Chapter 1 INTRODUCTION

1.1. Importance of plastics in modern society The Story of how the human race gained mastery over metals is well known and has been enshrined in terms of “Bronze age” and “Iron age”. Polymers are altogether more complex than metals, and the history of evolution of our familiarity with polymers, from the first wooden club to the latest carbon baseball bat, is still largely untold. Polymers have almost replaced materials such as metal, , wood, , fiber, ceramics etc. in packaging, automobiles, building construction, biomedical fields, electronics, electrical equipments, appliances, furniture, pipes and heavy industrial equipments. In a nutshell, from agriculture to transport and from aerospace to , the use of plastics has become an integral part of our modern daily living. [1] Polymers can be classified as either natural or man-made ‘ macromolecules ’ which are composed of smaller repeating units. Polymers, which are bio-synthesized in plants, animals, and micro and macro organisms, are called Natural polymers . Examples are polysaccharides, proteins, fats, nucleic acids and natural rubbers. The most easily recognized natural polymer is Cellulose , the most abundant organic polymer on earth. Polymers, which are man-made, are called synthetic polymers . Polyolefins are the most important group of synthetic polymers. Synthetic polymers are ubiquitous in our world finding diverse applications in many fields because of their useful properties (mechanical and physical properties) and inexpensive and therefore, contribute to enhancement of comfort and quality of life in our modern industrial society. One of the reasons for great popularity of plastics is due to tremendous range of properties exhibited by them because of their ease of processing. The properties of synthetic polymers like durability, resistance to weathering and photodegradation as well as biological attack and hydrophobicity, have contributed to their utility in different applications. Non-biodegradability of the polymers makes them to be applicable to diverse applications. Perhaps the most remarkable aspect of polymers derived from natural products is that such environmentally unstable materials should be the basis of environmentally durable industrial products. Rubber tyres before fabrication are among the least environmentally stable of all polymers and yet automotive tyres survive for many years in the outdoor environment long after use. The key applications of so called non-biodegradable thermoplastics are as follows:

A. In packaging: The major use of synthetic polymers has been as replacements for more traditional materials, particularly in packaging. Polymers are light in weight and yet have very good barrier properties against water and water-borne organisms. Compared with glass, they have much superior impact resistance and resilience, resulting in reduced product loss during transport. They protect not only perishable commodities from the environment but also the environment from corrosive or toxic chemicals. The production processes for plastics from crude oil are much less labor and energy intensive than traditional materials [2-3] (Table 1).

1 Table 1: Energy requirements for the production of materials used in packaging

Material Energy requirement kWhkg -1 Aluminium 74.1 Steel 13.9 Glass 7.9 Paper 7.1 Plastic 3.1

The fabrication of plastics by injection is also less energy intensive than the fabrication of traditional materials. The polymer is converted into useful product in a single rapid and repetitive process that does away with intermediate forming and joining procedure. When these factors are combined with lower density of polymers, the energy requirements for similar are found to be lower than for traditional materials. To compete with plastics, even if no energy were involved in the transport and cleansing of returnable , these returnable bottles would have to be recycled about twenty times. Further energy requirements for similar beverage containers in comparison of plastics are shown in Table 2.

Table 2. Energy requirements for similar beverage containers

Container Energy usage Weight (pounds) per (kWh) Aluminium can 3.00 1.41 Returnable soft drink 2.40 10.60 Returnable glass 2.00 8.83 Steel can 0.70 1.76 Paper 0.18 0.92 Plastic beverage container 0.11 1.23

B. In transportation : Polymers are light in weight compared with metals and ceramics. The modern plastic milk container is only a fraction of the weight of a similar bottle made from glass and this has a significant influence in transport costs. It is a popular belief that in returnable glass bottles is ecologically preferable to single-trip plastic containers. Non-biodegradability makes the synthetic polymers suitable to this purpose. Due to their long-term stability, plastics are also increasingly replacing traditional materials in automotive components, for example in motor vehicles, aircraft and boats. All the plastics materials have densities in the region of one, whereas even the lightest of the commonly used metals, aluminium, is considerably heavier. It has been estimated that over the past decade 200-300 kg of traditional materials have been replaced by 100 kg of plastics in the average car with consequent reduction of fuel consumption by about 750 liters over the useful life of the car (~ 150,000 km). In addition, since much more energy is used in the production of metals and glass than manufacture of the common plastics, it follows that the more plastics can be used to replace metals and glass in vehicles, the less fuel will be used in transport.

