Influence of carborane-containing compounds on the oxidative stability of products of pyrolysis of network polymers

Oxidative stability of products of pyrolysis of network polymers

2453

6. R. W. SEYMOUR and S. L. COOPER, ACS Polymer Preprints 14: No. 2, 1046, 1973 7. T. V. KOZLOVA, N. A. ]KOZLOV and V. V. ZHAItKOV, Zh. fiz. khimii 45: No. 8, 2110, 1971 8. V. V. ZHARKOV ~,nd N. K. RUDNEVSKII, Vysokomol. soyed. BI0: 29, 1968 9. R. ZBINDEN, I R spoktroskopiya vysokopolimorov (Ii~ Study of High Polymers). p. 360, "Mir", Moscow, 1960 10. X. IMADA, H. TADOKORO, A. UMEHARA and S. MURAHASHI, J. Chem. Phys. 42: No. 8, 2807, 1965

PolymerScienceU.S.S.R.Vol. 23, No. 10, pp. 2453-2459,1981 Printed in Poland

0032-3950/81/102453-07807.50/0 1982 PergamonPress Ltd.

INFLUENCE OF CARBORANE-CONTAINING COMPOUNDS ON THE OXIDATIVE STABILITY OF PRODUCTS OF PYROLYSIS OF NETWORK POLYMERS* L. A. ]~AICH-EI~, I). M. VALETSKII, S. V. VINOG-I{ADOVA, A. M. ZLATKIS a n d

V. V. KORSHAK A. N. Nesmeyanov I n s t i t u t e of Hetero-organic Compounds

(Received 24 June 1980) A study has been made of the influence of o-caxborane and its aryl derivatives on the oxidative stability of the products of pyrolysis of synthetic tbermoreactive polymers. I t is shown that. o-carborane increases the oxidative stability of the pyrolysis products more efficiently that phenyl- or diphenyl-o-carborane. To extend the range of applications of carbon materials prepared by thernml treatment of organic polymers the problem of increasing their mechazlical strength and oxidative stability must be resolved. The onset of oxidation is observed already at 400-450 ° for some existing materials that have an ultimate themvlal processing temperature of 2700-3000 °. The appli('ations of carbonized t carbon materials with a thelTnal processing temperature ()f 900-1200 °, and possessing higher mechanical strength, is limited by the fact that the oxidative stability ~,f the materials is inadequate [1, 2]. The oxidative stability of constructional carbon materiMs, consisting of carbon filler and resin (binder) is determined by the oxidative stability of tim latter [3], and so the problem to be resolved is that of developing a carbonized resin possessing enhanced oxidative stability, with particular reference to the use of antioxidant additions, l~igh-melting elements including Ti, Si, I-If, Zr, B, etc. [4], as welt as various phosphords compounds [5] are used ~s a n t i o x i d a n t additions. The most active additions are boron and phosph(,rus compomlds * Vysokomol. soyed. A23: No. 10, 2257-2262, 1981. * "Carbonized" is the term used for a carbon material prepared b y pyrolysis of the appropriate orga~aie compound from 200-250 ° to 1000-1500°. The carbonization product of the organic compound in question is referred to as "coke".

2454

L.A.

BAICHE~ et a/.

