Effect of light stabilizers on the breakdown of ester groups in UV-irradiation of polymethylmethacrylate films films

1920

et al. V. I. GOL’DENBERO

and coarse spherulitic specimens of polypropylene is evidence that tne noncrystalline material has different characteristics in each of these specimens. (3) The diffusion coefficients of oxygen in fine and coarse spherulitic specimens of polypropylene in the temperature range 25-67’C are equal to 42.0. exp { - 1l,OOO/ /RT) and lG*exp (--8700/RT)cm2/set respectively, the corresponding figure for polyethylene being 2*0*exp {-9200/RT}cm2/sec. Trawlat& by G. F. MODLEN REFERENCES 1. Z. P. KOSOVOVA and S. A. REITLJNGER, Vysokomol. soyed. A9: 415, 1967 (Translated in Polymer Sci. U.S.S.R. 9: 2, 468, 1967) 2. V. 6. NIKOL’SKII, I. I. CHfU-IEIBZE and N. Vs. BUBEN, Kin&&a i kataliz 5: 82, 1964 3. V. 6. NIKOL’SIUI, Dissertation, 1966 4. R. BARRER, Diffuzia v tverdykh t&&h (Diffusionin Solids). p. 29, Izd. inostr. lit., 1948 (Russian translation) 6. A. 6. SAWN, T. K. SBAPOSHNIKOVA, V. L. KARPOV, T. I. SOGOLOVA and V. A. KARGIN, Vysokomol. soyed. AlO: 1584, 1968 (Translated in Polymer Soi. U.S.S.R. 10: 7, 1834, 1968)

EFFECTOFIJGHTSTABILIZERSONTEEBREAKDOWNOFESTER GROUPSINW-IRRADIATIONOFPOLYMETI.IYLMETI3ACRYLATE FILMS* V. I. GOL’DENBEW, E. F. VBINSHTEM and V. YA. SHLYAP~TOKH Chemical Physics Institute, U.S.S.R. Academy of S&noes (Received25 November 1970) THE kinetics of the accumulation of free radicals has been studied [l] in pure films of polymethylmethacrylate (PMMA) and in tims containing additions of light-stabilizers and suppressors of excited states. It has thus been shown that additions lower the rate of accumulation of radicals to the same extent as that at which they absorb the photochemically active radiation (the screening effect). Suppression or any chemical effects of the additions on excited states were not observed, and neither were any processes observed to occur in the dark. With the aim of clarifying the mechanism by which light-stabilizers act, these investigations have been broadened in the present work to include the kinetics * Vysokomol. soyed. A14: No. 8, 1718-1726, 1972.

Effect of light stabilizerr*onbreakdown of ester groups

1921

of decomposition of ester groups in PMMA. The present paper presents the results of an investigation of the kinetics of decomposition of ester groups in unstabilized tims and also in films containing stabilizers.

Fm. 1. IR spectra (1=&6 p) of a specimen of PMMA Glm: u-before irradiation;&after irradiation for 20 min with the unfilteredlight from a DPSh-1000lamp; c-after 40 min; d-after

310 min irradiation.

The decomposition of ester groups in PMBLAhas not been studied previously by a direct method, although there are indications in the literature pointing to a change in the IR spectrum of the polymer in the region of the ester groups’ absorption under the action of light [2,3]. The composition of the gaseous products of photolysis, which are mostly methylformate and methanol [4, 51, is also evidence of the decomposition of ester groups. The quantum yields of methyl formate and methanol are respectively O-14 and O-48, that is, they are very high and approximate to the quantum yields in the photodecomposition of low-molecular esters [6]. Methyl radicals found in the low-temperature photolysis of PMMA are also formed from the ester groups of the polymer [7].

1922

V. I. GOL’DENBERG et al.

Figure la, shows the IR spectrum of the initial PlKMA film in the regions 700-1900 and 2800-3100 cm-l. Table 1 gives the interpretation of these bands according to Nagai [S] and Willis [9]. It may be seen from the Table that the absorption bands at 996, 1150, 1250 and 1270 cm-’ are connected with absorption by ester groups, the band at 1388 cm-l being connected with the absorption of a-CH, groups, and the bands at 1452,1465,2948 and 2995 cm-l being connected with the absorption of C-H bonds in ester, a-methyl and CH, groups. Figure 1 b-d, shows the spectra above a PMMA film irradiated in air with unfiltered light from a DRSh-1000 lamp. The absoprtion bands of the ester and C-H groups undergo the most pronounced changes. Thus the bands at 996, 1250 and 1270 cm-l, attributed to the absorption of only the ester groups, disappear completely. On the other hand, the bands at 1150 and 1200 cm-l, attributed by Willis to ether groups, are retained although their intensities are substantially reduced. This may be considered as an indication of the complex nature of the bands at 1150 and 1200 cm-l, in agreement with the interpretation of Nagai. The band at 2995 cm-l disappears simultaneously with the absorption bands of the ester groups. This band is chiefly determined by the absorption of C-H bonds in ester groups. The kinetics of decomposition of the polymer have been measured in the present work from the change in intensity of the bands at 1250 and 1270 cm-l, which are connected with the absorption of ester groups, and those at 1452 and 1465 cm-l, connected with the absorption of C-H groups in CH, and 0-CH, groups. The changes in the absorption band of a-CH, groups (1388 cm-l) are small and lie within the limits of experimental error. EXPERIMENTAL The IR spectra were reoorded on a UR-20 two-beam instrument with a N&l prism in the range 700-3100 cm-l. The experiments were carried out with &us 2-20 p thick, measured by means of a vertical optimeter type IKV (eocuracy of measurement, &-O-2 ,a) 3

Fx+. 2. Instrument for the irradiationof films: I-BUV-30P

lamp; Z--osss et; 3-frame for IIlm.

or measuredfrom the optical densitiesof the bands in the IR spectrum. The optical densities were determined by the base-line method, the procedure for oarrying this out being shown in Fig. la. Figure 2 shows a diagram of the irradiationrig. Irradiation was carried out with

Effect of light stabilizers on breakdown of ester groups

1923

light from a BUV-30P lamp, 80% of the radiation from which belongs to a wavelength of 253 m,c. The lamp voltage was supplied from a type S-O.5 ferro-resonancestabilizer. The light intensity was monitored by a type FS-Kl photo-resistance. The polymer Chns were secured in metal frames each with a 12 x 6 mm window and were installed in cassettes positioned at a distance of 5 cm from the axis of the lamp. The IR spectra of the flms were also recorded in the same frames. Special experiments had shown that no after-effect of the radiation could be observed. As shown in Fig. 2, the cassettes were separated from one another by opaque screenswhich did not transmit the light from remote sections of the lamp. With this design, it could be considered that the light was incident normally and that the film’s optical thickness was the same as its geometrical thickness. The absolute intensity of the light, as measured with a ferrioxalate actinometer, was 2.7 x lOIs cm-2 set-I. The PMMA used was the same as in [ 11.The methods of preparingthe films of introducing and determining the concentrations of the additions are also described in this reference.

The data shown in Fig. 1 and Effect of irradiation on absorption coeficients. in Table 1 indicate that the intensities of the different absorption bands decrease at different rates. This may be connected not only with changes in the concentrations of the corresponding groups, but also with changes in the absorption coefficients as a result in a change in the polymer’s structure under the effect of radiation, The effect of structure was investigated in two series of experiments. In the first series the spectrum of a film deposited on to a polished NaCl plate was recorded. The film was then melted directly on the plate, and its spectrum recorded once more after cooling. The melting caused practically no change in the values of the optical densities of the absorption bands. In the second series, the spectrum of a film prepared from a mixture of irradiated and unirradiated polymers was compared with the spectrum of an irradiated film. After 48 hr irradiation, the optical densities of the bands at 996, 1260 and 1270 cm-l had changed respectively by 18.8, 25.4 and 46*6%, whereas in the film prepared from a mixture (1 : 1) of irradiated and u&radiated polymers, the optical densities had changed by 9.2,16+3 and 23.1% of the corresponding optical densities of the unirradiated polymer. The optical densities thus change additively, and the transformation of the polymer during irradiation has no effect on the values of the absorption coefficients. This makes it possible to use the changes in the optical densities of the bands at 996, 1250 and 1270 cm-l as quantitative characteristics of the photo-transformation of the polymer’s ester groups. It was not possible to measure successfully the effect of structure on the optical density of the bands at 1465 and 2948 cm-l, since the absolute values of the optical densities of these bands were less well reproducible: however, in measurements of the kinetics, the relative changes in optical density were reproduced with adequate accuracy and have been used in following sections. Rate of degradation of ester groups. As an example, Fig. 3 shows kinetic curves for the disappearance of ester groups, as measured from the change in the optical density of the bands at 1250 and 1270 cm-l. It may be seen from the Figure that the optical density decreases linearly, that is, D=D,,--At, where

