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Plastics: The good the bad and the ugly

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(Hayden et al., 2013; Zhang et al., 2011;Teuten et al., 2009;Rios et al., 2013;Dyson, 1992),Global,
2008;(Yakuphanoglu et al., 2005;Schnabel, 1981;Henninger, 1992; Desriac, 1991;Desriac, 1991; Barahona and
Gomez-Vasquez, 1985;Henninger and Pedrazetti, 1988 are not in reference section

Full Length Research Paper

Plastics: The good the bad and the ugly

Kamweru Paul K.1* and Tindibale Edward L.2
1
Department of Physical Sciences, Chuka University, P. O. Box 109-60400, Chuka, Kenya.
2
Physics Department, Egerton University, P. O. Box 536-20115, Egerton, Kenya.
Received 29 August, 2016; Accepted 23 September, 2016

Plastics’ demand and consumption is on the increase since 1950s due to their unique properties. Their
high use coupled with challenges in end-of-use handling and their inherent resistance to degradation
has led to their accumulation in the environment, which is a matter of grave concern. This review
presents a general overview of the state of knowledge of the diverse faces of plastics with special
reference to polyethylene. This includes an outline of the polyethylene blends and grades put into most
use, disposal methods, polyethylene degradation and its effects on the environment. The Current state
of knowledge suggests that future trends and policies should be directed towards increased efforts to
recycling and minimizing the introduction of virgin materials into the cycle.

Key words: Polyethylene, degradation, polymers, plastics, environment.

INTRODUCTION

Plastics otherwise referred to as Polymer Based sort of biodegradation, though weathering and ultraviolet
Materials (PBMs) are macromolecules, formed by light can fragment large chunks. The extreme durability of
polymerization and having the ability to be shaped by the plastic defies the natural recycling process of the
application of reasonable amount of heat and pressure or biosphere. This in turn causes a major pollution menace,
any other form of forces. There is a wide range of obscuring the benefits of PBMs. Most PBMs as finished
applications of PBMs, ranging from their use in products are non-toxic, but in plastic products there may
aerospace industries to a simple shopping bag. Almost all be non-bound residual monomers, polymerization
aspects of daily life involve PBMs in some form or the chemicals, degradation products, and additives which
other. PBMs outclass all other materials such as metals have toxic properties (Lithner et al., 2009; DeMatteo,
and ceramics in their low density, strength to weight ratio, 2011). Although the advocates of plastics consider it as
low corrosion rate, ease of processing and excellent the most eco-friendly material saving natural resources
barrier and surface properties. Consequently, there has such as timber, the growing mountains of plastic garbage
been a worldwide increase in demand of PBMs since is now assuming dreadful proportions in many developed
1950s (Figure 1). societies.
Chemically, PBMS are the most non-biodegradable Polyethylene is the most demanded and produced
materials man has ever produced. They are beyond any plastic in the world (Plastics Europe, 2012). This is

*Corresponding author. Email: pkamweru@gmail.com or pkkamweru@chuka.ac.ke.

Author(s) agree that this article remain permanently open access under the terms of the Creative Commons Attribution
License 4.0 International License
 Figure 1. World Plastics Production 1950-2012. This figure has been
obtained and adopted from Plastics Europe (2013).

Table 1. Global and European plastic demand by resin type for the year 2011 (Plastics Europe 2012; Rappaport, 2011).

