Abstract
In this study, a novel compound lamination technique was applied to improve the mechanical properties of hemp fibre-reinforced polylactic acid composites. Polylactic acid fibres were blended with hemp fibres in a specific weight ratio in order to produce needled mats. Then, sections of the needled mat were stacked with several polylactic acid resin layers on either side, then formed hemp/polylactic acid composites through hot-pressing. The tensile and flexural properties of hemp/polylactic acid composites were tested according to ASTM standards. A multi-factor orthogonal analytical approach was adopted to discuss the effect of factors such as the hybrid ratio, forming temperature and pressure on mechanical properties of the developed green composites. The adhesion between the fibres and the matrix in the fracture surfaces and the thermal stability of the produced composites were observed via scanning electron microscopy and thermogravimetric analysis. The component analysis of composites was conducted by infrared spectra for confirming the contribution of polylactic acid. The results showed that adhesion between fibres and matrix was enhanced, as well as mechanical properties also improved, especially the tensile strength and flexural properties were obviously improved by utilizing this novel compounding technique.
Introduction
Recently, due to environmental issues and increasing concerns about global energy, natural plant fibres that can replace traditional synthetic fibres as reinforcements in degradable composite materials have received increased attention.1–3 Compared to synthetic fibres, plant fibres are more abundant and with greater output. 4 Moreover, as reinforcements in composites, they have a high specific strength, high modulus and are lightweight, cost-effective, as well as environmentally friendly. 5 Natural plant fibres that can be utilized as reinforcements in composites include but are not limited to fibres extracted from the hemp family (linen, ramie, sisal and hemp),6–8 banana fibres 9 and bamboo fibres. 10 While studies on other bast/leaf fibre-reinforced composites are common, limited research has been conducted on the applications of hemp fibres due to drug control requirements. In the last 30 years, with the development of low-toxicity or even non-toxic hemp plants, hemp fibres have become recognized as especially suitable for research and novel applications. Hemp is an annual herb that is easy to cultivate, highly adaptable, and fast-growing with few pest problems. Hemp also requires minimal resources and is degradable, pollution-free, and environmentally friendly. The main constituents of hemp fibres are cellulose and hemicellulose, which define the physical properties of the fibres. Cellulose is the stiffest and strongest organic structure in the fibres, resulting in mechanical properties of hemp fibres that are comparable to those of glass fibres. 11 Moreover, hemp fibres demonstrate a low density, delicate fineness, high toughness and good heat resistance, which are ideal characteristics for composite reinforcements.12–15
Recently, green composites have become the focus of many researchers. Polylactic acid (PLA) obtained by the polymerization of lactic acid (which is fermented from starches extracted from renewable plant resources and natural crops, such as corns, wheat and canes) is a representative matrix for green composites. PLA is a biodegradable material that has wide applications and demonstrates the highest strength among all currently available degradable polymers. Natural fibres reinforced with biodegradable polymers are truly ‘green’ composites. The advantages of using such composites include lower power consumption and lower greenhouse gas emissions.16–21 Life cycle assessment (LCA) studies have revealed the potentially significant benefits of green hemp fibre composites compared to glass fibre composites, especially in automotive and transportation products. Therefore, composites fabricated from hemp fibres and PLA are truly ‘green’ composites that fundamentally solve the shortage of raw materials caused by petroleum issues and address environmental problems caused by garbage pollution.22–25
The key points of hemp fibre-reinforced PLA composites are addressing the variability in their properties and improving their mechanical properties. The material properties of the final products greatly depend on aggregation structure of hemp fibres and the forming process. There are two major moulding methods involved in preparation of plant fibre-reinforced thermoplastic composites: injection moulding and laminate moulding. Injection moulding refers to a process in which natural fibres and thermoplastic polymers are mixed together and granulated before being poured into a mould where the mixtures cool and solidify to form composites. Laminate moulding is a process where a layered structure fabricated from natural fibres and polymer resins are heated under a specified forming temperature and pressure to obtain the final products. Generally, in laminate moulding, fibre mats (or arranged fibre yarns) and matrix films are alternately stacked and then hot-pressed. 26 Alternatively, matrix fibres and reinforcement fibres are blended, carded and needle-punched to produce a mat for further laminate structures. 27 Another method involves matrix fibres and reinforcement fibres that are blended and carded to form rough yarns, which are cut into short fibres and hot-pressed with the matrix to form the final shape. 28 However, all of these processing methods either damage the length of the fibres, which harms their mechanical properties, or reduces the interfacial strength between the reinforcement fibres and the matrix.
