Oxo-biodegradable plastics are environmentally friendly for certain category of products

Excerpt: Degradable materials are sensitive to various physical, chemical or biological factors which induce their gradual decomposition.

Abstract

Increasing use of plastics due to their merits in many aspects has invited a lot of problems to the environment which is causing massive destruction. In view of this, attempt has been made recently by various scientific organizations worldwide to use the plastics in less harmful way for the benefit of the society.

Various effective techniques have been evolved recently for using plastics in scientific way. They are recycling, biodegradation, and oxo-biodegradation. In the scope of this study, the pros and cons of oxo-bio degradation is discussed.

Introduction

THERE is a massive destruction of forests, pollution of seas and oceans and exploitation of fossil fuels. And we are increasingly impoverishing the resources of our planet. The environment is suffering as the process of globalization is quickly moving ahead. The Earth must be protected if life on planet is to be protected. The Earth can be saved by preventing the pollution, creating awareness and by sustainable development of suitable material of use by civilized society.

When it comes for pollution, “Plastic Pollution” is also damaging our environment rapidly. Though plastics have benefited us in many ways, they have made our cars lighter, protected food, insulated homes; but at the same time they contribute to major pollution on Earth since the waste plastic material is hard to dispose of. The major polluters are plastics which are used in packaging sector. Only about 14% of the plastic waste in packaging sector is recycled in USA[1]. Although scenario of recycling of plastics in India is better but globally only 19.5% of waste is recycled, 25.5% incinerated and 55% discarded[2]. It takes hundreds of years for degradation of plastics; therefore majority of the plastic waste goes to landfill and is also dumped into the oceans. It is therefore, forecasted that the way plastics use is growing, if it continue in the same rate by 2050, there can be plastics instead of fishes in the sea. This scenario can be well-imagined from the Fig. 1.

From the fig. 1, where it is projected that very less percentage of plastics is recycled in even in the advanced countries like USA. The composting data could not be collected because of non-availability of the data from the reliable resources.[3]

Therefore, it is the right time for taking this problem seriously and working towards eradicating it.

Although, biodegradable or bio-plastics (or controlled-life plastics) in lieu of synthetic plastics are been encouraged from the environmental point of view but the quantity of synthetic polymer is such a bigger than the quantity of bio-plastics and thus cannot be replaced by biodegradable or bio-plastics in many of the applications. In such a case, synthetic polymers which are plentifully used in thousands of applications need to be recycled when the adoption of circular economy needs to be done in the current scenario.[4]

Moreover, incineration and land filling are gradually been restricted due to environmental and land regulations. The recycling of synthetic plastics which particularly go to medical and sanitary, food packaging and agriculture applications has to be essentially managed by some technology which can handle the contamination appropriately. In such situation, it is probably oxo-degradation which has been recently claimed to be a more appropriate type of plastic waste management technology in some specific sectors.

Degradable materials are sensitive to various physical, chemical or biological factors which induce their gradual decomposition. Their classification is given in Table 1.[5]

The Biodegradation induced by catalyzing agent can be of the following types-[5]

  1. Photo- degradation--where the material reacts to UV light and degrades.
  2. Hydro-biodegradation-- where degradation requires moisture and biologically active environment.
  3. Oxo-biodegradation-- material degrades in the presence of oxygen.

Oxo-degradation is defined by TC 249/ WG 9 of CEN (the European Committee for Standardization) as “degradation identified as resulting from oxidative cleavage of macromolecules” and oxo-biodegradation as “degradation identified as resulting from the oxidative and cell- mediated phenomena, either simultaneously or successively”. [5]

The use of additives which sensitize the plastics to degrading factors, such as UV- radiation, oxygen, moisture, bacteria, etc. is gaining importance. These methods make use of complexes and salts of metal with variable valency, e. g. Fe, Mn or Co, which enhance the degradability of synthetic polymers under the action of sunlight and oxygen, as well as under bio-assimilation conditions, by catalyzing the oxidative degradation of the polymer chain which is then followed by degradation under the action of natural biological factors.[5]

Why oxo-biodegradation is the solution for waste management for synthetic plastics

The oxo-degradation has the merits in two aspects:

  1. Outdoor exposure or soil burial helps in oxo-degradation of plastics by making them brittle rapidly enough to disappear visually.
  2. Degradable material must be susceptible to eventual biological attack giving complete conversion to biomass over an appropriate time, without release of toxic gases.