2 C. In agriculture: Plastics have changed the face of rural environment by their wide range use in agriculture, and irrigation. Plastics have largely replaced glass in greenhouses and tunnels. They are much cheaper than glass in greenhouses but they have to be replaced more frequently. Photo-degradable polymers would be of not much use as longevity of usage is a major criteria in these application. D. At home and office: It is now taken for granted that for equipment operating at ambient temperatures, plastics are the modern materials of choice for items such as food mixers, vacuum cleaners, hair driers, television consoles, computers, word processors and other office equipments. E. In paintings and surface : Naturally occurring 'drying oils' based on polyunsaturated fatty acid esters have been used for centuries to protect metals from corrosion and wood from biological action. By far, the most important of the ‘environmentally compliant’ technologies to emerge were crosslinked coatings and based on oligomeric acrylate monomers. These could be crosslinked rapidly by UV light in the presence of photosensitizers. F. In building and civil engineering: A very visible and valuable contribution of polymers in the building industry is the replacement of wood in window frames and outdoor cladding. The advantage of plastics is their resistance to and this characteristic, coupled with reduced decoration costs, makes them the materials of choice as replacements for wood and iron. Pigmented rigid PVC (unplasticized) is the most widely used polymeric material for outdoors use where they are stabilized against the effects of weather by the use of synergistic stabilizers. G. In public utilities: Polymers have in recent years assumed an increasingly important role in underground applications. These include piping, ducting and underground chambers where previously steel or concrete were used. New uses include impermeable membranes in the contaminant of water in reservoirs and of effluents in sanitary landfill, in grids and nets in soil stabilization and in underground electricity cables. These sub-soil uses of polymeric materials make use of their resistance to biodegradation. The underground transport of oil, water and gas by pipeline is an ever-increasing aspect of utility supply to industrial and domestic destinations. Iron and steel were the main materials for pipe construction 50 years ago and but they fail due to corrosion. They are now replaced by plastics that do not corrode. Particularly favoured are HDPE, LLDPE, PP and to a lesser extent rigid PVC. H. In biology and medicine: The biological inertness and lightness of polymers make them very attractive in potential biomedical applications. Typical examples of this are dental applications. Replacement plastic prostheses are now state of the art. However, in this application the durability and biocompatibility of the polymer under the aggressive conditions to which they are subjected in use is a basic design parameter. Whatever the applications, there is often a natural concern regarding the durability of polymeric materials partly because of their relative newness and their useful life-time, maintenance and replacement. The Figures 1-2 illustrate the importance/awareness of the plastics in public life.

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Increase in Knowledge Quotient (KQ) 1970’s 1980’s 1990’ s

•Simple household •Technical article products •Basic design •Packaging •Intricate designs •No specific need requirements •Functional need •Low value •Specific designs •Higher value •Low KQ •Better value •Higher KQ •High KQ

Figure 1. Polymer articles

4 Higher performance & productivity

• Performance enhancement by means of – Alloys • Styrene-olefin / acrylic-olefin alloys for automotive, industrial/ consumer applications

– Nano-Technology Protective clothing – Fillers

Tough Industrial Films Filtration Systems Aseptic packaging

Figure 2. High performance polymeric articles

1.2. Plastic production Plastics are a product of this century. Originally, they were mimicking and replacing natural products but today they are largely synthetic materials and derived from extremely inexpensive but non-renewable resources fossil fuels (e.g. crude oil) [Figure 3].

