Degradation of Polyethylene

WHAT IS POLYMER DEGRADATION ?

We all have noticed the plastic bucket left for long run in the sun and rain loses its lusture and strength. This deterioration in properties is due to a phenomenon called " polymer degradation", which is characterized by an uncontrolled change in the molecular weight or constitution of the polymer.[1] Conventionally , the term 'degradation' is taken to mean a reduction in the molecular weight of the polymer . In day to day life , plastic articles have to face mechanical stresses , solar radiations , atmospheric oxygen , moisture etc and this can degrade the polymer .

Sample of Polycarbonate degraded [2]

[3]

Excellent properties of plastics made it irresistible component and part of any industry or household products. Their production is increasing at high rate , that calls the need to recycle them and degrade them to protect the environment . Some polymers degrade easily by simple micro-organisms but some take decades and centuries to degrade and persists in the environment thus polluting the environment like Polethylene.

Polyethylene

Polyethylene (abbreviated PE) or polythene (IUPACname polyethene or poly(methylene)) is the most common plastic. The annual production is approximately 80 million metric tons.[4]Its primary use is within packaging plastic bag,plastic films, containers including bottles etc.). Many kinds of polyethylene are known, but they almost always have the chemical formula(C2H4)nH2. Thus PE is usually a mixture of similar organic compounds that differ in terms of the value of n. ABS 4% PC 2% PP 25%

Global Plastics Consumption 2010 PET 7%

PVC 18%

PC 2% HDPE 17%

ABS 4% LLDPE 11%

PS 6%

PP 25%

LDPE 10%

2010 World Polymer Demand = 190 Million Metric Tons SPI Film & Bag – May 2011

[5]

Polyethylene is the most common and major polymer produce all over the world owing to its different uses and its good properties. But Polyethylene is the matter of concern.

Environmental issues of Polyethylene

One of the main problems of polyethelyne is that without special treatment it is not biodegradable, and thus accumulates. In Japan getting rid of plastics in an environmentally friendly way was the major problem discussed until the Fukushima Disaster in 2011. It was listed as a $90 billion market for solutions. Since 2008 Japan has rapidly increased the recycling of plastics, but still has a large rate of plastic wrapping which goes to waste.[6]] During the 1980s and 1990s it was shown that many endangered marine species including birds that live in the marine environment are at extra hazard, with thousands of cases of suffocation from swallowing plastic bags or plastic content.[7] In May 2008, Daniel Burd, a 16-year-old Canadian, won the Canada-wide Science Fair in Ottawa after discovering that Pseudomonas fluorescens, with the help of Sphingomonas, can degrade over 40% of the weight of plastic bags in less than three months.[8]

In 2009 it was discovered by a resident of Hawaii upon returning from a ship race that degraded plastics are a major cause for marine life destruction, being mixed in with plankton, comparable in size and weight but in much larger numbers.[9] In 2010 a Japanese researcher Akinori Ito released the prototype of a machine which creates oil from Polyethylene using a small, self-contained vapor distillation process.[10]

Why Polyethylene degradation is difficult ?

It is widely accepted that the resistance of polyethylene to biodegradation stems from its high molecular weight, its three-dimensional structure and its hydrophobic nature, all of which interfere with its availability to microorganisms," the researchers stated. Nevertheless, the researchers said several studies have demonstrated partial biodegradation of polyethylene [11] The researchers goal was to select a polyethylene-degrading microorganism and study the factors affecting its biodegrading activity. Some efforts has been made to degrade the polyethylene using different micro-organisms and pre-treatment of PE to facilitate degradation of PE as discussed in next sections .

1 . BIODEGRADATION OF POLYETHYLENE

INTROThe wide range of applications of polymers from sophisticated articles to disposable things implies their importance and significance in our day to day life . Thus enormous production of these polymers lead to accumulation in the environment since they are not easily degraded by micro-organisms they have become serious problems for both flora and fauna .

As reported by American Plastic Association, percentage distribution of HDPE ( high density polyethylene ) , LLDPE and LDPE are 17.4 & , 12! % and 8.2 % respectively in terms of sale and use in the year 2004 in US, Canada and Mexico . [12] Non-degradable plastics accumulate in the environment at a rate of 25 million tones per year . [13]

As polymer usage is unavoidable, ways have to be found to

1. Enhance the bio-degradability of the polymers by blending them with biodegradable natural polymers like starch or cellulose .

2. Mixing with pro-oxidants so that they are easily degraded

3. Isolate and improve micro-organisms that can efficiently degrade these polymers

Overview of Bio-degradation of Polymers

A general overview of biodegradable polymers over a period of time is schematically represented in Fig.1

Oxidation Chemical degradation Hydrolysis

Thermal degradation

Environmental polymer degrdation Physical /Physicochemical degrdation

Photo-degradation

Bio-degradtion

Mechanical Degradation

Fig.1

Polymeric materials released into the environment can undergo physical , chemical , and biological degradation or combination of all these due to the presence of moisture , air , temperature , light , high-energy radiation or microorganisms . The rate of chemical and physical degradation are higher when compared to biodegradation. Also physical and chemical degradation facilitates microbial degradation and complete mineralization of the polymer happen due to bio-degradation , which is the final step .

1.1 Mechanism of Biodegradation

1. Attachment of micro-organisms to the surface of the polymer

2. Growth of micro-organisms utilizing the polymer as the carbon source

3. Primary degradation of the polymer

4. Ultimate Degradation

Fig .Degradation of plastic mechanism [24]

Micro-organisms can attach to the surface of the polymer if the surface is hydrophilic . Since Polyethylene have only CH2 groups the surface is hydrophobic. Initial physical or chemical degradation leads to the insertion of hydrophilic groups on the polymer surface making it more hydrophilic , it also helps in decreasing the surface energy . Once the organism gets attached to the surface , it starts growing by utilizing the polymer as carbon source . In the primary degradation , the main chain cleaves leading to the formation

of low-molecular weight fragments (oligomers) , dimers or monomers . The degradation is due to extra cellular enzymes secreted by the organisms . These low molecular weight compounds are further utilized by microbes as carbon and energy sources . Small oligomers may also diffuse into the organisms and get assimilated . The ultimate products of degradation are C02 , H2O and biomass under aerobic conditions . Anaerobic conditions may also degrade these polymers under anoxic conditions . The primary products are C02 , H20, and CH4 and biomass under methanogenic condition or H2S , CO2 , AND H2O under sulfidogenic conditions . The environmental conditions decide the group of micro-organisms and the degradative pathway involved . Ultimate degradation of recalcitrant synthetic polymers may take several 100 years .

