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Elastomer TechnologyPE-4107Lecture 08,09
Types of Rubbers (GPE)
General Purpose Elastomer- Diene-Based Elastomers
Polybutadiene Structure and Properties
Effect of structure on Tg and Tm
Types of Polybutadiene
Uses
Styrene-Butadiene Rubber Monomers
Microstructure and MWD
Polymerization methods Cold method
Solution polymerization
Comparison
Processing and additives involved
Lecture Overview
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Polymerization of conjugated dienes Butadiene
Isoprene
Chloroprene
- They involves activation of either one or both double bonds to
give
1,2
3,4
1,4
For isoprene and chloroprene eight arrangements are
theoretically possible- The 1,2 and 3,4 polymers can be isotactic,
syndiotactic or atactic- While for 1,4 polymer both cis and trans
configurations are possible
The residual unsaturation in the polymer chains provides
convenient sites for the introduction of elastomeric network of
cross-links (vulcanization)
Conjugated dienes are the source of some of the most important
commercially available synthetic rubbers or elastomers
General Purpose ElastomerDiene-Based Elastomers
R Monomer Elastomer
H 1,3-butadiene Polybutadiene
Cl 2-chloro-1,3-butadiene Polychloroprene
CH3 2-methyl-1,3-butaiene Polyisoprene
General Purpose ElastomerDiene-Based Elastomers
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Polybutadiene is the second largest volume synthetic elastomer
next only to SBR
Polybutadiene can be produced by- Free radical addition
polymerization of butadiene
Resulting polymer has predominantly trans-1,4 units with only
about 20% 1,2 units
As the polymerization temperature is increased the proportion of
cis-1,4 units increases while that of 1,2 structure remains
unchanged
- Butadiene can also undergo anionic polymerization with
Lithium or organolithium initiators like n-butyllithium in
nonpolar solvents such as pentane or hexane
Resulting polymer has high cis-1,4 structure Which decreases as
either higher alkali-metal initiators or more polar
solvents are used
General Purpose ElastomerPolybutadiene (BR)
Polybutadiene can be produced by
- High molecular weight polybutadiene with a high content of
trans-1,4 polymers is prepared by solution polymerization of
butadiene
Using stereo-selective coordination Ziegler-Natta catalysts
Slight changes in catalyst composition can produce drastic
changes in polymer composition
General Purpose ElastomerPolybutadiene (BR)
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1,3-butadiene can be polymerized to produce a variety of isomers
and only some of them are elastomers
- The isomers differ in the position of insertion 1,2 versus
1,4
1,4 polymerized BR can exist in cis or trans form depending on
the orientation of the substituents across the enchained double
bonds
1,2-BR can be differentiated by the tacticity of the
substituents containing the pendant double bond
- Each of these isomeric forms are different BR elastomers with
unique physical, mechanical, and rheological properties
A mixtures of these isomers on a single chain can leads to
different elastomeric properties
Polybutadiene (BR)Microstructures
The 1,2-BR exists as three isomers- This orientation corresponds
to syndiotactic, isotactic and atactic
BR
Syndiotactic BR is made with cobalt Ziegler-Natta catalyst- It
is a crystalline thermoplastic melting at about 220 0C- It is
compatible with natural rubber (NR) and the blends are
excellent thermoplastic elastomers- Intra-chain mixtures of
syndiotactic and atactic isomers lead to
lower melting points
Amorphous 1,2-BR is made by anionic lithium alkyl polymerization
modified by chelating diamine- This leads to BR with ~99% of the
butadiene inserted at the 1,2
position- Intermediate structures having a greater amount of 1,4
insertion
are obtained by
Containing the ratio of chelating modifier to the anionic
lithium catalyst Increasing this ratio increases the vinyl content
and Tg of the BR
As well as polymerization temperature
MicrostructureStructure and Properties of 1,2-BR
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1,4-BR is in both cis and trans forms is made with transition
metal coordination catalyst- This catalyst is closely related to
Ziegler-Natta catalysts for polyolefins
Trans-1,4-BR has a melting point between 50 0C and 150 0C and is
not used as an elastomer
Alternate synthesis of amorphous trans-1,4-BR using alkoxide of
group II, reduced with organolithium or organomagnesium
compounds
Similar amorphous 1,4-BR containing 90% mixture of cis and
trans1,4-BR is made using living anionic catalyst- This polymer is
used in tyres due to its low hysteresis combined with
good wear characteristics- An advantage of anionic
polymerization is that these can be terminated
