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NATURAL FIBER BLENDS- FILLED ENGINEERING THERMOPLASTIC COMPOSITES FOR THE AUTOMOBILE INDUSTRY

Ertan Ozen1, 2 , Alper Kiziltas1, 3, Esra Erbas Kiziltas1 ,4 & Douglas J. Gardner1

1Advanced Engineered Wood Composite (AEWC) Center, University of Maine, Orono, ME 04469, USA

2Department of Woodworking Industrial Engineering, Faculty of Technology,

Mugla Sıtkı Koçman University, 48000 Mugla, TURKEY

3Department of Forest Industry Engineering, Faculty of Forestry, University of Bartin, 74100 Bartin, TURKEY

4The Scientific and Technological Research Council of Turkey (TÜBİTAK),

Tunus Cad, Kavaklıdere 06100, Ankara, TURKEY

Abstract

Natural fibers have several benefits over glass and mineral fibers: they are “green” and eco-friendly; they have high specific strength and modulus; they exhibit sound damping at lower cost and density. Since there is no one optimal fiber for all applications, the goal of this research is to develop blends of natural with optimal physical properties for specific applications in the automobile industry. In this study, engineering thermoplastic composites were prepared from natural fiber blends filled with nylon 6. Natural fiber blends, mixtures of kenaf, flax and hemp fibers, were added to nylon 6 matrices under mixing and temperature. The engineering thermoplastic composites with varying concentrations (from 5 to 20 wt. %) of natural fiber blends were prepared by injection molding and compression molding. Tensile, flexural and impact tests were used to evaluate the mechanical properties of the composites as well as to determine the composite densities. The composites reinforced with higher natural fiber blend loadings displayed enhanced tensile and flexural properties in comparison with the neat nylon6. Differential scanning calorimetry was used to determine thermal properties of the composites. Mechanical and thermal properties demonstrated that the natural fiber blends can be used as excellent reinforcing material for low cost, eco-friendly composites in the automotive industry as well as in other applications such as the building and construction industries, packaging, consumer products etc.

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Background

The collapse in prices of plastics both engineering (e.g.. polyamides and polyesters) and commodity (e.g. polyolefins), attempts to lessen the dependence on petroleum-based materials, and increasing environmental concerns are stimulating research to investigate more environmentally friendly, sustainable materials to replace the existing glass fibers and carbon fibers for the development of new materials for construction, furniture, packaging and automotive components [1-3]. Natural fibers have recently received much attention as reinforcements to substitute glass fibers and carbon fibers because of their excellent characteristics, such as low cost, low density, high specific strength and modulus, ease of fiber surface modification, relative non-abrasiveness, renewability and biodegradability, process friendly characteristics, good thermal and acoustic insulating properties, recyclability and world-wide availability [1-2, 4].

As a result of increasing demand for environmentally friendly materials and the aforementioned excellent characteristics of natural fibers, there is growing interest in the use of natural fibers in composite plastics in many areas and particularly the automotive industry for door panels, seat backs, headliners, package trays, dashboards trunk liners and interior parts [5-6]. Today nearly 50 percent of vehicle internals are made of polymeric materials; in developed countries, the average use of plastic in a vehicle is 120 kg, and the global average is around 105 kg which accounts for 10-12 percent of the total vehicle weight [6-7]. According to the estimation of the Corporate Average Fuel Economy (CAFE), the reduction for every 10 percent of the weight of a car can decrease fuel usage by 6 to 8 percent [7]. The higher volume fraction of lower density natural fibers in plastic composites can reduce the weight of the final component significantly [4]. The level of plastics usage in the automobile industry has also become a very important parameter for measuring the development level of a country's automobile industry [7]. Because of the latest developments and trends in the automobile industry including government regulations requiring increases in fuel economy and the need to reduce component costs and minimize overall vehicle weight, worldwide automotive industry demand for natural fibers is expected to grow in the future. The thermal stability of natural fibers is a key parameter in composite processing and the choice of plastic for the production of natural fiber reinforced composites is restricted to commodity plastics such as polypropylene and polyethylene, mainly because of the higher melting points of engineering plastics such as nylon 6 and polyethylene terephthalate which would cause thermal degradation of natural fibers [8]. However, certain ‘under-the-hood’ applications in the automobile industry are too harsh for commodity plastics to withstand [9]. Nylon composites are playing a growing role in ‘under-the-hood’ components for automobiles, where they are competing with incumbent materials - metals and thermoset plastics - because of their resistance to high temperatures, oils and corrosive chemicals, great tensile strength and modulus, relatively light weight, attractive surface appearance and the parts-consolidation possibilities they offer [10]. The use of nylon as the polymeric matrix in natural fiber composites presents some advantages: higher mechanical properties of nylon as compared with polyolefins, because of its hydrophilic nature, great compatibility between nylon and natural fibers. This great compatibility eliminates the need to use a compatibilizer or coupling agent [8]. Another advantage is that nylon is easier to recycle than thermosets, which remain competitive with nylon in under-the-hood applications in North American markets. This may give it an edge as environmental concerns in the plastics industry continue to mount [10]. Although recent efforts have demonstrated the feasibility of producing nylon/natural fiber composites, there is not enough information related to natural fiber reinforced nylon composites because of the

