1 Introduction

The development of carbon-reinforced composite materials and their application in additive manufacturing is an emerging field of research (Ghoshal 2017). Carbon derivatives, such as carbon black, carbon nanotubes, graphene, and carbon fibers have been implemented in various polymeric matrices in the material extrusion (MEX) additive manufacturing (AM) process (Acrylonitrile–butadiene–styrene—ABS (Vidakis et al. 2020), Polyamide 12—PA12 (Vidakis et al. 2023), and Polylactic Acid—PLA (Vidakis et al. 2021a) among others). The aforementioned PLA is a biocompatible, eco-friendly, and sustainable polymer (Rajeshkumar et al. 2021) with specifications that have made it popular for applications in packaging (Divakara Shetty and Shetty 2019) and in the medical field (Singhvi et al. 2019), while it is the most used polymer in MEX AM (Vidakis et al. 2022a). As expected, research on the PLA polymer has expanded to areas such as Hybrid Additive Manufacturing (HAM) to further expand its applicability, i.e., AM and laser cutting to upgrade the surface roughness and dimensional accuracy of 3D-printed workpieces (Petousis et al. 2023a), and AM combined with friction stir welding for the production of large parts (Vidakis et al. 2022c).

By varying the type of carbon, the carbon-to-matrix weight, and the resin type, the composite products exhibit improved mechanical, electrical, and thermal properties and can thus be engineered to address the requirements of a specific application (Blyweert et al. 2021; Casamento et al. 2023). A similar positive effect has been reported by the addition of such fillers in resins in the vat photopolymerization (VPP) AM process (Peerzada et al. 2020; Yang et al. 2022; Iervolino et al. 2022). Recent examples include the application of laser metal deposition (LMD) for the development of graphene aluminum matrix composite (Li et al. 2021), the preparation of an elastic carbon nanotube/polydimethylsiloxane compressive strain sensor (Liu et al. 2022a), and the application of fused filament fabrication (FFF) for the production of a carbon fiber/polyethylene terephthalate composite (García et al. 2022). Following fabrication, these composites can be used in an increasing number of industrial processes, including the aerospace, healthcare, wind energy, and energy storage sectors (Yuan et al. 2019; Aloqalaa 2022; Dashtbozorg et al. 2022). Tamez and Taha (2021) comprehensively reviewed the use of carbon fibers in composite materials for additive manufacturing. Such composites have shown positive behavior in lightweight applications, however, there are considerable issues to be solved. These include the high initial cost of obtaining graphene or carbon fibers, the lack of control over their alignment during extrusion, and the possibility of void formation which may lead to poor mechanical properties (Tamez and Taha 2021). Furthermore, their dependence on non-renewable fossil fuels, and the complex methodologies required to obtain the final carbon product from the raw material, largely increase their environmental footprint (Serrano-Luján et al. 2019; Yan et al. 2020; Ahmed and Mohamed 2023). Then again, the addition of metal-based fillers is a common practice to enhance the performance of resins in VPP and induce a multi-functional character on the parts built with resins with the specific AM method (Vidakis et al. 2022d). To achieve such characteristics along with an environmentally friendly profile, additives such as cellulose have been applied in VPP AM processes (Vidakis et al. 2022b). Consequently, researchers are looking for alternative, cost-efficient, and environmentally friendly approaches to obtain carbon materials suitable for additive manufacturing applications (Beloin-Saint-Pierre and Hischier 2021).

One such material is biochar, a high-carbon, solid material obtained through the pyrolysis of residual biomass and agricultural waste. Yearly, millions of tons of unexploited biomasses are disposed of in landfills throughout the world (AliAkbari et al. 2021; Sertolli et al. 2022). As part of the circular bio-economy, many types of biomasses can be converted to added-value materials such as biochar and used in a wide range of application (Gupta et al. 2022; Tan et al. 2023). The chemical and physical attributes of biochar are strongly dependent on the pyrolysis conditions (temperature and residence time) and the properties of biomass itself (lignin, cellulose, hemicellulose, and ash content). Additionally, post-production modifications through physical and/or chemical means can adjust these properties (Goswami et al. 2022; Liu et al. 2022b; Mehdi et al. 2022). Therefore, engineered biochar can be produced according to the requirements of a specific application.

As a multi-functional material, biochar has been utilized in various application. The majority of research has focused on its beneficial role as a soil amendment due to its water and nutrient-holding capacity as well as its porous network which delivers a habitat for soil microorganisms (Adhikari et al. 2022). Other established application include its utilization as a substrate in heterogeneous catalysis (Turunç et al. 2021; Qiu et al. 2021), as an electrode for energy storage devices (Tsubota et al. 2021), as an adsorbent for the removal of organic/inorganic contaminants from water and soil (Liang et al. 2021; Qiu et al. 2022), and as a filler in cement mixtures for the development of mortars with improved properties (Danish et al. 2021). However, the use of biochar in additive manufacturing composites is limited and only a few works have demonstrated the potential of the material in this field.

