Mixed post-consumer recycled polyolefins as a property tuning material for virgin polypropylene

Journal of Cleaner Production 239 (2019) 117978

Contents lists available at ScienceDirect

Journal of Cleaner Production journal homepage: www.elsevier.com/locate/jclepro

Mixed post-consumer recycled polyolefins as a property tuning material for virgin polypropylene Greg W. Curtzwiler a, b, c, *, Matthew Schweitzer a, e, Yifan Li d, Shan Jiang a, d, Keith L. Vorst a, b, c a

Polymer and Food Protection Consortium, Iowa State University, 536 Farmhouse Lane, Ames, IA, 50011, USA Department of Food Science and Human Nutrition, Iowa State University, 536 Farmhouse Lane, Ames, IA, 50011, USA Ideopak, LLC, 1568 Food Sciences Building, 536 Farmhouse Lane, Ames, IA, 50011, USA d Department of Materials Science and Engineering, Iowa State University, 528 Bissell Road, Ames, IA, 50011, USA e Department of Chemical and Biological Engineering, Iowa State University, 618 Bissell Rd, Ames, IA, 50011, USA b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 19 April 2019 Received in revised form 23 July 2019 Accepted 8 August 2019 Available online 12 August 2019

Polyethylene and polypropylene are widely used packaging materials that are commonly present in the same waste-streams (e.g., beverage bottle enclosures, laminated flexible packaging). Since these immiscible plastics are difficult to separate with current recycling practices, it is critical to understand the influence of mixed polyolefin post-consumer recycled feedstock composition on key performance properties of recycled blends for widespread industrial adoption. This research comprehensively characterized the optical, thermal, mechanical, morphological, and gas barrier properties of melt blended virgin polypropylene/mixed post-consumer recycled polyolefin materials at different blend ratios. The results strongly indicate that polyolefin beverage container enclosures can be melt blended with virgin polypropylene to tune physical performance properties (74% increased yield stress, 49% increased strain at yield, 160% increased UV blocking, 30e40% gas transmission reduction) and simultaneously reduce environmental contamination. Most measured properties followed the Law of Mixtures which enables highly predictable and tunable properties via precision melt blending of post-consumer recycled and virgin plastic. We hypothesize that increased compatibilization of polyolefin amorphous regions from plastic oxidation and structural changes in the plastic crystalline domains directly influence plastic physical properties. These data fill current knowledge gaps that post-consumer materials can provide value beyond sustainability to justify innovative landfill diversion strategies and realize a more circular economy. © 2019 Elsevier Ltd. All rights reserved.

Handling Editor: Baoshan Huang Keywords: Recycling Post-consumer recycled Polyolefin Polypropylene Polyethylene Plastic processing

1. Introduction As global demand for consumer goods expands, the production of virgin/first pass plastic feedstocks increases pressure on the global environment from natural resource consumption and environmental accumulation. An estimated 83,000 million metric tons of virgin plastics have been produced to date of which only 21% have been reclaimed for recycling or incineration (Geyer et al., 2017). Plastic packaging comprises the largest market of plastics and is currently one the largest contributors to solid waste

* Corresponding author. Department of Food Science and Human Nutrition, Iowa State University, 536 Farmhouse Lane, Ames, IA, 50011, USA. E-mail address: [email protected] (G.W. Curtzwiler). https://doi.org/10.1016/j.jclepro.2019.117978 0959-6526/© 2019 Elsevier Ltd. All rights reserved.

generation in the United States in landfills and marine environments (Geyer et al., 2017). In the United States (2015), 30% of the total generated municipal solid waste (238 million metric tons) was comprised of containers and packages (Anonymous, 2018a). As most common plastics utilized in packaging do not degrade in the environment, widespread contamination of the Earth's lands and oceans (4e12 million metric tons in 2010 alone) (Jambeck et al., 2015) will increase without commercially available alternatives. As a result, there is domestic and international urgency to utilize sustainable feedstocks and divert traditional plastic waste from landfills and marine environments through recycling and natural degradation (biodegradation, composting, marine degradation) mechanisms (Xanthos, 2012). This places a heavy burden on brand owners and manufacturers to utilize sustainably sourced feedstocks and consider end-of-life practices as packaging has the

2

G.W. Curtzwiler et al. / Journal of Cleaner Production 239 (2019) 117978

shortest service lifetime of all plastic products (Geyer et al., 2017). Recently available data (July 2018) from the Environmental Protection Agency indicated a decrease in the recycling rate from 9.5% to 9.1% between 2014 and 2015. In addition, the overall mass of plastic materials recycled also decreased: 2.85 million metric tons were recycled in 2015 compared to 2.88 million metric tons in 2014 resulting in 23.6 million metric tons of plastic entering landfills (Anonymous, 2018a). Although, the recycling rates of polypropylene are third to poly(ethylene terephthalate) (PET) and highdensity polyethylene (HDPE), 16 thousand metric tons of polypropylene (PE) were recycled in 2010 (Xanthos, 2012) which presents a significant source of plastic feedstock for a variety of applications. Polypropylene plastic is widely used in several industries, including, food (yogurt) packaging, automotive bumpers and battery cases, toothbrushes, and carpet materials (Xanthos, 2012). The wide variety of uses is attributed its low density, good thermal and chemical stability, stiffness, and low cost (Xanthos, 2012). One challenge in recycling polypropylene is thermal degradation during melt processing which is commonly mitigated by incorporating various stabilizers into the base polymer (Wypych, 2008; Xanthos, 2012). The initial degradation mechanism of PP is analogous to PE, i.e., oxygen absorption and attack of the polymer backbone followed by increased oxygenated functional groups, chain scission, and subsequent methyl ketones (Wypych, 2008). Furthermore, as with PE, the dominant degradation products are dependent upon the concentration and composition of residual polymerization catalysts and metal additives (Goss et al., 2003). However, oxygen absorption by polypropylene is significantly faster than other common plastics such as polyethylenes, polyamides, and polyesters thereby increasing the potential for additional degradation reactions (Wypych, 2008). The formation of hydroperoxides is considerably faster for PP than PE due to the stabilizing tertiary carbon in the PP molecular structure (Wypych, 2008; Xanthos, 2012). Therefore, it is expected that PP in postconsumer recycled polyolefin would demonstrate decreased thermal stability due to a higher concentration of oxygenated functional groups and lower molecular weight (from chain scission events) (Wypych, 2008). Another challenge for recycling is that PE and PP cannot be easily separated for recycling. The global flexible packaging market was valued at over 221,000 million USD in 2016 with plastics possessing approximately 80% of the market share (Anonymous, 2018b). It is well known that separation of laminated flexible structures (e.g., food packaging) for recycling is not economically viable, which results in excessive amounts of post-industrial and post-consumer recycled food packaging being sent to landfills. Although mixed material food packaging, such as PET water bottles and their polyolefin caps, are immiscible and are considered contaminants in their respective recycling streams, there are methods to sort the materials after the flaking/grinding process. However, laminated and flaked/ground structures that contain PE and PP cannot be easily separated. To address the challenges, the current study melt-blended virgin polypropylene with mixed stream recycled polyolefin packaging plastic. The objective of this research was to evaluate performance changes in melt blended post-consumer recycled polyolefin/virgin polypropylene. Specifically, the mechanical, optical, thermal, and gas barrier properties of different virgin polypropylene/postconsumer recycled polyolefin blend concentrations were investigated. Since polyolefin enclosure systems can be efficiently separated from PET bottles polymer during the recycling process using current technologies, a constant supply of post-consumer mixed polyolefin material can be collected. This creates a source of relatively consistent post-consumer polyolefin materials with similar

