The Making of Latex

WHAT IS LATEX MADE OF?

What is latex made of? The simplest answer: latex is rubber, or rather, the precursor to vulcanized rubber. Typically, the composition of latex is roughly fifty-five percent water and around forty percent rubber material.

Latex is the soft white substance found beneath the bark of a mature rubber tree. You may be surprised to think of latex as a natural material, given the strength and man-made feel of so many of its final applications, like tires, rubber gloves, and tennis shoes.

The truth is, natural rubber latex does, in fact, come from nature. Before it is processed, rubber tappers harvest latex from trees using a method that has been passed down for generations.

WHERE DOES LATEX COME FROM?

Most natural rubber latex comes from one species of rubber tree, Hevea brasiliensis. The composition of latex from different trees varies, but the most common one is native to South America. The tree also thrives in Southeast Asian plantations.

Latex lives just beneath the bark of these rubber trees. When tappers peel back the bark, they disrupt the plant ducts to reveal latex, a milky white substance. From there, they can harvest the latex and send it off to be processed.

HOW IS LATEX HARVESTED?

The latex harvesting process can only begin once a rubber tree is mature—about five years old. At this point, the tree is ready to start producing rubber.

To tap a rubber tree, tappers remove thin strips of bark. Doing so disrupts the plant ducts, which contain latex. Disrupting these ducts allows the latex material to flow down grooves that the tappers cut into the tree and into large cups.

Tappers tend to these cups, collecting the latex when they become full and replacing the collection cups to continue harvesting from the tree until it is appropriately tapped.

HOW IS LATEX PROCESSED INTO RUBBER?

The rubber creation process begins at the point of harvesting, when they take latex sap from rubber trees.

Next, companies filter the rubber latex so it can be packaged in drums for its next destination.

Once latex is ready to be made into smoked sheets of rubber, companies add acid to the latex. This causes the material to clump. The clumped fluid is then rolled into sheets in a mill. This process removes water from the latex material, so the sheets can be dried and smoked.

Finally, latex is prevulcanized. Prevulcanization involves chemical treatments and gentle heating at low temperatures. Once latex is prevulcanized, it is more convenient to transport. With more heating, the rubber latex material can now be made into regular rubber.

SYNTHETIC LATEX

Of course, not all latex materials are naturally occurring. Some latex is synthetic, composed of petroleum-based chemicals. These chemicals are used to create synthetic rubber polymers, which are clumped and dried, and then transported to a manufacturing plant.

The manufacturing plant then mixes the synthetic rubber polymers. Sometimes, they mix in additional ingredients, depending on the end product for the synthetic latex.

The plant rolls the rubber into sheets and cuts them into sheets for further processing. 

The most common synthetic rubber processing techniques are:

• Extrusion
• Injection Molding
• Compression Molding
• Latex Dipping

During extrusion, you feed the rubber polymer compound into the extruder, which heats it and compresses the latex material. This is done by the barrel and screw of the extruder. The polymer compound is then forced through a small opening to be vulcanized or cured.

Injection molding involves mixing rubber strips. Using high pressure, they are forced into a mold. Once in the mold, heat vulcanizes the rubber.  

Compression molding starts with preforms of rubber compound. The preforms are shaped into a mold with compression, pressure, and vulcanization.

Latex dipping involves dipping a specialized mold into a prepared latex solution repeatedly to achieve a specific thickness and shape. Latex dipping is a valuable method for producing specialized shapes and hollow products, such as handles and grips, gloves, medical products such as tubing and balloons, and more.

Alternative Natural Rubber Latex

The tire industry has invested considerable resources into investigating naturally occurring alternatives to natural rubber latex. Natural rubber latex is a tremendously valuable material with highly desirable properties, but there are some challenges and risks associated with relying too heavily on this material. The largest risk factors are:

• Vulnerability. Natural rubber latex is largely grown in areas where natural disasters and political instability can threaten the availability of the latex material. This makes for a fragile supply chain and severe consequences in the event of a major disruption.
• Growth timeline. Latex cannot be harvested from Hevea brasiliensis, the rubber tree, until it reaches full maturity, which takes six or seven years. In the event of a fire or other natural disaster, recovery takes many years, which can have long-term consequences on manufacturers and end users alike who rely on natural rubber latex.

