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Recycling Oxidized Model Polyethylene Powder as a Degradation Enhancing Filler for Polyethylene/Polycaprolactone Blends Michal Michalak, Minna Hakkarainen, and Ann-Christine Albertsson* Department of Fibre and Polymer Technology, KTH Royal Institute of Technology, S-10044 Stockholm, Sweden S Supporting Information *

ABSTRACT: Polyethylene (PE) powder with different degrees of oxidation was prepared to model end-of life polyethylene that has been oxidized during processing and/or service-life. The nonoxidized and preoxidized PE powders were blended with PCL to evaluate the possibility to recycle PE powder in PCL blends. Good elongation at break was reached for all 25/75 (w/w) PE/PCL films and the elongation at break correlated well with the carbonyl index of the original PE powder, indicating that the oxidation of PE increased the interfacial adhesion with PCL and improved the blend properties. The preoxidized PE powder in combination with surfactant and pro-oxidant greatly accelerated the degradation rate of PCL as measured by molecular weight decrease during low temperature thermo-oxidative aging. This coincided with the fast initial increase in PE carbonyl index. The degradation accelerating effect was larger when preoxidized PE was blended with PCL as compared to when nonoxidized PE was added. However, the degree of preoxidation had minor impact on PCL degradation rate. Without PE powder the degradation rate of PCL was not enhanced by the addition of pro-oxidant and surfactant alone. KEYWORDS: Polyethylene, Recycling, Waste, Polycaprolactone, Hydrolysis, Oxidation

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degradation of polyethylene films.20,21 The aim of this first step is to break polyethylene down to oxidized low molecular weight compounds that could be susceptible to further biodegradation.22,23However, realization of the first rate limiting step has proven difficult under accelerated or simulated laboratory tests24−26 and reaching the degree of oxidation required for total biodegradation under natural conditions27 seems extremely difficult without a more efficient design principle. The European Commission has communicated strategies on the prevention and recycling of waste. Already, in 1994 European Packaging Directive 94/62/EC was accepted setting up obligations and goals for recycling of packaging waste, it was further updated in 2004.28 In 2014, the European Commission reviewed the recycling and other waste-related targets in the EU Waste Framework Directive 2008/98/EC, the Landfill Directive 1999//31/EC, and the Packaging and Packaging Waste Directive 94/62/EC in order to “turn Europe into a circular economy, boost recycling, secure access to raw materials, and create jobs and economic growth”. Important plastic and/or packaging related elements of the proposal include increasing the recycling and reuse of packaging waste to

INTRODUCTION A significant part of plastics and in particular polyethylene (PE) products is designed for short-term applications leading to 25 million tons of plastic waste each year in Europe alone, a large part of which still ends up in landfills.1 The persistence of polyethylene and other volume plastics toward bio and/or environmental degradation2−5 has led to accumulation of plastic waste in landfills and natural environments like oceans.6,7 Mulching films and other agricultural plastics are examples of products, often made of polyethylene, that are problematic to recycle due to low quality, contamination. and problems with collection from the fields.8,9 A large part of mulching films is still landfilled or incorporated into soil by rototilling.10 This is a direct environmental problem due to extreme persistence of polyethylene in natural environments. The remaining plastic fragments can also interact and adsorbed contaminants like pesticides.11 At the same time, mulching films significantly benefit agriculture and mulching has dramatically increased worldwide.12,13 Biodegradable or so-called oxo-degradable films have been proposed as one possible solution.14,15 However, biodegradable films are often significantly more expensive than polyethylene films,16 whereas the degradation of oxo-degradable films has not been convincingly demonstrated under relevant environmental conditions.17−19 The principle of oxo-degradable films is through addition of pro-oxidants accelerate the inherent abiotic degradation mechanisms, i.e., thermo-and photo-oxidative © XXXX American Chemical Society

Received: August 11, 2015 Revised: November 27, 2015

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DOI: 10.1021/acssuschemeng.5b00858 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering Table 1. Composition of the Extruded PE/Masterbatch Films oxidation time PE-H

