FORSCHUNG 390 CHIMIA 47(1993) Nr. 10 (Oktobcr)
Chimia 47 (1993) 390-395 band for the polyolefins is in the region © Neue Schweizerisclze Chemische Gesellsclzaft below 200 nm, caused by a a-o* transi- 1SSN 0009-4293 tion. The absorption of light by synthetic rubbers based on butadiene or isoprene copolymers is associated with a n-rc* tran- sition and occurs in the wavelength region Degradation of Polymers of 180-240 nm, the longest absorption of polystyrenes are associated with the n-rc* by Photooxidation transition of the benzene ring and occurs in the wavelength region of 230-280 nm. The longest wavelenght absorption band Hans Zweifel* for polyacetals can be associated with a partially forbidden TJ-o* transition below 200 nm. Abstract. Most of the commercially important polymers are prone to deterioration The longest-wavelenght absorption caused by oxidation reactions. Formation and subsequent photolysis of hydro peroxides bands of polymers such as aliphatic polya- as key intermediates cause chain scission and transformation of the polymer's initial mides, aliphatic polyesters or poly(meth)- molecular structure. Suitable stabilizers are needed to impart to the polymer acceptable acrylics are in the region below 200 nm, service life time. UV -absorbers, triplet quenchers, radical scavengers, and hydroperox- associated with aTJ-rc* transition [1][2]. ide decomposers fulfill these requirements. However, there are many commercial polymers which absorb sunlight owing to 1. Introduction oxidation. Both processes involve fonna- the chromophoric groups that form part of tion of radicals followed by a breakdown the polymer structure, e.g. aromatic poly- Oxidative deterioration of polymers is of chemical bonds. esters and polyamides, polysulphides, pol- known to be the source of changes in the Fig. 1 shows the SEM picture of the yethersulphones, polycarbonate and other appearance of the polymer leading to the surface of a polypropylene-EPR-copoly- polymers containing aromatic moieties. loss, or a decrease, of mechanical and mer which was exposed to sunlight. The The absorption spectra of some virgin physical properties of the material. Heat destruction of the polymer surface can be polymers are shown in Fig. 2. and light are important physical factors in clearly seen. The first fundamental law of photo- With increasing temperature, ther- chemistry formulated by Grotthus and mooxidation may overlap photooxidation. Draper states that only light absorbed by Therefore, we have to discuss to some a molecule can be effective in producing extent the various factors influencing the photochemical change in the molecule. As oxidative deterioration of the polymers. shown, a number of polymers do not ab- sorb sunlight and no photochemically- 1.1. Absorption of Light induced reaction should, therefore, occur. The lower wavelength limit of sunlight However, a polymer undergoes different reaching the earth's surface is ca. 290 nm. steps of transformation processes in order Many of the commercially important to produce the 'end-use' article. Most of polymers, e.g. polyethylene or polypro- these transformation steps involve melt Fig. I. SEM Photo of the surface of a PP-EPR- pylene, should not absorb any sunlight processing such as extrusion, blow mold- copolymer. after being exposed to sunlight (Flor- since the longest wavelength absorption ing, injection molding, compression mold- ida. 3 years) ing and others. Shear, heat, and oxygen will cause
