On the problem of testing chemical compounds as stabilizers of organosilicon polymers

Testing chemical compounds as stabilizers of organosilicon polymers

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4. D. HAM (Ed.), Polimerizatsiya vinilovykh monomerov (Polymerizatior~ of Vinyl Monomers). 1973 5. N. S. NAMETKIN, V. M. VI)OVIN and V. N. ZAVYALOV, Dokl. A N SSSR 209: 621, 1973 6. V. A. POLETAYEV, V. M. VDOVIN and N. S. NAMETKIN, Dokl. A N SSSR 208: 112, 1973

ON THE PROBLEM OF TESTING CHEMICAL COMPOUNDS AS STABILIZERS OF ORGANOSILICON POLYMERS* V . I . SEVASTYA:NOV, YE. lE). OVCI-LA_RE:N'KO,O.A. SHUSTOVA and G. P. OLADYSHEV Chemical Physics Institute, U.S.S.R. Academy of Sciences (Received 20 November 1975)

One of the schemes that may be used for testing chemical compounds as stabilizers of heat stable polymers is put forward, and four main criteria are postulated in connection with the selection of potential stabilizers acting through a "nonchain inhibition" mechanism. The effectiveness of the proposed method of testing is demonstrated using a number of experimental methods. A METHOD of nonchain inhibition of tlmrmo-oxidative degradation of heat stable polymers was proposed relatively recently [1-4]. The effectiveness of the method was verified experimentally on a number of polymeric compositions [5-8]. I n some cases it can be said t h a t the method amounts to the stabilization of polymers by products of thermal decomposition of a compound introduced into the polymers in vie~" of the high rate of interaction of the foregoing products with oxygen, t h a t is, with an initiator of thermo-oxidative degradation processes. Oxalates were selected as stabilizers of heat stable polymers in order to verify the proposed scheme for testing the stabilizing capacity of chemical compounds. Testing was carried out. on the transition metal oxalates FeC~04" 2H20, NiC2Od" • 2H20, COC204"2H20 and CuC2Od"½H20, the pyrolysis of which in an oxygen free medium leads to the formation of finely dispersed metals and (or) lower valency oxides of the latter with a well developed surface (10-100 m2/g). The oxalates were of chemically pure grade, and were used without prior purification. The polyorganosiloxanes used as polymeric matrices with a view to final assessment of the efficiency of the potential stabilizers were trimethylcyclopolysiloxane (TMCPS) and polydimethylsiloxane (PDMS). Under the conditions adopted thermal degradation of the latter polymers is negligible within the temperature interval in question (300-450°). * Vysokomol. soycd. AI9: No. 5, 1094-1100, 1977.

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The following criteria may be taken as the main tests in the selection of :potential stabilizers. The temperature at which the compound decays with formation of an oxygen accepter (potential stabilizer) must not be below the temperature at which appreciable degradation of the polymer commences

Td
(1)

This requirement is due to the fact that the effectiveness of the initial compound is largely determined by how close its decay temperature T d is to the temperature at which oxidative degradation of the polymer that is to be protected starts To. d •

The thermodynamic stability of oxidation products of the potential stabilizer expressed in terms of oxygen pressure must satisfy the condition

po,lpg, I where Pc, is the pressure at which dissociation of inert products (oxides etc.) takes place, p~),, the partial oxygen pressure in the polymer. However, it should be noted that difficulty may be encountered in the use of expression (2), as in many cases the temperature dependence of the solubility o f oxygen in polymers at high temperatures is unkaown. To find the upper limit of stabilization we therefore selected the criterion

T i4o=o>800 °

C2')

For example, for metal oxides with temperature Tao=O for tlie equilibrium thermodynamic potential of oxide dissociation AG, condition (2) is normally invariably correct in the temperature interval 300-500 ° [9]. At the temperature o f 400 ° the vapeur pressure of oxygen in elastromers ~>50 torr [10] and elasticity of dissociation at this temperature of oxides F%08~ 10-80 torr, Cue ~ 10 -4 torr,. The reactive capability of the potential stabilizer and oxygen

