or the atmosphere would be premature in the absence of more extensive tests. Nevertheless, these findings indicate that in evaluating the potential air quality problem posed by MMT, the catalytic activity of M M T combustion products should be considered, in addition to increases in atmospheric Mn concentrations.
Literature Cited
(4) Otto, K., Sulak, R. J., Enciron. Sci. Techno/., 12, 181-4 (1978). ( ! 5 ) Holiday, E. P., Parkinson, M. C., “Another Look a t the Effects of Manganese Fuel Additive ( M M T ) on Automobile Emissions”, Preprint 78-54.2,71st Annual Meeting of the Air Pollution Control Association, Houston, Tex., June 25-30, 1978. (6) Shelef, M., Kummer, J. T., Chem. Eng. Prog. Symp. Ser., 67 ( l l j ) , 74-92 (1971). ( 7 ) Shelef, M., Otto, K., Gandhi, H., Atmos. Enuiron., 3, 107-22 (1969). (8)National Bureau of Standards Monograph 25, Section 10, 1972, p 38. (9) Joint Committee on Powder Diffraction Standards, Card No. 24-734. (10) Hougen, 0. A., Watson, K. M., “Chemical Process Principles”, Part 3 , Wiley, New York, 1947. ( 11) Swinbourne, E. S., “Analysis of Kinetic Data”, Nelson, London, 1971, p p 40-2.
(1) “Manganese”, National Academy of Sciences, Washington, D.C., 1973. ( 2 ) Ter Haar, G. L., Griffing, M. E., Brandt, M.. Oberding, D. G., Kapron, M., J . Air Pollut. Control Assoc., 25, 858-60 (1975). ( 3 ) Calabrese, E. J., Sorensen, A,, J . Air Pollut. Control Assoc., 25, 1254-5 (1975).
t l w r i t ~ e d/or recieii, August 28, 1978. Accepted December 27, 1978. This incestigation u’as supported b,y the Department fi/ Enuironmental Sciences, University of Lancaster, England, where H . W. Edwards spent the 1977-78 academic year on sabbatical leave from ‘olora do St a t e L‘niuersi ti..
Acknowledgments The authors are grateful for the technical assistance provided by P. D. E. Biggins and H. A. McCartney with the X-ray diffraction analyses and calibration of the NO, analyzer, respectively.
Rearrangement of Poly(dimethylsiloxane) Fluids on Soil Robert R. Buch” and Donald N. lngebrigtson Dow Corning Corporation, Midland, Mich. 48640
Poly(dimethylsi1oxane) (PDMS) fluids in intimate contact with many soils undergo siloxane bond redistribution and hydrolysis, resulting in the formation of low molecular weight cyclic and linear oligomers. Low molecular weight hydroxyfunctional hydrolysis products are water soluble, and the cyclics and trimethylsiloxy-end-blocked oligomers are volatile, thus providing materials which can partition from the soil t o the water and atmospheric environmental compartments. Clays were shown to be the catalytic soil component responsible for the above oligomerization and hydrolysis reactions. Although the activity of all types of clays was shown to be inversely related to the level of clay hydration, some were more susceptible to attenuation than others; e.g., montmorillonite was deactivated by hydration much more readily than kaolinite or halloysite. Furthermore, montmorillonite attack is preferentially upon trimethylsilicon sites, leading primarily t o linear oligomerization, whereas kaolinite attack, which is less discriminating, leads predominantly to cyclic oligomers. Silicones encompass an array of materials including resins, elastomers, and fluids of wide viscosity range. The fluids command a major portion of the silicone market. More specifically, a single type of fluid, poly(dimethylsiloxane), produced in a range of viscosities 0.65 to 60 000 cSt, is the dominant product of the silicone industry. These fluids have a unique combination of useful physical and chemical properties (low surface tension, thermal and oxidative stability, high dielectric constant), while manifesting virtually no biological activity or toxicity. Their exceptionally diverse spectrum of applications ranges from food and cosmetic additives to hydraulic fluids and electrical transformer fluids. Consequently, these fluids have many means of entry t o the environment. Estimated world production of PDMS fluids for 1978 is 100 million pounds. Emerging new markets are expected to increase this figure substantially over the next several years. In view of steadily increasing annual production and the absence of definitive information regarding the environmental 676
Environmental Science 8, Technology
persistence of PDMS, concern has developed over its possible adverse ecological impact ( I , 2). This concern stems from several well-known facts: (a) PDMS fluids are essentially nonreactive, except when catalyzed by strong acids and bases; (b) PDMS based elastomers are uniquely resistant to weathering (Le., hydrolysis and oxidation); (c) no evidence has been found for biodegradation of these polymeric fluids. An earlier report ( 3 ) from our laboratories was directed toward effective containment and cleanup procedures for spills of silicone fluids on water, roadways, and soils. The present paper describes the chemical effects of various soils on PDMS.
