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Abstract

This work explores a novel method for fabricating lightweight thermoplastic polymer-fiber-reinforced polymer (PFRP) composites, often referred to as self-reinforced composites (SRCs), using automated powder dispersion followed by compression molding. A representative PFRP was fabricated using plain-weave ultra-high-molecular-weight polyethylene (UHMWPE) fabrics embedded in a high-density polyethylene (HDPE) powder matrix. In the dispersion process, various parameters including sweeping speed, vibration voltage, sieve mesh size, and dispersion paths were studied for their effects on powder distribution, powder amount, and processing speed, aiming to achieve better control over fiber volume fraction ( $V_{f}$ ) and improve mechanical properties of PFRP composites. In addition, the tensile behavior of the PFRPs with a fiber volume fraction of 30% fabricated using the proposed method with different dispersion paths was investigated, and the highest average elastic modulus, tensile strength, and failure strain reached 7.2 GPa, 375 MPa, and 5.8%, respectively. Moreover, their specific tensile properties (normalized by composite density) are comparable to those of glass- or aramid-fiber-reinforced polymer composites. This work was motivated by the search for a diverse automated manufacturing method to fabricate lightweight thermoplastic PFRPs with enhanced mechanical properties.

Keywords

Polymer-fiber-reinforced polymers (PFRPs)self-reinforced composites automated powder dispersion ultra-high-molecular-weight polyethene (UHMWPE) fiber mechanical properties manufacturing

Introduction

Polymer-fiber-reinforced polymers (PFRPs), often referred to as single polymer composites or self-reinforced composites (SRCs), have gained increasing attention due to their excellent impact resistance,¹ lightweight nature,² and remanufacturing capability.³ These PFRP composites are composed entirely of polymer components, with both the fibers and matrix derived from the same or different types of thermoplastic polymer, including polyurethane (PU),⁴ polypropylene (PP),⁵ polyethylene (PE),⁶ polyethylene terephthalate (PET),⁷ polyamide (PA),⁸ polylactic acid (PLA),⁹ polymethyl methacrylate (PMMA),¹⁰ liquid-crystal polymer (LCP),¹¹ polyether ether ketone (PEEK),¹² and polyphenylene sulfide (PPS).¹³ Since the pioneering study conducted by Capiati and Porter half a century ago on the fabrication of PE-based PFRP composites,¹⁴ numerous studies have investigated the mechanical, physical, and morphological behaviors of PFRPs made from the aforementioned thermoplastics, In brief, these behaviors include, for instances, thermal stability,⁶ tension,¹⁵ compression,¹⁶ flexure,¹⁷ fiber-matrix interfacial bonding,^18,19 impact resistance,^20,21 quasi-static fracture,²² fatigue,²³ water absorption,²⁴ and micro-structural features.²⁵ The foregoing behaviors are closely related to the manufacturing methods and processing parameters (e.g., temperature,⁶ pressure,²⁶ cooling rate,²⁷ and consolidation time²⁸). In particular, for manufacturing methods, which are the focus of this work, a survey of the literature is presented to briefly summarize the different manufacturing methods used for the fabrication of PFRPs and their reported features.

