INTRODUCTION
POLYMER
The simplest definition of a polymer is
a useful chemical made of many repeating units. A polymer can be a
three-dimensional network (think of the repeating units linked together left
and right, front and back, up and down) or two-dimensional network (think of
the repeating units linked together left, right, up, and down in a sheet) or a
one-dimensional network (think of the repeating units linked left and right in
a chain). Each repeating unit is the “-mer” or basic unit with “poly-mer”
meaning many repeating units. Repeating units are often made of carbon and
hydrogen and sometimes oxygen, nitrogen, sulfur, chlorine, fluorine,
phosphorous, and silicon. To make the chain, many links or “-mers” are chemically
hooked or polymerized together. Linking countless strips of construction paper
together to make paper garlands or hooking together hundreds of paper clips to
form chains, or stringing beads helps visualize polymers. Polymers occur
in nature and can be made to serve specific needs. Manufactured polymers
can be three-dimensional networks that do not melt once formed. Such
networks are called THERMOSET polymers. Epoxy resins used in two-part adhesives
are thermoset plastics. Manufactured polymers can also be one-dimensional
chains that can be melted. These chains are THERMOPLASTIC polymers and
are also called LINEAR polymers. Plastic bottles, films, cups, and fibers are
thermoplastic plastics.
Polymers abound in nature. The
ultimate natural polymers are the deoxyribonucleic acid (DNA) and ribonucleic
acid (RNA) that define life. Spider silk, hair, and horn are protein
polymers. Starch can be a polymer as is cellulose in wood. Rubber tree
latex and cellulose have been used as raw material to make manufactured polymeric
rubber and plastics. The first synthetic manufactured plastic was Bakelite,
created in 1909 for telephone casing and electrical components. The first
manufactured polymeric fiber was Rayon, from cellulose, in 1910. Nylon was
invented in 1935 while pursuing a synthetic spider silk.
The Structure of Polymers
Other common
manufactured polymers have backbones that include elements other than
carbon. Nylons contain nitrogen atoms in the repeat unit backbone.
Polyesters and polycarbonates contain oxygen in the backbone. There are also
some polymers that, instead of having a carbon backbone, have a silicon or
phosphorous backbone. These are considered inorganic polymers. One of the more
famous silicon-based polymers is Silly Putty
Characteristics of Polymers
The majority of manufactured polymers
are thermoplastic, meaning that once the polymer is formed it can be heated and
reformed repeatedly. This property allows for easy processing and facilitates
recycling. The other group, the thermosets, cannot be remelted. Once these
polymers are formed, reheating will cause the material to ultimately degrade,
but not melt.
Every polymer has very distinct
characteristics, but most polymers have the following general attributes.
Polymers can be very resistant to chemicals. Consider
all the cleaning fluids in your home that are packaged in plastic. Reading the
warning labels that describe what happens when the chemical comes in contact
with skin or eyes or is ingested will emphasize the need for chemical
resistance in the plastic packaging. While solvents easily dissolve some
plastics, other plastics provide safe, non-breakable packages for aggressive
solvents.
Polymers can be both thermal and electrical insulators. A
walk through your house will reinforce this concept, as you consider all the
appliances, cords, electrical outlets and wiring that are made or covered with
polymeric materials. Thermal resistance is evident in the kitchen with pot and
pan handles made of polymers, the coffee pot handles, the foam core of
refrigerators and freezers, insulated cups, coolers, and microwave cookware.
The thermal underwear that many skiers wear is made of polypropylene and the
fiberfill in winter jackets is acrylic and polyester.
Generally, polymers are very light in weight with significant degrees of
strength. Consider the range of
applications, from toys to the frame structure of space stations, or from
delicate nylon fiber in pantyhose to Kevlar, which is used in bulletproof
vests. Some polymers float in water while others sink. But, compared
to the density of stone, concrete, steel, copper, or aluminum, all plastics are
lightweight materials.
Polymers can be processed in various ways. Extrusion
produces thin fibers or heavy pipes or films or food bottles. Injection
molding can produce very intricate parts or large car body panels. Plastics can
be molded into drums or be mixed with solvents to become adhesives or paints.
Elastomers and some plastics stretch and are very flexible. Some plastics are
stretched in processing to hold their shape, such as soft drink
bottles. Other polymers can be foamed like polystyrene (Styrofoam™), polyurethane
and polyethylene.
Polymers are materials with a seemingly limitless range of
characteristics and colors. Polymers
have many inherent properties that can be further enhanced by a wide range of
additives to broaden their uses and applications. Polymers can be made to
mimic cotton, silk, and wool fibers; porcelain and marble; and aluminum and
zinc. Polymers can also make possible products that do not readily come
from the natural world, such as clear sheets and flexible films.
Polymers are usually made of petroleum, but not always. Many
polymers are made of repeat units derived from natural gas or coal or crude
oil. But building block repeat units can sometimes be made from renewable
materials such as polylactic acid from corn or cellulosic from cotton linters.
Some plastics have always been made from renewable materials such as cellulose
acetate used for screwdriver handles and gift ribbon. When the building
blocks can be made more economically from renewable materials than from fossil
fuels, either old plastics find new raw materials or new plastics are
introduced.
Polymers can be used to make items that
have no alternatives from other materials. Polymers
can be made into clear, waterproof films. PVC is used to make medical
tubing and blood bags that extend the shelf life of blood and blood
products. PVC safely delivers flammable oxygen in non-burning flexible
tubing. And anti-thrombogenic material, such as heparin, can be
incorporated into flexible PVC catheters for open heart surgery, dialysis, and
blood collection. Many medical devices rely on polymers to permit
effective functiosection.
Types of polymers
1.
Thermoplastic
materials
2.
Thermosetting
materials
·
Thermoplastic Materials
Thermoplastics are plastic polymers that
soften when they are heated, allowing for molding, and solidify again as they
are cooled. Because of their unique chemical properties, thermoplastic
materials can be remolded and recycled without negatively affecting the
material’s physical properties. This makes thermoplastics an ideal material
for injection molding.
Thermoplastic materials are
used for a wide range of applications from consumer goods to medical equipment,
depending on the type of material. Commodity thermoplastics are the easiest to
process and are used to manufacture products in high volumes. These materials
are best for applications like packaging, clothing, food, and beverages. In
contrast, engineered and specialized thermoplastics have been blended to
enhance their characteristics. They are used for heavier-duty applications like
military, aerospace, and medical industries.
Each particular thermoplastic exhibits
different qualities, advantages, and disadvantages, making it critical to select
the right material for the application at hand. Additional fillers and
additives can also be used to provide specific characteristics that better meet
the application requirements. Stack Plastics will help you find the right
plastic that meets your needs.
Thermoplastics Differ from Thermoset Materials
Thermoplastics are very different from
thermoset materials. They are both used in the injection molding process, but
thermosets can only be heated and molded once. They cannot be changed or melted
back into their original shape. This makes them great for high-heat
applications, but also makes thermoset plastics less versatile than
thermoplastic materials.
During the thermoset plastic curing
process, polymers are linked together and form a permanent chemical bond. In
contrast, no chemical bonding takes place during the curing process of
thermoplastics, making it completely reversible. Thermoplastics can be remelted
and remolded whenever its necessary.