Conventional polyolefins are still far best solution for many applications requiring tough films, because they are cheaper to process and both mechanically tough and bio-inert. Although PE and PP will degrade naturally, the time scale is too long for them to be considered environmentally friendly and the increasing demand for such material requires way of converting them into water- wetable, mechanically weak material in short periods. The solution lies in accelerating the natural oxidative degradation of the polymer. In many applications the target is that the properties will deteriorate quickly at the end of the useful lifetime. Finally upon total mechanical degradation the residual plastic would be taken up into the bio-cycle without any negative influence on the environment. Since in the process useful biomass is generated, the material is not recovered in the usable form, but it becomes a part of circular economy and surely leading to a path for waste management of plastics used in specific sectors.[6]. It helps in designing out waste and pollution in specific applications if not all, although, it does not helps in retaining or keeping the products and materials in high- value use.[7]

Oxo-biodegradation

Oxo-biodegradation is a two-stage process of polyolefin degradation.[8]

Stage 1: Reaction of oxygen in the air with the polymer

Oxidation of polymer-carbon backbone takes place and smaller molecular fragments are formed. This stage is an Abiotic process. When oxygen is incorporated into the carbon chain, there is the formation of functional groups such as carboxylic or hydro-carboxylic acids, esters, aldehydes and alcohols. The behavior of hydrocarbon chain changes from hydrophobic to hydrophilic, and thus by absorbing water the polymer gets fragmented.

This degradation can be accelerated by UV light (Photodegradation) or by heat (thermal degradation) and also sometimes by mechanical stress.[8]

A. Photodegradation

The natural tendency of most of the polymers to undergo a gradual reaction with atmospheric oxygen in the presence of Ultraviolet radiation is known as Photodegradation. Mostly the photosensitizing agent is employed to accelerate the reaction. By the absorption of UV radiations, free radicals are generated; after which auto-oxidation occurs and the polymer starts disintegrating.

The instability of polyolefins is due to the presence of carbonyl and hydroperoxides groups which are formed during the fabrication or processing of the polyolefin products.

The thermally and photolytically unstable hydroperoxide group is the primary oxidation product which decomposes to produce two radicals, each of which participates in a chain reaction.

The presence of carbonyl groups in a degraded polymer is the indication of oxidation reaction that has taken place and also it shows that the material is vulnerable to further degradation since these groups are photolabile. Moreover, ketone photolysis also contributes to photodegradation of polymers which proceeds by Norrish I (free radical generation and no chain cleavage) or Norrish II (chain cleavage) reaction. Ketones are introduced onto the polymer by photo-oxidation and then on light exposure, absorb energy and break carbon-carbon bonds and lead to the formation of fragments.[8]

B. Thermal degradation

The oxidation initiated by heat is known as Thermal degradation. The mechanism and products are similar to those of photo-degradation except that ketone products are stable to heat but not to light.

Moreover, less branched polymers have a diminished permeability to gases and therefore in both the types of degradation, the resistance to oxidation increases with increasing density of the polymers. Also, rate of degradation is influenced by chain branching and chain defects such as saturation.

Oxidation susceptibility of polyolefins is in the following order-[8] iPP (isotactic polypropylene)> LDPE>LLDPE>HDPE

C. Mechanical stress

Degradation mechanisms are dependent on morphology and the polymer morphology gets changed on the application of mechanical stress; thus it accelerates the degradation of polymers.[8]

Stage 2: Biodegradation of the oxidation products by micro-organisms

Micro-organisms such as bacteria, fungi and algae consume the fragments formed above and form CO2, H2O and biomass. It has been reported to achieve significant biodegradation in a reasonable time period; the average molecular weight of oxidized polyolefin should be under 5000 Da. During the microbial degradation stage, there is a decrease in the number of carbonyl groups; this indicates that micro-organisms are growing which consume the carbonyl groups from the original oxidation products such as ketones, esters, etc.[8]

Prodegradant Technology

The natural degradation of polyolefins is far too long therefore the need is to convert them into biodegradable material in shorter periods of time, which is possible by accelerating the natural oxidative degradation using Prodegradant additive.[8]

The Prodegradant additives are of various types which are classified in Table 2.

The Prodegradant additives have been used in several patents[8] which are listed below in Table 3.

Conventional standards for oxo-degradation

Conventional plastics are enriched with additives for useful properties. The majority of these additivated plastics are oxo-degradable plastics. These conventional plastics are enriched with inorganic metal salt that should cause the plastic to degrade by a process initiated by oxygen and accelerated by light and/or heat.

For many years, the US guideline ASTM D 6954 was the only guide available for testing oxo- degradable plastics. However, since 2009, several other guides and standards were developed in Europe and the Middle East: XP T 54980 and AC T51-808 (France), UAE, 55009 (United Arab Emirates), BS 8472 (UK), SPCR (Sweden) and IS 2004 (Japan).

