polymers can degrade by exposure to high temperature shear action oxygen, ozone and chemicals...

Polymers can degrade by exposure to high temperature Shear action Oxygen, ozone and chemicals Electromagnetic ( , UV) Ultrasonic radiation Moisture Thermal Degradation Mechanical Degradation Chemical Degradation Light induced Degradation Hydrolysis Degradation of polymers

Figure. Electron micrograph of the surface of a HDPE beer crate after nine years of use and exposure to weathering. Deterioration of plastics to normal environmental conditions is called WHEATHERING. Often, multiple exposures, such as a combination of moisture heat or oxygen and light can result in accelerated deterioration. POLYMER moisture UV Heat Shear Ozone, oxygen

Thermal Degradation Depending upon the presence of oxygen, temperature and structure of polymer, degradation and/or oxidation reactions will occur. Theoretical point of view most commercial polymer systems should be relatively stable above their melting point in the absence of oxygen. It is interesting to note that saturated hydrocarbons are much more stable then polyethylene (PE) in the absence of oxygen as are chloroalkanes when compared with PVC. In some cases this temperature difference may be as high as 200 o C. There are mainly two reasons for this difference in behavior; - The first of which is simply that polymers by virtue of long chain nature are able to breakdown into smaller molecular fragments i.e. monomer formation via unzipping reactions

The second is that commercial polymer structures are more complex than their generic molecular formula indicates.

They may contain various structural irregularities, branches, unsaturated structures, carbonyl and hydroperoxide groups which will act as initiation sites for degradation to occur. For example for PVC, Impurities; Generally, transition Metals (from catalyst residue or other sources) The sequence of efficiency of metal ions to enhance degradation depends on its valence state and the type of its ligand, but may be postulated as follows: Cu > Mn > Fe > Cr > Co > Ni

The average dissociation energy of bonds forming the structure of a macromolecule appears thus to be a first criterion for estimating the thermal stability of a given polymer. The fraction of bonds that reaches the energy equal to dissociation energy D is determined by the Boltzmans factor Exp(-D/RT) where T stands for absolute temperature and R for universal gas constant. This may be exemplified as follows: the temperature at which in one mole of CC bonds at least one is dissociated into radicals is 486 o C, while in one mole of OO bonds it is only 30 o C.

Weak points/ links It is possible to emphasize chain scission by working at low temperature at which the evolution of volatile products is very slow. If chain scission occurs in polymer molecule in the absence of volatilization, then P t = P o (s+1) (1) in which P o and P t are the chain lengths of original polymer and after time at which s scissions have occurred on average per molecule. Thus S=(P o /P t )-1 (2) and the fraction of bonds broken, a, is given by equation (3) = s/P o = 1/P t -1/P o (3) If chain scission is random, that is, every interunit bond in every molecule is equally liable to break then = kt In which k is the rate constant for chain scission. Thus for purely random scission a plot of a against t should be linear and pass through the origin. On the other hands, if molecules contain some weak links in the molecules which break more rapidly at the beginning of reaction then = +kt (4) In which is the fraction of weak links in the molecules Figure 2.6. shows that PS obey equation 4 and does indeed incorporate weak links.

The differences can be accounted for in terms of known mechanisms of degradation, the lower temperature peak in radical polymer being the result of degradation through the unsaturated chain ends which are absent in the ionic polymer. Termination may occur by interaction of pair radicals in polymerization. Thus proportion of molecules have unsaturated chain terminal structures. The bond indicated is weakened by about 80 KJ which is resonance stabilization energy of the ally radical which would be formed by its scission. Initiation by this pathway then allows a limited amount of degradation at lower temperatures in radical-initiated polymers.

Depolymerization vs transfer reactions Thermal analysis shows that polystyrene degrades thermally in single step that monomeric styrene (approx. 40%) is the volatile product. A large cold ring fraction consists of decreasing amount dimer, trimer,tetramer and pentamer (oligomers).