1.2 Factors Affecting Bio-degradability

Bio-degradability of the polymer is essentially determined by the following important physical and chemical characteristics :-

1. Availability of functional groups that increases hydrophilicity 2. Size , molecular weight and density of the polymer

3. Amount of crystalline and amorphous regions

4. Structural complexity such as linearity or presence of branching in the polymer

5. Presence of easily breakable bonds such as ester or amide bonds as against carbon-carbon bonds .

6. Molecular composition (blend )

7. Nature and physical form of the polymer such as whether it is in form of films , pellets , powder or fibers.

1.3 Some research papers on the biodegradability of polyethylene

Title of Paper

Polymer

Organism

Mechanical behavior of HDPE/BLENDS

SOIL

biodegradable

MICRORGANISMS

polyolefins [14] DSC

FTIR polyethylene

Characterization

of

biodegradation

of

A.niger

polyethylene [15] Colonization , biofilm LDPE blends formation biodegradation

Rhodococus rubber

and of

polyethylene [16] Synergetic effect of UV LDPE /starch blends light

–soil

treatment

burial of

Soil microorganisms

biodegradation

of

polyethylene [17] Bio-degradation

of LDPE

Soil microorganisms

thermally oxidized low density

polyethylene

[18] Degradation

product LDPE/starch

pattern and morphology

Arthobacter parrafineus

as means to differentiate abiotic and biotic aged degradable polyethylene [19] Acquired

LDPE/HDPE

biodegradability

of BLENDS

R.rhodochrous

and

Nocardia asteroids

polyethylene containing pro-oxidant additives [20] Biodegradation

of LDPE/HDPE blends

Brevibacillus

polyethylene

by

borstelensis

thermophillic bacterium Brevibacillus borstelensis [21] Electrical analysis

thermal LDPE/Starch to

assess

biodegradation

of

Bacteria

Baccilus

clostridium and Fungi mucor , pencilium

polyethylene [22] Surface

Changes LDPE

FUNGUS

brought about by corona discharge treatment of polyethylene film and effect

on

microbial

subsequent colonization

[23]

Inference from the above studies

In general , poly-olefins are inert materials not susceptible to microbial attack because of the following reasons :-

1. Hydrophobic backbones consisting of long carbon chains that give high resistivity against hydrolysis .

2. Addition of anti-oxidants and stabilizers during their manufacture which keeps poly-olefins resistant to atmospheric oxidation .

3. High molecular weight ( from 10,000 to 40,000 )

4. High packing density

•

The decreasing order of susceptibility of polymers to degradation in soil is mixed with municipal refuse was PE >>>LDPE >>>HDPE as revealed by analyzing the weight loss of samples , C02 evolution , changes in tensile strength , changes in FTIR and bacterial activity in the soil .

•

Outdoor soil burial tests were done on the samples of HDPE blends with different biodegradable additives . DSC analysis of these polymers with different additives after a year showed no changed in melting temperature and fraction of crystalline region. Therefore , it was concluded that the biodegradation begins at the amorphous region rather than at the crystalline region. Biodegraded HDPE blends are more brittle in nature compared to non-degraded.

•

Fungi that include A.niger , Pencillium funiculosum , Fusarium redolens and A.vesicolor , and soil microorganisms are reported to degrade PE. DSC,FTIR and other mechanical and physical techniques were used to monitor the nature of biodegradation. Thermal , UV,photo and corona treated PE has been found to degrade faster than the untreated polymer .

•

Photo-oxidation is the triggering step in oxidative degradation of polyethylene . UV radiation leads to radical formation , followed by absorption of oxygen resulting in end products with carbonyl groups .Ultimately , photo-oxidation leads to the formation of molecular weight fragments and thus increases the hydrophilicity of the polymer. Later , the two carbon acetyl CoA , enters the TCA cycle and gets completely converted into carbon dioxide and water.

•

Cell homogenates from P.putida and Bacillus brevis were found to degrade PE films by oxidative degradation resulting in the formation of terminal hydroxyl , ketone and ester group . The presence of alcohol dehydrogenase was confirmed indirectly in the degradation reaction by inhibition studies .

•

The known lignin degrading bacteria S.virdosporos T7A , S.badius 252 and S.setonni 75vi2 and the fungus Phanerochaete chrysosporium were used to assess their ability to degrade biodegradable polyethylene .

•

Reduction in poly-dispersity and tensile strength were observed in biodegradable PE with bacterial treatment not with fungus . A.niger was reported to degrade commercially available PE. •

DSC analysis showed reduction in amorphous region of polymer. Biodegradation of LDPE was enhanced with Tween 80 in the

presence of P.aeuroginosa. This study explains the role of nonionic surfactant in biofilm formation , prerequisite for biodegradation process. •

The bacteria , Arthobacter parrafineus was found to degrade LDPE in 3 years by utilizing carboxylic acid formed during thermal oxidation. The utilization was through B-oxidation mechanism that yields the degradation products like acetyl CoA and propionyl CoA.

•

Corona discharge treatment was found to be more effective towards colonization of micro-organisms on food packaging grade LDPE films with little effect on the mechanical properties as compared to UV degradation . This suggest that corona discharge treatment is effecting the hydrophobicity of the surface of polymer and not penetrating it . A reduction in hydrophobicity of LDPE from 92 % to 66.6% was noted.

•

Thermal treatment of LDPE ( pro-oxidant additive ) in aerobic condition showed substantial polymer fragmentation with loss of mechanical properties in period of 18months with further treatment with soil microorganisms. Temperature is the crucial factor in determining the rate of thermo-oxidation whereas the effect of concentration of oxygen on the rate of thermo-oxidation is insignificant.

•

Thermally stimulated current of PE films were exposed to various aging conditions in soil were reported . After aging , new peaks were

detached on spectra . FTIR results show formation of functional groups. Reduction in melting point through DSC analysis . The degree of biological damage of films was function of starch content in composites. The predominant microbial taxa in composites were Bacillus, Micrococcus , Penicillum ,and Mucor . •

Rhodococcus rubber C208 was isolated from the surface of PE in polyethylene waste burial site by two step culture-enrichment protocol.Weight loss of 8% of photo-oxidised PE was observed in four weeks . This is higher than the rates already reported. The analysis of extra-cellular polysaccharides in the biofilm of C208 was 2.5 folds higher than protein , suggesting its tole in biofilm formation .

•

Cell surface hydrophobicity of R.ruber was studied and addition of mineral oil enhanced the degradation of PE films by about 50% after four weeks of incubation .

•

Brevibacillus borstelensis , a thermophillic bacterium , was found to degrade polyethylene better than R.rubber , although the biofilm forming capacity of the former was not found to be as good as on of latter. Still it was able to show considerable reduction in mass and molecular weight by 11-30% respectively for UV irradiated polyethylene.

• •

LDPE and HDPE films after photoxidation and thermal oxidation were incubated with R.rhodochrous and Nocardia asteroids. ATP (

adenosine triphosphate) assay was done to see metabolic activities of the cell in culture and those adhered to the surface of polymer. After 9months NMR analysis of this sample revealed the presence of ethanol and formate , which are end products of PE oxidation . This evidence support the initial fast growth of micro-organisms observed by ATP analysis.

1.4 Conclusion

Pre-treated polymers degrade more easily than untreated ones . Most of the examples deal with fungi and bacterial based degradation . Also, degradation is more facile with starch and cellulose blended polymers. Cell surface hydrophobicity and addition of surfactants showed an important role in biofilm formation, which is pre-requisite for biodegradation . Degradation leads to decrease in molecular weight , tensile strength and viscosity , formation of new functional groups such as carbonyl etc.

