Polymer Degradation and Performance - American Chemical Society

Chapter 24

Photolysis and Photooxidation in Engineering Plastics 1,2

2

Downloaded by CORNELL UNIV on July 30, 2012 | http://pubs.acs.org Publication Date: January 1, 2009 | doi: 10.1021/bk-2009-1004.ch024

Pieter Gijsman and Marjolein Diepens

1DSM Research, P.O. Box 18, 6160 MD Geleen, The Netherlands 2Laboratoryof Polymer Technology, Department of Chemical Engineering, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands

The most important UV-degradation mechanisms for polymers are photolysis and photo-oxidation. For polyolefins it is well known that photo-oxidation is the dominating mechanism. These polymers do not have an inherent absorption at wavelengths present in terrestrial sunlight (>290 nm) so photolysis cannot play an important role. In contrast to polyolefins, the majority of engineering plastics do have absorptions at these wavelengths. As a result, for these polymers, besides photo-oxidation, photolysis can play an important role. For polyamide 6, poly(butylene terephthalate), two different copolyetheresters and bisphenol-A polycarbonate, the importance of photolysis as well as that of photo-oxidation in polymer light-induced degradation is discussed.

© 2009 American Chemical Society

In Polymer Degradation and Performance; Celina, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2009.

287


288 The most important mechanisms causing weathering of polymers are photolysis and photo-oxidation (/, 2). If the absorption of light leads directly to chemical reactions causing degradation, this is called photolysis. Photooxidation is a result of the absorption of light which leads to the formation of radicals that induce oxidation of the material. For polyolefms it is well known that photo-oxidation is the dominating mechanism (2); these polymers do not have an inherent absorption at wavelengths present in terrestrial sunlight (>290300 nm), so that photolysis cannot play an important role. Nevertheless, irradiation of these polymers with terrestrial wavelengths results in accelerated degradation. This can be ascribed to impurities that are formed during storage and processing or to charge transfer complexes between the polymer and oxygen (2, 3). Due to photolytic reactions of these absorbing species, radicals are formed which initiate the photo-oxidation. In contrast to polyolefms, the majority of engineering plastics do have absorptions at wavelengths present in terrestrial sunlight, so that for these polymers photolysis can also play an important role. For these polymers in principle there are three mechanisms that can describe their light induced degradation: •

• •

Photolysis; absorption as a result of the inherent polymeric structure results in chemistry causing changes in the molecular weight distribution. Photo-oxidation initiated by photolysis reactions of the polymer itself as mentioned above. Photo-oxidation initiated by impurities not part of the inherent polymer structure.

For polyesters (e.g. poly(butylene terephthalate), PBT), polyamides (e.g. polyamide 6 (PA6) and polycarbonate (PC) it is known that they show photolysis as well as photo-oxidation (/, 4, 5). Figure 1 shows the UV absorption spectra of these polymers. The lowest wavelength present on earth lies between 290 and 300 nm (somewhat dependent on the location). PBT shows a large absorption above 290 nm (see Figure 1), meaning that for this polymer photolysis can play an important role. PA6 only shows a limited absorption. PC shows a small absorption between 290 and 300 nm, which implies that for this polymer the value of the lowest wavelength present can play a significant role. For this polymer a difference in degradation mechanism can be anticipated when the smallest wavelength present changesfrom290 to 300 nm. In the case of polyamides and polyesters the most important photolytic reactions are the Norrish I and II reactions (see Scheme 1). The Norrish I reaction leads to chain cleavage and radicals that might initiate oxidation, the Norrish II reaction only leads to chain cleavage. The main question for these polymers is: What is the relative importance of photolysis and photo-oxidation?


289

Ι

2 -ι 1,81,61,4-

I

PC

I

Ι

I

Ι

ι

1

1

ΡΒΤ

- - PC CH

ΡΒΤ

ι

J

3

-- é-O-CH?-CHr-CHr-CH7-0-é|0

—

ΡΑ6

-C -ΜΗ—C Η —C Η j—C H —C H —C H?7

?

?

ι

ο

•

ι

Absorbai

1

ι

ο


ι » .

0,40.2ΡΑ

^

ι.υ»

0 260

280

300

320

340

360

380

400

Wavelength (nm) Figure L UV absorption spectrum of Polycarbonate, Polybutylene terephtalate and Polyamide 6.

