Mechanical properties of surface-treated ramie fiber fabric/epoxy resin composite fabricated by vacuum-assisted resin infusion molding with hot compaction

Abstract

The optimization of composite manufacturing process is an important means for improving mechanical properties of natural fiber composites. In this paper, ramie fiber fabrics were, respectively, modified by NaOH, KMnO₄, and ammonium polyphosphate flame retardant, and then ramie fabric/epoxy resin composite laminates were prepared using vacuum-assisted resin infusion molding. In order to increase ramie fiber content in composite, vacuum pressure compaction and hot compaction with high pressure and temperature were, respectively, used to compress the fabric stack before molding. The effects of precompaction process on fiber compaction and mechanical properties of ramie fiber yarns and composites were studied. The comparison with the ramie fiber fabric without surface treatment was also done. It is found that surface-treated ramie fiber fabric has lower compressibility than the untreated one. Moreover, hot compaction with suitable conditions is effective in increasing fiber content and mechanical properties of all studied ramie fabric composites.

Keywords

Ramie fiber polymer matrix composite vacuum-assisted resin infusion molding surface treatment hot compaction mechanical property

Introduction

Natural plant fibers, especially bast fibers, have received a great deal of attention as reinforcement instead of synthetic fibers for composite because of their abundant source, low cost, easy recyclability, as well as high specific strength and modulus.^1–5 Among bast fibers, ramie fiber takes the leading position with high cellulose content, long length, and high strength. In addition, on account of its characteristics of antimicrobial and anticorrosion, ramie fiber is an ideal reinforcement material for resin matrix composites.^6,7 The forming processes for natural fiber reinforced resin matrix composites include hot-press forming and liquid composite molding, such as resin transfer molding (RTM) and vacuum-assisted resin infusion (VARI).^8–10 VARI is a kind of low-cost processing, whose main principle is exhausting the gas in dry fiber preform and meanwhile injecting low-viscosity resin into fiber preform under vacuum pressure. No complex molds and equipments are needed, thus cost is substantially cut in VARI.¹¹

Owing to low external pressure, fiber content of composite manufactured by VARI is generally lower than those in hot-press process and RTM, resulting in low mechanical properties, especially for fiber-dominated performances.¹² It has been found that the mechanical properties of natural fiber composite are directly related to fiber content.¹³ Some works have confirmed that the degree of increase in mechanical property of natural fiber reinforced composite is obviously larger than that in fiber content.^14,15 Thus, improving mechanical properties by increasing fiber content for natural fiber reinforced composite is more important than that for synthetic fiber composites.

Some works were done to study fiber compaction behavior in VARI. Duan et al.¹⁶ suggested that loading and unloading cycles on fiber preform during VARI process could achieve higher fiber volume fraction (V_f). Fumihito et al.¹⁷ found that after the densification of carbon fiber preform was accomplished by hot compaction, composite parts with high V_f could be obtained by VARI. Our previous study¹⁸ also indicated that the hot compaction process could increase fiber content of natural fiber composites in VARI. In the process of hot compaction, fiber preform was compacted under controlled pressure and temperature before performing vacuum bag assembly and injecting resin. Wu et al.¹⁹ studied the compaction behavior of bindered textile preforms and found compaction temperature was a main factor for determining fiber compaction degree. Francucci et al. and our studies^18,20,21 found that natural fibers were likely to be degraded if exposed to unsuitable high processing temperature and pressure. It can be seen that precompaction process is capable of increasing fiber content, and the processing parameters should be optimized for avoiding adverse effect on natural fiber composite.

For natural fiber reinforced composites, another common method for enhancing mechanical properties is surface treatment. In fact, there are a large number of polar hydroxyl and phenolic hydroxyl groups with high hydrophilia on the surface of natural fibers, giving rise to poor compatibility with nonpolar polymer. As a result, insufficient adhesion between fibers and polymer matrix happens, degrading mechanical properties of composites. Chemical and physical surface treatments can efficiently reduce fiber hydrophilia by decreasing the polar groups, so that the interfacial compatibility and mechanical properties are improved.^22,23

Alkali treatment is an important surface treatment which can be used in industrial production. It was found that interfacial compatibility was significantly improved after NaOH surface treatment. Meanwhile the impurities, pectin and lignin were removed, and fibers became more fluffy with larger specific surface area.^24,25 Potassium permanganate is used to modify natural plant fibers as a kind of oxidizing agent. The ingredients that link microfibers, for instance pectin, are removed through the oxidation process. For this reason, one fiber is divided into many microfibers, so the contact area between fibers and resin is increased. Moreover, the harsh surface of fibers resulting from oxidation creates mechanical interlock between fibers and resin.²⁶ Besides, as a kind of combustible material, the application of natural plant fibers is limited in flame-retardant requirement areas. Treating natural fibers with nitrogen–phosphorus flame retardant is an efficient method to improve flame retardancy of natural fiber composites. Li et al.²⁷ found that fiber surface was partly coated with flame retardant after the treatment of ammonium polyphosphate (APP) flame retardant. It can be seen that structure, composition, and surface morphology of natural fibers can be obviously changed by surface treatments.