PE-L

PE/PCL

0

30

60

120

0

30

60

120

100/0 75/25 25/75

PE100-H0 PE75-H0 PE25-H0

PE100-H30 PE75-H30 PE25-H30

PE100-H60 PE75-H60 PE25-H60

PE100-H120 PE75-H120 PE25-H120

PE100-L0 PE75-L0 PE25-L0

PE100-L30 PE75-L30 PE25-L30

PE100-L60 PE75-L60 PE25-L60

PE100-L120 PE75-L120 PE25-L120

biodegradation of polyethylene has also been shown previously.34 The temperature was set at 90 °C for PCL and 150 °C for PE. The screw speed was 100 rpm and extrusion time was 5 min. Procedure was repeated 5 times. The extruded material was cut into small pieces. Film Preparation. The mixing of the different oxidized and nonoxidized PE powders with PCL and additive masterbatches was achieved by twin-screw miniextruder (DSM Xplore 5 microcompounder, model 2012). For the PE-H powder samples, the temperature was set to 90 °C, for the PE-L the temperature was set to 80 °C. The screw speed was 100 rpm and the extrusion time was 5 min. Generally, 4 g of material containing 0.1 g of masterbatch was introduced to the miniextruder, which corresponded to 0.5% of Twin 80 and 0.5% of Mn stearate. Reference samples of nonoxidized polyethylene with (denoted H0 and L0) and without (denoted Hp and Lp) masterbatch were prepared in similar manner, but the temperature was set at 150 °C. PCL reference films with and without additives were also prepared and denoted PCL-a and PCL-p, respectively. The extruded materials were melt-pressed into films using a hot press (Fontijne Press). Three grams of material was used for each film and it was placed in a square shaped mold with dimensions of 15 × 15 cm and a thickness of 0.1 mm. The temperature was set to the same value as for extrusion and melt pressing was performed with a pressing force of 100 kN during 4 min. The abbreviations and compositions of PE films with masterbatch (Mn stearate and Tween 80) are summarized in Table 1. Thermo-Oxidative Degradation. Thermo-oxidation was carried out in an air oven maintained at 45 °C. The samples were removed at regular time intervals to monitor the degradation behavior. Thermooxidation of PE-H films was also carried out at 60 °C for IR microscopy evaluations of the oxidation. Degradation products formed during the thermo-oxidation were extracted by Soxhlet extraction utilizing water as an extraction medium. Around 0.8 g of the sample was extracted for 24 h, after which the residue was dried and weighted. The extract was then concentrated by a rotary evaporator. The extractable fraction was estimated from the weight loss of the sample. Characterization. Mechanical properties of the films were determined by an Instron 5566 Universal tester (Instron Corp., UK). Film samples were cut to strips of 25 mm in length and 10 mm in width. All specimens were placed at 23 °C, 50% humidity for 24 h before measurements were made. The crosshead speed was set to 25 mm/min with cell load of 100 N. At least five parallel tests were made on each material to ensure adequate statistics. Carbonyl index (CI) was investigated by PerkinElmer Spectrum 2000 FTIR with attenuated total reflectance (ATR) crystal accessory (Golden Gate). The PE/PCL 75/25 films were placed in chloroform for 24 h to dissolve the PCL fraction around the PE powder, followed with sample filtration and washing with CHCl3. The procedure was repeated once and then the sample was dried. All spectra were obtained by means of 16 individual scans at 4 cm−1 resolution in 4000−750 cm−1. Carbonyl index was defined as the ratio of the absorbance of carbonyl band around 1740 cm−1 and the internal thickness band at 2020 cm−1. The presented values are an average of three measurements. Number-average molar mass, weight-average molar mass and molar mass distribution (ĐM) of the water-soluble fractions were determined by size exclusion chromatography (SEC) conducted with chloroform as mobile phase at 30 °C with a flow rate of 1 mL/min, using PL GPC 50 Plus equipped with PL-RI detector with a set of two PLGel 5 μm MIXEDD ultrahigh efficiency columns. Polystyrene standards with narrow molar mass distributions were used for calibration.

80% by 2030 and phasing out landfilling by 2025 for recyclable waste.29 Direct recycling of postconsumer plastic materials generally leads to products with lower quality. This loss of properties originates from aging during the service-life and especially from the thermo-mechanical degradation taking place during reprocessing.30 Incorporation of biofibers or degradable components as fillers in thermoplastics is a popular concept for increasing the green nature of materials.31,32 Here we introduce a reverse design to recycle polyethylene as a cheap, resource saving, and property enhancing filler in biodegradable polycaprolactone, one potential application being mulching films. This could offer a viable route to recycling low quality polyethylene waste at the same time as the price of the biodegradable material is decreased by the cheap filler. The small size and large hydrophilic oxidized surface especially in combination with decreased molecular weight for the polyethylene powder offers increased possibilities for environmental degradation that should be further evaluated in future studies.

