SID2002 INDUSTRIAL POLYMER CHEMISTRY I LABORATORY REPORT

1. OBJECTIVE To determine the crosslink parameters of a vulcanised natural rubber 2. INTRODUCTION Natural rubber is an elastic substance derived from the latex sap of trees, especially trees belonging to the Hevea and Ficus genera. Natural rubber is either an elastomer or an elastic hydrocarbon polymer. Synthetic rubber is a man-made rubber produced at manufacturing plants by synthesizing it with petroleum and other minerals. Cross-like polymers are a type of polymer whose molecular chains are bound together. This bond is either a chemical or a physical bond, but usually when we talk about a cross-linked polymer, we mean chemical bonds. One of the most important properties of cross-linked polymers is that they are thermosetting, which ensures that they cannot be melted or dissolved. So, it could be harder for us to process this kind of polymer. Lots of processing methods, such as injection and extrusion, will not work. Nevertheless, cross-linked polymers do have some unique properties and are commonly used today. Extensive cross-linking results in a random three-dimensional interconnected chain network. Rubbery materials are normally elastic and stretchable. From the chemistry perspective, these materials need to meet several requirements in order to achieve these properties. Firstly, the molecules must be of sufficient long chain. When the molecular weight of a rubber is too low, it will be in liquid state at room temperature. Liquid natural rubber, for example, normally has an average molecular weight of below 20 000 g/mol. Secondly, rubbers must have molecular structures that allow segmental rotation along their backbone at room temperature. To achieve this, the molecular backbone must have non-collinear single bonds and that it must not be substantially crystalline at service temperature. The term rubber commonly refers to elastic materials that require vulcanisation before they can be used in products. This is because unvulcanised rubbers are normally weak when put under stress during use. Nevertheless, there are very few exceptions where vulcanisation is not required for product application. One of the examples is rubber glue where the rubber needs to be soluble in a solvent. Vulcanised rubber would not be soluble in any solvent. Other than these few exceptions, almost all rubber products require vulcanisation to provide the required strength for a longer designed service life. However, there are elastic polymers that do not require vulcanisation. The basis of elasticity in NR is that polymer chains without any external application of strain, will have the most appropriate configuration, statistically; i.e. randomly coiled. When strain is applied, the chain will be effectively extended to a longer, less appropriate configuration, followed by a decrease in entropy. If the strain is released, the chain will return to the most probable configuration and maximum entropy in accordance to the Boltzman expression: