Natural Rubber

 The natural rubber has a vegetative origin. It is extracted as latex from the bark of the Hevea tree. Natural rubber latex is a polymer. The monomer or building unit of natural rubber latex Isoprene. The molecule consists of thousands of Isoprene units, joined end to end resulting in poly Isoprene or Natural rubber. 

The natural rubber polymer is almost 100% Cis-1,4 poly Isoprene and the  average molecular weight (MW)  is in the 3.4 × 106   range

Natural rubber tends to crystallize spontaneously at low temperatures or when it is stretched, because of its high structural regularity. Low-temperature crystallization cause stiffening of the polymer, but it can be easily reversed by warming. Crystallization of natural rubber results in high tensile strength, and resistance to cutting, tearing, and abrasion.

Composition of Field Natural Rubber Latex

Material                                                     % By Weight

Total Solid Content(TSC)                                    36

Dry Rubber Content(DRC)                                 25-40

Protein Substances                                               1-1.5

Resinous Substances                                            1-2.5

Ash                                                                       Up to 1.0

Sugars                                                                   Up to 1.0

Water                                                                     Balance

Relative Density 0.97-0.99

PH                        6.5-7.0

Natural Rubber properties include,

• Easy processing
• Excellent dynamic properties with low hysteresis loss
• High-temperature resistance is limited (Maximum +70^oC)
• Good low-temperature properties (-500^oC)
• Good rubber to metal bonding property
• High resistance to tear and abrasion
• Can be self-healing
• Poor resistance to Sunlight, Oxygen, Ozone, solvents, and oils

How Natural Rubber is made

As mentioned earlier in this blog, Natural Rubber is a liquid sap, called latex which extracting from the Hevia brasiliensis tree or the aptly named rubber tree. These trees are indigenous to South America but are today abundantly found in Asia. 

The environmental  conditions that must be needed for the plantation of Heva tree are,

The altitude of land-about    -   300m

Annual rail fall                     -    not less than 2000mm

Temperature                          -    25-30

pH of the soil                        -    4.5-6.0  

Natural rubber products can be classified in two ways,

1. Latex-based products

These are produced directly by using the Natural Rubber Latex.

Examples:

Surgical Gloves

Industrial Gloves

Balloons

Condoms

Elastic Thread

Rubber Foam

Rubber Bands

Carpet Backing

    2. Coagulate the Natural Rubber Latex, making rubber sheets and producing rubber goods-These called "Dry Natural Rubber Products"

Examples:

Tires

Tubes

Hoses

Rubber floorings/Mats

Automotive Parts

Adhesives

Coating Materials

Rubber Toys

Natural Rubber sheets manufacturing process in brief

Latex is collected from the trees by making a cut in the bark and collecting the runny sap in cups.

This process is called tapping.

In order to prevent the sap from solidifying, anti coagulants are added

Examples of anticoagulants are,

Ammonia

Sodium Sulphite

Sodium Carbonate

Formalin

Why do we add anti-coagulants to Natural Rubber Latex?

NR Latex is a solution with biological origin

It's an ideal medium for the growth of bacteria

Bacteria dissociate the carbohydrate and produce simple organic acids

These acids denied the stability of latex and coagulate it

So, to prevent or stop the spontaneous coagulation of natural rubber latex, an anticoagulant is added just after extracting from the tree

The next step is the standardization of latex

This is done by adding pure water to Natural Rubber Latex

The primary objective of the standardization is to reduce the Dry Rubber Content(DRC) of the Natural Rubber Latex solution

For the production of Ribbed Smoked Sheets(RSS), the DRC value bring down to 12.5% 

What is the necessity of Standardization?

• To maintain the quality consistency of the final product
• It gives a softer coagulum which can be easily sheeted
• Faster setting of dirt and improve the clarity of dirt
• Bringing down the viscosity of latex results in, uniform distribution of chemicals(acids) added to the latex
• Easily remove the trapped air within the latex solution

The next step is straining the latex to remove impurities if any.

Acid is then added to the mix to extract the rubber, in a process called coagulation. This can take about 12 hours.

The commonly used acids for the coagulation are formic or acetic acids

The concentration of the acids is 1%

The coagulation process helps to separate the rubber portion from the latex solution

At about the pH value of 4.8, latex begins to change its colloidal nature and becomes a single mass of rubber

Then the rubber coagulum is removed from the medium

The coagulum is then passed through rollers to remove excess water.

There are two types of rollers is being employed. They are,

Smooth Rollers

Diamond Rollers

At the end of this stage, the thickness of the rubber sheet becomes 3.0mm

After the roller application, the sheets are washed with water to remove any foreign materials if any and the next sheets will be hanging on for dripping for about 4-6 hours

 Once this is complete, the layers of rubber are hungover racks in smokehouses or left to air dry.

The required smoke is provided by firewood or coconut shells

The burning is take place inside the smokehouses under the limited supply of air

The temperature inside the smokehouse is 60℃ and the smoking period is 5-6 days

After the smoking process, the moisture  content of the rubber sheets will be reduced to 0.8% and at this condition no growing of fungus on the sheets

The final step is inspection, sorting, grading, and storage of the sheets