Europe % demand (2011) of total Global % demand (2010) of total 190

Resin type
57 million metric tonnes million metric tonnes
Others 20.0 6
Polypropylene 19.0 25
Low density/linear low density (LDPE/LLDPE) 17.0 21
Polyethylene
Polyethylene-High density (HDPE) 12.0 17
Polyvinyl Chloride 11.0 18
Polystyrene solid/expandable 7.5 6
PolyurEthene 7.0 -
Poly(ethylene terephthalate) 6.5 7

because Polyethylene is strong, it is safe and it is (Table 1) of all plastics in Europe in 2011 and
versatile (Hayden et al., 2013). In this review paper, we approximately 38% of all plastics worldwide in 2010.
discuss the good, the bad and the ugly sides of PBMs By making polyethylene more or less "dense‖ (Table 2)
with special reference to Polyethylene. in the factory, there is a suitable type of material available
for every application (Vasile and Pascu, 2005). In
practice one of the following types/forms is used in 90%
POLYETHYLENE GRADES, BLENDS AND of the applications, low density (LDPE), linear low density
CONSUMPTION (LLDPE) and high density (HDPE) (Table 1). Other forms
of polyethylene include medium density (MDPE), ultra-
Polyethylene is a polymer of Ethene/ethylene, (C2H4)n, high molecular weight polyethylene (UHMwPE).
whose demand and production is approximately 30% Polyethylenes are frequently modified with other
 Table 2. Density of various polyethylene (PE) grades.

Polyethylene grade Density (kg/m3)

Linear low density (LLDPE) 920-930
Low density (LDPE) 910-935
Medium density (MDPE) 940
High density (HDPE) 955-977