- methylstyrene has no -hydrogen, therefore, depolimerize into its monomer completely. Table 2.1. clearly show that it is -hydrogen atom which is involved in transfer process

Depolymerization vs ester decomposition

Ester decompositon only becomes important when the monomer unit incorporates at least five hydrogen atoms on the - carbon and depolymerization is quantitative when there are at most one or two -hydrogen atoms. If a significant proportion of ester groups destroyed during early stage of heating then the residual methacrylic acid units (or methacrylic anhydride units formed by elimination of water) block unzipping process and thus inhibit formation of monomer. If the radical depolymerization reaction can be initiated at a lower temperature than ester decomposition even in poly(tert-butyl methacrylate) is replaced by quantitative production of monomer.

Poly(vinyl acetate) - non radical processes Ester decomposition also occurs in poly(vinylesters) but in this case carboxylic acids is liberated and olefinic double bonds appear in the polymer chain backbone. The - hydrogen atom is effectively interacting as a proton with oxygen atom, so that the reaction should be facilitated electron attracting group in vicinity.. The electron attracting properties of the carbon-carbon double bond causes the reaction in PVAc to pass from unit to unit along the chain by reaction the ultimate effect being to produce extended conjugation and colour.

Poly(ethylene terephthalate )

General Degradation Mechanism Chain scission can occur by one of three mechanism. These include 1-Random degradation where the chain broken at random sites. 2- Depolymerization where monomer units are released at an active chain end Terminal initiation Random initiation Depropagation Transfer Termination by disproportionation or combination

Characterization techniques for polymer degradation & stabilization

Understand the thermal degradation mechanism Frequently used techniques 1- Thermogravimetry (TG) Thermogravimetry (TG) is a thermal analysis method in which the mass change of a sample subjected to a controlled temperature programme is measured. The use of isothermal and dynamic TG for the determination of kinetic parameters in polymeric materials has raised broad interest during recent years Although TG cannot be used to elucidate a clear mechanism of thermal degradation, dynamic TG has frequently been used to study the overall thermal degradation kinetics of polymers because it gives reliable information on the activation energy, the exponential factor and the overall reaction order.

To establish a criterion for evaluating resin decomposition, the temperatures at which 10% decomposition [10% decomposition temperature (DT)] and 50% decomposition (50% DT) had occurred were noted. Temperatures were also recorded at which maximum rates of decomposition occurred. From Table 1.1, it can be seen that, based upon resin types 1, 2, 3, 4 and 7, the thermal stability of the resins decreases with increasing molecular weight of the meta-substituted phenol, i.e., stability decreases in the order phenol > m-cresol > m- isopropylphenol > cardanol > m-tert- butylphenol. The anomalous position of the m- tert-butylphenol indicates that branching of the side chain has a significant effect, particularly if branching occurs from the -alkyl carbon atom which is attached to the phenolic nucleus.

Evolved Gas Analysis (EGA) Thermal Volatilisation Analysis (TVA) In EGA, the sample is heated at a controlled rate under controlled conditions and the weight changes monitored (i.e., TGA). Reaction products are simultaneously led into a suitable instrument for identification and, in some cases, quantification. Many variants of this approach have been developed based on three methods for thermally breaking down samples: pyrolysis, linear-programmed thermal degradation (i.e., without recording weight change), and the thermogravimetric approach (i.e., continuously recording of sample weight). Using TVA experiment as a capstone, all products of degradation can be isolated for analysis by ancillary methods. At the end of the experiment, three main product fractions can be further examined: the volatile products condensable in liquid nitrogen; the tar-wax fraction that collected on the water-cooled surface beyond the hot zone (referred to as the cold ring fraction, CRF), and the non- volatile residue remaining in the sample boat. PIPA= Polyisocyanate polyols

Pyrolysis- GC-MS

EGA-MS

ThermogravimetryMass Spectroscopy (TG-MS) TG-MS features are high sensitivity and high resolution, which allow extremely low concentrations of evolved gases to be identified, together with overlapping weight losses that can be interpreted qualitatively This technique thus provides information about the qualitative aspects of the evolved gases during polymer degradation that is otherwise unavailable for TG-only experiments. This technique is therefore used for the structural characterisation of homopolymers, copolymers, polymeric blends and composites and also fi nds application in the detection of monomeric residuals, solvents, additives and toxic degradation products

DTA (differential thermal analysis) and DSC (differential scanning calorimetry) Measurement of oxidation induction times to study a stabilizers effectiveness and its diffusion within the solid

