PROCESSING OF PLASTICS
AND COMPOSITE
MATERIALS
UNIT-I PLASTICS AND COMPOSITE MATERIALS
A plastic material is any of a wide
range of synthetic or semisynthetic organic solids that are moldable. Plastics are typically organic
polymers of high molecular mass, but they often contain other substances.
They are usually synthetic, most commonly derived frompetrochemicals, but
many are partially natural.
Most plastics contain organic polymers. 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 the monomers are linked 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.
POLYMERISATION PROCESS OF PLASTIC
To understand how plastics are made, and why certain plastics are
suitable for some uses, and others not, you have to understand a little about
the structure of polymers. Polymers are large molecules made up of many
smaller molecules. 'Poly' means many and 'mer' means units. These smaller
units are called monomers (mono = one, mer = unit) and are joined together
through polymerisation to form polymers. A polymer contains hundreds of
thousands of monomers. Polymerisation, which means the linking of
monomers to form polymers results from two kinds of chemical reaction called
condensation and addition. In a monomer atoms are joined by double bonds,
and these must be broken and new bonds created between adjacent atoms to
form the long chain molecules of a polymer through polymerisation. If the
chains run parallel to each other the structure is said to be crystalline (made
of crystals). This contrasts with the disorder of tangled chains in an
amorphous (shapeless) structure. Many polymers have both crystalline and
amorphous regions, and the proportion of crystalline and amorphous regions
in a polymer depends on its chemical composition, molecular arrangement
and how it has been processed.
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 molded again and again. Examples
include polyethylene, polypropylene, polystyrene and polyvinyl
chloride. These chains are made up of many repeating molecular units,
known as repeat units, derived from monomers; 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.
Crystallisation is one of the two principles that have been used to produce
strong, stiff polymers (e.g. polythene and nylon), the other is the formation of
strong bonds between the chains which is a process known as cross linking.
Polymers such as bakelite and urea formaldehyde have many strong cross
links which do not soften with increased heat, but set once and for all after
their initial moulding. This is how we get the ‘set’ in thermosetting. These
plastics remain comparatively strong until over heating leads to a breakdown
of the cross links and chemical decomposition. By contrast, in thermoplastic
polymers only weak forces hold the chains together and these materials can
be softened by heating and if necessary remoulded. On cooling they recover
their original properties.
Figure 1.1 Thermoplastics
Figure 1.2 Thermosetting plastics
A thermoplastic, or thermosoftening plastic, is a polymer that becomes
pliable or moldable above a specific temperature, and returns to a solid state
upon cooling. Most thermoplastics have a high molecular weight. The polymer
chains associate through intermolecular forces, which permits thermoplastics
to be remolded because the intermolecular interactions increase upon cooling
and restore the bulk properties. In this way, thermoplastics differ from
thermosetting polymers, which form irreversible chemical bonds during the
curing process. Thermosets often do not melt, but break down and do not
reform upon cooling.
A thermosetting plastic, also known as a thermoset, is polymer material
that irreversibly cures. The cure may be induced by heat, generally above 200
°C (392 °F), through a chemical reaction, or suitable irradiation. Thermoset
materials are usually liquid or malleable prior to curing and
designed to be molded into their final form, or used as adhesives. Once
hardened a thermoset resin cannot be reheated and melted to be shaped
differently.Thermoset materials are generally stronger than thermoplastic
materials due to this three dimensional network of bonds (cross-linking), and
are also better suited to high-temperature applications up to the
decomposition temperature. However, they are more brittle. Since their shape
is permanent, they tend not to be recyclable as a source for newly made
plastic.
Figure 1.3 Thermosetting plastics classification
Properties of plastics
The properties of plastics are 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. While plastics can be made
electrically conductive, with the conductivity of up to 80 kS/cm in stretchorientedpolyacetylene, they are still no match for most metals like copper
which have conductivities of several hundreds kS/cm.
Some common thermo plastics
Acrylic
Acrylic, a polymer called poly(methyl methacrylate) (PMMA), is also known
by trade names such as Lucite, Perspex and Plexiglas. It serves as a sturdy
substitute for glass for such items as aquariums, motorcycle helmet visors,
aircraft windows, viewing ports of submersibles, and lenses of exterior lights
of automobiles. It is extensively used to make signs, including lettering and
logos. In medicine, it is used in bone cement and to replace eye lenses.
