Rubber Processing: Effects of the Mixing Process on Product Properties

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  1. At Goodyear, we consider the manufacture / mixing of custom compounds a core competency and big value addition for our customers
  2. In many cases, end product rejects and defects can be attributed to inadequate mixing quality not recognized during quality control in the mixing room.
  3. An approach to reduce overall (including end products and defects) reject rate is a systematic examination of quality influences in the mixing room
  4. In analyzing the compounding process we can define the factors influencing quality:
    1. Raw material and fluctuations in specified quality parameters
    2. Storage of raw materials and their transport to the mixer
    3. The mixing of the compound itself
  5. Quality Parameters of Raw Material – Numerous, so we focus on the high users
    1. EPDM Polymers
      1. Mooney Viscosity,
      2. Ethylene or Propylene concentration
      3. type and quantity of diene monomer
      4. type and quantity of extender oil
    2. Carbon Black – most commonly used filler
      1. DBP absorption, CTAB adsorption, pellet hardness, fines content, grit, bulk density
      2. Only pellet hardness has a tendency to vary delivery to delivery, all others show little distribution relative to defined specification limits
      3. 98% of carbon black is made using the furnace process in which hydrocarbons are thermally split or incompletely incinerated
      4. Aggregates are the smallest stable structures of carbon black
        1. High Structure Blacks have a high degree of branching and linkage
        2. Low Structure Blacks have weak aggregations
      5. Aggregates form loose agglomerates –
        1. Weak, fluffy, Van Der Waals interactions
      6. Void Volumes between Aggregates and Agglomerates is measured by DBP absorption
        1. DBP absorption defines ‘Structure’
        2. Structure reflects the number of primary particles in an agglomerate and their degree of branching
      7. Specific Surface Area is defined by CTAB adsorption
        1. correlates to surface are available for the rubber
        2. Specific Surface Area also is associated with primary particle diameter
          1. smaller the diameter, the larger the specific surface area
      8. Specific Surface Area (CTAB) and Carbon Black Structure (DBP) are the most important variables for Carbon Black
      9. Generally handled as Pelletized
        1. dry, wet, or oil pelletized
        2. Better than powder, low dust loading, good flow behavior, good filling, high bulk density.
        3. Mainly Wet Pelletized blacks are offered
      10. Pellet Hardness is variable delivery by delivery and it plays a decisive role in the mixing process
        1. Must be hard enough that they are not destroyed in handling
        2. Must be Soft enough for the Dispersion Process
        3. Pellets that are too hard can cause dispersion problems, particularly in soft compounds
      11. Dispersion
        1. Ideally, agglomerates are broken down in the Mixing Process
        2. The stronger the interaction (Van Der Waal) between aggregates forming agglomerates, the higher the required forces to break them again.
        3. It has been found that large agglomerates exist after very short mixing times.
          1. Agglomerates are destroyed bit by bit through shear force
        4. 4 Stages of Dispersion
          1. Incorporation – Pellets are broken up and compacted
            1. Big Agglomerates to Small Agglomerates
            2. Polymer is incorporated into aggregates
            3. Polymer is pressed into the voids in the agglomerates.
              1. is incised into the carbon black
          2. Dispersion
          3. Dispersion and Distribution
          4. Distribution
      12. Fines – created by abrasion in transport & handling process
        1. fines content reported (although not subject to transportations’ effect)
        2. fines content can be 60% higher depending on the conveying  / transport system
        3. Increasing fines content increases filling times