Internal wear of the batch mixer--part 2.

Both the Banbury mixer and the Intermix have acquired an excellent reputation as being rugged and effective compounding devices, providing years of service for mixing a broad range of both rubber and plastics compounds. Over the years, applications have emerged that have caused the rapid wearing of specific components of the mixer. New materials of construction and design changes have been necessary to assure an acceptable longevity.

With increased demands being placed on the performance of rubber and plastic products, new formulations, polymers, fillers, reactive and chemical additives have been developed. Many of these new materials and formulations are extremely abrasive to mix, and some contain potentially corrosive chemicals and emit corrosive gases during the mixing process. The compounding of these new products requires the mixer to be manufactured using specific materials of construction based on the application.

Besides the importance of selecting materials of construction that minimize mechanical wear and optimize corrosive resistance, the effect of these materials on the stick/ slip phenomenon required for efficient mixing and for a clean discharge from the batch mixer must also be considered (ref. 1). The compound must flow properly in the mixer to assure efficient mixing, and must release from the internal surfaces of the mixer when the mix is complete. Improper selection of materials of construction can have a significant adverse effect on the mixing process. It is important that, at the time of purchase of a new or rebuilt mixer, the application be carefully reviewed so the most appropriate design and materials of construction are specified.

Although not covered within the text of this article, the design of the dust and fume collection system is also important and can have a dramatic effect on the life of the mixer. It is important that generated corrosive gases be quickly and safely removed from the mixing machinery. It is equally important that the dust and fume collection system be able to capture and handle the effluents from the mixing process, but not be so aggressive that critical components of the mix, such as light fillers or minor concentrations of critical powdered chemicals, are removed from the mixing chamber.

In last month's Process Machinery column (January 2006), tribology was examined, including abrasive, adhesive, erosive, impact and corrosive-chemical wear.

The batch mixer and the mixing process

The Intermix and Banbury mixers are composed of a hopper assembly, a mixer body assembly and a bedplate assembly (figure 8).

[FIGURE 8 OMITTED]

The hopper assembly is made up of four vertical sides and is capped by a hopper cover plate at the top for containment of powders, gases and vapors (figure 9). The hopper has provision for connection to a fume and dust collection system for removal of unwanted gases and vapors generated during the mixing process.

[FIGURE 9 OMITTED]

Fume and dust removal

The hopper also contains a hinged door that opens and closes for the loading of materials to be mixed. On select sized mixers, the top of the hinged door is open to the atmosphere. Above the loading door of the hopper is provision for the placement of a hood for removal of gases and vapors. In the hopper assembly, there is a ram that can be raised or lowered pneumatically or hydraulically, forcing material added into the mixing chamber. There is a specific clearance between the ram/weight and the throat of the mixer that allows generated gases and air to be removed from the compound components as mixing progresses. The movement of the ram in the hopper caused by the circulation of material in the mixing chamber contributes to this removal process. It is quite common during the compounding operation for the ram to be raised to allow the batch to turn over. This action allows entrapped gas/fumes/volatiles contained within the mixing chamber to escape up the throat of the mixer into the fume and dust collection system. It is during this time that there is a continued surface generation of the product mix that allows an efficient removal of generated gases and vapors. Once the ram is placed down, the mixer relies on the weight-to-throat clearance for fume and vapor removal (figure 10).

[FIGURE 10 OMITTED]

It is in the hopper section of both the Banbury and Intermix where corrosive gases and vapors generated during the mixing process take their toll. Gaseous vapors generated can penetrate the mating surfaces between two adjoining parts and can attack the base metal of the mixer that is not protected by specialty coatings in these areas. The efficiency of the fume and dust collection system can have a dramatic effect on reducing the corrosion of the hopper assembly parts through speedy removal of corrosive gaseous vapors, or at least the lowering of the concentration of the corrodent. Table 2 identifies specific applications known to generate corrosive gaseous vapors or to have corrodents present during the mixing process. The identification of the corrodent is an ever changing target as new polymers and chemical additives continue to be developed. Also, the expanding use of reactive compounding applications is becoming an accepted practice.

The body assembly of the mixer is made up of two counter rotating rotors. The rotors turn in a dual bore cylindrical mixing chamber where the axis of rotation of the rotor is concentric in each rotor bore. The two cylindrical chambers are closed at either end by the rotor end plate. The rotors pass through the end plates on either side of the mixer. Each rotor is equipped with two dust stop assemblies that prevent mixing material from passing between the rotating rotors and the stationary rotor end plate. However, some generated gases can exit the mixing chamber in this area. The most common dust stop assembly has two oil circuits. One is referred to as the process oil circuit. It pumps oil between the rotor and the rotor end plate, purging any compound ingredients from entering the tight tolerance area. The second oil circuit, referred to as the lubrication circuit, delivers oil to the face of the stationary seal running against the wear ring of the rotating rotor.

