Polypropylene (PP) is currently the second largest general-purpose plastic. With the development of construction, automobile, household appliances and packaging industries, waste PP has become one of the waste polymer materials with large output in recent years. At present, the main ways to deal with waste PP are incineration for energy supply, catalytic cracking for fuel preparation, direct utilization and recycling. Considering the technical feasibility, cost, energy consumption, environmental protection and other factors in the process of waste PP treatment, recycling is the most commonly used, effective and most advocated way to deal with waste PP.
Due to the influence of light, heat, oxygen, external force and other factors in the use process, the molecular structure of PP will change, and the products will turn yellow, brittle, or even crack, resulting in obvious deterioration of PP toughness, dimensional stability, thermal oxygen stability, and processability. It is difficult to meet the requirements of the processing and use process to directly use waste PP to manufacture products.
Therefore, the recycling technology of waste PP continues to develop. Alloying with other polymers or compounding with fillers can significantly improve the processability, thermal properties, physical and mechanical properties of waste PP and realize the high-performance of waste PP.
Alloying
Alloying is the process of mixing waste PP with other polymer materials to prepare macroscopic homogeneous materials. The processability, physical and mechanical properties of waste PP can be improved by alloying different polymer materials. For example, the impact toughness of waste PP can be significantly improved by using elastomer.
The mechanical properties and thermal deformation behavior of waste PP/RU composite rubber (50% of natural rubber and 50% of styrene butadiene rubber) blends have been studied. It is found that the impact strength and elongation at break of waste PP can be significantly improved by first plasticizing RU composite rubber into small rubber particles, which can be uniformly dispersed in the continuous phase of waste PP, but the rigidity and thermal deformation resistance of PP will be reduced.
Because the vast majority of elastomers are incompatible with waste PP, the interface bonding is poor, and there is phase separation during processing and use, which affects its performance. In order to improve the interfacial compatibility of waste PP alloys and enhance the interfacial bonding, many scholars have carried out extensive research, and found two compatibilizers that can enhance the interfacial bonding of blends, and improve the storage modulus, loss modulus and system viscosity of blends.
Vulcanizing agent can improve the impact and tensile strength, melt viscosity, elongation at break and ductility of the blends; The addition of peroxide crosslinking agent can further improve the compatibility of the blend, improve the impact and tensile strength of the blend, but lead to a slight decline in elongation at break.
Compounding
Composite is the process of mixing waste PP and non polymer materials to prepare composite materials, which is the main way to realize the high performance and functionality of waste PP. The compounding of waste PP can improve its physical and mechanical properties such as rigidity, strength, thermal and electrical properties, and reduce its cost.
According to the composition of filler, it can be divided into inorganic filler and organic filler.
Inorganic filler compounding
Inorganic fillers commonly used for PP composite can be used to composite with waste PP, such as calcium carbonate, talc, montmorillonite, metal oxides, fly ash and glass fiber. It was found that although these inorganic fillers could significantly improve the rigidity of waste PP and reduce the cost, they had a large difference in polarity with waste PP, high surface energy and poor compatibility, which led to the decline of elongation at break and impact toughness of the composite.
Organic filler compounding
Common organic fillers include wood flour and wood fiber, starch, wheat straw, hemp fiber and waste newspapers. The microcellular foaming technology of waste PP filled with wood fiber was studied. The results showed that when the melting temperature was 180 ℃ and the holding pressure was 12.5 MPa, the microcellular structure was evenly distributed. Because the microporous structure can extend the propagation path of cracks, absorb external impact energy, and thus improve the impact strength.
Natural fiber is a new kind of waste PP filling material. In view of its high water absorption and incompatibility with waste PP, surface treatment is the main method to achieve the high performance of natural fiber filled waste PP composites. In addition, waste polyester can also be used to modify waste PP, which has been studied by some scholars β- Crystallization behavior of waste PP/waste polyester fabric composites β- Nucleating agent has heterogeneous nucleation effect on the crystallization of waste PP, increases the crystallization temperature of waste PP, and induces the formation of β Crystal.
Hybrid recombination
Hybrid composite is the process of preparing composite materials by filling polymer with more than two kinds of fillers. Due to the limitation of single filler, hybrid compounding can better improve the comprehensive properties of polymers through the complementary and synergistic effects of different fillers. Therefore, the research on the preparation and related properties of waste PP composites filled with hybrid fillers has attracted attention. The fillers involved mainly include the hybrid of different inorganic fillers and the hybrid of inorganic/organic fillers.
Alloy recombination
In order to give full play to the advantages of alloying and compounding, some researchers began to combine alloying and compounding to further improve and improve the physical and mechanical properties of waste PP, and realize the high-performance and industrialization of waste PP, such as the combination of organic fillers and elastomers, inorganic fillers and elastomers to modify waste PP.
The results show that the fracture behavior of waste PP and talc filled waste PP composites at low temperature is brittle, and the impact resistance of the composites can be significantly improved by adding EOC (ethylene octene copolymer); The dynamic mechanical behavior of EOC toughened talc filled waste PP composites did not change with the increase of recycling times.