Ind. Eng. Chem. Res. 1991,30,2531-2534
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Quantitative Structure-Activity Relationships for Fluoroelastomer/Chlorofluorocarbon Systems Kazimiera J. L. Paciorek,* Steven R. Masuda, and James H. Nakahara Ultrasystem, Znc., 2400 Michelson Drive, Zrvine, California 92715
Carl E. Snyder, Jr., and William M. Warner WRDCIMaterials Laboratory, Wright-Patterson Air Force Base, Ohio 45433
Swell, tensile, and modulus data were determined for a fluoroelastomer after exposure t o a series of chlorofluorocarbon model fluids. Quantitative structure-activity relationships (QSAR) were developed for the swell as a function of the number of carbons and chlorines and for tensile strength as a function of carbon number and chlorine positions in the chlorofluorocarbons. Introduction Chlorofluoroethylene telomers have been known for a long time (Barnhart, 1956; Miller, 1953; Barnhart and Wade, 1957). The materials of the general formula C1(CF2CFC1),C1 (with x = 3-4) are being considered for use as hydraulic fluids (Snyder et al., 1982) in view of their nonflammability, relatively low vapor pressure, acceptable fluid ranges (for the envisioned application), and capability to dissole additive packages. The telomerization process as such is not structure specific; consequently in the case of the trimer (x = 3) there are up to four positional isomers possible disregarding any rearrangement products. It is thus of practical interest to determine the effect of the positions and number of chlorine atoms on the general behavior of this class of compounds. Understanding of the relationships between rubber swell and the number of chlorines and their environment in chlorofluorocarbons is of importance in the development of optimum seals for systems using these types of fluids or conversely in designing optimum fluids for the available seals. Any considerations of this nature must include the effect of a given fluid on the mechanical properties of the elastomer. T o perform this study, a series of well-characterized model compounds was utilized. These materials were specifically synthesized for the development of structure-property correlations (Paciorek et al., 1988). The preliminary results of these investigations were reported earlier (Snyder et al., 1988).
Experimental Section Materials. The fluids are listed in Table I; all the materials were fully characterized and in the case of the model compounds the purities were >99% (Paciorek et al., 1988). The fluoroelastomer employed was a cured terpolymer of vinylidene fluoride/tetrafluoroethylene/perfluoro(methy1 vinyl ether) plus cure site monomer (Viton GLT, product of Du Pont). Formulation: 100 parts Viton GLT, 10 parts MT carbon black, 20 parts Austin Black, 4 parts calcium hydroxide, 4 parts triallylcyanurate, 4 parts 2,5-dimethyl-2,5-bis(tert-butylperoxy)hexane. Cure conditions: 10 min at 177 "C; 24-h post cure at 260 "C. The specimens employed were dog bone shaped, thickness 1.4 mm, overall length 25.4 mm, width of the test segment 1.5 mm, width at the ends 6.75 mm. Testing. For the rubber/fluid exposure the apparatus shown in Figure 1 was utilized; the specimens and fluid were sealed under nitrogen atmosphere a t 0 "C and then heated at 177 "C for 336 h. Under these conditions 99% swell is achieved in 24 h (Paciorek et al., 1983). In view of the limited quantities of fluids available, the assembly was designed to conserve the fluid used. The quantity of 0888-5885/91/2630-2531$02.50/0
fluid employed was -10 mL, which allowed the fluid to extend 1.5 cm above the uppermost swelled specimen at the conclusion of the test. The volume swell measurements were performed according to ASTM D471-79. The tube containing the specimens was cooled immediately after removal from the oven with running tap water. Swell measurements were performed within 60 min from the completion of the heat treatment. The standard procedure is to cool the specimens in fresh test fluid. Using the commercially available chlorotrifluoroethylene telomer (CTFE 3.1-cSt fluid, product of Halocarbon Inc.) and the Viton GLT elastomer, it was established that the cooling with water (as performed in our investigations) afforded comparable results as the standard procedure. It should be stressed that the measured swell is strongly dependent upon the residence time of the specimen a t room temperature. Accordingly, as mentioned above, the cooling process and the time lapsed, before the swell measurement was performed, was well within 60 min for all the samples. The tensile strength and modulus measurement were carried out using an Instron Model 1123 tensile tester according to AST-D-412. The specimen elongation was measured employing an extensometer. All the tests were performed in triplicate. Calculations were made using the original cross section. For the QSAR analysis SPSS statistical software package was employed. N
