Rubber Additives

Vulcanization Chemicals

• Vulcanization (curing) is the process in which the chains are chemically linked together to form a network, transforming the elastomeric liquid into an elastic solid. Strength and modulus increase while set and hysteresis decrease.
• Sulphur
• Peroxides
• Urethane crosslinkers
• Compounds that contain sulphur in a heat-labile form -> liberate sulphur at the vulcanization temperature (dithiodimorpholine, DTDM(I), caprolactamdisulfide, CLD(II), 2-morpholino-dithio-benzothiazole, MBSS(III), dipentamethylene thiuramtetrasulfide, DPTT(IV), tetramethyl thiuramdisufide and TMTD(VI) etc.)

Vulcanization Accelerators (CBS, TBBS, DPG etc.)

• Sulphur, by itself, is a slow vulcanizating agent. Large amounts of sulphur are necessary, high temperatures and long heating periods and one obtains an unsatisfactory crosslinking efficiency.
• To increase the rate and efficiency of sulphur crosslinking, accelerators are normally added.
• Guanidines
• Thiazoles
• Dithiocarbamates
• Xanthates
• Thiurams

Activators

• Almost all accelerators need activators for the development of their full activity
• Metal oxides (zinc oxide)
• Fatty acids (stearic acid)
• Salt of fatty acids

Vulcanization Inhibitors (PVI)

• Prevent premature vulcanization or scorching
• Chemicals based on phthalimide sulfenamides

Process Aids

• Lower viscosity and thus enable a rubber compound to be fabricated with less energy
• May also increase melt stability such that processing speed can be increased
• Two general kinds of process aid:
• chemical peptizers (renacit)
• physical plasticizers (oils)
• Chemical peptizers reduce molecular weight by increasing the rate of oxidative chain scission
• Physical plasticizers soften a compound by reducing entanglements and decreasing internal friction

Antidegradants

• Oxygen and ozone can react with elastomers and alter network structure by causing chain scission and/or crosslinking
• Antioxidants and antiozonats, which can work chemically or physically, have been developed to inhibit the action of oxygen and ozone
• Chemical protectants (6PPD, TMQ, DTPD) are capable of reacting with the degradant or interfering with the chain of reactions that otherwise would culminate in degration
• Physical protectants (various waxes) function by migrating (i.e. blooming) to the surface to provide a barrier to attack by degradants

Reinforcing Fillers

• Reinforcement is defined as the ability of fillers to increase the stiffness of unvulcanized compounds and to improve a variety of vulcanizate properties, e.g. tensile strength, abrasion resistance and tear resistance
• The reinforcement effect of a filler shows up specially in its ability to change the viscosity of a compound and also the vulcanizate properties with increasing amount of filler loading
• Fillers which only lead to small increases in the viscosity of the compound and otherwise to an worsening of the mechanical properties of the vulcanizate, are not reinforcing; they are called non-reinforcing or inactive fillers (such as calcium silicate, chalk powder and caoline)
• Fillers for which these property changing characteristics are only weakly developed are called semi-active fillers
• For a filler to cause significant reinforcement, it must posses high specific surface area; that is, the particles must be small (<= 1mm).
• Reinforcing fillers are carbon black and silica

Softeners

• Softeners are used to ease the processing
• most important mineral oils (paraffinic, aromatic)
• animal and vegetable oils (wool grease, castor oil, pine tar and soya oil)

Extenders

• Extenders are added to rubber compositions to reduce cost.
• A decrease in physical properties limits the amounts of extender that can be used.
• Oil extension of rubber, especially in tire treads, is widely practised. Oil levels of 30-40 phr are typical. Oil addition permits the use of elastomers with higher molecular weight and higher filler levels than would be possible otherwise.
• Other extender types are particulate solids, whose primary particle size is greater than 1 mm. These stiffen rubber compositions, but either have little influence on strength or diminish it (clay, calcium carbonate and ground coal).

Tackifiers (Manobond, coumarone resin etc.)

• Tack is the ability of two materials to resist separation after being in contact for a short time under light pressure
• Tackifiers are materials that are added to elastomers to improve tack
• Several types are available:
• Rosin derivatives, which are chemical mixtures of abietic and related acids that are usually esterified by polyhydric alcohols then hydrogenated, dimerized, or disproportionated to improve ageing and heat stability
• Coumarone-indene resins consisting of indene, coumarone, styrene, methyl styrene, methyl indene and other hydrocarbons, which are obtained from coal coke oven light oils
• Aliphatic petroleum resins made from unsaturates obtained while cracking crude oil.
• Terpene oligomers of a- or b-pinene obtained from pine tree stumps
• Alkyl-modified phenol-formaldehyde resins
• The function of tackifiers is twofold: to increase initial tack and to prevent tack degration, which can occur after a stock has been processed.