Effect of short basalt fibers on durability,mechanical properties,and thermal properties of polylactic acid composites

Abstract

The effect of basalt fiber (BF) content on the properties of BF-reinforced polylactic acid (PLA) composites was investigated. Composites with 10, 20, 30, 40, 50, and 60 wt% BF were fabricated. The results revealed that (1) the mechanical properties improved with increasing BF content. The maximum tensile strength and modulus of the composites (i.e. 140 and 5050 MPa, respectively) occurred at a BF content of 50%. The maximum flexural strength, that is, 159.5 MPa was two times larger than that of the pure PLA and was obtained at a BF content of 40%. However, the mechanical properties deteriorated at BF contents >50%. (2) BF can stop storage modulus loss and are effective in improving the crystallinity, as revealed by dynamic mechanical analysis and differential scanning calorimetry measurements. The crystallinity improved from 34.6% to 54.6% with BF addition. (3) After the accelerated aging test, pure PLA was too weak for testing. However, high values of the tensile modulus (i.e. 60% that of the nonaged samples) were maintained by the BF-reinforced PLA. This resulted possibly from the high crystallinity of the PLA composites. Therefore, suitable amounts of BF as reinforcements can yield improvements in the performance of PLA composites.

Keywords

Polymer–matrix composites basalt fiber mechanical properties thermal analysis durability

Introduction

Owing to environmental concerns, original equipment manufacturers have become increasingly interested in renewable material products. Polylactic acid (PLA) is one of the most extensively researched green materials.^1

-5 An increasing number of studies indicate that PLA composites reinforced with natural fiber can be developed and, owing to their environmental friendliness, may become the main type of composite in the automotive industry.^6
-8

Among natural reinforcements, basalt fibers (BFs) are regarded as one of the most suitable natural fibers, owing to their excellent mechanical properties and stability, although the BF is extracted from nonbiodegradable rocks.^9

-12 Young’s modulus of BF bundles, maximum stress, and strain at break are 80–110 GPa, 3200–4200 MPa, and 3.1%, respectively.^13
-15 These properties combined with the outstanding stability and commercial price render BF a very attractive alternative to glass fiber in structural applications.^16
-18 Ying et al.⁹ reported that pristine BF provided considerable reinforcement for PLA. A BF content of 20 wt% can increase the strength and modulus by approximately 30% and 47%, respectively. The surface treatment of BF with silane improves its affinity to the PLA matrix, reducing the interfacial energy in composite systems and leading to significant increases in both the tensile strength and the impact strength. Tábi et al.¹⁰ demonstrated that the creep strain of the composites can be substantially reduced using BF as reinforcements. Although the fiber length decreased with increasing fiber content, the creep resistance improved significantly due to strong fiber–matrix adhesion. Tábi et al. also found that the mechanical properties of PLA composites reinforced with 20 wt% BF (tensile strength 124 MPa and flexural strength 182 MPa) are superior to those of the other composites.¹⁹ The aforementioned studies indicate that these mechanical properties are promising for industrial applications of this new reinforcement.³

To date, studies have shown that BF as reinforcements can improve the stiffness and strength of the matrix considerably, with performance affected by the fiber length, weight fraction within the composite, and fiber/matrix interfacial adhesion.^16,20,21 The weight of basalt has a significant effect on the properties of the composite.^15,19 However, the correlation between the weight of BF and the properties of the composite (excessive amounts of fibers could be detrimental to the properties) has rarely been investigated. In particular, only a few reported studies have considered the thermal properties and aging behavior, which is required in automotive components. The relationship between the weight of fibers (i.e. the fiber content) and the properties is useful for future automotive design.

Therefore, in this work, we focused on the relationship between the weight of basalt and the mechanical, thermal as well as aging properties of the composites (these properties may be significant in promoting and developing automotive-component designs). Neat PLA and PLA composites with 10, 20, 30, 40, 50, and 60 wt% BF were manufactured through an injection process, which is the main manufacturing technique employed in the automotive industry. These materials were then characterized and compared. Three properties of the materials were investigated: (1) Mechanical properties including tensile, flexural, and impact properties were determined. (2) Thermal properties of the materials were evaluated via dynamic mechanical analysis (DMA) and dynamic scanning calorimetry (DSC) measurements. (3) Tensile properties were determined after the aging test. The results confirmed that BF played an effective role in improving the material properties, but an appropriate BF content (wt%) should be selected.

