Natural jute fiber treated with silane coupling agent KH570 reinforced polylactic acid composites: mechanical and thermal properties

Abstract

With the progress of society, fiber reinforced composites have become the subject of research in the field of composite materials, in which natural fibers are widely used in the enhancement and modification of thermoplastics due to their low density, biodegradation and most wide origins. However, the hydrophilicity of special fiber is the main factor that hinders its bonding with hydrophobic polymer matrix which leads to poor interface compatibility between the fiber/matrix interface. The coupling agent can increase the nonpolarity of the natural fiber and the compatibility between the fiber and the resin. In this study, the composites were fabricated by the film stacking method, and the surface of jute fiber was treated with silane coupling agent so as to prepare jute/polylactic acid composites with good mechanical and thermal properties. The mechanical and thermomechanical results show that the combination of the coupling agent can significantly improve the interfacial bond of the jute/polylactic acid composites, leading to an efficient enhancement of comprehensive performance.

Keywords

Natural fibers mechanical properties thermomechanical surface treatments textile composite nonwoven structure

As a kind of natural plant cellulose, jute fibers are low in price, biodegradable and have excellent mechanical properties.^1,2 Polylactic acid (PLA) is one of the most promising alternatives to plastics made from oil, which is attributed to its biodegradability, good transparency and good mechanical performance.^3,4 The interfacial compatibility of fiber and resin is crucial to the mechanical properties of composites,^5,6 and the main reason that affects the interfacial bonding strength of natural fiber reinforced composites is the high hygroscopic property of natural fiber, resulting in poor interfacial bonding performance of the fiber/matrix interface.⁷ Numerous studies have indicated that fiber surface modification has a significant effect on improving the interfacial compatibility of composite materials.

For the interface issue of natural fiber reinforced composites, most studies mainly focus on improving the interface bonding force of materials by modifying the fiber surface.⁸ Common reagents used in chemical treatments include alkali, acetylation, benzoylation, peroxides, maleic acid coupling agents, sodium chlorite, acrylonitrile grafting, isocyanate, stearic acid, permanganate, triazine and fungal agents.^9,10 Alkali and silane coupling agents are commonly used for fiber surface treatment.^11,12 The KH570 silane coupling agent is usually used as a crosslinking agent. It usually contains a siloxane group at one end, which is hydrolyzed to form silanol, and readily reacts with the oxygen-containing groups on the jute surface. The other end contains double bond groups, which can form covalent bonds with nonpolar PLA matrix. Therefore, the KH570 silane coupling agent can act as a bridge to improve the interface stability of fiber resin.¹³ For example, Lin et al.¹⁴ studied the preparation of rubber/aramid fiber composites by grafting aramid fiber with KH570 coupled with silane, and then in-situ forming of silica on the surface of aramid fiber. The experimental results show that the tensile properties of the composites are improved by 31.9%, and the interfacial adhesion of the composites is significantly improved.¹⁴ Liu et al.¹⁵ studied the effects of the silane coupling agent on the chemical properties, surface morphology and mechanical properties of the natural cellulose fiber extracted from natural corn straw and the results show that fibers treated with 5% silane coupling agent perform better with regard to tensile strength. At the same time, silane treatment improved the bond between the fiber and the resin matrix, and increased the impact strength of the composites.¹⁵ Georgiopoulos et al.¹⁶ studied by preparing PLA/flax fiber composites, it is found that the bending property of the composites is better when the concentration of the silane coupling agent solution is 2%. The properties of the composites also deteriorate when the silane concentration exceeds a certain amount.¹⁶

Joffre et al.¹⁷ also studied the effect of acetylation on adhesion between fibers and PLA matrix. Acetylation can significantly improve the strength of PLA/wood fiber composites. The fracture toughness of the composite samples immersed in water can be increased more than 30% by acetylation of the fibers.¹⁷ Acetylation can also efficiently improve the thermal stability of bamboo fibers. After acetylation, the water absorption of bamboo fibers decreases and the surface roughness of the fibers increases, which leads to a better adhesion between the fibers and the matrix in the composites.¹⁸ Majid et al.¹⁹ studied polyvinyl chloride/epoxidized natural rubber/hemp fiber composite, which was prepared by treating kenaf core powder with benzoyl chloride. The results show that the interfacial adhesion between the fibers and the matrix is improved, and the tensile strength, tensile elastic modulus, and elongation at break of the composites are all increased.¹⁹

