BANANA FIBER REINFORCED LDPE COMPOSITES FOR USE IN INJECTION MOLDED PARTS: PROPERTIES AND PROCESSING

Patrick Chester and William Jordan Baylor University

Abstract

As the automotive industry pushes to increase the fuel economy of vehicles, natural fiber reinforced polymeric composites present an advantageous option for injection molded parts that are lighter yet stronger. One type of natural fiber that is at times overlooked for these applications is the banana pseudo-stem, which has similar composition to other natural fibers already in use and is one of the most widely grown plants in the world.

This study looks at two different chemical treatments designed to promote the interfacial bonding between banana fibers and an LDPE matrix: peroxide treatment and permanganate treatment. The effects of the treatments on the tensile properties of individual banana pseudo-stem fibers were explored, with peroxide treatment enhancing the tensile properties and permanganate treatment having an inconclusive effect. Some interesting results from composite processing are briefly explored, leading to peroxide treated fibers being excluded from composite testing. The flexural and tensile properties of untreated and permanganate treated injection molded composite parts were then explored. Untreated banana pseudo-stem fibers provided a measurable increase in composite properties, especially in tensile stiffness. Permanganate treated fibers provided little to no advantage in composite properties compared to their untreated counterparts, even with post-fracture analysis showing enhanced interfacial bonding.

Background Many lignocellulosic fibers are well characterized in terms of composition and mechanical

properties. All of these fibers will contain a combination of cellulose, hemicellulose, and lignin, although their relative amounts vary greatly between different species of plants [1], [2]. Mechanical properties of lignocellulosic fibers may also vary greatly, even from the same plant. This is due to several factors during the plant’s life cycle, including the growing conditions, the ripeness of fibers during harvesting, the methods used to extract the fibers, and the transportation and storage of the fibers over time [3]. In general, however, plant fibers that come from the stems of their plant tend to exhibit better mechanical properties [4]. The following table provided by Bismarck et al. summarizes the mechanical properties of several common natural fibers [5] .

Table I: Mechanical properties and dimensions of some common natural fibers.

Fiber Tensile Strength (MPa)

Young’s Modulus

(GPa)

Elongation at Break

(%)

Density (g/cm^3)

Diameter (µm)

Jute 393-800 13-26.5 1.16-1.5 1.3-1.49 25-200 Hemp 690 70 1.6 1.47 25-500 Cotton 287-800 5.5-12.6 7-8 1.5-1.6 12-38 Sisal 468-700 9.4-22 3.7 1.45 50-200 Coir 131-220 4-6 15-40 1.15-1.46 100-460

Kneaf 930 53 1.6 - - Flax 345-1500 27.6 2.7-3.2 1.5 40-600

Banana 600-750 28-29 2-5 1.3 80-120

Natural fiber-reinforced composites have many appealing features for consumer applications, especially in the automotive industry (See Figure 1) [2], [6]. Common applications include interior paneling for aircraft and automobiles, household tables and chairs, window frames, laptop cases, and other consumer items [7], [8]. Natural fiber-reinforced composites are valued for their sustainability and light weight compared to similar conventional composites, which is what makes them especially popular in the automotive industry [8].

Figure 1: Automotive components made from natural fiber-reinforced composites [9] One of the biggest obstacles to widespread adoption of banana fiber-reinforced composites

is the relatively poor bonding between fiber and matrix. This poor bonding has to do with banana fibers’ tendency to absorb moisture. When the hydrophilic fibers are combined with a hydrophobic thermoplastic matrix, the result is relatively inefficient bonding between fiber and matrix. [9], [10] There are a couple of methods that have been used before to help improve this bonding. One method involves roughening the surface of the fiber to provide mechanical anchoring sites for the polymer to latch on to. Another method focuses on chemically changing

the surface of the fiber to reduce its hydrophilicity, thereby increasing its compatibility with the matrix [2] Some common chemical treatments used to achieve this are silane treatment, alkali treatment , peroxide treatment, permanganate treatment, isocyanate treatment, and a polymer coupling agent such as maleic anhydride [2], [9], [11] For this study, the main chemical treatments explored will be peroxide treatment and permanganate treatment.