0.400 oxidation, and hydroperoxides groups are formed. Subsequent chemical reactions ,',. ,',. .: .. - :. :. .', ,'. , . . lead to the formation of carbonyl groups . . which absorb sunlight. Some of these re- Polyrllothylmothncrylot : :•...... :...... •..... ~ •...... : .•.•••. : •••••.• :••••••• ~ ••.•.. actions are summarized in Scheme J. , ...... POlysulfon The stabilizers used (sterically hin- . -: : : , .. .. . ~ , -:' , ..:- .. , : , . , dered phenols, phosphites) that are added to the polymer to prevent degradation dur- .?~~.., ; : [ j -: : : . ing melt processing have in most instances A . , . . b aromatic groups. Furthermore, catalysts s 0.200 " " " > " :. "" .:" :. ".":" : ,, ':'" " ":. " " ..;- . : POlYC8rbonat are used to produce the polymers (transi- . . . , . , ,', , , , , , .." ,..' ~, , . , .. : , , . , , . '.' . , . . . ',' .. .. , ", . . , , , . tion metals, Ziegler-Natta catalysts [3]) . . , . and metals or metal compounds are, there-
. , . , , . :. . , , ...... , , ':' , . . , . ':' ., ..,:.,.... ~. . , . .. .: . , . . . . .:...... :' , . . . , .
...... , .. , ' .. ".,,'.,., *Correspolldellce: H. Zweifel Ciba-Geigy AG Polypropylen 350.0 500 Additives Division Wsvelengtn (nm.l Schwarzwaldallee Fig. 2. Absorption spectras of commercial polymers CH-4002 Basel FORSCHUNG 391 ClliMIA 47(1~9J) Nr. 10 (Okloh Scheme I, Photoreactions Leading to a Change in the Molecular Structure Scheme 3, Reactions of Polyolefin Macroalkyl RadiCllls lI'ith 302 and of the Polymer Subsequent Formation of Macroal/..:)'IHydroperoxides Photoreactions of Carbonyl Groups: Reaction of Molecular Oxygen with a Polyethylene Macroalkylradical: A. Norrish Type I: R o . .'0, 90' R~ 4 ' 90H II hu "CH2~Hl--CH-CH2- + ~ -CHz-CHz.-cH-CHz- ~ -CHl-CH1--CH~Ht" -CH2-C-CH2- - Reaction of Molecular Oxygen with a Polypropylene Macroalkylradlcal: B. Norrish Type II: ?OH -cH,4-<:H,--<; ••.. H O~ hu CH, CH, / ":C _ -CH=CH + /- 2 -~ or: CH2-CH2 Photo-Fries Rearrangement: o hu Lft~ -~~-o-©-- - 9C9 Scheme 2. Reactions of Molecular Oxygen and Ozone with Suitable Groups Scheme 4. Formation of Macroalkyl Hydroperoxides by Light Induced Present in Polyolefins and Rubbers Transfomation of a Polymer/Oxygen 'CT-Complex' 0-0' , I , PolymerlO, Charge Transfer Complex: 0.: .:~~: == 0-0 0, + -cH---9- H electrons unpaired C H C pOH 1 . 1 0, hu I. . 'C=C/ _ ~-c 0,: 0=0 0-0 : 0, + R'-r-RI'H """ [RI'R'i-H 0,] - [R'R'-rH 0, - R'-r'7' ·O,H]' - R'-r-DOH7' .:g-<;: == I \ I \ C c-c c- R, R, R, R, R, electrons paired C • T • Complex O 1\ o· "0'· 00 0-0 I I I I 0,: .'c),~ "0:.... - ..'·(l '0"•• '. 0, + -c=c- - -c-c- --c c- \I o fore, present in the resin. Pigments may be phase of the polymer. Photo-oxidative Molecularoxgen, 302, reacts similarto another source for impurities and atmos- degradation starts at the polymer's surface a radical (' diradicaloid state'). Macroalky 1- pheric pollution can contribute to the for- (see Fig. 1) and based on the work of radicals, RO, formed upon processing of mation of components or chromophoric Gillen and Clough [5] an oxidation profile polyolefins, are very easily transformed groups which absorb sunlight. can be established. The microcracks into macroperoxi-radicals ROO°. Singlet formed may propagate into the non-oxi- oxygen, 102, can react in an 'ene' -type 1.2. Diffusion and Solubility of Oxgen dized, ductile material when exposed to reaction in a-position to a C=C bond. It is in Polymers external stress orcaused by physical aging still a matter