type

bands

Ester

1150

1190

position

+ vs (CH,)

va (C&--O)

+ va (CH,)

vs (a-C&J + vu (a-Cl&) Iso- and syndioforms

vs (CH,-0)

Iso- and syndioforms

6 (CH,)

-

)

-

2995

2948

1465

1388

1150

1270

1240

998

v, cm-l

ester

in a-CH,

vs

ester

in 0-CH, in a-CH,

(C-H) (G-H)

va

va

in 0-CH,

in a-C%

(C-H)

(C-H)

va (CH,)

vs

to [9]

v. group

group

or (C-O)

of

or (G-O)

of

da (CH) in (O-CH,) 6, (C-H) in a-CH,

as(CH)

va (CO-C)

Va (C-O-C)

(C-C-O)

Trans-form

C-C-O

C&form

of vibration

rock vibrations

type

of bands according

0-CH,

Interpretation

Changes

in the PMMA

spectrum

during

2995

2948

1466

1452

1445‘

1388

1190

1150

1270

1250

996

v, cm-’

considerably

reduced

light

complete

310

disappears

is shifted (1170)

of bands consider-

Band

disappears

by

190 min

ably weakened by 310 min

Absorption

(dura310 min)

weakened

is shifted (1206)

tionof observation,

Considerably

maximum

Absorption gradually decreases,

maximum

Absorption gradually decreases,

Band

disappears

irradiation

after 40 min

min practically

after 190 min

Band

after

after 190 min and disappears

Band

by

DPSh- 1000

irradiation

irradiation: results from the present work+

* Changes in the apectrwn during irradiation. t Atmospheric oxygen does not affect the rate of change in the bands at 996, 1250, 1270, 1150, 1190 or 1138 cnrl, and increases the rate ofohange of the optical density of the bands at 1465,2948 and 2995 em-’ by a factor of 1.7-Z (253 mp radiation). Note:Y=and va are asymmetric and symmetric velency vibrations; 6. and &, corresponding deformation vibration&

2996

2948

Iso- and syndioforms

1465

6, @Hz-O)

Syndioform

1462

(CH,-0)

Isoform 6(CH,)

1445

stretch

super-

Iso- and syndioforms & (a-CH,

G-H

with

+

1138

of

skeletal

+W--o) Iso- and syndioforms

va (C-C-O)

Syndioform

[8]

(C-O)

together with

1270

vibrations

to

of vibration

according

Isoform (C-C-O)+vs

~a (CH,-0)

vs (C-O-C)

of

1250

996

v, cm-l

Interpretation

TABLE 1. INTERPRETATION OF BANDS IN THE IR SPECTRUM OF PMMA*

1925

Effect of light stabilizers on breakdown of ester groups

D is the current value of optical density and D,, is the optical density at time t=O. The rate of degradation w is characterized by the ratio w=A/D,,, which expresses the proportion of groups undergoing transformation in one hour in a film of unit thickness. The values of D, and A were determined by the method of least squares using a “Mir-1” computer. The experimental values of A, D, and the degradation rates, as well as the values of meansquare deviation 6 in the degradation rates, are shown in Table 2. TABLE 2. RATE OF BREAKDOWN OF ESTER UROUPS IN PUMA FILMS DURING IRRADIATION WITH LIGHT FROMA BUV-30 P LAMP IN AIR

