Waste treatment of bio-based compostable polymers

Chapter 11

Waste treatment of bio-­based compostable polymers Chapter Outline 11.1 Introduction 11.2 Mechanical recyling 11.3 Chemical recycling

349 350 357

11.4 Comparison of compostable polymer waste treatment methods363 References 366

11.1 Introduction In order to further develop the compostable polymers potentialities it is very important to carefully manage the compostable polymers waste disposal. Compostable polymers are designed to be disposed after their useful life by means of organic recycling, i.e. composting. Compostable polymers biodegradation is best carried out in industrial composting facilities to be efficient. It would be best not to rely only on its biodegradation either in the environment or in composting facilities. Therefore, to make compostable polymers an even more sustainable material compostable polymers recycling must be developed [1]. Environmental compatibility and recyclability are being considered during the designing of new polymer materials. Life cycle analyses and management are also being studied as tools for decision making [2]. A number of life cycle assessment (LCA) studies have been undertaken comparing end-­of-­life treatment options for post-­consumer plastic waste, including techniques such as: mechanical recycling, feedstock recycling, incineration with energy recovery and landfilling [3]. Recovery and recycling of plastic solid waste (PSW) can be categorized by four main routes, i.e. re-­extrusion, mechanical, chemical and energy recovery [4]. Reextrusion (primary recycling) utilizes scrap plastics by re-­introducing the reminder of certain extruded thermoplastics (mainly poly-­α-­olefins) into heat cycles within a processing line. When plastic articles are discarded after a number of life cycles, mechanical recycling techniques present themselves as a candidate for utilizing a percentage of the waste as recyclate and/or fillers. Collectively, all technologies that convert polymers to either monomers (monomer recycling) or petrochemicals (feedstock recycling) are referred to as chemical recycling. The technology behind its success is the depolymerization processes (e.g. thermolysis) that can result in a very profitable and Compostable Polymer Materials. https://doi.org/10.1016/B978-­0-­08-­099438-­3.00011-­2 Copyright © 2019 Elsevier Ltd. All rights reserved.

349

350  Compostable Polymer Materials

sustainable industrial scheme, providing a high product yield and a minimal waste. Nevertheless, due to their high calorific value and embodied energy, plastics are being incinerated solely or in combination with municipal solid waste (MSW) in many developed countries. Primary recycling, better known as re-­extrusion, is the re-­introduction of scrap, industrial or single-­polymer plastic edges and parts to the extrusion cycle in order to produce products of the similar material [1]. Currently, most of the PSW being recycled is of process scrap from industry recycled via primary recycling techniques. Primary recycling can also involve the re-­extrusion of post-­consumer plastics. Mechanical recycling, also known as secondary recycling, is the process of recovering plastic solid waste (PSW) for the re-­use in manufacturing plastic products via mechanical means [1]. Recycling PSW via mechanical means involves a number of treatments and preparation steps to be considered. Mechanical recycling technologies involved processes including the separation of polymer types, decontamination, size reduction, remelting and extrusion into pellets. Generally, the first step in mechanical recycling involves size reduction of the plastic to a more suitable form (pellets, powder or flakes). This is usually achieved by milling, grinding or shredding. Mechanical recycling is still the most preferred and viable recycling method. There are many factors that can be considered to facilitate and improve the mechanical recycling of bio-­based materials. For instance, additives can play an important role in improving bioplastics for durable uses by providing cost reduction, reinforcement, inhibiting degradability, increasing thermal resistance, reducing brittleness and increasing crystallization rate [5]. Another approach to the recycling of plastics wastes involves the generation of monomers and building blocks in high purity, from the plastic wastes, enabling the re-­manufacture of the original or new plastics [2]. Such novel recycling (e.g. glycolysis, ammonolysis, pyrolysis, etc.) represents a significant technological advancement that could supplement existing mechanical recycling techniques. Chemical recycling to make monomers, in the case of nylon and polyesters, has been established and disposal of very complex and contaminated mixtures of plastics by incineration has been developed. Products of chemical recycling have proven to be useful as fuel. Under the category of chemical recycling advanced process (similar to those employed in the petrochemical industry) appear e.g. pyrolysis, gasification, liquid–gas hydrogenation, viscosity breaking, steam or catalytic cracking and the use of PSW as a reducing agent in blast furnaces [1].

