WASTE REDUCTION AND MANAGEMENT INSTITUTE
SCHOOL OF MARINE AND ATMOSPHERIC SCIENCES
Degradable Plastics and Solid
Waste Management Systems
Prepared by:
David J. Tonjes
Department of Technology and Society
Center for BioEnergy Research and Development
Waste Reduction and Management Institute
Krista L. Greene
Department of Technology and Society
Center for BioEnergy Research and Development
November 2013
Stony Brook University
Table of Contents
Abstract.......................................................................................................................................... 1
1. Introduction .............................................................................................................................. 3
2. History of Degradable Plastics................................................................................................ 9
3. Degradable Plastics Labeling................................................................................................ 15
4. Current Degradable Resins................................................................................................... 17
4.1 Poly-Lactic Acid (PLA) ................................................................................................. 17
4.2 Polyhydroxyalkanoate (PHA) ........................................................................................ 18
4.3 Other Marketed Degradable Resins ............................................................................... 19
5. Compatibility of Degradable Plastics with Current Waste Management Processes ....... 29
5.1 Composting .................................................................................................................... 29
5.2 Recycling ........................................................................................................................ 33
5.3 Waste-to-Energy Incineration ........................................................................................ 35
5.4 Landfilling ...................................................................................................................... 36
5.5 Litter ............................................................................................................................... 39
6. Degradable Plastics and Effects on Proposed Solid Waste Management Technologies
and Strategies .................................................................................................................... 43
6.1 Expanded Recycling ....................................................................................................... 43
6.2 Expanded Organics Recovery ........................................................................................ 44
6.3 Innovative Energy Recovery .......................................................................................... 46
6.4 Bioreactor Landfills ........................................................................................................ 47
6.5 Extended Producer Responsibility ................................................................................. 48
6.6 Zero Waste ..................................................................................................................... 51
7.0 Conclusions ........................................................................................................................... 53
Acknowledgements ..................................................................................................................... 55
References .................................................................................................................................... 57
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Abstract
Plastics, which are woven into the fabric of modern life, have consequential impacts on
the environment. Many of these are associated with end-of-lifetime processes, and include
chemical contamination of the environment and effects from litter. Plastics also complicate waste
management processes, causing contamination in composting operations, and having poor
recovery rates through recycling. Plastics that are not as biologically recalcitrant, that decompose
when use is done, have been perceived as solutions to at least some of these problems. The first
generation of degradable plastics did not meet marketing claims; some of the more recent
formulations, partly as a consequence of third party certifications, are more compliant. However,
many plastics that are labeled as “degradable” do not decompose very readily, and it is not clear
that litter will be diminished to any great degree through their use. In addition, because not all
plastics are or will be degradable, user confusion is and will be common. Multiple formulations
mean not all degradable plastics address compost contamination, and most degradable plastics do
not address other problems associated with plastics waste management. Therefore it is not clear
that degradable plastics constitute a major technological advance; in fact, overall they may be
more harmful than helpful.
1
2
1. Introduction
Plastics, especially consumer plastics, are integral elements of modern life, and have been
in use for over 150 years (Friedel 1993; Andrady and Neal 2009). Their ubiquity is increasing;
one estimate was that 300 million tonnes of plastics were produced worldwide in 2012
(Rochman et al. 2013). The growth of plastics use is further reflected in US Environmental
Protection Agency US disposal estimates: in 2009, 30 million tons of plastics were disposed
(12% of US municipal solid waste), compared to 1960, when less than two million tons were
disposed (about 1% of the US waste stream) (USEPA 2010). Plastic materials can be created for
a near infinite variety of purposes, but packaging and single use consumer products capture most
of the general public’s attention (e.g., Sparke 1993; Livesey 1999; Viehover 2000). One estimate
is such items are 35%-45% of all plastics production (Chiellini et al. 2006). This implies that as
much as 100 million tonnes of single use plastics are made and disposed worldwide each year.
Plastics have replaced paper and other materials because they are superior in terms of
strength, durability, stability, lightness, and impermeability (Andrady and Neal 2009). These
same properties, however, impede their disappearance in the biosphere, creating continuing
concern over environmental impacts (Thompson et al. 2009; Rochman et al. 2013). Plastic litter,
especially in aquatic environments, can have noxious impacts on biota (Gregory 2009). Chemical
variation in resin types can make reuse and recycling difficult (Stein 1998; Kuswanti et al. 2003;
Goodship 2007). Conventional plastics may require decades (or longer) to degrade (Gruenwald
1993; Searle 2003; Koutny et al. 2006; Ammala et al. 2011), and the degradation process may
release additives and byproducts that pose threats to the health of organisms (including people)
(Talsness et al. 2009) to the degree that there has been a call to declare plastics hazardous
materials (Rochman et al. 2013).
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Negative aspects of plastics often continue as they are managed through solid waste
processes. Plastics cause problems at composting plants; collection of yard waste in plastic bags
causes residual plastics contamination at yard waste composting sites, and plastic disposable
utensils and other service ware complicate efforts to create clean compostable waste streams for
food and MSW composting (Katz 1993; Wiles and Scott 2006; USEPA 2010). Chemical
contaminants associated with plastics are often released to the environment through waste
management pathways. Additives that have sparked recent (and growing) concern, such as
bisphenol A (BPA) and phthalates (Meeker et al. 2009), have been found in landfill leachates
(Vandenberg et al. 2007; Zhang et al. 2009), reaching the environment if there are liner system
leaks. Another landfill leachate route to the environment is when leachates are treated at waste
water treatment plants and effluents are discharged, as not all of these chemicals are removed
through standard treatment (Auriol et al. 2006; Barnabe et al. 2008; Dargnat et al. 2009).
Plasticizers that are removed in sewage treatment contaminate the resulting sewage sludges
(Carbella et al. 2006; Barnabe et al. 2008; Dargnat et al. 2009; Barnabe et al. 2009), and the
trend toward greater reuse of sludges means wide distribution of them to agricultural soils
(Kinney et al. 2006; Barnabe et al. 2009). Incineration of chlorinated plastics has been linked by
some to enhanced dioxin generation (Thornton 2000; Belliveau and Lester 2004); polyvinyl
chloride (PVC), for instance, is more than 55% chlorine by weight (Scheirs 1998), and so may be
a substantial source of chlorine during combustion.
Litter (the improper disposal of goods) is an especially noxious problem due to plastics
(Wiles and Scott 2006; Barnes et al. 2009). Plastic bags are extremely mobile, with their high
surface area to weight ratio creating sail-like materials (FMI 2008). Most plastics are less dense
than water and are hydrophobic, so that they can be transported long distances after reaching
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water bodies because they float and do not settle (Thompson et al. 2004; Ryan et al. 2009
Browne et al. 2010). Although, like all organic matter, plastics are susceptible to damage from
UV radiation, the polymer structure of plastics rarely degrades entirely due to such effects
(Gruenwald 1993; Hakkarainen and Albertsson 2004); in addition, floating plastics may gain
fouling biofilm that inhibits further exposure to sunlight (Gregory and Andrady 2003). Few
microorganisms can use plastic polymers for sustenance, especially when the polymers are intact
(Gruenwald 1993; Witt et al. 1999). Thus, plastic litter, especially in marine settings, is notably
persistent as few other litter materials are, and often remains visible forever, seemingly (Derraik
2002; Moore 2008). Entanglement and envelopment in plastic debris affects organisms (Gregory
and Andrady 2003; Gregory 2009) and floatable material can serve as simulacra of prey, so that
surveys of charismatic marine species often document ingestion of plastic (Derraik 2002;
Gregory and Andrady 2003; Gregory 2009). The visible portion of litter may not be the greatest
problem, however, as a greater mass of plastic is present in the “microlitter” fraction (Thompson
et al. 2004). Organic carbon plastic chains are attractive sorption sites for other organic
molecules, including persistent organic pollutants, and so may serve as concentration sites for
contaminants of concern (Teuten et al. 2009). Marine plastics pollution has been documented to
have harmed individuals from 267 species, including 86% of sea turtles, 44% of seabirds, and
43% of marine mammals (Laist 1997), and impacts may be underestimated as many affected
organisms sink or are consumed by predators (Wolfe 1987). Terrestrial litter management is
expensive and often difficult (Mid Atlantic Solid Waste Consultants 2009). Marine litter is only
rarely addressed; one program was developed in New York Harbor after several notable beach
wash-up episodes (Swanson and Tonjes 2001).
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Solutions have been proposed to address the global challenges of plastic wastes. One
simplistic answer is to avoid plastic use altogether. The important role played by plastics in
modern materials (Katz 1993) makes this difficult to implement entirely. Minimization of
particular plastics use has been sought, so that some packaging uses (primarily polystyrene) were
banned in locations across the US in the 1990s (Swanson et al. 1993), or were voluntarily
foresworn (e.g., McDonald’s clamshells, see Roberts and Dennison 1994). Plastic bags have
been legislated against in various places, such as in Ireland in 2002 (Convery et al. 2007). The
plastics industry has responded by establishing and supporting recycling programs (Fisher 2003;
Kuswanti et al. 2003; Goodship 2007). Recycling diverts plastics from disposal, but rates for
most plastic items remain low, especially when compared to other items in commerce such as
newspaper or aluminum containers (Davis and Song 2006; Hopewell et al. 2009; USEPA 2010).
Packaging product stewardship programs (where manufacturers and/or distributors become
responsible for the end-of-life of packaging), since plastics constitute a major element of
packaging and are often perceived as the constituents causing the most problems (Viehover
2000), have been adopted in Germany (Fishbein 1994), generally across the European Union,
and in Japan, Taiwan, South Korea, Brazil, and Peru (Selke 2003), and in British Columbia,
Nova Scotia, and Ontario provinces, Canada (personal communication, H. Sanborn, Executive
Director, California Product Stewardship Council, 10 May 2011). Most recently, a position paper
suggested that because of the sum of impacts associated with their use and, especially, their
mismanagement, plastics should be classified with other products and chemicals that cause great
harm to people and other organisms, and receive an official label as a hazardous product
(Rochman et al. 2013).
