Effects of fiber loading and chemical treatments on properties of sisal fiber-reinforced sheet molding compounds

Abstract

In this study, the effects of fiber loading from 0 vol.% to 15 vol.% and the chemical treatments on the properties of the sisal fiber-reinforced sheet molding compounds were investigated. The chemical treatments were performed with alkali, γ-ammoniapropyl-triethoxy-silane (KH-550) and γ-methacryloxypropyl-trimethoxy-sliane (KH-570), respectively, to improve the interfacial adhesion between sisal fiber and the matrix (unsaturated polyester resin). The changes of surface morphology and sisal fiber functional groups were investigated by scanning electron microscopy and Fourier transform infrared spectroscopic analysis, respectively. Compared to the untreated sisal fiber-reinforced sheet molding compounds, the mechanical property of the treated sisal fiber-reinforced sheet molding compounds increased due to the interfacial enhancement between sisal fiber and the matrix, which could be observed through the scanning electron microscopy images of fracture surface. Moreover, the better interfacial adhesion was also assisted in water absorption resistance and the thermal stability of the treated sisal fiber-reinforced sheet molding compounds. In the chemical treatments, KH-570 treatment was proved to be an effective method to improve the interfacial adhesion between sisal fiber and the matrix.

Keywords

Natural fiber sheet molding compound mechanical properties thermal properties

Introduction

Nowadays, serious environmental problems caused by the utilization of non-renewable reinforcements in the composites have led to the investigation for more friendly and sustainable materials as reinforcement.^1,2 Natural fibers such as sisal, hemp, bamboo, and jute are widely used as reinforcement in polymer-based engineering composites due to their advantages of inherent biodegradability, low density, and low cost. The potential applications of natural fiber-reinforced composites include door, instrument panels, car upholstery, seat back, etc.^3,4

However, the poor quality of the interface between hydrophilic natural fibers and hydrophobic polymer matrix seriously limits the application of natural fibers in structural materials.⁵ In particular, the great moisture absorption of natural fibers adversely affects adhesion with hydrophobic matrix leading to loss of strength. Nowadays the issues of interfacial compatibility and water absorption have been gradually overcome through chemical surface modification of natural fibers using alkali, silane, or acetylation treatment and physical surface treatments such as heat treatment, corona treatment, and plasma treatment.^6,7 Chemical and physical treatments enhance interfacial adhesion between fibers and matrix, which leads to increasing the tensile, flexural and impact properties.⁸ Hill and Abdul Khali⁹ investigated the effect of chemical modifications such as acetylation, silane treatment, and titanate coupling on the mechanical properties of coir and oil palm fiber-reinforced polyester composites. A small increase in tensile strength, tensile modulus, and impact strength of the composites reinforced with modified fiber was noted. Rout et al.¹⁰ investigated the effect of alkali treatment on the performance of coin-polyester composites. As the concentration of sodium hydroxide increased, the mechanical properties decreased due to the cell wall thickening which led to poor adhesion with polyester resin. Felix et al.,¹¹ Belgacem et al.,¹² and Sapieha et al.¹³ utilized heat treatment, corona treatment, and plasma treatment to modify the surface of the fibers, respectively. They obtained better mechanical property of the composites than that of untreated fiber-reinforced composites.

There are many studies on the effect of surface treatment and fiber loading on the properties of fiber-reinforced polymer composites.^14–19 However, there are very few reports about the effect of surface treatment and fiber loading on the properties of fiber-reinforced sheet molding compounds (SMC). SMC consists mainly of a paste reinforced with short glass fiber. The paste is generally a polyester resin filled with calcium carbonate and other additives, whereas glass fiber is initially randomly oriented in the plane of the sheets and makes up for about 15–30% of the weight fraction. SMC has gained extensive applications in construction industry, electronics and transportation.

To the best of knowledge, few studies on natural fiber-reinforced SMC have been published. van Voorn et al.²⁰ discussed a non-automated SMC process for making bio-composites and researched the mechanical properties of the flax fiber SMC (FF-SMC). They concluded that the stiffness of FF-SMC was equal to or even slightly better than the stiffness of glass fiber-SMC with varying the fiber, filler percentages and fiber length. Geeta et al. ²¹ introduced novel bio-composites for low-cost housing panel applications. The bio-composites were made with various natural fibers including big blue stem grass, jute, and industrial hemp. They found that the natural fiber–UPE composites had almost the same specific strength and modulus as those of glass fiber–UPE composites. However, in the previous studies, the effect of chemical treatments on properties of sisal fiber (SF)-reinforced SMC has not been discussed.

