Unit 1 of the document provides an introduction to composite materials. It defines composites as materials made of two or more chemically different constituents combined macroscopically. Examples of natural composites include wood, bone, and granite. Man-made composites include concrete, plywood, fiberglass, and cermets. Composites provide advantages like strength, stiffness, corrosion resistance, and aesthetics. They are used in various industries such as automotive, aerospace, sports, transportation, and infrastructure. Composites are classified as particulate or fibrous, depending on the reinforcement material, and can have random or preferred orientation of constituents.

1. Unit – I
Introduction to
Composite Materials

2. Lecture 1
Introduction

3. Lecture Overview
• What are “composites”?
• Importance and areas of application
• Classification
• Advantages of fiber‐reinforced composites

4. What are “composites”?
• Composite: Two or more chemically different
constituents combined macroscopically to yield
a useful material.
• Examples of naturally occurring composites
– Wood: Cellulose fibers bound by lignin matrix
– Bone: Stiff mineral “fibers” in a soft organic matrix
permeated with holes filled with liquids
– Granite: Granular composite of quartz, feldspar
and mica

5. “ ”
What are composites ?
• Some examples of man‐made composites
– Concrete: Particulate composite of aggregates
(limestone or granite), sand, cement and water
– Plywood: Several layers of wood veneer glued
together
– Fiberglass: Plastic matrix reinforced by glass fibers
– Cemets: Ceramic and metal composites
– Fibrous composites: Variety of fibers (glass, kevlar,
graphite, nylon, etc.) bound together by a
polymeric matrix

6. These are not composites!
• Plastics: Even though they may have several
“fillers”, their presence does not alter the
physical properties significantly.
• Alloys: Here the alloy is not macroscopically
heterogeneous, especially in terms of physical
properties.
• Metals with impurities: The presence of
impurities does not significantly alter physical
properties of the metal.

7. Where are composites used?
• Automotive industry: Lighter, stronger, wear
resistance, rust‐free, aesthetics
– Car body
Brake pads
– Drive shafts
– Fuel tanks
– Hoods
– Spoilers

8. Where are composites used?
• Aerospace: Lighter, stronger, temperature
resistance, smart structures, wear resistance
– Aircraft: Nose, doors, struts, trunnion, fairings,
cowlings, ailerons, outboard and inboard flaps,
stabilizers, elevators, rudders, fin tips, spoilers,
edges
– Rockets & missiles: Nose, body, pressure tanks,
frame, fuel tanks, turbo‐motor stators, etc.
– Satellites: Antennae, frames, structural parts

9. Where are composites used?
• Sports: Lighter, stronger, toughness, better
aesthetics, higher damping properties
– Tennis
Bicycles
– Badminton
– Boats
– Hockey
– Golfing
Motorcycles …

10. Where are composites used?
• Transportation & Infrastructure: Lighter,
stronger, toughness, damping
– Railway coaches
Bridges
– Ships and boats
– Dams
– Truck bodies and floors
– RV bodies

11. Where are composites used?
• And many more industry sectors
– Biomedical industry
– Consumer goods
– Agricultural equipment
– Heavy machinery
– Computers
– Healthcare

12. Classification of Composites
Engineered
Composites
Particulate Fibrous
Random
Orientation
Preferred
Orientation
SingleLayer Multi‐Layer
Continuous &
Long Fibers
Discontinuous
& Short Fibers
Laminate
Hybrid
Laminate
Unidirectional Bi‐Directional
Random
Orientation
Preferred
Orientation

13. Classification of Composites
• Particulate composites have one or more
material particles suspended in a binding
matrix. A particle by definition is not “long”
in terms of its own dimensions.
• Fibrous composites have fibers of reinforcing
material(s) suspended in binding matrix.
Unlike particles, a fiber has high length to
diameter ratio, and further its diameter may
be close to its crystal size.

14. Classification of Composites
• Particulate composites:
– Random orientation: Orientation of particle is randomly distributed in all
directions (ex: concrete)
– Preferred orientation: Particle orientation is aligned to specific directions
(ex: extruded plastics with reinforcement particles)
Note: Particulate composites in general do not have high fracture
resistance unlike fibrous composites. Particles tend to increase
stiffness of the materials, but they do not have so much of an
influence on composite’s strength. In several cases, particulate
composites are used to enhance performance at high temperatures.
In other case, these composites are used to increase thermal and
electrical properties. In cemets, which are ceramic‐metal composites,
the aim is to have high surface hardness so that the material can be
used to cut materials at high speeds, or is able to resist wear.

