What Are the Properties of Polypropylene?

Polypropylene is a polymer produced by the addition polymerization of propylene. It is a white waxy material with a transparent and light appearance. The density is 0.89 0.91g / cm ³ , ^{[1] is} flammable, melting point is 165 ° C, softens at about 155 ° C, and the use temperature range is -30 to 140 ° C ^[2] It can resist the corrosion of acid, alkali, salt solution and many organic solvents below 80 , and can decompose under high temperature and oxidation. Polypropylene is widely used in the production of clothing, blankets and other fiber products, medical equipment, automobiles, bicycles, parts, pipelines, chemical containers, etc., and also used in food and pharmaceutical packaging. ^[3]

Polypropylene resin is one of the four general-purpose thermoplastic resins (polyethylene, polyvinyl chloride, polypropylene, and polystyrene). It is produced by polymerization using propylene as a raw material and ethylene as a comonomer. ^[10]

In view of the poor impact resistance, poor weather resistance, poor surface decoration, and the gap between electrical, magnetic, optical, thermal, and combustion functions and actual needs at low temperatures, polypropylene is modified to become Currently the most active areas for plastics processing development and the most fruitful areas. ^[11]

Polypropylene (PP) is currently the second-largest general-purpose plastic. With the development of construction, automotive, home appliances, and packaging industries, waste PP has become one of the waste polymer materials with a large output in recent years. At present, the main ways to deal with waste PP include: incineration for energy supply, catalytic cracking to prepare fuel, direct utilization and recycling. Considering the technical feasibility, cost, energy consumption and environmental protection in the process of disposing of used PP, re-recycling is currently the most commonly used, effective and advocated approach to dispose of used PP. ^[14]

Due to the effects of light, heat, oxygen, and external forces during use, the molecular structure of PP will change, and the product will turn yellow, brittle, and even crack, resulting in PP toughness, dimensional stability, thermal oxygen stability, and processability. Obviously deteriorate, it is difficult to meet the requirements of processing and use process by using waste PP products directly. ^[14]

Therefore, the re-recycling technology of waste PP has been continuously developed. The use of alloying with other polymers or compounding with fillers can significantly improve the processing performance, thermal properties, physical and mechanical properties of waste PP, and achieve high performance of waste PP. ^[14]

Polypropylene alloying

Alloying is the process of mixing waste PP with other polymer materials to prepare macroscopically homogeneous materials. By selecting different polymer materials for alloying, the processing properties, physical and mechanical properties of waste PP can be improved. If elastomers are used, the impact toughness of waste PP can be significantly improved. ^[14]

Research on the mechanical properties and thermal deformation behavior of waste PP / RU composite rubber (natural rubber and styrene-butadiene rubber each accounted for 50%) blended materials, found that the RU composite rubber was first plasticized into fine rubber particles, which were evenly dispersed in The continuous phase of waste PP can obviously improve the impact strength and elongation at break of waste PP, but it will cause the rigidity and heat distortion resistance of PP to decrease. ^[14]

Because most elastomers are incompatible with waste PP and have poor interface adhesion, phase separation exists during processing and use, which affects their performance. In order to improve the interfacial compatibility of used PP alloys and enhance interfacial adhesion, many scholars have carried out extensive research and found that two can enhance the interfacial adhesion of blended materials, increase the storage modulus, loss modulus and system viscosity of blended materials Compatibilizer. ^[14]

The vulcanizing agent can improve the impact and tensile strength, melt viscosity, elongation at break and ductility of the blended material; the addition of a peroxide crosslinking agent can further improve the compatibility of the blended material and improve the blended material Impact and tensile strength, but resulting in a slight decrease in elongation at break. ^[14]

Polypropylene composite

Compounding is the process of mixing waste PP with non-polymer materials to prepare composite materials. It is the main way to achieve high performance and functionalization of waste PP. Compounding waste PP can improve its rigidity, strength, thermal, electrical and other physical and mechanical properties, and reduce costs. ^[14]

According to the filler composition, it can be divided into inorganic filler and organic filler. ^[14]

Composite of inorganic filler

Inorganic fillers commonly used in PP compounding can be used to compound waste PP, such as calcium carbonate, talc, montmorillonite, metal oxides, fly ash and glass fiber. The research found that although these inorganic fillers can significantly improve the rigidity and reduce the cost of waste PP, they have large differences in polarity from waste PP, high surface energy, and poor compatibility, leading to a reduction in elongation at break and impact toughness of the composite. ^[14]

Organic filler composite

Common organic fillers include wood flour and wood fiber, starch, wheat straw, hemp fiber, and discarded newspapers. A study on wood fiber filled waste PP microcellular foaming technology has been conducted. The results show that the microcellular structure is uniformly distributed at a melting temperature of 180 ° C and a holding pressure of 12.5 MPa. Because the microporous structure can extend the propagation path of cracks, absorb external impact energy, thereby improving impact strength. ^[14]

Natural fiber is a new waste PP filling material. In view of its high water absorption and incompatibility with waste PP, surface treatment is the main method to achieve high performance of natural fiber filled waste PP composite materials. In addition, waste polyester can also be used to modify waste PP. Some scholars have studied the crystallization behavior of -nucleated waste PP / waste polyester fabric composites. The results show that waste polyester and -nucleating agent have different phases to the waste PP crystal Nucleation increases the crystallization temperature of waste PP and induces the formation of crystals. ^[14]

Hybrid composite

Hybrid composite is a process of preparing composite materials by filling polymers with two or more fillers. Due to the limitations of a single filler, hybrid composites can better improve the overall performance of polymers through the complementary and synergistic effects of different fillers. Therefore, research on the preparation and related properties of mixed filler filled waste PP composite materials has attracted attention. The fillers involved mainly include different inorganic filler mixtures and inorganic / organic filler mixtures. ^[14]

Polypropylene alloy composite

In order to give full play to the advantages of alloying and compounding, some researchers have begun to combine alloying and compounding to further improve and improve the physical and mechanical properties of waste PP, and to achieve high-performance and industrialization of waste PP, such as organic fillers and elastomers, inorganic Filler and elastomer are combined to modify waste PP and so on. ^[14]

The research results in this regard show that: the waste PP and talc powder-filled waste PP composites are brittle at low temperature, and the addition of EOC (ethylene-octene copolymer) can significantly improve the impact resistance of the composite; The dynamic mechanical behavior of ductile talc powder filled waste PP composites does not change with the increase of the number of recycling. ^[14]