{4, 6, 7]. I n contradistinction to phosphorus, it appears that boron added to a carbon material m a y interact with boundary defects in the lattice by analogy with the process described in [8]. Moreover, in tlm oxidation of a boronated carbon material boron oxidiz,3s and gives a B~O8 film blocking structural defects and preventing the penetration of oxygen into pores [9]. (b~ comparing schemes of oxidation inhibiting mechanisms associated with boron and phosphorus-containing compounds [6-9], it is reasonable to surmise t h a t boron and boron compounds are the most suitable as oxidation inhibitors in t h e case of carbonized carbon materials with highly defective structures. I n an earlier paper [10] it was shown that from this point of view the best of the boron compounds are o-carborane and its derivatives. For instance, compared with B and B4C (l l, 12] c~rboraae-12 compounds react readily with oxygen, and with oxygen-containing compounds on heating [13, 14]. This leads, in particular, to a marked increase in the coke yield from various carborane-containing polymers [13, 15, 16], whether in inert or in oxygencontaining media,. Among hydz~)carbon and oxygen-containing polymers the coke yields are highest for resins commonly used for the preparation of carbon materials [17, 18], i.e. resins such as natural pitch polymers and synthetic polymers of the furane and phenol-formaldehyde types. O u r p r e s e n t a i m w a s t o i n v e s t i g a t e t h e influence o f o - c a r b o r a n e a n d its d e r i v a t i v e s o n t h e c o k e y i e l d a n d o n t h e r e l a t i v e o x i d a t i v e stabilities o f c a r b o n i z a t i o n p r o d u c t s . T o d o so w e t o o k as s t u d y o b j e c t s resins n o r m a l l y u s e d for t h e p r e p a r a t i o n o f c a r b o n m a t e r i a l s , viz. t h o s e b a s e d o n t h e f o l l o w i n g c u r e d c o m p o u n d s : difurfurylideneacetone (DFA) oligomer with M~850-1050; monofurfurylidenea c e t o n e (FA) o l i g o m e r w i t h M = 9 0 0 - 1 1 0 0 ; f u r f u r y l a l c o h o l (FA1), p h e n y l f o r m a l d e h y d e resol (SF- 3021) a n d n o v o l a k c o n t a i n i n g u r o t r o p i n e . Boron and B~C were added dry to initial resins by mixing on a ball mill, while o-carborane and its derivatives were introduced in the form of solutions into the respective solutions o f thermosetting compounds (Table 1). The resulting modified resin solutions were dried TABLE

1. M E T H O D S OF M~}DIFICATIOI~ AND CURING CONDITIONS FOR PH~I~OL-FO]~MALDEHYD:E AND F U R A N E T Y P E T H E R M O S E T T I N G COlY~POUNDS

Initial products

D F A oligomer (40% solution in acetone) FA oligomer (40% solution in acetone) Furfuryl alcohol

Phenol-formaldehyde resol (40% solution in ethanol)

Method of introducing modifying additives 10% solution il~ aceton(. Ditto Dissolved in FAl 10% solution in ethanol

Hardening conditions* in air ] in reducing agent t tempera- [mean heat-I t ~ ~ t e ture in- ] ing rate, /tureinter- of heating, terval, °C I deg/hr ] val, °C deg/hr

50-200

10

200-260

8

50-200

10

200-260

8

80-120 120-180

1.5 10

180-260

8

50-170

10

170-260

8

* Curing conditions were the same for the unmodified products. t I n activated carbon surroundings.

Oxidative stability of products of pyrolysis of network polymers

2455

in air to a solvent content not exceeding 3-8%, and then underwent heat treatment in line with curing conditions outlined in Table 1. Carbonization was carried out in a reducing nxedium (annealing furnace with coke filling) with the temperature rising at an average rate of 10 deg/hr to I000°. All carbonization products were coraminuted to a particle size of 800-1000 pro. Weighed portions of the particles were placed in quartz cups; in this case they ~,ccupied one and the same volume (1.3 cma), the layer thickness being (~5/an. (Preliminary l,ests showed that a 0. 2-0.3 g spread in the size of weighed portions has no effect on the; shape of weight loss curves for one and the same coke). The portions were then oxidized in a quartz reactor at 6004-2.5 °, in an air current preheated to the same temperature (251./hr). Oxidation rates were determined from linear portions on curves of change in coke weight ],l the loss interval covering 10-20% [19]. Given the results in Table 2 we can n o w deduce the order in w h i c h coke o x i d a t i o n r a t e s are r e d u c e d for t h e unmodified resins [10]: p h e n o l f o r m a l d e h y d e resol -~ novolac ~ h i g h - t e m p e r a t u r e p i t c h ~ a v e r a g e - t e m p e r a t u r e p i t c h ~ D F A ~ FA~> :>FA1. I t is clear f r o m this order t h a t the best o x i d a t i v e s t a b i l i t y is possessed b y cokes o f f u r a n e c o m p o u n d s . T_kBLE 2. E F F I C I E N C Y

