Recycled Polypropylene Blends As Novel 3D Printing Materials [PDF]

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Additive Manufacturing 25 (2019) 122–130

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Full Length Article

Recycled polypropylene blends as novel 3D printing materials a,⁎

a

b

c

Nicole E. Zander , Margaret Gillan , Zachary Burckhard , Frank Gardea a b c

T

United States Army Research Laboratory, Weapons and Materials Research Directorate, Aberdeen Proving Ground, Aberdeen, MD, 21005, United States Montana Tech, Department of Mechanical Engineering, Butte, MT, 59701, United States United States Army Research Laboratory, Vehicle Technology Directorate, Aberdeen Proving Ground, Aberdeen, MD, 21005, United States

ARTICLE INFO

ABSTRACT

Keywords: Polymer additive manufacturing 3D printing Fused filament fabrication Recycled plastics Polymer blends Polypropylene

Consumer-grade plastics can be considered a low-cost and sustainable feedstock for fused filament fabrication (FFF) additive manufacturing processes. Such materials are excellent candidates for distributed manufacturing, in which parts are printed from local materials at the point of need. Most plastic waste streams contain a mixture of polymers, such as water bottles and caps comprised of polyethylene terephthalate (PET) and polypropylene (PP), and complete separation is rarely implemented. In this work, blends of waste PET, PP and polystyrene (PS) were processed into filaments for 3D printing. The effect of blend composition and styrene ethylene butylene styrene (SEBS) compatibilizer on the resulting mechanical and thermal properties were probed. Recycled PET had the highest tensile strength at 35 ± 8 MPa. Blends of PP/PET compatibilized with SEBS and maleic anhydride functionalized SEBS had tensile strengths of 23 ± 1 MPa and 24 ± 1 MPa, respectively. The noncompatibilized PP/PS blend had a tensile strength of 22 ± 1 MPa. PP/PS blends exhibited reduced tensile strength to ca. 19 ± 1–3 MPa with the addition of SEBS. Elongation to failure was generally improved for the blended materials compared to neat recycled PET and PS. The glass transition was shifted to higher temperatures for all of the blends except the 50–50 wt. % PP/PET blend. Crystallinity was decreased for the blends, but was highest in the 75–25 wt. % PP/PS and the 50-50 wt. % PP/PET blend with SEBS-maleic anhydride. Solvent extraction of the dispersed phase revealed polypropylene was the matrix phase in both the 50–50 wt. % PP/PET and PP/PS blends.

1. Introduction Plastics consumption for consumer products and packaging is continuing to grow despite environmental and sustainability concerns. In the United States alone, almost 32 million tons are generated annually, yet recycling rates are typically only at 5% [1,2]. More efficient reprocessing techniques and value-added applications are likely needed to change these recycling trends. One of the complications of plastic recycling is the need for sorting of different types after collection since single-stream recycling is common. In addition, some packaging contains more than one type of plastic, such as water bottles with polyethylene terephthalate (PET) bodies and polypropylene (PP) caps accounting for ca. 5% of the waste or yogurt tubs with PP bodies and high-density polyethylene (HDPE) lids accounting for ca. 5–10% of the total waste. The preparation of polymer blends of such recycled materials is potentially an attractive way to reuse mixed waste streams at a lower cost. In addition, polymer blending is a facile way to tailor mechanical properties of polymers or improve certain properties while maintaining

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others. Polypropylene has comparable toughness to ABS, but does not have sufficient strength to be classified as an engineering plastic [3]. Reinforcement with a rigid polymer such as polystyrene (PS) or PET can improve mechanical properties. However, most polymers suffer from incompatibility, leading to poor phase morphology and mechanical properties. There have been a number of studies evaluating the effect of different compatibilizers to improve the stability of the blends [4–6]. Of particular interest are studies of PP/PET and PP/PS blends, commonly found in many consumer waste streams. PP is considered immiscible and incompatible with both PS and PET. Many researchers have described such blend optimization through the use of compatibilizing copolymers such as SEBS or grafted PP in which one component can interact with each phase [7]. Wang et al. utilized supercritical CO2 to generate block and graft copolymer interfacial modifiers in-situ via reactive extrusion. Supercritical CO2 facilitated the grafting reaction due to its plasticizing effect [8]. Impact strength was improved most for the samples with the highest supercritical CO2 concentration [9]. Macaúbas et al. investigated PP/PS blends compatibilized with the triblock copolymers styrene butadiene styrene (SBS) and styrene ethylene

Corresponding author. E-mail address: [email protected] (N.E. Zander).

https://doi.org/10.1016/j.addma.2018.11.009 Received 1 October 2018; Received in revised form 5 November 2018; Accepted 6 November 2018 Available online 08 November 2018 2214-8604/ © 2018 Elsevier B.V. All rights reserved.