polyethylenes to improve processability, mechanical Indeed, in order to examine any PE/PE mixture as if it
performance and other material properties. Depending on were a solution, at least one of the components must
miscibility, such combinations can produce extremely have a molecular weight below this Mc. Wax has been
complex rheological results with behaviors and combined with or used as a modifier for LLDPE, LDPE,
concentrations spanning a range from those normally and HDPE, with studies performed targeting the
considered characteristic only of solutions, to those rheological (Yang et al., 2004), thermal (Hato and Luyt,
normally considered characteristic only of blends 2007), and solid-state mechanical properties (Lee et al.,
(Utracki, 2002). In a detailed review of the miscibility of 2010). Additionally, wide angle x-ray diffraction (WAXD)
polyethylene blends, Zhao and Choi (2006) points out has been employed to examine the crystallization of
that primarily it’s the branch content and not the wax/wax blends, with the intensity pattern used to
differences in molecular weight averages, molecular estimate the apparent degree of wax crystallinity (Lee et
weight distribution, and branch length of the two al., 2010). Small angle x-ray scattering (SAXS) has been
polyethylenes that governs their miscibility. used to study the phase structures in elongated
Though immiscible under flow, an LLDPE/LDPE blend HDPE/wax blends. (Ogino et al., 2006) Both WAXD and
nevertheless behaves as a compatible combination. SAXS (in-situ) have been employed to study shear-
(Utracki and Schlund, 1987) The assertion of immiscibility induced crystallization precursors in model PE blends
of LLDPE/LDPE is supported by previous thermal studies under flow conditions. (Yang et al., 2004).
of the combination under differential scanning calorimetry Polyethylene, its grades and blends mentioned, have
(DSC), generating the melting and recrystallization contributed much to society wellbeing. The LDPE or
temperatures and creating the associated enthalpies, and LLDPE form is preferred for film packaging/shopping
degrees and types of crystallinity (Neway and Gedde, bags and for electrical insulation. HDPE is blow-moulded
2004). These studies have yielded distinct phase to make containers for household chemicals such as
diagrams of the upper critical system temperature washing-up liquids and drums for industrial packaging. It
(UCST) form (Hill et al., 1992) Indeed, UCST proved the is also extruded as piping. Other uses of Polyethylene
applicable phase diagram type for all PE/PE blends. blends include medical implants, cable and marine ropes,
Additional studies examined the blends’ steady-state and sail cloth, sport equipment, fish nets, concrete
dynamic tensile mechanical properties in the solid state. reinforcements, protective clothing, ponds lining material
(Luyt and Hato, 2005) especially that contains industrial wastes, geotextile
Similarly to LLDPE/LDPE blends, LLDPE/HDPE and applications, making pipes for nuclear plants applications
LDPE/HDPE blends have been studied in melt under (Krishnaswamy, 2007) etc. However the good use of
conditions of oscillatory shear flow (Yamaguchi, 2006), polyethylene is almost obscured by its potential harm
elongation flow (Valenza et al., 1986), and steady state arising from challenges in handling the products after
tensile elongation in the solid state. HDPE/HDPE (a use, and its strength against degradation as discussed in
like/like combination) blends have been examined, the following sections.
specifically targeting the effects of different molecular
weights and the use of m-HDPE, using oscillatory shear
flow, DSC and solid-state mechanical testing (Bai et al., POLYETHYLENE DISPOSAL METHODS, RECYCLING
2010). The significance of this blend type is that it is AND ENERGY RECOVERY
considered miscible by all characterization techniques
across all compositions and conditions used. Worldwide most polyethylene products after use, ends up
A blend type of particular interest under the purview of in landfills, incinerated or in a recycling plant with higher
PE/PE mixtures is of PE with paraffin wax. This far, percentages in landfills. Each method of disposal has its
studies have considered only mixtures wherein each own limitations.
component possesses a molecular weight substantially
above the critical molecular weight for entanglement of
linear polyethylene (Mc = 3660-3800 g/mol) (Zang and Landfills
Carreau, 1991). However, the molecular weights of
paraffin oils and waxes are significantly below this value. The versatility and simplicity of landfills in terms of
technical requirements, environmental and socio- reduce the overall usefulness of incarnation. Plastic
economic aspects makes it popular than other known incineration overcomes some of the limitations placed on
techniques, for examples incinerate and biological landfill in that it does not require any significant space,
composting (Malek and Shaaban, 2008). There are also and there is even the capability for energy recovery in the
other major deficiencies of "dry tomb" landfilling as form of heat (Sinha et al., 2010). However, there is a