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One means of addressing some of these issues has been the production of plastics that are
intended to degrade once their service life is over (Stevens 2002). Degradable plastics are
intended to address litter problems (Koutny et al. 2006), and to coexist better with composting
efforts (Song et al. 2009); degradable plastics may also generate benefits when landfilled (Ress
et al. 1998), although it is unclear if degradation is always optimal in landfills (Barlaz 1998). The
compatibility of degradable plastics with conventional reuse and recycling programs remains a
problem (Al-Malaiki et al. 1995; Scott and Wiles 2001; ExcelPlas Australia et al. 2004;
Goodship 2007), and there has been little consideration of potential interactions with energy
recovery and other advanced waste processing systems.
Degradable plastics clearly are designed to address the end-of-life of plastic products, and
intend to reduce the environmental impacts associated with their use and management and
mismanagement. Are degradable plastics compatible with current waste management practices?
Can they serve as an element in future, more sustainable materials management systems? We
address these questions by surveying the development of degradable plastics, reviewing many
degradable plastics marketed today, and then considering whether these products have
appropriate specifications that are either compatible with or improve current and possible waste
technology systems.
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2. History of Degradable Plastics
Synthetic polymers are generally resistant to most degradation over human-scale time
frames, which is an important characteristic for their use. However, they do undergo
“weathering”: the sum of chemical reactions, such as oxidation or hydrolization and degradation
from radiation (Searle 2003), as particular bond lengths correspond to the radiation wave length,
and absorb energy, destabilizing the atomic bond through Norrish I or Norrish II reactions
(Gilead 1985). They can be consumed by organisms, too, although most polymers are not readily
digestible (Witt et al. 1999; McCarthy 2003). However, it must be understood that natural
polymers are common, such as lignin and coal; with very few exceptions, all polymers are
capable of being degraded by natural processes, although it may take a very long time
(Steinbuchel 1995; Scott 2000; Scott and Wiles 2001). It has been said that unweathered
polyethylene is “essentially a non-biodegradable material” (Koutny et al. 2006), but, in the
longest view, all plastics are in some sense “degradable.”
Plastics manufacturers use additives to enhance mechanical or optical properties of
polymer types. Stabilizing additives include hindered phenol antioxidants, thioethers, and metal
chelates, which reduce oxidation and hydrolization reactions by either competing for reactants or
blocking reaction sites. Ultra-violet (UV) light stabilizers, especially carbon black, convert UV
energy to heat preventing the energy from breaking atomic bonds; many other specialized
compounds are also used to retard degradation (Gruenwald 1993). Many plastics degrade under
visible light, which results in “yellowing” (Searle 2003). Halogen containing polymers, such as
poly vinyl chloride (PVC), are subject to loss of the halogens under high temperature or bright
light; metal compound additives suppress these reactions. Adding metals to polymers increases
concerns that their release could have environmental effects, and also affects the resulting
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material’s electrical conductivity. A common additive to PVC insulators is lead, since lead salts
are insoluble and so do not contribute to electrical conductance (Gruenwald 1993).
Intended degradation must only occur at the proper time under the right conditions
(Gruenwald 1993; Scott and Wiles 2001). Typically, one of two differing strategies is pursued:
stabilizers can be omitted, or additives can be included to promote desired reactions. Design
choices need to consider particular polymer properties and uses. For instance, some plastics are
especially resistant to UV degradation, such as acrylic polymers and polyolefins. However, if
polyolefins are oxidized, the resulting carbonyl groups are susceptible to UV absorption and
bond scission. Thus, photodegradable polyolefins can be synthesized by adding carbonyl groups.
The compounds do not degrade indoors because UV light is generally blocked by glass. The
carbonyl groups are reactive when exposed to full daylight, and so the overall polymer is cleaved
(the first step in eventual degradation of the product) (Gruenwald 1993).
Degradation potential of polymers is usually tested in laboratory experiments that
simulate long exposure times. These processes do not exactly match environmental conditions,
and the acceleration of the degradation processes by various manipulations in order for
experiments to be concluded timely means that determinations of when reactions occur under
ambient conditions are often inexactly estimated. The closer experiments mimic environmental
conditions, the better the likelihood of timing degradation effects correctly (Searle 2003),
although careful bench-scale tests in at least one instance considerably over predicted
degradation measured in field experiments (Farrell et al. 2001). Laboratory results can lead to
incorrect descriptions of degradation potentials, or widely varying estimates of environmental
persistence. Thus, many manufacturers claim their products undergo reactions faster or more
completely than they actually do. Contrarily, those observing products which remain more intact
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in the environment than product specifications outline then fear that these products will remain
undegraded for thousands of years or more (Swanson et al. 1993; Belliveau and Lester 2004).
In the late 1980s, several US plastics companies began to market products that were
“degradable”: they were intended to last in the environment for time periods of days-weeksmonths – something not well-defined, but certainly less than life-spans of normal plastics (Gross
and Kalra 2002). Degradation meant the loss of properties, not necessarily the total elimination
of polymeric structures. So, these kinds of degradable plastics might lose physical strength and
integrity faster than standard plastics. To achieve this, transition-state metals (Gilead 1985),
carbonyls, and carbon monoxide groups (Iwaki 1975) were inserted into some polymers, creating
greater photosensitivity, and degradation was expected to continue enough so that the remaining
fragments might be consumable by micro-organisms. A molecular weight of about 5,000 was
thought to be an important threshold (Scott 1973), although direct proof of microbial
consumption of polymer carbon has been a difficult task (Albertsson and Karlsson 1988; Witt et
al. 1999).
Because UV-sensitive plastics did not always meet consumer expectations of
“disappearing” after use (Krupp and Jewell 1991), other approaches, such as starch insertion into
polymer chains, were undertaken (Breslin and Swanson 1993). These plastics were commonly
made into films – both for “industrial” use (such as agricultural weed barriers and mulch
retainers) and consumer products (mostly as larger bags, such as for garbage cans). Some molded
products such as bowls and cups were also made. The degradable formulations lost mechanical
and physical properties faster than standard plastics, but generally failed to crumble into small or
microscopic pieces in “reasonable” amounts of time (seasons to a year) (Krupp and Jewell 1991;
Breslin and Swanson 1993). They thus lost favor with consumers, who did not appreciate the
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fine distinction made by manufacturers that loss of integrity was “degradation.” Scott (2000;
Scott and Wiles 2001, Wiles and Scott 2006) has argued this standard is not fair since many
naturally occurring organic materials (especially wood and rubber) do not degrade rapidly either.
This argument has not resonated with either the public or regulators; for instance, CSU Chico
(2007) suggests instead that “biodegradable” should mean the substance is consumed by
microorganisms, and that within 180 days the process will be completed, leaving no “small
pieces or residues” behind.
An alternative to modifying petroleum-based plastics is to make plastics from natural
polymers that are commonly used by organisms as food, such as starch or cellulose (Song et al.
2009), with the assumption that degradation is then likely to happen. These formulations have
had performance issues. Cellulose polymers lose important functionalities before the solid melts,
for instance, which limit their utility as plastics (McCarthy 2003). Cellophane, the best-known
cellulose “plastic,” is strong but tears easily when lightly damaged. Starch is somewhat better,
but tends to crystallize; its high water content makes its workability in manufacturing difficult
(Davis 2003), so that starch-based plastics tend to be brittle (Wang et al. 2003). Starch-based
products, often labeled as TPS (thermoplastic starch), are available; Earthshell is one prominent
brand (McCarthy 2003). Generally, usable plastic products have been realized with these two
base compounds by creating mixtures with synthetic polymers (Gross and Kalra 2002) or other
degradable resins, such as poly-lactic acid (PLA) (Wang et al. 2003; Schwach et al. 2008).
Starch-based plastics have consistently been found to degrade as expected (e.g., Breslin 1998).
Confusion has been further created by the development of industrial biotechnologies that
allow plant and other non-petroleum sources of carbon to serve as the basis of plastics (and other
products) (Bomgardner 2012). Coca-Cola recently announced that it would seek to use 100%
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plant-based plastics for its bottles (Tullo 2012), and Heinz, too, is seeking to use these kinds of
products (Reish 2012). These products, although non-petroleum, do not have any special
degradability features. Production rates were in the vicinity of 300,000 tonnes in 2010 (Darby
2012).
Bacterially-generated monomers, such as lactic acid and hydroxyalkonates, have
achieved better degradation results. These monomers are polymerized to produce the plastics
poly-lactic acid (PLA) and polyhydroxyalkanoate (PHA) (Stevens 2002). These plastics
generally meet compostable standards (see just below), with polymer chain breakdown often
initiated by a temperature signal (Farrington et al. 2005). Scott and Wiles (2001) describe these
polymers as “sitting on the knife-edge” between readily consumable polymers like starch and
cellulose that have little technical utility as plastics, and conventional polymers that have good
plastics attributes but poor degradability.
Certain petroleum-based plastics are marketed as being degradable. Many are
conventional plastics with bacterial plastic or starch inserts, but others are modified with metal or
other degradation prompters (Scott and Wiles 2001). Some of these polymers include
polybutylene succinate (PBS), polycaprolactone (PCL), polybutylene succinate coadipate
(PBSA) copolymers, polybutyrate adipate terephthalate (PBAT), adipic acid aliphatic and
aromatic copolyesters (AAC), modified polyethylene terephthalate (PET) ( Reverte and Biomax
brands), modified polyethylene (PE) (TDPA and Totally Degradable Plastic, Addiflex, Degrade,
ECO, PDQ, Bio-Solo, Reverte, Biobatch, and Entec brands), modified polypropylene (PP)
(Reverte brand), modified poly vinyl chloride (PVC) (BioSmart), polyvinyl alcohol (PVOH),
ethylene vinyl alcohol (EVOH), and ketone carbonyl additive packages used as a copolymer
(Ecolyte brand) (ExcelPlas Australia et al. 2004, Ammala et al. 2011). All of these are reported
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to generally not meet compostable standards (see just below) (Darby 2012), although at least two
of these polymers have been found to degrade in compost (Ammala et al. 2011).
