EBB 220/3ELASTOMER & LATEX

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

IntroductionIntroduction

Raw rubber – no crosslinking

Elastomer – crosslinking

Raw rubber vs ElastomerRaw rubber vs Elastomer

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 elastomer comes from ‘elastic polymer’.

Stretch

Retract

Function of crosslinksFunction of crosslinks

Rubber processingRubber 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 ProcessMastication 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 ProcessCompounding Process

Elastomer/ Raw rubber 100Fillers 50Softener 5Antioxidant 1Asid Stearic 1Zinc Oxide 5Accelerator 1Sulphur 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.

General rubber compounding formulationGeneral rubber compounding formulation

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, flame retardants,

antistatics agents retarders, peptizers

Compounding IngredientsCompounding 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 vulcanizationSulphur 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 systemsSulphur vulcanization systems

Conventional

vulcanization

Efficient vulcanizati

on

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 ProcessForming 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 vulcanizationOpen vulcanizationContinuous 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 ProcessVulcanization 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.

Compression Moulding ProcessCompression Moulding Process

Molding press

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.

Compression MouldingCompression Moulding

Before After

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.

Finishing ProcessFinishing Process

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.

Quality ControlQuality 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 processingLatex 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)

IntroductionIntroduction

Latices are classified as:

1. Natural latices obtain from plants

2. Synthetic latices produced from 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)

Classification of laticesClassification of latices

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. Concentration by creaming2. Concentration by centrifugation3. Concentration by evaporation

Natural rubber latex concentrateNatural 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 concentratePreservations 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 composition2. Tests related to colloidal stability 3. Tests related to physical properties

Investigation Latex PopertiesInvestigation 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 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. Surface active agents (surfactants)2. Liquid phase modifier3. Elastomer or rubber phase modifier

Latex CompoundingLatex Compounding

61.7% Elastomer/ Raw latex 10010% KOH 0.350% Sulphur 0.550% Zinc Oxide 0.2550% Accelerator 0.7550% Antioxidant 0.560% 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.

General latex compounding formulationGeneral 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 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. Surface active agents (surfactants)2. Liquid phase modifier3. Elastomer or rubber phase modifier

Latex CompoundingLatex 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 CuringLatex Curing

Latex StabilisersVulcanising agents +

other ingredients

Vulcanisation 55-80°C

Strained & bulked

Maturation

Processing

Mature for 7 days at normal ambient temperatureto 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. Preparation of sulphur-prevulcanised NR latex.

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

Latex Dipping ProcessLatex 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 questionExample of the exams question

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

Students ActivityStudents Activity