properties due to collection from the same application and waste stream (i.e., bottle enclosure and safety tear ring). The postconsumer polyolefin material was utilized as received from a commercial supplier without further pretreatment. Results of this study demonstrated increased performance and tunable material properties via precision melt blending establishing the potential for utilizing post-consumer recycled mixed polyolefin as a property tuning material in virgin polypropylene. The ability to tune physical properties utilizing waste-diverted materials is critical across multiple industries including food packaging, automotive, civil engineering, and radiation shielding (Curtzwiler and Vorst, 2018; Curtzwiler et al., 2017; Mahmoud et al., 2018a, b; Mahmoud et al., 2019; Singh et al., 2017). The increased value for mixed stream and contaminated postconsumer recycled polyolefin materials is anticipated to have a broad impact in recycling practice and promote increased sustainability efforts through waste stream diversion and overall reduction in global contamination. This study enables new approaches in recycling mixed stream polyolefin beverage containers, enclosure systems, and laminated flexible packaging to achieve a more circular economy. 2. Materials and methods 2.1. Fabrication of test specimens Post-consumer recycled polyolefin (PCRPO) material was received from a commercial domestic supplier of post-consumer recycled plastics which identified the source as beverage bottle enclosures. The composition was estimated to be 87.5% postconsumer recycled polypropylene and 12.5% post-consumer recycled polyethylene (see section 3.7). The PCRPO was mixed with virgin polypropylene (VPP) by hand to obtain post-consumer recycled polyolefin: virgin polypropylene (PCRPO:VPP) weight ratios of 0%, 20%, 40%, 60%, 80%, and 100% (Scheme 1). Each blend was extruded with a Wayne laboratory scale single screw (20 mm; L/D: 24:1) extruder (Wayne Extrusion; Totowa, NJ) using three-barrel heat zones, a coupler heat zone, and thermally controlled two zone coat hanger style die. The barrel heat zones, coupler heat zone, and die temperatures were set to 193
C, 204
C, and 204
C, respectively. The molten extrudate was collected on temperaturecontrolled J stack style rollers with a take up film winder. Sample thickness were between ~254 and 762 mm (10e30 mil). Each hand mixed blend was injection molded on a Boy 22-ton injection molder (Boy Model 22S; Boy Machine Inc., Exton, PA). The injection profile was 220e240
C, with a water-cooled ASTM D63814 injection molded Type I dog bone at 30e40
C (ASTM, 2014). 2.2. Thermogravimetric analysis The mixing efficiency and composition validation was quantified via investigation of the residual ash content. The postconsumer recycled polyolefin material possessed significantly higher residual ash content than the virgin polypropylene (see section 3.1), therefore, accurate and efficient mixing would be verified via regression analysis of residual ash content as a function of post-consumer recycled polyolefin content according to the Law of Mixtures (LOM) (Gooch, 2010). Thermal degradation properties (temperature at 5% mass loss and corresponding activation energy) of each PCRPO:VPP blend were quantified via modulated thermogravimetric analysis using a TA Instruments Q5000IR thermogravimetric analyzer (New Castle, DE) (Blaine and Hahn, 1998). Samples (5e10 mg) were loaded into a platinum pan then heated at 2
C/min with continuous modulation (amplitude ¼ ± 5
C; period ¼ 200 s) under a nitrogen atmosphere. The activation energy

G.W. Curtzwiler et al. / Journal of Cleaner Production 239 (2019) 117978

3

Scheme 1. Work flow process to fabricate extruded sheet and injection molded test specimens.

of degradation was determined at 5% mass loss according to ASTM E1641-16 (ASTM, 2016). 2.3. Melt flow index The melt flow index was determined for each virgin polypropylene/post-consumer recycled polyolefin blend according to Procedure A in ASTM D1238 e 13 (ASTM, 2013b) following the protocol for polypropylene (230
C; 2.16 Kg). The mass extruded in 5 min was recorded three times, converted into grams per 10 min, and the averages reported. 2.4. Electromechanical testing Mechanical properties were evaluated in tensile mode using a Shimadzu Autograph AGS-J (Shimadzu Corp.; Kyoto, Japan) electromechanical tester equipped with rigid clamps and a 5 kN load cell. All property evaluations were completed according to ASTM D638-14 with a gauge length of 50 mm and a grip separation rate of 50 mm/min (ASTM, 2014). Each blend was evaluated with ten individual specimens. 2.5. Gas permeation Oxygen and water vapor permeation properties were investigated according to ASTM D3985-17 and ASTM F1307-14, respectively (ASTM, 2013c, 2017). The thickness of each sample was recorded using a Mitutoyo IP 65 electronic digital micrometer (Kawasaki, Japan). For water vapor permeation, a Mocon W33/3 water vapor permeation instrument (Brooklyn Park, MN) was utilized with relative humidity and temperature setpoints of 90% RH and 38.7
C, respectively. The nitrogen flow rates for water vapor permeation analysis was 50 mL/min. For oxygen permeation, a Mocon OxTran 2/21 (Brooklyn Park, MN) was utilized employing a 10% certified oxygen in nitrogen concentration to avoid saturating the Coulox sensor from the low oxygen barrier properties associated with polypropylene materials. The flow rate of the nitrogen carrier gas and certified oxygen gas was 10 mL/min and 20 mL/min, respectively. Four specimens per material blend were evaluated for each test gas. 2.6. Ultravioletevisible spectroscopy Specimens were fitted into a clamp measuring 12.7 cm  8.5 cm x 2.3 cm and the absorbance spectra collected between 230 nm and 900 nm (Ultravioletevisible-Short Wave Near Infrared) using a

Tecan Safire microplate reader (Zurich, Switzerland). Specimen thicknesses were measured with a Mitutoyo IP 65 electronic digital micrometer (Kawasaki, Japan). Each absorbance value was divided by the thickness to account for thickness variations as noted previously (Curtzwiler et al., 2017). The UV absorption potential was calculated as described previously (Curtzwiler et al., 2017, 2018a) using the trapezoidal rule between 300 and 400 nm. 2.7. Fourier transform infrared spectroscopy Transmission Fourier Transform Infrared Spectroscopy (FTIR) spectra were collected with a Nicolet 6700 Infrared spectrometer at ambient temperature (DTGS detector, 32 scans, resolution of 2 cm1). All spectra were normalized to the thickness of the scanned region. The peak area of characteristic bands associated with carbonyl, nitrogen oxide, and vinylidene functional groups were quantified for each transmission spectrum by integrating the area under the thickness normalized intensity spectrum (Equations (1)e(3)) utilizing Omnic 8.3 software (Thermo Fisher, Waltham, MA) (ASTM, 2013a).