Wz+o,/%+o,] ~->r°.~>>1,

(3)

where Wz+o --interaction rate of Z accepter with oxygen, Wp+o --the reaction rate of chain initiation at thermooxidative destruction; To.d.--temperature of oxidative destruction. The "intensity" of the oxidation products of the potential stabilizer ZnOm in the initiation chain reaction

w~"°'lw, I ~>~0.: ~ l,

(4)

where wZ"°---initiation rate in the presence of the oxidation products, wi--the rate of spontaneous initiation. T h e current passing t h e Clarke electrode (Fig. la) w i t h t h e p l a t i u u m c a t h o d e (2) pot e n t i a l --0-75 V r e l a t i v e to t h e A g / A g C l - - a n o d e in a 0.1 ~ KC1 solution is (within 1%) a linear f u n c t i o n of t h e O3 c o n c e n t r a t i o n in t h e gas phase in t h e v i c i n i t y of t h e teflon m e m brane. T h e t e m p e r a t u r e controlled q u a r t z cell (Fig. lb) is designed go allow localized p y -

Testing chemical compounds as stabilizers of organosilicon polymers

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rolysis of the compounds in a current argon followed b y oxidation of the thermal decay products in the closed volume at low partial 0 , pressures (3-15 torr). The upper limit of the oxidation temperature was determined by the ability of the temperature controlling system a

FIG. 1. Clarke electrode (a) and quartz cell (b). to m a i n t a i n a constant temperature at the teflon membrane of the Clarke electrode (25°), while the lower limit was dictated b y the sensitivity of the apparatus. The metering accuracy for the initial O~ concentration introduced by micro syringe and recorded b y the Clarke TABLE

] . T E M P E R A T U R E I N T E R V A L S OF D E C A Y A ~ D C O M P O S I T I O N OF P Y R O L Y S I S P R O D U C T S OF T H E C O ~ P O U N D S B E I N G T E S T E D

Compounds being tested FeC204" 2H~O

Temperature intervals of decay (°C) in inret gas

air

315-365 [12]

330-420 [13] 175-220 [12] 280-300 [15] 255-300 [12] Onset of decomposition 230 [14] 220-260 [12] Maximum rate 250 [13] 310-350 [12] 270-310 [14] Maximum rate 300 [13]

CuC204" ½H,O

255-280 [12] Onset of decomposition 270 [14]

CoC~O," 2H~O

355-370 [12] Onset of decomposition 300 [13] 315-350 [12]

NiCzO4" 2I-I~O

Composition of products of pyrolysis in a n inert atmosphere Fe, CO2, H~O [13] Cu, CO2, H20 [14]

Co{~q-CoO (15--20~) CO, C02, H20 [13] N i + NiO

(5%)

COn,HzO[13, 14]

electrode was 1'5~o. For low current measurements (~ 10 -9 A) under potentiostatic conditions we used a n OH-102 type polarograph capable of automatic recording of the kinetic curves. The sensitivity of the apparatus was ~ 10 -6 mole/1. On, inertia 15-20 sec, current drift in 72 hr < l ~ o , relative error in measuring the rate of O~ absorption 5-10~o. A derivatographic method was used to test the stabilizers in the polymer matrix.

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al.

PDMS specimens ( M = 500,000) with a 10% content of the oxalates were prepared by mechanical mixing. In addition to derivatographic analysis of the polymeric compositions,

we also investigated the extent to which the compounds influence the moisture proof prop. T A B L E 2. C H E M I C A L COMPOSITION AND S T A B I L I T Y R E G I O N OF OXIDES OF T H E POTENTIAL STABILIZERS

Potential stabilizer

Oxides

Stability region of the oxides, °C [9]

Fe

Fee Fe304 aFe~O3 Cu,O Cue Co30, Co~O~ CoO NiO

1377 (m:p.)* 1600 (m.p.) 1457 (m.p.) 375-1229 (m.p.) 2Cue ~ Cu~O+ ½0, T~a=0= 1200 Co30, ~ 3COO+ ½0~ T~a=0=970 Co203 ~ 2Co30~+ ½02 T~a=0=350 2Co+O, ~ 2CoO 1810 (m.p.) 1960 (m.p.)

Cu

Co, CoO Ni

* The oxides are stable all the way up to the melting point.

erties of a varnish cqating based on TMCPS (M= 1200). Specimens with a 5% conten.t of the compounds being tested, prepared by an existing method [5], were subjected to thermal ageing at 400 ° for 200 hr, after which the specimens were placed in a compartment with 98% humidity. The water absorption of the specimens was given by the formula B-

m2--m 1

28

,

(5)

where B is the water absorption of the specimen, g/m"; m~, the mass of the specimen after remaining in the humidity compartment, g; rex, the mass of the initial specimen, g; S, the area of one side of the specimen, m 2.