Peroxide treatment is another treatment that has been studied with natural fibers, though not in the context of banana fiber/LDPE composites. A study led by Joseph et al. looked at the effects of several chemical treatments on the properties of short sisal fiber/LDPE composites, including peroxide treatment. It was found that randomly oriented peroxide treated fibers improved the tensile strength of the composite by as much as 50% [12]. It is believed this increase in strength is due to the peroxide initiated free radical reaction between LDPE and cellulose, described in figure 2 [12], [13]. It is also believed that peroxide treatment promotes crosslinking within the LDPE by itself, which would also contribute to its increase in strength [13], [16].

Figure 2: Chemical reaction for peroxide treatment [12].

A less common but still effective treatment used is permanganate treatment. Permanganate treatment modifies the surface of the fibers by interacting with highly reactive 𝑀𝑀𝑛𝑛3+ ions, which in turn induces the grafting of polyethylene onto the surface [21]. The entire process is illustrated in Figure 3 below [14]. In addition to improving interfacial bonding, the hydrophilicity of the fibers decreased as a result of the treatment [14]. As permanganate concentration increases, the properties will at first increase and then suddenly decrease after a certain threshold is reached, indicating the degradation of cellulosic fibers [12]. Therefore, the concentration and time of exposure to the treatment is very important to the fiber properties.

Figure 3: Graft copolymerization as a result of permanganate treatment [11].

Experimental Procedure Chemical Treatments The procedures for the chemical treatment of banana pseudo-stem fibers follow the method

described by Joseph et al. very closely [15]. For the peroxide treatment, untreated fibers are soaked in a 6% by weight solution of dicumyl peroxide in acetone for 30 minutes. The fibers were then decanted and allowed to air dry. For the permanganate treatment, untreated fibers were soaked in a solution of potassium permanganate in acetone (concentration varying from 0.005% to 0.2% by weight) for 1 minute. The fibers were then decanted and allowed to air dry.

Once the treatments were complete, fibers that were designated for thermal and single fiber testing were stored in a temperature and humidity controlled chamber. This was to ensure that all fibers contained the same moisture content. Fibers that were designated for composite processing were dried in an oven at 50°C overnight. This was meant to minimize swelling of the composite material during processing by expelling all moisture absorbed from the fibers.

Mechanical Testing In order to test the effects that the chemical treatments have on the individual strength of

banana fibers, a Dynamic Mechanical Analyzer (DMA) testing machine was used. With a tensile range of 0-18 N, the DMA allows for precise measurements of the loads subjected to individual banana fibers. For both treated and untreated cases, individual fibers were subjected to a tensile load from a constant rate of extension (0.5 mm/min) until fracture. The effects of idle time were also explored. Idle time refers to the amount of time treated fibers spent in storage in the temperature and humidity controlled chamber. Fibers with the same chemical treatment but different idle times were subjected to the same tensile test, and the results were compared. The

cross-sectional area of a sampling of each type of treated fiber was then determined using SEM imaging and the areas were averaged for each type.

Composite flexural properties were measured by performing a 3-point bend test according to standards set by ASTM D790-15. Injection molded composite beams (6.25x1.25x0.4 cm) were subjected to 3 point bend loading until attaining 5% strain, at which point the test was terminated. Any higher strain would lead to inaccurate results as mandated by the standard.

Composite tensile properties were measured by performing a simple tensile test according to ASTM D638-14. Injection molded ‘dog-bone’ samples were created using the dimensions shown in Table II. A constant rate of extension of 20 mm/min were used for these tests. Samples were loaded until failure.

Table II: Dimensions of tensile ‘dog-bone’ bar mold [9]

Mold Dimension Value (in) Width of narrow section 0.5 Length of narrow section 2.25

Total width 0.75 Total length 6.5

thickness 0.125 Radius of fillet 3

Results and Interpretation

Single Fiber Results As described earlier, several treated and untreated fibers were subjected to a single fiber

tensile test. The stresses experienced by the fibers were calculated using the cross-sectional area with the voids removed from the cross section. The error bars from each of the charts in this section represent the standard deviation. The values reported in this section represent the median of each sample set, which is less sensitive to outliers than the mean.