AxlO

13 P

1 D,

/$xlOS~

6x10* 1270 cm-1

1250 cm+ 2.4 2.9 3*1* 3-1 3.2 4-2 4.9 5.4.t

3.66 4.21 3.45 3.94 4.96 4-60 I.22 7.83

O-164 0.199 0.204 0.211 0.224 0.272 0.333 0.369

2.23 2.12 I.70 l-86 2-22 1.70 2.18 2.12

o-11 0.21 0.31 0.26 0.21 0.32 0.17 0.2

4.0 4.44 4.05 4.61 4.18 5.13 0.68 8.05

0.13 0.16 0,165 0.169 0.175 o-22 0.271 0.297

3.08 2.77 2.45 2.73 2.38 2.33 2.52 2.71

0.14 0.26 o-37 0.32 0.27 0.20 0.12 0.20

L

* Film preparedfrom B dioxanesolution. t Polymerpurifiedfrom the monomerby precipitationthreetimes fromacetonewith water. Note:Afeean values:A/D,=0.0202, 6=23x 1O-J(1250 cm-‘), .4/D,=O.O262,6=2.5x IO-’(1270cm-l).

It may be seen from Table 2 and 3 that, within the limits of experimental error, the degradation rates do not depend on the film thickness and are proportional to the light intensity. Table 2 shows that a change in the solvent used in preparing the films does not affect the rate of decomposition of the ester groups. TABLE 3. RELATIONSHIP BETWEEN THE RATE OF DEGRADATIONOF ESTER GROUPS AND THE LIGHT INTENSITY

;,

%*

lAx~ns/ D, /~~10"/6~10~/ z, %AxlO’ Ds / ($,~+~~0’ z, % / 1250 cm-l

100 28 10

4-23 2.53 0.438

0.199 0.419 0.195

2.12 6.05 0.225

1270 cm-l 0.22 0.31 0.14

100 28.6 10.6

4.44 2-40 0.38

O-16 O-326 0.157

2.78 0.736 0.242

O-26 0.38 0.10

100 26.6 8.7

* ZO is the intensity of the incidentradiation;Z is the intensity of the incidentradiationreducedby meansof B ealibrated screen.

According to the data in Table 2, t,he ester groups located in regions with a syndiotactic structure (band at 1270 cm-l) decompose more rapidly than the ester groups in isotactic regions (band at 1250 cm-l). These results agree with

1926

V. I. GOL'DENBEEW

et ad.

observations on the rate at which the molecular weight decreases: these indicated that the photodegradation of the syndioform in nitrogen occurs the more rapidly

WI.

Effect of the monomer and oxygen on the decompwition of ester groupa. The polymer specimen used in the present work contains about 1% of monomer [l]. At A=253-7 rnp, the molar absorption coefficients of the monomer and polymer are equal to 65-8 and 0.37 respectively [ll], that is, only about one-third of the light absorbed may be attributed to the polymer. In connection with this, it is important to assess the effect of the monomer on the rate of transformation of ester groups. The rate of decomposition of ester groups was measured in exper-

4 Time, hr

8

Time, hr FIU. 4

FIQ. 3

4 1.2 Time, hr

8

Fm. 5

FIU. 3. Kinetic curves for the change in the optical density for the absorption groups

in PMMA

films during

irradiation

12

by ester

by 263.7 rnp light in air; I--v=1250

cm-i;

2--v= 1270 cm-l. Fm. 4. Changes in the optical absorption tion with light A= 253.7 rnp; I-the

density (v-1466

initial polymer

three times, in air; 3-as

traces of the monomer by being reprecipitated Fm.

6. Kinetic

l’-polymer PCPbS

curves for the change in the optical density

with

(1.34%);

and 2948 cm-l)

the. addition 3-polymer

tion, in air. a-1466

of Tinuvin

P

(1%);

during irradia-

in air; 2-polymer,

of: l-the

2-polymer

with the addition of bisphenol

purified

2-but

with

from

in helium.

pure polymer; the addition

22-46 (4x),

cm-1 band; b-2948 cm-1 band, after the introduction for screening.

of

during irradiaof corrections

iments in films which had been prepared from polymer, the polymer having been purified from the monomer by precipitation of the initial polymer three times with water from a solution in acetone. As may be seen from Table 2, the

