synthesis and characterization of chemically

SYNTHESIS AND CHARACTERIZATION OF CHEMICALLY CROSSLINKED STYRENE-BUTADIENE RUBBER NANOGELS AND THEIR EFFECT ON VARIOUS PROPERTIES OF THE RUBBER

SUMAN MITRA, SANTANU CHATTOPADHYAY, ANIL K. BHOWMICK* RUBBER TECHNOLOGY CENTRE, INDIAN INSTITUTE OF TECHNOLOGY, KHARAGPUR 721302, INDIA

ABSTRACT Nano-sized styrene-butadiene rubber (SBR) latex gels were prepared by pre-vulcanizing SBR latex with different sulfur to accelerator ratios. These gels were characterized by swelling studies, dynamic light scattering, atomic force microscopy, mechanical and dynamic mechanical properties. With the increase in sulfur to accelerator ratio, the gels had higher amount of crosslink density and gel content. Particle size distribution did not alter much in the crosslinked gels. Incorporation of these nanogels into raw SBR led to the considerable drop in viscosity of the gel filled systems under capillary melt flow conditions. However, the reduction in viscosity was found to be dependent on the loading and crosslink density of the nanogels. Energy dispersive X-ray mapping of sulfur was used to check the dispersion of these gels into raw SBR matrix. The die swell values of gel filled SBR were much lower than that of the raw SBR. The reduction in principal normal stress difference values combined with the reinforcement effect of the gels was found to be responsible for the lowering of die swell values. Scanning electron photomicrographs of extrudates of gel filled systems showed much-improved surface smoothness compared to the unfilled SBR. The mechanical and dynamic mechanical properties also showed excellent improvement in modulus with the addition of gels in the raw rubber. A new empirical relationship was proposed to explain the reinforcement properties of nanogels as viscoelastic fillers.

INTRODUCTION Most virgin rubbers often possess poor mechanical strength and processing difficulty. Rubber compounds are formulated with various reinforcing fillers to improve their physical properties and flow behavior. Rheological properties are important in understanding flow behavior of polymeric materials during processing. These have been well documented and reviewed in the literature for various polymers.1-5 Particle filled polymer melts are even more important from industrial application point of view. The incorporation of solid particles can both improve product performance and cut down costs. The rheological behavior of carbon black filled styrene butadiene rubber (SBR) has been studied extensively. It has been found that shear viscosity of high abrasion furnace (HAF) carbon black filled SBR decreases with the increase in the degree of carbon black dispersion (state-of-mix), as determined from Mooney viscosity and capillary rheometry.6 The effect of filler volume fraction on the non-linear rheology of SBR filled with carbon black and silica has been studied systematically over a wide range of deformation rates. At finite deformation, filler network structure is destroyed and the compounds exhibit a nonlinear rheology quite similar to the unfilled rubber.7 In a similar work, non-linear viscoelastic properties of silica filled SBR have been investigated and occurrence of Payne effect at a very low loading has been reported.8 Leblanc et al. have reported the rheological behavior of carbon black filled SBR using torsional strain controlled rheometer (rubber process analyzer).9 Shanmugharaj and Bhowmick have investigated the rheological properties of SBR filled with electron beam surface modified dual phase fillers.10 Similarly, many researchers have also reported the influence of different fillers on the physical properties of various conventional rubbers including SBR.11-13 However, there is no work reported so far on the subject of this investigation. Like any other particulate fillers, physical gels are added into the raw rubber to improve various properties including processability.14 Kawahara et al.15 have reported the effect of gel on green strength of natural rubber. Bhowmick et al.16 have studied the influence of gels on crys* Corresponding author. Ph: (91-3222)-283180; Fax: (91-3222) –220312; email: [email protected]

842

CHEMICALLY CROSSLINKED STYRENE-BUTADIENE RUBBER NANOGELS 843 tallization, stress relaxation and orientation properties of natural rubber. Rheological behavior of gel filled raw rubber is also documented in the literature. The flow characteristics of polar nitrile rubber containing divinyl benzene crosslinked gel particles have been studied and it has been found that the viscosity as well as the die swell decrease with increasing gel content at intermediate and high shear rates.17 Montes and Ponce-Velez have investigated the effect of gel on the extrusion behavior of guayule rubber.18 It has been observed that the viscosity increases on incorporation of gel. A slight decrease in the rate of relaxation in raw natural rubber with addition of gels has also been reported.19 In most of the above work, the authors have mainly used the physically crosslinked or entangled network gels. However, the use of chemically crosslinked gels of varying size and crosslink densities has shown unique rheological behavior in the case of raw natural rubber.20-21 Viscosity and die swell properties of raw natural rubber reduce considerably with the addition of chemically crosslinked gels at a particular concentration. This behavior has been modeled. It has also been shown that the presence of gels in raw rubber matrix significantly alters the mechanical and dynamic mechanical properties. In continuation of our earlier work on the effect of chemically crosslinked gels in non-polar virgin rubber matrix, raw SBR latex has been sulfur pre-vulcanized at four different sulfur to accelerator ratios to generate nano-sized gels. These gels have been characterized by various sophisticated techniques. Nano clays and nano silica are known to give interesting rheological behavior with various polymer matrices including SBR,22-23 because of their high surface area and other unique properties. However, the nanoparticles have very high modulus (in GPa). It would be interesting to understand the effect of the modulus of nanoparticles on properties of the filled system. It is only possible by using synthesized nanogel of varying degrees of crosslink density. Hence, the present study is undertaken. Effect of these nano-sized SBR latex gels on various properties of raw SBR has been studied in detail with particular reference to the melt rheology by capillary rheometry. EXPERIMENTAL MATERIALS

