Rheology and morphology of latex-based SBR composites: An investigation on the effect of active fillers (aluminum triacrylate and aluminum trimethacrylate) and nanoclay

Abstract

Organic salts of aluminum triacrylate (ALTA) and aluminum trimethacrylate (ALTM) known as active fillers were made through a two-step synthesis and identified using Fourier Transform Infrared spectroscopy (FTIR). The resulting salts and a modified nanoclay (Cloisite 15A) were prepared in the form of slurry and mixed with the styrene-butadiene rubber (SBR) latex. The morphological and rheological properties of the composites were evaluated as a function of type, concentration of filler, and the preparation method of filler slurries, i.e. using magnetic stirrer and ball mill. The morphological investigations by Scanning Electron Microscope (SEM) images presented that ball milling led to finer size and more uniform distribution of fillers in the composites. The rheological measurements of composites containing ball milled ALTA and ALTM clearly exhibited significant improvement in storage modulus being comparable to that of clay. The complex viscosity showed a Power law behavior with an index about n = 0.23-0.25 for ball milled ALTA and ALTM being quite lower than that of magnetic stirred (n = 0.36) facilitating the processing of composites at high shear rates. The tan δ measurements displayed that ball milled active fillers have lower viscous behavior compared to magnetic stirred counterparts at medium frequencies offering higher elastic properties for polymers. The startup experiment indicated that composites loaded with ball milled active fillers presented higher elastic properties than those containing magnetic stirred active fillers.

Keywords

Styrene-butadiene rubber (SBR)active filler nanoclay rheology morphology

Introduction

Fillers and particularly nanofillers have greatly contributed to the development of polymers with higher physical and mechanical properties and have been the subject of numerous research.^1,2 Various fillers may be used to tune the desired properties of composites. Irrespective of the type of filler, the distribution and dispersion of filler particles along with interfacial adhesion between the polymer molecules and filler particle surfaces are commonly responsible for the enhancement of physical and mechanical characteristics of the polymer composites.^3–5 The adhesion may achieve by modifying the polymer or treating the filler surface using some low molecular weight chemicals.^6,7 A relatively new and less investigated methods of increasing polymer chain-filler adhesion is to use active fillers as they are able to make links with the polymer chains. Active fillers offer comprehensive mechanical reinforcement to rubber compounds through the self–polymerization and also physico-chemical interactions with polymer matrix. Nevertheless, they have received less attention. The reinforcing properties of active fillers start at small amounts of the fillers while those of common fillers and carbon black occur above a minimum concentration known as “critical volume fraction”. Below the critical volume fraction, the changes in matrix properties are either small or null.^8–10 Above this minimum, as the filler content is increased, typical properties such as tensile strength, hardness, elastic modulus and viscosity generally improve but elongation at break decreases. The high volume of filler also results in higher density of polymers. Contrary to the conventional reinforcing fillers, active fillers are able to increase modulus, tensile strength and elongation at break, simultaneously.^11–15 In recent years, rubbers have benefited from organic-metal salts of unsaturated carboxylic acid as reinforcing fillers.^16,17 In the presence of a peroxide curing agent, organic-metal salts of unsaturated carboxylic acid are polymerized during vulcanization process. The resulting polymerized molecules may be available as the separate homopolymers in the rubber matrix, or grafted to the rubber main chain molecules and create copolymers. The copolymer molecules play the role of a compatibilizer between the filler and polymer chain assisting in improving the mechanical properties of vulcanized rubber. Therefore, the mechanical properties of the resulting vulcanized system are highly affected by the salt cross-links. Organic-metal salts of unsaturated carboxylic acid may directly be added to the rubber matrix or be prepared through an in situ method by neutralizing oxides (hydroxides) of the metal and acid. Studies show that most of the rubber fillers, particularly aluminum compounds, can improve the processability of the matrix and positively affect the properties of final product.^11–13 Few research have focused on the synthesis of the organic metal salts of unsaturated carboxylic acid and investigation of the properties of resulting polymer composites, particularly elastomer composites, reinforced by this type of fillers.^18–21 In a study conducted by Yamada et al., thioglycolic acid modified with metal hydroxide was used as an active filler to improve the mechanical properties of NBR. A good distribution of active filler in raw rubber was reported.^14,22 Despite a decreased rubber cure state in the presence of large amounts of active filler, they reported a significant improvement in tensile strength and abrasion resistance for NBR samples containing metal thioglycolate active filler and silica particles compared to non-filled samples. In another study, Yamada et al., prepared the active aluminum carboxylate by the reaction between aluminum chloride and sodium carboxylate having mercapto group. The NBR and SBR compounds of the prepared active filler showed substantial enhancement in tensile properties of rubber vulcanizates with higher properties for thiosalicylate compared to aluminum thioglycolate.¹⁵ The properties of EVM (Ethylene Vinyl Acetate Monomer) vulcanizates filled with magnesium salt of methacrylic acid, added directly to the EVM rubber matrix, were studied by Du et al. They reported that the reinforcing properties of magnesium methacrylate active filler are higher than those of HAF carbon black.^23,24 Various reports have presented the improvement in properties of rubbers using organic-metal salts.^25–28 Researches have indicated to reach good mechanical properties, the in situ preparation method has priority over the direct addition of a metal salt.²⁹ The comparison of the effect of aluminum triacrylate active filler and nanoclay on mechanical properties of EPDM compounds exhibited higher properties for aluminum triacrylate vulcanizates. Compared to nanoclay, the increase in the active filler content led to higher abrasion resistance and tensile properties attributed to the polymerization of active filler during the vulcanization stage.^30,31

