EBB 220/3 ELASTOMER & LATEX
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EBB 220/3
ELASTOMER & LATEX
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The words rubber come from the materialsfrom the rubber tree name HaveaBrasiliensis
The different between raw rubber and
vulcanized rubber or elastomer:1. Raw rubber either natural rubber or synthetic
rubber are materials that has plasticproperties and can be reshaping at high
temperature and not sutaible for applications.1. Elastomer is the words that used for
vulcanized rubber, vulcanisate or crosslinkingrubber
Introduction
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Raw rubber no crosslinking
Elastomer crosslinking
Raw rubber vs Elastomer
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In beginning all product from rubber aremade from natural rubber that produced
from materials from natural rubber treecalled latex.
Synthetic rubber are produced fromreactions of low molecular weightmaterials called monomer to producedlong chain molecule called polymer
Elastic properties are produced by mixraw rubber with specific additives duringrubber compounding
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When rubber was heated the chemical reactionsoccur call vulcanization or curing.
Process were rubber molecules were tied togetherat specific place called crosslinks
The crosslinks will prevent the slippaged of
molecules.
Elastomer are elastic materials that can deformedwhen forced being applied and back to the originalshape when release the forced.
The words elastomer comes from elasticpolymer.
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Stretch
Retract
Function of crosslinks
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Rubber processingRaw Rubber
Vulcanize rubber/
End product
Mastication process
Compounding
Forming process
Vulcanization process
Rubber Compound
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Mastication is mechanical shearing process
using two roll mill or internal mixer) for
Reduced the molecular weight,
Reduced the viscosity and
To soften the raw rubber.
After mastication the processing will be
much easier and increased the effectiveness
of dispersions of compounding ingredients.
The mastication is compulsory for natural
rubber due to high molecular weight in nature
(around 105
-106
)
Mastication Process
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Rubber compounding is the way ofmaking useful products from raw
rubber
The process involved the addition ofadditives to change the masticated raw
rubber to rubber compound before a
forming process.
The compounding process used the
two roll mill and internal mixer
Compounding Process
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Elastomer/ Raw rubber 100Fillers 50
Softener 5
Antioxidant 1
Asid Stearic 1Zinc Oxide 5
Accelerator 1
Sulphur 2
Total 165** Parts per hundred rubber (pphr) All the ingredients
used in a compound formulation are normally given in
amounts based on a total of 100 parts of the rubber orcombination of rubber used.
General rubber compounding formulation
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Each ingredient has a specific function either inprocessing, vulcanization or end use of the products.
The various ingredients may be classified accordingto their specific functions in the following groups:
1. Fillers
Carbon black or non black fillers
2. Plasticizers or softeners
Extenders, processing aids, special plasticizers
3. Age resistors or antidegradants
Antioxidants, antiozonants, special age resistors
4. Vulcanizing or curing ingredients
vulcanization agents , accelerators and activator
5. Special-purpose ingredients
Coloring pigments, blowing agents, flame retardants,antistatics agents retarders, peptizers
Compounding Ingredients
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The crosslinking produced can have a monosulphideand polysulphide or both depending on the
vulcanization systemsused
Sulphur vulcanization systems can be divided into 3systems depending on the relative amount of sulphur
& accelerator used.
The three systems can be differentiate through thetypes of crosslinking produced and the main chain
modification after vulcanization
Sulphur vulcanization
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Network structure of sulphur vulcanizate
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System Sulphur content(pphr) Acceleratorcontents (pphr)
Conventional
vulcanization (CV)
2.0-3.5 1.0-0.5
Semi- Efficientvulcanization (semi-EV)
1.0-2.0(or sulphur donor)
2.5-1.0
Efficient vulcanization
(EV)
0.-1.0
(or sulphur donor)
6.0-2.0
Sulphur donor sulphur vulcanization
systems without sulphur but gives a sulphur
during vulcanization
Sulphur vulcanization systems
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Conventional
vulcanizationEfficient
vulcanization
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After all compounding ingredients havebeen properly mixed the compounded
green stock is tacky and thermoplastics
In this plastic condition, the stock can beshaped by the applications of force.
This can be accomplished for example, by
squeezing it betweens rolls (calendering) orpushing through an orifice having the desired
shape (tubing or extruding).
Forming Process
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After the green stock has been formed to the desiredshape, it needs to be converted to an elastic materials
vulcanization process usually under pressure at elevated
temperature using different techniques such as
Press vulcanizationOpen vulcanization
Continuous vulcanization
Chemically the process involves insertation of crosslinks
between the polymer macromolecules through the actionsof vulcanizing ingredients.
