PHOTO-CHEMICAL REACTIONS IN COMMERCIAL POLY(OXYMETHYLENE)

N. S. ALLEN & J. F. MCKELLAR

Department of Chemistry and Applied Chemistry, Unicersity of SalJbrd, SalJbrd M5 4WT. Lancs, Great Britain

(Received: 21 April, 1978)

ABSTRACT

Poly(oxymethylene) has been examined using infra-red, ultrariolet absorption and luminescence techniques. The first technique indicates that aldehydic carbonyl groups are present in the polymer whereas the latter two indicate the presence of ~,13- unsaturated carbonyl groups. The behat'iour of these impurity carbonyl species during irradiation under sunlight-simulated conditions has also been examined. Possible mechanisms for the participation of these chromophoric units in the photo- oxidation of the polymer are discussed.

INTRODUCTION

Earlier studies have shown that poly(oxymethylene) is susceptible to sunlight- induced degradation. 1- 3 These studies indicated that direct bond scission at the ~CH2--O ~ linkage is the primary process responsible for photo-degradation. However. this polymeric unit should not absorb light above 220 nm ~ indicating, therefore, that impurity chromophores are responsible for sunlight-induced degradation. In this paper we report on the nature of some of the potentially photo- reactive species and their participation in the photo-degradation of the polymer.

EXPERIMENTAL

Materials Poly(oxymethylene) in chip form (Duracon M90-02) was supplied by Dr K. Tsuji of

Sumitomo Chemical Co. Ltd, Japan. The polymer contains a few ---C---C-- bonds 47

Polymer Degradation and Stability (1) (1979)--~ Applied Science Publishers Ltd, England. 1979 Printed in Great Britain

48 Y. S. ALLEN, J, F. MCKELLAR

as comonomers, has a melt flow index of 9.0. specific gravity of 1-410 and a melting point of 165 °C. 5 The polymer chip was pressed into film of 50 and 200/~m thickness at 190°C for 1 rain.

Ultraviolet absorption spectra Ultraviolet absorption spectra were recorded using a Pye Unicam SP8-100

spectrophotometer. The instrument has a zero back-off for an absorbance of 3"0.

Infra-red spectra Infra-red spectra were recorded using a high resolution grating Perkin-Elmer

(Model 521) spectrometer.

Luminescence spectra Corrected fluorescence and phosphorescence spectra and phosphorescence

lifetimes were recorded using a double grating (1200 lines ram) Hitachi Perkin- Elmer MPF-4 spectrofluorimeter with a wavelength sensitivity of 220 to 950 nm.

Fully corrected excitation spectra below 250 nm were also obtained using the newly developed tar-ultraviolet spectrofluorimeter of Cundall et al. (see reference 10, instrumental details to be published).

Photo-oxidation The poly(oxymethylene) films were irradiated in a Xenotest-150 (supplied by

Original Hanau, Quartzlampen, GmbH) set up for natural sunlight-simulated exposure conditions (45 °C; 50 °/o relative humidity).

RESULTS

(1) Ultraviolet absorption spectra Figure 1 shows the ultraviolet absorption spectrum of poly(oxymethylene) film

(50 #m thickness) before and after irradiation. Before irradiation the ultraviolet absorption spectrum of the polymer film shows a broad band with a wavelength maximum at 280 nm and strong absorption below 240 nm. During irradiation the intensity of the absorption band at 280 nm gradually decreases. The absorption spectrum was measured using a zero back-off correction.

(2) Infra-red spectra Figure 2 shows the infra-red spectrum of poly(oxymethylene) film (50#m

thickness) in the region of 1800 to 1500 cm- ~ before and after irradiation. Before irradiation, the infra-red spectrum of the polymer shows an absorption band at 1735 cm- t and a much weaker band at 1640 cm- t. During irradiation the intensity

PHOTO-CHEMICAL REACTIONS IN COMMERCIAL POLY(OXYMETHYLENE) 49

2.5

2.0

1.5 I.iJ U Z

r r

O ~n (:9 <

It

\\ \ \

\ \ \ \ \ \ \ ' \ \ \"

\

\~ \ \ \ , \

\ \

\ \ i\

I 2~o 2~o 2~o 2~o 2~o 2~o 35o WAVELENGTH/nm

Fig. 1. Ultraviolet absorption spectrum ofpoly(oxymethylene) film (50/am thickness) before ( ) and after 45 h. (---), 120h (----) and 165 h (- × - x ) irradiation in a Xenotest-150,

of the band at 1735 cm - t increases significantly while the band at 1640 c m - t shows a much smaller change.