After the initial rush of enthusiasm for degradable plastics in the 1980s and early 1990s,
and the resulting consumer backlash when product degradation properties did not meet
expectations, degradable plastics production hit a lull. Output grew again in the late 2000s, with
one estimate being 300,000-400,000 tonnes sold per year (Song et al. 2009) (note that Song et al.
2009 estimated this was 0.2% of all plastics production, implying world-wide plastics production
of 200 million tonnes, 50% less than reported by Rochman et al. 2013).
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3. Degradable Plastics Labeling
The failure of early biodegradable plastics to degrade as completely as expected has led
to refined industry product descriptions so that degradable expectations are met by products
(Pagga et al. 1995; Krzan et al. 2006). These voluntary standards generally specify definitions,
testing guidelines, and acceptable advertising terminology (Kale et al. 2007), with the intent of
creating consistency, accountability, and reliability of behavior across different product lines.
Different but similar approaches have been put in place in the US, Germany, Japan, and the
European Union, and an international code has been developed by the International Organization
for Standardization (ISO) (Krzan et al. 2006).
The American Society for Testing and Materials (ASTM) promulgates acceptable usage
through Committee D20.96, “Environmentally Degradable Plastics and Biobased Products,”
leading to two ASTM standards addressing biodegradable plastics in composting environments:
D6400 (specification for compostable plastics) and D6868 (specification for biodegradable
plastics used as coating on paper and other compostable substrates) (BPI 2011). These standards
define compostable plastics operationally, based on conditions found at municipal and industrial
compost facilities (Krzan 2006), according to three tests:
1) conversion to CO 2 by organisms found in compost at an acceptable rate;
2) fragmentation; and
3) a determination that the resulting compost can support plant growth (including
elemental testing to meet standards for metals content) (Goldstein and Olivares 2007).
ASTM has also developed the standard D7081 for non-floating biodegradable plastics in the
marine environment which is used for biodegradable plastics that are designed to biodegrade in
marine waters and sediment. The primary European standards (from the Comite Europeen de
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Normilisation), EN 13432, as well as the companion standards (EN 14045, EN 14046, EN
14047, EN 14048), are similar to D6400 and D6868, and require that compostable plastics set in
an aqueous biotic environment be substantially (>90%) converted to CO 2 and biomass within six
months with visual disintegration, and result in a product that is “recognizabl[y]” compostabledegradable by the compost product “end user” without toxic by-products (Scott and Wiles 2001,
Wiles and Scott 2006, Krzan et al. 2006).
Certification programs are intended to guarantee certain materials or products meet
standards (Krzan et al. 2006; Kale et al. 2007). In the US, the Biodegradable Products Institute
(BPI) and the US Composting Council created the Compostable Logo program (CSU Chico
2007), so certified manufacturers can use a product or product packaging logo to identify a
compostable product to consumers, and, potentially, help ensure the product receives
appropriate end-of-life management. Many “degradable” plastic products cannot meet
compostable standards.
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4. Current Degradable Resins
Modern iterations of degradable plastics are used in packaging, disposable food utensils
and tableware, bags, mulch films, and diapers (Gross and Kalra 2002; Krzan et al. 2006). Only a
few durable goods are made from degradable plastics, as it can be difficult to suppress
degradability until disposal for long-lived products (Song et al. 2009).
Two resins used to make plastics conforming to certifications for compostable plastics
that have been tested to show good degradability at compost sites and are manufactured in
substantial quantities are poly-lactic acid (PLA) and polyhydroxyalkanoate (PHA) (Mohee and
Unmar 2007). They are typically manufactured as pellets, which can then be used to create a
variety of products, often with the use of additives to achieve specific materials properties
(Goldstein and Olivares 2007; BPI 2011). Other materials that are also marketed as degradable
plastics will be discussed, such as oxo-degradable plastics and other modified polymers.
4.1 Poly-Lactic Acid (PLA)
PLA has the largest share of the US bioplastic market due to its many applications, such
as thermoformed cups and containers, forks, coatings for paper cups, and candy wrappers
(Farrington et al. 2005, CSU Chico 2007). PLA is produced in a complex, two-step fermentation
and chemical polymerization process. Lactic acid is generated by bacterial fermentation of
various carbohydrates (often from agricultural byproducts, such as glucose from corn starch or
sucrose from cane sugars). Next, the lactic acid is oligomerized and then dimerized to make a
cyclic acid monomer, which is polymerized to yield PLA (Garlotta 2001).
PLAs have elastic properties that allow them to replace commodity plastics for packaging
or some film uses (Vert et al. 1995). However, PLA cannot be simply substituted into existing
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manufacturing processes in place of standard polymers because PLA seals, forms, and flows
differently than conventional plastics (Yepsen 2009b).
In compost piles, moisture and heat attack PLA polymer chains, splitting them apart
through hydrolysis, creating smaller polymer fragments, and, potentially, reducing the fragments
to lactic acid monomers. Microorganisms can consume smaller polymer fragments and lactic
acid (Farrington et al. 2005).
4.2 Polyhydroxyalkanoate (PHA)
Polyesters of hydroxylkanoates (PHAs) are a broad class of compounds naturally
synthesized by more than 300 species of bacteria as intracellular carbon and energy storage
compounds (Steinbuchel and Valentin 1995; Berlanga et al. 2006). PHA structure, physiochemical properties, monomer composition, and the number and size of granules vary depending
on the production species (Anderson and Dawes 1990; Ha and Cho 2002). PHAs accumulate in
the cytoplasm of bacterial cells under conditions of limited nutrients; the bacteria use PHAs as
carbon and energy sources when the nutrient limitation conditions end (Lee 1996). A wide
variety of substrates may be used to produce PHAs, especially agricultural materials such as
starches and sucrose (Reddy et al. 2003). To produce PHA commercially, unfavorable growth
conditions (limited nitrogen, phosphorus, or magnesium) are induced for particular bacteria,
causing them to synthesize and store PHAs (Suriyamongkol et al. 2007).
The wide diversity of potential PHA monomers results in a broad spectrum of polymers
with many different properties (Suriyamongkol et al. 2007). Extracted PHAs are processed
similarly to traditional plastics since specific PHA monomers confer specific properties (Byrom
1987). Outputs include both flexible and durable plastics, which are used primarily for products
such as containers, bottles, razors, and food packaging. PHA is expensive, however, costing five
18
to ten times traditional plastics (Suriyamongkol et al. 2007). The simplest and most common
PHA is polyhydroxybutyrate (PHB) (Leja and Lewandowicz 2009).
PHA plastics are attacked by microorganisms that secrete PHA depolymerizer, and most
products take three to nine months to degrade (Jendrossek 2001; Suriyamongkol et al. 2007).
These organisms naturally occur in soil, compost sites, and marine sediments (Reddy et al.
2003).
4.3 Other Marketed Degradable Resins
Polyolefins (commonly, polyethylene and polypropylene) can have quicker degradation if
modified by adding transition metals/compounds (iron, cobalt, manganese, ferrocene, and
titanium and zinc oxides), and other “pro-oxidants” (ketone copolymers, alkaline earth or
ammonium oxy-hydroxy groups, alcohols and esters, benzophenone, pyrone rings,
polyisobutylene, amines, and peroxides) into the polymer backbone. This results in UV or heatinitiated oxidation of the polymer (Ammala et al. 2011).
Oxidation of the polymer theoretically changes the plastic from hydrophobic to
hydrophilic (Scott and Wiles 2001), and makes the compounds digestible by microoganisms, if
molecular weights of the fragments are less than 5000 Daltons (Da) (Ammala et al. 2011),
although some polyethylene has been reported to be consumed by fungi at weights from 4 - 28
kDa (Yamada-Onerdera et al. 2001). Experiments reported by Koutny et al. (2006) showed that
shortened PE chains (up to 100 kDa, although often much shorter) served as substrate for
inoculated bacteria (either pure strains or extracts from environments such as compost piles), as
measured by either gas emissions or changes in the plastics. Placing shortened PE chains in
environmental conditions conducive to degradation (forest soils, compost piles) resulted in
continuing chain size reduction and CO 2 off-gassing.
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Koutny et al. (2006) were not convinced that the mechanisms for biotic decay have been
well-described. Despite finding some evidence of decay for longer polymers, they have endorsed
the view that reduction to ~5 kDa is necessary. Braunegg and Haage (2001), using analogies to
degradation of alkanes, thought it highly unlikely that chains longer than 44 carbon molecules
would be susceptible to microbial consumption because the molecules are too large to pass into a
cell, and no ligases exist for these anthropogenic polymers. They noted that polyethylene
molecules of 1-5 kDa would have 70-350 C-C bonds, and so discounted reports of
biodegradation of PE at 5 kDa. Thomas et al. (2012), while agreeing that fragmentation of
oxidizable plastics occurs in the environment (at time frames of 2-5 years in the UK), discounted
reports of subsequent biodegradation as unproven. Mohee et al. (2008), therefore, suggested that
fragmentation (“degradation”) should be distinguished from microbial uptake
(“biodegradation”).
Polyolefins require a mechanism for the initiation of degradation; this is said to be the
rate-determinant for the degradation process. The rates of degradation initiation can be
controllable by balancing inclusions of oxidant and anti-oxidant compounds in petroleum-based
polymers (Wiles and Scott 2006). Encouragement of biological degradation is often attempted by
blending a conventional, petroleum based polymer with starch. Many of the degradable plastic
products that use petroleum-based plastics have up to 50% starch content. Conventional
petroleum-based polymers that are sold mixed with large amounts of starch to either create
degradability or to reduce costs of the polymer include PBSA, PBAT, AAC, and Biomax brand
modified PET (ExcelPlas Australia et al. 2004). PHB, one of the bacterial PHAs generally
identified as a compostable plastic, can be mixed with polyethylenes as films or bottles (PBAT).
