Critical Review of Norms and Standards for Biodegradable Agricultural Plastics Part II: Composting

The critical review of norms and standards and corresponding tests to determine the compostability of biodegradable plastics, possibly applicable also to biodegradable agricultural plastics, shows that many norms concerning testing and labelling of compostable plastics have been established at the international level. Some of them are about plastic materials, some others are about products like packaging. The media and conditions of testing cover mainly the conditions designed for industrial composting facilities, and only a few concern home composting conditions. Considering that the end of life management of biodegradable agricultural plastic products will be done at the farm to reduce the management of the waste and also its cost, only a few of these norms are considered to be suitable for adaptation to cover also biodegradable agricultural plastic products. The biodegradability validation criteria under composting conditions, such as the threshold percentage of biodegradation and disintegration, the time and temperature, and the ecotoxicity, are presented for the main norms and standard testing methods. Based on these different norms and their content, a list of specs and technical requirements that could be adapted to meet farm composting conditions for agricultural compostable plastics is proposed. These requirements may be used as criteria for the establishment of a new integrative norm for agricultural compostable plastics.     

Determining biodegradability of plastic materials under controlled and natural composting environments

With the advent of recently promulgated Government regulations on plastics in Mauritius, a study was initiated to examine the biodegradability of two different types of plastic, namely Willow Ridge Plastics - PDQ-H additive (Plastic A) and Ecosafe Plastic - TDPA additive (Plastic B) under controlled and natural composting environments. The results obtained from the controlled composting environment showed that the cumulative carbon dioxide evolution for Plastic A was much higher than that for Plastic B. Plastic A therefore showed a higher level of biodegradation in terms of CO2 evolution than Plastic B. However, from the regression analysis, it was found that the level of CO2 varying with time fitted the sigmoid type curves with very high correlation coefficients (R2 values: 0.9928, 0.9921 and 0.9816, for reference material, inoculum and Plastic A, respectively). The corresponding F-values obtained from the ANOVA analysis together with significance levels of p<0.05 indicated that the three treatments analysed in the biodegradability experiment were significant. The other experiment was undertaken to observe any physical change of Plastics A and B as compared to a reference plastic, namely, compostable plastic bag (Mater-Bi product-Plastic C), when exposed to a natural composting environment. Thermophilic temperatures were obtained for about 3-5 days of composting and the moisture content was in the range of 60-80% throughout the degradation process. It was observed that after 55 days of composting, Plastic C degraded completely while Plastic A and Plastic B did not undergo any significant degradation. It can be concluded that naturally based plastic made of starch would degrade completely in a time frame of 60 days, whereas plastics with biodegradable additive would require a longer time.     

Behaviour of biodegradable plastics in composting facilities

Composting is a preferred treatment strategy for biodegradable plastics (BDPs). In this sense, the collection of BDPs together with organic household wastes is a highly discussed possibility. Under the aspect of the behaviour of BDPs in composting facilities, a telephone survey was carried out with selected composting facility operators. They were interviewed with respect to treated wastes, content of impurities, processes for impurity separation, experiences with biodegradable plastics and assumptions to the behaviour of biodegradable plastics in their facility. Forty percent of the facilities had some experiences with BDPs due to test runs, and also since the occurrence of BDPs in their waste was known. The majority of the operators expressed apprehension regarding an increase of impurities resulting from a combined collection of biowaste and BDPs. In the facilities, measures for the impurity separation from the biowaste were used in common practice - in 33% of the cases, separation of disturbing plastics was done before composting, in 33% after composting, and in 13% before and after composting. The most important separation processes for conventional plastics were sieving and manual sorting. In two cases air classification was also used. When asked about the separation possibility of the conventional but not of the biodegradable plastics in their facilities, the majority of operators were not in a position to comment or they replied that it was not an option. No problems were seen in most cases if the impurity separation follows composting. If impurity separation takes place before composting it was often assumed that the BDPs are mainly separated by sieving. In conclusion, in more than half of the cases, BDPs would not be composted if delivered to a composting facility. Under the actual conditions regarding the collection and the treatment/disposal possibilities, an application of BDPs seems to only be reasonable for clean (i.e., source separated on their own) fractions of BDPs.     

