CC BY
17
678.19:678.43
N. M. Livanova, A. A. Popov, G. E. Zaikov
STRUCTURE OF ELASTOMERS AND INTERPHASE INTERACTION IN THESE BLENDS
Keywords: acrylonitrile- butadiene rubbers, ethylene-propylene-diene elastomers, compatibility, phase structure, interphase layers,
stereoregularity, isotacticity propylene units, isomeric composition.
On the basis of experimentally measured deviations of the equilibrium degree of swelling in n-heptane from the additive value calculated for crosslinked heterophase blends composed of acrylonitrile- butadiene rubbers of different polarities and ethylene-propylene-diene terpolymers of the known comonomer composition and stereoregularity of propylene units, the density of interfacial layer and the amount of chemical crosslinks in it have been characterized. The effects of isomers of butadiene units, the ratio of comonomers in ethylene-propylene-diene terpolymers, and the degree of isotacticity of propylene units in the layer on the intensity of interfacial interaction in covulcanizates have been analyzed.
Ключевые слова: акрилонитрил- бутадиеновые каучуки, этилен-пропилен-диеновые эластомеры, совместимость, фазовая структура, межфазные слои, стереорегулярность, звенья изотактичного пропилена, изомерный состав.
На основе экспериментально измеренных отклонений от равновесной степени набухания в n-гептане от аддитивной величины, рассчитанной для сшитых гетерофазных смесей, состоящих из акрилонитрил-бутадиеновых каучуков различной полярности и этилен-пропилен-диеновых тройных сополимеров с известным составом сомономеров и стереорегулярностью пропиленовых звеньев, охарактеризованы плотность межфазного слоя и количество химических сшивок в нем. Исследованы влияния эффектов изомеров бутадиеновых звеньев, соотношения сомономеров в этилен-пропилен-диеновых тройных сополимерах и степень изотактичности пропиленовых звеньев в слое на интенсивность межфазного взаимодействия в совулканизатах.
Introduction
Formation of a strong interfacial layer is the key factor of the mechanism describing retardation of ozone degradation of a diene rubber by elastomer additives with a low degree of unsaturation [1-4]. The effect of comonomer ratio in ethylene-propylene-diene terpolymers (EPDMs) and stereoregularity of propylene units on the interfacial interaction and the amount of crosslinks in the interfacial layer was considered for heterophase crosslinked blends with acrylonitrile-butadiene rubbers (NBRs) of different polarities.
The density of the interfacial layer and the amount of crosslinks in it were determined via study of the swelling in the selective solvent «-heptane (the Zapp method [5, 6]) through deviation of the equilibrium degree of swelling from the additive value. It was proposed that the interfacial layer in the crosslinked blend of copolymers with different polarities may develop via diffusion penetration of EPDM units into the nonpolar regions of NBR [7-12].
We investigated also the density of the interfacial layer and the content of the formed crosslinks for EPDM samples with high contents of ethylene units and higher degrees of isotacticity of propylene chain fragments. It seems interesting to characterize the effect of the very low stereoregularity of propylene units, the content of diene groups, and the Mooney viscosity on the structure of interfacial region.
Experimental
The objects of research in this study were heterophase crosslinked BNR-EPDM (70:30) blends. At this content of the nonpolar component, a system of interpenetrating crosslinked networks appears. Commercial acrylonitrile- butadiene rubbers (trademarks BNKS-18, BNKS-28, and BNKS-40) were used. The AN-unit contents were 18, 28, and 40 wt %,
respectively, and the values of the Mooney viscosity (at 100°C) were 40-50, 45-65, and 45-70 rel. units, respectively. The content of trans-1,4-, 1,2-, and cis-1,4-units of butadiene was estimated via IR spectroscopy (bands at 967, 911, and 730 cm-1) [13] with the use of extinction coefficients from [14] (Table 1).