Carbonyl Area ¼

XA 1740cm1 t

Nitrogen Oxide Area ¼

Vinylidene Area ¼

XA 1555cm1 t

XA 875cm1 t

1

2

3

2.8. Differential scanning calorimetry Thermal transitions of each PCRPO:VPP blend were measured between 50
C and 210
C utilizing a heat/cool/heat protocol at a rate of 10
C/minute with a TA Instruments Q2000 differential scanning calorimeter (New Castle, DE) in a nitrogen atmosphere. A specimen of each blend (3e6 mg) was singularly loaded into a hermetically sealed T-zero DSC pan and crimped prior to analysis. 2.9. Inductively coupled plasma e optical emission spectroscopy procedure Three samples of 0.1500 ± 0.0005 g were taken from each virgin polypropylene/post-consumer recycled polyolefin blend. Samples

4

G.W. Curtzwiler et al. / Journal of Cleaner Production 239 (2019) 117978

were digested via microwave digestion (Milestone UltraWave) in 5 mL HNO3 (Thermo Scientific 67% v/v Trace Metal Grade) and 1 mL HCl (Thermo Scientific 34% v/v Trace Metal Grade). The digestion cycle is described in Table 1 with an initial pressure of 45 bar. This cycle was based on the polyurethane digestion protocol used by Dillner et al. (2007) and expanded for use in polypropylene by Upadhyay et al. (2009). Digested samples were diluted to 50 mL in a volumetric flask and filtered to remove undigested fillers and the filtrate mass measured. Inductively coupled plasma e optical emission spectroscopy (ICP-OES; Thermo Scientific iCap-7400 Duo) was used to determine inorganic element concentrations in each sample, using measurements of at least two wavelengths for each metal to increase the confidence of the measurement. Standards containing 1 mg/L, 5 mg/ L, 25 mg/L, 50 mg/L, and 100 mg/L of each metal of interest were used for calibration with a 5 mg/L yttrium internal standard. For each wavelength measured, the average metal concentration and standard deviation was determined by percent PCRPO was determined for the three repeated measures. 2.10. Scanning electron microscopy procedure Specimens were sectioned from injection molded parts then cryo-fractured to preserve microstructures. The polymer morphology was investigated by scanning electron microscopy operated at 10 kV (FEI Quanta-250 SEM, USA). Prior to the SEM studies, the cross sections of samples were coated with a thin iridium layer of 5 nm to prevent/minimize charging on the surface. Pre-tilted sample stage was used to hold the samples vertically under the electron beam, so the cross-sectional images could be obtained. 2.11. Statistical analysis Investigations for linear relationships and the statistical significance of trendline slopes between data sets were performed by calculating the Pearson product moment correlation coefficient (PPMCC) and linear regression analysis, respectively. Data sets containing more than two statistically equivalent values were not analyzed for trendline fits. Minitab 17 software (State College, PA) was used for statistical analysis utilizing a 95% confidence level (a ¼ 0.05) (Ellison et al., 2009). A one-way ANOVA utilizing a 95% confidence level was utilized to statistically evaluate mean values and grouped via Tukey's post-hoc analysis. Five repeated measures were utilized for all analyses unless otherwise noted.

is known to occur for material blends containing post-consumer recycled plastics that possess low bulk densities (e.g., beverage and shampoo bottles). The ash content was utilized to determine the overall mixing efficiency as the post-consumer recycled polyolefin utilized in this study possessed a significantly higher ash content compared to the virgin polypropylene (Fig. 1). Therefore, a linear increase in ash content is expected from melt blending the two materials according to the LOM (Gooch, 2010), which was observed for the blends in this study (R2 ¼ 0.9955; Fig. 1). These data indicate a high level of precision and mixing efficiency was achieved and that the observed trends presented herein can be attributed to differences in blend composition. Thermogravimetric analysis was used to determine thermal stability properties in a nitrogen atmosphere. A nitrogen atmosphere was selected over an oxygenated atmosphere as differences in thermal stability from the presence of oxygenated moieties (e.g., alcohols, ketones, hydroperoxides) and chain scission events would be more pronounced as further oxygen attack would be limited to oxygen impurities in the nitrogen purge gas. Generally, incorporation of post-consumer recycled polyolefin reduced the temperature associated with the 5% mass loss, which is considered the thermal degradation temperature according to ASTM E1641-16 (Fig. S1) (ASTM, 2016). The activation energy of thermal decomposition was determined according to the work of Blaine et al. and ASTM E1641-16 (5% mass loss) (ASTM, 2016; Blaine and Hahn, 1998). Interestingly, there was no clear trend in the activation energy although there was a general trend of decreasing thermal stability (Fig. S2). 3.2. Melt flow index The melt flow index of each virgin polypropylene/postconsumer recycled polyolefin blend was used to quantify changes in rheological properties as a function of PCRPO content. The melt flow rates calculated herein are comparable to those previously reported (ASTM, 2013b). The chain scission events known to occur

3. Results and discussion 3.1. Thermogravimetric analysis To ensure that measurable property changes in this study are attributed to post-consumer recycled polyolefin composition, the mixing efficiency and blend composition was verified. Separation and stratification in the hopper of extruders and injection molders

Fig. 1. Ash content of virgin polypropylene/post-consumer recycled polyolefin blends. Data points associated with the same letter are statistically equivalent.