IN

REDUCING THE

THERMOSETTING

COMPOUNDS

Iuitial thermosetting compounds

O X I D I Z A B I L I T Y OF MODIFIED

o-Carborane concentra tion, wt.%

~

COKES BASED

ON VARIOUS

O-CARBOBA.N~

Rate of coke oxidation at 600° (dog/rain) modified by unmodified o-carborane

V2

?)2

FA1 DFA oligomer

FA oligomer Phenol-formaldehyde resel

2 0

0.41 0.71

1

2 5 10 2 2

0"13

0"32

0"59 0"21

0"83

0.03

O-50 1.67

0.31 0.23 0-16

0"30 0.04 0.43

0.48 0.10

To d e t e r m i n e the most efficient a n t i o x i d a n t a d d i t i o n a m o n g the c a r b o r a n e c o n t a i n i n g c o m p o u n d s for the s y n t h e t i c resins we h a d first of all to be sure t h a t t h e y do n o t h i n g to i m p a i r so i m p o r t a n t a p a r a m e t e r of carbonizing p o l y m e r as t h e coke yield which, in the present case, is d e t e r m i n e d d u r i n g h e a t t r e a t m e n t o f the p o l y m e r in a r e d u c i n g m e d i u m . I n a d d i t i o n we d e t e r m i n e d the coke residues tbr the p o l y m e r s a t 950 ° in air in a c c o r d a n c e w i t h S t a t e S t a n d a r d 9951-73. C o m p a r i n g values of the coke residues a n d coke yields for t h e modified c a r b o r a n e c o n t a i n i n g c o m p o u n d s a n d the unmodified p o l y m e r s (Table 3) we conclude t h a t the i n t r o d u c t i o n o f c a r b o r a n e additions does n o t reduce the coke yield, a n d even increases it b y ~ 18% in the case of t h e o-carborane-modified f u r f u r y l alcohol. I t is also a p p a r e n t f r o m Table 3 t h a t t h e b o r o n c o n t e n t s in the h a r d e n e d D F A a n d FA1 are close to the calculated values, b o t h for t h e r a t h e r volatile

L. A. BAm~r~ e$ a/.

2456

o-carborane and for the nor~volatile diphenyl-o-earborane. H e a t treatment results in a ~ 15-20°/o loss of o-carborane introduced into the resin from the phenol-formaldehyde resol. In view of this we conclude that o-carborane is n o t sublimed from the resins based on furane thermosetting compounds, whereas sublimation does take place for the resin based on the PF-reso]. The furane thermosetting compounds probably form d e n ~ network structures already at low hardening temperatures, thus preventing sublimation of the o-carborane. In addition the formation of sufficiently stable H bonds in view of the acidic character of o-carborane C - - H bonds [20] and the high chemical reactivity of o-carborane with respect to oxygen and oxygen-containing compounds [11, 14] causes fixation of o-carborane in the structure of the hardened oxygen-containing multi-functional compounds.

100

80

I:1

qO

i

30

60 90 ?"[me, rain

120

Taking cokes based on the D F A oligomer as an example, we investigated the influence of earborane-containing compounds on the oxidative stability of the cokes in some detail. The Figure shows the curves of weight change for the D F A modified with various carborane-containing additions. When curves were plotted allowance was made for the weight increment due to the formation of a B~08 mineral residue. Thus for curve I in the Figure the increment was calculated on the assumption that some of the boron equal to part of the burnt-out coke was converted to B=03. The resulting plot (curve 1') practically coincides with the experimental curve. Oxidation rates for the cokes calculated on the basis of the curves of weight change (see Figure) are given in Table 2. It is seen from the Figure and from Table 2 that the addition of l~/o of o-carborane tO

