bio based polyamides reinforced with cellulose nanofibers—processing …€¦ · ·...

BIO-BASED POLYAMIDES REINFORCED WITH CELLULOSE NANOFIBERS—PROCESSING AND CHARACTERIZATION

Jennifer H. Zhu, Alper Kiziltas, Ellen C. Lee, Deborah Mielewski Materials Research and Advanced Engineering, Ford Motor Company, Dearborn, MI, 48124

Abstract

Bio-based polyamides are among the most promising families of bioplastics based on fully or partially derived renewable sources because of their low density, good mechanical and thermal properties, and durability. Bio-based polyamides (e.g., PA 11, PA 1010, and, to a lesser extent, PA 610) are also key resins for automotive applications because their continuous operating temperatures are comparable to the widely used PA6 and PA66. In this study, cellulose nanofibers (CNF) have been successfully dispersed in bio-based polyamide matrices (PA610 and PA1010) by conventional melt processing. The effects of CNF contents on the mechanical (tensile, flexural, and impact) and thermal (crystallization behavior and thermal stability) properties were investigated. The results indicate that these CNF fillers can be efficiently incorporated into the bio-based polymer matrices without the need for coupling agents, surface modifications or surfactants. The distribution and dispersion of the particles within the polymer matrix were studied using scanning electron microscopy. The composites produced here using bio-based polyamides have good mechanical and thermal properties and could be especially useful in applications within or near the engine.

Introduction

Traditional high performance polymer composites have been made in the past using inorganic fillers such as carbon fibers, glass fibers, and talc fillers. Although these composites have good mechanical and thermal properties, they are not renewable and not biodegradable. Current research focuses upon finding natural, environmentally-friendly, and renewable fibers to replace traditional fillers. The cellulose from natural fibers such as kenaf, flax, wood, and hemp has been successfully incorporated into polymer composites that demonstrate good mechanical behavior and are also lighter than traditional composites. Cellulose fibers are nontoxic materials that have a high specific strength and stiffness and a lower density than traditional glass or talc filled composites. However, cellulose fibers have high moisture absorption and have limited thermal stability at higher processing temperatures [1]. Beyond a temperature of 200 °C, degradation will start to occur under sustained exposure.

Polyamides are of particular interest as a polymer matrix due to their mechanical and thermal properties and relative ease of processing. It is used in a variety of packaging and auto applications. In addition, they pair well with cellulose because they both have hydrogen bonding, which can lead to stronger interactions between filler and matrix. The use of micro and nanocellulose as a reinforcing agent in polyamides has been a well researched subject. In 1984, Klason found that upon putting cellulose in PA 6 matrix, although the elastic modulus of the composites increased slightly, the strength and elongation decreased as a result of cellulose’s thermal instability [2]. Thermal degradation is the primary cause of difficulty when working with cellulose fibers, since its deterioration leads to reduced mechanical properties. In 1985, Zadorecki used reaction injection molding to make cellulose PA 6 composites, and although the lower temperature and pressure successfully reduced degradation, the molding method is fundamentally flawed and prevents highly loaded composites from being made [3]. In 2003,

Winata et al used Mucell technology to create a cellulose and PA 6 composite [4]. Although this successfully lowered the processing temperature, it allowed for cell nucleation to occur which worsened the mechanical properties. However, as Xu noted in 2008, if a workaround can be found to circumvent thermal degradation, cellulose fibers can be a much better reinforcing agent than traditional glass fibers due to its lower density, improved flexibility, and inherent renewability [5]. In 2014, Lee et al successfully incorporated microcrystalline cellulose (MCC) into a PA 6 matrix to increase the tensile strength of the composites by more than two times the neat polyamide [6]. Kiziltas et al also incorporated MCC in PA6 to create engineering thermoplastic composites with inmproved tensile and flexural properties [7].

The use of nanocellulose in polymer composites is a more recent topic of study. Although nanocellulose has been successfully used in different polymer matrices including polyethylene (PE), polymethylmethacrylate (PMMA), polypropylene (PP), and polystyrene (PS), most of these composites were created through methods other than melt blending, due to the challenges with thermal stability. Yousefian and Rodrigue successfully used melt compounding to create nanocrystalline cellulose (NCC) and PA 6 composites with improved mechanical properties [8]. The NCC was created via acid hydrolysis of a wood pulp. According to Yousefian, tensile strength and modulus increased by about 10% with only a 3% loading level of nanocrystalline cellulose (NCC) . However, after 3%, property improvement hit a plateau, likely due to poor filler dispersion at higher loading levels. Flexural modulus improved by 41% at 3% loading level. These NCC composites improved more than composites made from other natural fibers like hemp, flax, and kenaf. However, increasing loading level resulted in a reduction of composite impact strength [8].