Styrene-butadiene rubber (SBR) latex having 30% total solid content (T.S.C) as well as 30% bound styrene content, with a pH of 10.50, was generously received as gift sample from the Apar Industries, Ankeleswar, India. The raw SBR latex had an initial gel content of 20% (i.e. physical gel), which could be broken down upon extensive shearing. Sulfur (S), zinc oxide (ZnO) and zinc diethyl dithiocarbamate (ZDC), all in 50% aqueous dispersion, were obtained from the Rubber Board, Kottayam, India and used as received. All other chemicals of laboratory reagent grade (LR grade) and doubly distilled water were procured from different local sources. PREPARATION OF SULFUR PRE-VULCANIZED LATEX GEL AND GEL FILLED RUBBER

Virgin SBR latex was compounded with S, ZDC and ZnO dispersions and subsequently prevulcanized. The formulations of different mixes for sulfur pre-vulcanization are given in Table I. In the crosslinking recipes, sulfur to accelerator ratio was varied from 0.5 to 3. Pre-vulcanization of the compounded latex was carried out at 80 °C for 2 h using water bath with constant gentle stirring in a round bottom flask fitted with a condenser.20 After the vulcanization, the gelled latex was sieved through a 250 mesh screen to remove impurities. Films from pre-vulcanized latex were prepared by casting on a level glass plate and dried at ambient temperature (25±2 °C) to constant weight. Finally, the latex films were vacuum dried at 50 °C for 8h. These films were used for characterization of gelled rubber.

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TABLE I RECIPE FOR PRE-VULCANIZATION OF SBR LATEX

Ingredients (dry wt. basis, parts) 30% SBR latex 50% Sulfur dispersion 50% ZDC dispersion 50% ZnO dispersion

SB0.5 100.00 0.60 1.20 0.20

SB1 100.00 1.20 1.20 0.20

SB2 100.00 2.40 1.20 0.20

SB3 100.00 3.60 1.20 0.20

Gel mixed rubber samples were prepared by adding pre-vulcanized SBR latex to raw SBR base latex (SB) at different concentrations (2, 4, 8 and 16phr) and stirred at a slow speed for 1h. Then, these were cast and dried, following the aforementioned procedure. SAMPLE DESIGNATIONS

Control base SBR latex was denoted by SB. Sulfur pre-vulcanized latex gels were given SBa designation, where ‘a’ is the ratio of sulfur to accelerator used in pre-vulcanization; for example SB2 denotes the sulfur-prevulcanized gel of SB with sulfur to accelerator ratio of 2. Gel mixed raw SBR systems were designated as SBa/b where ‘a’ has the same notation as above and ‘b’ represents the phr of sulfur pre-vulcanized gel added into the base SBR (SB) latex. SB1/8 denotes a gel mixed SBR latex system, where 8phr of sulfur pre-vulcanized latex gel having sulfur to accelerator ratio of 1, added in raw SBR latex. Likewise, SB0.5/16 means that 16 phr of sulfur pre-vulcanized gel having sulfur to accelerator ratio of 0.50 added to neat SBR. CHARACTERIZATION OF GELS AND GEL FILLED LATEX SAMPLES Gel fraction of the pre-vulcanized gelled latex films was measured by immersing the samples in toluene at room temperature (25± 2 °C) for 48 h (equilibrium swelling time that was determined from the experiments), and calculated from the weight of the samples before and after swelling as follows: Gel fraction = W2/W1

(1)

where W1 is the initial weight of the polymer and W2 the weight of the insoluble portion of the polymer. The results reported here are the average of three samples. Crosslink density, which is the number of network chains per unit volume, was determined from initial weight, swollen weight and final deswollen weight of the samples. The samples were swollen in toluene. The number of crosslink points, ν per unit volume, was calculated using the famous Flory-Rehner equation:24 ⎤ ⎡ 2 −1 ⎢1n 1 − υ r + υ r + χ1υ r ⎥ ν= ⎥ ⎢ ν V ⎢ ⎥ ν 3r − r 2 ⎦ ⎣

(

)

(2)

where χ1 is the polymer–solvent interaction parameter, V the molar volume of the solvent and υr the volume fraction of the rubber in the swollen gel. υr was calculated using the following equation:25

CHEMICALLY CROSSLINKED STYRENE-BUTADIENE RUBBER NANOGELS 845

υr =

(D

S

(D

S

−

)

− Ff Aw ρr −1

)

Ff Aw ρr−1

+ As ρ s−1

(3)

