Mechanical behaviors of composites made of natural fibers through environmentally friendly treatment

Abstract

Natural fiber reinforced composites have garnered significant interests as potential substitutes for conventional materials because of their eco-friendly attribute and favorable physical and mechanical properties. Typically the natural fiber undergoes chemical treatment before processing with the matrix to produce composites, however, the chemical treatment can have a negative impact on the environment. This research work presents an environmentally friendly treatment method for hemp fibers by using boiling water and shear force for specific time periods. The purpose of the treatment is to break down the technical fiber bundles into elementary fibers, which creates a fourfold increase in bonding surface area between the fibers and matrix. The change in fiber length and size before and after the debundling treatment were analyzed using optical microscope, confocal microscope, and scanning electron microscopy. The treated fibers were then made into mats through a wet-laid process and compression molded with low density polyethylene via film stacking. The effects of different fiber treatment variables, including debundling time, on mechanical properties were compared with composites reinforced with conventional alkali treated fibers. The results presented show that the composites reinforced by hemp fiber using the new treatment method have equivalent or improved tensile, flexural and impact properties than the composite reinforced with alkali treated fibers.

Keywords

Hemp fiber elementary fiber composite materials mechanical behavior

Introduction

The current environmental concerns and the global energy crisis have drawn increasing attention to natural fiber reinforced polymer composites (NFPCs) in various applications, particularly in the automobile industry.^1–5 Compared to conventional materials, NFPCs have lower density, cost-effectiveness, and eco-friendliness, which can achieve weight and cost savings as well as enhanced environmentally friendliness.^2–4,6–8 A market survey, by Kline & Company, reported that almost 10 Kg of natural fibers have been used per automobile and this would increase by 15 % – 20 % per year in the future.² However, natural fibers have notable shortcomings such as incompatibility with polymer matrix.^1,9,10 To overcome the drawbacks of natural fibers, modifications of various natural fibers are emerging rapidly. The modification of natural fibers includes chemical,^11–14 physical^11,15–18 and biological methods.^19–22 Among these, chemical treatments such as alkali treatment have been widely used in various natural fibers to improve their compatibility with the polymeric matrix due to their simplicity, effectiveness, and economic efficiency as compared to other treatments. The downside of this approach is the usage of a substantial quantity of alkaline chemicals mainly NaOH, which can induce numerous environmental issues regarding their storage and disposal.^23,24 Hence, it would be very beneficial to develop a new environmentally friendly fiber treatment method to make natural fibers compatible with polymer matrices.

Physical methods are considered cleaner and simpler than chemical treatments to modify natural fibers. This type of treatment was typically utilized to alter the structural and surface properties of natural fibers.^22,25 Recently, this method was associated with heat treatment to obtain individual fibers from fiber bundles.^22,26 The impacts of introducing individual fibers within polymer composite formulations as well as the relevant influence on mechanical performance have been investigated through experimental studies.^27–30 The steam explosion process (STEX) was carried out by Vignon et al.³¹ with the aim of degrading the pectins from the middle lamella and producing individual fibers for fabrication of hemp fiber polyethylene (PE) composites. Similarly, the thermo-mechanical milling process was performed by Brendan et al.³² to yield individual fibers with increased compatibility with polypropylene matrix.

Based on their dimension and architecture, reinforcements made from plant fiber can be categorized into three major types at different scales, namely, technical fiber, elementary fiber, and nanofibril.²³ Utilization of plant fiber bundles for composites originates from the hemp stem which is referred primarily to as technical fiber.³³ A technical fiber consists of numerous elementary fibers (also referred to as individual or single fibers) and elementary fibers are surrounded by middle lamella which is a soft amorphous layer rich in lignin or pectin polymers.^34,35 Nanofibril is the fibrils at the nanoscale that exist in the elementary fiber. Among those reinforcements at different scales, elementary fibers have been regarded as excellent reinforcement due to their high flexibility, surface ratio, and relatively high mechanical properties.^36,37 Elementary fibers are bonded together to form a technical fiber by means of middle lamellae and this intricate structure of technical fiber results in the challenge of separating elementary fibers from technical fibers.