A variety of manufacturing methods for fabricating thermoplastic PFRPs have been investigated and reported in the literature. Among all the currently existing methods,^2,29 the film stacking and hot compaction methods are the most common methods due to their simplicity of production. The film stacking method leverages alternating polymer fabrics and polymer films as matrices, whereas the hot compaction method requires only polymer fabrics and partially melts their surfaces to create the matrix. As a result, the PFRPs fabricated by these two methods exhibit significant differences in fiber volume fraction ( $V_{f}$ ), with the maximum $V_{f}$ being less than 60% for film stacking method^2,30 and $V_{f}$ approximately 90% for hot compaction method (e.g., PE,³¹ PP,²² PA,³² PET³³). On the other hand, filament winding of polymer fibers wrapped with matrix films, followed by hot pressing, is also commonly used to exclusively fabricate uni-directional PFRPs. The filament-wound PFRPs with $V_{f}$ values exceeding 50% were demonstrated for different polymer fibers (e.g., PE,³⁴ PP,³⁵ LCP³⁶). Another common method for fabricating PFRPs is injection molding, which injects a heated polymer matrix over polymer fabrics or a mixture of polymer fibers and matrix under pressure to produce complex structures. However, this method typically does not produce the PFRP with a high $V_{f}$ value greater than 50%, as demonstrated for the PFRPs with different polymer fiber types, including PP,³⁷ PE,³⁸ and PET.³⁹ On similar grounds, solution impregnation and matrix melt impregnation also utilize the thermoplastic polymer matrix in the form of a flowable state to mix with polymer fibers. For instance, several researchers fabricated PE-based PFRPs with $V_{f}$ values below 50% by immersing polymer fibers into a polymer matrix solution.² In addition, Zhang et al.⁴⁰ produced PFRPs with a $V_{f}$ of approximately 40% by pouring flowable PMMA matrix into a mold containing UHWMPE fabrics. Moreover, Gong et al.⁴¹ fabricated PA-based PFRPs with a $V_{f}$ exceeding 60% by transferring flowable PA matrix into PA fabrics. On the other hand, fabricating PFRPs using powder coating or impregnation methods have also been reported in the literature to fabricate PA-based PFRPs⁴² and polyester-fiber-reinforced PP composites,⁴³ although these methods have been less studied compared to the aforementioned ones.

In the past decade, automation in manufacturing has been widely developed and applied to fabricate carbon-, glass-, and aramid-fiber-reinforced thermoset and thermoplastic polymers (CFRPs, CFRTPs, GFRPs, GFRTPs, AFRPs, and AFRTPs), as well as natural fiber composites, using techniques such as various 3D printing methods⁴⁴ and automatic fiber placement.⁴⁵ However, automated manufacturing methods for fabricating thermoplastic PFRPs remain relatively underexplored, despite the partial automation offered by processes such as injection molding and filament winding. Only a few studies on fully automated manufacturing of PFRPs have been reported in the literature. For example, Yadav et al.⁴⁶ demonstrated the fabrication of PLA-based PFRPs with a wide range of $V_{f}$ values by means of fused filament fabrication in 3D printing. Similarly, Zhang et al.¹³ developed an additive manufacturing approach and employed a supercooling method to successfully fabricate PPS-based PFRPs. The lack of including automation in manufacturing of thermoplastic PFRPs is a critical research gap, since these composites have potential applications^2,6 due to their lightweight features and excellent ductility but are often underexplored compared to the abundant studies in the literature for common carbon- or glass-fiber-reinforced polymer composites.

Therefore, this study introduces a novel method for fabricating UHMWPE-based woven PFRPs, as a representative case of thermoplastic PFRPs, by using automated powder dispersion enabled by a movable sieve with a vibration function, followed by compression molding. Various fabrication parameters were investigated and compared for their effects on powder dispersion as well as the subsequent morphology and mechanical properties of the PFRP composites. In addition, this work further compared the specific mechanical properties of such composites fabricated using the proposed method with the literature data of various PFRPs, as well as other common composites (CFRPs, CFRTPs, GFRPs, GFRTPs, AFRPs, AFRTPs). The proposed fabrication method, using automated powder dispersion, represents a significant advancement toward integrating automation into diverse manufacturing approaches for thermoplastic PFRPs, which have potential applications for transportation (e.g., heavy-truck exterior panels⁴⁷), aerospace (e.g., drone flight frames⁴⁸), and personal protection (e.g., ballistic helmets).⁴⁹

