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Ageing of butyl rubber against UV irradiation
PolymerPhou~c~m/stry2 (1982) 257-267
AGEING
OF BUTYL RUBBER UV IRRADIATION
AGAINST
R. P. SIr~GH
Polymer Chemistry Division, National Chemical Laboratory, Pune 411008, India and R. C~A~3RA
Regional Engineering College, Kurukshetra- 132119, India (Received: 28 August, 1981)
ABSTRACT
The photodegradation and stabilisation of butyl rubber have been studied in air in the temperature range of 258 to 318 K using a monochromatic light of 366 nm with a constant intensity flux of 1.68 × 10 -8 einstein s -~ cm -2 in the absence and presence of cuprous di-isopropyl dithiophosphate. Irradiations were carried out on films at 0, 2, 4, 6, 8, 10, 12 and 14 h. A n investigation into the nature and molecular changes during the irradiation with ultra-violet light (366 nm ) of butyl rubber has been conducted by light scattering technique. The quantum yields of the photolysis of the polymer have been determined using a potassium ferrioxalate actinometer. Light scattering data have been processed to yield the activation energies in the absence and presence of O. 1 wt. % stabiliser. The incorporation of 0.7 wt. % cuprous di-isopropyl dithiophosphate in the matrix of the rubber film exercises the maximum protective influence on the photolytic degradation of the rubber.
INTRODUCTION
The polymeric materials and rubbers in common use degrade on exposure to light. It has been demonstrated that the deterioration of rubber and polymers is due to the ultra-violet portion of sunlight reaching the earth. Because of the many advantages of rubber and plastic materials considerable efforts have been made to improve their light stability. The photo-oxidation and stabilisation in the solid state have been studied 1"2"3"4"5for only a few copolymers in air. A sound understanding of the photolytic processes can be obtained with quantum 257 Polymer Photochemistry 0144-2880/82/0002-0257/$2'75©Applied England, 1982 Printed in Northern Ireland
Science Publishers Ltd,
258
R . P . SINGH, R. CHANDRA
yield determinations alone. The commercially available salicylates, ohydroxybenzophenones, o-hydroxyarylbenzotriazoles, hindered phenols and amines have been used as ultra-violet stabilisers. 6 The investigation deals with a study of cuprous di-isopropyl dithiophosphate incorporated in the film of butyl rubber which has been irradiated by monochromatic light of 366 nm wavelength in the temperature range of 258 to 318 K and quantum yields for random chain scission processes were determined. The chemical methods for the estimation of carbonyl index and hydroperoxide groups in the irradiated butyl rubber have been described. The proposed mechanism of photodegradation and stabilisation of butyl rubber, containing different concentrations of the stabiliser has also been suggested.
EXPERIMENTAL
Methodology (i) Preparation of polymer film: Butyl rubber (BR), a copolymer of isobutylene and 3.5% isoprene supplied through the courtesy of M/s. Swastic Rubber Company, Poona, was cut into small pieces and subjected to acetone and methanol extraction for 75 h in a Soxhlet. The sample was kept under vacuum to a constant weight and then with petroleum ether (40-60°C) for four days, when a part of the rubber went into solution. The acetone extraction and petroleum ether dissolution were conducted in dark. The precipitation of rubber was done from 1% solution in petroleum ether with acetone. The precipitated rubber was dried in vacuum to a constant weight and preserved in a black painted desiccator at room temperature. The incorporation of cuprous di-isopropyl dithiophosphate (CDIP) into the matrix of films, was affected by dissolving the BR (5%) in cyclohexane containing 0.1, 0.7, 1.2 and 2.0 wt. % of CDIP. The uniform thickness films of B R (215/zm) were prepared by casting solutions (6 ml) of cyclohexane containing BR in the absence and presence of CDIP on quartz plates which were scaled with Pyrex glass plates having a 5 cm diameter bore. The films were dried in vacuo for 20 h to a constant weight. The chelate, cuprous di-isopropyl dithiophosphate (CDIP) was synthesised according to literature procedure. 7 (ii) Photoirradiation: The photoirradiation was carried out in air with a 125 W (230V) mercury vapour lamp whose glass casing was removed. The dried B R films in the absence and presence of CDIP were irradiated using monochromatic light of 366 nm wavelength for different intervals of times in the temperature range of 258-318 K. The temperature of the system was controlled within +1°C. The quantum yields for polymer chain scissions and
259
A G E I N G O F B U T Y L R U B B E R A G A I N S T UV I R R A D I A T I O N
intensity of radiation actinometer. 8
were
determined
using a potassium
ferrioxalate
Methods for characterisation (i) Refractive index increment (dn/dc): The refractive index increment (dn/dc) with concentration of B R and the optical constant (H) for the solution in cyclohexane were determined using a Brice-Phoenix differential refractometer (Phoenix Precision Instrument Co., Philadelphia, USA) and the values at 288 K are dn/dc = 0 . 1 2 4 3 , and 1 . 9 5 6 4 x 10 -~.