Following approaches can be adopted to enhance the bio-degradation of PE :-

1. Modify the polymer for microbial utility by i) addition of natural polymer like starch and pro-oxidants ii) Modification of polymers by protein hydrolysates and iii) Pretreatment of polymer

2. Modify the microbes to utilize the polymer by i) Modifying the medium composition and thus enhancing the utilization of polymer ii) genetically

modify the micro-organisms to utilize the polymers.

Both these strategies require the understanding of mechanism of microbial degradation of these polymers.

2 .Thermal Degradation

PE can be and is processed practically with all thermoplastic processing technologies including extrusion, injection molding, blow molding,

rotational molding, etc., but the largest amount of products are prepared by extrusion. The polymer is subjected to the effect of heat, shear and oxygen in all thermoplastic processing technologies. As an effect of these factors chemical reactions take place in the processing machine with a number of consequences. Although polyethylene is a simple polymer with an apparently well defined structure, its molecular structure often contains irregularities. The actual structure of PE depends on the polymerization technology used for its production. Polymerization under high pressure with radical initiation results in low density polyethylene with a large number of branches, while catalytic processes usually yield linear chains with more regular structures. However, even the catalyst used for the production changes chain structure, chromium catalyst used in the Phillips process create a double bond at the end of each chain, while the concentration of unsaturations is at least an order of magnitude smaller in Ziegler type polymers. Double bonds and other chain irregularities are potential reaction sites on which chemical reactions take place during processing. These reactions modify the chain structure of the polymer and also the properties of the final product. The thermal degradation of linear polyethylene in oxygen-free environment starts with random scission of the polymer chains followed by random intermolecular hydrogen abstraction and subsequent β-scission. R–R → R• + •R According to Holström and Sörvik [25] the thermal degradation of polyethylene is a radical process, and it starts with the scission of C–C bonds in allylic position

The primary radicals formed in the initiation reaction participate in the following further reactions under thermal conditions:  depolymerization through β-scission,  intramolecular hydrogen transfer,  intermolecular hydrogen transfer.
Decomposition reactions follow the intramolecular and intermolecular hydrogen transfer in the thermal degradation of polyethylene. 
β-scission of the radicals results in the formation of small molecular fragments, further alkyl radicals and vinyl groups:

Some of the primary radicals are expected to isomerize by intramolecular

hydrogen abstraction (back-biting) and to form secondary radicals, which are more stable. Intramolecular hydrogen transfer can occur from the fourth or fifth carbon atom to the first one :

In the case of 5 → 1 hydrogen transfer the reaction can proceed inside the polymer chain:

Hydrogen transfer can occur also in intermolecular reactions:

where R, R’ and R” are alkyl groups; R’” is hydrogen or alkyl group The intermolecular hydrogen abstraction can be followed by β-scission. The reactions of secondary radicals lead to the formation of vinyl groups, while those of tertiary radicals result in the formation of vinylidene and vinylene

groups:

Holström and Sörvik considered the intermolecular hydrogen abstraction followed by β-scission the most important propagation reactions in the degradation of polyethylene. These reactions result in a significant decrease of the average molecular mass and yield volatile products. On the basis of activation and bond dissociation energies Kuroki et al. [26] claimed that back-biting reactions and intermolecular radical-transfer reactions are much more likely to occur than depolymerization reactions. Intermolecular hydrogen abstraction followed by isomerization of a vinyl group results in the formation of vinylene group:

Recombination and disproportionation are the most important termination

reactions of thermal degradation.

Holström and Sörvik observed that the tendency to disproportionation increases with increasing temperature. From the trans-vinylene group/long chain branches content ratio of high density polyethylene Kuroki et al. concluded that the probability of the recombination termination reaction between primary and secondary macro radicals is 2-5 times larger than that of the disproportionation reaction. Although both types of termination reactions have zero activation energy, termination reactions are diffusion controlled in practice and the rate constant depends on the rate of diffusion of macro radicals in the media. If either of the radicals diffuses outside the field of reaction created by the surrounding polymer segments (cage), propagation (depolymerization, intra- and intermolecular radical transfer) and termination reactions take place, leading to the formation of volatiles and a decrease in molecular mass. Thus both the molecular mass increase and the degradation reactions become dependent on the rate of diffusion, in relation to the viscosity of the media, of the two radicals outside the cage.

2.1 Thermo-oxidative degradation The oxidation of hydrocarbons is a free radical-initiated autocatalytic chain

reaction. The reaction is slow at the start and accelerates with increasing concentration of the resulting hydroperoxides. The process can be regarded as proceeding in three distinct steps: chain initiation, chain propagation, and chain termination. Chain initiation:

Chain propagation:

Chain branching:

Chain termination:

Many schemes were proposed for the initiation step, but the origin of the primary alkyl radical R• is still controversial. It can be formed as an effect of heat, shear, catalyst residues, radical initiators, and/or impurities in the monomer. The alkyl radicals react with molecular oxygen practically without activation energy forming peroxy radicals. The rate constant for the reaction of most alkyl radicals with oxygen is of the order of 10 7-109 mol1s-1. The peroxy radicals form hydroperoxides upon abstraction of hydrogen from the polymer chain, which requires the breaking of a C-H bond, i.e. needs activation energy. Therefore this is the rate-determining step in autoxidation. The rate of the abstraction reaction decreases in the following order:

hydrogen in α-position to a C=C double bond (allyl) > benzyl hydrogen and tertiary hydrogen > secondary hydro- gen > primary hydrogen. Primary and secondary peroxy radicals are more reactive in hydrogen abstraction than the analogous tertiary radicals, and the most reactive are acylperoxy radicals. The peroxy radicals oxidize olefins ina way in which the oxidation of double bond is considered the most important reaction by Brill and Barone.

Hydroperoxides decompose fast to reactive oxy and hydroxyl radicals. The rate of decomposition increases with rising temperature. Metal ions and ultraviolet radiation catalyze hydroperoxide decomposition. The reactions with metal ions are described by reactions :

Oxy and hydroxyl radicals formed in the decomposition of hydroperoxides

are far more reactive than peroxy radicals, and lead to the branching of the reaction chain, i.e. auto-acceleration of the degradation process. β-scission of oxy macro-radicals yields carbonyl groups and other free alkyl radicals. Chain termination occurs by recombination or disproportionation of radicals. At high oxygen concentrations and moderate temperatures chain termination proceeds by the recombination of peroxy radicals. If the concentration of R • radicals is much higher than that of peroxy radicals (characteristic for polyethylene processing), chain termination is caused by recombination with other available radicals. The disproportionation of alkyl radicals leads to the formation of an unsaturated group but does not result in a decrease of molecular mass. The chain termination processes do not stop the thermooxidative reactions. The reaction products formed in the recombination reaction of peroxy radicals participate in further reactions. Norrish-II type breakdown of ketones results in the formation of vinyl and hydroxyl groups under irradiation:

The melt processing of polyethylene takes place in oxygen poor environment under shear. The high mechanical forces lead to C-C chain scission resulting in macro- radicals. The oxygen dissolved in the polymer reacts with the alkyl radicals forming peroxy radicals, subsequently hydroperoxides and new alkyl radicals. These hydro- peroxides decompose rapidly to the corresponding alkoxy and hydroxyl radicals. The latter can form inactive products (ROH and H2O) and further alkyl radicals through hydrogen abstraction, while ß-scission leads to the scission of the macromolecule. The number of weak sites in the polymer chain, the type

and amount of catalyst residues, as well as the processing conditions affect significantly the rate and direction of reactions. At high concentrations of unsaturated groups (Phillips type polyethylene) crosslinking reactions predominate during processing, while at low unsaturated group contents (Ziegler-type and metallocene polyethylenes) the direction of the reactions depends both on the number of vinyl groups and the processing conditions. Transition metal impurities, including catalyst residues, accelerate the decomposition of hydroperoxides leading to detrimental effects.

