Ind. Eng. Chem. Res. 1994,33, 1017-1021
1017
Molecular-Weight Parameters of Elastomeric Fluorocarbon Materials Mikio Zinbo’ and Ares N.Theodore Ford Motor Company, Research Laboratory, MD3061SRL, P.O.Box 2053, Dearborn, Michigan 48121
The key molecular-weight parameters of four different types of commercial fluorocarbon elastomers have been determined by size exclusion chromatography (SEC) for use in_automotive alternative Mw, and MZ) obtained fuel applications. The number-, weight-, and z-average molecular weights (Mn, for the soluble fraction (98.5-100% ) of each fluoroelastomer were compared with the Mooney viscosity provided by the suppliers. Correlations between the viscosity and any average molecular weight were generally poor. The best correlation coefficient found was naturally the value obtained between the viscosity and fiW.This paper also evaluates the molecular-weight degradation of some fluoroelastomeric m_aterials during the compound processing. Even at relatively low shearing an accelerated rate of M, degradation was observed in the early milling stages for a fluoroelastomer 2 10% The molecular-weight data provides in part a helpful with longer-chain molecules (M, insight in understanding the morphology of fluoroelastomer prototype parts which have been exposed to flexible fuel service conditions. Table 1. Physical Properties of Commercial Fluorwlastomers
Introduction Fluorocarbon elastomers are known for their excellent fuel, solvent, and chemical resistance and outstanding environmental stability over a wide temperature range. Currently, four types of fluoroelastomersare manufactured by the key domestic suppliers to meet the various automotive requirements with the exclusion of a lubricating environment (Crenshow and Tabb, 1990). The earliest types of fluorocarbon elastomers based on copolymerization of vinylidene fluoride (VFz) and hexafluoropropylene (HFP) are designated as type A fluoroelastomers. The A types were primarily developed for military applications (Montermose, 1961). Later the B types were developed. They are based on terpolymerization of vinylidene fluoride (VFz),hexafluoropropylene (HFP), and tetrafluoroethylene (TFE). To meet further application requirements for improved solvent resistance, multicomponent copolymers containing cure-site monomers and higher fluorine content were developed. These fluoroelastomers were designated as type C fluoroelastomers and contained a cure-site monomer such as bromoperfluoropropylene (BPFP) in addition to VFz, TFE, and HFP. Recently, type D fluoroelastomers were designed to provide low-temperature capability, replacing HFP units in the C types with.perfluoroviny1methyl ether (PVME). The C and D types are usually cured with peroxide systems. The type A, B, and C fluoroelastomers are prepared by fairly similar manufacturing processes consisting of emulsion polymerization (Arnold et al., 1973). However, a small variation in the polymerization conditions could result in elastomers of the same type which are not identical in terms of the key molecular-weight parameters, i.c, nu_mM,, ber-, weight-, and z-average molecular weights Cyn, and Mz)and the dispersity of the distribution (Mw/Mn). Usually the suppliers provide the specific gravity, fluorine content, and Mooney viscosity as a means of defining these fluoroelastomers. As a matter of fact, changes in Mooney viscosity are often implied to be changes in molecular weight (Kosmala and Tuckner, 1985). Although these two terms are related, there might not be direct correspondence between them. Furthermore, molecular-weight data of commercially available fluoroelastomers is very scarce in the open literature. Because of the Clean Air Act Amendments of 1990, mandating transition to alternative fuels to reduce vehicle 0888-5885/94/2633-1017$04.50/0
elastomer
Structural typea
Mooney F % d20 viscosity“
Abbreviations: (VF2) vinylidene fluoride, (HFP) hexatluoropropylene, (TFE) tetrafluoroethylene,(BPFP) bromoperfluoropropylene, (PVME) perfluorovinyl methyl ether. Measured by ML 1 10 at 121 O C (250 OF), ASTM D1646. (1
+
emissions, fluorocarbon elastomers have become one of the few automotive materials of choice (Gray and Alson, 1989). The change in physical and chemical properties with time and temperature is very important when service life of an engineering part is concerned. Fluorinecontaining elastomers have been the subject of extensive testing with methanol-gasoline fuel blends since the traditional elastomeric components experience difficulties. The need for a better understanding of the molecular properties of fuel-resistant elastomers has become more evident. As with any polymeric material, the molecularweight distribution directly affects the bulk properties of fluorocarbon elastomers. Such properties as average molecular weights are important because they are one of the parameters which relate to network structure and component performance. In this paper the size-exclusionchromatographic (SEC) determination of average molecular weights and the molecular-weight distribution of key fluorocarbon elastomers are presented. Also, changes in these parameters during compounding and processing are described.
