Elastomers and Rubber Elasticity - American Chemical Society

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Structural Factors and Tensile Properties of Ethylene—Propylene—Diene Terpolymers Prepared with Various Catalyst Systems BAO-TONG HUANG (PAO-TUNG HUANG), LI-TONG ZHAO, YU-LIANG LI, MIN YU, and BAO-YUAN HU Changchun Institute of Applied Chemistry, Academia Sinica, Changchun, Jilin Province, People's Republic of China Difference in vulcanizate tensile strength of EPDMs prepared with various catalyst systems, espe cially the high strength of EPDM prepared with vanadium carboxylates (v5-9), is examined against various structural parameters. Effect of [η], [C=C] and C3% respectively on tensile strength is noted. The catalyst systems are arranged in the decreasing order of tensile strength for samples with comparable [ η ] , [C=C] and C-3% as: V5-9-Et3Al2Cl3 >V(acac)3-Et2AlCl>V5-9-Et3Al2Cl3-ETCA (activator) >VOCl3-Et3Al2Cl3. Contributions of third monomer distribution and molecular weight distribution, though important as they should be, did not appear to be decisive. Variations in monomer sequence length distribution and long-chain branching, i f present, and their influence on vulcanizate tensile strength remain to be examined. Ethylene-propylene-diene terpolymers (EPDM), with their inherent complexity in structural parameters, owe their tensile properties to specific structures dictated by polymerization conditions, among which the controlling factor is the catalyst used in preparing the polymers. However, no detailed studies on correlation between tensile properties and EPDM structures have been published (l,2). An unusual vulcanization behavior of EPDMs prepared with vanadium carboxylates (typified by V^.o, carboxylate of mixed acids of C5-C9) has been recently reported Q). This EPDM attains target tensile properties in 18 and 12 minutes at vulcanization temperatures of I50 and l60°C respectively, while for EPDMs prepared with VOCl3-Et3Al2Cl3 or V(acac)3-Et2AlCl, about 50 and 4-0 minutes are usually required at the respective vulcani zation temperatures, a l l with dicyclopentadiene (DCPD) as the third monomer and with the same vulcanization recipe. This obser vation prompted us to inquire into the inherent structural factors 0097-6156/82/0193-0195$06.00/0 © 1982 American Chemical Society Mark and Lal; Elastomers and Rubber Elasticity ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

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that caused t h i s marked difference i n vulcanization behavior of EPDMs prepared with different catalyst systems. This paper reports some of the preliminary observations. EXPERIMENTAL Polymerizations were carried out batchwise i n ^ 0 0 - m l polymerization-grade hydrogenated gasoline or hexane. A f t e r saturating the solvent with the monomer mixture (C2/Cj=2.0 ± 0 . 1 , dried through consecutive NaOH, s i l i c a gel and ^A molecular sieve columns), DGPD (together with ETCA, ethyl trichloroacetate, an activator, when used), vanadium catalyst and alkylaluminum chloride solutions were syringed i n succession into the reaction mixture. Polymerization continued with s t i r r i n g under the monomer gas flow. With the exception of V O C l 3 - E t 3 A l 2 C l 3 polymerization, the 30~ inute polymerization was repealed f o r another cycle with further addition of catalyst components and diene i n order to boost the y i e l d i n one batch to get enough terpolymer f o r t e n s i l e tests. The terpolymer, a f t e r being precipitated from ethanol and washed with fresh portions of ethanol, with N-pheny1-β-rmphthylamine added i n the t h i r d washing, was vacuum dried at ^0°C to a constant weight. [yf\ was determined i n toluene at 30°C» degree of unsaturation, Cc=Cl (expressed i n mmole/gram polymer), by iodine chloride method (l£) and composition C3 mole% by infrared spectroscopy (Zeiss UR-10 or Perkin Elmer 5 7 7 ) using bands at 7 2 0 and I I 5 0 cm" 1 with pressed films employing a pre-constructed c a l i b r a t i o n curve ( j 5 ) . Tensile testing samples were of miniature size (ça. 1 . 5 x 1 . 8 mm i n cross-section and 5 · 5 cm i n gauge length. Long practice i n t h i s Laboratory has proven that samples of t h i s size give reproducible results and are e l i g i b l e f o r comparison purposes. The vulcanization recipe wasî EPDM 1 0 0 , stearic a c i d 1 . 0 , zinc oxide 5 . 0 , accelerator M 0 . 5 , TMTD 1 . 5 , HAF 5 0 , sulfur 1 . 5 phr. Peroxide-curing of E-P copolymers was carried out with a modified procedure of ( 6 ) : EPR 1 0 0 , dicumyl peroxide 3 , zinc oxide 3 , TMTD 0 . 5 , HAF 5 0 , s u l f u r 0 . 2 phr, the mixture being masticated f o r 2 0 minutes at ^ 0 - 5 0 ° C and cured at 1^0°G. Molecular weight d i s t r i b u t i o n (MWD) was rjetermined on a homemade GPG column (£), with toluene as the elua i t and turbidimetry i n methanol as means of detecting polymer concentrations i n the counts. m

