Question number 1
Section A:
Polymer properties are influenced not only by their chemical structure (constitution, molar
mass, configuration, microconformation), but also by their physical structure - interrelations
between processing, properties and structure forms is the cornerstone of materials
i.
ii.
i.
ii.
iii.
iv.
v.
vi.
vii.
viii.
ix.
x.
Use of the product in different temperature environments in the world - If Tg of
the polymer (product) is low compared to the temperature of the country (for
example Africa) then the product can shrink or warp.
chemical resistance
flexible or rigid - Parameters that can affect – Density, molar mass, side group,
linear or branched chain…etc.
Thermal degradation – for example, PVC since its processing temp is low, and
then it passes very quickly degradation (Dehydrochlorination).
Optical Properties - Color, gloss and surface texture
UV resistance - UV energy absorbed by plastics can excite photons, which then
create free radicals. Free radicals then react further with oxygen in the
atmosphere, producing carbonyl groups in the main chain. The exposed surfaces
of products may then discolour and crack, and in extreme cases, complete
product disintegration can occur.
Transparent or opaque
Flame Retardants
Shock resistant/impact resistance - dependent upon resin density, comonomer
type, molecular weight, and film fabrication conditions. Impact decreases as
density is increased.
Acoustic properties - Sound Absorption or Sound Reflection
Hydrophobic properties
Recyclability - thermoplastic or thermoset
Section B:
a)
Tg depends largely on the amount of thermal energy required to keep the
polymer chains moving. Since Tg is a temperature at which molecular rotation
about single bonds becomes restricted, therefore it can be concluded that
structural features are those which influence the ease of rotation. The factors
which affect the Tg:
i.
groups attached to the backbone of the polymer which increase the
energy required for rotation
ii.
rigid structure
iii.
the packaging of substituents around the main chain
iv.
secondary bonding between chains, hydrogen bonding
v.
primary bonding between chains, cross-linking
vi.
length of side chains
vii.
Molecular weight.
For example, Tg effects on Shrink or/and warps - the chemical structure of the
backbone chain and the side groups of the chain have an effect on the mobility
of the polymer chains thus on their physical properties, for example - Tg:
 Poly(vinyl chlorid) PVC, has a Tg of 85°C.
 Poly(ethylene glycol) PEG, has a Tg of -41°C.
The intermolecular forces in PVC are quite strong because of the C-Cl bond,
giving it a lower mobility, thus this raises the glass transition temperature while
PEG has a C-O which makes it very flexible (and giving it a higher mobility than
PVC) so it was a lower Tg.
b)
Chemical resistance - polymer morphology - Amorphous
(Polyvinylchloride,polystyrene) VS crystalline (polyethylene, polyester)
polymers.
The factors that effect on Chemical resistance of polymer:
1. Chemical nature of monomers, and
2. Their molecular arrangement
In order to answer the question, I will focus on Amorphous and Crystalline
Presence of branched chains prevents clos packing of the polymer chains so that
the density of the finished product is low. Crystalline regions of the polymer
consist of parallel chains of linked monomers, while amorphous regions are
randomly oriented irregular in configuration.
Factors affecting the ability to crystalline:
1. Symmetrical chain which allows the regular close packing required for
crystallite formation.
2. Chains possessing groups which encourage strong intermolecular
attraction, thereby stabilizing the alignment.
When the plastic is cooled down from a high temperature, the chains reentangle themselves. In the process they leave some spaces between the chains,
which are called the “free volume.” because of the orderly structures of the
crystalline areas in semi crystalline materials, the free volume is less and
therefore they are more chemically resistant than their amorphous
counterparts.
The degree of crystallinity has major effect on some of the additional properties:
Amorphous
Chains are distributed randomly
soft and elastic
broad range softening
Generally transparent
low chemical resistance
Crystalline
chains are distributed orderly
stiff and brittle
fixed melting point
generally opaque
High chemical resistance
C) Acoustic properties - Sound Absorption or Sound Reflection:
Sound waves, similar to light waves and electromagnetic waves, can be reflected, absorbed,
and transmitted when they strike the surface of a body.
The speed at which sound is transmitted through a solid barrier is proportional to Young’s
modulus of the material, E, but inversely proportional to its density, ρ.
The speed of sound through a material is dependent on the materials’ state.
For example: sound waves travel much slower through a polymer melt than through a
polymer in the glassy state and the speed of sound through a polymer in the rubbery state is
100 times slower than that through a polymer in a glassy state.
In the melt state, the speed of sound drops with increasing temperature because of density
increase. On the other hand, speed of sound increases with pressure.
Sound reflection - In order to obtain high sound reflection, the mass of the media 2 must be
high compared to the mass of media 1.
Materials that have the same characteristic impedance as air are the best sound absorbent
materials.
Sound waves that penetrate a polymer medium are damped out similar to that of
mechanical vibrations. Hence, sound absorption also depends on the magnitude of the loss
tangent tan δ, or logarithmic decrement Δ.
In conclusion:
Sound reflection
Foamed polymers have an impedance of the same
order as air, they are poor reflectors of acoustic
waves.
sound absorbent
Elastomers and amorphous polymers have the
highest sound absorption properties, whereas
metals have the lowest.
The mass of insulating sound walls can be increased
Foamed polymers ideal for eliminating multiple
with the use of fillers, such as plasticized PVC with
reflections of sound waves in acoustic or
barium sulfate or by spraying similar anti-noise
soundproof rooms.
compounds on the insulating walls.
Compared to wood, semi-crystalline polymers are considered sound-proof materials.
Materials with a glass transition temperature lower than room temperature are particularly suitable as
damping materials - thermoplastics and weakly cross-linked elastomers
Section C:
When we want to choose polymer, for our manufacturing process (extrusion twin screw/
single screw, injection molding, extrusion blow molding...etc.), product properties and
application, first thing we will do is classification polymer, each plastic will fall into one of
three areas:



High performance - more expensive due to their ability to withstand high
temperatures and maintain their strength and chemical resistance under
wearing conditions
As we move from high performance to engineering to commodity polymers, the
cost, temperature resistance and strength of the plastic drops.
Engineering
Commodity
Several questions need to be asked when choosing polymer for our process, etc.:
1. What is your budget?
2. How temperature resistant does your plastic need to be?
3. How strong does your plastic need to be?
4. Are there any particular characteristics the plastic needs to have?
Polymers that will choose from the literature ("Generic Properties" - will have range
characteristic) should just be used as an overall guideline, rather than being relied on.
This is because the final properties of any plastic material may be largely altered by the
processing conditions it is put through and/or by the addition of additives, and vary from
different suppliers and quality of the grades (polymer).
Question number 2:
Polyethylene produced by the polymerisation of ethylene gas, a derivative of the petroleum
industry. The polymer consists essentially of long-chain molecules of very high molecular
weight, made up of many thousands of the -CH2- repeating unit.
a)
Typical molecular structure:
Polyethylene type
LDPE (Low-density
Polyethylene)
MDPE (Medium density
)polyethylene
LLDPE (linear low-density
)polyethylene
Density [g/cm3]
Molecular structure
Molecular design
0.915 - 0.930
high degree of long-chain
branches
0.930 - 0.940
branched polyethylene having
a slightly lower density than
HDPE
0.915 - 0.940
linear molecule with a higher
level of short chain branching
than HDPE
mLLDPE (Metallocene Linear
Low Density Polyethylene)
0.915 - 0.940
HDPE (High-density
)Polyethylene
0.940 - 0.970
Short chain branching induced
by the comonomer type, and
little or no long chain branching
Linear molecule with a very low
level of short chain branching.
Copolymers of Polyethylene:
Homopolymers are composed of same types of monomers whereas copolymers are
composed of different monomers.
There are four copolymer structures:
Monomer are distributed randomly, and sometimes
Random (or Statistical)
unevenly
copolymer
Alternating copolymers
Block copolymers
Graft copolymer
Monomers are distributed in a regular alternating
fashion, with nearly equal molar amounts of each in
the chain
Monomers are segmented or blocked in a long
sequence
Branched copolymer with a backbone of one type of
monomer and one or more side chains of another
monomer
Copolymers are polymerized from two or more different monomers.
Some differences between Homopolymer and copolymers:
Homopolymers