where Ds, Ff, Aw, As, ρr and ρs are deswollen weight of the sample, fraction insoluble, sample weight, weight of the absorbed solvent corrected for swelling increment, density of rubber and density of solvent, respectively. Dynamic light scattering (DLS) technique was used for the measurement of particle size of gels and its distribution. Before testing, latex samples were diluted to 0.1g/l concentration level using doubly distilled water, which was filtered through a Millipore Millex syringe filter (Triton free, 0.22μm). The DLS studies were carried out in Zetasizer Nano-ZS (Malvern Instrument Ltd, Worcestershire, UK) with a He-Ne laser of 632.8nm wavelength. The data were analyzed by inbuilt machine software. The mean hydrodynamic particle diameter (Zavg) and polydispersity index (PDI) were directly obtained from the machine software (as per ISO 13321). The morphology of gel particles was analyzed by atomic force microscopy (AFM). AFM studies were carried out in air at ambient conditions (25 oC, 60% RH) using multimode AFM, from Veeco Digital Instruments, Santa Barbara, CA. Topographic height and phase images were recorded in the tapping mode AFM with the set point ratio of 0.9, using silicon tip having spring constant of 40N/m. The cantilever was oscillated at it resonance frequency of ~ 280 kHz. The samples were diluted several times before testing with doubly distilled water. A drop of this diluted sample was placed on a freshly cleaved mica surface and allowed to surface dry before taking the image. Tensile specimens were punched out from the cast sheets of 1mm thickness, using ASTM Die-C. The tests were carried out as per the ASTM D 412-98 method in a universal testing machine Zwick Roell Z010 (Zwick Roell, Ulm, Germany), at a crosshead speed of 500mm per min at 25±1 °C. TestXpert II software (Zwick Roell, Ulm, Germany) was used for data acquisition and analysis. The same procedure was followed in the case of gel filled raw SBR samples. The average of three tests is reported here. The experimental error was within ± 3% in the measurements of tensile strength and modulus, and within ± 5% for elongation at break values. Dynamic mechanical properties of the gels as well as the gel filled raw SBR samples were measured as a function of temperature using the Dynamic Mechanical Analyzer DMA 2980 (TA Instruments, Newcastle, Delaware). The measurements were taken under tension mode in the temperature range -60 to 60 °C at heating rate 3 °C/ min and at a frequency of 1Hz. The peak value of Tan δ curves was taken as the glass transition temperature (Tg). Thermal Advantage software (TA Instruments, Lukens Drive, Newcastle, DE, USA) was used for data acquisition and analysis. MEASUREMENT OF RHEOLOGICAL PROPERTIES

The melt flow properties of the SBR samples with and without gel were measured by means of a Monsanto Processability Tester (MPT) (Monsanto Company, Akron, OH, USA) (barrel radius, 9.53 mm) which is a fully automated capillary rheometer. The entire barrel and the capillary assembly were electrically heated with a microprocessor-based temperature controller. The capillary used had a length to diameter ratio equal to 30 (length 30.0 mm; diameter 1.0 mm). The Bagley correction factor was found to be negligible10 and the apparent shear stress was taken as equal to the true shear stress. This is due to the 45° and 60° compound entrance angles of the capillary used, which are known to minimize the pressure drop at entrance. The preheat time for each sample was 5 min. The extrusion studies were carried out at 130 °C, at four different shear rates of 61.3, 122.5, 306.3 and 612.5s-1, respectively. The rate of shear variation was achieved by

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changing the speed of the plunger automatically. The pressure at the entrance of the capillary was recorded automatically with the help of pressure transducer. The apparent shear stress (τapp), • apparent shear rate (γapp) and apparent shear viscosity (ηapp) were calculated using the standard equations26 and details are given in our earlier work.20 For non-Newtonian fluid, the apparent shear rate is different from that given above for the Newtonian fluid, and the Rabinowich correction was used to determine the true shear rate, as given below:27 3n ′ + 1 γ˙ 4n ′ app

γ˙ true =

(4)

•

where γtrue is the true shear rate and n' is the flow behavior index defined by the following equation (for Power law fluid): n′ =

d 1n τ app

(5)

d 1n γ˙ app

•

the value of n' was determined from the regression analysis of the graph of 1n τapp versus 1n γapp. • The apparent shear viscosity (ηapp) was related with the γapp by the following expression using the Power Law model:

−1 ηapp = kγ˙ napp

(6)

Logarithmic form for Equation (6) may be written as log ηapp = log k + (n ′ − 1) log γ˙ app

(7)

The values n' and k were calculated from the initial linear region observed at lower shear rate. The die swell measurement was directly obtained from the MPT instrument through a microprocessor controlled laser beam assembly as per the following equation:

Die swell=

de − dc dc

X100

(8)

where de is the extrudate diameter and dc is the capillary diameter respectively. Maximum recoverable deformation γm was calculated using the following equations:28

γm =

1 (α −4 + 2α 2 − 3) 2C

(9)

CHEMICALLY CROSSLINKED STYRENE-BUTADIENE RUBBER NANOGELS 847 where, C=

3n ′ + 1 4(n ′ + 1)

(10)

where α is the die swell and n' is the flow behavior index. The principle normal stress difference, σE in an elastic body, was calculated by using the following general equation:29 σE =

(2 + γ m )γ m 2 τ ( app ) 2(1 + γ m )

(11)

where τapp is the apparent shear stress, γm the maximum recoverable deformation as mentioned earlier. SCANNING ELECTRON MICROSCOPY (SEM) STUDIES AND ENERGY DISPERSIVE X-RAY MAPPING (EDX)

The surface topography of the extrudate profile was studied by using a JEOL JSM-5800 (JEOL Ltd., Tokyo, Japan) scanning electron microscope operating at an accelerating voltage of 20 kV. The samples were sputter coated with gold prior to SEM study. The X-ray sulfur (S) mapping of the gel filled raw SBR rubber systems was recorded in Oxford ISIS 300 EDX system (Oxford Instruments, Oxfordshire, UK) attached to the JEOL JSM 5800 scanning electron microscope. The white points in the figures denote sulfur signals. RESULTS AND DISCUSSION EFFECT OF SULFUR TO ACCELERATOR RATIO ON GEL PROPERTIES

Nano-sized SBR latex gels were synthesized by pre-vulcanizing raw SBR latex with sulfur as curing agent and ZDC as cure accelerator and ZnO as an accelerator activator, following the procedure already described in the experimental section. The weight proportion or ratio of sulfur to ZDC was varied from 0.5 to 3, keeping the amount of ZDC constant. Four different sulfur to ZDC ratios of 0.50, 1, 2 and 3 were used in pre-vulcanization recipe of SB to generate distinct variation in the amount and nature of crosslinks. Dynamic light scattering was used to determine the particle size and its distribution (PSD) for different latex gel particles as well as the control latex. These values are compared in Figure 1. The gels and the raw SB latex have relatively wide PSD with particle diameters ranging from 35nm to 139nm. These values are independent of the nature of gels (Figure 1). In all the cases, 70-80% of the particles have sizes ranging from 70-110nm. The Zavg, the mean hydrodynamic particle diameter, of all the gel systems as well as SB is listed in Table II.