2.2 Effects of thermal degradation The chemical reactions involved in thermal degradation lead to physical and optical property changes relative to the initially specified properties. Thermal degradation generally involves changes to the molecular weight (and molecular weight distribution) of the polymer and typical property changes include:  Reduced ductility and embrittlement  Chalking
  Color changes
  Cracking  General reduction in most other desirable physical properties The dominant mechanism of degradation and the degree of resistance to degradation depends on the application and the polymer concerned. The results are the same for most polymer families and significant property degradation can occur when thermal degradation does occur.

2.3 Protecting Plastics with Stabilizers Plastics can be protected from thermal degradation by incorporating stabilizers into them. Stabilizers are used to keep the polymer chains and the original molecular structure intact and therefore properties such as strength, stiffness and toughness can be retained over a longer period. Stabilizers can work in a variety of ways but in most cases they work by interrupting the thermal degradation cycle to slow down or prevent the cycle from completing.
Some stabilizers work by ‘mopping up’ the available free radicals (radical scavengers). In this case the stabilizer reacts rapidly with the available free radicals to produce another much less active free radical and thus slow the process down.

2.4 Protecting plastics with radical scavenger stabilizers A second group of stabilizers work by reacting with the hydroperoxide (ROOH) to produce inactive and stable products such as ROH and break the cycle at the hydroperoxide propagation step. Other groups of stabilizers exist for specific materials and applications and there are many different chemical families of stabilizers. In most cases a given plastic will incorporate a mix of stabilizers that are designed to work as a system to give the desired properties for the application. This mix will be designed specifically for the polymer being used and the requirements of the application. The mixture will also be

designed to be applied at a specific concentration – over dosing stabilizers can in fact be detrimental to the plastic and the effect of the stabilizer.

The given mechanisms can be substantiated using a report (33) aimed at studying the thermal and UV degradation of HDPE, LLDPE and metallocene grade materials, by means of the CL technique, and TGA in terms of determination of the frequency factor and the activation energy for the overall photooxidation process. For this purpose, their analysis was based on the kinetic analysis method developed by Park et al. [34], which allows calculation of A0 and Ea, and to predict the degradation of polyolefins at any time. The chain branching was expressed by the CH3 content determined by FTIR as the ratio A/d, where A is the absorption at 1378cm−1 corresponding to the deformation band of CH3 groups δ(CH3), and d the sample thickness. It was seen to increase in the following order: HDPE < m-PE < LLDPE. The results obtained indicate a clear relationship between activation energy and the chain branching of the polymers, Ea decreases as the chain branching increases. Such behavior can be explained by the mechanism described previously. An analysis of the three polymers before and after thermal ageing at 90 ◦ C was undertaken using FTIR. In general, the metallocene polymer exhibited the lowest initial concentration of oxidised products associated with a low oxidation

level

during

the

manufacturing

process.

Hydroperoxide

concentration for m-PE was seen to gradually increase up to 144h of ageing, whereas, for HDPE which gave the highest initial hydroperoxide values, and LLDPE, an autocatalytic oxidation process was observed. HDPE was the most unstable polymer, followed by LLDPE and m-PE. A similar order for the stability of the polyethylenes was found through the measurement of carbonyl index growth (see figure). Carbonyl index gives a measure of the amount of oxidized products in the system i.e. it tells us about the hydrophilicity. It is the ration between absorption intensities of C=O and CH2 .

Figure: Carbonyl index (1720 cm−1 ) vs. oven ageing time at 90 ◦ C for films of HDPE, LLDPE and m-PE polymers (d (= 290 × 10−4 cm) is the film thickness.

The m-PE and LLDPE samples showed an initial autoretarding effect, whereas, HDPE film exhibited a shorter induction period and the highest carbonyl levels up to 144 h heating. As already described, different vinyl types are generated in the thermal degradation of polymers. The analysis of the initial concentration and evolution of those species in HDPE, LLDPE and m-PE after thermal ageing, was undertaken using second order derivative UV analysis. In general, HDPE exhibited the higher initial concentration of vinyl groups compared to LLDPE and m-PE. HDPE had a very strong band at 199 nm (not present in LLDPE and m-PE), assigned to less substituted vinyl. m-PE showed a band at 205 nm corresponding to disubstituted vinyl, and was not present in HDPE and LLDPE. The three polymers have a band at 194 nm assigned to monosubstituted vinyli- dene groups, HDPE showed the highest absorbance of this band and a greater increase of those species with ageing time. From the results obtained, the order established for the oxidation susceptibility is as follow: HDPE > LLDPE > m-PE in accordance with carbonyl index results. CL spectra of the initial polyethylenes films were measured at 170 ◦ C (see figure).

Figure: CL spectra under nitrogen of HDPE, LLDPE and m-PE polymers at 170◦C. The CL in polymers is due to the light emission that accompanies the thermal decomposition of the thermooxidative degradation products (hydroperoxides), which are formed during processing or service life of the material under ambient conditions. By means of the CL technique, it is possible to determine the degree of oxidation, and even to predict the long term stability of the polymer. The metallocene polymer exhibited the lowest CL intensity, at 170 ◦ C, for initial sample which corresponds to the lowest concentration of oxidized products as determined by FTIR. For the three polyethylenes, the intensity of CL of the fresh and aged samples were measured from 25 to 250◦C. The intensity of CL of the aged

samples is very much higher than that of the initial polymers, due to the decomposition of the hydroperoxides generated during the thermal degradation. LLDPE and m-PE show a gradual increase of the intensity of CL with ageing time, whereas, for HDPE an autocatalytic oxidation process was observed; and the emission was detected at lower temperature than LLDPE and m-PE, which showed an inhibition period. This result would indicate that HDPE is more susceptible to oxidation, followed by LLDPE and m-PE, in agreement with the above commented results.

Taking the attention to the report on ‘Polyethylene degradation: effect of polymerization catalyst’, by Karmele Sanchez, Norman Allen, and Chris Liauw. They used several PE resins—e.g., high-density PE (HDPE), using a Phillips (Ph) metal-oxide catalyst, and linear low-density PE (LLDPE), based on Ziegler-Natta (ZN) and metallocene catalyst technology to acquire insights into the effect of different polymerization catalyst systems on degradation-product formation during melt processing. They used color measurement, IR spectroscopy, hydroperoxide determination, and meltflow-rate measurements to monitor degradation as a function of the number of passes though a twin-screw extruder. Relative to a range of additive-free PEs , they recorded unusually good thermo-oxidative stability during melt processing for PEs produced by

means of the metallocene route, especially medium- density PE. This relatively high stability is probably due to low levels of nocuous metalcatalyst residues, as well as very low vinyl-unsaturation content. The relatively high melt stability of the metallocene PEs was manifested as a weak tendency to chain extend/crosslink and the lowest recorded rate of carbonyl-group formation (see Figure).

