Recyclable degradable polyethylene

Modidus and impact strength can be increased by the use of recycled degraded polyethylene and, for blends with the same components, by increasing the mixing intensity (mixer vs. extruder). 

Y. Uemichi, K. Takuma, and A. Ayame, Chemical Recycling of Polyethylene by Catalytic Degradation into Aromatic Hydrocarbons using H-Ga-Silicate, Chem. Com-mun., 18, 1975-1976 (1998). 

Uemichi et al. [9], carried out chemical recycling of polyethylene by catalytic degradation into aromatic hydrocarbons using H-Ga silicate. This exhibits excellent catalytic activity towards the formation of aromatic hydrocarbons, mainly benzene, toluene and xylene. This catalyst is less acidic than H-ZSM-5, but the acid sites are significantly stronger than those on silica-alumina which exhibit low cracking activity. H-Ga silicate is highly effective as a catalyst for the production of aromatic hydrocarbons such as benzene, toluene and xylenes important raw material accounted for most of the aromatic products. 

Fig. 7 Changes in crystallinity near to the surface of a linear lowdensity polyethylene bar exposed to UV in the laboratory for 6 weeks. Samples taken at different depths from the exposed surface. Each sample was 0.1mm deep the corresponding result is plotted at the midpoint of the layer. (More details in Craig, I.H. White, J.R. Crystallization and chemicrystallization of recycled photo-degraded polyethylenes. Polym. Eng. Sci. 2005, 45, 588-595.)...

I. Jakubowicz, J. Enebro, Effects of reprocessing of oxobiodegradable and non-degradable polyethylene on the durability of recycled materials. Polym. Degrad. Stab. 97, 316 (2012)... 

BPEO means that in practice the same disposable product may end up in any one of the alternative options discussed above. Consequently the material used should ideally be accommodated in any of the procedures used. Thus for example, if a biodegradable product is to be mechanically recycled, it should be capable of being reprocessed at the same temperature as the rest of the polymeric waste. This has proved to be difficult in the case of many bio-based materials. Degradable polyethylene can be recycled normally at polyolefin processing temperatures [10] whereas most hydro-biodegradable polymers depolymerise or scorch at these temperatures and cannot be recycled with commercial synthetic polymers in standard reprocessing equipment. 

Obviously, there is no accumulation of natural polymers because they are recycled by biological degradation to CO2. This is then assimilated and reincorporated into biomass by photosynthesis. On the other hand, persistence is a well known property of most synthetic polymers, e.g. polyethylene (PE) and polystyrene (PS). 

Y. Sakata, Catalytic Degradation of Polyethylene and Polypropylene into Fuel Oil in Recycling of Polymers, J. Kabovec, ed., (Wiley-VCH, Weinheim Germany), pp 7-18 (1998). 

A. M. CunUffe and P. T. Williams, Characterisation of products from the recycling of glass fibre reinforced polyester waste by pyrolysis. Fuel, 82, 2223-2230, (2003). J. H. Harker and J. R. Backhurst, Fuel and Energy, Academic Press London, 1981. A. C. Albertson and S. Karlsson, Polyethylene degradation products, In Agricultural and Synthetic Polymers, ACS Symposium Series 433, J. E. Glass and G. Swift (eds), American Chemical Society, Washington DC, 60-64, 1990. 

M. L. Mastellone, F. Perugini, M. Ponte and U. Arena, Flnidized bed pyrolysis of a recycled polyethylene, Polymer Degradation and Stability, 76, 479-487 (2002). 

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... 

The importance of the [R-COO j as a preferred reaction pathway in the decomposition of pollutants on the surface [polyethylene-COO -Fe " ] is rationalized by the observation that these copolymer thin films should be attacked by Off, H02° and other oxidative radicals available in solution. But this was not the case for [polyethylene-COO -Fe ] even when used over long times (300 hours) and repeated recycling below the polyethylene flowing temperature (80°C). Moreover, the observation reported recently [8] that degradation of organic compounds is able to take place during Fenton photo-assisted treatment in the presence of 3000 ppm of Cl -ion... 

In Delaware, this oil (4500 gal/yr) is recycled at the oil refinery at Delaware City 306 Liquid carbon dioxide is being used to take motor oil off of high-density polyethylene containers, so that both can be recycled.307 Safety-Kleen Corp. offers nationwide collection for used oil and antifreeze from cars and other sources.308 Mobile on-site services are also available for removing toxic compounds and acid degradation products from ethylene glycol antifreeze, with addition of new corrosion inhibitors, so that the antifreeze can be reused.309 Organoclays have been used to remove oils from water.310... 

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. 

In addition to the large quantities of black plastic hay-wrap referred to above, which are discarded in the fields and farmyard, very large amounts of polyethylene and polypropylene are used in agricultural packaging of feedstuffs and fertiliser sacks. In spite of the fact that some of this is recycled and farmers find secondary uses for some of it, much of it finds its way into rivers and ditches where it degrades very slowly. This is the environmental down-side of plastics packaging. 

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