Scheme L Norrish I (top) and Norrish II (bottom) reactions for polyamides (X=NH) and polyesters (X=0).


290


For polycarbonate the main photolytic reaction is the photo-Fries reaction (see Scheme 2). For this polymer the main questions are: How important is the photo-Fries reaction and if it plays a role does it form free radicals that initiate the oxidation of the polymer? As mentioned above all of the discussed polymers can also undergo photooxidative reaction. The mechanism describing the photo-oxidation of polymers is shown in Scheme 3. In this paper the relative importance of photolysis and photo-oxidation during weathering of the above mentioned engineering plastics as well as two polyetherester copolymers will be discussed.

Experimental Materials The polyamide 6 (PA6) used was a cast film of 50 μπι thickness made from an unstabilized PA6. The polybutylene terephthalate (PBT) used was a cast film of 25 μπι thickness made of an unstabilized PBT. Two PBT based thermoplastic elastomers were studied. One was a PBTpolytetrahydrofuran (pTHF) copolymer (PECO-1) and the other one a PBTpoly(propylene glycol) (PPG) copolymer (PECO-2). In both cases the degradations were done on 30 μιη thick films. All of these polymers were supplied by DSM Engineering Plastics. Extruded films of unstabilized bisphenol A polycarbonate (PC) were supplied by GE Plastics. The thickness of these films was approximately 0.2 mm.

Weathering Weathering tests were done in different types of Atlas apparatus (Atlas Material Testing Technology b.v.) a standard Suntest CPS (CPS), a Suntest XXL+ (XXL+) and a Weather-O-meter Ci 65 (WOM) all using filtered xenon lamps. The light intensity level in the Suntest was 0.3 W/m /nm at 340 nm at a chamber temperature of 30-40°C. In the XXL+ the intensity was 0.4 W/m /nm at 340 nm, the chamber temperature 35°C and a relative humidity of 40%. In the WOM the intensity at 340 nm was 0.35 W/m /nm, the test chamber temperature 46°C and the relative humidity 55%. Due to the filters used there was a small difference in the wavelength cut off of both Suntesters (see Figure 2). 2

2

2



291

Scheme 2. Photo-Fries reaction of Bisphenol A Polycarbonate.

Chain Branching

Scheme 3. Photo-oxidation mechanism ofpolymers RH is most labile hydrogen.


292 The PA6, PBT and the two polyetherester copolymers were aged in closed boro-silicate vessels filled with air or nitrogen. The PC was weathered as a free standing film.

Analysis Oxygen uptake, CO and C 0 formation were determined by gas phase analysis after different oxidation times using gas chromatography (GC) (for details see ref 6). A Shimadzu UV-3102PC UV-VIS-NIR scanning spectrophotometer was used to record the UV-VIS spectra. Infrared spectra were recorded using a BioRad FTS 6000 spectrometer in the attenuated total reflection (ATR) mode for 200 scans at a resolution of 4 c m . The BioRad Merlin software was used to analyze the spectra. Fluorescence spectra were recorded on a Shimadzu RF-1501 spectrofluorimeter. The irradiated polycarbonate samples were dissolved in dichloromethane (0.001 wt% PC in DCM). Emission spectra were recorded using an excitation wavelength of 310 nm (7). The second-order Rayleigh scattering peak (located at 622 nm) was used to scale the recorded fluorescence spectra (8). The elongation at break was measured in triplicate using a Zwick 1445 at an elongation speed of 50 mm/min.


2

1

Results and Discussion PA6 and PBT The rate of oxygen uptake of PA6 and PBT was determined in duplicate (Figure 3). Both polymers oxidize and show a linear relation between weathering time and oxygen uptake without an induction time. The oxygen uptake rate is a little higher for PBT than for PA6. Oxygen uptake leads to several oxidation products. Two major gaseous products that are formed are CO and C 0 . The relation between the oxygen uptake and the CO and C 0 formed are shown in Figure 4 and Figure 5 respectively. In the case of PBT the conversion of oxygen into CO and C 0 is independent of the weathering time. For PA6, after an oxygen uptake of about 600 mmol/kg, which corresponds to a weathering time of 100-150 hours, an increase in the C 0 formation rate was observed. The conversion of oxygen into CO is about 5% and into C 0 about 10%. For both polymers CO can be formed by photolysis, but in the case of PA6, C 0 can only be formed by oxidation. 2

2

2

2

2

2



293

Figure 2. Irradiation wavelength spectra in Suntest CPSandXXL+ in comparison to the terrestrial solar spectrum.