Experimental Materials. Solvents used in this study were of reagent grade quality. Activated charcoal was commercial BPL grade, obtained from Pittsburgh Activated Carbon. Silicone fluids used i n this study included commercial grade Dow Corning 200 Fluids (MD,M, where M = (CHI)‘$i0,p2 and D = (CHI).Si01 of 10, 50, lo’, lo4, and loficSt viscosity; hydroxyl endblocked PDMS (HOD,H); linear decamethyltetrasiloxane, L4 (MD2M); linear dodecamethylpentasiloxane, L5 (MD3M); and octamethylcyclotetrasiloxane (D4). The 14C-tagged PDMS fluid as well as the dimethylsilanediol were specially prepared. Various local Michigan soil types representative of the local region’s surface and subsoils were obtained with the assistance of a local USDA soil advisor. Others, as well as pure clays, were obtained from Wards Mineral Supply House (Rochester, N.Y.). Procedures. Preparation of soils consisted of grinding (mortar and pestle) and sifting through an 80-mesh screen. Soils were dried by two techniques. For thorough drying, soils were placed in an air-circulating oven a t 80 “C for 7 days. Early work was done on samples dried for 2 h a t 105 “C. Less stringent drying was accomplished by spreading soil on Teflon sheets in the laboratory a t ambient conditions for 14 days. Soils having controlled levels of moisture were obtained by storing thoroughly dried soils in desiccators having constant 0013-936X/79/0913-0676$01.00/0
@
1979 American Chemical Society
/-Y
STARTING FLUID
Activated Charcoal Trap
+
Air Out
ioa.aoa cs
Meter Polypropylsns C."llt.l
---t
Figure 1. Apparatus for collection of volatiles from PDMS rearrangement on soil APPROXIMATE MOLECULAR WEIGHT
Figure 3. Gel permeation chromatograms of soil extracts. Soil extracted after 4 h and 15 days contact of PDMS (lo5 cSt)
g
\
-
PDMS
100 9 SOIL
1000 CS
'1 t
,
a a5L-----l 10
, 30
20
1
40
50
DAYS
Figure 2. Loss of PDMS (1000 cSt) from soil
humidities of 0, f!O,45, and 98%, achieved using appropriate saturated salt solutions. Equilibration was verified by gravimetrically monitoring the weight gains of the soils. Generally, 4 to 6 weeks was required for equilibration. The influence of moisture levels and/or temperature on the interaction of soils or clays with PDMS was evaluated as follows. Fluid and soil under investigation were added directly into a polyethylene bottle and sealed. Tumble mixing was utilized to distribute the fluid on the soil. Following storage a t the appropriate temperature, the extent of soil and fluid interaction was assessed by extracting the soil with hexane and analyzing the extract via gas-liquid chromatography (GLC). This analysis was facilitated in the early experiments by the use of relatively low molecular weight model compounds (D4, Lq, and L5) which are readily analyzed and identified. These were selected on the basis of their chemical and structural similarities to PDMS fluids. T h e consumption of original starting material and formation of rearrangement products were utilized as a measure of the soil-catalyzed PDMS reaction. An assembly as shown in Figure 1 was used to separate volatile and nonvolatile rearrangement products. The former are readily trapped on activated charcoal (Pittsburgh, BPL Grade). The siloxane bonds of PDMS were chemically inert toward activated charcoal; Le., Dq and L5 underwent no redistribution when adsorbed for extended periods of time. Trapped volatiles were desorbed with hexane or toluene, and identified by GLC analysis of the hexane extract. Identification was based on elution times for standard compounds and confirmed in some cases by mass spectrometry. Quantitation of the trapped volatiles was accomplished by atomic absorption (AA) analysis of the solvent extract. Soils were extracted with either toluene or methyl isobutyl ketone (MIBK) for nonpolar products, and acetonitrile or water for polar rearrangement products. Analysis of the solutions was done by AA, using typical solute-solvent combinations to establish cali-
bration curves. In all instances where extracts were assayed by AA, a distribution of oligomers was present which necessitated the selection of the most representative solute species for calibration. Dimethylsilanediol was the polar calibration standard used in both acetonitrile and water. Octamethylcyclotetrasiloxane (D4) and 50-cSt PDMS were used with MIBK and toluene. Identification of the lower molecular weight rearrangement products remaining on the soil necessitated the use of two different procedures. Nonpolar residual materials extract readily with hexane. Short chain linear oligomers containing silanol end groups do not elute quantitatively on GLC. Therefore, an in situ derivatization/extraction procedure using a 1:l mixture of acetonitrile and hexamethyldisilazane with a few drops of trifluoroacetic acid as catalyst was used to extract and convert these polar rearrangement products to trimethylsilyl end-blocked species. GLC analysis then provided identification of the derivatized polar rearrangement products. PDMS-siloxane bonds do not rearrange in the presence of the derivatizing mixture.