Despite the studies on the active fillers and their applications in polymers such as reactive aids in rubber curing systems, indicating their obvious priority over the common reinforcing fillers and nanofillers, little has been reported on the rheological behavior of active filler reinforced polymers, particularly rubber gum/active fillers. The present research studies and compares the effect of active fillers (aluminum salts of unsaturated carboxylates) and nanoclay on morphological and rheological properties of the SBR composites prepared from SBR latex and filler slurry mixtures. Two active fillers of aluminum triacrylate and aluminum trimethacrylate prepared in the laboratory along with a nanoclay, Cloisite 15A, were used. To our knowledge, this is the first time that the styrene butadiene rubber latex with no curing system (gum SBR) is reinforced with aluminum triacrylate (ALTA) and aluminum trimethacrylate (ALTM), and its rheological properties are investigated. Since the preparation of these active fillers is considered an effective step towards achieving cheaper and more effective fillers in the field of elastomer reinforcement, the investigation of the rheology of rubber gum/active fillers could shed light on how and how much these fillers affect the rheology-property relationship of the rubber compounds. The results and discussion section is divided into three main parts. The first part is devoted to the characterization of prepared active fillers. In the second part morphological results are presented. The third part discusses and compares the rheology of active filler composites and clay nanocomposites.

Experimental

Materials

The raw materials used in this research are listed in Table 1. Organic metal salts including aluminum triacrylate and aluminum trimethacrylate were prepared in the laboratory. The double-distilled water was used as the dispersing media.

Table 1.

Materials along with their sources.

Material	Characteristic	Manufacturer
Acrylic acid	Laboratory grade	Merck
Methacrylic acid	Laboratory grade	Merck
Sodium hydroxide	Laboratory grade	Merck
Aluminum chloride hexahydrate	Laboratory grade	Merck
Ethanol	Laboratory grade	Merck
Styrene butadiene rubber latex (SBR) latex 1502	Industrial (total solid content: 25% styrene content: 23%)	Basparan, Bandar Imam Petrochemical complex
Nanoclay	Cloisite 15A	Southern clay, USA
Sodium dodecyl sulfate (SDS)	Laboratory grade	Merck