At present, the researches on improving mechanical properties of natural fiber reinforced composites are mainly focused on surface treatment method, but less attention is paid to improving the fabrication process of surface-treated natural fiber reinforced composites. For example, by means of optimizing compaction process of VARI, increasing fiber content for improving mechanical properties of natural fiber composites might be realized. Due to the changes of structure, composition, and surface morphology of natural fibers after surface treatments, the effect of compaction process on the property of surface-treated natural fiber reinforced composites needs further study. In this paper, ramie fiber fabrics were, respectively, modified by NaOH, KMnO₄, and APP flame retardant. To study the effect of compaction process on fiber content and mechanical properties of these surface-treated ramie fiber/epoxy resin composites prepared by VARI, all kinds of ramie fabric stacks were compacted by two different precompaction processes. In order to optimize hot compaction conditions, the influence of hot compaction temperature on tensile strength (TS) of surface-treated ramie fiber yarns was investigated. Furthermore, mechanical properties of surface-treated ramie fiber fabrics reinforced composite laminates were compared with that of untreated one for evaluating the applicability of proposed composite processing method.

Experimental details

Materials

Ramie fiber biaxial plain weave fabric (Produced by Hunan Dongting Co. Ltd) was used in this paper. The major components of ramie fiber are cellulose, hemicellulose, pectin, and lignin.²⁸ Each yarn with diameter of 100–300 µm consists of many monofilaments with diameter of 20–40 µm. The volume density of ramie fiber is 1.56 g/cm³ while the areal density of fabric is 121 g/m². Figure 1 shows the morphology and structure of ramie fiber fabric. Three kinds of surface treatment methods were adopted to prepare surface-treated ramie fiber fabric, and the chemical reagents used to modify ramie fibers are listed in Table 1. A kind of DGEBA epoxy resin with a polyether amine-based curing agent (supplied by Tianjin Dasen Co. Ltd, named HJ-966) was used as the matrix of composite. The resin is suitable for fabricating composite in VARI process because of its low viscosity (less than 250 mPa s at 25℃) and good wettability.

Table 1.

Chemical reagents used for modifying ramie fiber fabric.

Surface treatment	Chemical reagent	Manufacturer
Alkali treatment	Sodium hydroxide (NaOH)	Sinophar Chemical Reagent Co. Ltd
Oxidation treatment	Potassium permanganate (KMnO₄)	Shanghai Meixing Chemical Co. Ltd
Flame retardant treatment	Ammonium polyphosphate (APP)	Shanghai Xusen Flame Retardant Co. Ltd

Surface treatments

For alkali treatment, ramie fiber fabrics were soaked in 5% mass concentration of NaOH aqueous solution for 2 h at room temperature.²⁵ Then the fabrics were washed several times to remove any NaOH solution sticking to the fiber surface and the fabrics were oven dried at 60℃ for 24 h. For oxidation treatment, ramie fiber fabrics were soaked in 0.055% mass concentration of KMnO₄ acetone solution for 5 min at room temperature,²⁶ cleaned with acetone, and dried at 60℃ for 24 h to remove excessive solvent. For flame retardant treatment, ramie fiber fabrics were soaked in 5% mass concentration of APP ethanol solution for 2 h at room temperature, pressed to squeeze solution, and dried at 60℃ for 24 h.²⁷

Compaction process

Ten layers of ramie fabric with 250 mm × 250 mm were cut and dried at 70℃ for 3 h in an oven, which were then unidirectionally stacked. The ramie fabric stack was compacted by either traditional vacuum pressure compaction or hot compaction in order to increase fiber content.

Vacuum pressure compaction was performed in vacuum bagging assembly. The compaction time was 15 min, and more than 0.095 kPa vacuum pressure was applied. After the first compaction cycle, the process of loading and unloading vacuum cycle was repeated two times for further compacting the stack.