(ii) Light scattering studies: The stock solutions of B R with and without CDIP were centrifuged to 15 000 rpm for 90 min to remove dust particles. The light scattering measurements were carried out with a light scattering photometer designed and calibrated by the present authors. 9 (iii) Quantum yield: The method for quantum yield determination with potassium ferrioxalate actinometry has been described elsewhere. 1°
(iv) Spectrophotometric measurement: Infra-red spectra of irradiated films of B R were recorded using a Perkin-Elmer (Model 21) infra-red spectrophotometer in the absence and presence of 0.1 wt. % CDIP at 288 K. (v) Carbonyl group formation: The carbonyl groups were determined by a base line method H from IR spectra. (vi) Hydroperoxide group formation: The hydroperoxides in irradiated B R films were determined by an iodometric method.~2
RESULTS A N D DISCUSSION
Plots of weight average molecular weight ]0/w as a function of irradiation time are shown in Fig. 1 for B R films irradiated in the absence and presence of 0.1 wt. % C D I P as a photostabiliser in air in the temperature range of 258 to 318 K with a light intensity flux of 1.68 x 10 -8 einstein s -1 cm -2 for different intervals of time. The plots show a rapid decrease in/(;/,~ initially which then slows down at each temperature. The weight loss is greater for the irradiated control film as compared to the corresponding values of B R stabilised films. This indicates that C D I P retards the degradation of the rubber. The initial rapid drop in ]firw is due to scission of bonds at various weak links that may be distributed along the polymer chain. Jellinek and Flagsman 13 considered that
260
R.P. SINGH, R. CHANDRA
5~
3 :<
3 I:E
ol
0 I R R A D . T I M E (h')
Fig. 1. Variations of weight-average molecular weight versus times of B R films in the absence and presence of 0.1 wt. % CDIP at various temperatures irradiated with 366 nm light in air. At 258 K--(3, BR; O, B R + C D I P . At 273 K--(~, BR; (]D, B R + C D I P . At 2 8 8 K - - A , BR; A, B R + C D I P . At 303 K----~, BR; I , B R + C D I P . At 318 K----~, BR; 1], B R + C D I P .
oxygen attacks only the weak links in such cases. Cameron and Grassie TM have also contended that the initial change in /(~w is due to the scission of weak links. The curve tends to increase at longer irradiations suggesting that crosslinking predominates at longer exposures. The specific rate constant, kl is calculated from the slope of a versus time curve, a = kit for a random chain degradation, where a is the degree of degradation. From the linear plots of log~ k~ versus I/T, the activation energy ( h E ) as well as the frequency factor (A) for degradation is calculated. The method of least squares can be used to evaluate the slope (-AE/R) and the intercept (In A ) from these data. The values of AE and A are substituted in eqns (1) and (2) xs with the activation energies in calories and frequency factor in s -1. k~ = 10.41 x 10 -3 exp
(-7240/RT) s -1 (BR)
(1)
261
AGEING OF BUTYL RUBBER AGAINST UV IRRADIATION
and
kl=12.63xlO-3exp(_9962/RT)s
1
( B R + 0 . 1 w t . % CDIP)
(2)
The values of kl were nearly constant with time indicating the reactions to be zero order. Simha and Wall a6 have pointed out that in random chain scission, the photodegradation is zero order with respect to polymer and oxygen concentration. For a zero order reaction AE may be equated to the heat of formation of the activated complex AH*, without any appreciable error. The free energy of activation AF* remains virtually constant at 39.21 kcal mo1-1 at 400 K in both cases which suggests that the rate determining step is the same in both the systems. The higher values of AH* indicate that CDIP retards the rate of degradation of BR. Figure 2 shows the variations of dissymmetry ratio Zd, a ratio of scattered intensity at 45 ° to that at 135 °, versus time for BR films irradiated in the absence and presence of 0.1, 0.7, 1.2 and 2.0wt. % CDIP in air for different time intervals at 288 K. Zd decreases monotonously with increasing time in the absence and presence of 0 . 1 w t . % CDIP but the values of Zd for the irradiated stabilised films are greater than those for the corresponding control films. This means that CDIP retards the photodegradation of BR. This figure also shows that at concentrations beyond 0.7 wt. % CDIP brings a saturation limit in photostabilisation of BR. Figure 3 also confirms that the saturation
3
Zd
0
2
4
6
8
t0
1"2
14
IR R AD. TIME ~h) Fig. 2. Variations of dissymmetry ratio versus time of B R films in the absence and presence of C D I P at 2 8 8 K irradiated with 3 6 6 n m light in air. O, B R ; O, B R + 0 . 1 % CDIP; A, B R + 0 - 7 % CDIP; V, B R + 1.2% CDIP.
262
R. P. SINGH, R. CHANDRA 1,0
"8
o ,6 A
_='4
\.
.2
'~2
6
(.
