Partial Biodegradable Blend with High Stability against Biodegradation for Fused Deposition Modeling

This research presents a partial biodegradable polymeric blend aimed for large-scale fused deposition modeling (FDM). The literature reports partial biodegradable blends with high contents of fossil fuel-based polymers (>20%) that make them unfriendly to the ecosystem. Furthermore, the reported polymer systems neither present good mechanical strength nor have been investigated in vulnerable environments that results in biodegradation. This research, as a continuity of previous work, presents the stability against biodegradability of a partial biodegradable blend prepared with polylactic acid (PLA) and polypropylene (PP). The blend is designed with intended excess physical interlocking and sufficient chemical grafting, which has only been investigated for thermal and hydrolytic degradation before by the same authors. The research presents, for the first time, ANOVA analysis for the statistical evaluation of endurance against biodegradability. The statistical results are complemented with thermochemical and visual analysis. Fourier transform infrared spectroscopy (FTIR) determines the signs of intermolecular interactions that are further confirmed by differential scanning calorimetry (DSC). The thermochemical interactions observed in FTIR and DSC are validated with thermogravimetric analysis (TGA). Scanning electron microscopy (SEM) is also used as a visual technique to affirm the physical interlocking. It is concluded that the blend exhibits high stability against soil biodegradation in terms of high mechanical strength and high mass retention percentage.


Introduction
The invention of additive manufacturing (AM) or 3D printing dates back to the 1990s and has introduced various new domains of research [1]. These include new AM technologies [2,3], materials [4,5], processing (pre, in situ, and post) [6], and applications [7]. Each new AM technology is designed for a particular material. For example, the fused filament fabrication (FFF) or fused deposition modeling (FDM) introduces the 3D printing of thermoplastics and elastomeric thermoplastics [8]. Stereolithography (SLA) registers its The drying was performed for approximately 1 h. A HAAKE TM rheomex single screw extruder (SCION, Rotorua, New Zealand) was used for the melt blending of all polymers. The melt blending resulted in a filament of 1.1 ± 0.2 mm in diameter that was pelletized into cylindrical pellets of 1.0 ± 0.2 mm length. Twin-screw extrusion has been reported with unwanted material degradation due to thermal shearing [32], which cannot be afforded in this research. Furthermore, the blend would be 3D-printed in a screw-based pellet 3D printer that may affect the original blend properties. Therefore, a single screw extruder was utilized for preparation of the blend.
The composition of the blend was based on the following three factors: (1) minimum nonbiodegradable polymer, (2) maximum physical interlocking, and (3) successful 3D printing of prepared composition.
Therefore, the blend compositions were continuously prepared until successful 3D printing was achieved. In this regard, the minimum reported composition of nonbiodegradable polymer (PP) was 20% [33][34][35][36][37] with a maximum of 75% of biodegradable constituent (PLA) [35][36][37]. The first composition (75: 5:20) resulted in unwanted extrusion swelling (die swelling), as shown in Figure 1. The swelling was probably caused by a high percentage of compatibilizer (5%) due to excessive maleic anhydride [38]. The first composition was thus rejected due to the unsuitable extruded diameter (2.3 mm) of extrudate for 3D printing. Therefore, the second blend composition was decided with the lowest nonbiodegradable polymer (7.5%) and compatibilizer (0.5%) based on no signs of swelling in the literature [37]. The second composition (92:0.5:7.5) was successfully extruded with the desirable extrudate diameter. The compositions are provided in Table 1.
China. Moplen HP400N polypropylene with a specific weight of 0.91 g/cm 3 and flow in dex of 11 g/10 min was provided by TCL-Hunt, New Zealand. The main aim of this re search was to achieve the maximum possible physical interlocking alongside partia chemical grafting; therefore, a high MFI grade of PP was particularly selected for blending [29,31].