Results and Discussion Non-cross-linked polymers on treatment with solvents dissolve slowly in two stages: f i i t a gel is formed and then, depending on the polymer, the gel gradually disintegrates into a true solution (Billmeyer, 1966). If the polymer is cross-linked, only swelling occurs. Chlorofluorocarbons are much better solvents for fluorinated elastomers than fluorocarbons. In our studies n-CBF18 was selected as a chlorine-free fluorocarbon reference inasmuch as the model compounds evaluated contained 7 and 8 carbons. It had been anticipated that, for chloroperfluorocarbons, only the chlorine(s) in the end group or groups is/are responsible for the increase in the rubber swell as compared to the chlorine-free fluorocarbon. The internal chlorines were expected to have a minimal effect. This assumption was based on the reports that the solubilizing action of chlorofluorocarbons is strongly, it not exclusively, dependent on the presence of terminal chlorines (Ferstandig, 1989). As evident from the data presented in Table I, at least for the series of compounds investigated, the internal chlorines appear to have comparable if not greater effect upon the rubber swell than the terminal chlorines. Thus comparing the data for n-C8F18 (test 13) with n-C7F15CF2Cl(test 17) and n-C5F11CFC1CF2CF2C1 0 1991 American Chemical Society
2532 Ind. Eng. Chem. Res., Vol. 30, No. 12, 1991 Table I. Swell and Mechanical Properties of Cured Viton GLT (VS-157)following Exposure to Various Fluids at 177 "C for 336 ha test no. test fluid % swell tensile psi 100% modulus psi elong % none" NA' 2260 1075 170 13 11.0 f 1.3 2005 f 75 800 f 65 130 n-CP18 14 30.4 i 0.7 830 f 140 n-C,F11CFClCFzCFClz 830 f 140 100 15 n-C7F15CC13 28.1 i 1.6 1705 f 50 885 f 90 170 16 650 f 10 n-C3F7CFClCFClCZF5 28.6 i 0.2 160 1950 f 100 17 n-CTF16CF2Cl 16.7 f 0.2 1130 f 60 1930 i 5 160 155 18 23.6 i 0.3 1395 f 420 n-C7F&FCI, 885 i 260 27.0 f 0.1 1840 f 30 1005 f 135 19 n-C,F11CFCICFzCI 190 20 n-C,F11CFClCF2CF2Cl 24.3 f 0.5 190 640 f 40 1885 f 110 21 180 960 i 15 n-C6F1&FClCFClz 27.3 f 0.9 1980 f 120 22 150 n-C6F&C12CF, 21.7 f 0.7 1715 i 340 990 i 90 160 48.5 i 1.5 1330 f 125 760 i 70 23 CF2ClCFClCFzCFClz 24 42.6 f 1.8 540 f 10 CFClzCFzCFzCFClz 70 25 44.8 i 0.2 1510 f 35 860 f 45 CF2ClCFClCFClCF2Cl 160 a Mechanical properties were determined on 'wet" samples; i.e., samples were surface dried and run immediately after removal from the test fluid. The results are averages of individual values of three dog bones from each test. bThese are the properties of the "as-received" dog bones. 'Not applicable.
_c
.t- 15
t
l"
OD
3 mm glass rod
T
10
PO
test specimen
90
40
so
Pmdlctmd K l u m S
Figure 2. Rubber swell calculated versus actual as a function of the number of carbons and chlorines in chlorofluorocarbons.
Ni-Cr wire (0.006")
Figure 1. Test assembly used for exposing the rubber specimens to liquids.
(test 20), one arrives at the increment value of 6.7% when the terminal fluorine is replaced by chlorine and 7.0% increment when an internal fluorine is replaced by chlorine. Accordingly, the degree of swell was found to be dependent to a greater degree on the total number of chlorines than the position of the chlorines. Related although not so well defined dependence became also apparent with respect to the mechanical properties as exemplified by the tensile strength. For a meaningful predictive capability a quantitative structure-activity relationship, QSAR, needed thus to be developed. Using multivariate linear regression analysis,
the simplest treatment is the expression of the calculated versus actual swell data as a function of number of chlorines (NOC1) and number of carbons (NOC). This is illustrated in Figure 2 and given by the equation swell = -2.76(NOC)
+ 5.66(NOCl) + 34.00
(1)
r = 0.987, s = 1.92, n = 13 where r = Pearson correlation coefficient, s = standard error of the estimate, and n = number of data points. The inclusion of the position descriptors for chlorine atoms, namely, number of terminal chlorines (TNOCl) and internal chlorines (INOCl), provides for somewhat better correlation as shown by Figure 3 and given by the equation swell = -2.75[NOC]
+ 5.51[TNOCl] +
6.32[INOCl] + 33.59 (2)
r = 0.988, s = 1.92, n = 13
However, neither this nor a further refinement incorporating differentiation of the internal chlorine positions, namely, internal adjacent (IANOC1) and internal nonad-
Ind. Eng. Chem. Res., Vol. 30, No. 12, 1991 2533
10
20
SO
40
50
600
760
1000
1260
1600
1760
2000
2260
P r r d i s t c d Vmlur t%)
Figure 3. Rubber swell calculated versus actual as a function of the number of carbons and number of terminal and internal chlorines in chlorofluorocarbons.