Figures 4 and 5 compare the single fiber tensile test results of peroxide treated fibers with untreated fibers. These results are also summarized in Table III. Peroxide treated fibers show dramatically enhanced properties compared to the untreated case.

Figure 4 Strength of peroxide treated banana fibers from single fiber tensile testing.

Figure 5. Modulus of peroxide treated banana fibers from single fiber tensile testing.

Table III Summary of Results for Peroxide Treated Single Fiber Tensile Testing

Treatment Ultimate Tensile Strength (MPa)

Strength Std. Dev.

(MPa)

Tensile Modulus (MPa)

Modulus Std. Dev.

(MPa) Untreated 99.0 52.0 7.0 2.1

Peroxide 6% 242.6 90.9 16.5 5.7

Figures 6 and 7 compare the single fiber tensile test results of permanganate treated

fibers with untreated fibers. These results are also summarized in Table IV. Permanganate fibers show widely varying results with no real identifiable trend in regards to concentration of treatment. However, the concentration with the highest strength is 0.05%, which is comparable with the permanganate treated composite strength from Joseph et al [12]. Therefore, the

permanganate concentration that will be used for further composite processing and testing is 0.05%.

Figure 6. Strength of permanganate treated banana fibers at various concentrations from single fiber tensile testing.

Figure 7: Modulus of permanganate treated banana fibers at various concentrations from single fiber tensile testing.

Table IV: Summary of Results for Permanganate Treated Single Fiber Tensile Testing

Treatment Ultimate Tensile Strength (MPa)

Strength Std. Dev.

(MPa)

Tensile Modulus (MPa)

Modulus Std. Dev.

(MPa) Untreated 99.0 52.0 7.0 2.1

Permang. 0.005% 70.4 84.7 2.0 4.1 Permang. 0.01% 78.7 21.0 3.4 1.0 Permang. 0.05% 294.6 126.5 11.5 4.7 Permang. 0.1% 135.2 73.7 6.9 4.5 Permang. 0.15% 70.4 30.8 3.4 1.9 Permang. 0.2% 258.4 104.5 12.6 4.6 Composite Processing In order to prepare composite samples, banana pseudo-stem fibers and LDPE pellets at

specific ratios were mixed together using a single screw extruder, forming a molten composite bead that was cooled in a water bath, pelletized, and dried in preparation for injection molding.

During the course of the extrusion process, the peroxide treated fibers caused the LDPE to

suddenly and dramatically swell at the outlet of the extruder, leading to the composite bead shown in Figure 8a. In addition to this swell, the molten characteristics of the LDPE also changed dramatically, becoming too viscous to be injection molded using the equipment available. As a result, the peroxide treated fibers have been excluded from the composite results.

Figure 8a: Extruded composite beads made with peroxide treated banana fibers (left) and untreated banana fibers (right)

While the untreated and permanganate treated fibers did not have such a dramatic effect

on the composite bead, there was still some swelling present during the injection molding process, illustrated in Figure 8b. The source of this swelling is not immediately apparent, as the composite pellets were dried prior to injection molding. Investigation into the source of this

phenomenon, while interesting, is beyond the scope of this study. The resulting composite parts did not show obvious voids within the part, and so they were accepted for composite testing.

Figure 8b: Untreated dried composite material swelling during the injection molding process. Composite Results Results from 3-point bend testing are shown in Figures 9 and 10 and summarized in Table

V. As may be expected, both flexural strength and flexural modulus increase as the fiber volume fraction increases. This is true both for the untreated and permanganate treated composite. At the lower fiber volume fraction of 10%, the permanganate treated fibers enhance both the flexural modulus and flexural strength. This is likely the result of enhanced interfacial bonding, combined with the relatively high alignment of fibers along the edge of the injection molded part. At the higher volume fraction of 20%, however, the permanganate treated fibers appear to degrade the strength and stiffness of the composite material. One possible explanation for this is at that fiber volume fraction the fibers are closely packed, increasing the likelihood that they would slide past each other during processing. The fibers, which were already roughened due to their treatment, would be more apt to break apart when they come into contact with another rough surface. As more and more fibers break apart, they could reduce to a size below their critical length. At this point the shortened fibers would act more as a defect than as a reinforcement, reducing the composite strength.