Table 1 Summary of digestion cycle parameters. Step

Step time (min)

Max. Microwave Energy (W)

Chamber Temperature (
C)

1 2 3 4 5 6

10 4 5 6 4 25

1500 1200 1200 1200 1000 1000

140 140 165 165 185 185

G.W. Curtzwiler et al. / Journal of Cleaner Production 239 (2019) 117978

5

Fig. 2. Melt flow index for virgin polypropylene/post-consumer recycled polyolefin blends. Data points associated with the same letter are statistically equivalent.

from reprocessing and in-service use are anticipated to increase melt flow rates (reduce viscosity) due to increases in molecular weight distribution and lower chain molecular weight. The data indicated that incorporating post-consumer recycled polyolefin material into virgin polypropylene increased melt flow rates as expected (Fig. 2), however, all material blends possessing postconsumer recycled polyolefin were determined to be statistically equivalent. This suggests that all material blends containing postconsumer recycled polyolefin materials will have similar rheological processing parameters and that processing conditions will not require significant variations from virgin parameters. 3.3. Electromechanical testing The tensile modulus and yield stress values of the virgin polypropylene used in this work are comparable to values previously reported for virgin polypropylene (Brachet et al., 2008; Ghabeer et al., 2012; Mehrabzadeh and Farahmand, 2001). There was no statistical difference (p > 0.05) in the calculated tensile modulus until the blend composition was 80% post-consumer recycled polyolefin which represented a 14% increase (Fig. 3). The modulus for the 80% post-consumer recycled polyolefin blend was statistically equivalent to the 100% post-consumer recycled polyolefin material. The increased modulus determined in this work for postconsumer recycled polyolefin plastics contradicts trends measured in previous work for blending virgin HDPE with virgin PP (Lin et al., 2015) which demonstrated a decreasing modulus as a function of HPDE content. Although our system is more complicated, i.e., possessing three distinct polymer types (virgin polypropylene,

Fig. 3. Tensile modulus of virgin polypropylene/post-consumer recycled polyolefin blends. Data points associated with the same letter are statistically equivalent.

Fig. 4. Stress (a) and strain (b) at the yield point for virgin polypropylene/postconsumer recycled polyolefin plastic blends. Data points associated with the same letter are statistically equivalent.

post-consumer recycled polypropylene, and post-consumer recycled polyethylene), it is possible that oxygenated functional groups (see section 3.6) provide synergistic interactions between postconsumer recycled plastics potentially increasing intermolecular interactions. Although the modulus of blended materials did not follow the LOM, the yield point properties followed the LOM as demonstrated by linear correlations between the stress/strain properties and post-consumer recycled polyolefin concentration. Both the strain and stress properties at yield increased as a function of postconsumer recycled content resulting in increases of 49% and 74%, respectively (Fig. 4). Furthermore, the maximum stress was increased from 21 to 31.4 MPa (50% increase) and the strain at maximum stress increased from 10.8 to 18.8% (74% increase) when the blend composition was 100% PCRPO (Fig. S3). We hypothesize that three different mechanisms contribute to the measured changes in the mechanical properties: 1) variation in the PP feedstock mechanical properties collected for the post-consumer recycled polyolefin as noted by previously (Jmal et al., 2018) (i.e., different grades and additives); 2) increased compatibilization between the amorphous regions of PP and PE phases due to increased oxygenated functional groups (see section 3.6). Previous reports have demonstrated linear relationships between the carbonyl index and tensile strength and strain properties which is agreement with our data (Fig. S4) (Wypych, 2008). These observations support our hypothesis that post-consumer recycled plastics can be utilized as a property tuning material and property changes are, in part, attributed to degradation events that occur during consumer use and the recycling process;

6

G.W. Curtzwiler et al. / Journal of Cleaner Production 239 (2019) 117978

3) the presence of phase separated domains comprised of a softer PE material as determined by the presence of two distinct crystalline melting peaks (see section 3.7). These softer domains may, to a certain extent, provide similar property gains as softer impact modifiers (Pesetskii et al., 2011). Pure recycled polypropylene has been demonstrated to possess inferior fracture properties compared to virgin material (Brachet et al., 2008). Such trends would be expected to correspond to reduced toughness (energy of break) in electromechanical analysis as observed here (Fig. S5) and smoother fractures surfaces. Plastic degradation is well known to occur during in-service use and melt-processing (Li et al., 2018) which produces oxygenated functional groups (as observed via FTIR in section 3.6), but additionally alters the molecular weight and molecular weight distribution (Kamleitner et al., 2017). Such changes in molecular structure can significantly influence realized physical properties (mechanical, thermal, optical, melt flow) as demonstrated here. Indeed, the measured improved mechanical properties can enable manufacturers to thin-gauge certain parts providing both increased sustainability and cost-savings from reduced material usage. 3.4. Gas permeation Our previous work has demonstrated that many physical properties vary according to the LOM for post-consumer recycled/ virgin plastic blends (Curtzwiler et al., 2011, 2017, 2018a, 2018b). Therefore, we sought to determine if permeation properties followed the Law of Mixtures as there is a lack of public disclosure on the permeation properties of post-consumer recycled/virgin plastic blends. Our results indicate that 100% post-consumer recycled polyolefin possess increased barrier properties compared to the virgin polypropylene utilized in this work for both water vapor and oxygen permeants (Fig. 5). However, blends of post-consumer recycled comprised of 40% and 60% deviate from the Law of Mixtures as the PCRPO blends possessed statistically equivalent water vapor and oxygen permeation values as virgin polypropylene. Surprisingly, the 20% PCRPO blend was statistically lower than the virgin, 40%, and 60% PCRPO blends and statistically the same as the 80% PCRPO blend for both permeant gasses. These observations agree with the reduced porosity observed via scanning electron microscopy (see section 3.9). 3.5. Ultravioletevisible spectroscopy Polypropylene is commonly characterized as an opaque plastic and even “water clear” grades are visually less transparent as other plastics such as PET and LDPE. The opaqueness of PP is generally attributed to the crystalline domain size and the overall crystallinity (Pritchard, 1964). Since the optical performance is closely related to crystalline structures (shape and size), changes in crystalline melting temperatures of PE and PP from the presence of post-consumer recycled polyolefin (see section 3.7: DSC) will influence the ultravioletevisible absorbance spectra due to differences in scattering efficiencies. Consistent with our previous work in PET and HDPE, incorporation of post-consumer recycled polyolefin increased the thickness normalized absorbance throughout the measured spectral range except for the short-wave near infrared region, i.e., greater than 700 nm (Figs. 6 and S6) (Curtzwiler et al., 2011, 2017, 2018a, 2018b). The increased absorption is most noticeable at lower wavelengths, specifically in the ultraviolet region, which may be a result of increased light scattering from changing polymer crystalline domain sizes and phase separated domains and interfaces. We acknowledge the difference between absorption and light scattering mechanisms with respect

Fig. 5. Water vapor (a) and oxygen (b) permeation properties of virgin polypropylene/ post-consumer recycled polyolefin plastic blends. Data points associated with the same letter are statistically equivalent.