11.2 Mechanical recyling To study the recyclability of the polymeric materials including bioplastics and their derivatives, it is a well-­tried practice to simulate the mechanical recycling by doing multiple extrusions and to find the durability or service life by

Waste treatment of bio-­based compostable polymers Chapter | 11  351

accelerated thermal and hydrothermal ageing. These methods make it possible to assess the effects of thermal, hydrothermal and thermomechanical degradation. Scientific findings concerning the recycling of bioplastics, their blends and thermoplastic biocomposites, with special focus on mechanical recycling of bio-­based materials were presented [5]. The effect of multiple injection-­moulding reprocessing of three biodegradable matrices (PLLA (poly(l-­lactide)), Mater-­Bi TF01 U/095R (bioplastic based on aliphatic polyester), Mater-­Bi YI014U/C (thermoplastic formulation based on starch)) on their mechanical properties, melt flow rate, molecular weight, phase transition and degradation temperature was studied [6]. It has been found that, with successive reprocessing, tensile, flexural and impact strength decreased. Drop in mechanical properties has been assigned to degradation of the matrices, as corroborated by melt flow and molecular weight analysis. Although reprocessing did not significantly affect the glass transition, it diminished the melting point and degradation temperature of polymers. The results showed that the capacity of the three matrices to sustain reprocessing differs to great extend. The reprocessing of the aliphatic polyester Mater-­Bi for up to 10 times caused no major changes in the mechanical properties of the polymers, whilst starch-­based Mater-­Bi wastes should be destinated to composting, since its recyclability is very poor. PLLA can be recycled for up to five times without suffering a drastic loss in mechanical and thermal properties. The behavior of PLA systems upon adding small amounts of commercial impact modifiers, and the effects of multiple recycling steps have been studied [7]. The use of impact modifiers can effectively improve mechanical properties. Recycling led to a significant reduction in the impact strength; however a relatively high fraction of the other mechanical properties was still retained. The results pointed out that the properties are affected by the change of crystallinity correlated with the decreased molecular weight, due to reprocessing. As a consequence, the recycled materials were stiffer, with reduced deformability, impact resistance and water absorption if compared to the reference one. The effects of multiple mechanical recycling on the structure and properties of amorphous polylactide was studied [8]. Polylactide (PLA) submitted to in-­ plant recycling simulation underwent thermo-­ mechanical degradation which modified its structure and morphology and induced changes on its thermal and mechanical properties (see Fig. 11.1). Although no significant changes were found from the observation of the functional groups of PLA, a remarkable reduction in molar mass was found, due to chain scission. PLA remained amorphous throughout the reprocessing cycles, but the occurrence of a cold-­crystallization during DSC and DMTA measurements, which enthalpy increased with each reprocessing step, suggested chain scission due to thermo-­ mechanical degradation. The use of poly(lactic acid) (PLA) industrial waste as a source of raw material for certain applications, as well as to understand the effects of the annealing on the fracture behavior of PLA was evaluated [9]. PLA waste has been

352  Compostable Polymer Materials

FIG. 11.1  Results from tensile and impact testing of PLA after reprocessing: (A) Young modulus and impact value. (B) Stress and strain at break. (Reprinted from Badia JD, Strömberg E, Karlsson S, Ribes-­Greus A. Material valorisation of amorphous polylactide. Influence of thermo-­mechanical degradation on the morphology, segmental dynamics, thermal and mechanical performance. Polym Degrad Stabil 2012;97:670.)

simulated by an initial step of extrusion in a single screw extruder and pelletizing. Specimens of virgin and reprocessed PLA were obtained by injection molding. An annealing treatment capable of increasing the percentage of crystallinity (determined by differential scanning calorimetry) was also analyzed in reprocessed and non reprocessed specimens. The fracture behavior was studied at slow and high testing speed, applying the linear elastic fracture mechanics (LEFM) on single edge notched bend (SENB) specimens. This study revealed that the fracture toughness of the reprocessed PLA was basically the same that the virgin PLA and also that the increase in the crystalline fraction produced an improvement on the fracture toughness, at slow loading rate. It was concluded that the recycling step resulted in some structural changes but did not