'
0
"~ I "2
,
I "4
STABILISER
Fig. 3.
Rate of protection of BR films irradiated with 366 nm light in air versus concentration of C D I P at 288 K.
protective action is reached at 0.7 wt % CDIP where kl and ko are the rate constants in the presence and absence of the stabiliser, respectively. The degree of protection is temperature dependent to some extent. Quantum yields are determined by eqn (3) 17 1
1
m
Pw.t Pw,o= w N ~bIat
(3)
where Pw.t and Pw,o are weight average degree of polymerisation at time t and zero, respectively; w is the weight of irradiated polymer, m is the molecular weight of the monomer, N is the Avogadro number, th is the quantum yield, Ia is the light absorbed in the film and t is the time. Ia is determined by potassium ferrioxalate actinometry: s The quantum yield ~b can be determined from eqn. (3) by plotting 1/pw.t versus irradiation time. Table 1 shows that
TABLE 1 QUANTUM YIELDS FOR PHOTO-OXIDATIVE S T A B I L I S A T I O N O F B R IN T H E A B S E N C E
DEGRADATION AND AND PRESENCE OF
0-1 wr.% cDw IN Am AT VARIOUSTEMPERATURES
Temp (K)
BR
258 273 288 303 318
0.20 1"16 2"18 4"37 5"14
~ x 10 -4
chain scissions per absorbed photon B R + O. 1 wt. % C D I P 0.009 0"12 0.43 0"91 0"99
263
AGEING OF BUTYL RUBBER AGAINST UV IRRADIATION
larger quantum yields are obtained for control BR at all temperatures and the smaller values are typical of stabiliser mixed BR films in which UV absorption occurs at CDIP. The quantum yields are less than unity, implying that in a macromolecule the energy is absorbed at one site which is partitioned over many bonds so that the probability of a single bond breaking is small, or the absorbed energy is dissipated by quenching reactions. The changes in carbonyl groups during the degradation of the polymer could be determined by IR spectral analysis. The rate of photo-oxidative degradation was measured by means of a carbonyl index at 1721 cm- 1 using an infra-red spectrophotometer: Carbonyl index = [log Io/It/d] × 100 where I0 is the intensity of incident light, It is the transmitted light intensity and d is the film thickness in microns. Figure 4 shows that the build-up in carbonyl index of the irradiated film is lower in the case of stabilised polymer compared to neat BR at each temperature. This clearly shows that CDIP retards the degradation process. The photo-oxidative degradation was also studied by the hydroperoxide concentration measurement by iodometry. 12 Figure 5 shows that the hydroperoxide concentration of CDIP incorporated rubber is lower than for base BR. These results suggest that CDIP stabilises the BR and destroys the 16 14
X
xlO _z8
°
~-6 Z
&
c~
~4 U 2 0
0
2.
4
6
8
10
12
14
IRRAD.T IME lh) Fig. 4. Variations of carbonyl index of BR films in the absence and presence of 0.1 wt. % CDIP irradiated with 366nm light in air. At 318K--C), BR; O, BR+CDIP. At 2 8 8 K - - A , BR; A, BR+CDIP. At 258 K---~, B R ; . , BR+CDIP.
264
R. P. SINGH, R. CFIANDRA
lo 2
0
2
4
6 8 IRRAD.TIME(h)
10
12
14
Fig. 5. Variations in hydroperoxide concentration of BR films in the absence and presence of 0.1wt.% CDIP irradiated with 366nm light in air. At 318K----O, BR; O, B R + C D I P . At 288 K--A, BR; i , B R + C D I P . At 258 K---~, BR; II, B R + C D I P .
unstable reaction intermediates, i.e. carbonyl and hydroperoxide to relatively stable products such as alcohols.
Mechanism of photodegradation and stabilisation of butyl rubber In polymers chain scission occurs via the formation of hydroperoxide, ROOH, or carbonyl groups as the primary products of oxidation and these decompose on irradiation to a variety of reaction intermediates depending upon the polymer structure and the reaction conditions. The hydroperoxide (ROOH) dissociates unimolecularly at the oxygen molecule diradical O w O link. The free energy of activation of the O - - O bond in a peroxide requires about 40kcalmo1-1 which is close to experimental free energy of activation (39.21 kcalmol-1). The polymer hydroperoxide can decompose when either energy transfer process o c c u r s 1 8 - 2 0 o r the sensitiser can propagate the freeradical-induced decomposition of the peroxide. 21 It can, therefore, be reasonably postulated that the rate controlling step for the photo-oxidative degradation process would be the initiation step. The isoprene units in butyl rubber are distributed at random along the polymer chain. The methylenic hydrogen in the a-position to the double bond
AGEING OF BUTYLRUBBER AGAINSTUV IRRADIATION
265
is a potential site for hydroperoxidation and this can lead to chain scission 22 as shown in Scheme 1. CH3
CH 3
I I --C--CH2--CH2--CH~C--CH2-I CH3
~ Hydroperoxidation CH 3
CH3
f I ~C--CH2--C--CH=C--CH2-I I CH 3 O--OH
Scheme 1.
