Biodegradation of Agricultural Plastic Films: A Critical Review

The growing use of plastics in agriculture has enabled farmers to increase their crop production. One major drawback of most polymers used in agriculture is the problem with their disposal, following their useful life-time. Non-degradable polymers, being resistive to degradation (depending on the polymer, additives, conditions etc) tend to accumulate as plastic waste, creating a serious problem of plastic waste management. In cases such plastic waste ends-up in landfills or it is buried in soil, questions are raised about their possible effects on the environment, whether they biodegrade at all, and if they do, what is the rate of (bio?)degradation and what effect the products of (bio?)degradation have on the environment, including the effects of the additives used. Possible degradation of agricultural plastic waste should not result in contamination of the soil and pollution of the environment (including aesthetic pollution or problems with the agricultural products safety). Ideally, a degradable polymer should be fully biodegradable leaving no harmful substances in the environment. Most experts and acceptable standards define a fully biodegradable polymer as a polymer that is completely converted by microorganisms to carbon dioxide, water, mineral and biomass, with no negative environmental impact or ecotoxicity. However, part of the ongoing debate concerns the question of what is an acceptable period of time for the biodegradation to occur and how this is measured. Many polymers that are claimed to be ‘biodegradable’ are in fact ‘bioerodable’, ‘hydrobiodegradable’, ‘photodegradable’, controlled degradable or just partially biodegradable. This review paper attempts to delineate the definition of degradability of polymers used in agriculture. Emphasis is placed on the controversial issues regarding biodegradability of some of these polymers.     

Miscibility and Biodegradability of Poly(Butylene Succinate)/Poly(Butylene Terephthalate) Blends

As one of the biodegradable polymers, the blend of poly(butylene succinate) and poly(butylene terephthalate) is dealt with in this study. In our previous work, it was demonstrated that PBS and PBT are immiscible not only from the changes of T _g but also from logG–log G plots. It is expected that the biodegradability of the blends could be improved by enhancing the miscibility. We tried to induce the transesterification reaction between two polyesters with various time intervals to enhance the miscibility of the blends. The extent of transesterification reaction was examined by ¹H-NMR. We utilized a dynamic mechanical thermal analyzer and a rotational rheometer to investigate the changes in miscibility. We also verified the biodegradability of PBS/PBT blends after the transesterification reaction by the composting method.     

Biodegradability of Poly (lactic acid), Preparation and Characterization of PLA/Gum Arabic Blends

In this study, the biodegradation of PLA films using microorganisms from Lake Bogoria (Kenya) were investigated. The biodegradation tests done using certain strains of thermophilic bacteria showed faster biodegradation rates and demonstrated temperature dependency. The biodegradation of the PLA films was studied using Gel Permeation Chromatography (GPC) and light microscopy. The biodegradation of PLA was demonstrated by decrease in molecular weight. The preparation and characterization of PLA/Gum Arabic blends were also investigated using DSC, TGA, TMA and NMR. In summary, the results obtained in this research show that PLA films undergo fast biodegradation using thermophiles isolated from Lake Bogoria. The PLA/GA blends studies show it is possible to prepare films of varying hydrophobic–hydrophilic properties for various applications.     

Biodegradability of a Selected Range of Polymers and Polymer Blends and Standard Methods for Assessment of Biodegradation

Synthetic polymers are important to the packaging industry but their use raises aesthetic and environmental concerns, particularly with regard to solid waste accumulation problems and the threat to wildlife. Some concerns are addressed by attention to problems associated with source reduction, incineration, recycling and landfill. Others are addressed by the development of new biodegradable polymers either alone or in blends. Materials used for biodegradable polymers include various forms of starch and products derived from it, biopolyesters and some synthetic polymers. Starch is rapidly metabolised and is an excellent base material for polymer blends or for infill of more environmentally inert polymers where it is metabolised to leave less residual polymer on biodegradation. This should help to improve the environmental impact of waste disposal. A number of standard methods have been developed to estimate the extent of biodegradability of polymers under various conditions and with a variety of organisms. They tend to be used mainly in the countries where they were developed but there is much overlap between the standards of different countries and wide scope for development of consistent and international standards.     

Biodegradability of Quaternized, Crosslinked Sugarcane Bagasse

Quaternized, crosslinked sugarcane bagasse can adsorb anionic dyes from textile wastewater. Disposal of dye-saturated adsorbent by composting or land application would require that modifications made to the bagasse do not interfere with its decomposition. The impact of quaternization and crosslinking on bagasse biodegradability was examined. Bagasse in varying states of modification was mixed with soil and monitored for carbon dioxide evolution for four weeks at 27°C. After subtracting the amount of carbon evolved from control soil samples, the net carbon evolved from the bagasse samples was determined and used as a measure of their extent of biodegradation. Biodegradability decreased in the order: bagasse (approx. 60% degraded after four weeks) > quaternized bagasse > quaternized, epichlorohydrin-crosslinked bagasse > quaternized, methylene-bis-acrylamide-crosslinked bagasse > epichlorohydrin-crosslinked bagasse (less than 5% degraded). Crosslinking severely impacted biodegration, probably by preventing the penetration of (hemi)cellulytic and lignolytic enzymes into the interior of the modified bagasse particles. It is concluded that the biodegradability of quaternized, crosslinked bagasse is too low for composting or land application.     

