Compounding

Compounding

1. Vulcanization and Curing
Vulcanization or curing is the process in which the chains are chemically linked together to form a network, thereby transforming the material from a viscous liquid to a tough elastic solid. Strength and modulus increase, while set and hysteresis decrease. Various curing systems are used to vulcanize different types of elastomers,
1.1 Sulfur Curing
The most widely used vulcanizing agent is sulfur. For sulfur to effectively crosslink a rubber, an elastomer must contain double bonds. General purpose diene elastomers such as BR, SBR, NR, and IR meet this basic requirement.
Two forms of sulfur are used in vulcanization: soluble (crystals of S8 rings) and insoluble (amorphous, polymeric sulfur). Sometimes, in compounds containing high levels of sulfur, insoluble sulfur is used to prevent sulfur blooming, a process by which the sulfur migrates to the surface of a compound and crystallizes there. Blooming can occur when large amounts of soluble sulfur are used, because at high mixing temperatures, the solubility of S8 is high, enabling large amounts to dissolve, but upon cooling the solubility decreases. When the solubility limit is reached, excess sulfur blooms to the surface. Sulfur bloom reduces the "tack" of a rubber compound, a necessary property if layers of rubber are to be plied up to make a composite structure, such as a tire. Insoluble sulfur does not bloom because it disperses in rubber as discrete particles, which cannot readily diffuse through the rubber. However, above 120°C, insoluble sulfur transforms into soluble sulfur. Thus, mixing temperatures must be kept below 120°C to take advantage of the bloom resistance of insoluble sulfur.
Crosslinking with sulfur alone is quite inefficient and requires curing times of several hours. For every crosslink, 40 to 55 sulfur atoms are combined with the rubber. The structure contains polysulfide linkages. Much of the sulfur is not involved in crosslinks between chains. Moreover, such networks are unstable and have poor aging resistance.
To increase the rate and efficiency of sulfur crosslinking, accelerators are normally added. These are organic bases
Often, a combination of accelerators is used to obtain the desired scorch resistance and cure rate. Generally, if two accelerators of the same type are combined, then cure characteristics are approximately the average of those for each accelerator alone. However, there is no general rule when combining accelerators of different types. Moreover, the type of accelerator is much more important than the level of accelerator in controlling scorch time. Although increased levels of accelerator increase the degree of crosslinking attained, generally accelerator concentration has only a small effect on scorch time.
Accelerated sulfur curing is more efficient when the activators zinc oxide and stearic acid are added. It is thought that these additives combine to create soluble zinc ions that activate intermediate reactions involved in crosslink formation.
1.2 Influence of Crosslink Density
Mechanical properties of an elastomer depend strongly on crosslink density. Modulus and hardness increase monotonically with increasing crosslink density, and the material becomes more elastic, less hysteretic. Fracture properties, such as tear and tensile strength, pass through a maximum as crosslinking is increased. To understand this behavior, it is helpful first to consider fracture in an uncrosslinked elastomer, and then to discuss changes in the mechanism of fracture as crosslinks are introduced.
When an uncrosslinked elastomer is stressed, chains may readily slide past one another and disentangle. At slow rates, fracture occurs at low stresses by viscous flow without breaking chemical bonds. The effect of a few crosslinks is to increase the molecular weight and creating branched molecules. It is more difficult for these branched molecules to disentangle and hence, strength increases. As crosslinking is increased further, the gel point is eventually reached when a three dimensional network forms. A gel cannot be fractured without breaking chemical bonds. Thus, strength is higher at the gel point, because chemical bonds must be ruptured to create fracture surface. However, strength does not increase indefinitely with more crosslinking.
When an elastomer is deformed by an external force, part of the input energy is stored elastically in the chains and is available (released upon crack growth) as a driving force for fracture. The remainder of the energy is dissipated through molecular motions into heat, and in this manner, is made unavailable to break chains. At high crosslink levels, chain motions become restricted, and the "tight" network is incapable of dissipating much energy. This results in relatively easy, brittle fracture at low elongation. Crosslink levels must be high enough to prevent failure by viscous flow, but low enough to avoid brittle failure.
Both the level and type of crosslinking are important. When curing with sulfur, the type of crosslinks depends on (1) sulfur level (2) accelerator type (3) accelerator/sulfur ratio and (4) cure time. Generally, high accelerator/sulfur ratio and longer cure time increase the number of monosulfidic linkages at the expense of polysulfidic ones. Vulcanizates containing predominately monosulfidic crosslinks have better heat stability, set resistance, and reversion resistance than those with polysulfidic links. This is attributed to greater stability of C-S bonds compared to S-S bonds. On the other hand, compounds containing a high proportion of polysulfidic crosslinks possess greater tensile strength and fatigue cracking resistance compared to compositions with monosulfidic links. This is thought to be due to the ability of S-S bonds in polysulfidic linkages to break reversibly, thereby relieving locally high stresses that could initiate failure.