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butylene styrene (SEBS) and the effect on interfacial tension. SEBS was found to be more efficient, but both copolymers served to reduce the size of the dispersed phase and the interfacial tension [10]. Brostow et al. found 5 wt. % SEBS to be optimal for improving PP/PS blend morphology, generating polystyrene-styrene and polypropylene-ethylene/butylene microphases and a significant size reduction of the dispersed phase in the matrix [11]. Huo et al. compounded PP/PS blends with a maleic anhydride grafted PP and amino terminated PS to improve miscibility [12]. PP/PET blend morphology can also be improved with the addition of SEBS. Inoya et al. found significant size reduction in the PP dispersed phase in recycled PP/PET blends with 5 phr compatibilizer [13]. Heino et al. observed improved stabilization and impact strength of PP/PET blends with functionalized SEBS containing maleic anhydride or glycidyl methacrylate likely due to the PET carboxylate end group reaction with the grafted SEBS midblocks. Average particle size was significantly reduced with the functionalized SEBS blends and the interfacial adhesion was improved [14]. Another approach to compatibilize PP/PET blends is with maleic anhydride grafted PP [15,16]. Silane coupling agents, which change interfacial adhesion typically between a polymer and inorganic substrate, can also serve as compatibilizers for PP/PET blends. The alkoxy silane groups react with PET end group hydroxyls, strengthening its interaction with PP [17]. Impact properties of PP/PET blends were particularly improved by the addition of a silane terminated polybutadiene (POLYVEST 25 by HÜLS-A VEBA Group Co). Singh et al. investigated the mechanical properties of ABS/high impact polystyrene (HIPS) blends [18]. In addition to the work by Inoya et al., there has been other work into blends and composites of recycled polymers. Mangat et al. embedded silk and sheep wool fibers into PLA to generate a bio-degradable composite for biological applications [19]. Mantia et al. investigated recycled PET/PLA blends and found that adding a small amount of PLA increased the thermal stability without compromising the mechanical properties [20]. Remili improved the recyclability of PS by adding an organophilic clay which served to increase the effective molecular weight due to crosslinking [21]. Bourmaud increased the properties of recycled PP by adding vegetal fiber [22]. Recently, Zander et al. and others have shown that recycled polymers can effectively be used to make fibrous membranes for use in a variety of applications such as filtration and tissue engineering [23–31]. Shin and Chase formed nanofibers from styrofoam using a solutionbased electrospinning technique [28], and Rajabinejad et al. made fibers from bottle-grade PET using melt-electrospinning [32]. Fibers were also successfully prepared from blends of PET, PP and PS. Blends with recycled PET showed improved properties compared to the neat PP, PS and PP/PS blends [30]. Recycled materials have also been used in additive manufacturing (AM). In this process, parts are made by the consecutive deposition of layers of material as directed by a three dimensional (3D) computer aided design (CAD) model. The process known as material extrusion or fused filament fabrication (FFF) is the most common polymer AM technique. Solid polymer filaments are melted in a liquefier and deposited on a temperature-controlled bed. ABS is a widely used polymer in FFF due to its amorphous nature and limited shrinkage and warpage upon cooling. While a few of other polymers such as PLA, polyamide, HIPS and polycarbonate are commonly used, FFF feedstocks are generally limited. Commodity polymers used in packaging and consumer products- polyolefins, PET and polystyrene- are rarely used in FFF. A few companies have commercialized filaments from recycled polymers such as ABS (Kickfly®) and PLA (Maker Geeks®). Refil® and B-PET® make filament from recycled PET. In addition, FFF printing using recycled HDPE, PLA, and low density polyethylene (LDPE) derived from meals-ready-to-eat (MRE) bags has been reported [33–35]. In previous work, we have demonstrated the use of 100% recycled and unmodified PET in the FFF of long-lead time military parts, and found parts printed from recycled PET had comparable properties to those made from ABS and polycarbonate-ABS [36]. To the best of our knowledge, we are the first to report 3D

printing of recycled blends from consumer-grade plastics. Here we report the use of recycled PP/PET and PP/PS blends, compounded with or without a SEBS compatibilizer for the purpose of fabricating feedstocks for FFF. While these polymers are commonly used in many applications, they are not widely utilized as feedstocks for FFF due to a variety of reasons including water absorption leading to molecular weight reduction (in PET), lack of control of crystallinity, and shrinkage/warpage, which can make printing difficult. Recycled polymers may contain contaminants and processing aids, and have likely been subjected to several thermal and mechanical stresses during processing cycles, potentially leading to lower performance than a virgin part due to deterioration of the polymer chains. However, the tensile strength of injection molded recycled PET (rPET) is similar to virgin PET and compares favorably to ABS (68 vs 55 MPa) [37,38]. Thus, it was predicted that the mechanical properties of printed blends of rPET with other recycled polymers could be at least comparable to prints made with commercial filament. Blends of rPP and rPET formed a consistent, flexible filament that was easily 3D printed. Blends of rPS and rPET were attempted, but yielded a brittle material that broke in the print head. In addition, recycled PS was quite brittle and could not spooled. Thus blends of PS with PP were pursued to improve the flexibility of the filament. Although tensile properties were reduced compared to recycled PET, elongation to failure was improved for PET and PS blended with PP, and tensile strengths were comparable to commercial filaments such as HIPS [39]. 2. Materials and methods 2.1. Preparation of polymer filament To fabricate the filament, feedstock was obtained from recycle bins, cleaned by rinsing with water and then ethanol and dried. The labels were removed before cutting in to pieces that could be fed into the paper shredder (Compucessory model CCS60075). Sources of polymer included clear polyethylene terephthalate plastic salad containers (rPET), opaque polypropylene yogurt containers (rPP), and clear polystyrene petri dishes (rPS). Different brands of each material were used to introduce realistic variation in the data. In addition, some of the yogurt containers had polyethylene in the lids, and this contamination was included in the dataset. The cleaning, removing of labels and glue, sorting and drying of the plastic was carefully controlled. However, some of the plastics contained a certain percentage of recycled content which was not controlled. Thus, the full thermal history of all feedstock was unknown, and this could potentially affect the material properties of the prints. Source and age or heat/light exposure of the plastics could also have an effect. Drying of PET and materials to be blended with PET is critical to prevent melt hydrolysis of PET. PET shreds were then dried under vacuum overnight at 120 °C, while the PP was dried overnight under vacuum at RT. The rPS was ground into powder in a blender, and was maintained under ambient conditions prior to use. SEBS (G1652) and SEBS grafted with maleic anhydride (1.4–2 wt. %, SEBS-MA, FG1901 Kraton®) were also dried overnight under vacuum at RT before use. Blends were prepared by weighing polymer in ratios of 25/75, 50/50, 75/25 and hand mixing before feeding into the extruder. Compatibilized blend ratios were fixed with equal weights of each polymer and 5 wt % SEBS or SEBS-MA (final ratio 47.5/47.5/5). A Thermo Scientific Process 11 Parallel Twin-Screw Extruder with 8 temperature zones and a conveying or non-mixing screw design was used for extrusion. The parameters were determined based on previous work [36]. The feed port was fixed at 140 °C and the adjacent zone to 170 °C to prevent clumping of the material while feeding. The following 5 zones were fixed at 260 °C. The die (2.5 mm) was set at 245 °C. Screw speed was held constant at 100 rpm. A conveyor belt (at RT) with speed set to 2 or ca. 1 m/min collected the filament (Pharma 11, Thermo Scientific). Shredded polymer was first extruded, pelletized (Varicut 123

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Pelletizer, Thermo Scientific), and then extruded a second time to improved uniformity of the filament diameter and blend mixing. The extruder speed for the second extrusion was set to 25 rpm. A combination of the conveyor belt and spooler (Filabot, speed set to 1.5 turns) was used for the rPP/rPS filament.

of 2 mm/min on a servohydraulic test frame with 2 kN grips and 5 kN load cell (Instron model 5000R). Four replicates were evaluated for each blend composition. Strain was determined from a digital image correlation (DIC) system consisting of one 1 megapixel monochrome digital camera (Point Grey) which streamed images (2–10 frames per second) to an attached computer. Camera recording settings were entered using Correlated Solutions Vic-SnapC software. Post-processing was done with Correlated Solutions Vic-2D software program, as has been described previously [36]. Before testing, the samples were spray painted to create a speckle pattern for DIC. The morphology of fractured tensile bars were imaged using a field-emission scanning electron microscope (SEM, Hitachi S-4700) after sputter coating with gold-palladium. Solvent extraction on fracture surfaces was conducted with hexafluoroisopropanol to remove the PET phase in the rPP/rPET 50-50 blend and chloroform to remove the PS phase in the rPP/rPS 50-50 blend.

2.2. 3D printing of recycled plastics Before printing, filament was dried under vacuum overnight to remove any excess moisture. Both Type V tensile bars (ASTM D638) and DMA bars (35 mm × 12.5 mm × 2 mm) were printed for characterization on a Lulzbot Taz 6® FFF printer. Recycled PP/PET blends were printed on a PET tape surface. Recycled PP/PS blends were printed on a polyetherimide surface. Simplify 3D was used to edit the STL files. For all samples, a100 °C bed temperature and a 260 °C nozzle temperature was used. A Y or flat on the bed build orientation was used, with 0.2 mm layer height, 2 shell layers and 100% infill [40]. 45/−45° orientation for the infill was used for tensile bars, and 0° for DMA bars. Tensile and DMA bar print speeds was 50 mm/s and 20 mm/s respectively.

3. Results and discussion 3.1. Chemical analysis

2.3. Fabrication of control tensile bars

Qualitative chemical characterization was performed using FTIR to verify extruded feedstock compositions (Fig. 1). Non-overlapping peaks were identified for each polymer and are denoted with dashed lines of matching color. The CH3 bending at 1376 cm−1 was utilized for PP, while the ester carbonyl C]O stretch at 1720 cm−1 and CH bending at 690 cm−1 was used to identify PET and PS, respectively. The PP identifier was present in all of the samples except the pure PET and PS, while the PS and PET identifiers were only present in the pure PET or PS and their blends with PP.

