The relationship between the toughening modification of polypropylene (PP) and the change of crystalline morphology was reviewed. The influence of different blending components on the crystallization behavior of PP was different. They were divided into fine fractions, co-crystals, and in-situ fiber-forming composites. The inorganic rigid particles and organic/inorganic nanoparticles play a role in heterogeneous nucleation and induce the formation of β-crystals that contribute to toughening. The influence of different β crystal nucleating agents on the crystallization of PP was discussed.
Keywords: Polypropylene Toughening Enhancement Crystallization Nucleating agent
Polypropylene (PP) is a typical crystalline polymer, and its crystalline morphology has a great influence on the mechanical properties. At low temperatures or high strain rates, the material exhibits brittleness due to the impermissible impact energy absorption, which limits the application of PP. Therefore, it is of practical significance to study the toughening modification of PP and its relationship with the change of crystal morphology in the system.
1 Crystallinity and changes in crystallite size During the cooling process of isotactic polypropylene melts, the resulting crystals are essentially α-spherulites, with a large spherulite size (about 130 μm in diameter) and a clear boundary with high crystallinity. Good surface hardness, elastic modulus, and tensile breaking strength, but poor adhesion between crystal grains cause poor toughness. The larger the spherulites, the more brittle the properties, and thus the size of the spherulites affects the impact strength of the material. Studies have shown that the refinement and incompleteness of spherulites are beneficial to improve the toughness of materials, and the increase of crystallinity is beneficial to increase the mechanical strength and stiffness. Therefore, the aim of refinement and division of crystal grains while minimizing the degree of crystallinity is an important focus of toughening polypropylene.
It has been reported that the two components of the PP/HDPE blend crystallize, interfering with each other during crystallization, and the melting points of the respective crystals are reduced, but the respective unit cell structures are not changed. With the insertion of HDPE, the morphology of PP spherulites became incomplete, and the spherulites were gradually divided into wafers, finally reaching the goal of refining PP crystals and improving the low temperature toughness of PP.
Xavier believes that glass fiber has a heterogeneous nucleation effect on PP. Wang Honggang et al. showed that when an acid anhydride-modified PP (MPP) was added to a composite system of glass fiber and PP treated with an amine silane coupling agent, Interfacial stress on the interface due to chemical bonding and shrinkage has led to the appearance of transverse crystals that cannot be produced in the case of normal physical bonding and weak chemical bonding. The analysis found that the addition of MPP acts as a nucleating agent, increasing the number of nuclei and crystallites, decreasing the size of the spherulites, and increasing the crystallinity, but does not change the crystal form.
2 Change in crystal form There are five isomorphic PP (iPP) crystal forms, of which the α and β crystal forms are more common. Under normal processing conditions, the crystallization of PP is dominated by the most stable α-monoclinic crystals, while the β crystal form belongs to the hexagonal crystal system. Its internal arrangement is much more evacuated than the α crystal form, and has a better absorption effect on impact energy. The impact strength of crystalline PP is 1 to 2 times higher than that of α, so the polypropylene can be toughened by converting the α-form to the β-form using appropriate processing techniques. Zhang et al. believe that the addition of heterogeneous nucleating agents can achieve high crystallization velocities. The increase in crystallization speed not only allows the crystallization process to be completed quickly, but also helps to shorten the molding cycle and ultimately maintain the dimensional stability of the product. It also promotes the refinement of the grain structure. In order to give the product a good physical and mechanical properties.
2.1 Effect of Inorganic Rigid Particles Liu et al. found that the wollastonite-filled PP (containing 22% of ethylene) forms the α- and β-type symbiotic crystals, and the alpha crystalline melting peak of PP splits into two peaks, two different α The crystal form is due to the different orientation of the helix. The appearance of β crystals makes the impact strength of the material high even at room temperature and even -40°C. Mitsuyoshi et al. found that talc is a polar inorganic filler but has a significant nucleating and crystallizing effect on PP.
Liu Jingjiang et al. showed that different PP/rare earth oxide (La2O3, Y2O3 and mixed rare earth oxides) composites have different crystal forms of PP, and Y2O3 and certain rare earth oxides are PP α and β crystal nucleating agents. La2O3 is a crystal nucleating agent.
