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Cellulose Nanofibril-Reinforced Polypropylene Composites for Material Extrusion: Rheological Properties.

Polypropylene (PP) is not typically utilized in 3D printing material extrusion because PP shrinks and warps during the printing process. Cellulose nanofibrils (CNF) have the potential to make PP 3D printer processable and also enhance mechanical properties of PP printed parts. The rheological behavior of CNF-PP composites during material extrusion requires study because it is different from injection molding and compression molding processes. This study revealed the effects of CNF contents (3 and 10 wt%) and maleic anhydride polypropylene (MAPP) coupling agent on the rheological properties of CNF-PP composites. Morphological analysis showed that CNF agglomerated during spray drying and a spherical structure was formed. Rheological tests showed that the elastic modulus, complex viscosity, viscosity, and transient flow shear stress of PP were increased by the addition of 10 wt% CNF, while the creep strain of PP was reduced. The damping factor and stress relaxation time remained the same when 10 wt% CNF was added to the PP. Incorporation of MAPP into the CNF-PP composites impacted the rheological properties of the CNF-PP composites. Flexural strength and modulus of PP were improved by 5.9% and 26.8% by adding 10 wt% CNF compared to the control.

INTRODUCTION

Material extrusion, also called fused layer modeling (FLM), is one of the additive manufacturing methods that is suitable for printing thermoplastics [1, 2]. A more familiar name for material extrusion is fused deposition modeling (FDM) which is a brand name coined by Stratasys Ltd. Compared to other additive manufacturing methods, material extrusion equipment is simple to operate, low in cost, and diverse in available materials. One of the disadvantages of material extrusion or most additive manufacturing techniques is that the mechanical properties of printed components are weaker than those from conventional manufacturing methods, for instance, injection molding [3]. All 3D printing techniques use a layer-by-layer deposition strategy. For material extrusion, the surface temperature of a polymer strand that allows the molecular diffusion to proceed decreases rapidly once the strand is laid down [4]. Complete diffusion cannot be achieved, resulting in the formation of interfaces and voids inside the components. Those interfaces and voids become the mechanical weak points that can initiate failures in printed parts. Several concepts have been tried to reinforce polymers made by the material extrusion process with different types of fillers including carbon fibers [5-8], thermotropic liquid crystalline polymers [9, 10], glass fibers [11, 12], tricalcium phosphate [13], copper and iron [14, 15], wood flour [16], and microcrystalline cellulose [17]. Nanofillers are outstanding in reinforcing polymers [18]. Recently, nanofillers also have been applied in polymers made by material extrusion for performance enhancements. Investigated nanofillers include graphene [19], carbon nanotubes [20], and nanoclay [21-23]. As opposed to other nanofillers, cellulose nanofibers are produced from renewable sources, biodegradable, low in cost, and light in weight [24]. Considerable research has been done to utilize cellulose nanofibers in reinforcing polymers with several review articles being published [24-27]. Cellulose nanofibers are generally categorized as cellulose nanofibrils (CNF), cellulose nanocrystals (CNC), and bacterial cellulose (BC), depending on their production process [24]. Unlike CNC and BC, CNF is lower in cost and better for mechanical reinforcing materials, thus have garnered more attention [28, 29].Well-dispersed CNF in a polymer matrix increases the modulus of the matrix, and also can maintain or increase the impact strength of the matrix which, on the other hand, has been reported to drop by the addition of natural fibers [30]. Polypropylene (PP), one of the most widely used thermoplastics, is difficult to print using material extrusion because of its shrinkage and warping behavior during printing [10, 31]. CNF has a low thermal expansion coefficient (0.1 ppm/k) [32]. Therefore, CNF can potentially restrain the shrinkage of PP, making PP printable by material extrusion.

Because of the hydrophilic nature of cellulose nanofibers, more studies have focused on solution or solvent mixing as the strategy for composites manufacturing as opposed to melt mixing [33]. However, melt compounding fibers with thermoplastics is the typical industrial method to make thermoplastic composites. To incorporate cellulose nanofibers into a polymer melt, a dry form is desired to facilitate polymer processing [34]. Spray drying is a method to dry cellulose nanofibers from suspensions. Spray drying appears to be a better method to dry CNF considering both the morphology of the spray-dried powder and the drying cost [35].