Properties of Thermoplastics
Thermoplastics are the only type of plastic
that can be welded. When the plastic is heated, the material becomes a paste or
a liquid that can then be molded into the desired shapes. Though each type of
thermoplastic offers its own characteristics and properties, they can all
withstand multiple re-shapings without any damage being done to the materials.
The Primary Benefits of Thermoplastic Materials:
- · Can be remolded and recycled without damage· Easy to mold and shape· Offers high strength and lightweight· Some can be used in place of metal· Relatively low processing costs· Easy to manufacture high volumes quickly· Able to maintain high precisionChemically retardant and impact resistant
·
Polyethylene terephthalate (PET or PETE),
A strong, stiff synthetic fibre and resin,
and a member of the polyester family
of polymers. PET is spun into fibers
for permanent-press fabrics, blow-molded into disposable beverage bottles, and
extruded into photographic film and magnetic recording tape.
The presence of a large
aromatic ring in the PET repeating units gives the polymer notable
stiffness and strength, especially when the polymer chains are aligned with one
another in an orderly arrangement by drawing (stretching). In this
semicrystalline form, PET is made into a high-strength textile fibre marketed
under such trademarked names as Dacron, by the American DuPont Company,
and Terylene, by the British Imperial Chemical
Industries PLC. The stiffness of PET fibres makes
them highly resistant to deformation, so they impart excellent resistance to
wrinkling in fabrics. They are often used in durable-press blends with other
fibres such as rayon, wool,
and cotton,
reinforcing the inherent properties
of those fibres while contributing to the ability of the fabric to recover from
wrinkling.
PET is also made into fibre
filling for insulated clothing and for furniture and pillows. When made in very
fine filaments, it is used in artificial silk, and in large-diameter
filaments it is used in carpets. Among the industrial applications of PET are
automobile tire yarns, conveyor belts
and drive belts, reinforcement for fire and garden hoses, seat belts (an
application in which it has largely replaced nylon), nonwoven fabrics for
stabilizing drainage ditches, culverts, and railroad beds, and nonwovens for
use as diaper topsheets and disposable medical garments. PET is the most
important of the man-made fibres in weight produced and in value.
At a slightly higher molecular weight,
PET is made into a high-strength plastic that can be shaped by all the common
methods employed with other thermoplastics. Magnetic recording tape and
photographic film are produced by extrusion of PET film (often sold under the
trademarks Mylar and Melinex). Molten
PET can be blow-molded into transparent containers of high strength and
rigidity that are also virtually impermeable to gas and liquid. In this form,
PET has become widely used in carbonated-beverage bottles and in jars for food
processed at low temperatures. The low softening temperature of
PET—approximately 70 °C (160 °F) prevents it from being used as a container for
hot-filled foods.
PET is the most
widely recycled plastic. PET bottles
and containers are commonly melted down and spun into fibres for fibrefill or
carpets. When collected in a suitably pure state, PET can be recycled into its
original uses, and methods have been devised for breaking the polymer down into
its chemical precursors for
resynthesizing into PET. The recycling code number for PET is #1.
PET was first prepared in
England by J. Rex Whinfield and James T. Dickson of the Calico Printers
Association during a study of phthalic acid begun
in 1940. Because of wartime restrictions, patent specifications for the new material
were not immediately published. Production by Imperial Chemical of its
Terylene-brand PET fibre did not begin until 1954. Meanwhile, by 1945 DuPont
had independently developed a practical preparation process from terephthalic
acid, and in 1953 the company began to produce Dacron fibre. PET soon became
the most widely produced syntheticfibre
in the world. In the 1970s, improved stretch-molding procedures were devised
that allowed PET to be made into durable crystal-clear beverage bottles—an
application that soon became second in importance only to fibre production.
MOULDING
Molding or molding (see
spelling differences) is the process of manufacturing by shaping liquid or
pliable raw material using a rigid frame called a mold or matrix.
This itself may have been made using a pattern or model of the final object.
... A mold is the counterpart to a cast
Types of
moulding
·
Blow Molding
The blow molding process follows
the same basic steps found in the art of glass blowing. To blow mold a part,
the manufacturer inflates a parison — a heated plastic mass, usually in the
shape of a tube — with air. The parison inflates until it fills the mold and
conforms to its shape. In this way, the plastic is blown into its desired form.
Once cooled, the newly formed plastic part is ejected from the mold.
Blow
molding is especially useful for economically manufacturing one-piece, hollow
objects in large volumes, as the process can quickly create uniform,
thin-walled containers — perfect for small objects like bottles, as well as
larger ones like storage containers and drums.
Depending
on the specific application, manufacturers can use a variety of thermoplastics
in blow molding to create a more customized product. Commonly worked materials
include low-density polyethylene, high-density polyethylene, polyethylene terephthalate,
polypropylene, and polyvinyl chloride.
·
Compression Molding
Compression
molding involves placing a heated plastic inside of a heated mold, then closing
it to compress the plastic into the desired shape. Once cooled, the part is
removed from the mold. The heating process, called curing, helps ensure that
the final product will maintain its integrity and shape.
Compression
molding offers many unique advantages; it’s both cost-effective and highly
efficient. The process is also quite versatile, allowing manufacturers to
create parts that vary greatly in thickness, length, and intricacy.
Because compression molding
often uses advanced composites for the plastic material, the process yields
stronger, more durable parts, making it popular across a range of different
industries. For example, compression molding often employs high-strength
materials, such as thermosetting
resins, fiberglass, and reinforced plastics, resulting in products that are
sturdier and more resilient than those offered by other molding processes.
Allowing
for the creation of high-strength parts, compression molding is used to produce
components for a vast range of applications, including automotive parts,
household appliances, clothing fasteners, and body armor.
·
Extrusion Molding
While
other forms of plastic molding use extrusion to insert the plastic resins into
the mold, extrusion molding extrudes the melted plastic material directly into
the die. This process is unique in that the shape of the die, not the mold,
determines the shape of the final product.
Extrusion
molding is ideal for manufacturing parts with continuous length and uniform
cross-sections. Similar to a plastic injection molding machine, the extrusion
molding machine has a screw that turns to feed the plastic resin into the
feeder. The molten plastic then moves through a die, creating a long, tubular
shape. The shape of the die determines the shape of the plastic tube. Once the
extrusion is cooled, it is removed from the machine.
Extrusion
molding is well-suited for long, hollow-formed applications, such as tubes,
pipes, and straws. Plus, manufacturers can create these parts in many different
shapes, including T-sections, U-sections, square sections, I-sections,
L-sections, and circular sections.
·
Injection Molding
Of
the various molding processes available, injection molding is considered to be
the most versatile, as it can be used to create a variety of parts, ranging in
both size and shape. Presses also come in different sizes, based on the
pressure they exert and their tonnage.
Injection
molding basic principles are fairly simple, but the actual process can be quite
complex when it comes to maintaining part consistency. The process
involves the injection of melted plastic into a mold, which is made of steel.
The mold itself has cavities that will form the parts; once injected, the
molten plastic fills the cavities and the rest of the mold. Once cooled, the
parts are ejected by pins.