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TABLE II PHYSICAL PROPERTIES OF RAW SBR LATEX AND DIFFERENT SBR LATEX GELS Sample S:ZDC type

Z-avg

PDI

ratio diameter

Gel content(%)

density

T.S. (MPa)

EB (%)

(×10-4(gmolcm-3)

(nm) SB

Crosslink

Modulus at

Tg

100% elongation

(oC)

(MPa)

Log E' (MPa) -50 °C 30 °C

-

85.0

0.17

20.0

-

0.23

700

0.28

-39.2

555

0.70

SB0.5

1:2

94.0

0.18

89.0

0.80

2.05

430

0.80

-38.0

711

0.80

SB1

1:1

92.0

0.17

92.5

1.50

2.75

405

1.05

-37.1

766

1.24

SB2

2:1

87.0

0.20

95.1

2.20

3.05

370

1.17

-35.9

859

2.24

SB3

3:1

90.0

0.18

97.0

2.40

3.16

360

1.37

-31.0

1405

2.61

FIG. 1. — Particle size distribution of various gels and control SBR latex as determined by DLS method.

The Zavg diameter of SB is about 85nm, which increases slightly on sulfur pre-vulcanization as evident from the Zavg values of the different gels, for example SB0.5 has a Zavg of 94nm. However, these values do not follow any particular trend. It is very interesting to note that the both Zavg and polydispersity index (PDI), which is a measure of homogeneity of particle distribution, show similar pattern after sulfur crosslinking (Table II). Sulfur crosslinking during prevulcanization is believed to occur inside the latex particles and does not alter the particle size and PDI to a great extent.30 With the help of tapping mode atomic force microscopy (AFM) technique, individual gel particles can be seen very clearly as illustrated in Figure 2. Here, SB3 gel system has been chosen as a representative system. The particle diameters vary from 40nm to 150nm with a broad range of size distribution, which corroborates the earlier DLS findings. Particle diameters of most of the single gel particles are around 100nm and are exactly spherical in shape.


FIG. 2. — Tapping mode AFM image showing morphology of SB3 gel particles (Scan size 1.48μm X 1.48μm).

The gel content and crosslink density values of all the different gelled lattices were determined from equilibrium solvent swelling method using dried gelled latex film. The values of the gel content and crosslink density of all the crosslinked SB lattices are also tabulated in Table II. With the increase in sulfur to accelerator ratio, both the gel content and crosslink density increase. SB0.5 has a gel content of 89%, which increases up to 97% in SB3. A similar trend is also observed for crosslink density (0.08×10-4gmol/cc for SB0.5 to 2.4×10-4gmol/cc for SB3). The increment in gel content and crosslink density values with increasing sulfur to accelerator ratio can be attributed to the formation of sulfide linkages between the molecules, which lead to a three dimensional network structure. However, as the sulfur to accelerator ratio increases from 2 to 3, the increase in the amount of crosslinking tends to level off as evident from gel content and crosslink density values of SB2 and SB3. This could be due to the saturation of sites available for sulfide linkages. The mechanical and dynamic mechanical properties of the gelled lattices are reported in Table II. The tensile strength (T.S.) of the control SBR latex (SB) shows many fold increase after sulfur crosslinking. The elongation at break (EB) values decrease steadily with increasing amount of sulfur in the system. Both the increase in T.S. and reduction in EB values are related to the introduction of greater number of crosslinks initiated by the sulfide linkages. On similar ground, modulus at 100% elongation increases steadily with sulfur loading. The effect of sulfur crosslinking is also strongly reflected on the dynamic mechanical properties of different gels as compared to that of raw SB. With the increase in sulfur to accelerator ratio, the tan δ peak (considered as Tg here) shifts toward higher temperature (Table II). The increase in Tg values with the progressive increase in sulfur to accelerator ratio can be ascribed to the restriction imposed on the chain movement due to the crosslinking, as there is lesser number of free chains available at Tg. The storage modulus (log E') value of SB3 is about 2.6 times higher than that of the SB in the glassy zone and 3.7 times higher in the rubbery zone, due to increased crosslink density (Table II). RHEOLOGICAL PROPERTIES OF SULFUR CROSSLINKED GEL FILLED RAW SBR •

The dependence of steady shear viscosity on the true shear rate (γtrue) at 130 °C for the raw SBR and the representative (SB1 and SB3) nanogel filled systems is shown in Figure 3a-b. In all the systems studied, entirely pseudoplastic or shear thinning behavior has been observed. It is very interesting to note that, with the addition of nanogel in the raw SBR matrix, viscosity of the

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gel filled systems reduces considerably. This unique rheological behavior of crosslinked gel filled raw rubber systems has been reported earlier by the same authors, for the natural rubber matrix.20-21 However, in the earlier work, quasi nano-sized gels (~220nm – 410nm) have been used. It can be seen here that introduction of even a very low amount (2phr) of nano-sized gels enables a considerable reduction in the shear viscosity of raw SBR. The effect is more prominent at relatively higher shear rates. With the increase in gel loading (beyond 2phr), viscosity of the nanogel filled systems increases gradually up to 8phr, although viscosity of the gel filled systems remains lower than the control SB. However, at a particular shear rate of 122.5s-1, as evident from Figure 4, viscosity of 16 phr gel loaded samples is higher than that of the control SBR (SB). This is true for other shear rates also. SB3 type nanogels having higher loadings of sulfur and consequently higher amount of crosslink density produce the maximum drop in viscosity. The probable reason for such behavior is explained in the later sections.

a.)

b.)

