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Silica ⋅ Dispersion ⋅ Filler-Filler Inter-action ⋅ Hydrophobation ⋅ Mechanical Properties ⋅ Interaction

The effect of addition of hydroxyl ter-minated polydimethylsiloxane (PDMS-OH) and polyisobutylene succinic anhy-dride (PIBSA) on the dispersion of silica in E-SBR (emulsion polymerised SBR) is investigated. E-SBR / silica compounds with hydroxyl terminated polydimethyl-siloxane (PDMS-OH) and poly -isobuty-lene succinic anhydride were prepared and characterized for static, dynamic and morphological properties. Addition of PDMS-OH and PIBSA in silica filled SBR compounds exhibit improved silica filler dispersion, reduced Payne effect, and improved tan δ properties which can lead to an increase in fuel efficiency of the vehicle without compromise on wet grip property.

Wirkung von Polydimethylsilo-xan und PolyIsobutylenber-steinäureanhydrid auf die me-chanischen und dynamischen Eigenschaften von mit Silica gefüllten SBR-Compounds Silica ⋅ Dispersion ⋅ Füller- Füller- Wechselwirkung ⋅ Hydrophobierung ⋅ Mechanische Eigenschaften ⋅ Wechsel-wirkungen

Die Wirkung der Zugabe von Hydroxyl-terminiertem Polymethylsiloxan (PDMS-OH) und Polyisobutylenbern-steinsäureanhydrid (PIBSA) auf die Dis-persion von Kieselsäure in ESBR (Emul-sions SBR) wird untersucht. E-SBR / Sili-ca-Compounds mit Hydroxyl-terminier-tem Polysiloxan (PDMS-OH) und Polyisobutylen-Bernsteinsäureanhydrid wurden hergestellt und hinsichtlich sta-tischer, dynamischer und morphologi-scher Eigenschaften charakterisiert. Die Zugabe von PDMS-OH und PIBSA in mit Silica gefüllten SBR-Compounds zeigt eine verbesserte Dispersion der Kiesel-säure, einen verringerten Payne-Effekt und verbesserte tan δ-Eigenschaften die zu einer Erhöhung der Kraftstoffeffi-zienz des Fahrzeugs ohne Kompromisse bei der Nasshaftungseigenschaft füh-ren können.

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

1. IntroductionIntroduction of tyre labelling regulation and the tyre approval regulation

(EU 1222/2009) by European Regula-tion from 2012 leads to increase in de-mands for fuel efficiency tyres with low rolling resistance and higher wet grip. This calls for increased use of silica as filler compared with carbon black. Silica, if dispersed well in a rubber, can impart better (lower) rolling resistance besides better wet grip to the tyre, than carbon black.

Silica which is amorphous, consists of silicon and oxygen tetrahedrally bound into an imperfect three dimensional structure .The surface of the silica com-prises of siloxane and the silanol groups and the surface chemistry depends upon the amount of silanol groups, degree of hydration, the amount of adsorbed wa-ter and the surface acidity. Silica surface has three different types of silanol groups: Isolated, geminal and vicinal, which is responsible for high polarity of the silica and it can be analysed by 29Si-NMR experiments [1-2] and by infrared spectroscopy [3-4].

Due to the high polarity of the silica surface and ability to make hydrogen bridging, the silica particles aggregate to form quite strong agglomerates. This re-sults in the formation of a strong silica – silica network causing poor dispersion in rubbers, which leads to increase in roll-ing resistance and reduction in wet trac-tion [5].

There has been number of approach-es to improve silica dispersion and en-hanced filler – rubber interactions , such as using coupling agents [6], silica sur-face treatment [7], the use of a newly developed dispersible silica [8] , the use of polar functionalized rubber as com-patibilisers [9] and in situ sol-gel synthe-sis [10]. In this the most widely used method to maximize the reinforcing effi-ciency of silica is the use of bifunctional organosilanes mainly TESPT as a cou-

pling agent. [11]. But TESPT has some disadvantages. As per recent analysis by Blume and Thibault – Starzyk on the sili-ca – silane reaction mechanism (charac-terised by in situ infra red spectroscopy) only 25% of Si- OH groups is reacted with the silane due to limited accessibility of these silanol groups to incoming silane molecules. So number of silanes grafted on to the silica surface is limited [12].

