Natural rubber - NR Rubber - definition, types, properties, applications

What is NR rubber?

NR rubber is an isoprene polymer, or elastomer. Its polymer chains reconnect when stretched, which gives the material its elasticity.

Unlike synthetic rubbers, which are made from petrochemicals, natural rubber is produced from the latex sap of rubber trees (although other plants also produce latex, rubber plants are the most efficient in this respect - which is why they account for the production of 99% of NR rubber).

Natural rubber is fully biodegradable and has excellent tensile, elongation and abrasion resistance over a wide range of hardnesses. It has many use cases that include medical devices, surgical gloves, aircraft and car tyres, dummies, clothing, toys, etc.

Is natural rubber latex rubber?

Yes, natural rubber is latex rubber. It is formed from the sap of rubber trees, which is what latex is.

What does NR stand for?

NR stands for Natural Rubber.

How is NR rubber produced?

NR rubber is produced from the sap of the rubber tree. To extract it, the tree is first incised and the sap is dripped into a cup. In this form, one third of the latex is rubber held in colloidal suspension and another third is water.

To process latex into rubber, it is mixed with formic acid. This coagulates the raw material into curds, which are then washed and pressed into blocks or sheets. This process also removes excess water from the material.

Later, the rubber sheets are subjected to smoking or drying, which eliminates mould.

When the raw material is ready, it goes into chewing machines. These increase the rubber's workability. The next step is mixing with chemicals that improve the properties of the material.

Later, the rubber is moulded using calendering or extrusion.

The final piece of the puzzle is the vulcanisation process, which makes the material more durable, flexible and resilient.

Vulcanisation involves heating the rubber with a certain amount of sulphur in a pressure cooker at around 160°C. However, the same effect is achieved by steam curing, heating in a microwave oven or sending through fluidised beds or molten metal salts.

Vulcanisation results in the cross-linking of the molecular chains of polyisoprene. The result is an increase in strength and chemical resistance, in addition to the elimination of the viscosity inherent in raw rubber.

What is the structure of NR rubber?

The structure of NR rubber is a polymer called "polyisoprene". It consists of isoprene monomer units, which have the chemical formula C5H8.

The repeating isoprene units are linked by carbon-carbon double bonds to form a long-chain polymer. The presence of the double bonds gives natural rubber its unique properties, such as high elasticity and flexibility.

What are the types of NR rubber?

There are four main types of NR rubber:

• TSR, 
• RSS,
• TPC, 
• special.

You can read more about each of these below.

TSR rubber (Technically Specified Rubber)

Natural rubbers of the Technically Specified Rubber (TSR) type are classified according to their source (field latex or cup lump) and specific technical parameters.

The main exporting countries (Malaysia, Thailand, Vietnam and Indonesia) produce their national standard TSR rubbers:

• SMR (Standard Malaysian Rubber), 
• STR (Standard Thailand Rubber), 
• SVR (Standard Vietnamese Rubber), 
• SIR (Standard Indonesian Rubber).

TSR grades 10, 20, 10CV and 20CV are derived from cups. They are dark and durable rubbers and are the most economical TSR grades.

TSR CV50 and CV60 rubbers, on the other hand, are derived from field latex and are classified according to the Mooney viscosity scale (CV = constant viscosity). These are light and soft rubbers, less economical due to higher processing costs.

TSR grade L has the lightest colour, classified according to the Lovibond scale.

RSS Rubber (Ribbed Smoked Sheet)

To produce RSS rubbers, water is removed from the field latex (before full coagulation) water is removed and sheets are formed. These are then dried by smoking and analysed visually to determine the percentage of impurities.

The finished product is graded on a scale of one to five, where five indicates the most impurities.

TPC rubber (Thick Pale Crepe)

TPC rubber is produced from a particular Hevea Brasiliensis maple tree that grows in Sri Lanka produces latex with a low pigment content. After coagulation, it is processed into sheets.

These are dried without smoking and then selected according to specific optical properties (similar to the RSS genus).

Special rubber types NR

There are other types of natural rubber, produced by changing the polymer chains or variations in the manufacturing process.  They are created to meet specific industrial parameters such as:

• resistance to ageing, 
• resistance to chemical agents, 
• resistance to high/low temperatures.

What are the properties of NR rubber?

The properties of NR rubber are: durability, abrasion resistance, vibration damping and low cost compared to many alternative polymers.

Its high tensile strength and elongation make it an extremely robust and resilient material. It also has high compressive strength, making it useful in dynamic engineering applications such as anti-vibration mounts, bumpers, dog toys, bushings and more.

In addition, it has some resistance to sunlight, atmospheric conditions, oils, petrol, lubricants, solvents, petroleum oils, animal and vegetable oils, silicone oil, refrigerants, ammonia, carbon dioxide, water and steam. However, there is no denying that synthetic materials are better suited to work in such conditions.

Like any other polymer, natural rubber also has some disadvantages.