Acrylic paint consists of PMMA particles suspended in water. Most schools
use it when teaching about thermoplastics.
Nylon
Nylon, belonging to a class of polymers called polyamides, has served as a
substitute for silk in products such as parachutes, flak vests and women's
stockings. Its fibers are useful in making fabrics, rope, carpets and musical
strings. Whereas in bulk form, nylon is used for mechanical parts including
machine screws, gear wheels and power tool casings. In addition, nylon is
used in the manufacture of heat-resistant composite materials.
Polyethylene
Polyethylene (or polyethene, polythene, PE) is a family of similar materials
categorized according to their density and molecular structure. For example,
ultra-high molecular weight polyethylene (UHMWPE) is tough and resistant to
chemicals, and it is used to manufacture moving machine parts, bearings,
gears, artificial joints and some bulletproof vests. High-density polyethylene
(HDPE), recyclable plastic no. 2, is commonly used as milk jugs, liquid laundry
detergent bottles, outdoor furniture, margarine tubs, portable gasoline cans,
water drainage pipes, and grocery bags. Medium-density polyethylene
(MDPE) is used for packaging film, sacks and gas pipes and fittings. Lowdensity polyethylene (LDPE) is softer and flexible and is used in the
manufacture of squeeze bottles, milk jug caps, retail store bags. and (LLDPE)
as stretch wrap in transporting and handling boxes of durable goods, and as
the common household food covering. XLPE or "PEX" (cross-linked
polyethylene) is a semi-rigid/flexible material which has gained wide use in
cold or hot water building heating/cooling applications (hydronic heating and
cooling) due to its exceptional resistance to breakdown from wide temperature
variations.
Polypropylene
Polypropylene (PP) is useful for such diverse products as reusable plastic
food containers i.e.) "microwave and dishwasher safe" plastic containers,
ropes, carpets, plastic moldings, piping systems, car batteries, insulation for
electrical cables and filters for gases and liquids. In medicine, it is used in
hernia treatment and to make heat-resistant medical equipment.
Polypropylene sheets are used for stationery folders and packaging and clear
storage bins. Polypropylene is defined by the recyclable plastic number 5.
Although relatively inert, it is vulnerable to ultraviolet radiation and can
degrade considerably in direct sunlight. It may be worthy to note that (PP) is
not as impact-resistant as the polyethlenes PP is also somewhat permeable
to highly volatile gases and liquids.
Polystyrene
Polystyrene is manufactured in various forms that have differing applications.
Extruded polystyrene (PS) is used in the manufacture of disposable cutlery,
CD and DVD cases, plastic models of cars and boats, and smoke detector
housings. Expanded polystyrene foam (EPS) is used in making insulation and
packaging materials, such as the "peanuts" and molded foam used to cushion
fragile products. Extruded polystyrene foam (XPS), known by the trade name
Styrofoam, is used to make architectural models and drinking cups for heated
beverages. Polystyrene copolymers are used in the manufacture of toys and
product casings.
Polyvinyl chloride
Polyvinyl chloride (PVC) is a tough, lightweight material that is resistant to
acids and bases. Much of it is used by the construction industry, such as for
vinyl siding, drainpipes, gutters and roofing sheets. It is also converted to
flexible forms with the addition of plasticizers, thereby making it useful for
items such as hoses, tubing, electrical insulation, coats, jackets and
upholstery. Flexible PVC is also used in inflatable products, such as water
beds and pool toys.
Teflon
Teflon is the brand name given by DuPont Corp. for a polymer called
polytetrafluoroethylene (PTFE), which belongs to a class of thermoplastics
known as fluoropolymers. It is famous as a coating for non-stick cookware.
Being chemically inert, it is used in making containers and pipes that come in
contact with reactive chemicals. It is also used as a lubricant to reduce wear
from friction between sliding parts, such as gears, bearings and bushings.
SOME COMMON THERMOSETTING PLASTICS
1. Epoxy resin
Used for casting and encapsulation, as an adhesive to bond other materials.