The work of Idrees et al. (2018) was among the first to demonstrate the effects of adding biochar to develop a sustainable and cost-effective composite with an upgraded thermomechanical response. The authors melt-mixed biochar with recycled PET [poly(ethylene terephthalate)] and produced 1.75 mm feedstocks using a filament extruder. The results also depicted that a 0.5% wt. loading of biochar in PET induced a 32% raise of the tensile strength, whereas a 5% wt. induction led to a 60% raise in tensile modulus compared to the raw PET matrix (Idrees et al. 2018). In a similar approach, Strongone et al. (2020) dispersed biochar from coffee waste in bisphenol-A-ethoxylate-diacrylate at various loadings in the range from 0.01 to 1.0% wt. and attempted to depict correlations between the thermal, optical, and rheological properties of the composites and the corresponding properties of the reference materials. Biochar was shown to be more readily and uniformly dispersed within the resin compared to the multi-walled carbon nanotubes (MWCNTs), thus leading to a predictable—and therefore tunable—effect on the thermal conductivity and transparency of the cured resin (Strongone et al. 2020). Interestingly, Alhelal et al. (2021) prepared biochar from the same biomass as Strongone et al. and used it as a reinforcement agent in epoxy resin to fabricate 3D printing specimens exploiting the direct-write (DW) approach with 1.0% and 3.0% wt. biochar grades. At the optimum biochar loading (1% wt.), the flexural modulus and strength of the composite reached a 55.5% and 43.4% raise, respectively. Nevertheless, increasing the 3.0% filler loading was found to have a negative impact on both the flexural and viscoelastic responses of the samples (Alhelal et al. 2021). The added value concept of sustainable waste management was referred to in the work of Mohammed et al. (2022). The authors hypothesized that biochar is not as effective as carbon nanomaterials in upgrading the thermomechanical properties of polymer materials, therefore they synthesized highly graphitized biochar from starch-derived sustainable leftovers. This modified biochar was studied as an effective reinforcement agent of polypropylene at quite small loadings between 0 and 1.0% wt. At the optimum found loading of biochar (0.75% wt.), the tensile elasticity modulus and strength experienced 34% and 46% increases, respectively (Mohammed et al. 2022). In an effort to reduce the environmental impact of the construction industry, biochar was combined with sand and cement to develop 3D printable mortars of the increased dimensional stability (Falliano et al. 2020, 2022). At biochar rates in the range of 5%–23% wt., compressive and flexural strengths higher than 60 and 8 MPa were maintained in the printed mortar, respectively, whereas up to 43% CO2 emission reduction was achieved. The performance of biochar composites using PLA as the matrix material has also been reported. Composites have been prepared with various methods with existing research not related to the AM processes (Arrigo et al. 2020; Aup-Ngoen and Noipitak 2020; Das et al. 2021).

On this basis, the rationale of the current investigation was to combine sustainable waste management, material science, and manufacturing processes to develop a biochar-based composite with improved mechanical and thermal properties, suitable for additive manufacturing applications. A standard-grade resin was selected as the matrix in the VPP process, whereas PLA was deployed in the MEX process for the development of an all-environmentally friendly composite with an enhanced mechanical response. To produce raw biochar, olive tree pruning was strategically selected as an abundant, renewable, and underutilized agricultural waste of the Mediterranean region. Yearly, vast quantities of this type of waste are burned in open fields to reduce their volume and eliminate the spread of diseases. However, such practice is a major organic aerosol and CO2 source for the Mediterranean region, whereas it has a negative impact on soil structure and increases the fire risk in adjacent areas (Kostenidou et al. 2013; García Martín et al. 2020; Romero-García et al. 2022). The hypothesis is that if olive tree biochar can be used to improve the thermomechanical scores of AM polymers, then a new exploitation pathway for a major agricultural residue may be developed. VPP composites were prepared with a high-shear stirring process, while MEX composites were prepared with a thermomechanical filament extrusion process. Samples were 3D-printed with both AM processes according to the corresponding international standards for polymers’ mechanical testing. Their mechanical performance was experimentally investigated. The thermal properties and compositional structure were also evaluated to verify the stability of the composites. SEM and AFM were utilized for the morphological and fractographic characterization of the samples.

Hereby, the key objectives of this research were to:

  • fabricate functional biochar through cost-efficient pyrolysis of olive tree pruning residuals and evaluate it through elemental and other characterization methods,

  • combine raw biochar milled powder, without any further post-production treatment, with a standard resin in the VPP AM process and PLA in the MEX AM process to produce 3D-printable composites for each AM process, and

  • evaluate the thermomechanical performance and further structural and printability features of the 3D-printed composites, to quantitatively depict the potential of biochar as a natural, eco-friendly reinforcement agent in AM processes.

2 Materials and methods

2.1 Materials

2.1.1 Biochar production and characterization

Olive tree pruning biomass was locally collected (Chania, Greece). To remove impurities, the prunings were washed and then air-dried. Olive tree pruning biochar was fabricated employing flame curtain pyrolysis. The design features, materials, dimensions, and other specifications of the pyrolysis kiln utilized in the present study, are already described in an author’s earlier paper (Tsubota et al. 2021). The pyrolysis reactor had a capacity of 0.5 m3.

More specifically, the prunings were divided randomly into three batches. Then flame-curtain pyrolysis was performed in triplicate. The pyrolysis of each batch lasted 1 h at a temperature of 540 ± 50 °C. The literature suggests that the variation of the temperature in this type of reactor can be very high (Jayakumar et al. 2023). To confront this issue, temperature was monitored through four thermocouples attached to 4 different spots on the external surface of the kiln. During the initial period of establishing the flame cap, considerable temperature fluctuations were observed. However, as the feeding rate of prunings (layering process) was stabilized, so did the temperature to a large extent. In flame cap pyrolysis, the uniformity of the feedstock and the optimum feeding rate is crucial to reduce the temperature fluctuations as much as possible.

After the first pruning started to pyrolyze, there was not any need for an external heating source, setting the process as self-sustained. After quenching with water, each batch of biochar was air-dried for 96 h and weighed. The 3 samples were then combined into one composite sample and homogenized. The sample was ground with the aid of a Sepor-type rod mill. After thorough sieving, the fraction of less than 100 μm in size was stored for further analyses and use. The distribution of the biochar particle size was depicted by means of a Malvern type-S Mastersizer (Malvern Instr., Malvern, UK) combined with laser diffraction. Elemental analysis (C, H, N, and S) was performed by deploying a ThermoFlash-2000 combustion analyzer (ThermoFisher Sc., UK). The content of the ash was determined after the European Biochar Certificate (EBC) Guidelines (EBC-2012, ver. 9.3E). The content of the oxygen was derived by difference. The sample chemical composition was accomplished through an X-ray fluorescence (XRF) spectrometer (Rigaku ZSX PrimuII, Japan).