Validation of an Automated Multiunit Composting System

Biodegradable packaging has high potential to help solve the crisis of non-biodegradable plastic waste causing an increase in the footprint of landfills. However, more research needs to be executed to develop a larger assortment of biodegradable plastics for numerous applications and to make them more economical to manufacture. This paper discusses the design and validation of an automated composting system (AMUCS) that fits the requirements of the American Society for Testing and Materials (ASTM) 5338-11 standard. The results of the experiments show that the AMUCS was able to create and maintain the conditions for biodegradation of biodegradable polymers in compost using microcrystalline cellulose. The biodegradation caused by the composting environment was observed visually with the naked eye and on the micro scale with an environmental scanning electron microscope. The magnitude of biodegradation was measured by calculating the carbon metabolized from the samples. The carbon metabolized from the three compost replicates was consistent and linear, and there was only an 8 % difference between the non-biodegradable low density polyethylene and the compost. For the biodegradation study according to ASTM D 5338-11, the experiment was validated with the use of cellulose as a reference material. Under controlled composting conditions, the mineralization of microcrystalline cellulose yielded 72.05 %, which is slightly higher than the 70 % mineralization requirement.     

Effect of the composting substrate on biodegradation of solid materials under controlled composting conditions

Biodegradability under composting conditions is assessed by test methods, such as ASTM D 5338-92, based on the measurement of CO₂ released by test materials when mixed with mature compost and maintained in a controlled composting environment. However, in real composting, biodegradation occurs in fresh waste. To clarify this point, the biodegradation of paper and of a starch-based biodegradable thermoplastic material, Mater-Bi ZI01U, was followed by measuring the weight loss of samples introduced either into a mature compost or into a synthetic waste. The weight loss in mature compost was higher at the beginning but tended to decrease; in synthetic waste a first lag phase was followed by an exponential phase. Complete degradation of paper was noticed simultaneously in the two substrates (after 25 days). The bulkier Mater-Bi samples were fully degraded after 20 days in fresh waste, but after 45 days in mature compost. Therefore, the test methods using mature compost as a substrate can possibly underestimate the biodegradation rate occurring in fresh waste, i.e., in real composting plants, and have to be considered as conservative test methods. The test procedure described in this paper seems very suitable as a screening method to verify the compostability of plastic materials in a composting environment.     

Modelling of organic matter dynamics during the composting process

Composting urban organic wastes enables the recycling of their organic fraction in agriculture. The objective of this new composting model was to gain a clearer understanding of the dynamics of organic fractions during composting and to predict the final quality of composts. Organic matter was split into different compartments according to its degradability. The nature and size of these compartments were studied using a biochemical fractionation method. The evolution of each compartment and the microbial biomass were simulated, as was the total organic carbon loss corresponding to organic carbon mineralisation into CO₂. Twelve composting experiments from different feedstocks were used to calibrate and validate our model. We obtained a unique set of estimated parameters. Good agreement was achieved between the simulated and experimental results that described the evolution of different organic fractions, with the exception of some compost because of a poor simulation of the cellulosic and soluble pools. The degradation rate of the cellulosic fraction appeared to be highly variable and dependent on the origin of the feedstocks. The initial soluble fraction could contain some degradable and recalcitrant elements that are not easily accessible experimentally.     

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.     

Biomass Carbon Ratio of Polymer Composites Measured by Accelerator Mass Spectrometry

The evaluation method of biomass carbon ratio of polymer composite samples including organic and inorganic carbons individually was investigated. Biodegradable plastics and biobased plastics can have their mechanical properties improved by combining with inorganic fillers. Polymer composites consisting of biodegradable plastics and carbonate were prepared by two different methods. Poly(lactic acid) (PLA) composite was prepared by synthesis from l-lactide with catalyst and calcium carbonate (CaCO₃) powders from lime. Poly(butylene succinate) (PBS) composite was prepared by hot-pressing the mixture of PBS powder and CaCO₃ powders from oyster shells. The mechanical properties of composite samples were investigated by a tensile test and a compression test using an Instron type mechanical tester. Tensile test with a dumbbell shape specimen was performed for PBS composite samples and compression test with a column shape specimen for PLA composite samples. Strength, elastic modulus and fracture strain were obtained from the above tests. Biomass carbon ratio is regulated in the American Standards for Testing and Materials (ASTM). In ASTM standards on biomass carbon ratio, it is required that carbon atoms from carbonates, such as CaCO₃, are omitted. Biomass carbon ratio was evaluated by ratio of ¹⁴C to ¹²C in the samples using Accelerator Mass Spectrometry (AMS). The effect of pretreatment, such as oxidation temperature and reaction by acid, on results of biomass carbon ratio was investigated. Mechanical properties decrease with increasing CaCO₃ content. The possibility of an evaluation method of biomass carbon ratio of materials including organic and inorganic carbons was shown.     

