Natural rubber latex is the oldest and most familiar material used in disposable gloves. In recent years it has increasingly been supplanted by nitrile and to a lesser extent vinyl gloves, but it remains popular in many uses, especially medical applications.
How are latex gloves made? It’s a process whose roots go back centuries.
A brief history of rubber
The basic raw material used in manufacturing is latex concentrate, which is a product of Hevea brasiliensis. Also known as the Pará rubber tree—after the Brazilian state of Pará—it is native to Brazil and the Guianas. Cultivation was common throughout pre-Columbian Central and South America, and such civilizations as the Aztecs and Mayans used it for everything from shoe soles to balls for playing games.
By the 1770s English theologian, philosopher, and chemist Joseph Priestley had coined the term “rubber” because the substance could be used to rub out pencil markings. (He also discovered oxygen, but that’s another story for another blog post.)
Rubber was slow to catch on globally because it could be temperamental to work with: In cold weather it became brittle, and in hot weather it was sticky. Enter Charles Goodyear, a dogged experimenter with no practical knowledge of chemistry. In 1841, Goodyear—after years of failure and poverty—discovered the process of vulcanization, in which sulfur- and lead-fortified rubber is heated at a low temperature to render it stronger, melt-resistant, and reliable. He patented the process in 1844 and essentially gave birth to the modern rubber industry. (He spent his later years fending off lawsuits and died poor in 1860, but in 1898 he was immortalized when the Goodyear Tire & Rubber Company was named for him.)
The rubber tree, which had been a closely guarded resource of the Amazon rainforest, was introduced to Southeast Asia, Africa, and elsewhere in the world in the 1870s after the fascinating and possibly apocryphal story of a notorious “biotheft” of 70,000 seeds by British explorer Sir Henry Wickham. This led to the British cultivating huge plantations in Singapore, Malaysia, and Ceylon (now Sri Lanka).
Today most of the world’s natural rubber comes from plantations in India, Indonesia, Thailand, Malaysia, and Vietnam.
Tapping the tree
Rubber trees are usually ready to be tapped after about seven years of growth. A steel tapping knife is used to remove thin strips of bark from the tree at a downward curve. This directs the milky-white sap to a spile, or spigot, which channels it into a cup affixed to the tree. Tapping is done in the early morning, because sap coagulates faster later in the day, when temperatures are higher, and reduces the flow.
After about six hours, the fluid stops flowing. In that six-hour period, a tree can usually fill a gallon bucket. The tree can be tapped again with another fresh cut, usually the next day. The latex is preserved with ammonia. Trees often are rested for a period after heavy tapping.
Because of its high water and non-rubber content—about 70% is water, protein, sterol glycosides, resins, ash, and sugars—the latex is concentrated and stabilized. The latex is mixed with processing chemicals including sulfur, zinc oxide, accelerators, pigments, stabilizers, a de-webbing agent, and antioxidants. The latex matures for 24 to 36 hours to become a compound ready for dipping.
The production line
Production uses ceramic or aluminum hand-shaped molds, or formers, that are first extensively washed in hot water and chlorine to ensure there is no residue from previous batches. Next the formers, suspended on a continuous moving chain, are dipped into a mixture of calcium nitrate solution and calcium carbonate—the nitrate is a coagulant, while the carbonate helps the gloves release from the formers.
After drying, the molds are dipped into the latex compound, with the duration of the dip determining the mil thickness of the gloves. The freshly molded gloves are next leached in a mixture of hot water and chlorine, which removes residual latex proteins and chemicals to help reduce the severity of any allergic reactions to latex.
The gloves are then dried and cured, which is where Charles Goodyear’s enormously important discovery enters the process. Vulcanization converts the gloves to an elastic state by causing a reaction between rubber molecules in the latex and chemicals that have been added, and gives gloves their elasticity so they are less likely to tear.
After drying, the gloves are rinsed again to leach out more latex proteins, then the cuffs are beaded, or rolled, to make them easier to don and doff. After a dip into cornstarch and a final drying, pneumatic air jets strip the finished gloves from the formers, or workers remove them by hand. The gloves are hot-air tumbled to remove any remaining powder.
The molds are given another thorough chemical wash and rinse, and the process begins anew.
The quality-control phase
Gloves are tested to ensure quality using methods from the American Society for Testing and Materials (ATSM). The U.S. Food and Drug Administration (FDA) regulates these standards. The pinhole leak test is one of these methods. Workers fill the gloves with one liter of water, then close and hang the gloves to check for leaks.
The tests adhere to guidelines regarding acceptable quality limits (AQLs). These standards designate a percentage to evaluate a batch of gloves. An AQL of 2.5% means that statistically, only 2.5 gloves for every hundred will fail a quality test. If a batch’s failed gloves exceed this percentage of the total batch, all the gloves in that batch fail. The results of these tests determine whether the gloves will be industrial or medical grade; the latter are subject to more rigorous testing.
Once the gloves are approved and packaged, they are shipped to the U.S. by ocean freight. AMMEX has distribution centers in Atlanta, Toronto, and Seattle.