Effect of light stabilizers on breakdown of ester groups rates agree, within

the limits

of experimental

error, with the corresponding

values for the unprecipitated polymer. Since the total optical density of the polymer monomer

1927

at il=253*7

and the polymer absorb light independently

nyl is small, the

and a change in monomer

concentration has no effect on the amount of light absorbed by the polymer. It therefore follows from the fact that the rates were the same that the decomposition

of ester groups occurs only under the action of light absorbed

by the

polymer. According to an assessment made with the assumption that the bands at 1250 and 1270 cm-l are connected only with vibrations of C-O-C groups, the quantum

yield for the decomposition

of ester groups is approximately

one.

The high values of quantum yield are in good agreement with the corresponding figures for low-molecular decomposition

esters. For example,

of ethyl propionate

of methylformate [S]. In order to clarify the effect of oxygen groups, experiments

the quantum yield for the photo-

is close to unity, and is equal to 0.8 in the case on the rate of degradation

of ester

were carried out in which PMMA films were irradiated

helium and in air*: these indicated no effect on the degradation rate.

that a change in the gaseous medium

in has

Effect of the monomer and oxygen on the decomposition of CH, groups. Figure 4 shows the kinetics of the change in optical density of the bands at 1465 and 2948 cm-1 occurring when films of the precipitated

polymer and of the initial polymer

containing 1% monomer were irradiated in air. It may be seen from the Figure that in the film containing the monomer, (curve I), the optical density changes much more rapidly than in the film made from the precipitated polymer (curve 2). The band at 1465 cm-l cording

to Nagai’s

was selected for further kinetic

interpretation,

measurements.

the strong band at 1465 cm-l

Ac-

is related to

vibrations of the CH, group [S,(CH,O)]. This band is, however, superimposed on another strong band at 1452 cm-l related to vibrations of the CH, group [s(CH,)]. to take

In measurements of the band at 1465 cm-l it is therefore necessary into account the contribution made by the vibrations of the two

groups. Since additions

of the monomer

or oxygen

do not affect the rate at which

the ester groups are consumed, it may clearly be assumed that the change in the rate of decrease of the optical density of the band at 1465 cm-l characterizes the removal of CH, groups in the presence of these additions. Thus, according to the data in Fig. 4, the difference in the rates of change of optical density (curves I and 2) the difference being characteristic

of the rate of removal of CH,

* The introduction of oxygen into the film under the given conditions of irradiation WM monitored from the EPR spectrum. The spectrum of the peroxide radical is observed at a sufficiently high rate of diffusion of oxygen. With insticient oxygen, the nine-component spectrum of the alkyl radical (CH,), C’COOCH, is superimposed on the spectrum of the peroxide radical.

V. I. GOL’DENBERQ et al.

1928

groups AwCH, is 6.5 x 10ls ~rn-~ see-l for a film i0 ,Uthick*. Based on the light absorbed by the monomer, the measured value wcII, corresponds to a quantum yield of approximately 0=15+. Figure 4 also shows that oxygen increases the rate of change of optical density in the band at 1465 cm-l. Thus the removal of CH, groups is accelerated by the presence of oxygen and the monomer. On the other hand, the presence of oxygen and the monomer does not affect the rate of degradation of ester groups in PMMA. EJect of aclditions on the degmihtiort of eater group. The ratio w,,Iw, where w,, and w are the rates of degradation in the absence and in the presence of additions respectively, was adopted Is a measure of the effect of additions on the rate of decomposition of ester groups. The rates were determined from the change AD in optical density AD in the appropriate band of the IR spectrum, w=, Dot

where D, is the initial optical density of the film and t is the duration of radiation. Any retardation of the photo-process connected with the fact that the addition absorbs part of the photochemically active radiation was taken into account by use of a screening coefficient i,=

1-lo-=P

D,+D,t DP

’ l_

lo-‘4+&t)

(Dp and Oat are the optical densities of the polymer and stabilizer respectively): the coefficient shows by what factor the addition reduces the amount of light absorbed by the polymer. Since Dp is very small at a wavelength of 263.7 rnp (0.0012 for a film 1 p thick), this expression may be used in the form i,=

2.30,+, l-10-%

Since w and w,, are proportional to the intensity of the light, the following cases arise: wJw*&= 1, if the reduction in rate is connected only with screening; wo w,Iw *$,p> 1, if an additional retardation mechanism is active; and -
we/w -i,

* The rate of change in optical density is given by wl= for curve 1,and for curve D,t 2 by w~=~S; WCE,=(W,- w,)n, where n is the number of CH, groups in one ems of the 0 film. t Estimated figure.