To solve these problems, a novel lamination technique for blending and carding PLA fibres and hemp fibres in a specific ratio is proposed herein. The PLA fibres and hemp fibres are lapped and needle-punched to form hybrid fibre mats in order to increase the mechanical properties and improve the bonding strength of the fibre/PLA matrix, and they are alternatively layered with PLA thin films to fabricate composites by hot-pressing. The forming conditions and the weight ratio of hemp fibre are also discussed by an orthogonal analytic approach to obtain the optimum proportion.
Experimental
Materials and processing
In this study, hemp fibres were provided by the Shenyang Beijing Hemp Industry Development Co., Ltd. (Shenyang, China). PLA fibres were manufactured by the Changshu Changjiang Chemical Fibre Co., Ltd. (Shangshu, China), which had an average length of 38 mm and a linear density of 1.67 dtex. The PLA resins were provided by the Zhejiang Hisun Group Co., Ltd.
First, hemp fibres and PLA fibres in various weight ratios were opened, blended and carded using a blowing-carding unit to form webs. Fibre webs were then lapped and needle-punched to produce needled hybrid fibre mats, which were cut into rectangular pieces of 20 mm × 175 mm. PLA resin film was formed using a hot-press machine. The PLA resin film was then cut into rectangular shapes (20 mm × 175 mm). Generally, the interfacial bonding strength between the hemp fibres and PLA matrix is weak, which is a disadvantage of natural fibre composites. Here, to improve the interfacial strength, PLA fibres and the needle-punch technique are introduced to form hybrid PLA/hemp fibre mats, in which the same type of PLA polymer with different shapes (fibre and/or film) together with the needle-punch effect may result in improved interfacial properties. The needle-punch process was conducted in two steps, that is, pre-needle punching (needling room: 3500 needles/m, needle frequency: 650 times/min, needle diameter: 1.83 mm) and main needle-punching (needling room: 5000 needles/m, needle frequency: 1000 times/min, needle diameter: 1.83 mm).
Finally, the PLA hybrid fibre mats and PLA resin films were dried in an oven at 105°C for 30 min. A lamination of hybrid mat and PLA film was prepared and then hot-press was conducted to obtain the hemp/PLA composite. The sample preparation process is shown in Figure 1. The weight percentage of the hybrid fibre mats in all composites was kept as 40 wt%, where the hybrid fibre ratios of the PLA fibres to hemp fibres within the 40 wt% hybrid fibre mats were varied. In addition, a series of contrast samples that consisted of 40 wt% hemp fibre composites by traditional laminating process, which is a method of lamination and hot-press with common hemp fibre mats and PLA resin films.
Diagram of preparation process of composite materials.
Characterization of composites
According to ASTM standards D638 and D790, experiments regarding the tensile and flexural properties of the composite samples were carried out using a DDL600 Universal Testing Machine (provided by the Changchun Research Institute for Testing Machines) with a cross-head speed of 2 mm/min.
A multi-factor orthogonal analytical approach was used to obtain the optimum manufacturing parameters. We considered the weight ratio of the PLA fibres to hemp fibres in hybrid fibre mats (shortened as the ‘PLA fibre ratio’), the forming temperature and the forming pressure in the hot-press process as the influencing factors, as listed in Table 1. According to the orthogonal analysis, three factors, namely, PLA fibre ratio, the form temperature and pressure, were changed. Nine groups of samples were prepared.
Sample preparation parameters and number.
PLA: polylactic acid.
An S-4800 scanning electron microscope (Hitachi, Japan) was used to observe the morphology of the tensile fracture surface in both the modified composites and the pure hemp/PLA composites with an accelerating voltage of 15 kV. All tests were carried out in a nitrogen atmosphere.
Thermal properties were tested using a DSC-60A automatic TG-DTA analyser (Shimadzu, Japan). The temperature ranged from 0°C to 600°C with a heating rate of 5°C/min. The whole process was conducted in nitrogen gas with a flow rate of 50 mL/min.
