Download Polymers - chemical engineering 2012

Engineering Materials
A polymer is a large
repeating structural units.
molecule
composed
of
The word polymer is derived from the Greek words polymeaning "many"; and - meros meaning "part".
Plastic and rubber materials are common polymers. Many of
them are organic compounds that are chemically based on
carbon, hydrogen, and other nonmetallic elements (O,N, and
Si). Some of the common and familiar polymers are
polyethylene (PE), nylon, poly vinyl chloride (PVC),
polycarbonate (PC), polystyrene (PS), and silicone rubber.
Properties
•These materials typically have low densities.
•They are not as stiff nor as strong as these other material types.
•Polymers are extremely ductile and pliable (i.e., plastic), which
means
they are easily formed into complex shapes.
•They are relatively inert chemically and un reactive in a large number
of environments. One major drawback to the polymers is their
tendency to soften and/or decompose at modest temperatures, which,
in some instances, limits their use.
•They have low electrical conductivities and are nonmagnetic.
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Non-resistant to heat or temperature
Sensitive to sunlight (radiations)
Environmental hazards (Nonbiodegradability)
Health hazards

Plasticizers used are mostly:
 Carbamates
 Pthalates
 Both of them are carcinogenics and hormones’
disrupters (e.g. estrogen hormone)
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Non-biodegradable: Not decomposed by
bacteria as other substances are decayed in
nature.
Biodegradable plastics are being developed
e.g. PHBV (Polyhydroxy Butyrate Valarate)
 Glucose
PHBV
Bacteria
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Some bacteria are cultured which can
decompose polymers like Nylon.
Flavo Bacteria and Pseudomonas are two
microbes that secrete Nylolase enzyme nd
are called Nylon Eaters, because they can
decompose nylon.
Similar are Oxo-biodegradable Plastics.
•Naturally
occurring polymers—those derived
from plants and animals—have been used for
many centuries; these materials include wood,
rubber, cotton, wool, leather, and silk.
•Other natural polymers such as proteins,
enzymes, starches, and cellulose are important
in biological and physiological processes in
plants and animals.
•It
is the process by which monomers are linked together to
generate long chains composed of repeat units.
•Two
general classifications according to reaction
mechanisms are
•Addition Polymerization
•Condensation Polymerization
Addition Polymerization (Chain Reaction)
•It is a process by which monomer units are attached one at a
time in chain-like fashion to form a linear macromolecule.
•Three distinct stages initiation, propagation, and
termination are involved in addition polymerization.
•During the initiation step, an active center capable of
propagation is formed by a reaction between an initiator
species and the monomer unit. Again, R represents the active
initiator, and is an unpaired electron.
•Propagation involves the linear growth of the polymer chain
by the sequential addition of monomer units to this active
growing chain molecule
•Propagation
may end or terminate in different ways. First,
the active ends of two propagating chains may link together
to form one molecule
•The other termination possibility involves two growing
molecules that react to form two “dead chains” thus
terminating the growth of each chain.
Condensation (or step reaction) polymerization
•It
is the formation of polymers by stepwise
intermolecular chemical reactions that may involve
more than one monomer species.
•There is usually a small molecular weight by product
such as water that is eliminated (or condensed). No
reactant species has the chemical formula of the
repeat unit, and the intermolecular reaction occurs
every time a repeat unit is formed. For example,
consider the formation of the polyester,
poly(ethylene terephthalate) (PET), from the reaction
between ethylene glycol and terephthalic acid;
Thermo plastics (thermo softening plastic)
•Thermoplastics
soften when heated (and
eventually liquefy) and harden when cooled—
processes that are totally reversible and may be
repeated.
•Thermoplastics are relatively soft.
•Examples of common thermoplastic polymers
include
polyethylene,
polystyrene,
poly(ethylene terephthalate), and poly(vinyl
chloride). e. g. luncheon box
Above its glass transition temperature, Tg, and below its
melting point, Tm, the physical properties of a
thermoplastic change drastically without an associated
phase change.
 Within this temperature range, most thermoplastics are
rubbery due to alternating rigid crystalline and elastic
amorphous regions, approximating random coils.
 Amorphous and semi-amorphous plastics are less
resistant to chemical attack and environmental stress
cracking because they lack a crystalline structure.
 Brittleness can be lowered with the addition of plasticizers
which interfere with crystallization to effectively lower Tg.