These polymers have same type of monomer units.
They can be classified as linear, branched, cross
linked and network homopolymers.
Linear homopolymers have linear long polymeric
chain of same types of monomer units. For
example; HDPE
Branched homopolymers have short or long
branches bonded on parent polymeric chain such
as LDPE.
Cross linked and network homopolymers have
braches on parent chain which are interconnected
with each other to form cross linked and network
polymer.
Short term stiffness
Short term impact strength
Copolymers
This is also known as heteropolymer as they are composed of two
different kinds of monomers.

Stereoblock Copolymers - copolymers with only one monomer.
Different sections of the macromolecule having varying tacticities.
containing sections of stereoregular tactic (usually isotactic) - may
also be regio-irregularities, that is, head-to-head and tail-to-tail
additions.
They can be classified as alternating copolymer, block copolymer, graft
copolymer and random copolymers.
In an alternating copolymer, the two monomers are arranged in an
alternative way and can be represented as ABABABABAB.
In random copolymer the monomers are arranged in any order such as
AABAAABBBBAB.
In block copolymers, two blocks of homopolymers are joined together.
It can be represented as AAAAAAABBBBBBB.
Long term stiffness
Long term impact strength
Ethylene undergoes polymerization with different molecules to form ethylene copolymers,
used to modify the properties to meet specific needs. It is a way of improving mechanical
properties.
Polymerizing copolymers of ethylene with other olefin monomers, including:
1. Propylene
2. Butene-1
3. Pentene-1
4. Hexene-1
5. Heptene-1
In linear polyethylenes (mLLDPE, LLDPE, MDPE and some HDPE grades), the branching in the
molecule is achieved through copolymerisation with comonomers such as:




butene-1
hexene-1
octene-1
4-methyl-1-pentene
These comonomers respectively give ethyl (C2), butyl (C4) and hexyl (C6) branches. The
degree of branching increases as the proportion of comonomer in the polymer is increased.
Linear polyethylene grades supplied by different manufacturers can have noticeably
different properties.
1. LLDPE is a copolymer of ethylene and another longer olefin, which is incorporated to
improve properties such as tensile strength or resistance to harsh environments. One of
four α-olefins (1-butene, 1-hexene, 4-methyl-1-pentene and 1-octene) is commonly
polymerized with ethylene to make LLDPE.
b)
Properties of polyethylene depend primarily on:
 molecular weight (or average length of molecular chains)
 molecular weight distribution (MWD) (or the distribution of different chain
lengths)
 degree of long chain branching
 degree of short chain branching (i.e. the number, length and distribution of
the short branches)
All of these make an impact on MFI, Density and Molecular Weight Distribution.
All of these factors can be controlled during the polymerisation process:
Microstructures of polyethylene depend upon type of catalyst, polymerization conditions,
comonomers used. Industrial polyethylenes are commonly classified and named using
acronyms that incorporate resin density or molecular weight:


Density –is directly related to crystalline content, crystallize depends on their
molecular structure. Extent and length of branching stem primarily from the
mechanism of polymerization and incorporation of comonomers. For example,
as branching increases, density decreases, lead to amorphous polymer.
Melt Flow Index – corresponds to the reciprocal of the viscosity and Molecular
weight and molecular weight distribution – measurement called the melt index
(MI), also known as melt flow index (MFI). polyethylenes that have very high
molecular weights, high load melt index (HLMI) is often used. Molecular weight
influence on:
 Melt flow characteristics
 Tensile strength
 Extensibility
 Toughness
Property
Melt Viscosity
Processability
Melting Point
Tensile Strength at Yield
Elongation at Break
Tensile Stiffness (Elastic
Modulus)
Impact Strength
Resistance to Environmental
Stress Cracking
Shrinkage
Warpage
Optical Properties
Transparency
Chemical Resistance
If the density (crystallinity)
is increased
no effect
slightly lower
much higher
much higher
Lower
If the MFI is increased (the
average mol wt. is lowered)
Much lower
Much better
Lower
slightly lower
much lower
If the molecular weight
distribution is narrowed
slightly higher
lower
slightly higher
no effect
no effect
much higher
slightly lower
no effect
much lower
much lower
slightly higher
Lower
much lower
slightly higher
higher
slightly higher
lower
lower
no effect
lower
lower
better
no effect
much higher
lower
no effect
The various types of polyethylene are made by different processes. Polymerisation process
used to produce the polyethylene, it can be a linear molecule or it can be highly branched.
For example, the effect of Polymerization on molecular structure:


Free radical polymerization (LDPE, EVA, EAA) - high pressure and temperatures,
contain both short chain and long chain branching and higher amorphous
content.
Transition metal catalysts - low pressures:
 Ziegler-Natta – for HDPE, will contains essentially no long chain
branching
 supported chromium catalysts (Phillips catalysts)
 single site catalysts
The range of suitable comonomers depends upon the nature of the catalyst or initiator,
each process and each comonomer leads to slightly different polymer structures. Each
comonomer type will effect on physical properties correlate to melt index and density.
Polyethylene Comonomers Commonly Used:
Polymer
Produced by
LDPE
produced only by free radical polymerization of ethylene
initiated by organic peroxides or other reagents that readily
decompose into free radicals
MDPE
LLDPE
mLLDPE
HDPE
Produced by copolymerization of ethylene with a-olefins
using:
 Ziegler-Natta
 supported chromium catalysts (Phillips catalysts)
 single site catalysts
produced by copolymerization of ethylene with a-olefins
using:
 Ziegler-Natta
 supported chromium catalysts (Phillips catalysts)
 single site catalysts
Comonomer
Is often blended with linear
low density polyethylene and
high density polyethylene to
improve processability.
similar to LLDPE, but
comonomer content is lower
Butene-1
hexene-1
octene-1
4-methyl- 1 - pentene
copolymerizing ethylene with selected a-olefins
Comonomers using metallocene based catalyst system.
Butene-1
hexene-1
octene-1
4-methyl- 1 - pentene
Produced by polymerization of ethylene using Ziegler-Natta
or supported chromium ("Phillips") catalysts.
Small amounts (4%of) a-olefin comonomers are used in
many of the commodity grades to introduce low
concentrations of short chain branching, primarily to
enhance processability but also to improve toughness and
environmental stress crack resistance
None - Small amount of aolefin incorporated to improve
polymer properties
c)
Chain branching in low density versions of polyethylene is common, Branching is
classified as:
1. LCB - long chain branching
2. SCB - short chain branching
The degree of branching increases as the proportion of comonomer in the polymer is
increased.
LDPE contains extensive LCB, branches on branches are also common in LDPE, and this
increases amorphous content.
In LLDPE, incorporation of relatively large quantities of alpha olefin comonomers results in
abundant SCB and lowering of density
The similarity in structure of the individual polyethylene molecules allows close packing of
parts of the chain, giving a regular, ordered, three-dimensional network. Presence of straight
chains with regularly spaced side groups facilitates crystallization.
The perfection of the crystallites and the overall crystallinity are mainly influenced by the
degree and distribution of branching in the molecule.
HDPE has higher crystallinity - 60 to 85%.
c)
Polymer
LDPE
MDPE
LLDPE
Applications
dispensing bottles
wash bottles
tubing
plastic bags for computer components
various molded laboratory equipment's
packaging industry for pharmaceutical and squeeze
bottles
 Caps
 Films for food packaging (frozen, dry goods, etc.)
 water pipes and hoses - due to Its plasticity and low
water absorption
Difference between HDPE and MDPE is that HDPE has a high
sensitivity to stress cracking whereas MDPE has better stress
cracking resistance when compared to HDPE.