Feedstock materials were cleaned as detailed above. Tensile bars from PET bottles and yogurt containers were cut using a punch press (Type C, ASTM D412). Injection molded tensile bars were made from clean, dry rPET, PP and PS shreds on an Xplore® Microcompounder (MC15) [36]. The polymer was compounded at 260 °C for 5 min and maintained in the transfer line at 260 °C prior to injection into a mold (Type V, ASTM D638) (temperature = 65 °C, pressure = 6 bar). 2.4. Chemical characterization

3.2. Rheology

Chemical information was obtained via analysis by Fourier transform infrared-attenuated total reflectance (FTIR-ATR) (Thermo Nicolet Nexus 870 ESP) using 256 scans and 4 cm−1 resolution over a range of 4000–400 cm−1.

The melt viscosity of the recycled polymer blends was measured to obtain information about the interaction of the polymers in the blend and effect of compatibilizers (Fig. 2). At low shear rates, the neat polymers had lower viscosities compared to the blends. Recycled PET and PS had lower viscosity than PP. For the PP/PET blends, the addition of SEBS reduced the viscosity relative to the blend without SEBS, however the viscosity of the blends remained higher than PP or PET alone. However, the addition of 5% SEBS-MA led to a marked increase in the viscosity. This is likely due to the hydrogen bonding interaction of the maleic anhydride with the PET chain ends and effective

2.5. Thermal characterization Differential scanning calorimetry (DSC) with a heat/cool program (Discovery DSC, TA Instruments) was used to evaluate thermal properties. Samples were heated at 20 °C per min to 300 °C and cooled at the same rate to −50 °C. TRIOS software (TA Instruments) was used to analyze the data. Crystallinity for the semi-crystalline PET and PP polymers was calculated as previously described (PET: 140 J/g, PP: 207 J/g) [36,41]. Crystallinity for each semi-crystalline polymer in the blends was calculated by multiplying the above equation by the mass fraction of each polymer. Dynamic mechanical analysis (DMA, Q800 TA Instruments) enabled characterization of the thermal mechanical properties using the single cantilever mode. The temperature was ramped from −50 °C to 200 °C at a rate of 2 °C per min and frequency of 1.0 Hz. The amplitude setpoint was 200 μm. The dimensions of the printed DMA bars were 35 mm x 12.5 mm x 2 mm with a span of 17.5 mm. Three replicates were tested for each blend composition. 2.6. Rheology Melt viscosity at 270 °C was measured using an AR2000 rheometer (TA Instruments) with 25 mm parallel aluminum plates. The gap was fixed at 1000 μm with the shear rate ramped from 0.1 to 10 s−1.

Fig. 1. Chemical characterization of recycled polymer blends using FTIR. Dashed lines with matching color denote marker peaks for each polymer. All blends were 50-50 by weight. Select blends were compatibilized with 5 wt. % unfunctionalized (SEBS) or maleic anhydride functionalized SEBS (SEBS-MA).

2.7. Uniaxial tensile measurements Uniaxial tensile experiments were performed at a displacement rate 124

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Fig. 2. Rheological behavior of recycled polymer blends. (A) Polyethylene terephthalate (rPET) and polypropylene (rPP), (B) polystyrene (rPS) and polypropylene (rPP). All blends were 50-50 by weight. Select blends were compatibilized with 5 wt. % unfunctionalized (SEBS) or maleic anhydride functionalized SEBS (SEBS-MA).

molecular weight increase, in addition to the SEBS interaction with PP and interfacial tension reduction, which has been documented by others [42]. This trend is not observed for the PP/PS blends; instead SEBS-MA addition leads to a sharp decline in viscosity with shear rate. Unfunctionalized SEBS, however, increases the viscosity due to its ability to interact with both the alkyl chains on PP and phase mix with PS. The viscosity of the uncompatibilized blend appears only slightly higher than neat rPP, suggesting minimal interaction. SEBS-MA was not expected to interact more strongly than SEBS with rPS phase due to the polarity mismatch, but it was thought there would be some improvement in interaction over the uncompatibilized blend. In addition, the viscosity of the rPP/rPS blend with SEBS-MA decreased with shear rate. It is possible that the domain sizes increased due to coalescence which served to reduce viscosity, although this was not studied in this work [43]. It is notable that the viscosities of the recycled polymers and blends were lower than that of polymers typically used in FFF processes such as ABS (zero-shear viscosity 18,000–75,000 Pa s) [44].

slightly decreased in the rPP/rPET and rPP/PS blends, with or without compatibilizer (except rPP/rPET SEBS). Cold crystalization peaks for rPET at ca. 120 °C are observed in all samples with PET except the blend with 25% rPET. The enthalpy increase of the cold crystallization peak corresponded nicely as expected with the increase in rPET concentration for the uncompatibilized blends and neat rPET, but the addition of SEBS and particularly SEBS-MA served to reduce the enthapy of the PET cold crystallization peak. The PET cold crystallization peak was reduced by half from the 75% rPET blend to the 50% blend, was non-apparent in the 25% rPET blend, and further reduced when SEBS and particularly SEBS-MA was added. Possibly chain movement for recrystallization was restricted by the PP phase, but the crystallization temperatures were largely unchanged for these rPP/rPET blends. It should be noted that the recycled polypropylene source had an additional melting peak at ca. 125 °C, attributed to low density polyethelene (LDPE) contamination, potentially masking rPET’s cold crystallization peak and complicating interpretation of the results. In most cases this peak was quite small and the recrystallization peak of rPET could be clearly seen with the exception of the 75-25 rPP/rPET blend, as discussed above. Upon cooling the sample, rPET and rPP crystallize at ca. 199 °C and 122 °C, respectively. Recycled PET’s crystallization temperature was largely unaffected by blending with rPP except for the 75% rPET blend and the SEBS-MA compatibilized blend. The Tc for the former was shifted up 9 °C possibly due to rPP nucleation reducing the free energy for crystal consolidation [45]. The Tc of the SEBS-MA blend was decreased by 5 °C potentially due to a plasticizing effect of the compatibilizer [46]. Chen et al. has reported this plasticization phenomenon in similar systems, leading to a facilitation of crystallization and better