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difficulties in producing the composites without considerable thermal degradation of natural fibers and high cost of nylon compared to commodity plastics [8, 11-16]. Numerous researchers have exploited the reinforcement potential of kenaf, flax, hemp, sisal and jute for developing thermoplastic and thermoset composites for the automobile industry using several different techniques such as compression molding and injection molding [17]. Most of the development work in the automobile industry is focused on single fiber in the literature and market. It is already known that there is no one perfect fiber for all applications. By using some blend of natural fibers, optimal physical properties for automotive applications can be achieved [18]. The objective of this paper is to establish and optimize a production process for engineering thermoplastic composites, based on natural fiber blends, which is the mixture of flax, hemp and kenaf fibers, and nylon 6, suitable for automotive applications especially under-hood applications. The present paper also deals with the effect of natural fiber blend reinforcement on mechanical and thermal properties of nylon 6 at different filler loading.

Matrix and Reinforcing Materials

The nylon 6 used as the thermoplastic matrix polymers was supplied by Entec Co., USA. Nylon 6 has a density of 1.13 g/cm3 and molding shrinkage 0.015 in/in. The long flax, hemp and kenaf fibers have been kindly supplied by Bast Fibers LLC. Fibers were cut into small size manually and milled into small particles using a lab-scale grinder and passed through 5 mm screen to obtain particles of uniform size. The same amount of natural fiber particles was then packed in an air tight container and mixed in high speed mixer (2000 rpm, 2min) to obtain a uniform mixture. The natural fiber blends and natural fiber particles were stored in sealed containers after being oven dried for at least 16 hours at 105°C to obtain low moisture content which is less than 1 percent. The lubricant (Struktol TPW 113) processing aid, being a blend of a complex, modified fatty acid ester and having a specific gravity of 1.005 and drooping point of 67-77°C respectively, was supplied by Struktol Company of America.

Sample Preparation

The mixture of natural fiber blends and Nylon 6 was dried to a moisture content of less than one percent using an oven at 105°C for 16-hrs. The matrix polymer, Nylon 6, was mixed with the natural fiber blends. The compounding was conducted with a Brabender Prep-mixer® equipped with a bowl mixer and the process temperature and torque changes were measured in real time. Melt temperature and torque changes for every run were recorded to determine optimum processability for the Nylon 6 - natural fiber blends composites. The temperature was set to 250°C and rotor speed at 60 rpm. Natural fiber blends were added to the mixer when the polymer melt appeared well mixed. Mixing was done for 15 min until torque became stabilized. The Nylon 6- natural fiber blends compounds were granulated using a lab-scale grinder. The ground particles were dried in an oven at 105°C for 16 hours before being injection molded into ASTM test specimens. All materials were injection molded using a barrel temperature of 250°C mold temperature of 250°C injection pressure of 2500 psi. The compositions of the composites are shown in Table 1.

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Table 1: Composition of natural fiber blends-filled nylon 6 composites

Sample Code Compositions

Nylon 6 Flax Hemp Kenaf NFB Lubricant

PA6 97 - - - - 3 5% M 92 - - - 5 3 10%M 87 - - - 10 3 20%M 77 - - - 20 3 20%F 77 20 - - - 3 20%H 77 - 20 - - 3 20%K 77 - - 20 - 3

Values are percentage by weight (wt. %) NFB = Natural fiber blends, mixture of flax, hemp and kenaf fibers.

Statistical Analysis

The tensile and flexural strength, tensile and flexural modulus, elongation at break and impact strength were compared using a one-way analysis of variance followed by Tukey-Kramer Honestly Significant Differences (HSD) test with JMP statistical analysis program [19].