Experimental Section Materials. The seven fluorocarbon elastomers examined in this work were obtained from two supplier sources (DuPont Co. and 3M Co.). The structural type, fluorine content, specific gravity, and Mooney viscosity (ASTM D1646) of each elastomer are given in Table 1. A test compounding recipe for fluoroelastomers A, A’, B, B’, and B” is shown in Table 2. Magnesium oxide and 0 1994 American Chemical Society
1018 Ind. Eng. Chem. Res., Vol. 33, No. 4, 1994 Table 2. Test Compounding Recipe for Fluoroelastomers A, A'. B,B', and Bff
comuound fluoroelastomer Maglite Dn calcium hydroxide" Culative No. 20b Culative No. 3OC
Parts
100 3 6 3 4
a Magnesium oxide and calcium hydroxide (194% ) obtained from C. P. Hall Co. Benzyltriphenylphosphonium chloride (33% in fluoroelastomer) marketed by E. I. DuPont de Nemours & Co. Bisphenol AF as 50% in fluoroelastomer provided by E. I. DuPont de Nemours & Co.
Table 3. Test Compounding Recipe for Fluoroelastomers C and D
compound fluoroelastomer Litharge No. 33O Diak No. 7b Luperco 101-XLc
Table 4. SEC Average Molecular Weights of the THF-Soluble Fractions of Commercial Fluoroelastomers*
elastomer
A A' B B' B"
C
D
solubility in THF, % w/w 98.5 98.6 99.1 99.7 100.0 99.1 100.0
a Sublimed lead oxide supplied by Akrochem Corp. Triallyl isocyanurate (coagent) marketed by E.I. DuPont de Nemours & Co. c 2,5-Dimethyl-2,5-di(tert-butylperoxy)hexane as45 % in an inert filler provided by Pennwalt Corp.
calcium hydroxide serve as neutralizers for hydrogen fluoride generated during the cure. Curative No. 20 serves as an accelerator, and Curative No. 30 provides the crosslinker of the vulcanization. In Table 3 is shown another test recipe for the vulcanization of fluoroelastomers C and D. The litharge serves as a neutralizer for hydrogen fluoride. Luperco 101-XL is a peroxide curing agent used with a coagent, Diak No. 7. Milling and Curing Procedure. Milling of each fluoroelastomer with the neutralizers and curatives was carried out on a 80 X 180-mm two-roll mill. The mill speed was 3.2 m/min, and the mill had a friction ratio of 1.4:l.O. The gap between the rolls varied from 0.15 to 70.0 mm during milling. Initially, the elastomer was banded on the rolls and the combined ingredients were added for 20 s. Then the compound was cut continuously at the edges of the rolls and placed in the center of the rolls for 120, 240, or 360 s. In general, test specimens were prepared from a 240-s milling of the ingredients by compression molding at 177 "C for 60 min (ASTM D3182). Cured compounds B" and D were further postcured in an air-circulating oven at 200 "C for 23 h to enhance some of the physical properties. Solubilityof Fluoroelastomers. The fluoroelastomer samples (ca. 50 mg) were placed in tared 4-dram vials (15 X 45 mm) and a 3-mL portion of tetrahydrofuran (THF) was added to each sample vial. The vials were sealed with PTFE septa assembled caps. The mixtures were stirred well with a vortex mixer (Fisher Model 232) to speed up solution and allowed to stand for 72 h a t room temperature. When the resulting mixtures contained white precipitates or gel particles, the clear top layers were carefully pipetted out (ca. 2.0 mL) and saved for size exclusion chromatographic analysis described in the following section. Another 2-mL portion of THF was added to each of the residual mixtures. The second mixtures were stirred well again and allowed to stand for 48 h at room temperature. Then, the clear supernatant layers of the mixtures were discarded, THF in the residual mixtures was stripped under a nitrogen flow, and the residues were dried in vacuo at 25 "C. The insoluble weight percent of each elastomer was determined gravimetrically with an electronic analytical balance (readability: 0.1 mg). Size Exclusion Chromatography (SEC). The num-
78.8b 91.OC 109.gc 70.3c 110.2c 63.4b 79.OC
Ax185.6b 308.4c 261.1c 261.4c 370.5c 246.gb 314.5c
ME 376.7b 995.3c 497.3c 671.W 850.8c 892.4b 1070.W
" SEC conditions: column, four ultrastyragel columns of 106,104, lo3, and 500 A; flow rate, 1.2 mL/min; sample size, 50 /rL; solvent, tetrahydrofuran; ARI detector sensitivity, -64 X 50 at 30 "C. An average of two SEC measurements. An average of three SEC measurements.
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ber-, weight-, and z-average molecular weights Mw, and az) and molecular-sizedistribution of tetrahydrofuran (THF)soluble fluorohydrocarbon elastomer fractions were determined by SEC using a Waters Model 1 5 0 4 ALC/ GPC, equipped with a differential refractometer (ARI) and an automatic injector. The SEC column set used consisted of four ultra-Styragel columns (30 cm X 7.8 mm i.d.) with permeability limits of lo5, lo4, 103, and 500 A. The mobile phase was tetrahydrofuran (THF),flowing at 1.2 mL/min and 30 "C. The relative sensitivity of the refractometer was set at 64. The elastomeric fluoropolymer samples (40-50 mg) and their well-mixed and cured compounds (ca. 100 mg) were dissolved in 3 mL each of THF. All of the THF supernatant solutions (ca. 2 mL) were filtered through 0.5-pm Millex SR filters. Each of the filtrates was diluted further with the same volume of THF for the analysis. An injection volume of 50 pL was used for SEC analysis of the filtered sample solutions throughout this work. The column set was calibrated with commercially available polystyrene standards, peak molecular weights (M,) ranging from 1.987 X lo6 to 1.77 X lo3. The correlation coefficient for the polystyrene column calibration curve of the logarithm of Mpvs retention time was 0.9995 with cubic polynomial fits. The standard error of the estimate was less than 4 % . Waters Maxima 820 GPC chromatography software was used for recording chromatograms, integrating sliced peak areas, and calculating average molecular weights. The average molecular weights cited in this work are, therefore, relative to polystyrene standards.
Results and Discussion Solubility of Fluoroelastomers. The mixtures of fluoroelastomers B" and D (ca. 50 mg each) with 3 mL each of THF were a single phase, clear and free from gel particles or cloudiness. The rest of the fluoroelastomer/ THF mixtures (five) contained a small amount of white particles that had precipitated. Therefore, the clear top layers of the sample solutions A, A', B, B', and C (ca. 2 mL each) were carefully pipetted out from the vials, filtered through 0.5-pm Millex-SR filters (Millipore Corp., Bedford, MA), and collected into clean 4-dram vials for size exclusion chromatographic (SEC) analysis. The solubility of each fluoroelastomer ( % w/w) in THF is included in Table 4, determined by gravimetry. An increase in molecular weight normally reduces solubility, and enhanced branching increases the solubility compared to a linear polymer of the same molecular weight. The white precipitated particles (trace to 1.5%)appeared to be slightly swollen in THF, possibly containing highly
Ind. Eng. Chem. Res., Vol. 33, No. 4,1994 1019
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MOONEY VISCOSITY Figure 2. Plota of Mooney viscosity vs average molecular weighta of seven commercial fluorocarbon elastomers.