Waste treatment of bio-­based compostable polymers Chapter | 11  353

alter significantly the mechanical and fracture properties. The annealing of PLA promoted an increase in the crystallinity with a raise in the HDT value, as well as an increase in the values of elastic modulus and tensile strength. The melt recycling of PLA plastic wastes to produce biodegradable PLA fibers by a melt spinning process was studied [10]. Melt spinning quality, mechanical, structural, thermal, surface morphological and dyeing properties of the PLA fiber samples were characterized. The results showed that melt spinning of PLA fibers from recycled wastes with acceptable properties is possible. The influence of processing conditions during melt extrusion on the degradation of poly(l-­lactide) (PLLA) has been investigated [11]. PLLA polymer was processed by melt extrusion in a double screw extruder at 210 and 240 °C. At the lowest processing temperature used, 210 °C, the loss in Mn is less dependent on the residence time in the melt compared to when processed at a temperature of 240 °C. The presence of moisture in the material affects the loss in Mn to a great extent when processing is done at 210 °C. The potential of close-­looped mechanical recycling of the PLA material that used in 3D printing was investigated [12]. Repeated 3D printing process can only last for two cycles, as significant deteriorations were detected in the viscosity values, which made the material unsuitable for further reprocessing. Though mechanical performance deteriorated to a limited extent, significant decreases in rheological property conformed with losses in molecular weights. Noticeable deteriorations have also been observed in the FTIR spectra and SEM micrographs; the presence of carbonyl groups and surface pinholes, indicating that thermomechanical degradation has taken places during melt processing and it is responsible for decrements in the molecular weight of PLA. Blending recycled PLA with virgin material improved the viscosity, which facilitates the close-­looped recycling process. It was concluded based on life cycle assessment approach that the environmental burdens associated with close-­looped recycling were lower than those of placing the 3D printed products in incineration or in landfill. The properties of parts 3D printed with virgin polylactic acid (PLA) to those printed with recycled PLA was compared [13]. Mechanical testing showed that 3D printing with recycled PLA is a viable option. With the recycled filament, tensile strength decreased 10.9%, shear strength increased 6.8%, and hardness decreased 2.4%. The tensile modulus of elasticity was statistically unchanged. Mechanical and chemical recyling of PLA blends, i.e. PLA/PS blends and polylactic acid/polyethylene (PLA/PE) and polylactic acid/poly (butylene succinate) (PLA/PBS) polymer blends were reported by Hamad et al. [14]. The effect of processing cycles (extrusion and injection) on the properties of PLA/ PS polymer blend was investigated. The results showed that intrinsic viscosity of the samples, which is directly related to the molecular weight, decreased after each processing cycle and it decreased steadily with increasing the processing number.

354  Compostable Polymer Materials

The degradation mechanisms engendered by multiple reprocessing of a plasticized PLA (pPLA) grade obtained by grafting/polymerizing acrylated poly(ethylene glycol) (acryl-­PEG) within PLA via reactive extrusion, and a neat PLA as reference was studied [15]. Up to five successive processing cycles including extrusion and compression-­molding, the tensile and impact properties drastically dropped indicating an embrittlement of pPLA. Structural analyses revealed that reprocessing caused these mechanisms: chain scission of PLA, crystallization of PLA, damaging of the inclusions, decrease of the size of poly(acryl-­PEG) phases within the inclusions, and cracking of PLA. Commercially available grades of pure PLA and PLA blended with PC and HDPE were investigated using methods that simulated post-­processing and post-­ consumer recycling [16]. Multiple processing of pure PLA did not affect impact strength or Tg, but caused crystallization and an increase in the MFI, which indicated degradation during processing. Multiple processing of the blends did not significantly affect the elastic modulus of the materials, but affected the elongation at break. However, although elongation at break increased with the number of extrusions for the PLA/HDPE blend, the trend was reversed for the PLA/PC blend. The results indicated that multiple processing of the PLA/HDPE blend caused increased dispersion and thus increased elongation at break, whereas the dominating mechanism in the PLA/PC blend was degradation, which caused a decrease in elongation at break. Post-­consumer recycling of the PLA/PC blend was simulated and the results clearly showed that ageing corresponding to one year of use caused a significant degradation of PLA. Pure PLA was severely degraded after only one ageing cycle. Although the PLA/PC blend showed some improved mechanical properties and resistance to degradation compared with pure PLA, one ageing cycle still caused a severe degradation of the PLA and even the PC was degraded as indicated by the formation of small amounts of bisphenol A. The recycling process of PLA within the context of packaging containing multiple types and amounts of production waste produced by extrusion was analyzed [17]. The analysis of the recycling behavior showed that internal PLA production waste is well suitable for recycling. The influence of the recycling on the molecular weight was reported negligible. The effect on the viscosity and thus on the extrusion process was higher. Packaging relevant properties like mechanical or optical properties were hardly influenced. Especially recycling with a recycling quota of up to 50% had an insignificant effect on the film properties. The effect of different mechanical recycling processes on the structure and some important properties, in food packaging applications, of PLA was studied [18]. A commercial grade of PLA was melt compounded and compression molded, then subjected to two different recycling processes. The first recycling process consisted of an accelerated ageing and a second melt processing step, while the other recycling process included an accelerated ageing, a demanding washing process and a second melt processing step. The intrinsic viscosity