Synthesis of active fillers

An amount of 40 g (1 mol) sodium hydroxide was added to a 500 mL flask containing 150 mL stirring double-distilled water kept in an ice and water bath. Using a burette, 72 g (1 mol) acrylic acid was slowly added to the above mixture, resulting in a solution of sodium acrylate. In another flask, 80 g aluminum chloride hydrate was dispersed in 250 g double-distilled water held in an ice and water bath. The slow addition of sodium acrylate solution to the flask of stirring aluminum chloride dispersion led to gradual formation of aluminum triacrylate precipitate. The resulting precipitate was washed with a solution of water/ethanol (70/30) several times and then dried in a vacuum oven (LVO-1003 - Lab. Tech. Korea) at 60°C for 24 hours. The same procedure was employed for the preparation of sodium trimethacrylate active filler; however the methacrylic acid was used instead of acrylic acid.³²

Preparation of SBR/ALTA, SBR/ALTM and SBR/Nanoclay composites

Three types of reinforcing filler i.e. nanoclay, aluminum triacrylate and aluminum trimethacrylate were used. A magnetic stirrer and a ball mill were used to disperse the aluminum triacrylate and aluminum trimethacrylate carboxylates and nanoclay in water. The ball mill was a small scale home-made one. Both the container and balls were selected from stainless steel to remove the possibility of rusting due to the presence of water. Details of this ball mill may be found elsewhere.³³ The amount of each filler added to the SBR latex was 1, 3 and 5 phr (part per hundred rubber) based on the dry content of latex. The fillers were prepared as the suspensions of 1, 3 and 5 phr in water using a magnetic stirrer. The filler suspensions at 3 phr were also made using the ball mill (Table 2). To prepare the filler suspensions, first a given amount of SDS surface active agent, was dissolved in water followed by the addition of filler and stirring the whole mixture for 48 hours. In case of suspensions prepared using the ball mill, the duration of milling was 24 hours. The ratio of the amount of surface active agent to that of filler was always selected as 3. Eventually, the SBR latex was slowly added to the stirring filler suspension and the mixture was additionally stirred for 5 hours using the magnetic stirrer. The resulting latex/filler suspension was cast and dried for 48 hours at 40°C. The dried samples were additionally mixed on a two-roll mill (Polymix. 200 L - Schwabenthan, Germany) at the temperature of 30-40°C and a roll speed of 20 r/min.

Table 2.

Type and amount of materials used in the preparation of composites^a,^**.

Number	Latex quantity (phr)	Aluminum Triacrylate (ALTA) (phr)	Aluminum Trimethacrylate (ALTM) (phr)	Nanoclay (phr)	Surface active agents (SDS) (phr)	Filler slurry production method
1	400	1	-	-	3	Magnetic stirrer
2	400	3	-	-	9	Magnetic stirrer
3	400	5	-	-	15	Magnetic stirrer
4	400	-	3	-	9	Magnetic stirrer
5	400	-	-	3	9	Magnetic stirrer
6	400	3	-	-	9	Ball mill
7	400	-	3	-	9	Ball mill
8	400	-	-	3	9	Ball mill

^aThe total dry content of the latex was 25%. The corresponding amount of filler was dispersed in 100 mL distilled water.

The samples are coded as follows: ALTA denotes aluminum triacrylate, ALTM stands for aluminum trimethacrylate, SBR represents styrene-butadiene rubber, the letter B shows ball mill preparation method, the letter M represents magnetic stirrer preparation method, and the numbers after the filler name denotes the amount of filler in composite in phr. For instance, SBR/ALTA3(B) stands for styrene-butadiene rubber composite filled with 3 phr aluminum triacrylate where the slurry of aluminum triacrylate has been prepared using ball mill. For brevity purposes, SBR was removed from the composite names, and henceforth, SBR/ALTA3(B) is shown as ALTA3(B) on the Figures and throughout the context. Table 3 gives the nomenclatures used in this paper.

Table 3.

Nomenclature used in this paper^a.

Material	Code
ALTA filler suspension prepared using magnetic stirrer	ALTA(M)
ALTA filler suspension prepared using ball mill	ALTA(B)
SBR/ALTA composite containing α phr ALTA(M)	ALTAα (M)
SBR/ALTA composite containing α phr ALTA(B)	ALTAα (B)

^aSimilar nomenclature used for ALTM filler.