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Figure 1. SEC separations of various commercial fluorocarbon elastomers: (a) vinylidenefluoride (VF2)/hexafluoropropylene(HFP) copolymere, (b) vinylidene fluoride (VFd/hexafluoropropylene(HEp)/ tetrafluoroethylene (TFE)terpolymers, and (c) peroxide-curable multicomponent copolymers.
networked structures. However, more than 50% of the THF-insoluble particles of elastomer A were identified as barium sulfate which was not completely removed by the post-polymerization washing procedures; the insoluble particles obtained from elastomer A contained ca. 35% ash, determined by a dual-gas thermogravimetric method (Siegl and Zinbo, 1985). SEC Analysis. The key molecular-weight parameters which define the molecular-weight distribution of a polymer can be readily obtained by SEC. Figure 1shows the SEC molecular-size distribution of the THF-soluble fractions of seven fluorocarbon elastomers. Two A types are in Figure la, three B typesin Figure lb, and one each of C and D types in Figure IC. Mn,M,, and &fz determined by SEC for seven fluoroelastomers are also listed in Table 4,relative to polystyrene standards. Although these values are not absolute, they are excellent relative values for studies investigating the changes in average molecular weights during the compound milling. The polydispersities (aw/&) of the type A and B fluoroelastomers ranged from 2.4 to 3.7,and those of the type C and D fluoroelastomers ranged from 3.9 to 4.0(cf. Table 4). Mean percent relative standard deviation (% RSD = 100 X s / X ) for &fn, &fw, and &fz are 9.21,5.42, and 12.02%,respectively, calculated from the SEC average molecular-weight data of seven fluoroelastomers. Relationship between Mooney Viscosity and Molecular Weight. The plots of the Mooney viscosity vs average molecular weights are shown in Figure 2. The mean viscosityvalues of 65and 74were used for elastomers A' and B', respectiyely. Correlation coefficients for the viscosity vs Mn,vs M,, and vs &fz were 0.1402,0.7085,and 0.6451,respectively, by linear regression analyses of the three relationships. The correlations are very poor, possibly because of the effects of the THF-insoluble elastomer sample fractions, small variations in elastomer
microstructure, and mean viscosity values used for fluoroelastomers A' and B'. The rheology of molten polymers is a function of molecular weight and of molecular-weight distribution as well (Hagen et al., 1966). The qualitative nature of such relationships must be recognized, however, since minor structural differences and process impurities may lead to large differences in viscosity, such as minor amounts of salts and very high molecular weight fractions. These results show that the Mooney viscosity of a fluoroelastomer is not always proportional to the average molecular weights determined by SEC. As a result, Mooney viscosities can only be a rough measure of &fwand provide no insight into the molecular-weight distribution of these materials. Effect of Milling Time and Shearing on Molecular Weight. To investigate the combined milling effects on key molecular-weight parameters, compounds B" and D were milled in three increasing time periods and shearing forces using a two-roll mill (8 X 18 cm), which has two rolls set parallel with a narrow opening. Each compound was rolled up and pressed out (i) for 120 s with an opening setting of 35 mm, (ii) for 240 s with an opening setting of 25 mm, and (iii) for 360 s with an opening setting of 10 mm. Molecular weight distributional changes of elastomers B" (terpolymer) and D (multicomponent copolymer) induced by the milling conditions are shown in Figures 3 and 4,respectively. Also, included are SEC chromatograms of each THF-extractable component from the postcured compounds B" and D for comparison. The postcured compound B" retained a relatively small amount of the THF-soluble fraction, i.e., ca. 3.5% relative to original elastomer B". On the other hand, the peroxide cured and postcured compound D retained a rather large amount of the THF-soluble fraction, i.e., ca. 11.5% relative to original elastomer D. Although some of this soluble fraction might be formed during the postcure period by oxidative-thermal depolymerization, it appears likely that a majority of the soluble fraction was left in the postcured compound by the lack of cure-site monomers in the shorter elastomer chains on the basis of ita molecular-size distribution. The plots af the average molecular weight vs milling time are shown in Figure 5. &fnvalues of elastomers B" and D decrease almost linearly with the milling time, but the decrease in &fwof elastomer B" is quite rapid even under a low shearing in the early milling stage, probably because of a relatively high initial M, of-elastomer B". The estimated initial rates of both&fwand Mndegradation
1020 Ind. Eng. Chem. Res., Vol. 33, No. 4,1994 n
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Figure 5. Effect of the compound milling conditions on the average molecular weightaof fluoroelastomersB" and D under the increasing time and shearing of 120s with a 35-mm opening, 240 s with a 25-mm opening, and 360 s with a 10-mm opening: 0,fiw of elastomer B"; 0 ,finof elastomer B"; 0,fi, of elastomer D; and M, finof elastomer D.