P-2-(2-hydroxy-6-methyl~henyl)benzo~ia~le:

0.5

1

1

S,6-dlohloroealieyli~ acid; DSR--diaalicylate reeoreln; A 63’99-

* Tlnwin

Psrene ChryS0XM3 p-Terphenyl

3

4

-

1.36 1.06 1.36 1.26 1.40 1.26

1.40 1.80 1.30 -

%

-

-

[ HO “-cH,-Fe-L x

IN

1.06 1.10 1.30 1.16 0.90 1.40

1.20 1.00 1.20 -

WJW *i,

i

0.20 0.26 0.23 0.10 0.11 0.26

6.1 2.30 2.77 1.69 3.22 3.62 0.488 2.67 3.20 3.40

4D t~108

-I

0.342 1.171 0.17 0.138 0.178 0.170 0.200 0.164 0.173 0.191

D,

-

WITH

1.76 1.96 1.60 2.16 1.46 1.26 1.06 1.66 1.40 1.46

1.26 1.10 1.26 1.16 1.06 1.20 0.80 1.30 1.00 1.16 PhESA-phenyl

1.40 1.80 1.30 1.90 1.36 1.06 1.36 1.26 1.40 1.26

-I

-

IRRADIATION

6

-!

_ eater of

0.18 0.26 0.32 0.32 0.20 0.20 0.10 0.18 0.18 0.24

-3

-r

263.7 rnp

1270cm-l; wO= 2.62x 10-a hr-1 (6=2.6x lo-‘)

FILMS DURINU

UV-12-2,4-dihy~xybensophenone;

-

-

-

1

_-

AIR

o-17 0.23 0.32 -

6

6= 2.3 x lo-s1

TEMPERA!lWRX

OF ESTER QROTJPS IN PMMA

dioysn- l,l-dicysn-2,2-diphenslene;

1.40 1.16 1.76 1.45 1.30 1.76

0.214 0.221 0.234 0.207 0.217 0.240

3.08 3.87 2.71 2.83 3.43 2.73

2

4

2

PhESA DSR w-12 A63’99 Naphthdene

1.05 1.76 1.65 -

wolw 0.418 0.226 0.216 -

-

-

ROOM

1250cm-l; w.=2*02 x 10-p hF’ 4D tX1O: 6.17 2.58 2.8 -

%

‘

AT

2 9

P

N

k&ion,

LIUFFJ!

ON THE RATE OF DEQRADATION

Zoncen-

OF ADDITIONS

Dioyan

Tinuvin

EFFECT

Addition*

TAFILE4.

kz 2

Q

d %

z g

% *

F 3

8

Z,

M

V. I. GOL’DENBER~et al.

1930

that the effect of the light stabilizers screening. Deviations in wJw*i, imental error, are insignificant. should be remembered

and suppressors investigated

reduces to

from unity, which exceed the limits of experIn an assessment of the results obtained, it

that the experiments

were carried out with thin films

for which the screening coefficient is small even with addition concentrations of 2-3%. The corrections for screening are therefore small and errors in determining the screening In addition,

coefficient

cannot

other mechanisms

at such high addition of a single mechanism,

for their activity

concentrations. namely,

make any essential change in the results. would

The conclusion

screening,

is therefore

have been apparent

about the predominance reliable.