Thermosetting
They become permanently hard during their formation, and do not soften
upon heating. Thermoset polymers are generally harder and stronger than
thermoplastics and have better dimensional stability. e.g. casing for
domestic light switch.
•A thermosetting plastic, also known as a thermoset, is polymer material that
irreversibly cures. The cure may be done through heat (generally above 200 °C
(392 °F)), through a chemical reaction (two-part epoxy, for example), or
irradiation such as electron beam processing.
•Thermoset materials are usually liquid or malleable prior to curing and designed
to be molded into their final form, or used as adhesives. Others are solids like that
of the molding compound used in semiconductors and integrated circuits (IC).
Once hardened a thermoset resin cannot be reheated and melted back to a liquid
form.
•According to IUPAC recommendation: A thermosetting polymer is a prepolymer
in a soft solid or viscous state that changes irreversibly into an infusible, insoluble
polymer network by curing. Curing can be induced by the action of heat or
suitable radiation, or both. A cured thermosetting polymer is called a thermoset.
•
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Curing is a term in polymer chemistry and
process engineering that refers to the
toughening or hardening of a polymer
material by cross-linking of polymer chains,
brought about by chemical additives,
ultraviolet radiation, electron beam or heat.
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They are materials that have low elastic moduli which show
great extensibility and flexibility when stressed but which returns
to their original dimensions, or almost so, when deformation
stress is removed. In essence, the molecules act like a series of
small helical spring. e.g. disposable surgical gloves.
The term, which is derived from elastic polymer, is often used
interchangeably with the term rubber. Each of the monomers
which link to form the polymer is usually made of carbon,
hydrogen, oxygen and/or silicon.
Elastomers are amorphous polymers existing above their glass
transition temperature, so that considerable segmental motion
is possible.
At ambient temperatures, rubbers are thus relatively soft
(E~3MPa) and deformable. Their primary uses are for seals,
adhesives and molded flexible parts.
Polyethylene (PE)
•Polyethylene (abbreviated PE) or polythene is the
most common plastic. The annual production is
approximately 80 million metric tons. Its primary use
is within packaging (plastic bag, plastic films).
•It is made by polymerization of ethene (ethylene).it
has excellent chemical resistance, that it is not
attacked by strong acids or strong bases, tough and
flexible over wide range of temp. Polyethylene burns
slowly with a blue flame having a yellow tip and gives
off an odour of paraffin. The material continues
burning on removal of the flame source.
•Polyethylene
terephthalate commonly abbreviated PET, OR
PETE is a thermoplastic polymer resin and is used in synthetic
fibre, beverage, food and other liquid containers.
•Its
monomer
can
be
synthesized
by
the
esterification
reaction
between
terephthalic
acid and ethylene glycol with water as a byproduct.
•PET consists of polymerized units of the monomer ethylene
terephthalate. PET in its natural state is a colorless, semicrystalline resin.
•Based on how it is processed, PET can be semi-rigid to rigid,
and very lightweight, good gas and fair moisture barrier, as
well as a good barrier to alcohol and solvents. It is strong
and impact-resistant.
•Polyvinyl
chloride, commonly abbreviated PVC, is
the third-most widely-produced plastic.
•PVC is widely used in construction because it is
durable, cheap, and easily worked. It can be made
softer and more flexible by the addition
of plasticizers.
•Pure polyvinyl chloride without any plasticizer is a
white, brittle solid. It is insoluble in alcohol.
•Polyvinyl chloride is produced by polymerization of
the monomer vinyl chloride. Used in production of
pipes for industrial and municipal applications. PVC is
commonly used as the insulation on electrical cables
Plasticizers: Added to decrease tensile strength of
PVC, e.g. Phthalates
Heat Stabilizers: To prevent thermal degradation
during processing and help to extend the life of
finished product. e.g. Sn, Pb, Ca, Zn.
Lubricants: Aid the melt flow of PVC compound
during processing. e.g. Waxes,
Fillers: They are mainly added to lower the cost of
PVC. About 80% of all the fillers used in PVC is
calcium carbonate. Titanium oxide is second.
•It
is a polymer containing monomers of amides.
They can occur both naturally and artificially,
examples being proteins, such as wool and silk, and
can be made artificially through step-growth
polymerization examples being nylons. Polyamides
are commonly used in textiles, automotives, carpet
and sportswear due to their extreme durability and
strength.
•The amide link is produced from the condensation
reaction of an amino group and a carboxylic
acid or acid chloride group. A small molecule,
usually water, or hydrogen chloride, is eliminated.
Structure of polyamide
•Polyester
is a category of polymers which contain
the ester functional group in their main chain.
Although there are many polyesters, the term
"polyester" as a specific material most commonly
refers to polyethylene terephthalate (PET).
Fabric balls knitted from polyester thread or yarn are
used extensively in apparel and home furnishings,
from shirts and pants to jackets and hats, bed sheets,
blankets, upholstered furniture and computer mouse
mats. Industrial polyester fibers, yarns and ropes are
used fabrics for conveyor belts, safety belts, coated
fabrics etc.
•Polyester
fiber is used as cushioning and insulating
material in pillows, comforters and upholstery
padding.
•The majority of the world's PET production is for
synthetic fibers (in excess of 60%)
•Polyester is a synthetic polymer made of purified
terephthalic
acid
(PTA),
dimethyl
terephthalate
(DMT)
and
monoethylene
glycol (MEG).
•Many
times, however, it is necessary to modify the
mechanical, chemical, and physical properties to a much
greater degree than is possible by the simple alteration of
molecular structure.
•Additives
Foreign substances are intentionally introduced to enhance
or modify many of these properties, and thus make a
polymer more serviceable.
•Fillers
Filler materials are most often added to polymers to improve
tensile and compressive strengths, abrasion resistance,
toughness, dimensional and thermal stability, and other
properties.
Materials used as particulate fillers include wood flour, silica
flour and sand, glass, clay, talc, limestone.
•Plasticizers
The flexibility, ductility, and toughness of polymers may be
improved with the help of plasticizers. Their presence also
produces reductions in hardness and stiffness. Plasticizers
are generally liquids having low vapor pressures and low
molecular weights.
•Heat Stabilizers
Additives that counteract deteriorative processes are called
stabilizers. One common form of deterioration results from
exposure to light. Another important type of deterioration is
oxidation. Sn, pb, ca,and zn.
•Colorants
Colorants impart a specific color to a polymer; they
may be added in the form of dyes or pigments.
Pigments are filler materials that do not dissolve, but
remain as a separate phase; normally they have a
small particle size. Dyes, Pigments, paints
•Flame Retardants
The flammability of polymeric materials is a major
concern, especially in the manufacture of textiles and
children’s toys. Most polymers are flammable in their
pure form. The flammability resistance of the
remaining combustible polymers may be enhanced
by additives called flame retardants. Aluminum
hydroxide, magnesium hydroxide.