Transmission Fourier-transform IR spectra illustrating the carbonyl region of (a) metallocene medium-density polyethylene (PE), (b) metallocene linear low-density PE (LLDPE), (c) Ziegler-Natta LLDPE, and (d) Phillips high-density PE. The absorbance was normalized to the internal standard at 2018cm -1. Upper group: Pass 0 samples. Lower group: Pass 5 samples.

In contrast, Ph-HDPE had a strong tendency to chain extend/crosslink and oxidize because of the abundant vinyl groups in this polymer. Vinyl groups are the origin of chain extension/crosslinking by addition of macro-radicals (see Figure ).

Normalized vinyl-group absorbance—Abs908/Abs2019— versus the number of extruder passes for m-MDPE (squares), m-LLDPE (circles), ZN-LLDPE (triangles), and Ph-HDPE (stars). The relative ease of melt oxidation of Ph-HDPE led to high hydroperoxide production levels, despite the presence of chromium catalyst residues that are believed to decompose hydroperoxides into free radicals. This results in acceleration of the auto-oxidation process. On the other hand, it was observed that significantly reduced hydroperoxide production within the metallocene and ZN PEs. This was entirely consistent with the carbonylindex trends, since these PEs generally featured less carbonyl growth during multipass extrusion. They recorded an increase in yellowness index with increased number of

extruder passes for all PEs examined (see Figure ).

Yellow index versus the number of extruder passes for m-MDPE (squares), m-LLDPE (circles), ZN-LLDPE (triangles), and Ph-HDPE (stars).

The increase was most and least significant for, respectively, ZN-LLDPE and metallocene LLDPE and Ph-HDPE. Previous work indicates that the increased color development in ZN-LLDPE may be caused by chelation of the titanium catalyst residues. In the other polymers, discoloration may be attributed to extensive formation of conjugated systems consisting of transvinylidene unsaturation and carbonyl groups. What next is needed to be examined is the effect of catalyst type on melt-

stabilization performance of single antioxidants, antioxidant combinations, melt- processing oxidation-product chemiluminescence, and the largely neglected area of the effect of stabilizer type on the production of lowmolar-mass (volatile) oxidation products.

Another report (35) on the effect of pro-oxidant on the lifetime of polymer by non-isothermal thermo- gravimetry has been investigated. The incorporation of these additives is expected to decrease the lifetime of polyethylene in general. This study is concerned with the degradation behaviour of a series of formulations containing cobalt stearate in the concentration

range

(0.05–

0.2%

w/w)

using

non-isothermal

thermogravimetric analysis in two different atmospheres: nitrogen and air. The kinetic parameters have been calculated which have been subsequently employed to predict the effect of cobalt stearate on the lifetime of LDPE. Air oven aging studies have also been performed at two different temperatures (70°C and 100°C) to demonstrate practically the pro-oxidant activity of cobalt stearate on LDPE.

TG/DTG traces for the thermal decomposition in nitrogen atmosphere at different heating rates a) 3°C/min, b) 5°C/min, c) 7°C/min

As is evident from the figure, all the samples exhibit single step decomposition in nitrogen atmosphere over a relatively short temperature range. In inert atmosphere, random scission has been reported to be the primary pathway for degradation in polyethylene. However, this is also accompanied by polymer branching. From the figures, it can be concluded that both, scission as well as branching, occur simultaneously resulting in a single mass loss step. The degradation temperature was found to increase with increase in the heating rate, which corresponds to the time temperature superposition principle. A shorter time is required for the sample to reach a given temperature at a faster heating rate. The onset temperature of

degradation (Tonset), temperature of maximum loss (Tmax), end temperature of degradation (Tend), temperature corresponding to 5% loss (T5l) and 50% loss (T50l) have been calculated from the DTG curves (3°C/min) and are presented in Table1. It was observed that Tonset shifts to lower temperatures with increase in the concentration of cobalt stearate, which also results in larger difference of Tonset and Tend. This also indicates that the degradation require relatively longer time periods. In air atmosphere, a slight increase in the weight due to heating till 160– 200°C, and this has been attributed to the formation of polymeric oxides(see figure). Contrary to the behavior in nitrogen atmosphere, the degradation temperature was not found to increase with increase in the heating rate in air. Table 2 reports the temperature at which 5% and 50% mass loss occurs as T5l

and

T50l

respectively.

TG/DTG traces for the thermal decomposition in air atmosphere at different heating rates a) 3°C/min, b) 5°C/min, c) 7°C/min

Table1

Table 2 On comparing Table 1 and 2, we observe that the degradation occurs at much lower temperatures in air than in nitrogen atmosphere. Addition of cobalt stearate to polyethylene leads to further lowering of these characteristic temperatures, which indicate its pro-oxidative nature.

3. Photo-degradation

The phenomenon of 'weathering' of polymeric materials is usually caused by a complex series of chemical reactions initiated by the absorption of ultraviolet light which ultimately result in the deterioration of the physical properties of the polymer.[37] All polyethylene (PE) is susceptible to degradation upon long-term exposure to sunlight. This degradation is brought about by physical changes, which occur in the polyethylene as a result of exposure to the ultraviolet (UV) portion of sunlight. UV light contains shorter wavelengths than visible light. The shorter the wavelength, the more energy it contains and thus, the more damage it does. Most commercial organic polymers undergo chemical reaction upon irradiation with ultraviolet light, because they possess chromophoric groups

(as regular constituents or as impurities) capable of absorbing UV light. Carbonyl groups play a prominent role among these chromophoric groups [23]. Therefore, the photochemistry of ketone polymers was selected to demonstrate how light-induced chemical reactions proceed. Here we take into consideration the Norrish reaction I and II.

We emphasis on photo-oxidation/degradation of polythene by following ways: • Accelerating the weathering conditions artificially. • Photo-catalytic

degradation

using

doped

or

un-doped

TiO2

nanoparticles.

Depending on the use of the particular polymer, as well as its identity, some mechanisms may be prominent, while others might not act at all. The degradation of LDPE films used as covering material is governed mainly by thermal, radiation, mechanical and chemical mechanisms.