Experimental

Materials

Injection molding-grade PLA (type 290 D made in Taizhou, Zhejiang Province, China) was supplied by Zhejiang Hisun Biomaterials Co., Ltd (China). This type of PLA is characterized as follows: density: 1.25 g cm⁻³, glass transition temperature (T _g): approximately 58°C, melting temperature (T _m): 175–180°C, number-average molecular weight (M _n): 6.5 × 10⁴ g mol⁻¹, weight-average molecular weight (M _w)/ M _n: 1.38, and the d-lactide content is from 0.3% to 0.5%. Chopped BF with an initial fiber length of 6 mm was purchased from Jilin Tong Xin Basalt Technology Co., Ltd (China). The average diameter of fibers provided by the supplier is 13 µm, and a silane treatment for asphalt paint was used.

Composite fabrication

Before fabrication of the composite, PLA pellets were dried in a vacuum oven at 70°C for at least 48 h. BF were dried in the vacuum oven for 24 h at 110°C to remove residual moisture from the surface of the fibers, although hydrolysis of the BF was difficult. PLA-based composites with a nominal BF content of 10, 20, 30, 40, 50, and 60 wt% were prepared using a torque rheometer (XSS-300, Shanghai Kechuang Rubber & Plastic Machinery Equipment Co. Ltd, Shanghai, China; working capacity: 50 g). The BF-reinforced PLA and pure PLA samples were injection molded with a Thermo Scientific HAAKE Mini Jet Pro injection molding machine (Key Laboratory of PLA, Ecological Building, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, China). An injection pressure, holding pressure, and holding time of 450 bars, 150 bars, and 17 s, respectively, were employed. The melt and mold temperatures were set to 205°C and 25°C, respectively.

Characterization

Fiber length distribution

Prior to composite fabrication, the fiber length distribution was measured via optical microscopy (Olympus BX 61). Fibers (mass: 5 g) were extracted repeatedly (five times) from raw materials, and 40 fibers were extracted from every 5 g BF. A total of 200 fiber lengths were measured in this test.

Mechanical tests

To explore the tensile properties of the materials, an ISO 527-2 standard was employed on a WSM-20 kN electronic universal testing machine (Changchun Intelligent Instrument Equipment Co., Ltd, China; max force: 20 kN, testing precision: ±0.1%). Testing was performed at 23 ± 2°C and 50 ± 5% relative humidity (RH). Based on the ISO 527-2 standard, dumbbell-type 5A samples with a 2 × 4 mm² cross section (total length: 75 mm, length of cross section: 25 mm) were subjected to testing at a velocity of 1 mm min⁻¹. At least five specimens of each material were tested. The tensile strength and tensile modulus were computed from the stress–strain curves.

Samples (10 × 4 × 80 mm³) manufactured via injection molding were subjected to three-point bending tests (Instron 5982 AVIC Touchstone Testing Innovation Cooperation, Beijing, China), performed in accordance with the ISO 14125 standard. Testing was performed at room temperature at a speed of 1 mm min⁻¹. At least five samples of each material were tested.

The impact energy was determined via an A-notch impact test and an unnotched test, performed in accordance with the ISO 179 standard. A pendulum impact tester was employed with an impact hammer of 4 J at a speed of 2.9 m s⁻¹. A Type-A notch was cut in the middle of the sample on the shortest side. The experiments were conducted at an ambient temperature of 23 ± 2°C with a normalized climate of 50% RH. The corresponding energy was calculated from a double integral formula. At least 10 repetitions were required for each composite.

Scanning electron microscopy

The morphology of the material after tensile testing was observed via scanning electron microscopy (SEM). Prior to SEM (Hitachi SU3500, Technical Institute of Physics and Chemistry, Hanzhou, China; acceleration voltage: 10 kV) examination, all sample surfaces were sputter coated with gold/palladium alloy for 2 min to avoid electrostatic charging.

