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SBR • silica • MPGMA • Curing • physi-cal properties • swelling properties

Methoxy polyethylene glycol methac-rylate (MPGMA)was synthesized by di-rect esterification reaction between (methoxy polyethylene glycol) and me-thacrylic acid and characterized by FTIR spectroscopy and proton nuclear mag-netic resonance (1H NMR) spectroscopy. Styrene butadiene rubber (SBR) samp-les loaded silica with different concent-rations of methoxy polyethylene glycol methacrylate (MPGMA) (2, 4, 6 and 8 phr) was investigated with respect to their Curing characteristics, physical, and swelling properties. The filler dis-persion, as revealed by SEM, has been found to be influenced by methoxy po-lyethylene glycol methacrylate. It was found that the addition of 4 phr MPG-MA into SBR compounds improved the properties of the rubber composite.

Einfluss von Methoxy-Poly-ethylenglykolmethacrylat auf die Eigenschaften von mit Sili-ca-gefüllten Styrol-Butadien-kautschuk-Kompositen SBR • Silica • MPGMA • Vulkanisation • Physikalische Eigenschaften • Quellver-halten

Methoxy-Polyethylenglykolmethacrylat (MPGMA) wurde durch die direkte Es-terreaktion zwischen Methoxy-Poly-ethylenglykol und Methacrylsäure syn-thetisiert und durch FT-IR Spektroskopie und Protonen-Kernspinresonanzspek-troskopie (1H NMR) charakterisiert. Sty-rol-Butadienkautschuk (SBR) wurde mit verschiedenen Konzentrationen von Methoxy-Polyethylenglykolmethacrylat (MPGMA) (2,4,6 und 8 phr) gefüllt und hinsichtlich der Vulkanisationscharakte-ristik, der physikalischen und Quel-lungs-Eigenschaften untersucht. Es wurde festgestellt, dass die Füllstoffdis-persion, wie sie durch REM beschrieben worden ist, durch Methoxy-Polyethy-lenglykolmethacrylat beeinflusst wird. Es wurde gefunden, dass die Zugabe von 4 phr MPGMA zu SBR Mischungen die Eigenschaften der Elastomerkompo-site verbessert.

Figures and tables: By a kind approval of the authors.

IntroductionSilica is commonly active filler used in rubber technology. Many products from silica-filled rubber compounds are known such as heavy service tire treads, passenger tire treads, shoe soles, engine mounts, wire coats, cables, and any ap-plication where a colored or clear (trans-parent) rubber. Because the polar silica molecule is not capable of forming strong filler-polymer bonds with a non-polar hydrocarbon polymer that are nec-essary for maximum reinforcement [1], due to the silanol groups on the surface of silica, which give rise to strong fill-er-filler interactions and adsorption of polar materials by hydrogen bonds. Since intermolecular hydrogen bonds between silanol groups on the surface of silica are very strong, it can aggregate tightly [2-4]. The improvement of the filler disper-sion and prevention of curative adsorp-tion on the silica surface are very impor-tant [5]. Silane coupling agents are used as modifiers, and the product obtained is the modified amorphous silica with bet-ter performance [6,7]. The aim of present study is to assess the reinforcing effect of a (MPGMA) as coupling agent on the curing characteristics, physical and swell-ing properties, of a conventional acceler-ator/ sulfur compound of styrene butadi-ene rubber loading with silica filler. The amount of MPGMA will be increased progressively from 0 to 8 phr.

Materials and experimental techniques

Materials ■ Styrene-butadiene rubber: (SBR 1502

non-staining) it has the following characteristics; styrene content 23.5% with specific gravity 1.17, Tg = -60 OC and Mooney viscosity ML (1 + 4), 52 ± 3 at 100 OC, was obtained from Transport and Engineering Company (TRENCO) Alexandria.

■ N-cyclohexyl-2-benzothiazole sulphe-namide (CBS), with a pale gray pow-der, with specific gravity of 1.27–1.31

at room temperature (25 ± 1OC), melting point 95–100°C. (2, 2, 4-tri-methyl-1,2-dihydroquinoline [TMQ] was used as a commercial grade.

■ Zinc oxide and stearic acid were used as activators with specific gravity at 15°C of 5.55–5.61 and 0.90–0.97, re-spectively.

■ Elemental sulfur, fine pale yellow powder, with specific gravity of 2.04–2.06 at room temperature.

■ Silica (Hi-Sil 233D) of surface area 150 m2/g, specific gravity 1.95, particle size 15nm PPG Industries Inc., Nether-lands, Europe.