3.1 The Details and Mechanisms

Poly-olefins rapidly lose most of their mechanical properties after severe processing or periods of thermal and light exposure. Low molecular weight hydrocarbon analogues are generally resistant to oxidation at ambient temperatures, but surprisingly the degree of degradation is unexpectedly high[2]. Degradation is a complicated non-linear time-dependent process which affects directly, or indirectly, several properties of the material related to its functional characteristics. In its final stage of degradation, a material does not meet its functional requirements and is easily prone to mechanical failure. As a practical rule, the useful life of a material is considered to be reached when its initial mechanical strength is reduced by 50% (Henninger & Pedrazetti, 1988). There are several factors to monitor and criteria to determine the degree of degradation (Randy & Rabek, 1983).[40] Poly-olefins should, in theory, be impervious to photo-degradation like pure aliphatic hydrocarbons, which do not absorb the UV radiation present in sunlight. It is generally assumed that impurities, or chromophores, which absorb UV light, initiate the photo-oxidation of commercial polyolefins.[39]

Also, relatively little chemical change is required to generate major changes in physical properties. The elongation at break (expressed as a %) appears to be a more sensitive &index' of degradation than the tensile strength, the stress at yield or the modulus of elasticity. Actually, the material becomes more brittle with degradation and so it cannot retain its initial elongation at break. This is especially important when the final product is exposed to weathering in outdoor applications, where degradation is initiated by the near-ultraviolet (UV) component of sunlight and oxygen. UV light alters the physical characteristics of polyethylene (PE).It does this by breaking the carbon and hydrogen bonds, creating free radicals which, in turn, break the PE into shorter molecules and thus, a more brittle polymer. Effectively, UV light creates a higher melt index polyethylene, especially on the exposed surface area. This shows up as a reduction in break elongation and impact properties, typical of higher melt index PE. The subsequent attachment of oxygen to these broken sites leads to further accelerated degradation and the formation of oxidized species such as carbonyl and car-boxyl structures, which are often used as analytical indicators of UV degradation.[41]

Coming to the mechanism of photo-oxidative degradation, the starting reaction is always bond scission in polymer chain or in some other molecule initiating degradation. This clarifies the fact that degradation follows a free radical mechanism initiated by scission of chromophores and other additive molecules. Subsequent reactions include crosslinking and the formation of double bonds. Polyethylene undergoes chain scission, branching and crosslinking, which occur as competitive reactions, whereas polypropylene undergoes chain scission predominantly, as shown below.[39]

Oxidation without UV involvement is much slower than photo-oxidation. After long exposure to UV light of short wavelength (254 nm) in a vacuum or in a nitrogen atmosphere, chain scission and hydrogen abstraction occur. Also Cross-linking and evolution of hydrogen are observed. [38]

The photo-oxidation happens mostly by the Norrish process I or II.

The Norrish

reaction in organic

chemistry describes

the photochemical

reactions taking place with ketones and aldehydes. The Norrish type I reaction is the photochemical cleavage or homolysis of aldehydes and ketones into two free radical intermediates. The carbonyl group accepts a photon and is excited to a photochemical singlet state. Through intersystem crossing the triplet state can be obtained. On cleavage of the α-carbon

carbon bond from either state, two radical fragments are obtained. A Norrish type II reaction is the photochemical intra-molecular abstraction of a γhydrogen (which is a hydrogen atom three carbon positions removed from the carbonyl group) by the excited carbonyl compound to produce a 1,4biradical as a primary photoproduct (IUPAC definition). [41] Copolymerization of ethylene monomer with small amount of carbon monoxide forms an ethylene-carbon monoxide copolymer, known to degrade on sunlight exposure by Norrish type I and II reactions. Both cases are equivalently dangerous to the durability of the polymer. If the polymer degrades according to Norrish I mechanism, two free radicals are formed simultaneously. When the Norrish II mechanism controls degradation, two molecules capable of absorbing sunlight are formed.[39] High-density PE and low-density PE contain unsaturations. The presence of these unsaturations (vinylidene groups) leads to formation of allylic hydroperoxides during the thermooxidative processes, and this becomes the major mechanism of initiation as shown in the figure below. [39]

This structure can be further converted by heat, UV, or other radicals to free radicals and/or to structures containing UV-absorbing groups (e.g., carbonyl). Unsaturations then usually predominantly lead to chain scission

reactions, but crosslink formation also occurs as shown in the next figure.

Other photosensitive groups introduced during processing and use are: (a) hydroperoxides and peroxides which are primary products of thermal oxidation during processing, are extremely photochemically active and are initiators of photo-reactions; (b) carbonyl groups produced by thermal oxidation during processing and storage and from photo-oxidation during use which are also well-known key initiators of the Norish I and II type photo-reactions (c) reactive forms of oxygen such as ozone (O3) which is present in the atmosphere and singlet oxygen (1O2) which can be produced inside the polymer by quenching of UV excited species by ground-state oxygen (3O2).

All these impurities lead to enhancement of photodegradation by either absorbing energy of the UV spectrum, or by being initiators of photooxidation reactions (in the case of carbonyl and hydroperoxide groups).[39] Once these impurities are present in the LDPE films, the photo-degradation proceeds, in two distinct parts: (a) photo-oxidation of the outer layers which can come in direct contact with the atmospheric oxygen, and proceeds rapidly mainly through radical chain oxidation reactions, while (b) the inner layers, which cannot under usual circumstances be reached by the atmospheric oxygen, degrade more slowly through photo-reactions of peroxy radicals or reaction of radical pairs. Usual products of these reactions are carbonyl (C=O), hydroxyl (OH) and vinyl groups, while trans-vinylene groups are produced in trace amounts.[39] Factors influencing polyolefin photodegradation include molecular structure, polymer morphology (degree of tacticity and crystallinity), internal impurities, specimen thickness, temperature, irradiation intensity and other climatic conditions. Since the additives' presence renders the polymers apparently useless, addition of stabilisers becomes inevitable avoiding which shall make the polymer material short lived. However, many stabilizers and antioxidants are lost over time through sacrificial transformations, evaporation, blooming or leaching. Consequently it is common practice to add further stabilizers to aged polymer material, a process known as restabilization [38]. Studies investigate mechanical, rheological behaviour of poly-olefins, including blends, intended for outdoor use that had been subjected to accelerated ageing by UV radiation for a variety of exposure

periods. Gloss and colour measurements are taken to observe any changes in surface characteristics. Gel permeation chromatography (GPC) is employed to observe any changes in chemical properties. Since in the laboratory, we create accelerated weathering conditions, so, comparisons are made in artificially degraded and naturally degraded samples of poly-olefins to calculate the similarities and hence application of the accelerated weathering conditions method. [38]

Now comes the catalytic photo-oxidation of polythene[42]. Titanium dioxide (TiO2) is one of the most well-known efficient photocatalysts. The capability of TiO2-based photocatalyst to degrade gaseous and aqueous contamination makes it a good candidate for use in air clean up and water purification. The most promising approach of activation of TiO2, in the visible light region, is modification of its chemical structure to shift the absorption spectrum to the visible light region. This type of modification involves introduction of doping with metal and non-metal species. The process of recycling polymers is expensive and time consuming; only a small percentage of the plastic waste is currently being recycled . Biodegradable plastics have shown considerable promise in this context. However, the biodegradable plastics till now cannot completely solve the problem due to their chemical stability and nonaffordable cost. More recently, photo degradation of plastics has also started receiving attention. The composition of plastic and TiO2 nanoparticles (NPs) has been proven to be a new and useful way to decompose solid polymer in open air. Investigations on the photo degradation of polyvinyl

chloride (PVC), polystyrene (PS), and polythene (PE) have been carried out . More specifically, a few recent reports describe the use of TiO2 and goethite and so forth as the photo-catalyst for oxidative degradation of PE with very encouraging. Currently we focus on solid phase photo-catalytic degradation of polyethylene plastic with TiO2 as photo-catalyst and Fe, Ag metals as dopants.[42]