Differential scanning calorimetry

The percentage of crystallinity characterizing the samples was determined via differential scanning calorimetry (NETZSCH DSC 214, Technical Institute of Physics and Chemistry, Hanzhou, China; temperature range: 0–210°C, heating rate: 5°C min⁻¹) with aluminum sample pans. The T _g, cold crystallinity of the material, and T _m were obtained from the tests. The sample was pulverized into scratch and seven types of composites (mass: 3–6 mg). A strain-free steel high-volume DSC pan was used to pack the composites. The composites were heated twice to assess the crystallinity levels of the material and sample, respectively. The crystallinity was calculated from the results of the first heating scan. Afterward, the samples were cooled to 0°C at the aforementioned rate (i.e. 5°C min⁻¹) to eliminate the thermal history and heated again at the same rate for calculation of the material crystallinity. All tests were performed under nitrogen atmosphere.

Dynamic mechanical analysis

The dynamic mechanical properties of the samples were evaluated via DMA testing (NETZSCH 242E, Germany) of rectangular 35 × 10 × 4 mm³ specimens tailored for the DMA test chamber. Impact test samples were used in this test with a dual cantilever mode. The experiment was conducted at a frequency, amplitude, minimum temperature, and maximum temperature of 1 Hz, 20 µm, −40°C, and 140°C, respectively. The temperature was increased linearly at a rate of 2°C min⁻¹.

Accelerated aging test

Accelerated aging was conducted at a temperature of 80°C and 95% RH in a WEISS environmental chamber (Key Lab of Body in White, College of Automotive Engineering, Jilin University, Jilin Sheng, China). The tensile properties of the aged samples were then determined. Samples were obtained after 12, 24, 33, and 48 h of continuous conditioning. The tensile experiments were performed after 24 h of sample exposure at room temperature.

Results and discussion

Determining fiber length

The length of the fiber is directly related to the key properties, including the mechanical properties and thermal properties, of the composites. The mechanical performance of long-fiber composites is superior to that of short-fiber composites and, hence, fiber-length determination is important. In this work, 6 mm basalt was blended with PLA to strengthen the composite. The distribution of 200 fibers (Figure 1) revealed a fiber content of 51.5%, with fibers having lengths of 551 and 650 µm dominating this content.

Figure 1.

Length distribution of the raw material.

Mechanical properties

The mechanical properties of the injection-molded specimens were analyzed via quasi-static tensile, flexural, notched, and unnotched impact tests. Figure 2 indicates that improved tensile properties could be obtained using BF-reinforced PLA with high weight. With increasing BF content of up to 50%, the tensile strength improved to a maximum (140 MPa), which was higher than the tensile strength of the pure PLA (72 MPa). The same trend was observed for the tensile modulus (Figure 2). For example, the tensile modulus of PLA with 50 wt% BF (PLA50) (i.e. 5050 MPa) was 2.7 times higher than that (1360 MPa) of the pure PLA, indicating that the tensile properties of the material improve with increasing BF content. However, the tensile strength of PLA60 (136 MPa) and the corresponding tensile modulus were lower than those of PLA50. The basis for this trend is explained in the following section.

Figure 2.

Tensile properties of PLA reinforced with different amounts of BF. PLA: polylactic acid; BF: basalt fiber.

The SEM photographs in Figure 3 show the surface characteristics of the PLA composites after the tensile test. Extensive fiber pullouts, and the corresponding holes, are seen in Figure 3(a). The pulled fibers occurred on a clean surface, indicative of the weak interfacial adhesion between each PLA and the BF. This weak adhesion resulted in inadequate transfer of loads between the matrix and the fibers and, consequently, degradation of the tensile properties. Compared with PLA10, PLA30 had superior tensile properties. The tensile strength and modulus reached values of 124 MPa and 3740 MPa, respectively. Figure 3(b) shows that, consistent with the weave-like behavior, a few fibers bundled to form a thick fiber. A few matrices were drawn out during fiber pullout, thereby increasing the force required for breaking the samples. Most of the PLA matrix was pulled out with massive fibers in PLA50. The SEM micrograph showed that these fibers collected into bundles or clouds, thereby enhancing the fiber/matrix adhesion. The results revealed that the interfacial bond behavior is excellent and PLA are reinforced and toughened with 50% fibers. However, in contrast to previous hypotheses, the tensile properties of the PLA60 declined, owing to insufficient resin around the fiber (i.e. the matrix used to bind the fibers was insufficient, as shown in Figure 3).

Figure 3.

SEM micrographs of PLA composites: (a) PLA10, (b) PLA30, (c) PLA50, and (d) PLA60. SEM: scanning electron microscopy; PLA: polylactic acid; PLA10: PLA with 10 wt% BF; PLA30: PLA with 30 wt% BF; PLA50: PLA with 50 wt% BF; PLA60: PLA with 60 wt% BF; BF: basalt fiber.