Thermomechanical and thermodynamic analysis of the compounds being tested. Much material has been published b y authors investigating t h e pyrolysis of b o t h organic a n d inorganic compounds. I n view of this it is possible t o c o n d u c t a critical analysis of our p r o b l e m using published data. I n our case we are dealing w i t h t h e t e m p e r a t u r e intervals of d e c a y of oxalates of iron, cobalt, copper a n d nickel, a n d w i t h t h e composition of p r o d u c t s o f d e c a y in a n inert atmosphere, as well as w i t h t h e stability o f t h e oxides a p p e a r i n g during s u b s e q u e n t oxidation processes. The results o f a n analysis of i n f o r m a t i o n published i n [9, 12-15] are p r e s e n t e d in Tables 1, 2. T a k i n g 350 ° to be t h e initial t e m p e r a t u r e of t h e r m o o x i d a t i v e d e g r a d a t i o n To.d. for t h e unstabilized polyorganosiloxanes u n d e r the e x p e r i m e n t a l conditions, one m a y conclude t h a t t h e oxalates of Fe, Cu a n d Ni satisfy t h e n e c e s s a r y condition o f stabilization (1), (2'). T h e nonfulfilment of cond i t i o n (2') for t h e cobalt o x a l a t e is due t o t h e ratio of oxides of Co203 a n d Co304 f o r m e d during o x i d a t i o n of t h e p r o d u c t s of pyrolysis (Co a n d COO). Since t h e degree o f solubility of 03 in polyorganosiloxanes is n o r m a l l y high, one ,must

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expect a predominance of the unstable oxide C%03, the dissociation tension o f which reaches an equilibrium value already at 350% Let us now go on to verify the sufficient condition of stabilization expressed in (3) and (4). TABLE 3. TEMPERATURES OF 10~o DECOMPOSITION FOR P D M S AI~D P D M S CONTAINING 10~0 OF TI~E OXALATES

T~ "°C

AT = T--

490°

Specimens

( ~ w = 10%)

(~w= ~o%)

PDMS PDMS-FeC~Od" 2H20 PDMS-CuC2Od- ½H20 PDMS-NiC20, •2H:O PDMS-CoC204 • 2H~O

490 540 530 510 485

5O 4O 20 --5

Analysis of the accepting capacities of the potential stabilizers with respect to oxygen, and of the "inertia" of the oxidation products. The kinetics of diffusion of heterogeneous reactions such as the oxidation of metals are described b y several empirical laws [9], which adds to the complexity of a comparative analysis o f the compounds under test. Moreover, despite the numerous investigations that have been reported on processes of interaction of 0 2 with metal surfaces, little or no study appears to have been made of the oxidation mechanism of finely dispersed metals. In view of this it seems advisable that the effectiveness of interaction of the metals with 02 should be characterized not b y the oxidation rate constant, b u t b y the initial rate of oxidation v0, the value of which is practically unrelated to the structure of a forming oxide layer, and the dimensionality o f which is independent of the oxidation mechanism. Preliminary tests showed that the rate of 02 absorption is proportional both to the initial partial pressure of the 02 vo~p "n, where n = l ~ : 0 - 1 (within the interval 3-15 torr), and to the amount of the compound (5-20 mg). I t is therefore convenient to take the specific rate of 0 2 absorption in units of mole/g.torr.scc as a quantitative criterion in assessing the accepting capacities of the stabilizers. Figure 2 shows typical kinetic curves of oxidation for the substances studied, plotted during decay of the oxalates under the conditions adopted. The integral quantity of' the O~ absorption is related mainly to the specific surface of the fine particle reagents, the magnitude of which decreases in the order Fe>Cu>Ni

(6)

The metals are in the same order as the magnitude of the specific rate v0. For instance, at 200 ° the respective values of v0 are (3.1±0.1) x 10-5; (2.0+0.1) x x 10-5; (0-8~0.05) x 10 -5 mole/g .torr.sec.

V.I. SE~'ASTYANOVet al.