The microstructures of the fracture surfaces were analysed using a KBr pellet technique in an IRPrestige-21 series Fourier Transform Infrared Spectrometer (Shimadzu, Japan). The scanning range was 0–4000 cm−1. Tests were carried out in standard conditions at a room temperature of 20°C and a relative humidity of 65%.
Results and discussion
Mechanical properties
To demonstrate the advantage of using the proposed novel compounding method (hybrid PLA fibres and hemp fibres, and needle-punching), the tensile strengths of the nine developed composites are shown in Figure 2 and are compared to that of the traditional lamination method (without PLA fibres or needle-punching). For all nine samples, the tensile strength of the developed composites by the novel compounding method clearly increased by 30%–52% compared with that of the traditional method, while in sample No. 3, the tensile strength increased by up to 52%. The flexural strength, as shown in Figure 3, has a clear improvement of 20%–100%, while in sample No. 6, the flexural strength is up to 100%.
Tensile strength of samples.
Flexural strength of samples.
The mechanical properties of the developed materials mainly depend on the fibre content and their interfacial strength. Generally, the interfacial bonding strength between the hemp fibres and the PLA matrix is weak, which is a disadvantage of the natural fibre composite. Here, to improve the interfacial strength, PLA fibres are introduced to form PLA/hemp fibre hybrid mats in which the same type of PLA polymer with varied shapes (fibre and/or film) is used to improve interfacial properties. The results outlined above indicate that the present techniques, the hybridization of the PLA fibres and hemp and the use of needle-punching are keys for improving the mechanical properties of green composites. Using a hybrid mat of PLA fibres and hemp fibres will significantly improve the bonding strength between the fibres and matrix. However, the interface between hemp fibre and PLA matrix is weak, and as seen with most green composites, the interfacial behaviour of the natural fibres/matrix is generally a disadvantage. However, using PLA fibres to hybridize with hemp fibres, the PLA fibres are the same polymer to PLA film, and this will significantly enhance the interfacial bonding properties of the fibres/matrix.
Optimization of hybrid fibre mat and forming process
The influence of the needle-punch technology can be explained by the fact that hemp fibre bundles have few tortuous surfaces and are difficult to insert into PLA fibres. In the laminating process, hemp fibres are not easily infiltrated and/or covered with PLA resin. As a result, when bearing a load, hemp fibres may fracture between fibre cells, which then leads to damage between layers and affects the mechanical performance of the green composites. However, when the hybrid fibres were needle-punched, a certain number of PLA fibres were introduced into the hybrid fibre mats, which enhanced the osmosis effect between PLA fibres and hemp fibre cells during the lamination process and improved adhesion between the hemp fibres and PLA resins. The PLA fibre ratio and forming conditions, such as the temperature and pressure in hot-pressing, are also important factors, and their optimum combinations need to be determined.
From Figure 2, it can be observed that there was an approximately 35% variation in the tensile strength for samples No. 1 and No. 3, where the forming conditions (temperature and pressure) are changed, but the PLA fibre ratio remains the same. The temperature seems to have the main influence. In the flexural strength (Figure 3), samples No. 4, 5, and 6 with a PLA fibre ratio of 20 wt% showed a high bending performance, approximately 40% larger than that of the group with a PLA fibre ratio of 10 wt% (No. 1, 2, 3) and approximately 35% larger than that of the group of PLA fibre ratio 30% (No. 7, 8, 9). All of the results show that the PLA fibre content and the forming conditions in hot-pressing have a great influence on the mechanical properties of the developed green composites.
Here, we want to use the multi-factor orthogonal analytic approach to obtain the optimum manufacturing parameters and clarify the contribution of each factor. The analysed results are shown in Table 2. Here, the correlation coefficient R is calculated to evaluate the contribution order of the PLA fibre ratio, temperature and pressure. Kij in the table and the correlation coefficient R are obtained as follows
where a larger correlation coefficient R indicates a larger contribution factor.
Multi-factor orthogonal analysis for mechanical properties.
PLA: polylactic acid.
Based on the obtained correlation coefficient R, we can find that for the tensile property of the developed green composites, the main contribution factor is the forming temperature during hot-pressing, followed by the forming pressure and PLA fibre ratio. The contribution factor order number is (2,3,3), and the optimum conditions in designing present green composites are a PLA/hemp fibre ratio of 20 wt%, a forming temperature of 195°C and a forming pressure of 11 MPa. For the flexural property, the main contribution factor is the PLA fibre ratio, followed by the forming temperature and the forming pressure. The contribution factor order number is (2,3,1), and the optimum conditions are a PLA/hemp fibre ratio of 20 wt%, a forming temperature of 195°C and a forming pressure of 7 MPa.