The rotors of the Banbury mixer mix primarily between the rotor tips and the cylindrical body bore of the mixer. High shear mixing occurs in this area, and this is where most of the mechanical wear is seen (especially on the leading edge of the rotor tips). This type of wear is also seen in the body bore. With the ram down, the rotors wipe the surface of both the drop door and weight bottom of the hopper assembly. Some minor mechanical wear is seen on the door top and weight bottom due to compound movement in the mixing chamber. The Intermix mixes primarily between the two rotors, and most of the mechanical wear is seen distributed around the surface of the rotor. Due to the squeeze flow and axial movement of material in front to the nog, there is more wear seen on the forward face and root of the nog. Due to the aggressive pumping of the compound against the rotor end plate by the hog, mechanical wear can be seen on the face of the rotor end plate and around the nog ends.

If the product being mixed contains or develops corrosive effluent gases during the compounding, the areas typically exposed to mechanical wear are also exposed to corrosive wear.

Batch mixers are used to mix various combinations of polymers, fillers, chemicals and plasticizers in a contained mixing chamber. The objective is to obtain a composite that meets specific physical and chemical properties required for fabrication and for the final product. There is a number of mixing procedures and operating parameters that must be implemented, controlled and monitored in order to achieve an acceptable product. The type and amount of filler, and the corrosive nature of the compound components, as well as the mixing procedure and operating parameters, can affect the rate of mixer wear.

During the mixing process, all of the components are added into the mixing chamber in a specific order dictated by the mixing procedure. It is the hard abrasive fillers in their raw form in high formulation concentration levels that cause the greatest damage. If the hard fillers are added first, as with an upside down mixing procedure, the effect of the filler on the mechanical wear of the mixer is most severe. Typically, polymers such as fluoroelastomers, natural and synthetic rubbers are introduced first into the mixing chamber. In some cases, a potentially corrosive chemical peptizer may also be added to help reduce the viscosity of a high viscosity polymer to facilitate mixing and later fabrication of the final product. In addition, the polymers contain residue amounts of chemicals and trace amounts of water from the polymerization or coagulation process. Halogen containing polymers, such as fluoroelastomers, emit corrosive acidic gases (even though there usually is an acid scavenger present) as mastication occurs. The fillers such as carbon black, clays, Ti[O.sub.2] or silica may be added with the polymer or after a degree of mastication or homogenization of the polymer(s) has occurred. When first added, the fillers come in direct contact with the internal working surfaces of the mixer. It is at this point during the mixing cycle that the moving rotors create the potential for aggressive mechanical wear of the internal working surfaces of the mixer. The hardness of the contacting filler compared to the hardness of the internal working surfaces of the mixer will determine the extent of the wear. The Mohr or the Knoop hardness scales have been used to compare the hardness of various materials for both materials of construction of the mixer, as well as for the fillers that are being mixed.

The Mohr scale and Knoop scales (ref. 4) are ways of identifying the relative hardness of a specific material (whether it be the working surface metal of the mixer or the fillers added into the mixing chamber). The Mohr scale is based on the principle that a harder mineral will scratch one that is softer. The scale is based on a simple scale of one to 10, with 10 being a diamond (the hardest). The Knoop scale is based on a test where the hardness of a particular material is determined by the depth to which the Knoop indenter (a specialty tool based on a diamond) penetrates a particular material's surface. Although arbitrary, both tests can relate the relative hardness of the working surface of the internal surfaces of the mixer as compared to the hardness of the fillers being mixed. Knowing what is to be mixed, one can choose a hard surfacing alloy with equal or greater hardness. The goal is to make the mixer's working surfaces in contact with the fillers resistant to mechanical wear by the fillers. Tables 3 and 4 present hardness values for various materials. The relative hardness of fillers used for compounding purposes, as well as that of various metals, is listed in tables 3 and 4 (ref. 5).

As one can see from an examination of these charts, the relative hardness of steel (steel Mohr 5-8.5) and iron (Mohr 4-5), as compared to the fillers being mixed, can have a dramatic effect on wear. The proper selection of the materials of construction for the working surfaces of the mixer is paramount.

Table 5 presents a few applications where mechanical wear is prevalent.

As mixing progresses, the polymer begins to melt or soften and the fillers are drawn into the polymer matrix, beginning the process of incorporation. It is at this point that the fillers are driven into and around the polymer matrix. As the incorporation progresses, less free filler is directly exposed to the working surface of the mixer, decreasing the exposure of the raw fillers directly on the working surface of the mixer.

