Enhancement of Thermal and Mechanical Properties: Nanofibrillated Cellulose - Reinforced PHB/PBAT 3D Filaments for Fused Deposition Modeling

doi:10.21203/rs.3.rs-3566630/v1

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Enhancement of Thermal and Mechanical Properties: Nanofibrillated Cellulose - Reinforced PHB/PBAT 3D Filaments for Fused Deposition Modeling

https://doi.org/10.21203/rs.3.rs-3566630/v1

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Melt extrusion was used to create three-dimensional (3D) filaments reinforced with nanofibrillated cellulose (NFC) in blends of polyhydroxybutyrate/polybutylene-co-adipate terephthalate (PHB/PBAT) in various ratios (0.5%, 1%, and 2%). A twin-screw extruder was used to extrude filaments with an approximate diameter of 3mm. Consequently, these filaments were analyses for their rheological, thermal, mechanical, and morphological properties. The PHB/PBAT/NFC blends and neat PHB were compared to assess the effect of blending PHB with PBAT and reinforcing it with NFC. The inclusion of PBAT and the reinforcing effect of NFC were investigated by thermal analysis, which resulted in an improvement in thermal behaviour and stability. However, as illustrated by the result of differential scanning calorimetry (DSC), the incorporation of PBAT reduced PHB’s crystallinity. This correspondingly affected its rheological and mechanical strength. The desired tensile properties necessary for three-dimensional (3D) printing can be maintained with the addition of NFC. The highest tensile properties were achieved by the lowest loading of NFC. Additionally, it is proved from the result of Scanning Electron Microscopy (SEM) analysis, for 0.5% NFC loading exhibited a better two-phase system and effective dispersion of NFC. This research expands the potential applications of PHB, including PHB/PBAT blends and PHB/PBAT/NFC composites.

Biopolymer

3D Printing

Nanocomposites

Nanocellulose

Polymer Blend

Additive manufacturing (AM), is a three-dimensional (3D) printing applied a cutting-edge technique to create physical objects of various depths and volumes. This technique also uses a digital model as a blueprint and deposited the materials layer by layer (Bhatia and Ramadurai 2017). The layering method used enables the formation of complex designs that might not be feasible using conventional manufacturing techniques. In particular, this method allows for the fabrication of objects with complex internal architectures, hollow spaces, and unique geometries that adhere to some specifications. The most widely used AM technology is fused deposition modeling (FDM), also known as material extrusion (Gonzalez et al. 2018). Figure 1 provides a visual representation of this fused deposition modelling (FDM) process.

The FDM manufacturing technique is based on the precise transformation of raw materials into intricate structures by melting and depositing them layer by layer. In this process, a continuous filament, typically rolled up on a spool, is drawn carefully through a set of drive wheels. It is then fed to a nozzle head equipped with precision temperature control mechanisms, where it is controlled heated until it reaches a semi-liquid state (Kristiawan et al. 2021). The nozzle precisely extrudes this semi-liquid material in ultrathin layers onto a build surface. These layers are added one by one to form the final 3D object. This rigorous layering procedure corresponds to the precise contours defined by a computer design, which is often developed in Computer-Aided Design (CAD) software. The CAD model is then split into a series of separate layers, which act as a roadmap for the FDM system, ensuring that each layer adheres to the design criteria. This layer-by-layer additive method is the fundamental foundation of FDM 3D printing and is essential to its ability to create extremely complex and bespoke physical items (Dizon et al. 2018, Mitchell et al. 2018).

It is critical in the context of FDM to go into the technical details of how the process is dependent on the thermoplastic nature of the filament. This method of 3D printing creates items by depositing and fusing layers of a thin thermoplastic filament. The success of this technology is dependent on the filament's thermoplasticity, which is defined as its ability to form strong connections between successive layers during the printing process and then solidify at ambient temperature after printing. Several major processing parameters have an impact on the mechanical properties of the final printed item. These characteristics are critical in defining the printed object's structural integrity and performance. According to Aumnate et al. 2018, the intricacy of FDM's needs has made the development of acceptable filament materials a tough issue. This highlights the importance of continual research and innovation in the realm of 3D printing materials in order to achieve the stringent criteria of FDM technology. This highlights the importance of continual research and innovation in the realm of 3D printing materials to meet the stringent requirements of FDM technology.

1.1 Filament manufacturing process

The filament is the primary raw material used in the FDM method. The manufacture of pure filament for FDM applications is a meticulous process that includes the extrusion of polymer pellets or unprocessed polymer components. This extrusion process is carried out utilising specialised extruders that exert pressure to force the material through perforations in a die, generating the necessary extrudate. Meanwhile, making filament from polymer composites or through reinforcing needs a different approach. Material blending takes primacy in this case, with each composition prepared beforehand to ease subsequent linkage. The process of mixing is determined by the specific properties of the constituent components. As indicated by Yetgin (2020), this can include mixing a solution, followed by a drying process prior to extrusion. Alternatively, the dry mixing process is used to produce the necessary material combination.

Figure 2 depicts a thorough overview of the extrusion process of extruded filament using an extruder. It outlines the stage process transformation of pellets into filament, beginning with their entrance into the hopper and going through the melting and eventual filament extrusion processes (Miron et al. 2017). Interestingly, a study by Miron et al. (2017) explores further into the quest for filament extrusion results with minimal deformation while assuring a systematic and consistent procedure. Their study elucidates the complex relationship between extrusion temperature and speed. They identify specific challenges caused by the interaction of extrusion factors through their analysis. For example, if the extruder temperature is set too low, the pellets do not melt sufficiently, resulting in screw rotation blockage. As a result, the extruder does not function properly, with spinning slowing. These difficulties arise as a result of low thrust and high material viscosity, resulting in an unfavorable increase in extrudate diameter. These discoveries can be put to practical use by accounting for the interaction of parameters in produce the desired filament output.

