Environmental Engineering Reference
In-Depth Information
HDPE bottles into a grade of recycled resin approved for food-contact
applications. Technically, this is a significant achievement. Compared to
PET waste, cleaning of used HDPE products free of contaminants, is more
complicated given its lower processing temperature. A complicating factor
is the contamination of HDPE intended for recycling by other bottles (such
as detergent bottles) made of HDPE or copolymers of PE. These cannot be
included in the closed-loop recycling stream for HDPE.
The cleaning in this case consists of multiple steps: washing, float/sink
separation, and/or hydrocycloning stages (APC, 1999). Float/sink
separationallowsdensity-basedremovalofmostnon-HDPEdebrisfromthe
granulated and the dried granules are pelletized. Design considerations that
help recycling include not using PP components (caps) on HDPE bottles as
theycannotbeeasilyseparated. Blendsof(>~5%PP)resultsinpoorquality
recyclate. The same is true of PVC labels.
9.6 DESIGNING FOR RECYCLABILITY
Designing products, especially plastic bottles, for easy recyclability is a key
sustainable development goal. For instance, avoiding any PVC components
in the bottle (such as labels or caps) is helpful as it simplifies cleaning
especially in PET waste streams; the two resins have overlapping densities
and PVC decomposes at a temperature lower than the melt temperature for
PET, releasing HCl gas.
The same applies to other resins such as PLA (compostable plastic), vinyl
alcohol copolymers (barrier layer and cap sealer), and poly(ethylene
naphthenate) (used for additional barrier properties). These do not disperse
well in the PET or HDPE base resins and can degrade the quality of the
recycled resin (Bolin and Smith, 2011). Dark colors, any metal layers,
adhesives not removed by water, non-plastic labels, and thermoset caps
make it more difficult for PET to be recycled into a high-grade product.
Recycling mixed plastics yields a low-grade “plastic lumber” that can be
used in a limited number of applications. Most classes of common plastics
are not miscible with each other and the commingled resin therefore has
poor mechanical properties (Chanda and Roy, 2007). This is avoided by
usingacompatibilizer(Ubonnutetal.,2007)at2-5%levelbutitaddstothe
cost of the product; compounds such as maleic anhydride-grafted PE or PP
(Lei and Wu, 2012; Leu et al., 2012) are often used (Nornberg et al., 2014).