As mixing progresses, the temperature of the polymer mix is increasing. In many cases, corrosive gases are being generated by the mixing process, and water that was entrapped within the polymer and absorbed on the surface of the fillers is driven out of the mixing compound. Ideally, with a good fume and dust collection system, most of these gases and vapors can be removed. However, in reality, some of the water vapor and corrosive gases condense in and around the mixing chamber and hopper of the mixer. In these areas, there is a significant potential for both oxidative and chemical attack of the mixer. Both the base metals and the hard surface coating are exposed to the potentially corrosive environment.

Once the mix is complete, the discharge door is opened and product is discharged. It is at this point in the cycle that the release characteristics of the internal metal surfaces of the mixer are obvious. If the internal metal surface of the mixer does not provide release of the product mix (either because of its own polymer release characteristic or its inability to maintain a specific metal temperature due to its heat transfer characteristics), the batch will not discharge cleanly and rapidly from the mixer. The product will either hang up within the mixing chamber or on the door top, as pictured in figure 11.

[FIGURE 11 OMITTED]

This article will be concluded in next month's Rubber World (April 2006), where wear observed in the mixer will be examined, including abrasive, corrosive-chemical, mechanical and impact types of wear. Wear protection options that are currently available will be reviewed. Conclusions will be drawn for extending the usable life of the mixer.

References

(1.) "Effect of full-slip condition along rotor in the mixing efficiency of internal mixers, " Antec 2003, University of Louvain & Michelin Clermont-Ferrand, France.

(2.) Merriam Webster On-Line.

(3.) Elements of Physical Metallurgy, A.G. Guy Addison, Wesley Publishing.

(4.) Friction and Wear of Engineering Materials, I.M. Hutchings, 1992.

(5.) Handbook of Chemistry and Physics, The Chemical Rubber Co., 2310 Superior Ave., Cleveland, OH.

(6.) Dictionary of Scientific and Technical Terms, McGraw-Hill.

Table 2--typical corrosive applications

Alloying fluorocarbon polymers

Compounding halogen containing polymers

Mixing-flame retardant compounds

PVC compounding applications

Compounding flame retardant formulations

Dewatering applications

Table 3--Mohr and Knoop scale
comparison

Mohr     Knoop    Material                            Other
value    value    description                         comparisons

   1       --     Talc
                  (oxide of magnesium silicate)

   2       32     Gypsum
                  (hydrated calcium sulphate)

  --       37     Cadmium

  --       60     Silver

   3      135     Calcite (hydrated silica)

   4      163     Fluorite

  --      163     Copper

  --      370     Magnesia

   5      430     Apatite (calcium phosphate)

  --      557     Nickel                              Carbon steel
                                                      (Mohr 5.0 to 8.5)

  --      530     Glass

   6      560     Feldspar (aluminum silicates)       Nitrided steels
                                                      (6 to 7)-
                                                      Ti[O.sub.2]

   7      820     Quartz (silicone dioxide-silica)    Similar to tool
                                                      steels

  --      935     Chromium

  --    1,160     Zirconia

  --    1,250     Beryllia

   8    1,340     Topaz (hydrous
                  aluminum fluorosilicate)

  --    1,360     Garnet

  --    1,400-    Tungsten carbide alloy
        1,800

  --    1,550     Zirconium boride
   9       --     Corundum                            Ruby or sapphire
                  (aluminum fluorosilicate)           aluminum oxides

   9    1,800     Titanium nitride

  --    1,880     Tungsten carbide

  --    2,000     Tantalum carbide

  --    2,100     Zirconium carbide

  --    2,100     Alumina

  --    2,410     Beryllium carbide

  --    2,470     Titanium carbide

  --    2,480     Silicone carbide

  --    2,500     Aluminum boride

  --    2,750     Boron carbide

  10    7,000     Diamond (carbon)

Table 4--hardness of fillers mixed in
batch mixers (ref. 3)

Mohr #       Knoop #      Filler description

3            135          Calcium carbonate/limestone
3.5-7.5                   Silica
1                         Carbon black
6                         Glass fibers
g                         Aluminum oxide
6-7                       Ti[O.sub.2]
3.5                       Barium sulfate
3.0                       Clay (kaolin)
3.0                       Mica
4.5                       Wollastonite (CaSi[O.sub.3])
3.5                       Aluminum hydroxide

Table 5--applications where mechanical
wear is prevalent

Application                    Abrasive agent

Floor tile                     Limestone (Ca[Co.sub.3]) and quartz
Specialty tire compounding     Silica (Si[O.sub.2])
  application
Color concentrates             Titanium dioxide Ti[O.sub.3]
Synthetic magnets flame        Ferrite and oxide powder
  retardant applications        (aluminum hydroxide)