of discussion whether singlet The oxidation of a polymer is a reac- processes. oxygen plays an important role in oxida- tion between a gas (oxygen) with a solid tive degradation processes [10]. The tri- (polymer). plet photosensitized production of singlet The solubility of oxygen in the poly- 2. Photooxidative Degradation of oxygen caused by carbonyl groups is a mer matrix and the rate of diffusion into Polyethylene and Polypropylene possible reaction and Gugumus [II]found the polymer play, therefore, an important improved photostability of polyethylene role [4-6]. The diffusion coefficients and 2.1. Formation of Macroalkyl in the presence of 1,4-diazabicyclo[2.2.2]- the solubilities of oxygen in some poly- Hydroperoxides octane (DAB CO), well known to act as a mers are summarized in Table 1. Oxygen is the key participant in the singlet oxygen quencher [12]. Ozone (03) Diffusion is typically about two orders photooxidative degradation of polyole- reacts easily with C=C bonds. These reac- of magnitude lower compared with the fins. tions lead finally to the formation of the values for liquid hydrocarbons. The con- Two Lewis structures may be proposed macroalkylhydroperoxides, as shown in centration of oxygen, in equilibrium with for molecular oxygen (Scheme 2). Scheme 3. which are the precursors for all the air, is ca. ten times lower then in liquid hydrocarbon, However, in semicrystal- line polymers, e.g. polyolefins, oxygen is Table 1. Diffusion Coefficients and Solubilities of Oxygen in Some Polymers only soluble in the amorphous part of the polymer. Therefore, the solubility of oxy- P I)mcr Dlffusi In olubilil)' Temp. Ref. gen is in simple linear dependence on I07cms1 101 • 11101 kg I 1° I crystallinity. Since the mechanical strength of a semicrystalline polymer depends on PE·LD -.4 O.M .5 171 the chain entanglement in the amorphous PE-IID 1.6 0.6 ::! [71 phase, the breakdown of the mechanical alUral rubber 16 5.0 25 (HI properties of a polyolefin is caused by P 0,56 7.34 35 19J oxydative degradation in the amorphous FORSCHUNG 392 CHIMIA 47 (1993) Nr. 10 (Oktober) further degradation reactions. For poly- tie work has been published on its photol- Therefore, the most probable mechanism propylene, a 'concerted' mechanism can ysis. The photolytic decomposition of t- of photodecomposition of hydroperoxide be formulated, based on abstraction of butyl hydroperoxide in an inert solvent groups is an energy transfer process from tertiary hydrogen. For this reason, poly- occurs at 313 nm with high quantum yield the exited carbonyl or aromatic hydrocar- propy lene is more prone to oxidation com- [14]. Fig. 3 shows the formation of carbo- bon groups to the hydroperoxide groups as pared with polyethylene. nyl groups on irradiation ofa polyethylene acceptors [15]. The resulting ketones can The formation ofhydroperoxide groups and a polypropylene film, using different photolyze according Non-ish Type-lor is also postulated by a 'polymer/oxygen- 'cut -off' filters_ Norrish Type-II reaction as shown in Fig. charge-transfer complex' which may be The light-induced oxidation of the pol- 3. Recently, Gugumus [11] has proposed transformed by excitation with light into ymers occurs at wavelengths around 300' intra- and intermolecular decomposition the corresponding macroalkyl hydroper- nm but even at wavelenghts of 360 nm, the mechanism based on the photolysis of the oxide [13], Scheme 4. yield of carbonyl groups is considerable. secondary and tertiary hydroperoxides as Polypropylene undergoes faster photoox- shown in Scheme 5. 