• The Rubber Mixings.

In the assembly known as the tire, we have a very complex product of which one demands many different, and sometimes, contradictory performances.

• Air tightness under pressure.
• Rigidity and mechanical resistance
• Flexibility
• Resistance, to wear …
• to oxidation
• Road holding
• Resistance to over heating
• Long Life

For example:

• Modulus: .

Particularly high for the product used in the bead zone of the tire.

• Perte: (energy loss) it is the lack of the rebound.
• Structure: distribution and size of the grains of the carbon black; important for the KM (wear resistance and road holding).

The criteria demanded of the mixing shop is that it must provide rubber mixings capable of satisfying the needs of the users:

• Production shop: for shaping, assembling and curing.
• The customer: for the performance requirements of the tire.

In order to assure the desired Quality and Regularity of the mixing, it is necessary to have the correct method, and the correct amounts of constituents witch are necessary for each mixing.

In other words:

• Blending.

This operation is carried out initially by the mixing shop on a large number of constituents requested by a mixing.

It consists of mixing several raw materials of the same nature and formula.

This technique facilitates the leveling out of the differences inherent in raw materials from different origins.

Blending improves the regularity of the quality of the fabrications produced.

Principle of Blending

Blending is continued after the manufacture of the rubber mixing, in order to even out the dispersions arising from their fabrication,

• COMPOSITION 0F THE MIXINGS.

The ingredients of a rubber mixing are determined in accordance with the performances it must satisfy. These are obtained using a precise recipe and method of fabrication:

-The specific properties of the mixing

-Its total price

-Its feasibility in the production department (processability)

and assure:

THE DESIRED QUALITY THE REGULARITY 0F THE QUALITY

The recipe is given by the formula of the rubber mixing example 18051X01 which indicates their nature and their distribution

Example of the formula for a KM mixing:

A) THE ELASTOMERS

Elastomers , Rubbers or gums can be:

NATURAL - SYNTHETIC - RECLAIMED

• NATURAL RUBBER

Natural rubbers are difficult to process due to their lack of plasticity. One could say that they are ?nervous?.

POSSIBLE NON-CONFORMITIES:

-Mouldy rubber

-Foreign matter

-Wet rubber

-Long exposure to the sun

-Excess coating of talc

• SYNTHETIC RUBBERS

Obtained from petro-chemicals

Known since 1876, the first patent on polymerization of the isoprene was taken out in 1910, by the English technicians Strange & Matthews.

During the war of 1939 --> 1945, Germany produced approximately 120,000 tons of synthetic rubber per year.

The Americans made 700,000 tons/year of GRS in 1945 (GRS = Government Rubber Styrolene).

The different synthetic rubbers are either used alone, mixed with each other or with natural rubber as a function of the different properties which characterise them.

Listed below are the main synthetic rubbers with their properties compared to natural rubber:

SBR = Styrene Butadiene Rubber (co-polymerization of butadiene with styrene)

• Better adherence
• Same wear characteristics.
• Bad before cure stick
• Better impermeability
• High level of over-heating
• Better resistance to cuts

PBR = Poly Butadiene Rubber

-Very good resistance to wear

-Very good resistance to overheating

-Very good resistance to flexing / bending

-Very good resistance to cuts (better than SBR)

-Bad in propagation of cuts

-Bad in adherence

-Bad in scaling / flaking

-Very permeable to gases

BUTYL

-Very good impermeability (1--> 10 times better than naturals)

-Slow curing speed: incompatibility with other elastomers

-Very resistant to oxidations

-High overheating to dynamic demands

POLY ISOPRENE

-Properties similar to those of natural rubber but more expensive

NEOPRENE

-Used generally in non-staining mixings

SILASTENE (silicon)

-Not used in the formula of a tire (very expensive and incompatible)

Used in the fabrication of rubber tools

EPDM (Ethylene Propylene Diene Monomere) replacing EPT

-Good resistance to ageing, abrasion, and to tearing

-Incompatible with other elastomers

-Elasticity the same as natural rubber

-Bad before cure tack

* RECLAIMED RUBBER

Sources:

Old tires) Only Michelin tires

Scrap tires) of known composition

Remains from retreating)

They are reduced to a fine powder, and all metal is extracted. They are then treated with saturated steam, plastified, filtered & packaged.

When incorporated with other elastomers their price is about the same, however they are valuable in the development of mixings for textile calandring (excellent before cure properties: tack, elasticity, flexibility & porosity).

B) THE FILLERS

Reinforcing fillers improve the qualities of the rubber mixing and reduce its cost.

* Reinforcing fillers: carbon black

Carbon blacks are the most common fillers used in the rubber mixing where they improve.

- General mechanical properties

- Abrasion resistance

- Electrical conductivity

- Resistance to oils

The properties of different carbon black are linked to their fineness (100 --> 1000 angstroms) and their structure.

The structure of the aggregate appears in the form of a microscopic ?BUNCH OF GRAPE? like particles whose liaisons are mechanically inseparable. This cons-truction explains certain differences in the behaviors of mixings in use, as a function of the order of incorporation of the constituents during their fabrication.

They are defined by their specific surface (SS) (free surface of all the grains contained in a gramme of carbon black, in m2.)

We use blacks of S.S. = 100, 200.

* Other fillers

Other fillings such as: chalk, kaolin, clay, silica... are used in order to:

• Reduce the cost
• Reduce the elasticity
• Increase the toughness, density or modulus (1)
• Colour the mixing

C) PROTECTION AGENTS

Anti-oxidants and anti-ozonants.

Objectives:

- Avoid ageing and alteration of the elastic quality

- Improve heat retention

- Protect from attacks of oxygen and heat

D) THE PLASTIFIERS (SOFTENERS)

These are used to facilitate processing:

* Physical softeners are oils, waxes, resins, fatty acids,... They locate themselves between the mole-cular chains and facilitate their displacement.

