The Mechanical Properties of Jute/PLA Composites Combining Nano Polymerized Styrene Butadiene Rubber Modified by Coupling Agents

ABSTRACT The jute/polylactic acid (PLA) composite material has been widely concerned in the field of composite materials because of its green, degradable and renewable properties, but its low mechanical properties limit the application field. In order to improve the mechanical properties of jute/PLA composites, styrene butadiene rubber (SBR) particles modified by different silane coupling agents were introduced into PLA matrix, and jute/PLA composites were prepared by solution casting and molding method. The changes in the mechanical properties of the jute/PLA composite were studied, and the toughening mechanism of the jute/PLA composite was analyzed. The results show that SBR particles can induce the crystallization of the PLA matrix, and the coupling agent plays a promoting role. The addition of SBR improves the bending and stretching of the jute/PLA composite. The silane coupling agent KH560 has the best modification effect on SBR particles. When the mass fraction of KH560-SBR particles in the composite is 1.0%, its bending strength and modulus achieve the highest, which are respectively 247.04% and 773.91% higher than the control group. When the mass fraction of KH560-SBR particles added in the composite is 2.0%, its tensile strength and modulus are the largest, 517.91% and 472.00% higher than the control group.


Introduction
With the continuous advancement of science and technology, fiber reinforced composite materials are increasingly widely used in the fields of aviation, automobiles and construction. As people pay more and more attention to the environment, natural fiber reinforced composite materials have become the nanoparticles are used as reinforcing materials, rubber particles can greatly improve the mechanical properties of fiber reinforced polymer composites.
In recent years, the improvement of jute/PLA composite performance mainly focuses on the modification of jute fiber. There are only few reports on the modification of PLA matrix. He et al. (2019) used nano silica and poly (butyl acrylate) rubber as rigid core and soft shell respectively, and designed a core-shell particle for modifying PLA matrix. The results show that this particle will cause local plastic deformation of PLA, thus improving the mechanical properties of jute/PLA composites. Compared with the unmodified composite, its flexural strength and modulus increased by 32% and 7.9%, and its tensile strength and modulus increased by 16.9% and 47.9%. In our previous research (Song et al. 2021), nano silica modified by silane coupling agent was introduced into PLA matrix. The results show that the addition of modified SiO 2 improves the interfacial properties of PLA and jute, and thus elevate the mechanical properties of the composite. Compared with the unmodified composite, when the mass fraction of KH560 modified silica is 4%, the flexural strength and modulus increase by 150.3% and 77.4%, and the tensile strength and modulus increase by 816.9% and 700.1% respectively. However, the report on the modification of PLA matrix by rubber particles is rare.
The physical and mechanical properties of styrene butadiene rubber (SBR) are close to those of natural rubber. SBR is economic, easy to be processed and of good abrasion resistance and thermal aging resistance (George et al. 2000). Due to the large specific surface area of SBR, the particles tend to agglomerate together. In order to overcome this problem, the coupling agent is usually used to modify the particles. Silane coupling agents can form a chemical bond with the oxygen-containing groups on the rubber surface. Meanwhile part of coupling agents may physically diffuse into the active points of the rubber surface voids and form physical adsorption, thus making the modified rubber more easily disperse in PLA and improving the interfacial action of rubber and PLA (Choi and Son 2016;Mei et al. 2016).
Different molding processes have a direct impact on the interfacial adhesion between the jute fiber and the PLA matrix, which in turn affects the transfer of the internal stress, and ultimately affects the performance and quality of the composite materials (Singh, Singh, and Dhawan 2018). Compression molding is one of the most widely used molding processes in the preparation of natural fiber composite materials. Huda et al. (2008) designed a layered structure with a layer of kenaf fiber sandwiched between every two layers of PLA film, and prepared kenaf/PLA composite materials by hot pressing. This method enhances the interface compatibility between kenaf fiber and PLA, and improves the mechanical and thermal properties of kenaf/PLA composite materials. The composite materials prepared by laminated hot pressing have high fiber content and excellent mechanical properties, leading to a wide application in the preparation of other fiber reinforced composites.
In this study, the PLA matrix was doped with SBR particles modified by different silane coupling agents, and jute/PLA laminated composite materials were prepared by hot pressing to improve the mechanical properties of the jute/PLA laminated composite materials. At the same time, the effect of SBR mass fraction modified by different kinds of silane coupling agents on the mechanical properties of jute/PLA composites was studied. The main purpose of this research is to develop a fully degradable laminated composite material with good mechanical properties and wide application.

Materials and methods
All material information, experimental procedures, standard techniques, sample numbers and instruments are provided in the supplementary information file. Modified SBR particles with different silane coupling agents KH550 (KH560 and KH570) were named SBR-550 (SBR-560 and SBR-570). The composites were numbered according to the coupling agent types and SBR concentration. For example, CS5-0.5, where C represents jute/PLA composite material, S represents added SBR particles, 5 represents treatment with KH550 silane coupling agent and 0.5 represents that the mass fraction of SBR.

The composition and physical property of modified SBR
The FTIR, shown in Figure 1, is adopted to verify the graft polymerization of silane coupling agents on SBR surface. The characteristic peak of Si-O bond in silane coupling agent can be observed at 1080 cm −1 (Das et al. 2019). As can be seen from the infrared spectrum of the modified SBR in Figure 1, there is an absorption peak near 1080 cm −1 , an asymmetric absorption peak of the Si-O-Si bond, and symmetric shrinking vibration of the Si-O-Si bond near 800 cm −1 , which indicates that the silane coupling agents have been grafted and modified on the surface of SBR. A strong absorption broad peak appears ranging from 3150 cm −1 to 3550 cm −1 , indicating the presence of the cross-linked hydroxyl group (Zhang et al. 2014). Meanwhile, compared with SBR, SBR-550 shows a sharp peak at 1565 cm −1 and 1489 cm −1 , corresponding to the characteristic peak of amino group, which further indicates that SBR has been successfully modified by KH550. The characteristic peak of stretching vibration of epoxy group appears at 910 cm −1 . Compared with the FTIR spectrum of SBR, the stretching vibration absorption peak of epoxy group appears near 910 cm −1 in the spectrum of SBR-560, indicating that the silane coupling agent KH560 grafted the surface of the SBR particles. The infrared spectrum of SBR-570 shows a stretching vibration absorption peak of carbon-oxygen double bond on the carbonyl group at 1718 cm −1 , which indicates the successful grafting of KH570 to SBR.
The particle sizes of original and modified SBR are shown in Figure 2. The surface morphology of original and modified SBR is shown in Figure 3. It can be seen that the particle size of original SBR is larger than the modified ones, indicating that silane coupling agents can modify SBR by reducing its particle size and effectively preventing their agglomeration. The KH560modified SBR has a special appearance and uniform particle size. This is probably because the hydrogen atoms of the methylene group in SBR are replaced by organic functional groups after modification, which reduces the number of free radicals and thus decreases the tendency of SBR particles to agglomerate. The SBR particle size after KH570 modification becomes smaller, and the particle uniformity decreases. Figure 4 shows FTIR spectra of PLA, PS, PS5, PS6 and PS7 blended film. It can be seen from Figure 4 that the -C=O tensile vibration peak in PLA is slightly shifted at 1758 cm −1 , which may be caused by the formation of hydrogen bonds between the hydroxyl groups in the modified SBR by the coupling agent and the hydroxyl groups in the PLA. It shows that with the addition of SBR, the hydrogen bonds in the composite membrane increase. The infrared spectra of samples PS, PS5, PS6 and PS7 showed a broad and strong characteristic peak at 3400 cm −1 which was the characteristic peak of -OH (Zhang et al. 2014). The peak intensity of modified SBR particles decreased significantly at 2890 and 1340 cm −1 . This indicates that the grafting reaction between PLA and the modified SBR particles successfully occurred, and the polarity of the particle surface was weakened.