In addition to using a renewably sourced filler, if the polymer (such as polyamide) used for the matrix was renewable, then the entire composite can be made in an environmentally friendly manner. Neat nylon 11 is an example of a biobased engineering thermoplastic that is currently used in 95% of Ford vehicles for fuel line applications due to the good mechanical, thermal, and processing properties of polyamides [9]. In addition, there are biobased polyamides such as PA 1010 and PA 610; the former of which is 100% based on castor oil, and the latter is 62% based on castor oil [10]. Both of these polyamides (and to a lesser extent, PA 610) have lower melting temperature, density, and moisture absorption than PA 6 and PA 66. Unfortunately, these materials are currently very expensive compared to tradition nylon materials. Costs can be reduced if less polymer material is used and reinforcing fillers such as cellulose nanofibers are added to strengthen the material. The resulting biobased composite has the potential of lowering costs while further strengthening the composite material. If these cellulose based composites can be successfully made with improved mechanical properties and without thermal degradation, then fully or partially biobased polyamide composites can be used to replace the glass-reinforced nylon composites that are currently used in engine-related vehicle applications, including engine intake manifolds and fuel-line applications. Preliminary results of using cellulose fibers to reinforce PA 1010 and PA 610 show that the addition of cellulose increased stiffness and strength of the composite material [11, 12, 13]. However, little to no published work could be found that explores the production and properties of ultrafine cellulose nanofiber reinforced PA 610 and PA 1010 composites.

Materials and Methods

Materials

Semicrystalline cellulose nanofibers, grade UFC 100, were kindly obtained from JRS Arbocel. According to JRS, it is the finest cellulose fiber available on the market, with an average particle size of 8 um and a diameter of approximately 2 um. It has a bulk density ranging from 150 to 220 g/L and a specific density of 1.4 g/cm3.

Two bio-based polyamides were studied as matrices for the cellulose nanofibers, PA1010 and PA610. Both are kindly supplied by Evonik Vestamid. PA1010 is 100% based on natural resources, as both of its monomers are derived from castor oil, giving it a carbon footprint of 4.0 kg CO2eq. It is a semicrystalline material with high mechanical strength and good for use at high temperatures.

Similarly, PA610 is also a biobased polyamide, but only 62% based on castor oil. It has a carbon footprint of 4.6 kg CO2eq, slightly larger than that of PA1010. Like PA1010, it is semicrystalline and has high mechanical strength (slightly higher than PA1010) and good thermal properties.

Sample Preparation and Processing

Conventional melt processing was used to disperse cellulose nanofibers into the bio-based polyamide matrices. Prior to extrusion, cellulose and polymer was dried in an oven at 70 °C overnight. Cellulose and polyamide were fed into the extruder hopper using K-Tron feeders to create a masterbatch. Single screw extrusion was carried out on a Davis-Standard machine. The melt temperature was set to 446 °F and decreased by 5 in each zone. The extrudant was solidified in a water bath and pelletized. Masterbatch pellets were mixed with pure polyamide pellets and extruded to create extrudant composites with 2, 4, 6, and 8 weight percent of cellulose nanofibers. In addition, 10 and 20 weight percent samples were made for the PA 610 matrix. A constant screw speed of 40 was used for all extrusion.

The pelletized extrudant was dried again in an oven at 80 °C overnight in preparation for injection molding. Samples were made according to ASTM standards for tensile, flexural, and impact testing in a Boy Machines Model 80M injection molder. The injection molding parameters used to create good PA1010 and PA610 samples are shown below in Table I.

Table I. Injection molding processing conditions for PA1010 and PA610.