Physical properties and biodegradability of blends containing poly(ε-caprolactone) and tropical starches

In order to assess feasibility of tropical starches (sago and cassava starches) as biodegradable plastic materials, blending with poly(-caprolactone) (PCL), a biodegradable polymer, was carried out. It was confirmed that the physical properties (tensile strength and elongation) of PCL/sago and PCL/cassava blends were similar to those of PCL/corn blend, suggesting that sago and cassava starches can also be blended with PCL for production of biodegradable plastic. However, the properties of all PCL/starch blends were still low compared with those of polyethylene. Enzymatic degradability evaluation showed that lipase degradation of PCL and-amylase degradation of starch increased as the starch content in the blend increased. Burial test of the blends for 1, 3, and 5 months was carried out and the rate of degradation of the PCL/sago blend was confirmed to be slower than those of PCL/corn and PCL/cassava blends. Observation of the film blends structure by scanning electron microscope revealed that the starch was dispersed in a PCL continuous phase. Furthermore, changes in the film surface before and after enyzme treatments were observed.     

Analysis of Biodegradability of Three Biodegradable Mulching Films

The development of biodegradable mulching films is a great direction for environment protecting and oil saving problems. In this paper, it was used three kinds of biodegradable mulching films named a, b and c (different ratio between modified starch and poly-CL with pro-oxidant additives) in microorganism culture test and soil burial test was investigated under laboratory conditions. The index of degradation was assessed by visual observation, weight loss and SEM analysis from quantitative and qualitative aspect. The results of both tests showed that these biodegradable mulching films were more readily degraded than the common plastic film. The percentage weight loss was in sequence of biodegradable mulching film c > biodegradable mulching film b > biodegradable mulching film a, while common plastic film basically had no changes. Weight loss was not as obvious as the visual degradation and suggested broader types of microbial attack. SEM analysis clearly indicated that the changes of surface morphology of these samples after the soil burial exposure.     

Biodegradation and Ecotoxicity of Polyethylene Films Containing Pro-Oxidant Additive

The worldwide accumulation of non-degradable plastic materials, such as plastic bags, is one of the most important environmental concerns nowadays. The use of degradable materials is an option to mitigate the environmental impact generated by the consumption of plastics. One of the technologies used for the manufacture and use of degradable plastics is the use of pro-degradant additives that are incorporated in conventional plastics to promote their degradation under certain conditions. The aim of this study is to evaluate the process of oxidation, biodegradation and potential ecotoxicity of polyethylene films containing an oxo-degradable additive, according to the standard ASTM D-6954. This method establishes a procedure in which the samples are subjected to consecutive steps of accelerated oxidation, biodegradation by composting and ecotoxicity assessment. Furthermore, the effect of the presence of printing ink in the polyethylene samples with oxo-degradable additive was evaluated, and the results were compared with those obtained for samples of conventional polyethylene and polylactic acid. After 180 days of laboratory controlled composting, the samples reached the following percentages of biodegradation: polylactic acid, 41 %; printed oxo-degradable polyethylene, 32.24 %; oxo-degradable polyethylene, 25.84 %; printed polyethylene, 18.23 % and polyethylene, 13.48 %. The cellulose sample used as a control was mineralized in 58.45 %. Ecotoxicity assessment showed that the products of biodegradation of the samples tested, did not generate a negative effect on germination or development of the vegetal species studied. Under proper waste management conditions, these plastics can be used as an option to decrease the environmental impact of plastic films.     