In this study, the effects of fiber loading ranging from 0 vol.% to 15 vol.% and the chemical treatments on the properties of SF-reinforced SMC were investigated. SF was treated by alkali, γ-ammoniapropyl-triethoxy-silane (KH-550), and γ-methacryloxypropyl-trimethoxy-sliane (KH-570), respectively. The changes of SF surface morphology and functional groups were investigated by scanning electron microscopy (SEM) and Fourier transform infrared (FTIR) spectroscopic analysis, respectively. Compared to untreated SF-SMC, the mechanical property of the treated SF-SMC increased due to the interfacial enhancement between SF and matrix. Moreover, the better interfacial adhesion was also assisted in water absorption resistance and the thermal stability of the treated SF-SMC.

Experimental

Materials

SF was supplied by Oriental Sisal Group Co. Ltd. Unsaturated polyester (UP), curing catalyst tert-butyl peroxybenzoate (TBPB), anti shrink agent (analytical reagent grade), zinc stearate (analytical reagent grade), calcium powder (analytical reagent grade), MgO powder (analytical reagent grade) were supplied by China National Chemical Corporation. NaOH (analytical reagent grade), acetic acid (analytical reagent grade), KH-550 and KH-570 were purchased from Sinopharm Chemical Reagent Co. Ltd. The properties of SF and unsaturated polyester are given in Tables 1 and 2, respectively.

Table 1.

Mechanical property of SF.

Fiber style	Density (g/cm³)	Strength (MPa)	Stiffness (GPa)
Sisal fiber	1.3–1.5	300–800	9–22

Table 2.

Mechanical property of unsaturated polyester resin.

Resin	Tensile strength (MPa)	Flexural strength (MPa)	Impact strength (KJ·m⁻²)
UP	46.46	72.64	7.18

Table 3.

Formulations of SF-SMC composites.

Designation	Code	Fibers (vol.%)	Resin (vol.%))	CaCO₃ (vol.%)	TBPB (vol.%)	PS (vol.%)	ZnSt (vol.%)	MgO (vol.%)
Untreated	U-S1	0	40.0	45.0	1.5	10.0	2.5	1.0
	U-S2	5.0	35.0	45.0	1.5	10.0	2.5	1.0
	U-S3	7.5	32.5	45.0	1.5	10.0	2.5	1.0
	U-S4	10.0	30.0	45.0	1.5	10.0	2.5	1.0
	U-S5	12.5	27.5	45.0	1.5	10.0	2.5	1.0
	U-S6	15.0	25.0	45.0	1.5	10.0	2.5	1.0
Alkaline treated	N-S2	5.0	35.0	45.0	1.5	10.0	2.5	1.0
	N-S3	7.5	32.5	45.0	1.5	10.0	2.5	1.0
	N-S4	10.0	30.0	45.0	1.5	10.0	2.5	1.0
	N-S5	12.5	27.5	45.0	1.5	10.0	2.5	1.0
	N-S6	15.0	25.0	45.0	1.5	10.0	2.5	1.0
KH-550 treated SF	550-S2	5.0	35.0	45.0	1.5	10.0	2.5	1.0
	550-S3	7.5	32.5	45.0	1.5	10.0	2.5	1.0
	550-S4	10.0	30.0	45.0	1.5	10.0	2.5	1.0
	550-S5	12.5	27.5	45.0	1.5	10.0	2.5	1.0
	550-S6	15.0	25.0	45.0	1.5	10.0	2.5	1.0
KH-570 treated SF	570-S2	5.0	35.0	45.0	1.5	10.0	2.5	1.0
	570-S3	7.5	32.5	45.0	1.5	10.0	2.5	1.0
	570-S4	10.0	30.0	45.0	1.5	10.0	2.5	1.0
	570-S5	12.5	27.5	45.0	1.5	10.0	2.5	1.0
	570-S6	15.0	25.0	45.0	1.5	10.0	2.5	1.0

Surface modification of SF

SF was initially cleaned with distilled water to remove the dirt on the surface. The washed SF was dried in an oven at 70℃ for 12 h. Then the SF was cut to 25 ± 1 mm in length.