15. Classification of Composites
• Fibrous Composites: In general, materials tend to have much better thermo‐
mechanical properties at small scale than at macro‐scale. This is shown in
the following table.
Material Fiber Tensile Strength (GPa) Bulk Tensile strength( GPa)
Glass 3.5 to 4.6 0.7 ‐ 2.1
Tungsten 4.2 1.1 ‐ 4.1
Beryllium 1.3 0.7
Graphite 2.1 to 2.2.5 Very low
At macro‐scale, imperfections in material have an accumulated effect of
degrading bulk mechanical properties of materials significantly. This is one
reason why fibrous composites have been developed to harness micro‐scale
properties of materials at larger scales. Man‐made fibers, have almost no
flaws in directions perpendicular to their length. Hence they are able to bear
large loads per unit area compared to bulk materials.

16. Classification of Composites
• Fibrous Composites:
– Single‐layer: These are actually made of several
layers of fibers, all oriented in the same direction.
Hence they are considered as “single‐layer”
composites. These can be further categorized as:
• Continuous and long fibers: Examples include filament
wound shells. These may be further classified as:
– Unidirectional reinforcement
– Bidirectional reinforcement

17. Classification of Composites
• Fibrous Composites (continued):
• Discontinuous and short‐fibers: Examples include fiber
glass bodies of cars. These may be further classified as:
– Randomly oriented reinforcement
– Reinforced in preferred directions
– Multi‐layer: Here, reinforcement is provided, layer‐
by layer in different directions.
• Laminate: Here, the constituent material in all layers is
the same.
• Hybrid laminates: These have more than one constituent
materials in the composite structure.

18. Reinforcements
The word “composite” means “consisting of two or more distinct
parts”. Thus a material having two or more distinct constituent
materials or phases may be considered to be a composite
material. Reinforcement and the matrix are the two phases of
the composite material.

19. Reinforcement in the Composites
Reinforcement can be fibers, fabric particles, or whiskers. these
reinforcements fundamentally used to increase the mechanical
properties of a composite.

20. The main purpose of the reinforcement is to
Provide superior levels of strength and stiffness to the composite.
Reinforcing materials (graphite, glass, SiC, alumina) may also provide
thermal and electrical conductivity, controlled thermal expansion, and wear
resistance in addition to structural properties.
The most widely used reinforcement form in high-performance composites
is fiber tows (untwisted bundle of continuous filaments).
Fiber monofilaments are used in PMCs, MMCs, and CMCs; they consist of a
single fiber with a diameter generally ≥100 μm.
In MMCs, particulates and chopped fibers are the most commonly used
reinforcement morphology, and these are also applied in PMCs.
Whiskers and platelets are used to a lesser degree in PMCs and MMCs

21. Fibers and Whiskers
• A fiber has:
– High length‐to‐diameter ratio.
– Its diameter approximates its crystal size.
• Modern composites exploit the fact that small scale samples of
most of the materials are much stronger than bulk materials. Thus,
thin fibers of glass are 200‐500 times stronger than bulk glass.
• Several types of fibers are available commercially. Some of the
more commonly used fibers are made from materials such as
carbon, glass, Kevlar, steel, and other metals.
• Glass is the most popular fiber used in composites since it is
relatively inexpensive. It comes in two principal varieties; E‐glass,
and S‐glass. The latter is stronger than the former.

22. Fibers and Whiskers
• Fibers are significantly stronger than bulk materials
because:
– They have a far more “perfect” structure, i.e. their crystals
are aligned along the fiber axis.
– There are fewer internal defects, especially in direction
normal to fiber orientation, and hence there are lesser
number of dislocations.
• At larger scales, the degree of structural perfection
within a material sample is far less that what is present
at small (micro and nano) scales. For this reason fibers
of several engineering materials are far more strong
than their equivalent bulk material samples.