3.3. Thermal analysis The thermal behavior of the recycled polymers was probed via differential scanning calorimetery (DSC) to determine the effect of the immiscible blends on crystallization and melting (Fig. 3, Table 1). Recycled PET has a melting peak at ca. 252 °C, which rPP melts at ca. 170 °C. Two distinct melting peaks are present in the rPP/rPET blends, as expected, confirming immiscibilty of the blends. The addition of the SEBS and SEBS-MA compatibilizer did not appear to improve miscibility. However, the melting temperatures of rPP and rPET were

Fig. 3. Representative differential scanning calorimetry curves for FFF printed recycled polymer blends. (A) Heating showing recrystallization and melting peaks, (B) cooling showing crystallization peaks. rPET (recycled polyethylene terephthalate), rPP (recycled polypropylene), rPS (recycled polystyrene). Select blends were compatibilized with 5 wt. % unfunctionalized (SEBS) or maleic anhydride functionalized SEBS (SEBS-MA). 125

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Table 1 Thermal analysis of FFF printed recycled polymer blends. Sample

rPET bulk rPP bulk rPET rPP rPP/rPS 25-75 rPP/rPS 50-50 rPP/rPS 75-25 rPP/rPS SEBS rPP/rPS SEBS-MA rPP/rPET 25-75 rPP/rPET 50-50 rPP/rPET 75-25 rPP/rPET SEBS rPP/rPET SEBS-MA

Melting

Crystallization

% Crystallinity

Tm (ºC)

ΔHm (J/g)

Tc (°C)

ΔHc (J/g)

ΔHcc(J/g)

rPET

rPP

248.5 169.9 252.3 167.9 161.5 163.5 165.6 162.4 161.4 164.1/249.3 164.6/250.5 164.7/250.4 165.5/253.3 162.3/247.9

47.8 94.6 45.9 67.6 7.1 21.2 37.2 27.5 34.9 8.9/36.3 33.4/24.3 23.3/11.3 19.4/19.3 33.5/19.8

188.2 110.7 198.8 121.8 107.1 117.1 119.4 113.1 114.4 113.1/207.9 116.3/200.1 118.3/199.1 112.2/198.3 112.8/194.2

46.5 92.0 43.8 81.7 4.4 29.3 58.6 42.7 43.7 25.4/32.0 41.4/19.9 82.6/10.9 24.9/15.8 39.8/16.1

—— —— 27.9 —— —— —— —— —— —— 20.4 10.6 —— 5.2 0.8

23.0 —— 12.9 —— —— —— —— —— —— 8.5 4.9 2.0 5.0 6.8

—— 45.7 —— 32.7 0.9 5.1 13.5 6.6 8.4 4.4 8.1 8.4 4.7 8.1

chain mobility [46]. The Tc of rPP in the rPP/rPS blends exhibited significantly more response to the blend formulation, resulting in a 2.4 °C decrease in the 75% PP blend to a 15 °C drop in the 25% rPP blend. Tc was lower with the compatibilizers compared to the 50–50 blend, and crystallinity was increased possibly due to plasticization from the copolymers, as discussed above. The crystallization temperature of rPP was also reduced by ca. 7–10 °C in the 25–75 rPP/rPET blend and all blends with either compatibilizer. The rPP crystallization peak had a small shoulder in most cases due to the contribution of crystallization from the LDPE impurity. Relative crystallinity for each polymer was reduced in the blends, particularly when the polymer was the minor component (25%). In general, the crystallinity of recycled polymers increases due to the reduction in molecular weight and chain entanglements from the repeated thermal processing cycles [47,48]. However for polymer blends, Itim et al. found a similar trend of decreased PET crystallinity in multiple extrusion cycles of PET bottles contaminated with PP caps [49]. The PP acted like a nucleation agent, reducing the interfacial free energy required for crystallization. Relative crystallinity tracks well with relative mass fractions for the blends, with the exception of the 75-25 rPP/rPET blend in which only the rPET crystallinity tracked well. The addition of SEBS appears to increase crystallinity over the uncompatibilized 50-50 blends, and compatibilization with SEBS-MA led to a further increase. Kuzmanović et al. found that PP crystallite size was reduced in PET blends due to a nucleation effect of PET, although this nucleation effect was not observed in our studies [50]. It is possible that the isolated or unreacted SEBS particles acted as nucleation sites increasing crystallinity [46,50]. The glass transition temperature (Tg) was determined using DSC and the apex of the tan delta peak from dynamic mechanical analysis (DMA). As expected, DMA results show higher glass transition temperature values compared to DSC, although the trends are generally similar. Results for the Tgs of rPET and rPS are reported in Table 2. (The Tg of polypropylene was nearly imperceptible and therefore was not included in the results). The Tg of rPET calculated from the tan delta peak was minimally impacted for the 50-50 and 75-25 rPP/rPET blends, but increased by 9 °C for the blends with SEBS and SEBS-MA. Trends in the Tg change for the 25–75 rPP/rPET blends are in disagreement between the DSC and DMA results, with DSC indicating a ca. 4 °C decrease, and DMA a ca. 4 °C increase. The Tg of rPS increased for all the blends with rPP by ca. 6–12 °C. This increase could be attributed to molecular confinement of PS’s amorphous domains by PP’s crystalline domains since rPS’s Tg is near rPP’s crystallization temperature [51]. The 75-25 rPP/rPS blend appeared to have the greatest effect on the Tg of rPS, which agrees with the above rationale. DMA was also used to investigate the viscoelastic properties and relaxations in the polymers. Plots of the tan delta, or the relationship between the elastic and viscous response, are displayed in Fig. 4.

