Latex sap comes from the tree Hevea brasiliensis. The natural latex sap is a white liquid, which is extracted from the tree sap. Roughly thirty to forty percent of the sap is actually latex; the rest is proteins, sugars, and water. Latex by itself is a clear liquid, but it is easily colored. Most colored latex remains translucent, however latex colored either black or white becomes opaque. Latex is a very desirable product due to this property, as well as its other physical characteristics.
Physical Properties
Elasticity:
Stretches 400-1000% of resting length
Boiling point:
77.3 ¡C
Molecular weight:
53.06
Density:
.8004
Odor threshold concentration:
21.4 ppm
When cured, latex is one of the most elastic polymers, stretching up to one thousand percent of its original length. It also has one of the best "memories", meaning that it can return to its original shape very easily after it has been distorted. It is a very strong rubber, even when it is cut in very thin layers, and maintains its form very well. It shrinks slightly when it cures, usually between three and four percent. It cures in a very short time, and at room temperature. Some forms of latex cure at slightly higher temperatures, but not more than fifty degrees Celsius. Commercial liquid latex will cure in a matter of minutes, but takes a few hours to set fully. This is the same with the latex sap, which will take up to twenty-four hours to set fully, but it solidifies much sooner. Latex has a very low resistance to most chemical stimuli, though. Ultraviolet light, oils, and ozone easily damage it, however. This is the reason that the preservatives are added, to decrease the sensitivity of the rubber.
The first step in preserving the latex, as well as the first step in the manufacturing process is to add ammonia, and small amounts of thiurams. These prevent the premature spoiling or curdling of the latex. The next step is to add proteolytic enzymes to break down any remaining proteins in the liquid, and create a more pure latex. Next, the latex is spun in a centrifuge to increase the purity even further. It serves to remove even more protein, as well as concentrate the rubber in the sap up to roughly sixty percent. Many times the sap is spun again to increase the concentration even more. After the rubber is removed from the centrifuge accelerators and antioxidants are added. The generally selected choices for accelerators are thiurams, mercaptobenzothiazole, carbamate, and thioureas. Some manufacturers are replacing the thiurams with dithiocarbamates, due to the nature of the thiurams to provoke allergic reactions to the latex. These accelerators are added to aid in the vulcanisation of the latex, while the antioxidants are added to as preservatives, which protect the final latex from heat, ultraviolet light, and ozone. In the case of products, which are to be manufactured to a certain shape, the next step is to dip a mold, coated with a coagulant, into the latex. The most common coagulant used is calcium nitrate, which converts the liquid latex into a sort of wet gel. Moving the gel through a warm oven completes the coagulation process. The next step is known as "Pre-vulcanisation leaching", which involves washing excess chemicals out of the latex, mainly the coagulant from the previous step. The latex is then vulcanised, by heating it and adding sulphur, which combines with the accelerators and improves the elasticity and physical durability of the finished product. The actual polymerization occurs when the latex molecules bond together with the sulfur molecules. This process takes several months to complete fully, however it cures to acceptable levels within hours. The latex is then leached again, for up to twenty four hours, to fully wash out all the chemicals that still remain in the latex, that are unnecessary to the finished product.
The molecular formula of latex is C3H3N. This is the pure form of the monomer, as it can be found in the tree sap, however the manufactured product makes use of the naturally occurring organic materials in the sap, as well as adding others to improve performance. As a polymer, latex is referred to as cis-1, 4-polyisoprene.
Due to its natural resiliency, latex is used in many products that are on the market today. Its most obvious application is in things such as rubber gloves, and balloons. Latex is also used in paints, due to the ease in which it is colored. It is used in many other, less obvious ways as well. It is used in adhesives, to allow them to remain flexible, and in surface coatings of anything that may need to be protected. It can be combined with other monomers to create strong acrylic fibers. In all cases the latex is combined with another polymer or another monomer, because of the low resistance to chemical fatigues possessed by latex.
There are two very strange uses for latex that deserve mentioning here. The first is that it can be used as a fumigant for grains, to kill maggots and insects living within the storage area. The other use is to combine it with acrylonitrile which polymerizes in wood pulp to create a synthetic soil compound. Both of these uses are relatively minimal, and are both disappearing, as better compounds are developed that perform the same tasks.