Mechanical Properties

All tension tests were conducted according to the American Society of Testing and Materials (ASTM) standard D 638-03. The tensile behaviors of composites were measured using an Instron 8801 with a 10 kN load cell. All the tension tests were tested at a rate of 0.2 in/min. At least six specimens were tested for each composition, and the results are presented as an average for tested samples. The flexure tests were conducted according to ASTM D 790-03, Test Method 1, Procedure A, i.e. three-point loading system utilizing centre loading, using an Instron 8801 with a 4.48 N load cell. The support span was 50 mm. and tests were run at a test speed of 0.05 in/min. At least six specimens were tested for each composition and the results are presented as an average for tested samples. The impact tests were conducted according to ASTM D 256-06. The notches were added using a NotchVIS machine manufactured by Ceast. The specimens were tested on a Resil 50 B impact test machine, manufactured by Ceast. The specimen was clamped in the bottom of the test fixture and the hammer was then released from a specified height. The depth under the notch and the specimen width was entered, and the machine then recorded the energy taken to break the specimen. All breaks must be completed breaks to count as a data point. A 2.75 J hammer was used to impact the specimens. At least ten specimens were tested for each composition.

Thermal Properties

Differential scanning calorimetry (DSC) analysis was carried out using a Perkin Elmer Instrument Pyris DSC with a sample weight of 8 to10 mg. All samples were held at 25°C for 5 min, heated at a rate of 10°C/min to 250°C, subsequently held for 5 min to erase thermal history, then cooled at a rate of 10°C/min to -20°C, subsequently held for 5 min and heated again at a rate of 10°C/min to 250°C under a nitrogen atmosphere. At least three randomly selected specimens from ground samples were tested for each composition, and the results are presented as an average for tested samples.

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Results and Discussions

Figure 1, Figure 2 and Table 2 show tensile strength and elongation at break of the neat Nylon 6 and natural fiber blends filled composites. It was observed that neat Nylon 6 exhibits a nonlinear elastic behavior with a tensile strength of 27.95 MPa and an elongation at break of 1.09%. None of composites including neat Nylon 6 showed signs of stress yielding. This led to the conclusion that the mechanism behind the elongation and rupture of the composites was quite similar compared to neat Nylon 6. The composite reinforced with natural fiber blends displayed enhanced tensile properties in comparison with the neat Nylon 6. Because of better compatibility as well as better stress- transfer properties, tensile strength of composites was larger (reaching values 61.99 MPa with the addition of 20% natural fiber blends. Tensile strength increased by 119% with 20% natural fiber blends addition. According to the literature, the tensile strength of Nylon 6 composites also increased with 20% Brazilian curaua fiber, alpha cellulose and microcrystalline cellulose additions [6, 13-14]. Similar to tensile strength, elongation at break of composites was longer (reaching values 2.92% with addition of 20% kenaf fibers).

Group Name

PA 6 5% M 10 % M 20 % M 20 % F 20 % H 20 % K

Ten

sile

Str

eng

th (

MP

a)

0

10

20

30

40

50

60

70

Figure 1: Effect of natural fiber blends loading on tensile strength of natural fiber blends filled Nylon 6 composites

.

Table 2: Summary of mechanical properties of nylon 6 and composites.

Properties PA6 5%M 10%M 20%M 20%F 20%H 20%K Density (g/cm

3) C C (NC) B (4.4%) B (5.5%) A (8.8%) B (5.2%) B (4.1%)

Tensile Strength (MPa) E D (22.6%) C (72.6%) A (121.7%) B (94.7%) A (119.7%) A (118.6%) TMOE (GPa) E D (11.4%) C (24.7%) A (50.4%) B (34.1%) A (45.9%) A (46.7%) EAB (%) C C (NC) B (57.9%) A (142.9%) A (152.4%) A (157.1%) A (167.4%) Impact Strength (J/ m) A B (-32.3%) BC (-35.9%) BC (-37.7%) BCD (-43.1%) CD (-45.5%) D (-50.1%) Flexural Strength (MPa) F E (5.7%) D (18.1%) A (33.9%) C (18.6%) C (24.2%) B (31.1%) FMOE (GPa) F E (10.6%) D (39.4%) A (84.2%) C (54.3%) C (62.6%) B (75.3%)

The same letters indicate no statistical difference between properties of composites and those around it. NC is no significant change upon the addition of natural fiber blends (α=0.05) and parenthesis show the effect of natural fiber blends loading on the mechanical properties of composites in comparison with the neat Nylon 6. FMOE: Flexural modulus of elasticity, TMOE: Tensile modulus of elasticity and EAB: Elongation at break.

