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27-Mar-15
1
Molecular weight of Polymers
MOLECULAR WEIGHT OF POLYMERS
Some natural polymers have molecules of same molecular weight (monodisperse)
Synthetic polymers are polydisperse (different polymer molecules have different molecular weights) as they have
distributed molecular weights.
Molecular weights are controlled during the synthesis of polymeric resins.
The properties of polymeric materials are strongly dependent on the molecular weights.
Properties of Polymers depend on the molecular
weight
Almost all properties of polymers have dependence on molecular weight.
Fig. Softening point of epoxy resin increases with increase of molecular weight
Properties of Polymers dependent on the molecular
weight
Fig. Polymer properties vs Polymer size
much lower molecular weight ; poor mechanical property much higher molecular weight ; too tough to process optimum molecular weight ; 15,000 - 100,000 g/mol
Methods for the measurement of molecular weight
of polymers
Mass spectroscopy
Colligative properties measurement
Viscosity measurement
Gel permeation chromatography (GPC)
Gel Permeation Chromatography
Complete molecular weight distribution of polymers can be determined.
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Gel Permeation Chromatograph
GPC can separate complex polymer parts.
Molecular weight distribution
Polymer does not contain molecule of the same size and, therefore does not have a single molecular weight.
Polymer contains large number of molecules- some are big, some small.
Variation in molecular size and weight is known as molecular weight distribution (MWD)
MWD exist in every polymeric system and this determines to a certain extent the general behaviour of polymers.
Molecular weight distribution
Average molecular weight of polymers is determined. Two ways of representing molecular weight of polymers:
Number-Average Molecular weight, Weight-Average Molecular weight
Number-average Molecular weight (Mn) Consider a sample of a polydisperse polymer of total weight W in which N=total
number of moles; Ni=number of moles of species i (comprising of the same
size); ni = mole fraction of species i ; Wi = weight of species i ; wi = weight
fraction of species i; Mi = molecular weight of species i; xi = degree of
polymerisation of species i
=
= =
=
=
=1
Molecular weight distribution
Number-average Degree of Polymerisation (DPn)
= 0=
Weight-average Molecular weight (Mw)
= =
=
2
Molecular weight distribution
Weight-average Degree of Polymerisation (DPn)
= 0=
Example A polymer sample containing 50 mol% of a species of molecular weight
10,000 and 50 mol% of species of molecular weight 20,000.
Mn= (0.5(10,000)+0.5(20,000))/1= 15,000
Mw = [(10,000)2+(20,000)2]/(10,000+20,000) = 17,000
Example Suppose that a polymer consists of 103 chains of M1 = 10
6 g/mol, and 103
chains of M2 = 104 g/mol. Then
Weight average molecular weight is greater than number average molecular weight
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Polydispersity Index
The ratio of weight-average molecular weight to number-average molecular weight is called the dispersion or polydispersity index (I).
It is a measure of width of the molecular-weight distribution curve.
Normal values of I are between 1.5 and 2.5 but may range to 15 or greater.
The higher the value of I , the greater is the spread of the molecular weight distribution of the polymer.
Example
A sample of PVC is composed according to the following fractional
distribution.
(a) Compute Mn, Mw, DPn and DPW
Wt. fraction
0.04 0.23 0.31 0.25 0.13 0.04
Mean mol. Wt. x103 g/mol
7 11 16 23 31 39
Solution
Display data in Table
Wt. Fraction
(wi)
Mean mol.
Wt. (Mi)
wi x Mi wi/Mi
0.04 7,000 280 0.57 x 10-5
0.23 11,000 2,530 2.09 x 10-5
0.31 16,000 4,960 1.94 x 10-5
0.25 23,000 5,750 1.09 x 10-5
0.13 31,000 4,030 0.42 x 10-5
0.04 39,000 1,560 0.10 x 10-5
Total 19,110 6.21 x 10-5
Solution Using equation
Using equation
Number of molecules per gram = (6.21 x 10-5) x (6.02 x 1023)
= 3.74 x 1019 molecules per gram
=1
=1
6.21 105= 16,100 g/mole
= = 19,110 g/mole
= 0=16,100 /
62.5 /= 258
= 0=19,110 /
62.5 /= 306
1 mer weight of vinyl chloride (C2H3Cl) = 62.5 g/mer
Effect of molecular weight and molecular weight
distribution on Physical and Mechanical properties
High molecular weight increases
Tensile strength
Impact toughness
Creep resistance
Melting temperature
Entanglement of chains
Effect of molecular weight and molecular weight
distribution on Mechanical properties
More entangled the molecules, the more tensile force would be required to cause them slide.