The oxidative-induction time/oxidation induction time (OIT) test is described in standard test methods ISO 11357-6 [2] and ASTM D3895 [3]. OIT is expressed as the time to onset of oxidation in a polymer test sample exposed to oxygen. The oxidative-induction time/oxidation induction time (OIT)

CL= chemiluminesans

Melt flow Index (MFI )

ThermogravimetryFourier Transform Infrared Spectroscopy (TG-FTIR ) The combination of TG and FTIR provides a very useful tool for the determination of the degradation pathways of a polymer, copolymer or the combination of one of these with an Additive. TG-FTIR makes it possible to assign the volatile components under investigation to the decomposition stages detected by TG during an experiment. Afterwards, a spectral range characteristic for a particular functional group can be selected and the infrared (IR) absorption bands in this range integrated and displayed as a function of time.

FT-IR One of the most informative and sensitive techniques to observe functional groups associated with oxygen is infrared (IR) spectroscopy, and many researchers have used mid-infrared spectroscopy to study and investigate degradation reactions and processes in polymers. For example, in low-density branched polyethylene photooxidation tends to lead to an increase in the level of the bands characteristic of the vinyl (CH=CH 2 ) end group, which is characterised by a pair of bands occurring at 990 and 910 cm1, whereas thermal-oxidation tends to lead to a reduction in relative intensity of the band attributed to vinylidene (>C=CH 2 )

Following a second compression moulding it was found that the hydroperoxide content, determined by an iodometric test, decreased by rapidly transforming to additional carbonyl groups (Figure 15).

Esters 1740 cm -1 Aldehydes 1730 cm -1 Ketones 1720 cm -1 Acids 1705 cm -1 Peracids 1785 cm -1 Peresters 1763 cm -1 The absorption bands due to hydroxy species are observed in the region 3600- 3200 cm -1

Carbonyl Index= Abs at 1710 cm -1 /Abs at 2820 cm -1 Carbonyl Index

Oxygen uptake The technique of oxygen uptake is an absolute quantitative technique; it affords a direct measure of oxygen consumption during polymer degradation

Thermo-oxidative Degradation of Polymers Eq.1. Initiation Eq.2. Chain Branching) EQ.8 and 9. Termination

During the processing operation considerable shear and heat are applied to the viscous polymer melt which causes some of polymer chains undergo homolytic scission at the carbon-carbon bonds with the formation of macro-alkyl radicals. The macro-alkyl radicals so produced are highly active species reacts with oxygen rapidly.( The rate constant of reaction with alkyl radicals is extremely high (k= 10 8 dm 3.mol -1.s -1 )) In the presence of oxygen the temperature of decomposition of most polymers decreases considerably and shift from 300- 600 o C for inert atmosphere to 100- 200 o C. This is due to macroradicals with oxygen to form hydroperoxides which themselves are unstable and breakdown rapidly forming more radicals, hence whole process becomes auotocatalytic. Free radicals P. generated during the initiation process (reaction 1) are, in the presence of oxygen, converted to peroxyl radicals PO 2. (reaction 2), and subsequently to hydroperoxides (reaction 3); intermediate hydroperoxides provoke further chain reaction unless stabilizers (InH or D) are used to interrupt it (reactions 12 and 13). Respective reaction of the scheme is completed by the method that monitors it. Thermal decomposition of dialkyl peroxides, diacyl peroxides, hydroperoxides and peracids depending on the structure of the peroxidic compound occurs in a measurable rate usually above 60 o C. Diacyl peroxides and peracids are considerably less stable than dialkyl peroxides and hydroperoxides.

Chain Branching Polymer oxy radicals (PO. ) undergo a number of other reactions including (i) - scission reactions which results in fragmentation of polymer chain together with formation of end carbonyl (ketone or aldehyde) groups and radicals

(ii) Formation of in-chain ketone groups (iii) Induced hydroperoxide decomposition

Some traces of metal and metal ions may initiate the decomposition of hydroperoxides even at room temperature. Traces of metal ions are present in almost all polymers and they may affect considerably the polymer oxidation and its subsequent degradation. The sequence of efficiency of metal ions to enhance degradation depends on its valence state and the type of its ligand, but may be postulated as follows: Cu> Mn> Fe> Cr> Co> Ni However, the mechanism for any particular ion may be more complex involving, for example, the reaction of a lower oxidation state of metal ion with peroxyl radicals, etc. Ions of Al, Ti, Zn and V usually reduce the rate of oxidation. Metal catalyzed hydroperoxide decomposition