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Group Name

PA 6 5% M 10 % M 20 % M 20 % F 20 % H 20 % K

Elo

ng

atio

n a

t B

reak

(%

)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Figure 2 : Elongation at break as function of natural fiber blends loading for natural fiber blends filled Nylon 6.

Figure 3 and Table 2 show the tensile modulus of neat Nylon 6 and natural fiber blends filled composites. Tensile modulus of elasticity of neat Nylon 6 was 2.91 GPa. The tensile modulus of elasticity of natural fiber blends filled composites systemically increased with increasing natural fiber blends loading (reaching values 4.38GPa with addition of 20% natural fiber blends). Xu and Kiziltas observed similar phenomena. Tensile modulus of elasticity of the composites increased with increasing alpha cellulose and microcrystalline cellulose content [13-14]. A similar effect was also reported by Caulfield et al. and dos Santos et al. for Nylon 66/hardwood and softwood fiber composites and Brazilian curaua fibers, respectively [6, 15].

Group Name

PA 6 5% M 10 % M 20 % M 20 % F 20 % H 20 % K

Ten

sile

Mo

du

lus

of

Ela

stic

ity

(GP

a)

0

1

2

3

4

5

Figure 3: Tensile modulus of elasticity of neat Nylon 6 and natural fiber blends filled composites.

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Figure 4 and Table 2 show flexure strength of the neat Nylon 6 and natural fibers filled composites. Similar to tensile strength, the composite reinforced with natural fibers displayed enhanced flexural properties in comparison with the neat Nylon 6. The flexural strength of natural fiber blends filled composites systemically increased with increasing natural fiber blends (reaching values 99.71 MPa with the addition of 20% natural fiber blends). Flexural strength increased by 34% with 20% natural fiber blends addition.

Group Name

PA 6 5% M 10 % M 20 % M 20 % F 20 % H 20 % K

Fle

xura

l S

tre

ng

th (

MP

a)

40

50

60

70

80

90

100

110

Figure 4: Effect of natural fiber blends loading on tensile strength of natural fiber blends filled Nylon 6 composites.

In Figure 5 and Table 2, the average values of the flexural modulus as a function of natural fiber blends are shown. As can be seen in the figure, the flexural modulus of composites was higher than neat Nylon 6. Flexural modulus of elasticity of neat Nylon 6 was 1.89 GPa. The modulus also increased with increasing natural fiber blends loading (reaching values 3.49GPa with addition of 20% natural fiber blends). Flexural modulus of elasticity increased by 84% with 20% natural fiber blends addition. A similar effect was also reported by Xu for Nylon 6/alpha cellulose fiber composites and Kiziltas for Nylon 6/microcrystalline cellulose [13-14]. Also Sears et al. used cellulose wood pulps as reinforcement for Nylon 6 composites and found that cellulose wood pulps increased flexural modulus compare to neat Nylon 6 composites [16].

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Group Name

PA 6 5% M 10 % M 20 % M 20 % F 20 % H 20 % K

Fle

xura

l Mo

du

lus

of

Ela

stic

ity

(GP

a)

0

1

2

3

4

Figure 5: Flexural modulus of elasticity of neat Nylon 6 and natural fiber blends filled composites

Figure 6 and Table 2 show the izod impact strength of neat Nylon 6 and natural fiber blends filled composites. The izod impact strength of composites decreases as the natural fiber blends loading increases and this observation is quite expected for filled polymer systems because of possibility of poor wetting of the particles by neat Nylon 6 poor interfacial adhesion creates between the natural fiber blends and the neat Nylon 6 and it cause weak interfacial regions [20]. Increasing natural fiber blends content only increase the interfacial regions which cause to crack propagation. In addition these, addition of natural fiber blends might cause polymer immobility. It can cause lower impact strength. Figure 6 also shows that the Izod impact strength of composites decreased from 48J/m for neat Nylon 6 to 24J/m for 20% kenaf fiber-filled composites.

Group Name

PA 6 5% M 10 % M 20 % M 20 % F 20 % H 20 % K

Imp

act

Str

eng

th (

J/m

)

0

10

20

30

40

50

60

Figure 6: Tensile modulus of elasticity of neat Nylon 6 and natural fiber blends filled composites.