Narrow MWD results in higher strength than broad MWD. Impact toughness increases with molecular weight (long chains would
transmit energy along the chain and share over more atoms, resulting
in dissipation of energy through vibrations, minor translations, and
heat).
Entanglement also allow some of the energy transport to other molecules, resulting in decrease in concentration of energy.
Broad MWD result in poor ability to transmit the energy between the chains.
Higher MW increases mechanical properties and broader MWD will decrease the properties.
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Effect of molecular weight and molecular weight
distribution on Melting point
More entangled the molecules, the more input energy required to get free movement and melting point increases.
High molecular weight enhances materials thermal stability and makes them suitable for high temperature applications.
Low molecular weight generally reduces melting point and improve ease of processing.
Crosslinks increase molecular weight, resulting in increased melting point and mechanical properties.
Melting
point
Mechanical
properties
Effect of molecular weight and molecular weight
distribution on processing
For some types of processes, a narrow MWD is preferred while in
others broad MWD give better processing.
Narrow MWD means that material will melt over a narrow range of temperature (same size molecules will require same amount of energy
to cause them to move freely)
Narrow MWD is suitable for injection moulding which depend on freezing of molten polymer.
Extrusion requires broad MWD. High melt strength (measure of ability of molten material to be
shaped) is achieved when MWD is broad. The small molecules melt
first and lubricate the entire mass giving some ease of sliding to large
molecules.
Broad MWD lowers effective melting temperature whereas the large polymers give strength to the melt because of their residual
entanglement.
Bimodal MWD provide ease of processing (UHMWPE).
Melt Index
A parameter used to describe polymer.
Flow characteristics of the polymer are strongly dependent on MW and MWD.
A melt index measurement is a very easy method of assessing the MW of polymers
(commonly used in industry).
Definition: It is defined as the mass of polymer, in grams, flowing in ten minutes through a capillary of a
specific diameter and length by a pressure applied via
prescribed alternative gravimetric weights for
alternative prescribed temperatures.
High melt index indicates a low molecular weight and vice versa. Melt index gives information about molecular weight but not MWD. Ease of melting of polymer can also be determined. High melt index numbers (ease of melting, lower energy input and easy
processing.
Length of Polymer chain
= ( ) .
For a polyethene molecule with Dp = 1000, the molecule
would have a maximum length by:
= . .
= 2,450
Length of Polymer chain
Anisotropic nature of polymer chains
Conformation
The change in shape of a given molecule due to torsion about single () bonds referred to as change in conformation.
A polymer molecule can take many different shapes due to its degree of freedom for torsion about bonds.
Rapid change in conformation
is responsible for the sudden
extension of rubber and high
flexibility of polymers.
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Shape (Steric) Effects
Effects of the shape or size of the atoms on the properties
of the polymer are called Steric effects.
Steric effects determine properties of polymers
Shape and size of the pendant groups have major effect on some of the important properties of polymers
Large pendant groups, crystallinity decreases (strength and thermal properties are lower for such polymers)
But
Pendant groups increase strength because of interference of the bulky groups of chains.
Shape (Steric) Effects
Bulky groups hinder the movement of the chains
Hindered movement increase mechanical and thermal properties similar to intermolecular forces or
entanglements
Influence of Bulky side groups (steric hinderance)
Crystallinity decreases Restrict the movement
of chains
Strength increases, elongation decreases and
thermal properties increases
Strength decreases, elongation increases and
thermal properties decreases
Net effect determines properties of polymers
Shape (Steric) Effects
Aromatic groups are very bulky and cause the steric hindrance effects
Some pendant groups are flexible such as aliphatic groups
Aliphatic groups do not inhibit translational, rotational or flexing movement.
Branched chain polyethene (LDPE) is very flexible material due to aliphatic side branches despite reduction
in crystallinity.
Aromatic pendant group decreases crystallinity,
increases strength and decreases elongation
whereas Aliphatic pendant groups decreases
crystallinity and strength but do not decrease
flexibility
Shape (Steric) Effects
Pendant groups (aromatic groups) in backbone affects physical properties considerably.