2.1.2 Resin material

Resin material from the Prima company and specifically Value UV Standard Resin white color (PrimaCreator, Malmö, Sweden) was procured. The specific resin was selected because it is affordable and high-quality. White color was selected to determine if biochar has been uniformly integrated into the resin matrix. The technical specification according to the resin datasheet is presented in Additional file 1: Table S1 in the supplementary material of the work. The composition of the resin, according to the safety data sheet of the manufacturer is presented in Additional file 1: Table S2 in the supplementary material of the work.

2.1.3 PLA polymer

The PLA used was of the 3052D grade. It was fabricated by Plastika Kritis SA (Heraklion, Crete, Greece). Its technical specifications are presented in Additional file 1: Table S3.

2.2 Methods

In the supplementary material of the work in Additional file 1: Figure S1, flow charts are presented from the experimental course followed for the preparation and characterization of the resin/biochar and the PLA/biochar compounds, along with corresponding screenshots from each step of the process.

2.2.1 Composite preparation and additive manufacturing Preparation of resin-biochar composites and 3D printing of specimens

The Phrozen Transform (Phrozen Technology, Hsinchu City, Taiwan, China) resin 3D printer was used to manufacture the composite samples. 3D printing started with raw material (without biochar) for the preparation of pure resin samples, which were the control samples in the study. The composites were then prepared with loadings as shown in Fig. 1. These ratios were chosen because it was not known how the resin would react to stirring with biochar (if it would be homogenized) and whether there would be a solidification of the prints with the addition of large amounts of biochar (high loadings) in the composite. For this reason, a more conservative approach to resin-biochar ratios was chosen and is the one presented below (Fig. 1). Specimens were 3D-printed at a 45° angle, and custom supports had been placed to facilitate the process. The 3D printing settings are presented in Table 1 and Fig. 1. Specimens were 3D printed with dimensions complying with the corresponding international norms (see Fig. 1) for mechanical testing. Five specimens were manufactured per test and composite.

Fig. 1
figure 1

MEX and VPP 3D printing settings and the biochar various loadings applied in each 3D printing process. On the right part, the geometry of the samples manufactured with the two AM methods is presented along with the corresponding international standards followed throughout the process of each mechanical test. The infill pattern used for the fabrication of the parts is also depicted in the samples

Table 1 PZSlice settings used for the specimens’ 3D printing with the VPP process, on the Phrozen Transform (Phrozen Technology, Hsinchu City, Taiwan, China) resin 3D printer

After the 3D printing, the specimens were cleaned up with isopropyl alcohol and placed inside in a UV light oven for 5 min on each side. To create the first composite material (Resin 99.75%—Biochar 0.25%), biochar was placed into the oven to get dried for 30 min before being used. Then, it was weighed, and the resin and the biochar were placed in different mixing vessels, one for each material, and stirred for 30 min. During the stirring process, the temperature of the mixture started to increase. The increase in temperature was due to the high speeds (16,000 rpm) of the stirring device. When the temperature exceeded 35 °C, the stirring was terminated to cool down the mixture for 10 min and then the stirring started again until the completion of the mixing process. By placing the resin-biochar of each mixture in the tank of the 3D printer (Phrozen Technology, Hsinchu City, Taiwan, China), the specimens of the corresponding mixture were 3D-printed. After the printing was completed, the samples were cleaned with isopropyl alcohol and inserted in the UV chamber to harden. The same process was followed for the other mixtures. Preparation of PLA-biochar composites and 3D printing of specimens

PLA-biochar composites were formed in filament for the MEX 3D printing, through a thermomechanical melt extrusion process. A two-step extrusion course was followed ensuring the scattering of the biochar in the polylactic acid matrix, to produce the composites in MEX 3D printing compatible filament form. The process is analytically presented in a former study by the authors (Petousis et al. 2023b). With the composite filament, correspondingly to the VPP method, specimens were 3D fabricated for mechanical testing. The MEX 3D printing control settings were selected in preliminary tests and in accordance with the literature for the 3D printing of the specific PLA grade (Petousis et al. 2023b) and are presented in Fig. 1 along with the biochar loading in the composite. The biochar loading was increased in the composite until processability issues in the extrusion or the 3D printing process restricted the further biochar loading increase and the mechanical properties started to decrease indicating saturation of biochar in the PLA matrix. Funmat-HT 3D printer from the Intamsys brand (Intamsys, Shanghai, China) was used for the fabrication of the test parts.

2.3 Field emission SEM and EDS

The morphological characteristics of the biochar powder and the 3D-printed parts were examined through field emission SEM (JSM-IT700HR, Jeol Ltd. Tokyo, Japan). Specimens were gold-plated whereas SEM photos were taken in a high-vacuum mode at 20 kV. The same apparatus was used for the elemental analysis of the samples (biochar powder and 3D printed samples) by Energy Dispersive Spectroscopy (EDS).

2.4 Thermogravimetric analysis

Thermogravimetric analysis (TGA) was accomplished to gauge the thermal stability and properties of the prepared composites. A PerkinElmer Diamond (PerkinElmer, Inc., Waltham, USA) device was deployed and the process followed is analytically presented in a previous work in which the same PLA grade was studied (Petousis et al. 2022).

2.5 Raman spectroscopy

Spectroscopic analysis was accomplished by means of an adapted LabRAM-HR Raman (HORIBA Sci., Kyoto, Japan) apparatus. Analytically, the methodology followed is presented in a previous study in which the same PLA grade was studied (Petousis et al. 2022).

2.6 Atomic force spectroscopy

To evaluate quantitively the surface topography of the VPP printed samples and the produced filament, Atomic Force Microscopy (AFM) was performed (Microscope Solver P47H-Pro device). Also, in this case, the methodology followed was presented in a previous work in which the same PLA grade was studied (Petousis et al. 2022). Surface roughness values on the measurement region were derived. Surface roughness provides a qualitative indication regarding the quality of produced filament. The filament quality is related to its processability in the MEX 3D printing process. Low-quality filaments with rough surfaces can cause issues in the MEX 3D printing process (Macedo et al. 2020).