Hot compaction was carried out in a universal testing machine (Instron 3382) using a 100 kN load cell, as shown in Figure 2. The compression clamps with sample were placed in a temperature controlling chamber installed on the machine, so that compression data of ramie fiber stack at certain temperature could be obtained. The preheating time for each test was 20 min. The fabric stack was compressed at a velocity of 200 N/s until the setting compressive load was reached, and then the applied pressure was kept constant for 30 min.

Composite laminate preparation

The composite laminate was manufactured using VARI process. It was divided into three stages as follows. (1) The precompacted dry fabric stack was placed on an one-side mold and then enclosed in a vacuum bag sealed on the mold, as shown in Figure 3. (2) The gas in the vacuum bag was exhausted by vacuum pump and at the same time HJ-966 epoxy resin was injected into the stack under vacuum pressure. The injection time was 5 min, and after that the vacuum pressure was kept for 20 min so that the resin fully infiltrated the fabric stack. (3) The assembly under constant vacuum pressure was put in an oven and cured at 70℃ for 6 h.

Characterization

Scanning electron microscopy (SEM

A CamScan-Apllo 300 SEM was used to observe the morphology of ramie fiber yarns extracted from ramie fabric before and after surface treatments. Ramie fiber composite samples cut from the cured laminate were also viewed by this instrument. The cross-sections of the samples were wet ground successfully with finer silicon sandpaper from 150 to 3000 grit and were wet polished with chromium oxide to obtain smooth surfaces. To obtain clear images, all samples were gold coated.

Dynamic mechanical analysis

The dynamic mechanical behavior of ramie fiber fabrics/epoxy resin composite was studied using T101423D DMA instrument. The experiment was carried out at frequency of 1 Hz and at a heating rate of 5℃/min. The testing temperature ranged from 30 to 150℃.

Measurement of fiber content

Fiber mass fractions of laminate (W_f) for all cured laminates were measured with a specimen of 100 mm × 50 mm cut from the laminate. W_f was calculated using the following equation

W_{f} = \frac{s_{0} \cdot n \cdot ρ_{s}}{m}

(1)

where s₀ was the area of specimen, n was the number of fabric layers, ρ_s was the areal density of the ramie fabric, and m was the mass of laminate.

Tensile testing for ramie fiber yarns

TS of ramie fiber yarns extracted from ramie fabric was tested according to ISO 11566:1996²⁹ using a universal testing machine (Instron 3344) with a 2000 N load cell and at a crosshead speed of 0.5 mm/min. The diameter of single fiber yarn was measured by an optical microscope for calculating cross-section area of yarn. At least 12 specimens were measured for each kind of ramie fabric.

Mechanical testing for composite laminate

For each type of cured composite laminate, tensile testing and three-point flexural testing were, respectively, performed on a universal testing machine (Instron 3382) at a crosshead speed of 2 mm/min. At least 12 specimens were measured for each testing. For TS and tensile modulus (TM) testing, specimens with 230 mm × 15 mm were tested according to GB/T3354-1999.³⁰ Specimens with 55 mm × 12.5 mm were tested for flexural strength (FS) and flexural modulus (FM) according to GB/T3356-1999.³¹

Results and discussion

Effect of hot compaction on fiber content of surface-treated ramie fiber fabric/epoxy resin composite

In order to study the effect of hot compaction on fiber content of ramie fiber composite, untreated and surface-treated ramie fabric stacks were densified by either vacuum pressure compaction or hot compaction. The hot compaction temperature was 100℃ and the pressure was 0.5 MPa. W_f of each kind of composite (labeled in Table 2) is presented in Figure 4.

Table 2.

Laminate code for ramie fabric stack with different surface treatment and precompaction.

Precompaction	Surface treatment
Precompaction	Untreated	NaOH	KMnO₄	Flame retardant
Vacuum pressure compaction	UV	NV	KV	FV
Hot compaction	UH	NH	KH	FH

It is clearly found that surface treatments led to obvious decrease of W_f. For vacuum pressure compaction cases, W_f of NaOH, KMnO₄, and flame retardant-treated composites are 30.2, 31.5, and 32.2%, respectively, which are 29, 26, and 24% lower than that of untreated composite, i.e. UV laminate. For NaOH- and KMnO₄-treated composites, ramie fibers become more loose under the chemical interaction of surface treatment.^25,26 For flame retardant-treated composites, the flame retardant on the surface of fiber yarns decreases the compressibility of ramie fabric stack. Figure 5 shows the cross-section morphology of ramie fiber yarns extracted from ramie fabric before and after surface treatments. It can be seen that NaOH and KMnO₄ treatments make ramie fiber yarns loosen. In addition, the ramie fiber yarn becomes tightened after flame retardant treatment, and meanwhile the surface of ramie fiber yarns is partly coated with flame retardant. Thus, after surface treatment, ramie fabric stacks are more difficult to be compressed under VARI, resulting in low fiber content.