Melt Blending
An HST China-based thermostat blast oven was used to dry PLA, PP, and PE-g MAH. The drying was performed for approximately 1 h. A HAAKE TM rheomex single screw extruder (SCION, New Zealand) was used for the melt blending of all polymers The melt blending resulted in a filament of 1.1 ± 0.2 mm in diameter that was pelletized into cylindrical pellets of 1.0 ± 0.2 mm length. Twin-screw extrusion has been reported with unwanted material degradation due to thermal shearing [32], which cannot be af forded in this research. Furthermore, the blend would be 3D-printed in a screw-based pellet 3D printer that may affect the original blend properties. Therefore, a single screw extruder was utilized for preparation of the blend.
The composition of the blend was based on the following three factors: (1) minimum nonbiodegradable polymer, (2) maximum physical interlocking, and (3) successful 3D printing of prepared composition.
Therefore, the blend compositions were continuously prepared until successful 3D printing was achieved. In this regard, the minimum reported composition of nonbiode gradable polymer (PP) was 20% [33][34][35][36][37] with a maximum of 75% of biodegradable con stituent (PLA) [35][36][37]. The first composition (75:5:20) resulted in unwanted extrusion swelling (die swelling), as shown in Figure 1. The swelling was probably caused by a high percentage of compatibilizer (5%) due to excessive maleic anhydride [38]. The first com position was thus rejected due to the unsuitable extruded diameter (2.3 mm) of extrudate for 3D printing. Therefore, the second blend composition was decided with the lowes nonbiodegradable polymer (7.5%) and compatibilizer (0.5%) based on no signs of swelling in the literature [37]. The second composition (92:0.5:7.5) was successfully extruded with the desirable extrudate diameter. The compositions are provided in Table 1. Based on the above-mentioned decided criteria for the preparation of compositions the third composition was not prepared, as the second composition was successfully ex truded and 3D-printed.  Based on the above-mentioned decided criteria for the preparation of compositions, the third composition was not prepared, as the second composition was successfully extruded and 3D-printed.

Pellet 3D Printing
Three-dimensional 3D printing or fused deposition modeling was performed on an in-house-built pellet 3D printer [39], as shown in Figure 2. The pellet 3D printer is built with a single lead screw that feeds the material into the heated barrel and also extrudes out melted material out of the 0.2 mm nozzle [39]. Unlike conventional lead screws in extruders, the lead screw in the customized pellet printer is designed with only one physical configuration, i.e., feeding [39]. The lead screw does not have any metering and compression configuration as found in normal injection molding or filament extruders [39]. The single physical feeding phase has been reported with low to negligible thermochemical shearing during 3D printing [32,39,40].

Pellet 3D Printing
Three-dimensional 3D printing or fused deposition modeling was perform in-house-built pellet 3D printer [39], as shown in Figure 2. The pellet 3D printe with a single lead screw that feeds the material into the heated barrel and also out melted material out of the 0.2 mm nozzle [39]. Unlike conventional lead s extruders, the lead screw in the customized pellet printer is designed with only o ical configuration, i.e., feeding [39]. The lead screw does not have any metering a pression configuration as found in normal injection molding or filament extrud The single physical feeding phase has been reported with low to negligible therm ical shearing during 3D printing [32,39,40].
For this particular research, a few modifications were made, such as a select sintering-based cone, Teflon insulative plate, and efficient area of slotted fluid c ( Figure 2). The three abovementioned modifications aimed to avoid pre-heatin novel polymer blend before reaching the heating barrel. Therefore, these modi enabled the printer's capability to avoid pre-thermal degradation of the novel bl ing 3D printing.  For this particular research, a few modifications were made, such as a selective laser sintering-based cone, Teflon insulative plate, and efficient area of slotted fluid channels ( Figure 2). The three abovementioned modifications aimed to avoid pre-heating of the novel polymer blend before reaching the heating barrel. Therefore, these modifications enabled the printer's capability to avoid pre-thermal degradation of the novel blend during 3D printing.
The computer-aided design (CAD) drawings were prepared in "SolidWorks version 2019" and saved in standard tessellation language (stl) format. The stl files were encoded into G-codes using slicing software (Slic3r) followed by 3D printing on the pellet 3D printer using "Pronterface".
Each new material required its own set of processing variables (parameters). These 3D processing parameters were set in the slicing software. In this regard, numerous combinations of different variables were investigated while making the layer thickness (0.2 mm) [41], extrusion width (0.3 mm) [41], nozzle diameter (0.4 mm), infill pattern (45/−45) [41], and infill density (100%) [41] as constants. The set of optimal values for the investigated variables are given Table 2.