Waste treatment of bio-­based compostable polymers Chapter | 11  355

measurements indicate that both recycling processes produce a degradation in PLA, which is more pronounced in the sample subjected to the washing process. DSC results suggest an increase in the mobility of the polymer chains in the recycled materials; however the degree of crystallinity of PLA seems unchanged. The optical, mechanical and gas barrier properties of PLA do not seem to be largely affected by the degradation suffered during the different recycling processes. These results suggest that, despite the degradation of PLA, the impact of the different simulated mechanical recycling processes on the final properties is limited. Therefore, it was concluded that the results revealed the potential of mechanically recycled PLA to be reused, even in demanding applications such as food packaging. The influence of small amounts of PLA on the recycling properties of post-­ consumer PET from bottles has been investigated by rheological, mechanical, morphological and thermogravimetric analysis [19]. The amount of PLA used was up to 5% by weight. Shear viscosity measurements revealed that the blends have lower viscosities than PET, and any presence of water can lead to a drop in the viscosity, due to significant hydrolytic chain-­scission. The results indicated that this presence could significantly affect the rheological properties under non-­isothermal elongational flow, while the mechanical properties were considerably affected only in some circumstances and the thermal stability was not significantly modified. The properties od starch-­based polymer (Mater-­Bi series Z), mainly composed of starch and caprolactone was reprocessed several times in an extruder to investigate the recyclability [20]. It was concluded that polymer was relatively stable to thermal degradation and to the applied mechanical stress. The effects of reprocessing on the mechanical properties of poly(butylene succinate) (PBS) were discussed along with the evaluation of the possibility of using PBS as a biodegradable material for material recycling in water-­usage environment [21]. The mechanical properties of PBS after immersion in water were investigated and it was found that the bending strength of PBS decreased as the immersion time and the immersion temperature (25 °C, 50 °C and 75 °C) increased due to the chemical degradation of PBS caused by hydrolysis. The degraded PBS was then reprocessed and re-­examined to investigate the mechanical properties. An increase both in the bending strength and in the molecular weight of PBS was observed. The improvement in the bending strength after reprocessing was possibly due to the removal of the cracks and voids observed on the surface of the degraded PBS. The higher molecular weight of PBS after reprocessing was explained by re-­synthesis. The effects of the number of reprocessing times was also reported. It was found that the molecular weight of PBS slightly decreased after reprocessing but remained almost constant up to even three times of reprocessing (see Fig. 11.2). The results indicate that there was no deterioration and hence no degradation in

356  Compostable Polymer Materials

FIG. 11.2  Molecular weight (Mw) of PBS against the number of reprocessing. (Reprinted from Kanemura C, Nakashima S, Hotta A. Mechanical properties and chemical structures of biodegradable poly(butylene succinate) for material reprocessing. Polym Degrad Stabil 2012;97:972.)

the mechanical properties of PBS just by reprocessing at least for three times under the given molding condition, which demonstrates that PBS is indeed a suitable polymer for the material recycling. Contrarily, it was found that the molecular weight of PLA was drastically decreased by molding, where PLA was critically damaged by high-­temperature molding during the reprocessing. The thermo-­mechanical degradation and recyclability of poly(butylene succinate) PBS was investigated via subjecting the material to five consecutive extrusion cycles under different temperature profiles, in order to simulate reprocessing intensive conditions [22]. It was found that PBS, when reprocessed at temperatures higher than 190 °C, suffers from branching/recombination degradation reactions, resulting in extrudates of higher solution viscosity, and of bimodal distribution of molar masses. In contrast to other aliphatic polyesters, such as PLA and PHBV, PBS degradation was governed by branching/ recombination reactions, resulting in increased solution and melt viscosity, weight-­average molecular weight and polydispersity index, and almost constant carboxylic end group concentrations. The incorporation of stabilizers at the level of 0.1% efficiently maintained polymer properties through reprocessing, while the higher concentration of 0.5% had a negative impact on extrudates quality. On thermal properties basis, degraded and stabilized PBS exhibited similar melting and degradation points compared to virgin material, showing its possibility for mechanical recycling. However, the induced degradation resulted in accelerated melt crystallization and lower degree of crystallinity, which might

Waste treatment of bio-­based compostable polymers Chapter | 11  357

be attributed to the nucleating effect of the formed branches. The addition of stabilizers restricted the increase of the melt crystallization rate, and the relevant melt behavior was found similar to virgin material. Recyclability of the bioplastic polyhydroxybutyrate-­co-­valerate (PHBV) was studied with multiple melt processing (five cycles), with their performances evaluated [23]. The reprocessability was confirmed with four different mechanical tests (tensile, flexural, impact, and DMA), GPC measurements, DSC, TGA, FTIR analysis, SEM images, and density measurements. All the measurements have shown that the material properties are maintained except for a slight decrease in the fifth cycle. Gel permeation chromatography studies revealed that the molecular weight of the polymer does not decrease drastically; however, a drop was observed after third, fourth, and fifth cycle (8.7% decrease after third cycle, 13.5% decrease after fourth cycle, and 16.6% decrease after fifth cycle). The differential scanning calorimeter showed that the glass transition and melting temperatures did not change upon reprocessing, but the degree of crystallinity was reduced as a consequence of melt processing. The thermal gravimetric analysis showed that the onset value of thermal decomposition decreased very slightly. It was observed by Fourier transform infrared spectroscopy that the chemical structure of PHBV was maintained without any side chain reaction during processing. The evolution of thermal and mechanical properties, as well as structural changes of poly(hydroxybutyrate) (PHB) up to three extrusion cycles were investigated [24]. Results indicated a significant reduction in mechanical properties already at the second extrusion cycle, with a reduction above 50% in the third cycle. An increase in the crystallinity index was observed due to chemicrystallization process during degradation by chain scission. FTIR results did not show any significant changes in polymeric structures associated to the formation of new chemical groups. Similarly, the thermal stability of PHB along processing cycles exhibited only a trend to decrease the thermal stability with extrusions paths. The reprocessing studies of compostable polymers are summarized in Table 11.1.