reviewed by Lee and Jones-Lee (1996). A major significant trade-off in that incineration of plastics leads to
drawback associated with landfill method is that the the formation of numerous harmful compounds, most of
landfill facilities occupy space that could be put into other which are released to the atmosphere (Zhang et al.,
uses such as for agriculture or human settlement (Zhang 2011). Heavy metals, toxic carbon- and oxygen-based
et al., 2004). With slow degradability of Polyethylene for free radicals, not to mention significant quantities of
example, landfill waste have been shown to persist for greenhouse gases, especially carbon dioxide, are all
more than 20 years (Tansel and Yildiz, 2011) meaning produced and released when plastics are incinerated
that the landfill space is unavailable for other uses for (Shen et al., 2010). The significant environmental
much longer time. The slow degradability is attributed to drawbacks of plastic disposal via both landfill and
the inherent nature of polymer degradation and incineration were the driving force behind the
compounded by limited availability of oxygen in landfills; development of plastic recycling processes.
the surrounding environment is essentially anaerobic
(Andrady, 2011; Massardier-Nageotte et al., 2006;
Tollner et al., 2011). Plastic debris in landfill also acts as POLYETHYLENE IMPACTS TO THE ENVIRONMENT
a source for a number of secondary environmental Among all the plastic debris collected on several debris
pollutants and these includes volatile organics, such as studies on Oceans, Polyethylene, mostly HDPE and
benzene, toluene, xylenes, ethyl benzenes and trimethyl LDPE is among the plastics that constitutes the highest
benzenes, released both as gases and contained in percentage (Thompson et al., 2004). This may infer that
leachate (Urase et al., 2008) and endocrine disrupting Polyethylene products are the highest plastic
compounds, in particular BPA (Xu et al., 2011; Zhang et environmental pollutants. Next to Polyethylene in use and
al., 2004). High concentrations of hydrogen sulphide are pollution quantities is Polypropylene (PP) (Morét-
potentially lethal (Tsuchida et al., 2011). In addition, a Ferguson et al., 2010; Corcoran et al., 2009; Barnes et
major drawback to landfills from a sustainability aspect is al., 2009). Such high levels of plastic debris in the
that none of the material resources used to produce the environment can be attributed to the high availability, high
plastic is recovered—the material flow is linear rather use of plastic products and its ability to persist in the
than cyclic (Hopewell et al., 2009). environment (Frost and Cullen, 1997; Ivar do Sul and
Costa, 2007). Polyethylene films from packaging and
green houses for example cause hazardous and
Recycling and reuse economically damaging effect to both marine and dry
land environments (Webb et al., 2013). They poses
Polyethylene can easily be recycled and reintroduced into threats to wildlife such as marine birds (Azzarello and
the production chain once more or reused e.g. waste Van Vleet, 1987), turtles (Barreiros and Barcelos, 2011),
polyethylene has been tested for reuse in road cetaceans (Baird and Hooker, 2000), fur seals
construction (Raju et al., 2007). However, low recycling (Pemberton et al., 1992), sharks (Sazima et al., 2002)
and reuse rates are often observed in conventional and filter feeders (Moore et al., 2001; Wright et al., 2013).
centralized recycling plants due to the challenge of The main dangers associated with the plastic objects are
collection and transportation for high-volume low-weight ingestion (Denuncio et al., 2011; Laist, 1997; Lazar and
polymers. The recycling rates decline further when low Gracan, 2011; van Franeker et al., 2011; Yamashita et
population density, rural and relatively isolated al., 2011) leading to internal and/or external abrasions,
communities are investigated because of the distance to ulcers and choking and animals entanglement restricting
recycling centers makes recycling difficult and both movements (Webb et al., 2013). Plastic particles in the
economically and energetically inefficient (Kreiger et al., ocean have been shown to contain quite high levels of
2013). Consequently the traded volume of waste plastic organic pollutants. Some of these compounds are added
globally is very minimal for example, less than 5% of the to plastics during manufacture while others adsorb to
new plastics produced in 2012 (Velis, 2014). plastic debris from the environment (Thompson et al.,
2009; Teuten et al., 2009). Toxic chemicals, such as
polychlorinated biphenyls (PCBs), nonylphenol (NP),
Incernation organic pesticides, such as
dichlorodiphenyltrichloroEthene (DDT), polycyclic
Incineration of waste offers a number of advantages aromatic hydrocarbons (PAHs), polybrominated diphenyl
including volume reduction of wastes and destruction of ethers (PBDEs) and bisphenol A (BPA) have been
pathogens. However, the discharge of air pollutants may consistently found throughout oceanic plastic debris
 Degradation Mineralization
- Biological
- Thermal
Material - Photo-induced Fragments
- Utrasonic
- Hydrolytic
- Chemical Residues
- Mechanical
Figure 2. Schematic steps in polyethylene degradation process.