Alkali treatment

The chopped SF was immersed in 10% alkaline solution for 24 h. The alkaline solution was made by dissolving the sodium hydroxide in aqueous solution. Then it was neutralized with acetic acid and washed with distilled water repeatedly. Finally, the SF was dried in an oven at 70℃ for 12 h.²²

Treatment of KH-550 silane

The treatment of the SF with KH-550 (0.05 M) was carried out in 80/20 (v/v) ethanol/water for 120 min under stirring. A pH of 4 of the solution was maintained to bring about the complete hydrolysis of the silane by the addition of acetic acid. After the treatment, the fiber was washed with distilled water and dried in oven at 70℃ for 12 h.

Treatment of KH-570 silane

The treatment of the SF with KH-570 (0.05 M) was carried out in 60/40 (v/v) ethanol/water for 90 min under stirring. A pH of 4 of the solution was maintained to bring out the complete hydrolysis of silane by the addition of acetic acid. Then the fiber was washed with distilled water and dried in an oven at 70℃ for 12 h.

Fabrication of SF-SMC composites

SMC was produced in a two-step way.²³ First, UP was mixed with an initiator (TBPB), an anti-shrink agent (PS), a thickener (MgO), zinc stearate, and calcium powder in the condition of stirring for 30 min. Then the paste was transformed onto two pieces of PE films. The treated fiber was evenly distributed onto the PE films and impregnated with the paste under a definite pressure. The second step was thickening stage. The prepreg was left to mature for one week at room temperature, then transformed to an oven at 40℃ for 48 h. The prepreg was moved to a mold (25 cm × 10 cm ×3 cm) at a temperature of 140℃ and a pressure of 6 MPa. The curing time was 4 min. The formulations of SF-SMC composites are listed in Table 3.

Characterization

Mechanical property measurements

Tensile property was investigated according to ASTM D638 standard. Flexural property was determined in a three-point bend mode according to ASTM D790 standard. The measurements were done by the universal testing machine (Shanghai Shenli Testing Machine Company, China) at a speed-head of 5 mm/min at room temperature. Izod impact measurement was performed by IMPats-15 impact tester (Chengde Jinjian instrument company, China) at room temperature. The impact strength was determined by striking the bar-shaped specimen with a hammer as per ASTM D256.

SEM analysis

The fiber surface and the impact fracture morphology of the SF-SMC composites were observed by means of SEM (FEI QUANTA FEG 250). The samples were sputter coated with a fine layer of gold in a sputter coater.

FTIR analysis

FTIR measurement was conducted on Nicolet 380 infrared spectrometer (Thermo Electron Corporation, USA) to obtain spectra of functional groups for the SF and unsaturated resin with the spectral range 4000–400 cm⁻¹.

Water absorption test

The specimens of water absorption test were cut with dimensions of 30 mm×28 mm×3 mm. The experiment was carried out to determine the moisture resistance of the SF-SMC according to ASTM D570. The specimens were dried in an oven for 24 h at 70℃. The specimens were denoted as W_original when they cooled down to the room temperature. The specimens were immersed in distilled water at room temperature. The amount of water absorbed by the specimens was measured when they were taken out of water each time, once every day for a week. The surface was wiped dry and weighted, and recorded as W_wet. The percentage water absorption (W%) was then calculated by the formula below

w % = \frac{w_{wet} - w_{original}}{w_{original}} \times 100 %

(1)

The thermal decomposition analysis

TGA was carried out to determine the thermal decomposition of the SF-SMC using TA D-09123 analyzer (TA Instruments, METTLER TOLEDO, China). The sample was placed in alumina pans and heated from 30℃ to 500℃ for 10 min⁻¹ under insert nitrogen atmosphere.

Results and discussion

The morphology of SFs

SEM images of the raw SF, alkaline-treated SF, KH-550-treated SF, and KH-570-treated SF are shown in Figure 1. It can be seen that the SFs comprise bundles of individual cells, which are bounded together by node-like substance. The surface of raw SF is covered with lots of wax. Alkali treatment was used to remove the lignin and created crack space on the SF,²⁴ as shown in Figure 1(b). After KH-550 or KH-570 treatment, the surfaces of the SFs seem to be cleaner than the raw SF. The reason might be that the fiber surface was completely covered by the silane coupling agents and a smooth layer of silane was formed on the fiber surface.