Results and Discussion P D M S Rearrangement-Volatiles Formation. Iowa top soil was treated with an MIBK solution of PDMS and the solvent evaporated. Residual PDMS was monitored as a function of time by extraction and AA analysis. Typical results are shown in Figure 2 and suggest a half-life of approximately 30 days. Direct evidence that PDMS rearrangement products were volatilizing was obtained by placing a 14C-tagged PDMS fluid on the same soil used previously and assaying the activity remaining as a function of time. Results were very similar to those shown in Figure 2. Evidence for the nature of the process occurring on soil is given in Figure 3, where gel permeation chromatograms of MIBK extracts are given. These extracts were obtained a t various time intervals following treatment of a soil with a 105-cSt PDMS fluid. The rearrangement of PDMS to lower molecular weight species is evident. Rearrangement studies with model compounds L j and D4 showed that L j formed an array of cyclic and trimethylsiloxy [(CH3)3SiO-] oligomers (Figure 4). Silanol end-blocked linears were also present. When Dd was the starting material, the resulting products were cyclic oligomers and hydroxy endblocked linear oligomers. Figure 5 shows the distribution of cyclic as well as the trimethylsilyl derivatized siloxanol oligomers. Determination of Key P a r a m e t e r s . Preliminary experiments indicated that the degree of catalytic activity was highly dependent upon the hydration of the soil. T o better establish this dependence, several soils were thoroughly dried and then equilibrated to various moisture levels by storing them at various constant humidities. It was found that as little as 1%moisture content (equilibration a t 20% relative humidity) significantly altered the soils catalytic activity. Soil temperature also influences activity (Table I). Volume 13, Number 6, June 1979
677
I
CH,CN/HMDS, _y-
MDM
MD2M
I I
Figure 4. GLC chromatogram of hexane extract. Rearrangement products resulting trom dodecamethylpentasiloxamer ( L ~ )
Table 1. Influence of Soila Hydration and Temperature on Siloxane Rearrangement humidity,
%
0 20 m L"
45
80 a
% water
soil temp, OC
% L5 remalnlng
contact time, h
0.13 1.44
.--
25 90 in
1 24
II Ad
20 20 fin ""
2.30 3.56
60 60
10
24 24
34
95
clay type
Loam sub-surface soil; Midland County, Mich. ~~~
Table II. Rearrangement Activity of Various Soil Types with D4 and L5 SOll
% Dqa
% L5a
silt (N.Y.) boulder clay (N.Y.) silt loam (N.Y.) sa 1y loam (N.Y.) dL 2 sand (Mich.) beach sand (N.Y.) muck (N.Y.)
