The comparison of thermal stability of some hydrofluoroethers and hydrofluoropolyethers

Journal of Fluorine Chemistry 125 (2004) 1081–1086

The comparison of thermal stability of some hydrofluoroethers and hydrofluoropolyethers G. Marchionnia,*, S. Petriccia, P.A. Guardaa, G. Spataroa, G. Pezzinb a b

Solvay Solexis R&D, viale Lombardia 20, 20021 Bollate, Milan, Italy Department of Physical Chemistry, University of Padova, Padua, Italy

Received 20 November 2003; received in revised form 16 January 2004; accepted 17 January 2004 Available online 30 April 2004

Abstract The thermal stability of representative hydrofluoropolyether (HFPE) and hydrofluoroether (HFE) compounds has been evaluated. The observed stability order appears to be correlated with the nature of the hydrogenated chain ends; in particular, molecules having fully hydrogenated chain ends (–OCH3 and –OC2H5) show a significantly lower stability compared with the –OCF2H terminated compounds. The main degradation products suggest, however, that the same primary reaction is responsible for the decomposition of all the compounds examined; this reaction involves the fragmentation of the RfO–CxHyFz bond with fluorine transfer between the two carbon atoms close to the oxygen, leading to the formation of a hydrofluorocarbon CxHyF(zþ1) and an acyl fluoride or a ketone. # 2004 Elsevier B.V. All rights reserved. Keywords: Hydrofluoroethers; Hydrofluoropolyethers; Thermal stability

1. Introduction Several hydrogen containing fluoroethers have been developed in the last few years as CFC substitutes to be used as solvents, foaming and fire extinguishing agents and in applications such as heat transfer agents and cleaning of electronic equipment [1–7]. Some of the physical properties of hydrofluoropolyethers (HFPEs) and of hydrofluoroethers (HFEs) have been reported recently [8–10]. These compounds are characterized by properties typical of perfluoropolyethers (PFPEs), in particular, a low vapor pressure, a wide liquid interval, and a low temperature dependence of viscosity [11,12]. The presence of hydrogen in the molecule, of course, strongly reduces the atmospheric lifetime; the above fluids, therefore, when compared with PFPEs, have a lower environmental impact in terms of global warming potential (GWP) [13–15]. The presence of hydrogen reduces also their thermal stability, in comparison with the perfluorinated analogues. Since in some applications, such as heat transfer, the stability of the fluid can be important, in the present work several

*

Corresponding author. Tel.: þ39-02-3835-6289; fax: þ39-02-3835-2152. E-mail address: [email protected] (G. Marchionni). 0022-1139/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jfluchem.2004.01.027

representative HFPEs and HFEs have been investigated and some degradation mechanisms have been proposed.

2. Results and discussion The following hydrofluoro-compounds have been studied: samples A–C are linear HFPEs of general formula HCF2O(CF2O)m(CF2CF2O)nCF2H (trade name H-Galden ZTTM) with different average molecular weight (MW); sample D is an HFPE having the structure CH3O(CF2CF2O)3CH3; sample E is the HFE ethoxy-nonafluorobutane (trade name HFE 7200TM) and consists of a mixture of the two isomers with essentially identical properties (CF3)2CFCF2OC2H5 (60%) and CF3CF2CF2CF2OC2H5 (40%); sample G is the HFE 2-trifluoromethyl-3-ethoxydodecafluorohexane (trade name HFE 7500TM). Some physical properties of these compounds are reported in Table 1. The thermal stability tests performed in the temperature range 160–300 8C clearly indicate different onset degradation temperatures for the hydrofluorocompounds investigated; the decomposition reaction yields both functional products (acyl fluorides or ketones) and partially hydrogenated (gaseous) fluorocarbons, as reported in Table 2. The quantities of the acidic degradation products (ketone for sample G) measured after the 8 h tests in the liquid phase

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Table 1 Structure, molecular weight, boiling point (Tb), density (r) at 25 8C (r25 8C) and kinematic viscosity at 25 8C (Z25 8C) of compounds investigated Sample

Formula

MW

Tb (8C)

r258C (g cm3)

Z25 8C (mPa s)

A B C D Ea Gb

HCF2O(CF2O)0.28(CF2CF2O)0.86CF2H HCF2O(CF2O)0.74(CF2CF2O)1.91CF2H HCF2O(CF2O)1.45(CF2CF2O)3.67CF2H H3CO(CF2CF2O)3CH3 C4F9OC2H5 C7F15OC2H5