Biodegradability of chitin- and chitosan-containing films in soil environment

The biodegradation of polyethylene-chitin (PE-chitin) and polyethylene-chitosan (PE-chitosan) films, containing 10% by weight chitin or chitosan, by pure microbial cultures and in a soil environment was studied. Three soil-inhabited organsims,Serratia marcescens, Pseudomonas aeruginosa, andBeauveria bassiana were able to utilize chitin and chitosan in prepared PE-chitin and PE-chitosan films after eight weeks of incubation at 25°C in a basal medium containing no source of carbon or nitrogen. In a soil environment, the biodegradation of those films was studied and compared with a commercial biodegradable film containing 6% by the weight of corn starch. In soil placed in the lab, 73.4% of the chitosan and 84.7% of the chitin in the films were degraded, while 46.5% of the starch in the commercial film was degraded after six months of incubation. In an open field, 100% of the chitin and 100% of the chitosan in the films were degraded, but only 85% of the starch in the commercial film was degraded after six months of incubation. The weight of controls, (polyethylene films), remained mainly stable during the incubation period. Both PE-chitin and PE-chitosan films degraded at a higher rate than the commercial starch-based film in a soil environment indicating the potential use of chitin-based films for the manufacturing of biodegradable packaging materials.     

Anaerobic Biodegradation of Blends Based on Polyvinyl Alcohol

The presented work deals with blends composed of polyvinyl alcohol (PVA) and biopolymers (protein hydrolysate, starch, lignin). PVA does not belong to biologically inert plastics but its degradation rate (particularly under anaerobic conditions) is low. A potential solution to the issue problem lies in preparation of blends with readily degradable substrates. We studied degradation of blow-molded films made of commercial PVA and mentioned biopolymers in an aqueous anaerobic environment employing inoculation with digested activated sludge from the municipal wastewater treatment plant. Films prepared in the first experimental series were to be used for comparing biodegradation of blends modified with native or plasticized starch; in this case effect of plasticization was not proved. The degree of PVA degradation after modification with native or plasticized starch increases in a striking and practically same manner already at a starch level as low as approximately 5 wt.%. Films of the second experimental series were prepared as additionally modified with protein hydrolysate and lignin. Only lignin-modified samples exhibited a somewhat lower degree of biodegradation but regarding the measure of lignin present in blend this circumstance is not essential. Level of biodegradation with all discussed films differed only slightly—within range of experimental error.     

Biodegradation of Waste Cellulose

Environmental issues such as the depletion of nonrenewable energy resources and pollution are very topical and would need more scientific attention in order to be addressed in a way beneficial to life. The extent of solid waste production is a global concern and development of its bioenergy potential can simultaneously address issues such as pollution control and renewable energy development. Various wastepaper materials, a major component of solid waste, have been treated with cellulase from Trichoderma viride to bioconvert its cellulose component into fermentable sugars that could be utilized as feedstock for bioproduct development. These paper materials exhibited different susceptibilities toward the cellulase and showed different sugarreleasing patterns when increasing amounts of each paper were treated with the enzyme. Bioconversion of paper with different enzyme concentrations and during various time intervals also resulted in nonsimilar sugar-releasing patterns. With all the paper materials, a general decline in efficiency was observed with increasing amounts of sugar produced during the different bioconversion variables investigated.     

Biodegradability: An assessment of commercial polymers according to the Canadian method for anaerobic conditions

The degradation of several biodegradable polymers was measured as a result of exposure to an anaerobic medium. The polymers investigated included materials based upon polylactic acid, polylactone, and poly(hydroxy butyrate/valerate) as well as those incorporating starch-based materials. The degradation was monitored by methane and carbon dioxide evolution. In addition, the physical and chemical changes were noted as a result of exposure. These measurements included changes in mass, dimension, and molecular weight. FTIR, UV-vis, proton, and¹³C NMR spectra were also recorded prior to and after exposure. The results clearly indicated that several biological and chemical degradation processes were occurring with the biodegradable polymers studied.Paper presented at the Bio/Environmentally Degradable Polymer Society—Second National Meeting, August 19–21, 1993, Chicago, Illinois.Issued as NRCC No. 37549.     