2.7 Mechanical testing

For both the VPP and the MEX 3D fabricated samples, tensile as well as flexural tests were performed, with the aid of an ImadaMX2 (Imada Inc., Northbrook, IL, USA) device with a suitable setup for each test, complying with the relative standards (ASTM D638-14 and ASTM D790-10 respectively). Charpy notched Impact tests were performed on a Terco MT220 (Terco AB, Kungens, Sweden), following ASTM D6110, while Vickers microhardness measurements were taken according to the ASTM E384 by means of an Innova Test 400 device (Innova BV, Maastricht, Netherlands). Analytically the mechanical testing methodology is presented in a previous work in which the same PLA grade was studied (Petousis et al. 2022).

3 Results

3.1 Biochar characterization

The average yield of the three runs was determined as 22% (on a dry basis), which is comparable to the yields reported for flame cap pyrolysis of woody residues (Jayakumar et al. 2023). The physicochemical characteristics of olive tree biochar are presented in Additional file 1: Table S4 in the supplementary material of the work (Nikolopoulos et al. 2023). The biochar used in this study was prepared by the current research team (Prof. D. Kalderis). The same biochar sample was used by Nikolopoulos et al. 2023. The biochar prepared herein was mechanically and manually homogenized and then characterized, therefore there was no need to perform all the characterization analyses again within the context of this study as they are reported in (Nikolopoulos et al. 2023).

The elemental content, pH, electrical conductivity, and ash content values were within the typical range of values reported earlier for woody biochars (Tu et al. 2022). The molar O/Corg ratio and H/C ratio indicate a high number of oxygenated functional groups on its surface (hydroxyls, phenols, ethers, carbonyls, and carboxyls), which practically means a high degree of hydrophilicity and therefore wettability of the material (Kalderis et al. 2019). However, a high concentration of oxygenated groups may lead to increased reactivity and thus degradability in the long term (Bakshi et al. 2020). Spokas (2010) portrayed the environmental constancy and stability for biochar with O/C ratios of ≤ 0.6 as highly stable (100–1000 years half-life range), whereas a C/O above 0.6 classifies biochar as rather unstable (half-life lower than 100 years). Furthermore, the O/Corg, H/Corg values and the heavy metal concentrations were well below the thresholds set by the European Biochar Certificate (EBC) for Class IV materials, which are materials that can be used in industrial application such as building composites, plastics, electronics, or textiles without risk to the environment and users (European Biochar Foundation (EBC) 2016). This is a positive first step towards the safe use of this biochar in composites. However, before a complete environmental risk assessment can be performed, the polycyclic aromatic hydrocarbon (PAHs) and dioxin content should also be determined. The BET surface area measurement was within the typical range reported for woody biochars (Tu et al. 2022).

The XRF analysis revealed a high content of CaO. The reason for the high level of CaO in biochar is the high concentration of CaCO3 found in the calcareous soil the olive trees are cultivated on. Statistical analysis of the data showed that d (0.9) and d (0.5) values for the biochar were 50.32 μm and 14.64 μm, respectively. Converting raw (crude) biochar to a fine powder is essential to achieve a uniform mixture with the resin at a later stage.

Figure 2C and D present two magnification images taken with SEM, of the biochar powder. The particle size was in the microscale, and they had a random wedge-type shape. Smaller and larger particles are shown in the images. Figure 2E shows the EDS graph acquired in the region shown in Fig. 2D. The elements identified were the expected ones and agreed with the XRF analysis presented above.

Fig. 2
figure 2

Biochar morphological and compositional characterization: (A) 2000 × SEM image, (B) particle size distribution, (C) 5000 × SEM photo, (D) 45,000 × SEM photo, (E) EDS plot depicting the elements identified in the region shown in D

3.2 TGA and spectroscopic analysis of the composites

The thermal properties of the VPP and the MEX biochar composites were determined with TGA. The produced graphs (Fig. 3A, weight loss vs. temperature), show that the biochar addition did not meaningly change the thermal response of pure materials in both the VPP and MEX process. The stability versus temperature of pure polymers was maintained through the inclusion of biochar filler, with minor differences between the curves of the composites having the same matrix material (VPP and MEX respectively). The residual mass by the completion of each measurement agreed with the biochar loading in each composite. Additionally, the composites with PLA as matrix polymer initiated measurable degradation at temperatures higher than 300 °C. This proves that the thermal setting applied to the filament melt extrusion and the 3D printing of the samples with the MEX method did not somehow affect the quality and integrity of the composites. The weight loss rate seems to slightly decrease with the addition of the biochar in both the resin and the PLA polymer. The temperature, in which the highest weight loss rate occurred was approximately the same in the resin-based composites (VPP composites). In MEX composites, the addition of biochar slightly decreased the temperature, in which the highest weight loss rate occurred, compared to the pure PLA polymer.

Fig. 3
figure 3

Thermal response of the compounds prepared herein, derived with TGA (A) and DTGA (B) plots for VPP and MEX biochar-composites

Figure 4 illustrates the Raman shift spectra for the resin and the PLA biochar composites. As seen in Fig. 4A the main Raman peaks arise in pure Resin. The biochar filler in resin induces insignificant differences (Table 2). As Fig. 4B shows, the major Raman peaks derive from PLA Pure. The loading of biochar in PLA specimens introduced two broad Raman curves, one between  1245−1 and 1410 cm−1, and the second between 1520−1  and 1660 cm−1. This was expected since they represent the D and G bands respectively of graphite structure in biochar as described earlier (Li et al. 2019b; Sousa et al. 2020) (Table 3). Every work cited in the following tables for the Raman spectra results has used different standards to generate the reference Raman signals and assignments. For example, Li et al. (2019b), have used standard phenanthrene (purity > 99%), Oxalic acid (purity > 97%), and others. The standards followed are mentioned in the corresponding literature works cited herein.