(3) Luminescence spectra Figure 3 shows the fluorescence and phosphorescence excitation and emission

spectra ob ta ined from commercial poly(oxymethylene) film (200/~m thickness). It is seen that the excitat ion spectrum for the fluorescence has a m a x i m u m at 285 nm

while that of the phosphorescence has a max imum at 290 n m wi th a shoulder at

50 N . S . ALLEN, J. F. MCKELLAR

Fig. 2.

W u z

I-- I-.

Z

I--

°° I 80 0&45 H

4 0 -

17~0 16100 WAV E NU M BER///C Ivl "f

Effect of irradiation in a Xenotest-I50 on the infra-red spectrum of poly(oxymethylene) film (50#m thickness) in the region 1800 to 1500cm- 1.

about 320nm. Using the far-ultraviolet spectrofluorimeter 6'7't° another, much stronger, excitation band is observed for the fluorescence with a wavelength maximum at 210nm (Fig. 4). The fluorescence and phosphorescence emission spectra are broad and structureless with wavelength maxima at 320 and 415 nm, respectively. The lifetime (zl/e) of the phosphorescence emission is about 1 sec. During irradiation the intensity of both fluorescence and phosphorescence emissions gradually decreases (Fig. 5). No change was observed in the phosphorescence emission lifetimes.

DISCUSSION

The results indicate that there are different types of impurity carbonyl groups present in poly(oxymethylene). The broad ultraviolet absorption band at 280 nm is

PHOTO-CHEMICAL REACTIONS IN COMMERCIAL POLY(OXYMETHYLEN'E) 51

FLUORESCENCE

I I

PHOSPHORESCENCE

WAVE L E NG T H i n m /

Fig. 3. Fluorescence (300K) and phosphorescence (77K) excitation and emission ~pectra of poiy(oxymethylene) film (200 um thickness).

52 N. S. ALLEN, J. F. MCKELLAR

usually associated with ketonic/aldehydic carbonyl chromophores 1 ~ while the infra- red band at 1735 cm- t is more specific and is associated with the aldehydic carbonyl groups, t - 3 The fluorescence and phosphorescence excitation spectra, on the other hand, exhibit similar excitation spectra to those of 2,/3-unsaturated carbonyl groups of the enone (or-al) and dienone (or-al) type, respectively. 6- 9.~2 Further, the long lifetime of the phosphorescence is consistent with the assignment of the emission to :~,/3-unsaturated carbonyl groups. This does not exclude the possibility that

Fig. 4.

), i -

l.-

z

L I t 160 2 0 0 2 4 0

WAVE.LE NG T H / n m /

Far-ultraviolet fluorescence excitation spectrum (300 K) of poly(oxymeth}lene) film (200~m thickness). Emission i = 320 nm.

saturated carbonyl groups are present in the polymer since these groups may be either weakly or non-phosphorescent.~3

Thus it would appear that there are two different types of impurity carbonyl chromophore in poly(oxymethylene). These are: saturated aldehydic carbonyl groups detected by infra-red and e,~-unsaturated carbonyl groups detected by luminescence spectroscopy. The fact that the absorption band at 280 nm decreases during irradiation suggests that it must be associated with the :~,~-unsaturated carbonyl groups and not with the saturated aldehydic groups detected by infra-red. Furthermore, the saturated aldehydic carbonyl groups appear to be one of the final products of photo-oxidation of the polymer (Fig. 2) as found by earlier workers, t - 3

PHOTO-CHEMICAL REACTIONS IN COMMERCIAL POLY(OXYMETHYLENE) 53

FLUOP, ESC ENCE PHOSPHOtRESC ENCE

I---

z

v-

' " ' I f ' . ' 7 - ."- , ,,. \ I rr . h ~ ' "40 ~ x "

EMISSION WAVELENG TH/nm

Fig. 5. Fluorescence (300 K) and phosphorescence (77 K) emission spectra of poly(oxymethylene) film (200,um thickness) before ( ) and after 45 h (- - -I. 120 h (--'1 and 165 h (- x - x ) of irradiation in a

Xenotest- 150.