Remarks
Susceptible to stress cracking
Low strength, stiffness and
maximum service
temperature. This limits its
usage in applications requiring
extreme temperatures
High gas permeability,
particularly carbon dioxide
Highly flammable

It has a low degree of scratch
resistance


Lower gloss than LDPE
Narrower temperature range
for heat sealing
Not as easy to process as
LDPE
used mainly in the production of:
 gas pipes
 sacks, fittings
 packaging films
 carrier bags







High performance bags
cushioning films
tire separator films
industrial liners
elastic films
ice bags
bags for supplemental packaging and garbage bags
narrower molecular weight distribution
(MWD) and more uniform comonomer distribution (CD) than
conventional LLDPE. These differences in molecular architecture
of the mLLDPE provide the polymer with significant improvement
Many LLDPE applications have been replaced with mLLDPE
polymers.
mLLDPE








Stretch film
Stretch film
Frozen food packaging
Liners
Shrink wrap
Lamination films

Due to its molecular characteristics:
 narrow distribution of
molecular mass with short
chain branching and
homogenous distribution of
the chain branching
The processability of mLLDPE is poor,
so a high melt pressure and a high
motor load during the extrusion are
required.
Molecular weight distribution is relatively narrow, has
applications in injection moldings or flat yarns
Molecular weight distribution is wide, is used to make film
products, hollow plastic products and pipes
HDPE










several packaging applications including crates
trays
pipes and fittings
wiring and cables - due to its excellent resistance to
chemical and hydrolysis
bottles for milk and fruit juices
caps for food packaging
jerry cans
Drums
industrial bulk containers
extruded pipe for potable water and gas distribution



Susceptible to stress cracking
High mold shrinkage
Poor UV- and low heat
resistance
Question number 3:
HDPE is produced at low pressures, the polymer is basically linear, with little or no
branching, depending on whether comonomer was used during the polymerisation process.
The overall size of the spherulites in the polymer crystals basically depends on the rate of
cooling and branch length.
Hence linear polyethylene will normally have a higher crystalline content than branched
polyethylene crystallised under the same conditions, while fast cooling from the melt will
normally yield lower crystallinities than slower cooling.
These differences in crystalline structure have important effects on both optical and
mechanical properties.
HDPE is poor conductors of heat, outer surface can be cooled and solidified while the inner
surface of the extruded pipe may remain hot, even as it exits the production line.
This results in a largely amorphous structure on outer side and a partially crystalline
structure on the inner surface.
In pipe this effect will cause a high stress in the wall that will reduce its physical properties,
particularly impact and stress-crack resistance.
Also, since the crystalline portion will have a higher density and more shrinkage than the
amorphous portion, there is a great deal of internal stress developed within the part as one
side shrinks more than the other.
For piping, the following properties should be considered:
Creep
Environmental
Stress
Cracking
Resistance
(ESCR)
CHEMICAL
RESISTANCE
Creep small deformations conditions is not very sensitive to
molecular weight, but is very dependent on polymer density.
The level of creep will increase with loading time, the applied
stress and temperature.
The %strain VS time will increase as density decreasing
compared to higher density
Must be taken into consideration, the crystallinity (and hence
density) of polyethylene depends on the rate of cooling and can
change.
Resistance to environmental stress cracking is a function of
molecular weight, density and structure of the polymer. The
probability of environmental stress cracking decreases when
both the MFI and the density decrease.
A polyethylene with a density of 0.918 g/cm3 is more resistant
than a grade with a similar MFI but density of 0.924 g/cm3
The lower the MFI and higher the density, the better is the
resistance to chemical and solvent attack. It is therefore
important that polyethylene grades of the lowest MFI consistent
with ease of fabrication are used for chemical applications.
Advantage of HDPE
Disadvantages of HDPE


Excellent resistance to
most solvents

High resistance to
Creep


high Stiffness (Elastic
Modulus)
Low frozen-in
orientation.