Rheological characterization of thermoplastics is critical to understand the fundamental flow behavior and provide information on processing. Several articles investigated the flow behavior of CNC/PP composites during melt compounding. The complex viscosity and storage modulus of PP increased dramatically with small addition of spray-freeze-dried CNC (5 wt%) [33]. The increase came from the web-like morphology of spray-freeze-dried CNC and a good dispersion of fillers inside the polymer matrix. Meanwhile, spray-dried CNC at 5 wt% addition level did not change the complex viscosity of PP significantly. This result can be attributed to CNC agglomeration and failure to form an interconnected web structure. Increasing the fiber content may be a solution to form such a structure inside the polymer matrix according to previous research on microcrystalline cellulose [36]. An increase in storage modulus and a decrease in transient flow stress of a PP melt with the addition of 1 wt% CNC were also reported [37]. On the other hand, the storage modulus, loss modulus, and complex viscosity were reported to decrease as CNC content in the polymer increased, attributed to a dilution effect [38]. It was reported that the addition of 20 wt% CNF increased the storage modulus of PP melt by a factor of three at 160[degrees]C using a parallel-plate rheometer [39]. In another study, 6 wt% CNF in PP was found to increase the melt flow index (MFI), but the MFI decreased once 2 wt% MAPP was incorporated [26]. Information on the rheological properties of CNF-PP composites needs further investigation.

In this study, CNF at two different addition levels (3 and 10 wt% based on the weight of the total material) with MAPP (2 wt% based on the weight of the total material) were compounded into PP to prepare composite filaments for material extrusion. For a bench-scale material extrusion device, the shear rate ([s.sup.-1]) involved in the printing process is much smaller compared to injection molding and extrusion. Because the diameter of the printing nozzle is smaller than that of the extrusion barrel, shear rate on the polymer at the nozzle is larger than that in the barrel. The shear rate at the printing nozzle was estimated to be in the range of 100-200 [s.sup.-1] [40]. Therefore, a parallel-plate rheometer is sufficient to provide useful rheology information. The objective of this work is to report on the parallel-plate rheological behavior of CNF/MAPP/PP composites for material extrusion.

MATERIALS AND METHODS

Materials

CNF (~3 wt% suspension) were provided by the Process Development Center of the University of Maine. The CNF suspension is produced by a disk refining method and produces a wide size distribution of fiber sizes. The suspension was diluted to 1.2 wt% for spray drying using a pilot-scale spray dryer (GEANiro). The drying was done at a temperature of 250[degrees]C, a disk spinning rate of 30,000 rpm, and a pump feeding rate of 0.4 L/min. Homopolymer PP (H19G-01) was purchased from Ineos Olefins & Polymers USA (League City, TX). Its density is 0.91 g/[cm.sup.3] with a melting point of 160[degrees]C and an MFI of 19 g/10 min (230[degrees]C/ 2.16 kg). Maleic anhydride polypropylene (MAPP) pellets (Polybond 3200) were obtained from Chemtura Corporation (Lawrenceville, GA). The MA content in the MAPP is about 1.0 wt%. Density, melting point, and MFI (190[degrees]C/2.16kg) of the MAPP is 0.91 g/[cm.sup.3], 190[degrees]C, and 115 g/10 min, respectively.

Morphology

Morphological information on the dried CNF was obtained by performing a particle size analysis using a microscope-based image analysis system (Morphologi G3S, Malvern, UK). Samples of 5 [mm.sup.3] were loaded into a special holder with both sides sealed by 25 [micro]m aluminum foil. The holder was placed in a dispersion unit and fibers were evenly dispersed on a glass plate with a pneumatic pressure of 0.5 MPa, injection time of 10 ms, and settling time of 60 s. A 50x objective lens was used for measuring the CNF. The software converted the 2D projection of a particle to a circle with the same area. The diameter of the circle is called circle equivalent diameter (CE diameter) [35]. The morphologies of the spray-dried CNF were visualized using a Hitachi Tabletop Microscope SEM (Hitachi High-Technologies Corporation, Tokyo, Japan) at an accelerating voltage of 5 kV. The environmental SEM does not require sputter coating for observation. The same SEM was also used to visualize the CNF distribution within PP by observing the impact-fracture surfaces of injection molded specimens.