oil fraction cracking crude oil oil refinery steam cracking separation products simple monomers e.g. ethylene, propylene,butadiene

compounding plastics consumer plastics and polymerization products processing

Figure 3. Actual production of plastics from crude oil

5 Less than 4% of the petroleum resources are used for polymer production, with the major portion being utilized as fuel. There are three ways by which polymers can be produced synthetically from simple starting materials (monomers). They are addition polymerization, condensation and rearrangement polymerization. The size and structure of the polymer molecule determines the properties of the plastic material. In their basic form, plastics are produced as powders or granules. The application of heat and pressure to these materials gives the final plastic product for specific end use applications. Although Indian plastics industry's foundation was laid during the fifties with the creation of facilities for the manufacture of PS and LDPE based on industrial alcohol, the industry flowered only after the establishment of petrochemical complexes by NOCIL., Bombay and I.P.C.L. Baroda in late sixties and mid seventies. The expansion of the existing petrochemical complexes and new gas and naptha based complexes at Nagothane (Maharashtra), Kawas (Gujarat), Auriya (U.P) and Haldia (West Bengal) would make available still large volume of existing products and new generation of thermoplastics such as new types of PP-homopolymers, PP-copolymers and linear low density . With a strong backing of supporting plastic processing, machine and tool industry and availability of variety of plastic raw materials, the Indian Plastics Industry has registered an impressive growth at a rate higher than plastics industry worldwide. The outlook for the Indian plastics industry is optimistic and the industry can certainly forward to a very promising future in the years to come, particularly since plastics have already established themselves firmly in the field of packaging, house- wares, consumer durables and are making rapid growth in the fields of electronics, automobiles and communication. High volume-low value commodity plastics such as LDPE, LLDPE, HDPE, PP, PS, PVC etc. have a substantial production capacity in the country today. The medium volume-medium priced engineering plastics such as , polyacetals modified poly(phenyl oxide) etc. are gradually entering the Indian market, even though entire quantity is imported. The low volume-high value speciality engineering polymers such as poly(phenylene sulphide), polysulfone and PEEK etc. have been marginally used for specific applications in space and defence but have not yet entered major markets. The actual production and demand of thermoplastics in India is given in Table 3.

6 Table 3. Current and projected capacities of thermoplastics in India Previous Previous Projected Projected Polymer capacity (TPA) capacities (TPA) capacities (TPA) capacities (TPA) 2000-01 2002-03 2005-2006 2009-2010 LDPE 240,000 300,000 520,000 970,000 LLDPE / HDPE 910,000 1,141,000 2,030,000 4,121,000 PP 675,000 875,000 1,754,000 3,820,000 PS/HIPS/ABS 175,000 245,000 384,000 790,000 PVC 685,000 815,000 1,190,000 2,100,000 Polyamide 11,000 15,000 25,000 75,000 PC 5,000 7,000 10,000 25,000 Modified PPO 1,500 2,000 3,500 5,000 Polyacetals 2,000 3,000 5,000 21,000 PET/PBT 13,000 25,000 40,000 125,000 TOTAL 2,717,500 3,428,000 5961,500 12,052,000

With high growth, high revenue earning potential and materials for the masses, the plastics have registered an exponential growth. The plastics industries have out paced all other industries in India and served the needs of the common man at lowest cost. Thus, there are substantial opportunities for the rapid growth of plastics industry in India in the coming years.

1.3. Environmental impact of plastics During the latter half of 20 th century, the production of synthetic plastics and fibres has grown so that the total volume of plastics produced worldwide now exceeds that of steel. Synthetic fibres and plastics have the lowest energy costs of nearly all comparable materials and cause less environmental pollution in their production and fabrication. They are easily recycled when not contaminated with foreign materials and can be manufactured in photo- or bio-degradable modifications tailored to highly -prone applications. In some cases, degradation may be a desirable goal. For example, it would be advantageous to be able to design plastic bottles and packaging film so that they rapidly degrade into environmentally safe by-products (e.g. carbon dioxide, water and biomass) that will occupy less volume in a landfill.