Soil Biodegradation Testing
Soil burial was performed to analyze the biodegradation ( Figure 3). The ASTM standard (D638 type IV) [42] of neat PLA and the blend was printed as per the literature [30]. Three samples were prepared for characterization at most. The weight of each as-prepared sample was measured and denoted as "m 0 ". The samples were buried in real soil at a depth of ≈1 m [43,44] for soil degradations analysis in Palmerston North ( Figure 3). The geographical coordinates for burial location of the samples were longitude 40 • 22 47.0 and latitude 175 • 36 49.9 . The samples were taken out after 45 days, washed with water, dried with paper towels, and left for 2 weeks for acclimatization at 25 ± 3 • C. The washing, drying, and acclimatization helped to remove soil debris/particles, additional moisture, and the settlement of sample temperature to room temperature, respectively. The weight of acclimatized dog bones was measured and denoted as "m 1 ". The weight retention factor (m R ) was calculated using the following relation [43], gated variables are given Table 2.

Soil Biodegradation Testing
Soil burial was performed to analyze the biodegradation (Figure 3 ard (D638 type IV) [42] of neat PLA and the blend was printed as pe Three samples were prepared for characterization at most. The weight sample was measured and denoted as "m ". The samples were bur depth of ≈1 m [43,44] for soil degradations analysis in Palmerston N geographical coordinates for burial location of the samples were longit latitude 175°36′49.9″. The samples were taken out after 45 days, washe with paper towels, and left for 2 weeks for acclimatization at 25 ± 3 °C ing, and acclimatization helped to remove soil debris/particles, addit the settlement of sample temperature to room temperature, respecti acclimatized dog bones was measured and denoted as "m ". The wei (m ) was calculated using the following relation [43], The effects of soil degradation were also analyzed in terms of me "general full factorial ANOVA with multiple levels" was used to an soil degradation on different levels of adhesion. As with soil burial levels of adhesion were achieved with the help of variable bed and pri Each combination of bed and printing temperature in the ANOVA d provided a different resistance to soil degradation. In this regard, th variables (bed and printing temperature) were selected based on rang The effects of soil degradation were also analyzed in terms of mechanical strength. A "general full factorial ANOVA with multiple levels" was used to analyze the effects of soil degradation on different levels of adhesion. As with soil burial treatment, different levels of adhesion were achieved with the help of variable bed and printing temperatures. Each combination of bed and printing temperature in the ANOVA design of experiment provided a different resistance to soil degradation. In this regard, three levels for both variables (bed and printing temperature) were selected based on ranges mentioned in the previous section (pellet 3D printing). The factors (parameters) and corresponding levels are given in Table 3. The combinations were analyzed for three samples at most. The ANOVA DoE analyzed the results of tensile strength using statistical means with the help of confidence level (alpha "α"). The 95% confidence level was selected, meaning only a 5% probability of the statistical model obtaining different mean tensile values of the 18 combinations, as shown in Table 4. Different tensile strength values are interpreted as a difference in the effects of the combination of bed temperature, printing temperature, and soil treatment. Table 3. General full factorial design of experiment (DoE) for soil degradation analysis.

Mechanical Testing
The mechanical testing (tensile) was performed on an Instron 5967 with a load cell capacity of 30 kN. The characterization was performed using a contact type clip-on-gauge extensometer with a 25 mm span length. The rate of extension for characterization was set at 5 mm/min.