23. Fibers and Whiskers
• The following table lists bulk as well as fiber properties for
different materials. It is seen from the table that the
difference between bulk and fiber strengths is significant.
Table 2.1: Properties of Some Common Engineering Materials in Bulk and Fiber Forms
Fiber Specific Gravity
Young's Modulus
(GPa)
Bulk Tensile Strength
(MPa)
Fiber Tensile
Strength (MPa)
Aluminium 2.7 78 140‐620 620
Titanium alloy/fiber 4.5 115 1040 1900
Steel 7.8 210 340‐212 4100
E‐Glass 2.54 72 70‐210 3500
S‐Glass 2.48 86 70‐210 4600
Carbon 1.41 190 very low 2100‐2500

24. Fibers and Whiskers
• Whiskers are similar in diameter to fibers, but in
general, they are short and have low length‐to‐
diameter ratios, barely exceeding a few hundreds.
• Thus, the difference in mechanical properties of a
whisker vis‐à‐vis bulk material is even more
pronounced. This is because the degree of perfection
in whiskers is even higher vis‐à‐vis that in fibers.
– Whiskers are produced by crystallizing materials on a very
small scale.
– Internal alignment within each whisker is extremely high.

25. Whiskers
• The following table lists bulk as well as whisker properties for
different materials. It is seen from the table that the difference
between bulk and whisker strengths is very significant.
Table 2.2: Properties of Some Common Engineering Materials
in Bulk and Whisker Forms
Bulk Tensile Strength Whisker Tensile
Fiber (MPa) Strength (MPa)
Alumina (Al2O3) 105‐107 19000
Silicon Carbide 3440 11000
Copper 220 3000
Iron whisker v/s bulk steel 525‐700 13000
Boron carbide 155 6700
Carbon very low 21000
• Modern composites derive much of their desired properties by
using fibers and whiskers as one of the constituent materials.
• Fibers made from carbon, E‐glass, S‐glass, and Kevlar are commonly
used in modern composite structures.

26. Problem Set
• Explore different types of fiber materials.
What fibers would you used with an objective
to:
– Improve thermal conductivity
– Improve electrical conductivity
– Improve mechanical strength
– Improve toughness

27. Glass Fibers
• Glass fibers are most commonly used fibers. They come in two forms:
– Continuous fibers
– Discontinuous or “staple” fibers
• Chemically, glass is sillicon di‐oxide (SiO2). Glass fibers used for structural applications come in
two “flavours”: E‐Glass, and S‐Glass. E‐glass is produced in much larger volumes vis‐à‐vis S‐
glass.
• Principal advantages:
– Low cost
– High strength
• Limitations:
– Poor abrasion resistance causing reduced usable strength
– Poor adhesion to specific polymer matrix materials
– Poor adhesion in humid environments
• Glass fibers are coated with chemicals to enhance their adhesion properties. These chemicals
are known as “coupling agents”.
– Many of coupling agents are silane compounds

28. How are Glass Fibers Made?
• Both, continuous and staple forms of glass fibers are produced by partially
similar method.
• Process of producing continuous fibers:
– Raw materials (sand, limestone, alumina) are mixed and melted in a furnace at
approximately 1260 C.
– Molten glass then :
• Either flows directly into a fiber‐drawing facility. This process is known as “direct‐
melt” process. Most of fiber glass in the world is produced this way.
• Or gets formed into marbles. These marbles are later fused, and drawn into fibers.
• For producing continuous fibers, molten glass passes through multiple
holes to form fibers. These fibers are quenched through a light spray of
water. Subsequently, fibers are coated with protective and lubricating
agents.

29. How are Glass Fibers Made?
• Next fibers are collected in bundles known as “strands”. Each strand may
have typically 204 individual fibers.
• Next, strands wound on spools. Fibers in these spools are subsequently
processed further to produce textiles.
• Staple fibers are produced by pushing high pressure air‐jet across fibers, as
they emanate from holes during the drawing process.
• These fibers, are subsequently collected, sprayed with a binder, and
collected into bundles known as “slivers”.
• These slivers may subsequently be drawn and twisted into yarns.

