A plastic material is any of a wide range of
synthetic or semi-synthetic organic solids
that are mold able.
Plastics are typically organic polymers of high molecular mass,
but they often contain other substances. They are usually synthetic, most
commonly derived from petrochemicals,
but many are partially natural.
Composition
Almost invariably, organic polymers mainly
comprise plastics. The vast majority of these polymers are based on chains of carbon atoms alone or with oxygen, sulfur, or nitrogen as well. 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).
The structure of these "side chains" influence the properties of the
polymer. This fine tuning of the properties of the polymer by repeating unit's
molecular structure has allowed plastics to become an indispensable part of the
twenty-first century world.
Additives
Most plastics contain other organic or inorganic compounds blended in. The amount of additives
ranges from zero percentage for polymers used to wrap foods to more than 50%
for certain electronic applications. The average content of additives is 20% by
weight of the polymer. Fillers improve performance and/or reduce production
costs. Stabilizing additives include fire retardants to lower the flammability of the
material. Many plastics contain fillers, relatively inert and inexpensive
materials that make the product cheaper by weight. Typically fillers are
mineral in origin, e.g., chalk. Some fillers are
more chemically active and are called reinforcing agents. Since many organic
polymers are too rigid for particular applications, they are blended with plasticizers,
oily compounds that confer improved rheology.
Colourants are of course common additives, although their weight contribution
is small. Many of the controversies associated with plastics are associated
with the additives.
Classification
Plastics are usually classified by their chemical structure of the polymer's backbone and side chains.
Some important groups in these classifications are the acrylics, polyesters, silicones, polyurethanes,
and halogenated
plastics. Plastics can also be classified by the chemical process
used in their synthesis, such as condensation, polyaddition,
and cross-linking.
Thermoplastics and
thermosetting polymers
There are two types of plastics: thermoplastics and thermosetting polymers. Thermoplastics are the
plastics that do not undergo chemical change in their composition when heated
and can be moulded again and again. Examples include polyethylene, polypropylene, polystyrene,polyvinyl chloride, and polytetrafluoroethylene (PTFE). 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 as repeat units, derived frommonomers;
each polymer chain will have several thousand repeating units.
Thermosets can melt and take shape once;
after they have solidified, they stay solid. In the thermosetting process, a
chemical reaction occurs that is irreversible. The vulcanization of rubber is a
thermosetting process. Before heating with sulfur, the polyisoprene is a tacky,
slightly runny material, but after vulcanization the product is rigid and
non-tacky.
Other
classifications
Other classifications are based on qualities
that are relevant for manufacturing or product design.
Examples of such classes are the thermoplastic and thermoset, elastomer, structural, biodegradable, and electrically conductive. Plastics can also be
classified by variousphysical properties, such as density, tensile strength, glass transition temperature, and
resistance to various chemical products.
Biodegradability
Main
article: Biodegradable plastic
Biodegradable plastics break down (degrade) upon
exposure to sunlight (e.g., ultra-violet radiation), water or dampness,
bacteria, enzymes, wind abrasion, and in some instances rodent pest or insect
attack are also included as forms of biodegradation or environmental degradation. Some modes of
degradation require that the plastic be exposed at the surface, whereas 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.
Natural vs synthetic
Main
article: Bioplastic
Most plastics are produced from petrochemicals.
Motivated by the finiteness of petrochemical reserves and possibility of global warming,
bioplastics are being developed. Bioplastics are made substantially from
renewable plant materials such as cellulose and starch.
In comparison to the global consumption of
all flexible packaging, estimated at 12.3 million tonnes, estimates put global
production capacity at 327,000 tonnes for related bio-derived materials.
Crystalline vs
amorphous
Some plastics are partially crystalline and partially amorphous in molecular structure, giving them both a melting point (the temperature at which the
attractive intermolecular forces are overcome) and one or more glass
transitions (temperatures above which the extent of localized molecular
flexibility is substantially increased). The so-called semi-crystalline plastics include polyethylene,
polypropylene, poly (vinyl chloride), polyamides (nylons), polyesters and some
polyurethanes. Many plastics are completely amorphous, such as polystyrene and
its copolymers, poly (methyl methacrylate), and all thermosets.
History
Early plastics were bio-derived materials
such as egg and blood proteins, which are organic polymers. Treated cattle
horns were used as windows for lanterns in the Middle Ages. Materials that
mimicked the properties of horns were developed by treating milk-proteins (casein) with lye.
In the 1800s the development of plastics accelerated with Charles Goodyear's
discovery of vulcanization as a route to thermoset materials derived from
natural rubber. Many storied materials were reported as industrial chemistry
was developed in the 1800s. In the early 1900s, Bakelite,
the first fully synthetic thermoset was reported by Belgian chemist Leo Baekeland.