~Chainscission CH3
CH3
I I ~C--CHzOH + ~H--CH~-----C--CH2~ I CH3
and other inert products Scheme 2. Thus, the isoprene units provide active centres for degradation and the process follows a random chain scission since these units are distributed at random (Scheme 2). The photo-oxidative degradation reaction scheme has been proposed by Chandra and Singh 3 as polymeric hydroperoxide formation (initiation), propagation, chain branching and termination. The chelate CDIP photostabilises the BR in two ways: first, it absorbs the photoenergy of 366 nm by resonating structures and chromophores; second, it converts the polymer peroxides and hydroperoxides to inert products. The peroxy radical is capable of abstracting an electron from the electron rich sulphur atom of CDIP and is converted into a peroxy anion (Scheme 3).
Cu
R02
H s O \ ~ "S+-]
~ RO2 +
[~3
/ f/
u
or
C3H70
\ / Scheme 3.
C3H7#
Cu
266
R.P. SINGH, R. CHANDRA
The peroxy radical may be stabilised by delocalisation (Scheme 4) FC IC3H70
\f
C3H70j
S Cu
RO--Oq
I
RO--O']
x//cu..
~S
c3.7%/s--s\/o.7c3 P~S C3H7 O /
F
P ~
.co
i + 2RO2 + Cu +
~OH7C 3
Scheme 4. The cuprous ion converts peroxy radicals into peroxy anions due to an electron transfer of the type shown in Scheme 5. Cu + + RO2 ~ Cu 2+ + R 0 2 Scheme 5. Thus CDIP stabilises BR by interference with the propagation reaction by absorption of light and decomposition of the macromolecular peroxides and hydroperoxides. By minimising the light energy absorbed by the polymer, the primary photochemical processes are inhibited. Due to the energy difference in the d-orbitals of CDIP, it absorbs UV incident light. Thus the chelate acts as a UV absorber and peroxide decomposer. CDIP may introduce additional terminal steps by acting as R', RO- and ROO- scavengers. In addition to this CDIP inhibits the initiation process by electron transfer and formation of inert charge-transfer complexes. ACKNOWLEDGEMENTS
The authors wish to thank Professor A. Syamal for providing facilities and for helpful discussions. The financial support for this work by CSIR, New Delhi is gratefully acknowledged. REFERENCES 1. 2. 3. 4. 5.
SCHMrrr,R. G. and HIRT, R. C., J. Appl. Polym. Sci., 7 (1963) 1565. CHANDRA,R. and BHATNAGAR,H. L., h'/d..1.. Chem., 14A (1976) 469. CHANDRA,R. and S~GH, R. P., Makromol. Chem., 181 (1980) 2737. CHANDRA,R. and SINGH,R. P., Ind. J. Technol., 18 (1980) 250. CHANDRA,R. and BHATNAGAR,H. L., Ind. J. Chem., 14A (1976) 272.
AGEING OF BUTYL RUBBER AGAINST UV IRRADIATION
267
HELLER, H. J., Europ. Polym. J., Suppl., 105 (1969). BURN, A. J., Tetrahedron, 22 (1966) 2153. 8. CALVERT,J. G. and PIT]S, N. J. Jr., Photochemistry, John Wiley, New York, 1966. 9. SINGH, R. P., Ph.D. Thesis. Submitted to Kurukshetra University, 1979, p. 79. 10. CHANDRA, R. and SINGH, R. P., Ind. J. Chem., 19A (1980) 849. 11. LUONGO, J. P., J. Polym. Sci., 42 (1960) 139. 12. WAGNER, C. D., SMITH, R. H. and PETER, E. D., Anal. Chem., 19 (1947) 976. 13. JELLINEK,H. H. G. and FLAGSMAN,F., J. Polym. Sci., 8 (1970) 71l. 14. CAMERON, G. G. and GRASSIE, N., Makromol. Chem., 53, (1962) 72. 15. GLASSTONE,S., EYRING, H. and LAIDER, K., The theory of rate process, McGraw-Hill, New York and London, 1941. 16. SI~IHA, R. and WALL, L. A., J. Phys. Chem., 56 (1952) 707. 17. JORTNER, J., J. Polym. Sci., 37 (1959) 199. 18. LUNER, C. and SZWARC,M., J. Chem. Phys., 23 (1965) 1978. 19. UBERREITER, K. and BRUNS, W., Macromolec. Chem., 68 (1963) 24. 20. WALLING, C. and GIBIAN, M., J. Am. Chem. Soc., 87 (1965) 3413. 21. SMITH, W. F. and ROSSITTER, B. W., Tetrahedron, 25 (1969) 2059, 2071. 22. BHATNAGAR,H. L. and SrNGH, M. M., Ind. J. Chem., 6 (1971) 218. 6. 7.