The majority of these guides and standards are composed of three so-called “Tiers”.[9]

  1. Abiotic degradation (Tier 1): using either accelerated or real time conditions samples are subjected to a combination of oxygen, heat and/ or light to reduce the molecular weight and/or mechanical properties.
  2. Biotic degradation (Tier 2): The residues from Tier 1 are retrieved for bio-degradation testing using the environment in which the material is intended to end-up after disposal (e.g. Compost soil, water, and landfill). In most cases the amount and rate of CO2 production, in case of aerobic bio-degradation, and additionally CH4 production, in case of anaerobic bio-degradation is measured.
  3. Ecotoxicity (Tier 3): By using a variety of living organisms, including plants, earthworm and aquatic organisms, the effect of the residues from tier 2, on the growth, survival and/or immobilization or fauna and flora can be determined.

Oxo-biodegradable plastics are neither certified for industrial or home comfortability nor for bio-degradability in soil or fresh water.

Nevertheless, several associations and institutes have created certification system and accompanying logos for oxo-degradable plastics based on the studies as per the above discussed points.

In this context, several oxo-degradable plastics are certified by the Emirates Authority for standardization and metrology (ESMA), conform UAE S5009. In other words, this means that these products were tested by an independent and accredited laboratory and fulfilled the criteria of UAE S 5009 (molecular weight level of 5000 Dalton or lower within 4 weeks and a bio- degradation value of at least 60%, within 6 months.

Mechanism of oxidative degradation

The mechanism of oxidative degradation of polymers has been studied by few authors. It is accepted that the key intermediates are hydroperoxides, which are always present because of oxidation during preparation or processing, and decompose under the influence of heat, light or transition metal catalysts to produce free radicals.[10]

Once radicals are produced, they enter a chain reaction with oxygen and C-H bonds in the polymer, to produce a range of oxidation products.[11] This can be expressed as the interlocking cycle or reaction depicted in Fig. 2.

The primary products of this cycle are hydroperoxides, so that oxidation generates its own initiator and has all the characteristics of an auto accelerating chain reaction. The decomposition of hydroperoxides yields alkoxy radicals which are responsible for many secondary products.

In particular, β- elimination by alkoxy radicals competes with H- abstraction, and leads to chain scission and formation of a variety of carboxylic products. Since linear polymers derive their mechanical properties from entanglement or their long chains limited chain scission causes a rapid change from tough to brittle materials. This is especially true of thin films, which require plastic of extreme toughness.

Since hydroperoxides decomposition to give free radicals is the key reaction in oxidation, additives which reduce their rate of formation or decomposition or which decompose hydroperoxides by non-radical routes will act as antioxidants, conversely, additives which act to accelerate hydroperoxides formation and decomposition to radicals are effective pro-oxidants since they accelerate the chain branching reactions.

Due to oxidation of polyolefin it loses its molar mass rapidly and hydrophilic surfaces are developed. Reduction of the molecular weight of PE to values of 40,000, continued with the introduction of oxygen containing functional groups, leads to biodegradable products.

In a natural environment micro-organisms colonizing a substrate form a biofilm, consisting of bacteria and fungi in a highly hydrated (85- 98%) matrix of extra cellular polymers.

Both hydrolysis and oxidation of the substrate can be mediated by the biofilm, by release of extracellular enzymes or free radicals. Fungi in particular can spread rapidly by secreting enzymes and free radicals. In addition, insoluble compounds that cannot cross a cell membrane are also susceptible to attack.

The mycelial growth habit of fungi also gives a competitive advantage over single cells, especially in the colonization of insoluble substrates.

Hyphal penetration provides a mechanical complement to the chemical breakdown and the high surface to cell ratio characteristic of the growing fungi maximizes both mechanical and enzymatic contact with the environment.

Cell enzymes, and particularly cytochrome P-450 which is produced by many bacteria, continue peroxidation by reducing ground state oxygen to the free radical superoxide (O2). When protonated; this species is converted to the much more reactive peroxyl radical and hydrogen peroxide, which can be reduced by transition metal ions in the polymer to give the highly reactive hydroxyl radical.

OH radicals initiate further peroxidation leading to continued biodegradation and ultimate bio-assimilation to bio-mass and CO2 as long as environmental oxygen and cell nutrients are available. Thus, the bio-assimilation of degraded polyolefins is a synergistic oxo- biodegradation. (Fig. 3)[12]

Thus, it is totally analogous to the two- stage, hydro- biodegradation by which linear polyesters are microbially assimilated.

The total potential interaction between microparticles and micro-organisms is shown in Fig. 4.

