Types of Rubbers

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Elastomer TechnologyPE-4107Lecture 06,07

Types of Rubbers

Lecture Overview

Introduction of general purpose elastomers

General purpose elastomers

Natural Rubber NR

Classification

Grades of NR

Modified grades of NR

Isoprene IR

Stereospecificity

Polymerization process

Applications

- Comparison of NR and IR

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IntroductionGeneral Purpose Elastomers

These elastomers are widely used in

- Tyres

- Mechanical rubber goods industry

Gaskets

Tear resistant rubber parts

Automobile rubber parts

Conveyor belts

Oil seals

Anti-Vibration Pads

Pump parts


General purpose elastomers are - Natural rubber (NR)

- Polyisoprene (IR)

- Polybutadiene (BR)

- Styrene Butadiene rubber (SBR)

- Nitrile Butadiene (NBR) *

- Ethylene propylene (EPM and EPDM) *

These rubbers have- Good physical properties

- Processability

- Compatibility

- Generally very economical

* Sometimes considered as General Purpose Elastomer

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For NR, IR, BR compounding, processing and curing are similar

They all contain unsaturation in their backbones- Can be vulcanized with sulfur Zinc oxide and stearic acid are almost always used to activate

organic accelerators

- Crosslinked with peroxides

These polymers all respond well to fillers such as- Carbon black- Silica- Clay

They can be extended with - Aromatic- Naphthenic- Sometimes paraffinic oils


Due to unsaturation in the backbone

- They are all susceptible to ozone and oxygen attack

- They perform best with the addition of

Antiozonants

Antioxidants

These polymers have good physical properties including

- Resistance to abrasion

- Tear

- Traction (Adhesive friction)

- Good low heat build-up

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These elastomers cannot be used

- In high temperature applications

- Where they come into contact with oils and solvents

Other elastomers can give

- Lower gas permeability (e.g. butyls and halobutyls)

- Better ozone resistance (e.g. EPR and EPDM)

- Better solvent resistance (e.g. NBR)

- Better heat resistance (e.g. Silicones and fluoro elastomers)

But none can compete the combination of overall performance and cost inherent in general purpose elastomers

General Purpose ElastomersNatural Rubber and Polyisoprene

Natural rubber and Polyisoprene have the same monomer chemistry

- Isoprene is the building block of these polymers

- Can polymerize in four different configurations

Cis-1,4

Trans-1,4

Trans-1,2

Trans-3,4

Polyisoprene can be polymerized with- Coordination catalyst (Ziegler Natta)

At a ratio around 1:1 the Al/Ti system can produce cis contents from 96-98%

- Alkyl lithium catalyst

Produces cis contents between 90-93%

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Isoprene Configurations

Cis 1,4

Trans 1,4

Trans 1,2

Trans 3,4

Isoprene Monomer


Natural rubber has a cis content of almost 100%- These microstructure gives natural rubber a glass

transition temperature Tg of approximately -75 0C

Due to the presence of trans-1,2 and 3,4 configurations- The glass transition temperature is slightly higher Tg -70

to -72 0C

There are three general forms of polyisoprene commercially available- High cis contents

It has the ability to undergo strain induced crystallization This phenomenon gives cis-polyisoprene very high tear strength

Exceptional tear strength translates into superb physical properties and performance in the end product

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- High trans contents

Gutta-percha or Balata contains high trans content polyisoprene

It is very hard at room temperature

Upon heating to 80 0C the crystallinity melts and the rubber becomes soft and workable

- High 3,4-trans contents

Commercially available polyisoprene with 60% 3,4-transcontent

It has the glass transition temperature Tg of about -5 0C

It is processable at normal operating temperatures

For most applications neither high trans nor the high 3,4-trans polymers can match the high cis polymers for performance

General Purpose ElastomersNatural Rubber

Present suppliers are Malaysia, Indonesia, Sri Lanka, Nigeria and others

Production of the natural product begins at the plantation where a slit is made into the bark of the rubber tree to allow the flow of a milky sap