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EXPERIMENTAL SECTION

Materials. Polycaprolactone (PCL) was obtained from SigmaAldrich (Mn Th = 80 000 g/mol according to Sigma-Aldrich and Mn SEC = 156 800 g/mol and Mw SEC = 223 800 g/mol measured with SEC against polystyrene standards). Polyethylene (PE) powder was prepared by modified Priest procedure33 from polyethylene without antioxidants received from Borealis and denoted H (CT7200, Mn = 90 000 g/mol and Mw = 170 000 g/mol). Polyethylene granulates were dissolved in toluene/xylene 1/1 mixture at 115 °C. The solution was cooled down to 60 °C and filtered. The obtained powder was washed 5 times with acetone and 3 times with water and dried in a vacuum. The shape of the white powder was spherical with an average diameter of 140 μm. Another polyethylene powder with unknown additives was purchased from Sigma-Aldrich and denoted L (Mn = 7700 g/mol and Mw = 35 000 g/mol). The shape of L resembled chips with rather large diameter between 200 to 300 and 70 μm thickness. 60 g of PE-H or PE-L polyethylene was oxidized in ozone/oxygen mixture (∼250 mg ozone/h) at 40 °C for 30, 60, or 120 min. The CI of the oxidized PE-H increased from the original value of 1 to 3, 6.5, and 58 after 30, 60, and 120 min of oxidation at 40 °C, respectively. At the same time the crystallinity of the sample increased to 38%, 41.7% and 43.9% from the original value of 36%. In the case of the PE-L the original CI was much higher, i.e., 9 and the changes caused by oxidation were much smaller. The CI of L increased to 10, 19.6, and 30.6 after the corresponding ozone oxidation times of 30, 60, and 120 min. In accordance the crystallinity, which was originally 31%, remained very similar to the starting crystallinity, i.e., 32.8%, 32.7%, and 33.6% after 30, 60, and 120 min. Probably the relatively big particle size of PE-L was the limiting factor for the oxidation and explains the higher CI index for the smaller PE-H particles. Masterbatch. Masterbatches with 20 wt % of Tween 80 surfactant, 20 wt % Mn stearate, and 60 wt % PCL or 60 wt % PE-H or PE-L were prepared. The blends containing 3 g of polymer, 1 g of Tween 80, and 1 g of Mn stearate were extruded in a twin-screw miniextruder (DSM Xplore 5 microcompounder, model 2012). Surfactant was added to enhance the wettability and hydrophilicity of the films and especially of the hydrophobic PE component. The enhancing effect of surfactant on B

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Figure 1. Elongation at break of PE and PE/PCL films containing nonoxidized and preoxidized PE.

Figure 2. SEM images of PE-H powders after the different oxidation times and the cross sections of the corresponding PE75-H and PE25-H films with composition of 75/25 and 25/75 PE/PCL.

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Thermogravimetry (TGA) was utilized to evaluate the thermal stability of the films. The instrument was a Mettler-Toledo TGA/ DSC1. The samples were heated at 10 °C/min from 25 to 600 °C in N2 atmosphere. The crystallinity was investigated by Mettler-Toledo DSC 820 module differential scanning calorimetry (DSC). The sample was first heated at a heating rate of 10 °C/min from 25 to 200 °C, after which it was cooled back to 25 °C and then heated again at 10 °C/min to 200 °C. The crystallinity was calculated with the help of equation

%crystallinity =

ΔHf(observed) ΔHf(100%crystaline)

RESULTS AND DISCUSSION

Polyethylene powder with different degrees of oxidation, to model end-of-life polyethylene that has been oxidized to different degrees during its service-life, was mixed with PCL to evaluate the possibility to recycle low quality PE in PCL blends. Furthermore, the effect of PE powder on the degradation rate of PCL was monitored. Mechanical Properties and Morphology of the Original PE/PCL Films. The elongation at break for the PE100, PE75, and PE25 series of films prepared from nonoxidized and preoxidized PE-H and PE-L is shown in Figure 1. Preoxidation slightly decreased the elongation at break of “pure” PE100-H films. On the contrary, preoxidation improved the elongation at break of PE25-H films, probably

× 100

where ΔHf(observed) is the measured enthalpy from the melting of the material and ΔHf(100% crystalline) is the enthalpy of 100% crystalline polyethylene (285 J/g35) or polycaprolactone (139.5 J/g36). C

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Figure 3. Molecular weight changes for PCL in PE-H/PCL and PE-L/PCL films as a function of thermo-oxidation time (a) PE75-H, (b) PE25-H, (c) PE75-L, and (d) PE25-L series. Compare also with Figure S2 in the Supporting Information.