A test of film made with PHB and mixes of PHB and TDPA found that soil burial for two
20
months resulted in measurable decreases in plastic properties, which have been associated with
chain length reductions. These effects were greatest when the samples had been heated prior to
burial (Martelli et al. 2009).
One exception to the relative lack of biodegradability of polyolefins (see below) appears
to be AAC (adipic acid aliphatic and aromatic copolyesters) produced by BASF. One test found
they met the German standards for biodegradation, based on CO 2 evolution after burial in a
natural medium. The rate of CO 2 production of shredded plastic in compost was measured and
approximately 95% of the carbon in three samples was converted to CO 2 over 100 days, at a
slightly slower rate than cellulose (Witt et al. 1999). In a laboratory experiment, an artificial
compost medium was inoculated with AAC powder and a microorganism isolated from compost
for 21 days. At that point no polymers were detectable – only monomers that compose the AAC.
A film of AAC under the same exposure conditions visually fragmented after four days (Witt et
al. 2001). It is thought that extracellular enzymes produced by naturally occurring fungi and
bacteria cause the initial cleavage of the polymer, allowing for microbial uptake (Witt et al.
1999). CSU Chico (2007) reports that the Ecoflex AAC products have been certified under the
ASTM and European standards.
Polycaprolactone (PCL) mixes well with other polymers, and this allows it to be added to
standard polymers to assist in their degradability. It is reported to degrade “readily” through
exposure to environmental micro-organisms (Woodruff and Hutmacher 2010), especially when
blended with starches, although this means losses in plastic properties (Koenig and Huang 1995).
A 12-day test in an artificial compost environment resulted in complete disappearance of a pure
PCL strip of compost bag; starch-synthetic mix and synthetic blend varieties had substantial
mass losses and noticeable physical changes (Day et al. 1997). In an extensive series of
21
laboratory and field testing, a PCL bag formulation was found to have mixed degradability; it
never degraded entirely as did some PLA and PHA bags, but it did better than polyethylene
mixes. Overall, its performance was equated to kraft bags (Farrell et al. 2001). PCL first was
used as a common dissolvable drug delivery implant beginning in the 1970s, but degradation
times of years in animals and humans meant it was supplanted by other more readily degradable
compounds. It is currently seeing more use in tissue implants, where it serves as a temporary
lattice to enable the body to regrow specific tissues (Woodruff and Hutmacher 2010). Because
PCL is only produced by Union Carbide, one brand (Mater-Bi) switched to other formulations
(unspecified) around 2001 (ExcelPlas Australia et al. 2004). Testing of the new formulation
showed more than 25% mass loss after 72 days of composting, and mixing with sludge to create
anaerobic conditions resulted in gas generation rates that were half of those generated from
cellulose, after 32 days (Mohee et al. 2008). CSU Chico (2007) reports that Mater-Bi materials
are certified under the ASTM and European standards, and by BPI.
The Korean government has encouraged manufacturers to use PCL-starch blended or
PBS plastics for retail and garbage bags. Laboratory testing of degradation of these bags under
aerobic and anaerobic conditions showed good degradation rates for the starch mixtures, as
measured by gas generation rates, but slower degeneration of the PBS (judged to be too slow for
effectiveness for compost applications). Both kinds of bags were physically degraded but still
recognizable after 90 days burial in a landfill (Cho et al. 2011).
TDPA is aggressively marketed as a degradable conventional plastic. It is licensed by
EPI, and used in many applications: mulch films, PE bubble wraps and bags, non-food contact
films, compostable bags, and shopping bags. The start of degradation ranges from several weeks
to several years, but the products are intended to degrade within 20-36 months, when disposed in
22
“appropriate” environments (Ammala et al. 2011). In laboratory testing, loss of properties
(mechanical strength, brittleness, chain shortening and compound oxidation) occurs within
weeks at 71ºC (Scott and Wiles 2006). After exposure to heat, molds and fungus can grow on
TDPA, causing surface pitting (Bonhomme et al. 2003). However, TDPA bag strips immersed in
0.6 m of seawater lost mass but did not lose structural integrity after 40 weeks of exposure,
although starch bag strips fell apart after 24 weeks (O’Brine and Thompson 2010). No
degradation was observed after 32 days inclusion in an anaerobic flask with sewage sludge;
methane gas generation was no different from a control (Mohee et al. 2008). Degradation begins
more quickly under dry and warm conditions (Chiellini et al. 2006), and is faster and more
complete in soil than in compost (Ammala et al. 2011). After 18 months in soil, heat-treated
samples were 60% mineralized, and degradation, as measured by CO 2 evolution, was found to
be continuing (Chielini et al. 2003). After 70 weeks of soil burial no recognizable soil fragments
could be recovered, a rate that was slightly slower but similar to how cellulose decomposed
(Ammala et al. 2011). A landfill cover film made from polyethylene and TDPA was buried for
three months and then found to no longer be intact, and another film, after 14 months exposure,
was reduced to average molecular weights of approximately four kDa (Swift and Wiles 2004).
For compost, however, a 12-day artificial compost exposure resulted in no measurable changes
to three different formulations (Day et al. 1997). Exposure of three other film formulations to 45
days of MSW composting resulted in weight losses but no change in the plastics’ appearance
(Agamathu and Faizura 2005). Exposure to 72 days of MSW compost showed no degradation of
the material, although some CO 2 generation was measured (Mohee et al. 2008). In compost at
55ºC, 25% of the carbon was respired after 14 months (Ammala et al. 2011). Testing in a
municipal composting plant found the product was biodegradable: that is, the product was
23
reduced to “particulate and partially biodegraded plastics” (Scott and Wiles 2006), which, since
the product passed toxicity tests and EPI rejects the compostable standards as unreasonably
demanding, was deemed sufficient (Billingham et al., 2000).
Reverte is added to polypropylene and polyethylene to make retail bags, garbage bags,
food containers and utensils, and trays, and also to polyethylene terephthalate (PET) for use in
beverage containers. Reverte plastics contain about 0.5% starch as well as proxidant additives.
These compounds add carbonyls on exposure to UV and are susceptible to microbial
degradation, as tested by in-house experiments, and also can be colonized by fungi. Up to 60%
of carbon was mineralized after two years of column compost testing (Ammala et al. 2011).
BioSmart plastics can be added to PE and PP like Reverte, but appear to have their use
limited to PVC shrink wraps. Its iron additive makes most products yellow-brown or darker
brown, although the company has worked to minimize these color effects. Additives prevent
degradation from starting for up to two months of UV exposure. Compost conditions (defined as
exposure to UV followed by 70ºC heat) resulted in greater brittleness (Ammala et al. 2011).
Addiflex makes degradable PE food packaging, food service goods, retail and garbage
bags, and drop cloths. Oven exposure (80ºC for eight days) resulted in greater brittleness.
Inclusion of film products in feedstock at a green waste composting facility showed the plastics
became brittle after two weeks, lost integrity after six weeks, and only 20% were detectable after
12 weeks (Ammala et al. 2011).
Symphony Environmental makes plastic additives that are used in plastic products for
companies including Nescafe, Pizza Hut, KFC and Walmart. These products do not degrade
under anaerobic conditions (Ammala et al. 2011).
24
Plastor and Plastigone products are used in mulch films. They are intended to begin
degrading under UV exposure (Ammala et al. 2011). No independent studies seem to have been
conducted regarding this or other degradation potentials.
ECO plastics are used for six-pack rings. They are intended to degrade due to UV
exposure, and have been shown to increase carbonyl groups after exposure to sunlight (but not
when under glass), becoming brittle after six weeks exposure on a rooftop or in seawater.
However, no test of microbial decay following embrittlement has been conducted (Ammala et al.
2011). Early compostable bags did not score well under testing (Farrell et al. 2001).
Ecolyte plastics are used in mulch films and packaging. They are designed to be sensitive
to UV, although increasing the additives to make degradation more probable reduces properties
as a polyethylene. It has been suggested they react more strongly to UV than ECO brand-type
plastics. No demonstration of degradability by microorganisms has been made (Ammala et al.
2011).
Biobatch is a developing product that is intended to be added to polyolefins to result in
degradation. It contains chemicals that are said to attract microorganisms, and also will use
sugars and other microbial substrates to make the plastic initially palatable. No field testing was
reported (Ammala et al. 2011).
Polyvinyl alcohol has unusual properties for a sealable plastic. It is water soluble, solvent
resistant, strong, resistant to oil and greases, and reportedly degrades readily in water and/or
common microorganisms. It is used in laundry applications (Davis 2003). It is reported that
degradation will be completed in 30 days (ExcelPlas Australia et al. 2004).
Certain consumer products are labeled as degradable without offering any overt or
independent certifications as to their actual performance, such as the organizational certifications
25
discussed above. We have noted that at sites where there are expectations for sound
environmental sensibilities (e.g., farmers’ markets and college campus retail locations), it is
common to receive goods in plastic bags that are stamped “degradable.” At the Huntington, NY,
farmers’ markets, buyers receive opaque film bags that are simply labeled “degradable plastic,”
with no other explanation. We have not been able to trace the manufacturer. At the Stony Brook
University (NY) Wolfmart, a white film plastic bag is used, labeled “Oxo-Biodegradable™
Bag.” Below that label was an explanation:
“[t]he plastic used in this bag will convert to water, carbon dioxide and biomass in
the presence of soil, moisture and oxygen. Like a fallen leaf, it will disappear over
time.”
There was a further label of “epi totally degradable plastic additives,” with the dot in the “i”
replaced with a stylized flower face. EPI is the licenser of TDPA. However, as related above,
there is little independent evidence that TDPA-treated plastic bags will “disappear” in a short
period of time.