Fourier Transform Infrared Spectroscopy (FTIR)
Intermolecular Chemical interactions were analyzed using a Thermo electron Nicolet 8700 FTIR spectrometer. OMNIC E.S.P 7.1 was used to perform the postprocessing of spectrums. The postprocessing included normalizing and baseline correction. The analysis aimed to detect the probable chemical interactions between different functional groups or elements as a result of polymer blending, 3D printing in-process thermal variables, and soil biodegradation. The analysis was performed in transmittance mode using an average of 30 spectrums measured in the wavelength range of 400-4000 cm −1 .

Differential Scanning Calorimetry (DSC)
Thermochemical analysis was performed on a NETZSCH 449-F1 Jupiter simultaneous thermal analyzer. The analysis aimed to obtain information associated with: (1) nature of chemical blending (physical interlocking or not), and (2) effects of soil biodegradation. The machine was operated in the temperature range of 25 • C-550 • C with a nitrogen purging of 50 mL/min. The rate of temperature increase was set at 10 • C/min.

Thermogravimetric Analysis (TGA)
Thermogravimetric analysis was also conducted in a NETZSCH 449-F1 Jupiter simultaneous thermal analyzer. The aim of the analysis was to obtain further information regarding physical interlocking and stability against soil biodegradation. The analyzer was operated within the range of 25 • C-550 • C under nitrogenous atmospheric conditions purged at 50 mL/min. The temperature was achieved in the main chamber at a rate of 10 • C/min.
The analysis of neat PLA was also performed for comparison with soil-degraded blends at two extreme printing combinations, i.e., "161 • C, 25 • C" and "171 • C, 85 • C". The analysis was performed in terms of mass loss in percent of the total mass of the original sample (≈15 g). In this regard, the corresponding temperatures for different mass losses (50%, 60%, 70%, 80%, 90%, 92%, and 95%) were noted and compared with neat PLA.

Scanning Electron Microscope (SEM)
Visual analysis of polymer blending and layer adhesion was performed in a Hitachi TM3030 Plus desktop SEM. The backscattered electrons (BSE) mode was applied to analyze different samples. Another main aim of SEM was to validate the results obtained in FTIR, DSC, and TGA analysis.

Soil-Based Biodegradation on Weight Retention
The effects of soil degradation on the weight retention of neat PLA and the blend are shown in Figure 4a. Overall, the weight retention ability improves with the increase in printing and bed temperature for neat PLA and the blend (Figure 4a). The neat PLA shows a significantly low weight retention (high biodegradation) as compared to the blend at all printed combinations. As a comparison, the range of 99.55-99.79% is observed for neat PLA for all combinations as compared to 99.89-99.76% of the treated blend. The high weight retention of the blend presents the high stability to soil degradation as compared to neat PLA. Polymers 2022, 14, x FOR PEER REVIEW 8 of 18

Soil-Based Biodegradation on Tensile Strength
The ANOVA analysis shows the effects of soil degradation on the tensile strength. The general full factorial ANOVA-based design of experiment is designed with 18 combinations, as shown in Table 4.
The tensile strength against all designed combinations is reported in Table 4. For treated blend combinations, the highest tensile strength of 42.79 MPa is noted for combination "25 • C, 171 • C" followed by 39.5 MPa of combination 55 • C, 161 • C. For nontreated blend combinations, the highest strength of 44.95 MPa is reported for "85 • C, 161 • C". This shows that the soil degradation does not result in a significant deterioration in tensile strength.
The statistical analysis reveals the printing temperature as the significant variable with a p-value of 0.032 (<"α = 0.05") in Figure 4b. The printing temperature is further confirmed by ANOVA analysis in the "main-effects plot" (Figure 4c) with the printing temperature of 171 • C being reported as the optimal temperature for high strength. The p-values for the remaining two nonsignificant variables are 0.937, 0.063 for bed temperature and soil treatment, respectively (Figure 4d).
The soil degradation treatment is detected as insignificant (p-value = 0.063) with a small decrease of just 3 MPa (40 MPa to 37 MPa) in the "main-effects plot" (Figure 4c). The small decrease in tensile strength is supported by high weight retention, which highlights the stability of PLA/PP/PE-g-MAH against soil degradation. The Minitab analysis is provided in "Supplementary File S1". The details of the design of experiment (DoE) and the corresponding tensile strength are presented in Table 4.