during polymerization with
Ketone
Aldehyde
Amine
Tin/Silicon halides
- Presence of this functionality leads to strong interaction
with fillers
MicrostructureStructure and Properties of 1,4-BR
Cis-1,4 Trans-1,4
MicrostructureEffect on Tg and Tm
Tg (0C) Tm (
0C)
Cis -106 2
Trans -107 97-125
Syndiotactic 1,2 -28 156
Isotactic 1,2 -15 126
Atactic -4 None
Microstructure dictates the glass transition temperature of the
polymer
- Which controls some of the performance of the compounds
Along with controlling microstructure different catalysts create
a wide range of microstructures
- This is very important because it controls the processability
of the polymer
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Polybutadiene (BR)Types of Polybutadiene
The catalyst used in the production determines the type of
polybutadiene product
Cis (%) Trans (%) Vinyl (%)
Neodymium 98 1 1
Cobalt 96 2 2
Nickel 96 3 1
Titanium 93 3 4
Lithium 10 to 30 20 to 60 10 to 70
High cis polybutadiene
- This is characterized by high proportion of cis and a small
proportion of vinyl
- Manufactured using Ziegler-Natta catalysts based on transition
metals
Low cis polybutadiene
- Using alkyllithium as catalyst produces polybutadiene called
low cis which contains 36% cis and 54% trans and 10% vinyl
Tyres- Polybutadiene is largely used in various parts of
automobiles tyres
The majority of it being high cis configuration
- Polybutadiene is used primarily in sidewall of truck tyres
This helps to improve fatigue to failure life due to the
continuous flexing during run
As a result tyres will not blow out in extreme service
conditions
Plastics- About 25% of the produced polybutadiene is used to
improve the
mechanical properties of plastics
It is used in particular of high-impact polystyrene (HIPS)
Golf balls- Most golf balls are made of an elastic core of
polybutadiene
surrounded by a layer of a harder material
- Polybutadiene is preferred to other elastomers due to its high
resilience
Polybutadiene (BR)Uses
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These are derived from petroleum oil- This applies to most
elastomers with the obvious
exception of NR
SBR represents half of all synthetic rubber production
It is mostly consumed in tyres, where - It competes with the
Natural Rubber
Sometimes matches with the properties and performance of Natural
rubber
There are many subgroups of the raw gum elastomer- Depending on
the method of synthesis of the polymer
Solution or emulsion polymerization
Ratio of the two major chemical building blocks Styrene and
Butadiene
General Purpose ElastomerStyrene-Butadiene Rubber
When gum vulcanized products are compared with NR and CR
- These have poor mechanical properties
Raw gum elastomer mush have reinforcing fillers Carbon black
Properties of SBR are broadly similar to NR- For chemical
- Solvent
- Weather resistance
Upper temperature heat aging resistance limit is a little
higher
Cost of the raw gum elastomer is low gum elastomer is low on the
relative scale for elastomers in general and is comparable with
NR
General Purpose ElastomerStyrene-Butadiene Rubber
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Butadiene- The largest use of butadiene is for styrene-butadiene
rubber
representing 28% of the total volume
- Butadiene is manufactured in several different ways
The key industrial process include recovery of butadiene from
ethylene production as a by-produce
Dehydrogenation or using oxidative dehydrogenation
Butadiene is also produced from ethanol
Styrene- Styrene monomer is widely used for polystyrene and
also
used for SBR rubber
- Styrene monomer is produced by two processes
Ethylbenzene dehydrogenation is most common process
Using propylene oxide coproduce route
Styrene-Butadiene RubberMonomers
Styrene-Butadiene rubber (SBR) is polymerized using two
processes
- Emulsion Polymerization
This process gave primarily 1,4-cis microstructure in the final
product
- Solution Polymerization
This process gave a lower level of 1,4-cis level typically
around 45%
1,2-vinyl content could be modified
Better control of branching and molecular weight distribution
attainable with anionic process made solution SBR This elastomer is
suitable for tire application
Development in SB block copolymers led to new materials which
were thermoplastic in character
Styrene-Butadiene RubberMicrostructure and MWD
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Butadiene and styrene can be copolymerized in a number of
ways
- Ionic and free-radical initiations can be used for this
purpose
Initially the free-radical process was entirely based on hot
emulsion process
- Utilizing a polymerization temperature of 50 0C
- Later developments showed that much improvement can be made to
the product if the polymerization temperature can be lowered to
around 5 0C