FIG. 3. — Plots showing the variation in Log ηapp with Log γ true for (a) SB3 gel filled and (b) SB1 gel filled systems, respectively.


FIG. 4. — Dependence of viscosity on nanogel loading at representative shear rate of 122.5s-1.

As mentioned above, all the systems studied show well-defined shear thinning behavior and are represented by the power-law model for non-Newtonian flow regime. The pseudoplasticity index (n') and the consistency index (k) for SB0.5, SB1 and SB2 nanogel filled systems have been calculated from the linear logarithmic plots of viscosity vs. shear rate and are listed in Table III. The consistency index, k, which is essentially the viscosity at unit shear rate, shows a similar trend as viscosity. The pseudoplasticity index, n', which is a measure of non-Newtonian flow behavior, does not follow any definite trend, and varies from 0.38 to 0.44. However, it seems to increase with the addition of crosslinked gels. TABLE III CONSISTENCY INDEX (k) AND PSEUDOPLASTICITY INDEX (n’) OF DIFFERENT GEL FILLED SYSTEMS

Sample SB SB0.5/2 SB0.5/4 SB0.5/8 SB0.5/16 SB1/2 SB1/4 SB1/8 SB1/16 SB2/2 SB2/4 SB2/8 SB2/16

k (kPa.sn) 5.85 5.60 5.63 5.76 5.99 5.79 5.90 5.93 5.96 5.30 5.62 5.86 5.92

-n' 0.40 0.38 0.39 0.40 0.44 0.40 0.40 0.38 0.40 0.41 0.44 0.43 0.44

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In order to check the distribution of the nanogel particles into the raw SBR matrix, energy dispersive X-ray dot mapping (EDX) of sulfur has been carried out. Figure 5a-d illustrates the representative EDX micrographs of SB containing 2, 4, 8 and 16phr of SB2 gel respectively. It is apparent that the gels are uniformly distributed into the raw SBR matrix. As the concentration of gel increases beyond 4phr, the nano-sized gel particles start to show some sign of aggregation. This type of aggregation offers greater resistance to flow and is probably responsible for the gradual increase in melt viscosity beyond 4phr of gel loading in all the systems studied.

FIG. 5. — EDX sulfur mapping of (a) SB2/2, (b) SB2/4, (c) SB2/8 and (d) SB2/16 systems.

This unique rheological behavior of chemically crosslinked gel-filled raw rubbers is dependent on several inter-related factors. Introduction of crosslinks in the raw polymer matrix reduces the number of dangling chain ends, which accounts for the drop in the resistance to flow i.e. viscosity.21 In the presence of nano-sized gel particles, large polymer chains (large sized flow unit of the uncrosslinked polymer chains) can roll over or slip past the comparatively rigid nanogel particles leading to a substantial decrease in the melt viscosity under capillary flow condition.17 Due to the large interfacial area of the nano-sized gel particles, these are very effective in assisting the melt flow even at a very low concentration of 2phr (Figure 4). Similar effects were observed in our earlier work with crosslinked NR gels in NR matrix at relatively higher loadings.20 Persisting with the above line of reasoning, one can expect that, the increase in the crosslink density of the gel particles would further enhance the reduction in raw SBR viscosity. Indeed this has been found to be the case, as shown in Figure 6, where for a representative 2phr gel loading, viscosity of the gel filled raw SBR drops almost linearly with the increase in the crosslink density of gels used. The relative rigidity of the crosslinked gels plays a vital role in the chain roll over effect. It has been shown in our earlier publication that depending on a particular gel’s crosslink density which in turn governs the rigidity or flexibility, gels can either help chains to flow past over them or these can resist flow just like any other particulate inclusions.20 Also

CHEMICALLY CROSSLINKED STYRENE-BUTADIENE RUBBER NANOGELS 853 gel particles behave like viscoelastic fillers in rubber matrix, whose sizes are smaller than the size of the flow units.17 In the present case, although these effects are responsible for reducing the viscosity of the gel filled SBR at relatively low gel loadings, agglomeration of nanogel particles at relatively higher loadings counterbalances them. This aggregation of nano-sized gel particles after a loading of 4phr, as evident from EDX study, and increased gel particle-gel particle interaction offer greater resistance to flow and viscosity of the gel filled raw SBR rises gradually with gel loading. This explains the steady rise in the viscosity of gel filled raw SBR, after 4phr gel loading, in all the systems studied. However, it is apparent that the difference in the particle sizes of these gels does not influence the viscosity of the gel filled systems.