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 polymers were
polystyrene (PS) and polyvinyl chloride (PVC). The development of plastics has
come from the use of natural plastic materials (e.g., chewing gum, shellac)
to the use of chemically modified natural materials (e.g., rubber,nitrocellulose, collagen, galalite)
and finally to completely synthetic molecules (e.g., bakelite, epoxy, polyvinyl
chloride).
Parkesine
The plastic material, parkesine,
was patented by Alexander Parkes,
In Birmingham, UK in 1856. It was unveiled at the 1862 Great
International Exhibition in London. Parkesine won a bronze medal at the 1862 World's fair in London. Parkesine
was made from cellulose (the major component of plant cell walls) treated with nitric acid as a solvent. The output of the
process (commonly known as cellulose nitrate or pyroxilin) could be dissolved
in alcohol and hardened into a transparent and
elastic material that could be molded when heated. By incorporating pigments into the
product, it could be made to resemble ivory.
Bakelite
Main
article: 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-born
American living in New York state.
Baekeland was looking for an insulating shellac to coat wires in electric
motors and generators. He found that combining phenol (C6H5OH)
and formaldehyde (HCOH) formed a sticky mass and later found that the material
could be mixed with wood flour, asbestos, or slate dust to create strong and
fire resistant "composite" materials. The new material tended to foam
during synthesis, requiring that Baekeland build pressure vessels to force out
the bubbles and provide a smooth, uniform product, as announced his in 1909, in
a meeting of the American Chemical Society.Bakelite
was originally used for electrical and mechanical parts, coming into widespread
use in consumer goods in the 1920s. Bakelite was a purely synthetic material,
not derived from living matter. It was also an early thermosetting plastic
Representative
polymers
Polystyrene
Plastic piping and firestops being installed in Ontario.
Certain plastic pipes can be used in some non-combustible buildings, provided
they are firestopped properly and that the flame spread ratings comply with the
localbuilding code.
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
Polyvinyl chloride (PVC, commonly called
"vinyl")[13]incorporates
chlorine atoms. The C-Cl bonds in the backbone are hydrophobic and resist
oxidation (and burning). PVC is stiff, strong, heat and weather resistant,
properties that
Styrene polymerization
recommend its use in devices for 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.
Styrene polymerization
recommend its use in devices for 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
Vinylchloride polymerization
occurs.[14] Because hydrogen chloride readily combines with water vapor in the air to form hydrochloric acid,[15] polyvinyl chloride is not recommended for long-term archival storage of silver, photographic film or paper (mylar is preferable).[16]
occurs.[14] Because hydrogen chloride readily combines with water vapor in the air to form hydrochloric acid,[15] polyvinyl chloride is not recommended for long-term archival storage of silver, photographic film or paper (mylar is preferable).[16]
Nylon
Main
article: Nylon
The plastics industry was revolutionized in
the 1930s with the announcement of 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 fortoothbrushes.
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.
General condensation polymerization reaction for nylon
It took DuPont twelve years and US$27 million
to refine nylon, and to synthesize and develop the industrial processes for
bulk manufacture. With such a major investment, it was no surprise that Du Pont
spared little expense to promote nylon after its introduction, creating a
public sensation, or "nylon mania".
Nylon mania came to an abrupt stop at the end
of 1941 when the USA entered World War II.
The production capacity that had been built up to produce nylon stockings,
or just nylons, for
American women was taken over to manufacture vast numbers of parachutes for
fliers and paratroopers. After the war ended, DuPont went back to selling nylon
to the public, engaging in another promotional campaign in 1946 that resulted
in an even bigger craze, triggering the so called nylon riots.
Subsequently polyamides 6, 10, 11, and 12
have been developed based on monomers which are ring compounds; e.g. caprolactam.
Nylon 66 is a material manufactured by condensation polymerization.
Nylons still remain important plastics, and
not just for use in fabrics. In its bulk form it is very wear resistant,
particularly if oil-impregnated, and so is used to build gears, plain bearings,
and because of good heat-resistance, increasingly for under-the-hood
applications in cars, and other mechanical parts.
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 was 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.
Synthetic rubber
Main
article: 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, Caltechresearchers
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.
Properties of plastics
The properties of plastics is defined chiefly
by the organic chemistry of the polymer. such as hardness, density, and resistance
to heat, organic solvents, oxidation,
and ionizing radiation. In particular, most plastics
will melt upon heating to a few hundred degrees celsius.[18]While
plastics can be made electrically conductive, with the conductivity of up to 80
kS/cm in stretch-oriented polyacetylene, they are still no match for most
metals like copper which have conductivities of several
hundreds kS/cm.