Oxidative stability of products

of pyrolysis

of network polymers

2457

D F A (which is e q u a l to 1.00% of B in t h e coke) results in a 1.2-fold r e d u c t i o n in t h e o x i d a t i o n r a t e of its p y r o l y s a t e , w h e r e a s t h e a d d i t i o n of 2 % o f d i p h e n y l a n d p h e n y l - o - c a r b o r a n e (1.00 a n d 1.34% of B in t h e cokes, respectively) n o t o n l y fails to r e d u c e t h e o x i d a t i o n r a t e of t h e modified p y r o l y s a t e , b u t , on t h e c o n t r a r y , increases it c o m p a r e d w i t h t h e u n m o d i f i e d cokes. On a p r e v i o u s occasion [21] it w a s f o u n d t h a t o - c a r b o r a n e h e a t e d in t h e presence of o x y g e n i n t e r a c t s w i t h 02 m u c h m o r e r e a d i l y t h a n d i p h e n y l - o - c a r b o r a n e , g i v i n g in t h e process m a i n l y h y d r o g e n along w i t h inconsiderable a m o u n t s o f CO a n d C02. I t a p p e a r s t h a t t h e higher r e a c t i v i t y of o - c a r b o r a n e c o m p a r e d w i t h its d e r i v a t i v e s , e.g. w i t h d i p h e n y l - o - c a r b o r a n c [21] m e a n s t h a t w h e n i n t r o d u c e d TABLE 3. INFLUENCE COKE YIELD

Initial thermosetting compound

DFA

FA1

Phenol-formaldehyde resol

OF

AND

CARBORANE-COI~TTAIiN-ING C O M P O U N D S BORON

Modifying additive

CONTENT

Amount of the additive,

IN

THE

THERMAL

ON

THE

PROCESSING

B content based on elemental analysis,* %

COKE

RESIDUE,

PRODUCTS

Coke

%

residue, in initial I in coke compound _ % ealc. found calc. [ found

No additive o-Carborane

1.0

0-75

0"77 0"78 0"79

1-15

1.00 0-99 1-01

Ditto

2.0

1.50

1"35 1"39 l"37

2'04

Phcnyl-o-carborane

2.0

0.98

0"89 0'88 0"92

Diphenyl-oearborane

2.0

0. 73

No additive o-Carborane

2.0

additive o-Carborane

2.0

Coke yield ,$

%

55"2 56"5

64'3 65'4

1"86 1"84 1"83

57"0

68"4

1"35

1"34 1"36 1"32

57"8

66"2

0'75 0"76 0"74

1-10

1"00 0'98 0'97

56"2

67"8

1.50

1"48 1.45 1.47

1.73

1.74 1.75 1.72

1.50

1.21 1-18 1.20

1.82

1.53 1.50 1.51

m

No

* Average standard deviation based on 36 variants of S o = 0 . 0 4 [23]. t l~ethod of deformation in accordance with State Standard 9951-73 . $ Cokes prepared b y heat treatment in reducing medium.

49"3

67"5 85"5

55"3 58" 1

63' 1 65"0

2458

L . A . BA*Cm~R ~ u2.

into oxygen-containing resins, such as DFA, it interacts readily with oxygencontaining groups. This interaction makes for compaction of the three-dimensional network of the hardening polymer, and during pyrolysis results in a bigger change in the structure of coke based on the o-earborane-modified resin than in the case of the less reactive diphenyl- and phenyl-o-carboranes. The optimum amount of o-carborane for m a x i m u m increase in the stability of the carbonized resin towards oxidation was determined experimentally, taking the DFA oligomer as an example. The results obtained arc presented in Table 2. An optimum amount is ~ 5% of o-carborane. In this case the oxidation rate for the carbonized polymer modified with 3% of o-carborane is reduced by a factor of 28.6 compared with the unmodified polymer. I t can be seen t h a t o-carborane is an efficient antioxidant for cokes of other thermosetting resins as well (Table 2). Reductions in oxidation rates of the carbonized modified resins m a y be placed in the order: P-F r e s o I > D F A > F A I > F A . I t is probable t h a t the above order coincides with the order of change in the defectiveness of resin cokes following the introduction of o-carborane. Certainly, it is clear from the results of ESR reported in [22] t h a t P-F resite-type network polymers, whose structure includes carborane rings, are structurally considerably more stable under pyrolysis, and yield cokes of a structurally denser type, and are consequently less defective. This is substantiated by the fact t h a t no oxygen effect is observed for the cokes over the entire temperature interval examined in the pyrolysis. I t is probable t h a t the addition of o-carborane rhakes for less defective cokes, thereby reducing their oxidation rate more effectively, proportionally to the number of oxygen-containing functional groups in the modified resins, and proportionally to their reactivity. It follows from these findings that, for cokes from furane type resins, o-carborane is the most efficient of the antioxidant additives examined, and t h a t ~ 5°'/o o-carborane addition, introduced into the original resin, is the optimum amount. Translated by R. J. A. I-IENDt$Y REFERENCES 1. A. F. P A ~ V - L ~ C ~ O Y , Ye. M. C H E R E D ~ K , V. S. 0 S T R 0 ¥ S ~ [ and S. A. A S A T ~ O V ,

