Sunday, June 1, 2014

Latex Compounding

To improve difference properties of latex, generally emulsions, dispersion and solutions are added as compounding ingredients.

Surface active agents are used to stabilize the latex by reducing the surface free energy. The stabilization means delay the coagulation of latex. There are different types of surfactants as follows,

  • Anionic surfactants: carboxylates, Sulfonates  
  • Cationic surfactants: Acetyl tri methyl ammonium bromide
  • Amphoteric surfactants: Water soluble proteins
  • Nonionic surfactants: Poly ethylene fatty acids

Property modifiers

  • Vulcanizing system: is used to increase the strength. Generally the vulcanizing agent is S (sulfur). Primary activators like ZDC (zinc dithio carbonate) and secondary activators like thiurum, thiosole are also used.

  • Viscosity modifiers: are used to control skin thickness.

  • Softeners: are used to increase the softness of rubber. Examples are vegetable oils, organic esters, petroleum oils etc.

  • Fillers: are used to increase stiffness of rubber. Examples are carbon black, TiO2, CaCO3, ZnS etc.

  • Antioxidants:  there are two types called Staining Antioxidants and non-staining Antioxidants. Examples are staining- Amines, non staining- phenolic compounds  

Auxiliary ingredients

They are coagulants, heat sensitizers, flame retardants, colorants etc. 
The equipment like Ball mills, Attrition mill,   and colloidal mills are used in latex compounding.   

Thursday, April 3, 2014

Natural Rubber Latex

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)
Non rubber content
5-6 %
Water content

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  

  • 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.
  1. Total Solid Content (TSC): Measure how much of solid are present in latex
  2. Dry Rubber Content (DRC): Measure the pure rubber content. This is very important parameter  
  3. Alkalinity: Measure the free alkali content of latex
  4. KOH number: How much KOH is consumed in neutralizing volatile and non-volatile acids in latex
  5. Mechanical Stability Time (MST): Latex is stirred very rapidly (about 13000 rpm) then time taken to stabilization is observed as MST
  6. Volatile Fatty Acid number (VFA): Measures volatile fatty acid content in latex