EPDM of the Royalen brand (Uniroyal, USA), of the Keltan brand the DSM 778, 714, and 712 brands (DSM N.V., Netherlands) and the domestic EPDMs having different relative amounts of ethylene, propylene, and ethylidene norbornene (ENB) units and different degrees of microtacticities of the propylene sequences respectively, were used [15-17]. The composition, the molecular-mass characteristics, the Mooney viscosity, the isotacticity of EPDM propylene units according to IR data [13, 18, 19] are given in Table 2 and Table 3.
Table 1 - Isomeric composition of butadiene units in different AN-butadiene copolymers
Copolymer Content of units, %
trans-1,4- 1,2- ds-1,4-
BNKS -18 82.0 8.2 9.8
BNKS -28 76.4 14.4 9.2
BNKS -40 93.0 4.4 2.6
For domestic EPDMs the data on the Mooney viscosity; the content of ethylene, propylene, and ethylidenenorbornene units according to the manufacturer's data.
A vulcanizing system for NBRs had the following composition, phr: stearic acid, 2.0; Sulfenamide Ts (^-cyclohexylbenzothiazole-2-sulfenamide), 1.5; zinc oxide, 5.0; and sulfur, 0.75. EPDM of the Royalen brand, the DSM and domestic
EPDM was vulcanized with supported Peroximon F-40 taken in an amount of 5.5 phr. Each rubber was mixed with its vulcanizing system by roll milling at 40-60°C for 15 min. Then, a rubber blend was prepared under the same conditions. The blends were vulcanized at 170°C within 15 min.
Table 2 - Composition and basic characteristics of ethylene-propylene-diene elastomers
EPDM Ethylene: Isotac- ENB Mwx
brand propylene, ticity, content, 10-5
wt % % wt % [2]
R 512 68:32 20 4 1.95
R 505 57:43 24 8 -
R 521 52:48 22 5 2.11
778 65:35 13 4.5 2.0
714 50:50 12 8 -
712 52:48 11 4,5 3.01
EPDM Mnx MwMn Mooney
brand 10-5 viscosity
[2] at 125°C
R 512 - 1.50 57
R 505 - Narrow 55
R 521 1.31 1.61 29
778 1.35 1.48 63
714 - - 63
712 1.60 1.88 63
Table 3 - Characteristics of domestic EPDM
EPDM n Ethy Iso ENB
trademark Mooney lene: tac content,
viscosity, rel. units propy lene ticity, % wt %
EPDM-40 36-45 70/30 29 4
Elastokam 67 74/26 9,5 5,4
6305
EPDM- 60 60/40 13 4
60(I)
EPDM- 62 60/40 13 6,7
60(II)
Elastokam 83 60/40 9,5 5,1
7505
The density of the interfacial layer and the amount of crosslinks in this layer were characterized by calculation of the deviation of the equilibrium degree of swelling Qeq from additive values toward increase in the nonpolar solvent «-heptane [5, 6]. The deviation is related to a weak interfacial interaction between thermodynamically incompatible polymer components, one of which contains polar units. In such systems, as was suppose, only local segmental solubility of nonpolar chain portions is possible [7-12].
The Flory-Huggins interaction parameter x for polybutadienes and EPDM with «-heptane and solubility parameters 5 for c/s-PB, EPDM, and BNKS were reported in [20, 21].
The deviation of Qeq from additive values -a was calculated by the formula [6]: a= [(Qad - Qeq)/(Qad-Q2)] * 100%,
where Qeq is the equilibrium degree of swelling of a covulcanizate; Qad is the additive value of swelling in a given solvent, as calculated from the equilibrium degree of swelling of vulcanizates for each rubber; and Q2 is the share of swelling of the second elastomer (NBR).
