Types of Latex Rubber
Before the Second World War, virtually all latex used in production was natural. During WW2 supplies of latex from the Far East were very restricted, hence the search for a synthetic alternative. Scientists tried to copy natural latex (derived from cis-polyisoprene) and largely failed: the latex produced was inferior to the real thing.
Instead, they developed a latex based on a synthetic polymer that behaved in a similar way. The word ‘polymer’ simply means a compound made up of long chains of molecules, each link in the chain being derived from simple chemicals known as ‘monomers’. A number of synthetic latices were developed, the most useful one being made by polymerizing Styrene (a liquid) with Butadiene (a gas) to give Styrene-Butadiene rubber, abbreviated to SBR.
Natural and synthetic rubbers have differing properties. Natural rubber is very soft and elastic, synthetic rubber gives the foam good hardness and processes better (is easier to work with) on production. Compounders tend to use blends of natural & synthetic to get the best overall properties, and to stabilize prices.
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, aminoacids). These unique ingredients are very important in explaining the behavior of natural latex.
Soaps – Potassium Oleate
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 mould. 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 mould. The mould is closed and the Talalay cycle begins. The mould is cooled and a vacuum is applied, which causes the foam to expand to fill the mould completely. A disposable paper gasket prevents latex entering the vacuum lines and a rubber gasket seals the mould 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 Dunlop Process, the foam is set by addition to the wet foam of 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 & set it.
This means that on warming up again the foam does not revert to liquid. The foam at this stage is however very weak and could not possibly be removed from the mould intact. The strength is built in during the next stage – vulcanization.
Sulphur and Vulcanization
Sulphur is added to the mix during compounding. Without sulphur 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 sulphur, 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 sulphur 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 mould 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 sulphur 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 (& 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.
Moulds and Heat transfer
Moulds 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.