Figure 4. Amount of water-soluble products in (a) PE75-H, (b) PE25-H, (c) PE75-L, and (d) PE25-L films as a function of thermo-oxidation. Compare also with Figure S1 in the Supporting Information.

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compared to the PE25 films indicating that a major part of the water-soluble products originates from the PE part of the blends. The extraction profiles also correlated well with the molecular weight profiles, showing generally higher extractable fractions for the blends containing preoxidized PE as compared to the blends containing nonoxidized PE. The almost constant molecular weight for plain PCL films with and without additives (pro-oxidant and surfactant) during the 160 days of thermo-oxidative aging explains the relatively low amount of water-soluble extractables from these materials and further confirms that most of the water-soluble products originate from PE part of the blends. The water extractable fractions after 160 days of thermo-oxidation were only 0.6% and 0.7% for PCL-p and PCL-a, respectively. Moreover, already after 20 days of thermo-oxidation the amount of extractables reached a plateau and no further changes were observed during the remaining time period. This shows that PCL films without PE did not degrade enough to form significant amounts of low molecular weight product during the thermo-oxidative aging at 45 °C.37 Changes in Carbonyl Index during Long-Term Thermo-Oxidation. There was a significant difference in the evolution of carbonyl index for the nonoxidized PE powder and the preoxidized PE powders in the different PCL blends (Figure 5). Only a small CI increase was observed in the case of

due to the improved interfacial adhesion between oxidized PE particles and PCL. The elongation at break of PE25-H films increased from 300% for the blends with nonoxidized PE-H to 550% for the blends containing the most oxidized PE-H120 powder. The PE100-L films all had relatively low elongation at break, probably due to the low molecular weight of PE-L. Opposite to this, the PE25-L blends exhibited by far the best elongation at break of all the studied material combinations, demonstrating values between 800 and 900%, i.e. close to the elongation at break of 1160% for pure PCL films. The difference between the materials containing nonoxidized and preoxidized PE powders was not as clear as in the case of PE25H films, which correlates with the higher original CI of PE-L powder and the small additional CI increase during the preoxidation of PE-L. The elongation at break of all PE75-H and PE75-L films was poor. SEM images of the different nonoxidized and oxidized PE-H powders and the cross sections of the corresponding PE75-H and PE25-H films are presented in Figure 2. Effect of Oxidized Polyethylene Powder on Polycaprolactone. The influence of PE powder with different degrees of preoxidation on the degradation rate of PCL was explored by following the molecular weight of PCL as a function of low temperature thermo-oxidative aging. The SEC chromatograms of the chloroform soluble part of the PE/PCL films exhibited two peaks. One relatively high molecular weight peak corresponding to the main PCL fraction and a second peak in the low molecular weight range possibly corresponding to a mixture of low molecular weight PCL and chloroform soluble short chain PE. This low molecular weight peak had original Mn around 800 g/mol and dispersity around 2.2 in the case of PE-H and for PE-L Mn was around 1000 g/mol and the dispersity around 3.0. The molecular weights of PE75-H and PE25-H are shown in Figure 3a,b. The molecular weight of PE75-L and PE25-L are shown in Figure 3c,d. A similar trend is observed in all cases. The PCL in contact with preoxidized PE exhibits clearly faster molecular weight decrease as compared to the PCL in the blends with nonoxidized PE. However, the actual degree of oxidation did not have significant further effect on PCL degradation rate. In addition, the PCL molecular weight decreases faster when the PE content in the blends was higher. This was especially clear in the case of PE-H series, whereas the composition effect was smaller for PE-L series. The numberaverage molecular weight of PCL films containing pro-oxidant but no polyethylene is relatively constant or even increases slightly during the 160 days of thermo-oxidation, possibly due to migration of low molecular weight compounds from the material. Figure S1 and Figure S2 in the Supporting Information demonstrate almost identical hydrolysis rates for PCL with and without pro-oxidant and surfactant. Comparison with the PE/PCL blends further shows that even nonoxidized PE increases the degradation rate of PCL even though the accelerating effect is smaller as compared to the effect of preoxidized PE powder. Water Extractable Fractions. The amount of watersoluble compounds formed in PE75 and PE25 films as a function of thermo-oxidation time is illustrated in Figure 4. Even though significant molecular weight decrease was observed for the PCL fraction (Figure 3), the water-soluble product fractions only increased very moderately during the thermo-oxidative aging. A somewhat higher amount of watersoluble products was extracted from the PE75 films as