In order to increase the hydrophoba-tion of the silica surface and to reduce the process issues various other chemi-cals agents such as alcohols, amines, polymer based chemicals has been inves-tigated in recent times.

Silicones and organo modified silox-anes can be of great help in processing thermoplastics. [13] The effect of such additives on PP filled with 40% (by wt.) of calcium carbonate filler was studied. The general conclusions in this study were: the silicones help in reducing the energy consumed during mixing and increased throughputs in extrusion etc., but, with a little loss in stiffness but better scratch resistance (due to slipping of the sharp sliding object on the polymer surface) and notched impact strengths.

Effect of a Polydimethylsiloxane and Polyisobutylene Succinic anhydride on the mechanical and dynamic Properties of Silica filled SBR Compounds

AuthorsP. Indumathy, B. Kothandaraman,Chennai, India Corresponding Author:B. KothandaramanDepartment of Rubber & Plastic TechnologyAnna University, MIT Campus, Chennai 600 044, IndiaE-Mail: bkraman@mitindia.edu

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Addition of polysiloxane with various functional groups to silica compound has been reported to have reduced the viscosity and improve in processibility of diene rubber based silica compounds [14]. In this patent, the silicones used, contain many functional groups which entail multiple synthetic steps.

PolyIsobutylene succinic anhydride is known dispersing agent in engine oil [15] and its metal functionalized version is applied as a process aid in silica com-pound. [16] Polyisobutylene succinic an-hydride(PIBSA) is used to make disper-sants that are used in tailor-made formu-lations to meet the challenging demands of engine oils. It is a critical engine oil additive which keep engines clean by dis-persing harmful debris generated during operation that can thicken the oil, cause wear, and plug the filter. It helps in reduc-tion of the formation of deposits on metal surfaces and inhibit soot agglomeration via stable micelle formation. Hence it will be interesting to see whether such an ad-ditive can help in obtaining a uniform dispersion of silica in a non-polar rubber.

The effect of a silicone with a simple structure like hydroxyl terminated poly dimethylsiloxane (PDMS-OH) and Poly-isobutylene succinic anhydride( PIBSA) on silica dispersion and hence, on the static and dynamic mechanical proper-ties has been rarely reported in litera-ture. This study can provide an improved understanding of, the application of hyd-roxyl terminated polysiloxanes and PIB-SA in tyre tread compounds, to obtain lower filler – filler interaction there by achieving low rolling resistance with no sacrifice in wet grip properties.

The present work explores the possi-bility of using hydroxyl terminated poly-dimethylsiloxane (PDMS-OH) and poly-isobutylene succinic anhydride (PIBSA) to reduce the filler – filler interaction and thereby, improving the silica dispersion and better mechanical properties.

2. Experimental

2.1 MaterialsSBR 1502 with a styrene content of 23.5 wt % and silica was provided by re-nowned commercial suppliers1. Hydroxyl terminated polydimethyl-

siloxane from M/s Sigma Aldrich.2. Polyisobutylene succininic anhdydride

M/s Mohini Organics Pvt Ltd., Mum-bai, India

All other rubber chemicals were industri-al grade scale and were used as received.

2.2 Modification of Silica Surface with TESPT/ PDMS-OH & PIBSATo investigate the effect of PDMS-OH & PIBSA in silica –TESPT compounds , silica was surface treated with the following chemicals:

2.2.1 Silica with TESPT10 gms of silica is dispersed in 100 ml of toluene. Then TESPT in toluene, was add-ed, as per formulation, and heated to 140°C for 2 hrs. The unreacted TESPT is removed by the extraction with ethanol. The compound was dried in vacuum ov-en for 24 hrs. @ 50 °C.