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for elastomer B" were 50.9 and 2.3 X lo3 Da/min, and those of &fwand &f, degradation for elastomer D were 21.9 and 1.6 X lo3Da/min, respectively. When the rate of Mw degradation is appreciably higher than that of Mn degradation for a polymer, it is likely that chain scissions will occur preferentially on the longest chain molecules, which is clearly shown in Figure 3 by a series of molecular-sizedistribution changes of elastomer B" treated under progressively severe milling conditions. This compound milling experiment shows that when the initial weight-average molecular weight, (Mw)o,of a fluoroelastomer is higher than ca. 3 X 106, the initial rate of the ifdw degradation can be accelerated even at a relatively low shearjng li.e., 35-mm opening). The polydispersity values (Mw/M,)of compounds B" and D also decrease with increasing compound milling time. Similar effects have been reported previously with 1:4 transpolyisoprene due to mechanical degradation (Arvanitoyannis et al., 1992). Effect of Milling Time under a Constant Shearing on Molecular Weight. To obtain only the milling-time effect on key molecular-weight parameters, each of compounds A (copolymer), B" (terpolymer), and D (multi-
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MILLNG TIME (s) Figure 6. Effect of the compound milling time on the average molecular weights of fluoroelastomersA, B", and D under a constant shearing (10-mmopening): A,fi, of elastomer 4A,finof elastomer A; 0,fi, of elastomer B"; 0 ,finof elastomer B"; 0,fiw of elastomer D, and m, finof elastomer D.
component copolymer) was milled in the same three increasing time periods, i.e. (i) for 120 s, (ii) for 240 s, and (iii) for 360 s, but with a constant opening setting of 10 mm. The plots of the average molecular weight vs milling time are shown in Figure 6. The average molecular weights plotted for the 0-s milling time were obtained with uncom ounded elastomers A, B", and D, because of the rapid degradation during the 20-s premilling of each compound at a relatively high shearing (i.e., 10-mm opening). The estimated initial rates of &fw and ifdn degradation for elastomers A, B", and D under a constant shearing are summarized in Table 5. This compound milling experiment under a constant shearing also shows that the initial rates of Mwdegradation are further accelerated for both elastomers B" and D j u t not for elastomer A, whose value is as low as those of M, degradation for elastomers A, B", and D. Effect of Methanol-Gasoline Blends on 0-Ring Parts. The molecular-weight data obtained from a series of compound-D samples, i.e., average molecular weights, molecular-weight distribution, and amounts of non-
Jw
Ind. Eng. Chem. Res., Vol. 33, No. 4, 1994 1021 Table 5. Initial Rates of t h e Average Molecular Weight Degradation during the Compound Milling under a Constant Shearinrt.
elastomer A B” D
rate of fi, degradation* X 109, Da/min 3.4 59.3 29.0
rate of fin degradationc X 109, Da/min 1.5 2.0 1.2
The opening of a two-roll mill (8 X 18 cm) was at the setting of a 10-mm opening. The initial weight-average molecular weights, I@&,, for elastomers A, B”, and D were 182.9,403.6, and 330.6 X 105, respectively. c The initial number-average molecular weights, (fi,)~, for elastomers A, B”, and D were 76.1, 125.1, and 83.0 X 105, respectively.
crosslinked polymer chains (cf. Table 4 and Figure 4), is of real interest because it provides a helpful insight into the understanding of the morphology of fluoroelastomer O-rings exposed to an alternate fuel environment. Although these O-ringsperformed adequately in a methanolgasoline blend (80:20) for 6 months, inspection of the prototype test parts indicated that some type of material was squeezed out of the O-ring surfaces. Initial assessment was that the exuded material could be attributed to the process aids or impurities which were accidentally incorporated into the elastomer compound D. However, SEC data indicates that noncrosslinked material most likely comes from the elastomer itself, and some of this material could be a byproduct of the curepostcure process. In addition, the data in Figures 3 and 4 indicates that the peroxide-cured system (cf. Table 3) contains more noncrosslinked material than the bisphenol AF compound (cf. Table 2). The O-ring-test observation further suggests the need to understand the mechanism of network formation and the structure of the network itself for reducing or eliminating possible morphological, dimensional stability, and catastrophic failure problems.
