Mechanical properties and thermal stability of high-temperature (cooking temperature)- resistant PP/HDPE/POE composites

: The heterogeneous nucleation process of polypropylene (PP)/high density polyethylene (HDPE)/thermoplastic elastomer (POE) composites was realized through blending modi ﬁ cation, and characterization techniques, including scanning electron microscopy, di ﬀ erential scanning calori-metry, X-ray di ﬀ raction, and dynamic mechanical analysis, were used to analyze the pattern of the impact of modi ﬁ ed dicyclohexyl-terephthalamide (TMB-5) on the mechanical properties and thermal stability of the PP/HDPE/POE composites. The results indicated that the modi ﬁ ed TMB-5 was advantageous to the improvement of the mechanical properties and thermal stability of the high-temperature (cooking temperature)-resistant PP/HDPE/POE composites. When the amount of added modi ﬁ ed TMB-5 was 0.4%, the impact strength and tensile strength of the PP/HDPE/POE composites increased to 36.3 kJ/m 2 and 31.7 MPa, respectively, which were, respectively, 99.5% and 8.5% higher than those of the materials prepared when the amount of added modi ﬁ ed TMB was 0.1%. The materials had higher storage modulus in room-temperature and high-temperature environments: 2,438.2 MPa (room temperature), 1,103.9 MPa (120°C), and 1,054.8MPa (140°C). In addition, the capability of the PP/HDPE/ POE composites to crystallize was improved continuously, and


Introduction
With the groundbreaking research of the plastics industry and medical technology and the growing concern about health problems, plastics have been extensively used in the medical field because of their excellent properties, convenient molding process, and relatively low cost.Polypropylene (PP) has characteristics of lightweight, low cost, hygienic properties, and easy molding process and is mainly used in the fields of small appliances, toys, washing machines, and auto parts.However, the traditional PP materials fail to produce the sterilization environments of high-temperature-resistant medical materials and sterile bags for agricultural products and meet the usage requirements of high-temperature (cooking temperature)-resistant materials, such as aluminum plastic film.Furthermore, the poor mechanical properties, along with the aforementioned drawbacks, severely restrict the promotion and application of PP in high-end industrial and agricultural fields.
At present, domestic and foreign scientific and technological workers have made a lot of research to improve the mechanical properties and thermal stability of high-temperature cooking-grade PP composites, mainly using blending modification, filler modification, and nucleating agent heterogeneous nucleation modification.Among them, blending modification is mainly based on polypropylene (PP)/high density polyethylene (HDPE), polypropylene (PP)/thermoplastic elastomer (POE), and PP/PE blends, while filling modification is mainly based on inorganic fillers, mainly SiO 2 , CaCO 3 , BaSO 4 , etc. Whether blending modification or filling modification can improve the mechanical properties of PP composites to a certain extent, but the improvement of thermal stability is still insufficient, and there are problems of dispersion (1)(2)(3)(4)(5)(6).Nucleating agent modification can be used to improve the mechanical properties and thermal stability of PP composites.Nucleating agent modification can improve the mechanical and thermal stability properties by regulating heterophase nucleation, refining grain size, and enhancing interfacial adhesion (7,8).Kang et al. (9) modified PP using a combination of β nucleating agent and nano-scale CaCO 3 to form a state in which αand β-crystals coexist, enhancing the material toughness and heat resistance and realizing the high stiffness and high toughness of the modified material.Yi et al. investigated the effects of different catalyst systems on the PP structure.Li et al. (10) conducted detailed studies on the morphological structure, thermal stability, and mechanical properties of PP and propyleneethylene copolymer blends with different ratios.Dong et al. (11) studied the effect of three α-nucleating agents on the mechanical and crystallization properties of PP.However, the aforementioned studies have all indicated that the thermal stability or mechanical properties of PP materials can be improved to a certain extent.However, the research on the ideal mechanical properties with high thermal stability in the field of high-temperature cooking is still lacking and not systematic but excellent mechanical properties with high thermal stability are still a technical challenge.
Therefore, in this study, PP was used as the base material, a dispersing agent was used to modify the nucleating agent, and the blending modification technique was used to prepare high-temperature (cooking temperature)-resistant PP/HDPE/POE composites and explore the pattern of the impact of modified nucleating agent dicyclohexyl-terephthalamide (TMB-5) on the mechanical properties and thermal stability of the PP/HDPE/POE composites.As presented in Table 1, N,N′-ethylenebis(stearamide) and TMB-5 were placed into a high-speed mixer at a specific ratio to carry out the modification, the duration was 30 min, the temperature was 100°C, and the rotation speed was 100 rpm.