FTIR spectroscopy of SFs

FTIR spectra of raw and treated SFs are shown in Figure 2. The bond around 1740 cm⁻¹ is ascribed to the acetyl and ester groups of hemicellulose and aromatic components of lignin. The band around 1640 cm⁻¹ is related to the bending of water absorbed into cellulose fiber structure and the absorption band at 1380 cm⁻¹ is associated with C–H₃ bending of cellulose. The band at 1260 cm⁻¹ is ascribed to the C–O stretching vibration of hemicellulose component.^25–27 After the alkali treatment, the peaks at 1740 cm⁻¹ and 1260 cm⁻¹ disappear indicating that the alkali treatment removed hemicellulose and a portion of lingin. KH-550 silane-treated SF shows new absorption band at 1560 cm⁻¹ and 1150 cm⁻¹ assigned to NH₂ and Si–O–Si bending vibration of silane agent indicating that the KH-550 silane was successfully grafted on the SF. As a result of KH-570 silane treatment, the new peaks at 1700 cm⁻¹ and 1160 cm⁻¹ relating to the C=C and Si–O–Si stretching vibration of KH-570 silane agent appear, which indicates that the KH-570 was also successfully grafted on the SF.

Mechanical and morphological properties of SF-SMC composites

The effects of fiber loading and chemical treatments on the tensile strength of the SF-SMC composites are displayed in Figure 3. It can be observed that the tensile strength increases with the increase of fiber loading, up to 10 vol.%. After that, the tensile strength slightly decreases at 12.5 vol.% fiber loading for all chemical treatment SF-SMC composites. The flexural strength and the impact strength were also investigated, as shown in Figures 4 and 5. The results show the same trend as that of the tensile strength. It is generally considered that it is more difficult for the resin to infiltrate the fiber at high fiber loading level as well as weaken the interfacial adhesion between SF and matrix.^28–30 From the studies of the tensile, flexural and impact properties, it can be concluded that the optimum fiber loading is at 10 vol.% for all SF composites.

Meanwhile, the KH-570 silane-treated SF-SMC composite shows the best tensile strength 68.46 MPa which improves about 18.70% compared with that of the raw SF-SMC composite at the optimum fiber loading (10 vol.%). The KH-550 silane-treated SF-SMC composite shows the tensile strength of 64.96 MPa which improves by about 12.62%. The alkaline-treated SF-SMC composite shows the tensile strength 61.54 MPa which improves by about 6.70%. As for the flexural and impact strength of the KH-570 silane-treated SF-SMC composite, they were obtained as 91.44 MPa and 19.82 KJ/m², respectively, which improve by about 15.75% and 18.43% compared to the untreated SF-SMC composite. The flexural and impact strength of the KH-550 silane-treated SF-SMC composite were obtained as 87.54 MPa and 18.37 KJ/m², respectively, which improve by about 10.81% and 12.84%. The flexural and impact strength of the alkaline-treated SF-SMC composite were obtained as 83.68 MPa and 16.8 KJ/m², which improve by about 5.92% and 7.96%, respectively.

The SEM images of the impact fracture surfaces of the SF-SMC (10 vol.% SF) composites with the different chemical treatments are shown in Figure 6 to further identify the effect of chemical treatments on mechanical properties of the SF-SMC composites. The improvement in the interfacial adhesion between the treated SF and the matrix can be clearly seen from the SEM images. Figure 6(a) shows that the interface between the untreated SF and the matrix has obvious cracks leading to the weak interfacial bond. Figure 6(b) shows that the interfacial adhesion between the alkaline-treated SF and the matrix is closer than that of Figure 6(a). After KH-550 and KH-570 silane treatments, the interfaces between SF and matrix combine more closely than the alkaline treatment, as shown in Figure 6(c) and (d).

For the alkali treatment, NaOH reacted with hydroxyl groups of the cementing material hemicellulose. It brought the destruction of the cellular structure. Thereby the SF split into filaments, increasing the effective contacting surface area with the matrix.

