12.elastomer(rubber) 2016 (1)

8/17/2019 12.Elastomer(Rubber) 2016 (1)

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Raw rubber – no crosslinking

Elastomer –

crosslinking

Raw rubber vs Elastomer


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• In beginning all products from rubber aremade from natural rubber that areproduced from materials from naturalrubber tree called latex.

• Synthetic rubber are produced from

reactions of low molecular weightmaterials called monomer to producelong chain molecule called polymer

• Elastic

properties are produced by mixing

raw rubber with specific additives duringrubber compoundingLatex being collected from atapped rubber tree


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• When rubber was heated the chemical reactions occur, the process

called vulcanization or curing.

• Process where rubber molecules were tied together at specific placecalled crosslinks

• The crosslinks will prevent the slippage of molecules.

• Elastomer are elastic materials that can deformed when forced beingapplied and back to the original shape when release the forced.

• The words elastomer comes from ‘elastic polymer’ .


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Stretch

Retract

Function of crosslinks


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Rubber processing

Raw 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 orinternal mixer) to

Reduce the molecular weight,Reduce the viscosity and

soften the raw rubber.

After mastication the processing will be much easier andincreased the effectiveness of dispersions of compoundingingredients.

The mastication is compulsory for natural rubber due to highmolecular weight in nature (around 10 5-10 6 )

Mastication Process


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Rubber compounding is the way of making useful products fromraw rubber

The process involved the addition of additives to change the

masticated raw rubber to rubber compound before a formingprocess.

The compounding process used the two roll mill or internal mixer

Compounding Process


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Elastomer/ Raw rubber 100

Fillers 50Softener 5

Antioxidant 1

Stearic Acid 1

Zinc Oxide 5Accelerator 1

Sulphur 2

Total 165

** Parts per hundred rubber (pphr) – All the ingredients usedin a compound formulation are normally given in amountsbased on a total of 100 parts of the rubber or combination of rubber used.

General rubber compounding formulation


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Each ingredient has a specific function either in processing,vulcanization or end use of the products.

The various ingredients may be classified according to theirspecific functions in the following groups:

1. Fillers• As reinforcement or cost reduction. Eg. 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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Sulfur is the oldest vulcanization agent for unsaturated rubbersused in rubber practice

The crosslinking produced can have a monosulphide and

polysulphide or both depending on the vulcanization systemsused

Sulphur vulcanization systems can be divided into 3 systems

depend ing on the relative amount of sulphur & accelerator used.

The three systems can be differentiated through the types of crosslinking produced and the main chain modification aftervulcanization

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- 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 vulcanizationsystems without sulphur but gives a sulphur

during vulcanization (for example sulphur from accelerator)

Sulphur vulcanization systems

Depends on product:

Good Strength- CV systemGood Ageing- EVSemi EV- in between


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Conventionalvulcanization

In CV- there are many S-S link, EV has many C-S link. Depend on bondenergy, for ageing C-S is more difficult to break compared to S-S (thusEV shows good ageing properties).

Efficientvulcanization


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Organic peroxides as curing agent.

Especially for saturated rubbers which do not contain any reactivegroup capable of forming x-links.

Peroxide does not enter into the polymer chains but produces radicals

which form C-to-C linkages with adjacent polymer chains

The peroxide decompose at vulcanization temp & form free radicals.These free radicals would extract H atom from polymer chains andform bonding, leaving polymer radicals. The polymer radical will reactwith adjacent polymer radical to form crosslink.

Eg. dicumyl peroxide, zinc peroxide, benzoyl peroxide, 2,4-chlorobenzoyl peroxide

Peroxide vulcanization


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After all the compounding ingredients havebeen properly mixed the compounded greenstock is tacky and thermoplastics

In this plastic condition, the stock can beshaped by the applications of force.

This can be accomplished for example, bysqueezing it betweens rolls ( calendering ) orpushing through an orifice having the desiredshape (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 elevatedtemperature using different techniques such as

Press vulcanizationOpen vulcanization/ autoclaveContinuous 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 aprocess 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.

Examples of press vulcanizationCompression MoldingTransfer MoldingInjection Molding

Press Vulcanization


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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) atelevated temperature (140-200 °C) using the cure timeobtained from rheometer curve .

After mould is closed the stock will flow and completelyfill the mould cavity

The mould is maintained closed under pressure for aprescribed time at particular moulding temperature themould is then removed from the press and opened toremove the moulded part.

Compression Moulding


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Molding press


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Before After


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This is a variation on compression moulding and involvesloading a pre-formed blank of rubber in a cavity connectedto the mould cavity by a runner.

The blank is compressed when the mould closes and isforced under pressure into the mould cavity.

Transfer Moulding

Before After


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Autoclaves, known as 'curing pans' or 'vulcanisers ‘

It’s a pressure vessels filled with steam that are used to cure

rubber articles not contained in moulds (hose, extrudedsection, coated cloth, retread tyres, and small cablebatches).

Most are horizontal with vertical autoclaves now rare. Theycan have either quick opening doors or multi-bolted doors.

Open Vulcanization/Autoclaves


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Products produced on a continuous process such as rubbercovered cable and strip are continuously cured , rather thanon a batch basis.

There are various methods but each one is sited just afterthe exit die of the extruder or calender.

Continuous Vulcanization


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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 out byhand trimming using scissors or knives.