Alkali treatment is one of most widely used methods for fiber modification, among which sodium hydroxide (NaOH) is a desirable candidate to remove weak components such as lignin on the surface of natural fibers, improve the surface roughness of natural fibers, and thus enhance the adhesion with the resin matrix.^20
–22 Plasma processing requires the fibers to be in a low vacuum chamber, and then gas is introduced into the chamber and ionized to produce plasma. The gases, including oxygen, nitrogen, helium, and air, impinge on the surface of the fiber, causing physical and chemical changes of the fiber structure.^23
–25 The surface roughness of the fibers is increased by plasma treatment, which increases the wettability of the fibers. Therefore, plasma can successfully improve the wettability of hemp fibers and even replace chemical treatment.

In this work, the effect of the concentration of silane coupling agent KH570 treatment on the mechanical and thermal performance of jute/PLA composites has been systematically investigated. Based on the basic composition of jute fiber, the main performance of KH570 treated jute fibers under investigation have been obtained. The mechanical and thermal properties of jute/PLA biodegradable composites were also summarized in terms of tensile, flexural, scanning electron microscopy (SEM), thermogravimetric analysis (TGA) and dynamic mechanical analysis (DMA), which indicate that the jute/PLA composite material has the potential possibility to replace some traditional resin matrix composite materials in construction, automobile, aerospace and other fields.

Materials and methods

Materials

PLA was supplied by Nature Works Company, USA. The selected grade 4032D has a density of 1.24 g/cm³. Jute fiber with a density of 1.45 g/cm³ was purchased from Jiang Ke Linyi Co., Ltd., Shandong, China. Acetic acid was provided by Qiang Sheng Co., Ltd., Jiangsu, China. Silane coupling agent (KH570) and absolute ethyl alcohol were supplied by Sinopharm Co., Ltd., Suzhou, China. All chemicals were AR grade and were used as received. KH570 (CH₃CCH₂COO(CH₂)₃Si(OCH₃)₃) belongs to methacryloxylpropyltrimethoxysilane.

Treatment of jute nonwovens

Jute nonwovens and PLA membranes were prepared in our previous research.²⁶ The preparation process of jute nonwovens is shown in Figure 1. The ethanol aqueous solution was first prepared in the ratio of 2:3. Then a certain amount of KH570 silane coupling agent was added to it to prepare solutions at different concentrations of 1 wt%, 3 wt%, 5 wt%, 7 wt%, and 9 wt%, respectively. The intact jute nonwovens were directly soaked in the solution of silane coupling agent, in the proportion of 1:30, under room temperature for 1 h. The treated jute nonwovens were then washed with deionized water with acetic acid several times, followed by being dried in the oven at 80°C. The jute nonwovens in the control group were just prepared with the final washing procedure.

Figure 1.

The preparation process of jute nonwoven fabric.

Preparation of jute/PLA composite materials

The preparation of jute/PLA composites was performed according to the procedures reported previously.²⁷ Two layers of treated jute nonwovens and three layers of PLA films were cross-arranged in parallel by the method of film stacking. This use of sandwich-like lamination structure layer can mix PLA and jute as evenly as possible. After the composites were completely covered with polytetrafluoroethylene (PTFE) film, they were placed in the customized mold (150 mm × 150 mm) before the release agent was uniformly sprayed. Figure 2 shows the preparation process of the composite material. The final thickness of prepared composites is ∼2 mm.

Figure 2.

The preparation process of composite materials.

Fourier transform infrared test

The Fourier transform infrared (FTIR) test was carried out by a Fourier transform infrared spectrometer (5700; Thermo Nicolet Corporation, USA). After drying, the jute fiber sample was ground into powder, and the dried potassium bromide powder was put into a quartz dish to be fully ground, so that the sample was mixed evenly. After about 10 s of pressing, the sample was tested at the scanning wave number of 400∼4000 cm⁻¹, resolution of 4 cm⁻¹ and scanning number of 32 times.

X-ray diffraction test

X-ray diffraction (XRD) was carried out by an X-ray diffractometer (D8 Advance, Bruck, Germany). All the fiber samples were ground into powder, and appropriately placed in a circular groove until the groove was completely covered. The specific test was performed at the scanning speed of 4°/min in the range of 5∼60°.