to light transmittance through an object, however, we do not specifically differentiate these mechanisms in this work. Future work aims to elucidate the contribution of each mechanism to changes in the measured spectra. Interestingly, a characteristic peak in the ultraviolet region for the 100% virgin polypropylene and 20% postconsumer recycled polyolefin blend was detected but diminishes as the heterogeneity of the plastic blend increases, i.e., post-consumer recycled polyolefin concentration of 40% and 60% (Fig. 6). The average thickness normalized ultravioletevisible absorption curve associated with the 100% virgin polypropylene plastic (0% PCRPO) was subtracted from spectra containing post-consumer recycled polyolefin plastic to specifically investigate trends in absorption potential throughout the spectrum (Fig. S6). There was a

Fig. 6. Representative thickness normalized absorbance polypropylene/post-consumer recycled polyolefin blends.

spectra

of

virgin

G.W. Curtzwiler et al. / Journal of Cleaner Production 239 (2019) 117978

noticeable peak in the subtracted absorption spectra ~670 nm which is of similar size as the solidified PE domain sizes of virgin PE/PP blends in the literature (Lin et al., 2015). However, it is important to note that the refractive index of PE and PP are both ~1.5 (Schael, 1964, 1968), thus, significant scattering from the presence of different materials is not anticipated and this peak is likely attributed to a different mechanism. Light scattering from moieties of ~670 nm would be most noticeable of radiation with a wavelength of ~2 mm, and, efficient scattering of light at 670 nm would possess a size of ~220 nm (Koleske, 2012). Interestingly, the thickness normalized absorption for the 20%, 40%, and 60% PCRPO blends were lower than virgin PP plastic for radiation in the shortwave near infrared region (>700 nm) (Workman and Lois, 2012). Our previous work demonstrated increased ultraviolet absorption potential for post-consumer recycled PET and HDPE plastics (Curtzwiler and Vorst, 2018; Curtzwiler et al., 2017, 2018a, 2018b) compared to virgin, first pass, plastic. These results have been reproducible for post-consumer recycled plastics melt-processed in our facilities as well as utilizing commercial processing equipment. Similar to PE and PET, incorporation of post-consumer recycled polyolefin with virgin polypropylene increased the UV absorption potential (Fig. 7) approximately 160% of the virgin plastic. Interestedly, the increased absorption properties deviated from the Law of Mixtures as there was a sharp increase in the absorption potential then remained statistically constant until the material was comprised of 100% post-consumer recycled polyolefin. It should be noted that there was increased variability in the measured UV absorption for both the 40% and 60% PCRPO blends (Fig. 7) compared to all other blend compositions. Elucidating the mechanism(s) of reduced light transmission (i.e., scattering versus absorption) will enable controlled tailoring of material properties by blend composition. One possibility is that the large amount of molecular heterogeneity due to the relative concentrations of virgin polypropylene and post-consumer recycled plastic influences the size and structure of crystalline domains, and therefore influences scattering and absorption properties. We are currently building and procuring instrumentation to determine the mechanism(s) of the measured light transmittance properties. The ultravioletevisible absorption properties are important in food packaging applications to 1) enable consumers to visually inspect products before making a purchase decision and 2) protect food from light-induced degradation reactions that can produce off-flavors and catalyze color changing reactions in retail display environments (Curtzwiler and Vorst, 2018; Curtzwiler et al., 2017, 2018a; Vorst et al., 2017). Our previous work demonstrated increased UV absorption potential for plastic blends containing

Fig. 7. Thickness normalized ultraviolet absorbance potential for virgin polypropylene/ post-consumer recycled polyolefin blends. Data points associated with the same letter are statistically equivalent.

7

post-consumer recycled materials (Curtzwiler et al., 2011, 2017, 2018a, 2018b). As the absorption spectrum is normalized to the thickness, it is possible to directly compare the UV absorption properties between materials. In general, the UV absorption potential trends PP > PE > PET which manufacturers can utilize to customize the UV and visible light transparency through precision blending and material selection.

3.6. Fourier transform infrared spectroscopy Degradation of both PE and PP generate carbonyl compounds in the polymer backbone, thus characterization of the carbonyl content is commonly utilized to investigate the extent of polymer degradation (Wypych, 2008). The carbonyl area was calculated via integrating the thickness normalized characteristic band associated with carbonyl functional groups (~1740 cm1). There was a linear increase in the measured carbonyl area with increasing postconsumer recycled polyolefin content (Fig. 8) which follows the Law of Mixtures. These data suggest a higher degree of degradation for blends containing post-consumer recycled material as would be expected without adding stabilizers during melt blending. A significant increase in the absorbance band near 1555 cm1 was determined with increasing post-consumer recycled polyolefin (Fig. 9). Characteristic bands of infrared absorption in near 1555 cm1 are often attributed to nitrogen oxide compounds (Coates, 2000; Naranjo et al., 2008; Socrates, 2001). The increased concentration of measured nitrogen oxide compounds in this region can be attributed to the consumption of hindered amine light stabilizers which are often melt blended with PE and PP to mitigate oxygen induced radical attack of the polymer backbone (Step et al., 1994; Wypych, 2008). The presence of increased vinylidene functional groups can be attributed to increased number of polypropylene chain ends from chain scission degradation reactions (Socrates, 2001; Wypych, 2008). Increased vinylidene content is expected, as observed here (Fig. 10), with increasing post-consumer recycled plastic due to the higher percentage of composition that possesses exhausted stabilizers as shown in Fig. 9. As the infrared spectroscopy measurements were collected via transmission experiments and thickness normalized, it is possible to compare the carbonyl content directly with previous studies. The measured carbonyl area is several orders of magnitude lower than previously measured for cast extruded and blown film polyethylene (Curtzwiler et al., 2018a, 2018b). This is presumably a result of incorporation of stabilizers in polypropylene (Step et al., 1994) as demonstrated by the increased observance of NeO containing

Fig. 8. Measured carbonyl area of virgin polypropylene/post-consumer recycled polyolefin. Data points associated with the same letter are statistically equivalent.

