Adding silicas tends to increase viscosity quicker than other fillers.

- Dec 11, 2019-

Adding silicas tends to increase viscosity quicker than other fillers.
This is especially with high surface area (small particle size- hard) silicas
The most frequently used accelerator systems are deactivated by silicas
both optimum cure and rate of cure are reduced
Early addition of zinc oxide and silica results in lower Mooney Viscosity, greater die swell and lower modulus.
Withholding zinc oxide until the silica is dispersed raises viscosity.
Certain activators used with silica-filled rubber reduces viscosity greatly
Diethylene glycol (DEG) and Triethanolamine (TEA) at 2 phr can reduce viscosity by 30%
Polyethylene glycol (PEG) is only half as effective
Viscosity can also be reduced by using hydrocarbon process oils
In Natural Rubber with Silicas, Plasticizers of vegetable origin have special characteristics, 5 phr of Tall Oil is equivalent to 30 phr of napthenic oil.
In SBR, aromatic resins have advantages with silica fillers.
Coumarone-indene resins aid in incorporation rate and dispersion
Tensile, tear, abrasion and extrusion smoothness are superior with 10-20 phr aromatic resin as process oil
Curing Systems
The surface chemistry of precipitated silicas differ significantly from other fillers.  This leads to a unique set of compounding variations.  Sulfur-cured rubbers containing precipitated silica must be modified to obtain optimum performance.
With fine particle silicas above about 20phr, diethylene glycol (DEG) or polyethylene glycol (DEG) reduces accelerator requirements.  The glycols also buffer the variable moisture and are an economic advantage.
Efficient Vulcanization systems (EV) rely on sulfur donors or accelerators with available sulfur and little or no elemental sulfur.
General guidelines:
Combine two or more accelerators, one from the thiazoles or sulfonamides, the second from the guanidines, thiurams, or dithiocarbamates
Add glycol activators (TEA, DEG, PEG) to lessen accelerator demand and buffer variable moisture
Sulfur Donor (EV) systems that provide efficient crosslinking, property optimization and heat-aging
Vulcanazite Properties – assume good state of cure
low modulus compared to carbon black in similar particle size
Tensile strength is similar in above
Higher elongation at break and lower resistance to abrasion
Resistance to Abrasion increases with surface area
High Tear is outstanding with silica and it increases with increasing surface area
Rubber filled with small-particle silicas tears in an irregular “saw-tooth” process often called ‘knotty tear’.
Important that optimum state of cure is reached
Reinforcement is enhanced with coupling agents
mercaptopropyltrimethoxysilane sis the most effective
produces strong adhesion between silica and rubber, resulting to substantial increase in reinforcement
With silanes, it is important to allow the silica, rubber and silane to mix thoroughly before adding other ingredients
It is particularly important that zinc oxide not be present during this phase
Rather, add zinc oxide late in 1st stage or in 2nd stage to avoid interference with coupling action
Only ingredients essential for processibility (plasticizers) be present during early mixing cycle
Silicone-Rubber Compounding
Three types of silica used:
Fumed – highest strength and best electrical properties
Precipitated – moderately good tensile strength, tear strength, and water absorption and electrical properties at an intermediate cost.
Special grades offer better electrical properties and reduced water absorption
Water Volume swell after 24 hrs is 1-2% with fumed and 5-10% with precipitated