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Properties of Natural Rubber

Natural rubber is a naturally occurring latex with the long-chain polymer of cis polyisoprene. Natural latex is a white color fluid that is collected by tapping the rubber tree. There are different Natural rubber species more than 200. But Hevea brasiliensis is the commercially significant source of natural rubber.

Advantages of natural rubber

  • Good processability
  • Excellent elastic properties
  • Good tensile strength
  • High elongation
  • Good tear resistance
  • Good wear resistance
  • Heat build-up in the dynamic stress is very low. Little dissipation factor.
  • Excellent cold resistance
  • Good electrical insulator
  • Due to the non-polar properties of natural rubber, it shows high resistance to water and acids.

Disadvantages of natural rubber

  • Poor weather and ozone resistance
  • Without adding specific ingredients, natural rubber has poor heat-resistant properties. The typical working temperature range of natural rubber is between -55℃  and +82℃.
  • Natural rubber does not dissolve in oils or water. But it will swell in oils and fuels.
  • Because natural rubber is nonpolar, it is unsuitable for use with organic liquids in general even though vulcanization considerably improves swelling resistance. But NR can be used with low molecular weight alcohols.

General properties of natural rubber

01. Molecular weight

Natural rubber is a high molecular weight polymer. The molecular weight can be up to 6000000 g/mol. Generally, synthetic rubber has a molecular weight range between 5000 – 10000 g/mol. The molecular weight of natural cannot be controlled and the molecular weight of synthetic rubber can be controlled. Polymers with high molecular weight have high physical properties.

Because the polymer molecules are entangled and connected to each other well. Therefore, the intermolecular interactions are high. So, the physical properties such as tensile strength, impact strength, shear stress, viscosity are increasing. But the commercially important range of molecular weight is 10000 – 100000 g/mol.

Variation of physical properties with respect to molecular weight
Figure 01: Variation of physical properties with respect to molecular weight

02. Solubility in liquids

Natural rubber (Cis polyisoprene) molecules are very nonpolar. Therefore, it cannot dissolve in polar solutions. However natural rubber is very difficult to dissolve in nonpolar solutions also. Because natural rubber has long-chain molecules, they are highly entangled. Vulcanized rubber does not dissolve in liquids either and it swells in solvents.

Cis polyisoprene
Figure 02: Cis polyisoprene

03. Change of properties with temperature

Polymers change their properties from a glassy state to a rubbery state when increasing the temperature. In the glassy state, the polymer is brittle, and it is hard to work with glassy polymers. The temperature at which polymer changes from a glassy state to a rubbery state is called “Glass transition temperature (Tg)”. The Tg of natural rubber is around -67 ℃ and the melting point is approximately 180 ℃. In this region, the natural rubber maintains its rubbery properties. So, the natural rubber is in a rubbery state in a high range of temperatures. After vulcanizing the Tg will increase.

04. Strain-induced crystallization

Natural rubber is an amorphous polymer. When it is applied stress, the rubber molecules are arranged along the direction of the force. Therefore, some regions of the rubber are crystalized. This phenomenon is called strain-induced crystallization. This is a unique property for natural rubber. With the crystallization, the Tg and melting temperature (Tm) also increased. As well as the tensile strength of the polymer will increase. Therefore, natural rubber shows high tensile properties even without fillers.

Strain-induced crystallization of natural rubber
Figure 03: Strain-induced crystallization of natural rubber

05. Ability to be vulcanized

Vulcanization is the process that forms crosslinks between rubber chains. This will enhance the elastic properties of natural rubber. When un-vulcanized rubber (raw rubber) is applied a force, it will stretch, and then remove the force the rubber will not be completely recovered to its original dimensions. So, un-vulcanized rubber simply behaves like plastic. But the vulcanized rubber will stretch when a force is applied and will recover to its original state (or very close to its original state) when force is removed. This is because of the cross-links in vulcanized rubber. Vulcanized rubber is an example of elastomers.

Crosslinks formation of natural rubber
Figure 04: Crosslinks formation of natural rubber

Physical, mechanical, and thermal properties of natural rubber

PropertyNatural rubber
Density1100Kg/m3
Hardness30- 90 shore A
Tensile strength5 MPa
Young’s modulus50 MPa
Elongation500% - 900%
Compression set10-30%
Specific gravity0.92-0.93
Thermal conductivity0.5 W/mK
Thermal expansion coefficient220 ×10–6 m/mK)
Glass transition temperature (Tg)–72 ℃  
Melting temperature (Tm)177 ℃
Heat capacity1300 J/gK
Table 01: Physical, mechanical, and thermal properties of natural rubber

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References and Attributes

timcorubber.com - Natural Rubber: A Tough Material for Physically Demanding Applications 

material-properties.org - Rubber – Density – Strength – Melting Point – Thermal Conductivity

Figures:

The cover image was designed by using an image by Slashme, licensed under CC BY-SA 4.0, via Wikimedia Commons.


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