Toxicity
Due to their insolubility in water and
relative chemical inertness, pure plastics generally have low toxicity. Some
plastic products contain a variety of additives, some of which can be toxic.
For example, plasticizers like adipates and phthalates are often added to brittle plastics
like polyvinyl chloride to make them pliable enough for use in food packaging, toys, and many other
items. Traces of these compounds can leach out of the product. Owing to
concerns over the effects of such leachates, the European Union has restricted the use of DEHP (di-2-ethylhexyl phthalate)and other
phthalates in some applications. Some compounds leaching from polystyrene food
containers have been proposed to interfere with hormone functions and are
suspected human carcinogens.
Whereas the finished plastic may be
non-toxic, the monomers used in the manufacture of the parent polymers may be
toxic. In some cases, small amounts of those chemicals can remain trapped in
the product unless suitable processing is employed. For example, the World Health Organization's International Agency for Research on
Cancer (IARC) has
recognized that vinyl chloride,
the precursor to PVC, as a humancarcinogen.
BPA controversy
Some polymers may also decompose into the
monomers or other toxic substances when heated. In 2011, it was reported that
"almost all plastic products" sampled released chemicals with
estrogenic activity, although the researchers identified plastics which did not
leach chemicals with estrogenic activity.
The primary building block of polycarbonates, bisphenol A (BPA), is an estrogen-like endocrine disruptor that may leach into food.Research
in Environmental Health Perspectives finds that BPA leached from the lining
of tin cans, dental sealants and polycarbonate bottles can increase
body weight of lab animals' offspring. A more recent animal study suggests
that even low-level exposure to BPA results in insulin resistance, which can
lead to inflammation and heart disease.
As of January 2010, the LA Times newspaper
reports that the United States FDA is spending $30 million to investigate
indications of BPA being linked to cancer.
Bis(2-ethylhexyl) adipate, present in plastic wrap based on PVC, is also of concern, as
are the volatile organic compounds present in new car smell.
The European Union has a permanent ban on the
use of phthalates in toys. In 2009, the United States
government banned certain types of phthalates commonly used in plastic.
Environmental issues
Further
information: Marine debris and Great Pacific Garbage Patch
Plastics are durable and degrade very
slowly; the chemical 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.Perhaps
the biggest environmental threat from plastic comes from nurdles, which are the raw material from which
all plastics are made. They are tiny pre-plastic pellets that kill large
numbers of fish and birds that mistake them for food.
Prior to the ban on the use of CFCs in
extrusion of polystyrene (and general use, except in life-critical fire
suppression systems; seeMontreal Protocol), the production of polystyrene
contributed to the depletion of the ozone layer;
however, non-CFCs are currently used in the extrusion process.
Incineration of
plastics
Plastic can be converted as a fuel since they
are usually hydrocarbon-based and can be broken down into liquid hydrocarbon.
One kilogram of waste plastic produces a liter of hydrocarbon. In some cases, burning plastic can
release toxic fumes. Burning the plastic polyvinyl
chloride (PVC) may create dioxin.
Recycling
Thermoplastics can be remelted and reused,
and thermoset plastics can be ground up and used as filler, although 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.
The greatest challenge to the recycling of
plastics is the difficulty of automating the sorting of plastic wastes, making
it labor intensive.
Typically, workers sort the plastic by looking at the resin identification
code, although common containers like soda bottles can be sorted from memory.
Typically, the caps for PETE bottles are made from a different kind of plastic
which is not recyclable, which presents additional problems to the automated
sorting process. Other recyclable materials such as metals are easier to
process mechanically. However, new processes of mechanical sorting are being
developed to increase capacity and efficiency of plastic recycling.
While containers are usually made from a
single type and color of plastic, making them relatively easy to be sorted, a
consumer product like a cellular phone may have many small parts consisting of
over a dozen different types and colors of plastics. In such cases, the
resources it would take to separate the plastics far exceed their value and the
item is discarded. However, developments are taking place in the field ofactive disassembly, which may result in more
consumer product components being re-used or recycled. Recycling certain types
of plastics can be unprofitable, as well. For example, polystyrene is rarely
recycled because it is usually not cost effective. These unrecycled wastes are
typically disposed of in landfills, incinerated or used to produce electricity at waste-to-energy plants.
In 1988, 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"
Plastics type marks:
the resin identification code
1.
PET (PETE), polyethylene terephthalate
2.
HDPE, high-density polyethylene
3.
PVC, polyvinyl chloride
4.
LDPE, low-density polyethylene,
5.
PP, polypropylene
6.
PS, polystyrene
7.
Other types of plastics
Common plastics and uses
§ Polyethylene terephthalate (PET) – Carbonated drinks bottles,
peanut butter jars, plastic film, microwavable packaging.