However, the amounts formed by photolysis are limited as shown by weathering experiments in a nitrogen atmosphere (Figure 6). In this case for both polymers a constant formation rate of CO (PBT and PA6) and C 0 (PBT only) was observed. Based on the comparison of the amount of CO formed in PBT and PA6, it can be calculated that the photolysis rate in PBT is 3 times higher than in PA6. However, for PBT, when the amount of C 0 formed by photolysis is also taken into account, the difference in photolysis rate between PA6 and PBT becomes even more pronounced. In PBT three possible Norrish 1 reactions can take place, two lead to C 0 and one to CO (see Scheme 4). The ratio between the amount of CO and C 0 formed in PBT is about 1:2, thus it seems that all three Norrish 1 reactions are of equal importance. For applying engineering plastics in an outdoor environment, the change of the mechanical properties is one of the most important factors. During weathering in oxygen and nitrogen, the drop of the elongation at break (EAB) was determined. For PA6 the results are shown in Figure 7 and for PBT in Figure 8. In the case of PA6 there is a large influence of the environment on the decline of the mechanical properties. In air the degradation is much faster than in nitrogen {Figure 7). In nitrogen PA6 shows some decrease in mechanical properties. This drop corresponds to the formation of CO (Figure 9) and is caused by photolysis. However, the decrease of the EAB in air is much faster, 2

2

2

2



294

Figure 3. Oxygen uptake of PA6 and PBT (in duplicate) during weathering in borosilicate vessels filled with dry air in the WOM.

Figure 4. CO formed versus Oxygen uptake ofPA6 and PBT (in duplicate) aged in borosilicate vessels filled with dry air in the WOM.



295

Figure 5. C0 formed versus Oxygen uptake of PA6 and PBT (in duplicate) aged in borosilicate vessels filled with dry air in the WOM. 2

Figure 6. CO (PBT and PA6) and CO (PBT) formed during weathering in borosilicate vessels filled with dry nitrogen as a function of time in a WOM. 2



1

3

-C-rO-r-Ch^^*"

1

ο ; . il !

2

Η ;

·/

#

2

2

\—C—Ο* +*CH ^

Ο

#

Ο

w v

\—C + 0—CH *"~

Scheme 4. Possible Norrish 1 reactions in PBT.

1 "


•

^

Λ^ν/

\

\ ·

+ co

· + CO

2

297 which means that for PA6, photo-oxidation is much more important than photolysis. For PBT the situation is different. In this case there is no large difference in decline in EAB in nitrogen and in air (Figure 8). This means that the loss of the mechanical properties is mainly due to photolysis. In this case the decrease in EAB corresponds to the formation of CO and C 0 (Figure 9). It was shown that PBT oxidizes, but this oxidation does not lead to a faster decrease of the mechanical properties, which means that oxidation does not contribute to the breaking of chains and thus has to be a result of the oxidation of chains broken by photolysis.


2

PBT and its Polytetrahydrofuran (PECO-1) or Poly(propylene glycol) (PECO-2) Copolymer As was shown above the drop of the elongation at break of PBT is mainly due to photolysis. For polyether-ester copolymers containing pTHF or PPG this is different. In a nitrogen atmosphere a drop in the elongation at break was found, but for both copolymers the drop in the elongation at break in air is faster (Figure 9). As expected the PPG containing copolymer shows a faster decrease in the elongation at break in air than the pTHF containing copolymer (Figure 9), which can be ascribed to the higher oxidizability of PPG in comparison to pTHF, as is shown in Figure 10. In this graph the oxygen uptake of these polymers and PBT are shown during ageing in closed borosilicate vessels. The reproducibility of the oxygen uptake of both PECOs is quite good. At the beginning the oxidation rate of all polymers is about comparable. After about 5 hours PECO-2 starts to deviate from the PBT curve and after 20 hours PECO-1. It seems that sufficient radicals have to be produced in the PBT before the polyether part starts to oxidize. For PBT and its copolymers CO and C 0 can be formed by photolysis as well as by oxidation. The CO and C 0 formation rate of PBT and its copolymers in nitrogen is depicted in Figure 11 and Figure 12. These gases can only be formed in the PBT part of the copolyetheresters. However, although the amount of PBT in both copolymers is lower than in the PBT homopolymer the formed amount of CO and C 0 is not. It seems that the incorporation of polyether leads to a higher CO and C 0 formation rate, especially for PPG. It is known that the incorporation of this polyether into PBT leads to a change in morphology of the amorphous PBT part, causing an increased mobility of this phase (9), which might be the reason for the higher CO and C 0 formation rate for the PPG containing copolyetherester. For PBT and its copolymers the formation rate of CO and C 0 in air (Figure 13 and Figure 14) is much larger than in nitrogen. Thus, in air the majority of the CO and C 0 are formed by oxidation. 2

