Mateo Gonzalez de Gortariab,
Arturo Rodriguez-Uribeb,
Manjusri Misra*ab and
Amar K. Mohantyab
aSchool of Engineering, University of Guelph, Thornbrough Building, Guelph, N1G 2W1, Ontario, Canada. E-mail: mmisra@uoguelph.ca
bBioproducts Discovery and Development Centre, Department of Plant Agriculture, University of Guelph, Crop Science Building, Guelph, N1G 2W1, Ontario, Canada
First published on 21st July 2020
Reducing greenhouse gas emissions (GHG) in vehicles requires the use of lighter-weight materials. One possible strategy is using biomass-derived carbons (biocarbon), which have a lower density compared to traditional mineral based fillers. In this study, novel composites reinforced with 20 and 30 wt% of a biocarbon produced at high temperature (950 °C) were melt compounded with polyphthalamide (PPA), followed by injection molding, and compared to talc-filled composites. Mechanical tests were performed with ASTM standard samples for tensile, flexural and impact properties, alongside thermal, spectroscopic and morphological characterizations. Surface area and elemental composition of the biocarbon and talc particles were also determined. The biocarbon and talc composites had matching mechanical properties in most of the tests (3.7 GPa for the Young's modulus of the 20 wt% talc-filled composite versus 3.7 GPa for both 20 wt% biocarbon-filled composites), with all the properties surpassing those of the unfilled, neat PPA (Young's modulus of 2.4 GPa), and the biocarbon-filled composites have a lower density than the talc-filled ones (1.277 g cm−3 for the 20 wt% talc-filled composite versus 1.176 g cm−3 for both 20 wt% biocarbon-filled composites). The main influencing factors for the better performance of the biocarbon-PPA composites were found to be the similarity of particle size between the talc and the biocarbon.
A widely used strategy to reduce weight is to replace metallic components in the interior and under the hood of cars with plastics and polymer composites. Nowadays, these materials are some of the most studied and employed alternatives for transport applications and weight reduction initiatives. Most composites are compounded with heavy fillers and, in this respect, bio-fillers can be successfully used to reduce the overall weight of these materials. Use of plastics parts in vehicles has increased significantly since the 1960s. Currently, the average plastic weight in a conventional vehicle is 351 pounds, a significant increment compared to 20 pounds or less of plastic material.5 Due to the replacement of metallic parts by plastic parts, the average fuel economy in North America of a light-duty vehicle has increased from 14.3 miles per gallon in the 1960s to 23.5 miles per gallon in 2010 [Original Equipment Manufacturer (OEM)].6,7
Replacing more specialized metallic parts by using plastics is a challenging task. The challenges arise from the thermal and mechanical conditions in which the plastics must be used. In this scenario, so-called high-performance engineering plastics can take the lead. These polymers are capable of retaining exceptional mechanical and performance characteristics above 150 °C,8 and some of these polymers do not start decomposing until a temperature range of 480 to 500 °C is reached.9 The current size of the market for these polymers is a small fraction of the global plastics market. The market of high performance polyamides was valued at USD $2642.1 million in 2018,10 whereas the market for commodity plastics (the most commonly used polymers in industry for general applications) is expected to reach USD $493 billion by 2022.11 Using Nylon price as a reference point, the price of these polymers can be 3 to 20 times higher, although the use of inexpensive and bio-based fillers can overcome the economic hurdle by reducing the cost of the composites produced.
Polyphthalamide (PPA), is found among these high-performance engineering plastics. The ASTM standard D5336-15a specifies that, for a polymer to be called PPA, there must be a minimum of 55 molar% content of isophthalic or terephthalic acid in the dicarboxylic portion of the polymer chain,12 with the rest of the molar weight being composed by a diamine co-monomer such as decamethylene diamine or hexamethylene diamine.13 This means that PPA is not a term for one specific polymer, rather PPA can be considered an umbrella term for a family of polymers. Commercial samples that only include terephthalic acid and hexamethylene cannot be used in injection molding applications, as such a polymer has a degradation temperature below its melting temperature, that is, it cannot be processed by melt compounding techniques.13 Instead, in order to produce a PPA that can be processed through standard plastic processing techniques, manufacturers use a mix of comonomers, using different ratios of diamines and isophthalic and terephthalic acid (and some other acids such as adipic acid) to create commercial products.13 Thus, PPAs' high mechanical performance has a range of values, depending on the specific comonomers used to produce the polymer: the tensile strength at break can be between 70 to 90 MPa,13 the heat deflection temperature (HDT) is around 120 °C (measured with a force of 1.8 MPa),13 and the melting point is approximately 300 °C.13
PPA has been traditionally used and studied with mineral fillers such as talc,13 as well as short and long glass fiber.14–16 Studies exist in which other materials are used, such as mesoporous silica,17 boehmite and phenylphosphinic acid,18 graphene oxide,19 boron nitride nanosheets,20 fullerenes21 and nano carbon tubes,22 among others.