The sodium-CaCO3 was introduced into the PP by two-step melt blending, so that the matrix PP grew both alpha crystals and a small amount of beta crystals. The addition of sodium-CaCO3 easily induces the yield deformation of the matrix, thus increasing the dissipation of impact energy. At the same time, sodium-CaCO3 has obvious heterogeneous nucleation effect on the crystallization of PP, which makes the crystals fine and small, and has a large degree of crystallinity. The toughening effect on notched impact strength and non-notch impact strength of the material is obvious, and is 4% in CaCO3. It reaches the maximum value.
The spherulites of PP in PP/L-CaCO3 (light CaCO3) composites studied by Liao Kairong et al. are small (3~5μm) and a large amount of β-crystals are generated. When L-CaCO3 is treated with dinuclear aluminate, it is beneficial for L-CaCO3 particles to act as nucleating agents for PPβ-crystals (the relative content of β-crystals is up to 87.6%). In the PP/L-CaCO3>75/25 (coupling agent is 2DH-306), the impact strength of the blend is 10kJ/m2 higher than that of the pure PP, which is nearly 30kJ higher than that of the unmodified PP/L-CaCO3. /m2.
2.2 The role of nucleating agents commonly used beta crystal nucleating agents are: quinochrome ketone red dye, pimelic acid / calcium stearate complex, low-melting metal powder, ultra-micro oxide, etc.. In PP/POE (ethylene-propylene copolymer), PP/BR (polybutadiene rubber), and PP/LLDPE systems, when the β nucleating agent is added, the tensile strength, bending strength, and heat distortion temperature of the system decrease, but Normal temperature and low temperature notch impact strength and melt flow rate have greatly improved. Therefore, the use of β crystal nucleating agent in toughened PP can reduce the amount of toughening agent and greatly improve the impact performance and processing performance.
The low-melting-point metal LMPM is compounded with PP by an appropriate mixing process, and the amount of LMPM and the coupling agent is increased, and the β-crystalline content of PP is increased. The formation of β-crystals is attributed to the heterogeneous nucleation of LMPM and the large temperature gradient inside the composite during the cooling process. The number of seed crystals with homogeneous nucleation in the system did not change much, while the growth of β-crystals limited the growth of α-crystals. As a result, the content of β-crystals increases, the size of α-crystals decreases, and the crystallinity does not change much.
The content of β crystal nucleating agents (pimelic acid and calcium stearate) has certain influence on the crystallization and melting behavior of PP under isothermal and non-isothermal conditions. When the nucleating agent content is 0.005%, both the melting enthalpy ΔHmβ and the melting point T mβ of the crystallization 焓ΔHc, β crystals are the maximum, and the relative content of the α crystals is the smallest. The relative intensity of the (301) diffraction peak of the crystal face (301) of the sample with the highest nucleating agent content decreased, indicating that the longitudinal ordering of the molecular chain arrangement was reduced. It is believed that the β nucleus of polypropylene is formed by epitaxial crystals on the β nucleating agent crystal as a substrate, and only at a higher crystallization temperature, a less rapid crystallization rate can form a stable, highly ordered crystallization. Beta crystals.
When isotactic polypropylene is crystallized at a higher temperature (above 130°C), it is mainly controlled by the nucleation process. Adding less than 1% of yttrium ultrafine powder can increase the crystallization rate constant of polypropylene by an order of magnitude. The PP crystal form (partial β crystals), but also increased the initial and final crystallization temperature of PP, reducing the crystallization temperature range. All these indicate that the ultrafine powder of yttria has obvious nucleation effect on the alpha crystal and beta crystal of PP, and these will all contribute to improve the impact resistance of polypropylene.
3 The eutectic crystals of the same two crystals can produce eutectic phenomena, and each other enters the other's lattice. The eutectic helps to improve the compatibility and strengthens the interaction between the two phases of the material.