The extrusion process is a complicated process in which several critical parameters have a substantial impact on the produced filament. These parameters include die temperature, roller puller speed, spindle speed, and inlet temperature, all of which influence the diameter of the filament cable. The interaction of these parameters is essential because they have a direct impact on the viscosity of the material, which determines whether the extruded material aligns with the specified diameter at the nozzle die. Furthermore, the winding screw design is crucial in understanding the regularity of the filament. This is especially important in the context of composite filaments, because the thread configuration influences the orientation of the filler elements inserted into the filament. This complex interaction of elements emphasises the significance of monitoring programs and precision in the extrusion process, where deviations in any of these aspects can result in considerable differences in filament characteristics and performance (Geng et al. 2019, Haq et al. 2017).

1.2 Biopolymer filament

The bio-polyester polyhydroxybutyrate (PHB), which exhibits characteristics resembling those of non-biodegradable polymers, has attracted considerable attention as an environmentally friendly material. PHB’s high degree of crystallinity adds to its good mechanical properties, however, it has a lower elongation at break than Polypropylene (PP) (Strong et al. 2016). It is also important to consider that PHB has certain limitations. PHB is stiff and brittle owing to its high crystallinity, resulting in very poor mechanical properties and limits its range of application (Saska et al., 2018; Tang et al., 2012). PHB’s crystallinity level also reflects its slow crystallisation rate, which leads to large crystal sizes and a brittle structure with sticky properties (Mekonnen et al. 2013). Handling filament breakage during the extrusion process of 3D printing will be challenging due to poor processability and limited processing. This will cause an inconsistent flow and incomplete deposition of melted PHB, causing the printing process to be hampered. To address these issues, most research suggests polymer blending (Ferreira et al., 2001). PBAT is a random copolymer composed of butylene adipate and terephthalate units with both amorphous and crystalline units. Unlike PHB, PBAT has favourable qualities in terms of flexibility, toughness, and excellent processability. This blending strategy has the advantage of lowering processing temperatures. Simultaneously expands the processing window to address PHB’s drawbacks (Avella et al., 2000; Beber et al. 2018).

Amidst the biodegradability of PHB and PBAT, they formed an immiscibility phase when blended owing to differences in their chemical structures and molecular interactions (El-Taweel and Al-Ahmadi, 2021; Garcia-Garcia et al., 2016; Lim et al., 2013). As illustrated in Fig. 3, this phase separation occurs when distinct regions of each polymer form within the blend. These distinct regions affect the properties of the blend, such as mechanical strength, thermal stability, and processability. Differences in their chemical structures caused limited molecular interactions between PHB and PBAT. PHB has a crystalline structure with strong intermolecular forces such as hydrogen bonding (Sudesh et al., 2000), whereas PBAT has both amorphous and crystalline regions with weaker intermolecular interactions. This mismatch in molecular interactions prevents the complete mixing of the polymers at the molecular level (Deng et al. 2018). These domains can be visualised under a microscope, showing a two-phase or multiphase structure. The size, distribution, and shape of these domains depend on factors such as the composition of the blend and the conditions during processing (Quental et al. 2010).

The challenge of the polymer blend, along with the absence of desired filament and printed blend characteristics, can also be addressed by incorporating biologically derived reinforcements. Nanofibrillated cellulose (NFC), due to its unique properties and sustainability, is an attractive additive for polymer blends. In general, the addition of NFC to the polymer matrix results in composite filaments with biodegradable attributes, reduced crystallisation, and enhanced mechanical and thermal properties (Lee et al. 2014). However, the key hurdles in manufacturing polymer composites lie in achieving a homogeneous dispersion of NFC and ensuring good interfacial adhesion between the hydrophilic NFC and the polymer matrix. Perić et al. (2020) and Gregor-Svetec et al. (2021) emphasise the importance of considering various factors that influence the improvement of properties in nano-reinforced composites. These factors include matrix and filler properties (particularly the percentage, shape, and size of particles, as well as their orientation within the matrix), compatibility, and interactions between the filler and matrix, and the dispersion of NFC within the PLA matrix in fibre-reinforced systems.

Here, a high-consistency NFC was produced from a never-dried oil palm trunk (OPT) pulp through mechanical fibrillation using a high-speed blender. NFC was then used to reinforce PHB/PBAT to form filaments for 3D printing. The impact of NFC content (0.5%, 1%, and 2%) on the rheological, thermal, and mechanical properties of the composite filaments was investigated. Various characterisation techniques such as scanning electron microscopy (SEM), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), melt flow index (MFI), and tensile testing were utilised to evaluate the NFC and PHB/PBAT/NFC composites.

2.1 Materials

Polyhydroxybutyrate (PHB) and polybutylene-co-adipate terephthalate (PBAT) were employed in this study, utilising commercial-grade materials (Biomer P226 for PHB and Ecoflex F Blend C1200 for PBAT). The PHB was sourced from Germany, while the PBAT was obtained from BASF in Malaysia. Both materials were in pellet form. Nanofibrillated cellulose (NFC) was produced from oil palm trunk fibres using a high-speed blender for a minimum of 60 minutes. Subsequently, the pulp was homogenised for 30 minutes to facilitate enhanced fibrillation.

2.2 Preparation of PHB/PBAT/NFC composites

2.2.1. Production of NFC

Nanofibrillated cellulose (NFC) is made from never-dried oil palm trunk (OPT) pulp fibre. The pulp fibre was prepared using the soda pulping procedure (Hassan et al. 2020; Rodriguez et al. 2010). NFC is manufactured through the utilization of a high-speed blender and a homogenizer, following a procedure closely aligned with the method introduced by Uetani and Yano (2011), with minor adjustments. The pulp is first soaked in distilled water for 24 hours. Following that, the pulp slurry is transferred to a high-speed blender (Jouyoung L4-L971XK, stirring power : 160 W) and blended for 60 minutes at a 3% consistency. Following the blending, the slurry is transferred to a beaker and homogenised for 30 minutes using a homogenizer (IKA 3720001 T18). The NFC is ultrasonically dispersed before used it to disperse more of the fibres and reduce agglomeration.

