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MCAT

Unit 3: Lesson 1

Chemical and physical sciences practice passage questions

Elasticity and kinetics of vulcanized rubber

Problem

Vulcanization is a process which adds more elasticity to natural rubber. This is achieved by chemically cross-linking the cis-polyisoprene strands that make up natural rubber with sulfur, as shown in figure 1. These bridges of sulfur remove natural rubber’s inherent plasticity to create a more elastic and consistent structure. Non-vulcanized rubber is easily deformable due to the polymer strands moving independently from one another. By introducing sulfur bridges, the rubber may still deform under strain, but upon release the rubber will return to its previous shape.
Varying the amount of sulfur and temperature involved with vulcanization can affect the overall durability of the rubber product: ranging from the mostly non-vulcanized rubber latex gloves used in medicine, to the hard, heavily cross-linked rubber used in bowling balls. To test the amount of cross-linking achieved in a polymer, scientists can use what is known as a swelling experiment, in which a set volume of liquid is added within a polymer container. The swelling the polymer undergoes can then be used to calculate the amount of cross-linking.
When more sulfur is used, more cross-links are created, which leads to a harder rubber product. Generally, higher temperatures will also result in higher cross-link density as well. Vulcanization can be performed at temperatures anywhere between 120°–180°C. Improper maintenance of these variables can result in an inferior product. This is especially important for medical supplies such as tubing and latex gloves. Much effort has gone into testing the vulcanization process of latex gloves to prevent glove failure during surgery. Table 1 shows the various temperatures at which gloves are vulcanized compared to the force required to cause glove rupture.
Figure 1. Sulfur cross-links between cis-polyisoprene strands (blue and green)
Temperature of Vulcanication (°C)Force Upon Rupture (N)
1209
1309.9
14010.7
15011.2
16011.8
17012.4
18013.0
Table 1. Temperatures of latex glove vulcanization and their tensile strength limit
Attributions: Chemical structure of vulcanized rubber from public domain.
Information related to vulcanization process from: Rabindra Mukhopadhyay, Sadhan K. De, S.N. Chakraborty Effect of vulcanization temperature and vulcanization systems on the structure and properties of natural rubber vulcanizates Polymer Volume 18, Issue 12, December 1977, Pages 1243–1249
Latex 2006: Frankfurt, Germany, 24-25 January 2006 By Rapra Technology Limited
In trials, researchers were able to stretch a latex glove 10 cm using 5N of force. Assuming the glove elasticity matches that of an ideal spring, what is the spring constant (k) of this glove?
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