Experimental preparation and characterizations

Materials

The plain-weave ultra-high-molecular-weight polyethylene (UHMWPE) fabrics were used as the reinforcement, and high-density polyethylene (HDPE) powder was used as the matrix for fabricating thermoplastic PFRPs. Single UHMWPE fibers exhibited tensile strengths ranging from 2.9 to 3.7 GPa and tensile moduli ranging from 90 to 135 GPa, as provided by the manufacturer (Honeywell Spectra). HDPE powder has an average particle size of approximately 20 μm, with a particle size distribution ranging from 5 to 50 μm. Both the UHMWPE fabric and HDPE powder have a similar density of 0.95 g/cm³. As shown in Figure 1(a), the UHMWPE fabric exhibits multiple melting peak temperatures ranging from 145°C to 156°C, corresponding to the melting and transition between different crystal phases in the UHMWPE fabric.^50,51 In contrast, the HDPE powder has a lower melting peak temperature of 127°C. The aforementioned temperature difference provides a suitable processing window for fabricating UHMWPE-based PFRPs using the approach described in the following section. In addition, based on the heat capacity as a function of temperature as shown in Figure 1(a), the crystallinities of the investigated UHMWPE fabric and HDPE power were 89% and 56%, respectively.

Figure 1.

(a) Heat capacity versus temperature during heating process for both HDPE powder and UHMWPE fabric; (b) automated motion-controlled and vibratory powder dispersion system modified from a Prusa 3D printer; (c) an illustration of dispersed HDPE powder over the UHMWPE fabric in an aluminum mold; (d) schematic layup of the fabric and powder layers in the compression mold with the cavity of 127 mm $\times$ 178 mm. The fabric layer is approximately 100 μm thick, and the powder layer is approximately 10 μm thick at a fiber volume fraction of 30%.

Fabrication of PFRPs

Step 1: Automated powder dispersion

An automated powder dispersion system (Figure 1(b)–(c)) was developed by modifying a Prusa MK3S 3D printer to uniformly deposit HDPE powder over woven UHMWPE fabrics. The print head was replaced with a custom-designed sieve assembly, enabling programmable motion and vibratory powder dispensing controlling powder flow. The custom sieve head consists of a 60 mm $\times$ 60 mm stainless steel mesh screen, surrounded by 25 mm high walls to form a powder reservoir. Several interchangeable mesh screens (m_e = 800 µm, 500 µm, 381 µm, 228 µm, and 89 µm) were selected to investigate the powder dispensing rate and effectiveness. A Vibronic 6.4G DC vibration motor was mounted next to the powder reservoir to assist powder flow through the mesh during motion. The vibratory motor was connected to a controller that initiated vibration only at the beginning of each programmed dispersion path, minimizing powder waste and improving consistency. Three voltages (V_o = 3.3 V, 5 V, and 9 V) were investigated to achieve different vibration intensities (i.e., frequency and amplitude) in this work. To enable motion-controlled deposition, the system used the original Prusa motion platform and firmware, operating at three selected sweeping speeds (V_s = 25 mm/s, 50 mm/s, and 100 mm/s) and a fixed sieve height of 125 mm (measured from the bottom of the sieve to the mold) modified through custom G-code instructions. Three dispersion paths were also implemented in the G-code scripts, as shown in Figure 3(d): (1) a single path across the mold area; (2) a one-directional raster pattern with 20 mm step spacing; and (3) a crosshatched raster pattern with 20 mm step spacing, involving sweeping in both horizontal and vertical directions. Vibration was initiated for approximately 2 s to achieve a stabilized condition before conducting powder dispersion along the designed path.

Powder was dispensed directly into a mold cavity of 127 mm $\times$ 178 mm where the woven UHMWPE fabric was pre-positioned and fixed in place during deposition. Each pass deposited powder onto the fabric surface in a layer-by-layer manner prior to consolidation. In this work, composites were fabricated using two layers of UHMWPE fabric and three layers of HDPE powder as illustrated in Figure 1(d). The final step to fabricate the composites requires compression molding, which will be discussed in the next section. It is worth mentioning that, in this work, composites with only one fiber volume fraction ( $V_{f}$ = 30%) were fabricated and tested to demonstrate the proposed manufacturing capability, while higher $V_{f}$ values of up to 50% were achieved by varying the voltage, mesh size, and dispersion time. However, the powder distribution uniformity at higher $V_{f}$ values was relatively lower than that at 30%. This aspect is considered by the authors as future work to further improve the uniformity of powder distribution for composites with higher fiber volume fractions. Here, the fiber volume fraction was defined as the weight fraction of the fabric layers divided by the total weight of the composite, since the densities of the UHMWPE fabric and HDPE matrix are nearly the same.

