Raw gum elastomer
This is the key ingredient (the one which is actually cross-linked) on which depend many of the properties of the final product. It is the therefore always at the top of the formulation list and is expressed as 100 parts by weight of the total recipe.

Sulfur
It reacts chemically with the raw gum elastomer forming cross-links between the polymer chains, resulting in a more dimensionally stable and less heat-sensitive products. Its cost is relatively low but its function is essential. It is available in different particle sizes (fineness) as rubbermakers sulfur, and can also have a small quantity of oil added to reduce its dust in the air during handling. Rubbermakers sulfur is sulfur suitable for vulcanizing rubber; it has a low ash content, low acidity and sufficient fineness for adequate dispersion and reaction. The finer particle sizes, coated with magnesium carbonate, assist its dispersion in elastomers such as nitrile. Sometimes, as the sulfur level in a compound is increased, some of it can slowly bloom to the surface. For example Heinisch mentions that sulfur levels as low as around 1 phr (at room temperature) might bloom. Blooming occurs if an additive dissolves totally in the polymer at the processing temperature but is only partially soluble on cooling collecting on the surface of the polymer mass, causing a bloom. In this case, a highly 'polymeric' (amorphous) form of sulfur, known as insoluble sulfur, is available to reduce this problem, although dispersion in the compound can be more difficult. Although bloom does not generally affect a product's performance it is aesthetically displeasing. In the uncured compound bloom can reduce tack needed in building operations(such as plying up uncured sheets of rubber to obtain thicker sheets).

Zinc oxide and stearic acid
Theses two materials, together with sulfur and accelerator, constitute the 'cure system' for the formulation. Zinc oxide reacts with stearic acid to form zinc stearate (in some cases zinc stearate is used in place of zinc oxide and stearic acid) and together with the accelerator they speed up the rate at which sulfur vulcanization occurs. With sulfur alone, the curing process might take hours. With this curing system, it can be reduced to minutes. Zinc oxide + Stearic acid --> Zinc stearate

Accelerators
The accelerator (not to be confused with a catalyst, which remains fully available at the end of a chemical reaction), is usually understood to mean an organic chemical, and as the name implies, it speeds up the rate of vulcanization. Some have a built in delay time, so that when heat is applied to the compound at the beginning of the curing process, no vulcanization(cross-linking) takes place for a specified initial period of time. They are appropriately called delayed action accelerators. An example would be the sulfonamides. This delay is highly beneficial if a compound takes a long time to completely fill a cavity in a heated mold.

Some accelerators are able to provide sulfur from their own chemical structure, so that the need for elemental sulfur might be reduced or eliminated in the formulation. They are called sulfur donors. Sulfur donors provide monosulfidic cross-links which impart improved compression set and heat resistance.

The rubber compounder also needs to consider the shelf life of chemicals during storage of the raw material prior to compounding

Other cross-linking systems
Peroxides: Peroxides are suitable for curing rubber but are not recommended for some elastomers such as IIR or CIIR. Peroxides can be used to cure many elastomers, since, unlike sulfur, they do not need unsaturated bonds in the polymer. Thus they may be used to cure ether-type polyurethanes, certain fluoroelastomers, silicones, and all of the previously mentioned saturated elastomers. Peroxides can also be used to cross-link CR.

Although not nearly as popular as sulfur, peroxides have a distinct place in rubber compounding, and are a major curative for silicone rubber. In the basic rubber compound formulation, the zinc oxide, stearic acid, sulfur and accelerator can all be replaced by a single material, the peroxide. Some care must be taken in compound formulation, to avoid unwanted interaction with peroxide. This applies, for example to antioxidant selection. Contact with oxygen (air) should be avoided during vulcanization (such as in hot air ovens or autoclave curing). Some ingredients, which are not part of the cure system, which are common in sulfur systems can interact with the peroxide in peroxide cure systems and thus interfere with cure. Use of peroxides as curing agents can confer some advantages. First an improvement in the heat aging resistance of the vulcanizate, thus upper temperature limits can be pushed up a little or the lifetime extended. Compression set is also improved. On the other hand, tensile strength, tear, tear strength, tear strength, and fatigue (dynamic deformation such as constant flexing) life are reduced. A post cure (continued cure outside of the mold) is sometimes undertaken with peroxide cured vulcanizates, to complete the cure and remove unwanted byproducts.

The cross-link density of a peroxide cured compound can be increased by addition of chemicals called coagents, of which methacrylates are a good example. This result in a higher state of cure with improvements in properties such as compression set.

Electron beam curing: Using a beam of electrons are not widely used throughout the industry and such a process has found a place in partially cross-linking components of tires, as an aid to tire production, using a radiation dosage of about four megarads.

Antioxidants, age resistors and antidegradants
In the human body free radicals (which play a part in the aging process) are neutralized by antioxidants ( in the form of some vitamins). In the same way antioxidants are also necessary to protect other organic materials, such as most elastomers' from aging. Many vulcanizates become brittle when they age. Aging can be caused by the ravages of oxygen, accelerated by heat. Antioxidants are designed to slow down this process and can act as free radical scavengers. Like accelerators, there are many antioxidants available, grouped into a number of chemical classes. The chemist also needs to be careful when choosing age resistors, for example, in light colored compounds or where the product comes into contact with a surface that can not tolerate a stain. The chemist also needs to be aware of volatility of some antioxidants (a material is not much use, if it evaporates during high temperature mixing of a compound). Some antioxidants excel in applications involving a high level of flexing of the product (anti-flex cracking antioxidants).

Fillers
This section explains why so many rubber products are black. It is much more than just putting black color into them. While the cured raw gum elastomers of NR and CR are mechanically strong, most gums are weak when vulcanized and they need reinforcing fillers. As the term implies, there is a reinforcement effect, the empirical results of which are to increase mechanical strength (for example tensile strength and resistance to tearing) in the vulcanizate, and to increase stiffness. Addition of filler increases hardness of the cured product. All fillers are not created equal, so that there is a range of reinforcement from very high to very low, corresponding to the primary size of the filler particle, from around 10 nm for very fine particle carbon blacks giving high reinforcement, to greater than 300 nm for some calcium carbonates which give low reinforcement. Use of the latter reduces compound cost. The shape and surface chemistry of the fillers particle also play an important part in reinforcement. Some popular fillers are, in order of decreasing reinforcement, carbon blacks and silicas, clays and then whitings (calcium carbonate, otherwise known as chalk).

Carbon black
This is a material of major significance to the rubber industry, so it is no surprise that most rubber products we see in the market place are black in color. We have moved a long way from collecting carbon from smoky oil flames, which produced a material called lampblack. The next historical step was to burn natural gas against iron channels, then scrape off the carbon to produce a highly reinforcing material called channel black. Both the use of this black in the rubber industry and its source of supply is currently limited and its cost is somewhat high. There are two common methods of producing carbon black today. Heating natural gas in a silica brick furnace to form hydrogen and carbon, produces a moderately reinforcing material called thermal black. Alternatively, if we incompletely burn heavy petroleum fractions, then furnace blacks are produced.