Analysis for Intermolecular Interactions
The FTIR spectrum of all neat polymers are confirmed with the literature [45][46][47]. Figure 5 shows the FTIR spectrum of neat PLA and PP.
The intensities in percentage of numerous functional groups in the nontreated blend (161 • C, 25 • C) are also varied after melt blending as compared to neat PLA. For example, a 5% increase in C=O groups [48] is noted in nontreated blends (161 • C, 25 • C) as compared to neat PLA (90%). The significant rise in C=O is due to the observed intermolecular synchronization of similar groups of two different polymers. The synchronization is highlighted in Figure 5 with a magnified image of the hump that includes the C=O groups of both PLA and maleic anhydride at 1705 cm −1 .
Another sign of intermolecular interactions is noted as a new C-H peak (2950 cm −1 ) in the nontreated blend (161 • C, 25 • C), which is not found in neat PLA ( Figure 5). The new C-H peak is merged with three peaks of the nontreated blend (161 • C, 25 • C) originally inherited by the polypropylene. However, the interesting fact is the prominent reduction in intensity (98%) of new the C-H peak as compared to the one found in neat PLA (88%). The literature interprets the new distinct peak as a physical interlocking or phase separation and decrease in intensity as obstructed intermolecular movement [48].
The abovementioned FTIR confirms the melt blending to form physical interlocking along with signs of chemical interactions.
The comparison of soil-degraded samples at low-and high-temperature combinations is analyzed with the nontreated combination at the corresponding temperatures (low or high) in Figure 5. The low-temperature comparison reveals a decrease of about 2.3% (87.3-85%) of C-O-C and 5.5% (90.6-85%) of C=O groups in soil-degraded samples. The small depletion (2.3%) of C-O-C and high depletion of C=O show low chain scission and high chemical grafting [48], respectively, which provide stability to the novel blend against soil degradation. This is also verified with the insignificance of soil degradation in ANOVA analysis (Figure 4). On the contrary, the high printing temperatures (171 • C, 85 • C) decrease the C-O-C groups by 7.9% (95.9-88%) and the C=O by 5.5% (96.7-91.2%), as shown in Figure 5. The drastic decrease in C-O-C groups presents severe chain scission. The chain scission describes the decrease in tensile strength noted in the "main-effects plots" with the increase in printing temperature for soil-degraded samples (Figure 4). However, it is necessary to find more evidence to investigate the in-depth relation of chain scission with tensile strength.  The comparison of soil-degraded samples at low-and high-temperature combinations is analyzed with the nontreated combination at the corresponding temperatures (low or high) in Figure 5. The low-temperature comparison reveals a decrease of about 2.3% (87.3−85%) of C-O-C and 5.5% (90.6−85%) of C=O groups in soil-degraded samples. Until this point of discussion, the physical interlocking and chemical grafting stand out as notable phenomena. However, the analysis requires DSC to analyze the exact nature of the polymer blend.

Analysis for Nature of Blending and Effects of Degradation Mechanisms
The analysis for effects of melt blending is performed on thermographs of neat PLA and as-prepared blend pellets (nonprinted), as shown in Figure 6. Two notable variations are observed, i.e., glass transition and melt crystallization. scission with tensile strength.
Until this point of discussion, the physical interlocking and chemical grafting stand out as notable phenomena. However, the analysis requires DSC to analyze the exact nature of the polymer blend.