Composites Manufacturing

Before compounding, CNF and PP pellets were oven dried for 2 h at 105[degrees]C. CNF was added into PP pellets and mixed by hand. A masterbatch containing 30 wt% CNF was first made by starve-feeding the mixture into a twin-screw co-rotating extruder (C. W. Brabender Instruments, South Hackensack, NJ) attached to a drive system (Intelli-Torque Plastic-Corder). The material feeding rate was 8 g/min. The L/D of this extruder is 40/1. Previous work in our research group used a C. W. Brabender Prep Mixer (C. W. Brabender Instruments, South Hackensack, NJ) to prepare a PP masterbatch [30]. The mixing method resulted in good distribution of CNF into the PP matrix. Increasing the screw rotational speed from 200 to 1,000 rpm was found to improve the dispersion of nanoclay in PP [41]. However, better exfoliation of nanoclay did not create higher mechanical properties possibly attributable to more chain scission at the higher screw speed. Therefore, a screw speed of 250 rpm was adopted in this study and the process is referred to as a "fast masterbatch production process." The extrusion temperature was set at 200[degrees]C for all the five zones of extruder barrel. After exiting the extruder, the masterbatch was ground using a granulator (Hellweg MDS 120/150, Hackensack, NJ). The composite pellets, fresh PP, and MAPP were oven dried at 105[degrees]C before the second extrusion with the formulations in Table 1. During the second extrusion, the masterbatch was diluted with fresh PP pellets to the desired CNF filler contents. The extrusion temperature was 200[degrees]C and the screw speed was 250 rpm. The composite extrudate passed through a two-nozzle die with a nozzle diameter of 2.7 mm. The extrudate was carried by a 2200 Series End Drive Conveyor (Domer, Hartland, WI) and finally chopped by a pelletizer. Pellets were made into flexural bars using an injection molder (Model #50 "Minijector") with a ram pressure of 17 MPa at 200[degrees]C. A mold with dimensions of 180 x 55 x 75 mm (length x width x height) was used. Samples were held in the mold at ambient temperature for 10 s before demolding. To make the control sample, the as-received PP pellets went through the same extrusion and injection molding process for manufacturing.

Rheological Tests

Rheological tests were done using a stress-controlled Bohlin Gemini rheometer (Malvern Instruments, UK) at a temperature of 200[degrees]C under air. Parallel plates with a diameter of 25 mm were selected. Sheet-shaped samples were cut from flexural bars and placed between the plates. A gap size of 1 mm was chosen for all tests. Before the small amplitude oscillation shear (SAOS) test, a stain sweep test was performed to check the linear viscoelastic regime of all specimens and strain amplitude of 1% was selected. The elastic modulus (G'), viscous modulus (G"), and complex viscosity ([[eta].sup.*]) were recorded at a frequency range of (0.1, 100) Hz. A steady shear flow test was conducted in the 0.001 to 5 [s.sup.-1] range to investigate the nonlinear behavior of the samples. A transient flow test was performed at a shear rate of 0.5 [s.sup.-1]. The relationship between flow time and shear stress was recorded. A stress relaxation test was done at a shear strain of 1%. Elastic modulus was recorded as a function of time. Finally, creep/creep recovery tests were conducted with a shear stress of 10 Pa and a creep time of 60 s. After that, the stress was removed and the strain recovery was recorded for 30 s. All tests were performed at two replicates to ensure repeatability.

Flexural Testing

The applicability of the "fast masterbath production process" method was demonstrated by producing injection molded flexural bars that were tested according to ASTM D 790-10. Flexural bar dimensions were 125 x 12.7 x 3.2 mm. The span-to-depth ratio is 16:1. Tests were conducted at room temperature of 23 [+ or -] 2[degrees]C and relative humidity of 50 [+ or -]10% RH. A universal testing machine (Instron 5966) with a 10 kN load cell was used for the tests. The span of the flexural test was 52 mm. With an outer fiber strain rate of 0.01/min, the flexural test speed was 1.4 mm/min. Flexural strength and Young's modulus of the specimens were determined. Five replicates of each sample were tested. The flexural properties were analyzed using a two-way analysis of variance (ANOVA) along with a student test at [alpha] = 0.05. The analysis was done in JMP statistical analysis program (JMP Statistical Discovery Software Version 8 2008). A statistical model was used to represent the properties of PP.

[Y.sub.ijk] = [mu] + [[alpha].sub.i] + [[beta].sub.j] + [([alpha][beta]).sub.ij] + [e.sub.ijk] (1)

Where i = 1, 2, 3; j = 1, 2; and k = 1, 2, 3. [Y.sub.ijk] is the mean of flexural property; [mu] is the population mean of flexural property of pure PP. The effects of filler content and coupling agent on flexural property were represented by [[alpha].sub.i] and [[bet]a.sub.j]. Effects of interaction between two factors on flexural property was represented by [([alpha][beta]).sub.ij]. The [e.sub.ijk] is the error for this model.

RESULTS AND DISCUSSION

Morphology

The morphology of the dried cellulose nanofibers is critical to modifying the rheological properties of resulting polymer composites. Spray-freeze-dried CNC from 1 wt% suspension produced a web-like structure after drying [33]. The microscopic features gave the CNC/PP composites a percolation threshold (2.5 wt%) above which the rheological properties changed dramatically. The change is caused by improved particle-particle and particle-polymer interactions. Similar results were obtain by studying freeze-dried CNC-reinforced PLA via solution casting [42]. Unlike spray-freeze drying and freeze-drying, spray drying tends to generate spherical particles attributable to agglomeration [35]. As seen in Fig. 1, both spherical and fibril CNF particles are created during spray-drying. Most CNF particles lose nanoscale dimensions because of the agglomeration from capillary forces, hydrogen bonding, and van der Waals forces during drying [33]. Unlike spray-freeze-dried CNC, no high-porosity or web-like structure is created inside the spray-dried CNF [33]. Based on the frequency curve in Fig. 2, a certain portion of the dried particles remain in the nanoscale dimension (smaller than 1 [micro]m). The majority of the CNF particles is smaller than 10 [micro]m. A few percentage of the particles are in the millimeter length scale. Morphological properties of the spray-dried CNF particles are listed in Table 2. Mean diameter of the CNF particles is in the micron scale dimensions. High-sensitive (HS) circularity depicts how close the shape is to a perfect circle. A perfect circle has a circularity of 1 while a spike or irregular object exhibits circularity closer to 0 [34]. Convexity is the measurement of the edge roughness of a particle. A smooth shape has a convexity of 1 while a spike or irregular object has a convexity closer to 0. Because of the aspect ratio, HS circularity and convexity are closer to 1, and the shape of the spray-dried CNF is more spherical than fibril. The smooth surface indicated by large convexity value implies that the spray-dried CNF has less porosity.

The distribution of CNF within PP after injection molding is shown in Fig. 3. Higher magnification graphs were taken, zooming into possible aggregation areas in the low-magnification micrographs. In general, CNF powders distribute fairly well in the PP matrix. This indicates the "fast masterbatch production" method is of high efficiency in distributing CNF in PP. The addition of MAPP does not change the distribution of CNF in the PP. Also observed are large agglomerates of CNF and many finer CNF particles, indicating a lower degree of dispersion of CNF. The morphology of CNF embedded in the PP is similar to that of CNF powder before compounding, meaning no significant dispersion of CNF can be achieved with our method. This is because the shear forces involved in the compounding cannot disrupt the forces which produce the agglomeration of CNF during spray drying.

Rheology

SAOS. Figures 4 and 5 show the development of complex viscosity ([[eta].sup.*]) and elastic modulus (G') as a function of frequency for the CNF-PP composite samples. In general, the change in [[eta].sup.*] and G' of PP after adding CNF is modest compared to previous studies where percolation was formed [33]. The reason is CNF agglomeration caused by spray drying diminishes the particle-particle and particle-polymer interaction [33]. As seen in Fig. 4, the [[eta].sup.*] decreased as frequency increased for pure and filled-PP, implying a non-Newtonian behavior over the entire tested frequency range [36]. The PP-10% composite has a higher [[eta].sup.*] while the PP-3% composite has a similar [[eta].sup.*] compared to pure PP. For instance, at a frequency of 0.1 Hz, the [[eta].sup.*] of the PP-10% is 25% larger than the pure PP and PP-3%. The higher addition percentage of CNF imparts the composite with more CNF-CNF and CNF-PP contacts, which increases the [[eta].sup.*]. No significant difference was found for the shear thinning behavior among all samples during the SAOS test. The MAPP can be a lubricant and a coupling agent, determined by its weight percentage in PP composites [43]. At a moderate addition level, MAPP is an effective compatibilizer that improves the interfacial adhesion between PP molecules and CNF, impeding the disentanglements of PP molecules. When MAPP addition is excessive, it acts as a lubricant that facilitates the disentanglement and reptation of PP molecules [37]. These conclusions are confirmed by this study. For PP-MA, the [[eta].sup.*] decreases compared to pure PP. For PP-10%-MA, the addition of MAPP increases the [[eta].sub.*] of the composite melt.

