Natural
rubber (NR) latex is serum of Havea Brazilienzis tree which is commonly used as
rubber tree. Its composition is as follows,
|
Rubber (dry rubber content)
|
30-40%
|
|
Non rubber content
|
5-6 %
|
|
Water content
|
55-65%
|
Non
rubbers present in NR are sugar, proteins (amino acids), phospholipids, metals
etc. Not only that its contain unreacted COO-, NH3+, NH4+ also there. Generally field latex
has a PH of 6-7. But with the time PH is reduced as Bacterial activities
present. That means H+ percentage is increased. Because of
that latex coagulates. To avoid coagulation
PH should be increased by adding bases. It is known as preservation of
NR
Eg:
- Ammonia of PH 8
High ammonia (0.7-0.8 w/w)
Law ammonia (0.2w/w)
- Non-ionic surfactants
- Cationic surfactants
Since
dry rubber content (DRC) is law in NR latex or since high water content it is
not economically in transporting, storing etc. So to increase the DRC different concentration
methods are used. Commonly used methods are listed below.
- Creaming
If latex
is kept in rest rubber rich layer will lie on top of water rich layer due to
the density difference
- Centrifuging
Centrifugation
is used to separate rubber compounds from water
- Electro decantation
This
is a method which rely on the fact that latex particles carry a negative charge
so they will migrate towards a positive electrode through a semi permeable membrane
- Evaporation
This
method was developed with the means of stabilizers and controlled evaporation
because uncontrolled evaporation leads to skin formation in latex.
Characterization
of Natural Rubber latex
There
are different methods to do characterization of NR as given below.
- Total Solid Content (TSC): Measure how much of solid are present in latex
- Dry Rubber Content (DRC): Measure the pure rubber content. This is very important parameter
- Alkalinity: Measure the free alkali content of latex
- KOH number: How much KOH is consumed in neutralizing volatile and non-volatile acids in latex
- Mechanical Stability Time (MST): Latex is stirred very rapidly (about 13000 rpm) then time taken to stabilization is observed as MST
- Volatile Fatty Acid number (VFA): Measures volatile fatty acid content in latex