Its a good electrical insulator. Very hard and brittle when not reinforced. Has
the ability to resist chemicals. Has low shrinkage during curing. Provides
better adhesion between fibre and matrix. The major disadvantages include
the toxicity of the material with limited temperature application range upto 175
degree celcius. It absorbs moisture affecting its dimensional properties. It has
high coefficient of thermal expansion.
2. Melamine Formaldehyde
These laminates are used for work surfaces, electrical insulation, table ware
etc., It is stiff, hard, strong and resists chemicals.
3. Polyester resin
Used for casting, encapsulation and bonding of materials. Its stiff, hard and
brittle, good electrical insulator and resists chemicals.
4. Urea formaldehyde
Stiff, hard, brittle and good electrical insulator
ENGINEERING PROPERTIES OF PLASTICS
Characteristic
Thermoplasti
c
Thermoset
Effect on product
High
temperature
mechanical
Poor to good
Excellent
Increasessurvivability
during high temperature
performance
Coefficient of
friction
Very
low to Moderate
moderate
very high
to Affects pulling
tension
and wire processing
Elongation
Poor to good
to Stretching
property
before
breaking.
Its
encountered
during
installation,
processing
etc.,
Tensile
strength
Poor
excellent
to Poor
excellent
to Has impact on durability
Colourability
Good
excellent
to Poor
excellent
to Improved aesthetic look
and protection of surface
Heat
deformation
Poor to Good
Cold
temperature
compatibility
Poor
excellent
Good
excellent
Good
excellent
to Good
excellent
to Becomes soft when
nears melting point
it
to Affects,
storage,
handling,
installation,
and usage at extreme
cold conditions
DIFFERENCE BETWEEN THERMOPLASTICS AND THERMOSETTING
PLASTICS
Thermo plastics
Thermosetting plastics
1
Thermoplastic
pellets soften
when heated and become
more fluid as additional heat
is applied. The curing process
is completely reversible as no
chemical
bonding
takes
place.
This
characteristic
allows thermoplastics to
be
remolded
and
recycled
without negatively
affecting
the
material’s
physical
properties.
Thermoset plastics contain polymers
that cross link together during the
curing process to form an irreversible
chemical bond. The cross linking
process eliminates the risk of the
product remelting when heat is
applied, making thermosets ideal for
high heat applications such as
electronics and appliances.
2
There
are
multiple
thermoplastic resins that offer
various performance benefits,
but most materials commonly
offer high strength, shrinkresistance
and
easy
bendability. Depending on the
resin, thermoplastics
can
serve low stress applications
Thermoset
plastics
significantly
improve the material’s mechanical
properties,
providing
enhances
chemical resistance, heat resistance
and structural integrity.
Thermoset
plastics are often used for sealed
products due to their resistance to
deformation.
such as plastic bags or high
stress mechanical parts
3
Advantages
Advantages
1. Highly recyclable
1. More
resistance
to
high
temperature
than
thermoplastics
2. Aesthetically superior finish
2. High flexible design
3. High impact resistance
3. Thick to thin wall capabilities
4. Remolding/Reshaping
capabilities
4. Excellent aesthetic appearance
5. Chemical resistance
5. High levels of dimensional stability
6. Hard crystalline or rubbery
surface options
6. Cost effective
7. Eco friendly manufacturing
4
Disadvantages
Disadvantages
Generally more expensive
than thermoset
Cannot be recycled
Can melt if heated
Cannot be remoulded or reshaped
More difficult to get a finished surface
COMPOSITE MATERIALS
What is composite?
• A material which is composed of two or more materials at a microscopic scale and
have chemically distinct phases
• Heterogeneous solid consisting of 2 or more different materials that are
mechanically or metallurgically bonded together.
• Constituent materials have significantly different properties and retains its identity in
the composite and maintains its characteristic structure and properties.
Need for composites
To enhance certain desired properties like-*
Strength
• Stiffness
• Toughness
• Corrosion resistance
• Wear resistance
• Reduced weight
• Fatigue life
• Thermal/Electrical insulation and conductivity
• Acoustic insulation
• Energy dissipation
Major constituents of a composite material
1. Matrix- base material
2. Reinforcement- added material to give some desired properties
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