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ISSN 2522-1841 (Online) AZERBAIJAN CHEMICAL JOURNAL № 4 2023 ISSN 0005-2531 (Print)
UDC 678.762.2.134
EFFECT OF HEATING ON STRUCTURAL PARAMETERS OF NITRILE BUTADIENE
RUBBER IN THE PRESENCE OF CHLORINE-CONTAINING COMPOUNDS
111 2 G.G.Azizova , F.A.Amirov , I.G.Movlayev , G.A.Mammadova ,
R.F.Khankishiyeva2, Sh.M.Mammadov2
1 Azerbaijan State Oil and Industry University Institute of Radiation Problems of the Ministry of Science and Education of the
Republic of Azerbaijan
shiraz.mamedov@gmail. com
Reseived 24.01.2023 Accepted 03.04.2023
A new class of active organic crosslinking agents for the vulcanization of NBR based on halogen-methyl-containing compounds has been found, allowing improvement significantly in the rheological and structural parameters of NBR. Gel content and Mooney viscosity in binary systems after plastification on rollers were studied. The sol fraction of cross-linked NBR (SKN-40+TCMCC) was found to be characterized by a bimodal MWD. Abnormally high value of M<v/Mns of sol fractions is associated with the bimodal nature of their MWD and indicates the presence of gel in the elastomer. The value of M n of the sol fractions in cross-linked NBR was found to be insignificant. The sol fraction of cross-linked HCPX elastomer was shown to have lower values of the specified parameters and lower content of high-molecular-weight fractions. The branching density p of macromolecules of sol fraction of cross-linked NBR was 1.90-2.3 •lO-3, i.e. on average corresponded to one monomer unit and two hundred branching nodes per molecule.
Keywords: nitrile butadiene rubber, hexachloroparaxylene, Mooney viscosity, sol fraction, rheology, crosslinking, vulcanization, gel, plasticity, dispersion.
doi.org/10.32737/0005-2531-2023-4-91-96 Introduction
To obtain homogeneous mixtures with additive properties, it is necessary to have: elastomers with close compatibility features, giving mixtures with one glass-transition point; elastomers with similar viscosities, elastomers with similar vulcanization characteristics, the possibility of some variation in the plasticity of blended elastomers to overcome small differences and viscosities in compatibility features [1-3]. It is necessary to use a displacement method that promotes their uniform distribution in elastomers.
The cross-linking of NBR with tri-chloromethyl containing compounds (TCMCC) generally consists of components of different structures. In principle, any technological characteristic of NBR will be a function of the relative content of features in the structure of each of the components.
In this regard, one of the most important
rheological characteristics of NBR was studied - the ability to plasticize on rollers and the effect of heating on the structural parameters of the spatial network of NBR polymer systems (SKI-40 + TCMCC) cross-linked after heating in an electric press at a mode of 423K x 30 min.
Experimental part
The objects of study were NBR (SKN-40M) and organic crosslinking agents (OCA) -the trichloromethyl-containing compounds hex-achlorparaxylene (HCPX), ro-octachlor-4.4-dimethyl diphenylmethane (OCDMDPM) and ro-decachlor-4.4-dimethyl-1.4-dibenzylbenzene (DCDMDB) [4, 5].
Mechanical (cold) plastification of NBR was carried out on laboratory rollers with a size of 160x320 mm at 203 K ± 283K. The rollers load was 100g with a fraction f=1:2, elastomer mixtures (wt.% per 100 wt.% of rubber) were prepared with the following composition. 1. SKN-40M-100 + HCPX - 3.0;
2. SKN-40M-100 + OCDMDPM;
3. SKN-40M-100 +DCDMDB-3.0
After mechanical plastification on rollers, it was vulcanized in an electric press at 423K*40 min. Plasto-elastic properties of mixtures were determined according to GOST 41592, 10722-87. Rheological properties of the samples were determined on an Ubbelohde capillary viscometer [4] in toluene at 293 K; the calculation was carried out according to Mark-Houwink equation at a value of solution constant K = 1.16x104 and a = 0.7 [7].
Molecular characteristics of the elastomer cross-linked by different OCAs were determined by sol-gel analysis [4, 5], equilibrium swelling [6], molecular sieve chromatography [7], and viscometry [8, 9]. The content of sol fraction S in the elastomer, macrogel G, its degree of equilibrium swelling Qe, number average molecular weight of the chain segment between cross-links Afc, the resistance of the mac-rogel to shear deformations 0 [8], the number average Mns, mass average Mw, and viscosity average Af,, molecular weights, degree of poly-dispersity, Afn, MWD and branching density p of the sol fraction macromolecules have been estimated.