* Peptisants: are catalysers which speed up the process of oxidation which in turn promotes the rupture of the molecular chains.

Objectives:

-Reduce the costs of mechanical work (additional working)

-Facilitates dispersion of the fillers

-Reduces temperature elevations during mixing (rises in temperature during mixing)

-Facilitates processing

-Improves before cure tack

Disadvantages:

-Reduces resistance to wear, modulus, etc

-Increases over-heating

E) THE VULCANIZING AGENTS

* Sulphur

Promotes the passage of the mixings from a plastic to an elastic state during the curing of the tire

It is the best-known vulcanizing agent for elastomers with double bonds.

For butyl, which has few double bonds, we use resins.

* Accelerators

These are used to regulate the time necessary for vulcanization.

They can be: ultra-rapid, rapid, slow, or retarded.

* Activaters

These are used to improve the action of the accelerators. The best know is zinc oxide together with stearic acid.

F) OTHER PRODUCTS

These are specific to each applications, and have:

* No particular influence on the mixing

-Colours used for identification of accelerator blocks

* A defined action on the mixing:

-Powder Y:provides the adhesion to metal

-Peptisants: plastifiers

-Retarders

-Diverse resins: improve certain mechanical resistances

NON- CONFORMITIES & THEIR CONSEQUENCES

• Rheological characteristics.

• Rheology:

Study of physical characteristic : Fluidity (viscosity), Plasticity, Elasticity,……in rubber technology.

• Fluidity :

The fluidity is the value of cylinder dip (unit: 1 point= 1/100 mm), during a constant of time (11’). This value is given with a pressure value. Example: 80 (T:75)

Factors of non-conformity.

• Ageing.
• Processing and storage temperature.
• % of hot and cold chutes.
• Working time.
• Temperature, speed, state and machine adjustment.
• Blending.

• Plasticity or (Mooney viscosity).

The value is a value of torque (during a fixed time 1+4). The Mooney unit = 0.083 N.m

Depending on the different diameter of rotor ( ? ), we get ML (Mooney Large) or MS (Mooney Small).

Example: ML (1+4): 95

Factors of non-conformity: The same as Fluidity.

• Homogeneity :

This is the correct distribution of the ingredients contained in a rubber mixing as well as its transfer properties.

• Elasticity :

In rubber technology, this characterises the state of a mixing, which permits rubber products to deform themselves under the action of a force; then to retake their initial shape on the suppression of this force.

• Scorching of the mixings. (FIXATION)

Also known as : - Premature curing

Even at ambient temperatures, from the moment a mixing contains the curing agents, there is a risk of premature curing. In other words, the mixing is changing and vulcanisation has commenced. The mixing will become harder (therefore less plastic), rough and quilted with scorch lump (cured and elastic particles) of various sizes.

Note: The higher the temperature, the faster the process.

The mixing is then no longer suitable for use, and if it is not segregated in time, there is the risk that it will contaminate other mixings during processing.

If the mixing is used in the production of tires, after cure anomalies can be experiencedwhich may not alwaysbe detected.

In service, after low mileage, these tires can deteriorate, with sometimes, disastrous results.

IMPORTANT:

• The temperature and times experienced by a mixing are cumulative.
• As far as possible, one must limit the evolution of a mixing during its transformation in the manufactu-ring process.

A MIXING ALWAYS REMEMBERS THE TREATMENT IT HAS RECEIVED.

This risk of premature curing will be reduced by taking the following precautions.

·The incorporation of vulcanisation agents should be left to the last possible moment.

·Use maximum cooling on the mills (water circulation)

·Cool the mixings and the products made from the mixings, avoiding the stacking of large and compact masses of rubber at above room temperature.

·Respect ageing tolerances.

·Respect the percentage of chutes being recycled.

·Limit the working of the mixings to that which is just necessary, especially if the work causes a high increase in temperature, even for a short time.

·Forbid the ?mixing?, by chance, of two mixings and thereby give a ?third mixing? which could increase in temperature very rapidly.

• Ageing.

This phenomena effects different qualities of mixings both before and after curing. There are four principal areas:

• BLOOMING

This is the migration of sulphur towards the surface of an uncured mixing, on which it forms a crystalline deposit. This creates a reduction in before and after cure tack.

• REARRANGEMENT

This is the changing in position of the molecular chains, which tend to attach themselves to one another, and therefore reduce the plasticity of the uncured mixing.

• SENSIBILISATlON

This is the beginning of premature curing under elevated temperatures. The calories provided at ambient temperature are sufficient to produce this effect.

• OXIDISATlON

This is the de gradation of the molecular chains by the action of certain atmospheric agents of which the most important are:

• OXYGEN

Aside from its importance during plastification, it is the oxygen in the air which is the cause of ageing of natural rubber (always by the degradation of molecular chains).

This reveals small cracks and a change in the mechanical properties (ex- : resistance to flexing, to tearing...).

It is also the oxygen which is responsible for ?virage au gras? which renders the rubber sticky. This degradation is accelerated when the temperature is higher (it can double with a temperature increase of 10°C).

• OZONE

“Overactive oxygen” it also degrades natural rubber but with a violence clearly much higher (with only 8 parts per 100,000,000 at ground level in normal condition)

Therefore one must be careful with electric welding (in particular) which emits enormous amounts of ozone: 100 à 200 parts per 100,000,000 in several hours work in a workshop.

• ULTRA-VIOLETS (sun, artificial light)

According to the length of their waves, the ultra-violets act as accelerator of oxidisation and therefore assist the degradation of the bonds between the molecular chains.

Fortunately the carbon black in the rubber absorbs the biggest part of these rays, which limits the damage.