3.1 Simple eutectic addition of maleic anhydride (MAH) grafted PP (MAPP) in the PP/paper powder blend system, both the PP and MAPP, or the PP/MAPP crystals are present in the alpha form, the three have the same Cell parameters. The PP segment of the coupling agent MAPP can be co-crystallized with the matrix PP, thereby improving the compatibility of the paper powder with the PP matrix and improving the properties of the blend. In researching the brittle-ductile transition of the PP/EPDM/CaCO3 ternary blend system, Zhang Yuncan et al. found that the EPDM part that diffuses, infiltrates, or co-crystallizes with the PP matrix around the dispersed particles of the system is toughened by PP. Active ingredients.
3.2 Composite eutectic Early 80s Kiss proposed the concept of in situ fiber-forming composites. In recent years, this concept has been broadened to the blending of non-liquid crystal materials (such as PA-66, UHMWPE, etc.) and general heat-like plastic resins such as PP.
PP and UHMWPE blends can form in situ fiber-forming composites. The DSC diagram shows that UHMWPE partially crystallizes with the ethylene block of PP1330, and UHMWPE can crystallize prior to PP and become a PP nucleus at a faster cooling rate, forming a composite eutectic. The co-crystallization behavior of PP and UHMWOE is one of the main reasons that this composite material can significantly enhance toughness at the same time.
3 Microcrystalline structure Shen Kaizhi etc. extruded PP sheet and pipe under special process conditions through melt deformation method to obtain a self-reinforcing material, which made the PP melting point drift up to 13.9°C, and the crystallinity was also significantly improved. The tandem interlocking structure (pull-out structure) in the specimen observed by the scanning electron microscope significantly increased the tensile strength of the sheet by 10 times in the longitudinal direction and 50% in the transverse direction.
4 Conclusion In summary, during the course of toughening and strengthening of PP, the addition of other components causes changes in the crystalline morphology. The research work is expected to have the following developments: (1) In-depth study of the mechanism of eutectics, improvement of the compatibility of fillers with the matrix from the crystal itself (2) Development of a more economical and efficient nucleating agent for PPβ crystals (3) in processing The introduction of a vibration force field not only disperses and breaks the filler, but also possibly refines the crystal grains, orients the molecular chains in a certain direction, or even the orientation of the crystal regions, thereby achieving a toughening effect. (4) Manufacturing of special toughening processing The equipment is further applied to PP and its blends. (Cao Yurong Li Huilin) ​​(State Key Laboratory of Polymer Materials Engineering, Institute of Polymer Science, Sichuan University, Chengdu 610065)
Keywords: Polypropylene Toughening Enhancement Crystallization Nucleating agent
Polypropylene (PP) is a typical crystalline polymer, and its crystalline morphology has a great influence on the mechanical properties. At low temperatures or high strain rates, the material exhibits brittleness due to the impermissible impact energy absorption, which limits the application of PP. Therefore, it is of practical significance to study the toughening modification of PP and its relationship with the change of crystal morphology in the system.
1 Crystallinity and changes in crystallite size During the cooling process of isotactic polypropylene melts, the resulting crystals are essentially α-spherulites, with a large spherulite size (about 130 μm in diameter) and a clear boundary with high crystallinity. Good surface hardness, elastic modulus, and tensile breaking strength, but poor adhesion between crystal grains cause poor toughness. The larger the spherulites, the more brittle the properties, and thus the size of the spherulites affects the impact strength of the material. Studies have shown that the refinement and incompleteness of spherulites are beneficial to improve the toughness of materials, and the increase of crystallinity is beneficial to increase the mechanical strength and stiffness. Therefore, the aim of refinement and division of crystal grains while minimizing the degree of crystallinity is an important focus of toughening polypropylene.
It has been reported that the two components of the PP/HDPE blend crystallize, interfering with each other during crystallization, and the melting points of the respective crystals are reduced, but the respective unit cell structures are not changed. With the insertion of HDPE, the morphology of PP spherulites became incomplete, and the spherulites were gradually divided into wafers, finally reaching the goal of refining PP crystals and improving the low temperature toughness of PP.
Xavier believes that glass fiber has a heterogeneous nucleation effect on PP. Wang Honggang et al. showed that when an acid anhydride-modified PP (MPP) was added to a composite system of glass fiber and PP treated with an amine silane coupling agent, Interfacial stress on the interface due to chemical bonding and shrinkage has led to the appearance of transverse crystals that cannot be produced in the case of normal physical bonding and weak chemical bonding. The analysis found that the addition of MPP acts as a nucleating agent, increasing the number of nuclei and crystallites, decreasing the size of the spherulites, and increasing the crystallinity, but does not change the crystal form.