The value of Mc was calculated using nomograms obtained by the graphical solution of the Flory-Rehner equation [10], and the values of Mns and p were calculated from the known relations of sol-gel analysis [6]:
The MWD of sol fractions was determined by molecular sieve chromatography [9, 11, 12]. This method allows us to simultaneously separate elastomers into fractions and to relatively quickly determine their molecular characteristics. The analysis of cross-linked elastomer by molecular sieve chromatography was performed using a glass column with a length of 800 mm and a diameter of 8 mm. A modified silochrome S-2 with a grain size of 0.16-0.25 mm was used as a nozzle. The flow rate of eluent (carbon tetrachloride) was 1 ml/min. The detector was an ITR-2 interferometer. The column was calibrated using narrow fractions of NBR with Mw/ Mn < 1.1.
Results and discussion
The experiments have shown that plastification on cold rolls (293-300 K) has significantly higher effect on non-cross-linked elastomers than on cross-linked ones. Thus, the Mooney viscosity of non-cross-linked SKN-40 decreases by 50 and 70 % respectively after 20 minutes of plastification, and the viscosity of elastomers cross-linked with TCMCC by 18 and 30 % of the initial viscosity (Figure 1).
Fig. 1. Dependence of Mooney viscosity on plastification time
1. SKN-40M + DCDMDB;
2. SKN-40M + OCDMDPM;
3. SKN-40M + HCPX;
4. SKN-40M
The data in Table 1 below show that all the mixtures containing TCMCC in their composition have a high content of dense gel in SKN-40, while those without organic cross-linking agents (OCA) do not. The quasi-binary system (SKN-40-TCMCC+ZnO) has a higher viscosity than the binary mixtures. The gel content during mechanical plastification of cross-linked NBR also decreases slightly and even after plastification for 30 min it becomes 50 %. A less drastic change in the properties of cross-linked NBR during plastification can be explained by the presence of labile cross-links in them, which easily break down with the formation of active fragments, which then recombine again, crea-ting new cross-links [1, 13, 14].
Table 1. Changes in gel content, density of binary and quasi-systems and viscosity after plastification on rolls
Composition Gel content, G% Gel density, g/cm Mooney viscosity, c.u.
SKN-40 78 - 25
SKN-40 + HCPX 78 3.8 43
SKN-40 + OCDMDPM 84 4.4 46
SKN-40 + DCDMDB 87 4.5 49
SKN-40 + HCPX + ZnO 85 4.1 53
SKN-40 +OCDMDPM+ZnO 90 4.7 56
SKN-40 + DCDMDB + ZnO 92 4.8 60
The destruction of the main polymer chains obviously occurs during mechanical plastification of non-cross-linked NBR, as evidenced by a decrease in the intrinsic viscosity of the toluene solution of the polymer (Figure 2).
*1s dl/g
0 "I-T-T-T-r
0 2 4 6 8
Plastification time, min
Fig. 2. Dependence of intrinsic viscosity on
plastification time
1. SKN-40M + DCDMDB;
2. SKN-40M + OCDMDPM;
3. SKN-40M + HCPX;
4. SKN-40M
The observed decrease in the intrinsic viscosity of NBR polymer systems during their mechanical plastification indicates chain degradation in the samples.
Thus, the behavior of polymer systems (SKN-40 + TCMCC) during processing on rollers has different rheological properties.
It was natural to expect that the nature of the interaction of binary mixtures containing chlorine-containing compounds with zinc oxide would be different.
Indeed, the introduction of carbon black P324 drastically increases the Mooney viscosity
of the binary mixture during heating at 423 K (Figure 3).
M, c.u
0 i-1-1-1-1-r-
0 10 20 30 40 SO HfitNiift time, miii
Fig. 3. Dependence of Mooney viscosity on
the heating time
1. SKN-40M + DCDMDB;
2. SKN-40M + OCDMDPM;
3. SKN-40M + HCPX;
4. SKN-40M
Consequently, the introduction of carbon black in binary elastomeric mixtures creates strong bonds between the polymer and the carbon black as a result of the interaction of the carbon black particles with the active polymer fragments that form cross-links in the elastomer.
The introduction of carbon black into the binary mixtures seems to result only in a weak physical interaction of the binary mixtures with the carbon black [2]. The radicals, arising during the destruction of the main polymer chains, probably cannot react with the carbon black because SKN-40-P324 does not form carbon in the elastomeric gel, although the destruction of its molecular chains does occur (Table 2).
