Evaluation of morphological,mechanical,and rheological properties of vulcanized styrene–butadiene rubber/modified corn starch blends

Abstract

The starch-filled styrene–butadiene rubber (SBR) was prepared using a laboratory-sized two-roll mill. Starch was modified by yeast fermentation for 1 day before it was blended with SBR. The hydrophilicity of SBR was enhanced by grafting with modified starch (MST) by utilizing tetramethyl thiuram disulfide as a catalyst. The effect of modified corn starch loading on morphological, mechanical, and rheological properties of vulcanized SBR blends was investigated. Scanning electron microscope result revealed that the adhesion between the MST and SBR was weak, and the starch pulled out due to poor interfacial bonding. The lowest ultimate tensile strength, elongation at break, and tensile modulus of the SBR-g-MST were found in the sample containing 150 phr of starch. The variation of the percentage elongation of neat rubber and MST/rubber composites was 91.34%. The significant decrease in cure times was observed with the loading of MST in all blends up to 100 phr starch, while no significant change in scorch time was observed. The maximum torque, minimum torque, and cross-linking density increased as the starch loading increases up to 100 phr MST. The water absorption by the composite increases with immersion time and MST loading, although the rate of absorption decreases with increased time. The current product could be especially advantageous in agricultural and horticultural applications as a good controlled fertilizer release and for water retention.

Keywords

Morphological mechanical rheological styrene–butadiene rubber starch

Introduction

Elastomeric blends are technologically important materials in the rubber industry as they achieve the best compromise in physical properties, processability, and cost. Their chemical and physical properties recommend them as engineering materials for chemical industry, electric insulators, and many other uses.^1

–6 The wide range of applications of rubber from footwear to automobile tires comes from their individual mechanical characteristics, such as good flexible attitude even at great distortion and good energy absorbing capability.

Styrene–butadiene rubber (SBR) is widely used in different industries for different applications.^6,7 The tire industry depends on the SBR for its meritorious properties such as good incision impedance, humidity, and weather resistance.^6,8 Moreover, it is used for other various applications, including membranes, wires, and cables.^9
–11

Starch is a natural polymer produced from large different harvests and is a promising crude material for biodegradable material resources.^12

–15 Its structure was specified by the huge amount of glucose molecules that connect to each other by glycoside bonds. It is produced from many foods such as potatoes, wheat, corn, and rice.^16,17 The compositions of starch are linear and helical amylose and branched amylopectin molecules. It is considered a natural renewable material^18,19 with low cost, characteristics that enable it to be a potent concurrent to petroleum-based thermoplastics in many applications.^20,21 Blending starch with synthetic polymers is widely used.²² The results of thermoplastic starch/natural rubber (NR) polymer blends revealed a decrease in the modulus and in tensile strength and the phase separation was also shown.²³ Several types of natural and synthetic polymers can be blended with starch. Some of these blends are designed for several particular aims.^7,24,25

The modification of starch can be done by physical methods, chemical methods, enzymatic degradation techniques, and genetic modifications, which involve the transgenic techniques targeting the various enzymes involved in starch biogenesis.²⁶ The starch conversions were used in different applications, such as the food, pharmaceutical, pulp, textile, and other branches of industry. Some industries are concerned with the fermentation of starch to ethanol and other products and to the production of cyclodextrins.²⁷ In this research, yeast has been chosen for the modification of starch to explore the effect of yeast curing on characteristics of starch/SBR blends.

In this study, blends of SBR possessing different amounts of modified starch (MST) (25–150 phr) by yeast solution have been prepared. A two-roll mill method is used for blending components in SBR. To the best of the author’s knowledge, the properties of SBR/MST (by yeast solution) blends using tetramethyl thiuram disulfide (TMTD) as an accelerator have not been systematically investigated in the literature. Therefore, this research is aimed to determine the effect of content of MST on morphological, mechanical, rheological, and swell behavior properties of SBR.

Experiment

Materials

The materials used in the experiments are listed in Table 1 by chemical name and supplier.

Table 1.

Materials and suppliers.