Several government-sponsored reports on degradable plastics (CSU Chico 2007,
ExcelPlas Australia et al. 2004) found little evidence in either scientific literature or through
independent testing that “oxo-degradable” materials biodegrade. Scott has argued that the
definitions applied to degradable plastics are unfair, since many organic polymers do not degrade
in the environment in short time frames; he is partial to comparing rubber and wood to
oxidizable polyolefins, and claims that the plastic polymers lose properties at similar or faster
rates than the natural polymers (Scott 2000; Scott asnd Wiles 2001; Wiles and Scott 2006).
However, this means that the oxo-degradable plastics remain in the environment for years, so
that consumer expectations do match their experiences.
Tables 1 and 2 summarize some findings on compostable and degradable plastics.
26
Table 1. Compostable plastics
Independent
Testing
Much
Much
Much
A little
A little
Manufacturer Claims of
Degradability
Yes
Yes
Yes
Yes
Yes
Plastic
Certified
Starch-based
Generally
PLA
Generally
PHA
Generally
AAC
Some
PLC
Some
Starch-inserted
Generally
conventional
not
Some
Oxo-degradable
No
No*
* negative results ** often offer alternative definition of compostable
Yes
Yes**
Table 2. Degradable plastics
Degradability –
Degradability –
Plastic
Independent Testing Manufacturer Claims
Starch-based
Completely
Completely
PLA
Completely
Completely
PHA
Completely
Completely
AAC
Completely*
Completely
PLC
Completely
Completely
Starch-inserted conventional
Mixed results
Completely
UV-initiated
Mostly not
Completely
Oxo-degradable
Not
Completely**
* not shown directly ** often offer alternative definition of degradable
27
28
5. Compatibility of Degradable Plastics with Current Waste Management Processes
5.1 Composting
Four kinds of wastes are composted in the US: sewage sludges, yard wastes, food wastes,
and MSW (Epstein 2001). The latter two are collected curbside. MSW is always contained, and
most yard wastes are too in suburban and urban environments. Paper and plastic bags are often
used for these purposes, even when reusable garbage cans are set out. Specialized paper bags for
yard wastes are expensive, although they are degradable. Conventional plastic bags cause output
contamination (Epstein 2001).
Degradable plastics are primarily intended to address composting contamination (and
litter issues). Compostable plastics require specific levels of moisture and oxygen for initial
reactions to occur to make the polymers consumable by bacteria (Song et al. 2009). These
conditions are usually only found in larger, industrial-commercial facilities, where materials are
regularly turned, and usually have been pre-processed (often shredded) (Kale et al. 2007).
Initiation of degradation either requires hydrolyzation (for PLAs) or reactions with enzymes
from microorganisms (PHAs), making large polymers smaller and simpler. These smaller
molecules can pass through semi-permeable cell membranes to be used as energy sources,
nominally creating wastes of water and CO 2 . In composting, there is an intention to produce
some quantum of residual organic matter (humus), at least some of which is biomass associated
with the microbial and macrobiota consumers (Shah et al. 2008): some portion of compost
organic matter is expected to be indigestible (Scott 2000; Scott and Wiles 2001). Thus,
performance standards for degradable (compostable) plastics do not require all polymers to be
consumed, so that absolutely no plastic remains. In the UK, plastics must be 90% consumed in
laboratory testing (the test is assumed to have 10% variability on average); in other jurisdictions,
29
the typical requirement is to “degrade to the degree that compost inputs do” (Song et al. 2009).
Standards often add an element of toxicity testing (CSU Chico 2007, Wiles and Scott 2006),
minimizing the potential for the compost product to cause harm to plants, animals and humans
(Tosin et al. 1998).
Compostable plastics under standard, large-scale composting practices have been found
to degrade well, with different kinds of substrates, such as yard waste, manure, and food waste
(Kale et al. 2007), or using different technologies, such as turned windrow or in-vessel (CSU
Chico 2007). These results have led to endorsement of their wider use. For instance, San
Francisco encourages the use of compostable liner bags in its mandatory curbside organic waste
separation program. The bags are intended to be compatible with the City’s compost process and
provide users with acceptable container sanitation (Chester et al. 2008; Kale et al. 2007). Mulch
films are another area where compostable plastics are perceived as technological advances, as
dirt adhesions to the films make them difficult to recycle (Scheirs 1998) but may actually
enhance compostability.
However, reports of failure to perform by compostable-labeled plastics in at-home
composting environments are common. Inadequate temperatures in these smaller piles, so that
the key hydrolysis reaction for PLAs is not initiated, are thought to be the reason for much of the
poor results (Song et al. 2009; Farrington et al. 2005). This has reignited controversies associated
with earlier degradable plastic products, due to the mismatch between producer claims and
consumer experiences. The Sustainable Packaging Alliance (Australia) (2009) stated that the use
of compostable bags is limited because jurisdictions need to ensure formulations are compatible
with the system accepting the waste (and bags).
30
Composting plastics minimizes the amount of waste which goes to landfills. Reducing
the landfilling of waste has been a major public policy initiative for decades (Sidique et al. 2010;
Loughlin and Barlaz 2006), but more than half of all US solid waste is currently landfilled
(USEPA 2010; van Haaren et al. 2010). Yard and food waste, according to sampling across the
US, constitute at least 10% (and sometimes much more) of disposed wastes, even in crowded
cities or rural areas where yard waste is not disposed (Table 3). USEPA (2010) found food
wastes to be 20.3% and yard wastes 8.3% of disposed wastes in 2009. Thus, those seeking to
increase waste recovery see organic wastes as a great opportunity for additional composting
(NYSDEC 2010). Contamination of yard wastes with plastic bags is a major operational
inconvenience, and institutional food waste composting requires removal of unwanted plastic
cutlery and the like (see Figure 1). Compostable plastics are perceived as means to address these
issues.
Table 3. Percent yard and food waste in disposed wastes (sampling results)
Location
Year
Yard waste Food waste Total Source
Vermont1
2001
0.7
21.3
22.0 DSM Environmental Services 2002
Georgia2
2003-2004
7.6
16.4
24.0 RW Beck 2005
2
Iowa
2005
1.6
10.6
12.2 RW Beck 2006a
California2
2008
3.8
15.5
20.3 Cascadia Consulting Group 2009
New York1 2004-2005
9.9
15.9
24.9 RW Beck 2006b
2
3
Chicago
2007
1.2
20.7
21.9 CDM 2010
Phoenix4
2003
28.13
16.8
44.9 Cascadia Consulting Group et al. 2003
1
residential wastes; 2 residential and commercial wastes; 3 compostable yard waste only; 4 single family
residential wastes only
31
Figure 1. Plastic contamination of yard waste compost (photo by D. Tonjes)
However, there are concerns that composting plastics invalidates the resulting product for
organic certification and subsequent use on organic farms (Sullivan 2011). Tentative organic
certification rules for degradable plastics in organic compost may require specification of the
source of the feedstock for the plastic, with petroleum-based plastics not qualifying as organic,
but plant-based degradable plastics meeting this organic standard (Fernandez-Salvador 2012), a
process which may be complicated to implement. A primary purpose of degradable plastics is to
support greater composting use; however, it is not clear that these degradable plastics will win
32
widespread acceptance if the resulting compost product may not be considered organic, and/or
there continue to be widespread failures in at-home usage.
5.2 Recycling
Recycling is the primary method used to minimize waste in landfills and it is perceived to
be the most preferable means of managing plastics (e.g., Brandrup 2003; Lazarveric et al. 2010).
However, many resins are difficult to recycle (Kuswanti et al. 2003); one cause of problems is
certain resins are intolerable contaminants for other resins (Goodship 2007); PVC is often
singled out in this regard (Belliveau and Lester 2004; Park et al. 2007). The high volume-weight
ratios for some plastics, especially blown polystyrenes, makes collection and transport of some
products very difficult and/or expensive (Goodship 2007). Sorting plastics to general resin
categories can be challenging (Kuswanti et al. 2003); many plastics products look similar but are
of different compositions (Park et al. 2007), and some plastic wastes are small and difficult to
handle (Richard et al. 2011; Kinoshita et al. 2006). Different polymers can be similar in density,
so that float sortation is not efficient (polypropylene and HDPE are examples) (Scheirs 1998).
Styrofoam peanuts are notoriously difficult to handle, due to extremely low density and static
cling. Contaminants adhering to plastics can cause processing issues for certain means of
materials recovery (Lazarevic et al. 2010), and some chemicals attached to plastic products, such
as adhesives, are difficult to manage and cause production quality control issues (Scheirs 1998).
Generally, in order for plastics to be recycled, well-characterized, single material product
streams need to be created (Goodship 2007). Thermoset plastics, which are mostly rigid,
structural forms (Gruenwald 1993), are difficult to recycle because the thermosetting process
changes the plastics’ structure, so that it is not possible to recover the underlying poly/monomer
through most processes (Pickering 2006). Recycling of thermoplastics requires remelting them;
33
early quality issues with secondary products from recycling, such as plastic lumber, resulted
from differences in melt temperatures with different polymers. Remelting thermoplastics leads to
loss of polymerization and thus to some deterioration of materials properties so that even
collections of some single polymers are not indefinitely recyclable (Goodship 2007). For
instance, intrinsic viscosity of PET decreases with each re-melt. For a 50% recycled content PET
bottle, due to accumulating extrusion histories, one calculation was there will be a decrease in
intrinsic viscosity of the resulting product of over a third. Technology adaptations and additives
exist that can minimize or eliminate the reductions in polymer strength. HDPE tends not to
degrade through recycling, although tinting of resins from included colored caps causes
reductions in the utility of recycled products, and “black speck” inclusion (resulting from flakes
that have too long residence time in an extruder) is another aesthetic problem. High milk
container content reduces crack resistance, as well, which limits recycled content in many
applications. For PVC, as little as 1% contamination levels can cause problems in the
reformation process (Scheirs 1998).
