Page 185 - Materials Chemistry, Second Edition

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156 Waste Management Practices: Municipal, Hazardous, and Industrial
flakes measure about 1 cm (3/8 in.) across. The flakes are washed with hot water and detergents to
remove labels, adhesives and dirt, and floated to remove any heavy contaminants. The HDPE is placed
into a spin dryer to remove free water. Flakes are dried with hot air, reducing moisture content to about
0.5% (Tchobanoglous et al., 1993). The dried flakes may be sold in that form. More sophisticated
plants reheat the flakes, add pigment, and pass them through a pelletizer, which produces small plas-
tic beads that are used in injection molding presses to create new products (Figure 6.16)(CMI, 2002).
Resin may also be fluidized using an extruder. Flakes are fed into the extruder and compressed
as they are forced forward toward the die. The combined heat from flow friction and supplemental
heating causes the resin to melt. Volatile contaminants are vented from the mixture. The melted
resin mixture may pass through a fine screen to remove any remaining solid impurities
(Tchobanoglous et al., 1993).
PET is a form of polyester that is extremely tough and versatile. Soft drink and water bottles
are made from this resin as are many plastic jars and “clamshell” packages (e.g., salad containers).
Recycling PET is similar to that for HDPE. Bottles may be color-sorted and are then ground and
washed. Unlike polyethylene, however, PET sinks in the wash water while the plastic caps and
labels float off. The clean chips are dried and pelletized. PET bottles may contain aluminum caps,
and granulated aluminum may contaminate PET chips. Electrostatic precipitation is used to remove
the aluminum (Tchobanoglous et al., 1993). Recycled PET has many uses and there are well-estab-
lished markets for this resin. The largest usage of recovered PET is in textiles. Carpet companies
often use 100% recycled resin to manufacture polyester carpets in a variety of colors and textures.
PET is also spun into fine filaments to make fiber filling for pillows and jackets. A substantial quan-
tity of recycled PET returns to the bottle market (CMI, 2002).
Regardless of the reassuring numbers of collection programs, however, the overall recovery of
plastics for recycling is quite small, totaling 1.3 million tons or 5.6% of plastics generation in 1999
(Table 6.7). However, recovery of some plastic containers has generally increased. PET soft drink

FIGURE 6.16 Recycled polyethylene beads ready for extrusion.