2.2.2. Composite filament preparation

The optimal formulation for 3D printing applications involved using a PHB/PBAT blend in an 80/20 weight ratio. For this investigation, NFC loadings of 0.5%, 1%, and 2% were employed. The weights of each material were measured after formulating the three different NFC loadings, as indicated in Table 1. Subsequently, the materials were mixed and subjected to a drying process in a vacuum oven at 40°C for 24 hours ( as shown in Fig. 4 [a]). To ensure the removal of excess moisture, the material was further dried in the oven at 80°C for an additional 3 hours before proceeding to the melt extrusion process (Perić, et al., 2020).

Table 1

List of PHB/PBAT and PHB/PBAT/NFC composite code names and formulation
Formulation	PHB (wt.%)	PBAT (wt. %)	NFC (wt. %)
100PHB	100	-	-
80PHB20PBAT	80	20	-
80PHB20PBAT0.5NFC	80	20	0.5
80PHB20PBAT1NFC	80	20	1
80PHB20PBAT2NFC	80	20	2

The melt compounding process was conducted using a laboratory-scale twin-screw extruder (Fig. 4[b]), specifically the LabTech Model. The materials were supplied gravimetrically through the primary feeder. The temperature profile for the melt extrusion process ranged from 180 to 190°C (Fig. 5). The extruder was equipped with a 3-mm diameter nozzle, which corresponds to the desired filament size for 3D printers. The auto-feed hopper speed was set at 14.5 rpm, while the screw speed was set at 45 rpm. A customised electric spooling machine was integrated into the lab-scale extruder and was operated at a speed of 15 rpm to ensure filament size control within the range of 2.75 mm to 3 mm.

Using the obtained filaments, the testing specimens were fabricated using an Ultimaker 2 + 3D printer. A printing nozzle with a diameter of 0.8 mm was utilised. The layer height was set to 0.3 mm, and the printing speed was set to 60 mm/s while maintaining a temperature of 210°C. To ensure consistency in filament samples, a neat PHB sample was produced using the same method as PHB/PBAT and labelled as 100 PHB.

2.3 Characterisation Methods

2.3.1 Scanning Electron Microscope (SEM)

SEM was used to conduct a thorough examination of the freeze-dried NFC's morphological characteristics. To facilitate gold coating, these samples were thoroughly coated with POLARON SC515 prior to SEM scanning. Following that, the microstructure images of the samples were thoroughly examined at various magnification levels.

A similar approach was used to examine the morphology of fragmented PHB, blend, and composite filaments, specifically for following melt flow index (MFI) and tensile testing.

2.3.2 Transmission Electron Microscopy (TEM)

The TEM samples were prepared in the form of a solution. Prior to TEM investigation, each sample of NFC was thoroughly sonicated for 15 to 30 minutes to ensure homogeneous dispersion. Following that, each produced sample was carefully dropped as a small droplet onto a copper-carbon composite grid.

NFC samples were negative stained with uranyl acetate for about one minute to improve TEM contrast and enable imaging. Finally, TEM images were taken, with a specific magnification level of 100 nm used to capture detailed features. To increase its reliability, ten measurements were taken for ten separate acquired NFC photos.

2.3.3 Differential Scanning Calorimetric (DSC)

The crystallinity of the PHB/PBAT blend and PHB/PBAT/NFC composite was determined using a Perkin-Elmer DSC instrument. A 10-mg sample was subjected to an isothermal double heating cycle, with temperatures ranging from 40°C to 200°C. The sample was heated at a rate of 30°C/min for 5 minutes, followed by cooling to 40°C at a rate of 10°C/min. It was then held at 40°C for 5 minutes before returning to 200°C at the same cooling rate to check the calorimetric values.

Equation 1 was used to determine the degree of crystallinity (X_c), where H represents the melting enthalpy (J/g) of the sample, Hm is the melting enthalpy for 100% crystalline PHB, and W is the weight fraction of PHB.

\(\text{X}\text{c}=\frac{\varDelta \text{H}\text{m}- \varDelta \text{H}\text{c}\text{c}}{\varDelta \text{H}\text{˚}\text{m}}\text{x} \frac{100}{\text{W}}\) (Eq. 1)

2.3.4 Thermogravimetric analysis (TGA)

The thermal degradation and stability of PHB, the blend, and the composite were examined using the TGA model 2050 from TA Instrument in New Castle, DE. The experiment involved using a sample weighing approximately 5–10 mg placed in an aluminium pan. The heating rate used was at 10°C/min, and the temperature range from 30°C to 800°C.

2.3.5 Melt Flow Index (MFI)

MFI measurements were carried out using a melt flow index instrument that complies with ASTM D1238 standards. The samples were initially pre-heated to 190˚C, and the die used had a diameter of 8 mm and a length of 2.09 mm.

2.3.6 Tensile properties

Tensile tests were conducted to assess the strength, modulus, and elongation at the break of the filaments, as demonstrated in Fig. 6. A 50-kN load cell was utilised, and the deflection was measured using an extensometer at a crosshead speed of 5 mm/min. To prevent filament slippage during testing, masking tape was applied to both ends.

3.1 Morphology of NFC produced

SEM images of freeze-dried NFC are shown in Fig. 7 (a), (b), and (c). Fig. 7 displays the surface of a fibre exhibiting fibrillation, which is the procedure of generating delicate fibrils or nanofibres on the surface of NFC particles. This process encompasses additional mechanical aimed at augmenting surface roughness and improving the fibrillar configuration of NFC. The used of high-speed blender method has been adopted due to its ability to achieve an equivalent level of fibrillation while inflicting minimal damage to the NFC, corroborated by the findings of Uetani and Yano (2011). It is essential to note that this mechanical fibrillation process leads to the breakdown of the rigid structure of the OPT fibre, a phenomenon vividly illustrated in Fig. 7 (b) and (c). The act of peeling microfibrils was achieved through the application of mechanical stress to the OPT pulps. According to Zimmermann et al. (2010), fibrillation occurs frequently when shearing pressures are applied to the longitudinal fibre axis, which is also influenced by disintegration time and passes through the homogenizer. They observed noticeable separation of fibrils (shown in Fig. 7 [a]) when the pressure generated the appropriate shear stress when the material flowed through a small gap in a homogenizing valve.

This specific approach is designed to perturb the outer layers of NFC particles, consequently revealing and disentangling the individual fibrils on the surface, as depicted with precision in Fig. 7 (c), showcasing the distinctive coarse surface of NFC. The application of energy to microfibrils leads to their breakdown, which serves as a fundamental step. This is done with the clear aim of surface fibrillation to augment the available surface area, ultimately enhancing the interaction between NFC and various materials. This heightened interaction is critical and applicable, particularly in reinforcing composites. Elevated surface roughness and the development of a fibrillar structure bestow NFC with enhanced attributes, notably amplifying adhesion, dispersibility, and reinforcement potential. To quantify the extent of fibrillation, especially in complex structured pulp fibres transitioning into uniform NFC, TEM analysis was done as a precise measurement tool.

3.2 Size morphology analysis of NFC

The TEM images (Fig. 8) provided a clear depiction of the extent to which OPT pulp fibres had undergone transformation into minute nanofibrils, thus confirming the successful fibrillation achieved during NFC production. Additionally, the TEM analysis facilitated the precise measurement of their nanoscale dimensions and the visualization of NFC's morphological characteristics. The image offered valuable insights into the creation of the nanofibril network within the NFC. These images unveiled the interrelations between individual nanofibrils, resulting in the formation of a 3D network structure (Kalia et al., 2014).

The TEM images, as shown in Fig. 9, revealed that the NFC produced had an average diameter of 8-11 nm. This relatively small average diameter is likely due to the absence of hemicellulose, which usually binds the fibre bundle together. Removal of hemicellulose and lignin can reduce the fibre diameter. Additionally, the size distribution of NFC nanofibrils appeared to be quite uniform, with a significant proportion falling within a specific size range. Gaining insights into the size distribution of NFC nanofibrils through TEM results is crucial for comprehending their physical characteristics, including aspect ratio, surface area, and potential performance in various applications.

3.1 Melt processing temperature of filaments

The results of the first heating, cooling, and second heating scans in the DSC measurements are displayed in Fig. 10 and Table 2. The DSC curves exhibited three typical temperature transitions observed in PHB polymer: (i) the first melting peak, (ii) the cold crystallisation peak, and (iii) the second melting peak. The absence of a glass transition (T_g) peak in the initial heating scan as shown in Fig. 10 (i) can be attributed to multiple factors. First, the temperature range for the T_g of PHB is 6°C, as reported by Pradhan et al. (2018). However, due to the highly crystalline nature of PHB, its T_g may not be detected in this specific DSC analysis, even at lower temperatures. Meanwhile, PBAT has a glass transition temperature of -30°C as reported by Carbonell-Verdu et al. (2018), which falls outside the applicable range of the current analysis. Shifting our focus to the initial melting peak observed in the first heating scan, the combination of PBAT and PHB influences the melting temperature of PHB. The occurrence of the phenomenon, where the addition of PBAT affects the T_m¹ of PHB, is primarily linked to the rearrangement of PHB crystals. When PBAT is combined with PHB, it influences the crystalline structure of PHB and triggers crystal reorganisation (Xiao et al., 2009). During this process, the lamellar structures within the PHB crystals thicken. As the lamellae thicken, the packing of polymer chains becomes more compact and ordered. The thickening of lamellar structures increases in the T_m¹ of the blends. The thicker lamellae require more energy to break their intermolecular interactions and shift from a solid to a molten state (Shen et al. 2020). As a result, the T_m¹ shifts to higher values.

A study conducted by Beber et al. (2018) also observed changes in the melting peak of PHB/PBAT blends. The researchers suggested that the heat transmission systems in the blends were likely hindered. The PHB and PBAT macromolecules, as well as their crystallites, are in a mixed-interlaced-solid state within the blend before heating (Blanco and Siracusa 2021). Once PBAT starts to melt, it undergoes a phase transition from a solid to a molten state. Fortunately, the PHB chains and crystals remain solid at this stage. The solid PHB components perform as heat-transfer barriers, restricting the free mobility of the macromolecular chains within the blend (Hoffmann et al. 2019). This increase in energy required to melt PBAT is reflected in the higher T_m observed for the PHB/PBAT blends. Table 2 displayed the specific increase in T_m¹values for different PHB/PBAT blend compositions. The effect of NFC on the thermal and kinetic properties of PHB/PBAT blends was examined. However, the results indicated that the presence of NFC did not result in a substantial shift in the T_m of the PHB/PBAT composite. This might be owing to the possibility that the NFC particles did not strongly interact with polymer matrices. The NFC may also have a little impact on the blend’s crystallisation and melting kinetics, thereby preserving the overall T_m.