As seen in Fig. 5, the G' of the PP-10% is larger than the pure PP and PP-3%. For example, at a frequency of 0.1 Hz, G' of PP-10% is 33% higher than the PP and PP-3%. This is mainly attributable to the rigid nature of the CNF which restricts the deformation of PP [44]. No nonterminal behavior (pseudo solid-like) of CNF-filled PP was observed, indicating that no 3D microstructure is formed at those filler content levels. Three reasons account for this: (1) there is no strong CNF-PP interaction because of their different polarities; (2) the spherical structure of spray-dried CNF prevents the formation of an effective CNF network inside the PP matrix; and (3) the low porosity of spray-dried CNF cannot facilitate the polymer melt infiltration to improve dispersion and particle-polymer interaction. The lack of 3D microstructure of cellulose nanofibers reinforced polymer systems was also reported by previous research [44]. The PP-10%-MA samples show improved G' compared to PP-10% samples. At a higher CNF to MAPP ratio, the interfacial bonding is enhanced by MAPP [45]. Stress transfer from the PP molecules to CNF is more efficient. Therefore, CNF exhibits a better reinforcing effect in PP.

The damping factor (tan 6) is the ratio of viscous modulus (G") to elastic modulus (G'). The material acts as viscous liquid when tan [delta] 5 > 1 and appears elastic solid when tan [delta] [less than or equal to] 1 [46]. As seen in Fig. 6, the melts are viscous liquid below 20 Hz. The addition of CNF marginally changes the behavior of the PP melt. The crossover point (tan [eta] = 1) is the transition from liquid-like to solid-like behavior. The inverse of the crossover frequency is the characteristic relaxation time of a polymer chain [33]. The crossover frequencies for all the samples without MAPP are identical (24 Hz), corresponding to a characteristic relaxation time of 0.042 s for PP chains and is comparable to a previous finding on pure PP melts [33]. This indicates that PP chain relaxation was not significantly affected by the addition of CNF because the particle-polymer interaction is weak. This interaction is incapable to retard the relaxation of the PP molecules significantly. When MAPP is added into pure PP, it facilitates the relaxation of PP chains as indicated by a higher crossover frequency (25 Hz). When MAPP is introduced into the composite melt, the damping factor decreases. The crossover frequency of PP-10%-MA (23 Hz) is lower than the PP-10%. This indicates that PP chain relaxation is slightly retarded by the presence of MAPP. The nature of the MAPP is to couple PP molecules with CNF, thus more restriction is applied to the mobility of PP molecules.

Steady Shear Flow. Nonlinear rheological properties of the polymer melt can be obtained by performing steady-state shear tests up to high shear rates. As seen in Fig. 7, at low shear rates, all samples show Newtonian behavior. The PP-10% possesses higher viscosity than the PP and PP-3%. For example, the viscosity of PP-10% at 0.001 [s.sup.-1] is 15% higher than pure PP. The increase in viscosity stems from the fact that larger filler content offers more hindrance to the movement of polymer chains [47]. At higher shear rates, all samples display shear thinning behavior. The higher the fiber content is, the more shear thinning the sample exhibits. For a fiber-reinforced polymer, the gradual alignment of fibers to the flow direction at high shear rate was reported to account for the increased shear thinning [48]. However, spray-dried CNF is nearly spherical thus does not show considerable orientation even under fluid flow. The change in shear thinning behavior results from the disrupted CNF-PP interaction at higher shear rates. The addition of MAPP to the PP-3% and PP-10% composite improves the interaction between CNF and PP molecules and increases the viscosity of the composite melt. Because the difference between the viscosity of CNF-PP composites and PP becomes much smaller after a shear rate of 5 [s.sup.-1], the composites will have no difficulty to flow in a typical material extrusion device where the shear rate is normally above 100 [s.sup.-1] [40].