(Mato et al., 2001; Rios et al., 2013; Hirai et al., 2011). As frequencies inducing vibrations and eventually breaking
mentioned earlier, most polyethylene products in their of the chains (Suslick and Price, 1999),
pure form are not toxic. However, when the material v) hydrolytic –occurs in polymers containing functional
interacts with the environment, degradation occurs which groups which are sensitive to the effects of water (Gewert
may lead to formation of toxic and irritant Acrolein, et al., 2015),
Aldehydes and acids (Hakkarainen and Albertsson, vi) chemical-corrosive chemicals, such as ozone or the
2004). sulphur in agrochemicals, may attack the polymer chain
causing bond breaking or oxidation (Gaca et al., 2008)
and
DEGRADATION OF POLYETHYLENE vii) Biological-specific to polymer with functional groups
that can be attacked by microorganisms e.g. bacterial,
Ultimate degradation of synthetic polymers may take fungi and algae (Restrepo-Flórez et al., 2014; Shah et al.,
several hundred years (Vasile, 1993; Matsumura, 2005; 2008; Leja and Lewandowicz, 2010).
Gu, 2003; Gilan et al., 2004). Generally degradation is
the irreversible process, that affects directly, or indirectly, Just like for general polymers, polyethylene
several properties of the material related to its functional degradation initiation depends on where the material is
characteristics as a result of environmental factors (such being used and the type of environmental exposures it is
as light, heat and moisture etc.), chemical condition or prone to. For example, degradation of LDPE films used
biological activity (Pospisil and Nespurek, 1997; Dilara as greenhouse covering materials is governed mainly by
and Briassoulis, 2000). Ultimately, degradation makes thermal, radiation, mechanical and chemical mechanisms
the materials susceptible to mechanical failure, leading to and not the other mentioned mechanisms (Dilara and
fragmentation and formation of residues or mineralization Briassoulis, 2000).
(Figure 2). It is the susceptibility to mechanical failure that
scientists normally rely upon when monitoring the degree
of degradation e.g. tensile strength, elongation at break Thermal-oxidative degradation of polyethylene
(expressed as a %) stress at yield or the modulus of
elasticity (Randy and Rabek, 1983). As mentioned above, Most polymeric molecules are only stable below 100 -
degradation of polymers is induced by different external 200°C. Above some critical temperature, bond scission
factors and mechanisms. Corresponding to the various may occur with high frequency leading to quick
environmental induction, the various polymer degradation deterioration of the polymer structure and properties. This
types are; critical temperature is usually higher than 400-600°C, and
beyond which, the temperatures are able to provide
i) Thermal occurs- due to exposure to high temperatures sufficient energy for bonds scission. Typical bonds have
(Singh et al., 2008), a dissociation energy around 150-400 kJ per mole at
ii) photo-induced - on exposure to the UV radiation or any 25°C. Polyethylene is practically stable up to 100°C in
other high energy radiation, the polymer or its inert atmosphere (Schnabel, 1981) with a very low glass
morphological defects or impurities within the polymer transition temperature of below 125K (Fakirov and
absorb the radiation, inducing degradative chemical chain Krasteva, 2000). Practically most polyethylene is used at
reactions (Singh et al., 2008), ambient temperatures, the most extreme cases being in
iii) Mechanical- due to an application of mechanical green houses that do not exceed 80°C. This means that
stress /strain (Caruso et al., 2009). degradation by chain scission due to thermos processes
iv) Ultrasonic-due to the application of sound at certain is rare and given little attention. However, elevated
temperatures can significantly increase the rate of biodegradation of the polymers (Andrady, 1994, 2000;
various chemical reactions, such as oxidation, and Arutchelvi et al., 2008; Zaikov and Gumargalieva, 2010).
therefore lead in an indirect way to degradation of the
polymer. This effect is discussed further in the course of a) Accumulation of biomass (experimentally determine
this research on photo-degradation. the growth rate of microorganisms with the polymer as
the sole carbon source)
b) Oxygen uptake rate
Biodegradation of polyethylene c) Carbon dioxide evolution rate
d) Products of reaction using chemical analysis
For degradation to be termed biodegradation and to e) Surface changes
occur, the following elements are indispensable. f) Changes in the mechanical and physical properties of
the polymer.
1) Existences of microorganisms and their attachment to
the surface of the polymer. These microorganisms should
be able to grow utilizing the polymer as the source of Photodegradation