Several different methods to generate elementary fibers from technical fiber bundles have been studied so far. Amongst them, the wet beating technique is widely used throughout the field of the papermaking industry for achieving the desired dimension and fibrillation of natural fibers.^38,39 It uses a high-intensity mechanical action such as shear, crushing, stretching, and friction to break the fiber bundle structure and generate elementary fibers.³⁹ However, this approach is associated with excessive mechanical action, which is unfavorable for maintaining the integrity of the individual fiber structure and can result in a negative reinforcement effect for the composite.^40–42 Therefore, the intensity and condition of wet beating treatment are crucial in achieving good separation of fiber bundles.³⁸

Hemp is one of the world’s oldest cultivated plants and has been currently considered a sustainable and high yielding industrial fiber crop that is widely used for automobile applications by incorporating into a polymer matrix.^{1–3,9,43–45} Low density polyethylene (LDPE) is a suitable choice as the matrix, due to its low processing temperature, good specific properties, and low price.⁴⁶ Although research work has studied the effect of the wet beating process on morphologies and chemical characteristics of hemp fibers, very little knowledge is available on the reinforcement effect of debundled elementary hemp fibers on the mechanical behavior of their composites.^39,47

In this paper, we focused on developing an elevated temperature aqueous debundling method for converting technical fibers to elementary fibers and using the elementary fibers and LDPE for producing elementary hemp fiber reinforced thermoplastic composites. We also studied the influence of the interfacial bonding area between the elementary fiber and LDPE on the mechanical properties of their composites. The dimensional and morphological changes of the hemp fiber were investigated by optical microscope, confocal microscope, and SEM. The elementary hemp fiber reinforced LDPE composites were tested on their tensile, flexural and impact properties and the reinforcement effect of the elementary hemp fiber on the mechanical behavior of its composites was evaluated.

Materials and methods

Materials

Technical hemp fibers (Cannabis sativa L.) were obtained from Composites Innovation Center, Canada. Prior to alkali treatment and elevated temperature aqueous debundling treatment, unwanted woody core pieces were removed manually and then the fibers were cleaned with distilled water to completely remove dust particles. Commercial grade LDPE sheets were obtained from Wrap’s Company, USA. Sodium hydroxide pellets with 98 % purity were supplied by LabChem company, USA. The sodium hydroxide pellet was used to treat the hemp fiber in this work for comparison purpose.

Elevated temperature aqueous debundling treatment

Once cleaned, the hemp fibers were placed in a beaker containing boiling water (100°C) and this mixture was then transferred to a food blender (Figure 1(a)). The lowest speed setting (3000 r/min) on the food blender was utilized for this mechanical debundling treatment. This mechanical debundling process was maintained for 10 s, 30 s, 60 s and 90 s, respectively, with the purpose of evaluating the duration effect on fiber length and diameter. Thereafter, the treated fiber and water mixture was then transferred to the hydroentanglement unit that is comprised of a stainless steel container and a stainless steel wire mesh (35 mesh) placed at the bottom of the container for filtering (Figure 1(b)). The dispersion of fiber was achieved by manually rearranging and adjusting the position of fibers in water. The water was drained out of the hemp fiber suspension by gravity through the 35 mesh filter, while hemp fibers were left on the surface of the filter (Figure 1(c)). The fiber mat was then placed in a dryer (EMERSON SPEED DRYER MODEL 145) at a temperature of 80°C for about 12 h to remove the remaining water (Figure 1(d)).

Figure 1.

The steps of elevated temperature aqueous debundling treatment process, hydroentanglement process, and compression molding process; (a) debundling process of hemp fiber under elevated temperature aqueous condition; (b) hydroentanglement process of the debundled hemp fiber; (c) hemp fiber mat made from hydroentanglement process; (d) dried debundled hemp fiber mat; (e) compression molding process; (f) debundled hemp fiber/LDPE composite panel.

Alkaline treatment

Technical hemp fibers were alkaline treated for comparison purpose. After being cleaned, the hemp fibers were soaked in a 5 wt% NaOH aqueous solution at ambient temperature. The solution to fiber ratio was 30:1 (by weight) for all fiber treatments. The fibers were immersed in the sodium hydroxide solution for 2 h. After treatment, the fibers were thoroughly rinsed with water to remove any traces of sodium hydroxide. The treated fibers were then transferred to the hydroentanglement unit and a fiber mat was obtained by manually rearranging fibers in water and removing the water using the metal mesh. The formed fiber mat was then dried in the air dryer at 80°C for 12 h.

Composite preparation

Four different variants of hemp/LDPE composites were fabricated. These were LDPE reinforced with undebundled hemp fiber, LDPE reinforced with alkali treated hemp fiber, and LDPE reinforced with debundled hemp fiber with different treatment times. A neat LDPE panel was also compression molded with the same processing conditions and tested for comparison purpose.