Mechanical Property and Biodegradability of Cast Films Prepared from Blends of Oppositely Charged Biopolymers

Biodegradable cast films of about 50 m thickness were fabricated by blending oppositely charged biopolymers such as anionic starch–chitosan, and cationic starch–pectin. The tensile strength and elongation at break (%) of films were evaluated as well as their capacity to degrade in compost. Films recovered from soil every 48 h showed consistent degradation (weight loss), diminution of the polymers characteristic peak absorbance in the carbohydrate fingerprint region of the FTIR, and changes in the surface morphology via scanning electron microscopy (SEM). Anionic starch–chitosan films had much superior tensile strength and elongation compared to cationic starch–pectin, suggesting that the ionic bonds formed between anionic-starch and positively charged groups in chitosan polymer were much more stable and stronger. Initially, both films lost about 36% weight within 96 h, which also correlated well with the loss in the characteristic absorption peaks in the region of the infrared spectrum typical of biopolymers. The total mineralization of films by microorganisms in compost soil was also measured using respirometric techniques. Though the rate of mineralization differed for two formulations, total mineralization (extent) for both films were achieved within 45 days.     

Study of the Determination of the Ultimate Aerobic Biodegradability of Plastic Materials Under Controlled Composting Conditions

Several ISO standards for determining the ultimate aerobic/anaerobic biodegradability of plastic materials have been published. In particular, ISO 14855-1 is a common test method that measures evolved carbon dioxide using such methods as continuous infrared analysis, gas chromatography or titration and others (ISO 14855-1(2005.9)). This method is a small-scale test for determining the ultimate aerobic biodegradability of plastic materials, where the amounts of compost inoculum and test sample in one tenth comparing with that of ISO 14855-1. This method is well versed in ISO/DIS 14855-2 which the carbon dioxide evolved from test vessel is determined by gravimetric analysis of carbon dioxide absorbent. The focus of this study is to elucidate statistically the results of round robin test by seven countries used MODA, which were various deviations among the experiments.     

Method of Producing Biodegradable Reference Material and its Biodegradability Based on International Standard Evaluation Method (ISO 14855-2)

Returnable cups made of poly(lactic acid) (PLA) are employed as an example of products made of biodegradable plastics. Two kinds of PLA samples plates and powders with different shapes were prepared from PLA cups. The plates were cut from a cup using nippers. Powders were prepared using a rotation mechanical mixer for 45 min. PLA powders were separated by sieves with 60 meshes (250 μm) into a size ranging from 0 to 250 μm. An average diameter of powders separated by a sieve is 169 μm. Biodegradation tests of PLA plates, PLA powders and cellulose powders in controlled compost at 58 °C were performed using a Microbial Oxidative Degradation Analyzer (MODA) instrument according to ISO 14855-2. PLA powders showed almost the same biodegradation curve as that of cellulose powders. PLA plates biodegraded at a slower rate than PLA powders.     

Biodegradable Blends with Potential Use in Packaging: A Comparison of PLA/Chitosan and PLA/Cellulose Acetate Films

Poly(lactic acid) (PLA) is a biodegradable polymer that exhibits high elastic modulus, high mechanical strength, and feasible processability. However, high cost and fragility hinder the application of PLA in food packaging. Therefore, this study aimed to develop flexible PLA/acetate and PLA/chitosan films with improved thermal and mechanical properties without the addition of a plasticizer and additive to yield extruder compositions with melt temperatures above those of acetate and chitosan. PLA blends with 10, 20, and 30 wt% of chitosan or cellulose acetate were processed in a twin-screw extruder, and grain pellets were then pressed to form films. PLA/acetate films showed an increase of 30 °C in initial degradation temperature and an increase of 3.9 % in elongation at break. On the other hand, PLA/chitosan films showed improvements in mechanical properties as an increase of 4.7 % in elongation at break. PLA/chitosan film which presented the greatest increase in elongation at break proved to be the best candidate for application in packaging.     

Thermodynamic and Diffusion Properties of Biodegradable Systems Based on Starch and Cellulose Derivatives

The interval sorption and diffusion of water vapor were studied for two systems: methylcellulose (MC)/starch and carboxymethylcellulose (CMC)/starch. The diffusion coefficient of water vapor and the Gibbs free energy of swelling of these blends in water were estimated. The Gibbs free energy of mixing starch with the cellulose derivatives was determined using the thermodynamic cycle. CMC/starch was shown to be more compatible than MC/starch. Biodegradation of these systems in the water–soil environment was measured and found to increase with the concentration of starch in its blends with cellulose derivatives.     