PA1010 PA610

Shot size 24 mm 24 mm

Injection Pressure 800|1300 psi 2000|2000 psi

Hold Pressure 450|550|750|1000 psi 750|850|950|1000 psi

MPP Back Pressure 50|50 psi 50|50 psi

Mold Temperature 160 °F 180 °F

Temperatures 470|470|465|460|455 °F 510|510|505|500 °F

Injection Speed 2|2|2 mm/s 100|100|100 mm/s

Screw Speed 50|50 1/min 100|100 1/min

Cooling Time 30 s 30 s

Decompression 1 mm 1 mm

Testing and Characterization

After the injection molded samples have rested for a week to allow samples to reach equilibrium, their mechanical, thermal, rheological, and morphological properties were evaluated. An Instron 3366 was used to perform tensile testing and 3 point bend flexural testing. Tensile properties were determined according to ASTM D638-10 with Type 1 specimens. The Instron was equipped with a 5-kN load cell and a travel extensometer. The tensile tests used a strain rate of 5 mm/min and a gage length of 2 in, and the test recorded the stress at max load, strain at max load, strain at yield, and the Young’s modulus. The test would be stopped when either the specimens failed or the strain reached 0.23 mm/mm, whichever came first. A three-point bend flexural test was used to determine flexural properties according to ASTM D790-10. As with tensile testing, the Instron was equipped with a 5-kN load cell. Flexural testing was conducted at a strain rate of 1 mm/min, recording the modulus and the stress and stopping the test at 5% strain or failure. Izod impact testing was conducted according to ASTM D256. Undeformed flexural bars were cut to a 2.5 inch length and notched. A 10 lb pendulum was used to hit the notched sample on a Testing Machines Inc. 43-02-03 model impact test machine. At least six samples per loading level were tested for tensile, flexural, and impact properties.

In addition to mechanical testing, differential scanning calorimetry was performed on a Mettler Toledo analyzer. The sample was placed into an aluminum crucible and sealed, where it was first heated from 25 °C to 300 °C at a rate of 20 °C/min to remove any thermal history, then held at 300 °C for 5 min, cooled from 300 °C to 0 C at a rate of -10 °C/min, held at 0 C for 5 min, and heated again to 300 °C at 10 °C/min. The rate of heat flow in mW was plotted against time to find and integrate the peaks. The temperatures and enthalpies of melting and crystallization were recorded. Scanning electron microscopy was also performed using a JEOL 6610 SEM to visually study cellulose nanofiber dispersion and orientation after injection molding. Fracture surfaces from impact testing were carbon coated to prepare the nonconducting nanocomposite samples for SEM. Thermal gravimetric analysis (TGA) was also performed on a Mettler Toledo

analyzer on samples of about 10 mg. Each sample was scanned from 25 °C to 600 °C at a heating rate of 10 °C/min under nitrogen with a flow rate of 30 mL/min to avoid sample oxidation. Three randomly picked samples per loading level were used for TGA analysis and the averaged results are presented here.

Results and Discussion

SEM

Figure 1a shows an SEM micrograph of carbon coated raw cellulose nanofibers. It appears that the fibers have a diameter of approximately 1-3 um. Neat PA1010 and PA610 micrographs are shown in Figure 1b and 1c respectively.

When CNF is mixed into nylon to create composites, the cross-sectional area of the fibers appears in SEM as bright spots with 1-3 um diameter. This can be seen in Figure 2a and 2b, which shows 8 wt% CNF in PA1010 and PA610 respectively. It appears that the CNF is fairly well dispersed by the single screw extruder.

Figure 2. SEM micrographs of 8 wt% CNF in PA1010 (a, left) and PA610 (b, right).

In addition, an injection molded sample of 8 wt% CNF in PA1010 was fractured length-wise to verify how fibers were oriented within the matrix. The micrograph shown in Figure 3 shows that the fibers appear to stretch out, some to lengths of at least 90 um, and orient themselves with the flow of the injection molder. It is typical for the fibers in short fiber reinforced polymer composites to orient themselves due to the flow in injection molding, resulting in anisotropic final

Figure 1. SEM micrographs of raw CNF (a, left), neat PA1010 (b, middle), and neat PA610 (c, right).

50 um 100 um 100 um

100 um 100 um

products [14]. However, it is difficult to both quantitatively study fiber alignmenment in experimental samples and to analytically model flow behavior during injection molding. With that said, fiber alignment should improve the mechanical properties of the composite material, given sufficient filler-matrix interactions [15].