Results and Discussion
Calculations showed that the value of -a for BNRS-28 is somewhat lower than that for BNRS-18, even though the polarity of the former rubber is higher. This phenomenon was attributed to the effect of various butadiene isomers on compatibility with ethylene-propylene-diene elastomers (Table 1).
t, %
Fig. 1 - The plots the value of -a100% for covulcanizates EPDM with DSM (1) 778, (2) 714, (3) 712, (7) EPDM-60(I); -a (4) for R 521; -acis+1,2 for (5) R 505 and (6) Elastokam 6305 as a function of the total content of 1,4-cis- and 1,2-butadiene isomers in NBR
The value of -a depends on the amount of crosslinks in the interfacial layer and its volume. Let us assume that the major fraction of polar units of NBR is uninvolved in its formation and the value of -a was recalculated to the 100% content of butadiene units -a100% (Fig. 1) and used to characterize the structure of the interfacial interaction zone and the amount of crosslinks contained in it. In such a manner, the effect of the interfacial layer volume could be minimized. As will be shown below, this situation may not be attained in all cases.
It is seen that a linear decrease in the value of -a100% with an increase in the total content of 1,4-c/s and 1,2 units is observed for the crosslinked blends of NBR with all DSM EPDM samples and EPDM-60(I) characterized by a low isotacticity of propylene (Table 2 and 3) . The fact that the value of -a100% decreases in proportion to the amount of 1,4-c/s and 1,2 isomers of butadiene units for EPDM-based blends provides evidence that the density of the transition layer increases. This circumstance implies that the mutual solubility of EPDM comonomers and butadiene units that occur for the most part in the 1,4-trans
configuration in the neighborhood of these isomers is improved.
For DSM and EPDM-60(I) the region of interfacial interaction is bounded by nonpolar NBR units. The compatibility of chain portions of these EPDM samples with the polar acrylonitrile groups is ruled out. As the proportion of propylene units in EPDM is increased, the compatibility of the components, the density of the interfacial layer, and the amount of crosslinks in it drop sharply (the absolute value of -a100% increases). Thus, the higher the content of atactic propylene units in EPDM, the lower the adhesion interaction of the components and the worse its compatibility with NBR. The strengthening of interfacial interaction with an increase in £(1,4-cis and 1,2 units) for EPDM 778, EPDM 714, and EPDM 712 by 2.0, 1.4, and 1.3 times, respectively, is apparently explained by the loosening effect of these isomers on the structure of the nonpolar phase of NBR comprising for the most part 1,4-trans units (Table 1), which show the tendency toward ordering at the low content of acrylonitrile and other isomers [22-24]. Ordered structures worsen compatibility of polymers even to a higher extent [5-10, 24]. In [25] is shown that sorption radicals and other low molecular substance occurs in defect structure containing isomers of butadiene units available in small quantities. From the data (Fig. 1) obtained it follows that when combined with the polymers forming the interfacial layer also occurs mainly in the defective domains BNK with sufficient free volume.
When the Uniroyal EPDM, which is characterized by a high stereoregularity of propylene sequences, was tested in blends, no proportionality between the values under consideration was observed (Fig. 1) [26].
For NBR covulcanizates with EPDM R 512 containing a large proportion of ethylene units and distinguished by the presence of stereoregular propylene sequences, the value of -a100% linearly decreases with the content of butadiene units by a factor of 2.6 (Fig. 2).
50 60 70 80 90
c,%
Fig. 2 - (1) (-a) and (2,3,4) (-a 100%) vs. overall content of butadiene units c for (1) EPDM-40, (2) EPDM-60 (II), (3) Elastokam 7505, (4) EPDM R 512 in their covulcanizates with NBR
This fact leads us to infer that, firstly, ethylene units adjoining predominantly short isotactic propylene sequences are well compatible with all isomers of butadiene units. Secondly, an increase in the density of the interfacial layer and in the amount of crosslinks in it (a decrease in -a100%) with a rise in the proportion of nonpolar units in NBR implies that EPDM molecular fragments may penetrate into NBR regions that apparently contain single polar acrylonitrile groups.
Figure 2 presents the (-a) value plotted against the content of butadiene units in NBR for the crosslinked blends with EPDM-40 (curve 1). As compared with other commercial EPDMs (table 2 and 3), EPDM-40 is characterized by a very high degree of isotacticity (29%) of propylene chain fragments (about 90% of the isotactic fraction). As was shown propylene fragments with a high stereoregularity are able to penetrate into the NBR regions that are likely to contain single polar groups so as for EPDM R 512.
