EBB 220/3

ELASTOMER & LATEX

The words rubber come from the materialsfrom the rubber tree name “HaveaBrasiliensis”

The different between raw rubber andvulcanized rubber or elastomer:

1. Raw rubber either natural rubber or syntheticrubber are materials that has plasticproperties and can be reshaping at hightemperature and not sutaible for applications.

1. Elastomer is the words that used forvulcanized rubber, vulcanisate or crosslinkingrubber

Introduction

Raw rubber – no crosslinking

Elastomer – crosslinking

Raw rubber vs Elastomer

In beginning all product from rubber aremade from natural rubber that producedfrom 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

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 ofmolecules.

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’.

Stretch

Retract

Function of crosslinks

Rubber processing

Raw Rubber

Vulcanize rubber/

End product

Mastication process

Compounding

Forming process

Vulcanization process

Rubber Compound

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

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

Compounding Process

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 ingredientsused in a compound formulation are normally given inamounts based on a total of 100 parts of the rubber orcombination of rubber used.

General rubber compounding formulation

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

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

Sulphur vulcanization

Network structure of sulphur vulcanizate

System Sulphur content

(pphr)

Accelerator

contents (pphr)

Conventional

vulcanization (CV)

2.0-3.5 1.0-0.5

Semi- Efficient

vulcanization (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

Conventionalvulcanization

Efficient vulcanization

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).

Forming 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.

Vulcanization Process

Press vulcanization takes place in press thatsupply heat and pressure.

A vulcanizing press consists essentially of twoor more plates that can be bought togetherand separated by hydraulic pressure heatedby steam or electricity

The rubber articles are vulcanized in variousmoulds between the heated plates underpressure.

In its simplest form, a mould consists of twometal plates with cavities conforming to theoutside shape of the desired finished part.

Compression Moulding Process

Molding press

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) atelevated temperature (140-200°C) 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

Before After

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 fluorinein 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

Quality control is necessary to ensure thequality of final products

During various stages, checking will beperformed on incoming materials, in processmaterials and finished products.

The rubber manufacturer usually test theraw 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 therubber compound resulting from errors.

Quality Control

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 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

Vulcanize latex/

End product

Latex compounding

Latex curing

Latex processing

Compounded Latex

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)

Introduction

Latices are classified as:

1. Natural latices obtain from plants

2. Synthetic latices produced fromemulsion polymerization process

3. Artificial latices produced by dispersingthe appropriate bulk polymer in an aqueousdispersion medium

4. Modified latices produced bymodification of existing type of latex ( bygrafting, crosslinking etc)

Classification of latices

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

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

Preservations for latex concentrate

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. Tests related to chemical composition

2. Tests related to colloidal stability

3. Tests related to physical properties

Investigation Latex Poperties

Process of addition of chemicals to the latex

compounding

Latex containing these chemicals compounded 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 intothree general classifications:

1. Surface active agents (surfactants)

2. Liquid phase modifier

3. Elastomer or rubber phase modifier

Latex Compounding

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 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

Process of addition of chemicals to the latex

compounding

Latex containing these chemicals compounded 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 intothree general classifications:

1. Surface active agents (surfactants)

2. Liquid phase modifier

3. Elastomer or rubber phase modifier

Latex Compounding

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.

Latex Curing

Latex StabilisersVulcanising agents +

other ingredients

Vulcanisation

55-80°CStrained & 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 attained

latex is cooled and run off

into container

Pre-vulcanised latex

Preparation of sulphur-prevulcanised NR latex.

Dipping process is used to make thin and hollow latexproducts 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

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.

Example of the exams question

In 5 minutes try to list how many points

that you remember the differences

between elastomer and latex

Students Activity