11.3 Chemical recycling Two main processes have been used for chemical recycling of PLA. The first one is hydrolysis of PLA at high temperatures to obtain lactic acid, and the second one is thermal degradation of PLA to prepare l,l-­lactide, which is a cyclic dimer and can be used for polymerization of new PLA [5,12]. Two routes to selective chemical recycling of poly(l-­lactic acid) (PLLA)-­ based polymer blends, PLLA/polyethylene (PE) and PLLA/poly(butylene succinate) PBS were carried out using environmentally benign catalysts, clay catalysts and enzymes [25]. One is the direct separation of PLLA and PE first by their different solubilities in toluene, followed by the chemical recycling of PLLA

Polymer

Reprocessing conditions

Properties measured

Number of cycles

Reference

Poly(butylene succinate) PBS

140 °C

Molecular weight Bending strength

3

[21]

Poly(butylene succinate) PBS

190, 200 and 210 °C

Molecular weight Thermal (by DSC method) Carboxyl end group concentrations

5

[22]

Poly(lactic acid) PLA

160–170–190–200–190 °C

Tensile properties Impact properties

5

[8]

Poly(l-­lactic acid) PLLA

170–195 °C

Tensile properties Flexural properties Impact properties

8

[6]

Poly (lactic acid) PLA

180–210 °C

Thermal (by DSC method) Tensile properties Fracture properties

1

[9]

Poly(lactic acid) PLA

180–200–200 °C

Flexural properties Tensile properties

3

[7]

Poly(lactic acid) PLA

210 °C (3D printing)

Flexural properties Tensile properties Rheological properties Thermal properties (DSC and TGA)

2

[12]

Poly(lactic acid) PLA

210 °C (3D printing)

Tensile properties, shear testing, hardness

1

[13]

358  Compostable Polymer Materials

TABLE 11.1  Reprocessing studies of compostable polymers.

180 °C

Tensile properties Impact properties Thermal (by DSC) Microstructure (SEM, AFM methods)

5

[15]

Poly(lactic acid) PLA

-­ 

Accelerated ageing and a second melt processing step; -­ Accelerated ageing, a washing process and a second melt processing step

Optical, mechanical (hardness, Young modulus) and gas barrier properties; Thermal properties (DSC, TGA)

1

[18]

Mater-­Bi TF01 U/095R (bioplastic based on aliphatic polyester),

120–140 °C

Tensile properties Flexural properties Impact properties

10

[6]

Mater-­Bi YI014U/C (thermoplastic formulation based on starch)

165 °C

Tensile properties Flexural properties Impact properties Melt flow index

2

[6]

Materi Bi series Z (ZIO1U) (bioplastic based on corn starch and polycaprolactone)

100–130–170–170 °C

Tensile properties Rheological measurements

5

[20]

Polyhydroxybutyrate-­co-­ valerate (PHBV)

170 °C

Tensile properties Flexural properties Impact properties GPC, DSC, TGA

5

[23]

Poly(3-­hydroxybutyrate) (PHB)

165–170–170 °C

Tensile properties, FTIR, TGA

3

[24]

Waste treatment of bio-­based compostable polymers Chapter | 11  359

Poly(lactic acid) PLA

360  Compostable Polymer Materials

using montmorillonite K5 (MK5). The other is the selective degradation of PLLA in the PLLA/PE blend by MK5 in a toluene solution at 100 °C for 1 h forming the LA oligomer with a molecular weight of Mn = 200–300 g/mol, which is the best Mn for repolymerization. Thus regenerated PLLA had a Mw of greater than 100,000 g/mol. The PE remained unchanged and was quantitatively recovered by the reprecipitation method for material recycling. In a similar procedure, chemical recycling of PLLA/PBS blend was also carried out and compared by two routes. One is the direct separation of PLLA and PBS by solubility in toluene. The other route is the sequential degradation of PLLA/PBS blend using a lipase first to degrade PBS into cyclic oligomer, which was then repolymerized to produce a PBS. Next, PLLA was degraded into repolymerizable LA oligomer by MK5. The former procedure was carried out using a single solvent; however, the latter required mixed solvents, which decreased the efficient recycling use of solvents. The chemical recycling by solvolysis of polylactic acid (PLA) and PET waste in either methanol or ethanol was investigated [26]. Zinc acetate as a catalyst was found to be necessary to yield an effective depolymerization of waste PLA giving lactate esters, while with the same reaction conditions PET remains as an unconverted solid. By taking advantage of the different reactivity to alcoholysis of the two plastics a selective depolymerization process has been developed. It not only represents a promising process for the chemical recycling of PLA but also for the treatment of mixed PLA and PET as the separation is facilitated via the reaction sequence converting PLA to a liquid monomer from which the unreacted PET can be filtered. New method of poly(lactic acid) chemical recycling by reactions with diols, diamines, and macrodiols (such as polyesterdiols and polyetherdiols) leading to potentially useful products was proposed [27]. Controlled chemical degradation processes of commercial poly(lactic acid) in the presence of small molecules, such as diols, dipentaerythriol, diamines and adipic acid or oligo(ethylene glycol) as well as polyesterodiols were studied. The processes were found to be efficient and in the presence of a catalytical amount of tin(II) octanoate lead to the formation of homo-­or block diol type copolymers or oligomers of lactic acid. In order to ensure the sustainable development, the chemical recycling of polyhydroxyalkanoates (PHAs) is proposed aiming at reducing the amount of wastes, saving the material resources and reusing the recovered monomer for producing other types of polymers [28]. At the same time, cascade flow of the PHAs could be introduced before it is finally released to the environment. It was reported that PHB and PHBV were chemically converted into vinyl monomers such as crotonic acid via pyrolysis. The polymers were fully degraded during pyrolysis to obtain pyrolyzates which consisted 70–75% of monomeric compounds. By adding certain metallic catalysts during the thermal degradation, e.g. Mg compounds; the degradation temperature can be lowered by 40–50 °C and hence, lowering the activation energy, Ea of PHB thermal degradation by 11–14 kJ mol−1. By the catalytic degradation, the polymers were selectively converted into the monomers and interestingly, almost 100% of the monomers were trans-­crotonic acid. The crotonic acid formed during the pyrolysis was further

Waste treatment of bio-­based compostable polymers Chapter | 11  361

copolymerized with a co-­monomer, i.e. acrylic acid in order to produce functional copolymers. The copolymers produced were hydrophilic, with higher content of crotonic acid increased the thermal stability of the copolymers. Lipase can act as a powerful catalyst for both green polymer production (polymerization) and chemical recycling (depolymerization). The enzyme-­ catalyzed synthesis and chemical recycling of biodegradable aliphatic polyesters and poly(carbonate ester)s was discussed [29,30]. These chemical recycling systems using an enzyme was proposed as a novel methodology for sustainable polymer recycling. Lipase catalyzes the condensation polymerization of a hydroxy acid, diacid with diol, diacid anhydride with oxirane, and polyanhydride with diol, or the ring-­opening polymerization of lactones of small to large rings, and a cyclic diester to produce the corresponding polyesters. Also, lipase catalyzes the condensation polymerization of a dialkyl carbonate with diol, and the ring-­opening polymerization of a cyclic carbonate to produce the corresponding polycarbonates. These polyesters and polycarbonates were selectively degraded by lipase to produce repolymerizable oligomers. There are two routes for the chemical recycling of PCL [31]. One is the enzymatic conversion of PCL into repolymerizable 6-­hydroxyhexanoate oligomers. This oligomer route can be used for the reproduction of PCL polymer that equals the original polymer. The other pathway is the selective ring-­closing depolymerization of PCL into the cyclic dimer, dicaprolactone (DCL), which can be used as a monomer for the same polymer and also as a comonomer for improving the polymer properties. Furthermore, DCL may become a versatile intermediate for the production of various compounds. However, the enzymatic transformation of PCL into the cyclic dimer DCL generally requires large amounts of organic solvents, such as 0.2% toluene solutions, in order to facilitate intramolecular cyclization. The lipase-­catalyzed selective transformation of poly(ε-­caprolactone) (PCL) into dicaprolactone (DCL: 1,8-­dioxacyclot etradecane-­2,9-­dione) and the repolymerization of DCL in supercritical carbon dioxide (scCO2) were carried out for the establishment of a sustainable green polymer chemistry for PCL [31]. PCL with a number-­average molecular weight of Mn = 110,000 was selectively transformed into repolymerizable cyclic DCL in 90% yield using immobilized Candida antarctica lipase (lipase CA) in scCO2 fluid by compression to 18 MPa in the presence of a small amount of water at 40 °C. The DCL obtained such was readily polymerized by lipase CA in scCO2 to produce a PCL with an Mn of 33,000 after 6 h. The enzymatic transformation of poly(ε-­caprolactone) (PCL) into repolymerizable oligomers in supercritical carbon dioxide (scCO2) using an enzyme was carried out in order to establish a sustainable chemical recycling system for PCL, which is a typical biodegradable synthetic plastic [31]. The enzymatic conversion of PCL beads having an Mn of 110,000 using Candida antarctica lipase (lipase CA) in scCO2 containing small amounts of water quantitatively afforded CL oligomers at 40 °C. The CL oligomers were readily repolymerized using the same enzyme to produce high-­molecular weight PCL. Chemical recyling of compostable polymers are summarized in Table 11.2.

Polymer

Process

Products

Reference

Poly(lactic acid) PLA

Thermal degradation

L,L-­lactide (cyclic dimer)

[5,14]

Poly(lactic acid) PLA

Hydrolysis

Lactic acid

[5,14]

Poly(lactic acid)

Controlled degradation with protic (macro)molecules

Oligomeric or polymeric materials comprising two terminal secondary hydroxyl groups (homo-­or block diol type copolymers or oligomers of lactic acid)

[29]

PLA/PBS blends

Direct separation by toluene, then degradation using a lipase

Lactic acid oligomer, cyclic butylene succinate (BS) oligomer

[26]

PLA/PBS blends

Sequential degradation using a lipase

Lactic acid oligomer, cyclic BS oligomer

[26]

PHB

Pyrolysis

Crotonic acid

[28]

PHBV

Pyrolysis

Crotonic acid

[28]

Polycaprolactone (PCL)

Enzymatic conversion

6-­Hydroxyhexanoate oligomers

[29–31]

Polycaprolactone (PCL)

Selective ring-­closing depolymerization (enzymatic transformation)

Dicaprolactone (DCL) (cyclic dimer)

[29–31]

362  Compostable Polymer Materials

TABLE 11.2  Chemical recycling of compostable polymers.

Waste treatment of bio-­based compostable polymers Chapter | 11  363

11.4 Comparison of compostable polymer waste treatment methods Carbon and energy footprints of the waste treatment phase of compostable polymers were compared [32]. The best waste treatment option for biodegradable materials has been discussed, including home and industrial composting, anaerobic digestion and incineration. The following materials were considered in the study: paper, cellulose, starch, polylactic acid (PLA), starch/polycaprolactone (Mater Bi), polybutylene adipate-­terephthalate (PBAT, Ecoflex) and polyhydroxyalkanoates (PHA). Not all materials are suitable for all types of biological treatment. For example PLA does not degrade in home composting in temperate climates and PBAT and mechanical pulp do not degrade in anaerobic digestion. Of the discussed waste treatment options, digestion was found to be the most favorable for biodegradable materials for the time being because it combines energy recovery with the production o of digestate, which can be used as a soil conditioner. The results show that anaerobic digestion has the lowest footprint for the current level of technology, but incineration may become better in the future if energy efficiency in waste incineration plants improves significantly. Home composting is roughly equal to incineration with energy recovery in terms of carbon and energy footprint when carbon credits are considered. The same applies to industrial composting if carbon credits are assigned for compost to replace straw. Recent developments in the utilization of bio-­waste show that in the future it will be more and more treated by a combination of energetic and material utilization in fermentation plants with a subsequent composting plant or a combination of both [33]. Optimization opportunities to ferment biodegradable polymers or to optimize the hydrolysis as pre-­step prior fermentation were investigated. Materi Bi™, a compound based on starch and polycaprolactone (PCL) and BioFlex®F1110, based on polylactic acid (PLA) compounded with an aliphatic-­aromatic copolyester named Ecoflex® were tested under anaerobic conditions. It was reported that thermoplastic starch biopolymers were faster anaerobically degradable than polylactic acid biopolymers and based on the input mass of material they also produce higher amounts of biogas. Results of hydrolysis experiments that the hydrolysis of biopolymers, their solubility in water and the availability for microorganisms are possible at high temperatures, but still require further research. Like in the anaerobic preliminary tests, the starch-­based biopolymers are faster and more quickly solvable than biopolymers based on lactic acid. Based on the results the selection of a temperature range of min. 90 °C is currently recommended. Life cycle assessment (LCA) model comparing the waste management options for starch-­polyvinyl alcohol (PVA) biopolymers including landfill, anaerobic digestion, industrial composting and home composting to identify the environmentally superior disposal option for the specific biopolymer