Figure 9: Banana fiber/LDPE composite flexural strength.

Figure 10: Banana fiber/LDPE composite flexural modulus.

Table V: Summary of Banana Fiber/LDPE Composite Flexural Properties

Treatment Fiber Vol. Frac.

(%)

Flexural Strength (MPa)

Strength Std. Dev.

(MPa)

Flexural Modulus (MPa)

Modulus Std. Dev.

(MPa) Neat LDPE N/A 6.60 0.17 140.22 1.66 Untreated 10 13.07 0.39 242.03 1.98 Untreated 15 18.38 0.55 339.07 13.46 Untreated 20 21.99 0.96 405.62 55.65

Permang. 0.05% 10 14.84 0.20 271.34 3.09 Permang 0.05% 20 19.05 0.4 405.62 17.96

Results from composite tensile testing are shown in Figures 11 and 12 and summarized in

Table VI. As may be expected, both tensile strength and tensile modulus increase with increasing fiber volume fraction. This is true both for the untreated and permanganate treated composite. In terms of composite strength, untreated banana fibers only marginally increase the overall tensile strength, and the permanganate treated fibers perform no differently than their untreated counterparts. In terms of composite tensile modulus, however, there is a much greater effect, with permanganate treated fibers either performing the same or a little less than their untreated counterparts. These trends with permanganate treated fibers do not follow other studies of permanganate treatment found in literature [14], [18]. One possible reason for this is that in other studies the fibers used in permanganate treatment were first subjected to alkali treatment, whereas the fibers in this treatment were only subjected to permanganate treatment. This combination of alkali and permanganate treatment may have a greater effect on the properties and interfacial bonding of natural fibers than each treatment by itself.

Figure 11: Banana fiber/LDPE composite tensile strength.

Figure 12: Banana fiber/LDPE composite tensile modulus.

Table 6: Summary of Banana Fiber/LDPE Composite Tensile Properties

Treatment Fiber Vol. Frac.

(%)

Tensile Strength (MPa)

Strength Std. Dev.

(MPa)

Tensile Modulus (MPa)

Modulus Std. Dev.

(MPa) Neat LDPE N/A 9.79 0.49 128.36 5.04 Untreated 10 11.45 0.23 443.7 7.99 Untreated 15 11.88 0.19 616.34 20.15 Untreated 20 13.16 0.4 873.4 34.29

Permang. 0.05% 10 11.67 0.2 441.63 22.48 Permang 0.05% 20 12.92 0.27 799.68 18.45

Figure 13 looks at a side view of a couple of fracture surfaces from untreated composite

tensile tests. As can be seen, there are several exposed fibers sticking out from the fracture surface, indicating that the fibers were pulled out of the matrix without breaking, a consequence of poor interfacial bonding.

Figure 13: Side view of fracture surface for untreated banana fiber/LDPE composites with 10% fiber volume fraction (left) and 20% fiber volume fraction (right).

One sample broke in an unusual manner (Figure 14), allowing the chance to see adjacent fracture surfaces close together. It can be seen that the crack propagation is mostly through the LDPE matrix, leaving many fibers intact. This can be seen even more clearly at the microscopic level (Figure 15), where fibers can be seen clearly jutting from the fracture surface, and the LDPE matrix that surrounds it exhibit clamshell markings indicative of crack propagation.

Figure 14: Unusual dual fracture surface from 15% fiber volume fraction untreated banana fiber/LDPE composite. Left: macroscopic crack propagation. Right: more detailed photo of one fracture surface.