7 1.4. Definitions The definition of biodegradation is not always clear and is open to a large diversity of interpretations. The term does not give any information about the specific environment where the biodegradation is supposed to happen, the rate which will regulate the process and extent of biodegradation. Here are the definitions of some key words according to the ASTM D20-96 : [4] Degradable Plastic: A plastic designed to undergo a significant change in its chemical structure under specific environmental conditions resulting in a loss of some properties that may vary as measured by standard test methods appropriate to the plastic and its applications in a period of time. : A degradable plastic in which the degradation results from the action of naturally occurring microorganisms such as bacteria, fungi and algae. Photodegradable Plastic: A degradable plastic in which the degradation results from the action of natural daylight. Thermal degradable Plastic: A degradable plastic in which the degradation results from the action of heat. Compostable Plastics: A plastic that undergoes degradation by biological processes during composting to yield carbon dioxide, water, inorganic compounds and biomass at a rate consistent with other compostable materials and leaves no visible, distinguishable or toxic residue (ASTM D 6400-99). Although a number of standard committees have sought to produce definitions of biodegradable plastics, each gives its own definition of biodegradable polymer. In conclusion it can be said that the intrinsic capacity of a material to be degraded by the action of microorganism is called biodegradability. More specifically there are two definitions depending on the final fate of the polymer in the environment. Compost: Compost is an organic soil conditioner obtained by biodegradation of a mixture consisting principally of various vegetable residues, occasionally with other organic material and having a limited mineral content. Compost quality has to be defined by the relevant national standards. Compostability: Compostability is a property of a packaging to be biodegraded in a composting process. To claim compostability it must have been demonstrated that a packaging can be degraded in a composting system as can be shown by the standard methods. The end product must meet the relevant compost quality criteria. -Primary Biodegradability (Partial Biodegradability) is the alteration in the chemical structure of the material and loss of specific properties. -Ultimate Biodegradability (Total Biodegradability): The material is totally degraded by the action of microorganisms with the production of carbon dioxide (under aerobic conditions) and methane (under anaerobic conditions), water, few mineral salts and biomass. Disintegration is the falling apart into very small fragments of packaging or packaging material caused by environmental degradation mechanisms. Very often disintegration is misunderstood and is claimed as biodegradation, especially in the of polyolefins. Many blend compositions of polyolefins (especially with starch) disintegrate and do not biodegrade.

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1.5. Types and Mechanisms of degradation Degradation is an irreversible process, resembling the phenomenon metal corrosion. Almost all the commercial polymers degrade in air when exposed to sunlight as the energy of sunlight is sufficient to cause breaking of polymeric bonds. Table [4] shows the different types of degradation and their environmental factors which are responsible for different kind of polymer degradation. [5]

Table [4]. Environmental factors and types of degradation

Environmental factors Types of degradation Susceptible examples Light (UV, Visible) Photo-chemical degradation Polyacetals, polycarbonates X-, γ-rays, fast High – energy radiation induced Poly (methyl methacrylate), electrons polymer degradation. Polyisobutylene, Laser light (pulsed Ablative photo-degradation, - mode) involving photo-thermal and/or photochemical processes and laser flash photolysis. Electric field Electric aging - Plasma Corrosive degradation, etching - Microorganisms. Biodegradation or biological Polyurethanes, Polyethers, degradation , Co-polyesters, and polyamides. Enzymes in vivo, Bio-erosion Short-chain polymers, nitrogen- Complex attack: containing polymers, polyesters hydrolysis ionization Stress Forces Mechanical degradation, fatigue Rubbers Abrasive Forces Physical degradation, physical Polyurethane wear, environmental stress, cracking Ultrasound Ultrasonic degradation Chemicals Chemical All thermoplastics degradation/decomposition Heat Thermal Vinyl polymers degradation/decomposition Oxygen, Ozone Oxidative Polyisoprene, Polybutadiene degradation/decomposition Light and Oxygen Photo-oxidation Photo-sensitive polymers

1.5.1. General mechanism of degradation The Polymer degradation comprises of initiation, propagation and termination steps [6].

9 a.Initiation: The formation of the radical (R ·) is necessary for rapid polymer oxidation i.e. hydrogen abstraction:

hν/∆ RH R + H trace metals

Where RH: macromolecule/polymer This reaction may be initiated by physical factors, e.g., UV radiation, ionizing radiation, heat, ultrasonic, mechanical treatment etc., or by chemical factors such as catalyst, direct reaction with oxygen, singlet oxygen or ozone. The generated radical is called macroradical. b.Propagation: Propagation step is the actual degradation process where the degradative chain is prolonged by following steps: i. Formation of Polymer hydro-peroxides: The macroradicals generated during the initiation step can easily react with atmospheric oxygen molecule producing peroxy radicals

ROO R + O2

This peroxy radical can abstract hydrogen from another molecule to form polymer hydroperoxide while another macroradical will be generated.