Some rubber parts are painted to give them an attractive finish ormore often to protect against ozone, oils, acids, chemicals ant thelike. The paints are usually rubber based.

Items such as seals and windshield wiper blades may requiresurface treatment with chlorine, bromine or fluorine in water.The halogenated surface will have lower friction than theuntreated 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 the qualityof final products

During various stages, checking will beperformed on incoming materials, in process

materials and finished products.The rubber manufacturer usually test the rawmaterials to make sure that they are uniformand of adequate quality

Control test are applied to various steps in themanufacturing process. The test employed aresensitive to any change in the rubber compoundresulting 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 addition5. too short or too long mixing cycles.

The control scheme – utilize mostly standardized physicaltests on both unvulcanized and vulcanized rubbercompounds.

Unvulcanized compound tests – measurement of the viscosity, scorch time andcure rate.

Vulcanized compound tests – measurements density, hardness, tensilemodulus, tensile strength and elongation at break.


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Process Function

Mastication Mechanical shearing process to reduced themolecular weight, viscosity and to soften therubber.

Compounding Process of addition the rubber additives toconvert the raw rubber into rubber compound.

Forming Process of convert the rubber compound into arequired size and shape with a forced.

Vulcanization Process of convert the shape product to anelastic materials with formation a crosslinkingunder temperature and pressure

Final Operation Final operation after vulcanization such astrimming, paint or surface treatment

Quality Control To ensure the quality of final product


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Latex processing

Raw Latex

Vulcanize latex/End product

Latex compounding

Latex curing

Latex processing

Compounded Latex


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Latex is defined as a dispersion of polymer in an aqueousmedium which is a dispersion of polymeric solids in water

Latex has two phases:

1. The dispersed phase or discontinues phase consisting of smallparticles of polymers (particle size < 5 micrometer in diameter)

2. The dispersion medium or the continuous phase or serum (a diluteaqueous solution)

Introduction


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Latices are classified as:

1. Natural latices obtained from plants

2. Synthetic latices produced from emulsion polymerization

process

3. Artificial latices produced by dispersing the appropriate bulkpolymer in an aqueous dispersion medium

4. Modified latices produced by modification of existing typeof latex ( by grafting, crosslinking etc)

Classification of latices


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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 beused to make a latex products

The rubber content of the field latex need to be increased before it isused 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 rubberlatex:

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

Natural rubber latex concentrate


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Preservation is necessary to prevent micro-organism from attacking thenon-rubber components of latex.

Latex which has been attacked by micro-organism has lower colloidalstability and bad odour.

Concentrated latex is preserved for long-term storage . The most popularpreservative 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 contentin 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 suitability of the latex for a particularapplication.

• To assure the general quality of the latex

The test methods to evaluate latex properties can beclassified into three groups:

1. Tests related to chemical composition2. Tests related to colloidal stability3. Tests related to physical properties

Investigation Latex Poperties


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• Process of addition of chemicals to the latex compounding

• Latex containing these chemicals known as compounded latex.

• Most chemicals added to the latex ~ in solid form therefore thesechemical need to be dispersed in water first before added to thelatex.

• The dispersed chemicals prepared by grinding them togetherwith the dispersing agent in water.

• The chemicals used in latex compounding can be divided into threegeneral classifications:

1. Surface active agents (surfactants)2. Liquid phase modifier3. Elastomer or rubber phase modifier

Latex Compounding


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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 usedin a compound formulation are normally given in amountsbased on a total of 100 parts of the rubber or combination of rubber used.

General latex compounding formulation


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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 involvingsulphur as cross-linking agent.

Two types of vulcanisation in latex:

pre-vulcanisation when the vulcanisation is done while the latex isstill 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 PVlatex (for pre-vulcanised natural rubber PVNR).

Appearance PVNR very similar to unvulcanised NR with maintainedfluidity.

The cross-linking only takes place in each individual particles.particles in PVNR have the same shape, size & size distribution as thosein initial unvulcanised latex.

Latex Curing


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Latex StabilisersVulcanising agents +

other ingredients

Vulcanisation55-80 °C

Strained & bulked

Maturation

Processing

Mature for 7 days at normal

ambient temperatureto improve uniformity

Depends onformulation

Latex is constantly stirredavoid formation of skin &sedimentation of ingredients.Various tests to check degreeof vulcanisation

To get desired degreeof vulcanisation

After desired degree of

vulcanisation attainedlatex is cooled and run offinto container

Pre -vu l can i sed l a t ex

Preparation of sulphur-prevulcanised NR latex.


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How is a glove produced?

• Collecting the raw material• After workers tap the rubber trees the raw material is

centrifuged with the effluent latex

• After a 25 day maturity and stabilization period , the latexis sent to the glove factory

• Combined with chemical compounds according to theglove specifications.

(formulation plays a major role in determining the quality ofthe final product. )

• On a single or double line, glove formers go throughstages of dipping, leaching, rinsing and drying.

(The speed of the line determines the thickness andstrength, the formers the texture and size of the glove)

• After the forming process is completed, the gloves needto remove harsh chemicals through a vulcanization andleaching process. During the leaching phase protein isminimized.

• Washed and dried before they are inspected for testing.


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• Gives the rubber processing flow chart including the simpledefinition 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


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

Students Activity


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12.elastomer(rubber) 2016 (1)

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