Thanks
to its excellent versatility, injection molding can be used to create everything
from large automotive parts to small, intricate parts used in surgical
equipment. Injection molding also allows for a high level of customization, as
various plastic resins and additives can be used, allowing designers and
engineers to create unique parts to meet highly complex or unusual application
needs. And there are some enhancements and techniques available — such as an
array of resin and finish options — for manufacturers looking to create even
more specialized parts.
Though
it can be expensive to initially make the molds themselves, once built, the
production costs become quite low. In fact, injection molding is best-suited
for the creation of very high volumes of precise parts; once production begins,
the cost per part drops significantly, making the process very economical for
high-volume runs.
·
Plastic
injection molding is a highly reliable solution for producing large numbers of
precise, consistent components. It’s also more efficient and cost-effective
than other molding styles, in that it produces much less waste. As a result,
injection molding is most often used for the manufacture of high-quality parts
in high volumes.
·
Rotational Molding
Rotational
molding, also known as rotomolding, uses high temperatures and rotational
movement to coat the inside of a mold to form a part’s desired shape. First,
the mold is filled with a polymer powder. The heated mold then rotates on two
perpendicular axes so that the powder adheres to the entire interior of the
mold. As it continues to rotate, the mold eventually cools and is removed,
ultimately forming an even-walled component.
Rotational
molding is best suited for the creation of large, hollow, one-piece containers,
such as tanks. Though cost-effective, it is not a fast-moving process. However,
rotomolding wastes little material, and what excess material is produced can
often be reused, making it an economical and environmentally friendly
manufacturing process.
Chemical
Structure of Plastics
Common thermoplastics range from
20,000 to 500,000 amu,
while thermosets are assumed to have infinite molecular weight. These chains
are made up of many repeating molecular units, known asrepeat units, derived
from monomers;
each polymer chain will have several thousand repeating units. The vast
majority of plastics are composed of polymers of carbon and hydrogen alone
or with oxygen, nitrogen, chlorine or sulfur in
the backbone. (Some of commercial interests are silicon based.)
The backbone is that part of the chain on the main "path" linking a
large number of repeat units together. To customize the properties of a
plastic, different molecular groups "hang" from the backbone (usually
they are "hung" as part of the monomers before linking monomers
together to form the polymer chain). This fine tuning of the properties of the
polymer by repeating unit's molecular structure has allowed plastics to become
an indispensable part of twenty first-century world.
Classifications of Plastics
Plastics
Families
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Amorphous
|
Semi-crystalline
|
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Ultra polymers
|
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High performance polymers
|
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Other polyamides
|
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Mid range polymers
|
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Commodity polymers
|
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There exists terrific variety among
the many hundreds of different types of plastics. In order to simplify
the organization of their similarities and differences, it is useful to
classify plastics according to certain specific criteria; we can classify
plastics to:
·
Fossil-based plastics.
·
Biodegradable plastics.
·
Special purpose plastics.
Fossil-based plastics
a) Bakelite
The first so called plastic based on
a synthetic polymer was made from phenol
and formaldehyde,
with the first viable and cheap synthesis methods invented in 1907, by Leo Hendrik Baekeland, a
Belgian living in New York state. Baekeland was
looking for an insulating shellac to coat wires in electric motors and
generators. He found that mixtures of phenol (C6H5OH) and
formaldehyde (HCOH) formed a sticky mass when mixed together and heated, and
the mass became extremely hard if allowed to cool. He continued his
investigations and found that the material could be mixed with wood flour,
asbestos, or slate dust to create "composite" materials with
different properties. Most of these compositions were strong and fire
resistant. The only problem was that the material tended to foam during
synthesis, and the resulting product was of unacceptable quality.
Baekeland built pressure vessels to
force out the bubbles and provide a smooth, uniform product. He publicly
announced his discovery in 1912, naming it Bakelite. It was originally used for
electrical and mechanical parts, finally coming into widespread use in consumer
goods in the 1920s. When the Bakelite patent expired in 1930, the Catalin
Corporation acquired the patent and began manufacturing Catalin plastic
using a different process that allowed a wider range of coloring.
Bakelite was the first true plastic.
It was a purely synthetic material, not based on any material or even molecule
found in nature. It was also the first thermosetting plastic.
b) Polystyrene and Polyvinyl Chloride
After the First World War, improvements
in chemical technology led to an explosion in new forms of plastics. Among the
earliest examples in the wave of new plastics were polystyrene (PS) and
polyvinyl chloride (PVC).
Polystyrene is a rigid, brittle,
inexpensive plastic that has been used to make plastic model kits
and similar knick-knacks. It would also be the basis for one of the most
popular "foamed" plastics, under the name styrene
foam or Styrofoam.
Foam plastics can be synthesized in an "open cell" form, in which the
foam bubbles are interconnected, as in an absorbent sponge, and "closed
cell", in which all the bubbles are distinct, like tiny balloons, as in
gas-filled foam insulation and flotation devices. In the late 1950s, high impact styrene
was introduced, which was not brittle. It finds much current use as the
substance of toy figurines and novelties.
Polyvinyl Chloride (PVC, commonly
called "vinyl"). has side chains incorporating chlorine
atoms, which form strong bonds. PVC in its normal form is stiff, strong, heat
and weather resistant, and is now used for making plumbing,
gutters, house siding, enclosures for computers and other electronics gear. PVC
can also be softened with chemical processing, and in this form it is now used
for shrink-wrap,
food packaging, and rain gear.
All PVC polymers are degraded by
heat and light. When this happens, hydrogen chloride is released into the
atmosphere and oxidation of the compound occurs. Because
hydrogen chloride readily combines with water vapor in the air to form
hydrochloric acid, polyvinyl chloride is
not recommended for long-term archival storage of silver, photographic film or
paper (Mylar is preferable).
a)
Nylon
The real star of the plastics industry in the 1930s was polyamide (PA),
far better known by its trade name nylon.
Nylon was the first purely synthetic fiber, introduced by DuPont Corporation at
the 1939 World's Fair in New York City.
In 1927, DuPont had begun a secret
development project designated Fiber66, under the direction of Harvard chemist Wallace Carothers and
chemistry department director Elmer Keiser Bolton.
Carothers had been hired to perform pure research, and he worked to understand
the new materials' molecular structure and physical properties. He took some of
the first steps in the molecular design of the materials.
His work led to the discovery of
synthetic nylon fiber, which was very strong but also very flexible. The first
application was for bristles for toothbrushes. However, Du
Pont's real target was silk,
particularly silk stockings.
Carothers and his team synthesized a number of different polyamides including
polyamide 6.6 and 4.6, as well as polyesters.
b)
Rubber
Natural rubber is an
elastomer (an elastic hydrocarbon polymer) that was originally derived from latex, a
milky colloidal suspension found
in the sap of some plants. It is useful directly in this form (indeed, the
first appearance of rubber in Europe is cloth waterproofed with unvulcanized
latex from Brazil) but, later, in 1839, Charles Goodyear
invented
vulcanized rubber; this a form of natural rubber heated with, mostly, sulfur
forming cross-links between polymer chains (vulcanization),
improving elasticity and durability.
c)
Synthetic rubber
The first fully synthetic rubber was
synthesized by Sergei Lebedev in
1910. In World War II, supply blockades of natural rubber from South East Asia caused
a boom in development of synthetic rubber, notably styrene-butadiene rubber. In
1941, annual production of synthetic rubber in the U.S. was
only 231 tonnes which increased to 840,000 tonnes in 1945. In the space race and nuclear arms race,Caltech researchers
experimented with using synthetic rubbers for solid fuel for rockets.