Figure 3. SEM micrograph of an 8 wt% CNF in PA1010 composite fractured lengthwise that shows how fibers are oriented as a result of the injection molding flow.

Tensile

Adding cellulose nanofibers to both PA1010 and PA610 increased the Young’s modulus of the material as shown in Figure 4. This observation is in agreement with other studies that have reported an increase of Young’s modulus in polyamide composites with increasing micro and nanocellulose content [8, 11]. Xu also found similar behavior in cellulose reinforced composites made from PA 6 and PA 66 [5]. Neat PA1010 has a Young’s modulus of 1900 MPa and neat PA610 has a Young’s modulus of 2185.9 MPa. Neat PA610 has a better Young’s modulus (and all other mechanical properties) than neat PA1010 because it is a more crystalline material. The smaller monomer in PA610 allows it to have a more closely packed structure than PA1010. Young’s modulus for PA1010 increases with increased loading level up to 6% (at which point, the modulus is 2063.2 MPa, but then decreases for 8%, although it remains 6% better than the neat material. It is unclear if modulus will continue to decrease as loading level increases beyond 8%. On the other hand, the Young’s modulus for PA610 composites continues to increase monotonically. At 8%, modulus improved by 10% and at 20% loading level, modulus improved by 36%.

100 um

Figure 4. Young’s modulus as a function of CNF loading level in PA1010 and PA610.

On the other hand, the strain at yield drastically decreases with loading level for both of the PA1010 and PA610 matrices. The average strain at yield is 4.8% for neat PA1010 and 4.3% for neat PA610. This decreases to 2.8% for 8% CNF in PA1010 and 2.4% for 8% CNF in PA610, resulting in a 42% decrease and 43% decrease from their respective neat materials. As CNF loading increases to 20% in PA610, the strain at yield continues to decrease to 60% of the neat value. This data can be seen in Figure 5. The worsened elongation suggests that the incorporation of cellulose nanofibers acts as a mechanical reinforcement on the polymer chains but also makes the composites very brittle, due to a rigid interface between the filler and matrix material. Polyamide composites reinforced by cellulose and flax fillers also show a decrease in the elongation at break upon increased loading level [12, 17]. Santos et al also reported similar changes to elongation behavior in composites made from Brazilian curacua fibers in polyamide [18].

Figure 5. Tensile strain at yield for CNF in PA1010 and PA610.

Neat PA1010 reaches a maximum stress of 48.8 MPa, whereas neat PA610 reaches a maximum stress of 50 MPa. For CNF in PA1010, maximum stress initially increases to 50 MPa at 2% loading, but drops off for 4 to 8%. At 8%, the maximum stress decreased by 15% from the neat polymer. For CNF in PA610, maximum stress immediately drops, and continues to decrease up to the tested 20% loading level. At 8%, the stress at max load has decreased by 11% from the neat polymer, and at 20%, the stress has decreased by 27%. This can be seen in Figure 6. Kuciel et al has found an improvement in maximum stress as filler loading increases in biopolyamide composites reinforced by flax, glass, and carbon [17]. It appears that those composites experience a large enough improvement in modulus to counteract their decreased elongation. If measures are taken to lessen the decrease in elongation and further improve the Young’s modulus, it seems that CNF reinforced biopolyamide composites would also be capable of withstanding higher stresses as filler loading increases.

Figure 6. Tensile stress at maximum load as a function of CNF loading level in PA1010 and PA610.

Flexural

The effect of cellulose nanofiber loading on the flexural strength is shown in Figure 7. Both the neat PA1010 and PA610 experience lower stresses (62.4 MPa and 68.7 MPa respectively) compared to the CNF -reinforced composites. From 0 to 4 wt% of CNF in PA1010, there is no obvious increase in the stress required for 5% strain, but it increases for 6 to 8 wt% by up to 2.7%. PA 610 shows bigger changes in in the flexural strength. It monotonically increases as cellulose loading level increases—the stress at 5% strain for 8 wt% is 74.9 MPa (9% increase) and 76.2 MPa (10.8% increase) for 10 wt%. The flexural samples at 20 wt% broke before 5% strain was reached.

Figure 7. Flexural stress at 5% strain for various CNF loading levels in PA1010 and PA610.