2.2. Photochemistry of idation compared with polyethylene be- Hydroperoxides cause of the concerted mechanism of hy- 2.3. Thermo-oxidative Reactions The polymeric hydroperoxides are the droperoxide formation. The photolysis of Involving Peroxy, Alkoxy, key intermediate in the breakdown of the hydroperoxide groups under solar irradia- and Alkyl Radicals polymer molecule. The thermal decompo- tion is a slow process, the average lifetime The photolysis of the hydroperoxide sition of polyolefin hydroperoxides has of an -OOH group under constant irradi- group yields aprimary or secondary alkoxy been frequently investigated, however, lit- ation is reported to be -25 h [15-17]. radical, RO·. The alkoxy radical are im- Scheme 6. Reactions of Secondwy and Tertiary Alkoxy Radicals PP PE-HD O' -A- all UV -- all UV I -c- -0-- 320 nm -II- 320 nm H -+-- 360 nm --+- 360 nm +0, ~ 395 nm - 395 nm FurtherOxidation 0' o carbonyl index I II +R' 800 - ~-CH,-CH,- P·Sclsslon. _ CH + 'CH,-GH,- -t---.Sranching/Crosslinking L-+C_H....:.,_C-,H,,.-Disproportionantion o o H /I 600 - C-CH -C-CH -~- + 'CH, I 2 2 I H O· H CH, CH, - C--CH -e-GH -c _fl· Scission I ' I 'I H,,\ yH, / CH, CH, CH, - ~-CH, + C-CH,-C - I II R CH, 0 Scheme 7. Formation of Acid-, Ester-, Peracid-, Perester and y-Lacrone Groups 1) Oxidation of Aldehydes 120 160 200 o 0 0 0 0 exposure time (days) -J!H 0, or ROOH. -J!-OH, -J!-OR , -J!-OOH, ""g-OOR 2) Oxidation of sec. Alcohols and Hydroperoxides (Ref.21) Fig 3. Build-up of carbonyl groups upon irradiation of a polyethylene and a polypropylene film, using different 'cut-off' filters ?H R' RH ?H ?H ?H P -Scission ""C¥ ~ Y\oC¥ ~ Y\oC'" .. ""C +."" I I II H 00' a Scheme 5. Intramolecular Decomposition of Secondary and Tertiary Hy- ?OH R' RH ?OH ?OH ?OH ~-Scisslon droperoxides ""C'" ~ Y\oC'V ~ Y\oC'" " Y\oC + ."" I I II a H 00' a hu II oNvC"'" + H,O - 3} Reac1ions of Ketones with Radicals (Ref. 22) a 0 II II HO·. + ""C¥ _ ""C-OH + 1/ o 0 oNvCH° + CH,=CH"'" + H,O II II RO' + Y\oC'V _ ""C-oR + a a II II ROO'+ Y\oC¥ _ ""C-OOR + o CH, 4) Formation of y-Lactones (Ref. 23, 24) II I + oNvC-CH, + CH,=C-- H,O ~O "'-C-C ~C-C-C-C - O~";C I \ C OH OH oII FORSCHUNG 393 CHI MIA 47 (1993) Nr. III (Oktnhcr) portant in the autoxidation of hydrocar- bons [18]. They may, as shown in Scheme O.Uti 6, abstract hydrogen from the substrate, 1 x combine with available free radicals or 3x undergo f3-scission to give a ketone and an 2x alkyl radical which will be transformed by 5x further oxidation reactions into various oxidation products [19][20]. Ul I- Z Further oxidation of the intermediate ::l degradation products occurs and the for- w ~ 0.029 mation of acid, ester, peracid, pere- 2.4. Changes in Molecular Weight Ul I- 212 h and Molecular- Weight Z ::l Distribution of Polypropylene upon w 160 h Processing, Thermal Aging or ~ 0.95 140h Table 2. Changes in Molecular Weight, Mw,Molecular- Weight Distribution, M,jMn, of Unstabilized All these reactions occur when the PP-Homopolymer, after Multiple Extrusion Passes at 2600 polymer is exposed to high temperature, shear, and light and during the polymer I~.LrU\lon Pas. It life time. The changes in molecularweight and molecular-weight distribution of un- 17 000 2.99 stabilized polypropylene after multiple 106000 _A extrusion passes, after oven aging at 135° 7:! 