Rubber Technology: Activators, Accelerators, Ingredients and Fillers

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1. Accelerator Activators

• Inorganic compounds – mainly metal oxides-zinc oxide, hydrated lime
• Organic Acids – Normally in combination with metal oxides
  • Stearic, oleic
• Alkaline substances – will increase ph of rubber
  • Usually increases cure rate

1. Age Resistors (Antidegredants) – All rubbers are affected by aging

• Prevent or slow polymer breakdown caused by
  • Ozone, oxygen, heat, light, weather
• Chemical Protectants – 3 types
  • Amines – tend to discolor, “staining”
  • Phenolics – nonstaining
  • Phosphites – used as stabilizers for SBR
• Physical Protectants – waxy materials that bloom to surface forming a protective coat
  • Shields the part from ozone, oxygen etc.
• Effects of oxygen on elastomers
  • Softens (or usually softens) – NR, IR, IIr
  • Hardens ( or usually hardens)- CR, SBR, NBR, BR, EPDM, CSM, FKM
• PHR – usually 2-3 phr
• Amines are about 65% of the age resistors

1. Softeners (Physical plasticizers)
   1. Used as processing aid (2-10 phr) or to alter the finished part (up to 100 phr)
   2. Do not react chemically with the compound, modify physical characteristics
   3. Must be compatible with rubber and other compounding ingredients. Incompatibility will result in “bleeding” in the final product or poor processing characteristics or both.
   4. Typical for plasticizers to act a dual purpose ingredients, increase elongation, reduce hardness, improve tack; depending on the amount and type used and the rubber involved
2. Miscellaneous ingredients – used when a particular effect or property are required in a vulcanizite. For example:
   • Abrasives – Erasers, Grinding and Polishing wheels
     1. Mineral ingredients such as ground silica and pumice
   • Blowing Agents – gas-generating chemical needed for sponge and microporous rubber
     1. Release gas during vulcanization
     2. Azo compounds and carbonates
   • Colorants – inorganic pigments or organic dyes
     1. Must be stable, color-fast and inexpensive
   • Flame Retardants
     1. Chlorinated hydrocarbons, phosphate and antimony
   • Homogenizing Agents – aid in blending two or more elastomers
     1. Reduce processing time and unify blends
   • Odorants – screen out or mask odors in compounds
   • Retarders – reduce accelerator activity during processing and storage
     1. Should decompose or not interfere with accelerator during normal vulcanization
     2. Generally, these are organic acids that function by lowering ph of compound, thus retarding vulcanization

Fillers – used to extend, color, reinforce and cheapen compounds

1. Blacks
2. Clays – semi reinforcing
   1. Many types but Kaolins are used exclusively in rubber
   2. Fine Particle (<2 microns) – Hard Clays – Hard Duro imparted
   3. Soft clays lead to soft durometer
   4. Calcining (removing bound water) can impart good di-electric strength
      1. Increases cost and decreases reinforcement
   5. Silane Coupled Clays have increased modulus and tensile
   6. Duro is starting point for deciding how much phr
      1. rule of thumb is 5-6 phr hard clay or 7-8 phr soft clay
   7. Main factor to consider with clays is the reduction in cure rate.  Accelerators or Activators needed
   8. Hard clay at equal loadings to N-990 produces similar properties in Tensile, Tear and Abrasion but is deficient in Modulus and compression set.
3. Calcium carbonate – extending fillers
4. Precipitated silica (reinforcing)
   1. used alone reinforces significantly less due to its high polarity and that rubber and silica have differing polarities
   2. Therefore, organosilanes are used (organic silica compounds) to act as coupling agents between rubber and silica.
   3. Chemical Reinforcement: Covalent bonds between fillers and rubbers – Mineral Fillers
      1. Silica-Silane – the Magic Triangle
      2. Roling Resistance, Wear Resistance , Wet Grip
      3. Reduces Rolling Resistance by 25% and fuel consumption by 5% over carbon black
         1. However, Processing is more difficult because the chemical reaction must take place.  Now time, temperature are crucial.
      4. Bonding Fillers to Rubber – Bifunctional Organosilanes use two groups:
         1. Trialkoxy Silyl Group – builds stable siloxane bonds (filler modifier)
         2. Rubber Active Group – reacts with polymer during vulcanization and develops covalent filler-rubber bonds.  These bonds are responsible for the high reinforcing potential of the silica-silane filler system.
         3. Both chemical reactions have to be carefully controlled.
            1. ensure coupling agents are thoroughly mixed
            2. Also ensure a reaction with the polymer matrix doesn’t occur in mixing
   4. The highest reinforcement of non-black fillers
      1. Noted for unique combination of tear strength, abrasion resistance, adhesion, age resistance, color and economics.
      2. Manufactured by the controlled precipitation from sodium silicate with acid or alkaline earth metal salt.
      3. Particle size is controlled by the conditions of precipitation.
      4. Variety of highly to medium reinforcing grades
   5. Vary in ph from slightly acidic to slightly alkaline
   6. Since they are hydrated, they absorb ambient moisture.
5. Fumed Silicas – aka “pyrogenic’ or ‘anhydrous’
   1. smallest particle-size silicas
   2. High Priced
   3. Only really used in high quality silicone rubber products
   4. The only fillers combining high reinforcement, high-temperature stability, electrical properties, and the transparency for which silicones are known.

Compounding with Silicas

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

1. Titanium dioxide – pigmenting filler