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The values of Tg, Tm, Tc and corresponding melting enthalpies (∆Hm), crystallization enthalpies (∆Hc) and degree of crystallinity are presented in Table 3. The DSC measurements indicate that the presence of natural fiber blends does not affect the Tg of the Nylon 6 matrix, which occurs approximately at 42°C in Figure 7. A similar behavior was observed in microcrystalline cellulose-filled Nylon 6 composites [21]. There are also small peaks at around 35°C which is the effect of lubricant. Figure 7 illustrates that adding natural fiber blends does not change the Tc of the composites or it has a slight effect at most. However, increasing the natural fiber blends content, in all cases, results in lower heat of crystallization, ∆Hc in Table 3. Similar phenomena were also observed for the addition of microcrystalline cellulose in nylon 6 composites [21]. The melting point was taken as the main peak of the endothermic curve. The melting point of neat Nylon 6 is 219°C. The effect of natural fiber blends on Tm of the composites is presented in Figure 7. It can be seen for the Figure 7 and Table 3 that the melting points of composites are between 216°C and 219°C. The addition of the natural fiber blends to composites does not have significant influence on the Tm of the composites. From these results, it can be concluded that the Tg and Tm values of the composites were strongly influenced by the matrix polymer. The degree of crystallinity of composites was calculated using the heat of fusion determined from DSC measurements and the one corresponding to a 100% crystalline nylon reported by Wu et al. [22]. Increasing the natural fiber blends content does not have significant change in percent crystallinity as seen in Table 3.

Figure 7: DSC graph of neat nylon 6 and natural fiber blends filled composites.

Temperature (°C)

0 50 100 150 200 250

Heat

Flo

w E

xo

Up

(m

W)

-10

0

10

20

PA 6

5% M

10% M

20% M

20% F

20% H

20% K

Crystallization Temperature.

Glass Transition Temperature.

Melting Temperature

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Table 3: Summary of Tm, Tc, ∆Hm (J/g), ∆Hc (J/g and crystallinity for the nylon 6 and composites.

Sample Code Tm (0C) TC(0C) ΔHm(J/g) ΔHc(J/g) Xc (%)

PA6 218.81 (1.26) 193.93 (1.05) 79.76(0.01) 79.67(0.17) 41.98 (0.01) 5%M 217.71 (0.49) 192.54 (1.61) 76.07(2.02) 77.71(0.17) 42.14 (1.12) 10%M 216.56 (4.10) 190.69 (4.96) 69.56 (2.37) 71.82 (0.11) 40.68 (1.39) 20%M 217.60 (1.23) 192.99 (0.34) 64.88(3.83) 65.15(1.00) 42.68 (2.52) 20%F 217.79 (1.47) 193.17 (0.21) 64.29 (1.75) 67.40 (1.29) 42.30 (1.15) 20%H 216.46 (2.97) 192.39 (0.24) 64.93 (1.99) 64.49 (1.39) 42.72 (1.32) 20%K 216.11 (0.37) 192.97 (0.11) 57.27 (4.31) 66.10 (1.45) 37.68 (2.83)

Parenthesis indicates standard deviation. ΔHm and ΔHc are calculated based on total area of endothermic peak and exothermic peak respectively.

Conclusions

It is possible to produce composites of natural fiber blends in high melting engineering thermoplastics, PA-6 with melt compounding followed by injection molding without compatibilizers and other additives. The composites reinforced with natural fiber blends displayed enhanced tensile and flexural properties in comparison with the neat Nylon 6. Overall the addition of 20% of natural fiber blends shows comparable or higher mechanical properties than the addition of 20% of individual fibers. These results show that using natural fiber blends; we can achieve the optimal physical and mechanical properties for particular applications in the automobile industry. The development of engineering thermoplastic composites with natural fibers is just beginning. Development of surface modifications for natural fibers, new compounding techniques and the use of new engineering thermoplastics, which have higher melting temperatures than Nylon 6, will extend the application of natural fiber blends filled engineering thermoplastic composites not only in automotive but also many other applications because of both environmental and economical benefits. It is also believde that the knowledge gained from this work and future studies will result in natural fiber blends replacing glass and mineral filers in automobile industry.

Acknowledgements

The authors would like to acknowledge the contributions of Alex Nash and Chris West whose hard work made this paper possible. The authors also wish to thank Roberta Laverty for her editorial suggestions.

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