The latex consists of

- Water

- Polyisoprene

- Small quantities of other ingredients such as proteins and carbohydrates

It is collected and coagulated with formic acid in large tanks

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General Purpose ElastomersNatural Rubber

Coagulum is squeezed between rollers

- This removes excess water

- The obtained mass is then dried

Much of is then baled and shipped off to the rubber processor

Some areas of rubber industries use the latex directly for

Medical gloves

Contraceptive industries

- In this case the latex is concentrated and mixed with ammonia for mechanical stabilization at the plantation and shipped out

Natural RubberClassification

There are two generic classifications of natural rubber

- Crepe NR

Is a crude form of NR that is characterized by a crinkled texture

Natural rubber in the form of colorless or pale yellow crinkled sheets prepared by pressing bleached coagulated latex through corrugated rollers

Used to construct shoe soles

- Sheets

Natural rubber sheets are used in the applications requiring elasticity and flexibility combined with resistance to abrasion, tearing and low temperatures

It is tough, organic material that resists alkalis, salts, mild acids and other types of chemicals

It is common material for tyres and gaskets

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Natural RubberGrades of Natural Rubber

Type Source

Ribbed Smoked Sheet Coagulated field latex

Pale Crepe Coagulated field latex

Estate Brown Crepe Estate cuplump, tree lace

Compo Crepe Cuplump, tree lace, wet slab, RSS cuttings

Thin Brown Crepe Cuplump, tree lace, wet slab, unsmoked sheet

Thick Brown Crepe Cuplump, tree lace, wet slab, unsmoked sheet

Flat Bark Crepe Cuplump, tree lace, earth scrap

Pure Smoked Blanket Crepe Remilled RSS and RSS cuttings

Wet slab: Still wet from the coagulation tanks

RSS cutting: Ribbed smoked sheets


Technically specified rubbers (TSR)- Technical specifications suits the requirements of the manufacturing

sector- TSR include assurance of quality for important technical parameters

Consistency in quality

Minimum space for storage

Clean and easy to handle packing

Different methods are developed to produce TSR all these processes involves certain common steps such as- Coagulation of latex - Drying- Processing of field coagulum - Baling- Size reduction - Testing- Grading - Grading

Difference among commercial processes lie in - The method of coagulation- Machinery used for crumbling the coagulum

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Grades of Natural RubberTechnically Specified Rubber

Country Nomenclature

Malaysia Standard Malaysian Rubber (SMR)

Indonesia Standard Indonesian Rubber (SIR)

Thailand Standard Thai Rubber (STR)

India Indian Standard Natural Rubber (ISNR)

Sri Lanka Sri Lanka Rubber (SLR)

Singapore Standard Singapore Rubber (SSR)

Nomenclature of TSR in different countries


Modification of NR

Physical

Incorporation of compounding ingredients

OENR

Masterbatches

Latex stage compounds

Polymer blends NR/SR,

NR/plastics

TPNR

Impact modified plastics

Other forms

Deproteinized NR

Powdered NR

Chemical

Intramolecular changes

Cyclization

Cis-trans isomerization

Depolymerization

Peptization

Grafting

Styrene

Methyl Methacrylate

Acrylonitrile

Maleic anhydride

Acrylamide

Attachment of functional groups

Vulcanization

Epoxidation

Halogenation

Hydro halogenation

Hydrogenation

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Natural RubberModified Grades of Natural Rubber

Modified natural rubbers are treated usually performed at the latex stage. These include

1. Epoxidized Natural Rubber (ENR) Typical formation of ENR from NR

Cis-1,4-isoprene employing peracetic acid

Isoprene and epoxidized isoprene act as monomer units that are randomly distributed along polymer chain

Various degree of epoxidation of NR is commercially available ENR-25 contains 25% of isoprene units

ENR-50 contains 50% of isoprene units

ENR-75 contains 75% of isoprene units

Epoxidation changes several physical properties Increase in Tg Higher damping

Lower gas permeability

Increased polarity This reduces swelling in non-polar oils and increase compatibility with polar

polymers such as Polyvinyl chloride

Damping is an effect that reduces the amplitude of oscillations in an oscillatory system