2.2.2 Silica with TESPT & PDMS-OH/ PIBSA

5 gms of TESPT treated silica and PDMS-OH was separately dispersed in toluene. Both the solutions were mixed and left for 16 hrs. stirring at ambient tempera-ture. The unreacted TESPT and PDMS-OH were removed by ethanol extraction. Then the specimen was dried in vacuum oven for 24 hrs. at 50°C. The same proce-dure was followed for PIBSA.

On these treated filler specimens, BET surface area measurement (for checking change in surface area), FTIR spectrosco-py (to analyse the formation of any new bonds) and TGA (to check for any water bound by the filler) were done.

2.3 Preparation of SBR/ Silica / TESPT/PDMS-OH /PIBSA compoundsTo examine the effect of PDMS-OH and PIBSA on the rubber compound, SBR/ silica compounds were prepared as per table 1 and mixing carried out according to table 2. The compounds were mixed in an internal mixer with the initial temper-ature setting to 60°C and final tempera-ture up to 150°C to complete the silani-sation of silica and silane.

3. Characterization

3.1.FT-IRFourier transform infrared (FTIR) meas-urement in ATR ( Attenuated total re-flection ) mode in Perkin Elmer spec-trometer was carried out to study the effect of PDMS- OH and PIBSA on Silica

1 Formula for SBR/ Silica / TESPT/PDMS-OH /PIBSA compounds.Materials Amount ( PHR)a

A (No TESPT b)

B (TESPT b)

C (TESPT b/PDMS-OH c)

D (TESPT b/PIBSAd)

SBR -1502 100 100 100 100SILICA 40 40 40 40TESPT 0 3.2 3.2 3.2PDMS-OH 0 0 3 0PIBSA 0 0 0 3Wax 1 1 1 16PPD 2 2 2 2Oil 7 7 7 7Stearic acid 2 2 2 2TBBS 1 1 1 1Zinc Oxide 3 3 3 3Sulphur 2 2 2 2a Parts per hundred parts of rubbers. d PolyIsobutylene succinic anhydrideb bis[3-(triethoxy silyl)propyl]-tetrasulfide e N-Tertiary butyl -2-benzothiazole sulfenamide c Hydroxy terminated Poly (Dimethyl Siloxane)

2 Formulation of the experimental compoundsTemperature & Time

Step 1-SBR Mastication-Addition of Silica + TESPT-Addition of PDMS-OH-other ingredients - mixing-Dump

60°C110°C120°C130°C140°C -145°C145°C

Step 2Mixing of step 1 compound 150°CStep 3Addition of Zinc oxide, TBBS, Sulphur 60°C for 3min

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–TESPT compound. The wavelength re-gion scanned was 4000 cm-1 - 600cm-1 with 4 cm-1 spectral resolution was ap-plied.

3.2 Nitrogen physisorptionNitrogen physisorption in silica coated with TESPT /PDMS-OH and PIBSA, was determined with Nova series BET analyser. The specific surface area of the specimen was calculated by fitting the BET isotherm from p/p0 = 0.06 to p/po = 0.25 of the measured BET iso-therms.

3.3 Thermo gravimetric AnalysisThermo gravimetric analyser of TA In-struments was used to analyse the bound rubber and the effect of coating of silica by the addition of PDMS-OH and PIBSA on Silica –TESPT compound at a heating rate of 10°C / min in nitrogen atmosphere.

3.4 Measurement of Mooney Viscosity and Payne effectMooney viscosity [ML(1+4), 100°C] was tested by using a MV 2000 (Alpha Tech-nologies) according to ASTM D1646.