RESULTS AND DISCUSSION Among the main molecular structural variables i n EPDMs that are stipulated by catalyst systems and that affect the vulcanizate tensile properties we may mention: molecular weight (MW) and MWD, degree of unsaturation (LG=C1) and i t s d i s t r i b u t i o n i n the polymer, composition ( C ^ ) and monomer sequence length d i s t r i b u t i o n along molecular chains, and long-chain branching i f present. Effect of

Mark and Lal; Elastomers and Rubber Elasticity ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

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Ethylene-Propylene-Diene Terpolymers

carbon black reinforcement and of chemical structure of crosslinking bonds on tensile properties can be neglected when samples are processed and vulcanized under same conditions. Preliminary Observations I t was observed i n the early stage of this work that EPDMs prepared with different catalysts had varying vulcanizate tensile properties. For example, i t was noted that V^_o-EPDMs had higher vulcanizate tensile strength than VOCl^-EPDMs with comparable and [C=C] (Figure 1A) or with even higher [ y j ] and [c=C] (Figures IB and 1C), even though E t ^ l g C l o + Et2AlCl should have given lower vulcanizate tensile strength than E t ^ ^ C l ^ alone Q ) . Similar tensile difference existed between vulcanizates of V ( a c a c ) 3 - E t o A l C l and Ve Q - E t 2 A l C l EPDMs of comparable [/(] and [C=C] (Figure I D ) . " D

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The effect of MW on tensile properties of other elastomers that, before a MW limit i s reached, tensile strength increases with MW was well established. Due to complexity i n MW^fjll rela tionship for EPDMs Q ) , as an expedient measure, C^D used as a practical scale of MW. Figure 2 shows examples of E P D M samples having same Cc=Cl but varying prepared with V r ^ - E t ^ A ^ C l o + Et2AlCl (lîl). Thus, the effect of [η] i s n u l l i f i e d by using samples having comparable [y\] when comparing samples prepared with different catalyst systems. Under comparable [yQ, samples with higher Cc=Cl showed higher tensile strength and JOOfo modulus and lower elongation at break for EPDMs obtaianed with a l l the catalysts studied V(acac)oE t 2 G l (Figures 3 and 4), 9 ^ 3 Α 1 2 0 ΐ 3 (Figure 5 ) and V O C I 3 E t 3 A l 2 C l o (Figure 6A). The importance or the degree of unsatura tion i n determining tensile strength i s emphasized i n the failure of the contribution of higher to overcome the effect of lower [C=Cl (Figure 6B). Composition of the terpolymers as expressed i n 0j mole%, of course, must be kept i n Ithe range that yields good elastomeric properties. E-P polymers of higher ethylene content naturally result i n higher tensile strength as a consequnece of formation of partial c r y s t a l l i n i t y due to short ethylene blocks (8) . C ^ of EPDMs studied i n this work a l l abide by the usual EPDM require ments . i s

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Systematic Comparison I t G=became apparent that comparison of samples with similar G D'il» and C^£ would reveal the degree of tensile difference brought about by other structural factors. The matching samples with three structural factors comparable simultaneously were

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Figure 1. Tensile differences in EPDMs. Conditions for A: (1) V . Et Al Cl ; Et AlCl, 1:1; (2) VOCl -Et Al Cl ; [ ] (dl/g), 2.05; [C=C] (mmol/g), 0.49 (1) 0.43 (2); 150°C. Conditions for Β: V . Et Al Cl , Et AlCl, 1:1; (2) VOCl Et Al Cl ; [η] (dl/g), 1.83 (1) 1.91 (2); [C=C] (mmol/g), 0.42 (1) 0.51 (2); 150°C. Conditions for C: (1) V . Et Al Cl , (2) VOCl Et Al Cl ; [η] (dl/g), 1.85 (1)2.01 (2); [C=C] (mmol/g), 0.73 (1) 1.01 (2); 150°C. Conditions for D. (1) V(acac) Et AlCl; V . Et AlCl (2); [η] (dl/g), 1.18 (1) 1.14 (2); [C=C] (mmol/g), 0.35 (1) 0.37 (2); 160°C. 5 9

