of lignin modification to achieve a good interaction with polymers to get good mechanical properties as in the work published by Majid Kiaei et al. [36]. Other study to consider is the one published by R.R.N Sailaja et al. [37] where improvements in different mechanical properties of LDPE (low density polyethylene) were achieved by blending this polymer with up to 40%wt loadings of esterified lignin. If research continues improving compounding techniques, the inclusion of nanolignin in polyethylene will acquire a key role in the development of greener, environmentally friendly polymeric multifunctional materials in the future. Indeed, the results presented below cover the growing necessity of developing sustainable materials to protect environment and contribute positively to the global climate crisis. By incorporating biobased additives as lignin, the use of fossil-fuel derived and widely use PE is reduced. Nanolignin is presented as well 6 as a green alternative to other non-green silica, clay or carbon black, the improvement of certain properties, explained below, is an important achievement. Concerning the innovation and the novelty of this research, the conventional lignin particles present problems to integrate in polymers. Due to high surface area nanolignin interacts more effectively with PE leading to better interfacial adhesion and to improved mechanical and thermal properties. The presented control on particle size, dispersion, and loading percentage of nanolignin allows fine-tuning of composite performance which is a key innovation area with potential important applications in packaging materials with antimicrobial properties or sustainable composites for construction or automotive uses among others. 2. Materials & Methods High-density polyethylene PE-Liten-PEHex (purchased from Orlen Unipetrol) was supplied by SILON plastic compounding company and used as received. Kraft lignin (BioPiva 100) was supplied by UPM (Finland); all other chemicals were purchased from commercially available vendors within the EU and used without further purification. RO water was used for the preparation of nanolignins. The two types of nanolignins were provided by Creative Nano PC, prepared from commercially available Kraft lignin either via chemical modification (LigAc) or via an in-house developed physico-mechanical method using ultrasonication (NK) [38]. FTIR Fourier-Transform Infrared Spectroscopy (FTIR) was recorded on a Tensor 27 spectrometer, a Bruker instrument fitted with a diamond Attenuated Total Reflection (ATR) accessory at a specific resolution of 4 cm⁻¹ in the wavenumber range from 600 to 4000 cm⁻¹ in transmittance mode. The baseline of all presented spectra was corrected, and the spectra was normalized. DLS Dynamic Light Scattering (DLS) was used to evaluate the particle size of NK and LigAc additives using a Litesizer 500 particle analyzer (Anton Paar, Graz, Austria). Prior to analysis, a dispersion in water was prepared at approximately 70-200 ppm to obtain 30-40% transmittance. All measurements were performed no less than three times. TGA A Q50 TGA model from TA Instruments was used to perform Thermogravimetric Analysis (TGA). The samples were heated from ambient temperature to 750°C in a 40 mL/min flow of air at a heating rate of 10°C/min. DSC 7 A Q20 DSC model from TA Instruments with Heat Flux was used to perform Differential Scanning Calorimetry analysis (DSC). It is equipped with Tzero® Technology for enhanced signal and baseline performance, dual digital mass flow controllers and compatibility with autosamplers and multiple cooling systems. Important thermal transitions, including crystallization temperature (Tc) and melting temperature (Tm) are achieved. In the experiments presented in this work, a double heating ramp from 20ºC to 240ºC with double cooling at a heating/cooling rate of 10ºC/min was used. SEM The morphology and dispersion of fillers within the HDPE polymeric matrix of the developed composites were studied by Scanning Electron Microscopy (SEM). Samples were embedded in an epoxy resin before being carefully cut using a low-speed saw and subsequently polished to preserve the fractured surfaces. The samples were then mounted onto the sample holder using double-sided carbon tape. To ensure the required conductivity of HDPE-LigAc & HDPE-NK composites necessary for SEM high-quality imaging, all composites were gold coated using a SC7620 ‘Mini’ Sputter/Glow Discharge System (Quorum Technologies LTD, UK). SEM images were recorded on a TESCAN VEGA COMPACT instrument, which provided detailed surface images for understanding the distribution of components and interactions within the HDPE-lignin hybrids. Contact angle Water contact angle measurements were performed using an optical tensiometer, One Attention (Biolin Scientific, Espoo, Finland). Measurements were performed in quadruple and averaged. UV-vis An Agilent Cary 60 UV-vis instrument run in absorbance mode was used to record UV-vis spectra. The antioxidant activity of the HDPE composites with NK and LigAc fillers at different filler loadings were evaluated by monitoring the reduction rate of the DPPH radical in the antioxidant’s (NK/LigAc) presence via UV-Vis spectroscopy. This technique measures the ability of a substance to scavenge free radicals by observing the decrease in absorbance of a DPPH solution after incubation with the test sample. A 0.079 mM DPPH solution in EtOH was prepared and stored in the dark for 16 h at room temperature. The prepared films with the same dimensions (1 cm × 1 cm) were immersed in 3 mL of the