Table 2 Glass transition temperatures of FFF printed recycled polymers. Sample

rPET rPS rPP/rPSa 25-75 rPP/rPSa 50-50 rPP/rPSa 75-25 rPP/rPS SEBSa rPP/rPS SEBS-MAa rPP/rPET 25-75 rPP/rPET 50-50 rPP/rPET 75-25 rPP/rPET SEBS rPP/rPET SEBS-MA a

Glass Transition (ºC) DSC

Tan δ

72.5 94.8 100.2 103.4 106.2 103.4 104.7 68.1 73.2 69.9 77.8 80.0

83.5 104.0 110.6 114.5 114.1 110.9 111.3 87.9 83.7 84.7 92.5 92.5

Tg reported for rPS.

Generally high values correspond to viscous behavior while low values to elastic behavior. The value of the neat rPET peak and blend with 25% rPP was nearly identical at 0.36 and 0.37, respectively. The height of the peak dropped to 0.20 in the 50-50 blend and further to ca. 0.11 for the 75% rPP and blends with compatibilizer. Thus, the tan delta values decreased with increasing rPP content, reflecting the viscous nature of the PET and elastic behavior of blend [42]. The value of the neat rPS tan delta peak was significantly higher than rPET at 1.73 due to the amorphous nature of the polymer. The 25–75 rPP/rPS blend had a similar tan delta value (1.69), but the value was reduced to 0.93 for the 50-50 blend. Further addition of rPP (75-25 blend) or compatibilizer reduced the value to 0.33. The addition of rPET, rPS and SEBS compatibilizers changed the ability of rPP to store energy, which is reflected by changes in the storage modulus (Fig. 5). Neat rPET had a storage modulus of 1433 MPa at 40 °C, while neat rPP’s storage modulus was much lower (798 MPa). There was little change in the storage modulus with the addition of 50% rPET (747 MPa), while 75% rPET served to slightly reduce the modulus (667 MPa). This reduction in the modulus with 75% PET seems counterintuitive particularly since the crystallinity is increased, but Yoon et al. found similar results in blend systems with 50–90% PET and attributed it to poor adhesion between PET and PP. The largest changes came from the addition of 25% rPET (870 MPa) and for the rPP/rPET blend with SEBs-MA (980 MPa). In the former system, the Tc for rPP and rPET were closest together of all the blends and the Tg of rPET was lower than most of the blends, potentially an indication of increased miscibility. For the rPP/rPET SEBS-MA, the total crystallinity was highest compared to the other rPP/rPET blends, which served to 126

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Fig. 4. Tan delta of FFF printed recycled polymer blends determined by dynamic mechanical analysis. (A) Recycled polypropylene/polyethylene terephthalate blends, (B) recycled polypropylene/polystyrene blends. Select blends were compatibilized with 5 wt. % unfunctionalized (SEBS) or maleic anhydride functionalized SEBS (SEBS-MA).

increase the stiffness. However, all values are significantly less than neat rPET. This is due to the ductile nature of rPP and the SEBS copolymers, which led to a reduced stiffness in the blends and weak mechanical interlocking between the immiscible phases. The reduction in modulus could also be related to the phase connectivity, in which PP domains are connected forming the matrix phase and hence modulus is closer to that of PP. The storage modulus behavior for the rPP/rPS blends generally followed the rule of mixtures law, with modulus reduced in proportion to amount of rPP added. The addition of 25% rPP to rPS reduced the storage modulus from 1739 MPa to 1459 MPa. Further additions of 50 and 75% rPP reduced the modulus to 1245 and 1040 MPa, respectively. SEBS and SEBS-MA additions in the 50-50 blend yielded the lowest storage moduli of ca. 830 MPa, similar to neat rPP. This may be due to the rubbery nature of the SEBS elastomers or PP phase connectivity as discussed above.

