EBB 2203 ELASTOMER

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EBB 220/3ELASTOMER LATEX
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Introduction

• The words rubber come from the materials from the
  rubber tree name Havea Brasiliensis
• The different between raw rubber and vulcanized
  rubber or elastomer
• Raw rubber either natural rubber or synthetic
  rubber are materials that has plastic properties
  and can be reshaping at high temperature and not
  sutaible for applications.
• Elastomer is the words that used for vulcanized
  rubber, vulcanisate or crosslinking rubber

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Raw rubber vs Elastomer
Raw rubber no crosslinking
Elastomer crosslinking
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• In beginning all product from rubber are made
  from natural rubber that produced from materials
  from natural rubber tree called latex.
• Synthetic rubber are produced from reactions of
  low molecular weight materials called monomer to
  produced long chain molecule called polymer
• Elastic properties are produced by mix raw rubber
  with specific additives during rubber compounding

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• When rubber was heated the chemical reactions
  occur call vulcanization or curing.
• Process were rubber molecules were tied together
  at specific place called crosslinks
• The crosslinks will prevent the slippaged of
  molecules.
• Elastomer are elastic materials that can deformed
  when forced being applied and back to the
  original shape when release the forced.
• The words elastomer comes from elastic polymer.

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Function of crosslinks
Stretch
Retract
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Rubber processing
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Mastication Process

• 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 )

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Compounding Process

• Rubber compounding is the way of making useful
  products from raw rubber
• The process involved the addition of additives to
  change the masticated raw rubber to rubber
  compound before a forming process.
• The compounding process used the two roll mill
  and internal mixer

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General rubber compounding formulation

• Elastomer/ Raw rubber 100
• Fillers 50
• Softener 5
• Antioxidant 1
• Asid Stearic 1
• Zinc 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 or combination of rubber
  used.

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Compounding Ingredients

• Each ingredient has a specific function either in
  processing, vulcanization or end use of the
  products.
• The various ingredients may be classified
  according to their specific functions in the
  following groups
• Fillers
• Carbon black or non black fillers
• Plasticizers or softeners
• Extenders, processing aids, special plasticizers
• Age resistors or antidegradants
• Antioxidants, antiozonants, special age resistors
• Vulcanizing or curing ingredients
• vulcanization agents , accelerators and
  activator
• Special-purpose ingredients
• Coloring pigments, blowing agents, flame
  retardants, antistatics agents retarders,
  peptizers

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Sulphur vulcanization

• The crosslinking produced can have a monosulphide
  and polysulphide or both depending on the
  vulcanization systems used
• Sulphur vulcanization systems can be divided into
  3 systems depending on the relative amount of
  sulphur accelerator used.
• The three systems can be differentiate through
  the types of crosslinking produced and the main
  chain modification after vulcanization

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Network structure of sulphur vulcanizate
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Sulphur vulcanization systems
Sulphur donor ? sulphur vulcanization systems
without sulphur but gives a sulphur during
vulcanization
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Conventional vulcanization
Efficient vulcanization
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Forming Process

• After all compounding ingredients have been
  properly mixed the compounded green stock is
  tacky and thermoplastics
• In this plastic condition, the stock can be
  shaped by the applications of force.
• This can be accomplished for example, by
  squeezing it betweens rolls (calendering) or
  pushing through an orifice having the desired
  shape (tubing or extruding).

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Vulcanization Process

• After the green stock has been formed to the
  desired shape, it needs to be converted to an
  elastic materials ? vulcanization process usually
  under pressure at elevated temperature using
  different techniques such as
• Press vulcanization
• Open vulcanization
• Continuous vulcanization
• Chemically the process involves insertation of
  crosslinks between the polymer macromolecules
  through the actions of vulcanizing ingredients.
• The crosslinking of rubber also referring as
  curing ? it a process whereby a raw materials is
  converted into a useful product.
•  

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Compression Moulding Process

• Press vulcanization takes place in press that
  supply heat and pressure.
• A vulcanizing press consists essentially of two
  or more plates that can be bought together and
  separated by hydraulic pressure heated by steam
  or electricity
• The rubber articles are vulcanized in various
  moulds between the heated plates under pressure.
• In its simplest form, a mould consists of two
  metal plates with cavities conforming to the
  outside shape of the desired finished part.

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Molding press
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Compression Moulding

• Most of rubber products produced using this
  method.
• Rubber compound is placed in each cavity of the
  mold and closed and placed in hydraulic press.
• Under the applied of hydraulic pressure (4-6 MPa)
  at elevated temperature (140-200C) using the
  cure time obtained from rheometer curve.
• After mould is closed ? the stock will flow and
  completely fill the mould cavity
• The mould is maintained closed under pressure for
  a prescribed time at particular moulding
  temperature ? the mould is then removed from the
  press and opened to remove the moulded part.

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Before
After
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Finishing Process

• Many rubber articles require finishing operations
  after vulcanization.
• For example, the flash attached to moulded parts
  must be removed before the parts are ready for
  use ? carried out by hand trimming using scissors
  or knives.
• Some rubber parts are painted to give them an
  attractive finish 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
  may require surface treatment with chlorine,
  bromine or fluorine in water. The halogenated
  surface will have lower friction than the
  untreated surface while the bulk will be
  unaffected.
• The finishing operations of extruded articles
  include coiling, cutting to specific length and
  washing.

