There are several physical and chemical methods to investigate the degradation of polymers, from methods without change of physical and chemical properties of polymers to degradation processes for the recovery of monomers. The initiation of degradation may be considerably different: heat (thermal degradation), heat and catalyst (thermocatalytic degradation), oxygen (oxidative degradation), heat and oxygen (thermo-oxidative degradation), radiation (photochemical degradation), radiation and oxygen (photo-oxidative degradation), chemicals (chemical degradation), microorganisms or enzymes (biodegradation or bioerosion), etc.
The common characteristic of these methods is that they cause irreversible changes in the structure of polymers. The decomposition of the framework of polymers results in decreasing molecular weight and significant changes of physical and chemical properties.
The widely known and thoroughly studied methods are thermal and thermocatalytic degradation, which are referred to in the literature as chemical recycling. Chemical recycling and chemical degradation are not the same, because chemical degradation means degradations caused by chemicals (acids, solvents, alkalis, etc.)
The study of chemical recycling and degradation of waste plastics has developed since the 1980s. During chemical recycling hydrocarbon macromolecules of polymers break down into smaller molecules owing to thermal or thermal and catalytic effects. According to researchers, the possibility of further utilization of cracked products is their fuel-like use; therefore the objective of researches is the production of hydrocarbons with similar properties to refinery streams (e.g. gasoline, kerosene, diesel oil, etc.). For this reason the formation of volatile products (gases, liquids) with suitable yield is a key issue during investigations.
There are some advantages of the application of catalysts, on the other hand it may cause difficulties (feeding of catalysts into the reactor, activity loss, etc.). Usually batch experiments were carried out on laboratory scale (1-5 g) using one or two types of polymers, which were virgin and not waste polymers, therefore the confrontation with problems of greater amounts of catalysts is probable in the immediate future.
Many polymers with different structure and properties may be used as raw materials in degradation (HDPE, LDPE, PP, PS, PVC, PET, PA, PUR, etc.) although polyolefins (HDPE, LDPE, PP) and polystyrene (PS) have the best properties from the point of view of their further utilization. Several reports have described the thermal and catalytic cracking of waste polymers. Two types of polymers have been widely investigated: polyethylene and polypropylene, because they represent 60-65% of all plastic wastes. The degradation of plastics means heating to high temperatures where macromolecules break into smaller fragments. Valuable mixtures of hydrocarbons (gas, liquid and residue) are obtained [1-6].
The structure of the hydrocarbons produced can be modified by the use of catalyst. Catalytic cracking consumes less energy than the noncatalytic process and results in formation of more branch-chain hydrocarbons. On the other hand the addition of the catalyst can be troublesome, and the catalyst accumulates in the residue or coke. There are two ways to contact the melted polymer and catalysts: the polymer and catalyst can be mixed first, then melted, or the molten plastics can be fed continuously over a fluidized catalyst bed. The usually employed catalysts are US-Y, and H-ZSM-5. Catalyst activity and product structure have been reported [7-11]. It was found that the H-ZSM-5 and FCC catalysts provided the best possibility to yield hydrocarbons in the boiling range of gasoline.
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