Although the tensile properties improved with BF addition, the fibers separated the matrix, and perpendicular fibers were observed (Figure 3(c)). Fiber addition resulted in a decrease in the elongation and, in fact, the ductility of the composites was negatively correlated with the BF content. The BF content (as discussed above) has a positive effect on the tensile properties of the composites, except for the elongation.

Similarly, the flexural properties improved with BF addition. As shown in Figure 4, the maximum value of the flexural strength (i.e. 159.5 MPa), which occurred at a BF content of 40%, was considerably higher than that (63.5 MPa) of the pure PLA. The composites with BF high content consisted of perpendicular fibers (see previous section), which have a negative effect on the flexural strength. In addition, the flexural modulus increased (in general) with increasing BF content. The maximum value of the modulus (i.e. 18,200 MPa) was 4.5 times that of the pure PLA.

Figure 4.

Flexural property results.

PLA is considered a brittle material and is therefore used on a limited basis in industrial engineering, especially in automotive applications.¹⁹ To use PLA as an engineering material, high strength and stiffness as well as high impact strength of the material are required. The impact strength of the PLA composites is shown in Figure 5. As the figure shows, PLA30 had the largest impact energy, and the notched impact strength of PLA20 (4.9 kJ m⁻ ²) was higher than that of neat PLA (1.5 kJ m⁻ ²). However, the notched impact strength and unnotched impact strength of each composite decreased with increasing BF content, for content ≥30%. The unnotched impact strength decreased significantly, to values even lower than that of neat PLA. This is attributed to the fact that the perpendicular fibers destroyed the continuity of the matrix. The fibers had different orientations, and those that were vertical to the major axis (Figure 6) had a negative effect on the impact strength. This perpendicular arrangement was correlated primarily with the addition of fibers.

Figure 5.

Notched and unnotched impact strength of material.

Figure 6.

SEM micrographs of material: (a) PLA 40 and (b) PLA50. SEM: scanning electron microscopy; PLA: polylactic acid; PLA40: PLA with 40 wt% BF; PLA50: PLA with 50 wt% BF; BF: basalt fiber.

The results indicated that BF can yield considerable improvement in the mechanical properties of PLA. For example, tensile strength values were higher than that of the PP that is usually used in automotive products.^22,23 BF plays an active role in promoting the application of PLA in industrial engineering and is essential for the eco-design of automotives based on renewable materials.

Thermal analyses

The crystallinity levels of the PLA composites are directly related to their mechanical properties, including brittleness, toughness, stiffness, and modulus. Therefore, thermal property investigation of PLA composites used in engineering applications is essential.²⁴ Rapid crystallization (i.e. a high rate of crystallization) typically results in low crystallinity. During the manufacture of the material at a cooling rate of even 500–1000°C min⁻ ¹, the samples remain mainly amorphous and transparent. The percentage of crystallinity was calculated from DSC measurements. Moreover, thermal properties such as the T _g, T _m), and cold crystallization temperature were obtained from the curves. The heat of melting (H _m) and heat of crystallization (H _c) of each composite were also computed. The degree of crystallinity, based on the BF content, was calculated as follows:

X_{c} (% crystallinity) = \frac{Δ H_{m} - Δ H_{c}}{Δ H_{f} (1 - α)} \times 100

where X _c is the crystallinity, $Δ H_{m}$ (J g⁻ ¹) and $Δ H_{c}$ (J g⁻ ¹) are computed via integration of the corresponding endothermic and exothermic peak of the melting and cold crystallization process, respectively, $Δ H_{f}$ (J g⁻ ¹) is the enthalpy of fusion for 100% crystalline PLA, and α is the mass fraction of BF. A $Δ H_{f}$ value of 93 J g⁻ ¹ has been reported in the literature.²⁵ These calculations are based on the assumption that only a small and unknown part of the amorphous PLA becomes crystallized.