A stiff backbone results in high strength, impact toughness and higher thermal properties.
Aramid fibres (Kevlar and Nomex) have aromatic groups in backbone.
Aliphatic chains are more flexible.
Effect of Substituent group/bulky groups/pendant groups
Bond rotation
become difficult
Bulky side groups
n
CH2 CH2
CH CH2
CH3
n
CH2 CHn
Strength, rigidity, modulus
Flexibility, elongation, crystallinity
Decreases Increases
CH2 C
CH3
C O
O CH3
PE
PP
PS
PMMA
Classification of polymers on the basis of steric effect
Steric structure of chains
High packing ( increased crystallinity),
decreased elongation, high strength, High melting points,
good resistance against solvents and chemcials
Thermoplastics
Small branches decreases viscosity and
Improves workability in melt Long branches increase viscosity,
Increase ductility and low strength
3-D polymers, Increased strength
and rigidity, Highly brittle, No ductility,
Donot melt instead Degrade and char
Thermoset
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Geometrical Isomerism/Chain configurations
Isoprene exhibit geometrical Isomerism
Cis-Isoprene Trans-Isoprene
Natural rubber is cis-polyisoprene
(has elsotmeric nature)
Trans-polyisoprene (Gutta Percha)
has rigid nature
Head to Tail and Head to Head or Tail to Tail configurations
Polymer Crystallinity Packing of molecular chains
to produce ordered structure
Polymers are:
Semi-crystalline
Amorphous
LDPE = 50 % crystallinity
HDPE = 75 % crystallinity
Any chain disorder or
misalignment (twisting,
kinking, coiling) result
in amorphous regions
Morphology of polymers Structure in the solid state,
size and shape of crystal and
crystal aggregates
Polymers have small
crystalline regions which are
dispersed in amorphous
regions
TEM of single crystal of polyethene
having thickness 10-20 nm and 10 m
long
Multilayered structure
Fig. Schematic drawing of single crystal with regular chain folding
Fringed Micelle Model of Semi crystalline polymer
Chain folded model
Crystalline region
Amorphous region
Morphology of crystallites from Polymer melts
Spherulites lead to grain
boundaries
Spherulite structure in natural rubber
Morphology of crystallites from Polymer melts
Spherulites are similar to grain boundaries in polycrystalline materials.
PE,PP,PVC,PTFE and Nylon form spherulitic structure when they
crystallise from the melt.
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Kinetics of Crystallization
Formation of nuclei
Growth of crystallites ( spheurlites formation in all direction depending on
external conditions, High supercooling
creates many small crystallites while
crystallization at higher temperature
produces fewer spherulites)
Factors controlling degree of crystallinity
1. Rate of crystallization
2. Controlling chemistry
3. Side groups
4. Tacticity
5. Branching
6. Cross-linking
Pre-deformation by Drawing
Heat treating
Increasing crystallinity of polymers after solidification
Crystallinity Increases Tensile strength, Melting point,
Decreases ductility, Increase resistance
to dissolution
Determination of crystallinity
By measuring specific
volume/densities
DSC
NMR
XRD
c= density of 100 % crystalline material
a= density of 100 % amorphous material
s= density of unknown sample
Thermal Transitions in Polymers
Heating polymer Polymer chains move internally to absorb energy input
Molecular twisting, vibrating, stretching, translation and other movement
Heat distortion
temperature, HDT
Glass transition temperature,Tg
Melting
point, Tm
Decomposition
temperature, Td
Increasing
temperature
Hard
Stiff
Glass like
Limited atomic
movements
small volume
increases
Moderately hard and stiff
Creep
Slightly higher atomic
movements
Small volume
increases
Limit for structural
applications
Pliable, leathery
Larger, longer-range
and
coordinated
movements
Liquid
Entire polymer
molecules
move
independently
Degradation
Chain breakage
Gas release
Char formation
Dramatic and non-reversible
change in
properties
Color change
Heat Distortion Temperature, HDT
weight
Height gauge
Stirrer
Heat transfer
fluid sample
Thermometer
ASTM, D648, Deflection under load test to determine heat distortion temperature
Sample dimensions
= 5 x x inch
Maximum structural use temperature ,
especially for any application in which
the part will be loaded mechanically
Glass Transition Temperature The thermal transition that occurs when the
polymer molecule begins to make
coordinated long-range movements is called
glass transition temperature (Tg). Polymer
becomes pliable and leather like.
