1. Compounding Ingredients and Formulation Construction
1.5 Vulcanizing Chemicals
1.5.5 Crosslinking Agents
Sulfur is the most commonly used crosslinking agent for all general-purpose rubbers (NR, BR, SBR, NBR, etc.). This is because:
• Sulfur is inexpensive and widely available.
• NR and most of the low-cost synthetic rubbers contain sufficient unsaturation for crosslinks.
• For soft rubber goods 0 to 5 phr of sulfur is used depending on the amount of accelerator used and properties of the vulcanizate required. The product derived from the use of sulfur level from 5 to 15 phr results in a leathery product (practi- cally not useful). For hard rubber articles (ebonite) sulfur is used up to 25 phr.
During crosslinking of rubber with sulfur, the sulfur gets chemically bound with the rubber network in different ways.
1.5.5.1.1 Soluble and Insoluble Sulfur
• Soluble sulfur is the rhombic form of elemental sulfur and exists as a cyclic (ring) structure of eight atoms of sulfur as S8. This form of sulfur causes blooming in the vulcanizate, which reduces building tack of compounds. This form is more in use as compared to its insoluble counterpart as it is much cheaper.
• The insoluble sulfur is the amorphous form of elemental sulfur, which is insoluble in CS2 and is polymeric in nature with a molecular weight of 100,000 to 300,000.
This form of the sulfur is insoluble in most of the solvents and rubbers, and hence the name is insoluble. Because of its insolubility, it resists migration to the surface prior to cure, and hence blooming of sulfur does not take place. It is used in com- pounds where blooming will not occur to deteriorate building tack. Precautions must be taken while mixing insoluble sulfur so that the mixing temperature (dump temperature) does not exceed 100°C, otherwise this costlier insoluble sul- fur will be converted to soluble sulfur.
1.5.5.1.2 Effect of Vulcanization of Rubber with Sulfur Alone
• The rate of reaction is very slow, taking a long time.
• The cure rate varies leading to undercure or overcure—poor quality of a product.
• At optimum cure conditions the original (initial/unaged) physicals and aged physicals are far from satisfactory.
• Maximum crosslinks are polysulfidics (number of sulfur atoms is more than 2) and therefore aging characteristics are very poor.
• The vulcanizates are dark in color and show sulfur blooming.
1.5.5.1.3 The Chemistry of Accelerated-Sulfur-Vulcanization
• The accelerator reacts with sulfur to give monomeric polysulfides of the type:
AC-SX-AC.
• The polysulfides can interact with the rubber to give polysulfides of the type:
Rubber-SX-AC.
• The rubber polysulfides then react, either directly or through a reactive intermedi- ate to give crosslinks of rubber polysulfides of the type: Rubber-SX-Rubber.
1.5.5.2 Sulfur Donor Vulcanization
In this case, vulcanization is done without elemental sulfur, by the use of sulfur donor sys- tems. When the sulfur level is below 0.5 phr, it is not possible to match the modulus simply by increasing the accelerator level, unless the accelerator is a sulfur donor type. Sulfur donors liberate sulfur at vulcanization temperatures and can be directly substituted for sulfur. They can be subdivided into two types:
1. One type can act as a sulfur donor as well as an accelerator. An example is TMTD (tetramethyl thiuram disulfide).
2. The other type does not act as an accelerator and must always be used in con- junction with an accelerator. An example is DTDM (dithiodimorpholine). Because of toxicity DTDM is not used. A suitable alternative with no toxicity is available in the market. CLD 80 is one such alternative, manufactured by Rhein Chemie (Mannheim, Germany).
Generally, sulfur donors are used when the free sulfur level is reduced with the objective that the vulcanizates produced will primarily have mono- and disulfidic crosslinks and hence higher thermal and oxidative aging.
1.5.5.3 Peroxide Vulcanization
Saturated rubbers (Ethylene Propylene Rubber (EPR), silicone rubbers, etc.) cannot be crosslinked by sulfur and accelerator. Organic peroxides are useful for vulcanizing these rubbers. Peroxides are the most common crosslinking agents after the sulfur because of their ability to cure a number of diene and non-diene containing elastomers. When perox- ides decompose, free radicals are formed that, in turn, react with polymer chains to pro- duce free radicals in the polymer matrix that then combine to form crosslinks. Crosslinks of this type only involve carbon-to-carbon bonds and are quite thermally stable (very good aging resistance). The main disadvantage of the system is the higher cost involvement and products having low abrasion and tear properties.