30. Surface Treatment of Glass Fibers
• During production, glass fibers are treated chemically . These
treatments are known as sizes.
• There are two types of sizes: Temporary and Compatible.
– Temporary sizes are used to reduce degradation of fiber strength attributable
to abrasion of fibers due to inter‐fiber friction during fiber drawing process.
They are also used to bind fibers for easy handling. They are made from
starch‐oils (starch, gelatin, polyvinyl alcohol, etc.). These sizes inhibit good
resin‐fiber adhesion. They also promote moisture absorption.
– During composite fabrication, these sizes are removed by heating the fibers at
340 C for 15‐20 hours. Post their removal, these fibers are coated with
coupling agents (also known as finishes), which promote resin‐fiber adhesion.
These agents also inhibit deteriorating effects of humidity on the fiber‐resin
bond. Many of these agents are organo‐functional silanes.

31. Composition & Properties of Glass Fibers
Typical Chemical Composition of E & S Glass in %
SiO2 54.3 64.2
Al2O3 15.2 24.8
CaO 17.2 0.01
B2O3 8.0 0.01
MgO 4.7 10.3
Na2O 0.6 0.27
BaO 0.2.0
FeO 0.21
Others 0.03
Important Properties of Glass Fibers
Property E‐Glass S‐Glass
Specific gravity 2.54 2.49
Tensile strength (MPa) 3450 4590
Tensile modulus (GPa) 72 86
Diameter range (microns) 3 to 20 8 to 13
CTE (per million per C) 5 2.9

32. Graphite Fibers
• Graphite and carbon fibers are extensively used in high‐strength, high‐
modulus applications.
– Graphite fibers have carbon content in excess of 99%.
– Carbon fibers have carbon content in the range 80‐95%
• Fiber’s carbon content depends on processing method for these fibers.
• Significantly more expensive than glass fibers.
• Key application areas include aerospace, sporting, railway, infrastructure,
automotive, oil drilling, as well as consumer sector industries.
• Graphite structure consists of hexagonally packed carbon atoms in layers,
and several such layers are interconnected through weak van der Waals
forces. Thus, such a structure generates:
– High inplane modulus
– Significantly less modulus in out‐plane direction

33. How are Graphite Fibers Produced?
• A precursor material, which is rich in carbon, is subjected to pyrolysis to
extract its carbon content.
o Pyrolysis: Thermo‐chemical decomposition of organic material when it is subjected to
elevated temperatures, but no oxygen. Through such a process, the precursor organic
material breaks down into gases, liquids, and a solid residue which is rich in carbon.
o Precursor: It is a carbon‐rich chemical compound, used as “raw” material for pyrolysis.
• Currently, three materials are used as precursors. These are:
o Polyacrylonitrile (PAN)
o Pitch: It is a viscous substance produced by plants, and also extracted from petroleum.
o Rayon: It is regenerated cellulose fiber produced from naturally occurring polymers.
• A good precursor material should have following characteristics.
o Sufficient strength and handling properties so that it can hold together fibers during
carbon fiber production process.
o Should not melt during production process.
o Should not be completely volatile, as it will drastically reduce yield of carbon fiber.
o Carbon atoms should self‐align in graphite structure during pyrolysis, as this will
enhance fiber’s mechanical properties.
o Inexpensive

34. How are Graphite Fibers Produced?
• A precursor material, which is rich in carbon, is subjected to pyrolysis to
extract its carbon content.
o Pyrolysis: Thermo‐chemical decomposition of organic material when it is subjected to
elevated temperatures, but no oxygen. Through such a process, the precursor organic
material breaks down into gases, liquids, and a solid residue which is rich in carbon.
o Precursor: It is a carbon‐rich chemical compound, used as “raw” material for pyrolysis.
• Currently, three materials are used as precursors. These are:
o Polyacrylonitrile (PAN)
o Pitch: It is a viscous substance produced by plants, and also extracted from petroleum.
o Rayon: It is regenerated cellulose fiber produced from naturally occurring polymers.
• A good precursor material should have following characteristics.
o Sufficient strength and handling properties so that it can hold together fibers during
carbon fiber production process.
o Should not melt during production process.
o Should not be completely volatile, as it will drastically reduce yield of carbon fiber.
o Carbon atoms should self‐align in graphite structure during pyrolysis, as this will
enhance fiber’s mechanical properties.
o Inexpensive

35. Production of Graphite Fibers from PAN
• PAN precursor material is initially spun into fiber form.
• These precursor fibers are then stretched through application of tensile load.
• During stretching, they are also subjected to high temperatures (200 ‐ 240 C), for
approximately 24 hours in an oxidizing atmosphere. This process is called
“stabilization”.
• These stabilized fibers are next subjected to pyrolysis at 1500 C in inert
atmosphere. This process is called “carbonization”. During this process, most of
non‐carbon elements are driven out of PAN fibers.
• Next, these fibers are “graphitized” by heating them at 3000 C in inert
environment. This improves tensile modulus of fibers as graphite crystals develop
in carbon.