All the fiber mats were made into the same mass (around 15 g) and the same dimension (254 mm by 254 mm). The fiber mats and LDPE sheets were dried at 50°C in an oven for 12 h to remove any moisture. A film stacking and compression molding process was used to consolidate the LDPE/hemp fiber composite (Figure 1(e)). The mold was heated to 130°C at a pressure of 2.5 MPa and subsequently hot-pressed for 1 h. After that, the mold was air cooled and the panel was de-molded when the mold reached room temperature (Figure 1(f)).

Mechanical testing

Tensile testing was conducted based on ASTM D3039 using an MTS 810 Material Test System, equipped with an MTS extensometer (25.4 mm gauge length) for strain measurement. Five specimens from each material variant were tested and their average tensile strength and modulus are reported. Flexural specimens were prepared and tested based on ASTM D790 using an INSTRON SATEC APEX T5000 testing frame. Five specimens for each material variant were tested and the average value of flexural strength and flexural modulus were calculated. The impact strength of the composite was determined according to ASTM D256 by using a Tinius Olsen Model Impact 104 unit. All the specimens were machined with an 1-mm-deep notch and average values of impact strength were calculated from 10 specimens from each material variant.⁴⁸

Fiber length and diameter measurements

To measure the diameter of treated hemp fibers, an optical microscope (Olympus) equipped with a SPOT IMAGING camera was used. The treated hemp fibers were examined at 25x, 50x, and 100x magnifications. The samples were prepared by producing 3 g hemp fiber suspension with a small amount of water. The suspension was placed into a petri dish and suspended fibers were manually dispersed. Images of fiber diameter and fibrilization were captured and evaluated using ImageJ software. The fiber diameter was determined by measuring and averaging the diameters at three different locations on one fiber and 100 fiber specimens were measured for each variant. For the length measurement, hemp fibers were placed into an aluminum pan and water was used to disperse the hemp fibers. Images were then captured by a Cannon EOS camera. The ImageJ software was used to measure the individual fiber length. The averaged fiber length was obtained from 200 fiber specimens. Fiber aspect ratio, defined as the fiber length over its diameter, was obtained based on the measurement of 75 different fiber specimens from each variant. Measurements of the fiber length and diameter were performed using the aforementioned methods.

The surface morphology of the fiber mat were studied using a digital microscope (Keyence; VHX-5000) with a magnification ranging from 20x to 100x. Images were collected in a quality improved mode. The fiber length, fiber diameter, and defibrillation were studied to evaluate the debundling effect between different treatment times.

Scanning Electron Microscopy (SEM)

Examination of the fracture surfaces of the sample after impact testing was carried out using a Quanta FEG 650 scanning electron microscope. Regular technical (as received), alkali treated, and debundled fiber composite samples after tensile testing were examined under the SEM at an accelerating voltage of 5 kV and micrographs at different magnifications were captured.

Results and discussion

Optical microscopic analysis of treated hemp fibers

The optical microscopic images (Figure 2) show that the modifications applied to hemp fibers altered their morphology and resulted in separation of elementary fibers from technical fiber bundles. The extent of the fiber splitting differs with different treatment durations. An as-received technical hemp fibers is shown in Figure 2(a) and it has a large diameter around 150 μm as compared to the diameter around 50 μm for a 30s debundled fiber, as shown in Figure 2(b). In contrast, debundling treatment of fibers led to splitting of fiber bundles, as shown in Figure 2(c). The increase in debundling time generated more fiber bundle separation and resulted in smaller elementary fibers. Figure 2(d) shows a single elementary fiber of about 10 μm size.

Figure 2.

Optical microscopic images of (a) as-received technical hemp fiber, (b) hemp fiber after 30s debundling, (c) separation of elementary fibers after 30s debundling, and (d) elementary hemp fiber.

Separation of the elementary fibers can be attributed to the plasticizing effect of water molecules and high temperature, which strongly weakened the bonding among elementary fibers and made it easier for the mechanical forces to separate the elementary fibers. The presence of moisture content in the hemp fiber has a distinct influence on the glass transition temperature of some constituents in the fiber, such as hemicelluloses and lignin. Many researchers have demonstrated that the hemicelluloses and lignin showed comparatively low Tg under equilibrium moisture conditions (around 50 °C–80°C).^49–59 According to the structure of hemp fiber bundles, all elementary fibers were glued together by middle lamellae made of lignin and pectin within technical fiber bundles.^60,61 The introduction of boiling water induced a weak bond in the technical fiber bundle which consisted of pectin and lignin area. Therefore, the separation of elementary fibers from technical fiber bundles was easily achieved by means of applying a relatively low intensity mechanical treatment onto the softened technical fiber bundles.