Step 2: Compression molding

The mold containing the powder and polymer fabrics was then transferred to a hot press (Carver, Indiana, USA) set at a temperature of 140°C and a pressure of 0.2 MPa. The temperature was maintained for 15 min, followed by cooling to room temperature at a rate of approximately 0.8°C/min. It is worth mentioning that the processing temperature of 140°C exceeded the melting peak of the HDPE powder but remained below that of the UHMWPE fiber as shown in Figure 1. Such a processing temperature has often been reported and extensively studied in the literature for the fabrication of thermoplastic PFRPs,^52,53 as it facilitates interfacial bonding between the polymer fiber and matrix without significantly weakening the reinforcing properties of the polymer fiber. Using a temperature below the matrix melting peak can result in insufficient matrix melting, potentially causing defects in self-reinforced composites. Therefore, different processing temperatures were not studied in this work, as the focus was placed on the automated powder dispersion process and the associated composite mechanical properties.

Thermal, mechanical, and morphological characterizations

The melting behavior of the UHMWPE fabric and HDPE powder (Section - Materials) was measured using a Differential Scanning Calorimeter (DSC, TA Instruments Q2000). Powder samples were placed in a Tzero pan, while fiber samples were cut from UHMWPE fabric and crimped into another pan. Three samples were tested for repeatability. The samples and reference pans were subjected to heating at 10°C/min from 40°C to 200°C. Nitrogen flow was supplied at a flow rate of 50 mL/min to prevent unwanted reactions, and improve stable measurements.

To characterize the tensile strength and modulus, representative properties of the composites, tensile tests were conducted on UHMWPE-based PFRPs fabricated using the proposed approach (Section - Fabrication of PFRPs). Rectangular specimens were tested using an electromechanical Instron 5582 load frame under displacement control at a rate of 2 mm/min. Digital Imaging Correlation (DIC) analysis using a DIC system by Correlated Solutions was used to obtain the stress-strain relationship of the specimens. The specimens measured 125 mm × 12.5 mm × 0.6 mm, with a gauge area of 65 mm × 12.5 mm. Five specimens for each scenario were tested for repeatability.

The surface morphology of the composite was characterized using a Keyence VR-5000 3D optical profilometer, while the cross-sectional morphology was examined using an Olympus DSX1000 optical microscope. For cross-sectional analysis, square specimens (12.5 mm × 12.5 mm) cut from the fabricated composites were sequentially ground with sandpapers (180–1200 grit), rinsed with ethanol, polished using dedicated pads and solutions, and dried. A detailed preparation procedure for the samples used for cross-sectional optical microscopy is available in a recent publication by the authors.⁶

Results and discussion

Powder dispersion versus parameters

The dispersion of HDPE powder as a function of time (t), mesh size (m_e), and vibration voltage (V_o) was initially investigated under conditions where the sieve vibrated without translational motion. To evaluate the effect of m_e, the contour plot in Figure 2(a) presents the powder dispersion amount (W_g) while maintaining a fixed combination of t and V_o. At a lower vibration voltage (V_o = 3.3 V), variations in m_e did not produce noticeable changes in powder dispersion. However, as the vibration voltage increased to V_o = 9 V, mesh size began to play a more significant role, with larger m_e values resulting in increased powder dispersion. For instance, after approximately 15 s of powder dispersion at V_o = 9 V, the W_g values are 0.10 g, 1.83 g, 8.95 g, and 15.82 g for wire mesh sizes of 89 μm, 228 μm, 381 μm, and 500 μm, respectively. On the other hand, the contour plot in Figure 2(b) clearly shows the effect of V_o on the variation in W_g while t and m_e are held constant. In this figure, a strong relationship between V_o and m_e is observed, similar to that shown in Figure 2(a), highlighting the increasingly significant role of V_o in powder dispersion at larger m_e values. After approximately 15 s of powder dispersion using m_e = 500 μm, the W_g values are 11.74 g, 15.36 g, and 15.82 g for vibration voltages of 3.3 V, 5 V, and 9 V, respectively. Overall, the powder dispersion rates for m_e values of 89 μm, 228 μm, and 381 μm were approximately 0 g/s, 0.14 g/s, and 0.63 g/s, respectively, across all investigated V_o values. However, the powder dispersion rate for m_e values of 500 μm and 800 μm were approximately 0.85–1.1 g/s and 2.06–4.7 g/s, respectively, for V_o values ranging from 3.3 to 9 V.