Analysis for Nature of Blending and Effects of Degradation Mechanisms
The analysis for effects of melt blending is performed on thermographs of neat PLA and as-prepared blend pellets (nonprinted), as shown in Figure 6. Two notable variations are observed, i.e., glass transition and melt crystallization.
Regarding the glass transition phase, the TG of the as-prepared blend pellets is increased to 63.2 °C as compared to the 59.89 °C of neat PLA ( Figure 6). The enthalpy of glass transition is also noted with a significant increase (≈1.7 J/g) for blend pellets as compared to neat PLA (0.026 j/g). The increases in TG and ΔHG present the re-orientation of polymeric chains and thus show the improved formation of crystallites [49]. Regarding the melt crystallization phase, the as-prepared pellets are found with a bimodal peak ( Figure 6). The multiple peaks validate the phase separation or the physical interlocking [33] of PP in the PLA matrix. Furthermore, the TM of the as-prepared blend Regarding the glass transition phase, the T G of the as-prepared blend pellets is increased to 63.2 • C as compared to the 59.89 • C of neat PLA ( Figure 6). The enthalpy of glass transition is also noted with a significant increase (≈1.7 J/g) for blend pellets as compared to neat PLA (0.026 j/g). The increases in T G and ∆H G present the re-orientation of polymeric chains and thus show the improved formation of crystallites [49].
Regarding the melt crystallization phase, the as-prepared pellets are found with a bimodal peak ( Figure 6). The multiple peaks validate the phase separation or the physical interlocking [33] of PP in the PLA matrix. Furthermore, the T M of the as-prepared blend pellets (155.5 • C) is observed with a minor increase as compared to 154.7 • C of polylactic acid, which is probably due to chemical grafting [48].
The soil degradation after printing at the high-temperature combination (171 • C, 85 • C) shows a visible decrease in almost all parameters as compared to the low-temperature combinations (161 • C, 25 • C). For example, the ∆H G , ∆H C , ∆H M , and ∆H D decrease to 1.57 J/g, 10.56 J/g, 9.75 J/g, and 647.5 J/g from 2.086 J/g, 13.94 J/g, 11.96 J/g, and 787.3 J/g, respectively. Similarly, the T C , T M , and T D decrease to 104.1 • C, 156 • C, and 365.8 • C from 106.9 • C, 156.2 • C, and 369.8 • C, respectively. The decrease in properties is clearly related to the chain scission [48], as found in FTIR analysis ( Figure 5). Furthermore, the decrease in the aforementioned DCS parameters for soil-degraded samples at high temperature provides a thermo-chemical justification for the two results obtained in ANOVA analysis: (1) significant printing temperature ( Figure 4b) and (2) decrease in strength after soil biodegradation (Figure 4c).
Based on the decrease in thermal properties in DSC and chain scission in FTIR, a suitable reason for mechanical stability after soil degradation is the physical interlocking. However, it requires TGA analysis to further validate the effects of interlocking.

Measurement of Interlocking and Chemical Grafting
Thermogravimetric analysis (TGA) is used to validate the FTIR and DSC results regarding physical interlocking. The analysis also aims to analyze the thermal stability to the degradation after soil biodegradation and hydrolytic degradation.
The thermogravimetric analysis of neat PLA, PP, and printed blends is shown in Figure 7. The physical interlocking is confirmed from the distinct step that occurs above 400 • C. However, the mass percentage of the step associated with PP occurs at 6.19% and 6.75% for soil-degraded samples of "161 • C, 25 • C" and "171 • C, 85 • C", respectively. Both (6.19% and 6.75%) are less than the added percentage of PP in the blend (7.5%). The mass percentage less than 7.5% shows minor chemical grafting [48]. Hence, the desired characteristics of the blends are achieved and validated.
The soil-degraded samples at the low-temperature combination (161 • C, 25 • C) reveals a near-similar onset temperature (348.9 • C) as compared to 350.3 • C of neat PLA (Figure 7). The T END for most degradation percentages (50% to 90%) is also noted similar with a maximum of 0.59% difference for 70% mass loss ( Table 5). The high-temperature combination (171 • C, 85 • C) provides a near-3% difference in the temperature for mass losses of 50% to 70% ( Table 5). The decrease in temperature in TGA is explained by (1) Figure 6). Based on the aforementioned decrease in different parameters of the FTIR, DSC, and TGA results, the drop (but insignificant) in tensile strength with the soil degradation in the "main-effect plots" of ANOVA ( Figure 4) is thermochemically verified. However, the statistically insignificant decrease proves the stability of the blend against soil degradation. A suitable reason for such a low strength loss is the physical interlocking of PP (6.19% and 6.75%) in the PLA matrix as found with a phase separation in TGA graphs (Figure 7).   The soil-degraded samples at the low-temperature combination (161 °C, 25 °C) reveals a near-similar onset temperature (348.9 °C) as compared to 350.3 °C of neat PLA (Figure 7). The TEND for most degradation percentages (50% to 90%) is also noted similar