HDT is not formal thermal transition HDT is easier to determine than Tg
Measurement of Glass transition temperature
1.By measuring changes in specific
volume with temperature
Chain
mobility
freezes
Chain
mobility
increases
Tg
Increase in
temperature
Tg is a temperature at which
frozen free volume of 2.5 %
appears and stays constant at
low temperature
2. Differential Scanning
Calorimetery
DSC monitors heat effects associated with phase transitions and
chemical reactions as a function of temperature
In DSC difference in heat flow to the sample and a reference at
same temperature is recorded as a function of temperature. The
difference in heat flow can be positive or negative.
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DSC curve
The integral under the DSC
peak , above the base line,
gives the total enthalpy
change for the phase
transition
Determination of Crystallinity by
using DSC curve
100material same of sample ecrystallin totally theofEnthalpy
Sample theofEnthalpy % Crystallinity =
Multiple Transitions
Polycarbonate Gamma
transition ,
T= -100 oC
Gamma transitions
took place due to the
presence of certain
side groups ( may
increase toughness
of polymer)
Factors effecting glass transition temperature
Bulky side
groups
Polar side
groups
Double chain bond or
aromatic groups
Crystallinity
Molecular weight
Branching and
crosslinks
LDPE Tg = -110 oC
HDPE Tg = -90 oC
Tg = 0.5-0.8 Tm (K)
Tg of different vinyl polymers
Melting of polymers Melting is simply the process of a polymer chains
gaining sufficient energy to move independently
HDT Tg
Tm Td
Hard, stiff Leathery Liquid
Thermoplastic (Amorphous)
Thermoplastic (crystalline)
HDT
Tm
Temperature Char
Tm Td
Hard , stiff Liquid Char
Thermosets
HDT Tg Td
Hard, stiff Semi-rigid
General behaviour of thermoplastics and thermosets
Decomposition temperature, Td When input energy has localized in the bond and equals the bond energy (breaking of
covalent bonds occur), the corresponding
temperature is called decomposition
temperature. Thermoplastics decomposition occur
in liquid state release gas and may
form crosslinks.
Thermosets decomposition occur in
solid state forming char ( by product
of gases are often released and the
polymer may begin to change color,
often yellowing or blackening.
Measurement of decomposition temperature DSC or DMTA
TGA ( easier and most commonly used
method) TGA Sample is progressively heated
and changes in the weight of the
material are recorded
Processing Temperature this is a temperature at which plastic
material can be conveniently
molded TP = Tm + 50 to 100 oC
Non-thermal energy inputs energy input from mechanical source
or from any other sources (sound ,
light, x-ray)
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Thermogravimetric analysis Onset of degradation
TGA curve of a polymer
Differential thermal analysis (DTA)
Schematic illustration of DTA cell
Changes in sample which lead
to absorption or evolution of
heat can be detected relative
to the inert atmosphere
exothermic
DTA curves
Diffusion in Polymers
Permeability and absorption characteristics related to the degree to which foreign substances diffuse into the material.
Penetration of these foreign substances can lead to swelling and chemical reactions with the polymer molecules, which often leads to
degradation of material and loss of physical properties.
Diffusion rates are greater through amorphous substances than crystalline.
Size of foreign molecule affect the diffusion process. Small and inert molecules diffuse faster.
Diffusion in polymers according to Ficks law is defined as
J = diffusion flux of a gas through the membrane (cm3 STP)
PM = Permeability coefficient = DS, (D= diffusion coefficient and S= solubility of
diffusing species in polymer
x = Membrane thickness P = Difference in pressure of gas across the membrane
Diffusion in Polymers
Applications requiring low permeability rates:
Automobile Tires and inner tubes, beverage and food packaging.
Application requiring high permeability rates:
Polymer membranes used as filter to separate one chemical from
another (desalination of water)
Polymer electrolyte fuel cells.
Example Problem Mechanical and Thermomechanical Behaviour of Polymers
Stress-strain behaviour
Brittle (thermosets and some thermoplastics)
Ductile (thermoplastics)
Totally elastic (elastomers)
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Deformation of Semicrystalline Polymers
Mechanism of Elastic deformation
Elongation of the chain molecules from their stable conformation in the direction of the applied stress by bending and stretching of the
strong chain covalent bonds.