The crosslinking of rubber also referring as curing it a
process whereby a raw materials is converted into a useful
product.
Vulcanization Process
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Press vulcanization takes place in press thatsupply heat and pressure.
A vulcanizing press consists essentially of twoor more plates that can be bought together
and separated by hydraulic pressure heatedby steam or electricity
The rubber articles are vulcanized in variousmoulds between the heated plates under
pressure.
In its simplest form, a mould consists of twometal plates with cavities conforming to theoutside shape of the desired finished part.
Compression Moulding Process
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Molding press
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Most of rubber products produced using this method.
Rubber compound is placed in each cavity of the moldand closed and placed in hydraulic press.
Under the applied of hydraulic pressure (4-6 MPa) at
elevated temperature (140-200C) using the cure timeobtained from rheometer curve.
After mould is closed the stock will flow andcompletely fill the mould cavity
The mould is maintained closed under pressure for aprescribed time at particular moulding temperature the mould is then removed from the press and openedto remove the moulded part.
Compression Moulding
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Before After
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Many rubber articles require finishing operations aftervulcanization.
For example, the flash attached to moulded parts must beremoved before the parts are ready for use carried outby hand trimming using scissors or knives.
Some rubber parts are painted to give them an attractivefinish or more often to protect against ozone, oils, acids,chemicals ant the like. The paints are usually rubber based.
Items such as seals and windshield wiper blades mayrequire surface treatment with chlorine, bromine or fluorine
in water. The halogenated surface will have lower frictionthan the untreated surface while the bulk will be unaffected.
The finishing operations of extruded articles include coiling,cutting to specific length and washing.
Finishing Process
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Quality control is necessary to ensure thequality of final products
During various stages, checking will beperformed on incoming materials, in process
materials and finished products. The rubber manufacturer usually test the
raw materials to make sure that they areuniform and of adequate quality
Control test are applied to various steps inthe manufacturing process. The testemployed are sensitive to any change in the
rubber compound resulting from errors.
Quality Control
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Example of errors1. faulty compounding ingredients,
2. poor dispersion,3. improper temperature control,
4. wrong sequence of addition
5. too short or too long mixing cycles.
The control scheme utilize mostly standardizedphysical tests on both unvulcanized and vulcanized
rubber compounds.
1. Unvulcanized compound tests measurement of the
viscosity, scorch time and cure rate.
1. Vulcanized compound tests measurements density,
hardness, tensile modulus, tensile strength and
elongation at break.
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Materials Function
Elastomer/raw rubber Basic components in rubber compounding
Fillers Used for reinforced or modified the mechanical
properties and also to reduced the cost
Plasticier To ease the processing, to modify the specific
properties and also as extender
Anti oxidant To protect the rubber from ageing
Accelerator To increase vulcanization process and reduced the
time of vulcanization
Activator To increased the accelerator efficiency
Vulcanization agent Needed during vulcanization process to produced a
crosslinking
Other ingredients Used for specified purpose such as for coloring, as
retarder etc
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Process Function
Mastication Mechanical shearing process to reduced the
molecular weight, viscosity and to soften the
rubber.Compounding Process of addition the rubber additives to
convert the raw rubber into rubber compound.
Forming Process of convert the rubber compound into a
required size and shape with a forced.
Vulcanization Process of convert the shape product to an
elastic materials with formation a crosslinking
under temperature and pressure
Final Operation Final operation after vulcanization such as
trimming, paint or surface treatment
Quality Control To ensure the quality of final product
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Latex processingRaw Latex
Vulcanize latex/
End product
Latex compounding
Latex curing
Latex processing
Compounded Latex
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Latex is defined as a dispersion of polymerin an aqueous medium which is a
dispersion of polymeric solids in water
Latex has two phases:
1. The dispersed phase or discontinues phase
consisting of small particles of polymers
(particle size < 5 micrometer in diameter)
2. The dispersion medium or the continuous phase
or serum (a dilute aqueous solution)
Introduction
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Latices are classified as:1. Natural laticesobtain from plants
2. Synthetic latices produced from
emulsion polymerization process
3. Artificial laticesproduced by dispersingthe appropriate bulk polymer in an aqueous
dispersion medium
4. Modified latices produced bymodification of existing type of latex ( by
grafting, crosslinking etc)
Classification of latices
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The NR latex obtained from the plantation is calledfresh or field latex
The field latex contains about 33% NR and thus noteconomical to be used to make a latex products
The rubber content of the field latex need to beincreased before it is used in making latex products.
This is achieved by doing concentration process tothe field latex
There are few methods that can be used toconcentrate natural rubber latex:
1. Concentration by creaming
2. Concentration by centrifugation
3. Concentration by evaporation
Natural rubber latex concentrate
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Preservation is necessary to prevent micro-organismfrom
attacking the non-rubber componentsof latex.