ROO + RH ROOH + R

These reactions are sensitive to steric and polar effects of attacking radical and dependent on temperature too. These peroxy radicals are strongly stabilized by resonance and are relatively electrophilic species which abstract tertiary bonded hydrogen in preference to secondary or primary bonded. ii. Decomposition of Polymer Hydroperoxide: The polymer hydroperoxides are decomposed by light irradiation according to the following equations: hν/∆ ROOH R + OOH

hν/∆ ROOH RO + OH

The decomposition of hydroperoxides can also be induced by raising temperature and is promoted by metal catalyst. Free radical formed during the above mentioned reaction could also take part in radical induced decomposition of hydroperoxides.

10 - ROOH + Mn RO + HO + Mn+1

ROOH + R RO + ROH

ROOH + OH ROO + H2O

iii. Formation of Hydroxyl Group: The hydroxyl groups are formed in the reaction between alkoxy macroradical (RO’) and other polymer molecules:

RO + RH ROH

The hydroxyl groups may be formed along the polymer chain or on its end group. The carbonyl and aldehyde groups are formed from the scission of alkoxy radicals or by the decomposition of hydroperoxy radicals. The reaction between the polymer alkoxy radicals also produces a carbonyl and hydroxyl group by disproportionation:

R O R R O R

CH CH CH CH CH CH CH 2 + CH 2

R O R R OH R

CH C CH CH 2 CH CH CH CH 2 iv. Decomposition of Carbonyl group: The carbonyls groups thus formed are further decomposed by a Norrish type reactions I or II. Norrish type I: In the primary process, the bond between the carbonyl group and adjacent α- Carbon is unimolecularly cleaved to produce radicals: RCOR 1 RCO + R 1

Norrish type II: It is a non-radical intramolecular process. The abstraction of a H-atom from the γ-carbon results in its subsequent decomposition into an olefin and alcohol or an aldehyde. This reaction may also involve intramolecular β-hydrogen atom transfer:

11 O OH γ β α 1 CR R C C R2CH CR 2 CR 2 C R R2C 2 + 2 R1 c.Termination: The termination of the radical chain is due to reactions of free radicals with each other by combination, in which inactive products are formed:

ROO + ROO

ROO + RO ROO + R Photo-oxidized products R + R RO + R }

When the oxygen pressure is high, the termination reaction almost exclusively is followed by above equations. In the solid state, when sufficient oxygen concentration can not be maintained in the system, the termination reaction becomes significant. The polymer radicals may be coupled mutually and form crosslinks with polymer radicals. These processes are dependent on the chemical and physical structure of irradiated polymers.

1.5.2. Depolymerization Depolymerization is the reverse of the propagation step in chain polymerization in which monomer units are released successively from the main polymer chain or at weak links end. This process is termed as unzipping also. Most of the polymers give negligible yield of monomers during thermal degradation but some of them such as polytetrafluoroethylene and poly(methyl methacrylate) give close to quantitative yields. The depolymerization reactions are initiated thermally, photochemically or ultrasonically etc. When the degradation produces a chain fragment smaller than the zip length, the entire chain fragment depolymerizes completely to monomers before chain transfer occurs and no small fragment is left to decrease the average molecular weight of the residue. The depolymerization occurs mainly by the scission of main chain bonds followed by a reverse of polymerization, which results in a high molecular yield, or by the evaporation of low molecular weight of the residue. The depolymerization occurs mainly by the scission of main chain bonds followed by a reverse of polymerization, which results in a high monomer yield, or by the evaporation of low molecular weight homologues, which is produced by random degradation of the polymer. Depolymerization has a number of practical applications such as polymer modifications, synthesis or recovery of small molecules, waste treatment, and lower molecular weight oligomers. The polymers, which undergo thermal depolymerization without formation of non- volatile residues, are employed as temporary binders for propellants and explosives. Plastic waste is depolymerized to produce chemicals that can be used as feedstock for manufacturing other plastics.