Table 2. Effect of carbon black (P 324) on Mooney viscosity and formation of carbon elastomer gel in them
Indicators SKN-40M + P324 SKN-40M + HCPX + P324 SKN-40M + OCDMDPM + P324 SKN-40M DCDMDB + P324
Color of the mixture Dark Dark Dark Dark
Amount of carbon elastomeric gel,% At 298K At 343 K 32 22 75 53 90 69 86 72
Increase in Mooney viscosity of the mixture by the introduction of carbon black (P 324) 12 12 22 28 75 35 81 31
The physical nature of the interaction of elastomers with carbon black is confirmed by equal values of the Mooney viscosity of the SKN-40M elastomer and its carbon mixture at temperatures above 343 K, when weak intermolecular interaction between the elastomer and carbon black is eliminated. At the same time, the Mooney viscosity of SKN-40M + TCMCC and its carbon mixture at all studied temperatures differ significantly (Table 2).
Thus, when using TCMCC as an organic crosslinking agent (OCA) for crosslinking NBR, binary and quasi-binary vulcanized samples are characterized by significantly better processing properties than non-cross-linked elastomer mixtures.
The presence of cross-links (C-C, C-Cl) in the NBR structure helps to reduce its destruction during plastification on rollers and improves the interaction of the polymer with carbon black, which leads to an increase in the viscosity of raw binary and quasi-binary cross-linked vulcanizates.
Molecular weight distribution (MWD) parameters of sol fractions were established by
molecular sieve chromatography [9]. This method allows the simultaneous separation of elastomers into fractions and relatively fast determination of their molecular characteristics. Analysis of cross-linked elastomer by molecular chromatography was performed using a glass column for narrow fractions of NBR with
Mn /Mn.
It has been found that the sol fractions of NBR cross-linked with various TCMCC are characterized by a bimodal MWD (Figure 1). This agrees with the results obtained for cross-linked elastomers by nephelometric titration [6]. At the same time, fractions with relatively high molecular weights (< 3.8 105) prevail in the NBR sol fractions. At the same time, the sol-fractions of the cross-linked NBR, especially SKN-40, contain more fractions compared to the standard NBR with low molecular weights (< 3.0-104). The anomalously large value of Mw/Mns, of the sol fractions of NBR cross-linked by various OCAs (Table 3) is associated with the bimodal nature of their MWD and indicates the presence of a macrogel in the elastomer [12, 13].
Table 3. Molecular characteristics of the sol fractions of the cross-linked NBR
Composition S, % -'■-Y10-4 ;.,nsio-4 ."■V10-4 p103 G, % Qe, % 0 ."■."c -10-4 McMo-4
SKN-40M+ HCPX 14 12.1 3.5 16.2 4.2 1.9 86.0 21 1 8.7 6.80
SKN-40M+ OCDMDPM 9.7 15.0 3.4 18.0 5.3 2.1 90.3 17.3 1 5.5 4.85
SKN-40M+ DCDMDB 5.8 16.1 3.2 19.5 6.0 2,3 94.2 13.0 1 4.0 3.89
*- for rubber without OCA
Table 3 shows the values of the structural parameters of macrogels of NBR cross-linked with trichloromethyl-containing compounds, obtained from the data of equilibrium swelling, as well as by calculation using the equations of sol-gel analysis and molecular sieve chromatography.
The values of M,, of the sol-fractions of the studied NBR cross-linked with trichloromethyl-containing compounds differed insignificantly.
Molecular parameters of sol fractions of cross-linked NBR by HCPX, OCDMDPM, and DCDMDB systems were similar due to the same way of their preparation. The sol fraction of elastomer cross-linked with hexachloroparaxylene had lower values of the above parameters and lower content of high-molecular-weight fractions. This explains the relatively low degree of polydispersity.
The branching density p of the macromolecules of the sol fractions of the cross-linked NBR was 1.9010-3; 2.1+2.3 10-3, i.e. on average, one link out of two hundred was cross-linked. This corresponded to the presence of about two hundred branching nodes per macromolecule in the elastomer. The obtained data correlate satisfactorily with the results of viscometric measurements available in the literature [14, 15].
Cross-linked NBR - SKN-40M + OCDMDPM and SKN-40M + DCDMDB - is characterized by the highest content of macrogel G and a lower degree of its equilibrium swelling Qe compared to SKN-40M + HCPX. The 0 value of cross-linked elastomer macrogels of various compositions was 1.0, which indicates a high strength of the macrogel [16].