Considering that mechanical properties of natural fiber composite are directly related to fiber content,¹³ to increase W_f of surface-treated composite is of great importance. As Figure 4 illustrates, hot compaction can effectively increase W_f of surface-treated composites. W_f of NaOH, KMnO₄, and flame retardant-treated composites processed by hot compaction increases by 16, 14, and 16%, respectively, than that of the corresponding one processed by vacuum pressure compaction.

Figure 6 shows the SEM images of the cross-section of laminate. There are no obvious defects in these laminates, demonstrating the advantage of VARI in fabricating ramie fabric composites. Moreover, it is clearly observed that for each kind of surface-treated composite, the fabric yarns using hot compaction are denser than those using vacuum pressure compaction. This difference is attributed to the irreversible cross-section deformation of ramie yarn and nesting of ramie fiber yarns resulted from hot compaction. Consequently, the hot compaction is effective in improving the consolidation of ramie fabric composite and increasing the fiber content.

Effect of hot compaction temperature on mechanical properties of surface-treated ramie fiber yarns

High temperature can lead to the degradation of natural fiber.^20,21 For this reason, surface-treated ramie fabric stacks were processed by hot compaction with different temperatures and 0.5 MPa hot compaction pressure, and the TSs of these ramie yarns were measured, as shown in Figure 7.

Figure 7(a) shows that the TS of NaOH-treated ramie yarn changes little before hot compaction temperature reaches 120℃, varying from 272 MPa at 70℃ to 260 MPa at 120℃. However, the TS is 222 MPa with 140℃ hot compaction temperature, which is 19% decreased compared with the one under 70℃. The significant decrease is associated with the fact that degradation may happen under high temperature. For KMnO₄-treated ramie yarn, it can be seen from Figure 7(b) that the TS decreases slightly from 313 MPa at 70℃ to 283 MPa at 100℃, and decreases distinctly when the temperature exceeds 100℃. As shown in Figure 7(c), the TS of flame retardant-treated ramie yarn has maximum value at 100℃ with value of 248 MPa, and decreases as hot compaction temperature increases. From this investigation, it is clear that for the three kinds of surface-treated ramie yarns, TS decreases under high compaction temperature resulting from the damage of ramie fiber. Saheb and Jog³² found that the thermal degradation of natural fibers was a two-stage process, first hemicellulose began to degrade and then it was lignin with the gradually increased temperature.

Although the change trend of TS varies with the type of surface treatment, the TSs of NaOH, KMnO₄, and flame retardant-treated fibers with 100℃ hot compaction are relatively high. Therefore, the hot compaction temperature of 100℃ was chosen for fabricating composites laminates.

Effect of hot compaction on mechanical properties of surface-treated ramie fiber fabric/epoxy resin composite

With the purpose of understanding the influence of hot compaction on properties of surface-treated ramie fiber fabric/epoxy resin composite, both dynamic mechanical behavior and static mechanical properties of surface-treated composites were studied. The static mechanical properties include TS, TM, FS, and FM.

Figures 8 and 9 show the influence of hot compaction on dynamic mechanical behavior of NaOH, KMnO₄, and flame retardant surface-treated composites. As Figure 8 illustrates, for all the surface-treated composites, storage modulus (E′) increases as a result of the increments of W_f caused by hot compaction. The storage modulus–temperature curve gives valuable insight into the stiffness of a material as a function of temperature. The increase in storage modulus is due to high W_f allowing more stress transfer from matrix to fibers. This is consistent with the research of Joseph et al.,³³ who found that the E′ of natural fiber reinforced polypropylene composites augmented as fiber content increased.

Figure 9 clearly illustrates that tan δ of the composite decreases with hot compaction treatment, indicating that high fiber content lowers the damping capacity. Mechanical damping term (tan δ) is the ratio of the loss modulus to storage modulus and is related to the degree of molecular mobility in the polymer material. It has been reported³³ that the incorporation of stiff fibers reduces the tan δ peak height by restricting the movement of polymer molecules.