Ultimately, all large military rockets and missiles would use synthetic rubber
based solid fuels, and they would also play a significant part in the civilian
space effort.
Biodegradable (compostable) plastics
Research has been done on biodegradable plastics
that break down with exposure to sunlight (e.g., ultra-violet radiation),
water or dampness, bacteria, enzymes, wind abrasion and some instances rodent
pest or insect attack are also included as forms of biodegradation or environmental degradation. It
is clear some of these modes of degradation will only work if the plastic is
exposed at the surface, while other modes will only be effective if certain
conditions exist in landfill or composting systems. Starch
powder
has been mixed with plastic as a filler to allow it to degrade more easily, but
it still does not lead to complete breakdown of the plastic. Some researchers
have actually genetically engineered bacteria
that synthesize a completely biodegradable plastic, but this material, such as Biopol, is
expensive at present. The German chemical
company BASF
makes
Ecoflex, a fully biodegradable polyester for food packaging applications.
Special purpose plastics
Special purpose plastics include Melamine formaldehyde (MF) – One of the aminoplasts,
and used as a multi-colorable alternative to phenolics, for instance in
moldings (e.g., break-resistance alternatives to ceramic cups, plates and bowls
for children) and the decorated top surface layer of the paper laminates (e.g.,
Formica), Plastarch material – Biodegradable and heat resistant, thermoplastic
composed of modified
corn starch.,
Phenolics (PF) or (phenol formaldehydes) – High modulus, relatively heat resistant, and excellent fire resistant
polymer. Used for insulating parts in electrical fixtures, paper laminated
products (e.g., Formica), thermally insulation foams. It is a thermosetting
plastic, with the familiar trade name Bakelite that can be molded by heat and
pressure when mixed with filler-like wood flour or can be cast in its unfilled
liquid form or cast as foam (e.g., Oasis). Problems include the probability of
moldings naturally being dark colors (red, green, brown), and as thermoset it
is difficult to recycle, Polyetheretherketone (PEEK) – Strong, chemical- and
heat-resistant thermoplastic, biocompatibility allows for use in medical implant applications, aerospace moldings. One of the most
expensive commercial polymers, Polyetherimide (PEI) (Ultem) – A high
temperature, chemically stable polymer that does not crystallize, Polylactic acid (PLA) – A biodegradable, thermoplastic found converted
into a variety of aliphatic polyesters derived from lactic acid which in turn can be made by fermentation of various
agricultural products such as corn
starch, once
made from dairy products, Polymethyl
methacrylate (PMMA)
– Contact lenses, glazing (best known in this form by its various trade names
around the world; e.g., Perspex, Oroglas, Plexiglas), aglets, fluorescent light
diffusers, rear light covers for vehicles. It forms the basis of artistic and commercial
acrylic paints when suspended in water with the use of other agents, Polytetrafluoroethylene (PTFE) – Heat-resistant,
low-friction coatings, used in things like non-stick surfaces for frying pans,
plumber's tape and water slides. It is more commonly known as Teflon, also
special purpose plastics include Urea-formaldehyde (UF) – One of the aminoplasts
and used as a multi-colorable alternative to phenolics. Used as a wood adhesive
(for plywood, chipboard, hardboard) and electrical switch housings.
Plastics Environmental Issues
Plastics are durable and degrade very
slowly; the molecular bonds that make plastic so durable make it equally
resistant to natural processes of degradation. Since the 1950s, one billion
tons of plastic have been discarded and may persist for hundreds or even
thousands of years. In some cases, burning
plastic can release toxic fumes.
Burning the plastic polyvinyl chloride (PVC) may create dioxin. Also, the
manufacturing of plastics often creates large quantities of chemical
pollutants.
Prior to the ban on the use of CFCs in
extrusion of polystyrene (and general use, except in life-critical fire
suppression systems), the production of polystyrene contributed to the
depletion of the ozone layer; however,
non-CFCs are currently used in the extrusion process.
By 1995, plastic recycling programs
were common in the United States and elsewhere. Thermoplastics can be remelted
and reused, and thermoset plastics can be ground up and used as filler, though
the purity of the material tends to degrade with each reuse cycle. There are
methods by which plastics can be broken back down to a feedstock state.
Plastic can be converted as a fuel.
Plastic is made from crude oil, so it can be broken down to liquid hydrocarbon.
One kilogram of waste plastic produces a liter of hydrocarbon. Plastic wastes
are used in cement plants as a fuel.
To assist recycling of disposable
items, the Plastic Bottle Institute of the Society of the
Plastics Industry devised
a now-familiar scheme to mark plastic bottles by plastic type. A plastic
container using this scheme is marked with a triangle of three "chasing arrows",
which encloses a number giving the plastic type:
Other types of
plastics
Polyethylene Terephthalate (PET)
Polyethylene terephthalate (sometimes written poly(ethylene
terephthalate)), commonly abbreviated PET, PETE, or the obsolete PETP or PET-P, is a thermoplastic
polymer resin
of terephthalic acid and ethylene glycole (A thermoplastic, also known as
thermo-softening plastic, is a polymer that
turns to a liquid when heated and freezes to a very glassy state when cooled
sufficiently) PET is a strong, rigid and light material. PET physical
properties make it an ideal substance for using in various fields, such as
production of packing (bottles, cortexes, etc.), film, constructions elements, PET is also used in synthetic fibers; beverage, food
and other liquid containers;
thermoforming applications; and engineering resins often in combination with
glass fiber. The term 'polyethylene terephthalate' is a source of confusion
because this substance, PET, does not contain polyethylene. Thus, the alternate
form, ‘poly(ethylene terephthalate)' is often used in scholarly journals for
the sake of accuracy and clarity.
Depending on its processing and
thermal history, polyethylene terephthalate may exist both as an amorphous
(transparent) and as a semi-crystalline polymer. The
semi-crystalline material might appear transparent (particle size <
500 nm) or opaque and white (particle size up to a few microns) depending
on its crystal structure and particle size. Its monomer
(bis-β-hydroxyterephthalate) can be synthesized by the
esterificationreaction
between terephthalic acid and ethylene glycol with
water as a byproduct, or by transesterification reaction
between ethylene glycol and dimethyl terephthalate
with methanol as a
byproduct. Polymerization is through a polycondensation
reaction
of the monomers (done immediately after esterification/transesterification)
with water as the byproduct.
The majority of the world's PET
production is for synthetic fibers (in excess of 60%) with bottle production
accounting for around 30% of global demand. In discussing textile applications,
PET is generally referred to as simply "polyester"
while "PET" is used most often to refer to packaging applications.
The polyester industry makes up about 18% of world polymer production and is
third after polyethylene (PE)
and polypropylene (PP).
