The Polymer School
Would you like to learn more about the basic materials used in products manufactured by Trelleborg? Here you can read about, for example, the history of rubber, how it is manufactured and its areas of application.
The Crying Tree
We do not know the name of the lady and we do not know why she was out in the vast South American rain forest one day, thousands of years ago. Probably, she was out collecting fruit, berries and roots, which were important foods for the Indian tribes living a nomadic life. She stopped at “the curious crying tree” which she had passed several times before and observed that there was damage to the trunk. From the cut a “tacky” liquid was dripping. Using a fruit shell formed as a bowl, she collected the “tears” that came from the tree.
She felt there was something unique about the tree, and presented the filled bowl to the Chief on her return to the camp fire. He investigated the content. Maybe he was carrying out a religious ceremony when he spilled “the tears” onto a piece of wood and moved it closer to and above the hot fire.
The result was sensational. “The tears” changed consistency and turned into an elastic substance. The Chief formed a ball of the mass between his palms. Being careless for a short moment, he dropped the ball. To the delight of the children present, the ball bounced up and down several times. If the Chief had not suspected something magic about the ball, the party would have gone on the whole night. The immediate conclusion was, that a material possessing such properties must be very special. Something of an understatement, on hindsight!
The Rubber tree (Hevea brasiliensis) can be 20m tall. Tapping is normally started seven years after planting. On average, one tree produces 250 kg rubber during its lifetime. Tapping means that a cut is made in the trunk to a very accurate depth. The latex that subsequently drips out is collected in a cup. A rubber tree can be tapped about 170 times a year.
“That stuff could be useful!”
As time went by, the Indians became increasingly interested in the newly discovered material from the rubber tree (Hevea brasiliensis) which they named “cahu-chu” – the crying tree. By mixing the new material with various parts of local plants, they found that the properties changed and new ways of using the material were developed. When the Europeans arrived in South America, they learned how the Indians used the rubber material to make clothing and waterproof boots and soon realized themselves that this could be a very useful material. The Indian word “cahu-chu” was changed to “kaut-schuk” and shortly that was the name of whole new industry.
The main part of Natural Rubber is a polymeric hydrocarbon, where every molecule contains thousands of isoprene units. The formula is (C5H8)n. where n is the number of units in the molecule
It is possible to extract sap with similar properties from other plants and trees, but none has been found to have the same importance as the rubber tree.
The sap, called latex is a mixture of water (62%), rubber particles (35%) and several other ingredients, of which protein is one of the most important. The quality of the latex can vary depending on the type of tree (clones), climate and so on. The latex is a suspension of isoprene polymers. A polymer is built up of many units into very long polymer chains. The word “polymer” means many parts. Each unit is called a “monomer.”
First step of natural rubber production
In the introduction, it was noted that “the tears” were moved towards and above a fire whereby the material changed and became tough. What was happening?
By analyzing latex freshly tapped from the rubber tree, one finds many small rubber particles, many less than a thousandth of a millimeter.
Around the rubber particle is a very thin layer of proteins, which prevent the rubber from forming a lump. If the protein layer is broken, the polymers will coagulate. Today, that is achieved by adding acid to the latex but long ago coagulation was achieved by exposure to hot smoke, as in our story. The material was brittle at low temperatures, and tacky at high temperatures. For this reason, it was to take until 1839 before rubber started to become a commercially viable industry. The man behind the increased interest in rubber was Charles Nelson B. Goodyear who discovered that the addition of sulfur improved the material’s properties.
The discovery was really a fluke, since Goodyear accidentally left a piece of rubber on a hot oven. There just happened to also be sulfur on the oven. A subsequent investigation of the rubber showed that the material had changed to become elastic. The procedure was called “vulcanization.”
In 1888, J.B. Dunlop invented a tire that could be filled with pressurized air (the pneumatic tire). The invention was an important step in the development of cars and bicycles.
Vulcanization signaled start of the commercial rubber industry
It was earlier explained that rubber has long molecule chains. In nonvulcanized rubber, the molecules can move against each other, and the material has a plastic behavior, similar to chewing gum. Through the vulcanization process, also called curing, sulfur is added and the material is heated under pressure. The sulfur forms bonds between the polymer molecules called “crosslinks”.
The crosslinks (sulfur bridges) give rubber its typical properties:
- The material becomes elastic (for example, a rubber band will resume its normal shape after having been extended).
- Tensile strength and resistance to wear and tear increase substantially.
- Swelling in solvents decreases.
- Elasticity and other properties remain over a greater temperature interval.
The addition of other properties, such as tightness and resistance to air and water and possibilities of combining rubber with other materials, confirms that rubber is a fantastic material, in both natural and synthetic form.