Length and diameter distributions of hemp fibers

The hemp fiber length and diameter distributions before and after the elevated temperature aqueous debundling treatment (0 s, 10 s, 30 s, 60 s, and 90 s) were investigated and compared in Figure 3. The hemp fibers treated for 10 s, 30 s and 60 s exhibited appreciable difference in length and diameter as compared with their initial state (0 s). The increase in debundling time reduced long fibers and resulted in a shorter length and smaller diameter. The hemp fibers after 90 s debundling treatment had the largest percentage of short fibers (75 %, length less than 5 mm) and abundant elementary fibers (87 %, diameter less than 0.05 mm). The presence of a substantial number of short fibers with extremely small diameters indicated separation of elementary fibers from technical fiber bundles by using the elevated temperature aqueous debundling treatment.

Figure 3.

(a) Hemp fiber length distributions with different debundling times; (b) histogram of hemp fiber length with different debundling times; (c) distribution of hemp fiber diameters with different debundling times; (d) histogram of fiber diameters with different debundling times.

The curves of average length and diameter are plotted in Figure 4. It is observed that the fiber length and diameter were substantially altered at the beginning of treatment and the efficiency of treatment decreased with the increment of debundling time. There were no significant quantitative changes in fiber dimension between 60 s and 90 s treatment groups, although the latter one experienced 50% more debundling time than the former. A possible explanation for this reduction of the treatment effectiveness after 60 s might have originated from the lower water temperature (Table 1) which was less than the glass transition temperature of lignin and hemicellulose and has a less plasticizing effect during treatment.

Figure 4.

Change of the average fiber length (a) and diameter (b) with debundling time; error bar showing the standard deviation (SD) of the fiber length or diameter at different debundling times.

Table 1.

The water temperatures after different debundling durations.

Debundling time (s)	Water temperature (°C)
0	95.4
10	93.2
30	88.7
60	81.5
90	74.4

Aspect ratio and specific surface area

The debundling treatment resulted in an alteration of the fiber length and diameter of hemp fiber. After 90 s debundling treatment, the aspect ratio (length/diameter ratio) of hemp fibers improved appreciably from 119.8 to 155.9. This is primarily due to the dramatic decrease of fiber diameter with increasing debundling treatment time. Additionally, specific surface area also increased with debundling treatment time. Specific surface area is a common method of characterizing fillers and it can be correlated to various performance characteristics of filled systems. Table 2 presents the aspect ratio and specific surface area for different treatment times. The result was an estimate by using average fiber length, diameter, and density.⁴⁶ The 90 s debundling treatment generated a substantial amount of elementary fibers and thereby led to a fourfold increase of surface area. This higher surface area was beneficial for the formation of a larger matrix-fiber interfacial bonding area and better load transfer between LDPE and fibers.⁴⁷

Table 2.

Average fiber length, average fiber diameter, fiber aspect ratio, 95 % confidence interval, specific surface area of hemp fibers for different debundling treatment times.

	Debundling time
	0 s	10 s	30 s	60 s	90 s
Average fiber length (mm)	14.4	10.6	7.1	4.4	3.9
Average fiber diameter (μm)	139	98	65	40	35
Aspect ratio (L/D)	119.8	125.7	137.2	153.4	155.9
95 % CI	107–133	114–138	120–154	136–172	138–174
Specific surface area (m²/g)	0.019	0.027	0.041	0.067	0.076

Morphology of hemp fiber mat after debundling treatment

Figure 5 shows the images of the as received technical fiber and debundled hemp fiber mats made by the hydroentanglement process. Figure 5(a) illustrates the surface of technical fiber mat that is consisted of a substantial quantity of hemp fiber bundles with a broad range of fiber diameter and a small amount of individual fibers. In contrast, the debundling treatment led to the breakdown of the fiber bundle and release of individual fibers as shown in Figures 5(b) and (c). In addition, less fiber bundles were present in the fiber mat with the increment of treatment time, which also possesses a better surface finish (Figure 5(d)) due to reduction of large fiber bundles and abundant elementary fibers with a more uniform diameter. This variation in fiber dimension resulted in the formation of a highly dense fiber network and most fibers were entangled tightly by other short fibers. The abundant fiber-fiber joint formation within the fiber mat can promote better strength of the dry fiber mat.⁴⁸ However, this structure could cause reduced impregnation of the polymer matrix.