Carbon materials-catalyzed hydroliquefaction of flame-retardant plastics in organic solvents

Flame-retardant plastics, such as desktop and laptop personal computer bodies, could be completely liquefied by carbon materials-catalyzed hydroliquefaction in tetralin without using H₂ as a hydrogen source. Active carbons with larger surface areas (1450–3450 m²/g) acted as superior catalysts in transferring tetralin hydrogens to plastics. On the other hand, carbon blacks and fullerene-rich soot were less active catalysts. Graphite and mesocarbon microbeads did not show any catalytic effects. Benzene, toluene, and ethylbenzene were obtained as recyclable hydrocarbons; their total amounts varied from 4 wt% to 12 wt% depending on the types of plastics and the carbon materials used. Organic bromides such as polybromodioxins were not contained in the gases and oils of the product. Received: July 19, 2000 / Accepted: September 17, 2000     

Evaluation of the Biodegradability of Polyvinyl Alcohol/Starch Blends: A Methodological Comparison of Environmentally Friendly Materials

Polyvinyl alcohol (PVA) and starch are both biodegradable polymers. These two polymers can be prepared as biodegradable plastics that are emerging as one of the environmental friendly materials available now. In this study, after reacting with sodium trimetaphosphate (STMP), modified corn starch was blended with PVA in different ratios by a barbender. Test samples were prepared for mechanical and thermal properties measurements. The surface roughness and morphology of fractured surface of the samples were observed by an atomic force microscopy (AFM) and scanning electron microscope (SEM) measurements. Aqueous degradation by enzyme, water absorption and biodegradability behavior were evaluated for the degradability. The biodegradability of these materials was followed by bio-reactivity kinetics models. Results showed that the addition of modified starch could enhance its water uptake. With an addition of 20 wt% of modified starch, the blend had a maximum weight loss during enzymatic degradation. It was found that the degradability was enhanced with the addition of the starch. Analyzing the results of the biodegradability based on the kinetic models, the growth rate of the microorganism was found to be increasing with the increase of the content of starch in the PVA/starch blends in the first order reaction fashion. In our biodegradability analysis, i.e., based on the China national standards (CNS) 14432 regulations, we estimated the decomposition behavior based on the mentioned first order reaction. We found that the PVA/starch blends would take 32.47, 16.20 and 12.47 years to degrade by 70% as their starch content 0, 20 and 40 wt%, respectively.     

A kinetic analysis of solid waste composting at optimal conditions

This discussion explores the possibility of having a measure of the biodegradable organic carbon content in solid wastes. Currently, indirect measures for determining the concentration of biodegradable organic matter are being used and most of them are based on respiration indices (oxygen consumption or carbon dioxide production) or chemical parameters (volatile solids or total organic carbon). The results obtained for the cumulative carbon dioxide production in composting experiments can be expressed as "aerobic biodegradable carbon" for the wastes that were studied. The calculation of a useful biodegradable C/N can also be obtained from the aerobic biodegradable carbon content. A comparison with some results obtained in measuring the concentration of "anaerobic biodegradable carbon" also is presented.     

Biodegradable kinetics of plastics under controlled composting conditions

This study models and evaluates the kinetics of C-CO₂ evolution during biodegradation of plastic materials including Polyethylene (PE), PE/starch blend (PE/starch), microcrystalline cellulose (MCE), and Polylactic acid (PLA). The aerobic biodegradation under controlled composting conditions was monitorated according to ISO 14855-1, 2004. The kinetics model was based on first order reaction in series with a flat lag phase. A non-linear regression technique was used to analyze the experimental data. SEM studies of the morphology of the samples before and after biodegradation testing were used to confirm the biodegradability of plastics and the accuracy of the model. The work showed that MCE and PLA produced the high amounts of C-CO₂ evolution, which gave readily hydrolysable carbon values of 55.49% and 40.17%, respectively with readily hydrolysis rates of 0.338 day⁻¹ and 0.025 day⁻¹, respectively. Whereas, a lower amount of C-CO₂ evolution was found in PE/starch, which had a high concentration of moderately hydrolysable carbon of 97.74% and a moderate hydrolysis rate of 0.00098 day⁻¹. The mineralization rate of PLA was 0.500 day⁻¹ as a lag phase was observed at the beginning of the biodegradability test. No lag phase was observed in the biodegradability testing of the PE/starch and MCE. The mineralization rates of the PE/starch and MCE were found to be 1.000 day⁻¹, and 1.234 day⁻¹, respectively. No C-CO₂ evolution was observed during biodegradability testing of PE, which was used for reference as a non-biodegradable plastics sample.     