DPPH/EtOH solution at room temperature and kept in the dark. The composites’ antioxidant capacity was determined by measuring the absorption decay at 517 nm at regular time intervals via UV-Vis. The residual DPPH content in the solution was calculated using equation 1: Equation 1 Residual DPPH content (%) = 100 − 100 (A0 − A1/A0) 8 where A0 is the absorbance of the control sample and A1 is the absorbance in the presence of the films. Rheometer Rheometric measurements were taken using an Anton Paar MCR 102e rheometer operating in both oscillatory and rotational modes. It was used to determine both deformation and fluidity in all types of tests. In this case, a plateto-plate geometry with a diameter of 25 mm and a gap between plates of 1 mm was used. The samples were tested in oscillatory mode, always below the viscoelastic limit, with a constant strain of 0.1%. Both storage modulus and viscosity values were acquired using rheometric measurements. Mechanical tests Tensile tests were performed in a universal machine, SHIMADZU AG-X PLUS 5 kN, following standard UNE-EN ISO 527-2:2012. Fabricated specimens were subjected to tensile testing at a speed of 1 mm/min up to 2% strain to determine elastic modulus and at a speed of 20 mm/min until failure to determine tensile strength. The same machine was used to perform flexural tests under standard UNE-EN ISO 178:2020 under a force load of 1.5 N. Extrusion process 15 Figure 4: SEM images of cross sections of composite tensile coupons. Neat HDPE (top left), NK35 (bottom left), LigAc35 (top right) and LigAc15 (bottom right). Respective scale bars are plotted in the bottom-left corner. Figure 4 illustrates SEM images of the cross section of the composite tensile coupons. For neat HDPE (top left) a smooth surface is observed whereas the HDPE-nanolignin composites (all other SEM images) seem to have a ‘roughened’ surface. It is noteworthy that even at high filler loadings (NK35 & LigAc35) a homogeneous dispersion of the nanolignin fillers in the HDPE matrix was observed. 3.4 Contact Angle Considering the potential industrial applications of HDPE composites with improved features, contact angle analysis provides insights into the surface properties of the material, such as wettability, which is critical for various applications. To produce thin film samples for contact angle measurements (Figure 5a), a hot press machine was used following the procedure described in section 2. The sessile water droplet method was used to investigate the hydrophilicity of all HDPE composite films. Measurements illustrated in Figure 5b were performed in quadruple. 16 Figure 5: a) Hot press sample corresponding to HDPE integrating 15% wt of LigAc b) Contact angle measurements of HDPE-NK and HDPE-LigAc composites at different loadings. Although there is certain standard deviation in some of the measurements, a general trend of increasing hydrophilicity with increasing lignin filler loading may be deduced. Neat HDPE is more hydrophobic than all the composites with a contact angle of 95.8°. HDPE-LigAc composites are more hydrophobic compared to HDPE-NK composites, which may be explained by the partial alteration of hydroxyl groups in LigAc, which occurs during its synthesis. Since hydroxyl groups enhance hydrophilicity, HDPE-NK has more free hydroxyls and thus more wetting capability. Obtaining HDPE-based composites with improved hydrophilicity might be of special interest in the coating industry. 3.5 Antioxidant activity Antioxidant activity of HDPE composites containing different loadings of nanolignin fillers was studied using the DPPH method (Section 2). The results are shown in Figure 6. 17 Figure 6: Antioxidant activity of HDPE-NK and HDPE-LigAc composites at different loadings, measured by monitoring the reduction rate of DPPH radical using UV-vis spectroscopy. It is apparent that the neat HDPE sample has negligible antioxidant activity; however, a substantial increase is observed when NK and LigAc lignins are incorporated into HDPE as additives, which in turn further increases with higher additive loading. Unsurprisingly, the HDPE-NK35 composite shows the greatest antioxidant activity, followed by HDPE-NK25 and HDPE-Lig-Ac35. The antioxidant activity of nanolignins arises from the free phenolic hydroxyl groups, which are abundantly present; the partial alteration of hydroxyl groups in LigAc could explain its reduced antioxidant activity compared to NK. 3.6 Viscosity and thermal properties To obtain a better understanding of how the integration of green fillers affects the properties, such as the thermal stability of the final composites, some additional studies were performed. DSC tests were conducted following the procedure described in section 2. The most relevant results are displayed in Figures 7a and b, which compile values corresponding to the melting temperatures (Tm) and the crystallization temperatures (Tc) of all composites prepared. It is apparent that Tm slightly increases for 15% filler loading in both cases, whereas further increasing the filler loading to 25% results in composites with a Tm like neat HDPE. Interestingly, at 35% NK loading, the Tm is increased to 138.4°C, while in contrast, 35% LigAc loading decreases the Tm to 131.8°C. This may be due to weak interactions with the polymer or minimal nucleation effects. These results 18 are relevant for applications in the injection molding and/or extrusion fields. On the other hand, Tc remains constant with increasing filler loading up to 25% in both cases. After this, LigAc35 shows a slight increase in Tc while NK35 shows a