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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