~1266

To determine the rate of interaction of the potential stabilizers with oxygen dissolved in the polymer wz+o, (mole/1.-sec) we assume t h a t the linear relation:ship between the rate of the reaction of 02 absorption and Po, and the amount of stabilizer m (g/1.) is preserved in the polymer matrix, i.e. Wz+o,=G'Po," m

(7)

Now, if we take, for example, Po~----50 torr at 400 ° on the basis of equation (7), we o b t a i n the following values of wz+o, for 10~o oxalate in the polymer: Fe ~ 14.5 × ×10 -~, Cu~9.4×102, N i ~ 3 . 5 × 1 0 -2 [16]. With an initiation rate constant of 10-a l./mole- sec [ 17], [RH]---- 10 mole/1., and [02]--~ 10-3 mole/1, we have WRH+v, ----10-5 mole/1., sec. I t follows t h a t the condition

WZ+o,/Wl~H+O,I T= 4000 >>1 holds for oxalates selected in accordance with tests (1) and (2).

~z Lee/.un#s/ 1 ^

0"5 ~ 2

I

•

2 ,, ~

_

,..-.--3 I00

200%~ec

Fro. 2

Ioo

56 I

lOO

I

I

4O0

FIG. 3

ggo 7:,'O

I0

F

I

3o 5O T/me,daus Fro. 4

Criterion (4) is analogous to a test used in the selection of chemical compounds us oxidative degradation inhibitors using model reactions, e.g. chain oxidation ~of low molecular compounds [18]. For high temperatures ( T > 3 5 0 °) it is at present difficult to select a model reaction for the oxidative degradation of heat :stable polymers t h a t would be satisfactory since the reaction mechanism has not been adequately examined and understood, and for this reason it is better to verify criterion (4) for simplicity directly in the polymer matrix. I n verifying the "inertia" of the oxidation products of Fe, Cu and Ni we carried out a derivaCographic analysis of PDMS specimens with 10% oxide which were prepared by oxidation of the respective metals generated from the oxalates. As a means of evaluating the effect of oxides on the degradation of PDMS we tbok a temperature corresponding to 10% decay of the specimens. Whereas this temperature is 490 ° for the pure polymer, for specimens containing oxides of Fe, Cu and Ni the temperatures are 505, 500 and 500 ° respectively. It appears therefore t h a t oxides formed after oxidation of the stabilizers do not initiate degrada-

Testing chemical compounds as stabilizers of organosilicon polymers

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tion of the polymer, and that the oxalates of Fe, Cu and Ni satisfy both the necessary (1, 2') and the sufficient (3), (4) stabilization conditions. Testing the effectiveness of potential stabilizers in polymeric matrices. Although the proposed tests (1-4) are the main ones to be used in selecting thermo-oxidative degradation inhibitors, one has still to verify the effectiveness of these compounds by direct methods in the polymers or polymeric compositions. This involves consideration of a number of other physico-chemical factors, the most important being: the chemical stability of the starting substance and the stabilizer; the compatibility of the ste,rting compound and the stabilizer with the polymer; the effect of solid phase on the rectivity of the stabilizer. Figure 3 shows characteristic TGA and DTG curves of the stabilized and unstabilized polymers (PDMS). The two stage character of the TGA curve of the stabilized polymer is due to dehydration and decomposition of the oxalate. The onset of decomposition of the polymer is displaced to the higher temperature region on account of oxidation of the metal under the action of O3 diffusing into the specimen. As a simple quantitative characteristic of the effectiveness of stabilizers we took a temperature corresponding to 10% decomposition of the specimens (T, °C at AW=10~o). Table 3 gives the results of analysis of the TGA curves taking the thermal decomposition of the oxalates into account. These results show that in respect of their efficiency as PDMS stabilizers the metals follow the sequence of (6) obtained with the aid of the postulated tests (1-4). Moreover, cobalt oxide, which fails to satisfy criterion (2) with respect to the stability of Co~Oa at high temperatures, has a negative stabilization effect. The problem of how the compounds under test influence the water absorption of coatings based on TMCPS was investigated. Water absorption is closely related to the mechanical properties of paint and varnish coatings (Fig. 4). The metals follow the order of sequence of (6) (with B values of 95, 75 and 20 g/m 2 for Fe, Cu and Ni respectively) when examined on the basis of the difference in the total amount of absorbed water AB=Bunstab--Bstab (Bunstab, Bstab are amounts of absorbed water per unit surface of an unstabilized and a stabilized coating respectively). Thus it is clear from the experimental results obtained in the framework of the proposed test scheme that there is unambiguous agreement between the stabilization criteria adopted for heat stable polymers and the test results for chemical compounds in the specimens. The proposed tests (1-4) may therefore be used for the rapid selection of compounds capable of stabilizing polymers by a "nonchain inhibition" mechanism followed by verification of the effectiveness of the compounds on typical representatives of heat stable polymers of various types. It should be noted that the experimentally observed correlation between the rate of interaction of the m~tals with oxygen and the stabilization effect holds only in case swhere thermo-oxidative degradation predominate. At high temperatures, when the role of thermal degradation is considerable, there may well be