AGEING
OF BUTYL RUBBER UV IRRADIATION
AGAINST
R. P. SIr~GH
Polymer Chemistry Division, National Chemical Laboratory, Pune 411008, India and R. C~A~3RA
Regional Engineering College, Kurukshetra- 132119, India (Received: 28 August, 1981)
ABSTRACT
The photodegradation and stabilisation of butyl rubber have been studied in air in the temperature range of 258 to 318 K using a monochromatic light of 366 nm with a constant intensity flux of 1.68 × 10 -8 einstein s -~ cm -2 in the absence and presence of cuprous di-isopropyl dithiophosphate. Irradiations were carried out on films at 0, 2, 4, 6, 8, 10, 12 and 14 h. A n investigation into the nature and molecular changes during the irradiation with ultra-violet light (366 nm ) of butyl rubber has been conducted by light scattering technique. The quantum yields of the photolysis of the polymer have been determined using a potassium ferrioxalate actinometer. Light scattering data have been processed to yield the activation energies in the absence and presence of O. 1 wt. % stabiliser. The incorporation of 0.7 wt. % cuprous di-isopropyl dithiophosphate in the matrix of the rubber film exercises the maximum protective influence on the photolytic degradation of the rubber.
INTRODUCTION
The polymeric materials and rubbers in common use degrade on exposure to light. It has been demonstrated that the deterioration of rubber and polymers is due to the ultra-violet portion of sunlight reaching the earth. Because of the many advantages of rubber and plastic materials considerable efforts have been made to improve their light stability. The photo-oxidation and stabilisation in the solid state have been studied 1"2"3"4"5for only a few copolymers in air. A sound understanding of the photolytic processes can be obtained with quantum 257 Polymer Photochemistry 0144-2880/82/0002-0257/$2'75©Applied England, 1982 Printed in Northern Ireland
Science Publishers Ltd,
258
R . P . SINGH, R. CHANDRA
yield determinations alone. The commercially available salicylates, ohydroxybenzophenones, o-hydroxyarylbenzotriazoles, hindered phenols and amines have been used as ultra-violet stabilisers. 6 The investigation deals with a study of cuprous di-isopropyl dithiophosphate incorporated in the film of butyl rubber which has been irradiated by monochromatic light of 366 nm wavelength in the temperature range of 258 to 318 K and quantum yields for random chain scission processes were determined. The chemical methods for the estimation of carbonyl index and hydroperoxide groups in the irradiated butyl rubber have been described. The proposed mechanism of photodegradation and stabilisation of butyl rubber, containing different concentrations of the stabiliser has also been suggested.
EXPERIMENTAL
Methodology (i) Preparation of polymer film: Butyl rubber (BR), a copolymer of isobutylene and 3.5% isoprene supplied through the courtesy of M/s. Swastic Rubber Company, Poona, was cut into small pieces and subjected to acetone and methanol extraction for 75 h in a Soxhlet. The sample was kept under vacuum to a constant weight and then with petroleum ether (40-60°C) for four days, when a part of the rubber went into solution. The acetone extraction and petroleum ether dissolution were conducted in dark. The precipitation of rubber was done from 1% solution in petroleum ether with acetone. The precipitated rubber was dried in vacuum to a constant weight and preserved in a black painted desiccator at room temperature. The incorporation of cuprous di-isopropyl dithiophosphate (CDIP) into the matrix of films, was affected by dissolving the BR (5%) in cyclohexane containing 0.1, 0.7, 1.2 and 2.0 wt. % of CDIP. The uniform thickness films of B R (215/zm) were prepared by casting solutions (6 ml) of cyclohexane containing BR in the absence and presence of CDIP on quartz plates which were scaled with Pyrex glass plates having a 5 cm diameter bore. The films were dried in vacuo for 20 h to a constant weight. The chelate, cuprous di-isopropyl dithiophosphate (CDIP) was synthesised according to literature procedure. 7 (ii) Photoirradiation: The photoirradiation was carried out in air with a 125 W (230V) mercury vapour lamp whose glass casing was removed. The dried B R films in the absence and presence of CDIP were irradiated using monochromatic light of 366 nm wavelength for different intervals of times in the temperature range of 258-318 K. The temperature of the system was controlled within +1°C. The quantum yields for polymer chain scissions and
259
A G E I N G O F B U T Y L R U B B E R A G A I N S T UV I R R A D I A T I O N
intensity of radiation actinometer. 8
were
determined
using a potassium
ferrioxalate
Methods for characterisation (i) Refractive index increment (dn/dc): The refractive index increment (dn/dc) with concentration of B R and the optical constant (H) for the solution in cyclohexane were determined using a Brice-Phoenix differential refractometer (Phoenix Precision Instrument Co., Philadelphia, USA) and the values at 288 K are dn/dc = 0 . 1 2 4 3 , and 1 . 9 5 6 4 x 10 -~.