Latex which has been attacked by micro-organism has lowercolloidal stabilityand bad odour.
Concentrated latexis preservedfor long-term storage. Themost popular preservative is ammonia.
There are two types ammonia preservation systems: Low -ammonia latex (LA latex)
0.2 % m/mammonia.
short term preservation
suitable for certain applications such as latex foam requires lowammonia content in order for to gel properly.
secondary preservatives are added to LA latex for long term storage.
High ammonia latex (HA latex): 0.6 - 0.8 % m/m of ammonia
long term preservation
Preservations for latex concentrate
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The properties of the latex are of interest because: To gain better understanding of the physical and chemical
nature of the latex.
To determine the fitness of the latex for a particularapplication.
To assure the general quality of the latex
The test methods to evaluate latex properties canbe classified into three groups:
1. Tests related to chemical composition
2. Tests related to colloidal stability
3. Tests related to physical properties
Investigation Latex Poperties
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Process of addition of chemicals to the latex compounding
Latex containing these chemicalscompounded latex.
Most chemicals added to the latex ~ in solid form thereforethese chemical need to be dispersed in water first beforeadded to the latex.
The dispersed chemicals dispersion and prepared bygrinding them together with the dispersing agent in water.
The chemicals used in latex compounding can be divided into
three general classifications:
1. Surface active agents (surfactants)
2. Liquid phase modifier
3. Elastomer or rubber phase modifier
Latex Compounding
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61.7% Elastomer/ Raw latex 10010% KOH 0.3
50% Sulphur 0.5
50% Zinc Oxide 0.25
50% Accelerator 0.7550% Antioxidant 0.5
60% Fillers 15
Total 117.3
** Parts per hundred rubber (pphr) All the ingredientsused in a compound formulation are normally given inamounts based on a total of 100 parts of the rubber orcombination of rubber used.
General latex compounding formulation
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Process of addition of chemicals to the latex compounding
Latex containing these chemicalscompounded latex.
Most chemicals added to the latex ~ in solid form thereforethese chemical need to be dispersed in water first beforeadded to the latex.
The dispersed chemicals dispersion and prepared bygrinding them together with the dispersing agent in water.
The chemicals used in latex compounding can be divided into
three general classifications:
1. Surface active agents (surfactants)
2. Liquid phase modifier
3. Elastomer or rubber phase modifier
Latex Compounding
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Curing of latex process of introducing cross-links in the
rubber molecules. The term vulcanisation used to describe cross-linking
process involving sulphur as cross-linking agent.
Two types of vulcanisation in latex:
pre-vulcanisationwhen the vulcanisation is done while
the latex is still in liquid state.
post-vulcanisationif the vulcanisation is done on latexfilm.
Latex that has been pre-vulcanised referred as pre-vulcanised latex or PV latex (for pre-vulcanised natural rubber PVNR).
Appearance PVNR very similar to unvulcanised NR withmaintained fluidity.
The cross-linking only takes place in each individualparticles.particles in PVNR have the same shape, size
& size distribution as those in initial unvulcanised latex.
Latex Curing
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Latex StabilisersVulcanising agents +
other ingredients
Vulcanisation55-80C
Strained & bulked
Maturation
Processing
Mature for 7 days at normal
ambient temperature
to improve uniformity
Depends on
formulation
Latex is constantly stirred
avoid formation of skin &sedimentation of ingredients.Various tests to check degree
of vulcanisation
To get desired degree
of vulcanisation
After desired degree of
vulcanisation attainedlatex is cooled and run off
into container
Pre-vulcanised latex
Preparation of sulphur-prevulcanised NR latex.
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Dipping process is used to make thin and hollow latex
products such as gloves, catheters and toys Theseproducts are called dipped products.
In principle dipping process involves dipping cleanedformers into compounded latex (prevulcanized orpostvulcanized compound)
Latex film will be formed around the former and productobtained by drying and curing the films.
Latex dipping process can be classified into three methods:
1. Straight or simple dipping no stabilize agency isused to form the films
2. Coagulant dipping a direct coacervant is used topromote film formation
3. Heat-sensitized dipping a heat sensitizer is used topromote film formation
Latex Dipping Process
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Gives the rubber processing flow chartincluding the simple definition of the
processes involved.
Gives the general formulation to produceelastomer products with simple
elaborations ingredients used.
Briefly explain the vulcanization process.
Example of the exams question
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In 5 minutes try to list how many points
that you remember the differences
between elastomer and latex
Students Activity
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