FIG. 6. — Effect of crosslink density of the gels on viscosity of the gel filled systems. EFFECT OF GEL ON THE DIE SWELL OF RAW SBR

The extrudate swell or die-swell behavior is manifestation of elastic characteristic of polymer melts during processing operations, such as extrusion and injection molding. Usually, the degree of swell of the extrudate is expressed using the die-swell ratio or simply die-swell as given in Equation (8) for a circular die. Figure 7a-b exhibits the variation in die swell with the experimental shear rates at 130 °C for representative two different gel systems, SB3 and SB1. Because of the unfilled and virgin nature of the SBR and its high molecular weight, all the systems including the control SBR show large values of die swell. Initially, with the increase in shear rate, die swell remains constant or reduces slightly. After that, the die swell increases almost linearly with the experimental shear rate. For example, SB has a die swell value of 87% at 61.3s-1 shear rate, which increases up to 93% at 612.5s-1. Generally, the deformation of the melt in extrusion increases with increasing shear rate due to the increase in the recoverable elastic energy.31 However, with the introduction of small amount of crosslinked gel into the raw SBR matrix, the die swell reduces to a great extent. It can be seen easily that for any given shear rate, the die swell values of 16phr gel filled SBR are more than 10% lower than that of unfilled raw SBR (e.g. SB3/16 and SB1/16 compared to SB).

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a.)

b.)

FIG. 7. — Die swell vs. Log γ° app plots for (a) SB3 and (b) SB1 gel filled systems.

Unlike the viscosity, the die swell decreases exponentially with the increase in gel loading at a representative shear rate of 122.5 s-1 as evident from Figure 8. In this case, gels having higher crosslink density e.g. SB2 and SB3 seem to be more effective in reducing the die swell than their low crosslink density counterparts. The presence of crosslinked gels in raw polymer matrix prevents the extensive chain uncoiling and recoiling during flow and lowers the ability of the chains to store and recover elastic energy.28 As a result, gel filled systems show much lower die swell values compared to the raw SBR.


FIG. 8. — Variation in die swell with the gel loading at a representative shear rate of 122.5s-1.

In order to understand the elastic effect within these systems more distinctively, the first or principal normal stress difference (σE) has been calculated within the experimental shear rates using Equation (11) and is related with the apparent shear stress by Equation (12). The logarithmic plot of principal normal stress difference against apparent shear stress, for the representative SB2 system, is shown in Figure 9.

( )

σ E = I τ app

m

(12)

Or

( )

log σ E = log( I ) + m log τ app

(13)

Principal normal stress difference values increase steadily with the increase in apparent shear stress. Addition of gels considerably reduces the principal normal stress difference values of neat SBR up to about 8phr of gel loading. After that it starts to increase at 16phr loading, although the values never cross the neat SBR level. Other gel filled systems also show similar trend. To further validate this outcome, the principal normal stress difference values for all the systems were calculated by the well known Tanner's equation32 as given below:

σ E = 2 τ app

[2{(α) − 1}] 6

(14)

where σE is the principal normal stress difference and α is die swell. Table IV lists the σE values calculated using Equation (14) and Equation (11) at two different shear rates of 61.3s-1 and 612.5s-1 for neat SBR and various gel filled systems. Increase in the shear rates leads to the increase in the principal normal stress difference. It can be seen that ini-

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tially with the addition of the gels the σE value decreases considerably. For example, at 61.3s-1 shear rate, SB has a σE value of 575.4 kPa as calculated using the Equation (14), which decreases to 457.1 kPa and 478.6 kPa in SB1/4 and SB1/8, respectively. However, it increases thereafter with further addition of gels i.e. SB1/16 has a σE value of 549.5 kPa. Principal normal stress difference values further reduce with the increase in crosslink density of the gels used at a particular gel loading (c.f. SB1/4, SB2/4 and SB3/4 have principal normal stress difference values of 457.1 kPa, 436.5 kPa and 426.6 kPa, respectively). The trend appears to be the same from both the methods. Thus, addition of gels reduces the principal normal stress difference values upto 8phr gel loading and is primarily responsible for the reduction in die swell values. However, the reduced die swell values observed at higher gel loading (16phr) is probably due to the reinforcement effect of the crosslinked nanogels which outweighs the increase in principal normal stress difference. As more and more crosslinked gel molecules are introduced in the virgin matrix, reinforcement effect of these gels prevents the extended rubber molecules from relaxing at the die exit. As a result die swell values show a decrease. Again, because of the roll-over or slip effect of these nanogels, gel filled polymer chains experience less amount of elastic strain compared to their unfilled counterpart which facilitates in bringing down the die swell value. This lowering of elastic strain33 contributes to the overall improvement in raw SBR’s processability.

FIG. 9. — Log-log plot of principal normal stress difference as a function of apparent shear stress at 130 °C.

CHEMICALLY CROSSLINKED STYRENE-BUTADIENE RUBBER NANOGELS 857 TABLE IV VALUES OF σE FOR DIFFERENT SYSTEMS AT TWO SHEAR RATES CALCULATED USING EQUATION (14) AND EQUATION (11) AT 130 °C

σE (kPa)* at shear rates of

Sample

SB SB1/2 SB1/4 SB1/8 SB1/16 SB2/4 SB2/16 SB3/4 SB3/16

61.3s-1 575.4 (93.3) 467.7 (75.9) 457.1 (75.9) 478.6 (79.4) 549.5 (95.5) 436.5 (72.4) 524.8 (93.3) 426.6 (72.0) 501.2 (89.1)

612.5 s-1 1621.8 (251.2) 1230.3 (190.5) 1202.3 (186.2) 1288.2 (204.2) 1584.9 (257.0) 1174.9 (185.4) 1548.8 (251.2) 1148.2 (182.0) 1513.6 (245.5)

* Values in the parenthesis have been calculated using the Equation (11). EFFECT OF GEL ON EXTRUDATE ROUGHNESS