P r r d l c t r d Vaiur (PSI)
Figure 5. Tensile strength calculated versus actual as a function of number of carbons and chlorines in chlorofluorocarbons.
61
A 0
t U
a I 91
8 W
e I
I
21
1 10
20
so
40
SO
P m d i o t r d Value (%)
Figure 4. Rubber swell calculated versus actual as a function of the number of carbons and number of terminal, nonadjacent (internal), and adjacent (internal) chlorines in chlorofluorocarbons.
jacent (INANOCl) chlorines, as shown by Figure 4 and given by the equation swell = -2.56[NOC] + 5.62[TNOCl] + 5,84[INANOCl] + 6.75[IANOCl] + 32.06 (3) r = 0.989, s = 1.94, n = 13 improves significantly the agreement between the actual and calculated values. The above treatments show clearly that at least for the 13 compounds studied the QSAR model, utilizing just the number of carbon atoms and the number of chlorines, provides for good swell prediction in the C8 to C4 carbon chains having up to 4 chlorines. It is actually surprising that such a relationship could be established including the four-carbon system, which in view of its short chain length would be expected to exhibit disproportionately high swell data. In view of the above results it was of interest to explore the possibility of uti-
600
760
1000
1260
1600
1760
2000
2260
P r r d l c t r d Valur (p.1)
Figure 6. Tensile strength calculated versus actual as a function of number of carbons and number of terminal, nonadjacent (internal), and adjacent (internal) chlorines in chlorofluorocarbons.
lizing a parallel approach for the prediction of tensile strength. The expression relating the tensile strength to the number of carbons and chlorines is tensile strength = 37[NOC] - 205[NOC1] + 1831 (4) r = 0.639, s = 388, n = 13 Further defining the position of chlorines into terminal, internal nonadjacent, and internal adjacent, i.e. tensile strength = 117[NOC] - 202[TNOC1] 225[INANOCl] + 15[1ANOCl] + 1090 (5) r = 0.833, s = 312, n = 13 provides for definite improvement in the agreement between the calculated and actual values. This is evident in comparison of Figures 5 and 6. On the basis of the
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Ind. Eng. Chem. Res. 1991,30, 2534-2542
above evaluations it is clearly apparent that the position, not the actual number of chlorines in a given chlorofluorocarbon, affect the mechanical properties of the elastomer. The much larger spread observed in the tensile strength values (triplicates) as compared to the swell data is not surprising. The very small diameter of the test segment will amplify, in this type of measurement, any nonuniformity or defect in the rubber specimen. Furthermore, the high temperature, 177 OC, involved in these tests in conjunction with the complexity of the elastomer and the reactive nature of the chlorofluorocarbons, in particular the compounds having more than one chlorine present, most likely results in chemical reaction/degradation. The latter conclusion is supported by the finding that the position, not the number of chlorine atoms present, has the most pronounced influence on the mechanical properties. Considering all these factors, it is encouraging that at least some correlations could be derived. No identifiable trends were evident in the modulus data. The computational evaluations failed to uncover any trends even when the four-carbon compounds were excluded.
Acknowledgment The support of this investigation by the U.S.Air Force Wright Research and Development Center through Contract F33615-85-(2-5089is gratefully acknowledged. Registry No. (F3COCF=CF2)(F2C=CF2)(F2C=CH2) (copolymer), 56357-87-0; n-C8F18,307-34-6; n-CbFl1CFClCF2CFCl2, 136489-77-5; n-C7F15CC13,88639-57-0; n-C3F7CFC1CFC1C2F, 136489-78-6;n-C7F&F2C1, 307-33-5; n-C7F15CFC12,135941-34-3;
n-C$11CFClCF2Cl, 662-65-7; n-C,FllCFCl(CF2)2Cl, 136489-79-7; n-C6F13CFCICFC12,135941-33-2; n-C6F,3CC12CF3,123426-17-5; CF2ClCFCICF&FC12,423-38-1;CFC12(CF2)2CFC12,72765-11-8; CF2ClCFClCFClCFZC1, 375-45-1.
