Transcript EBB 220/3 ELASTOMER & LATEX - USM :: Universiti Sains Malaysia

EBB 220/3
ELASTOMER & LATEX
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:
1. 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.
1. Elastomer
is the words that used for
vulcanized rubber, vulcanisate or crosslinking
rubber
Raw rubber vs Elastomer
Raw rubber – no crosslinking
Elastomer – crosslinking

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

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
polymer’.
elastomer
comes
from
‘elastic
Function of crosslinks
Stretch
Retract
Rubber processing
Raw Rubber
Mastication process
Compounding
Rubber Compound
Forming process
Vulcanization process
Vulcanize rubber/
End product
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 )
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
General rubber compounding formulation
Elastomer/ Raw rubber
Fillers
Softener
Antioxidant
Asid Stearic
Zinc Oxide
Accelerator
Sulphur
Total
** Parts
100
50
5
1
1
5
1
2
165
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.
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:
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,
antistatics agents retarders, peptizers
flame
retardants,
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
Network structure of sulphur vulcanizate
Sulphur vulcanization systems
System
Sulphur content
(pphr)
Accelerator
contents (pphr)
Conventional
2.0-3.5
vulcanization (CV)
Semi- Efficient
1.0-2.0
vulcanization (semi-EV) (or sulphur donor)
Efficient vulcanization
0.-1.0
(EV)
(or sulphur donor)
Sulphur donor  sulphur vulcanization
systems without sulphur but gives a sulphur
during vulcanization
1.0-0.5
2.5-1.0
6.0-2.0
Efficient
vulcanization
Conventional
vulcanization
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).
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.
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.
Molding press
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 (140-200°C) 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.
Before
After
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.
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.

Example of errors
1.
2.
3.
4.
5.

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.
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.
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
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
Latex processing
Raw Latex
Latex compounding
Compounded Latex
Latex curing
Latex processing
Vulcanize latex/
End product
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:
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)
Classification of latices

Latices are classified as:
1.
Natural latices  obtain from plants
2.
Synthetic latices  produced
emulsion polymerization process
3.
Artificial latices  produced by dispersing
the appropriate bulk polymer in an aqueous
dispersion medium
4.
Modified
latices  produced by
modification of existing type of latex ( by
grafting, crosslinking etc)
from
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:
1.
2.
3.
Concentration by creaming
Concentration by centrifugation
Concentration by evaporation
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
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:
1.
2.
3.
Tests related to chemical composition
Tests related to colloidal stability
Tests related to physical properties
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:
1.
2.
3.
Surface active agents (surfactants)
Liquid phase modifier
Elastomer or rubber phase modifier
General latex compounding formulation
61.7% Elastomer/ Raw latex
10% KOH
50% Sulphur
50% Zinc Oxide
50% Accelerator
50% Antioxidant
60% Fillers
100
0.3
0.5
0.25
0.75
0.5
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.
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:
1.
2.
3.
Surface active agents (surfactants)
Liquid phase modifier
Elastomer or rubber phase modifier
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 prevulcanised 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.
Preparation of sulphur-prevulcanised NR latex.
Latex
Stabilisers
Depends on
formulation
Vulcanising agents +
other ingredients
To get desired degree
of vulcanisation
After desired degree of
vulcanisation attained 
latex is cooled and run off
into container
Strained & bulked
Vulcanisation
55-80°C
Pre-vulcanised latex
Mature for 7 days at normal
ambient temperature
to improve uniformity
Maturation
Processing
Latex is constantly stirred 
avoid formation of skin &
sedimentation of ingredients.
Various tests to check degree
of vulcanisation
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:
1.
Straight or simple dipping  no stabilize agency is
used to form the films
2.
Coagulant dipping  a direct coacervant is used to
promote film formation
3.
Heat-sensitized dipping  a heat sensitizer is used to
promote film formation
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.
Students Activity

In 5 minutes try to list how many points
that you remember the differences
between elastomer and latex