Figure 2.

HDPE powder dispersion as a function t, m_e, and V_o for the sieve vibrated without translational motion (V_s = 0): (a) illustration of the effect of m_e while maintaining a fixed t and V_o, and (b) illustration of the effect of V_o while maintaining a fixed t and m_e.

When the sieve vibrated and moved, the dispersion rate of the HDPE powder did not change significantly compared to the stationary vibration condition for the investigated sweeping speeds (V_s), but the dispersion path can affect the power distribution and the quality of the composites as will be discussed in the next section. Only the mesh sizes of 228 μm, 381 μm, and 500 μm were investigated, as the smaller m_e value (89 μm) does not disperse powder effectively, and larger m_e value (800 μm) can release all the powder from the container before dispersion is completed. As plotted in Figure 3, the dispersion of HDPE powder as a function of V_s, m_e, and V_o was summarized for three different dispersion paths. For lower m_e and V_o values (Figure 3(a)–(c)), the V_s does not significantly affect the amount of the powder after the dispersion is completed. However, the amount of powder increases noticeably as the V_s decreases when m_e or V_o is higher. For the second and third dispersion paths in Figure 3(b) and (c), a mesh size of 500 μm was not included since all the powder, with an amount of approximately 44 g, can be dispersed before the dispersion is completed. For the third dispersion path in Figure 3(c), all the powder was just dispersed completely from the container under the conditions of m_e = 381 μm, V_o = 9 V, and V_s = 25 mm/s. It is worth mentioning that the amount of powder dispersion depends on the size of the container, and the amount investigated is suitable for fabricating UHMWPE-based PFRPs with the aluminum mold size described in Section - Fabrication of PFRPs. Based on the experiments summarized in Figure 3, a certain combination of manufacturing parameters (V_s, m_e, and V_o) in the sweeping powder dispersion process can be selected to fabricate a customized V_f value of the composite.

Figure 3.

HDPE powder dispersion as a function V_s, m_e, and V_o for the sieve vibrated with translational motion. This figure includes three different dispersion paths: (a) path 1, (b) path 2, and (c) path 3. Details of dispersion paths were plotted in (d).

Tensile properties of UHMWPE-based PFRPs

The effects of the powder dispersion path on the uniaxial tensile properties of the composites with a constant fiber volume fraction (V_f ≈ 30%) were further investigated, as shown in Figure 4(a) and (b) and Table 1. Among the three dispersion paths, the failure strains of the UHMWPE-based PFRP composites fabricated using automatic powder dispersion are similar, ranging from 5% to 5.8%. However, the elastic modulus and tensile strength of those composites show more pronounced differences. The composites fabricated using the third automatic powder dispersion path exhibit the highest elastic modulus and tensile strength, reaching approximately 6.4 to 7.2 GPa and 290 to 375 MPa, respectively, compared to those fabricated using the other two paths. The differences among the three powder dispersion paths were also reflected in the morphologies of the fractured composites after testing, as shown in Figure 5. Composites fabricated using the first and second paths exhibited more localized fracture compared with those produced using the third path, which showed more distributed deformation and damage. The underlying reasons causing the difference in tensile properties of the UHMWPE-based PFRP composites between the third path and the other two paths are explained in Figure 4(c). In this figure, a representative surface morphology of the UHMWPE-based PFRP composite fabricated using the first path is shown, where the powder matrix is unevenly distributed on the polymer fabrics, revealing areas without powder coverage. In contrast, for the composite fabricated using the third path, the powder matrix is uniformly distributed on the surface of the polymer fabric, as illustrated in Figure 4(c), and well impregnated into the polymer fabrics, as evident from the cross-sectional morphology in Figure 4(d). It is worth mentioning that fabric architecture, such as the spacing between tows, can also play an important role in the wettability of powders over fabrics during the manufacturing. For the investigated UHMWPE plain-weave fabric, the spacing between tows ranges from approximately 80 to 150 μm, which allows successful penetration of the investigated HDPE powders (5 to 50 μm) through the fabric layers. This promotes wettability and helps eliminate potential cavities near the fabrics. If the spacing between tows is smaller than the powder particle size range, wettability during the manufacturing can be adversely affected, potentially leading to cavities within the composites.

Figure 4.

(a) Engineering stress-strain curves of the investigated UHMWPE-based PFRP composites (V_f ≈ 30%) under un-axial tension, (b) Average engineering tensile strengths of the composites based on the results from (a), (c) Surface morphologies of these untested composites, and (d) Representative cross-sectional morphology of an un-tested UHMWPE-based PFRP composite (V_f ≈ 30%) with well-impregnated HDPE powder matrix. Note, $h$ represents surface height measured from the three-dimensional optical profilometer.

Table 1.

Tensile properties of the investigated UHMWPE-based PFRP composites ( $V_{f}$ ≈ 30%) fabricated using different parameters.

	Path #1	Path #2	Path #3	Manual
Tensile strength (MPa)	224.05 $\pm$ 24.11	264.1 $\pm$ 18.85	332.66 $\pm$ 43.49	292.88 $\pm$ 30.13
Elastic modulus (GPa)	4.35 $\pm$ 0.18	5.16 $\pm$ 0.45	6.76 $\pm$ 0.49	6.31 $\pm$ 0.59
Failure strain	0.055 $\pm$ 0.003	0.056 $\pm$ 0.002	0.054 $\pm$ 0.004	0.052 $\pm$ 0.004

Figure 5.

Normalized maximum principal strain ( $ε_{p, n o r}$ ) field and fracture morphologies for the investigated UHMWPE-based PFRP composites (V_f ≈ 30%) fabricated using different parameters.

Interestingly, as illustrated in Figure 4(a), the UHMWPE-based PFRP composites with the same V_f value but fabricated using the manual powder dispersion method, as reported by the author in a recent publication,⁵⁴ exhibit slightly lower elastic modulus (5.7 to 6.9 GPa), tensile strength (263 to 323 MPa), and failure strain (5% to 5.6%) compared to those fabricated using the third automatic powder dispersion path. A slight difference was also observed in the morphologies of the fractured composites fabricated using the third path and manual powder dispersion, as both showed similar deformation and damage bands but with different sizes (Figure 5). The improvement achieved using automatic powder dispersion can be attributed to a more homogeneous distribution of powder on the polymer fiber surface and its penetration into the fiber tows (Figure 4(d)). Automated powder dispersion via the third path took 24 s and produced the highest mechanical performance in the investigated PFRP composites, whereas manual dispersion in the same time could not guarantee uniform distribution of the HDPE powder and show lower mechanical performance (Figure 4). It is worth mentioning here that determining the optimized powder dispersion path will require further quantitative studies, which is beyond the scope of this work, as the aim here is to demonstrate the feasibility of the powder dispersion method for the fabrication of PFRPs.

Comparison with other polymer composites

Having discussed the mechanical performance of the UHMWPE-based PFRP composites fabricated using the proposed method, it is insightful to compare their properties with those of other types of fiber-reinforced polymer composites. Therefore, various woven thermoplastic PFRPs, as well as carbon-, glass-, and aramid-fiber-reinforced polymer composites reported in the literature, were selected for comparison. The comparison focuses on the specific elastic modulus (E/ $ρ$ ) and specific tensile strength ( $σ_{c} / ρ$ ), both normalized by composite density ( $ρ$ ).