8

G.W. Curtzwiler et al. / Journal of Cleaner Production 239 (2019) 117978

Fig. 9. Area of characteristic bands associated with nitrogen oxide containing bonds of virgin polypropylene/post-consumer recycled polyolefin plastic blends. Data points associated with the same letter are statistically equivalent.

indicting that more structured crystals formed with diminishing amounts of PP in the blend. In contrast, the PP melting temperature was measured to decrease from 166
C of the pure virgin PP to ~160
C for the 100% post-consumer recycled polyolefin material (Fig. 11). The shifting melting temperatures of the PE and PP crystals is contrary to what has been observed in previous reports for blending virgin HDPE with virgin PP where the peak melting temperatures remained constant (Lin et al., 2015). The decreased PP melting temperature could be attributed to the presence of phase separated PE domains that serve as nucleating agents for the PP crystals thereby reducing the crystallite size (Lin et al., 2015). As the polymer cools from the melt, the PP spherulites form around phase separated molten PE domains which constrains the PE plastic chains to a specific volume for subsequent crystallization when the temperature is sufficiently low. Furthermore, the constrained volume forces the molten PE polymer chains to adopt conformations that minimizes the surface area of molten droplets to reduce the droplet surface energy. This may contribute to increased PE melting temperatures with increasing PCRPO content in the blend since processing conditions remained the same. Previous studies determined that the crystallization peaks of PP and HDPE became indistinguishable with increasing concentrations of HDPE, which is similar to what was observed here (Lin et al., 2015). However, the crystallization temperatures of PP and HDPE in that work were similar prior to blending under the testing conditions utilized. In the present work, there was approximately a 20
C difference in crystallization temperatures which decreased to ~5
C in the 100% post-consumer recycled polyolefin material (Fig. 12, S7, and S8).

Fig. 10. Area of characteristic bands associated with vinylidene functional groups in virgin polypropylene/post-consumer recycled polyolefin blends. Data points associated with the same letter are statistically equivalent.

functional group characteristic bands in the infrared spectra (Fig. 9) (Wypych, 2008). We hypothesize that the PE in the post-consumer recycled polyolefin source was stabilized to a greater extent than in our previous investigations as a PE composition of ~12.5% alone cannot account for the several order of magnitude reduction in carbonyl content. Therefore, it would be interesting to investigate the potential for stabilizers to migrate from the polypropylene phase to the polyethylene phase during melt processing. 3.7. Differential scanning calorimetry Differential scanning calorimetry was utilized to investigate the melting and crystallization properties of each post-consumer recycled polyolefin/virgin polypropylene blend. Two noticeable melting peaks were observed for material blends that contained post-consumer recycled polyolefin with one peak within the range normally associated with polypropylene (~165
C) and another peak in the range associated with polyethylene (~125
C), which has been observed previously for post-consumer recycled polyolefin materials (Brachet et al., 2008). The presence of two melting peaks indicates the presence of two different crystallizable species, and therefore two distinct phases, that coexist in the material. Utilizing the approach of Brachet et al., the composition of the 100% post-consumer recycled polyolefin was ~12.5% PE and the remainder PP (Brachet et al., 2008). Interestingly, the melting temperature of the PE crystals increased from ~124
C to ~127
C

Fig. 11. Polyethylene (a) and polypropylene (b) crystalline melting temperature in virgin polypropylene/post-consumer recycled polyolefin blends. Data points associated with the same letter are statistically equivalent.

G.W. Curtzwiler et al. / Journal of Cleaner Production 239 (2019) 117978

The decrease in the crystallization temperature measured by DSC for polypropylene would likely increase the cycle times of injection molded parts which would slow production. Therefore, additional materials, such as nucleating agents, may need to be added to increase the crystallization temperature of the continuous PP phase to reduce the cycle time and ensure similar production rates. 3.8. Inductively coupled plasma e optical emission spectroscopy Select inorganic elements that contribute to the ash content measured via TGA were analyzed via inductively coupled plasmaoptical emission spectroscopy. These data suggest that the recycling process may concentrate intentionally added, and currently unregulated, catalyst metals (iron, titanium, aluminum) in addition to unintentionally added regulated metals such as cadmium, chromium, and lead (Anonymous, 2018c) (Table 2). Furthermore, we can deduce that the polyethylene was polymerized via a Ziegler Natta catalyst system instead of a Phillips catalyst due to the low coefficient of determination for chromium and high coefficient of determination for titanium and iron. This is further supported by the increased aluminum concentrations for post-consumer recycled polyolefin blends. For packaging applications, heavy metal regulations such as those set forth by the Toxics in Packaging Clearinghouse (Anonymous, 2018c) can limit certain postconsumer recycled feedstocks for direct food contact due to elevated levels of regulated metals: lead, mercury, hexavalent chromium, and cadmium. Therefore, to determine the potential for specific diverted waste streams to comply with these regulations, it is critical to quantify potentially toxic metals in each material blend. Generally, the metal concentrations increased according to the Law of Mixtures. Aluminum levels rose linearly from an average of 163 mg/g in virgin resin to 202 mg/g in 100% PCRPO resin (R2 ¼ 0.95). Similarly, average titanium concentrations rose linearly from 1.38 mg/g to 67.4 mg/g (R2 ¼ 0.92) and iron concentrations rose linearly from 10.3 mg/g to 35.7 mg/g (R2 ¼ 0.92). Lead levels increased slightly from 4.19 mg/g to 12.8 mg/g (R2 ¼ 0.71), and cadmium levels increased from 0.27 mg/g to 0.53 mg/g (R2 ¼ 0.66). Chromium concentrations did not increase with post-consumer recycled polyolefin content but ranged from 0.28 to 0.94 mg/g. We hypothesize that the increasing concentration of metals associated with polymerization catalysts (Table 2) as a function of post-consumer polyolefin is attributed to utilization of older generation, less efficient polymerization catalysts during manufacturing of the postconsumer material. Indeed, source identification of increased concentrations measured here are critical for long-term

Fig. 12. Crystallization properties of virgin polypropylene/post-consumer recycled polyolefin blends upon cooling from melt.

9

Table 2 Selected inorganic element composition of virgin polypropylene/post-consumer recycled polyolefin plastic blends. All values are reported in mg/g. Sample

Al

Cd

Cr

Fe

Pb

Ti

0% PCRPO 20% PCRPO 40% PCRPO 60% PCRPO 80% PCRPO 100% PCRPO MLOD x 103 MLOQ x 103

163.61 174.04 174.03 181.17 195.60 202.30 5.84 19.4

0.27 0.17 0.29 0.33 0.32 0.53 0.185 0.617

0.94 0.28 0.19 0.77 0.31 0.87 1.33 4.44

10.32 16.32 20.77 25.00 24.08 35.68 0.649 2.16

4.19 7.75 6.89 5.68 12.32 12.81 29.4 98.1

1.38 24.85 24.47 30.39 48.13 67.41 0.275 0.915

Note: the method limit of detection (MLOD) and method limit of quantification (MLOQ) concentrations are three orders of magnitude lower than the measured sample concentrations.