The filler – filler interaction or Payne effect of the final batch compounds with the curatives, was analysed by using a Rubber Process Analyser (RPA 2000, Al-pha Technologies, USA) at 100°C, fre-quency 1 Hz and strains varying from 0 to 100%. The difference of storage shear modulus (G’) at 0.28 and 100% was cal-culated and reported as Payne effect.

3.5 Cure characteristics, Vulcanization, Tensile properties, Heat build-up (∆T), Dynamic set % and Abrasion resistance measurementsA Moving Die Rheometer (MDR) (MDR 2000, Alpha Technologies. USA) was used for this experiment. The specimens was prepared as per ASTM D5289 and tested

at 160°C for 60 minutes. Stress – Strain properties were measured on an universal testing machine (Zwick, Z005) with Ex-tensometer for 2mm thickness vulcanized sheets which were cut in to dumbbell specimen using Die type C, at a cross head speed of 500 mm/ min as per ASTM D412.

The Heat build-up and Dynamic set % of the specimens were analysed by Doli flexometer as per ASTM D623

3.6 Bound Rubber contentAnalysis of bound rubber (BdR) content was carried out, by swelling followed by TGA.

A specimen about 0.5 g was taken from the 1st stage compound before the curatives are added. The uncured speci-men was cut in to small pieces and im-mersed in toluene at ambient tempera-ture for 72 hrs. Renewal of toluene was done for every 24 hrs. The specimen on-ce again dried at 50°C for 24 hrs. and then analysed by TGA. For TGA approxi-mately 10 - 12 mg of this specimen was analysed using a thermo gravimetric analyser, heating the specimen from 50°C - 600°C, with heating rate 10°C /min, in nitrogen atmosphere. The bound rubber content was calculated according to the equation (1) [17]

%BdR = phr filler * ∆ extracted / Residue (1)

3.7 Cross link DensityThe crosslink density and the crosslink structure of the compound was analysed by using Flory – Rehner Equation (2)

𝛾𝛾 =1

2𝑀𝑀𝑀𝑀=

ln(1 − 𝑉𝑉𝑟𝑟) + 𝑉𝑉𝑟𝑟 + Х𝑉𝑉𝑟𝑟2

2𝜌𝜌𝑉𝑉𝜌𝜌(𝑉𝑉𝑟𝑟13 − 𝑉𝑉𝑟𝑟 /2)

(2)

Fig. 1: Schematic diagram of interaction of Silica with TESPT/ PDMS-OH / PIBSA

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Fig. 2: FTIR (a) and TGA (b) graph of pure silica(A) / silica coated TESPT(B)/ PDMS-OH(C) &PIBSA(D).

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Where V is the crosslink density , X is the polymer – solvent interaction parameter, Vr is the volume fraction of polymer in the swollen gel, Mc is average molecular weight between crosslinks, Vs is molar volume of the solvent and ρ is the densi-ty of the polymer.

3.8 Analysis of dynamic mechanical propertiesDynamic mechanical properties was an-alysed by MTS system. In MTS system, a sinusoidal stress was applied and the strain in the material was measured al-lowing one to determine the complex Modulus. Storage Modulus , loss modu-lus and tan δ were measured at 0°C, 30°C at 60°C using MTS system.

3.9 Surface Topography analysis by microscopyTo get a wider picture about the disper-sion of silica in rubber compounds , spec-imens were tested in an optical micro-scope ( Carl Zeiss, Germany).

To get a greater insight into the rub-ber – filler interactions, the specimens were tested in SEM ( EVO 18 model, Carl Zeiss, Germany).

4. Results and DiscussionPDMS-OH reacts with silica by hydrogen bonding. Strong multiple hydrogen bonds of 1 - 2 nm thickness are formed between –OH groups of the silica and polydimethylsiloxane chain, irrespective of its end group [18].

PIBSA has long alkyl chain which is non-polar which covers the silica surface and makes the silica hydrophobic and anhydride group which is polar, can in-teract with silica, which is polar.