EPDM-40 with a higher degree of isotacticity of propylene units (as compared with EPDM R 512) is characterized by a far more intensive penetration of propylene chain fragments in their rigid isotactic configuration into NBR regions containing single polar groups. An increased volume of the interfacial layer and its local density reduction are confirmed by the fact that the value of (-a) for blends with EPDM-40 is higher than that for the EPDM R 512-based blends. This increase comes to 1.3-, 1.5-, and 1.7-folds for BNKS-18, BNKS-28, and BNKS-40, respectively.
For the covulcanizates of R 521 EPDM, which contains a large amount of isotactic propylene sequences [15, 16, 26], the proportional decrease in the value of -a by a factor of 1.85 is observed in the -a-E(1,4-cis and 1,2 units) coordinates (Fig. 1). Consequently, at a high content of isotactic propylene units, EPDM shows better compatibility with NBR if the butadiene comonomer is enriched with 1,4-cis and 1,2 units (a reduction in the value of -a). The values of -a do not take into consideration restrictions related to the mutual interpenetration of segments of dissimilar chains associated with the presence of polar groups in NBR. As a result, it is not improbable that NBR portions containing polar groups may be involved in the interfacial interaction region. This circumstance is related to the specific features of NBR interaction with EPDM containing a large proportion of isotactic propylene chain fragments. This observation may be attributed to the rigidity of isotactic propylene sequences arising from hindered conformational transitions. The potential barrier to transitions between rotational isomers of monomer units for the isotactic PP is 21 kJ/mol, while for PE, the potential barriers of T-G and G-G transitions are 2.5 and 8.8-10 kJ/mol, respectively [27].
For blends with EPDM R 505, which contains a large amount of ENB, the linear dependence is attained when the values of -a100% are recalculated to the 1% of the sum of 1,4-cis and 1,2 units in NBR (-acis + 12) (Fig. 1) [2]. An analysis of the -acis + 1,2 versus E(1,4-cis and 1,2 units) curves demonstrates that -acis + 1,2 decreases by a factor of 5.3 with an increase in the total amount of these butadiene unit isomers. The data
presented above suggest that the chain fragments of this EPDM are well compatible only with those portions of butadiene chains that contain 1,4-c/s and 1,2 isomers, while in the case of 1,4-trans units, compatibility is much worse. It appears that the bulky diene group, as in EPDM 714, significantly hinders the incorporation of EPDM chain portions into butadiene regions of NBR.
For Elastokam 6305 with a high content of ethylene units (74%), with a higher amount of ENB (5,4%) and a very low degree of isotacticity of propylene sequences (9.5%) (Table 3), as follows from Fig. 1, the dependence on the overall content of c/s-1,4-and 1,2-butadiene units in NBR is linear if the (-a) value is normalized to the 100% content of nonpolar units and 1% content of c/'s-1,4-and 1,2-isomers (-ac/s-1,4 + 12). Therefore, one can conclude that this copolymer is compatible only with butadiene chain fragments of BNR containing preferably c/s-1,4- and 1,2-isomers. Hence, when propylene units are characterized by a very low microtacticity (9.5%), compatibility between the components decreases. As was shown in [26], the high content of ethylene units, whatever the configuration of propylene sequences, provides a better compatibility with butadiene copolymer BNKS. However, as atactic configuration in the blend dominates, the depth of penetration of EPDM segments is limited by the butadiene part of the copolymer.
Because, in EPDM-60 (I), half of all propylene units exist in the isotactic configuration and the content of diene groups is lower, its segments are able to diffuse into nonpolar NBR regions to a greater depth than segments of Elastokam 6305.
EPDM-60 (II) with a higher content of ENB (6.7%) is compatibilized only with butadiene units (Fig. 2, curve 3) and (-a100%) increases by a factor of 1.8 as their content grows. This tendency suggests that this interfacial layer is characterized by an increased friability and the number of crosslinks in this layer is low. The higher the volume of this layer (the lower the polarity of NBR), the higher the (-a100%) values.