Degradable versions of products differ from conventional plastics in either base polymers
or additive mixtures; this means that the increase in input heterogeneity will reduce the quality of
the recovered plastic. A test mix of 5-10% of a variety of degradable and compostable plastics
with HDPE and LDPE resulted in decreases in mechanical and aesthetic properties (CSU Chico
2007). Reports from Australia suggest that recyclers there do not want to accept degradable
plastics for two reasons. One, the resulting products exhibit loss of plastic properties. Secondly,
there are concerns regarding potential later degradation of these end products (ExcelPlas
Australia et al. 2004, The Sustainable Packaging Alliance 2009). Clearly, degradation of the
resulting recyclate would not be desired. This issue would be magnified if segregated
34
accumulations of degradable plastics were to be made, since any partial degradation of feedstock
would make the resulting recycled product less suitable for reuse (Goodship 2007; Song et al.
2009). In addition, if degradable plastics constitute appreciable amounts of the plastics
recyclables stream, there is concern regarding the stability of long-life (construction) and
medium life (outdoor furniture) applications (Thomas et al. 2012). However, the current
consensus appears to be that degradable plastics can be considered to be recyclable, as they
constitute only a very small part of overall feedstock, not enough to make a measurable impact
given other concerns in recycling plastics (Al-Malaiki et al. 1995; Song et al. 2009).
5.3 Waste-to-Energy Incineration
The processes in waste-to-energy (WTE) incineration would not be affected by whether
input plastic is degradable or not. However, the use of bio-based resins would reduce fossil CO 2
emissions. Current estimates are 56% of all energy resulting from WTE incineration comes from
biogenic organic MSW, and so combustion of MSW produces energy that is at least half-derived
in a way that does not increase the amount of CO 2 in the biosphere (Energy Information Agency
2007). The amount of fossil carbon in MSW (and its percentage of the energy content) is
increasing with growing use of petroleum-stock plastics, however. WTE incineration has been
identified as a means of producing electricity with fewer climate change impacts than the
standard US grid mix of generation sources (Bahor et al. 2009), so more bioplastics use would
increase the environmental benefits of this process. Still, producing degradable plastics with the
aim of improving the performance of WTE incinerators is not especially efficient, although it
could be an unintended, pleasant side effect. Many degradable plastics are made from renewable
feedstocks (Suriyamongkol et al. 2007); the production of conventional plastics uses 4% of the
world’s annual petroleum production (Andrady and Neal 2009). Therefore, increasing the market
35
share of degradable plastics would slightly reduce demands on diminishing petrochemical
reserves (Suriyamongkol et al. 2007), and, if offsetting uses for the petroleum were not found,
could somewhat reduce the potential for global climate change due to CO 2 emissions (Dornburg
et al. 2004; Gross and Kalra 2002).
5.4 Landfilling
Most waste in the US is disposed in landfills (USEPA 2010, van Haaren et al. 2010).
Landfills are ranked lowest on the USEPA hierarchy because of their potential for environmental
impacts. The primary impacts from landfills are landfill leachate generation (which threatens
water quality) and gas generation (escaping methane can cause local explosion dangers and is a
potent greenhouse gas). Landfill mesocosm and other laboratory experimental work (e.g.,
Pohland et al. 1993; Eleazar et al. 1997) support the notion that biological degradation processes
are the primary control on leachate quality (Christensen and Kjeldsen 1989). This suggests
degradation of organic matter is an important process in landfills. In a Swedish test cell, ~50% of
paper, animal, and vegetable matter degraded over 20 years (Flyhammar et al. 1998), and
samples from several landfills indicated that degradation of more labile organic matter was
common (Bookter and Ham 1982). However, some other experimental work and excavations of
landfills found that most organic wastes will not degrade even after decades. Wall and Zeiss
(1995) estimated 20% of organic wastes would degrade over a landfill lifetime (degradation was
estimated to be log-linear with time), and Rathje (as reported in Rathje and Miller 2001) found
only 2-5% of an Arizona’s landfill contents degraded after 15 years. Bozkurt et al. (1999)
modeled landfill processes, and suggested most organic matter may not be affected by landfill
residence except on time scales of centuries or more.
36
Gas generation is not always perceived as a negative feature of landfilling. If the gas can
be efficiently captured with little losses to the atmosphere, then it represents an environmental
benefit if it can be used to generate electricity. The benefits accrue because landfill methane is
produced from non-fossil carbon, and so is classified as “green” energy (Jaramillo and Matthews
2005). However, methane is more potent a greenhouse gas than CO 2 , so depending on the
portion of methane that escapes collection, gains may or may not be realized by displacing fossil
fuels at conventional electricity sources. One estimate is that between 35% and 90% of methane
can be captured, depending on the landfill cover system (Spokas et al. 2006). If there is little to
no fugitive gas from a standard landfill gas to energy system, and if degradable plastics were
consumed by microbes in landfills, then the use of degradable plastics would increase potential
reductions in overall greenhouse gas emissions associated with the technology (Levis and Barlaz
2011). Wiles and Scott (2006) and ExcelPlas Australia et al. (2004) hypothesize that loss of
mechanical strength and other degradation of plastic bags could enhance gas production, on the
theory that intact plastic bags prevent access by microorganisms to their contents and so either
prevent or slow the degradation of labile carbon in landfills.
Replacing recalcitrant plastics with plastics that have greater potential to degrade may
result in greater degradation of the plastics themselves – if the degradable plastics encounter
conditions that result in depolymerization. Burial of UV-sensitive plastics is not likely to result
in any early plastics decay. Plastics where degradation is initiated by higher temperatures are
more likely to start decomposing in most landfills. For instance, landfill cover film made of
polyethylene and TDPA lost integrity in one three month trial, and average molecular weight
was reduced to less than five kDa after 14 months at another site (Swift and Wiles 2004). Most
degradable plastics that are “compostable” generally require moisture and oxygen for the process
37
to proceed very far, however. Moisture may or may not be available in particular landfills or
areas in landfills, but landfills generally are known to be lacking in oxygen. No studies of
compostable plastics in landfill environments were located (per ExcelPlas Australia et al. 2004),
although some starch-based plastics have degraded in simulated anaerobic digesters (CSU Chico
2007). The first generation of degradable plastics, which did not meet compostable standards,
showed no notable degradation in landfills (Breslin and Swanson 1993). Although Swift and
Wiles (2004) suggest anaerobic decay of degradable plastics will occur (albeit slowly), it seems
more likely that degradable plastics will not behave very differently from petroleum-based
plastics in most landfills.
Some have identified the lack of degradation of organic material in a landfill as a benefit.
This is because no decay of organic matter represents a sequestration of carbon (Staley and
Barlaz 2009), especially if retarded for centuries or more (Bozkurt 1999). In this view, however,
even potentially degradable organic compounds reduce environmental benefits.
Thus degradable plastics in landfills offer the following potential effects:
1) decay and release of more methane – which is a benefit if enough gas is captured and
used as an alternative energy source, but otherwise causes more environmental
problems
2) decay and production of higher strength leachate, which poses an environmental
problem
3) sequestration of carbon, which reduces overall climate change impacts and so is an
environmental benefit. If the degradable plastics are biobased, this benefit would
be greater than burying petroleum-based plastics, as petroleum-based plastic
sequestration represents prevention of the release of old carbon, while
38
sequestration of biobased plastics represents a drawdown in current stocks of
circulating CO 2 .
Since it seems most likely that degradable plastics will not decay readily in landfills, use of these
products likely would lead to a small environmental benefit due to enhanced sequestration
effects.
5.5 Litter
Their persistence when inappropriately strewn into the environment makes plastics the
poster-child for litter issues (Rochman et al. 2013) (Figure 2). It has been argued that if plastics
were to be degradable, even at timescales of several years, it would reduce the impact of litter
tremendously (Guillet et al. 1995). It has been asserted that most compostable plastics do not
degrade very well outside of compost piles (Song et al. 2009). Scott argues that this highlights
the value of UV-sensitive degradables, as they will be affected by the environment if left in the
open, as most litter is (Scott 2000; Scott and Wiles 2001; Wiles and Scott 2006). Certainly UV
sensitivity would appear to be a better attribute for plastics than compostability if persistence of
litter is the issue at hand.
One test of PHB materials found that the coated cups would either entirely degrade or
almost entirely degrade within a year in laboratory tests designed to simulate key attributes of
marine settings. Greater degradation occurred in bacteria-inoculated salt water when additional
nutrients and sediments were added; in the absence of additional nutrients, even readily labile
materials often did not degrade entirely, and neither did the PHB-coated cups. PHB films had
approximately similar results (Ratto et al. 2001).
39
Figure 2. Marine plastic litter (Caribbean Sea) (photo courtesy RL Swanson)
UV-sensitive plastics require exposure to sufficient radiation for degradability to be
initiated. If plastics accumulate in the open or float on the water, then they are likely to receive
significant UV exposure. However, certain plastics have sufficient density (or retain not enough
air) to sink below the ocean’s surface, and these plastics may not receive enough UV energy to
cause initiation of decay. In that case, since they lack any means to initiate decomposition, they
are functionally the same as conventional plastics. O’Brine and Thompson (2010) found that bag
strips set in 0.6 m of water were fouled by macro-organisms and algae after eight weeks, which
also would impede UV exposure; perhaps consequentially, the UV-triggered degradable bag
formulation they tested was still cohesive after 40 weeks of exposure, although it had lost some
mass. However, that study found a starch-based compostable plastic degraded enough to lose its
integrity before fouling occurred. A marine exposure test, over 14 and 21 day test periods, of a
40
range of compostable, UV-sensitive, and oxidative degradable bags and materials by CSU Chico
(2007) found that UV-sensitive six-pack rings became brittle, and the PHA-based plastic lost 3660% of its mass, but none of the other plastics had any detectable degradation.