This is the reason this process is called cold emulsion
process
Advances in anionic polymerization showed further advantages
leading to the introduction of solution process
Styrene-Butadiene RubberPolymerization Process
Cold SBR is produced by emulsion process In this process an
emulsion of monomers (styrene and
butadiene) is formed in water- Emulsion is made using an
emulsifying agent which is
usually soap
The monomer in the emulsion are polymerized by a water-soluble
initiator fragment- A free radical generated from a hydroperoxide
or from an
oxidation reduction process
Free radical enters the emulsion droplet, a micelle, it
polymerizes the monomers present
The reaction occurs in three stages- Stage I: Nucleation stage
Free radical is generated from the initiating species in water
phase
Conversion of monomer is typically 15-20%
Polymerization ProcessCold SBR
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The reaction occurs in three stages- Stage II: Growth stage
Micelles becomes polymer particles with a thin layer of
surfactant on its surface
Polymerization reaches a steady state inside the swollen polymer
particles
This stage ends when most of monomer is converted to polymer,
reaching a conversion level of around 65%
- Stage III: Final stage
Characterized by complete disappearance of monomer droplets
Polymerization rate declines
Unreacted monomer continues to polymerize at a declining
rate
Polymerization ProcessCold SBR
Polymerization ProcessCold SBR
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It is based on the anionic process The most common initiator for
copolymerization of styrene and
butadiene is butyl lithium- Once the polymerization process is
initiated the tendency of the
anionic species to add to butadiene or styrene monomer will be
different
Polybutyadienyl anion will prefer to react with another
butadiene monomer rather than a styrene monomer
- Styrene can be used as a solvent for anionic polymerization of
butadiene This is used to when the polymerization reaction is
terminated as soon as
the butadiene monomer is depleted
Special precautions are required otherwise block copolymer of
butadiene and styrene will be obtained- In tire applications it is
very important to have a random copolymer- Special additives must
be employed to assure random distribution
of styrene units in the copolymer
Polymerization ProcessSolution SBR
Polymerization ProcessSolution SBR
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There are some key differences between emulsion SBR and solution
SBR- This is the basis for the difference between two product
groups
- In free-radical polymerization process polymer chain is
initiated by the initiator fragment
In a fraction of a second several thousand monomer units are
added to the growing chain
- The polymer chain is terminated through one of three
mechanisms
1. Through coupling reaction with other radical
2. Through disproportionation, where an electron is transferred
from one radical to another
3. Through chain transfer reaction
- Termination leads to a inactive polymer molecule which remains
inactive until the end of polymerization process
- In emulsion polymerization the polymer chains grow in a very
short while and then terminate
Polymerization ProcessComparison
In anionic polymerization the initiator is typically lithium
which initiates the polymerization process
- Initiated monomer reacts rapidly with the available
molecules
- The active polymer chains remain active for the duration of
the polymerization process
- Growing polymer chains must be deactivated specifically by the
addition of a deactivator at the end of the process
- The growing chains are active until the monomers are consumed
or a terminating agent is added
The chains produced tends to be uniform in length
- Solution SBR process can control the MWD and branching
Polymerization ProcessComparison
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Typical processing of SBR is done in two stages- In first stage
non-reactive components are mixed with the
rubber to obtain a good dispersion
At this stage to prevent any cross-linking, crosslinking agents
and accelerators are not added
This stage is called nonproductive stage
SBR and optionally a second rubber is mixed with additives such
as fillers, oils, antioxidants, stabilizers and antiozonants as
well as other specific additives
Oil extenders include naphthenic, aromatic and paraffinic oils
Their function is to soften the rubber for processing
- SBR contains active double bonds on the polymer backbone
Crosslinking reactions is carried out
Without the help of acceleration this reaction will require long
times and high temperatures
Styrene-Butadiene RubberProcessing of SBR
Styrene-Butadiene RubberAdditives used for SBR processing and
their function