The ratio between ethylene and propylene units in Elastokam 7505 is similar to that in EPDM-60 (II), but the degree of isotacticity of propylene units is lower (9.5%) and the Mooney viscosity is very high (Table 3 ). Elastokam 7505 is characterized by the minimum compatibility with all NBR samples. As follows from Fig. 2 (curve 3), the friability of the interfacial layer of this EPDM is maximum; as a result, (-a100%) markedly increases with a decrease in the polarity of NBR (by a factor of ~3). The worst results were observed for EPDM-60 (II) with an increased content of diene units and for Elastokam 7505 with a high Mooney viscosity.
Thus, the structure of interfacial interaction zone depends on the comonomer composition of EPDM, the stereoregularity of propylene units, and the isomerism of butadiene units.
Owing to conformational restrictions, stereoregular propylene units can penetrate into NBR regions that apparently contain single polar acrylonitrile units. As a result, the volume of the interfacial zone increases but its density and the amount of crosslinks contained in it decrease locally. The compatibility of propylene fragments of EPDM chains that
predominantly occur in the atactic configuration with butadiene units is much worse. The interfacial layer is looser, and the region of diffusion penetration of phases is confined by the presence of nonpolar units. The proportional growth of the density of interfacial layer and the amount of crosslinks with an increase in the total amount of isomers of butadiene units contained in NBR in a smaller amount is explained by disordering of 1,4-trans units. As a result, their compatibility with portions of EPDM molecules is facilitated.
The ability of EPDM that contains a large amount of diene units and stereoregular propylene units to compatibilize with butadiene units of NBR is close to that of EPDM with a low degree of isotacticity of the propylene comonomer and is determined by steric hindrances related to the presence of the bulky diene. However, a higher content of diene groups ensures better crosslinking of EPDM with the matrix. As a result, the total amount of crosslinks between phases is higher than that for blends with EPDM having the same ratio of ethylene and propylene units but a smaller amount of diene (cf. EPDM R 505 and R 521, EPDM 714 and EPDM 712).
The largest amount of crosslinks in the interfacial layer forms when EPDM with a high content of ethylene units is used despite the moderate amount of diene contained in it.
Conclusion
The structure of interfacial interaction zone depends on the comonomer composition of EPDM, the stereoregularity of propylene units, and the isomerism of butadiene units. It is shown the proportional growth of the density of interfacial layer and the amount of crosslinks with an increase in the total amount of 1,4-c/s and i,2-isomers of butadiene units contained in NBR in a smaller amount with sufficient free volume.and disordered of 1,4-trans units. At a high content of isotactic propylene units, EPDM shows better compatibility with NBR. The largest density of interfacial layer and amount of crosslinks in the interfacial layer forms when EPDM with a high content of ethylene units is used despite the moderate amount of diene contained in it. The low density of interfacial layer are for EPDM with mean stereoregularity of propylene units and .small amount of ethylene units.
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© N. M. Livanova - Ph.D., senior researcher, Emanuel Institute of Biochemical Physics, Russian Academy of Sciences, Moscow, Russia, A. A. Popov - Doctor of Chemistry, Full Professor of Plekhanov Russian University of Economics, Head of Laboratory, Deputy Director of Emanuel Institute of Biochemical Physics, Russian Academy of Sciences, Moscow, Russia, G. E. Zaikov - Doctor of Chemistry, Full Professor of Plastics Technology Department, Kazan National Research Technological University, Kazan, Russia, [email protected].
© Н. М. Ливанова - кандидат химических наук, старший научный сотрудник Института биохимической физики им. Н.М. Эмануэля РАН, Москва, Россия, А. А. Попов - доктор химических наук, профессор Российского университета экономики им. Г.В. Плеханова, заведующий лабораторией, заместитель директора Института биохимической физики им. Н.М. Эмануэля РАН, Москва, Россия, Г. Е. Заиков - доктор химических наук, профессор кафедры Технологии пластических масс, Казанский национальный исследовательский технологический университет, [email protected].