carbon, that is, have an appropriate metabolic pathway to
synthesize enzymes specific for the target substance to Mechanical properties of many plastic materials/polymers
initiate depolymerization and mineralization of monomers degrade upon exposure to high energy radiation (Amin et
and oligomers. al., 1995; Hamid et al., 1992; Hollander and Behnisch,
2) The environmental conditions such as oxygen, 1998). When the energetic UV radiation, 290-400 nm
temperature, moisture, salts and pH should render the (Khan and Hamid, 1995) is absorbed by the polymer,
biodegradation processes possible. direct photolysis could occur i.e. bond cleavages and
3) The material structure also influences the degradation depolymerization. On the other hand, the free radicals
process. Therefore, chemical bonds, degree and type of produced in this way may then react with the atmospheric
branching, degree of polymerization (DP), degree of oxygen and lead to further degradation of the plastic,
hydrophobicity, stereochemistry, molecular weight which is called photo-oxidation (Dilara and Brissoulis,
distribution (MWD), crystallinity and morphological 2000). Generally, initiation and progression of photo
aspects. degradative processes depends on the types of bonds
present in the material (Dyson, 1992), radiation energy
As a first step of biodegradation, cleavages of side available at the earth’s surface, presence of absorptive
chains or backbone lead to an increasing contact chromophores (mainly as impurities) in the polymer
interface between microorganisms and polymers. (Kroschwitz, 1990; Global, 2008; Dilara and Briassoulis,
Microorganisms can attach to the surface, if the polymer 2000; Schnabel, 1981), thickness of the material
surface is hydrophilic. Since Polyethylene has only CH2 (Yakuphanoglu et al., 2005) etc. Light absorbed by
groups, its surface is hydrophobic. The initial degradation chromophores especially the carbonyl groups can induce
(e.g. mechanical or photo-induced) that cleaves the side bond scission by either Norrish type I and type II
chains or backbones therefore, it is necessary to allow processes. Whenever the carbonyl groups are on the
the insertion of hydrophilic groups on the polymer surface polymer backbone both Norrish I and Norrish II
making it more hydrophilic (insertion of hydrophilic groups processes cause main chain ruptures (Schnabel, 1981).
also decreases the surface energy). The second step in Therefore the mechanism of photodegradation in PE is
biodegradation, which often is referred as the primary one of thermo oxidative or photo oxidative degradation
biodegradation, involves fragmentation of the material, rather than of direct photolysis. The photo oxidative
aided by extra cellular enzymes (endo-enzymes) degradation of macromolecules is initiated by the
secreted by the organism. The resultant low molecular absorption of light quanta by chromophoric groups and
weight compounds are further utilized by the microbes as the products of thermo oxidative transformations of
carbon and energy sources. The last step, mineralization, macromolecules (Goldade et al., 2004). The photo
occurs after the sufficiently small oligomeric fragments oxidative mechanism proceeds when free radicals that
are bio assimilated by the microbes, resulting to the final are formed by photo illumination react with molecular
products CO2, CH4, H2O, N2, H2, salts, minerals and new oxygen, the chemical quantum yield in presence of
biomass (BM). Under sulfidogenic conditions, the end oxygen being rather high. Peroxides are formed
products could also be H2S, CO2 and H2O. The according to the conventional mechanisms of
environmental conditions decide the group of autoxidation
microorganisms and the degradative pathway involved.
Additives, antioxidants and other stabilizers added to
commercial polyethylene may be toxic to the organisms
or may slow down the rate of biodegradation. The
following strategies are used to assess and monitor the Electronspin Resonance (ESR) measurements in a study
of long-lived free radicals in gamma – irradiated scission less so (Igarashi and De Vries, 1983). However,
UHMWPE, PE showed changes from alkyl to peroxy LDPE still suffers to some extent of de-gradation due to
radicals in air due to oxygen reaction, which did not mechanical loading. For linear macro-molecules, such as
change in vacuum (Choon et al., 2004). The auto PE, it has been shown that the probability of scission is
oxidative process has three important steps namely, higher in the middle of the chain (Schnabel, 1981). In
initiation, propagation and termination. Initiation in this another work (Popov et al., 1983), increased degradation
case is by photo irradiation and there is formation of has been observed for LDPE held under tension (from 0