In the degradation of plastic polymers, no matter the mechanism or process, a point can
be reached where “fragments” are created. At this stage, either these residues prove to be
recalcitrant (on meaningful time scales) or the fragments decompose further. With further
decomposition, either the compounds become incorporated into biomass (in a sense, functionally
reduced to CO 2 ) or a residue will be created. The recalcitrant residues need to be characterized,
both chemically and in terms of their potential environmental effects (Karlsson and Albertsson
1995; Swift 1995), although this is rarely done. Fragmentation of plastics eliminates the visual
blight of plastic litter and would seem likely to reduce ingestion of plastic by organisms that
search for food using visual clues (ExcelPlas Australia et al. 2004). However, microlitter, with its
greater surface area, serves as ready sorption sites for organic pollutants, and can be consumed
by filter-feeding organisms in the ocean or earthworms on land (Thomas et al. 2012; Rochman et
al. 2013). Therefore, plastics that only partially degrade still represent substantial environmental
problems if they become litter (Breslin and Swanson 1993).
The government of Australia mandated that all “bait bags” be made of degradable
plastics to address one marine litter issue. A particular formulation by Mater-Bi has been the
choice of manufacturers (it is a starch-conventional plastic mix). Testing appears to show 10%
weight loss over three weeks seawater exposure, and an estimated six month degradation time in
total. Tests have been conducted to mimic the effect of turtle digestive juice on the bags
(ExcelPlas Australia et al. 2004) (but results were not reported).
41
Table 4 summarizes potential impacts on current waste management practices from the
use of compostable and degradable plastics.
Table 4. Impacts of degradable and compostable plastics on current waste management
practices
Waste
Management
Practice
Compostable Plastics
Reduce-eliminate contamination at
Composting
industrial sites; cause contamination of
end product at small and backyard sites
Increase resin complexity, cause
Recycling
recyclate quality issues
WTE Incineration No impact
Landfilling
No apparent impact beyond some C
sequestration
Litter
Starch-based resins will degrade; PHB
may degrade; others may not
42
Degradable Plastics
Cause contamination of end
product
Increase resin complexity, cause
recyclate quality issues
No impact
May have slight beneficial effects
if enhanced gas generation is
sought; potential C sequestration
Visible effects may be decreased
(for UV-sensitive plastics);
otherwise, no enhanced
degradation
6. Degradable Plastics and Effects on Proposed Solid Waste Management Technologies
and Strategies
6.1 Expanded Recycling
Most US municipal recyclables collection programs were established in the 1980s and
1990s, and can now be considered to be mature. National evaluations of recycling have generally
shown relatively flat rates over the first decade of the new millennium (USEPA 2010; van
Haaren et al. 2010). Still, expectations are (and associated plans call for) increased recovery rates
over time (see NYSDEC 2010, for instance).
One proposed mechanism to increase recycling rates is to inject economic factors into
collection processes, using an idea known as “Pay as You Throw” (PAYT). PAYT means that
waste generators pay variable amounts depending on waste management practices, with
separated wastes for recycling generally being free and those requiring disposal being charged
either by weight or volume. Although the impact of PAYT is difficult to isolate from other
programmatic changes, one analysis of multiple locations that attempted to control for
extraneous factors suggested as much as 5% more of the waste stream will be diverted to
recycling (Skumatz 2008). Plastics, by best available estimates, are becoming a larger proportion
of recyclable products, replacing glass, metal, and paper alternatives (USEPA 2010), so that the
recovery of plastics is likely to be a more substantial component of recycling statistics over time.
As discussed above, degradable plastics have the potential to reduce the recyclability of plastics,
especially if their share of the plastics market increases. Therefore, increasing use of degradable
plastics may hinder efforts to increase conventional recycling rates.
43
6.2 Expanded Organics Recovery
Organic wastes such as food wastes are often considered to be the best target to make
appreciable increases in solid waste recovery rates (USEPA 1999; Cuellar et al. 2010; NYSDEC
2010). Although food reuse and waste reduction programs have received some attention, the
primary recovery of food wastes (and related organic matter such as soiled paper) is to be
through either composting or anaerobic digestion (enclosed decomposition of matter by
organisms producing gases such as methane and CO 2 , and a solid organic residue).
Key to the success of either process is the capture of food waste from the waste stream,
with minimal inclusion of materials that are not degradable by organisms. Processes where
organic wastes are produced in large quantities in a controlled fashion – food processing plants,
industrial kitchens, institutions (such as prisons and offices with on-site cafeterias), even
supermarkets – seem to generate waste streams with less contamination potential than residential
or front-of-the-restaurant collection programs, where differing perceptions of proper
participation cause quality control issues. Degradable plastics are generally identified as an
important means to reduce contamination effects, although the degradable plastic in the wastes
needs to be matched with the degradation process to be utilized (Mohee et al. 2008).
Compostable plastics, if they require oxic conditions to complete degradation, may not be
consumed by anaerobic organisms in digesters, for instance. So, for instance, PHA plastic
underwent 38% degradation under anaerobic conditions in a 50:50 mix with food waste, but PLA
only had minor degradation (6%), as did other formulations that were tested (such as UVsensitive or oxidative degradable plastics) (CSU Chico 2007).
Along with bags and agricultural films, plastic utensils and packaging that currently
contaminate compostable waste streams are being targeted by compostable plastics
44
manufacturers as primary market opportunities. Organizations that want to appear to be
progressive in waste management practices are adopting compostable plastic materials for use
although they do not currently have a food waste or other organic waste diversion program (such
as at our home institution). However, incomplete market penetration may result in, say,
compostable plastics being disposed instead of separated for composting, or conventional
plastics included in the compostable waste stream, due to consumer confusion.
Compost sites often create objectionable odors (Buckner, 2002; Goldstein, 2002; Gage,
2003, Goldstein, 2007). This has made it difficult to operate the cheapest kind of compost site,
one with outdoor composting, in regions where the most wastes are generated. Anaerobic
digestion, which requires enclosed degradation with gas capture, and can generate electricity
from these gases, may be the preferred future technology. Anaerobic digestion is a proven
technology for organic wastes generally, and has had commercial success in treating sourceseparated organic wastes from the solid waste stream. Anaerobic digestion requires that waste
compounds be amenable to decomposition by well-characterized bacteria (Demirbas et al. 2011).
As was discussed above in connection with landfills, it is not clear that compostable plastics can
provide the proper substrates for decomposition under anaerobic conditions. Mater-Bi
degradable plastic degraded about half as well as cellulose over a 32 day simulated digester
exposure (Mohee et al. 2008), which is either a hopeful or poor result, depending on perspective.
If anaerobic digestion is to be the preferred technology for managing organic portions of solid
waste, degradable plastics may not be appropriate to include in the feedstock, as they appear
likely to add to residues.
Contaminants are best removed prior to composting or digestion, as pre-processing to
increase efficiency often includes grinding or other processes that reduce particle sizes, which
45
results in residues that are difficult to identify and separate because they are so small. If
conventional and degradable plastics are not easily distinguishable, then incomplete market
penetration will mean that it is better to exclude all plastics (if possible) from feedstocks. This
may be possible: at an Italian plant, all degradable plastics were removed in pre-processing
through a technology that targeted (conventional) plastics (Garaffa and Yepsen 2012). In Europe,
where one element of organic waste processing is to produce a largely inert material to meet
landfilling requirements (Demirbas et al. 2011), the creation of a not-marketable solid residue is
not an impediment to use of this technology, and the poor degradability of degradable (or
conventional) plastic formulations under anaerobic conditions would not seem to be an issue. If
the digestate or residues are to have some future commercial use, as is often the case in the US,
then allowing materials that tend to be recalcitrant into processes is not beneficial.
6.3 Innovative Energy Recovery
Over the past several decades various technologies have been considered as alternatives
to mass-burn incineration, primarily because of its general inefficiencies in energy recovery
(Murphy and McKeogh 2004). Pyrolysis (the degradation of organic molecules under low-no
oxygen conditions) (Demirbas and Balat 2007) and gasification (conversion of organic solids to
gases under high heat and pressure conditions) (Arena 2012) have been receiving more attention
in recent years, as underlying technologies have been refined and the potential for improved
performance over incineration has been appreciated. A third alternative technology, plasma arc
gasification, while potentially netting more energy with fewer residuals (Mountouris et al. 2006),
seems further from commercial application.
Whether plastic feedstocks are degradable or not should be meaningless in the context of
these technologies. They degrade molecular bonds through the application of energy, and there is
46
no biological component to be considered. Source separated plastics are currently successfully
processed through these technologies (Williams and Williams 1997, Panda et al. 2010); the
success or failure of these more delicate processes when applied to general municipal solid waste
will not be affected by minor differences in plastics chemistry, but rather by more global issues
of solids-residues quality and contaminants in fuel stocks.
6.4 Bioreactor Landfills
Bioreactors landfills are operated, in contravention of standard landfilling practices, to
enhance degradation processes. This is usually accomplished by increasing the liquids content of
the fill, primarily by not removing leachates and instead re-circulating them through the wastes.
Enhanced degradation increases the generation of methane, which means there is greater
recovery of the inherent energy in the wastes (the necessary assumption is there will be effective
methane capture and the subsequent production of electricity or liquid fuels). Enhanced
degradation also implies faster settling of wastes, allowing for airspace to be (in a sense) reused
or resold. Quicker, more complete degradation of wastes may also mean that a site is determined
to have “stabilized” sooner, reducing long-term monitoring costs and responsibilities (Warith
2002).
Degradable plastics that depend on exposure to UV radiation will not degrade faster than
conventional plastics under bioreactor landfill conditions. Plastics affected by higher
temperatures could have degradation processes initiated by the elevated temperatures found in
bioreactors (akin to composting sites), but compostable plastics require moisture and oxygen.
Bioreactor landfills are intended to be very moist, but not aerobic. Therefore, degradable plastics
should behave in a bioreactor as they do in a standard landfill: most probably not degrade any
47
more quickly than conventional plastics, negating the reasons for selecting them (but causing no
negative consequences, either).