the highest tensile strength, nearly 5 times that of polypropylene and polystyrene materials. The rPET bottles had yielding followed by strain hardening, with substantial strain before failure. In comparison, the injection molded rPET failed at much lower strain value. This may be due to the additional extrusion cycle and molecular weight reduction due to moisture uptake or loss of chain orientation from the blow molding process. Fig. 6B displays representative stress-strain curves for 3D printed tensile bars. Average values for the ultimate tensile strength of each material are displayed in Fig. 7. The tensile strength of printed rPET was nearly twice that of rPS and rPP (35.1 ± 8 vs. 19.9 ± 3.9 and 20.1 ± 2.3 MPa, n = 4). All of the blends had reduced tensile strengths compared to the neat rPET, which was expected due to the weaker mechanical properties of rPP and immiscibility of the blends. The differences between the neat rPP and rPS and the blends were not significant except for the rPP/rPET SEBS-MA which was significantly higher than neat rPP. SEBS-MA likely can interact with the PET chains, accounting for the increased tensile strength. But SEBS copolymers are elastomers with low tensile strength, and are typically used to improve impact rather than tensile strength. The rPP/rPET 50-50 blend had the lowest tensile strength of all the blend formulations evaluated at 17.2 ± 3.6 MPa, significantly lower than the compatibilized rPP/rPET blends (23.1 ± 1.1 MPa rPP/rPET SEBS, 24.2 ± 1.3 rPP/rPET SEBSMA). Based on the fracture surfaces (Fig. 8), the 50-50 blend had the weakest interfacial adhesion. Inoyen et al. attributed the weak tensile properties of a 50-50 virgin PP/PET blend to the phase separation, with the voids acting as stress concentration regions. The tensile strength of

3.4. Mechanical testing To get a better understanding of different recycled polymer feedstocks and the best properties to expect from such materials, mechanical testing was performed. Tensile dogbones were cut using a die out of soda bottles (rPET) and yogurt containers (rPP), and the shredded polymers were injection molded. (Polystyrene materials were too brittle to punch out). Representative stress-strain curves are displayed in Fig. 6A. Table 3 reports the peak stress and strain for the die-cut, injection molded and 3D printed materials. The soda bottles (die-cut) had

Fig. 5. Storage moduli of FFF printed recycled polymer blends determined by dynamic mechanical analysis of recycled polymers. (A) Recycled polypropylene/ polyethylene terephthalate blends, (B) recycled polypropylene/polystyrene blends. Select blends were compatibilized with 5 wt. % unfunctionalized (SEBS) or maleic anhydride functionalized SEBS (SEBS-MA). 127

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Fig. 6. Representative stress-strain curves of recycled polymers. (A) Die-cut and injection molded, (B) FFF printed. All blends were 50-50 by weight. Select blends were compatibilized with 5 wt. % unfunctionalized (SEBS) or maleic anhydride functionalized SEBS (SEBS-MA).

elongation to failure compared to injection molded samples (60 and 18% compared to 14 and 10%, respectively). Printed rPS was extremely brittle with elongation to failure less than 1% compared to 14% for injection molded. Interestingly, the elongation to failure was significantly increased for the uncompatibilized rPP/rPET blend (47%), potentially due to the weak interface and the ability of the rPET tubes to pull out from the matrix (Fig. 8). The elongation to failure was reduced to 3–4% with the addition of the SEBS copolymers, which served to increase interfacial bonding but also reduce pull out of the PET tubes or spheres. All of the rPS blends exhibited an increase in elongation to failure due to the addition of the ductile rPP phase (9–21%). The largest increase was observed for the rPP/rPS SEBS blend, likely due to the improved interaction between polymer phases. As discussed above, the rPS is brittle and doesn’t undergo any significant elongation before breaking. Thus unlike in the case for the more ductile rPET, strong interfacial adhesion with rPP improves the elongation for rPP/rPS blends. Fractured 3D printed tensile bar cross-sections are displayed in Fig. 8. Recycled polypropylene exhibited ductile fracture, and there was evidence of fibril deformation and stretching (Fig. 8A). Recycled polystyrene and PET cross-sections showed generally brittle fracture, although some tear fractures generated during deformation of the polymer are visible (Fig. 8B, C). The rPP/rPET 50-50 blend has a cylinder or fractured sphere in matrix morphology with the fracture generally brittle in nature, but there is evidence of cylinder/sphere pullout and regions of ductile failure like those seen in neat rPP (Fig. 8D). Macroscopically, this sample showed ductile behavior with incomplete breakage in the tensile test. The adhesion of the two phases is generally poor, with some regions of gaps between the cylinders/spheres and the matrix. Cylinder/sphere diameters were not uniform in size and ranged from 2 to 5 μm. To determine composition of the dispersed and matrix phases, solvent extraction in which the rPET and rPS phases were removed from the rPP/rPET and rPP/rPS 50-50 blends was done (Fig. 9). After extraction and rinsing, the matrix for both materials is intact with holes where the rPET and rPS once were. Thus, for these compositions, the rPET and rPS are represented in the dispersed or cylinder/sphere phase while the matrix is composed of rPP, corroborating moduli values discussed above. The volume fraction for the rPP/rPS 50-50 blend was 1.1, and thus well matched to the feed weight fraction. However, the rPP/rPET 50-50 blend volume fraction was 1.4, which agrees well with Fig. 9A showing a greater percentage of the volume in the matrix or PP phase. The rPP/rPET 50-50 blend compatibilized with 5 wt. % SEBS had larger cylinders/spheres on the order of 10 μm in addition to 1–2 μm sized cylinders/spheres (Fig. 8E). There is evidence of the SEBS particles at the interface between phases and matrix adhesion is somewhat improved. The presence of phase-separated SEBS particles could indicate that 5 wt. % SEBS exceeds the useful threshold, and