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V. I. SEVASTY~OV et al.

departures from the order of (6) obtained on the basis o f the necessary and the sufficient stabilization conditions. For instance, finely dispersed metals may react with primary radicals and form stable compounds, i.e. these may also be thermal degradation inhibitors. Translated by R. J. A. ttENDItY REFERENCES

1. G. P. GLADYSHEV, P u t i stabilizatsii termostoikikh polimerov (Methods for Stabili. zation of Heat-stable Polymers). Chem. Phys. Inst. U.S.S.R. A c a d e m y of Sciences, 1972 2. O. A. SHUSTOVA and G. P. GLADYSHEV, Dokl. A N SSSR 221: 399, 1975 3. G. P. GLADYSHEV, Dokl. A N SSSR 216: 585, 1974 4. G. P. GLADYSHEV, Vysokomol. soyed. A17: 1257, 1975 (Translated in Polymer Sei. LT.S.S.R. 17: 6, 1441, 1975) 5. Ye. N. OVCHARENKO and O. A. SHUSTOVA, Vysokomol. soyed. B17: 864, 1975 (Not t r a n s l a t e d in Polymer Sci. U.S.S.R.) 6. G. P. GLADYSHEV, K. Z. GUMAI~GALIYEVA, V. I. SEVASTYANOV and O. A. SHUSTOVA, Vysokomol. soyed. B17: 862, 1975 (Not translated in Polymer Sci. U.S.S.t¢.) 7. G. P. GLADYSHEV, J. P o l y m e r Sei., P o l y m e r Chem. Ed. 14: 1753, 1976 8. O. A. SHUSTOVA and G. P. GLADYSHEV, Uspekhi khimii 45: 1695, 1976 9. Zh. BENAl~ (Ed.), Okisleniye m e t a l l o v (Oxidation of Metals). vol. 1, "Metallurgiya", 1968 I0. S. A. REITLINGER, Pronitsayemost' polimernykh materialov (Permeability of Polymeric Materials). Izd. " K h i m i y a " , 1974 11. V. I. SEVASTYANOV and Ye. D. BARSUKOV, Zh. fiz. khimii 50: 2710, 1976 12. D. DOLLIMORE and D. L. GREFFITHS, J. Thermal Areal., N2, 229, 1970 13. A. BOULLE and J. L. DOLLIMIEUX, Compt. rend. 248: 2211, 1959 14. Le Van MAY, G. PERINET and P. BIANCO, Bull. Soc. Chim. France, 361, 1961 15. Ye. N. OVCHARENKO and E. B. ZEINALOV, V I N I T I , Dep. No. 1445-75, 1975 16. V. I. SEVASTYANOV, Ye. N. OVCHARENKO and T. V. LYASHIK, ¥ysokomol. soyed. BI8: 790, 1976 (Not translated in Polymer Sci. U.S.S.R.) 17. Ye. T. DENISOV, K o n s t a ~ t y skorosti gomoliticheskikh zhidkofaznykh reaktsii (Rate Constants for Homolytie Liquid-phase Reactions). Izd. " N a u k a " , 1971 18. N. M. EMANUEL, G. P. GLADYSHEV, Ye. T. DENISOV, V. F. TSEPALOV, V. V. K H A RITONOV and K. B. PIOTROVSKII, Testirovaniye khimicheskikh soyedinenii k a k stabilizatorov polimernykh materialov (Testing of Chemical Compounds as Stabilizers of Polymeric Materials). Chem. Phys. Inst. U.S.S.R. :Academy of Sciences, 1973