sustainability and environmental health. Further research with techniques such as X-ray Diffraction is needed to definitively identify the physical state of the detected metals (i.e., salts or oxides) to better understand the source of the metals in the postconsumer polyolefin materials. 3.9. Scanning electron microscopy (SEM) In order to examine the plastic morphology in detail, scanning electron micrographs of samples with different blend ratios were collected. It is interesting to note, as the concentration of PCRPO increases, the plastic becomes less porous (Fig. 13). Increased porosity has been observed for PP/HDPE blends containing paper impurities and the potential for paper contamination in the postconsumer recycled plastics used in this study cannot be ignored (Mehrabzadeh and Farahmand, 2001). There are also small white particles observed with the PCRPO samples although the exact particle composition cannot be determined from SEM analysis alone. However, previous publications showed similar images of mixed polyethylene with polypropylene, where white particles are small polyethylene crystal domains (Brachet et al., 2008; Lin et al., 2015; Mehrabzadeh and Farahmand, 2001). The morphology study supports the previous mechanical results, i.e., melt blending virgin PP with PCRPO decreases defects of the plastic and improves the mechanical performance. 4. Conclusions We demonstrate herein the ability to tune physical performance properties of virgin polypropylene utilizing mixed post-consumer recycled polyolefin materials from beverage bottle enclosure waste streams. The data indicated that melt-blending post-consumer recycled, mixed polyolefin material (~12.5% post-consumer recycled PE, 87.5% post-consumer recycled PP) significantly altered the measured physical properties of virgin polypropylene. Further research is required to understand the effect of postconsumer PE on the physical properties of 100% mixed postconsumer polyolefin material and the resulting polymer blend with virgin PP as presented here. Generally, the property changes followed the Law of Mixtures (linear trends), resulting in highly predictable and tunable properties. Specifically, increased ultravioletevisible absorption, barrier to water vapor and oxygen, and tensile strength properties were measured which is consistent with other plastic types. To some extent, altered properties can be attributed to the presence of phase separated domains (i.e., polyethylene discontinuous phases in a polypropylene continuous phase) and increased compatibility from oxygenated functional groups. This research is in direct response to requests from private and

10

G.W. Curtzwiler et al. / Journal of Cleaner Production 239 (2019) 117978

Fig. 13. Representative scanning electron micrographs (left: 1.5k x, scale bar ¼ 30 mm; right: 35k x, scale bar ¼ 1 mm) of cryo-fractured virgin polypropylene/post-consumer recycled polyolefin blends.

public entities to increase the understanding of property changes from incorporating more post-consumer recycled materials in commercially available products. These data fill current knowledge gaps and demonstrates increased value of contaminated mixed stream polyolefin plastics to justify innovative landfill diversion strategies. As consumers demand more sustainably sourced materials, it is critical to demonstrate increased value of post-consumer waste beyond sustainability. Funding This study was supported by the Polymer and Food Protection Consortium at Iowa State University and the Iowa Agriculture and Home Economics Experiment Station, Ames, Iowa and Project No. IOW03902 by the Hatch Act and State of Iowa. The authors would like to thank Aaron Industries for supplying the virgin polypropylene and post-consumer recycled polyolefin materials. S.J. would like to thank Iowa State University for the Start-up Fund and 3M for the Non-tenured Faculty Award. Conflicts of interest The authors declare no conflict of interest. The funding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jclepro.2019.117978.

References Anonymous, 2018a. Advancing Sustainable Materials Management: 2015 Fact Sheet. United States Environmental Protection Agency. Anonymous, 2018b. Flexible packaging market size, share & trends analysis report by raw material (paper, aluminum foil, plastics, bioplastics), by application (F&B, pharmaceutical, cosmetics), and segment forecasts, 2012 - 2022. https:// www.grandviewresearch.com/industry-analysis/global-flexible-packagingmarket. (Accessed 18 April 2018). Anonymous, 2018c. Toxics in packaging Clearinghouse. http://www.coneg.org/tpch. (Accessed 15 August 2018). ASTM D5576-00, 2013a. Standard Practice for Determination of Structural Features in Polyolefins and Polyolefin Copolymers by Infrared Spectrophotometry (FTIR). ASTM International, West Conshohocken, PA, 2013. ASTM D1238-13, 2013b. Standard Test Method for Melt Flow Rates of Thermoplastics by Extrusion Plastometer. ASTM International, West Conshohocken, PA. ASTM F1249-13, 2013c. Standard Test Method for Water Vapor Transmission Rate through Plastic Film and Sheeting Using a Modulated Infrared Sensor. ASTM International, West Conshohocken, PA. ASTM D638-14, 2014. Standard Test Method for Tensile Properties of Plastics. ASTM International, West Conshohocken, PA. ASTM E1641-16, 2016. Standard Test Method for Decomposition Kinetics by Thermogravimetry Using the Ozawa/Flynn/Wall Method. ASTM International, West Conshohocken, PA. ASTM D3985-17, 2017. Oxygen Gas Transmission Rate through Plastic Film and Sheeting Using a Coulometric Sensor. ASTM International, West Conshohocken, PA. Blaine, R.L., Hahn, B.K., 1998. Obtaining kinetic parameters by modulated thermogravimetry. J. Therm. Anal. Calorim. 54 (2), 695e704. Brachet, P., Høydal, L.T., Hinrichsen, E.L., Melum, F., 2008. Modification of mechanical properties of recycled polypropylene from post-consumer containers. Waste Manag. 28 (12), 2456e2464. Coates, J., 2000. Interpretation of infrared spectra, a practical approach. In: Meyers, R.A. (Ed.), Encyclopedia of Analytical Chemistry. John Wiley & Sons Ltd, Chichester, pp. 10815e10837. Curtzwiler, G., Vorst, K., Danes, J.E., Auras, R., Singh, J., 2011. Effect of recycled poly(ethylene terephthalate) content on properties of extruded poly(ethylene terephthalate) sheets. J. Plastic Film Sheeting 27, 65e86. Copyright (C) 2013 American Chemical Society (ACS). All Rights Reserved. Curtzwiler, G.W., Vorst, K.L., 2018. Methods of Optimizing Polymer Composition for Sustainable Light Filters and Protectors in Retail Display Case Applications and