Figure 15: Untreated banana fiber/LDPE composite fracture surfaces. Top: 10% and 15% fiber volume fraction surfaces with clear clamshell marking in the LDPE. Bottom: 10% fiber volume fraction surfaces with protruding fibers.

Figure 16 looks at a side view of a couple of fracture surfaces from permanganate treated composite tensile tests. As can be seen, there are some exposed fibers sticking out from the fracture surface (an indication of poor interfacial bonding), although the amount of exposed fibers is decidedly less than that of the untreated case in Figure 13. This provides some evidence of an improvement of interfacial bonding.

Figure 16: Side view of fracture surface for 0.05% permanganate treated banana fiber/LDPE composites with 10% fiber volume fraction (left) and 20% fiber volume fraction (right).

Further effects of the permanganate treatment can be seen in Figure 17. The top two micrographs show a fiber with enhanced bonding to the LDPE matrix, causing the fiber to tear when subjected to a shearing force. This behavior is contrary to the untreated case where the fracture pattern goes around the fibers (see Figure 14). Additionally, a change in the physical appearance can be detected from the bottom micrographs of Figure 17. This is attributed to the effects of the chemical treatment, and may be a sign of degradation of the fiber surface.

Figure 17: Permanganate treated banana fiber/LDPE composite fracture surfaces. Top: 10% fiber volume fraction surfaces with enhanced interfacial bonding. Bottom: comparison of fiber cross sections for untreated (left) and treated (right) fibers.

Conclusions Banana pseudo-stem fibers provide a unique opportunity for reinforcement of

thermoplastics such as LDPE. Peroxide and permanganate treatment serve to enhance the interfacial bonding of banana pseudo-stem fibers to their LDPE matrix. Peroxide treatment has the additional effect of enhancing the tensile properties of individual fibers, whereas permanganate treatment has an inconclusive effect on the tensile properties of individual fibers. During processing, peroxide treated fibers caused the LDPE matrix to swell considerably and increased the viscosity of the molten composite material to the point of being unable to make injection molded parts. The untreated and permanganate treated fibers experienced some swelling during processing, though not to the degree exhibited by the peroxide treated fibers.

In terms of composite flexural properties, untreated banana pseudo-stem fibers enhanced

the strength and stiffness considerably, with an increasing effect with increasing fiber volume fraction. The permanganate treated composite behaved similarly to the untreated composite, with a slight enhancement in properties compared to the untreated composite at 10% fiber volume fraction and a slight reduction in properties compared to the untreated composite at 20% fiber volume fraction. This may be due to permanganate treated fibers breaking apart as they rub past each other during processing due to their roughened surface.

In terms of composite tensile properties, untreated banana pseudo-stem fibers slightly

enhanced composite strength and greatly enhanced composite stiffness with an increasing effect with increasing fiber volume fraction. The permanganate treated composite behaved similarly to their untreated composite counterparts at equivalent fiber volume fractions, either matching or slightly underperforming the untreated properties. These trends for the permanganate treated composite do not match the trends shown in other literature, and it may be due to the fact that in other literature the fibers were alkali pre-treated before the permanganate treatment.

Post fracture analysis revealed that the path of fracture in the untreated composite mostly

occurred in the matrix, moving around the fibers and leaving them intact. This resulted in several fibers sticking out from the fracture surface, an indication of poor interfacial bonding. The permanganate treated composite showed some improvement to interfacial bonding, with some fibers included along the fracture path and fewer fibers sticking out from the fracture surface overall. SEM imaging revealed differences in physical appearance between untreated and permanganate treated fibers, providing evidence of the treatment changing the composition of the fibers.

In conclusion, banana pseudo-stem fibers provide some measurable enhancement to

LDPE properties, especially in terms of tensile stiffness. Permanganate treatment appeared to enhance the interfacial bonding but otherwise appeared to provide little to no advantage over the untreated composite in terms of tensile and flexural properties. This may be due to the permanganate treated fibers not being alkali pre-treated prior to permanganate treatment.

Acknowledgments We wish to thank the other students and faculty members of the Baylor University Sic’em

research group. Dr. David Jack was very helpful in several important areas.

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