(ii) Light scattering studies: The stock solutions of B R with and without CDIP were centrifuged to 15 000 rpm for 90 min to remove dust particles. The light scattering measurements were carried out with a light scattering photometer designed and calibrated by the present authors. 9 (iii) Quantum yield: The method for quantum yield determination with potassium ferrioxalate actinometry has been described elsewhere. 1°
(iv) Spectrophotometric measurement: Infra-red spectra of irradiated films of B R were recorded using a Perkin-Elmer (Model 21) infra-red spectrophotometer in the absence and presence of 0.1 wt. % CDIP at 288 K. (v) Carbonyl group formation: The carbonyl groups were determined by a base line method H from IR spectra. (vi) Hydroperoxide group formation: The hydroperoxides in irradiated B R films were determined by an iodometric method.~2
RESULTS A N D DISCUSSION
Plots of weight average molecular weight ]0/w as a function of irradiation time are shown in Fig. 1 for B R films irradiated in the absence and presence of 0.1 wt. % C D I P as a photostabiliser in air in the temperature range of 258 to 318 K with a light intensity flux of 1.68 x 10 -8 einstein s -1 cm -2 for different intervals of time. The plots show a rapid decrease in/(;/,~ initially which then slows down at each temperature. The weight loss is greater for the irradiated control film as compared to the corresponding values of B R stabilised films. This indicates that C D I P retards the degradation of the rubber. The initial rapid drop in ]firw is due to scission of bonds at various weak links that may be distributed along the polymer chain. Jellinek and Flagsman 13 considered that
260
R.P. SINGH, R. CHANDRA
5~
3 :<
3 I:E
ol
0 I R R A D . T I M E (h')
Fig. 1. Variations of weight-average molecular weight versus times of B R films in the absence and presence of 0.1 wt. % CDIP at various temperatures irradiated with 366 nm light in air. At 258 K--(3, BR; O, B R + C D I P . At 273 K--(~, BR; (]D, B R + C D I P . At 2 8 8 K - - A , BR; A, B R + C D I P . At 303 K----~, BR; I , B R + C D I P . At 318 K----~, BR; 1], B R + C D I P .
oxygen attacks only the weak links in such cases. Cameron and Grassie TM have also contended that the initial change in /(~w is due to the scission of weak links. The curve tends to increase at longer irradiations suggesting that crosslinking predominates at longer exposures. The specific rate constant, kl is calculated from the slope of a versus time curve, a = kit for a random chain degradation, where a is the degree of degradation. From the linear plots of log~ k~ versus I/T, the activation energy ( h E ) as well as the frequency factor (A) for degradation is calculated. The method of least squares can be used to evaluate the slope (-AE/R) and the intercept (In A ) from these data. The values of AE and A are substituted in eqns (1) and (2) xs with the activation energies in calories and frequency factor in s -1. k~ = 10.41 x 10 -3 exp
(-7240/RT) s -1 (BR)
(1)
261
AGEING OF BUTYL RUBBER AGAINST UV IRRADIATION
and
kl=12.63xlO-3exp(_9962/RT)s
1
( B R + 0 . 1 w t . % CDIP)
(2)
The values of kl were nearly constant with time indicating the reactions to be zero order. Simha and Wall a6 have pointed out that in random chain scission, the photodegradation is zero order with respect to polymer and oxygen concentration. For a zero order reaction AE may be equated to the heat of formation of the activated complex AH*, without any appreciable error. The free energy of activation AF* remains virtually constant at 39.21 kcal mo1-1 at 400 K in both cases which suggests that the rate determining step is the same in both the systems. The higher values of AH* indicate that CDIP retards the rate of degradation of BR. Figure 2 shows the variations of dissymmetry ratio Zd, a ratio of scattered intensity at 45 ° to that at 135 °, versus time for BR films irradiated in the absence and presence of 0.1, 0.7, 1.2 and 2.0wt. % CDIP in air for different time intervals at 288 K. Zd decreases monotonously with increasing time in the absence and presence of 0 . 1 w t . % CDIP but the values of Zd for the irradiated stabilised films are greater than those for the corresponding control films. This means that CDIP retards the photodegradation of BR. This figure also shows that at concentrations beyond 0.7 wt. % CDIP brings a saturation limit in photostabilisation of BR. Figure 3 also confirms that the saturation
3
Zd
0
2
4
6
8
t0
1"2
14
IR R AD. TIME ~h) Fig. 2. Variations of dissymmetry ratio versus time of B R films in the absence and presence of C D I P at 2 8 8 K irradiated with 3 6 6 n m light in air. O, B R ; O, B R + 0 . 1 % CDIP; A, B R + 0 - 7 % CDIP; V, B R + 1.2% CDIP.
262
R. P. SINGH, R. CHANDRA 1,0
"8
o ,6 A
_='4
\.
.2
'~2
6
(.
'
0
"~ I "2
,
I "4
STABILISER
Fig. 3.