Characterization

Fourier transform infrared spectroscopy (FTIR) (EQUINOX55 – Bruker, Germany) was used to analyse the chemical structure of synthesized unsaturated carboxylates of aluminum triacrylate and aluminum trimethacrylate over a wave number range of 400-4000 cm⁻¹. The dispersion state of nanoclay, aluminum triacrylate and aluminum trimethacrylate particles in polymer matrix was examined using a scanning electron microscope (SEM) (AIS2100 model - Seron Technology Co. Korea). Morphological images were prepared from the cryo-fractured sections of casted samples.

Rheological tests

The rheological measurements were carried out on a rheometer (MCR 501- Anton Paar. Austria) at 70°C and 100°C. A parallel-plate geometry with a plate diameter of 25 mm was selected. The experiment was performed both in frequency sweep (0-600 rad/s) and rate sweep (0-100 s⁻¹) modes. The frequency sweep experiments were carried out at a strain of 0.5% to assure of remaining in linear regime.

Results and discussion

Characterization of synthesized organic metal salts of aluminum triacrylate (ALTA) and aluminum trimethacrylate (ALTM)

The prepared aluminum triacrylate and aluminum trimethacrylate salts were characterized and identified using FTIR spectrometer (Figure 1(a) and 1(b)). Figure 2 exhibits chemical structures of the aluminum triacrylate and aluminum trimethacrylate. As it is observed, the $π -$ electrons of double bond between the carbon and oxygen atoms, C = O, in the structure of carboxylate anion are in resonance between the single and double bonds through which the two oxygen atoms connected to the carbon atom, C = O and C-O. In other words, the two oxygen atoms linked to the carbon atom benefit from the same bond strength, owing to the delocalization of $π -$ electrons (Figure 2).³⁴ The peaks assigned to carbon-oxygen bonds appear between 1550 and 1650 cm⁻¹, more precisely at 1648 cm⁻¹ and 1580 cm⁻¹, and the 1450 cm⁻¹ peak is attributed to the asymmetric and weak symmetric stretching vibrations of oxygen atoms, respectively. The bending type vibration of = CH₂ group appears at 980 cm⁻¹ wavenumber. The small peak appeared around 1270 cm⁻¹ assigned to the stretching vibration of unsaturated carboxylate group. It should be noticed that in the spectrum of ALTM active filler, this peak has been masked by the adjacent 985 cm⁻¹ one. The chemical structure of aluminum trimethacrylate is discriminated from aluminum triacrylate by a methyl group, -CH₃, connected to alpha carbon atom of the acrylate base. The presence of this group commonly leads to vibrations near 2950 cm⁻¹, 1450 cm⁻¹ and 1380 cm⁻¹. It is worth noting that the vibration corresponding to = CH₂ may also be appeared about the same range. Consequently, the FTIR spectrum of ALTM is almost the same as that of ALTA.

Morphological studies

In this section, the results of SEM micrographs of the samples are provided. Figure 3 clearly shows aggregates of the filler particles having a length of about 50 microns and a thickness of 10 microns. As would be shown later, these aggregates are broken to smaller different particle sizes depending on the slurry preparation method.

The images of the scanning electron microscopy (SEM) of composites reinforced with 1, 3 and 5 phr aluminum triacrylate (ALTA) filler, taken from the cryo-fractured cross section of the composites, are presented in Figure 4. The ALTA slurries prepared using the magnetic stirrer. As the images indicate, the dispersed filler has irregular shape covering a relatively wide range of particle size and aspect ratio. Taking into account the particle size, as the percentage of filler is increased, no specific difference between the images is observed. However, the images clearly show the higher particle density of filler for composites with higher filler content.

Figure 5 illustrates and compares the morphological images of SBR composites reinforced with 3 phr of various fillers. The filler slurries were prepared using two different methods of ball milling and magnetic stirring. The images demonstrate more uniform dispersion and decreased particle size of the fillers for the samples reinforced with ball milled filler slurries compared to those reinforced with magnetic stirred slurries, as would be confirmed by rheological experiments later. Experimental studies indicate that a perfect distribution can be achieved using ball mill for the materials which were poorly distributed.³⁵ The sub-micron size of fillers after ball milling is evident in the image. The higher energy transferred to the ball mill charge during the milling time is explained as the main cause of particle size reduction.³³ The ball mill delivers significant stresses to the filler slurry for a long time which is not absolutely comparable with the magnetic stirrer.