364  Compostable Polymer Materials

waste was presented by Guo et al. [34]. Materials modeled include three starch–PVOH based biopolymers i.e. wheat based foam (WBF) and two additional potato/maize starch-­based foams (PSBF/MSBF). The ranking of biological treatment routes for starch–PVOH biopolymer wastes depended on their chemical compositions. Anaerobic digestion represents the optimum choice for starch–PVOH biopolymer containing N and S elements in global warming potential (GWP100), acidification and eutrophication but not on the remaining impact categories, where home composting was shown to be a better option due to its low energy and resource inputs. For those starch–PVOH biopolymers with zero N and S contents, i.e. MSBF and PSBF foams home composting delivered the best environmental performance amongst biological treatment routes in most impact categories (except for GWP100). Study focusing on the analysis of the best final disposition of bioplastic wastes in order to maximize the energy saving was carried out by using the Life Cycle Assessment (LCA) methodology for PLA and Mater Bi with reference to composting, incineration, anaerobic digestion and mechanical recycling processes [35]. It was concluded that mechanical recycling of a bioplastic leads to an overall reduction of the environmental impact associated to the production and disposal of the bioplastic itself. Life cycle assessment (LCA) comparing three forms of poly(lactic acid) (PLA) disposal: mechanical recycling, chemical recycling and composting was presented [36]. The LCA data was taken from lab scale experiments for composting and hydrolysis steps. Polymerization data in chemical recycling was obtained from computer simulation. Mechanical recycling data from lab scale were combined with the data from a plastics commercial mechanical recycling plant. The analysis considered two different product systems based on the input of the recycled PLA in the product system. Considering the categories: climate change, human toxicity and fossil depletion, the LCA showed that mechanical recycling presented the lowest environmental impact, followed by chemical recycling and composting. Among the forms of recycling, the most important input was the electricity consumption. The end-­ of-­ life options of bio-­ based (made of polyhydroxyalkanoate (PHA)) and polyethylene plastic bags were compared within the context of Singapore [37]. Life cycle assessment (LCA) was used for the investigation. The following disposal options were considered: (1) land filling at Singapore’s offshore Semaku Island, (2) incineration, and the (3) composting of bio-­bag. While the energy from incineration of both bags are fed back into the LCA production stage, compost from bio-­bags can be used as a peat substitute, thus generating carbon dioxide savings from reduced peat production. The end-­of-­ life environmental impacts were generated for global warming potential, acidification, and photochemical ozone formation. The highest end-­of-­life impacts were observed from land filling of bio-­bags. Next highest disposal impacts were

Waste treatment of bio-­based compostable polymers Chapter | 11  365

from incineration, and least of all from the composting of bio-­bags. The greenhouse gas savings from peat substitutes derived form the compost material were found to be rather insignificant. It was reported that the environmental burdens generated from any of the disposal options were less significant compared to those from both products life cycle production stages. From the trend of the final cradle-­to-­grave results, it can be claimed that the life cycle production of bio-­bags from PHA can only be considered as environmentally friendly alternatives to conventional plastic bags if clean energy sources are supplied throughout its production process. A high water content is one of the major drawbacks for the utilization of bio-­ oil [38]. One technology which shows the potential to satisfy the demand for bio-­oil with a reduced water content is the flash co-­pyrolysis of biomass with biopolymers. The influence of biopolymers on the pyrolysis yield of a biomass waste stream is investigated with a semi-­continuous home-­built pyrolysis reactor. Polylactic acid (PLA), corn starch, polyhydroxybutyrate (PHB), Biopearls, Eastar, Solanyl and potato starch are the biopolymers under investigation. All biopolymers show their specific benefits during flash co-­pyrolysis with willow (target biomass) at 723K. Each (co-­)pyrolysis of pure willow (reference) and all 1:1 (w/w) ratio willow/biopolymer blends was evaluated based on five predefined criteria. The flash co-­pyrolysis of biomass and biopolymers resulted in improved pyrolysis characteristics. The flash co-­pyrolysis of 1:1 willow/PHB was the most performance option, while 1:1 willow/PLA, 1:1 willow/Biopearls and 1:1 willow/potato starch showed increased potential as well. The fact that biopolymers, despite their biodegradability, should be considered as waste, further increases the appealing features of the flash co-­pyrolysis of biomass and biopolymers. Economic assessment of flash co-­pyrolysis of short rotation coppice and biopolymer waste streams was presented by Kuppens et al. [39]. Flash co-­pyrolysis of biomass and waste of biopolymers synergistically improves the characteristics of the pyrolysis process, e.g. decrease in the amount of pyrolytic water, more bio-­oil, and less char production and an increase of the higher heating value (HHV) of bio-­oils. The economic consequences of the synergistic effects of flash co-­pyrolysis of 1:1 w/w ratio blends of willow and different biopolymers waste streams were investigated via cost-­ benefit analysis and Monte Carlo simulations. Flash co-­pyrolysis leads to better economic results as compared to flash pyrolysis of pure willow: the net present value (NPV) of cash flows has been increased with at least 0.405 MEUR (for the willow-­corn starch blend). Of all the biopolymers under investigation, polyhydroxybutyrate (PHB) is the most promising, followed by Eastar (aliphatic-­aromatic polyester), Biopearls (bioplastic granulates based on PLA), potato starch, polylactic acid (PLA), corn starch and Solanyl (bio-­based plastics made out of reclaimed potato starch) in order of decreasing profits.

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