PET consists of polymerized units
of the monomer ethylene terephthalate, with repeating C10H8O4 units. PET is commonly recycled, and has the
number "1" as its recycling symbol.
The first PET was patented in 1941
by John
Rex Whinfield, James Tennant Dickson and their
employer the Calico Printers' Association of Manchester. The
PET bottle was patented in 1973 by Nathaniel Wyeth. PET is used as substrate in thin film and
solar cell.
Because PET is an excellent barrier
material, plastic bottles made
from PET are widely used for soft
drinks. For certain specialty bottles,
PET sandwiches an additional polyvinyl alcohol to
further reduce its oxygen permeability.
Biaxially oriented PET film (often known by one of its trade
names, "Mylar") can be aluminized by evaporating a thin film of
metal onto it to reduce its permeability, and to make it reflective and opaque
(MPET). These properties are
useful in many applications, including flexible food packaging and thermal insulation, such as "space
blankets". Because of its high mechanical strength, PET film is often used
in tape applications, such as the carrier for magnetic
tape or backing for pressure sensitive adhesive tapes.
Non-oriented PET
sheet can be thermoformed to make packaging trays and blisters.
If crystallizable PET is used, the trays can be used for frozen dinners,
since they withstand both freezing and oven baking temperatures. When filled
with glass particles or fibers, it becomes significantly stiffer and more durable. PET is also used as substrate
in thin film and solar cell.
2:4:1 Chemical
Properties of PET
Polyethylene
terephthalate (PET), is a condensation polymer produced from the monomers
ethylene glycol, HOCH2CH2OH, a dialcohol, and dimethyl
terephthalate, CH3O2C–C6H4–CO2CH3,
a diester. By the process of transesterification, these monomers form ester
linkages between them, yielding polyester.
PET chemical resistance is good for
concentrated acids, dilute acids, Alcohols, greases and oils, halogens, and
ketones, and it has a poor chemical resistance for Alkalis and a fair one for
aromatic hydrocarbons.
(Polyethylene Terephthalate)
Physical
properties of PET:
PET can be
semi-rigid to rigid, depending on its thickness, and it is very lightweight. It
makes a good gas and fair moisture barrier, as well as a good barrier to alcohol (requires additional
"barrier" treatment) and solvents. It is
strong and impact-resistant.
It is naturally colorless with a high transparency.
Some of PET physical properties:
·
Molecular formula: (C10H8O4)n
·
Density amorphous: 1.370 g/cm3
·
Density
crystalline: 1.455 g/cm3
·
Young’s modulus: 2800-3100 MPa
·
Tensile strength (σt): 55-75 MPa
·
Elastic
limit: 50-150%
·
Notch test:
3.6 KJ/m2
·
Thermal
conductivity: 0.24 W/(m.K)
·
Linear
expansion coefficient (α): 7×10-5 /K
·
Water
absorption (ASTM): 0.16
·
Refractive
Index: 1.5750
Intrinsic viscosity
One of the most
important characteristics of PET is referred to as intrinsic viscosity (IV). The intrinsic viscosity of the
material, measured in deciliters per gram (dâ„“/g) is dependent upon the
length of its polymer chains. The longer the polymer chains, the more
entanglements between chains and therefore the higher the viscosity. The
average chain length of a particular batch of resin can be controlled during polycondensation.
The intrinsic viscosity range of PET
Fiber grade
0.40
– 0.70 dâ„“/g Textile
Film grade
Bottle grade
I.
Drying
PET is hygroscopic,
meaning that it absorbs water from its surroundings. However, when this 'damp'
PET is then heated, the water hydrolyzes the PET, decreasing its resilience.
Thus, before the resin can be processed in a molding machine, it must be dried.
Drying is achieved through the use of a desiccant or dryers before the PET is fed into
the processing equipment.
Inside the dryer,
hot dry air is pumped into the bottom of the hopper containing the resin so
that it flows up through the pellets, removing moisture on its way. The hot wet
air leaves the top of the hopper and is first run through an after-cooler,
because it is easier to remove moisture from cold air than hot air. The
resulting cool wet air is then passed through a desiccant bed. Finally the cool dry air leaving
the desiccant bed is re-heated in a process heater and sent back through the
same processes in a closed loop. Typically, residual moisture levels in the
resin must be less than 5 parts per million (parts of water per million parts
of resin, by weight) before processing. Dryer residence time should not be
shorter than about four hours. This is because drying the material in less than
4 hours would require a temperature above 160 °C, at which level hydrolysis would begin inside the pellets before
they could be dried out.
PET can also be
dried in compressed air resin dryers. Compressed air dryers do not reuse drying
air. Dry, heated compressed air is circulated through the PET pellets as in the
desiccant dryer, then released to the atmosphere.
II.
Copolymers
In addition to pure
(homopolymer)
PET, PET modified by copolymerization is also available.
In some cases, the
modified properties of copolymer are more desirable for a particular
application. For example, cyclohexane dimethanol (CHDM) can be added to the polymer
backbone in place of ethylene. Since this building block is much larger (6
additional carbon atoms) than the ethylene glycol unit it replaces, it does not
fit in with the neighboring chains the way an ethylene glycol unit would. This
interferes with crystallization and lowers the polymer's melting temperature.
Such PET is generally known as PETG (Eastman Chemical and SK Chemicals are the
only two manufacturers). PETG is a clear amorphous thermoplastic that can be injection
molded or sheet extruded. It can be colored during processing.
Replacing
terephthalic acid (right) with isophthalic acid (center) creates a kink in the
PET chain, interfering with crystallization and lowering the polymer's melting
point.
Another common
modifier is isophthalic
acid, replacing some of the 1,4-(para-) linked terephthalate units. The 1,2-(ortho-) or 1,3-(meta-) linkage produces an angle in the
chain, which also disturbs crystallinity.
Such copolymers are
advantageous for certain molding applications, such as thermoforming,
which is used for example to make tray or blister packaging from co-PET film,
or amorphous PET sheet (A-PET) or PETG sheet.
On the other hand, crystallization is important in other applications where
mechanical and dimensional stability are important, such as seat belts.
For PET bottles, the use of small amounts of isophthalic acid, CHDM, DEG or
other comonomers can be useful: if only small amounts of comonomers are used,
crystallization is slowed but not prevented entirely. As a result, bottles are
obtainable via stretch blow molding ("SBM"), which are both
clear and crystalline enough to be an adequate barrier to aromas and even
gases, such as carbon dioxide in carbonated beverages.
III.
Crystals
Crystallization of polymers occurs when polymer chains fold up on
themselves in a repeating, symmetrical pattern. Long polymer chains tend to
become entangled on themselves, which prevents full crystallization in all but
the most carefully controlled circumstances. PET is no exception to this rule;
About 60% crystallization is the upper limit for commercial products, with the
exception of polyester fibers. Besides, about 60% crystalline polymer about
40 % of the polymer chains remaining amorphous. Therefore PET is commonly
called a semi-crystalline polymer.
Like most materials, PET tends to produce spherulites containing many small crystallites when crystallized from an amorphous
solid, rather than forming one large single crystal. Light tends to scatter as
it crosses the boundaries between crystallites and the amorphous regions
between them. This scattering means that crystalline PET is opaque and white in
most cases. Fiber
drawing is among the few
industrial processes that produce a nearly single-crystal product.
IV.
Degradation
PET is subject to
various types of degradations during processing. The main degradations that can
occur are hydrolytic, thermal and, probably most important, thermal oxidation.
When PET degrades, several things happen: discoloration, chain scissions
resulting in reduced molecular weight, formation of acetaldehyde and cross-links ("gel" or
"fish-eye" formation). Discoloration is due to the formation of
various chromophoric systems following prolonged thermal treatment at elevated
temperatures. This becomes a problem when the optical requirements of the
polymer are very high, such as in packaging applications. The thermal and
thermooxidative degradation results in poor processibility characteristics and
performance of the material.
One way to
alleviate this is to use a copolymer.
Comonomers such as CHDM or isophthalic
acid lower the melting
temperature and reduce the degree of crystallinity of PET (especially important
when the material is used for bottle manufacturing). Thus the resin can be
plastically formed at lower temperatures and/or with lower force. This helps to
prevent degradation, reducing the acetaldehyde content of the finished product
to an acceptable (that is, unnoticeable) level. See copolymers,
above. Other ways to improve the stability of the polymer is by using
stabilizers, mainly antioxidants such as phosphites. Recently, molecular level
stabilization of the material using nanostructured chemicals has also been
considered.
Degradation may cause acetaldehyde:
Acetaldehyde is a colorless, volatile substance
with a fruity smell. Although it forms naturally in some fruit, it can cause an
off-taste in bottled
water. Acetaldehyde forms by degradation of PET through the mishandling of
the material. At high temperatures, (PET decomposes above 300 °C or 570 °F),
high pressures, extruder speeds (excessive shear flow raises temperature) and
long barrel residence times all contribute to the production of acetaldehyde.
When acetaldehyde is produced, some of it remains dissolved in the walls of a
container and then diffuses into the product stored inside,
altering the taste and aroma. This is not such a problem for non-consumables
(such as shampoo), for fruit juices (which already contain acetaldehyde), or
for strong-tasting drinks like soft drinks.
For bottled water, however, low acetaldehyde content is quite important,
because if nothing masks the aroma, even extremely low concentrations (10–20
parts per billion in the water) of acetaldehyde can produce an off-taste.
The
Advantages and Disadvantages of PET
a) The
advantages of PET:
PET
provides very good alcohol and essential oil barrier properties, generally good
chemical resistance (although acetones and ketones will attack PET) and a high
degree of impact resistance and tensile strength. The orienting process serves
to improve gas and moisture barrier properties and impact strength. This
material does not provide resistance to high temperature applications—maximum
temperature 160 °F (71 °C).
Generally PET offers numerous
advantages because it is easily moldable and thus allows production of
individual bottle shapes, specifically designed as non-returnable items.
Further advantages for the consumer are its stability, lightweight, inexpensive
and shatter-resistant. In addition bottles can be made completely from PET,
which simplifies recycling considerably.
Also recycled polyethylene terephthalate
(RPET) can be used to make many new products, including fiber for polyester
carpet; fabric for T-shirts, athletic shoes, luggage, upholstery and sweaters;
fiberfill for sleeping bags and winter coats; industrial strapping, sheet and
film; automotive parts such as luggage racks, fuse boxes, bumpers, grills and
door panels; and new PET containers for both food and non-food products.
b)
The disadvantages of
PET:
I.
Its influence on
beverages:
Gas permeability is a major
difficulty here, as it can lead to problems within the beverage. Because these
processes occur as diffusion, independent of pressure, even a carbonated
beverage takes up oxygen and at the same time releases carbon dioxide. The
intruding oxygen can damage beverage ingredients, in particular vitamins and
flavors.
In addition, PET can absorb flavor
components of the beverage. This is a result of the structure of the plastic.
The long polymer molecules are tangled within each other like a sponge. In this
"sponge", flavor components are stored and later released again. When
the crystallization level increases, this sponge structure becomes
"untangled" and less foreign material may be absorbed. This has made
the development of returnable PET bottles possible, despite the possibility of
flavor absorption. At the same time, crystallization of the material also
improves its resistance to heat, so that hot-fill PET bottles can also be
produced using the same technology.
However, these disadvantages mean
that the shelf life of a beverage in PET is usually shorter than in glass
bottles. But through appropriate recipe design, the manufacturer of flavor
systems can offset many of the disadvantages of PET.
II.
Toxicity
of PET:
Commentary
published in Environmental Health
Perspectives in April 2010 suggested that PET
might yield endocrine disruptors under conditions of common use and
recommended research on this topic. Proposed
mechanisms include leaching of phthalates as well as leaching of antimony.
·
Injection Molding
Injection molding is one of the most
popular processing operations in the plastics industry. In recent years, more
than half the processing machinery manufactured was injection-molding machines.
The equipment is basically designed to achieve the melting of the resin,
injecting the melt into a cavity mold, packing the material into the mold under
high pressure, cooling to obtain solid product, an8d ejecting the product for
subsequent finishing. It is different from extruders in that a mold is used
instead of a die, requiring a large force to pack the melt into the mold. A
machine is typically classified by the clamping force (which can vary from 1 to
10,000 tons!) and the shot size determined by the size of the article to be
manufactured. Other parameters include injection rate, injection pressure,
screw design, and the distance between tie bars.
The machine is generally made of a
hydraulic system, plasticating and injection system, mold system, and a
clamping system. The hydraulic system delivers the power for the operation of
the equipment, particularly to open and clamp down the heavy mold halves. The
injection system consists of a reciprocating screw in a heated barrel assembly
and an injection nozzle.
The system is designed to get resin from
the hopper, melt and heat to correct temperature, and deliver it into the mold
through the nozzle. Electrical heater bands placed at various points about the
barrel of the equipment allow close control of the melt temperature. The mold
system consists of platens and molding (cavity) plates typically made of
tool-grade steel. The mold shapes the plastic melt injected into the cavity (or
several cavities). Of the platens, the one attached to the barrel side of the machine
is connected to the other platen by the tie bars. A hydraulic knock-out system
using ejector pins is built into one of the platens to conveniently remove the
molded piece.
The machine operates in an
injection-molding cycle. The typical cycle sequence is, first, the empty mold
closes, and then the screw movement delivers an amount of melt through the
nozzle into it. Once the mold is full, the pressure is held to “pack” the melt
well into the mold. The mold is then cooled rapidly by a cooling medium
(typically water, steam, or oil) flowing through its walls, and finally the
mold opens to eject the product. It is common for this cycle to be closely
monitored and to be mostly automated by the use of sophisticated control
systems. Figure 2.3 shows a diagram of a simple injection molding
machine indicating the hydraulic, injection, and mold systems. The mold filling
(a), compaction (b), cooling (c), and ejection (d) steps are also illustrated
in Figure 2.3
Figure
2.3 Diagram of a simple injection-molding machine indicating the
hydraulic, injection, and mold systems
When a multicavity mold designed for
several “parts” is used, the ejected product is complex, consisting of runners,
a spruce, and flashing that needs to be removed (and recycled) to obtain the
plastic product. Figure shows a
molding with one of the product “parts” removed from it. [10]
Figure: Injection-molded piece.