000 1.92 and after irradiation in a XENO 1200 ex- 60000 1.71 posure device are listed in Tables 2-4. The results clearly demonstrate that ") Powder. upon exposure of the polymer, chain scis- h) 2nd - 5th Extrusion: pellets. sion reaction occur. Since the values of Tables 2-4 can be directly related to oxi- dation as shown in Figs. 4-6, the follow- Table 3. Changes in Molecular Weight, Mw,Molecular- Weight Distribution, M"IMn' of Unstabilized ing statements can be made: PP-Homopolymer") after Oven Aging at i 35° (draft air oven) - Upon thermal- and photoaging of fol- lowing unstabilized PP-homopolymer, Ilour\ at l.l5 JI, M.J In polymer-chain scission reactions caused by oxidative degradation oc- 0 17 000 2.99 cur. The polydispersity slightly increas- 121000 2.92 es upon exposure, thus indicating that 2 106000 2.9-1 the oxidation takes place randomly. 9 000 3._7 - During processing, the level of oxida- 9- 000 .+.32 tion is much lower owing to oxygen- deficient conditions. However, signif- ") I mm compression molded plaques. icant chain scission takes place. Mo- lecular weight as well as polydispersi- ty decrease, indicating that chain scis- Table 4. Changes in Molecular Weight, Mw,Molecular- Weight Distribution, M,;Mn' of Unstabilized sion is caused by thermo-mechanical PP-Homopolymer") after Irradiation ina XENOTEST 1200 Exposure Device, b.p. Temperature: 55° processes, involving high shear [26]. The low level of oxidation products, II urs M. M.,1 tll especially carbonyl groups, can act at a () J7 000 2.99 later stage as 'sensitizers' for the pho- tolysis of the hydroperoxide groups. 5 16 000 3.06 Thus, thermal history is of great im- 3.3_ 24 133000 portance for the polymer's service life M 102 ()(Kl 2.55 time . IflO 77 000 .3 ") 1 mm compression molded plaques. 3. Photooxidative Degradation of Bisphenol-A Polycarbonate Scheme 8. Photo-Fries Reaction involving Bisphenol-A Polycarbonate 3.1. Photo-Fries Pathway o Unstabilized bisphenol-A polycar- II bonate, when exposed to sunlight, under- "Concerted" goes discoloration, cross-linking occurs ~~ - and cracks are formed on the surface. Since the polymer, by its structure, can absorb light in the region above 295 nm, photon-induced processes can occur. Early investigations on the photoaging a of BPA-PC suggested that the 'photo- r:r Fries' reaction, as shown in Scheme 8, is the key mechanism [27][28]. Recent investigations by Factor and coworkers [29] indicate that upon expo- sure to sunlight, small amounts of photo- Fries products, some products assigned to ring oxidation [30] and large amounts of side-chain oxidation products are found in the aged material. A general scheme for the photooxidative reaction pathway is @-o-@+ shown in Scheme 9. + The photooxidation mechanism, pro- +@--oH + H--@+ moted by light absorption in the region of 310-350 nm, caused by photo-Fries prod- FORSCHUNG 395 CHIMIA./7 (IW3) Nr. 10 iOkloher) Scheme 9. Photooxidative Reactions Involving Bisphenol-A Polycarbonate [8] GJ. Van Amerongen, J. Polym. Sci. 1950, 5,307. [9] N. Muruganadam, WJ. Karas, D.R. Paul, -O~CH3~ ° l. Polym. Sci., Part B: Polym. Phys. 1987, 25,1999. + ° [10] J.R. Mac Callum, in 'Developments in -O~OH~ Polymer Degradation', Ed. N. Grassie, Ap- plied Science Publishers, London, 1977, + Vol. I, p. 237. -O-@-oH [II] F. Gugumus, Macromol. Chem .. Macro- mol. Symp. 1989,27,25. [12] P.R. Ogilby, M.P. Dillon, M. Kristiansen. R.L.Clough, Macromolecules 1992, 25, 3399. ~ rH3~ °II R'~ ARH ~ rH;~• °II hu/02 ~ rHi~. °II -O~OC ~ -0-\Q;-;t;\Q;-0c- -- -0-\Q;-;t;\Q;-0C- [13] DJ. Carlsson, D.M. Wiles, 1. Macromo/. CH CH3 CH3 Sci., Rev. Macromol. Chem. 1976, C I4( 1J, 3 65. ---!I"L _ ACIDS [14] J.T. Martin, R.G.W. Non'ish, Pmc. R. Soc. ~l ESTERS London [Ser.] A 1953,220,322. 