2

2

2

2

2

2



298

Figure 7. Relative drop of the elongation at break ofPA6 weathered in borosilicate vesselsfilledwith dry air or dry nitrogen as a Junction of time in a WOM.

Figure 8. Relative drop of the elongation at break of PBT (in duplicate) weathered in borosilicate vesselsfilledwith dry air or dry nitrogen as a function of time in a WOM.


299


APBTCO

Elongation at break (%)

Figure 9. Amount of CO and C02 formed versus drop in elongation at breakfor PA6 and PBT weathered in borosilicate vesselsfilledwith dry nitrogen in a WOM.

The oxygen uptake of PECO-2 starts to deviate from the PBT curve after about 5 hours and PECO-1 after about 20 hours. For the formation of CO and C 0 this is different. For PECO-2, the formation of CO starts to deviate from the PBT line at about the same time as for the oxygen uptake, but for the amount of C 0 this takes more than 20 hours. For PECO-1, the formation of C 0 starts to deviate from the PBT line at around the same time as for the oxygen uptake, but for the formation of CO this deviation appears already earlier. From the oxygen uptake and CO/C0 formation in nitrogen, it can be concluded that the main degradation mechanism of both PECOs is photooxidation, which is initiated by the photolysis of the PBT phase. 2

2

2

2

Bisphenol A Polycarbonate Another engineering plastic that can degrade by photolysis and photooxidation is polycarbonate. It was shown that the relative importance of these mechanisms depends on the irradiation wavelengths used; the photo-Fries rearrangement reaction is more likely to occur when light with wavelengths shorter than 300 nm is used, whereas photo-oxidation reactions are more important when light of longer wavelengths (> 340 nm) is used (4, 10, 11). It was also postulated that the photolytic reaction products can initiate the photooxidation (72), although other oxidation initiating reactions were postulated too (13). PC samples were irradiated with differently filtered xenon light (Figure 2).



300

Figure 9. Relative drop of the elongation at break of PECO-1 and PECO-2 (in duplicate) aged in borosilicate vessels filled with dry air or dry nitrogen as a function of time in a WOM

Figure JO. Oxygen uptake of PECO-J and PECO-2 (in duplicate) and PBT in borosilicate vessels filled with dry air as a function of time in a WOM



301

Figure IL CO formed as a function of ageing time of PECO-1 and PECO-2 and PBT (in duplicate) aged in a borosilicate vessel filled with dry nitrogen in a WOM.

Figure 12. CO formed as a function of time of PECO-1 and PECO-2 and PBT (in duplicate) aged in borosilicate vesselsfilledwith dry nitrogen in a WOM. 2



302

Figure 13. CO formed as a function of ageing time of PECO-1 and PECO-2 (in duplicate) and PBTaged in borosilicate vessels filled with dry air in a WOM.

Figure 14. C0 formed as a function ageing time of PECO-1 and PECO-2 (in duplicate) and PBTaged in borosilicate vessels filled with dry air a WOM. 2