4.1 FTIR Analysis & TGA analysisThe broad band in Figure 2(a) at 3700cm-1 – 3000 cm-1 due to stretching vibration of Si-OH indicates the presence of surface hydroxyl groups which are in hydrogen bonding with one another [19]. Decrease in vibration peak at 3700 cm-1 – 3000 cm-1 indicates the increase hydrophobation of the silica surface by the coating of TESPT , PDMS-OH and PIBSA on Silica.

Transmittance peak at 1066 cm-1 & 793 cm-1 is originated due to asymmetric and symmetric stretching of Si-O-Si in Silica. [20] Increase in the peak intensity in specimen B & C is due to coating of silica with TESPT and PDMS-OH which has Si- O-Si stretching. The FT-IR results indicate that PDMS-OH has interacted with the silica surface and increased its hydrophobation.

TGA was carried out to analyse the increase in hydrophobation of the silica by the addition of PDMS-OH and PIBSA. For silica specimens in TGA, a weight loss below 200°C is due to removal of adsor-bed water molecules, and the weight loss in the region from 200 - 700°C is consistent with the dehydration of sila-nol groups on silica surface [21]. In Figure 2(b) the weight loss from 50°C to 200°C corresponding to adsorbed water is much reduced for specimen C and D in comparison with specimen B which indi-

cates that the surface of silica becomes more hydrophobic by the addition of PDMS-OH and PIBSA (it leads to lesser amount of moisture absorbed by the fil-ler surface).

4.2 BET AnalysisThe interaction of the PDMS –OH and PIBSA on silica surface can be evaluated by BET( Brunauer – Emmett – Teller) analysis.

Coating of the silica with the coupling agents/covering agents reduces the sur-face area of the silica due to the binding of these agents on silica [22]. BET surface area of silica grafted with PDMS-OH and PIBSA in table 3 shows decrease in sur-face area compared with pure silica and TESPT treated silica, which is due to the interaction of PDMS-OH / PIBSA on silica surface.

4.3 Mooney Viscosity, Cure characteris-tics, Payne effect & Bound Rubber Figure 3(a) shows the Mooney viscosity of the compounds. Decrease in Mooney viscosity indicates good silica dispersion of the rubber compounds [23] and easy processing. The Compounds with PDMS-OH and PIBSA show a decrease of Moon-ey viscosity, compared to the regular TESPT compound. This shows that filler – filler interaction is reduced in the PDMS – OH and PIBSA compounds through the interaction of PDMS-OH and PIBSA with the silica surface.

The mixing graph in Figure 3(b) gives an insight to what happens during mixing.The energy consumption between 125 sec to 220 sec is less for specimen C com-pared with Specimens A, B and D. This may be due to increase in hydrophoba-

3 BET Analysis ResultsS.N0 Specimen m2/g1 Silica 2852 Silica grafted with TESPT 2093 Silica grafted with TESPT /

PDMSOH164.7

4 Silica grafted with PIBSI 190.54

Fig. 3: (a) Mooney Viscosity of speci-mens .(b) Power consumption curve with no TESPT(A)/ TESPT(B)/PDMS-OH(C)/PIBSA(D).

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tion effect of silica compound which re-duces filler – filler interaction by the ad-dition of PDMS-OH, which is in line with the Mooney viscosity values.

The cure characteristics of silica com-pounds which was analysed using MDR are shown in Figure 4 (a) and Table 4. The minimum torque (Tmin), which is related to the degree of filler agglomeration (i.e flocculation ) [24] is lower in specimen C and specimen D compared to specimen B, which has only TESPT.

This indicates that the addition of PDMS-OH and PIBSA causes an increase of the hydrophobation of silica and lowers

the filler – filler interaction compared with the compound containing TESPT alone.

In addition to this, lower T2(min) ,T90 (min) and higher ∆T (Tmax - Tmin i.e., change in torque) which represent the high cross link density [25] is observed in Speci-mens C& D against Specimen B. This is probably because of lesser levels of ab-sorption of the acclerators by silica due to covering of the filler surface by PDMS-OH and PIBSA.