As illustrated in Figure 6(a) and (b), the specific tensile properties of the investigated UHMWPE-based PFRPs (E/ $ρ$ = 4.44 to 7.47 GPa/[g/cm³] and $σ_{c} / ρ$ = 217 to 382.1 MPa/[g/cm³]) are higher than various thermoplastic PFRPs, including PP,^55–58 PET,^16,59 PA,³² PLA,⁶⁰ and PHA,⁶¹ at similar fiber volume fractions, On the other hand, if the E/ $ρ$ and $σ_{c} / ρ$ values of the PE-based PFRP composites ( $V_{f}$ $\approx$ 5 to 80%) fabricated using hot compaction or film stacking methods in the literature^62,63 were linearly normalized to a fiber volume fraction of $V_{f}$ $=$ 30%, the resulting E/ $ρ$ and $σ_{c} / ρ$ values range from 4.1 to 9 GPa/(g/cm³) and 176 to 333 MPa/(g/cm³), respectively. While the E/ $ρ$ values of the investigated UHMWPE-based PFRP composites for the same $V_{f}$ value fall within this range, the average $σ_{c} / ρ$ value can be higher by roughly 18%. It is worth mentioning here that normalizing the PE-based PFRP composites from $V_{f}$ $\approx$ 80% to $V_{f}$ $=$ 30% provided only a rough estimation. This approximation may be considered reasonable given the significant scatter in the literature data arising from differences in manufacturing methods and processing parameters, as well as the observed trends of mechanical properties with respect to fiber volume fraction shown in Figure 6(a) and (b) and the summarized literature data.²

Figure 6.

Specific tensile strength ( $σ_{c} / ρ$ ) and specific elastic modulus (E/ $ρ$ ) versus fiber volume fraction for various woven or cross-ply polymer composites: (a and b) comparison this work and other thermoplastic PFRPs (PP, PE, PET, PA, PLA, and PHA), and (c and d) comparison between this work and common polymer composites (CFRPs, CFRTPs, GFRPs, GFRTPs, AFRPs, and AFRTPs). The lower and upper bound data of the investigated UHMWPE-based PFRPs fabricated using the three different powder dispersion paths were used for plotting. Note, CFRPs or CFRTPs mean carbon-fiber-reinforced thermoset or thermoplastic polymers, GFRPs or GFRTPs mean glass-fiber-reinforced thermoset or thermoplastic polymers, and AFRPs or AFRTPs mean aramid-fiber-reinforced thermoset or thermoplastic polymers.

Similarly, the specific tensile properties of the investigated UHMWPE-based PFRPs were further compared with that of carbon-, glass-, and aramid-fiber-reinforced polymers (CFRPs,^64–66 CFRTPs,^67,68 GFRPs,⁶⁴ GFRTPs,⁶⁹ AFRPs,^64,70,71 and AFRTPs^72,73), as shown in Figure 6(c) and (d). The definitions of these abbreviations are provided in the caption of Figure 6. As it can be noted from Figure 6(c) and (d), the specific properties of the investigated UHMWPE-based PFRPs were comparable to that of glass- or aramid-fiber-reinforced polymers at similar fiber volume fractions. More specifically, by linearly normalizing the experimental data of carbon-, glass-, and aramid-fiber-reinforced polymers from $V_{f}$ $\approx$ 50–60% to $V_{f}$ $=$ 30%, while the specific tensile properties of the investigated UHMWPE-based PFRPs are lower than carbon-fiber-reinforced composites (E/ $ρ$ = 19.2 to 35 GPa/[g/cm³] and $σ_{c} / ρ$ = 214.8 to 621.3 MPa/[g/cm³] for CFRPs, as well as E/ $ρ$ = 17.4 to 29.2 GPa/[g/cm³] and $σ_{c} / ρ$ = 147 to 391 MPa/[g/cm³] for CFRTPs), they are close to those of glass-, and aramid-fiber-reinforced polymers, with E/ $ρ$ = 6.3 to 14.7 GPa/(g/cm³) and $σ_{c} / ρ$ = 83.4 to 257.8 MPa/(g/cm³) for GFRPs and GFRTPs, as well as E/ $ρ$ = 3.7 to 15.9 GPa/(g/cm³) and $σ_{c} / ρ$ = 66.6 to 297.5 MPa/(g/cm³) for AFRPs and AFRTPs. Of course, the foregoing linear normalization provided only a rough estimation. The comparisons in Figure 6 highlight the differences in mechanical properties between the investigated UHMWPE-based PFRPs and other types of fiber-reinforced polymer composites, providing insights for readers to consider potential applications of thermoplastic PFRPs.

Prospects and future directions

This study represents a critical step toward developing diverse automated fabrication methods for thermoplastic PFRPs, an area with limited research compared to carbon- and glass-fiber composites, and offers a promising lightweight, ductile material solution for the automotive industry and beyond. Several future research extensions have been identified:

(1) Fabricate thermoplastic PFRPs with higher fiber volume fractions using automated powder dispersion with optimized dispersion parameters, or other approaches, to achieve uniform powder dispersion;

(2) Investigate the underlying dispersion mechanisms through computational modeling;

(3) Identify the challenges in scaling the automated dispersion process. In the current form, this approach may be more suitable for fabrication of PFRP prepregs rather than complex-shaped components;

(4) Beyond basic tensile properties, explore mechanical properties that are important to applications of UHMWPE-based PFRPs, specifically the impact and fracture resistance. In addition, cooling rate and strain rate, important processing and loading parameters, should be explored for self-reinforced thermoplastic composites;

(5) Extend the automated powder dispersion method to other fiber reinforcements, such as glass, carbon, and aramid fibers, to evaluate the versatility and adaptability of this approach across different composite systems.

Conclusions

This work presents a novel method for fabricating thermoplastic polymer-fiber-reinforced polymers (PFRPs) via automatic powder dispersion followed by compression molding, and investigates the effects of various processing parameters within this method on the mechanical behavior of the composites. Plain-weave UHMWPE fabrics embedded in a HDPE powder matrix was used as a representative PFRP composite. Based on the experimental results, the following conclusions can be summarized:

(1) In the automated powder dispersion process, a significant and non-linear increase in powder amount was observed with decreasing dispersion speed when using a larger wire mesh and higher vibration voltage. In contrast, with a smaller mesh and lower voltage, the powder amount increases linearly and less significantly;

(2) The dispersion path incorporating both vertical and horizontal movements (third Path) results in a more uniform powder distribution on the polymer fabric compared to the other two paths. Consequently, the UHMWPE-based PFRPs ( $V_{f}$ = 30%) fabricated using this path exhibit the highest tensile strength, elastic modulus, and failure strain, reaching up to 375 MPa, 7.2 GPa, and 5.8%, respectively, with the tensile strength and elastic modulus being approximately 10% higher than that of composites produced using manual powder dispersion;

(3) The lower tensile strength and elastic modulus of the UHMWPE-based PFRPs fabricated using the first and second paths likely result from the uneven distribution of the powder matrix on the polymer fabrics, leading to regions lacking matrix coverage and consequently causing defects;

(4) The specific tensile properties of the investigated composites fall within the range of PE-based PFRP composites, while being noticeably higher than those of other PFRP composites (PP, PET, PLA, PA, and PHA) and comparable to glass- and aramid-fiber-reinforced polymer composites.

Footnotes

Author contributions

Jose L Ramos: Investigation, Methodology, Data curation, Formal analysis, Writing – review & editing; Yao Qiao: Investigation, Methodology, Data curation, Formal analysis, Writing – original draft, Writing – review & editing, Conceptualization, Project administration, Funding acquisition; Yelin Ni: Investigation, Writing – review & editing; Ethan K Nickerson: Investigation; Kevin L Simmons: Investigation, Funding acquisition.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work is supported by the US Department of Energy, Office of Energy Efficiency and Renewable Energy, Vehicle Technologies Office. PNNL is operated by Battelle Memorial Institute for US Department of Energy under contract DE-AC06-76RLO 1830.

ORCID iD

Yao Qiao

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Fabrication of thermoplastic polymer-fiber-reinforced polymer composites through automated powder dispersion and compression molding

Abstract

Keywords

Introduction

Experimental preparation and characterizations

Materials

Fabrication of PFRPs

Step 1: Automated powder dispersion

Step 2: Compression molding

Thermal, mechanical, and morphological characterizations

Results and discussion

Powder dispersion versus parameters

Tensile properties of UHMWPE-based PFRPs

Comparison with other polymer composites

Prospects and future directions

Conclusions

Footnotes

Author contributions

Declaration of conflicting interests

Funding

ORCID iD

References