10 85 95 97 100 100 100
10 15 30 40 95 100 100
After 24-h soil/PDMS contact at 21 'C
Insight into the soil component responsible for the rearrangement of PDMS was then sought. Various soil types were oven-dried prior t o contacting them with PDMS model compounds. After addition of the model compounds, samples were sealed in vials for 24 h a t ambient temperatures before assaying. These results (Table 11) revealed that soils high in humus or sand display no significant rearrangement activity. In sharp contrast, soils with a substantial clay component demonstrate activity with both compounds and generally are more reactive with linear than cyclic structures. T o confirm the role of clays, several representative clays were evaluated with PDMS in a manner similar to that used with soils. In addition, catalytic rearrangement activity was evaluated with both D4 and L5 model compounds. Results are shown in Table 111. The activity of clays is dependent on hydration, as with tile
Table 111. Influence of Moisture Content on Rearrangement Activities of Clays for Cyclic and Linear Siloxanes at 21 "C
-7
I"
~~
a
Figure 5. GLC chromatogram of extracted/derivatized rearrangement products resulting from octamethylcyclotetrasiloxamer (04)
tnvironmenrai science & I ecnnoiogy
kaolinite no. 9 halloysite no. 29 halloysite no. 13 montmorillonite no. 27
ambient air dried oven dried at 80 OC, (RH = 3 5 % ) , % siloxane remaining % siloxane remaining after 1 h after 24 h 04
L5
D4
L5
15 85 20 98
3 5 5 5
90 98 60 100
75 95 40
100
soils, but also varies considerably with clay type. In addition, discrimination in activity toward cyclic and linear PDMS structures is apparent. All dry clays degrade linear materials, but clays such as montmorillonite and halloysite no. 29 exhibit markedly lower activity with cyclic structures, even when preconditioned by oven drying at 100 "C. Several observations suggest that permanent "active sites" in the clays are responsible for this catalytic behavior: (a) the dramatic reduction in catalytic activity with low levels of hydration; (b) the apparent complete reversibility of their catalytic behavior with moisture content; (c) washing of the clays or soils with water or solvents does not destroy their activity, i.e., with appropriate drying, their catalytic activity is restored. The enhanced rate of rearrangement for linear siloxanes rather than cyclic siloxanes further suggests that the catalytic sites of montmorillonite and halloysite are acidic in character, since the susceptibility of the trimethylsiloxy end groups to attack by acids is well established in organosilicon chemistry. Extensive earlier work ( 4 - 7 ) describes the use of clays as effective polymerization catalysts for cyclic organosiloxanes. In these earlier studies relatively pure naturally occurring and/or acid-treated clays were utilized. T o characterize more fully the rearrangement resulting from PDMS fluid and the dependence of product distributions on clay type, we oven dried kaolinite and montmoril-
Table V. Partitioning of 50-cSt PDMS Rearrangement Products
Table IV. Residual PDMS and Rearrangement Products on Clays a kaolinite extraction solvent
MlBK
acetonitrile water a
montmorillonite
PDMS extracted Oh
17 17 11
extraction solvent
MlBK
acetonitrile toluene water
% PDMS extracted
69 71 23 71
soil
silt
lonite clays a t 80 "C for several days and then loaded them with approximately 1%by weight 50-cSt PDMS. T h e assembly shown in Figure 1 was used to collect volatile species. After 4 weeks contact time, individual samples of clays were separately extracted and assayed via AA for silicon content. The charcoal traps were extracted with hexane with the extracts subsequently analyzed via GLC to identify volatile species. The clay extraction data (Table IV) suggest a markedly different distribution of rearrangement products resulting from the two clays. Kaolinite-catalyzed rearrangement resulted in the loss of approximately 80% of the original PDMS via volatilization. T h e remaining 10-20% consists largely of material soluble in polar solvents, Le., siloxanols. As shown by GLC, the volatile species consist primarily of cyclic species with some trimethylsiloxy end-blacked linears. T h e results from montmorillonite suggest a quite different distribution of rearrangement products. The data of Table IV indicate that most of the PDMS remains on the montmorillonite clay. In situ extraction/derivatization and analysis by GLC confirmed the presence of substantial amounts of siloxanol oligomers. Extraction and GLC analysis of the charcoal-trapped volatiles revealed mainly hexamethyldisiloxane. The trimethylsiloxy site is more prone to attack by acids than dimethylsiloxy sites and is