The thermal stability of PPC prepared from homogeneous catalysts may be compromised during the preparation process due to its poor thermal stability, which is produced at around 230 °C in industrial production of normal industrial polymers. Moreover, due to the poor mechanical properties of PPC, the above defects seriously limit its application in industrial production. Therefore, PPC needs to be modified to improve its performance and expand its industrial applications. In order to broaden the field of application of PPC, other materials are usually selected to be fused with PPC to prepare composite materials. The main focus here is on the modification of PPC with green materials (inorganic materials, natural organic polymers, biodegradable polymers, etc.). The use of such degradable materials allows the modified PPC to retain its degradability, which is still in line with the green development route. The modified materials introduced in this paper are shown in Figure 7
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The main common blending methods are melt blending and solution blending. Melt blending is a common method of modifying many thermoplastics and is technically mature. Melt blending comprises the use of thermoplastics to achieve a molten state when blended with other materials. Melt blending is industrially easy to implement, simple to operate and does not require solvent recovery operations. Solution blending is a more complex procedure whereby PPC is mixed with other materials in a solvent state, resulting in a more complete blend. Both melt blending and solution blending methods require a second substance or substances to blend with PPC to improve the properties of the composite.
Inorganic materials are diverse, abundant and inexpensive, and mineral materials are often used as filler materials to enhance the mechanical and thermal properties of other materials. Moreover, a variety of inorganic mineral materials have special properties that can often contribute a certain functionality to the material. The main inorganic materials presented here are carbon fibers, graphene oxide, laponite, montmorillonite (MMT), modified silica nanoparticles, activated white clay (AC), aluminum hydroxide (ATH), elokite nanotubes (HNTs), calcium carbonate and other materials. Furthermore, different processing methods and additives are used in the preparation of composites. The inorganic materials used to modify PPC and their modification effects are shown in Table 6
Modification of PPC using polymeric materials, especially natural ones, often achieves better results [ 81 82 ]. In this regard, natural degradable polymers, such as cellulose, lignin, starch, chitosan (CS) and wool, are renewable, degradable, widely available and inexpensive [ 83 ]. Because natural polymers are inherently biodegradable, the composites produced by mixing PPC with natural polymers are still biodegradable. The complete degradability of natural polymer-modified PPC does not put pressure on the ecological environment.
Of the natural polymers, cellulose, the main component of the cell walls of plants, is simple and easy to obtain and is the natural polymer with the largest reserves. The twin-screw extruder method is commonly used for melt blending; therefore, the melt blending process of PPC/cellulose composites can be achieved with the help of a twin-screw extruder. The addition of cellulose enhances the tensile properties and energy storage modulus of the composite. However, the compatibility between PPC and cellulose is poor, and the reinforcing effect is somewhat limited. The addition of cellulose hinders the molecular-chain movement of PPC to some extent, which can enhance the thermal stability and Tg of composites [ 84 ].
Once a material reaches the nanoscale, some superior functionality often emerges. Nanocellulose (NCC) has more excellent properties than natural cellulose, such as high specific surface area, high crystallinity, high strength and ultra-fine structure [ 85 ]. Some studies have shown that the tensile strength of PPC increases with the increase in NCC content when the NCC addition does not exceed 1.5%. As the NCC content exceeds 1.5%, agglomeration of NCC occurs, which leads to a decrease in the tensile strength of PCC. The composite material changes from ductile fracture to brittle fracture.
Lignin is also a common natural polymer, and byproducts from the paper industry are often include alkaline lignin (AL), which is difficult to utilize. Lignin is a renewable, degradable and heat-resistant material with a Tg of between 100 and 180 °C. A considerable amount of research has been carried out to make effective use of lignin, not least of which is the preparation of composites from AL mixed with PPC. PPC/AL composites can also be prepared using melt-blending methods, and for composite studies the blending ratio is the first issue of concern. The thermal and mechanical properties of PPC/AL composites were investigated at 10% and 40% AL content [ 86 ]. The Tg of PPC/AL composites was found to be 30.9 °C at 10% AL content, which was 8.9 °C higher than that of pure PPC; the tensile strength of PPC/AL composites was 13.44 MPa at 40% AL content, which was 213% higher than that of PPC, and the elongation at break was 115%. However, AL is poorly dispersed, and therefore, modification is often required. Chemical modification of AL using formaldehyde enhances the dispersibility of the lignin. Formaldehyde-modified black liquor lignin (BLF) is then blended with PPC using a melt-blending method. The tensile strength, modulus of elasticity, thermal stability and processing stability of composites were significantly improved after the addition of small amounts of BLF due to its good dispersion [ 87 ].