No.	Material	Supplier
1	Styrene–butadiene rubber	Hoa Thuan Co. Vietnam
2	Corn starch	Arab Food Industries Co. Ltd, Amman, Jordan
3	Zinc oxide	ChemTAL (Malaysia) Sdn-Bhd Malaysia
4	Stearic acid	Acidchem-International Co. Malaysia
5	TMTD	Al-Kiiubar Co. KSA
6	Yeast	Akmaya Yeast Industry, Turkey

TMTD: tetramethyl thiuram disulfide.

Preparation of blends

Initially, MST was prepared by the starch fermentation in the solution of yeast (5 g/L). The curing time of fermentation was 1 day at starch loading of 10 wt% of total yeast solution. Then, the MST was separated from the solution using a centrifuge and was washed with distilled water. The product was dried in a furnace at a temperature of 80°C for 5 h. The mixing processes were carried out in State Company for Tire Industry in Najaf according to ASTM D 3182. Starch, zinc oxide, stearic acid, and TMTD were first mixed in a plastic bag until the materials became homogeneous. In the second stage, SBR and homogenized materials were mixed by two-roll mill (Comerio Ercole Busto Arsizo, Italy, with a dimension of 15 × 30 cm²), according to ASTM D 3182. The formulation of compounds is present in Table 2. Vulcanization processes were carried out using compression molding (Moore, England) at a temperature of 185°C, pressure of 4 MPa, and time period of 15 min.

Table 2.

Formulation of MST/SBR compounds.

Material	A1	A2	A3	A4	A5
SBR (wt%)	100	100	100	100	100
MST (wt%)	0	25	50	100	150
ZnO (phr^a)	3	3	3	3	3
Stearic acid (phr)	2	2	2	2	2
TMTD (phr)	3	3	3	3	3

MST: modified starch; SBR: styrene–butadiene rubber.

^aphr (parts per hundred) of total blend (SBR + MST).

Scanning electron microscope

The morphology of SBR and its blends with different amount of MST were examined by a scanning electron microscope (SEM, FEI, Quanta model, Holland). The fracture surfaces were coated with a thin film of gold to avoid charging during SEM testing.

Ultimate tensile strength, elongation at break, and tensile modulus tests

The mechanical properties were estimated according to ASTM D412 in State Company for Tire Industry using Monsanto, T10 Tensometer (England). The result was taken from an average of five tests.

Rheometer measurement

A rheometer experiment was executed on each individual batch of the blend. The rheometer illustrates accurately and speedily the curing and processing properties of vulcanizable rubber blends. Curing speeds and the optimal curing time of SBR rubber dough and its blends were determined using a Monsanto Rheometer (ODR-2000) in accordance with ASTM D2084 standard. The sample taken from the prepared semifinished product was placed on the rheometer device and cured at 185°C.

Swelling ratios

The samples were cut with measures of 1.5 × 1.5 × 0.25 cm³. After immersion in water, the specimens were pulled out at various time periods, swept with filter paper to take off the excess surface water, and weighed with a precision balance with 0.0001 g resolution. The swelling ratio (SR) was calculated using equation (1):

SR % = \frac{m_{t} - m_{0}}{m_{0}} \times 100

where m_o and m_t are the weights of the sample before and after a time t of immersion, respectively.

Results and discussion

Scanning electron microscope

SEM investigations were undertaken to estimate the MST/SBR interactions. SEM gave a general review of the construction of the blends and the investigation of the fracture attitude of blends. In the morphological test, the morphological alterations take place according to the interfacial binding between different MST loadings and SBR polymer. Figure 1 shows SEM test at ×110 to ×119 magnification of fractured tensile of MST/SBR blends with various starch loadings. From Figure 1(a) to (d), it was observed that there were starch separation and weak interfacial binding. These bad behaviors of blend contributed to reduce ultimate tensile strength even at lower MST loadings, as shown in mechanical properties results. Riyajan et al.²⁸ reported that the interfacial binding between starch and rubber is poor and that they also reduced the characteristics of mechanical blends. For the case of high starch loadings, the starch were accumulated in the matrix, which contributed to a reduction in mechanical characteristics.

Figure 1.

SEM photograph of MST/SBR blends: (a) 25 phr, (b) 50 phr, (c) 100 phr, and (d) 150 phr MST.