Transient Flow. During a transient flow test, polymer chains disentangle and reptate [49]. For an entangled material, the shear stress first experiences a climbing then a drop and eventually reaches a steady state. The peak shear stress depends on how easily the disentanglement and reptation occur. Figure 8 shows the shear stress as a function of shear time. All samples rapidly disentangle their polymer chains and start to flow after 1.2 s. PP-10% has a higher shear stress than PP and PP-3%. Shear stress during transient flow was reported to increase as fiber content increased in polymer composites [42, 50]. The occurrence of a stress peak is mainly created by the fiber alignment to the flow direction of polymer molecules [42]. As discussed before, fiber alignment is not the reason for the stress peak observed. When a certain amount of CNF is present in the PP melt, the frictional force between CNF and PP molecules or the attachment of PP chains to the CNF surface reduces the mobility of the PP molecules [37]. Therefore, higher stress is required to disentangle the PP chains. In addition, the shear stress of the PP-10% at steady state is larger than PP and PP-3%. The steady state shear stress depends on the equilibrium of disentanglement and entanglement of polymer chains [37]. In the PP-10%, the disentanglement is more difficult because of increased CNF-PP and CNF-CNF interactions. Finally, the shear stress at peak and steady state of PP-3%-MA and PP-10%-MA is higher than PP-3% and PP-10% because of the enhanced interaction between the CNF and PP molecules that impedes the disentanglements of the PP chains.

Stress Relaxation. The stress relaxation test is another way to detect the interaction between polymer chains and fillers. In a system where particle-polymer interaction is strong, polymer chains will relax much slower than pure polymer because the particles retard the movement of the polymer chains [33]. Based on the results shown in Fig. 9, all samples relax rapidly within the first second, again implying that the CNF-PP interaction is weak. The PP and PP-3% samples reach the zero-stress state at 9 s while PP-10% sample achieved zero-stress state at 12 s. The PP-10% has more CNF-PP interactions because the higher filler content results in more contact among CNF and PP molecules. The PP-3%-MA and PP-10%-MA relaxed slightly slower than the PP-3% and PP-10% attributable to improved CNF-PP interactions. Neither the addition of CNF nor MAPP significantly affects the stress relaxation behavior of PP. This confirms the finding from tan d results.

Creep/Creep Recovery. Creep tests are used for measuring the elasticity of polymer melts [44]. As seen in Fig. 10, the strains of all samples increase almost linearly with creep time. The addition of CNF decreases the strain of pure PP, indicating a higher elasticity induced by the intrinsic rigidity of CNF that restricts the movement of the polymer melt [44]. In the creep recovery test, elastic deformation is restored. The recoverable strain yR is defined as follows [51]:

[[gamma].sub.R] = [[gamma].sub.r]/[[gamma].sub.c] x 100% (2)

Where [[gamma].sub.c] is the strain at the end of creep test and yr is the strain developed at the end of recovery test. The strain from creep recovery test further confirms the finding from the creep tests. The recoverable strains for PP, PP-MA, PP-3%, PP-3%-MA, PP-10%, and PP-10%-MA are 1.5%, 1.2%, 1.9%, 1.4%, 2.6%, and 1.9%. For groups without MAPP, pure PP displays the smallest recovery strain. These strain values from the recovery test are close to what was reported for polyethylene containing similar CNF content using a creep stress of 200 Pa [44]. The recovery strain of pure PP is smaller than those (~10%) obtained in previous study using a creep stress of 10 Pa [51]. That PP had an MFI of 3.8 g/min (230[degrees]C/2.16 kg) which is five times lower than our PP. The low MFI indicates a higher molecular weight, larger molecular entanglement density, and higher elasticity, which leads to higher recovery strain. The recoverable strain of the PP melt increases with the incorporation of CNF attributed to the increase of elasticity caused by CNF. MAPP slightly increases the strain of composite melts during the creep test and decreases recoverable strain during the recovery test. This is the only situation in this study where the rheological property of CNF filled-PP is adversely affected by MAPP and the results are contradictory to a previous finding on wood/PP/ PE composites [52]. For the other tests in this study, polymer chains undergo large disentanglement under the applied test conditions. However, a small shear force (10 Pa) was applied during the creep test. The deformation of the melt was modest and no significant chain sliding occurred. The rule of mixtures explains that the elastic modulus of the composite is roughly the sum of the elastic modulus of each component multiplied by their volumetric percentage in the composite [53]. As MAPP is a low-molecular-weight polymer compared to PP, it is mechanically less stiff than PP and CNF. Consequently, the incorporation of MAPP to CNF-PP would decrease the elastic modulus.

