Polyethylene and polypropylene (iPP) are the two most widely used commodity plastics in the world, with an annual output of 700 million tons and 500 million tons respectively, making up nearly two-thirds of the world's plastics with excellent physical and chemical properties And durable and lightweight, beautiful, cheap and so on, replaced a large number of traditional packaging materials, contributed to a revolution in the packaging industry.However, polyethylene and polypropylene despite similar hydrocarbon composition, but not with each other Compatible, limiting the treatment of mixed waste, reducing the value of recycling products, creating a large number of durable non-rotting plastic garbage Let our ocean has become a veritable white landfill, is one of the greatest threats to humanity in the 21st century !
It is no exaggeration to say that the recycling of plastics (polyethylene / polypropylene) will affect the economy and sustainable development and will never be tolerated At present, the vast majority of polyethylene and polypropylene preparation using heterogeneous chromium and titanium catalyst heterogeneous olefin polymerization catalyst has many active sites, each molecule has its own reactivity differences, resulting in different molecular weight (MW ), Molecular weight distribution and microstructure polymers.With polyethylene and polypropylene, for example, these differences and phase separation inhibit the interfacial adhesion and reduce the mechanical properties of the melt blend.When these plastics are recycled, only about 5% performance is retained as a low value product.If the compatibilizer is used for polyethylene / polypropylene recycling, it is expected to convert polyethylene / polypropylene into equivalent or higher value materials while reducing the sorting The cost for the plastic recycling has brought the dawn.
Increasing the compatibility of polyethylene with polypropylene relies on the addition of large amounts of amorphous polymer (≥ 10%) additives, and block copolymers of polyethylene and polypropylene are expected to function as additives. Busico et al. C2 symmetric zirconium catalyst catalyzed the synthesis of the diblock copolymer PE-b-iPP.The experimental results show that the catalyst through β-hydrogen elimination, the chain growth is limited, the structure is simple, iPP block proved to be significant Stereoscopic .Continue to improve the catalyst to have a longer life expectancy, to produce higher molecular weight polyethylene / polypropylene diblock copolymer and multi-block copolymer is the goal pursued by scientists.
Recently, U.S. scientists found that the use of pyridylaminotrifluoro compounds as olefin polymerization initiators can synthesize high molecular weight PE-b-iPP diblock copolymers and multiblock copolymers with precise control of block length.First, hafnium catalysts And B (C6F5) 3, and then start the propylene polymerization reaction to obtain isotactic polypropylene.When the propylene is completely consumed, the reactor can be formed by adding ethylene PE-b-iPP diblock copolymer Mn ( The number average molecular weight (MWW) value increases linearly with the monomer conversion, which is consistent with the theoretical prediction and is consistent with the characteristics of the living polymerization.The MW (weight average molecular weight) of the ethylene block can be controlled by the reaction time and the propylene block MW consists of propylene And the proportion of catalyst to control, so as to achieve precise control of polymer molecular weight purposes.
Most commercial polypropylenes and polyethylenes generally exhibit poor interfacial adhesion, usually due to thermodynamic incompatibility, weak van der Waals interactions, and the accumulation of amorphous polymers at the interface of the melt-forming material. The researchers used a simple peel The test evaluated the adhesion of polyethylene and polypropylene laminate combinations with and without block copolymers.The results show that the laminate composition without the block copolymer is easily peeled off .
Pure polypropylene and polyethylene exhibit ductility and mechanical hardness when subjected to tensile forces at room temperature, however, mixing the two materials results in a significant reduction in the phase separation of the two components and strain at the fracture point resulting in poor mechanical properties. The catalyst system developed by scientists enables the synthesis of semi-crystalline polyethylene / polypropylene block copolymers with precise control of the length and structure of the blocks. By rational design, these macromolecules can be formed with commercial polyethylenes and polypropylenes Firm interface The interface strength can be enhanced to control the morphology and mechanical toughness of commercial polyethylene and polypropylene blends to improve the impact of the mixture and crack resistance, recycling of waste plastics to produce higher value materials, plastic Recycling garbage provides an effective solution ...