2 Change in crystal form There are five isomorphic PP (iPP) crystal forms, of which the α and β crystal forms are more common. Under normal processing conditions, the crystallization of PP is dominated by the most stable α-monoclinic crystals, while the β crystal form belongs to the hexagonal crystal system. Its internal arrangement is much more evacuated than the α crystal form, and has a better absorption effect on impact energy. The impact strength of crystalline PP is 1 to 2 times higher than that of α, so the polypropylene can be toughened by converting the α-form to the β-form using appropriate processing techniques. Zhang et al. believe that the addition of heterogeneous nucleating agents can achieve high crystallization velocities. The increase in crystallization speed not only allows the crystallization process to be completed quickly, but also helps to shorten the molding cycle and ultimately maintain the dimensional stability of the product. It also promotes the refinement of the grain structure. In order to give the product a good physical and mechanical properties.
2.1 Effect of Inorganic Rigid Particles Liu et al. found that the wollastonite-filled PP (containing 22% of ethylene) forms the α- and β-type symbiotic crystals, and the alpha crystalline melting peak of PP splits into two peaks, two different α The crystal form is due to the different orientation of the helix. The appearance of β crystals makes the impact strength of the material high even at room temperature and even -40°C. Mitsuyoshi et al. found that talc is a polar inorganic filler but has a significant nucleating and crystallizing effect on PP.
Liu Jingjiang et al. showed that different PP/rare earth oxide (La2O3, Y2O3 and mixed rare earth oxides) composites have different crystal forms of PP, and Y2O3 and certain rare earth oxides are PP α and β crystal nucleating agents. La2O3 is a crystal nucleating agent.
The sodium-CaCO3 was introduced into the PP by two-step melt blending, so that the matrix PP grew both alpha crystals and a small amount of beta crystals. The addition of sodium-CaCO3 easily induces the yield deformation of the matrix, thus increasing the dissipation of impact energy. At the same time, sodium-CaCO3 has obvious heterogeneous nucleation effect on the crystallization of PP, which makes the crystals fine and small, and has a large degree of crystallinity. The toughening effect on notched impact strength and non-notch impact strength of the material is obvious, and is 4% in CaCO3. It reaches the maximum value.
The spherulites of PP in PP/L-CaCO3 (light CaCO3) composites studied by Liao Kairong et al. are small (3~5μm) and a large amount of β-crystals are generated. When L-CaCO3 is treated with dinuclear aluminate, it is beneficial for L-CaCO3 particles to act as nucleating agents for PPβ-crystals (the relative content of β-crystals is up to 87.6%). In the PP/L-CaCO3>75/25 (coupling agent is 2DH-306), the impact strength of the blend is 10kJ/m2 higher than that of the pure PP, which is nearly 30kJ higher than that of the unmodified PP/L-CaCO3. /m2.
2.2 The role of nucleating agents commonly used beta crystal nucleating agents are: quinochrome ketone red dye, pimelic acid / calcium stearate complex, low-melting metal powder, ultra-micro oxide, etc.. In PP/POE (ethylene-propylene copolymer), PP/BR (polybutadiene rubber), and PP/LLDPE systems, when the β nucleating agent is added, the tensile strength, bending strength, and heat distortion temperature of the system decrease, but Normal temperature and low temperature notch impact strength and melt flow rate have greatly improved. Therefore, the use of β crystal nucleating agent in toughened PP can reduce the amount of toughening agent and greatly improve the impact performance and processing performance.
The low-melting-point metal LMPM is compounded with PP by an appropriate mixing process, and the amount of LMPM and the coupling agent is increased, and the β-crystalline content of PP is increased. The formation of β-crystals is attributed to the heterogeneous nucleation of LMPM and the large temperature gradient inside the composite during the cooling process. The number of seed crystals with homogeneous nucleation in the system did not change much, while the growth of β-crystals limited the growth of α-crystals. As a result, the content of β-crystals increases, the size of α-crystals decreases, and the crystallinity does not change much.