Differential scanning calorimetry (DSC)
Table 1 lists the thermal performance parameters of PLA/SBR blends measured and calculated from DSC, namely crystallization temperature (T c ), melting temperature (T m ), enthalpy of fusion (ΔH m ), enthalpy of crystallization (ΔH c ) and crystallinity (X c ). It can be concluded from Table 1 that with the addition of SBR, the glass transition temperature of the PLA/SBR blend system is stable, while the crystallinity increases in some extent.
The crystallization temperature and crystallinity of the PLA/SBR blend system reached the maximum at a mass fraction of 2%. Comparing the modification effect of the coupling agent with the same mass fraction, it is found that KH560 increases the crystallization temperature and crystallinity of the system the most.
The addition of SBR into PLA plays a role of heterogeneous nucleation, which increases both the crystallization temperature and the crystallinity degree of prepared composite materials. However, during the crystallization process of PLA, SBR will occupy a certain spatial position in its structure, thereby preventing the folding and rearrangement of the molecular chain of PLA, resulting in a decrease in the crystallinity of the polymer (Lonkar et al. 2009). From Table 1, a small amount of SBR can significantly improve the crystallinity of the blend system. As the mass fraction of SBR continues to increase, the contact area between the particles and the matrix is increased correspondingly, resulting in the continuous decrease of the crystallinity. When the SBR particle content is higher than 2%, the crystallinity will not increase any more even with the increase of the particle concentration, probably resulting from the agglomeration of SBR particles in the system. At the same time, the modification of SBR by the coupling agent reduces the agglomeration of SBR particles in the PLA matrix, and is more conducive to its heterogeneous nucleation, thereby further promoting the crystallization of PLA. As the degree of crystallinity increases, the strength, modulus and thermal properties of the polymer all increase (Al-Itry, Lamnawar, and Maazouz 2012; Fan et al. 2015).  modulus of jute/PLA composites is relatively low but increases more or less by the addition of SBR. With same coupling agent, and the overall change trend of the composite material's flexural strength and flexural modulus with the increase of the SBB mass fraction is to increase first and then decrease.

Bending property
A similar phenomenon can be observed in terms of the bending strain and fracture energy which is calculated by integrating the bending stress-strain curve. As shown in Figure 5(c,d), the addition of SBR improves the bending strain and fracture energy of the jute/PLA composites.
The addition of a small amount of SBR promotes the jute/PLA interfacial bonding and the bending property of the composites increases with the increase of SBR mass fraction until reaching a peak at about 1%. After that, the agglomeration of SBR particles will affect the interfacial adhesion and the bending property becomes poorer. With the continuous increase of the SBR content, the flexural properties are mainly influenced by the mechanical properties of the SBR particles themselves, and the flexural property is enhanced. When the mass fraction of KH560-SBR particles added to jute/PLA composite is 1.0%, the flexural strength and modulus of the composite are the best, which increases to 149.27 MPa and 2.01 GPa, respectively, 247.04% and 773.91% higher than the control group, and 645.18% and 123.33% higher than the composite added with unmodified SBR.

Tensile property
The tensile properties of jute/PLA composites are shown in Figure 6. It is concluded from Figure 6 that the cooperation of nano-SBR can effectively improve the tensile tolerance of composite materials. With the same SBR mass fraction, the KH560-modified sample has the best tensile strength, which can be attributed to the smaller particle size of the SBR-560.
With less content, SBR can be more uniformly distributed in the matrix, therefore enhancing efficient load transfer between jute and PLA, improving the tensile properties of the composites. When the mass fraction of KH560-SBR particles added in jute/PLA composite is 2.0%, the tensile strength of jute/PLA composite increases to 26.57 MPa, and the modulus is 2.86 GPa, 517.91% and 472.00% higher than the control group, and 374.46% and 176.67% higher than the composite added with unmodified SBR respectively. The tensile strength is highest with 2% SBR mass fraction while adding more SBR will worsen the tensile properties, probably due to the increase of SBR-jute contact points which reduces adhesion between jute and PLA Li et al. 2014;Sanivada et al. 2020;Yang et al. 2009). When the mass fraction of SBR particles is 8%, the tensile properties of the composite materials are mainly affected by the mechanical properties of SBR, the elongation at break increases and the tensile strength also increases. Figure 7 shows the tensile fracture morphology of the jute/PLA composites. The tensile fracture surface of neat jute/PLA composite, illustrated in Figure 7(a), shows large amounts of fiber extraction leading to a lower mechanical resistance to some extent. Figure 7(b,c) shows that there are mainly fiber fracture and cracks in the matrix with SBR mass fraction of 0.5% and 1%. With the further increase of SBR mass fraction, as shown in Figure 7(d), a large number of small cracks and holes appeare on the fractured surface. The cracks and holes produced during the stretching process consume a lot of energy. Figure 7(e) shows the tensile fracture surface of CS6-4, and it can be found that the fracture occurs near the SBR particles. A large number of SBR particles aggregate which reduces the adhesion of the interface, resulting in a decline in the tensile properties of CS6-4. When the mass fraction of SBR increased to 8%, a large number of small cracks appeared on the fracture surface again, indicating that more energy is needed to break the sample, leading to a slight increase of tensile tolerance, as shown in Figure 7(f).

