The Cure System

Selection of the CURE SYSTEM involves many of the same considerations involved in the selection of the elastomer.  While cost is a consideration, it is usually secondary compared to the importance of obtaining the desired rate and state of cure, since “cure” is the process which gives the elastomer its desired physical properties.

Cure is often called vulcanization or cross-linking.  It is an intermolecular reaction caused by the introduction of chemicals (usually sulfur and zinc oxide or Morfax) which link or tie independent chain molecules together causing the polymer to form molecular networks.  These chemical cross-links between polymer chains may be chains of sulfur atoms, single sulfur atoms, carbon to carbon bonds, polyvalent organic radicals, or polyvalent metal ions.

  The critical parameters relating to the curing process are: (1) the time elapsed before the curing process starts, (2) the rate at which the process occurs, and (3) the extent to which the cross-linking occurs.  There must be sufficient time before the process begins to allow the mixing of all of the ingredients of the rubber compound, the forming of the ultimate product and the molding.  Thereafter, the process should be rapid, but controlled.  Accelerators (such as zinc oxide) reduce the time required for cure while at the same time resisting premature vulcanization (called scorch).  Retarders, on the other hand, inhibit cross-linking or delay cure.  It is desirable for a retarder to inhibit cross-linking at processing temperatures, but not at curing temperatures, thus providing more processing time without affecting the cure rate.  It is the compounder's task to choose the proper cure system ingredients to accommodate the time required for mixing, forming, molding, and processing the selected polymer into the product or component being manufactured, while at the same time minimizing the time necessary to cure the material to its optimal physical properties.  A strong background in chemistry (preferably rubber chemistry) is a highly desirable qualification for a compounder.

  Fillers

Apart from the polymer and the cure system, CARBON BLACK is the most important ingredient which can alter the properties of a rubber compound.  Carbon black is a FILLER which  can actually assist in the curing of the compound, reducing the need for curatives and cure time because of its activating effect.  Carbon black also serves as reinforcement and imparts abrasion resistance to all rubbers.

The particle size of the carbon black determines its abrasion resistance.  The smaller the particle, the greater the resistance of the polymer to tread wear during moderate service.  However, carbon black with smaller particle size is more expensive, and the smaller-size blacks require more work on the rubber batch to achieve adequate dispersion of ingredients than coarser blacks.  The smaller blacks also generate more heat during mixing which can lead to scorch (premature vulcanization).  Though different elastomers have widely different molecular structures, the incorporation of fillers (specifically carbon black) in appropriate amounts greatly reduces such differences in both cured and uncured stock.  This has generally been referred to as the leveling action of carbon black.

  Pigments

Other fillers, generally referred to as non-black PIGMENTS, are divided into three groups:  reinforcing, non-reinforcing, and clays.  The introduction of reinforcing pigments such as Hi-Sil (hydrated silica) produces a dry, boardy stock.  Non-reinforcing pigments such as titanium dioxide are used for extreme whiteness or to mask the inherent color of the rubber or other ingredients.  Clays are generally regarded as slightly reinforcing pigments and are used to aid in extrusion and to improve abrasion resistance.  They also reduce mold shrinkage more than any other known material and are effective in reducing air or gas permeability.

  Processing Aids

PROCESSING AIDS generally consist of plasticizers, extenders, and other oils which aid the mixing, extruding, and calendering processes.  They contribute flexibility at ambient temperatures, increasing the softness or reducing the stiffness of the raw stock with only modest changes in other physical properties.  Processing aids may be either completely or partially soluble.  Some are soluble when the stock is hot and bleed to the surface when the stock cools.  Waxes used for ozone resistance fall into this category.  As the wax migrates (or blooms) to the surface, it forms a protective coating which acts as a barrier against ozone attack (oxidation).

  As stated previously, the most important compounding ingredient in a high quality rubber product is the polymer, followed closely by the curing system.  The carbon black is the next most important ingredient.  Selection of the remainder of the components is important, but generally not critical.  Cost, in all cases, must be weighed against the intended function of the product, public safety issues, and the competitive environment in which the product is marketed.

Tire Manufacture

The Sumerians invented the wheel over 5000 years ago.  R.W. Thomson invented the tire in 1846 - though he called it “an air tube device” in his patent, and his invention went unnoticed until 1888.  We've come a long way in slightly more than 100 years, but there are still only three basic tire types:

•THE BIAS OR CROSS-PLY TIRE which is constructed with reinforcing cords extending diagonally across the tire from bead to bead in successive layers called plies;

•THE BIAS BELTED TIRE which is constructed of a bias-angle carcass and a restricting belt around the circumference under the tread; and

•THE RADIAL TIRE which has plies of reinforcing cords extending across the tire from bead to bead, overlaid by a belt composed of several more layers of cord.

All tires consists of the following components:

Diagram

The Tread, which is the wear resistant component consisting of ribs designed for noise suppression and traction and grooves designed for traction, directional control, and cool running;

The Sidewalls, which are the portions of the tire between the beads and the tread compounded of rubber with high flex and weather resistance to control the ride and provide support;

The Shoulder, which is the upper portion of the sidewall just below the tread edge and affects tire heat behavior and cornering characteristics;

The Bead, which is a structure composed of high tensile strength steel wire formed into hoops which function as anchors for the plies and hold the tire assembly onto the rim of the wheel;

The Plies, which are layers of fabric cord extending from bead to bead to reinforce the tire;

The Belts, which are narrow layers of coated tire cord or rubber encased steel cords located directly under the tread in the crown of the tire to resist deformation in the footprint (i.e., the tire's contact patch on the road) and to restrict the carcass plies;

The Liner, which is a thin layer of rubber inside the tire which contains compressed air; and

The Chafer, which consists of narrow strips of material around the outside of the bead that protect the cord against wear and cutting by the rim, distributes flex above the rim, and prevents dirt and moisture from getting into the tire.

Tire manufacturing is still a very labor intensive process.  It consists of:

• Mixing the elastomers, carbon blacks, and other chemicals to form the rubber compound;

•Fabricating the various fabrics and Coating them with rubber in a calender;

•Tubing the rubber treads and sidewalls by extruding the rubber from a tuber;

•Assembling all of those components (i.e., the calendered and cut carcasses and belts, and the extruded tread, sidewall, and beads) on a tire building machine;

•Curing the assembled tire under heat and pressure (called vulcanization);

•Inflating the tire; and

•Finishing it (which involves trimming, buffing, balancing, and inspecting).

  Prior to vulcanization (cure), the rubber components are in their “green” (uncured) state.  As stated above, the tire is built or assembled at the tire building machine in the green state.  Basically tire building involves the application of plies and other components onto a cylindrical drum or barrel, commencing with the inner liner.  Next, the beads are set in place, and the plies are turned up around the beads.  In the case of a belted tire, the belts are applied next, followed by the tread and sidewall.  Then, the drum is collapsed, and the tire is removed and loaded onto an automatic tire press for vulcanization at high temperatures and pressures.

  After cure, the tire is mounted on a rim and permitted to cool while being inflated.  Thereafter, it is finished and shipped to a warehouse for distribution.