Figure 5. Effect of preoxidation on the changes in carbonyl index during thermo-oxidative degradation of PE75-H0, PE75-H30, PE75H60, and PE75-H120.

nonoxidized PE powder in the PE75-H0 blends, whereas the CI of PE75-H30, PE75-H60, and PE75-H120 rapidly increased during the first 40−60 days of thermo-oxidation and then continued to increase at more moderate rate during the remaining period until 160 days. The CI of polyethylene fractions correlated well with the thermo-oxidative stability of PCL in the blends and the period of fast CI increase for PE coincided with the fast molecular weight decrease for PCL. PE75-H0 with nonoxidized PE powder had by far the lowest CI increase and the PCL in this blend decreased less in molecular weight. All PEs with preoxidized PE experienced fast CI increase and faster molecular weight decrease for PCL fraction. Effect of Preoxidation and Thermo-Oxidative Aging on Thermal Stability. The thermal stability of the films was investigated by TGA to show the influence of nonoxidized vs preoxidized PE powder and the additive packages on the maximum temperature of degradation (Tmax) (Figure 6). Tmax temperature for polycaprolactone without additives was 411 °C and it was not influenced by the thermo-oxidative aging time. The addition of pro-oxidant and surfactant greatly reduced the thermal stability of PCL and PCL-a originally had Tmax = 316 E

DOI: 10.1021/acssuschemeng.5b00858 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Figure 6. Effect of preoxidation on thermal stability (a) PE75-H0, (b) PE75-H30, (c) PE75-H60, and (d) PE75-H120 as a function of thermooxidation time.

°C. Tmax of PCL-a increased during the first 40 days of thermooxidation and stabilized then at Tmax= 331 °C. In the case of PE/PCL formulations, the thermal stability of polycaprolactone was decreased, but the level of decrease depended on the PE particle preoxidation time. Interestingly, the correlation was opposite to what was observed during low temperature thermooxidative aging. Lowest thermal stability was originally observed for PCL in the PE75-H0 blends with nonoxidized PE particles. As a function of low temperature thermo-oxidation time multiple peaks moving toward higher Tmax values were observed for the PCL component. In the case of PE75-H30 and PE75H60, the Tmax value of PCL in the blends was originally lower, but reached a value close to that of pure PCL after 20−40 days of thermo-oxidation. Finally further increasing the PE preoxidation time to reach PE75-H120 blends, resulted in Tmax values close to those of pure PCL. This could indicate some kind of immobilization of the pro-oxidant on the preoxidized PE particles, which could lead to lower effect on the thermal stability of PCL component in the blends.

blended with PCL as compared to when nonoxidized PE was added. No effect on PCL degradation was observed when prooxidant and surfactant were added alone without PE powder. In addition, the small size and large hydrophilic oxidized surface especially in combination with decreased molecular weight for the polyethylene powder offers increased possibilities for environmental degradation that should be further evaluated in future studies, as we anticipate that the developed films could have great potential in mulching applications including active mulching with nutrients.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.5b00858. Comparison of hydrolytic degradation of PCL with (PCL-a) and without (PCL-p) additives (PDF).

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CONCLUSIONS We successfully demonstrated that oxidized model polyethylene powder is recyclable as a property enhancing filler in polycaprolactone. Films with excellent elongation at break were obtained when the film composition was 25% PE and 75% PCL. The elongation at break was generally higher for the blends containing oxidized PE powder as compared to blends with nonoxidized PE powder, probably due to better interfacial adhesion, but the elongation at break decreased when PE content increased. Interestingly, PE powder in combination with pro-oxidant and surfactant acted as a degradation accelerating additive for PCL leading to faster molecular weight decrease. This coincided with fast CI increase for the PE fraction. This effect was larger when preoxidized PE was

AUTHOR INFORMATION

Corresponding Author

*A.-C. Albertsson. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the ERC Advance Grant PARADIGM (Grant agreement no. 246776) for the financial support of this work.

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REFERENCES

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