The flexural modulus demonstrated similar behavior to the strength, shown in Figure 8. Although modulus increased slightly for increased loading in PA1010, there is no statistically significant difference between 0, 2, and 4 wt%, but at 8 wt%, the modulus increased by 4% from 1.70 GPa to 1.77 GPa. Similarly, neat PA610 has a modulus of 1.84 GPa, and this value increases monotonically with loading level. At 8 wt%, the modulus is 2.14 GPa (16% increase), and as loading level increases to 20 wt%, the modulus increases to 2.24 GPa for a 21% increase from the neat modulus. This behavior is to be expected due to the inherent stiffness of the cellulose nanofibers that would increase the stiffness of the composite material. A well-formed interface should lead to better stress transfer from the matrix to the cellulose fibers. Because both cellulose and nylon are polar in nature, a well-formed interface should form and create a composite material with high modulus and strength [19]. The research done by Kiziltas et al on incorporating cellulose fibers in polyamides shows similar results [11]. Bledzki et al also found a similar trend in flexural behavior when incorporating natural fibers and man-made cellulose into PA610 and PA1010 matrices [19].

Figure 8. Flexural modulus for various CNF loading levels in a PA1010 and PA610 matrix.

Impact

To study how impact behavior of the CNF composites change with loading level, a notched Izod impact test was performed at room temperature. The impact strength of neat PA1010 and PA610 are 59 J/m and 53 J/m respectively. As Figure 9 shows, the impact strength appears to decrease once cellulose is added, but due to the large error in testing, it is hard to definitively say how much it decreases and how it changes as loading level increases. For PA1010, it appears that for any CNF loading level, the impact strength is approximately 20-30% lower than the neat material, corresponding to an impact strength of approximately 38-48 J/m. For PA610, the impact strength for 2 to 6 wt% appears to be 30-40% lower than the neat material (32-36 J/m), and then it may drop off to a 43% decrease (30.3 J/m) for 8 wt%. For 10 wt% and 20 wt% loading level, the impact strength continues to be lower than for the neat samples, yielding values of 35.9 J/m and 22.3 J/m respectively. However, a decrease in impact strength after the addition of natural fillers has been found in other studies, namely Amintowlieh for microcrystalline cellulose composites and Xu cellulose-polyamide composites [20, 5]. This is because first, increasing filler content increases the interfacial regions which cause crack propagation, and second, increasing filler content can reduce polymer chain immobility [21]. The decrease in impact strength in composites can potentially be improved by using surface functionalizations and impact modifiers [21].

Figure 9. Impact strength of CNF composites in PA1010 and PA610.

Differential Scanning Calorimetry

Figure 10 shows the results of a sample DSC experiment for a set of CNF -PA1010 composites, which shows one small crystallization peak at approximately 115 °C and a second big peak at 175 °C, and two similar sized melting peaks at approximately 190 and 200 °C. The peak at 190 °C is from the melt of the previously formed crystal, whereas the peak at 200 °C is from the melt of the re-crystallized melt [22]. Similarly, the peak at 175 °C is from the main crystallization whereas the peak at 190 °C is due to the crystallization of the re-melted crystals. The positions of these peaks change with cooling rate and blend composition [22].

Figure 10. DSC for various CNF loading levels in a PA1010 matrix.

In comparison, Figure 11 shows the results of a sample DSC experiment for a set of CNF -PA610 composites. PA610 exhibits one crystallization peak at 192 °C and one melt peak at 223 °C. The higher crystallization and melting temperatures of PA610 compared to PA1010 is also due to its higher crystallinity, similar to its mechanical behavior.

Figure 11. DSC for various loading levels in a PA610 matrix.

Table II shows how the melting and crystallization temperatures and enthalpies change as a result of cellulose nanofiber loading level for PA1010 and PA610. For CNF in PA1010, it is apparent that as loading level increases, the main crystallization peak at 175 °C shifts to slightly higher temperatures, and the main melting peak at 190 °C shifts to slightly lower temperatures. The secondary crystallization peak shifts to slightly lower temperatures and the second melting peak shifts to slightly higher temperatures. A similar trend can be seen for CNF in PA610—as loading level increases, crystallization temperature increases and melting temperature decreases. However, it is important to note that these shifts are all very slight—at most the peaks shift by 2 °C even at 8 wt% loading level. In addition, the enthalpies of crystallization and melting for both polyamide matrices decrease as loading level increases. This behavior is all consistent with expectations. As filler loading increases, then the increased interaction between matrix and filler restricts chain movement. This causes the number of crystallites that form to decrease and become more imperfect, thus lowering the enthalpy required for both crystallization and melting [22].