Poor resistance to
hydrocarbons
(aliphatic, aromatic,
halogenated)
Low Resistance to
Environmental Stress
Cracking
high Shrinkage and
Warpage
Melt Elasticity
and Memory
Melt emerges from the die, the molecules will tend to relax
elastically to their initial randomly coiled state; it is this elastic
recoil which causes die swell.
In addition to die swell, be associated with extrudate defects
such as distortion and “sharkskin”, frozen-in orientation and
melt drawdown.
Frozen-in orientation can have a dramatic deleterious effect on
mechanical properties.
If relaxation time is longer gives higher frozen-in orientation.
The larger and more highly branched molecules take longer than
the shorter molecules.
Question number 4:
Crosslinked polyethylene XLPE (PEX) can be based on HDPE as well as MDPE and on LDPE.
In the crosslinking process the molecules are linked together by strong chemical bonds, and
chemical and physical crosslinking methods are both available.
We use crosslinked polyethylene in applications that may experience high temperatures or
where exceptional toughness is required.



The crystallinity level of a crosslinked polyethylene is lower than that of its
precursor resin, because the crosslinks impede molecular re-organization during
crystallization.
Crosslinked polyethylenes are tougher than their precursors, because their
chains are bound together to form a network.
crosslinked polyethylenes do not flow when their crystallites melt
Crosslinking of polyethylene results in:
Advantage
improved impact resistance
improved abrasion resistance
improved resistance to high temperature
improved environmental stress crack
resistance
improved weathering resistance
improved chemical resistance
Disadvantages
Tensile strength of crosslinked one is lesser than
the tensile strength of un cross linked
polymers.
formation of insoluble and infusible polymers (not
recyclable)
Because of molecules are linked together, the
mobility of molecules is restricted, especially in the
amorphous area of polymer, which results in
higher polymer elasticity.
At the same time, molecular weight is significantly
increased and the flow behavior is also
considerably changed, resulting in increase of:
 viscosity,
 mixing torque
 Reduction of melt flow index and elongation
at break.




Applications
insulation surrounding
high voltage electricity
distribution cables
chemical storage tanks
whitewater kayaks
Different procedures may be employed for the initiation of PE crosslinking, the main
crosslinking methods are:

Crosslinking by radiation - The radiation splits carbon hydrogen bonds to produce
free radicals, when two free radicals meet, they combine to form a covalent bond
between the carbon atoms creating a crosslink between adjacent chains
Radiation crosslinking of polyethylene:
a) Scission of C-H bond,
b) Migration of radicals and
c) Formation of covalent C-C crosslink


Crosslinking by peroxides - The peroxy radicals abstract hydrogen atoms from the
polyethylene chains to create free radicals, crosslinking takes place when two
radicals react to form a covalent bond.
Peroxide crosslinking of polyethylene:
a) Decomposition of dicumyl peroxide,
b) Abstraction of hydrogen from polyethylene chain, and
c) Formation of covalent C-C crosslink
Crosslinking by silane compounds - Crosslinking chemical reactions consist of two
reaction steps:
1. Hydrolysis of alkoxy group to a silanol group in the presence of water and an
alcohol is released as a by-product.
2. Condensation of two silanol groups into siloxane crosslink and regenerate
water.
Crosslinking by radiation
Crosslinking by peroxides
radiation
 post-processing
crosslinking
 one step process in solid
Advantage
or molten state
 variable crosslinking
conditions
 difficult to crosslink thick
article with irregular
shapes
 heterogeneous network
Disadvantages
 free radical accumulation
Crosslinking by silane compounds
peroxides
 the most uniform and
homogeneous crosslink
distribution in whole volume
 high gel content
 crosslinking of thick-walled
articles
 crosslinking proceeds only in
molten state
 possible side reactions
 decrease of crystallinity
 Energy intensive processes, high
scrap rates and low outputs.
Various methods for crosslinking of PE:
silane compounds
 post-processing crosslinking in
solid state
 variable crosslinking conditions
depending on composition
 crosslinking is without free
radical formation
 heterogeneous network
 degree and uniformity of
crosslinks is highly dependent
on catalysts and water diffusion
 curing time is very high
 possibility of premature
crosslinking by moisture pellets
during storage
Question number 5:
d) From the curve it can be seen that the longer the time and the higher the
temperature, then the failure stress decrease. As noted before (Question
number 3), the level of creep will increase with loading time, the applied
stress and temperature.
Also, the thickness and diameter of the pipe effects on failure stress.