SEM of jute fiber

To ensure the sample drying, the fibers were placed in an oven at 70°C and dried for 20 min. The morphology of 5% KH570 treated jute fibers was examined using SEM (S-4800; Hitachi Co., Japan). The tested samples were all gold coated and observed with a field emission gun at an acceleration voltage of 3.0 kV.

Tensile properties

The tensile test was carried out using the universal tensile testing machine (Instron 3365; USA) in accordance with the ASTM D3039 standard. The tensile force is characterized by a dynamometer, which is attached on the upper jaw that is driven hydraulically. The displacement is recorded by the movement of the lower jaw. The composite samples were cut into the size of 125 mm × 12.5 mm. The upper and lower ends of the sample were vertically clamped with a clamping length of 75 mm and stretched at a tensile speed of 2 mm/min by the moving lower jaw. The tensile stress is obtained by, respectively, dividing the tensile force by the initial cross-sectional area of the corresponding sample. The tensile strain of the specimen is obtained by dividing the relative displacement of the jaws byh the clamping length (75 mm). Each sample was measured five times.

Fracture SEM analysis of composites

The cross-sectional morphology of the specimen after the tensile test was observed by a desktop scanning electron microscope (TM3030; Hitachi, Japan). The samples were scanned and observed by firing the gun under the acceleration voltage of 3.0 kV after 90 s of gold sprayed by ion sputtering.

Flexural test

The flexural test was carried out on an electronic universal material tester (Instron 336, USA) with a three-point bending fixture. According to the GBT1449-2005 standard, the sample was cut into a size of 50 mm × 25 mm with a cutting machine and measured at a speed of 2 mm/min. The span distance between two supporting points was set at 24 mm. Each sample was measured five times.

Thermogravimetric analysis

A thermogravimetric analyzer (SDTQ800; TA Instruments, USA) was used to test the TGA of the treated jute/PLA composite samples, and the same procedure was performed as the treated fiber samples. Each sample was measured five times.

Dynamic mechanical thermal analysis

Dynamic thermomechanical analyses were performed using a thermomechanical analyzer (SDTQ800; TA Instruments, USA) in dual cantilever mode. The sample was cut into rectangular splines of 51 mm × 10 mm and tested in air atmosphere under heating from room temperature to 180°C at a heating rate of 3°C/min. Each sample was measured five times.

Results and discussion

The infrared spectrum of jute nonwovens before and after being treated with the silane coupling agent is shown in Figure 3. In the spectrum, the absorption peak at 1750 cm⁻¹ is C = O functional group which is from the carboxyl group contained in the acetic acid dripping during the experiment. The absorption peak at 1695 cm⁻¹ is C = C from the silane coupling agent. The significant enhancement of the peak at 1100 cm⁻¹ is mainly due to the Si-O produced by the reaction between the coupling agent and the fiber.¹⁴ However, as it coincides with the C-O in the jute fiber, no significant new peak can be observed. The obvious enhancement of the absorption peak at approximately 2924 cm⁻¹ is attributed to the C-H stretching vibration of the CH₃ and CH₂ groups in the silane coupling agent. The peak at 3412 cm⁻¹ corresponds to Si-O-H, and the gradual distinctiveness of these two peaks is due to the increase inf the concentration of coupling agent.^14,15 This shows that the silane coupling agent is attached to the surface of the fiber, and also improves the water absorption of the jute fiber.

Figure 3.

Infrared spectrogram before and after silane coupling agent treatment of jute nonwovens.

As can be seen from the XRD patterns of jute nonwovens in Figure 4, two obvious diffraction peaks 2θ = 16.7° and 2θ = 22.3° can be observed, respectively. The crystal shape of the jute fibers is not changed after the KH570 treatment because the positions of the diffraction peaks have not shifted. With the increase in the coupling agent concentration, the intensity of the diffraction peak gradually weakened, which indicates that the crystallinity of the fiber decreased with the addition of the coupling agent. The coupling agent can penetrate into the cellulose molecular segments and destroy the internal hydrogen bonds of the fibers, reducing the crystallinity of the jute fibers. With the decrease in the crystallinity of jute fibers, the mechanical properties of the fibers are weakened, so that the mechanical properties of the composite material decrease.¹³

Figure 4.

X-ray diffraction spectrum before and after silane coupling agent treatment of jute nonwovens.