■ Other chemicals: methacrylicacid, methoxy polyethylene glycol, hydro-quinone, sulfuric acid and toluene.

All chemicals were supplied by Sig-ma-Aldrich, Germany.

Methods of preparation:Synthesis of the methoxy polyethyl-ene glycol methacrylate (MPGMA) [8]

The methoxy polyethylene glycol meth-acrylate was synthesized as shown in scheme 1 by direct etherification reac-tion between methoxy polyethylene gly-col and methacrylic acid, which produce MPGM and water.

Influence of Methoxy Polyethy- len Glycol Methacrylate on the Properties of Silica filled Styrene Butadiene Rubber Composites

AuthorsA. I. Khalaf, D. E. El-Nashar, A. M. Ellaban, A. M. Naser, H. Abdel-Wahhab, Cairo, Egypt Corresponding Author: Dr. Aman Ibrahim Khalaf Department of Polymers and PigmentsNational Research Centre, Dokki, Cairo, EgyptFaculty of Science, Al-Azhar University, Cairo, Egypt E-Mail: aman2502003@yahoo.com

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Fig. 2: 1HNMR spectrum of MPGMA.

2

Chemical composition of (MPGMA) sample for instance can be determined by Fourier transform infrared (FTIR) spectros-copy as in Figure 1. The peaks at 2885, 1712, 1466 and 1111cm-1 were associated

with C–H of alkane, C=O, and C=C stretch-ing frequencies of the methoxy polyethyl-ene glycol moiety and MAA moiety, re-spectively. The 1HNMR peak assignments are as follow, the three protons (1 Figure

2) of [ CH3-O] appear at ppm 3.2, two pro-tons of PEG moiety (2 Figure 2) of [CH3-O-CH2] appear at ppm 3.7 and two protons (3 Figure 2) of [O-CH2-CH2-O-C=O] appear at ppm 4.2,while three protons (4 Figure 2) of [CH3-C=CH2] appear at ppm 1.8 and two protons (Figure 2) of [CH3-C=CH2] ap-pear at ppm 5.6 and 6.

Preparation of rubber compositesRubber was pre-mixed with all com-pounding ingredients according to ASTM: D3182-0 (2018). Mixing was done on a laboratory of two-roll mill and the speed of the slow roll is 24 rev/min with a gear ratio of 1:1.4. The compounded rubbers were left overnight before vulcanization. The vulcanization was carried out at 152 ± 1°C for SBR in an electrically heated press under a pressure of about 4 MPa to get vulcanized rubber sheets of 2 mm thickness. The rubber formulations are tabulated in Table (1).

Techniques ■ Fourier transform infrared spectrome-

ter (FTIR) using a JASCO FTIR-6100E (Japan) operated in the absorption mode, in the wave number range 4000 to 400 cm-1 after mixing with potassium bromide and pressed in the form of discs. The spectra were collected at a resolution of 4 cm-1.

■ Proton nuclear magnetic resonance 1HNMR was measured in dimethyl sulfoxide using BRUKER ECA 400 MHz NMR spectrometer instrument.

■ The surface morphology of the samp-les was investigated by environmen-tal scanning electron microscope (SEM) (Quanta FEG-250).

■ Curing characteristics such as ML (mi-nimum Torque), MH (maximum Torque), Tc90 (optimum cure time), Ts2(Scorch time), and cure rate index (CRI) were determined according to

Fig. 1: FTIR for (MPGMA).

1

Scheme (1): synthesis of methoxy poly ethyl-englycol (MPGM).

1 Formulations of silica filled SBR compoundscontaining different concentrations of (MPGMA).Sample No. SB0 SB1 SB2 SB3 SB4 SB5 SB6ingredients (phr) silica 0 20 20 20 20 20 20( MPGMA) 0 - 2 - - - -( MPGMA) 0 - - 4 - - -( MPGMA) 0 - - - 6 - -( MPGMA) 0 - - - - 8 -( MPGMA) 0 - - - - - 10Base recipe: SBR, 100; stearic acid, 2; zinc oxide, 5 ; sulfur, 2; CBS,1

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ASTM:D 2084-07 using a Monsanto Oscillating Disc Rheometer model 100 (Akron, OH,USA). The measurements were carried out at 152 ± 1°C.

■ Tensile strength, tensile modulus at 100 % and 200% elongation and elon-gation at break were measured at room temperature on a tensile testing machine (Zwick /Roell Z010) accor-ding to ASTM: D412-06.

