Recycling plastics Subject

This subject effects designers since many products have the requirement by regulations or otherwise to use recycled plastics. Different methods are used to recycle materials to provide plastics with a continuing life. Method used is influenced by factors such as costs, quantity involved, weight involved, size and shape, complexity of mixed types of plastics, extended of contamination such as metallic particles, continued availability of material, etc. (Recognize that they can also be used as energy sources through incineration that can be combined with production of electricity and/or hot water for example). 

Plastic, virgin A plastic material in the form of pellets, granules, powder, flock, liquid, etc. that has not been subjected to use or processing other than what was required for its initial manufacture. It is not recycled plastics. 

In 1976 plastic became the most widely used material in the United States. It represents a 140-billion-per-year industry8, greater than steel and aluminum combined. Unfortunately the increase in the use of plastic has affected the problem of waste plastic disposal at about the same rate. The need to recycle plastics to reduce the use of virgin resins and to reduce the amount of waste materials has become a subject of considerable interest. 

When discussing many subjects it is important that they be properly identified by definitions such is the case in recycling. Different definitions exist to meet different industry and commercial requirements. ASTM defines a recycled plastics as those plastics composed of postconsumer material and recovered material only, or both, that may or may not have been subjected to additional processing steps of the types used to make products such as recycled regrind, or processed or reconstituted plastics. The industry scrap includes what is commonly referred to as trim or regrind in plastic production, is not considered recycled material. 

Much of thermodynamics concerns the causes and consequences of changing the state of a system. For example, you may be confronted with a polymerization process that converts esters to polyesters for the textile industry, or you may need a process that removes heat from a chemical reactor to control the reaction temperature and thereby control the rate of reaction. You may need a process that pressurizes a petroleum feed to a flash distillation unit, or you may need a process that recycles plastic bottles into garbage bags. In these and a multitude of other such situations, a system is to be subjected to a process that converts an initial state into some final state. 

While the effects of material and processing variations on polymer properties are quite well-documented [7-10], reliable prediction techniques for the evolution of properties during the service life of a part are still lacking. In a typical application, a polymer component may be simultaneously subjected to mechanical, thermal and chemical stresses suA that the characterisation of the durability of such a part is a complex task. Polymers retain memory of preceding life-cycle steps so the durability not only depends on what happens during service, but also on what happened before, during manufacture (Figure 2.5). Hence, the durability of recycled plastics will depend on the full history... 

Moreover, PP/LDPE blends are the products of plastic recycling. Since separation of PP and PE in sohd domestic waste (SDW) is very expensive, many recycling companies sell mixtures of reworked PE and PP ( polymer mix ) [13]. Thus, the study of the stmcture of blends of these materials will increase the field of application of recycled plastics. This is the reason for why PP-LDPE blends are the main subject considered in this chapter. 

The recycling of homogeneous post-consumer plastics presents many problems mainly because of degradation of the polymeric materials during processing and lifetime. The degradation processes, which can become very important in recycled materials subjected to many repeated processing operations, lead to drastic reductions of the mechanical properties of the secondary materials. 

I) Scrap in the form of offcuts, rejects, sprues, etc., arising in the manufacture of plastics products. Most of this waste material is recycled by blending it with virgin plastic, subject to the careful control of the levels of contamination in the plastic regrind, the proportion of regrind to virgin materials, and the deterioration in the p%sical properties which may be caused by repeated thermal and mechanical processing. 

These two major classifications of thermoplastics (TPs) and thermosets (TSs) in turn have different classifications such as virgin or recycled plastics. Virgin plastics have not been subjected to any fabricating process. NEAT plastics identify plastics with Nothing Else Added To. They are true virgin polymers since they do not contain additives, fillers, etc. However they are rarely used since they do not provide the best performances. Thus the technically correct term to identify the materials is plastics. Of the 35,000 types available worldwide there are about 200 basic types or famihes that are commercially recognized with less than 20 that are popularly used. Examples of these plastics are shown in Table 1.2. 

Brief details are given of two proposed resolutions on the subject of pyrolysis of waste plastic. The first states that pyrolysis and other methods of chemically reprocessing post-consumer plastics is a suitable way of diverting waste from landfills. The second resolution, supported by environmentalists, states that pyrolysis only recovers plastic s energy value, and should not be viewed as recycling. 

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