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| Sorting
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Sorting
Sorting Methods | Reclamation
| Granulation & Air Classification
| Washing | Separation
| Rinsing/Drying | Melt
Filtering | End Markets
Sorting of whole bottles by resin type is essential to effective
plastic bottle recycling and generally takes place at the
materials recovery facility (MRF), or at an intermediate processing
center (IPC) that consolidates and processes source separated
materials. Some plastic reclaimers accept mixed plastic bottle
types and separate them at their facility; however, for the
purpose of this general discussion, the same sorting principles
apply, regardless of where the materials are sorted.
Sorting of commingled recyclable materials is, in general,
a labor-intensive effort, even where automated systems are
used. Bear in mind that MRFs and IPCs separate and prepare
several materials for reclamation including glass, aluminum,
steel, paper, and plastic. This discussion is focused on the
sorting of rigid plastic bottles.
Two basic methods are currently used to sort plastic bottles
for recycling. Sorting of whole bottles is carried out manually
(by visual inspection) or automatically (detection systems that
use sensors to analyze one or more properties of the plastic
bottles passing by).
Manual Identification
Sorting of whole bottles by visual inspection is done by bottle
shape, color and/or product recognition. However, this method
can lead to inaccurate identification and separation due to
human error or distorted containers. In addition, complications
arise when bottles of the same design are made of different
plastic polymers. These are called ‘look-alike’
bottles. Most plastic bottles manufactured in the United States
are stamped on the bottom with a resin identification code
(SPI code). However, this numbering system has limited value
to sorting personnel, as the volume at which cost-effective
sorting must be done precludes looking at the bottom of every
passing bottle.
Automated Identification
Automated sorting (auto-sorting) systems employ one or
more detection systems that use sensors to analyze the physical
or chemical properties of plastic bottles passing by and separate
them into categories by resin type, color or both. Auto-sorting
was initially developed to separate PVC from a stream of plastic
bottles, using x-ray systems that could easily identify the
chorine element in the PVC. However, these systems were limited
to detecting PVC only. More recently, techniques based on
near-infrared analysis can identify and separate plastic bottles
of multiple resin types, providing automated, ‘positive’
selection. Techniques based on optical scanning that employ
one or more cameras can separate plastic bottles by color.
Although it has limitations, auto-sorting greatly improves
the quality and efficiency of the separation process.
Regardless of the method employed to sort whole plastic bottles,
it is in the best interest of the MRF or IPC to create products
with the greatest market value. A few MRFs currently accept
all rigid plastic packaging for recycling into mixed resin
products like plastic lumber. However, most separate incoming
materials by plastic resin type to maximize the value of the
specific categories of materials collected for recycling.
The Guidelines will discuss five plastic bottle types that
presently represent more than 98 percent of the plastic bottles
used to package consumer products. Sorting these materials
properly offers the best opportunity to maximize their value
for recycling into new products.
Reclamation facilities employ a series of processing stages
using a wide range of systems and technologies to prepare plastic
bottles for recycling. It would be impractical to describe all
of them in detail, therefore, only the most common processing
stages will be discussed. These stages include Granulation and
Air Classification, Washing, Separation, Rinsing/Drying and
Melt Filtering.
The two most important factors in all reclamation operations
are yield and quality. Any attachment to a plastic bottle,
such as closures, closure liners, base cups, inserts, labels,
pour spouts, handles, sleeves, safety seals, coatings and
layers can impact the recovery rates of the base resin (i.e.,
the resin the bottle is made from) by reducing yield and increasing
recycling costs. These attachments, when not compatible with
the base resin being recovered, represent a significant cost
to the processor in terms of separation, recovery and waste
disposal, and can have an adverse effect on the quality of
the PCR (post-consumer resin) produced.
Average Reclaimer Yield Values
| Bottle Type |
Base Resin Yield (%) |
| Two-piece
PET soda bottles (w/base cup) One-piece |
65-75 (PET) |
| PET soda bottles and custom
PET bottles |
75-85 (PET) |
| Natural HDPE
bottles (e.g., milk, water) |
85-95 (HDPE) |
| Pigmented HDPE bottles (e.g.,
soap, detergent) |
75-85 (HDPE) |
| PVC bottles
|
85-92 (PVC) |
| PP bottles |
85-95 (PP) |
The table above shows typical yields reported by plastic
reclaimers for a variety of base resins. This table shows
a significant difference in the yield of the base resin due
to attachments. Compare for example, two-piece PET soda bottles
with a base cup vs. a one-piece PET soda bottle, or an HDPE
milk jug vs. an HDPE detergent container.