4.4 Payne effectThe magnitude of filler – filler interac-tion was ascertained by RPA with strain

amplitude from 0 to 100% . The variation of the G’ at 0 and 100 % strain amplitude (∆G’) can be related to the filler-filler networking . The values are as shown in Figure 4(b).

Specimen C and specimen D is show lower Payne effect compared with specimen B with TESPT and specimen A with no coupling agent. This lower Payne effect indicates lower filler – fil-ler networking which is due to impro-ved dispersion and distribution of sili-ca filler [26] by the addition of PDMS-OH and PIBSA to silica TESPT com-pound.

Fig. 4: (a) Cure characteristics of specimen A, B, C & D and (b) Pay-ne effect of Silica with Nil TESPT(A)/ TESPT(B) /PDMS-OH(C) /PIBSA(D).

4

4 Cure characteristics of the compoundsSpecimen T2 (min) T90 (min) Tmin (N-m) Tmax (N-m) ∆T (Tmax - Tmin) (N-m)A 3.06 33.23 0.67 2.46 1.79B 3.61 24.06 0.29 1.79 1.50C 4.02 13.55 0.16 1.80 1.64D 3.54 12.58 0.16 1.76 1.60

5 Bound rubber % by TGACompound Bound Rubber %A 17.8B 19.1C 32.5D 36.1

Fig. 5: TGA curve of bound rubber %

5

Fig. 6: Vulcanizate structure of specimen A, B, C & D

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4.5 Bound Rubber % By TGAThe Bound rubber content (in %) by TGA was calculated according to the equa-tion(1).

Bound rubber content of the speci-mens are displayed in Table 5 and the re-spective TGA graph is shown in figure 5.

Specimen C & D shows high bound rubber content when compared to speci-men A and Specimen B. The high bound rubber content is due to less filler floccu-lation and improved silica dispersion [27] by the addition of PDMSOH and PIBSA along with TESPT. 4.6 Cross Link densityThe crosslink density, along with the types of cross links in the vulcanizate (Poly sulphide crosslink, Di- sulphide crosslink and Mono sulphide) has influ-ence on the physical and dynamic prop-erties of rubber vulcanizate [28].

The vulcanizate structure of com-pounds A, B, C and D are shown in Figure 6 and Table 6.

The results reveal that the compound C with PDMS-OH is showing significantly higher total crosslink density compared with specimens B and A.

In addition to total crosslink density, the specimen C shows significantly high-er mono sulphide and di- sulphide cross-links which can improve ageing perfor-mance.

Remarkable improvement is noticed in polysulphide crosslinks which is respon-sible for very good mechanical properties such has flex resistance and lower heat built up. This result is in line with the cu-ring characteristics, which impart higher ∆ torque value for specimen C.

The analysis confirms that the additi-on of PDMS-OH to the TESPT silica com-pound improves Silica filler dispersion by hydrophobating the silica surface. This increase in hydrophobation reduces the silica agglomerate formation which may inhibit the adsorption of vulcani-zing chemicals [29] leading to improved crosslink density and crosslink struc-ture.

Not much difference in crosslink den-sity and crosslink structure is noticed in specimen D (which contains PIBSA) when compared to the specimen B which is having only TESPT

4.7 Stress – strain propertiesStress strain properties of the compound by the addition of PDMS-OH and PIBSA are depicted in Table 7 and Figure 7. Higher value of M100% / 300% in speci-men C indicates better rubber – filler interaction by the addition of PDMS-OH [30].