For the total evaluation of both the tensile and flexural properties, the contribution factor order number is (2,3,3), and the optimum conditions in designing the present green composites are a PLA/hemp fibre ratio of 20 wt%, a forming temperature of 195°C and a forming pressure of 11 MPa. The optimum conditions suggested by multi-factor orthogonal analysis can be confirmed in our present experimental data, as shown in Figure 2. As for the influence of the PLA fibre ratio (PLA fibres to hemp fibres) on the performance of the developed composites, the PLA fibre ratio 20% will present the best tensile and flexural strengths. Since reinforcements are the main load bearers in composites, as the percentage of reinforcement fibres increases, the mechanical properties of composites are enhanced. Among all the composites fabricated in this study, the total fibre content percentage was fixed at 40 wt%, while the PLA fibre ratio varied. The hemp fibres have much a higher tensile strength than the PLA fibres do and will bear more of the load in the developed materials. When the PLA fibre ratio increased from 10 wt% to 30 wt%, the tensile strength of the composites increased first and then decreased. This finding indicates that when the percentage of PLA fibres in the hybrid needled mats is within a certain range, a certain amount of PLA fibres is needed to improve mechanical properties because an increase in PLA fibre content leads to better impregnating effects in the hemp fibre bundles, which enhances the adhesion between the hemp fibres and PLA fibres and/or PLA films. The experimental result of sample No.3 with the largest tensile strength among the nine kinds of samples shows that the factor order number of sample No. 3 is (1,3,2), with a PLA fibre ratio of 10%, a temperature of 195°C and a pressure of 9 MPa. A potential improvement in the mechanical properties requires a PLA fibre ratio of 20 wt% under a higher forming pressure than that listed above.
As for the influence of the forming temperature on the performance of the developed composites, the forming temperature of 195°C presents the best mechanical properties. The forming temperature had a large influence on the mechanical properties, and mechanical properties increased with the increase of forming temperature. The main reason for this occurrence is that at a relatively low forming temperature, only some of the PLA fibres melt, resulting in a weak adhesion strength between the matrix and the hemp fibre bundles. With the increase in forming temperature, PLA materials will enhance the osmosis and impregnation into the reinforced structures during hot-pressing and improve the mechanical properties of the developed composites, while much higher temperatures may reduce the adhesion conditions of the interface.
As for the influence of the forming pressure on the performance of the developed composites, the forming pressure of 11 MPa results in enhanced mechanical properties. Compared with the influences of other two factors, the forming pressure had a relatively small influence on the mechanical properties. However, the tensile strength increased with an increase in forming pressure, although the corresponding amplitude increment decreased since an increase in forming pressure enhanced the mobility of the melted matrix as well as the effective coverage of the reinforcement fibres. This, in turn, improved the mechanical properties of the developed composites. However, a further increase in forming pressure may hinder the mobility of the melted matrix and the osmosis of the reinforcement fibres, thus decreasing adhesion between the matrix and reinforcement fibres and preventing further improvement in the mechanical properties.
Thermal stability
Figure 4 depicts the thermogravimetric (TG) weight-loss curves and differential thermal analysis (DTA) curves of sample No. 5 (PLA fibre ratio 20 wt%, pressure temperature 190°C and pressure 11 MPa) compared to that of the traditional lamination method (without PLA fibres and no needle-punched). Three weight-loss stages under continuous heating conditions for both composite samples were observed. The first weight-loss stage occurred at a temperature of approximately 130°C and was mainly caused by the pyrolysis of water and small molecules. The second weight-loss stage occurred at a temperature of approximately 280°C and was mainly caused by the pyrolysis of cellulose, partial lignin, hemicellulose from the hemp fibres and the pyrolysis of most of the PLA. The third weight-loss stage occurred at a temperature of approximately 410°C. Weight losses in this stage were mainly caused by the pyrolysis of the remaining lignin and PLA. Nine samples were developed using the proposed novel compounding method (hybrid PLA fibres and hemp fibres, and needle-punching), and there are no obvious differences in their thermal properties and no obvious differences in their weight-loss curves compared to those of the traditional lamination method (without PLA fibres and no needle-punching). The results showed that the developed PLA/hemp hybrid fibre composites have high thermal stability.