Rate of protection of BR films irradiated with 366 nm light in air versus concentration of C D I P at 288 K.
protective action is reached at 0.7 wt % CDIP where kl and ko are the rate constants in the presence and absence of the stabiliser, respectively. The degree of protection is temperature dependent to some extent. Quantum yields are determined by eqn (3) 17 1
1
m
Pw.t Pw,o= w N ~bIat
(3)
where Pw.t and Pw,o are weight average degree of polymerisation at time t and zero, respectively; w is the weight of irradiated polymer, m is the molecular weight of the monomer, N is the Avogadro number, th is the quantum yield, Ia is the light absorbed in the film and t is the time. Ia is determined by potassium ferrioxalate actinometry: s The quantum yield ~b can be determined from eqn. (3) by plotting 1/pw.t versus irradiation time. Table 1 shows that
TABLE 1 QUANTUM YIELDS FOR PHOTO-OXIDATIVE S T A B I L I S A T I O N O F B R IN T H E A B S E N C E
DEGRADATION AND AND PRESENCE OF
0-1 wr.% cDw IN Am AT VARIOUSTEMPERATURES
Temp (K)
BR
258 273 288 303 318
0.20 1"16 2"18 4"37 5"14
~ x 10 -4
chain scissions per absorbed photon B R + O. 1 wt. % C D I P 0.009 0"12 0.43 0"91 0"99
263
AGEING OF BUTYL RUBBER AGAINST UV IRRADIATION
larger quantum yields are obtained for control BR at all temperatures and the smaller values are typical of stabiliser mixed BR films in which UV absorption occurs at CDIP. The quantum yields are less than unity, implying that in a macromolecule the energy is absorbed at one site which is partitioned over many bonds so that the probability of a single bond breaking is small, or the absorbed energy is dissipated by quenching reactions. The changes in carbonyl groups during the degradation of the polymer could be determined by IR spectral analysis. The rate of photo-oxidative degradation was measured by means of a carbonyl index at 1721 cm- 1 using an infra-red spectrophotometer: Carbonyl index = [log Io/It/d] × 100 where I0 is the intensity of incident light, It is the transmitted light intensity and d is the film thickness in microns. Figure 4 shows that the build-up in carbonyl index of the irradiated film is lower in the case of stabilised polymer compared to neat BR at each temperature. This clearly shows that CDIP retards the degradation process. The photo-oxidative degradation was also studied by the hydroperoxide concentration measurement by iodometry. 12 Figure 5 shows that the hydroperoxide concentration of CDIP incorporated rubber is lower than for base BR. These results suggest that CDIP stabilises the BR and destroys the 16 14
X
xlO _z8
°
~-6 Z
&
c~
~4 U 2 0
0
2.
4
6
8
10
12
14
IRRAD.T IME lh) Fig. 4. Variations of carbonyl index of BR films in the absence and presence of 0.1 wt. % CDIP irradiated with 366nm light in air. At 318K--C), BR; O, BR+CDIP. At 2 8 8 K - - A , BR; A, BR+CDIP. At 258 K---~, B R ; . , BR+CDIP.
264
R. P. SINGH, R. CFIANDRA
lo 2
0
2
4
6 8 IRRAD.TIME(h)
10
12
14
Fig. 5. Variations in hydroperoxide concentration of BR films in the absence and presence of 0.1wt.% CDIP irradiated with 366nm light in air. At 318K----O, BR; O, B R + C D I P . At 288 K--A, BR; i , B R + C D I P . At 258 K---~, BR; II, B R + C D I P .
unstable reaction intermediates, i.e. carbonyl and hydroperoxide to relatively stable products such as alcohols.
Mechanism of photodegradation and stabilisation of butyl rubber In polymers chain scission occurs via the formation of hydroperoxide, ROOH, or carbonyl groups as the primary products of oxidation and these decompose on irradiation to a variety of reaction intermediates depending upon the polymer structure and the reaction conditions. The hydroperoxide (ROOH) dissociates unimolecularly at the oxygen molecule diradical O w O link. The free energy of activation of the O - - O bond in a peroxide requires about 40kcalmo1-1 which is close to experimental free energy of activation (39.21 kcalmol-1). The polymer hydroperoxide can decompose when either energy transfer process o c c u r s 1 8 - 2 0 o r the sensitiser can propagate the freeradical-induced decomposition of the peroxide. 21 It can, therefore, be reasonably postulated that the rate controlling step for the photo-oxidative degradation process would be the initiation step. The isoprene units in butyl rubber are distributed at random along the polymer chain. The methylenic hydrogen in the a-position to the double bond
AGEING OF BUTYLRUBBER AGAINSTUV IRRADIATION
265
is a potential site for hydroperoxidation and this can lead to chain scission 22 as shown in Scheme 1. CH3
CH 3
I I --C--CH2--CH2--CH~C--CH2-I CH3
~ Hydroperoxidation CH 3
CH3
f I ~C--CH2--C--CH=C--CH2-I I CH 3 O--OH
Scheme 1.