3.2 Effect of morphology on photodegradation

Degradation of PE is influenced by a great extent of its crystallinity. It has long been known that branched polyethylene oxidizes faster than linear polyethylene, and it has been discovered that its oxidation rate is roughly proportional to the amount of amorphous fraction present. This suggests that the oxidation of semi-crystalline polyethylene is restricted to its amorphous region. It was subsequently discovered that the crystalline region absorbs practically no gas which implies that oxygen is simply not available in the crystalline region. [39] Based upon these facts, polyethylene of low crystallinity has a high rate of carbonyl formation and a low concentration of radicals. Crystallinity changes during the course of degradation. In the initial stages of

photodegradation, chain scission prevails which reduces molecular weight. Shorter chains are more mobile and are thus able to crystallize more readily. Therefore, embrittlement of PE is controlled by two associated processes: reduction of molecular weight and increased Crystallinity. Characterization Characterization of TiO2 Nanoparticles X-Ray Diffraction Analysis: Crystal size of the prepared photocatalyst was studied by powder XRD technique. X-ray diffraction patterns were obtained on JEOL JDX-II X-ray diffractometer using Cu-Kα radiation at an angle of 2θ from 10◦ to 80◦. The crystallite size was determined fromthe X-ray diffraction patterns, based on the Scherer equation.

where k is a shape factor = 0.9, λ is the radiation wavelength= 1.54051◦A, θ is the Bragg angle, β = full width of a diffraction line at one half of maximum intensity in radian. [42] SEM Study. SEM study of doped and undoped TiO2 NPs was conducted with JEOL JSM-6460 scanning electron microscope to see the distribution of metal on the surface of TiO2 in doped species.

EDS Analysis. Energy dispersive spectroscopic (EDS) analysis was conducted with Oxford INCA X-sight 200 to perform the quantitative analysis of the TiO2 both in doped and undoped conditions.[42]

3.3 Experimental

Conclusions The photo degradation proceeds mainly by photo initiated oxidation reactions. Although photo oxidation mechanisms of poly-olefins are very similar, their weather resistance is different. The photo degradation of poly-olefins is strongly influenced by their morphology. Oxidation rate is roughly proportional to the amount of amorphous fraction present, which suggests that the oxidation of more crystalline polymer is slower than that of lower crystallinity. The effect of polymorphism on the degradation is also observed. In summary, three major functional groups are accumulated during degradation: ketones, carboxylic acids, and vinyl groups. During photolysis, ketones and vinyl groups increase linearly with time of exposure, whereas carboxylic acids accumulate exponentially.

If polyethylene is exposed for long periods outdoors, it is degraded by the concerted action of atmospheric oxygen and radiation at the UV end of the solar spectrum. The degradation leads to a deterioration in impact resistance and ultimate elongation, and possibly, to discoloration. Elongation of HDPE especially is reduced to almost zero.

Method 1. The effects of accelerated weathering conditions as seen experimentally can be bulleted in the following points:  The effect of UV exposure, whether in the field or in the artificial environments, was not significant up to 1000 h as far as the mechanical properties of the materials are concerned. Any surface effects would not compromise the mechanical integrity of the product. However, surface appearance may be an issue.  It is likely that the difference in results for field-aged and artificially aged materials was due to variability in the source materials. This was demonstrated by the molecular weight and viscosity analysis.  The field-aged material had a higher overall molecular weight in comparison with the new material. This increase could be due to degradative crosslinking. Method 2.

The analysis of photocatalytic degradation using TiO2

nanoparticles highlights the following points:  Doping of TiO2 NPs by Liquid Impregnation method alters its

characteristics such as particle size and surface morphology.  The effect of mix doping is midway between that of the doping effect by a single metal alone. This indicates that metal ratios can be adjusted to get a desired impact for a particular requirement.  This idea was implied and verified in the photo degradation of PE under UV and artificial light irradiation.  Photo degradation of PE-TiO2 films occurred at faster rate and was more complete than the simple photo degradation of pure PE films under UV and artificial light irradiation.  Overall degradation trend can be represented as o PE-doped TiO2 > PE-undoped TiO2 > simple PE.  Catalytic trend among the doped TiO2 NPs under UV irradiation can be represented as o Fe/Ag mix doped TiO2 > Ag doped TiO2 ∼= Fe doped TiO2.

It is an observation that development of this kind of composite polymer can lead to an environmental friendly polythene product.

4 . POLYETHYLENE STARCH BLENDS

Although

several

microorganisms

facilitate

the

biodegradation

of

hydrocarbon , the biodegradation polyethylene is somewhat slow. Studies show that biodegradation of polyethylene is extremely slow, the rate of conversion was 2% per year . This suggest that the PE films could not be degraded significantly . This is because the degradation mechanism for linear hydrocarbon involves the oxidation of terminal methyl group to carboxylic acid group and the degradation of resulting fatty acid by stepwise B-oxidation ( two carbon at same time ). In high molar mass linear polyethylene , there are only two methyl groups , which are not located in the bulk of hydrophobic medium and there are not readily accessible to the micro-organisms . On the other hand , if ester groups could be introduced into backbone of polyethylene , it would become biodegradable .

Incorporation of starch into a polyolefin matrix was first proposed as an effective means of accelerating the deterioration of plastics under biotic environment exposure conditions .The inclusion of starch , a readily available biodegradable polymer , into the synthetic polymer believed to

result in rapid enzymatic hydrolysis of starch under biotic exposure conditions , leading to void-containing matrix , the reduced mechanical integrity of the ensuing void -containing matrix leads to facile deterioration and perhaps even promotes subsequent biodegradation of synthetic polymer , due to increased surface area available for interaction with the microorganisms. [43]

Physical incorporation of granular starch derivatives as functional additives and fillers into poly-olefins during polymer processing example :extrusion ,injection molding or film blowing is well known and had been used for many years . In the absence of additives , films made from starch or amylose are brittle and sensitive to water .

A technique for blending gelatinized starch and poly( ethylene-co-acrylic acid ) ( EAA ) to produce flexible blown films that contain high levels of starch was introduced . Ammonia and 2% moisture were essential ingredients for obtaining uniform films . The inclusion of polyethylene in film formulation improved the economics and increased the UV stability and rate of biodegradability of films . The films have potential applications as agriculture mulches and packaging , especially where bio-degradation is important.

The biodegradation of PE films containing ( by weight ) 40% gelatinized cornstarch and 15% EAA was studied in variety of aqueous environments . In the laboratory , some amylotic bacteria degraded starch in the film more rapidly and to a greater extent than others during 60 day incubation . Loss of starch was accompanied by a concomitant decrease in film tensile strength , facilitating the disintegration of film from mechanical stresses. Films composed of PE-EAA and PE-EAA-starch were exposed to three fresh water ecosystems for 90days . The surfaces of all 3 films were rapidly coated by a complex biofilm containing bacteria , algae , fungi , protozoa and diatoms . These data suggest that microbial starch degradation , mechanical disintegration , bio-disintegration are all factors that influence the environmental fate of starch containing plastics .[44]

The effect of marine environment on un-stabilized PE-starch composites ,with or without a metal catalyst ( MC ) and auto-oxidants (AO) has been studied . Starch tends to absorb water. For PE-starch composites containing metal catalyst and anti-oxidants exposed under plain sea-water , there appears to be practically no microbial activity as indicated by no surface erosion and no change in tensile properties . However , the decrease in molecular weight on the surface indicates ongoing chemical degradation due to presence of MC and AO .[45]

The use of starch additives in PE and EAA has been applied commercially in manufacture of bags. Compatibility problems are minimized by use of silane coupling agents. In theory the polymers in contact with soil or water are

attacked by microbes which ingest the starch additives in the polymer matrix. This leaves behind a porous sponge like structure with a high interfacial area and a low structure integrity. Thus the starch component is ingested first , followed by enzymatic attack on the polymer. This attack consists of many consecutive enzymatic reactions . In each cycle of attack , one base unit , usually an acetic acid molecule , is split off . Thus the average molecular weight tends to decrease relatively slowly .

During irradiation , the tensile strength decreased to almost the same extent for all three materials - a mean value of 70% of the initial value was obtained after irradiation . The elongation at break for pure LDPE decreased during irradiation from 650 to 500% . After irradiation the decrease was considerable - a mean value of 60% being recorded for the elongation at break LDPE .[46]

5 CHARACTERISATION OF DEGRADED POLYTHENE The level of polythene degradation can be determined by the various methods as well as analytical techniques and the detail is given. At topographical level, the Scanning Electron Microscopy (SEM) are being used to see the level of scission and attachment of the microbes on the surface of the polythene before and after the microbial attack [47]. The microdestruction of the small samples is widely analyzed by an important tool such as Fourier Transform Infrared spectroscopy (FT-IR), and due to the recent up-gradation of this instrument the map of the identified compounds on the surface of the sample can be documented via collection of large number of FT-IR spectra [47]. To measure the physical changes of the polythene after the microbial attack various parameters are usually used to determine the weight loss, percentage of elongation and change in tensile strength .The products from polythene degradation are also characterized using various techniques such as Thin Layer Chromatography (TLC), High Performance Liquid Chromatography (HPLC) and Gas ChromatographyMass Spectrometry (GC-MS).

5.1 OXIDATIVE DEGRADATION OF HDPE It has long been known that PE materials are susceptible to oxidative degradation in certain environments. Wholesale oxidative degradation of PE leads to a reduction in the molecular weight of the polymer with a consequent loss of mechanical properties .Oxidized PE material eventually can become so degraded that it will respond to an applied stress in a very brittle fashion with an elongation to break of only a few percent - as opposed to PE material in its normal form where the elongation to break can be from

600 – 1000%. The steps that are normally taken to prevent oxidative degradation from occurring in PE materials include adding various chemical stabilizers, antioxidants (AO's) of various types, to the PE resin. Generally carbon black is added at 2 – 3 weight percent to protect the material against oxidative degradation from exposure to UV radiation, processing stabilizers (antioxidants to prevent oxidation during pipe extrusion at 350o – 400oF) and other antioxidants intended to provide protection against oxidation caused by long-term exposure to water containing dissolved air (oxygen) and other oxidative agents such as water disinfectants.

5.2 FOURIER TRANSFORM INFRARED SPECTROSCOPY (FTIR) Micro-FTIR spectroscopy and ATR-FTIR spectroscopy are utilized to measure the degree of oxidation on the PE film samples. Oxidation of PE results in the formation of carbonyl groups onto the PE molecules. These groups have characteristic infrared absorption frequencies. Among these groups, the strongest absorption peak is observed at about 1710 to 1720 cm1. Weaker peaks are seen at about 1735 and 1775 cm-1. The stabilizer compounded into the PE has a carbonyl group with a characteristic infrared absorption peak at 1740 cm-1. When oxidation occurs, a ketone carbonyl peak near 1710 cm-1 to 1720 cm-1 is formed, which progressively increases in intensity as the degree of polymer oxidation increases. The carbonyl index (C.I.) is defined as the ratio of this carbonyl absorbance to that of a polymer absorption band at 1465 cm-1. The use of this ratio

compensates for any differences in sample thickness and serves as an internal standard. ATR-FTIR is used to measure the C.I. right at the inner surface of the sample. The sample is clamped against the diamond crystal and then analyzed. The depth of penetration for this method is very small, on the order of 5 microns, yielding another infrared spectra measurement of oxidation at the inner surface of the sample. In past experience in characterizing oxidation of polyolefins, C.I. values less than 0.02 were not considered as significantly oxidized.[48]

OXIDATIVE INDUCTION TIME (OIT)

This test is a relative measure of the amount of antioxidant still remaining in the film after extrusion and service. OIT was measured at both the inner surface and at the interior core of each pipe sample. Most of the samples tested exhibited very low OIT values at the inner surface, which correlates well with the FTIR and bend back test results. It is not uncommon for the inner surface OIT value to be low in polyolefin films that has been exposed to an oxidizing environment for an extended period. [48]

5.3 CHARACTERIZATION OF POLYMERS USING TGA Thermogravimetric analysis (TGA) is one of the members of the family of thermal analysis techniques use to characterize a wide variety of materials. TGA provides complimentary and supplementary characterization information to the most commonly used thermal technique, DSC. TGA measures the amount and rate (velocity) of change in the mass of a sample as a function of temperature or time in a controlled atmosphere. The measurements are used primarily to determine the thermal and/or oxidative stabilities of materials as well as their compositional properties. The technique can analyze materials that exhibit either mass loss or gain due to decomposition, oxidation or loss of volatiles (such as moisture). It is especially useful for the study of polymeric materials, including thermoplastics, thermosets, elastomers, composites, films, fibers, coatings and paints. TGA measurements provide valuable information that can be used to select materials for certain end-use applications, predict product performance and improve product quality. The technique is particularly useful for the following types of measurements: • Compositional analysis of multi-component materials or blends • Thermal stabilities • Oxidative stabilities • Estimation of product lifetimes • Decomposition kinetics • Effects of reactive atmospheres on materials • Filler content of materials

• Moisture and volatiles content

TGA decomposition information can be used to predict the useful product lifetimes of some polymeric materials, such as the coatings for electrical or telecommunication cables. The sample is heated at three or more different heating rates. The use of the different heating changes the time scale of the decomposition event. The faster the applied heating rate, the higher the given decomposition temperature becomes. This approach establishes a link between time and temperature for the polymer decomposition and this information can be used to model the decomposition kinetics. Shown in graph 1 are the TGA results generated on a sample of polyethylene at heating rates ranging from 1 to 40 C/min. As the heating rate is increased, the onset of decomposition is moved to higher temperatures. The kinetics analysis provided by the software provides valuable predictive information on polymeric materials, including lifetime estimations. Displayed in graph 2 are the isoconversion curves, which presents the time to achieve a particular level of conversion as a function of temperature. These are particularly useful for product lifetime assessments. If the desired level of critical conversion is known, then the time to achieve this critical level at a particular operating or end use temperature can be predicted.[49]

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