Due to the inhomogeneity of cross-linked elastomer (NBR) macrogels of different compositions [13, 17], the estimation of Mc using the Flory-Rehner equation is only rational in the case of densely cross-linked macrogels, i.e., when the number of bonds within particles is so much greater than the number of bonds between them that the latter can be neglected. The Mc values of the macrogels cross-linked
with different trichloromethyl-containing compounds of NBR calculated by the sol-gel analysis formulas using molecular sieve chromatography data and the Flory-Rehner equation correlate satisfactorily with each other.
Based on the obtained data, a macrogel in a cross-linked elastomer can be schematically represented as consisting of large aggregates of highly cross-linked microgel particles.
In this case, the number of bonds between particles and individual agglomerates is negligible. The macrogel in these rubbers is formed directly during processing by the mechanism considered in the theory of gel formation, which seems to be close to the continuous cross-linking of macromolecules in an unbounded isotropic system. The results of the study show that molecular sieve chromatography is a promising method for determining the molecular parameters of the cross-linked TCMCC elastomer. This method can be used in conjunction with other methods to directly control the molecular characteristics of nitrile elastomers to assess their standards under production conditions.
Thus, the sol fractions of the investigated cross-linked NBR are characterized by a bimodal MWD, high values of Aiw/Mns and p,
indicating the presence of a gel in it. The macrogel in the investigated cross-linked NBR has greater resistance to shear deformations.
Conclusion
Based on the above and based on experimental data, we will try to explain the position of the crosslinking agents TCMCC we studied concerning SKN-40M. In the process of plastification at 343K, a decrease in the intrinsic viscosity of the sol fraction of NBR during their mechanical plastification indicates the destruction of the main chains. The introduction of carbon black into the system SKN-40M+ TCMCC +P324 causes weak physical interaction of the elastomer with the carbon black. Likely, the radicals arising from the degradation of the basic polymer are not capable of reacting with the carbon black.
Crosslinking of NBR with TCMCC at 423K*30 min occurs as a result of the decomposition of TCMCC with the elimination of chlorine from both trichloromethyl groups and the resulting biradical accentuates the hydrogen of NBR macromolecules.
The molecular parameters of sol fractions of NBR cross-linked with TCMCC systems are similar, which is due to the same way of their preparation. Cross-linked NBR is characterized by the highest macrogel G content and a lower degree of its equilibrium swelling Qe as compared to SKN-40M+HCPX. Due to the inhomogeneity of macrogels of cross-linked NBR of different compositions, the values of Ms were estimated.
The large Mw /Mn value of the sol fractions cross-linked with various TCMCC is associated with the bimodal nature of their MWD and indicates the presence of a microgel in the elastomer.
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XLOR TORKlBLi BIRLO§MOLORIN I§TIRAKI ILO BUTADIEN-NITRIL KAUCUKUNUN QURULU§
PARAMETRLORlNO QIZDIRILMANIN TOSiRi
G.H.Ozizova, F.A.Omirov, i.H.Mövlayev, G.A.Mamm3dova, R.F.Xanki$iyeva, ^.M.Mammadov
BNK-nin reoloji va struktur parametrlarini ahamiyyatli daracada yax§ila§dirmaga imkan veran metilhalogen tarkibli birla§malar asasinda BNK-nin vulkanla§masi ügün aktiv üzvi garpaz birla§diricilarin yeni sinfi a§kar edilmi§dir. Rulon-larda plastifikasiyadan sonra binar sistcmlordo gel torkibi va Mooney özlülüyü övronilmisdir. Tikilmis BNK-mn (SKN-40+TCMCC) sol fraksiyasinin bimodal MKP ilo xarakteriza olundugu askar cdilmisdir. Sol fraksiyalann Mw/Mn, qey-ri-nonnal yüksak dovori onlann MKP-ninbimodal tobioti ib olaqolondirilir vo clastomcrdo gel varligini göstorir. Molum olmusdur ki, tikilmis BNK-da sol fraksiyalannin MN qivmoti ohomiyyotsi/dir. Göstorilmisdir ki, tikilmis HXPK elas-tomerinin sol-fraksiyasinda bu parametrlarin a§agi qiymatlari va yüksak molekulyar gaki fraksiyalann tarkibi daha azdir. Tikilmi§ BNK-nin sol fraksiyasinin makromolekullarinin §axalanma sixligi p=1.90-2.3 10-3 idi, yani orta hesabla bir monomer vahidina va har molekula iki yüz §axalanma nöqtasina uygun galirdi.
Agar sözlzr: butadien-nitril kauguku, heksaxloroparaksilen, Mooney özlülük, sol fraksiya, reologiya, tikilma, vulkan-la§ma, gel, plastiklik, dispersiya