Figure 10a-e compares the SEM photomicrographs of the extrudate of raw SBR and SB3 gel filled systems at a representative shear rate of 122.5s-1 taken at 35X magnification. It is apparent from the SEM photomicrographs that surface irregularity of the extrudates improves tremendously with the incorporation of crosslinked gel in the raw SBR. Due to the high elastic recovery or memory effect, the extrudate surface of raw SBR is very rough, irregular and rippled (Figure 10a). This type of behavior, which is commonly observed in various high molecular weight polymers, is better known as melt fracture. Melt fracture is the manifestation of the normal stress generated during extrusion through the capillary. Compared to the raw SBR (SB), SB3/2 and SB3/4 exhibit much reduced surface roughness. However, in the case of SB3/8 and SB3/16, even smoother extrudate surface with almost no sign of any melt fracture can be seen. Similar trends are observed in all the other systems. For any given system, extrudate roughness increases with the increasing shear rate (not shown here), indicating generation of higher degree of principal normal stress difference. Incorporation of crosslinked gel in the raw rubber reduces the principal normal stress difference values (as given in Table IV) upto 8phr loading, which leads to the lowering of die swell values and consequently giving rise to smoother extrudate surface. At still higher gel loading, it is the reinforcing effect of these nanogels, which overcome the disadvantage caused by the increase in σE values at this loading, thus greatly reducing the melt distortion.

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FIG. 10. — SEM photomicrographs of the extrudates for (a) SB, (b) SB3/2, (c) SB3/4, (d) SB3/8 and (e) SB3/16 systems.

EFFECT OF GEL ON THE MECHANICAL PROPERTIES OF RAW SBR

The effect of sulfur crosslinked gels on the maximum stress (σmax), modulus at 50% elongation and elongation at break (EB) of the raw SBR has been studied. The stress vs. elongation plots of raw SBR and a representative SB1 gel filled samples are shown in Figure 11. Raw SBR has a very low value of σmax of only 0.29MPa, which increases considerably with the addition of gels. It is 0.51MPa for SB1/16 sample (an increase of about 76%). In all the systems studied, maximum stress is witnessed at around 100% strain level and after that, the stress falls steadily with the increase in strain and hence the maximum stress values are used for discussion. The values

CHEMICALLY CROSSLINKED STYRENE-BUTADIENE RUBBER NANOGELS 859 of σmax, EB and modulus at 50% elongation for SB0.5, SB1 and SB2 gel filled raw SBR systems are tabulated in Table V. It is evident that the incorporation of sulfur crosslinked nanogels in the raw rubber matrix increases the σmax and the modulus with accompanying decrease in EB. As the crosslinked gels having higher tensile strength and modulus are introduced into the raw rubber, these values of gel filled systems rise proportionately. The reinforcing effect of the gel particles as well as their three dimensional network structure is responsible for the increase in maximum stress level. The moderate decrease in elongation at break with the increase in gel loading is due to the restricted mobility of the chains under tensile forces in presence of the crosslinked gels. The increase in σmax and 50% modulus is because of higher resistance to tensile deformation provided by the network structure of the gels, which is very prominent at 50% strain level. TABLE V MECHANICAL PROPERTIES OF VARIOUS GEL FILLED SYSTEMS AT 25 °C

System

SB SB0.5/2 SB0.5/4 SB0.5/8 SB0.5/16 SB1/2 SB1/4 SB1/8 SB1/16 SB2/2 SB2/4 SB2/8 SB2/16

Maximum stress (σmax) (MPa) 0.29 0.31 0.32 0.35 0.42 0.36 0.39 0.46 0.51 0.40 0.44 0.51 0.55

Elongation at break (EB) (%) 700 590 515 500 450 530 525 505 455 545 525 490 440

Modulus at 50% elongation (MPa) 0.25 0.27 0.28 0.30 0.38 0.32 0.35 0.40 0.46 0.34 0.38 0.45 0.49

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FIG. 11. — Tensile stress vs. elongation plot for the control and different SB1 gel filled systems at 25 °C.

In order to understand the reinforcement behavior of these nano-sized gels in raw SBR matrix, tensile properties of these gel filled systems are analyzed in detail following the GuthGold model of particulate reinforcement. Introduction of particulate fillers in a rubber matrix leads to an increase in modulus of the composite material. Smallwood introduced,34 for the first time, the following equation, using an analogy to the Einstein viscosity equation, viz.,

Ec = Em (1+2.5Φ)

(15)

where Ec is the modulus of the filled matrix and Em is that of the virgin matrix, and Φ is the volume fraction of the fillers. The constant 2.5 is applicable for spherically shaped particles. Later, Guth and Gold35 modified the above equation by taking into account the polymerfiller interaction, and proposed the following equation,

Ec = Em (1+2.5Φ+14.1Φ2)

(16)

In subsequent years, many more modifications have been introduced considering deviations in different systems. In the case of fillers, the particulate inclusions are basically considered as rigid, which have a considerable modulus difference with the virgin matrix. As an example, carbon black has a Young’s modulus of 1.0GPa36 and that of dickite clay is around 6.20GPa37 as obtained from atomic force microscopy (AFM) measurements, whereas modulus of elastomer matrix ranges between ~0.5MPa to ~20MPa in most of the cases. However, in the case of present systems, sulfur crosslinked nanogels have been used which are partially deformable and their moduli are slightly higher than that of the virgin polymer. For such viscoelastic particulate inclusions (fillers), which are at most semi-rigid, the improvement in the reinforcing ability of the nanogels could not be explained fully by the above equation. A new generalized equation has been proposed here correlating the Young’s Modulus and volume fraction of the gels. Ec = Em (1+2.5aΦ+14.1bΦ2) (17) where ‘a’ and ‘b’ are constants for a particular type of gel. The Young’s moduli of different gel