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1.5.3. Thermal degradation Thermal degradation of polymers can be defined as the deterioration of properties of polymers by the action of heat. The thermal oxidative degradation occurs frequently during processing of polymers at elevated temperature. Since the solid polymers are complex micro-heterogeneous systems, their thermo-oxidation cannot take place uniformly in the bulk. The reason of this non-uniformity is that the polymer is composed of crystalline and amorphous regions which differ widely for oxygen penetrability and accessibility with respect of impurities, stabilizers and additives. Thermal oxidation initially generates free radical by thermal energy (heat) at high temperature and/or under simulated injection molding conditions with varying amount of oxygen. Shear in the extruder also causes rapid degradation. Thermal degradation leads to a very rapid drop in molecular weight by random scission processes and cross-linking. The yellowing, deepening to black extreme cases is observed in thermal degradation.

1.5.4. Photodegradation Polymeric materials undergo degradation on exposure to the normal environmental conditions. The sun emits radiation of all wavelengths from X-rays to the infrared (IR) light. Fortunately, the ozone layer of the outer atmosphere protects the earth’s surface from the high-energy radiation below 290nm. The deleterious effects of sunlight on polymeric materials has been associated to a complex set of reactions in which both the absorption of UV-light and the participation of oxygen are the main events. As a result, this process has been generally termed as photo-degradation or photo- oxidation.

1.5.5. Biodegradation Biodegradation refers to the process of chemical breakdown of a substance due to the action of living organisms. Generally, it is the action of microorganisms present in soil, water or special environments such as compost heaps that is responsible for biodegradation. These environments support large populations of different varieties of bacteria, fungi and actinomycetes species [7] The biological degradation is caused by: (a) Macro-organism (b) Micro-organism © Marine organism (both micro and macro) In case of macro organism such as insects, rodents and marine borers, the attack is physical in nature but the micro- organisms attack the polymeric materials chemically.

1.5.5.1. Enzymatic : Biodegradation refers to the chemical degradation caused by biochemical reactions, especially those catalyzed by enzymes produced by microorganisms and such reactions can occur by either random attack along the polymer chain backbone or by specific attack at the polymer chain ends. The microorganisms that can provide the enzymes for such processes include bacteria, fungi, algae and others. In addition, it should be noted that microorganisms can also secrete very reactive reagents into the environment that can cause degradation and some enzymes can catalyze the

13 reaction to secrete those reactive reagents in the environment that can degrade polymers ultimately.

1.5.5.2. Hydrolytic: Many polymers are susceptible to degradation due to the effect of water, particularly in acidic conditions. The polymers which are composed of the functional groups can be degraded by the hydrolytic method. The polymers, which are containing hetero-atoms for example polyurethanes, polyethers and polyesters are readily hydrolyzed biotically. Factors that influence the susceptibility of a given polymer to hydrolysis include water permeability and solubility, which are influenced by the chemical structure of the polymer and its physical state.

1.5.5.3. Photo : Photo-oxidation is the important process for biodegradation. The functionality in polymer backbone is one of the precursors for bio-assimilation. The photo-oxidation process can create the pathway for bio-assimilation by generating long chain fatty acid esters in the case of homopolymers like polyolefins. So photo-oxidation generates low molecular weight compounds by different photo-chemical decomposition steps causing chain scission of the polymer backbones.

1.5.5.4. Composting : A plastic that undergoes degradation by biological processes during composting to yield carbon dioxide, water, inorganic compounds and biomass, compostable materials and leaves visually no disintegrable or toxic residue. Compost is an organic soil conditioner obtained by biodegradation of a mixture consisting principally of various vegetable residues, occasionally with other organic material and having a limited mineral content.

1.5.6. Mechanical degradation Polymer degradation can also result from the application of stress such as high shear deformation of polymer solutions or melts. In the case of solids, stress-induced degradation may result from comminution (grinding, milling or crushing), machining, stretching, fatigue, tearing, abrasion or wear. This mechano-degradation is particularly severe for high-molecular-weight polymers, which exist in a highly entangled state. The result of stress-induced degradation is the generation of macroradicals originating from random chain rupture. In a process called mastication, natural rubber is softened by passing between spiked rollers, which also serve to disperse and other additives such as accelerators, vulcanizers and antioxidants.

1.5.7. High energy degradation The ionizing radiation is important environmental factor in biomedical applications, as its effect is useful in sterilization of medical equipments, controlled cross-linking and radiation-initiated graft polymerization. The high-energy penetrating radiation (energy range 10 9eV to 10 15 eV) in the environment impinges on the polymeric materials of the spacecraft, transverse with very little interaction and causes ionization of the material. These charged particles like electrons and protons transfer all their energy

14 by inducing ionization. Thus the spacecraft passes through the belt of charged particles trapped in the earth’s magnetic field. The trapped particles execute spiraling motion about the magnetic field line, bouncing back and forth between reflection points at high geomagnetic latitude and drift slowly in longitude. These particles are confined in a magnetic shell, which is specified by two parameters- the magnitude of the magnetic field and the distance from the center of the earth of the shell at the geomagnetic equator. The photochemistry ceases to around 90-100 nm and the region belongs to the radiation chemistry i.e., the chemistry induced by absorption of α,β,γ or X rays which causes ionization in polymers. Applications of high-energy radiations include nuclear power plants, particle accelerators, radiation equipments, sterilization systems (medical equipment, hospital clothing and food stuffs packaging) etc. The high-energy radiations to which to which polymers generally exposed are γ rays, X rays and electron beams. The pioneering work on high-energy radiations on polymers was carried out in early sixties. The more recent studies of radiation studies of radiation chemistry on elastomers and polymers were also reviewed [8]. The initial interaction of γ rays with the polymers yields high-energy ion by collision (similar to those of e- beam radiation) that in turn gives rise to a complex cascade (ejection of secondary electrons) such as:

γ rays + P P P , e excitation ionization (electron ejection)

The electron ejection during the ionization will loose its excess kinetic energy by initiation of further ionization and excitation until the electron finally reaches thermal energy. At this point, the neutralization of the electron with a cation or its attachment to a neutral molecule can occur:

- + e + P P

- - e + P P

The highly excited macromolecule may decompose further by one of the two mechanisms:

P . + P . P * . P + H .

15 Subsequently, at room temperature ion – electron recombination occurs immediately to give highly excited state (P •), but these may be trapped in the polymer matrix at low temperature ( ≤ - 100 0C). But the recombination of the two macro- radicals may also be facilitated in a solid matrix. The free radicals produced in above reactions lead to many chemical products associated with the post- irradiation reactions.

. . C + H C + H 2

(Hydrogen abstraction)

2 C. HC CH

(macroradical combination causing crosslinking)

C. (CH ) C. C. . (CH ) 2 n C 2 n

H H

(CH ) C C 2 n H H

These reactions are generalized in the absence of oxygen only but when in air and/or oxygen are in equilibrium with the exposed polymer, the oxygen reacts with carbon- centered radicals to give peroxy radicals, which produces hydro peroxide by abstracting hydrogen from the polymer matrix. The hydroperoxide is thermally and photolytically unstable and readily decomposes at ambient temperature slowly to radicals, leading to branching and oxidation products during storage even after irradiation.

. C C +O2 O2

. . C O2 + C HC OOH+ C

16 Thus, the polymeric materials are subjected to a variety of conditions in natural environment and the combination of two or more factors described above are more serious towards the degradation of polymers. The intensity of these factors also varies with the time of day, location and weather conditions.

Reference:

1. R.P.Singh, P.N.Thanki, S.S. Solanky and S.M.Desai., (2001). ‘ Photo-degradation and Stabilization of Polymers’ Chapter 1 in Advanced Functional Molecules and Polymers ed. H.S.Nalwa. 2. J.E.Guillet (ed.), (1973) Polymers and Ecological Problems , Plenum Press. 3. R.M Koerner (1989) Durability and Aging of Geosynthetics , Elsevier Applied Science. 4. R.W.Lenz (1993) Adv.Polym.Sci, 107;1 5. R.Mani. Ph.D Thesis , University of Pune, 1994. 6. R.P.Singh and S.Sivaram., (1991) Adv.Polym.Sci, 101;169 7. E.A.Paul. and F.E. Clark (1989) Soil microbiology and Biochemistry , Academic press New York). 8. G.C.A. Bohm, J.O. Tveerkrem, (1982) Rubber Chem. and Technol . 55; 669.

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