Figure 5 shows the SEM images of the jute fiber surface treated with 5% KH570. It is found that part of the wax, pectin and lignin on the jute fiber surface is removed by the KH570 treatment, and many uneven protrusions are observed in the form of thin mist. This is due to the fact that the silane coupling agent has two functional groups with different properties, one end is the resin-philic CH = CH₂ functional group and the other end is the fiber-philic C₂H₂ functional group. Figure 6 is the schematic diagram of the reaction between jute fiber and KH570. It can be seen from the diagram that the surface of silanized fiber can form a resin-philic molecular layer, as clearly shown on the fiber surface in the SEM images.

Figure 5.

The scanning electron microscopy (SEM) images of 5% KH570 treated jute fiber.

Figure 6.

The proposed reaction course of hydroxyl groups of jute fibers with silane coupling agent KH570.

Figure 7 shows the tensile properties of jute fiber reinforced PLA composites treated with KH570. Compared with the untreated composite, the tensile strength of the material shows a gradual increasing trend with the increase in the treatment concentration. This is mainly due to the formation of active functional groups on the fiber surface by KH570 treatment, which can increase the nonpolarity and hydrophobicity of jute fiber as well as increasing the interfacial compatibility between fiber and resin. In addition, the tensile strength of the material is all nearly 10 MPa at treatment concentrations of 5%, 7%, and 9%. Figure 7(b) shows the tensile strength and specific strength histograms of jute/PLA composites before and after silane coupling agent treatment. The tensile modulus was calculated according to tensile stress–strain curves with the strain ranging from 0.05% to 0.2%. The initial stage under 0.05% was not considered, because in this period the sample was trying to straighten the buckling which was caused by the curing process. From Figure 7(b) it is also shown that with the increase in the KH570 concentration, the overall tensile strength and specific strength increase first and then basically remain unchanged, indicating that the improvement of the tensile strength of the material treated with 5% KH570 has reached the optimum results, and the further increase in KH570 concentration has no obvious effect on the tensile properties of the material. As can be seen from Figure 7(c), after 3% KH570 treatment, the composite material has the maximum tensile modulus and specific modulus, and then the modulus gradually decreases with the increase in the concentration. This is because after treatment, the interface between jute fiber and PLA resin is improved, enhancing the mechanical properties of the composite, and the tensile modulus increases. However, when the coupling agent content is excessive, the crystallinity of jute decreases seriously, which weakens the mechanical properties of the material.

Figure 7.

(a) Stress–strain curves of jute/polylactic acid (PLA) composites before and after silane coupling agent treatment; (b) tensile strength and specific strength histograms of jute/PLA composites before and after silane coupling agent treatment; (c) tensile modulus and specific modulus histograms of jute/PLA composites before and after silane coupling agent treatment.

SEM images of tensile fracture sections of composites are shown in Figure 8. Figure 8(b) shows the tensile fractured morphologies of 5% KH570 treated jute/PLA composites. There are basically no small round holes left by fiber pull-out compared with the untreated composite material shown in Figure 8(a). It shows that the interfacial bonding of the fibers with the resin is greatly improved after the coupling agent treatment, which is consistent with the analyses of the tensile properties of the fibers in the last part.

Figure 8.

The tensile fracture morphologies of jute/polylactic acid (PLA) composites. (a) The tensile fracture morphologies of untreated jute/PLA composites; (b) the tensile fractured morphologies of 5% KH570 treated jute/PLA composites.

The flexural properties of composites treated with different concentrations of KH570 are shown in Figure 9. As can be seen from the stress–strain curves in Figure 9(a), the flexural strength of the material increases gradually with the increase in KH570 treatment concentration, and the bending strength of the sample treated with 9% KH570 is the maximum reaching up to 14 MPa. Compared with untreated composite material, it is found that the increase in flexural strength is more obvious than that in the tensile strength of the composite treated with KH570. Moreover, with the increase in treatment concentration, the flexural strength of the composites also increases further, which is different from the gradually unchanging tensile properties, indicating that silanization of the jute fibers has a more significant effect on the flexural properties of the composites. In addition, it can be seen from the histograms of flexural strength and modulus in Figure 9(b) that the flexural modulus of the material does not show an obvious increasing tendency with the increase in concentration of KH570.

Figure 9.

(a) Flexural stress–strain curves of jute/polylactic acid (PLA) composites before and after silane coupling agent treatment; (b) llexural strength and modulus histograms of jute/PLA composites before and after silane coupling agent treatment.