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Oxidative stability of products of pyrolysis of network polymers

2459

8. M. A. AVI)EYEN]KO, (book) Konstruktsionnye matorialy n a osnovo grafita (Constructional Materials Based on Graphite). ed. b y S. Ye. Vyatkin, No. 3, p. 63, Metallurgiya, Moscow, 1967 9. D. J. ALLARDICE and P. L. WALKER, Carbon 8: No. 3, 375, 1970 10. L. A. BAICHER, P. M. VALETSKII, V. V. KULA]KOV and G. V. SERKINA, (book) Konstruktsionnye materialy n a osnove ugloroda (Constructional Materials Based on Carbon). ed. b y V. I. Kostikov, No. 13, p. 21, Metallurgiya, Moscow, 1978 11. L. I. ZAKHARKIN, Vestnik Akad. Nauk SSSR, No. l l , 16, 1974 12. O.A. OSIPOV a n d V. I. MINKIN, Spravochnik po dipol'nym m o m e n t a m (Dipole Moments Handbook), Vysshaya shkola, Moscow, 1971 13. V. V. KORSHAK, S. V. VINOGKADOVA, P. M. VALETSKII a~d N. I. BEKASOVA, Vestnik Akad. Nauk SSSR, No. 11, 23, 1974 14. S. R. RAFIKOV, S. A. PAVLOVA, I. V. ZHURAVLEVA, R. S. AYUPOVA, P. M. VALETSKH, A. I. KALACHEV, S. V. VINOGRADOVA and V. V. KORSHAJK, Dokl. Akad. Nauk SSSR 207: No. 5, ] 133, 1972 15. V. V. KORSILkK, P. M. VALETSKII, S. V. VINOGRADOVA, A. I. KALACHEV and V. I. STANKO, Vysokomol. soyed. A13: 848, 1971 (Translated in Polymer Sci. U.S.S.R. 13: 4, 957, 1971) 16. A. I. AKSENOV, G. A. ~KOLOMOYETS, L. I. GOLUBENKOVA, P. M. VALETSKII, V. I. STANI~O, S. V. VINOGRADOVA and V. V. KORSItAK, Dok]. Ak~l. Nauk SSSI~ 227: No. 3, 603, 1976 17. E. F I T Z E R and B. TERWIESH, Carbon 10: /No. 4, 383, 1972 18. A. Ya. YIALKOV, Uglegrafitovye materialy (Carbon and Graphite Materials). p. 14, Energiya, Moscow, 1979 19. E. T. TURKDOGEN, R. G. OLSSON and J. VINTERS, Carbon 8: No. 4, 545, 1970 20. L. Ye. VINOGRADOVA, L. A. LEITES, V. N. KALININ and L. I. ZAKHARKIN, Izv. Akad. Nauk SSSR, Seriya khimich., 2847, 1969 21. G. A. KOLOMOYETS, L. I. KOMAROVA, A. B. BLYUMENFEL'D, L. I. GOLUBENKOVA, B. M. KOVARSKAYA, P. M. VALETSKII, S. V. VINOGRADOVA and V. V. KORSHAK, Vysokomol. soyed. A18: ]386, 1976 (Translated in Polymer Sci. U.S.S.R. 18: 6, 1589, 1976) 22. L. S. STREL'CHENKO, P. M. VALETSKII, P. A. PSHENICHI(IN, T. M. ABRAMOVA, L. I. GOLUBENKOVA, S. V. VINOGRADOVA and V. V. KORSHAK, Vysokomol. soyed. B21: 135, 1979 (l~ot translated in Polymer Sci. U.S.S.R.) 23. R. V. FENNEL and T. S. WEST, Zh. analit, l~limii 26: No. 5, 1021, 1971