It has limited heat resistance, making it only able to withstand temperatures up to 82°C. Furthermore, due to the presence of double bonds in the polymer backbone, natural rubber is susceptible to ozone.

Exposure to ozone or oil-based fluids will result in material degradation over time. The rubber's tensile strength, elongation and other physical properties will decrease.

As a final note, it is worth adding that the price of natural rubber although low can fluctuate due to the collection and production process.

What is the hardness of NR rubber?

The hardness of NR rubber ranges from 30 to 95 on the Shore scale. However, know that it depends on factors such as the specific composition, processing conditions and intended use. The hardness of NR rubber can be adjusted as required.

Is natural rubber waterproof?

Yes, natural rubber is waterproof.

What is the temperature range of NR rubber?

The temperature range of NR rubber is wide, especially when it comes to resistance to low temperatures. The material retains its properties in conditions ranging from -46°C to 82°C.

What are the applications of NR rubber?

NR rubber's applications cover many industries. Some of the most common use cases include: refrigeration gaskets, air conditioning, engine mounts, engine coolants, petroleum and chemical tank linings, automotive gaskets and seals, anti-vibration mounts or weather seals.

Other examples of NR rubber applications include:

• Automotive - Natural rubber is used together with synthetic polymers in tyres for cars, buses and even aircraft. Its durability and abrasion resistance are the main features that help tyres to last for a long time. In addition to this, rubber is used to manufacture many other automotive parts, such as brake pads, airbags and window seals.
• Dog toys - Natural rubber is an excellent choice for chew toys. Both because it is non-toxic (which is important in case it is swallowed) and because it is extremely wear-resistant (the toys last despite constant chewing).
• Clothing - Natural rubber is often used in clothing (such as shorts and foam for water sports) because of its ability to stretch. 
• Flooring - Many commercial buildings and public spaces (such as gyms, kitchens and playgrounds) use rubber flooring because of its durability. Natural rubber serves for a long time. In addition, it creates a surface that is easy to clean and maintain.

Is natural rubber still used today?

Yes, natural rubber is still widely used in various industries and applications.

What are the trade names of NR rubber?

The trade names of NR rubber are as follows:

• RSS, 
• TSR, 
• SMR,
• CREPE,
• DYNAT, 
• Heveaplus MG.

However, the material is most commonly referred to simply as natural rubber.

Who invented NR rubber?

There is no single inventor of NR rubber. The material has been used by indigenous peoples in Mesoamerica for hundreds of years, with evidence of its use dating back to the Olmec culture. However, the vulcanisation process that greatly improved the properties of rubber was developed by Charles Goodyear in the mid-19th century.

The man experienced first-hand the limitations of natural rubber.

One day in 1834, he went to a shop to buy a rubber lifebelt and by chance found his new hobby. He became interested in the natural material and its potential uses.

A little later, the US government awarded Goodyear a contract to produce rubber mailing bags, which he fulfilled.  However, the bags quickly broke down and created problems in extreme weather conditions.

Charles looked for a way to solve this problem. He experimented for so long until he finally developed vulcanisation.

This is the process of heating and curing an entire mass of rubber to make it extremely strong and durable.

While Goodyear was perfecting vulcanisation in the US, another Englishman, Thomas Hancock, invented the same thing overseas. As a result, Goodyear is credited with inventing the process itself, while Hancock developed the term "vulcanisation", which is named after the Roman god of fire.

What is the difference between NR and NBR?

The main difference between NR and NBR is that NR rubber is a natural isoprene polymer, while NBR rubber is a synthetic copolymer of acrylonitrile and butadiene.  Therefore, the two rubbers have different properties. While NR is fully biodegradable and has excellent tensile strength, elongation and abrasion resistance, NBR is an oil-resistant synthetic rubber with the ability to capture and retain gas and can be used to produce rubber hoses.

Is natural rubber a silicone?

No, natural rubber is not silicone. They are two different materials with different properties and composition.

Natural rubber is a polymer called polyisoprene, which is naturally produced from the sap of the rubber plant. Silicone, on the other hand, is an industrial material with a polymer structure containing silicon, carbon, hydrogen and oxygen.

Is natural rubber better than plastic?

Natural rubber is better than plastic in some use cases, but the two materials are very different, so it is difficult to compare them.

Natural rubber is an elastomer characterised by its flexibility, strength and resistance to certain chemicals and environmental conditions. It is also biodegradable.

Plastic, on the other hand, is a synthetic polymer that can be moulded into different shapes and sizes. In addition, depending on the type, it has different levels of resistance to chemicals, heat and impact.

Why is natural rubber better?

Natural rubber is better in several respects, such as high elasticity coefficient, high tensile strength, adhesion or biodegradability. However, it also has its disadvantages, such as lower resistance to light, UV rays or heat compared to synthetic rubbers.

Is natural rubber expensive?

No, natural rubber is not expensive compared to most synthetic rubbers.  However, take into account that its price can fluctuate due to the capriciousness of the harvesting and production process.