Talc in particular, has been used to boost the mechanical, thermal and impact properties, such as having a higher HDT, increasing the stiffness or improving the impact resistance, among other improvements, such as surface scratching resistance and appearance.23 However, the use of talc can increase CO2 emissions when a composite is manufactured, especially compared to natural fibers and fillers.24,25
As previously mentioned, the melting and processing temperature of PPA starts at 300 °C.13 Obvious alternatives to talc, like natural fibers or biofillers (such as sawdust, etc.) cannot withstand such high temperatures. One viable alternative to bio-fillers is biocarbon. Made from almost unlimited sources of biomass, biocarbon is the solid residue left after pyrolysis. Its production costs and characteristics change depending on the temperature at which it is produced. Temperatures of pyrolysis can reach as a high as 1200 °C.26 The decomposition behavior in a thermal ramp of a biocarbon depends, mainly, on the temperature at which the pyrolysis was carried out. The thermal stability of a biocarbon produced at 500 °C can reach up to 600 °C,27 a bio-oil pyrolyzed at 900 °C remains thermally stable up to 900 °C.28 Due to these properties, biocarbon has attracted a lot of research and commercial interest in the past decade, and many biocarbon-filled composites have been developed, optimized and are currently used in commercial products.29–32 The price of biocarbon depends on the temperature at which it was produced (USD $0.0748–$0.109 per kg, for a range of 500 °C to 1000 °C).33 Even with these prices, biocarbon is still cheaper to produce than talc (USD $0.15 per kg to US$ 0.66 per kg).23 Particularly in automotive applications, biochar can play a role in reducing environmental impact of composites, as a Life Cycle Analysis (LCA) study performed by Tadele et al., showed that in the case of polypropylene (PP) composites, substituting talc with Miscanthus biochar results in reduced environmental impacts, mainly through the weight reduction achieved by the lower-weight density of the PP composites.34
This study posits that biocarbon/PPA composites could produce materials that are a viable alternative to talc-filled ones. One of the main research objectives is to investigate if this new composite can match the overall performance of a talc-filled PPA composite. As a result of the reduction in weight of the composites, this would also have a direct impact in the overall transportation costs, fuel efficiency and reduction in the emissions of GHG. Within this framework, a discussion of the advantages of biocarbon over the mineral filler is presented. To the best of the knowledge of the authors, no work using biocarbon in high performance composites with any type of PPA has been previously studied or published, adding to the significance of this research.
Coding | Wt% of PPA | Wt% of talc | Wt% of BioCM/1Hr | Wt% of BioCM/4Hr |
---|---|---|---|---|
PPA | 100 | 0 | 0 | 0 |
80PPA/20Talc | 80 | 20 | 0 | 0 |
70PPA/30Talc | 70 | 30 | 0 | 0 |
80PPA/20BioCM/1HR | 80 | 0 | 20 | 0 |
70PPA/30BioCM/1HR | 70 | 0 | 30 | 0 |
80PPA/20BioCM/4HR | 80 | 0 | 0 | 20 |
70PPA/30BioCM/4HR | 70 | 0 | 0 | 30 |
The composites were melt compounded at 325 °C (the micro-extruder has three heating zones, the temperature was set to be the same in all of them), with a 100 rpm for a total time of 2 min. The resulting melts were then collected and transported via a cylindrical injector (Xplore Instruments) and injected to molds heated to 100 °C (the manufacturer recommends these conditions in order to produce the best quality PPA parts). According to the instructions provided, this ensures the maximum crystallinity of the injected samples and produces the best mechanical properties. The injection process was carried out with 16 bar pressure for 10 seconds of holding time, with an additional 12 seconds at 10 bar pressure. Tensile, flexural, and impact specimens were produced following the dimensions specified by the ASTM International standards.
Density was measured using an electronic Densimeter MDS-3000 (AlfaMirage, Osaka Japan), which has a resolution of 0.01 g cm−3. Two replicate samples were employed.
Pore volume and surface area of the biocarbon and talc were characterized using an Autosorb-iQ (Quantachrome Instruments, Boynton Beach, Florida United States). Samples between 200 to 250 mg were degassed for 3 hours at 150 °C. Nitrogen gas was employed as the adsorbent, and the temperature was kept constant by using a liquid nitrogen bath at its boiling temperature at room standard pressure conditions (−196.15 °C). The linear region of the sorption part of the curve, along with the last sorption point, were used with the Brunauer–Emmett–Teller (BET) model to calculate the dimensions of the average pore and the total volume of pores, as well as the surface area.
The degradation, as well as the thermal stability of the composites was measured in a thermogravimetric analysis unit, TGA Q500 (TA Instruments, New Castle, DE, United States). For each material tested, samples were cut from an injection molded sample. A 50 mL min−1 flow of nitrogen was used, heating all samples at a constant rate of 10 °C min−1 from near room temperature to reach a final temperature of 700 °C.
The heat deflection temperature (HDT) of each composite produced was tested in a DMA Q500 unit (TA Instruments, New Castle, DE, United States). A bar of the size, shape and dimensions as specified in ASTM D256 was used. A force calculated to exert 0.455 MPa of stress on the bar was applied. Samples were heated from an initial temperature of 0 °C to a final temperature of 270 °C, at a heating rate of 2 °C min−1.
Table 2 shows the results of the surface area analysis of the two biocarbons and talc, while Table 3 shows their particle size analysis. The results in both tables (Tables 2 and 3) show that using the ball-milling to reduce the size of the biocarbon particles was successful, the reduction depending on the time the particles spent in the ball-mill. As the surface area of BioCM/4HR increased, its average pore volume and the average particle size decreased. The results for talc of surface area properties and particle size distribution are consistent with the material data sheet provided by the manufacturer. The talc particles, as expected for a manufactured product, have a narrow size range, with 90% of the particles found within 3 to 10 μm, while the BioCM has a wider distribution range. The BioCM/1HR has ∼80% of is particles within 2–10 μm and the BioCM/4HR has ∼87% within 2 to 10 μm, similar to the talc.
Filler | Surface area (m2 g−1) | Total pore volume (cc g−1) | Average pore radius (nm) |
---|---|---|---|
Talc | 11.00 | 0.00821 | 14.92 |
BioCM/1HR | 6.55 | 0.00141 | 4.30 |
BioCM/4HR | 9.09 | 0.00177 | 3.89 |
Sample | % of particles within the size range | Average particle size (μm) | Aspect ratio | ||||
---|---|---|---|---|---|---|---|
<2 μm | 2–3 μm | 3–5 μm | 5–10 μm | ≥10 μm | |||
Talc | 0 | 4.52 | 54.35 | 38.51 | 2.63 | 5.15 | 0.666 |
BioCM/1HR | 7.02 | 17.82 | 30.05 | 35.02 | 10.09 | 5.54 | 0.628 |
BioCM/4HR | 7.75 | 32.39 | 36.17 | 19.35 | 4.37 | 4.21 | 0.617 |
The FTIR spectra of the unfilled PPA and the composites are shown in Fig. 2. Previous studies39 show similar characteristic peaks of PPA: the peaks at 1628 cm−1 and 3293 cm−1 are related to vibrations of the amide group, the 3075 cm−1, 1495 cm−1 and 1537 cm−1 peaks can be attributed to the motions of the benzene rings present in the polymer matrix, and the 2850 cm−1 and 2919 cm−1 and peaks are associated with the CH2 groups present in the PPA's polymer chain. The FTIR spectra of the biocarbon composites have the same characteristic peaks as the neat PPA, at a reduced peak height, while the talc composites all have the three characteristic peaks found in the talc filler. These results indicate that both the talc and the biocarbons are not generating any significant amount of new or already present chemical bonds with the polymer matrix that can be detected through FTIR, and that mechanical properties of the composites could be attributed purely to polymer-filler interactions.