Structure of Polymers The structure of polymers is again an important factor in controlling its relative stability. The presence of a labile hydrogen atom is particularly important in this regard and ease of oxidation to form peroxides. Thus in structure below the rate of oxidation of polymers decreases from polyethylene to polyisoprene. The electron delocalizing effect of attached group is primary important here which controls the stability of subsequent carbon centered radical after hydrogen abstraction. In the solid state PS anomalous since it is more stable than predicted here due to the shielding effect of bulky phenyl group.

Hydrogen abstraction Any type of free radical may participate in the hydrogen abstraction from a polymer macromolecule. Depending on the polymer chain structure,, the hydrogen atoms can be abstracted in order of primary < secondary < tertiary C-H sites and this process is independent of the nature of the attacking radical. Hydrogen abstraction occurs principally the tertiary carbon atoms It may also occur from the secondary carbon atoms in methylene groups.

Degradation of polyolefins during processing Processing of polyolefins requires high temperatures (150- 300 o C) depending on the type of polymer. At these temperatures thermal oxidative and mechanochemical degradation occur. The main processes observed during the thermal oxidation of polymers are the formation of hydroperoxy groups and carbonyl groups. Figures 3.13 and 3.15 show that the maximum rate of initial carbonyl formation in polyethylene (LDPE).

Figures 3.14 and 3.16 show that the hydroperoxide (as primary oxidation product) concentration rises to a maximum and then decays with heating time both in melt and the solid phase and that the maximum concentration achieved increases with decreasing temperature. In the absence of oxygen, hydroperoxide concentration decayed to zero in less than 20 h at 110 o C. Polyethylene undergoes crosslinking reactions, leading to an decrease in MFI where polypropylene under similar conditions undergoes chain scission, leading to an increase in MFI. In both cases, the initial reaction occuring is chain scission due to shearing forces acting on the polymer when its viscosity is high.

Photodegradation of Polymers Photodegradation (chain scission and/or crosslinking) occurs by activation of the polymer provided by absorption of a photon of light by the polymer. In the case of photoinitiated degradation light is absorbed by photoinitiators (or chromophore groups) which are photocleaved into free radicals, which further initiate degradation (in non-photochemical processes) of the polymer. In photo-thermal degradation both photodegradation and thermal degradation processes occur simultaneously and one of these can accelerate another. Photoageing is usually initiated by solar UV radiation, air, and pollutants, whereas water, organic solvents, temperature and mechanical stress enhance these processes.

Molecular Orbital Theory and Electronic Transitions Hints: *The energy of the electrons is determined by their particular orbital. Each element has its own particular orbitals so that the energy values of its electrons are characteristic of it and different from those electrons of other elements. * Normally electrons in an atom occupy those orbitals which are essentially nearest to the nucleus to form most stable arrangement. * If the most loosely bound electron is moved to an orbital which is farther away from the nucleus then energy must be supplied i.e. the electron must absorb energy which corresponds exactly to the difference in energy of starting orbital from that of final orbital, therefore, electronic transitions will involve definite amount of energy.

Atomic orbitals can combine and overlap to give more complex standing waves. We can add and subtract their wave functions to give the wave functions of new orbitals. This process is called the linear combination of atomic orbitals (LCAO). The number of new orbitals generated always equals the number of orbitals we started with. Thus, when a chemical bond forms, the outer orbitals can be divided into three types: 1- Antibonding orbitals (of higher energy) 2- Bonding orbitals (of lower energy) 3- orbitals not involved in bonding and are therefore, referred to as non-bonding orbitals and are at a different energy than the other two types. They occur in heteroatoms such as nitrogen and oxygen. Covalent bonds in organic molecules are formed by the overlap of atomic orbitals to form molecular orbitals in which, the electrons are, on average, closer to the atomic nuclei than they were in the atomic orbitals and therefore have lower energy.

The possible of electronic transitions between the orbitals are presented by the vertical arrows. Electronic transitions occur upon absorption of light of energy equal to the energy difference between the orbital from which the electron originates and the orbital into which the electron is promoted.

