Elasticity of latex

Latex is a natural resource and the elasticity can be increased by heating it with sulfur. My question is why they are have different elasticity than that of latex? I mean like rubber gloves have a good elasticity while rubber bands not really elastic. Is it cause by the amount of sulfur that we heat?




The 'elasticity' of a material is property which describes how it changes shape in response to a tensile stress. Specifically, when a material is placed under a tensile stress, it experiences some sort of elongation (strain). If removing the stress results in a return to the original size, the material is described as 'elastic' over this range of stress (note: under sufficiently high stress, all materials will eventually 'yield' to permanent deformation and ultimately 'break').


In 'engineering materials terms' the ratio of tensile stress (in MPa) to strain (in mm) within its elastic region is called the 'modulus of elasticity' of the material (aka 'elastic modulus' or 'Young's modulus').


In the case of latex rubber, it has an elastic modulus of about 0.1 GPa.


Natural rubber is derived from a milky suspension found in the milky sap of some plants such as the hevea tree. The sap which contains a polymer of isoprene called 2-methyl-1,3-butadiene, as well as a small percentage of proteins and acetone-soluble materials such as resins, fatty acids, and sterols. 


Most of the isoprene consists of the long-chained stereoisomer, cis-1,4-polyisoprene. 


![cis-1,4-polyisoprene](https://i.stack.imgur.com/oh1Qb.gif)


The 'cis-' indicates that substituent groups are on the same side of the C=C double bond, as shown above. The high cis- content (~99%) of natural rubber latex is what gives its elastic properties. Natural rubber in this form is a thermoplastic, that is, it is soft at higher temperatures but becomes hard and brittle when cooled.


Vulcanization involves heating the natural rubber with sulfur at $140-180~\mathrm{^\circ C}$ which encourages the formation of cross-links between the long-chained polymers, resulting in a 'thermosetting' elastopolymer. Other additives are also sometimes used, such as N-phenyl-P-naphthylamine antioxidants to reduce deterioration by atmospheric oxygen or ozone, or 'accelerators' such 2-mercaptohenzothiazole, used to speed up the reaction or allow it to occur at lower temperatures. Reinforcing agents such as carbon or silica can also be introduced to added the 'stiffness' and tensile strength as well as resistance to abrasion. Softeners such as fatty acids, wood resins, pine oils or coal tar byproducts are also sometimes added. Finally, colouring agents and fillers (barium sulfide, calcium carbonate) are also sometimes used for aesthetic or to reduce the cost, but sometimes at the expense of reduced strength. 


So by combining small amounts of other chemicals to latex, chemists are able to adjust the properties of the rubber in subtle ways to achieve the most desirable mechanical characteristics. 


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