Figure 7 shows the DSC thermograms obtained for PLA composites with different fiber contents. The exothermic peak of cold crystallization was apparent in all seven curves. During crystallization, the amorphous part of the PLA transformed into a polymer (for the sake of comparison, the curve of neat PLA is shown in the middle of the graph). The area of the exothermic peak decreased gradually with increasing fiber content. This result indicated that BF aids the crystallization process (the surface topography and roughness of the fibers have a positive effect on crystallization) by acting as a nucleating agent. Figure 8 shows the degree of crystallinity calculated from equation (1). The degree of crystallinity improved [from 34.6% (pure PLA) to 54.6% (PLA60)] with increasing BF content, and the relationship between the crystallinity and the fiber content was approximately bilinear. However, this type of neat PLA exhibited higher crystallinity than other PLA (a crystallinity of approximately 20% has been reported in the literature). In general, PLA composites with a high level of crystallinity have better mechanical properties than amorphous PLA. The fibers had no effect on T _g. The values of T _g ranged from 56°C to 59°C, and a T _m of approximately 185°C was obtained. The materials were cooled after first heating and then reheated to eliminate the thermal history. Furthermore, the crystallinity of the materials was calculated from equation (1), and the values obtained (i.e. 63.5–72.3%) revealed that the materials were more crystalline than the samples (Figure 8). Consistent with a previous study,²⁶ our results suggest that a post-molding heating process can improve the crystallinity of materials.

Figure 7.

DSC thermograms of PLA composites with different fiber content. DSC: dynamic scanning calorimetry; PLA: polylactic acid.

Figure 8.

Crystallinity of the basalt fiber composites before and after eliminating the thermal history.

Dynamic mechanical thermal analysis

Automotive products operate in a harsh environment, and hence, the component materials should exhibit durability over a wide range of temperatures. Excellent mechanical properties must be maintained for temperatures ranging from −40°C to (at least) 60°C. Dynamic mechanical thermal analysis is essential for establishing the wide range of temperature-dependent material data required and determining the relationship between BF and the dynamic mechanical properties.²⁷

The effect of temperature on the mechanical properties of materials including polymers and composites can be investigated via dynamic mechanical thermal analysis. The storage modulus represents the energy stored in a material in each cycle of deformation. Figure 9 shows that the storage modulus loss of the PLA composites, that is, approximately 100% before 80°C, is higher than that of the other composites. The factor tan δ represents damping and is a measure of energy dissipation. The energy dissipation of the PLA composites is shown in Figure 10. Similar to a previous study,²⁸ a tan δ value of >2 was obtained for pure PLA. The results in Figure 10 show that, with increasing fiber content, tan δ decreased to a quarter of the original value. This resulted probably from the presence of BF, which prevents the chain mobility of the pure PLA.

Figure 9.

Storage modulus of PLA composites with different fiber content. PLA: polylactic acid.

Figure 10.

Tan δ of PLA composites with different fiber content. PLA: polylactic acid.

The storage modulus of each sample increased after 80°C, except for the modulus of pure PLA (Figure 9). This indicates that BF yielded an increase in storage modulus of the composites after the T _g temperature and is associated with cold crystallization behavior.²⁷ No such transition was observed in the pure PLA. The properties improved significantly after crystallization of the amorphous part of the thermoplastic PLA. In conclusion, the addition of fibers resulted in increased stiffness and a considerable decrease in energy dissipation. These data provide quantitative and qualitative information about the behavior of the material.

Accelerated aging test

Durability of PLA composites in hot and humid environments is crucial for their long-term use in automotive products. The performance of these composites in harsh conditions is significant. In this study, the principle of accelerated aging was employed; this method is applied in the automotive industry to evaluate the internal components of cars. The accelerating aging test included continuous conditioning at 80°C and 95% RH. The samples were evaluated via tensile tests after 12, 24, 33, and 48 h of continuous conditioning.

Exposure of the samples to moisture and heat resulted in a slight deterioration of the matrix, as evidenced by the occurrence of a milky color on the surface. This environment was too harsh for pure PLA, and the material degraded. The decline in mechanical properties due to accelerated aging for different time periods is shown in Figure 11. As the figure shows, the tensile strength and tensile modulus decrease with increasing aging time. The pure PLA was too weak to be tested. The tensile strength of the PLA composites was considerably lower than that of the pure PLA. For example, in the first 12 h, the tensile strength of (i) PLA10 decreased (from 89.1 MPa to 18.6 MPa) to 20% of the original, (ii) PLA20 decreased to (40.0 MPa) nearly 36% of the primitive value, and (iii) PLA30, PLA40, and PLA50 remained at 41% of the original value. The results showed that BF may retard the degradation of the composites. As the BF content increased to 60%, the tensile strength decreased to 24.8% of the value corresponding to the pure PLA. This was attributed to the fact that the matrix provided inadequate connection between the fibers. After 24 h, the tensile strength of each composite was 50% that of the corresponding 12-h value (Figure 11). The tensile strength of PLA10 and PLA20 after 33 and 48 h of aging was similar to the strength after 24 h of aging. The tensile strength of PLA30, PLA 40, PLA50, and PLA60 decreased only slightly (from 3 MPa to 8 MPa) relative to that of the 24-h aged sample.