In particular,

it follows from this that, in the systems investigated, there are no chemical mechanisms for the protection of ester groups and the disruption of the polymer structure caused by the additions

has no effect on the rate of the photo-trans-

formation. Effect of adti%tima on the rate of change of optical den&y in CH,-group abaorption banda(v=1465, 2948 cm-l). It has been shown [l] that additions of the monomer and oxygen, which do not affect the rate of removal of ester groups, selectively accelerate the removal of CH, groups. It has been found that additions of PCPhS and 22-46* also affect the kinetics of change of optical density in the 1465 and 2948 cm-l bands. After corrections for screening, the rates were found to be approximately the same in a film without additions and in a film with the addition of Tinuvin P (Fig. 5). The rates were found to be considerably

greater in the films containing

bisphenol 22-46 and PCPhS after corrections for screening. It is interesting to note that PCPhS and 22-46 also accelerate the accumulation of radicals [l], but do not affect the rate of decomposition

bf ester groups, as the experiments

have shown. I

CONCLUSIONS

(1) A method has been developed for the measurement of the efficiency of light stabilizers in their effect on the rate of decomposition of ester groups in polymethylmethacrylate (PMMA) under the action of UV light. (2) The rate of decomposition of ester groups is proportional to the intensity of the light and the quantum yield approximates to unity. (3) It has been established that the light stabilizers and the suppressors of excited states investigated reduce the rate of decomposition of ester groups by a screening mechanism. The monomer, oxygen, the antioxidant bisphenol 22-46 and one of the light stabilizers, namely, pentachlorphenylsalicylate, aocelerate the decomposition of CH, groups but do not affect the rate of photo-transformation of the ester groups in PiKMA. !l’radated * PCPhS, pentachlorphenylselicylate;2246, phenol.

by G. F. MODLEN

2,2’-methylene-bis-(4-methyl-6-teti.butyl)

Effect of stabilizers on kinetics of accumulation of free radicals

1931

REFERENCES 1. Ye. V. BYSTRITSKAYA, V. I. GOL’DENBERG, 6. V. PARIISKII, L. V. SAMSONOVA and V. Pa. SHLYAPINTOKH, Vysokomol. soyed. &4: 1727, 1972 2. 0. A. GUNDBR, V. 6. VLASOV, L. I. KOVAL’ and B. M. KRASOVSKII, Plast. massy, No. 6, 3, 1968 3. Ya. M. WS, V. D. SHCHERBA and A. N. TYNNYI, Fiz.-khimich. m&h. materialov 6: 114, 1970 4. R. B. FOX and S. STOKES, J. Polymer Sci. Al: 1079, 1963 5. J. P. ALLISON, J. Polymer Sci. 4, A-l: 1209, 1966 6. J. 6. CALVERT and J. N. PITTS, Fotokhimiya (Photochemistry). Izd. “Mb”, 1969 (Russian translation) 7. V. 6. VIN’OGRADOVA, Dissertation, 1966 8. H. NAGAI, J. Appl. Polymer Sci. 5: 1697, 1963 9. H. A. WILLIS and P. J. HEUDBA, Polymer 10: 737, 1969 10. D. GARDNER, J. Polymer Sci. B5: 101, 1967 11. M. WARNOCK and D. GARDNER, J. Appl. Polymer Sci. 12: 2325, 1968

EFFECTOFSTABlLIZERSONTHEKINETICSOFACCUMULATION OFFREERADICALSDURINGTHEUV-IRRADIATIONOF POIXMETHYLMETHACRYLATEFILMS* YE. V. BYSTRITSKAYA, V. I. GOL’DENBERG, G. B. PARIISKII, L. V. SAMSONOVA

and V. YA. SHLYAPINTOKH Chemical Physics Institute, U.S.S.R. Academy of Sciences (Received 26 November 1970)

THE action of light on polymers is one of the principal reasons for their decomposition under natural conditions. Specially introduced additions of light stabilizers, whose concentration rarely exceeds a few per cent, make it possible to increase the light-stability substantially, making it at least several times greater. The mechanism by which light-stabilizers act is, however, as yet little studied in quantitative respects, and the selection of the most effective light-stabilizers is chiefly made empirically by the consecutive testing of a large number of substances. Having set ourselves the task of investigating the mechanism by which lightstabilizers exert their protective action, we studied their effect on the kinetics of the following factors, namely, the accumulation of free radicals, the decomposition of ester groups and the decrease in the molecular weight of polymethylmethacrylate (PMMA) which was selected as a model polymer. Results from an * Vysokomol. soyed. A14: No. 8, 1727-1736, 1972.