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Vulcanization is the process that forms crosslinks between rubber chains. The pre-vulcanization reaction takes place in each latex particle without altering its shape, and appearance. Pre- vulcanized latex is very similar to un-vulcanized latex in appearance. It is possible to pre-vulcanize the dispersed phase of natural rubber latex without any prior coagulation.

Sulphur Pre-vulcanization of NR

Necessary Compounding ingredients in a sulfur vulcanizing system are sulfur as the cross-linking agent, Activators, Accelerators, Stabilizers, pH builders, Fillers, Pigments, Thickeners, etc. Latex compound is prepared by dispersing these compounding agents with preserved natural rubber latex. latex compound can be either dispersion or emulsion

The typical compounding formula of the sulfur vulcanizing system

IngredientParts by weight
DryWet
NR latex (60%)100167
KOH (10%)0.33
Stabilizer (10%)0.22
Sulfur (50%)24
ZnO (50%)12
ZDEC (50%)12
Table 01: The typical compounding formula of the sulfur vulcanizing system

Sulfur vulcanization of NR latex occurs at many low temperatures when compared to sulfur vulcanization of dry rubber. Sulfur vulcanization in the latex stage occurs much more rapidly than the vulcanization of dry rubber at the same temperature using the same vulcanizing ingredients.

Effects of sulfur pre-vulcanization upon properties of NR latex and films derived from NR latex

Sulfur pre-vulcanized NR latex is slightly yellow. The density of latex increases with pre-vulcanization.

Tensile strength vs Time of Prevulcanization
Figure 01: Tensile strength vs Time of Prevulcanization

Level of Network combined vs Time of Prevulcanization
Figure 02: Level of Network combined vs Time of Prevulcanization

Similar results have been obtained for synthetic cis 1,4-polyisoprene. This indicates 30% of the water in the latex may be responsible for the occurrence of sulfur pre-vulcanization at rather a low temperature.

Mechanism of sulfur vulcanization of NR latex

It is assumed that the vulcanization reaction starts by a mechanism similar to sulfur vulcanization of dry natural rubber.

General mechanism of sulfur vulcanization

First, the vulcanizing activator reacts with the S8 ring and forms an active sulphurating agent.

vulcanizing activator reacts with the S8 ring

Active sulphurating agent reacts with natural rubber chains and forms activator + sulfur (Ac-Sx) pendent groups. (Polysulphidic pendant groups)

Active sulphurating agent reacts with natural rubber chains

The polysulfidic pendent sulphurating group further react with another rubber chain and form sulfur crosslinks. The activator is removed.

The polysulfidic pendent sulphurating group further react with another rubber chain

Some sulfur atoms are removed from pendent sulphurating groups. (De-sulphuration)

De-sulphuration

By removing sulfur atoms from polysulphidic crosslinks mono-sulfidic and di-sulfidic crosslinks are formed.

5

The rearrangement reactions are as follows.

6

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References and Attributes

Figures:

The cover image was created using Image by flockine from Pixabay


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