Figure 10 shows the thermogravimetric results of composite materials processed by KH570 treated jute fiber. As can be seen from the thermogravimetric curve in Figure 10(a), the composites show no significant weight loss at the initial stage, which indicates that the composites have no obvious moisture loss at 100°C for the hydrophilicity of the composites decreases effectively after the treatment with KH570. As can be seen from the derivative thermogravimetry (DTG) curve in Figure 10(b), with the increase in the KH570 concentration, the DTG peaks gradually shift to the right, indicating that the temperature at the fastest weightlessness of the material increases. Meanwhile, the peak value of the DTG curve gradually increases, indicating that the mass loss rate becomes faster at about 345°C and the thermal stability of the material decreases gradually. Therefore, the high concentration of silane coupling agent treatment will have a negative impact on the thermal stability of the composites. Table 1 lists the initial decomposition temperature (T_onset), the maximum decomposition temperature (T_max) and the char residual of the composites untreated and treated with KH570. It can be seen from Table 1 that the T_onset and the T_max of the composite without coupling agent treatment are 281°C and 360°C, respectively. With the increase in the coupling agent concentration after treatment, the T_onset of treated composites stays relatively stable except for 9% KH570, while T_max shows a slightly increasing trend, which is consistent with previous analysis.

Figure 10.

(a) Thermogravimetric plots of the KH570 treated composites; (b) DTG plots of the KH570 treated composites.

Table 1.

The T_onset, T_max and the char residual of the composites treated with KH570

Sample	T_onset (°)	T_max (°)	The char residual (%)
Original	281	360	6.0
1% KH570	278	362	6.2
3% KH570	281	364	7.6
5% KH570	276	365	9.4
7% KH570	282	366	7.0
9% KH570	292	369	7.1

T_onset: initial decomposition temperature; T_max: maximum decomposition temperature.

Figure 11 shows the dynamic thermomechanical properties of composites treated with KH570 at different concentrations. It can be seen from the curves of the storage modulus in Figure 11(a) and loss modulus in Figure 11(b) that there are two obvious state changes of the material, which are mainly caused by the three-state transition of PLA. The glass transition occurs at 68°C and then the material enters a high-elastic state. At 160°C, due to the melting of PLA, the molecular chain movement is intensified and the material thus enters the viscous state. It can be clearly seen from the tangent value of the loss angle curve of Figure 11(c) that the peak value of the loss factor decreases gradually with the increase in the KH570 concentration, indicating that the internal crosslinking degree and the rigidity of the composites increase while the damping decreases. With the increase in KH570 concentration, the improvement of the crosslinking degree between fiber and resin also leads to the increase in internal friction of fiber and resin, so the change in storage modulus of the material is actually not obvious.

Figure 11.

(a) Storage modulus of the produced composites under different KH570 treatment concentrations (E′); (b) loss modulus (E′′) of the produced composites under different KH570 treatment concentrations; (c) loss factor tan δ (E′′/E′) of the produced composites under different KH570 treatment concentrations.

Conclusions

A series of KH570 treated jute nonwovens sandwiched in PLA membrane with different treatment concentrations were effectively processed. The effect of fiber surface treatment on mechanical and thermomechanical properties of jute/PLA degradable composites was first analyzed in detail.

It was found that after being treated with coupling agent, interface interaction between jute fibers and PLA matrix enhances quite a little. The coupling agent KH570 enhances the tensile properties of composites by improving the surface nonpolarity of fibers. At low concentrations, the tensile strength of the material increases gradually with the increase in KH570 concentration. When the concentration of KH570 increases to 5%, the tensile strength and flexural strength of the material tend to be stable. Due to the treatment of KH570, the amorphous proportion in the composite increases accordingly and results in a decrease in the storage modulus of the composite to varying degrees. Given a comprehensive consideration of the mechanical and thermomechanical response, jute/PLA composites have great potential in the field with a high toughness requirement.

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) disclosed receipt of the following financial support for the researech, authorship, and/or publication of this article: This work was supported by National Natural Science Foundation of China (grant nos. 11802192 and 11602156), Science and Technology Guiding Project of China National Textile and Apparel Council (grant no. 2020064), and Foundation Project of Jiangsu Advanced Textile Engineering Technology Center (grant no. XJFZ/2021/3).

ORCID iDs

Yuan-yuan Li

Ping Wang

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