TG and DTA curve of samples: (a) traditional lamination and (b) novel compounding.
Fracture surface morphology of composites
Figure 5 shows scanning electron microscopy (SEM) images of tensile fracture surfaces of composites that were prepared by the two compounding methods. Figure 5(a) and (b) shows the fracture surface morphology of pure hemp/PLA composites fabricated using a traditional compounding method and the blended fibre composites using the proposed novel compounding method (PLA and hemp hybrid fibres, and needle-punching). In Figure 4(a), the fractured fibres of the pure hemp/PLA composites vary and have loose structures. The adhesion between the fibres and resins is relatively weak, and the resins do not sufficiently cover the fibres. In contrast, in Figure 4(b), the fibres in the fracture of the PLA/hemp hybrid fibre composites with needle-punching, the fractured fibres are tightly bound, and the fractured cross-section is almost normal to the tensile direction. It can be clearly observed that the adhesion between the fibres and the matrix in the fracture process is strong, and the reinforcement fibres are fully infiltrated and covered by the matrix material.
Fracture surface morphology of samples: (a) traditional lamination and (b) novel compounding.
Contribution of PLA fibres
Figure 6(a) and (b) shows the infrared spectra of the PLA resins and the PLA fibres, respectively. The representative characteristic absorption peaks, such as an –OH vibrational absorption peak at 3460 cm−1, an apparent C = O stretching vibration peak at 1747 cm−1, a C–O vibration absorption peak at 1076 cm−1 and a C–C stretching peak near 858 cm−1, appeared in both the PLA resin and PLA fibres. However, the intensities of the absorption peaks are obviously different in the PLA resins and PLA fibres, and the intensity of the absorption peaks in the PLA resin is much higher than that in the PLA fibres. Using the compounding method with hybrid PLA fibres and hemp fibres, as shown in Figure 6(b), the infrared spectra confirmed that PLA fibres exist in the developed materials, and this also shows that the control temperature condition is suitable for preventing the melting of PLA fibres during hot-pressing, which may be a main reason that the hybrid of PLA fibres enhance the mechanical properties in the developed PLA/hemp hybrid fibre composites with high tensile strength and improved interfacial adhesion properties. The SEM images of the tensile fracture surfaces in Figure 5 also support the existence of PLA fibres in the developed PLA fibre and hemp fibre hybrid composites.
Infrared spectra of samples: (a) traditional lamination and (b) novel compounding.
Conclusion
In this article, a novel compounding method (PLA and hemp hybrid fibres, and needle-punching) was proposed to fabricate non-woven laminated green composites. In addition, the optimum hybrid ratio of PLA fibres to hemp fibres and the forming conditions in the hot-press process were investigated. The experimental results show that:
The tensile strength of the developed composites by the novel compounding method increased by up to 52%, and the flexural strength increased up to 100%.
A multi-factor orthogonal analytic approach was adopted to obtain the optimum manufacturing parameters and clarified the contribution of each factor (hybrid ratio of PLA fibre/hemp fibre, forming temperature and pressure). The correlation coefficient R calculated using the multi-factor orthogonal analytical approach confirmed that the PLA fibre/hemp fibre hybrid in the hybrid needled mats had an optimum improvement value for the mechanical properties. The PLA fibre contribution leads to better impregnating effects in the hemp fibre bundles, which enhances adhesion between the hemp fibres and PLA fibres and/or PLA films. The contribution order of the three factors is clarified as the hybrid ratio of PLA fibre/hemp fibre, the forming temperature and the forming pressure.
From the analysis, we found that in addition, an enhancement of the adhesion between fibres and matrix, the mechanical properties, especially tensile strength and flexural properties, were obviously improved, and this novel needle-punched compounding method has clear potential to improve the mechanical properties of PLA fibre/hemp fibre hybrid green composites.
Footnotes
Author’s note
Qignqing Ni is also affiliated to Anhui Polytechnic University, Wuhu, China.
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 financially supported by the Anhui Province International Science and Technology Cooperation Program (1704e1002213), Anhui Province University Natural Science Research: Major Program (KJ2017ZD13), as well as the pre-research project of national science foundation of Anhui Polytechnic University (2017yyzr05).