Literature Cited Barnhart, W. S. Telomerization with Sulfuryl Halide and Products Thereof. US. Patent 2,770,659, 1956. Barnhart, W. S.; Wade, R. H. Polyfluorohalogenated Organic Compounds and Method of Preparation Thereof. U.S. Patent 2,806,865, 1957. Billmeyer, W., Jr. Textbook of Polymer Science; Wiley: New York, 1966; pp 25-26. Ferstandig, L. L. Private communication, April 1989. Miller, W. T. Manufacture of Hydrocarbon Oils. US. Patent 2,636,907, 1953. Paciorek, K. L.; Nakahara, J. H.; Ahmed, S. R.; Smythe, M. E.; Kratzer, R. H. M140 Gun Mount Seal Degradation in MIL-H46170 Hydraulic Fluid. U S . Army ARDC Report AD-E401092, November 1983. Paciorek, K. L.; Podejko, K. G.; Nakahara, J. H.; Johri, K. K.; Eapen, K. C., Chen, L. S. Chlorofluorocarbon Model Compounds and Fluids. Part I. Synthesis. Presented at the 12th International Symposium on Fluorine Chemistry, Santa Cruz, CA, Aug 7-12, 1988. Snyder, C. E., Jr.; Gschwender, L. J.; Campbell, W. B. Development and Mechanical Evaluation of Nonflammable Aerospace (-54O to 135 "C) Hydraulic Fluids. Lubr. Eng. 1982,38,41. Snyder, C. E., Jr.; Gschwender, L. J.; Fultz, G . W.; Paciorek, K. L.; Podejko, K. G.; Johri, K. K. Chlorofluorocarbon Model Compounds and Fluids. Part 11. Molecular Structure-Property Relationships. Presented at the 12th International Symposium on Fluorine Chemistry, Santa Cruz, CA, Aug 7-12, 1988. Received for review April 1, 1991 Revised manuscript received July 19, 1991 Accepted August 13, 1991
Free-Radical Graft Polymerization of Vinylpyrrolidone onto Silica Mark Chaimbergt and Yoram Cohen* Department of Chemical Engineering, University of California, Los Angeles, Los Angeles, California 90024
The free-radical graft polymerization of vinylpyrrolidone onto silica was studied in an aqueous slurry batch reactor. The graft polymerization process consisted of first activating the silica surface with vinyltriethoxysilane followed by a free-radical-polymerization step. The progress of the homopolymerization and graft polymerization reactions was followed by UV and thermogravimetric analyses, respectively. In order to determine the molecular weight of the grafted poly(viny1pyrrolidone) (PVP)chains, the surface-grafted polymer was cleaved from the silica substrate by acid hydrolysis with hydrofluoric acid, and the polymer molecular weight was determined by size exclusion chromatography (SEC). The study revealed that the polymer graft yield and monomer conversion increased with increasing initial monomer concentration and reaction temperature, and that the rate of graft polymer formation is correlated to the rate of monomer consumption. The weight-average molecular weight of the surface-grafted polymer was shown to be higher than that of the homopolymer for the graft polymerization reactions performed a t low values of the initial monomer concentration and low reaction temperatures. In contrast, for graft polymerization reactions performed a t high initial monomer concentration and temperature, the weight-average molecular weight of the grafted polymer was as much as 60% lower than that of the corresponding homopolymer.
Introduction Chemical grafting of polymer chains to solid substrates leads to the formation of composite materials that possess distinct surface and structural properties. The grafted polymer outer layer modifies the surface chemistry of the *Author to whom correspondence should be addressed. 'Present address: Institute of Gas Technology, 3424 $3. State St., Chicago, IL 60616. 0888-5885/91/2630-2534$02.50/0
substrate while the basic geometry and mechanical strength of the solid substrate remain intact. Of particular interest in this work is the process of surface modification via graft polymerization The method of graft polymeriof polymer chains from active zation relies on the sites on the surface of a substrate (e.a., by a step or chain polymerization reaction mechanism).-The surface-grafted chain grows as monomers diffuse to the surface where they are added to the growing chain. In contrast with graft polymerization, the polymer grafting method involves the 0 1991 American Chemical Society