§ Polyethylene (PE) – Wide range of inexpensive uses
including supermarket bags, plastic bottles.
§ High-density polyethylene (HDPE) – Detergent bottles, milk jugs,
and molded plastic cases.
§ Polyvinyl chloride (PVC) – Plumbing pipes and guttering,
shower curtains, window frames, flooring.
§ Polyvinylidene chloride (PVDC) (Saran)
– Food packaging.
§ Low-density polyethylene (LDPE) – Outdoor
furniture, siding, floor tiles, shower curtains, clamshell
packaging.
§ Polypropylene (PP) – Bottle caps, drinking straws,
yogurt containers, appliances, car fenders (bumpers), plastic pressure pipe systems.
A chair made with a polypropylene seat
A chair made with a polypropylene seat
§ Polystyrene (PS) – Packaging
foam/"peanuts", food containers, plastic tableware, disposable cups,
plates, cutlery, CD and cassette boxes.
§ High impact polystyrene (HIPS) -: Refrigerator liners, food
packaging, vending cups.
§ Polyamides (PA) (Nylons) – Fibers,
toothbrush bristles, fishing line,
under-the-hood car engine moldings.
§ Acrylonitrile butadiene styrene (ABS) – Electronic equipment cases
(e.g., computer monitors, printers, keyboards), drainage pipe.
§ Polycarbonate (PC) – Compact discs, eyeglasses, riot shields,
security windows, traffic lights, lenses.
§ Polycarbonate/Acrylonitrile Butadiene
Styrene (PC/ABS) – A blend of PC and ABS that creates a stronger plastic. Used
in car interior and exterior parts, and mobile phone bodies.
§ Polyurethanes (PU) – Cushioning foams, thermal
insulation foams, surface coatings, printing rollers (Currently 6th or 7th most
commonly used plastic material, for instance the most commonly used plastic
found in cars).
Special purpose plastics
§
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 a
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 implantapplications, 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
commercialacrylic 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.
§
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.
Material properties of some thermoplastics
Properties of some thermoplastic
materials
|
|||||||||
name
|
Symbol
|
Density
[g/cm3] |
Tensile strength
[MPa] |
Flexural strength
[MPa] |
Elastic modulus
[GPa] |
Elongation at rupture
[%] |
Thermal stability
[°C] |
Expansion at 20°C
[10−6/°C] |
|
High DensityPolyethylene
|
HDPE
|
0.95
|
31
|
40
|
1.86
|
100
|
120
|
126
|
|
Low DensityPolyethylene
|
LDPE
|
0.92
|
17
|
14
|
0.29
|
500
|
90
|
160
|
|
PVC
|
1.44
|
47
|
91
|
3.32
|
60
|
80
|
75
|
||
PP
|
0.91
|
37
|
49
|
1.36
|
350
|
150
|
90
|
||
PET
|
1.35
|
61
|
105
|
1.35
|
170
|
120
|
70
|
||
PMMA
|
1.19
|
61
|
103
|
2.77
|
4
|
100
|
65
|
||
PC
|
1.2
|
68
|
95
|
2.3
|
130
|
120
|
66
|
||
ABS
|
1.05
|
45
|
70
|
2.45
|
33
|
70
|
90
|
||
Nylon 6
|
1.13
|
60
|
91
|
2.95
|
60
|
110
|
66
|
||
PI
|
1.38
|
96
|
143
|
3.1
|
7
|
380
|
43
|
||
PSF
|
1.25
|
68
|
115
|
2.61
|
75
|
160
|
56
|
||
Polyamide-imide,
electrical grade
|
PAI
|
1.41
|
138
|
193
|
4.1
|
12
|
260
|
30
|
|
Polyamide-imide,
bearing grade
|
PAI
|
1.46
|
103
|
159
|
5.5
|
6
|
260
|
25
|
|
PTFE
|
2.17
|
24
|
33
|
0.49
|
300
|
260
|
95
|
||
1.27
|
105
|
151
|
2.9
|
60
|
210
|
31
|
|||
PEEK
|
1.32
|
100
|
3.6
|
50
|
343
|
||||
Polyaryletherketone(strong)
|
PEAK
|
1.46
|
136
|
213
|
12.4
|
2.1
|
267
|
||
Polyaryletherketone(tough)
|
PEAK
|
1.29
|
87
|
124
|
3
|
40
|
190
|
||
Self-reinforcedpolyphenylene
|
SRP
|
1.19
|
152
|
234
|
5.52
|
10
|
151
|
||
PAI
|
1.42
|
152
|
241
|
4.9
|
15
|
278
| |||
NOTE: Bulk properties of pure cast or hot formed materials. Properties could change considerably by mechanical treatment and cold forming. Fiber and foils are not considered