36. Overview of Different Types of Graphite Fibers
• PAN based carbon fibers:
– Low cost
– Reasonable mechanical properties
– Very popular in aircraft, missile and space applications
• Pitch‐based carbon fibers
– Higher stiffness
– Higher thermal conductivity: This makes them particularly useful in thermal
management systems and satellite structures
• Rayon‐based carbon fibers:
– Not used much in structural applications
– Low thermal conductivity: Useful for insulation materials, and heat shields
– Used in rocket nozzles, missile re‐entry nose cones, heat insulators

37. Important Properties of Graphite Fibers
Important Properties of Graphite Fibers
Property PAN Pitch Rayon
Fiber diameter (microns) 5 to 8 10 to 11 6.5
Specific gravity 1.71 to 1.96 2.0 to 2.2 1.7
Tensile modulus (GPa) 230 to 595 170 to 980 415 to 550
Tensile strength (MPa) 1925 to 6200 2275 to 4060 2070 to 2760
Elongation at failure (%) 0.40 to 1.20 0.25 to 0.70
CTE (Axial, X 1E‐06/C) ‐0.75 to ‐0.40 ‐1.6 to ‐0.90
Thermal conductivity (W/m‐K) 20‐80 400‐1100

38. Aramid Fibers
• Aramid is short for “aromatic‐polymide”. Aramids are a class of polymers,
where self repeating units contain large phenyl rings, linked together by
amide groups.
• As per US based FTC, aramid fibers are manufactured fibers where“the
fiber‐forming substance is a long‐chain synthetic polyamide in which at
least 85% of the amide linkages, (‐CO‐NH‐) are attached directly to two
aromatic rings”.
• Important properties of these fibers are:
– High resistance to abrasion
– High resistance to organic solvents
– Tough as well as strong
– Non‐conductive
– No melting point (they start degrading at 500 C)
– Low flammability
– Sensitive to acids, and solvents

39. Properties of Aramid Fibers
• Kevlar is a very well known and widely used aramid fiber.
– Invented by DuPont
– Widely used in ballistic applications
– Comes in different flavors.
Important Properties of Kevlar Fibers
Property Kevlar 29 Kevlar 49 Kevlar 129 Kevlar 149
Diameter (microns) 12 12
Specific gravity 1.45 1.45 1.5 1.45
Tensile modulus (GPa) 62 124 96.0 186
Tensile strength (MPa) 2760 3620 3380.0 3440
Elongation (%) 3.4 2.8 3.3 2.5
Axial CTE (per million per C)
Radial CTE (per million per C)
‐2
‐60
‐2
‐60
‐2 ‐2

40. Boron Fibers
“
”
• Boron fibers are relatively more popular in composites, vis‐à‐vis other
fibers (aluminum, steel, etc.).
• These fibers are made using a chemical vapor deposition (CVD) process.
o Here, boron tri‐chloride is chemically reduced in a hydrogen environment on a tungsten
or carbon filament substrate.
o The tungsten or carbon filament is resistively heated at temperatures in excess of 1500
C. Due to application of temperature, boron‐tri‐chloride interacts with hydrogen, and
reduces to pure boron.
o This boron gets deposited on the tungsten or carbon filament. As the filament is
continuously pulled out of reduction chamber, a well controlled boron layer deposits on
the substrate wire. These wires have a boron outside and a tungsten or carbon core.
Property
Important Properties of Boron‐Tungsten Fibers
Dia = 100 microns Dia = 140 microns Dia = 200 microns
Specific gravity 2.61 2.47 2.4
Tensile modulus (GPa) 400 400 400.0
Tensile strength (MPa) 3450 3450 3450.0
CTE (per million per C) 4.9 4.9 4.9

41. Ceramic Fibers
• Ceramic fibers are used in high temperature applications. These fibers have high
strength, high elastic modulus, as well as the ability to withstand high
temperatures without getting chemically degraded.
• Commonly used fibers for such applications are made from alumina, and SiC.
• Alumina fibers are made spinning a slurry of alumina and firing of the slurry.
These fibers retain their strength up to 1370 C.
• Silicon carbide fibers are produced either by a chemical vapor deposition (CVD)
process, or through pyrolysis.
• SiC fiber retain their tensile strength up to 650 C.
• Alumina and SiC fibers work well in metal matrices, unlike carbon and boron fibers,
since the latter react with metal matrices. Further, due to their resistance to high
temperatures, these fibers are also used in turbine blades.