Some displacement of adjacent molecules resisted by intermolecular forces.
Elastic modulus is combination of the moduli of crystalline and amorphous phases.
Elastic deformation in polymers occur at low stress level.
Mechanical and Thermomechanical Behaviour of Polymers
Effect of temperature on mechanical properties
Decrease in modulus
Reduction in tensile strength
Increase in ductility
Effect of rate of deformation
Slower deformation result in decrease in modulus, decrease in strength and higher elongation.
Deformation of Semicrystalline Polymers
Stages of Elastic deformation
Two adjacent chain-folded
lamellae and interlamellar
amorphous material before
deformation
Elongation of amorphous tie
chains during the first stage
of deformation
Increase in lamellar crystallite
thickness (which is reversible)
due to bending and stretching of
chain in the crystallite
Deformation of Semicrystalline Polymers
Mechanism of Plastic deformation
Interactions between lamellar and intervening amorphous regions in response to applied tensile load.
Two adjacent chain-folded
lamellae and interlamellar
amorphous material before
deformation
Tilting of lamellar chain folds
during second stage
Separation of crystalline
block segment
Orientation of block segments and tie chains with the tensile
axis in the final deformation stage
Heating specimen at
some arbitrary stage of
deformation will allow
material to regain
spherulitic structure and
also shrink (depends on
the annealing
temperature and also the
degree of deformation)
Deformation of Semicrystalline Polymers Deformation of Semicrystalline Polymers
Macroscopic deformations of polymers
Methods of increasing strength of polymers:
Molecular weight , Degree of Crystallinity, predeformation by drawing, heat treating
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Elastomers
Criteria for polymer to be called as Elastomers Rubber like elasticity (ability to bounce back) Ability to undergo large deformations and then elastically spring back to
original form
Ability to spring back arises due to light cross-linking between the molecular chains.
Highly amorphous and composed of cross-linked chains that are highly twisted, coiled and kinked.
Chain bond rotation must be relatively free for the coiled chains to readily respond to an applied force.
The onset of plastic deformation must be delayed. Elastomer must be above glass transition temperature.
Properties: Low elastic modulus and vary with increase of strain. Non-
linear stress-strain curves.
Deformation of Elastomers
Highly coiled, twisted and
cross-linked chains
Chains straighten up upon application of
tensile force
Driving force for elastic deformations
Entropy (measure of degree of disorder within the system) Elastomer experiences rise in temperature and modulus increases with
increase of temperature unlike to metals.
Ordered chain
(low entropy)
Disordered chain
(High entropy)
Increase in Temperature
Elastomers
Vulcanisation The process of crosslinking in elastomers is called vulcanisation.
Useful elastomer is obtained by reacting 1 to 5 parts of sulphur with 100 parts of rubber (1 crosslink for every 10 to 20 repeat units)
Cross linking of natural rubber (cis-isoprene)
Comparison between Vulcanised and Unvulcanised rubber
Unvulcanised rubber with very few cross links is soft, tacky and has poor resistance to abrasion.
Modulus of elasticity, tensile strength and resistance to degradation by oxidation are enhanced by vulcanisation.
Types of Elastomers
Aliphatic Thermoset elastomers
These are the most common elastomers. These have a double bond after polymerization has occurred. These are noncrystalline. These are highly flexible.
Natural rubber (extracted from tree)
Types of Elastomers
Synthetic Polyisoprene or Isoprene rubbery (IR)
Produced to meet the shortage of natural rubber during WW. Mixture of both cis and trans molecular form. Used for tire for light weight vehicles such as bicylces and early automobiles. Natural rubber is used extensively because of its low cost.
Butadiene rubber (BR)
No cis or trans isomers Lower mechanical strength because of no pendant methyl group but also more
flexibility
Lower cost (all synthetic from cheap monomer) Improvement of low-temp flexibility Compatibility with other polymer materials Poor oil resistance and sensitivity to oxidation and UV radiation.
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Types of Elastomers
Oil resistant elastomers
Nitrile Butadiene rubber (NBR)
Copolymerization of butadiene and acrylonitrile More expensive than SBR or BR Excellent oil and oxidation resistance. Has good abrasion resistance but poor low temperature elasticity. Typical applications include: oil and fuel lines, gaskets, seals, conveyor belts and
coatings for printer rolls.