Table II. Melting and crystallization behavior of PA1010 and PA610 composites during DSC.

Group Name

Tc1 (°C) Tc2 (°C) Enthalpy (mJ/mg)

Tm1 (°C) Tm2 (°C) Enthalpy (mJ/mg)

PA1010 173.72 119.61 82.42 189.45 198.32 128.48

2% CNF 175.51 115.58 81.62 188.72 198.96 112.88

4% CNF 174.23 116.47 75.74 188.62 198.58 123.69

6% CNF 175.22 116.24 79.58 188.51 198.49 116.39

8% CNF 174.36 115.46 79.48 188.36 198.82 113.18

PA610 192.84 - 103.08 223.33 - 101.61

2% CNF 192.99 - 99.01 222.75 - 106.66

4% CNF 193.29 - 100.56 222.58 - 102.53

6% CNF 193.67 - 100.30 222.30 - 99.71

8% CNF 193.69 - 90.36 221.89 - 99.05

Thermogravimetric Analysis

The TGA and DTGA curves for neat polyamides and their CNF reinforced composites are shown in Figures 12 and 13 for PA1010 and PA610 respectively. They indicate that as CNF loading level increases in both PA1010 and PA610 matrices, the onset temperature of rapid thermal degradation decreases. Neat PA1010 and PA610 both exhibit a single stage degradation with a peak at approximately 460 °C. CNF composites show two degradation peaks, where the first (cellulose) peak is at approximately 360 °C and the second (polyamide) peak is at 460 °C.

Figure 12. TGA and DTGA curves for neat and CNF-reinforced PA1010.

Figure 13.TGA and DTGA curves for neat and CNF-reinforced PA610.

Table III shows the results of TGA for CNF in PA1010 and PA610. The temperature at 10% mass loss decrease as loading level increases. This shows that as filler loading increases, the thermal stability of the composites decrease due to the lower thermal stability of CNF compared to the neat polyamides. In addition, as CNF content increases from 0 to 8 wt%, the final ash content increases from around 1.1 to 2.3% for PA1010 and 1.8 to 2.5% for PA610. The increase in final ash content as filler loading increases shows that cellulose increases the fire retardancy as the composite. These findings match those of Kiziltas for TGA of macro-cellulose reinforced PA1010 and PA610 composites [11].

Table III. Thermogravimetric data for neat polyamides and CNF composites.

Temp at 10% mass loss (°C)

Final Ash Content @ 600 °C (%)

PA1010

0% 430.23 1.08

2% 432.79 1.17

4% 423.85 1.48

6% 418.87 1.75

8% 424.46 2.19

PA610

0% 435.25 1.50

2% 430.37 1.38

4% 422.01 2.07

6% 420.70 1.97

8% 423.18 2.73

10% 397.70 3.29

20% 369.11 4.99

Summary and Next Steps

This study shows that it is possible to successfully produce well-dispersed composites made from cellulose nanofiber and biobased polyamides with single screw extrusion and injection molding. Overall, the composites showed improvement in some mechanical properties (specifically in tensile modulus and flexural strength and modulus) for both polyamide matrices. However, tensile strength, strain at yield, and impact strength showed less promising results. Although SEM shows that the cellulose nanofibers were evenly dispersed, twin screw extrusion can be used to further improve dispersion. This could result in more enhanced mechanical properties, especially for tensile properties and impact strength. The incorporation of a coupling agent or a surface treatment to the cellulose may also improve mechanical properties. Differential scanning calorimetry shows that even at up to 8 wt% loading level, the addition of cellulose nanofibers did not make a big impact on the melt and crystallization behavior of the composites. The thermal stability of the composites will be studied in more depth by using thermogravimetric analysis. Although biobased polyamides currently cost considerably more than the polyamides conventionally used in the automotive industry, this cost will drop as production costs decrease. In addition, with further work to improve the matrix-cellulose interface and dispersion, cellulose nanofibers can be used to successfully strengthen, lightweight, and reduce the cost of biobased polyamides.

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