■ A swelling test was carried out by so-aking a specific weight of rubber sam-ples in toluene at room temperature for 24 h. The equilibrium swelling in toluene was determined according to the standard method (ASTM: D471-06).

Results and discussionsFor silica-filled rubber compounds, im-provement of the filler dispersion and prevention of curative adsorption on the silica surface are very important. Be-cause silica exhibits strong filler–filler interactions, the filler dispersion of a silica-filled rubber compound is worse than that of a carbon black-filled rubber compound [9-11]. The polar surface of silica makes hydrogen bonds with polar materials in a rubber compound. Be-cause the silica surface is acidic, it forms a strong hydrogen bond with basic ma-terials. The influences of the MPGMA concentrations on the dispersion, cure characteristics and physical properties of silica-filled SBR compounds were studied. The effect of MPGMA on the morphological, cure characteristics and of physical properties of SBR filled with silica was studied.

Dispersion of SilicaTo investigate the MPGMA effect on the dispersion of silica, we investigated the degree of filler dispersion with conven-tional SEM. Figure 3 shows scanning electron microscopy of the vulcanizate surfaces of compounds (SB 1 and SB 3). Compound SB 1 without MPGMA has a very poor dispersion of silica. Whereas compound SB 3 contain 4.0 phr MPGMA. For the silica-filled compound without MPGM, silica agglomerates are clustered, as shown in Figure 3a. For the silica-filled compounds containing MPGMA, silica is dispersed to some extent, as shown in Figure 3b. This can be explained by the adsorption of MPGMA onto the silica. The MPGMA adsorbed onto the silica surface reduces the filler–filler interac-tions so that the silica is dispersed well in the rubber.

Curing CharacteristicsCuring characteristics of the compounds were investigated at 152°C. Variations of the minimum torque, maximum torque, and ∆t (maximum torque -minimum torque) as a function of the MPGMA con-centrations are collected in Table 2. The most important of these parameters however, are the surface characteristics and the chemical active sites which de-termine the interaction between filler and polymer chains [12]. Table 2 shows that the maximum torque increased and minimum torque decreased respectively in all composites by the addition of MP-GMA. The rubber composite (SB3) con-tains 4 phr MPGMA has the highest MH and lowest value ML. The difference be-tween the maximum MH and minimum torque ML is a rough measure of the crosslink density of the samples and usu-ally known as ∆t. From this table, can be seen that ∆t increased with addition of MPGMA. This implies that the crosslink density of composites increases with in-creasing MPGMA concentration. The ∆t of a rubber compound mainly depends on the amounts of free curatives in the compounds. Therefore, it can lead to the conclusion that the MPGMA incorporat-ed in rubber mixes prevents the adsorp-tion of curatives. This is an indication of the improvement in the filler- polymer interaction. The ∆t of the mix containing MPGMA with 4 phr is higher than other

rubber mixes. The scorch time and cure time results in Table 2 show a decreasing value by addition of MPGMA.

Also, the changes in rheometric torque with filler can be used to charac-terize the filler-matrix interaction or re-inforcement, that is reinforcement factor αf calculated from the rheographs [4,13,14] and is given by:

∆Lmax(filled)- ∆Lmax (gum)∆Lmax (gum)

αf =

where ∆Lmax (filled) and ∆Lmax (gum ) are the changes in torque during vulcan-ization for the filled and gum compounds respectively.

From αf values listed in Table 2, it is clearthat the best mechanical properties can be expected from αf values in the case of silica with MPGMA. The optimum cure time TC90 and scorch time decrease by the incorporation of MPGMA. This may be attributed to the methoxy group of the MPGMA which s a precondition for the secondary reaction that takes place between the polar hydroxyl groups on the silica surface and moisture which in-creases the dispersion time of silica into the rubber. All these findings clearly indicate that the 4 Phr MPGMA is the most suitable concentration and can act as coupling agent for silica and secondary accelera-tor to rubber [15].

Fig. 3: Scan-ning elec-tron micros-copy (a) SB1 and (b) SB3.