The influence of soil macroinvertebrates on primary biodegradation of starch-containing polyethylene films

The primary biodegradability of polyethylene (PE) films containing different percentages of cornstarch (0–50%) and other additives (prooxidant, oxidized polyethylene) was tested using four species of earthworms (Eisenia fetida, Lumbricus terrestris, Aporectodea trapezoides, Aporectodea tuberculata), three species of cockroaches (Periplaneta americana, Blaberus sp.,Blattella germanica), termites (Reticulotermes flavipes), sowbugs (Porcellio laevis), and crickets (Acheta domesticus). These studies were conducted to elucidate the potential role of soil macroinvertebrates in degrading starch/PE biodegradable plastics. The results of the macroinvertebrate bioassays indicate that crickets, cockroaches, and sowbugs consumed starch-containing PE films most readily. In addition, the degree to which the films were attacked and consumed was directly related to the starch content of the film. Films with oxidized polyethylene and those containing prooxidant (vegetable oil and a transition metal catalyst) were also consumed. None of the four species of earthworms tested or the termites showed any activity toward the starch/polyethylene films. These results have important implications for determining the fate of novel plastic formulations which claim to be biodegradable in natural environments. Studies such as these, coupled with studies on microbial degradation, will help provide the type of information needed to assess the environmental fate of biodegradable starch/PE plastics and fill the voids in the scientific database regarding this rapidly developing field.     

Biodegradable Composite Foams of PLA and PHBV Using Subcritical CO<Subscript>2</Subscript>

One key strategy for increasing the application potential for biodegradable plastics lies in improving the physical and mechanical characteristics, which can be attained by inducing a cellular morphology in the pure polymer with the aid of a blowing agent, as well as by blending two or more polymers with the desirable properties. This paper examines the effect that blending two biodegradable polymers has on the thermal properties and morphology of the resultant foams blown with carbon dioxide (CO₂). Polylactic acid (PLA), polyhydroxybutyrate-co-valerate (PHBV) and blends of both were foamed and characterized in terms of thermal characteristics, relative density, cell size, and foam morphology. The results indicate that although PLA and PHBV are immiscible, the presence of small quantities of PHBV (25 wt%) could lead to low density foams with finer, more uniform cells. Furthermore, the crystallinity of PHBV was found to be unaffected by the presence of PLA in the composite, which supports the immiscibility of PLA and PHBV.     

Engineering and Processing of Poly(HydroxyButyrate) (PHB) Modified by Nano-sized Graphene Nanoplatelets (GNP) and Amino-Functionalized Silica (A-fnSiO<Subscript>2</Subscript>)

Manufacturing of plastics by biodegradable polymers can reduce the environmental impact and limit the recourse to non-renewable resources. Physical, chemical and mechanical properties of biodegradable polymers cannot often be comparable with petroleum-based plastics. However, the modification of bioplastics by physical and chemical routes can improve their performance. In this work, Poly(HydroxyButyrate) PHB is reprocessed by the dispersion of graphene nano-platelets as well as by the dispersion of Amino-Functionalized Nano-Silica (A-fnSiO₂). Modification of the PHBs after compounding and pelletizing was evaluated by Fourier Transform Infrared. Thermal analysis was performed by Differential Scanning Calorimetry after extrusion and compression moulding. Hardness and scratch performance of the polymers were measured on compression molded substrates by pencil and progressive and constant load scratch tests. Chemical resistance was evaluated on compression molded substrates by dipping in aggressive acidic, basic and saline aqueous solutions. In conclusion, the PHBs modified by the graphene nanoplatelets were found to be very promising, featuring good chemical inertness, hardness and mechanical strength.     

Influence for Soil Environment by Continuing use of Biodegradable Plastic

The influence on soil environment by continuing use of the biodegradable plastic films (biodegradable mulching films) in farmland was investigated. The difference was not seen in the amount of soil bacteria between mulching film plowing sections and non-plowing sections. The total bacteria amount did not increase by the effect of plowing the biodegradable mulching film. Poly-(butylene succinate and adipate) (PBSA) and poly-(ε-caprolactone) (PCL) decomposing bacteria did not increase in PBSA and PCL mulching film plowing sections comparing polyethylene covering section (PE) and no-film section. Polylactic acid (PLA) decomposing bacteria were not detected in all sections. Total denaturing gradient gel electrophoresis (DGGE) band patterns did not show a clear transition of the bacterial community structure in both the cultivating and promoting sections. In usual usage condition of the biodegradable plastic films, it was hardly influence to the soil environment such as bacterial community structure in farmland.     

国内降解塑料的生产现状及发展中应注意的问题

通过对几家生产企业典型降解塑料产品及产品降解性的分析 ,提出了在研究开发和推广应用降解塑料时应综合考虑生产成本及产品降解性两个重要因素。