·
Practices in Collection
and Recycling of PET
Collection
There are four basic ways in which
communities around the world offer recycling collection services for PET
plastic bottles and containers (in addition, to other recyclable materials) to
their residents. The first method is not up to individual communities but is
created as a result of statewide laws known as Returnable Container
Legislation, or “Bottle Bills.” Many states around the United States have
passed such legislation, which establishes a redemption value on carbonated
beverage (and, in some cases, non-carbonated beverage) containers. These
containers, when returned by the consumer for the redemption value, facilitate
recycling by aggregating large quantities of recyclable materials at beverage
retailers and wholesalers to be collected by recyclers, while simultaneously
providing the consumer with an economic incentive to return soft drink
containers for recycling.
The second, and most widely
accessible, collection method is curbside collection of recyclables. Curbside
recycling programs are generally the most convenient for community residents to
participate in and yield high recovery rates as a result. In the United States,
research conducted by the Center for Plastics Recycling Research at Rutgers
University estimates that curbside collection gathers 70%-90% of available
recyclables. In addition, estimates by the National Association for Plastic
Container Recovery (NAPCOR) indicate that approximately 55% of all the PET
plastic containers collected for recycling are generated through curbside
programs.
Communities that provide curbside
collection generally request residents to separate designated recyclables from
their household garbage and to place them into special receptacles or bags,
which are then set out at the curb for collection by municipal or
municipally-contracted crews. Some communities allow their residents to
commingle recyclables, that is, mix recyclable materials of different kinds
into the same receptacle. Others require some level of material segregation
known as “source separation.” For example, many curbside collection programs
require that newspapers and cardboard be bundled separately and placed
alongside the receptacle containing their commingled recyclable containers.
Some communities will collect recyclables on the same day as normal garbage
collection, while others have separate days for trash collection and collection
of recyclables.
The third collection method is known
as drop-off recycling. In this method, containers for designated
recyclable materials are placed at central collection locations throughout the
community, such as parking lots, churches, or other civic associations. The
containers are generally marked as to which recyclable material should be
placed in them. Residents are requested to deliver their recyclables to the
drop-off location, where recyclables are separated by material type into their
respective collection containers. Drop-off centers require much less investment
to establish than curbside programs, yet do not offer the convenience of
curbside collection. However, drop-off collection centers work well in rural
locations where curbside collection is impractical.
The last collection method employs the
use of buy-back centers. While communities do not provide this service per
se, as most buy-back recycling centers are operated by private companies, they
often provide incentives, through legislation or grants and loan programs,
which can assist in the establishment of buy-back centers for their residents.
Buy-back centers pay consumers for recyclable materials that are brought to
them. Most buy-back centers have purchasing specifications that require
consumers to source separate recyclable materials brought for sale, in addition
to other requirements they may have (for example, removal of caps from
bottles). These purchase specifications can greatly reduce contamination levels
and allow the buy-back center to immediately begin processing the recyclables
they purchase, while providing consumers with an economic incentive to comply
with the specifications.
Finally, many communities that offer
curbside recycling collection services will augment this service with drop-off
and buy-back centers where curbside is not as effective, such as near
multi-family housing units. While buy-back centers may not be as convenient as
curbside collection, they offer an economic incentive to the public that
curbside collection does not.
PET plastic wastes are also
collected by the following ways:
·
Private Collection:
This type of collection is done in restaurants, hotels, business
establishments, supermarkets and fast food chains.
·
Household Consumer: The
household consumers segregate and sell their plastic waste to eco-aids.
However, some of them dispose their commingled solid waste to garbage bins or
containers for pick- up by dump trucks or garbage collectors.
·
Junk Shops: There are
many junk shops collecting recyclable items and separate them. They buy from
scavengers and household consumers and sell their scrap to the recyclers/ processors.
PET bottles are sold after sorting and cleaning (removal of cover and label)
from the commingled waste.
·
Middleman: The
middleman or consolidators operates in the following ways:
a) Collects
and grinds PET industrial waste "on- site".
b) Collects and grinds PET industrial and post
consumer waste in their own plant.
c) Collects
PET industrial/ consumer waste and sell them to PET recyclers.
Sorting and grinding
After PET plastic containers are
collected they must be sorted and prepared for sale. Each subsequent step in
the recycling process adds value to the post-consumer PET and puts it into
marketable form for other processors and end-users that will use them to
manufacture new products. The amount and type of sorting and processing
required will depend upon purchaser
specifications and the extent to
which consumers separate recyclable materials of different types and remove
contaminants.
Collected PET plastic containers are
delivered to a materials recovery facility (MRF) or a plastics intermediate
processing facility (IPC) to begin the recycling process. The value of the
post-consumer PET plastic and its ability to be economically remanufactured
into new products is dependent on the quality of the material as it passes
through the recycling process.
MRFs accept commingled curbside
collected recyclables and separate them into their respective material
categories. PET plastic bottles and containers are separated from other
recyclables and baled for sale to IPCs, plastics recycling facilities (PRFs),
or reclaimers.
Unlike MRFs and IPCs, plastic
recycling facilities only accept plastic containers, either commingled or
source separated from other plastic containers. PRFs will generally accept
plastics in both loose and baled form. Very often, these materials are supplied
by drop-off and buy-back centers, which require source separation of recyclable
materials that are brought to them. Once again, PET plastic bottles and
containers are sorted from other plastic containers at PRFs and, in most cases,
further processed by color sorting and granulating PET for shipment to
reclaimers as “dirty” regrind. Some PRFs merely separate PET and other plastic
containers by resin category and bale them for shipment to reclaimers or end-users.
However, IPCs shall generally refer
to recycling facilities that take in loose; source separated plastic bottles
and densifies them for shipment to PRFs, reclaimers or end-users. And, PRFs
will be used to describe sorting, baling, and/or grinding facilities.
Cleaning and drying
Sorting and grinding alone are not
sufficient preparation of PET bottles and containers for remanufacturing. There
are many items that are physically attached to the PET bottle or containers
that require further processing for their removal. These items include the
plastic cups on the bottom of many carbonated beverage bottles (known as “base
cups”), labels and caps.
Dirty regrind from PRFs is then sent
to reclaimers that process post-consumer PET plastic into a form that can be
used by converters. Converters process the recycled PET plastic into a
commodity-grade form that can then be used by end-users to manufacture new
products. At a reclaiming facility, the dirty flake passes through a series of
sorting and cleaning stages to separate PET from other materials that may be
contained on the bottle or from contaminants that might be present. First,
regrind material is passed through an “air classifier” which removes materials
lighter than the PET such as plastic or paper labels and “fines” -- very small
PET particle fragments that are produced during granulating. The flakes are
then washed with a special detergent in a “scrubber.” This step removes food
residue that might remain on the inside surface of PET bottles and containers,
glue that is used to adhere labels to the PET containers, and any dirt that
might be present.