Natural RubberModified Grades of Natural Rubber

2. Deproteinized Natural Rubber (DNR)

Natural rubber is treated with proteolytic enzymes that removes the allergenic protein from fresh natural rubber

3. Heveaplus MG Rubber

Natural rubber with grafted polymethyl methacrylate side chains

4. Thermoplastic Natural Rubber (TNR)

Blends of natural rubber and polypropylene

Oil extended natural rubber (OENR)- This contains 20-25 phr of aromatic or naphthalenic oil and is

produced either in Latex stage or Dry rubber stage

Increasing oil content reduces tensile strength and resilience

But the vulcanizate retain good tear resistance and possess high wear resistance when blended with butadiene rubber

- OENR shows good skid resistance on wet surfaces when used in tyre tread

Proteolytic enzymes break the long chainlike molecules of proteins into shorter fragments (Peptides)

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General Purpose ElastomersPolyisoprene

IR can be polymerized to produce a variety of isomers depending on the polymerization conditions

These differ in position of the insertion- 1,2- 1,4

Exist in cis or trans forms depending on the orientation of the substituents across the double bond

Cis 1,4-IR is a synthetic analogue of NR

Trans 1,4-IR is a tough semicrystalline polymer

- 3,4

Stereospecific (cis versus trans) and regio specific (1,2 versus 1,4) catalysts include- Anionic lithium-alkyl system- Coordination catalysts similar to Ziegler-Natta catalyst

All synthetic IR elastomers are produced by solution polymerization using solvents either- C5-C7 alkanes or- Aromatic compounds

PolyisopreneStereospecificity

Cis 1,4-IR- Was first made using Ziegler-Natta catalyst consisting of

TiCl4 in conjugation with trialkyl aluminum

- Later Cis 1,4-IR was made by alkali metal polymerization to give >90% Cis 1,4-IR

- Cis 1,4-IR of predictable molecular weight can also be prepared by anionic polymerization

Trans 1,4-IR- Is a thermoplastic

- Is made using Ziegler-Natta catalyst with vanadium as the transition metal

Mixed (cis/Trans) isomer 1,4-IR- This is made using free radical emulsion polymerization

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Isoprene Configurations

Cis 1,4

Trans 1,4

Trans 1,2

Trans 3,4

Isoprene Monomer

PolyisoprenePolymerization Process

Before entering the reactors the solvent, catalyst and isoprene monomer must be free of catalyst poison- Chemical impurities- Moisture- Air

Purified stream enter a chain of reactors in series into which the catalyst is injected and polymerization begins

At desired extent of polymerization short stop or catalyst deactivator

Non-staining antioxidant is added to protect the polymer during finishing and storage

The cement mixture passes the stripping section which recovers solvent and polymer cement is converted to crumb by hot water and steam

Crumb slurry passes through extruders to remove water before it is cooled, baled, packaged and placed in storage ready for shipment

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General Purpose ElastomersUses of Polyisoprene

Trans 1,4-IR

- Crystalline thermoplastic with a melting point around 60 0C

- Resists abrasion, scuffing and cutting

- Application

Mainly used in high quality golf ball covers

Orthopedic devices

Transmission belts

Cable covering

Adhesives

Cis 1,4-IR

Tyres and Tyre products

Belts and gaskets

Hoses

Foam rubber, molded and mechanical goods

Baby bottle teats, gloves, caulking, sealants, footwear and sportinggoods

Natural Rubber and PolyisopreneComparison

Consists of 99% rubber hydrocarbons (RHC)

Lighter in color and more consistent in chemical and physical properties

Its lower molecular weight leads to easier processing

- Mw ranges between 755 thousand and 1.25 million

- MWD vary from less than 2.0 for lithium-catalyzed polyisoprene to almost 3.0 for Al/Ti-catalyzed polyisoprene

Consists of 93% rubber hydrocarbons (RHC)

Higher green strength and modulus

- Especially at higher strain levels and temperatures

The Mw ranges from 1 million to 2.5 million

- MWD is greater than 3.0 and widely variable

Synthetic Polyisoprene Natural Rubber

Types of Rubbers

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