1. EBP 200/3 POLYMER DEGRADATION DR AZURA A.RASHID Room 2.19 School of Materials And Mineral Resources Engineering, Universiti Sains Malaysia, 14300 Nibong Tebal, P. Pinang Malaysia

2. Polymers are susceptible to attack by a wide variety of naturally occurring and man made agent. Polymer degradation is a change in the properties – tensile strength, colour, shape, etc - of a polymer or polymer based product under the influence of one or more environmental factors such as heat, light or chemicals. These changes may be undesirable, such as changes during use, or desirable, as in biodegradation or deliberately lowering the molecular weight of a polymer. Such changes occur primarily because of the effect of these factors on the chemical composition of the polymer. In a finished product such a change is to be prevented or delayed. However the degradation process can be useful from the view points of understanding the structure of a polymer or recycling/reusing the polymer waste to prevent or reduce environmental pollution. Introduction

3. Polymers molecules are very large on the molecular scale which derive their unique and useful properties from their size. Today there a primarily six commodity polymers in use, namely PE (polyetylene), PP (polypropylene), PVC (Polyvinyl chloride), PET (Polyethylene terephthalate), PS (polystyrene) and PC (polycarbonate). These make up nearly 98% of all polymers and plastics encountered in daily life. Each of these polymers has its own characteristic modes of degradation and resistances to heat, light and chemicals. For example,  polyethylene usually degrades by random scission - that is by a random breakage of the linkages (bonds) that hold the atoms of the polymer together.  When this polymer is heated above 450 C it becomes a complex mixture of molecules of various sizes which resemble gasoline.  Other polymers - like polyalphamethylstyrene - undergo 'specific' chain scission with breakage occurring only at the ends. they literally unzip or depolymerize to become the constituent monomers

4. Plastics are frequently preferred to metals for use in structural applications because of their resistance to corrosion However there are number of ways in which polymeric materials may degrade over a period of time. These are as follows : 1. Oxidative degradation 2. Radiation degradation 3. Mechanical degradation 4. Microbiological degradation 5. Chemical attack Classification of polymer degradation

5.  Oxidative degradation is an autocatalytic process of attack on the hydrogen atoms to form hydroperoxides  The stability of polymers is inversely proportional to the number of hydrogen atoms of the carbon atoms present in the polymer chain.  The degradation is catalyzed by heavy metals such as copper.  The degradative reaction can be inhibited by the presence of hydrogen-donating compounds such as hindered phenols or by peroxide decomposers.  Mixtures of different types of stabilisers appear to be synergistic in their effect.  Natural rubbers or other elastomers can be protected against attack by ozone by the addition of microcrystalline wax. More permanent stabilisation is obtain by addition of derivatives of the phenol p-phenylediamene. Oxidative degradation

6.  Engineering plastics are commonly used out doors and it is recognised that sunlight with a wavelength less than 290 nm is responsible for the photo-oxidation of polymer surfaces.  Long wavelengths have insufficient energy to break covalent bonds in organic compounds but UV wavelengths will selectively excite electrons in the polymer chain.  The increases the vibration & rotational energy of covalent bonds  leading to degradation by bond cleavage  Typical types of degradation include  yellowing, surface embrittlement, loss of tensile or impact strength & cracking  The process usually proceeds from the surface layers & weakened surface can act as a site for crack nucleation.  Such cracks may be propagate into un-degraded materials beneath & causing a failure Radiation degradation

7.  Mechanical degradation may occur when stress is imposed on a polymer through machining, stretching or ultrasonic.  Bond cleavage may occur forming macro radicals which can add oxygen and produce compounds which will undergo degradation reactions Mechanical degradation

8.  Most widely used plastic materials are inert in the presence of microbes & this stability is important in applications.  Only short term performance is required in certain situations before the materials is discarded  e.g in fast food packaging  It is considered an advantage if the discarded plastics degrades when exposes to microbes  It is a challenge to a scientist to develop plastics that poses the requisite properties for their anticipated service life but capable of degrading in safe manner  We may define biodegradable plastics as those whose physical integrity is lost upon contact with microbial & invertebrate activity in natural environment within a limited period of time.  In the limit  conversion of the material to carbon dioxide, water, inorganic salts, microbial cellular components & miscellaneous by products. Microbiological degradation

9. Biodegradation is the process by which organic substances are broken down by living organisms. The term is often used in relation to ecology, waste management, environmental remediation (bioremediation) and to plastic materials, due to their long life span. Organic material can be degraded aerobically (with oxygen) or anaerobically ( without oxygen). A term related to biodegradation is biomineralisation in which organic matter is converted into minerals. Biodegradable matter is generally organic material such as plant and animal matter and other substances originating from living organisms, or artificial materials that are similar enough to plant and animal matter to be put to use by microorganisms Biodegradation

10.  Plastics are susceptible to environmental failure when exposed to certain organic chemicals and this limits their use in many applications.  Diffusion of species into plastics is common and adverse effects can arise which are not chemical in nature  In most interactions of water with structural plastics no chemical bonds are altered but damage known as physical corrosion may occur.  Absorbed moisture has shown to act as plasticizer  reducing the Tg and strength of polymer  These effects are essentially reversible but the irreversible effects may encounter such as microcracking or crazing as well as chemical degradation of the polymer structure Chemical attack

11.  Organic liquids Plastics are susceptible to environmental failure when exposed to certain organic chemicals and this limits their use in many applications.  The most serious problems arises when a material is exposed to aggressive fluids when it is under stress.  Organic liquids may interact both chemically and physically with polymer.  Chemical interactions involve a decrease in the molecular weight by chain breakage  may cause reduction in mechanical properties such as tensile strength, stiffness and fracture toughness.  Swelling of the polymer results in high stresses which can cause crazing or cracking. Fracture arising from physical effects may be observed in many glassy plastics such as PS and PC. Chemical attack

12. There are a number of ways in which polymeric materials may degrade over a period of time. Explain two of any of polymer degradation below: 1. Oxidative degradation 2. Radiation degradation 3. Mechanical degradation 4. Microbiological degradation Example of the exams question

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