328 480 641 394 264 414

88 133 178 161 78 130

1.62 1.68 1.72 1.52 1.43 1.61

0.56 0.92 1.50 1.07 0.50 0.84

a b

Sample HFE is a mixture of the two isomers (CF3)2CFCF2OC2H5 (60%) and CF3CF2CF2CF2OC2H5 (40%) as shown by Sample HFE is (CF3)2CFCF(OC2H5)CF2CF2CF3.

as a function of temperature are collected in Table 3. For samples A and B no evidence of gaseous products, in the temperature range 160–230 8C was found. Treatment at 300 8C resulted in the presence of CO2 (or COF2) and CHF3 as main degradation products in the vapor phase, together with low amounts of CO. Similarly, the acidimetric analyses of the liquid phase gave values close to the analytical threshold for the samples treated at 230 8C, whereas measurable values were obtained at 300 8C. The acidimetric data were confirmed by the 19 F NMR analysis, since the acyl fluoride RfOCF2COF was detected in the liquid phase only in the sample treated at 300 8C, together with traces of HF and chain ends of the –OCF2CF2H type. For compound D, only tested at 230 8C, the analysis of the liquid phase indicated a measurable acidity and the GC analysis showed CH3F as the main degradation product present in the vapor phase, whereas CO and CO2 (or COF2) were present only in traces. The acidimetric analyses performed on the liquid phase of sample E showed a value close to the analytical threshold after the test at 160 8C, and values rapidly increasing for the higher temperatures (it has to be noted that the test at 300 8C had to be stopped after two hours, due to high pressure). By 19 F NMR analysis of the liquid phase, the acyl fluoride C3F7COF was identified, together with traces of HF. In the gas phase C2H5F and C2H4 were found as main products (they were detected in low amount also at 160 8C); moreover the gas chromatography-mass spectrometry (GC/MS) analysis revealed trace amounts also of CO, CO2, C3F7H, C3F6, and C3F8. The 19 F NMR analysis of sample G treated at 230 8C showed the formation of the ketone C3F7COC3F7 in appreciTable 2 Main degradation products identified in liquid and gas phases after the thermal stability tests and corresponding onset degradation temperatures Sample

Temperature (8C)

Main degradation products Liquid phase

Gas phase

A–C D E G

>230 230a >160 >160

RfCOF RfCOF C3F7COF C3F7COC3F7

CHF3 CH3F C2H5F and C2H4 C2H5F and C2H4

a

Compound D tested only at 230 8C.

19

F NMR.

able amount, with traces of HF. In the vapor phase, the same main products of sample E (C2H5F and C2H4) were found, with traces of CO, CO2, C3F7H, C3F6, and C3F8. In the case of product G the acidimetric titration gave unreliable results, since the ketone C3F7COC3F7 is unstable under the conditions of the analysis; therefore, this product was quantified by 19 F NMR analysis (detection limit ¼ 0:4 mmol kg1), as reported in Table 3. Since the differences of thermal stability among the investigated compounds are clearly appreciable at 230 8C, at this temperature specific tests for longer times (16 and 24 h) were performed. Fig. 1 reports the amount of the acidic degradation products (ketone for compound G) measured in the liquid phase as a function of time, whereas in Table 4 the corresponding data of the total gas production referred to the amount of compound treated (the sum of C2H5F and C2H4 for samples E and G; CH3F for sample D) are summarized. The longer time tests confirm the 8 h test data: the HFPE compounds A and B after 24 h treatment at 230 8C show acidity values close to the analytical threshold and non quantifiable traces of gaseous degradation products; in the tests on the HFEs, the quantities of the degradation products after 24 h are about 20 times higher than the corresponding values measured after 8 h. Compound D, after tests carried out for 16 and 24 h, has a significantly lower degradation compared with the HFEs. To define better the degradation temperature limit for the HFEs, specific tests were performed at 160 8C as a function of time. After 48 h compound E shows a degradation about 20 times higher than that observed in the 8 h test. Compound Table 3 Quantification of the acidic degradation products measured by acidimetric titration in the liquid phase after the 8 h tests Temperature (8C) 160 200 230 300 a b

A (mmol kg1)

B (mmol kg1)

– <0.4 <0.4 250

– – <0.4 173

D (mmol kg1)

E (mmol kg1)

– – 5.5 –

0.6 6.4 5.3 190a

G (mmol kg1) <0.4b <0.4b 12b 370b

Test stopped after 2 h owing to high pressure produced. Values referred to the amount of ketone obtained by 19 F NMR analysis.