BLF could also be modified with hydroxypropylation to make modified hydroxy black liquor lignin (HBL) [ 88 ]. The unmodified lignin is prone to agglomeration, whereas HBL has a lower thermal transition temperature and higher melt flow, resulting in better dispersion of HBL in the PPC matrix. As a result, the tensile properties, thermal stability and processing stability of the PPC/HBL composites are improved. In addition, the hydrophilicity of the composites is improved, which accelerates the natural degradation rate of the composites.
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Starch is a natural polymer that is completely degradable and has the advantages of being non-toxic and inexpensive. Starch can be melted and blended with PPC to enhance its performance. However, there are two problems with this treatment method. First, starch is a polysaccharide polymer compound with a large number of hydroxyl groups in its molecules, which can form a large number of intra- and intermolecular hydrogen bonds and complete granules with a microcrystalline structure, resulting in a Tm much higher than the degradation temperature. As a result, common starch does is not thermoplastically processable. PPC is also less compatible with starch, which leads to limited modification. To overcome these problems, there are usually two solutions. On the one hand, reactive bulking agents, such as succinic anhydride, diphenylmethane diisocyanate, maleic anhydride, etc., can be added to the blend of PPC and starch. On the other hand, starch can be pretreated by grafting and esterification before being blended with PPC to prepare composites. It has been shown that oxidation of starch can improve compatibility with biodegradable materials [ 89 90 ].
In the first type of solution, one step of using maleic anhydride as a reaction capacitor is as follows. PPC was first grafted to prepare maleic-anhydride-grafted PPC (PPCMA), and then PPCMA was melt-blended with TPS, thermoplastic starch oxide (TPOS) and aluminate-pretreated starch oxide (DL-TPOS) to compare the modification effect of each of the three [ 91 92 ]. Hydrogen bonding is formed between PPCMA and starch, so the tensile strength of PPCMA/TPS composites is increased compared to that of pure PPC. For PPCMA/TPOS composites, the partial conversion of hydroxyl groups in TPOS to carbonyl groups enhances the compatibility between PPCMA and TPOS, and the PPCMA/TPOS composites have improved tensile and impact strength, energy storage modulus, loss modulus and complex viscosity. DL-TPOS reduces the exposure of hydroxyl groups on the starch surface, reduces the probability of starch granule agglomeration, improves dispersion and further enhances the tensile strength of the composite. At a DL-TPOS content of 40%, the tensile strength of a PPC/DL-TPOS composite was increased by 460% compared to pure PPC. A maximum value of 13.72 MPa was reached.
The presence of a large number of hydrogen bonds between starch molecules is not conducive to melting operations. PPC is hydrophobic, whereas common starch is hydrophilic and rigid, resulting in poor compatibility between the two materials. Core-shell starch nanoparticles (CSS NPs) can be used as the core, and the shell is prepared using a poly (methyl acrylate) (PMA), which is produced in the soap-free emulsion copolymerization of methyl acrylate. The combination of the flexibility of PMA shells and the flexibility of CSS NPs resulted in improved compatibility of microcapsules with PPC, which significantly enhanced the mechanical and thermal properties of the PPC/CSS composites. The improved compatibility of starch with PPC, which has a core-shell structure, enhanced the mechanical and thermal properties of the PPC/CSS composite [ 93 ].
CS is the second most abundant natural organic polymer after cellulose. In the PPC/CS composites, the compatibility, Tg, thermal weight, loss temperature and tensile properties of the PPC/CS blends were examined for different levels of CS, and the effect of different levels of CS on the abovementioned properties of the PPC/CS blends was analyzed [ 94 ]. The tensile properties of PPC/CS increased when CS content was increased to 20%, and the tensile properties of PPC/CS composites increased from 4.7 MPa to 12.5 MPa when the CS content reached 20%. Moreover, the addition of CS also inhibited unzipping-type degradation in the low-temperature region of PPC and improved the heat resistance of PPC. O-lauroyl chitosan and PPC are blended using the solution casting method [ 95 ]. Hydrogen bonding interactions are formed between PPC and OCS. The tensile properties, elongation at break and Young&#;s modulus of the PPC/OCS blended film increase.
Wool is a widely used animal fiber, and its specific surface area is significantly increased when it is ground into granules to make wool powder (WP). WP can be used as an effective additive to modify PPC, and the thermal and mechanical properties of PPC can be significantly improved by blending PPC with WP using the solution-blending method and making a film by stirring and drying [ 96 ].
Besides the abovementioned common natural polymers, many researchers have blended agricultural residues containing the above components with PPC, such as straw [ 97 ], waste tea leaf powder [ 98 ], sisal [ 99 ], eggshell powder [ 100 ] and waste wood powder.
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