303 UV absorptions at 320 nm were measured as an indication for photo-Fries rearrangement reactions and IR absorptions at 1713 cm" were used as an indication for photo-oxidation. Irradiation in the CPS leads to an increased absorption at 320 nm compared to irradiation in the XXL+ (Figure 15), which might be an indication for more photolytic reactions in the CPS. Fluorescence spectroscopy can also be used to detect photo-Fries rearrangement products in polycarbonates (14,15). Emission spectra of solutions in DCM of polycarbonate films irradiated in the CPS and XXL+ were recorded and normalized to the second-order Rayleigh scattering peak at 622 nm. The normalized intensity at 480 nm increases with increasing irradiation times. The change in fluorescence intensity is shown in Figure 16. This figure shows that in the CPS this intensity increases more rapidly with increasing irradiation times than in the XXL+. This also means that the formation rate of photo-Fries products in the CPS is higher than in the XXL+. The increased amount of photo-Fries reactions can be attributed to the presence of shorter wavelengths in the CPS. The rate of carbonyl formation (absorption at 1713 cm" ) is comparable in both cases (Figure 17), from which can be concluded that in both cases the photo-oxidation rate is similar. Thus, although in the CPS the photo-Fries reaction is more important, this does not lead to a faster oxidation. From this, it can be concluded that the initiation of the photo-oxidation results from reactions other than the photo-Fries reaction.


1

1

Conclusions Polymers that absorb terrestrial sunlight wavelengths can degrade under environmental conditions according to different mechanisms. •

• •

Photolysis, light absorption as a result of the inherent polymer structure results in chemistry causing changes in the molecular weight distribution. Photo-oxidation initiated by photolysis reactions. Photo-oxidation initiated by impurities not being part of the inherent polymer structure.

For PBT it was demonstrated that loss of mechanical properties in an inert atmosphere occurs with a comparable rate as in air, from which it can be concluded that photolysis is the most important degradation mechanism. For polyether copolymers of PBT the situation changes. Due to the high oxidizability of the polyether moiety, the most important UV-degradation mechanism for these polymers is photo-oxidation, which is probably initiated by photolysis products of the PBT. The same mechanism is important for PA6; the



304

Figure 15. Absorbance at 320 nm as a function of irradiation time in the andXXL+.

Figure 16. Fluorescence emission at 480 nm (excitation wavelength: 310 as a function of irradiation time in the CPS and XXL+.



305

1

Figure 17. Absorbance at 1713 cm' as a function of irradiation time in the CPS andXXL+.

main degradation mechanism is photo-oxidation, which is initiated by photolytic reactions. PC also shows photolytic (in this case photo-Fries) and photo-oxidation reactions, but when PC is irradiated with wavelengths above 300 nm, photolysis only plays a minor role. Photo-oxidation is the major degradation mechanism; it was shown that the initiation of this photo-oxidation is not due to the photoFries rearrangement reaction.

References 1. Gijsman, P.; Meijers, G.; Vitarelli, G. Polym. Degrad. Stab. 1999, 65, 433441. 2. Rabek, J. F. Polymer Photodegradation: Mechanisms and Experimental Methods; Chapman and Hall: Cambridge, 1995. 3. Gijsman, P. Polym. Degrad. Stab. 1998, 60, 217-219. 4. Lemaire, J.; Gardette, J. L.; Rivaton, Α.; Roger, A : Polym. Degrad. Stab.l9S6,15,1-13. 5. Diepens, M . ; Gijsman, P. Polym. Degrad. Stab. 2007, 92, 397-406. 6. Gijsman, P.; Hennekens, J.; Tummers, D. Polym. Degrad. Store. 1993, 39, 225-233. 7. Hoyle, C. E.; Sha, H.; Nelson, G. L. J. Polym. Sci. A: Polym. Chem 1992, 30,1525-33.


306 8. 9. 10. 11. 12. 13. 14.


15.

Potyrailo, R.; Lemmon, J. U. S. Patent 6,193,850 Β1, 2001. Lange, R. F. M.; Gabrielse, W. e-Polymers 2004, no 064. Rivaton, Α.; Sallet, D.; Lemaire, J. Polym. Photochem. 1983, 3, 463-481. Torikai, Α.; Mitsuoka, T.; Fueki, K. J. Polym. Sci. A: Polym. Chem. 1993, 31, 2785-2788. Factor, Α.; Chu, M. L. Polym. Degrad. Stab. 1980, 2, 203-223. Factor, A. Handbook Polycarbonate Science and Technology; Marcel Dekker: New York, 2000; pp 267-292 Hoyle, C. E.; Sha, H.; Nelson, G. L. J. Polym. Sci. A: Polym. Chem. 1992, 30, 1525-33. Shah, H.; Rufus, I.; Hoyle, C. Macromolecules 1994, 27, 553-61.


Polymer Degradation and Performance - American Chemical Society

Recommend Documents