radicals. Propagation step can be divided into six steps to 34 kg/mm2) in an ozone environment, that could not be
and leads to the scission process of polymer alkoxy explained by changes in the degree of crystallinity,
radicals with the formation of aldehyde end groups and orientation or chain mobility. The rate of oxidation
end polymer alkyl radicals (Rabek, 1987). decreased with increasing orientation and a linear
relationship was observed between the level of stress
and the degradation. Finally, increased reactivity of the
stretched macromolecules, especially at the parts of the
The initiation process reactions are independent of chains which are held in high tension, was put forth as a
temperature and cease when irradiation is removed. The plausible explanation of the observed behavior.
secondary reactions, which includes oxidations are
temperature dependent and can proceed without further
irradiation (Birley et al, 1992). It’s therefore hypothesized Chemical degradation
that initiation can be done pre- consumer use (for
example during manufacture) of these short term use The effect of a solvent on the structure of the polymer
products. Once they are exposed to sunlight, propagation material can be significant at times. Most thermoplastics
of the photodegradation take place with consequent are soluble in several solvents. Usually, a swelling stage
degradation. preceeds the dissolution. Polymeric materials capable of
forming crystallites, such as LDPE, tend to be rather
resistant to physical interaction with solvents. Dissolution
Mechanical degradation is impeded by the strong intermolecular interactions
between macromolecules. Only when these interactions
PBMs respond to mechanical forces on the molecular are overcome by thermal activation will the polymer swell
level by changing conformation, chain slippage, and eventually dissolve. However, apart from the physical
segmental alignment, disentanglement, and ultimately action of dissolution, solvents can also chemically attack
bond scission. These molecular scale events evolve to such polymers. Immersion tests show that while PE had
the macroscale, resulting in the formation of crazes and satisfactory resistance at ambient temperature to
cracks, ending in catastrophic failure (Caruso et al., methanol and only limited resistance to acetone, it
2009). PBMs, because of having higher molecular exhibits unsatisfactory resistance to other saturated
weights, free radicals are produced by chemical bond hydrocarbons, benzene, carbon disulphide and carbon
rupture during mechanical treatment (Stoeckel et al., tetrachloride (Schnabel, 1981).
1978). Smaller molecules are generally free to change Environmental pollution can also be harmful to the
positions and accommodate the applied mechanical structural integrity of the polyethylene due to chemical
stress that lead to rupture of chemical bonds are attack of the polymer bonds. Atmospheric pollutants such
produced in macromolecules which possess lower as nitrogen oxides, sulphur oxides, hydrocarbons and
mobility (Mills, 1993). Although bond scission is a particulate can enhance the degradation of the polymers
relatively rare event, studies has shown that the rupture (Ranby and Rabek, 1983; Schnabel, 1981; Dilara and
of bonds due to mechanical loading depends on the Briassoulis, 1998), especially when combined with
amount of elastic energy that a macromolecule is capable applied stress (Igarashi and De Vries, 1983) and must
of storing and on the time the macromolecule remains also be taken into account. For instance, infrared studies
under strain (Dilara and Briassoulis, 2000). Mostly energy have revealed that polyethylene reacts with NO2 at
is dissipated through non-chemical processes such as elevated temperatures, and that chemical attack is
slippage of the chains, changes in chain conformation observed even at 25°C, probably due to the presence of
and crystallinity. Those two categories, chemical and impurity olefinic bonds which react readily with NO2
non-chemical energy dissipation, compete with each (Schnabel, 1981). Similarly, SO2 is rather reactive,
other. In cases where the non-chemical processes are especially in the presence of UV irradiation, which it
inhibited, such as in stiff polymers, e.g. nylon, the bond readily absorbs and forms triplet excited sulphur dioxide
scission occurs with higher frequency. In contrast, more (3SO2*). This species is capable of abstracting hydrogen
supple polymers, such as polyethylene, where strong from the polymer chains leading to the formation of
bonding between macro- molecules does not exist, macroradicals in the polymer structure, which in turn can
slippage of chains occurs more frequently and bond undergo further depolymerization (Schnabel, 1981).
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