The talc-filled composites are shown in Fig. 3E and F. Both talc composites show good dispersion of the talc particles, with some small agglomerations. A stratified structure can be observed within the polymer matrix of the talc-filled composites, and larger agglomerations occur, as well as delamination of the talc particles, as has also been previously reported.40 The 20 and 30 wt% filled composites with BioCM/1HR and BioCM/4HR are very similar. However, as most biocarbon particles cannot be easily observed through SEM, and the particle size analysis showed that the majority are within a narrow range of sizes, the images were taken in locations in which larger particles were observed in order to highlight specific morphological features not easily seen otherwise. In Fig. 3G–J, it can also be observed that both the matrix and particles are not easily distinguished, suggesting good interaction and compatibility between the biocarbon particles and the PPA polymer matrix. In Fig. 3G, H and J, the pores previously observed in BioCM/1HR and BioCM4/HR are clearly visible in the images of all the biocarbon-filled composites, and no phase separation of the particles and the PPA can be seen. 80PPA/20BioCM/4HR shows smaller pores within the structure of the biocarbon, and 70PPA/30BioCM/4HR shows part of the internal structure within the biocarbon, revealing layers of sheets.
Fig. 4 (A) Tensile properties, (B) flexural properties and (C) impact energy and density of the composites. |
Both tensile and flexural properties suggest that the addition of biocarbon increases the stiffness of the material. Previous studies performed with different polymer matrices,37,40,43,44 show that the addition of a high temperature biocarbon increases the tensile modulus, although in some cases it reduces the tensile strength. As for the reduction in particle size, it did not significantly affect any of the mechanical tests, as both biocarbons had a similar distribution range.
This increase in stiffness in the filled composites, in contrast with the unfilled PPA, is also suggested by the impact energy decrease that occurs with the addition of the fillers, with the lowest impact energy being that of the composites with 30 wt% of biocarbon, as can be seen in Fig. 4C, although the differences between all the composites do not seem to be significant. Above a certain critical size of 30 nm, the size of the particles does not significantly affect the mechanical properties.45 As the size of all the employed fillers is bigger than 30 nm, it explains why the two different biocarbon composites have very similar properties. It should be noted that the neat PPA matrix did not fully break so, according to the ASTM standard, it cannot be directly compared. However, it has been included for the sake of completeness, and to show how the addition of BioCM and talc significantly decreases the impact energy.
As discussed previously, the SEM images of the biocarbon-filled composites show that the biocarbon has no phase separation between the PPA matrix in the four composites, as there is no gap or void between the biocarbon and the polymer. This adhesion could then be explained by a favorable interaction between the PPA matrix and the biocarbon particles. The FTIR spectra of the composites, shown in Fig. 2, suggest that this interaction is not through a chemical bond, but rather through a van der Waals-type interaction.
In a PPA-blend with carbon nanotubes, the good compatibility between the nanofillers and the PPA has been suggested as a possible example of π stacking or aromatic–aromatic interaction, due to the presence of benzene rings in both the polymer and the carbon nanotubes.22 The data in this study could be pointing to a similar phenomenon, with further research necessary to confirm or deny this possibility.
Fig. 4C also shows that the biocarbon composites have a lower density than the talc filled ones, an 8% decrease for the equivalent 20 wt% composite and an 11% decrease for the equivalent 30 wt% composite. This is a key finding in the search for lightweight materials, especially for transport applications, as having similar mechanical properties at a lower weight is key for a material for transport applications.
Ec(u) = EmVm + EpVp | (1) |
(2) |
The elastic modulus of the biocarbon-filled composites has been calculated in this study. The elastic modulus of the biocarbon has been measured in previous studies and has been estimated as 15.78 GPa.36 The volume fraction that the matrix and the particles occupy can be estimated using the fact that, in a composite, the sum of the mass of the matrix and the particle is the mass of the composite:
mc = mm + mp | (3) |
Using the relationship that m = vρ:
vcρc = vmρm + vpρp | (4) |
Dividing by the volume of the composite:
(5) |
The ratios of and are the volume fractions Vm and Vp, respectively:
ρc = Vmρm + Vpρp | (6) |
As the sum of the volume fractions must be 1, so (1 − Vm) = Vp:
ρc = (1 − Vp)ρm + Vpρp | (7) |
Rearranging the terms gives the particle volume fraction in terms of the densities of the composite, matrix and particles:
(8) |
The density of the particles, ρp, has also been previously calculated at 1.7 g cm−3.46 The bounds, as well as the volume fractions of each composite and their elastic (Young's) moduli, are shown on Fig. 5. It is apparent that the biocarbon-filled composites have elastic moduli above what the upper bound of the rule of mixtures predicts. This suggests that there is an interaction between the polymer and the biocarbon that cannot be explained solely by adding the mechanical properties of the PPA polymer matrix and the biocarbon particles.