G.W. Curtzwiler et al. / Journal of Cleaner Production 239 (2019) 117978 Methods Thereof. Iowa State University, United States. Curtzwiler, G.W., Williams, E.B., Hurban, E., Greene, J., Vorst, K.L., 2018a. Certification markers for empirical quantification of post-consumer recycled content in extruded polyethylene film. Polym. Test. 65, 103e110. Curtzwiler, G.W., Williams, E.B., Maples, A.L., Davis, N.W., Bahns, T.L., Eliseo De n, J., Vorst, K.L., 2017. Ultraviolet protection of recycled polyethylene tereLeo phthalate. J. Appl. Polym. Sci. 134 (32), 45181. Curtzwiler, G.W., Williams, E.B., Vorst, K.L., 2018b. Multivariable regression analysis for empirical quantification of post-consumer recycled content in extruded polyethylene film. In: SPE International Polyolefins Conference. Society of Plastics Engineers, Houston, TX. Dillner, A.M., Shafer, M.M., Schauer, J.J., 2007. A novel method using polyurethane foam (PUF) substrates to determine Trace element concentrations in sizesegregated atmospheric particulate matter on short time scales. Aerosol Sci. Technol. 41 (1), 75e85. Ellison, S., Barwick, V., Trevor, F., 2009. Practical Statistics for the Analytical Scientist: A Bench Guide, second ed. The Royal Society of Chemistry, Cambridge. Geyer, R., Jambeck, J.R., Law, K.L., 2017. Production, use, and fate of all plastics ever made. Sci. Adv. 3 (7). Ghabeer, T., Dweiri, R., Al-Khateeb, S., 2012. Thermal and mechanical characterization of polypropylene/eggshell biocomposites. J. Reinf. Plast. Compos. 32 (6), 402e409. Gooch, J.W., 2010. Encyclopdic Dictionary of Polymers, second ed. Springer, New York, NY. Goss, B.G.S., Nakatani, H., George, G.A., Terano, M., 2003. Catalyst residue effects on the heterogeneous oxidation of polypropylene. Polym. Degrad. Stab. 82 (1), 119e126. Jambeck, J.R., Geyer, R., Wilcox, C., Siegler, T.R., Perryman, M., Andrady, A., Narayan, R., Law, K.L., 2015. Plastic waste inputs from land into the ocean. Science 347 (6223), 768e771. Jmal, H., Bahlouli, N., Wagner-Kocher, C., Leray, D., Ruch, F., Munsch, J.-N., Nardin, M., 2018. Influence of the grade on the variability of the mechanical properties of polypropylene waste. Waste Manag. 75, 160e173. Kamleitner, F., Duscher, B., Koch, T., Knaus, S., Schmid, K., Archodoulaki, V.-M., 2017. Influence of the molar mass on long-chain branching of polypropylene. Polymers 9 (9), 442. Koleske, J.V., 2012. Paint and Coating Testing Manual, fifteenth ed. ASTM International, West Conshohocken. Li, Y., Jia, S., Du, S., Wang, Y., Lv, L., Zhang, J., 2018. Improved properties of recycled polypropylene by introducing the long chain branched structure through reactive extrusion. Waste Manag. 76, 172e179. Lin, J.-H., Pan, Y.-J., Liu, C.-F., Huang, C.-L., Hsieh, C.-T., Chen, C.-K., Lin, Z.-I., Lou, C.W., 2015. Preparation and compatibility evaluation of polypropylene/high density polyethylene polyblends. Materials 8 (12), 5496. Mahmoud, M.E., El-Khatib, A.M., Badawi, M.S., Rashad, A.R., El-Sharkawy, R.M., Thabet, A.A., 2018a. Fabrication, characterization and gamma rays shielding

11

properties of nano and micro lead oxide-dispersed-high density polyethylene composites. Radiat. Phys. Chem. 145, 160e173. Mahmoud, M.E., El-Khatib, A.M., Badawi, M.S., Rashad, A.R., El-Sharkawy, R.M., Thabet, A.A., 2018b. Recycled high-density polyethylene plastics added with lead oxide nanoparticles as sustainable radiation shielding materials. J. Clean. Prod. 176, 276e287. Mahmoud, M.E., El-Khatib, A.M., El-Sharkawy, R.M., Rashad, A.R., Badawi, M.S., Gepreel, M.A., 2019. Design and testing of high-density polyethylene nanocomposites filled with lead oxide micro- and nano-particles: mechanical, thermal, and morphological properties. J. Appl. Polym. Sci. 136 (31), 47812. Mehrabzadeh, M., Farahmand, F., 2001. Recycling of commingled plastics waste containing polypropylene, polyethylene, and paper. J. Appl. Polym. Sci. 80 (13), 2573e2577. Naranjo, A., del Pilar Noriega, E.M., Osswald, T.A., Rold an-Alzate, A., Sierra, J.D., 2008. Plastics Testing and Characterization, Plastics Testing and Characterization. Carl Hanser Verlag GmbH & Co. KG, pp. iexi. Pesetskii, S.S., Jurkowski, B., Filimonov, O.V., Koval, V.N., Golubovich, V.V., 2011. PET/ PC blends: effect of chain extender and impact strength modifier on their structure and properties. J. Appl. Polym. Sci. 119 (1), 225e234. Pritchard, R., 1964. The transparency of crystalline polymers. SPE Trans 4 (1), 66e71. Schael, G.W., 1964. Determination of polyolefin film properties from refractive index measurements. J. Appl. Polym. Sci. 8 (6), 2717e2722. Schael, G.W., 1968. Determination of polyolefin film properties from refractive index measurements. II. Birefringence. J. Appl. Polym. Sci. 12 (4), 903e914. Singh, N., Hui, D., Singh, R., Ahuja, I.P.S., Feo, L., Fraternali, F., 2017. Recycling of plastic solid waste: a state of art review and future applications. Compos. B Eng. 115, 409e422. Socrates, G., 2001. Infrared and Raman Characteristic Group Frequencies: Tables and Charts. John Wiley & Sons, LTD, West Sussex, England. Step, E.N., Turro, N.J., Gande, M.E., Klemchuk, P.P., 1994. Mechanism of polymer stabilization by hindered-amine light stabilizers (HALS). Model investigations of the interaction of peroxy radicals with HALS amines and amino ethers. Macromolecules 27 (9), 2529e2539. Upadhyay, N., Majestic, B.J., Prapaipong, P., Herckes, P., 2009. Evaluation of polyurethane foam, polypropylene, quartz fiber, and cellulose substrates for multielement analysis of atmospheric particulate matter by ICP-MS. Anal. Bioanal. Chem. 394 (1), 255e266. Vorst, K.L., Brown, W., Danes, J., Curtzwiler, G.W., 2017. Method for Optimizing Plastic Compositions Used in Packaging to Increase Shelf-Life of Perishable Products and a System Thereof. Iowa State University, United States. Workman Jr., J., Lois, W., 2012. Practical Guide and Spectral Atlas for Interpretive Near-Infrared Spectroscopy, second ed. CRC Press, Boca Raton. Wypych, G., 2008. Handbook of Material Weathering, 4 ed. ChemTec Publishing, Toronto. Xanthos, M., 2012. Recycling of the #5 polymer. Science 337 (6095), 700.