The content of β crystal nucleating agents (pimelic acid and calcium stearate) has certain influence on the crystallization and melting behavior of PP under isothermal and non-isothermal conditions. When the nucleating agent content is 0.005%, both the melting enthalpy ΔHmβ and the melting point T mβ of the crystallization 焓ΔHc, β crystals are the maximum, and the relative content of the α crystals is the smallest. The relative intensity of the (301) diffraction peak of the crystal face (301) of the sample with the highest nucleating agent content decreased, indicating that the longitudinal ordering of the molecular chain arrangement was reduced. It is believed that the β nucleus of polypropylene is formed by epitaxial crystals on the β nucleating agent crystal as a substrate, and only at a higher crystallization temperature, a less rapid crystallization rate can form a stable, highly ordered crystallization. Beta crystals.
When isotactic polypropylene is crystallized at a higher temperature (above 130°C), it is mainly controlled by the nucleation process. Adding less than 1% of yttrium ultrafine powder can increase the crystallization rate constant of polypropylene by an order of magnitude. The PP crystal form (partial β crystals), but also increased the initial and final crystallization temperature of PP, reducing the crystallization temperature range. All these indicate that the ultrafine powder of yttria has obvious nucleation effect on the alpha crystal and beta crystal of PP, and these will all contribute to improve the impact resistance of polypropylene.
3 The eutectic crystals of the same two crystals can produce eutectic phenomena, and each other enters the other's lattice. The eutectic helps to improve the compatibility and strengthens the interaction between the two phases of the material.
3.1 Simple eutectic addition of maleic anhydride (MAH) grafted PP (MAPP) in the PP/paper powder blend system, both the PP and MAPP, or the PP/MAPP crystals are present in the alpha form, the three have the same Cell parameters. The PP segment of the coupling agent MAPP can be co-crystallized with the matrix PP, thereby improving the compatibility of the paper powder with the PP matrix and improving the properties of the blend. In researching the brittle-ductile transition of the PP/EPDM/CaCO3 ternary blend system, Zhang Yuncan et al. found that the EPDM part that diffuses, infiltrates, or co-crystallizes with the PP matrix around the dispersed particles of the system is toughened by PP. Active ingredients.
3.2 Composite eutectic Early 80s Kiss proposed the concept of in situ fiber-forming composites. In recent years, this concept has been broadened to the blending of non-liquid crystal materials (such as PA-66, UHMWPE, etc.) and general heat-like plastic resins such as PP.
PP and UHMWPE blends can form in situ fiber-forming composites. The DSC diagram shows that UHMWPE partially crystallizes with the ethylene block of PP1330, and UHMWPE can crystallize prior to PP and become a PP nucleus at a faster cooling rate, forming a composite eutectic. The co-crystallization behavior of PP and UHMWOE is one of the main reasons that this composite material can significantly enhance toughness at the same time.
3 Microcrystalline structure Shen Kaizhi etc. extruded PP sheet and pipe under special process conditions through melt deformation method to obtain a self-reinforcing material, which made the PP melting point drift up to 13.9°C, and the crystallinity was also significantly improved. The tandem interlocking structure (pull-out structure) in the specimen observed by the scanning electron microscope significantly increased the tensile strength of the sheet by 10 times in the longitudinal direction and 50% in the transverse direction.
4 Conclusion In summary, during the course of toughening and strengthening of PP, the addition of other components causes changes in the crystalline morphology. The research work is expected to have the following developments: (1) In-depth study of the mechanism of eutectics, improvement of the compatibility of fillers with the matrix from the crystal itself (2) Development of a more economical and efficient nucleating agent for PPβ crystals (3) in processing The introduction of a vibration force field not only disperses and breaks the filler, but also possibly refines the crystal grains, orients the molecular chains in a certain direction, or even the orientation of the crystal regions, thereby achieving a toughening effect. (4) Manufacturing of special toughening processing The equipment is further applied to PP and its blends. (Cao Yurong Li Huilin) ​​(State Key Laboratory of Polymer Materials Engineering, Institute of Polymer Science, Sichuan University, Chengdu 610065)
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