Fig. 5 Rule of mixtures showing the predicted behavior of the composites versus the experimental results. |
Fig. 6 TGA analysis of the composites and the unfilled PPA, with an inset showing the early stages of thermal degradation. |
Material | 2% weight loss temperature [°C] | 5% weight loss temperature [°C] | 10% weight loss temperature [°C] | Maximum degradation temperature [°C] | Glass transition temperature (tanδ) [°C] | HDT at 0.455 MPa, at 0.2% strain [°C] |
---|---|---|---|---|---|---|
PPA | 404.86 ± 8.10 | 429.71 ± 0.33 | 441.72 ± 0.01 | 471.55 ± 1.22 | 140.77 ± 9.84 | 122.30 ± 17.97 |
80PPA/20Talc | 409.34 ± 1.16 | 432.42 ± 1.19 | 443.47 ± 1.27 | 472.04 ± 1.05 | 138.45 ± 10.80 | 141.70 ± 21.32 |
70PPA/30Talc | 414.11 ± 5.55 | 434.88 ± 2.09 | 445.98 ± 1.32 | 473.34 ± 1.10 | 144.99 ± 2.14 | 225.70 ± 2.04 |
80PPA/20BioCM/1HR | 411.46 ± 4.24 | 433.33 ± 0.70 | 444.88 ± 0.01 | 470.36 ± 0.57 | 143.47 ± 2.13 | 139.56 ± 7.83 |
70PPA/30BioCM/1HR | 416.28 ± 1.58 | 435.99 ± 1.70 | 447.07 ± 1.78 | 471.69 ± 1.73 | 144.87 ± 0.10 | 161.92 ± 0.89 |
80PPA/20BioCM/4HR | 413.15 ± 4.18 | 433.93 ± 1.72 | 445.15 ± 2.61 | 470.94 ± 3.03 | 134.16 ± 1.66 | 139.85 ± 13.67 |
70PPA/30BioCM/4HR | 425.28 ± 0.81 | 439.75 ± 1.00 | 449.74 ± 0.96 | 473.31 ± 0.84 | 149.42 ± 0.08 | 155.79 ± 5.82 |
Fig. 7 to Fig. 9 show the results of the DSC analysis, while Table 5 shows the relevant enthalpy information (see also Fig. 8). The crystallinity of polymer composites can be calculated from the measured enthalpies of the composite, which requires ΔH0f (the enthalpy of crystallization of a theoretical 100% crystalline form of the unfilled PPA). However, this data has not been calculated.47 The Tg cannot be clearly seen for the neat PPA or the composites from the DSC curve, although it has been reported at 117 °C for the unfilled PPA.22 A small exothermic peak is also observed in all the composites, which decreases in height in more highly loaded composites. The Tm does not change significantly from the unfilled PPA to the composites. The decrease in the ΔHf in comparison with the unfilled PPA indicates that the fillers are affecting the energy required for a crystallization process within the polymer matrix. Although there is a small increase for the ΔHf, going from 80PPA/20BioCM/1HR to 70PPA/30BioCM/1HR, the difference is small enough to be attributable to an error when measuring the sample, as the ΔHc of the composites follows the same trend for all composites and fillers, it decreases as the wt% of the filler increases. The addition of the BioCM particles increases the Tc which, according to previous studies performed with other polymer matrices, means the particles could act as nucleating agents and enhance the crystallization process.37,40,43,48
Material | Tm [°C] | ΔHf [J g−1] | Tc [°C] | ΔHc [J g−1] |
---|---|---|---|---|
PPA | 299.20 | 14.08 | 259.22 | 27.50 |
80PPA/20Talc | 300.09 | 9.731 | 277.44 | 16.44 |
70PPA/30Talc | 300.66 | 8.176 | 280.74 | 12.95 |
80PPA/20BioCM/1HR | 298.84 | 11.35 | 268.72 | 18.81 |
70PPA/30BioCM/1HR | 299.07 | 12.34 | 272.45 | 16.53 |
80PPA/20BioCM/4HR | 298.65 | 12.62 | 269.55 | 18.53 |
70PPA/30BioCM/4HR | 299.29 | 7.766 | 269.59 | 15.68 |
The addition of talc also aided the crystallization process in both talc-filled composites, as can be seen from the increase of the cold crystallization temperature. Studies performed with a polypropylene matrix show that talc can perform the role of a nucleating agent for the polymer matrix to form spherulites,42,49 suggesting the same possibility for PA. These results coincide with the morphology observed, in which the talc was seen to influence the structure of the polymer matrix, especially for the 30 wt% filled composites.
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