Conclusions Correlations between Mooney viscosity and average molecular weights of the THF-soluble fraction of seven commercial fluorocarbon elastomers were generally quite poor, possibly because of the presence of solvent-insoluble particles in some of the fluoroelastomers and ranged Mooney viscosity values provided by the manufacturers. In addition, the key molecular-weight parameters of fluoroelastomers of the same type but from a different manufacturing source are not quite the same, Le., small variation in elastomer microstructure. In the early stages of the compound milling, the rates of Mwdegradation found for elastomers B” and D were 50-60 and 22-29 X lo3 Da/min, respectively. However, the initial rate of Mw degradation for elastomer A was
only 3.4 X lo3Da/min under a relatively high shearing. A probable cause of the fast initial rates of Mw degradation is likely the shear-degradation reduction of longer elas1 108)on the basisof moleculartomer chain molecules (M, size distributional changes induced by the compound milling. The estimated shear effect on the initial rate of Mwdegradation is ca. 300Da/min/mm of opening for both elastomers B” and D based on the two milling experiments performed. Loss of the noncrosslinked elastomeric material under flexible fuel service conditions would alter the physical and dynamic mechanical properties of the part and result in performance and durability changes (Nagdi, 1988). Actually, SEC measurements of the molecular-weight distribution of solvent-extractable components in “cured” and further “postcured” elastomeric parts can be extremely valuable in providing information on the degrees of curing and oxidative thermal resistance.
Literature Cited Arnold, R. G.; Barney, A. L.;Thompson, D. C. Fluoroelastomers. Rubber Chem. Technol. 1973,46,619-651. Arvanitoyannis, I.; Blanchard, J. M. V.; Kolokuris, I. Effects of Mastication on the Distribution of Molecular Weights and on the Mechanical Properties of Native and Commercial Dental Gutta Percha (trans-Polyisoprene). Polym. Znt. 1992,27,7-15. ASTM D3182. “Rubber-Materials, Equipment, and Procedures for Mixing Standard Compounds and Preparing Standard Vulcanized Sheets”; 09.01; American Society for Testing and Materia l ~Philadelphia, 1989. ASTM D1646. ‘Rubber-Viscosity and Vulcanization Characteristics (Mooney viscometer)”;09.01; American Society for Testing and Materials: Philadelphia, 1992. Crenshow, L.E.; Tabb, D. L. Fluoroelastomers. In The Vanderbilt Rubber Handbook; Ohm, R. F., Ed.; R. T. Vanderbilt Co., Inc.: Nowalk, CT, 1990; pp 211-222. Gray, C. L.,Jr.; Alson, J. A. The Case of Methanol. Sci. Am. 1989, Nov., 108-114. Hagen, R. S.; Thomas, D. P.; Schlich, W. R. Application of the Rheology of Monodisperse and Polydisperse Polystyrenes to the Analysis of Injection Molding Behavior. Polym. Eng. Sci. 1966, 6, 373-376. Kosmala, J. L.; Tuckner, P. F. Fluoroelastomers: Polymers, Applications and Properties. Abstracts of Papers, 127th Meeting of the American Chemical Society, Rubber Division, Loa Angeles, CA, April 1985; American Chemical Society: Washington, DC, 1985; Educational Symposium, paper g. Montermose, J. C. Fluorine-Containing Elastomers. Rubber Chem. Technol. 1961,37,1521-1551. Nagdi, K. Kautsch. Gummi, Kunstst. 1988,41,717-722. Siegl, W. 0.;Zinbo, M. On the Chemical Composition and Origin of Engine Deposits. In Chemistry ofEngine Combwtion Deposits; Ebert, L.B., Ed.; Plenum: New York, 1986; pp 53-70. Received for review August 24, 1993 Revised manuscript received December 7, 1993 Accepted December 21, 1993. Abstract published in Advance ACS Abstracts, February 15, 1994. @