6.5 Extended Producer Responsibility
“Extended producer responsibility” (EPR) describes policies which legislate incentives
for manufacturers to incorporate environmental considerations in the design of products, and
shift physical and economic post-consumer responsibility to the producer or seller of the product
(Fishbein 1998; Lifset 1993; Nash and Bosso 2013; OECD undated). EPR programs intend to
create links between product design and end-of-life management (Lewis 2005; Hickle 2007;
Quinn and Sinclair 2006; Lifset and Lindhqvist 2008). EPR generally requires that the specific
users of products bear financial responsibility for end-of-life management, usually by explicitly
incorporating end-of-life costs into the product’s price, which could create consumer cost
preferences (Lifset 1993; Palmer and Walls 1999). This, and the fostering of reuse and
recyclability through performance standards, are intended to create an impetus for Design-forthe-Environment (DfE) (Lifset 1993; Lifset and Lindhqvist 2008). DfE broadly includes
considering the environmental impact of input material resources and end of life effects in the
design of the product, moving beyond performance/materials availability and cost bounds that
are central to traditional design. Notionally, DfE increases the use of recycled products and
reuse, minimizes environmental effects from resource extraction and production, and promotes
more sustainable materials management (Lifset 1993; Lifset and Lindhqvist 2008). When DfE is
extended to post-consumer arenas, it should encourage redesign of products to minimize end-oflife environmental impacts and increase post-consumer value (Quinn and Sinclair 2006;
Lindhqvist and Lifset 1999). EPR makes it essential that product designers understand the
48
environmental, social, or health impacts associated with the product life cycles (Veleva 2008;
Nash and Bosso 2013).
Not all EPR initiatives share equally in all of these rationales. To this end, compostable
and degradable plastics fit many but not all of EPR criteria. Compostable plastics from biomass
feedstock seem to more sustainable, in that they are made from renewable materials and are to be
managed through an environmentally favored waste management method. Thus, they fit with
some DfE concepts, as clearly their design involves concern about their impact on the
environment. Another important element of EPR is to simulate technological change and
innovation toward an environmental benefit (Tojo 2004), as has been the case with degradable
plastics. However, EPR economic concepts do not fit as well: manufacturers of degradable
plastics have no explicit involvement in their end-of-life management, especially in terms of
financial support; and these products are much more expensive for consumers, sending a
negative price signal. Toffel (2003) noted EPR imposes various combinations of economic,
physical, informative, or legal responsibilities upon manufacturers. However, degradable plastics
manufacturers assume only the informative burden (depending upon the perception of where the
burden of additional costs associated with creating the degradable products falls). Degradable
manufacturers do not explicitly pay for any end-of-life costs, unless the extra manufacturing
costs are seen as implicitly accounting for some end-of-life expenses.
Ultimately, the only mechanism to increase compostable products use will be through
more extensive use of composting facilities which accept them (Gross and Kalra 2002). This
may only be possible through governmental mandates to build such facilities. Until infrastructure
is available, conversion from conventional to compostable plastics will have no environmental
49
benefit if most waste continues to be landfilled. Thus, the DfE purpose of compostable plastics is
difficult to achieve under current and foreseeable conditions.
The second aspect spurring EPR development is usually less explicitly discussed. Targets
of EPR (Table 5) are often the more difficult to manage or expensive elements of the waste
stream (Driedger 2002). Plastics (especially plastic packaging), by some accountings, are more
expensive to manage than other elements of the waste stream, especially through recycling
(Nakajima and Vanderburg 2006). If this is indeed the case, which is far from clear, then even
though revenues from recycling would be lost, shifting plastics management to composting
might be an overall cost reduction – but not if in-vessel management is required, as is the case in
the UK (Song et al. 2009).
Table 5. Targets for EPR in Beyond Waste (the NY State solid waste management plan)
Target
Legislation Passed? Introduced?
Electronics
Effective 4/11
Reusable Batteries
Effective 7/11
Pharmaceuticals
Introduced, 2011 session
Household hazardous waste (HHW)
Packaging
Mercury-containing products
Introduced, 2011 session
Paint
Automobiles
Carpet
Office Furniture
Roofing shingles
Appliances
Tires
EPR also offers waste managers the potential to shift costs from all taxpayers to those
who use particular materials. Normally, this assessment is based on the standard waste
management costs associated with collection from waste generation points and subsequent
management of the materials. However, the management of litter, which has such a close
50
association with plastics (especially plastics packaging), is no one’s explicit responsibility,
although various levels of government do assume it. Estimates of annual terrestrial litter
management costs in the US were $11.5 billion (Mid Atlantic Solid Waste Consultants 2009),
and, in Australia, Aus$200 million/yr (ExcelPlas Australia et al. 2004). Marine litter
management, if undertaken, would seem likely to cost at least as much, with one estimate for
cleanups along the California coast alone being $500 million (Rochman et al. 2013). Considering
degradable plastics as an acceptable means of addressing EPR could allow manufacturers to
avoid further end-of-life responsibilities. Those working toward packaging EPR legislation in the
US (Gardner 2013) understand that packaging EPR may provide a financing mechanism for
litter, particularly litter that might reach the marine environment (personal communication, H.
Sanborn, Executive Director, California Product Stewardship Council, 10 May 2011). Thus,
degradable plastics appear technically unable to address marine litter, and their adoption as an
EPR alternative could impede assigning the costs of clean-up to manufacturers.
6.6 Zero Waste
“Zero waste” is a waste (and materials) management planning approach that has been
adopted by a number of governments around the world; the most prominent among them are
New Zealand and San Francisco (Greyson 2007). Zero waste requires changing materials
management from a linear process (extraction-processing-use-discard) to a circular process,
where inputs for new products come from those things that are no longer needed. The infused
paradigm is to mimic how natural systems reprocess nutrients (McDonough and Braungart
2002).
Re-use and waste avoidance are primary elements of a zero waste system in order to
minimize energy losses. Recycling is de-emphasized, as recycling requires energy supplements
51
and also tends to produce products that have reduced value (“downcycling”) (McDonough and
Braungart 2002). Composting is welcomed in one sense, as the recovery of organic matter back
into organic matter that may support agriculture is clearly part of a sustainability program;
however, losing the embedded energy in manufactured goods and re-purposing them as compost
does not jibe with the essential program of maintaining materials in the form they are in, as is
possible. In addition, re-use and creative remanufacturing using discards depends upon waste
stream simplification (McDonough and Braungart 2002); at least initially, increasing use of
degradable plastics creates materials complications (unless/until all plastics were made from
degradable polymers).
Table 6 summarizes the impacts of degradable and compostable plastics on future and
potential waste management systems.
Table 6. Impacts of degradable and compostable plastics on future and potential waste
management systems
Waste
Management
System
Expanded
Recycling
Expanded
Organics
Recovery
Innovative
Energy Recovery
Bioreactor
Landfill
Compostable Plastics
Increase resin complexity, cause
recyclate quality issues
If aerobic, would decrease-eliminate
product contamination; cause product
contamination in anaerobic environments
(perhaps not starch-based resins)
Degradable Plastics
Increase resin complexity, cause
recyclate quality issues
Cause product contamination
No impact
No impact
Slight positive effect from bag
failures
Not compliant with many key
elements; may conflict with
opportunities to recover marine
litter control costs
Conflicts with precepts to
minimize losses of embedded
energy
No positive effects
Extended
Producer
Responsibility
Not compliant with many key elements;
may conflict with opportunities to
recover marine litter control costs
Zero Waste
Conflicts with precepts to minimize
losses of embedded energy
52
7. Conclusions
Although all polymers will degrade under certain conditions, plastics that are specifically
designated as degradable have been manufactured to do so in an enhanced way. Degradable
plastics are supposed to lose important materials properties within days-weeks-months after
intended usage has been completed. The extent to which the plastic physically changes and then
is consumed by biota varies for different formulations, sometimes intentionally. The first plastics
marketed as degradable tended not to meet user expectations within expected timeframes. Thus,
around the turn of the century various groups and agencies established sets of standards that
manufacturers were expected to meet in order to make certain product claims.
Degradable and compostable plastics have been created to primarily address two issues
associated with conventional plastics: their process contamination of compost, and the
persistence of plastics as litter. Compostable plastics, a subset of degradable plastics, are
manufactured so as to become indistinguishable from other organic compounds treated through
managed aerobic decomposition. Visible and chemical pollution of the humus that results from
composting has been a process issue since large-scale composting began to widely adopted in the
1980s. It is thought the use of plastics that meet compostable standard will eliminate these
problems. However, conditions that lead to the degradation of compostable plastics are not
sustained at most small and/or backyard compost piles, and so certified compostable plastics
again are not meeting user expectations in many cases.
Most litter does not accumulate at sites where industrial compost conditions exist, so
compostable plastics tend not to be identified as the means to eliminate litter effects. It has been
suggested that plastics where degradation can be initiated under less stringent conditions may
help to minimize litter impacts. Those plastics that begin degrading due to UV light exposure
53
appear to have the most promise in causing less litter effects, since much objectionable litter is
found outside. However, it is not clear that UV-initiated degradation results in fragments that are
palatable to organisms. It also is not clear that "oxo-degradable" plastics break down quickly or
completely enough to affect litter, either.
At best, degradable plastics only create small, insignificant benefits in other waste
management processes; more generally, they just seem to create complications. They may
become substantial impediments to plastics recycling if they grow to be a substantial portion of
plastics markets. Some see degradability, in and of itself, as a necessary feature of increased
materials sustainability, although the reasoning for this position is not well articulated.
Therefore, it is difficult to perceive of degradable plastics as being much more than an
ineffective solution addressing a subset of the significant issues associated with modern
materials' selection and management processes.
54
Acknowledgements
The authors have received support from the Town of Brookhaven through the Center for
BioEnergy Research and Development. Although the Town of Brookhaven financially supported
some of the research described in this article, it does not necessarily reflect the view of the Town
and no official endorsement should be inferred. The Town makes no warranties or
representations as to the usability or suitability of the materials and the Town shall be under no
liability for any use made therefrom.
55
56
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