1 [15] J.F. Rabek, 'Photostabilization of Poly- hut 02/'OH RING° OXIDATION° mers, Principles and Applications', Elesvi- --- er Applied Science, London, 1990. [16] DJ. Carlsson, A. Garton, D.M. Wiles, in 'Developments in Polymer Stabilization', Ed. G. Scott, Applied Science Publishers, London, 1979, Vol. I, p. 219. Table 5. Changes in Molecular Weight, Mw. and Elongation of BPA-Polycarbonate after Irradiation [17] DJ. Carlsson, D.M. Wiles, J. Polym. Sci .. in a 340 FL Exposure Device Polym. Chem. 1974, /2, 2217. [18] J.L. Bolland, G. Gee, TraIlS. Faraday Soc. \lOlli' M ••• % Elongation 1946,42,236. [19] S. AI-Malaika, in 'Atmospheric Oxidation () _9900 9 and Antioxidants', Ed. G. Scott, Elsevier _85 25000 42 Science Publishers B.V., London, 1993, Vol. 1. 35 _'" 550 2 [20] G. Scott, 'Atmospheric Oxidation and Anti- (){) 102000 15 oxidants', Elsevier Publishing Company, 970 77000 0 London, 1965. [21] J. Lacoste, DJ. Carlsson, S. Falicki, 25 J..Un films D.M.Wiles, Polym. Degrad. Stab. 1991, 34,309. [22] G. Geuskens, M.S. Kabamba, Polym. De- Table 6. Changes in Yellowness Index (YI) of BPA Polycarbonate. after Irradiation in a 340 FL grad. Stab. 1982,4,69. Exposure Device [23] F. Tdos, M. Iring, Acta Polym. 1988, 39, 19. !lour VI [24] J.H. Adams, J. PolYIIl. Sci. 1970.8, 1077. [25] D.J. Carlsson, D.M. Wiles, Macromole- cules 1969, 2, 597. () .00 [26] H. Hinsken, S. Moss, J.R. Pauquet, H. 285 4. '" Zweifel, Polym. Degrad. Stab. 1991, 34, 535 6.:-4 279. [27] D. Bellus, P. Hrdlovic, Z. Manasek, Polym. ()() 8 Lett. 1966, 4, I. 97() 11 [28] K.B. Abbas, J. Appl. Polym. Sci., Polym. Symp. 1979,35, 345. [29] A. Factor, W.V. Ligon, R.J. May, Macro- molecules 1987, 20, 2461. [30] D.T. Clark, H.S. Munro, Polym. Degrad. ucts [29], defects and impurities in the [I] J.F. Mc Kellar, N.S. Allen, 'Photochemis- Stab. 1982, 4, 441. polymer [31] leads finally to colored pho- try of Man-Made Polymers', Applied Sci- [31] A.L. Andrady, N.D. Searle, L.F.E. Crewd- toproducts. The low amount of photo- ence Publishers, London, 1979. son, Polym. Degrad. Stab. 1992,35,225. Fries products may be explained by the [2] J. Guillet, Polymer, 'Photophysics and Pho- [32] A. Rivaton, D. Sallet, J. Lemaire, Polym. tochemistry', Cambridge University Press, work of Lemaire [32][33] who demon- Photochem. 1983,3,463. Cambridge, 1985. strated that such products are easily photo- [33] A. Rivaton, D. Sallet, J. Lemaire, PolY'''' [3] P. Gijsman, J. Hennekens, J.Vincent, Pol- degraded. Degrad. Stab. 1986, 14, 1. ym. Degrad. Stab. 1993,39, 27l. [34] P. Klemchuk, personal communication. BPA-PC undergoes chain scission re- [4] N. Billingham, in 'Oxidation Inhibition in actions upon irradiation, leading to a de- Organic Materials', Eds. I.Pospisil and P. crease in molecular weight [28]. Results Klemchuck, CRC Press, Boca Raton, Flor- of changes in molecular weight and dis- ida, 1990, Vol. II, p. 250. coloration are listed in Tables 5 and6 [34]. [5] K.T. Gillen, R.L. Clough, Polymer 1992, The water content in the resin may 33,4358. [6] C.R. Boss, J.C.W. Chien, 1. Polym. Sci., influence the deterioration of the polymer. Part A-11966, 4, 1543. Therefore, reactions caused by pho- [7] A.S. Michaels, HJ. Bixler, J. Polym. Sci. tooxidation of BPA-PC need further elu- 1961, 50, 393. cidation.