Figure 5.

Images of fiber mat made of hemp fibers (a) before debundling and after being debundled for (b) 10 s, (c) 60 s, and (d) 90 s.

Tensile properties

Tensile strength and modulus for all of the samples with different treatment methods are presented in Figure 6. The sample consisting of hemp fibers treated with NaOH (“Sample NaOH”) shows an obvious increase in tensile properties compared to the untreated hemp fiber group (“Sample Untreated”), which implies that the alkali treatment provided better compatibility between the hemp fiber and the matrix. This is interpreted by the change of the surface polarity by removing impurities, pectin, reducible hemicelluloses and lignin from the fiber surfaces.^62,63 Moreover, the alkaline treatment also caused a rougher fiber surface, which provided better fiber wetting and interfacial bonding with matrix.⁶⁴

Figure 6.

(a) Tensile strength comparison of LDPE, 20 wt% and 30 wt% hemp fiber reinforced LDPE composites; (b) tensile modulus comparison of LDPE, 20 wt% and 30 wt% hemp fiber reinforced LDPE composites.

The elevated temperature aqueous debundled samples exhibited better mechanical properties compared with the NaOH treated sample. The application of 90 s debundling process (“Sample ETAD90”) led to an increase in the tensile strength of 30 wt% debundled fiber reinforced composites (25.5 MPa) by about 11 % in comparison to the alkali treated samples (22.8 MPa) with the same weight fractions. The tensile modulus of 30 wt% debundled hemp fiber reinforced composites (2.18 GPa) demonstrated an enhancement of 10% than alkali treated hemp fiber reinforced composites (1.98 GPa). A similar observation was made for 20 wt% debundled hemp fiber composites and both their tensile strength and tensile modulus were improved over alkali treated hemp reinforced composites due to the debundling process. This debundling process allowed an increase of the tensile strength and modulus properties by 12%, and 15%, respectively, compared to alkali treated samples. This difference is attributed to the appreciable increase in fiber surface area after the debundling process. The change of the fiber morphology resulted in better interfacial bonding between fiber and matrix and thereby led to an improvement of tensile strength. Several researchers have mentioned the positive effect of surface area on the mechanical properties of the composite.^31,65,66 Mohammad et al.⁵⁹ studied the effect of surface nanoengineering treatment on the mechanical properties of glass fiber reinforced composites. It was found that the silica-based nanostructured coatings significantly improved the fiber surface area and provided higher tensile and flexural properties (up to 31 %).

Flexural properties

The comparisons of flexural strength and flexural modulus for all of the samples with different fiber treatments are shown in Figures 7(a) and (b). The alkali treatment and debundling process resulted in an increase in flexural properties. This difference was attributed to the alteration of the fiber surface caused by alkaline treatment as mentioned earlier. Several researchers have reported similar results in their work.^13,64,67 The reinforcement effect was much more pronounced in debundled hemp fiber reinforced LDPE composite. It can be seen that the debundling process provided a higher reinforcing effect than the alkali treatment by comparing composites with 20 wt% and 30 wt% fiber contents. The flexural strength for the composites with 20 and 30 wt% debundled fibers increased by 15 % and 21 % compared to the composite with alkali treated at the same weight fractions. The flexural modulus values showed a similar trend as flexural strength. The flexure modulus increased by 21 % and 17 % for the composites reinforced by 20 to 30 wt% debundled fibers relative to the alkali treated fibers composites.

Figure 7.

(a) Flexural strength comparison among LDPE, 20 wt% and 30 wt% hemp fiber reinforced LDPE composites; (b) flexural modulus comparison among LDPE, 20 wt% and 30 wt% hemp fiber reinforced LDPE composites.

It has been reported that the flexural failure of the laminates was determined mainly by the strength of interfacial adhesion between fibers and matrix rather than the fiber strength.⁶⁸ In this case, the elevated temperature aqueous debundling treatment contributed to the increase of fiber surface area and resulted in better bonding between matrix and fibers, which in turn markedly improved the flexural properties of the composite. The better interfacial bonding between hemp fibers and polymer matrix by optimizing the surface area of fibers was in a good agreement with other research work.^65,69,70