Flexural Properties

Concern may rise when the mixing time is dramatically reduced with a fast extrusion speed during the masterbatch production procedure. Improper mixing is detrimental to the mechanical properties of the composites. The SEM graphs proved the efficiency of our method visually. As a supplemental support, the flexural properties of injection molded CNF-PP composites from the "fast masterbatch production process" method were tested. Analysis of variance on the flexural properties of PP and its composites are shown in Table 3. The interactive effect of CNF content and MAPP is important to the flexural strength. Different combinations of CNF content and MAPP content will generate CNF-PP composites with varied flexural strength. While for flexural modulus, only CNF content is critical, meaning the MAPP cannot improve the flexural modulus of CNF-PP composites. The finding on flexural modulus further confirmed the results from the creep/creep recovery test.

As can be seen from Table 4, the addition of 3 wt% CNF into PP insignificantly affects the flexural properties of pure PP regardless of the presence of MAPP. After 10 wt% CNF is incorporated into PP, the flexural strength and modulus of the composite are 5.9% and 26.8% higher than the pure PP. Further addition of MAPP improves the flexural strength of PP by 12.9% compared to the pure PP. Those results are comparable to what were found before using a slower extrusion speed and a longer mixing time [30]. Therefore, this "fast masterbatch production process" method is efficient for producing CNF-PP compounds. The improvement in Young's modulus of PP components by the addition of 10 wt% CNF is mainly attributed to the rigidity of CNF itself [44, 54]. The mechanisms of improved strength in short fiber-filled polymer composites include (1) enhanced the stress transfer at interface, (2) lowered stress concentration at fiber ends, and (3) crack deflection [55]. Meanwhile, short fibers can degrade a polymer matrix with fiber ends which initiates cracks. Whether the strength will increase or decrease depends on which factors dominate [55]. Apparently, the enhancement of the flexural strength of CNF-PP composite by MAPP is attributed to the improved stress transfer at interfaces as demonstrated by the rheological tests [45]. At higher CNF content, stress transfer at the interface is more effective attributed to increased fiber-polymer contact. The stress around a fiber is affected by other fibers. The stress concentration is reduced once the fibers are closer to each other which can be a result of higher fiber content. The addition of 10 wt% CNF increases the crack initiation. At the same time, the larger number of CNF fibers increases the stress transfer at the interface and reduces the stress concentration at the fiber ends. The overall result is a slight increase in flexural strength of the PP.

CONCLUSIONS

The effects of CNF content and MAPP coupling agent on the rheological properties and flexural properties of CNF-PP composites for 3D printer filaments were studied. SEM showed that CNF agglomerated during drying and a spherical structure with low porosity was formed. Spray-dried CNF can be well distributed into PP using a "fast masterbatch production" method. Rheological tests showed that elastic modulus, complex viscosity, viscosity, and transient flow shear stress of PP were increased by approximately 33%, 25%, 15%, and 27% at the chosen frequency and shear rates after 10 wt% CNF was added into the PP. The increase came from enhanced particle-polymer interaction at higher filler content. Creep strain was reduced with the addition of 10 wt% CNF because of increased rigidity. The damping factor and stress relaxation time remained the same even at 10 wt% CNF addition because the CNF-PP interaction is weak. MAPP increased the complex viscosity, elastic modulus, viscosity, transient flow shear stress, and creep strain of CNF-filled PP but decreased the stress relaxation of these composites. The flexural strength and modulus of PP were increased by 5.9% and 26% after 10 wt% CNF was added into PP. This further confirms the efficiency of the "fast masterbatch production process" method used in this study. Adding 3 wt% CNF into PP changed neither the rheological properties nor the flexural properties of PP significantly. In summary, the addition of CNF into PP, through a "fast masterbatch production process" method, marginally changed the rheological properties from a practical consideration. The small change in rheological properties at lower shear rate brought by CNF makes the resulting PP composites filament process friendly to material extrusion devices.