Preparation of PP composite materials
As presented in Table 2, the PP/HDPE/POE composites were prepared using a twin-screw extruder, and the extrusion process was as follows: temperatures, 150°C, 190°C, 220°C, 220°C, 220°C, and 190°C; melting temperature, 220°C; main screw speed, 20 rpm.The extruded materials were dried in a heating blast drying oven at 60°C for 24 h and then cooled down for future use.The obtained materials were then placed in an injection molding machine to prepare the molded specimens.The injection molding process temperatures were as follows: 200°C, 220°C, 220°C, and 200°C.After being cooled down, the sample was characterized.

Measurement of the mechanical properties
The tensile properties were measured according to GB/T 1040.2-2006.The impact strength was measured according to GB/T 1843-2008.

Crystallization behavior
According to GB/T 19466.1-2004,3-5 mg of the specimen was cut and placed into an aluminum crucible.A differential scanning calorimeter (DSC) was used for the measurement, which was performed in a nitrogen environment with a heating rate of 10°C•min −1 .The method to calculate the crystallinity X DSC is as follows (12): where X DSC is the crystallinity, ΔH m is the heat enthalpy of the melting process, H Δ m 0 is the melting enthalpy corresponding to the sample with a crystallinity of 100%, and its value was 177 J•g −1 (13).

Crystalline structure
The samples were cut in blocks, and a Cu-K α X-ray diffraction (XRD) system was used for the measurement, with a scan rate of 5°•min −1 , X-ray wavelength of 0.154 nm, a voltage of 40 kV, a current of 40 mA, and a scan range of 10-30°.The method to calculate the crystallinity X XRD is as follows ( 14): where X XRD is the crystallinity, B cryst represents the inte- grated intensity of crystal sharp diffraction peaks, and B amorp represents the integrated intensity of the amorphous diffuse scattering peak.
The crystallinity of the β-crystal form was calculated according to Eq. 3 (15): where R β represents the relative content of the β-crystal form.R β was calculated according to Eq. 4 ( 16):

Dynamic mechanical analysis
In the nitrogen atmosphere, the measurement was taken under dual cantilever bending mode at a heating rate of 5°C•min −1 , the measurement range was −120°C to 140°C, the measurement frequency was 1 Hz, and the specimen size was 60 mm × 10 mm × 2 mm.

Characterization of the microstructure
The PP/HDPE/POE material sample was chilled in liquid nitrogen and then placed on an impact strength tester to carry out a rapid fracture.Subsequently, the fracture surface was sprayed with gold, and a field emission scanning electron microscope was used to record the fracture surface morphology of the sample under specific acceleration voltage conditions.

Results and discussion
4.1 Discussion on the mechanical properties decreasing trend after the initial increase, and the tensile strength exhibited a small increase.When the amount of added modified TMB-5 was 0.1%, the impact strength was 18.2 kJ•m −2 and the tensile strength was 29.2 MPa (Specimen1# in Figure 1).When the amount of added modified TMB-5 was 0.4% (Specimen4# in Figure 1), the impact strength and the tensile strength were 36.3 kJ•m −2 and 31.7 MPa, respectively, which were, respectively, 99.5% and 8.5% higher than those of the specimen obtained when the amount of added modified TMB-5 was 0.1%.This is mainly because HDPE/POE elastomer particles can serve as stress concentration points.Furthermore, an increase in the amount of added modified TMB-5 promotes the generation of numerous shear bands, consuming the impact energy and enhancing the impact strength (17,18).Additionally, the nucleating agent not only plays a role in blocking, diverting, and terminating the growth of small cracks but also can enhance the compatibility of the composite material, form strong interfacial adhesion and refined particles, and promote the composite material to possess higher impact strength while maintaining excellent tensile properties (19,20).As the amount of added modified TMB-5 increased to 0.5% (Specimen5# in Figure 1), the impact strength decreased and the tensile strength increased.The aforementioned results suggested that the use of the heterogeneous nucleation effect of modified TMB-5 could achieve a relatively ideal result of enhancing the strength and toughness of the PP/HDPE/POE composites.Among them, when the amount of added modified TMB-5 was 0.4%, the most ideal comprehensive mechanical properties of the PP/HDPE/ POE composites were obtained.