Fig. 4
figure 4

Raman spectra for all samples tested, (A) Pure resin and its composites, (B) pure PLA and its composites

Table 2 Major Raman peaks identification along with their corresponding assignments for pure resin and its biochar composites
Table 3 Major Raman peaks identification along with their corresponding assignments for pure PLA and the related composites with biochar

3.3 Surface morphology examination with AFM

rface topography and roughness measurements were obtained with AFM. Roughness is a quality characteristic of composite parts. The topography of the top surface parts 3D-printed with the VPP process was examined for specimens made with the composites prepared and the measurements results are presented in Fig. 5B–E. Surprisingly, the addition of biochar reduced the surface roughness of the resin composites compared to the pure resin. No specific pattern was identified. Surface roughness was reduced at the 0.25 wt.% biochar loading and then increased with the increase of the biochar loading in the composites, but even at the resin composite with the highest biochar loading, the surface roughness was lower than that of the pure resin. Corresponding surface quality measurements were obtained on the external surface of the produced composite filament and are inserted (shaded) in Fig. 5F–I. The same trend was observed in the PLA/biochar composites, with the Ra roughness declining over the addition of biochar to the pure PLA and then increasing with the increase of the biochar loading in the composites. Again, surface roughness measurements are lower than the pure PLA, even at the highest biochar loading composite of 8 wt.%.

Fig. 5
figure 5

Surface topography derived with AFM (A) pure resin, (B) resin/biochar 0.25 wt.%, (C) resin/biochar 0.50 wt.%, (D) resin/biochar 0.75 wt.%, and the external surface of the produced composite filament (E) PLA/biochar 2.0 wt.%, (F) PLA/biochar 4.0 wt.%, (G) PLA/biochar 6.0 wt.%, (H) PLA/biochar 8.0 wt.%

3.4 MEX filament quality checks and tensile tests

The produced filament with the extrusion process for the 3D printing of the samples with the MEX method was inspected on the optical microscope (Fig. 6A and B). No defects or voids were located at the side surface, indicating a successfully completed extrusion process. During the melt extrusion course, the filament diameter was continuously monitored with a sensor embedded in a 3devo extruder. The diameter records of randomly selected periods for two different composites are shown in Fig. 6A and B, respectively. As seen, the measurements are within the same range and acceptable for the MEX 3D printing process zone, in both types of composites.

Fig. 6
figure 6

Produced filament for the MEX process examination: microscopy examination and filament diameter monitoring during its extrusion course (A) PLA/biochar 4.0 wt %., (B) PLA/biochar 8.0 wt %., (C) filament tensile testing, (D) filament sample after it failed in the tensile test (E) average values of tensile strength and modulus along with their measurement deviations as they were derived from experimental procedures

The filament was also tested for its mechanical strength in tensile tests. These tests were not performed according to a standard procedure, still, they provided significant comparative information on the characteristics of the filament and can be correlated with the corresponding results on the 3D-printed samples. Fig. 6D depicts the tensile test results on the extruded filament for every composite prepared herein. The tensile strength and elasticity modulus are presented (average values and deviation). A clear reinforcement effect is shown in the results up to the 6 wt.% biochar loading. Beyond this point, the mechanical performance started to decline until the highest loading of 8 wt.% indicating that the saturation threshold has been reached for the biochar in the PLA matrix (marginally lower than the pure PLA at this loading, significantly reduced compared to the 6 wt.% composite). The 4 wt.% loading composite showed the highest reinforcement with a 19.5% rise in the tensile strength, as well as 17.3% in the elasticity modulus, in comparison to the pure PLA.

3.5 Mechanical testing

The tensile test experimental results for both the resin-based and the PLA-based composites proved an evident improvement of sample strength terms, by adding biochar. In the resin/biochar composites, a 3.6% improvement was achieved in the tensile strength (Fig. 7B) and a 5.3% in the tensile elasticity modulus (Fig. 7C), through a rather low 0.50 wt.% biochar loading in the composite. In the stress vs. strain graphs (Fig. 7A), the strain was not significantly altered as a consequence of adding biochar in the resin composite, which is an indication that the ductility of the samples was not affected. This needs to be verified on the inspection with SEM that followed. The reinforcement effect on the PLA/biochar composites was more intense, probably due to the higher biochar loading in the composites. All the composites, except the one containing the maximum loading of 8 wt.% showed improved tensile properties in comparison to the PLA pure grade. The 8 wt.% biochar samples also failed at lower strain (Fig. 7A) showing that the specific loading compromised the performance of 3D-printed samples. This is an indication that the saturation threshold is between the 6.0 and 8.0 wt.% loading for the biochar additive in the PLA matrix. An increase of 20.9% to the tensile strength was achieved by the 2.0 wt.% biochar loading composite (Fig. 7B), compared to the pure control PLA, while the 4.0 wt.% loading composite achieved a 25.8% rise in the elasticity modulus (Fig. 7C). The reinforcement trend was similar to that of the filament, which is the expected behavior. The tensile property values were higher than the respective ones for the filament. Such differences can be attributed to the fact that the filament was not tested according to a standard procedure, so the derived experimental values are not directly comparable. This is because, to the authors’ best knowledge, there is no standard available for the tensile testing of filaments. Therefore, to evaluate the mechanical performance of the produced filament and how it is affected by the biochar concentration in the composites, tensile tests were conducted using the same conditions as the 3D printed samples, which were tested according to the corresponding international standard (ASTM D638), but in the filament tests, the authors cannot claim that a standard was followed.