Figure 10 demonstrates the effect of hot compaction on tensile properties of different surface-treated composites. As expected, the hot compaction brings about obvious improvements of tensile properties of all surface-treated composites in contrast with the cases with vacuum pressure compaction. The effect of hot compaction on flexural properties of different surface-treated composites is presented in Figure 11. It is observed that flexural properties are improved by hot compaction as well. The increasing degrees of tensile properties and flexural properties change with different kinds of surface-treated ramie fabric stack, as shown in Table 3. In addition, the improvements by hot compaction in mechanical performances of composites without surface treatment are also listed in Table 3.

Table 3.

Percentage change of mechanical properties of composite laminate caused by hot compaction (with temperature of 100℃ and pressure of 0.5 MPa) compared with those with vacuum pressure compaction for different kinds of surface-treated fabric.

Mechanical property	Percent change (%)
Mechanical property	Untreated	NaOH	KMnO₄	Flame retardant
Tensile strength	30	29	5	13
Tensile modulus	25	28	9	16
Flexural strength	17	14	6	12
Flexural modulus	32	3	12	9

After tensile tests, for untreated and flame retardant-treated ramie fiber fabric reinforced composites, there are many fiber yarns pulled out from the fractured surfaces, which is an indication of low adhesion between ramie fiber and matrix. However, for the NaOH- and KMnO₄-treated ramie fiber fabric reinforced composites, the phenomenon of pulled-out fibers occurs in much smaller extent, and most fibers are still embedded in the resin. This indirectly indicates better adhesion.

From Table 3, it is clear that the increasing degree of strength for NaOH-treated composite by hot compaction is the largest among the three surface-treated composites, which has the increments of 29% of TS and 14% of FS, in close proximity to those of the untreated one. The improvement caused by hot compaction on flame retardant-treated composite is also obvious. The TS and TM of the flame retardant-treated composite under hot compaction are found to have 13 and 16% improvements, respectively, while the increases of FS and FM are 12 and 9%. The increasing degrees in mechanical properties are relatively lower than those of raw ramie fiber fabric composite. The reason may be that the APP flame retardant left on the surface of ramie fiber yarns after surface treatment has a negative effect on the interfacial bonding between fiber yarns and resin. For the KMnO₄-treated composites, the improvement is relatively not evident, and the TS, TM, FS, and FM are increased by 5, 9, 6, and 12%, respectively. This may be related to the fact that the strong oxidation of KMnO₄ may cause damage to ramie fiber though it can improve composite interfacial compatibility. In a word, the method of hot compaction can effectively improve mechanical properties of surface-treated ramie fiber fabric reinforced composites.

Conclusions

This paper investigated the effect of hot compaction process on mechanical properties of surface-treated ramie fiber fabric/epoxy resin composites fabricated by VARI. The surface-treated fiber fabric stack was densified under vacuum pressure compaction and hot compaction before injecting resin in order to increase fiber content. The composite laminates were manufactured using the precompacted fabric stack with VARI process, and the mechanical properties of composite were measured for evaluating the precompaction method. The following points are drawn from the experimental results:

The composite laminates using surface-treated ramie fiber fabrics by NaOH, KMnO₄, and flame retardant have lower fiber contents than that using untreated fabric. Hot compaction with high temperature and high pressure is very effective in increasing fiber content of dry surface-treated ramie fiber fabric/epoxy resin composites in VARI. This result is attributed to the cross-section deformation of ramie yarn and nesting of ramie fiber yarns during hot compaction.

The TSs of NaOH, KMnO₄, and flame retardant-treated ramie fiber yarns decrease after hot compaction with excessive high temperature. It is explained by physical damage of fiber structure during hot compaction. Considering the effect of temperature on TS of yarns and compaction degree of fabric stack, 100℃ is believed to be the suitable hot compaction temperature for fabricating NaOH, KMnO₄, and flame retardant-treated ramie fabric/epoxy resin composites.

In contrast with vacuum pressure compaction, hot compaction can significantly improve mechanical properties of NaOH, KMnO₄, and flame retardant-treated ramie fabric composites resulting from the increment of fiber content. For NaOH-treated composite, the TS and TM are, respectively, increased by 29 and 28%. The increasing degree of mechanical properties varies with the type of surface treatment for ramie fiber fabric.

Footnotes

Acknowledgments

The authors thank Professor Li Yan at School of Aerospace Engineering & Applied Mechanics in Tongji University for providing ramie fabric.

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 was supported by the National 973 Program of China (Project No. 2010CB631100).

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