G. Marchionni et al. / Journal of Fluorine Chemistry 125 (2004) 1081–1086

1083

-1

Degradation products (mmol kg )

120

100

80

60

40

20

0 0

4

8

12

16

20

24

28

Time (h) Fig. 1. Acidic degradation products (ketone for sample G) as a function of time, measured in the liquid phase, after thermal tests at 230 8C of samples A (*), B ( ), D (~), E (&), G (*).

G after 48 h does not show the ketone signal on 19 F NMR analysis but some gas production was found in the corresponding gas phase. An indication of the degradation limit temperature for the –OCF2H terminated HFPEs was obtained from thermal tests independently performed on the HFPE sample C, which was heated at 230 and 250 8C for different times in an autoclave, the residual pressure after cooling at room temperature being recorded. The results obtained showed that the fluid is stable at 230 8C at least for 110 h and can withstand short periods (about 30 h) also at 250 8C. The experimental results reported above indicate the following thermal stability scale for the compounds investigated: AC > D > G > E. From the quantification of the main degradation products (see Tables 3 and 4) it appears that the acidic (or ketone) compounds and the gaseous products (partially hydrogenated fluorocarbons) are formed in roughly equal amounts from the HFE samples E and G. For the –OCF2H terminated HFPEs the amount of CHF3 found after the 8 h tests at 300 8C was of the same order of the acidity value measured on the liquid phase. In the case of sample D the amount of CH3F was far below the corresponding acidity value. Table 4 Quantification of the main degradation products revealed in the gas phase after the thermal tests at 230 8C for samples D, E, and G Time (h)

D (CH3F) (mmol kg1)

E (C2H5F and C2H4) (mmol kg1)

G (C2H5F and C2H4) (mmol kg1)

8 16 24

0.1 0.1 0.2

4.7 24 97

1.9 16 37

On the basis of the main degradation products it is possible that the HFPE and HFE compounds studied here undergo the same primary degradation reaction (Eq. (1)) Rf CF2 OCx Hy Fz ! Rf COF þ Cx Hy Fðzþ1Þ

(1)

where x ¼ 1 or 2. The reaction results in fragmentation of the C–O bond leading to the elimination of the partially hydrogenated moiety CxHyF(zþ1) (CHF3, CH3F, or C2H5F in case of the compounds examined here) and to the formation of the acyl fluoride (or ketone in case of compound G). The mechanism, which involves a fluorine transfer between the two carbon atoms close to the oxygen, is presumably heterolytic, since the homolytic C–F bond scission is expected to have a very high activation energy. It is well known that these reactions are promoted by Lewis acids, both on the perfluoropolyether structures [11,12,16] and on partially hydrogenated fluoroalkyl alkyl ethers [17]. Therefore, one should not exclude the possibility that the observed degradation could be promoted by metal fluorides generated on the metal surface of the vessel. Such conditions are of course representative of a typical heat transfer application. Based on the analytical data (see Table 2), and taking into account the minor products, a simplified degradation mechanism is proposed for the above reactions in Schemes 1–4. In the thermal degradation of the HFPE compounds A and B (see Scheme 1) the main degradation reaction produces the acyl fluoride RfOCF2COF and CHF3; the presence of CO can be explained by decarbonylation reactions of the acyl fluoride to form –OCF3 chain ends (identified in trace amounts by 19 F NMR analysis on samples treated at 300 8C). Hydrolysis due to humidity from the air can be present, giving HF and the acid RfCOOH (traces).

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G. Marchionni et al. / Journal of Fluorine Chemistry 125 (2004) 1081–1086 Rf-O(CF2CF2O)m(CF2O)nCF2CF2OCF2H Rf-OCF2COF

Rf-O(CF2CF2O)m(CF2O)nCF2COF + CHF3

Rf-OCF3 + CO

Rf-OCF2COF + H2O

Rf-OCF2COOH + HF

Rf-OCF2H + CO2 Rf-OCF2CF2O(CF2O)nCF2H COF2 + H2O

Rf-OCF2CF2H + (n+1) COF2

CO2 + 2 HF

Scheme 1. Reaction pathway for the thermal degradation of samples A–C.