ABBREVIATIONS

FDM Fused deposition modeling

FLM Fused layer modeling

PP Polypropylene

MAPP Maleic anhydride polypropylene

CNF Cellulose nanofibrils

MFI Melt flow index

CE diameter Circle equivalent diameter

SAOS Small amplitude oscillation shear

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Lu Wang (iD),(1,2) Douglas J. Gardner, (1,2) Douglas W. Bousfield (3)

(1) Advanced Structures and Composites Center, University of Maine, Orono, Maine 04469-5793

(2) School of Forest Resources, University of Maine, Orono, Maine 04469-5755

(3) Department of Chemical and Biological Engineering, University of Maine, Orono, Maine 04469-5737

Correspondence to: L. Wang; e-mail: lu.wang@maine.edu

Grant sponsor: Maine Agricultural and Forest Experiment Station (MAFES); Grant number: ME0-M-8-00527-13; Grant sponsor: USDA ARS Forest Products Research Agreement; Grant number: 58-0202-4-003.

DOI 10.1002/pen.24615

Caption: FIG. 1. SEM micrographs of spray-dried CNF.

Caption: FIG. 2. Particle size distribution frequency curves of spray-dried CNF. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 3. Spray-dried CNF distribution within PP after melt compounding.

Caption: FIG. 4. Complex viscosity of specimens as a function of frequency.

Caption: FIG. 5. Elastic modulus of specimens as the function of frequency.

Caption: FIG. 6. Damping factor of specimens as the function of frequency.

Caption: FIG. 7. Viscosity of specimens as a function of shear rate.

Caption: FIG. 8. Shear stress of specimens as a function of time.

Caption: FIG. 9. Elastic modulus of specimens as a function of relaxation lime.

Caption: FIG. 10. Strain development of specimens as a function of creep/creep recovery time.
TABLE 1. CNF-PP composites formulations.

Samples                 Labels       PP      CNF     MAPP

PP                    PP             100      0       0
PP + MAPP             PP-MA          98       0       2
PP + 3% CNF           PP-3%          97       3       0
PP + 3% CNF + MAPP    PP-3%-MA       95       3       2
PP + 10% CNF          PP-10%         90      10       0
PP + 10% CNF + MAPP   PP-10%-MA      88      10       2

TABLE 2. Morphological properties of spray-dried CNF.

          CE diameter      Aspect          HS
Sample    ([micro]m)       ratio       circularity    Convexity

CNF       9.58 (0.90)   1.25 (0.007)   0.84 (0.03)   0.96 (0.009)

TABLE 3. Selected values from the ANOVA for flexural properties of
CNF-PP composites.

                                    Sum of squares

Source                  DF (a)     FS (b)     FM (c)

Corrected total           29       263.04      1.45
model                      5       228.49      1.36
CNF content                2        36.28      0.71
MAPP                       1        10.36      0.003
CNF content x MAPP         2        46.05      0.001
Error                     24        34.54      0.09

                             F value                 p value

Source                    FS         FM         FS          FM

Corrected total
model                   31.75      72.95
CNF content             12.60      95.21      0.0002 *   <0.0001 *
MAPP                     7.20       0.75      0.013 *     0.3944
CNF content x MAPP      15.60       0.18     <0.0001 *    0.8403
Error

(a) Degree of freedom.

(b) Flexural strength.

(c) Flexural modulus.

* Significance level at [alpha] = 0.05.

TABLE 4. Flexural properties of specimens from injection molding.

                                              Young's
                Strength                      modulus
Samples          (MPa)        Significance     (GPa)     Significance

PP          48.14 (1.09) (a)     CD (b)     1.68 (0.03)       B
PP-MA       45.91 (1.72)         E          1.68 (0.05)       B
PP-3%       47.09 (0.54)         DE         1.66 (0.04)       B
PP-3%-MA    49.26 (1.15)         BC         1.63 (0.09)       B
PP-10%      50.79 (1.43)         B          2.13 (0.08)       A
PP-10%-MA   54.35 (0.91)         A          2.09 (0.05)       A

(a) Values in the parentheses stand for standard deviation.

(b) Capital letters represent for statistical differences. Values
with different letters are significantly different.
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Author:Wang, Lu; Gardner, Douglas J.; Bousfield, Douglas W.
Publication:Polymer Engineering and Science
Article Type:Report
Date:May 1, 2018
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