The non-spherical shape of the active fillers is quite clear (Figure 5(a)–(d)). The results corresponding to nanoclay slurry prepared using both the ball mill and magnetic stirrer (Figure 5(e) and (f)), however, give no clear priority of former method over the latter one possibly due to smaller particle size of the clay (nano scale) compared to active fillers (ALTA and ALTM). It is worth emphasizing that due to the nano scale particle size of nanoclay, the main function of ball mill may mainly be limited to breaking the nanoclay aggregates, and ball milling probably has slight influence on the particle size of the nanoclay (Figure 5(e) and (f)). The large stresses created between the balls and container wall of the ball mill facilitate breakup of the filler aggregates resulting in a filler slurry with less aggregates and more uniform particle size and dispersion and consequently higher properties for the resulting composite. As would be seen later in the rheological section, SBR latexes reinforced with filler slurries prepared by ball mill exhibit increased rheological properties.

Rheological measurements

Frequency sweep test

To investigate how the particle size and type of the filler affect properties of the SBR composites, rheological experiments were performed on composites. Figure 6 shows the variation of storage and loss moduli against frequency at 70°C for the neat SBR prepared from the latex and corresponding composites reinforced with three types of filler, i.e. ALTA at 3 and 5 phr, ALTM and nanoclay both at 3 phr. To remain in linear viscoelastic regime, the strain is chosen as low as 0.5%. The letters M and B indicate that either a magnetic stirrer or a ball mill was used to prepare the filler slurry, respectively.

Figure 6 shows a continuous increase in storage and loss moduli as the frequency approaches higher values. The loss modulus of the composites show plateau at high frequencies while the storage modulus still persists an increasing trend. Furthermore, no crossover occurs between loss and storage moduli. This may need frequencies smaller than 0.01 rad/s as the storage and loss moduli of each composite reinforced with a given filler show an approaching trend with the decrease in frequency. Figure 6(a) and (b) also show the ball-milled fillers have a discriminating effect on rheological properties of the composites compared to magnetic stirred fillers. This priority of ball milled fillers over magnetic stirred ones is also observed at high frequencies. The composites reinforced with ball-milled fillers exhibit smaller changes compared to those reinforced with magnetic stirred fillers throughout the frequency range. It is also observed that the elastic and viscous moduli of the composites show a rational increase for the ALTA(M) filler content of 3 and 5 phr. The 5-phr magnetic stirred ALTA(M) presents the highest modulus throughout the frequency range. This clearly indicates the capability of active fillers in improving the composite properties. A part of this improvement may be attributed to the non-spherical shape of filler particles. It can be predicted that the ball milled active filler at 5 phr loading will greatly enhance composite properties. The ALTA3(B) and ALTM3(B) reinforced composites represent almost similar (with a slight difference) trends of modulus over the whole range of frequency. The higher modulus of ball milled fillers over magnetic stirred counterparts may be attributed to the elevated surface area resulting from smaller particle size of the fillers after being ball milled. An increased posed surface area of filler particles to the polymer molecules provides higher potential for possible interactions with polymer chains resulting in a more efficient stress transfer from a low modulus polymer matrix to rigid and high modulus filler particles. The shape and aspect ratio of filler particles may be taken into account as the additional cause of property enhancement for active filler composites. Furthermore, the ability of active fillers in creating the interactions with polymer chains facilitates the stress transfer, and therefore, enhances the composite properties. The smaller increase observed for the nanoclay-reinforced composites may be due to the fact that ball milling has no or slight influence on the nano-size particles and large surface area of the nanoclay compared to other fillers. Notice that at low frequencies the reinforcing effect of the ball milled nanoclay on the corresponding composites is even higher than that of ball milled ALTA at the same filler content of 3 phr, however, as the frequency approaches higher values, a comparative drop in the nanoclay reinforcing property is observed. This may be attributed to the slippage of polymer molecules on the filler platelets (very large aspect ratio and surface area) at high frequencies (high shear rates) as the composite reinforced with virgin nanoclay (before ball milling) show approximately the same behavior. It is worth noting that, the nanoclay used here is Closite 15A with a particle surface modified with quaternary ammonium salt rendering less polarity to the filler. On the one hand, the relative changes and the possible loss in quaternary ammonium salt content during the ball milling may lead to higher polarity and less chemical compatibility with the SBR molecules. Furthermore, the breakup of the clusters results in a better dispersion of the clay particles and increasing the particle surface area and therefore, increasing the nanocomposite properties. The balance between the above-mentioned effects determines the final properties of the clay composite. Consequently, the property enhancement observed for the nanoclay after ball milling may be lesser compared to other fillers, i.e. ball milled ALTA and ALTM. The effect of the type of filler and slurry preparation method (ball milling or magnetic stirring) are exhibited in Figure 7(a)–(d) in detail, where ALTA(B) and ALTM(B) present a superior properties compared to magnetic stirred ones. The effect of ball milling on the rheological properties of the ALTA active filler is more evidently shown in Figure 7(d) where 3phr SBR/ALTA(B) composite presents higher modulus than 3 phr SBR/nanoclay(B) composite, particularly at high frequencies. Since the solid-state behavior at high frequencies is attributed to the conformation changes,^5,36 the comparison obviously exhibits that ALTA(B) imposed further hindrance to conformation variations compared to clay(B). Recalling that nanoclay features as a nanofiller with a large capability of reinforcing character attributed to the nanosize and platelet particle shape. In fact, the active filler presents rheological properties as high as the nanoclay at low frequencies and better properties at medium to large frequencies thanks to the interactions the active filler makes with SBR matrix.

