Degradation of polymers

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

2. Often, multiple exposures, such as a combination of moisture heat or oxygen and light can result in accelerated deterioration. Shear Ozone, oxygen Heat POLYMER UV moisture 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.

3. 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 200oC. 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

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

5. 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

6. 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 Boltzman’sfactor 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 C–C bonds at least one is dissociated into radicals is 486oC, while in one mole of O–O bonds it is only 30oC.

8. 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 Pt= Po(s+1) (1) in which Po and Pt are the chain lengths of original polymer and after time at which s scissions have occurred on average per molecule. Thus S=(Po/Pt)-1 (2) and the fraction of bonds broken, a, is given by equation (3) a= s/Po = 1/Pt-1/Po (3) If chain scission is random, that is, every interunit bond in every molecule is equally liable to break then a= 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 a= b+kt (4) In which b is the fraction of weak links in the molecules Figure 2.6. shows that PS obey equation 4 and does indeed incorporate weak links. b

9. 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.

10. Depolymerizationvstransfer 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).

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

12. Depolymerizationvs ester decomposition

13. Ester decompositon only becomes important when the monomer unit incorporates at least five hydrogen atoms on the b- carbon and depolymerization is quantitative when there are at most one or two b-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.

14. 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 b-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.

15. Poly(ethylene terephthalate)

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

19. Characterization techniques for polymer degradation & stabilization

20. 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.

22. 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 a-alkyl carbon atom which is attached to the phenolic nucleus.

24. 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= Polyisocyanatepolyols

27. Pyrolysis- GC-MS

28. EGA-MS

36. Thermogravimetry–Mass 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

37. DSC

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

39. The oxidative-induction time/oxidation induction time (OIT) 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.

40. CL= chemiluminesans

41. Melt flow Index (MFI)

42. Thermogravimetry–Fourier 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.

43. 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=CH2) 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=CH2)

44. 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).

45. 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- 3200cm -1

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

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