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2 Curing characteristics of silica filled SBR compounds containing different concentra-tions of MPGMA.Sample No. SB0 SB1 SB2 SB3 SB4 SB5 SB6

Cure characteristics at 152 ±1 °CMinimum torque (ML) (dN) 3 5 5 6 12 10 8Maximum torque (MH) (dN) 17.5 21 35 70 59 48 43Maximum torque - Minimum torque (∆ torque) (dN)

14.5 16 30 64 47 38 35

Reinforcement factor (αf) 0 0.2 1 3 2.37 1.47 1.46Optimum cure timeTc90 (min) 13.75 11 8.5 7 7.5 7.5 8.5Scorch time Ts2 (min) 9 5.5 4 3 2.5 2.5 3Cure rate index CRI(min-1) 21 18.2 22.22 25 20 20 18.2

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Physical PropertiesThe mechanical properties are improved by the addition of MPGMA to the sili-ca-filled SBR compound. Figure (1-4) gives the mechanical properties of silica filled SBR with different concentrations of (MPGMA). From this Figures, one can be seen that the values of tensile strength, elongation at break and modu-lus at 100% elongation of vulcanizates containing MPGMA are higher than that

of the vulcanizate without MPGMA. The tensile strength, elongation at break and modulus at 100% elongation of vulcani-zates increases with the addition of MP-GMA up to 4 phr and then decreases slightly with increasing MPGMA content. This improvement in tensile strength, elongation at break and modulus at 100% elongation can be explained by fa-vorable interactions at the phase bound-aries by incorporation of the MPGMA, as

a result of the silica dispersion, even for intercalated morphologies [16].

Swelling propertiesFigure 5 showed that the swelling prop-erties of the investigated compounds were incorporated in SBR formulations (SB0–SB6) according to the usual regime and are presented in Table 2. It can be seen from this Figure that the sample with silica and without silica had higher equilibrium swelling than other samples containing MPGMA. Also it can be no-ticed that the sample (SB 4) with 4 phr MPGMA has the highest efficiency. This made it difficult for the solvent to pene-trate to the gaps between the rubber molecules and hence decreased the equi-librium swelling. Lower percentage of swelling indicates lower polymer toluene interaction and greater resistance to tol-uene due to the structure of the net-works formed during vulcanization. The rubber-filler interaction can be studied according to Lorenz and Parks [17];

Qf

Qr

= ae-z + b

where f and r refers to filled and unfilled vulcanizates. Q is defined as the grams ofsolvent per grams of hydrocarbon and is given by

Swollenweight - driedweightoriginalweight - formulaweight x 100

Q =

The crosslinking density (ν), mol/cm3 of SBR vlucanizates was determined on the basis of solvent-swelling measurements (toluene solvent for 24 h at 25 + 1°C) us-ing the Flory–Rehner equation [18,19].

12Mc

ν =

where: Mc is the molecular weight be-tween crosslinks (g/mol)

-ρVsVr1/3[ln (1-Vr) + Vr + χVr2]

Mc =

where: ρ is the density of rubbers (ρ SBR is 0.933 g/cm3),Vs is the molar volume of the solvent (toluene) = 106.35 cm3/mol, χ is the interaction parameter of rubbers where the of (SBR) is 0.446 and Vr is the volume fraction of swollen rubber that can be obtained from the mass and den-sity of rubber samples and the solvent. The filler-rubber interaction of sili-ca-filled styrene butadiene rubber (SBR) composites at various concentrations from MPGMA is shown in Table 3. It was found that Qf/Qr values were lower than unfilled and filled silica especially at 4

Fig. 4: The variation of (a) Tensile strength , (b) Elongation at break and (c) modulus at 200% elongation with the MPGM concentration.

4

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phr MPGM which is the optimum con-centration from MPGMA. Also, the crosslink density increases with addition of MPGMA.

Conclusion ■ The curing characteristics of silica

filled SBR composites showed that the scorch time (Ts2) and cure time (Tc90) decreased but maximum torque (MH) increased with the presence of MPG-MA in the composites.

■ The tensile strength, elongation at break and tensile modulus increased with addition of MPGMA.

■ The crosslink density and rubber-filler interaction were also improved. It can be concluded that MPGMA overall en-hance the mechanical properties of NR/PKS composites. 4 phr MPGMA is the optimum concentration in silica-SBR composites

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3 Results of the swelling measurements of silica filled SBR compounds containing different concentrations of MPGMA.Sample No. SB0 BS1 BS2 BS3 BS4 BS5 BS6Qf/Qr 1.0 0.986 0.635 0.601 0.636 0.632 0.640Vr 0.305 0.308 0.402 0.422 0.408 0.409 0.407Mc (g/mol) 2979 2903 1332 1238 1377 1359 1395ν x 10-4 (mol/cm3) 1.679 1.72 3.75 4.04 3.63 3.68 3.58Q, equilibrium swelling; Vr, volume fraction of rubber; Mc, molecular weight between the crosslink points; ν, crosslink density.

Fig. 5: The equilibrium swelling of the SBR formulations (SB0–SB6).

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