Morphological Analysis Using SEM
Scanning electron microscopy (SEM) further confirms the physical interlocking through visual analysis (Figure 8). The blend appears with a clear phase separation of PP in the PLA matrix. The distinct PP fiber can be noted in Figure 8c. Therefore, the SEM analysis proves the overwhelming physical interlocking as mentioned in FTIR, DSC, and TGA analysis. The physically interlocked PP is the reason for high stability against biodegradation and moisture hydrolytic degradation.

Conclusions
This work presents the detailed analysis of effects of in-process temperatures and soil biodegradation on an FDM blend with excess physical interlocking and reasonable chemical grafting. The polymer blend stands out among contemporary FDM blend systems due to the lowest percentage (7.5%) of a nonbiodegradable polymer (PP) in a biodegradable polymer (PLA). The approach of physical interlocking helps to achieve high tensile strength and soil degradation. The research includes a detailed design of the experiment consisting of mixed-level general full factorial ANOVA. The DoE includes the variables capable of causing in-process thermal variations and post-printing biodegradation. In this regard, the bed temperature and printing temperature are selected as in-process thermal variables, and the interval of soil burial is considered as a variable for analyzing soil degradation. The statistical results are supported with post-mechanical characterizations including FTIR, DSC, TGA, and SEM. The research concludes with the following outcomes. The novel blend system is statistically stable against 45 days of biodegradation with a p-value greater than 5% in confidence level.
The in-process 3D printing (nozzle) temperature is the significant variable with a pvalue less than the 5% confidence level.
The blend reports the highest weight retention of 99.89% after 45 days of biodegra-

Conclusions
This work presents the detailed analysis of effects of in-process temperatures and soil biodegradation on an FDM blend with excess physical interlocking and reasonable chemical grafting. The polymer blend stands out among contemporary FDM blend systems due to the lowest percentage (7.5%) of a nonbiodegradable polymer (PP) in a biodegradable polymer (PLA). The approach of physical interlocking helps to achieve high tensile strength and soil degradation. The research includes a detailed design of the experiment consisting of mixed-level general full factorial ANOVA. The DoE includes the variables capable of causing in-process thermal variations and post-printing biodegradation. In this regard, the bed temperature and printing temperature are selected as in-process thermal variables, and the interval of soil burial is considered as a variable for analyzing soil degradation. The statistical results are supported with post-mechanical characterizations including FTIR, DSC, TGA, and SEM. The research concludes with the following outcomes. The novel blend system is statistically stable against 45 days of biodegradation with a p-value greater than 5% in confidence level.
The in-process 3D printing (nozzle) temperature is the significant variable with a p-value less than the 5% confidence level.
The blend reports the highest weight retention of 99.89% after 45 days of biodegradation, which is far higher than that of neat PLA. This shows high stability against biodegradation.
The FTIR reveals the chain scission of C-O-C bonds in the blend due to which the tensile strength shows a statistically insignificant decrease of 4 MPa, i.e., 42 MPa to 38 MPa.
The low decrease in tensile strength after biodegradation presents high stability against biodegradation. The reason for the observed stability, even with chain scission, is due to physical interlocking confirmed in DSC, TGA, and SEM characterizations. The physical interlocking of at least 6% is found for both low-temperature (161, 25) and high-temperature (171, 85) combinations.