Dynamic mechanical thermal analysis (DMTA)
The dynamic changes of storage modulus(E'), loss modulus(E'') and loss factor tanδ(E'"/E") of jute/PLA composites before and after modification from room temperature to 180°C are shown in Figure 8. It can also be found that there is a same varying pattern for different variables; that is, with the increase of SBR content, the storage modulus and loss modulus of jute/PLA composites first increase and then decrease. The increase in storage modulus indicates an improvement of the jute-PLA interfacial adhesion, leading to greater stress transfer between them (Porras and Maranon 2012). The result shows that increasing the SBR content can improve the interfacial adhesion. The loss modulus represents the  dissipated energy of jute/PLA composites under stress whose maximum value is known to relate to the composite's glass transition temperature (Tg). The Tg of untreated jute/PLA composites is around 64°C while the Tg of the SBR-modified jute/PLA composites gets higher with a maximum of about 76°C. The loss factor tanδ, the ratio of loss modulus and storage modulus of the composite, represents the damping performance of the materials. The decrease in tanδ value indicates the inhibition of the matrix mobility by the fibers. Known from Figure 8(c), the jute/PLA composite has the highest tanδ value and the CS6-1 has the lowest tanδ value.
A small amount of SBR can improve the PLA-jute interfacial bonding and reduce the mobility of PLA macromolecules in the composites while increasing SBR will instead reduce the interfacial adhesion. The excessive SBR particles will get into the PLA matrix, blocking the mobilization of polymer chains at the interface of jute/PLA composites, which can enhance the interfacial adhesion. Figure 8(d) compares DMTA between jute/PLA and CS6-1 sample, and the glass transition temperature of the latter is 18.7% higher than that of the former. This is because the addition of an appropriate amount of SBR particles can act as a cross-linking point in some crystal regions, so that the inter-chain cross-links can form a three-dimensional network structure. The chain segments are constrained due to cross-linking, resulting in restricted relaxation of the molecular chain, the temperature gradually increases, and the glass transition temperature of the polymer shifts to high temperature.

Conclusions
In order to enhance the mechanical properties of jute/PLA composites, SBR particles modified by different coupling agents were added into the PLA matrix. The effect of coupling agent type and the mass fraction on the mechanical properties of the composites were comprehensively investigated. The results show that SBR can promote the crystallization and alpha crystal formation of PLA, which increases the tensile strength of jute/PLA composites and therefore reduces its impact properties. The addition of SBR can also improve the interfacial properties between PLA and jute and thus promote the mechanical properties of the composites. In addition, the smaller the SBR particle size, the better the mechanical properties of the composite materials. The analysis also shows that adding SBR can improve the thermomechanical properties and glass transition temperature of the composites. Moreover, the fracture morphology of the composites indicates that the toughness of SBR particles with their induced matrix microcracks and cavitation are the main reasons for toughening the composites. The severe environmental pollution situation faced by the world requires the development of environment-friendly composites as far as possible. However, the difficulty lies in how to make environment-friendly composites have high mechanical properties to meet the application needs. This study provides a feasible way to prepare fiber reinforced composites with high mechanical resistance and degradable properties. In the future work, fiber reinforced phase and PLA matrix can also be modified at the same time to further improve the mechanical properties of the composite.

Highlights
• The polymerized styrene butadiene rubber particles were introduced into jute fiber reinforced polylactic acid composites, and the properties of the composites were experimentally studied. • The addition of polymerized styrene butadiene rubber particles can dramatically improve the mechanical and thermal properties of composites. • The toughening mechanism of jute/polylactic acid composites was analyzed. • Silane coupling agent modified polymerized styrene butadiene rubber particles can enhance the interface properties between polylactic acid and jute fiber.

Disclosure statement
No potential conflict of interest was reported by the author(s).

Funding
The work was supported by the Major Project of Natural

Human or animal rights
This article does not contain any studies with human participants or animals performed by any of the authors.