Additionally, Beber et al. (2018) discovered that the addition of NFC to a PHB/PBAT blend reduced the degree of crystallinity in the composites. Data presented in Table 2 follow the same pattern as their previous observation. A decrease in crystallinity caused the enhanced mechanical response of the NFC-incorporated PHB/PBAT composites (Perić et al., 2019; Platnieks et al. 2021). As reported by Srithep et al. (2012), NFC acts as a nucleating agent that promotes early crystallisation in a blend. In their previous study, the presence of NFC facilitated more rapid crystallinity growth through two-dimensional or three-dimensional PHB crystal growth with heterogeneous nucleation. Wong et al. (2015) claimed that the broad range of NFC widths was found to play a role in favouring the formation of densely packed fibre networks within a PHB/PBAT matrix. The interweaving of NFC with PHB/PBAT macromolecular chains can also be a factor. The hard structure of NFC was found to have an inhibitory effect on the mobility of molecular chains during crystallisation, leading to a reduction in the temperature of cold crystallisation (T_cc), as shown in Fig. 10(ii) of the study. This indicates that the presence of NFC restricts the movement and mobility of PHB/PBAT chains during the crystallisation process.

Table 2 Temperature of melting (T_m¹) (T_m²), cold crystallisation (T_cc), and enthalpy by DSC analysis

PHB/PBAT/NFC	Melting Temperature, T_m¹ (˚C)	Crystallization Temperature, T_cc (˚C)	Melting Temperature, T_m² (˚C)	Melting Enthalpy, ∆H_m (J/g)	Crystallization Enthalpy, ∆H_cc (J/g)	Crystallinity, Xc (%)
100/0/0	171	93	162-172	64.00	146.74	53
80/20/0	175	84	163-170	52.17	124.34	44
80/20/0.5	177	83	162-170	48.35	110.06	28
80/20/1	173	82	158-170	42.21	104.56	38
80/20/2	176	78	156-168	38.00	84.00	34

Fig. 10(iii) reveals a twofold melting peak, indicating an uneven crystal area. For the 100PHB formulation, the given crystallisation period was insufficient to generate perfect crystals. During the heating-cooling process, chain scission and degrading activities also contributed to a reduction in the T_m²across all formulations. The wide dispersion of NFC in the blend served as nucleation sites with various sizes and nucleating abilities, potentially leading to irregular crystal formation (as demonstrated in Fig. 10[iii]). This irregular crystal morphology is likely a consequence of the heterogeneous nucleation provided by the NFC particles throughout the blend. The 80PHB20PBAT0.5NFC formulation exhibited a significant shift of 2˚C. This shift can be attributed to the well-distributed NFC within the matrix, which facilitated substantially higher nucleation of nanoscale NFC. This enhanced nucleation led to improved stability in the crystallisation process of the PHB/PBAT/NFC composite. T_m² showed a slight drop from 100PHB to composites with higher NFC content. Also, the presence of NFC particles may constraint polymer chain mobility and hinder the rearrangement of crystals, resulting in a lower T_m².

3.2 Thermal degradation of filaments

This study aimed to enhance various characteristics of PHB such as its crystallinity index, processability, and mechanical properties to make it more suitable for a wide range of applications. To assess the thermal stability, TGA was employed. Table 3 presents the findings related to the thermal stability of different formulations. None of the materials exhibited significant mass loss at 200°C, indicating that they can be processed at this temperature without undergoing degradation. A similar observation was conducted by Arrieta et al. (2014). However, above 200°C, all the composites exhibited signs of early degradation (T_on), indicating that this temperature represents the upper limit for the production of polymers in FDM 3D printing. The TGA and DTG curve (Fig. 11) clearly illustrates the degradation phase of the PHB/PBAT/NFC, which occurs in both a single and double decomposition step. At an initial temperature of 267.85°C, weight loss was observed in 100PHB. It was confirmed that random chain scission, known as cis elimination, is the primary reaction during the thermal breakdown of PHB (Li et al. 2002). This chain scission process leads to the formation of crotonic acid and its oligomers by breaking the ester groups (Aoyagi et al., 2002).

Previous research has explored the thermal stability of PHB/PBAT blends (Hoffmann et al., 2019). The analysis of these blends using TGA revealed a two-stage degradation process. The DTG thermogram illustrated that the first stage of mass loss occurred due to the degradation of PHB within a temperature range of 256.25°C to 281.03°C, while the second stage was attributed to the degradation of PBAT within a range of 330°C to 450 °C. From Table 3, T_on and T_max values for the composites showed the highest among the tested materials. The inclusion of PBAT in the blend system enhanced the thermal characteristics of PHB, thereby expanding the temperature range at which the polymer containing PHB can be processed (Lin et al. 2018). Additionally, the incorporation of NFC improved the thermal behaviour of PHB/PBAT blends and enabled their utilisation at higher temperatures, as clearly illustrated in Fig. 11.

Table 3 Temperature of T_on, T_50% and T_max and residue amount at the end (%) of neat PHB/PBAT/NFC composites

PHB/PBAT/NFC	Degradation Temperature (˚C)			Residual Weight (%) at 800 ˚C
PHB/PBAT/NFC	T_on	T₅₀	T_max	Residual Weight (%) at 800 ˚C
100/0/0	275.86	300.98	526.28	2.46
80/20/0	280.11	305.89	414.81	5.65
80/20/0.5	287.17	384.91	547.00	1.62
80/20/1	290.40	390.34	525.92	0.86
80/20/2	282.51	384.91	559.55	0.54

As the NFC content increased, the T_on of the composite filament shifted to higher values, ranging from 280.11°C to 290.40°C. This shift highlights the positive effect of NFC on the thermal performance of PHB/PBAT blends, even at low NFC loadings. The improvement can be attributed to the presence of NFC likely results in strong interaction with the PHB/PBAT matrix. The interactions may include physical entanglements, intermolecular forces, or interfacial adhesion between NFC and the PHB/PBAT matrix. Furthermore, the formation of hydrogen bonds between the carbonyl groups of PHB and the hydroxyl groups of NFC could play a significant role. These hydrogen bonds have the potential to hinder the random chain scission process of PHB, thus improving its thermal stability as well as that of the PHB/PBAT blend. By slowing down the chain-breaking reactions, the presence of NFC contributes to enhanced thermal stability. Support for these findings can be found in the work of Zhang et al. (2019), who also demonstrated that NFC is more effective than nanocrystalline cellulose (NCC) in improving the thermal stability of PHB. This further validates the positive impact of NFC on PHB's thermal behavior. Another proposed mechanism to explain the improved thermal stability is the formation of a reticular cross-structure at the microlevel due to the presence of NFC. This cross-structure, which arises from the interaction between NFC and the PHB/PBAT matrix, can act as a protective barrier thus minimising PHB degradation. This proposition aligns with the research conducted by (Mokhena et al. 2018), who suggested that the reticular cross-structure formed by NFC can help mitigate PHB degradation.