CRChloroprene rubber (neoprene)
Thermal stability Non-flammable Applications : fuel hoses, boots, shoe soles, and coatings for fabrics where oil
resistance and nonflammability are important.
Example Problem
How much sulphur must be added to 100 g of polyisoprene rubber to cross
link 5 percent of the mers? Assume all available sulphur is used and that
only one sulphur atom is involved in each cross linking bond.
MW (polyisoprene) of mer unit = 68 g/mol
Mole of polyisoprene = 100 g/68 g/mol = 1.47 mol of polyisoprene
For 100 % cross-linking with sulphur we need 1.45 mol of S or
1.47 mol x 32 g/mol = 47 g of sulphur
For cross-linking 5 percent of the bonds we need only
0.05 x 47 g = 2.35 g of S
Types of Elastomers
Thermoplastic elastomers
Not cross linked. Noncrystalline. Aliphatic. Ease of processing and recycle able More temperature sensitive. Not well developed.
Ethylene Propylene Monomer rubber (EPR)
Amorphous Improved oxidation resistance and improved acid and alkali resistance over
natural rubber but have poor compatibility with other rubbers, poor creep
resistance, relatively poor resilience and poor resistance to hydrocarbon solvents.
Applications: bumpers, hoses, seals, mats, wire insulation , appliance parts , gaskets and coated fabrics.
Types of Elastomers
Thermoplastic elastomers
Copolymers such as SBR and SIS are thermoplastic elastomers
Types of Elastomers
Silicones
Three forms--- oil, elastomers, moulding compound, sealants, adhesives Oils are used as mould releases, coolants, lubricants, hydraulic fluids, etc. Silicone elastomers have high molecular weight and crosslinked chains. Much higher molecular weights than former are called silicone moulding
compounds.
Advantages of silicone elastomers and moulding compounds: Low surface tension, nonionic/nonpolar characteristics , hydrophobicity, high thermal stability
(250 C) , oxidation resistance, high degree of flexibility at low temperatures, room
temperature curing (RTV),etc.
Biomedical implants and tubing ,etc make excellent use of its superior biocompatiblity.
Poly dimethyl siloxane
Different forms of degradation of polymers
Degradation
(An Irreversible process leading to a significant change in
the structure of a material, typically characterized by a
loss of properties and/or fragmentations)
Stabilization
(The protection of polymeric materials
from which lead to deterioration of
properties)
Photo-degradation
(Degradation preceded by light (UV) Photo-stabilization
Screening radiation Absorption of radiation Radical Scavenging
Bio-degradation
(Degradation processes in which at least
one step is mediated by biological agents) Bio-stabilization
Chemical inertness Coating of anti-microbial agents
Thermal Degradation (Degradation caused by heat and temperature)
Thermal Stabilization Flame Retardancy Introduction of thermal stabilizers
Ultrasonic Degradation (Degradation caused by Ultrasonic sounds)
High Energy Degradation (Degradation caused by high energy radiations like X-ray, ,, rays)
High Energy Introduction of radiation protectors
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Polymer Additives/Compounding ingredients
Functions of Additives
Facilitate processing without degradation or decomposition To cater the requirements of a specific application.
Desirable Properties of Additives
Additives can be solids, liquids or rubbery materials.
Stable under the processing conditions and also under the conditions of their applications.
Efficient, i.e, small quantity should do the required job. Compatible i.e, non-bleeding Non-toxic and not impart any taste and odour to the polymer and should not
negatively effect the inherent properties of the polymer
Cheap.
Types of Additives
Fillers
Functions:
Improvement of physical properties. Making the material cost less.
Fillers may be organic or inorganic, mineral, natural or synthetic in nature. They may
be particulates, fibrous, resinous or rubbery materials.
Examples, addition of saw dust or wood flour in phenol formaldehyde.
Reinforcing Fillers
Improve the mechanical properties of the polymer.
Can be called as composites.
Types of Additives
Plasticisers and softeners
Functions:
Improvement in ductility/flexibility Reduce glass transition temperature.
External Plasticisation
Addition of glass micro-spheres bring about plasticisation effect by acting as a
spacer between the molecules of the polymer.
Internal Plasticisation
Incorporation of polymer molecules in the polymer so that they become part of the polymer structure.
Addition of various types of esters can bring about internal plasticisation effect. Examples trioctyl phosphate, phthalic acid ester , etc.