Analysis of the microstructure
Figure 2 displays the microstructural characteristics of the PP/HDPE/POE composites.As can be observed from the figure, with an increasing amount of added modified TMB-5, the fracture surface of the material was rough, and the thicknesses of the stress-whitening region and interface layer increased, but a smooth fracture surface also existed and exhibited the characteristics of coexistence of ductilebrittle fracture that was dominated by ductile fracture.These findings demonstrated that modified TMB-5 could promote the conversion of the fracture characteristics of the PP/HDPE/POE composites from complete brittle fracture to a certain ratio of ductile fracture.Among them, when the amount of added modified TMB-5 was 0.4% (Specimen(c) in Figure 2), the impact sections all exhibited strong interfacial adhesion and stress-whitening areas.Therefore, more energy dissipation is induced when the material is subjected to external impact force, thus greatly enhancing the comprehensive mechanical properties of the material.However, when the amount of added modified TMB-5 continued to increase to 0.5% (Specimen(e) in Figure 2), the degree of interfacial adhesion of the PP/HDPE/POE composites substantially weakened or even disappeared, and the characteristics of coexistence of ductile-brittle fracture changed, which was consistent with the result of the analysis of the mechanical properties.Therefore, when the amount of added modified TMB-5 was 0.4%, the best comprehensive mechanical properties of the PP/HDPE/POE composites were obtained.
The underlying mechanism is that because of the addition of a nucleating agent, the interfacial adhesion and stresswhitening phenomenon of the material can induce extensive crazing and shear yielding when the external force is applied, thus promoting the energy dissipation of the polymer (21,22).Furthermore, an increase in the amount of added modified nucleating agent and an enhancement in dispersity can effectively promote the appearance of interfacial adhesion and stress-whitening phenomenon, which plays a critical role in improving the comprehensive mechanical properties of the material.However, there is a maximum dispersity value of the modified nucleating agent, so it has a limited value on the interfacial adhesion and stress-whitening.This indicated that when the amount of added modified TMB-5 was 0.4%, the PP/HDPE/POE composites could possess relatively ideal comprehensive mechanical properties, which was consistent with the aforementioned results.3, with increasing amount of added modified TMB-5, the crystallization temperature (T c ) increased by 7.8%, manifested by an increase from 86.9°C to 93.7°C, whereas the half-peak width exhibited a gradient decrease (1#∼4# in Table 3).This is mainly because, with an increasing amount of added modified TMB-5, the crystallization rate further increases, which promotes an increase in the capability of crystallization, a decrease in intermolecular entanglement ability, increases in molecular chain flexibility and lattice stacking regularity, and optimization in the ratio of the crystalline region and amorphous region.As a result, crystallization is improved and comprehensive mechanical properties are enhanced (23).However, as the amount of added modified TMB-5 increased to 0.5% and T c decreased (5# in Table 3).The main reason is the metastability of polymer platelets.At a higher amount of added nucleating agent, the thickness of the material platelets gradually increased, thus leading to a decrease in T c (24).Additionally, as can be observed from Figure 3(b), as the amount of added modified TMB-5 increased, the melting temperature of the material exhibited an increasing trend.When the amount of added modified TMB-5 was 0.4%, the melting temperature of the material increased to 150.9°C (4# in Table 3).The heterogeneous nucleation effect generated from the nucleating agent optimizes the relaxation process and rearrangement intensity of the internal chain structure of the material, refines the grains, and improves the crystallization process of the material, thus increasing the melting temperature and improving the mechanical properties (25).When the amount of added modified TMB-5 continued to increase to 0.5%, the melting temperature greatly increased to 151.3°C, but the crystallization temperature decreased to 91.7°C (5# in Table 3).These findings suggest that an excessive amount of nucleating agent inhibits the increases in the crystallization rate and capability of crystallization so that the proportion of amorphous region increases, which reduces the mechanical properties of the material to a certain extent.Therefore, an excessive amount of nucleating agent was not advantageous to the enhancement of the comprehensive properties of the material.

Analysis of the crystallization behavior
In addition, as can be observed from Table 3, with an increasing amount of added modified TMB-5, the crystallinity of the PP/HDPE/POE composites increased substantially.When the amount of added modified TMB-5 was 0.4%, X DSC was 35.9%, which was 85.1% higher than 19.4% when the amount of added modified TMB-5 was 0.1% (1# and 4# in Table 3).This is mainly because an increase in the amount of added nucleating agent weakens the intermolecular forces, optimizes the internal molecular conformation of materials, and increases the molecular chain relaxation degree and reconstruction regularity so that the crystallization of the material tends to be complete, and the crystallinity increases (26,27).When the amount of added modified TMB-5 was 0.5%, the crystallinity decreased by 1.3% or 15.2% compared with that of the material obtained when the amount of added modified TMB-5 was 0.4% or 0.1% (1#, 4#, and 5# in Table 3).The main reason is that a further increase in the amount of added nucleating agents accelerates the intermolecular motion, causing a more compact stacking of molecular chains.Consequently, the molecular distance is restricted, and the material loses its crystallization structure, inhibiting a further increase in crystallinity (28).When the amount of added modified TMB-5 was 0.4%, the crystallization behavior of the material was good.This indicated that when the amount of added modified TMB-5 was 0.4%, the comprehensive mechanical properties were optimal, which was consistent with the result of the analysis of the mechanical properties.

XRD analysis
Figure 4 presents the XRD patterns of the PP/HDPE/POE composites.As can be observed from the figure, with the amount of added modified TMB-5 gradually increasing from 0.1% to 0.4% (1#∼4#in Figure 4), obvious diffraction peaks appeared at 2θ = 14°, 16.7°, 18.4°, and 21.6°in the PP

Table 3: Crystallization and melting values of the PP/HDPE/POE composites
X DSC /% composites, and they corresponded to the diffraction peaks of different crystal planes, which were α(110), α(040), α(130), and α(111), respectively.More importantly, a diffraction peak of β(300) crystal plane occurred at 2θ = 16°, suggesting that the addition of the nucleating agent induced the conversion of the α-crystal form to the β-crystal form.When the amount of added modified TMB-5 was 0.5%, the diffraction peak intensity of β(300) crystal plane decreased.In combination with Table 4, it can be observed that the crystallinity exhibited a decreasing trend after the initial increase as the amount of added modified TMB-5 increased, which was consistent with the result of the DSC analysis.Furthermore, when the amount of added modified TMB-5 was 0.4% (4# in Table 4), the maximum R β value (42.2%) was obtained.An increase in the content of the β-crystal form can largely increase the impact energy dissipation, thus enhancing the toughness of the material.However, when the amount of added modified TMB-5 increased to 0.5% ((5# in Table 4), the content of the β-crystal form decreased dramatically, which further confirmed that the addition of an excessive amount of nucleating agent was not advantageous to the improvement of the crystal structure of the material but decreased the toughness of the material.This was consistent with the result of the analysis of the mechanical properties.

Dynamic mechanical analysis
Figure 5 presents the storage modulus-temperature curve of the PP/HDPE/POE composites.As can be observed, with an increasing amount of added modified TMB-5, the storage modulus first decreased slowly as the temperature rose and then decreased sharply before finally reaching the equilibrium state.One possible reason for this phenomenon is that when the initial low temperature increases to a certain temperature, the change in the environment in which the specimen exists leads to a slow decrease in the storage modulus of the composite material (29).The subsequent sharp decrease in the storage modulus is because during the glass transition process, the composite material shifts from a rigid state to an elastic state, which is accompanied by the release of modulus.Finally, the equilibrium state is reached.At this time, the rigid state weakens, manifested by the elastic state (29).As can be observed from Figure 5 and Table 5, when the amount of added modified TMB-5 was 0.1% ((1#in Figure 5), the stiffness of the material in a low-temperature environment was poor.As the environmental temperature increased to room temperature and high temperature, the modulus exhibited a large decrease, and the stiffness in the corresponding   environment was not sufficient.When the amount of added modified TMB-5 was 0.4% (4# in Table 5), the glass transition temperature (T g ) was −51°C, which was 32°C higher than that when the amount of added modified TMB-5 was 0.1% (1# in Table 5).The storage modulus in the room-temperature environment was higher (2,438.2MPa); the storage moduli at high temperatures of 120°C and 140°C remained higher and were 1,103.9 and 1,054.8MPa, respectively.These results indicated that the material still maintained excellent stiffness in a high-temperature environment, and the high-temperature stability was considerably improved.According to the Kelvin-Voigt model, when the storage modulus increases, the viscosity of the material increases, elastic deformation also becomes larger, and the stiffness increases (30).As can be observed from the results, when the amount of added modified TMB-5 was 0.4%, the specimen maintained a high storage modulus in a hightemperature environment, and an ideal high-temperature stability was obtained.When the amount of added modified TMB-5 was 0.5% (5# in Table 5), T g decreased to −61°C; the storage moduli at different environmental temperatures were lower than those of the specimen obtained when the amount of added modified TMB-5 was 0.4% (4# in Table 5).These findings indicated that as the amount of added nucleating agent increased, the thermal stability and stiffness of the material decreased and the tensile strength decreased, which was consistent with the result of the analysis of the mechanical properties.