CHEMICALLY CROSSLINKED STYRENE-BUTADIENE RUBBER NANOGELS 861 loaded samples have been curve fitted using the above equation for all four types of nanogels used in this study and are shown in Figure 12. Here the nanogel particles are considered as spherical in shape and these exist in the matrix forming well-defined interfaces. These curves generate a set of ‘a’ and ‘b’ values (a = 3.71, 8.06, 12.03, 14.84 and b= 0.22, -4.05, -7.5, -10.22 for SB 0.5, SB1, SB2 and SB3 respectively) which are linear functions of crosslink density of the gels used. These can be represented as follows: a = 6.60X – 1.73

(18)

b = 5.26 – 6.19X

(19)

where X is crosslink density of the respective nanogels. It can be seen quite clearly that except SB0.5, all other gel filled systems initially give linear rise in Young’s modulus with gel loading, which then tends to level off (attains a steady state) at comparatively higher loadings. This onset of steady state shifts towards lower loading with increasing crosslink density of the gels used. This could be because of the higher reinforcement capacity of highly crosslinked gels. It may be noted here that, the values of ‘a’ and ‘b’ can be used to predict the reinforcement capability of any particular gel. Higher the value of ‘a’ and lower the value of ‘b’ for any particular gel, greater is its reinforcing capability. For SB0.5, which has the lowest reinforcing capacity among all, steady state region could not be attained within the experimental loadings.

FIG. 12. — Plots showing curve fitting of Young’s moduli of gel-loaded samples as a function of volume fraction of different nano-gels.

EFFECT OF GEL ON THE DYNAMIC MECHANICAL PROPERTIES OF RAW SBR

Figure 13a-b shows the loss factor and storage modulus as a function of temperature for raw SBR and a representative SB3 gel filled samples. Addition of crosslinked gels in the raw matrix shifts the glass transition temperature (Tg), from -39.2 °C in SB to -36.5 °C in SB3/16 (Figure 13a). The tan δ peak height reduces considerably and shows broadening effect with the increase in gel loading. Other gel filled samples also give similar trends. The increase in Tg of the gel filled samples is attributed to the restricted mobility imparted by the cross-linked nanogel particles. The storage modulus also increases with increasing gel content (Figure 13b). This follows the same trend of static tensile modulus as described earlier. As the temperature is increased, the modulus decreases sharply in the transition zone, beyond which its decrement slows down, which is known as the rubbery plateau zone. In the glassy zone, the storage modulus of SB3/16

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(1190 MPa) is almost double that of SB (650 MPa). The difference in modulus of gel filled rubber to that of raw rubber is even more pronounced in the plateau zone. The presence of crosslinks in the raw SBR matrix hinders the segmental motions of the polymer chains and therefore, Tg is shifted to higher temperature. Similarly, the increase in storage modulus upon gel loading is manifested due to the greater resistance to dynamic deformation imparted by the crosslinked network.

a.)

b.)

FIG. 13. — DMA plots showing temperature variation of (a) tan δ and (b) storage modulus for SB3 gel filled systems.

CONCLUSIONS In this research work, sulfur crosslinked nano-sized SBR latex gels were prepared and characterized by various methods. The rheological, mechanical and dynamic mechanical properties of the raw SBR and SBR containing sulfur crosslinked nanogels were investigated. From the above study, the following conclusions are drawn: The variation in sulfur to accelerator ratio during the prevulcanization of SBR latex has generated a variable crosslink density in the gels, with higher dose of sulfur ratio producing higher

CHEMICALLY CROSSLINKED STYRENE-BUTADIENE RUBBER NANOGELS 863 amount of crosslink density with concurrent improvement in mechanical and dynamic mechanical properties. Under capillary melt flow condition, all the gel filled and raw SBR systems show shear thinning or pseudoplastic behavior, which follows the power law model. Addition of these nanogels (even at a very low loading of 2phr) considerably reduces the apparent shear viscosity of raw SBR. These nanogels affect the shear viscosity in different ways depending on their nature, concentration and crosslink densities. The probable reasons for this unique behavior have been attributed to several factors such as reduction in dangling chains, roll over effect and nanogel agglomeration, as revealed from EDX study. However, the particle size of these gels does not cause any variation in property. The sulfur crosslinked nanogels also tremendously improve the die swell and surface roughness of the raw SBR. The die swell decreases progressively with the increase in gel loading. This is explained by calculating normal stress difference for all the systems. The decrease in principal normal stress difference values combined with the reinforcing ability of the nanogels is responsible for the improvement in die swell property. Reduction in die swell could also be due to the lesser amount of elastic strain experienced by the polymer chains. The addition of chemically crosslinked nanogels also improves the tensile strength and modulus of the gel filled rubbers compared to the pristine one. σmax and modulus at 50% elongation increase, whereas EB decreases with the increase in nano-gel loading. Presence of nano-gels shifts the Tg of raw SBR towards higher temperature with an accompanying increase in storage modulus. With the help of a new empirical relation, the reinforcement behavior of these nano sized viscoelastic fillers can be described. ACKNOWLEDGEMENT The authors would like to acknowledge the financial assistance provided by Department of Atomic Energy (DAE), Board of Research in Nuclear Sciences (BRNS), Mumbai vide sanction no. 2005/35/4/BRNS/516 dated 08-06-2005. We also greatly appreciate the help of Mr. K. Dinesh Kumar in the AFM study. REFERENCES 1N.

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