These transitions are seen as absorption bands in the UV-Visible spectrum of an organic compounds. For example, the UV absorption spectrum of formaldehyde exhibits three bands has shown in figure below, due to n *, n * and *. In general, - *transition is most difficult and the absorptions lie below the limit of 180 nm. These are associated with all organic molecules containing bonds such as C-C or C-H.

- * transition is usually associated with multiple bonds of C, N, O and S such as C=N, C=S or C=O. n- * transition occurs in all covalently bonded compounds containing heteroatoms with non- bonding electrons such as C-O-C, C-Cl or C-N. n- * transition is associated with multiple bonds containing heteroatoms such as C=N, C=O or C=S. When two molecular orbitals of two conjugated double bonds delocalise, the energy of highest occupied orbital is rised and that of the lowest unoccupied antibonding orbital is lowered.

Whats happen when molecule absorbs light quanta ? When a molecule absorbs UV or visible light, an electron can be promoted to a higher energy orbital. The resulting excited state is transient and in many cases the excess energy is lost as HEAT when the molecules return to its ground state. However, excited states of some substances return to the ground state with emission of radiation. LUMINESCENCE is a general term for such behavior.

Excitation: the time required for singlet-singlet (S 0 -S 1 ) transition is only about 10 -15 s. S 0 -T 1 is forbidden transition (10 -6 times less probable then (S 0 -S 1 ) transition). Lifetime of an excited singlet is about 10 -9 - 10 -6 s. Internal Conversion (IC): Molecule may lose energy in the S 1 state by vibrational relaxation to the lowest vibrational level of the excited singlet state ( for this reason, the wavelengths of the fluorescence band are always longer than the wavelengths of the exciting photons) or the ground state through the evolution of heat. Fluorescence is a type of Luminescence in which the emission from a photoexcited state (S 1 ) occurs within nanosecond to a microsecond (10 -6 - 10 -11 s) after excitation. Intersystem crossing (ISC) is radiationless transition in which S 1 state in its lower vibrational level is transformed to T 1 state. Phosphorescence is a type of Luminescence in which there is a delay from 10 -4 10 2 s or more before emission occurs (from T 1 ). Lifetime of an excited triplet is about 10 -4 10 2 s. The triplet life time is so long that there is a good opportunity for the loss of excitation energy by collision with oxygen and with solvent molecules. This process is referred as quenching. For this reason phosphorescence is rarely observed at room temperature in solutions.

The excitation energy of a molecule in its excited state may be dissipated by the following processes: 1- Radiative processes: LUMINESCENCE ( fluorescence and phophorescence) 2- Radiationless processes: Intersystem crossing(ISC) and Internal conversion (IC) 3- Bimolecular deactivation processes: Quenching 4- Energy transfer processes 5- Dissociation (cleavage) processes. Lifetime of an excited singlet state (10 -6 - 10 -15 s) and triplet state (10 2 -10 -3 s) is an important factor deciding the dissociation (cleavage) processes (5) of an excited state (S 1 and/or T 1 ) into free radicals. If the lifetime is very short, the above reaction is less probable. When a molecule (polymer molecule) absorbs electromagnetic radiation (light) its energy increses by an amount egual energy of the absorbed photon (E): E= E 2 - E 1 = h According to Grothus and Draper law only the light which is absorbed by a molecule can be effective in producing photophysical process (bond dissociation) or photochemical process (e.g. photo-rearrangement) in that molecule However absorbed light has to have enough energy for example to cause a bond dissociation( see table) Most pure polymers contain only C-C, C-H, C-O, C-Cl, C-N and C-P bonds and are not, therefore, expected to absorb light at wavlengths longer than 200nm. The fact that photodegradation occurs even with ligth > 300nm indicates some kinds of chromophoric groups must be present in these polymers.

For example carbonyl groups exhibiting n- * type absorption bands in the range 300- 360 nm can be responsible for the absorption of radiation from spectral region in which many poymers themselves do not absorb (>300 nm). The extended absorption of many polymers can result from formation of charge Transfer (CT) complexes between polymer and molecular oxygen.