42. HPPE Fibers
• HPPE stands for High Performance Polyethylene .
• HPPE fibers are have a density slightly less than that of water. Thus, even
though their modulus and strength are slightly less than Kevlar fibers, on a
specific strength, and specific modulus are 30‐40% more than that for
Kevlar fibers.
• HPPE fibers have very high energy absorption characteristics. Thus they
are widely used in ballistic armor applications.
• HPPFE fiber’s modulus and strength increases significantly with increasing
strain rates. Thus HPPFE composites work very well when subjected to
high‐velocity impacts.
• HMPE (high modulus polyethylene) and ECPE (extended chain
polyethylene) are other materials with chemical structure similar to HPPE
material. Their fibers are also used in composites.

43. Properties of Ceramic and HPPE Fibers
Important Properties of Ceramic and HPPE Fibers
SiC
Property Alumina SIC (CVD) (Pyrolysis) HPPE
Diameter (microns) 15‐25 140 10‐20 38
Specific gravity 3.95 3.3 2.6 0.97
Tensile modulus (GPa) 379 430 180 62‐120
Tensile strength (MPa) 1380 3500 2000 2180‐3600
Elongation (%) 2.8‐4.4

44. Matrix Materials
• Fibers and whiskers in composites are held together by a binder
known as matrix. This is required since fibers by themselves:
– Given their small cross‐sectional area, cannot be directly loaded.
– Further, they cannot transmit load between themselves.
• This limitation is addressed by embedding fibers in a matrix
material.
• Matrix material serves several functions, the important ones being:
– Binds fibers together.
– Transfers loads and stresses within the composite structure.
– Support the overall structure
– Protects the composite from incursion of external agents such as
humidity, chemicals, etc.
– Protects fibers from damage due to handling.

45. Matrix Materials
• Matrix material strongly influences composite’s overall transverse
modulus, shear properties, and compression properties.
• Matrix material also significantly limits a composite’s maximum
permissible operating temperature.
• Most of the matrix materials are relatively lighter, more compliant, and
weaker vis‐à‐vis fibers and whiskers.
• However, the combination of fibers/whiskers and matrix can be very stiff,
very strong, and yet very light.
– Thus most of modern composites have very high specific strengths, i.e. very
high strength/density ratios.
– This makes them very useful in aerospace applications, where weight
minimization is a key design consideration.

46. Matrix Materials
• Matrix materials can be broadly classified on the basis of their usable
temperature ranges.
Different Classes of Materials and
Usable Temperature Ranges
Matrix Material Usable Temperature Range (C)
Polymers < 260
Metals 260 ‐750
Glass 750 ‐ 1150
Ceramic and carbon 1150 ‐ 1400

47. Choosing the Right Matrix Materials
• While selecting matrix material for a composite system,
several considerations have to be factored into, principal ones
being:
– Physical properties such a specific gravity.
– Mechanical properties such as modulus, strength, CTE, conductivity, etc.
– Melting of curing temperature for the matrix material
– Viscosity: It strongly affects processing attributes of the composite, and also
uniform flow of matrix material into the composite system.
– Reactivity with fibers: One would certainly not desire possibility of chemical
reactions between fibers and matrix material.
– Fabrication process compatible with matrix and fibers
– Reactivity with ambient environment
– Cost

48. Polymers as Matrix Materials
• Polymers: Most widely used matrix materials
– Common examples: Polyesters, vinylesters, PEEK, PPS, nylon, polycarbonate, polyacetals,
polyamides, polyether imides, polystyrene, epoxies, ureas, melamines, silicones.
• Advantages:
– Low cost
– Easy to process
– Low density
– Superior chemical resistance
• Limitations:
– Low strength
– Low modulus
– Limited range for operating temperature
– Sensitivity to UV radiation, specific solvents, and occasionally humidity