Conclusions
1.With an increasing amount of added modified TMB-5, the impact strength exhibited a decreasing trend after the initial increase and the tensile strength exhibited a small increase.When the amount of added modified TMB-5 was 0.4%, the impact strength and the tensile strength were 36.3 kJ•m −2 and 31.7 MPa, respectively, which were, respectively, 99.5% and 8.5% higher than those of the specimen obtained when the amount of added modified TMB-5 was 0.1%.2. When the amount of added modified TMB-5 was 0.4%, the impact sections all exhibited strong interfacial adhesion and stress-whitening areas.Therefore, more energy dissipation is induced when the material is subjected to external impact force, thus greatly enhancing the comprehensive mechanical properties of the material.3.With increasing amount of added modified TMB-5, the crystallization temperature (T c ) increased by 7.8%, manifested by an increase from 86.9°C to 93.7°C, whereas the half-peak width exhibited a gradient decrease.When the amount of added modified TMB-5 was 0.4%, the melting temperature of the material increased to 150.9°C, and the comprehensive mechanical properties were optimal, which was consistent with the result of the analysis of the mechanical properties.4. When the amount of added modified TMB-5 was 0.4%, the maximum R β value (42.2%) was obtained.An increase in the content of the β-crystal form can largely increase the impact energy dissipation, thus enhancing the toughness of the material.5.When the amount of added modified TMB-5 was 0.4%, the specimen maintained a high storage modulus in a high-temperature environment, and an ideal high-temperature stability was obtained.When the amount of added modified TMB-5 was 0.5%, T g decreased to −60.8°C; the storage moduli at different environmental temperatures were lower than those of the specimen obtained when the amount of added modified TMB-5 was 0.4%.These findings indicated that as the amount of added nucleating agent increased, the thermal stability and stiffness of the material decreased, and the tensile strength decreased, which was consistent with the result of the analysis of the mechanical properties.
Acknowledgements: I would like to express my gratitude to all those who helped me during the writing of this thesis.I sincerely thank corresponding author Yi Zhang and Xiao-Xiao Huang for his help.In addition, I would like to thank Guizhou Institute of Metallurgy and Chemical Engineering for providing a learning platform.Finally, I would like to thank the editors of e-polymers for their help.

Figure 1 Figure 1 :
Figure 1 presents the curves of changes in the impact strength and tensile strength of the PP/HDPE/POE composites with different amounts of added modified TMB-5.As can be observed from the figure, with an increasing amount of added modified TMB-5, the impact strength exhibited a

Figure 3
Figure3displays the effect of modified TMB-5 on the crystallization behavior (a) and melting behavior (b) of the PP/HDPE/POE composites.As presented in Figure3(a) and Table3, with increasing amount of added modified TMB-5, the crystallization temperature (T c ) increased by 7.8%, manifested by an increase from 86.9°C to 93.7°C, whereas the half-peak width exhibited a gradient decrease (1#∼4# in Table3).This is mainly because, with an increasing amount of added modified TMB-5, the crystallization rate further increases, which promotes an increase in the capability of crystallization, a decrease in intermolecular entanglement ability, increases in molecular chain flexibility and lattice stacking regularity, and optimization in the ratio of the crystalline region and amorphous region.As a result, crystallization is improved and comprehensive mechanical properties are enhanced(23).However, as the amount of added modified TMB-5 increased to 0.5% and T c decreased (5# in Table3).The main reason is the metastability of

Funding information:
The authors express their sincere thanks to the Guizhou Province Science and Technology Support Plan of China (Grant No. [2020] 4Y063), the Guizhou Basic Research Program of China (Nos.[2019]1448 and [2020] 1Z043), and the Guiyang Science and Technology Plan Project

Table 1 :
Preparation of PP composites