Fig. 7
figure 7

Resin/biochar and PLA/biochar composites tensile test results (A) tensile stress vs. strain, (B) average tensile strength and experimental deviations, (C) average tensile modulus of elasticity and experimental deviations

The flexural experimental course per sample was terminated at 5% strain, following the directives of the standard procedure (ASTM D790) (Fig. 8A). A reinforcement effect, comparable to the tensile test trend, was observed in the flexural tests. A 4.8% reinforcement in comparison to the pure control resin was reported for the 0.50 wt.% biochar loading composite. In these tests, it was not possible to 3D print a good-quality sample with the resin/biochar 0.75 wt.% composite, thus the corresponding results are missing (Fig. 8B). In the inset image, the defects of the 3D printed samples from the resin/biochar 0.75 wt.% composite can be seen. These defects are attributed to insufficient photopolymerization of the composite. The effect of carbonaceous materials in the curing process is known and studied in the literature (Hoppe et al. 2010; Tomal et al. 2020). Thermal properties, such as enthalpy, are affected. The curing temperature is expected to be decreased with the addition of carbonaceous materials in the resins (Khan et al. 2020). Still, in the work, the curing temperature was kept at a constant value for all compounds, to have comparable results.

Fig. 8
figure 8

Resin/biochar and PLA/biochar composites flexural test results (A) flexural stress vs. strain, (B) flexural strength findings, (C) flexural elasticity modulus findings

Regarding the PLA/biochar composites produced with the MEX process, a 14.1% rise of the flexural strength in comparison to the pure control PLA is reported for the composite with 4.0 wt.% biochar loading, whereas a 10.7% increase in the flexural elasticity modulus for the composite with 6.0 wt.% biochar loading. At the 8 wt.% loading, the flexural properties started to decline, but still, they are marginally higher than the control PLA pure.

The toughness metric was determined as the integral of the stress over strain plots, and it shows the absorbed energy by the sample throughout the corresponding experiment. Table 4 presents the average calculated toughness and deviation per composite for the tensile and flexural experiments, respectively. In the tensile tests, the resin/biochar composites exhibited in general, a decreased toughness, when compared to the pure control resin. This agrees with the tensile test results and can be attributed to the deformation of the resin composites before they fail in the tensile tests. As the composites showed a less ductile behavior (reduced elongation before their failure) the area below the stress vs stain curve from which the toughness is calculated, is reduced. The toughness shows the energy absorbed by the material before it failed the tests. As the sample fails at lower strain values, the energy absorbed in the experiment is lower.

Table 4 Resin/biochar and PLA/biochar composites experimental results: average tensile toughness and deviation, average flexural toughness, and deviation, average Charpy impact strength and deviation, average Vickers microhardness and deviation

Only the 0.50 wt.% loading composite showed increased toughness by 7.3% compared to the reference samples. This was the only resin/biochar composite with increased tensile strength. The flexural toughness (Table 4) is decreased in the resin/biochar composites, in comparison to the resin pure. Furthermore, the toughness of the PLA/biochar composites followed a similar pattern to the corresponding tests, still, the biochar composites with the highest toughness values were different from those that showed the highest increase in strength in the tests. This can be attributed to a stiffer behavior (higher slope of the Hooke region) or a more ductile behavior (higher strain at failure). A 22% increase in the tensile toughness was observed for the PLA/biochar with a 2.0 wt.% biochar loading, whereas a 7% increase was determined in the flexural roughness for the PLA/biochar with 4.0 wt.% biochar loading.

The addition of biochar is reducing the impact toughness (Table 4), as well as the Vickers microhardness (Table 4) of the resin/biochar grades. On the other hand, an impressive 140% increase was achieved in the impact strength of the 4.0 wt.% loading PLA/biochar composite, compared to pure PLA. Microhardness was not influenced significantly by adding biochar in the PLA matrix, with values being similar to the pure PLA in all loadings studied. The 2.0 wt.% loading PLA/biochar composite achieved slightly higher microhardness values, increased by a marginal 0.8% compared to the pure PLA polymer.

3.6 Morphological characterization

The fractographic and morphological features of the 3D-printed samples were inspected by SEM investigations. Figure 9 presents SEM images for the resin/biochar composites. Figures 9A, D, and G present the images from the side surface of pure resin, resin/biochar 0.50 wt.%, and resin/biochar 0.75 wt.%, respectively. In the image for the pure resin, (Fig. 9A) the 3D-printed structure does not show any defects or voids. The addition of biochar in the resin matrix has a negative effect on the 3D printing structure, with minor voids visible on the sample (0.50 wt.% biochar composite, Fig. 9D). Further increase of the biochar loading in the composite to 0.75 wt.% (Fig. 9G) significantly increased the voids that are visible on the side surface of the sample, indicating that the quality of the composite worsened. This has also a negative consequence on the mechanical performance of the parts for the highest biochar loading. Figures 9B, C E, F, and H, and I present at two different magnifications the fracture surface of the resin/biochar grades. The fracture surface in all samples indicates a brittle fracture mechanism, as minimum deformation can be observed. The characteristic striation fans, porosity, and dimples (Ratnesh Raj et al. 2022) in the fracture surface of the VPP 3D-printed can be observed in the samples.

Fig. 9
figure 9

SEM images taken from the resin/biochar composites. In each column are presented: top image 150 × magnification of the sample side surface, middle image 27 × magnification of the sample fractured surface, bottom image 5000 × magnification on the sample fractured surface, (AC) pure resin, (DF) resin/biochar 0.50 wt.%, (GI) resin/biochar 0.75 wt.%

Figure 10 includes corresponding SEM photos for the PLA/biochar composites. The side surface of the pure PLA sample (Fig. 10A) and the PLA/biochar 4.0 wt.% (Fig. 10D) shows a defect and void-free layer fusion, with layers having a uniform and stable shape. In the sample built with PLA/biochar 8.0 wt.% composite, porosity, voids, and a non-uniform layer shape can be observed (Fig. 10G). This caused an influence on the mechanical properties of this grade case, as presented above. Figure 10B, C ,E ,F ,H and I present at two magnification levels the fractured face of the PLA/biochar composites. Higher deformation can be observed in the pure PLA sample (Fig. 10B, and C) on the fracture surface, although the specimen overall is not significantly deformed. In the PLA/biochar 4.0 wt.% sample, the deformation in the fracture surface was minimum, still, the specimen seemed to be deformed more than the pure PLA one (change in the shape of the section is visible). Additionally, the porosity appeared to be higher in this sample than in the corresponding one made with pure PLA. Microcracks were also visible at the higher magnification image (Fig. 10F). The sample with the highest biochar loading (8 wt.%) showed the highest deformation in the shape of the specimen, although the fracture surface itself was not significantly deformed. Additionally, the porosity was increased with more pores visible and micro-pores in the fracture surface (Fig. 10H and I, respectively). The increased porosity negatively affects the mechanical performance of the 3D-printed parts (Li et al. 2019a), as was confirmed by the mechanical tests conducted. Still, such geometrical characteristics are expected in the 3D-printed structure of the parts built with the MEX process (Song et al. 2017).