3.3 Flowability of filaments

Fig. 12 depicts the MFI melt flow index of PHB and PHB/PBATNFC composites. The MFI evaluates the flowability of a polymer melt under heat and pressure. According to the hypothesis, the higher the MFI values, the lower the melt’s molecular weight and viscosity, and vice versa (Chaiwutthinan et al. 2019). Fig. 12 shows that the MFI of 80PHB20PBAT blends (33.076 g/10 min) was greater than that of pure PHB (26.116 g/10 min). The MFI value of 80PHB20PBAT increased, resulted in increased mobility of the polymer chains and a drop in blend strength and modulus. The flowability of these polymers determines the printability of the PHB/PBAT combination. MFI is important in determining the level of difficulty throughout the printing process. It is worth noting that PHB exhibits significant pseudoplastic behaviour (Boonprasertpoh et al., 2017), which is highly sensitive to variations in processing conditions. PBAT also demonstrates non-Newtonian fluid behaviour characterised by enhanced sustained shear viscosity and elasticity at low frequencies (Jian et al., 2020). Therefore, the inclusion of PBAT at a weight ratio of 20% increased the melt viscosity and elasticity of the PHB/PBAT blends. The addition of PBAT resulted in a reduction of the Newtonian region in the rheological behaviour of the blends. At higher shear rates, the PHB/PBAT blends exhibited lower shear viscosity compared to the pure PHB.

Fig. 12 provides insights into the effect of NFC content on the MFI for PHB/PBAT/NFC blends. The results showed that adding 0.5% and 1% NFC reduced the MFI of the composites. The NFC-reinforced 80PHB20PBAT blend exhibited lower viscosity (25.458 - 30.368 g/10 min) compared to the neat blend (33.076 g/10 min). Specifically, the MFI value of 80PHB20PBAT2NFC (41.736 g/10 min) was higher than that of 80PHB20PBAT (33.076 g/10 min). This finding suggests that adding NFC increased the viscosity of the blend. The MFI value increased due to the lubricating effect of the natural filler on the polymer chains. This is attributed to factors such as higher volume percentage, particle size, and surface roughness of the NFC, which hampered polymer chains' mobility and resulted in increased viscosity. The MFI values obtained provide an insight into the challenges encountered during the printing process and can also act as a measure of the printed samples' quality. High NFC contents are expected to result in a decrease in the viscosity of the extruded filament at the printer nozzle. However, it is crucial to consider the potential issues that may arise with excessively low MFI values, such as nozzle clogging. As mentioned, the MFI of approximately 26.544 g/10 min for the pure 100PHB sample already poses a risk of nozzle clogging. This problem may persist even with higher NFC loadings as the increased heat impact can disrupt the flowability of the polymer. It is important to highlight the lightweight nature and large surface area of NFC particles. Including 2% NFC in the 80PHB20PBAT composite is a significant factor when comparing component weights.

Fig. 13 provides valuable insights into the blend morphology of the PHB/PBAT system thus explaining the miscibility between the two polymers and indicating the presence of NFC on the surfaces of the blend. When comparing 100PHB in Fig. 13(a) with the blends containing PBAT in Fig. 13(b), it is evident that the surface of PHB appears smoother surface, indicating a brittle nature, while the presence of PBAT introduces roughness due to its ductility. A similar observation was reported by Astner et al. (2019). The micrograph analysis of the blends showed surface roughness, owing to the phase separation between PHB and PBAT. In Fig. 13(b), the PBAT phase is observed, and the PHB domains display beads of varying sizes and distributions. The formation of these beads represents the presence of PBAT. By dispersing within the PHB domain, PBAT creates a discrete phase, resulting in a co-continuous phase structure. These observations emphasise the immiscibility of the PHB and PBAT components in the blends. The phase separation between the two polymers leads to the formation of distinct domains and highlights the challenges associated with achieving a homogeneous blend. However, at lower NFC content of 0.5%, the bead appearance was minimised. The tiny thread-like cellulose fibre bundles can be detected in Fig. 13(c), (d) and (e). Fig. 13(d) depicted fibre aggregation, suggesting unequal dispersion in the matrix. The surfaces of filaments of 1% and 2% NFC exhibit a rougher appearance compared to those with 0.5% NFC. The dispersion of NFC within the PHB/PBAT matrix is not uniform, as observed in the fractured surface images. These visual representations provide concrete evidence of the influence of the nanofiller on the material.

3.4 Tensile properties

The mechanical properties of 100PHB and PHB/PBAT/NFC filaments can be determined from their tensile strength. Fig. 14 and Fig. 15 exhibit tensile strength, modulus, and elongation at break data for 100 PHB and PHB/PBAT reinforced with various NFC loadings. The tensile strength of 100PHB (8.26 MPa) is relatively high and comparable to that of 80PHB20PBAT (10.97 MPa) because PHB exhibits good mechanical properties. It has a high tensile strength, which refers to its ability to resist deformation or breakage under tensile (pulling) forces. On the other hand, the blend of 80PHB20PBAT consists of a combination of PHB and PBAT. The presence of PBAT in the blend enhances its elongation and impact resistance, while still maintaining a relatively high tensile strength due to the combination of PHB and PBAT properties. This is expected because PBAT possesses improved excellent mechanical properties due to its aliphatic unit in the molecular chain. Consequently, the elongation at the break of the samples was observed to increase with higher PBAT concentration. This behaviour aligns with previous studies by Boonprasertpoh et al. (2017) and John et al. (2002) in which they also reported an increase in elongation at break with increasing PBAT concentration in blends.

