Natural rubber comes from the paragum tree, the paracca tree, or Hevea brasiliensis as it’s known in Latin. The tree grows naturally in the Amazon in South America but is also planted for rubber production in Southeast Asia and Western Africa.
The history of rubber is surrounded by myth. It’s told that it was discovered in the middle ages, when Colombus saw natives in Haiti play games with elastic balls that bounced a lot better than those used in Europe. In fact, the local population of the West Indian islands and parts of Central and South America knew about, and used, rubber for various purposes long before the rest of the world knew anything about the material. Before rubber made its entrance into the modern world, cork plates and other material was used for dampening shocks and blows, and as insoles in shoes.
Rubber as a useful material developed slowly between the 15th and 18th century as a fundamental problem remained to be solved. The challenge was to tackle the great disadvantage of rubber – that it became hard and brittle when cold, and soft and sticky in heat. In the 19th century several failed attempts were made to try to improve the qualities of rubber to make it stable, dry and bendy rather than soft and sticky.
In 1839 – 100 years before the second world war – an American named Charles Nelson Goodyear discovered that if you mix a small amount of sulfur with the rubber and heat it, the rubber will maintain the desired qualities – something we nowadays call vulcanization.
Vulcanization is a process during which the rubber is transformed into an elastic state by changing its chemical structure, using crosslinking. During vulcanization, the elastic qualities are improved and expanded so that they apply in a greater temperature range. After his fundamental discovery, Charles Nelson Goodyear patented vulcanization in 1844 – the same year Sweden introduced their national anthem for the first time.
The best quality of rubber is that it’s elastic and works a little like a steel spring, which dampens a blow or vibration. The difference between rubber and a steel spring is that rubber has a long natural frequency, which makes it ideal to use for the dampening of vibrations.
Hardness or stiffness is a quality in rubber which describes the power needed to deform the material. Hardness is not the same as density. Density is a measurement of tightness of the material, meaning the mass of the material per volume quantity.
There are two groups for measuring hardness:
The hardness or stiffness of rubber is determined by measuring how far a blunt measuring tip can be pressed into the rubber.
Originally there was a number of methods for testing hardness, but nowadays it’s primarily two different methods that are being used: Shore and IRHD.
The oldest method for measuring hardness is the Shore method from 1915, where a truncated cone is pushed into the rubber using a spring, and a reading is made after 3 seconds. Prior to this method the time interval “within one second” was used, but this has been changed to 3 seconds in ISO 7619 to enhance accuracy.
The Shore method comes in a number of scales (A, B, C, D, AO, DO, O, OO and AM). Of these the four most common are specified in the ISO standard. Shore A is used for standard rubber stiffness, Shore D for hard rubber, and Shore AO for soft rubber.
All of the Shore scales range from 0 to 100, where 0 is “infinitely” soft and 100 is “infinitely” hard. There’s special Shore measuring equipment to measure the hardness of rubber.
The IRHD method (International Rubber Hardness Degrees) was an enhancement of the Shore method within ISO, to improve accuracy when measuring hardness.
Natural rubber is what we call the rubber extracted from plant juice (the latex) of the rubber tree, Hevea Brasiliensis.
Vulcanized rubber is cheap, durable, elastic and comes with a high abrasion resistance. It’s suitable for use in temperatures between -30 and 80° C, but is not oil or gas resistant. To enhance resistance levels, lampblack is often added. That’s why natural rubber is often black, despite the rubber in its pure form being yellowish.
IR is based on synthetically produced cis-polyisoprene. It has similar mechanical qualities to natural rubber, but is a much more pure material that is better suitable for the industries of food and medicine.
SBR was developed in Germany in the 1930’s. It’s similar to natural rubber in quality, but is somewhat more durable and resistant to wear and tear, heat and weather conditions (ozone). Car tires are often made of a combination of natural rubber and SBR.
Thanks to the excellent ability of nitrile rubber to withstand gasoline and oil it’s often used in oil hoses, but also food packaging for products in contact with fats. NBR can withstand prolonged temperatures of up to 90° C, but is not particularly weather resistant. To work around this, it is later mixed with about a third of PVC.
Butyl rubber stands out primarily thanks to its ability not to let any gas through. Therefore, it’s often used on the inside of hoses for cars and bike tires. It’s also quite resistant to heat, with a temperature range of -30 to 120° C during prolonged periods, and -40 to 140°C for shorter periods.