slight decrease. The physical dispersion of particles within the polymeric matrix explains the obtained thermal stability. Lignin contains aromatic rings and crosslinked structures, which are thermally stable. When nanolignin is welldispersed, it can function as a thermal barrier, slowing down polymer degradation by acting as a radical scavenger during decomposition and delaying heat transfer through the matrix. Figure 7: Evolution obtained for Tm (a) and Tc (b), viscosity (c), and enthalpy of crystallization (d) for the two types of lignin studied: NK (blue lines) and LigAc (red lines). Figure 7c depicts the evolution of the viscosity of the HDPE composites as nanolignin filler loading increases gradually. These values have been estimated by performing rheometric measurements as described in section 2. Even though adding reinforcements usually increases viscosity, a good dispersion of fillers in the polymer may allow the polymer chains to move more freely, which can reduce viscosity. However, if large clumps of nanofillers form, this could increase flow resistance and raise viscosity, especially when the filler content exceeds 30% by weight, as observed in the case of LigAc. Figure 7d shows the decrement of enthalpy of crystallization, estimated from DSC measurements previously explained, with the addition of nanofillers. In both cases, the evolution is quite abrupt, meaning that the crystallization degree of the composites is reduced, thus 19 losing the capacity of being compressed, which might affect different polymeric processes. Dispersibility also affects the crystallization properties since nanolignin can function as a nucleating agent, promoting polymer crystallization if dispersed homogeneously. This explains the stability of the obtained crystallization temperature. On the other hand, good dispersibility should lead to slightly higher crystallization enthalpy (ΔHc) due to more polymeric chains organizing around dispersed particles. In the case of the presented work, this magnitude decreases with the filler load. This might indicate the presence of some particle agglomerates in the samples subjected to the study or maybe due to the amorphous nature of nanolignin. 3.7 Mechanical properties Both flexural and tensile coupons were fabricated by injection molding following the procedure described in section 2. Figure 8 depicts a) tensile strength, b) elastic modulus, c) flexural strength, and d) flexural modulus of the prepared composites at different filler loadings. The error bars plotted in each case correspond to the standard deviation of the mean value corresponding to a set of 5 coupons evaluated under the exact same conditions. These error bars represent the variability of the represented magnitudes that each point summarizes. The small bars indicate the composite blends are homogeneous and consistent; thus, these bars provide a direct qualitative measure of the good compounding homogeneity systematically obtained by the developed methodology. The analyzed mechanical properties follow a similar trend with both types of fillers, but in all cases, the composites with LigAc presented slightly lower values. Tensile strength decreases as filler content increases (Figure 8a) due to the progressive agglomeration of fillers with increasing the filler content. The formed agglomerates may function as stress concentrators, and this can result in the premature failure of the specific part under tensile stress. Furthermore, the compatibility between the hydrophilic fillers, such as the lignin, and hydrophobic polymers like HDPE is crucial to the resulting mechanical properties. If compatibility is poor, the weak interfacial interactions limit the stress transfer between fillers and matrix, which can result in reduced tensile strength. Contrary to the tensile strength, the flexural strength increases with higher filler content (see Figure 8b). More specifically, an increase of up to ~60% in flexural strength was observed for NK25 and ~30% for LigAc25. Flexural loads involve a combination of tensile and compressive stresses. The fillers have made a strong contribution in increasing the compression strength of the material in the compression dominated zones. This reinforcing effect counteracts the weakening observed in the tensile properties and results in an increase of the overall flexural strength. A significant increase (~60%) was also observed in elastic modulus for composites NK35 and LigAc35 (see Figure 8c). Finally flexural modulus showed a similar trend with increasing filler load, an increase of ~60% for NK35 and ~50% for LigAc25 was observed (see Figure 8d). This is a common observation in 20 polymer nanocomposites and is often attributed to the rigid nature of the fillers, which constrain the mobility of the polymer chains and make the material stiffer. Some studies [35] reported increments of flexural strength when lignin is chemically modified up to +30% vs unmodified-lignin blends when dispersed in polyethylene at 50 wt% loading of lignin. This value matches the one reported in the present work for the case of LigAc25 but is far from the 60% enhancement obtained for NK25%. This result suggests the importance of the particle size of nanolignin to get good dispersibility within a polymeric matrix and the beneficial effect on the compounding properties. To better understand the influence of the two diverse types of nanolignin on the mechanical properties of the manufactured composites, the ANOVA method has been applied in a straightforward way for each of the magnitudes analyzed. Analysis of the values