Conversion Processes

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The techniques used to convert polymer granules to final products are well established, and the main inputs are electricity, thermal fuels, and cooling water. Table 3.3 gives values of these parameters for a selection of processes. At the conversion stage various additives and fillers are usually incorporated into the product. In Table 3.3 these are included in the column headed resin.

The data of Table 3.3 cannot be regarded as definitive because of the nature of the conversion industry. In particular, note should be taken of the following points:

1. Unlike polymer production, the conversion industry is fragmented and ranges from large modern plants, continuously producing a single product such as sheet or PET bottles, to small jobbing molders that produce short runs of many different products, usually by injection molding. For these small molders, the frequent stopping, starting, and mold changes on machines that are not always tailored to the job in hand means that they are usually less energy efficient than the large-volume producers who use machines specifically matched to their purpose.

2. Materials losses are typically 1-2% and mainly arise from contaminated waste generated during startup, maintenance, and machine purging. For injection molding there is also a loss arising from the removal of flashing and sprues; such waste can only sometimes be recycled back into the process.

3. Few operators monitor separately the energy consumption of the machines and the energy used in space heating and lighting. Consequently, the data of Table 3.3 refer to plant performance rather than machine performance.

ENERGY AND POLYMER RECYCLING 133 Table 3.3 Data for Production of 1 kg of Selected Products

Thermal

Resin Electricity Fuels Water

Thermal

Resin Electricity Fuels Water

Polymer

Product

Process

(kg)

(MJ)

(MJ)

(kg)

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