types of rubbers
DESCRIPTION
Rubber typesTRANSCRIPT
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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
IntroductionGeneral Purpose Elastomers
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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IntroductionGeneral Purpose Elastomers
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
IntroductionGeneral Purpose Elastomers
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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IntroductionGeneral Purpose Elastomers
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
General Purpose ElastomersNatural Rubber and Polyisoprene
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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General Purpose ElastomersNatural Rubber and Polyisoprene
- 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
Natural RubberGrades of Natural Rubber
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
Natural RubberGrades of Natural Rubber
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