Bimolecular deactivation and Energy transfer processes An electronic energy transfer is the one-step transfer of electronic excitation energy from excited donor (D*) to an acceptor (A) molecule in separate molecules (intermolecular energy transfer) or in a different part of the same molecule ( intramolecular energy transfer). Electronic energy transfer process may occur only if the absorption spectrum of an acceptor (A) overlaos an emission spectrum of an excited donor (D*).

Typical Energy transfer In solid state the rate or efficiency of an energy transfer above glass transition temperature(T g ) is the same order of magnitude as in solution. In polymer having aromatic groups (e.g. polystyrene, polyvinylnaphthalene etc) energy transfer process may occur via formation of excimers (excited dimer). Electronic energy transfer process may also occur via formation of exciplexes. An exciplex (excited charge transfer complex) is a well defined complex which exits in electronically excited states. An exciplex is formed between an excited donor (D*) and/or an excited acceptor (A*) and donor (D) molecules.

Effect of Free volume When polymer is cooled to 0 K no motion of the groups or constituents is possible. As the temperature is increased from 0 K the specific volume of the polymers increases. Since there is very little change in the bond lengths with temperature the observed increases in specific volume must be due to the formation of small holes or voids in the system which collectively increases in size and/or number as the temperature rises. As the free volume in the polymer increases, various types of molecular motion can begin to occur and these are identified by transitions (e.g. the crank shaft motion in polyethylene observable at -85 o C and the transition associated with movement of the phenyl ring polystyrene at -80 o K.

Effect of The Glass Transition Temperature In generally, dissociation of free radical pairs can be relatively efficient in the solid-state if one component is a small free radical but will be significantly inhibited if both are polymeric radicals. Reaction which can be considered to be associated with caged radicals, such as the photo-Fries, will require very little free volume and can be expected to be quite efficient in solid polymers below transition, whereas photochemical process such as Norrish Type II process are substantially reduced in glassy polymers below the glass transition temperature (T g ). Bimolecular reactions which require the diffusion of a small molecule reagent to a species in a polymer matrix will depend on both the diffusion constant and solubility of the material in the matrix. Solid polymers generally have internal viscosities only two to three orders of magnitude less than those for simple liquids such as benzene or hexane so that under suitable conditions quite efficient bimolecular reactions can be induced to occur by diffusional processes in polymer materials.

This is because of the rate of diffusion of oxygen will be faster in the amorphous material which will oxidize faster followed by moulded film and the single crystals. Morphology of polymer In the solid state amorphous polymers are more susceptible to oxidation than crystalline polymers.

Oxygen diffuses only into amorphous regions of a polymer making them more susceptible to photo-oxidative degradation. The presence of crystalline domains in a polymer matrix has two effects on oxygen diffusion and solubility behavior. 1- At temperatures well below the melting point (T m ) crystalline regions are generally inaccessible to oxygen and to most penetrants. 2- Crystalline domains require penetrant migration around them, which increases the average pathlength relative to nominal dimensions of the sample.

Role of Mechanical stress Mechanical stress causes changes in the physical Properties of polymers. Macroscopic extension of a polymer film causes anisotropic orientation and extension of polymer chains. Stress can cause chain breaks and introduce radicals which can initiate degradative processes Such as oxidation or microcraking. In solid polymers in the glassy state, where mobility of the macromolecules is limited, chain end radicals formed by mechanical stress may only abstract hydrogen atoms from adjacent molecules. Following the chain reaction neighbouring macromolecules may be degraded in fast reactions, and local sites of disintegration are formed in a stressed polymer. Such local degraded sites may be considered as submicrocracks. Microcracks are generally formed at the weakest links in polymeric materials. The overall embrittlement of material is a result of the formation of microcracks.

In this study, the data trend shown in Figure 5 correlated well with activation energies derived from the thermal analysis, which showed that the thermal oxidative stability followed the order LLDPE> mPE>HDPE, whereas the trend for photo-oxidative stability was mPE>HDPE>LLDPE. The thermal-oxidative results were also in accord with CL measurements, although the CL data for the photo-oxidative series demonstrated a higher light stability for the LLDPE compared to HDPE and mPE, both of which exhibited CL emission below their melting points.

polymers can degrade by exposure to high temperature shear action oxygen, ozone and chemicals...

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