The formation of the hydrogenated chain ends, –OCF2CF2H and COF2, observed at high temperature (300 8C), is indicative of homolytic C–H bond breaking reactions, previously reported to be significant over 260 8C [15]. In Scheme 2 the degradation paths suggested for compound D, show in the primary reaction the formation of the acyl fluoride CH3O(CF2CF2O)2CF2COF and of CH3F; as in the previous case, the secondary reactions of the acyl fluoride could explain CO, HF, and CO2 production (through decarbonylation, hydrolysis, and decarboxylation reactions, respectively). The primary degradation reactions of the HFE compounds (E and G), shown in Schemes 3 and 4, produce C2H5F and the acyl fluoride or the ketone, respectively. Fluoroethane is known easily to undergo dehydrofluorination to form ethene [18]; this appears to be confirmed also in our experiments, since the ratio C2H5F/C2H4 progressively decreases with time. The secondary reactions of the acyl fluoride in Scheme 3 are responsible for the formation of CO, CO2, C3F7H, C3F6, and C3F8. The products observed in traces amounts in the treatment of compound G (C3F7H, CO, CO2, C3F6, C3F8) can be explained by the partial degradation of the ketone to form the acyl fluoride and hexafluoropropene, followed by the degradation reactions of the acyl fluoride, as in compound E. The experimental data clearly show that ethers containing a fully hydrogenated methyl or ethyl group (–ORH) have lower thermal stability compared with ethers having a

CF3CF2CF2CF2OCH2CH3

CF3CF2CF2COF

+

CH3CH2F

CH2=CH2 + HF CF3CF2CF2COF + H2O

CF3CF2CF2COOH + HF

C3F7H + CO2

C3F6 + HF CF3CF2CF2COF

C3F8 + CO

Scheme 3. Reaction pathway for the thermal degradation of sample E.

partially hydrogenated group (such as –OCF2H); it is well known that the thermal stability of perfluorinated ethers is even higher. The same scale of stability has been recently reported in radiolysis experiments performed on some CF3

CF3

OCH2CH3

CFCFCF2CF2CF3

O

CFCCF2CF2CF3

+

CH3CH2F

CF 3

CF3

CH2=CH2 + HF CF3

O

CFCCF2CF2CF3

C3F7COF + C3F6

CF3 H3CO(CF2CF2O)2CF2CF2OCH3 Rf-OCF2COF Rf-OCF2COF + H2O

H3CO(CF2CF2O)2CF2COF + CH3F

C3F7COF + H2O

C3F7COOH + HF

Rf-OCF3 + CO C3F7H + CO2

Rf-OCF2COOH + HF

Rf-OCF2H + CO2

Scheme 2. Reaction pathway for the thermal degradation of sample D.

C3F7H

C3F6 + HF

C3F7COF

C3F8 + CO

Scheme 4. Reaction pathway for the thermal degradation of sample G.

G. Marchionni et al. / Journal of Fluorine Chemistry 125 (2004) 1081–1086

HFPEs and HFEs compared with PFPEs and CFCs; in particular, molecules containing –OCF2H chain ends were found to have higher radiolytic stability compared with other hydrofluoro compounds [19]. Literature data show that the substitution of fluorine by hydrogen decreases the C–O bond strength in HFEs [20,21]. Moreover, it is known [17] that fluorine atoms attached to a carbon which also bears an alkyl ether group (as in HFEs) are labile with respect to electrophilic attack. Therefore, it seems reasonable to assume that the thermal stability order observed for compounds A–G is correlated mainly with the nature of the hydrogenated end group. However, the structure of the perfluoroalkyl moiety can also influence the ether stability (compare compounds D, E, and G). In particular, a stabilization effect of the perfluoropolyether backbone chain can perhaps explain the good stability of the experimental compound D, despite the presence of the fully hydrogenated –OCH3 end groups.

3. Conclusions The hydrofluoropolyether and hydrofluoroether compounds examined in this work have significantly different thermal stabilities. In particular the HFPE compounds bearing –OCF2H end groups (A–C) are found to be stable at temperatures as high as 230 8C, whereas the HFE compounds E and G show appreciable degradation above 160 8C. The HFPE compound D is more stable than the HFEs, despite the presence of two –OCH3 groups in the molecule. The degradation products identified suggest that the same primary reaction is responsible for the decomposition of all the products examined; this reaction involves the fragmentation of the RfO–CxHyFz bond with fluorine transfer between the two carbon atoms close to the oxygen, leading to the elimination of a hydrofluorocarbon CxHyF(zþ1) (Eq. (1)) and to the formation of an acyl fluoride or a ketone. The thermal stability order observed can be correlated with the chemical structure of the molecule; in particular the presence of fully hydrogenated chain ends seems to reduce significantly the thermal stability. In case of the HFPE– OCH3 terminated (compound D) a stabilization effect due to the PFPE backbone compared with the perfluoroalkyl chain can be envisaged.