For the silane treatments, there was a possibility of a direct condensation reaction between the silanol groups (Si–OH) and hydroxyl groups of the SF. The silanol formed strong covalent bonds or hydrogen bonds with the O–H groups of cellulose. The remaining silanol groups were capable of forming hydrogen bonding or condensing with adjacent silanol groups (Si–O–Si). The scheme for the interaction of silane with natural fibers by hydrolysis is shown in Figure 7. The long hydrophobic polymer chain of polymerized silane could adhere to the matrix mainly due to the van der Waals attractive forces. As a result, silane-coupling agent formed a bridge at the interface and caused a good fiber–matrix interaction.³¹ For KH-570 silane, the methacrylic groups of KH-570 silane were capable of reacting with the double bonds of the UP matrix. Thus, a tightly combined interface between SF and matrix was formed, as shown in Figure 6(d). The dominant co-polymerization mechanism is shown in Figure 8.

Water absorption test

The water absorption of the SF-SMC composites is shown in Figure 9. It can be observed that the water absorption of all the SF-SMC composites increases with the growth of time. However, the water absorption of the SF-SMC composites decreases obviously due to the chemical treatments. The alkaline treatment could remove hydrophilic components (lingin and wax) of the SF. KH-550 and KH-570 treatments could form a compact coating on the SF surface, which prohibited the water infiltration effectively. As a result, the untreated SF-SMC composite shows the highest water absorption whereas the KH-570-treated SF-SMC composite shows the lowest water absorption.

Thermal decomposition of SFs and SF-SMC composites

TGA was used to identify the thermal degradation process of the SFs and the SF-SMC composites. In Figure 10(a), TGA curves were obtained during the heating of the SFs from 30℃ to 500℃. Thermal degradation takes place in two stages for both the untreated SF and the treated SF. The initial 6–8% of weight loss is attributed to the inherent moisture in the SF. But after 250℃, the SF decomposes rapidly and the SF completely degrades at around 375℃. This second stage degradation involves the degradation of hemicellulose, lignin and cellulose. The temperatures are in the similar range as analyzed by Oza et al.³² In the case of the treated SFs, the thermal stability is higher due to the removal of organic impurities such as pectin and wax on the surface of SFs. For the alkaline-treated SF, it shows higher thermal stability because the treatment removed most of the impurities and exposed higher amount of cellulose molecules. After the silane treatments, the surface coating weakened SF thermal stability compared to the alkaline-treated SF.

Compared to KH-550 and KH-570 silane-treated SF-SMC composites (Figure 10(b)), the alkali-treated SF-SMC composite has the lowest thermal stability from 25℃ to 250℃ because the coupling reaction happened once the SF was brought in contact with the matrix. The coupling reaction resulted in strong covalent bonds, such as the Si–O–Si. However, the alkaline-treated SF resulted in only hydrogen bonding with the matrix and these bonds were not as strong as the covalent bonds created by the silane treatment. Comparison of the two silanes shows that the methacrylic group in the KH-570 was capable of reacting with the double bonds of the UP matrix. However, the KH-550 silane combined with the matrix attributing to van der Waals attractive forces. As a result, the KH-570 silane-treated SF-SMC composite showed the better thermal stability than that of KH-550 silane-treated SF-SMC composite.

Conclusions

In this study, the effects of fiber loading and different fiber treatments on the properties of SF-SMC were investigated. The optimum fiber loading was found to be 10 vol.%. The KH-570-treated SF-SMC with 10 vol.% SF loading possessed the best interfacial adhesion, showing an increase of 18.70% tensile strength, 15.75% flexural strength, 18.43% impact strength, respectively. It was good enough to transfer the stress from the matrix to the SF at 10 vol.% fiber loading. When the fiber loading exceeded 10 vol.%, fiber clusters appeared and the mechanical properties decreased. The KH-570 silane treatment could induce a compact coating on the SF surface, resulting in the largest reduction in water absorption. Meanwhile, the KH-570 silane-treated SF-SMC possessed the best thermal ability because the coupling reaction could result in strong covalent bonds. Therefore, the SF-SMC containing 10 vol.% of KH-570-treated SF was selected as the composite with optimal properties. The results also promise a simple and effective approach to achieve green composite for great application potentials.

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 research, authorship, and/or publication of this article: This work is supported by the Special Program for Independent Innovation and Achievements Transformation of Shandong Province, China (grant no. 2014ZZCX05302).

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