Figure 8 shows the changes in tan δ as a function of frequency for composite samples. Three distinct regions are evident in Figure 8. First, all samples present a sharp decrease in tan δ at very low frequencies. The curves then continue through a pseudo-plateau region as the frequency increases and finally a declining trend observed at high frequencies. ALTA3(M) and ALTM3(M) reinforced composites show a very close behavior to neat SBR throughout the whole frequency range at the test temperature indicating the slight influence of fillers on the damping properties of the composites.

The large particle size along with uneven distribution of fillers may be taken into account as the main cause of the similar behavior of the above mentioned composites to neat SBR. The second group of the curves corresponds to the composites reinforced with ALTA3(B), ALTM3(B) and ALTA5(M). Compared to the first group, the second one shows higher damping at starting frequency and high frequencies as well; however, lower damping is observed at medium frequencies. Except for the ALTM5(M), the ALTA3(B) and ALTM3(B) reinforced composites benefit from smaller particle size and more uniform distribution of particles compared to the magnetic stirred prepared composites resulting in more interaction between the active filler particles and the polymer matrix. At the same time, more barriers and frictions to the polymeric chain movements are also created resulting in a longer relaxation times for corresponding composites. At very low frequencies, the presence of active fillers causes higher loss and viscous chain motions. At medium frequencies, elastic movements are dominated resulting in a lower damping for the second group samples.

The lowest damping property is presented by the third group including the nanoclay3(M) and nanoclay3(B) reinforced composites. A comparison between the nanoclay3(M) reinforced composite and the nanoclay3(B) counterpart indicates that the ball milling of filler leads the latter composite to presenting higher damping at low and high frequencies compared to the former one, a typical trend observed for the second group of composites reinforced by ball milled active fillers. In other words, compared to SBR/ magnetic stirred fillers composites, ball milling either by changing the particle size or particle shape of the nanoclay provides composites with higher damping particularly at low and high frequencies.

The effect of filler type, filler content and filler slurry preparation method on complex viscosity changes against frequency is exhibited in Figure 9. A shear thinning behavior which follows the Power law trend is observed for all composites throughout the whole investigated frequency range. The composites SBR/ALTM(M) and SBR/ALTM(B) both at 3 phr filler exhibit an onset of approaching the zero shear viscosity, though smaller frequencies are required to clearly observe the zero shear viscosity.

A closer look at the curves reveals that the composite reinforced with 3 phr ALTA(M) shows slightly lower viscosity compared to composite reinforced with 3 phr ALTM(M) while, the ball- milled counterparts present an opposite trend, i.e., 3 phr SBR/ALTA(B) composite presents slightly higher viscosity than the 3 phr SBR/ALTM(B), indicating a more viscous behavior of ball-milled ALTA/matrix system. This case was previously mentioned where the damping behavior of composites was discussed. The ball milling leads to a further non-Newtonian behavior for the composites reinforced with ball milled filler providing a smaller Power law index (n). Considering the Cox-Merz rule^37,38:

η (\dot{γ}) = | η^{*} (ω) |; w i t h ω = \dot{γ} C o x - M e r z r u l e

(1)

It is possible to use the Power law equation as follows:

η = {K (\dot{γ})}^{n}

(2)

where K is the consistency coefficient, and n is the Power law index indicating a measure of thinness (0 < n < 1) or thickness (n > 1) of the fluid due to an applied shear, ω is the frequency in rad/s and

\dot{γ}

is the shear rate. The more the shear thinning the flow behavior, the further approaches n the zero. Moreover, higher viscosity is observed for the composites used ball milled slurry compared to those used magnetic stirrer slurries (Table 4).

Table 4.

Power law index for various SBR composites.

law parameter	Neat SBR	SBR/ALTA3 (M)	SBR/ALTA5 (M)	SBR/ALTM3 (M)	SBR/nanoclay3 (M)	SBR/ALTA3 (B)	SBR/ALTM3 (B)	SBR/nanoclay3 (B)
n	0.37	0.36	0.22	0.36	0.29	0.23	0.25	0.18

Having a look at the Table 4, it is observed a Power law index of 0.37 for neat SBR followed by 0.36 and 0.36 for ALTA3(M) and ALTM3(M), respectively, signifying the two latter composites exhibit nearly similar Power law indices to SBR. This clearly shows that the composites still present the same viscous behavior as neat SBR, indicating poor reinforcing characteristics for the fillers prepared by magnetic stirrer. For the filler slurries prepared using the ball mill, the Power law indices decrease from 0.36 to 0.23 and 0.25 for the ALTA3(B) and ALTM3(B) composites, respectively. Indeed, smaller size of filler leads to more non-Newtonian behavior, i.e. further shear thinning, for the composites. An almost similar drop in Power law index value, n, is also observed for the nanoclay3(B) composites after ball milling the filler.

Start up test

This part discusses the results of carrying out a startup test on the composites and nanocomposites. The experiment was performed at various shear rates starting from 0.1 s⁻¹ and increased till 100 s⁻¹ at 70°C. However, due to the instabilities observed in the experiment carried out at high shear rates leading to strong fluctuations in the rheo-graphs, only results corresponding to shear rate 0.1 s⁻¹ are presented. Before performing the main test, to remove the history of composites, a preashear of 0.1 s⁻¹ was applied to the samples for a duration of 300 seconds, allowing them to reach their equilibrium states. The composites were then left to rest at the test temperature for 5 min. and finally the main experiment was carried out. When a material is subjected to a startup test, a sudden rate of strain is applied to the material resulting in a response, which depending on the amount of the strain rate may be different. Below the critical strain, the elastic part of the material is stretched in the shear field. When such elastic elements approach their critical strain, the structure begins to break down, causing shear thinning (strain softening) and follows the flow. As the rate of strain is increased (not shown), the material response departs from linear behavior at shorter times. The results of startup test for shear stress growth coefficient, $η^{+} (t, \dot{γ}) = σ (t, \dot{γ}) / \dot{γ}$ , of various composites are presented in Figure 10.

The comparison of the shear stress coefficient, $η^{+} (t, \dot{γ})$ , of the reinforced SBR composites with that of neat SBR at the strain rate of 0.1 s⁻¹ shows that $η^{+} (t, \dot{γ})$ for all composites begins to grow till it achieves a steady value. Considering the type, filler loading and the method used to prepare the slurry, i.e. magnetic stirrer and ball mill, following explanation may be given for the plateau zone. Applying a shear rate causes the polymer to flow, resulting in molecular chain orientation and chain slippage on those of vicinity in the flow direction. The easier the chain slippage, the lower is the shear stress coefficient. As it is observed, the biggest difference is between the ALTM3(M) and ALTM3(B) composites which may be attributed to the higher surface area and uniform particle size and distribution of ALTM(B) as explained previously. The increased surface area results in more interaction of filler particles with polymer chains leading to further difficult chain motions.^5,39 The same explanation could be applied to the ALTA(M) and ALTA(B) corresponding composites, however, very surprisingly ALTA3(M) and ALTA3(B) composites show almost similar shear stress coefficient in the plateau region locating below the shear stress coefficient of neat SBR. Nevertheless, ALTA3(B) composite presents stronger overshoot, i.e. passing through a maximum while the shear rate is constant, compared to ALTA3(M). A closer look at the curves reveals that, despite the high shear stress coefficient observed for ALTM3(B) and ALTA5(M) composites, they display no overshoot, while ALTA3(B) and clay3(B) composites present higher overshoot compared to other composites. The highest overshoot is exhibited by ALTA3(B) composite, followed by clay3(B) and clay3(M) composites. Very interestingly, the overshoot corresponding to ALTA3(B) composite takes longer time to occur compared to other composite’s overshoots and, particularly, to its ALTA3(M) counterpart. This obviously indicates that ALTA3(B) composite needs higher strain to reach the critical strain (and so required larger elastic energy) which is explained by the more efficient interactions created between the polymer chain and ball milled ALTA(B) filler. The active filler ALTA is capable of forming stronger network with polymer chains particularly when the filler is ball milled. The overshoots corresponding to both clay3(B) and clay3(M) composites are believed to be due to nano size and the platelet shape of nanoclay particles creating higher barrier to the molecular movements. In fact, the active filler ALTA(B) despite the larger particle size has surpassed the ball milled nanoclay.

Conclusion

• It was shown that two active fillers of ALTA and ALTM (aluminum triacrylate and aluminum trimethacrylate salts), made and characterized in the laboratory, are able to use as the reinforcing agent in preparing gum SBR composites through latex method.

• The evaluation of morphological images indicated a more uniform dispersion and distribution of fillers in the samples prepared by the ball mill.

• The improvements in properties of the prepared composites and nanocomposites were examined using small angle oscillatory shear (SAOS) rheological experiments. For brevity purposes, partilce loadings were limited to 3 phr of the reinforcing agents. All rheological properties such as storage and loss moduli, viscosity, etc. indicated composites reinforced with the ball milled active fillers presented comparable properties with the nanocomposites made by using the nanoclay (closite 15A). This is while, nanoclay originally benefits platelet particles and lower particle size compared to active fillers of ALTA and ALTM. The improvement in rheological properties was particularly further pronounced for ball-milled fillers due to the active sites, lower particle size and higher surface area. The rheological improvements would positively influence the mechanical properties. Furthermore, the lower Power law index facilitates the processing of the resulting composites

• The resulsts of startup test performed at a steady shear rate of 0.1 s⁻¹ indicated that the ALTM3(B) presented a shear stress coefficient considerably higher than that of corresponding ALTM3(M) counterpart . Both ALTA3(B) and clay3(B) showed an stress overshoot compared to ALTA3(M) and clay3(M) counterparts, respectively. The observed behaviors are attributed to higher surface area, uniform particle size and distribution of ball milled active fillers compared to magnetic stirred ones.

Footnotes

Author contributions

Negar Hosseini Darabi, as a Ph.D student (first author), produced samples, analyzed the results, and wrote the manuscript. Dr. Foroud Abbasi-Sourki, as the corresponding author (second author) and supervisor, was responsible for conceptualization, editing, final review of the article, management and supervision of the project. Also, Dr. Gholamreza Bakhshandeh (the third author) was involved in the conceptualization and supervision of this research work.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Foroud Abbassi-Sourki

Data availability statement

All data generated or analyzed during the current study are included in this published article*.

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