When natural rubber is tapped from a tree it is very dilute, the rubber content being only about 30%. It has to be concentrated before use to above 61.5% solids. Of these solids 60.0% is rubber, the remaining 1.5% are compounds that are unique to natural latex (proteins, phospholipids, carbohydrates, amino acids). These unique ingredients are very important in explaining the behavior of natural latex.
Fatty Acid Soaps
This stabilizes the mix, i.e. it prevents it from coagulating until we are ready for it to do so, when the foam is in the mold. Soaps also assist the latex mixture to foam up when air is introduced in the foaming machine.
The latex compound is foamed up to the correct foam density, then the required amount metered into the mold. The mold is closed and the Talalay cycle begins. The mold is cooled and a vacuum is applied, which causes the foam to expand to fill the mold completely. A disposable paper gasket prevents latex entering the vacuum lines and a rubber gasket seals the mold from the outside world.
This is the key step in the foam making process. It is at this point that a phase change occurs and liquid foam becomes “solid” foam, and the foam sets or “gels.” In the original (standard) Dunlop Process, the foam is set by adding to the wet foam a small amount of gelling agent (sodium silicofluoride or SSF). In the Talalay process the foam is frozen at 0°F then carbon dioxide gas (an acidic gas) is passed through the foam to lower its pH and set the foam.
This whole process is done so that when the foam begins to warm up again, it does not revert to a liquid. The foam at this stage is however very weak and could not possibly be removed from the mold intact. The strength is built in during the next stage – vulcanization.
Sulfur and Vulcanization
Sulfur is added to the mix during compounding. Without sulfur in the production process, the foam would resemble chewing gum and would have little resilience. The double bonds in the rubber molecule are utilized by sulfur, which forms bridges with adjacent molecules, known as cross-linking. This process gives the product its familiar properties of elasticity and resilience.
The process of heating rubber with sulfur is called vulcanization or “curing,” and was discovered by Charles Goodyear in 1839. This is a fairly slow process, even at a temperature of +240°F so certain accelerators are required in the production process to make this happen quickly. A very small addition of these reduces the time required for curing from about 25 minutes to about 8 minutes. At the end of this time the mold is cooled, opened, and the product is removed and sent to the washer.
This removes soaps, ammonia and anything else water soluble, which have served their purpose and are no longer required or desirable. If they were not removed they would contribute to discoloration, odor and could leave the product feeling tacky.
This removes all water from the block and completes the vulcanization process, thus giving the product satisfactory physical properties (compression set, tensile strength, elongation at break, pounding and indentation set). The dried products then arrive at Inspection for weighing, hardness checks and grading.
Any double bonds in the rubber which are not used up by the sulfur are at risk from attack by oxygen and ozone in the atmosphere, particularly when catalyzed by the presence of UV light. This is why latex will deteriorate in sunlight. A small amount of ‘antioxidant’ is added to the latex during compounding. This is a substance which is preferentially oxidized (and therefore sacrificed), thus affording some protection to the rubber. Eventually however it becomes depleted and deterioration of the rubber then occurs. Latex foam must never be cleaned with solvents (dry-cleaning): this would remove any antioxidant completely; deterioration would then be very rapid.
Molds and Heat Transfer
Molds are made from aluminum (very good heat transfer properties) and are hollow, with channels within their walls so that a heat transfer fluid can circulate through them.
Since latex foam is a very poor conductor of heat a large number of “pins” are present to enable heat/cold to get into the heart of the foam. The resulting pinholes then play another very useful role in that they make it much easier to remove moisture during the drying process.
In the end, natural latex is still a foam, but an almost perfectly natural one:
1. Natural Latex Rubber 90-95%
2. Zinc Oxide 2- 3%
3. Fatty Acid Soaps 1-2%
4. Sulfur 1-2%
5. Sodium 1-2%
Item 1 is pure, natural rubber harvested exclusively from the “Hevea Brasiliensis” tree, which grows primarily in South-East Asia. Ours is from Sri Lanka.
Items 2 thru 5 are essential to the vulcanization, foaming and curing process that all latex cores must go though. The finished core is then washed a minimum of 3 times to remove any residuals that may be left over after curing.
Things to Consider
You will note from the above information that all the ingredients in our 100% natural latex mattresses are natural – this is why it is listed as 100% natural. You should also note that NO fillers are used in these mattresses. It is very common for other sellers and manufacturers to use clay and other natural fillers in a 100% natural latex mattress, these ingredients make the mattress less costly to manufacture but also hinder the quality, comfort and longevity of the mattress as well.
While the term “natural latex” has been used throughout this article, since organic latex is made of natural latex, the process above also applies to organic latex.
The process above outlines the making of any natural latex foam product, both Dunlop and Talalay processes, however one can get more detailed in explaining the two processes. I have done exactly that in the article here Dunlop vs. Talalay. While our 3″ organic latex slab product says it all, to note it here, we sell Dunlop processed latex.