The specimen C shows increase tensile strength value compared to specimen B with TESPT due to increase in filler - rub-ber interaction because of improvement in silica dispersion. The increase in stiff-ness leads to small decrease in elongation at break. Specimen D does not show much

6 Cross link density measurementsspecimens Mono Sulphide

(Mol/cc)Di sulphide Mol/cc

Poly Sulphide Mol/cc

Total cross link ( Mol/cc)

A 1.89 0.45 0.87 3.21B 2.86 1.1 0.98 4.94C 3.18 2.07 2.05 7.3D 2.34 1.01 1.09 4.44

7 Stress strain properties of the specimen with Nil TESPT/ TESPT/ PDM-OH and PIBSASpecimens M100% (MPa) M300% (MPa) TS (MPa) EB % Tear (KgF/cm)A 1.5 3.60 13.89 717 51.28B 1.56 5.18 16.89 572 59.92C 1.89 7.58 17.06 464 59.05D 1.51 5.03 16.41 584 53.85

Fig. 7: (a) Stress strain properties of the specimens and Figure 7(b) Tear reistance of specimen A, B, C & D.

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8 Heat Built Up / Pprmanent set % and volume loss of the speciment A/B / C& DSpecimen Heat Built up°C Permanent set% Volume loss (mm3)A -* -* 258B 38.0 9.5 122C 21.2 2 100D 20.7 1.8 131*specimen deformed more than the machine’s limit and hence the testing could not be done in the machine

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9 Dynamic mechanical properties of the compoundsG’ at 0°C G’ at 60°C G’’ at 0°C G’’at 60°C tan δ at 0°C tan δ at 60°C

A 14.1 7.87 2.74 1.89 0.1942 0.2407B 12.4 7.09 2.67 1.29 0.2149 0.1817C 12.27 7.03 3.10 1.07 0.2524 0.1518D 11.45 6.57 3.03 1.05 0.2646 0.16

improvement in tensile strength and mo-dulus compared to specimen C.

Better tear resistance is observed for the compounds with PDMS- OH and PIB-SA in Table 7 & Figure 7(b). This may be due to better filler dispersion and impro-ved crosslink density.

4.8 Heat Build up/ Permament set and abrasion resistanceCompounds C with PDMS OH exhibit lower ∆T (change in temperature of com-pound with time) compared to com-pound B. Compound D also exhibits slightly lower heat built up compared

with compound B. Lower ∆T ( heat built up) indicates reduced filler – filler inter-action [31] i.e improved silica dispersion, which leads to lower heat dissipation. Figure 8 & Table 8 shows the ∆T and ∆T vs Time.

Permanent set % of the compound depends on higher crosslink density , in-creased rubber –filler interaction and less filler – filler interaction. Lower per-manent set in % of compound C and compound D indicates that addition of PDMS-OH and PIBSA reduces filler – filler interaction. Permanent set values are shown in Figure 9 and Table 8.

4.9 Abrasion resistanceTable 8 and Figure 10 reveal that speci-men C has low volume loss compared to A, B and D. The abrasion loss depends on modulus and filler dispersion [32]. The improved dispersion and increase in modulus in specimen C also results in good abrasion resistance of the com-pound. In contradiction with this, Speci-men D shows higher volume loss inspite of having good filler dispersion. This needs to be investigated.

4.10 Dynamic PropertiesFor high performance tyres ,the com-pound should exhibit high tan δ value at 0°C and low tan δ value at 60°C which specify better wet grip and low rolling resistance [33]. It is observed that for a 2.5% strain, compound C and D exhibit 17.5% and 23.1% higher tan δ value at 0°C shows better wet grip properties and 16.4% and 12% lower tan δ value at 60°C

Fig. 8: (a) Heat Built up of Speci-men B, C &D(b) Delta T°C with respect to time of speci-men A, B, C &D is displayed.

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Fig. 9:(a) Permanent set% of specimen with respect to time.(b) Permanent deformation with respect to time.

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showing low rolling resistance against compound B with TESPT as depicted in table 9 & Figure 11. This signifies better filler dispersion and lower heat dissipa-tion by the addition of PDMS-OH and PIBSA to silica along with TESPT.