The total process of interaction between micro-plastics and micro-organisms is such that according to Prof. Ignacy Jakubowicz, the biodegradation process can be understood as, “An entire change of the material from a high molecular weight polymer to monomeric and oligomeric fragments and from hydrocarbon molecules to oxygen containing molecules which can be bio-assimilated.” According to Prof. since the material has been fragmented to monomeric and oligomeric fragments, they are not considered as micro-plastics. [7, 13]

The role of totally degradable plastic additives (TDPA) in oxo-degradation

TDPA is designed to control and manage the lifetime of products made from the most common plastics used by modern society. This technology enables products made from PE, PP and PS to degrade and in most cases bio-degradable when discarded into environmentally benign products within a few months or few years as compared to decades or longer for products made without benefit of the technology.[10, 12] Control of the rates of the two degradation stages is achieved through a balance of appropriate additives. In this way, end-use performance can be altered to fit specific markets without altering the normal degradation pathways and products.

After use, the plastic film or product part does not need to be recollected, transported to a recollection centre and disposed of by burial, landfill or incineration.

The other benefit is that these additives can be used with plastics and processed with standard processing equipments and conditions without affecting mechanical or the optical properties of the plastics and with only minimal implications for recycling.[14.15,16]

Generation of so-called micro-plastics due to oxo-degradation of plastics

Oxo-degradable plastics, sometimes known as Oxo-plastics, also in better sense called Oxo-bio-degradable plastics, facilitate the rapid degradation of polymer materials in-to very small particles and may potentially contribute to micro-plastic pollution.[17]

When the small particle's dimension is below 5mm then this is considered as microplastics category. This is a particular concern for the aquatic environment. The potential impact of microplastics on the environment (aquatic) and human health have generated concerns in member States of the European Union and worldwide.

It is stated that any microplastics larger than 150 micron or 0.15 millimeter (size of fine sand grains) should be able to pass through the human body without any issues. The problem occurs when we get to even smaller i.e. below 10 nanometers (or 0.00001 millimeters) when they are not bio-degradable.[17]

That is why, second stage of oxo-degradation i.e. bio-assimilation step is very important in oxo-degradation to avoid such problems.[18]

Merits of oxo-biodegradable plastics

Oxo-degradable plastic is not normally meant for composting [18] and also not it is designed for anaerobic digestion nor for degradation in deep landfill. Oxo-degradable plastic is not designed to merely fragment- it is designed to be completely bioassimilated by naturally occurring micro-organisms in a time-scale longer than that required for composting but shorter than for nature's waste such as leaves, etc. and much shorter than for normal plastics.

In oxo-degradable process, plastics will eventually become embrittled and will fragment and be bioassimilated. The difference made by oxo-biodegradation technology is that the process is accelerated.

Therefore, in order to keep the oxo-biodegradable material as distinguished material, it should be properly labeled as per code of practice.[18]

Whereas compostable plastics are designed to be deliberately destroyed in the composting process, but oxo-biodegradable plastics can be re-used many times and recycled if collected during their useful life span.

So far as recycling is concerned, oxo-biodegradable plastics can be recycled (before starting of degradation) in the same way as ordinary plastics. By contrast, compostable plastics cannot be recycled with ordinary plastic and will ruin the recycling process if it gets into the waste stream.

It is worth mentioning that where recycling could be a more expensive process for producing new plastics, the Oxo-biodegradation technology could be a viable option.

In this regard, the term bio-assimilation should not be confused with the term biodegradation because bio-degradation process is a natural process by which organic chemicals in the environment are converted to simpler compounds, mineralized and re-distributed through elemental cycles such as carbon, nitrogen, sulphur and others.

The oxo-biodegradable plastics are recently also known as PAC (Pro-oxidant Additive Containing) Plastics, since they contain oxidative additives.[19] They are basically aromatic ketones and/or transition metal complexes.

Areas for use of oxo-degradable plastics

The existing product time of disposable plastics can be the application area of oxo-degradable plastics. They can be used primarily in packaging materials and agriculture plastics.[18] These plastics can be also suitable for retail carrier bags, foam packaging and shrink wraps and plastic service-ware i.e., all single-use plastic-products. [20…24]

References

  1. http://www.theguardian.com/sustainable-business/2017/feb/22/plastics-recycling-trash-chemicals-styrofoam-packaging Feb22, 2017
  2. https://ourworldindata.org
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  4. http://sustainability.com/our-work/insights/creating-a-circular-economy-forplastics/Nov 12, 2018
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Author Details

Dr. SS,ubhash C. Shit and Uzma Fatima

CIPET: CST Raipur (C.G.) 493221