49. Polymers as Matrix Materials
• Polymer classification
– Thermoplastics
• Soften or melt when heated. This process is reversible.
• Their structure has long chains of molecules with strong intra‐molecular bonds, but
weak inter‐molecular bonds.
• When exposed to heat, these inter‐molecular bonds breakdown, and the material
starts “flowing”.
• Semi‐crystalline of amorphous in structure
• Examples: polyethylene, PEEK, polyamides, polyacetals, polysulfone, PPS, nylon,
polystyrene.
– Thermosets
• These polymers do not melt, but breakdown (decompose) when heated.
• Amorphous structure
• They have networked structures with strong covalent bonds linking all molecules.
• These networks permanently breakdown upon heating. Hence, these polymers,
once “set”, cannot be reshaped.
• Examples: epoxies, polyesters, phenolics, urea, melamine, silicone, polyimides.

50. Polymers as Matrix Materials
• Polymers behave significantly differently vis‐à‐vis metals, and ceramics.
– Performance of polymers is highly sensitive to several environmental variables. For
instance, while mechanical properties of metals are temperature sensitive only in
proximity of melt temperature, polymers’ mechanical properties are highly sensitive to
heat.
• Following table depicts sensitivity of various polymer properties to
external variables.
Sensitivity of Different Polymer Properties to External Variables
Strength Stiffness CTE
Thermal
Conductivity
UV
Degradation
Melting
Point
Tg
Heat High High High High High High High
Environment High High High
Strain Rate High High

51. Temperature Sensitivity of Polymers
• Polymers have significant behavioral sensitivity to increased temperatures.
This sensitivity is strongly dependent on the structure of a polymer.
• As mentioned earlier, polymers may either be thermoplastics, or
thermosets. While thermosets have amorphous structure, thermoplastics
may have either semi‐crystalline structure, or amorphous structure.
• Temperature sensitivity of amorphous thermoplastics
– When these plastics are heated, their specific volume slowly increases somewhat linearly
with increasing temperature. However, if the temperature exceeds their glass transition
temperature Tg, their specific volume increases at a faster rate. This is accompanied with a
significant change in their mechanical properties.
– Hence, these maximum use temperature for these materials should not exceed Tg.
– If these materials are heated beyond Tg, then the material melts at Tm.
– Examples of these materials are polystyrene, polycarbonate, and polymethylmethacrylate.

52. Temperature Sensitivity of Polymers
• Temperature sensitivity of semi‐crystalline thermoplastics
– When these plastics are heated, their specific volume slowly increases somewhat linearly
with increasing temperature.
Further, if the temperature exceeds their glass transition temperature Tg, their specific
volume increases at a somewhat faster rate.
– This is so, because presence of crystalline structure in these materials tends to limit the
extent of changes in material’s mechanical properties.
– It is only when the temperature exceeds their melting point Tm, that their material
properties change significantly, and this change is accompanied by very significant increase
in specific volume. This happens because at melting point, the crystalline bonds in the
material breakdown, and all properties of the material undergo sudden and large changes.
– Thus, maximum use temperature for semi‐crystalline thermoplastics is determined more
by their melting point, and not so‐much by their glass transition temperature.
– Examples of these materials linear polyethylene (PE), polyethylene terephthalate (PET),
polytetrafluoroethylene (PTFE) or isotactic polypropylene (PP).

53. Temperature Sensitivity of Polymers
• Temperature sensitivity of thermosets
– Unlike thermoplastics, thermosets do not melt upon heating. Rather, they decompose
when they are heated beyond a certain threshold.Hence, thermosets polymers are
associated only with glass transition temperature, Tg, and have no melting point.
– When these plastics are heated, their specific volume slowly increases somewhat linearly
with increasing temperature. However, if the temperature exceeds their glass transition
temperature Tg, their specific volume increases at a faster rate.
– However, the change in mechanical properties for these materials at corresponding to
glass transition temperature, is much less vis‐à‐vis amorphous thermoplastics.Their relative
reduced sensitivity to temperature at Tg, is attributable to high degree of cross‐linked
bonds, which sustain material’s mechanical properties even at Tg.
– Even then, maximum use temperatures for these materials are dictated by Tg.
– Common examples of these materials include epoxies, polyesters, and phenolics.