4. Experimental 4.1. Materials Structure, MW, boiling point (Tb), density (r) and kinematic viscosity (Z) of the compounds investigated here are reported in Table 1. The structures were determined by 1 H and 19 F NMR spectroscopy and by GC/MS analysis. HFPEs A–C have been obtained by fractional distillation of a polydisperse mixture having boiling points from 35 to

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200 8C. Details of the synthesis and purification processes are reported in the patent literature [22]; the HFPE sample D was synthesized on a lab scale and its preparation has been also described [23]. The HFEs E and G are commercial products from 3 M and were used as received. 4.2. Thermal tests The thermal stability tests were performed for 8 h at 160, 200, 230, and 300 8C in a stainless steel 250 ml vessel, equipped with temperature and pressure control, on 50 g of compound, without elimination of air. The values of the actual pressure during the test, as well as the pressure after cooling at room temperature, were recorded. The gas phase was then analyzed by GC and GC/MS and the liquid was discharged and analyzed by acidimetric titration and 19 F NMR. Longer time tests were carried out at 160 and 230 8C on 4 g of each product in a stainless steel cylindrical 25 ml vessel heated with an oil bath. The liquid and gas phases were analyzed at 8, 16, 24, 48 and 8, 16, 24 h, respectively. 4.3. Analytical methods GC/MS was used to identify the degradation products in the gas phase. The analyses were performed on a HP 5890 gas chromatograph equipped with Porabond Q column (diameter 0.32 mm, length 25 m) coupled with a HP 5989 mass spectrometer. The quantitative analyses of the gas phase products were carried out on a CE 8000 Top gas chromatograph, equipped with an HWD detector and a capillary column Plot Ultimetal (diameter 0.5 mm length 25 m), coating Paraplot Q (DF 20 UM). Gas chromatographic response factors were available for CHF3, CO, and CO2; for CH3F, C2H5F, and C2H4 simple area percentage calculations were performed. It must be emphasized that CO2 and COF2 are indistinguishable under the analytical conditions used. The degradation products, measured both in the liquid and in the gas phases, have been referred to the amount of the starting product and reported in mmol kg1 of treated compound. 19 F NMR spectroscopy was also used to identify the degradation products in the liquid phase. Spectra were recorded on a Varian Mercury 400 NMR spectrometer, working at 376 MHz frequency. The acidity of the liquid phases was measured by potentiometric titration with NaOH (in acetone=water ¼ 40=10) on a Mettler Toledo DL 55 titrator, using a DG 115-SC electrode (detection limit of the analysis ¼ 0:4 mmol kg1).

Acknowledgements The authors wish to thank Dr. U. De Patto and Dr. M. Avataneo for the synthesis of HFPE sample D; Dr. F.

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Morandi, Dr. G. Geniram and Dr. E. Barchiesi for the analytical characterizations and Solvay Solexis for permission to publish this work. References [1] M. Visca, R. Silvani, G. Marchionni, Chemtech 27 (2) (1997) 33–37. [2] J.A. Abusleme, R. Silvani, P. Maccone, US Patent No. 5,654,263, 1997 [3] M. Visca, G. Spataro, G. Marchionni, US Patent No. 5,856,587, 1999. [4] R. Silvani, G. Spataro, G. Marchionni, US Patent No. 6,020,298, 2000. [5] R. Silvani, G. Spataro, G. Marchionni, US Patent No. 5,780,414, 2000. [6] B. Wang, J.L. Adcock, S.B. Mathur, W.A. Van Hook, J. Chem. Thermodyn. 23 (1991) 699–710. [7] A. Sekiya, S. Misaki, J. Fluorine Chem. 101 (2000) 215–221. [8] G. Marchionni, R. Silvani, G. Fontana, G. Malinverno, M. Visca, J. Fluorine Chem. 95 (1999) 41–50. [9] G. Marchionni, P. Maccone, G. Pezzin, J. Fluorine Chem. 118 (2002) 149–155. [10] J. Murata, S. Yamashita, M. Akiyama, S. Katayama, T. Hiaki, A. Sekiya, J. Chem. Eng. Data 47 (2002) 911–915.

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