Mechanical properties

The influence of MST on the ultimate tensile strength of SBR is illustrated in Figure 2(a). The ultimate tensile strength of SBR-g-MST decreased at the loadings of MST in the specimen between 25 phr and 150 phr of starch. The decrease in the ultimate tensile strength of SBR-g-MST with loading MST was due to separation and weak interfacial binding of starch in the blends, as shown in Figure 1(a) to (d) and it was confirmed in the SEM results. Similar results were reported by Riyajan et al.²⁸ The lowest ultimate tensile strength of the SBR-g-MST was observed in a specimen of 150 phr starch, and its ultimate tensile strength was about 1.4 MPa. As the MST loading was increased further, the individual granules tend to restack together to form agglomerates, as shown in the SEM results. This sort of arrangement weakens the interface between starch and the rubber component, causing a drop in the properties.²⁹ Therefore, the blend of 150 phr MST loading led to the loss of mechanical characteristics due to poor interaction and adhesion between starch and the SBR.

Figure 2.

Influence of MST on (a) ultimate tensile strength and (b) elongation at break of the SBR-g-MST.

Figure 2(b) shows the elongation at break of the modified SBR. The elongation at break of SBR-g-MST decreased with increasing of loading MST in the sample ranging from 25 phr to 150 phr of MST. Percentage elongation at break amounts indicate the maximum expansion of the specimens while under stress. Results showed that the elongation at break of the SBR was 347%, while the elongation at break of SBR-g-MST in the presence of 150 phr of MST was 30%, as shown in Figure 2(b). The elongation at break amounts relied on the starch/rubber interaction. It was observed from Figure 2(b) that the difference of the percentage elongation of virgin rubber and 150 phr starch/rubber composites is 91.34%. It is well-known that the proportion elongation at break is a significant item characterizing the laceration attitude of the blend materials. The reduction in proportion elongation from 347% at 0 phr MST to 30% at 150 phr MST may be attributed to the introduction of MST to the rubber matrix, which causes interruption in the rubber segment mobility and thus causing the composite to fail. It was observed that the blend of 100 phr MST has the highest % elongation at break. This may be attributed to 100 phr MST blend having highest cross-linking density, as confirmed in the “Curing characteristics” section.

Young’s tensile modulus of elasticity for neat rubber and MST/rubber blends is illustrated in Figure 3 at 200% and 300% elongation. The neat rubber has Young’s tensile modulus of elasticity of 1.377 and 1.883 MPa at 200% and 300% elongation, respectively. Increasing the MST from 25 phr to 100 phr in the matrix causes to decrease the tensile modulus values linearly. On the other hand, the addition of 150 phr MST deteriorated the tensile modulus of elasticity of the blend to 0.096 MPa. Therefore, Young’s tensile modulus of elasticity lost 93% and 95% of its original value at 200% and 300% elongation, respectively, as compared to the neat rubber matrix. The reduction in Young’s tensile modulus can be attributed to the weakest adhesion between the starch surface and rubber at the starch–matrix interface; the polarity of starch and nonpolarity of rubber causes the repulsion for two materials and resulting starch decreases the strength of the blends by the deterioration of the orientation in the rubber chain.

Figure 3.

Influence of MST on Young’s tensile modulus of elasticity of the SBR/MST blends.

Curing characteristics

Table 3 presents the influence of MST loading on the curing behavior of MST-loaded SBR as specified with a Monsanto rheometer. As presented in Table 3, there is a considerable reduction in cure times with the loading of MST in all blends up to 100 phr MST, and the value further increases at MST loading of 150 phr. The shown decrease in cure times is a sign of the improvement of the cure rate. The cure improvement is a result of filler features such as surface area, surface reactivity, particle volume, its humidity, and the existence of metal oxides.³⁰ In addition, in our research, the cure improvement was perhaps due to the generation of binds attributed to the interaction between the starch functional groups (–OH in starch) and TMTD accelerator, confirming that the starch works as an activator for TMTD accelerator.²⁹ Furthermore, the increase in curing time at the loading of 150 phr MST may be due to the agglomeration of MST in the blend, as revealed by the SEM results. The agglomeration of MST leads to missing the contact between the starch, TMTD, and rubber, and the subsequent lack of reaction between them. Therefore, the result gives an increasing curing time at 150 phr MST. These results are in agreement with tensile modulus results. Moreover, no significant changes were observed in scorch time (t_s2), but it is slightly increased, as shown in Table 3. The increase in viscosity of blends may be attributed to MST loading.

Table 3.

Influence of MST content on the curing and scorch times (min) of SBR blends.