Fig. 10
figure 10

SEM images taken from the PLA/biochar composites. In each column are presented: top image 150 × magnification of the sample side face, middle image 27 × magnification of the sample fractured surface, bottom image 3000 × magnification of the sample fractured surface, (AC) pure PLA, (DF) PLA/biochar 4.0 wt.%, (GI) PLA/biochar 8.0 wt.%

4 Discussion

Herein, the potential of using an ecological, biocompatible, nature-sourced carbon-based material in powder form as a filler for enhancing the performance of pure polymeric grades used in AM, is investigated. This additive was the biochar prepared from olive tree prunings. A standard-grade resin in VPP and PLA in MEX 3D printing was used as the matrix materials. Composites were prepared with high-shear stirring for the VPP process and with extrusion in the MEX process. From the experimental process, it was found that although biochar enhanced the performance of the resin even at low loadings (0.50 wt.%) it is not a suitable reinforcement additive, due to the processability issues that prohibited the production of higher biochar loading composites, which were expected to achieve a sufficient increase in the mechanical performance. The mechanical testing findings are collectively plotted in radar (spider) charts provided in the supplementary material of the work for the resin/biochar composites and the PLA/biochar composites. The grey area of plots represents the pure polymers’ experimental results. It is shown that only the 0.50 wt.% biochar composite had upgraded mechanical properties, in comparison to the pure control resin. On the other hand, the reinforcement effect of the biochar addition in the PLA matrix is evident, with improved mechanical response in all the tests conducted. The biochar reinforcement mechanism on polymeric materials has been investigated in the literature. It is reported that the filler structure, i.e., the aggregation structure and particle size, and the interactions on the surface between the biochar and the matrix highly affect the reinforcing mechanism of biochar (Jiang et al. 2020). The reinforcing mechanism of biochar is attributed to Vander wall interactions between the biochar and the polymer. Additionally, the reinforcing mechanism of biochar is attributed to the interlocking of polymer within micro-size pores of biochar (Mohammed et al. 2022).

In the resin/biochar composites, the increase in the tensile strength reached 3.6%, which might not be impressive, but it should be considered that this was achieved with a very low biochar loading of 0.50 wt.%. PLA/biochar composites reached a 20.9% increase in the tensile tests, with a rather low biochar loading of 2.0 wt.%, still the 4 wt.% composite had a similar response in the tensile test and improved flexural properties. So, a 4 wt.% biochar loading should be considered as the optimum loading for the PLA/biochar composite. The performance in the mechanical tests of this composite reached the mechanical performance of high-performance polymers, such as Polycarbonate (PC) in MEX 3D printing (Vidakis et al. 2021b). This is a high-merit outcome, considering that such a performance was achieved with a composite that both the matrix and the additive materials were biocompatible, environmentally friendly, and obtained from natural resources. In the PLA/biochar composite, in the 8 wt.% biochar loading, a saturation of the biochar in the composite occurred, as the mechanical properties were decreased. The exact saturation threshold was not identified, as it was not within the scope of the work. It should be noted that the two different composite grades were prepared with different methods, in order to produce composites compatible with each AM process (VPP and MEX) and, in both methods, reinforcement occurred.

By correlating the SEM images on the fracture surfaces (Fig. 10) and the mechanical properties found in the experiments, it can be assumed that the mechanical performance of the samples was affected by the changes in the internal structure of the parts, caused by the increase of the biochar concentrations in the composites. As shown in the SEM images of the side surface of the sample in Fig. 10, as the biochar concentration increases in the samples, the 3D printing quality is reduced, as the processability of the composites worsens. This leads to an increased number of defects in the 3D printing structure in the form of increased pores in shape and size, as shown in the fracture surface images. The reinforcing effect of biochar as the concentration increases overlaps the negative effect of the increased porosity, having an overall positive effect on the mechanical performance of the samples. Further increase of the biochar concentration leads to a 3D printing structure with an increased number of defects and reduced sample area due to the increased porosity, therefore the mechanical performance was reduced.