The terminal aldehydic carbonyl groups may initiate photo-oxidation of the polymer by the following bond scission process: ~ -3

--CH2--O--CH2--O--CHO , ~CH:--O--CH2--O" + . C H O - (1)

The .CHO radical may then abstract a hydrogen atom from the polymer to form formaldehyde: 1 - 3

~CH2--O--CH2--O~ + " C H O - ~ ~CH:--O~H--O +

H2C:O

(2)

The polymer radical ( ~ CH2- -O- ) may also abstract a hydrogen a tom from the

5 4 N . S . ALLEN, J. F. MCKELLAR

polymer to form a terminal hydroxyl group as found earlieH -3 or, alternatively, it may form an aldehydic end group:

~ C H 2 - - O - - - C H 2 - - O -

P - - H , ~ CH2--O--CH2--OH + P.

I , ~CH2--O--CH + H.

O

(3)

The :t,/3-unsaturated carbonyl groups, on the other hand, are gradually consumed during irradiation. These chromophoric units are normally photolysed initially via an isomerisation process to give fl-y-unsaturated carbonyl groups, 8 e.g.:

I I L h,. I I I ~ C----C==C--C ~ , ~ C = C - - - C - - C -,-

I II I II O O

(4)

In the case of poly(oxymethylene), however, the presence of an alternate oxygen atom in the chain would inhibit this process, indicating that photo-reaction must occur by another mechanism. Cross-linking between adjacent :t,fl-unsaturated carbonyl groups would appear unlikely due to the low concentration of these groups in the polymer. For the present, it appears that two scission processes would account for the results. These are:

I I h,. [ I ~ O--C==C--C--CH2--O ~ , -,- O--C==C--C • + 'CH~--O ~

I~ k[ 0 0

(5)

I I hv [ I ... O - - C ~ C - - C - - O - - C H 2 ~ , ~ O - - - C = = C - - - C . + . O - - - C H 2

O O

(6)

Both reactions would be consistent with the formation of known products.1 - 3

ACKNOWLEDGEMENT

The authors would like to thank Dr K. Tsuji of Sumitomo Chemical Co. Ltd, Japan, for supplying the poly(oxymethylene) used in this study.

PHOTO-CHEMICAL REACTIONS IN COMMERCIAL POLY(OXYMETHYLENE) 55

REFERENCES

1. N. GRASS~E and R. S. ROCHE, Makron¢ol. Chemie., 112, 34 (1964). 2. P. G. KELLEHER and L. B. JASSlE, J. AppL Polym. Sci., 9, 2501 (1965). 3. O. R. HUGHES and L. C. COARD, J. Polym. Sci., Polym. Chem. Ed., 7, 1861 (1969). 4. B. RANBY and J. F. RABEK, Photodegradation, photooxidation and Photostabilisation o]'polymers,

New York, John Wiley Interscience, Publishers (1976). 5. K. TsuJI, Private Communication. 6. N. S. ALLEN, R.. B. CUND^LL, M. W. JONES and J. F. MCKELLaR, Chem. Ind., (London), 110 (1976). 7. N. S. ALLEN, J. HOMER and J. F. MCKELLaR, Chem. Ind., (London), 692 (1976). 8. N. S. ALLEN, J. HOMER and J. F. MeKELLAR, J. Appl. Polym. Sci.. 21, 2261 (1977). 9. N. S. ALLEN, J. HOMER, J. F. MCKELLAR and D. G. M. WOOD, J. Appl. Polym. Sci., 21, 3147 (1977).

10. R. B. Ct,'NDALL, D. GRIEFIrHS, M. W. JONES, R. C.' M. LERNER and J. E. G, WHEATON, to be published.

I1. R. B. Fox, L. G. ls/~Acs and S. STOKES, J. Polym. Sci., Polym. Chem. Ed., 1. 1079 (1963). 12. N. S. ALLEN, J. F. MCKELLaR and D. WILSON, J. Polym. Sci.. Polym. Chem. Ed., 15, 2793 (1977). 13. A. CHARLESS¥ and R. H. PARTRIDGE, Proe. Roy. Soe. (London), A283, 312. 329 (1965).