vulcanisation

Vulcanisation of rubbers

Vulcanization or vulcanisation is a chemical process for converting natural rubber or related polymers into more durable materials by heating them with sulfur^[1] or other equivalent curatives or accelerators. These additives modify the polymer by forming cross-links (bridges) between individual polymer chains.^[2][3] Vulcanized materials are less sticky and have superior mechanical properties. The term vulcanized fiber refers to cellulose that has been treated in a zinc chloride solution to cross-link the cellulose fibers. The term "vulcanization" is named after Vulcan, the Roman god of fire. Hard vulcanized rubber is sometimes sold under the brand name Ebonite or the genericized former brand term "vulcanite", and is used in making articles such as clarinet and saxophone mouth pieces, bowling balls, and ice hockey pucks.

Other definitions of vulcanisation

A chemical reaction of sulfur (or other vulcanizing agent) with rubber or plastic to cause cross-linking of the polymerchains; it increases strength and resiliency of the polymer. Also known as cure.

An irreversible process during which a rubber compound, through a change in its chemical structures, becomes lessplastic, more resistant to swelling by organic liquids, and more elastic (or the elastic properties are extended over agreater range of temperature).

Advantages of vulcanized rubber:

·         Good tensile strength and extensibility

·         Excellent resilience i.e.it returns to the original shape, when the deforming load is removed

·         Low water absorption tendency

·         Higher resistance to oxidation, wear and tear abrasion

·         Better electrical insulator

·         Resistant to organic solvents (petro, benzene), fats and oils

·         Higher useful temperature range unlike uncured rubber which becomes soft at high temperature (>335 K) and brittle at low temperatures (<283 K)

rubber compounding

In developing a rubber compound, it is  essential to mix the raw dry rubber with various compounding ingredients.  The process of sequentially adding the ingredients into the raw rubber is termed ‘compounding’ and the resulting final homogeneous mix is referred to as the ‘rubber compound’. 

The most common  compounding ingredients  added into the rubber are vulcanizing agents, vulcanization accelerators, activators for the accelerators, fillers, processing aids, softeners, antidegradants and other miscellaneous ingredients for specific purposes.  Each compounding ingredient has specific role in the compound and some give significant effects on the final properties of the resulting products.Therefore a rubber compound has to be formulated with an expectation to incorporate the desired properties required in the finished rubber product as well as in the production process.  In view of this, the important factors that need to be considered when designing a rubber compound are price, processability and functional properties. 

      There are 5 main ingredients in a rubber compound formula and each are measured in parts per hundred (phr):

1.       Rubber/Polymer

2.       Filler

3.       Protectant

4.       Processing Aid

5.       Curing System

Below is an example of a compounding recipe.

A typical rubber formation based on parts per hundred is:

Polymer          100 phr

Filler                30 – 60 phr
Antioxidant     1 – 3 phr
Antiozonants    1 – 3 phr
Oil                       5 – 30 phr
Cure                  5 – 10 phr

hydrocarbon skill