~Chainscission CH3
CH3
I I ~C--CHzOH + ~H--CH~-----C--CH2~ I CH3
and other inert products Scheme 2. Thus, the isoprene units provide active centres for degradation and the process follows a random chain scission since these units are distributed at random (Scheme 2). The photo-oxidative degradation reaction scheme has been proposed by Chandra and Singh 3 as polymeric hydroperoxide formation (initiation), propagation, chain branching and termination. The chelate CDIP photostabilises the BR in two ways: first, it absorbs the photoenergy of 366 nm by resonating structures and chromophores; second, it converts the polymer peroxides and hydroperoxides to inert products. The peroxy radical is capable of abstracting an electron from the electron rich sulphur atom of CDIP and is converted into a peroxy anion (Scheme 3).
Cu
R02
H s O \ ~ "S+-]
~ RO2 +
[~3
/ f/
u
or
C3H70
\ / Scheme 3.
C3H7#
Cu
266
R.P. SINGH, R. CHANDRA
The peroxy radical may be stabilised by delocalisation (Scheme 4) FC IC3H70
\f
C3H70j
S Cu
RO--Oq
I
RO--O']
x//cu..
~S
c3.7%/s--s\/o.7c3 P~S C3H7 O /
F
P ~
.co
i + 2RO2 + Cu +
~OH7C 3
Scheme 4. The cuprous ion converts peroxy radicals into peroxy anions due to an electron transfer of the type shown in Scheme 5. Cu + + RO2 ~ Cu 2+ + R 0 2 Scheme 5. Thus CDIP stabilises BR by interference with the propagation reaction by absorption of light and decomposition of the macromolecular peroxides and hydroperoxides. By minimising the light energy absorbed by the polymer, the primary photochemical processes are inhibited. Due to the energy difference in the d-orbitals of CDIP, it absorbs UV incident light. Thus the chelate acts as a UV absorber and peroxide decomposer. CDIP may introduce additional terminal steps by acting as R', RO- and ROO- scavengers. In addition to this CDIP inhibits the initiation process by electron transfer and formation of inert charge-transfer complexes. ACKNOWLEDGEMENTS
The authors wish to thank Professor A. Syamal for providing facilities and for helpful discussions. The financial support for this work by CSIR, New Delhi is gratefully acknowledged. REFERENCES 1. 2. 3. 4. 5.
SCHMrrr,R. G. and HIRT, R. C., J. Appl. Polym. Sci., 7 (1963) 1565. CHANDRA,R. and BHATNAGAR,H. L., h'/d..1.. Chem., 14A (1976) 469. CHANDRA,R. and S~GH, R. P., Makromol. Chem., 181 (1980) 2737. CHANDRA,R. and SINGH,R. P., Ind. J. Technol., 18 (1980) 250. CHANDRA,R. and BHATNAGAR,H. L., Ind. J. Chem., 14A (1976) 272.
AGEING OF BUTYL RUBBER AGAINST UV IRRADIATION
267
HELLER, H. J., Europ. Polym. J., Suppl., 105 (1969). BURN, A. J., Tetrahedron, 22 (1966) 2153. 8. CALVERT,J. G. and PIT]S, N. J. Jr., Photochemistry, John Wiley, New York, 1966. 9. SINGH, R. P., Ph.D. Thesis. Submitted to Kurukshetra University, 1979, p. 79. 10. CHANDRA, R. and SINGH, R. P., Ind. J. Chem., 19A (1980) 849. 11. LUONGO, J. P., J. Polym. Sci., 42 (1960) 139. 12. WAGNER, C. D., SMITH, R. H. and PETER, E. D., Anal. Chem., 19 (1947) 976. 13. JELLINEK,H. H. G. and FLAGSMAN,F., J. Polym. Sci., 8 (1970) 71l. 14. CAMERON, G. G. and GRASSIE, N., Makromol. Chem., 53, (1962) 72. 15. GLASSTONE,S., EYRING, H. and LAIDER, K., The theory of rate process, McGraw-Hill, New York and London, 1941. 16. SI~IHA, R. and WALL, L. A., J. Phys. Chem., 56 (1952) 707. 17. JORTNER, J., J. Polym. Sci., 37 (1959) 199. 18. LUNER, C. and SZWARC,M., J. Chem. Phys., 23 (1965) 1978. 19. UBERREITER, K. and BRUNS, W., Macromolec. Chem., 68 (1963) 24. 20. WALLING, C. and GIBIAN, M., J. Am. Chem. Soc., 87 (1965) 3413. 21. SMITH, W. F. and ROSSITTER, B. W., Tetrahedron, 25 (1969) 2059, 2071. 22. BHATNAGAR,H. L. and SrNGH, M. M., Ind. J. Chem., 6 (1971) 218. 6. 7.