1.5.5.4 Vulcanization with Resins
Certain di-functional compounds form crosslinks with elastomers by reacting with two polymer molecules to form a bridge. Epoxy resins are used with NBR, quinone di-oximes and phenolic resins are used with butyl rubber, and dithiols and diamines are used with fluorocarbons. One of the most important curing agents for butyl rubber is phenolic resin.
This cure system is widely used for bladders and the curing bags used in tire curing and the retread industry. The low levels of unsaturation of butyl rubber require cure activation by halogen-containing materials like SnCl2, CR, etc.
1.5.5.5 Metal Oxide Vulcanization
Polychloroprene rubber (CR or Neoprene) and chlorosulfonated polyethylene (CSM) are vulcanized with metal oxides. The reaction involves active chlorine atoms. Usually a com- bination of zinc oxide and magnesium oxide is used for the purpose of controlling the vulcanization rate and absorbing the chloride formed.
Table 1.21 provides a summary of the above discussion.
1.5.5.6 Radiation Curing
Radiation curing is a process where the final curing is being carried out at ambient tem- perature under closely controlled conditions, such as radiation dose, irradiation dose rate, penetration depth (in case of electron beam curing), etc. Radiation can produce a degree of crosslink like normal sulfur curing. The type of crosslink formed by the radiation tech- nique is mainly carbon-carbon (-C-C-), as compared to -C-Sx-C- link in the case of sul- fur cure. The -C-C- crosslink gives better thermal aging properties at high temperature.
However, the flexing properties are reduced with -C-C- crosslink. Other properties like compression set, abrasion resistance, etc., are also improved with radiation cure. Radiation cure is widely used in the tire industry.
1.5.5.7 Conventional, Semi-Efficient, and Efficient Vulcanization Systems
• For the most frequently used conventional vulcanization (CV) systems for a NR-based compound, roughly 2.0 to 3.5 phr of sulfur and 0.4 to 1.2 phr of accelera- tor are used. These typically provide high initial physical properties—tensile, tear, and good fatigue properties—but with a greater tendency to lose these properties after heat aging.
• When accelerator dosage is increased to 1.2 to 2.5 phr, less sulfur (1.0 to 1.7) is required to achieve the same crosslink density. This results in the formation of crosslink with a lower number of sulfur atoms. This system is known as the semi- efficient vulcanization (Semi EV) system and produces heat and reversion resis- tant vulcanizates.
• A further increase in accelerator dose and reduction in sulfur content (or even in the absence of elemental sulfur and in the presence of a sulfur donor) results in a sys- tem called the efficient vulcanization (EV) system. This leads to mono- and disul- fidic crosslink structures and the result is very good heat and reversion resistance.
Table 1.22 briefly illustrates the above systems in natural rubber:
• For conventional vulcanization of NR, a slightly higher sulfur dose and a some- what lower accelerator dose are employed than for synthetic rubbers.
• SBR in particular can take advantage of EV curing. If a conventional system is employed, it does not exhibit fatigue loss as observed in natural rubber, and the TABLE 1.21
Crosslinking Agents for Different Elastomers
Class Elastomers Advantage Disadvantage
Accelerated sulfur
and sulfur donor Diene; i.e., NR, SBR, BR,
EPDM Versatile Heat resistance and set
properties Peroxides Especially saturated
types like silicone, EPR Excellent heat resistance
and set properties Control of cure rate and poor fatigue resistance Resins Primarily butyl rubber Heat resistance, stable
modulus Slow cure
Metal oxides Halogenated elastomers Water resistance
average number of sulfur atoms per crosslink is significantly lower in SBR than that in NR. The use of SEV and EV systems is advantageous for SBR.
• For butyl and EPDM rubbers having very limited unsaturations, larger amounts of sulfur and accelerators are required than for other diene rubbers.