Polyethylene thermal recycling

Because of the resemblance to the 1,4-butane diol in the above example, it is worth mentioning that polyethylene glycol) (PEG) and poly(propyleneglycol) (PPG) have attracted interest as novel solvents for catalytic processes (see Fig. 7.2 for examples). They are both relatively inexpensive and readily available materials. They are essentially non-toxic (PPG is often used as a solvent for pharmaceutical and cosmetic preparations and both are approved for use in beverages) and have good biodegradability. Moreover, they are immiscible with water, non-volatile, thermally robust and can, in principle, be readily recycled after removal of the product. 

High-temperatnre pyrolysis and cracking of waste thermoplastic polymers, such as polyethylene, polypropylene and polystyrene is an environmentally acceptable method of recycling. These type of processes embrace both thermal pyrolysis and cracking, catalytic cracking and hydrocracking in the presence of hydrogen. Mainly polyethylene, polypropylene and polystyrene are used as the feedstock for pyrolysis since they have no heteroatom content and the liquid products are theoretically free of sulfur. 

Recently the pyrolysis of polymer mixtures has become a focus of interest due to the increasing role of plastics recycling. Many researchers have investigated the thermal decomposition of various polymers in the presence of PVC. Kniimann and Bockhom [25] have studied the decomposition of common polymers and concluded that a separation of plastic mixtures by temperature-controlled pyrolysis in recycling processes is possible. Czegfny et al. [31] observed that the dehydrochlorination of PVC is promoted by the presence of polyamides and polyacrylonitrile however, other vinyl polymers or polyolefins have no effect on the dehydrochlorination. PVC generally affects the decomposition of other polymers due to the catalytic effect of HCI released. Even a few per cent PVC has an effect on the decomposition of polyethylene (PE) [32], HCI appears to promote the initial chain scission of PE. Day et al. [33] reported that PVC can influence the extent of degradation and the pyrolysis product distribution of plastics used in the... 

Thermal processes are mainly used for the feedstock recycling of addition polymers whereas, as stated in Chapter 2, condensation polymers are preferably depolymerized by reaction with certain chemical agents. The present chapter will deal with the thermal decomposition of polyethylene, polypropylene, polystyrene and polyvinyl chloride, which are the main components of the plastic waste stream (see Chapter 1). Nevertheless, the thermal degradation of some condensation polymers will also be mentioned, because they can appear mixed with polyolefins and other addition polymers in the plastic waste stream. Both the thermal decomposition of individual plastics and of plastic mixtures will be discussed. Likewise, the thermal coprocessing of plastic wastes with other materials (e.g. coal and biomass) will be considered in this chapter. Finally, the thermal degradation of rubber wastes will also be reviewed because in recent years much research effort has been devoted to the recovery of valuable products by the pyrolysis of used tyres. 

PE = polyethylene PP = polypropylene PS = polystyrene ASR = automobile shredder residue VGO = vacuum gas oil LCO = light cycle oil. SA = Si02/ AI2O3 MOR = mordenite. TD/CD = thermal degradation followed by catalytic degradation COMB = mixed polymer and catalyst in a batch reactor COMS = mixed polymer and catalyst in a semibatch reactor FB = fixed bed flow reactor BIRR = Berty internal recycle reactor. 

Three commodity addition polymers, polyethylene, polypropylene and polystyrene, were the focus of this review. Although thermal decomposition of these polymers is one strategy used for tertiary recycling, it is inherently non-selective... 

N.K. Madi, Thermal and mechanical properties of injection molded recycled high density polyethylene blends with virgin isotactic polypropylene. Mater. Des. 46, 435-441 (2013)... 

K.A. Tawab, S.M. Ibrahim, M.M. Magida, The effect of gamma irradiation on mechanical, and thermal properties of recycling polyethylene terephthalate and low density polyethylene (R-PET/LDPE) blend compatibilized by ethylene vinyl acetate (EVA). J. Radioanal. Nucl. Chem. 295, 1313-1319 (2013)... 

Suarez, J. C. M., Mano, E. B., Pereira, R. A., Thermal behavior of gamma-irradiated recycled polyethylene blends. Polymer Degradation and Stability 2000, 69(2), 217-222. 

Due to its low thermal stability, wood flour is usually used as filler only in plastics that are processed at temperatures lower than about 200 °C. The majority of wood-plastic composites use polyethylene as the matrix (Figure 15.2). This is, in part, due to that fact that much of the early wood-plastic composites were developed as an outlet for recycled film. Polypropylene is more commonly used in automotive applications, and polyethylene is more commonly used in exterior building applications. 

Cellulose materials are good candidates to use as fillers in polymers, especially with their recent importance in recycling. There are many different types of materials available, such as wood flour, recycled newspaper, nut shells, and starch. These materials have several advantages, including low cost, low density, low abrasiveness, and they are a renewable resource. However, they do have a number of disadvantages, such as low thermal stability, high moisture absorption, and poor interfacial adhesion. Despite these problems, there are a number of products available in the marketplace. One of the most visible is plastic lumber. Most plastic lumber is made using polyethylene as the base resin and contains up to 50% wood. Much of the time, both resin and wood feedstocks come from recy-... 

With ever-increasing oil prices, recycled plastics are becoming an economical alternative for the production of a wide range of commodity plastic parts. Polyolefinic polymers, such as polyethylene (PE) and polypropylene (PP), contain approximately 14% hydrogen these materials could provide the hydrogen required for thermal coprocessing with biomass, which could lead to an increase of the liquid production of oligomers or short chain polymeric materials. 

---