About Latex (2)

Compounding of Latex

Rubber tapper pouring latex into
a mould at a plantation

Latex to be used for dipping, coating, and rug-backing must be mixed with compounding agents which include: 

1. stabilizers including surfactants 
2. vulcanizing agents
3. vulcanization accelerators
4. antioxidants 
5. fillers
6. viscosity modifiers (thickeners) and 
7. gel sensitizers

These ingredients provide the physical characteristics required in the final product. This requires a uniform, fine dispersion of these chemicals in the latex, which can be accomplished with the homogenizer.

Growing of Latex Particles

In the manufacture of foam rubber, it is desirable to have a latex solids content of 60 to 65% and still have a workable viscosity of not over 2000 cP, prior to foaming. Lower solids content will result in cells of insufficient wall thickness, causing collapse of the foam with light loads.

There are a number of methods of preparing the latex, so that it may be concentrated without acquiring an excessive viscosity. These methods are:

1. solvent addition
2. soap neutralization
3. electrolyte addition
4. freeze agglomeration
5. chemical agglomeration 
6. pressure agglomeration by homogenization.

Only the latter three are used commercially, and homogenization is the preferred method. Various parameters will affect the extent of agglomeration: pH, solids content, temperature, soap-to-rubber ratio and homogenizing pressure. 

To increase agglomeration one must make one or more of the following changes:

• decrease pH,
• increase solids,
• decrease temperature,
• decrease soap-to-rubber ratio and increase homogenizing pressure.
• The use of the homogenizer for agglomeration is covered by a number of patents.

Thread

Thread from Latex

Thread is usually made from natural latex, although there is an increasing use of some of the synthetics in this application. Thread is manufactured by dispersing into this latex: vulcanizing agents, antioxidants, fillers, opacifiers, etc. Frequently, when these materials have been incorporated in latex, the viscosity of the latex is increased. Since the latex thread is manufactured by gravity flow of latex through a spinnerette and into a coagulating bath, any change in viscosity will result in a change in the thread diameter or, in extreme cases, breakage of the thread. It has been found that passing the latex through the homogenizer stabilizes the viscosity and ensures a uniform compound.

Master-Batching

In normal master-batching, a latex is coagulated, washed, dried and then mixed in a rubber mill with carbon black or other reinforcing agents to provide a fine, uniform dispersion of the reinforcing agent to obtain maximum strength in the elastomer. There are processes for mixing carbon black dispersions with liquid latex prior to coagulation, but these methods do not achieve the same tensile strength as the dry mixing method. By mixing a crude carbon black aqueous dispersion with a latex and homogenizing the mixture, it is possible to obtain an intimate combination of the carbon black and the latex, which results in a rubber having higher tensile strength than rubber made by the normal methods of master-batching.

PAGE 2 OF 3: PAGE  1   -   2   -   3