Nanocomposites of Cellulose Acetate Butyrate Reinforced with Cellulose Nanocrystals

Nanocrystals were prepared by acid hydrolysis of bacterial cellulose microfibrils. These were topochemically trimethylsilylated, in an attempt to reduce their hydrophilicity. Composites were made by dispersing either native or silylated crystals in cellulose acetatebutyrate matrixes and solution casting of the dispersions. Particles were characterized by transmission electron microscopy. The composites were characterized by differential scanning calorimetry and dynamic mechanical analysis. The unmodified cellulose crystals exhibited better reinforcement characteristics than the trimethylsilated crystals.     

Abiotic Hydrolysis of Some Poly-D-glucaramides and Subsequent Microbial Utilization/Degradation

Carbohydrate acid amides, diamides and polyamides have been proposed to be utilized as nitrogen plant fertilizers or fertilizer components, and experiments with Brassica rapa demonstrated a positive biological response when these compounds were used as the only source of fixed nitrogen for plant growth. The present study was carried out with the aim of elucidating the mechanism of degradation of these polymers in both soil/compost and in liquid media and the role of microorganisms in this process. The results obtained suggest that a major route of degradation of polyglucaramides in the environment is their abiotic hydrolysis/release of the diacid and diamine building block units of these polymers, which are then utilized for growth by microorganisms. In cell-free crude extracts from enrichment cultures obtained with different poly-D-glucaramides, no enzyme activities catalyzing the release of diamines from these compounds were detected.     

Environmental Degradation of Blends of Atactic Poly[(R,S)-3-hydroxybutyrate] with Natural PHBV in Baltic Sea Water and Compost with Activated Sludge

Degradation of atactic poly[(R,S)-3-hydroxybutyrate] (a-PHB) binary blends with natural poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV, 12 mol% of 3HV units), has been investigated and compared with plain PHBV in the compost containing activated sludge and under marine exposure conditions in the dynamic water of the Baltic Sea. Characteristic parameters of compost and the Baltic Sea water were monitored during the incubation period (6 weeks) and their influence on the degree of biodegradation is discussed. After specified degradation times of the experiments the weight loss of the samples, surface changes, changes in molecular weight and polydispersity as well as changes of the composition and thermo-mechanical properties of the blends have been evaluated. Macroscopic observations of the samples were accompanied by investigations using optical microscopy, size-exclusion chromatography (SEC), nuclear magnetic resonance (NMR), differential scanning calorimetry (DSC) and tensile testing. The degree of degradation of blends of a-PHB with PHBV depends on the blend composition and environmental conditions. In both environments studied the weight loss of plain PHBV was more significant than changes the molecular weight. In both environments only enzymatic degradation of the blends, which proceeds via surface erosion mechanisms, was observed during the incubation period.     

Valorization of regenerated LDPE by blending with EPDM in the presence of peroxide

The present work aims to the valorization of regenerated low density polyethylene (LDPE) by blending with small quantities of ethylene-propylene-diene monomer (EPDM). Three types of regenerated LDPE (rLDPE) from different waste sources (greenhouses, milk pouches,...) were characterized in terms of physico-chemical (density, melt flow index, water absorption, melting temperature and structure by Fourier transform infrared (FTIR) spectroscopy) and mechanical properties (tensile properties and hardness). The optimization of the peroxide content required for the crosslinking of the LDPE/EPDM blends was due by measuring torque and tensile strength. Once the peroxide content was optimized, different blends were obtained by varying the EPDM content. Then they were characterized physically (density, water absorption) and mechanically (tensile properties and hardness). Finally, the blends behavior under the conjugated effect of heat and water was determined at 70 °C for 7 days. The obtained results showed that this kind of blending has contributed in improving the performance of regenerated LDPE.     

Estimation on Biodegradability of Poly (3-hydroxybutyrate-<Emphasis Type="Italic">co</Emphasis>-3-hydroxyvalerate) (PHB/V) and Numbers of Aerobic PHB/V Degrading Microorganisms in Different Natural Environments

To assess the capacity of the natural environment for degrading PHB/V, the film-MPN method proposed previously was modified to estimate the numbers of PHB/V degrading microorganisms (degraders) in various environments. The First-Order Reaction (FOR) model was used to determine the appropriate incubation period for the method. Numbers of aerobic PHB/V degraders were estimated in garden soil, paddy field soil, farm soil, river bank soil, infertile garden soil, river water, activated sludge, and seawater by the film-MPN method. Results were compared with those estimated by the clear-zone technique and showed that the film-MPN method was suitable for estimating the numbers of PHB/V degraders in the environments tested. On the other hand, biodegradability of injection molded PHB/V samples was investigated in several kinds of environments. The changes of weight were studied and results showed that biodegradability of PHB/V related to the numbers of PHB/V degraders in similar ecosystem in different regions. In different environments the biodegradability of PHB/V not only related to the number of PHB/V degraders, but also depended on whether there were conditions for the PHB/V degraders to grow and proliferate easily in the environment.