Figure 11.

Effect of accelerated aging on tensile properties of PLA composites with different fiber content: (a) tensile strength and (b) tensile modulus. PLA: polylactic acid.

The reduction in tensile modulus varied with the tensile strength. Figure 11(b) shows the variation in the modulus. In the first 12 h, the values remained unchanged, but after 24 h, the modulus increased slightly, owing to the heating process. After 48 h of accelerated aging, the modulus was approximately 60% of the value corresponding to the un-aged sample. A meaningful tensile modulus (in contrast to the tensile strength) is maintained by the PLA composites.

The results revealed that BF has a positive effect on the antiaging performance. Compared with pure PLA, which can only be tested in the first 12 h, the BF-containing PLA composites exhibit superior mechanical properties during the accelerated aging test. This may be attributed to: (i) the beams formed between BF are stronger than those formed in pure PLA, which could reduce wicking of moisture and penetration of the composites, leading possibly to smaller reductions in structural integrity²⁷ and (ii) crystallization. As discussed in the previous paragraph, cold crystallization behavior occurred at temperatures <80°C, thereby contributing to an increase in the storage modulus, that is, a mechanical property. The heating process in this test promoted crystallization of the amorphous portions of the material. Crystalline regions are impermeable to oxygen and, hence, as reported in other studies, degradation can occur in amorphous regions via chain scission. In general, compared with the pure PLA, the PLA composites with BF exhibited better overall resistance to a hot and humid environment. Therefore, the addition of BF to PLA should be considered as a means of improving the durability of PLA.

Conclusions

The mechanical properties, thermal properties, and durability of PLA with different BF contents were investigated for potential use of these composites in load-bearing applications in the automotive sector. The relationship between fiber content and mechanical properties was emphasized. The major conclusions can be summarized as follows.

The results of the tensile test demonstrated that the chopped BF had a positive effect on the tensile properties. With increasing BF content of up to 50%, the tensile strength and modulus increased significantly to 140 MPa and 5 GPa, respectively. The SEM micrograph showed that fibers twisted into a beam can improve the fiber/PLA matrix adhesion, with the best adhesion occurring at a BF content of 60%. Similarly, the maximum flexural strength and impact strength occurred at BF contents of 40% and 30%, respectively. Therefore, suitable amounts of BF can yield improvements in the mechanical properties of PLA composites.

The addition of BF had little influence on the T _g and T _m of the PLA composites. However, the degree of crystallinity was correlated with the BF content. The level of crystallinity increased with increasing BF content, reaching a maximum of 54.6%, which was 20% higher than that of pure PLA. DMA revealed the deterioration of the composites, as evidenced by the occurrence of a tan δ peak at temperatures ranging from 67°C to 72°C. The decrease in the storage modulus was delayed, owing to an increase in the BF content. After 80°C, the PLA composites exhibited cold crystallization behavior, which played a significant role in retarding the degeneration of the material.

Continuous conditioning at 80°C and 95% RH was used to accelerate aging, which is extremely harsh against PLA. Twelve hours of exposure to heat and humidity rendered the pure PLA too weak for testing. Although the tensile strength decreased significantly, high values of the tensile modulus were maintained, owing to the improved crystallinity resulting from BF addition.

This study allows us to establish a correlation between the fiber content and material properties. These results demonstrate that the weight of BF (i.e. the BF content) should be the most important factor in selecting BF as reinforcement. In future work, fatigue properties as well as noise, vibration, and harshness performance that are of interest to the automotive materials field will be investigated.

Footnotes

Acknowledgements

The authors thank the Key Laboratory of Polymer Ecomaterials (Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, China) for access to laboratory facilities.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the National Key Research and Development Program of China [2016YFB0101601] and Special Project of Jilin Province and Jilin University (SXGJQY2017-7 and SXGJQY2017-2-1-5).

ORCID iD

Lu Han

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