1. OBJECTIVE
To determine the crosslink parameters of a vulcanised natural rubber
2. INTRODUCTION
Natural rubber is an elastic substance derived
... [Show More] 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:
𝑆 = 𝑘 𝑙𝑜𝑔𝑊
Although rubber does not behave like solid, gas or liquid, part of its behaviour can be understood from the analogy of the properties of a perfect gas. Pressure exerted by gas on the wall of a vessel is caused by the continuous and random bombardment of the gas molecules; tension on an extended rubber is caused by the random thermal movement of segments of the polymer chain. Gas molecules are inclined towards a most probable distribution in space. Similarly, the segments of the polymer chains are inclined towards the most probable, randomly coiled condition.
We can also produce an equation for rubber, similar to the ideal gas equation, P = nRT/V. the equation for the situation is,
𝐹 = 𝑑𝑅𝑇 (λ − λ−2)
𝑀𝐶
where F is the tensional force (i.e. the force per cross-sectional area of the unextended rubber sample, kg cm-2), d is the density of rubber (g cm-3), R the gas constant, T the absolute temperature (K), Mc is the molecular weight between crosslinks, and 𝜆 the ratio of the extended length to its original.
Just as no gas behaves as ideal gas, similarly, rubber will display a deviation from its ideal behaviour, such that the equation is modified to fit actual observation
𝐹 = 𝑔𝑑𝑅𝑇 (1 − 2 𝑀𝐶) (λ − λ−2)
𝑀𝐶 𝑀
where g is a correction factor for the additional (apparent) crosslinks caused by the physical entanglement of polymer chains, M the molecular weight of the elastomer, and the expression (1 – 2 Mc/M) is a modification which takes into consideration the existence of free ends in a 3-dimensional network.
3. METHODOLOGY
Part A: Stress-Strain Method
Two positions of 5 cm distance marked in the rubber sample. The sample gripped with 2 clips and the sample hung as shown in Figure 8.1
Figure 1 Experimental set-up
The container which will be attached to lower clip weighed. The distance between the markings measured. Weights successively added and the corresponding lengths (l) of the sample measured until it was extended to about 25 cm.
Part B: Swelling Method
(1) Determination of rubber density
A portion of the rubber sample (~10 cm) cut and its weight determined accurately. A 5 mL measuring cylinder filled with distilled water to its 3.00 ± 0.05 mL. The rubber was cut into smaller pieces and put into the measuring cylinder. Make sure there were no air bubbles on the rubber samples. The new water level in the flask was read accurately. The volume (V) of the rubber determined and hence, the density of the rubber calculated.
(2) Determination of swelling of the rubber sample
About 3 cm of the rubber sample was cut and weighed accurately. The sample placed in a conical flask containing 25 mL of toluene. The flask stopped and placed in a dark corner of a cupboard for at least 12 hours. The rubber took out and placed on a wire mesh for 5 seconds. The rubber transferred to a weighing bottle of known weight, closed and the weight was determined.
4. RESULT & DISCUSSION Part A: Stress-Strain Method
Table 1 The weight of load and length of elongation readings of addition.
Addition
Individual
weight (g) (±6.6 g) Cumulative weights, L/ kg Length (cm) λ λ -1 λ – λ-2 L/
(λ – λ-2)
0.00 0.00 5.00 1.00 1.0000 0.0000 0.0000
0.1450 0.1444 5.5 1.10 0.9091 0.2736 0.5278
0.0198 0.1642 5.6 1.12 0.8929 0.3228 0.5087
0.1057 0.2699 6.0 1.20 0.8333 0.5056 0.5338
0.1000 0.3699 6.2 1.24 0.8065 0.5896 0.6274
0.1471 0.5170 6.5 1.30 0.7692 0.7083 0.7300
0.0494 0.5664 7.0 1.40 0.7143 0.8898 0.6365
0.1415 0.7079 7.5 1.50 0.6667 1.0556 0.6706
0.1025 0.8104 8.0 1.60 0.6250 1.2094 0.6701
0.2893 1.0997 12.0 2.40 0.4167 2.2264 0.4939
0.2894 1.3891 16.0 3.20 0.3125 3.1023 0.4478
0.0502 1.4393 16.7 3.34 0.2994 3.2504 0.4428
0.2900 1.7293 19.2 3.84 0.2604 3.7722 0.4584
0.2890 2.0183 21.0 4.20 0.2381 4.1433 0.4871
0.2899 2.3082 22.5 4.50 0.2222 4.4506 0.51862
0.2960 2.6042 24.0 4.80 0.2083 4.7566 0.54749
0.1471 2.7513 25.0 5.00 0.2 4.9600 0.55470 [Show Less]