The addition of NFC to the PHB/PBAT blend showed improvements in the tensile strength, modulus, and elongation at the break of filaments. This trend aligns with findings reported by Tian et al. (2022) who studied NFC reinforced in PLA biocomposites. NFC forms a networked structure with PHB/PBAT matrix, promoting physical entanglements and interlocks inside the composites (Xu et al. 2021) . Furthermore, with the addition of NFC, the trend in the modulus of the composites was similar to that of the strength. Among the compositions examined, 100PHB had the lowest modulus, at 145 MPa. The composite with the composition 80PHB20PBAT0.5NFC, on the other hand, had the highest modulus, reaching 991.47 MPa. These entanglements and interlocks add more points of contact and interactions within the composite materials, which contributes to the improved mechanical characteristics. The interactions between the NFC and the polymer matrix promote load transmission and stress distribution within the composite. As a result, there is a strengthening effect and an increase in tensile strength. Additionally, the presence of NFC in the composite adds to material toughening by absorbing and dissipating energy during deformation. Zaaba et al. (2020) found that NFC facilitates the establishment of a networked structure and improves interactions between NFC and polymer matrix.

However, as the NFC content in the PHB/PBAT matrix increases, the tensile strength and modulus of the filament decrease. The dispersion of NFC particles gets more extensive as the NFC content increases, generating a higher degree of disturbance in the polymer structure. The NFC particles cause weak points in the matrix, acting as stress concentrators that can cause cracks and fractures when subjected to applied loads. The insufficient NFC dispersion within the PHB/PBAT filaments could explain the drop in trend. Limited NFC dispersion is caused by agglomeration and increased filler-matrix interaction (Chun et al., 2013; Shil’ko et al. 2021). Hence, NFC’s reinforcing ability is limited, leading to low mechanical properties. Moreover, entanglements between NFC and the polymer matrix can limit the polymer's molecular mobility, resulting in a drop in composite modulus.

Contradicting the observed trends in tensile strength and modulus, elongation at break showed a different pattern. 2% NFC had the largest elongation at break, 0.5% and 1% NFC exhibited lower values of 2.26% and 2.57%, respectively. The increased elongation at break at higher NFC content can be attributed to the enhanced flexibility and ductility provided by the NFC particles. NFC enhances the movement of polymer chains, allowing them to slide and reorient more freely. The increased mobility allows composites to absorb and distribute stress more efficiently. It also minimises the chance of premature failure by preventing localised stress concentrations. Hence, composites exhibit improved elongation at break, allowing them to stretch and deform further before breaking. Filaments must be able to withstand elongation and deformation, as filaments are subjected to pulling forces during the extrusion and layering process (Aho et al., 2019; Kristombu et al., 2021). High NFC content enables filament to withstand these forces and undergo greater elongation without breaking. This is beneficial as it reduces the possibility of filament breakage during printing, resulting in increased printing reliability and success.

Tensile strength was examined further using SEM micrograph results. Tensile test filament composed of 100 PHB exhibited a smooth surface. This is attributed to the fact that 100 PHB is a pure polymer with inherent crystallinity ( Zhang and Thomas, 2011), which results in higher tensile strength and modulus. Besides, 80PHB20PBAT filament also has a better surface suggesting PHB and PBAT are compatible at this ratio (Fig. 16[b]). All the filament samples showed plastic deformation, indicating their ability to undergo elongation before fracture. The combination of PHB and PBAT enhances fracture toughness (Lu et al. 2017), as supported by the tensile data and SEM images (Fig. 16).

The SEM images in Fig. 16 show the surface characteristics of PHB/PBAT blends with different NFC loadings. The SEM image of PHB/PBAT with 0.5% NFC (Fig. 16[c]) displays a smooth surface with notable ductility (Lu et al. 2017). However, micro gaps can be observed, indicating the immiscibility of PHB and PBAT polymers (Jandas et al., 2014). Due to the low loading at 0.5% NFC, the filler is neither visible. Furthermore, it demonstrated minimal fibre pull-out. In Fig. 15 (c) and (h), the NFC fibre pull-out is plainly visible and designated. This signifies that the NFC fibres have become partially or entirely detached from the surrounding matrix and are emerging from the fractured surface. The presence of NFC improves the interfacial adhesion of the polymer matrix components. The enhanced adhesion was suggested because there is less tendency for the fibres to tear away from the matrix, resulting in a smoother surface. Because of their nano-sized dimensions, NFC are more uniformly spread at lower percentages which may be compatible with the blends. Additionally, the presence of dimples at the NFC during tensile loading, as shown in Fig. 16(h), suggests efficient stress transmission at low NFC content. The strong bonding between the filler and matrix material at the fractured interface (Fig. 16[c] and [h]) indicates optimal bonding and prevents NFC from slipping out of the polymer matrix, as evidenced by the absence of visible pull-out.

In contrast, the fractured surfaces of PHB/PBAT with 1% and 2% NFC depicted in Fig. 16(d)(i) and Fig. 16(e)(j), respectively, exhibit a rougher appearance compared to the 0.5% content. As the filler content increased to 1% and 2%, the NFC became visible and unevenly dispersed within the PHB/PBAT matrix. When the filler percentage is increased to 2%, the presence of NFC becomes more visible and embedded within the PHB/PBAT matrix. Fig. 16(i) and (j) depict fibre pull-out, indicating limited NFC dispersion. The fractured surface images demonstrate the impact of the nanofiller, revealing a correlation between the fracture pattern and the NFC loading percentage. Furthermore, the distribution of NFC with the PHB/PBAT matrix is not uniform. The fractured surface images clearly demonstrate the nanofiller's influence on the material. The variations in fractured surface morphology suggest a link between the way the material is fragmented and the proportion of filler loading. The rougher appearance and presence of fibre pull-out in Fig. 15 (i) and (j) compared to the other images indicate that increasing NFC loading has a more significant effect on the material's fracture behaviour.

3.5 Printing of PHB/PBAT/NFC filament

To demonstrate the 3D printability of PHB/PBAT/NFC composites, FDM 3D printing has been conducted (demonstrated in Fig. 17). Initially, we opted for a straightforward design suitable for specimen testing, as the inherent properties of these materials posed challenges in printing complex shapes. To achieve successful 3D printing using the FDM method, filaments with a stable stretching capability must be used to avoid disturbances in the printing process. For example, Lau et al. (1998) investigated how the melt strength of polypropylene (PP) can ease sagging difficulties during thermoforming procedures (Wittman and Drummer 2022). Similar factors were discovered in this research. As discussed in MFI test, the effectiveness and quality of filament utilization in the printing process are consistently related to their MFI value.

In a prior investigation, it was noted that when using an 80PHB20PBAT blend, successful 3D printing was achievable but often resulted in warping issues. This warpage phenomenon could not be effectively mitigated by introducing a nanofiller. According to reports, the successful elimination of warping was greatly dependent on the use of a compatibilizer (Spreeman et al. 2019), a key factor in guaranteeing compatibility between the different components of the composite. However, when compared blend with composite, warpage problem can be reduced. As proven by Fig. 17 visually conveys the promising results of employing an 80PHB20PBAT0.5NFC formulation. This composite exhibited excellent printability, manifesting itself in a smooth surface finish and minimal discernible agglomeration of NFC particles during the 3D printing process. As a result, we were able to successfully extrude consistent filaments, broadening the 3D printing operational range and allowing us to print blends at higher temperatures.

In conclusion, achieving successful fused deposition modelling (FDM) requires a polymer filament with favourable characteristics. The blending of PHB with a compatible polymer like PBAT enhances PHB’s ability to withstand higher temperatures without degrading, thereby improving its thermal stability. This, in turn, leads to enhanced mechanical properties, particularly the tensile strength of the PHB/PBAT filament. Good tensile properties are crucial to avoid brittleness and breakage during the printing process. Additionally, the flowability of the PHB/PBAT blend is vital for the printing procedure and affects the quality of the printed samples. The incorporation of NFC generally improves the properties of the polymer blend, although the degree of improvement depends on the NFC loading in each formulation. DSC analysis shows that NFC reduces crystallinity while increasing strength and modulus at a low NFC loading of 0.5%. However, higher NFC concentrations can lead to agglomeration, which reduces polymer flowability and the interaction between the PHB/PBAT matrix and NFC. Moreover, higher NFC content results in lower tensile strength and modulus due to material incompatibility. SEM images reveal distinct phases in the PHB/PBAT blend, particularly with 20% PBAT, as well as agglomeration and fibre clusters within and on the surface of the PHB/PBAT matrix. The mechanical characteristics of the filament differed significantly from those of the composite filament. However, the addition of NFC does improve the thermal stability of the composite filament. Overall, both PBAT and NFC contribute to the potential applications of 3D-printed objects by providing enhanced filament properties.

Acknowledgements This work was supported by the Malaysian Ministry of Higher Education Malaysia as part of the Science and Technology Research Partnership for Sustainable Development (SATREPS) (203/PTEKIND/67811002).

Authors Contribution NIJ: Conceptualization, Investigation, Writing- Original Draft. NFMR: Supervision, Methodology, Formal Analysis, Writing-Review & Editing. MHMK: Supervision, Validation, Formal Analysis. LCP: Supervision, Project Administration, Funding Acquisition, Formal Analysis. CKA: Investigation, Resources, Formal Analysis. AK: Validation, Visualization, Funding Acquisition. TA: Validation, Visualization, Funding Acquisition

Funding This research was supported by the Malaysian Ministry of Higher Education Malaysia as part of the Science and Technology Research Partnership for Sustainable Development (SATREPS) (203/PTEKIND/67811002). The authors greatly acknowledge their support of this work.

Data availability Data is available from the authors upon request.

Conflict of Interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests. Leh Cheu Peng reports financial support was provided by Japan Science and Technology Agency and Japan International Cooperation Agency.

Consent for publication All authors agreed to the publication in the submitted form.

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Competing interest reported. The authors declare the following financial interests/personal relationships which may be considered as potential competing interests. Leh Cheu Peng reports financial support was provided by Japan Science and Technology Agency and Japan International Cooperation Agency.

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Enhancement of Thermal and Mechanical Properties: Nanofibrillated Cellulose - Reinforced PHB/PBAT 3D Filaments for Fused Deposition Modeling

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Abstract

Figures

1. Introduction

1.1 Filament manufacturing process

1.2 Biopolymer filament

2. Experimental

2.1 Materials

2.2 Preparation of PHB/PBAT/NFC composites

2.2.1. Production of NFC

2.2.2. Composite filament preparation

2.3 Characterisation Methods

2.3.1 Scanning Electron Microscope (SEM)

2.3.2 Transmission Electron Microscopy (TEM)

2.3.3 Differential Scanning Calorimetric (DSC)

2.3.4 Thermogravimetric analysis (TGA)

2.3.5 Melt Flow Index (MFI)

2.3.6 Tensile properties

3. Results and discussions

4. Conclusions

Statements and Declarations

References

Additional Declarations

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