Types of Additives
How Plasticisers work?
Types of Additives
Anti-aging Agents
Functions:
To minimise the structural changes (chain rupture, cross-linking, chromophoric groups, polar groups) occur due to chemical reactions such as oxidation, ozone
effect, UV light attack ,etc.
To increase the service life of polymers
Antioxidants (thiobisphenol,alkylphenol), UV-light abosrobers (hydroxybezophenone)
etc are added.
Types of Additives
Antistatic agents
Functions:
To stop the build up of electrostatic charges on the surface of polymeric materials which can lead to fire and dust catching.
Compounds such as fatty amides, quaternary ammonium compounds, glycol
esters and sulphates are very common agents. They are usually incompatible
with the polymer due to which they slowly migrate to the surface and give a
protective layer.
Polyethylene glycol is a very widely used antistatic agent for PVC conveyor belting.
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Types of Additives
Blowing agent
Functions:
To produce cellular polymers (foams,sponges)
Blowing agent at the manufacturing temperature turn to a gaseous form to pervade
the mass of the polymer.
Common blowing agents are azo-dicarbonamide (for PVC and polyolefins)
which decomposes at 190-230 C giving off N2, CO and CO2 gases.
Types of Additives
Lubricants and flow promoters
Functions:
To prevent sticking of the polymer to the metal walls.
External lubricants have low solubility and carbon chain length C22-C32. Examples
are stearic acid, myristic acid, paraffin wax, etc.
To improve the flow of the polymer melt.
Internal lubricants remain within the mass of the polymer reduce the cohesive forces
of the molecular interfaces and thus improve the flow of polymer melt. Examples are
stearyl alcohol, metal stearates, monoglycerin esters, etc.
To reduce friction in the final products.
MoS2 and graphite are added in small quantities (1-2 %) to reduce friction in
applications such as nylon gears and bearings, etc.
Types of Additives
Colourants
Functions:
To give aesthetic appeal. To give a means of identification.
Four methods of imparting colours to Polymers
Coating the surface Dyeing the surface Incorporation of the materials which would provide chromophoric groups in the
polymer
Colourant fall into two groups- dyes and Pigments
Dyes give colour by dissolving in the polymer (much better against fading but
sensitive to light)
Pigments disperse throughout the mass of the polymer (tend to migrate more)
Types of Additives
UV-degradants
Functions:
To facilitate the disposal of the polymers after they have been used.
Photodegradants
On absorbing light, the heat generates a highly reactive chemical intermediate which
destroys the polymer.
Iron di-thio-carbamate is an example of such an agent. The photo activator reduces
the molecular weight of the polymer below 9,000 at which polymer becomes
biodegradable.
Polymer P* (reactive intermediate) Polymer of lower mol. wt UV-radiation
Liquid Crystal Polymers
LC polymers have liquid crystalline state LCP have ability to be aligned in highly ordered configuration As solids form domain structures having characteristics intermolecular
spacing.
Partially crystalline aromatic polyesters based on p-hydroxybenzoic acid and related monomers.
LCP have outstanding mechanical properties at high temperatures, excellent chemical resistance, inherent flame retardancy and good
weatherability.
Liquid Crystal Polymers
LCP are used in LCDs on digital watches, televisions, monitors, etc.
Cholestric LCPs are used for LCDs as they are liquid at room temperature, transparent and optically anisotropic.
LCPs are sandwiched tween two glass sheets. The outer surface is coated with conductive film. The characters are etched into the film on the
side to be viewed. Application of voltage will cause disruption in the
orientation of the LCPs molecules resulting in darkening of the LCP
material and in the formation of a visible character.
27-Mar-15
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Fracture in Polymers
Thermoset have brittle fracture
Thermoplastic experience both ductile and brittle fracture
Brittle fracture occurs due to
Reduction in temperature Increase in strain rate Presence of notch in the sample Any modification in polymer which increases glass transition temperature
Fracture in Polymers
Crazing
Some thermoplastic experience crazing Formation of microvoids, growth and coalescence
results in crack formation
Fracture in Polymers
Thermoset have brittle fracture
Thermoplastic experience both ductile and brittle fracture
Brittle fracture occurs due to
Reduction in temperature Increase in strain rate Presence of notch in the sample Any modification in polymer which increases glass transition temperature