The tire industry is currently focused on two primary alternatives to natural rubber latex and the Hevea brasiliensis plant: guayule and Russian dandelion.

The guayule is a perennial woody shrub that grows naturally in the southwestern United States and northern Mexico. Unlike Hevea brasiliensis, guayule can be harvested after just two years. Rubber materials experts have been experimenting with guayule for nearly a century, but the tire industry started investing more heavily in guayule research in the early 2010s. In 2015, Bridgestone made a tire with guayule-derived natural rubber, the first of its kind. Since that milestone, most major tire manufacturers have begun experimenting with guayule. Firestone even debuted a guayule racing tire in 2022.

At this stage, the future of guayule-derived rubber in the tire industry remains uncertain. The material shows promise, but it may not be able to replace synthetic rubber for a long time. Additionally, farming and harvesting guayule requires different equipment than is currently being used for any other crop, which can be prohibitive.

The other alternative to natural rubber latex currently being explored is the Russian dandelion. The Russian dandelion is native to Kazakhstan, Kyrgyzstan, and Uzbekistan and thrives in temperate climates. It’s also quite tolerant of low-quality soil. Like guayule, the Russian dandelion has been a subject of experimentation for nearly a century. In fact, 30% of the rubber used by the USSR during World War II was derived from Russian dandelion. Today, Continental appears to be leading Russian dandelion experimentation efforts in the tire industry and has already debuted bicycle and truck tires using it. Like guayule, Russian dandelion cultivation needs further development, but the future is exciting. 

LATEX MATERIAL PROPERTIES

After processing, rubber latex is an impressive material.

Rubber latex is known for its tensile and elongation abilities, as well as tear resistance and overall resilience. Most common abrasives will not bother latex.

Low temperatures are not a threat to rubber latex, but high temperatures can pose an issue. Above eighty-two degrees, latex can begin to corrode. Manufacturers and compounders can add treatment chemicals to protect latex rubber from corrosion due to heat, sunlight, and oxygen.

It is also best not to use latex rubber alongside petroleum products and solvents. Excess exposure to these materials can cause the latex material to break down.

How Latex is Tested

Latex testing is an excellent tool for assessing the quality and purity of latex. Common latex tests include:

• Viscosity testing. Viscosity testing is used to quantify the pre-vulcanization characteristics of latex. This is especially important given the fact that latex behaves like a non-Newtonian fluid.
• Mechanical stability testing (MST). MST assessments are vital for understanding the stability of latex prior to shipping. This testing is used to determine whether the latex material is immature or starting to coagulate, which is important for understanding shelf life. ISO and ASTM standards have been developed for MST assessments of latex.
• % solids testing. % solids testing is a method for understanding the composition of latex and quantifying the total solid content of a latex material, which is crucial to understand before further processing can take place.
• KOH analysis via titration. This test method used titration to determine “the number of grams of potassium hydroxide equivalent to the acid radicals combined with ammonia in latex containing 100 g of total solids.”
• Metals analysis via ICP. This method quantifies metals found in latex materials, which is critical for understanding the composition of latex.

LATEX APPLICATIONS

Latex has a wide variety of applications, ranging from everyday items to more specialized uses.

Natural rubber latex is most commonly used to make items like gloves, swim caps, chewing gum, mattresses, catheters, rubber bands, balloons, tennis shoes, and many other sporting goods. The world’s largest consumer of natural rubber latex, however, is the tire industry.

Synthetic latexes are common in coatings, like latex paint. You’ll also find them in glues, thanks to their ability to solidify as water evaporates from the polymer particles in the latex. Synthetic latex can also be added to cement used for resurfacing and patching cracks in cement surfaces.

HAVE MORE QUESTIONS ABOUT LATEX?

ACE Laboratories is proud to have a team of in-house latex experts who can support you at any stage of the latex material and product development process. From material properties testing to research and development support, our team can help you collect the data you need to make confident decisions and stay competitive in the market.

ACE offers on-site latex dipping services for creating prototypes and test samples and specimens. We offer a full suite of testing services for both raw latex materials and prepared specimens.

Ace Laboratories is excited to answer your questions and meet your challenges. Contact us to find out what we can do for your latex.