Table 3 Peak stress and strain of recycled polymers. Sample

Peak Stress (MPa)

Peak Strain (mm/mm)

rPP Die-cut rPET Die-cut rPP Injection Molded rPS Injection Molded rPET Injection Molded rPP 3D Printed rPS 3D Printed rPET 3D Printed rPP-rPET rPP-rPET SEBS rPP-rPET SEBS-MA rPP-rPS rPP-rPS SEBS rPP-rPS SEBS-MA

25.9 ± 6 103.2 ± 23 34.5 ± 1 32.2 ± 9 67.8 ± 2 20.1 ± 2 19.9 ± 4 35.1 ± 8 17.2 ± 4 23.1 ± 1 24.2 ± 1 23.0 ± 1 19.0 ± 3 22.9 ± 1

0.09 ± 0.04 0.5 ± 0.06 0.07 ± 0.01 0.008 ± 0.002 0.03 ± 0.002 0.2 ± 0.02 0.007 ± 0.003 0.02 ± 0.009 0.4 ± 0.2 0.03 ± 0.008 0.04 ± 0.0006 0.1 ± 0.05 0.2 ± 0.09 0.1 ± 0.1

Fig. 7. Ultimate tensile strength of FFF printed recycled polymers. All blends were 50-50 by weight. Select blends were compatibilized with 5 wt. % unfunctionalized (SEBS) or maleic anhydride functionalized SEBS (SEBS-MA).

the rPS/rPP blends were all fairly similar, ranging from 19.0 ± 3.4 to 22.9 ± 1.4 MPa for the SEBS and SEBS-MA compatibilized to the uncompatibilized. Based on the rheological data, there was less of an interaction between the phases even with the addition of the SEBS copolymers. While the SEBS-MA was not expected to interact strongly with the PS, the unfunctionalized SEBS with its styrene groups should have formed a close interaction. Potentially, the effect on the impact strength would be more apparent. Recycled polypropylene and PET printed samples had a longer 128

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Fig. 8. Scanning electron microscopy images of FFF printed recycled polymer fracture surfaces. (A) Recycled PP, (B) recycled PS, (C) recycled PET, (D) rPP/rPET 5050, (E) rPP/rPET SEBS, (F) rPP/rPET SEBS-MA, (G) rPP/rPS 50-50, (H) rPP/rPS SEBS, (I) rPP/rPS SEBS-MA. Scale bar denotes 10 μm.

lower amounts may be more effective. The rPP/rPET 50-50 blend compatibilized with 5 wt. % SEBS-MA has similar cylinder/sphere sizes to the uncompatibilized blend, but there is a marked improvement in the matrix adhesion, with almost no gaps between phases perceptible (Fig. 8F). The rPP/rPS blends have smaller cylinders/spheres compared to the rPP/rPET blends, which are in the size range of 0.5 to 1.5 μm (Fig. 8G). There are many voids present in the sample and the gaps between the matrix and dispersed phase due to the poor adhesion. The size of the dispersed phase is significantly reduced with the addition of SEBS (0.37 ± 0.1, n = 40) and further with SEBS-MA (0.30 ± 0.1, n = 40). Neither appear to have the same sized voids as in the uncompatibilized blend, but there is still evidence of gaps between the dispersed phase and the matrix (Fig. 8H, I).

4. Summary and conclusions Blending of polymers can be a cost-effective way to reduce plastic waste and increase re-use of these materials. Blends of recycled polypropylene with PET or polystyrene represent novel and viable feedstocks for FFF 3D printing, with tensile strengths comparable to some lower-end common commercial filaments such as HIPS. While compatibilization with SEBS elastomers did not greatly increase tensile strength, morphological analysis showed improved bonding between phases. In addition, compatibilization led to an increase in the glass transition temperature, increasing the performance window of the materials. Recycled polypropylene blend filaments could be used in distributed manufacturing, in which 3D printed parts are made at or

Fig. 9. Scanning electron microscopy images of FFF printed recycled polymer fracture surfaces with disperse phase extracted with solvent. (A) rPP/rPET 50-50, (B) rPP/rPS 50-50. Scale bar denotes 50 μm. 129

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near the point of need using locally available materials. In addition to reduction in cost and carbon footprint, the ability to manufacture parts as needed could be life-changing chiefly for those in rural and isolated locations. Future work will involve optimization of the compatibilization of the polymers through alternative processing methods such as solid state shear pulverization to improve miscibility in the blend. In addition, research will be conducted into the incorporation of a reinforcing agent such as glass or cellulose fibers to increase mechanical properties to match or exceed mid-grade polymers used in additive manufacturing such as ABS.

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