The findings of the current research are not directly comparable to others existing in the literature, since there is not found any similar work for the specific composites, their fabrication processes, and their feasibility and performance in AM technologies. Commercial, certified biochar derived from various natural resources, but not from olive tree prunings used herein, was introduced to resins at 2 wt.%. The preparation process was different to the one followed herein, and the biochar properties differed, and the resins investigated were different to the current study (Bartoli et al. 2019). Composites were not in a form suitable for use in the AM process. Still, the reported reinforcement effect in the work of Bartoli et al. (Bartoli et al. 2019) was rather impressive, exceeding 100% in comparison to the pure-grade resin. Differences in the performance of the composites can be attributed to the different material specifications used in the current study, with the corresponding ones in the study of Bartoli et al. (2019), as mentioned above. In a different study, maple-derived biochar was used as reinforcement in epoxy resins with loadings of up to 20 wt.%. Composites were prepared with a different methodology than the current study and they were not compatible with the AM processes. The highest reinforcement was achieved with the 1 wt.% biochar loading composite, with an increase of about 38% compared to pure resin (Giorcelli et al. 2019a). Again, differences can be attributed to the different resin and preparation processes. Biochar has also been introduced to epoxy resins to enhance conductivity properties, due to its high carbon content (Giorcelli et al. 2019b). PLA/biochar composites have been presented in the literature, again, not in a form compatible with the AM processes, and a different preparation method was implemented. Arrigo et al. (2020) employed solvent casting and melt mixing for PLA/biochar composites development with a loading of up to 7.5 wt.%. Biochar was sourced from coffee beans. In both methods, the composites showed a stiffer response than the pure PLA, but the tensile strength was not improved with the addition of biochar. To the best knowledge of the authors, there is just one published work, which involves biochar in the MEX AM process. More specifically, biochar was derived from packaging waste, and composites were prepared from recycled Polyethylene terephthalate (PET) polymer (Idrees et al. 2018). Composites for up to 5 wt.% biochar loading was prepared and the increase in the tensile strength reached about 11% compared to the neat PET polymer (recycled without biochar additive). Differences between the results presented herein may be attributed to the dissimilar polymers studied and biochar properties. Additionally, since recycled PET was used, its exact composition was not known. Idrees et al. (2018) used peanut-shaped packaging material to produce biochar, the composition of which was very different from the olive tree prunings we used. Furthermore, they used a Ni-based superalloy high-pressure/temperature reactor, whereas herein a much simpler and cost-efficient flame pyrolysis kiln was employed. Idrees et al. (Idrees et al. 2018) combined their biochar with recycled PET, not commercial resins as the PLA used herein as the matrix material. Overall, their approach was purely from a material science point of view and did not include any environmental aspects, whereas the wider scope of the current study was not only to produce a novel composite but to develop a new valorization pathway for a major agricultural waste. Additionally, in the work recycled poly(ethylene terephthalate) was used as the matrix material, while in the current study virgin PLA was the matrix material. The biochar concentrations were similar in the two studies. In the work of Idrees, et al. tensile and DMA tests were conducted for the evaluation of the composites' mechanical performance, while in the current study, four different tests were implemented. The work of Strongone et al. (2020) used coffee waste to produce biochar and not agricultural waste, so their biochar had very different properties. Furthermore, their pyrolysis apparatus was a commercial quartz reactor, and their biochar was produced at a much higher temperature (800 °C), compared to the process followed herein. Additionally, a different resin was the matrix material, the work focused on films and coatings, and no mechanical tests were conducted to assess the reinforcing effect of biochar, which is the main purpose of the current study. The biochar loadings on the prepared composites were similar to the ones implemented herein. Such literature investigation results verify the novelty of the current study and presented findings.

It should be noted that higher biochar concentrations on the resin matrix were not possible to be achieved. The resin/biochar suspension were not easy to prepare, and 3D print. Moreover, the addition of biochar in the resin matrix made the produced composite darker, and as the concentration increased, the photopolymerization process was not possible to be achieved. The solidification of the resin and the change of phase from liquid to solid is achieved in VPP 3D printing through the photopolymerization principle. Since this was not possible to be achieved at higher biochar concentrations, it was not possible to 3D print test samples with compounds with higher biochar concentrations with the VPP 3D printing process. So, as the biochar concentration increased, photopolymerization issues probably lead to this inferior performance of the resin/biochar composites. Carbonaceous materials absorb UV light and affect the process (Barzaegari et al. 2023). As a result, the curing time was affected (Chiu et al. 2015). In the current work, the curing time is the same in all compounds for comparison purposes. The MEX 3D printing process is based on a different operating principle (thermomechanical extrusion) and in this method higher biochar concentrations in the compounds are possible to be achieved. Regarding the Raman results, unfortunately, they do not show any significant changes in the chemistry of the specimens to explain the material strength increase. Small changes are present at 770 cm−1 and 781 cm−1 for PLA. In the STD Resin specimens, only the gradual increase of biochar is present as seen between 1245 and 1410 cm−1, and between 1520 and 1660 cm−1. This represents the D and G bands respectively of graphite structure in biochar. This increase is not gradual in PLA specimens.

5 Conclusions

Herein, the hypothesis of using the biocompatible, environmental-friendly biochar as reinforcement in AM polymers for two different processes, i.e., VPP and MEX was proven. This was an effort to develop biocompatible, environmental-friendly, and low-cost composites for the two most often used AM methods. A low-cost resin of standard grade was selected as the matrix in the VPP process and the biocompatible, derived from natural resources, PLA was the matrix material in the MEX 3D printing process. Biochar was collected and produced from olive tree prunings within the context of the work. Its composition and morphological characteristics were investigated and reported. The biochar addition into the two matrices did not cause any thermal instability and properties of the matrix materials. The reported structural properties were also the expected ones.

The reinforcement effect of biochar in both types of composites (resin/biochar and PLA/biochar) was found to be sufficient considering the loadings investigated, as explained above. In the VPP technique, the resin with 0.5 wt.% biochar exhibited the highest reinforcement of 3.6% in the tensile tests, while in the MEX 3D printing method the PLA with 2.0 wt.% biochar exhibited a sufficient 20.9% increase in tensile strength, compared to the unfilled PLA thermoplastic. Similar results are reported for the flexural tests as well. In the VPP 3D printing method, processability and printability issues occurred at rather low biochar loading, prohibiting the further increase of the biochar loading in the composites. This can be attributed to the specific resin grade, the VPP 3D printer used, and the known literature curing issues carbon additives induce in the composites, as explained above. Therefore, based on the current study results, it cannot be recommended as a reinforcement agent in the VPP process, at least for the specific resin. On the other hand, PLA/biochar can be considered an ecological, sustainable, and biocompatible composite with improved mechanical properties, reaching the response of high-performance polymers, as mentioned. In the MEX 3D printing method, processability issues occurred at the upper loading of 8 wt.% prepared, in which the saturation threshold for the biochar additive was exceeded. In future work, the composite preparation methods and parameters can be further optimized, in view of scaling up the process. Given the wide range of biochars and their multi-functionality, different samples should be investigated in order to determine which of their physicochemical properties play a role in the curing process and affect the properties of the composites. Such information will allow the production of engineered biochars, suitable for VPP and MEX additive manufacturing.