apparently removed early in the rearrangement process. With its removal, the formation of less volatile hydroxy endblocked linears via hydrolysis appears to be favored. These findings indicate that clay type can influence to a high degree the rearrangement products resulting from PDMS. T h a t is, a kaolinite-based soil will promote the formation of predominantly volatile rearrangement products, whereas a montmorillonite based soil will promote the formation of predominantly water-soluble hydrolysis products. Further experiments using PDMS fluid on several types of soils were conduc1,ed to simulate the effect of rainfall on soil activity. Air-dried soils were coated with 50-cSt PDMS fluid and stored in a protected but ventilated area. Periodically, the total volume of each soil was washed with about lox its weight of distilled water, which was subsequently analyzed for water-soluble silicon species by AA. J u s t prior to the water wash, a small port,ion of this soil was extracted with toluene to give a measure of the nonpolar polymer remaining. As shown in Table V the water-soluble species constitute a relatively small fraction of the silicone. In all cases, extensive loss of PDMS is noted. Environmental Implications. These studies indicate that PDMS rearrangement products partition largely to the atmosphere. Volatile species are cyclic structures and trimethylsiloxy end-blocked linears. Approximately 10 to 25% of the original PDMS may partition to waterways as shortchain siloxanols. However, soil composition and hydration have a marked influence on the amount and distribution of rearrangement products. T h e kinetics of PDMS rearrangement on soil are difficult to define because of the retarding influence of hydration on the activity of soils (clays). These studies suggest the half-life of PDMS on soil can range from several minutes to weeks or
9 30 53
clay loam
5
silty loam
26 47 6 24 45
Clays oven dried; 4 weeks contact time. Original loading was 1.0 g of
PDMSllOO g of clay.
contact time, days
clay
5 26 49
Oh
Oh
%
extracted (toluene)
extracted (water)
PDMS volatiles
62 38 14 88 78 39 70 20 10 99 29 16
5 5 6 3 3 3
5 3 4 8 12 10
32 55 78 9 19
58 25 77 81 0 59 74
substantially longer. Other modes of degradation based on hydrolytic or photolytic reactions could significantly alter these estimates. It is clear, however, that relatively moist or wet soils exhibit significantly less catalytic rearrangement activity toward PDMS than dry soils. Similarly, soils with low or no clay content exhibit minimal rearrangement activity. Since dry soils are most active, most PDMS rearrangement will occur a t or near the uppermost layer of soil. In the work of Cox and Ingebrigtson ( 3 ) ,PDMS migration was noted to occur both upward and downward. Thus, PDMS located several inches below the earth's surface on moist soil can migrate to drier surface soil and degrade via clay-catalyzed rearrangement. These findings underscore the importance of additional studies aimed a t elucidating the aqueous chemistry of PDMS hydrolysis products and the atmospheric chemistry of the volatile oligomers. Such studies are currently in progress and will be reported later.
Acknowledgments The efforts of C. L. Hanson, David Helmreich, and Alan Snodgrass in the gathering of laboratory data are gratefully acknowledged. Helpful suggestions were received from Thomas Lane, Robert C. Smith, David Spielvogel, and Paul J. Garner in regard to analytical and synthetic aspects of this study. Finally, the assistance and guidance of Mr. Ken Mettert (USDA) and Professors Lee Jacobs and Max Mortland of Michigan State University in preliminary discussions on soil composition and chemistry were very helpful and appreciated. Literature Cited (1) Brown, S. L., Chan, F. Y., Jones, J. L., Liu, D. H., McCaleb, K. E.,
Mill, T., Sapios, K. N., Schendel, D. E., Research Program on Hazard Priority Ranking of Manufactured Chemicals, Phase-11, Report No. P B 247 778 (available from National Technical Information Service, Springfield, Va.). ( 2 ) Howard, P. H., Durkin, P. R., Hanchett, A,, Environmental Hazard Assessment of Liquid Siloxanes (Silicones), Report No. P B 263 162 (available from National Technical Information Service, Springfield, Va.). (3) Cox, T. S., Ingebrigtson, D. N., Rnuiron. Sci. Technol., 10, 598 (1976). (4) Britton, E. C., White, H. C., Moyle, C. L., U.S.Patent 2 460 805 (1949). (5) Ishizuka, J., Aihara, T., Kogyo Kagahu Zasshi, 59, 1198 (1956). (6) Andrianov, K. A., Krasovskaya, T . A., Khim. Promst. (Moscow), 8,462 (1956). ( 7 ) Sakiyama, M., Okawara, R., J . Organomei. Chem., 2, 473 (1964). Received for review September 1 , 1978. Accepted December 28, 1978.
Volume 13, Number
6, June 1979
679