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Figure 2. Effect of [η] on vulcanizate tensile properties: V . -Et Al Cl , Et AlCl, (1:1), 150°C; [C=C] (mmol/g), 1—0.43, 2—0.42; [ ] (dl/g), 1—2.37, 2—1.83. 5 9

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300 20 30 40 50 60 Vulcanization time (min.) Figure 3. Effect of [C=C] on tensile properties: V(acac) -Et AlCl, 160°C; [C=C] (mmol/g), 1—0.23, 2—0.35, 3—0.53; [ ] (dl/g), 1—1.19, 2—1.18, 3— 1.16. 3

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Figure 5. Effect of [C=C] on tensile properties: V . -Et Al Cl , 150°C; [C=C] (mmol/g), 1—0.46, 2—0.32; [ ](dl/g), 1—2.54, 2—2.45. 5 9

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Vulcanization time (min.) Figure 6. Effect of [C=C] on tensile strength on EPDMs for VOCl EtAl Cl . Conditions for A: [η] (dl/g), 2.01 (1) 2.05 (2); [C=C] (mmol/g), 1.01 (1) 0.43 (2); Conditions for Β: [η] (dl/g), 1.69 (1) 1.88 (2) 2.30 (3); [C=C] (mmol/g) 0.81 (1) 0.42(2) 0.24(3). 3


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assorted from samples from large amount of polymerization work. Six matched sets of samples, each comparable i n \J\] Zc^cl and 0ψ> (Table I ) , were chosen from terpolymers obtained with V^_QE t ^ A ^ C l o , V O C l o - E t o A ^ C l o , V i a c a c U - E ^ A l C l and the activated V^_g-Et3Al2Gl3-ETGA"; respectively. Comparison of vulcanizate t e n s i l e properties of the s i x sets of comparable samples (Figures 7A-F) gave results (Table I , l a s t column) pointing to the a b i l i t y of the systems i n giving decreas ing t e n s i l e strength i n the following orders 9

V r ^ - E t y i l g C l ^ > V (acac ) 3 - E t ^ l C l > V _ -Etya 2 Cl3-ETCA> 5

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I t i s seen that, i n some cases, samples having the same modulus (Figures 7C and F) or even lower modulus (Figures 7D and E ) had even higher t e n s i l e strength. Taking 300^ modulus as a measure of cross-linking density, i t may mean that, besides [V/l» degree of unsaturation, composition and cross-linking density as symbolized by 300^ modulus, there are other factors that a f f e c t the t e n s i l e strength. As supporting evidence, i t may also be mentioned that t90 (time needed f o r 90^ vulcanization) and V (vulcanization rate) values on a eurometer showed the followed orders e

V5_9-Et3Al Cl > VOC^-Ety^Cl^ ; 2

3

V(acac)3-Et AlCl> V5_9-Et3Al2Cl3-ETCA> V O C O ^ - E t ^ ^ C l j . 2

In the case of V ^ ^ - E t o A l g C l o system, the vulcanizate tensile strength i s lowered on addition of the a c t i v a t o r ETCA, apparently because of the change i n molecular structure of the r e s u l t i n g polymer as a result of change i n the nature of the polymerizationactive centers. But this catalyst s t i l l y i e l d s vulcanizate strength higher than that of VOCI3-EPDM. Effect of D i s t r i b u t i o n of Double Bonds The d i s t r i b u t i o n of the t h i : monomer i n molecular chains or i n the whole polymer should affect the perfection of the v u l c a n i zate network, free chain ends or the uncross-linked parts i n the polymer making no contribution to the t e n s i l e strength but acting as a p l a s t i c i z e r of l i k e structure as the polymer. So f a r i t has not been possible to determine the distribution! of the t h i r d monomer units i n molecular chains. Yet i t i s possible to follow the rate of third monomer incorporation i n polymerization so as to estimate the heterogeneity of i t s d i s t r i bution i n the whole polymer. We have previously reported the marked difference i n incorporation of DCPD i n polymerization with V ( a c a c ) 3 - E t A l C l and V q . g - E t ^ l p G l ^ - E T G A . ( 3 ) . Figure 8 shows that Vr_« i n combination with various alkylaluminum halides and V O C L ^ Et A l C I are not noticeably different i n influencing the incorροϊα,υχυιι of DCPD during EPDM polymerization. Thus, difference i n 2


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Elastomers and Rubber Elasticity - American Chemical Society

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