Next, the flakes pass through what
is known as a “float/sink” classifier. During this process, PET flakes, which are
heavier than water, sink in the classifier, while base cups made from high
density polyethylene plastic (HDPE) and caps and rings made from polypropylene
plastic (PP), both of which are lighter than water, float to the top. The
ability of the float/sink stage to yield pure PET flakes is dependent upon the
absence of any other plastics that might also be heavier than water and sink
with the PET. It should be noted that some reclaimers use a different device
known as a “hydro cyclone” to perform this same step. This device essentially
operates like a centrifuge and separates materials based on their weight
(density) differences. Following the float/sink stage the flakes are thoroughly
dried.
After they have dried, the PET
flakes pass through what is known as an electrostatic separator, which produces
a magnetic field to separate PET flakes from any aluminum that might be present
as a result of bottle caps and tennis ball can lids and rings. Some reclaimers
use a number of different particle separation technologies where PET flakes are
further processed to remove any residual contaminants that may still be
present, such as x-ray separation devices for PVC removal, or optical sorting
devices to remove other contaminants. The purity level to which PET flakes are
processed depends on the end-use applications for which they are intended.
Once all of these processing steps
have been completed, the PET plastic is now in a form known as “clean flake.”
In some cases reclaimers will further process clean flake in a “repalletizing”
stage, which turns the flake into “pellet.” Clean PET flake or pellet is then
processed by reclaimers or converters which transform the flake or pellet into
a commodity-grade raw material form such as fiber, sheet, or engineered or
compounded pellet, which is finally sold to end-users to manufacture new
products.
End-use categories recycled PET:
There
are five major generic end-use categories for recycled PET plastic:
1)
Packaging applications
(such as new bottles).
2)
Sheet and film
applications (including some thermoforming applications, such as laundry
scoops).
3)
Strapping.
4)
Engineered resins
applications (such as reinforced components for automobiles).
5)
Fiber applications
(such as carpets, fabrics and fiberfill).
There are a number of emerging
technologies that are generically referred to as depolymerization processes.
These processes -- like glycolysis and methanolysis -- break down the PET
plastic into its individual chemical components, which can then be recombined
back into PET plastic. While not used extensively, these technologies are
employed when the economics warrant and offer yet another market opportunity
for post-consumer PET plastic containers.
One of the highest value end-uses
for recycled PET plastic is to manufacture new PET bottles and containers.
Recycled PET can be made into numerous other products including: Belts,
blankets, boat hulls, business cards, caps, car parts (bumpers, distributor
caps, and exterior panels), carpets, egg cartons, furniture, insulation,
landfill liners, overhead transparencies, paint brush bristles, pillows,
polyester fabric for (upholstery, T-shirts, sweaters, backpacks, athletic wear
and shoes), recycling bins, sails, scouring pads, strapping, stuffing for ski jackets,
cushions, mattresses, sleeping bags and quilts, tennis ball cans, tennis ball
felt, twine and welcome mats.
Designing Community for PET Recycling
Collection Program
Properly designed PET
recycling collection programs greatly increase the quantity and quality of PET
collected and can reduce overall recycling system costs. In order to maximize
the recovery and value of PET plastic containers our community recycling
collection program. Two best practices should be followed when designing program.
The first is to establish an effective and ongoing consumer education program.
The second best practice is to designate all PET bottles with
screw-neck tops as acceptable for recycling.
There are seven basic messages that
should be included in any consumer education or promotional program aimed at
the collection of PET bottles:
1) Only
PET bottles with screw-neck tops to be placed for collection or brought to a
collection location. PET can be identified by looking for the #1 code, any
non-bottle PET should be excluded.
2) Only
PET bottles that are clear or transparent green should be included for
recycling, other colors to be excluded.
3) Consumers
should remove lips, caps and other closures from PET bottles placed for
recycling.
4) All
PET bottles that are set out for recycling should be completely free of
contents and rinsed clean.
5) Consumers
should flatten PET bottles prior to setting out for collection.
6) Consumers
should never place any material other than the original content into PET
bottles for recycling.
Hypodermic needles are increasing safety
concern at recycling facilitiesneatnes
Basic Benefits of PET Recycling
The greenhouse gas emissions from
management of selected materials in municipal solid waste’s report issued by
the EPA, states: “recycling of PET and other materials positively infects the
environment by helping to reduce green house gas emissions and global warming”.
[18]
Recycling of PET plastic waste can
help reduce waste disposal costs (since the PET Plastic is removed from the
waste stream) and it can generate revenues from the sale of the recycled PET,
it can also reduce labor costs associated with the handling of PET during the
waste disposal process, further more the recycling of PET can help with
streamlining overall waste processing operations, it can help free up space
(used for the temporary storage of PET), and also it can help improve workplace
safety and neatness..
Hazards at plastics
recycling facilities can be divided into three general categories:
1)
Health and hygiene
hazards (noise, dust, climate, EMFs - electromagnetic frequencies).
2)
Safety hazards (vehicle
and machine hazards)
3)
Ergonomic hazards
(fatigue and musculoskeletal).
Compliance with worker safety
regulations and proper system design and maintenance are the best practices to
be followed to minimize the incidence of workplace hazards. In addition,
reducing fatigue through proper ergonomic design can increase worker
productivity and improve material quality at PET processing facilities.
While a discussion of regulatory
compliance for all OSHA regulations is beyond the scope of this document, there
are a number of major safety issues and safety best practices at PET recycling
facilities that should be discussed. Once again, it is the responsibility of
the facility operator to ensure that all safety regulations that apply to their
specific operations are complied with at all levels of government. And, the
Best Practices presented below are not intended as a comprehensive listing for
regulatory compliance.
General Safety Best
Practices:.
·
Make sure all
conveyors, balers, grinders, and other processing equipment are equipped with
emergency power-cut-off switches (often referred to as “kill” switches) and
machine guards. This will allow plant personnel to react to safety hazard or
emergency situations or to ensure worker safety during normal equipment
operation and when performing equipment repair or maintenance.
·
Make sure all grinders
and regrind evacuation systems (blowers) are insulated or enclosed in a
separate room to maintain noise levels below the OSHA regulated noise exposure
level for workers.
·
Make sure that all
cyclone discharges from grinders are properly exhausted into baghouse or other
dust collection systems, or are otherwise properly filtered in compliance with
regulatory requirements, to maintain ambient dust levels within OSHA
guidelines.
·
Ensure that all
equipment is properly maintained for safe and efficient operation through the
implementation of a regular and preventative maintenance schedule for all
equipment within the facility.
·
Provide adequate space
for vehicle and worker activities.
·
Ensure that adequate
lighting is provided for general plant visibility and to prevent worker eye
fatigue.
·
Ensure that adequate
fire protection equipment is in place based on the nature of the materials
being processed and stored, the types of equipment being used, local fire codes
and insurance requirements.
·
Make sure that proper
signage is maintained throughout the facility.
·
If the facility is
equipped with sorting systems that use electromagnetic frequencies (EMFs), like
x-rays or ultraviolet light, as the detection signal, make sure that the
equipment is properly shielded to eliminate worker exposure to EMFs.
·
Ensure that only
trained personnel operate specific equipment and that designated operators have
any required operating certificates or licenses for that type of equipment.
·
Provide adequate
disposal containers that are in regulatory compliance for the disposal of oily,
hazardous, or combustible wastes.
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