Granulation and air classification are generally the first steps
in the reclamation process. Following sorting by resin type,
whole bottles are ground to a particle size that best suits
the reclamation process. The ground plastic resin is referred
to as regrind or flake. Most granulation systems employ an air
classifying technique, after size reduction, to separate "light"
materials, such as labels, from the heavier base resin being
recovered. Granulation loosens plastic and paper labels, and
begins to free other attachments from a bottle. Excess glue
on labels or attachments can be problematic as it has a detrimental
impact on granulation and "lights" removal. This increases
the cost of reclamation by decreasing the wash cycle yield.
Washing the ground flake is the next step in most reclamation
operations. The washing may be done at ambient or elevated temperatures
and can include the addition of detergents or surfactants to
aid the cleaning process. Washing efficiency and effectiveness
are affected by labels, label inks, adhesives, base cups, closures,
closure liners, inserts, layers, coatings, or other attachments
that may be present in or on the bottle. Labels, label inks
and label adhesives should all be chosen carefully in order
to improve recovery of the base resin. Labels can contaminate
the base resin material; label inks can bleed into the wash
water tinting the PCR products; and label adhesives that can’t
be removed can coat the plastic regrind and embed unwanted contaminants.
Adhesives used to affix other attachments can be difficult to
remove and should be applied sparingly. Washing is often followed
by another air classification step to remove lighter materials
that have been liberated in the wash system.
Most conventional reclamation systems employ water in sink/float
or hydrocyclone systems to separate the base resin from attachments
and contaminants based on differences in the densities of the
materials used. Plastic resins with densities greater than 1.0
can be separated from resins with densities less than 1.0 (see
table) in water. However, resins with similar or overlapping
densities are difficult to separate in these systems. For example,
resins with densities greater than 1.0 cannot be easily separated
from each other (i.e., PVC from PET). Likewise, resins with
densities less than 1.0 cannot be separated from each other
either (i.e., PP from HDPE). It is therefore important when
selecting plastic resins for attachments or components in a
bottle design to avoid any such overlap, or to make them from
the base resin in the same color as the bottle.
Rinsing is done to remove residual dirt and detergent from plastic
regrind after it is washed and separated. The material is then
dried to a level necessary for remanufacturing applications
or for further processing.
An additional processing stage that may take place at a reclaimer,
but is generally done at a converter, is called melt filtering.
Melt filtering removes non-melting, particulate contaminants
in plastic regrind that may remain after sorting, washing and
separation stages. Melt filtration occurs inside an extruder,
where the plastic regrind is melted and filtered through one
or more screens as it is passed through the extruder to make
pellets. Converting plastic regrind to pellets provides for
a more uniform feedstock for remanufacturing applications and
lowers transportation costs for the reclaimer or converter.
Recovered materials are used in a wide variety of end-product
applications. When a material is recycled back into the same
product it is referred to as a product-to-product, or ‘closed
loop’ recycling application (e.g., remanufacturing new
plastic bottles from old plastic bottles). When product-to-product
applications are not possible, the resin recovered from recycled
plastic bottles is made into a different kind of product (e.g.,
remanufacturing PET soda bottles into carpet fiber). When reclaimed
plastic bottles are remanufactured into products other than
new bottles, it is referred to as ‘open loop’ recycling.
Higher value recycling applications require higher quality PCR
with excellent performance characteristics. Plastic reclaimers
strive to produce the highest quality PCR to supply the highest
value recycling applications.
The Association of Postconsumer Plastic Recyclers supports
PCR utilization in the widest range of recycling applications
possible to ensure the economic viability of plastic recycling.
Better bottle design that incorporates design for recyclability
criteria will help improve opportunities for economically
viable recycling by reducing the processing costs associated
with removing incompatible contaminants and improving the
quality of the PCR. The following pages were developed as
design guidelines for the specific bottle resin types described.
Description of Terms
APC - American
Plastics Council
APR - Association of Postconsumer
Plastic Recyclers
EVA - Ethylene vinyl acetate
EVOH - Ethylene vinyl alcohol
HDPE - High density polyethylene
LDPE - Low density polyethylene
LLDPE - Linear low density polyethylene
MDPE - Medium density polyethylene
MRF - Material recovery facility
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NAPCOR - National
Association for
PET
Container Resources
OPP - Oriented polypropylene
PCR - Post-consumer resin
PET - Polyethylene terephthalate
PP - Polypropylene
PS - Polystyrene
PVC - Polyvinyl chloride
SPI - Society of the Plastics Industry |
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