4.11. Dispersion MorphologyFigure 12 represents the measurement of dispersion of the vulcanizate in opti-

cal and SEM microscopy. As anticipated, the dispersion of the silica is improved in specimen C and specimen D with the addition of PDMS-OH and PIBSA compa-red with the compound with only TESPT. The dispersion results are in line with the Payne effect values. The increase in hydrophobicity of silica by the addition PDMS-OH and PIBSA along with TESPT, which reduces filler – filler interaction,

is responsible for the enhanced disper-sion.

The interactions between silicone rubber and silica filler were discussed in [34]. The study involved measuring the changes in rheological properties of silicone-silica mixes over long peri-ods of time. The authors conclude that polymer adsorption onto the surface plays an important role in determining

Fig. 12: (a) Optical microscope photographs of specimens (b) SEM micrographs of specimen with Nil TESPT(A), TESPT( B), PDMS-OH(C) & PIBSA(D).

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Fig. 10: Abrasion loss of specimen

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Fig. 11: Dynamic mechanical properties of the SBR composites.

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the overall stability of the silica-silicone systems. Significant changes in the dis-persion of filler were observed over a period of time and these changes were found to be dependent on polymer mo-lecular weight and filler surface treat-ment.

The addition of a surface treating agent (either physically adsorbed or co-valently bound to the silica surface) which inhibits the adsorption of poly-mer was found to significantly increase the stability of the dispersion of filler and therefore the rheological properties. Since the process times for mixing a rubber-silica compound are of the order of seconds to hours and the time to reach equilibrium for the adsorption process can be months or years, one should expect corresponding changes in the rheological behaviour of these sys-tems as the amount of adsorbed poly-mer changes, especially if the entangle-ment and bridging interactions are im-portant. Here, the silicone taken has a low molecular weight. This can lead to the attainment of equilibrium very quickly. The adsorption of silicone oil (low molecular wt PDMS) by the silica was believed to be accompanied by for-mation of bridges between silica partic-les through the silicone chains. However, for very low molecular weight systems or in systems with the surface sites arti-ficially covered by a surface treating agent (hexamethyl disiloxane, in their study), the adsorption could reach equi-librium very quickly.

This could have lead to reduction of the Mooney viscosity values and torque on the rotors in mixing equip-ment and in the MDR, as seen in our study.

A few publications have discussed the possibilities of enhanced interactions between silica based fillers like fly ash and solid glass spheres with epoxy resins if the resin was modified by amine con-taining PDMS, from mechanical property measurements and free volume measu-rements [35-36]

Literature on the use of PIBSA as a dis-persant for inorganic fillers in rubbers is rare. As noted earlier, the patent by Wang and Foltz [15] describes the use of metal (aluminium) functionalised PIBSA in sili-ca filled rubber compounds and the in-ventors claim that they could reduce the use of plasticisers in these compounds using this additive. They could also im-prove the wet traction of such com-pounds.

5. ConclusionIn conclusion, it may be said that the addition of PDMS-OH and PIBSA increas-es the hydrophobicity of silica by reacting with the surface of the silica. The in-crease in hydrophobation and reduction of the filler – filler interaction may be the reason for improved filler dispersion (hence lower Payne effect). The absorb-ance of the curatives may also lead to in-creased cross link density. These results may reflect in improved tyre properties such as wet grip, rolling resistance , and abrasion resistance besides improving the processibility of the compound with PDMSOH. For specimen D with PIBSA, improvement is noticed in properties re-lated to better wet grip but not in abra-sion resistance. The findings also indi-cate that the improvements in mechani-cal properties and processing parame-ters, of the compounds is more with the silicone (PDMS-OH) rather than with PIB-SA. This may be due to greater level of hydrophobicity of the filler surface caused by the silicone compared with PIBSA.

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AcknowledgementsThe authors thank:

■ MRF Ltd, Chennai, India. for providing the experimental facilities for carrying out this work.

■ Department of Science and Technolo-gy, Government of India(FIST Scheme) for funding and facilities