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Quality Control

• Quality control is necessary to ensure the
  quality of final products
• During various stages, checking will be performed
  on incoming materials, in process materials and
  finished products.
• The rubber manufacturer usually test the raw
  materials to make sure that they are uniform and
  of adequate quality
• Control test are applied to various steps in the
  manufacturing process. The test employed are
  sensitive to any change in the rubber compound
  resulting from errors.

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• Example of errors
• faulty compounding ingredients,
• poor dispersion,
• improper temperature control,
• wrong sequence of addition
• too short or too long mixing cycles.
• The control scheme utilize mostly standardized
  physical tests on both unvulcanized and
  vulcanized rubber compounds.
• Unvulcanized compound tests measurement of the
  viscosity, scorch time and cure rate.
• Vulcanized compound tests measurements density,
  hardness, tensile modulus, tensile strength and
  elongation at break.

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Latex processing
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Introduction

• Latex is defined as a dispersion of polymer in an
  aqueous medium ? which is a dispersion of
  polymeric solids in water
• Latex has two phases
• The dispersed phase or discontinues phase
  consisting of small particles of polymers
  (particle size lt 5 micrometer in diameter)
• The dispersion medium or the continuous phase or
  serum (a dilute aqueous solution)

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Classification of latices

• Latices are classified as
• Natural latices ? obtain from plants
• Synthetic latices ? produced from emulsion
  polymerization process
• Artificial latices ? produced by dispersing the
  appropriate bulk polymer in an aqueous dispersion
  medium
• Modified latices ? produced by modification of
  existing type of latex ( by grafting,
  crosslinking etc)

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Natural rubber latex concentrate

• The NR latex obtained from the plantation is
  called fresh or field latex
• The field latex contains about 33 NR and thus
  not economical to be used to make a latex
  products
• The rubber content of the field latex need to be
  increased before it is used in making latex
  products.
• This is achieved by doing concentration process
  to the field latex
• There are few methods that can be used to
  concentrate natural rubber latex
• Concentration by creaming
• Concentration by centrifugation
• Concentration by evaporation

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Preservations for latex concentrate

• Preservation is necessary to prevent
  micro-organism from attacking the non-rubber
  components of latex.
• Latex which has been attacked by micro-organism
  has lower colloidal stability and bad odour.
• Concentrated latex is preserved for long-term
  storage. The most popular preservative is
  ammonia.
• There are two types ammonia preservation systems
• Low -ammonia latex (LA latex)
• 0.2 m/m ammonia.
• short term preservation
• suitable for certain applications such as latex
  foam ? requires low ammonia 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

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Investigation Latex Poperties

• 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
  particular application.
• To assure the general quality of the latex
• The test methods to evaluate latex properties can
  be classified into three groups
• Tests related to chemical composition
• Tests related to colloidal stability
• Tests related to physical properties

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Latex Compounding

• Process of addition of chemicals to the latex ?
  compounding
• Latex containing these chemicals ? compounded
  latex.
• Most chemicals added to the latex in solid form
  therefore these chemical need to be dispersed in
  water first before added to the latex.
• The dispersed chemicals ? dispersion and prepared
  by grinding them together with the dispersing
  agent in water.
• The chemicals used in latex compounding can be
  divided into three general classifications
• Surface active agents (surfactants)
• Liquid phase modifier
• Elastomer or rubber phase modifier

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General latex compounding formulation

• 61.7 Elastomer/ Raw latex 100
• 10 KOH 0.3
• 50 Sulphur 0.5
• 50 Zinc Oxide 0.25
• 50 Accelerator 0.75
• 50 Antioxidant 0.5
• 60 Fillers 15
• Total 117.3
• 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 or combination of rubber
  used.

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Latex Compounding

• Process of addition of chemicals to the latex ?
  compounding
• Latex containing these chemicals ? compounded
  latex.
• Most chemicals added to the latex in solid form
  therefore these chemical need to be dispersed in
  water first before added to the latex.
• The dispersed chemicals ? dispersion and prepared
  by grinding them together with the dispersing
  agent in water.
• The chemicals used in latex compounding can be
  divided into three general classifications
• Surface active agents (surfactants)
• Liquid phase modifier
• Elastomer or rubber phase modifier

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Latex Curing

• 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-vulcanisation ? when the vulcanisation is
  done while the latex is still in liquid state.
• post-vulcanisation ? if the vulcanisation is done
  on latex film.
• 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
  with maintained fluidity.
• The cross-linking only takes place in each
  individual particles. ? particles in PVNR have
  the same shape, size size distribution as those
  in initial unvulcanised latex.

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Preparation of sulphur-prevulcanised NR latex.
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Latex Dipping Process

• Dipping process is used to make thin and hollow
  latex products such as gloves, catheters and toys
  ?These products are called dipped products.
• In principle? dipping process involves dipping
  cleaned formers into compounded latex
  (prevulcanized or postvulcanized compound)
• Latex film will be formed around the former and
  product obtained by drying and curing the films.
• Latex dipping process can be classified into
  three methods
• Straight or simple dipping ? no stabilize agency
  is used to form the films
• Coagulant dipping ? a direct coacervant is used
  to promote film formation
• Heat-sensitized dipping ? a heat sensitizer is
  used to promote film formation

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Example of the exams question

• Gives the rubber processing flow chart including
  the simple definition of the processes involved.
• Gives the general formulation to produce
  elastomer products with simple elaborations
  ingredients used.
• Briefly explain the vulcanization process.

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Students Activity

• In 5 minutes try to list how many points that you
  remember the differences between elastomer and
  latex

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