obtained for tensile strength allowed several important conclusions to be drawn NK particles result in a decrease from 20 to 15.2 MPa as filler content increases, and LigAc causes a tensile strength to decrease from 20 to 15.9 MPa, a slightly less steep decrease. NK shows a steeper decline in tensile strength with increasing filler content compared to LigAc, so increasing filler content tends to reduce tensile strength for both fillers, due to stress concentration points or poor interfacial bonding at higher loading. LigAc maintains slightly better tensile performance at higher concentrations, indicating it may be more compatible or less disruptive to the matrix at high loadings. On the other hand, the ANOVA analysis on elastic modulus shows that it increases with filler content for both types, indicating that both fillers effectively reinforce the polymer. But NK is more efficient than LigAc at low and intermediate concentrations (15– 25%), producing higher modulus values while at high concentration (35%), both fillers achieve similar modulus values, suggesting saturation of the reinforcement effect. Regarding flexural strength, NK provides stronger reinforcement at 15– 25% but may show a slight reduction at 35% while LigAc is more consistent and gradually improves the property across all concentrations. These results might suggest that the inclusion of NK fillers could be a preferable option for maximum enhancement at moderate filler levels and LigAc may be a better option for higher filler loadings or better stability. Finally, some conclusions may be taken from the ANOVA analysis of flexural modulus. NK consistently provides higher performance than LigAc. Differences become more pronounced at higher concentrations, indicating NK is better suited for applications requiring maximum reinforcement. Therefore, the use of NK for high-strength composites should be preferable; nevertheless, LigAc may be considered for moderate reinforcement where other factors (cost, processing…) are important. These mechanical properties are strongly affected by the main mechanism between the interaction of filler particle-polymer: physical dispersion of particles within the polymeric matrix, good dispersion ensures efficient stress transfer from the HDPE matrix to the nanolignin particles since the elastic modulus improves due to the rigid structure of nanolignin. 21 Figure 8: Evolution with the load of LigAc (red lines) and NK (blue lines) of four significative magnitudes: a) tensile strength, b) elastic modulus, c) flexural strength and d) flexural modulus. Error bar in each case corresponds to the standard deviation of the mean value corresponding to a set of 5 coupons evaluated under the exact conditions. Considering injection molding, it would be useful in future studies to understand the evolution of key plastic properties such as thermal conductivity (k), specific heat capacity (Cp), and density (ρ) of polymer nanocomposites with the temperature (T) and the nanofiller concentration (ϕ). In the Supplementary Information, the equations proposed are shown. In summary, it is foreseen that density decreases with temperature and increases with filler loading improving dimensional stability and structural integrity under heat. Additionally, the specific heat should increase with temperature (mostly from polymer) but decrease with filler loading which might affect processing conditions such as heating rates and cycle times. Finally, an increment of thermal conductivity with filler loading is expected, moderately influenced by temperature and sensitive to filler connectivity and interface quality. The estimation of these properties will undoubtedly lead to better designing of advanced and multifunctional polymer nanocomposites to be applied in different important industrial areas. 4. Conclusions HDPE-based composites reinforced with different loadings of two types of nanolignin green fillers (NK and LigAc), ranging from 15 wt% to 35 wt%, were developed. Rheological analysis identified an optimal filler loading of approximately 35 wt%, ensuring good dispersion within the polymer matrix. The 22 inclusion of nanolignin fillers provided several notable benefits. Contact angle measurements indicated increased hydrophilicity with higher filler content, while antioxidant activity also improved proportionally. However, the most significant enhancement was observed in mechanical performance: flexural strength increased by approximately 60% and 30% for NK25 and LigAc25 composites, respectively, compared to neat HDPE. Similarly, the elastic modulus increased by about 60% in the NK35 and LigAc35 samples. Melt extrusion and injection molding parameters were optimized to fabricate specimens for mechanical testing. Results highlight the complex balance between additive fillers and polymer matrix. Emphasizing the importance of optimizing the dispersion of the fillers, selecting appropriate processing conditions which are essential to maximize filler efficiency and achieve an optimal balance between stiffness and strength. Both nanolignin fillers exhibited comparable performance, with no clear advantage of one over the other. Acknowledgements This research was funded by the European Union’s Horizon Europe Framework Programme under Grant Agreement No. 101058449. References [1] www.grandviewresearch.com/industry-analysis/lignin-based-biopolymersmarket-report [2] J.K. Fink, Reactive polymers: fundamentals and applications: a concise guide to industrial polymers. William Andrew (2017) [3] K.B. 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