Samples	Curing time, t₉₀ (min)	Scorch time, t_s2 (min)
A1	2.15	0.85
A2	2.03	0.89
A3	2.02	0.78
A4	1.85	0.89
A5	2.35	0.84

MST: modified starch; SBR: styrene–butadiene rubber.

Figures 4, 5 and 6 show the maximum torque (M_H), minimum torque (M_L), and cross-linking density (ΔM = M_H – M_L) values, respectively, for all blends of MST/SBR. The maximum torque, minimum torque, and cross-linking density increased as the MST loading increases up to 100 phr MST. The increase in the maximum torque, minimum torque, and cross-linking density amounts suggests that the existence of fillers in the blends causes to decrease the mobility and flexibility of the polymer molecules of the rubber.^31

–34 Also, it indicates that the addition of MST causes an enhancement in the cross-linking efficiency and the cross-linking density of SBR blends. It can be concluded that the –OH groups in MST take part in curing SBR/MST blends, and it works as an activator for TMTD accelerator, which leads to an increase in cross-link density, as shown in Figure 6, as compared to pure SBR, confirming the MST participated in the curing process. A similar result was observed for NR matrix cured by starch.²⁹ On the other hand, the blend containing 150 phr MST causes the maximum torque, minimum torque, and cross-linking density to deteriorate and the reason may be that the accumulation of MST in the blend at 150 phr MST hinders the reaction between reactant materials.

Figure 4.

Influence of MST content on the maximum torque of SBR blends.

Figure 5.

Influence of MST content on the minimum torque of SBR blends.

Figure 6.

Influence of MST content on the cross-linking density of SBR blends.

Swelling ratio

Blends were immersed in distilled water at room temperature for 9 days. The weight gain curves and rate of absorption curve as a function of time are shown in Figures 7 and 8. It was observed from the graph that the water absorption by the composite increases with immersion time although the rate of absorption decreases with increased time. It was also shown that the composite attains equilibrium after 9 days. The amount of water absorbed increases with MST content. The water absorption property of composite reinforced with biomaterials and their derivatives is dependent on the amount of the biomaterials, biomaterials orientation, immersion temperature, area of the disclosed surface to water, the permeability of biomaterials, void existence, and hydrophilicity of the particular ingredients (in this case, the MST).³⁵ Therefore, the swelling proportion of SBR-g-MST dramatically boosted with raising MST loadings attributed to its hydrophilic starch. In the case of the composites produced, exposure to water causes the hydrophilic starch to swell. As a result of starch swelling, microcracking of the blend occurs particularly along the starch/rubber interface, which gives room for further water penetration. The swelling stresses that develop under these circumstances can result in composite failure. The 150 phr composition of the MST has the highest water absorbed. The starch content contributes to its absorptivity rate. In the beginning, it absorbs water at an increased rate before it attains the maximum, after which the rate drops drastically until reaching the saturation point. The 150 phr of the MST was a higher rate of absorption than the other specimens. This illustrates that the addition of MST displays good water solubility and produces excellent swelling proportion in water.

Figure 7.

Swelling ratio % against time (days).

Figure 8.

Rate of absorption against time.

Conclusions

MST was blended with SBR using a two-roll mill, and the grafting reaction between them occurred by using tetramethylthiuram disulfide (TMTD) as a catalyst. From this study, the following conclusions can be drawn. The interfacial bonding between the MST and SBR matrix is weak, and they also decreased the ultimate tensile strength, elongation at break, and tensile modulus properties of composites. As the MST loading was increased further, the individual granules of MST tend to restack together to form agglomerates, which lead to a drop in the mechanical properties. The addition of MST to rubber reduces its percentage elongation from 347% at 0 phr MST to 30% at 150 phr MST. On the other hand, the cure rate was enhanced with the loading of MST materials up to 100 phr MST. The addition of 100 phr MST gave a maximum enhancement in the cross-linking efficiency and the cross-linking density of SBR blends. It was concluded that the –OH groups in MST take part in curing SBR/MST blends, and it works as an activator for TMTD accelerator. The increased MST in blend exhibits good water absorption and leads to excellent swelling ratio (SR) in water. It can be suggested that this product could be especially useful in agricultural and horticultural applications.

Footnotes

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

Ismaeel Moslam Alwaan

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