Impact strength

Figure 8 illustrates the results of the Izod impact strength of hemp fiber reinforced LDPE composites. It is evident that the neat LDPE sample had a higher impact strength (36.1 kJ/m²). When hemp fiber was added, the impact performance was decreased. Apart from that, the impact strength of LDPE/hemp composites decreased with an increasing fiber content. The composites with 20 wt% hemp fiber showed higher impact strength than the ones with 30 wt% hemp fiber. This is probably because the hemp fiber possessed a higher modulus than LDPE and a higher percentage of hemp fiber reduced the impact strength of the material. In the research work reported by Chen et al.,⁷¹ similar observations were made with respect to the impact strength of a LLDPE/wood fiber composite. It was reported that the decreasing trend of the impact strength was obtained as increased fiber loading. The mechanisms of energy absorption during impact can be segmented into three major stages such as debonding, pull-out and breakage of fibers. A poor adhesion between matrix and fibers can easily cause the occurrence of debonding and lead to a higher energy absorption.^72–74 Alkali treated fiber composite samples showed lower impact strength than the untreated fibers since the alkali treatment provided stronger interfacial bonding between the fiber and LDPE matrix. Due to this, the fiber breakage can be more easily triggered instead of the occurrence of fiber debonding and pull-out, which led to less energy absorption and lower impact resistance of the alkali treated fiber composites. Therefore, the impact resistance of the alkali treated fiber composites decreased. Similar observations have been made by other researchers.^73,75,76 For 20 wt% and 30 wt% composites reinforced by debundled hemp fibers, the impact strength of Sample ETAD90 increased by approximately 14.1 % and 11.3 %, respectively, compared with the alkali treated fiber composite. The elementary fibers contributed to a large bonding area between fibers and matrix. As a result, the pull-out and debonding behavior of the elementary fibers absorbed more energy and resulted in an enhanced toughness. A similar finding was reported by Hua et al.⁷⁰ The effect of surface area on the impact properties of carbon fiber composite was investigated and the results showed that carbon fiber oxidation and surface modification can enlarge the specific surface area and noticeably increase the impact performance of the composite.⁷⁰

Figure 8.

Impact strength comparison of LDPE, 20 wt% and 30 wt% hemp fiber reinforced LDPE composites.

SEM analysis of debundled fiber reinforced composite

The SEM images in Figure 9 show the fracture surfaces of the hemp fiber reinforced LDPE composites after impact. Figures 9(a) and (c) show the fracture surface of untreated hemp fiber reinforced composite. The protruding hemp fibers indicated poor bonding between the LDPE matrix and the fibers (Figure 9(a)). Although impregnation of LDPE matrix into the hemp fiber mat was observed from SEM images, the impregnation of LDPE into untreated hemp fiber bundles was observed to be challenging due to the integrity of hemp fiber bundle structure. Figure 9(c) shows splitting of hemp fiber bundles after the fiber breakage, indicating the weak bonding between each elementary fiber within the technical fiber bundle structure. Figures 9(b) and (d) show the fractured surface of the debundled fiber reinforced composite with 30 wt%. It is evident from Figure 9(d) that abundant elementary fibers were observed with different sizes. In contrast to the dimensions of hemp fiber bundles, a large number of elementary fibers with small diameters and lengths were present. The small-size fibers greatly improved the fiber-matrix contact area and ensured a better bonding between hemp fibers and polymer matrix without use of chemicals.

Figure 9.

SEM images of fracture surfaces of 30 wt% hemp fiber reinforced LDPE composite; (a) untreated (90x), (b) 90s debundling treatment (90x), (c) untreated (400x), (d) 90s debundling treatment (400x).

Conclusions

An environmentally friendly fiber debundling method has been developed to separate elementary fibers from hemp fiber bundles. A combination of elevated temperature, moisture, and shear force have been proved to be adequate to break the bonding among elementary fibers and produce fibers with a significantly small size (average diameter of 35 μm when debundled for 90 s). It was found that when the debundling time increased from 0 s to 90 s, the fiber aspect ratio (length/diameter) increased. The largest fiber aspect ratio was found to be 156 compared with 120 when not treated, a 30 % increase. Furthermore, separation of the elementary fibers from the hemp technical fiber bundles resulted in a fourfold increase of the surface area, which offered better bonding between the hemp fiber and matrix and more efficient load transfer from matrix to fiber. Compared to the traditional alkali treatment method, the debundled hemp fiber reinforced composite showed increases of tensile, flexural, and impact strength by 11, 17 and 14 %, respectively. Results suggest that the hemp fiber treated with an environmentally friendly process does not compromise its composite properties compared with those of the composites reinforced with alkali treated fibers. The same approach can be extended to other plant-based natural fibers to improve the load transfer efficiency in natural fiber polymer matrix composites while avoiding use of chemicals such as NaOH.

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) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Haibin Ning

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